JOURNAL OF POLYMER SCIENCE Polymer Chemistry Edition VOL. 14, 2621-2629 (1976)

Studies on Propagating Species in Cationic Polymerization of Styrene Derivatives by Acetyl

Perchlorate or Iodine. 11. Salt Effect on the Relative Reactivity in the Cationic Copolymerization of Styrene Derivatives with 2-Chloroethyl Vinyl Ether Initiated

by Iodine

KENJI YAMAMOTO and TOSHINOBU HIGASHIMURA, Department of Polymer Chemistry, Faculty of Engineering, Kyoto University, Kyoto 606,

Japan

Synopsis

To determine the effect of the dissociation of propagating species on the relative reactivity of monomers, 2-chloroethyl vinyl ether was copolymerized with p-methoxystyrene or with p-methyl- styrene by using iodine in various solvents a t 0°C. A common-ion salt (tetra-n-butylammonium iodide or tetra-n-butylammonium triiodide) was added to these copolymerization systems in a polar solvent to depress the dissociation of the propagating species. The addition of a common-ion salt increased the vinyl ether content in the copolymer. The more the dissociation of propagating species was depressed, the more the vinyl ether content in the copolymer increased. This effect of com- mon-ion salt was in agreement with that of decreasing solvent polarity which yielded vinyl ether-rich copolymer as well. Therefore, the change of the monomer reactivity ratio by the solvent polarity, which used to be explained in terms of a selective solvation, must be reconsidered from the viewpoint of varying degrees of the dissociation of propagating species.

INTRODUCTION Two kinds of propagating species exist in the cationic polymerization of styrene

and its derivatives initiated by iodine,l acetyl per~hlorate,'-~ perchloric a ~ i d , ~ , ~ and other strong proton acids.6 This conclusion is based on the bimodal mo- lecular weight d i ~ t r i b u t i o n l - ~ , ~ , ~ and the steric structure4 of the polymers ob- tained. Therefore, it is of interest to know the difference of the reactivity or the selectivity between the two propagating species. At the present, we cannot compare the absolute propagation rate constants, since no reliable method to determine the propagation rate constant has been established in the field of cationic polymerization. However, an investigation of the dependence of the monomer reactivity ratio in the copolymerization reaction on the nature of propagating species will give us information concerning the relative reactivity of propagating species.

The stability-selectivity relationship of carbocations involved in solvolysis reactions seems to have been e1ucidated.'-l0 However, there have been only a few reports concerning the relationship between the dissociation of carbocation

2621

0 1976 by John Wiley & Sons, Inc.

2622 YAMAMOTO AND HIGASHIMURA

and the selectivity (the dissociation-selectivity problem).l1J2 Investigations of the dissociation-selectivity problem covering a wide range of carbocations will give us valuable information on the nature of the reactive intermediate in the cationic polymerization as a field of the carbocation chemistry.

In the present study, cationic copolymerization of vinyl ethers with styrene derivatives initiated by iodine was investigated in order to clarify the relationship between the nature of propagating species and the relative reactivity of mono- mers. When a common-ion salt for iodine, which depresses the dissociation of propagating species, was added to the copolymerization system in a polar solvent, copolymerization behavior similar to that in a nonpolar solvent was observed. These results show that the degree of the dissociation of propagating species affects the relative reactivity of monomers.

EXPERIMENTAL

Materials

Commercial 2-chloroethyl vinyl ether (CEVE), isobutyl vinyl ether (IBVE), and p-methylstyrene (pMS) were washed with 10% aqueous solution of sodium hydroxide and with water and distilled from calcium hydride before use. p - Methoxystyrene (PMOS) was synthesized by the dehydration of p -methoxy- phenylmethyl carbinol with KHSO4, which was synthesized by the reduction of p-methoxyacetophenone with LiA1H4.13J4 The crude product was washed with 10% aqueous solution of sodium hydroxide and with water and distilled under reduced pressure. The purity of the monomers used was found to be more than 99.0% through gas chromatography.

