cationic polymerization of styrenes by protonic acids and their derivatives, 2. two propagating...

Makronzol. Chein. 177, 2995-3007 ( I 976)

Department of Polymer Chemistry, Faculty of Engineering, Kyoto University, Yoshida Sakyo-ku, Kyoto 606, Japan

Cationic Polymerization of Styrenes by Protonic Acids and Their Derivatives, 2*)

Two Propagating Species in the Polymerization by CF3S03H

Mitsuo Sawamoto, Toshio Masuda, and Toshinobu Higashimura

(Date of receipt: October 17, 1975**))

SUMMARY: The bimodal molecular weight distribution (MWD) of the polymers and the polymeriza-

tion rate in the cationic polymerization of styrene by CF3S03H were studied under a variety of conditions. Both the decrease of the dielectric constant of the reaction mixture and the addition of a common-ion salt (Bu,N+S03CF ;) reduced the polymerization rate and the formation of the higher molecular weight portion of the polymers (the high polymer). It appears that of the two propagating species the one which forms the high polymer is more ionically dissociated and more reactive in propagation. Salt effects indicate that in nitrobenzene the propagating species is a solvent-separated ion pair and/or a free ion. The effects of monomer concentration on the bimodal MWD have shown that there are different chain-breaking reactions for the two propagating species. The possibility that the monomer is complexed with one of the growing species is also discussed.

ZUSAMMENFASSUNG: Bei der Polymerisation von Styrol durch CF3S03H unter verschiedenen Bedingungen

haben wir die Geschwindigkeit und die Molekulargewichtsverteilung (MWD) untersucht. Wenn die Dielektrizitatskonstante der Reaktionslosung herabgesetzt wird oder ein Salz mit einem gemeinsamen Ion (Bu4N+S03CF;) hinzugefiigt wird, werden die Geschwindig- keit und die Bildung des Hochpolymeren verringert. Es scheint als o b die wachsende Spezies welche das Hochpolymere bildet mehr ionisch dissoziiert und reaktiver ist. Der Salzeffekt deutet darauf hin, daB in Nitrobenzol die wachsende Spezies ein losungsmit- telgetrenntes Ionenpaar und/oder ein freies Ion ist. Der EinfluB der Monomerkonzentra- tion auf die bimodale MWD zeigt, da13 die zwei wachsenden Spezies an verschiedenen

*) Part 1 : cf.'). **) Revised manuscript of January 16, 1976.

2995

M. Sawamoto, T. Masuda, and T. Higashirnura

Abbruchreaktionen beteiligt sind. Die Frage ob eine der wachsenden Spezies mit dei Monomer einen Komplex bildet, wird auch behandelt.

Introduction

In the preceding paper’) we have reported that strong protonic acids (CF3S03H, FS03H, and C1SO3H) effectively polymerize styrene in methylene chloride to give polymers with a bimodal molecular weight distribution (MWD), and that the shape of the MWD depends strongly on solvent polarity. Recent studies have revealed that a wide variety of monomer/initiator combina- tions yield polymers with a bimodal MWD under suitable conditions. As to polymers of styrene derivatives, the bimodal MWD has been observed in the following monomer/initiator systems: styrene + CH3COC1042,3),

(C6H5)3CC1045), CF3COzH6); p-methylstyrene + HC1045), iodine7), CF3S03H8); p-methoxystyrene + iodine”; p-chlorostyrene + HC104’), CH3COC1047), CF3S03H8). Therefore, the formation of polymers with a bimodal MWD seems to be not a special phenomenon but rather general in cationic polymerizations.

The bimodal MWD studied in the styrene polymerization by acetyl perchlo- rate ( A C C I O ~ ) ~ ~ ~ . ~ ) indicates that the simultaneous and independent propagation of two propagating species is responsible for this MWD, and that one of the two species which forms the higher molecular weight portion of the polymer is more ionically dissociated than the other. Similar bimodal MWDs were also found in the polymerizations by strong sulfonic acids’), so that the nature of the propagating species generated by these acids seems to resemble that formed by AcC104; little is known about that, however. In a kinetic investigation of the styrene polymerization by CF3S03H by Chme- h”), a detailed discussion is given on the initiation mechanism, but little on the propagating species, unfortunately.

In the present work we investigated the nature and reactivity of the propagating species in the polymerization of styrene with CF3S03H by studying the polymerization rate and the MWD of the polymers.

