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Design and Prediction of High Potent ansa-Zirconocene for Olefin Polymerizations: Combined DFT Calculations and QSPR Approach
Abstract
Homopolymerization of ethene (ET), propene (PP), 1-butene (BT), 1-hexene (HX), and styrene (ST) by rac [Zr{1-Me2Si(3-Pr-(η5-C9H5))(3-H-(η5-C9H5))Cl2] ansa-zirconocene catalyst were investigated using Density Functional Theory (DFT) calculations. This study unveiled the following: (i) Ansa-zirconocene is regio and stereoselective catalyst for PP, BT, HX, ST polymerization; (ii) Regio and stereoselectivities depend on the orientation of the growing chain and incoming olefin monomers; (iii) Primary (1,2) insertion is preferred over secondary (2,1) insertion for linear olefin ( PP, BT, and HX) while it is the opposite for ST; (iv) The activity of olefin polymerization by ansa-zirconocene is controlled by the thermodynamics of π-complex formation and the kinetics of insertion. Our results revealed that the regio and stereoselectivity of the catalyst can be described by the steric interaction between the incoming monomer and isobutyl group of the growing chain. For ET polymerization, the effect of catalyst structure on the productivity of the process was examined and QSPR models were constructed based on experimental activities and theoretical calculations. The model suggests that potent ansa-zirconocene catalysts should have less value of the dipole moment and the more positive charge on the Zr metal atom. This information will provide clues for the design of more potent ansa-zirconocene catalysts for ethylene polymerization.
... When In recent years, computational chemistry has been widely applied to studying catalytic mechanisms and structure-activity relationships of transition metal catalysts [16][17][18][19][20][21]. Ratanasak et al. [22] used density functional theory (DFT) to investigate the olefin polymerization process catalyzed by ansa-Zr catalysts. By calculating the energies of various structures in the reaction pathway, they found a significant correlation between the intrinsic activation energy of the ethylene insertion process and the experimental activity. ...
This theoretical study investigates the high molecular weight (Mw) production in copolymerization of ethylene and 1-octene using heteroatom-substituted constrained geometry catalysts (CGCs). The research explores the correlation between chain termination reactions and polymer molecular weight, revealing that the Gibbs free energy barrier of the chain termination reactions is positively linked to the molecular weight. Quantitative structure–property relationship models were constructed, indicating that molecular descriptors such as atom charge, orbital energy, and buried volume significantly influence the polymer molecular weight.
... DFT calculations of the regioselectivity observed in the polymerization of propene and 1-butene, is still emphasizing the similarity between the two monomers (Ratanasak et al., 2021). The ΔE (ΔG) # regio values in Table 1 underscore the consistent regioselectivity in both cases. ...
Isotactic poly (1-butene) (iPB) is an interesting semi-crystalline thermoplastic material characterized by notable physical and mechanical attributes encompassing superior creep and stress resistance, elevated toughness, stiffness, and thermal endurance. These distinctive features position iPB as a viable candidate for specific applications; however, its widespread utilization is hindered by certain inherent limitations. Indeed, iPB manifests an intricate polymorphic behavior, and the gradual and spontaneous transition of the kinetically favored form II to the thermodynamically favored form I during aging introduces alterations to the material’s properties. Despite its potential, the attainment of iPB with an exceedingly high molecular mass remains elusive, particularly when employing homogeneous catalysts renowned for their efficacy in propene polymerization. In this study we analyze the mechanistic aspects governing 1-butene polymerization by using DFT calculations modelling the regioselectivity of 1-butene insertions and the termination reactions occurring after primary (1,2) and secondary (2,1) insertions. Finally, the isomerization pathways leading to the formation of 4,1 units in iPB samples synthesized by homogenous catalysts is also discussed. All these aspects, furnish a mechanistic picture of the main drawbacks of an “old” but still interesting material.
... In addition, our calculations revealed that all product structures still provided a strong β-agostic interaction which is supported by the elongated C R -H β bond distance (1.13-1.15 Å) as compared to the normal C-H bond distance of 1.10 Å [47,48]. ...
The reaction mechanism of ethylene (ET) polymerization catalyzed by the phenoxy-imine (FI) ligands using DFT calculations was studied. Among five possible isomers, isomer A which has an octahedral geometry and a (cis-N/trans-O/cis-Cl) arrangement is the most stable pre-reaction Ti-FI dichloride complex. The isomer A can be activated by MAO to form the active catalyst and the active form was used for the study of the mechanism for Ti-FI. The second ethylene insertion was found to be the rate-determining step of the catalyzed ethylene polymerization. To examine the effect of group IVB transition metals (M = Ti, Zr, Hf) substitutions, calculated activation energies at the rate-determining step (EaRDS) were compared, where values of EaRDS of Zr < Hf < Ti agree with experiments. Moreover, we examined the effect of substitution on (O, X) ligands of the Ti-phenoxy-imine (Ti-1) based catalyst. The results revealed that EaRDS of (O, N) > (O, O) > (O, P) > O, S). Hence, the (O, S) ligand has the highest potential to improve the catalytic activity of the Ti-FI catalyst. We also found the activation energy to be related to the Ti-X distance. In addition, a novel Ni-based FI catalyst was investigated. The results indicated that the nickel (II) complex based on the phenoxy-imine (O, N) ligand in the square-planar geometry is more active than in the octahedral geometry. This work provides fundamental insights into the reaction mechanism of M − FI catalysts which can be used for the design and development of M − FI catalysts for ET polymerization.
... [5] Hence, homogeneous single-site catalysts have been developed to overcome this problem. Many studies have been conducted, and new homogeneous metallocene [6][7][8][9] and post-metallocene/nonmetallocene [10][11][12][13][14] have been synthesized. New ligands and transition metal centers particularly with group IVB transition metals such as titanium (Ti), zirconium (Zr), and hafnium (Hf) or late transition metals such as nickel (Ni) and palladium (Pd) have been introduced. ...
