Cationic polymerization of cyclic sulfides, 5 Polymerization of propylene sulfide with triethyloxonium tetrafluoroborate

Time‐conversion curves show that the polymerization of propylene sulfide ( 1 ) by triethyloxonium tetrafluoroborate (TEFB) in methylene chloride consists of an initial very fast, non‐stationary stage A and a much slower subsequent stage B which leads to quantitative conversion. The termination reaction of stage A is of first order with respect to initiator concentration and is independent of monomer concentration. The proposed mechanism for this termination is the formation of a (mainly) 12‐membered cyclic sulfonium salt at the end of the polymer chain. The ratio of the rate constant of propagation to the rate constant of termination at 0°C in CH 2 Cl 2 is 9 dm 3 mol −1 . From the kinetics, it is concluded that stage B is initiated by slow formation of a three‐membered cyclic sulfonium salt by an intramolecular nucleophilic attack of the last sulfide function of the polymer chain on the α‐carbon next to the sulfonium salt. Further evidence for this kind of re‐initiation is obtained from the observation that simple trialkylsulfonium salts are very poor initiators or completely incapable of initiating the polymerization, whereas under identical conditions 2‐thia‐alkylsulfonium salts give polymerizations without stage A which are similar to the stage B of the TEFB‐initiated reactions. A backbiting reaction, similar to the termination reaction, is responsible for the degradation of polymer (mainly) into cyclic tetramer. The molecular weights of the polymers, measured by gel permeation chromatography, are in reasonable agreement with the proposed mechanism.