Polymeric systems, electron spin resonance

This article is an attempt to review possibilities in a quantum chemical treatment of open-shell systems. In order to cut down the extent of this review, we disregard some problems, especially those concerning macromolecules, polymerization reactions, and open-shell transition-metal complexes. Electron spin resonance is mentioned only briefly, because it has been a topic of many reviews. 

Several investigations dealing with the characterization of polymeric membrane systems have been reported lately. A. Kusumi 99> studied phase transitions, fluidity and polarity properties of polymerized and nonpolymerized methacryloyl-derivatized phosphatidylcholine (57) vesicles by DSC and electron spin resonance (ESR). 

For chainwise polymerizations, the analysis of model systems implies consideration of the homopolymerization or copolymerization of bifunctional monomers. Kinetic results cannot be directly extrapolated to the case of networks, because very important features such as intramolecular cycliza-tion reactions are not present in the case of linear polymers. However, the nature of initiation and termination reactions may be assessed. For example, using electron spin resonance (ESR), Brown and Sandreczki (1990) identified different types of radicals produced during the homopolymerization of a monomaleimide (a model compound of bismaleimides). 

Tt is well known that the presence of precipitated polymer can influence the course of polymerization. In bulk acrylonitrile polymerization the effects are most dramatic and have been the subject of many studies. The literature on this subject has been reviewed by Bamford et al. (4) by Thomas (29), and by Peebles (23). Under conditions where the system becomes heterogeneous owing to precipitation of small particles of polymer, a protracted acceleration period is observed at the start of polymerization, and the final rate is found to depend on the 0.8 power of the concentration of free radical initiator. Unusual post-polymerization effects are observed in photoinitiated polymerization of acrylonitrile, owing to the presence of trapped radicals which can be detected by electron spin resonance. None of the detailed mechanisms proposed to... 

The term free radical is often used in the context of a reactive intermediate, as in the case of polymerization of vinyl monomers, but the same structure (unpaired electron) can and does exist in a kind of immobilized environment. For example, a bulk-polymerized (monomer and initiator only in the polymerization system) poly(methyl methacrylate) (PMMA) contains an appreciable number of free radicals that can be detected by electron spin resonance (ESR) [1]. When the polymerization system becomes highly viscous toward the end of the bulk polymerization, gel formation occurs and immobilizes the growing end of free radical chain growth polymerization, preventing recombination of two free radical ends of growing chains. 

Grassi, A. Pellecchia, C. Oliva, L. Laschi, F. A combined NMR and electron spin resonance investigation of the (C5(CH3)5)Ti(CH2C6H5)3/B(C6F5)3 catalyst system active in the syndiospecific styrene polymerization. Macromol. Chem. Phys. 1995,196, 1093-1100. 

Much discussion centres on whether plasma polymerization takes place by a predominantly radical or ionic mechanism. Both species will undoubtedly be present in a gas discharge the pre-eminence of either will depend upon the monomer and the reaction configuration. Evidence for the existence of radicals trapped in plasma polymer film from electron spin resonance (e.s.r.) studies is common, and the effect of ions in polymerization is demonstrated by polymerizations in electroded systems, where polymerization is predominantly at an electrode (the cathode in d.c. discharges), although this is dependent upon the frequency. Polymerization may take place at a surface, in the gas phase, or by a combination of both. 

To better understand the mechanism of the dimethylbutadiene-MA copolymerization, low-temperature (-196 to -60°C) initiated polymerizations were studied with y rays and spectral changes observed by electron spin resonance.The study shows that liquid-phase copolymerization of MA and dimethylbutadiene takes place with the formation of a copolymer with regular alternation of monomer units, even while heating a glassy monomer mixture exposed to y radiation at -196°C. From an analysis of the electron paramagnetic resonance spectra and calorimetric data, it was clearly established that the mechanisi of copolymerization for this system consists of the stepwise addition of monomer molecules to a growing radical. This is contrary to a CTC homopolymerization model proposed earlier.It is assumed that the cause of the almost exclusive addition of monomers to foreign radicals... 

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