Addition of aluminum or zinc alkyls (AlR3 or ZnR2, R = Me or Et) lowers the molar masses of norbornene-ethene (N/E) copolymers obtained with the catalyst systems Me2C(Ind)(Cp)ZrCl2/MAO or rac-C2H4(2-(BuMe2SiO)-Bu-t-Ind)(2)ZrCl2/MaO to different degrees. These chain-shortening effects and changes in relative end-group abundance indicate the degree to which E/N copolymers growing at a Zr catalyst center are transferred to Al or Zn centers by alkyl-polymeryl exchange. Increased molar masses of copolymers obtained in the presence of added Al(Bu-i)(3) indicate that this reagent suppresses polymer chain exchange between Zr catalyst centers and methylaluminum species contained in the MAO activator. These observations are interpreted in terms of reversible adduct formation between Al or Zn alkyls and cationic zirconocene alkyl species.