Download Olefin polymerization

Olefin polymerization
In an attempted distillation of ethyl lithium, Ziegler observed α-olefins. He reasoned that
the following process was occurring involving partial decomposition of C2H5Li and
formation of ethylene :
β-H elimination
insertion
Li
+ LiH
Li
Organoaluminum compounds such as Et2AlH displayed even higher activities towards
ethylene resulting in higher aluminum alkyls that could be readily hydrolyzed to produce
higher alcohols.
excess
Al
H
Al
n
Al
m
1
Ziegler-Natta polymerization
Ziegler wondered what other metals may do. An exploration of this curiosity led to the
TiCl3/Et2AlCl catalyzed Zeigler-Natta polymerization (Nobel Prize, 1963) which is currently
used commercially to produce ~15 million tons of polyethylene and polypropylene
annually. Ziegler's original process for ethylene polymerization:
TiCl4 / AlR3
n
ethylene
polyethylene
Natta extended this to propylene polymerization. He found that a greater stereospecificity
could be achieved by using crystalline TiCl3, with the amount of the desired isotactic
polypropylene increased to 90%.
TiCl3 / AlR3
n
propylene
Ziegler, Angew. Chem. 1955, 67, 541.
Natta, Angew. Chem. 1956, 68, 393.
polypropylene
2
General features of TM-catalyzed polymerization
Oligomer, n = 3 – 100
Polymer, n > 1000
Termination
via β-H elimination
n
LnM R
R
R = H, alkyl
R
H
LnM
Ln M R
insertion
n
Propagation
via insertion
R
LnM
R
LnM
R
R
LnM
LnM
insertion
3
Isotactic polypropylene
Polymerization of alkenes is of great industrial importance, and one key issue is the
production of stereoregular polymers. The most important isotactic polypropylene has a
higher melting point, density and tensile strength than the soft and elastic atactic form.
Also of commercial importance is syndiotactic polypropylene.
isotactic: stereoregular material, long
sequences having the same
stereochemistry at adjacent carbons.
Physical properties: crystalline with a
melting temperature ~165°C.
syndiotactic: long sequences having
the opposite stereochemistry at
adjacent carbons. Physical
properties: semi-crystalline with a
melting temperature ~100°C.
atactic: stereo-random polymer that
behaves as an amorphous gum
elastomer.
4
Cossee mechanism for polypropylene
According to the Cossee mechanism, propagation of the polymer occurs at the Ti centers.
The role of the alkyl aluminum species is that of an initiator, by alkylating TiCl3. The δ-form
of TiCl3 is stereoselective below 100°C. In the more recent form of the catalyst (used since
the 1980s), TiCl4 is supported on MgCl2, and AlEt3 may be used for alkylation.
Cl
Ti
Cl
Cl
Cl
Polymer
Ti
Cl
Cl
Ti
disfavored
Me H
Ti
Cl
Cl
Cl
Polymer
Ti
Cl
Cl
Ti
Me HMe H
Ti
Cl
Cl
Ti
Cl
Cl
Ti
Polymer
favored
Representation of a Ti center on the edge of a TiCl3 crystal. The
growing polymer occupies one open site. Propene preferentially
binds with the methyl substituent anti to the polymer chain.
Cossee TL 1960 (17) 17.
Brintzinger Angewandte 1995 (34) 1143.
5
Metallocene polymerization catalysts
The development of Ziegler-Natta catalysts has, since the 1980’s, included the use of
metallocenes – complexes of early transition metals with Cp ligands or ligands derived
from cyclopentadiene.
Ti
Cl
Cl
n
polyethylene
Et2AlCl
ethylene
Natta JACS 1957 (79) 2975.
Breslow JACS 1957 (79) 5072.
Mechanism:
Ti
Cl
Cl
Et2AlCl
Ti
Cl
Al Cl
Cl
Cl
Ti
Al Cl
Cl
alkylation
Ti
Cl
migratory
insertion
Unlike the heterogeneous Ziegler-Natta
polymerization catalysts, the Ti catalysts are
ineffective for propylene polymerization.
