Small-Angle Neutron Scattering Analysis of Bottlebrush Polymers Prepared via Grafting-Through Polymerization

Bottlebrush polymers are highly branched macromolecules with potential applications in antifouling coatings, rheological modifiers, and drug delivery systems. However, the solution conformation of bottlebrush polymers has been studied in only a limited set of materials made primarily by grafting-from polymerization. Here we present small-angle neutron scattering (SANS) measurements on a series of polystyrene bottlebrush polymers with varying sidechain and backbone lengths in d8-toluene to analyze their size, shape, and conformation. Bottlebrush polymers with 2−7 kg mol−1 polystyrene side chains (degree of polymerization DP = 14− 54) and poly(oxanorbornene) backbones (DP = 10−264) were synthesized using reversible addition−fragmentation chain transfer (RAFT) followed by a ring-opening metathesis polymerization (ROMP) grafting-through synthesis scheme. Analysis by Guinier−Porod, rigid cylinder, and flexible cylinder models provided estimates of the bottlebrush polymer length, radius, and stiffness. The bottlebrush polymer cross-sectional area depends primarily on side-chain DP, and the radius of gyration Rg exhibits a power-law dependence with side-chain DP. We also observe a sphere-to-cylinder transition with increasing backbone DP, with the transition occurring at a backbone DP of approximately 120 for the polystyrene bottlebrush polymers studied. The maximum molecular dimension for the series studied varies from 25 to 350 nm. ■ INTRODUCTION Bottlebrush polymers are highly branched macromolecules with polymeric side chains attached to a main-chain polymer backbone (Figure 1) and are promising for applications that rely on large, highly extended macromolecules and/or densely packed polymeric chains. These include stimuli-responsive surfaces, polymer photonics, and drug delivery systems. The development of bottlebrush polymers for applications such as drug delivery requires quantitative knowledge of their structure in solution, but the majority of work with bottlebrush polymers has relied on the characterization of bottlebrush polymers adsorbed on a surface or in the melt state. This can provide an estimate of the single-molecule size but not quantitative information on their solution-state structure. Here, we present a detailed analysis of the conformation of a model series of bottlebrush polymers in solution through small-angle neutron scattering (SANS) measurement and analysis. Three general techniques to synthesize bottlebrush polymers include grafting-from, grafting-to, and graftingthrough. The grafting-from approach has been the most widely used and can give large, synthetically diverse bottlebrush polymers but with imperfect side-chain attachment. The grafting-to technique yields varying degrees of grafting efficiencies. Of the three methods, grafting-through is the only technique that guarantees 100% side-chain attachment. Recent work has demonstrated that living ringopening metathesis polymerization (ROMP) of norbornenyl macromonomers can be used to prepare well-defined bottlebrush polymers with controlled backbone and sidechain molecular weights. While a variety of bottlebrush polymers made by the ROMP grafting-through technique have been reported recently, a study of their solution size and conformation has not been reported. Prior small-angle X-ray scattering (SAXS) and SANS studies have focused on bottlebrush polymers prepared by the grafting-from technique. Received: June 16, 2013 Revised: August 2, 2013 Published: August 22, 2013 Figure 1. Schematic of a bottlebrush polymer, depicting an extended, cylindrical bottlebrush polymer with fully stretched polymeric side-