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Crosslinked Polymers
Chapter 15
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Crosslinked Polymers R.V. Law ~ and D.C. Sherrington 2 1Department of Chemistry, Imperial College of Science, Technology and Medicine, London, SW 7 2AZ, UK; 2D.C. Sherrington, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, UK
15.1
Introduction
Crosslinked polymers are key materials in a wide range of technological applications ranging from the "high-tech" aerospace area to the "low-tech" use of wood-derived products. Inevitably, the bulk properties and materials performance are controlled in the final analysis by the polymer molecular structure, although to relate the backbone structure to performance in applications is a complex exercise. This often involves the requirement for numerous intermediate correlations e.g., molecular structure with micromorphology, micromorphology with phase separation, etc. Evaluating the molecular structure of linear polymers has become relatively straightforward in recent years and solution phase ~H and ~3C NMR techniques have played an important role in this context. When linear polymers are crosslinked to form an infinite network, either during the polymerisation process, or as a post-polymerisation chemical treatment, then analysis by routine solution NMR spectroscopy rapidly becomes impossible as a result of signal broadening. When the level of crosslinking remains low, solution phase methodology applied to highly solvent swollen polymer can still be useful (see Sections 4.3 and 4.4). In general though, most crosslinked polymers are relatively highly crosslinked, and become amenable to analysis by NMR spectroscopy only by using solid phase techniques.
15.2 Solid-state ~3C and ~SN cross-polarisation/magic-angle spinning (CP/MAS) NMR 15.2.1
Background
Despite the obvious benefits of quantitative solid-state NMR via single pulse excitation methodology (SPE) (see Section 3), cross-polarisation combined with magic-angle spinning (CP/MAS) [1] has many intrinsic advantages which
510
R.V. LAW AND D.C. SHERRINGTON
have proved very valuable in evaluating the structure of crosslinked polymers. The principle benefit of CP is that it enables a spectrum to be acquired very quickly, as more scans can be obtained in any given time, giving a better signal-to-noise (S/N) ratio. The increase in S/N ratio is due to the fact that in many organic materials the proton spin-lattice relaxation mechanism, by which the system can relax back to equilibrium, is approximately an order of magnitude shorter that for 13-carbon. Though it is desirable to use SPE it is often impractical to carry out a fully quantitative analysis because experimental conditions e.g., MAS at high temperatures, means that there is limited time available and therefore a compromise, using only CP, has often to be made. If time permits an ideal approach is to use a combination of both SPE and CP as these techniques complement each other well (see Section 3). The other advantage that CP brings is important information about the molecular dynamics within the polymer from CP variable contact studies. The parameters typically obtained include the cross relaxation time between 1H and 13C, TcH, and the relaxation time in the rotating frame, ~H Tip. Furthermore, there is also a large (--~ factor 4 for 13C) sensitivity enhancement for ~3C when carrying out CP as polarisation is transferred from the isotopically abundant 1H (100%) to the rare 13C (1.1%). This section will deal principally with crosslinked polymers which have been characterised with CP combined with MAS. Earlier monographs and reviews have dealt, in part, with the application of CP/MAS to crosslinked polymer systems. These include reviews by Yu and Guo [2], Andreis and Koenig [3], books by Komorski [4], Mathias [5], McBrierty and Packer [6], Ibbett [7], Bovey and Mirau [8] and the annual reports by Webb [9]. This section will focus on more recent papers dealing with the application of solid-state CP/MAS NMR spectroscopy to the analysis crosslinked polymer systems.
15.2.2
Phenol-formaldehyde resins
Phenol-formaldehyde (PF) resins have been used as model compounds for the study of pyrolysis and combustion reactions that occur in solid fuels [10]. Utilising these resins it is possible to incorporate a wide range of heteroatomic and hydrocarbon moieties to simulate compounds that arise naturally in the solid fuels. A series of phenol resins crosslinked with thiophene, dibenzothiophene, diphenylsulfide, benzyl phenyl sulfide, thioanisole, 8-hydroxyquinoline and 2-hydroxycarbazole were synthesised. These samples were then cured at 200~ (Fig. 15.2.1) and the resulting resins examined by solid-state NMR spectroscopy. The ~3C CP/MAS spectra of a standard PF resin is shown
CROSSLINKED POLYMERS
511
OH '---'- S ~
OH
Excess CH20 "OH
/
CH20H
S
CH2
Curing
/
CH2
S
CH, -
Fig. 15.2.1. Incorporation of diphenyl sulfide into the structure of the co-resite and the resultant
resite after curing.
in Fig. 15.2.2. This was compared to the partially and fully cured resins. (Figs. 15.2.3 and 15.2.4). By curing the standard resins at increasing temperatures (up to 200~ it was possible to show that peaks at 35 and 70 ppm, attributable to methylene and alkyl ether bridges respectively, were converted from the ether to methylene bridges at the final cure temperature. The aryl ether peak at 160 ppm possibly arose from the condensation of two phenolic moieties. These resins were studied in order to obtain the optimum cure conditions. The sulphurcontaining resins show a wide range of ether, ethylene and methylol constituents. The peaks at 38, 55-80 and 90 ppm arise from methylene and methylol carbons attached to the ortho and para positions of the phenol ring and hemiacetal carbons, respectively. Fully cured resins contain only aliphatic peaks at 18 and 38 ppm with no remaining alkyl ether linkages. The aryl ether peak at 160 ppm also increases slightly in intensity. A major side reaction that was
512
R.V. L A W A N D D.C. S H E R R I N G T O N
't
j .......
I
. . . .
200
90~ (48 hours) ~._
I. . . . .
100 PPM
-
--I
. . . . . .
0
Fig. 15.2.2. Solid state 13C C P / M A S N M R spectra of a normal PF resin after various cure periods. SB = spinning side band.
identified for both the normal and sulfur containing resins was the formation of an arylmethyl moiety which gave a peak at 18 ppm. Understanding the curing process in PF resins and the analogues employed in this work gave a useful insight into the mechanisms that occur in solid fuels. Though useful as model compounds, PF resins have many commercial uses in their own rights, these include thermal insulation, mouldings, and use as wood resins. Therefore the systematic understanding of their molecular structure gives an insight into their physical properties. One important physical property is the resistance of the resin towards acid and bases; understanding this enables a better approach to the correct formulation of the resins.
513
CROSSLINKED POLYMERS
(a)
(c)
200
1SO
leo PPH
50
0
Fig. 15.2.3. Solid state 13C CP/MAS NMR spectra of the partially cured (130~ containing (a) dibenzothiophene (b) thioanisole (c) phenyl benzyl sulfide.
resites
This has been studied by 13C CP/MAS NMR where the degradation of PF resins in the presence of acid (oxidising and nonoxidising), base and formalin [11] has been monitored (Fig. 15.2.5). The main structural components believed to be present in the cured resin are shown in Fig. 15.2.6. The proportions of each species depends upon the initial P : F ratio, p H value, catalyst and temperature. At relatively low concentrations (---1 M) exposure to acid and base simply leads to neutralisation or formation of the phenoxide salt of the resin. This is shown by the change in intensity of the peak at ca. 152 ppm. Also in the presence of alkali, peaks at 73 ppm disappeared giving a proportional rise in intensity at 65 ppm. This was explained by the cleavage of dimethylene ether linkages to produce the corresponding methylol groups. Treatment with formalin also produced methylol groups at positions ortho or para with respect to the phenolic linkage, producing peaks
514
R.V. LAW A N D D.C. S H E R R I N G T O N
(b)
(c)
(d)
200
150
I00 PPM
50
0
Fig. 15.2.4. Solid state 13C CP/MAS NMR spectra of the fully cured (200~ resites containing
(a) dibenzothiophene (b) phenyl benzyl sulfide (c) diphenyl sulfide (d) thioanisole.
at 65 ppm. There was also evidence, from the presence of peaks at 40 ppm, of the formylation of p,p'-methylene linkages formed. Under stronger nonoxidising acidic conditions (sulphuric acid 36 N) (Fig. 15.2.7) all of the methylol and dimethylene ether linkages are cleaved. Some of this cleavage gives rise to CHO (194ppm) and CH3 (18ppm) moieties. There is also evidence that there is sulphonation of the aromatic ring in the ortho and para position to the phenolic hydroxide group (ca. 152 ppm). The use of strong oxidising acid (15 N nitric acid) brought about more major structural changes
CROSSLINKED P O L Y M E R S
515
Phenol i c ring carbons
herl•rbons /
~
~C_H2OAr'
ArCHO
ArCH2Ar'
b
C 9
i"~'~'T
200
,"'i
1
'
150
'
'
i 1 "" 100
'
'
I
50
""'~ ' I 0
'
PPM
Fig. 15.2.5. 15.1 MHz 13C CP/MAS N M R spectra of (a) resole type resin (b) cured resin after treatment with 1.0 N sodium hydroxide under N2(g) for 65~ for 3 days, and (c) cured PF resin after treatment with 36.8% formal under N2(g) at 25~ for 1 day.
(Fig. 15.2.8). These included the conversion of phenolic rings into cyclic ketones and nitration of the rings. Methylol and methyl groups are also similarly nitrated. 15.2.3
Melamine-formaldehyde resins
These are common materials that have found many applications e.g., as hard, durable and chemically resistant surfaces. A series of uncured and cured
516
R.V. LAW AND D.C. SHERRINGTON OH
OH
OH
0
~
OH
Fig. 15.2.6. Main moieties present in a PF cured resin.
melamine-formaldehyde (M-F) resins have been examined by solution state (1H, 13C) and solid-state (13C, 15N) NMR respectively [12]. The resins were either laboratory synthesised or obtained from an industrial manufacturing process. The solid-state NMR studies involved four resins, two laboratory synthesised and two obtained from industry, all of which had different M : F ratios. The 13C solid-state NMR spectra of the synthetic resins are shown in Fig. 15.2.9. The peak at 166 ppm in readily assigned to azine ring carbons. The more interesting region, however, is the methylene region (40-60 ppm) in which there are five components at 48, 52, 58, 66, and 72 ppm. These have been assigned to the following structures,
--NHCHzNH m,
=NCH2NH--,
--NCH2N=,
mNHCH2OCH2~,
This indicated that resins synthesised with M ' F ratios of 1.0" 1.5 contain principally methylene linkages where as higher ratios of M ' F (1.0" 3.0) contain equal amounts of methylene and methylene ether linkages. The spectra, however, showed no evidence for methylol groups which should give peaks at 64 and 69 ppm. The was also confirmed by examining the 15N solid-state NMR spectra (Fig. 15.2.10). The 15N solid-state NMR spectra of model compounds containing methylolmelamines showed peaks at 87 and 107 ppm (indicative of ~ N H C H 2 O H and ~N(CH2OH)2 which were absent in the spectra of the resins. What the 15N spectra did indicate, however, was a peak at 77 ppm and a shoulder (for the M ' F 1.0" 3.0 resin) at 93 ppm indicative of ~ N H C H 2 ~ and - - N C H 2 ~ linkages. The industrially synthesised resins showed very similar spectra indicating the presence of high concentrations of branched and linear methylene and methylene ether linkages.
517
CROSSLINKED P O L Y M E R S
Phenolic ri ng carbons ot--~hercarbons
I
COCH2Ar'
J!
ArCH2OAr' ~ AI.CH2OCH2Ar'
C_OH ArCHO
ArCH2OH
\
- ArCH2Ar
a
b
C
200
!50
100
50
0
PPI4
Fig. 15.2.7. 15.1 MHz 13C CP/MAS NMR spectra of three residues of cured PF resin after treatment with sulphuric acid under N2(g) and three different conditions (a) 1.0 N sulphuric acid solution at 65~ for 3 days (b) 36 N sulphuric acid at 25~ for 1 day (c) 36 N acid at 65~ for 1 day.
15.2.4
Urea-formaldehyde resins
Currently, one of the most important commercially available materials today are the urea-formaldehyde (U-F) resins. Their applications include coatings, adhesives, castings, moulding compounds and textiles. Maciel et al. have produced a series of extensive papers in this area concentrating on both 13C and 15N CP/MAS [13-16].
518
R.V. LAW A N D D.C. S H E R R I N G T O N
a
b 0
d
-
'"
1.-
' 1"
2S0
'
' ' I '''
200
' I '''''''~
150
'
'1"
100
'""
I '
50
''
~1
~
0
'
1 '
-50
PP~I
Fig. 15.2.8. 13C CP/MAS NMR spectra of the residue of cured PF 50 resin after treatment with 15 N nitric acid in air at 25~ for 1 day (a) 50.3 MHz (b) 22.6 MHz (c) 15.1 Mhz and (d) 15.1 MHz 50-1~s dipolar dephasing. Spinning sidebands are marked with an asterisk.
A systematic 13C CP/MAS study was undertaken of a large array of different U-F resins synthesised under a wide range of conditions including pH, concentration, and U : F ratio [13]. Catalyzed by both acid and base a great variety of reactions can occur leading to a large and complex range of moieties depending upon the synthetic conditions. At high pH the principal linkages present are methylol urea (65, 72 ppm) and dimethylene ether (69, 70 ppm), also present under the basic conditions is a small amount of methylene methyl ethers (55 ppm). Under acidic conditions other reactions predominate. In addition to the formation of linear methylene linkages (47, 54,
519
CROSSLINKED POLYMERS
Ca)
L
1
300
J- .
200
.
.
.
100
.
1
. . . . . . .
0
ppm
I
-100
(b)
L
300
~
:
200
..........
l
100
.
.
.
.
ppm
__1
0
.j
-100
Fig,. 15.2.9. Solid state 13C CP/MAS NMR spectrum of M-F resin.
60ppm) resins contained a substantial amount of crosslinking methylene linkages (69, 76ppm) which increase as the F : U ratio increases. At very high F : U ratios methylene dimethylene ethers, methylol and hemi-formals occurred (69-72 ppm). Also present in large quantities were disubstituted urons (75, 79 ppm) which increased as pH increased. In an attempt to formulate alternative [14] resins N,N'-dimethylolurea or paraformaldehyde was used as a different source of formaldehyde to crosslink the urea molecules. The resins produced by these methods generally exhibited similarities to those previous synthesised using formaldehyde [13]. Small
520
R.V. LAW A N D D.C. S H E R R I N G T O N
(a)
r
(b)
i
I
I
400
300
,
J_
_
200
1
t
100 ppm 0
,
.1
-100
Fig. 15.2.10. Solid state 15N CP/MAS N M R spectrum of M-F resin.
differences between the resins produced by the different synthetic route appeared to be due principally to the problems of solubility of the N,N'dimethylolurea or paraformaldehyde. A large volume rotor MAS system was used to examine the natural abundance 15N present in urea-formaldehyde resins [15]. Increasing the amount of material which is examined has enabled the investigation of the isotopically low 15N present (0.37%) in the resins without having to resort to synthesising 15N enriched materials. There are four possible interaction sites between urea and formaldehyde (Fig. 15.2.11).
CROSSLINKED POLYMERS O II
0 II
~--N--C--NCH2OH + H20
~-~N--C--NH + HOCH2OH
I
521
I
O II ,~,N--C--NCH2(OCH2)nOH
I
I
0 II 9,,,,,,~N--C--NCH20H
I
0 Ii
+HOCH2OH
,,~N--C--NCH2(OCH2)n+IOH + H20
0 II + ,,---N--C--NH
I
I
~
O O II II ,~-N--C--NCH2N--C--NH.'~ + H20
I
I
I
I
I
O O II II ,~,N--C--NCH2OCH2N--C--NH,w~ + H20
0 I!
~-~N--C--NCH2OH
I
I
I
I
O
",,
o II
--N--C--N--N [ ~H2 CH2 I
OH
~
/
N
~ jN
+ H20
O
OH
Fig. 15.2.11. Structural units present in UF resins.
For resins synthesised under acidic conditions tertiary amides initially seen by 13C CP/MAS were confirmed by 15N CP/MAS. Under neutral or basic conditions the main constituents of the resin are N,N'-dimethylolurea (102 ppm), monomethylolurea (102 and 78 ppm) and dimethylene ether linkages (90 ppm). Using dipolar dephasing and cross-polarisation times it was possible to distinguish primary, secondary and tertiary substituted nitrogens. These results confirmed the existence of many moieties postulated by 13C CP/MAS. The widespread application of U-F resins has meant understanding the mechanism of the degradation process is important if an improvement in resin stability is to be obtained [16]. Therefore the way in which U-F resins change when they undergo hydrolysis was examined. The resins have been described previously [13]. There are a number of possible mechanisms which involved the hydrolysis of the moieties in the U-F resins. A typical degraded resin is shown in Fig. 15.2.12.
522
R.V. LAW AND D.C. S H E R R I N G T O N
A
b)---J -I-
i ....
I"
i
"!
I
i ....
200 180 160 140 120 100 80
I'
I
I"
I --
60
40
20
0 PPM
Fig. 15.2.12. (a) 50.3 MHz 13C CP/MAS NMR spectrum of a UF resin sample prepared from formalin (F) and urea (U) with an equivalent F/U/water ratio of 2.00/1.00/1.07 at pH 3 and (b) its solid residue after hydrolytic treatment at pH 4 and 86~ for 20 h. Spinning side bands are marked with asterisks.
It was demonstrated that resins prepared with an equivalent molar ratio of F : U gave the highest stability towards hydrolytic treatment. Resins which contained higher F : U ratio (2.0: 1.0) contained a wide range of moieties which were more readily susceptible to hydrolysis, the products formed include dimethylene ether linkages, poly(oxymethylene glycols) and methylols attached to tertiary amine groups. These moieties are the sources of formaldehyde when the resin degrades. Resins of different composition showed similar degradation patterns. 15.2.5
Isocyanurate based resins
In a series of papers cured resins based upon ~SN enriched 4,4'-methylenebis(phenyl isocyanate) (MDI) have been examined by utilising ~3C and 15N
CROSSLINKED POLYMERS RNCO + H20 R'NCO + RNH2 ~ ~ - ' P "
RNCO + R'NHCONHR"
523
~-" RNH2 + CO 2
RNHCONHR' ~" R"NHCON(R")CONHR
Fig. 15.2.13. Reactions of isocyanate units that occur in MDI-polyisocyanurate resins.
CP/MAS NMR. In the first of these papers the structures within the resins were examined as a function of cure temperature [17]. The chemistry of the resins is very complex but one of the principle reactions is the formation of stable isocyanurate structures from three isocyanate units. Other species are also present e.g., amine, urea and biuret. These are formed by the reactions shown in Fig. 15.2.13. 13C CP/MAS NMR spectra of the resins indicated that the optimum cure temperature was 120~ at which most of the isocyanate groups were converted to isocyanurate (Fig. 15.2.14). The peak at 150ppm is due to the isocyanurate carbonyl carbon, the benzylic substituted aromatic carbons para- to an isocyanurate moiety are shown by a peak at 145 ppm, the 130ppm peak is due to the protonated aromatic carbons, the shoulder at 125 ppm is due to both isocyanate and ortho aromatic carbons. These signals were confirmed by using dipolar dephasing spectra. However, because of the complexity of the spectra ~SN CP/MAS NMR was used to clarify the structures present at the different cure temperatures. A summary of the moieties found in the resin is given in Fig. 15.2.15. In addition Duff et al. also undertook a quantitative analysis by utilising the large difference in cross-polarisation time for protonated and nonprotonated nitrogen. In a related study a series of resins were examined in which biuret linkages predominated [18]. These were formed by the reaction of formic acid and 4,4'-methylenebis(phenyl isocyanate) (MDI). The principal reactions that occur in the resins are shown in Fig. 15.2.16. The 13C and 15N CP/MAS spectra showed that when the formic acid" MDI ratio increased the biuret linkage predominates. The pathway for this was initially the formation of MDI-based urea and formic anhydride moieties which further reacted with isocyanate groups to form the biuret linkages and possibly diformyl imide groups. In a further study the same resins were analysed straight after curing and again after a 7 months exposure to air [19]. Three different cure temperatures were used, 100~ 120~ and 160~ A typical example of the 15N CP/MAS spectra is given in Fig. 15.2.17. The predominant structure in these resins is the isocyanurate linkage. These are relatively stable and it is the chemistry of the residual isocyanate groups that dominate the formation of new bonds in the system during the
524
R.V. LAW AND D.C. SHERRINGTON
160 *C
140 ~
120 ~
100 ~
80 ~ 160
140
120
100 PPM
Fig. 15.2.14. 50.3 MHz 13C CP/MAS spectra of MDI-polyisocyanurate resins prepared at different temperatures.
exposure to the air. Using ~SN CP/MAS it was possible to identify clearly the products of isocyanate hydrolysis which involved principally the formation of amines and the urea-linkage condensation products, there was no significant formation of biuret linkages. Structural assignment for these resins was further substantiated by 13C CP/MAS. These results aided the identification of the decrease and increase in concentrations of isocyanate groups and urea linkages, respectively. Duff et al. also attempted a quantification experiment using the ~SN CP/MAS results to determine the relative concentrations of the
CROSSLINKED POLYMERS
525 15N Chemical Shift
Isocyanate
ArNCO
46
Amine
ARNH 2
53
Urea
ArNHC(O)NHAr
104
114 ArNHC(O)N(Ar)C(O)NHAr
Biuret
141
.0
L Uretidione
Ar--N
\]l/
(NH) (N)
145
N--At
O
Isocyanurate
O
'~N
N/Ar 149
I
Ar Fig. 15.2.15. Structures and 15N chemical shift data pertinent to MDI-based resins.
RNCO + R'COOH
~"
RNCO + 2R'COOH RNCO + R'NHC(O)R" RNCO + R'C(O)OC(O)R'
RNHC(O)R'
+ C02
RNHC(O)NHR + R'C(O)OC(O)R' ~--
RNHC(O)NR'C(O)R"
~
R'C(O)NRC(O)R' + C02
Fig. 15.2.16. Reactions that occur between formic acid and 4,4'-methylenebis(phenyl isocyanurate) (MDI).
526
R.V. LAW AND D.C. S H E R R I N G T O N
B
A l
200.00
J_
1
| ~ , ~0
J .....
!
i013.=
I
_
l
50,00
,,.J
!
E, O0 PPM
Fig. 15.2.17. (a) 20.3 MHz 15N CP/MAS spectra of MDI-polyisocyanurate cured at 100~ (b) Same resins after 7-month exposure to air.
different nitrogen containing moieties before and after prolonged exposure to air [20]. The differing extents to which hydrolysis occurred in the samples was interpreted in terms of the structural effect that the cure conditions had. The possibility that the different morphologies present could be responsible for the amount of hydrolysis was further investigated by study of the 1HT~p of the samples. Finally the thermal degradation of the samples was examined. In this study it was possible to show that the degradation of all biuret and uretidione linkages occurred at 230~ a decrease of residual isocyanate took place on heating to temperatures up to 240~ an increase in urea linkages occurred in samples heated up to 240~ followed by a decease in these from 250-260~ A steady increase in amine groups and a decrease in isocyanurate groups was also observed. The peaks in both the ~3C and ~SN CP/MAS NMR
CROSSLINKED POLYMERS
527
spectra broadened with increasing temperature. This was attributed to the formation of free radicals and substantiated by ESR spectroscopy. The thermal stabilities of the relevant groups were in the order biuret, uretidione < urea < isocyanurate < urea', where urea' is a urea-type more stable than isocyanurate. 15.2.6
Synthetically crosslinked natural polymers
Natural polymers crosslinked by synthetic molecules represent many resins used for industrial applications. These are now being more closely examined by solid-state N M R spectroscopy to try to understand more fully what occurs in these systems and how it is possible to improve them. Of industrial importance are the polyphenolic tannin resins crosslinked by hexamethylenetetramine. These principally contain flavan-3-ols (Fig. 15.2.18) in the tannin [21] and have been examined by ~3C CP/MAS solid-state N M R spectroscopy. Hexamethylenetetramine was used in preference to formaldehyde as it has showed a much faster rate of reaction. The intermediates in this reaction are tribenzyl-, dibenzyl-4~, and monobenzylamines some of which rearrange to give the dihydroxydiphenylmethane crosslinking bridges in the resin. The exact nature of the crosslinking process, however, is still in debate and the study was undertaken to try and clarify the issue. To examine this process fully, a comparison was made between pine tannin (high in flavan-3-ol) (Fig. 15.2.19) pine tannin hardened with paraformaldehyde (Fig. 15.2.20) and pine tannin hardened with hexamethylenetetramine (Fig. 15.2.21). For the paraformaldehyde cured species three new peaks were observed that were representative of the C4 unsubstituted flavonoid site (38 ppm) and (OH) OH H
OH
H
(OH)
Fig. 15.2.18. A typical flavonoid structure. The parentheses indicate variations in flavonoid structure with some hydroxyl groups absent.
528
R.V. LAW A N D D.C. S H E R R I N G T O N
..[
......
, ....
~00
Fig. 15.2.19. 13C C P / M A S N M R
t. . . . . . . . . .
150
l . . . . . . . . .
I00
ppm
I
. . . . . . . . .
50
! . . . . . . . . . .