Methylene chloride and carbon tetrachloride as solvents, and bromobenzene, toluene, and tetralin as internal standards for gas chromatography were purified by the usual method. Iodine (Merck, guaranteed reagent) was used without further purification. Tetra-n-butylammonium iodide (n-Bu4NI) (guaranteed reagent) was used after drying under vacuum overnight. Tetra-n-butylam- monium triiodide (n-Bu4NIs) was synthesized from n-BudNI and iodine by the method reported by Buckles and Yuk15 and recrystallized three times from ethanol (mp 70.5-71.0"C; lit15 mp 70.0-70.5"C).

ANAL. Calcd for C&36NI3: C, 30.8% H, 5.8%; N, 2.3%. Found: C, 31.0%; H, 5.9%; N, 2.3%.

Procedures

Copolymerizations were carried out under dry nitrogen in the same manner as reported previously.16 The concentration of residual water in the system was determined by Karl Fisher titration to be 0.2-0.3 mmoleh. Tetralin (5 vol-%) was used as an internal standard for gas chromatography of CEVE-PMOS and PMOS-pMS copolymerization systems. Toluene and bromobenzene (5 vol-%) were used for CEVE-IBVE and CEVE-pMS copolymerization systems, re- spectively. Copolymer compositions were determined from the amounts of re- sidual monomers measured by gas chromatography. Monomer reactivity ratios were calculated according to the improved Fineman-Ross method17 and the curve-fitting method. The formation of the copolymers was confirmed by NMR spectroscopic and gel-permeation chromatographic measurements.

POLYMERIZATION OF STYRENE DERIVATIVES. I1 2623

M1 in Monomer

Fig. 1. Effect of solvent on the copolymer composition in the copolymerization of CEVE (MI) withpMOS (M2): (0) CHzClz; (A) CH2CI&C14 (2/1 v/v); (0) CCl+ For other reaction conditions, see Table I.

RESULTS

Copolymerization of CEVE(M1) with pMOS(M2)

CEVE was copolymerized with PMOS by iodine in various solvents at O°C in order to study the effect of solvent polarity. Figure 1 shows the copolymer composition curves and the monomer reactivity ratios are summarized in Table I. The copolymer composition curves are S-shaped, as already observed in the cationic copolymerization of styrene derivatives with vinyl ethers.18J9 However, the products were found to be true copolymers, not mixtures of homopolymers. The NMR spectrum of the copolymer showed a different feature from the mix- ture of poly(CEVE) and poly(pMOS), that is, a new peak was observed at T 7.25 and three peaks a t T 3.50,6.30, and 8.25 became broad based on the difference of the neighboring monomer unit. These changes of the NMR spectra suggest the formation of a true copolymer, as described in the previous paper.lg Moreover, the molecular weight distribution of the copolymer was entirely dif- ferent from that of the mixture of each homopolymer, and no formation of the oligomers was observed, which also supports the true copolymer formation. Similar results were observed for all the other copolymers studied.

M1 in Monomer

Fig. 2. Effect of n-Bu4NI on the copolymer composition in the copolymerization of CEVE (Mlj ' withpMOS (Mz) in CHzCIz/CC14 (2/1 v/v) by Iz a t O°C a t various [n-BudNI]o: (A) 0; (0) 0.04rnM; (0) 3.3rnM; (- - -) cc14, salt-free (from Fig. 1). [I210 = 6.6rnM.

2624 YAMAMOTO AND HIGASHIMURA

M1 in Monomer

Fig. 3. Effect of n-BuNI3 on the copolymer composition in the copolymerization of CEVE (MI) with PMOS (Mz) in CHZCldCC4 (2/1 v/v) by Ip at O°C a t various [n-Bu4NI3]0: (A) 0; (0) 3.3mM; (- - -) cc14, salt-free (from Fig. 1). [I210 = 6.6mM.

A common-ion salt for iodine was added to the copolymerization system in a polar solvent to investigate the effect of a common anion on the monomer re- activity ratios. It is uncertain whether the counteranion is in the form of I- or 13- in the cationic polymerizations initiated by so both n-BQNI and n-Bu4NI3 were used as common-ion salt in the present study.

