Cyclic Gradient COPolymers Debut

Copolymers can take a variety of forms, among them versions with alternating monomers, alternating blocks of monomers, and gradients in which the monomer gradually switches from predominantly one type to another. Now there’s a new class to add to the list: cyclic gradient copolymers (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201101853). To create the novel species, Stanford University’s Robert M. Waymouth and colleagues copolymerized δ-valerolactone and ε-caprolactone using an N-heterocyclic carbene catalyst. Since δ-valerolactone reacts faster, the polymer starts with that monomer and gradually switches over to ε-caprolactone as δ-valerolactone runs out. The growing polymer is zwitterionic, with a negatively charged alkoxide on the growing end and a positively charged imidazole on the other. The attraction between the charges promotes eventual cyclization, producing a ring with both an abrupt and a gradual structural switch. In linear copolymers, those with gradients have different properties from those with sharp interfaces. Circular polymers also behave differently from their linear counterparts. Combine a cyclic polymer with both a gradient and a sharp interface, and “all bets are off,” Waymouth says. His group is now studying the polymers’ morphology and functional properties to see exactly how they might behave.