Solution vinyl polymerization radiation initiation

Radiation initiation for solution vinyl polymerization is more complex. A special feature of radiation is that it attacks all components of the system including the solvent. With a monomer-solvent mixture, therefore, the rate of initiation can be represented by a linear equation as follows ... 

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.

The radiation initiated polymerization of vinyl monomers in pure liquid or in solution was initially thought to be exclusively free radical in nature. In 1957 Davison et al. (7) demonstrated that isobutylene could be polymerized both v/ith high energy electrons and with cobalt 60 radiation. Because this monomer pciynerizes only by a cationic mechanism it was clear that radiation could, indeed, initiate ionic polymerization in the liquid state. Since then there has been considerable research with a number of monomers. The field has, however, continued to be dominated by studies of free radical processes. Almost all the ionic studies have been concerned with cationic processes although a few anionic polymerizations have been reported, notably with nitrcethylene (8). 

Considerable work has been done on the initiation of the vinyl fluoride by ionizing radiation much as y-radiation from a °Co source. A selection of references on this research includes Usmanov and other authors [4,48-62], Of these, Usmanov et al. [4] deal with the graft copolymerization of vinyl fluoride to some natural and synthetic polymers. Usmanov et al. [53] discuss the formation of branched polymers during radiation-induced polymerization. Gubareva et al. [54] deal with solution polymerizations. Nakamura et al. [58, 59, 61] deal with emulsion polymerizations of vinyl fluoride by radiation initiations. Usmanov et al. [60,61] discuss the effects of chain-transfer agents during radiation-initiated polymerization. Some copolymerization studies are described in Usmanov et al. [55]. 

In general, the polymerization of vinyl chloride may be carried out in bulk, solution, suspension, and in emulsion. Free-radical initiators are most commonly used although organometallic initiators and radiation initiation have been considered. Since the monomer is a gas at ordinary temperatures and pressures, suitable equipment is required for VCM polymerization. Sealed tubes and capped bottles have been used for this experimental work. In the use of bottles, safety precautions should be considered both from the standpoint of explosion hazards and the problems of exposure of personnel to VCM. 

Regarding the initiation process of polymerization, it can be started by y-radiation. It is a method that has been used for the synthesis of hydrogels of PEO as well as hydrogels based on vinyl monomers " in this latter case, azo-compounds such as 2,2-azo-isobutyroni-trile (AIBN)f or 2,2 -azobis (2-amidine-propane) dihydrochloride or V-SO, and aqueous salt solutions such as aqueous ammonium peroxodisulfate are also used. Among the monomers most used in the preparation of hydrogels through free-radical polymerization are 2-hydroxyethyl methacrylate (HEMA) and A-vinyl-2-pyrrolidone (VP). ... 

For example, compositions of 10 ml of a 3.75 gm per 100 ml aqueous solution of sodium dioctylsulfosuccinate (Aerosol OT or Manoxol OT) and 1.25 ml of vinyl acetate with small additions of sodium dihydrogen phosphate, after exclusion of oxygen by at least four degassings, were sealed in glass ampoules. The samples were exposed, at 15°C, to a Co source (100 curie). Conversion was nearly 75% after 30 min. Polymers of MW as high as 10 were formed. The polymers contained trapped free radicals capable of initiating graft polymerization with methyl methacrylate after removal from the radiation source [185]. 

Graft copolymerization is the most effective method utilized in the chemical modification of natural fibers. According to Gassan et al. [99], the reaction is initiated by free radicals on the fiber surface. Ionization polymerization of fibers is carried out in an aqueous solution, followed by exposure to a high-energy radiation. Fiber molecule cracks and radicals are subsequently formed. Next, the radical sites of the fiber are treated with a suitable solution compatible with a polymer such as vinyl monomer, acrylonitrile, methyl methacrylate, or polystyrene. Finally, this leads to the formation of a graft copolymer, which possesses fiber and graft polymer characteristics. 

