Other
Scientific paper
Jun 1975
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975rspsa.344....1m&link_type=abstract
Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, Volume 344, Issue 1636, pp. 1-19
Other
Scientific paper
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. These results may be accounted for by the fact that the free bromine atoms liberated by the primary reaction are scavenged by the water and so cannot catalyse the formation of a conjugated chain. Thus the free polymer radicals cross-link to form a colourless network structure and the bromine is eliminated as HBr. The reaction of Br atoms with water is somewhat dot{B}r + H2O(aq) -> H+(aq) + Br-(aq) + dot{O}H endothermic, but is facilitated by the excess energy of the photolysing quantum. This is greater than 460 kJ, while the strength of the C-Br bond is only 272 kJ and thus yields an excess of energy of at least 188 kJ. With the aid of a mercury resonance lamp, the quantum yield of bromine elimination in the early stage of reaction was concluded to be unity in aqueous THF, and approximating to 2 in the dry solvent. As the reactions proceed, however, the apparent yields decline due to light scattering by the gelatinous products. In the presence of monomeric acrylonitrile or ethylene in dry THF, graft copolymers could be separated. No blackening occurred. These grafts arose from initiation by the free PVB radicals. Homopolymer of acrylonitrile was also produced at the same time, initiated by the photolysed Br atoms; in aqueous THF, however, the homopolymer was reduced to a minimum due to the scavenging of Br atoms by the water. Some homopolymer was still produced, however, which was attributed to the termination of the growing graft polymer chains by transfer to monomeric acrylonitrile or THF. The mechanism of the generation of conjugated ethylenic unsaturation in the PVB by the catalytic action of Br atoms in the dry solvent is discussed.
Mullik S. U.
Norrish G. W. R.
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