Functional polymers supported on porous silica

ReactivePolymers, 16 (1991/1992) 41-49 Elsevier Science Publishers B.V., Amsterdam

41

FUNCTIONAL POLYMERS SUPPORTED ON P O R O U S SILICA I. GRAFTING ACTIVE PRECURSORS ONTO POROUS SILICA ERIC CARLIER a, ALAIN GUYOT a, ANDRI~REVILLON a, MARIE-FRANCE LLAURO-DARRICADES ~ and ROGER PETIAUD b

a CNRS--Laboratoire des Matjriaux Organiques,BP 24-69390 Vemaison (France) b CNRS--Service Centrald'Analyse, BP 22-69390Vernaison(France) (Received April 26, 1991; accepted in revised form July 20, 1991)

A set of porous silica has been grafted with trialkoxysilane coupling agents carrying either a T-propyl methacryloyl group, a T-propyl thiol or a T-propyl amine group. The grafts have been characterized by diffuse reflectance Fourier transform infrared spectroscopy and 29Si and 13C cross-polarizalion magic-angle-spinning nuclear magnetic resonance; the grafting yields were estimated from elemental analysis. The grafted amine was further reacted with azobiscyanovaleric acid. The grafted materials are to be used as co-monomer, transfer agent or initiator for a further grafting of a functional polymer via a radical polymerization. Keywords: functional polymers; silica supports; silica grafted polymers; grafting characterization; silica coupling agents

INTRODUCTION

Catalysts and reagents supported on a solid support have gained a large audience, reviewed in a few recent books [1-3]. In most cases the supports are organic polymers, mainly styrene-divinyl benzene macroporous or gel-type copolymers. However these systems suffer two main drawbacks: lack of accessibility due to the viscous character of the

polymeric m e d i u m when swollen by suitable solvents, and lack of thermal, mechanical a n d / o r chemical stability. In our laboratory we have been very much involved in trying to improve the accessibility [4,5]. Very recently we have given our attention to pellicular resins based on silica [6]. Porous silica may have a morphology well suited for a good accessibility but, in addition, it confers to the supported catalysts both thermal and me-

0923-1137/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

42

chanical stability. The main drawbacks can be a rather small volume capacity and some lack of stability at extreme pH conditions. A lot of work has been published concerning the grafting of various organic compounds onto silica, mainly for applications in the field of chromatography [7]. However, most often, very few data about the characterization of the grafting are available, the attention being given to the performances of the material for the given application. Only a few studies have been published concerning catalysts supported on polymers grafted onto silica [8]. The most extensive study was carried out by Challa et al. [9-11], using mainly nonporous silica grafted with vinylpyridine polymers or copolymers as ligands for copper catalysts used in phenol oxidative coupling. Such catalysts show a very limited loss of activity, as compared to similar homogeneous catalysts and were recycled more than 20 times without noticeable loss of activity. This paper is the first of a series describing our work in this field. Our strategy was to attach to porous silica a thin layer of functional polymers, so as to combine good accessibility, due to both the pore size distribution of the silica and the small thickness of the grafted polymer layer, with an acceptable capacity (up to about 2 m e q / g ) due to the increase in the number of functional groups initially carried by the silica (silanol groups) through the polymeric chains of the grafts carrying the functional groups attached to the silica after reaction with the silanol groups. Two main routes can be followed. The first is to attach a preformed polymer suitably functionalized to react with the silanols. Such a route has already been followed to attach polyethylene oxide with applications in phase transfer catalysis [12,13]. The main interest is the easier possibility of characterizing the polymer to be grafted. On the other hand the accessibility of pores to large polymer molecules should be limited, so that the grafting yield, and thus the capac-

