Propene Insertion Research Articles

Structural dependence of thermodynamics of alkene binding to yttrium alkyl complexes and of kinetics of alkyl migration to coordinated alkenes.

Agostic interactions in yttrium alkyls are structure dependent. Primary alkyl yttrium complexes have beta-CH(2) agostic interactions at low temperature, but a shift toward alpha-agostic interactions occurs on warming. For the more crowded beta-disubstituted yttrium alkyls, an alpha-CH(2) agostic interaction is seen. The thermodynamics of alkene binding to the primary alkyl yttrium complex Cp(2)YCH(2)CH(2)CH(CH(3))(2) (2) depend strongly on the structure of the alkene. A single allylic substituent on the alkene has a small effect on alkene binding, but a second allylic substituent has a large destabilizing effect. Propene binding to yttrium alkyls is largely independent of the nature of the alkyl ligand. Equilibrium constants for propene binding to n-, gamma-substituted, beta-substituted, and secondary alkyl yttrium complexes are similar. The rate of migration of an alkyl group to a coordinated alkene depends strongly on the structure of the alkyl group: n-alkyl approximately gamma-substituted >> beta-substituted >> alpha-substituted. The approximately 200-fold slower insertion of propene into Cp(2)YCH(2)CH(CH(3))(2) (6) than that into Cp(2)YCH(2)CH(2)CH(CH(3))(2) (2) is therefore due to kinetically slow migration of the beta-disubstituted alkyl group of 6 and not to differences in the equilibrium binding of propene. Processes related to chain transfer and site epimerization at the metal center are also reported.

Propene−Norbornene Copolymers: Synthesis and Analysis of Polymer Structure by 13C NMR Spectroscopy and ab Initio Chemical Shift Computations

A series of propene−norbornene (P−N) copolymers were synthesized in the presence of rac-Et(indeny)2ZrCl2/MAO in toluene at 30 °C. P−N copolymers were characterized by 13C NMR spectroscopy, SEC, and DSC. Results were compared with those of a series of P−N copolymers synthesized in the presence of rac-Me2Si(Indenyl)2ZrCl2/MAO under the same experimental conditions. Polymerization activity appears to be quite low in comparison with ethene−norbornene (E−N) copolymerization. A first assignment of the main 13C NMR signals of P−N copolymers containing isolated N units was obtained on the basis of distortionless enhancement by polarization transfer (DEPT) 13C spectra and by comparison with isotactic polypropene (i-PP) and E−N copolymer spectra. Ab initio theoretical 13C NMR chemical shifts, computed for the most relevant conformers of a rather simple model compound and averaged using the RIS conformer populations estimated for an isotactic chain (P4−N)x, gave important detailed indications for the assignment of the complex 13C spectra of P−N copolymers with N content up to 35 mol %. Such assignments were used to estimate the N copolymer content. The discrepancy between the values obtained from the areas of methyl signals and from the areas of the signals assigned to norbornene carbons was found to be partially due to the presence of 1,3-propene misinsertions, which are formed in significant amount when increasing the [N]/[P] ratio of the feed. Our results confirm that, despite the relatively lower polymerization activity, at low norbornene/olefin ratio it is possible to obtain P−N copolymers that are relatively richer in norbornene than the E−N copolymers prepared in similar conditions. However, at higher norbornene/olefin feed ratios the great amount of 1,3-propene misinsertions clearly reveals that the steric hindrance of the Mt−tertiary carbon bond when N is the last inserted unit makes difficult the next propene insertion, causing low polymerization activities, molecular masses, and Tg.

A proposal on an index of activities and selectivities of olefin polymerization catalysts by using 'paired interacting orbitals (PIO)' analysis

Olefin insertion and chain transfer to monomer on [Oh-CH3MCl4]m(M = Ti, Zr, m = –1, –2: models of Ziegler-Natta catalysts) and [Td-CH3ML2]m(M = Ti, Zr; L = Cl, Cp m = 0, +1: models of metallocene catalysts) were analyzed by paired interacting orbitals (PIO) proposed by Fujimoto et al. Polymerization activities, molecular weights and regioselectivities of propylene insertion were easily predicted by using the total overlap population of all PIOs as an index of catalytic reactions.

