Alternating and Random Copolymerization of Isoprene and Ethylene Catalyzed by Cationic Half-Sandwich Scandium Alkyls

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.