Dinuclear Pentacoordinated Organoyttrium Biphenolates as Initiators for the Synthesis of High-Molecular Weight Isotactic Poly(2-vinylpyridine)

Dinuclear organoyttrium biphenolato complexes have been synthesized for the first time starting from Y­(CH₂SiMe₃)₃(THF)₂ and readily available biphenols with bulky substituents at the 3,3′-positions. XRD analysis revealed that these complexes are formed as homochiral dimers even though racemic biphenols were employed. Such dinuclear penta-coordinated yttrium complexes were evaluated as initiators in the polymerization of 2-vinylpyridine (2-VP). High activity and excellent isotacticity (mm > 99%) were observed. DSC analysis of poly­(2-VP) showed a T_m = 221 °C, which is the highest value reported to date. Attempts to improve the low initiation efficiency of the present system were made by co-use with several organo-main-group metal reagents. Specifically, good molecular weight control within the high M_n region was achieved by the addition of Et₃Al (Al/Y = 10).