Hydrogen effects in propylene polymerization reactions with titanium‐based Ziegler–Natta catalysts. I. Chemical mechanism of catalyst activation

AbstractThe hydrogen activation effect in propylene polymerization reactions with Ti‐based Ziegler–Natta catalysts is usually explained by hydrogenolysis of dormant active centers formed after secondary insertion of a propylene molecule into the growing polymer chain. This article proposes a different mechanism for the hydrogen activation effect due to hydrogenolysis of the Tiiso‐C3H7 group. This group can be formed in two reactions: (1) after secondary propylene insertion into the TiH bond (which is generated after β‐hydrogen elimination in the growing polymer chain or after chain transfer with hydrogen), and (2) in the chain transfer with propylene if a propylene molecule is coordinated to the Ti atom in the secondary orientation. The TiCH(CH3)2 species is relatively stable, possibly because of the β‐agostic interaction between the H atom of one of its CH3 groups and the Ti atom. The validity of this mechanism was demonstrated in a gas chromatography study of oligomers formed in ethylene/α‐olefin copolymerization reactions with δ‐TiCl3/AlEt3 and TiCl4/dibutyl phthalate/MgCl2–AlEt3 catalysts. A quantitative analysis of gas chromatography data for ethylene/propylene co‐oligomers showed that the probability of secondary propylene insertion into the TiH bond was only 3–4 times lower than the probability of primary insertion. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1353–1365, 2002

Mechanism of propylene insertion using bis(phenoxyimine)-based titanium catalysts: an unusual secondary insertion of propylene in a group IV catalyst system.

A highly regioselective secondary enchainment of propylene in a group IV catalyst system is reported. End-group analysis of polypropylene formed using the phenoxyimine-based titanium catalysts revealed a reversal in the regioselectivity of insertion for this class of catalysts. To the best of our knowledge, bis(phenoxyimine)-based titanium complexes are the only known group IV catalysts that insert propylene with exclusive 2,1-regiochemistry. Insertion of propylene into the initiating titanium hydride occurs with high 1,2-regiochemistry. Subsequent insertions into primary titanium alkyls are regiorandom, while insertions into secondary titanium alkyls proceed with high 2,1-regioselectivity. Cyclopolymerization and ethylene/propylene copolymerization strategies are employed to support this proposal.

Influence of regio- and stereoregularity of propene insertion on crystallization behavior and elasticity of ethene-propene copolymers.

Melting enthalpies of several ethene-propene samples, obtained by different single-center catalytic systems and having widely different constitutions and compositions in the range of industrial interest, are compared. For the first time, a clear evidence of influence of the regio- and stereoregularity of propene insertion on crystallization behavior of ethene/propene copolymers is provided. This influence is in qualitative agreement with the results of simplified energy calculations relative to models of pseudo-hexagonal packing of ethene-propene copolymers.

Quantum mechanical modelling of alkene hydroformylation as catalyzed by xantphos-Rh complexesBased on the presentation given at Dalton Discussion No. 4, 10–13th January 2002, Kloster Banz, Germany.

Fundamental issues concerning the hydroformylation of 1-alkenes as catalyzed by Rh complexes ligated with the xantphos diphosphine ligand are explored using ONIOM calculations. In this study xantphos serves as a prototype of the large bite-angle ligands that are associated with high regioselectivity and rates in catalytic hydroformylation. Computations have been used to explore the thermodynamics of 56 unique isomers (e.g., cis vs. trans isomers of square planar complexes, diequatorial vs. axial-equatorial five-coordinate complexes) and conformers for intermediates along the reaction pathway. More than 20 transition states relevant to the catalyst mechanism have been determined. In terms of realistically modelling experiment, the computational results are mixed. In agreement with experiment, the computations yield a mixture of diequatorial and axial-equatorial isomers of HRh(xantphos)(CO)2 as the catalyst resting state. Dissociation of CO from these complexes is computed to be barrierless leading to a computed free energy for exchange of CO ligands around 15 kcal mol−1, somewhat lower than the value of ca. 20 kcal mol−1 derived from experimental data. The computed ratios of rates of propene insertion to form n-propyl and i-propyl Rh-alkyl (42 ∶ 1) is in good agreement with experimental ratios of n-nonanal to i-nonanal (52 ∶ 1) for 1-octene hydroformylation. Nonetheless, the computations dramatically overestimate the overall activation free energies for catalytic hydroformylation. Thus, at this stage computations do not provide useful insight into the the kinetics of hydroformylation and detailed mechanistic issues. It appears that much of this discrepancy between computed and experimental activation energies originates from the underestimation of propene bonding energies.

