A polymer comprising a first fraction and a second fraction, wherein the first fraction comprises from 90 to 100 wt % polymerized C3-C6 carboxylic acid monomer units; and the second fraction comprises from 30 to 80 wt % polymerized C3-C6 carboxylic acid monomer units and from 20 to 70 wt % polymerized sulfonic acid monomer units; wherein the first fraction is from 10 to 40 wt % of the polymer and the second fraction is from 60 to 90 wt % of the polymer; the polymer has Mw from 3,000 to 30,000; and monomers are randomly distributed within each fraction.