Thermally Induced Cross-Linking of Polymers via C,H Insertion Cross-Linking (CHic) under Mild Conditions

Exploiting the Reversible Dimerization of N-Heterocyclic Carbenes to Access Dynamic Polymer Networks with an Organocatalytic Activity

The capability of some N-heterocyclic carbenes (NHCs) to reversibly dimerize is exploited to access dynamic polymer networks. Benzimidazolium motifs serving as NHC precursors have thus been supported onto copolymer chains by reversible addition-fragmentation chain transfer (RAFT) copolymerization of styrene and up to 20 mol % of 4-vinylbenzyl-ethyl-benzimidazolium chloride. Molecular versions of 1,3-dialkyl benzimidazolium salts have been synthesized as models, the deprotonation of which with a strong base yields the NHC dimers in the form of tetraaminoalkenes. The crossover reaction between two distinct NHC homodimers, forming heterodimers, is then evidenced. Applying this deprotonation method to the RAFT-derived copolymers leads to polymer networks with cross-links consisting of labile dimerized NHC motifs. These networks can be subsequently decross-linked using two distinct triggers, namely, a monofunctional NHC precursor as competitor and carbon dioxide (CO2). In the latter case, regeneration of the network occurs by chemically fueling the linear copolymer bearing benzimidazolium motifs with tBuOK in the presence of trace amounts of water. The same networks can also be used as latent precursors of transient polyNHCs to catalyze the benzoin condensation upon heating. The polymer-supported organocatalysts can thus be used in successive catalytic cycles.

Carbene-Mediated Polymer Modification Using Diazo Compounds under Photo or Thermal Activation Conditions

Based on the characteristics of commodity polymers in large quantities and low costs, modification of existing commodity polymers emerges as the most effective approach for exploring novel materials. Nevertheless, conventional modification methods typically involve high-energy processes (e.g., high temperature, high-energy radiation), which may lead to irreversible detrimental effects on the polymers, contradicting the desired performance enhancement through modification. In this work, we propose a carbene-mediated postpolymerization modification (PPM) strategy utilizing diazo compounds. Under photochemical or thermal activation conditions, insertion of the C-H bond can be achieved without compromising the performance of polymers. These diazo compounds can be easily synthesized in just two steps and applied to all C-H-containing polymers. This practical and effective modification strategy offers new opportunities and possibilities for enhancing the value and expanding the applications of polymers.

Art of Cross-Linking In Situ Bulk Perovskites for Efficient and Stable Photovoltaics

Perovskites are hybrid materials containing templating organic linkers and inorganic halides with efficiencies that have superseded the efficiency of silicon-based photovoltaic devices (PVs) in a very short period of 10 years. Nevertheless, low ambient stability due to traps and ion migration caused hysteresis to remain the bottlenecks on the way to achieving higher operational stability with bulk perovskite-based PVs. In this context, herein we highlight the prospects of in situ cross-linking of linkers within the perovskite lattice either mediated by thermal means or attained photochemically that can maneuver the ambient as well as operational stability for enhanced power conversion efficiency for PV applications or could improve the conductivity of this hybrid semiconductor. Additionally, some important studies of additive engineering via in situ cross-linking that can affect the structure of perovskite in addition to defect passivation to endow ambient environment stability are highlighted herein.

Thermal/Blue Light Induced Cross-Linking of Acrylic Coatings with Diazo Compounds

The thermal curing of industrial coatings (e.g., car painting and metal coil coatings) is accompanied by a substantial energy consumption due to the intrinsically high temperatures required during the curing process. Therefore, the development of new photochemical curing processes-preferably using visible light-is in high demand. This work describes new diazo-based cross-linkers that can be used to photocure acrylic coatings using blue light. This work demonstrates that the structure of the tethered diazo compounds influences the cross-linking efficiency, finding that side reactions are suppressed upon engineering greater molecular flexibility. Importantly, this work shows that these diazo compounds can be employed as either thermal or photochemical cross-linkers, exhibiting identical crosslinking performances. The performance of diazo-cross-linked coatings is evaluated to reveal excellent water resistance and demonstrably similar material properties to UV-cured acrylates. These studies pave the way for further usage of diazo-functionalized cross-linkers in the curing of paints and coatings.