Ultra‐High Molecular Weight Polydimethylsiloxane Bottlebrushes From Norbornene‐Pyrrolidine Capped Macromonomers

ABSTRACT Bottlebrush (BB) polymers with polydimethylsiloxane (PDMS) side‐chains exhibit supersoft moduli ( E < 100 kPa) without solvent, making them promising tissue‐mimetic materials for biomedical applications. While larger backbones and side‐chains enhance softness, synthetic limitations have constrained their molecular weight. We report a graft‐through method to access ultra‐high molecular weight PDMS BB polymers (~1–20 million g mol −1 ). Central to this was the design of a norbornene‐pyrrolidine aryllithium initiator for the anionic ring‐opening of hexamethylcyclotrisiloxane (D3), yielding monofunctionalized PDMS macromonomers (MMs) with molecular weights from 1–30 kg mol −1 . Subsequent ring‐opening metathesis polymerization (ROMP) produced BB homopolymers within minutes ( t ≤ 5 min) and with low dispersity ( Đ < 1.3). The ‐C‐Si‐O‐ linkage between the backbone and side‐chains provided improved chemical stability over earlier ‐C‐O‐Si‐ analogues. These MMs were also used to create a linear‐bottlebrush‐linear block copolymer, forming a supersoft ( E = 79 kPa), thermoplastic elastomer with excellent elastic recovery (~96%). This platform enables the synthesis of ultra‐high molecular weight graft (co‐)polymers, advancing the study of extreme structure–property relationships and supporting the development of novel soft materials for bio‐interfacing.