Surface‐Coordinated Gas–Solid Polymerization Enables Quasi‐2D Polyphenylacetylene Single‐Crystalline Nanoribbons

Two‐dimensional (2D) conjugated polymer single crystals are highly desirable for optoelectronics, yet their realization is limited by the intrinsic coupling between molecular‐weight buildup and chain entanglement, which disrupts long‐range order. Here, we report a gas–solid interfacial polymerization strategy that decouples monomer diffusion from chain propagation, enabling the growth of quasi‐2D polyphenylacetylene (PPA) single‐crystalline nanoribbons on Cu/CuO x surfaces. This process is likely enabled by surface coordination‐assisted activation of phenylacetylene and directional π‐stacking‐driven assembly, which suppress chain entanglement and promote crystallographic ordering. As a result, micrometer‐scale PPA single‐crystalline nanoribbons with left‐handed cis‐transoidal helical chains and long‐range crystallinity are obtained. Under an ideal fully extended‐chain model, the PPA chains exhibit a geometrically estimated upper‐bound molecular weight of up to ∼3.7 × 10 9 g mol − 1 . The resulting PPA nanoribbons exhibit efficient charge transport and broadband photodetection from 375 to 1550 nm, with a responsivity of 30.1 A W − 1 and a detectivity of 8.65 × 10 1 2 Jones. This interfacial synthesis strategy provides a route to overcoming the trade‐off between molecular‐weight buildup and structural order in conjugated polymers, offering opportunities for quasi‐2D polymer semiconductors in flexible optoelectronics and programmable chiral materials.