A polymerization process comprising contacting under polymerization conditions ethylene and at least one alpha olefin of formula CH2=CHA wherein A is a C2-C20 alkyl radical to obtain a copolymer containing from 95% by mol to 50% by mol of ethylene derived units in the presence of a catalyst system obtainable by contacting: b) at least a metallocene compound of formula (I) b) alumoxane or a compound capable of forming an alkyl metallocene cation; and optionally c) an organo aluminum compound; wherein the groups R1-R4, L, M and W are described in the text.