Earth-abundant catalysts based on transition metal phosphides (TMPs) such as CoxP have recently gained a lot of attention in the field of electrocatalysis and are usually acquired by chemical synthesis.

Formation of site‐specific deposition of polymeric particles and films with controllable parameters via enzyme‐mediated addressing is investigated. The polymers are synthesized in situ by combining the addressing process with enzyme‐mediated radical polymerization, using the enzyme horseradish peroxidase. As a model system, the technically important poly(methyl methacrylate) is formed and addressed on both glass‐ and polyethylene supports.The key to the controllability is the influence of different reaction parameters on particle formation, namely, the concentration of H2O2 and the pH show the most significant impact. With higher concentrations of H2O2, size and amount of gained particles increases, due to H2O2 being a limiting factor to the reaction. Moreover, since acetyl acetone forms the end‐group of the polymer chains, the pH directly influences the particle stability in addition to affecting the enzyme activity. This effect can be utilized to gain either films or particle assemblies at will. Deposition occurs highly site‐specific, exclusively on enzyme‐functionalized areas, enabling patterned coating structures within a safe, energy efficient, and easy‐to‐apply process.