30. Furan-Based Adhesives

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
1 H 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.
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