Cationic and free-radical polymerization of optically active 1-olefins

The AlCl₃-initiated cationic polymerization of optically active 1-olefins yields polymers of varying optical rotatory power. Polymers of (+)-3-methyl-1-pentene and (?)-4-methyl-1-hexene prepared between ?78 and ?55°C. in CH₂Cl₂ or n-heptane are almost completely optically inactive. Under identical reaction conditions (+)-5-methyl-1-heptene gives polymers of significant optical rotatory power. Alternating SO₂copolymers of the same olefins, formed in reactions which proceed through free-radical intermediates, yield optically active products with specific rotations similar to those of low molecular weight analogs. These results are consistent with a cationic polymerization mechanism in which the growing chain undergoes intramolecular hydride shift and the asymmetric carbon atoms are converted into carbonium ions. The data also provide evidence for the lack of rearrangement in free-radical polymerization. By comparing the specific rotations of the cationic and free-radical polymers, the extent of rearrangement during cationic polymerization can be estimated. The calculations show that the 1,2-polymer in cationic poly-3-methyl-1-pentene is less than 2%, the sum of 1,2- and 1,3-polymer in cationic poly-4-methyl-1-hexene is less than 4%, and the sum of 1,2-, 1,3-, and 1,4-polymer in cationic poly-5-methyl-1-heptene is 14–20%.