N-Phenylglycine (NPG) has been used for many years as efficient biocompatible co-initiator in initiating radical photopolymerization. Apart of that, very few amino acid co-initiators have been proposed to exhibit the comparable ability with NPG. Unlike NPG with free carboxyl group, we found that several glycine derivatives with closed carboxyl groups as co-initiators could compete or even work better than NPG in initiating photopolymerization in the presence of camphorquinone, while the counterparts with free carboxyl groups were invalid. In this work, the performances of these glycine derivatives as co-initiators were evaluated by photopolymerization of hydroxyethyl acrylate aqueous solution, and NPG was used as reference. The effects of pH values of formula on their co-initiation behaviors were investigated. Although the same photo-induced electron transfer pathway for radical generation, NPG underwent decarboxylation to produce radical species, while the investigated derivatives leaded to sterically hindered radicals only by hydrogen-abstraction on a-methylene, which were confirmed by 1H NMR spectra, ESR spectra and polymerization experiments. The derivatives combined with CQ could efficiently initiate photopolymerization of other monomers, indicating the wide substrate adaptability and being a class of robust co-initiators with potential application in the preparation of biohydrogels. [ABSTRACT FROM AUTHOR]