Alkene polymerization catalysts

The ability of transition-metal complexes to activate substrates such as alkenes and dihydrogen with respect to low-barrier bond rearrangements underlies a large number of important catalytic transformations, such as hydrogenation and hydroformy-lation of alkenes. However, activation alone is insufficient if it is indiscriminate. In this section we examine a particularly important class of alkene-polymerization catalysts that exhibit exquisite control of reaction stereoselectivity and regioselec-tivity as well as extraordinary catalytic power, the foundation for modern industries based on inexpensive tailored polymers. 

This section focuses on group 4 metallocenes, which have been the most widely and thoroughly investigated among the homogeneous alkene-polymerization catalysts. These will also serve as useful reference standards in the following discussions regarding non-metallocene catalysts. 

Colorimetric assays, with organometallic complexes, 1, 912 Combinatorial approaches, for alkene polymerization catalyst discovery, 11, 727 Complexation studies... 

B.l. Comparison with Other Alkene Polymerization Catalysts The Mystery of the Polymerization Mechanism in the Phillips Catalyst... 

Azaborolyl Zirconium(iv) Complexes as Alkene Polymerization Catalysts... 

Early metal-metallocene-alkene polymerization catalysts permit the synthesis of highly isotactic polypropylene . They rely on controlling the stereochemistry of alkene insertion by the use of chiral C2 symmetric metallocenes . Late metal systems for alkene polymerization , and copolymerization of alkenes and CO , have also been developed. 

Complexes of Crlv with alkyl,216 alkylidene,217 or r/-carborane218 ligands are extensively studied as model compounds for Cr-based alkene polymerization catalysts (Section 4.6.5.1.2). These complexes are described in detail in the Comprehensive Organometallic Chemistry series. A CrIV complex with isocyanide ligands is described in Section 4.6.4.2.I. 

Other studies on mixed Ti-Al systems as alkene polymerization catalysts have afforded examples of bridging between these two metals by cyclopentadienyl residues (69), by carbidic carbon atoms (70, 71, 72), " 5 and by methyl groups (74). The carbide systems 70, 71, and 72 were prepared by reactions between phosphinimido-titanium methyl compounds and trimethylaluminum. The coordination at the carbide carbon atoms in 70 and 72 is flattened tetrahedral. However, the coordination in 71 is distorted trigonal bipyramidal, with a near-linear Ti—C—Al vector (178.5°) (cf. 65 and 66) and bonding as in 67. 

Modified MAOs (MMAO) that incorporate Bu and other alkyl groups to improve stability and optimize catalytic activity are commercially available. The use of additional modifiers such as boranes, boroxines, and pentafluorophenyl derivatives has been investigated in recent years. MAO has also been supported on silica and other materials for the preparation of heterogeneous alkene polymerization catalysts. These developments are detailed in several reviews. ... 

Examples given below illustrate synthesis of alkene polymerization catalysts, but these catalysts are simpler than the supported metallocenes used in industry, because they lack the promoter methylaluminoxane (MAO), an ill-defined material that greatly complicates characterization. Other examples given below illustrate (a) details of the surface chemistry of conversion of an organometallic precursor into a supported catalyst (b) synthesis of metal clusters of various sizes and compositions on a family of supports from metal carbonyl precursors and (c) synthesis of supported bimetallic clusters with combinations of noble (e.g., Pt) and oxophilic (e.g., W) metals that give quite stable catalysts with extremely high metal dispersions. 

The Phillips catalyst is prepared from relatively inexpensive chromium salts it is robust, but structurally complex, and the catalytic sites are not identified. To make a structurally simpler silica-supported alkene polymerization catalyst, Ajjouet al. used the precursor bis(neopentyl)chromium(IV). The synthesis chemistry was represented as follows ... 

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