Living polymerizations of propylene sulfide initiated with potassium xanthates characterized by unprecedentedly high propagation rates

Abstract

In this paper we describe the original thiol-free approach towards the polymerization of propylene sulfide (PS) under various conditions (bulk, solution, and emulsion) initiated with potassium xanthates. The most impressive results were obtained for the reactions initiated with potassium O-methyl xanthate in complex with 18-crown-6 ether (PMX[18C6]). It was demonstrated that using this complex one can steadily prepare polymers with high M_n values (up to 250k Da) and relatively narrow molecular weight distributions (1.1–1.4) in a matter of minutes at 0 °C. Considering the obtained molecular weight distributions the striking observation was the absence of even a hint of linear dependence between the experimental M_n values and the initial [M]/[I] ratios for the processes carried out in both homogeneous solutions and emulsions. Quantum-chemical calculations revealed that the energetic barrier for the propagation reaction associated with the nucleophilic attack of “naked” thiolate (K⁺ in complex with 18-crown-6 ether) on the PS molecule was almost twice as low in comparison with that for the initial attack of PMX[18C6] on the first PS molecule (initiation step). It was therefore deduced that these notable differences lead to the tremendous boost of the rate of chain propagation to the extent that polymerization practically tends to complete prior to the exhaustion of the residual initiator.