Styrene radiation-induced polymerization

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. 

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). 

In order to verify the formation of ion radicals from styrene and to correlate them with the radiation-induced polymerization, ESR spectra and the polymerization of styrene in the irradiated glass matrices were studied. The experimental results obtained are summarized in the following two sections. 

When the 2-methyltetrahydrofuran and n-butylchloride glasses with about 10 mole-% of styrene in them are irradiated to the dose of 1 X 107 rad, polystyrene is obtained only from the latter glass, though the polymer yield is low (1 2%). This indicates that radiation-induced polymerization occurs in the glass where the cation radicals are formed by the radiation. The experimental results of the polymerization in glass matrices are consistent with the previous report that the radiation-... 

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. 

It was found in this experiment that both anionic and cationic species reacted efficiently with methanol in bulk styrene. The bonded dimer cations and the radical anions were converted to long-lived benzyl radicals, which initiated the radical polymerization. The G value of the propagating benzyl radical was only 0.7 in pure styrene, but it increased up to 5.2 in the presence of methanol. A small amount of methanol converted almost all the charge carriers to propagating free radicals this explains why the mechanism of radiation-induced polymerization is changed drastically from cationic to radical processes on adding methanol. 

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],... 

Mah et al. demonstrated the effect of counterions on the cationic polymerization of styrene [35-37]. The radiation-induced polymerization is much more sensitive to impurities than the catalytic polymerization, as the former involves the cationic species in a free ion state. Thus, one can expect, in the presence of stable anions, the promotion of the cationic polymerization because of the ion-pair formation between the dimer cation and the counterion. The effect was... 

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. 

The evidence in the case of styrene, where both modes of radiation-induced polymerization can be conveniently studied, is quite convincing that reduction of the concentration of water changes the predominating mode of propagation from purely free radical to essentially ionic. Evidence for an ionic propagation initiated by radiation has also been obtained in pure a-methylstyrene (3, 24), isobutylene (12, 32), cyclopenta-diene (5), / -pinene (2), 1,2-cyclohexene oxide (II), isobutyl vinyl ether (6), and nitroethylene (38), although the radical process in these monomers is extremely difficult, if not impossible, to study. 

Recently, Garreau et al. (1979) reported a careful and rather detailed study of the kinetics of the radiation-induced polymerization of styrene in emulsion with sodium lauryl sulfate under conditions found earlier to lend to close agreement with simple Case 2 Smilh-Ewart kinetics (Smith, 1948). Most of the normal reaction variables were studied, and the rates of polymerization were found to be independent of the luonomer-to-water... 

Fifl. 4. Temperature dependence of the rates and degrees of polymerization for the radiation induced polymerization of styrene. Dose rate OJ093 Mrad/hr (Oarreau er al. 1979 reproduced with permission of Journal of Colloid and Inlerfoct Science.)... 

Application of these arguments to those data which are unequivocally free ion values does seem to work, however. For example the radiation induced polymerizations produce pre-exponential factors of 10 —10 1 mole sec , slightly larger than the accepted range for the free radical polymerization of styrene. Similarly the work on iV-vfnylcarbazole [29] produces Ap — 10 1 mole" sec", somewhat larger than the free radical value [106], and presumably affected by solvation contributions. The data for isobutyl vinyl ether [31] indicate a value for Ap of 10 —10 , consistent with the idea that free cations are the main contributors. 

In the case of photo-initiated polymerizations bimodal distributions can occur as a result of concurrent free-radical and cationic growth, these intermediates not being in chemical equilibrium. This arises in the bulk polymerizations of styrene photo-initiated in the presence of tetracyanobenzene and also in the radiation-induced polymerization of alkyl-substituted styrenes.In the former system the higher molecular weight fraction is attributed to a cationic... 

Figure 2. Relative rates for radiation-induced polymerization of styrene in dibutyl...

First, suitable monomers are required for radiation-induced polymerization proceeding by a cationic mechanism. Isobutylene, vinyl ethers, cyclopentadiene and p-pinene polymerize only by a cationic mechanism, whereas a-methyl styrene polymerizes by both cationic and anionic mechanisms. Second, it is necessary to use the conditions of the existence of ions M+ (M—>M+ + e) and the stabilization of secondary electrons capable of neutralizing M+. This is achieved (a) by carrying out polymerization at low temperatures when the lifetime of ions increases and the activity of free radicals drastically decrease, and (b) by using electron-accepting solvents or additives. 

