Kinetics and mechanism of metallocene‐catalyzed olefin polymerization: Comparison of ethylene,propylene homopolymerizations,and their copolymerization

A series of ethylene, propylene homopolymerizations, and ethylene/propylene copolymerization catalyzed with rac‐Et(Ind)₂ZrCl₂/modified methylaluminoxane (MMAO) were conducted under the same conditions for different duration ranging from 2.5 to 30 min, and quenched with 2‐thiophenecarbonyl chloride to label a 2‐thiophenecarbonyl on each propagation chain end. The change of active center ratio ([C^*]/[Zr]) with polymerization time in each polymerization system was determined. Changes of polymerization rate, molecular weight, isotacticity (for propylene homopolymerization) and copolymer composition with time were also studied. [C^*]/[Zr] strongly depended on type of monomer, with the propylene homopolymerization system presented much lower [C^*]/[Zr] (ca. 25%) than the ethylene homopolymerization and ethylene–propylene copolymerization systems. In the copolymerization system, [C^*]/[Zr] increased continuously in the reaction process until a maximum value of 98.7% was reached, which was much higher than the maximum [C^*]/[Zr] of ethylene homopolymerization (ca. 70%). The chain propagation rate constant (k_p) of propylene polymerization is very close to that of ethylene polymerization, but the propylene insertion rate constant is much smaller than the ethylene insertion rate constant in the copolymerization system, meaning that the active centers in the homopolymerization system are different from those in the copolymerization system. Ethylene insertion rate constant in the copolymerization system was much higher than that in the ethylene homopolymerization in the first 10 min of reaction. A mechanistic model was proposed to explain the observed activation of ethylene polymerization by propylene addition. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 867–875     

Polymer‐supported zirconocene catalyst for ethylene polymerization

The use of crosslinked poly(styrene‐co‐4‐vinylpyridine) having functional groups as the support for zirconocene catalysts in ethylene polymerization was studied. Several factors affecting the activity of the catalysts were examined. Conditions like time, temperature, Al/N (molar ratio), Al/Zr (molar ratio), and the mode of feeding were found having no significant influence on the activity of the catalysts, while the state of the supports had a great effect on the catalytic behavior. The activity of the catalysts sharply increased with either the degree of crosslinking or the content of 4‐vinylpyridine in the support. Via aluminum compounds, AlR₃ or methylaluminoxane (MAO), zirconocene was attached on the surface of the support. IR spectra showed an intensified and shifted absorption bands of C N in the pyridine ring, and a new absorption band appeared at about 730 cm⁻¹ indicating a stable bond Al N formed in the polymer‐supported catalysts. The formation of cationic active centers was hypothesized and the performance of the polymer‐supported zirconocene was discussed as well. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 37–46, 1999     

A DFT Study on the Mechanism of Rh2II,II‐Catalyzed Intramolecular Amidation of Carbamates

The potential‐energy surfaces of the reactions of dirhodium tetracarboxylate (Rh₂^II,II) catalyzed nitrene (NR) insertion into C H bonds were examined by a DFT computational study. A pure Becke exchange functional (B88) rather than a hybrid exchange functional (B3, BHandH) was found to be appropriate for the calculation of the energy difference between the singlet and triplet Rh₂^II,II–NH nitrene species. Rh₂^II,II–NR¹ (R¹=(S)‐2‐methyl‐1‐butylformyl) is thermodynamically more favorable with a free energy lower than that of Rh₂^II,II–N(PhI)R¹. The singlet and triplet states of Rh₂^II,II–NR¹ have similar stability. Singlet Rh₂^II,II–NR¹ undergoes a concerted NR insertion into the C H bond with simultaneous formation of the N H and N C bonds during C H bond cleavage; triplet Rh₂^II,II–NR¹ undergoes H atom abstraction to produce a diradical, followed by subsequent bond formation by diradical recombination. The singlet pathway is favored over the triplet in the context of the free energy of activation and leads to the retention of the chirality of the C atom in the NR insertion product. The reactivities of the C H bonds toward the nitrene‐insertion reaction follow the order tertiary>secondary>primary. Relative reaction rates were calculated for the six reaction pathways examined in this work.     

