AbstractThe helix–coil transition for poly(β‐benzyl‐L‐aspartate) [poly(Asp[OBzl])]in solvent mixtures of trifluoroacetic acid/deuterated chloroform (F3AcOH/CDCl3) was studied by means of proton and carbon‐13 nmr. Conformational fixation of the side chain occurs before the coil–helix transition of the backbone, when neighboring phenyl rings face each other. Another type of conformational fixation occurs in the side chain after the coil–helix transition of the backbone. These conformational changes of the side chain are due to the changes of the strength of the interaction between the side‐chain ester group and the F3AcOH molecule. In the absence of F3AcOH (coil‐forming solvent), the polymer has a rather rigid structure in which the side chain may wrap around the backbone. These conformational changes of the polymer are closely related to the changes of the interaction between the polymer an