In this work, we explored the reaction mechanism of heterobimetallic nickel phenoxyphosphine polyethylene glycol (Ni‐PEG) with alkali metals (M⁺=Li⁺, Na⁺, K⁺, and Cs⁺) catalysts for ethylene polymerization using the DFT calculations. The activation energy of the necessary step shows the following trend, Li⁺<Na⁺<K⁺<Cs⁺, which corresponds to experimentally observed activities. Roles of secondary metals (M⁺) in Ni‐PEG catalysts were clarified. Our findings suggest that the active catalyst should contain strong cooperative metal‐metal/metal‐ligand interactions and less positive charge on M⁺ cation. Besides, the key role of M⁺ is to control the PEG group which stabilizes the catalyst structure. In addition, we found two key factors (shorter M‐O1 and M‐OPEG distances) for designing new catalysts from the pre‐reaction state of the Ni‐PEG(M⁺) catalysts. Finally, Ni‐PEG(M²⁺) catalysts with Be²⁺, Mg²⁺, Co²⁺, and Zn²⁺ were suggested for candidates of highly active catalysts for ethylene polymerization.
The quantitative structure-property relationship (QSPR) of constrained geometry catalyst (CGC) has been analyzed by combining density functional theory (DFT) and multivariate linear regression (MLR). QSPR models for the reaction energy...
The kinetics and mechanism of ethylene and 5-ethylidene-2-norbornene copolymerization catalyzed by rac -Et(Ind) 2 ZrCl 2 were investigated using 2-thiophenecarbonyl chloride.
50 years ago, Karl Ziegler and Giulio Natta were awarded the Nobel Prize for their discovery of the catalytic polymerization of ethylene and propylene using titanium compounds and aluminum-alkyls as co-catalysts. Polyolefins have grown to become one of the biggest of all produced polymers. New metallocene/methylaluminoxane (MAO) catalysts open the possibility to synthesize polymers with highly defined microstructure, tacticity, and steroregularity, as well as long-chain branched, or blocky copolymers with excellent properties. This improvement in polymerization is possible due to the single active sites available on the metallocene catalysts in contrast to their traditional counterparts. Moreover, these catalysts, half titanocenes/MAO, zirconocenes, and other single site catalysts can control various important parameters, such as co-monomer distribution, molecular weight, molecular weight distribution, molecular architecture, stereo-specificity, degree of linearity, and branching of the polymer. However, in most cases research in this area has reduced academia as olefin polymerization has seen significant advancements in the industries.
Therefore, this paper aims to further motivate interest in polyolefin research in academia
by highlighting promising and open areas for the future.
A number of 2-methyl-4-aryl-5-methoxy-6-alkylindenes and C2-symmetric Me2Si-bridged ansa-zirconocenes based on them were synthesized. Zirconocenes were obtained by means of highly effective and scalable racemo-selective synthetic approach based on the use of a Zr tert-butyl amide complex. The structure of μ-(bis-[η5-6-tert-butyl-5-methoxy-2-methyl-4-tert-butylphenyl-1H-inden-1-yl]dimethylsilanediyl)dichlorozirconium(IV) (14) has been established with X-ray analysis. The introduction of a methoxy group into the indenyl fragment of zirconocene significantly improved its catalytic performance (i.e., its activity, stereoselectivity, molecular mass potential, and thermal stability) in the polymerization and copolymerization of propylene in comparison with the benchmark Spaleck-zirconocene. The role of the methoxy group is proposed to stabilize the cationic catalytic intermediates, which was confirmed using DFT calculations.
We present a quantitative structureeactivity relationship study performed over a set of bis(imino)pyridine iron catalyst used for olefin po-lymerisation. The experimental results were previously obtained by our research group. The structural variability of this catalyst set is mainly focused on the substituents of the aryl rings. We managed to find a statistically robust model which correlates the experimentally determined polymer's molecular weight with the steric and electrostatic fields. As a result the main contribution comes from the steric contribution which amounts up to 75% of the model. The predictive capability of the model was successfully probed with a test set of catalyst reported in the literature by other research group.
We present a quantitative structureeactivity relationship study performed over a set of bis(imino)pyridine iron catalyst used for olefin po-lymerisation. The experimental results were previously obtained by our research group. The structural variability of this catalyst set is mainly focused on the substituents of the aryl rings. We managed to find a statistically robust model which correlates the experimentally determined polymer's molecular weight with the steric and electrostatic fields. As a result the main contribution comes from the steric contribution which amounts up to 75% of the model. The predictive capability of the model was successfully probed with a test set of catalyst reported in the literature by other research group.
In the search for more efficient single-center ethene/α-olefin copolymerization catalysts, metallocenes bearing a 2-indenyl substituent pattern have largely been ignored in the past. Here, we show that such a structural motif yields competent linear low-density polyethylene (LLDPE) catalysts. They are also relatively easy to synthesize, allowing for a wide structural amplification. A screening of 28 catalysts reveals that the lead catalyst in this study displays high comonomer affinity and molecular weight capability at industrially relevant temperatures. QSAR models show that steric factors likely contribute stronger than electronic factors to the observed substituent trends, both for comonomer affinity and MW capability.
A computational study of olefin polymerization has been performed on 51 zirconocene catalysts. The catalysts can be categorized into three classes according to the ligand framework: Class I - Cp2ZrCl2 (ten catalysts), Class II - CpIndZrCl2 (thirty-eight catalysts), and Class III - Ind2ZrCl2 (three catalysts), Ind = 5-indenyl. Detailed reaction pathways, including chain propagation and chain termination steps, are modeled for ethylene polymerization using Class II catalysts. Optimized structures for reaction coordinates indicated the presence of α-agostic interactions in the transition states (TSs) for both the 1st and 2nd ethylene insertions as well as in the ethylene π-complex of the ZrnPr cation. However, βagostic interactions predominate in the cationic n-propyl and n-pentyl intermediates. The calculated activation energy barriers energies show that the TS for insertion of ethylene into the Zr-CH3+ bond is the highest point on the computed reaction coordinates. Quantitative structure-activity relationships (QSAR) studies were also performed for 38-mixed zirconocene dichlorides. This study, in concert with previous work, suggests that the type of ring attached to Zr (Cp vs. Ind) affects the reaction kinetics and thermodynamics less significantly than the type of substituents attached to the Cp and indenyl rings, and that substituent effects are even greater than those arising from changing the metal (Zr vs. Hf).
