Organometallics of rare earths such as (C
5Me
5)
2LnR (Ln=Y, Sm, Yb, Lu; R =H, Me) initiated the living polymerizations of alkyl methacrylates and alkyl acrylates for the first time to afford high molecular weight polymers (
Mn>500, 000) with extremely narrow molecular weight distribution (
Mw/
Mn<1.05). The 1 : 2 reaction of SmH (C
5Me
5)
2 with MMA provides a single crystal and its X-ray analysis revealed the formation of an 8-membered ring complex where one end is bound with enolate form and the other end by the coordination of ester group of MMA. Based on these findings we propose coordination anionic polymerization mechanism for this reaction. SmMe(C
5Me
5)
2(THF) and YOMe(C
5Me
5)
2(THF) were also able to conduct the living polymerizations of β-propiolactone, δ-valerolactone and ε-caprolactone. Organo rare earth metal (II) complex with bulky auxiliary ligand, Me
2Si(2-Me
3Si-4-
tBu-C
5H
2)
2Sm, as well as rare earth metal (III) complexes, Me
2Si[2, 4-(Me
3Si)
2C
5H
2]
2LnCH(SiMe
3)
2(Ln_=La, Sm), were found to conduct the polymerization of ethylene to give high moleculr weight polymers with narrow molecular weigt distribution.
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