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Mechanistic Aspects of Alkene Polymerization

Mechanistic Aspects of Alkene Polymerization. Clark R. Landis Dow Chemical Company March, 2002. Doug Sillars Kim Rosaaen Curtis White Dr. Zhixian Liu. Funding Dow Cooperative Research Department of Energy. Plastics Industry: Prediction vs. Reality. What Makes a Catalyst Impressive?.

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Mechanistic Aspects of Alkene Polymerization

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  1. Mechanistic Aspects of Alkene Polymerization Clark R. Landis Dow Chemical Company March, 2002 Doug Sillars Kim Rosaaen Curtis White Dr. Zhixian Liu Funding Dow Cooperative Research Department of Energy

  2. Plastics Industry: Prediction vs. Reality

  3. What Makes a Catalyst Impressive? “ The use of chiral catalysts to obtain high optical yields … representsone of the most impressive achievements to date in catalytic selectivity,rivaling the corresponding stereoselectivity of enzymic catalysts.”These catalyst systems are impressive … also for their very highactivities. … in respect of both selectivity and rate, the behavior of these syntheticrivals, to an unprecedented degree, that of enzymic catalysts.” Halpern, J. Science1982, 217, 401-407.

  4. Metallocene Single Site Catalysts Enzyme-Like Behavior Industrially Significant Rates ≈ 104 insertions/sec. Stereospecificity > 99% Regiospecificity > 99.5% Linear-Low Density PE US Annual Production > 1,000 Metric Tons Exquisite Ligand-Based Control of Selectivity

  5. Goal: To develop a fundamental understanding of the mechanistic details of alkene polymerization through detailed kinetics Our Research Activities Ion-Pair Dynamics via NMR and high sensitivity conductivity studies Creation of new active site counting methods Fabrication of novel time-resolved calorimeters and quenched-flow reactors Determination of rate laws for initiation, propagation, and termination. Counter-ion influences on reaction mechanisms Heavy-Atom Kinetic Isotope Effects: Exp. And Ab Initio Computations

  6. Systems Under Investigation Catalyst Precursors (EBI)Zr(CH3)2 (Me4Cp)Zr(CH3)2 CGC-1 Catalyst Activators B(C6F5)3, R3NH+ B(C6F5)4-, Ph3C+ B(C6F5)4-, MAO Alkenes Ethene, Propene, 1-Hexene

  7. Active Site Counting Methods Quenching with 14CO Non-stoichiometric, very sensitive, radioactive, does not indicate type of alkyl Labeling with CH3OT stoichiometric, very sensitive, radioactive, Kinetic Isotope Effect, does not indicate type of alkyl Labeling with CS2 incomplete labeling Marques, M. M. et alia, J. Polym. Sci.: Part A, Polym. Chem1998, 36, 573-585.

  8. product Quench agent Berger Ball Mixers Catalyst monomer Quench Flow Reactor

  9. CH2D Bu Bu n Initiation Kinetics, Active Site Counts by CH3OD Quench Timed Reaction Interval (t) Quench “Count” D-terminated Chains by 2H NMR asa function of time Average of 5 runs CDCl3 Fraction Active Sites Time (s) Information: The fraction of Zr centers that are attached to polymers at the time of quench.

  10. Active Site Counting with CD3 Conditions 0C, Toluene Solution 1M 1-hexene 8 x 10-4M (EBI)ZrMe2 8 x 10-4M B(C6F5)3 Comparison of Two Labeling Methods Fraction Active Sites Information: The fraction of Zr centersthat produced polymer atsome time before quench. Time (s)

  11. Active Sites and Polypropene 2H NMR of MeOD quenched product 40s reaction time 1.45 M propene 4x10-4 M (EBI)ZrMe2 4x10-4 M B(C6F5)3 20°C, Toluene 15% active sites Label found only at terminal methyl groups Int. Std. Solvent Labeled Polymer

