Structure and Physical Properties of Syndiotactic Polypropylene from Living Polymerization with Bis(phenoxyimine)-Based Titanium Catalysts

Abstract

A study of the structure and mechanical properties of syndiotactic polypropylene prepared with non-metallocene catalysts, based on titanium chlorides bearing phenoxyimine ligands, is reported. This catalyst promotes living polymerization of propylene, producing high molecular weight syndiotactic polypropylene, through a secondary (2,1) mechanism of enchainment of monomer and chain-end control of stereoselectivity. The effect of the presence of m dyads defects of stereoregularity, consistent with the chain-end mechanism, and defects of regioregularity, due to (1,2) misinsertions, on the crystallization behavior and mechanical properties is analyzed. As-prepared samples crystallize in conformationally disordered modifications of the isochiral helical form II containing kink-bands defects. This is probably due to the presence of regiodefects, producing pairs of vicinal methylene groups along the polymer chains, which favors, locally, the formation of trans-planar sequences that, in turn, induces the crystallization of the sample in the kink-bands disordered modification of form II. This also explains the observed stability of the trans-planar form III obtained in stretched fibers, which does not transform completely in helical forms by releasing the tension or by annealing, as instead occurs in metallocene-made syndiotactic polypropylene. The kink-bands disordered modifications are metastable and transform into the most stable antichiral helical form I by crystallization from the melt. However, disordered modifications of form I are obtained, even at high crystallization temperatures. The presence of m dyads defects of stereoregularity prevents the crystallization of the fully ordered antichiral helical form I. These defects may induce inversions of the helical chirality along the chains so that correlations at long distances between the chirality of the helices along a and b axes of the unit cell are destroyed. The particular microstructure also influences the mechanical properties. The analyzed sample presents values of elastic modulus lower than that of metallocene-made syndiotactic polypropylene of similar stereoregularity, due to higher flexibility of chains and the presence of higher degrees of disorder in crystals induced by the microstructural defects. Unoriented compression-molded samples show a remarkable elastic recovery even though the sample is crystalline and experiences plastic deformation during stretching. As a consequence, oriented fibers present good elastic properties in a large deformation range. The elasticity is associated with polymorphic transitions occurring in the crystals during the mechanical stress−relaxation cycles.