Abstract
4‐Propenoylbenzil (PCOCO) obtained by dehydrochlorination of 4‐(3‐chloropropanoyl)benzil (CICOCO) was homo‐ and copolymerized with styrene (St) and methyl methacrylate (MMA). The polymers strongly absorb UV light at wavelengths between 300 and 400 nm. The triplet‐triplet absorption spectra of the homo‐ and copolymers and of CICOCO are bathochromically shifted relative to the spectrum of unsubstituted benzil triplets. The triplets of the homopolymer have a lifetime shorter than that of CICOCO and the copolymers, which is attributed to self‐quenching. Similar conclusions were arrived at from emission measurements. Irradiation of the copolymer PCOCO/St in dilute solution at λ~inc~ = 366 nm results in simultaneous crosslinking and mainchain degradation. Crosslinking is accomplished via combination of ketyl radicals formed by hydrogen abstraction of 1,2‐dicarbonyl groups in side chains. Therefore, crosslinking is the predominant process in solutions of hydrogen‐donating liquids such as tetrahydrofuran. Main‐chain scission is a secondary photoreaction. It primarily affords the photooxidative decomposition of 1,2‐dicarbonyl groups. Main‐chain scission predominates over crosslinking in solvents not acting as hydrogen donors such as benzene. Generally, O~2~ retards crosslinking and enhances main‐chain degradation.