Synthesis and Rheology of Bulk Metallo-Supramolecular Polymers from Telechelic Entangled Precursors

Effect of Counterions on the Physicomechanical Properties of Copper-Nitrogen-Coordinated Metallosupramolecular Elastomers

Metallosupramolecular elastomers have attracted much attention due to their excellent mechanical properties, flexible tailoring of performance, and responsiveness to photo and thermal stimuli. The physicomechanical properties of metallosupramolecular elastomers are highly dependent on metal salts and ligand units; however, the role of counterions lacks practical exploration. To this end, we synthesized a simple acrylate copolymer model and introduced copper salts with different counterions to construct dynamic copper-nitrogen coordination cross-linked networks. This approach generated a series of elastomers with a tensile strength of over 10 MPa and a laser self-healing efficiency of over 90% within 2 min. In particular, we studied the effects of counterions on the thermodynamic, viscoelastic, mechanical, photothermal, and self-healing properties of the materials. Therefore, this work can provide instruction for the preparation and performance tailoring of metallosupramolecular elastomers.

Plasticization of a Semicrystalline Metallosupramolecular Polymer Network

The assembly of ligand-functionalized (macro)monomers with suitable metal ions affords metallosupramolecular polymers (MSPs). On account of the reversible and dynamic nature of the metal-ligand complexes, these materials can be temporarily (dis-)assembled upon exposure to a suitable stimulus, and this effect can be exploited to heal damaged samples, to facilitate processing and recycling, or to enable reversible adhesion. We here report on the plasticization of a semicrystalline, stimuli-responsive MSP network that was assembled by combining a low-molecular-weight building block carrying three 2,6-bis(1'-methylbenzimidazolyl) pyridine (Mebip) ligands and zinc bis(trifluoromethylsulfonyl)imide (Zn(NTf₂)₂). The pristine material exhibits high melting (T _m = 230 °C) and glass transition (T _g ≈ 157 °C) temperatures and offers robust mechanical properties between these temperatures. We show that this regime can be substantially extended through plasticization. To achieve this, the MSP network was blended with diisodecyl phthalate. The weight fraction of this plasticizer was systematically varied, and the thermal and mechanical properties of the resulting materials were investigated. We show that the T _g can be lowered by more than 60 °C and the toughness above the T _g is considerably increased.

Bond strength regime dictates stress relaxation behavior

Reconfigurable polymer networks are gaining interest for their potential applications as self-healing, recyclable, and stimuli-responsive smart materials. Relating the bond strength of dynamic interactions to material properties including stress relaxation time and modulus is crucial for smart material design. In this work, in situ crosslinked transition metal-terpyridine reconfigurable networks were utilized to modulate the characteristic network stress relaxation time, τ_R. The use of stress relaxation experiments rather than oscillatory frequency sweeps allowed for the measurement of network bond dynamics across a wider dynamic range than has been previously reported. The stress relaxation time was shown to be tunable by metal center, counterion, and crosslink density. Remarkably, the network crosslinked with covalent-like ruthenium chloride-terpyridine interaction, while having a longer τ_R, was qualitatively similar to the other metal-ligand networks. Furthermore, the relaxation time was independent of crosslink density in strongly bonded networks, allowing for independent tunability of modulus and τ_R. In contrast, increasing crosslink density reduced τ_R in networks crosslinked with weaker interactions.

