Ethene polymerization mechanism

Many questions remain about the initiation, propagation, and termination steps of the ethene polymerization mechanism. The most important models proposed to date are the Cossee model, which requires a vacant coordination site on the metal center in the position adjacent to the growing alkyl chain, where ethene is coordinated before insertion into the chain (628), and the Green-Rooney model, which requires the presence of a metal-carbene species and a vacant site where ethene is coordinated prior to insertion (629). 

I. Tritto, L. Boggioni, and D.R. Ferro, Metallocene catalyzed ethene-and propene co-norbomene polymerization Mechanisms from a de-... 

Exercise 10-30 Write a reasonable mechanism for termination of ethene polymerization by disproportionation. Calculate AH° values for termination of the chain reaction by combination and disproportionation. Which is the more favorable process ... 

The aim of this Section is to discuss the experimental methods, problems, and recent improvements in the determination of the exact nature of the precursor species Yi, Y2, Y3, etc. (i.e., the determination of the initiation mechanism for the ethene polymerization). Facing this topic, we must be aware that, besides the problems related to the determination of the Cr(II) structure (vide supra Section VI.A.2), the identification of the species formed during the initial stages of the reaction has been prevented so far for two other reasons (a) only a fraction of the Cr(II) sites are active in the polymerization under the usually adopted conditions (225), so that almost all the characterization techniques give information about the inactive majority Cr sites and (b) the active sites are characterized by a very high polymerization rate (high turnover frequency, TOF). It is thus clear that any experimental efforts devoted to the detection of the precursor and/or intermediate species must solve these two problems (vide infra Section VI.C). 

Some Co(l) macrocyclic complexes have been implicated in C-H activation reactions. The observation of N-H exchange with solvent CD3CN in solutions of Co(l) cyclam derivatives has been interpreted as evidence for C-H activation by a mechanism involving oxidative addition of CD3CN to the Co(l) center. Co(I) Schiff-base complexes have been proposed to be important intermediates in ethene polymerization. Treatment of Co(ll) complexes containing 2,6-bis(imino)pyridyl ligands with MAO leads to highly... 

In at least one case, there is an example of ethene polymerization using a Ta-carbene complex in which there is strong evidence for a metathesis-based mechanism. See H. W. Turner and R. R. Schrock, J. Am. Chem. Soc., 1982,104, 2331, a paper that describes oligomerization of up to 35 ethene units in the presence of Ta[=CH(f-Bu)](H)(PMe3)3l2. It seems clear that although the Cossee mechanism is operative when polymerization occurs in the presence of Z-N-type catalysts, some polymerizations may involve metathesis, especially when hydrido-metal carbenes can form readily. 

Several important homogeneous catalytic reactions (e.g. hydroformylations) have been accomplished in water by use of water-soluble catalysts in some instances water can act as a solvent and as a reactant for hydroformylation. In addition, formation of aluminoxanes by partial hydrolysis of alkylaluminum halides results in very high activity bimetallic Al/Ti or Al/Zr metallocene catalysts for ethene polymerization which would be otherwise inactive. Polymerization of aryl diiodides and acetylene gas has recently been achieved in water with palladium catalysts. Finally, nickel-containing enzymes, such as carbon monoxide dehydrogenase (CODH) and acetyl-CoA synthase, operate in water with reaction mechanisms comparable with those of the WGSR or of the Monsanto methanol-to-acetic-acid process. ... 

Thus, the mechanism of the activating action of hydrogen in propene polymerization, as well as the mechanism of reduction of catalyst activity in ethene polymerization in the presence of hydrogen, are still not completely clear and need further confirmation. 

Busico, V. Cipullo, R. Talarico, G Stevens, J. C. New high activity group 4 Ci-symmetric catalysts for isotactic-selective high temperature solution polymerization of propene, and copolymerization of propene with ethene. 2-Polymer microstructure and polymerization mechanism. Abstract of Papers, European Polymer Conference on Stereospecific Polymerization and Stereoregular polymers. Milano, Italy, June 8-12, 2003 pp 81-82. 

Fig. 12.10 Proposed mechanism for Ziegler-Natta polymerization of ethene (Cossee mechanism).

Scheme 6.20.2 Back-biting mechanism and vinylidene formation in the free-radical ethene polymerization process leading to the characteristic structural features of LDPE.

Cyclopentene can be copolymerized with ethene or propene by heterogeneous and homogeneous Ziegler-Natta catalysts. Crystalline or elastomeric copolymers are obtained depending on the cyclopentene content and the part of ring-opening or vinyl-type polymerization mechanism. Metallocene/MAO catalysts are very active in the copolymerization of cyclopentene with ethene. In contrast to the homopolymerization of cyclopentene, the cyclic olefin is incorporated into the copolymer chain by 1,2-enchainment. 

Butene is used in the plastics industry to make both homopolymers and copolymers. Polybutylene (1-polybutene), polymerized from 1-butene, is a plastic with high tensile strength and other mechanical properties that makes it a tough, strong plastic. High-density polyethylenes and linear low-density polyethylenes are produced through co-polymerization by incorporating butene as a comonomer with ethene. Similarly, butene is used with propene to produce different types of polypropylenes. 

Most technically important polymerizations of alkenes occur by chain mechanisms and may be classed as anion, cation, or radical reactions, depending upon the character of the chain-carrying species. In each case, the key steps involve successive additions to molecules of the alkene, the differences being in the number of electrons that are supplied by the attacking agent for formation of the new carbon-carbon bond. For simplicity, these steps will be illustrated by using ethene, even though it does not polymerize very easily by any of them ... 

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