Statistically Syndioselective Coordination (Co)polymerization of 4‑Methylthiostyrene

The homopolymerization of a polar monomer, 4-methylthiostyrene (MTS), was successfully achieved by a rare-earth metal based catalyst in the highest activity of 45.1 × 10⁴ g mol_Y^–1 h^–1 and the excellent syndioselectivity (<i>rrrr</i> > 99%). The polymerization was rather controllable that the resultant poly­(methyl­thiostyrene)­s (PMTS) had molecular weights comparable to the theoretic ones reaching up to 1.7 × 10⁵ while the molecular weight distributions were narrow (PDI = 1.3–1.9). Moreover, the copolymerization of this polar MTS with the nonpolar styrene (St) performed fluently under various MTS-to-St ratios in a quasi-living mode. The monomer reactivity ratios were <i>r</i>_MTS = 1.08 and <i>r</i>_St = 0.77, following the first Markov statistics, and was close to the ideal random copolymerization. Therefore, a series of unprecedented statistical random copolymers, P­(St-<i>r</i>-MTS)­s, where the compositions were strictly closed to the monomer fed ratios, had been accessed. Strikingly, both monomer sequences remained highly syndiotactic as their homopolymers regardless of the compositions, thus endowing P­(St-<i>r</i>-MTS)­s variable glass transition temperatures and melting points. The shortest number-averaged sequence length for these copolymers P­(St-<i>r</i>-MTS) crystallizing from the melts was <i>n̅</i>_St = 5.75 for PS sequences and <i>n̅</i>_MTS = 8.11 for PMTS

NNN-Tridentate Pyrrolyl Rare-Earth Metal Complexes: Structure and Catalysis on Specific Selective Living Polymerization of Isoprene

The acid–base reactions of NNN-tridentate pyrrolyl ligands (HL¹: 2,5-bis­((pyrrolidin-1-yl)­methylene)-1<i>H</i>-pyrrole; HL²: 2,5-bis­((piperidino)­methylene)-1<i>H</i>-pyrrole) with rare-earth metal tris­(alkyl)­s, Ln­(CH₂SiMe₃)₃(THF)₂, afforded the corresponding bis­(alkyl) complexes L¹Ln­(CH₂SiMe₃)₂(THF)_<i>x</i> (Ln = Sc, <i>x</i> = 0 (<b>1a</b>); Ln = Y, <i>x</i> = 1 (<b>1b</b>); Ln = Lu, <i>x</i> = 1 (<b>1c</b>)), L²Sc­(CH₂SiMe₃)₂ (<b>2a</b>), and L²₂Ln₂(CH₂SiMe₃)₄ (Ln = Y (<b>2b</b>); Lu (<b>2c</b>)) in moderate to high yields. X-ray diffraction analysis revealed that the scandium complexes <b>1a</b> and <b>2a</b> are THF solvent-free monomers where the ligands coordinate to the Sc³⁺ ion in a κ¹:κ² mode, while the yttrium and lutetium complexes <b>1b</b> and <b>1c</b> have the same ligand coordination geometry to that of the scandium complex but are one-THF solvates; complex <b>2b</b>, however, is a dimer bridged by two anionic L² fragments that coordinate to the two yttrium ions in mixed η⁵:η⁵/κ¹:κ¹ coordination modes. Upon activation with an organoborate, all these complexes initiated the controlled polymerization of isoprene. In general, complexes <b>2a</b>–<b>c</b>, bearing ligand L², exhibited higher activity than the analogous complexes <b>1a</b>–<b>c</b>, attached to the L¹ ligand. Complex <b>2b</b>, in which the L² ligand adopts the mixed η⁵/κ¹ coordination mode, showed the highest activity and livingness mode toward the polymerization of isoprene with high <i>cis</i>-1,4-selectivity (94.1%), and both scandium complexes <b>1a</b> and <b>2a</b> exhibited high 3,4-selectivity (87%) irrespective of the ligand type; in contrast, the lutetium complexes initiated the atactic isoprene polymerization. The influences of thell ligand structural factors, the coordination solvent, and the central metal ion on the catalytic activity and selectivity are discussed

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Isoprene Polymerization with Iminophosphonamide Rare-Earth-Metal Alkyl Complexes: Influence of Metal Size on the Regio- and Stereoselectivity

