Site Epimerization Research Articles

Octahedral Zirconium Salan Catalysts for Olefin Polymerization: Substituent and Solvent Effects on Structure and Dynamics.

Group 4 metal-Salan olefin polymerization catalysts typically have relatively low activity, being slowed down by a pre-equilibrium favoring a non-polymerization active resting state identified as a mer-mer isomer (MM); formation of the polymerization active fac-fac species (FF) requires isomerization. We now show that the chemistry is more subtle than previously realized. Salan variations bearing large, flat substituents can achieve very high activity, and we ascribe this to the stabilization of the FF isomer, which becomes lower in energy than MM. Detailed in situ NMR studies of a fast (o-anthracenyl) and a slow (o-tBu) Salan precursors, suitably activated, indicate that preferred isomers in solution are different: the fast catalyst prefers FF while the slow catalyst prefers a highly distorted MM geometry. Crystal structures of the activated o-anthracenyl substituted complex with a moderately (chlorobenzene) and, more importantly, a weakly coordinating solvent (toluene) in the first coordination sphere emphasize that the active FF isomer is preferred, at least for the benzyl species. Site epimerization (SE) barriers for the fast catalyst (ΔS > 0, dissociative) and the slow catalyst (ΔS < 0, associative) in toluene corroborate the solvent role. Diagnostic NMe 13C chemical shift differences allow unambiguous detection of FF or MM geometries for seven activated catalysts in different solvents, highlighting the role of solvent coordination strength and bulkiness of the ortho-substituent on the isomer equilibrium. For the first time, active polymeryl species of Zr-Salan catalysts were speciated. The slow catalyst is effectively trapped in the inactive MM state, as previously suggested. Direct observation of fast catalysts is hampered by their high reactivity, but the product of the first 1-hexene insertion maintains its FF geometry.

Optical Purity as a Programmable Variable for Controlling Polyolefin Tacticity in Living Coordinative Chain Transfer Polymerization: Application to the Stereomodulated LCCTP of α,ω-Nonconjugated Dienes

Two-state, stereomodulated living coordinative chain transfer polymerization (LCCTP) of 1,6-heptadiene has been achieved to produce a series of cis-poly(methylene-1,3-cyclohexane) (cis-PMCH) with stereochemical microstructures that vary between two limiting forms as a function of the optical purity of the chiral C1-symmetric hafnium preinitiator 1 when activated by the borate co-initiator B1 in the presence of a fixed excess amount of ZnEt2. Optical purity as a programmable variable was demonstrated by premixing the pure homochiral (SC,SHf) and (RC,RHf) enantiomers of 1 in different ratios prior to activation. In contrast, the LCCTP of 1,5-hexadiene using a “racemic” mixture of 1 to produce cis/trans-poly(methylene-1,3-cyclopentane) (cis/trans-PMCP) was less successful due to the loss of stereocontrol for insertion under chain transfer conditions that arises with site epimerization at the transition metal center. A correlation is proposed between the diastereoselectivity and efficiency for the cyclization step in propagation and the magnitude of site epimerization due to reversible chain transfer. These results serve to validate the strategy of using the optical purity of an initiator as a programmable parameter for controlling polyolefin tacticity as well as to better establish the criteria that are required for achieving successful stereomodulated LCCTP for the production of next generation polyolefin materials.

A Theoretical Outlook on the Stereoselectivity Origins of Isoselective Zirconocene Propylene Polymerization Catalysts.

The first three insertion steps of propylene for isoselective metallocenes from the one-carbon-bridged cyclopentadienyl-fluorenyl {Cp/Flu} and silicon-bridged ansa-bis(indenyl) {SBI} families were computed by using a theoretical method implementing the B3PW91 functional in combination with solvent corrections incorporated with the Solvation Model based on Density (SMD) continuum model. For C1 -symmetric {Cp/Flu}-type metallocenes, two mechanisms of stereocontrol were validated theoretically: more facile and more stereoselective chain "stationary" insertion (or site epimerization backskip) and less stereoselective alternating mechanisms. For the C2 -symmetric {SBI}-type system, the computation results were 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 by using three-parameter and one-parameter statistical models for the two metallocene systems, respectively. The calculated meso/rac pentad distributions were found to be in good agreement with those determined experimentally for isotactic polypropylene samples obtained at different polymerization temperatures.