ity, might be rather low The second route is to proceed to the polymerization of the functional monomer inside the pores. Then, the polymer may be fixed simply by adsorption, but it seems better to fix it through a covalent bond. Radical polymerization is of interest for that route for two reasons: first, it allows the polymerization and the copolymerization of a very large variety of functional monomers; second, it is versatile with respect to covalent fixation. One can use the initiation reaction if the radical generator is anchored onto the silica, as well as the propagation reaction by copolymerization with an anchored monomer, or a transfer reaction if the silica is carrying a transfer agent. We call these various anchored compounds the active precursors in radical polymerization. All of them are directly or indirectly available as coupling agents commonly used in composite materials. Their general formula is (RO)3Si-(fn2)n-X where R is a methyl or ethyl group, n = 3 and X may be NH 2 (aminopropyl siloxane, APS), an acrylic or a methacrylic moiety (MPS, as methacryloylpropyl siloxane) or a thiol (propyl thiol triethoxy silane, PTTS). This route is summarized in scheme 1. MPS is used for coupling the functional polymer by copolymerization initiated by azobisisobutyronitrile (AIBN) or eventually a peroxide. PTTS is used as a transfer agent. In the case of APS a further reaction with azobiscyanovaleric acid (ACVA) is needed to fix a radical generator. Very recently Challa and

MP5

~IOH

l 5 i. o-5 i ~ ' ~ ' ~ 0.,

P T T5

AP5

i-o-si.--.~N,~

Scheme 1. The functionalization of silica.

43 TABLE 1 Physical and chemical properties of silica Silica

Diameter (tzm)

Surface area (m2/g)

Porous volume (cma//g)

SiOH (/zmole / m 2)

XOC005 F 1000 EC 299 RP 1 ED 3

142 63 1500 28 5.6

18 29 78 89 400

1 1.1 0.98 0.9 1.8

4.8 7.9 5.3 3.3

his group followed that route using nonporous silica [14] and other nonporous mineral supports [15]. This first paper is devoted to the fixation of the active precursors only. Further papers will describe the polymerization, grafting of preformed polymers, the physical study of accessibility and some applications to catalysis.

RESULTS AND DISCUSSION Spherical porous silica, varying in diameter, surface area, porous volume and surface silanol concentration, was used in this study.

The data from their characterization are reported in Table 1. Before use, the silica was dried at 120 o C, in order to remove physically adsorbed water. Then samples of each one were reacted with various amounts of MPS, APS or PTTS. Most of the data are gathered ifi Table 2. Grafting yields were obtained from elemental analysis of carbon (for MPS), sulfur (for PTFS) and nitrogen (for APS). They are relative to the silanol groups, assuming the reaction of one molecule of the coupling agent per silanol group. ~Si--OH

+ (RO)3Si~X OR !

,

+

I

OR Because of the possible reaction of two alkoxy groups of the same molecule with two vicinal SiOH groups of the silica, the yields indicated in Table 2 may be underestimated. On the other hand if some traces of moisture had been present, some condensation of the coupling agent might have taken place, and the yields then would be overestimated. Anyway, it is clear that the yield increases with the amount of coupling agent used, but tends

I

~/i-°"s~ ~1 Iw

~.$

°~k,° 19.0

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c.s

7.,. ,

,

$?oo

,

,

,

CfC ,

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2600 4SO0 WAVENUMEER

Fig. 1. FTIR spectrum of silica (ED3) after reaction with MPS showing the bands assigned to characteristic organic structures.

Z < n~ I-

8°$

~,

6.4

:

:

~

;

:

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Fig. 2. FTIR spectrum of silica (ED3) after reaction with PTTS.