A Joint Experimental/Theoretical Investigation of the Statistical Olefin/Conjugated Diene Copolymerization Catalyzed by a Hemi‐Lanthanidocene [(Cp*)(BH4)LnR]

Statistical copolymerization of ethylene and isoprene was achieved by using a borohydrido half‐lanthanidocene complex. Under copolymerization conditions, activation of [(Cp*)(BH₄)₂Nd(thf)₂] (Cp*=η⁵‐C₅Me₅) by an appropriate alkylating agent affords trans‐1,4‐poly‐isoprene‐co‐ethylene. Analysis of the microstructure of the copolymer revealed the presence of successive short sequences of ethylene/ethylene, trans‐1,4‐isoprene/ethylene, and trans‐1,4‐isoprene/trans‐1,4‐isoprene. A small amount of 1,2‐insertion of isoprene was observed, and no cyclic structures within the chain were characterized. Test runs showed that these catalysts are unable to copolymerize α‐olefins (such as hex‐1‐ene) with isoprene. The probable initial steps in the copolymerization have been computed at the DFT level of theory. Analysis of the energy profile provides insight into the catalyst’s activity and selectivity. Our theoretical results highlight the key role played by the allyl intermediate, in which diene insertion, and to a lesser extent olefin insertion, is the rate‐determining step of the process. These results also illustrate the coordination behavior of the allyl ligand during the insertion of an incoming monomer, which directly inserts, after pre‐coordination to the metal center, into the η³‐allyl ligand without inducing an η³ to η¹ haptotropic shift. Finally, the inactivity of this family of catalysts towards the copolymerization of hex‐1‐ene was rationalized on the basis of the free‐energy profile of the copolymerization.     

Hydride Ligands Make the Difference: Density Functional Study of the Mechanism of the Murai Reaction Catalyzed by [Ru(H)2(H2)2(PR3)2] (R=cyclohexyl)

The catalytic cycle for the Murai reaction at room temperature between ethylene and acetophenone catalyzed by [Ru(H)₂(H₂)₂(PMe₃)₂] has been studied computationally at the B3PW91 level. The active species is the ruthenium dihydride complex [Ru(H)₂(PMe₃)₂]. Coordination of the ketone group to Ru induces very easy C H bond cleavage. Coordination of ethylene after ketone de-coordination, followed by ethylene insertion into a Ru H bond, creates the Ru ethyl bond. Isomerization of the complex to a Ru^IV intermediate creates the geometry adapted to C C bond formation. Re-coordination of the ketone before the C C coupling lowers the energy of the corresponding TS. The highest point on the potential energy surface (PES) is the TS for the isomerization to the Ru^IV intermediate, which prepares the catalyst geometry for the C C coupling step. Inclusion of dispersion corrections significantly lowers the height of the overall activation barrier. The actual bond cleavage and bond forming processes are associated to low activation barriers because of the presence of hydrogen atoms around the Ru center. They act as redox buffers through formation and breaking of H H bonds in the coordination sphere. This flexibility allows optimal repartition of the various ligands according to the change in stereoelectronic demands along the catalytic cycle.     

Isomeric effect of the Et(H4Ind)2Zr(CH3)2 catalyst on the copolymerization of ethylene and styrene: A computational study

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     

Dihydrophilic block copolymers with ionic and nonionic blocks. I. Poly(ethylene oxide)‐b‐polyglycidol with OP(O)(OH)2, COOH,or SO3H functions: Synthesis and influence for CaCO3 crystallization

Dihydrophilic block copolymers of poly(ethylene oxide)‐b‐polyglycidol were prepared and polyglycidol blocks converted into ionic blocks containing  OP(O)(OH)₂,  COOH, or  SO₃H groups. Although phosphorylation of polyhydroxy compounds with POCl₃ usually leads to insoluble products, phosphorylation of poly(ethylene oxide)‐b‐polyglycidol using a POCl₃/ OH ratio equal to 1/1 gave soluble products, predominantly monoester of phosphoric acid (after hydrolysis) (provided that the reaction was conducted in triethyl phosphate as solvent). All copolymers were characterized by ¹H NMR, ¹³C NMR, and/or ³¹P NMR spectra for confirming their structure. The degree of substitution was determined from quantitative ¹³C NMR spectroscopy (inverted‐gate decoupling‐acquisition mode). Preliminary results indicate that from these three groups of block copolymers the phosphoric acid esters are the most effective ones at least in controlling the growth of CaCO₃ crystals in aqueous solution. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 955–963, 2001     

