the instant invention provides a process for substantially reducing the polymer cement viscosity or reducing the molecular weight distribution of the a conjugated diene polymer synthesized by using a charge of catalytically effective amounts of a rare earth catalyst and an organo magnesium component by adding an organo zinc component to the charge in an amount and under conditions sufficient to reduce the polymer cement viscosity . the invention also contemplates a catalyst compostion for substantially reducing the polymer cement viscosity of a polymer product as well the resultant polymer product .