Ring-expansion polymerization (REP) of l-lactide with cyclic tin(II) bisphenoxides

[Display omitted] •A new synthetic method for a convenient preparation of novel tin(II) catalysts.•Two of the new catalysts are extraordinarily efficient for REPs of l-lactide.•Yield colorless, optically pure, cyclic poly(l-lactide)s having Mw’s up to 250,000 g mol−1. Five new cyclic catalysts were prepared by a new synthetic method from tin(II)-2-ethyhexanoate and silylated catechols, silylated 2,2′dihydroxybiphenyl or silylated 1,1′-bisnaphthol. These catalysts were compared with regard to their usefulness as catalysts for the ring expansion polymerization (REP) of l-lactide in bulk at 160 °C, and with two different tin(IV) derivatives of 1,1′bisnaphthol. Best results were obtained using seven-membered cyclic tin(II)bisphenoxides, which yielded colorless cyclic poly(l-lactide)s free of racemization with weight average molecular weights (Mw) up to 305,000 g mol−1. Furthermore, these catalysts were active even at a lactide/catalyst ratio of 20,000/1. Our new results were superior to those obtained from all other previously published catalysts yielding cyclic poly(l-lactide). The seven-membered cycles also proved to be more active than tin(II) 2-ethylhexanoate with and without the addition of alcohol.