Substitution Reactions of Saturated Polymeric Hydrocarbons

It is often desirable to replace hydrogens with halogens. Fluorination of polyethylene can be carried out in the dark by simply exposing the polymer, either in sheet or in powder form, to fluorine gas. The reaction is exothermic and it is best to dilute the gas with nitrogen 9:1 to allow a gradual introduction of the fluorine and avoid destruction of the polymer [132, 133]. In this manner, however, only the surface layers are fluorinated and the substitutions occur only a few molecular layers deep. In surface fluorination with vacuum ultraviolet glow discharge plasma, the photon component of the plasma enhances the reactivity of the polymer surfaces toward fluorine gas [134]:

The surface free-radicals can also cause elimination of hydrogen radicals and formation of double bonds. Whether as free-radicals or through unsaturation, the units are now more reactive toward f luorine. Afilm that is 3 mil thick can be completely fluorinated on a 100-mesh phosphor bronze gauze, if the reaction is allowed to proceed for several days [135]. Fluorination can also be carried out with mercuric or cupric fluorides in hydrofluoric acid. The reaction must be carried out at 110C for 50 h. As much as 20% of fluorine can be introduced [136]. In direct fluorination of powdered high-density polyethylene with the gas, diluted with helium or nitrogen, the accompanying exotherm causes partial fusion. In addition, there is some destruction of the crystalline regions [137]. On the other hand, fluorination of single crystals of polyethylene can result in fluorine atoms being placed on the carbon skeleton without disruption of the crystal structure. The extent of cross-linking, however, is hard to assess [138]. The reaction has all the characteristics of free-radical mechanism [139]:

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 100Corhigher, 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 TiCl4 [141]. Such trace quantities can even be residual titanium halide from a Ziegler-Natta catalyst left over in the polymer from the polymerization reaction. When it is carried out at 50C in chlorobenzene, –CHCl– groups form [142]:

This slows the chlorination of adjacent groups. Trace amounts of oxygen catalyze chlorinations in the presence of visible light [140]. The same reaction in ultraviolet light is accompanied by cross-linking. The photochemical process can be illustrated as follows:

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. Commercial chlorinations of polyethylene are usually conducted on high-density (D > 0.96) linear polymers. The molecular weights of the starting materials vary. High molecular weight polymers form tough elastomers. Low molecular weight materials, however, allow easier processing of the products. The reactions are carried out in carbon tetrachloride, methylene dichloride, or chloroform at reflux temperatures of the solvents and at pressures above atmospheric to overcome poor solubility. The solubility improves with the degree of chlorination. Industrially, two different procedures are used. In the first one, the reactions are conducted at 95–130C. When the chlorinations reach a level of about 15%, the polymers become soluble and the temperatures are lowered considerably [143]. In the second one, the reactions are conducted on polymers suspended in the solvent. When the chlorine content reaches 40% and the polymers become soluble, chlorinations are continued in solution. By continuing the reaction, a chlorine content of 60% can be reached. The products from the two processes differ. The first one yields a homogeneous product with the chlorine atoms distributed uniformly throughout the molecules. Chlorination in suspension, on the other hand, yields heterogeneous materials with only segments of the polymeric molecules chlorinated. Some commercial chlorinations are conducted in water suspensions. These reactions are carried out at 65C until approximately 40% levels of chlorine are achieved. The temperatures are then raised to 75Cto drive the conversions further. In such procedures, agglomerations of the particles can be a problem. To overcome that, water is usually saturated with HCl or CaCl₂ [144]. Problems with agglomeration are also encountered during suspension chlorinations in solvents, like CCl4. Infra-red spectra of chlorinated polyethylenes show presence of various forms of substitutions. There are–CHCl–CHCl as well as–CCl₂– groups present in the materials [145, 146]. Surface photo chlorination of polyolefin films [146] considerably improves the barrier properties of the films to permeations of gases. Chlorinations of polypropylene usually result in severe degradations of the polymer. When TiCl4 is the chlorination catalyst, presumably, less degradation occurs [140]. Studies of bromination of polypropylene (atactic) show that when the reaction is carried out in the dark, in CCl₄ at 60C, the substitution reactions proceed at the rate of 0.5%/h [147].

Chlorosulfonation or polyethylene is a commercial process. The reaction resembles chlorination in the step of hydrogen abstraction by chlorine radicals. It is catalyzed by pyridine [148–150] and can be pictured as follows [147]:

The amount of SO2 vs. chlorine in the reaction mixture affects the resultant ratios of chlorosul fonation vs. chlorination of the polymer. These ratios and the amounts of conversion vary with the temperature [149]. Commercially produced chlorosulfonated polyethylene contains approximately 26–29% chlorine and 1.2–1.7% sulfur [151]. The material is an elastomer that remains flexible below 50C. It is commonlycross-linked (vulcanized) through the sulfonyl chloride groups. Heating it, either as a solid or in solution, to 150C results in loss of SO₂ and HCl. If the material is heated for 2 h at 175C, all SO₂Cl groups are removed. In gamma radiation-induced chlorosulfonations and sulfoxidations of polyethylene powders [153] at room temperature, the ratios of SO₂Cl groups to Cl groups decrease with increases in radiation. Polypropylene can be chlorosulfonated to the extent of containing 6% chlorine and 1.4% sulfur without embrittlement. The reaction can be done in CCl4 at 55C. There is apparently less degradation than in a direct chlorination reaction [148, 152].

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مواضيع ذات صلة

Chloromethylation Reactions

Reactions of Polystyrene

Substitution Reactions of Halogen-Bearing Polymers

Cyclizations and Intramolecular Rearrangements

Isomerization Reactions

Epoxidation Reactions

Diels–Alder Condensations

1,3-Dipolar Additions

Electrophilic Additions of Aldehydes

Addition of Carbenes

Hydrogenation

Halogenation