Abstract:

Ring-opening polymerization of cyclic sulfur compounds is one of the important methods to synthesize sulfur-containing polymer. Traditional anionic polymerization techniques, however, face challenges due to the high reactivity of sulfur-centered anions, which frequently results in chain transfer side reactions that yield broad molar mass distributions and structural variations. This has the effect of undermining the controllability and reproducibility of material properties. To overcome this issue, this study presents a synergistic catalytic system that combines triethylborane (Et₃B), the phosphazene base ^tBuP₁, and a thiol initiator to facilitate efficient and controlled anionic ring-opening polymerization of propylene sulfide. Experimental characterization showed that adding Et₃B completely blocked chain transfer reactions to monomers and prevented the formation of disulfide bonds, as evidenced by ¹H NMR spectroscopy and size exclusion chromatography (SEC), when the reaction occurred at 0 °C. Density functional theory (DFT) calculations further revealed that Et₃B stabilizes the active sulfur-centered anion intermediates through strong B-S coordination, significantly reducing their reactivity and allowing for precise control over polymerization. Building on this methodology, a “one-pot, two-step” strategy that merges anionic polymerization with reversible addition-fragmentation chain transfer (RAFT) polymerization has been established. An alternating copolymer of carbon disulfide and propylene sulfide was initially synthesized to serve as a macro-chain transfer agent, followed by the introduction of styrene to create a well-defined polystyrene-co-poly(carbon disulfide-alt-propylene sulfide) terpolymer. This work presents a facile strategy for the controlled synthesis of high-performance or functional sulfur-rich polymers.

Key words: propylene sulfide; triethylborane, anionic ring-opening polymerization, copolymerization