Self-Activated Coordination Polymerization of Alkoxystyrenes by a Yttrium Precursor: Stereocontrol and Mechanism

Synthesis of bis(oxazoline)-based rare-earth metal complexes and their catalytic performance in the polymerization of isoprene and polar ortho-methoxystyrene

A series of bis(oxazoline) rare-earth metal dialkyl complexes [(L)Ln(CH2SiMe3)2(THF)n] (L = L1 (dimethyl-substituted bis(oxazoline) ligand), Ln = Y (1-Y), Lu (1-Lu), Sc (1-Sc), n = 1; L = L2 (phenyl-substituted bis(oxazoline) ligand), Ln = Y (2-Y), Lu (2-Lu), Sc (2-Sc), n = 2) was successfully prepared. NMR spectroscopy and X-ray diffraction indicated that all the complexes ligated with a C2 symmetric bis(oxazoline) and two trimethylsilylmethyl ligands. In the presence of borate and triisobutyl aluminium, these complexes exhibited high catalytic activity for the polymerization of isoprene, yielding the polymer with high cis-1,4-regularity (up to 99.9%) and high molecular weight. Moreover, these ternary catalytic systems also served as efficient initiators for the polymerization of polar ortho-methoxystyrene. However, atactic polymers in all the cases were isolated despite the C2 symmetric geometry of bis(oxazoline) ligands.

Origins of Ligand-Controlled Stereoselective Polymerization of ortho-Methoxystyrene by Rare-Earth Catalysts: A Theoretical Perspective

The stereoselective polymerization of polar vinyl monomers has recently received much attention due to their excellent physicochemical properties. Over the past decade, breakthroughs have been achieved in this field by rare-earth catalysts. However, the mechanistic origins of those stereoselective polymerizations still remain unclear. Herein, stereoselective polymerization of ortho-methoxystyrene (oMOS) by several representative rare-earth catalysts bearing different ligands (i.e., η5-C5Me5, pyridinyl-methylene-fluorenyl, quinolyl-anilido, β-diketiminato) were systematically investigated by density functional theory (DFT) calculations. After achieving agreement between the calculations and experiments, we focused on discussing the role of ligands in controlling stereoselectivity. Our results reveal that the stereoregularity of oMOS polymerization is mainly controlled by the steric effect of the catalyst-monomer structures. Specifically, the type of ligand influences the orientation and configuration of the inserting monomer, thereby affecting the tacticity of the polymers. In the cases of η5-C5Me5-, pyridinyl-methylene-fluorenyl, and quinolyl-anilido-ligated yttrium catalysts, we observe consistent insertion directions and alternating insertion sides of oMOS monomers, leading to syndiotactic selectivity. The opposite insertion directions and the alternating insertion sides of oMOS monomers were observed in the case of the β-diketiminato yttrium catalyst, leading to isotactic selectivity. These findings reported here offer valuable insights into the role of ligands in controlling stereoselectivity in rare-earth catalyzed coordination polymerization of polar vinyl monomers, thus providing guidance for the rational design of new ligands for stereospecific polymerization of polar monomers in the future.

Alpha-metalated N,N-dimethylbenzylamine rare-earth metal complexes and their catalytic applications

