Thermal characterization of poly(ethylene glycol)-poly(D,L-lactide) block copolymer micelles based on pyrene excimer formation

Thermal properties and physicochemical behavior in aqueous solution of pyrene-labeled poly(ethylene glycol)-polylactide conjugate

A fluorescence-labeled bioresorbable polymer was prepared by a coupling reaction of poly(ethylene glycol)-polylactide (PEG-PLA) with carboxyl pyrene, using N,N’-diisopropylcarbodiimide/1-hydroxy-7-azabenzotriazole (DIC/HOAt) as a coupling agent and 4-dimethylaminopyridine (DMAP) as a catalyst. The obtained copolymer, termed PEG-PLA-pyrene, was characterized using various analytical techniques, such as gel permeation chromatography (GPC), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), proton nuclear magnetic resonance (¹H-NMR), infrared spectroscopy (IR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA), to identify the molecular structure and to monitor the thermal property changes before and after the reaction. The presence of a pyrene moiety at the end of polylactide (PLA) did not alter the crystallization ability of the poly(ethylene glycol) (PEG) blocks, indicating that the conjugate preserved the inherent thermal properties of PEG-PLA. However, the presence of PEG-PLA blocks strongly reduced the melting of pyrene, indicating that the thermal characteristics were sensitive to PEG-PLA incorporation. Regarding the physicochemical behavior in aqueous solution, a higher concentration of PEG-PLA-pyrene resulted in a higher ultraviolet-visible (UV-vis) absorbance and fluorescence emission intensity. This is of great interest for the use of this conjugate as a fluorescence probe to study the in vivo distribution as well as the internalization and intracellular localization of polymeric micelles.