A catalyst is prepared by combining a first compound consisting of a bis(cyclopentadienyl)metal compound having one of the following general formulae: 1. (A-Cp)MX1X2 2. (A-Cp)M @@@@'2 3. (A-Cp)ML 4. (Cp*)(Cp @@@X1 Wherein: M is a metal selected from the Group consisting of titanium, zirconium and hafnium; (A-Cp) is either (Cp)(Cp*) or Cp-A'-Cp* and Cp and Cp* are the same or different substituted or unsubstituted cyclopentadienyl radicals; A' is a covalent bridging group; L is an olefin, diolefin or aryne ligand; X1 and X2 are, independently, selected from the group consisting of hydride radicals, hydrocarbyl radicals, substituted-hydrocarbyl radicals, organometalloid radicals and the like; ; X'1 and X'2 are joined and bound to the metal atom to form a metallacycle, in which the metal atom, X'1 and X'2 form a hydrocarbocyclic ring containing from about 3 to about 20 carbon atoms; and R is a substituent on one of the cyclopentadienyl radicals which is also bound to the metal atom. With a second compound which is an ion exchange compound comprising a cation which will irreversible react with a ligand on said first component and an anion which is a single coordination complex comprising a plurality of lipophilic radicals covalently coordinated to and shielding a central charge-bearing metal or metalloid atom, which anion is bulky and stable to reactions involving the cation of the second component.; Upon combination of the first and second components, the cation of the second component reacts with one of the ligands of the first component, thereby generating an ion pair consisting of a Group IV-B metal cation with a formal coordination number of 3 and a valence of +4 and the aforementioned anion, which anion is compatible with and noncoordinating towards the metal cation formed from the first component.; Suitable second components may be represented by the following general formula: [(L'-H)]d[(M')Q1Q2...Qn] Wherein: L' is a neutral Lewis base; H is a hydrogen atom; [L'-H] is a Bronsted acid; M' is a metal or metalloid selected from the Groups subtended by Groups V-B to V-A of the Periodic Table of the Elements; ie., Groups V-B, VI-B, VII-B, VIII, I-B, II-B, III-A, IV-A and V-A; Q1 to Qn are selected, independently, from the Group consisting of hydride radicals, dialkylamido radicals, alkoxide and aryloxide radicals, hydrocarbyl and substituted-hydrocarbyl radicals and organometalloid radicals and any one, but not more than one, of Q1 to Qn may be a halide radical - the remaining Q1 to Qn being, independently, selected from the foregoing radicals; m is an integer from 1 to 7; n is an integer from 2 to 8; and n - m = d.; Many of the catalysts thus formed are stable and isolable and may be recovered and stored. The catalysts may be preformed and then used to polymerize olefins, diolefins and/or acetylenically unsaturated compounds either alone or in combination with each other or with other monomers or the catalysts may be formed in situ during polymerization by adding the separate components to the polymerization reaction. The catalyst will be formed when the two components are combined in a suitable solvent or diluent at a temperature within the range from about -100 DEG C to about 300 DEG C. The catalysts thus prepared afford better control of polymer molecular weight and are not subject to equilibrium reversal. The catalysts thus produced are also less pyrophoric than the more conventional Ziegler-Natta olefin polymerization catalysts.; Certain of the catalysts also yield homopolymers of alpha -olefins having relatively high molecular weights. Certain of these catalysts also yield copolymers containing significantly more comonomer, many of which copolymers will be elastomeric.