Method of selectively preparing phosphorus containing flame retardants and their use in thermoplastics

A process for producing a phosphorus-containing flame retardant, comprising reacting at a reaction temperature a mixture comprising a metal or suitable metal compound and a stoichiometric excess relative to the metal or suitable metal compound of an unsubstituted or alkyl or aryl substituted phosphonic acid, wherein:

• the metal is capable of forming a polycation or the suitable metal compound is represented by the formula Mp(+)yXq where M is a metal, (+)y represents the charge of the metal cation, y is 2 or higher, X is an anion, and the values for p and q provide a charge balanced metal compound;
• the molar ratio of the unsubstituted or alkyl or aryl substituted phosphonic acid to the metal or suitable metal compound in the mixture is higher than 4:1;
• the reaction temperature is 105° C. or higher;
• and
• the unsubstituted or alkyl or aryl substituted phosphonic acid is in a molten state at the reaction temperature.