Supramolecular polymer networks are three-dimensional structures of crosslinked macromolecules connected by transient, non-covalent bonds; they are a fascinating class of soft materials, exhibiting properties such as stimuli-responsiveness, self-healing, and shape-memory. This critical review summarizes the current state of the art in the physical–chemical characterization of supramolecular networks and relates this knowledge to that about classical, covalently jointed and crosslinked networks. We present a separate focus on the formation, the structure, the dynamics, and the mechanics of both permanent chemical and transient supramolecular networks. Particular emphasis is placed on features such as the formation and the effect of network inhomogeneities, the manifestation of the crosslink relaxation dynamics in the macroscopic sample behavior, and the applicability of concepts developed for classical polymer melts, solutions, and networks such as the reptation model and the principle of time–temperature superposition (263 references).

Many polymer gels display network defects and crosslinking inhomogeneity. This review reflects and interrelates investigations on the characterization of such polymer-network heterogeneity and on its impact on the swelling, elasticity, and permeability of polymer gels.

Polymer-network gels often display nano- to microstructural spatial inhomogeneity of their polymer-segmental and crosslinking densities, especially if they are formed by uncontrolled free-radical crosslinking copolymerization of mono- and multifunctional monomers and crosslinkers. This structural complexity markedly affects their optical clarity, mechanical strength, and permeability, which is of central relevance for their performance in everyday-life and high-tech products. This review summarizes and inter-relates a rich amount of existing studies on the origin of this inhomogeneity. It arcs from early fundamental work in the 1990s, development and assessment of highly ideal and super tough model-network gels in the 2000s, up to more recent work on hierarchically structured gels.

Mono- and bifunctional supramolecular poly(isobutylene)s (PIBs) bearing hydrogen-bonding motifs (barbituric acid or a Hamilton wedge) are prepared by a combination of living carbocationic polymerization (LCCP) and azide–alkyne “click” reactions to investigate their dynamics and self-healing behaviour. Barbituric acid (7) or Hamilton wedge (8) functionalized polymers (3a–c, 4a–d, 5a–c, 6a) with molecular weights of ∼3000 up to 30 000 g mol⁻¹ exhibit complete end group transformation as proven by NMR and MALDI methods. Temperature-dependent rheology in the melt reveals thermoreversible formation of supramolecular clusters. Stoichiometric mixing of the polymers by solution blending affects the extent of clustering by specifically interacting barbituric acid/Hamilton wedge moieties. Frequency-dependent measurements on bifunctional barbituric acid functionalized PIBs reveal a strong rubbery plateau and terminal flow, caused by the formation of dynamically bridged clusters. In addition, fluorescence recovery after photobleaching (FRAP) measurements on the same supramolecular polymers reveal a multitude of different chain dynamics. Small discs of these polymers show self-healing at room temperature after being cut and brought into contact at the fractured surface.

Defect engineering is a success story in crystalline hard matter; this review summarizes its parallels in amorphous soft matter.