Scandium Half-Metallocene-Catalyzed Syndiospecific Styrene Polymerization and Styrene−Ethylene Copolymerization: Unprecedented Incorporation of Syndiotactic Styrene−Styrene Sequences in Styrene−Ethylene Copolymers

On treatment with 1 equiv of [Ph3C][B(C6F5)4], the scandium half-sandwich bis(alkyl) complex (C5Me4SiMe3)Sc(CH2SiMe3)2(THF) showed extremely high activity (up to 1.36 × 104 kg of sPS/(mol Sc·h)) and syndiospecificity (rrrr > 99%) for the polymerization of styrene at room temperature in toluene. More remarkably, this catalyst system could also effect the syndiospecific copolymerization of styrene with ethylene to yield styrene−ethylene copolymers having syndiotactic styrene−styrene sequences. The styrene content in the copolymers could be easily controlled by changing the styrene feed and could reach higher than 80 mol %. This is the first example of formation of such types of styrene−ethylene copolymers, which are expected to show novel properties

Scandium-Catalyzed Syndiospecific Copolymerization of Styrene with Isoprene

Scandium-Catalyzed Syndiospecific Copolymerization of Styrene with Isopren

[(SiMe₃)₂NC(N<i>i</i>Pr)₂]₂Ln(μ-Me)₂Li(TMEDA) (Ln = Nd, Yb) as Effective Single-Component Initiators for Styrene Polymerization

[(SiMe3)2NC(NiPr)2]2Ln(μ-Me)2Li(TMEDA) (Ln = Nd, Yb) as Effective Single-Component Initiators for Styrene Polymerizatio

QM/MM Studies on Scandium-Catalyzed Syndiospecific Copolymerization of Styrene and Ethylene

The copolymerization of styrene and ethylene by the cationic half-sandwich scandium alkyl species (η5-C5Me5)Sc(CH2SiMe3) has been computationally investigated by using the quantum mechanics/molecular mechanics (QM/MM) method. It has been found that the initiation of styrene polymerization both kinetically and energetically prefers 2,1-insertion (secondary insertion, free-energy barrier of 12.6 kcal/mol, and exergonic by 19.1 kcal/mol) to 1,2-insertion (primary insertion, free-energy barrier of 19.0 kcal/mol, and exergonic by 8.9 kcal/mol). This is in contrast to a titanocene-based catalyst system, in which the initiation of styrene polymerization was computationally found to prefer 1,2-insertion, while the subsequent styrene insertion (polymerization) proceeds in a 2,1-insertion pattern. In the current Sc-based catalyst system, although the insertion of styrene into the metal–alkyl bond of the active species is kinetically slower than that of ethylene, the formation of a styrene π-complex is more favorable than that of an ethylene complex. Also, the insertion of styrene into an ethylene-preinserted species is more energetically favorable than continuous ethylene insertion into the ethylene-preinserted species. These thermodynamic factors could add to a better understanding of styrene–ethylene copolymerization. The thermodynamic preference for the insertion of styrene rather than that of ethylene into the active species with an ethylene end group was not reported for group 4 catalyst systems. It is also found that the syndiospecific selectivity is inherently determined by the substituent of the ancillary ligand η5-C5Me5

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

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

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

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

Syndiospecific Copolymerization of Styrene with <i>para</i>-Methoxystyrene Catalyzed by Functionalized Fluorenyl-Ligated Rare-Earth Metal Complexes

The development of efficient catalysts for the copolymerization of nonpolar monomers with polar monomers remains a great challenging task in polymer synthesis. A one-pot reaction of anhydrous LnCl3 with pyridyl-methylene-functionalized octamethylfluorenyl lithium OctFlu-CH2PyLi in a 1:1 molar ratio, followed by alkylation with 2 equiv of LiCH2SiMe3 in THF afforded the fluorenyl-ligated rare-earth metal bis­(alkyl) complexes (OctFlu-CH2Py)­Ln­(CH2SiMe3)2(THF) [Ln = Sc (1), Y (2)]. Both complexes were isolated as neutral species and were characterized by NMR spectrum and elemental analysis. Complex 2 was subjected to single-crystal X-ray diffraction, which showed that the whole modified fluorenyl ligand was coordinated to Y3+ in the η5/κ1 mode to form a constrained geometry configuration. In the presence of excess AliBu3, and on activation with 1 equiv of [Ph3C]­[B­(C6F5)4] in toluene, complexes 1 and 2 became active for both styrene (St) and para-methoxystyrene (pMOS) polymerization, giving polymers with high syndiotacticity (rrrr > 99%) without solvent extraction. Moreover, the ternary catalyst system composed of complex 2/AliBu3/[Ph3C]­[B­(C6F5)4] was highly effective for the syndiospecific copolymerization of styrene with pMOS, producing random copolymers with high molecular weights and narrow molecular weight distributions. The contents of pMOS in the copolymers could be easily tuned in a wide range (11–93 mol %) by simply changing the pMOS-to-St feed ratios

Rare-Earth Metal Complexes Supported by A Tridentate Amidinate Ligand: Synthesis, Characterization, and Catalytic Comparison in Isoprene Polymerization

