It has been discovered that certain compounds serve as particle size regulators when employed together with peroxide initiators for the polymerization of vinylaromatic monomers in the presence of diene polymers. Suitable compounds that can control particle size include, but are not necessarily limited to, cobalt naphthenate, zinc naphthenate, iron octoate, calcium octoate, zinc octoate, zinc neodecanoate, potassium octoate, copper naphthenate, calcium versalate, and the like. Use of these metal carboxylates when vinylaromatic monomers are polymerized in the presence of polybutadienes and peroxide initiators results in reduced polybutadiene particles. Grafting and morphology characteristics may also be improved.