Novolac Intramolecular hydrogen bond

Figure 7.10 Intramolecular hydrogen bonding of high ortho novolacs.

Dielectric measurements were used to evaluate the degrees of inter- and intramolecular hydrogen bonding in novolac resins.39 The frequency dependence of complex permittivity (s ) within a relaxation region can be described with a Havriliak and Negami function (HN function) ... 

Novolac resins, as the oldest synthetic polymers, have played an important role 1n microelectronic Industry as positive photoresists. Studies of novolac dissolution have populated the literature a recent survey shows that the rate of dissolution 1s influenced by the concentration of the alkali, size of the cation, addition of salt, and the presence of dissolution Inhibitors (1-6). The voluminous experimental results, however, have not led to a clear understanding of the dissolution phenomena. Arcus (3) proposed an 1on-permeab1e membrane" model while Szmanda (1) and Hanabata (6) emphasized the Importance of secondary structures of novolac molecules, for Instance, Inter- or Intramolecular hydrogen bonding and the various isomeric configurations of the resins. These important contributions nevertheless point to a need for additional studies of the mechanism of dissolution. 

The secondary structure model, proposed by Templeton et al., relates the secondary structures of novolacs [see Figs. 11.36(a) and (b)] to their dissolution behavior. The authors distinguished between structures where intermolecular bonds between novolac molecules predominate and those with predominantly intramolecular hydrogen bonds and they correlated these to dissolution behavior. They found that, for example, for novolacs made from p-cresol, the secondary structure of the resin brings the OH groups of the phenols together to such an extent that... 

As discussed earlier in the secondary structure model, Templeton et al. have shown that the steric arrangement of the methylene links in the novolac resin can have a profound effect on its dissolution rate and on lithographic performance. Using molecular mechanics, these authors have calculated the equilibrium secondary structures of cresol-formaldehyde oligomers. They found that the secondary structure of these molecules determines the relative positions of the hydroxyl groups in the novolac matrix, and hence the possibility of intramolecular hydrogen bonding. ... 

Another way to synthesize novolacs is under weak acidic conditions. Typical reaction conditions are a pH-range between 4 and 7 and the use of bivalent metal salts (e.g. Ca, Zn, Mg, Pb, Co, Cu, Ni) [56,57]. These conditions are highly selective for ortho,ortho -novolacs that are of increasing interest These compounds possess only a vacant para position for further reactions. Such materials have increased importance as molding compounds and foundry resins because ortho-linked novolacs can only react at the para position and they possess a hyperacidity [58]. The hyperacidity can be easily explained by the formation of intramolecular hydrogen bondings that stabilize the phenoxy anion. Complete methylene bridging at the ortho position of phenolic compounds can lead to calixarenes. 

Templeton et al 30) used semi-empirical molecular modeling calculations and x-ray crystal structures of novolac oligomeric residues to show that ortho-ortho linkages in novolac result in an extensive network of intramolecular hydrogen bonding. Templeton proposed that this hydrogen bonding causes the phenols to form... 

Scheme 3.18. Hypothetical intramolecular hydrogen bonds in novolac resins.

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