Transformations, mechanisms radiation-initiated

Chojnowski and co-workers have studied the polymerization of octamethyltetrasila-l,4-dioxane, a monomer more basic than cyclosiloxanes, which is capable of forming more stable oxonium ions, and thus being a useful model to study the role of silyloxonium ions.150-152 In recent work, these authors used Olah s initiating system and observed the formation of oxonium ion and its transformation to the corresponding tertiary silyloxonium ion at the chain ends.153 The 29Si NMR spectroscopic data and theoretical calculations were consistent with the postulated mechanism. Stannett and co-workers studied an unconventional process of radiation-initiated polymerization of cyclic siloxanes and proposed a mechanism involving the intermediate formation of silicenium ions solvated by the siloxane... 

The basic mechanisms of radiation-initiated transformations in polymers are far from being completely understood. Most reactions are commonly interpreted on the basis of free radical processes, but other species—e.g., ions, and other reactive intermediates—may play a significant role. A better understanding of the basic reactions occurring in irradiated polymers is required, and this would undoubtedly spur further developments and industrial applications. This paper is devoted to a survey of the different species found in irradiated polymers. Atten-... 

White beam synchrotron radiation was used in a topographical study [64] of the phase transition in [Ni(en)2 (NCS)2J single crystals. The short exposure times possible using this intense radiation enabled the strained centre of the crystal to be identified as the most imperfect zone, within which the nucleation and growth process was initiated. The phase transition commenced at defect sites. The role of generation and relaxation of mechanical stresses in influencing kinetics of solid state transformations was later reviewed [65]. 

The great variation in the types of active centres generated in the irradiated monomer makes it possible to initiate polymerization by different mechanisms. In each specific case, the nature of the monomer determining the formation of a certain type of active centre which ensures effective initiation and the polymerization conditions, mainly the temperature and the medium (solvents), are of the greatest importance. Hence, the polymerization process usually occurs by a certain definite mechanism. Since in the course of secondary radiation-chemical transformations, in practice, particles with a longer lifetime form free radicals, the free-radical mechanism is the simplest process of radiation-induced initiation. 

The synthetic method utilizes the three dimensional periodicity of a crystalline monomer phase as a template to determine the molecular and crystallographic structures of a polymer(4-12). In the ideal case, solid state reaction (initiated either thermally, mechanically, or by exposure to actinic radiation) transforms a monomer single crystal to a polymer single crystal with nearly the same dimensions and similar structural perfection. G. Wegner first demonstrated that certain diacetylene reactions closely approximate this ideal case(4) ... 

When high-energy radiation is directly absorbed by polysaccharides, electronically excited moieties, radical cations and electrons are generated. To date, little is known of the mechanism of any subsequent processes leading to the formation of radiolysis products, but results obtained with low-molar-mass models have helped to provide an understanding to some extent of the radiation-induced chemical processes in polysaccharides [80,83]. Apparently, free-radical processes play an important role, as confirmed by ESR studies [83,95]. It is assumed that radical cations generated initially are rapidly transformed to free radicals by deprotonation, as illustrated in Scheme 5.20. 

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