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Ind. Eng. Chem. Res. 2006, 45, 4884-4891
Negative-Working Lithographic Diazo Resins: The Structure of 4-Diazodiphenylamines (Diazo S and Diazo MS) and Their Polymeric Formaldehyde Condensates David H. Jones* PJS Chemicals, Unit 8, Guardian Park, Station Road Industrial Estate, Tadcaster, LS24 9SG, North Yorkshire, United Kingdom
Jenny M. Jones Energy and Resources Research Institute, School of Process, EnVironmental and Materials Research, UniVersity of Leeds, Leeds, LS2 9JT, United Kingdom
The structure of negative-working diazo resins has been investigated by means of quantum mechanical modeling of models for the polymerization processes leading to their formation. Comparison of the predicted heats of formation, the ultraviolet-visible (UV-Vis) spectra, and the infrared spectra with experimental results leads to the conclusion that the diazonium structure of 4-diazodiphenylamines is most stable in acidic media. The regioselectivity for the condensation polymerization reaction of such species with O-protonated formaldehyde cation was examined using atomic charge (electrostatic potential (ESP)), frontier molecular orbital, and product development approaches. Although methylol substitution at both rings seems possible, a reaction that leads to ortho- and para-substitution of the non-diazo-containing ring (β-ring) seems to be favored, according to product development and frontier orbital considerations. During subsequent polymerization, similar conclusions are reached regarding the electrophillic attack on 4-diazodiphenylamine by diazo carbocations derived from dehydration of 4-diazodiphenylamine methylol derivatives. In contrast, atomic charge considerations suggest the feasibility of the reaction involving the R-ring. Introduction Since 1934, when a patent application was filed concerning the preparation of diazo condensates from diazonium salts and compounds containing a carbonyl group,1 4-diazodiphenylamines (see Figure 1) have maintained their importance in the manufacture of negative-working presensitized lithographic printing plates.2 The photosensitivity of these plates can be affected by the method of preparation of the diazo resin. Accordingly, prodigious patent literature has appeared concerning the preparation and application of diazo resins.3 These are typically formed by the condensation of 4-diazodiphenylamine (Diazo S) or 3-methoxy-4-diazodiphenylamine (Diazo MS) with mainly formaldehyde or 4,4′-methoxymethyldiphenyl ether.3,4 The composition of the diazo resin can show wide variation, even under well-controlled conditions.5 This implies that the diazo resin structure is dependent on its preparation method. Therefore, it is surprising that very little research has been published that addresses the likely structure and reactivity of these systems.5-11 The limited research has been published primarily by Russian and U.S. groups in the 1960s and reviewed very briefly in 1994 by Baumann and Timpe.2 These latter authors also commented on the lack of reliable data concerning the structure of these important polymeric materials. Scheme I shows a possible mechanism for the condensation polymerization of Diazo S with formaldehyde under the acidic conditions that have been used. The reaction sequence continues until ultimately forming a three-dimensional cross-linked polymersthe diazo resinswith a distribution of molecular * To whom correspondence should be addressed. Tel.: +44 113 343 2477. Fax: +44 113 246 7310. E-mail address: j.m.jones@ leeds.ac.uk.
Figure 1. Numbering scheme for diazodiphenylamines (the p-iminoquinone diazide version shown).
weight that is dependent on the reaction conditions.4 The equilibrium between the diazonium and the diazide is dependent on pH, and both species may participate in the condensation reaction.11 The published work has involved the infrared (IR) characterization of species designated as the diazo-monomer, diazo-dimer, diazo-tetramer,9 and the resultant diazo resin.5 Baumann and Timpe2 mentioned nuclear magnetic resonance (NMR) work, but this has not been published in full. Ortho/ para condensation of the β-ring (see Figure 1) has generally been implied in the published work; however, no conclusive evidence has been presented. The present work provides a theoretical study of the reaction of diazonium and diazide versions of 4-diazodiphenylamine (Diazo-S) and 3-methoxy-4-diazodiphenylamine (Diazo-MS) with formaldehyde, to evaluate the condensation at ortho/meta/ para positions of the β-ring and possible sites in the R-ring. It also investigates the progress of the reaction, as far as the dimers. The objective has been to identify likely structural units in the diazo resins using a computational chemistry approach. To this end, we have applied density functional and semiempirical quantum mechanical methods (AM1, PM3, and ZINDO/S with limited configuration interaction). The results comprise
10.1021/ie058002l CCC: $33.50 © 2006 American Chemical Society Published on Web 06/08/2006
Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006 4885 Scheme 1. Possible Mechanism for the Condensation Polymerization of Diazo S with Formaldehyde under Acidic Conditions
optimized bond lengths and bond angles, heats of formation, infrared and UV-Vis spectra of starting materials, as well as heats of formation for arenium ion intermediates and condensed diazo resin species. Where possible, these data were compared with published or measured values. Methodology Gaussian 03W software12 was used for the density functional (DFT) calculations. Structures of interest were geometrically optimized at the restricted B3LYP DFT level (6-31G(d) basis set in each case). The ground state of each system has been reported. The semiempirical PM3 quantum mechanical calculations were also conducted using the Gaussian program at the RHF level of SCF.12 The semiempirical AM1 quantum mechanical calculations13 were conducted using Chem Plus, which is a Microsoft Windows form of the HyperChem molecular modeling package. For all calculations, the AM1 algorithm was utilized at the RHF level of SCF. The AM1 method is a semiempirical (MNDO) quantum mechanical method, which has been parametrized to predict the heats of formation of smaller organic molecules accurately. The AM1 method was chosen for the majority of the calculations that have been performed for this study, after the predicted geometries for the starting materials had been favorably compared (relative to PM3) with those from density
functional calculations. Initial coordinates were obtained using a chemical model builder that was supplied with the software package. From such approximate models, geometry optimized structures were derived using either the Polak-Ribiere Conjugate Gradient procedure or an Eigenvector Following method; an energy gradient of
