Location of a Hydrophobic Load in Poly(oligo(ethylene glycol) methyl ether methacrylate)s (PEGMAs) Dissolved in Water and Probed by Fluorescence

Abstract

Two series of pyrene-labeled poly(oligo(ethylene glycol) methyl ether methacrylate)s referred to as PyEG 5 −PEG n MA and PyC 4 −PEG n MA were prepared to probe the region surrounding the polymethacrylate backbone by using the fluorescence of the dye pyrene. PyEG 5 −PEG n MA and PyC 4 −PEG n MA were prepared by copolymerizing the EG n MA methacrylate monomers with penta(ethylene glycol) 1-pyrenemethyl ether methacrylate or 1-pyrenebutyl methacrylate, respectively. In organic solvents, the much longer 18 non-hydrogen atom linker connecting the pyrene moieties to the polymethacrylate backbone in the PyEG 5 −PEG n MA samples enabled the deployment of the pyrenyl labels into the solution. In water, however, an excited pyrene for PyEG 5 −PEG n MA was found to probe a same volume as for the PyC 4 −PEG n MA samples where a much shorter 6 nonhydrogen atom spacer connected pyrene to the backbone. Another surprising observation, considering that the hydrophobicity of pyrene induces strong pyrene aggregation for many pyrene-labeled water-soluble polymers (Py−WSPs) in water, was the little pyrene aggregation found for the PyEG 5 −PEG n MA and PyC 4 −PEG n MA samples in water. These effects could be related to the organic-like domain (OLD) generated by the oligo(ethylene glycol) side chains densely arranged around the polymethacrylate backbone of the polymeric bottlebrush (PBB). Additional fluorescence experiments conducted with the penta(ethylene glycol) 1pyrenemethyl ether derivative indicated that the cylindrical OLD surrounding the polymethacrylate backbone had a chemical composition similar to that of ethylene glycol. Binding of hydrophobic pyrene molecules to unlabeled PEG n MA bottlebrushes in water further supported the existence of the OLD. The demonstration, that PEG n MA samples form an OLD in water, which can host and protect hydrophobic cargoes like pyrene, should lead to the development of improved PEG n MA-based drug delivery systems.