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Emulsion polymerization of allyl sulfide copolymers for enhanced molar mass
Summary
Researchers used emulsion polymerization to synthesize polystyrene copolymers incorporating cleavable carbon-sulfur backbone bonds, achieving molar masses nearly five times higher than bulk free radical methods — demonstrating a viable route to high-performance, chemically recyclable vinyl plastics that degrade on demand when treated with allyl dithiol.
Existing recycling approaches for vinyl-derived polymers, which constitute approximately 50% of global plastic production, rely heavily on energy-intensive incineration and pyrolysis, whereas laboratory-scale chemical recycling is material specific. To enable more universal chemical recyclability in vinyl polymers, ring-opening copolymerization with the cyclic comonomer 3,7-bis(methylene)-1,5-dithiacyclooctane (BMDTO) is an emerging solution that involves the integration of cleavable C–S bonds in the backbone. However, bulk free radical polymerization (FRP) with BMDTO limits the molar mass because of undesired radical transfer, prohibiting access to the desired mechanical properties. In this study, we employ emulsion polymerization to synthesize PS-BMDTO (PSB) copolymers with 1–7 mol% BMDTO. Compared with bulk FRP, size exclusion chromatography reveals that emulsion polymerization yields a greater molar mass (81 kg mol−1 vs. 17 kg mol−1) at 2.4 mol% BMDTO. The incorporation of BMDTO is observed by 1H nuclear magnetic resonance spectroscopy and further confirmed by a chain scission reaction with allyl dithiol. The molar mass of 3.9 mol% PSB decreases from 33 kg mol−1 to 10 kg mol−1, while the molar mass of PS without BMDTO remains unchanged. These results reveal that emulsion polymerization effectively prevents undesired radical transfer and extends radical lifetime, offering a suitable synthesis route for high-performance, recyclable vinyl copolymers. Ring-opening copolymerization using the cyclic comonomer 3,7-bis(methylene)-1,5-dithiacyclooctane (BMDTO) introduces cleavable C–S bonds for chemical recyclability; however, it suffers from low molar mass via bulk free radical polymerization due to undesired radical transfer. To this end, this study leverages emulsion polymerization to produce polystyrene-BMDTO copolymers (1–7 mol% BMDTO), achieving higher molar mass (81 vs. 17 kg/mol) at 2.4 mol% BMDTO. Nuclear magnetic resonance spectroscopy and size exclusion chromatography confirm the incorporation and cleavability of BMDTO, establishing emulsion polymerization as a viable method for producing recyclable, high-performance vinyl copolymers.