Formation and cure of novolacs: NMR study of transient molecules

High-resolution magnetic resonance spectroscopy has been used to investigate three areas of novolac resin chemistry: first, the lifetimes of glycol species in formalin under acid and base conditions and the reactivity of methylene glycol towards alcohols; second, the in situ identification of novolac resin intermediates; third, the novolac–hexamethylenetetramine cure mechanisms. It is shown that the mean lifetime of a given glycol species is an order of magnitude shorter under basic conditions than under an acid of equal normality. The ratio of the equilibrium constants for the addition of methylene glycol to methanol, benzyl alcohol, and phenol was measured as 70:20:1. New reactive intermediates were discovered while monitoring the reaction of phenol and formalin. The reactions of various phenols with hexamethylenetetramine were studied, and short-lived benzoxazines were detected at 100–120°C whenever ortho hydrogen was available on the phenolic nucleus. The NMR peak assignments in the reaction sequence: benzoxazine, dibenzylamine, tribenzylamine, diphenylmethane are presented. Spectra of the very elusive p-benzylamines of 2,6-xylenol were recorded at 100–155°C. At 170°C, the lifetimes of these p-benzylamines are too short to afford proton resonance spectra. No evidence of hexahydrotriazine or (CH₂ = N-CH₂)₃N type structures was found in 100 MHz NMR spectra of reaction mixtures of a phenol and hexamethylene-tetramine.