Initiation by Ionizing Radiation

Radioactive sources and particle accelerators are used to initiate polymerizations. Electrons, neutrons, and a-particles (He ) are particulate radiations, while gamma and X rays are electromagnetic radiations. The interactions of these radiations with matter are complex [Chapiro, 1962 Wilson, 1974]. The chemical effects of the different types of radiation are qualitatively the same, although there are quantitative differences. Molecular excitation may occur with the subsequent formation of radicals in the same manner as in photolysis, but ionization of a compound C by ejection of an electron is more probable because of the higher 

The initially ejected electron may be attracted to the cation B+ with the formation of another radical  

Radicals may also be produced by a sequence of reactions initiated by the capture of an ejected electron by C 

The radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals as described above. It is then possible for these species to initiate chain polymerizations. Whether a polymerization is initiated by the radicals, cations, or anions depends on the monomer and reaction conditions. Most radiation polymerizations are radical polymerizations, especially at higher temperatures where ionic species are not stable and dissociate to yield radicals. Radiolytic initiation can also be achieved using initiators, like those used in thermally initiated and photoinitiated polymerizations, which undergo decomposition on irradiation. 

The kinetics of radiation-initiated polymerizations follow in a relatively straightforward manner those of photolytic polymerization. The initiation rate is determined by the radiation intensity and the concentration and susceptibility of the compound that radiolyzes to yield the initiating species (ionic and/or radical). The final expression for Rp is determined by the exact details of the initiation, propagation, and termination steps. 

10-7M), one observes a bimodal molecular weight distribution. This indicates the simultaneous occurrence of cationic and radical polymerizations since the two are known to yield different-molecular-weight polymers. With decreasing water concentration ( l() 7 

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The actual species responsible for cationic polymerizations initiated by ionizing radiation is not established. The most frequently described mechanism postulates reaction between radical-cation and monomer to form separate cationic and radical species subsequently, the cationic species propagates rapidly while the radical species propagates very slowly. The proposed mechanism for isobutylene involves transfer of a hydrogen radical from monomer to the radical-cation to form the r-butyl carbocation and an unreactive allyl-type radical ... 

Cationic polymerization initiated by ionizing radiation is markedly different from other cationic polymerizations in that the propagating species is a free ion remote from a counterion. Overall electrical neutrality is maintained by electrons trapped by the monomer. 

The expressions (Eqs. 5-34 and 5-42) for Rp in cationic polymerization point out one very significant difference between cationic and radical polymerizations. Radical polymerizations show a -order dependence of Rp on while cationic polymerizations show a first-order depenence of Rp on R,. The difference is a consequence of their different modes of termination. Termination is second-order in the propagating species in radical polymerization but only first-order in cationic polymerization. The one exception to this generalization is certain cationic polymerizations initiated by ionizing radiation (Secs. 5-2a-6, 3-4d). Initiation consists of the formation of radical-cations from monomer followed by dimerization to dicarbo-cations (Eq. 5-11). An alternate proposal is reaction of the radical-cation with monomer to form a monocarbocation species (Eq. 5-12). In either case, the carbocation centers propagate by successive additions of monomer with radical propagation not favored at low temperatures in superpure and dry sytems. 

Polymerizations initiated by ionizing radiation or stable carbocation salts such as trityl or tropylium hexachloroantimonate are useful for evaluating the free-ion propagation rate constant. Ionizing radiation yields free ions (in the absence of ion pairs) whose concentrations... 

There is sometimes a question as to whether a particular initiator or initiator system initiates polymerization by radical, cationic, or anionic means. Such a question can easily arise, for example, in polymerizations initiated by ionizing radiation. The mode of initiation of a particular initiator can be distinguished by a consideration of its characteristics compared to those of known radical, cationic, and anionic initiators ... 

Initiation by Ionizing Radiation. See Initiation by Fission Fragments... 

Macrocellulosic radicals initiated by ionizing radiation can be divided into two classes one class in which the radiant energy is selectively absorbed by the cellulose molecule and another class in which the radiant energy is randomly absorbed by the cellulose molecule. In both cases, after localization of the absorbed radiant energy, oxidative depolymerization of cellulose is initiated, and macrocellulosic radicals are formed (3). 

