Metallocenes, catalysts molecular symmetry

In catalysts obtained from achiral non-bridged metallocenes of class I with C2v molecular symmetry (double helical), such as Cp2MtX2, the positions of the coordinated monomer and of the alkyl ligand are not chirotopic and, therefore, the catalyst control is completely lacking (Table 3.1) [68],... 

In catalysts obtained from chiral stereorigid metallocenes of class III with C2 molecular symmetry (helical), such as racemic isomers of ansa-metallocenes, e.g. rac.-(IndCH2)2MtX2 (Table 3.1) and rac.-(ThindCH2)2MtX2, the two coordination positions available for the incoming monomer and the growing... 

Two examples clearly illustrate the relationship between molecular structures of the metallocene catalysts on the one hand, and the tacticity of the resultant polymers on the other. As shown in Fig. 6.9, complexes 6.32, 6.33, and 6.34 have very similar structures. In 6.33 and 6.34 the cyclopentadiene ring of 6.32 has been substituted with a methyl and a f-butyl group, respectively. The effect of this substitution on the tacticity of the polypropylene is remarkable. As already mentioned, 6.32, which has Cs symmetry, gives a syndiotactic polymer. In 6.33 the symmetry is lost and the chirality of the catalyst is reflected in the hemi-isotacticity of the polymer, where every alternate methyl has a random orientation. In other words, the insertion of every alternate propylene molecule is stereospecific and has an isotactic relationship. In 6.34 the more bulky t-butyl group ensures that every propylene molecule inserts in a stereospecific manner and the resultant polymer is fully isotactic. 

Ewen first demonstrated a correlation between a catalyst s molecular symmetry and the stereoregularity of the polymer produced by studying the two isomers (rac and meso) of the ansa-metallocene Et(Ind)2TiCl2 (Ind = indenyl). Ewen found that the achiral meso form (1) produced atactic polypropylene, whereas the C2-symmetric rac form (2) produced polypropylene with moderate isotacticity (Eigure 3.1). Both (1) and (2) suffered from poor catalytic activity owing to the instability of the titanium species. 

The single-crystal X-ray molecular structure of the complex p-(Me2Si) (3,6di- BuFlu)( BuN)TiCl2 10, is depicted in Fig. 17. It shows a striking similarity to crystal structure of complex 9 with respect to its overall symmetry, despite the exchange of one of the aromatic rings, the cyclopentadienyl, in the molecule and its replacement with an amido group. Complex 10 exemplifies one of the rare examples of a titanium-based syndiotactic-specific metallocene catalyst systems. 

Metallocene Molecular Symmetry and the Catalyst s Syndiotactic Specificity... 

All syndiotactic-specific catalysts that have been discussed so far had one principal element in common their precatalyst s metallocene molecules in solid phase all presented bilateral molecular symmetry or a vertical plane of S5mimetry (ffy) in the solid state. However, from the discussions in Sect. 4.3 it is clear that the whole notion of a perfect molecular symmetry, bilateral or otherwise, in the solution phase should be handled with caution and not t taken very literally. We saw that even with initially perfect bilateral symmetric metallocene molecules like 1 (2) and 6 (7), the perfect symmetry may be lost, at least in part, in solution and during different stages of the polymerization due to incessant t, and possibly t ... 

In this study, we examined the copolymerization of propylene with 1-butene and 1-pentene in liquid propylene medium in the presence of a highly active isospecific homogeneous a 5o-metallocene catalyst with the C -symmetry, rac-Me2Si(4-Ph-2-MeInd)2ZrCl2, activated by methyl-aluminoxane (MAO). The effects of the type of comonomer on the rate of copolymerization and the molecular mass characteristics, microstructure, thermophysical, and mechanical properties of the copolymers are investigated. The results of this study are compared with the data obtained with the same system used for the copolymerization of propylene with ethylene, 1-hexene, and l-octene. ... 

S. a,(a-IHolefin Polymers (cychpofymerization) Cyclopolymerization of 1,5-hexadi-ene using metallocene catalyst (5,5)-l in the presence of MAO gives an optically active polymer 20 (Scheme 11.4) [58,59]. The polymerization proceeded exclusively via the cyclization mechanism, and the obtained polymer 20 was rich in rrans-units (73%) having no plane of symmetry and it showed molecular rotation of [ ]4os -51.2°. 

The chiral metallocene-based catalysts are characterized by a defined active center, forming a sound basis for establishing relationships between the molecular structure of the catalyst and the microstmcture of the resulting polymer. Catalyst-polymer correlations are addressed using approaches ranging from symmetry-based rules [30,31] (as illustrated in Fig. 18) to more elaborate computational approaches that attempt to accurately predict the polymer microstmcture (see below). 

This chapter will discuss all known group 3 and 4 doubly bridged ansa-metallocenes made to date. When polymerization data is unavailable, comments will be made on the perceived viability of the compounds as precatalysts for a-olefin polymerization based on their structure and symmetry. For the a-olefin polymerization precatalysts described herein, the correlation between catalyst structure and polymer tacticity will be discussed. Further, the correlation between catalyst structure and regiocontrol, polymerization activity, and polymer molecular weight will be addressed when there is pertinent data present for a given precatalyst. 

---