Cyclopentadienyl compounds styrenes

Although the zirconium-catalyzed cyclization of a,cu-dienes is suitable only for cyclization of 1,6-hepta-and 1,7-octadienes, it has been shown that by using a catalytic amount of the tantalocene (/] -Me5C5)TaCl2(styrene) cyclization of 1,8-nonadiene to l-methylene-2-methylcycloheptane can be achieved in up to 90% yield [56]. Also, cyclopentadienyl compounds of the late transition metals such as (rj -Me5C5)RuCl(COD) 106 are capable of catalysis of cyclization of a,cu-dienes [57]. 

The reactive nature of compound 22 is illustrated by the series of transformations shown in Scheme 7.12, in which its Zr—C bond reacts selectively with electrophilic reagents to produce a-haloboronates 36—38. Compound 22 also catalyzes the polymerization of styrene. The polymers thus obtained had weight-average molecular masses in the range 75000—100000 with polydispersities of 1.8—2.1. An X-ray analysis of 22 confirmed it to be a four-coordinate Zr complex with two cyclopentadienyl rings, chlorine, and the aliphatic C-l carbon atom as the ligands (Fig. 7.4). 

Cyclopentadienyl ligands have become extremely important in catalysis for metal such as Ti, Zr, and Hf (Chapter 10) and in academic studies of related elements such as Ta. Ethylene polymerisation with the use of CpiTiCE (alkylated with aluminium alkyl compounds) has been known for many decades, but the intensive interest in derivatives of these compounds started in the early 1980 s following the discovery of MAO (methaluminoxane - see chapter 10) which boosted metallocene catalyst activities by several orders of magnitude. Commercial interest focussed on ethylene copolymers (LLDPE where more homogeneous comonomer incorporation resulted in greatly improved copolymer properties) and in enantiospecific polymerisations for propene, styrene, etc. 

Some half sandwich titanium compounds with cyclopentadienyl ligands have proven to be most active, but soluble tetraethyoxytitanium also shows a certain amount of activity. In contrast to olefin polymerization, titanocenes are more active than zirconocenes and fluoro ligands are better than chloro ligands. Table 24 [207] compares some catalysts for the polymerization of styrene. 

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]. 

Some of the more active catalysts for the polymerization of styrene are shown in Figure 22.17. Monocyclopentadienyltitanium halides, such as CpTiXj, CpTiXj, and CpTiXj, in combination with MAO, were more active catalysts than simple tetrahalides. Of these Cp-ligated trihalide complexes, the fluoride complexes were most active, followed by alkoxides and then chlorides. Moreover, cyclopentadienyl-ligated alkyltitanium compounds, such as Cp TiRj (R = hydrocarbyl) activated by boron compounds B(C Fj)j... 

Stille and co-workers devised an elegant method for attaching cyclopentadienyl ligand onto polystyrenes. Thus, reaction of p-styryl-MgBr with norbomen-7-one gave syn-7-(p-styryl)norbom-2-en-7-ol, which was converted into 7-chloro-7-(p-styryl)norbom-2-ene. Polymerization of this compound with styrene and divinylbenzene gave polymers... 

Syndiospecific catalyst systems for styrene polymerization which are composed of several titanium or zirconium compounds and methylalumoxane (MAO) as a cocatalyst have been reported by several authors (i.e., Ti(0R), >2) zr(0R)4, TiCl4,2) Cp2TiCl2 (Cp=cyclopentadienyl), CpTiCl3,2) TiBz " (Bz=benzyl), or Zr(Bz)4. " However, as far as we know, the copolymerization of styrene and olefin in the presence of these catalyst systems has not yet been reported. 

The different transition metals for the syndiospecific polymerization of styrene were summarized. Compounds of titanium with one cyclopentadienyl ligand show a high performance for the SPS production. Transition metals are stabilized by cyclopentadienyl ligands with electron-releasing substituents, and bulky substituents decrease the catalytic activities. o-Bonded groups at the transition metal complex are substituted by other groups by MAO or TIBA and showed comparable catalytic activities in the syndiospecific styrene polymerization [31]. 

In case of borate as cocatalyst, the catalytic activity of the titanium complex with a pentamethylcyclopentadienyl hgand is high, but a titanium complex with a cyclopentadienyl ligand without any substituents is not active for the syndiospecific styrene polymerization. The reason is that the reaction product of the borate and the cycopentadienyltitanium compound is unstable. The stability of the active site with the borate compound is lower in comparison to that with MAO. The reaction of CH2(Cp)2Ti(Me)2 with dimethylanilinium tetrakis(pentafiuorophenyl)borate or tris(pentafluorophenyl)borane in an equimolar mixture has been examined by Miyashita, Nabika, and Suzuki [11]. Two types of methylene bis(cyclopentadienyl)titanium ion complexes were isolated (see Fig. 3.6). These complexes were active in the polymerization of styrene, but only atactic polystyrene was formed. 

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