A kind of novel nonmetallocene catalysts for ethylene polymerization

A kind of novel bridged nonmetallocene catalysts was synthesized by the treatment of N,N‐imidazole and N,N‐phenylimidazole with n‐BuLi, and MCl₄ (M = Ti, Zr) in THF. Those catalysts were performed for ethylene polymerization after activated by methylaluminoxane (MAO). The effects of polymerization temperature, Al/M ratio, pressure of monomer, and concentration of catalysts on ethylene polymerization behaviors were investigated in detail. Those results revealed that the catalyst system was favorable for ethylene polymerization with high catalytic activity. The polymer was characterized by ¹³C NMR, WAXD, GPC, and DSC. The result confirmed that the obtained polyethylene featured broad molecular weight distribution around 20, linear structure, and relative low melting temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 33–37, 2008     

Ethylene polymerization reactions with Ziegler–Natta catalysts. III. Chain-end structures and polymerization mechanism

Ethylene polymerization reactions with many Ziegler–Natta catalysts exhibit a number of features that differentiate them from polymerization reactions of α olefins: (1) a relatively low ethylene reactivity, (2) markedly higher polymerization rates in the presence of α olefins, (3) a high reaction order with respect to ethylene concentration, and (4) a strong reversible rate depression in the presence of hydrogen. A detailed kinetic analysis of ethylene polymerization reactions¹ provided the basis for a new kinetic scheme that postulates the equilibrium formation of Ti C₂H₅ species with the H atom in the methyl group β-agostically coordinated to the Ti atom in an active center. This mechanism predicts several new features of ethylene polymerization reactions, one being that chain initiation via insertion of any α-olefin molecule into the Ti H bond should proceed with an increased probability compared to that via ethylene insertion into the same bond. As a result, a significant fraction of ethylene/α-olefin copolymer chains should contain α-olefin units as the starting units. This article provides experimental data supporting this prediction on the basis of both a detailed structural analysis of co-oligomers formed in ethylene/1-pentene and ethylene/4-methyl-1-pentene copolymerization reactions and a spectroscopic analysis of chain ends in the copolymers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4281–4294, 1999     

Synthesis of a new zirconium catalyst for ethylene polymerization

A novel complex dichlorobis(2‐ethyl‐3‐hydroxy‐4‐pyrone)zirconium(IV) (ZrCl₂(ethylpyrone)₂) was synthesized. Complexation of the pyrone ligand to the zirconium was confirmed by UV, ¹H and ¹³C‐NMR, and electrochemical studies. NMR showed the presence of four isomers and density functional theory calculations indicated that the main isomer had a cis configuration. The catalyst was shown to be active in ethylene polymerization in the presence of the cocatalyst methylaluminoxane. The highest catalyst activity for the zirconium complex was achieved at Al/Zr = 2500, 70 °C and when a small concentration of catalyst was used (1 μmol). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3830–3841, 2008     

Structure characterization of copolymers of ethylene and 1‐octadecene

Four ethylene‐1‐octadecene copolymers and the corresponding polyethylene homopolymer, synthesized with a metallocene catalyst, have been analyzed by using three characterization techniques in the solid state: differential scanning calorimetry, wide‐angle X‐ray diffraction, and Raman spectroscopy. Very important annealing effects are observed in the copolymers with higher comonomer content while standing at room temperature, in such a way that the enthalpies of melting derived from the first and second melting are different. The X‐ray diffractograms have been analyzed in terms of amorphous and crystalline components, determining both the crystallinity and the position of the different reflections. The variation of the unit cell parameters has been calculated from those reflections. No indication of a possible participation of the relatively long 1‐octadecene branches in the crystallization can be deduced from the X‐ray data. The degree of crystallinity has also been determined from the Raman spectra, following two procedures. The results indicate that the crystallinities deduced from the band at 1416 cm⁻¹ are much lower than those derived from the other two characterization techniques. On the contrary, the data from the 1060 cm⁻¹ band are practically coincident with the X‐ray determinations. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1440–1448, 2000     

Olefin polymerization and copolymerization with soluble transition‐metal complex catalysts

