Synthetic Polyisoprenes

In following natural rubber, the synthetic efforts are devoted to obtaining very high cis-1,4 polyisoprene and to forming a synthetic “natural” rubber. Two types of polymerizations yield products that approach this. One is through use of Ziegler–Natta type catalysts and the other through anionic polymerization with alkyllithium compounds in hydrocarbon solvents. One commercial process, for instance, uses reaction products of TiCl4 with triisobutylaluminum at an Al/Ti ratio of 0.9–1.1 as the catalyst. Diphenyl ether or other Lewis bases are sometimes added as catalyst modifiers [113–116]. The process results in an approximately 95% cis-1,4 polyisoprene product. Typically, such reactions are carried out on continuous basis, usually in hexaneandtake2–4h. Polymerizations are often done in two reaction lines, each consisting of four kettles arranged in series. The heat of the reaction is partially absorbed by precooling the feed streams. The remaining heatis absorbed on cooled surfaces. When the stream exits, the conversion is about 80%. Addition of a shortstop solution stabilizes the product. Alkyllithium-initiated polymerizations of isoprene yield polymers with 92–93% cis-1,4 content. One industrial process uses butyllithium in a continuous reaction in two lines each consisting of four reaction kettles. The heat of the reaction is removed by vaporization of the solvent and the monomer. The catalyst solution is added to the solvent stream just before it is intensively mixed with the isoprene monomer stream and fed to the first reactor. After the stream leaves each reactor, small quantities of methanol are injected between stages into the reaction mixture. This limits the molecular weight by stopping the reaction. Fresh butyllithium catalyst is added again at the next stage in the next reactor to initiate new polymer growth [117–119]. As is described in Chaps. 3 and 4, the monomer placement into the polyisoprene chain can occur potentially in nine different ways. These are the three tactic forms of the 1,2 adducts, two 1,4 adducts, cis and trans, and three tactic forms of 3,4-adducts. In addition, there is some possibility of head-to-head and tail to tail insertion, though the common addition is head to tail. Table 6.8 presents the various microstructures that can be obtained in polymerizations of isoprene with different catalysts.

Cationic polymerizations of isoprene proceed more readily than those of butadiene, though both yield low molecular weight liquid polymers. AlCl₃ 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-1,4 structures. Alfin catalysts yield polymers that are higher in trans-1,4 structures than free-radical emulsion polymerizations [121].

Chromium oxide catalysts on support polymerize isoprene-like butadiene to solid polymers. Here too, however, during the polymerization process, polymer particles cover the catalyst completely within a few hours from the start of the reaction and retard or stop further polymer formation. The polymerization conditions are the same as those used for butadiene. The reactions can be carried out over fixed bed catalysts containing 3% chromium oxide on SiO₂-Al₂O₃. Conditions are 88°C and 42 kg/cm2 pressure with the charge containing 20% of isoprene and 80% isobutane [122]. The mixed molybdenum-alumina catalyst with calcium hydride also yields polyisoprene.

Lithium metal dispersions form polymers of isoprene that are high in cis-1,4 contents as shown in Table 6.8. These polymers form in hydrocarbon solvents. This is done industrially and the products are called Coral rubbers. They contain only a small percentage of 3,4-structures and no trans-1,4 or 1,2 units. The materials strongly resemble Hevea rubber.

Use of Ziegler-Natta catalysts, as seen from Table 6.8, can yield an almost all cis-1,4-polyisoprene or an almost all trans-1,4-polyisoprene. The microstructure depends upon the ratio of titanium to aluminum. Ratios of Ti: Al between 0.5:1 and 1.5:1 yield the cis isomer. A 1:1 ratio is the optimum. Ratios of Ti: Al between 1.5:1 and 3:1 yield the trans structures [123]. The titanium to aluminum ratios also affect the yields of the polymers as well as the microstructures. There also is an influence on the molecular weight of the product [124]. Variations in catalyst compositions, however, do not affect the relative amounts of 1,4 to 3,4 or to 1,2 placements. Only cis and trans arrangements are affected. In addition, the molecular weights of the polymers and the microstructures are relatively insensitive to the catalyst concentrations. The temperatures of the reactions, however, do affect the rates, the molecular weights, and the microstructures.

Use of Ziegler-Natta catalysts, as seen from Table 6.8, can yield an almost all cis-1,4-polyisoprene or an almost all trans-1,4-polyisoprene. The microstructure depends upon the ratio of titanium to aluminum. Variations in catalyst compositions, however, do not affect the relative amounts of 1,4 to 3,4 or to 1,2 placements. Only cis and trans arrangements are affected. In addition, the molecular weights of the polymers and the microstructures are relatively insensitive to the catalyst concentrations. The temperatures of the reactions, however, do affect the rates, the molecular weights, and the microstructures.

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الآخبار الصحية

الصحة العالمية تكشف حقيقة انتشار فيروس نيباه خارج الهند

متى يصبح محيط الخصر "مؤشر خطر"؟

دراسة تكشف دور "العوامل الوراثية" في تحديد عمر الإنسان

رائحة كريهة في الجسم.. "علامة خفية" أخطر مما تتصور

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الآخبار التكنلوجية

مايكروسوفت تكشف عن الجيل الثاني من شرائحها للذكاء الاصطناعي

خرافة "ثمانية أكواب يوميا".. كم من الماء يحتاج جسمك فعليا؟

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المزيد

مواضيع ذات صلة

High Molecular Weight Polybutadiene

Liquid Polybutadiene

Polybutadiene

Ionomers

Copolymers of Ethylene with a-Olefins and Ethylene with Carbon Monoxide

Ethylene and Propylene Elastomers

Poly (4-methyl pentene-1)

Polyisobutylene

Syndiotactic Polypropylene

Manufacturing Techniques

Polypropylene

Materials Similar to Polyethylene