Propene Polymerization with C1-Symmetric Fluorenyl-Metallocene Catalysts

Propene homopolymers have been produced by employing three C1-symmetric metallocene molecules (1, 2 and 3), each having t-butyl substituent(s) on the Cp, on the fluorenyl or on both aromatic moieties activated with methylaluminoxane at different polymerization temperatures and monomer concentrations. Polymers’ microstructures determined by 13C NMR spectroscopy suggest that the otherwise dominant alternating mechanism governed by the chain migratory insertion is largely replaced by the competing site epimerization mechanism, as a direct result of the imposing steric bulk of the t-butyl substituent on one of the distal positions of the Cp moiety. This phenomenon is more pronounced with 3 when a second t-butyl is present in the same half-space of the molecule making the site epimerization mandatory. The lower activity of catalyst 3 with respect to catalyst 2 is also in line with the necessity for the polymer chain to back-skip (or the site to epimerize) to its original position before the subsequent monomer insertion. Chain end group analyses by 1H NMR spectroscopy have revealed that the formation of vinylidene end groups, either via β-H elimination or as a result of direct chain transfer to the monomer after an ordinary 1,2-insertion is the prevailing chain termination route. A correlation between the relative concentrations of vinylidene end groups of polypropene (PP) polymers produced with each catalyst and the corresponding polypropenes’ molar masses was found, indicating the lower the relative concentrations of vinylidene end groups, the higher the molar masses.

Ligand Coordination Driven by Monomer and Polymer Chain: The Intriguing Case of Salalen–Ti Catalyst for Propene Polymerization

Salalen–Ti systems for propene polymerization catalysis were originally developed under the assumption of a “directional site epimerization promoted electronically” strategy and allowed the synthesis of polypropylene samples with extraordinarily high isotacticities. Our DFT calculations revealed an alternative and more intriguing interpretation of polymerization behavior of such systems. Ligand coordination around the metal center driven by the monomer and the polymer chain dictates the formation of active species different from the ones expected from X-ray structures of precursors. The “synergic” role of R1 and R3 substituents is responsible of the extraordinarily high isotacticities reported in propene polymerization.

DFT Investigation of the Tacticity Control during Styrene Polymerization Catalyzed by Single-Component Allyl ansa-Lanthanidocenes {(C5H4CMe2(9-C13H8)}Ln(C3H5)

Theoretical methods (DFT) were used to investigate the syndiospecificity of the styrene polymerization catalyzed by single-site, single-component allyl ansa-lanthanidocenes {(C5H4CMe2(9-C13H8)}Ln(C3H5). Two limiting chain-end stereocontrol mechanisms were studied, namely, frontside “migratory” insertion through a site epimerization and site stereoconfiguration independent backside insertion on a “stationary” polymer chain. Four consecutive insertions of styrene were computed to reveal that (i) backside insertions are more favorable than, or at least as favorable as, frontside insertions, and (ii) the formation of a syndiotactic polymer is controlled by the thermodynamics. Moreover, the odd (first and third) insertions are of 2,1-down-si-type and are kinetically favored over the 2,1-up-re-ones. This control is the conjunction of two effects: minimization of styrene−styrene and styrene(phenyl ring)−fluorenyl repulsions and (iii) the steric hindrance of the polymer chain induces for the fourth insertion an e...

