Impact of Helical Chain Shape on Polypeptoid Block Copolymer Self-Assembly*

Controlling the self-assembly of block copolymers with variable chain shape and stiffness is important for both driving the self-assembly of functional materials containing non-ideal chains, as well as for developing materials with new mesostructures. In sequence-defined polypeptoids the backbone secondary structure can be controlled by the choice and tacticity of the side chains. Bulky, homochiral side chains are introduced at specific locations along the chain to form a helical secondary structure, or a racemic mixture of side chains forms a disordered chain shape. In a series of poly(peptoid-b-(n-butyl acrylate) polymers, block copolymers with a helical block display larger domain spacings than those with a Gaussian coil block. The increases in domain spacing are similar expectations from introducing a stiffer polymer chain that fills a larger conformational volume and has larger penalties for chain stretching. However, while the polypeptoid helix has a persistence length longer than that of the unstructured polymer chain, it is actually less space-filling. We attribute the changes in domain structure to a combination of packing frustrations and increased penalties for stretching the helix relative to the disordered chain.