REACTION-KINETICS OF POLYMER SUBSTITUENTS - MACROMOLECULAR STERIC HINDRANCE EFFECT IN QUATERNIZATION OF POLY(VINYLPYRIDINES)

The retardation in the quaternization of poly(vinylpyridines) is of steric origin. Using very voluminous reagents, a steric effect also induces an important limitation to the extent of quaternization which is not included in theoretical treatments of neighboring group effects. A kinetic expression based on the classical second-order equation is proposed. It includes the rate constant k0 for reaction of a pyridyl group and a macromolecular steric factor-beta lowering the effective pyridine group concentration capable of reacting. Beta-characterizes the effect of the size and nature of the substituents on the accessibility of the pyridine groups. The expression describes very well the experimental kinetics published and associates to each alkylating agent a steric factor related to its bulkiness. Cessation of the reaction at a given temperature is predicted, in full agreement with experimental results. Possible reasons for such a limitation are discussed, and the change in the polymer conformation from a free-draining coil to a scarcely permeable globule is shown to account for the effect.