44 TABLE 2 Grafting yield of precursor onto silica Silica

SiOH (/zmol/m 2)

Nature

Precursor amount (/~mol/m 2)

Amount grafted (/zmol/m 2)

Grafting yield (%)

XOC005

4.8

MPS

1.6 4 8.1 40.7

1.5 2.1 3.2 3.4

31 53 66 70

EC 299

7.9

MPS APS PTrs

8 8 8

4.2 4.4 4.5

53 55 57

RP 1

5.3

MPS

9.3 39.5 8 8

1.4 1.9 3.2 2.8

26 35 60 52

7.8 24 8 8 8

2.7 2.8 1.9 2.7 2.0

82 84 58 a 82 b 62

APS FITS ED 3

3.3

MPS APS PTTS

a In water, b In toluene.

to level off rather rapidly when a two times excess of silane is used. The maximum yield tends to increase with the surface area of the silica, but it is not safe to establish direct relationships. The yield seems not dearly dependent on the nature of the coupling agent. Qualitative proof of the grafting may be obtained from infrared spectroscopy [16,17]. The diffuse reflection FTIR spectra are shown in Figs. 1-3, using the same silica with each of the three agents. In the case of MPS (Fig. 1) the following bands can be observed: at 1637 c m - ] (C=C), 1708 and 1719 cm -1 (C=O), free and hydrogen bonded, 2922 and 2944 cm -1 (CH 3) and 2937 cm-1 (CH3).A very similar spectrum is observed in the case of PTTS (Fig. 2), except for the absence of the C=O bands and the presence of a S - H elongation band at 2570 cm-1. The spectrum obtained with APS (Fig. 3) is very similar to one already published by Kaas and Kardos [16], showing a very broad

OH band and two small NH 2 bands at 3301 and 3352 cm -1, which, according to the authors can be interpreted as evidence for a high degree of hydrogen bonding between the amine group and unreacted silanol groups. It can be suggested that these silanol groups hydrogen bonded with the NH 2 groups of APS, are not available for the OEt t4.0

0 z < I-u~ z < "~' Io~.

-Si ~

--

~.6

NH2

O£t

¢!.2

$.8 8.4 7.0

: 4800

• 370O

:¢H 3

: 2600

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Fig. 3. FTIR spectrum of silica (ED3) after reaction with APS.

45 reaction with the alkoxy silane groups, so giving an a c c o u n t for the limitation in the yield. G r a f t i n g has also b e e n studied using solid 29Si a n d 13C N M R [18,19]. F i g u r e 4 shows the 29Si spectra. As cross polarization is used

(29Si or 13C m a g n e t i z a t i o n occurs via the p r o t o n m a g n e t i z a t i o n ) , the observed 29Si atoms are only those n e a r e n o u g h to the H atoms, i.e., those located at the surface. T h e s e surface Si atoms b e a r e i t h e r two or o n e or no O H groups (a, b, c of s c h e m e 2 a n d Fig. 4 a ) .

b

b

d

e

b

, 2'0

'

.20 '

'

.60 '

'PP" M

C

-'120

Fig. 4.29Si CP-MAS NMR spectra of silica (ED3): (a) before grafting; (/3) reacted with MPS; (y) reacted with APS. Resonances a, b, c are relative to surface Si atoms bearing two, one or no silanol OH groups, respectively. Resonances d, e, f (for MPS) and d', e', f' (for APS) are relative to Si atoms from the graft (Si) with two, one or no alkoxy OR groups, respectively (for precise chemical shift values see scheme 2).

46 OH I

0

OH I

b

I

0 (a)-91.7 ppm

R' I ~Si ,,CR

o

' S'-o 0 d (d')

(b)-101 ppm

Rk ,CR o.-SLo

,

,

..Si

..Si

?

?o,

o6 "od "o e (e')

f (f')

-58 ppm

Those from the grafts are polarized through the S i - C H 2 bonds of the SiR' group. They have either one or two or three S i - O - S i bonds, respectively (d, e, f of scheme 2 and Fig. 4/3 in the case of MPS, and d', e' and f' in Fig. 4y for the case of APS). It may be tempting to derive quantitative results from the data shown in Fig. 4. However some remarks must be taken into account: (i) the rates of polarization are not the same for Si atoms from the silica and from the grafts; (ii) when the grafting increases, the surface Si atoms carrying no silanol (c in Fig. 4) may be

S

hCH . " b

ql

~/ao ORCH3

ioo

~o

~io

~o

~

b

Fig. 5. Solid state 13C N M R spectrum of silica (ED3) reacted with MPS (see scheme 3 for band assignments).