I
0
spectrum of pine tannin.
the formation of methylene bridges between two phenolic rings (36.8-37 and 33 ppm). Unsurprisingly methylene bridges were formed exclusively. For the hexamethylenetetramine cured species peaks assigned to the formation of methylene bridges (identical to the first reaction) together with peaks assigned to the formation of tribenzyl- (57.5 ppm), dibenzyl- (51.0 ppm) and monobenzylamines (45 ppm) linkages were observed. In the latter case it appeared that 40-50% of the crosslinks were the benzylamine type (with tri- and monobenzylamine predominating) the remaining being methylene bridges between phenolic type structures. Further evidence for this was shown from the peak at 98 and 105-110 ppm representative of the free and reacted C6/C8 sites respectively, the former decreasing and the latter increasing in both cases with reaction with the formaldehyde and the hexamethylenetetramine. Pecan nut tannin with another predominant flavonoid form of differing reactivity was also reacted with hexamethylenetetramine. In this case the dibenzylamine and tribenzylamine units were the dominate moieties present in the
CROSSLINKED POLYMERS
.!
.........
200
f .........
150
! .........
I00
ppm
f .....
50
....
529
1 .........
f
0
Fig. 15.2.20. 13C CP/MAS NMR spectrum of pine tannin extract hardened with paraformal-
dehyde.
system, also the level of methylene bridging was much lower representing only 20% of the total crosslinks. In two closely related studies Wendler and Frazier examined, by ~SN NMR, the interaction between both model cellulose compounds [22] and wood with 15N enriched polymeric diphenylmethane diisocyanate (pMDI) [23]. The resin formed is used commercially as a wood adhesive. Previous work [17] had shown that this reaction is sensitive to moisture, the formation of different products depending upon the degree of moisture present (Fig. 15.2.22). Urea and biuret type linkages were all characterised. Biuret structures were predominant when the moisture content was low, gradually being replaced by urea linkages when there was higher moisture content, the formation of urethane and amine moieties also occurred at intermediate moisture contents. This is in contrast to the previous study [17] where only a small amount of biuret linkages were detected. This may be due to the presence
530
R.V. LAW AND D.C. SHERRINGTON
! .....
200
, ....
! ....
150
~ ....
1 .....
, ....
100
I .....
50
, ....
t ..........
1
0
ppm Fig. 15.2.21. 13C CP/MAS NMR spectrum of pine nut tannin extract hardened with hexamethylenetetramine.
of the large amounts of hydroxyl groups available from the cellulose to react further with the urea linkages. 15.2.7
Polyacenicpolymers
The potential applications for conducting polymers are enormous and this has stimulated a large amount of research into this area. Not surprisingly, solid-state NMR spectroscopy has been applied to study these amorphous, insoluble and in many cases crosslinked materials [24]. Looking at the 13C CP/MAS spectra of a series conducing polyacenic polymers, some of which were doped with iodine, it was possible to see the effect of the halogen upon conductivity. These resins were prepared by a conventional procedure for the preparation a Novolak-type phenol-formaldehyde resin. After synthesis, the phenol-formaldehyde resin were dissolved and solutions were cast as a film and heat treated to between 590-670~ in a Ne atmosphere to form the polyacenic film. The electrical conductivity of the films was shown to increase
531
CROSSLINKED POLYMERS
1
t
1
250
200
! ....
150
, ....
! . . . . . . . . .
;0 . . . . . . . . .
I
100
PPM
Fig. 15.2.22. ~SN CP/MAS spectrum of wood/15N-pMDI composite as a function of precure moisture content.
with higher temperature. The addition of iodine to one of the films gave rise to substantial increase in electrical conductivity. The postulated structures are shown in Fig. 15.2.23. Typical 13C CP/MAS spectra are shown in Fig. 15.2.24. By using the reference spectrum of a phenol-formaldehyde resin the peaks for the polyacenic films were assigned. The main peak at 127-130 ppm moves upfield and broadens with increasing temperature indicating an increased amount of polyacenic-type structures. The peak at 150 ppm assigned to quaternary aromatic carbon substituted by hydroxy groups and the peak at 40 ppm assigned to methylene carbons decrease in intensity with increasing temperature but are never completely removed. Using dipolar dephasing spectra the aromatic region also revealed further signals and it was possible
o
o
~..,.
o
!
o
o_
P~
j /)
/
// ,, /
~ ~
i'"
~
~,
7
J
i
0 Z
~ N
Z C~
h
.'~
t~
CROSSLINKED
533
POLYMERS
2,3 5,6
i
~-
i
~)o
~
~
i
9
15o
, .
100
.
.
.
.
50
Fig. 15.2.24. 13C C P / M A S spectra of a polyacenic film.
to assign them using chemical shifts from model compounds. The peaks at 129.5 and 151ppm were due to protonated/nonprotonated and hydroxy substituted carbons respectively in the residual phenol-formaldehyde resin structure. The peaks at ca. 125 and 138 ppm are the methine and quaternary carbon in the polyacene. The ratios of two of the aromatic peaks, obtained by deconvolution, were related to the degree of electrical conductivity. For the iodine doped sample it was shown that the iodine interacts significantly with the polyacene part and not with the phenol formaldehyde part. This was shown by peak broadening of the polyacene type peaks. 15.2.8
Polyethers
A semi-crystalline poly(1,3-dioxolane) was examined by solid-state NMR spectroscopy looking at both linear and crosslinked polymers [25]. The
534
R.V. LAW AND D.C. SHERRINGTON
OCH2CH20
OCH20
(b)
,'l",~']"'"'i
le5
100
....
95
I ....
$0
! .... "T"'"I
85
80
....
75
I' '"" i '''~:i . . . .
7e
65
i
6a ~p~
Fig. 15.2.25. (a) 13C single pulse excitation and (b) 13CCP/MAS spectra of poiy(1,3-dioxolane).
crosslinks were formed by introduction and reaction of acrylate groups allowing the formation of a network and control of the molecular weight. The systems were examined by 13C MAS and typical spectra appear in Fig. 15.2.25 where two characteristics peaks at 67.5 ppm ( O C H 2 0 ) and 96.0 ppm ( O C H 2 0 ) are diagnostic. For the crosslinked polymer the CP spectrum, being more sensitive to static molecular motion, revealed a further peak at 93.4 ppm (the O C H 2 0 region) which was assigned to a less mobile phase. To clarify these results variable contact CP and cross-depolarisation experiments were carried out and by deconvolution of the peaks in the NMR spectrum three regions, a crystalline, an interfacial and elastomeric one, were indicated. Further study carried out using 1HTlp which is indicative of kHz motion in
CROSSLINKED POLYMERS
535
the polymer, also suggested that there were three phases present in the polymers. 15.2.9
Epoxide based resins
In a high temperature study [26] of two epoxide resins, the samples were heated to above Tg whilst still carrying out magic-angle spinning to remove residual line broadening interactions. At these temperatures (ca. 260-290~ the molecular motion of the system had increased to such an extent that it was possible to use conventional one (~H, DEPT) and two dimensional (HECTOR) solution state NMR experiments on the samples. The networks looked at were the oligomer of diglycidyl ether of bisphenol A (DGEBA) cured with 100 and 66% of diaminodiphenyl sulphone (DDS) (Fig. 15.2.26). The spectra of the 100% cured resin at ambient and high temperatures are shown in Fig. 15.2.27. The principal reactions are between the epoxy and the primary and secondary amines, further reactions between hydroxyl groups are also possible. The principal reaction moiety for the 100% cured polymer is indicated in Fig. 15.2.28. For the 66% cured polymer the situation was more complex the spectrum is shown in Fig. 15.2.29 and the major structural moiety present is indicated in Fig. 15.2.30. The increase in resolution at higher temperature is clearly evident and the possibility of using conventional solution state editing techniques is advantageous as they greatly aid peak assignment. Unfortunately this technique may be applicable only where there is a substantial increase in motion above Tg. Many highly crosslinked polymers do not show a discrete Tg and therefore would not be expected to show any substantial decrease in line broadening if they were heated to high temperatures. There is also the question as to whether further post-curing reactions may occur when polymers are heated to such temperatures. Epoxide resins made from 2,2-[4-(2,3-epoxypropyl)phenyl]propane (DGEBA) polycondensed with 4,4'-sulphonyl-dianiline (DSS) produce a three-dimensional insoluble network which was examined by CP/MAS NMR spectroscopy [27]. In this study the chemical structures and the cure kinetics were determined. A cured epoxy synthesised from a mixture of the diglycidyl ether of bisphenol A (DGEBA) and 1,3-phenylenediamine was studied by ~H NMR spectroscopy including multiple pulse techniques and spin-lattice relaxation in the rotating frame, T~o. The study [28] focused on the water distribution based upon possible variation in the cross-link density measured by spin diffusion. From the analysis involving a combination of T~p and multiple
t~ ta~
/o~
c.,---c.c.,o
o.
Y" /~
I
~---~xk ..../f--oc~c.c.,o
]/----x
I /--x.
/o~ X~ <
>, Z
DGEBA
h :z:
./
N
sch -
o u
It
N
Z
\ H
DDS Fig. 15.2.26. Structure of diglycidyl ether of bisphenol A and 4,4'-diaminodiphenylsulphone.
0 Z
537
CROSSLINKED POLYMERS
II _ ~L_L_=_
I
180
I
I
lz~O
i
I
100
~
i
I
60
I
I
20
~c/ppm
Fig. 15.2.27. 13C MAS spectra of DGEBA-DDS with 100% stiochiometry (a) 23~ with crosspolarisation and (b) 290~ without CP (c) expanded.
HO
0
\
I
'
I
0
\
/OH
I
o
Fig. 15.2.28. Major structure present in the 100% cured DGEBA-DDS resin.
pulse it was possible to postulate that the water was molecularly dispersed in the epoxy rather than aggregated in the voids. Also there seemed to be an absence of two distinct sites for water affinity. The presence of accelerators, either magnesium perchloroate or N,Ndimethylbenzylamine (DMBA) [29], on the curing of bisphenol A diglycidyl ether with butane-l,4-diol (BADGE-BD) were studied by CP/MAS (Fig. 15.2.31). Magnesium perchlorate was shown to induce the consumption of all the diol whereas the DMBA showed only approximately 50% consumption
538
R.V. LAW AND D.C. SHERRINGTON
q I
--
I
180
I
,
,
,
'I
,
I
L
.....
1/.,0
I
,
I
100
9
9
9
I
60
LJ ,
I
c/ppm
20
Fig. 15.2.29. 13C MAS spectra of DGEBA-DDS with 66% stoichiometry (a) 23~ with crosspolarisation; (b) 260~ without CP and (c) expanded.
as indicated by the residual primary alcohols from the butanediol (Fig. 15.2.32). 15.2.10
Methacrylate-based resins
Polymer composites are increasingly used for dental applications [30], the durability and aesthetic appeal has made them ideal substitutes for the more traditional amalgam fillings. The dental polymer composites are principally composed of an organic matrix and a powdered ceramic phase. The organic matrix is composed of an aromatic or urethane dimethacrylate such as 2,2bis[4-(2-hydroxy-3-methacryloyl propoxy) phenyl]propane (bis-GMA) with
539
CROSSLINKED POLYMERS
O
o
Fig. 15.2.30. Major structure present in the 66% cured DGEBA-DDS resin.
I _
s
150
,,,
!
.....
!
110
I
,
r
70
1,
!
....
30 ppm
Fig. 15.2.31. 13C CP/MAS spectrum of BADGE-BD system with accelerator Mg(C104)2.
another monomer such as triethylene glycol dimethacrylate (TEGDMA) to alter viscosity. This system has the further advantage that it can be photopolymerised. ~3C solid-state NMR spectroscopy has been used to study the extent of the crosslinking reaction. A series of commercial and laboratory synthesised resins were examined by CP/MAS and SPE to determine more accurately the relative amounts of unreacted resin present (Fig. 15.2.33). In the synthesis of a methacrylate-based metal chelating resin, ~3C CP/MAS spectroscopy has been used to confirmed that the target resin had been made [31]. Here the imidazole ligand bis(imidazo-2-yl)methylaminomethane (bimam) was attached to a glycidyl methacrylate-co-trimethylolpropane trimethylacrylate (pGMT) resin. The peaks in the ~3C NMR spectrum were
540
R.V. LAW AND D.C. SHERRINGTON
_
t
150
!. . . . .
!
110
....
s,
t. . . .
70
! ....
t
30 ppm
Fig. 15.2.32. 13C MAS spectrum of BADGE-BD system with accelerator DMBA.
assigned as follows: 7.3 ppm, hindered methyl in trimethylolpropane residue; 24.3 ppm, backbone methyl; 41.4 ppm, ~ C H N in ligand; 46.0 and 56.1 ppm, polymer backbone; 67.5 ppm, ~ O C H 2 ~ in epoxy linkage; 127.5 and 145.6, imidazole carbons; and 176.3, carbonyl carbon. In an attempt to provide alternative supports to styrene-divinylbenzene resins [32] for use in reactive chemistry, poly(hydroxyethylmethylacrylate) (poly(HEMA)) has been employed. In this study poly(HEMA) was crosslinked with ethyleneglycol dimethacrylate and the CP/MAS recorded. The peaks were assigned as follows: 18.6 ppm is due to the methyl attached to the aliphatic backbone; 56.0 and 45.4 ppm are due to the methylene and quaternary of the backbone; the ~ O C H 2 ~ forming the crosslink are at 63.0ppm; the conjugated and unconjugated carbonyls are at 167.2 and 177.6 ppm and finally the methylene and quaternary carbons of the unreacted vinyl group are at 126.3 and 137.0 ppm. Another alternative is the use of crosslinked ethylene dimethacrylate [33]. Here, the unreacted double bonds in resins were used as a graft point for further reaction. The level of unreacted double bonds was determined by the relative areas of the carbonyl peaks at 176.3 and 166.3 ppm (Fig. 15.2.34) determined by CP/MAS before and after reaction with glycidyl methacrylate. These results were in good agreement with data from Raman spectroscopy. Spin diffusion is a valuable method by which it is possible to examine the heterogeneity of a polymer [34]. Spin-lattice relaxation times in the rotating
541
CROSSLINKED POLYMERS
260
P-20
180
140
100
60
20
0
ppm
Fig. 15.2.33. 13C CP/MAS spectra of commericially available dental acrylate resins. (a) Tetric; (b) Zl00; (c) Duo Bond; (d) Coltene bonding agent; (e) TEGMA the labels r and u indicate carbonyl peaks from reacted and unreacted methacrylate groups.
flame have been used to determine the rate of spin diffusion. Tip data from three solid polyacrylate networks made by photopolymerisation of poly(ethylene glycol) diacrylate (PEGA), trimethylolpropane triacrylate (TMPTA), and dipentaerythritol pentaacrylate (DPHPA) have been used in this way. The photopolymerisation was carried out by a laser and this was related to the degree of crosslinking that occurred which was quantified in terms of the signals from the residual double bonds in the CP/MAS NMR spectra. The level of heterogeneity in these resins was measured by the 1HTlp and was related to the degree of crosslinking.
542
R.V. LAW AND D.C. SHERRINGTON
1
190
I
I
180
170
I
160
PPM
Fig. 15.2.34. Part of the 13C CP/MAS n.m.r, spectrum of poly(ethylene dimethacrylate) showing the peaks used for the determination of the double bond content.
In an attempt to obtain high surface area glycidyl methacrylate-co-trimethylolpropane trimethacrylate resins were synthesised [35] with a variety of porogens. The degree of unreacted double bonds was determined by CP/MAS NMR spectroscopy. Oligomers containing ether-ester groups were synthesised [36] in order to obtain a crosslinking agent that gavegood cure kinetics and was uniformly distributed in the network structure. The crosslinking agents were modified so that vinylidene groups were incorporated to enable them to be polymerised free radically with styrene or methyl acrylate. The oligomer was incorporated (5-50%) in the polymer to give clear hard resins and these were characterised by CP/MAS NMR (Fig. 15.2.35). The peak at 30 ppm is due to the tert-butyl group, the broad peaks for
CROSSLINKED POLYMERS
543
/ V
200
160
ASO
t40
t20
100 ~X
60
40
2O
0
Fig. 15.2.35. Solid state 13C CP/MAS spectra of three samples of poly(methyl methacrylate)
crosslinked with 5 (lower), 20 (mid) and 50 (top) % of t-butyl acrylate end-capped oligomer containing ca. 2-3 HDDA repeat units.
the ethyl and ester carbons bonded to the oxygen are at 65-80 ppm, and the carbonyl carbon on the upfield side of the P M M A carbonyl at 178 ppm. Polyacrylamides can be synthesised by two methods, either polymerisation of an acrylamido monomer or chemical modification of another polymer e.g., poly(methylmethacrylate). In the latter case 13C CP/MAS N M R spectra show clearly the loss o f - - O C H 3 groups as these are replaced b y - - N H C H z - groups (Fig. 15.2.36). The latter approach has the advantage that polymethacrylates like P M M A can be obtained easily in a bead form of uniform size and are physically convenient for further exploitation. CP/MAS was used in a study [37] of the kinetics of reaction between P M M A and a series of amines. This was carried out by taking aliquots of the reaction mixture at certain times and recording the solid-state spectrum after the reaction had been quenched. The degree
544
R.V. LAW AND D.C. SHERRINGTON
_JA
~"-'~
720 rain.
360 min.
240 rain.
120 rain. PMA PpM ,
J 200.00
~
I 150.00
,
1 100.00
, 50.00
-0.00
Fig. 15.2.36. 13C CP/MAS spectra of PMMA reacted after various times with 1,6-diaminohexane.
of reaction was determined by 13C CP/MAS and further structural evidence was provided by 15N~ CP/MAS. 15.2.11
Styrene-basedpolymers
The degree of unsaturation in styrene cured polyesters was investigated [38] by using dipolar dephased 13C CP/MAS NMR data. The commercial resins contained fumarate, isophthalate and propylene glycol structural units and were cured with styrene. The use of dipolar dephasing (see Fig. 15.2.37) suppressed the strong phenyl peak at 129 ppm and therefore allowed the determination of the degree of unsaturation in the resin. The signal at 131 ppm was attributed to the isophthalate units, and the peak at 144 ppm to the quaternary substituted carbon from the styrene. This peak also showed
545
CROSSLINKED POLYMERS
180
160
lt, O
Chemico[ shift
(ppm}
12O
Fig. 15.2.37. Part of a 13C CP/MAS spectrum of a solid polyester obtained (a) without dipolar dephasing and (b) with dipolar dephasing. Chemical shift are shown by the numbers.
partial resolution into two peaks at ---142 and --~146ppm which may have been due to styrene units in sequences of differing lengths. The two carbonyl peaks at 165 and 172 ppm were ascribed to unreacted fumarate/isophthalate and reacted fumarate carbonyls, respectively. The degree of residual unsaturation was calculated by determination of the relative ratios to these peaks. From this it was possible to determine the optimum level of styrene (47%) needed to give the lowest degree of unsaturation in the resin. Polyesters derived from maleic anhydride and 2,2-di(4-hydroxyphenyl)propane were copolymerised with styrene and then studied by CP/MAS NMR [39] spectroscopy. The three dimensional-crosslinked network formed by the polymerisation was examined using spin-lattice relaxation times in the rotating frame. A correlation between reaction conditions and the structure of the resulting material was found. The degree of residual unsaturation was determined by subtraction of two relaxation times from a linear additivity model used for crosslinked polymer systems. In two closely related papers [40, 41] CP/MAS was used to examine a series of styrene-divinylbenzene (St-DVB) and chloromethylated resins. In the first part of this study the authors were concerned with trying to determine the residual amount of unreacted vinyl groups present in St-DVB resins (see Section 3). In order to increase the sensitivity of the method the authors used 13C-labelled divinylbenzene (labelled in the methine position) and combined this with unlabelled styrene (1-20% by weight). The final resins were examined by CP/MAS and it was found that even for a very lightly crosslinked
546
R.V. LAW A N D D.C. S H E R R I N G T O N
species (1% DVB-St) at 70~ residual unreacted vinyl groups were present. Post-curing reactions, carried out by swelling the resin in a solvent and then heating the resin to 155~ in the presence of initiator, showed it was possible to remove the residual unreacted vinyl groups. For resins with higher amounts of DVB (10 and 20%) it was impossible to react all the residual vinyl groups by using this method. This showed that the unreacted vinyl groups are trapped in inaccessible locations within the polymer network. The second part of the study was concerned with studying resins in the unswollen glassy state and the solvent swollen state using CDC13. St-DVB and a chloromethylated resin were examined. For samples with low levels of crosslinking (<6%) swelling in a good solvent whilst spinning at the magic angle (54.74 ~ gave the best resolution (see Section 4). 4-Acryloxy benzophenone (APB) was copolymerised with 4% divinylbenzene (DVB) and ~3C CP/MAS and other techniques were used to characterise the resin [42]. The purpose was to examine the nature of polymer-supported initiators. Data from solution state NMR analysis of the linear uncrosslinked poly(APB) was used as a model for assigning the ~3C CP/MAS NMR spectrum. The solution state spectrum of poly(ABP) gave 12 well resolved lines. The CP/MAS of the crosslinked poly(APB-DVB) copolymer gave six broad lines, and the keto carbonyl at 192.5 ppm and the ester carbonyl at 173.2 ppm. The peak at 153.8 ppm arose from the quaternary aromatic carbon attached to the oxygen atom of the ester group. The remaining quaternary and protonated aromatic carbons were visible as two broad overlapping signals at 128.8 ppm and 122.0 ppm. The signal from the DVB probably also contributes to this region and to the methylene-methine (ca. 40.9 ppm) signal but the authors did not comment upon this. The signal at 40.9 ppm was similar to the broad feature seen in the solution state NMR and is representative of the methylene and methine peaks of the backbone unit. The line broadening was attributed to the distribution of chemical shifts commonly seen in crosslinked resins. A series of copolymer hydroquinone diacrylate (HyDA) resins were characterised by using 13C CP/MAS NMR spectroscopy [43]. The series include styrene-HyDA, glycidyl methacrylate-HyDA, phenylmethacrylate (PhMA)-HyDA, 2,4,6-tribromophenyl acrylate-HyDA, and 4-acetylphenylmethacrylate-HyDA. The 13C CP/MAS spectrum of the PhMA-co-HyDA copolymer is shown in Fig. 15.2.38 with the 13C-{~H} solution NMR spectrum of poly(PhMA). Comparison of the these makes the peak assignment unambiguous. The a-methyl group of phenyl methacrylate appears as a broad peak at 16.4 ppm, the backbone methylene group shows a sharp intense line at 44.0 ppm and the small broad quaternary carbon peak is at 54.0 ppm. The O ~ C aromatic
547
CROSSLINKED POLYMERS
! 3C&SC 2C&6C /
4C
1C
(Ca)
_
I
200
t
!
160
!
,
I
1
120
!
80
1
I,
t,O
l
{~
PPM
Fig. 15.2.38. (a) 13C-{1H} NMR solution spectrum of poly(PhMA) and (b) 13C CP-MAS NMR solid-state spectrum of the PhMA-co-HyDA polymer.
peak (C1) is at 149.2 ppm, the protonated aromatic peaks are at 127.6 (C2 and C6) and 119.6 ppm (C3 and C5) and the para carbon (C4) is at 124.9 ppm. The carbonyl peak is at 172.2 ppm. Hypercrosslinked resins have also been the subject of an N M R study [44] (see Section 3.4). 13C CP/MAS NMR has been used to estimate the degree of crosslinking in a series of these species. The resins were examined in a solvent swollen and a nonswollen state to determine if with swelling in a deuterated solvent it was possible to narrow the line lines further. Determination of the level of crosslinking was carried out by deconvolution of the quaternary aromatic peak at ca. 146 ppm (Figure 15.2.39). This was broken down into two peaks, one centred at 146 and one at 140 ppm. This is a difficult undertaking as there appears to be no substantial asymmetry present in the peak. Use of SPE methodology is probably more quantitative in the context (see Section 3.4). The levels of crosslinking were shown to be high as there was no substantial narrowing of the static proton line widths even at temperatures of 200~
548
R.V. LAW AND D.C. SHERRINGTON
8
b _
-''''
J
'26o ' ' '
~
6o' :'''
Fpr~
Fig. 15.2.39. 13C CP/MAS spectra of (a) crosslinked polystyrene (0.3% DVB) and (b) hypercrosslinked sample. Side bands are indicated by the presence of an asterisk.
