High molar mass cyclic poly(l-lactide) via ring-expansion polymerization with cyclic dibutyltin bisphenoxides

Abstract

Two new catalysts (SnNa and SnBi) were prepared from dibutyltin oxide and 2,2′-dihydroxybiphenyl or 2,2′dihydroxy(1,1′-binaphtyl). These catalysts enabled rapid polymerizations of l-lactide at 160 or 180 °C in bulk, whereby almost exclusively cyclic polylactides were formed. These polymerizations were free of racemization and yielded pol(l-lactide)s having weight average molecular weights (Mw's) up to 140 000 g mol−1. The Mw's varied little with the Lac/Cat ratio as expected for a ring expansion polymerization (REP). Polymerizations performed in bulk at 140, 120 and 102 °C yielded cyclic polylactides with lower molecular weights. At 102 °C a strong predominance of even-numbered cycles was found with SnNa as catalyst. SnNa can also catalyze alcohol-initiated ROPs yielding linear poly(l-lactide) free of cyclics.