Cationic polymerization of cyclic sulfides

The mechanism of the cationic polymerization of cyclic sulfides, initiated with triethyloxonium salts is reported. Differences due to changing the counter‐ion from BF to SbCl are discussed. Initiation consists of the alkylation of monomer forming a cyclic sulfonium ion which is the active species in the propagating step. A generally occurring termination reaction is the formation of non‐strained (linear or cyclic) sulfonium ions. In the case of epithiopropane (propylene sulfide) these can slowly re‐initiate the polymerization by an intramolecular reaction forming the 3‐membered cyclic sulfonium salt. This re‐initiation is not possible with the thietanes because of the greater difficulty to form 4‐membered cyclic sulfonium salts. When SbCl is the counter‐ion, an additional, more drastic termination can occur most probably by reaction of the growing chain with the counter‐ion, thereby forming an alkyl chloride and SbCl5. After the polymerization of propylene sulfide with BF as the counter‐ion, the polymer degrades to low molecular weight oligomers, predominantly the cyclic tetramer. This degradation is a back‐biting reaction occuring via sulfonium salts. With SbCl as the anion, the sulfonium salts are destroyed by the termination reaction and degradation does not occur. It was possible to follow the concentration of the growing species (4‐membered cyclic sulfonium salt) during the polymerization of 3,3‐dimethylthietane by means of 300 MHz NMR spectroscopy. By measuring the rate constants of propagation for different initiator concentrations or in the presence of different amounts of an indifferent electrolyte, it was possible to calculate separate rate constants for propagation via free ions (k ) and via ion‐pairs (k ). The ratio k /k was about 60 when BF was the anion and 35 when it was SbCl .