Лако-красочные материалы — производство

The acid — or heat-initiated cross-linking mechanisms of 1 were extensively studied for decades, but because of the complexity of the reactions involved and the effect of atmospheric conditions (e. g., light, oxygen, and water vapor) intermediate products were not identified until 1975. In that study, 1 was polymerized at 100-250°C in the absence of air and the following intermediates were isolated [5,6]:

The polycondensation of 2 in acidic media has also been studied for a long time, but only recently was a clear-cut reaction mechanism established from a study in our

laboratory [2,7,8]. The success of this investigation stemmed from the fact that a large number of model compounds were synthesized which helped to establish the mechanisms of both cross-linking and color formation in this process. The use of mild catalysts confirmed that the first steps of the polymerization reactions occurred as follows:

This initial mechanism does not explain, however, these anomalies since both macromolecular structures should give rise to colorless and thermoplastic materials.

It was then shown that only several units actually condensed following this mechan­ism, since the average degree of polymerization (DP) never exceeded about 5, and cross­linking and color formation rapidly took place thereafter. In the mechanism of color formation, sketched in Scheme 1, we postulated that the formation of highly conjugated sequences resulted from successive hydride-ion/proton abstraction cycles [7]. This mechanism was confirmed by using different model compounds which were treated with an excess of hydride-ion (H“) abstractors (such as dioxolenium or triphenylmethyl cations) and the ensuing reactions followed by both ultraviolet (UV)-visible and 1H nuclear magnetic resonance (NMR) spectroscopies. This mechanism also explained the presence of methyl groups already observed by several authors [9-11]. The reaction of poly2 (obtained at early stages of the polycondensation) with hydride-ion abstractors was again followed by UV-visible spectroscopy and the results confirmed the proposed mechanism. Thus, the presence of conjugated sequences of different lengths was established, since the corresponding carbenium ions absorbed at different wavelengths, namely around 420, 450, 540, 600, and 800 nm.

Having solved the long-standing puzzle related to color formation, we switched to the problem of the occurrence of branching and/or cross-linking reactions [2,7,8]. It was argued that these events could start either from the ‘‘irregular’’ units formed by the mechanism shown in Scheme 1, as illustrated in Scheme 2, and/or by Diels-Alder reactions between two chains, as proposed in Scheme 3. In fact, since the participation of furanic hydrogen atoms at C3 and C4 and those of methylene bridges had been clearly excluded on the basis of model reactions, it seemed reasonable to attribute the branching and cross-linking reactions to these two mechanisms. The second alternative, involving the cross-linking through Diels-Alder reactions, was recently confirmed by using 2,5-dimethyl furan as a solvent for the acid-catalyzed polycondensation of 2. In this experiment, the large excess of dimethyl furan played the role of predominant diene trap for the exo-dihydrofuran dienophiles and thus prevented their coupling with the regular units of poly2 (Scheme 3). The fact that in these conditions the polymers remained soluble up to long reaction times and high yields was taken as clear evidence of the validity of Scheme 3.