Polymer and Materials Science
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The polymerization kinetics in water of acrylylglycinamide (AG) initiated by K2S2O8 was studied over the temperature range 40.0 to 60.0°C. Monomer concentration was varied from 7.8 × 10-3 to 31.2 × 10-3M and catalyst from 1.85 × to 11.10 × 10-5M. The rate expression is -d[M]/dt = Rp, = k1.22[K2S2O8]0.5[M]1.22, and the overall empirical rate constant, k1.22 = 1.14 × 1011e-15,800/RT 1.0.72 mole-0.72 min-1. To explain the dependence on monomer, a kinetic scheme which includes a bimolecular reaction (k2) between monomer and initiator is suggested. The simplified expression which describes the initial rate of polymerization is: -d[M]/dt = Rp, = k4(2[I]/k5)1/2[M](k1 + k2[M])1/2, where k1, k2, k4 and k5 are rate constants for S2O8 = decomposition, a bimolecular reaction between monomer and initiator, propagation, and termination, respectively. Individual bimolecular rate constants are expressed in liter/mole-min. The equation predicts a dependence on monomer concentration between 1.0 and 1.5 with 1.5 being approached a t high monomer concentrations. Plots of RP2/[M]2 versus [M] are linear, as predicted by the postulated reaction route and values for k2 and k4/k51/2 were obtained from the slopes and intercepts of these plots. The temperature dependence of the bimolecular monomer-initiator reaction is k2 = 5.19 × 1021e-36,000/RT. Instead of the usual behavior, the k4/k51/2 ratio was found to decrease with temperature and the difference of activation energies, (E4 - E5/2), is -1.50 kcal. The temperature dependence of the propagation to square root of the termination rate constant ratio is k4/k51/2 = 6.16e1500/RT. These rather unusual results may be related to the ability of AG polymers in water to form thermally reversible gels; even above the gel melting points, the polymers are considerably aggregated in solution. This would tend to make the bimolecular termination reaction more temperature dependent and also account for the high values (59-69) for the k4/k51/2 ratios. For similar temperatures, the overall rate constants for AG are approximately four times those for acrylamide.
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