Different initiators and solvents. Our research group previously reported the radical homopolymerization of the thionolactone dibenzo[c,e]oxepine-5(7H)-thione (DOT) and found significant retardation associated with the process. After 27 days of heating with several sequential additions of the radical initiator AIBN, only 22% monomer conversion to pDOT was achieved. Herein, the cationic ring-opening homopolymerization of DOT was explored, Scheme 1. Gratifyingly, DOT reacted readily upon addition of several cationic initiators leading to the disappearance of its bright yellow colour and rapid formation of pDOT homopolymer, as observed by 1 H NMR spectroscopy, Figure . Of six initiators tested (BF3•Et2O, MeOTf, MeOTs, TfOH, SnCl4, and PCCP) effective polymerization was observed with four (BF3•Et2O, MeOTf, TfOH, and SnCl4). Notably, polymerizations occurred at ambient conditions at RT, without the use of a glovebox or anhydrous solvents and led to nearly 100% conversion of the monomer (Table , entries . The isolated polymer, pDOT, presented as a white solid. 1 H NMR analysis gave the same signals as the polymer obtained through radical ring-opening polymerization, see Figure . 13 C, 1 H- 13 C HSQC and 1 H- 13 C HMBC NMR spectra (Figure -S5) and FT-IR analysis (characteristic band at ν = 1656 cm -1 assigned to SC=O stretch, Figure ) of a sample made using BF3•Et2O as initiator confirmed the expected structure of the polymer and S-O isomerization. ) and were likely due to back-biting, a common phenomenon found in cationic polymerizations . The amount of formed DOO was typically around 0-5 mol-% and was assumed to be caused by attack of water onto a cationically activated DOT monomer, although, surprisingly, our results showed little correlation between the observed amount of DOO formation and the use of anhydrous solvents. Gratifyingly, the use of BF3•Et2O with (non-)anhydrous DCM or toluene reproducibly led to no or very low (1-2 mol-%) formation of side products. DOO and DTO, as well as the S,S-isomer dibenzo[c,e]thiepin-5(7H)-thione (DTT) were synthesised separately and subjected to react with BF3•Et2O under similar conditions. No conversion was found for any isomer, indicating the importance of the thionolactone-thioester S-O isomerization in driving the ring-opening and that the formation of the side products, while lowering the yield, did not lead to the formation copolymers, see Scheme 1.