Formation and Structures of Hafnocene Complexes in MAO- and AlBui3/CPh3[B(C6F5)4]-Activated Systems

The formation of cationic species relevant to olefin polymerization based on (SBI)HfCl2, Me2C(C5H4)(Flu)HfCl2, Ph2C(C5H4)(Flu)HfCl2, and L′HfCl2 activated by MAO, AlMe3/CPh3[B(C6F5)4], and AlBui3/CPh3[B(C6F5)4] (SBI = rac-Me2Si(Ind)2; L′ = C2H4(Flu)(5,6-C3H6-2-MeInd)) was studied by 1H, 13C, and 19F NMR spectroscopy. Thermally stable heterobinuclear intermediates of the type [LHf(μ-Me)2AlMe2]+[MeMAO]− and [LHf(μ-Me)2AlMe2]+[B(C6F5)4]− were identified when using MAO and AlMe3/CPh3[B(C6F5)4] as activators, respectively. The stability of these species explains the low productivity of hafnocene catalysts in the presence of AlMe3-containing activators, compared to zirconocenes. By contrast, in the ternary systems LHfCl2/AlBui3/CPh3[B(C6F5)4] hydride species were detected that must be responsible for the formation of the highly active sites in olefin polymerization. The ionic hydrido species differ significantly in stability. The formation of the mixed-alkyl complex L′Hf(Me)CH2SiMe3 proceeds with surprisingly high diastereoselectivity; the sterically more hindered isomer is produced preferentially. It reacts with CPh3[B(C6F5)4] to afford the ion pair [L′Hf-CH2SiMe3]+[B(C6F5)4]− as two diastereomers that exist in dynamic equilibrium. The rates of site epimerization of this ion pair indicate only small energy differences between the two isomers.

Ligand Mobility and Solution Structures of the Metallocenium Ion Pairs [Me2C(Cp)(fluorenyl)MCH2SiMe3+···X−] (M = Zr, Hf; X = MeB(C6F5)3, B(C6F5)4)

The mixed-alkyl metallocene complexes (IPCF)M(Me)(CH2SiMe3) (M = Zr, Hf; IPCF = Me2C(C5H4)(C13H8)) were synthesized by the reaction of (IPCF)M(Me)Cl (M = Zr, Hf) with Me3SiCH2MgCl. The crystal structures of (IPCF)Zr(CH2SiMe3)2, (IPCF)HfMe2, and (IPCF)Zr(Me)Cl were determined by X-ray diffraction. The kinetics of site epimerization of the ion pairs (IPCF)M(CH2SiMe3)(μ-Me)B(C6F5)3 and [(IPCF)MCH2SiMe3+···B(C6F5)4−] (M = Zr, Hf) were studied by variable-temperature NMR spectroscopy, while the solution ground-state structures of the ion pairs [LZrCH2SiMe3+···B(C6F5)4−] (L = SBI, IPCF; SBI = rac-Me2Si(Ind)2) were probed experimentally by 19F,1H HOESY NMR spectroscopy and theoretically by DFT and molecular dynamics calculations. They reveal differences in the strength of anion interactions between the SBI and IPCF systems which may be significant for their catalytic activity. The tetraarylborate salts are stabilized by agostic interactions to ligand Si−Me moieties, with Hf > Zr. The exchange rates of both the M...

Catalyst Site Epimerization during the Kinetic Resolution of Chiral α-Olefins by Polymerization

A new enantiopure C1-symmetric olefin polymerization precatalyst, (1,2-SiMe_2)_2{η^5-C_5H_2-4-((S)-CHEtCMe_3)}{η^5-C_5H-3,5-(CHMe_2)_2}ZrCl_2, (S)-2, was synthesized, and its use for the kinetic resolution of 3-methyl-substituted racemic α-olefins was investigated. Upon activation with methyl aluminoxane (MAO), selectivity factors for most olefins were greater when (S)-2 was used as the catalyst as compared to its previously reported methylneopentyl analogue, (1,2-SiMe_2)_2{η^5-C_5H_2-4-((S)-CHMeCCMe_3)}{η^5-C_5H-3,5-(CHMe_2)_2}ZrCl_2, (S)-1. Pentad analysis of polypropylene produced by the two catalysts at various propylene concentrations indicates that (S)-2 undergoes more efficient site epimerization (polymeryl chain swinging prior to subsequent monomer enchainment) at intermediate propylene concentrations compared to (S)-1. At high and low propylene concentrations, however, the two catalysts behave similarly. On the other hand, polymerization of 3,5,5-trimethyl-1-hexene at different olefin concentrations and temperatures illustrated that selectivity differences between the two catalysts are likely not a consequence of inefficient site epimerization for (S)-1.

