Universal Approach to Photo-Crosslink Bottlebrush Polymers

We demonstrate a universal approach to form bottlebrush polymer networks in both bulk and thin films by photo-crosslinking mixtures of well-defined bottlebrush precursors and bis-benzophenone-based additives. This strategy is compatible with a wide variety of different side-chain chemistries including poly(acrylates), poly(ethers), poly(esters), and poly(siloxanes) due to the indiscriminate C-H abstraction behavior of benzophenone upon exposure to ultraviolet light. The appropriate choice of molecular "linker" that bridges benzophenones is critical to solubilize the additive in a given bottlebrush precursor at room temperature without a solvent. Importantly, homogeneous mixtures can be achieved using two distinct types of linkers: Telechelic polymers matched to the bottlebrush side-chain chemistry or small-molecule branched alkyl derivatives that are often synthetically more accessible. As evidenced by in situ UV shear rheology, the curing kinetics and mechanical properties of these amorphous or semicrystalline networks are controlled by bottlebrush precursor chemistry, architecture, and crosslinker loading. The influence of elastically effective and ineffective crosslinks, which arise in tandem from the statistical nature of benzophenone-induced radical reactions, is quantitatively captured by introducing a general model that relates crosslinker concentration and shear modulus. These results provide a conceptual framework that can be used to conveniently synthesize bottlebrush networks with tailored properties.