  12. Kinetic Data: Initiation • Kinetics at 0°C in Toluene • Each observed k is average of three runs • Initiation rate is unaffected by excess borane Rate = ki [Zr][1-hexene] ki = 2.1 x 10-2 M-1s-1 at 0°C = 0.25 M-1s-1 at 24°C DH‡= 11.2(1.5) kcal/mol DS‡= -24(5) cal/mol-K kIobs (s-1)

  13. Catalytic Kinetics:[(EBI)Zr(Me)](MeB(C6F5)3]-Catalyzed Polymerization of 1-Hexene General Observations General Conditions • [Zr]: 2x10-4 - 2x10-3 M • [1-Hexene]: 0.15 M - 3.0 M • Temperatures: -40 - 60°C • Activator: 1-5 equiv. • Solvent : Toluene • Clean, Reproducible Kinetics • Exotherm < 1°C • Polymer Molecular weights: 1,000 - 30,000 depending on quench time

  14. Convolution of Initiation and Propagation Kinetics Polymer mass(t) = 84.16kp [Zr]tot[1-hexene](t+(e-ki[1-hexene]t)) + C Conditions 0˚C, Toluene Solution 1M 1-hexene 8 x 10-4M (EBI)ZrMe2 8 x 10-4M B(C6F5)3 • kp : propagation rate constant • ki : initiation rate constant • [Zr]tot = concentration of all Zr species • C= constant of integration = -84.16 kp[Zr]tot/ki kp = 2.1 M-1s-1

  15. Complicated Kinetics Are Good “There is no such thing as a free lunch” Milton Friedman “There is no such thing as free information” Jack Halpern, Kinetics Course, Spring 1980

  16. Propagation Kinetics-High Conversion 0°C, Hexene Disappearance (IR) 50°C, Polymer Mass vs. Time kp = 2.2 M-1s-1 Propagation Rate = kp[Zr][1-hexene] kp = 8.1 M-1s-1 at 25°C DH‡= 6.4(1.5) kcal/mol DS‡=-33(5) cal/mol-K At 0°C, propagation is 70-times faster than initiation!

  17. B/Zr=1 Weight of polymer(g) B/Zr=2 B/Zr=4 Reaction time(s) Excess B(C6F5)3 or PhNMe3+ BMe(C6F5)3-: No Effect on Propagation Rate • Reactions with excess B(C6F5)3 indicate no “double activation effect” • Reactions with added BMe(C6F5)3- are ambiguous: the lack of an inhibitory effect contradicts the schemeshown above only if all the ions are free ions. In low dielectric mediaone anticipates tight ion-pairing and no common ion effect.

  18. Interception of the Propagating Species [Zr]0 = 8 mM [1-hexene]0= 0.6 M Temp. =-40°C 1-hexene 1-Hexene Polymerization Followed by 1H NMR 1 spectrum every 2 minutes

  19. Characterization of the Propagating Species Initiation and Propagation Kinetics -40°C obs. Previous (extrapolated) kinit(M-1s-1) 11.5  10-4 8.78  10-4 kprop(M-1s-1) 0.256 0.299 • Other evidence… • Resonances disappear in 0 to -1 region with (EBI)Zr(CD3)2. • 19F NMR exhibits new ortho peak upon initiation. • 1H and 19F NMR shifts suggest coordinated -CH3B(C6F5)3. • 1H{11B} NMR demonstrates CH3-B topology of peaks at-0.62 and -0.85 ppm.

  20. Ion-Pair Dynamics of Propagating Species Using 1D-Pulse Field Gradient Spin Echo NOESY, irradiate one of the indenyl peaks

  21. Effect of Excess B(C6F5)3 on Exchange Rates ksym (s-1) 8.2 mM (EBI)Zr(CH3)2 -40 °C Concentration of free B(C6F5)3 (mM) • Measurements demonstrate: • Similar symmetrization rates in the limit of no free borane. • Free borane does not promote symmetrization of the propagating species. 8.4 mM (EBI)Zr(CH3)2 -36 °C

