The polyaddition of 4,4'-bis[(3-ethyl-3-oxetanyl)methoxy]biphenyl (4,4'-BEOBP) and phenylphosphonic dichloride (PPDC) with quaternary onium salts as catalysts proceeded under mild reaction conditions to afford a polymer containing phosphorous atoms in its main chain. A polyphosphonate with a high number-average molecular weight (10,300) was obtained by the reaction of 4,4'-BEOBP and PPDC in the presence of tetraphenylphosphonium chloride (TPPC) in o-dichlorobenzene at 130 degreesC for 24 h. The structure of the resulting polymer was confirmed with IR, H-1 NMR, and P-31 NMR spectroscopy. Furthermore, it was proved that the polyaddition of certain bis(oxetane)s with phosphonic dichlorides proceeded smoothly to give corresponding polyphosphonates with TPPC as the catalyst.