Compressing and Swelling To Study the Structure of Extremely Soft Bottlebrush Networks Prepared by ROMP

To fully explore bottlebrush polymer networks as potential model materials, a robust and versatile synthetic platform is required. Ring-opening metathesis polymerization is a highly controlled, rapid, and functional group tolerant polymerization technique that has been used extensively for bottlebrush polymer generation but to this point has not been used to synthesize bottlebrush polymer networks. We polymerized a mononorbornene macromonomer and dinorbornene cross-linker (both poly(n-butyl acrylate)) with Grubbs' third-generation catalyst to achieve bottlebrush networks and in turn demonstrated control over network properties as the ratio of macromonomer and cross-linker was varied. Macromonomer to cross-linker ratios ($\left[\text{MM}\right]/\left[\text{XL}\right]$) of 10 to 100 were investigated, of which all derivative networks yielded gel fractions over 90%. Because of its amenability toward small samples, contact adhesion testing was used to quantify dry-state shear modulus $G$, which ranged from 1 to 10 kPa, reinforcing that bottlebrush polymer networks can achieve low moduli in the dry state compared to other polymer network materials through the mitigation of entanglements. A scaling relationship was found such that $G\sim {\left(\right[\text{MM}]/[\text{XL}\left]\right)}^{-0.81}$, indicating that macromonomer to cross-linker ratio is a good estimator of cross-linking density. The swelling ratio in toluene, $Q$, was compared to dry-state modulus to test the universal scaling relationship for linear networks $G\sim Q^{-1.75}$, and a measured exponent of -1.71 indicated good agreement. The synthetic platform outlined here represents a highly flexible route to a myriad of different bottlebrush networks and will increase the accessibility of materials critical to applications ranging from fundamental to biomedical.