Preparation of phosphonic acid derivatives

A method of preparing a compound of formula (I):

• formula (I)
• wherein:
• each R2 is independently selected from the group consisting of hydrogen, alkyl groups, alkali metals, alkaline earth metals and nitrogen-containing groups;
• R3 and R4 are independently selected from the group consisting of hydrogen and alkyl groups;
• R5 is selected from the group consisting of hydrogen, alkyl groups and
• wherein said method comprises the steps of:
• (a) azeotropically removing water from a reactant of formula (II):
• wherein:
• R1 is CHR3R4;
• R2, R3, R4 and R5 are as defined above;
• and X is a number between 0 and 20;
• to form a dehydrated reactant (II);
• (b) pyrolysing said dehydrated reactant (II) at a temperature of from 170° C. to 300° C., and thereafter
• (c) optionally, converting any anhydrides formed by the pyrolysing step (b) to the corresponding acid or salt having the formula (I).