One of the aspects of polymer-supported reactions (see Section 4) is the ability to separate reactive centres from each other. The extent to which a benzoin condensation reaction occurs on a crosslinked polymer was examined [45]. The starting material, a polymeric benzaldehyde, was prepared by incorporation of vinyl benzaldehyde into a resin using either divinylbenzene or tetraethyleneglycol diacrylate as a crosslinker. The product was examined using ~3C CP/MAS. The spectra showed two important peaks at 86.2, 126.0 and 166.0 ppm. These were attributed to the c~-hydroxy carbon, the protonated aromatic carbons and the carbonyl carbon of the a-hydroxy ketone. This demonstrated that in the polymer the benzoin condensation reaction had occurred to a significant extent. Another metal chelating resin was looked at using 13C CP/MAS [46] (Fig. 15.2.40). This resin was based on an salicylaldehyde acrylate monomer
CROSSLINKED POLYMERS
549
OHC
Fig. 15.2.40. Structure of salicylaldehyde-containing polymer resin.
polymerised with a small amount of crosslinker, divinylbenzene. This was then further modified to form the oxime. The final product, the aldehyde and oxime, were also examined by 13C CP/MAS (Fig. 15.2.41). Only some of the peaks were assigned: the peak at 40.2ppm was the backbone from the methylene in the styrene and the peak at 126.0 ppm the protonated aromatic carbons. The phenyl ester carbonyl occurred at 160.6 ppm and the aldehydic carbonyl of the salicylaldehyde which appeard at 186.3 ppm in the initial polymer was not present in the oxime. The authors did not comment on the ratio of the peaks found between 55-105 ppm though this may be due to the poor signal-to-noise of the spectrum in this region. 15.2.12
Miscellaneous
Though it now possible to make diamond film by chemical vapour deposition, this technique has limitations and it would still be extremely advantageous to be able to make diamond or a diamond-like material synthetically. Towards this end crosslinked adamantane have been synthesised [47] by reacting 1,3,5,7-tetrakis(4-iodophenyl) adamantane with 2-methyl-3-butyn-2-ol and phenylacetylene to give two corresponding tetracetylene derivatives. These materials were then cured and the final products were examined with solidstate NMR spectroscopy. Typical spectra are shown in Fig. 15.2.42. In these the phenylacetylene crosslinked adamantane shows peaks at 79 and 84 ppm. These arise due to the free acetylene. Upon partial and full curing these lines broaden to form a single peak at 79 ppm. The adamantane carbons are at 39.5 and 50.8 ppm and the phenyl carbons appear as three peaks at 120-132 and one at 148 ppm. On curing the adamantane peaks
0
9
m
0
.~-~
m ~~
Cr
o 5"~~
=" ~ ~ "
e-+
"~
,e
m
m N
.. m ~._
|
~i.,.
X
o~
N
8i
"
I::::I
O
9
9
-o
I~"
g
~
8
o
..~_~
~..r- ' ~
9
CROSSLINKED POLYMERS
551
ssb
7/~---k / 3,8
.8.9,10
ssb
I
180
160
'
I
140
'"' ...... I 120
'
'i 100
' '
1 80
'
! 60
'
1 40
'
I
20
ppm
Fig. 15.2.42. '3C CP/MAS spectra of adamantane-alkyne condensate before (bottom) and after curing (top).
containing adamantane cores to yield a material which shows excellent thermal stability. The crosslinked insoluble polymers obtained by vapour deposition techniques have been studied [48] by NMR spectroscopy. In poly(furyleneethylene) the ~3C CP/MAS spectra showed additional peaks at 106.4 and 107.5 ppm which indicated that the chains were crosslinked with furyleneethylene units (Figs. 15.2.43 and 15.2.44). A series of polymers (including polysulphones, polyethers) containing the monomer trans-l,2-diphenylcyclopropane have been synthesised [49] by solution polycondensation. 13C CP/MAS spectroscopy was used, among other techniques, to characterise the cyclopropane ring opening reaction which had occurred in the final thermally crosslinked product. One such polymer (Fig. 15.2.45) showed characteristic peaks in its CP/MAS spectrum (Fig. 15.2.46).
552
R.V. LAW AND D.C. S H E R R I N G T O N
.../~
3
2~o.
2
3
ff-----a // \\
!
1
I
Fig. 15.2.43. Structure of poly(furyleneethylene) film. Numbers indicate carbon atoms corresponding to resonances shown in the spectrum in Fig. 15.2.44.
Those at 162.7, 152.9, 136.9ppm are from the nonprotonated aromatic carbon bound to the sulfonyl group, oxygen and cyclopropane ring, respectively. The protonated aromatic signals arise at 130.3, 121.7 and 117.0 ppm. The peak at 28.7 ppm is attributed to the aliphatic cyclopropane ring carbons. Upon crosslinking the cyclopropane carbons disappear, and three new aliphatic peaks appear. This change also corresponds with a change in chemical shift of the peak at 136.9 ppm which shifts to higher field upon crosslinking. A crosslinked polymer synthesised [50] by crosslinking ethylene-vinyl acetate copolymer with dicumyl peroxide gave two large signal at 31.8 and 33.8 ppm in its 13C CP/MAS NMR spectrum These corresponded to crystalline and amorphous methylene units in the ethylene polymer.
15.2.13
Summary comments
For the characterisation of crosslinked polymer systems it has been shown that solid-state 13C and 15N CP/MAS NMR spectroscopy offers a unique insight into the structure of these insoluble polymeric materials. Often it is the only way possible to develop a fundamental understanding of these generally structurally very complex systems. The power and utility of these NMR techniques are demonstrated by their increasing application. Some authors have adopted the approach of using 13C and 15N labelled
CROSSLINKED POLYMERS
553
c~
c3',c3
C1'
CZ
side b
t side band
180
160
140
IZ0
100
80
side band l
60
40
ZO
0
Fig. 15.2.44. Solid state 13C CP/MAS NMR spectrum of a poly(furyleneethylene) film.
O
, o-
xO///-V
% //
I!_
jo
Fig. 15.2.45. Structure of polysulphone containing the 1,2-diphenylcyclopropane moiety.
compounds for the starting monomeric materials, and this can greatly aid the analysis of the final spectra. This is of course costly, time consuming, and requires the synthesis of the labelled model networks; nevertheless the benefits achieved can be very worthwhile. Conventional CP/MAS methodology applied to unlabelled polymer networks is itself very powerful, however, and further advances and developments in both software and hardware will provide greater understanding and widen the applicability of this technique.
554
R.V. LAW AND D.C. SHERRINGTON
I '
-
-
=
--
200
'
;
150
. . . .
100
50
0
PPM Fig. 15.2.46. 13C CP/MAS NMR spectra of noncrosslinked and cross-linked polymer poly(ether sulfone).
15.3 Solid-state ~3C CP and single pulse excitation (SPE) MAS NMR analysis of crosslinked particulate polystyrene-based resins
15.3.1
Background
Spherical particulate crosslinked resins based on styrene and (meth)acrylate monomers prepared by suspension polymerisation methodologies [51] have become extremely important in a number of technological areas. Perhaps the most important are ion exchange resins [52] for purification of surface, ground and waste water, for steam turbine condensation polishing, for hydrometallurgy, for nuclear process and reprocessing, for de-ionised water production, for ultra-pure water production and for use in various purification (e.g., sugar) and separation processes. The closely related sulphonic acid resins have also become important industrial acid catalysts and are now used in a growing number of large scale acid-catalysed reactions [53]. High surface area nonfunctional styrene-divinylbenzene resins are also finding increasing
CROSSLINKED POLYMERS
555
use as sorbents for both liquid and gas phase components and contaminants [54], and in this context hypercrosslinked resins [55, 56] are now becoming available from commercial sources [57] for industrial exploitation. Most resins are based on crosslinked polystyrene and generally the level of crosslinking is higher (5% upwards) than the very lightly crosslinked gel-type species used in solid phase combinatorial synthesis (see Section 4). Analysis of such resins by NMR spectroscopic techniques was and remains problematical, but solidstate CP and SPE MAS methodologies have moved the area forward significantly in recent years. The high level of crosslinking generally means that the associated increased line widths of signals cannot be reduced simply by solvent swelling although this aspect is currently being re-visited [58]. Solid-state ~3C MAS NMR spectroscopy has had much success in examining amorphous insoluble polymers [59]. In recent years, however, there has been some debate on the reliability of quantitative data derived from CP experiments [60] and work on fossil fuels in particular has highlighted the problem [61, 62]. Undoubtedly, the issues arise in the analysis of polymers as well [63-66]. While CP results in signal-to-noise enhancement and hence reduced accumulation times, carbon atoms present with no proximal protons tend to have their peak intensities reduced relative to other signals. Quaternary aromatic carbons are likely to suffer badly in this respect. The modulation of the dipolar interactions by the motion of some moieties can also introduce quantitative errors [67]. The rotation of the methyl group about its 3-fold axis of symmetry is a good example of this. Single pulse excitation (SPE) [60] however overcomes the problems that are associated with CP, i.e., that the dynamics may alter the CP rates and may therefore discriminate for or against some types of carbon [61, 68] but the long delay time needed between each acquisition (typically 5 times the ~3C spin-lattice relaxation time (30-100s)) means that the technique is time-consuming. In addition, the signal enhancement brought about by the CP sequence is lost.
15.3.2 13C MAS NMR studies of anion exchange resins and their precursors Typically anion exchange resins are synthesised by the sequence shown in Fig. 15.3.47. The chloromethylation reaction is a potentially hazardous one [69] and an important side-reaction is methylene bridging. The latter effectively increases the level of crosslinking and manufacturers have to take account of this (by experience) in tailoring the final physical properties of resins. Important issues in manufacture, in addition to the level of methylene bridging, are the degree of chloromethylation achieved and the efficiency of
R.V. LAW AND D.C. SHERRINGTON
556
//'(
CMME Lewis acid CH2CI
~~N(CH3) 3
c~
MethyleneBridging
[~CH2~(CH3)3CIChloromethylation and amination of poly(styrene-divinylbenzene) resins to form anion exchange resins.
Fig. 15.3.47.
amination of these groups in generating quaternary ammonium ion anion exchange sites. Ford and his co-workers have used swollen gel-phase [70, 71] and solidstate CP MAS 13C NMR spectroscopy [72] to probe the products of the chloromethylation reaction, while a very detailed study and analysis of all steps in the synthesis of the final anion exchangers have been reported by Sherrington and his collaborators [73] using both CP and SPE MAS 13C NMR methods. In the latter work four precursor resins were employed, two gel-type with 3.5 and 0.5 wt% divinylbenzene crosslinkers, and two macroporous with 7.5 and 3.5 wt% divinylbenzene crosslinker. Each was chloromethylated using chloromethyl methyl ether and in the case of resin 1 ZnCla as the Lewis acid catalyst, and for resins 2-4, FeC13. 13C CP MAS NMR spectra were recorded at 25.2 MHz and the ~CHaC1 resonance was seen clearly at 46.8 ppm. The enhanced intensity at ---40 ppm confirmed the occurrence of methylene bridging but overlap with backbone CH2 and CH signals precluded even tentative quantification. CP spectra of the anion exchange resins formed by reaction of trimethylamine with the chloromethylated species showed signals at 69.0 and 53.2ppm due to and ~ N + C H 3 carbons and corresponding loss of intensity around 47 ppm. However, a significant feature remained at 46 ppm and exhaustive study of this suggested its assignment to a weak base function ~N(CH3)2 probably generated from impurity dimethylamine in the trimethylamine. Interestingly such functionality had been detected from the ion exchange behaviour of these species much earlier by resin manufacturers. Using SPE techniques with the anion exchange resin samples it did prove possible to quantify for the first
557
CROSSLINKED POLYMERS
1
.
.
.
.
.
.
.
!
Iso
.
.
.
.
!
.
1o0 PPn
.
.
.
1
so
.
.
.
.
,.,
I
.
o
Fig. 15.3.48. 13C SPE NMR spectra of: top, anion exchange resin derived from, bottom, styrene-divinylbenzene resin 1(3.5% DVB, gel-type) (see Ref. 73).
time the level of methylene bridging arising on chloromethylation. Figure 15.3.48 shows the 13C SPE MAS NMR spectra of precursor polystyrene resin 1 and its derived anion exchange form, while Fig. 15.3.49 shows the corresponding spectra for resin 2.
558
R.V. LAW AND D.C. SHERRINGTON
1
|
9
9
9
~n
1
1S0
~.
~.
.
.
1
loo PPn
I
J
a
-
|
J
$o
-
9
_a
|
, j
o
Fig. 15.3.49. 13C SPE NMR spectra of: top, anion exchange resin derived from, bottom, styrene-divinylbenzene resin 2 (0.5% DVB, gel-type). (see Ref. 73).
The signal from aromatic quaternary carbon attached to the ~CH2N+(CH3)3 group (originally the--CH2C1) moves upfield to overlap with the protonated aromatic carbon signals. The residual downfield aromatic quaternary carbon signal therefore contains only two components, that due to the carbon atom connecting to the polymer backbone (quantitatively identical to the corresponding signal in the original nonfunctional resin) and that from carbon atoms attached to methylene bridges. Careful assessment of peak areas showed that on average nearly all aromatic groups are substituted with mCH2N+(CH3)3 groups (confirming elemental analytical data) but rather surprisingly ---53% of aromatic groups are methylene bridged in the anion exchange resin derived from resin 1, and --~63% in the case of resin
CROSSLINKED POLYMERS
559
CH~(CH3}~ Fig. 15.3.50. Structural units most likely present in anion exchange resins.
2. While resin manufacturers had suspected significant levels of methylene bridging, the quantitative data is nevertheless surprising. This means of course that the predominant anion exchange site is structure (2) rather than structure (1) (Fig. 15.3.50).
15.3.3
~3C MAS NMR studies of highly crosslinked poly(divinylbenzene)
resins High surface area polystyrene and polymethacrylate-based resins without additional functionality are useful sorbents [74]. Typically they are prepared by suspension polymerisation using relatively high levels of crosslinker (usually divinylbenzene, >50 vol%) in the presence of a solvating porogen such as toluene [74, 75]. It has been known for many years that polymerisation of all the vinyl groups in these systems is not complete, and indeed residual groups can be used for chemical modification. In addition, achieving adequate detailed molecular structural characterisation of these materials has proved very difficult primarily because of their highly crosslinked nature, but also because of the complex composition of the comonomer mixtures used in polymerisations. Commercially sourced "divinylbenzene" is prepared from diethylbenzene and the grade employed contains both meta- and para-isomers as well as smaller levels of other components. Consequently dehydrogenation to yield "divinylbenzene" yields a mixture of at least four components: metaand para-divinylbenzene and meta- and para-ethylstyrene, the latter arising from incomplete dehydrogenation. Restricting the conversion to divinylbenzene isomers seems deliberate since 100% divinylbenzene is rather unstable and gels readily on storage or in transport. Two grades of commercial divinylbenzene are widely available, one containing ---50-55% divinylbenzene isomers and the other ---80% of these. Widely known resins prepared with high levels of crosslinkers are the XAD series from Rohm and Haas, XAD2 and XAD-4 being particularly much used styrene types. Recently Sherrington and his collaborators [76] have prepared model high surface area resins by polymerisation of both grades of commercially available
560
R.V. LAW AND D.C. SHERRINGTON
0 Q
ClinCH
c
/
H
!
H3
200
150
100 PPM
50
0
Fig. 15.3.51. 13C CP/MAS NMR spectra of poly(divinylbenzene) resins: top, 100% p-DVB" middle,---50% commercial DVB" bottom---80% commercial DVB. (see Ref. 76).
divinylbenzene using toluene as the porogen, and a third model species from ---100% para-divinylbenzene prepared in-house. Fig. 15.3.51 shows the ~3C CP MAS N M R spectra of the three resins obtained at 25.2 MHz. The presence of ethyl groups from ethylstyrene in the commercial divi-
CROSSLINKED POLYMERS
561
nylbenzene is seen clearly, along with signals from unreacted vinyl groups. In the case of the resin from ---100% para-divinylbenzene, ethyl groups are absent and small signals from a carbonyl containing contaminant are also apparent. The corresponding 13C SPE MAS NMR spectra for the three resins are shown in Fig. 15.3.52. Unlike the CP spectra, the latter allow quantification of the level of residual unreacted vinyl groups by integration of appropriate peak areas. For the resin prepared from ---100% para-divinylbenzene ---44% of vinyl groups remain unreacted i.e., the effective crosslink ratio is ---56%. For the resin prepared with the ---80% grade of commercial divinylbenzene --~45% of vinyl groups remain i.e., the effective crosslink ratio is ---45%, while for the resin prepared with the ---50% grade of commercial divinylbenzene ---32% of vinyl groups remain i.e., the resin is ---35% crosslinked. The levels of residual double bonds are therefore remarkably high. 13C CP MAS NMR spectra for XAD-2 and XAD-4 are shown in Fig. 15.3.53, and corresponding SPE spectra in Fig. 15.3.54. Even before attempting any quantitative analysis the clear visual similarity of these spectra with the spectra of the two model resins (from ---50% and ---80% grade divinylbenzenes) is apparent. Since the manufacturers have not disclosed the comonomer composition used to produce XAD-2 and XAD-4 the quantitative analysis of these spectra was more difficult. However, by examining the ratio of aromatic carbons to aliphatic ones it seems that no styrene comonomer is used in the polymerisation feeds (confirmed from FTIR spectra). For XAD-2 the ethylstyrene and divinylbenzene contents seem to be ---49 and ---51% respectively. Likewise the figures for XAD-4 are ---21 and 79% respectively. With this data in hand the SPE spectra were then reanalysed as for the model resins to determine the levels of unreacted double bonds. For XAD-2 this turns out to be ---47% i.e., an effective crosslink ratio of ---27%, and for XAD-4, 49% unreacted vinyl groups i.e., an effective crosslink ratio of ---40%. Thus it seems that XAD-2 and XAD-4 are manufactured directly from the two widely available commercial grades of divinylbenzene, and the high levels of unreacted vinyl groups correspond very closely to the levels found in the two model resins prepared in-house. 15.3.4
13C MAS NMR studies of hypercrosslinked polystyrene resins
Davankov and his co-workers [55, 56] first discovered these remarkable resins. They are prepared either by chemically crosslinking linear polystyrene in a post-polymerisation treatment, or similarly post-treating lightly crosslinked (with ---0.3-2% divinylbenzene) polystyrene resins. Reagent quantities are chosen to allow exhaustive Friedal-Crafts alkylation of aromatic
562
R.V. LAW AND D.C. SHERRINGTON
,
1
200
.
a
.a
.
I
IS0
a
I
9
9
1
100
PPN
~
J
~
~
1
.
~
SO
i
.
1
.
0
Fig. 15.3.52. 13C SPE NMR spectra of poly(divinylbenzene) resins" top, 100% p-DVB; middle, ---50% commercial DVB; bottom, ---80% commercial DVB (see Ref. 76).
563
CROSSLINKED POLYMERS
I
9
I
a
I
I
150
Fig. 15.3.53.
13C
,
9
.
9
|
I00 PPH
I
,a
I
J
!
SO
J
9
,
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l
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CP/MAS NMR spectra of: top, XAD-4; bottom XAD-2 (see Ref. 76).
groups and the final product resins display some remarkable properties. Firstly, surface areas measured by N2 sorption can be as high as 8001000 mZg -1, some 200-300 mZg -1 higher than conventional high surface area styrene-divinylbenzene resins, and secondly, the resins, though "totally hydrophobic" visibly swell in nonsolvents such as alcohols and even water. Two solid-state 13C NMR spectral studies of these resins have now been published [58, 77] in an attempt to evaluate their molecular structures. The results of the two studies may not be directly comparable since the source of the resins differs in each case, and hence the structures might differ. In addition, one of the studies [58] uses CP spectra and a peak deconvolution procedure and it seems likely that this approach is less quantitative than the other which utilises SPE spectra [77]. Figure 15.3.55 shows the basic chemical steps in
564
R.V. LAW AND D.C. SHERRINGTON
I
'
9
I
9
|
IsO
9
9
a
!
|
"
Ioo PPH
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'
'
~
50
, a
a
a
'
|
J
0
Fig. 15.3.54. 13C SPE NMR spectra of: top, XAD-4; bottom XAD-2 (see Ref. 76).
synthesising the resins, where the methylene bridging side-reaction described earlier in the context of anion exchange resins is deliberately exploited. The 13C NQS and CP MAS NMR spectra of a hypercrosslinked resin are shown in Fig. 15.3.56 together with the spectral assignments. Residual ~CHzC1, and possibly ~ C H z O H groups are apparent, the former being confirmed by amination with trimethylamine. Figure 15.3.57 shows the corresponding ~3C SPE MAS NMR spectrum. The peak area ratios can be used here to quantify the various types of carbon atom present. The theoretical ratio of total aromatic carbons to total
565
CROSSLINKED POLYMERS CHsOCHtCI SnCI 4
SnCl,/
,/
\
t
CH,CI
,( rl
Fig. 15.3.55. Synthesis of hypercrosslinked resins via deliberate exploitation of methylene bridging reaction.
aliphatic carbons based on the model methylene-bridged structure in Fig. 15.3.55 is 2.40:1.00. The experimental value is 2.05:1.00. This confirms the very high level of methylene bridging, but since the ratio is even less than predicted from the simple structure, it suggests that additional aliphatic carbon functionality arises, yielding trialkyl-substituted aromatic rings. This is only possible if a very high level of double methylene bridging of aromatic groups occurs, and there appears to be a statistical distribution of such bridges between all aromatic groups rather a regular structure. In addition, elemental microanalysis confirms the presence of residual mCHzC1 groups which are located on --~10% of the aromatic groups. These must, of course, be highly hindered species. Two additional hypercrosslinked resins from a commercial source reportedly prepared from conventionally heavily crosslinked divinylbenzene resins [78] appear in one instance to exploit unreacted double bonds (and possibly additional divinylbenzene) to generate the secondary crosslinks, and in another to use SOCl2/Lewis acid to introduce very high levels of sulfoxide secondary crosslinks. Interestingly dynamic NMR experiments suggest that the latter two hypercrosslinked species retain more flexibility and mobility than that prepared via the more conventional methylene bridging technique.
R.V. LAWAND D.C. SHERRINGTON
566
_CH,O(?)
CICH,Ar I
HOCHIAr ~ .
.
.
/
CH,CH,Ar
.
H
ClC.~
~CH,CHAr
~]/~ CHsCH2Ar
l_
Fig. 15.3.56.
Ref. 77). 15.4 15.4.1
13C
~1
9
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NQS and CP/MASNMR spectra of hypercrosslinkedpolystyreneresin (see
Use of NMR spectroscopic analysis in solid phase synthesis
Background
Professor Bruce Merrifield first disclosed the use of a crosslinked polystyrene support as a macroscopic protecting group in oligopeptide synthesis in 1963 [79], and heralded the arrival of "solid phase synthesis". Some twenty years later he received the Nobel Prize for Chemistry for developing and exploiting this concept. Since his original work the use of polymer-, and indeed inorganic oxide-based supports, has expanded enormously to include polymeric reagents, polymeric protecting groups and auxiliaries and a wide range of polymer-supported catalysts [80]. The vast majority of applications employ crosslinked insoluble polymers as the support, usually in the form of spherical
567
CROSSLINKED POLYMERS
1
1
ISO
IO0 PPn
St
0
Fig. 15.3.57. 13C SPE NMR spectrum of hypercrosslinked polystyrene resin (see Ref. 77).
particulates or beads, --~20-500 Ixm in diameter, and this format provides the convenience of handling and manipulation which has led to the popularisation of the methodology. Right from the start a major weakness in solid phase synthesis, especially in applications where quite complex molecules were to be synthesised attached to a resin support, was the lack of molecular structural characterisation techniques comparable in scope and sensitivity to the methodologies available for soluble molecules. In particular, the organic synthetic chemists most powerful analytical tool, high resolution 1H and 13C NMR spectroscopy was initially not applicable. Over the last five years or so there has been an explosion in the use of solid phase synthesis when it was realised that the method could be adopted in combinatorial synthesis to allow rapid, even automated, synthesis of large libraries or mixtures of organic molecules [81-86]. This possibility has been seized upon by drug discovery groups in pharmaceutical companies as a rapid method of preparing and screening large numbers of compounds in the search
568
R.V. LAW AND D.C. SHERRINGTON
for new lead compounds. This in turn has led to a stimulation of this work in small entrepreneurial companies and in academic research laboratories worldwide [86]. The involvement of a broad group of synthetic chemists has made the need for better structural analysis more urgent, and this has brought effort and resources to bear on improving the analysis of supported reactions by NMR spectroscopy. In practice the type of support used most widely is a lightly crosslinked poly(styrene-divinylbenzene) system usually in the form of spherical beads ---50-500 l~m in diameter produced by suspension polymerisation [51]. Typically the level of the crosslinking comonomer divinylbenzene used is only ---0.5-2.0 vol%. This is crucial in terms of structural analysis by NMR, since such lightly crosslinked systems can swell considerably in suitable solvents (up to ---15 fold), such that the local environment around a functional group attached to the polymer network can approach closely to that in isotropic solution (see later).
15.4.2
Solution phase NMR studies
Structural analysis of linear polymers molecularly dissolved in a suitable solvent using 1H and 13C solution phase NMR spectroscopy is long established [87-89]. Not surprisingly therefore when a linear soluble polymer is used as a support in solid phase synthesis 1H and X3C solution phase NMR spectroscopy can be a powerful tool in following the chemical synthesis on the support [90]. Figure 15.3.58, for example, shows a series of 1H NMR spectra of dissolved linear polymer samples taken at various stages in the solid phase synthesis of oligoethers on soluble polystyrene [91]. The various chemical steps Fig. 15.3.59 are clearly demonstrated. Even in the case of linear polymers, however, if the solution concentration is increased to allow the local and macroscopic viscosity to rise significantly the mutual dipole-dipole interactions of magnetic 1H nucleii give rise to broadening of their NMR resonances, though the effect with the very much lower natural abundance of 13C nucleii is considerably less. In the case of crosslinked polymers therefore even when the level of crosslinking is very low attempts to record "solution" 1H NMR spectra of solvent swollen samples generally result only in broad signals. However, ~3C NMR spectra from swollen lightly crosslinked polymer gels can show remarkably fine line-width resonances.