Syndiospecific Polymerization of Propene Promoted by Bis(salicylaldiminato)titanium Catalysts: Regiochemistry of Monomer Insertion and Polymerization Mechanism

Syndiotactic polypropylene samples were synthesized under proper conditions using homogeneous catalytic systems based on bis(iminophenoxy)Ti(IV) derivatives and methylaluminoxane. 13C NMR analysis of the natural abundance end groups suggests that the regiospecificity of propene insertion during the propagation is prevailingly 2,1, although 1,2 insertion is favored in the initiation step. A copolymer of propene with a small amount of 1-13C-ethene was also prepared with one of the title catalysts, and its stereochemical structure was analyzed by NMR, confirming 2,1 monomer insertion as the main mode of propagation.

Alternating Copolymerization of Ethylene and Propylene: Evidence for Selective Chain Transfer to Ethylene

C1-symmetric ansa-metallocenes Me2E(Ind)(Flu)ZrCl2 (E = Si (1) and C (2)), upon activation with methylaluminoxane (MAO), produce atactic alternating ethylene−propylene copolymers (up to 92% EP dyads) with low molecular weight (less than 20K). Quantitative 13C NMR analysis of copolymer chain ends produced with 2/MAO indicates that the chain initiation/termination process is highly selective to generate a vinylidene as the unsaturated end group (terminating end) and an ethyl group as the saturated end group (initiating end). The high selectivity for sec-butyl end groups reveals a highly selective chain-initiation process where ethylene insertion is followed by propylene insertion at the initiating end. Chain transfer from a propylene-terminated polymer chain to a coordinated ethylene monomer is proposed to explain this selectivity. The facile cross-monomer chain-transfer process explains the much lower molecular weights obtained for the EP copolymers relative to the ethylene or propylene homopolymers.

Hybrid QM/MM study of propene insertion into the RhH bond of HRh(PPh 3) 2(CO)(η 2-CH 2CHCH 3): the role of the olefin adduct in determining product selectivity

A hybrid QM/MM computational investigation of propene insertion into the RhH bond of HRh(PPh 3) 2(CO)(η 2-CH 2CHCH 3) was performed to address the issue of kinetic (transition state) versus thermodynamic (ground state) determination of regioselectivity in hydroformylation catalysis. Two propene adduct isomers; one having an equatorial–axial arrangement of the two PPh 3 co-ligands (ea- 3) and the other having a bis-equatorial arrangement (ee- 1), were predicted to be the most stable. The adduct ea- 3 was predicted to be more stable than ee- 1, by 1.0 kcal mol −1, and based on the computed Boltzmann populations ea- 3 is expected to be present in roughly a three-fold excess over ee- 1. Based on the computed energy barriers leading to the corresponding linear and branched Rh–propyl products, ea- 3 generates the linear insertion product almost exclusively, while ee- 1 produces primarily the branched product. The 1.6 kcal mol −1 difference in their respective activation energies translates into a fifteen-fold greater reactivity for ee- 1 than ea- 3. Hence, two separate reaction channels exist, one leading to the branched insertion product which is derived from the more active, minor propene adduct (ee- 1), and one leading to the linear insertion product which originates from the less active, major propene adduct (ea- 3). Thus the regioselectivity in hydroformylation catalysis may be rationalized in terms of ground state discrimination between the two most stable isomers of the propene adducts.

DFT Studies on Substituent Effects in Palladium-Catalyzed Olefin Polymerization

Gradient-corrected density functional theory has been used to study substituents effects on the cationic N∧N−Pd(II) diimine catalyst in ethene and propene polymerization. Here N∧N = −C(R)−N(Ar)−N(Ar)−C(R)− with R = H, −CH3, −An and Ar = H, C6H5, −2,6-C6H3(Me)2, −2,6-C6H3(iPr)2. Calculations have been performed on the [N∧N-Pd(II)−P]+ (P = n-propyl and isopropyl) alkyl complexes (1) and the corresponding [N∧N-Pd(II)−P(η2-CH2CHRo)]+ π-complexes of ethene (Ro = H) and propene (Ro = CH3), as well as the ethene and propene (1,2- and 2,1-) insertion transition states. The results show that an increase in the size of the substituents on the Pd(II) catalyst enhances the preference of 1 for the isomer with the branched isopropyl alkyl group P, while for the olefin complexes [N∧N-Pd(II)−P(η2-CH2CHRo)]+ the isomer with the linear n-propyl group P becomes preferred. Further, an increase in the size of the substituents affects the relative binding of ethene and propene. Thus, the electronic preference of propene comple...