Janus Cross-links in Supramolecular Networks

Thermosets composed of cross-linked polymers demonstrate enhanced thermal, solvent, chemical, and dimensional stability as compared to their non-cross-linked counterparts. However, these often-desirable material properties typically come at the expense of reprocessability, recyclability, and healability. One solution to this challenge comes from the construction of polymers that are reversibly cross-linked. We relied on lessons from Nature to present supramolecular polymer networks comprised of cooperative Janus-faced hydrogen bonded cross-links. A triazine-based guanine-cytosine base (GCB) with two complementary faces capable of self-assembly through three hydrogen bonding sites was incorporated into poly(butyl acrylate) to create a reprocessable and recyclable network. Rheological experiments and dynamic mechanical analysis (DMA) were employed to investigate the flow behavior of copolymers with randomly distributed GCB units of varying incorporation. Our studies revealed that the cooperativity of multiple hydrogen bonding faces yields excellent network integrity evidenced by a rubbery plateau that spanned the widest temperature range yet reported for any supramolecular network. To verify that each Janus-faced motif engages in multiple cross-links, we studied the effects of local concentration of the incorporated GCB units within the polymer chain. Mechanical strength improved by colocalizing the GCB within a block copolymer morphology. This enhanced performance revealed that the number of effective cross-links in the network increased with the local concentration of hydrogen bonding units. Overall, this study demonstrates that cooperative noncovalent interactions introduced through Janus-faced hydrogen bonding moieties confers excellent network stability and predictable viscoelastic flow behavior in supramolecular networks.

Using Coupling Motion of Connecting Ions in Designing Telechelic Ionomers

Conventional telechelic ionomers have one ion fixed at each end, enabling the chains to form a physical network. Here we report a type of telechelic ionomers with a distribution of the number of ions at the chain ends, which endows them with very rich rheological properties. We synthesized the ionomer samples via a two-step polymerization. Namely, we synthesized a precursor chain first and then polymerized a few ion-containing monomers at its two ends. An average number of ion-containing monomers per chain end, m, varies from 0 to 3.0. Linear viscoelasticity of these samples can be well explained through considering the Poisson distribution of m, and the hierarchical relaxation of the chains ends according to the number of connecting ions that exhibit the coupling motion.

Living coordination–insertion copolymerization of 1-hexene and ligated α-olefins using an α-diimine nickel catalyst and preparation of metal–ligand coordination crosslinked polymers

Metal–ligand coordination crosslinked polymers were prepared by coordination–insertion copolymerization of 1-hexene and ligated α-olefins.

A Critical Approach to Polymer Dynamics in Supramolecular Polymers

Over the past few years, the concurrent (1) development of polymer synthesis and (2) introduction of new mathematical models for polymer dynamics have evolved the classical framework for polymer dynamics once established by Doi-Edwards/de Gennes. Although the analysis of supramolecular polymer dynamics based on linear rheology has improved a lot recently, there are a large number of insecurities behind the conclusions, which originate from the complexity of these novel systems. The interdependent effect of supramolecular entities (stickers) and chain dynamics can be overwhelming depending on the type and location of stickers as well as the architecture and chemistry of polymers. This Perspective illustrates these parameters and strives to determine what is still missing and has to be improved in the future works.

Enhancement of metallo-supramolecular dissociation kinetics in telechelic terpyridine-capped poly(ethylene glycol) assemblies in the semi-dilute regime

The dynamics of supramolecular polymer assemblies is governed by that of their polymeric building blocks and that of the transient bonds between them. Entrapment of such bonds by topological crowding often causes renormalization of the bond lifetimes towards prolonging. In the present study, by contrast, we show that this effect can also be inverse in the case of telechelic metallo-supramolecular polymers in semi-dilute solution. We focus on linear poly(ethylene glycols) capped by terpyridine binding motifs at both ends that can form metal-ligand coordinative bonds with various transition metal ions, thereby creating transient metallo-supramolecular assemblies of varying length and binding strength. Oscillatory shear rheology measurements along with theoretical modelling of the mechanical spectra of these samples reveals a pronounced enhancement of the complex dissociation kinetics that is dependent on the length of the polymeric chain segment, with longer segments yielding faster dissociation times up to six orders of magnitude shorter than described for the free complexes. This finding indicates that the dynamic activity of the polymer chain itself causes complex destabilization.

Bottom-up design of model network elastomers and hydrogels from precise star polymers

Well-defined high-molecular weight star polymers based on low-T_g water-soluble polymers enable bottom-up design of model network elastomers and functional hydrogels.

A novel supramolecular polymer network based on a catenane-type crosslinker

A novel supramolecular mechanically interlocked crosslinker was designed and used to prepare a supramolecular polymer network.

Terpyridine-functionalized stimuli-responsive microgels and their assembly through metal–ligand interactions

We developed a terpyridine-functionalized microgel (tpy-mG) for its supramolecular assembly.