Alkene polymerization, metallocene

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. 

The goal of precise synthesis of supported mononuclear and polynuclear metal complexes can be traced to the early work of Yermakov [1], Ballard [2], and others. Their work stimulated the hvely field referred to as surface organometalhc chemistry [3-6]. The success and importance of precise synthesis of supported molecular catalysts are illustrated by the apphcation of supported metallocene catalysts for industrial alkene polymerization [7j. 

Chain-end controlled isospecificity and syndiospecificity for 1-alkene polymerizations at low temperatures with achiral metallocenes have also been reported.2,163 81131135 The polymerization with these catalysts is highly regio-specific in favor of primary monomer insertion. 

Cationic zirconocenes serve as useful reagents in such diverse fields as alkene polymerization, carbohydrate chemistry, asymmetric catalysis, and so on. Reagents that were originally developed for polymerization reactions (MAO, ansa-metallocenes, non-nucleophi-lic borate counterions) have now found use in organic synthesis and are being employed for carbometalation reactions, hydrogenation, and Diels—Alder catalysis. 

Landis and coworkers [140] have developed an active-site counting method based on H-labelling, for the metallocene-catalyzed alkene polymerization. After quenching the reaction by addition of methanol, the polymer is analyzed by NMR, which allows the quantification of Zr-alkyl sites. A typical NMR of quenched polymer is shown in Scheme 1.7 (label is found at terminal positions only). This technique has been applied to the polymerization of 1-hexene catalyzed by [Zr(rac-C2H4(l-indenyl)2)Me][MeB(QF5)3], 91. As shown in Scheme 1.7, there are two possible approaches ... 

Casey et al. have studied complex 98 as a model for intermediates in metallocene catalyzed alkene polymerization, by means of LSA [166]. 

Block copolymers have been successfully synthesized because many metallocene polymerizations of MMA proceed as living polymerizations, and it is possible to have a single one-way crossover from carbanion (alkene) polymerization to MMA (enolate) polymerization with metallocene and related initiators, especially when group 3 transition metal initiators are used [Boffa and Novak, 2000 Desurmont et al., 2000a,b Jin and Chen, 2002 Yasuda et al., 1992],... 

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. 

The simplest supported catalysts are mononuclear metal complexes, exemplified by industrial supported metallocene catalysts, used (with promoters) for alkene polymerization these are the so-called single-site catalysts that are finding wide industrial applications (Kristen, 1999 Kaminsky, 1999 Roscoe et al., 1998). The most common supports are metal oxides and zeolites. The metals in these complexes range from oxophilic (e.g., Zr and Ta) to noble (e.g., Rh). Supported metal complexes are stabilized by ligands—in addition to those provided by the support—such as hydride (H), hydrocarbons, and carbonyl (CO). In a typical supported metal complex, the metal is present in a positive oxidation state. Although some such complexes are relatively stable, most are, befitting their roles as catalysts, highly reactive and air- and moisture-sensitive. 

Two major mechanisms have been proposed for alkene polymerization. These are the Cossee-Arlman mechanism and the Green-Rooney mechanism. A modified version of the latter has also been considered to explain the behavior of homogeneous, metallocene catalysts. The original Cossee-Arlman mechanism was proposed for the TiCl3 based heterogeneous catalyst. In the following sections we discuss these different mechanisms in some detail. In the following discussion in accordance with the results obtained from the metallocene systems, the oxidation states of the active surface sites are assumed to be 4+. 

The structure of ligands in metallocene complexes determines activity, stereoselectivity, and molecular weight of 1-alkene polymerizations, by controlling the preferential conformation of the growing polymer chain which in turn controls the stereochemistry of monomer coordination ( enantiomorphic site control ). The difference between this and the chain-end control mechanism mentioned earlier is that stereo errors due to misinsertions can be repaired.101,106... 

Methylaluminoxane (MAO), sometimes referred to as polymethylaluminoxane (PMAO), currently gamers the most industrial interest of aU the alkylaluminoxanes owing to its activity as a cocatalyst for metallocene-catalyzed alkene polymerization (see Oligomerization Polymerization by... 

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. 

An important development, especially with respect to mechanistic and stereochemical investigations of alkene polymerization, was the use of homogeneous polymerization catalysts, such as metallocene dihalides of titanium, zirconium and hafnium (Cp2MX2), usually activated by alumoxanes as cocatalysts10. 

Over the last two decades, organometallic complexes have been at the heart of many of the key advances in metal-mediated alkene polymerization technology, with many examples now reaching the early stages of commercialization. While early transition metal complexes (e.g., metallocenes, constrained-geometry catalysts) have led the way, the advent of late transition metal catalysts has presented a rich library of highly active systems that can be employed... 

Alkene polymerization Ziegler-Natta catalysis and metallocene catalysts... 

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