Secondary syndiotactic-specific propene insertion in the presence of homogeneous V-based catalysts

This paper shows, by terminating the polymerization through deuterolysis with CF 3CH 2OD, that the propyl end groups of syndiotactic polypropylene obtained in the presence of homogeneous catalysts, based on vanadium salts and alkylaluminium halides, such as VCl 4–Al(C 2H 5) 2Cl–Anysole, arise, at least mostly, from termination, thus confirming the Coleman–Fox type of the statistical model of the propagation already proposed in previous papers. Direct evidence is also provided that secondary insertion of propene is the only syndiotactic specific polymerization step. In view of this, the early hypotheses concerning the structure of the active species promoting syndiotactic specific polymerization are critically reconsidered.

Synthesis, Fluxionality, and Propene Insertion Reactions of Zirconium Boryldiene Complexes with Sterically Undemanding Cp Ligands

The reduction of CpRZrCl3(dme) (CpR = Cp (a), C5H4SiMe3 (b), C5H4Me (c), Ind (d)) with sodium amalgam in the presence of isoprene, followed by the addition of allylmagnesium chloride, gives the diene complexes CpRZr(η3-allyl)(η4-isoprene) (2a−d). The preparations are conveniently carried out as one-pot reactions. The reaction of 2a−d with B(C6F5)3 in toluene solution at −78 °C proceeds quantitatively to give the thermally unstable zwitterionic complexes CpRZr(η3-allyl){η1:η3-CH2CMeCHCH2B(C6F5)3} (3a−d), which on warming decompose under C−H activation and propene elimination to give CpRZr(C6F5){η4-CH2CMeCHCHB(C6F5)2} (4a−d). The complexes are stabilized by the coordination of one o-F atom of a boryl−C6F5 ring to the metal center. Compounds 4 are fluxional. The rotation of the Zr−C6F5 ligand is influenced by the steric demand of the Cp ligands (ΔG⧧ = 43−49 kJ mol-1), while there is little variation in the rotational barriers of the B−C6F5 substituents (ΔG⧧ = ca. 47 kJ mol-1). Recrystallization of 4a from di...

High molecular weight atactic polypropene synthesized with (η5-pentamethylcyclopentadienyl)tribenzyl titanium activated with MAO

The half-titanocene (η5-pentamethylcyclopentadienyl)tribenzyl titanium (Cp*TiBz3) with methylaluminoxane (MAO) as the cocatalyst was employed to catalyze propene polymerization at ambient pressure. A novel atactic polypropene elastomer with a high molecular weight (M̄w = 2 − 8 × 105) was produced. The effects of the polymerization conditions on the catalytic activity and polymer molecular weight are discussed. 13C NMR analysis confirmed that the catalyst system Cp*TiBz3/MAO produced atactic polypropenes, and the polymerization mechanism was in agreement with the Bernoullian process. The triad sequence distribution of the polymer was measured and found to be as follows: mm = 6.15%, mr = 40.87%, and rr = 52.98% (Bernoullian factor B = 1.03); this indicated that the insertion of propene with the catalyst system followed a chain-end control model. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 411–415, 2000

High molecular weight atactic polypropene synthesized with (?5-pentamethylcyclopentadienyl)tribenzyl titanium activated with MAO

The half-titanocene (η5-pentamethylcyclopentadienyl)tribenzyl titanium (Cp*TiBz3) with methylaluminoxane (MAO) as the cocatalyst was employed to catalyze propene polymerization at ambient pressure. A novel atactic polypropene elastomer with a high molecular weight (Mw = 2 − 8 × 105) was produced. The effects of the polymerization conditions on the catalytic activity and polymer molecular weight are discussed. 13C NMR analysis confirmed that the catalyst system Cp*TiBz3/MAO produced atactic polypropenes, and the polymerization mechanism was in agreement with the Bernoullian process. The triad sequence distribution of the polymer was measured and found to be as follows: mm = 6.15%, mr = 40.87%, and rr = 52.98% (Bernoullian factor B = 1.03); this indicated that the insertion of propene with the catalyst system followed a chain-end control model. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 411–415, 2000

Zirconocene Allyl Complexes: Dynamics in Solution, Reaction with Aluminum Alkyls, B(C6F5)3-Induced Propene Insertion, and Density-Functional Calculations on Possible Formation and Reaction Pathways

The complex (C5H5)2Zr(Me)(methallyl) (1) was prepared and studied as a model for Zr-bound allyl species which are likely to arise in zirconocene-based polymerization systems. 1 undergoes allyl−alky...

