Ultrahigh-molecular-weight tadpole polymers with statistically symmetric head-to-tail geometry: the key topological intermediate bridging linear and cyclic architectures.

Polymer topology profoundly influences material properties by regulating chain arrangement, conformational entropy, and interactions, yet understanding topological entanglement constraints like threading and knotting remains a fundamental challenge in polymer science, partly due to the scarcity of structurally precise, ultrahigh-molar-mass ring-containing polymers as model systems. Herein, we address this synthetic challenge with a dual-terminal intramolecular cyclization strategy to synthesize tadpole polymers, the key architectural intermediate bridging linear and monocyclic topologies. This approach leverages synergistic dual-end groups and viscoelastic effects to maintain chain-end functionality, enabling efficient synthesis of tadpole polymers with ultrahigh molar masses (0.4-1.0 MDa) and statistically symmetric head-to-tail geometry. Such precise structural control facilitates direct visualization of individual tadpoles via advanced microscopy techniques. Physical characterization shows their distinct properties: an average ∼10% reduction in intrinsic viscosity (signaling compact packing) while the fractal dimension (d f ≈ 1.7) remains unchanged; in the melt, rheology exhibits an entanglement plateau extended by 1-2 orders of magnitude, indicative of long-lived, threading-stabilized transient networks. Additionally, we develop a versatile post-polymerization strategy to build functional tadpole polymer libraries. This work provides a valuable experimental platform for examining the linear-to-cyclic topological transition and establishes a robust platform for investigating constrained dynamics and engineering advanced soft materials in future work.