Kinetics and mechanism of the quaternization of poly(4-vinyl pyridine) with alkyl iodides

Abstract

The preparation of pure samples of poly(4-vinyl pyridine), P4VP, and poly(4-vinyl pyridine-co-styrene), P4VP-S, suitable for quantitative quaternization reactions is described. Reactivity ratios at 343 K are r₁= 0.69 ± 0.01 and r₂= 0.46 ± 0.10 (1 is 4VP, 2 is styrene).

Quaternization reactions of P4VP with ethyl, n-butyl and n-hexyl iodides gave extent, ξ, against time plots which were very well fitted using the neighbouring-group model. The parameters are k₀, K=k₁/k₀ and L=k₂/k₀, where k_i is the rate constant for reaction of a pyridyl groups with i already reacted neighbours. K and L pairs (±0.03) are: ethyl 0.96, 0.51; n-butyl 0.70, 0.30; n-hexyl 0.55, 0.27. The retardation compared with overall second-order behaviour, K=L= 1, increases with alkyl-group size. Quaternization of P4VP + n-butyl iodide in sulpholane, with and without 4-methyl-N-ethyl pyridinium iodide, showed that added salt had no effect on the kinetics. The neighbouring-group effect was absent from the quaternization of P4VP-S containing 0.2 mole fraction vinyl pyridine. The results of retardation are discussed in terms of a steric rather than an electrostatic mechanism, although the precise mechanism is not known. The results and conclusions are comparable with those found previously with alkyl and arylalkyl bromides.