cationic polymerization of styrenes by protonic acids and their derivatives, 4. homo- and...

Makromol. Chem. 178,1497-1505 (1977)

Department of Polymer Chemistry, Faculty of Engineering, Kyoto University, Kyoto 606, Japan

Cationic Polymerization of Styrenes by Protonic Acids and Their Derivatives, 4*)

Homo- and Co-polymerizations of paraSubstituted Styrenes by CF3S03H

Mitsuo Sawamoto, Toshio Masuda, and Toshinobu Higashimura

(Date of receipt: May 25, 1976)

SUMMARY: p-Methyl- and p-chlorostyrenes (pMeSt and pC1St) were polymerized by CF3S03H

in methylene chloride at 0°C and gave polymers with a bimodal molecular weight distribution (MWD). This indicates that the polymerizations involve two independent propagating species: a dissociated and a non-dissociated species. Their selectivities toward monomers were compared in the copolymerizations of pMeSt and pClSt with styrene (St) by CF3S03H in benzene, methylene chloride, and nitrobenzene solvents. Decreasing solvent polarity and the presence of a common-ion salt ( ( B - C ~ H ~ ) ~ N + S O ~ C F ; ) greatly increased the reactivity difference between the monomers. Thus the non-dissociated species is more selective than the dissociated species. In contrast, the composition of pMeSt-St copolymers produced by BF30(C2H5)2 did not change with the solvent polarity. The difference in nature between the propagating species generated by CF3S03H and by BF30(C2H5)* is discussed.

Introduction

We have recently studied the dissociation state of the two independent propagating species which are involved in the cationic polymerization of styrenes by protonic acids (CF3S03H1’, CF3C02H2’, and others3’) or their derivative (CH3COC1044-7)). One of the two species is as dissociated as free ions and gives high molecular weight polymers; the other is a non-dissociated species which forms low molecular weight polymers. They propagate simultaneously under suitable conditions and yield polymers with a bimodal

*) Part 3: cf.2’.

1497

M. Sawamoto, T. Masuda, and T. Higashimura

molecular weight distribution (MWD). Pepper839' has reported similar results for the polymerization of styrene by perchloric acid.

A closely related subject of interest is the reactivity and selectivity of the two propagating species in polymerization. From this viewpoint the copolymerizations of styrene with p-methylstyrene and with 2-chloroethyl vinyl ether by acetyl perchlorate initiator were studied recently"'. Large effects of solvent and common ion on the copolymerization indicated that the two propagating species have different selectivities toward the two monomers. Early results on similar copolymerizations by metal halides are in sharp contrast to these, showing minor solvent"' and catalyst 12) effects.

The present work is an extension of our studies on the two propagating species generated by trifluoromethanesulfonic acid (CF3SO3H)'g3). p-Methyl- and p-chlorostyrenes (pMeSt and pC1St) were employed as monomers which are more and less reactive than styrene (St), respectively. First their homopolymerizations by CF3S03H were examined and the nature of the propagating species derived from each monomer was discussed. They were then copolymerized with styrene to investigate the monomer selectivity of the propagating species and its dependence on the para-substituents in the monomers. Copoly- merization initiated by boron trifluoride etherate (BF30(C2H5),) was also studied for comparison.

Experimental Part

Materials

Commercial pMeSt was purified by distillation over calcium hydride before use. pClSt was prepared in the same manner as described el~ewhere'~'. Gas chromatograms showed more than 99,O-% purities of these monomers. BF30(C2H5)2 was used after distillation of commercial products. Other materials were used as previously reported lJ.

Procedures

Homo- and co-polymerizations were performed under dry nitrogen at 0°C. Initial monomer concentrations ([MI,) in homo- and co-polymerizations were 1 ,O mol/l and 10 vol% (in total), respectively. The monomer conversion and the copolymer composition were determined by measuring the residual monomer concentration by gas chromatography. Bromobenzene (4-5 ~01%) as internal standard was added to the reaction mixture in advance. Monomer reactivity ratios were calculated by an improved Fineman-Ross

1498

Cationic Polymerization of Styrenes by Protonic Acids

? 100 3- I I I I

4 pMeSt /; 5 b B /O

/t

- 5 >

- 50 - -

0

method14'. Treatment of the results according to the method of Kelen and Tiidij~'~' showed the reliability of the values thus obtained. Polymerization products were precipi- tated into excess of methanol, recovered by evaporating the solution, and then subjected to MWD measurements by gel permeation chromatography (GPC) as previously described'). Their molecular weight was estimated from the GPC curve by using a calibration curve for polystyrene.

