Three vinyl-functionalized phosphine oxide photoinitiating monomers have been synthesized: 4-vinylbenzoyldiphenylphosphine oxide (VBPO), 2,6-dimethyl-4-vinylbenzoyldiphenylphosphine oxide (DMVBPO), and 2,4,6-trimethylbenzoylphenyl-4-vinylphenylphosphine oxide (TMBVPO). VBPO was copolymerized with vinylpyrrolidone or vinyl acetate (PPI- 1a) and dimethylacrylamide (PPI- 1b). DMVBPO and TMBVPO were both copolymerized with dimethylacrylamide (PPI-2 and PPI-3, respectively). The choice of vinylphosphine oxide and comonomer(s) had a significant influence on the properties of the resulting PPI. PPI-1a was not stable in solution in 2-hydroxyethyl methacrylate (HEMA), whereas the VBPO-dimethylacrylamide (DMA) copolymer (PPI-1b) was stable in HEMA but not stable in aqueous solutions. PPI-2 was both soluble and stable in water up to 22 months. PPI-1a was as effective as trimethylbenzoyldiphenylphosphine oxide (TPO, BASF Lucirin). PPI-2 was more effective in the polymerization of HEMA/ water mixtures than PPI-3. PPI-2 and PPI-3 acted as self-cross-linking species, resulting in the formation of hydrogels; PPI 3 was more effective in this. PPI-2 was very effective in forming hydrogels based on poly(ethylene glycol) diacrylate.