Since its inception in 2019 with the publication of dibenzo[c,e]oxepine-5(7H)-thione (DOT), thiocarbonyl addition ring-opening (TARO) polymerization has gained much popularity. TARO polymerization is used to grant degradability to polymers by installing thioester linkages into their backbone via the radical ring-opening polymerization of thionolactones. TARO polymerization has been shown to be compatible with the three main methods of reversible-deactivation radical polymerization (RDRP); nitroxide mediated polymerization, atom transfer radical polymerization, and reversible addition-fragmentation chain transfer polymerization. Additionally, thionolactones made for TARO polymerization have been copolymerized with vinyl monomers of all types, ranging from unreactive vinyl monomers (such as methacrylates) to reactive vinyl monomers (such as vinyl acetate).

However, there is still scope for TARO polymerization to synthesize stimuli-responsive smart polymers. As such, the work performed herein details the conditions for the synthesis, polymerization, and degradation of a novel TARO monomer, benzo[5,6]oxepino[4,3-c]pyridine-5(7H)-thione (BOPT). BOPT contains a pyridine ring in its structure, making it pH-responsive and allowing it to change its solubility/reactivity upon protonation. BOPT was shown to be polymerizable with both more and less activated monomers and it has a wide range of solubility in greener solvents such as alcohols.

BOPT presents as stable orange crystals but isomerizes to the unreactive thiolactone upon heating. Upon protonation, “H-BOPT” is unstable, as the proton is extremely labile and it deprotonates to BOPT. As such, polymerizations for H-BOPT must be performed in an acidic medium.