Figure 2 shows the copolymer composition curves for the CEVE-PMOS co- polymerization in CH2Clz/CC14(2/1 v/v) in the presence of n-Bu4NI. As is clear from Figure 2, CEVE content in the copolymer increased by adding a small amount of n-Bu4NI. This tendency was more clearly revealed when an in- creasing amount of n-Bu4NI was added. In methylene chloride, the addition of n-BurNI also changed rl and r2 (Table I). Figure 3 shows the effect of the addition of n-Bu4NI3, which is similar to that observed with the addition of n-

To sum up, in the cationic copolymerization of CEVE with PMOS initiated by iodine, the CEVE content in the copolymer increased either by the decrease of the solvent polarity or by the addition of a common-ion salt for the initiator in a polar solvent.

Bu~NI.

TABLE I

pMOS(M,) Initiated by Iodine a t 0"Ca Monomer Reactivity Ratios in the Cationic Copolymerization of CEVE(M,) with

Solvent

CH,Cl, CH, Cl,/CCl,(2/1 )

CH,Cl, CH,Cl,/CCl,(2/1 ) CH,Cl,/CCl,( 2/1) CH,CI,/CCl,( 2/1) CH,Cl,

CCl,

a[M], = 10 VO~-%.

3.3 6.6

10.0 3.3 6.6 6.6 6.6 3.3

[Salt1 0 ,

Salt mM rl

0.83 f 0.23 1.36 k 0.29 1.99 f 0.26

n-Bu,NI 0.01 0.99 f 0.15 n-Bu,NI 0.04 1.87 f 0.18 n-Bu,NI 3.3 1.88 f 0.33 n-Bu,NI, 3.3 2.00 f 0.23 n-Bu,NI, 6.6 1.72 f 0.02

- - - - - -

l.2

9.28 f 1.40 8.78 t 1.54 3.92 k 0.63 9.04 k 1.08 8.73 * 0.67 6.02 * 0.90 6.50 f 0.65 6.65 * 0.08

POLYMERIZATION OF STYRENE DERIVATIVES. I1 2625

M1 in Monomer

Fig. 4. Effect of solvent on the copolymer composition in the copolymerization of CEVE (MI) with pMS (M2): (0) CH2C12; (A) CHzCldCC14 (2/1 v/v); (0) CCld. For other reaction conditions, see Table 11.

Copolymerization of CEVE(M1) with pMS(M2)

For another example of the cationic copolymerization between a vinyl ether and a styrene derivative, C E W p M S copolymerization was investigated. CEVE was copolymerized with pMS by iodine in various solvents a t 0°C. Figure 4 shows the copolymer composition curves, and the monomer reactivity ratios are summarized in Table 11. CEVE was more reactive than pMS irrespective of the kind of solvent, and the CEVE content in the copolymer increased markedly as the solvent polarity decreased.

Figure 5 shows the copolymer composition curves for the copolymerization in CH&lz/CCk (2/1 v/v) when n-BwNI or n-BmN13 (halfas much as the amount of the initiator, iodine) was added. The monomer reactivity ratios are sum- marized in Table 11. When n-Bu4NI or nBurNI3, both of which can depress the dissociation of propagating species, was added to the copolymerization in a polar solvent, the copolymer composition curve tended to approach that observable for the copolymerization in a nonpolar solvent. It is particularly noteworthy that the copolymer composition curve obtained in CH&lz/CC14 (2/1 v/v) with n-BuqNI3 almost coincided with that obtained in carbon tetrachloride without the salt (Fig. 5 ) .

It is concluded that CEVE content in the copolymer increases either on de- crease of the solvent polarity or on addition of a common-ion salt for the initiator

TABLE I1 Monomer Reactivity Ratios in the Cationic Copolymerization of CEVE(M,) with

pMS(M,) Initiated by Iodine at 0"Ca

[I, l o ,

CH,CI, 3.3

CCI, 10.0

Solvent mM

CH,Cl,/CCl,(2/1) 6.6

CH,Cl,/CCl,(2/1) 6.6 CHzC12/CC1,(2/1) 6.6

[Salt 1 o, Salt mM rl r2

7.62 * 0.35 0.66 t 0.05 0.22 c 0.10 0.00 t 0.06

n-Bu,NI 3.3 15.5 f 1.49 0.03 * 0.06

- - 13.5 * 1.66 22.9 f 2.96

- - - -

n-Bu,NI, 3.3 25.9 t 3.33 0.00 t 0.11

a [MI ,, = 10 VO~-%.

