Interchain exchange reactions in radical polymerization

Abstract

AbstractInterchain exchange reactions with the formation of “hybrid” polymers take place in mixed solutions of two polymers or in solutions of polymers dissolved in “foreign” monomers, in the presence of free radical initiators. A number of methods, namely, turbidimetric titration, ultraviolet spectroscopy, refractometry, and pycnometry were used to identify and analyze the hybride polymers. A qualitative method for determining the “hybridity” of polymers was developed. This consisted in determining the optical and mechanical (relaxation) properties of polymer films obtained from various solvents (poor solvents for one and good solvents for the other type of block or branch of the hybrid polymer). For the quantitative investigation of systems containing the hybrid polymers a new method employing an ultracentrifuge has been developed. This is a modification of Meselson's method (equilibrium centrifugation in a concentration gradient) and consists in centrifuging a mixture of two homopolymers and their hybrid polymer in a mixture of two solvents of which the density has been so chosen that the homopolymers give the Boltzmann distribution at the bottom and at the meniscus of the cell. The hybrid polymer accumulates in a band in the central part. Theoretically the method allows one to determine simultaneously the molecular weight distribution of all three polymers and the composition of the “hybrid” polymer. The present experiments are important in throwing light on the secondary reactions taking place in radical homopolymerization and leading to unusual multimodal or “tailed” molecular weight distributions.