A polymer latex suitable for use in gypsum wallboard or other applications. The polymer latex is a styrene butadiene latex that is substantially stable against divalent ions. The polymer latex includes styrene, butadiene, and an ionic monomer comprised of a 2-acrylamido-2-methyl propanesulfonic acid salt. The polymer latex may also include hydrophilic adjunct comonomers. The latex includes at least 0.25 wt. % of the ionic monomer.