Poly photo-rearrangement

Scheme 7.25 Photo-rearrangement of poly(4-acetoxy styrene).

The combination of photo-scission, photo-oxidation and photo-rearrangement means that even very high-performance polymers, such as epoxy resins, aromatic polyethers polyketones and polysulfones, and the aromatic polyamides poly(l,3-phenylene isophthalamide) and poly(l,4-phenylene terephthalamide) (Nomex and Kevlar respectively) will degrade significantly on solar exposure. For example, Davis (1977) showed that poly(phenylene... 

Poly(orr/jo-nitrobenzaldehyde acetal) (4.31) under UV irradiation is photo-rearranged to polyglycols and o-nitrobenzoate ester [1711] ... 

For the initial phase of the photo-Fries rearrangement, IA can be determined approximately by subtracting the absorption due to the strongest absorbing product of the reaction mixture, provided its concentration and molar extinction coefficient are known. By means of this initial rate approach Humphrey8 determined the quantum yields of the photorearrangement of 13 (913-14 = 0.14), of 14 (914-.18 = 0.056), and of poly-2,2-propanebis(4-phenyl carbonate) ([Pg.138]

A considerable amount of attention has also been paid to the photo-Fries rearrangement of polymer pendant groups. For example, the rearrangement of poly (phenyl acrylate) (10,11) in solution or in the solid-state, is usually incomplete and results in the formation of both the ortho and the para-hydroxyphenone rearranged products in amounts which vary with the conditions of the photolysis. A concurrent side-reaction, which we term the Fries degradation, also results in the liberation of small amounts of phenol (Scheme 2). Similar results have been obtained with poly (phenyl methacrylate) and other substituted aryl acrylates (4,9,12). 

The sharp increase in adsorbance which is observed near 256 and 340 nm is characteristic of the o-rearranged product. Similar results are also obtained when the irradiation experiments are carried out under nitrogen atmosphere, indicating that the photo-oxidation reported by Hiraoka and Pacansky (23) for poly(p-hydroxystyrene) is not a factor in this instance. 

Although the acetate ester of poly (p-hydroxystyrene) is more readily prepared than the corresponding formate (IV), the occurrence of a photostabilizing rearrangement during photolysis of III makes it ultimately ill-suited for use as an imaging system. A survey of the literature on the photo-... 

Fries rearrangement of aromatic formate esters suggests that phenols are the major products (.24) obtained in the reaction. As poly(p-hydroxystyrene) is remarkably clear in the deep UV, it is likely that poly(p-formyloxystyrene) will not suffer from the same problem of photostabilization upon exposure as was the case with poly (p-acetoxystyrene). This expectation was confirmed by our study of the photo-Fries reaction of p-cresyl formate no ortho rearranged product was isolated after reaction while p-cresol and a small amount of starting material were obtained. 

A thorough study of the photo-Fries rearrangement of poly(p-acetamido styrene) was made difficult by the lack of solubility of the polymer in solvents other than DMSO or DMF. Nevertheless, photoexposure of thin films of V cast from DMF solution confirmed the lack of reactivity of the polymer as only minimal conversions could be achieved even after long periods of exposure. 

The hydroxylated copolymers, poly(acenaphthylene-co-vinyl alcohol), were reacted with the following acyl chlorides 1-naphthoyl, 2-naphthoyl, benzoyl and cinnamoyl. The latter polymers contain an ester group as a spacer between the main chain and one of the fluorophores. The choice of an ester group as a connecting unit between the polymer chain and fluorophore is probably a poor one since it is known that carbonyl groups easily form triplets and photo-Fries rearrangements can occur (2). 

Quantum yield values measured in solution may not necessarily apply to polymer films, the usual environment for practical application of this photochemistry. McKean et al. have adapted the indicator dye method to the measurement of quantum yields for Bronsted acid photogeneration in poly-(4-tert-butoxycarbonyloxystyrene) [20], As with the solution photochemistry of diphenyliodonium salts [71], an inverse dependence of quantum yield on exposure intensity was observed absolute quantum yields from 0.26 to 0.40 were measured at 254 nm, which extrapolate to approximately 0.45 at zero intensity, comparable to the value estimated by Dektar and Hacker [82b] in solution. McKean et al. [20b] note that similar quantum yields in solution and polymer films below Tg have also been reported for photo-Fries rearrangements [84] and photodissociation of diacyl peroxides [85]. 

Salicylate-type additives are largely imsuitable for use in aromatic polyesters, but some UV-stabilising ability has been shown by polyesters which contain moieties capable of transformation in the same way as the salicylates the so-called Photo-Fries rearrangement [20-22]. More recently, re-arrangeable polymers such as poly(phenyl acrylate) and poly(p-methylphenyl acrylate) have been proposed... 

The mechanism is believed to be that of a photo-induced Fries rearrangement. It may be similar to one catalyzed by Lewis acids. Another example is a spontaneous rearrangement in the solid state of poly(cx-phenylethylisonitrile) [128] ... 

Under UV irradiation of poly(phenyl acrylate) (3.44), the photo-Fries rearrangement to polymeric ortho- 3.45) and para-(3.46) hydroxybenzophenones occurs [874, 1333] ... 

The photoyellowing of poly(4,4 -diphenylolpropane isophthalate) (4.58) has been attributed to a photo-Fries rearrangement in a polymer containing o-hydroxybenzophenone as part of the chain structure (4.59) [1391] ... 

Fluorenone based polyesters such as poly(9,9-bis(4-hydroxyphenyl) lluorene isophthalate) (4.61), poly(9,9-biscresol fluorene isophthalate) and poly(9,9-bis(3,5-dimethyl-4-hydroxyphenyl)fluorene isophthalate) undergo, under UV irradiation, typical photo-Fries rearrangement, during which linear o-hydroxybenzophenone moieties can be formed [1346] ... 

Gu, W. Q., Bi, S. G., and Weiss, R. G., Photo-Fries rearrangements of 1-naphthyl esters in the glassy and melted states of poly(vinyl acetate). Comparisons with reactions in less polar polymers and low-viscosity solvents, Photochem. PhotobioL Sci., 1, 52, 2002. 

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