Polyisobutylene

The original commercial methods for preparing high molecular weight polyisobutylene by cationic polymerization in good yields were reported in 1940. The reaction was carried out at 40 to 80Cin a diluent with BF₃ catalysis [72]. This developed into current commercial practices of polymerizing isobutylene at 80 to 100C, using liquid ethylene or methyl chloride as a diluent [73, 74]. Even at these low temperatures the reaction is quite violent. Methods were developed, therefore, to dissipate the heat. In one of them, called “flash polymerization process” the catalyst (a Lewis acid, like BF3 or AlCl3, for instance) is added in solution to the cooled isobutylene solution. The polymerization takes place very rapidly and is complete in a few seconds with the heat of the reaction being removed by vaporization of the diluent. Such reactions, however, are very difficult to carry out in conventional batch reactors. Two types of procedures were, therefore, adopted [75]. The first one is built around a moving stainless steel belt contained inside a gas-tight reactor housing. Isobutylene and liquid ethylene from one source and a Lewis acid in ethylene solution (0.1–0.3% based on monomer) from another source are fed continuously onto the moving belt where they are mixed and moved. The movement of the belt is adjusted at such a speed that the polymerization is complete before the polymer arrives at the end of its travel, where it is removed with a scraper and further processed. In the second process, the polymerization is carried out in multiple kneaders or mixers. These are arranged in a series of descending steps. Here the reaction mixture is carried from one kneader to another with the temperature being raised at each station and completed at the last one. All commercially important polyisobutylenes are linear, head to tail polymers, with tertiary butyl groups at one end of the chains and vinylidene groups at the other:

The differences lie in molecular weights. They range from 2,000 to 20,000 for viscous liquids to between 100,000 and 400,000 for high molecular weight elastomers that resemble unmilled crepe rubber. The polymers degrade readily from thermal abuse. They can be stabilized effectively, however, by adding small quantities (0.1–1.0%) of such stabilizers as aromatic amines, phenols, or sulfur compounds. Polyisobutylenes are soluble in many hydrocarbons and are resistant to attacks by many chemicals. Coordination polymerizations with Ziegler–Natta catalysts yield similar polymers that range from viscous liquids to rubbery solids. At 0C, a catalyst with a 1:16 Ti to Al molar ratio yields a polymer with a molecular weight of 5,000–6,000 [76]. The molecular weight, however, is dependent upon the reaction time. This contrasts with polymerizations of ethylene, propylene, and 1-butene by such catalysts, where the molecular weights of the products are independent of the reaction time. In addition, there are some questions about the exact molecular structures of the products [76]. Bochmann and coworkers [77] carried out polymerizations of isobutylene and copolymerizations with isoprene using cationic zirconocene hydride complexes. The combination of [Cp2ZrH] with various trityl salts of weakly coordinating anions gives binuclear cationic hydrides [Cp₄Zr₂H (u-H)₂]+X which are powerful initiators for the polymerization of isobutene and its copolymerization with isoprene. The temperature dependence of M is indicative of a cationic mechanism. The highest molecular weights are obtained only under scrupulously dry conditions. High molecular weight polyisobutylene has fair tensile strength but suffers from the disadvantage of considerable cold flow. A copolymer of isobutylene with some isoprene for cross-linking is, therefore, used as a commercial elastomer and called “butyl rubber.” The isoprene is present in the copolymer in only minor proportions (1.4–4.5%). The uncross-linked material is very similar to polyisobutylene. Copolymers of isobutylene with other dienes are also called butyl rubbers. They can also be terpolymers, where the third component may be cyclopentadiene for improved ozone resistance. The molecular weights of the copolymers vary inversely with the quantities of isoprene incorporated, the polymerization temperatures, and amount of impurities present during polymerization. Impurities like n-butene or water act as chain transferring agents [79]. To maintain uniform molecular weights, the conversions are usually kept from exceeding 60%.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة: نهج البلاغة مصدر صافٍ للفكر والمعرفة ومحصِّنٌ من الشبهات

العتبة العباسية المقدسة تطلق محاضرات محفل نهج البلاغة الأسبوعي في ذي قار

العتبتان المقدستان الحسينية والعباسية تدعوان للمشاركة في مهرجان الشموع بنسخته السادسة والعشرين

بالتعاون مع مستشفى الكفيل.. قسم الشؤون الطبّية يواصل إقامة البرنامج التدريبي لملاكاته النسوية

المزيد

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

المزيد

الآخبار التكنلوجية

مايكروسوفت تكشف عن الجيل الثاني من شرائحها للذكاء الاصطناعي

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

هل توجد حياة خارج كوكبنا؟.. اكتشاف جديد يثير اهتمام العلماء

اكتشاف الخلايا المسؤولة عن فقدان ذاكرة الطفولة

المزيد

مواضيع ذات صلة

Synthetic Polyisoprenes

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)

Syndiotactic Polypropylene

Manufacturing Techniques

Polypropylene

Materials Similar to Polyethylene