These catalysts are metallocenes together with a cocatalyst, typically MAO, −[O−Al(CH)]−. The idealized metallocene catalysts have the composition CpMCl (M = Ti, Zr, Hf) such as titanocene dichloride. Typically, the organic ligands are derivatives of cyclopentadienyl. In some complexes, the two cyclopentadiene (Cp) rings are linked with bridges, like −CH−CH− or >SiPh. Depending on the type of their cyclopentadienyl ligands, for example by using an ansa-bridge, metallocene catalysts can produce either isotactic or syndiotactic polymers of propylene and other 1-alkenes.