Metallocene catalysts half-sandwich

Zirconocene and Half-Sandwich Zirconium Derivatives The development of a single-site heterogeneous catalyst for metallocene-based polymerization catalysis has also been explored extensively with zirconocene and half-sandwich zirconium derivatives [32, 75, 91, 92]. 

Some of the drawbacks of the metallocene catalysts are their limited temperature stability and the production of lower-molecular-weight materials under commercial application conditions. It follows that they have a limited possibility for comonomer incorporation due to termination and chain-transfer reactions prohibiting the synthesis of block copolymers by sequential addition of monomers. This led to the development of half-sandwich or constrained geometry complexes, such as ansa-monocyclopentadienylamido Group IV complexes (67) 575,576... 

Syndiotactic polystyrene was first obtained only recently by Ishihara et al. [5] in polymerisation with a homogeneous catalyst derived from a transition metal compound such as monocyclopentadienyltitanium trichloride and methylalu-minoxane in toluene. Since then, several authors have reported on the synthesis of syndiotactic polystyrene promoted by different catalysts based on metal hydrocarbyls such as benzyl compounds, half-sandwich metallocenes (e.g. monocyclopentadienyl, monopentamethylcyclopentadienyl and monoindenyl metal derivatives), metal alkoxides, metallocenes and some other compounds. These catalysts are commonly derived from titanium or zirconium compounds, either activated with methylaluminoxane or aluminium-free, such as those activated with tris(pentafluorophenyl)boron, and promote the syndiospecific polymerisation of styrene and substituted styrenes [5-10,21,48-70], Representative examples of the syndiospecific polymerisation of styrene using catalysts based on various titanium compounds and methylaluminoxane are shown in Table 4.2 [6,52,53,56,58],... 

When supported on AI2O3 as a carrier, half-sandwich metallocenes such as CpTiCl3 and Cp TiCl3 also gave rise to suitable stereospecific catalysts that could even be activated by Al(z -Bu)3 [69]. However, in contrast to the respective homogeneous catalysts (yielding syndiotactic polystyrene), polymerisation with these heterogeneous catalysts afforded isotactic and syndiotactic polystyrenes. 

What are the advantages of half-sandwich metallocene-based catalysts as compared with heterogeneous Ziegler-Natta catalysts in styrene polymerisation What are the possible consequences of this for developing industrial processes ... 

Let us recall that half-sandwich metallocene-based catalysts, either activated with methylaluminoxane or aluminium free, such as those activated with tris(per-fluorophenyl)boron, also promote the syndiospecific polymerisation of styrene. 

Conjugated dienes have been polymerised using supported half-sandwich metallocene catalysts. For instance, catalysts derived by supporting CpTiCl3 on alumina-silica gels, containing—0-Ti(Cp)Cl2 species, displayed activity in isoprene polymerisation without the addition of any other activator. Depending on the alumina-silica gel composition, the kind of polymerisation medium and the temperature, these catalysts exhibited various activities and selectivities polyisoprenes with a predominant 3,4 structure and mixed 1,2/ trans-1,4 structure were obtained [118,119],... 

In contrast to heterogeneous Ziegler-Natta catalysts, homogeneous catalysts based on biscyclopentadienyl derivatives of group 4 transition metals, which contain cationic metallocene species of formally d° 14-electronic structure, hardly promote the polymerisation of conjugated dienes, since the diene can act as a donor of four electrons rather than of two electrons as in monoolefin polymerisation (let us recall that the polymerisation of conjugated dienes is catalysed by half-sandwich metallocene-based catalysts). However, it has been reported [162] that statistical copolymers of ethylene and butadiene were obtained with the Cp2ZrCl2— [Al(Me)0]x catalyst. 

Unbridged, bridged, substituted, and half-sandwich complexes have been used as metallocenes for ethylene polymerization (Figs. 1 and 2). To compare the activities and molecular masses, the polymerizations are carried out under the same conditions (30°C, 2 bar ethylene pressure, with toluene as a solvent) (105). Table IV shows the polymerization behavior of various met-allocene/alumoxane catalysts. Generally, zirconium-containing catalysts are... 

