Isoprene results

Includes isoprene results which are indistinguishable from those for butadiene Newer values not included by Mayo and Walling.i Vinyl isobutyl ether. 

The product 303 from disulfur dichloride and 2,3-dimethylbuta-l,3-diene rearranges spontaneously to the tetrahydrothiophene 304 (equation 159)150. The reaction of liquid sulfur dioxide with conjugated dienes 305 (e.g. butadiene, isoprene) results in cyclic sulfones which dissociate into their components on heating (equation 160)151 152. 

Isoprene was tested for carcinogenicity in male mice and in male rats by inhalation exposure in one-year studies. In mice, exposure to isoprene resulted in increased incidence of benign and malignant tumours of the lung, liver and forestomach and of Harderian gland adenomas. The study by inhalation in rats was inadequate for an assessment of carcinogenicity (lARC, 1994). 

Reaction of 3a with butadiene or isoprene results in the formation of the 7i-allyl complexes 7a and 7b, respectively, which are stable in benzene solution for days at 25 °C.18 Hydrocarbon solutions of 3a react with 2,3-dimethylbutadiene to initially form the titanacyclopent-3-ene complex 8, isolated as a purple crystalline solid, which then subsequently reacts with generated free ethylene to form a mixture of the isomers 9 and 10 (Scheme 5). In an earlier paper, Rothwell and co-workers19... 

A mutagenic metabolite, isoprene dioxide, was tentatively identified in all examined tissues following exposure to isoprene. It is believed that the formation of reactive epoxides following exposure to isoprene results in tumor induction. 

Palladium-catalysed reaction of trans-j8-farnesene, formed by catalytic tri-merization of isoprene, resulted in regioselective head-to-head coupling to a dimer which yielded squalene on hydrogenation. A detailed n.m.r. study of functionalized squalane derivatives and model compounds suggests the presence of precoiled conformations. ... 

Nickcl-calaly/.ed hydromagnesiation of conjugated diene s with Grignard reagents seems less fruitful from the synthetic point of view. Attempted reactions of butadiene and isoprene resulted in a formal hydromagnesiation of the corresponding... 

Reaction of an zinc-copper couple on alumina (N2) in the presence of isoprene results in formation of 2, 3, and 4 in the approximate ratio 2 2 1. The product (2) is a natural terpene, karahaenone. The zinc oxyallyl cation (a) is... 

The application of these procedures to 1,3-dienes has presented problems. The rates of polymerization were observed to decrease and then stop due to a buildup of excess free nitroxide (Keoshkerian et al., 1998). An effective procedure for the controlled polymerization of isoprene at 145°C involved the addition of a reducing sugar such as glucose in the presence of sodium bicarbonate to react with the excess nitroxide (Keoshkerian et al., 1998). After 4 h, polyisoprene with M = 21,000 and My,/M = 1.33 was obtained in 25% yield. The reaction of TEMPO-terminated polystyrene with either butadiene or isoprene resulted in the formation of the corresponding diblock copolymers that were characterized by NMR and SEC (Georges et al., 1998). No evidence for either polystyrene or polydiene homopolymers was reported. 

At high conversions, cationic polymerizations of isoprene result in formations of some crosslinked material. The soluble portions of the polymers are high in tram A A structures. Alfin catalysts yield polymers that are higher in trans-lA structures than free-radical emulsion polymerizations. ... 

Cationic polymerizations of isoprene proceed more readily than those of butadiene, though both yield low molecular weight liquid polymers. AICI3 and stannic chloride can be used in chlorinated solvents at temperatures below 0°C. Without chlorinated solvents, however, polymerizations of isoprene require temperatures above 0°C. At high conversions, cationic polymerizations of isoprene result in formations of some cross-linked material [120]. The soluble portions of the polymers are high in trans-, A structures. Alfin catalysts yield polymers that are higher in trans-l,A structures than free-radical emulsion polymerizations [121]. 

Like substituted ethylene, isoprene undergoes free radical polymerization. Polymerization of isoprene results in the polymer polyisoprene, which is a simple alkene having one double bond in each unit (Figure 9.3). Polymerization of isoprene may follow one of two pathways cii-polymerization or trans-polymerization. 

Polyisoprene is made by solution polymerization of isoprene (2-methyl-l,3-butadiene). The isoprene monomer, the structural unit of the natural rubber polymer, can polymerize in four isomeric forms trans 1,4 addition, cis 1,4 addition, 1,2 addition, leaving a pendant vinyl group, and 3,4 addition. The production of a synthetic analogue to natural rubber was stymied for over 100 years because polymerization of isoprene resulted in mixtures of isomeric forms. In the 1950s, rubber-like elastomers with >90% cis 1,4 isoprene configuration were finally produced using stereospecific catalyts. 

The final type of isomerism we take up in this section involves various possible structures which result from the polymerization of 1,3-dienes. Three important monomers of this type are 1,3-butadiene, 1,3-isoprene, and 1,3-chloroprene, structures [X]-[XII], respectively ... 

