Novel phosphonamidates, O-tert-butyl P-phenyl 1-piperidinyl phosphonamidate (1), O-tert-butyl P-phenyl N,N-diethyl phosphonamidate (2), O-tert-butyl P-phenyl 1-morpholinyl phosphonamidate (3), O-tert-butyl P-phenyl N-methyl-N-phenyl phosphonamidate (4), O-2-phenylpropyl P-phenyl 1-piperidinyl phosphonamidate (5), and O-1-phenylethyl P-phenyl 1-piperidinyl phosphonamidate (6) were synthesized from phenylphosphonic dichloride with the corresponding amines, followed by reaction with 2-methyl-2-propyl alcohol, 2-phenylpropyl alcohol, or 1-phenylethyl alcohol in the presence of sodium hydride. Their initiator activities were examined in the polymerization of GPE. GPE did not react with 1-4 below 110 degreesC, but it smoothly polymerized above 110 degreesC to afford the polymer with M-n of (0.7-1.4) x 10(3). The polymerization of GPE did not proceed with 5 and 6 below 90 and 170 degreesC, respectively, but proceeded rapidly above those temperatures. The phosphonamidates 1-6 served as thermally latent anionic initiators in the polymerization of GPE. The activities of 1-6 were affected by the decomposition rates of the ester parts and the basicity of the corresponding amines. The initiating species were identified as the amines by a N-15-labeled NMR experiment.