''Link-functionalized'' polymers: An unusual macromolecular architecture through bifunctional initiation

Bimetallic complexes of the type (C5Me5)(2)Sm-R-Sm(C5Me5)(2) were used for the living bis-initiated polymerization of methyl methacrylate and epsilon-caprolactone, giving polymers with discrete functionalities at the center of the backbone (''link-functionalized''). Poly(methyl methacrylate) containing olefin units was prepared from [(C5Me5)(2)Sm](2)(mu,eta(3)-CH2CHCH-)(2) and related allyl compounds, although slow and/or inefficient initiation of polymerization is observed for complexes in which R is a bulky, heteroatom, or coordinating group. In contrast, the polymerization of epsilon-caprolactone may be carried out with good control in the presence of both sterically hindering and electron-rich moieties. Polylactones incorporating aromatic, cumulene, and amine/imine groups were synthesized through one-step bimetallic initiation in this manner. Differences between lanthanide(III) methacrylate and lactone polymerizations, differences between lanthanide(III) and aluminum(III) lactone polymerizations, and the presence of transesterification in the lactone system are discussed.