The present invention relates to the field of polymerisation catalysts, and systems comprising these catalysts for polymerising carbon dioxide and an epoxide, a lactide and/or lactone, and/or an epoxide and an anhydride. The catalyst is of formula (I): wherein at least one of M1 or M2 is selected from Ni(II) and Ni(III)-X. A process for the reaction of carbon dioxide with an epoxide; an epoxide and an anhydride; and/or a lactide and/or a lactone in the presence of the catalyst is also described.