2-Methoxyethylamino-bis(phenolate)yttrium Catalysts for the Synthesis of Highly Isotactic Poly(2-vinylpyridine) by Rare-Earth Metal-Mediated Group Transfer Polymerization

Theoretical investigations of 2-vinylpyridine stereoselective polymerization catalyzed by cationic yttrium complexes with different ancillary ligands

The polymerization mechanism of 2-vinylpyridine catalyzed by cationic yttrium complexes with diverse ancillary ligands, specifically [L1Y(CH2SiMe3)(THF)]+ [L1 = (2,6-Et2C6H3)NC(Me)CHC(Me)N(2,6-Et2C6H3)] (Y-1), [L2Y(CH2SiMe3)(THF)]+ [L2 = (2,6-Cl2C6H3)NC(Me)CHC(Me)N(2,6-Cl2C6H3)] (Y-2), and [L3Y(CH2SiMe3)(THF)]+ [L3 = (2,6-C6H5)NC(Me)CHC(Me)N(2,6-iPr2C6H3)] (Y-3), was studied using density functional theory (DFT) calculations. Having achieved an agreement between theory and experiment, it is found that isotactic selectivity induced by Y-1 or Y-2 results from a combination of smaller deformation of the catalyst and stronger electronic effects. Conversely, the Y-3 complex exhibits comparable energy barriers for proceeding via either isotactic or syndiotactic pathways, aligning with the production of atactic polymers as seen experimentally. To examine the steric effects on the kinetic and thermodynamic properties, a computational model of an analogue complex [L4Y(CH2SiMe3)(THF)]+ [L4 = (2,6-Cl2C6H3)NC(Me)CHC(Me)N(iPr2C6H3)] (Y-4), featuring increased steric hindrance, was analyzed. Distortion-interaction and topographic steric map analyses further affirmed that steric hindrance significantly influences stereoselectivity. A direct relationship was identified between the energy barriers of isotactic insertion transition states and the bulkiness of ancillary ligands; greater distortion energy of the catalyst correlates with higher barriers for isotactic polymerization. These findings enhance the mechanistic comprehension of 2-vinylpyridine polymerization and are expected to contribute valuable insights for the improvement of catalytic polymerization systems of 2-vinylpyridine.

Ancillary ligand effects on α-olefin polymerization catalyzed by zirconium metallocene: a computational study

The polymerization of α-olefins catalyzed by zirconium metallocene catalyst was systematically studied through experiments and density functional theory (DFT) calculations. Having achieved an agreement between theory and experiment, it was found that the effect of the catalyst ligand on the C[double bond, length as m-dash]C insertion reaction was significantly greater than that on the β-H elimination reaction. Therefore, the molecular weight of polymers can be increased by improving the activity of the C[double bond, length as m-dash]C insertion. In addition, in comparison with propylene, the chain length of α-olefins can directly affect the stereotacticity of polymerization products, owing to steric hindrance between the polymer chain and monomer.

Zirconium Complexes with Bulkier Amine Bis(phenolate) Ligands and Their Catalytic Properties for Ethylene (Co)polymerization

A series of new zirconium complexes bearing bulkier amine bis(phenolate) tetradentate ligands, Me₂NCH₂CH₂N{CH₂(2-O-3-R-5-^tBu-C₆H₂)}₂ZrCl₂ [R = CPhMe₂ (1); CMePh₂ (2); CPh₃ (3); Ph (4); 3,5-Me₂C₆H₃ (5); 3,5-^tBu₂C₆H₃ (6); 4-^tBuC₆H₄ (7)], were synthesized and characterized by ¹H nuclear magnetic resonance (NMR), ¹³C NMR, and elemental analyses. The molecular structures of complexes 1 and 3 were determined by single-crystal X-ray diffraction analysis. The X-ray crystallography analysis reveals that these complexes display a slightly distorted octahedral geometry around their metal centers. Upon activation with methylaluminoxane (MAO), dry-MAO, MAO/butylated hydroxytoluene (BHT), or AlⁱBu₃/CPh₃B(C₆F₅)₄, these zirconium complexes exhibit high catalytic activity for ethylene polymerization [up to 1.07 × 10⁷ g PE (mol Zr)^-1 h^-1] and ethylene/1-hexene copolymerization [up to 2.78 × 10⁷ g polymer (mol Zr)^-1 h^-1], affording (co)polymers with moderate to high molecular weights and good comonomer incorporations. The zirconium complexes with bulkier R groups show higher catalytic activities and longer lifetimes and produce polymers with higher molecular weights, while the zirconium complexes with aryls as R groups demonstrate relatively good comonomer incorporation ability for the copolymerization reactions. These catalytic systems also show moderate catalytic activities for the polymerization reactions of propylene, 1-hexene, and 1-decene. Upon activation with MAO, the zirconium complexes also show moderate catalytic activities for the copolymerization reaction of ethylene with 3-buten-1-ol (treated with 1 equiv of AlⁱBu₃), affording copolymers with the incorporation of 3-buten-1-ol up to 1.05%.