Olefin polymerizations catalyzed by Cp′TiCl₂(O‐2,6‐ⁱPr₂C₆H₃) ( 1 – 5 ; Cp′ = cyclopentadienyl group), RuCl₂(ethylene)(pybox) { 7 ; pybox = 2,6‐bis[(4S)‐4‐isopropyl‐2‐oxazolin‐2‐yl]pyridine}, and FeCl₂(pybox) ( 8 ) were investigated in the presence of a cocatalyst. The Cp*TiCl₂(O‐2,6‐ⁱPr₂C₆H₃) ( 5 )–methylaluminoxane (MAO) catalyst exhibited remarkable catalytic activity for both ethylene and 1‐hexene polymerizations, and the effect of the substituents on the cyclopentadienyl group was an important factor for the catalytic activity. A high level of 1‐hexene incorporation and a lower r_E · r_H value with 5 than with [Me₂Si(C₅Me₄)(N^tBu)]TiCl₂ ( 6 ) were obtained, despite the rather wide bond angle of Cp Ti O (120.5°) of 5 compared with the bond angle of Cp Ti N of 6 (107.6°). The 7 –MAO catalyst exhibited moderate catalytic activity for ethylene homopolymerization and ethylene/1‐hexene copolymerization, and the resultant copolymer incorporated 1‐hexene. The 8 –MAO catalyst also exhibited activity for ethylene polymerization, and an attempted ethylene/1‐hexene copolymerization gave linear polyethylene. The efficient polymerization of a norbornene macromonomer bearing a ring‐opened poly(norbornene) substituent was accomplished by ringopening metathesis polymerization with the well‐defined Mo(CHCMe₂Ph)(N‐2,6‐ⁱPr₂C₆H₃)[OCMe(CF₃)₂]₂ ( 10 ). The key step for the macromonomer synthesis was the exclusive end‐capping of the ring‐opened poly(norbornene) with p‐Me₃SiOC₆H₄CHO, and the use of 10 was effective for this polymerization proceeding with complete conversion. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4613–4626, 2000     

Structure of Charged Poly(propylene imine) Dendrimers: Theoretical Investigation

A mean‐field model for charged dendrimers has been elaborated and applied to Astramol dendrimers of 5^th generation in salt‐free solution. The free energy of a dendrimer molecule was minimized with respect to the dendrimer size and to the profile of counterion distribution. The model of highly stretched freely jointed chain was used to describe the elasticity of long branches, the dissociated groups were assumed to be localized mostly on the periphery of the molecule, and the electrostatic interactions were described in the Poisson‐Boltzmann approximation. It was found that the osmotic pressure of counterions leads to moderate expansion of dendrimer molecules upon charging, and a significant fraction of counterions is localized within the dendrimer molecule under typical experimental conditions.

The schematic structure of poly(propylene imine) dendrimers for the 4^th generation.     

DFT mechanistic study of the H2‐assisted chain transfer copolymerization of propylene and p‐methylstyrene catalyzed by zirconocene complex

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?PP_n facilely, which displace functionalized pMS?PP_n 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. 2015 , 53, 576–585     

Role of the anomeric effect in methanediamines in the gas phase and aqueous solutions

An ab initio study of methylenediamine and several methylated derivatives in the gas phase and aqueous solution was performed. The conformational preferences can be considered adequately described at the HF/6‐31G**//HF/6‐31G** level, because these results agree with those obtained using larger basis sets and including ZPE and electron correlation. The energy ordering is clearly dependent on the number and position of the methyl groups present in a molecule. For a first set of the compounds, the energies obtained were interpreted in terms of the anomeric effect because the favored conformers show two or one anti orientation between the nitrogen lone pair and the C N polar bond. Reverse anomeric effects were found for a second set of compounds. The NBO analysis was used to interpret these energetic tendencies and the rotational barrier around the N C bonds. Thus, the preference for the anti‐Lp N C N orientations is mainly due to charge delocalization, always stronger than the electrostatic and steric contributions included in the Lewis term. However, the origin for the reverse anomeric effect may be related to the steric hindrance associated with the methylation. The influence of water on the conformational preferences was evaluated by means of the PCM method. Contrary to expectation, the anomeric effect is not reduced in water, and the most stable conformers in the gas phase are maintained in solution. The electrostatic term of the free solvation energy is the main responsible of the energetic changes, and depends strongly on local solute–solvent interactions. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 135–150, 2001     

A DFT Quantum‐Chemical Study of the Structure of Precursors and Active Sites of Catalyst Based on 2,6‐Bis(imino)pyridyl Fe(II) Complexes

