ted from the group consisting of hydrogen and methyl, R2 is a member selected from the group consisting of hydrogen, alkyl having 1 to 4 carbon atoms and --CH2 --CHR1 --COOMe, X is a member selected from the group consisting of (1) a direct bond between the carbon and the phosphorus, ##EQU12## and Me is a member selected from the group consisting of hydrogen and a cation capable of complex formati