Polyolefins are a class of polymers derived from simple olefins. Known methods of making polyolefins involve the use of Ziegler-Natta polymerization catalysts. These catalysts polymerize vinyl monomers using a transition metal halide to provide a stereoregulated polymer.
Numerous Ziegler-Natta polymerization catalysts exist. The catalysts have different characteristics and/or lead to the production of polyolefins having diverse properties. For example, certain catalysts have high activity while other catalysts have low activity, and similarly certain catalysts have a long life while other catalysts have a short life. Moreover, polyolefins made with the use of Ziegler-Natta polymerization catalysts vary in stereoregularity, molecular weight distribution, impact strength, melt-flowability, rigidity, heat sealability, isotacticity, and the like.
Impact copolymers are made of polyolefins and rubbers. While higher amounts of rubber in an impact copolymer increase the impact resistance, the higher amounts undesirably lower the flow properties of the copolymer. Lower amounts of rubber in an impact copolymer undesirably increase stiffness or rigidity, which in turn lowers the impact resistance.
Useful Ziegler-Natta polymerization catalysts made through a precipitation method are made using an organic magnesium compound starting material. The organic magnesium compound leads to the formation of a desirable spherical catalyst particle. Replacing the organic magnesium compound starting material with a markedly less expensive magnesium halide results in a catalyst particle with a morphology that is difficult to control and aspherical.