Nature and degree of substitution patterns in novolaks by carbon-13 NMR spectroscopy

The nature and degree of substitution patterns on different ring carbons in novolak (cresol- formaldehyde) resins have been determined by carbon-13 NMR spectroscopy. The acquisition of carbon-13 NMR spectra of novolak resins yielded improved S/N ratio through selective irradiation of protonated carbons while suppressing quaternary carbons. For quantitative estimations, carbon nuclear spin-relaxation rates were measured to ensure that carbon nuclei of interest fully relax after each pulse train. NMR signals from quaternary carbons in the aromatic region have been selectively suppressed employing distortionless enhancement via polarization transfer (DEPT) pulse sequence, thus greatly improving the quantitative estimations. An experimental parameter, Un, where n is the carbon position in the ring, has been defined for chain propagation at any given carbon position. For example, in a pure meta cresol novolak resin, polymerization mainly proceeds via 4,6-disubstitution, and the C2 ortho carbons do not seem to participate in the polymerization process as much as C4 and C6 carbons do. However, in novolak resins prepared by mixing meta and para cresols, the chain propagation step greatly involves C2 carbons ortho to the hydroxyl group in meta cresol. The extent of such involvement can easily be determined relative to a given standard sample. The polymerization preference and differences in chemical structure of various polymers can easily be compared by determining their respective Un parameters. The bridged methylene linkages yield weaker NMR signals compared to those in the pure meta or para cresol resins, and thus, the estimations based on these carbons are only approximate. The experimental parameters as defined in this paper are discussed in relation to the physical and lithographic properties of these polymers.