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Isomerization of double bonds from an allylic to propenyl position is generally mediated by expensive metal catalysts, demanding an additional synthetic step, thereby reducing sustainability of the reaction. However, such functionalities are inherent in naturally occurring compounds, enabling a versatile protocol for their industrial utility. Herein, we report the synthesis of benzoxazine monomers based on biosourced isomeric phenols, eugenol (E) and isoeugenol (IE), and biobased amine, furfurylamine (fa) to form E-fa and IE-fa monomer, respectively. The structural variation in the phenols revealed a differential chemical reactivity, during both the synthesis of the monomer and the polymerization reaction, confirming a significant influence of isomerism. The monomers only differ in the position of the double bond in the para-substituted propylene unit forming nonconjugated vs conjugated alkylene chain with the benzene ring containing benzoxazine in E-fa and IE-fa, respectively. The structure of the monomers was confirmed by 1H NMR, 13C NMR, FTIR, XRD, and mass spectrometry. The high purity of monomer was further affirmed by HPLC and DSC to demonstrate the effect of isomerization on the polymerization behavior. The extended conjugation of the double bond in IE-fa with the proximal benzoxazine ring showed a higher reactivity toward ring-opening polymerization, polymer conversion, and cross-linking reactions as supported by FTIR, NMR, and DSC-based kinetic studies. Thermal stability, mechanical properties, and adhesive analysis by TGA, DMTA, and lap shear strength measurements further supported the effect of structural isomerism of monomers with a higher potential of PIE-fa over the PE-fa network. Current work illustrates an economic, one-step, microwave-assisted, and VOC’s- and catalyst-free synthesis with a simultaneous solventless processing of synthesized monomers using renewable materials as feedstocks for high-performance polymers.
Amarnath et al. (Tue,) studied this question.