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Systematic Insights from Medicinal Chemistry To Discern the Nature of Polymer Hydrophobicity

Magenau, Andrew J. D., Richards, Jeffrey A., Pasquinelli, Melissa A., Savin, Daniel A., Mathers, Robert T.
Macromolecules 2015 v.48 no.19 pp. 7230-7236
alcohols, chemical structure, contact angle, drug design, hydrophilic polymers, hydrophobicity, octanol-water partition coefficients, physical properties, polymerization, prediction, screening, solvents, surface area
Predicting polymer hydrophobicity based on monomer structure is an ill-posed problem. Generally, the hydrophobicity of a polymer or a series of polymers has been determined through indirect methods (i.e., contact angle) after polymerization. This sequence presents a problem for the systematic design and rapid evaluation of specialty polymers synthesized via controlled polymerization methods. Here, we propose an approach inspired by medicinal chemistry to predict polymer hydrophobicity based on octanol–water partition coefficients (LogPₒcₜ) determined through simple computational approaches. We envisioned that LogPₒcₜ, analogous to what is used in drug design, could provide a rational methodology to translate molecular structures of monomers and oligomers into quantifiable hydrophobicity values for polymers. A combination of critical design criteria and the predictive power of LogPₒcₜ values, normalized by surface area (LogPₒcₜ/SA), accurately assess polymer hydrophobicity. Experimental corroboration with a polarity-sensitive dye (i.e., Nile Red), advancing water contact angles measurements, and swelling ratio experiments verify the method represents a dramatic improvement. A direct and quantitative correlation existed between spectral shifts of Nile Red and calculated LogPₒcₜ/SA values, confirming a quantifiable metric for predicting polymer hydrophobicity. Computationally predicted values also resulted in a first approximation of advancing contact angle measurements over a broad spectrum of common polymers providing a basis for estimating contact angles, a screening tool to enhanced monomer design a priori, and a criterion to understand polymer physical properties. Furthermore, swelling ratio measurements elucidated boundary limits for swelling of relatively hydrophobic and hydrophilic polymers in water and hexanes, in addition to alternative alcohol derivative solvents.