Rate Constant of Initiation Reaction in Cationic Polymerization of Vinyl Monomers. I. Polymerization of Styrene Derivatives Catalyzed by Triphenylmethyl Stannic Pentachloride

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Abstract
To investigate the method of measurement of the initiation rate constant in cationic polymerization, styrene and α-methylstyrene are polymerized by triphenylmethyl stannic pentachloride (Ph3CSnCl5). Adding these monomers to a solution of Ph3CSnCl5, the strong absorption of triphenylmethyl cation at 400 to 450 mμ disappears. The rate of disappearance of the absorption at 430 mμ is proportional to the concentration of Ph3CSnCl5 and monomer. It is confirmed that this disappearance of the absorption is due to the conversion of triphenyl cation to styryl or α-methylstyryl cation. Therefore, the rate of consumption of triphenylmethyl cation corresponds to that of the addition of triphenylmethyl cation to the olefinic double bond in a monomer, that is, the rate of the initiation reaction. Considering the dissociation constant of Ph3CSnCl5, the initiation rate constants of styrene and α-methylstyrene in ethylene chloride solution at 30°C are 11.6 × 10−2 and 2.85 liters/mole-min, respectively. These values seem to be much smaller than the propagation rate constant of each monomer. However, the effect of polymerization conditions, for example, the kind of a monomer and the polarity of a solvent, on the initiation rate constant is the same as in the propagation reaction. This fact suggests the similarity of a reaction mechanism in both elementary reactions.

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