Polyisoprene halogenation

The standard polymers used for rubber linings consist of materials that are cross-linkable macromolecules which, on mixing with suitable reactants that form strong chemical bonds, change from a soft deformable substance into an elastic material. These polymers include natural rubber and its corresponding synthetic, c/s-polyisoprene, styrene-butadiene rubber, polychloroprene, butyl rubber, halogenated butyl rubbers, acrylonitrile-... 

Natural rubber Styrene-butadiene rubber Polybutadiene Polyisoprene Nitrile rubber Halogenated nitrile rubber Ethylene-propylene rubber EPDM... 

Figure 7. Proton noise-decoupled 22.6-MHz C-13 NMR spectra of the hydro-halogenated 1,4-polyisoprene samples. Deuteriochloroform solutions at 25° C with TMS as internal reference. Approximately 5000 pulses with an acquisition time of 0.7 sec and a flip angle of 30°.

As with many polymers, polyisoprene exhibits non-Newtonian flow behavior at shear rates normally used for processing. The double bond can undergo most of the typical reactions such as carbene additions, hydrogenation, epoxidation, ozonolysis, hydrohalogena-tion, and halogenation. As with the case of the other 1,4-diene monomers, many copolymers are derived from polyisoprene or isoprene itself. 

Some of the applications of the organometallic compounds of lanthanides are as catalysts for (i) stereo specific polymerization of diolefins and in particular to obtain high yields of 1,4-ci.v-polybutadiene and 1,4-cw-polyisoprene and copolymer of the two monomers. The order of effectiveness of the rare earths as catalysts is Nd > Ce, Pr < Sm, Eu. The nature of halogen of the Lewis acid affecting the catalytic activity is in the order Br > Cl > I > F. Detailed work on the activity of cerium octanoate-AlR3-halide showed stereo specificity with cerium as the primary regulator. Cerium is thought to form jr-allyl or 7r-crotyl complexes with butadiene. 

Polyenes (i.e., unsaturated aliphatic polymers) such as polyisoprenes, and polybutadiens may be hydrogenated, halogenated, hydrohalo-genated, cyclized, and epoxidized. 

Anionic Vinyl Polymerization. The carbanionic terminals in living anionic polymerization can be transformed into carbon—halogen bonds suited for radical generation. The backbones utilized thus far for this approach include polystyrene (B-58 to B-62)215,374 and polyisoprene (B-63 and B-64),374,375 although the former segment can also be prepared by the living radical polymerization (Figure 19). 

CHEMICAL MODIFICATION The following chemical modifications of cis-1,4-polyisoprene are employed as a convenient way of altering physical and mechanical properties hydrohalogenation, halogenation, oxidation, ozonolysis, hydrogenation, carbene addition, cyclization. ... 

Halogenation reactions of unsaturated polymers follow two simultaneous paths, ionic and free radical. Ionic mechanisms give soluble products from chlorination reactions of polybutadiene." The free-radical mechanisms, on the other hand, cause crosslinking, isomerization, and addition products. If the free-radical reactions are suppressed, soluble materials form. Natural rubber can be chlorinated in benzene with addition of as much as 30% by weight of chlorine without cycliza-tion. " Also, chlorination of polyalkenamers, both cis and trans, yields soluble polymers. X-rays show that the products are partly crystalline. The crystalline segments obtained from 1,4-trans-polyisoprene are diisotactic poly( 0 /rw-dichlorobutamer)s while those obtained from the 1,4-cis isomer are diisotactic polyOAfieo-l,2-dichlorobutamer)s. ... 

Since 1,4-polyisoprene has a secondary carbon atom at the double bond it follows that it is generally more reactive to both free radicals and to carbonium ions than 1,4-poly butadiene. The typical addition reactions associated with the double bond suggest that the ultimate hydrogenated, halogenated, hydrohalogenated and isomerized diene polymers would have the same structure irrespective of the initial cis-ltrans- ratio. 

Like the very important diene rubbers (polyisoprene, polybutadiene and SBR) the ethylene-propylene rubbers are hydrocarbons. They are therefore resistant to polar solvents but dissolve (when unvulcanized) or swell (when vulcanized) in hydrocarbons. Being saturated they are somewhat inert chemically and therefore have good resistance to oxygen, ozone, acids and alkalies. Besides being attacked by peroxide radicals they may also be halogenated. 

This definition is usually extended to embrace polymers in which one or more of the hydrogen atoms in the basic polyalkenamer structure are substituted by alkyl, aryl, halogen or other groups. Thus polybutadiene, polyisoprene and polychloroprene may be considered as particular members of this group. 

Representative diene-based polymers include natural rubber (NR), polyisoprene (PIP), PBD, styrene—butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR), which together compose a key class of polymers widely used in the rubber industry. These unsaturated polyolefins are ideal polymers for chemical modifications owing to the availability of parent materials with a diverse range of molecular weights and suitable catalytic transformations of the double bonds in the polymer chain. The chemical modifications of diene-based polymers can be catalytic or noncatalytic. The C=C bonds of diene-based polymers can be transformed to saturated C—C and C—H bonds (hydrogenation), carbonyls (hydrofbrmylation and hydrocarboxylation), epoxides (epoxidation), C—Si bonds (hydrosilylation), C—Ar bonds (hydroarylation), C—B bonds (hydroboration), and C—halogen bonds (hydrohalogenation). ... 

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