Disclosed herein is a thermally curable mixture, comprising at least one phosphonic diester (A), at least one diphosphonic diester (A), or at least one phosphonic diester and at least one diphosphonic diester (A); and at least one compound (B) which can be reacted by transesterification, transamidation, self-condensation of N-hydroxyalkylamino groups, self-condensation of N-alkoxyalkylamino groups, transacctalization of N-alkoxyalkylamino groups, acctalization of N-hydroxyalkylamino groups, or a combination thereof. Also disclosed is a process for making the thermally curable mixture, and a cured material comprising the product of thermally curing the mixture.