A set of 19 silicon-bridged C2-symmetric zirconocenes rac-R′2Si(2-Me-4-R-indenyl)2ZrCl2 of varying steric demand in position 4 were synthesized and screened in propene homopolymerization in a high-throughput experimental setup. The size and accuracy of the experimental data set allow to identify surprisingly good correlations among stereoselectivity, regioselectivity, and molecular weight capability (R² ≈ 0.8–0.9) over a broad range. We rationalize this trend by assuming that steric tuning in the 4-position affects both preferred insertion and stereoerror formation similarly but leaves other barriers largely unaffected. A quantitative structure–activity relationship based on one single computational descriptor, Δ%VBur—using the difference in the percent of buried volume between the “blocked” and “open” quadrants of the catalyst precursor—is established. Provided that a large sphere of 5.0 Å is used, stereoselectivity can be predicted with unprecedented accuracy, i.e., a mean average deviation (MAD) of 0.18 kcal/mol (ΔΔG‡enantio), 0.0007 (σ, probability that the preferred propene enantioface is selected at an active site of given chirality), or 0.3% (mmmm pentads). On the basis of this empirical model, we predicted that the catalyst with R = o-tolyl is an ideal candidate for high stereoselectivity/high MW capability. Ad hoc synthesis and testing of the precursor confirmed the expectations: the catalyst shows the highest stereoselectivity reported so far (σ = 0.9999) for metallocenes at 60 °C, while maintaining a high MW capability (Mw > 1 MDa) and relatively high regioselectivity.
The first three insertion steps of propylene for isoselective metallocenes from the {Cp/Flu}‐ and {SBI}‐families were computed using a theoretical method implementing the B3PW91 functional in combination with solvent corrections incorporated with the SMD continuum model. For the C1‐symmetric {Cp/Flu}‐type metallocenes, two mechanisms of stereocontrol were validated theoretically: more facile and more stereoselective chain "stationary" insertion (or site epimerization back‐skip) and less stereoselective alternating mechanisms. For the C2‐symmetric {SBI}‐type system, computation results are in complete agreement with the sole operating chain migratory insertion mechanism. The thermochemical data obtained through the study were used to predict microstructures of polypropylenes using three‐parameter and one‐parameter statistical models for the two metallocene systems, respectively. The calculated meso/rac pentad distributions were found in good agreement with those determined experimentally for iPP samples obtained at different polymerization temperatures.
ansa metallocene complexes of Ti, Zr and Hf were synthesized, characterized and tested as catalysts for homogeneous ethylene polymerization. The ligand system comprises two indenyl moieties tethered at the 1,1′-positions via a 2,2′-dimethyl biphenylene bridge. The ligand precursor was obtained by the reduction of diphenic acid using lithium aluminum hydride (LiAlH4), followed by the reaction with phosphorus tribromide, and, finally, the reaction with indenyllithium. The corresponding group (IV) metal complexes were synthesized by deprotonation of the ligand precursors using n-BuLi followed by reactions of the corresponding metal tetrachloride. After activation with methylaluminoxane (MAO), the zirconium and hafnium complexes proved as highly active catalysts for ethylene polymerization. The zirconium complex 6 showed the best performance with 12,460 kg PE/mol cat. h. The titanium complex 5 showed no catalytic activity obviously because of decomposition reactions.
The synthesis and characterization of a new class of ansa bis(indenyl) complexes of zirconium and hafnium is described. Two indenyl moieties are linked at the 1-,1′-positions via a 1,2-bis(dimethylsilyl)benzene unit. The ligand precursor was prepared via three reaction steps including Grignard coupling of chlorodimethylsilane and 1,2-dibromobenzene, PdCl2-catalyzed chlorination, and the reaction with indenyllithium. The zirconium and hafnium complexes were obtained by deprotonation of the corresponding bis-(indenyl) compound with n-BuLi followed by metalation reactions of MCl4 (M = Zr, Hf) in tetrahydrofuran. The solid state molecular structures of both complexes were established by single crystal X-ray diffraction analyses. In ethylene polymerization reactions, both complexes exhibited high activities. The zirconocene catalyst 4 showed a higher activity (7610 kg PE/mol cat. h) compared to the hafnium catalyst 5 (3590 kg PE/mol cat. h).
Zirconium and hafnium complexes bearing cycloheptane- or cyclononane-fused [OSSO]-type bis(phenolato) ligands ([C7] and [C9], respectively) were prepared and subjected to the polymerization of 1-hexene as the precatalyst. The polymerizations produced poly(1-hexene)s with high activities and high isospecificity, where complexes bearing [C9] were more reactive than those bearing [C7]. Their activities were compared with those of the corresponding complexes bearing cyclohexane- and cyclooctane-fused ligands ([C6] and [C8], respectively), which we reported previously, to show the order of activity [C8] > [C9] > [C7] > [C6]. The ring-size effect on the activity was investigated with the help of DFT calculations on active and dormant cationic zirconium species, π complexes of the active species with propene, and transition states for propene insertion into the Zr–C(iBu) bond. The order of activity speculated from the activation energy, that is the energy difference between the π complex and the corresponding transition state, was [C8] > [C7] > [C9] ≈ [C6]. However, calculations on active and dormant cationic zirconium complexes including [B(C6F5)4]⁻ as the counteranion revealed that the active species are more stable than the dormant species by 9.1 kcal mol–1 for [C8] followed by 7.4 kcal mol–1 for [C9] and 3.1 kcal mol–1 for [C7] and, in contrast, that the active species with [C6] is less stable by 1.0 kcal mol–1 than the corresponding dormant species. Thus, the abundances of active species bearing [C6] and [C7] are reduced, which leads to the reversal of the order of [C7] and [C9] on the basis of activation energy to reproduce the order observed experimentally.
A series of copolymers, based on benzo[1,2-b:4,5-b′]dithiophene (BDT) as the electron donor and 2,1,3-benzothiadiazole (BT)/diketopyrrolo[3,4-c]pyrrole (DPP) as the electron acceptors, were synthesized for highly efficient polymer solar cells. By changing the BT/DPP ratio in the conjugated backbone, the absorption, energy levels, molecular aggregation and carrier mobility could be finely tuned. With increased DPP content, the absorption range was extended to the longer wavelength region with narrower bandgaps. The highest occupied molecular orbital (HOMO) levels were also raised up and the molecular aggregation was enhanced. The balance of these factors would afford a remarkable device performance enhancement. Polymer P3 with BT:DPP = 0.7:0.3 (molar ratio) exhibited the highest power conversion efficiency (PCE) of 9.01%, with open circuit voltage (Voc = 0.73 V, short current density (Jsc = 18.45 mA·cm−2, and fill factor (FF) = 66.9%. The PCE value was improved by 48.7% compared to P1 and by 117.6% compared to P7, respectively, indicating a great potential in photovoltaic application.
