Halogenation

Hydro chlorination of natural rubber is often accompanied by cyclization [35, 36]:

trans-1,4 and 1,2-Polybutadiene can be hydro halogenated under mild conditions with gaseous HCl [51]. The same is true of copolymers of butadiene with piperylene and also of isotactic trans-1,4 piperylene. The addition of HCl to the asymmetric double bond is trans for polypiperylene and occurs in a stereoselective way, judging from the ¹³CNMR [51] spectra. Polysilanes with alkene substituents add HCl and HBr in the presence of Lewis acids [58]. The products are the corresponding chlorine and bromine containing polymers with little degradation of the polysilane backbone:

Chlorinations of rubber, however, are fairly complex, because several reactions occur simultaneously. These appear to be: (1) additions to the double bond; (2) substitutions; (3) cyclizations; and (4) cross-linkings. As a result, the additions of halogens to the double bonds are only a minor portion of the overall reaction scheme [37, 38]. In CCl4, the following steps are known to occur:

Halogenation reactions of unsaturated polymers follow two simultaneous paths, ionic and free radical. Ionic mechanisms give soluble products from chlorination reactions of polybutadiene [42]. The free-radical mechanisms, on the other hand, cause cross-linking, isomerization, and addition products. If the free-radical reactions are suppressed, soluble materials form. Natural rubber can be chlorinated in benzene, however, with addition of as much as 30% by weight of chlorine without cyclization [39, 40]. Also, chlorination of polyalkenamers both cis and trans yields soluble polymers. X-rays show that the products are partly crystalline [43, 44]. The crystalline segments obtained from 1,4-trans polyisoprene are diisotactic poly (erythro dichlorobutamer) s, while those obtained from the 1,4-cis isomer are diisotactic poly (threo 1,2-dichlorobutamer) s [45]. Additive type chlorination of natural rubber can also be carried out with phenyl iododichloride or with sulfuryl chloride [39, 40]. Traces of peroxides must be present to initiate the reactions. This suggests a free-radical mechanism. Some cyclization accompanies this reaction as well [40]. In CCl₄, for the first 25 chlorine atoms that add per each 100 isoprene units, 23 double bonds disappear and only a small quantity of HCl forms. Subsequent 105 chlorinations, however, cause a loss of only 53 double bonds. Rubber can be brominated at 30C. If traces of alcohol are present, the reaction appears to go on entirely by addition [39, 40]. Without alcohol, substitutions take place rapidly and simultaneously with the additions to the double bonds [41]. Exomethylene groups and intramolecular cyclic structures form in the process. Slow additions of bromine to vinylidene double bonds result in formations of tri bromides, –C₅H₇Br₃. Also, cis and trans isomers of polyisoprene [41] brominate differently. Substitution reactions take place in brominations with N-bromosuccinimide. They are accompanied by cyclizations [39]. Brominations of polybutadienes with N-bromosuccinimide yield a-brominated polybutadienes [46, 47] that may also contain butane diylidene units. The products act as typical alkyl halides and can undergo Grignard-Wurz reactions:

The bromination reaction is accompanied by shifts of the double bonds that are coupled with the sites of substitution. Several different substituents can form. The polymers may contain pentane diylidene, hexane diylidene, and heptane diylidene units [46, 47]. By contrast, chlorination of polybutadiene in benzene is a straightforward addition reaction of the halogens to the double bonds [48, 49]:

Very little HCl is liberated until all the double bonds are consumed. When CCl4 is used in place of benzene, some substitutions occur during the latter stages of the reactions. If cross-linking occurs at the same time, the substitutions may not be extensive. The cross-linking reactions are believed to involve carbocationic intermediates. Polybutadiene can be halogenated readily in tetrahydrofuran with iodine chloride or bromine [49]. The products are glassy polymers. These products dehalogenate in reactions with organolithium compounds, like n-butyllithium, sec-butyllithium, and polystyryllithium in tetrahydrofuran solution. Dehalogenation of poly (iodo-chlorinated butadiene) with n-butyllithium yields product with different cis/trans ratios. Also, this is accompanied by partial cross-linking. The reactions may involve [49] halogen-metal exchanges that are followed by intra- and intermolecular elimination of lithium halide. In brominations of polybutadienes, both couplings and eliminations take place. Both iodo-chlorinated and brominated polybutadienes form graft copolymers when reacted with polystyryllithium in tetrahydrofuran [50]. Gel formation, however, accompanies the grafting reaction.

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مواضيع ذات صلة

Reactions That Favor Large Molecules

Effects of Changes in Solubility

Effect of Molecular Size

Paired Group and Neighboring Group Effects

Reactivity of Macromolecules

Polyalkanoates

Synthetic Methods for the Preparation of Nucleic Acids

DNA and RNA

Nucleic Acids

Chemical Modification of Proteins

Synthetic Methods for the Preparation of Polypeptides

Structures and Chemistry of Proteins