Results and Discussion

Homopolymerizations of p-methyl- and p-chlorostyrenes

CF3S03H polymerized pMeSt and pClSt at low initiator concentrations around 10-4mol/l in methylene chloride (CH2C12) as Fig. 1 shows. The polymerization rate increased, as expected, with increasing electron-donating property of the para-substituent in the monomers. Similar results were obtained in benzene and nitrobenzene.

Fig. 2 illustrates the MWD of polymers produced in three solvents of different polarities. In CH2C12 all the polymers showed a bimodal MWD

Time in rnin

Fig. 1

Solvent pMeSt St p a s t

IIIIIIIIIIIIIII 30 40 30 40 30 40

Elution volume in counts - Fig. 2

Fig. 1. Polymerizations of para-substituted styrenes (p-methylstyrene (pMeSt) and p- chlorostyrene(pC1St))and styrene"(St) by CF3S03H in CH2C12at 0°C: [MI,= 1.0 mol/l; IO4.[CF3SO3H],/(mol l - ' ) = 1,0 (pMeSt); 2.0 (St. pC1St)

Fig. 2. MWD(GPCcurves)of poly(para-substituted styrene)s and polystyrene3' produced byCF3S03HatO"C: [MI,= 1.0 mol/l; 104~[CF3S03H]o/(moll~')= l.O(pMeSt); 2.0-5,0 (St); 2.0-25 (pC1St)

1499


Poly(St) Poly(pC1St)

at any conversion. This was reduced to a unimodal MWD with a high molecular weight peak alone in nitrobenzene, and to a unimodal MWD with a low molecular weight peak in benzene. These trends of MWD are virtually the same as that in the styrene polymerization by CF3S03H’,3). Therefore, two independent propagating species are also involved in the polymerizations ofpMeSt and pClSt in CH2C12; one of them forming the high polymers is more dissociated and more favored in the polar solvent (nitrobenzene) than the other which is predominant in the nonpolar solvent (benzene).

The weight fraction of the high polymers (W(H)) in CH2C12 increased in the following order: pMeSt > St >pClSt, i.e. a more electron-donating substituent increased W(H). Changing the initiator from acetyl perchlorate (AcC104) to CF3S03H also increased W(H) in CH2C12 at 3 M O % conversions as summarized below :

w (HI 1 (AcC104 (CF3S03H)

0,41 5 , 0,75” 0,247) 0,67

These results suggest that the propagating species exists more in its dissociated form when it is stabilized by an electron-donating substituent or its counterion is derived from a stronger acid16), and thus its dissociation is facilitated.

Copolymerizations of p-methyl- and p-chlorostyrenes with styrene

pMeSt and pClSt were copolymerized with St (MI) by CF3S03H under such conditions that only one of the two propagating species exists; i.e. in nitrobenzene or in benzene. For comparison, a pMeSt-St copolymerization in CH2C12 was also studied. Fig. 3 shows the results obtained. Corresponding monomer reactivity ratios are given in Tab. 1. The copolymerization products were of fairly high molecular weight (> 1,5. lo3 (by GPC) even in benzene), and the conventional composition equation can be applied reasonably. The monomer reactivity ratios calculated by the two methods’4,’ 5 , agreed closely with each other.

The copolymer compositions in both the pMeSt-St and pC1St-St copolymerizations depended strongly on the solvent polarity. The composition curves are far away from the diagonal in benzene, while close to it in nitrobenzene. This means that the non-dissociated propagating species, predominant in

1500

Tab.

1.

Mon

omer

rea

ctiv

ity r

atio

s rl

and

r2

for

the

copo

lym

eriz

atio

ns o

f st

yren

e (M

I) w

ith p

ara-

subs

titut

ed s

tyre

nes

at 0

°C

([M

]o=1

0 ~

01

%)

Initi

ator

M

2 So

lven

t 10

3. [C

],"'

103.

[S]b

) r

l ~

-

mol

l-'

mol

l-'

r2

BF3

0(C

2H5)

2 pM

eSt

C6H

6 20

4 pM

eSt

CH2C

12

1,O

pMeS

t C

sHsN

02

2,O

a)

Initi

ator

con

cent

ratio

n.

b,

Con

cent

ratio

n of

(n-

C4H

9)4N

+S03

CF;

as

a co

mm

on-i

on s

alt.

d,

[H20

],=5

,0.

mol

/l.

