Insensitivity of Sterically-Defined Helical Chain Conformation to Solvent Quality in Dilute Solution*

The interplay between polymer–solvent interaction and interactions that impose secondary structures determines polymer chain conformation in dilute solution. Polypeptoids–poly(N-substituted glycines) can form helical secondary structures primarily driven by steric interactions from chiral, bulky side chains, while a racemic mixture of the same side chains results in unstructured coil chains. Small-angle neutron scattering (SANS) reveals that the helical polypeptoids are locally stiffer but overall flexible. However, the radii of gyration (R_gs) of both helical and coil polypeptoids do not increase with improved solvent quality (A₂), and the chain expansion deviates from the universal swelling of polymer chains in dilute solution. Potential effects from solvents disrupting the steric interactions of chiral side chains are excluded by comparing to another chemically analogous coil polypeptoid devoid of side chain chirality. The results indicate that chain conformation change of these polypeptoids cannot be simply captured by excluded volume interactions, nor the steric interactions imposing the helical secondary structure. It is likely that the specific interactions between polypeptoid segments dominate the chain shape as opposed to polypeptoid–solvent interactions.