Ethylene zirconium dichloride

The first metallocene which could polymerize pro-pene to highly isotactic polypropylene was bis(tetrahydroindenyl)-ethylene-zirconium-dichloride (i) and was published in 1985 by Brintzinger and Kaminsky It s x-ray structure shows a perfect C2 sym-metry (fig. 1). 

Metallocene Catalysts. Polymerization of cycloolefins with Kaminsky catalysts (combinations of metallocenes and methylaluminoxane) produces polymers with a completely different stmcture. The reactions proceeds via the double-bond opening in cycloolefins and the formation of C—C bonds between adjacent rings (31,32). If the metallocene complexes contain bridged and substituted cyclopentadienyl rings, such as ethylene(hisindenyl)zirconium dichloride, the polymers are stereoregular and have the i j -diisotactic stmcture. 

In the 1980s, researchers introduced bis-cyclopentadienyl zirconium dichloride, catalysts that maintained their high level of activity for long periods of time. These catalysts became the favorites for copolymerizing ethylene with alpha olefms to produce LLDPE. 

These catalysts were used in combination with methylaluminoxane (MAO) for ethylene-norbornene co-polymerization and compared with isopropylidene[9-fluorenylcyclopentadienyl]zirconium dichloride catalyst activity under identical conditions. 

For example, ethylene bis(indenyl) zirconium dichloride can be prepared by the reaction of zirconium tetrachloride with bis(inden-yl)ethane lithium salt in tetrahydrofuran (4). 

An example of such a catalyst system is racemic isopropylene bis(l-indenyl) zirconium dichloride in combination with an alumi-noxane (21). The reaction is carried out in hydrocarbon solvents, e.g., toluene. A solution of norbornene in toluene with the catalyst is degassed and then pressurized with ethene. The polymerization is carried out while stirring at 70°C under constant ethylene pressure at 18 bar. After completion, the polymer is precipitated in acetone and filtered (21). The cycloolefin copolymers obtained in this way have a high thermal shape stability and it is possible to use the polymers as thermoplastic molding compositions. 

When bis(cyclopentadien)zirconium dichloride (Cp2ZrCl2) and MAO are used for ethylene polymerization, yields as high as 40 x 106 g of polyethylene (grams of zirconium x h) 1 are obtained. 

Under the same conditions, syndiospecihc (Cs-symmetric) metallocenes are more effective in inserting a-olefins into an ethylene copolymer than isospecific (C2-symmetric) metallocenes or unbridged metallocenes. In particular, hafnocenes are more efficient than zirconocenes. An interesting effect is observed for the polymerization with ethylene(bisindenyl)zirconium dichloride and some other metallocenes. The catalytic activity for the homopolymerization of ethylene is very high, and it increases when copolymerization with propylene occurs (114) (Fig. 12). Munoz-Escalona et al. (125) observed similar effects in the copolymerization of ethylene with 1-hexene. 

An interesting effect is observed for the polymerization with ethylene(bisin-denyl) zirconium dichloride and some other metallocenes (Fig. 5). Although the activity of the homopolymerization of ethene is very high, it increases when copolymerizing with propene [66]. 

The metallocene pre-catalyst, ethylene-bis2-(4-butenyldiisopropylsiloxy)-1 -indenyl) zirconium dichloride, has been prepared. When blended with co-catalyst methyla-lumoxane forming an aluminum/zirconium ratio of 300 1, respectively, the catalytic mixture had very high ethylene polymerization activity. 

Preparation [Ethylene-bis-2-(4-Butenyldiisopropylsiloxy)-l-Indenyl)] Zirconium Dichloride... 

The use of the boratabenzene heterocycle as a ligand for transition metal complexes dates back to 1970 with the synthesis of (C H5B-Ph)CpCo+ (1) (Cp = cyclopentadienyl).1 Since boratabenzene and Cp are 6 it electron donors, 1 can be considered isoelectronic to cobaltocenium. Many other transition metal compounds have been prepared that take advantage of the relationship between Cp and boratabenzene.2 In 1996, the synthesis of bis(diisopropylaminoboratabenzene)zirconium dichloride (CsHsB-NPr ZrCh (2) was reported Of particular interest is that 2 can be activated with methylaluminoxane (MAO) to produce ethylene polymerization catalysts with activities similar to those characteristic of group 4 metallocenes.4 Subsequent efforts showed that, under similar reaction conditions, (CsHjB-Ph ZrCh/MAO (3/MAO) gave predominantly 2-alkyl-1-alkenes5 while (CsHsB-OEt ZrCh/MAO (4/MAO) produced exclusively 1-alkenes.6 Therefore, as shown in Scheme 1, it is possible to modulate the specificity of the catalytic species by choice of the exocyclic group on boron. 

Related Reagents. (—)-[Ethylene-l,2-bis(t -4,5,6,7-tetra-hydro-l-indenyl)]zirconium (/ )- ,l -Bi-2,2 -naphtholate ( )-1, l -Ethylenebis(4,5,6,7-tetrahydro- l-indenyl)zirconium Dichloride. 

The regio- and stereoselective zirconocene-catalyzed addition of alkylmagnesium halides to alkenes, a process which has been described previously (see Section 7.5.2, Scheme 7-79) was investigated with ethylene-l,2-bis( M,5,6,7-tetrahydroind-l-enyl)zirconium dichloride [(EBTHI)ZrCl2l) [118] as chiral zirconocene. Thus, treatment of the latter with alkylmagnesium halides leads to the formation of the derived zirconocene-alkene complex 88, characterized by NMR [119], which reacts with cyclic ethers or amines to lead to the corresponding homoallylic alcohol or amine, respectively, in > 95% ee and good overall yield [120] (Scheme 7-103). 

Using bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2 Structure 6) and MAO, up to 40 000 000 g polyethylene/g Zr per h are obtained. Every zirconium atom forms an active complex and produces about 46 000 polymer chains per h. The time of insertion of one ethylene unit is only 3 X 10 s. Table 2 shows the polymerization behavior of different metallocene/alumoxane catalysts. 

It is also possible to copolymerize ethylene with a-olefins such as propene, 1-butene, 1-pentene, 1-hexene, and 1-octene, forming linear low-density polyethylene (LLDPE). The product of copolymerization parameters V2 obtained by using ethylenebis(l-indenyl)zirconium dichloride (11) indicates random incorporation of the comonomer [38]. 

The heptane-insoluble portion of polypropylene prepared with the ethylene-bis(l-indenyl)zirconium dichloride (11) catalyst at 0°C was found to crystallize in the y (70 %) and a modifications (30 %). The formation of the y modification is attributed to a small portion of 2,1-regioerrors and the quite unusual 1,3-misin-sertions [47, 48]. 

Salt metathesis was employed to synthesize half-sandwich zirconium and hafnium dichloride complexes 331 incorporating the bidentate, mono-anionic benzamidinate ligand (Equation (26)). The corresponding zirconium dimethyl and dibenzyl complexes have also been prepared using appropriate alkylating reagents.260 The zirconium dichloride complex (R= H), upon activation with MAO, are active for both polymerizations of ethylene... 

---