Mechanisms of
Stereoselective Polymerizations
Luigi Cavallo
Università di SalernoItaly
Modeling Lab for NanostructuresAnd Catalysis
http://www.chem.unisa.it/groups/molnac
Outline
• Introduction
• Examples of Site Stereocontrol
• Examples of Chain-End Stereocontrol
• About Regiomistakes
• Ion-Pairs
• Final Remarks
Outline
• Introduction
• Examples of Site Stereocontrol
• Examples of Chain-End Stereocontrol
• About Regiomistakes
• Ion-Pairs
• Final Remarks
Milano, Italy, 53 years and 4 days ago
Nobel prize to Karl Ziegler and Giulio Natta in 1963“Nature synthesizes many stereoregular polymers, for example cellulose and rubber. This ability has been so far thought to be a monopoly of Nature operating with biocatalysts known as enzymes. But now Prof. Natta has broken this monopoly… The scientific and technical consequences of your discovery are immense and cannot even now be fully estimated.”
Presentation Speech, Stockholm, December 10 1963
Polypropylene made
Milano, Italy, 53 years and 4 days ago
Nobel prize to Karl Ziegler and Giulio Natta in 1963“Nature synthesizes many stereoregular polymers, for example cellulose and rubber. This ability has been so far thought to be a monopoly of Nature operating with biocatalysts known as enzymes. But now Prof. Natta has broken this monopoly… The scientific and technical consequences of your discovery are immense and cannot even now be fully estimated.”
Presentation Speech, Stockholm, December 10 1963
Polypropylene made
Consequences of the discovery
• About 100*106 tons/year of PE & PP in 2005
• 1 m3 blocks would wrap the equator 2.5 times
The Catalysts
Resconi Cavallo Fait PiemontesiChem Rev 2000, 100, 1253
CoatesChem Rev 2000, 100, 1223
Brintzinger, Fischer, Mülhaupt, Rieger, WaymouthAngew Chem Int Ed 1995, 34, 1143
EwenScientific American May 1997
TiCl3/AlR3 VCl4/AlR3
1954 19881962 2000 20011985
Ewen JACS 1984, 106, 6355Kaminsky, Külper, Brintzinger, Wild ACIE Engl. 1985, 24, 507.
Ewen, Jones, Razavi, FerraraJACS 1988, 110, 6255
Fujita, Kashiwa et al. Macromol. Rapid Commun. 2001, 22, 1072
Tshuva, Goldberg, KolJACS 2000, 122, 10706
The Catalysts
TiCl3/AlR3 VCl4/AlR3
1954 19881962 2000 20011985
CoatesChem Rev 2000, 100, 1223
Brintzinger, Fischer, Mülhaupt, Rieger, WaymouthAngew Chem Int Ed 1995, 34, 1143
EwenScientific American May 1997
Resconi Cavallo Fait PiemontesiChem Rev 2000, 100, 1253
The Catalysts
TiCl3/AlR3 VCl4/AlR3
1954 19881962 2000 20011985
CoatesChem Rev 2000, 100, 1223
Brintzinger, Fischer, Mülhaupt, Rieger, WaymouthAngew Chem Int Ed 1995, 34, 1143
EwenScientific American May 1997
Resconi Cavallo Fait PiemontesiChem Rev 2000, 100, 1253
Mechanism of Chain Growth
+ CH2=CH2
Cossee, P. J. Catal. 1964, 3, 80
The Chain-Migratory Mechanism
Step (i+1) Step (i)
The Three Polypropylenes
m m m m m m m m m
m r r r r r m r m
r r r r r r r r r
Isotactic polypropylene (i-PP)
Syndiotactic polypropylene (s-PP)
Atactic polypropylene (a-PP)
n
Same faceenchainment
Opposite facesenchainment
Random facesenchainment
m r r r r r m r m
The Three Polypropylenes
m m m m m m m m m
r r r r r r r r r
Isotactic polypropylene (i-PP)
Syndiotactic polypropylene (s-PP)
Atactic polypropylene (a-PP)
n
Same faceenchainment
Opposite facesenchainment
Random facesenchainment
Possible Stereomistakes
TiCl3/AlR3
VCl4/AlR3
Cp2TiCl2 at low T
Enantiomorphic-sitesterocontrol
Chain-endSterocontrol
Outline
• Introduction
• Examples of Site Stereocontrol
• Examples of Chain-End Stereocontrol
• About Regiomistakes
• Ion-Pairs
• Final Remarks
The Catalysts
1954 19891962 2000 2001
TiCl3/AlR3 VCl4/AlR3
1985
Ewen JACS 1984, 106, 6355Kaminsky, Külper, Brintzinger, Wild ACIE Engl. 1985, 24, 507.