Cl
Al Cl
Cl
Ti H
Ti
H
Cl
n
n
propagation
Al Cl
Cl
Ti
n
Breslow JACS 1959 (81) 81.
Al Cl
Cl
β-H elimination
H
Cl
Al Cl
Cl
n
6
Metallocene polymerization catalysts
Trace amounts of water cause a significant increase in the rates of ethylene polymerization
by Cp2TiEtCl / AlEt2Cl system. It was later found that water also activated analogous Zr
complexes (which are unreactive in the pure form) to highly active catalysts for both
ethylene and propylene polymerization.
Zr
ethylene
n
polyethylene
X
X = Cl or Me
AlR3
n
atactic polypropylene
propylene
Hydrolysis of AlMe3 results in the formation of a mixture of oligomeric methylaluminoxanes.
Preformed MAO is equally effective as an activator of Cp2ZrMe2 and Cp2ZrCl2 catalysts
towards olefin polymerization.
Al Me
Me
Me
H2O
Me
Al O
n
or
MAO
Barron JACS 1995 (117) 6465.
Me
O Al
Me Al
O
O
Al
Al O Me
Me
n
7
Activation by MAO
It is postulated that the highly Lewis acidic Al centers in MAO "abstract" CH3 resulting in a
cationic Zr complex and a [CH3-MAO]− counterion that may be weakly associated with the
metal.
Zr
Cl
Cl
MAO
polymer
β-H elimination
H3
C
Zr
Zr
Me
Me
MAO
H3
C
Al(MAO)
Zr
Al(MAO)
Me
n
propagation
H3
C
Zr
CH3
Al(MAO)
Al(MAO)
Zr
Me
insertion
Kaminsky ACIEE 1976 (15) 630.
Kaminsky ACIEE 1980 (19) 390.
Brintzinger ACIEE 1995 (34) 1143.
8
Cationic metallocene catalysts
[Cp2ZrMe(THF)]+ was one of the first cationic complexes used for ethylene polymerization.
The low polymerization activity was attributed to the coordinated THF which competes with
ethylene for binding.
Cl
Zr
Me
AgBPh4 (1 eq)
Zr
THF
O
Me
BPh4
Jordan JACS 1986 (108) 7410.
First well-characterized cationic zirconocene catalyst
capable of propylene polymerization at high rates:
Zr-C 2.25, 2.56 Å
Zr
Me
Me
B(C6F5)3 (1 eq)
Zr
C6H6
Marks JACS 1991 (113) 3623.
H3
C B(C6F5)3
Me
9
C2-symmetrical metallocene catalysts
Brintzinger's C2-symmetric catalysts:
1-Naphthyl
Me
Cl
Zr
Cl
Ethylene-bis-(indenyl)
zirconium dichloride
MAO
50 ºC
Cl
Zr
Cl
Ethylene-bis(tetrahydroindenyl)
zirconium dichloride
MAO
60 ºC
Me2Si
Me Cl
Zr
Cl
Naphthyl-1
MAO
50 ºC
n
78% isotacticity,188 activity
(kg pol/mol Zr·h),
Mw=24,000.
91% isotacticity, 7700 activity
(kg pol/mol Zr·h), Mw = 12,000.
>99% isotacticity, 875 activity
(kg pol/mol Zr·h), Mw = 920,000.
Brintzinger ACIEE 1985 (6) 507.
Paulus OM 1994 (13) 954.
Paulus OM 1994 (13) 954.
10
C2-symmetrical metallocene catalysts
Proposed model for isospecific polymerization: The C2-symmetrical ligand controls the
stereochemistry of monomer addition. Olefin binds with the Me-substituent anti to the growing
polymer chain. Stabilizing C-H agostic interaction is thought to rigidify the TS thereby
increasing the stereospecificity of insertion.
polymer is in
open quadrant
Polym
Polym
Zr
H
H
H
Me is down in
open quadrant
H
Zr∗
H
Zr
H
H
n
C-H agostic TS
The meso form of the catalyst gives
atactic polypropylene
Ewan JACS 1984 (106) 6355.
Grubbs Acc. Chem. Res. 1996 (29) 85.
