Copolymers of styrene and acrylonitrile, and methacrylic add and acrylonitrile have been prepared by free radical initiation, with the purpose of examining the mechanisms of copolymerization. The composition and monomer sequence structure of the copolymers have been determined using 1H NMR, 13C NMR, the DEPT spectral editing technique, and 2D NMR HETCOR techniques.
The reactivity ratios for the terminal and penultimate copolymerization models have been determined by fitting the models to the copolymer composition and triad monomer sequence data using non-linear least squares analysis. The uncertainty in the reactivity ratios is illustrated by 95% joint confidence surfaces.
The styrene-acrylonitrile copolymerization in toluene, examined to low conversions, has been shown to be better represented by the penultimate model than the terminal model. The penultimate model reactivity ratios, determined by the fit to the triad fractions (rSS = 0.242, rAS = 0.566, rSA = 0.109, rAA = 0.133) are regarded as more precise than those obtained from the fit to the copolymer compositions, following examination of the 95% joint confidence surfaces for each set of reactivity ratios.
These reactivity ratios have been used to predict the changes in copolymer composition and triad fractions with increasing conversion. The predicted values compare favourably with the experimental values, indicating that the penultimate model provides a good description of the copolymerization at increased conversions (0 — 90%).
The styrene-acrylonitrile copolymerization has also been analysed in bulk, butanone and acetonitrile. In each of the solvents, the copolymerization is again better represented by the penultimate model than the terminal model. However, a solvent effect is evident. The terminal and penultimate models in their simplest forms cannot account for this effect.
Copolymers of methacrylic acid and acrylonitrile, prepared in bulk, have been analysed by NMR to examine the monomer sequence structure. The methine resonances in the 1H NMR spectra of the copolymers yield monomer sequence information on a to triad level. The carbonyl and nitrile resonances in the 13C NMR spectra similarly yield triad monomer sequence information. From the 13C NMR DEPT experiments, diad and tetrad monomer sequence information is available from the methylene resonances, and triad monomer sequence information is available from the methine resonances. Two dimensional NMR HETCOR experiments have enabled the assignment of the resonances in the aliphatic region of the 13C NMR spectra.
The copolymer compositions and triad fractions have been used to estimate the reactivity ratios for the terminal and penultimate models. The terminal model was found to provide an adequate description of the copolymerization, with no significant improvement observed by considering the penultimate unit. The reactivity ratios determined from the fit to the triad fractions (rA = 0.184, rM = 3.68) again, were more precise than those calculated from the copolymer compositions.
Comparison between the copolymerizations of methacrylic acid and acrylonitrile in bulk and DMSO, indicate that the solvent effect is not consistent with that described by the "bootstrap" model.
The radiation degradation of the poly (acrylonitrile-co—styrene) and poly (acrylonitrile-co—methacrylic acid) systems have been investigated. The effect of γ-radiation on the polymers has been examined by (i) electron spin resonance (ESR) spectroscopy to analyse the free radical intermediates at 77K and 300K, and (ii) determination of the yields of main chain scission and crosslinking through a Charlesby—Firmer analysis of the soluble fractions of polymer (above the gel dose) and through changes in the molecular weight (below the gel dose) as observed by gel permeation chromatography (GPC).
Examination of polyacrylonitrile which has been T-irradiated at 300K in vacuum, indicates a strongly crosslinking system. G-values for crosslinking and scission have been determined to be 0.59 and 0.00 respectively. ESR spectroscopy has been used to examine the irradiated polymer. The ESR spectra suggest that the major radicals present at 77K and 300K are (a) the backbone radical formed as a result of H abstraction from the methylene group, and (b) polyimine radicals. Crosslinking probably results from backbone radical addition to the nitrile group on a neighbouring polymer chain, thus yielding the polyimine radical. The G-value for radical formation at 77K is 2.8, while the value at 300K is 4.2.
The ESR analysis of the styrene-acrylonitrile copolymers γ-irradiated at 300K indicates the presence of significant radiation protection in the formation of radicals. This protection has been attributed to the aromatic group on the styrene units. The yield of acrylonitrile—based radicals in the γ-irradiated copolymers (300K) has been related to the monomer sequence structure of the copolymer; with the presence of acrylonitrile—based radical being directly proportional to the AAA triad sequence fraction in the copolymer. The styrene units are regarded as acting as energy sinks, providing protection against radiation damage. This feature is particularly evident where an acrylonitrile unit has at least one styrene unit as a next neighbour.
The copolymers exposed to γ-radiation at 77K show no evidence, from ESR analysis of the total radical yield, of any radiation protection.
The methacrylic acid — acrylonitrile copolymer has been irradiated at 77K and examined by ESR spectroscopy. The results indicate that there is an enhancement in radiation damage in the copolymer compared with either of the homopolymers.