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Li X, Nishiura M, Hu L, Mori K, Hou Z: Alternating and random copolymerization of isoprene and ethylene catalyzed by cationic half-sandwich scandium alkyls. J Am Chem Soc. 2009 Sep 30;131(38):13870-82.
The acid-base reactions between the scandium trialkyl complex Sc (CH (2) SiMe (3))(3)(THF)(2) and 1 equiv of Cp'-H afforded straightforwardly the corresponding mono (cyclopentadienyl) scandium dialkyl complexes Cp'Sc (CH (2) SiMe (3))(2)(THF) (Cp' = C (5) H (5) (1), C (5) MeH (4) (2), C (5) Me (4) H (3), C (5) Me (5) (4), C (5) Me (4) SiMe (3) (5)) in 65-80% isolated yields. The analogous half-sandwich complexes having a heteroatom-containing side arm, (C (5) Me (4) R) Sc (CH (2) SiMe (3))(2) (R = CH (2) CH (2) PPh (2) (6), C (6) H (4) OMe-omicron (7)), were obtained by the one-pot metathetical reactions of ScCl (3)(THF)(3) with 1 equiv of the potassium salts of the ligands and 2 equiv of LiCH (2) SiMe (3). The similar reactions of ScCl (3)(THF)(3) with KC (5) Me (4)(C (6) H (4) NMe (2)-omicron) and LiCH (2) SiMe (3) gave a methylene-bridged binuclear complex [{C (5) Me (4)(omicron-C (6) H (4) N (Me) CH (2)-mu}Sc (CH (2) SiMe (3))](2) (8). Complexes 1-8 were fully characterized by (1) H, (13) C NMR, X-ray, and microelemental analyses. The reactions of 5 and 7 with 1 equiv of [PhMe (2) NH][B (C (6) F (5))(4)] in THF afforded quantitatively the structurally characterizable cationic monoalkyl complexes [(C (5) Me (4) SiMe (3)) Sc (CH (2) SiMe (3))(THF)(2)][B (C (6) F (5))(4)] (10) and [(C (5) Me (4) C (6) H (4) OMe-omicron) Sc (CH (2) SiMe (3))(THF)(2)][B (C (6) F (5))(4)] (11), respectively. In the presence of an activator such as [Ph (3) C][B (C (6) F (5))(4)], [PhMe (2) NH][B (C (6) F (5))(4)], or B (C (6) F (5))(3), all of the half-sandwich dialkyl complexes 1-7 were active for isoprene polymerization and isoprene-ethylene copolymerization, with the activity and selectivity being significantly dependent on the substituents at the cyclopentadienyl ligands to yield the corresponding homo- and copolymer materials with different microstructures and compositions. In the homopolymerization of isoprene, the less sterically demanding complexes 1 and 2 showed high cis-1,4 selectivity (up to 95%), whereas the more sterically demanding complexes 3-5 yielded 3,4-polyisoprene (51-65%) as a major product. The ether side arm coordinated complex 7 preferred trans-1,4-polyisoprene formation (60-79%), whereas the phosphine analogue 6 showed high cis-1,4 selectivity (84-90%) under the same conditions. In the copolymerization of isoprene and ethylene, complexes 1 and 2 afforded the random copolymers with high isoprene contents (85-92 mol %) and predominant cis-1,4-microstructures (up to 90%), thus constituting the first example of cis-1,4-selective copolymerization of isoprene with ethylene. In contrast, the copolymerization of isoprene and ethylene by 3, 4, 6, and 7 gave, for the first time, almost perfect alternating isoprene-ethylene copolymers. Possible mechanisms of the polymerization and copolymerization processes were proposed on the basis of the DFT calculations. |
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