Porous crystals possess micropores that facilitate various reactions. Among porous crystals, metal–organic frameworks (MOFs) incorporate an organic molecular framework that allows externally introduced molecules to react with the components of the crystal. Leveraging these characteristics, polymer synthesis utilizing the components of crystals has been actively pursued, resulting in the production of polymers with unique structures that are unattainable through conventional polymer synthesis. This chapter provides an overview of these studies and elaborates on several distinct patterns.