High T m linear poly(l-lactide)s prepared via alcohol-initiated ROPs of l-lactide.

Alcohol-initiated ROPs of l-lactide were performed in bulk at 160 °C for 72 h with variation of the catalyst or with variation of the initiator (aliphatic alcohols). Spontaneous crystallization was only observed when cyclic Sn(ii) compounds were used as a catalyst. Regardless of initiator, high melting crystallites with melting temperatures ( T m ) of 189-193 °C were obtained in almost all experiments with Sn(ii) 2,2'-dioxybiphenyl (SnBiph) as catalyst, even when the time was shortened to 24 h. These HT m poly(lactide)s represent the thermodynamically most stable form of poly(l-lactide). Regardless of the reaction conditions, such high melting crystallites were never obtained when Sn(ii) 2-ethylhexanoate (SnOct 2 ) was used as catalyst. SAXS measurements evidenced that formation of HT m poly(l-lactide) involves growth of the crystallite thickness, but chemical modification of the crystallite surface (smoothing) seems to be of greater importance. A hypothesis, why the "surface smoothing" is more effective for crystallites of linear chains than for crystallites composed of cycles is discussed.