Graft copolymer formation during the polymerization of vinyl acetate in the presence of polyvinyl alcohol

Abstract

AbstractWhen aqueous dispersions of polyvinyl acetate are prepared by polymerizing vinyl acetate monomer, emulsified in water in the presence of polyvinyl alcohol and an alkali, with a water‐soluble persulfate catalyst, polyvinyl acetate‐polyvinyl alcohol graft copolymers are formed. The presence of graft copolymers in the dispersions was first established by the techinques of turbidimetric titration and adsorption and partition chromatography on paper. Subsequently the graft copolymers were separated according to species and at the same time the polyvinyl acetate was fractionated on a molecular weight basis, by using a continuous gradient elution chromatographic fractionation technique. The results obtained show that in the fractionation of polymers by chromatographic methods, solubility in the developing solvent is a more important differentiating property than adsorption on the supporting medium. Three dispersions prepared using identical quantities of vinyl acetate monomer and catalyst, but with varying quantities of polyvinyl alcohol, have been fractionated. Two different species of graft copolymer, one water‐soluble and of high polyvinyl alcohol content, and the other water‐insoluble and of low polyvinyl alcohol content, have been isolated from each of the dispersions. The relative amounts of the two species of graft copolymer and the moleculare weight distributions of the polyvinyl acetate were found to differ in the three dispersions. Mechanisms for the reactions giving rise to graft coplymer formation have been postulated which explain these differences. It is suggested that the watersoluble species of graft copolymer are formed in the initial stages of the reaction by the radicals from the decomposing catalyst which first abstract hydrogen atoms from the polyvinyl alcohol molecules to form polymer radicals which then initiate polymerization of vinyl acetate monomer. The water‐insoluble species of graft copolymer are formed at later stages in the reaction by transfer from growing polyvinyl acetate chains on to polyvinyl alcohol molecules, forming polymer radicals which can again initiate polymerizaion of vinyl acetate. In the infrared spectra of both species of graft copolymer an absorption maximum occurs at a wavelength of 8.8μ. An absorption maximum occurring at this wavelength is characteristic of a tertiary alcohol group, and suggests that the grafted polyvinyl acetate chains are attached to the chain carbon atoms of the polyvinyl alcohol molecules to which the hydroxyl groups are also attached.