Ring-ring equilibration (RRE) of cyclic poly(L-lactide)s by means of cyclic tin catalysts

With 2,2-dibutyl-2-stanna-1,3-dithiolane (DSTL) and 2-stanna-1,3-dioxa-4,5,6,7-dibenzoxepane (SnBiPh) as catalysts ring-expansion polymerizations (REP) were performed either in 2 M solution using three different solvents and two different temperatures or in bulk at 140 and 120 degrees C. A kinetically controlled rapid REP up to weight average molecular masses (M-w's) above 300 000 was followed by a slower degradation of the molecular masses at 140 degrees C, but not at 120 degrees C Furthermore, a low molecular mass cyclic poly(L-lactide) (cPLA) with a M-n around 16 000 was prepared by polymerization in dilute solution and used as starting material for ring-ring equilibration at 140 degrees C in 2 M solutions. Again, a decrease of the molecular mass was detectable, suggesting that the equilibrium M-n is below 5 000. The degradation of the molecular masses via RRE was surprisingly more effective in solid cyclic PLA than in solution, and a specific transesterification mechanism involving loops on the surface of crystallites is proposed. This degradation favored the formation of extended-ring crystallites, which were detectable by a "saw-tooth pattern" in their MALDI mass spectra.