Evidence for Mixed-Ion Clusters in Metallocene Catalysts: Influence on Ligand Exchange Dynamics and Catalyst Activity

Alkylzirconium ion pairs [L2Zr−CH2SiMe3+···B(C6F5)4-], as models of catalytic zirconium polymeryl species, form mixed-ion clusters [{LZrR+}n{CPh3+}m{X-}(n+m)], as shown by pulsed field-gradient spin−echo NMR measurements. Higher ion aggregation leads to enhanced rates of site epimerization. This rate enhancement is used to probe previously inaccessible kinetic barriers for the “chain swinging” process. The presence or absence of such a barrier has implications for catalytic activity and response to ionic activators.

Insertion vs. site epimerization with singly-bridged and doubly-bridged metallocene polymerization catalysts

A statistical model has been employed to determine the unidirectional site epimerization probability, ε, during propylene polymerization with the following C 1-symmetric metallocene precatalysts activated with MAO (MAO = methylaluminoxane): doubly-bridged rac-(1,2-SiMe 2) 2{η 5-C 5H 2-4-(CHMe(CMe 3))}{η 5-C 5H-3,5-(CHMe 2) 2}ZrCl 2 ( 1) and (1,2-SiMe 2) 2{η 5-C 5H 2-4-(1 R,2 S,5 R-menthyl)}{η 5-C 5H-3,5-(CHMe 2) 2}ZrCl 2 ( 2); and singly-bridged Me 2C(3-(2-adamantyl)-C 5H 3)(C 13H 8)ZrCl 2 ( 3) and Me 2Si(3-(2-adamantyl)-C 5H 3)(C 13H 8)ZrCl 2 ( 4). For 1/MAO a steep tacticity dependence on monomer concentration was found, as ε increased from 0.114 to 0.909 as [C 3H 6] decreased from 12.5 M to 0.5 M; similarly, ε increased for 2/MAO from 0.177 to 0.709. For 3/MAO, ε was moderately responsive to an increase in polymerization temperature, as ε increased from 0.000 to 0.485 from T p = 0–90 °C ([C 3H 6] = 1.1 M). Similarly, ε increased for 4/MAO from 0.709 to 0.913 from T p = 0–40 °C; at higher temperatures, bidirectional site epimerization was implicated.

Mechanism of Activation of a Hafnium Pyridyl−Amide Olefin Polymerization Catalyst: Ligand Modification by Monomer

We have investigated the olefin polymerization mechanism of hafnium catalysts supported by a pyridyl-amide ligand with an ortho-metalated naphthyl group. Ethylene-alpha-olefin copolymers from these catalysts have broad molecular weight distributions that can be fit to a bimodal distribution. We propose a unique mechanism to explain this behavior involving monomer modification of the catalyst, which generates multiple catalyst species when multiple monomers are present. More specifically, we present evidence that the hafnium alkyl cation initially undergoes monomer insertion into the Hf-naphthyl bond, which permanently modifies the ligand to generate new highly active olefin polymerization catalysts. Under ethylene/octene copolymerization conditions, a plurality of new catalysts is formed in relative proportion to the respective monomer concentrations. Due to the asymmetry of the metal complex, two "ethylene-inserted" and eight "octene-inserted" isomers are possible, but it is a useful approximation to consider only one of each in the polymerization behavior. Consequently, gel permeation chromatography data for the polymers can be fit to a bimodal distribution having a continuous shift from a predominantly low molecular weight fraction to predominantly higher molecular weight fraction as [octene]/[ethylene] is increased. Theoretical calculations show that such insertions into the Hf-aryl bond have lower barriers than corresponding insertions into the Hf-alkyl bond. The driving forces for this insertion into the Hf-aryl bond include elimination of an eclipsing H-H interaction and formation of a stabilizing Hf-arene interaction. These new "monomer-inserted catalysts" have no beta-agostic interaction, very weak olefin binding, and olefin-insertion transition states which differ on the two sides by more than 4 kcal/mol. Thus, the barrier to site epimerization is very low and high polymerization rates are possible even when the chain wags prior to every insertion. Experimental evidence for aryl-insertion products is obtained from reactions of ethylene (13C2H4 NMR studies) or 4-methyl-1-pentene (4M1P) using relatively low monomer/catalyst ratios. Quantitative generation of monomer-inserted products is complicated by slow initiation kinetics followed by fast polymerization kinetics. However, NMR evidence for reaction with 13C2H4 was observed in situ at low temperature, and the attachment of monomer to ligand was confirmed by GC/MS and 13C NMR after quenching. Furthermore, a 4M1P-appended ligand was isolated from a polymerization reaction (50:1 monomer:catalyst) by column chromatography followed by multiple recrystallizations. One isomer was characterized by X-ray crystallography, which unequivocally shows a 4-methylpentyl substituent at the 2-position of the naphthyl, consistent with 1,2-insertion into the Hf-aryl bond. NMR suggests a second diastereomer (not isolated) is formed from a 1,2-insertion of opposite stereoselectivity.