"([6 "J~ oos) dols uo!looloJdop puooos 'd '.dols ~u!Idnoo puooos JO uo!legUOlO ' 3 '.dols UOgeA!lOe 'O '.dols uo!loo~mdop 'D '.do~s $u!ldnoo :to luotuqoelle '~l '.ouoJg~Sglod POl~IgqlotuoJolqo olqnlos 'V :laoddns oIqnIos Jeou!I e uo (6~'17"~I "g!d) sJoqloo$!IO jo s!soqluds oseqd p!los jo oSels qoeo le potuaoj slonpoJd jo ealoods ~IIAIN H~ uo!lnIos "8_c'P'~I "8!d
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CROSSLINKED POLYMERS
571
(••=•CH2CI coupling
Nail H(OCH2CH2),OCPh3
(~~CH2(OCH2CH2),OCPh3 deprotection
CF,C 0 H H20
(~~-~CH2(OCH2CH2),OH activation ( ~ - ~
CH3SO2CI DMAP/TEA
C H ~ ( O C H ~ C H ~ ) , O S O~CH3
elongation
NaH H(OCH2CH2),OCPh 3
deprotection
CF3C O2H H,O
@--~CH2(OCH2CH~).OH Fig. 15.4.59. Scheme for solid phase synthesis of oligoethers on a soluble linear polystyrene support.
15.4.3 Swollen gel-phase 13C NMR studies Early work on the application of 13C NMR conventional techniques to the study of solvent swollen crosslinked polymers was reported by Stenlicht and co-workers [92] and Schaefer [93]. The first detailed work on the chloromethylated polystyrene resins used in solid phase synthesis appears to be by Manatt and co-workers [94]. They employed a 0.095% crosslinked species supplied by the Dow Chemical Co. and chloromethylated this to varying d e g r e e s . 1H noise decoupled 13C NMR spectra were recorded in the usual
572
R.V. LAW AND D.C. SHERRINGTON
200
160
120
ppm
80
40
6%DVB
2%
!
o~
Fig. 15.4.60. 13C NMR spectra of chloromethylated polystyrene resins of 0-6% crosslink ratio swollen in CDC13 (see Ref. 97).
way at 15.1, 25.1 and 45.2 MHz using resin slurries in CHC13 or CDC13 in 10 mm o.d. sample tubes. The chemically modified Dow resin yielded spectra at various--CHzC1 loadings essentially as sharp as those of a model soluble linear polymer while the spectrum of a chloromethylated ---1% crosslinked polystyrene already started to show broadening. Interestingly the analysis showed that chloromethylation occurred almost exclusively in the 4-position of styryl residues, and significant levels o f - - C H z O H groups were also detected as a result of hydrolysis of the - - C H z C 1 groups, confirming an earlier report by Merrifield and co-workers [95]. Manatt and co-workers also described the use of a 19F NMR approach for studying peptide synthesis on swollen gels [96]. In many respects, however, Ford and his collaborators brought to a wider audience the potential scope for using conventional X3c NMR techniques for analysing solvent swollen lightly crosslinked functional polymers [70, 71, 97]. Figure 15.4.60 shows the gated decoupled 13C NMR
CROSSLINKED POLYMERS
573
spectra of polystyrenes containing 25 wt% chloromethylstyrene and 0-6% divinylbenzene crosslinker. The spectra were recorded at 25.2 MHz with the samples fully swollen in CDC13 in 12 mm o.d. tubes. Extensive relaxation experiments and attempts to quantify relative peak areas demonstrated a number of artefacts to arise in the generation of these spectra (see later). Nevertheless the work set the scene for the expansion in the use of so-called "gel-phase" 13C NMR spectroscopy applied to solid phase reactions on crosslinked polymer supports. Major contributions to gel phase 13C NMR spectral analysis in the solid phase synthesis of oligopeptides have been made by the groups of Epton [98-101] and Girault [102-104]. Epton and his co-workers [105] pioneered the use of "ultra-high load" methods in peptide synthesis using a phenolic resin, such that in the final support-peptide assemblies the anchored peptide chains comprise the major mass component. In these reactions the group have monitored N-terminal Boc and Fmoc removal by gel phase 13C NMR spectroscopy [99, 100], investigated the lability and stability of some common side-chain protecting groups [101] and characterised a number of polymerpeptide assemblies [99, 100]. Remarkably detailed 13C NMR spectra (e.g., Fig. 15.4.61) are reported in these works. Girault's group have examined the mobility of pendant groups on polymer resins and shown the importance of using the correct solvent, not only for swelling the resin, but also for minimising any intrapolymeric interactions of immobilised groups that can give rise to major line broadening in the 13C NMR spectrum [106]. The association of peptide chains during solid phase synthesis has also been studied by quadruple echo deuterium NMR spectroscopy [107]. As well as utilising lightly crosslinked gel-type resin supports Giraults group has also examined macroporous polystyrene resins, Kel-F-gstyrene, polyacrylamide and controlled pore glass supports, and have applied both 19F and 13C NMR gel-phase spectroscopy to correlate the microenvironmental mobility of protected amino acids with the differences in reactivity observed in peptide synthesis [103]. Other state-of-the-art work from the group includes the stage-by-stage gel-phase 13C NMR spectroscopic characterisation of a growing peptide chain during stepwise synthesis [102]; 31p_ NMR spectroscopic studies of oligonucleotide synthesis on lightly crosslinked polystyrene; and 2D-1H-13C correlated NMR spectra of oligopeptides bound to similar supports. In each case the crosslinked gel was swollen in CDCI3 [104]. During the above development of gel-phase NMR analysis of supported reactions, advances were also in progress with regard to the solid support itself. The most widely used lightly crosslinked polystyrene systems had con-
c.,--- co--..--
c.--co--..--c.--co-N.--c.--co-!
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/~ C=NHI NH SO,
o I
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r~4
148
r~!'
]
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104
ppm
l0
tl
Z C~ -] 0 Z
1J 40
20
0
Fig. 15.4.61. Gel phase 13C NMR spectrum of peptide synthesised on a high load resin with the matrix swollen in (CD3)2SO (see Ref. 101).
CROSSLINKED P O L Y M E R S
575
siderable restrictions with regard to the solvents which could be employed in reactions, and this led to a number of strategies to try to overcome the problem. Perhaps the one which has had the greatest impact is the development of lightly crosslinked polystyrene gels with long polyethylene glycol (PEG) derived sidechains, where the solid phase synthesis is carried out on the free terminus of the PEG chains [108, 109]. These materials have been termed "tentacle" polymers [110, 111] and form the basis of the TentaGel | range of supports available from Rapp Polymere, Tfibingen, Germany (also known as the "Rapp resin"). These materials are unusual because of the broad solvent compatibility range they offer, but also because of the flexibility and accessibility of the functional endgroups. This is reflected in the gelphase 13C NMR relaxation times, T1, of peptides bound to the PEG terminus, and to the sharp NMR signals which are recorded [112]. When a poor solvent for the PEG tentacles is employed however, e.g., diethyl ether, broad NMR signals are obtained typical of measurements on solids in suspension. Tentagel | resins have rather low capacities (per gramme) as a result of the long PEG spacers used and this also, no doubt, contributes to the efficiency of solid phase synthesis and to the quality of 13C NMR spectra which can be obtained. However, recently Brown and Ramsden [113] have used gel-phase ~3C NMR spectroscopic analysis to demonstrate high fluidity and homogeneity in the case of 2% crosslinked polystyrene resin functionalised to a relatively high level (--~60% pendent group) with an eleven carbon atom spacer. Again the solvent employed is crucial and for a tethered tetrapeptide [2H6]DMSO proved very effective, presumably as a result of minimising interchain aggregation. Typically spectra were obtained at 126 MHz in --~1 hour at 50~ A further development in gel-phase ~3C NMR spectroscopic analysis has been the use of ~3C enriched building blocks in the synthesis [114]. This allows the use of much smaller quantities of resin (--~20 mg) containing less than 1 mg of the moiety to be analysed. In the case of synthesis on a Tentagel | resin, rapid analysis is possible because of the enhanced mobility of the system and the reduced number of transients that need to be accumulated. 15.4.4
Swollen gel-phase MAS 1H NMR studies
Although analysis of solid phase reactions by ~3C NMR spectroscopy is a very powerful methodology the move towards synthesising a diverse range of organic molecules on polymer supports had led to an increased demand for ~H NMR spectra comparable to those achievable for small molecules dissolved in a solvent. As early as 1974 Doskocilov~ and co-workers [115] reported that high resolution ~H NMR spectra of crosslinked poly(methylme-
576
R.V. LAW AND D.C. SHERRINGTON
thacrylate) gels swollen in chloroform could be achieved by spinning the sample at the "magic angle" relative to the applied stationary field, in a similar way to the acquisition of solid-state spectra. The method was effective for crosslinking levels in the range 0-1%. Despite a further report in 1985 indicating 1H line narrowing in polystyrene-divinylbenzene resins [116] by magic-angle spinning of swollen gels, this development was not picked up by those involved in solid phase synthesis. The effect was confirmed and exploited by Mashelkar and his co-workers studying super-absorbing polymer gels [117], before the much broader relevance was realised by Fr6chet and his group [118, 119]. These researchers showed that in the case of a CDC13 swollen functionalised 1% crosslinked polystyrene support with 12% of the aromatic groups with ephedrine moieties, direct polarisation 13C and 1H NMR spectra with high resolution could be obtained by spinning the sample in a normal MAS probe. Figures 15.4.62 and 15.4.63 show the 13C and 1H spectra respectively obtained using a standard 7 mm MAS probe in a Bruker AF-300 spectrometer operating at 75.47 MHz for ~3C. Spinning rates were 2000 Hz and 2350 Hz respectively for 13C and 1H spectra. In this instance the ephedrine moiety was attached directly to the polymer backbone and in retrospect it is clear that structural units attached by flexible spacers were likely to yield even better spectra. Since this approach did not require the auxiliary power amplifiers typically needed for cross-polarisation (CP) MAS in the solid-state the authors argued [118] that the method could become rather routine from both an experimental and instrumental point of view. More detailed work [119] by the group on 1-2% crosslinked resins highly swollen with solvent, as examples of polymeric systems that undergo rapid but anisotropic reorientation on the molecular scale, confirmed that the spectral line broadening due to the residual dipolar coupling (1H NMR) or chemical shift anisotropy (13C NMR) can be removed by moderate-rate magic-angle spinning, to yield highly resolved ~H and 13C NMR spectra. Furthermore, well-resolved signals attributable to the crosslink junctions themselves (in the case of a 5% dimethylsilane crosslinker) were recorded. A further advance was reported by Keifer and co-workers [120] in terms of reduced sample size. They utilised a Varian Nano-NMR probe with a 500 MHz instrument to record 1H NMR spectra of Tentagel | bound substrates. Typically 10 mg samples swollen in 30 ~L of DMSO-d6 and spun at 2000 Hz at the magic angle yielded better spectra than 100 mg of sample in a more routine gel-phase experiment not involving magic-angle spinning. Shapiros group have used a more conventional 7 mm o.d. probe to obtain 1 3 C ~ 1 H correlated NMR spectra of substrates attached to both Tentagel | [121] and Wang [122] resins. Again spinning at the magic angle yielded much better resolved signals than those from simple gel-phase experiments, and 2D corre-
577
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lations approaching those of solution phase spectra. Reduction in sample size to the ultimate analysis of a single resin bead has been reported by both Keifer and co-workers [123], and Rapp and his collaborators [124]. The former group employed the Varian Nano-NMR probe with a single bead of Wang resin carrying a 3,5-dimethoxy-benzoic residue. The bead was swollen in 30 ~l of CHzClz-d2 and the 1H NMR spectrum recorded at 500 MHz with the sample spun at the magic angle at 2000 Hz. In practice the resulting
578
R.V. LAW AND D.C. SHERRINGTON
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Fig. 15.4.63. Gel phase IH NMR spectra of ephedrine immobilised on a 1% crosslinked polystyrene resin: A, "solution state" spectrum" B, sample spun at the magic angle (see reference 118).
spectrum was complicated by large signals arising from solvent backgrounds, impurities and fingerprints on the outside of the sample cell, as well as smaller peaks due to the polystyrene backbone itself. To achieve usable spectra it was necessary to synthesise the corresponding species 13C enriched on the methyl groups of the methoxy substituents. To some extent therefore this requirement negates the value of the single bead analysis. In contrast the Rapp team [124] took a single macro bead of Tentagel | with a diameter in the range 400-750 ~m swollen in DMSO-d6 or CDC13 fixed into an insert
CROSSLINKED POLYMERS
579
and placed in a standard 4 mm o.d. solid-state NMR rotor. The detection volume was 200-400 I~L. The XH NMR spectra were recorded at 300 MHz with magic-angle spinning at 3000 Hz. Very detailed high resolution spectra (Fig. 15.4.64) of the hydantoin reaction sequence on the resin were obtained in this way. Since the above reports appeared, the use of magic-angle spinning of solvent swollen lightly crosslinked gel-type polymer supports has expanded, particularly for monitoring the progress of solid phase syntheses [125, 126]. ~H NMR spectra at 600 MHz show remarkably detailed features [126] and magic-angle spinning in acquisition of 19F NMR spectra has also proved very valuable [127]. The in-house use of the Varian Nano-NMR probe has been expanded [128] to investigate the influence of the resin structure, tether length and solvent on the quality of the high resolution 1H NMR spectra of bound peptides. Perhaps, not surprisingly, all of these factors have been shown to contribute to spectral quality. Despite the improvement in the narrowing of XH NMR signals by spinning solvent swollen gels at the magic angle, this approach still treats the signals from the polystyrene support in exactly the same way as the signals from any anchored species. Wekler and Westman [126] have used the Varian Nanoprobe with spinning at the magic angle in a 600 MHz instrument to record ~H NMR spectra of a diphenylsulphone attached to CDC13 swollen Tentagel | By combining presaturation of an upfield linker signal at 3.5 ppm, with a 180~ ~ double pulse experiment, the broad signals from the aromatic protons were almost totally suppressed (Fig. 16.4.65). Sarkar and co-workers have argued that the complication of the presence of the large peaks from the matrix could be reduced by judicious choice of the r value in the spin echo pulse sequence (90~176 [129]. This should be possible because the matrix signals are much broader presumably because of a short T2. The use of the spin echo sequence to distinguish between narrow and broad lines has been reported before [130, 131]. Figure 15.4.66 shows the 500 MHz ~H MAS NMR spectra of species (3) bound to a Wang resin swollen in CD2C12. The spectra were recorded from ---6 mg resin in a Nano-NMR probe. It is clear that by appropriate choice of the r value the effect of the polystyrene resonances can indeed be reduced significantly. At the time of writing Shapiro and his group have shown further that the loss of coupling information which accompanies the spin-echo experiment, can be restored by utilising a 2D J-resolved procedure [132]. Similarly S0rensen and his co-workers have reported the results of 2D ~H homonuclear correlations to reduce the inhomogeneous line broadening [133]. Clearly the various specialised instrumental techniques described above are yielding increasingly better quality 1H NMR spectra of resin bound
580
R.V. LAW AND D.C. SHERRINGTON I
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Fig. 15.4.64. Gel phase 1H NMR spectra of products at each stage of a solid phase synthesis of an hydantoin on a single lightly crosslinked Rapp macro-bead swollen in CDC13 and spun at the magic angle (see Ref. 124).
581
CROSSLINKED POLYMERS
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Fig. 15.4.65. Gel phase 1H NMR spectra of aromatic sulfone species attached to Tentagel | swollen in CDC13 and spun at the magic angle, showing manipulation of the pulse sequence to suppress the signals from the support hydrogen atoms (see Ref. 126).
substrates, and it is by no means clear yet how far these developments can go and where the levelling in technology will occur. Nor is it yet clear what can be regarded as routine. Much of the instrumentation and some of the techniques described are costly in terms of both hardware and man-hours, and this will restrict the number of groups capable of applying these methods, and indeed potentially limit the throughput of samples within these groups. There is little doubt however that those wishing to participate at the leading edge of solid phase synthesis methodology will have to assimilate some of these procedures. In all of these developments it is also important to bear in mind two other factors. Firstly, it is not clear yet how quantitative the various specialised methods are likely to be since application of complex chemical-physics methodologies often involve a compromise in terms of the generation of artefacts, especially with regard to the level of quantification that is possible (see Solid State SPE MAS 13C NMR analysis). The second factor to be borne
582
R.V. LAW AND D.C. SHERRINGTON
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CROSSLINKED POLYMERS
583
in mind is the dramatic limitation all these developments involve with regard to the level of crosslinking and local mobility of the support. Essentially all the work reported has involved very lightly crosslinked gel-type supports typically nominally --~1% crosslinked. While this is fine in terms of combinatorial solid phase synthesis no advantage has been reported to date in those areas (e.g., polymer-supported catalysts) using highly crosslinked macroporous resins. No doubt some forward movement will be seen here in due course, but for now it remains an area which offers a considerable challenge to NMR specialists!
15.5
Swollen-state N M R studies of crosslinked elastomers
Elastomeric polymers i.e., polymers whose glass-transition temperatures, Tg, lie well below ambient temperature generally have very low tensile and tear strength. This is simply because the intermolecular forces are very weak, allowing individual polymer chains to be physically pulled apart. For practical application therefore it is necessary to vulcanize or crosslink elastomers to provide toughness and strength. Although in recent years a number of thermoplastic block copolymer elastomers have been developed in which the interbonding of elastomer chains is provided by discrete microphases of glassy polymer, in practice the large majority of rubbers use crosslinking as the method of forming a tough infinite network of elastomeric chains. The level of crosslinking is not surprisingly therefore a key parameter in defining and controlling the bulk properties of an elastomer. Despite this pivotal position, it has proved very difficult to develop routine and simple methods for quantifying the level of crosslinking in a polymer. An important methodology established early on is an equilibrium volume swelling approach and application of the Flory-Rehner equation [134, 135]. While this is satisfactory for simple one component elastomer networks it is not particularly convenient experimentally and is of dubious value for multicomponent elastomers. Paradoxically the "problematical" line broadening, arising in the NMR spectra of polymers which are crosslinked (see Section 4), has been exploited to provide a simple instrumental technique for quantifying crosslinking. Although ~H NMR spectral line broadening on crosslinking had been observed earlier, Marchenkov and Khitrin [136] first published a mathematical explanation of the observation that line widths in the spectra of polymer gels are quantitatively related to the concentration of crosslinks under defined conditions. Loadman and Tinker [137] took vulcanizates of a blend of natural rubber and acrylonitrile-butadiene rubber swollen in CDC13 and recorded
584
R.V. LAW AND D.C. SHERRINGTON
CW ~H NMR spectra at 90 MHz. While the peak broadening of the methyl protons in natural rubber alone could be monitored, this proved impossible in blends because of spectral overlap. As a result the proton signals from the olefinic group were found to be more generally useful. Peak broadening (H%) was assessed as the ratio of the signal strength at a reference point on the high field side of the peak to the signal strength at the peak. Good correlation was found between H% and the level of crosslinking in each component rubber as measured by a separate solvent swelling experiments. Blends of natural rubber and cis-polybutadiene have been similarly investigated [138]. The analytical procedure has also been developed for 1H NMR spectra of swollen elastomers recorded on a 300 MHz FT instrument [139]. In this case a progressive downfield shift of the olefin proton peaks with increasing crosslink ratio is observed and this complication has to be accounted for in the peak broadening analysis. Similar studies have been reported using a 200 MHz instrument [140]. Although most studies have employed ~H NMR spectra, analogous phenomena are seen with ~3C NMR spectra obtained from solvent swollen elastomers, and similar correlations with crosslink ratio are possible [139]. This approach broadens the applicability of the technique to include elastomers whose ~H NMR spectra do not lend themselves to this analysis. Overall the use of advanced NMR spectral techniques is becoming more widespread in the study of intractable elastomers [141, 142].
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CROSSLINKED POLYMERS
587
84. K.S. Lam, S.E. Salmon, E.M. Hersh, g.J. Hruby, W.M. Kazmiersky and R.J. Knapp, Nature (London) 354 (1991) 82. 85. M.A. Gallop, R.W. Barrett, W.J. Dower, S.P.A. Fodor and E.M. Gordon, J. Med. Chem. 37 (1994) 1233. 86. F. Balkenhopl, C. von dem Bussche-Ht~nnefeld, A. Lansky and C. Zechel, Angew. Chem. Int. Edn., Engl. 35 (1996) 2288. 87. F.A. Bovey, High Resolution NMR of Macromolecules, Academic Press, New York, 1972, 118. 88. K. Matsuzaki, T. Uryu, K. Osada and T. Kawarmura, Macromolecules 5 (1972) 816. 89. A. Allerhand and R.K. Hailstone, J. Chem. Phys. 56 (1972) 3718. 90. W. Schoknecht, K. Albert, G. Jung and E. Bayer, Liebigs Ann. Chem. (1982) 1514. 91. H.C. Henderson, D.C. Sherrington and A. Gough, Polymer 35 (1994) 2867. 92. H. Sternlicht, G.L. Kenyon, E.L. Packer and J. Sinclair, J. Am. Chem. Soc. 93 (1971) 199. 93. J. Schaefer, Macromolecules 4 (1971) 110. 94. S.L. Manatt, D. Horowitz, R. Horowitz and R.P. Pinnell, Anal. Chem. 52 (1980) 1532. 95. S.B.H. Kent, A.R. Mitchell, M. Engelhard and R.B. Merrifield, Proc. Natl. Acad. Sci., USA 76 (1979) 2180. 96. S.I. Manatt, C.F. Amsden, C.A. Beltison, W.T. Frazer, J.T. Gudman, B.E. Lenk, J.F. Lubetich, E.A. McNelly, S.C. Smith, D.J. Templeton and R.P. Pinnell, Tetrahedron Lett. 21 (1980) 1397. 97. W.T. Ford and T. Balakrishnan, Macromolecules 14 (1981) 284. 98. R. Epton, P. Goddard and K.J. Ivin, Polymer 21 (1980) 1367. 99. F.G.W. Butwell, R. Epton, J.V. McLaren, P.W. Small and D.A. Wellings in Peptides-Structure and Function, C.M. Deber, V.J. Hruby and K.D. Kopple (Eds.), 9th American Peptide Symposium Proceedings, Pierce Chem. Co., Rockford, USA, 1985, 173. 100. R. Epton and D.A. Wellings in Peptides 1986, D. Theodoropoulos (Ed.), 19th European Peptide Symposium Proceedings, Walter de Gruyter, Berlin, Germany, 1987, 151. 101. F.G.W. Butwell, R. Epton, E.J. Mole, N. Muzaffar and S. Phillips in "Innovation and Perspectives in Solid Phase Synthesis", R. Epton (Ed.), SPCC UK Ltd., Birmingham, U.K., 1990, 121. 102. E. Girault, J. Rizo and E. Pedroso, Tetrahdron 40 (1984) 4141. 103. F. Albericio, M. Pons, E. Pedroso and E. Girault, J. Org. Chem. 54 (1989) 360. 104. E. Girault, F. Albericio, F. Bardella, R. Eritja, M. Feliz, E. Pedroso, M. Pons and J. Rizo in Innovation and Perspectives in Solid Phase Synthesis, R. Epton (Ed.), SPCC UK Ltd., Birmingham, UK, 1990, 111. 105. R. Epton, D.A. Wellings and A. Williams, Reactive Polymers 6 (1987) 143. 106. M. Feliz, E. Girault, J.M. Ribo and F.R. Trull, Makromol. Chem. 189 (1988) 1551. 107. A.G. Ludwick, L.W. Jelinski, D. Live, A. Kintannar and J.L. Dumais, J. Am. Chem. Soc. 108 (1986) 6493. 108. E. Bayer, Angew. Chem. Int. Edn. Engl. 30 (1991) 113. 109. G. Barany, F. Alberico, S. Biancalana, S.L. Bontems, J.L. Chang, E. Eritja, M. Ferrer, C.G. Fields, M.H. Lyttle, N.A. So16, Z. Tian, R.J. Van Abel, P.B. Wright, S. Zalipsky and D. Hudson, in J.A. Smith and J.E. Rivier (Eds.), Peptides: Chemistry and Biology (Proc. 12th American Peptide Symp.), ESCOM, Leiden, The Netherlands, 1992, 603. 110. E. Bayer and W. Rapp, German Patent DE-A 3714258 (1988). 111. W. Rapp, Dissertation, Universit~it Ttibingen, Germany, 1985. 112. E. Bayer, K. Albert, H. Willisch, W. Rapp and B. Hemmasi, Macromolecules 23 (1990) 1937.
