Improved methods for preparing sulfonated block polymers with acyl sulfates in non-halogenated aliphatic solvents are provided. The methods include the sulfonation of a precursor block polymer with an acyl sulfate in a reaction mixture further comprising at least one non-halogenated aliphatic solvent to form a sulfonated block polymer, wherein the initial concentration of the precursor block polymer is in the range of from about 0.1 wt % to a concentration that is below the limiting concentration of the precursor block polymer based on the total weight of the reaction mixture. A sulfonation degree of greater than about 1.0 milliequivalent sulfonic acid per gram sulfonated block polymer can be achieved substantially free of polymer precipitation and free of disabling gelation.