Characterization of the active centers in the cationic polymerization of p‐substituted α‐methylstyrenes

Polymer tacticity, molecular weight and molecular weight distribution were used as the basis for characterizing the type of active centers present in the cationic polymerization α‐methylstyrene monomers. The affect of reaction solvent polarity and the resonance effects of para‐substituents were used to deduce the nature of, and changes in, the ion pair character of the carbocation active center. For this purpose, the polymer yield, tacticity and molecular weight parameters were determined in the cationic polymerization reactions of p‐chloro and p‐fluoro‐α‐methylstyrene. The dielectric constant of the reaction medium was varied by using mixtures of methylene chloride and either hexane, a non‐solvent for the polymers, or toluene, a good solvent for the polymers. For both solvent systems, the yields and molecular weights of the polymers from both of the monomers were strongly effected by reaction time and solvent composition. Also, for both monomers, tacticity did not change with reaction time, and for the p‐fluoro monomer tacticity was constant with changes in reaction solvent composition in both mixed solvent systems. In contrast, the tacticities of the polymers from the p‐chloro monomer decreased with increasing amounts of the non‐polar solvent in the mixture. The tacticity effects induced by reaction solvent changes are interpreted on the basis of the electronic effects of the para‐substituents on changing the ion pair character of the carbocation active center. The effect of solvent composition on polymer yield and molecular weight may be a function of changes in the rates of initiation, propagation and monomer transfer, with the latter two also determined by the ion pair character of the carbocation active end group.