| Abstract: | The acid‐catalyzed urea–formaldehyde reactions were reexamined in detail by using quantum chemistry method and 13C‐NMR determinations. Some issues in the synthesis theory that were not well understood previously have been addressed and clarified. The identified reaction mechanisms and calculated energy barriers suggest that the competitive formations of methylene and methylene ether linkages are kinetically affected by both reaction energy barriers and steric hindrance effect. The thermodynamic properties determine that the methylene linkages are dominant at the late condensation stage. The theoretical results well rationalized the observed different changing processes of resin structures with different F/U molar ratios. The previously proposed mechanism for transformation of methylene ether linkage to methylene linkage cannot explain the structural changes during condensation, and thus, other mechanisms were proposed. The calculated results for uron explained the fact that the formation of such structure is much slower than other structures under weak acidic condition. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44339. |