Coates Chem. Rev. 2000 (100) 1223.
∗
Cl
Zr
Cl
11
Torsional isomers
Removal of the bridge between the indenyl ligands allows rotation about the metal ligand
bond and formation of C2- and meso- isomers, which have similar energies. The phenyl
substituents were incorporated to slow down the ligand rotation. The result of this was
production of an isotactic-atactic stereoblock copolymer.
R
Zr
C2 -isomer
isotactic block, < 28 %
Waymouth Science 1995 267 217.
R
Zr
meso-isomer
atactic block
12
Shell higher olefin process - SHOP
SHOP process is operated on a 1 million ton capacity and constitutes one of the largest
applications of homogeneous catalysis.
Ph Ph
Ph
P
Ni
PPh3
Ph O
activity: 6,000 mol ethylene/ mol Ni
ethylene,
40 atm
Mechanism:
n
50 ºC, toluene
Ph Ph
P
Ph
Ni
PPh3
O
Ph
-PPh3
Ph Ph
P
Ph
Ni
O
Ph
insertion
Ph Ph
P
H
Ni
O
Ph
β-H elimination
Keim and Kruger Angewandte 1978 (17) 466.
Keim Angewandte 1990 (29) 235.
H
Ph
Ph
Ph Ph
P
Ni H
O
Ph
Ph Ph
P
Ni
O
Ph
n
Ph Ph
P
Ni
O
Ph
Ph Ph
P
Ni
O
Ph
β-H elimination
Ph Ph
P
H
Ni
O
Ph
associative
displacement
99% linear
98% α-olefins
up to C30
H
propagation
n
13
Brookhart’s cationic Ni(II) catalyst
The rate of associative displacement leading to chain termination is retarded in these systems
by the steric bulk of the ligand which blocks the axial positions above and below the plane of
the Ni complex, and thus hinders addition of C2H4 and formation of 5-coordinate species.
N
N
Ni
Br
71 methyl branches per
1000 carbon atoms
Br
MAO
ethylene,
1 atm
n
25 ºC, toluene
m
branched polyethylene
Mw = 410,000
Catalyst activation:
Ar
N
N
Ar
Ni
Br
Br
MAO
Ar
N
N
Ar
Ni
Ar
N
Me
Brookhart JACS 1995 (117) 6414.
Brookhart Chem. Rev. 2000 (100) 1169.
N
Ar
+
Ni
Me
14
Mechanism of branching
catalyst activation
N
N
Ni
MAO
Br
N
N
Br
Ni
N
N
CH3
Ni
insertion
propagation
Linear high
Mw polymer
N
N
CH3
H
N
Ni
N
N
Ni
N
Ni
branched high
Mw polymer
n
branching
β-H elimination
N
N
re-insertion
Ni
propagation
N
H
n
N
Ni
N
n
N
H
Ni
m
n
15
Grubbs’ neutral Ni(II) catalyst
Unlike heterogeneous Ziegler-Natta and homogeneous cationic metallocene polymerization
catalysts (poisoned by O, N, and S heteroatom functionality), the neutral Ni(II) catalyst is
highly tolerant of oxygenated functionality. Olefin polymerizations can be run in the presence
of ether, ketone, and ester additives without significantly inhibiting catalyst activity.
O Ph
Ni
3
N PPh
Pri
Pri
ethylene,
7 atm
10 ºC, toluene
n
linear PE
Mw > 250,000
activity: 3,700 kg PE/h⋅mol Ni
Grubbs Science 2000 (287) 460.
16
Proposed catalytic cycle
O Ph
Ni
3
N PPh
Pri
The rate of associative displacement of the polymer leading to chain
termination is retarded in these systems (as in the Brookhart system)
by the steric bulk of the ligand, which blocks the axial positions above
and below the plane of the Ni complex.
Pri
N
O
Ni
Ph
N
PPh3
O
-PPh3
N
O
associative
displacement
N
O
Ni
Ni
Ph
O
Ni
O
O
n
O
Ni
Ph
Ph
N
H
N
N
H
insertion
H
β-H elimination
N
Ni H
Ni
propagation
H
Ni
n
17