Polymerization kinetics of monomers of importance in biomedicine investigated by γirradiation

Gamma irradiation was used to initiate the polymerization of NIPAM in water below the LCST. Conversion as a function of time was followed with automated tracking dilatometry. The data were used to show that classical kinetics were obeyed. Relaxation experiments (i.e. where the dilatometer is removed from the γ source) then provide reliable values for the reactivity ratio, ki/kp, the ratio of the termination to propagation rate coefficients. It was found at a monomer concentration of 0.442 M, kikp had a value of 8.1 at 27 deg C . This suggested that either kt was very low, perhaps because the radicals are constrained to some complexation or aggregation mechanism, or kp is extraordinarily high. Pulsed laser polymerizations (in conjunction with size exclusion chromatography, SEC) were performed to obtain accurate kp values for NIPAM in water over a temperature range below the LCST (2 - 27 deg C ). The main difficulty with most functional polymers is obtaining quantitative data from SEC. PNIPAM is no exception to the rule, and meticulous care in molecular weight analysis was necessary. Special insights were developed into sample preparation to obtain accurate molecular weight distributions (MWD). The value kp was found to be high, but not extraordinarily so, and hence the high value of kt must be due to a constrained polymer radical