Abstract: A polymer having a backbone and a side unit of 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide bonded thereto, where the backbone of the polymer includes at least one heterocyclic moiety. There is also provided a process for producing the polymer as defined herein as well as a flame retardant polyolefin formulation including the polymer as defined herein and a moulded article including the flame retardant polyolefin formulation as defined herein.