Implementation of meta-Positioning in Tetrafunctional Benzoxazines: Synthesis, Properties, and Differences in the Polymerized Structure

Implementation of meta-positioning in intermediate tetrafunctional benzoxazines instead of para-positioning was done in an attempt to obtain low curing and weight loss-free benzoxazine monomers before/during ring-opening polymerization. Three different meta-positioned tetrafunctional benzoxazine monomers (mBZ4s) were synthesized by the tandem reaction method using a starting phenol/amine, which has an aminomethylphenol backbone at the meta-position, aromatic diamines such as 4,4'-diaminodiphenyl methane (DDM-D) and m-phenylene diamine (mPDA) or bisphenols such as 4,4'-dihydroxydiphenyl methane (DDM-B), and paraformaldehyde. All mBZ4s showed a low curing temperature (mBZ4-mPDA (209 degrees C) < mBZ4-DDM-D (219 degrees C) < mBZ4-DDM-B (226 degrees C)), no weight loss before/during ROP and good thermal stability for resulting polybenzoxazines (initial degradation temperature at 10% weight loss (T-d10), 376-411 degrees C; char yield at 600 degrees C, 61-66%). The major structural differences between these monomers are the atoms present at the meta-positioning, N,O in mBZ4-DDM-D, N,N in mBZ4-DDM-B and a combination of m-N,O and m-N,N in mBZ4-mPDA. The ring-opening polymerization in these monomers proceeded via different pathways, intramolecular electrophilic substitution of iminium ion in m-N,O-positioned BZ4s (mBZ4-DDM-D and mBZ4-mPDA) due to presence of main active sites, ortho-position to the oxygen of oxazine ring within the molecule itself, and formed four-membered AZA cyclic rings along with phenolic Mannich bridges in the networked structure, whereas in m-N,N-type positioning, polymerization proceeded intermolecularly and resulted in traditional phenolic Mannich bridges. Solid-state C-13 NMR analysis of poly(mBZ4)s obtained at 200 and 250 degrees C revealed the presence of AZA cyclic rings in the cured product of m-N,O-positioned tetrabenzoxazines and their further ring opening to form additional cross-links. The nature of cross-links was determined with the help of conducting a model study using N-phenylbenzazetine, which has a similar four-membered cyclic ring; the second part of this artide is devoted to proving those aspects.