A series of self-assembled monomethoxypoly-(ethylene glycol)-b-poly(2-hydroxypropyl methacrylate)/poly(2-hydroxypropyl methacrylate) (mPEG(113)-PHPMA/PHPMA) blends are prepared by aqueous photoinitiated polymerization-induced self-assembly (photo-PISA) of HPMA at room temper- ature mediated by a binary mixture of mPEG(113)-CEPA and CEPA CEPA = 4-cyano-4-(ethylthiocarbonothioylthio)pentanoic acid). High rates of polymerization are observed in all cases, and quantitative monomer conversions can be achieved within 15 min of 405 nm visible light irradiation. Gel permeation chromatography (GPC) measurement confirms good control is maintained under aqueous photo-PISA conditions at different [mPEG(113)-CEPA]/[CEPA] ratios. A morphological phase diagram is constructed for the first time by changing the degree of polymerization (DP) of PHPMA and [mPEG(113)-CEPA]/[CEPA] ratio. The introduction of homopolymer promotes the formation of higher order morphologies and larger spherical micelles. The obtained AB/B assemblies can be used as seeds for preparing ABC/BC assemblies by seeded polymerization of hydrophilic or hydrophobic monomers. This method provides a facile platform for in situ preparation of various AB/B and ABC/BC assemblies in water at room temperature.