Latent catalyst-containing naphthoxazine: Synthesis and effects on ring-opening polymerization

Abstract

A naphthoxazine containing a latent catalyst based on -OH as part of its monomer structure has been synthesized through a simple one-pot reaction at room temperature. The chemical structure of this naphthoxazine monomer has been confirmed by 1H NMR, 13C NMR, and FT-IR and its purity by elemental analysis. Differential scanning calorimetry (DSC) and in situ FT-IR have been used to investigate the active and inactive conditions for the latent catalysis. DSC and thermogravimetric analysis (TGA) studies indicate that this naphthoxazine polymerizes quickly before the monomer evaporation, as is often the case for conventional naphthoxazines, minimizing the monomer loss during polymerization. The intramolecular interactions between the -OH group and oxazine ring and pyrrolidine ring of the monomer have been investigated in detail by using the homonuclear two-dimensional (2D) NMR technique 1H-1H nuclear Overhauser effect spectroscopy (NOESY). The -OH interacts with the N in the pyrrolidine ring and oxazine ring through stable intramolecular hydrogen bonds instead of presenting free -OH at room temperature, leading to the enhanced shelf life of the monomer. The free phenolic -OH initiates and catalyzes the polymerization once the hydrogen-bonded interactions are weakened or disrupted upon increasing temperature, showing a latent catalytic effect.