Stereoselective Polymerization of Styrene with Cationic Scandium Precursors Bearing Quinolyl Aniline Ligands

Abstract

The novel N-R-quinolinyl-8-amino ligands HL1−5 (R = 2,6-Me2C6H3 (HL1), 2,4,6-Me3C6H2 (HL2), 2,6-Et2C6H3 (HL3), 2,6-iPr2C6H3 (HL4), C6H5 (HL5)) reacted with Sc(CH2SiMe3)3(THF)2 to afford the well-defined complexes (L1−5)Sc(CH2SiMe3)2(THF) (1−5), which were fully characterized by NMR spectral and X-ray diffraction analyses. Complexes 1−3 combined with organoborates to establish binary systems that exhibited high activity for the polymerization of styrene, while 4 was less active and 5 was almost inert. The cationic complex [L1Sc(CH2SiMe3)(DME)2][B(C6F5)4] (6) was successfully isolated by treatment of 1 with [PhMe2NH][B(C6F5)4], and represents probably the structural model of the initiation active species. Remarkably, upon addition of aluminum trialkyls to the binary systems, distinguished improvement in catalytic performances was achieved, among which the ternary system 1/5AliBu3/[Ph3C][B(C6F5)4] displayed the highest activity (1.56 × 106 g mol−1 h−1) and syndioselectivity (r = 0.94) via a chain-end control mechanism governed by the concerted steric effect of the ligand and the aluminum alkyls. This represents the first non-cyclopentadienyl stabilized rare-earth metal based catalyst showing both high activity and specific selectivity for the polymerization of styrene, which might shed new light on designing more efficient precursors and further investigation of the mechanism for this polymerization.