Phosphorus-containing flame retardants

A flame-retardant comprising one or more polymers comprising a unit represented by formula (I), (Ia) or (II):

• where A is selected from O, S, SO2, a single bond, alkyl, and —CH2—P1;
• P1 is a phosphorus-containing group of the formula:
• wherein R1 and R2 together with a group Y form a group R1—Y—R2, which together with the phosphorus atom bound to R1 and R2 form a ring:
• in which R1 and R2 are the same or different and each is selected from O-alkyl, O-aryl, alkyl and aryl, and Y is a linking group selected from direct bond, alkylene and —O—;
• R3 is H or alkyl;
• each a is an integer individually selected from 0 to 4, provided that at least one a is at least 1;
• n is an integer from 4 to 100,000, and m is an integer from 0 to 100,000.