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Absolute reactivity in the cationic polymerization of N-vinylcarbazole
Absolute reactivity & the cationic polymerization of N-vinylcarbazole P. M. BOWYER,A. LEDWITH and D. C. SHERRINGTON
The cationic polymerization of N-vinylcarbazole initiated by tropylium hexachloroantimonate and tropylium perchlorate has been studied in detail. Initiation was rapid and complete and conversion to polymer was quantitative in all cases. Reaction rates were measured by an adiabatic calorimetric technique and half lives were of the order of 1-2 s. Catalyst concentrations employed were sufficiently low ( < 10 s M) for essentially complete dissociation into free ions, as indicated by the ion-pair dissociation constants. The rate coefficient for propagation by free cations in dichloromethane at 0°C is estimated as ~3 × 10+5M-ts i, approximately five orders of magnitude greater than that for the corresponding free radical polymerization. Molecular weights were higher when SbCIs- was the counter-ion than with C104 and it is suggested that monomer transfer reactions occur more readily with the small equilibrium concentrations of ion pairs (allowing an effect of counter-ion), whereas propagation occurs predominantly via free cations. Initiation is shown to occur by addition of tropylium ion to the monomer double bond and detailed mechanisms for this, together with propagation and transfer reactions, are considered.
N-VINYLCARBAZOLE (NVC)is a useful crystalline monomer, easily purified, and readily polymerized by free radicaP, z cationic 3-7 and organometallic 8 induced polymerization processes. Ionizing radiation has also been used to initiate polymerization of NVC particularly in the solid state 9. The radical or ionic nature of the propagating species was readily demonstrated in most cases by copolymerization studies and/or the effect of suitable additives 1-9. Some years ago we embarked on a study of reactivity and mechanism in cationic polymerization ~° and designed catalyst systems H and reaction conditions which permit ready evaluation of absolute rate coefficients for propagation processes. A central feature of these studies was the use of stable carbonium ion salts as initiators~, 12, notably hexachloroantimonate salts of cycloheptatrienyl (tropylium) and triphenyl methyl cations. The very stability of the initiator salts meant that only reactive monomers could be initiated and this has been confirmed by polymerization of alkyl vinyl ethers, p-methoxy styrene, indene, NVC and certain cyclic ethers such as tetrahydrofuran. Full details of the kinetics and mechanism of cationic polymerization of isobutyl vinyl ether have been published 13 as part of a series14; the present paper reports in detail similar studies of the cationic polymerization of NVC. For any ionic process (including polymerization) in solvents of low dielectric constant, reaction rates and products will be seriously affected by the nature of the ionic reactant, especially its degree of association or aggregation with counter-ions (gegen-ions) and other ion-pair species. The effects of 509
P. M. BOWYER, A. LEDWITH AND D. C. SHERRINGTON
ion-pair dissociation equilibria on organic reactions in solvents of low polarity were first characterized by Winstein and his collaborators 15 and for anionic polymerization, elegant work by Szwarc and his associates as well as other groups 16, has dramatized the vast differences in reactivity between propagating free ions and corresponding ion-pairs. Effects of ion-pair equilibria in cationic polymerizations have been discussed by Plesch 17, and characterized recently by Pepper and collaborators~s for the perchloric acid initiated polymerization of styrene. As noted previously, determination of absolute reactivity in the polymerization systems under study, requires knowledge of the ion-pair dissociation equilibria for both initiating and propagating species. Consequently a full study of the initiators was made using conductance measurements, and details have now been publishedlZ, ~9.
EXPERIMENTAL
Materials Methylene dichloride was purified and stored as described~3; solvent remaining on the vacuum line after three days was discarded and replaced by a freshly purified batch. Acetonitrile was purified2° by passage over a column of molecular sieves followed by fractionation from phosphorus pentoxide (b.p.82.5°C at 760 mmHg). It was stored, under vacuum, over fresh calcium hydride and transferred using normal vacuum techniques. Tropylium hexachloroantimonate was prepared and purified as previously described13. Tropylium perchlorate 21 was made by oxidation of cycloheptatriene using 2,3-dichloro-5,6-dicyano-p-benzoquinone in the presence of perchloric acid. The crude product was washed several times with dry ether, dissolved in the minimum volume of acetonitrile and reprecipitated by addition of ether. It was dried and finally stored under vacuum. The compound exploded during elemental analysis. N-Vinylcarbazole was recrystallized three times from A.R. methanol. In each case the flasks and solutions were flushed with dry nitrogen so that crystallization occurred in the absence of oxygen. The crystals were dried by pumping on a high vacuum line for two days, and stored in vacuo in the dark, m.p. 64-0°C. Analysis: found, C, 86.88, H, 5.75, N, 7.56~; required for C14HllN, C, 87.04, H, 5.70, N, 7.25~. Kinetic technique Polymerizations were investigated using the adiabatic calorimetric technique previously described in detaiP 3. A slightly modified calorimeter was used in which the lower sectionl~ was made longer so as to reduce heat losses, the stirrer now being mounted in a Teflon bush in the calorimeter base in order to improve the efficiency of mixing. The solvent metering system18 was changed to a conventional cold distillation arrangement, and the Biddulph-Plesch tap discarded. With these modifications, the previously described procedurO 3 for solvent scavenging was found to be superfluous. Catalyst solution was introduced into the polymerization mixture by the use of the vacuum phial technique13. In the case of the hexachloroantimonate salt the solvent for catalyst phials was methylene dichloride but for tropylium 510
C A T I O N I C P O L Y M E R I Z A T I O N OF N - V I N Y L C A R B A Z O L E
perchlorate it was found necessary to use acetonitrile in order to attain the required catalyst solubility. It should be stressed however that during polymerization the solvent was still ~ 9 9 % methylene dichloride. Polymer was recovered by addition of excess filtered methanolto the reaction mixture, previously reduced in volume to ~ 2 5 ml. Samples were collected in pre-weighed glass sinter crucibles and after drying overnight in a vacuum oven were reweighed to determine polymer yields. Molecular weights were determined by viscometry on benzene solutions and intrinsic viscosities, ['q], were converted to molecular weights, M, using the expression 22 [r/]--~ 3.35 × 10-aM °'s8 This relationship has been confilmed by more recent work 23 and differs substantially from the equation derived by North and Hughes 1. Preliminary investigations showed that Nvc has a substantial enthalpy of polymerization; polymerizations were extremely rapid and thus introduced considerable limitations on the concentration ranges employed. Polymer yields were invariably quantitative even at the lowest initiator concentrations employed ( < 10 -5 M). Recorde- traces from kinetic runs showed a general similarity to those obtained la in the parallel study of isobutyl vinyl ether (roVE) polymerization. One major difference however is that initiation in the present case appears to be virtually instantaneous since, even at 25°C, recorder traces were found to rise sharply as soon as the initiator phial was broken (see Figure 1). Rapid
20 o
13
10~ c-
A t i
i
i
I
20
15
10
I
5 T~rne (s)
II
0
Figure 1 Typical recorder trace for polymerization of Nvc by C7HT+SbCI6- in CH2CI2 at O°C
initiation and the apparent absence of termination permitted direct evaluation of the polymerization rates Rp from the maximum slopes (essentially the initail slopes) of recorder traces. Propagation rate coefficients (k~) were then obtained from the expression R~ = k~ [C7H7+]0[NVC]0 as indicated below. 511
P. M. BOWYER, A. LEDWITH AND D. C. SHERRINGTON
Typical kinetic run f o r polymerization o f N v c in methylene dichloride at O°C [C7HT+SbC16-]0 = 0.987× 10 -6 M ; [yvc]0 = 5.39 × 10 -2 M Polymer yield = 1.026g ~ 5.31 × 10 -3 mol (99-8 ~o) Initial slope of recorder trace ~ 1.85/0.60 chart divisions per second. Total rise of trace corrected for breaker c o n t r i b u t i o n == 6.30 chart divisions E q u a t i n g the total rise to the p o l y m e r yield, 6.30 chart divisions ~ 5.31 × 10 -2 M • initial rate -
1.85
()~6() chart divisions/second 2.60 × 10 -2 M s -1.
