A photosensitizing metal-organic layer (MOL), IrBPY-MOL, based on hafnium-oxo clusters and cyclometalated iridium-complex-derived organic linkers, was synthesized and used as an efficient catalyst for photopolymerization of methyl methacrylate and other monomers to afford polymers with high-number-averaged molar masses and low polydispersity indices. The corresponding metal-organic framework (MOP) failed to photopolymerize or exhibited low catalytic efficiency under identical conditions. This work highlights the advantages of MOLs over their MOF counterparts in overcoming pore-size and diffusion limitations in photopolymerization reactions.