-66 ppm

Scheme 2. Structure of the surface Si groups and the corresponding chemicalshifts observed by 29Si CP-MAS NMR. The silicium resonances from the silica are a, b and c, and those from the grafted Si are d, e and f for MPS, and d', e' and f' for APS. See Fig. 4 a, ~ and y. A good fit was observed compared with previous assignments [14,16].

~i

g lla

R\ /R' • Si ",,-, ~SL--O'-. ', • u

Si Si Si,. Si"o o','o',. 0 0 / '-'

-48 ppm

C

e f

(c)-111 ppm

°

3/~f C H H g H MPS

OCH2CH3 I S1i - - ~ "Si~ ..~v IORCH~CH3 a2

a= 7.6 ppm b= 22.1 ppm

e= 137.3 ppm f= 123 ppm

c= 66.7 ppm d= 166.9 ppm

g= 16.5 ppm h= 50 ppm

~= 8.3 ppm [3= 27 ppm

NH2 "Y

underestimated as compared with the two others (a, b in Fig. 4); (iii) any determination is complicated by the fact that, for instance, if one group appears as e (Fig. 4fl), it means that two b groups must disappear and, at the same time, two c groups appear; similar complex features are valid for the formation of d or f groups. Anyway, the important differences between the relative intensities of d, e, and f (Fig. 4/3) and of d', e' and f' (Fig. 4y), lead to the conclusion that there are more multibridged grafted Si atoms with APS (Fig. 43,) than with MPS (Fig. 4/3). Solid state 13C CP-MAS N M R spectrum of silica reacted with MPS shows all the bands expected from the reaction. The corresponding assignments, are reported in scheme 3. Grafting with APS is an intermediate step for anchoring a radical generator onto the

APS

CH2 = 58 ppm CH3 = 13 ppm

7= 43.8 ppm

Scheme 3. Chemical shifts observed in solid state 13C N M R spectra of silica reacted with MPS and APS (good fit with previous assignments [15,16]).

47 I

-St--O" I St_

o..SlvvNH&~; +

/ I

~

OH

v

NC CRy,

I xx..,_

-,rI-

/

" Sl

-..

r,-,,.

(cH~

H3C--I---CN

o

.CH3

O~N~N

v

N

H~C---~N CN

OH /I

(CH2)2

-I

~c

N

8.9

0 o'~

I

Scheme 4. Fixation of the radical generator ACVA onto APS grafted silica. TABLE 3 Fixation of ACVA onto APS grafted silica Silica

Amount of APS grafted (/~mol/m 2)

N before reaction (%)

N Reaction after yield reaction (%) (%)

F 1000 EC 299 EC 299 ED 3

4.9 4.4 4.5 2.8

0.17 0.48 0.49 1.6

0.44 1.07 1.02 3.23

73 75 71 69

silica. A P S must be reacted with A C V A according to scheme 4. As shown in Table 3, the yield of that reaction, estimated from elemental analysis, is around 70% and does

7.0 l

.

4800

.

. 3/00

.

.

CON., 2600

t500

, 400

WAVENUMI~I~.R

Fig. 6. FFIR spectrum of silica (ED3) reacted first with APS and then with ACVA showing the bands assigned to characteristic organic structures.

not d e p e n d on the silica initially used, or on the amount of A P S grafted. In their recent paper, Boven et al. [14] used the acid chloride of A C V A , which results in a slightly higher yield of 90%, but requires preparation of the acid chloride. Hopefully the two carboxylic groups of the A C V A molecules should react with the amine groups of grafted APS, so that, upon thermal decomposition, the two radicals p r o d u c e d are grafted onto the silica. Actually, this is not the case as some carboxylic groups remain unreacted. The same observation was m a d e by Boven et

-_CHr::"

-CH~

- S; - 5H~-

-- ~-N'N-

--CON -

I1<+~11 - ..~. ~ cl4 3

PPM Fig. 7. Solid state 13C NMR spectrum of silica (ED3) reacted first with APS and then with ACVA. (see scheme 5 for band assigi~i~ents).

