Carbocationic polymerization: Mechanisms and kinetics of propagation reactions

Abstract

Propagation rate constants k p in carbocationic polymerizations can be obtained through two general methods. The first one, used for decades, calculated k p from the polymerization rates and from the ionic species concentrations (ISC) measured or estimated in various ways. The second one, used during the last 10 years, is based on the diffusion-clock (DC) assumption, in which competitive reactions between propagation with the monomer and termination with another nucleophile N permit to calculate k p if termination is a diffusion-controlled reaction (with e.g. k N= k diff=3×10 9 L mol −1 s −1 in CH 2Cl 2 solution). A problem arises since the k p obtained by this last method with, e.g. styrene and isobutylene are 10 4 to 10 5 times larger than those obtained earlier in solution by the ISC method, and the aim of this article is to try to explain this discrepancy. The different methods of measurement of the second-order rate constants of propagation k p + or k p ± , respectively, on unpaired ions and ion-pairs are examined in Sections 2 and 4 and compared in Section 3 with the rate constants of model reactions. The validity of the k p + and k p ± determinations by the two methods are compared ( Section 6), but results are unfortunately obtained only by the DC method for styrene, p-chlorostyrene and p-methylstyrene with k p ± ∼ 10 9 L mol − 1 s − 1 , and by the ISC method for most other monomers with k p ± between 10 4 and 10 5 L mol −1 s −1. It is shown that the large difference between these two sets of values as well as that between the parameters of ionization K i, k i and k −i of the terminal halides in living polymerizations ( Section 5) cannot be explained quantitatively by the large electrophilicity of the carbocation of these poly(styrene)s. Diffusion-controlled propagation for styrene is also in contradiction with reactivity ratios and rates of copolymerization with various monomers. The recent measurements of k p ± in living polymerizations of several monomers have confirmed the validity of the k p ± obtained earlier from non-living systems and based on the ionic species concentration. It is concluded that k p ± for styrene should be of a similar order of magnitude. In order to have a comprehensive view interpreting all experimental results, the hypothesis has been made of competitive termination (and possibly propagation) occurring as two-steps reactions, the first step being a complexation of the growing carbocation with the nucleophile, giving a resonance stabilized complex, and the second step a unimolecular rearrangement of the complex.