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113. J.M. Brown and J.A. Ramsden, J. Chem. Soc., Chem. Comm. (1996) 2117. 114. G.C. Look, C.P. Holmes, J.P. Chinn and M.A. Gallop, J. Org. Chem. 59 (1994) 7588. 115. D. Doskocilovfi, B. Schneider and J. Trekoval, Coll. Czech. Chem. Comm. 39 (1974) 2943. 116. D. Bahneider, D. Doskocilov~i and J. Dybal, Polymer 26 (1985) 253. 117. S. Ganapathy, M.V. Badiger, P.R. Rajamohanan and R.A. Mashelkar, Macromolecules 22 (1989) 2025. 118. H.D.H. Stover and J.M.J. Frdchet, Macromolecules 22 (1989) 1574. 119. H.D.H. Stover and J.M.J. Fr6chet, Macromolecules 24 (1991) 883. 120. W.L. Fitch, G. Detre, C.P. Holmes, J.N. Shoolery and P.A. Keifer, J. Org. Chem. 59 (1994) 7955. 121. R.C. Anderson, M.A. Jarema, M.J. Shapiro, J.P. Stokes and M. Ziliox, J. Org. Chem. 60 (1995) 2650. 122. R.C. Anderson, J.P. Stokes and M.J. Shapiro, Tetrahedron Lett. 36 (1995) 5311. 123. S.K. Sarkar, R.S. Garigipati, J.L. Adams and P.a. Keifer, J. Am. Chem. Soc. 118 (1996) 2305. 124. M. Pursch, G. Schlotterbeck, L.H. Tseng, K. Albert and W. Rapp, Angew. Chem. Int. Edn., Engl. 35 (1996) 2867. 125. I.E. Pop, C.F. Dhalluin, B.P. D6prez, P.C. Melnyk, G.M. Lippens and A.L. Tartar, Tetrahedron 52 (1996) 12209. 126. T. Wehler and J. Westman, Tetrahedron Lett. 37 (1996) 4771. 127. M.J. Shapiro, G. Kumaravel, R.C. Petter and R. Beveridge, Tetrahedron Lett. 37 (1996) 4671. 128. P.A. Keifer, J. Org. Chem. 61 (1996) 1558. 129. R.S: Garigipati, B. Adams, J.L. Adams and S.K. Sarkar, J. Am. Chem. Soc. 61 (1996) 2911. 130. D.L. Rabenstein, K.K. Mills and E.J. Strauss, Anal. Chem. 60 (1988) 1380A. 131. P.F. Agris and I.D. Campbell, Science 216 (1982) 1325. 132. M.J. Shapiro, J. Chin, R.E. Marti and M.A. Jarosinski, Tetrahedron Lett. 38 (1997) 1333. 133. A. Meissner, P. Bloch, E. Humpfer, M. Spraul and O.W. SCrensen, J. Am. Chem. Soc. 119 (1997) 1787. 134. P.J. Flory and J. Rehner, J. Chem. Phys. 11 (1943) 521. 135. P.J. Flory, J. Chem. Phys. 18 (1950) 108. 136. V.V. Marchenkov and A.K. Khitrin, J. Chem. Phys. 3 (1986) 2214. 137. M.J.R. Loadman and A.J. Tinker, Rubber Chem. Technol. 62 (1989) 234. 138. P.S. Brown and A.J. Tinker, J. Nat. Rubber Res. 8 (1993) 1. 139. P.S. Brown, M.J.R. Loadman and A.J. Tinker, Rubber Chem. Technol. 65 (1992) 744. 140. V.A. Shershnev, I.K. Shundrina, V.D. Yulovskaya and I.A. Vasilenko, Polymer Science 35 (1993) 1428. 141. C.D. Hull, K.D.O. Jackson and M.J.R. Loadman, J. Nat. Rubber Res. 9 (1994) 23. 142. I. Fontao, L. Gonzalez, M.L. Jimeno and A. Marcos, Kautsch. Gummi Kunstst. 46 (1993) 431.
Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved
Crosslinked Polymers R.V. Law ~ and D.C. Sherrington 2 1Department of Chemistry, Imperial College of Science, Technology and Medicine, London, SW 7 2AZ, UK; 2D.C. Sherrington, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, UK
15.1
Introduction
Crosslinked polymers are key materials in a wide range of technological applications ranging from the "high-tech" aerospace area to the "low-tech" use of wood-derived products. Inevitably, the bulk properties and materials performance are controlled in the final analysis by the polymer molecular structure, although to relate the backbone structure to performance in applications is a complex exercise. This often involves the requirement for numerous intermediate correlations e.g., molecular structure with micromorphology, micromorphology with phase separation, etc. Evaluating the molecular structure of linear polymers has become relatively straightforward in recent years and solution phase ~H and ~3C NMR techniques have played an important role in this context. When linear polymers are crosslinked to form an infinite network, either during the polymerisation process, or as a post-polymerisation chemical treatment, then analysis by routine solution NMR spectroscopy rapidly becomes impossible as a result of signal broadening. When the level of crosslinking remains low, solution phase methodology applied to highly solvent swollen polymer can still be useful (see Sections 4.3 and 4.4). In general though, most crosslinked polymers are relatively highly crosslinked, and become amenable to analysis by NMR spectroscopy only by using solid phase techniques.
15.2 Solid-state ~3C and ~SN cross-polarisation/magic-angle spinning (CP/MAS) NMR 15.2.1
Background
Despite the obvious benefits of quantitative solid-state NMR via single pulse excitation methodology (SPE) (see Section 3), cross-polarisation combined with magic-angle spinning (CP/MAS) [1] has many intrinsic advantages which
510
R.V. LAW AND D.C. SHERRINGTON
have proved very valuable in evaluating the structure of crosslinked polymers. The principle benefit of CP is that it enables a spectrum to be acquired very quickly, as more scans can be obtained in any given time, giving a better signal-to-noise (S/N) ratio. The increase in S/N ratio is due to the fact that in many organic materials the proton spin-lattice relaxation mechanism, by which the system can relax back to equilibrium, is approximately an order of magnitude shorter that for 13-carbon. Though it is desirable to use SPE it is often impractical to carry out a fully quantitative analysis because experimental conditions e.g., MAS at high temperatures, means that there is limited time available and therefore a compromise, using only CP, has often to be made. If time permits an ideal approach is to use a combination of both SPE and CP as these techniques complement each other well (see Section 3). The other advantage that CP brings is important information about the molecular dynamics within the polymer from CP variable contact studies. The parameters typically obtained include the cross relaxation time between 1H and 13C, TcH, and the relaxation time in the rotating frame, ~H Tip. Furthermore, there is also a large (--~ factor 4 for 13C) sensitivity enhancement for ~3C when carrying out CP as polarisation is transferred from the isotopically abundant 1H (100%) to the rare 13C (1.1%). This section will deal principally with crosslinked polymers which have been characterised with CP combined with MAS. Earlier monographs and reviews have dealt, in part, with the application of CP/MAS to crosslinked polymer systems. These include reviews by Yu and Guo [2], Andreis and Koenig [3], books by Komorski [4], Mathias [5], McBrierty and Packer [6], Ibbett [7], Bovey and Mirau [8] and the annual reports by Webb [9]. This section will focus on more recent papers dealing with the application of solid-state CP/MAS NMR spectroscopy to the analysis crosslinked polymer systems.
15.2.2
Phenol-formaldehyde resins
Phenol-formaldehyde (PF) resins have been used as model compounds for the study of pyrolysis and combustion reactions that occur in solid fuels [10]. Utilising these resins it is possible to incorporate a wide range of heteroatomic and hydrocarbon moieties to simulate compounds that arise naturally in the solid fuels. A series of phenol resins crosslinked with thiophene, dibenzothiophene, diphenylsulfide, benzyl phenyl sulfide, thioanisole, 8-hydroxyquinoline and 2-hydroxycarbazole were synthesised. These samples were then cured at 200~ (Fig. 15.2.1) and the resulting resins examined by solid-state NMR spectroscopy. The ~3C CP/MAS spectra of a standard PF resin is shown
CROSSLINKED POLYMERS
511
OH '---'- S ~
OH
Excess CH20 "OH
/
CH20H
S
CH2
Curing
/
CH2
S
CH, -
Fig. 15.2.1. Incorporation of diphenyl sulfide into the structure of the co-resite and the resultant
resite after curing.
in Fig. 15.2.2. This was compared to the partially and fully cured resins. (Figs. 15.2.3 and 15.2.4). By curing the standard resins at increasing temperatures (up to 200~ it was possible to show that peaks at 35 and 70 ppm, attributable to methylene and alkyl ether bridges respectively, were converted from the ether to methylene bridges at the final cure temperature. The aryl ether peak at 160 ppm possibly arose from the condensation of two phenolic moieties. These resins were studied in order to obtain the optimum cure conditions. The sulphurcontaining resins show a wide range of ether, ethylene and methylol constituents. The peaks at 38, 55-80 and 90 ppm arise from methylene and methylol carbons attached to the ortho and para positions of the phenol ring and hemiacetal carbons, respectively. Fully cured resins contain only aliphatic peaks at 18 and 38 ppm with no remaining alkyl ether linkages. The aryl ether peak at 160 ppm also increases slightly in intensity. A major side reaction that was
512
R.V. L A W A N D D.C. S H E R R I N G T O N
't
j .......
I
. . . .
200
90~ (48 hours) ~._
I. . . . .
100 PPM
-
--I
. . . . . .
0
Fig. 15.2.2. Solid state 13C C P / M A S N M R spectra of a normal PF resin after various cure periods. SB = spinning side band.
identified for both the normal and sulfur containing resins was the formation of an arylmethyl moiety which gave a peak at 18 ppm. Understanding the curing process in PF resins and the analogues employed in this work gave a useful insight into the mechanisms that occur in solid fuels. Though useful as model compounds, PF resins have many commercial uses in their own rights, these include thermal insulation, mouldings, and use as wood resins. Therefore the systematic understanding of their molecular structure gives an insight into their physical properties. One important physical property is the resistance of the resin towards acid and bases; understanding this enables a better approach to the correct formulation of the resins.
513
CROSSLINKED POLYMERS
(a)
(c)
200
1SO
leo PPH
50
0
Fig. 15.2.3. Solid state 13C CP/MAS NMR spectra of the partially cured (130~ containing (a) dibenzothiophene (b) thioanisole (c) phenyl benzyl sulfide.
resites
This has been studied by 13C CP/MAS NMR where the degradation of PF resins in the presence of acid (oxidising and nonoxidising), base and formalin [11] has been monitored (Fig. 15.2.5). The main structural components believed to be present in the cured resin are shown in Fig. 15.2.6. The proportions of each species depends upon the initial P : F ratio, p H value, catalyst and temperature. At relatively low concentrations (---1 M) exposure to acid and base simply leads to neutralisation or formation of the phenoxide salt of the resin. This is shown by the change in intensity of the peak at ca. 152 ppm. Also in the presence of alkali, peaks at 73 ppm disappeared giving a proportional rise in intensity at 65 ppm. This was explained by the cleavage of dimethylene ether linkages to produce the corresponding methylol groups. Treatment with formalin also produced methylol groups at positions ortho or para with respect to the phenolic linkage, producing peaks
514
R.V. LAW A N D D.C. S H E R R I N G T O N
(b)
(c)
(d)
200
150
I00 PPM
50
0
Fig. 15.2.4. Solid state 13C CP/MAS NMR spectra of the fully cured (200~ resites containing
(a) dibenzothiophene (b) phenyl benzyl sulfide (c) diphenyl sulfide (d) thioanisole.
at 65 ppm. There was also evidence, from the presence of peaks at 40 ppm, of the formylation of p,p'-methylene linkages formed. Under stronger nonoxidising acidic conditions (sulphuric acid 36 N) (Fig. 15.2.7) all of the methylol and dimethylene ether linkages are cleaved. Some of this cleavage gives rise to CHO (194ppm) and CH3 (18ppm) moieties. There is also evidence that there is sulphonation of the aromatic ring in the ortho and para position to the phenolic hydroxide group (ca. 152 ppm). The use of strong oxidising acid (15 N nitric acid) brought about more major structural changes
CROSSLINKED P O L Y M E R S
515
Phenol i c ring carbons
herl•rbons /
~
~C_H2OAr'
ArCHO
ArCH2Ar'
b
C 9
i"~'~'T
200
,"'i
1
'
150
'
'
i 1 "" 100
'
'
I
50
""'~ ' I 0
'
PPM
Fig. 15.2.5. 15.1 MHz 13C CP/MAS N M R spectra of (a) resole type resin (b) cured resin after treatment with 1.0 N sodium hydroxide under N2(g) for 65~ for 3 days, and (c) cured PF resin after treatment with 36.8% formal under N2(g) at 25~ for 1 day.
(Fig. 15.2.8). These included the conversion of phenolic rings into cyclic ketones and nitration of the rings. Methylol and methyl groups are also similarly nitrated. 15.2.3
Melamine-formaldehyde resins
These are common materials that have found many applications e.g., as hard, durable and chemically resistant surfaces. A series of uncured and cured
516
R.V. LAW AND D.C. SHERRINGTON OH
OH
OH
0
~
OH
Fig. 15.2.6. Main moieties present in a PF cured resin.
melamine-formaldehyde (M-F) resins have been examined by solution state (1H, 13C) and solid-state (13C, 15N) NMR respectively [12]. The resins were either laboratory synthesised or obtained from an industrial manufacturing process. The solid-state NMR studies involved four resins, two laboratory synthesised and two obtained from industry, all of which had different M : F ratios. The 13C solid-state NMR spectra of the synthetic resins are shown in Fig. 15.2.9. The peak at 166 ppm in readily assigned to azine ring carbons. The more interesting region, however, is the methylene region (40-60 ppm) in which there are five components at 48, 52, 58, 66, and 72 ppm. These have been assigned to the following structures,
--NHCHzNH m,
=NCH2NH--,
--NCH2N=,
mNHCH2OCH2~,
This indicated that resins synthesised with M ' F ratios of 1.0" 1.5 contain principally methylene linkages where as higher ratios of M ' F (1.0" 3.0) contain equal amounts of methylene and methylene ether linkages. The spectra, however, showed no evidence for methylol groups which should give peaks at 64 and 69 ppm. The was also confirmed by examining the 15N solid-state NMR spectra (Fig. 15.2.10). The 15N solid-state NMR spectra of model compounds containing methylolmelamines showed peaks at 87 and 107 ppm (indicative of ~ N H C H 2 O H and ~N(CH2OH)2 which were absent in the spectra of the resins. What the 15N spectra did indicate, however, was a peak at 77 ppm and a shoulder (for the M ' F 1.0" 3.0 resin) at 93 ppm indicative of ~ N H C H 2 ~ and - - N C H 2 ~ linkages. The industrially synthesised resins showed very similar spectra indicating the presence of high concentrations of branched and linear methylene and methylene ether linkages.
517
CROSSLINKED P O L Y M E R S
Phenolic ri ng carbons ot--~hercarbons
I
COCH2Ar'
J!
ArCH2OAr' ~ AI.CH2OCH2Ar'
C_OH ArCHO
ArCH2OH
\
- ArCH2Ar
a
b
C
200
!50
100
50
0
PPI4
Fig. 15.2.7. 15.1 MHz 13C CP/MAS NMR spectra of three residues of cured PF resin after treatment with sulphuric acid under N2(g) and three different conditions (a) 1.0 N sulphuric acid solution at 65~ for 3 days (b) 36 N sulphuric acid at 25~ for 1 day (c) 36 N acid at 65~ for 1 day.
15.2.4
Urea-formaldehyde resins
Currently, one of the most important commercially available materials today are the urea-formaldehyde (U-F) resins. Their applications include coatings, adhesives, castings, moulding compounds and textiles. Maciel et al. have produced a series of extensive papers in this area concentrating on both 13C and 15N CP/MAS [13-16].
518
R.V. LAW A N D D.C. S H E R R I N G T O N
a
b 0
d
-
'"
1.-
' 1"
2S0
'
' ' I '''
200
' I '''''''~
150
'
'1"
100
'""
I '
50
''
~1
~
0
'
1 '
-50
PP~I
Fig. 15.2.8. 13C CP/MAS NMR spectra of the residue of cured PF 50 resin after treatment with 15 N nitric acid in air at 25~ for 1 day (a) 50.3 MHz (b) 22.6 MHz (c) 15.1 Mhz and (d) 15.1 MHz 50-1~s dipolar dephasing. Spinning sidebands are marked with an asterisk.
A systematic 13C CP/MAS study was undertaken of a large array of different U-F resins synthesised under a wide range of conditions including pH, concentration, and U : F ratio [13]. Catalyzed by both acid and base a great variety of reactions can occur leading to a large and complex range of moieties depending upon the synthetic conditions. At high pH the principal linkages present are methylol urea (65, 72 ppm) and dimethylene ether (69, 70 ppm), also present under the basic conditions is a small amount of methylene methyl ethers (55 ppm). Under acidic conditions other reactions predominate. In addition to the formation of linear methylene linkages (47, 54,
519
CROSSLINKED POLYMERS
Ca)
L
1
300
J- .
200
.
.
.
100
.
1
. . . . . . .
0
ppm
I
-100
(b)
L
300
~
:
200
..........
l
100
.
.
.
.
ppm
__1
0
.j
-100
Fig,. 15.2.9. Solid state 13C CP/MAS NMR spectrum of M-F resin.
60ppm) resins contained a substantial amount of crosslinking methylene linkages (69, 76ppm) which increase as the F : U ratio increases. At very high F : U ratios methylene dimethylene ethers, methylol and hemi-formals occurred (69-72 ppm). Also present in large quantities were disubstituted urons (75, 79 ppm) which increased as pH increased. In an attempt to formulate alternative [14] resins N,N'-dimethylolurea or paraformaldehyde was used as a different source of formaldehyde to crosslink the urea molecules. The resins produced by these methods generally exhibited similarities to those previous synthesised using formaldehyde [13]. Small
520
R.V. LAW A N D D.C. S H E R R I N G T O N
(a)
r
(b)
i
I
I
400
300
,
J_
_
200
1
t
100 ppm 0
,
.1
-100
Fig. 15.2.10. Solid state 15N CP/MAS N M R spectrum of M-F resin.
differences between the resins produced by the different synthetic route appeared to be due principally to the problems of solubility of the N,N'dimethylolurea or paraformaldehyde. A large volume rotor MAS system was used to examine the natural abundance 15N present in urea-formaldehyde resins [15]. Increasing the amount of material which is examined has enabled the investigation of the isotopically low 15N present (0.37%) in the resins without having to resort to synthesising 15N enriched materials. There are four possible interaction sites between urea and formaldehyde (Fig. 15.2.11).
CROSSLINKED POLYMERS O II
0 II
~--N--C--NCH2OH + H20
~-~N--C--NH + HOCH2OH
I
521
I
O II ,~,N--C--NCH2(OCH2)nOH
I
I
0 II 9,,,,,,~N--C--NCH20H
I
0 Ii
+HOCH2OH
,,~N--C--NCH2(OCH2)n+IOH + H20
0 II + ,,---N--C--NH
I
I
~
O O II II ,~-N--C--NCH2N--C--NH.'~ + H20
I
I
I
I
I
O O II II ,~,N--C--NCH2OCH2N--C--NH,w~ + H20
0 I!
~-~N--C--NCH2OH
I
I
I
I
O
",,
o II
--N--C--N--N [ ~H2 CH2 I
OH
~
/
N
~ jN
+ H20
O
OH
Fig. 15.2.11. Structural units present in UF resins.
For resins synthesised under acidic conditions tertiary amides initially seen by 13C CP/MAS were confirmed by 15N CP/MAS. Under neutral or basic conditions the main constituents of the resin are N,N'-dimethylolurea (102 ppm), monomethylolurea (102 and 78 ppm) and dimethylene ether linkages (90 ppm). Using dipolar dephasing and cross-polarisation times it was possible to distinguish primary, secondary and tertiary substituted nitrogens. These results confirmed the existence of many moieties postulated by 13C CP/MAS. The widespread application of U-F resins has meant understanding the mechanism of the degradation process is important if an improvement in resin stability is to be obtained [16]. Therefore the way in which U-F resins change when they undergo hydrolysis was examined. The resins have been described previously [13]. There are a number of possible mechanisms which involved the hydrolysis of the moieties in the U-F resins. A typical degraded resin is shown in Fig. 15.2.12.
522
R.V. LAW AND D.C. S H E R R I N G T O N
A
b)---J -I-
i ....
I"
i
"!
I
i ....
200 180 160 140 120 100 80
I'
I
I"
I --
60
40
20
0 PPM
Fig. 15.2.12. (a) 50.3 MHz 13C CP/MAS NMR spectrum of a UF resin sample prepared from formalin (F) and urea (U) with an equivalent F/U/water ratio of 2.00/1.00/1.07 at pH 3 and (b) its solid residue after hydrolytic treatment at pH 4 and 86~ for 20 h. Spinning side bands are marked with asterisks.
It was demonstrated that resins prepared with an equivalent molar ratio of F : U gave the highest stability towards hydrolytic treatment. Resins which contained higher F : U ratio (2.0: 1.0) contained a wide range of moieties which were more readily susceptible to hydrolysis, the products formed include dimethylene ether linkages, poly(oxymethylene glycols) and methylols attached to tertiary amine groups. These moieties are the sources of formaldehyde when the resin degrades. Resins of different composition showed similar degradation patterns. 15.2.5
Isocyanurate based resins
In a series of papers cured resins based upon ~SN enriched 4,4'-methylenebis(phenyl isocyanate) (MDI) have been examined by utilising ~3C and 15N
CROSSLINKED POLYMERS RNCO + H20 R'NCO + RNH2 ~ ~ - ' P "
RNCO + R'NHCONHR"
523
~-" RNH2 + CO 2
RNHCONHR' ~" R"NHCON(R")CONHR
Fig. 15.2.13. Reactions of isocyanate units that occur in MDI-polyisocyanurate resins.
CP/MAS NMR. In the first of these papers the structures within the resins were examined as a function of cure temperature [17]. The chemistry of the resins is very complex but one of the principle reactions is the formation of stable isocyanurate structures from three isocyanate units. Other species are also present e.g., amine, urea and biuret. These are formed by the reactions shown in Fig. 15.2.13. 13C CP/MAS NMR spectra of the resins indicated that the optimum cure temperature was 120~ at which most of the isocyanate groups were converted to isocyanurate (Fig. 15.2.14). The peak at 150ppm is due to the isocyanurate carbonyl carbon, the benzylic substituted aromatic carbons para- to an isocyanurate moiety are shown by a peak at 145 ppm, the 130ppm peak is due to the protonated aromatic carbons, the shoulder at 125 ppm is due to both isocyanate and ortho aromatic carbons. These signals were confirmed by using dipolar dephasing spectra. However, because of the complexity of the spectra ~SN CP/MAS NMR was used to clarify the structures present at the different cure temperatures. A summary of the moieties found in the resin is given in Fig. 15.2.15. In addition Duff et al. also undertook a quantitative analysis by utilising the large difference in cross-polarisation time for protonated and nonprotonated nitrogen. In a related study a series of resins were examined in which biuret linkages predominated [18]. These were formed by the reaction of formic acid and 4,4'-methylenebis(phenyl isocyanate) (MDI). The principal reactions that occur in the resins are shown in Fig. 15.2.16. The 13C and 15N CP/MAS spectra showed that when the formic acid" MDI ratio increased the biuret linkage predominates. The pathway for this was initially the formation of MDI-based urea and formic anhydride moieties which further reacted with isocyanate groups to form the biuret linkages and possibly diformyl imide groups. In a further study the same resins were analysed straight after curing and again after a 7 months exposure to air [19]. Three different cure temperatures were used, 100~ 120~ and 160~ A typical example of the 15N CP/MAS spectra is given in Fig. 15.2.17. The predominant structure in these resins is the isocyanurate linkage. These are relatively stable and it is the chemistry of the residual isocyanate groups that dominate the formation of new bonds in the system during the
524
R.V. LAW AND D.C. SHERRINGTON
160 *C
140 ~
120 ~
100 ~
80 ~ 160
140
120
100 PPM
Fig. 15.2.14. 50.3 MHz 13C CP/MAS spectra of MDI-polyisocyanurate resins prepared at different temperatures.
exposure to the air. Using ~SN CP/MAS it was possible to identify clearly the products of isocyanate hydrolysis which involved principally the formation of amines and the urea-linkage condensation products, there was no significant formation of biuret linkages. Structural assignment for these resins was further substantiated by 13C CP/MAS. These results aided the identification of the decrease and increase in concentrations of isocyanate groups and urea linkages, respectively. Duff et al. also attempted a quantification experiment using the ~SN CP/MAS results to determine the relative concentrations of the
CROSSLINKED POLYMERS
525 15N Chemical Shift
Isocyanate
ArNCO
46
Amine
ARNH 2
53
Urea
ArNHC(O)NHAr
104
114 ArNHC(O)N(Ar)C(O)NHAr
Biuret
141
.0
L Uretidione
Ar--N
\]l/
(NH) (N)
145
N--At
O
Isocyanurate
O
'~N
N/Ar 149
I
Ar Fig. 15.2.15. Structures and 15N chemical shift data pertinent to MDI-based resins.
