Factors Affecting the Solid-State Polymerization of 1,4-Bis(1,3-octadecadiynyl)benzene to a Polydiacetylene

In an attempt to produce a polydiacetylene (PDA) with larger third order nonlinear optical properties, the polymer precursor, 1,4-bis(1,3-octadecadiynyl)benzene, was synthesized. It was found, however, that neither thermal treatment (up to 343 K) nor y-ray irradiation at above 300 K was able to promote solid-state polymerization. A detailed analysis of the molecular motion was conducted using differential scanning calorimetry, solid-state cross-polarization magic-angle-spinning 13C, and solid-state ²H NMR and solution-state 13C NMR. It was found that below 300 K the monomer undergoes small-amplitude motions only. Above 300 K the sp-bonded phenyl ring undergoes π flips around the long axis of the molecule at an intermediate rate with respect to the NMR time scale (<2 × 10^-5 Hz^-1) with an activation energy of 60 ± 1 kJ mol^-1. The poor polymerization above 300 K was attributed to the monomers being unable to obtain the proper alignment with respect to each other necessary for the polymerization process. The results of this study lead to the successful polymerization of the monomer by using γ-ray irradiation but at 273 K. The structure of the polymer was determined using solid-state ¹³C NMR.