High Molecular Weight Polybutadiene

High molecular weight polybutadiene homopolymers are prepared commercially with anionic catalysts and with coordination ones. Polybutadiene formed with sodium dispersions was prepared industrially in the former USSR, and perhaps might still be produced in that area today. This sodium- catalyzed polybutadiene contains 65% of 1,2-adducts [112]. Many of the preparations by others, however, utilize either alkyllithium or Ziegler-Natta type catalysts prepared with titanium tetra iodide or preferably containing cobalt. Because high molecular weight polybutadiene can be prepared by different catalytic systems, the choice of catalyst is usually governed by the desired microstructure of the product. Alfin catalysts yield very high molecular weight polymers with a large amount of trans-1,4 structures. Both the molecular weight and microstructure can be affected significantly, however, by variations in the Alfin catalyst components. These can be the alkyl groups of the organometallic compounds or alkoxide portions. When butadiene is polymerized with lithium metal or with alkyllithium catalysts, inert solvents like hexane or heptane must be used to obtain high cis-1,4 placement (see Chap. 4). Based on 13C NMR spectra, 1,4-polybutadiene formed with n-butyllithium consists of blocks of cis-1,4 units and trans-1,4 units that are separated by isolated vinyl structures [113]:

The quantity of such units in the above polybutadienes is approximately 48-58% trans-1,4, 33-45% cis-1,4, and 7-10% 1,2 units [112]. There is little effect of the reaction temperatures upon this composition. As described in Chap. 3, however, addition of Lewis bases has a profound effect. Reactions in tetrahydrofuran solvent result in 1,2 placement that can be as high as 87%.

The microstructures of polybutadienes prepared with Ziegler-Natta catalysts vary with catalyst composition. It is possible to form polymers that are high either in 1,2 placement or in 1,4 units. The catalysts and the type of placement are summarized in Table 6.7. Butadiene can be polymerized with chromium oxide catalyst on support to form solid homopolymers. The products, however, tend to coat the catalyst within a few hours after the start of the reaction and interfere with further polymerization. Polybutadiene can also be prepared in the presence of molybdenum catalyst promoted by calcium hydride. The product contains 80% of 1,4 units and 20% of 1,2 units. Of the 1,4 units, 62.5% are cis and 37.5% are trans [110]. Cobalt oxide on silica-alumina in the presence of alkyl aluminum also yields high cis-1,4 structure polymers. An all 1,2 polybutadiene can be prepared with n-butyllithium modified with bis-piperidino ethane. The atactic polymer can be formed in hexane at -5 to +20°C temperature [111]. The 100% 1,2 placement was postulated to proceed according to the following scheme [111]. First a complex forms between the base and butyllithium:

The above complex reacts with butadiene to form a new complex:

This is followed by insertion of butadiene into the carbon–lithium bond:

Annunziata et al. [114] reported that Group 4 metals complexes bearing anilido methylpyridine ligands were prepared by them. After activation by AlBu2H and methylalumoxane, the catalysts were tested in 1,3-butadiene and a-olefin polymerization. The zirconium complexes showed higher activity than the titanium analogous. Polymerization of ethylene resulted in all cases in the production of high molecular weight linear polyethylene. On the other hand, propylene polymerization tests provided substantially atactic polypropylene. 1,3-Butadiene polymerizations produced cis-1,4 polybutadiene. Use of zirconium complexes produced polymers with a content of cis-1,4 units higher than 99.9% were claimed.

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متى يصبح محيط الخصر "مؤشر خطر"؟

دراسة تكشف دور "العوامل الوراثية" في تحديد عمر الإنسان

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المزيد

مواضيع ذات صلة

Synthetic Polyisoprenes

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