RNCO + R'COOH
~"
RNCO + 2R'COOH RNCO + R'NHC(O)R" RNCO + R'C(O)OC(O)R'
RNHC(O)R'
+ C02
RNHC(O)NHR + R'C(O)OC(O)R' ~--
RNHC(O)NR'C(O)R"
~
R'C(O)NRC(O)R' + C02
Fig. 15.2.16. Reactions that occur between formic acid and 4,4'-methylenebis(phenyl isocyanurate) (MDI).
526
R.V. LAW AND D.C. S H E R R I N G T O N
B
A l
200.00
J_
1
| ~ , ~0
J .....
!
i013.=
I
_
l
50,00
,,.J
!
E, O0 PPM
Fig. 15.2.17. (a) 20.3 MHz 15N CP/MAS spectra of MDI-polyisocyanurate cured at 100~ (b) Same resins after 7-month exposure to air.
different nitrogen containing moieties before and after prolonged exposure to air [20]. The differing extents to which hydrolysis occurred in the samples was interpreted in terms of the structural effect that the cure conditions had. The possibility that the different morphologies present could be responsible for the amount of hydrolysis was further investigated by study of the 1HT~p of the samples. Finally the thermal degradation of the samples was examined. In this study it was possible to show that the degradation of all biuret and uretidione linkages occurred at 230~ a decrease of residual isocyanate took place on heating to temperatures up to 240~ an increase in urea linkages occurred in samples heated up to 240~ followed by a decease in these from 250-260~ A steady increase in amine groups and a decrease in isocyanurate groups was also observed. The peaks in both the ~3C and ~SN CP/MAS NMR
CROSSLINKED POLYMERS
527
spectra broadened with increasing temperature. This was attributed to the formation of free radicals and substantiated by ESR spectroscopy. The thermal stabilities of the relevant groups were in the order biuret, uretidione < urea < isocyanurate < urea', where urea' is a urea-type more stable than isocyanurate. 15.2.6
Synthetically crosslinked natural polymers
Natural polymers crosslinked by synthetic molecules represent many resins used for industrial applications. These are now being more closely examined by solid-state N M R spectroscopy to try to understand more fully what occurs in these systems and how it is possible to improve them. Of industrial importance are the polyphenolic tannin resins crosslinked by hexamethylenetetramine. These principally contain flavan-3-ols (Fig. 15.2.18) in the tannin [21] and have been examined by ~3C CP/MAS solid-state N M R spectroscopy. Hexamethylenetetramine was used in preference to formaldehyde as it has showed a much faster rate of reaction. The intermediates in this reaction are tribenzyl-, dibenzyl-4~, and monobenzylamines some of which rearrange to give the dihydroxydiphenylmethane crosslinking bridges in the resin. The exact nature of the crosslinking process, however, is still in debate and the study was undertaken to try and clarify the issue. To examine this process fully, a comparison was made between pine tannin (high in flavan-3-ol) (Fig. 15.2.19) pine tannin hardened with paraformaldehyde (Fig. 15.2.20) and pine tannin hardened with hexamethylenetetramine (Fig. 15.2.21). For the paraformaldehyde cured species three new peaks were observed that were representative of the C4 unsubstituted flavonoid site (38 ppm) and (OH) OH H
OH
H
(OH)
Fig. 15.2.18. A typical flavonoid structure. The parentheses indicate variations in flavonoid structure with some hydroxyl groups absent.
528
R.V. LAW A N D D.C. S H E R R I N G T O N
..[
......
, ....
~00
Fig. 15.2.19. 13C C P / M A S N M R
t. . . . . . . . . .
150
l . . . . . . . . .
I00
ppm
I
. . . . . . . . .
50
! . . . . . . . . . .
I
0
spectrum of pine tannin.
the formation of methylene bridges between two phenolic rings (36.8-37 and 33 ppm). Unsurprisingly methylene bridges were formed exclusively. For the hexamethylenetetramine cured species peaks assigned to the formation of methylene bridges (identical to the first reaction) together with peaks assigned to the formation of tribenzyl- (57.5 ppm), dibenzyl- (51.0 ppm) and monobenzylamines (45 ppm) linkages were observed. In the latter case it appeared that 40-50% of the crosslinks were the benzylamine type (with tri- and monobenzylamine predominating) the remaining being methylene bridges between phenolic type structures. Further evidence for this was shown from the peak at 98 and 105-110 ppm representative of the free and reacted C6/C8 sites respectively, the former decreasing and the latter increasing in both cases with reaction with the formaldehyde and the hexamethylenetetramine. Pecan nut tannin with another predominant flavonoid form of differing reactivity was also reacted with hexamethylenetetramine. In this case the dibenzylamine and tribenzylamine units were the dominate moieties present in the
CROSSLINKED POLYMERS
.!
.........
200
f .........
150
! .........
I00
ppm
f .....
50
....
529
1 .........
f
0
Fig. 15.2.20. 13C CP/MAS NMR spectrum of pine tannin extract hardened with paraformal-
dehyde.
system, also the level of methylene bridging was much lower representing only 20% of the total crosslinks. In two closely related studies Wendler and Frazier examined, by ~SN NMR, the interaction between both model cellulose compounds [22] and wood with 15N enriched polymeric diphenylmethane diisocyanate (pMDI) [23]. The resin formed is used commercially as a wood adhesive. Previous work [17] had shown that this reaction is sensitive to moisture, the formation of different products depending upon the degree of moisture present (Fig. 15.2.22). Urea and biuret type linkages were all characterised. Biuret structures were predominant when the moisture content was low, gradually being replaced by urea linkages when there was higher moisture content, the formation of urethane and amine moieties also occurred at intermediate moisture contents. This is in contrast to the previous study [17] where only a small amount of biuret linkages were detected. This may be due to the presence
530
R.V. LAW AND D.C. SHERRINGTON
! .....
200
, ....
! ....
150
~ ....
1 .....
, ....
100
I .....
50
, ....
t ..........
1
0
ppm Fig. 15.2.21. 13C CP/MAS NMR spectrum of pine nut tannin extract hardened with hexamethylenetetramine.
of the large amounts of hydroxyl groups available from the cellulose to react further with the urea linkages. 15.2.7
Polyacenicpolymers
The potential applications for conducting polymers are enormous and this has stimulated a large amount of research into this area. Not surprisingly, solid-state NMR spectroscopy has been applied to study these amorphous, insoluble and in many cases crosslinked materials [24]. Looking at the 13C CP/MAS spectra of a series conducing polyacenic polymers, some of which were doped with iodine, it was possible to see the effect of the halogen upon conductivity. These resins were prepared by a conventional procedure for the preparation a Novolak-type phenol-formaldehyde resin. After synthesis, the phenol-formaldehyde resin were dissolved and solutions were cast as a film and heat treated to between 590-670~ in a Ne atmosphere to form the polyacenic film. The electrical conductivity of the films was shown to increase
531
CROSSLINKED POLYMERS
1
t
1
250
200
! ....
150
, ....
! . . . . . . . . .
;0 . . . . . . . . .
I
100
PPM
Fig. 15.2.22. ~SN CP/MAS spectrum of wood/15N-pMDI composite as a function of precure moisture content.
with higher temperature. The addition of iodine to one of the films gave rise to substantial increase in electrical conductivity. The postulated structures are shown in Fig. 15.2.23. Typical 13C CP/MAS spectra are shown in Fig. 15.2.24. By using the reference spectrum of a phenol-formaldehyde resin the peaks for the polyacenic films were assigned. The main peak at 127-130 ppm moves upfield and broadens with increasing temperature indicating an increased amount of polyacenic-type structures. The peak at 150 ppm assigned to quaternary aromatic carbon substituted by hydroxy groups and the peak at 40 ppm assigned to methylene carbons decrease in intensity with increasing temperature but are never completely removed. Using dipolar dephasing spectra the aromatic region also revealed further signals and it was possible
o
o
~..,.
o
!
o
o_
P~
j /)
/
// ,, /
~ ~
i'"
~
~,
7
J
i
0 Z
~ N
Z C~
h
.'~
t~
CROSSLINKED
533
POLYMERS
2,3 5,6
i
~-
i
~)o
~
~
i
9
15o
, .
100
.
.
.
.
50
Fig. 15.2.24. 13C C P / M A S spectra of a polyacenic film.
to assign them using chemical shifts from model compounds. The peaks at 129.5 and 151ppm were due to protonated/nonprotonated and hydroxy substituted carbons respectively in the residual phenol-formaldehyde resin structure. The peaks at ca. 125 and 138 ppm are the methine and quaternary carbon in the polyacene. The ratios of two of the aromatic peaks, obtained by deconvolution, were related to the degree of electrical conductivity. For the iodine doped sample it was shown that the iodine interacts significantly with the polyacene part and not with the phenol formaldehyde part. This was shown by peak broadening of the polyacene type peaks. 15.2.8
Polyethers
A semi-crystalline poly(1,3-dioxolane) was examined by solid-state NMR spectroscopy looking at both linear and crosslinked polymers [25]. The
534
R.V. LAW AND D.C. SHERRINGTON
OCH2CH20
OCH20
(b)
,'l",~']"'"'i
le5
100
....
95
I ....
$0
! .... "T"'"I
85
80
....
75
I' '"" i '''~:i . . . .
7e
65
i
6a ~p~
Fig. 15.2.25. (a) 13C single pulse excitation and (b) 13CCP/MAS spectra of poiy(1,3-dioxolane).
crosslinks were formed by introduction and reaction of acrylate groups allowing the formation of a network and control of the molecular weight. The systems were examined by 13C MAS and typical spectra appear in Fig. 15.2.25 where two characteristics peaks at 67.5 ppm ( O C H 2 0 ) and 96.0 ppm ( O C H 2 0 ) are diagnostic. For the crosslinked polymer the CP spectrum, being more sensitive to static molecular motion, revealed a further peak at 93.4 ppm (the O C H 2 0 region) which was assigned to a less mobile phase. To clarify these results variable contact CP and cross-depolarisation experiments were carried out and by deconvolution of the peaks in the NMR spectrum three regions, a crystalline, an interfacial and elastomeric one, were indicated. Further study carried out using 1HTlp which is indicative of kHz motion in
CROSSLINKED POLYMERS
535
the polymer, also suggested that there were three phases present in the polymers. 15.2.9
Epoxide based resins
In a high temperature study [26] of two epoxide resins, the samples were heated to above Tg whilst still carrying out magic-angle spinning to remove residual line broadening interactions. At these temperatures (ca. 260-290~ the molecular motion of the system had increased to such an extent that it was possible to use conventional one (~H, DEPT) and two dimensional (HECTOR) solution state NMR experiments on the samples. The networks looked at were the oligomer of diglycidyl ether of bisphenol A (DGEBA) cured with 100 and 66% of diaminodiphenyl sulphone (DDS) (Fig. 15.2.26). The spectra of the 100% cured resin at ambient and high temperatures are shown in Fig. 15.2.27. The principal reactions are between the epoxy and the primary and secondary amines, further reactions between hydroxyl groups are also possible. The principal reaction moiety for the 100% cured polymer is indicated in Fig. 15.2.28. For the 66% cured polymer the situation was more complex the spectrum is shown in Fig. 15.2.29 and the major structural moiety present is indicated in Fig. 15.2.30. The increase in resolution at higher temperature is clearly evident and the possibility of using conventional solution state editing techniques is advantageous as they greatly aid peak assignment. Unfortunately this technique may be applicable only where there is a substantial increase in motion above Tg. Many highly crosslinked polymers do not show a discrete Tg and therefore would not be expected to show any substantial decrease in line broadening if they were heated to high temperatures. There is also the question as to whether further post-curing reactions may occur when polymers are heated to such temperatures. Epoxide resins made from 2,2-[4-(2,3-epoxypropyl)phenyl]propane (DGEBA) polycondensed with 4,4'-sulphonyl-dianiline (DSS) produce a three-dimensional insoluble network which was examined by CP/MAS NMR spectroscopy [27]. In this study the chemical structures and the cure kinetics were determined. A cured epoxy synthesised from a mixture of the diglycidyl ether of bisphenol A (DGEBA) and 1,3-phenylenediamine was studied by ~H NMR spectroscopy including multiple pulse techniques and spin-lattice relaxation in the rotating frame, T~o. The study [28] focused on the water distribution based upon possible variation in the cross-link density measured by spin diffusion. From the analysis involving a combination of T~p and multiple
t~ ta~
/o~
c.,---c.c.,o
o.
Y" /~
I
~---~xk ..../f--oc~c.c.,o
]/----x
I /--x.
/o~ X~ <
>, Z
DGEBA
h :z:
./
N
sch -
o u
It
N
Z
\ H
DDS Fig. 15.2.26. Structure of diglycidyl ether of bisphenol A and 4,4'-diaminodiphenylsulphone.
0 Z
537
CROSSLINKED POLYMERS
II _ ~L_L_=_
I
180
I
I
lz~O
i
I
100
~
i
I
60
I
I
20
~c/ppm
Fig. 15.2.27. 13C MAS spectra of DGEBA-DDS with 100% stiochiometry (a) 23~ with crosspolarisation and (b) 290~ without CP (c) expanded.
HO
0
\
I
'
I
0
\
/OH
I
o
Fig. 15.2.28. Major structure present in the 100% cured DGEBA-DDS resin.
pulse it was possible to postulate that the water was molecularly dispersed in the epoxy rather than aggregated in the voids. Also there seemed to be an absence of two distinct sites for water affinity. The presence of accelerators, either magnesium perchloroate or N,Ndimethylbenzylamine (DMBA) [29], on the curing of bisphenol A diglycidyl ether with butane-l,4-diol (BADGE-BD) were studied by CP/MAS (Fig. 15.2.31). Magnesium perchlorate was shown to induce the consumption of all the diol whereas the DMBA showed only approximately 50% consumption
538
R.V. LAW AND D.C. SHERRINGTON
q I
--
I
180
I
,
,
,
'I
,
I
L
.....
1/.,0
I
,
I
100
9
9
9
I
60
LJ ,
I
c/ppm
20
Fig. 15.2.29. 13C MAS spectra of DGEBA-DDS with 66% stoichiometry (a) 23~ with crosspolarisation; (b) 260~ without CP and (c) expanded.
as indicated by the residual primary alcohols from the butanediol (Fig. 15.2.32). 15.2.10
Methacrylate-based resins
Polymer composites are increasingly used for dental applications [30], the durability and aesthetic appeal has made them ideal substitutes for the more traditional amalgam fillings. The dental polymer composites are principally composed of an organic matrix and a powdered ceramic phase. The organic matrix is composed of an aromatic or urethane dimethacrylate such as 2,2bis[4-(2-hydroxy-3-methacryloyl propoxy) phenyl]propane (bis-GMA) with
539
CROSSLINKED POLYMERS
O
o
Fig. 15.2.30. Major structure present in the 66% cured DGEBA-DDS resin.
I _
s
150
,,,
!
.....
!
110
I
,
r
70
1,
!
....
30 ppm
Fig. 15.2.31. 13C CP/MAS spectrum of BADGE-BD system with accelerator Mg(C104)2.
another monomer such as triethylene glycol dimethacrylate (TEGDMA) to alter viscosity. This system has the further advantage that it can be photopolymerised. ~3C solid-state NMR spectroscopy has been used to study the extent of the crosslinking reaction. A series of commercial and laboratory synthesised resins were examined by CP/MAS and SPE to determine more accurately the relative amounts of unreacted resin present (Fig. 15.2.33). In the synthesis of a methacrylate-based metal chelating resin, ~3C CP/MAS spectroscopy has been used to confirmed that the target resin had been made [31]. Here the imidazole ligand bis(imidazo-2-yl)methylaminomethane (bimam) was attached to a glycidyl methacrylate-co-trimethylolpropane trimethylacrylate (pGMT) resin. The peaks in the ~3C NMR spectrum were
540
R.V. LAW AND D.C. SHERRINGTON
_
t
150
!. . . . .
!
110
....
s,
t. . . .
70
! ....
t
30 ppm
Fig. 15.2.32. 13C MAS spectrum of BADGE-BD system with accelerator DMBA.
assigned as follows: 7.3 ppm, hindered methyl in trimethylolpropane residue; 24.3 ppm, backbone methyl; 41.4 ppm, ~ C H N in ligand; 46.0 and 56.1 ppm, polymer backbone; 67.5 ppm, ~ O C H 2 ~ in epoxy linkage; 127.5 and 145.6, imidazole carbons; and 176.3, carbonyl carbon. In an attempt to provide alternative supports to styrene-divinylbenzene resins [32] for use in reactive chemistry, poly(hydroxyethylmethylacrylate) (poly(HEMA)) has been employed. In this study poly(HEMA) was crosslinked with ethyleneglycol dimethacrylate and the CP/MAS recorded. The peaks were assigned as follows: 18.6 ppm is due to the methyl attached to the aliphatic backbone; 56.0 and 45.4 ppm are due to the methylene and quaternary of the backbone; the ~ O C H 2 ~ forming the crosslink are at 63.0ppm; the conjugated and unconjugated carbonyls are at 167.2 and 177.6 ppm and finally the methylene and quaternary carbons of the unreacted vinyl group are at 126.3 and 137.0 ppm. Another alternative is the use of crosslinked ethylene dimethacrylate [33]. Here, the unreacted double bonds in resins were used as a graft point for further reaction. The level of unreacted double bonds was determined by the relative areas of the carbonyl peaks at 176.3 and 166.3 ppm (Fig. 15.2.34) determined by CP/MAS before and after reaction with glycidyl methacrylate. These results were in good agreement with data from Raman spectroscopy. Spin diffusion is a valuable method by which it is possible to examine the heterogeneity of a polymer [34]. Spin-lattice relaxation times in the rotating
541
CROSSLINKED POLYMERS
260
P-20
180
140
100
60
20
0
ppm
Fig. 15.2.33. 13C CP/MAS spectra of commericially available dental acrylate resins. (a) Tetric; (b) Zl00; (c) Duo Bond; (d) Coltene bonding agent; (e) TEGMA the labels r and u indicate carbonyl peaks from reacted and unreacted methacrylate groups.
flame have been used to determine the rate of spin diffusion. Tip data from three solid polyacrylate networks made by photopolymerisation of poly(ethylene glycol) diacrylate (PEGA), trimethylolpropane triacrylate (TMPTA), and dipentaerythritol pentaacrylate (DPHPA) have been used in this way. The photopolymerisation was carried out by a laser and this was related to the degree of crosslinking that occurred which was quantified in terms of the signals from the residual double bonds in the CP/MAS NMR spectra. The level of heterogeneity in these resins was measured by the 1HTlp and was related to the degree of crosslinking.
542
R.V. LAW AND D.C. SHERRINGTON
1
190
I
I
180
170
I
160
PPM
Fig. 15.2.34. Part of the 13C CP/MAS n.m.r, spectrum of poly(ethylene dimethacrylate) showing the peaks used for the determination of the double bond content.
In an attempt to obtain high surface area glycidyl methacrylate-co-trimethylolpropane trimethacrylate resins were synthesised [35] with a variety of porogens. The degree of unreacted double bonds was determined by CP/MAS NMR spectroscopy. Oligomers containing ether-ester groups were synthesised [36] in order to obtain a crosslinking agent that gavegood cure kinetics and was uniformly distributed in the network structure. The crosslinking agents were modified so that vinylidene groups were incorporated to enable them to be polymerised free radically with styrene or methyl acrylate. The oligomer was incorporated (5-50%) in the polymer to give clear hard resins and these were characterised by CP/MAS NMR (Fig. 15.2.35). The peak at 30 ppm is due to the tert-butyl group, the broad peaks for
CROSSLINKED POLYMERS
543
/ V
200
160
ASO
t40
t20
100 ~X
60
40
2O
0
Fig. 15.2.35. Solid state 13C CP/MAS spectra of three samples of poly(methyl methacrylate)
crosslinked with 5 (lower), 20 (mid) and 50 (top) % of t-butyl acrylate end-capped oligomer containing ca. 2-3 HDDA repeat units.
the ethyl and ester carbons bonded to the oxygen are at 65-80 ppm, and the carbonyl carbon on the upfield side of the P M M A carbonyl at 178 ppm. Polyacrylamides can be synthesised by two methods, either polymerisation of an acrylamido monomer or chemical modification of another polymer e.g., poly(methylmethacrylate). In the latter case 13C CP/MAS N M R spectra show clearly the loss o f - - O C H 3 groups as these are replaced b y - - N H C H z - groups (Fig. 15.2.36). The latter approach has the advantage that polymethacrylates like P M M A can be obtained easily in a bead form of uniform size and are physically convenient for further exploitation. CP/MAS was used in a study [37] of the kinetics of reaction between P M M A and a series of amines. This was carried out by taking aliquots of the reaction mixture at certain times and recording the solid-state spectrum after the reaction had been quenched. The degree
544
R.V. LAW AND D.C. SHERRINGTON
_JA
~"-'~
720 rain.
360 min.
240 rain.
120 rain. PMA PpM ,
J 200.00
~
I 150.00
,
1 100.00
, 50.00
-0.00
Fig. 15.2.36. 13C CP/MAS spectra of PMMA reacted after various times with 1,6-diaminohexane.
of reaction was determined by 13C CP/MAS and further structural evidence was provided by 15N~ CP/MAS. 15.2.11
Styrene-basedpolymers
The degree of unsaturation in styrene cured polyesters was investigated [38] by using dipolar dephased 13C CP/MAS NMR data. The commercial resins contained fumarate, isophthalate and propylene glycol structural units and were cured with styrene. The use of dipolar dephasing (see Fig. 15.2.37) suppressed the strong phenyl peak at 129 ppm and therefore allowed the determination of the degree of unsaturation in the resin. The signal at 131 ppm was attributed to the isophthalate units, and the peak at 144 ppm to the quaternary substituted carbon from the styrene. This peak also showed
545
CROSSLINKED POLYMERS
180
160
lt, O
Chemico[ shift
(ppm}
12O
Fig. 15.2.37. Part of a 13C CP/MAS spectrum of a solid polyester obtained (a) without dipolar dephasing and (b) with dipolar dephasing. Chemical shift are shown by the numbers.
partial resolution into two peaks at ---142 and --~146ppm which may have been due to styrene units in sequences of differing lengths. The two carbonyl peaks at 165 and 172 ppm were ascribed to unreacted fumarate/isophthalate and reacted fumarate carbonyls, respectively. The degree of residual unsaturation was calculated by determination of the relative ratios to these peaks. From this it was possible to determine the optimum level of styrene (47%) needed to give the lowest degree of unsaturation in the resin. Polyesters derived from maleic anhydride and 2,2-di(4-hydroxyphenyl)propane were copolymerised with styrene and then studied by CP/MAS NMR [39] spectroscopy. The three dimensional-crosslinked network formed by the polymerisation was examined using spin-lattice relaxation times in the rotating frame. A correlation between reaction conditions and the structure of the resulting material was found. The degree of residual unsaturation was determined by subtraction of two relaxation times from a linear additivity model used for crosslinked polymer systems. In two closely related papers [40, 41] CP/MAS was used to examine a series of styrene-divinylbenzene (St-DVB) and chloromethylated resins. In the first part of this study the authors were concerned with trying to determine the residual amount of unreacted vinyl groups present in St-DVB resins (see Section 3). In order to increase the sensitivity of the method the authors used 13C-labelled divinylbenzene (labelled in the methine position) and combined this with unlabelled styrene (1-20% by weight). The final resins were examined by CP/MAS and it was found that even for a very lightly crosslinked
546
R.V. LAW A N D D.C. S H E R R I N G T O N
species (1% DVB-St) at 70~ residual unreacted vinyl groups were present. Post-curing reactions, carried out by swelling the resin in a solvent and then heating the resin to 155~ in the presence of initiator, showed it was possible to remove the residual unreacted vinyl groups. For resins with higher amounts of DVB (10 and 20%) it was impossible to react all the residual vinyl groups by using this method. This showed that the unreacted vinyl groups are trapped in inaccessible locations within the polymer network. The second part of the study was concerned with studying resins in the unswollen glassy state and the solvent swollen state using CDC13. St-DVB and a chloromethylated resin were examined. For samples with low levels of crosslinking (<6%) swelling in a good solvent whilst spinning at the magic angle (54.74 ~ gave the best resolution (see Section 4). 4-Acryloxy benzophenone (APB) was copolymerised with 4% divinylbenzene (DVB) and ~3C CP/MAS and other techniques were used to characterise the resin [42]. The purpose was to examine the nature of polymer-supported initiators. Data from solution state NMR analysis of the linear uncrosslinked poly(APB) was used as a model for assigning the ~3C CP/MAS NMR spectrum. The solution state spectrum of poly(ABP) gave 12 well resolved lines. The CP/MAS of the crosslinked poly(APB-DVB) copolymer gave six broad lines, and the keto carbonyl at 192.5 ppm and the ester carbonyl at 173.2 ppm. The peak at 153.8 ppm arose from the quaternary aromatic carbon attached to the oxygen atom of the ester group. The remaining quaternary and protonated aromatic carbons were visible as two broad overlapping signals at 128.8 ppm and 122.0 ppm. The signal from the DVB probably also contributes to this region and to the methylene-methine (ca. 40.9 ppm) signal but the authors did not comment upon this. The signal at 40.9 ppm was similar to the broad feature seen in the solution state NMR and is representative of the methylene and methine peaks of the backbone unit. The line broadening was attributed to the distribution of chemical shifts commonly seen in crosslinked resins. A series of copolymer hydroquinone diacrylate (HyDA) resins were characterised by using 13C CP/MAS NMR spectroscopy [43]. The series include styrene-HyDA, glycidyl methacrylate-HyDA, phenylmethacrylate (PhMA)-HyDA, 2,4,6-tribromophenyl acrylate-HyDA, and 4-acetylphenylmethacrylate-HyDA. The 13C CP/MAS spectrum of the PhMA-co-HyDA copolymer is shown in Fig. 15.2.38 with the 13C-{~H} solution NMR spectrum of poly(PhMA). Comparison of the these makes the peak assignment unambiguous. The a-methyl group of phenyl methacrylate appears as a broad peak at 16.4 ppm, the backbone methylene group shows a sharp intense line at 44.0 ppm and the small broad quaternary carbon peak is at 54.0 ppm. The O ~ C aromatic
547
CROSSLINKED POLYMERS
! 3C&SC 2C&6C /
4C
1C
(Ca)
_
I
200
t
!
