Bulk polymerization of (L,L)-lactide using non-organometallic triazolium carbene: limited advantage

The ring-opening polymerization (ROP) of L,L-lactide (L-LA) has been studied in bulk using alcohol (either methanol or benzyl alcohol) and 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene carbene, 1, respectively as initiator and non-organometallic catalyst, over a range of temperature and alcohol-to-1 molar ratios. Although 1 is known to perfectly control the polymerization of L-LA in solution at 90°C, its efficiency in bulk is drastically decreased since its thermal instability limits the possibility to reach high molecular weight poly(L-lactide) (PL-LA). Under optimum conditions, PL-LA chains with average molecular weight up to 10,000 g.mol-1 have been obtained within 15 minutes and interestingly characterized by very narrow polydispersity indices. Compared to PL-LA conventionally prepared with stannous octoate as catalyst, the thermal stability proved remarkably enhanced when the ROP was promoted by 1.