The mechanism of the BF3•Et2O-initiated CROP of DOT (Scheme 2) was presumed to involve the reaction of BF3 with (adventitious) water to generate a proton as the actual initiating species, followed by activation of the C=S double bond of DOT generating a cationic species stabilised by the aromatic ring and resonance with the thionoester oxygen (Scheme 2). Based on the other ring being able to stabilise a ringopened cation, the formation of a benzylic cation intermediate through irreversible ringopening (and concurrent thionolactone-thioester isomerization) was considered likely. After attack of the next DOT monomer, propagation via benzylic cations was presumed, until termination is expected to happen through (reversible) recombination with a hydroxide anion. Influence of polymerization temperature and monomer-to-initiator ratio. BF3•Et2O-initiated polymerization kinetics were examined at RT, 4 °C, and -10 °C in anhydrous toluene. Expectedly, the polymerization rates decreased and SECmeasured molar masses decreased with decreasing temperature, see Figure , entry 4). Dotted lines were added as a guide. All polymerization discussed above were done with an initial ratio [M]0:[I]0 of 50:1. Additional ratios of 25:1, 100:1, 200:1, and 1000:1 were trialled (Table , entries . The ratio had a strong effect on the resulting degree of polymerization with SECmeasured molar masses increasing with a decreasing initiator concentration. The series of polymers had molar masses ranging from 4.0 to 26 kg/mol with similar dispersities between 1.5 < Ð < 1.7, Figure . Notably, this range included polymers with a much greater length than the pDOT species prepared through radical polymerization (Mn = 4.9 kgmol -1 with Đ = 2.0). Interestingly, fewer to no side products were detected at lower initiator concentrations. Attempts at living cationic polymerization. We attempted to polymerize DOT through "living" cationic methods using cationic RAFT and through 1,2,3,4,5pentakis(carbomethoxy)cyclopentadiene (PCCP)-initiated/mediated polymerization. Two RAFT agents, S-benzyl-S'-propyl carbonotrithioate and S-benzyl-N,Ndiethyl dithiocarbamate were used. To the best of our knowledge neither had been used for cationic polymerizations, but were expected to be suitable for DOT based on the presumed formation of a benzylic cation, similar to the DOT monomer. BF3•Et2O and triflic acid were used as initiators (Table , entries . While polymer products were formed with poor to moderate conversions, the SEC-measured dispersities (1.5 < Ð < 2.4) were not lower than in polymerizations without RAFT agents. Indeed, the very high SEC-measured molar mass of 50.4 kg/mol for a polymerization of DOT (1000 eq) initiated by BF3•Et2O (1 eq) in the presence of S-benzyl-N,N-diethyl dithiocarbamate (20 eq) suggested no sharing of the cations between different chains. Unfortunately, attempted PCCP-initiation failed with no conversion of DOT to polymer observed (Table , entry 9). In an attempt to use PCCP as mediator but not initiator, a combination of PCCP and MeOTf was employed (Table , entry 25). While polymer was formed, its relatively high SEC-measured dispersity of Ð = 2.03 suggested no control.