Abstract: A process and catalyst for polymerizing 1-olefins, for example ethylene, the process comprising (a) supporting a chromium compound on a refractory oxide support material, (b) adding one or more tetravalent titanium compounds, (c) heating the product under such conditions that an active polymerization catalyst is formed, (d) and contacting the monomer with the active polymerization catalyst in the presence of (6) one or more organometallic compounds having wherein the metal is of groups 1A, 2A, 2B and 3A of the Periodic Table (Mendeleef), and (7) a polyene modifier. The refractory oxide support material is preferably silica and the titanium compound is preferably a tetraalkyl titanate. The organometallic compound can be, for example, a triakyl aluminium and the polyene can be for example a conjugated or unconjugated diene.