thus kp = 4.9 × 10 +5 M -1 s -1 M o l a r heats of polymerization, AHp, were calculated as outlined previously a n d in this example AH~ = --22.9 kcal tool -1. RESULTS Details of the kinetic a n d molecular weight data o b t a i n e d for t r o p y l i u m salt initiated p o l y m e r i z a t i o n o f NVC are shown in Tables 1 a n d 2. Averaged values for the enthalpy of p o l y m e r i z a t i o n of y v c were --22.7 1.5 kcal mo1-1 a n d --25.0 ± 1.5 kcal mo1-1 at 0°C a n d - - 2 5 °C respectively. Conceivably this difference m a y represent experimental error b u t it is n o w a p p a r e n t that enthalpies of solution for poly(N-vinyl carbazole) in CH2C12 show a similar v a r i a t i o n with t e m p e r a t u r e 24. A full discussion of these
Table 1 Polymerization of NVC by C7H7+ SbCI6- in CH2C12 Temp.
INVC]o
(°C)
(10aM)
0 0 0 0 0 0
5.39 5.40 5.40 5.40 2.70 4-04
[C]0 (106M)
102Rp (M s-1)
10-~ kp (M -1 s-1)
0.987 0.987 0.987 0.750
2.60 2.42 2.39 1.97
1.00 1.00
1.15 1.87
4.9 4.5 4.5 4.9 4.3 4.6
10-5 Mol. Wt. 2.58 1.68 1.49 ! .92 1.38 1-84
average 4"6 25 --25 --25 --25 --25 --25 --25
5"40 5'39 5-40 5'39 2-69 4"05 4'05
2.00 1.75 1"50 1.00 1-50 1.25 1.75
1.42 1'90 1"09 0.703 0-577 0.951 1.48
1-3 2.0 1"4 1'3 1.4 1.8 2.1
average 1.6 512
10"2 9.00 8.83 8.57 7.59 8-77 11.3
C A T I O N I C P O L Y M E R I Z A T I O N OF N - V I N Y L C A R B A Z O L E
Table 2 Polymerization of NVC by C7H7+CIO4 in CH2CIz*
Rp
Temp.
[NVC]
(c'C)
(102M)
[C]0 (10~iM)
(M s -1)
5.52 5-51 5.50 5.53 5.52 5-53 5.52 2-76 4.07
2.64 2.79 3-30 3"96 4.18 4.62 5.57 2.79 2-79
3.64 2.34 5.06 3.25 5.40 4.75 3.60 2.21 3-32
0 0 0 0 0 0 0 0 0
10"
10 5 kp
10 ~ Mol. Wt.
(M I s a) 2.5 1.5 2"8 1"5 2"3 1.9 1.2 2-9 2"9
0"893 0"908 0-743 0"710 l "14 0"715 0.632 0.640 0'719
average 2"2 25 --25
5.70 5.70
3'40 4.08
- 25
5.71
5.45
1.84
25 25 25 - 25 25
5.69 5.70 5.70 2.98 4.13
5.74 8.62 11.5 5.74 5.74
2.54 2-12 2.50 1.47 1.89
0'70 0"74 0.59 0.78 0"43 0"38 0.86 0"79
1.35 1.72
1-52 1 "12 1"16 0"830 0-820 0.726 0"849 0"860
average 0.66 *Catalyst added as solution in C H z C N giving a polymerization solvent composition, approx. 99 ~ CHzCI,2, 1 ~o
CHaCN.
anomalies, and their possible significance, will be presented separately. Values of AHp at the appropriate reaction temperatures are needed for evaluation of kp but any errors introduced by apparent variation in AH,,. for the polymer would be comparatively small, with little effect on the order of magnitude of kp. Polymer yields and molecular weights were invariably high but, because the carbazole rings in alkyl carbazoles [including poly(N-vinylcarbazole)] are highly reactive towards electrophilic reagents eS, it is at least possible that propagating cations alkylate carbazole rings of monomeric segments. Accordingly N-ethylcarbazole (NEe) was chosen as a model for monomer segments in poly(Nvc), without the steric hindrance of polymeric substituents, and its effect on polymerization studied as indicated in Table 3. Table 3 Polymerization of NVC in CH2C12 at 20°C in the presence of NE(: [NVC]
(M)
[CTH7+SbCI6 ] (105M)
Polymer yield
10 4 Mol. Wt.
(M)
[NEC]
0.102 0'102
0 0'417
1 "58 1'58
93 97
1"54 1"54
(%)
R e a c t i o n o f NVC with tropylium hexachloroantimonate in methanol N V C ( 0 . 6 3 g ) was dissolved in m e t h a n o l (50 ml) and added t o a s o l u t i o n o f t r o p y l i u m h e x a c h l o r o a n t i m o n a t e ( 1 . 1 7 g ) in m e t h y l e n e dichloride ( 2 0 0 m l ) . A 513
P. M. BOWYER, A. LEDWITH AND D. C. SHERRINGTON
transient red colour was formed before the solution became clear, whereupon the solution was extracted with aqueous alkali and twice with distilled water. The methylene dichloride layer was separated and dried over anhydrous sodium sulphate. Removal of the solvent on a rotary evaporator yielded a tacky product (0.95 g, 95~). I.r. analysis of this showed the presence of cycloheptatrienyl (702 cm -1) and methoxy groups (1120 cm-1). The crude product was separated into five fractions ~by dissolving in hot methanol and adding successive amounts of distilled water. The fraction most soluble in methanol (0.26 g, 25 ~) gave a very sharp, well resolved, i.r. spectrum with the cycloheptatriene absorption at 702 cm -1 more intense than the carbazole peaks (720 and 760 cm-1). This material was a white crystalline solid, m.p. 112°C. Analysis: found, C, 83.22, H, 6.66, N, 4.61 }o~; required for C22H21ON, C, 83.75, H, 6.71, N, 4.41 ~, which is entirely consistent with the anticipated structure:
I
CTHTCH2CHOCH3
The other fractions gave broader but similar i.r. spectra with the cycloheptatriene absorption becoming progressively weaker relative to the carbazole ring absorbances as solubility in methanol decreased. Melting points also tended to increase in the range 113-116°C in the same sequence. Presumably these materials consist of similar oligomeric adducts containing more than one yvc segment per cycloheptatrienyl unit.
