The copolymerization of SO2 with propylene oxide mediated by organic ammonium salts: a comprehensive study of the main-chain structure, living polymerization character and regioselectivity

The utilization of sulfur dioxide (SO2) for constructing sulfur-containing polymers is of much significance in terms of both environmental issues and obtaining high-value materials. The copolymerization of SO2 with epoxides is deemed to be an efficient way to meet this desire. However, several problems, including issues involving mixed polymer segments (polysulfite versus polyether), cyclic by-products, uncontrolled molecular weights, and undetermined stereochemistry, have largely limited the development of this reaction. This study investigates these problems via studying the organic ammonium salt catalyzed copolymerization of SO2 with propylene oxide (PO). Main-chain analysis via nuclear magnetic resonance (NMR) spectroscopy, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), and electrospray ionization-mass spectrometry (ESI-MS) proved that the copolymer was an alternating copolymer. The co-present polyether segments had a cyclic structure, which was the result of the cationic homopolymerization of PO. The linear increase in molecular weight with PO conversion and chain extension experiments indicated the living polymerization character of this copolymerization. Furthermore, the low enantiomeric excess (ee) of the copolymer hydrolysis products, derived from the copolymerization of SO2 with optically pure PO, suggested the poor regioselectivity of copolymerization, and this was further proved via density functional theory (DFT) calculations. Additionally, the cyclic sulfite by-products turned out to be generated via alkoxide back-biting at the sulfite units in the polymer chains.