HC104394), CF3S03H1’, FS03H’). ClS03H1), CH3S03H1’, H2S04536),

2996

Cationic Polymerization of Styrenes by Protonic Acids, 2

Experimental Part

Materials

Tetrabutylammonium trifluoromethanesulfonate (Bu4NfS03CF ;) was prepared by adding dropwise 50% aqueous CF3S03H to a 10% aqueous tetrabutylammonium hydroxide solution (Nakarai Chemicals, guaranteed reagent) at ice temperature. The precipitated white powder was filtered off, dried, and purified by several reprecipitations of the salt from ethyl acetate solution into hexane. Final yield: 85%, mp 114-115"C.

'H-NMR (CH2C12): b=1,07 (d; 3H; J=6Hz) , 1,47 (m; 4H), and 3,20ppm (bt; 2H).

C17H36N03F3S (391,5) Calc. C 52,15 H 9,27 N 3,58 F 14,56 Found C 52,16 H 9,03 N 3,51 F 14,27

Other materials were purified and used as described').

Procedures

Polymerizations, purification of products, and MWD measurements of the polymers were done as reported". The bimodal MWD curves were assumed to be composed of two Gaussian curves, and the weight fractions of the high and low molecular weight polymers were determined by means of a Dupont curve resolver.

Results

In this paper, as in a previous one3), the polymer corresponding to the higher molecular weight peak of a bimodal MWD curve is called the high polymer (H) and vice versa with the low polymer (L). The symbols W( ) and P( ) denote the weight fraction and the molecular weight at the peak in a MWD curve, respectively, of each polymer.

Effects of monomer concentration and of the dielectric constant of the reaction mixture on M W D

The initial monomer concentration ([MIo) was varied from 2,0 to O,lOmol/l by the following two methods, because the dielectric constant ( E ) of the reaction mixture increases as [MIo is decreased in methylene chloride solutions: (a) [MIo was varied in pure methylene chloride (uncompensated system). In these experiments E changed from 7,11 ([M],,=2,Omol/l) to 8,84

2997

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

([M]o=O,lOmol/l); (b) [MIo was varied with E fixed at 7,11 by use of benzene for the dielectric compensation (compensated system). The E of the reaction mixture was calculated as usual on the assumption that it is the volume- weighted mean of that of the components.

(A) Uncompensated (6) Compensated 4 Molecular weight

lo5 lo4 13 lo2 105 lo4 lo3 102 I I I I I I I I

I I I I I I I I I I

25 30 35 4 0 45 25 30 35 40 45 Elution volume in counts -

Fig. 1. Effect of monomer conc. (in mol/l as indicated) on MWD of polystyrenes produced by CF3S03H at 0°C. [C]o=2,0.10-4 mol/l, conversions are 30-50%: (A) the uncompensated system in CH2C12; (B) the compensated system at e=7,11, solvent: a: CH2C12, b-d: CH2CIz/C6Hrj

Fig. 1 shows the MWD of polystyrenes obtained in both the compensated and uncompensated systems. The bimodal MWD was observed in all experiments. In the uncompensated systems W(H) increased and P(H) decreased with decreasing [MI,, while P(L) remained almost constant. On the other

2998

Tab

. 1.

M

olec

ular

wei

ght d

istr

ibut

ion

of p

olys

tyre

nes

prod

uced

by

CF

3S03

H at

var

ious

mon

omer

con

cent

ratio

ns"'

Unc

ompe

nsat

ed s

yste

mb)

[M

I0

Com

pens

ated

sys

tem

")

A

A

I

3

IO-~

.P(H

)~)

~o

-~.P

(L)

W(H

~

mol

I-'

Ed)

IO

-~.P

(H)

10

-3.q

~)

W(H

;

7,ll

4,

9 1

2

0,37

2,O

7,

l 1 b,

4,9

12

0,

37

8,02

1,8

5 -1

,O

0,75

1 ,o

7,

l 1

2,6

- l,o

0,33

8,

47

1,4

- l,o

0,78

0,

50

7,11

1 ,

I - l,o

0,

32

8,70

0,

48

- l,o

0,88

0,

25

7,lt

0,

54

-l,o

0,

41

-

-

-

-

8,84

0,

23

- l,o

0,91

0,

lO

a)

Initi

ator

con

c., [

ClO

=2,O

. b,

So

lven

t: C

H2C

12.

') So

lven

t: C

H2C

12/C

6H6 m

ixtu

res.

d

, /:=

diel

ectr

ic co

nsta

nt.

e'

P(H

), P

(L):

mol

ecul

ar w

eigh

t of t

he h

igh

and

low

pol

ymer

s, re

sp.,

W(H

): w

eigh

t fra

ctio

n of

the

hig

h po

lym

er.

mol

/l, t

emp.

