Isotactic Polymerization Research Articles

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.

Vectorial On‐Surface Synthesis of Polar 2D Polymer Crystals

AbstractThe asymmetric introduction of functional groups into polymers is promising due to its potential to provide novel electronic and magnetic properties. Though traditional on‐surface bottom‐up synthesis has succeeded in creating various types of polymers, it struggles at realizing asymmetry due to the difficulty of stereoregular polymerization and the tendency for overall polarity cancellation during agglomeration. Here, enabled by the low halogen‐contaminated metal surfaces provided by two‐zone chemical vapor deposition, “compass” precursors possessing three independent bonding, edge, and dipole vectors undergo isotactic polymerization via the vectorial self‐assembly of chiral precursor diradicals without the need for chiral catalysts required in conventional in‐solution polymerization. The isotactic polymers exhibit polar 2D crystalline structures due to the hetero‐edge CH–π interaction between the standing phenyl and butoxy group surpassing the homo‐edge interactions of π–π and CHCH. The developed vectorial on‐surface synthetic technique not only paves the way to the realization of stereoregular control but also unlocks unprecedented crystal engineering.

Strong and Confined Brønsted Acid Catalyst Enables Isotactic Polymerization of Vinyl Ethers

Key words imidodiphosphorimidates - polymerization catalysts - isotactic polymers - vinyl ethers

Beyond the Active Site. Cp*ZrMe3/Sulfated Alumina‐Catalyzed Olefin Polymerization Tacticity via Catalyst⋅⋅⋅Surface Ion‐Pairing

AbstractSurface‐bound organometallic molecules have recently enabled the development of single‐site heterogeneous catalysts, advancing the atomic scale understanding and diversity of heterogeneous catalysis. Here we report that supporting Cp*ZrMe3 (Cat1) on acidic sulfated‐alumina (AlS) affords the surface catalyst Cat1/AlS, which was characterized by multi‐dimensional solid‐state NMR spectroscopies, and is active in ethylene homo‐ and copolymerizations, as well as propylene and 1‐hexene homopolymerizations. In contrast to propylene (or 1‐hexene) polymerization by homogeneous Cp*ZrMe2+ B(C6F5)4− which yields atactic polyolefins, Cat1/AlS promotes remarkable isotacticity with mmmm >95 %. Complementary DFT analysis argues that the restrictive local Cat1/AlS C1‐symmetry favors activation and enchainment at the propylene re enantioface, promoting isotactic polymerization via a “back‐skip‐like” mechanism.

Mechanistic Study on Effect of Electron Donors in Propylene Polymerization Using the Ziegler–Natta Catalyst

TiCl4/MgCl2 catalysts are still the main industrial catalyst for isotactic polypropylene at present. However, the mechanism of isotactic polymerization of propylene has not been fully understood. D...

A novel catalytic way of comprising a β-nucleating agent in isotactic polypropylene: Catalyst design strategy and polymerization-assisted dispersion

This article demonstrates a catalytic as well as efficient method to comprise a β-nucleating agent N,N’-dicyclohexyl-terephthalamide (DCTPA) into isotactic polypropylene (iPP) for the synthesis of β-crystalline-specified iPP (β-iPP). A novel DCTPA-based Ziegler-Natta catalyst was designed via a first modification step of DCTPA with TiCl4 and a subsequent supporting step of MgCl2-TiCl4-donor components on DCTPA, by which the final catalyst composition and morphology were facilely controlled, and the propylene polymerization was endowed with high activity as well as high stereospecificity. The chemical structure of the DCTPA supported Ti-Mg catalyst was investigated using FTIR, XPS and XRD, and the morphology of the catalyst particle and its polymers was monitored by SEM. Moreover, uniform dispersion of DCTPA fragments in iPP resin was realized through the in-reactor polymerization, benefiting from the catalyst-polymer morphological replication effect, followed by inducing the homogeneous β-modification of iPP with high efficiency after a melting-crystallization treatment of the polymers. This novel difunctional catalyst proved to be effective for both propylene isotactic polymerization and polymer β-modification.

