Stereoselective Synthesis of High Trans-1,4-Regularity Polyisoprene via Controlled Polymerization with Amidinate...

Synthetic trans-1,4-polyisoprene is a crystalline thermoplastic with wide applications, but catalysts enable one to achieve high-selectivity and living polymerization remain scarce. In this work, two novel nonsymmetric amidine proligands (2,6-Me₂C₆H₃)NC(Ph)NH(C₆H₄-2-OMe) (HL¹) and (2,6-Me₂C₆H₃)NC(Ph)NH(CH₂C₆H₄-2-OMe) (HL²) were prepared. Amine-elimination reactions between HL¹/HL² and one equivalent of Ln[N(SiHMe₂)₂]₃(THF)_x afforded the amidinate rare-earth metal bis(silylamide) complexes [(2,6-Me₂C₆H₃)NC(C₆H₅)N(C₆H₄-2-OMe)]Ln[N(SiHMe₂)₂]₂(THF)_n (Ln = Sc, n = 0 (1); Ln = Y, n = 1 (2); Ln = Nd, n = 1 (3); Ln = Sm, n = 1 (4)) and [(2,6-Me₂C₆H₃)NC(C₆H₅)N(CH₂C₆H₄-2-OMe)]Sc[N(SiHMe₂)₂]₂ (5). On activation of [Ph₃C][B(C₆F₅)₄] in the presence of excess AlMe₃, all complexes were active for isoprene polymerization. Notably, the catalytic system of 4/[Ph₃C][B(C₆F₅)₄]/AlMe₃ achieved good control over the polymerization process, giving poly(isoprene) > 99% trans-1,4-selectivity. This catalytic system was able to prepare diblock copolymers comprising highly trans-1,4-poly(isoprene) and trans-1,4-poly(myrcene) sequences. The heterometallic Ln-Al methyl complexes like [(2,6-Me₂C₆H₃)NC(C₆H₅)N(C₆H₄-2-OMe)]Sm[(μ-Me)₂AlMe₂]₂ (6) were experimentally verified as precatalyst for this cationic polymerization. These findings advance the rational design of regioselective polymerization catalysts for constructing tailored polyisoprene-based materials, offering promising prospects for both academic and industrial exploration.