Ring-opening alternating copolymerization of epichlorohydrin and cyclic anhydrides using single- and two-component metal-free catalysts

摘要

Epichlorohydrin (ECH), a chloromethyl-carrying epoxide, is subject to alcohol-initiated ring-opening copolymerization with cyclic anhydrides using a mild phosphazene base (t-BuP1) or a Lewis pair comprising t-BuP1 and triethylborane as an organic/metal-free catalyst. The products exhibit controlled molar mass and strictly alternating sequence distribution without ether linkages though ECH is added in excess. The single-component organobase catalyst favors elevated reaction temperature (60 degrees C) and leads to broad molar mass distribution, especially at a relatively high catalyst loading, due to the nucleophilic substitution (NS) of the ECH chloromethyl groups by the chain-end carboxylate. This side reaction suspends the propagation of some chains and generates chloride anions that can reinitiate the copolymerization, thus leads to low-molar-mass byproducts. Meanwhile, the epoxy-ended copolymers may undergo chain-end ring opening to form high-molar-mass byproducts. On the other hand, the two-component catalyst allows for a wider range of reaction temperature (25-60 degrees C) and higher copolymerization rate. NS can be significantly delayed at room temperature with optimized catalyst loading and composition, resulting in narrowly distributed molar mass, ranging from 2 to 30 kg mol(-1), at high or complete anhydride conversion. Interestingly, NS still occurs at the end of the copolymerization so that pendent- and end-group bifunctionalized polyester can be achieved in one synthetic step.