: OT

, con

vers

ion:

30-

50%

.


hand, when E was held constant (the compensated systems), W(H) did not change and P(H) decreased as [MIo was decreased. Small peaks at elution volume count ca. 42, which appeared at: low [MIo in the compensated systems, are due to a styrene dimer"). Va1ut:s of W(H), P(H), and P(L) obtained with both systems are summarized in Tab. 1.

Molecular weight lo5 to4 id id , 1 I I

I I , I I

25 30 35 40 45 Elution volume in counts

7,i 1

7,48

7,86

8,04

a,47

0,32

0,42

0,50

Fig. 2. Effect of the dielectric constant ( E ) of the reaction mixture on MWD ofpolystyrenes produced by CF3S03H at 0°C. [MIo=

mol/l, conversions are 30 to 50%;solvent: a-d : CH2C12/ C6H6; e: CH2C12

039

0,78 0,50 mol/l, [ClO=2,O.

Changes in the MWD of the polymers with E are depicted in Fig. 2; E was varied by adding benzene to niethylene chloride in various proportions at a constant [MIo (0,50 mol/l). W(H) increased monotonically with increasing E, but both P(H) and P(L) were independent of E, provided that [MIo was kept constant. The dimer formation' was also observed in these experiments. The effect of [MI, on P(H) (Fig. 1 (B)) and that of E on W(H) (Fig. 2) are seen overlapping in Fig. 1 (A).

3000


The effects of monomer concentration and dielectric constant on the polymerization rate

The reaction was markedly accelerated when [MIo was decreased from 2,O to 0,25 mol/l in the uncompensated systems, as Fig. 3 shows. In contrast, in the compensated systems, where E was fixed at 7,11 ( E at [MIo = 2,O mol/l in methylene chloride), the time-conversion curves did not change with [MIo

Fig. 3. Effect of monomer conc. on the rate of polymerization of styrene by CF3S03H at 0°C. [C],= 2,O. mol/l; (-): the uncompensated system in CH2C12; (----): the com- pensatedsystem in CH2C12/ C6H6 mixture of ~=7,11. [M]o/(mol I - ' ) : (0) 2,O; (a, A) 1,o; (0, .) 0 3 ; (v, 0,25

Time in min

and were almost the same as that at [Ml0=2,0 mol/l (dashed line in Fig. 3). Under the conditions employed here, the polymerization rate (R,) is expressed as''

R - - d[Ml= k[C]o[M] '- dt

where k is the overall rate constant of polymerization and [C], the initiator concentration. The results in the compensated system mean that k is independent of [MIo provided that E is kept constant.

The acceleration with decreasing [MI observed in the uncompensated system may be accounted for either by an increase in E with decreasing [MIo or by the inactivation of the propagating species and/or of the initiator by n-complexation with styrene (or monomer solvation), or both.

3001


0 loo + I I I

c

C

.-

.o t? %!

6 c

:/- 5 0 -

I I I

5 6 8 10 15 I I I I l l I

Fig. 5. Effect of the dielectric compensation by CCI, on the rate of polymerization of styrene by CF3S03H at 0°C. [C],= 2,O. I O P mol/l, E = 7, I I : (0): [M]o=2,0mol/l,solvent CH2CI2; other points: CH2CI2/CCl4, [MIo/ (mol 1- ’): (A) 1,O; ( 0 ) 0,50; (A) 0,25

I I I I I I I I I I I I I I I I I

I 0

I I 0

I

I I

I

I i

&

Fig. 4. Effect of the dielectric constant ( E ) of the reaction mixture on the rate-constant k of the polymerization of styrene by CF3S03H at 0°C. [MIo= 0,50 mol/l, [C]o=2,0.10-4 molj1:solvent: (0 ) : CH2CI2/

CH2Cl2/C6H5NO2. The upper part of the Fig. shows the obtained MWDs schematically (L, “low”, H, “high” polymer)

C,H,; (A): CHpC12; (0) :

(& -1)/(2&*1)

First: to clarify the effect of E on R,, polymerizations were carried out at various E at constant [MIo (0,50 mol/l). The E was varied from 5 to 17 by adding benzene or nitrobenzene to methylene chloride. Fig. 4 shows the relation between k and E. The rate-constant k remained small and virtually unchanged when E was small (< 6), increased sharply in the range of E from 6 to 10, and then levelled off again when E was large (> 10). In Fig. 4 the


variation of the MWD of the polymers with E is also illustrated schematically (cf. Fig. 2).

