Polymerization and Selective Oligomerization of Alkenes

Homogeneous Catalysis Mechanisms and Industrial Applications, Second Edition. Sumit Bhaduii and Doble Mukesh. 

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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. 

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]. 

A number of reviews with relevant material address K-complexes of various kinds and iiKlude reviews on the syntheses of cationic 7t-complexes of the transition metals, die activation of C-C bonds in unsaturated hydrocarbons by metal complexes, a review on phosphorus heterocycles and phosphorus analogues of unsaturated hydrocarbon transition metal n-omiplexes J metal-complex catalyzed polymerizations of a-alkenes, coordination equilibria and coupling leactitms of unsaturated hydrocarbons at a metal centre, oligomerization of a-alkenes to higher oligomers, an annual survey on the chemistry of selected n-complexes for the year 1989, an annual survey on the chemistry of ferrocene for the year 1989, and in a number of articles in a journal issue devoted to alkene metathesis. 

Athene Oligomerization. Bu2MeP has been used as a ligand in metal-catalyzed alkene oligomerization or polymerization with various influence of the ligand(s) on reactivity and selectivity. 

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. 

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)... 

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. 

While rare-earth metals are proven powerful olefin polymerization catalysts [21-24], there are only limited reports on controlled olefin oligomerizations or selective olefin dimerizations utilizing these elements [204,207,208], An ansa-scandocene [207] and the bis(indenyl)yttrium complex 41 (Fig. 25) [204] were reported to produce head-to-tail dimers from monosubstimted aliphatic alkenes (57). Complex 41 produces predominantly the tail-to tail adduct with styrene. The codimerization of an aliphatic alkene (including substrates containing various functionalities) with styrene affords tran -tail-to-tail dimers, apparently as a result of 1,2-insertion of the a-olefin followed by 2,1-insertion of styrene directed by the phenyl group (58). 

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