Radiation-induced crosslinking

These processes compete with radiation-induced crosslinking, scission and, for case (c), polymerization. 

Elastin-mimetic protein polymers have been fabricated into elastic networks primarily via y-radiation-induced, radical crosslinking of the material in the coacervate state [10]. Although effective, this method cannot produce polymers gels of defined molecular architecture, i.e., specific crosslink position and density, due to the lack of chemoselectivity in radical reactions. In addition, the ionizing radiation employed in this technique can cause material damage, and the reproducibility of specimen preparations may vary between different batches of material. In contrast, the e-amino groups of the lysine residues in polymers based on Lys-25 can be chemically crosslinked under controllable conditions into synthetic protein networks (vide infra). Elastic networks based on Lys-25 should contain crosslinks at well-defined position and density, determined by the sequence of the repeat, in the limit of complete substitution of the amino groups. 

Considerable disagreement still prevails as to the mechanism of crosslinking in polymers. Is it an ionic or a radical process While the author admits the existence of some ionic species (as revealed by ESR at low temperature and by radiation-induced conductivity), his present view is that the ionic contribution to crosslinking in solids and liquids is only minor. This attitude is based inter alia on the following evidence. 

Radiation-Induced Crosslinking. In the absence of oxygen, the predominant effect of ionizing radiation on UHMWPE is crosslinking (27). Crosslinking of UHMWPE forms covalent bonds between the polymer chains, which inhibit cold flow or creep of the individual polymer chains. 

Dizdaroglu M, Simic MG (1984a) Radiation-induced formation of thymine-thymine crosslinks. Int J Radiat Biol 46 241-246... 

Dizdaroglu M, Simic MG (1985b) Radiation-induced crosslinking of pyrimidine oligonucleotides. Radiat Phys Chem 26 309-316... 

Distel L, Distel B, Schussler H (2002) Formation of DNA double-strand breaks and DNA-protein crosslinks by irradiation of DNA in the presence of a protein. Radiat Phys Chem 65 141-129 Dizdaroglu M (1986) Chemical characterization of ionizing radiation-induced damage to DNA. Bio Techniques 4 536-546... 

Wojcik A, Bochenek A, Lankoff A, Risowska A, Padjas A, von Sonntag C, Obe G (2005) DNA interstrand crosslinks are induced in cells prelabelled with 5-bromo-2 -deoxyuridine and exposed to UVC radiation, in preparation... 

Very primary events in the chemical effect of radiations on matter are excitation and ionization of molecules, which result in the formation of neutral free radicals and radical ions. These reactive species play vital roles in the radiation-induced chemical reactions. As they are paramagnetic with an unpaired electron, electron spin resonance (ESR) spectroscopy has been a useful method for elucidating the mechanism of radiation-induced reactions in solid matter where radical species can be trapped temporarily. Since the early days of the chemical application of ESR, this method has been applied very often to the identification and quantification of free radicals in polymers irradiated by radiation [1]. This is probably because, from the view-point of fundamental research, a variety of free radicals are readily trapped in solid polymers and, from the view-point of applied research, these free radicals have close correlation with radiation-induced crosslinking and degradation of polymers. 

Polyethylene is known as one of the typical polymers which crosslink under the influence of ionizing radiation. The radiation-induced crosslinking (the formation of C-C covalent bonds between polymer chains) of polyethylene has long attracted the interest of a large number of research workers, because this polymer has the most simple chemical structure for the fundamental study of radiation effects on polymers, and also because the irradiation with ionizing radiation is a practically important means of modifying the mechanical and thermal properties of polyethylene. 

The recombination between two alkyl radicals is believed to be the main source of radiation-induced crosslinks in polyethylene [21], so that it is important to study the mechanism for formation and reaction of the alkyl radical. We have applied the ESE method to elucidate the paramagnetic relaxation mechanism and the spatial distribution of the alkyl radical, in order to get further insight into the radiation-chemical reactions of polyethylene resulting in the formation of crosslinks. 

This reaction mechanism predicts a small separation distance between the two alkyl radicals in pair determined by the proton-transfer distance. This small separation distance was thought to favor the high yield of the radiation-induced crosslinks. However, the above reaction mechanism seems not to accommodate the results of the ESE study. 

Pulse radiolysis studies concerning the polymerization as well as the degradation, crosslinking and radiation resistance of polymers are surveyed. Initiation mechanisms of the radiation-induced polymerization of styrene and other monomers are discussed on the basis of the direct measurements of the reaction intermediates. Optical and kinetic data on the short-lived chemical intermediates produced in the solution of polymers and in the rigid polymers are surveyed and discussed with special reference to the degradation mechanism of polymers. 

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