Polymerization catalyst

A supported Ziegler catalyst prepared by the following steps:

• (A) heating together at a temperature in the range 250.degree. to 1100.degree. C. a refractory oxide support material having surface hydroxyl groups and one or more halogen-free metal derivatives which are hydrides and/or organic derivatives of the metal, said metal being selected from the group consisting of titanium, aluminum, nickel, vanadium, zirconium, boron, magnesium, silicon, zinc, and tin
• (B) reacting the product from (A) with one or more organometallic compounds having the general formula MR.sub.a.sup.1 Q.sub.b-a wherein M is a metal atom, R.sup.1 is a hydrogen group, Q is halogen or an oxyhydrocarbyl group, b is the valency of M and a is an integer from 1 to b and wherein the metal atom H is aluminium, boron, lithium, zinc or magnesium,
• (C) impregnating the solid product from step (B) with one or more halogen-containing transition metal compounds wherein the transition metal or metals comprise titanium and/or vanadium and/or zirconium.