In this chapter, we highlight the use of horseradish peroxidase (HRP) as a catalyst to initiate free radical polymerizations of vinyl monomers under benign reaction conditions. A variety of vinyl monomers, including 4-acryloylmorpholine (AM), 2-hydroxyethyl methacrylate (HEMA), and poly(ethylene glycol) methyl ether acrylate (PEGA) were polymerized. The enzyme converts exogenous hydrogen peroxide into a usable radical source, which when coupled with a β-diketone, yields a radical that initiates chain growth in the presence of monomers. The resulting polymers were characterized using nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC). By using enzymatic free radical polymerizations, polymers can be generated in a sustainable, environmentally-friendly, and scalable fashion.