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Design and synthesis of supramolecular copolymers for responsive self-assembled photonic structures.

Periodic Reporting for period 1 - SUPRACOPHS (Design and synthesis of supramolecular copolymers for responsive self-assembled photonic structures.)

Reporting period: 2016-09-01 to 2018-08-31

SUPRACOPHS was aimed at introducing a new way of preparing materials with optical properties through the self-assembly of supramolecular block copolymers. The ultimate goal of the project was to overcome limitations related to the typically used high molecular weight block copolymers and create a versatile and reproducible approach. With a growing interest in photonic materials across disciplines, the ability to prepare materials that interact with light and alter its properties in a cost-effective and potentially renewable fashion would drastically affect the study of their properties and optimization of the spatial periodic variation of the dielectric permittivity.
SUPRACOPHS sought a solution to the inherent limitations of currently used block copolymers by eliminating drawbacks of the current approach, such as low diffusivity and the difficult variation of the spatial periodicity across multiple length scales. By connecting the polymer blocks through directive supramolecular interactions, the molecular weight of the components is decreased, while the blocks can still form periodic domains owing to their chemical immiscibility.
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.
While no beyond the state of the art achievements were disseminated, it has become obvious that one should balance the association of the supramolecular interactions of a polymer system with its tendency to demix, when seeking certain properties of the resulting material (such as its mechanical properties, as outlined in the review article). These findings will be further investigated upon securing of relevant funding. It is expected that upon finding suitable homopolymer pairs that are of suitable molecular weight to allow association without macrophase-separation taking place, a new type of phase diagram can be prepared, one that takes into account the supramolecular motif. Such a phase diagram should broaden the understanding of the scientific community of the behavior of non-covalently bound polymer blends that have some affinity through supramolecular interactions, and thus allow the designing of complex materials with simple starting materials, eventually leading to the reduced cost of the end product (if such a product currently requires the use of covalently bound block copolymers), such as solar cells.
schematic outline of SUPRACOPHS