Hydrogen Bonding-Driven Ring-Opening Polymerization of β-Lactones to Polyhydroxyalkanoates Using Aminocyclopropenium Carboxylates

Abstract

Although the chemical synthesis of polyhydroxyalkanoates by ring-opening polymerization (ROP) of four-membered β-lactones has attracted wide interest for sustainable materials, research on polyhydroxyalkanoates by organocatalysis remains rare and challenging. In particular, high activity of organic initiators/catalysts is always accompanied by acidic abstraction, resulting in low molecular weights and broad distribution. Herein, we report hydrogen bonding-driven ROP of β-butyrolactone and β-propiolactone by a series of aminocyclopropenium catalysts featuring tunable hydrogen bond donors for the preparation of poly(3-hydroxybutyrate) (P3HB) and poly(3-hydroxypropionate) (P3HP), respectively. The optimal catalyst achieved the desired reactivities and high molecular weights (up to 216.7 kg/mol for P3HP). Mechanism studies, ¹H NMR titrations, and in situ FTIR proved that hydrogen bonding prompted fast polymerization via activation of monomers. In propagation, hydrogen bonding between the N–H moiety and carboxylate stabilized active chain ends, limiting elimination/transesterification. The hydrogen bond donor aminocyclopropenium catalysts extend organocatalyzed polymerization of β-lactones to produce polyhydroxyalkanoates with high molecular weights.