Summary: A DFT method has been applied for quantum‐chemical calculations of the molecular structure of charge‐neutral complex LFeMe(μMe)₂AlMe₂ which is formed in system LFeMe₂ + AlMe₃ (L = 2,6‐bis(imino)pyridyl). Calculations suggested the formation of highly polarized complex LFeMe(μMe)₂AlMe₂ ( II ) in system LFeMe₂ + AlMe₃, characterized by r(Fe μMe) = 3.70 Å and r(Al μMe) = 2.08 Å and deficient electron density on fragment [LFeMe]^Q (Q = +0.80 e). Polarization of the complex progresses with the bounding of two AlMe₃ molecules (complex LFeMe(μMe)₂AlMe₂ · 2AlMe₃ ( III )) and with replacement of AlMe₃ by MeAlCl₂ (complex LFeMe(μMe)₂AlCl₂ ( IV )). The activation energy of ethylene insertion into the Fe Me bond of these complexes has been calculated. It was found that the heat of π‐complex formation increases with increasing of polarization extent in the order II < III < IV . Activation energy of the insertion of coordinated ethylene into Fe Me bond decreases in the same order: II > III > IV .

Calculated model complex (NH₃)₃FeMe₂; tridentate bis(imino)pyridyl ligand was substituted by three coplanar NH₃ groups.     

Activity and stability spontaneously enhanced toward ethylene polymerization by employing 2‐(1‐(2,4‐dibenzhydrylnaphthylimino)ethyl)‐6‐(1‐(arylimino)ethyl) pyridyliron(II) dichlorides

A series of 2‐(1‐(2,4‐dibenzhydrylnaphthylimino)ethyl)‐6‐(1‐(arylimino)ethyl)pyridyliron(II) complexes ( Fe1 ? Fe5 ) was synthesized and characterized. The molecular structure of the representative Fe2 was determined by single‐crystal X‐ray diffraction, revealing a distorted pseudo‐square‐pyramidal geometry around the iron center. On activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all these iron complex precatalysts performed with high activities (up to 1.58 × 10⁷ g (PE) mol^?1 (Fe) h^?1) toward ethylene polymerization, producing highly linear polyethylenes with high molecular weight and bimodal distribution, which was in accordance with high temperature ¹³C NMR, high T _m values (T _m ～130 °C) and the GPC curves of the obtained polyethylenes. Meanwhile, DFT calculation results also showed the good correlation between net charges on iron and experimental activities. Compared with previous bis(imino)pyridyliron analogues, the current iron complexes containing the benzhydrylnaphthyl groups exhibited relatively higher activities and better thermal‐stability at elevated temperatures, especially at 80 °C as the industrial operating temperature, and still showed high activities toward ethylene polymerization up to 8.57 × 10⁶ g (PE) mol^?1 (Fe) h^?1 in the presence of co‐catalyst MMAO. In addition, these iron complex precatalysts all exhibited long lifetimes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 988–996     

Preparation of linear low‐density polyethylene by the in situ copolymerization of ethylene with an iron oligomerization catalyst and rac‐ethylene bis(indenyl) zirconium (IV) dichloride

An iron oligomerization catalyst, [(2‐ArN?C(Me))₂C₅H₃N]FeCl₂ [Ar = 2,6‐C₆H₃(F)₂], was combined with rac‐ethylene bis(indenyl)zirconium (IV) dichloride [rac‐Et(Ind)₂ZrCl₂] to prepare linear low‐density polyethylene (LLDPE) by the in situ copolymerization of ethylene. A series of LLDPEs with different properties were prepared by the alteration of the reaction temperature, Fe/Zr molar ratio, Al/(Fe + Zr) molar ratio, and reaction time. The structures of the polymers were characterized with differential scanning calorimetry, ¹³C NMR, gel permeation chromatography (GPC), and so forth. The melting points, crystallizations, and densities of the resulting products increased, and the average branching degree decreased, as the reaction temperature, Al/(Fe + Zr) ratio, and reaction time increased. The melting points, crystallizations, and densities of the polymers decreased, and the average branching degree increased, when the Fe/Zr ratio increased. The ¹³C NMR and GPC results showed that there were no unreacted α‐olefins remaining in the resulting polymers because the percentage of low‐molar‐mass sections (C₄–C₁₀) of the oligomers obtained with this catalyst was very high (>70%). In addition, the formation of polymers with two melting points under different reaction conditions was examined in detail, and the results indicated that the two melting points of the polymers could be attributed to polyethylene with different branches. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 984–993, 2005