Abstract: A polymer supported catalyst comprises Ziegler-Natta catalysts immobilized on a magnesium-modified polymer support. The catalyst support is prepared by (1) dissolving a carboxyl group-containing polymer in a solvent and precipitating the polymer in a polar non-solvent, (2) wet-grinding the precipitated polymer, and (3) mixing the ground polymer with an organomagnesium compound or a complex of an organomagnesium compound and an organoaluminum compound to give a magnesium-modified polymer support. Optionally, the resulting magnesium-modified support is treated with a halogen-containing silicon compound. The catalysts may be loaded onto the support by reacting the magnesium-modified support with a transition metal compound to form a catalyst constituent, and then combining the catalyst constituent with an appropriate organo-metallic compound.