Polylactones 54: ring-opening and ring-expansion polymerizations of ϵ-caprolactone initiated by germanium alkoxides

Et3GeOMe-initiated polymerizations of ϵ-caprolactone were studied at six different temperatures, but even at 180 °C the conversion was below 50% after two days. Ge(OEt)4 proved to be more reactive and a temperature of 120 °C was found to allow for a nearly quantitative conversion after two days. All four ethoxy groups were active in the initiation process, but a broad molecular weight distribution was found and a low percentage (presumably <1 mol%) of cyclic oligomers was formed. Ring-expansion polymerization initiated with a spirocyclic germanium alkoxide was feasible at 140 °C and yielded a molecular weight distribution quite different from that obtained with Ge(OEt)4. The monomer/initiator (M/I) ratio was varied, and it was found that the average degree of polymerization (DP) did not agree with the M/I ratio for M/Iʹs above 200. Addition of 4-nitrobenzoyl chloride to the hot virgin reaction mixtures yielded poly(ϵ-CL) chains having functional endgroups but the degree of functionalization never exceeded 80%.