2626 YAMAMOTO AND HIGASHIMURA

MI in Monomer

Fig. 5. Effect of n-BurNI or n-Bu4NI3 on the copolymer composition in the copolymerization of CEVE (MI) withpMS (M2) in CH2Clz/CC14 (2/1 v/v) by I2 at 0°C: (A) salt-free; (0) [n-BudNI]o = 3.3mM; 1.1 [ ~ - B u ~ N I ~ ] o = 3.3mM. [I210 = 6.6mM.

MI in Monomer

Fig. 6. Effect of solvent and salt on the copolymer composition in the copolymerization of CEVE (Mi) with IBVE (Mz): (0) CH2C12, salt-free; (A) CH2CIz/CC14 (2/1 v/v), salt-free; (0) CH2C12/CCI4 (2/1 v/v), [I210 = 6.6mM, [n-Bu4NI]o = 3.3mM; (0) c c 4 , salt-free.

) M1 in Monomer

Fig. 7. Effect of solvent and salt on the copolymer composition in the copolymerization of PMOS (Mi) with pMS (M2): (0) CH2C12, salt-free; (0) CHzC12, [I210 = 3.3mM, [n-BurNI& = 3.3mM; (A) CH2C12/CC14 (2/1 v/v), salt-free; (0) cc14, salt-free.

POLYMERIZATION OF STYRENE DERIVATIVES. I1 2627

in a polar solvent. This is true equally for the CEVE-pMS and CEVE-pMOS copolymerization systems.

Copolymerization between Vinyl Ethers and between Styrene Derivatives

Cationic copolymerizations between monomers having similar structures were examined. Figures 6 and 7 show the effects on copolymer composition curves of the solvent polarity and the addition of a common-ion salt for the cationic copolymerizations of PMOS-pMS and of IBVE-CEVE initiated by iodine, re- spectively. The monomer reactivity ratios are summarized in Table 111. The relative reactivities of the monomers were not greatly affected either by the solvent polarity or by the addition of a common-ion salt.

TABLE I11

between Styrene Derivatives Initiated by Iodine at 0 ’ C” Monomer Reactivity Ratios in the Cationic Copolymerization between Vinyl Ethers and

Monomer

M, M,

CEVE IBVE CEVE IBVE CEVE IBVE CEVE IBVE

PMOS pMS PMOS p M S

PMOS pMS

PMOS pMS

[ L l m [ S a l t l o . Solvent rnM Salt m M rL

0.30 t 0.05 0.27 t 0.03 CH,CI,/CCI,(2/1) 6.6

CH,Cl,/CCl,( 2/1) 6.6 n-Bu,NI 3.3 0.23 t 0.03 0.35 t 0.05

- - CH,CI, 3.3

CCI, 10.0 CH,CI, 3.3

CCI, 10.0

- -

- - 11.0 2 0.96

CH,CI, 3.3 n-Bu,NI, 3.3 26.3 t 6.50 26.5 f 6.49 CH,CI,/CCI,( 211 ) 6.6 28.7 f 7.40

- -

- - - -

r ,

4.25 t 0.41 5.14 t 0.28 4.59 t 0.39 4.46 t 0.37 0.56 t 0.30 0.41 t 0.25 0.28 t 0.06 0.22 t 0.05

a [MI, = 10 v01-R

DISCUSSION

In cationic copolymerization between monomers having similar structures, such as copolymerization between vinyl ethers or between styrene derivatives, the monomer reactivity ratios are not greatly affected by the solvent polarity or by the nature of the i n i t i a t ~ r . ~ ~ ? ~ ~ The insensitiveness of the monomer re- activity ratios to the reaction condition has been explained in terms that the solvent and the counteranion affect the properties of the two competing propa- gating species having similar structures to a similar extent. On the contrary, in the cationic copolymerization of a polar monomer with a nonpolar monomer, as the vinyl ether-styrene d e r i ~ a t i v e l g * ~ ~ , ~ ~ or p-chlorostyrene-isobutene sys- t e m ~ , ~ ~ it is well known that the monomer reactivity ratios are greatly affected by polarity of the solvent. This phenomenon has been explained in terms of a selective solvation of propagating species by a polar monomer in a nonpolar ~ o l v e n t . ~ ~ , ~ ~ This concept of “selective solvation” has been widely accepted, because it appears to explain most of the experimental results.

