Radiation-Initiated Polymerizations

Usually, free-radical initiators such as azo compounds or peroxides are used to initiate the polymerization of acrylic monomers. Photochemical (72—74) and radiation-initiated (75) polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of acrylic monomers is first order with respect to monomer concentration and one-half order with respect to the initiator concentration. Rate data for polymerization of several common acrylic monomers initiated with 2,2 -azobisisobutyronittile (AIBN) [78-67-1] have been determined and are shown in Table 6. The table also includes heats of polymerization and volume percent shrinkage data. 

Radiation-Induced Polymerization. In 1956 it was discovered that D can be polymerized in the soHd state by y-irradiation (145). Since that time a number of papers have reported radiation-induced polymerization of D and D in the soHd state (146,147). The first successhil polymerization of cychc siloxanes in the Hquid state (148) and later work (149) showed that the polymerization of cycHc siloxanes induced by y-irradiation has a cationic nature. The polymerization is initiated by a cleavage of Si—C bond and formation of silylenium cation. 

Polymerization in aqueous solution of acrylamide can also be fulfilled in thin layers (up to 20 mm) applied on a steel plate or a traveling steel band. Polymerization is initiated by persulfates, redox system, UV or y radiation. Polymerization proceeds in isothermal conditions as the heat of polymerization is dissipated in the environment and, additionally, absorbed by the steel carrier. Nonadhesion of the polymer to the carrier is ensured by the addition of glycerol to isopropyl alcohol or by precoating the steel band with a film based on fluor-containing polymers. This makes polymerization possible at a high concentration of the monomer (20-45%) and in a wider process temperature range. This film of polyacrylamide is removed from the band, crushed, dried, and packed. 

The creation of active sites as well as the graft polymerization of monomers may be carried out by using radiation procedures or free-radical initiators. This review is not devoted to the consideration of polymerization mechanisms on the surfaces of porous solids. Such information is presented in a number of excellent reviews [66-68]. However, it is necessary to focus attention on those peculiarities of polymerization that result in the formation of chromatographic sorbents. In spite of numerous publications devoted to problems of composite materials produced by means of polymerization techniques, articles concerning chromatographic sorbents are scarce. As mentioned above, there are two principle processes of sorbent preparation by graft polymerization radiation-induced polymerization or polymerization by radical initiators. We will also pay attention to advantages and deficiencies of the methods. 

The theory of radiation-induced grafting has received extensive treatment [21,131,132]. The typical steps involved in free-radical polymerization are also applicable to graft polymerization including initiation, propagation, and chain transfer [133]. However, the complicating role of diffusion prevents any simple correlation of individual rate constants to the overall reaction rates. Changes in temperamre, for example, increase the rate of monomer diffusion and monomer... 

Chojnowski and co-workers have studied the polymerization of octamethyltetrasila-l,4-dioxane, a monomer more basic than cyclosiloxanes, which is capable of forming more stable oxonium ions, and thus being a useful model to study the role of silyloxonium ions.150-152 In recent work, these authors used Olah s initiating system and observed the formation of oxonium ion and its transformation to the corresponding tertiary silyloxonium ion at the chain ends.153 The 29Si NMR spectroscopic data and theoretical calculations were consistent with the postulated mechanism. Stannett and co-workers studied an unconventional process of radiation-initiated polymerization of cyclic siloxanes and proposed a mechanism involving the intermediate formation of silicenium ions solvated by the siloxane... 

The radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals as described above. It is then possible for these species to initiate chain polymerizations. Whether a polymerization is initiated by the radicals, cations, or anions depends on the monomer and reaction conditions. Most radiation polymerizations are radical polymerizations, especially at higher temperatures where ionic species are not stable and dissociate to yield radicals. Radiolytic initiation can also be achieved using initiators, like those used in thermally initiated and photoinitiated polymerizations, which undergo decomposition on irradiation. 

The kinetics of radiation-initiated polymerizations follow in a relatively straightforward manner those of photolytic polymerization. The initiation rate is determined by the radiation intensity and the concentration and susceptibility of the compound that radiolyzes to yield the initiating species (ionic and/or radical). The final expression for Rp is determined by the exact details of the initiation, propagation, and termination steps. 

High pressure can have appreciable effects on polymerization rates and polymer molecular weights. Increased pressure usually results in increased polymerization rates and molecular weights. Figure 3-17 shows these effects for the radiation-initiated polymerization of styrene [Moore et al., 1977]. 

Fig. 3-17 Effect of pressure on the polymerization rate (o) and polymer molecular weight (A) of radiation-initiated polymerization of styrene at 25° C. After Moore et al. [1977] (by permission of Wiley-Interscience, New York.)...

Cationic photoinitiators are compounds that, under the influence of UV or visible radiation, release an acid, which in turn catalyzes the desired polymerization process. Initially, diazonium salts were used, but they were replaced by more thermally stable iodonium and sulfonium salts. Examples of cationic initiators are in Table 4.3. 

Usually, free-radical initiators such as azo compounds or peroxides are used to initiate the polymerization of acrylic monomers. Photochemical and radiation-initiated polymerizations are also well known. Methods of radical polymerization include bulk, solution, emulsion, suspension, graft copolymerization, radiation-induced, and ionic with emulsion being the most important. 

