Monomers, pulse radiolysis

Guldi D M 1997 Capped fullerenes stabilization of water-soluble fullerene monomers as studied by flash photolysis and pulse radiolysis J. Phys. Chem. A 101 3895-900... 

Pulse Radiolysis. Surprisingly few studies have been carried out using the powerful technique of pulse radiolysis to study radiation reactions in polymers although an increasing number of studies on organic systems (67) and on monomers (68,69) are appearing. 

Figure 5 Transient absorption spectra of irradiated additive-free solid PMMA containing MMA monomers observed by the authors improved nanosecond pulse radiolysis system over a wide range of temperatures and with the monitoring wavelength region from 300 to 1600 nm.

Similarly, fluorescence detected magnetic resonance effects observed during the pulse radiolysis of anthracene-dio in the presence of 2,3-dimethyl-l-butene support the presence of 8 equivalent methyl groups. Because the splitting, Odi = 0.82 mT, was approximately one-half that of the monomer splitting, Omon =1.71 mT, the sandwich dimer 91 + was invoked. ... 

A method for preparing a-methylstyrene to investigate its radiation-induced polymerization yields samples which exhibit reproducible kinetics. The kinetic results are interpreted as indicating that free radicals, carbonium ions, and carbanions can all propagate simultaneously, the relative importance of each species depending upon the dryness of the monomer and all associated glassware. This viewpoint is further supported by data from a preliminary investigation of the transients formed in a-methylstyrene, as studied by the pulse radiolysis technique. 

CMS and Polystyrene Solutions in Cyclohexane. Both monomer and excimer fluorescences were observed in the pulse radiolysis of polystyrene solution in cyclohexane. The decay curves of monomer and excimer fluorescences at 287 and 360 nm are shown in Figures 7(a) and (b), respectively. Energy migration on the polymer chain has been discussed elsewhere (15). The dependences of the decay of monomer fluorescence and the rise of excimer fluorescence on the... 

Figure 7. The decay curves obtained from pulse radiolysis of polystyrene solution in cyclohexane (a) monomer and (b) excimer fluorescence monitored at 287 nm and 360 nm, respectively.

Marcus RA (1999) Electron transfer past and future. In Jortner J, Bixon M (eds) Electron transfer -from isolated molecules to biomolecules, part 1. Wiley, New York, pp 1-6 Martin RF, Anderson RF (1998) Pulse radiolysis studies indicate that electron transfer is involved in radioprotection by Floechst 33342 and methylproamine. Int J Radiat Oncol Biol Phys42 827-831 Maruthamuthu P (1980) Absolute rate constants for the reactions of sulfate, phosphate and hydroxyl radicals with monomers. Macromol Chem Rapid Commun 1 23-25 Maruthamuthu P, Neta P (1977) Reactions of phosphate radicals with organic compounds. J Phys Chem 81 1622-1625... 

Fluorescence detected magnetic resonance effects observed during the pulse radiolysis of anthracene-d10 in the presence of tetramethylethene portray an additional facet of the cyclobutane radical cation system [345, 346]. The spectra [eight ( ) equivalent methyl groups, ad = 8.2 G approximately one half of the monomer splitting, am = 17.1 G] are compatible with a dimer cation. In analogy to the benzene dimer radical cation [347, 348] they were interpreted as evidence for a sandwich , one molecule above the other [346],... 

Pulse radiolysis studies concerning the polymerization as well as the degradation, crosslinking and radiation resistance of polymers are surveyed. Initiation mechanisms of the radiation-induced polymerization of styrene and other monomers are discussed on the basis of the direct measurements of the reaction intermediates. Optical and kinetic data on the short-lived chemical intermediates produced in the solution of polymers and in the rigid polymers are surveyed and discussed with special reference to the degradation mechanism of polymers. 

Pulse radiolysis is a powerful tool for the creation and kinetic investigation of highly reactive species. It was introduced to the field of radiation chemistry at the end of the 1950s and became popular in the early 1960s. Although the objects of this modern technique were, at first, limited to solvated electrons and related intermediates, it was soon applied to a variety of organic and inorganic substances. As early as 1964, ionic intermediates produced by electron pulses in vinyl monomers were reported for the first time. Since then, the pulse radiolysis method has achieved considerable success in the field of polymer science. 

Application of pulse radiolysis to polymers and polymerization was motivated at first by the success of radiation-induced polymerization as a novel technique for polymer synthesis. It turned out that a variety of monomers could be polymerized by means of radiolysis, but only a little was known about the reaction mechanisms. Early studies were, therefore, devoted to searching for initiators of radiation-induced polymerization such as radicals, anions and cations derived from monomers or solvents. Transient absorption spectra of those reactive intermediates were assigned with the aid of matrix isolation technique. Thus the initiation mechanisms were successfully elucidated by this method. Propagating species also were searched for enthusiastically in some polymerization systems, but the results were rather negative, because of the low steady state concentration of the species of interest. 

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. 

Apart from the relevance to the radiation-induced polymerizations, the pulse radiolysis of the solutions of styrene and a-methylstyrene in MTHF or tetrahy-drofuran (THF) has provided useful information about anionic polymerization in general [33]. Anionic polymerizations initiated by alkali-metal reduction or electron transfer reactions involve the initial formation of radical anions followed by their dimerization, giving rise to two centers for chain growth by monomer addition [34]. In the pulse radiolysis of styrene or a-methylstyrene (MS), however, the rapid recombination reaction of the anion with a counterion necessarily formed during the radiolysis makes it difficult to observe the dimerization process directly. Langan et al. used the solutions containing either sodium or lithium tetrahydridoaluminiumate (NAH or LAH) in which the anions formed stable ion-pairs with the alkali-metal cations whereby the radical anions produced by pulse radiolysis could be prevented from rapid recombination reaction [33],... 

Only a few monomers, other than styrene and related monomers, have been investigated by pulse radiolysis. The studies on methyl methacrylate and related monomers have shown the formation of the associated dimer anion of the... 

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. 

Alternatively, pulse radiolysis suggests that the radical cations react with monomer to form dimeric radical cations as in Eq. (64). 

In the pulse radiolysis of acrylamide, methacrylamide, and some other monomers, Chambers et ah have observed broad absorption bands with peaks near 370 m/x (9), which can plausibly be attributed to OH adducts at the /3 position—e.g.,... 

Pure Monomers. The first pulse radiolysis study of a polymerizing system was with isobutylene, which gave an absorption with a peak at 297 mfi, which disappeared in a fast first-order reaction (6). The absorption may be attributed to the trimethylcarbonium ion, which, from independent work, appears to have an absorption maximum at 292 m/ and an extinction coefficient close to 6.3 X 103M 1 cm. 1 (28). 

For solutions of naphthalene in cyclohexane the yield of triplet states of naphthalene, the intensity of monomer and excimer fluorescence, and the yield of singlet state naphthalene have been measured as a function of naphthalene concentration by pulse radiolysis methods. The influence of a number of electron scavengers on these yields has been determined... 

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