Cationic Polymerization of Cyclic Sulfides

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Abstract
The mechanism of the cationic polymerization of several thietanes and of propylene sulfide under the influence of triethyloxonium tetrafluoroborate in methylene chloride is described. The thietane polymerizations stop at limited conversions because of a termination reaction occurring between the reactive chain ends (cyclic sulfonium salts) and the sulfur atoms of the polymer chain. The maximum conversions obtained under identical conditions differ markedly for the different monomers. Ratios of rate constants of propagation (kp) to rate constants of termination (kt) have been calculated. The differences in k p/kt. values for the different monomers are explained in terms of differences in basicity and differences in steric hindrance of the monomers compared to the corresponding polymers. In the case of propylene sulfide it is proposed that the main termination reaction is the formation of 12-membered ring sulfonium salts by an intramolecular reaction of the third sulfur of the growing polymer chain with the reactive chain end (three-membered ring sulfonium salt). This terminated polymer is able to reinitiate the polymerization, for example, by reaction of a monomer molecule at the exocyclic carbon atom of the sulfonium salt function. The cyclic tetramer of propylene sulfide is formed in this reaction. After complete polymerization, formation of cyclic tetramer continues, probably via a backbiting mechanism. In methylene chloride as solvent, the absolute value of the rate constant of propagation for 3,3-dimethylthietane changes with changing concentration of initiator and by adding different amounts of indifferent electrolyte to the reaction mixture. From these changes, and assuming that the value of the dissociation constant of the growing chain-ends is close to values of dissociation constants of low molecular weight sulfonium salts, separate rate constants for propagation via free ions and ion-pairs were calculated. The propagation constant of free ions is about 70 times higher than that of ion pairs in methylene chloride at 20°C. Free ions and ion pairs are nearly equally reactive in nitrobenzene.