Paul W. O'Sullivan

Determination of reactivity ratios from analysis of triad fractions—analysis of the copolymerization of styrene and acrylonitrile as evidence for the penultimate model

Abstract A numerical analysis of the relationship between the composition of the monomer feed and that for the resultant copolymer (at low conversion) is the method usually adopted for determination of copolymerization reactivity ratios. However, the six triad fractions provide much more information about a copolymer than does the single copolymer composition. In most of the cases where both sets of information can be reliably obtained, the copolymer composition can be determined with slightly greater experimental accuracy than the triad fractions. Despite the somewhat lower accuracy of the individual triad fractions, in some cases the “best” model of the copolymerization and the corresponding reactivity ratios can be determined with higher precision from the triad fractions, because of the extra information provided by these data. A direct numerical analysis of the triad fractions for the styrene-acrylonitrile copolymerization in terms of the penultimate model has yielded the reactivity ratios and their joint confidence regions for the bulk polymerization at 60°. The analysis shows that the data can be satisfactorily described by the penultimate model, whereas statistical tests demonstrate that the terminal model will not adequately represent the polymerization. The reactivity ratios are determined with higher precision from the triad fractions than from the copolymer compositions.

Radiolysis of model polypeptides in the solid state: Radiation sensitivity and side chain structure

Abstract Homopolymers of the amino acids gly, ala, val, phe and tyr have been used as models to investigate the effect of molecular structure on the degradation of polypeptides by ionizing radiation. The main reactions were—(i) scission of the entire side chain leading to its hydrogen adduct, (ii) fragmentation of the side chain, (iii) main-chain scission and fragmentation leading to a reduction in molecular weight and elimination of a segment of the chain. ESR studies confirmed hydrogen abstraction to form the α-carbon radical (except for polytyrosine) with G(R·) comparable with G (total ammonia), but these radicals were shown to be stable up to the temperature at which the volatile products were removed (100°C). The radicals formed from polytyrosine were unstable and decayed rapidly at room temperature. It is proposed that main-chain scission of the N-C α bond occurs to form polymer cations and radicals, and that fragmentation occurs to form polymer cations and radicals, and that fragmentation occurs via the cation. High yields of aliphatic acids reported by previous workers were found to be due to residual, hydrogen-bonded water in the polymer. Aromatic protection was observed in polyphenylalanine and more effectively with the phenolic group in polytyrosine.

A kinetic study of crosslinking vinyl polymerization by laser Raman spectroscopy - Free radical polymerization of diethylene glycol bis(allyl carbonate)

SummaryKinetic studies of crosslinking vinyl polymerizations are usually only applicable to the initial stages of the reaction, prior to the gel point, due to the subsequent insolubility of the polymer and the great increase in viscosity. We have demonstrated the suitability of laser Raman spectroscopy for quantitative analysis over the entire conversion range in an investigation of the polymerization of diethylene glycol bis (allyl carbonate) in bulk, with benzoyl peroxide initiation, between 70 and 85°C. The rate of consumption of vinyl bonds could be approximated by first-order kinetics up to ca. 80% depletion. Lower rates were observed at higher conversions.

Sequence distribution and intramolecular excimer formation in styrene-acrylonitrile copolymers

Styrene-acrylonitrile copolymers, like many other copolymers containing styrene, exhibit both normal and excimer fluorescence. We have shown that the ratio of the excimer to monomer fluorescence intensities in random styrene-acrylonitrile copolymers is linearly dependent upon the concentration of styrene-styrene bonds in the copolymer. This observation is consistent with a photophysical model which allows the energy absorbed by styrene units to migrate freely along the copolymer chain. Some of the energy is emitted in the form of normal fluorescence; some of the energy, trapped by neighbouring styrene-styrene pairs suitably oriented to allow excimer formation, is emitted as excimer fluorescence. The fluorescence characteristics of acrylonitrile-styrene copolymers are contrasted with those of methyl methacrylate-styrene copolymers, in which the methylmethacrylate sequences are believed to present partial barriers to energy migration along the copolymer chains.

