Perfectly Alternating Copolymerization of CO and Epoxides to Aliphatic Polyester Oligomers via Cooperative Organoboron–Cobalt Complexes

Abstract

The carbonylative polymerization of epoxides provides a promising but challenging strategy to synthesize polyhydroxyalkanoates (PHAs) which are of high commercial value in the field of biomedical materials and engineering plastics. Herein, a suite of well-defined bifunctional organoboron−cobalt catalysts, featuring simple preparation, high yields, and low metal content, are exploited for the carbonylative polymerization of epoxides to PHA oligomers. The organoboron−cobalt-mediated carbonylative polymerization exhibits high polyester selectivity (over 95%, four epoxide examples), and the obtained PHAs have more than 99% ester linkages on the polymer main chain. Detailed investigations based on in situ reactIR and 1 H NMR analyses first found the concurrence of the direct alternating copolymerization of epoxides/CO, the ring expansion carbonylation of the epoxides to afford β-lactones, and the in situ homopolymerization of β-lactones under a single catalyst. This rare discovery provides a fundamentally different mechanism in the preparation of PHAs from CO/epoxides, extending the arsenal of catalytic systems that can copolymerize CO with epoxides.

Carbonylative Polymerization of Epoxides Mediated by Tri‐metallic Complexes: A Dual Catalysis Strategy for Synthesis of Biodegradable Polyhydroxyalkanoates

“…Epoxide-ring opening occurred at methylene or methine positions, leading to the formation of two possible regioisomers, while only one that resulted from nucleophilic attack at the methylene site could undergo ring closure to form β-lactones. We envisage that both of the resting species isomers could undergo intra-and/or intermolecular chain shuttling via alkoxide species (D), [15,21] producing metal carboxylate dimers (E) and regenerating [Lewis acid] + [Co(CO) 4 ] À (M). The dimers (E) should undergo chain propagation to afford polyester P2 with a regioirregular structure and lower molecular weight (Figure 3, cycle 2 and Scheme 3, route I).…”

Self-Switchable Polymerization: A Smart Approach to Sequence-Controlled Degradable Copolymers

“…Because isocyanates are a type of extremely reactive monomers, they may play a dominating role in the self-switchable polymerizations. Explorations are required for the amenable cyclic monomers with isocyanates in the future. Carbon monoxide (CO) was reported to undergo alternating copolymerization with epoxides to produce polyesters. − Although there is no precedent of using CO in self-switchable polymerizations, the much higher nucleophilicity of CO over CO 2 may render this combination as an efficient switching agent. In recent years, there has been a surge of interests in developing sulfur-rich cyclic monomers, many of which realized efficient modulation between polymerization and depolymerization. To name a few, the monomers include thionolactones, , S -carboxyanhydride, episulfides/thioanhydrides, or isothiocyanates, etc.…”

Modular Organoboron Catalysts Enable Transformations with Unprecedented Reactivity

“…Although organoboron catalysis has made marked progress, catalysts with higher efficacy and wider application range are still needed . We believe that a large pool of catalysts that exhibit diverse built-in reactivity will become accessible as their synthesis involves only the simple joining of amines, halohydrocarbons, and boranes; as a bonus, these components are generally commercially available on a large scale.…”