Abstract
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[c,e]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 H-1-H-1 COSY, H-1-C-13 HMBC, and H-1-P-31 HMBC NMR spectroscopy revealed that DEVP was inserted as single units and enabled the identification of DEVP repeat units sandwiched between two DOT groups (delta(P) = 24.5 ppm) and those located at the alpha end group between an AIBN-derived fragment and a DOT repeat unit (delta(P) = 23.7 ppm). The proportion of single DEVP repeat units at the alpha 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 degrees C, with a higher DEVP content found to increase the formation of dibenzo[c,e]thiepin-5(7H)-one (DTO) thiolactone product during thermal degradation.