Synthesis of novel thermoresponsive micelles by graft copolymerization of N-isopropylacrylamide on poly(ε-caprolactone-co-α-bromo-ε-caprolactone) as macroinitiator via ATRP

Abstract

α-Bromo-e-caprolactone (α-BrCL) was synthesized from α-bromocyclohexanone by using 3-chloroperoxybenzoic acid. α-BrCL was then used as a comonomer in the ring-opening polymerization of e-caprolactone (CL) initiated with aluminum isopropoxide to synthesize poly(CL-co-α-BrCL) copolymer. This copolymer was used as the macroinitiator in the atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAAm) for the synthesis of stimuli-responsive and biodegradable PCL-g-PNIPAAm copolymer. A core-shell type nano-structure was formed with a hydrophilic outer shell and a hydrophobic inner core from these copolymers, which exhibited a phase transition temperature around 31 °C. The copolymers were characterized by 1H-NMR and FT-IR spectroscopies. Number-average molecular weight of the poly(CL-co-α-BrCL) and PCL-g-PNIPAAm copolymers was calculated from corresponding 1H-NMR spectra to be 5770 and 6810 gmol−1, respectively. Thermal stability of the copolymer was investigated by thermogravimetric analysis (TGA) and crystallization behavior was studied by differential scanning calorimetry (DSC). Transmission electron microscopy (TEM) showed that the self-assemble micelle aggregates had well defined spherical shape. From the fluorescence spectra, fluorescence intensity of pyrene in the copolymer micelles increased and red-shifted as the copolymer concentration increases, indicating the formation of self-assemble polymeric micelles in water. The critical micelle concentration was found to be 3.2 × 10−3 mg/mL. TEM results showed that micelles have spherical shapes with a diameter of about 70 nm. The obtained micelles can be desirable for potential applications in biomedical fields, such as drug delivery systems.