DISCUSSION
Polymerizations of NVC initiated by tropylium hexachloroantimonate and tropylium perchlorate showed identical features. For both catalysts there was a virtually instantaneous initiation reaction followed by very rapid propagation. Yields of polymer were invariably quantitative and, as in the case of the polymerization of isobutyl vinyl ether13, the evidence points to absence of termination during the kinetic lifetimes. Molecular weights of poly(N-vinylcarbazole) from the perchlorate initiation were slightly lower than those from the hexachloroantimonate catalysis and did not show the same temperature variation. From both catalysts however the molecular weight data showed clear evidence of a transfer reaction, characteristic of homogeneous cationic systems17,26,27, though the degree of transfer is significantly less than in the case of isobutyl vinyl ether polymerization13. Mechanistic features of the initiation, propagation and transfer processes will now be considered in detail.
Initiation Although reactions between tropylium salts and NVC were instantaneous under most conditions, at temperatures below --50°C the two reagents 514
CATIONIC POLYMERIZATION OF N-VINYLCARBAZOLE
reacted to form only a pink coloured solution from which polymer could not be precipitated on addition of methanol. However, if the solutions were allowed to warm to around --30°C before addition of methanol, polymer formed rapidly. It seems likely therefore that initiation involves pre-equilibrium formation of a charge transfer complex which, at temperatures above --50°C, collapses rapidly to yield a propagating cation such as I:
CTHTX
+
CH_,~CH
~]transfer complex
~
CTHTCH,-CH X-
-
I
1 CH ~OH
I C7 HTCH2CH
II CH 2
I
CH Xetc
CTHTCH_,CHOCH3
Charge transfer complexes between tropylium ion and non-polymerizable carbazole derivatives, e.g. N-alkyl carbazoles, have been recorded previously 2s and the reaction of tertiary amines with tropylium salts studied by McGeachin 29 provides independent evidence for the addition process with enamines. In the present work overwhelming evidence for the proposed addition mechansim was provided by the isolation and characterisation of the cycloheptatrienyl derivative I[, from reactions carried out in the presence of methanol. Ion-pair equilibria for the initiation reaction are considered in the next section.
Propagation reactions Ion-pair dissociation equilibria for both tropylium salts have been investigated by conductance measurements, i.e., Kj C7H7 + X - ~ C7H7 + q- X -
Data for the hexachloroantimonate salt in methylene dichloride have been reported previouslylZ,19 and the data for the perchlorate salt in ~ 9 9 ~ v/v CH2C12/CH3CN are very similar; values of Ka were 0.33 × l0 -4 and 0.50 × 10 _4 M at 0°C and --45°C respectively. It follows that, at the catalyst concentrations employed in the present work, the initiating salts will be predominantly dissociated into their free ions and initiation, in both cases, will involve addition of free organic cation to monomer. Arguments in support of the assumption that the propagating species will be similarly dissociated at these low concentrations have already been presented 13 and, since conditions in the present work are very similar, the discussion need not be repeated. Values of kp obtained therefore represent 515
P. M. BOWYER, A. LEDWITH AND D. C. SHERRINGTON
estimates of the rate coefficient for the reaction of free propagating cation with monomer, i.e. they are a measure of the absolute reactivity of the cation derived from NVC. The data should be independent of the initiating salt used and should correlate with values of rate constants for similar fundamental processes. Average values of k~ obtained are summarized in Table 4 together with estimated activation enthalpies. Table 4 Polymerization of NVCby tropylium salts in CHzCI2 Counter-ion
Temperature
(°C)
10-5 kp (M 1 s-l)
Activation enthalpy
(kcal/mol) C104-
0
2"2
SbCI6-
--25 0
0'66 4"6
--25
1'6
6"5
5"7
Data for the two catalysts are in good agreement, although it is not possible to decide whether the variations are due to slightly different ionpair dissociation equilibria, arising because of different counter-ions, or merely represent the experimental limitations of the fast reaction technique. Nevertheless the agreement is such as to provide adequate support for the assumptions made, in the computation of kp, and, that largely free ions are involved in these systems. In recent years the polymerization of NVC initiated by electron transfer reactions to organic molecules has been widely reported and the literature reviewed by Ellinger 3°. Much of this work is qualitative in nature although it is clear that cationic polymerization results in almost every case, e.g. (A = acceptor molecule) NVC + A ~ [C.T. complex] ~ NVC ~- A ; NVC
2 (NVC$ A : ) -~ (NVC)2++ (A:)2
> Polymer
Suitable acceptor molecules include aromatic and aliphatic polynitro compounds, quinones and tetracyanoethylenea°. Pac and Plesch a and Szwarc and his collaborators 4 independently studied polymerization of NVC in nitrobenzene initiated by tetranitromethane. Both groups found the overall rate of polymerization to be represented by an expression of the form: --d [NVC] __ k [C]o [NVC] dt where [C]0 is the initial concentration of added initiator. A similar expression was derived for the polymerization of NVC in CH2C12 initiated by tetracyanoethylene 5 and for all these systems, estimated values of k were similar (k = 11 M -1 s --1 at 34°C 3, 4-3 M -1 s -1 at 10°C 4 and 13 M -1 s -1 at 30°C5). It follows therefore that cationic polymerizations of NVC initiated by these 516
C A T I O N I C P O L Y M E R I Z A T I O N OF N - V 1 N Y L C A R B A Z O L E
organic electron acceptors do not involve free cations as propagating species. This is not too surprising since the nature of counter-ions derived from organic electron acceptors is uncertain and, in many cases, complex ion-pairs may well be involved. It is interesting to compare data from the free radical polymerization of NVC in tetrahydrofuran reported by North and Hughes with the present results. Values of the free radical propagation rate constant, kp, were 6-0, 2.2 and 0.84 M -1 s -1 at +10, --10 and --30°C, respectively, many orders of magnitude below the reactivity of free NVC cation now reported. Data for the absolute reactivity of free styryl (3.5 × 10 +6 M -1 s -~ at 15°C) and a-methyl styryl (4.0 × 10 +~ M -1 s -1 at 0°C) cations in their respective bulk monomers is also available 3~ and in the light of these the present value for NVC is very reasonable. For NVC the activation enthalpy is slightly larger than that determined for the propagation of free styryl cation 31 but very similar to that obtained for isobutyl vinyl ether lz,13.
