Initial work was aimed at choosing the most suitable conditions for the preparation of homopolymers that are end-functionalized with moieties capable of forming complementary hydrogen bonds. The unsuitability of the initially proposed supramolecular motif was made clear from existing literature that showed the preferential self-association of the motif, as opposed to the desired formation of complementary hydrogen bonds with a second motif. As such, a new system was found that would allow the formation of a complementary quadruple hydrogen-bonding pair and limited self-association of the individual motifs. The new system consisted of a guanosine urea (UGy) and a diamido naphthyridine (NaPy) that were prepared after multi-step syntheses. Each motif was then used to end-functionalize the corresponding homopolymer: poly(isoprene) for UGy and poly(styrene) for NaPy. These two homopolymers were shown to phase-separate in the absence of supramolecular motifs. Upon functionalization, the two polymers were mixed in different ratios in solution and their absorption was studied by spectroscopical means. Both UGy and NaPy absorptions were identifiable, while upon reaching a 1:1 ratio for the two homopolymers, and thus the two motifs, a new absorption peak was observed which was ascribed to the formation of the hydrogen-bonded pair of UGy-NaPy. Upon drying of the solution, the anticipated formation of a homogeneous film was not observed; instead macrophase separation was imminent, thus suggesting that the hydrogen-bonded polymer end groups had dissociated. It was hypothesized that this was the result of polymer demixing forces surpassing the strength of the hydrogen bonds keeping the polymers connected. As such, lower molecular weight homopolymers were obtained and the synthesis of UGy and NaPy, as well as the functionalization of the homopolymers was sought. Unfortunately, the latter was hindered by incomplete reactions which was found to be a result of a poor quality starting material (i.e. the two homopolymers did not fully contain the end group handle needed to attach the supramolecular motifs). Therefore, a new strategy was designed whereby the two supramolecular motifs were converted into polymerization initiators, therefore ensuring their presence on the polymer chains. This strategy required the choice of a new homopolymer to end-functionalize with UGy, as the synthesis of poly(isoprene) in the presence of a functional group, such as UGy, is very demanding and beyond the scope of this project. As such, poly((2-dimethylamino)ethyl methacrylate) was chosen, as it is also immiscible with poly(styrene) and can be readily synthesized in the presence of functional groups, such as UGy. The kinetics of the two homopolymerizations were established and the initiators were synthesized, to an extent, within the course of the funding period.
During the evaluation of the significance of the balance between immiscibility and supramolecular interactions, an interesting gap in the existing literature was observed which instigated the preparation of a review article that points at the importance of supramolecular interactions on the (stimuli-responsive) mechanical properties of polymers. This review article was published in Chemical Reviews and has received 15 citations and over 5,000 reads.
