A catalyst system including a procatalyst, a co-catalyst and an external electron donor of Formula I′: Si(L)n(OR11)4-n-m(R12)m  Formula I′ wherein, Si has a valency 4+; O has a valency 2− and O is bonded to Si via a silicon-oxygen bond; n is 1-4; m is 0-3; n+m≦4; each R11 and R12 group is independently an alkyl or aromatic substituted or unsubstituted hydrocarbyl; each L group is independently of Formula II: wherein, L is bonded to the silicon atom via the nitrogen-silicon bond; L has a single substituent on the nitrogen atom, where this single substituent is an imine carbon atom; and X and Y are independently a hydrogen atom; a heteroatom of Groups 13-17; an alkyl, optionally containing a heteroatom of Groups 13-17 or an aromatic substituted and unsubstituted hydrocarbyl, optionally containing a heteroatom of Groups 13-17.