Isoprene sites

Halogenated Butyl Rubber. Halogenation at the isoprene site ia butyl mbber proceeds by a halonium ion mechanism leading to a double-bond shift and formation of an exomethylene alkyl haUde. Both chlorinated and brominated mbber show the predominate stmcture (1) (>80%), by nmr, as described eadier (33,34). Halogenation of the unsaturation has no apparent effect on the isobutylene backbone chains. Cross-linked samples do not crystallize on extension due to the chain irregularities introduced by the halogenated isoprene units. 

Butyl and Halobutyl Rubber. Butyl mbber is made by the polymerization of isobutylene a small amount of isoprene is added to provide sites for curing. It is designated HR because of these monomers. Halogenation of butyl mbber with bromine or chlorine increases the reaction rate for vulcanization and laminates or blends of halobutyl are feasible for production of mbber goods. It is estimated that of the - 100 million kg of butyl (UR) and halobutyl (HIIR) mbber in North America, over 90% is used in tire apphcations. The halogenated polymer is used in the innerliner of tubeless tires. Butyl mbber is used to make innertubes and curing bladders. The two major suppHers of butyl and halobutyl polymers in North America are Exxon and Bayer (see ELASTOLffiRS,SYNTHETIC-BUTYLrubber). 

Cure Systems of Butyl Rubber and EPDM. Nonhalogenated butyl rubber is a copolymer of isobutjiene with a small percentage of isoprene which provides cross-linking sites. Because the level of unsaturation is low relative to natural mbber or SBR, cure system design generally requites higher levels of fast accelerators such as the dithiocarbamates. Examples of typical butyl mbber cure systems, thein attributes, and principal appHcations have been reviewed (26). Use of conventional and semi-EV techniques can be used in butyl mbber as shown in Table 7 (21). 

Butyl Rubber. Butyl mbber was the first low unsaturation elastomer, and was developed ia the United States before World War II by the Standard Oil Co. (now Exxon Chemical). It is a copolymer of isobutylene and isoprene, with just enough of the latter to provide cross-linking sites for sulfur vulcanization. Its molecular stmcture is depicted ia Table 1. 

Butyl rubber and other isobutylene polymers of technological importance iaclude various homopolymers and isobutylene copolymers containing unsaturation achieved by copolymerization with isoprene. Bromination or chlorination of the unsaturated site is practiced commercially, and other modifications are beiag iavestigated. 

Chlorobutyl (CIIR) and bromobutyl (BIIR) are modified types containing 1.2% wt of chlorine or bromine, the isoprene unit being the site of halogenation. Introduction of the halogen gives greater cure flexibility, and enhanced cure compatibility in blends with other diene rubbers. It also confers increased adhesion to other rubbers and metals. 

Cymopolia barbata elaborates a unique series of (20 prenylated bromohydroqui-nones called the cymopols (Hoegberg et al. 1976). Monobromination is found exclusively para (directly across) to the site of prenylation as seen in cymopolone (Fig. 1.7f). The isoprene side chains all originate from C10 geranyl units with different... 

The simplest way to achieve three-dimensional cross-linking is to use monomers with three or more reactive sites. Examples are maleic anhydride, butadiene, isoprene, epichlorohydrin, pyromellitic dianhydride, and tri-methylol propane. 

Based on this approach Schouten et al. [254] attached a silane-functionalized styrene derivative (4-trichlorosilylstyrene) on colloidal silica as well as on flat glass substrates and silicon wafers and added a five-fold excess BuLi to create the active surface sites for LASIP in toluene as the solvent. With THF as the reaction medium, the BuLi was found to react not only with the vinyl groups of the styrene derivative but also with the siloxane groups of the substrate. It was found that even under optimized reaction conditions, LASIP from silica and especially from flat surfaces could not be performed in a reproducible manner. Free silanol groups at the surface as well as the ever-present impurities adsorbed on silica, impaired the anionic polymerization. However, living anionic polymerization behavior was found and the polymer load increased linearly with the polymerization time. Polystyrene homopolymer brushes as well as block copolymers of poly(styrene-f)lock-MMA) and poly(styrene-block-isoprene) could be prepared. 

Butyl rubber, polyisobutylene, is an example of cationic polymerization with an acid. Review Chapter 14, Section 2.3. A small amount of isoprene is added to enable cross-linking during vulcanization through the allylic sites. 

With Ga-Beta it was found that, when the Si/Ga ratio increased from 10 to 40, the number of strong sites decreased drastically for Si/Ga between 10 and 25 and then reached a plateau above Si/Ga = 25 [53], The strength and density of acid sites in H(Ga, La)-Y were also found to be lower than those in HY crystals of the type used in FCC preparation (LZY-82) [250], Similar catalytic selectivities were obtained for both Ga-ZSM5 and A1-ZSM5 in Prins condensation of isobutylene with formaldehyde. Catalytic tests coupled with microcalorimetric measurements have shown that medium to weak acid strength sites favor the selectivity to isoprene [254],... 

Lewis base adducts, 25 64, 68-69 metal exchange reactions, 25 57 NMR spectra, 25 93-95 pyrolysis, 25 107 silicide formation, 25 110 tetracarbonylsilyl hydride reaction with isoprene, 25 75 reductive elimination, 25 81 site, formation, ribonucleotide reductase, 43 372-375... 

Helmig, D., J. Greenberg, A. Guenther, P. Zimmerman, and C. Geron, Volatile Organic Compounds and Isoprene Oxidation Products at a Temperate Deciduous Forest Site, . 1. Geophys. Res., 103, 22397-22414 (1998b). 

The most obvious reaction is simply to add more styrene and convert the isoprene anion to a styryl anion and grow it to a desired size and form the styrene-isoprene-styrene triblock of which we are more familiar in all the theoretical work that has been reported (ji, 7). The last example employs a bifunctional monomer, divinylbenzene, to form miniblocks of divinylbenzene as it reacts with a number of diblocks into what is called a star-block configuration (8). The active anionic sites are now on the divinylbenzene. These materials are then terminated by protonating agents to obtain the final product. The number of arms or diblocks that united into a starblock of this type is controlled by the relative amounts of the diblock to the divinylbenzene. 

Naphthalene-terminated polyvinyl aromatics and polyisoprene were obtained successfully. These functional polymers were metalated by potassium in THF at 25°C. The formation of a stable dinegative ion is observed unless the naphthalene is directly attached to the end of the polyvinyl aromatics, in which case a few isoprene units can be advantageously inserted between the naphthalene end group and the polyvinyl aromatics. The polymeric and stable dinegative ion polymerizes oxirane by both anionic sites and forms three-branched starshaped block copolymers. 

Investigating a bismuth molybdate catalyst with Bi Mo = 1 oxidizing pentenes to isoprene, Watanabe and Echigoya [344] found that isomers of pentenes were less reactive than those of the corresponding butenes in flow experiments, but the reverse was true in pulse experiments. Heat of adsorption measurements make it clear that the active sites are not uni-... 

P1D and P4/D have non-negligible values if a donor is present in the 2-position, because 4 1 and 4 have significant coefficients at this site. As a consequence, every 4t orbital contributes to the HOMO (see scheme e below). The coefficients at the central carbons are difficult to evaluate because some of the contributions cancel, but there is no doubt that the coefficient at C4 is much greater than that at C4. Catalog examples are isoprene and 2-amino- and 2-methoxybutadiene. 

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