Firstly, we focused our attention on the synthesis of the target monomers to obtain the iminoboronate polymers. The target homocomplementary (AB-type) and heterocomplementary (AA- and BB-type) monomers’ structures are shown in Figure 1.
The synthetic route to the monomers 1, 2 and 3 (Figure 1) consisted of successive protection of the anilines, Palladium cross coupling reactions (Suzuki-Miyaura, Sonogashira and Heck, respectively), triflation, borylation and final selective deprotection. Following this route, protected derivatives were obtained for the three AB-type monomers. Several deprotection reactions were tested, employing a wide range of conditions and/or altering the order of the selective deprotections (starting by either the amine or the boronate ester). These include among others:
After thoroughly studying this process, we found that these deprotections always result in an insoluble red solid that, unfortunately, we were unable to characterize. A possible explanation for this unexpected solubility is that uncontrolled oligomers are being formed during the deprotection step by a fast imine condensation reaction.
We envisaged different approaches to overcome this problem:
A) Pre-formation of the aryl-boronate ester employing a catechol bearing solubilizing alkyl chains to increase the solubility of the monomer.
B) Employing a photo-responsive masked aldehyde to avoid the uncontrolled reaction between the amine and the aldehyde.
C) Preformation of the iminoboronate moiety and synthesis of the polymer by a Palladium crossed coupling reaction.
Next, we moved to AA- and BB- heterocomplementary monomers, where two different monomers are employed in the polymerization reaction. In this particular case, shorter co-polymers are expected. On the other hand, systems of increasing complexity can be easily obtained by simply combining two species with different electronic properties. An optimization of the reaction conditions for the formation iminoboronate oligomers was carried out by 1H-NMR by reacting monomers and terminating groups in different ratios, aiming to monitor the changes in the NMR spectrum upon increasing the length of the polymeric species. We analysed the impact of different factors in the reaction such as the concentration, solvent, temperature and the nature of the catechol. Chlorinated and aromatic solvents were designated as best solvents to stabilize the iminoboronate moiety. After a thorough study, we were able to obtain polymeric red films by imine condensation reaction of AA- BB- type monomers in tetachloroethane at 70 °C for 24h. However, the characterization of these polymeric films was exceptionally challenging due to its solubility features. Solvents such as chlorobenzene or dichlorobenzene led to the formation of shorter polymers/oligomers in the same reaction conditions. The presence of these shorter units was confirmed by 1H-NMR in deuterated tetrachloroethane, where the disappearance of the aldehyde proton signal and appearance of the imine signal were observed.
