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

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 (similar to 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 (D < 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 (similar to 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.