Resconi Cavallo Fait PiemontesiChem Rev 2000, 100, 1253
CoatesChem Rev 2000, 100, 1223
Brintzinger, Fischer, Mülhaupt, Rieger, WaymouthAngew Chem Int Ed 1995, 34, 1143
EwenScientific American May 1997
System with well defined chemistry!
C2-symmetric systems(isospecific symmetry)
(S)
(S)
C2-axis
(R)
(R)
C2-axis
Mirror plane
C2-symmetric systems(isospecific symmetry)
(+)-Chain
(–)-Chain
(S)
(S)
(S)
(S)
C2-symmetric systems(isospecific symmetry)
(+)-Chain
(–)-Chain
(S)
(S)
(S)
(S)
Messenger!
-chain
–-chain
monomer monomer +
–
Mt Mt
P
P
Orientations of the growing chain
C2-symmetric systems(isospecific symmetry)
(+)-Chain
(–)-Chain
Favored !
(S)
(S)
(S)
(S)
C2-symmetric systems(isospecific symmetry)
or?
re si
(+)-Chain
(S)
(S)
C2-symmetric systems(isospecific symmetry)
(S)
si
(+)-Chain
(S)
re
C2-symmetric systems(isospecific symmetry)
(S)
(S)
(+)-Chain
si reFavored !
(+)-Chain
(S)
(S)
si
The Chain-Migratory Mechanism
Step (i+1) Step (i)
C2-symmetric systems(isospecific symmetry)
(+)-Chain
(+)-Chain
si si
(S)
(S)
(S)
(S)
C2-axis
C2-symmetric systems(isospecific symmetry)
(+)-Chain
(+)-Chain
si si
(S)
(S)
(S)
(S)
C2-axis
Homotopic coordination positions:the same monomer enantioface is favored at each step
(i.e. an isotactic polymer is formed)Cavallo, Corradini, Guerra, Vacatello
Polymer 1991, 32, 1329.
The Catalysts
1954 19881962 2000 2001
TiCl3/AlR3 VCl4/AlR3
1985
Solved!
The Catalysts
TiCl3/AlR3 VCl4/AlR3
1954 19881962 2000 20011985
Solved!
Other systems
Mechanism of the chiral orientation of the growing chainCorradini, Guerra, Cavallo Acc. Chem. Res. 2004, 37, 231
Metallocene1985
Heterogeneous1954
Post-Metallocene2000
The Catalysts
TiCl3/AlR3 VCl4/AlR3
1954 19881962 2000 20011985
Solved! Solved!Solved ?
The Catalysts
1989
TiCl3/AlR3 VCl4/AlR3
1954 19881962 2000 20011985
Solved! Solved!Solved ?
