A double hydrophilic block copolymer composed of poly(acrylic acid) (PAA) and poly(4-vinyl pyridine) (P4VP) was obtained through hydrolysis of diblock copolymer of poly(tert-butyl acrylate) (PtBA) and P4VP synthesized using atom transfer radical polymerization. Water-soluble micelles with PAA core and P4VP corona were observed at low (acidic) pH, while micelles with P4VP core and PAA corona were formed at high (basic) pH. Two metalloporphyrins, zinc tetraphenylporphyrin (ZnTPP) and cobalt tetraphenylporphyrin (CoTPP), were used as model compounds to investigate the encapsulation of hydrophobic molecules by both types of micelles. UV-vis spectroscopic measurements indicate that micelles with P4VP core are able to entrap more ZnTPP and CoTPP as a result of the axial coordination between the transition metals and the pyridine groups. The study found that metalloporphyrins encapsulated by the micelles with PAA core could be released on pH increase, while those entrapped by the micelles with P4VP core could be released on pH decrease. This behavior originates from the two-way pH change-induced disruption of PAA-b-P4VP micelles. (c) 2006 Wiley Periodicals, Inc.