Provided is a collection of polymer beads, wherein the volume distribution expressed as a function of bead diameter comprises (A1) a first peak having a maximum of PM1 at diameter PD1 and having full width at half maximum of 75 μm or less, (A2) a second peak having a maximum of PM2 at diameter PD2 and having full width at half maximum of 75 μm or less; and (B1) a valley having a minimum value of VM1 at a diameter between PD1 and PD2, wherein VM1 is less than 0.25*PM1 and VM1 is also less than 0.25*PM2. wherein the polymer beads comprise polymerized units of one or more monofunctional vinyl monomer and one or more multifunctional vinyl monomer.