160
!
,
I
1
120
!
80
1
I,
t,O
l
{~
PPM
Fig. 15.2.38. (a) 13C-{1H} NMR solution spectrum of poly(PhMA) and (b) 13C CP-MAS NMR solid-state spectrum of the PhMA-co-HyDA polymer.
peak (C1) is at 149.2 ppm, the protonated aromatic peaks are at 127.6 (C2 and C6) and 119.6 ppm (C3 and C5) and the para carbon (C4) is at 124.9 ppm. The carbonyl peak is at 172.2 ppm. Hypercrosslinked resins have also been the subject of an N M R study [44] (see Section 3.4). 13C CP/MAS NMR has been used to estimate the degree of crosslinking in a series of these species. The resins were examined in a solvent swollen and a nonswollen state to determine if with swelling in a deuterated solvent it was possible to narrow the line lines further. Determination of the level of crosslinking was carried out by deconvolution of the quaternary aromatic peak at ca. 146 ppm (Figure 15.2.39). This was broken down into two peaks, one centred at 146 and one at 140 ppm. This is a difficult undertaking as there appears to be no substantial asymmetry present in the peak. Use of SPE methodology is probably more quantitative in the context (see Section 3.4). The levels of crosslinking were shown to be high as there was no substantial narrowing of the static proton line widths even at temperatures of 200~
548
R.V. LAW AND D.C. SHERRINGTON
8
b _
-''''
J
'26o ' ' '
~
6o' :'''
Fpr~
Fig. 15.2.39. 13C CP/MAS spectra of (a) crosslinked polystyrene (0.3% DVB) and (b) hypercrosslinked sample. Side bands are indicated by the presence of an asterisk.
One of the aspects of polymer-supported reactions (see Section 4) is the ability to separate reactive centres from each other. The extent to which a benzoin condensation reaction occurs on a crosslinked polymer was examined [45]. The starting material, a polymeric benzaldehyde, was prepared by incorporation of vinyl benzaldehyde into a resin using either divinylbenzene or tetraethyleneglycol diacrylate as a crosslinker. The product was examined using ~3C CP/MAS. The spectra showed two important peaks at 86.2, 126.0 and 166.0 ppm. These were attributed to the c~-hydroxy carbon, the protonated aromatic carbons and the carbonyl carbon of the a-hydroxy ketone. This demonstrated that in the polymer the benzoin condensation reaction had occurred to a significant extent. Another metal chelating resin was looked at using 13C CP/MAS [46] (Fig. 15.2.40). This resin was based on an salicylaldehyde acrylate monomer
CROSSLINKED POLYMERS
549
OHC
Fig. 15.2.40. Structure of salicylaldehyde-containing polymer resin.
polymerised with a small amount of crosslinker, divinylbenzene. This was then further modified to form the oxime. The final product, the aldehyde and oxime, were also examined by 13C CP/MAS (Fig. 15.2.41). Only some of the peaks were assigned: the peak at 40.2ppm was the backbone from the methylene in the styrene and the peak at 126.0 ppm the protonated aromatic carbons. The phenyl ester carbonyl occurred at 160.6 ppm and the aldehydic carbonyl of the salicylaldehyde which appeard at 186.3 ppm in the initial polymer was not present in the oxime. The authors did not comment on the ratio of the peaks found between 55-105 ppm though this may be due to the poor signal-to-noise of the spectrum in this region. 15.2.12
Miscellaneous
Though it now possible to make diamond film by chemical vapour deposition, this technique has limitations and it would still be extremely advantageous to be able to make diamond or a diamond-like material synthetically. Towards this end crosslinked adamantane have been synthesised [47] by reacting 1,3,5,7-tetrakis(4-iodophenyl) adamantane with 2-methyl-3-butyn-2-ol and phenylacetylene to give two corresponding tetracetylene derivatives. These materials were then cured and the final products were examined with solidstate NMR spectroscopy. Typical spectra are shown in Fig. 15.2.42. In these the phenylacetylene crosslinked adamantane shows peaks at 79 and 84 ppm. These arise due to the free acetylene. Upon partial and full curing these lines broaden to form a single peak at 79 ppm. The adamantane carbons are at 39.5 and 50.8 ppm and the phenyl carbons appear as three peaks at 120-132 and one at 148 ppm. On curing the adamantane peaks
0
9
m
0
.~-~
m ~~
Cr
o 5"~~
=" ~ ~ "
e-+
"~
,e
m
m N
.. m ~._
|
~i.,.
X
o~
N
8i
"
I::::I
O
9
9
-o
I~"
g
~
8
o
..~_~
~..r- ' ~
9
CROSSLINKED POLYMERS
551
ssb
7/~---k / 3,8
.8.9,10
ssb
I
180
160
'
I
140
'"' ...... I 120
'
'i 100
' '
1 80
'
! 60
'
1 40
'
I
20
ppm
Fig. 15.2.42. '3C CP/MAS spectra of adamantane-alkyne condensate before (bottom) and after curing (top).
containing adamantane cores to yield a material which shows excellent thermal stability. The crosslinked insoluble polymers obtained by vapour deposition techniques have been studied [48] by NMR spectroscopy. In poly(furyleneethylene) the ~3C CP/MAS spectra showed additional peaks at 106.4 and 107.5 ppm which indicated that the chains were crosslinked with furyleneethylene units (Figs. 15.2.43 and 15.2.44). A series of polymers (including polysulphones, polyethers) containing the monomer trans-l,2-diphenylcyclopropane have been synthesised [49] by solution polycondensation. 13C CP/MAS spectroscopy was used, among other techniques, to characterise the cyclopropane ring opening reaction which had occurred in the final thermally crosslinked product. One such polymer (Fig. 15.2.45) showed characteristic peaks in its CP/MAS spectrum (Fig. 15.2.46).
552
R.V. LAW AND D.C. S H E R R I N G T O N
.../~
3
2~o.
2
3
ff-----a // \\
!
1
I
Fig. 15.2.43. Structure of poly(furyleneethylene) film. Numbers indicate carbon atoms corresponding to resonances shown in the spectrum in Fig. 15.2.44.
Those at 162.7, 152.9, 136.9ppm are from the nonprotonated aromatic carbon bound to the sulfonyl group, oxygen and cyclopropane ring, respectively. The protonated aromatic signals arise at 130.3, 121.7 and 117.0 ppm. The peak at 28.7 ppm is attributed to the aliphatic cyclopropane ring carbons. Upon crosslinking the cyclopropane carbons disappear, and three new aliphatic peaks appear. This change also corresponds with a change in chemical shift of the peak at 136.9 ppm which shifts to higher field upon crosslinking. A crosslinked polymer synthesised [50] by crosslinking ethylene-vinyl acetate copolymer with dicumyl peroxide gave two large signal at 31.8 and 33.8 ppm in its 13C CP/MAS NMR spectrum These corresponded to crystalline and amorphous methylene units in the ethylene polymer.
15.2.13
Summary comments
For the characterisation of crosslinked polymer systems it has been shown that solid-state 13C and 15N CP/MAS NMR spectroscopy offers a unique insight into the structure of these insoluble polymeric materials. Often it is the only way possible to develop a fundamental understanding of these generally structurally very complex systems. The power and utility of these NMR techniques are demonstrated by their increasing application. Some authors have adopted the approach of using 13C and 15N labelled
CROSSLINKED POLYMERS
553
c~
c3',c3
C1'
CZ
side b
t side band
180
160
140
IZ0
100
80
side band l
60
40
ZO
0
Fig. 15.2.44. Solid state 13C CP/MAS NMR spectrum of a poly(furyleneethylene) film.
O
, o-
xO///-V
% //
I!_
jo
Fig. 15.2.45. Structure of polysulphone containing the 1,2-diphenylcyclopropane moiety.
compounds for the starting monomeric materials, and this can greatly aid the analysis of the final spectra. This is of course costly, time consuming, and requires the synthesis of the labelled model networks; nevertheless the benefits achieved can be very worthwhile. Conventional CP/MAS methodology applied to unlabelled polymer networks is itself very powerful, however, and further advances and developments in both software and hardware will provide greater understanding and widen the applicability of this technique.
554
R.V. LAW AND D.C. SHERRINGTON
I '
-
-
=
--
200
'
;
150
. . . .
100
50
0
PPM Fig. 15.2.46. 13C CP/MAS NMR spectra of noncrosslinked and cross-linked polymer poly(ether sulfone).
15.3 Solid-state ~3C CP and single pulse excitation (SPE) MAS NMR analysis of crosslinked particulate polystyrene-based resins
15.3.1
Background
Spherical particulate crosslinked resins based on styrene and (meth)acrylate monomers prepared by suspension polymerisation methodologies [51] have become extremely important in a number of technological areas. Perhaps the most important are ion exchange resins [52] for purification of surface, ground and waste water, for steam turbine condensation polishing, for hydrometallurgy, for nuclear process and reprocessing, for de-ionised water production, for ultra-pure water production and for use in various purification (e.g., sugar) and separation processes. The closely related sulphonic acid resins have also become important industrial acid catalysts and are now used in a growing number of large scale acid-catalysed reactions [53]. High surface area nonfunctional styrene-divinylbenzene resins are also finding increasing
CROSSLINKED POLYMERS
555
use as sorbents for both liquid and gas phase components and contaminants [54], and in this context hypercrosslinked resins [55, 56] are now becoming available from commercial sources [57] for industrial exploitation. Most resins are based on crosslinked polystyrene and generally the level of crosslinking is higher (5% upwards) than the very lightly crosslinked gel-type species used in solid phase combinatorial synthesis (see Section 4). Analysis of such resins by NMR spectroscopic techniques was and remains problematical, but solidstate CP and SPE MAS methodologies have moved the area forward significantly in recent years. The high level of crosslinking generally means that the associated increased line widths of signals cannot be reduced simply by solvent swelling although this aspect is currently being re-visited [58]. Solid-state ~3C MAS NMR spectroscopy has had much success in examining amorphous insoluble polymers [59]. In recent years, however, there has been some debate on the reliability of quantitative data derived from CP experiments [60] and work on fossil fuels in particular has highlighted the problem [61, 62]. Undoubtedly, the issues arise in the analysis of polymers as well [63-66]. While CP results in signal-to-noise enhancement and hence reduced accumulation times, carbon atoms present with no proximal protons tend to have their peak intensities reduced relative to other signals. Quaternary aromatic carbons are likely to suffer badly in this respect. The modulation of the dipolar interactions by the motion of some moieties can also introduce quantitative errors [67]. The rotation of the methyl group about its 3-fold axis of symmetry is a good example of this. Single pulse excitation (SPE) [60] however overcomes the problems that are associated with CP, i.e., that the dynamics may alter the CP rates and may therefore discriminate for or against some types of carbon [61, 68] but the long delay time needed between each acquisition (typically 5 times the ~3C spin-lattice relaxation time (30-100s)) means that the technique is time-consuming. In addition, the signal enhancement brought about by the CP sequence is lost.
15.3.2 13C MAS NMR studies of anion exchange resins and their precursors Typically anion exchange resins are synthesised by the sequence shown in Fig. 15.3.47. The chloromethylation reaction is a potentially hazardous one [69] and an important side-reaction is methylene bridging. The latter effectively increases the level of crosslinking and manufacturers have to take account of this (by experience) in tailoring the final physical properties of resins. Important issues in manufacture, in addition to the level of methylene bridging, are the degree of chloromethylation achieved and the efficiency of
R.V. LAW AND D.C. SHERRINGTON
556
//'(
CMME Lewis acid CH2CI
~~N(CH3) 3
c~
MethyleneBridging
[~CH2~(CH3)3CIChloromethylation and amination of poly(styrene-divinylbenzene) resins to form anion exchange resins.
Fig. 15.3.47.
amination of these groups in generating quaternary ammonium ion anion exchange sites. Ford and his co-workers have used swollen gel-phase [70, 71] and solidstate CP MAS 13C NMR spectroscopy [72] to probe the products of the chloromethylation reaction, while a very detailed study and analysis of all steps in the synthesis of the final anion exchangers have been reported by Sherrington and his collaborators [73] using both CP and SPE MAS 13C NMR methods. In the latter work four precursor resins were employed, two gel-type with 3.5 and 0.5 wt% divinylbenzene crosslinkers, and two macroporous with 7.5 and 3.5 wt% divinylbenzene crosslinker. Each was chloromethylated using chloromethyl methyl ether and in the case of resin 1 ZnCla as the Lewis acid catalyst, and for resins 2-4, FeC13. 13C CP MAS NMR spectra were recorded at 25.2 MHz and the ~CHaC1 resonance was seen clearly at 46.8 ppm. The enhanced intensity at ---40 ppm confirmed the occurrence of methylene bridging but overlap with backbone CH2 and CH signals precluded even tentative quantification. CP spectra of the anion exchange resins formed by reaction of trimethylamine with the chloromethylated species showed signals at 69.0 and 53.2ppm due to and ~ N + C H 3 carbons and corresponding loss of intensity around 47 ppm. However, a significant feature remained at 46 ppm and exhaustive study of this suggested its assignment to a weak base function ~N(CH3)2 probably generated from impurity dimethylamine in the trimethylamine. Interestingly such functionality had been detected from the ion exchange behaviour of these species much earlier by resin manufacturers. Using SPE techniques with the anion exchange resin samples it did prove possible to quantify for the first
557
CROSSLINKED POLYMERS
1
.
.
.
.
.
.
.
!
Iso
.
.
.
.
!
.
1o0 PPn
.
.
.
1
so
.
.
.
.
,.,
I
.
o
Fig. 15.3.48. 13C SPE NMR spectra of: top, anion exchange resin derived from, bottom, styrene-divinylbenzene resin 1(3.5% DVB, gel-type) (see Ref. 73).
time the level of methylene bridging arising on chloromethylation. Figure 15.3.48 shows the 13C SPE MAS NMR spectra of precursor polystyrene resin 1 and its derived anion exchange form, while Fig. 15.3.49 shows the corresponding spectra for resin 2.
558
R.V. LAW AND D.C. SHERRINGTON
1
|
9
9
9
~n
1
1S0
~.
~.
.
.
1
loo PPn
I
J
a
-
|
J
$o
-
9
_a
|
, j
o
Fig. 15.3.49. 13C SPE NMR spectra of: top, anion exchange resin derived from, bottom, styrene-divinylbenzene resin 2 (0.5% DVB, gel-type). (see Ref. 73).
The signal from aromatic quaternary carbon attached to the ~CH2N+(CH3)3 group (originally the--CH2C1) moves upfield to overlap with the protonated aromatic carbon signals. The residual downfield aromatic quaternary carbon signal therefore contains only two components, that due to the carbon atom connecting to the polymer backbone (quantitatively identical to the corresponding signal in the original nonfunctional resin) and that from carbon atoms attached to methylene bridges. Careful assessment of peak areas showed that on average nearly all aromatic groups are substituted with mCH2N+(CH3)3 groups (confirming elemental analytical data) but rather surprisingly ---53% of aromatic groups are methylene bridged in the anion exchange resin derived from resin 1, and --~63% in the case of resin
CROSSLINKED POLYMERS
559
CH~(CH3}~ Fig. 15.3.50. Structural units most likely present in anion exchange resins.
2. While resin manufacturers had suspected significant levels of methylene bridging, the quantitative data is nevertheless surprising. This means of course that the predominant anion exchange site is structure (2) rather than structure (1) (Fig. 15.3.50).
15.3.3
~3C MAS NMR studies of highly crosslinked poly(divinylbenzene)
resins High surface area polystyrene and polymethacrylate-based resins without additional functionality are useful sorbents [74]. Typically they are prepared by suspension polymerisation using relatively high levels of crosslinker (usually divinylbenzene, >50 vol%) in the presence of a solvating porogen such as toluene [74, 75]. It has been known for many years that polymerisation of all the vinyl groups in these systems is not complete, and indeed residual groups can be used for chemical modification. In addition, achieving adequate detailed molecular structural characterisation of these materials has proved very difficult primarily because of their highly crosslinked nature, but also because of the complex composition of the comonomer mixtures used in polymerisations. Commercially sourced "divinylbenzene" is prepared from diethylbenzene and the grade employed contains both meta- and para-isomers as well as smaller levels of other components. Consequently dehydrogenation to yield "divinylbenzene" yields a mixture of at least four components: metaand para-divinylbenzene and meta- and para-ethylstyrene, the latter arising from incomplete dehydrogenation. Restricting the conversion to divinylbenzene isomers seems deliberate since 100% divinylbenzene is rather unstable and gels readily on storage or in transport. Two grades of commercial divinylbenzene are widely available, one containing ---50-55% divinylbenzene isomers and the other ---80% of these. Widely known resins prepared with high levels of crosslinkers are the XAD series from Rohm and Haas, XAD2 and XAD-4 being particularly much used styrene types. Recently Sherrington and his collaborators [76] have prepared model high surface area resins by polymerisation of both grades of commercially available
560
R.V. LAW AND D.C. SHERRINGTON
0 Q
ClinCH
c
/
H
!
H3
200
150
100 PPM
50
0
Fig. 15.3.51. 13C CP/MAS NMR spectra of poly(divinylbenzene) resins: top, 100% p-DVB" middle,---50% commercial DVB" bottom---80% commercial DVB. (see Ref. 76).
divinylbenzene using toluene as the porogen, and a third model species from ---100% para-divinylbenzene prepared in-house. Fig. 15.3.51 shows the ~3C CP MAS N M R spectra of the three resins obtained at 25.2 MHz. The presence of ethyl groups from ethylstyrene in the commercial divi-
CROSSLINKED POLYMERS
561
nylbenzene is seen clearly, along with signals from unreacted vinyl groups. In the case of the resin from ---100% para-divinylbenzene, ethyl groups are absent and small signals from a carbonyl containing contaminant are also apparent. The corresponding 13C SPE MAS NMR spectra for the three resins are shown in Fig. 15.3.52. Unlike the CP spectra, the latter allow quantification of the level of residual unreacted vinyl groups by integration of appropriate peak areas. For the resin prepared from ---100% para-divinylbenzene ---44% of vinyl groups remain unreacted i.e., the effective crosslink ratio is ---56%. For the resin prepared with the ---80% grade of commercial divinylbenzene --~45% of vinyl groups remain i.e., the effective crosslink ratio is ---45%, while for the resin prepared with the ---50% grade of commercial divinylbenzene ---32% of vinyl groups remain i.e., the resin is ---35% crosslinked. The levels of residual double bonds are therefore remarkably high. 13C CP MAS NMR spectra for XAD-2 and XAD-4 are shown in Fig. 15.3.53, and corresponding SPE spectra in Fig. 15.3.54. Even before attempting any quantitative analysis the clear visual similarity of these spectra with the spectra of the two model resins (from ---50% and ---80% grade divinylbenzenes) is apparent. Since the manufacturers have not disclosed the comonomer composition used to produce XAD-2 and XAD-4 the quantitative analysis of these spectra was more difficult. However, by examining the ratio of aromatic carbons to aliphatic ones it seems that no styrene comonomer is used in the polymerisation feeds (confirmed from FTIR spectra). For XAD-2 the ethylstyrene and divinylbenzene contents seem to be ---49 and ---51% respectively. Likewise the figures for XAD-4 are ---21 and 79% respectively. With this data in hand the SPE spectra were then reanalysed as for the model resins to determine the levels of unreacted double bonds. For XAD-2 this turns out to be ---47% i.e., an effective crosslink ratio of ---27%, and for XAD-4, 49% unreacted vinyl groups i.e., an effective crosslink ratio of ---40%. Thus it seems that XAD-2 and XAD-4 are manufactured directly from the two widely available commercial grades of divinylbenzene, and the high levels of unreacted vinyl groups correspond very closely to the levels found in the two model resins prepared in-house. 15.3.4
13C MAS NMR studies of hypercrosslinked polystyrene resins
Davankov and his co-workers [55, 56] first discovered these remarkable resins. They are prepared either by chemically crosslinking linear polystyrene in a post-polymerisation treatment, or similarly post-treating lightly crosslinked (with ---0.3-2% divinylbenzene) polystyrene resins. Reagent quantities are chosen to allow exhaustive Friedal-Crafts alkylation of aromatic
562
R.V. LAW AND D.C. SHERRINGTON
,
1
200
.
a
.a
.
I
IS0
a
I
9
9
1
100
PPN
~
J
~
~
1
.
~
SO
i
.
1
.
0
Fig. 15.3.52. 13C SPE NMR spectra of poly(divinylbenzene) resins" top, 100% p-DVB; middle, ---50% commercial DVB; bottom, ---80% commercial DVB (see Ref. 76).
563
CROSSLINKED POLYMERS
I
9
I
a
I
I
150
Fig. 15.3.53.
13C
,
9
.
9
|
I00 PPH
I
,a
I
J
!
SO
J
9
,
J
l
,
0
CP/MAS NMR spectra of: top, XAD-4; bottom XAD-2 (see Ref. 76).
groups and the final product resins display some remarkable properties. Firstly, surface areas measured by N2 sorption can be as high as 8001000 mZg -1, some 200-300 mZg -1 higher than conventional high surface area styrene-divinylbenzene resins, and secondly, the resins, though "totally hydrophobic" visibly swell in nonsolvents such as alcohols and even water. Two solid-state 13C NMR spectral studies of these resins have now been published [58, 77] in an attempt to evaluate their molecular structures. The results of the two studies may not be directly comparable since the source of the resins differs in each case, and hence the structures might differ. In addition, one of the studies [58] uses CP spectra and a peak deconvolution procedure and it seems likely that this approach is less quantitative than the other which utilises SPE spectra [77]. Figure 15.3.55 shows the basic chemical steps in
564
R.V. LAW AND D.C. SHERRINGTON
I
'
9
I
9
|
IsO
9
9
a
!
|
"
Ioo PPH
'
'
'
~
50
, a
a
a
'
|
J
0
Fig. 15.3.54. 13C SPE NMR spectra of: top, XAD-4; bottom XAD-2 (see Ref. 76).
synthesising the resins, where the methylene bridging side-reaction described earlier in the context of anion exchange resins is deliberately exploited. The 13C NQS and CP MAS NMR spectra of a hypercrosslinked resin are shown in Fig. 15.3.56 together with the spectral assignments. Residual ~CHzC1, and possibly ~ C H z O H groups are apparent, the former being confirmed by amination with trimethylamine. Figure 15.3.57 shows the corresponding ~3C SPE MAS NMR spectrum. The peak area ratios can be used here to quantify the various types of carbon atom present. The theoretical ratio of total aromatic carbons to total
565
CROSSLINKED POLYMERS CHsOCHtCI SnCI 4
SnCl,/
,/
\
t
CH,CI
,( rl
Fig. 15.3.55. Synthesis of hypercrosslinked resins via deliberate exploitation of methylene bridging reaction.
aliphatic carbons based on the model methylene-bridged structure in Fig. 15.3.55 is 2.40:1.00. The experimental value is 2.05:1.00. This confirms the very high level of methylene bridging, but since the ratio is even less than predicted from the simple structure, it suggests that additional aliphatic carbon functionality arises, yielding trialkyl-substituted aromatic rings. This is only possible if a very high level of double methylene bridging of aromatic groups occurs, and there appears to be a statistical distribution of such bridges between all aromatic groups rather a regular structure. In addition, elemental microanalysis confirms the presence of residual mCHzC1 groups which are located on --~10% of the aromatic groups. These must, of course, be highly hindered species. Two additional hypercrosslinked resins from a commercial source reportedly prepared from conventionally heavily crosslinked divinylbenzene resins [78] appear in one instance to exploit unreacted double bonds (and possibly additional divinylbenzene) to generate the secondary crosslinks, and in another to use SOCl2/Lewis acid to introduce very high levels of sulfoxide secondary crosslinks. Interestingly dynamic NMR experiments suggest that the latter two hypercrosslinked species retain more flexibility and mobility than that prepared via the more conventional methylene bridging technique.