Res

ults

of

two

repl

icat

e ex

perim

ents

.

1,91

f0,1

5 1,

81 &

0,36

0,

27 F 0,0

2 0,

52 f0

,02

0,59

f 0,

02

0,19

& 0,

03

0,53

f0,0

5

0,50

i: 0,

05

033 f 0,

03

0,46

k 0,

07

0,33

f0,W

0,

65 f 0,

08

2,85

+0,1

9 1,

80 f 0,

04

1,30

+ 0,0

6 2,

44&

0,19

1,

31 k0

,14

496

+0,1

2 1,

99 &

0,lO

1,

96 &

0,16


0

-0,2

Mole fraction of St in monomer

Fig. 3

p - M e H p-CI

-0,4 -0,2 0 0, (

Fig. 4

Fig. 3. Solvent effects on the copolymerizations of styrene (St) with para-substituted styrenes (pC1St. open symbols; pMeSt, filled symbols) by CF3S03H at 0°C; (0, 0 ) :

C,H,; (A): CH2C12; ( 0 , m): C b H 5 N 0 2 . See Tab. 1 for reaction conditions

Fig. 4. with para-substituted styrenes by CF3S03H at 0°C. Data from Fig. 3

Hammett plots of log(l/rl) against CJ’ for the copolymerizations of styrene (MI)

benzene, has a greater monomer selectivity than the dissociated species, predominant in nitrobenzene. In CH2C12 where the two species coexist, an inter- mediate composition curve was obtained.

The monomer selectivity difference between the two species can also be demonstrated by the Hammett plots shown in Fig. 4, in which log(l/vl) is plotted against Brown’s cr’. The Q’ value for the non-dissociated species in benzene is negatively greater than that for the dissociated species in nitrobenzene. This shows the higher selectivity of the former species. Such selectivity difference may be accounted for by the conventional “selectivity-reactivity’’ relationship; that is, the less reactive” non-dissociated species should be more selective toward monomers. More data may be necessary, however, to interpret the difference generally.

Figs. 5 and 6 show the effects of a common-ion salt ((n-C4H9)4N+S03CF;) on the copolymer composition and the MWD, respectively, of pMeSt-St copolymers (see also Tab. 1). The copolymer composition in CH2Clz shifted to that in benzene when the dissociation of propagating species was suppressed

1502


by the salt (open circles in Fig. 5). The copolymer formed in a salt-free system in CH2CI2 showed a bimodal MWD (s. Fig. 6), again indicating the coexistence of differently dissociated propagating species. The presence of the salt reduced the high molecular weight peak and the resultant MWD was similar to that of the copolymer formed in benzene. Thus, the salt effect on the MWD just corresponds to that on the copolymer composition, which

Elution volume in coun 25 30 35 40 45

I I

Mole fraction of St in monomer

Fig. 5 Fig. 6

Fig. 5. Effect of (n-C4H9)4N+S03CF: on the pMeSt-St copolymerization by CF3S03H at 0°C. Salt present: (0) CHZCl2; ( 0 ) C6H5N02. Salt free (from Fig. 3): (-.-.-) ChHh; (-- -) CH2Cl2; (-) C h H S N 0 2 . See Tab. 1 for reaction conditions

Fig. 6. MWD of pMeSt-St copolymers produced by CF3S03H or BF30(CzH5)2 in CH2CI2 at 0°C: [pMeSt]o/[St]o=3/5; [MI,= 10 volx; lO3.[1nitiator],/(mol I - ' ) : (a): 0.20; (b): 030; (c): 1.0. [(n-C4H9),N+SO3CF.T] = 1.0. mol/l ((b) only)

shows that the large solvent effect on the copolymerizations resulted from changes in the dissociation state of the propagating species with the solvent polarity. The same conclusion has been reached in our recent work on the pMeSt-St copolymerization by acetyl perchlorate"'.

In nitrobenzene, in contrast, the salt did not affect the copolymer composition as shown in Fig. 5 (filled circles) and Tab. 1. Therefore, in the presence of the salt, different non-dissociated propagating species must be involved in the copolymerizations in CHzC12 and in nitrobenzene, as we previously concluded'-'').

