UK Patent 1,306,044 relates inter alia to a process for polymerising alpha-olefins using a catalyst comprising an organometallic compound and the solid product obtained by reacting silica or alumina with an excess of a compound of the formula MRn Xm-n wherein M is aluminium or magnesium, R is a hydrocarbon radical, X is hydrogen or halogen, m is the valency of M and n is a whole number not greater