In the cationic copolymerization of CEVE with pMOS or with pMS initiated by iodine, the content of vinyl ether in the copolymer was found to increase when a common-ion salt was added to the copolymerization system in a polar solvent. This means that a dissociated propagating species tends to react perferentially

2628 YAMAMOTO AND HIGASHIMURA

with a styrene derivative and that a nondissociated one tends to react prefer- entially with a vinyl ether.

In the cationic homopolymerization of PMOS or p M S initiated by iodine, the dissociation of propagating species seems to be depressed either by the decrease of the solvent polarity or by the addition of a common-ion salt (n-Bu4NI or n- BuqNI3) to the polymerization system in a polar solvent. This was deduced from the molecular weight distribution of the polymers.1.28 Therefore, both the de- crease of the solvent polarity and the addition of a common-ion salt seem to have the same effect on the dissociation of propagating species produced from a sty- rene derivative as well as that from a vinyl ether.

As the structure of the counteranion in cationic polymerization initiated by iodine is not certain (I- or 13-),20-22 the effect of the common-ion salt was ex- amined with n-BurNI and n-Bu4NI3 in the present study. Both of these salts affected the relative reactivity of monomers, as clear from the copolymer com- position curves. The polymerization rate was reduced by the addition of either n-Bu4NI or n-Bu4NI3. The experimental findings suggest that the likely counterions for propagating carbocations could be I3-, or I- and 13- concurrently. If, for analternative possibility, I- is,the onlycounterion inthe present copolym- erizations, the addition of a small amount of n-Bu4NI3 wouldcause the coun- terion exchange, and the more dissociated propagating species carrying 13- as a counterion would give rise to an increased polymerization rate. However, this was not actually observed. Whatever the nature of counterion may be, it is safe to conclude that the addition of n-Bu4NI and n-Bu4NI3 depressed the disso- ciation of propagating species.

In conclusion, in the cationic copolymerization of CEVE with PMOS or with p M S initiated by iodine, the CEVE content in the copolymer increased either with a decrease in the solvent polarity or on addition of a common-ion salt to the copolymerization in a polar solvent. Thus, it is clear that the dissociation of the propagating species affects the relative reactivity of monomers in the cationic copolymerization of vinyl ethers with styrene derivatives. Therefore, the change of the monomer reactivity ratios in the cationic copolymerization, which used to be explained in terms of a conventional “selective” solvation, has to be re- considered from the standpoint of varying degrees of the propagating species. The present investigation may be the first to show that propagating species having various degrees of dissociation shows a different relative reactivity, i.e., selectivity, towards a pair of monomers in the same solvent.

In the field of the solvolytic displacement reactions, there have been the reports concerning changes in selectivity according to the degree of dissociation of the carbocation.11J2 Harris et a1.12 concluded from a study on ethanolysis of 2- adamantyl arenesulfonates that the reaction product depends on the degree of dissociation of the intermediate ionic species. This dependence was explained by the consideration that the location of a water molecule and an ethanol mole- cule with respect to a carbocation differs according to the degree of dissociation of the ionic species. Although this explanation seems attractive, it is difficult to apply it to the present cationic polymerization without hesitation, because in the latter little is known about the exact nature of the reaction intermediate and location of reagents. Further investigation is necessary to elucidate the change of the relative reactivity of monomers in the cationic copolymerization in connection to the dissociation of propagating species.

POLYMERIZATION OF STYRENE DERIVATIVES. I1 2629

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Received November 3,1975 Revised January 5,1976