Alkene Polymerization and Oligomerization

Alkene polymerization is one of the most important catalytic reactions in commercial use. The Ziegler-Natta catalysts, for which Ziegler and Natta won the Nobel Prize in 1963, account for some 15 million tonnes of polyethylene and polypropylene annually. These catalysts are rather similar to the metathesis catalysts in that mixtures of alkylaluminum reagents and high-valent early metal complexes are used. The best known is TiCl3/Et2AlCl, which is active at 25°C and 1 atm this contrasts with the severe conditions required for thermal polymerization (200 C, 1000 atm). Not only are the conditions milder, but the product shows much less branching than in the 

Mechanism Cossee was the first to propose a mechanism in which the polymer chain (P in Eq. 11.58) grows by successive insertion of ethylene.  

There is an obvious problem with this route Why does the polymer chain not chain-terminate by p elimination The answer seems to be that the high-valent d metal has insufficient ability to back-donate in order to break the C—H bond recall that 3.7 failed to -eliminate for the same reason. A second difficulty is that ethylene insertion into an alkyl group is rather rare (see Section 3.3). 

Schrock has found an ethylene oligomerization catalyst, Ta(=CH/-Bu)-Hl2(PMe3)3, which does appear to go via metalcycles (Eq. 11.61). After 20-50 ethylene units have been inserted, the chain -eliminates to give a 1-alkene. Since the alkyl form of the catalyst is d, the unmodified Green-Rooney mechanism is allowed. 

In her studies on the /-block metals, Patricia Watson found a remarkable system in which successive alkene insertions into a Lu—R bond can be observed step by step (Eq. 11.62). Not only do the alkenes insert, but the reverse reaction, P elimination of an alkyl group, as well as the usual p elimination of a hydrogen, are also observed. For the d-block this p elimination of an alkvl erouo would normally not be possible it is probably the larger M—R 

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One of the major areas of application of iron alkene complexes that has been discovered in recent years is their use as catalyst precursors for alkene polymerization and oligomerization reactions. There has been great interest in this area with a very large number of papers published in the last 10 years. This trend is likely to continue, with more efficient and selective catalysts being discovered in the future. 

Ziegler-Natta-type catalysts, which are active in polymerization and oligomerization of alkenes. are also influenced by adding CO2 to the reaction mixture. The addition of CO2 changes the molecular we t and crystallinity of the products or the activity and selectivity of the catalyst, both in polyethylene [307,308] and in polypropylene production [309-312]. 

The polymerization and oligomerization of alkenes has been one of the most successful applications of organometallic chemistry to the synthesis of organic products on a large scale. As noted in the introduction to this chapter, organometallic complexes are involved in the synthesis of close to, or in excess of, fifty to one hundred million metric tons of polyolefins and a-olefins per year. In most cases, these products are formed by a series of alkene insertions into metal alkyl complexes in competition with p-hydrogen elimination processes. In other cases, selective dimerization or trimerization of alkenes occurs by the intermediacy of metallacyclic intermediates. 

In the previous chapters we discussed alkene-based homogeneous catalytic reactions such as hydrocarboxylation, hydroformylation, polymerization, and oligomerization. In this chapter we discuss a number of other homogeneous cataljTic reactions where an alkene is the only or one of the principal reactants. Some of the industrially important reactions that fall under the former category are selective di-, tri-, and tetramerization of ethylene, dimerization of propylene, and di-and trimerization of butadiene. 

Acid-catalyzed dimerization and oligomerization of 1,2,4-trioxolanes will be covered in Section 4.16.5.2.1. In general, ozonides are not prone to spontaneous polymerization. Polymeric products can be obtained from the ozonolysis of alkenes but most likely arise from reaction of the primary ozonide. Bicyclic 1,2,4-trioxolanes such as 2,5-dimethylfuran endoperoxide can dimerize on warming in CCI4 (Section 4.16.5.1.1). 1,2,4-Trithiolane tends to polymerize at room temperature especially if left open to air, whilst more highly substituted ring systems are stable. 

Along with enabling technological advances in alkene polymerizations, the application of new ancillary ligands or cocatalysts has become significant in the field of alkene oligomerizations to obtain a-olefms. This research exploits a new phase in the development of known catalyst systems containing Gr, Ni, Pd, Ti metal centers, as well as Fe and Co as newcomers. 

Fast side reactions under the conditions of acid-catalyzed alkylation are oligomerization (polymerization) and conjunct polymerization. The latter involves polymerization, isomerization, cyclization, and hydrogen transfer to yield cyclic polyalkapolyenes. To suppress these side reactions, relatively high (10 1) alkane alkene ratios are usually applied in commercial alkylations. 

The most important representatives are the lowest 1-alkenes, ethylene and propene. Ethylene is not particularly easily polymerized by radical or ionic mechanisms. Its importance as a monomer was greatly enhanced by the discovery of coordination polymerizations. Propene is oligomerized by radical and ionic initiators. This explains the importance of Natta s modification [1] of Ziegler [2,3] catalysts, enabling inferior raw materials to yield high-quality polymers. 

Alternatively, Lewis acids such as SbCl5 may initiate oligomerization directly by electron transfer from extremely reactive alkenes such as 1,1-diphenylethylene and 1,1 -di(p-methoxyphenyl)ethylene [28,143,144]. The dimeric tail-to-tail carbenium ion of 1,1-diphenylethylene shown in Eq. (32) was observed, and its formation explained by a radical cation intermediate. Because 1,1-diarylethylenes can not polymerize, only oligomerization was observed. 

Polyalkenes are invariably made by polymerization of alkenes. Why is an analogous route, the polymerization of disilenes, R2Si=SiR2, not used to make polysilanes The reason is that the barrier to polymerization of disilenes is simply too low, so that in most cases they polymerize or oligomerize as soon as they are generated. Stable disilenes can be made and isolated, but only with very bulky substituents at the silicon which make the polymer less stable than the disilene. An example is tetramesityldisilene (7), a highly reactive compound that undergoes many novel chemical reactions but does not polymerize. 

Olefin additions to bridging alkylidenes yield dimetallacyclopentanes . These reactions also provide a mechanism for olefin metathesis, a topic not discussed here. Although addition of an olefin to a metal carbone, a 2n + In addition, would be symmetry forbidden in organic chemistry, ab initio calculations " of the conversion of a metal carbene-alkene to a metallocyclobutane show it to be a barrierless reaction. Metal d orbitals relax the symmetry restrictions for the In + 2n addition. The mechanism of reaction (p) has not been widely considered for the olefin polymerization, but it may be relevant to olefin dimerization and oligomerization—reaction (s), for example ... 

An analogous scheme can be advanced to rationalize the stereospecificity with which alkyl migration proceeds in metal alkyl-transition metal-catalyzed alkene oligomerization and polymerization reactions such as Ziegler-Natta polymerization and alkene metathesis. " ... 

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