T r a n s f e r reactions Tables 1 and 2 show that molecular weights of poly(N-vinylcarbazole) are limited by transfer reactions though the degree of transfer is considerably less than that found in many cationic systems. The slightly lower molecular weights of polymers prepared using the perchlorate salt and the relatively small dependence on temperature compared with polymers from the hexachloroantimonate salt catalysis indicate a definite molecular weight dependence on counter-ion*. Therefore, despite propagation occurring largely by the reaction of free ions, the small equilibrium fractions of ion-pairs appear to affect the transfer step. Apparently the activation energy for transfer from a perchlorate ion pair is lower than that from a hexachloroantimonate ionpair. Recent work by Tazuke 3z provides evidence for the effect of ion-pairs containing perchlorate ion, in facilitating chain transfer by methanol, during polymerization of NVC initiated by protic acids. However these systems are not so well characterized as in the present work and it may be that the apparent common effect of perchlorate ion arises from quite different phenomena. Transfer to monomer is the most common molecular weight controlling process in cationic polymerization17,26, 27 although mechanistic details are not clear, and may vary for different monomers 13. For a polymerizable olefin, R C H = C H 2 , transfer to monomer may be represented as a simple proton equilibrium between the double bond of the monomer and that of the terminal olefinic linkage formed in the polymer chain i.e. :
~CHzCH-CH~--CH + + CHz=CH k ~ # C H z C H C H - C H - CH~CH+ I R
q R
I R
R
kp
I R
I R
~CHeCH--CH2CH--CHz--CH + [ ] I R R R
*Since the systems containing perchlorate ion also contain a small amount of CH3CN, transfer by the latter cannot be completely ruled out However data (unpublished) from perchlorate salts in other solvents makes this possibility less likely. 517 P-2K
P. M. BOWYER, A. LEDWITH AND D. C. SHERRINGTON
Clearly, the transition state for transfer to monomer will be similar to that for propagation, and in many systemsa3, is values of k , and ktr do not differ greatly at room temperature and above. The relative importance of free ions and ion pairs in transfer to monomer has not previously been discussed, but it is at least plausible that the enthalpy of activation (and hence the rate) for monomer transfer may be lowered, relative to the propagation step, by participation of certain counter-ions. A possible transition state, showing the counter-ion having a stabilizing effect on both leaving and developing charges, for monomer transfer, is indicated below: R
H
.CH - - R
A preferential effect of certain counter-ions on proton transfer between carbanions has been noted in several systemszz and the structurally related hydride ion transfer between tetrahydrofuran and triphenyl methyl cation occurs more rapidly with PhzC+SbC16 - ion-pairs than with the free cationlL In the present work it seems clear that transfer (presumably to monomer) is favoured for the systems having perchlorate counter-ions and a highly plausible explanation would be that ion-pairs present, in low equilibrium concentrations, dominate the monomer transfer reactions - - the effect being greatest for ion-pairs containing perchlorate ions. The possibility that ionpairs are mainly responsible for monomer transfer reactions in cationic polymerization would be of much wider significance, if established, and further work is now in progress to test the idea. In cationic polymerization of olefins having aromatic substituents there is always the possibility of transfer occurring via intra- or intermolecular electrophilic aromatic substitution by the growing polymeric cations. Such effects would be expected to be manifest in the case of NVC polymerization, on account of the enormously high reactivity of carbazole derivatives in this type of reaction zS,3a. However polymerizations carried out in the presence of excessive amounts of N-ethylcarbazole (NEC) - - used as a model for poly(Nvc) - showed no effect on molecular weight, or yield, of poly(NVC) (Table 3). It may be concluded therefore that alkylation of aromatic rings is not an important transfer process for cationic polymerization of NVC at the low concentrations of active sites used in the present work. It must be noted however that polymerization of NVC by cationic initiators at significantly higher active site concentrations leads to crosslinked polymers, especially in chlorinated hydrocarbon solvents 44.
Termination reactions True termination processes are difficult to envisage for polymerizations of a monomer like NVC, having a propensity for reaction with both protic and Lewis acids. No evidence of significant termination during kinetic lifetimes 518
CATIONIC POLYMERIZATIONOF N-VINYLCARBAZOLE was obtained in the present work, but previously we have shown 6,13 that side reactions o f SbC16 ions may lead to chloride termination, especially at higher temperatures. Covalent b o n d formation with C104- counter-ions is an apparent termination process in cationic polymerization o f styrene 17,18 but does not appear to be significant with NVC. F o r the latter however there is the additional, remote, possibility that loss of active centres might result from quaternization o f the carbazole nitrogen atoms of segmental units. The unshared pair of electrons on the nitrogen a t o m o f a carbazole ring comprise part of a 14 7r-electron aromatic system, isoelectronic with anthracene. This undoubtedly accounts for the comparative acidity of the N - H bond in carbazole and makes quaternization of alkyl carbazoles unlikely. Recently Hellwinkel and Seifert have synthesized salts of the N,N-dimethyl carbazole cation by an indirect method and the great reactivity of this quaternary salt alkylating agent makes it likely that any similar species, formed during cationic polymerization of NVC, would react immediately with m o n o m e r to initiate further chains. Finally it should be noted that the effects of added NEC referred to above (see Table 3) clearly argue against termination by quaternization. ACKNOWLEDGEMENT We wish to thank Professor C. E. H. Bawn for his constant interest and encouragement, and British Petroleum (P.M.B.) and SRC (D.C.S.) for maintenance awards. Donnan Laboratories, University o f Liverpool, Liverpool, U K
(Received 1 March 1971)
