Methyl methacrylate hypochromic effect

More quantitative studies have shown that, although some hypochromism effects exist, they are small. For example, Monnerie I9) examined the integrated molar extinction coefficient of atactic PS in chloroform and found only a 2 % decrease for the polymer relative to ethyl benzene. In addition, Vala and Rice 15 reported a 10% decrease in absorption for the 260 nm band of isotactic PS relative to atactic PS, which was qualitatively confirmed by Longworth18>. Similarly, Cantow 8> observed a 4% hypochromism of the 261.5 nm band of isotactic PS in dioxane relative to atactic PS. Finally, Cantow 8) observed strong hypochromism (relative to atactic PS) ranging from 19% at 262 nm to 32% at 269 nm for an alternating styrene-methyl methacrylate copolymer and for random copolymers having a low styrene content. 

So far, we have proposed a correlation between hypochromic effect and copolymer composition. In order to explicate this dependence, it is convenient to analyze hypochromism in terms of copolymer structure—that is in terms of sequence distribution. In an approach suggested by Stiitzel et al. (12), the molar extinction coefficient (e) of the random copolymer at a given wavelength can be written as follows (A = styrene, B = methyl methacrylate) ... 

It is evident that the coefficient of cBAA (a) is higher at copolymer compositions characterized by larger amounts of styrene whereas the opposite is valid for the coefficient of eBAB (b). Solvent effects on the hypochromism of styrene-methyl methacrylate random copolymers seem to be correlated, at least qualitatively, to the variation in a and b as functions of styrene content in the copolymer. Specific solvent effects on the extinction coefficients eAAA, baa> and bab account for the quantitative aspects of hypochromism. Full details will be given (II). 

Solvent effects on the optical densities of alternating and random copolymers of styrene and methyl methacrylate can explain the variation in hypochromism at 269.5 nm (see Figure 1 and Table II). Actually, the hypochromism seems to depend on specific interactions between methyl methacrylate mers and styrene mers. Good correlations between hypochromism and chemical structure of the copolymers can be deduced on the basis of the sequence length distribution. 

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