100th Anniversary of Macromolecular Science Viewpoint: The Past, Present, and Future of Stereocontrolled Vinyl Polymerization

The thermomechanical properties exhibited by synthetic macromolecules can be directly linked to their tacticity, or the relative stereochemistry of repeat units. The development of stereoselective coordination-insertion polymerization, for example, led to the discovery of isotactic polypropylene, now one of the most widely produced commodity plastics in the world. Widespread interest in controlling polymer tacticity has led to a variety of stereoselective polymerization methodologies; however, this area of polymer science has lagged behind when compared to the ability to control molecular weight, dispersity, and composition. Despite decades of advancements, many stereoregular vinyl polymers remain unknown, particularly those comprised of polar functionality or derived from renewable resources. This Viewpoint provides an overview of recent developments in stereocontrolled polymerization, with an emphasis on propagation mechanism, and highlights successes, limitations, and future challenges for continued innovation.

Mechanistic Insight into the Stereoselective Cationic Polymerization of Vinyl Ethers

The control of the tacticity of synthetic polymers enables the realization of emergent physical properties from readily available starting materials. While stereodefined polymers derived from nonpolar vinyl monomers can be efficiently prepared using early transition metal catalysts, general methods for the stereoselective polymerization of polar vinyl monomers remain underdeveloped. We recently demonstrated asymmetric ion pairing catalysis as an effective approach to achieve stereoselective cationic polymerization of vinyl ethers. Herein, we provide a deeper understanding of stereoselective ion-pairing polymerization through comprehensive experimental and computational studies. These findings demonstrate the importance of ligand deceleration effects for the identification of reaction conditions that enhance stereoselectivity, which was supported by computational studies that identified the solution-state catalyst structure. An evaluation of monomer substrates with systematic variations in steric parameters and functional group identities established key structure-reactivity relationships for stereoselective homo- and copolymerization. Expansion of the monomer scope to include enantioenriched vinyl ethers enabled the preparation of an isotactic poly(vinyl ether) with the highest stereoselectivity (95.1% ± 0.1 meso diads) reported to date, which occurred when monomer and catalyst stereochemistry were fully matched under a triple diastereocontrol model. The more complete understanding of stereoselective cationic polymerization reported herein offers a foundation for the design of improved catalytic systems and for the translation of isotactic poly(vinyl ether)s to applied areas.

From isoselectivity to syndioselectivity: Lewis base regulates stereochemistry in 2-vinylpyridine polymerization

We have developed an effective synthesis of syndiotactic poly(2-vinylpyridine) by a novel catalytic system. Switching from isoselective to syndioselective polymerization was achieved by adding Lewis base.

Regioselective, stereoselective, and living polymerization of divinyl pyridine monomers using rare earth catalysts

The first regioselective, stereoselective, and living polymerization of divinyl pyridine monomers, mediated by simple rare earth catalysts, is reported.

Origin of stereoselectivity and multidimensional quantitative analysis of ligand effects on yttrium-catalysed polymerization of 2-vinylpyridine

Through a combination of density functional theory (DFT) calculations and multivariate regression analysis, the origin of the stereoselectivity of yttrium-catalysed polymerization of 2-vinylpyridine (2VP) has been investigated.

Metal-Catalyzed Group-Transfer Polymerization: A Versatile Tool for Tailor-Made Functional (Co)Polymers

Accommodating the increasing demand for tailor-made polymers is a major goal in polymer chemistry. Therefore, the investigation of polymerization techniques, which allow the precise synthesis of macromolecules is of exceptional interest. Ionic or controlled radical polymerization are capable living-type methods for the generation of uniform polymers. However, even these approaches reach their limits in certain issues. In the last decades, group-transfer polymerization (GTP) and especially metal-catalyzed GTP have proven to give access to a plethora of tailor-made homo- and copolymers based on α,β-unsaturated monomers. Thereby, GTP has established its potential in the development of functional and smart polymers. This concept article highlights the most significant progress in metal-catalyzed GTP with a focus on functional (co)polymers including different polymeric architectures and microstructures.

Controlled iso-specific polymerization of 2-vinylpyridine catalyzed by arylamide-ligated rare-earth metal aminobenzyl complexes

Arylamide rare-earth metal aminobenzyl complexes were employed as highly active catalysts for iso-specific polymerization of 2-vinylpyridine.

Perfectly isoselective polymerization of 2-vinylpyridine promoted by β-diketiminato rare-earth metal cationic complexes

Highly isotactic poly(2-vinylpyridine)s were produced with rare-earth metal complexes supported by a symmetric β-diketiminate ligand in the presence of borate.

Effects of nucleophilic ligands on the chain initiation efficiency of polar monomer polymerizations catalyzed by 2-methoxyethylaminobis(phenolate)yttrium complexes: a DFT study

DFT calculations suggest that less steric hindrance and a good orbital match induced by nucleophilic ligands are beneficial to the chain initiation of ROP and GTP, respectively.