1503


It should be noted that in both the pMeSt-St and pC1St-St copolymerizations a less reactive monomer increased in reactivity in a polar solvent (Fig. 3). This result is opposite to the expectation based on Ouerberger’s view”’ that a more dissociated cation in a polar solvent would react preferentially with an electron-rich and hence more reactive component. The “selective solvation” concept18’ appears also unattractive to explain the above result.

Copolymerization by BF 3 0 ( C2 H 5 ) 2

pMeSt-St copolymerization by BF30(C2H5)2 initiator was studied for comparison. Fig. 7 shows the results obtained in three solvents (see also Tab. 1). Copolymer compositions were essentially unchanged by the solvent polarity. The MWD of the copolymer formed in CH2C12 was unimodal (Fig. 6, (c)). These results are in sharp contrast to those obtained with CF3S03H shown in Figs. 3 and 6. The composition curve for BF30(C2H5)2 lies between the two extremes for CF3S03H in benzene and nitrobenzene.

Fig. 7. pMeSt-St copolymerizations by BF30(C2Hs)2 at 0°C: (0 )

See Tab. 1 for reaction conditions C6H6; (A) CHZC12; (a) C6H5N02.

“0 03 100 Mole fraction of St in monomer

That the copolymer composition is independent of solvent polarity indicates that the propagating species derived from BF30(C2H5)2 always has a similar selectivity toward the monomers. This is in contrast to the fact that in the copolymerization by CF3S03H the monomer selectivity of the propagating species depended definitely on solvent polarity. This difference may be due to the different nature of the counterions, i.e. one from a metal halide and the other from a superacid.

1504


In conclusion, the two propagating species generated by CF3S03H were found to differ from each other not only in dissociation state and reactivity but also in selectivity in copolymerization. They yielded polymers with a bimodal MWD in homo- and co-polymerizations of styrenes. It is a characteris- tic of the CF3SO; anion to cause the bimodal MWD and the large solvent effect on the copolymerizations.

M. Sawamoto, T. Masuda, T. Higashimura, Makromol. Chem. 177, 2995 (1976) 2 , M. Sawamoto, T. Masuda, T. Higashimura, S. Kobayashi, T. Saegusa, Makromol.

Chem. 178,389 (1977) 3, T. Masuda, M. Sawamoto, T. Higashimura, Makromol. Chem. 177, 2981 (1976) 4, T. Masuda, T. Higashimura, J. Polym. Sci., Part B, 9, 783 (1971) 5, T. Higashimura, 0. Kishiro, J. Polym. Sci., Polym. Chem. Ed. 12, 967 (1974) ') T. Higashimura, 0. Kishiro, K. Matsuzaki, T. Uryu, J. Polym. Sci., Polym. Chem.

7, T. Higashimura, 0. Kishiro, T. Takeda, J. Polym. Sci., Polym. Chem. Ed. 14, 1089 Ed. 13, 1393 (1975)

(1976) D. C. Pepper, Makromol. Chem. 175, 1077 (1974)

9, D. C. Pepper, J. Polym. Sci., Symp. 50, 51 (1975) l o ) T. Higashimura, K. Yamamoto, J. Polym. Sci., Polym. Chem. Ed., in press

For example, see, C. G. Overberger, L. H. Arnold, J. J . Taylor, J. Am. Chem. SOC. 73, 5541 (1951)

1 2 ) For example, see, C. G. Overberger, R. J. Ehrig, D. Tanner, J. Am. Chem. SOC. 76, 772 (1954)

13) C. S. Marvel, G. L. Schertz, J. Am. Chem. SOC. 65, 2056 (1943); R. F. Nystrom, W. G. Brown, J. Am. Chem. SOC. 69, 1197 (1947)

14) A. I. Ezrielev, E. L. Brokhina, E. S. Roskin, Vysokomol. Soedin. 11, 1670 (1969) 1 5 ) T. Kelen, F. Tudos, J. Macromol. Sci.-Chem. A9, 1 (1975) ") T. Gramstad, Tidsskr. Kjemi, Bergves., Metall. 19, 62 (1959); C. A. 54, 12739 (1960) 1 7 ) C. G. Overberger, T. M. Chapman, T. Wojnarwski, J. Polym. Sci., Part A, 3, 2865

( 1965) C. G. Overberger, V. G. Kamath, J. Am. Chem. SOC. 81, 2910 (1959); 85, 446 (1 963)

1505

cationic polymerization of styrenes by protonic acids and their derivatives, 4. homo- and...

Documents