Y-Radiation-induced polymerization

The y-radiation-induced polymerization requires an extremely high purity reaction system. Trace amounts of water can terminate a cationic reaction and inhibit polymerization. Organic bases such as ammonia and trimethylamine also inhibit polymerization. The y-radiation-induced polymerization of a rigorously dried D obeys the Hayashi-WilHams equation for completely pure systems (150). 

At least initially, the y-radiation-induced polymerization in the solid state is quite different from that of liquid A -vinylpyrrolidone. The rate in the solid state is said to increase after several hours until it reaches values comparable to those found in the liquid phase. This autoacceleration may be related to the regularity of the crystal structures of the frozen monomer, the temperature, and the quantity of preformed polymer present [83]. The activation energy for the polymerization of liquid A -vinylpyrrolidone was found to be 9 0.5 kcal/mole while that for the solid state polymerization is 18.0 kcal/mole [84]. 

Radiation-Induced Polymerization. In 1956 it was discovered that D can be polymerized in the soHd state by y-irradiation (145). Since that time a number of papers have reported radiation-induced polymerization of D and D in the soHd state (146,147). The first successhil polymerization of cychc siloxanes in the Hquid state (148) and later work (149) showed that the polymerization of cycHc siloxanes induced by y-irradiation has a cationic nature. The polymerization is initiated by a cleavage of Si—C bond and formation of silylenium cation. 

Nakase, Y., Kurijama, I. and Odajima, A. Analysis of the Fine Structure of Poly(Oxyme-thylene) Prepared by Radiation-Induced Polymerization in the Solid State. Vol. 65, pp. 79-134. 

The most important feature of ionizing radiations is, as the term implies, ionization to give ionic intermediates in irradiated systems. Though radiation-induced radical polymerization had long been studied, it is only a decade since radiation-induced ionic polymerization was first found. In 1957, Davison et al. obtained polymer from isobutene, which is known not to be polymerized by radical catalysts, by irradiating at low temperature with y-rays (7). Before long, the radiation-induced polymerization of styrene was proved to proceed as an ionic mechanism in suitable solvents (2,3,4). Since these pioneering researches, the study of the chemical kinetics of radiation-induced ionic polymerization has been extended to several vinyl, diene and cyclic monomers. 

Tabata and coworkers studied extensively the radiation-induced polymerization of vinylcarbazole (VC) in various solvents27-32. In one paper27 they studied the initial species formed in the polymerization of VC in benzonitrile solutions. To identify the products, they used the optical spectrum obtained for glassy solutions of vinylcarbazole in butyl chloride and 2-methyltetrahydrofuran irradiated at 77 K with y-rays. Irradiation in butyl chloride matrix is known, similarly to that in other organic chlorides, to give... 

Alcock and coworkers studied the polymerization of butadiene (as well as of monoolefins, acetylene and aromatic olefins) trapped within the tunnel clathrate system of tris((9-phenylenedioxy)cyclotriphosphazene, induced by Co-y-radiation. The host was used in order to find if the concatenation and orientation of the monomer molecules under the steric forces generated within the host crystal lattice will lead to stereospecific polymerization. The clathrate was prepared by addition of liquid butadiene to the pure host at low temperature. The irradiation was conducted at low temperatures. Irradiation of pure butadiene (unclathrated bulk monomer) leads to formation of a mixture of three addition products f,2-adduct, cis- and trons-f,4-adducts. In contrast, the radiation-induced polymerization within the tunnel system of the host yielded almost pure trans-1,4-polybutadiene. A small percentage of f, 2-addition product was observed, but no evidence for the formation of c/s-f,4-adduct was found, confirming the earlier observation by Fin ter and Wegner. The average molecular weight was about 5000,... 

A major difference between the two methods of initiation is that the solvent in y-ray studies is almost inevitably the monomer itself, and these generally have lower dielectric constants than the chlorocarbon solvents most often used in the chemically initiated systems. As a result, it is not possible to compare the values of kp +) obtained from each technique without accounting for this difference in solvation. Classically, propagation involves charge dispersion in forming the transition-state complex and hence a reduction in the polarity of the system. Thus media of lower solvation power should favourably influence the process. (See reference 114 for more detailed discussion.) Experimentally the values of kp(+) from radiation-induced polymerizations are consistently higher than those obtained using stable salts as initiators, and this simplistic picture therefore seems to be confirmed. Dunn has recently carried out a detailed compilation of the available data on / p(+) and readers will find this an excellent distillation of the current position. 

Living polymerization under a constant source of y-radiation in the presence of thiocarbonylthio compounds was first reported in 2001 by Pan and co-workers [15, 16], Scheme 4 shows the mechanism used by these authors to account for the living behaviour observed under a constant source of y-radiation. Under this scheme, y-radiation induces sequential homolytic cleavage of the carbon sulfur bond in the dithioester, yielding a stable thiocarbonylthiyl radical. The other half of the thiocarbonylthio compound (R1) initiates polymerization, and the resulting chains are then reversibly terminated by the stable radical. Pan and co-workers base this mechanism on the fact that the thiyl group of the Z-C(S)-S- is always attached to the head of the monomer. However, this explanation cannot differentiate between the two mechanisms, as polymers generated via RAFT will share the exact same structure. 

The development of solid-state polymerization was largely due to the use of high-energy radiation such as y-rays 115,123), x-rays 124>, electron beams 125,126) or a-partic-les ll6). y-Rays are used most frequently. It is generally accepted that y-ray-induced polymerization of TXN proceeds by a cationic mechanism. Ions or radical-ions are formed by electron transfer from TXN, the loss of hydrogen atoms or the heterolytic cleavage of the ring 127>. 

Considerable work has been done on the initiation of the vinyl fluoride by ionizing radiation much as y-radiation from a °Co source. A selection of references on this research includes Usmanov and other authors [4,48-62], Of these, Usmanov et al. [4] deal with the graft copolymerization of vinyl fluoride to some natural and synthetic polymers. Usmanov et al. [53] discuss the formation of branched polymers during radiation-induced polymerization. Gubareva et al. [54] deal with solution polymerizations. Nakamura et al. [58, 59, 61] deal with emulsion polymerizations of vinyl fluoride by radiation initiations. Usmanov et al. [60,61] discuss the effects of chain-transfer agents during radiation-initiated polymerization. Some copolymerization studies are described in Usmanov et al. [55]. 

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