Abstract
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(OCH2tBu)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 nLn:nCl ratio of 1:2. The alkoxide-based tris-TMA adducts gave narrower molecular weight distributions than the aryloxide-based bis-TMA adduct complexes (Mw/Mn = 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.
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