Discrete Lanthanide Aryl(alk)oxide Trimethylaluminum Adducts as Isoprene Polymerization Catalysts

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 (AlMe 3 ) x /Et 2 AlCl (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(OAr i Pr ) 3 (AlMe 3 ) 2 and tris-TMA adducts Ln(OCH 2 t Bu) 3 (AlMe 3 ) 3 (Ln = La, Nd) produced highly reactive initiators, whereas the mono-TMA adducts Ln(OAr t Bu ) 3 (AlMe 3 ) 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.