Roles of a novel dibenzoyl sulfide donor in Ziegler–Natta (ZN) catalyzed propylene polymerization were examined using DFT calculations. The adsorption mode for dibenzoyl sulfide and diisobutyl phthalate electron donors on the MgCl2(110) surface is the chelate coordination mode, similar to malonate electron donors. The dibenzoyl sulfide/diisobutyl phthalate donor provides regio- and stereo-selectivity to the ZN catalyst. Factors that control the isotacticity and activity of the ZN catalyst are steric repulsion and π-complex stabilization by electron donors. The steric repulsion signifies intrinsic activation energy and π-complex formation energy for π-complex stabilization. The two factors are combined in the apparent activation energy and dibenzoyl sulfide gives the lowest apparent activation energy. Therefore, dibenzoyl sulfide shows the best catalytic enhancement as compared to diisobutyl phthalate and di-n-butyl-2-cyclopentyl malonate, which is in good agreement with the results obtained from the experiments.
The mechanism of styrene polymerization catalyzed by five analogous cationic rare-earth-metal complexes [(RCH2-Py)Y(CH2SiMe3)]⁺ (R = C5Me4 (Cp′), 1⁺ R = C9H6 (Ind), 2⁺ R = C13H8 (Flu), 3⁺), [(Flu-Py)Y(CH2SiMe3)]⁺ (4⁺), and [(Flu-CH2CH2-NHC)Y(CH2SiMe3)]⁺ (5⁺) has been studied through DFT calculations. Having achieved an agreement between theory and experiment in the activity discrepancy and selectivity, it is found that styrene polymerization kinetically prefers 2,1-insertion to 1,2-insertion. The free energy profiles for the insertion of a second monomer molecule have been computed for both migratory and stationary insertion manners, and the former resulting in a syndiotactic enchainment indicates obvious kinetic preference. The current results suggest that the coordination of styrene to the active metal center could play an important role in the observed activity difference. Interestingly, the charge on central metal of the cationic species accounts for the activities of 1⁺, 2⁺, and 3⁺: the higher the charge on the central metal, the higher the activity. The coordination of a THF molecule to the central metal and more difficult generation of the active species could be responsible for the low activity of 4⁺. For species 5⁺, the resulting product of the first styrene insertion is quite stable, and the ancillary ligand and styryl group hamper the insertion of the incoming styrene molecule. This could be responsible for the absolute inertness of 5⁺ toward styrene polymerization. The calculated results also suggest that a longer alkyl chain of the side arm of the ancillary ligand could deter monomer coordination and thus decrease the polymerization activity.
Advances in theory and processing power have established computation as a valuable interpretative and predictive tool in the discovery of new asymmetric catalysts. This tutorial review outlines the theory and practice of modeling stereoselective reactions. Recent examples illustrate how an understanding of the fundamental principles and the application of state-of-the-art computational methods may be used to gain mechanistic insight into organic and organometallic reactions. We highlight the emerging potential of this computational tool-box in providing meaningful predictions for the rational design of asymmetric catalysts. We present an accessible account of the field to encourage future synergy between computation and experiment.
We report here the synthesis of two new ansa-bridged permethylindenyl zirconocenes, their reaction with solid polymethylaluminoxane (sMAO) and their use in slurry phase ethylene polymerisation. Meso-(EBI*)Zr(CH2Ph)2 and meso-(EBI*)Zr(CH2tBu)Cl, (EBI* = ethylenebis[1-(2,3,4,5,6,7-hexamethylindenyl)]) were synthesised from meso-(EBI*)ZrCl2 and KCH2Ph and LiCH2tBu respectively. The new zirconocenes were characterised by NMR spectroscopy and X-ray crystallography, and density functional theory calculations were carried out. Solid precatalysts were obtained when these compounds were reacted with the polymethylaluminoxane support. Ethylene polymerisation activities of up to 6000 kgPE/molZr/h/bar were obtained in the slurry polymerisation of ethylene. The polyethylenes showed molecular weights, Mw, above 200 000 kg/mol and low polydispersities, Mw/Mn < 3.
Developing more efficient catalysts remains one of the primary targets of organometallic chemists. To accelerate reaching this goal, effective molecular descriptors and visualization tools can represent a remarkable aid. Here, we present a Web application for analyzing the catalytic pocket of metal complexes using topographic steric maps as a general and unbiased descriptor that is suitable for every class of catalysts. To show the broad applicability of our approach, we first compared the steric map of a series of transition metal complexes presenting popular mono-, di-, and tetracoordinated ligands and three classic zirconocenes. This comparative analysis highlighted similarities and differences between totally unrelated ligands. Then, we focused on a recently developed Fe(II) catalyst that is active in the asymmetric transfer hydrogenation of ketones and imines. Finally, we expand the scope of these tools to rationalize the inversion of enantioselectivity in enzymatic catalysis, achieved by point mutation of three amino acids of mononuclear p-hydroxymandelate synthase.
The dimerization of 1-hexene was catalyzed by ten zirconocenes. High catalytic productivity was achieved via a two-step activation process, namely, the treatment of the zirconocene with triisobutylaluminum (TIBA) followed by methylaluminoxane (MAO). The zirconocene [(C5H4SiMe2)2O]ZrCl2 (10) exhibits a higher productivity and selectivity to dimer formation than the unsubstituted (C5H5)2ZrCl2 (1) complex. For the ansa-[Z(C5H4)2]ZrCl2 complexes, the catalytic activity increases as the angle between the cyclopentadienyl rings decreases. The dimerization selectivity of 10 reaches 94% when the reaction is performed using Et2AlCl as the chlorine source needed to form the catalytic species. The possible mechanism of selective α-olefin dimerization is discussed.
Quantitative structure-property relationships in group 4 metallocene olefin polymerization catalysts were constructed for a dataset of 8 zirconocenes in a combined experimental and quantum chemical study. A total of 45 descriptors measuring the steric and electronic influence of the rac-SiMe2[Ind]2 based ligand framework were correlated with the observed polymerization activities, together with the molecular weights, melting temperatures, stereoerrors and regioerrors of the produced isotactic polypropene. Direct correlations between the calculated descriptors and observed catalytic properties turned out to be relatively weak due to the combined steric and electronic effects of multiple descriptors. Multiple regression models based on ordinary least squares linear regression and principal component regression techniques typically led to significant improvements, particularly for the molecular weights, melting temperatures and regioerrors, providing R2 of up to 0.92. The results provide a solid starting point for an extension of the methodology for a greater variety of catalyst structures, and thereby for the development of new high-performance catalysts for this industrially important process.
