Metallocenes, catalysts bridged

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. 

Table 8. Isotactic poly(propylene) polymerized by bridged- and unbridged-metallocene catalysts... 

Chien already postulated that C,-symmetric ansa-bridged complexes exist in two isomeric states, which interconvert during the course of the polymerization reaction [14, 15, 21, 22], Different stereoselectivities for monomer coordination and insertion are found for the two coordination sites of the asymmetric metallocene catalysts (Fig. 6,1 and IV). The migration of the polymer chain to the monomer, coordinated at the isoselective site f I—>11), followed by a consecutive chain back-skip (at higher temperatures) to the sterically less hindered side (II >111) leads to isotactic [mmmm] sequences [11],... 

For example, it is possible to synthesize isotactic as well as syndiotactic polypropylene in high configurational purity and high yields. The same holds for syndiotactic polystyrene. Furthermore, metallocene catalysts open the possibility to absolutely new homopolymers and copolymers like, e.g., cycloolefin copolymers (COG) and even (co)polymers of polar monomers.The simplest metallocene catalyst consists of two components. The first one is a n-complex (the actual metallocene) that can be bridged via a group X and therefore can become chiral ... 

In the case of co-polymers with cyclic olefins other than NB, Naga et al. performed the co-polymerizations of ethylene or propylene with cyclopentene by using a number of bridged metallocene catalysts, and then investigated the peak melting temperatures and crystalline structures of the resultant co-polymers, all containing cyclopentane units. In 2004, Waymouth and Lavoie reported on the catalytic properties of a series of... 

In 1983 the Exxon Chemical Company reported the discovery that the properties of the metallocene catalyst (e.g., molecular weight, comonomer selectivity, and activity) could be controlled by the choice of metal (Ti, Zr, or Hf) and by substituting and/or bridging the two cyclopentadienyl rings on the catalyst precursor [21]. 

Patents have been reported for a series of bimetallic metallocene catalysts based on lanthanide and group IV metals. The bridging ligand is a substituted ethyl-linked fluorenyl indenyl bearing a substituent of varying length [26]. The complexes are reported to act as good olefin polymerization catalysts in the presence of MAO.52,53... 

Effective ethylene-styrene co-polymerization with the post-metallocene catalysts has been achieved with the pentacoordinated complexes 195 and 196.159,1245 Pseudo-random ethylene-styrene co-polymers (i.e., co-polymers with no regioregularly arranged styrene-styrene units)642 were synthesized. The synthesis of an alternating crystalline co-polymer with isotactic styrene units and a Tm of 116 °C was also claimed. In these first studies, the importance of the S atom was noted, since the CH2-bridged complex 197 showed no co-polymerization activity. However, further studies demonstrated that the S atom is not fundamental, since complex 198 also promotes ethylene-styrene co-polymerization. Moreover, it was shown that complexes 195 and 196 incorporate a rather minor amount of styrene units (about 6-10 mol%) in the co-polymer relative to complex 198 (about 35 mol%), but they are roughly 1-2 order of magnitude more active than 198. 

Metallocene catalysts can be classified into several categories based on ligand symmetrical structures. This includes Cj, Ci-v, Cs, Ci, oscillating structures, and bridged half-metallocenes. The main features of these... 

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