Dimerization, Oligomerization, and Polymerization of Alkenes

Treatment of 2-methylpropene with hot aqueous sulfuric acid gives two dimers 2,4,4-trimethyl-1-pentene and 2,4,4,-trimethyl-2-pentene. This transformation is possible because 2-methylpropene can be protonated undOT the reaction conditions to furnish the 1,1-dimethylethyl (tert-butyl) cation. This species can attack the electron-rich double bond of 2-methylpropene with formation of a new carbon-carbon bond. Electrophihc addition proceeds according to the Markovnikov rule to generate the more stable carbocalion. Subsequent deprotonation from either of two adjacent carbons furnishes a mixture of the two observed products. 

The two dimers of 2-methylpropene tend to react further with the starting alkene. For example, when 2-methylpropene is treated with strong acid under more concentrated conditions, trimers, tetramers, pentamers, and so forth, are formed by repeated electrophilic attack 

The title reactions are related in relying on chain extension by repeated 1,2-insertion of an alkene into the catalyst M-C bond, but the extension proceeds to different extents (Eq. 12.14) depending on the catalyst and conditions. Dimerization requires one such insertion, oligomerization up to 50, and beyond that point, the product is considered a true high polymer. 

Better defined, homogeneous versions of the catalysts often have the general form [LL MCl2] (M = Ti, Zr, or Hf), where L and L are a series of C- or N-donor ligands. Initially, L and U were Cp groups, hence the term metallocene catalysts. Later improvements involved a much wider 

Metallocenes produce polyethylene that is strictly linear, without side branches, termed LLDPE (linear low-density polyethylene). Other processes tend to produce branches and hence a lower quahty product. If shorter chains are needed, H2 can be added to cleave them via heterolysis (Eq. 12.15). 

Polypropylene has an almost perfectly regular head-to-tail structure when produced with metallocenes. The arrangement of the methyl 

Syndiotactic polypropylene has no chirality and is formed by catalysts lacking chirality. It tends to adopt a planar zigzag conformation (12.9) of the main chain. 

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The insertion of alkenes into M-H bonds has been examined in Chap. 4. This reaction is very important because, it leads to the dimerization, oligomerization and polymerization of alkenes. It is broad and concerns not only transition metals, but also main-group metals (group 13 Lewis acids), lanthanides and actinides. For instance, AlEt3 is an excellent initiator of olefin polymerization. This reaction can also be considered as the hydrometallation or the hydroelementation of an olefin, and stoichiometric examples have been shown. If the element E does not have the property of a Lewis acid allowing olefin pre-coordination onto a vacant site and thus facilitating insertion, the insertion reaction is not possible without a catalyst. 

Alkenes.— A text on Ziegler-Natta catalysts has been published and polymerization by transition-metal hydrides, alkyls, and allyl compounds heis been reviewed. A reaction model for Ziegler-Natta polymerization has also been formulated. The conventional mechanism for alkene dimerization, oligomerization, and polymerization has, however, been questioned because there are no unambiguous examples of metal-alkyl-alkene compounds which unda go alkene insertion into the metal-alkyl bond. Also, catalysts which effect Ziegler-Natta polymerization are often active for alkene metathesis reactions and so a similiar mechanism for both has been proposed (Schrane 2). ... 

Dimerization, oligomerization, and polymerization all rely on the Cossee-Arlman mechanism that consists of repeated alkene 1,2-insertion into the M-C bond of the growing polymer chain (Fig. 

In the initial reports NHC-Ni only displayed moderate activities, compared with literature data, in the polymerization of alkenes. In addition, low selectivity due to higher molecular weight distributions was observed. Similar results were obtained for dimerization of ethylene. In many cases, the decomposition of the eatalyst (increased by temperature) competed with the dimerization or oligomerization processes. Interestingly, Wassercheid, Cavell and co orkers showed that this decomposition could be partially or totally inhibited in ionic liquids. On another hand, cis bis(aryloxide-NHC)... 

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. 

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. 

The 7r-back donation stabilizes the alkene-metal 7c-bonding and therefore this is the reason why alkene complexes of the low-valent early transition metals so far isolated did not catalyze any polymerization. Some of them catalyze the oligomerization of olefins via metallocyclic mechanism [25,30,37-39]. For example, a zirconium-alkyl complex, CpZrn(CH2CH3)(7/4-butadiene)(dmpe) (dmpe = l,2-bis(dimethylphosphino)ethane) (24), catalyzed the selective dimerization of ethylene to 1-butene (Scheme I) [37, 38]. 

Polymerization reactions follow an insertion mechanism, that is, alkene coordination to a vacant site on the active metal species, followed by a migratory alkyl transfer step. The addition of donor molecules which can compete with the alkene for coordination sites is therefore a means of reducing the rate of propagation and allows /3-H elimination to take place, so that a polymerization reaction might be converted to oligomerization or dimerization. On the other hand, metals which... 

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. 

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