THE DEVELOPMENT OF METALLOCENE CATALYSTS

The development of metallocene catalysts has made it possible to synthesize syndiotactic polypropylene (sPP). sPP has nearly the same melting temperature as aiPP, but its industrial development seems hindered by its lower overall crystallization rate. As a consequence, the search for appropriate nucleating agents may be essential. sPP has been shown to interact with low molecular weight organics and polymers, and notably aiPP. 

Group NIB and f-BIOCk Metallocenes. The need for expensive cocatalysts has always hampered the development of metallocene catalysts for olefin polymerization. It was recognized early on that substitution of the Group IVB metal with a lanthanide or Group IIIB element in the -1-3, state would represent a cocatalyst-free analogue of the Kaminsky system. Active catalysts are indeed obtained from bis-Cp lanthanocenes and yttrium- and scandium-based congeners. The lutetium dimer 29 has an activity of >7 kg/mmol(Lu)/h/atm for ethylene polymerization in cyclohexane (95). (This remarkable compound can also break the C—H bonds of alkanes.)... 

Randomly incorporated ethylene introduces defects along the backbone. These defects dismpt crystallization, reducing the modulus, melting point, and heat of fusion. The incorporation of random ethylene also reduces haze. Butene has also been used as a comonomer in PP. With the development of metallocene catalysts, even higher a-olefins such as hexene could be incorporated. While these alternative copolymers are now technically feasible, they have not seen commercial... 

Although metallocene types of structures have been known for several decades [18,19], their potential as commercial catalysts remained unrealized until 1980, when Kaminsky and coworkers [20,21] discovered that methylalumoxane improved their catalytic activity dramatically. Since that discovery, massive and intense research programs have been undertaken to bring metallocene products to coimnercial fruition. Reviews of the development of metallocene catalysts can be found in papers by Horton [22] and Kaminsky [23]. The reason for this enormous interest lies in the ability of metallocene catalysts to provide well-dehned polymer products, opening the way to the molecular engineering of resins with properties tailored to the precise needs of the end user. Particular effort has been expended to replace conventional (Ziegler-Natta-type) linear low density poly-... 

The focus of commercial research as of the mid-1990s is on catalysts that give desired and tailored polymer properties for improved processing. Development of metallocene catalyst systems is an example. Exxon, Dow, and Union Carbide are carrying out extensive research on this catalyst system for the production of polyethylene and polypropylene. 

The molecular design of stereospecific homogeneous catalysts for polymerization and oligomerization has now reached a practical stage, which is the result of the rapid developments in early transition metal organometallic chemistry in this decade. In fact, Exxon and Dow are already producing polyethylene commercially with the help of metallocene catalysts. Compared to the polymerization of a-olefins, the polymerization of polar vinyl, alkynyl and cyclic monomers seems to be less developed. 

Dienes are less reactive toward transition metals than enynes and diynes, and perhaps for this reason, the development of effective catalyst systems for the cyclization/hydrosilylation of dienes lagged behind development of the corresponding procedures for enynes and diynes. The transition metal-catalyzed cyclization/hydrosilylation of dienes was first demonstrated by Tanaka and co-workers in 1994. Reaction of 1,5-hexadiene with phenyl-silane catalyzed by the highly electrophilic neodymium metallocene complex Cp 2NdCH(SiMe2)3 (1 mol%) in benzene at room temperature for 3 h led to 5- ///76 -cyclization and isolation of (cyclopentylmethyl)phenylsilane in 84% yield (Equation (15)). In comparison, neodymium-catalyzed reaction of 1,6-heptadiene with phenylsilane led to 5- X(9-cyclization to form (2-methylcyclopentylmethyl)phenylsilane in 54% yield as an 85 15 mixture of trans. cis isomers (Equation (16)). 

Last, but not least, the development of ternary catalyst systems consisting of a metallocene, an organoborate, and an aluminium compound will reduce the manufacturing costs of mPE and will improve its ability to compete with other polymers. 

Since the discovery of Ziegler-Natta catalysts, polyolefin industries have been developed mainly by using titanium-based catalysts. However, after the appearance of metallocene catalysts, much emphasis has been placed on those based on zirconium owing to the superiority of their performance over titano-cene catalysts. 

The next important step was made using the ansa metallocenes synthesized by Brintzinger et al. in 1982 [28] for the stereospecific polymerization of propene. Ewen et al. [29] succeeded 1988 in synthesizing a Cs-symmetric zireonoeene ([Me2C(Flu)(Cp)]ZrCl2) which produces syndiotactic polypropylene in high quantities. Since 1985, a rapid, worldwide industrial and academic development began in the field of metallocene catalysts which continues today. 

Early work at Mitsui Petrochemicals concentrated on copolymerization of the multicyclic olefin dimethano-octahydronaphthalene (DMON, structure II (Ri = R2=H) in Fig. 4.1), using soluble vanadium catalysts [4] that eventually led to the commercialization of Apcl polyolefins [5]. Later, the utility of metallocene catalysts for cyclic olefin copolymerization was recognized by both Mitsui and Hoechst [6]. This led to the joint development of the Topas line of polyolefins [7], now being marketed by Ticona. 

One of the most exciting and active areas of actinide research involves the development of novel catalysts. Thoriiun and uranium metallocene complexes have been shown to react in highly specific manners that in some cases parallel those of early transition metals, and in others the reactions are unique to the actinides. M. Sharma and M.S. Eisen s chapter details metallocene organoac-tinide chemistry with a special focus on novel reaction pathways that have in some cases been deduced from thermochemical studies. 

One of the most exciting recent advances in organic and organometallic chemistry has been the development of new catalysts that produce polypropylene with high stereochemical purity. Both isotactic and syndiotactic polypropylene are now made commercially with a new class of metallocene catalysts, prototypes of which are shown below. The mechanism of the polymerization reaction is discussed in Chapter 17. Here we will focus on the stereochemistry, because symmetry principles of the sort we discussed above were crucial in the design of this chemistry. 

Structural features depicting the evolution of metallocene catalysts from the 1950s to 1990s are illustrated [68] in Table 13. Bis-Cp structure when activated with methyl alumoxane could be commercialized in a limited way. Activity and MW capability of substituted bis-Cp metallocenes were substantially higher than that of the unsubstituted structure. The mono-Cp structure was further developed [68] which produced high MW ethylene copolymers with higher comonomer content. Bridged, substituted bis-Cp structures could polymerize propylene to an isotactic or syndio-... 

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