Frustrated Lewis Pair Polymers as Responsive Self-Healing Gels

In Situ Formation of Frustrated Lewis Pairs in a Water-Tolerant Metal-Organic Framework for the Transformation of CO2

Frustrated Lewis pairs (FLPs) consist of sterically hindered Lewis acids and Lewis bases, which provide high catalytic activity towards non-metal-mediated activation of "inert" small molecules, including CO₂ among others. One critical issue of homogeneous FLPs, however, is their instability upon recycling, leading to catalytic deactivation. Herein, we provide a solution to this issue by incorporating a bulky Lewis acid-functionalized ligand into a water-tolerant metal-organic framework (MOF), named SION-105, and employing Lewis basic diamine substrates for the in situ formation of FLPs within the MOF. Using CO₂ as a C1-feedstock, this combination allows for the efficient transformation of a variety of diamine substrates into benzimidazoles. SION-105 can be easily recycled by washing with MeOH and reused multiple times without losing its identity and catalytic activity, highlighting the advantage of the MOF approach in FLP chemistry.

Photoinduced Regulation of the Heat Resistance in Polymer Networks with Diarylethene-Conjugated Reversible Covalent Cross-Links

A cross-linker with a diarylethene-conjugated Diels-Alder adduct (DAE/DA) was synthesized and applied in a radical polymerization system to afford polymer networks whose dynamic nature can be changed reversibly by photoirradiation. Free-radical polymerization of hexyl methacrylate and the DAE/DA-based cross-linker furnished an insoluble and transparent poly(hexyl methacrylate) network with DAE/DA moieties at their cross-linking points, which undergo de-cross-linking via a thermally induced retro-DA reaction upon heating. The photoregulation of such a thermal de-cross-linking reaction in DAE/DA-based polymer networks was demonstrated by swelling experiments and tensile tests, revealing drastic changes in the heat resistance and mechanical properties upon exposure to UV-vis irradiation.

Alkylammoniotrifluoroborate functionalized polystyrenes: polymeric pre-catalysts for the metal-free borylation of heteroarenes

Three polymeric versions of ansa-N,N-dialkylammoniumtrifluoroborate ambiphilic molecules based on the styrene motif (poly(1-NMe2H+-2-BF3--4-styrene) (P-Me), poly(1-NEt2H+-2-BF3--4-styrene) (P-Et) and poly(1-piperidinyl-H+-2-BF3--4-styrene) (P-Pip)) were synthesized, characterized and tested as heterogeneous pre-catalysts for the borylation of electron-rich heteroarenes. These heterogeneous versions of previously reported pre-catalysts show similar reactivity patterns and represent the first examples of solid-supported FLP metal-free catalysts for the C-H borylation of heteroarenes.

Controlling interfacial interactions of supramolecular assemblies by light-responsive overcrowded alkenes

A light-responsive supramolecular polymer was constructed by an AB-type monomer containing a light-responsive overcrowded alkene. The primary assemblies of the supramolecular polymer can further undertake secondary self-assembly by interfacial host-guest connections, which can be manipulated by light stimuli to convert into discrete primary assemblies.

Fabrication of Poly(acrylic acid)/Boron Nitride Composite Hydrogels with Excellent Mechanical Properties and Rapid Self-Healing Through Hierarchically Physical Interactions

Many living tissues possess excellent mechanical properties and self-healing ability. To mimic these living tissues, a series of novel composite hydrogels, poly(acrylic acid)/surface-modified boron nitride nanosheets (PAA/BNNS-NH2) were fabricated simply through hierarchically physical interactions: molecular-scale metal coordination interaction between -COOH of PAA and Fe3+ and nanoscale H-bond between -COOH of PAA and -NH2 of BNNS-NH2. The composite hydrogels exhibit both excellent mechanical properties (including enhanced fracture stress, elongation, toughness, Young's modulus, and dissipated energy) and rapid healing ability without any external stimulus. Especially, the B0.5P70 (the hydrogel with BNNS concentration of 0.5 mg mL- 1, the water content of 70 wt%) exhibits a fracture stress of ~ 1311 kPa and toughness of ~ 4.7 MJ m- 3, almost ~ 3 times and ~ 8 times to B0P70, respectively. The excellent properties, combined with the simple preparation method, endow these composite hydrogels with potential applications.

