This study investigated the synthesis of the biocatalyst, magnesium 2,6-di-tert-butyl-4-methylphenoxide (Mg(BHT)(2)) complex, and the ring-opening polymerization (ROP) of epsilon-caprolactone (CL). The complex demonstrates high catalytic activity and controllable of molecular weight for the ROP of CL in tetrahydrofuran at room temperature, even when polymerization was performed under air. Before this study, the polymerization of CL had never been performed using a magnesium catalyst under air at room temperature. Various forms of alcohols with different purposes were also used as initiators with Mg(BHT)(2). The results show that the magnesium complex acts as a perfect catalyst because of its high catalytic activity and control ability without any cytotoxicity in the polymerization of CL, making it suitable for biomedical applications. In addition, nanoparticle formation, cytotoxicity, and phototoxicity of tri-2-hydroxyethyl ester [Ce6-(CH2CH2OPCL)(3)] were also studied in this article and Ce6-(CH2CH2OPCL)(3) formed nanoparticle can act as a nanophotosensitizer for photodynamic therapy. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 2697-2704, 2012