In hopes of extending the existing knowledge on the chemistry of phospholyl and boratabenzene complexes of zirconium, which have shown potential notably as polymerization catalysts, this study aims at exploring the synthesis of mono boratabenzene and mono phospholyl zirconium complexes and at studying their reactivity towards the formation of mixed (boratabenzene)(phospholyl)zirconium complexes. Several derivatives of (η5-phospholyl)Zr(NMe2)xCl3−x and (η6-boratabenzene-NMe2)Zr(NMe2)xCl3−x were synthesized and used as precursors for the formation of mixed (boratabenzene)(phospholyl)zirconium complexes.
This contribution describes the synthesis and characterization of a series of zirconium metallocenes derived from partially alkylated s-indacene. The complexes [Cp∗ZrCl2-s-Ic″H] 1, [Cp∗ZrCl2-s-Ic″H] 3 and [CpZrCl2-s-Ic″H] 4 (s-Ic′H = 2,6-diethyl-4,8-dimethyl-1-hydro-s-indacene; s-Ic″H = 2,6-dibutyl-4,8-dimethyl-1-hydro-s-indacene) were synthesized from the monolithiated salts of s-indacene (s-Ic′H or s-Ic″H) with one equivalent of C5R5ZrCl3 (R = H, CH3). All complexes here reported were characterized by means of 1H and 13C NMR, mass spectrometry, elemental analysis. Complexes [Cp∗ZrCl2-s-Ic′H] 1 and [CpZrCl2-s-Ic′H] 2 (previously reported by NMR analysis) were characterized by X-ray diffraction. In order to gain further knowledge about their catalytic behavior, the complexes were tested in the catalysis of ethylene polymerization showing a highest activity. Complex 4 presents the highest ethylene polymerization activity than complexes 1 and 2, and 3 under the same working conditions.
We have carried out a systematic computational study on olefin polymerization by metallocene/borate catalysts, using three metallocenes: Cp2ZrMe2 (Cp), rac-SiMe2-bis(1-(2-Me-(4-PhInd))ZrMe2 (4-PhInd), and rac-SiMe2-bis(1-(2-Me-(4,5-BenzInd))ZrMe2 (4,5-BenzInd). Detailed reaction pathways, including the structure of the catalytically active ion pair, anion displacement, chain propagation, and chain termination steps, are reported for ethene homopolymerization, alongside with investigation of ethene-propene copolymerization reactions. Initially, all catalysts form inner-sphere ion pairs ([L2ZrMe]+-[B(C6F5)4]−) with a direct Zr-F interaction, which is weak enough to be displaced by the incoming monomer. In comparison to Cp, the bulky and electron-rich 4-PhInd and 4,5-BenzInd show higher barriers for anion displacement but lead to relative stabilization of the resulting π complexes. 4-PhInd enables the most feasible propene uptake, and both catalysts suppress the chain termination reactions relative to Cp. The borate counterion is shown to have a minor influence after the catalyst activation step.
Two deactivation pathways of Ti and Zr half-metallocene complexes activated with B(C6F5)3 in toluene solvent were studied using Density Functional Theory (DFT) with dispersion corrections: (a) H transfer from counterion to Me initiating group to release methane and (b) C6F5 transfer from counterion to metal. Transition state geometries and energies were computed for twenty-seven complexes, and the barrier height for the C6F5 transfer pathway was linearly correlated to the amount of steric congestion near the metal. Unimolecular rate constants for catalyst deactivation were predicted for all 27 catalysts by constructing a DFT-based quantitative structure activity relationship (QSAR). This QSAR was constructed by using the DFT-computed energy barrier (ΔV0) and vibrational frequency along the reaction coordinate (v‡) as chemical descriptors and fitting QSAR parameters to experimental data for reference systems. The computed rate constants were in excellent agreement with available experimental data. Specifically, the dominant deactivation pathway for each catalyst and the relative deactivation rates of different catalysts were correctly predicted. Of note, the IndTi(OC6H-2,3,5,6-Ph4)Me2/B(C6F5)3 system is predicted to have a good combination of slow deactivation and high olefin polymerization rates.
The ansa indenyl ligand precursors CH2CH2CH2(C9H6R)(2) (R = Me (7), Et (8), Pr (9)) have been prepared by the reaction of the corresponding lithium indene, Li(C9H6R-1) (R = Me (4), Et (5), Pr (6)), with 1,3-dibromopropane. Compounds 7-9 were converted, by the reaction with butyllithium, to the dilithium compounds, Li-2{CH2CH2CH2(C9H5R-3)(2)} (R = Me (10), Et (11), Pr (12)). The three carbon atom bridge ansa-bis(indenyl)zirconium complexes, [Zr{CH2CH2CH2(eta(5)-C9H5R-3)(2)}Cl-2] (R = Me (13), Et (14) Pr (15)) were synthesized from the reaction of 10-12 with zirconium tetrachloride. Compounds 13-15 have been tested as homogeneous catalysts in the polymerization of ethylene. The molecular structures of 14 and 15 have been determined by single-crystal X-ray diffraction studies.
A combined experimental and quantum chemical study has been performed on rac- and meso-[Zr{1-Me2Si(3-η5-C9H5Et)2}Cl2] (rac- and meso-1) and their hydrogenated forms (rac- and meso-2) to understand ligand effects and guide ligand design for more active ansa-bis(indenyl) zirconocenes for the polymerisation of ethylene. The rac-ansa-zirconocene rac-[Zr(1-Me2Si{3-Et-(η5-C9H9)}2)Cl2] (rac-2) has been prepared and fully characterised by NMR spectroscopy and elemental analysis. The molecular structure of rac-2 has also been determined by single-crystal XRD. The behaviour of the catalysts was analysed in the polymerisation of ethylene and higher activities were obtained for rac-1 and its hydrogenated form rac-2. The influence of the stereochemistry and hydrogenation of the indenyl ligand on the experimental activities has been evaluated by computational studies. The differences along the reaction pathway are dominated by changes in the relative stabilities of the catalytic intermediates. A hybrid density functional B3LYP study, in the presence of toluene as the solvent, indicates that the rac forms give rise to more active species than their meso counterparts. The hydrogenation of the rac forms is a very promising approach to increase activities in polymerisation, in contrast to the meso forms. Finally, the global mechanism rate constants for the polymerisation reaction for each metallocene were calculated by using the thermodynamic formulation of transition-state theory to complement the computational study.
