Polybenzoxazines (PBz), an emerging class of thermosets, offer superior properties at a comparatively lower cost. The advantages of PBz comprise but are not limited to excellent thermal and mechanical properties, near-zero shrinkage upon polymerization, good chemical and water resistance, high char yield, molecular design flexibility, etc. Furthermore, benzoxazine (Bz) monomers can be easily processed under catalyst-free thermal ring-opening polymerization (ROP), thus enabling industrial sustainability. Tailoring new structural motifs in monomer to enhance the performance of PBz and thus bringing suitability to a vast domain of applications is the recent trend among polymer scientists. The inclusion of phenyl-substitution at the oxazine ring is an effective strategy to regulate properties of monomers and polymers is discussed in this piece of work, as shown in abstract Figure. Monomers are characterized by 1D and 2D NMR spectroscopy, HRMS, and X-ray crystallography. Polymerization temperature and thermal stability of monomers and polymers are determined using DSC and TGA respectively. Intramolecular interaction and stereo-electronic effect of phenyl ring plays an active role in controlling mass-loss and thus thermal stability during polymerization, as corroborated from TGA-GC-MS, crystal structure analysis, and NBO calculation. Mass loss during polymerization, a major problem associated with the first generation un-substituted monomer, is overcome largely by the substitution at the oxazine ring, i.e., with the latest fourth-generation di-substituted benzoxazine. Hence, structural engineering appears as an attractive strategy to design the latest formaldehyde-free 4th generation high-performance PBz thermosets.

Elucidating the stereo-electronic effect of phenyl substituents on thermal properties: A study on Oxazine-Ring un-substituted and di-substituted benzoxazine monomers & polymers

ACS Spring 2022

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