Abstract
The advancement of polymeric materials relies heavily on the innovation in polymerization reactions. In this study, we have discovered alternating copolymerization of isothiocyanate (ITC) and epoxide, which results in a nearly unexploited sulfur-containing polymer, polythioimidocarbonate (PTC). Provided with a simple two-component catalyst, i.e., a Lewis pair consisting of triethylborane (Et3B) and excess phosphazene base (PB), the copolymerization starts from an alcohol and proceeds in a strictly alternating and highly chemoselective manner, yielding PTC with controlled molar mass and low dispersity, free of cyclic byproducts and ether linkages. The method applies well to a variety of ITCs and epoxides. It is also found with great excitement that the reaction on ITC is fully inhibited when the catalyst composition is inverted to have Et3B in excess, while homopolymerization of epoxide occurs selectively in this case. Density functional theory (DFT) calculation reveals that Et3B-alkoxide complexation is the key to suppressing the back-biting reaction during the copolymerization ([Et3B] < [PB]) and inhibiting the copolymerization ([Et3B] > PB]). This unique "biased" feature is harnessed to develop a catalyst switch strategy for one-pot block copolymerization from the mixture of ITC and epoxide with either copolymerization or homopolymerization conducted first, resulting in tailor-made PTC-polyether block copolymers with reversible sequence structures. On the other hand, sequence-selective terpolymerization occurs from a mixture of phthalic anhydride, ITC, and epoxide, allowing the one-step synthesis of polyester-PTC block terpolymer. These results have highlighted the versatility of the method for exploring this uncharted area of polymers.