The protonolysis reaction of β-iminophosphonamine ligand (NPN^dipp = Ph₂P­(NC₆H₃^<i>i</i>Pr₂-2,6)₂) with one equivalent of rare-earth-metal tris­(alkyl)­s afforded the corresponding bis­(alkyl) complexes NPN^dippLn­(CH₂SiMe₃)₂(THF) (Ln = Sc (<b>1</b>), Lu (<b>2</b>), Y (<b>3</b>), Er (<b>4</b>)). The bis­(4-methylbenzyl) complexes NPN^dippLn­(CH₂Ph-4-Me)₂(THF) (Ln = Nd (<b>5</b>), La (<b>6</b>)) were prepared by treatment of the tris­(4-methylbenzyl) compounds Ln­(CH₂Ph-4-Me)₃(THF)₃ with β-iminophosphonamine ligand. The small-size rare-earth-metal-based complexes <b>1</b>–<b>4</b> upon activation with Al^<i>i</i>Bu₃ and [Ph₃C]­[B­(C₆F₅)₄] showed high 3,4-selectivities up to 98.1% for isoprene polymerization. When the larger size rare-earth-metal-based 4-methylbenzyl complexes <b>5</b> and <b>6</b> were employed instead, moderate 3,4-selectivities were obtained since the opening coordination environment facilitated the 1,4-enchainment (Nd³⁺: 76.1%; La³⁺: 62.9%). Replacing Al^<i>i</i>Bu₃ by AlEt₃, the <b>5</b> and <b>6</b> systems exhibited high activity and excellent <i>trans</i>-1,4 selectivity for both isoprene (96.5%, 0 °C) and butadiene (92.8%, 20 °C) polymerizations

Binuclear Rare-Earth-Metal Alkyl Complexes Ligated by Phenylene-Bridged β‑Diketiminate Ligands: Synthesis, Characterization, and Catalysis toward Isoprene Polymerization

Deprotonation of <i>m</i>-phenylene-bridged bis­(β-diketiminate) ligands (PBDI^<i>i</i>Pr-H₂ = [2,6-^<i>i</i>Pr₂C₆H₃NHC­(Me)­C­(H)­C­(Me)­N]₂-(<i>m</i>-phenylene); PBDI^Et-H₂ = [2,6-Et₂C₆H₃NHC­(Me)­C­(H)­C­(Me)­N]₂-(<i>m</i>-phenylene); PBDI^Me-H₂ = [2,6-Me₂C₆H₃NHC­(Me)­C­(H)­C­(Me)­N]₂-(<i>m</i>-phenylene)) by rare-earth-metal tris­(alkyls) Ln­(CH₂SiMe₃)₃(THF)₂ (Ln = Y, Lu, Sc) gave a series of rare-earth-metal bis­(alkyl) complexes: PBDI^<i>i</i>Pr-[Y­(CH₂SiMe₃)₂]₂(THF)₂ (<b>1</b>), PBDI^Et-[Ln­(CH₂SiMe₃)₂]₂(THF)_<i>n</i> (<b>2a</b>, Ln = Y, <i>n</i> = 2; <b>2b</b>, Ln = Lu, <i>n</i> = 2; <b>2c</b>, Ln = Sc, <i>n</i> = 1), and PBDI^Me-[Y­(CH₂SiMe₃)₂]₂(THF)₂ (<b>3</b>). All these complexes were fully characterized by NMR spectroscopy, X-ray diffraction, and elemental analyses, adopting binuclear structures with the two rare-earth-metal ions taking <i>trans</i> positions versus the phenyl ring. Complexes <b>1</b>, <b>2a</b>,<b>b</b>, and <b>3</b> coordinate two solvated THF molecules, while the scandium complex <b>2c</b> incorporates only one THF molecule, owing to the steric crowding. Upon activation with 2 equiv of organoborate, the yttrium systems showed higher catalytic activity toward isoprene polymerization in comparison to those based on lutetium, and the scandium system was less active. Addition of aluminum alkyls to the above binary systems accelerated dramatically the polymerization rate irrespective of the central metal type through scavenging impurities in the systems and abstracting the solvated THF molecules in the precursors. The resultant polyisoprene had higher 3,4-regularity (20% vs 5%) as well as higher molecular weights in comparison with the mononuclear systems, which might be attributed to the steric bulky effect of the binuclear systems

Copolymerization of ε‑Caprolactone and l‑Lactide Catalyzed by Multinuclear Aluminum Complexes: An Immortal Approach

