This manuscript describes the synthesis of poly(methacrylic acid) (PMAA) brushes via surface-initiated atom transfer radical polymerization (SI-ATRP) of sodium methacrylate (NaMA) and their use as substrates for the mineralization of calcium carbonate. A CuBr/CuBr2/bipyridine catalyst system in aqueous solution at room temperature allowed the synthesis of brushes with thicknesses of up to 300 nm. Using substrates modified with mixtures of an ATRP-initiator modified trimethoxysilane and an "inert" pivaloyl-modified trimethoxysilane to initiate the ATRP of NaMA, a series of brushes with varying chain density could be prepared. Subsequent mineralization experiments revealed that, while low-density brushes promoted the formation of calcite crystals, high-density brushes were covered with a thin layer of amorphous CaCO3 (ACC). This is of interest because ACC can serve as a metastable precursor for different crystalline CaCO3 polymorphs and offers attractive perspectives for the bottom-up fabrication of well-defined CaCO3 crystal architectures.