The synthesis of a series of amphiphilic AB diblock copolymers in which the A block comprises poly(2-(methacryloyloxy)ethyl phosphorylcholine) [PMPC] and the B block comprises poly(2-hydroxypropyl methacrylate) [PHPMA] by atom transfer radical polymerization (ATRP) is reported. The aqueous solution properties of these new diblock copolymers were examined using dynamic light scattering and variable temperature H-1 NMR spectroscopy. Copolymers with shorter thermoresponsive PHPMA blocks formed relatively large aggregates, while copolymers with longer PHPMA blocks formed smaller aggregates. This apparently "anomalous" self-assembly behavior occurs because the PHPMA block becomes more hydrophobic as its degree of polymerization is increased. Therefore, shorter PHPMA blocks lead to the formation of loose highly hydrated aggregates, whereas longer blocks formed more compact dehydrated aggregates. In addition, these new PMPC-PHPMA diblock copolymers are highly biocompatible and can mediate the relatively rapid efficient uptake of a fluorescent dye by human dermal fibroblast cells. Interestingly, dye uptake kinetics appear to depend on the hydrophobic/hydrophilic balance of the copolymer. This not only bodes well for in vitro imaging of live cells for biomedical applications but also highlights the importance of copolymer design to ensure efficient drug delivery.