48 COOH = t73 ppm

CH~, =

CN =

~'18 ppm

CH,~a(': 34.ppm

CH2~' :

29 ppm

NC

U~N~OH

24ppm

fluence on the adsorption of organic compounds. In order to reduce these effects, a further treatment of the MPS grafted silica was performed with hexamethyldisilazane. The data reported in Table 4 show that 12 to 24% of the initial silanol groups of the silica remain unreacted; the limitation of the silylation reaction is probably due to steric hindrance.

C-N--N: 75ppm

C~43

0

~OH NC CHS

Scheme 5. Assignments of chemical shifts for 13C NMR spectra of ACVA.

al. [14]. Proof of the presence of unreacted carboxylic groups is given both by the infrared (Fig. 6) and the NMR CP-MAS 13C spectra (Fig. 7). Assignments of the 13C spectra of Fig. 7 are made from the NMR spectra of both grafted APS and free ACVA, as shown in scheme 5. The small band at 17001705 cm -1 shown in Fig. 6 is attributed to the carboxylic group, while the band at 1672 cm -1 corresponds to the amide group. Unfortunately it is not easy to make a safe quantitative estimate of the proportion of ACVA molecules grafted by only one of its two carboxylic end groups. A large portion of the silanol groups remains unreacted with the alkoxysilane groups. These residual silanol groups (20 to 50%) confer to the grafted silica a polar character which may have a deleterious inTABLE 4 Elimination of residual silanol groups upon reaction of grafted silica with hexamethyldisilazane (HMDS) Silica

Amount of MPS grafted (/~mol / m 2)

Residual silanol groups Before After HMDS treatment

Yield (%)

XOC005

2.1 3.2 1.9 2.7

2.7 1.6 3.4 0.6

76 81 78 88

1.1 0.9 1.2 0.4

EXPERIMENTAL

The silica, coupling agents and other chemicals are all commercial products and used without further purification. A typical procedure for grafting of the coupling agent (MPS, PTTS or APS) is as follows: 5 g of silica, dried at 120 °C under vacuum (0.1 mmHg), was placed u n d e r a stream of nitrogen in a glass flask equipped with a reflux condenser, together with 50 ml of a dry solvent (CC14 or toluene, previously distilled and stored under molecular sieve 3 A). A given amount of coupling agent was then introduced (typically to prepare a solution with a concentration corresponding to 8 /zmol/m 2 of silica). The solution was heated at reflux temperature for 10 h under nitrogen. The silica was then washed 6 times with methylene chloride and dried for 4 h at 40 ° C under vacuum. For the reaction between azobiscyanovaleric acid (ACVA) with APS modified silica, a 250 ml flask was filled with 150 ml of dry tetrahydrofuran (THF). The flask was then cooled by a dry-ice/acetone mixture ( - 7 8 ° C ) ; 3.275 × 10 -3 mole ACVA (0.918 g) was added and then 0.4 ml (4.18 × 10 -3 mole) of ethylchloroformiate and 1 ml of triethylamine. After 15 min, the silica (for instance 17.3 g of EC 299 grafted with APS, with 6.54 m e q / g of N), previously dried, was added and after 1 h of reaction, the temperature was allowed to reach - 1 0 °C and kept overnight at that temperature. After filtraO

49

tion and successive washing with THF, water and methanol, the silica was dried at room temperature and stored at less than 10 ° C. The treatment of MPS modified silica with hexamethyldisilazane (HMDS) was carried out in CCI 4 for 2 h at room temperature. The silica was then washed with CH2C12 and THF and dried. The diffusion reflection FTIR was carried out on the powder using a NICOLET 20 SX apparatus. 29Si and 13C CP-MAS N M R spectra were obtained using a B R U K E R AC 200 apparatus, equipped with a solid accessory, working at 39.76 MHz and 50.3 MHz, respectively. The compounds were spun at 5 kHz. Pulse interval times were 2 ms. The contact times were 7 ms (29Si) and 2 ms (13C). The amount of sample used was typically 100-200 mg. 200-500 FIDS were accumulated; all the chemical shifts were referenced to tetramethylsilane.

