Modelling Resin Swelling in Mixed Solvents and Its Applications

The resin swelling of Merrifield and HypoGelTM200 resins in different volume/volume ratios of binary solvents was investigated. The results show that there is usually not a linear variation of resin swelling on changing the ratio of two solvents. High, medium and low swelling regions for each resin can be determined within the Hansen solubility parameter (HSP) 3D space model. If the line connecting two solvents goes through the high resin swelling region in HSP space, then there will be mixtures of the two solvents that will make the resin swell more than either pure solvent.
To illustrate the significance of this effect, binary mixtures of two green solvents were predicted which could replace the traditional organic solvents used in solid-phase peptide synthesis. The experimental results confirm that appropriate ratios of the solvent mixtures give better synthetic results than either pure solvent. In another application of this model, binary solvents were used to dissolve polystyrene. Generally, mixtures of two solvents at an appropriate ratio were found to more rapidly dissolve polystyrene than the parent solvents. Comparison of the resin swelling results obtained on the same resin backbone with different functional groups show that the functionality does not influence the magnitude of resin swelling. However, it could influence the optimised ratio of two solvents needed to optimally swell the resin. In addition, the resin swelling experiments demonstrate that some binary mixtures of two solvents have strong solvent-solvent interactions which prevent the mixed solvent from interacting with the resin matrix.