Preparation and photochemical properties of 4‐propenoylbenzil polymers

Abstract

4-Propenoylbenzil (PCOCO) obtained by dehydrochlorination of 4-(3-~hloropropanoyl)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 ClCOCO are bathochromically shifted relative to the spectrum of unsubstituted benzil triplets. The triplets of the homopolymer have a lifetime shorter than that of ClCOCO 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 Ainc = 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, 0, retards crosslinking and enhances main-chain degradation.

Benzil Photoperoxidations in Polymer Films and Crosslinking by the Resultant Benzoyl Peroxides in Polystyrene and Other Polymers

“…Subsequent combination and disproportionation reactions of the ketyl radical and radical formed from the hydrogen donor are responsible for the eventual photoproducts. Additionally, recombination of two polymeric pendant ketyl radicals has been postulated to be responsible for the photocrosslinking of copolymer-bearing BZ structures in deaerated THF solution [25]. The lowest triplet states and ketyl radicals were detected by transient absorption measurements in this system.…”

“…565 nm ( E T = 212 kJ mol −1 ) in various solvents as well as in BZ / p - tert -butylcalix [ 6 ] and calix [ 8 ] arenes complexes [ 22 ]. The maximum of the phosphorescence from unsubstituted BZ in a glassy PMMA film (prepared by solution casting) is at 531 nm ( E T = 225 kJ mol −1 ) at 77 K and at 544 nm ( E T = 219 kJ mol −1 ) at 296 K [ 25 ]. This value indicates that the emitting species has an average −CO−CO− dihedral angle that is not completely relaxed; it is skewed with an angle greater than 72° but smaller than 180°.…”

“…The phosphorescence maxima of BZ and its derivatives in polymer films of PS, PMMA, polyvinylchloride, polypropylene and polyethylene or BZ structures appended to the main chains in a homopolymer or copolymer have maxima at ca. 520–550 nm and are 10–20 nm lower at 77 K than at 296 K; the phosphorescence lifetimes were 3.2−5.5 ms at 77 K and 0.1−0.5 ms at 296 K [ 25 , 27 , 28 ].…”

“…Additionally, recombination of two polymeric pendant ketyl radicals has been postulated to be responsible for the photocrosslinking of copolymer-bearing BZ structures in deaerated THF solution [ 25 ]. The lowest triplet states and ketyl radicals were detected by transient absorption measurements in this system.…”

“…Later, a considerable amount of peroxybenzoic acid was also identified [ 51 ]. When an aerated benzene solution of styrene copolymer with pendant BZ groups was irradiated at λ = 366 nm, low-molecular-weight cleavage photoproducts, as well as BZ fragments bound to the copolymer, were found [ 25 ].…”