Single-Unit Comonomer Insertion Initiates Radical Polymerization of Thionolactone to Give Chemically and Thermally Degradable Polythioesters

Polythioesters are promising as sustainable materials with improved properties. But, despite the industrial importance of radical polymerization, polythioester homopolymers are not readily accessible through this method. Using dibenzo-[<i>c</i>,<i>e</i>]-oxepin-5-(7H)-thione (DOT) as a thionolactone monomer, this study shows that the attempted radical homopolymerization suffers from low (3-13%) conversion. Conversely, addition of diethyl vinylphosphonate (DEVP), a judiciously chosen comonomer, led to near-quantitative DOT conversion at low (<22%) DEVP incorporation. Thorough analysis of the products by ¹H-¹H COSY, ¹H-¹³C HMBC, and ¹H-³¹P HMBC NMR spectroscopy revealed that DEVP was inserted as single units and enabled the identification of DEVP repeat units sandwiched between two DOT groups (δ_P = 24.5 ppm) and those located at the α end group between an AIBN-derived fragment and a DOT repeat unit (δ_P = 23.7 ppm). The proportion of single DEVP repeat units at the α end group increased with a decreasing DEVP copolymerization feed, demonstrating the importance of the secondary DEVP-based radical in effectively initiating the polymerization of DOT. Polymers that carried, on average, less than one internal DEVP unit and consisted mostly of AIBN-DEVP-initiated DOT homopolymers were prepared by tuning the DEVP feed ratio. The polymers were fully degradable through aminolysis and by heating (in bulk) to 140 °C, with a higher DEVP content found to increase the formation of dibenzo-[<i>c</i>,<i>e</i>]-thiepin-5-(7H)-one (DTO) thiolactone product during thermal degradation.