Radiolysis of N-acetyl amino acids as model compounds for radiation degradation of polypeptides

Radiation chemical yields of (i) the volatile radiolysis products and (ii) the trapped free radicals from the y-radiolysis of the N-acetyl derivatives of glycine, L-valine, L-phenylalanine and L-tyrosine in the polycrystalline state have been determined at room temperature (303 K). Carbon dioxide was found to be the major molecular product for all these compounds with G(CO2) varying from 0.36 for N-acetyl-L-tyrosine to 8 for N-acetyl-L-valine. There was evidence for some scission of the N-Cα bond, indicated by the production of acetamide and the corresponding aliphatic acid, but the determination reaction was found to be of much lesser importance than the decarboxylation reaction. A protective effect of the aromatic ring in N-acetyl-L-phenylalanine and in N-acetyl-L-tyrosine was indicated by the lower yields of volatile products for these compounds. The yields of trapped free radicals were found to vary with the nature of the amino acid side chain, increasing with chain length and chain branching. The radical yields were decreased by incorporation of an aromatic moiety in the side chain, this effect being greater for the tyrosyl side chain than for the phenyl side chain. The G(R·) values showed a good correlation with G(CO2) indicating that a common reaction may be involved in radical production and carbon dioxide formation.

The mechanism of radiation degradation of polypeptides

It is proposed that γ-irradiation of polypeptides causes ionisation to give the polymeric radical cation, which undergoes chain fragmentation to produce an amino cation and an alkyl radical. This is followed by fragmentation of the cation with elimination of small chain segments and substituent branches. Thus, in poly(alanine) methane and acetamide are major products. The variety of products formed is influenced by the presence of hydrogen-bonded water in the polymer. In particular, when the polymer is carefully dried prior to radiolysis no aliphatic carboxylic acids are formed.

Methyl methacrylate-chloroprene copolymerization: an evaluation of copolymerization models

The composition and triad fraction data of Ebdon (Polymer 1974, 15, 782) for the copolymerization of methyl methacrylate with chloroprene have been re-evaluated using non-linear least squares curve-fitting procedures and statistical testing of conclusions. Using the compositions and triad fractions separately, best estimates of reactivity ratios together with the joint confidence intervals for these parameters for terminal and penultimate models have been calculated. It has been shown that the terminal model cannot describe adequately the experimental compositions or triad fractions. While the allowance of a penultimate effect for methyl methacrylate chain end radicals provides a significant improvement over the fit of the terminal model to the copolymer compositions, there is only a small probability that both the triad fraction and the composition data can be described adequately by one set of penultimate reactivity ratios.

Anionic copolymerization of butadiene and isoprene: Applicability of the terminal model to high conversion

SummaryThe compositions of copolymers of butadiene (B) and isoprene (I), produced by anionic copolymerization with sec-butyl lithium initiator in hexane at 20–40°C gave a good fit at low conversions to the terminal model for copolymerization. Non-linear, “best estimates” of the reactivity ratios rB and rI were 2.82 and 0.42, 1.72 and 0.36, 2.18 and 0.35 at 20, 30 and 40°C. The rates of copolymerization at conversions up to 98% of total monomer gave a smooth relationship with the instantaneous feed composition, calculated using the terminal model, at each temperature and independent of the initial feed composition. These results are in accord with the terminal model being applicable to this anionic copolymerization from low to hiqh conversions.

Scission Efficiencies of Short Chain Branches in the γ-Radiolysis of Ethylene-α-Olefin Copolymers

Abstract γ-Irradiation of copolymers of ethylene with propene, 1-butene, and 1-hexene, containing from 1 to 6 alkyl short chain branches per 1000 carbon atoms, at 25°C in vacuum, produced small amounts of n-alkanes with a maximum yield for the alkane corresponding to the alkyl branch of the α-olefin unit. A multilinear regression analysis showed a highly significant dependence of G(Cn alkane) on the frequency of alkyl branches containing n carbon atoms, determined by 13C-NMR. The corrections to the G(Cn alkane) yields from other fragmentation processes were substantial but no dependence for G(Cn alkane) on fragmentation of chain ends or fragmentation of the chain following branch elimination could be deduced from the data. The scission efficiencies = G(alkane) divided by the branch frequency per 1000 carbon atoms ± 95% confidence limits were (0.7 ± 0.7) × 10−3, (2.7 ± 0.8) × 10−3, and (1.5 ± 0.3) × 10−3, for methyl, ethyl, and butyl branches, respectively. These factors can be used to determine the short-...

An ESR Study of the UV Photolysis of Styrene and Maleic Anhydride at 90 K

Abstract Free radicals produced in styrene and maleic anhydride mixtures and in solutions in acetone and chloroform by UV photolysis at 90 K have been studied by electron spin resonance and changes observed on warming. A doublet spectrum observed in all systems containing maleic anhydride has been assigned to the radical formed by H addition to a carbonyl group in the monomer, and not to the corresponding radical on maleic anhydride units in the copolymer or to the maleic anhydride propagating radical. Interpretations of copolymerization mechanisms based on radicals produced in frozen comonomers in bulk or in solution by photolysis or radiolysis must therefore be viewed with caution.