Mechanism of Isotactic Polypropylene Formation with C1-Symmetric Metallocene Catalysts

The mechanism for isotactic polypropylene formation by the C1-symmetric catalyst system [Me2C(3-tert-butyl-C5H3)(C13H8)]ZrCl2/MAO (MAO = methylaluminoxane, C13H8 = fluorenylidene) has been examined. Evidence supports an alternating mechanism, where both sites of the metallocene wedge are utilized for monomer insertion, rather than the previously proposed site epimerization (inversion at Zr) following each monomer insertion. As the polymerization temperature increases (0 to 60 °C) with lower concentrations of propylene, the site epimerization mechanism begins to compete, as evidenced by an increase in isotacticity. The alternating mechanism also accounts for polypropylene microstructures obtained with Me2C(3-R-C5H3)(Oct)ZrCl2/MAO, where Oct = octamethyloctahydrodibenzofluorenylidene and R = methyl, cyclohexyl, diphenylmethyl, and with Me2C(3-tert-butyl-4-Me-C5H2)(Oct)ZrCl2/MAO. For an Oct-containing catalyst system with R = 2-methyl-2-adamantyl, unprecedentedly high (for a fluorenyl-based metallocene catalyst) isotacticity ([mmmm] > 99%) is obtained; the polymer prepared at 0 °C has Tm = 167 °C and Mw = 370 000.

Site epimerization in ansa-zirconocene polymerization catalysts

Metallocene alkyl cations for polymerization of olefins possess two active sites involved in migratory insertion. Site epimerization, with an inversion at the metal atom, is considered to be one of the major causes for break-down of the alternating propagation model, resulting in stereoerrors whenever the two catalytic sites have substantially different enantioface selectivities. Density functional theory has been used to determine the intrinsic reaction coordinate that connects the optimized minima and transition states of inversion in the parent ansa-zirconocene [{H 2C(Cp) 2}Zr–Pr] + (Pr = n-propyl). These calculations yield a three-step reaction path for site epimerization. Starting from the pyramidal β-agostic complex, an activated rotation around the Zr–Pr bond first produces an α-agostic conformation. Continued rotation leads to an equivalent second α-agostic intermediate and then finally to the inverted β-agostic complex. The second step is rate-determining and proceeds through a planar three-coordinate transition state. In the case of [{H 2C(Cp) 2}Zr– i Bu] + ( i Bu = iso-butyl), the situation is more complicated, because there are several interconvertible α-, β- and γ-agostic intermediates, but the rate-limiting step is again an inversion process connecting two different α-agostic conformers with the alkyl group on opposite enantiosides. For both ansa-zirconocene catalysts, the computed free-energy barriers for epimerization are around 11–12 kcal/mol and almost independent of temperature, while those for insertion increase with temperature due to the entropic cost of association. According to the computational results for the isolated catalysts, insertion remains favored over epimerization for the experimentally relevant temperature range in the n-propyl case, whereas both processes are competitive in the iso-butyl case. Inclusion of bulk solvent effects by a continuum solvation model does not affect the results much, while explicit consideration of a coordinating counterion causes larger changes. The present model calculations on the role of site epimerization should thus be most relevant for propene polymerization with non-coordinating counterions.

