Previously, we reported that some types of olefin polymers and their small-molecule models were unexpectedly resistant to sulfonation in hydrocarbon solvents. This substantial reactivity difference between chlorinated and hydrocarbon solvents was attributed to the solubility characteristics of the sulfonating agent. Aromatic polymers such as polystyrene are inherently less reactive than olefin polymers and are even more resistant to sulfonation in hydrocarbon solution. We postulated that hydrocarbon-soluble acyl sulfates, unlike acetyl sulfate, would be efficient and rapid reagents for the sulfonation of polymers that are unreactive, and this has been substantiated. Even small improvements in hydrocarbon solubility were found to be beneficial, and completely homogeneous sulfonation was possible with longer chain acyl sulfates. As a result, polystyrene was sulfonated effectively in cyclohexane solution. Long-chain phosphate ester-sulfur trioxide complexes were also soluble in cyclohexane and are suitable reagents for completely homogeneous sulfonation.
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