Polymerization, activation radiation induced

Solution polymerization of VDE in fluorinated and fluorochlorinated hydrocarbons such as CEC-113 and initiated with organic peroxides (99), especially bis(perfluoropropionyl) peroxide (100), has been claimed. Radiation-induced polymerization of VDE has also been investigated (101,102). Alkylboron compounds activated by oxygen initiate VDE polymerization in water or organic solvents (103,104). Microwave-stimulated, low pressure plasma polymerization of VDE gives polymer film that is <10 pm thick (105). Highly regular PVDE polymer with minimized defect stmcture was synthesized and claimed (106). Perdeuterated PVDE has also been prepared and described (107). 

The creation of active sites as well as the graft polymerization of monomers may be carried out by using radiation procedures or free-radical initiators. This review is not devoted to the consideration of polymerization mechanisms on the surfaces of porous solids. Such information is presented in a number of excellent reviews [66-68]. However, it is necessary to focus attention on those peculiarities of polymerization that result in the formation of chromatographic sorbents. In spite of numerous publications devoted to problems of composite materials produced by means of polymerization techniques, articles concerning chromatographic sorbents are scarce. As mentioned above, there are two principle processes of sorbent preparation by graft polymerization radiation-induced polymerization or polymerization by radical initiators. We will also pay attention to advantages and deficiencies of the methods. 

In this volume, we present five articles-two papers on the behavior of free radicals and active intermediates produced by irradiation in the polymeric matrix are studied one article is on radiation-induced polymerization as a process for biomedical application, and finally there are two papers concerned with the radiation effects on polymeric materials from ion beams and in relation to their use for fusion reactors. 

There seems little doubt that in radiation induced polymerizations the reactive entity is a free cation (vinyl ethers are not susceptible to free radical or anionic polymerization). The dielectric constant of bulk isobutyl vinyl ether is low (<4) and very little solvation of cations is likely. Under these circumstances, therefore, the charge density of the active centre is likely to be a maximum and hence, also, the bimolecular rate coefficient for reaction with monomer. These data can, therefore, be regarded as a measure of the reactivity of a non-solvated or naked free ion and bear out the high reactivity predicted some years ago [110, 111]. The experimental results from initiation by stable carbonium ion salts are approximately one order of magnitude lower than those from 7-ray studies, but nevertheless still represent extremely high reactivity. In the latter work the dielectric constant of the solvent is much higher (CHjClj, e 10, 0°C) and considerable solvation of the active centre must be anticipated. As a result the charge density of the free cation will be reduced, and hence the lower value of fep represents the reactivity of a solvated free ion rather than a naked one. Confirmation of the apparent free ion nature of these polymerizations is afforded by the data on the ion pair dissociation constant,, of the salts used for initiation, and, more importantly, the invariance, within experimental error, of ftp with the counter-ion used (SbCl or BF4). Overall effects of solvent polarity will be considered shortly in more detail. 

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. 

First, suitable monomers are required for radiation-induced polymerization proceeding by a cationic mechanism. Isobutylene, vinyl ethers, cyclopentadiene and p-pinene polymerize only by a cationic mechanism, whereas a-methyl styrene polymerizes by both cationic and anionic mechanisms. Second, it is necessary to use the conditions of the existence of ions M+ (M—>M+ + e) and the stabilization of secondary electrons capable of neutralizing M+. This is achieved (a) by carrying out polymerization at low temperatures when the lifetime of ions increases and the activity of free radicals drastically decrease, and (b) by using electron-accepting solvents or additives. 

Hence, in order to ensure a relatively high rate of radiation-induced polymerization in the emulsion, low dose rates (0.1-1 Gy/s) and low absorbed doses are required. For all these reasons, radiation emulsion polymerization is particularly advantageous from an economical standpoint. Its activation energy, just as for other processes of radical radiation-induced polymerization, is 15-35 kJ/mol. The molecular weight of polymers increases with temperature, as in the case of typical free-radical processes (to a certain extent). This increase is due to an increase in kp with temperature, whereas k, does not depend on the temperature. 

D-Glucose isomerase has been immobilized with retention of enzymic activity by water soluble carbodi-imide aided reaction with chitosan. By reaction with/adsorption on to a range of new polyphenolic resins, and as Strepto-myces phaeochromogenes cells in fine-particle form using radiation-induced polymerization at low temperatures with previously salted out hydrophilic monomers such as 2-hydroxyethylacrylate and 2-hydroxyethylmethacrylate. ... 

Radiation-induced grafting is a process where, in a first step, an active site is created in the preexisting polymer. This site is usually a free radical, where the polymer chain behaves like a macroradical. This may subsequently initiate the polymerization of a monomer, leading to the formation of a graft copolymer structure where the backbone is represented by the polymer being modified, and the side chains are formed from the monomer (Fig. 1). This method offers the promise of polymerization of monomers that are difficult to polymerize by conventional methods without residues of initiators and catalysts. Moreover, polymerization can be carried out even at low temperatures, unlike polymerization with catalysts and initiators. Another interesting as-... 

There are other phenomena that do not present unconditional proof of the mechanism. The activation energies of polymerizations in the solid state are often unusually low (see below). Since no activation energy is needed with a radiation-induced start reaction, these low activation energies can also be due to the special conditions in the crystal. The same applies to solid state polymerizations of monomers that can only be polymerized ionically in the fluid phase. 

The only way to apply chemically bonded thin perfluorosulfonic acid layer onto the surface of an inert support is to graft perfluorinated functional monomers onto perfluorinated polymers. Some features of radiation-induced graft copolymerization of PFAVESF onto fluoropolymers were investigated. The studies showed that neither irradiation of a fluoropolymer-PFAVESF mixture (direct grafting) or interaction of PFAVESF with previously irradiated fluoropolymers (preirradiation grafting) yielded the grafted copolymers. It was assumed that this is connected with the low activity of PFAVE in radical polymerization. A special method has been developed for the synthesis of grafted copolymer. Previously irradiated fluoropolymer powders were used to prevent waste of PFAVESF. 

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