Cationic Polymerization of Styrene by Metal Halide and Nonmetal

Derived from the Two Categories of Initiators Halide Initiators: Difference between the Propagating Species

INTRODUCTION

Initiators for the cationic polymerization of vinyl compounds can be divided into two categories: metal halide and nonmetal halide initiators.* Metal halides, for example, BF30(CzH5)2 and SnC4, are well-studied, conventional initiators that can yield high polymers effectively. The “nonmetal halide” initiators may include molecular halogens, protonic acids, and their derivatives. Our recent investigations are concerned with the polymerization of styrenes initiated by acetyl perchlorate (AcCIO~),’.~ trifluoromethanesulfonic acid (CF3S03H),3.4 and the like.5*6 We have found that these nonmetal halide initiators generate two independent propagating species that grow simultaneously to give polymers with a characteristic “bimodal” molecular weight distribution (MWD).

The polymerization of styrene by CF3SO3H depends largely on the initial monomer concentration ([MI01 in CH2C12 a t 0°C.3 The reaction rate greatly increases with decreasing [M]o. The MWD of produced polymers, being bimodal, changes its shape; that is, as [MI0 decreases, the high-molec- ular-weight peak increases in proportion and shifts toward the low-molecular-weight side. We have related these trends to the coexistence of two propagating species that differ in nature, the mole fractions of which are under the strong influence of the dielectric constant (e) of the reaction medium.3 Chmelir7.8 also studied the effect of [MI0 on the same polymerization and explained his results in a different manner from ours. For the styrene polymerization by perchloric acid in CH2CI2 Pepper9.10 examined the effect of [MI0 on the molecular weight of polymers to discuss mainly transfer reac- tions.

It seems to us that this dependence on [MI0 and e is a common characteristic of polymerization by nonmetal halide initiators. Therefore we compared the styrene polymerizations by nonmetal halide (AcC104, CFaSOsH, etc.) and metal halide (BF30(CzH5)2) initiators under identical condi- tions.

EXPERIMENTAL

Commercial CF3S03H (Sumitomo 3M Co., purity 198%) was used as reported: and AcC104 was prepared as described elsewhere.” Methanesulfonic acid (CH3SOa) and boron trifluoride etherate (BF30Et2) were purified by distillation (under reduced pressure for the former) of commercial products. Trifluoroacetic acid (CF3C02H Haloarbon Products Co., purity 199%) was used without further purification. Other materials were used as previously r e p ~ r t e d . ~

Polymerizations were carried out under dry nitrogen at 0°C.5 The MWD of polymers was mea- sured by gel permeation chromatography on a Shimazu GPC-700 chromatograph equipped with columns SG-1,2,3,4, and 5 (eluent, 2-butanone). Monomer consumption was followed by gas chromatography.

RESULTS AND DISCUSSION

Effect of the Initial Monomer Concentration

The rates of styrene polymerization by AcC104 (a nonmetal halide initiator) and by BF30Et2 (a metal halide initiator) were compared at various [MI0 from 0.25 to 2.0M in CH2C12 (Fig. 1). The polymerization rate by AcC104 increased steadily with decreasing [Mlo, as observed with CF3SO3H i n i t i a t ~ r . ~ In sharp contrast the time-conversion curves for polymerization by BF30Et2 were in- dependent of [Mlo.

Figure 2 shows the MWD of polystyrenes obtained with AcC104 and BF30Et2 at various [MI0 in

* For a brief review from this viewpoint, see T. Higashimura, M. Sawamoto, and T. Masuda, Preprints of the Polymer Colloquium (the Society of Polymer Science, Japan), p. 26, September 1977.

Journal of Polymer Science: Polymer Chemistry Edition, Vol. 16,2675-2678 (1978) 0 1978 John Wiley & Sons, Inc. 0360-6376/78/0016-2675$01.00

2676 J. POLYM. SCI.: POLYM. CHEM. ED. VOL. 16 (1978)

100- I I I 100 I I I I

Time, min Time, min

Fig. 1. Effect of monomer concentration on the rate of polymerization of styrene by AcC104 and BF30Et2 in CHZClz at 0°C. [C]O: AcC104,0.20 aM, BF30Et2,3.0 mM.

Fig. 2. Molecular weight distribution of polystyrenes produced by AcC104 and BF30Et2 at various [MI0 in CH2Cl2 a t 0°C. [C],: AcC104,0.20 mM; BF30Et2,3.0 mM.

