Alkenes polymerization, homogeneous catalysis

The importance and relevance of homogeneous catalysis in polymerization reactions have increased tremendously in the past few years for two reasons. First, from about the beginning of the early 1990s a special class of sandwich complexes has been used as homogeneous catalysts. These catalysts, often referred to as metallocene catalysts, can effect the polymerization of a wide variety of alkenes to give polymers of unique properties. Second, the molecular mechanism of polymerization is best understood on the basis of what is known about the chemistry of metal-alkyl, metal-alkene, and other related complexes. 

Polymerizations of alkenes, with or without functional groups, are very important industrial processes. The industrial manufacture of polyethylene (PE) and polypropylene (PP), two of the largest plastics by tonnage, are based mainly on heterogeneous catalysts. However, the importance and relevance of homogeneous catalysis in polymerization reactions have increased significantly for two reasons. 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

We have already alluded to the diversity of oxidation states, the dominance of oxo chemistry and the cluster carbonyls. Brief mention should be made too of the tendency of osmium (shared also by ruthenium and, to some extent, rhodium and iridium) to form polymeric species, often with oxo, nitrido or carboxylato bridges. Although it does have some activity in homogeneous catalysis (e.g. of m-hydroxylation, hydroxyamination or animation of alkenes, see p. 558, and occasionally for isomerization or hydrogenation of alkenes, see p. 571), osmium complexes are perhaps too substitution-inert for homogeneous catalysis to become a major feature of the chemistry of the element. The spectroscopic properties of some of the substituted heterocyclic nitrogen-donor complexes may yet make osmium an important element for photodissociation energy research. 

Phosphine-modified cobalt hydroformylation is only used commercially by Shell. It is tightly coupled to Shell s Higher Olefins Process (SHOP, see Metathesis Polymerization Processes by Homogeneous Catalysis) that produces a C4 through C20 mixture of linear, internal alkenes for hydroformylation to detergent-grade alcohols. 

Metallacyclopentanes are of considerable importance as intermediates in catalytic alkene dimerization (see Oligomerization Polymerization by Homogeneous Catalysis). 

W(NAr)(=CHBuO(OR)2 (R = CMe(CF3)2, 50) are highly active catalysts for the metathesis of internal alkenes (equation 16), and also effect the stereoselective olefmation of hydroxy ketones (equation 17). The reactivity of these catalysts can be tuned by varying the aUcoxide ligands for example, when R = Bn, the complex acts only upon strained cyclic alkenes and is a highly effective ring-opening metathesis polymerization (ROMP) catalyst (see Metathesis Polymerization Processes by Homogeneous Catalysis). 

In the field of homogeneous catalysis the metathesis of unsaturated hydrocarbons offers many intriguing possibilities for producing important intermediates and end products from alkenes and alkynes. Through major advances in catalyst design in recent years, metathesis has become an important synthetic route to be considered in chemical laboratories whenever a special organic product has to be obtained. In fine chemistry metathesis is a very valuable reaction for the synthesis of natural compounds. Metathetical polymerization has been accepted by the industry as a viable means for producing polymers. 

Zr and Hf hydrides have been proposed as intermediates or by-products in heterogeneous and homogeneous catalysis of alkene isomerization, polymerization, and trimerization reactions.388 657-659 For example, heating the well-characterized silica-bound Zr(neopentyl)ra (n 1, 2) with dihydrogen afforded silica supported Zr-hydrides as indicated by IR spectroscopy. 58 Interestingly, this silica-supported Zr hydride exchanged via cr-bond metathesis with alkanes to produce the corresponding Zr alkyl derivatives.388,660,661... 

Since cationic titanium complexes have been proposed as intermediates in alkene polymerization by soluble Cp2TiCl2/AlR3-based Ziegler-Natta catalysts see Ziegler-Natta Catalysts), there has been considerable interest in the synthesis and reactivity of cationic titanium complexes (see Oligomerization Polymerization by Homogeneous Catalysis). 

Homogeneous Catalysis for Ziegler-Natta Coordination Polymerization (Section 29.6B) Initiation of Anionic Polymerization of Alkenes (Section 29.6D)... 

An Organometallic Com pound That Occurs Naturally Coenzyme B,2 591 Organocopper Reagents 592 Palladium-Catalyzed Cross-Coupling 595 Homogeneous Catalytic Hydrogenation 597 Olefin Metathesis 600 Ziegler-Natta Catalysis of Alkene Polymerization 603 Summary 606 Problems 608... 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

---