Polybenzoxazine Aerogels. 1. High-Yield Room-Temperature Acid-Catalyzed Synthesis of Robust Monoliths, Oxidative Aromatization, and Conversion to Microporous Carbons

We describe a new room-temperature HCl-catalyzed method for the synthesis of polybenzoxazine (PBO) aerogels from bisphenol A, formaldehyde, and aniline that cuts the typical multiday high-temperature (≥130 °C) route to a few hours. The new materials are studied comparatively to those from heat-induced polymerization, and both types are evaluated as precursors of carbon (C-) aerogels. In addition to the ortho -phenolic position of bisphenol A, the HCl-catalyzed process engages the para position of the aniline moieties leading to a higher degree of cross-linking. Thereby, the resulting aerogels consist of smaller particles with higher mesoporosity, higher surface areas (up to 72 m 2 g –1 ), and lower thermal conductivities (down to 0.071 W m –1 K –1 ) than their thermally polymerized counterparts (corresponding best values: 64 m 2 g –1 and 0.091 W m –1 K –1, respectively). It is also reported that the carbonization efficiency (up to 61% w/w), the nanomorphology, and the pore structure of the resulting C-aerogels depend critically on a prior curing step of as-prepared PBO aerogels at 200 °C in the air. According to spectroscopic evidence and CHN analysis, curing at 200 °C in air oxidizes the −CH 2 – bridges along the polymeric backbone and subsequently fuses aromatic rings (see Abstract Graphic) in analogy to transformations during carbonization processing of polyacrylonitrile. C-aerogels from cured PBO aerogels are microscopically similar to their respective parent aerogels; however, they have greatly enhanced surface areas, which, for C-aerogels from HCl-catalyzed PBOs, can be as high as 520 m 2 g –1 with up to 83% of that attributed to newly created micropores. The acid-catalyzed route is used in the next article for the synthesis of iron oxide/PBO interpenetrating networks as precursors of iron(0) aerogels.