Ansa-metallocene Et

The copolymerization of ethylene with 1-olefins catalyzed by homogeneous Cp2ZrCl2—MAO catalysts and catalysts supported on HY zeolite shows a loss in activity on heterogenization, an increase in molecular weight, and a decrease in comonomer incorporation (Table 7). The ansa-metallocenes Et-(Ind)2MCl2 (M = Zr, Hf) display no attenuation of comonomer incorporation. The authors hypothesize that the smaller metallocenes are in the pores of the zeolite, into which diffusion of the comonomer is difficult, and the ansa-metallocenes, not adsorbed inside the pores, are located on the exterior and are therefore more accessible to comonomer. 

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. 

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