Ewen, Jones, Razavi, FerraraJACS 1988, 110, 6255
CS-symmetric systems(syndiospecific symmetry)
Mirror plane
CS-symmetric systems(syndiospecific symmetry)
C2-axisMirror plane
CS-symmetric systems(syndiospecific symmetry)
(+)-Chain
si
CS-symmetric systems(syndiospecific symmetry)
(+)-Chain (–)-Chain
resi
Mirror plane
CS-symmetric systems(syndiospecific symmetry)
(+)-Chain (–)-Chain
resi
Enantiotopic coordination positions:opposite monomer enantiofaces are favored in successive steps
(i.e. a syndiotactic polymer is formed)
Mirror plane
Cavallo, Corradini, Guerra, Vacatello Macromolecules 1991, 24, 1784.
The Catalysts
1989
TiCl3/AlR3 VCl4/AlR3
1954 19881962 2000 20011985
Solved! Solved!Solved? Solved!
Chain Orientation as Messenger(1,2 propagation, site stereocontrol)
Flow of Information
Configuration of thecatalytic site
Configuration of monomer insertion
Stereoregularpolymer
Chain Orientation as Messenger(1,2 propagation, site stereocontrol)
Flow of Information
Configuration of the chirallyoriented growing chain
Configuration of thecatalytic site
Configuration of monomer insertion
Stereoregularpolymer
-chain –-chain
Messenger!monomer monomer
+ –
Experimental Validation of the Mechanism
If the chiral orientation of the chain is the key element, without a long chain propene insertion shouldn’t be enantioselective…
P
Experimental Validation of the Mechanism
NMR study of propene insertion into the M-CH3 bondZambelli Locatelli Sacchi Tritto Macromolecules 1982, 15, 831.
CH3
Experimental Validation of the Mechanism
13CH2CH3
13CH2CH3
++
(10%)
(90%)
13CH3
13CH3
++
(50%)
(50%)
Enantioselectivity of propene insertion into M-13CH3
Enantioselectivity of propene insertion into M-13CH2CH3
The mechanism of the chiral orientation of the growing chain is confirmed
Outline
• Introduction
• Examples of Site Stereocontrol
• Examples of Chain-End Stereocontrol
• About Regiomistakes
• Ion-Pairs
• Final Remarks
The Catalysts
1989
TiCl3/AlR3 VCl4/AlR3
1954 19881962 2000 20011985
Solved! Solved!Solved? Solved!
Fujita, Kashiwa et al. Macromol. Rapid Commun. 2001, 22, 1072
Primary (1,2) Insertion
Secondary (2,1) Insertion
Regiochemistry of Insertion
11
22
12 12
TiCl3/AlR3
VCl4/AlR3
Stereorigid Catalysts
HeterogeneousTi-catalysts
Interconversionimpossible
The bridge confers stereorigidity
(R)
(R)
(S)
(S)
Stereoflexible Catalysts
PossibleInterconversion
Withouth a bridge stereoflexible complexes
HomogeneousV-catalysts
E = 5.8re-chain/-site/si-propene(r-diad)
Stericstress!
Stericstress!
Chain-Site-Monomer Interaction
E in kcal/mol
E = 5.8re-chain/-site/si-propene(r-diad)
Stericstress!
Stericstress!
E = 3.8re-chain/-site/re-propene(m-diad)
Stericstress!
Chain-Site-Monomer Interaction
E in kcal/mol
E = 5.8re-chain/-site/si-propene(r-diad)
Stericstress!
Stericstress!
E = 3.8re-chain/-site/re-propene(m-diad)
Stericstress!
E = 1.9re-chain/-site/re-propene(m-diad)
Stericstress!
Chain-Site-Monomer Interaction
E in kcal/mol
E = 5.8re-chain/-site/si-propene(r-diad)
Stericstress!
Stericstress!
E = 3.8re-chain/-site/re-propene(m-diad)
Stericstress!
E = 1.9re-chain/-site/re-propene(m-diad)
Stericstress!