Some half-sandwich titanium compounds with cyclopentadienyl ligands have been shown to be the most active catalysts for synthesis of these polymers. Fluorinated half-sandwich metallocenes, synthesized by Roesky et al. (263), have activities of up to a factor of 30 greater than those of chlorinated compounds. Polymerization has been carried out within a temperature range of 10-70°C (264). 

As was found for the polymerization of styrene, CpTiCT/M AO and similar half-sandwich titanocenes are active catalysts for the polymerization of conjugated 1,3 dienes (Table XX) (275). Butadiene, 1,3-pentadiene, 2-methyl-l,3-pentadiene, and 2,3-dimethylbutadiene yield polymers with different cis-1,4, trans-1,4, and 1,2 structures, depending on the polymerization temperature. A change in the stereospecificity as a function of polymerization temperature was observed by Ricci et al. (276). At 20°C, polypen-tadiene with mainly ds-1,4 structures was obtained, whereas at -20°C a crystalline, 1,2- syndiotactic polymer was produced. This temperature effect is attributed to a change in the mode of coordination of the monomer to the metallocene, which is mainly cis-rf at 20°C and trans-rj2 at -20°C. 

Half-sandwich metallocenes (120) (see Half-sandwich Complexes) play important role in the copolymerization of ethene and various a-olefins. They are distinguished by a sterically accessible active site. These catalysts are remarkably stable up to polymerization temperatures of 160 °C. 

For the production of ethylene/l-octene copolymers, metallocenes in combination with oligomeric methylalumoxanes or other compounds are now used [31, 63]. Half-sandwich transition metal complexes such as [(tetramethyl- / -cyclopentadienyl) (A-/-butylamido)dimethylsilyl]titanium dichloride are applied to synthesize linear low-density copolymers and plastomers, called constrained geometry catalysts [31]. Ethylene and styrene can be copolymerized to products ranging from semicrystalline mbber-like elastomers to highly amorphous rigid materials at room temperature [64]. 

Other catalytic systems that generate long chain-branched polyolefins have been reported such as Dow s half-sandwich constrained geometry titanium catalysts (CGC) [14e], and Bayer s novel donor/acceptor metallocenes [14f]. This polymer structural feature improves material and processing properties. 

Figure 2.16 depicts another important type of metallocene catalyst monocyclopentadi-enyl complexes, also called constrained geometry catalysts (CGC) or half-sandwich catalysts. Their most important property is a very high reactivity ratio toward a-olefin incorporation, allowing the easy copolymerization of ethylene with long a-olefins and polymer chains having a vinyl terminal group. The latter are called macromonomers, and. 

Copolymers of ethylene and styrene are not readily prepared by free radical polymerization, due to the extremely high reactivity of styrene relative to ethylene. Further, copolymers produced using traditional Ziegler-Natta catalysts typically contain less than 1% styrene [28]. However, random ethylene/styrene copolymers with varied styrene content have recently been prepared using homogeneous half-sandwich and metallocene catalysts, activated by MAO, [29-31] but detailed studies of the effect of the bulky phenyl pendant group have yet to be reported. 

Abstract The use of methylaluminoxane (MAO) as cocatalyst for the polymerization of olefins and some other vinyl compounds has widely increased the possibilities for more precisely controlling the polymer composition, polymer structure, tacticity, and special properties. Highly active catalysts are obtained by different transition metal complexes such as metallocenes, half-sandwich complexes, and bisimino complexes combined with MAO. These catalysts allow the synthesis of polyolefins with different tacticities and stereoregularities, new cycloolefins and other copolymers, and polyolefin composite materials of a purity that cannot be obtained by Ziegler-Natta catalysts. The single-site character of metaUocene/MAO or other transition metal/ MAO catalysts leads to a better understanding of the mechanism of olefin polymerization. 

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