G-5—G-9 Aromatic Modified Aliphatic Petroleum Resins. Compatibihty with base polymers is an essential aspect of hydrocarbon resins in whatever appHcation they are used. As an example, piperylene—2-methyl-2-butene based resins are substantially inadequate in enhancing the tack of 1,3-butadiene—styrene based random and block copolymers in pressure sensitive adhesive appHcations. The copolymerization of a-methylstyrene with piperylenes effectively enhances the tack properties of styrene—butadiene copolymers and styrene—isoprene copolymers in adhesive appHcations (40,41). Introduction of aromaticity into hydrocarbon resins serves to increase the solubiHty parameter of resins, resulting in improved compatibiHty with base polymers. However, the nature of the aromatic monomer also serves as a handle for molecular weight and softening point control. 

In the process of thermal dimerization at elevated temperatures, significant polymer is formed resulting in seriously decreased yields of dimer. Dinitrocresol has been shown to be one of the few effective inhibitors of this thermal polymerization. In the processing of streams, thermal dimerization to convert 1,3-cyclopentadiene to dicyclopentadiene is a common step. Isoprene undergoes significant dimerization and codimerization under the process conditions. 

The photosensitized dimerization of isoprene in the presence of henzil has been investigated. Mixtures of substituted cyclobutanes, cyclohexenes, and cyclooctadienes were formed and identified (53). The reaction is beheved to proceed by formation of a reactive triplet intermediate. The energy for this triplet state presumably is obtained by interaction with the photoexcited henzil species. Under other conditions, photolysis results in the formation of a methylcydobutene (54,55). 

Other Compounds. Primary and secondary amines add 1,4- to isoprene (75). For example, dimetbylamine in ben2ene reacts with isoprene in the presence of sodium or potassium to form dimetby1(3-metby1-2-buteny1)amine. Similar results are obtained with diethylamine, pyrroHdine, and piperidine. Under the same conditions, aniline and /V-metbylaniline do not react. Isoprene reacts with phenol in the presence of aluminum phenoxide (76) or concentrated phosphoric acid (77) to give complex products. 

The first successhil use of lithium metal for the preparation of a i7j -l,4-polyisoprene was aimounced in 1955 (50) however, lithium metal catalysis was quickly phased out in favor of hydrocarbon soluble organ olithium compounds. These initiators provide a homogeneous system with predictable results. Organ olithium initiators are used commercially in the production of i7j -l,4-polyisoprene, isoprene block polymers, and several other polymers. 

Propylene Dimer. The synthesis of isoprene from propjiene (109,110) is a three-step process. The propjiene is dimeri2ed to 2-methyl-1-pentene, which is then isomeri2ed to 2-methyl-2-pentene in the vapor phase over siUca alumina catalyst. The last step is the pyrolysis of 2-methyi-2-pentene in a cracking furnace in the presence of (NH 2 (111,112). Isoprene is recovered from the resulting mixture by conventional distillation. 

The first sulfur curable copolymer was prepared ia ethyl chloride usiag AlCl coinitiator and 1,3-butadiene as comonomer however, it was soon found that isoprene was a better diene comonomer and methyl chloride was a better polymerization diluent. With the advent of World War II, there was a critical need to produce synthetic elastomers in North America because the supply of natural mbber was drastically curtailed. This resulted in an enormous scientific and engineering effort that resulted in commercial production of butyl mbber in 1943. 

Polyisobutylene and isobutylene—isoprene copolymers are considered to have no chronic hazard associated with exposure under normal industrial use. Some grades can be used in chewing-gum base, and are regulated by the PDA in 21 CPR 172.615. Vulcanized products prepared from butyl mbber or halogenated butyl mbber contain small amounts of toxic materials as a result of the particular vulcanization chemistry. Although many vulcanizates are inert, eg, zinc oxide cured chlorobutyl is used extensively in pharmaceutical stoppers, specific recommendations should be sought from suppHers. 

Third Monomers. In order to achieve certain property improvements, nitrile mbber producers add a third monomer to the emulsion polymerization process. When methacrylic acid is added to the polymer stmcture, a carboxylated nitrile mbber with greatly enhanced abrasion properties is achieved (9). Carboxylated nitrile mbber carries the ASTM designation of XNBR. Cross-linking monomers, eg, divinylbenzene or ethylene glycol dimethacrylate, produce precross-linked mbbers with low nerve and die swell. To avoid extraction losses of antioxidant as a result of contact with fluids duriag service, grades of NBR are available that have utilized a special third monomer that contains an antioxidant moiety (10). FiaaHy, terpolymers prepared from 1,3-butadiene, acrylonitrile, and isoprene are also commercially available. 

Until the mid-1950s the only polyolefins (polyalkenes) of commercial importance were polyethylene, polyisobutylene and isobutylene-isoprene copolymers (butyl rubber). Attempts to produce polymers from other olefins had, at best, resulted only in the preparation of low molecular weight material of no apparent commercial value. 

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