Propagation rate coefficient of poly(N-isopropylacrylamide) in water below its lower critical solution temperature

Pulsed laser polymerization (PLP) of N-isopropylacrylamide (NIPAM) in water was performed over the range 2-20 degreesC (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 similar to0.4-0.8 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.