Nature Nanotechnology: Precision polymer synthesis provides unlimited possibilities!

Olefin metathesis, as a powerful metal-catalyzed carbon-carbon bond formation method, has made great progress in recent years. However, the complexity arising from multicomponent interactions has long hindered the complete understanding of the olefin metathesis mechanism, thereby hindering further optimization of the reaction.

In view of this, Professor Xuefeng Guo and Professor Fanyang Mo of Peking University , Professor Kendall N. Houk of the University of California, Los Angeles, and Professor Yanwei Li of Shandong University used a sensitive single-molecule electrical detection platform to elucidate the production pathway and hidden degenerate pathway of ring-closing metathesis by focusing on a catalyst . In addition to visualizing the entire pathway, they also found that the traditionally unwanted degenerate pathway has an unexpected constructive coupling effect on the production pathway, and both types of pathways can be regulated by external electric fields . Then, they advanced this capability to ring-opening metathesis polymerization involving more interacting components. Through monomer insertion event resolution, precise synthesis of single polymers was achieved on the device by online manipulation of monomer insertion kinetics, intramolecular chain transfer, stereoregularity, degree of polymerization, and block copolymerization . These results provide a comprehensive mechanistic understanding of olefin metathesis and provide unlimited opportunities for practical precision manufacturing. The relevant research results were published in the latest issue of Nature Nanotechnology with the title "Full on-device manipulation of olefin metathesis for precise manufacturing".

The authors constructed a single catalyst junction and focused on two types of olefin metathesis reactions: ring-closing metathesis (RCM) and ROMP (Figure 1) . Real-time single-molecule electrospectroscopy with high temporal resolution enabled visualization of reaction trajectories, revealing hidden degradation pathways in RCM and intramolecular back-connections in ROMP. In addition, with the help of an external electric field (EEF), we achieved precise online manipulation of monomer insertion kinetics, intramolecular chain transfer, stereoselectivity, DP, and block copolymerization.

Figure 1. Schematic diagram of a single catalyst structure focusing on RCM and ROMP.

Using advanced photolithography techniques, the authors constructed nanographene dot electrodes on a silicon chip. They integrated a molecular bridge with a second-generation Hoveyda-Grubbs catalyst (HG2) into the device. The devices allow real-time electrical measurements, providing reaction kinetics and pathway data. The authors explored the complexity of ring-closing metathesis (RCM), a reaction commonly used to synthesize cyclic structures . Due to the molecular complexity involved, degenerate pathways (unproductive side reactions) emerge. Using diethyldiallylmalonate as a model substrate, the team studied RCM with a single HG2 catalyst linker, observed multiple conductance states and identified a productive pathway and two degenerate pathways (a and b) . The productive pathway : initiated by the binding of a ruthenium methylene complex to the substrate, forming an α-substituted metal cyclobutane intermediate, which then undergoes cyclization to give the desired cyclopentene product. The degenerate pathway (a) : generates a β-substituted metal cyclobutane intermediate, which reverts to the initial complex and starting substrate. Degradation pathway (b) : involves the formation of α,α-disubstituted metal cyclobutanes (trans and cis isomers) and then returns to the initial complex. About 22% of the devices showed a current-voltage (IV) response, indicating the presence of a functional molecular bridge . Statistical analysis showed that the probability that a single molecular connection was responsible for the electrical signal was about 90%, confirming the integrity of the device and its effective single-molecule measurement capability . The reliable molecular connection rate of this platform enables accurate real-time tracking of the catalytic process, laying a solid foundation for observing and controlling metathesis reactions.

Figure 2. Electrical characteristics and signal distribution of RCM at a single catalyst junction

Manipulating backbiting in ROMP

The authors analyzed the ring-opening metathesis polymerization (ROMP) of cyclooctene (COE) to control the formation of cyclic oligomers caused by intramolecular backbiting . The electrical signal recorded during ROMP can track individual monomer insertion events, with each square pulse in the It trace representing an insertion. The electric field can restrict backbiting by affecting the chain conformation of the polymer. Figure 3b shows an It curve where polymerization proceeds in a stepwise manner, with each step reflecting the addition of a COE monomer. The arrows indicate a sudden drop in the current, corresponding to backbiting events with shortened polymer chain length. Figures 3c-d combine optical tracking and electrical signals to confirm polymerization. Shorter polymer trajectories are associated with observed backbiting, which is confirmed by a sudden drop in the current signal. Figure 3e illustrates the effect of higher EEF, which reduces backbiting by extending the chain conformation and reducing intramolecular entanglements. Overall, the suppression of backbiting by EEF is evident in the continued extension of the polymer chain without the formation of cyclic oligomers. This controlled suppression enables the formation of high-purity polymers and minimizes backbiting at optimized electric field strengths. The ability to maintain linear polymer growth significantly increases the yield of desired polymer structures.

Summary

Here we demonstrate an advanced platform capable of monitoring and controlling olefin metathesis at the single-molecule level . By exploiting single catalyst ligation and precise electric field tuning, the researchers achieved a new level of control over metathesis reactions. Major achievements include decoding degenerate pathways in RCM, suppressing backbiting in ROMP, controlling stereoselectivity, and facilitating the precise synthesis of block copolymers. The application of external electric fields becomes a versatile tool capable of dynamically controlling reaction pathways and polymer configurations, which could have a significant impact on the fields of precision polymerization and organic synthesis . The insights gained here provide a comprehensive understanding of the metathesis mechanism and open new avenues for customized molecular synthesis.