Chlorination of polyethylene

The first patent on the chlorination of polyethylene was taken out by ICI in 1938. In the 1940s scientists of that company carried out extensive studies on the chlorination process. The introduction of chlorine atoms onto the polyethylene backbone reduces the ability of the polymer to crystallise and the material becomes rubbery at a chlorine level of about 20%, providing the distribution of the chlorine is random. An increase in the chlorine level beyond this point, and indeed from zero chlorination, causes an increase in the Tg so that at a chlorine level of about 45% the polymer becomes stiff at room temperature. With a further increase still, the polymer becomes brittle. 

Chlorination of natural rubber, involving both addition and substitution (with some cyclization), yields a product with improved chemical and corrosion resistance. Chlorination of polyethylene in the presence of sulfur dioxide results in substituting both chloride and sulfonyl chloride groups into the polymer. A commercially useful material is one which contains about 12 chlorides and one sulfonyl chloride per 40-45 repeating units. This extensive substitution converts the polyethylene, a plastic, into an elastomer by destroying crystallinity. 

Chlorination of polyethylene (15-40% Cl2) produces a rubbery polymer which has lost most of the crystallinity. With... 

Exhaustive studies on well-defined systems are rather scarce (4) nevertheless 3 systems thoroughly analyzed by independant research groups are of outstanding interest a) the quaternization of polyvinylpyridines by alkyl halogenides (20-25) b) the chlorination of polyethylene (13,26-28) c) the basic or acid hydrolysis of PMMA (29-31). On the other hand, neighbouring groups effects have been quantitatively taken into account for the kinetic analysis of periodate oxidation of amylose (32,33). 

Chlorinated polyethylene is produced by chlorination of polyethylene and has the structure shown in Figure 4.31. 

Abnormal behavior can occur in the reaction of an initially homogeneous system if there is a change in the physical nature of the system on reaction of the polymer. Partial reaction may yield a polymer that is no longer soluble in the reaction medium or that forms a highly viscous system. The solubility changes can be quite complex, as shown in the chlorination of polyethylene when carried out in solution using aliphatic or aromatic hydrocarbon solvents... 

The chlorination of polyethylene, poly(vinyl chloride), and other saturated polymers has been studied [Favre et al., 1978 Lukas et al., 1978 McGuchan and McNeil, 1968 Stoeva and Vlaev, 2000]. The reaction is a free-radical chain process catalyzed by radical initiators. 

Chlorination is one of the most interesting processes for polymer modification, and is usually carried out by means of catalysts or by UV irradiation. Since 1960, radiation-induced chlorination of polyethylene and polypropylene has been studied, especially by Soviet workers (1-3). As the polymers used in that work are insoluble in the usual solvents at normal temperature, chlorination was done in the heterogeneous phase— for example, by leading continuously a stream of chlorine over the finely ground polymer or through an aqueous suspension of the polyolefin. It is, therefore, difficult to compare the results obtained under the different conditions used. 

The moderate random chlorination of polyethylene suppresses crystallinity and yields chlorinated polyethylene elastomer (CPE), a rubber-like material that can be crosslinked with organic peroxides. The chlorine (Cl) content is in the range of 36 to 42%, compared to 56.8% for PVC. Such elastomer has good heat and oil resistance. It is also used as a plasticizer for PVC. They provide a very wide range of properties from soft/elastomeric too hard. They have inherent oxygen and ozone resistance, resist plasticizers, volatility, weathering, and compared to PEs have improved resistance to chemical extraction. Products do not fog at high temperatures as do PVCs and can be made flame retardant. 

Low chlorination of polyethylene, causing random substitution, reduces chain order and thereby also the crystallinity. The low chlorine products (22-26% chlorine) of polyethylene are softer, more rubber-like, and more compatible and soluble than the original polyethylene. However, much of the market of such materials has been taken up by chlorosulfonated polyethylene (Hypalon, Du Pont), produced by chlorination of polyethylene in the presence of sulfur dioxide, which introduces chlorosulfonyl groups in the chain. 

In the absence of oxygen, the chlorination of polyethylene, with or without a catalyst, can be controlled to provide products with varying chlorine content. The chlorination process is statistically random so that chlorination of polyethylene to the same chlorine content as poly(vinyl chloride) (60%) gives a product that is chemically different from PVC yet fiilly compatible with it. This random chlorination of polyethylene destroys its crystallinity. At a degree of chlorination corresponding to the loss of all its crystallinity, the chlorinated product becomes soluble at room temperature. The p-bromination of polyethylene follows a similar course to yield a rubberlike polymer at 55% bromine content. 

We discussed above only the sim dest model, i ., the irreversible reaction of the first order. Let us now consider how one can take into accoimt some comjdications in the reaction. One such complication is to discuss non-first order reactions, for example, the chlorination of polyethylene which under certain conditions proceeds as a (l/2)order reaction This resulting order can be exidained by the following sdieme ... 

Chlorination of polyethylene can result in varying amounts of hydrogen atoms being replaced by chlorine. It is possible to form a product that contains 70% by weight of chlorine. The amount of chlorination affects the properties of the product. At low levels of substitution the material still resembles the parent compound. When, however, the level of chlorine reaches 30-40%, the material becomes an elastomer. At levels exceeding 40% the polymer stiffens again and becomes hard. 

The mechanism of UV-radiation iniliated chlorination of polyethylene primarily involves the fi ee radical rcactions shown in Figure 15. 

Chlorinations of polyethylene can be carried out in the dark or in the presence of light. The two reactions, however, are different, though both take place by free-radical mechanism. When carried out in the dark at 100°C or higher, no catalyst is needed, probably because there are residual peroxides from oxidation of the starting material. Oxygen must be excluded because it inhibits the reaction and degrades the product [140]. The reaction is catalyzed by traces of TiCU [141]. Such trace quantities... 

Ethylene/vinyl chloride (EVC) copolymers are typically prepared indirectly by chlorination of polyethylene [32]. These copolymers are of technological interest and, consequently, their microstructure/morphology relationships have been studied intensively [33-36]. In several of these studies, the importance of the sequence distribution of the pendant chloro substituents was particularly emphasized. 

A recent paper describes a mathematical model for the chlorination of polyethylene in a bubble column reactor, the model was used to optimize product quality in the continuous chlorination of polyethylene. Another theoretical treatment deals with the change in polymer reactivity during the course of a macromolecular reactions in solution or in the melt. The reactivity of a transforming unit in the polymer depends on its microenvironment, including nearest neighbours on the same chain and on other chains, as well as small molecules in the reacting system. The equations derived describe the kinetic curve, the distribution of units, and the compositional heterogeneity of the products. 

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