Since the cell wall structure of the wood is not swollen by the vinyl monomer, there is little opportunity for the monomer to reach the free radical sites generated by the gamma radiation on the cellulose to form a vinyl polymer branch. From this short discussion, it is reasonable to conjecture that there should be little if any difference in the physical properties of catalyst-heat initiated or gamma radiation initiated in situ polymerization of vinyl monomers in wood. 

Fig. 11 Nascent radiation initiated PTFE dispersion particles. The polymerization conditions were (a) 0% emulsifier, 90 min, (b) 0.5% emulsifier, 60 min, (c) 1% emulsifier, 60 min, (d) 2% emulsifier, 90 min, all for essentially the same radiation dose rate at 25 °C in water (with 1.3% hexadecane and ammonium perfluorooctanoate emulsifier) at 30-kg/cm2 pressure. The measured molecular weights, the corresponding extended chain lengths (eel) the and dimensional characteristics are, respectively, (a) Mn = 230 x 104, edn = 6.0 pm, particle volume approximately 7 x 108 Da (b) Mn = 50 x 104, eel = 1.3 pm, rod diameter approximately 100 molecules (c) Mn = 20 x 104, ed = 0.52 pm rod length (d) Mn=2xl04, ed = 520A. (Reprinted from Ref. [14] with permission from Wiley-Interscience)...

Abstract. Water-soluble polymers of acrylamide and acrylic acid with high extent ( 90%) of Ceo consumption are obtained by technique of low-temperature radiation living radical polymerization. In absorption spectra of these copolymers one can see gradually descended unstructured absorption in the range 240-700 mn, characteristic for fullerene covalent-bound or its klasters. The way of radiation initiation is used to obtain the products of high purity, because it is not necessary to embed into the system any initiators or catalyst. Latter is very important in the case of synthesis of polymers for medical purposes. Also at radiation initiation a rate of initiation reaction does not depend on the temperature and the sterilization of products takes place simultaneously. 

Williams et al. studied the radiation-initiated cationic polymerization of styrene. By conductivity measurements they were able to determine very small amounts of ions and thus directly determine the propagation rate constant [381] under otherwise comparable conditions, k+ is about 30times larger than k. ... 

There exist communications on chemical reactions catalyzed by magnetic fields in connection with electromagnetic radiation. A photoresist was cross--linked by Panico under such conditions [10]. Teffal and Gourdenne polymerized (without initiator) 2-hydroxyethyl methacrylate by means of microwaves in a waveguide [11]. Actually this should be regarded as thermal polymerization, with the heat generated by dipole vibrations of the polar groups the monomer. 

Figure 11. Solutions of the Smith-Ewart recursion equation for the case of no aqueom propagation or termination. Dotted line m = 0 (Smith-Ewart Case II). Curve 1 (m = 10 ) depicts typical styrene-like polymerization. Curve 2(m = 0.01) depicts radiation initiated emulsion polymerization of vinyl chloride. Curve 3 (m > 1.0) depicts chemically initiated emulsion polymerization of vinyl chloride.

Radiation Monomer structure for addition polymerization No initiator Liquid or solid phase... 

Polymerization initiated by ionizing radiation such as y rays or high-energy electron beams is somewhat similar to plasma polymerization. Understanding of radiation polymerization is very helpful in elucidating mechanisms of plasma polymerization. In radiation polymerization, no initiator is employed, and the chain-carrying species are created by the ionization of monomer. In this respect, radiation polymerization... 

Hoigne and O Neil (1972) studied several features of the y radiation-initialed polymerization in emulsion. Sodium lauryl sulfate, dioctyl and dibutyl sodium sulfosuccinates, and two nonionic, polyoxyethylene-type emulsifiers were used. Sodium lauryl sulfate gave, by far, the hipest rates and most stable lattices and was used for all of the results reported. The rates were found to be 0.43 order on the dose rate and 0.53 on the... 

Wang (1962) was the first to report studies on the y-radiation initiated polymerization of vinyl chloride in emulsion at room temperature. Rapid rates to high conversions were obtained after rather long induction periods of 1 to 3 hr. The degrees of polymerization were constant within experimental error at about 2000, in keeping with termination being dominated by chain transfer to monomer. Little or no dependence of the rale on the emulsifier concentration or the monomer concentration was observed. However, tbe rates were proportional to the 1.22 power of the dose rate. 

Panajkar and Rao (1979) have reported a ratber extensive study of the y radiation-initiated polymerization of vinylidene chloride in emulsion. With sodium lauryl sulfate as the emulsifier smooth polymerization-time curves at high rates were obtained, up to more than 9 conversion. Between 45 and 60% conversion, the linear region, the rate was 0.3 order with respect to tbe emulsifier concentration. The molecular weights were found to increase with conversion and values up to 79,000 were obtained. Some reasons for the departure from Smith Ewart behavior were suggested. Earlier. Hummel el al. (1967) had presented some interesting data on a closely related system, a similar rate-time behavior was observed and a tentative explanation proposed. Both discussions were based on tbe insolubility of the polymer in its own monomer. 

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