Molecular Weight Dependence of Zero-Shear Viscosity in Atactic Polypropylene Bottlebrush Polymers

A series of bottlebrush polymers with atactic polypropylene side chains were synthesized via ring-opening metathesis polymerization using Grubbs' third-generation catalyst (H₂IMes)(3-BrPy)₂(Cl)₂Ru═CHPh. Polymers were prepared with fixed side chain length and variable backbone degree of polymerization (DP) ranging from 11 to 732 (M_w = 22-1500 kg/mol) and include the largest reported polyolefin-based bottlebrush polymers. The zero-shear viscosity of each polymer sample was measured using small-amplitude oscillatory shear measurements. Power law fits of zero-shear viscosity (η₀) versus weight-average molar mass (η₀ ∼ M_w^α) revealed distinct regimes with differing molecular weight dependences. A weak dependence (α < 0.5) was observed at low molecular weight due to the increasingly compact nature of short bottlebrush polymers with the continued addition of side chains. The scaling transitioned to Rouse-like dynamics (α = 1.2) at high molecular weight as a consequence of a sphere-to-cylinder conformational change with increasing DP of the bottlebrush backbone.