AbstractThis paper reports a new copolymerization based on the novel concept, i.e., ring‐opening‐closing alternating copolymerization. The reaction of a cyclic monomer A and a noncyclic monomer B takes place without added initiator or catalyst to give a copolymer of structure having a ring‐opened unit from monomer A and a ring‐closed unit from monomer B in alternating arrangement. As monomer A, five, six and seven‐membered cyclic phosphonites are employed. As monomer B, on the other hand, are used muconic acid and dialdehydes. Combinations of these monomers A and B produced in all cases ring‐opened and closed 1 : 1 alternating