Two-dimensional correlation spectroscopy has been applied to study PMPCS (poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]-styrene}), a representative example of mesogen-jacketed liquid crystalline polymers. With the precise analysis of a series of Fourier transform infrared (FTIR) spectra of PMPCS recorded at varied temperatures, a reasonable mechanism of the development of liquid crystalline (LC) phase is proposed. Before the phase transition, the conformational change of individual side chains occurs sooner than that of the backbone due to the larger motional freedom of the side chains. After the phase transition, however, the readjustment of still somewhat mobile backbone occurs before the ordered, rigid, and mutually interacting side chains. That is, phase transition leading to the LC phase formation brings in a new cooperative restriction of motions to the segments.