Propylene-Ethylene Copolymers from Supported Metallocene Catalysts: A Route to Span Properties from Polypropylene to Polyethylene

Propylene-ethylene copolymers with isotactic propylene sequences were synthesized across the entire range of comonomer composition, from 0.4 to 84 wt % of ethylene (C2), using a stereoselective supported metallocene catalyst. Propylene-rich copolymers crystallize in the alpha or gamma forms of isotactic polypropylene (iPP), whereas ethylene-rich copolymers with C2 concentrations ranging from 65 to 84 wt % crystallize in the stable orthorhombic form of polyethylene (PE). Samples with C2 concentrations between 20 and 60 wt % are amorphous. Copolymers with C2 concentrations lower than 1-2 wt % are stiff and brittle materials that transform into highly flexible and ductile materials with variable strength as the C2 concentration increases up to 16 wt %. Amorphous samples with C2 concentration ranging from 20 to 60 wt % show easy deformability and low strength, sometimes associated with viscous flow. However, strength and ductility increase again with increasing C2 concentrations from 60 to 84 wt % in samples showing PE crystallinity. Remarkable elastic properties develop for C2 concentrations between 10 and 16 wt % and between 50 and 65 wt %. The outstanding mechanical properties and elastic behavior are attributed to phase transformations and crystallization occurring during deformation. Amorphous samples crystallize upon stretching in the pseudohexagonal form of PE, and these crystals act as knots within the amorphous network, preventing viscous flow and inducing the development of elastic properties. The synthetic strategy, based on the use of a supported metallocene catalyst, enables the retention of high molecular mass and uniform comonomer concentration across the entire composition range. This approach allows for the definition of materials with properties spanning from polypropylene to polyethylene and from rigid plastomers to elastomers. The crystallization behavior and mechanical properties of these materials are compared with those of industrially practiced systems produced using Ziegler-Natta catalysts, providing evidence for the unique or superior performance of the metallocene-based copolymers within specific composition ranges.