Influence of substituent on equilibrium of benzoxazine synthesis from Mannich base and formaldehyde

Abstract

N-Substituted aminomethylphenol (Mannich base) and 3,4-dihydro-2H-3-substituted 1,3-benzoxazine (benzoxazine) were synthesized from substituted phenol (p-cresol, phenol, p-chlorophenol), substituted aniline (p-toluidine, aniline, p-chloroaniline) and formaldehyde to study influence of substituent on equilibrium of benzoxazine synthesis from Mannich base and formaldehyde. (1)H-NMR and charges of nitrogen and oxygen atoms illustrate effect of substituent on reactivity of Mannich base, while oxazine ring stability is characterized by differential scanning calorimetry (DSC) and C-O bond order. Equilibrium constants were tested from 50 °C to 80 °C, and the results show that substituent attached to phenol or aniline has same impact on reactivity of Mannich base; however, it has opposite influence on oxazine ring stability and equilibrium constant. Compared with the phenol-aniline system, electron-donating methyl on phenol or aniline increases the charge of nitrogen and oxygen atoms in Mannich base. When the methyl group is located at para position of phenol, oxazine ring stability increases, and the equilibrium constant climbs, whereas when the methyl group is located at the para position of aniline, oxazine ring stability decreases, the benzoxazine hydrolysis tends to happen and equilibrium constant is significantly low.

Improvement of aldehyde‐functional polybenzoxazine processability and mechanical properties achieved by 5‐aminoindole/benzoxazine copolymerization

“…There is one sharp exothermic peak in the DSC curve of PHB‐a due to thermal‐induced ring‐opening polymerization of benzoxazine. Due to the electron‐withdrawing effect of aldehyde groups, the exothermic peak temperature at 200 °C was observed, the curing peak temperature being decreased compared to that of conventional benzoxazine 34,35 . With the addition of 5‐aminoindole, two onset curing temperatures for PHB‐a‐indole were observed, decreased to 70 and 167 °C, respectively, demonstrating the occurrence of two types of reactions.…”

“…Due to the electron-withdrawing effect of aldehyde groups, the exothermic peak temperature at 200°C was observed, the curing peak temperature being decreased compared to that of conventional benzoxazine. 34,35 With the addition of 5-aminoindole, two onset curing temperatures for PHB-aindole were observed, decreased to 70 and 167°C, respectively, demonstrating the occurrence of two types of reactions. The heat enthalpy decreased from 347 to 166 J g -1 after prepolymerization, indicating the high efficiency of 5-aminoindole towards catalytic polymerization of benzoxazine monomers.…”

Multifunctional Benzoxazines Feature Low Polymerization Temperature and Diverse Polymer Structures

“…Studies on the electronic effects of phenol-based N -phenylbenzoxazines have revealed that electron-withdrawing substituents on the benzene ring at the para position to the oxygen lowered curing temperatures when compared to unsubstituted N -phenylbenzoxazine (onset curing temperature T c = 263 °C). In contrast, electron-withdrawing groups on the aniline ring in the para position increased the curing temperatures of benzoxazines [1,18].…”

Effect of phenol on the synthesis of benzoxazine

“…18 Finally, benzoxazines are formed via the dehydration reaction between mannich bases and formaldehyde (Scheme 1, step 3). 19,20 For this route, N,N-dihydroxymethyl amine or N-hydroxymethyl amine which belongs to the formaldehyde-amine derivatives, are generally considered as the key intermediates. 14 Nevertheless, formaldehyde-amine derivatives are generated very fast, and no clear correlations between the consumption of primary amine (or the formation of formaldehyde-amine derivatives) and the formation of benzoxazine are established.…”