Ring‐opening polymerizations of L‐Lactide catalyzed by zinc caprylate: Syntheses of cyclic and linear poly(L‐lactide)s

Abstract Alcohol‐initiated ring‐opening polymerizations (ROPs) of L‐Lactide (LA) were conducted in bulk at 130 °C catalyzed by Zn acetate (ZnAc 2 ), lactate (ZnLac 2 ), or caprylate (ZnCap 2 ). 11‐undecenol (UND), 1‐hydroxymethylnaphtalene (HMN), and 4‐nitrobenzylalcohol (4NB) were used as alcohols. Whereas variation of the alcohols had little effect, the usefulness of the catalysts increased in the order: acetate < lactate < caprylate. Hence, further alcohol‐initiated polymerizations were performed with ZnCap 2 alone and with variation of the lactide/catalyst (LA/Cat) ratio. With increasing LA/Cat ratio higher fractions of cyclic poly(L‐lactide) (PLA) were found, so that the measured degree of polymerization (DP) is considerably lower than the theoretical value (i.e., 2 x LA/alcohol). With neat ZnCap 2 cyclic PLAs were the largely prevailing reaction products. For these cyclic PLAs weight average molar masses (M w ) up to 134,000 were obtained and an optical purity around 99% was indicated by 13 C NMR spectroscopy and DSC measurements even after 48 h at 150 or 160 °C.