REFERENCES 1 Hughes, J. and North, A. M. Trans. Faraday Soc. 1966, 66, 62 2 Ellinger, L. P. J. Appl. Polym. Sei. 1965, 9, 3939 3 Pac, J. and Plesch, P. H. Polymer, Lond. 1967, 8, 237 4 Gumbs, R., Penczek, S., Jagur-Grodzinski, J. and Szwarc, M. Macromolecules 1969, 2, 77 5 Bawn, C. E. H., Ledwith, A. and Sambhi, M. Polymer, Lond. 1971, 12, 209 see also Nakamura, T., Soma, M., Onishi, T., and Tamaru, K. Makromol. Chem. 1970, 135, 241 6 Cowell, G. W., Kocharyan, K., Ledwith, A and Woods, H. J. European Polymer J. 1970, 6, 561 7 Barrales-Rienda, J. M., Brown, G. R. and Pepper, D. C. Polymer, Lond. 1969, 10, 327 8 Heller, J., Tieszin, D. O. and Parkinson, D. B. J. Polym. Sei. (A) 1963, 1, 125 9 Chapiro, A. and Hardy, G. J. Chim. Phys. 1962, 59, 993 10 Bawn, C. E. H., Fitzsimmons, C. and Ledwith, A. Proc. Chem. Soc. 1964, 19391 ; Bawn, C. E. H., Bell, R. M. and Ledwith, A. Polymer, Lond. 1965, 6, 95 11 Ledwith, A. J. Appl. Chem, 1967, 17, 344 12 Ledwith, A. Amer. Chem. Soc. Advances in Chemistry Series, 1969, 91, 317 13 Bawn, C. E. H , Fitzsommons, C., Ledwith, A., Penfold, J., Sherrington, D. C. and Weightman, J. A. Polymer, Lond. 1971, 12, 119 14 Ledwith, A. and Sherrington, D. C. Polymer, Lond. 1971, 12, 344; Eckard, A., Ledwith, A. and Sherrington, D. C. Polymer, Lond. 1971, 12, 444 519
P. M. BOWYER~ A. LEDWlTH AND D. C. SHERRINGTON
15 For a review see Winstein, S., Appel, B., Baker, R. and Diaz, A. Chem. Soc. (London), Spec. Publ. 1965, 19, 109 16 For an excellent and comprehensive review, see Szwarc, M. 'Carbanions, living polymers and electron-transfer processes,' Interscience, New York, 1968 17 Plesch, P. H. Progress in High Polymers 1968, 2, 137 18 Darcy, L. E., Millrine, W. P. and Pepper, D. C. Chem. Comm. 1968, p 1441 McCarthy, B., Millrine, W. P. and Pepper, D. C. ibid. 1968, p 1442 19 Bowyer, P. M., Ledwith, A. and Sherrington, D. C. J. Chem. Soc. (B) 1971, in press. 20 Coetzee, J. F., Cunningham, G. P., McGuire, D. K. and Padmanabhan, G. R. Analyt. Chem. 1962, 34, 1139 21 Reid, D. H., Frazer, M., Molloy, B. B. and Payne, H. A. S. TetrahedronLetters 1961, 15, 530 22 Ueberrieter, K. and Springer, J. Z. Phys. Chem. (Frankfurt) 1963, 36, 299 23 Sitaramaiah, G. and Jacobs, D. Polymer, Lond. 1970, 11, 165 24 Eckard, A., unpublished work 25 Bruck, P., Ledwith, A. and White, A. C. J. Chem. Soc. (B) 1970, p 205; Bruck, P. J. Org. Chem. 1970, 35, 2222; lies, D. H. and Ledwith, A. Chem. Comm. 1969, p 364; Sumpter, W. C. and Miller, F. M. 'The chemistry of heterocyclic compounds', Vol. 8 (A. Weissberger, Ed.), Interscience, New York, 1954 26 Eley, D. D., 'The chemistry of cationic polymerisation', (P. H. Plesch, Ed.) Pergamon, Oxford, 1963, p 375 27 Zlamal, Z. in 'Vinyl Polymerisation', Vol. 1, Pt. II (G. Ham, Ed,) Dekker, New York, 1970, p. 231 28 Sambhi, M., PhD Thesis, Univ. of Liverpool, 1966 29 McGeachin, S. G. Canad. J. Chem. 1969, 47, 151 30 Ellinger, L. P. Advances in Makromol. Chem. 1968, 1, 169 31 Williams, F., Hayashi, K., Ueno, K., Hayashi, K. and Okamura, S. Trans. Faraday Soc. 1967, 63, 1501 ; Ueno, K., Hayashi, K. and Okamura, S. J. Macromol. Sci. 1968, 2, 209; Metz, D. J. Amer. Chem. Soc. Advances in Chemistry Series 1961, 91, 202 32 Tazuke, S. Chem. Comm. 1970, p 1277 33 Hogan-Esch T. E. and Staid, J. J. Amer. Chem. Soc. 1967, 98, 2764; Hunter, D. H. and Lin, Y-T. J. Amer. Chem. Soc. 1968, 90, 5921 34 Ledwith, A., North, A. M. and Whitelock, K. E. European Polymer J. 1968, 4, 133 35 Hellwinkel, D. and Seifert, H. Chem. Comm. 1968, p 1683
520
The cationic polymerization of N-vinylcarbazole initiated by tropylium hexachloroantimonate and tropylium perchlorate has been studied in detail. Initiation was rapid and complete and conversion to polymer was quantitative in all cases. Reaction rates were measured by an adiabatic calorimetric technique and half lives were of the order of 1-2 s. Catalyst concentrations employed were sufficiently low ( < 10 s M) for essentially complete dissociation into free ions, as indicated by the ion-pair dissociation constants. The rate coefficient for propagation by free cations in dichloromethane at 0°C is estimated as ~3 × 10+5M-ts i, approximately five orders of magnitude greater than that for the corresponding free radical polymerization. Molecular weights were higher when SbCIs- was the counter-ion than with C104 and it is suggested that monomer transfer reactions occur more readily with the small equilibrium concentrations of ion pairs (allowing an effect of counter-ion), whereas propagation occurs predominantly via free cations. Initiation is shown to occur by addition of tropylium ion to the monomer double bond and detailed mechanisms for this, together with propagation and transfer reactions, are considered.
N-VINYLCARBAZOLE (NVC)is a useful crystalline monomer, easily purified, and readily polymerized by free radicaP, z cationic 3-7 and organometallic 8 induced polymerization processes. Ionizing radiation has also been used to initiate polymerization of NVC particularly in the solid state 9. The radical or ionic nature of the propagating species was readily demonstrated in most cases by copolymerization studies and/or the effect of suitable additives 1-9. Some years ago we embarked on a study of reactivity and mechanism in cationic polymerization ~° and designed catalyst systems H and reaction conditions which permit ready evaluation of absolute rate coefficients for propagation processes. A central feature of these studies was the use of stable carbonium ion salts as initiators~, 12, notably hexachloroantimonate salts of cycloheptatrienyl (tropylium) and triphenyl methyl cations. The very stability of the initiator salts meant that only reactive monomers could be initiated and this has been confirmed by polymerization of alkyl vinyl ethers, p-methoxy styrene, indene, NVC and certain cyclic ethers such as tetrahydrofuran. Full details of the kinetics and mechanism of cationic polymerization of isobutyl vinyl ether have been published 13 as part of a series14; the present paper reports in detail similar studies of the cationic polymerization of NVC. For any ionic process (including polymerization) in solvents of low dielectric constant, reaction rates and products will be seriously affected by the nature of the ionic reactant, especially its degree of association or aggregation with counter-ions (gegen-ions) and other ion-pair species. The effects of 509
P. M. BOWYER, A. LEDWITH AND D. C. SHERRINGTON
ion-pair dissociation equilibria on organic reactions in solvents of low polarity were first characterized by Winstein and his collaborators 15 and for anionic polymerization, elegant work by Szwarc and his associates as well as other groups 16, has dramatized the vast differences in reactivity between propagating free ions and corresponding ion-pairs. Effects of ion-pair equilibria in cationic polymerizations have been discussed by Plesch 17, and characterized recently by Pepper and collaborators~s for the perchloric acid initiated polymerization of styrene. As noted previously, determination of absolute reactivity in the polymerization systems under study, requires knowledge of the ion-pair dissociation equilibria for both initiating and propagating species. Consequently a full study of the initiators was made using conductance measurements, and details have now been publishedlZ, ~9.
