The microstructural dynamics of polyisobutylene have been investigated using a new rheooptical technique, two-dimensional Raman scattering (2D Raman). 2D Raman employs intrinsic bond vibrations to selectively resolve the segmental dynamics within a material subjected to small-amplitude oscillatory deformation. In addition, interspecies interactions are probed through a crosscorrelation of the orientation-induced response from distinct Raman peaks. It has been observed that the motions of the main chain and side groups in polyisobutylene are affected independently as the mechanical frequency is increased from the rubbery plateau into the glassy regime. In particular, at low frequencies (<10 s(-1)) and room temperature, the motions of the side group and main chain are highly cooperative; however, at higher frequencies (>10 s(-1)), the groups reorient independently as the mechanical glass transition is approached. The results are in accord with the observed frequency dependence of the stress-optical coefficient.