Catalyst compositions comprising metallocene complexes having polymerisable olefinic groups substituent on an organic group containing a cyclopentadienyl nucleus may be used for the preparation of polyolefins. The catalyst compositions may be in the form of polymers comprising the metallocene complex and may be suitably supported on inorganic supports. Polymers having a broad range of density and melt indices as well as low hexane extractables and excellent powder morphology and flowability may be obtained by use of the catalyst compositions. Preferred metallocene complexes are zirconium complexes in which the polymerisable olefinic group is vinyl.