Bismuth halide‐catalyzed polymerizations of ε‐caprolactone

In contrast to BiF 3 , the other three Bi‐halides catalyzed the ring‐opening polymerization of ε‐caprolactone (ε‐CL) in bulk. A temperature of 140 °C was found to be advantageous for rapid polymerization and optimum molecular weights. At this temperature, the reactivity of the catalysts increases in the order BiCl 3 < BiBr 3 < BiJ 3 . Variation of the monomer‐catalyst ratio (M/C) yielded number‐average molecular weights ( M n s) up to 80,000 Da (corrected SEC data, 120,000 Da uncorrected), but a proper control of the M n s was not achieved. In addition to CH 2 OH endgroups, CH 2 Cl, CH 2 Br, and CH 2 J endgroups were detected, but no evidence for a cationic polymerization mechanism was found.