Revisiting the Stereochemistry of Propylene Isotactic Polymerization Reaction Mechanism on C2 Symmetric [SiH2(Ind)2ZrCH3]+ and [SiH2(Ind)2ZrCH3]+[CH3B(C6F5)3

Abstract The [SiH2(Ind)2ZrCH3]+ (Ind = indenyl) catalyzed stereoregularity of propylene polymerization mechanism has been investigated at M06 level of theory. Four different approaches of propylene to the reactive catalyst lead to four isomeric products due to the C2 symmetry of bridged Ind ligand of the catalyst. Consequently, various possibilities of propylene attack as well as orientation of polymer chain yield numerous stereoisomers. The calculations of the first and second insertions with various conformers clarified the most favorable reaction pathway and showed that isotactic propagation is more favorable (3.5 kcal mol−1) than syndiotactic propagation. The structures of resting state catalysts displayed various agostic interactions of the CH bond with the Zr center which stabilize the catalytic systems and play important roles in determining the favorable reaction pathway. The influence of counter anion [CH3B(C6F5)3]− on the reactivity of the catalyst was also studied. The results also confirm that the trans orientation of the counter anion with respect to propylene is more favorable than its cis orientation and clarify the most favorable reaction pathway in the first and second insertion. Because agostic interactions are involved in various aspects, AIM analysis has been used to find the bonding nature of agostic interactions as well as ion-pair bonds. The overall results suggest that rigidity of ansa-zirconocene, unique structure of C2 symmetric ansa ligand, influence of [CH3B(C6F5)3]− and β agostic interaction may restrict the attack of propylene only to isotactic polymerization and not to syndiotactic polymerization.

Asymmetric synthesis from terminal alkenes by cascades of diboration and cross-coupling

Amongst prospective starting materials for organic synthesis, terminal (monosubstituted) alkenes are ideal. In the form of α-olefins, they are manufactured on enormous scale and they are the core product features from many organic chemical reactions. While their latent reactivity can easily enable hydrocarbon chain extension, alkenes also have the attractive feature of being stable in the presence of many acids, bases, oxidants and reductants. In spite of these impressive attributes, relatively few catalytic enantioselective transformations have been developed that transform aliphatic α-olefins in >90% ee and, with the exception of site-controlled isotactic polymerization of α-olefins,1 none of these processes result in chain-extending C-C bond formation to the terminal carbon.2, 3, 4, 5, 6 Herein, we describe a strategy that directly addresses this gap in synthetic methodology and present a single-flask catalytic enantioselective conversion of terminal alkenes into a range of chiral products. These reactions are enabled by an unusual neighboring group participation effect that accelerates Pd-catalyzed cross-coupling of 1,2-bis(boronates) relative to nonfunctionalized alkyl boronate analogs. In tandem with enantioselective diboration, this reactivity feature connects abundant alkene starting materials to a diverse array of chiral products. Importantly with respect to synthesis utility, the tandem diboration/cross-coupling reaction (DCC reaction) generally provides products in high yield and high selectivity (>95:5 enantiomer ratio), employs low loadings (1–2 mol %) of commercially available catalysts and reagents, it offers an expansive substrate scope, and can address a broad range of alcohol and amine synthesis targets, many of which cannot be easily addressed with current technology.

Computational Studies on Isospecific Polymerization of 1-Hexene Catalyzed by Cationic Rare Earth Metal Alkyl Complex Bearing a C3iPr-trisox Ligand

1-Hexene polymerization catalyzed by dicationic rare earth metal alkyl species [Ln(iPr-trisox)(CH2SiMe3)]2+ (Ln = Sc and Y; trisox = trisoxazoline) has been computationally studied by using QM/MM approach. It has been found that the initiation of 1-hexene polymerization kinetically prefers 1,2-insertion (free energy barrier of 17.23 kcal/mol) to 2,1-insertion (free energy barrier of 20.05 kcal/mol). Such a preference of 1,2-insertion has been also found for chain propagation stage. The isotactic polymerization was computed to be more kinetically preferable in comparison with syndiotactic manner, and the dicationic system resulted in lower insertion free energy barrier and more stable insertion product in comparison with the monocationic system. The stereoselectivity was found to follow chain-end mechanism, and the isospecific insertion of 1-hexene is mainly controlled by kinetics. In addition, the current computational results, for the first time, indicate that the higher activity of Sc species toward 1-h...

Isospecific Cobalt Catalyst for Propylene Oxide Polymerization

The authors report an extremely active catalyst for the highly isotactic polymerization of propylene oxide. When the CoII(salph) precursor was reacted with air and oxidized to the active CoIII catalyst, TOF’s of 220 (h-1) were achieved when the reaction was performed at 0 °C in toluene. Raising the temperature (25 °C and 40 °C) reduced the TOF, while lowering the temperature (-20 °C) reduced the yield of polymer.

