Propagation Rate of the Cationic Polymerization of 2,4,6-Trimethylstyrene: A Linear Free Energy Approach

Kinetics of the reactions of benzhydryl cations with 2,4,6-trimethylstyrene have been performed to determine its nucleophilicity parameters N = 0.68 and s = 1.09, which are comparable to styrene and considerably less nucleophilic than 4-methylstyrene, indicating the retarding effect of the ortho methyl groups. Kinetic investigations of the reactions of the 2,4,6-trimethylstyryl cation 6 + and the corresponding dimer 7 + with the reference nucleophile 2-chloropropene allow us to calculate the closely similar electrophilicity parameters E ( 6 + ) = 6.04 and E ( 7 + ) = 6.16 which are considerably lower than that of the styryl cation ( E = 9.6). As a consequence, the propagation rate constant of the cationic polymerization of 2,4,6-trimethylstyrene is considerably lower than that of styrene. Substituting these values into the correlation log k 20 ° C = s ( N + E ) yields a propagation rate constant of 3 × 10 7 M -1 s -1, which has to be considered an upper limit because extra steric strain generated in the reaction of 7 + with 2,4,6-trimethylstyrene is not included in this correlation.