A series of aluminum complexes L^aAl₂Me₄ (<b>1</b>), L^b₂Al₄Me₄ (<b>2</b>), and L^cAl₂Me₄ (<b>3</b>) have been prepared from the reaction of AlMe₃ with Salan- and Salen-type ligands (L^aH₂ = [2-OH-3,5-<i>^t</i>Bu₂-C₆H₂CH₂N­(CH₃)]₂-(<i>m</i>-phenylene); L^bH₄ = [2-OH-3,5-<i>^t</i>Bu₂-C₆H₂CH₂NH]₂-(<i>m</i>-phenylene); L^cH₂ = [2-OH-3,5-<i>^t</i>Bu₂-C₆H₂CHN]₂-(<i>m</i>-phenylene)), respectively. All these complexes were characterized by NMR spectroscopy, X-ray diffraction, and elemental analyses, with complexes <b>1</b> and <b>3</b> adopting binuclear structures, while complex <b>2</b> being tetranuclear. In the presence of alcohol, the binuclear complexes <b>1</b> and <b>3</b> catalyzed controlled ring-opening homopolymerizations of both ε-CL and l-LA. In the copolymerization experiments, complexes <b>1</b> and <b>2</b> produced tapered copolymers of ε-CL and l-LA, while complex <b>3</b> was able to provide ε-CL-<i>co</i>-l-LA with tendentially random structure indicated by the average lengths of the caproyl and lactidyl sequences (<i>L</i>_CL = 1.4; <i>L</i>_LA = 2.6). Particularly, addition of excess alcohol into the catalytic system of complex <b>3</b> established the first “immortal” copolymerization of ε-CL/l-LA, which accelerated the polymerization rates of both monomers and, thus, afforded random copolymers with predictable molecular weights and narrow molecular weight distributions

Magnesium and Zinc Complexes Supported by <i>N</i>,<i>O</i>-Bidentate Pyridyl Functionalized Alkoxy Ligands: Synthesis and Immortal ROP of ε-CL and l-LA

The <i>N</i>,<i>O</i>-bidentate pyridyl functionalized alkoxy ligands 2-(6-methyl-2-pyridinyl)-1,1-dimethyl-1-ethanol (<b>L¹–H</b>) and 2-(6-methyl-2-pyridinyl)-1,1-diphenyl-1-ethanol (<b>L²–H</b>) have been prepared by treatment of acetone and benzophenone with monolithiated 2,6-lutidine. Deprotonolysis of the ligands <b>L¹–H</b> and <b>L²–H</b> with 1 equiv of Mg^<i>n</i>Bu₂ and ZnEt₂ in toluene by releasing butane and ethane, respectively, gave the corresponding dimeric metal-monoalkyl complexes [L¹Mg^<i>n</i>Bu]₂ (<b>1</b>), [L²Mg^<i>n</i>Bu]₂ (<b>2</b>), [L¹ZnEt]₂ (<b>3</b>), and [L²ZnEt]₂ (<b>4</b>). Complexes <b>1</b>–<b>4</b> were characterized by ¹H and ¹³C NMR spectroscopy analysis, and the molecular structures of <b>1</b>, <b>3</b>, and <b>4</b> were further confirmed by X-ray diffraction analysis. The investigation of the catalytic behavior of these complexes toward ε-caprolactone (ε-CL) and l-lactide (l-LA) polymerizations showed that the Mg-based complexes gave higher activity than those attached to zinc metal, probably owing to the greater ionic character of the magnesium metal. Remarkably, the magnesium complex <b>2</b> exhibited a striking “immortal” nature in the presence of primary alcohols where up to 500 PCL chains grew from each Mg active center when benzyl alcohol was employed, while, in particular, in the presence of triethanolamine, complex <b>2</b> also displayed an immortal mode for the polymerization of l-LA

Highly 3,4-Selective Living Polymerization of Isoprene and Copolymerization with ε‑Caprolactone by an Amidino N‑Heterocyclic Carbene Ligated Lutetium Bis(alkyl) Complex

The amidino-modified N-heterocyclic carbene ligated lutetium bis­(alkyl) complex <b>1</b>, (Am-NHC)­Lu­(CH₂SiMe₃)₂, was synthesized by treatment of (AmH-NHC-H)Br ((2,6-^<i>i</i>PrC₆H₃NC­(C₆H₅)­NHCH₂CH₂(NCHCHN­(C₆H₂Me₃-2,4,6)­CH)­Br) with ((trimethylsilyl)­methyl)­lithium (LiCH₂SiMe₃) and lutetium tris­(alkyls) (Lu­(CH₂SiMe₃)₃(THF)₂) via double-deprotonation reactions and characterized by NMR spectroscopy and X-ray diffraction analysis. Under activation of an organoborate, complex <b>1</b> exhibited distinguished catalytic performance for the polymerization of isoprene with respect to high activity, 3,4-regioselectivity (99.3%), and livingness mode. In contrast to the systems reported to date, this system seemed not to be affected obviously by the polymerization temperature (0–80 °C), solvents, monomer-to-initiator ratios (500–5000), and type of organoborate. The resultant polymers have high glass-transition temperatures (38–48 °C) and moderate syndiotacticity (racemic enchainment triad <i>rr</i> 45%, pentad <i>rrrr</i> 20%). In addition, the living lutetium–polyisoprene active species could further initiate the ring-opening polymerization of ε-caprolactone to give selectively the poly­(3,4-isoprene)-<i>b</i>-polycaprolactone block copolymers with controllable molecular weight (from 4.9 × 10⁴ to 10.2 × 10⁴) and narrow polydispersity

