Ziegler-Natta Polymerization: Synthesis of tacticity specific polypropylene S.C.S. Lai ([email protected]) Leiden University April 8th, 2004

Ziegler-Natta Polymerization:

Synthesis of tacticity specific polypropylene

S.C.S. Lai ([email protected])

Leiden University

April 8th, 2004

S.C.S. Lai, April 8th 2003

Ziegler-Natta Polymerization 2

Table of contents Overview

Mechanism (general)

Structure of catalyst

Stereospecifity\

Role of ß-TiCl3

Conclusion



Overview, polymerization (1) Three possible polymer syntheses mechanisms:

Free radicals ions metalorganic complexes

Polymers of specific tacticity wanted in industries:

Isotactic Syndiotactic Atactic



Overview, polymerization (2) Linear vs. branched polymers

Ziegler-Natta catalyst generally used to produce linear, isotactic polypropylene!



Overview, history (1) First report in September 1955 using “purple phases” of

TiCl3 (α-TiCl3 and γ-TiCl3) and AlEt3 (higher activity) or

AlEt2Cl (higher stereoselectivity).

Solvay 1973: Added TiCl4, which acted as a catalyst to convert β-TiCl3 into an active phase of TiCl3 (higher activity due to smaller particles).



Overview, history (2)

Shell 1980: TiCl4 supported on MgCl2 in presence of AlEt3 or

AlEt2Cl. Active species still TiCl3 .

Other remarks: Awarded Nobel price in 1963. 1980’s: Process attributed to Robert Banks and J. Paul Hogan

Cerutti, L; International Journal for Philosophy of Chemistry, 1999 (5), 3-41



Mechanism

Two complications

Why Cl-vacancy? Why

stereospecific?

Cossee-Arlman postulate (1964)



Structure of the catalyst, overview

Three phases of TiCl3

Color Stucture Activity

α-TiCl3 Purple Hexagonal layered structure

Isotactic

β-TiCl3 Brown Needle structure Little stereospecifity

γ-TiCl3 Purple Cubic layered structure

Like α-TiCl3



Structure of the catalyst, overview

Schematic view of the structures of α-TiCl2, α-TiCl3

and ß-TiCl3



Structure of the catalyst, Cl-vacancies (1)

Sheet of α-TiCl2, consisting of 2 layers of Cl with Ti in the octahedral holes.

Ion count:

(2m2 – 2) Cl-

(m - 1)2 Ti2+

-----------------------------

Surplus of 4(m - 1) negative charges

Offsetting by Cl- vacancies



Structure of the catalyst, Cl-vacancies (2)

Thus: Surplus of 4 (m – 1) Cl- on (m – 1)2 Ti2+

Number of vacancies:

Typical crystal of ~1μm has about than 1-2 vacancies per 1000 Ti2+-ions.

Analogous calculation for α-TiCl3 yields the same result.

2

2 .

4

1

4

)1(

)1(4

TiClmole

sequivalent

mmm

mh



Structure of the catalyst, active site (1) Cl-vacancies on the edges of the crystal.

Electron Microscopy: active sites are on the edges

Ti at the active sites in a square of Cl



Structure of the catalyst, active site (2)

Square makes an angle of 55° with the base plane.

Cl-’s not equivalent: 3 stuck in crystal 1 bound by 2 Ti3+

1 loosely bound (to 1 Ti3+)

Vacancy and L not equivalent sites



Stereospecifity, bonding of propylene

Two possibilities: 1. Alkalyne moves back to vacancy

2. Alkalyne doesn’t move back

Ti

L F

B B

V

B

Ti

B

LB

V

F

B= = Ti L

V

F

AlEt3Ti V

Et

F

CH2

CH3

Ti -

Et

F CH2

HCCH3

Ti CH2

V

F

CHEt

CH3



Stereospecifity, Polymerization (1)

Polymer moves back to vacancy isotactic polypropylene

Ti CH2

V

F

CHEt

CH3

TiF

V

H2C

CHEt

CH3

CH2

CH3

Ti -

H2C

F

CHEt

CH3

CH2

HCCH3

Ti CH2

V

F

CH2

CH3CH2

CHEt

CH3

CH3 CH3

R R



Stereospecifity, Polymerization (2)

Polymer doesn’t back to vacancy syndiotactic polypropylene

Experimental: Some syndiotactic PP at -70°

Ti CH2

V

F

CH

EtCH3

Ti CH2

|

F

CH

EtCH3

CH3

CHCH2

CH2

CH3

Ti

H2C

F

CH3

HCCH2

HCEt

CH3

V

CH3 CH3

R R



ß-TiCl3, Structure (1)

β-TiCl3 has a needle structure:

Cl Cl Cl Cl

Cl Ti3+ Cl Ti3+ Cl Ti3+ Cl

Cl Cl Cl Cl

Actual structure

Cl Cl Cl Cl

Cl Ti3+ Cl Ti3+ Cl Ti3+ �� Cl Cl �

ß1 ß2

m

Charges:3(m+2) +3(m+9) - 3 vacancies per chain

m



ß-TiCl3, Structure (2)

ß1 site: TiCl3FCl2L � TiCl3FClLR � Charge - 1/2

ß2 site: TiCl3FClL �2 TiCl3FR �2 Charge +1/2



ß-TiCl3, Reactivity

Reactive sites for

diene-polymerization:

ß1 site: 1 vacancy, limited space 1,4 trans-polymers

ß2 site: 2 vacancies, both forming pi-bonds with diene 1,4 cis-polymers

Experimental: butadiene: mixture of trans

and cis isoprene: only cis



Conclusion Three phases of TiCl3

Only α-TiCl3 and γ-TiCl3 active in stereospecific Ziegler-Natta polymerization

Active sites are the Cl--vacancies, located at the edges of the catalyst.

Stereospecifity are due stereometric interactions, forcing the same orientation for each propagation step

ß-TiCl3 has 2 different active sites, one forcing dienes to polymerize 1,4-cis, one 1,4-trans, if molecule is flexible.



Final remarks

Slides: http://home.wanadoo.nl/scslai Questions?

Documents

Ziegler-Natta Polymerization: Synthesis of tacticity specific polypropylene S.C.S. Lai ([email protected]) Leiden University April 8th, 2004