Utilization of a Combination of Weak Hydrogen-Bonding Interactions and Phase Segregation to Yield Highly Thermosensitive Supramolecular Polymers

Abstract

Supramolecular polymerization, i.e., the self-assembly of polymer-like materials through the utilization of the noncovalent bond, is a developing area of research. In this paper, we report the synthesis and investigation of nucleobase-terminated (N6-anisoyl-adenine and N4-(4-tert-butylbenzoyl)cytosine) low molecular weight poly(THF) macromonomers (<2000 g mol(-1)). Even though the degree of interaction between the nucleobase derivatives is very low (<5 M(-1)) these macromonomers self-assemble in the solid state to yield materials with film and fiber-forming capability. While the mechanical properties of films of both materials show extreme temperature sensitivity, resulting in the formation of very low viscosity melts, they do behave differently, which is attributed to the nature of the self-assembly controlled by the nucleobase. A combination of FT-IR, WAXD, and rheological experiments was carried out to further investigate the nature of the self-assembly in these systems. The studies demonstrate that a combination of phase segregation between the hard nucleobase chain ends and the soft poly(THF) core combined with aromatic amide hydrogen bonding is utilized to yield the highly thermosensitive supramolecular polymeric materials. In addition, analysis of the data suggests that the rheological properties of these supramolecular materials is controlled by the disengagement rate of the nucleobase chain ends from the "hard" phase, which, if shown to be general, provides a design criteria in the development of more thermally responsive materials.

Model Transient Networks from Strongly Hydrogen-Bonded Polymers

“…It has been demonstrated previously 13,69 that, in addition to crystallization of hydrogen-bonding units in functional copolymers, microphase separation can play a major role in the bulk properties of such materials. Only a weak scattering peak is observed by small-angle X-ray scattering for each of the triblocks (Figure 8a), and the intensity of the peak increases with increasing concentration of the UPy's toward the chain ends.…”

“…1 H NMR (500 MHz) and 13 C NMR (125 MHz) were performed on a Bruker AVANCE500 spectrometer at room temperature. Proton chemical shifts are reported in ppm downfield from tetramethylsilane (TMS).…”

“…The reversible junctions in transient networks typically contain areas of hydrophobic associations, [7][8][9][10] hydrogen bonding, [11][12][13][14][15][16][17] metal-ligand interactions, 18,19 or ionic associations. 20,21 In many cases such junctions are poorly defined aggregates with unknown association strength and kinetics, but in a few transient network materials bulk properties have been directly correlated to defined polymer microstructure or well-characterized reversible interactions leading to important insights into the properties of these systems.…”

“…29,30 To broaden the scope and applicability of supramolecular polymer chemistry, many new strongly hydrogen-bonding moieties have been synthesized [31][32][33][34][35][36][37] and incorporated into networkforming materials. [12][13][14][15][38][39][40][41] The strength and specificity of multiple-hydrogen-bonded (MHB) groups vary widely, from weakly complementary pyridine-phenol pairs 42 to extremely strong, self-complementary 6-H-bonded dimers. 31 The 2-ureido-4[1H]-pyrimidinone (UPy) group first reported by Sijbesma et al 12 was developed as a synthetically accessible, exceptionally strong (K dim = 6 Â 10 7 M -1 in CDCl 3 ) quadruple-hydrogenbonded dimer in order to create highly thermally responsive polymeric materials.…”

Mechanochemistry in Polymers with Supramolecular Mechanophores

“…[64][65][66][67][68][69] In the last 15 years, a large number of hydrogen-bonding supramolecular polymers has been investigated. [70,71] Interestingly, however, only few reports have described mechanically active hydrogen bonds in supramolecular polymers with unambiguous experimental evidence that show dissociation and formation of hydrogen bonds in response to mechanical stimulation. [72][73][74][75] Some of the most compelling examples of mechanoactive hydrogen bonds in polymers have been inspired by the natural protein titin.…”