A modifier material and method for making the same is characterized by the addition of a partitioning agent to a polymer material such as synthetic rubber to prevent reagglomeration of the rubber. The partitioning agent reduces the time required to disperse the modifier material in a material to be modified and also reduces the viscosity of the modified material so that the modified material is easier to handle. The partitioning agent preferably includes a mixture of phenyl formaldehyde resin and precipitated silica gel.