Ziegler-Natta Polymerization:
Synthesis of tacticity specific
polypropylene
S.C.S. Lai (s.lai@chem.LeidenUniv.nl)
Leiden University
April 8th, 2004
Table of contents
 Overview
 Mechanism (general)
 Structure of catalyst
 Stereospecifity\
 Role of ß-TiCl3
 Conclusion
S.C.S. Lai, April
8th 2003
Ziegler-Natta Polymerization
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Overview, polymerization (1)
 Three possible polymer syntheses mechanisms:
 Free radicals
 ions
 metalorganic complexes
 Polymers of specific tacticity wanted in industries:
Isotactic
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Syndiotactic
Ziegler-Natta Polymerization
Atactic
3
Overview, polymerization (2)
 Linear vs. branched polymers
Ziegler-Natta catalyst generally used to produce linear,
isotactic polypropylene!
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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).
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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
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Mechanism
 Two
complications
 Why Cl-vacancy?
 Why
stereospecific?
Cossee-Arlman postulate (1964)
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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
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Structure of the catalyst, overview

Schematic view of the structures of α-TiCl2, α-TiCl3
and ß-TiCl3
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Structure of the catalyst, Clvacancies (1)
Ion count:
(2m2 – 2) Cl
-
(m - 1)2 Ti2+
-----------------------------
Surplus of 4(m - 1) negative
charges
Sheet of α-TiCl2, consisting of 2
layers of Cl with Ti in the
octahedral holes.
S.C.S. Lai, April
8th 2003
-
Offsetting by Cl vacancies
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Structure of the catalyst, Clvacancies (2)
-
2
2+
Thus: Surplus of 4 (m – 1) Cl on (m – 1) Ti
Number of vacancies:
4(m  1)
4
4  equivalents 

h

 
2
(m  1)
m  1 m  mole.TiCl2 
Typical crystal of ~1μm has about than 1-2 vacancies per
2+
1000 Ti -ions.
Analogous calculation for α-TiCl3 yields the same result.
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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
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Structure of the catalyst, active
site (2)

Square makes an
angle of 55° with the
base plane.

Cl ’s not equivalent:
-




S.C.S. Lai, April
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3 stuck in crystal
1 bound by 2 Ti3+
1 loosely bound (to 1
Ti3+)
Vacancy and L not
equivalent sites
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Stereospecifity, bonding of
propylene
V
L
V
V
F
=
Ti
B
B
B
F
Ti
B
L
=
Ti
Et
AlEt 3
L
F
Ti
V
F
B
B
H3C
CH3
Et
CH2
Ti
F
Two possibilities:
H3C
V
HC
CH2
Ti
CH
Et
CH 2
F
1. Alkalyne moves back to vacancy
2. Alkalyne doesn’t move back
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Stereospecifity, Polymerization
(1)
H3C
CH
H3C
V
Ti
CH
Et
Et
Ti
F
CH
H3C
H2C
CH 2
H3C
V
H2C
CH2
Ti
F
H3C
CH
H3C
V
Ti
CH 2 CH 2
F
Et
H3C
R
Et
CH3
HC
CH2
H3C
R
CH 2
F
Polymer moves back to vacancy  isotactic polypropylene
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Stereospecifity, Polymerization
(2)
CH3
CH3
H3C Et
V
CH
Ti
CH 2
H2C
H3C
CH
|
H3C Et
HC
CH
H2C
CH2
Ti
F
CH 2
F
H3C
R
Ti
CH 2
CH3
HC
Et
V
F
H3C
R
Polymer doesn’t back to vacancy  syndiotactic polypropylene
Experimental: Some syndiotactic PP at -70°
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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
m
Charges:
3(m+2) +
3(m+9)  3 vacancies per chain
Actual structure
Cl
Cl
Cl
Cl
Cl Ti3+
Cl Ti3+
Cl Ti3+
•
•
Cl
Cl
•
ß1
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8th 2003
m
ß2
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ß-TiCl3, Structure (2)
ß1 site: TiCl3FCl2L•
 TiCl3FClLR•
Charge - 1/2
ß2 site: TiCl3FClL•
2
Charge +1/2
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TiCl3FR•
2
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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
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Conclusion
 Three phases of TiCl3
 Only α-TiCl3 and γ-TiCl3 active in stereospecific ZieglerNatta 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.
S.C.S. Lai, April
8th 2003
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Final remarks
 Slides: http://home.wanadoo.nl/scslai
 Questions?
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8th 2003
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