Polymeric (steric and electrosteric) stabilizers, based on water-soluble polymers such as poly(acrylic acid) and poly(ethylene oxide), are widely used to make surface coatings by emulsion polymerization and form a "hairy layer" which provides colloidal stability to the latex particles. It is shown by a combination of NMR and rate studies that the growth of these polymer colloids is dominated by a hitherto unsuspected combination of mechanisms: an abstraction reaction in the "hairy layer" which results in a radical on the water-soluble polymer which is slow to propagate but quick to terminate (Macromolecules 2006, 39, 6495-6504) and can also undergo beta-scission reactions. Termination with radicals which would otherwise lead to particle growth leads to radical loss, i.e., a slower polymerization rate, while beta-scission leads to radicals which can enter the water phase and cause secondary particle formation. Both of these effects are undesirable from a manufacturing point of view, and the newly discovered mechanism could be used to mitigate these undesirable features.