Highly Entangled Bottlebrush Polymer Networks

Entanglements are topological constraints that govern dynamic mechanical behavior of polymer networks. Linear polymers readily entangle at low molecular weights, whereas bottlebrush polymers—consisting of a long backbone densely grafted with many short side chains—suppress entanglements, enabling solvent-free networks with tissue-like softness. Yet, the same steric crowding pre-strains the backbone and makes such networks brittle. Here, we report entangled bottlebrush elastomers that combine extreme softness and toughness. Using short poly(ethylene glycol) side chains, we synthesize high molecular weight bottlebrush polymers that remain amorphous at room temperature. We identify an entanglement modulus of ~1.3 kPa, nearly 1000 times lower than that of linear counterparts. Unentangled networks exhibit strain-stiffening, whereas entangled networks display pronounced strain-softening followed by delayed stiffening due to entanglement slippage. Despite their low moduli (~10³ Pa), these elastomers stretch up to ~1800% and show a fatigue threshold of ~46 J/m², comparable to natural rubber. Their intrinsic fatigue strength—fatigue threshold normalized by modulus—surpasses that of highly entangled linear polymer networks by >40-fold. These results establish a new class of soft yet tough materials and provide a model system for nonlinear mechanics in complex polymer architectures.