  22. Two types of vinyl end groups are found via proton NMR: Vinylidene 4.7, 4.78 ppm (singlets) Termination Kinetics Internal Alkene 5.4 ppm (broad multiplet) 1.5 M 1-hexene 0.15M 1-hexene Vinylene:vinylidene ratio depends on [1-hexene]

  23. Termination Rate Measurements All runs conducted with < 10% 1-Hexene conversion

  24. 50°C 20°C Log(kvinylidene) 10°C 0°C [1-hexene] Vinylidene and Internal Alkene Formation Have Different Rate Laws Internal Alkene (vinylene) Vinylidene Rate=kvinylene[Zr][1-hexene] kvinylene=9.7x10-3M-1s-1 DH‡=9.7(12)kcal,mol, DS‡=-35(4)cal/mol-K Rate = kvinylidene[Zr] kvinylidene=1.3x10-3s-1(25°C) DH‡=16(3)kcal,mol, DS‡=-13(6)cal/mol-K kvinyleneobs (s-1)

  25. Are Internal Alkenes Formed by Chain Transfer to Monomer? Conventional Wisdom* •Vinylidene = Mononuclear -Hydride Elimination • Internal Alkene = Bimolecular Chain Transfer to Monomer? Why must secondary alkyls wait for a monomer whereas primary alkyls do not? *Resconi et al. Chem. Rev.,2000, 100, 1253-1345.

  26. Alternate Model: Every 2,1-Insertion Leads to Termination The steady-state concentration of the secondary alkyl (shownabove) resulting from a 2,1-insertion is proportional to the[1-hexene] because it is formed by occasional misinsertion of 1-hexene from the catalyst resting state (a primary alkyl). The rate of termination is really the rate of 2,1-propagation

  27. Steady-State Analysis: Vinylenes = Rate of 2,1 insertion

  28. Polymer Microstructure via 13C NMR Strategy: Use 13C label in 1-position of 1-hexene to look for enchained regioerrors and to examine microstructure Analyze by 1D 13C NMR, INADEQUATE, HMBC, DEPT, 1HNMR

  29. Trans hexenyl 13C NMR Spectrum of Labeled Polymer: 106-134 ppm vinylidene Cis hexenyl Termination after 2,1 insertion Termination after 1,2 insertion

  30. 13C NMR Spectrum of Labeled Polymer: 10-50 ppm C1 C3 C4 C5 C6 Cis hexenyl C2 pentenyl Trans hexenyl

  31. Analysis of Polymer Microstructure Reveals • No enchainment of 2,1 regioerrors: every misinsertion leads to termination of polymer growth • Several end groups can be identified • cis and trans hexenyl (after 2,1 insertion) • cis and trans pentenyl (after 2,1 insertion) • vinylidene (after 1,2 insertion) • hexyl endgroup (from first insertion into Zr-H) • D-labeled methyl (from MeOD quench) • After a misinsertion: • Elimination to form hexenyl end group 4-times more frequent than pentenyl end group formation • cis-hexenyl end group 2.4-times more frequent than trans • >99% isotacticity (mmmm pentads)

  32. Anion Effects on Polymerization Mechanism: MeB(C6F5)3- vs. B(C6F5)4- How does anion coordination affinity affect active site counts, propagation, and termination kinetics? Approximately 35% active sites [Ph3C]+ [B(C6F5)4]-=5.0x10-4M [(EBI)Zr(CH3)2]=5.0x10-4M [1-Hexene]=1.0M, Toluene t=20oC

  33. B(C6F5)4-: Propagation Kinetics [CPh3]+ [B(C6F5)4]-=5.0x10-4M [(EBI)Zr(CH3)2]=5.0x10-4M T=60oC Toluene • Initiation period is not observed • Same rate law as for MeB(C6F5)3- kp= 125 M-1s-1 at 20°C

  34. Termination Products and Rate Laws 1H NMR of vinyl region vinylene 20°C, toluene 1.25 M 1-Hexene 2.8 sec, 9% conv. vinylidene tri-substituted alkene Mono-substituted alkene

  35. Comparison of Rate Constants: MeB(C6F5)3- vs. B(C6F5)4- (EBI)ZrMe2 + Activator, 20°C, Toluene Solvent • Propagation and Termination to yield vinylene endgroups (i.e. 2,1 propagation) involve significant ion-pair separation (20-40 fold increase)• -Hydride Elimination does not require ion-pair separation (same rate).

