Precise Microstructure Control in Poly(hydroxybutyrate-co-lactic Acid) Copolymers Prepared by an Yttrium Amine Bis(phenolate) Complex

A diamino-bis(phenolate) yttrium amide complex, 1, was used as an initiator in the ring-opening copolymerization of rac-lactide (rac-LA) and rac-β-butyrolactone (rac-β-BL) in toluene at ambient temperature. Copolymers were prepared across a wide composition range (10–89% LA content), and the copolymer composition closely matched the monomer feed ratio. The copolymers had Tg values ranging from 3.8 °C (for a copolymer with 10% LA content) to 44.0 °C (for a copolymer with 89% LA content). Tm ranged from 132 to 144 °C for copolymers with LA composition 48–10%. The copolymer microstructure was determined using 1H and 13C{1H} NMR spectroscopy of copolymers and through reaction monitoring by NMR spectroscopy. This revealed that no BB diads are formed in the copolymer in the presence of any rac-LA monomer, even at very high concentrations of rac-β-BL. However, when [LA] = 0, rac-β-BL is rapidly polymerized. The copolymers thus have a microstructure consisting of two distinct regions, one containing both LA and BL units in an even distribution and one containing solely BL or LA units, depending on the monomer feed ratio.