Regioselectivity of the insertion of propene with achiral Pd(ii) catalysts to highly isotactic poly[1-oxo-2-methylpropane-1,3-diyl]. Is the syndiotactic structure accessible?

Achiral cationic palladium catalysts, modified with 1,2-bis(diarylphosphinomethyl)benzene ligands with different electronic properties, can efficiently produce poly(propene-alt-CO) with essentially complete regioregularity and high isotacticity, depending on the aryl substituent; the stereochemistry of the copolymers obtained is barely, if at all, influenced by the regiochemistry of the olefin insertion.

Regioirregular Propene Insertion in Polypropenes Synthesized with Unbridged Bis(2-aryl)indenyl Zirconium Dichloride Catalysts: Implications on Activity

Catalysts of the type bis(2-(3,5-R 2 C 6 H 3 )indenyl)zirconium dichloride/MAO (2, R = H; 3, R = CF 3 ) were investigated for regioregularity of monomer insertion in bulk propene polymerization and compared to the isospecific catalyst rac-(ethene)bis(indenyl)zirconium dichloride/MAO (1). The amount of regioirregular propene insertions were in the range of 0.1-0.5 mol % and followed the trend 2 < 3 < 1. Comparison of polymerizations in the presence and absence of hydrogen revealed a 4-10-fold increase in productivity for catalysts 2 and 3 in the presence of hydrogen but only a slight increase in productivity with 1. For catalysts 2 and 3, only 2,1-erythro internal regioerrors were observed, whereas 1 produced both 2,1-erythro and 2,1-threo misinsertions. Evidence for ethene insertion following a 2,1-regioerror was obtained by copolymerization of 13 C 2 H 4 with propene using catalyst 3. The correlation between number of 2,1-misinsertions and catalyst activity is discussed.

Hartree–Fock and density functional theory analysis of propylene insertion in Al(CH 3) 3/TiCl 3/TiO 2 (red.) in the presence of a Lewis base

Titanium tetrachloride heterogenized on reduced TiO 2 was recently described as an active catalyst for ethylene polymerization. This catalytic system also permits propylene polymerization with lower activity, however, with low polydispersity, high molecular weight and with the good isotacticity of 78%. The isotacticity of this polymer can be increased by addition of a Lewis base. The behavior of the active site, in the presence of a Lewis base, was studied by ab initio Hartree–Fock and density functional theory. The atomic charge values obtained suggest that methyl benzoate used as a Lewis base makes the active site more susceptible to coordination with propylene.

Propylene polymerization with Ph2C(3-RCp)(Flu)ZrCl2 [R = Me, i-Pr, PhCH2, Me3Si] catalysts activated with MAO and Me2PhNH·B(C6F5)4/i-Bu3Al

Propylene polymerization was conducted with Ph 2 C(R-Cp)(Flu)ZrCl 2 [R = Me, i-Pr, PhCH 2 , Me 3 Si] catalysts in combination with methylaluminoxane (MAO) and dimethylanilinium tetrakis(pentafluorophenyl)borate (Me 2 PhNH. B(C 6 F 5 ) 4 ) as cocatalyst; the dependence of the stereoregularity of poly(propylene) on cocatalysts and bulkiness of the substituents in β-position of the cyclopentadienyl ligand was studied. Methyl and i-propyl substituted metallocene catalysts produce hemi-isotactic poly(propylene). These results are in good agreement with the results of the isopropylidene bridged metallocene analogue. The benzyl substituted metallocene catalyst produces syndiotactic poly(propylene) regardless of the cocatalyst. This means that this substituent group does not affect migration insertion of propylene. Stereoregularity of poly(propylene) obtained with diphenylmethylidene(3-trimethylsilylcyclopentadienyl)(fluorenyl)zirconium dichloride (Ph 2 C(Me 3 SiCp)(Flu)ZrCl 2 ) as a catalyst was significantly influenced by the cocatalyst. Me 2 PhNH. B(C 6 F 5 ) 4 /triisobutylaluminium(i-Bu 3 Al) produces poly(propylene) with 65% racemic and 23% meso pentads at 40°C, whereas the MAO activated catalyst produces isotactic rich poly(propylene). Fractionation experiments indicated that Me 2 PhNH. B(C 6 F 5 ) 4 /i-Bu 3 Al forms two active sites, one of them being the same as that of the MAO activated catalyst, the other one producing syndiotactic rich poly(propylene).