Second: the possibility of the monomer complexing was examined by using carbon tetrachloride instead of benzene for the dielectric compensation at various [MI,. Both solvents possess similar dielectric constants, but their complexing power is probably very different. As Fig. 5 shows, the dielectric compensation by carbon tetrachloride showed just the same effect on R, as that by benzene (Fig. 3). The polymerization rate was thus found to be greatly dependent on E (s. also Discussion).

Salt effects on the polymerization rate and on M W D

To obtain information on the nature of the two propagating species, salt effects were investigated in three solvents. A common-ion salt, Bu4N+S03CF j, was added to reaction mixtures at several concentrations ([S]).

1 /+r

I ' P

Fig. 6. Effect of Bu4N+S03CF: (S) on the rate ofpolymerization of styrene by CF3S03H at 0°C; k and ko are the overall rate constants in the presence and the absence of the added salt, resp. [MIo= 1,0 mol/l; 104.[C]o/ (mol/l): (0. A ) 2,O; (0) $0

OO O S 1 ,o' 10

[sl/ lcl,

Fig. 6 shows the salt effects on the rate-constant k . It should be noted that a small amount of the salt caused a great rate depression in methylene chloride and in benzene, whereas in nitrobenzene k was decreased more gradually by increasing [S].

The salt effects on MWD of polymers are illustrated in Fig. 7. In methylene chloride the formation of the high polymer was suppressed by the added

3003


WSW CH2Cl2 h - Molecular weight

lo5 10' 10' lo2 lo5 lo4 13 1 6 lo5 to4 1d lo2 I I I I I I I I I I 1 I

1 I I 1 I I I I I I I I 1 I

25 30 35 40 45 25 30 35 40 45 30 35 40 45 Elution volume in counts c

Fig. 7. Effect of Bu4N+S03CF; (S) on MWD of polystyrenes produced by CF3S03H at 0°C. [MIo= 1,0mol/l; in C6H5N02 and CHzC12: [c]0=2,0.10-~, in C6H6: [C]O= 5,O. 10-4mol/l

salt. On the other hand, in nitrobenzene the MWD of the polymers was unchanged and only the high polymer was produced even in the presence of the salt. In benzene no change appeared in the molecular weight of the low polymer, but a considerable amount of dimer") was obtained as a by- product.

Discussion

Nature and reactivity of the two propagating species

Large changes in R, and in MWD of polymers were induced by change in E as well as by the addition of the common-ion salt. Both the decrease of E and the addition of the salt in methylene chloride reduced R, and

3004


suppressed the formation of the high polymer. It is therefore concluded, as for the polymerization by A c C ~ O ~ ~ ) , that dissociated and non-dissociated propagating species coexist under suitable conditions, and that the former is more reactive in propagation to give the high polymer.

The other interesting result is the close correlation between R , (or k of Eq. (1)) and the MWD of polymers which is clearly shown in Fig. 4. Fig. 4 indicates that the bimodal MWD appears only in a narrow range of E

(&lo) where k changes drastically, and that k remains almost constant when the MWD is unimodal. That is, the relative populations of the two propagating species are very sensitive to small changes in E, and k increases greatly as the dissociated propagating species becomes predominant with increasing E. These results also support strongly the coexistence of the two propagating species and the higher reactivity of the dissociated species.

Chmelir"'has reported large values of R, in the polymerization by CF3S03H at low [MIo (below 0,2 mol/l) in methylene chloride at - 15 "C. His time-conversion curves were S-shaped and R , increased with decreasing [MI,. He empha- sized the significance of monomer complexation of the initiator to account for these results. However, the great acceleration produced by the small increase in E which accompanies a decrease in [MIo in methylene chloride was not considered in his discussion. Further, we showed that the rate-constant k is independent of [MIo when the increase in E due to decreasing [MIo is compensated by a nonpolar solvent such as benzene or carbon tetrachloride (Figs. 3 and 5). If the inactivation of the propagating species and/or of the initiator by .n-complexation does operate, replacing styrene with carbon tetrachloride (dielectric compensation by carbon tetrachloride) should increase k , because carbon tetrachloride does not possess n-electrons for complexing. It therefore seems that n-complexation does not play an important role in these polymerizations, and the increase of R, with decreasing [MIo can be attributed mainly to the increase in E.

