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

Abstract

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.