A process for preparing a polydiene, the process comprising the steps of (i) providing conjugated diene monomer; (ii) charging a nickel-based catalyst system to the conjugated diene monomer; and (iii) charging a modulating Lewis base to the conjugated diene monomer, to thereby polymerize the conjugated diene monomer in the presence of the modulating Lewis base, where said step of charging a modulating Lewis base is separate from said step of charging a nickel-based catalyst, and where said steps of providing conjugated diene monomer, charging a nickel-based catalyst system and charging a Lewis base form a polymerization mixture that includes less than 20% by weight of organic solvent based on the total weight of the polymerization mixture.