The rate of polymerization in the constant rate period is shown in Figure 5 as a function of AIBN concentration, both on logarithmic scales. At low concentrations of initiator (I) the rate varies as I0 89, but the slope drops as low as 0.33 at high rates of initiation. Similar results have been reported by Chapiro and Sebban-Danon (12) for polymerizations initiated by ionizing radiation at 19 °C. Initiator exponents significantly higher than the normal value of 0.5 have been reported by many workers (6, 10, 17, 25, 30, 31) for the polymerization of acrylonitrile under heterogeneous conditions. 

Polymerization initiated by ionizing radiation such as y rays or high-energy electron beams is somewhat similar to plasma polymerization. Understanding of radiation polymerization is very helpful in elucidating mechanisms of plasma polymerization. In radiation polymerization, no initiator is employed, and the chain-carrying species are created by the ionization of monomer. In this respect, radiation polymerization... 

The studies by the methods of radiolysis provided a wealth of knowledge about the kinetic and thermodynamic controls of radical reactions, the importance of which is no more questioned. Indeed, it is known that the chemical events initiated by ionizing radiation, are the same as those that take place in normal and deleterious events of every day s life. In this review we focus on some ofthe major knowledge that was acquired by the use of pulse radiolysis and steady-state gamma radiolysis of aqueous solutions of amino acids, peptides and proteins (Inset). The potential role of pulse radiolysis (Chapter 2) for studying biomolecules has been acknowledged rather early. In most cases, pulse radiolysis... 

Time resolved electronic absorption and emission spectroscopy has been extensively used for solution phase kinetic studies of fast chemical processes initiated by ionizing radiation. A wealth of information on rate parameters and reaction mechanisms on a variety of chemical reactions has been obtained by this technique. As valuable as these techniques are, they have limitations. In particular, the electronic spectra in solution are often broad and featureless and offer little structural information. As a consequence, the identification of a reaction intermediate is based on chemical intuition and not on its spectral characteristics. Moreover, when more than one transient is present in the system with overlapping electronic absorption, the kinetic monitoring of the individual concentration becomes difficult. Vibra-... 

Polymerization initiated by ionizing radiation can be carried out by both a free-radical and an ionic (cationic or anionic) mechanism. The determining factor is the nature of the end of the growing chain. 

This paper is dedicated to the memory of S. C. Lind (1879-1965) whose active interest in radiochemistry (the name he preferred for what is now termed radiation chemistry) spanned over half a century and to my precepter, Harry Essex, who was the first to use an electric-field technique in studies of reactions initiated by ionizing radiations. 

There have been many studies of graft copolymer formation initiated by ionizing radiation, and methods have been discussed.88 Among the systems studied are styrene onto polyethylene,88 styrene onto polypropylene,100 styrene onto Nylon-6,101 styrene onto ethylene-vinyl acetate copolymers,102 styrene and MMA onto natural rubber,103 pentafluorostyrene onto Nylon and polyethylene,104 acrylamide onto Nylon-6,105 acrylamide onto starch,100 vinyl monomers... 

Chaplin, R.R, Dwoijanyn, P.A., Gamage, N.J.W. et al. 1996. The role of partitioning of reagents in grafting and curing reactions initiated by ionizing radiation and UV. Radiat Phys Chem. Al 435-437. 

The reactions induced in polymer matrix with sulfur crosslinking system in composition are probably multistage. To comprehend mechanisms of the reactions initiated by ionizing radiation, further investigation within this area is needed. 

Polymer destruction initiated by ionizing radiation is accompanied by the formation of double bonds and by the evolution of gaseous products (hydrogen, low-molecular-weight hydrocarbons, hydrochloric acid, tetrafluoroethylene, etc.). In the presence of oxygen, oxidized structures such as hydroperoxides. 

Grafting or IPN polymerization (e.g., initiation by ionizing radiation, UV, 0 3, or RFGD, followed by polymerizations)... 

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