Complementary hydrogen bonded cross-linked polymer networks based on two distinct hydrogen bonding recognition motifs have been synthesized by using a combination of ring-opening metathesis polymerization and hydrogen bonding interactions and were subsequently characterized in solution using rheometry. The hydrogen bonding recognition units were based on either three-point cyanuric acid-2,4-diaminotriazine or six-point cyanuric acid-Hamilton wedge interactions. Through the addition of "ditopic cross-linking agents", the polymer scaffold, which was functionalized with cyanuric acid functional groups, was noncovalently cross-linked in solution through complementary interchain hydrogen bonding interactions. The extent of cross-linking could be controlled by varying the amount of the cross-linking agent added. These networks are thermally reversible and have highly tunable mechanical properties that are controlled by the molecular structure of the cross-linking agent. While the addition of the Hamilton wedge cross-linking agent to the polymer solution led to high-viscosity fluids, the 2,4diaminotriazine cross-linking agent produced highly viscoelastic gels. It is hypothesized that this is due to a higher degree of connectivity between the cross-linking agent and the polymer in spite of the inherently weaker hydrogen bonding (three-vs six-point). The study shows that the microstructure plays an important role in the macroscopic mechanical properties of these hydrogen bonded networks in solution. By varying the hydrogen bonding motif, materials with tunable rheological properties were obtained from the same parent polymer backbone. Such a strategy will allow for materials design by tailoring the network microstructure via the molecular architecture of the cross-linking agents.

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Side-chain-functionalized polymers containing hydrogen bonding and metal coordination sites have been synthesized using ring-opening metathesis polymerization. These polymers were cross-linked reversibly either selectively by using hydrogen bonding or metal coordination or simultaneously using both interactions through the addition of small molecule cross-linking agents. The hydrogen bonding motifs utilized for reversible cross-linking are based on cyanuric acid residues hydrogen bonded to 2,4-diaminotriazine-based cross-linking agents. The metal coordination motifs are based on palladated SCS pincer complexes coordinated to bispyridine cross-linking agents. By controlling the reversible cross-linking strategy, we were able to modulate (1) the rheology of the polymer networks from a free-flowing liquid to a highly elastic gel and vice versa and vary the dynamic moduli over 10 orders of magnitude and (2) the responsiveness of the networks to external stimuli such as temperature and ligand displacement agents. The hydrogen bonded cross-linking resulted in polymer networks that were thermally reversible whereas the metal coordinated cross-linked networks mainly showed chemoresponsive behavior. Since both interactions are fully orthogonal to each other, we successfully cross-linked the polymer using both interactions to obtain multiresponsive networks that exhibited both thermal and chemoresponsiveness. We were also able to selectively de-cross-link the hydrogen bonded cross-links of the multifunctionalized networks through competitive interactions at room temperature via the addition of a monotopic end-capping agent without affecting the metal coordinated cross-links. In contrast, the metal coordination could be de-cross-linked completely using a ligand displacement agent such as triphenylphosphine again without affecting the hydrogen bonded cross-links.

Block copolymers containing both hydrogen bonding and metal coordination sites have been synthesized by ring-opening metathesis polymerization and subsequently functionalized using noncovalent interactions. The resulting block copolymers can be viewed as "universal polymer backbones", as a wide variety of polymers with varying functionalities can be prepared by altering the noncovalent functionalization strategy of the same polymer backbone. The effect of degree of polymerization, block copolymerization, block copolymer composition, and metal coordination on the hydrogen bonding interaction has been investigated. In general, none of these variables have a profound effect on the strength of the hydrogen bonding interactions along the polymer backbones, suggesting that the metal coordination and hydrogen bonding are orthogonal to each other in block copolymers. Finally, the effect of the noncovalent functionalization on the thermal properties of the polymers was investigated. We found that the noncovalent functionalization of all copolymers via hydrogen bonding and/ or metal coordination reduced the glass-transition temperature and the thermal stability of all copolymers.