A proton magnetic nuclear resonance study was performed on co‐condensation reactions of resorcinol, 5‐methylresorcinol and 2,5‐dimethylresorcinol with methylol compounds, including ortho‐ and para‐methylolphenol, N‐methylol‐caprolactam, methylol‐N,N‐diethylurea, methylolurea and N,N′‐dimethylolurea. Spectral assignments, reaction kinetics and composition of products are discussed. The reaction in melt (120°C) with methylolphenols occurs as co‐condensation in the presence of all catalysts studied. In resorcinols C4C6 substitution is favored. The rate constants of methylol disappearance clearly indicate the preferable influence of acid and alkaline catalyst (not zinc acetate) on para‐methylol. The reaction with N‐containing methylol compound does not give any co‐condensate in the presence of alkaline catalyst. The optimum conditions for co‐condensation mainly depend on reactivity of co‐reagents with formaldehyde and stability of methylol compound. The quantitative amount of co‐condensate with methylolcaprolactam can be obtained in melt‐condensation (70°C) in the presence of acid catalyst. Because of the higher reactivity of urea, the reaction in melt (100°C) in the presence of acid catalyst does not lead to quantitative co‐condensation. The condensation of methylol compound or resorcinols with formaldehyde can be avoided substantially by performing the reaction in aqueous solution at lower temperature.

An attempt is made to describe the complicated rheokinetic behaviour of urea-formaldehyde resins during storage, in close connection with the changes in their chemical structure. The alkali-promoted non-linear polycondensation during resin storage is evaluated by pre-gel viscosity measurements. Viscosity changes are described by a power law function using extrapolation to zero shear and infinitely high shear rates. During most of the period of viscosity increase, the rheological behaviour of studied resins is similar. The deviation from the power law dependence occurring in the initial low-rate growth of viscosity takes place because of formaldehyde redistribution, which does not lead to covalent network build-up. In the presence of methanol this period is extended due to the formation of stable methoxymethylene groups.

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