EXPERIMENTAL
Materials Methylene dichloride was purified and stored as described~3; solvent remaining on the vacuum line after three days was discarded and replaced by a freshly purified batch. Acetonitrile was purified2° by passage over a column of molecular sieves followed by fractionation from phosphorus pentoxide (b.p.82.5°C at 760 mmHg). It was stored, under vacuum, over fresh calcium hydride and transferred using normal vacuum techniques. Tropylium hexachloroantimonate was prepared and purified as previously described13. Tropylium perchlorate 21 was made by oxidation of cycloheptatriene using 2,3-dichloro-5,6-dicyano-p-benzoquinone in the presence of perchloric acid. The crude product was washed several times with dry ether, dissolved in the minimum volume of acetonitrile and reprecipitated by addition of ether. It was dried and finally stored under vacuum. The compound exploded during elemental analysis. N-Vinylcarbazole was recrystallized three times from A.R. methanol. In each case the flasks and solutions were flushed with dry nitrogen so that crystallization occurred in the absence of oxygen. The crystals were dried by pumping on a high vacuum line for two days, and stored in vacuo in the dark, m.p. 64-0°C. Analysis: found, C, 86.88, H, 5.75, N, 7.56~; required for C14HllN, C, 87.04, H, 5.70, N, 7.25~. Kinetic technique Polymerizations were investigated using the adiabatic calorimetric technique previously described in detaiP 3. A slightly modified calorimeter was used in which the lower sectionl~ was made longer so as to reduce heat losses, the stirrer now being mounted in a Teflon bush in the calorimeter base in order to improve the efficiency of mixing. The solvent metering system18 was changed to a conventional cold distillation arrangement, and the Biddulph-Plesch tap discarded. With these modifications, the previously described procedurO 3 for solvent scavenging was found to be superfluous. Catalyst solution was introduced into the polymerization mixture by the use of the vacuum phial technique13. In the case of the hexachloroantimonate salt the solvent for catalyst phials was methylene dichloride but for tropylium 510
C A T I O N I C P O L Y M E R I Z A T I O N OF N - V I N Y L C A R B A Z O L E
perchlorate it was found necessary to use acetonitrile in order to attain the required catalyst solubility. It should be stressed however that during polymerization the solvent was still ~ 9 9 % methylene dichloride. Polymer was recovered by addition of excess filtered methanolto the reaction mixture, previously reduced in volume to ~ 2 5 ml. Samples were collected in pre-weighed glass sinter crucibles and after drying overnight in a vacuum oven were reweighed to determine polymer yields. Molecular weights were determined by viscometry on benzene solutions and intrinsic viscosities, ['q], were converted to molecular weights, M, using the expression 22 [r/]--~ 3.35 × 10-aM °'s8 This relationship has been confilmed by more recent work 23 and differs substantially from the equation derived by North and Hughes 1. Preliminary investigations showed that Nvc has a substantial enthalpy of polymerization; polymerizations were extremely rapid and thus introduced considerable limitations on the concentration ranges employed. Polymer yields were invariably quantitative even at the lowest initiator concentrations employed ( < 10 -5 M). Recorde- traces from kinetic runs showed a general similarity to those obtained la in the parallel study of isobutyl vinyl ether (roVE) polymerization. One major difference however is that initiation in the present case appears to be virtually instantaneous since, even at 25°C, recorder traces were found to rise sharply as soon as the initiator phial was broken (see Figure 1). Rapid
20 o
13
10~ c-
A t i
i
i
I
20
15
10
I
5 T~rne (s)
II
0
Figure 1 Typical recorder trace for polymerization of Nvc by C7HT+SbCI6- in CH2CI2 at O°C
initiation and the apparent absence of termination permitted direct evaluation of the polymerization rates Rp from the maximum slopes (essentially the initail slopes) of recorder traces. Propagation rate coefficients (k~) were then obtained from the expression R~ = k~ [C7H7+]0[NVC]0 as indicated below. 511
P. M. BOWYER, A. LEDWITH AND D. C. SHERRINGTON
Typical kinetic run f o r polymerization o f N v c in methylene dichloride at O°C [C7HT+SbC16-]0 = 0.987× 10 -6 M ; [yvc]0 = 5.39 × 10 -2 M Polymer yield = 1.026g ~ 5.31 × 10 -3 mol (99-8 ~o) Initial slope of recorder trace ~ 1.85/0.60 chart divisions per second. Total rise of trace corrected for breaker c o n t r i b u t i o n == 6.30 chart divisions E q u a t i n g the total rise to the p o l y m e r yield, 6.30 chart divisions ~ 5.31 × 10 -2 M • initial rate -
1.85
()~6() chart divisions/second 2.60 × 10 -2 M s -1.
thus kp = 4.9 × 10 +5 M -1 s -1 M o l a r heats of polymerization, AHp, were calculated as outlined previously a n d in this example AH~ = --22.9 kcal tool -1. RESULTS Details of the kinetic a n d molecular weight data o b t a i n e d for t r o p y l i u m salt initiated p o l y m e r i z a t i o n o f NVC are shown in Tables 1 a n d 2. Averaged values for the enthalpy of p o l y m e r i z a t i o n of y v c were --22.7 1.5 kcal mo1-1 a n d --25.0 ± 1.5 kcal mo1-1 at 0°C a n d - - 2 5 °C respectively. Conceivably this difference m a y represent experimental error b u t it is n o w a p p a r e n t that enthalpies of solution for poly(N-vinyl carbazole) in CH2C12 show a similar v a r i a t i o n with t e m p e r a t u r e 24. A full discussion of these
Table 1 Polymerization of NVC by C7H7+ SbCI6- in CH2C12 Temp.
INVC]o
(°C)
(10aM)
0 0 0 0 0 0
5.39 5.40 5.40 5.40 2.70 4-04
[C]0 (106M)
102Rp (M s-1)
10-~ kp (M -1 s-1)
0.987 0.987 0.987 0.750
2.60 2.42 2.39 1.97
1.00 1.00
1.15 1.87
4.9 4.5 4.5 4.9 4.3 4.6
10-5 Mol. Wt. 2.58 1.68 1.49 ! .92 1.38 1-84
average 4"6 25 --25 --25 --25 --25 --25 --25
5"40 5'39 5-40 5'39 2-69 4"05 4'05
2.00 1.75 1"50 1.00 1-50 1.25 1.75
1.42 1'90 1"09 0.703 0-577 0.951 1.48
1-3 2.0 1"4 1'3 1.4 1.8 2.1
average 1.6 512
10"2 9.00 8.83 8.57 7.59 8-77 11.3
C A T I O N I C P O L Y M E R I Z A T I O N OF N - V I N Y L C A R B A Z O L E
Table 2 Polymerization of NVC by C7H7+CIO4 in CH2CIz*
Rp
Temp.
[NVC]
(c'C)
(102M)
[C]0 (10~iM)
(M s -1)
5.52 5-51 5.50 5.53 5.52 5-53 5.52 2-76 4.07
2.64 2.79 3-30 3"96 4.18 4.62 5.57 2.79 2-79
3.64 2.34 5.06 3.25 5.40 4.75 3.60 2.21 3-32
0 0 0 0 0 0 0 0 0
10"
10 5 kp
10 ~ Mol. Wt.
(M I s a) 2.5 1.5 2"8 1"5 2"3 1.9 1.2 2-9 2"9
0"893 0"908 0-743 0"710 l "14 0"715 0.632 0.640 0'719
average 2"2 25 --25
5.70 5.70
3'40 4.08
- 25
5.71
5.45
1.84
25 25 25 - 25 25
5.69 5.70 5.70 2.98 4.13
5.74 8.62 11.5 5.74 5.74
2.54 2-12 2.50 1.47 1.89
0'70 0"74 0.59 0.78 0"43 0"38 0.86 0"79
1.35 1.72
1-52 1 "12 1"16 0"830 0-820 0.726 0"849 0"860
average 0.66 *Catalyst added as solution in C H z C N giving a polymerization solvent composition, approx. 99 ~ CHzCI,2, 1 ~o
CHaCN.
anomalies, and their possible significance, will be presented separately. Values of AHp at the appropriate reaction temperatures are needed for evaluation of kp but any errors introduced by apparent variation in AH,,. for the polymer would be comparatively small, with little effect on the order of magnitude of kp. Polymer yields and molecular weights were invariably high but, because the carbazole rings in alkyl carbazoles [including poly(N-vinylcarbazole)] are highly reactive towards electrophilic reagents eS, it is at least possible that propagating cations alkylate carbazole rings of monomeric segments. Accordingly N-ethylcarbazole (NEe) was chosen as a model for monomer segments in poly(Nvc), without the steric hindrance of polymeric substituents, and its effect on polymerization studied as indicated in Table 3. Table 3 Polymerization of NVC in CH2C12 at 20°C in the presence of NE(: [NVC]
(M)
[CTH7+SbCI6 ] (105M)
Polymer yield
10 4 Mol. Wt.
