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(CH2SiMe3)3(THF)2 and 1 equiv of Cp′−H afforded straightforwardly the corresponding mono(cyclopentadienyl)scandium dialkyl complexes Cp′Sc(CH2SiMe3)2(THF) (Cp′ = C5H5 (1), C5MeH4 (2), C5Me4H (3), C5Me5 (4), C5Me4SiMe3 (5)) in 65−80% isolated yields. The analogous half-sandwich complexes having a heteroatom-containing side arm, (C5Me4R)Sc(CH2SiMe3)2 (R = CH2CH2PPh2 (6), C6H4OMe-ο (7)), were obtained by the one-pot metathetical reactions of ScCl3(THF)3 with 1 equiv of the potassium salts of the ligands and 2 equiv of LiCH2SiMe3. The similar reactions of ScCl3(THF)3 with KC5Me4(C6H4NMe2-ο) and LiCH2SiMe3 gave a methylene-bridged binuclear complex [{C5Me4(ο-C6H4N(Me)CH2-μ}Sc(CH2SiMe3)]2 (8). Complexes 1−8 were fully characterized by 1H, 13C NMR, X-ray, and microelemental analyses. The reactions of 5 and 7 with 1 equiv of [PhMe2NH][B(C6F5)4] in THF afforded quantitatively the structurally characterizable cationic monoalkyl complexes [(C5Me4SiMe3)Sc(CH2SiMe3)(THF)2][B(C6F5)4] (10) and [(C5Me4C6H4OMe-ο)Sc(CH2SiMe3)(THF)2][B(C6F5)4] (11), respectively. In the presence of an activator such as [Ph3C][B(C6F5)4], [PhMe2NH][B(C6F5)4], or B(C6F5)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.