Polypropylene synthesis in liquid monomer with titanium–magnesium catalyst: effect of different alkoxysilanes as external donors

Abstract

The effect of a set of methoxy- and ethoxysilanes as external donors (ED) on propylene polymerization in liquid monomer with the supported titanium–magnesium catalyst (TMC) and on properties of the produced polypropylene (PP) are studied. Addition of the studied donors to the polymerization system significantly increases the PP isotacticity compared to polymerization over TMC without ED (up to 92–98% vs. 66%, respectively). It is found that activity and stereospecificity of the catalytic system, as well as the molecular weight of the produced PP, decline as the number and size of alkoxy groups increase and the size (branching) of alkyl substituents decreases. Bulky substituents in alkoxysilanes have a positive effect on activity and stereospecificity of TMC. However, the double bond in the substituent moiety reduces the catalyst activity. Nitrogen atom in the substituent (with the same alkoxy groups) increases isotacticity and crystallinity of PP, its flexural modulus and strength characteristics. Varying electron donors with different number and size of alkoxy groups and different substituents (aliphatic, aromatic, alicyclic, amino, and vinyl) at silicon atom allows one to control the catalyst activity and isotacticity, as well as the molecular and thermal characteristics and impact strength properties of PP.