Marked counteranion effects on single-site olefin polymerization processes. Correlations of ion pair structure and dynamics with polymerization activity, chain transfer, and syndioselectivity.

Counteranion effects on the rate and stereochemistry of syndiotactic propylene enchainment by the archetypal C(s)-symmetric precatalyst [Me(2)C(Cp)(Flu)]ZrMe(2) (1; Cp = C(5)H(4); Flu = C(13)H(8), fluorenyl) are probed using the cocatalysts MAO (2), B(C(6)F(5))(3) (3)(,) B(2-C(6)F(5)C(6)F(4))(3) (4)(,) Ph(3)C(+)B(C(6)F(5))(4)(-) (5), and Ph(3)C(+)FAl(2-C(6)F(5)C(6)F(4))(3)(-) (6), offering greatly different structural and ion pairing characteristics. Reaction of 1 with 3 affords [Me(2)C(Cp)(Flu)]ZrMe(+) MeB(C(6)F(5))(3)(-) (7). In the case of 4, this reaction leads to formation the micro-methyl dinuclear diastereomers [([Me(2)C(Cp)(Flu)]ZrMe)(2)(micro-Me)](+) MeB(2-C(6)F(5)C(6)F(4))(3)(-) (8). A similar reaction with 6 results in diastereomeric [Me(2)C(Cp)(Flu)]ZrMe(+) FAl(2-C(6)F(5)C(6)F(4))(3)(-) (10) ion pairs. The molecular structures of 7 and 10 have been determined by single-crystal X-ray diffraction. Reorganization pathways available to these species have been examined using EXSY and dynamic NMR, revealing that the cation-MeB(C(6)F(5))(3)(-) interaction is considerably weaker/more mobile than in the FAl(2-C(6)F(5)C(6)F(4))(3)(-)-derived analogue. Polymerizations mediated by 1 in toluene over the temperature range of -10 degrees to +60 degrees C and at 1.0-5.0 atm propylene pressure (at 60 degrees C) reveal that activity, product syndiotacticity, m and mm stereodefect generation, and chain transfer processes are highly sensitive to the nature of the ion pairing. Thus, the complexes activated with 4 and 5, having the weakest ion pairing, yield the highest estimated propagation rates, while with 6, having the strongest pairing, yields the lowest. The strongly coordinating, immobile FAl(2-C(6)F(5)C(6)F(4))(3)(-) anion produces the highest/least temperature-dependent product syndiotacticity, lowest/least temperature-dependent m stereodefect abundance, and highest product molecular weight. These polypropylene microstructural parameters, and also M(w), are least sensitive to increased propylene pressure for FAl(2-C(6)F(5)C(6)F(4))(3)(-), but highest with MeB(C(6)F(5))(3)(-). In general, mm stereodefect production is only modestly anion-sensitive; [propylene] dependence studies reveal enantiofacial propylene misinsertion to be the prevailing mm-generating process in all systems at 60 degrees C, being most dominant with 6, where mm stereodefect abundance is lowest. For 1,3-dichlorobenzene as the polymerization solvent, product syndiotacticity, as well as m and mm stereodefects, become indistinguishable for all cocatalysts. These observations are consistent with a scenario in which ion pairing modulates the rates of stereodefect generating processes relative to monomer enchainment, hence net enchainment syndioselectivity, and also dictates the rate of termination relative to propagation and the preferred termination pathway. In comparison to 3-6, propylene polymerization mediated by MAO (2) + 1 in toluene reveals an estimated ordering in site epimerization rates as 5 > 4 > 2 > 3 > 6, while product syndiotacticities rank as 6 > 2 > 5 approximately 4 > 3.

Syndiotactic Poly(propylene) from [Me2Si(3,6‐di‐tert‐butyl‐9‐fluorenyl)(N‐tert‐butyl)]TiCl2–Based Catalysts: Chain‐End or Enantiotopic‐Sites Stereocontrol?

