Heterophase polymerization of 4- vinylpyridine with butyllithium

Abstract

The butyllithium-initiated polymerization of 4-vinylpyridine in hydrocarbon media proceeded by an anionic addition mechanism. The colorless, soluble reactants rapidly formed a granular, intensely colored, polymer precipitate at temperatures below −30°C. in heptane and toluene diluents. When the reaction was carried out in an impurity-free environment, the highly colored reaction mixture remained unchanged for indefinite periods of time, and further addition of monomer produced continued growth of polymer chains. A dilatometric technique was employed to follow the rate of polymerization in toluene media. The experimental methods that were used for manipulation of solutions in vacuo resulted in polymerization (30–50% conversion) during an initial unsteady temperature period followed by a constant temperature interval during which rate measurements were obtained. Initial monomer and initiator concentrations were varied over a fourfold range. Rate measurements were made in the temperature interval −30°C. to +30°C. The dilatometric kinetic experiments together with additional, independent observations established that the rate was proportional to monomer concentration, and to the one half power of the initiator concentration, suggesting an equilibrium complex formation of growing polymeric anions. Rate constants were computed from the values of the rates obtained from the dilatometric kinetic experiments and the square root of the effective initiator concentration. A calculation of the temperature dependence of the rate constants produced an activation energy of 12.7 kcal./mole. As predicted by anionic polymerization theory, the polymer formed in these reactions had a number-average molecular weight equal to the ratio, (weight of polymer)/(equivalents of initiator). The polymer was fractionated by elution chromatography with tert -butyl alcohol and benzene solvent–nonsolvent mixtures. The molecular weight distributions of the polymer, constructed from fractionation data, showed a bimodal distribution in two fairly narrow molecular weight ranges. This uncommon molecular weight distribution is due to occlusion of a portion of the active growing chain ends in the polymer precipitate. These trapped reaction sites (which in these experiments amounted to 95% and up of the total initiator concentration) no longer produced chain growth due to the absence of available monomer. The remaining “effective” reaction sites thus grow to high molecular weight and are responsible for the observed kinetics.