A method of preparing a multiarm polymer includes reacting a branched polyol with one or more functionalizing reagents to effect substitution of an ionizable functional group or a protected ionizable functional group, Y, to form a mixture comprising (i) unsubstituted branched polyol containing no Y groups (ii) a monosubstituted polyol comprising one Y group, and (iii) a multisubstituted polyol (e.g., a disubstituted polyol comprising two Y groups) followed by purifying the mixture to separate the monosubstituted polyol from other species Thereafter, a water-soluble and non-peptidic polymer segment is attached to the monosubstituted branched polyol at the site of at least one of the hydroxyl groups. The invention also provides purified monosubstituted branched polyols and multiarm polymers prepared by the method and polyol precursors for use in the method.
