Styrene pulse radiolysis

One the other hand, short-lived intermediates formed from styrene by radiations were studied by the pulse radiolysis technique by Metz et al. (43). They observed the anion radicals of styrene as an optical absorption band with the maximum at 370 mp, but could not find cationic intermediates. Shida and Hillma irradiated the 2-methyltetrahydrofuran glass and butylchloride glass, both containing styrene, and observed the absorption bands due to added styrene at 410 mp and 350 mp, respectively. The former band was assumed to be due to the anion-radicals and the latter to the cation radicals (44). 

Ito O, Matsuda M (1982) Polar effects in addition reactions of benzenethiyl radicals to substituted styrenes and a-methylstyrenes determined by flash photolysis. J Am Chem Soc 104 1701-1703 Janata E, Veltwisch D, Asmus K-D (1980) Submicrosecond pulse radiolysis conductivity measurements in aqueous solutions. II. Fast processes in the oxidation of some organic sulphides. Ra-... 

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. 

Egusa et al. disclosed the behaviors of cationic intermediates in the initiation stage of the polymerization of styrene and a-methylstyrne by low temperature pulse radiolysis [24, 25], Figure 3 shows the transient absorption spectra for styrene in a mixture of isopentane and n-butyl chloride (4 1 by volume) irradiated with 4 /is pulses at — 165 °C. M, and M2 refer the absorption bands... 

The dimer cation was supposed to have a sandwich structure in which the orbitals of one molecule overlapped with those of the other molecule. The band at 450 nm (B) is due to the bonded dimer cation (St—St T) the formation of this species corresponds to the initiation step of the polymerization. The bonded dimer cation may be formed by the opening of the vinyl double-bonds. Egusa et al. proposed that the structure was a linked head-to-head type I or II, by the analogy of the dimeric dianions of styrene and a-methylstyrene. Table 1 summarizes the assignment of absorption bands observed in pulse radiolysis of 1,1-diphenylethylene in dichloromethane, which is a compound suitable for studying monomeric and dimeric cations [28],... 

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],... 

Another investigation along this line is the pulse radiolysis study of the electron transfer reactions from aromatic radical anions to styrene this type of reaction is commonly used to initiate anionic polymerization of styrene [35], The electron transfer rates from the unassociated biphenyl radical-anions to styrene derivatives in 2-propanol were found to increase along the... 

Next to the styrene compounds, /V-vinylcarbazol has been most extensively studied by pulse radiolysis. When IV-vinylcarbazole is irradiated in aerated benzonitril, the cyclodimer of the Af-vinylcarbazole is formed the polymer is formed in both aerated and deaerated nitrobenzene. Tagawa et al. have proved that the radical cation of Af-vinylcarbazole plays an important role in both cyclodimerization and polymerization processes [39-43]. 

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... 

Despite of this inherent limitation, several spectacular results have been obtained. It should be noted that the initiation mechanism of the cationic polymerization of styrene described above was also deduced from the results of pulse radiolysis experiments. The pulse radiolysis combined with other techniques, such as the matrix isolation technique, the electron spin resonance technique and usual polymerization techniques, definitely provides a powerful means for investigating fundamentals of polymerization. 

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. 

Pulse Radiolysis. Several investigations of styrene (32) and -methyl-styrene (20, 24) under rigorous drying conditions, have been published. Other data (31) on less rigorously dried systems have also been reported, but because of the uncertainties of the effects of impurities, it is difficult to compare the latter with the former. This part of the discussion will be confined to the very pure systems. 

The role of cations in the pulse radiolysis of styrene and a-methyl-styrene is not yet clear. Using frozen glasses, Shida and Hamill (26) and Williams (27) have seen absorptions with peaks at 350 and 650 m/x for styrene and a-methylstyrene. Bands have also been seen previously (24) at 460 or 475 m/x. Perhaps the species responsible for the shoulders seen by Schneider and Swallow between 340 and 370 m/x is a cation, whose absorption overlaps that of the anion radical. This could explain the diminution of the absorption intensity round 390 m/x when n-butylamine was added. However, there was no evidence for an absorption band at longer wavelengths. 

Table I summarizes some of the results on the pulse radiolysis of styrene and a-methylstyrene obtained by the different authors. Although...

The difference spectrum results from the absorbance of the intermediates and bleaching of the dimers. Hence, the spectrum of the intermediates is constructed by adding the known spectrum of the photolyzed dimers to the observed difference spectrum. Such a procedure is illustrated in Fig. 3. The resulting spectrum of the intermediates closely resembles that of ot-methyl styrene radical-anions reported by three independent groups (4), who used pulse radiolysis in their studies, it follows that the photolysis leads to direct or indirect photo-dissociation of the dimeric-dianions into radical-anions of ot-methyl styrene, ot, i.e.,... 

