The photolysis of polyvinyl bromide

Polyvinyl bromide in solution, in dry tetrahydrofuran (THF) yields a black polymer upon exposure to u.v. light of 260 nm or less. This is shown to be the result of (a) the elimination of bromine atoms in the primary reaction, and (b) the catalysed formation of a conjugated chain by the eliminated bromine atoms. The black polymer is bleached by visible light with the formation of a cross-linked product. This blackening takes place in the full light of the mercury lamp, but by the introduction of a suitable filter passing only visible light, the colour changes can be arrested and ultimately reversed though the viscosity continues to increase due to cross-linking. In dry solutions the liberated bromine is absorbed by the THF solvent and no free Br2 or HBr is produced. In the presence of water, the products are completely changed. No blackening occurs, free HBr is eliminated, and the colourless PVB becomes cross-linked and ultimately yields a gel. On irradiating the black polymer in aqueous THF by visible light, it becomes bleached, yielding a cross-linked colourless polymer which ultimately gels.