Polyethyleneglycols (PEGs) have two terminal hydroxy groups separated by a long distance. It is thus difficult to selectively modify one of these two groups as the transformation of one group has no influence on the reactivity of the second one. The iteroselective mono-tosylation of PEGs was however successfully achieved in presence of silver(I) oxide particles and potassium iodide (Figure ). The iteroselectivity was rationalized by an activation of one of the two terminal OH groups through its chemisorption on the surface of the silver particles, the other group remaining inactivated due to the entropically unfavorable backfolding of the PEG chain. For 1.0 equiv. of tosyl chloride, the authors obtained a ratio of starting, mono-tosyl, and di-tosyl PEGs (i = 0, 1, 2) of 179:733:88 (PEG-1500) and 158:762:79 (PEG-2000). The normal distribution for these iteromers under these conditions is 318:364:318 (see Table the Supporting Information). Accordingly, the iteromeric excess ite the mono-tosylated products are 58% (PEG-1500) and 63% (PEG-2000).