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

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.