Solid polymers, Fries reaction

Another reaction which occurs readily in solid polymers is the Fries reaction which can be observed in phenyl esters, particularly in phenyl acrylate and phenyl methacrylate polymers. The course of the reaction can be followed very... 

The photo-Fries reaction occurs readily in solid polymers and is observable in phenyl esters, particularly in poly(phenyl acrylate) and poly (phenyl methacrylate) and their derivatives. The course of the reaction can be followed very easily by ultraviolet spectroscopy, since the product hydroxy ketones have strong absorbance at 260 and 320 nm (Fig. 10). Reaction occurs with equal efficiency in small model compounds in solution and in the polymers in the solid phase (44). An Arrhenius plot of the quantum yield for the para product (Fig. 11) shows a linear increase up to 294 K, above which no further change in quantum efficiency is observed, either above or below the glass transition temperature. 

Figure 11. Arrhenius plot of the formation of hydroxyphenone groups as measured by absorbance changes at 265 nm. The transition tenqieratures of the polymer are also included. Reprinted with permission from S. K. L. Li and J. E. Guillet, Studies of the photo-Fries reaction in solid poly(phenylacrylate). Macromolecules, 10, 840 (1977). Copyright 1977, American Chemical Society.

This reaction is important in polymer photochemistry, because it can occur with significant efficiency even in glassy, solid polymers. It is believed to contribute significantly to photo-induced yellowing in aromatic polymers, particularly polycarbonates (Kobsa, 1964 Li and Guillet, 1977). The photo-Fries rearrangement has also been identified as a source of photo-yellowing in aromatic polyamides (Wiles and Carlsson, 1980). 

Reactions which can be considered to be associated with caged radicals, such as the photo-Fries, will require very little free volume and can be expected to be quite efficient in solid polymers below the glass transition, whereas photochemical processes such as the Norrish Type II process are substantially reduced in glassy polymers below the glass transition temperature (Tg), unless the geometry of the cyclic six-membered ring is particularly favoured by steric factors in the chain, so that the most stable conformation corresponds to that required for reaction [874]. 

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). 

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