I. Introduction 4071 II. The Term Supramolecular Polymers 4071 III. General Aspects of Supramolecular Polymers 4073 IV. Supramolecular Polymers Based on Hydrogen Bonding 4073 A. Strength of Hydrogen Bonds 4073 B. Hydrogen Bonding Enforced by Liquid Crystallinity 4075 C. Hydrogen Bonding Enforced by Phase Separation 4076 D. Strong Dimerization of Multiple Hydrogen-Bonding Units 4077 E. Ureidopyrimidinone-Based Polymers 4079 V. Supramolecular Polymers Based on Discotic Molecules 4081 A. Arene−Arene Interactions 4082 1. Triphenylenes 4082 2. Phthalocyanines and Porphyrins 4083 3. Helicenes 4084 4. m-Phenylene Ethynylene Oligomers 4084 5. Other Systems 4085 6. Chromonics 4086 B. Hydrogen Bonding 4086 C. Arene−Arene Interactions and Hydrogen Bonding 4087 1. Guanine and Pterine Derivatives 4087 2. Hydrogen-Bonded Pairs 4089 3. Complexation of Tetrazoles with 1,3,5-Tris (4,5-dihydroimidazol-2-yl)benzene 4089 4. C 3 -Symmetrical Discotic Molecules 4090 VI. Supramolecular Coordination Polymers and Miscellaneous Systems 4091 VII. Conclusions and Outlook 4094 VIII. Acknowledgments 4094 IX. References and Notes 4094

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Units of 2-ureido-4-pyrimidone that dimerize strongly in a self-complementary array of four cooperative hydrogen bonds were used as the associating end group in reversible self-assembling polymer systems. The unidirectional design of the binding sites prevents uncontrolled multidirectional association or gelation. Linear polymers and reversible networks were formed from monomers with two and three binding sites, respectively. The thermal and environmental control over lifetime and bond strength makes many properties, such as viscosity, chain length, and composition, tunable in a way not accessible to traditional polymers. Hence, polymer networks with thermodynamically controlled architectures can be formed, for use in, for example, coatings and hot melts, where a reversible, strongly temperature-dependent rheology is highly advantageous.

Elastomers are widely used because of their large-strain reversible deformability. Most unfilled elastomers suffer from a poor mechanical strength, which limits their use. Using sacrificial bonds, we show how brittle, unfilled elastomers can be strongly reinforced in stiffness and toughness (up to 4 megapascals and 9 kilojoules per square meter) by introducing a variable proportion of isotropically prestretched chains that can break and dissipate energy before the material fails. Chemoluminescent cross-linking molecules, which emit light as they break, map in real time where and when many of these internal bonds break ahead of a propagating crack. The simple methodology that we use to introduce sacrificial bonds, combined with the mapping of where bonds break, has the potential to stimulate the development of new classes of unfilled tough elastomers and better molecular models of the fracture of soft materials.

6-Methyl-2-butylureidopyrimidone dimerizes via four hydrogen bonds in the solid state as well as in CHCl3 solution via a donor−donor−acceptor−acceptor (DDAA) array of hydrogen bonding sites in the 4[1H]-pyrimidinone tautomer. An intramolecular hydrogen bond from the pyrimidine NH group to the urea oxygen atom preorganizes the molecules for dimerization. The dimerization constant of the dimer in CHCl3 exceeds 106 M-1. In CHCl3 containing DMSO, the dimer is in equilibrium with the monomeric 6[1H]-pyrimidinone tautomer. In 6-phenyl-2-butylureidopyrimidone, the 4[1H]-pyrimidinone tautomer coexists with the pyrimidin-4-ol form, which dimerizes with similar high dimerization constants via four hydrogen bonds in a DADA array. The latter tautomer predominates in derivatives with electronegative 6-substituents, like 6-nitrophenyl- and 6-trifluoromethyl-2-butylureidopyrimidone. Due to its simple preparation and high dimerization constant, the ureidopyrimidone functionality is a useful building block for supramolecular chemistry.

Nature uses mechanochemical transduction processes to achieve diverse and vital functions, such as hearing, cellular adhesion and gating of ion channels. One fascinating example of biological mechanotransduction is the emission of light on mechanical stimulation. However, molecular-level transduction of force into luminescence in a synthetic system remains a challenge. Here, we show that bis(adamantyl)-1,2-dioxetane emits visible light when force is applied to a polymer chain or network in which this unit is incorporated. Bright-blue luminescence was observed on sonication of solutions of dioxetane-containing linear polymers and on the straining of polymer networks with dioxetane crosslinkers. Light is emitted from the adamantanone-excited state that forms on opening of the four-membered dioxetane ring. Increased sensitivity and colour tuning were achieved by energy transfer to suitable acceptors. High spatial and temporal resolutions highlight the potential to study the failure of polymeric materials in unprecedented detail.

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2-Ureido-4[1H]-pyrimidinones are known to dimerize via a strong quadruple hydrogen bond array. A detailed study of the dimerization constant and lifetime of the dimer is presented here. Excimer fluorescence of pyrene-labeled 2-ureido-4[1H]-pyrimidinone 1b was used to determine a dimerization constant K dim of 6 × 10 7 M -1 in CHCl 3 , 1 × 10 7 M -1 in chloroform saturated with water, and 6 × 10 8 M -1 in toluene (all at 298 K). Under these conditions, the preexchange lifetime of the similar dimers of both 1d and 1e is 170 ms in CDCl 3 , 80 ms in wet CDCl 3 , and 1.7 s in toluene-d 8 , as determined by dynamic NMR spectroscopy. Association rate constants were calculated from the K dim values and the preexchange lifetimes. The resulting values are significantly lower than the diffusion-controlled association rate constants calculated using the Stokes-Einstein and the Debeije equations. This difference is ascribed to a tautomeric equilibrium of the monomer between the dimerizing 4[1H]-pyrimidinone and nondimerizing 6[1H]-pyrimidinone tautomers, which is unfavorable for dimerization.