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

Many copolymers of ethylene with a-olefins are prepared commercially. Thus, ethylene is copolymerized with butene-1, where a comonomer is included to lower the regularity and the density of the polymer. Many copolymers are prepared with transition metal oxide catalysts on support. The comonomer is usually present in approximately 5% quantities. This is sufficient to lower the crystallinity and to markedly improve the impact strength and resistance to environmental stress cracking. Copolymers of ethylene with hexene-1, where the hexene-1 content is less than 5%, are also produced for the same reason. In most cases, the monomers that homopolymerize by Ziegler–Natta coordination catalysts also copolymerize by them [94]. In addition, some monomers that do not homopolymerize may still copolymerize to form alternating copolymers. Because the lifetime of a growing polymer molecule is relatively long (can be as long as several minutes), block copolymerization is possible through changes in the monomer feeds. Also, the nature of the transition metal compound influences the reactivity ratios of the monomers in copolymerizations. On the other hand, the nature of the organometallic compound has no such effect [95]. It also appears that changes in the reaction temperature between 0 and 75C have no effect on the r values. Copolymers can be formed using either soluble or heterogeneous Ziegler–Natta. One problem encountered with the heterogeneous catalysts is the tendency by the formed polymers to coat the active sites. This forces the monomers to diffuse to the sites and may cause starvation of the more active monomer if both diffuse at equal rates. Many different block copolymers of olefins, like ethylene with propylene and ethylene with butene-1, are manufactured. Use of the anionic coordination catalysts enables variations in the molecular structures of the products. It is possible to vary the length and stereoregularity of the blocks. This is accomplished by feeding alternately different monomers into the reactor. When it is necessary that the blocks consist of pure homopolymers, then after each addition the reaction is allowed to subside. If any residual monomer remains, it is removed [96]. This requires a long lifetime for the growing chains and an insignificant amount of termination. The stability of the anion depends upon the catalyst system. One technique for catalyst preparation is to form TiCl₃ by reducing TiCl4 with diethyl aluminum chloride followed by careful washing of the product of reduction to remove the by-product Al(C₂H₅) Cl. Other reports describe using the a-form of TiCl₃ or heat treating it to form the b or g-forms that yields more stereospecific products. The transition metal oxide catalysts on support, such as the CrO3/silica–alumina (Phillips) and MoO₃/Al₂O₃ (Standard Oil), are used to copolymerize minor quantities of a-olefins with ethylene. Such copolymerizations introduce short pendant groups into polymer backbones. Ethylene and other olefins can also be copolymerized with carbon monoxide to form polymers of aliphatic ketones, using transition metal catalysts, like palladium (II) coupled with non-coordinating anions. There are numerous reports of such catalysts in the literature. One example is a compound composed of bidentate diarylphosphinopropane ligand and two acetonitrile molecules coordinating Pd2+ coupled with BF3 counterions. This compound, bis(acetonitrile)palladium (II)-1,3-bis (diphenyl phosphino) propane-(tetrafluoborate), can be illustrated as follows [97]:

When the tetrafluoroborate is replaced with a perchlorate, the compound is a very active catalyst [97]. One copolymer of ethylene and carbon monoxide are available commercially. The material offered under the trade name of Carilon is actually a terpolymer, because it contains a small quantity of propylene. It is reported [98] that use of a palladium catalyst permits formation of perfectly alternating interpolymer.08 The product is reported to be a tough, chemical resistant material. Hustad et al. [99] developed a technique to make polydisperse polyethylene diblock copolymers with 1-octene with a distribution of block lengths. When melted and compressed into films, the distinct polymeric segments self-assemble into layered patterns of semi-crystalline and hard and amorphous phases. Because each phase has a different refractive index, the block copolymer, shown below, can function as a photonic crystal and scatter visible light.

Nomura and coworkers [100] studied copolymerization of ethylene with various pentenes:

where n = 8, 12. The polymerizations were carried out with titanium catalysts illustrated below. Titanium compound were combined with methyl aluminoxane:

Their results show that the monomer reactivities are influenced not only by substituents on the olefins but also by the nature of the catalytically active species. Derlin and Kaminsky [101] reported copolymerizations of ethylene and propylene with a sterically hindered monomer, 3-methyl-1-butene, using titanium and zirconium metallocenes with methyl aluminoxane cocatalyst. Tritto and coworkers [102] reported that the complex [Pd (k²-P, O-{2-(2-MeOC₆H₄)₂P}C₆H₄SO₃) Me (DMSO)] was investigated as a single-component catalyst for the copolymerization of ethylene with norbornene. The catalyst was illustrated as follows:

The copolymers were obtained in very good yields and molar masses were significantly higher than those of polyethylene. Three copolymers were formed:

Determination of microstructure and reactivity ratios revealed a strong inherent tendency to form alternating copolymers. Jordan [103] described copolymerization of ethylene with vinyl ethers and with vinyl fluoride. The catalyst used was (ortho-phospheno-arene sulfonate) PdMe (pyridine). The reaction was illustrated as follows:

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

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

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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

المزيد

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

Synthetic Polyisoprenes

High Molecular Weight Polybutadiene

Liquid Polybutadiene

Polybutadiene

Ionomers

Ethylene and Propylene Elastomers

Poly (4-methyl pentene-1)

Polyisobutylene

Syndiotactic Polypropylene

Manufacturing Techniques

Polypropylene

Materials Similar to Polyethylene