Catalyzed Chemical Synthesis of Unnatural Aromatic Polyhydroxyalkanoate and Aromatic–Aliphatic PHAs with Record-High Glass-Transition and Decomposition Temperatures

Abstract

The catalyzed chemical synthesis of polyhydroxyalkanoates (PHAs) via stereoselective ring-opening polymerization (ROP) of eight-membered cyclic diolides (8DLR, R denotes the two substituents on the ring) has shown its ability to synthesize a variety of stereoregular aliphatic PHAs, but its utility for the synthesis of aromatic PHAs has not yet been demonstrated. Here, we report that the controlled ROP of meso-8DLBn (Bn = benzyl)—catalyzed by metal-based complexes supported by C2-Salen ligands—affords syndiotactic ([rr] = 92%) poly(3-hydroxy-4-phenylbutyrate) (st-P3H4PhB) with a high molar mass (Mn up to 147 kg mol–1) and the highest glass-transition temperature (43 °C) reported in the PHA family, whereas the ROP of rac-8DLBn leads to essentially pure isotactic ([mm] > 99%) it-P3H4PhB. With judicious selections of catalysts, monomers, and procedures, copolymerizations of meso-8DLBn with rac-8DLR (R = Me, nBu) produces aromatic–aliphatic random, stereotapered, or crystalline stereo-diblock PHA copolymers. In particular, the copolymer of meso-8DLBn with rac-8DLBu—P3H4PhB-co-P3HHp (Mn = 205 kg mol–1)—is a strong, hard, but ductile (∼191% elongation at break) material, displaying perhaps the highest decomposition temperature (281 °C) reported for PHAs to date.