Mechanisms of Stereoselective Polymerizations Luigi Cavallo Università di Salerno Italy...

Mechanisms of

Stereoselective Polymerizations

Luigi Cavallo

Università di SalernoItaly

[email protected]

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

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


1954 19881962 2000 20011985





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




• Ion-Pairs

• Final Remarks

The Catalysts

1954 19891962 2000 2001


1985

Ewen JACS 1984, 106, 6355Kaminsky, Külper, Brintzinger, Wild ACIE Engl. 1985, 24, 507.





System with well defined chemistry!

C2-symmetric systems(isospecific symmetry)

(S)

(S)

C2-axis

(R)

(R)

C2-axis

Mirror plane


(+)-Chain

(–)-Chain

(S)

(S)

(S)

(S)


(+)-Chain

(–)-Chain

(S)

(S)

(S)

(S)

Messenger!

-chain

–-chain

monomer monomer +

–

Mt Mt

P

P

Orientations of the growing chain


(+)-Chain

(–)-Chain

Favored !

(S)

(S)

(S)

(S)


or?

re si

(+)-Chain

(S)

(S)


(S)

si

(+)-Chain

(S)

re


(S)

(S)

(+)-Chain

si reFavored !

(+)-Chain

(S)

(S)

si

The Chain-Migratory Mechanism

Step (i+1) Step (i)


(+)-Chain

(+)-Chain

si si

(S)

(S)

(S)

(S)

C2-axis


(+)-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


1985

Solved!

The Catalysts


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


1954 19881962 2000 20011985

Solved! Solved!Solved ?

The Catalysts

1989


1954 19881962 2000 20011985

Solved! Solved!Solved ?

Ewen, Jones, Razavi, FerraraJACS 1988, 110, 6255

CS-symmetric systems(syndiospecific symmetry)

Mirror plane


C2-axisMirror plane


(+)-Chain

si


(+)-Chain (–)-Chain

resi

Mirror plane


(+)-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


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



-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


NMR study of propene insertion into the M-CH3 bondZambelli Locatelli Sacchi Tritto Macromolecules 1982, 15, 831.

CH3


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




• Ion-Pairs

• Final Remarks

The Catalysts

1989


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


Stericstress!

Stericstress!

E = 3.8re-chain/-site/re-propene(m-diad)

Stericstress!


E in kcal/mol


Stericstress!

Stericstress!


Stericstress!


Stericstress!


E in kcal/mol


Stericstress!

Stericstress!


Stericstress!


Stericstress!


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



-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


1954 19881962 2000 20011985

Solved! Solved!Solved? Solved! Solved!Solved!

Outline

• Introduction




• 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



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



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.



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




• 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




• 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

Mechanisms of Stereoselective Polymerizations Luigi Cavallo Università di Salerno Italy...

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