A computational study within the framework of density functional theory is presented on the oligomerization of ethylene to yield 1-hexene using [(η5-C5H4CMe2C6H5)]TiCl3/MAO] catalyst. This study explicitly takes into account a methylaluminoxane (MAO) cocatalyst model, where the MAO cluster has become an anionic species after having abstracted one chloride anion, yielding a cationic activated catalyst. Hence, the reaction profile was calculated using the zwitterionic system, and the potential energy surface has been compared to the cationic catalytic system. Modest differences were found between the two free energy profiles. However, we show for the first time that the use of a realistic zwitterionic model is required to obtain a Brønsted-Evans-Polanyi relationship between the energy barriers and reaction energies.
A family of group 4 metallocene complexes based on the hexamethylindenyl ligand (C9Me6H; Ind#) have been prepared and fully characterised. The complexes rac- ZrCl2 (rac-1), meso- ZrCl2 (meso-1), rac- HfCl2 (rac-2), meso- HfCl2 (meso-2) were prepared by the reaction of Ind#Li with the corresponding MCl4 (where M = Zr, Hf); and rac- Zr(CH2Ph)2 (rac-3) was derived from rac-1 using two equivalents of potassium benzyl (KCH2Ph). All five species were characterised by NMR spectroscopy, single crystal X-ray diffraction and density functional theory. The zirconocenes were tested for their activity as solution-phase ethylene polymerisation catalysts and rac-1 was found to outperform the meso-1 at most temperatures. The benzyl analogue, rac-3, peaked at more than double the activity reported for the dihalide species.
DFT computations have been performed to investigate the mechanism of H2-assisted chain transfer strategy to functionalize polypropylene via Zr-catalyzed copolymerization of propylene and p-methylstyrene (pMS). The study unveils the following: (i) propylene prefers 1,2-insertion over 2,1-insertion both kinetically and thermodynamically, explaining the observed 1,2-insertion regioselectivity for propylene insertion. (ii) The 2,1-inserion of pMS is kinetically less favorable but thermodynamically more favorable than 1,2-insertion. The observation of 2,1-insertion pMS at the end of polymer chain is due to thermodynamic control and that the barrier difference between the two insertion modes become smaller as the chain length becomes longer. (iii) The pMS insertion results in much higher barriers for subsequent either propylene or pMS insertion, which causes deactivation of the catalytic system. (iv) Small H2 can react with the deactivated [Zr]−pMS−PPn facilely, which displace functionalized pMS−PPn chain and regenerate [Zr]H active catalyst to continue copolymerization. The effects of counterions are also discussed. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014
Electron donors enhance the productivity and isotacticity of products of the Ziegler-Natta catalyzed propylene polymerization. Using the fact that adsorption energies of electron donors to catalyst surface are linearly related to activities, the Quantitative Structure Property Relationship (QSPR) for adsorption energies was performed for a set of 24 compounds from 3 different groups, i.e., phthalates, 1,3-diethers and malonates using the multiple linear regression (MLR) analysis. The QSPR model shows high correlation (R2 = 0.84, R2CV = 0.83) between adsorption energies and 3 descriptors, i.e., the radius of gyration (50%), the dipole moment (16%), and the forcite bond energy (34%). Consequently, the catalyst activity of propylene polymerization mainly depended on steric hindrance. The predictive ability of the model was successfully validated with a set of five electron donors which randomly selected from three different groups. Predictive R2 for the test set was 0.77, indicating good predictive ability of the model. The QSPR model provided the valuable information for the design of better electron donors for propylene polymerization.
Copolymerizations of ethylene with 1,3-butadiene in the presence of catalytic systems based on C-2-symmetric zirconocenes rac-(CH3)(2)Si(2-R-4-R'-1-indenyl)(2)ZrCl2 (where R = CH3- or H and R' = C6H5- or H) are compared. The chemical nature and the relative amount of constitutional comonomer units from butadiene (1,4-trans, methylene-1,2-cyclopentane and methylene-1,2-cyclopropane) are strongly affected by the bulkiness of the substituent on the indenyl ligands. The unsubstituted indenyl zirconocene system rac-(CH3)(2)Si(indenyl)(2-) ZrCl2/methylalumoxane (MAO) inserts 1,3-butadiene leading to both cyclopentane and 1,4-trans units, whereas exclusively cyclopentane constitutional units are obtained from rac-(CH3)(2)Si(2-methyl-1-indenyl)(2)ZrCl2/MAO. The catalytic system rac-(CH3)(2)Si(2-methyl-4-phenyl-1-indenyl)(2)ZrCl2/(MAO) is able to incorporate about 30% of butadiene units into polymer chains and to form up to 10% of cyclopropane units. An unusual insertion mechanism for conjugated diolefins, that involves a butadiene 12 primary coordination and insertion leading to formation of cycloalkane units, is largely predominant for all used catalytic systems. Mechanistic studies and DFT calculations indicate that the chemoselectivity of the reaction depends on the bulkiness of the substituents on the indenyl ligands of catalyst.
This paper reviews the findings of quantitative structure–activity relationship (QSAR) studies focusing on single-site polymerization catalysts, with special attention paid to the use of 3D-QSAR tools. Such tools reveal the fine details of catalyst structure that may be correlated with polymerization activity or the properties of the synthesized polymer. The introduction of effective single-site polymerization catalysts, in addition to allowing scientists to synthesize new tailor-made polymers, has enabled a detailed theoretical analysis of the synthesis process. The benefits of single-site polymerization for theoretical studies include easy elucidation of the catalyst structure, a well-defined mechanism of action, and the fact that experiments can be systematically conducted on catalyst series featuring different substitution patterns. Using QSAR methods, experimental results can be related to theoretical measurements through statistical or chemometric tools. These tools have been extensively and successfully used in the field of drug design.
The effect of the metal on elementary chain propagation and termination reactions in hafnocene- and zirconocene-catalyzed olefin copolymerization processes has been systematically studied by quantum chemical methods. Two consecutive monomer insertions, parallel with the competing chain termination reactions, were studied for copolymerization of ethene with propene, 1-butene, and 1-hexene. For the purpose of a comparative study, analogous species along the reaction pathway were studied for each metallocene/monomer combination. Effects due to incorporation of a comonomer were analyzed as a function of both the central metal and the comonomer size. In accordance with the general experimental observations of zirconocenes producing lower molecular weight polymer than the hafnocenes, differences in activation energies for chain propagation and termination are smaller for the zirconocene. For the zirconocene, the activation energy is particularly low for β-hydrogen elimination after secondary comonomer insertion.
