This paper theoretically explores possible phase diagrams of the associating polymers in which macro- and microphase separation transitions coexist. Our model binary mixture consists of long chains (A) carrying f associative groups and short chains (B) carrying one associative group on their ends. Comb-shaped block copolymers with A as a main chain bearing a different number of side B chains are formed in equilibrium. When A and B unfavorably interact, macro- and microphase separations compete. If the population of the formed block copolymers is sufficiently large, miscibility between A/B chains is so improved that a stable microphase appears in a restricted region on the phase plane near the stoichiometric concentration where the concentration of the mixture agrees with the molecular composition of the fully associated cluster A . B-f. This macrophase reveals new properties since it is formed by reversible bonds instead of covalent ones. Relation to the recently observed microphase in hydrogen-bonded polymer/surfactant systems is discussed.