In this study, amphiphilic diblock copolymers (PMPCm-PTSMn) composed of hydrophilic poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) and hydrophobic poly(3-(tris(trimethylsiloxy)silyl)propyl methacrylate) (PTSM) were prepared via RAFT-controlled radical polymerization. The subscripts m and n represent the degrees of polymerization of the PMPC and PTSM blocks, respectively. Thus, the m/n ratio represents the hydrophilic/hydrophobic balance of the block copolymer. We prepared PMPCm-PTSMn with m/n ratios of 12, 1.8, and 0.3 and studied their association behaviors in water. PMPCm-PTSMn with m/n ratios of 12 and 1.8 could dissolve in water. However, PMPCm-PTSMn with an m/n ratio of 0.3 did not directly dissolve in water. First, PMPCm-PTSMn with an m/n ratio of 0.3 dissolved in ethanol, whose solution was dialyzed in water to prepare the aqueous solution. The solubilizing state of these polymers in water was examined by light scattering, a fluorescence probe method, and TEM. These polymers took interpolymer aggregation state with various morphologies. The m/n ratios of 12, 1.8, and 0.3 yielded spherical shapes, cylindrical micelles, and vesicle interpolymer aggregate shapes, respectively. (c) 2019 Wiley Periodicals, Inc.