Lanthanide borohydrido complexes for MMA polymerization: syndio‐ vs iso‐ stereocontrol

This paper presents an extensive study of the polymerization of MMA with borohydrido lanthanide complexes for the first time. Catalytic systems are made from a lanthanide derivative bearing zero one, or two bulky ligands: substituted cyclopentadienyl (Cp^*′ = C₅Me₄nPr, Cp⁴ⁱ = C₅HiPr₄, Cp^Ph3 = H₂C₅Ph₃‐1,2,4), and/or diketiminate ([(p‐tol)NN] = [(p‐CH₃C₆H₄)N(CH₃)C]₂CH), in the presence of variable quantities of alkylating agent. With BuLi in apolar medium, highly isotactic polymer (up to 95.6%) is formed. In THF, syndiotactic‐rich PMMA is obtained whatever the nature of the co‐catalyst (BuLi or MgⁿBu₂). The presence of an electron‐withdrawing ligand such as Cp^Ph3 allows high syndioregularity, up to 81.8% at 0 °C, together with the highest conversion. There is quite good concordance between calculated and experimental molecular data in THF. Divalent Cp^*′₂Sm^II(THF) and (Cp^Ph3)₂Sm^II(THF) are active as single‐component initiators; the former affords PMMA 88% syndiotactic at 0 °C. Copyright © 2005 John Wiley & Sons, Ltd.