Abstract: A transition metal compound containing at least one .eta..sup.6 -arene is contacted with a solid material of the formula MX.sub.2 nL where X is a halogen atom, n is such that 0<n.ltoreq.6 and L is an organic Lewis Base compound. The transition metal compound can be a compound such as TiCl.sub.2.2AlCl.sub.3 -arene. The solid material is preferably of high surface area and this may be inherent in the solid material or can be achieved by a grinding procedure. The solid material may be MgCl.sub.2 2.0 C.sub.2 H.sub.5 OH; MgCl.sub.2 2.0 CH.sub.3 COOH or MgCl.sub.2 1.0 CH.sub.3 COOC.sub.2 H.sub.5. The solid material can be treated with a halogen-containing compound either before, during or after the contacting with the transition metal compound. The product of the contacting is useful either as catalyst for the polymerization of unsaturated hydrocarbon monomers or as a component of such a catalyst.

Abstract: A transition metal compound containing at least one .eta..sup.6 -arene is contacted with a compound of the formula MX.sub.n Y.sub.2-n mL; or Z.sub.3 Al[MgZ.sub.2 ].sub.y AlZ.sub.3 where L, M, X, Y, Z, m, n, and y are defined. The transition metal compound can be a compound such as TiCl.sub.2.2AlCl.sub.3 -arene. The compound of the defined formula may be a solid compound which may be insoluble in hydrocarbon liquids when it may be of high surface area which may be inherent in the solid or can be achieved by a grinding procedure. The compound of the defined formula may be Mg(OC.sub.2 H.sub.5).sub.2 ; Mg(CH.sub.3).sub.2 ; Mg(CH.sub.2 C.sub.6 H.sub.5).sub.2 ; (C.sub.2 H.sub.5).sub.3 Al[Mg(C.sub.2 H.sub.5).sub.2 ]-Al(C.sub.2 H.sub.5).sub.3 or (iC.sub.3 H.sub.7 O).sub.3 Al(Mg(OiC.sub.3 H.sub.7).sub.2 ]Al(OiC.sub.3 H.sub.7).sub.3. During or after the contacting with the transition metal compound, treatment with a halogen-containing compound may be effected.