3,4-Polymerization of Isoprene by Using NSN- and NPN-Ligated Rare Earth Metal Precursors: Switching of Stereo Selectivity and Mechanism

Abstract

The rare-earth metal complexes bearing NSN-bidentate β-diimidosulfonate ligands (RNSN dipp )Ln-(CH 2 SiMe 3 ) 2 (THF) n (R = Ph, Ln = Lu (1a), n = 1, Y (1b), n = 2, Sc (1c), n = 1; R = PhNMe 2 , Ln = Lu (1d), n = 1) were synthesized by treatment of the ion-pairs [Ln(CH 2 SiMe 3 ) 2 (THF) x ][BPh 4 ] with equimolar amount of the ligand lithium salts (RNSN dipp )Li(THF) 2 (NSN dipp = S(NC 6 H 4 i Pr 2 -2,6) 2 ). Addition reaction between lutetium tris(alkyl)s, Ln(Z) 3 (THF) n and NSN dipp gave the corresponding dialkyl complexes (ZNSN dipp )Lu(Z) 2 (THF) n (Z = CH 2 SiMe 3 , n = 1 (1e); Z = o-CH 2 C 6 H 4 NMe 2 , n = 0 (1f)). Deprotonation of β-imidophosphonamido ligands H−NPN dipp and H−NPN Et (NPN dipp = Ph 2 P(NC 6 H 3 i Pr 2 -2,6) 2 , NPN Et = PPh 2 (NC 6 H 3 i Pr 2 -2,6)(NC 6 H 4 -Et-2)) with Lu(CH 2 SiMe 3 ) 3 (THF) 2 yielded the corresponding dialkyl complexes (NPN dipp )Lu(CH 2 SiMe 3 ) 2 (THF) ( 2) and (NPN Et )Lu(CH 2 SiMe 3 ) 2 (THF) (3). All the complexes had been structurally well-defined, and 1a, 1b, 1e, 2, and 3 were further characterized by X-ray diffraction analysis where the almost planar NSN rare-earth metal unit is C s (or pseudo C s ) symmetry with the two alkyl groups arranging on both sides and a coordinated THF against it. Upon activation with [PhMe 2 NH][B(C 6 F 5 ) 4 ] and Al i Bu 3 , all these complexes exhibited high 3,4-regioselectivity (ranging from 91% to >99%) for the polymerization of isoprene. Moreover, the excellent isospecific selectivity up to mmmm > 99% have been achieved with complexes 1 depending on the electronics of the sulfur substituents to give crystalline polyisoprene with the highest T m (170 °C) reported to date. The NPN-bidentate β-imidophosphonamide ligated rare-earth metal complexes provide both high syndio-and iso-3,4-selectivities (3,4 > 99%, rr = 66%, mmmm = 96%) depending on the frameworks, steric environment and geometry of the ligands. The regio-and stereo-selective mechanisms proceeded in these systems were explicated by DFT simulation.

The behaviour of β-triketimine cobalt complexes in the polymerization of isoprene

“…33,64 The activities are broadly in line with most other Co complexes, once the superior effect of EASC over DEAC is taken into account, which is to say, in the range of hundreds to a few thousands of mol IP mol À1 catalyst h À1 . It becomes clear that catalysts 5-7 are moderately active; much more active than cationic 61 or anionic polymerizations, 62 more active that nickel salts, 20 more active than some lanthanum and lanthanide catalysts, 63 but less active than most.…”

“…Historically and currently, Ziegler-Natta catalysts have oen been used in the polymerization of 1,3-dienes; 9-11 however, molecular catalysts have also met with wide interest. It is now possible to obtain high trans-1,4 polymer, 16,28-30 high 3,4isotactic polymer [31][32][33] and high 3,4 atactic or syndiotactic polymer, 34 mostly with lanthanides, but it is also possible to obtain unique cis-1,4-alt-3,4 polyisoprene with cobalt phosphine complexes. 12,13 The key modes of enchainment of polyisoprenes are shown in Chart 1.…”

Iminoimidazole‐based Co(II) and Fe(II) complexes: Syntheses, characterization, and catalytic behaviors for isoprene polymerization

“…For example, CpTiCl 3 was used for cis ‐1,4 living polymerization of butadiene and its biphenolate derivatives for copolymerization of isoprene with styrene . Lanthanide complexes reported by Hou and Cui group had been successfully used for cis ‐1,4, trans ‐1,4, syndiotactic 3,4‐polymerization of isoprene . As Brookhart and Gibson reported their seminal work of α‐diimine‐Ni/‐Pd ligands and bis(imino)pyridine‐Co/Fe catalysts, researchers have revisited the first‐row of d‐block elements due to their long‐term availability .…”

Precisely Controlled Polymerization of Styrene and Conjugated Dienes by Group 3 Single‐Site Catalysts

“…Cui and co‐workers found that the complexes 113 Lu and 117 (Figure ), upon the activation with [PhMe 2 NH][B(C 6 F 5 ) 4 ] and Al i Bu 3 , provided PIP with high 3,4‐selectivities, but the isoselectivities were different . The complex 113 Lu gave a distinguished isospecific selectivity ( mm =96 %, mmmm =96 % in chlorobenzene at −30 °C), while the more bulky NPN‐ligated complex 117 gave moderate syndiotactic PIP ( rr =59 %, rrrr =32 %) under the same conditions.…”