(M)
[NEC]
0.102 0'102
0 0'417
1 "58 1'58
93 97
1"54 1"54
(%)
R e a c t i o n o f NVC with tropylium hexachloroantimonate in methanol N V C ( 0 . 6 3 g ) was dissolved in m e t h a n o l (50 ml) and added t o a s o l u t i o n o f t r o p y l i u m h e x a c h l o r o a n t i m o n a t e ( 1 . 1 7 g ) in m e t h y l e n e dichloride ( 2 0 0 m l ) . A 513
P. M. BOWYER, A. LEDWITH AND D. C. SHERRINGTON
transient red colour was formed before the solution became clear, whereupon the solution was extracted with aqueous alkali and twice with distilled water. The methylene dichloride layer was separated and dried over anhydrous sodium sulphate. Removal of the solvent on a rotary evaporator yielded a tacky product (0.95 g, 95~). I.r. analysis of this showed the presence of cycloheptatrienyl (702 cm -1) and methoxy groups (1120 cm-1). The crude product was separated into five fractions ~by dissolving in hot methanol and adding successive amounts of distilled water. The fraction most soluble in methanol (0.26 g, 25 ~) gave a very sharp, well resolved, i.r. spectrum with the cycloheptatriene absorption at 702 cm -1 more intense than the carbazole peaks (720 and 760 cm-1). This material was a white crystalline solid, m.p. 112°C. Analysis: found, C, 83.22, H, 6.66, N, 4.61 }o~; required for C22H21ON, C, 83.75, H, 6.71, N, 4.41 ~, which is entirely consistent with the anticipated structure:
I
CTHTCH2CHOCH3
The other fractions gave broader but similar i.r. spectra with the cycloheptatriene absorption becoming progressively weaker relative to the carbazole ring absorbances as solubility in methanol decreased. Melting points also tended to increase in the range 113-116°C in the same sequence. Presumably these materials consist of similar oligomeric adducts containing more than one yvc segment per cycloheptatrienyl unit.
DISCUSSION
Polymerizations of NVC initiated by tropylium hexachloroantimonate and tropylium perchlorate showed identical features. For both catalysts there was a virtually instantaneous initiation reaction followed by very rapid propagation. Yields of polymer were invariably quantitative and, as in the case of the polymerization of isobutyl vinyl ether13, the evidence points to absence of termination during the kinetic lifetimes. Molecular weights of poly(N-vinylcarbazole) from the perchlorate initiation were slightly lower than those from the hexachloroantimonate catalysis and did not show the same temperature variation. From both catalysts however the molecular weight data showed clear evidence of a transfer reaction, characteristic of homogeneous cationic systems17,26,27, though the degree of transfer is significantly less than in the case of isobutyl vinyl ether polymerization13. Mechanistic features of the initiation, propagation and transfer processes will now be considered in detail.
Initiation Although reactions between tropylium salts and NVC were instantaneous under most conditions, at temperatures below --50°C the two reagents 514
CATIONIC POLYMERIZATION OF N-VINYLCARBAZOLE
reacted to form only a pink coloured solution from which polymer could not be precipitated on addition of methanol. However, if the solutions were allowed to warm to around --30°C before addition of methanol, polymer formed rapidly. It seems likely therefore that initiation involves pre-equilibrium formation of a charge transfer complex which, at temperatures above --50°C, collapses rapidly to yield a propagating cation such as I:
CTHTX
+
CH_,~CH
~]transfer complex
~
CTHTCH,-CH X-
-
I
1 CH ~OH
I C7 HTCH2CH
II CH 2
I
CH Xetc
CTHTCH_,CHOCH3
Charge transfer complexes between tropylium ion and non-polymerizable carbazole derivatives, e.g. N-alkyl carbazoles, have been recorded previously 2s and the reaction of tertiary amines with tropylium salts studied by McGeachin 29 provides independent evidence for the addition process with enamines. In the present work overwhelming evidence for the proposed addition mechansim was provided by the isolation and characterisation of the cycloheptatrienyl derivative I[, from reactions carried out in the presence of methanol. Ion-pair equilibria for the initiation reaction are considered in the next section.
Propagation reactions Ion-pair dissociation equilibria for both tropylium salts have been investigated by conductance measurements, i.e., Kj C7H7 + X - ~ C7H7 + q- X -
Data for the hexachloroantimonate salt in methylene dichloride have been reported previouslylZ,19 and the data for the perchlorate salt in ~ 9 9 ~ v/v CH2C12/CH3CN are very similar; values of Ka were 0.33 × l0 -4 and 0.50 × 10 _4 M at 0°C and --45°C respectively. It follows that, at the catalyst concentrations employed in the present work, the initiating salts will be predominantly dissociated into their free ions and initiation, in both cases, will involve addition of free organic cation to monomer. Arguments in support of the assumption that the propagating species will be similarly dissociated at these low concentrations have already been presented 13 and, since conditions in the present work are very similar, the discussion need not be repeated. Values of kp obtained therefore represent 515
P. M. BOWYER, A. LEDWITH AND D. C. SHERRINGTON
estimates of the rate coefficient for the reaction of free propagating cation with monomer, i.e. they are a measure of the absolute reactivity of the cation derived from NVC. The data should be independent of the initiating salt used and should correlate with values of rate constants for similar fundamental processes. Average values of k~ obtained are summarized in Table 4 together with estimated activation enthalpies. Table 4 Polymerization of NVCby tropylium salts in CHzCI2 Counter-ion
Temperature
(°C)
10-5 kp (M 1 s-l)
Activation enthalpy
(kcal/mol) C104-
0
2"2
SbCI6-
--25 0
0'66 4"6
--25
1'6
6"5
5"7
Data for the two catalysts are in good agreement, although it is not possible to decide whether the variations are due to slightly different ionpair dissociation equilibria, arising because of different counter-ions, or merely represent the experimental limitations of the fast reaction technique. Nevertheless the agreement is such as to provide adequate support for the assumptions made, in the computation of kp, and, that largely free ions are involved in these systems. In recent years the polymerization of NVC initiated by electron transfer reactions to organic molecules has been widely reported and the literature reviewed by Ellinger 3°. Much of this work is qualitative in nature although it is clear that cationic polymerization results in almost every case, e.g. (A = acceptor molecule) NVC + A ~ [C.T. complex] ~ NVC ~- A ; NVC
2 (NVC$ A : ) -~ (NVC)2++ (A:)2
> Polymer
Suitable acceptor molecules include aromatic and aliphatic polynitro compounds, quinones and tetracyanoethylenea°. Pac and Plesch a and Szwarc and his collaborators 4 independently studied polymerization of NVC in nitrobenzene initiated by tetranitromethane. Both groups found the overall rate of polymerization to be represented by an expression of the form: --d [NVC] __ k [C]o [NVC] dt where [C]0 is the initial concentration of added initiator. A similar expression was derived for the polymerization of NVC in CH2C12 initiated by tetracyanoethylene 5 and for all these systems, estimated values of k were similar (k = 11 M -1 s --1 at 34°C 3, 4-3 M -1 s -1 at 10°C 4 and 13 M -1 s -1 at 30°C5). It follows therefore that cationic polymerizations of NVC initiated by these 516
C A T I O N I C P O L Y M E R I Z A T I O N OF N - V 1 N Y L C A R B A Z O L E
organic electron acceptors do not involve free cations as propagating species. This is not too surprising since the nature of counter-ions derived from organic electron acceptors is uncertain and, in many cases, complex ion-pairs may well be involved. It is interesting to compare data from the free radical polymerization of NVC in tetrahydrofuran reported by North and Hughes with the present results. Values of the free radical propagation rate constant, kp, were 6-0, 2.2 and 0.84 M -1 s -1 at +10, --10 and --30°C, respectively, many orders of magnitude below the reactivity of free NVC cation now reported. Data for the absolute reactivity of free styryl (3.5 × 10 +6 M -1 s -~ at 15°C) and a-methyl styryl (4.0 × 10 +~ M -1 s -1 at 0°C) cations in their respective bulk monomers is also available 3~ and in the light of these the present value for NVC is very reasonable. For NVC the activation enthalpy is slightly larger than that determined for the propagation of free styryl cation 31 but very similar to that obtained for isobutyl vinyl ether lz,13.
