Synthesis and Characterization of High-Density and High Degree of Polymerization Bottlebrush Block Copolymers for Photonic Applications

Synthesis of high-purity bottlebrush block copolymers (BBCPs) with high degrees of polymerization via graft-through ring-opening metathesis polymerization of norbornene-based macromonomers faces difficulties due to the lower reactivity of these macromonomers compared to smaller monomers. Herein, we report a scalable synthetic methodology to access polystyrene-b-polylactide BBCPs with a high degree of polymerization (PS₇₉₆-b-PLA₁₁₁₄) and high brush densities (2 brushes per norbornene repeat unit, PS₂₀₁-b-diPLA₂₂₉) with high purity (as illustrated by the monomodal molecular weight distribution). This methodology combines graft-through (GT) polymerization of macromonomer polymerization and multifunctional monomer polymerization to synthesize a bottlebrush-linear block copolymer as an intermediate. The brushes of the second block are then synthesized using a graft-from (GF) polymerization. The synthesized BBCPs self-assemble into periodic structures with photonic properties showing wavelengths of reflection nearing the IR region (∼1500 nm). No difference between the traditional GT and this combined GT-GF polymerization was identified spectroscopically or in their self-assembled structures. However, the viscoelastic behaviors suggest that GT-GF-based BBCPs are more flexible in comparison to purely GT-based polymers. This difference in molecular flexibility is putatively attributed to a difference in chemical structure at the interface between the two blocks. Moreover, we noted that BBCPs with higher brush density exhibited greater elasticity when compared to those with lower density. In summary, we successfully created a range of BBCPs characterized by a high degree of polymerization and high brush density and examined how the variations in their topology influenced the structural, photonic, and viscoelastic properties of the materials.