Non-linear optical properties of polydiacetylene films would be enhanced by optimizing the effective conjugation length and the orientation of the polymer chains, for these parameters affect the third order polarizability and susceptibility. We investigated monolayers and Langmuir-Blodgett multilayers of two batches of poly-3-butoxy-carbonyl-methyl-urethane with different molecular weight. One batch was polymerized by gamma-ray irradiation (gamma/P3BCMU); the other was obtained by UV polymerization (UV/P3BCMU) and its molecular weight was ten times lower. From the thermodynamic parameters we evaluated that the phase transition occurring on compression of monolayers formed at the air-water interface leads to a larger increase of order in the polymer chains of gamma/P3BCMU. The visible and Raman spectra of multilayers confirmed the better arrangement of the conjugated chains in gamma/P3BCMU. Though the number of repeated units in both polymer batches was much larger than the value corresponding to the optimum effective conjugation length, a better arrangement of the conjugated chains was found in the films of the polymer with higher molecular weight.