The development of polymers that can spontaneously repair themselves after mechanical damage would significantly improve the safety, lifetime, energy efficiency, and environmental impact of manmade materials. Most approaches to self-healing materials either require the input of external energy, or need healing agents, solvent or plasticizer. Despite intense research in this area, the synthesis of a stiff material with intrinsic self-healing ability remains a key challenge. Our laboratory has recently succeeded in a design of multiphase supramolecular thermoplastic elastomers that combine high modulus and toughness with spontaneous healing capability. In one design, H-bonding brush polymers (HBPs) self-assemble into hard-soft microphaseseparated system, combining the enhanced stiffness and toughness of hybrid polymers with the self-healing capacity of dynamic supramolecular assemblies [1]. In another design, supramolecular ABA triblock copolymers formed by dimerization of 2-ureido-4-pyromidone (UPy) end-functionalized polystyrene-b-poly(n-butylacrylate) (PS-b-PBA) AB diblock copolymers are synthesized, resulting in a self-healing material that combines the advantageous mechanical properties of thermoplastic elastomers and the dynamic self-healing features of supramolecular materials [2]. In contrast to previous self-healing polymers, our systems spontaneously self-heals as a single-component solid material at ambient conditions without the need of any external stimulus, healing agent, plasticizer, or solvent.

Traditionally the dispersion of particles in polymeric materials has proven difficult and frequently results in phase separation and agglomeration. We show that thermodynamically stable dispersion of nanoparticles into a polymeric liquid is enhanced for systems where the radius of gyration of the linear polymer is greater than the radius of the nanoparticle. Dispersed nanoparticles swell the linear polymer chains, resulting in a polymer radius of gyration that grows with the nanoparticle volume fraction. It is proposed that this entropically unfavorable process is offset by an enthalpy gain due to an increase in molecular contacts at dispersed nanoparticle surfaces as compared with the surfaces of phase-separated nanoparticles. Even when the dispersed state is thermodynamically stable, it may be inaccessible unless the correct processing strategy is adopted, which is particularly important for the case of fullerene dispersion into linear polymers.

Despite numerous strategies involving dynamic covalent bond exchange for dynamic and self-healing materials, it remains a challenge to be able to tune the malleability and self-healing properties of bulk materials through simple small molecule perturbations. Here we describe the use of tunable rates of boronic ester transesterification to tune the malleability and self-healing efficiencies of bulk materials. Specifically, we used two telechelic diboronic ester small molecules with variable transesterification kinetics to dynamically cross-link 1,2-diol-containing polymer backbones. The sample cross-linked with fast-exchanging diboronic ester showed enhanced malleability and accelerated healing compared to the slow-exchanging variant under the same conditions. Our report demonstrates the possibility of transferring small molecule kinetics to dynamic properties of bulk solid material and may serve as a guide for the rational design of tunable dynamic materials.

Ethylene pressure has been used to control the competition between isomerization (chain walking) and monomer insertion processes for ethylene coordination polymerization catalyzed by a palladium-alpha-diimine catalyst. The topology of the polyethylene varies from linear with moderate branching to "hyperbranched" structures. Although the overall branching number and the distribution of short-chain branching change very slightly, the architecture or topology of the polyethylene changes from linear polyethylene with moderate branches at high ethylene pressures to a hyperbranched polyethylene at low pressures.