The influence of hydrogen bonding on the propagation rate coefficient in free-radical polymerizations of hydroxypropyl methacrylate

The propagation rate coefficient k(P) was determined for hydroxypropyl methacrylate by applying pulsed laser initiated polymerizations and subsequent analysis of the polymer by size-exclusion chromatography. k(P) data were derived for polymerizations in bulk and in several solvents: toluene, tetrahydrofuran (THF), benzyl alcohol, and supercritical CO2. With the exception of THF, no solvent influence on k(p) was observed. For polymerizations in THF k(p) values 40% below the corresponding bulk data were obtained. In addition, the activation energy of kp for polymerizations in THF is higher than for the other systems. The results are explained by a complexation of the OH group contained in the ester group with THF. As a consequence, H bonds between OH groups and carbonyl O atoms, which occur in the other systems, are not formed in the presence of THF. This explanation is supported by Raman spectra, which show that. association of carbonyl groups does not occur for systems containing THF, whereas for all other systems the occurrence of two peaks at 1 703 cm(-1) and 1720 cm(-1) is indicative of the vibrations of two different - associated vs. not associated - types of carbonyl groups. Based on the change in activation energy it is suggested that a true kinetic solvent effect occurs.