Nitrous oxide-saturated solutions of styrene and a-methylstyrene also give sharp peaks at about 305 and 320 m/x, after pulse radiolysis. These resemble the characteristic double peaks of benzyl at 306 and 317 m/x (16). The peaks are therefore attributed mainly to adducts of OH radicals at the / -position in the vinyl group, to give radicals resembling benzyl—e.g.,... 

Attempts have been made to study the formation of polymer under pulse radiolysis conditions. Metz et al. have examined the ionic polymerization which should predominate with their very dry styrene (26). 

Styrene and a-Methylstyrene in Organic Solvents. Pulse radiolysis studies have been made on styrene and a-methylstyrene dissolved in methanol, benzene, carbon tetrachloride, dioxane, tetrahydrofuran, hexane, and cyclohexane (9, 24, 29, 30, 31). The results are easiest to understand for the aliphatic hydrocarbons and especially for the styrene in cyclohexane, which has been studied the most (31). For such solutions, two absorption bands were seen after the pulse by Keene, Land, and Swallow (24) and Schneider and Swallow (30) with peaks at 320 and 390 m/. The absorption at 320 m/u disappeared slowly by complex kinetics, and the 390-m/x absorption was very short lived, decaying by second-order kinetics with k/c = 4-7 X 10 cm. sec.-1. The relative intensities of the two peaks were quite variable. Chambers et al. saw the long lived absorption at 320 m/, but did not see a separate peak at 390 m/a, although it was observed that the absorption at 375 mfi decayed rapidly with k/e = 2.6 X 106 cm. sec.-1. 

A number of alkene radical cations have been generated in matrices at low temperature and have also been studied by ESR, CIDNP, and electrochemical methods. However, until recently very little absolute kinetic data have been available for the reactions of these important reactive intermediates in solution under conditions comparable to those used in mechanistic or synthetic studies. In a few cases, competitive kinetic techniques have been used to estimate rates for nucleophilic additions or radical cation/alkene cycloaddition reactions. In addition, pulse radiolysis has been used to provide rate constants for some of the radical cation chemistry relevant to the pho-topolymerization of styrenes. More recently, wc and others have used laser flash photolysis to generate and characterize a variety of alkene radical cations. This method has been extensively applied to the study of other reactive intermediates such as radicals, carbenes, and carbenium ions and is particularly well-suited for kinetic measurements of species that have lifetimes in the tens of nanoseconds range and up and that have at least moderate extinction coeffleients in the UV-visible region. 

Pulse radiolysis has been used to generate alkene radical cations for kinetic studies. " In this case ionizing radiation produces radical cations of the solvent which, in the case of alkanes and chloroalkanes, have lifetimes that are sufficient to be scavenged by mM concentrations of an appropriate donor. The method has been used to generate styrene radical cations in nonpolar solvents, as indicated in Eq. 13 (RH = cyclohex-ane). . . It has also been used in polar solvents, in which case the alkene is oxidized by a strong transient oxidant such as SO "orTP produced by pulse radiolysis. ... 

As noted in Section 3, the lifetimes of arylalkene radical cations in the absence of added nucleophiles or alkenes are frequently determined by their reaction with excess neutral alkene in solution to generate an adduct radical cation that ultimately gives the 1,2-diarylcyclobutane or substituted naphthalene products described above. Rate constants for the addition of the radical cation to its neutral precursor have been measured recently for both styrene and diarylethylene radical cations using either flash photolysis or pulse radiolysis methods. - ... 

Several pulse radiolysis studies have provided evidence that the 450-500-nm transients assigned to 1,4-acyclic radical cations react with the parent styrenes in nonpolar solvents. The rate constants for these reactions are generally in the 10 -10 M" s range, several orders of magnitude slower than the intial addition of the monomer radical cation. The reactions have been attributed to the trimerization reaction that is the first step in the chain growth in cationic polymerizations (Eq, 27). 

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. 

Most studies have dealt either with the free radical polymerization of hydrophobic monomers—e.g., styrene [56-89], methyl methacrylate (MMA) [68,73,74,84,86,90-93] or derivatives [2,94,97], and butyl acrylate (BA) [98-100]—within the oily core of O/W microemulsions or with the polymerization of water-soluble monomers such as acrylamide (AM) within the aqueous core of W/O microemulsions [101-123]. In the latter case, the monomer is a powder that has to first be dissolved in water (1 1 mass ratio) so that the resulting polymer particles are swollen by water, in contrast with O/W latex particles, where the polymer is in the bulk state. The polymerization can be initiated thermally, photochemically, or under )>-radiolysis. The possibility of using a coulometric initiation for acrylamide polymerization in AOT systems was also reported [120]. Besides the conventional dilatometric and gravimetric techniques, the polymerization kinetics was monitored by Raman spectroscopy [73,74], pulsed UV laser source [72,78], the rotating sector technique [105,106], calorimetry, and internal reflectance spectroscopy [95]. 

---