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

Two series of pyrene-labeled poly(oligo(ethylene glycol) methyl ether methacrylate)s referred to as PyEG(5)-PEG n MA and PyC4-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 PyC4-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 PyC4-PEG n MA samples where a much shorter 6 non-hydrogen 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 PyC4-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) 1-pyrenemethyl 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.