R.V. LAWAND D.C. SHERRINGTON
566
_CH,O(?)
CICH,Ar I
HOCHIAr ~ .
.
.
/
CH,CH,Ar
.
H
ClC.~
~CH,CHAr
~]/~ CHsCH2Ar
l_
Fig. 15.3.56.
Ref. 77). 15.4 15.4.1
13C
~1
9
i
6,
J IS0
100 PPN
SO
0
NQS and CP/MASNMR spectra of hypercrosslinkedpolystyreneresin (see
Use of NMR spectroscopic analysis in solid phase synthesis
Background
Professor Bruce Merrifield first disclosed the use of a crosslinked polystyrene support as a macroscopic protecting group in oligopeptide synthesis in 1963 [79], and heralded the arrival of "solid phase synthesis". Some twenty years later he received the Nobel Prize for Chemistry for developing and exploiting this concept. Since his original work the use of polymer-, and indeed inorganic oxide-based supports, has expanded enormously to include polymeric reagents, polymeric protecting groups and auxiliaries and a wide range of polymer-supported catalysts [80]. The vast majority of applications employ crosslinked insoluble polymers as the support, usually in the form of spherical
567
CROSSLINKED POLYMERS
1
1
ISO
IO0 PPn
St
0
Fig. 15.3.57. 13C SPE NMR spectrum of hypercrosslinked polystyrene resin (see Ref. 77).
particulates or beads, --~20-500 Ixm in diameter, and this format provides the convenience of handling and manipulation which has led to the popularisation of the methodology. Right from the start a major weakness in solid phase synthesis, especially in applications where quite complex molecules were to be synthesised attached to a resin support, was the lack of molecular structural characterisation techniques comparable in scope and sensitivity to the methodologies available for soluble molecules. In particular, the organic synthetic chemists most powerful analytical tool, high resolution 1H and 13C NMR spectroscopy was initially not applicable. Over the last five years or so there has been an explosion in the use of solid phase synthesis when it was realised that the method could be adopted in combinatorial synthesis to allow rapid, even automated, synthesis of large libraries or mixtures of organic molecules [81-86]. This possibility has been seized upon by drug discovery groups in pharmaceutical companies as a rapid method of preparing and screening large numbers of compounds in the search
568
R.V. LAW AND D.C. SHERRINGTON
for new lead compounds. This in turn has led to a stimulation of this work in small entrepreneurial companies and in academic research laboratories worldwide [86]. The involvement of a broad group of synthetic chemists has made the need for better structural analysis more urgent, and this has brought effort and resources to bear on improving the analysis of supported reactions by NMR spectroscopy. In practice the type of support used most widely is a lightly crosslinked poly(styrene-divinylbenzene) system usually in the form of spherical beads ---50-500 l~m in diameter produced by suspension polymerisation [51]. Typically the level of the crosslinking comonomer divinylbenzene used is only ---0.5-2.0 vol%. This is crucial in terms of structural analysis by NMR, since such lightly crosslinked systems can swell considerably in suitable solvents (up to ---15 fold), such that the local environment around a functional group attached to the polymer network can approach closely to that in isotropic solution (see later).
15.4.2
Solution phase NMR studies
Structural analysis of linear polymers molecularly dissolved in a suitable solvent using 1H and 13C solution phase NMR spectroscopy is long established [87-89]. Not surprisingly therefore when a linear soluble polymer is used as a support in solid phase synthesis 1H and X3C solution phase NMR spectroscopy can be a powerful tool in following the chemical synthesis on the support [90]. Figure 15.3.58, for example, shows a series of 1H NMR spectra of dissolved linear polymer samples taken at various stages in the solid phase synthesis of oligoethers on soluble polystyrene [91]. The various chemical steps Fig. 15.3.59 are clearly demonstrated. Even in the case of linear polymers, however, if the solution concentration is increased to allow the local and macroscopic viscosity to rise significantly the mutual dipole-dipole interactions of magnetic 1H nucleii give rise to broadening of their NMR resonances, though the effect with the very much lower natural abundance of 13C nucleii is considerably less. In the case of crosslinked polymers therefore even when the level of crosslinking is very low attempts to record "solution" 1H NMR spectra of solvent swollen samples generally result only in broad signals. However, ~3C NMR spectra from swollen lightly crosslinked polymer gels can show remarkably fine line-width resonances.
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CROSSLINKED POLYMERS
571
(••=•CH2CI coupling
Nail H(OCH2CH2),OCPh3
(~~CH2(OCH2CH2),OCPh3 deprotection
CF,C 0 H H20
(~~-~CH2(OCH2CH2),OH activation ( ~ - ~
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elongation
NaH H(OCH2CH2),OCPh 3
deprotection
CF3C O2H H,O
@--~CH2(OCH2CH~).OH Fig. 15.4.59. Scheme for solid phase synthesis of oligoethers on a soluble linear polystyrene support.
15.4.3 Swollen gel-phase 13C NMR studies Early work on the application of 13C NMR conventional techniques to the study of solvent swollen crosslinked polymers was reported by Stenlicht and co-workers [92] and Schaefer [93]. The first detailed work on the chloromethylated polystyrene resins used in solid phase synthesis appears to be by Manatt and co-workers [94]. They employed a 0.095% crosslinked species supplied by the Dow Chemical Co. and chloromethylated this to varying d e g r e e s . 1H noise decoupled 13C NMR spectra were recorded in the usual
572
R.V. LAW AND D.C. SHERRINGTON
200
160
120
ppm
80
40
6%DVB
2%
!
o~
Fig. 15.4.60. 13C NMR spectra of chloromethylated polystyrene resins of 0-6% crosslink ratio swollen in CDC13 (see Ref. 97).
way at 15.1, 25.1 and 45.2 MHz using resin slurries in CHC13 or CDC13 in 10 mm o.d. sample tubes. The chemically modified Dow resin yielded spectra at various--CHzC1 loadings essentially as sharp as those of a model soluble linear polymer while the spectrum of a chloromethylated ---1% crosslinked polystyrene already started to show broadening. Interestingly the analysis showed that chloromethylation occurred almost exclusively in the 4-position of styryl residues, and significant levels o f - - C H z O H groups were also detected as a result of hydrolysis of the - - C H z C 1 groups, confirming an earlier report by Merrifield and co-workers [95]. Manatt and co-workers also described the use of a 19F NMR approach for studying peptide synthesis on swollen gels [96]. In many respects, however, Ford and his collaborators brought to a wider audience the potential scope for using conventional X3c NMR techniques for analysing solvent swollen lightly crosslinked functional polymers [70, 71, 97]. Figure 15.4.60 shows the gated decoupled 13C NMR
CROSSLINKED POLYMERS
573
spectra of polystyrenes containing 25 wt% chloromethylstyrene and 0-6% divinylbenzene crosslinker. The spectra were recorded at 25.2 MHz with the samples fully swollen in CDC13 in 12 mm o.d. tubes. Extensive relaxation experiments and attempts to quantify relative peak areas demonstrated a number of artefacts to arise in the generation of these spectra (see later). Nevertheless the work set the scene for the expansion in the use of so-called "gel-phase" 13C NMR spectroscopy applied to solid phase reactions on crosslinked polymer supports. Major contributions to gel phase 13C NMR spectral analysis in the solid phase synthesis of oligopeptides have been made by the groups of Epton [98-101] and Girault [102-104]. Epton and his co-workers [105] pioneered the use of "ultra-high load" methods in peptide synthesis using a phenolic resin, such that in the final support-peptide assemblies the anchored peptide chains comprise the major mass component. In these reactions the group have monitored N-terminal Boc and Fmoc removal by gel phase 13C NMR spectroscopy [99, 100], investigated the lability and stability of some common side-chain protecting groups [101] and characterised a number of polymerpeptide assemblies [99, 100]. Remarkably detailed 13C NMR spectra (e.g., Fig. 15.4.61) are reported in these works. Girault's group have examined the mobility of pendant groups on polymer resins and shown the importance of using the correct solvent, not only for swelling the resin, but also for minimising any intrapolymeric interactions of immobilised groups that can give rise to major line broadening in the 13C NMR spectrum [106]. The association of peptide chains during solid phase synthesis has also been studied by quadruple echo deuterium NMR spectroscopy [107]. As well as utilising lightly crosslinked gel-type resin supports Giraults group has also examined macroporous polystyrene resins, Kel-F-gstyrene, polyacrylamide and controlled pore glass supports, and have applied both 19F and 13C NMR gel-phase spectroscopy to correlate the microenvironmental mobility of protected amino acids with the differences in reactivity observed in peptide synthesis [103]. Other state-of-the-art work from the group includes the stage-by-stage gel-phase 13C NMR spectroscopic characterisation of a growing peptide chain during stepwise synthesis [102]; 31p_ NMR spectroscopic studies of oligonucleotide synthesis on lightly crosslinked polystyrene; and 2D-1H-13C correlated NMR spectra of oligopeptides bound to similar supports. In each case the crosslinked gel was swollen in CDCI3 [104]. During the above development of gel-phase NMR analysis of supported reactions, advances were also in progress with regard to the solid support itself. The most widely used lightly crosslinked polystyrene systems had con-
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CROSSLINKED P O L Y M E R S
575
siderable restrictions with regard to the solvents which could be employed in reactions, and this led to a number of strategies to try to overcome the problem. Perhaps the one which has had the greatest impact is the development of lightly crosslinked polystyrene gels with long polyethylene glycol (PEG) derived sidechains, where the solid phase synthesis is carried out on the free terminus of the PEG chains [108, 109]. These materials have been termed "tentacle" polymers [110, 111] and form the basis of the TentaGel | range of supports available from Rapp Polymere, Tfibingen, Germany (also known as the "Rapp resin"). These materials are unusual because of the broad solvent compatibility range they offer, but also because of the flexibility and accessibility of the functional endgroups. This is reflected in the gelphase 13C NMR relaxation times, T1, of peptides bound to the PEG terminus, and to the sharp NMR signals which are recorded [112]. When a poor solvent for the PEG tentacles is employed however, e.g., diethyl ether, broad NMR signals are obtained typical of measurements on solids in suspension. Tentagel | resins have rather low capacities (per gramme) as a result of the long PEG spacers used and this also, no doubt, contributes to the efficiency of solid phase synthesis and to the quality of 13C NMR spectra which can be obtained. However, recently Brown and Ramsden [113] have used gel-phase ~3C NMR spectroscopic analysis to demonstrate high fluidity and homogeneity in the case of 2% crosslinked polystyrene resin functionalised to a relatively high level (--~60% pendent group) with an eleven carbon atom spacer. Again the solvent employed is crucial and for a tethered tetrapeptide [2H6]DMSO proved very effective, presumably as a result of minimising interchain aggregation. Typically spectra were obtained at 126 MHz in --~1 hour at 50~ A further development in gel-phase ~3C NMR spectroscopic analysis has been the use of ~3C enriched building blocks in the synthesis [114]. This allows the use of much smaller quantities of resin (--~20 mg) containing less than 1 mg of the moiety to be analysed. In the case of synthesis on a Tentagel | resin, rapid analysis is possible because of the enhanced mobility of the system and the reduced number of transients that need to be accumulated. 15.4.4
Swollen gel-phase MAS 1H NMR studies
Although analysis of solid phase reactions by ~3C NMR spectroscopy is a very powerful methodology the move towards synthesising a diverse range of organic molecules on polymer supports had led to an increased demand for ~H NMR spectra comparable to those achievable for small molecules dissolved in a solvent. As early as 1974 Doskocilov~ and co-workers [115] reported that high resolution ~H NMR spectra of crosslinked poly(methylme-
576
R.V. LAW AND D.C. SHERRINGTON
thacrylate) gels swollen in chloroform could be achieved by spinning the sample at the "magic angle" relative to the applied stationary field, in a similar way to the acquisition of solid-state spectra. The method was effective for crosslinking levels in the range 0-1%. Despite a further report in 1985 indicating 1H line narrowing in polystyrene-divinylbenzene resins [116] by magic-angle spinning of swollen gels, this development was not picked up by those involved in solid phase synthesis. The effect was confirmed and exploited by Mashelkar and his co-workers studying super-absorbing polymer gels [117], before the much broader relevance was realised by Fr6chet and his group [118, 119]. These researchers showed that in the case of a CDC13 swollen functionalised 1% crosslinked polystyrene support with 12% of the aromatic groups with ephedrine moieties, direct polarisation 13C and 1H NMR spectra with high resolution could be obtained by spinning the sample in a normal MAS probe. Figures 15.4.62 and 15.4.63 show the 13C and 1H spectra respectively obtained using a standard 7 mm MAS probe in a Bruker AF-300 spectrometer operating at 75.47 MHz for ~3C. Spinning rates were 2000 Hz and 2350 Hz respectively for 13C and 1H spectra. In this instance the ephedrine moiety was attached directly to the polymer backbone and in retrospect it is clear that structural units attached by flexible spacers were likely to yield even better spectra. Since this approach did not require the auxiliary power amplifiers typically needed for cross-polarisation (CP) MAS in the solid-state the authors argued [118] that the method could become rather routine from both an experimental and instrumental point of view. More detailed work [119] by the group on 1-2% crosslinked resins highly swollen with solvent, as examples of polymeric systems that undergo rapid but anisotropic reorientation on the molecular scale, confirmed that the spectral line broadening due to the residual dipolar coupling (1H NMR) or chemical shift anisotropy (13C NMR) can be removed by moderate-rate magic-angle spinning, to yield highly resolved ~H and 13C NMR spectra. Furthermore, well-resolved signals attributable to the crosslink junctions themselves (in the case of a 5% dimethylsilane crosslinker) were recorded. A further advance was reported by Keifer and co-workers [120] in terms of reduced sample size. They utilised a Varian Nano-NMR probe with a 500 MHz instrument to record 1H NMR spectra of Tentagel | bound substrates. Typically 10 mg samples swollen in 30 ~L of DMSO-d6 and spun at 2000 Hz at the magic angle yielded better spectra than 100 mg of sample in a more routine gel-phase experiment not involving magic-angle spinning. Shapiros group have used a more conventional 7 mm o.d. probe to obtain 1 3 C ~ 1 H correlated NMR spectra of substrates attached to both Tentagel | [121] and Wang [122] resins. Again spinning at the magic angle yielded much better resolved signals than those from simple gel-phase experiments, and 2D corre-
577
CROSSLINKED POLYMERS
t
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'
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Fig. 15.4.62. Gel phase 13C NMR spectra of ephedrine immobilised on a 1% crosslinked polystyrene resin" A, "solution state" spectrum" B, sample spun at the magic angle (see Ref. 118).
lations approaching those of solution phase spectra. Reduction in sample size to the ultimate analysis of a single resin bead has been reported by both Keifer and co-workers [123], and Rapp and his collaborators [124]. The former group employed the Varian Nano-NMR probe with a single bead of Wang resin carrying a 3,5-dimethoxy-benzoic residue. The bead was swollen in 30 ~l of CHzClz-d2 and the 1H NMR spectrum recorded at 500 MHz with the sample spun at the magic angle at 2000 Hz. In practice the resulting
578
R.V. LAW AND D.C. SHERRINGTON
'
. . . .
I
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16.9
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Fig. 15.4.63. Gel phase IH NMR spectra of ephedrine immobilised on a 1% crosslinked polystyrene resin: A, "solution state" spectrum" B, sample spun at the magic angle (see reference 118).
spectrum was complicated by large signals arising from solvent backgrounds, impurities and fingerprints on the outside of the sample cell, as well as smaller peaks due to the polystyrene backbone itself. To achieve usable spectra it was necessary to synthesise the corresponding species 13C enriched on the methyl groups of the methoxy substituents. To some extent therefore this requirement negates the value of the single bead analysis. In contrast the Rapp team [124] took a single macro bead of Tentagel | with a diameter in the range 400-750 ~m swollen in DMSO-d6 or CDC13 fixed into an insert
CROSSLINKED POLYMERS
579
and placed in a standard 4 mm o.d. solid-state NMR rotor. The detection volume was 200-400 I~L. The XH NMR spectra were recorded at 300 MHz with magic-angle spinning at 3000 Hz. Very detailed high resolution spectra (Fig. 15.4.64) of the hydantoin reaction sequence on the resin were obtained in this way. Since the above reports appeared, the use of magic-angle spinning of solvent swollen lightly crosslinked gel-type polymer supports has expanded, particularly for monitoring the progress of solid phase syntheses [125, 126]. ~H NMR spectra at 600 MHz show remarkably detailed features [126] and magic-angle spinning in acquisition of 19F NMR spectra has also proved very valuable [127]. The in-house use of the Varian Nano-NMR probe has been expanded [128] to investigate the influence of the resin structure, tether length and solvent on the quality of the high resolution 1H NMR spectra of bound peptides. Perhaps, not surprisingly, all of these factors have been shown to contribute to spectral quality. Despite the improvement in the narrowing of XH NMR signals by spinning solvent swollen gels at the magic angle, this approach still treats the signals from the polystyrene support in exactly the same way as the signals from any anchored species. Wekler and Westman [126] have used the Varian Nanoprobe with spinning at the magic angle in a 600 MHz instrument to record ~H NMR spectra of a diphenylsulphone attached to CDC13 swollen Tentagel | By combining presaturation of an upfield linker signal at 3.5 ppm, with a 180~ ~ double pulse experiment, the broad signals from the aromatic protons were almost totally suppressed (Fig. 16.4.65). Sarkar and co-workers have argued that the complication of the presence of the large peaks from the matrix could be reduced by judicious choice of the r value in the spin echo pulse sequence (90~176 [129]. This should be possible because the matrix signals are much broader presumably because of a short T2. The use of the spin echo sequence to distinguish between narrow and broad lines has been reported before [130, 131]. Figure 15.4.66 shows the 500 MHz ~H MAS NMR spectra of species (3) bound to a Wang resin swollen in CD2C12. The spectra were recorded from ---6 mg resin in a Nano-NMR probe. It is clear that by appropriate choice of the r value the effect of the polystyrene resonances can indeed be reduced significantly. At the time of writing Shapiro and his group have shown further that the loss of coupling information which accompanies the spin-echo experiment, can be restored by utilising a 2D J-resolved procedure [132]. Similarly S0rensen and his co-workers have reported the results of 2D ~H homonuclear correlations to reduce the inhomogeneous line broadening [133]. Clearly the various specialised instrumental techniques described above are yielding increasingly better quality 1H NMR spectra of resin bound
580
R.V. LAW AND D.C. SHERRINGTON I
@
OCH2CH~OC~N~
/
POE
[D6]DMSO
TMS \
O OCH3 @-'~OCI'hCH2)nOCH~2 H2NH--~--CH~--@ H2OH
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OCH3
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Fig. 15.4.64. Gel phase 1H NMR spectra of products at each stage of a solid phase synthesis of an hydantoin on a single lightly crosslinked Rapp macro-bead swollen in CDC13 and spun at the magic angle (see Ref. 124).
581
CROSSLINKED POLYMERS
r
8.G
8.2
7.8
7.4
7.0
G.6
iopm
Fig. 15.4.65. Gel phase 1H NMR spectra of aromatic sulfone species attached to Tentagel | swollen in CDC13 and spun at the magic angle, showing manipulation of the pulse sequence to suppress the signals from the support hydrogen atoms (see Ref. 126).
substrates, and it is by no means clear yet how far these developments can go and where the levelling in technology will occur. Nor is it yet clear what can be regarded as routine. Much of the instrumentation and some of the techniques described are costly in terms of both hardware and man-hours, and this will restrict the number of groups capable of applying these methods, and indeed potentially limit the throughput of samples within these groups. There is little doubt however that those wishing to participate at the leading edge of solid phase synthesis methodology will have to assimilate some of these procedures. In all of these developments it is also important to bear in mind two other factors. Firstly, it is not clear yet how quantitative the various specialised methods are likely to be since application of complex chemical-physics methodologies often involve a compromise in terms of the generation of artefacts, especially with regard to the level of quantification that is possible (see Solid State SPE MAS 13C NMR analysis). The second factor to be borne
582
R.V. LAW AND D.C. SHERRINGTON
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CROSSLINKED POLYMERS
583
in mind is the dramatic limitation all these developments involve with regard to the level of crosslinking and local mobility of the support. Essentially all the work reported has involved very lightly crosslinked gel-type supports typically nominally --~1% crosslinked. While this is fine in terms of combinatorial solid phase synthesis no advantage has been reported to date in those areas (e.g., polymer-supported catalysts) using highly crosslinked macroporous resins. No doubt some forward movement will be seen here in due course, but for now it remains an area which offers a considerable challenge to NMR specialists!
15.5
Swollen-state N M R studies of crosslinked elastomers
Elastomeric polymers i.e., polymers whose glass-transition temperatures, Tg, lie well below ambient temperature generally have very low tensile and tear strength. This is simply because the intermolecular forces are very weak, allowing individual polymer chains to be physically pulled apart. For practical application therefore it is necessary to vulcanize or crosslink elastomers to provide toughness and strength. Although in recent years a number of thermoplastic block copolymer elastomers have been developed in which the interbonding of elastomer chains is provided by discrete microphases of glassy polymer, in practice the large majority of rubbers use crosslinking as the method of forming a tough infinite network of elastomeric chains. The level of crosslinking is not surprisingly therefore a key parameter in defining and controlling the bulk properties of an elastomer. Despite this pivotal position, it has proved very difficult to develop routine and simple methods for quantifying the level of crosslinking in a polymer. An important methodology established early on is an equilibrium volume swelling approach and application of the Flory-Rehner equation [134, 135]. While this is satisfactory for simple one component elastomer networks it is not particularly convenient experimentally and is of dubious value for multicomponent elastomers. Paradoxically the "problematical" line broadening, arising in the NMR spectra of polymers which are crosslinked (see Section 4), has been exploited to provide a simple instrumental technique for quantifying crosslinking. Although ~H NMR spectral line broadening on crosslinking had been observed earlier, Marchenkov and Khitrin [136] first published a mathematical explanation of the observation that line widths in the spectra of polymer gels are quantitatively related to the concentration of crosslinks under defined conditions. Loadman and Tinker [137] took vulcanizates of a blend of natural rubber and acrylonitrile-butadiene rubber swollen in CDC13 and recorded
584
R.V. LAW AND D.C. SHERRINGTON
CW ~H NMR spectra at 90 MHz. While the peak broadening of the methyl protons in natural rubber alone could be monitored, this proved impossible in blends because of spectral overlap. As a result the proton signals from the olefinic group were found to be more generally useful. Peak broadening (H%) was assessed as the ratio of the signal strength at a reference point on the high field side of the peak to the signal strength at the peak. Good correlation was found between H% and the level of crosslinking in each component rubber as measured by a separate solvent swelling experiments. Blends of natural rubber and cis-polybutadiene have been similarly investigated [138]. The analytical procedure has also been developed for 1H NMR spectra of swollen elastomers recorded on a 300 MHz FT instrument [139]. In this case a progressive downfield shift of the olefin proton peaks with increasing crosslink ratio is observed and this complication has to be accounted for in the peak broadening analysis. Similar studies have been reported using a 200 MHz instrument [140]. Although most studies have employed ~H NMR spectra, analogous phenomena are seen with ~3C NMR spectra obtained from solvent swollen elastomers, and similar correlations with crosslink ratio are possible [139]. This approach broadens the applicability of the technique to include elastomers whose ~H NMR spectra do not lend themselves to this analysis. Overall the use of advanced NMR spectral techniques is becoming more widespread in the study of intractable elastomers [141, 142].
References
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
J. Schaefer, E.O. Stejskal and R. Buchdahl, J. Am. Chem. Soc. 10 (1977) 384. T. Yu and M.M. Guo, Prog. Polym. Sci. 15 (1990) 825. M. Andreis and J.L. Koenig, Ad. Polym. Sci. 124 (1995) 191. R.A. Komoroski (ed.), High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk, VCH, 1986. L.J. Mathias (ed.), Solid State NMR of Polymers, Plenum Press, 1991. V.J. McBrierty and K.J. Packer, Nuclear Magnetic Resonance in Solid Polymers, Cambridge University Press, 1993. R.N. Ibbett (ed.), NMR Spectroscopy of Polymers, Blackie Academic and Professional, 1993. F.A. Bovey and P.A. Mirau, NMR of Polymers, Academic Press (2nd edn.), 1996. G.A. Webb (ed.), Annual Reports on NMR Spectroscopy, Academic Press. K. Ismail, O. Sirkecioglu, J.M. Andresen, S.D. Brown, P.J. Hall, C.E. Snape and W. Steedman, Polymer 37 (1996) 4041. I.-S. Chuang and G.E. Maciel, Macromolecules 24 (1991) 1032. J.R. Ebdon, B.J. Hunt, W.T.S. O'Rourke and J. Parkin, Brit. Polym. J. 20 (1988) 327.
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