Stereospecific post-metallocene polymerization catalysts: the example of isospecific styrene polymerization

In the context of developing single-site stereoselective post-metallocene catalysts, the case for isospecific styrene polymerization catalysts based on methylaluminoxane-activated group 4 metal bis(phenolato) complexes is summarized. Ligands derived from the 1,4-dithiabutanediyl-linked bis(phenol)s have been found to induce stereochemical rigidity by the presence of the hemi-labile sulfide donor functions. Isospecific styrene polymerization was achieved using easily accessible catalyst precursors of the type [MX 2(OC 6H 2- t Bu 2-4,6) 2{S(CH 2) 2S}] (M = Ti, Zr, Hf; X = Cl, O i Pr, CH 2Ph). Activating the dibenzyl titanium complex [Ti(CH 2Ph) 2(OC 6H 2- t Bu 2-4,6) 2{S(CH 2) 2S}] with B(C 6F 5) 3 and Al i Bu 3, controlled isotactic polymerization became possible at lower temperatures. A remarkable dependence of both the activity and stereoselectivity on the ligand substitution pattern was observed. Analogous precursors with the 1,5-dithiapentanediyl-linked bis(phenolato) ligand gave syndiotactic polystyrene with lower activity.

A density functional study on the formation of stereoerrors in the stereoselective propene polymerization with zirconocene catalysts

Abstract Recent labeling experiments have detected stereoerrors in the isotactic polymerization of propene with zirconocenium catalysts, which are characterized by a deuterium transfer from the polymer chain to a methyl group. Using Cp 2 Zr- iso -butyl + as a model system, two alternative mechanisms leading to the observed products are investigated using fully nonlocal levels of density functional theory (DFT). The direct hydrogen transfer starting from a γ-agostic intermediate requires an overall activation relative to the β-agostic resting state in excess of 200 kJ mol −1 and can be excluded. The sequential mechanism involves a (i) β-elimination to yield an initial iso -butene π-complex; (ii) a subsequent internal rotation of this π-complex; and (iii) reinsertion of the olefin into the ZrH bond via two consecutive formations of β-agostic interactions, with maximum relative energies along this path between 35 and 54 kJ mol −1 (depending on functionals and basis sets employed). The subsequent steps are the reverse of the first three steps and can lead to an inverted iso -butyl complex with migrated deuterium. Although certain barrier heights are somewhat sensitive to the density functional employed (BP86 vs. B3LYP), the sequential mechanism is much more favorable energetically. Loss of olefin from the intermediate π-complex is endothermic by 104–114 kJ mol −1 and should therefore not occur.

Synthesis of MgCl2-supported dichlorobis(4,4,4-trifluoro-1-phenyl-1,3-butanedionato)titanium catalysts with different titanium contents and their application to propene polymerization

Various MgCl2-supported Ti(4,4,4-trifluoro-1-phenyl-1,3-butanedionato)2Cl2 catalysts with different Ti contents (0.017 ∼ 0.002 mmol/g) were synthesized and applied to propene polymerization using triethylaluminium and di-i-propyldimethoxysilane (DIPDMS) as the cocatalyst and external donor, respectively. When polymerization was conducted in the absence of DIPDMS, neither the activity nor the isospecificity of catalyst were dependent upon the Ti content. However, the effect of DIPDMS on the catalyst performance was found to be markedly dependent upon the Ti content, i.e., the addition of DIPDMS to the catalyst with low Ti content caused a prominent increase in the activity for isotactic polymerization. As a result, the present catalyst with low Ti content gave a highly isotactic polypropene with Tm = 169.3°C and [mmmm] > 99% in high selectivity (I.I. = 97.7%).

New Ligands for the Living Isotactic Anionic Polymerization of Methyl Methacrylate in Toluene at 0 °C. 2. 7Li NMR Analysis of sec-Butyllithium Ligated by Lithium Silanolates

The mixed associated species {xsBuLi, (y − x)sBuMe2SiOLi} formed by reaction of sec-Butyllithium (sBuLi) with octamethylcyclotetrasiloxane (Me2SiO)4 in toluene have been used to initiate the anioni...