In nitrobenzene R, was slightly reduced by the salt but only the high polymer was obtained even in the presence of a large amount of the salt. This means that a reactive propagating species forming the high polymer can exist even in the presence of the salt in nitrobenzene. Since the concentration of free ions should be decreased by a large amount of added common-ion salt, the above reactive species seems to be an ion-pair with a reactivity similar to that of a free ion, and so it is probably a solvent-separated ion pair. The involvement of a solvent-separated ion pair seems reasonable because nitrobenzene is a strong cation solvator. On the other hand, in methylene

'

3005


3 -

2 -

1 -

chloride, since its solvating power is not so large, a solvent-separated ion pair cannot exist when the salt is added. Therefore, the addition of the salt in this solvent increases the concentration of the non-dissociated species, hence R, was greatly reduced and the formation of the high polymer was suppressed by the salt.

However, salt effects on R, are complicated and not so simple as those in the polymerizations by A c C ~ O ~ ~ ' or perchloric acid 12). In methylene chloride, the decrease in k with increasing [S] was larger than that expected from the usual linear relation between k and [S]-'/2, and in benzene in which the concentration of free ions should be negligible, k was also reduced by increasing [S]. The aggregation of ion pairs may be one reason for these facts, which need detailed future investigation.

The nature of the propagating species forming the low polymer (the non-dissociated species) is also difficult to discuss at present because of lack of information, but it may be that it is a pseudo-cationic one13). This problem is now being investigated by using weak protonic acids as initiators6).

Fig. 8. Plot of the Schdz- Harborth equation for the high polymer produced by CF3S03H at 0°C. All the P (H) values given in Tab. 1 are

I I 1 plotted

Different chain-breaking reactions for the two propagating species

As Fig. 1 shows, the molecular weights of the high and low polymers showed different dependences on [MI,. The molecular weight of the high polymer increased with increasing [MI,; that of the low polymer was independent of [MI,. The P(H) values were plotted according to the Sckulz-Harbortk e q ~ a t i o n ' ~ ) and gave a straight line through the origin (Fig. 8). The P(H)

- 5

*: 0 4 [

3006


was used instead of AT, because this is difficult to determine exactly from a “resolved component” MWD curve. The plots obtained by Pepper” for the styrene/HC104 system at 0°C were straight lines nearly through the origin for the high polymer and one with an intercept for the low polymer. Fig. 8 indicates that the propagating species forming the high polymer does not transfer to monomer, but that there is one or more chain-breaking reactions with some other component of the reaction mixture. In contrast, the constant molecular weight of the low polymer implies that for the propagating species forming the low polymer monomer transfer is dominant. We believe that ours is the first report concerning the difference between the chain-breaking reactions of the two propagating species occurring in the polymerizations initiated by protonic acids.

The authors wish to thank Dr. M. Hosono at the Institute for Chemical Research of Kyoto University for his kind permission to use the curve resolver. This investigation was supported by the financial fund from the Ministry ofEducation, Japanese Government, 1975.

’) T. Masuda, M. Sawamoto, T. Higashimura, Makromol. Chem. 177, 2981 (1976), preceding paper T. Masuda, T. Higashimura, J. Polym. Sci., Part B, 9, 783 (1971)

3, T. Higashimura, 0. Kishiro, J . Polym. Sci., Polym. Chem. Ed. 12, 967 (1974) 4, D. C. Pepper, Makromol. Chem. 175, 1077 (1974) 5 , D. C. Pepper, J. Polym. Sci., Polym. Symp., 50, 51 (1975)

M. Sawamoto, T. Takeda, T. Masuda, T. Higashimura, presented at the 24th Sympo- sium of the Society of Polymer Science, Japan, Nov. 1975

’) T. Higashimura, 0. Kishiro, T. Takeda, J. Polym. Sci., Polym. Chem. Ed. 14, 1089 (1976) M. Sawamoto, T. Masuda, T. Higashimura, paper presented at the 24th Annual Meeting of the Society of Polymer Science, Japan, May 1975

9, T. Higashimura, 0. Kishiro, K. Matsuzaki, T. Uryu, J. Polym. Sci., Polym. Chem. Ed. 13, 1393 (1 975)

lo) M. Chmelir, Makromol. Chem. 176, 2099 (1975) 1 1 ) M. Sawamoto, T. Masuda, H. Nishii, T. Higashimura, J. Polym. Sci., Polym. Lett.

”) B. MacCarthy, W. P. Millrine. D. C. Pepper, Chem. Commun. 1968, 1442 1 3 ) A. Gandini, P. H. Plesch, J. Polym. Sci., Part B, 3, 127 (1965); J. Chem. SOC. 1965,

1 4 ) G. V. Schulz. G. Harborth. Makromol. Chem. 1 , 104 (1947)

Ed. ’13, 279 (1975)

4826; Eur. Polym. J. 4, 55 (1968)

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cationic polymerization of styrenes by protonic acids and their derivatives, 2. two propagating...

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