Isomeric Speciation of Bisbenzoxazine Intermediates by Ion Spectroscopy and Ion Mobility Mass Spectrometry

Abstract

Bisbenzoxazines (BisBz) are a relevant model for the diverse bifunctional benzoxazines that are used to increase the polybenzoxazines cross-linking extensions and modulate the final resin properties for various usages. The presence of side products and intermediates during monomer formation can influence the resin characteristics by inducing chain termination and ramifications, affecting the polymerization and cure processes. This work investigated the diverse isomeric intermediates and side products that are present during the BisBz formation from bisphenol A, aniline, and formaldehyde by ion mobility coupled to tandem mass spectrometry (MS/MS) and ion spectroscopy techniques. The species detected in this work suggest that these multifunctional phenols open diverse concurrent reaction pathways based on two main reactive steps: (i) the imine/iminium phenol attack to form a phenylamino intermediate and (ii) the formaldehyde attack followed by dehydration to form the oxazine ring. The species observed also support previous studies of the benzoxazine formation mechanism and showcase the application of advanced analytical techniques in studying complex chemical systems.

Figure 1

Simplified representation of the (a) benzoxazine synthesis from aniline, phenol, and formaldehyde and (b) the formation of MonoBz and BisBz from aniline, bisphenol A (BPA), and formaldehyde. BPA opens different polymerization possibilities, enhancing the molecular diversity as represented in (c) for the Bz and (d) for MonoBz and BisBz.

Figure 2

(a) Typical positive mode nanospray ionization MS of a stoichiometric reaction of BPA, formaldehyde, and aniline after 1 h of reaction at 120 °C. (b) Possible species observed for the stoichiometric reaction of BPA, aniline, and formaldehyde after 1 h at 120 °C, as observed by the positive mode ESI–MS. Isomeric species are not represented and will be discussed throughout the text. Ions marked with † and * are, respectively, in-source fragments and reaction byproducts not relevant for the reaction mechanisms discussed in this work. Their proposed structure can be found in Table S1.

Figure 3

(a) IRMPD spectrum of the species with m/z 346, (b) calculated spectra of the N-MonoBz (black), O-MonoBz (red), and ROP-MonoBz (blue) isomers, (c) IRMPD spectrum of the species with m/z 463, and (d) calculated spectra of the N-BisBz (black), O-BisBz (red), and ROP-BisBz (blue). The simulated absorption spectra were calculated at the B3LYP/6-311+G(d,p) level of theory using 0.95 as a scale factor.

Figure 4

(a) IRMPD spectrum of the species with m/z 451 (blue), (b) calculated spectra of the NH-PA-MonoBz (green) and ROP-PA-MonoBz species (red), and (c) calculated spectrum of the N-PA-MonoBz (pink) and O-PA-MonoBz (black) species at the B3LYP/6 311+G(d,p) level of theory using 0.95 as a scale factor.

Figure 5

FAIMS spectra for the ion (a) m/z 334 (PA-BPA); (b) m/z 346 (MonoBz); (c) m/z 439 (PA-BPA-PA); (d) m/z 451 (PA-MonoBz); and (e) m/z 463 (BisBz). Each spectra represents the fragments observed for each population (Figures S5–S7). Highlighted species indicate population-specific fragments.

Figure 6

Representation of the most relevant species for the BisBz monomer formation as discussed in the text. All o-PA intermediates represented are identified by PA only for clarity.