Solvent effects on TEMPO-mediated radical polymerizations: behaviour of 3-vinylpyridine in a protic solvent

Solvents can strongly influence the equilibrium between dormant and active species which is involved in 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-mediated radical polymerizations. At 110°C, the overall polymerization rate of 3-vinylpyridine in pyridine was logically found lower than that in bulk, due to dilution. However, in ethylene-glycol, the opposite behaviour was observed. According to infrared spectroscopy, this unexpected increase in the overall polymerization rate was attributed to hydrogen bonding between ethylene-glycol and the pyridine rings, which weakens the C–O–N bond of the TEMPO-terminated polymers. Semi-empirical molecular orbital calculations were used to support the FTIR results and to calculate the decrease in the dissociation enthalpy of the polymer–nitroxide bond.