Title：Simple and Precision Approach to Polythioimidocarbonates and Hybrid Block Copolymer Derivatives

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.