  36. Heavy Atom Kinetic Isotope Effects in1-Hexene Polymerization • More weakly coordinating anions appear to be correlated with • higher catalytic activities • more stereoerrors in syndiotactic polymerizations • rates with greater than 1st order dependence on [propene]? Do catalysts resulting from all activators share a common first irreversible step for 1-hexene incorporation? • Hypothesis: • Changes in the nature of the alkene insertion step could be revealed by changes in the Kinetic Isotope Effect (KIE). • Interpretation of heavy atom KIE’s do not depend on well-determined active site counts  KIE’s provide empirical bridge from MeB(C6F5)3- to other anions

  37. Measurement of 1-Hexene KIE 0 °C ca. 5% toluene activator 3 M Activators 2 x 10-4 M ca. 95% B(C6F5)3 Recover unreacted 1-hexene, quantitateconversion, and integrate (carefully) 13C NMR Al(C6F5)3 C-2 MAO [PhNMe2H]+ [B(C6F5)4]- C2 C4 C6 R/Ro = (1-F)(1/KIE)-1 C1 C3 C5 •R : minor isotopic component in recovered material •Ro : minor isotopic component in the original material •F : fractional conversion of reactants •KIE : relative rate of major/minor isotopic components Singelton, D. A.; Thomas, A. A. J. Am. Chem. Soc. 1995, 117, 9357.

  38. Empirical 1-Hexene KIEs Average of 3 independent runs, 3 spectra/run 0°C, (EBI)ZrMe2 + 2 eq. Activator,Toluene C2 C4 C6 C1 C3 C5 C1 C2 C3 C4 C5 B(C6F5)3 1.009(4) 1.019(6) 0.999(1) 1.001(1) 1 toluene 1.010(2) 1.017(3) 1.000(0) 1.000(2) 1 Al(C6F5)3 1.009(1) 1.017(1) 1.001(2) 1.001(1) 1 PhNMe2H+ B(C6F5)4- MAO 1.007(4) 1.018(1) 1.000(1) 1.000(2) 1 B(C6F5)3 1.003(1) 1.013(2) 0.999(1) 1.000(1) 1 chlorobenzene • KIE(C2)>KIE(C1) • Weaker Ion-Pairs yield smaller KIE’s?

  39. Do KIE’s Reveal More? Computational Model k2 k1 k-1 2 3 1 Why? What is Computed? • • ClMe as an anion substitute • • computationally accessible • • ca. thermoneutral association • • Free Energy: Association and Insertion • • Both 1,2- and 2,1-insertion pathways • • 3 trajectories for alkene association • • KIE for k1 and k2 • • EIE for K1 (=k1/k-1) • • B3LYP/LANL2DZ 1 +propene ca. 10 kcal/mol 2 G 3

  40. Computation:Association C1 C2 C3 Averages Results • Small KIE EIE 1.003(6) 0.995(7) 0.987(16) KIE 1.009(9) 1.001(4) 0.996(16)

  41. Computation:Insertion Results • KIE(C2)>KIE(C1) C1 C2 C3 Averages KIE 1.020(7) 1.044(6) 1.007(5)

  42. Does Alkene Bind Reversibly? Scenario 1: Irreversible Alkene Association KIE fixed at the alkene association step. C1 C2 C3 KIE = KIE1 1.009(9) 1.001(4) 0.996(16) Scenario 2: Reversible Alkene Association KIE fixed at the alkene insertion step. KIE = EIE1xKIE2 1.023(7) 1.039(7) 0.993(16) Experiment 1.08(7) 1.018(4) 1.000(2) Data are NOT Compatible with Scenario 1

  43. Working Mechanism slow fast

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