Overcoming synthetic challenges in developing High-Performance polybenzoxazine from Diamine-Functionalized Double-Decker silsesquioxane (DDSQ) cage

A new diamine-functionalized double-decker silsesquioxane (DDSQ-NH₂) derivative was first synthesized from the phenyltrimethoxysilane as the starting material by following sol–gel reaction in NaOH to form DDNa, corner capping with methylvinyldichlorosilane to form the DDSQ-CH=CH₂ derivative and finally reacted with 4-bromoaniline by using Heck reaction to obtain the target compound. Various synthetic approaches, including one-pot Mannich condensation and three-step synthesis based on this new diamine-functionalized DDSQ (DDSQ-NH₂) compound, were explored but proved unsuccessful due to low selectivity and DDSQ structural degradation. The novel DDSQ-based benzoxazine monomer (DDSQ-BZ (III) in this study) was successfully synthesized through a Schiff-base reaction between DDSQ-NH₂ and 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine-6-carbaldehyde (CHO-BZ) monomer, maintaining the fully closed-cage DDSQ structure, which is thoroughly characterized using FTIR, NMR, DSC, and TGA analyses, confirming the retention of the DDSQ cage and the formation of the benzoxazine ring. Thermal polymerization behavior was analyzed, revealing record-high stability (T_d10 = 644 °C, char yield = 82.8 wt%). TEM, SEM, and EDX analyses demonstrated homogeneous dispersion of the DDSQ cage structure within the polybenzoxazine matrix, ensuring enhanced thermal stability.