Magnesium and Zinc Complexes Supported by <i>N</i>,<i>O</i>-Bidentate Pyridyl Functionalized Alkoxy Ligands: Synthesis and Immortal ROP of ε-CL and l-LA

The <i>N</i>,<i>O</i>-bidentate pyridyl functionalized alkoxy ligands 2-(6-methyl-2-pyridinyl)-1,1-dimethyl-1-ethanol (<b>L¹–H</b>) and 2-(6-methyl-2-pyridinyl)-1,1-diphenyl-1-ethanol (<b>L²–H</b>) have been prepared by treatment of acetone and benzophenone with monolithiated 2,6-lutidine. Deprotonolysis of the ligands <b>L¹–H</b> and <b>L²–H</b> with 1 equiv of Mg^<i>n</i>Bu₂ and ZnEt₂ in toluene by releasing butane and ethane, respectively, gave the corresponding dimeric metal-monoalkyl complexes [L¹Mg^<i>n</i>Bu]₂ (<b>1</b>), [L²Mg^<i>n</i>Bu]₂ (<b>2</b>), [L¹ZnEt]₂ (<b>3</b>), and [L²ZnEt]₂ (<b>4</b>). Complexes <b>1</b>–<b>4</b> were characterized by ¹H and ¹³C NMR spectroscopy analysis, and the molecular structures of <b>1</b>, <b>3</b>, and <b>4</b> were further confirmed by X-ray diffraction analysis. The investigation of the catalytic behavior of these complexes toward ε-caprolactone (ε-CL) and l-lactide (l-LA) polymerizations showed that the Mg-based complexes gave higher activity than those attached to zinc metal, probably owing to the greater ionic character of the magnesium metal. Remarkably, the magnesium complex <b>2</b> exhibited a striking “immortal” nature in the presence of primary alcohols where up to 500 PCL chains grew from each Mg active center when benzyl alcohol was employed, while, in particular, in the presence of triethanolamine, complex <b>2</b> also displayed an immortal mode for the polymerization of l-LA

Synthesis and Stereospecific Polymerization of a Novel Bulky Styrene Derivative

A novel vinylbiphenyl monomer, 2-methoxy-5-phenyl­styrene (MOPS), was designed and efficiently synthesized to investigate the stereospecific polymerization of bulky and polar styrenic derivative. Regardless of its large side group and electron-donating <i>o</i>-methoxy substituent, this compound showed a high polymerizability and was readily converted to the corresponding polymers with moderate to high molecular mass through radical, anionic, and coordination polymerizations. The resultant polymers were characterized by a combination of ¹H/¹³C NMR spectrometry, thermal analysis, and wide-angle X-ray diffraction. Radical polymerization initiated by AIBN in toluene at 60 °C produced a syndiotactic-rich (<i>rr</i> = 0.37) polymer as most bulky vinyl monomers, whereas anionic polymerizations induced by <i>n</i>-BuLi yielded only isotactic-rich polymers no matter if polar tetrahydrofuran (−78 °C, <i>mm</i> = 0.54) or apolar toluene (−40 °C, <i>mm</i> = 0.78) was employed as the solvent. The isotactic-rich microstructure obtained by anionic polymerization in polar solvent at low temperature, the condition that usually leads to syndiotactic-rich polymer, manifested the strong interactions between the <i>o</i>-methoxy groups of the growing chain end and the penultimate unit with the lithium counterion. Highly isotactic (<i>mm</i> = 0.95) and perfect syndiotactic (<i>rr</i> > 0.99) polymers were obtained via coordination polymerizations in toluene at ambient temperature with the β-diketiminato­yttrium precursor (<b>I</b>) and the heterocyclic-fused cyclo­pentadienyl­scandium complex (<b>III</b>) as the catalytic precursor, respectively. All the polymers were thermally stable with 5% weight loss temperatures above 360 °C. They underwent glass transitions in the temperature range of 124–140 °C depending on the tacticity, much higher than polystyrene, implying the dominant role of congestion effect of large side groups on the segment movement restriction of polymer chain. Both isotactic and syndiotactic polymers were crystalline and had melting points higher than 300 °C, although the atactic and less stereoregular polymers were amorphous. The facile synthesis in conjunction with stereostructure tailorability, high thermal stability, glass transition temperature, and melting point makes the polymer a promising candidate for not only helical functional material but also engineering plastics