CH2C12. AcC104 gave polymers with a bimodal MWD at every [Mlo, as observed at [MI0 = 1.0 A4.l~~ The bimodal MWD changed systematically with [Mlo: with decreasing [MI0 the high polymer was formed with greater ease, whereas its molecular weight steadily decreased. However, the molecular weight of the low polymer was almost entirely independent of [M]o.

The polymers formed by BF30Et2, on the other hand, showed a unimodal MWD that shifted to the low-molecular-weight side as [MI0 decreased. This is the usual dependence on [MI0 of cationically prepared polystyrenes.

Figure 2 shows that the molecular weight of the high polymer formed by AcC104 and that formed by BF30Et2 similarly change with [Mlo. Plots based on the Schulz-Harborth equationI2 for the two systems were straight lines nearly through the origin; their slopes (k tr /kp) were 3 X M for AcC104 and 2 X 10-5M for BF30Et2, respectively. A similar plot for the high polymer produced by CF3S03H also gave a straight line through the origin3 and a kt,/kp value of 3 X 10-5M. This similarity in transfer constants suggests that one of the two propagating species that forms the high polymer is similar in nature to that derived from the metal halide.

Effect of the Dielectric Constant of the Reaction Medium Because a decrease in [MI0 in CHpCl2 [the reduction of a nonpolar component (styrene) in a polar

solvent] causes an increase in c (and this may be the simplest explanation for the strong effect of [MI0 on polymerizations by AcC104 and CF3S03H3), effects of t on styrene polymerization were studied with three protonic-acid initiators (CF~SOSH, CH$303H, and CF3C02H). These acids are similar in structure but differ in acidity (relative acidity based on the conductivity in anhydrous acetic acid13: CF3S03H, 427; CH3S03H, 17; CF~COZH, 1). The e of the reaction medium was varied by adding

NOTES 2677

E 3 4 6 8

5.0

Fig. 3. Effect of the dielectric constant (e) of the reaction medium on the rate constant k of the polymerization of styrene by three protonic acids at OOC. Solvent: benzene, benzene/CHzCla (l/l, 1/2, and 1/3; v/v), and CHzClz; [MI0 = 1.OM; [C]O (mM): CF3S03H, 0.20; CH3S03H, 30; CF~COZH, 200. H and L, see text.

benzene to CHzClz in various proportions ([MI0 = 1.OM). Figure 3 illustrates the effect of c on tb’e polymerization rate expressed by the constant k in eq. (1):

- d [M]/dt = k[C]o[M] (1)

where [C]O is the initial concentration of initiator. The three acids gave very different k values; for example, the value for CF3S03H was larger by about five orders of magnitude than that of

The MWD of produced polystyrenes was examined under the conditions in Figure 3. All three acids gave polymers with a bimodal MWD in CHzClz or its mixture with benzene, which indicated that two independent propagating species are commonly involved in polymerizations by these ini- tiators. The high polymer formed more readily in a polar solvent. The changes in MWD with e are shown schematically in Figure 3 (H, the high polymer, L, the low polymer). By contrast, a uni- modal MWD was always observed for polymers formed by BF30Etz.

Figure 3 shows that, although the absolute value of k depends on the acidity, k changes with t as it does with the three protonic acids; that is, the constant greatly increased with e in a narrow range o f t in which the bimodal MWD of polymers appeared. The polymerization rate for BF30Eh re- mained almost unchanged in the range of t in Figure 3. Comparison of Figure 3 with Figures 1 and 2 indicates that decreasing [MI0 in CHzCIp has the same effect as increasing t, and therefore the effect of [MI0 on polymerization by Ad2104 can be explained simply by a change in t accompanying a change in [M]o. An alternative explanation concerning the inactivation of initiator by styrene m o n ~ m e r ~ . ~ seems to be less likely, as we have already pointed The independence of the rate for BF30Etz on [MI0 (Fig. 1) also excludes the possibility that the propagating species are inactivated by monomer molecules.

Thus a clear difference was revealed between polymerizations by nonmetal halide (AcCl04 and protonic acids as CF3S03H) and metal halide (BF30Etz) initiators. This difference should reflect the nature of counterions derived from these two categories of initiators. The sensitivity to solvent polarity of the counterions derived from nonmetal halide initiators may result from their strong interaction with the propagating ends, which allows ester formation in an extreme case.6

CF3COzH.

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MITSUO SAWAMOTO TOSHIO MASUDA TOSHINOBU HIGASHIMURA

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

Received August 30,1977 Revised October 6,1977