Apart from the fluoro monomers vinyl fluoride (VF), vinylidene fluoride (VF2), and tetrafluoroethylene (TFE), only chlorofluoroethylene has found commercial use as homopolymer. It is applied as thermoplastic resin based on its vapor-barrier properties, superior thermal stability (Tdec > 350 °C), and resistance to strong oxidizing agents [601]. Chlorofluoroethylene is homo- and copolymerized by free-radical-initiated polymerization in bulk [602], suspension, or aqueous emulsion using organic and water-soluble initiators [603,604] or ionizing radiation [605], and in solution [606]. For bulk polymerization, trichloroacetyl peroxide [607] and other fluorochloro peroxides [608,609] have been used as initiators. Redox initiator systems are described for the aqueous suspension polymerization [603,604]. The emulsion polymerization needs fluorocarbon and chlorofluorocarbon emulsifiers [610]. 

The polymerization of vinyl chloride has also been initiated by y-radiation in bulk [45, 186], in solution [187, 188], and in emulsion [189]. 

Hydrogels can be prepared conveniently with the aid of yrays, starting with aqueous solutions of the respective monomer [16]. The process consists of two radiation-induced steps (i) polymerization of the monomer and (ii) crosslinking of the polymer formed in the first step (see Section 5.2.1). In fact, the process can commence with the OH radical-induced polymerization of monomers. With prolonged irradiation, the initially formed macromolecules such as poly(vinyl pyrrolidone) and polyacrylamide undergo crosslinking by OH radical attack [15,115]. 

Vinylpyridinium ions, 1, like other vinyl monomers are subject to thermal and light initiated polymerization. A recent study in our laboratory has confirmed that dilute ( < 0.20 M) aqueous solutions of 1 (R = CH3, C2H5) and sonicated suspensions of 1 and water are stable at temperatures below 70 °C in normal laboratory light. Suspensions of water-insol monomers, e.g. 1(R = C12H25), polymerize rapidly at temperatures above 80 °C. Dilute aqueous solutions of the water-soluble monomer 1 (R = CH3 or C2H5) resist polymerization at 80-100 °C but polymerize when subjected to UV radiation (300 nm). 

All attempts to copolymerize 1 with other vinyl monomers were unsuccessful until Salamone and coworkers obtained a 1,2-addition copolymer 4 of 1 (R = H] and 4-vinylbenzenesulfonate, 20, obtained by spontaneous polymerization oi the two monomers as a melt or in concentrated aqueous solution [61]. Vinyl-type copolymers of 1 and 20, and 1 and 21 were also obtained by initiation with radicals (e.g. from ACVA), light or y-radiation. These experiments are summarized in Table 6.3.1. Copolymerization of 1 (R = H) with AMPS 21 is complicated when carried out as a spontaneous (i.e. absence of an initiator) process in water, methanol, or iV,iV-dimethylformamide (DMF). Two homopolymers derived from 1 (R = H), the 1,2-addition polymer, 2 (R = H), and the polyionene, 7. are obtained along with poly AMPS. Similar attempts to form copolymer from 1 (R = H) and vinylsulfonate have been unsuccessful. In fact, the latter monomei resisted all conditions of polymerization and 1 (R = H) formed the ionene 7 from a melt or in dilute aqueous solution (< 1.0 m) and yielded 2(R = H) in concentrated aqueous solution. Table 6.3.1. Recall that this behavior duplicates that reported for 1 (R = H) by Ringsdorf and others. Ringsdorf attributed the change in structure of homopolymer to micellization of 1 (R = H) in concentrated aqueous solution. 

Free radical copolymerization is the most commonly used method to synthesize hydrogel from vinyl monomers. The ionic or neutral monomers and the cross-linking agents are copolymerized in bulk, in solution, or in suspension. The reaction is normally initiated with chemical initiator, heat, or radiation, e.g. UV light or gamma ray. After polymerization the hydrogels need to be washed with water in order to remove the residual monomers and homopolymers not incorporated into the network. 

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