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

摘要

The acid-base reactions between the scandium trialkyl complex Sc(CH_2SiMe_3)_3(THF)_2 and 1 equiv of Cp'-H afforded straightforwardly the corresponding mono(cyclopentadienyl)scandium dialkyl complexes Cp'Sc(CH_2SiMe_3)_2(THF) (Cp' = C_5H_5 (1), C_5MeH_4 (2), C_5Me_4H (3), C_5Me_5 (4), C_5Me_4SiMe_3 (5)) in 65-80% isolated yields. The analogous half-sandwich complexes having a heteroatom-containing side arm, (C_5Me_4R)Sc(CH_2SiMe_3)_2 (R = CH_2CH_2PPh_2 (6), C_6H_4OMe-o (7)), were obtained by the one-pot metathetical reactions of ScCI_3(THF)_3 with 1 equiv of the potassium salts of the ligands and 2 equiv of LiCH_2SiMe_3. The similar reactions of ScCI_3(THF)_3 with KC_5Me_4(C_6H_4NMe_2-o) and LiCH_2SiMe_3 gave a methylene-bridged binuclear complex [{C_5Me_4(o-C_6H_4N(Me)CH_2-μ}Sc(CH_2SiMe_3)_]2 (8). Complexes 1 -8 were fully characterized by ~1H, ~(13)C NMR, X-ray, and microelemental analyses. The reactions of 5 and 7 with 1 equiv of [PhMe_2NH][B(C_6F5)_4] in THF afforded quantitatively the structurally characterizable cationic monoalkyl complexes [(C_5Me_4SiMe_3)Sc(CH_2SiMe_3)(THF)_2][B(C_6F5)_4] (10) and [(C_5Me_4C_6H_4OMe-o)Sc(CH_2SiMe_3)(THF)_2][B(C_6F_5)_4] (11), respectively. In the presence of an activator such as [Ph_3C][B(C_6F_5)_4], [PhMe_2NH][B(C_6F_5)_4], or B(C_6F_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 frans-1,4-polyisoprene formation (60-79%), whereas the phosphine analogue 6 showed high c/s-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 c/s-1,4-microstructures (up to 90%), thus constituting the first example of c/s-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.