Dimethylsilylene dichloride

Extensive efforts have also been made to develop olefin polymerisation catalysts based on metallocenes with only one ligand of the cyclopentadienyl type. Ethylene-,dimethylsilylene- or tetramethyldisilylene-bridged mono(l-tetra -methylcyclopentadienyl), mono(l-indenyl) or mono(9-fluorenyl)-amidotita-nium complexes, such as dimethylsilylene(l-tetramethylcyclopentadienyl)(t-butyl)amidotitanium dichloride [Me2Si(Me4Cp)N(/-Bu)TiCl2] (Figure 3.10), have recently attracted both industrial and scientific interest as precursors for methylaluminoxane-activated catalysts, which polymerise ethylene and copolymerise ethylene with 1-butene, 1-hexene and 1-octene [30,105,148-152]. 

Figure 3.10 Monocyclopentadienyl amidotitanium procatalyst, dimethylsilylene(l -tetra -methylcyclopentadienyl)( -butyl)amidotitanium dichloride (Me2Si(Me4Cp)N( -Bu) TiCE, for obtaining low-density polyethylene. Front side view and side view respectively...

Various bridged boratabenzene zirconium complexes have been synthesized by salt metathesis, including ethylene-bridged bis(A,A-diisopropylaminoboratabenzene)zirconium complexes 1235, dimethylsilylene-bridged bis(A,A-diisopropylaminoboratabenzene)zirconium dichloride 1236, and dimethylmethylene-bridged (cyclopenta-dienyl)(A,A-diisopropylaminoboratabenzene)zirconium complexes 1237.944 As the crystal structures of the dimethyl... 

Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilylene/7w(2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

Preparation of elastomeric polypropylenes was also reported by Chien et al. [64]. Two metallocene catalysts of different stereospecificities were used. The isospecific catalyst precursors were either rac-ethylene bis-(l-r -indenyl)zircoifium dichloride or rac-dimethylsilylene bis(l-T] -indenyl)zirco-nium dichloride. The unspecific one was ethylene bis(9-T] -fluorenyl)zirconium dichloride. The precursors were activated with triphenyl carbenium tetrakis(pentafluorophenyl)borate and triisobutylaluminum. The resultant catalysts exhibit very high activity, yielding products that range from tough plastomers to weak elastomers [64]. 

FIGURE 7.9 As studied by Spaleck et al., 4-aryl substitution impacts catalyst performance in prodncing isotactic polypropylene. Shown are precatalysts dimethylsUylene bis(2-methylindenyl)zirconinm dichloride (1) and dimethylsilylene bis(2-methyl-4-naphthylindenyl)zirconium dichloride (2). Liqnid propylene polymerization at 70 °C, Zr Al 1 15,000, methylalnminoxane cocatalysL... 

In 1997, Schneider et al. reported that benzannelation of the indenyl ring in 1 forming rac-dimethylsilylene bis(2-methylbenz[e]indenyl)zirconium dichloride (7) enhances the incorporation of octene in ethylene-octene copolymerization. In order to explain this remarkable observation, they carried out a MM study and indeed found that 7 has a smaller transition state energy difference between ethylene insertion and octene insertion than the parent complex 1. 

However, only the introduction of methyl substituents in the 2-position, by the former Hoechst AG in 1990 [20, 50], led to polypropylenes with sufficiently high molar mass under industrially relevant polymerization conditions. In 1990, rac-dimethylsilylene-bis(2-methylindenyl)zirconium dichloride thus became the first an a-metallocene to combine all these improvements and be suitable for industrial use [50, 51]. Since then, the basic concept of dimethylsUylene-bridged,... 

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