Photolysis of vinyl ketone copolymers. 4: Macromonomeric products from photolyses of copolymers of styrene and methyl acrylate with methyl vinyl ketone

Some statistical copolymers of styrene and methyl vinyl ketone, prepared by free radical copolymerization, have been photolyzed in benzene solution using u.v. light of wavelength 300 or 350 nm. It is confirmed by n.m.r. spectroscopy that the oligomeric products so obtained have unsaturated end-groups with structures resembling that of -methyl styrene, as would be expected on the basis of a predominantly Norrish type II photolysis mechanism. The molecular weights of the oligomers slightly exceed those expected from the copolymerization statistics, indicating probably that some slight cross-linking or repolymerization of end-groups accompanies the chain scission. Although not homopolymerizable, the unsaturated styrene oligomers produced by photolysis can be free-radically copolymerized with methyl acrylate, demonstrating that they behave as macromonomers. A copolymer of methyl acrylate with methyl vinyl ketone also gives fragments with unsaturated end-groups on photolysis, but here the accompanying cross-linking or end-group repolymerization is more dominant, leading to oligomeric products with extremely broad molecular weight distributions.