High-Tg PLA copolymers via base-catalyzed transesterification of PLA with 2,5,7-trioxabicyclo[2.2.2]octan-6-one

A new bicyclic lactone, 2,5,7-trioxabicyclo[2.2.2]octan-6-one (TOB), was synthesized by direct condensation reaction of glycerol and glyoxylic acid. The ring-opening polymerization (ROP) of TOB and its copolymerization with lactide (LA) under different conditions were systematically investigated. Both acid and base could catalyze the ROP of TOB, but only oligomers were obtained in low yields. Three discrete oligomers (O1, O2, and O3) of TOB were separated and characterized, confirming that the high crystallinity and poor solubility of PTOB in the polymerization media caused the low monomer conversion and low molar mass of PTOB. Copolymerization of LA and TOB was investigated with benzyl alcohol as the initiator and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as the catalyst. It was confirmed that LA was consumed very quickly to form PLA, followed by slow incorporation of TOB into the formed PLA chains. By varying the feed ratios of LA to TOB, a series of random copolymers with the incorporation ratio of TOB up to 49 mol% were obtained. These copolymers exhibited high glass transition temperatures (Tg) up to 100 oC. The TBD-catalyzed copolymerization mechanism was revealed to be a base-catalyzed transesterification of PLA and TOB. Finally, PLA-co-PTOB with variable Tg could thus be simply prepared by direct transesterification of PLA with TOB in the presence of TBD.