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

Two new catalysts (SnNa and SnBi) were prepared from dibutyltin oxide and 2,2'-dihydroxybiphenyl or 2,2'dihydroxy(1,1'-binaphty1). These catalysts enabled rapid polymerizations of L-lactide at 160 or 180 degrees 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 degrees C yielded cyclic polylactides with lower molecular weights. At 102 degrees 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.