Abstract: Polymerization of ethylene using a transition metal hydrocarbyl complex, especially zirconium tetrabenzyl, supported on alumina consisting of substantially spherical particles having diameters in the range 20-150 .mu.m. The particulate polymer produced by the process contains substantially no "fines" (i.e. particles <100 .mu.m diameter) and has improved pourability.

Abstract: Organometallic compounds of the general formula L.sub.r M(CH.sub.2 SiR.sup.1 R.sup.2 R.sup.3).sub.m (1) where M is a transition metal of Groups IIIA to VIA of the Periodic Table of Elements, m is an integer from 1 to the prevailing valency of the metal M, L.sub.r represents the sum of any other incidental ligands attached to the metal M, r being 0 or an integer, and R.sup.1, R.sup.2 and R.sup.3 represent substituted or unsubstituted hydrocarbyl groups or hydrogen atoms, at least one of said groups having the general formula ##SPC1##Where n is an integer from 1 to 5 and Y is a group capable of donating electrons to the metal M are useful as catalysts for olefin polymerization. The compounds may be used as catalysts alone or when supported upon a matrix for example of alumina. Particularly useful compounds are those in which the transition metal M has a valency lower than the maximum valency for that metal.