Ultra-High-Molecular-Weight Polymers Produced by the Immortal Phosphine-Based Catalyst System

A strong organophosphorus superbase, N-(diphenylphosphanyl)-1,3-diisopropyl-4,5-dimethyl-1,3-dihydro-2H-imidazol-2-imine (IAP3) was combined with a sterically encumbered but modestly acidic Lewis acid (LA), (4-Me-2,6-^t Bu₂ -C₆ H₂ O)Alⁱ Bu₂ ((BHT)Alⁱ Bu₂ ), to synergistically promote the frustrated Lewis pair (FLP)-catalyzed living polymerization of methyl methacrylate (MMA), achieving ultrahigh molecular weight (UHMW) poly(methyl methacrylate) (PMMA) with M_n up to 1927 kg mol^-1 and narrow molecular weight distribution (MWD) at room temperature (RT). This FLP catalyst system exhibits exceptionally long lifetime polymerization performance even in the absence of free MMA, which could reinitiate the desired living polymerization after the resulting system was held at RT for 24 h.

Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs

The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or regioselectivity, as well as its unique application in materials chemistry. These advances made in LPP are comprehensively reviewed, with the scope of monomers focusing on heteroatom-containing polar monomers, while the polymerizations mediated by main-group LAs and LBs separately that are most relevant to the LPP are also highlighted or updated. Examples of applying the principles of the LPP and LP chemistry as a new platform for advancing materials chemistry are highlighted, and currently unmet challenges in the field of the LPP, and thus the suggested corresponding future research directions, are also presented.

Efficient sampling of reversible cross-linking polymers: Self-assembly of single-chain polymeric nanoparticles

We present a new simulation technique to study systems of polymers functionalized by reactive sites that bind/unbind forming reversible linkages. Functionalized polymers feature self-assembly and responsive properties that are unmatched by the systems lacking selective interactions. The scales at which the functional properties of these materials emerge are difficult to model, especially in the reversible regime where such properties result from many binding/unbinding events. This difficulty is related to large entropic barriers associated with the formation of intra-molecular loops. In this work, we present a simulation scheme that sidesteps configurational costs by dedicated Monte Carlo moves capable of binding/unbinding reactive sites in a single step. Cross-linking reactions are implemented by trial moves that reconstruct chain sections attempting, at the same time, a dimerization reaction between pairs of reactive sites. The model is parametrized by the reaction equilibrium constant of the reactive species free in solution. This quantity can be obtained by means of experiments or atomistic/quantum simulations. We use the proposed methodology to study the self-assembly of single-chain polymeric nanoparticles, starting from flexible precursors carrying regularly or randomly distributed reactive sites. We focus on understanding differences in the morphology of chain nanoparticles when linkages are reversible as compared to the well-studied case of irreversible reactions. Intriguingly, we find that the size of regularly functionalized chains, in good solvent conditions, is non-monotonous as a function of the degree of functionalization. We clarify how this result follows from excluded volume interactions and is peculiar of reversible linkages and regular functionalizations.

Polysiloxane/Polystyrene Thermo-Responsive and Self-Healing Polymer Network via Lewis Acid-Lewis Base Pair Formation

The use of thermo-reversible Lewis Pair (LP) interactions in the formation of transient polymer networks is still greatly underexplored. In this work, we describe the synthesis and characterization of polydimethylsiloxane/polystyrene (PDMS/PS) blends that form dynamic Lewis acid-Lewis base adducts resulting in reversible crosslinks. Linear PS containing 10 mol % of di-2-thienylboryl pendant groups randomly distributed was obtained in a two-step one-pot functionalization reaction from silyl-functionalized PS, while ditelechelic PDMS with pyridyl groups at the chain-termini was directly obtained via thiol-ene “click” chemistry from commercially available vinyl-terminated PDMS. The resulting soft gels, formed after mixing solutions containing the PDMS and PS polymers, behave at room temperature as elastomeric solid-like materials with very high viscosity (47,300 Pa·s). We applied rheological measurements to study the thermal and time dependence of the viscoelastic moduli, and also assessed the reprocessability and self-healing behavior of the dry gel.

Semi-solid and solid frustrated Lewis pair catalysts

This review presents the strategies for the construction of heterogeneous frustrated-Lewis-pair catalysts, their catalytic applications and future challenges and opportunities.