Polystyrene photolysis products

Product III has not been identified. It has a weak, unstructured fluorescence maximum ca. 390 nm, with an excitation maximum at 332 nm. This product was fully extractable from a photolyzed film with methanol, suggesting that it may be a secondary photolysis product arising from Product II. If nonaltemating sequences exist within the copolymer, Product III might be thought to correspond to the diene observed in the photolysis of polystyrene. However, Product III formed more rapidly in vacuum than in air, the reverse of the manner in which diene appeared to form in polystyrene. 

Polystyrene (PS) yields benzene (I = 600) as the major photolysis product with apparently smaller amounts of toluene, ethylbenzene, and styrene formed. Quantitation of the last three materials is complicated by variable amounts of residual monomer in the samples available and a small amount of thermal decomposition occurring in the injection port. 

Photolysis of poly (methyl methacrylate) proceeds primarily by formation of the monomer, corresponding to the product observed at I = 720. As in the case of polystyrene, small amounts of residual monomer and monomer formed fi om thermal decomposition were observed in non-irradiated sampled. Methanol and methyl formate, which have also been reported as photolysis products are not separated on SE-30 but analysis on Carbowax 20 M give two peaks corresponding to these substances. 

Scheme 6.192 shows the o-nitrobenzyl moiety used as a photolabile linker in the solid-phase oligosaccharide synthesis on a polystyrene (PS) support.1208 After the synthesis of the protected oligosaccharide 404, PS attached to the photoremovable group is removed by photolysis and the final product is obtained by hydrogenolysis. Such a strategy could be promising for combinatorial synthesis of oligosaccharide libraries. 

It was Zehavi [16] who first used the nitroveratryl linker (4) in 1973 to synthesize an oligosaccharide using a photolabile solid phase strategy. Starting with isovanillin, 4 was prepared by nitration, alkylation with the Merrifield polystyrene resin and reduction of the aldehyde. Photolysis using UV irradiation (>320 nm) in dioxane produced product in 13% yield after 32 h of irradiation (Scheme 17.4). 

When polystyrene films are exposed to ultraviolet radiation of 235.7 nm in vacuo, crosslinking occurs and the sample becomes partly insoluble [41]. Grassie and Weir [42] have performed a detailed investigation of the photolysis of polystyrene in vacuo as a preliminary step to the study of the photo-oxidation of that polymer. They found that the only gaseous product formed is hydrogen with a quantum yield of 4.3 x 10 2. The rate of the reaction increases linearly with the intensity of the incident light... 

Photolysis of oxidative products (hydroperoxide groups, chain peroxides and ketonic groups) formed during the preparation of polystyrene. If hydroperoxides are present (Fig. 3.51) the oxidation rates are increased, and degradation can also be induced in the anionic polystyrene. 

The gas chromatogram of the products formed in polystyrene peroxide photolysis is shown in Figure 6.5. The GC peaks 1, 3, 4 and 6 are attributed to the products formaldehyde, benzaldehyde, a-hydroxy acetophenone and phenyl glycol, respectively, and were identified by their characteristic MS [44, 45]. The MS of two of the photodecomposition products (GC peaks 4 and 7 due, to a-methoxy acetophenone and a-benzoyloxy acetophenone), respectively, are given in Figure 6.6. 

Figure 6.5 GC of the products formed in polystyrene peroxide photolysis in chloroform. Reproduced with permission from K. Subramanian, European Polymer Journal, 2002, 38, 1167. 2002, Elsevier [43]...

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