A series of ethylene-bridged C-1-symmetric ansa-(3-R-indenyl)(fluorenyl) zirconocene complexes 3a-i (R = 2[2-(4-methylphenyl)propyl], 3a; R = 2-[2-(3,5-dimethylphenyl)propyl], 3h; R = 2-(2-benzylpropyl), 3c; R = 2-methylbenzyl, 3d; R = 2-(2-cyclohexylpropyl), 3e; R = 2-[2-(1-cyclohexenyl)propyl], 3f; R = 2-(2-n-butylpropyl), 3g; R = cyclohexyl, 3h; R = Pr-i, 3i) were synthesized by a salt metathesis method and characterized by NMR spectroscopy, elemental analysis (or HRMS) and X-ray diffraction (3e and 3h). Upon activation with methylaluminoxane, most of these zirconocene complexes exhibited sufficient catalytic activities up to 2.5 x 10(5) g C-6/(mol-Zr.h) and high selectivities up to 99% toward propylene dimerization, affording 2-methyl-1-pentene as the major isomer which was confirmed by gas chromatography. Remarkably, the selectivity and activity of complexes 3a-i were significantly influenced by the structural features of the substituent on the 3-position of indenyl ring: a pendant aryl or alkyl group linked by a quaternary carbon bridge provided the complex with high selectivities in the range of 89.9-99.0% for 2-methyl-1-pentene and low to moderate catalytic activities; the lack of a quaternary carbon bridge within the substituent would lead to mainly polypropylenes of low molecular weight. The steric hindrance around the active metal center induced by the pendant group might be responsible for the catalytic dimerization behavior, and the presumed mechanism was discussed. In addition, for complexes 3h and 3i, the selectivity for propylene dimerization could also be enhanced with the increase of reaction temperature. Noticeably, most of these ansa-zirconocene complexes exhibit excellent thermal stability at 100 degrees C, which is important with regard to industrial application.
Ethylene polymerization was carried out by immobilization of rac-ethylenebis(1-indenyl)zirconium dichloride (Et(Ind)2ZrCl2) and rac-dimethylsilylbis(1-indenyl)zirconium dichloride (Me2Si(Ind)2ZrCl2) preactivated with methylaluminoxane (MAO) on calcinated silica at different temperatures. Polymerizations of ethylene were conducted at different temperatures to find the optimized polymerization temperature for maximum activity of the catalyst. The Me2Si bridge catalyst showed higher activity at the lower polymerization temperature compared to the Et bridge catalyst. The highest catalytic activities were obtained at temperatures about 50 °C and 70 °C for Me2Si(Ind)2ZrCl2/MAO and Et(Ind)2ZrCl2/MAO catalysts systems, respectively. Inductively coupled plasma-atomic emission spectroscopy results and polymerization activity results confirmed that the best temperature for calcinating silica was about 450 °C for both catalysts systems. The melting points of the produced polyethylene were about 130 °C, which could be attributed to the linear structure of HDPE.
The discovery of methylaluminoxane (MAO) was the start for investigations and innovations of new classes of highly active olefin polymerization catalysts. Different transition metal complexes together with MAO as cocatalyst allow the synthesis of polymers with a highly defined microstructure, tacticity, and stereoregularity as well as new cycloolefin, long chain branched, or blocky copolymers with excellent properties. These new polyolefins could not be obtaind with such a purity before by Ziegler–Natta catalysts. The single site catalyst character of metallocene and other transition metal complexes activated by MAO leads to a better understanding of the mechanism of the olefin polymerization.
The energies of formation and the heterolytic dissociation energies for the ion pairs Cp′2ZrMe+A− (active sites in olefin polymerization) in the presence of Al- and B-containing activators including low-molecular-weight ones X(C6F5)3 (X = Al, B) and a number of models for Al-sites in polymethylaluminoxane (MAO) were obtained from DFT calculations. The reaction mechanisms were thoroughly studied and the energy characteristics of the reactions of the ion pairs Cp′2ZrMe+A− with ethylene molecule (Cp′ = η5-C5H5, η5-C5Me5, A− = MeB(C6F5)3−, MeAl(C6F5)3−, and [(C6F5)3Al–Me–Al(C6F5)3]− or three models for anions in MAO-containing systems [Me–AOTMA]−, [Me–2AOTMA]−, [Me–3AOTMA]−, AOTMA = Me2AlO(Al2Me5)) were calculated. Heterolytic dissociation energy (energy needed for complete separation of counterions) is found to be a crucial parameter which determines the energy characteristics of the polymerization reaction. We propose that the involvement of the second and third Lewis acid Al-sites in the formation of the ion pair could explain why high Al:Zr ratios (>1 for Al(C6F5)3 and ≫1 for MAO) are necessary for a high catalytic activity of zirconocenes, whereas for B(C6F5)3 an equimolar B:Zr ratio is quite sufficient.
The stereospecificity of the polymerization of propene under the influence of α-TiCl3 and alkyl aluminum compounds is explained. The explanation rests exclusively on quantum chemical principles and the crystal chemistry of transition metal trichloride layer structures, subjects described in two preceding papers.The active centers formed by alkylation of titanium ions at those faces of the surface where chlorine vacancies are available are shown to possess the asymmetry necessary for the appropriate orientation of the α olefin in the π complex with the exposed titanium.The nonequivalence of the crystallographic sites of vacancy and alkyl group at the active center ensures that the growing alkyl group will move back to its original position after each incorporation of a new monomer.Thus the polymerization consists of a sequence of sterically identical steps which leads to isotacticity of the product.
As modifications of the effective catalyst precursor [dimethylsilanediylbis(benz[e]indenyl)]zirconium dichloride, the ansa-zirconocene complexes Me2Si(cyclopenta[c]phenanthryl)2ZrCl2, Me2Si(cyclopenta[l]phenanthryl)2ZrCl2, Me2Si(2-methylcyclopenta[l]phenanthryl)2ZrCl2, and Me2Si(2-methyltetrahydrobenz[e]indenyl)2ZrCl2 have been synthesized. When activated by methylaluminoxane (MAO), these complexes give highly active catalysts for the polymerization of propene to polymers with high isotacticities and molecular weights.
