(5) Recent Progress in Polycondensation

This commentary deals with progress in polycondensation along with theories. Flory formulated a theory of irreversible polycondensations, in which the formation of cyclic polymer and the change of the reactivity of polymer were not considered. Enhancement of the reactivity of AB monomer connected to the polymer end enabled to change the polymerization mechanism from step-growth to chain-growth polymerization. Enhancement of the reactivity of AA monomer connected to BB monomer made it possible to undergo unstoichiometric polycondensation. Jacobson and Stockmayer developed a theory of reversible polycondensation, in which back-biting induced the formation of low-molecular-weight cyclic polymer and ring-chain, ring-ring, and chain-chain equilibration. However, Kricheldorf found that quantitative formation of cyclic polymer through intramolecular reaction of the polymer ends, not back-biting, and that intermolecular exchange reaction of the polymer backbone took place. Accordingly, selective synthesis of linear polymer is a recent project in polycondensation. Cyclic polyesters, obtained by conventional irreversible polycondensation, were converted into linear polymers by means of reversible exchange reaction of the cyclic polymer with a small molecule having the same linkage of the polymer. Furthermore, reversible unstoichiometric polycondensation afforded selectively linear polymer capped with excess monomer at both ends. These backbone exchange reactions were applicable for adhesion.