Chain-Site-Monomer Interaction
E in kcal/mol
Favored!re-chain/-site/si-propene
(r-diad)
A Possible Overall PictureE in kcal/mol
Ti-Pn
re-chain/-site
Ti-Pn+1
si-chain/-site
Ti-Pn+1
si-chain/-site
Ti-Pn+2
re-chain/-site
SiteIsomerization
SiteIsomerization
si-propene
re-propene
Stereoerror
re-propeneE‡ = 1.9
Stereoerror
si-propeneE‡ = 1.9
Proposed Mechanism
Dissociative Mechanism on the basis of NMR-studies
Bickley & Serpone Inorg. Chem. 1979, 18, 2002Bei, Dale, Swenson, Jordan OM 1997, 16, 3282
-complex -complex
G‡racemization = 10 ÷ 20 kcal/mol (Mt = Ti, Zr)
Monomer Assisted Dissociation
DFT calculations
Edissociation = 14.9 kcal/mol
Dissociation
Site Chirality as Messenger(1,2 propagation, site stereocontrol)
Flow of Information
Configuration of the fluxionaloctahedral active species
Configuration of thegrowing chain
Configuration of monomer insertion
Stereoregularpolymer
-site -site
Messenger!
Other systems
re-chain/-site/si-propene(r-diad)
re-chain/-site/si-propene (r-diad)
Mechanism of the chiral orientation of the growing chainMilano Guerra Cavallo JACS. 2004, 37, 231 First proposed:
Corradini, Guerra, Pucciariello Macromolecules 1985, 13,42
The Catalysts
1989
TiCl3/AlR3 VCl4/AlR3
1954 19881962 2000 20011985
Solved! Solved!Solved? Solved! Solved!Solved!
Outline
• Introduction
• Examples of Site Stereocontrol
• Examples of Chain-End Stereocontrol
• About Regiomistakes
• Ion-Pairs
• Final Remarks
Enantioselectivity of Regiomstakes
C2-symmetric metallocene
CS-symmetric metallocene
Regiomstakes areenantioselective
Guerra Cavallo Moscardi Vacatello CorradiniJ. Am. Chem. Soc. 1994, 116, 2988.
P
Favored
re
P
si
P
re
P
Favored
si
Enantioselectivity in 1,2 vs 2,1 insertion
C2-symmetric metallocene
CS-symmetric metallocene
Opposite enantiofaces are favored in propagation and regiomistakes
The same enantioface is favored in propagation and regiomistakes
Guerra Cavallo Corradini Longo ResconiJ. Am. Chem. Soc. 1997, 119, 4394.
P
re
P
si
P
si
P
si
Selectivity in 2-butene/ethene Copolymerization
C2-symmetric metallocene
CS-symmetric metallocene
P
re
P
si
P
si
P
si
P
cis
P
trans
+ =
+ =
Ethene/2-butene Copolymerization Tests
P
cis
P
trans
cis-butene is selectively copolymerized
trans-butene is selectively copolymerized
Guerra Longo Corradini CavalloJ. Am. Chem. Soc. 1999, 121, 8651.
C2-symmetric metallocene
CS-symmetric metallocene
Regiochemistry in Octahedral Systems
SecondaryInsertion
E‡Regio
kcal/mol
Octahedral-systems : Ligand Effect
PrimaryInsertion
0.3 3.6
Slightly favored Strongly favored
Octahedral-systems : Chain Effect
Growing Chain
E‡Regio
kcal/mol 3.6
-2.0
1.4
i-Bu i-Pr
0.3
Growing Chain
i-Bu i-Pr
Steric stress!
A secondary chain pushes insertion towards another secondary insertion
Origin of Different Regiochemistry
Phenoxy-imine
Phenoxy-amine
Talarico Busico Cavallo JACS 2003, 125, 7172.
