Molecular Bottlebrushes with Bimodal Length Distribution of Side Chains

The structural details of bottlebrush polymers, specifically the grafting density and molecular weight distribution of side chains, influence their physical properties; however, they are difficult to analyze using conventional techniques. Herein we report the synthesis, chatacterization, and molecular imaging of bottlebrush macromolecules with both uniform and bimodal length distributions of poly(n-butyl acrylate) (PnBA) side chains. The densely grafted copolymers were prepared via the "grafting from" approach using atom transfer radical polymerization (ATRP). Bottlebrush macromolecules with both shorter and longer grafted chains were prepared by removal of a fraction of the bromine chain ends of the initial densely grafted brush by selective capping with 4-butoxy-TEMPO and subsequent chain extension of remaining active chains forming longer PnBA grafts. This procedure provided bottlebrush macromolecules with two distinct degrees of polymerization of the grafted side chains herein called bimodal grafts. AFM imaging of individual macromolecules confirmed the formation of wormlike structures with a distinct halo of diffuse side chains originating from bottlebrushes with bimodal PnBA grafts, To quantify the grafting density and dispersity of the initial monomodal side chains, the side chains were cleaved froth the backbone and independently characterized. Utilizing a combination of AFM molecular imaging and the Langmuir-Blodgett technique, the grafting density of monomodal bottlebrushes was measured. The distance between macromolecules is linearly proportional to the weight-average degree of polymerization of the side chains for both the monomodal and bimodal brushes.