Azopyridine-functionalized benzoxazine with Zn(ClO₄)₂ form high-performance polybenzoxazine stabilized through metal-ligand coordination

In this study, we prepared a benzoxazine monomer (Azopy-BZ) that features azobenzene and pyridine units through the reaction of paraformaldehyde, aniline, and 4-(4-hydroxphenylazo)pyridine (Azopy-OH), which is obtained through a diazonium reaction of 4-aminopyridine with phenol in the presence of sodium nitrite and NaOH. The azobenzene and pyridine groups in the benzoxazine monomer play the following two roles: (i) allowing photoisomerization between the planar trans form and the nonplanar cis form of the azobenzene unit (characterized using UV-vis spectroscopy and contact angle analyses) and (ii) serving as a catalyst that accelerated the ring opening polymerization of the benzoxazine units, which was characterized by the exothermic peak shifting to a lower temperature during differential scanning calorimetry (DSC) analyses. The curing temperature of the model benzoxazine 3-phenyl-3,4-dihydro-2H-benzoxazine (Pa-type) was 263 °C; it decreased to 208°C for Azopy-BZ, presumably because of the basicity of the azobenzene and pyridine groups. Blending with zinc perchlorate [Zn(ClO₄)₂] not only improved the thermal properties, as determined through dynamic mechanical analysis (DMA), due to physical crosslinking of the pyridine units through zinc cation coordination in a metal-ligand bonding mode, but also further facilitated the ring opening polymerization to occur at a temperature of only 130°C (DSC). Thus, the presence of Zn(ClO₄)₂ overcame the problem of high temperature curing (ca. 180-210°C) required for traditional polybenzoxazines. Introducing the azobenzene and pyridine units and the zinc salt into this polybenzoxazine system provided a multifunctional material that exhibited photoisomerization-based tuning of its surface properties, accelerated ring opening polymerization of its oxazine rings, and increasing physical crosslinking density, through metal-ligand interactions, to enhance its thermal properties.