Using polymeric photocatalytic materials for visible lightdriven heterogeneous conversion of organic substances in environmentally benign reaction media as in water, in particular, has gained much attention recently. Most of the current conjugated polymer photocatalysts are often unsuitable for applications in water because of their highly hydrophobic nature. Classical polymer-based photocatalysts possess advantages that include precise control of properties through facile synthesis and direct access to the toolbox of classical polymer chemistry. However, the effect of monomer composition remains unclear, and a systematic investigation of structure−property relationships is still missing. In this work, we design water-compatible poly(methyl methacrylate) (PMMA)-based polymer photocatalysts by precise control of comonomers with specific functions. The electronic and optical properties and water compatibility of PMMA photocatalysts could be tuned, and they showed direct dependence on the monomer composition. Mathematical as well as density functional theory simulation as hydration enthalpy calculations supported the observed effects. The photocatalytic degradation of 2,4-dichlorophenol in water as model reaction showed an optimum of polymer composition and solubility for the photocatalytic reactivity. More remediation of important contaminants was successfully conducted as diethyl phthalate, acetophenone, and bisphenol-A, which are common products from plastic degradation detected in groundwater.