The subjects of reports on radiation-induced polymerization have included the following monomers ethylene,70 tetrafluoroethylene,71 acrylonitrile,72 acrylic acid,78 and methacrylonitrile,74 alkyl acrylates and methacrylates,76 styrene,78 other vinyl monomers,77 78 acrylamide,79 vinylcarbazole,80 maleimide,81 pentenes,82 aminoalkyl monomers,83 isobutyl vinyl ether,84 and buta-1,3-diene.85... 

Radiation-induced curing of plastic coatings has been discussed,8 and radiation-induced polymerization in the solid state reported.87 The radiation chemistry of epoxy-containing electron resists88 and polycondensation induced by ionizing radiation in the urea-formaldehyde system88 have been described. Radiation-induced copolymerization of the following pairs of monomers has been achieved ethylene-hexafluoropropylene,80 tetrafluoroethylene-propylene,81 tetrafluoroethylene-hexafluoropropylene,82 hexafluoroacetone-a-olefins,83 MMA-di- and tri-methacrylates,84 styrene-acrylonitrile,85 buta-1,3-diene-acrylonitrile,88 and acenaphthylene-vinylcarbazole.87... 

When a miniemulsion polymerization initiated by Co y-rays was carried out to synthesize polystyrene particles [65], the dose rate and total dose were each found to aflect the size of the latex particles. Y-Hke branched surfactants were synthesized and used for y-ray miniemulsion polymerization at room temperature [66] moreover, 2wt% polyurethane used as a hydrophobe for the miniemulsion polymerization of styrene was suflBdent to ensure a shelf-Hfe of one year for the miniemulsion [67]. In both cases [66, 67], the particle size and distribution were preserved throughout the polymerization. The copolymerization of styrene with l-vinyl-2-pyrrolidone as a polar monomer in the presence of dodecane in the oil droplets, also led to the production of nanocapsules [68]. Subsequent H NMR spectroscopic analyses showed that graft copolymers had been obtained via radiation-induced polymerization, rather than random copolymers. Hence,... 

TABLE 8.16 Radiation Induced Polymerization of Styrene in Bulk and in Emulsion... 

Fukano K and Kageyama E, Study on radiation-induced polymerization of vinyl monomers adsorbed on inorganic substances. I. Radiation-induced polymerization of styrene adsorbed on several inorganic substances , J Polym Sci Polym Chem... 

It is important to note that values are of the same order of magnitude whether conventional UV or laser radiation were used (Table I). This result is in contrast with the recent work of Sadhir et al. (7) on the photopolymerization of maleic anhydride and styrene, induced by the 363.8 nm emission of an argon ion laser. That work showed to be the laser initiation 1000 times more energy efficient than the UV-induced polymerization. This discrepancy may arise from two factors (i) the lower light-intensity used in the laser irradiation that should favor chain propagation and (ii) differences in the way of comparing the... 

The formation of ion radicals from monomers by charge transfer from the matrices is clearly evidenced by the observed spectra nitroethylene anion radicals in 2-methyltetrahydrofuran, n-butylvinylether cation radicals in 3-methylpentane and styrene anion radicals and cation radicals in 2-methyltetrahydrofuran and n-butylchloride, respectively. Such a nature of monomers agrees well with their behavior in radiation-induced ionic polymerization, anionic or cationic. These observations suggest that the ion radicals of monomers play an important role in the initiation process of radiation-induced ionic polymerization, being precursors of the propagating carbanion or carbonium ion. On the basis of the above electron spin resonance studies, the initiation process is discussed briefly. 

H. Hopff and N. Balint developed a copolymerization process for tetrachloroethylene with ethylene. Radiation-induced chlorination of polyisobutylene is the subject of the chapter of C. Schneider and P. Lopour. M. Litt, V. T. Stannett, and E. Vanzo show that the polymerization of vinyl caproate follows the kinetics of styrene. 

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 unique capabilities of chemical vapor deposition are clearly demonstrated by the thin films belonging to this class of polymers. Yasuda et al first studied and reported the polymerization of organic compounds in glow discharge. Polymerization of organic compounds in the presence of plasma is quite different from the conventional chemically or radiatively initiated polymerization. For instance, polymerization of styrene in conventional polymerization can be done using several means of initiation such as radiation, pyrolysis induced, etc., to create the free radical species. But the propagation is... 

Another application in macromolecular chemistry is radiation-induced graft polymerization, by which favourable properties of two polymers can be combined. In this process, copolymers of A and B are produced by irradiation of the polymer A in the presence of the monomer B. Examples are graft polymers of polyethylene and acrylic acid or of polyvinyl chloride and styrene. The properties of textiles (cellulose, wool, natural silk, polyamides, polyesters) can also be modified by graft polymerization, for example for the production of weatherproof products. 

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