CoopHeal research line initially focused on linking a cooperative, multivalent supramolecular motif, like our G-C dinucleoside able to form H-bonded cyclic tetramers, to both ends of soft polymers with low Tg, such as poly(dimethylsiloxane) (PDMS). A protocol was optimized in which a dinucleobase molecule was endowed with a reactive pentafluorophenol ester to be coupled to telechelic diamino-substituted silicones of diverse molecular weights. The final amide-linked polymers were subjected to different studies by 1H NMR and CD spectroscopies to confirm the formation of cyclic tetramer cross-linked networks, which influences enormously materials’ properties: the amino-terminated PDMS pristine material is a liquid, but forms relatively resistant transparent films upon functionalization with the dinucleoside. Preliminary thermomechanical experiments with some of these materials were also very promising, but we soon learned that we needed to change two important elements in our original design: 1) the mechanical properties of the final material are too “soft”, and we need to use polymer chains of higher Tg, and 2) the dinucleoside motif is too “expensive” and it takes too many steps and synthetic efforts to arrive to 1 gram of material, which is only enough to produce a few polymer samples for complete rheological and thermomechanical analysis.
Therefore, we planned to develop a new improved version of these (potentially) self-healing thermoplastics. On one hand, we substituted the polymer chains by polyurethanes based on hydroxyl-terminated PDMS and a diisocyanate, frequently used polymer in the coating industry. The introduction of urethane groups in the main polymer chain brought as expected additional intermolecular H-bonding interactions that increased Tg and hence reinforced thermomechanical properties. On the other, we changed our terminal dinucleoside motif able to form cyclic tetramers with guanosine units able to associate in G-quadruplexes in the presence of alkaline salts. The main advantages are that guanosine is an extremely cheap compound that is bought in kg amounts, whose functionalization is simple and straightforward, and that mechanical properties could be modulated as a function of the salt employed.
The polymers produced have been characterized by common H-NMR, FT-IR, MS, GPC, DSC and TGA measurements, whereas the cross-linking process was studied by NMR and optical spectroscopy as a function of temperature. The rheological and mechanical properties, as well as the qualitative evaluation of the self-heling properties, have been carried out in collaboration with the group of Dr. Daniel López at the Institute of Polymer Science and Technology (Spanish Research Council) in Madrid. The results of all these techniques showed the formation of a reversible network based on G-quadruplex assemblies. The network is practically stable until ca. 150 °C where the network dissociation starts, in agreement with the disassembly of a supramolecular polymer network into lower molecular weight species. The network structure is completely recovered when the material is cooled down to room temperature, as well as their mechanical properties. This behaviour has already been observed in other supramolecular polymers (for example, polymers carrying UPy motifs). However, the mechanical properties are clearly improved with lower functionalization degree, as well as more stable at higher temperatures due to the steady quadruplex assembly.
