Polyvinyl chloride radicals

When growing polyvinyl chloride radicals react with polyalkene macromolecules, transfer to polymer occurs according to scheme (40) (see Sect. 1.2). A polyalkene-gra/l-poly vinyl chloride results. This polymer is already being... 

The simplest linear-chain polymer is polyethylene (Fig. 22.3a). By replacing one H atom of the monomer by a side-group or radical R (sausages on Fig. 22.3b, c, d) we obtain the vinyl group of polymers R = Cl gives polyvinyl chloride R = CIT3 gives... 

Polyvinyl chloride can also be produced in emulsion. Water is used as the emulsion medium. The particle size of the polymer is controlled using the proper conditions and emulsifier. Polymers produced by free radical initiators are highly branched with low crystallinity. 

When many molecules combine the macromolecule is termed a polymer. Polymerization can be initiated by ionic or free-radical mechanisms to produce molecules of very high molecular weight. Examples are the formation of PVC (polyvinyl chloride) from vinyl chloride (the monomer), polyethylene from ethylene, or SBR synthetic rubber from styrene and butadiene. 

Addition polymers, which are also known as chain growth polymers, make up the bulk of polymers that we encounter in everyday life. This class includes polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Addition polymers are created by the sequential addition of monomers to an active site, as shown schematically in Fig. 1.7 for polyethylene. In this example, an unpaired electron, which forms the active site at the growing end of the chain, attacks the double bond of an adjacent ethylene monomer. The ethylene unit is added to the end of the chain and a free radical is regenerated. Under the right conditions, chain extension will proceed via hundreds of such steps until the supply of monomers is exhausted, the free radical is transferred to another chain, or the active site is quenched. The products of addition polymerization can have a wide range of molecular weights, the distribution of which depends on the relative rates of chain grcnvth, chain transfer, and chain termination. 

Polyvinyl chloride Vinyl chloride Free radical ... 

The vinyl chloride monomer polymerizes via addition polymerization to form polyvinyl chloride. The final polymer has the chemical composition shown in Fig. 22.1. The polymer exhibits limited crystallinity, though this property is not often considered as important in defining its performance. It tends to be atactic or regionally syndiotactic, surrounded by extended atactic runs. When exposed to temperatures above 100 °C, polyvinyl chloride decomposes, creating free radicals that further attack the polymer chain, as we shall discuss in more detail later. For this reason, the degradation of polyvinyl chloride is autocatalytic... 

Vinyl chloride polymerization occurs via an exothermic radical reaction. In fact, the reaction is approximately 25% more exothermic than polyethylene polymerization. The highly exothermic nature of the reaction and the strong molecular weight dependence on temperature make heat transfer, and its control, critical to the manufacture of polyvinyl chloride. 

Figure 22.4 Radical chain mechanism for the degradation of polyvinyl chloride a) initiation, b propagation and c) termination...

On commercial scale this technique is used to obtain polyvinyl chloride, polyvinyl acetate, etc. In this the free radical initiators are used. 

Examples of important commercial products obtained by free radical polymerisation of substituted ethenes are polypropene (polypropylene). Polyphenylethene (polystyrene), poly-1 chloroethene (polyvinyl chloride) and poly 1-methoxy carbonyl-1 methylethene (polymethalmethacrylate). 

After the demonstrations of preparation of stereoregular polymers having novel properties by means of special ionic methods, die possibilities of free radical methods were examined extensively. It must be concluded that in free radical systems the structures of homopolymers and copolymers can be little influenced by specific catalysts and other reaction conditions, but are determined largely by monomer structure. This is consistent with the relative uniformity of comonomer reactivity ratios in radical copolymerizations. However, it has been found possible to obtain somewhat more syndiotactic structure, dldl. than normally obtained by radical reactions, at low temperatures and by selecting solvents. Examples are polyvinyl chlorides of higher than usual crystallinity from polymerizations at low temperature e.g.. —50°C under ultraviolet light... 

Considering now reactions (5 a) and (5 b) (p. 176), it was found that the addition of monomers to macroradicals produced by chain transfer depends directly on the reactivity and polarity of both the radical and the monomer (203) and that the Q—e scheme of Alfrey and Price can be applied to these graft copolymerizations by chain transfer (227). In this way some unsuccessful attempts for grafting were interpreted, e. g. vinyl acetate on polystyrene and methyl methacrylate on polyvinyl acetate and polyvinyl chloride. 

Polyvinyl chloride. Organic peroxides are used to catalyze the free radical polymerization of vinyl chloride monomer in water. The organic peroxide is selected to generate free radicals thermally at the temperature of polymerization. 

Vinylene carbonate is one of the few 1,2-disubstituted ethylenes that is known to undergo facile radical initiated homopolymerization. Initiation may be by oxygen, peroxides or cobalt-60 y-radiation. Such polymers are reportedly useful as coatings and films. Vinylene carbonate also copolymerizes with ethylene under high pressure to yield a material with about 10% vinylene carbonate content. This polymer, when blended with polyvinyl chloride, is suitable for injection molding. 

Polyvinyl chloride is produced by the free-radical polymerization of vinyl chloride. Bulk, emulsion, solution, and suspension polymerization processes have been used. 

Polyvinyl chloride-montmorillonite nanocomposites were prepared either by in-situ suspension polymerisation using initiator- and comonomer-modified montmorillonite in the presence of free-radical initiators, such as AIBN, and compared. It was found that monomer conversion was low when the nanocomposites were prepared using initiator-modified montmorillonite. Exfoliated nanocomposites... 

The effectiveness of such cheap termination by oxygen is, of course, low. In high-pressure polyethylene, polyvinyl chloride, polytetrafluoroethylene, etc., a large amount of frozen radicals was detected by ESR [126]. Actually, even after oxygen addition to a radical, the resulting ROO is sufficiently unstable. Research in this direction could lead to an acceptably cheap and efficient deactivation. 

The radical copolymeiization of N-vinyl-4,5,6,7-tetrahydroindole with vinyl chloride was accompanied by dehydrochlorination of polyvinyl chloride blocks (Equation (56)) (08MI315). 

Even though vinyl chloride was discovered in 1835, polyvinyl chloride was not produced until 1912. It is now one of our most common polymers production in 1984 was over 6 billion pounds. The monomer is made by the pyrolysis of 1,2-dichloroethane, formed by chlorination of ethylene. Free radical polymerization follows Markovnikov s rule to give the head-to-tail polymer with high specificity ... 

In addition polymerization, monomers react to form a polymer chain without net loss of atoms. The most common type of addition polymerization involves the free-radical chain reaction of molecules that have C = C bonds. As in the chain reactions considered in Section 18.4, the overall process consists of three steps initiation, propagation (repeated many times to build up a long chain), and termination. As an example, consider the polymerization of vinyl chloride (chloro-ethene, CH2 = CHC1) to polyvinyl chloride (Fig. 23.1). This process can be initiated by a small concentration of molecules that have bonds weak enough to be broken by the action of light or heat, giving radicals. An example of such an initiator is a peroxide, which can be represented as R—O—O—R, where R and R represent alkyl groups. The weak 0—0 bonds break... 

In the polymer industry phenolic compounds are important stabilizers they are used to prevent the free-radical induced polymerization of monomers (e.g. methyl methacrylate, styrene) during transit and as stabilizers for polymer systems where radical induced decomposition and decay mechanisms operate. Examples of the latter include polyolefins, such as polyethylenes or polypropylenes, and polyvinyl chlorides. However, these uses of phenols and phenolic polymers are not considered in this chapter (refer to Chapter 12). 

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