Samarium chloride and borohydride complexes of the diamide-diamine ligands (2-C5H4N) CH2N(CH2CH2NR)(2) (R = SiMe or mesityl) are described; the borohydride compounds are the first polydentate amide-supported single component lanthanide catalysts for the controlled polymerisation of polar monomers, and also represent the first lanthanide borohydride complex for the polymerisation of methyl methacrylate.