Mechanism aspects of th e ring‐opening polymerization of the episulfides compared to epoxides

The cis ‐ and trans ‐2‐butene episulfides polymerize with cationic catalyts differently than reported for the corresponding oxides. Where the cis ‐oxide gave amorphous disyndiotactic polymer, the cis ‐sulfide gives crystalline racemic diisotactic polymer since this polymer could be asymmetrically synthesized in optically active form. Also the same crystalline polymer was obtained with coordination catalysts. Where the trans ‐oxide gave only crystalline, meso ‐diisotactic polymer, the trans ‐sulfide gives mainly amorphous polymer which, in one case, did slowly crystallize. The difference between the trans forms appears due to the longer CS bond which lowers steric hindrance and thus isomer selection in the attack of episulfide on the growing sulfonium ion to give less steroregular polymer. The difference in the cis forms may result from the sulfur atom in the last chain unit coordinating with the counterion. The greater hindrance around oxygen in the comparable oxide polymers may prevent the same mechanism from being utilized. The cationic polymerization of isobutylene sulfide gives both crystalline and amorphous polymer. NMR evidence indicates that the amorphous polymer results from substantial head‐to‐head, tail‐to‐tail polymerization, along with the expected head‐to‐tail polymerization. The same phenomenon occurs, but to a lesser extent, in cationic isobutylene oxide polymerizations. The preparation and properties of high molecular weight, head‐to‐tail isobutylene oxide and sulfide polymers from R 2 Mg‐NH 3 coordination catalysis are described.