We report opal photonic crystals that are self-assembled from functional polymer particles. We randomly copolymerized functional side-chain monomers containing motifs that form homodimers or heterobridges. These include ether or methylene bridges, hydrazone bridges, acids for anhydride formation, low-T-g copolymers or physical cross-links by hydrogen bonds and/or polarity. To generate particles that are monodisperse, spherical, and functionalized, we combined emulsifier-free synthesis with swelling synthesis steps. Laser diffraction from centimeter-sized beams, white-light interferometry, and atomic force microscopy demonstrates symmetry and homogeneity across the entire crystal without the loss of interstitial volume. Compared to the stability of nonfunctional particles, the stability of the crystal against immersion in water and isopropanol was enhanced from 10 to a perfect 100%. One of the successful approaches (methylene bridges from N-methylolmethacrylamid) is triggered by thermal activation, but as shown, this is operative far from the trivial regime of sintering. We demonstrate successful infiltration with and solvation of a laser-polymerizable resin, thus enabling the processing of 3D photonic waveguide structures.