Lactide polymerization activity of alkoxide, phenoxide, and amide derivatives of yttrium(III) arylamidinates

In quest of new, single-site catalysts for cyclic ester polymerizations, a series of mononuclear yttrium(III) complexes of N,N'-bis(trimethylsilyl)ben zamidinate ([L-TMS](-)) and hindered N,N'-bis-(2,6-dialkylaryl)toluamidinat es ([L-Et] aryl = Et2C6H3, and [L-iPr](-), aryl = (Pr2C6H3)-Pr-i) were synt hesized and characterized by X-ray diffraction: (L2Y)-Y-TMS(mu -Cl)(2)Li(TM EDA) (1), (L2Y)-Y-TMS((OC6H2Bu2Me)-Bu-t) (2), (L2Y)-Y-TMS(OC6H3Me2)(2)Li(TH F)(4) (3), (L2Y)-Y-TMS(mu -(OBu)-Bu-t)(2)Li(THF) (4), (LY)-Y-iPr[N(SiMe2H)( 2)](2)(THF) (5), (L2Y)-Y-Et(THF)(Cl)(mu -Cl)Li(THF)(3) (6), and (L2Y)-Y-Et[ N(SiMe2H)(2)] (7). Coordination numbers ranging from five to seven were obs erved, and they appeared to be controlled by the steric bulk of the support ing amidinate and alkoxide, phenoxide, or amide coligands. Complexes 2-5 an d 7 are active catalysts for the polymerization of D,L-lactide (e.g., with 2 and added benzyl alcohol, 1000 equiv of D,L-lactide were polymerized at r oom temperature in less than 1 h, with polydispersities less than 1.5). The neutral complexes 2, 5, and 7 were more effective than the anionic complex es 3 and 4. In addition, the presence of the more hindered amidinate ligand s [L-Et] and [L-iPr](-) On yttrium-amides slowed the polymerizations (7 < 5 < Y[N(SiMe2H)(2)](3)). (C) 2000 John Wiley & Sons, Inc.