Polylactones, 37. Polymerizations of L‐lactide initiated with Zn(II) L‐lactate and other resorbable Zn salts

Zn(II) L ‐lactate (ZnLac 2 ) was prepared either from ZnO and ethyl L ‐lactate or with slightly higher optical purity from ZnO and L ‐lactide. Using water‐free ZnLac 2 L ‐lactide was polymerized in bulk at 120°C or 150°C. Higher yields and higher molecular weights were found at 150°C. The highest number average molecular weights ( M̄ n around 70 000) were obtained at monomer/initiator (M/I) ratios of 4000. Despite the high reaction temperature the isolated poly( L ‐lactide)s were 100% optically pure. Analogous polymerizations were also conducted with Zn(II) L ‐mandelate or Zn(II) stearate with inferior results. Poor yields and molecular weights were found, when zinc glycolate salt was used as catalyst. Furthermore, numerous polymerizations were conducted with ZnCl 2 , ZnBr 2 or ZnI 2 as initiators. Again poly( L ‐lactide)s with 100% optical purity were isolated, but most molecular weights were lower and never higher than those obtained with ZnLac 2 . Therefore, ZnLac 2 proved to be the most favorable and fully resorbable (biocompatible) initiator of this study. Finally, the combination of ZnLac 2 with a primary alcohol, which plays the role of a coinitiator, allows a broad variation of the molecular weight and the introduction and modification of an ester endgroup. This approach also allows the incorporation of bioactive alcohols such as α‐tocopherol, stigmasterol or testosteron in the form of covalently bound ester endgroups.