Studies on the propagation reaction in the free radical polymerization of ethyl α-hydroxymethacrylate

The free radical propagation kinetics of ethyl α-hydroxymethacrylate have been investigated using pulsed-laser polymerization experiments. Experimental conditions were not optimised for successful experiments in bulk monomer, where transfer caused complications in the interpretation of the data. However, in solution and at low temperatures (below 45°C), successful experimental conditions were identified and an extensive data set of propagation rate constants is reported. A remarkable dependence of the propagation rate coefficient, k_p, on the solvent was observed. The variation in k_p is exemplified by the range obtained at 15°C where k_p was found to fall between 580 (THF) and 1860 (xylene) L/mol/s. The Arrhenius parameters for the propagation reaction were obtained in three solvents (tetrahydrofuran, toluene and ethanol) at three different dilutions. The samples with the highest concentration of EHMA yielded values for the activation energy of 15-16 kJ/mol and pre-exponential factors 5-10 times lower than those previously reported for ethyl methacrylate. Both Arrhenius parameters were strongly dependent on solvent type and concentration. A hypothesis for these results is tendered - suggesting direct interactions in the transition state between the monomer and the propagating radical. These specific interactions are ameliorated to a greater or lesser extent by the inclusion of different solvents.