Strategies for preventing gelation in preparation of hyperbranched benzoxazines with high performance

The gelation of linear oligomeric benzoxazine precursors (OBPs) and high-molecular-weight benzoxazine precursors (HMWBPs) have been noticed and addressed by various effective actions, while no similar reports on hyperbranched benzoxazoles have been available. A series of hyperbranched benzoxazines were synthesized by A2 + B3 method using 1,3-bis(3-aminopropyl)tetramethyldisiloxane (BATSi) and 1,1,1-tris(4-hydroxyphenyl) ethane (THPE) and paraformaldehyde, namely HB-TSis, to examine the conditions for preventing gelation. We found that in most cases, increasing the feeding molar ratio of-OH to NH2 group, prolonging the reaction time and increasing the reactant concentration were all favorable for the formation of soluble HB-TSis. Except when the molar ratio of-OH to NH2 was 2:3, a lower reactant concentration was more likely to avoid gel formation. A sufficiently high vacuum degree during drying was reported for the first time as a factor affecting the gelation of polybenzoxazine precursors. In addition, gelation-free HB-TSis exhibited high degrees of branching (DBs) and weight-average molecular weight (Mw) in the range of 0.58-0.69 and 6100-10,100 Da, respectively. Their cured resins, poly(HB-TSi)s, both perform well in terms of thermal stability with high char yield (Yc) varying from 37.9 to 56.2%, and flame retardancy with low the heat release capacity (HRC) varying from 49.9 to 81.9 J/(g K), which are capable of being applied as nonignitable materials.

Automated summary

The conditions for synthesizing soluble polybenzoxazine precursors were discovered, and the influence of vacuum degree on gelation of polybenzoxazine precursors was found for the first time. The synthesized hyperbranched benzoxazole polymers exhibited good thermal stability and flame retardancy, making them suitable for nonignitable materials.