This perspective summarizes our group's extensive research in the realm of organometallic lanthanide complexes, while also placing the catalytic reactions supported by these species within the context of known lanthanide catalysis worldwide, with a specific focus on phosphorus-based catalytic reactions such as intermolecular hydrophosphination and hydrophosphinylation. α-Metalated N,N-dimethylbenzylamine ligands have been utilized to generate homoleptic lanthanide complexes, which have subsequently proven to be highly active lanthanum-based catalysts. The main goal of our research program has been to enhance the catalytic efficiency of lanthanum-based complexes, which began with initial successes in the stoichiometric synthesis of organometallic lanthanide complexes and utilization of these species in catalytic hydrophosphination reactions. Not only have these species supported traditional lanthanide catalysis, such as the hydrophosphination of heterocumulenes like carbodiimides, isocyanates, and isothiocyanates, but they have also been effective for a plethora of catalytic reactions tested thus far, including the hydrophosphinylation and hydrophosphorylation of nitriles, hydrophosphination and hydrophosphinylation of alkynes and alkenes, and the heterodehydrocoupling of silanes and amines. Each of these catalytic transformations is meritorious in its own right, offering new synthetic routes to generate organic scaffolds with enhanced functionality while concurrently minimizing both waste generation and energy consumption. Objectives: We aim for the research summary presented herein to inspire and encourage other researchers to investigate f-element based stoichiometric and catalytic reactions. Our efforts in this field began with the recognition that potassium salts of benzyldimethylamine preferred deprotonation at the α-position, rather than the ortho-position, and we wondered if this regiochemistry would be retained in the formation of lanthanide complexes. The pursuit of this simple idea led first to a series of structurally fascinating homoleptic organometallic lanthanide complexes with surprisingly good stability. Fundamental studies of the protonolysis chemistry of these complexes ultimately revealed highly versatile lanthanide-based precatalysts that have propelled a catalytic investigation spanning more than a decade. We anticipate that this summative perspective will animate the synthetic as well as biological communities to consider La(DMBA)3-based catalytic methods in the synthesis of functionalized organic scaffolds as an atom-economic, convenient, and efficient methodology. Ultimately, we envision our work making a positive impact on the advancement of novel chemical transformations and contributing to progress in various fields of science and technology.

Unveiling the Detailed Mechanism and Origins of Chemo-, Regio-, and Stereoselectivity of Rare-Earth Catalyzed Alternating Copolymerization of Polar and Nonpolar Olefins

The direct copolymerization of polar and nonpolar olefins is of great interest and significance, as it is the most atom-economical and straightforward strategy for the synthesis of functional polyolefin materials. Despite considerable efforts, the precise control of monomer-sequence and their regio- and stereochemistry is full of challenges, and the related mechanistic origins are still in their infancy to date. Herein, the mechanistic studies on the model reaction of Sc-catalyzed co-syndiospecific alternating copolymerization of anisylpropylene (AP) and styrene were performed by DFT calculations. The results suggest that the subtle balance between electronic and steric factors plays an important role during monomer insertions, and a new amino-dissociated mechanism was proposed for AP insertion at chain initiation. AP insertion follows the 2,1-si-insertion pattern, which is mainly controlled by steric factors caused by the restricted MeO···Sc interaction. As for styrene insertion, it prefers the 2,1-re-insertion manner and its regio- and stereoselectivities are influenced by steric repulsions between the inserting styrene and the polymer chain or the ligand. More interestingly, it is found that the alternating monomer-sequence is mainly determined by the "steric matching" principle, which is quantitatively expressed by the buried volume of the metal center of the preinserted species. The concept of steric pocket has been successfully applied to explain the different performances of several catalysts and other alternating copolymerization reactions. The insightful mechanistic findings and the quantitative steric pocket model present here are expected to promote rational design of new rare-earth catalysts for developing regio-, stereo-, and sequence-controlled copolymerization of specific polar and nonpolar olefins.

Isospecific Polymerization of Halide- and Amino-Substituted Styrenes Using a Bis(phenolate) Titanium Catalyst

Isospecific polymerization of polar styrenes is a challenge of polymer science. Particularly challenging are monomers bearing electron-withdrawing substituents or bulky substituents. Here, we report the coordination polymerization of halide- and amino-functionalized styrenes including para-fluorostyrene (pFS), para-chlorostyrene (pClS), para-bromostyrene (pBrS), and para-(N,N-diethylamino)styrene (DMAS) using 2,2′-sulfur-bridged bis(phenolate) titanium precursor (1). The combination of 1 and [Ph3C][B(C6F5)4] and AliBu3 provides crystalline poly(pFS)s with perfect isotacticity (mmmm > 95%) and high molecular weights (≤16.0 × 104 g mol−1). Upon activation with a large excess of DMAO, 1 reaches polymerization activity of 5.58 × 105 g molTi−1 h−1 producing isotactic poly(pFS)s featuring higher molecular weights (≤39.6 × 104 g mol−1). The distinguished performance of the 1/DMAO system has been extended to the polymerization of pClS and pBrS, both usually involve halogen abstraction during the polymerization, to produce isotactic and high molecular weight (Mn = 32.2 × 104 vs. 13.7 × 104 g mol−1) polymers in good activities (2.18 × 105 vs. 1.31 × 105 g molTi−1 h−1). Surprisingly, 1/DMAO is nearly inactive for DMAS polymerization, on contrary, the system 1/[Ph3C][B(C6F5)4]/AliBu3 displays isoselectivity (mmmm > 95%) albeit in a moderate activity.