REFERENCES 1 P. Hodge and D.C. Sherrington, Polymer Supported Reactions in Organic Synthesis, Wiley, New York, 1980. 2 W.T. Ford, Polymeric reagents and catalysts, ACS Symp. Ser., 308 (1986). 3 D.C. Sherrington and P. Hodge, Synthesis and Separations Using Functional Polymers, Wiley, New York, 1988. 4 A. Guyot, Polymer supports with high accessibility, Pure AppL Chem., 60 (1988) 365. 5 A. Revillon, A. Guyot, Quing Yuan and P. Da Prato, Reagents on styrene divinylbenzene supports with improved accessibility, React. Polym., 10 (1989) 11. 6 A. Guyot, Polymer supported catalysts with improved accessibility, in: R. Epton (Ed.), Innovations and Perspectives in Solid Phase Synthesis, SPCC (UK) Ltd., Birmingham, 1990, pp. 87-100.

7 See, for instance, K.K. Unger, Porous Silica, Elsevier, Amsterdam, 1979. 8 P. Tundo, P. Venturello and E. Angeletti, Phasetransfer catalysts immobilized and adsorbed on alumina and silica gel, J. Am. Chem. Soc., 104 (1982) 6551. 9 G. Challa, Polymer chain effects in polymeric catalysts, J. Mol. Cat., 21 (1983) 1. 10 J.P.J. Verlaan, J.P.C. Bootsma and G. Challa, Immobilization of a homogeneous macromolecular copper catalyst for the oxidative coupling of phenols, J. Mol. Cat., 14 (1982) 211. 11 C.E. Koenig, B.L. Hiemstra, G. Challa, M. van de Velde and E.J. Goethals, Investigation and immobilization of the copper complex of poly [(N-ethoxycarbonylethyl) iminotrimethylene] as a catalyst for the oxidation coupling of 2,5-dimethylphenol, J. Mol. Cat., 32 (1985) 309. 12 R.A. Sawicki, Phase transfer catalysts polyethylene glycols immobilized onto metal oxide surfaces, Tetrahedron Lett., 23 (1982) 2249. 13 G.E. Totten and A. Clinton, Poly(ethylene glycol) derivatives as phase transfer catalysts and solvents for organic reactions, J. Macromol. Sci. Rev. Macromol. Chem. Phys., C28 (1988) 293. 14 G. Boven, M.L.C.M. Oosterling, G. Challa and A. J. Schouten, Grafting kinetics of polymethylmethacrylate on microparticulate silica, Polymer, 31 (1990) 2377. 15 G. Boven, R. Folkersma, G. Challa and A. Jan Schouten, Radical grafting of polymethylmethacrylate onto silicon wafers, glass slides and glass beads, Polymer Commun., 32 (1991) 50. 16 R.L. Kaas and J.L. Kardos, The interaction of alkoxy silane coupling agents with silica surfaces, Polym. Eng. Sci., 11 (1967) 11. 17 J.W. de Haan, H.M. van de Bogaert, J.J. Ponjee and L.M. van de Ven, Characterization of modified silica powders by Fourier Transform Infrared Spectroscopy and cross-polarization magic angle spinning NMR, J. Colloid Interface Sci., 110 (1986) 591. 18 A.M. Zaper and J.L. Koenig, Application of solid carbon-13 NMR spectroscopy to chemically modified surfaces, Polym. Composites, 6 (1985) 156. 19 W.T. Ford, S. Mohanray and M. Periyasamg, Nuclear magnetic resonance spectral analysis of polymer-supported reagents and catalysts, Br. Polym. J., 16 (1984) 179.