Radiation-induced cationic polymerization

The kinetics of radiation-induced polymerization of bulk nitroethylene was also studied at 10° C by the use of hydrogen bromide as an anion scavenger (27). The value of Gt (yield of the initiation by 100 eV energy absorbed) was found to be about 3, which was much larger than the value obtained for many radiation-induced cationic polymerizations. The propagation rate constant, kp, was estimated to be 4 x 107 M-1 sec-1. The large kp value was attributed to the concept that the propagating chain ends were free ions in contrast to the existence of counter ions in catalytic polymerization. 

Since 1965, Ueno and the present author (K. H.) have extensively studied the radiation-induced polymerization of rigorously dried styrene in bulk (39, 40, 41). They found that the rate of polymerisation was increased remarkably by drying the monomer. This was attributed to the radiation-induced cationic polymerization which was enhanced by the removal of water, a cation scavenger, from the polymerization system. Potter et al. reported the same findings independently (42). 

The cation radical is believed to be the primary cationic intermediate involving the monomer molecule in the initiation process of radiation-induced cationic polymerization. Now, the question is how the car-bonium ions are formed from the cation radicals. 

The behavior of cationic intermediates produced in styrene and a-methyl-styrene in bulk remained a mystery for a long time. The problem was settled by Silverman et al. in 1983 by pulse radiolysis in the nanosecond time-domain [32]. On pulse radiolysis of deaerated bulk styrene, a weak, short-lived absorption due to the bonded dimer cation was observed at 450 nm, in addition to the intense radical band at 310 nm and very short-lived anion band at 400 nm (Fig. 4). (The lifetime of the anion was a few nanoseconds. The shorter lifetime of the radical anion compared with that observed previously may be due to the different purification procedures adopted in this experiment, where no special precautions were taken to remove water). The bonded dimer cation reacted with a neutral monomer with a rate constant of 106 mol-1 dm3s-1. This is in reasonable agreement with the propagation rate constant of radiation-induced cationic polymerization. 

The very straightforward results concerning the mechanism of propagation and cyclization in the polymerization of cyclosiloxanes were obtained by studying the radiation-induced cationic polymerization of 6-, 8-, and 10-membered cyclic siloxanes (D3, D4, Ds) [252,253]. 

The first coupling of a LINAC with infrared spectroscopy has been performed by Palmese et al. in order to study in situ kinetics of radiation-induced cationic polymerization of epoxy systems. The aim of the study is to understand the curing behavior of polymers under irradiation. A UV light source and an electron beam (10 MeV pulse width of the beam from 2.5 to 10 pm) are coupled to a portable near infrared (NIR) instrument. Briefly, a portable NIR spectrometer (Control Development Incorporated, South Bend, IN, USA) is used,... 

Mascioni M, Sands JM, Palmese GR. (2003) Real time in situ spectroscopic characterization of radiation induced cationic polymerization of glycidyl ethers. Nucl Instr and Meth in Phys Res B 208 353-357. 

Radiation-Induced Cationic Polymerization of Limonene Oxide, o-Pinene Oxide, and - ene Oxide... 

AIKINS AND WILLIAMS Radiation-Induced Cationic Polymerization... 

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

The main reason for the limited industrial application of radiation-induced cationic polymerization is that oligomers that offer high cure rates are not commercially available. So far, due to their excellent combination of chemical, physical and electrical properties, only multifunctional epoxy oligomers have been used, but these have poor cure speeds compared to acrylate based systems. This is a serious limitation. Interesting alternatives to the epoxys are oligomers based on highly reactive vinyl ethers (21. 22) and the recently developed distsrrene ethers (23). which are as reactive as vinyl... 

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. 

Adur A.M., Williams F., Radiation-induced cationic polymerization of beta-pinene, J. Polym. Sci. A Polym. Chem., 19(3), 1981, 669-678. 

Despite improvements in experimental techniques, the fundamental processes in radiation-induced cationic polymerizations remain largely hypothetical. Pulse-radiolysis studies - on styrene solutions have led to the conclusion that charge transfer from the solvent produces a styrene cation-radical which then dimerizes to form both associated dimer cation-radicals and bonded dimer cation-radicals. These initial steps are thought to be sev al orders of magnitude faster than the subsequrat prop tion reactions. The presence of trace impurities can dictate the course of polymerization, and rate studies provide circumstantial evidence for the theory that nucleophiles can neutralize the cations in these systems and allow free-radical polymerization to occur alone. 

Mah S, Yamamoto Y, Hayashi K. Radiation-induced cationic polymerization of a-methylstyrene enhanced by diphenyliodonium hexafluorophosphate. Macromolecules 1983 16 681-685. 

Solid-state polymerization of TOX can be initiated by different kinds of radiation, including y-rays, X-rays, electron beams, or a-particles.The mechanism of initiation is not well understood. It is, however, generally accepted that radiation induces cationic polymerization. Ions or radical ions are generated by electron transfer, the loss of hydrogen ions, or the heterolytic cleavage of TOX rings. 

Scheme 5.7 High-energy-radiation-induced cationic polymerization of tetrahydrofuran [21].

The radiation-induced cationic polymerization in the presence of onium salts has attained practical importance for the EB curing of systems containing epoxides or vinyl ethers [22,23]. The chemical structures of typical compounds were presented in Table 3.23. THE does not play a role in this context, because of its very low propagation rate constant (kp 4 x 10 lmol s ). A reaction mechanism for the polymerization of vinyl ethers in the presence of an iodonium salt, as proposed by Crivello [22], is shown in Scheme 5.8. 

Yamamoto, Y. (1993) Pulse radiolysis of onium salts and radiation-induced cationic polymerization, in Radiation Curing in Polymer Science and Technology, vol. II (eds J.P. Fouassier and J.F Rabek). Elsevier Applied Science, London. 

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