Structure and Conformation of High Molecular Weight Novolak Resin

A considerable high molecular weight novolak resin could be obtained from the reaction of phenol with paraformaldehyde in acetic acid by HCI catalysis. The average molecular weights of each fraction were estimated by gel permeation chromatography (GPC) from the universal calibration curve. The molecular conformation in N, N-dimethylformamide (DMF), tetrahydrofuran (THF) and methanol was determined by solution viscosity measurements and the molecular structure was discussed in detail by ¹H- and ¹³C-NMR measurements. It has been found that the resin obtained in acetic acid was a random novolak with high molecular weight and the conformation is compact in solution. The fraction of O-O′, O-P′ and p-p′ methylene linkage changed slightly with increasing molecular weight. However, the fraction of terminal, linear and branch phenolic unit strongly depended on the molecular weight, i.e., the fractions of terminal and linear unit were decreasing and the branch unit fraction was increasing with increasing molecular weight. This suggested that the novolak resin formed a network structure by intramolecular reaction as the molecular weight was increasing. The compact conformation of the novolak was due to the branching and network structure rather than the hydrogen bond among phenolic hydroxyl groups. The crosslinking of this network structure was loose and the networks expanded or shrunk by the interaction with solvents in solution.