T r a n s f e r reactions Tables 1 and 2 show that molecular weights of poly(N-vinylcarbazole) are limited by transfer reactions though the degree of transfer is considerably less than that found in many cationic systems. The slightly lower molecular weights of polymers prepared using the perchlorate salt and the relatively small dependence on temperature compared with polymers from the hexachloroantimonate salt catalysis indicate a definite molecular weight dependence on counter-ion*. Therefore, despite propagation occurring largely by the reaction of free ions, the small equilibrium fractions of ion-pairs appear to affect the transfer step. Apparently the activation energy for transfer from a perchlorate ion pair is lower than that from a hexachloroantimonate ionpair. Recent work by Tazuke 3z provides evidence for the effect of ion-pairs containing perchlorate ion, in facilitating chain transfer by methanol, during polymerization of NVC initiated by protic acids. However these systems are not so well characterized as in the present work and it may be that the apparent common effect of perchlorate ion arises from quite different phenomena. Transfer to monomer is the most common molecular weight controlling process in cationic polymerization17,26, 27 although mechanistic details are not clear, and may vary for different monomers 13. For a polymerizable olefin, R C H = C H 2 , transfer to monomer may be represented as a simple proton equilibrium between the double bond of the monomer and that of the terminal olefinic linkage formed in the polymer chain i.e. :
~CHzCH-CH~--CH + + CHz=CH k ~ # C H z C H C H - C H - CH~CH+ I R
q R
I R
R
kp
I R
I R
~CHeCH--CH2CH--CHz--CH + [ ] I R R R
*Since the systems containing perchlorate ion also contain a small amount of CH3CN, transfer by the latter cannot be completely ruled out However data (unpublished) from perchlorate salts in other solvents makes this possibility less likely. 517 P-2K
P. M. BOWYER, A. LEDWITH AND D. C. SHERRINGTON
Clearly, the transition state for transfer to monomer will be similar to that for propagation, and in many systemsa3, is values of k , and ktr do not differ greatly at room temperature and above. The relative importance of free ions and ion pairs in transfer to monomer has not previously been discussed, but it is at least plausible that the enthalpy of activation (and hence the rate) for monomer transfer may be lowered, relative to the propagation step, by participation of certain counter-ions. A possible transition state, showing the counter-ion having a stabilizing effect on both leaving and developing charges, for monomer transfer, is indicated below: R
H
.CH - - R
A preferential effect of certain counter-ions on proton transfer between carbanions has been noted in several systemszz and the structurally related hydride ion transfer between tetrahydrofuran and triphenyl methyl cation occurs more rapidly with PhzC+SbC16 - ion-pairs than with the free cationlL In the present work it seems clear that transfer (presumably to monomer) is favoured for the systems having perchlorate counter-ions and a highly plausible explanation would be that ion-pairs present, in low equilibrium concentrations, dominate the monomer transfer reactions - - the effect being greatest for ion-pairs containing perchlorate ions. The possibility that ionpairs are mainly responsible for monomer transfer reactions in cationic polymerization would be of much wider significance, if established, and further work is now in progress to test the idea. In cationic polymerization of olefins having aromatic substituents there is always the possibility of transfer occurring via intra- or intermolecular electrophilic aromatic substitution by the growing polymeric cations. Such effects would be expected to be manifest in the case of NVC polymerization, on account of the enormously high reactivity of carbazole derivatives in this type of reaction zS,3a. However polymerizations carried out in the presence of excessive amounts of N-ethylcarbazole (NEC) - - used as a model for poly(Nvc) - showed no effect on molecular weight, or yield, of poly(NVC) (Table 3). It may be concluded therefore that alkylation of aromatic rings is not an important transfer process for cationic polymerization of NVC at the low concentrations of active sites used in the present work. It must be noted however that polymerization of NVC by cationic initiators at significantly higher active site concentrations leads to crosslinked polymers, especially in chlorinated hydrocarbon solvents 44.
Termination reactions True termination processes are difficult to envisage for polymerizations of a monomer like NVC, having a propensity for reaction with both protic and Lewis acids. No evidence of significant termination during kinetic lifetimes 518
CATIONIC POLYMERIZATIONOF N-VINYLCARBAZOLE was obtained in the present work, but previously we have shown 6,13 that side reactions o f SbC16 ions may lead to chloride termination, especially at higher temperatures. Covalent b o n d formation with C104- counter-ions is an apparent termination process in cationic polymerization o f styrene 17,18 but does not appear to be significant with NVC. F o r the latter however there is the additional, remote, possibility that loss of active centres might result from quaternization o f the carbazole nitrogen atoms of segmental units. The unshared pair of electrons on the nitrogen a t o m o f a carbazole ring comprise part of a 14 7r-electron aromatic system, isoelectronic with anthracene. This undoubtedly accounts for the comparative acidity of the N - H bond in carbazole and makes quaternization of alkyl carbazoles unlikely. Recently Hellwinkel and Seifert have synthesized salts of the N,N-dimethyl carbazole cation by an indirect method and the great reactivity of this quaternary salt alkylating agent makes it likely that any similar species, formed during cationic polymerization of NVC, would react immediately with m o n o m e r to initiate further chains. Finally it should be noted that the effects of added NEC referred to above (see Table 3) clearly argue against termination by quaternization. ACKNOWLEDGEMENT We wish to thank Professor C. E. H. Bawn for his constant interest and encouragement, and British Petroleum (P.M.B.) and SRC (D.C.S.) for maintenance awards. Donnan Laboratories, University o f Liverpool, Liverpool, U K
(Received 1 March 1971)
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