Propagation Rate Coefficient of Poly(<i>N</i>-isopropylacrylamide) in Water below Its Lower Critical Solution Temperature

Abstract

Pulsed laser polymerization (PLP) of N -isopropylacrylamide (NIPAM) in water was performed over the range 2−20 °C (below its lower critical solution temperature), to obtain propagation rate coefficients ( k p ). While the value of k p deduced from these data obeyed some of the consistency criteria for PLP (e.g., that the multiple points of inflection give the same apparent value of k p ), the apparent k p so obtained depended on monomer and initiator concentrations. For monomer concentration ∼0.4−0.8 M, the temperature dependence is approximated by k p (apparent)/dm 3 mol -1 s -1 = 10 8.7 exp(−24.5 kJ mol -1 / RT ). FTIR and osmometry measurements were used to infer the presence of significant amounts of dimer and to deduce the equilibrium constant for dimer formation. A model based on dimerization was derived to account for k p variations with monomer concentration but did not fit the experimental data, implying that a more complex treatment taking into account complexation with propagating chain ends or a bootstrap effect is required.