Amido-trihydroquinoline ligated rare-earth metal complexes for polymerization of isoprene

In combination with a borate, the amido-trihydroquinoline ligated rare-earth metal complexes (Ln = Y, Lu) showed moderate catalytic activities for isoprene polymerization to generate 1,4-enriched polyisoprenes.

Isospecific (co)Polymerization of Unmasked Polar Styrenes by Neutral Rare-earth Metal Catalysts

Syndioselective polymerization of unprotected polar styrenes has achieved great success via specially designed catalysts and establishment of "self-assisted" theory. On contrary, isospecific polymerization of polar styrenes has remained less explored, which needs to break the dilemma of high selectivity and activity of the involved catalysts. Herein, we present new racemic ansa-bis(benz[e]indenyl) rare-earth metal complexes and their high activity and perfect isoselectivity ( mmmm > 99 %) for the polymerization of unmasked polar styrenes without any activators. Moreover, the copolymerizations of para / meta -methoxystyrenes with styrene give gradient and random copolymers, respectively. The insertion rate of polar monomers could be readily tuned in the range of 0-100 mol % by changing their loading ratios. The resultant isotactic polar polystyrenes are quantitatively transformed into hydroxyl or methylsulfonyl polystyrenes with high T g s. DFT calculations reveal the isospecific mechanim.

Direct Synthesis of Functional Thermoplastic Elastomer with Excellent Mechanical Properties by Scandium-Catalyzed Copolymerization of Ethylene and Fluorostyrenes

Herein, we report copolymerizations of ethylene (E) and ortho-/meta-/para-fluorostyrenes (S^F =oFS/mFS/pFS) by using quinolyl methylene fluorenyl scandium complex (Flu-CH₂ -Qu)Sc(CH₂ SiMe₃ )₂ . The copolymerizations proceed in a controlled fashion to afford copolymers composed of "soft" ethylene-fluorostyurene (E-S^F ) random segments (T_g =-22.2-5.1 °C) and "hard" crystalline ethylene-ethylene (E-E) segments (T_m =42.3-130.2 °C). The copolymers behave like thermoplastic elastomers at room temperature by showing high stress values up to 39.5 MPa under elongation-at-break above 774 % with elastic recovery over 75 %. The excellent mechanical properties are mainly attributed to the microphase separation of the nanoscale crystalline E-E domain from the amorphous E-S^F copolymer matrix proved by AFM, WAXD and SAXS. The mechanism investigation by the density functional theory (DFT) simulation reveals that the steric bulky and electron-withdrawing ligand of the catalytic precursor prefers E propagation to generate long E-E segments, while the incorporation of S^F is thermodynamic control.

Syndiospecific coordination (Co)polymerization of carbazole-substituted styrene derivatives using the scandium catalyst system

Perfect syndiospecific polymerization of carbazole-substituted styrene derivatives was achieved using a rare-earth metal catalyst. Also copolymerization of FSt with styrene afforded copolymers with gradient sequence distributions and easily tunable incorporation rate.

Highly syndioselective coordination (co)polymerization of vinyl heteroaromatic monomers using rare-earth-metal complexes

This paper demonstrates the coordination polymerization of vinylbenzothiophene and its copolymerization with styrene with high activity and perfect syndioselectivity to give high molecular weight products by using rare-earth-metal precursors.

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.