To systematically investigate the dependence of the initiating group and metal size on polymerization performance, a family of rare-earth metal bis(alkyl)/bis(benzyl)/bis(amide) complexes supported by a monoanionic tridentate amidinate ligand [(2,6-iPr2C6H3)NC(Ph)N(C6H4-2-OMe]− (HL) were synthesized and well-characterized. Treatment of rare-earth metal tris(alkyl)/tris(benzyl)/tris(amide) complexes Y(CH2C6H4NMe2-o)3 or Y(CH2SiMe3)3(THF)2 or Ln[N(SiHMe2)2]3(THF)x (Ln = Sc, x = 1; Ln = Y, La, Sm, Lu, x = 2) with 1 equiv of HL gave the corresponding mono(amidinate) rare-earth metal bis(alkyl)/bis(benzyl)/bis(amide) complexes [(2,6-iPr2C6H4)NC(Ph)N(C6H4-2-OMe)]Y(CH2C6H4NMe2-o)2 (1), [(2,6-iPr2C6H4)NC(Ph)N(C6H4-2-OMe)]Y(CH2SiMe3)2(THF) (2), and [(2,6-iPr2C6H4)NC(Ph)N(C6H4-2-OMe)]Ln[N(SiHMe2)2]2(THF)n (Ln = Y, n = 1 (3); Ln = La, n = 1 (4); Ln = Sc, n = 0 (5); Ln = Lu, n = 0 (6); Ln = Sm, n = 0 (7)) in good isolated yields. These complexes were characterized by elemental analysis, NMR spectroscopy, and single-crystal X-ray diffraction. In the presence of excess AlMe3 and on treatment with 1 equiv of [Ph3C][B(C6F5)4], these complexes could serve as precatalysts for cationic polymerization of isoprene, in which the dependence of the polymerization activity and regioselectivity on the initiating group and metal size was observed

Unusual Si–H Bond Activation and Formation of Cationic Scandium Amide Complexes from a Mono(amidinate)-Ligated Scandium Bis(silylamide) Complex and Their Performance in Isoprene Polymerization

Amine elimination of scandium tris­(silylamide) complex Sc­[N­(SiHMe₂)₂]₃(THF) with 1 equiv of the amidine [PhC­(N-2,6-^<i>i</i>Pr₂C₆H₃)₂]H in toluene afforded the neutral mono­(amidinate) scandium bis­(silylamide) complex [PhC­(N-2,6-^<i>i</i>Pr₂C₆H₃)₂]­Sc­[N­(SiHMe₂)₂]₂ (<b>1</b>) in 93% isolated yield. When <b>1</b> was activated with 1 equiv of [Ph₃C]­[B­(C₆F₅)₄] in the presence of THF, the unexpected cationic amidinate scandium amide complex [{PhC­(N-2,6-^<i>i</i>Pr₂C₆H₃)₂}­ScN­{SiHMe₂}­{SiMe₂N­(SiHMe₂)₂}­(THF)₂]­[B­(C₆F₅)₄] (<b>2</b>) was generated. Treatment of <b>1</b> with excess AlMe₃ gave the Sc/Al heterometallic methyl complex [PhC­(N-2,6-^<i>i</i>Pr₂C₆H₃)₂]­Sc­[(μ-Me)₂AlMe₂]₂ (<b>3</b>). All these complexes were well-characterized by elemental analysis, NMR spectroscopy, and X-ray crystallography. The combination <b>1</b>/[Ph₃C]­[B­(C₆F₅)₄] in toluene showed activity toward isoprene polymerization. Addition of excess AlMe₃ to the <b>1</b>/[Ph₃C]­[B­(C₆F₅)₄] catalyst system switched the regioselectivity of isoprene polymerization from 3,4-specific to cis-1,4-selective

Unusual Si–H Bond Activation and Formation of Cationic Scandium Amide Complexes from a Mono(amidinate)-Ligated Scandium Bis(silylamide) Complex and Their Performance in Isoprene Polymerization

Amine elimination of scandium tris­(silylamide) complex Sc­[N­(SiHMe₂)₂]₃(THF) with 1 equiv of the amidine [PhC­(N-2,6-^<i>i</i>Pr₂C₆H₃)₂]H in toluene afforded the neutral mono­(amidinate) scandium bis­(silylamide) complex [PhC­(N-2,6-^<i>i</i>Pr₂C₆H₃)₂]­Sc­[N­(SiHMe₂)₂]₂ (<b>1</b>) in 93% isolated yield. When <b>1</b> was activated with 1 equiv of [Ph₃C]­[B­(C₆F₅)₄] in the presence of THF, the unexpected cationic amidinate scandium amide complex [{PhC­(N-2,6-^<i>i</i>Pr₂C₆H₃)₂}­ScN­{SiHMe₂}­{SiMe₂N­(SiHMe₂)₂}­(THF)₂]­[B­(C₆F₅)₄] (<b>2</b>) was generated. Treatment of <b>1</b> with excess AlMe₃ gave the Sc/Al heterometallic methyl complex [PhC­(N-2,6-^<i>i</i>Pr₂C₆H₃)₂]­Sc­[(μ-Me)₂AlMe₂]₂ (<b>3</b>). All these complexes were well-characterized by elemental analysis, NMR spectroscopy, and X-ray crystallography. The combination <b>1</b>/[Ph₃C]­[B­(C₆F₅)₄] in toluene showed activity toward isoprene polymerization. Addition of excess AlMe₃ to the <b>1</b>/[Ph₃C]­[B­(C₆F₅)₄] catalyst system switched the regioselectivity of isoprene polymerization from 3,4-specific to cis-1,4-selective