AbstractThe present paper describes the ring‐opening‐closing alternating copolymerization of cyclic phosphonites (2‐phenyl‐1,3,2‐dioxaphosphelane (1a), 2‐phenyl‐1,3,2‐dioxaphosphorinane(1b), 2‐phenyl‐1,3,2‐dioxaphosphepane(1c), and 2‐phenyl‐4H‐1,3,2‐benzodioxa phosphorin‐4‐one(1d)) with muconic acid(2a), succinaldehyde(2b), glutaraldehyde(2c), adipinaldehyde(2d), o‐phthalaldehyde(2e), acrylic anhydride(2f), methacrylic anhydride(2g), N‐methyl diacrylamide(2h), and N‐methyldimethacrylamide(2i). The reaction of a 1:1 monomer feed ratio, explained by a mechanism of propagation via zwitterion intermediates, proceeds without any added catalyst to give an alternating copolymer3involving ring‐opening of1and ring‐closing of2. The structure of copolymer3was determined by1H,13C NMR,31P NMR and IR s