Response to comments by V. Busico and R. Cipullo on the paper titled “Error formation in ansa-zirconocene-catalyzed isotactic propylene polymerization”

Response to comments by V. Busico and R. Cipullo on the paper titled “Error formation in ansa-zirconocene-catalyzed isotactic propylene polymerization”

New bridged zirconocenes for olefin polymerization: Binuclear and hybrid structures

Four newly synthesized bridged bisindenyl zirconocenes which are derivatives of known structures have been tested in polymerizations of liquid propylene with methylaluminoxane as cocatalyst. They include two asymmetric and two binuclear systems which are suitable for isotactic polymerization. Surprising differences from the behavior of similar systems (regiospecificity, chain termination) are found and discussed.

Polymerization of 3‐methyl‐1‐butene promoted by metallocene catalysts

Abstract3‐Methyl‐1‐butene was polymerized in the presence of a number of homogeneous metallocene catalysts (co‐catalyst methylalumoxane). Contrary to literature reports, it was found that even the simplest C2‐symmetric metallocenes promote the isotactic polymerization of this monomer with reasonable productivities. Quite surprisingly, a prevailingly isotactic polymer was also obtained in the presence of Cs‐symmetric metallocenes, which are instead syndiotactic‐specific in propene polymerization.

Isotactic polymerization of propylene with zirconocene catalysts in the presence of a lewis base

Polymerization of propylene was carried out at 0°C in the presence of methyl methacrylate(MMA) or ethyl benzoate(EB) using rac-Et(Ind)2ZrMe2 or rac-Me2Si(Ind)2ZrMe2/Ph3CB(C6F5)4 as catalyst, which resulted in highly isotactic poly(propylene) with [mmmm]>98%, Tm=160–161°C and very few 1,3- or 2,1 regioirregular units. With the use of an achiral zirconocene Cp2ZrMe2, a polymer with Tm=139°C was resulted as well. Base on these experimental fact that the zirconocene can form a C1-symmetrical complex with a Lewis base.

Effect of tetraalkylammonium salts on the tacticity of poly(methyl methacrylate), 1. Initiation by ethyl α‐lithioisobutyrate

AbstractThe anionic polymerization of methyl methacrylate (MMA) was carried out in the presence of ethyl α‐lithioisobutyrate (α‐LiEtIB)/quaternary ammonium salts (QAS) in toluene and tetrahydrofuran/toluene (vol. ratio 75/25) at −60°C. It was found that the tacticity of PMMA strongly depends on size and shape of QAS. A highly isotactic polymerization in toluene was observed, when using the system α‐LiEtIB/trimethylhexadecylammonium bromide. The initiator system α‐LiEtIB/tetrahexylammonium chloride produces polymers with a high (>50%) syndiotactic content and relatively low polydispersity. The influence of QAS on the mode of polymerization is due to specific salt effect resulting from the cation exchange reaction, from the participation of the bulky substituent in the primary solvation shell, and ionic aggregation between the growing anion, Q+ and LiX.

Isospecific Polymerization of Methyl Methacrylate Initiated by Chiral Zirconocenedimethyl/Ph3CB(C6F5)4 in the Presence of Lewis Acid

Polymerization of methyl methacrylate (MMA) was conducted with the catalyst systems of rac-Et(Ind) 2 Zr(CH 3 ) 2 /Ph 3 CB(C 6 F 5 ) 4 , rac-Et(IndH 4 ) 2 Zr(CH 3 ) 2 /Ph 3 CB(C 6 F 5 ) 4 , and rac-Me 2 Si(Ind) 2 Zr(CH 5 ) 2 /Ph 3 CB(C 6 F 5 ) 4 . These catalysts only were found to be inactive. However, the addition of suitable Lewis acids like alkylzinc and alkylaluminum compounds gave highly isotactic PMMA. The isotactic pentad [mmmm] in PMMA as well as the propagation rate constant (k p ) depended significantly upon the zirconocene compounds. Both the microstructure ([mmmm]) and propagation rate constant (k p ) increased in the following order. Me 2 Si(Ind) 2 CH 2 CH 3 > (CH 2 ) 3 CH 3 . When the polymerization was initiated by the rac-Et(Ind) 2 Zr(CH 3 ) 2 /Ph 3 CB(C 6 F 5 ) 4 /Zn(C 2 H 4 CH=CH 2 ) 2 catalyst system at 0°C in toluene, the molecular weight of PMMA increased in proportion to the conversion of MMA. The increase of temperature from 0 to 80°C caused a decrease in isotactic pentad content by approximately 10%. The isotactic polymerization of MMA was revealed to proceed by an enantiomorphic site-controlled mechanism