Reaction of the arylimine-amidinate ligand[NNN]H (1) ([NNN]=[2-C(H)NDippC₆H₃NHC(Ph)NDipp], Dipp=2,6-i-Pr₂C₆H₃) with Ln(CH₂SiMe₃)₃(THF)₂ (Ln=Y, Sm) in n-hexane at -78℃ followed by stirring the mixture for 3 h at room temperature afforded[NNN]Ln(CH₂SiMe₃)₂ (Ln=Y, 2; Ln=Sm, 3) in 75% and 50% yields after work up. 2 and 3 have been characterized by ¹H NMR, ¹³C NMR, IR spectroscopy, elemental analysis. The molecular structure of 2 has been confirmed by X-ray analysis. Complexes 2 and 3 catalyzed the polymerization of isoprene in the presence of[Ph₃C][B(C₆F₅)₄] (1 equiv.) and AlEt₃ (10 equiv.) at 25℃. 2 converted 250 equivalents of isoprene into polyisoprene with a narrow molecular weight distribution (M_w/M_n=1.20) and a 3,4-rich microstructure (3,4-selectivity: 78%, rr=50%) in 12 h. A significant increase of 3,4-selectivity was observed in the presence of one equivalent of AlEt₃ (3,4-selectivity: 87%). When i-Bu₂AlH (10 equiv.) was used instead of AlEt₃, the catalytic activity and selectivity did not change noticeably while the resulting polyisoprenes feature a broad molecular weight distribution (M_w/M_n=2.45). When the amounts of i-Bu₂AlH decreased from 5 to 1 equivalent, a significant increase of 3,4-selectivity and number-average molecular weight have been observed while the molecular distributions became small. The catalyst is active within a wide range of temperatures from low temperature to 40℃. At 40℃, a similar selectivity (3,4-selectivity: 78%, M_w/M_n=1.43) to that found at 25℃ but the increased reaction rate has been observed. High stereo- and regioselectivity and narrow molecular weight distributions were obtained at -20℃ (3,4-selectivity: 88%, rr=52%, M_w/M_n=1.15). The samarium alkyl 3 exhibited a high activity but low selectivity (25℃, 3 h, 3,4-selectivity: 67%, M_w/M_n=1.71) compared to the yttrium alkyl 2.

Key words: rare-earth metal, alkyl complex, arylimine-amidine, isoprene polymerization, 3,4-selectivity