Synthesis and characterization of polyisobutylene-polycaprolactone block copolymers

The preparation and characterization of a polyisobutylene-b-poly(ε-caprolactone) (PIB-b-PCL) diblock copolymer is reported. Terminally unsaturated PIB (> 85%) was hydroxy functionalized by hydroboration followed by oxidation to give hydroxyl terminated PIB(PIB-OH). The reaction of PIB-OH with ε-caprolactone, using stannous octoate as catalyst, gave the resulting PIB-b-PCL copolymer. ¹H and ¹³C nuclear magnetic resonance (n.m.r.) spectroscopy was used to characterize the copolymers as well as determine their composition. In each case, the degree of polymerization (DP) of poly(ε-caprolactone) as well as the PIB/PCL ratio are reported. It was found that while copolymerization data essentially agree with the theoretical values based on PIB and ε-caprolactone stoichiometry, the amount of PIB in the diblocks deviates from the expected amounts with increasing caprolactone content. ¹H n.m.r. also shows that the reactivity of PIB-OH with the ε-caprolactone monomer is restricted to only those polymer chains possessing a primary hydroxyl chain end since secondary hydroxyl functionality does not initiate lactone polymerization. Model studies involving the ring opening of ε-caprolactone by primary and secondary alcohols also support this finding.