Kinetics and mechanism of the quaternization of poly(4-vinyl pyridine) with ethyl, n-propyl, n-butyl, n-hexyl and benzyl bromide in sulpholane

Abstract

Kinetic results for quaternization reactions of four n-alkyl bromides (C₂, C₃, C₄, C₆) and benzyl bromide, with poly(4-vinyl pyridine) in sulpholane, at a variety of concentrations and temperatures, have been analysed according to a neighbouring-group model. A particular system is characterized by the rate constants k_i for reaction of a pyridyl group having i= 0, 1 or 2 already reacted neighbours. Values of k₀ under comparable conditions are given. The ratios K=k₁/k₀ and L=k₂/k₀ for the series of reagents: C₂(0.95, 0.54), C₃(0.80, 0.37), C₄(0.70, 0.31), C₆(0.55, 0.27) and Bz(0.75, 0.34), indicate that retardation as reaction proceeds, in excess of that expected from second-order kinetics, is due predominantly to steric (rather than electrostatic) effects. All of the results for the time dependence of extent, ξ, of reaction are corrected on the supposition that 5 % of the polymer consists of unreactive impurity; previous studies have shown evidence of impurities, but these have not been allowed for in the few attempts at kinetic analysis. Activation energies E₀ are fairly constant for C₂ to C₆: 68.8 ± 3.3, 66.2 ± 4.3, 65.8 ± 4.0 and 66.0 ± 3.5 kJ mol^–1; cf. 52.6 ± 4.9 kJ mol^–1 for benzyl bromide. The k₁ and the k₂ show the same temperature dependence, within experimental error, as the k₀, i.e.E₀=E₁=E₂.