Structure-Activity Relationships for Bis(phenolate-ether) Zr/Hf Propene Polymerization Catalysts

A series of 20 bis(phenolate-ether) ligands, in combination with Zr and Hf, was tested for performance in propene polymerization, focusing on molecular weight, stereoregularity and regioregularity of the resulting polymer. Ligand variation covers length of the aliphatic linker between the ligand halves, as well as steric bulk of the groups ortho to the phenolate oxygen. The linker length has a dramatic effect on MW: two-carbon linkers produce oligomers (Mn < 2.5 kDa) while three- and four-carbon linkers generate much higher MW (Mn typically 50–500 kDa). Stereoselectivity can be tuned using large, flat substituents in the o-phenolate position; tuning of regioselectivity is much harder. Hf catalysts are slower than their Zr analogs and do not work well with MAO/BHT (BHT = 2,6-di-tert-butyl-4-methylphenol); they are generally more selective (MW, stereo and regio). Density functional calculations agree fairly well with observed selectivities, supporting the involvement of a fac/fac coordinated active species. These O4 catalysts are considerably more flexible than e.g. metallocenes, making accurate prediction of PP microstructure a significant challenge.