Photodegradable polymers-Side-Chain

SCHEME 14.1 The general stmcture of an acrylic polymer and the estahhshed photodegradation mechanism via Norrish I a-cleavage of the carhonyl side chain, leading to main-chain polymeric radical a and oxo-acyl radical b. The secondary P-scission rearrangement reaction leading to the propagating radical c is also shown. 

The TREPR experiments and simulations described here have provided an enormous amount of structural and dynamic information about a class of free radicals that were not reported in the hterature prior to our first paper on this topic in 2000. Magnetic parameters for many main-chain acrylic radicals have been established, and interesting solvent effects have been observed such as spin relaxation rates and the novel pH dependence of the polyacid radical spectra. It is fair to conclude from these studies that the photodegradation mechanism of acrylic polymers is general, proceeding through Norrish 1 a-cleavage of the ester (or acid) side chain. Recently, model systems have... 

The majority of the work on the fundamental processes of photodegradation of acrylic polymers has been carried out by Grassie and co-workers (5-7). For radiation with wavelength A > 300.0 nm photodegradation is initiated by absorption of a photon by the carbonyl group of the ester side chains. Subsequent fragmentation depends on local stereochemistry and temperature. The principal modes of fragmentation are described by the Norrish I and Norrish II processes (8). 

Natural Polymers.—Two comprehensive review articles have appeared on wool" and cellulose photodegradation. Holt and Milliganhave examined the photo-oxidation of serine, threomine, and cystine side chains in wool. Serine is converted into a-carboxyglycine, cystine to a-formylglycine, and threomine to a-acetylglycine. Waters et have shown that the ph oto-... 

The deliberate introduction of carbonyl groups into polymers, either in the main chain by copolymerization with carbon monoxide or in the side chains by copolymerization with vinyl ketones, is the basis of at least one type of plastic designed to photodegrade rapidly in outdoor exposure (115). These copolymers are widely used in the United States for the six-pack ring packaging of soft drinks, whose damaging effect on marine life has been of great concern. 

The photodegradation of poly(alkylacrylate)s and poly(methacrylate)s under UV irradiation (248 nm) in solution was studied for the first time by TR EPR by Harbron et Well-resolved spectra of oxo-acyl radicals from the ester side chain and of main-chain polymeric alkyl radicals were used to show the side-chain cleavage via the Norrish I process. The methacrylate spectra are strongly influenced by the stereoregularity of different polymer tacticity, the temperature and the solvent. The relations of these dependences on the conformational motion in the polymer chain are discussed. 

Four main mechanisms of the photoalignment are known (i) reversible photochemical trans-cis isomerization in azo-dye-containing polymers, monolayers, and pure dye films (ii) pure photophysical reorientation of the azo-dye chro-mophore molecules (iii) topochemical crosslinking in cinnamoyl side-chain polymers (iv) photodegradation in polyimide materials. 

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