A comparative investigation into the ethene polymerisation behaviour of several mono-substituted metallocene catalysts ((CpR)2ZrCl2/MAO; R=H, Me, Et, iPr, tBu, SiMe3, CMe2Ph) was performed. The activity of the catalysts was found to be dependant on both steric (quantified using the Tolman cone angle, analytical solid angle and numerical solid angle measurement methods) and electronic effects (Hammett substituent parameters). The highest activity was found for the catalysts with R=Et and SiMe3.
Rac and meso ansa-zirconocenes [Zr{1-Me2Si(3-R-(η5-C9H5))(3-R′-(η5-C9H5))}Cl2] (R=Et, R′=H; R=Pr, R′=H; and R=Et, R′=Pr) 4–9 have been prepared by the reaction of ZrCl4 with the corresponding dilithiated derivatives from the ligands {1-Me2Si(3-R-(η5-C9H5))(3-R′-(η5-C9H5))} (R=Et, R′=H 1; R=Pr, R′=H 2; and R=Et, R′=Pr 3) in diethyl ether/toluene at −78°C. The molecular structure of rac [Zr{1-Me2Si(3-Et-(η5-C9H5))(3-Pr-(η5-C9H5))}Cl2] 8 has been determined by single crystal X-ray diffraction studies, which show a pseudotetrahedral environment formed by the two chlorine ligands and two η5-coordinated indenyl ligands. The activity in homogeneous and heterogeneous polymerization is compared and discussed.
A density functional theory (B3LYP) computational study of the ethylene–styrene copolymerization process using meso-Et(H4Ind)2Zr(CH3)2 as the catalyst is presented. The monomer insertion barriers in meso species are evaluated and compared with previously obtained barriers in rac diastereoisomers. Differences related to ethylene homopolymerization and ethylene–styrene copolymerization activities as well as styrene incorporation into the copolymer are found between the meso and rac diastereoisomers. Nevertheless, a migratory insertion mechanism seems to hold for both diastereoisomeric species. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4752–4761, 2006
Single site olefin polymerisation catalysts are suitable candidates for modelling purposes. Their well-defined structure and the almost complete elucidation of their polymerisation mechanisms, make these organometallic complexes ideal for studies based on quantitative structure-activity relationships (QSAR). Although the QSAR technique is extensively used in drug design, there are very few reports on its application to metallocene-based polymerisation catalysis. This probably has something to do with the difficulties inherent in controlling experimental conditions during the polymerisation process. In the present study, we obtained ethylene polymerisation data using a number of zirconocene catalysts under carefully controlled experimental conditions, i.e. keeping all polymerisation variables constant except catalyst structure. The catalytic activity and molecular weight of the resulting polyethylenes were experimentally determined. We then applied 3D-QSAR methodology to explain the experimental data in terms of three-dimensional (3D) field descriptors related to the structure of the metallocene catalyst. Our results provide useful correlations between experimental ethylene polymerisation activities and the steric, LUMO and local softness fields of the catalysts. The molecular weights calculated from comparative molecular field analysis (CoMFA) models including LUMO and local softness fields correlate well with the experimental ones. The predictive capacity of the models was also tested. Based on the proposed models, steric and electronic factors affecting polymerisation performance are discussed.
We have applied 3D-QSAR chemometric tools to analyze the polymerization activity of a series of analogous zirconocene catalysts with different substitution patterns in the cyclopentadienyl rings. The selection of the most stable conformers was performed by means of a conformational analysis of the different catalysts in a step previous to the QSAR study. The calculated QSAR model has been assessed in robustness following a progressive scrambling protocol that helps to check redundancy in the descriptor space. This model presents a good predictive ability as tested with a set of four catalysts. The final model is controlled mostly by steric contributions, which can be related to the weakness of the catalyst–cocatalyst interaction.
ansa-Zirconocene complexes have been studied by Three-Dimensional Quantitative Structure–Activity Relationship (3D-QSAR). Using the comparative molecular field analysis (CoMFA) method, the experimental results obtained for catalytic activity and polymer molecular weight have been successfully correlated with the steric and electrostatic 3D structural descriptors, which were calculated by density functional theory (DFT) methods.
Nonrelativistic and quasirelativistic ab‐initio pseudopotentials representing the Ne‐like X(Z−10)+ cores (X=Sc–Zn) of the first row transition metals and optimized (8s7p6d1f )/[6s5p3d1f ]‐GTO valence basis sets for use in molecular calculations have been generated. Excitation and ionization energies of the low lying states of Sc through Zn from numerical HF‐ as well as SCF‐ and CI(SD)‐pseudopotential calculations using the derived basis sets differ by less than 0.1 eV from corresponding all‐electron results.
The productivity of a number of bis-(phenoxyamine)Zr(IV)-based catalysts (bis(phenoxyamine) = N,N′ -bis(3-R1 -5-R2 -2-O-C6H2CH2 )-N,N′ -(R3 )2 -(NCH2CH2N)) in ethene and propene polymerization was
evaluated for different R1/R2/R3 combinations. In previous studies on this class we demonstrated that the cations that form upon precatalyst activation (e.g., by methylalumoxane) adopt a “dormant” mer-mer geometry, and an endothermic isomerization to the active fac-fac geometry is the necessary first step of the catalytic cycle. Herewith we report a clear correlation between catalyst activity and the DFT-calculated energy difference ΔEi between the active and dormant state. The correlation only holds when the calculations are run on ion pairs, which is less obvious than it may appear because the anion in these systems is not at the catalyst front. This finding provides a comparatively simple and fast method to predict the activity of new catalysts of the same class.
Styrene was copolymerised with ethylene using the catalyst systems [rac-Et(H4Ind)2TiCl2]/MAO and [rac-Et(H4Ind)2ZrCl2]/MAO. Keeping other experimental variables under control, we tested different styrene/ethylene ratios in the reactor feed. It was found that the titanium-based catalyst showed very low activity even for ethylene homopolymerisation. In contrast, the zirconium system achieved monomer polymerisation, incorporating small amounts of styrene. When the styrene/ethylene ratio was increased, both catalyst activity and the molecular weight of the resulting copolymers decreased. However, styrene incorporation into the copolymer increased as the styrene/ethylene ratio rose. To gain insight into the copolymerisation mechanisms at play, we undertook a computational study using a high-level hybrid DFT method (B3LYP). Agreement between the experimental and theoretical results was generally good, indicating the usefulness of combined experimental/theoretical studies for clarifying mechanisms of α-olefin copolymerisation using organometallic systems.