Single-center vs. multi-center post-metallocene catalysts for propylene polymerization

The structural characteristics of polypropylene samples prepared with two post-metallocene catalysts based on complexes bis-{M-(3,5-di-tert-butyl-salicylidene)-4-[bis-(5-methyl-2-furyl)methyl]aniline}titanium dichloride and [(4R,5R)-2,2-dimethyl-α,α,α′,α′-tetra(pentafluorophenyl)-1,3-dioxalan-4,4-dimethanol)titanium dichloride are investigated by GPC, 13C NMR, IR, DSC, and XRD methods. A combination of the first complex and MAO forms a single-center catalyst which polymerizes propylene to a nearly perfectly atactic polymer. A combination of the second complex and MAO forms a multi-center catalyst system producing polymer mixtures with broad molecular weight distributions containing five to six Flory components with different average molecular weights. Relative contents of the Flory components strongly depend on the type of solvent in the polymerization reactions. Some of the active centers produce high molecular weight, highly isotactic crystalline material with the melting point over 154 °C. The nature of steric errors in these polymer fractions (determined by 13C NMR) can be explained by a variant of stereocontrol similar to that exerted by metallocene catalysts of the C1 symmetry.

The article describes structural characteristics of polypropylene prepared with two post-metallocene catalysts containing bidentate Ti complexes activated with MAO. Analysis of polymers and their fractions by a combination of GPC, 13C NMR, IR, DSC, and XRD methods shows that different bidentate complexes can produce either single-site or multi-site catalysts systems. The multi-site catalysts are characterized by very broad molecular weight distributions (see GPC curves). Some of the active centers in them produce high molecular weight, highly isotactic crystalline materials with the melting points >150 °C. The nature of steric errors in these polymer fractions determined by 13C NMR can be explained by a variant of stereocontrol similar to that exerted by metallocene catalysts of the C1 symmetry.Figure optionsDownload as PowerPoint slide