A method for preparing a polydiene polymer, the method comprising: (i) preparing an active polymerization mixture by introducing conjugated diene monomer, and optionally monomer copolymerizable therewith, and a lanthanide-based catalyst system; (ii) adding a nitrile compound containing a protected amino group to the active polymerization mixture; and (iii) adding a co-functionalizing agent to the active polymerization mixture.