On-device programmable synthesis of polymers
Olefin metathesis underpins precision polymer synthesis, yet ensemble-averaged measurements obscure real-time catalytic trajectories and transient intermediates, impeding mechanistic resolution and sequence control. This study exploits a graphene-molecule-graphene single-molecule junction to interrogate ring-opening metathesis polymerization (ROMP) at single-event fidelity. Application of an external electric field (EEF) suppresses backbiting 2.7-fold via chain disentanglement, yielding >99% ROMP selectivity at 10 mM monomer concentration. Moderate EEFs (0.5–0.8 V) elevate Z-selectivity to 97%, with density functional theory attributing this to stabilization of the syn-configuration of Ru–Cl ligands; low fields favor anti-configuration and E-isomer formation. Fluorescent tracking confirms restricted chain-end mobility in Z-selective chains due to tighter helical packing. EEFs function as an on/off switch: at 1 V, polymerization ceases abruptly, enabling precise block copolymer synthesis. Alternating cyclooctene (COE) and norbornene (NBE) monomers under EEF regulation produces sequence-defined block copolymers, validated by fluorescent tags (f-COE emission at 500 nm; f-NBE at 550 nm). Quantized photon counts during stepwise polymerization (e.g., 20 COE units → 1 f-COE → 20 COE units) demonstrate single-monomer resolution. This platform bridges single-molecule behavior and ensemble phenomena, offering deterministic control over polymer architecture and stereochemistry, with implications for biomimetic and defect-free materials.