Origin of Different Regiochemistry
Phenoxy-imine
Phenoxy-amine
Talarico Busico Cavallo JACS 2003, 125, 7172.
weak antibondinginteraction
strong antibondinginteraction
GrowingChain
GrowingChain
Alkene
AlkeneMe
Me
Outline
• Introduction
• Examples of Site Stereocontrol
• Examples of Chain-End Stereocontrol
• About Regiomistakes
• Ion Pairs
• Final Remarks
Ion-Pairs
Metallocene/BorateH2Si(Cp)2ZrMe+/B(C6F5)4
-
Metallocene/BoraneH2Si(Cp)2ZrMe+/MeB(C6F5)3
-
Excellent NMR study on Metallocenium Ion-Pairs Zuccaccia, Stahl, Macchioni, Chen, Roberts, Marks
J. Am. Chem. Soc. 2004, 126, 1448
Energetic and structure of ion-pairs fundamental to understandactivity, copolymerizations, “microstructure”
Static methods (i.e. classical QM calculations) not particularly suited
Dynamics of Ion-Pairs
System simulated
1 Metallocenium ion-pair swollenin roughly 1100 benzene molecules
Conditions : P = 1 atm, T = 25 °C
Sampling time : 5 ns(can be longer)
Correa and CavalloJ. Am. Chem. Soc. 2006, 128, 10952
Classical Molecular Dynamics Simulations(i.e. no Quantum Mechanics)
Performance of the Model
Excellent agreement with several structural properties
Correa and CavalloJACS 2006, 128, 10952
The Metallocene/Borane Ion-pair
Insight into the Metallocene/Borate Ion-Pair
F2 F3
(ps)
The unclear NMR data are due to a continous fluctuation between different geometries (as found in our MD simulations)
Correa and Cavallo JACS 2006, 128, 10952
Dynamic Behavior of the Metallocene/Borate Ion-pair (Forming and breaking of M…F interactions)
Future Applications of Classical Molecular Dynamics of the Catalyst/Counterion Pair :
Moving to other metallocenes and to non-metallocenesPolymer chain extended to 10, 100, 1000 monomeric unitsCounterion displacement by a coordinating monomerKey to explain activity and copolymerizations ?
Outline
• Introduction
• Examples of Site Stereocontrol
• Examples of Chain-End Stereocontrol
• About Regiomistakes
• Ion-Pairs
• Final Remarks
Understanding the Mechanics…a previous teaching
Flying machineCodex B, f.74 (1485)
Aerial screwCodex B, f.83 (1489)
Leonardo da Vinci(1452-1519)Self-portrait
Stereospecific Olefins Polymerization aka
The best understood organometallic reactionTheoreticians
Paolo CorradiniGaetano GuerraTom ZieglerTom WooKeiji MorokumaGiuliano Lanza
Experimentalists
Karl ZieglerGiulio NattaJohn EwenTobin MarksMaurice Brookhart Jim StevensBob WaymouthRichard JordanHans-Herbert BrintzingerWalter KaminskyGerhard FinkNorio KashiwaTominaga KeiiKazuo SogaTerunori FujitaAdolfo ZambelliUmberto GianniniLuigi ResconiVincenzo BusicoMoshe Kol
All these people synergically contributed tounderstand the mechanisms of this reaction
Absolutely not a comprehensive list!
Final Slide
Why to understand ?
It is key for the rational design of new catalystsIt allows to develop new catalysts fasterIt is an intellectual pleasure
Computational chemistry
It is a powerful tool for the rationalization and prediction of chemical behavior
The New Lab
Strong interaction between theoreticians and experimentalists
Acknowledgments
The Crew
Dr. Computers U of Salerno Dr. G. Milano U of Salerno Dr. A. Correa U of Salerno Dr. C. Costabile U of Salerno Dr. G. Talarico U of Napoli Dr. L.Caporaso U of Salerno Dr. G. Moscardi Basell Polyolefins Dr. H. Jacobsen U of Tulane Dr. J. M. Ducéré U of Salerno Dr. J. Budria U of Eindhoven
Financial
Basell Polyolefins U of Salerno MURST PRIN-2004 project MURST FISR project Cineca Grant SuperCalcolo INSTM Italia
dōmo arigatō
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