The structure-reactivity relationship of the rare-earth metal aryl(alk)oxide-promoted coordination polymerization of isoprene was investigated using binary initiating systems Ln(OR)(3)(AlMe3)(x)/Et2AlCl (Ln = La, Nd, Y). Depending on the degree of the rare-earth metal aryl( alk) oxide prealkylation (x = 1, 2, 3), such discrete trimethylaluminum (TMA) adduct complexes of rare-earth metal alkoxide and aryloxide components displayed a distinct initiating capability. The heterobimetallic bis-TMA adducts Ln(OAriPr)(3)(AlMe3)(2) and tris-TMA adducts Ln(OCH(2)tBu)(3)(AlMe3)(3) (Ln = La, Nd) produced highly reactive initiators, whereas the mono-TMA adducts Ln(OArtBu)(3)(AlMe3) were catalytically inactive. The highest reactivities and stereoselectivities (> 99% cis) were obtained for a n(Ln):n(Cl) ratio of 1:2. The alkoxide-based tris-TMA adducts gave narrower molecular weight distributions than the aryloxide-based bis-TMA adduct complexes (M-w/M-n 1.74-2.37 vs 2.03-4.26). A plausible mechanistic activation/deactivation scenario of the formation of the catalytically active/inactive species is presented.