Initiators, high-energy radiation-induced

High-Energy-Radiation-Induced Cationic Polymerization of Vinyl Ethers in the Presence of Onium Salt Initiators... 

The present paper reports a study of the initiation mechanism for high energy radiation-induced cationic polymerization of divinyl ethers in the presence of various onium salts. Although there is a great difference in the dose rates of y-radiation and electron beam, the radiation chemistry is essentially the same. 

The initiation of free radical and ionic polymerization will be described in Section 5.2, while chemical transformations in neat linear synthetic polymers and biopolymers, and in polymers both in the solid state and in solution, will be treated in Sections 5.3, 5.4, and 5.5, respectively. The importance of high-energy radiation-induced processes for technical applications will be discussed in Section 5.6. 

The kinetics of high-energy radiation-induced polymerization in homogeneous liquid phase can be treated conventionally in terms of initiation, propagation and... 

Unlike ionic polymerizations, radical chain polymerizations have so far been found to occur only with unsaturated compounds. In some cases they can be induced purely thermally, or by means of light or high-energy radiation generally, however, radical initiators such as peroxo compounds, azo compounds, and redox systems are used. 

DNA injury can initiate apoptosis by a powerful, early activated mechanism mediated by the nuclear phosphoprotein p53. This protein is activated by both transcriptional and posttranslational means and is critical in the cellular response to double-strand DNA breaks, which can be induced by high-energy radiation such as UV light. Although the detailed mechanisms are not well understood, p53 apparently plays a regulatory role whereby the cell is directed either toward the completion of repair or to apoptosis (W20). In fact, p53 was proven to be essential for the induction of apoptosis of some cells treated with DNA alkylating agent (W19). 

In the liquid phase there are many examples of ionic polymerizations of olefinic compounds induced by high energy radiation. In some, such as propylene,77 1-hexene,78 1-octene,18 and n-hexadecene-1,75 the initiating ionic species is believed to be the parent ion radical while in others such as isoprene,79 isobutylene,80 butadiene,81 and 2-methylstyrene,82 it is thought to be the carbonium ion. 

Low energy initiation techniques [179, 180, 181] (near infrared, ultrasonic radiation and line tuneable pulse laser) have lately emerged to be better alternatives to the high-energy radiations (y-irradiation and e-beam). Laser-induced polymerisation of monomers have attracted significant attention in recent years generating a considerable literature published on both pulsed... 

A basic requirement of the ESR technique is the presence of molecules or atoms containing unpaired electrons. Such species can be generated in polymeric systems by homolytic chemical scission reactions or by polymerization processes involving unsaturated monomers. These reactions can be initiated thermally, photochemically, or with a free-radical initiator, and, in the case of scission, by mechanical stress applied to the system. Therefore, ESR can be used to study free-radical-initiated polymerization processes and the degradation of polymers induced by heat, light, high-energy radiation, or the application of stress. 

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

Free radicals are produced by the dissociation of excited molecules. High-energy-irradiation-induced polymerization is especially important for graft polymerization and polymerization in the solid state. Because of high investment costs, high-energy-radiation-initiated polymerization of gaseous, liquid, or dissolved monomers has not become established. However, to a small extent ( 2000 t/a), the polymerization of methyl methacrylate is radiation initiated. 

To stabilize PP to high-energy radiation requires a solution to the problems of postirradiation embrittlement, discoloration and thermal instability. To control both autoxidation and heat-initiated oxidation, special additives, such as free radical scavengers, peroxide decomposers and other stabilizers are incorporated in the usual polymer formulation. As the radiation-induced radicals and peroxides formed in solid polymers are long lived, the stabilizer systems should also keep their activity for a long period. 

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