Low molar mass cyclic poly(l-lactide)s: separate transesterification reactions of cycles and linear chains in the solid state

Abstract

L-Lactide (LA) was polymerized with neat tin(II) 2-ethylhexanoate (SnOct₂) in toluene at 115 °C at low concentration with variation of the LA/Cat ratio. Cyclic polylactides (cPLAs) with number average molecular weights (M_n) between 7000 and 17 000 were obtained. MALDI-TOF mass spectrometry also revealed the formation of a few percent of linear chains. Crystalline cPLAs with M_n around 9000 and 14 000 were annealed at 140 °C in the presence of ScOct₂ or dibutyl-2-stanna-1,3-dithiolane (DSTL). Simultaneously, crystallites of extended linear chains and crystallites of extended cycles were formed regardless of the catalyst, indicating that transesterification reaction proceeded different for linear chains and for cycles, governed by thermodynamic control. The formation of extended chain crystallites with low dispersity indicates the existence of symproportionation of short and long chains. A complementary experiment was carried out with a PLA ethyl ester composed mainly of linear chains with a small fraction of cycles.