Catalyst component for the polymerization of olefins comprising Mg, Ti and an electron donor of formula (I) In which X and Y are selected from, R1, and —OR1 and —NR2, B is oxygen or sulphur S is sulphur, R1 is selected from C1-C15 hydrocarbon groups, optionally contain a heteroatom selected from halogen, P, S, N, O and Si, which can be fused together to form one or more cycles, R is hydrogen or R1 and A is a bivalent bridging group with chain length between the two bridging bonds being 1-10 atoms.