Insight into the Mechanism of Reversible Ring-Opening of 1,3-Benzoxazine with Thiols

Abstract

The reversible ring-opening addition and fragmentation reaction of p-cresol-based N-phenylbenzoxazine with aliphatic and aromatic thiols was investigated in solvent-mediated and solvent-free reactions. Independently of the used thiol, N-phenylbenzoxazine and the thiols reacted to equilibrium with comparable amounts of reactants and products in aprotic solvent, whereas in protic solvent almost full conversions were reached. In contrast, thiol reactivity was a crucial factor in solvent-free reactions yielding fast and complete conversions for a more acidic thiol and balanced equilibrium concentrations in case of thiols with high pK values. The strong influence of thiols with low pK values emphasizes the relevance of the protonation step in the ring-opening reactions of 1,3-benzoxazines with thiols in absence of solvents where acidity predominates nucleophilicity. The reverse reactions, namely adduct dissociation and benzoxazine recovery, were successfully conducted at elevated temperatures and reduced pressure facilitated by the removal of the formed thiols yielding up to 95% recovered 1,3-benzoxazine. These results provide deeper understanding of the reversible ring-opening reaction mechanism of 1,3-benzoxazine with thiols.

Review on the Accelerated and Low-Temperature Polymerization of Benzoxazine Resins: Addition Polymerizable Sustainable Polymers

“…Besides monomer, main-chain type PBz was polymerized at room temperature with various thiols, namely, thiophenol, 2-ethanethiol and 1-butanthiol in CH 3 OH/CHCl 3 for 24 h. Successful incorporation of the thiol compounds to PBz was confirmed by spectral and molecular weight characterizations [279]. Urbaniak et al [281] proposed that reversible ring-opening of 1,3-benzoxazine with thiols proceeds via an iminium ion intermediate rather than the cyclic six-membered transition state, as shown in Figure 40 [278]. p-Nitrothiophenol and thiophenol promotes a substantial increase in % conversion of monomer as can be seen from Figure 40b, which illustrates the strong influence of thiols with low pK a values.…”

A cardanol-based polybenzoxazine vitrimer: recycling, reshaping and reversible adhesion

“…41 It is noteworthy that the catalytic opening of the lateral benzoxazine rings by thiols (COLBERT) could also happen. [42][43][44] However, the absence of both thiols in the monomer, as revealed by Raman analysis (see Fig. S4 in the ESI †), and benzoxazine/thiol adducts in the polymer, according to the NMR analysis of the soluble part (see Fig.…”

Ring Opening Reaction of 3,4‐Dihydro‐2H‐1,3‐Benzoxazine with Amines at Room Temperature

“…firstly found that benzoxazine can copolymerize with polyfunctional thiol at room temperature. After that, Takeshi and Semiha et al . conducted a series of studies on the reaction conditions and applications of catalytic opening of the lateral benzoxazine rings by thiols (COLBERT reaction).…”