AbstractThe origin of the syndiotactic selectivity in propene polymerization of a typical bridged fluorenyl‐amido catalyst, namely [Me2Si(3,6‐di‐tert‐butyl‐9‐fluorenyl)(N‐tert‐butyl)]TiCl2 (1), activated with methylaluminoxane or [HMe2N(C6H5)][B(C6F5)4]/Al(i‐butyl)3, was investigated by means of 150 MHz 13C NMR spectroscopic microstructural polymer analysis. The asymmetric induction was traced unambiguously to enantiotopic‐sites control. Compared with the better‐known cyclopentadienyl‐fluorenyl ansa‐zirconocenes, 1 turned out to be almost identically enantioselective (in agreement with computer modeling), but less stereoselective because of a higher propensity to undergo site epimerization. This results in a chain microstructure with large predominance of m over mm stereodefects, deceptively similar to that of syndiotactic poly(propylene) produced under chain‐end control.Methyl region of the 150 MHz 13C NMR spectrum of a poly(propylene) sample prepared with catalyst system 1/borate/TIBAl.magnified imageMethyl region of the 150 MHz 13C NMR spectrum of a poly(propylene) sample prepared with catalyst system 1/borate/TIBAl.

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.

Syndiospecific Propylene Polymerization Using C1-Symmetric ansa-Metallocene Catalysts: Substituent and Bridge Effects

A series of syndiospecific C1-symmetric ansa-metallocenes ([Me2X(Cp)(2-R1-3-R2-Ind)]ZrCl2, X = C, Si; R1 = H, Me; R2 = Me, Et, CH2SiMe3) have been synthesized, and their catalytic behavior in the polymerization of propylene has been studied. Upon activation with MAO, these carbon- or silicon-bridged cyclopentadienyl/indenyl systems afford polypropylene with various degrees of syndiotacticity ([rrrr] = 28−66%), as a function of the substituent size, substitution pattern, and bridging moiety. The silicon-bridged systems produced PP samples of higher molecular weight, lower syndiotacticities, and comparable productivities with respect to their carbon-bridged analogues. Statistical modeling of the experimental pentad distributions suggests that site epimerization is the major cause of stereoerrors within both sets, while a decreased site stereospecificity can be implied for the silicon-bridged systems.

Metallocene-Catalyzed Propene Polymerization: From Microstructure to Kinetics. 1. C2-Symmetric ansa-Metallocenes and the "Trigger" Hypothesis

A microstructural method for the determination of the reaction order with respect to the monomer in coordinated 1-alkene polymerizations is proposed. The method is applicable whenever enantioselective monomer polyinsertion competes with an intramolecular process introducing stereodefects of known and recognizable nature in the growing polymer chains at a rate independent of monomer concentration, [M] (e.g., chain or site epimerization for C2-symmetric and Cs-symmetric ansa-metallocene catalysts, respectively). Indeed, in such cases, the stereoregularity of the polymer obtained at a given temperature results from the balance between the rates of the two competing processes, and is obviously a function of [M], because so is the rate of polyinsertion, 〈vp〉. Therefore, from the way polymer stereoregularity changes with [M], the corresponding change of 〈vp〉 can be inferred. Compared with the conventional macroscopic approach based on measurements of monomer consumption, the microstructural route is more reprod...

On the Connection between Microstructure and Kinetics of (Pseudo)copolymerization

The connection between microstructure and kinetics in the case of irreversible growth of long (pseudo)copolymers has been discussed, extended and established. The theoretical results have been cast in simple matrix equations which enable one to compute the probability to find any given sequence in terms of ratios of kinetic constants, rather than in terms of stochastic parameters. As an application, the case of regioregular polymerization of vinyl monomers by C1-symmetric coordination catalysts has been considered. In addition to propagation, first order in monomer concentration, chain and site epimerization, both zeroth-order in monomer concentration, have been taken into account. The resulting equations have been tested against a set of literature data obtained by five different catalysts, each operated at five different concentrations, resulting in the evaluation of ratios of kinetic constants. For all catalysts the quantitative analysis of the kinetics confirms that the contribution of chain epimeriza...