The order - disorder phase transition in the linear chain compound (TMMC) was investigated by means of x-ray diffraction. The hexagonal structure at room temperature (space group with Z = 2) is characterized by an orientational disorder of the organic group TMA. A weakly first-order phase transition occurs at 126 K which stabilizes a monoclinic low-temperature phase (space group with Z = 4) characterized by a doubling of the hexagonal unit cell along the b direction. The cell parameters were determined in a large range of temperature including the two phases (from 5 to 300 K) and the extent of lattice distortion was measured in the ordered monoclinic phase. The temperature dependences of both the spontaneous strain component and the intensity of superstructure reflections were analysed by a Landau type free energy expansion involving two coupled order parameters necessary to account for this `triggered' phase transition.