Novel self curable phosphorus- and spiro phosphorus-based maleimides: synthesis, characterization, cure behavior and thermal properties

Novel phosphorus- and spiro phosphorus-based maleimides such as tris[4-(2,5-dioxopyrrol-1-yl)phenyl] phosphite (TDPPI), tris[4-(2,5-dioxopyrrol-1-yl)phenyl] phosphate (TDPPA), 3,9-di[4-(2,5-dioxopyrrol-1-yl)phenoxy]-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane (TDU), and 3,9-di[4-(2,5-dioxopyrrol-1-yl)phenoxy]-2,4,8,10-tetraoxa-3λ⁵,9λ⁵-diphosphaspiro[5.5]undecane 3,9-dioxide (TDUD) have been synthesized. The structural characterization of the synthesized maleimides was performed using elemental analysis, Fourier transform infrared (FTIR) spectroscopy, and ¹H, ¹³C and ³¹P nuclear magnetic resonance spectral data. Melting behavior and heat changes during the materials’ physical and chemical changes as a temperature function were studied using differential scanning calorimetry (DSC). Mass loss rate, residual mass and the gases evolved during thermal degradation were examined using a thermogravimetric (TG) analyzer coupled with FTIR. All four maleimides having a low enthalpy of curing and high char residue (about 50 wt%) display better thermal stability than the parent N-(4-hydroxy phenyl)maleimide (N4HPMI). This was owing to the presence of phosphorus or spiro phosphorus moiety in conjunction with the maleimide unit. The combined impact of phosphorus and maleimide units makes the materials suitable for high-performance applications.