Transcript Olefin Polymerization Organometallic Catalysis 11/6/2015 Olefin Polymerization What is a polymer?  [ -monomer- ]n  A polymer is anything that is hard to get.

Olefin Polymerization
Organometallic Catalysis
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What is a polymer?
 [ -monomer- ]n
 A polymer is anything that is hard to get out of a Schlenk
tube
 A polymer is anything that gives broad NMR spectra
 Plastics, rubbers, superabsorbent materials, starch,
cellulose, peptides, DNA
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Atactic polypentene - 1H NMR
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Atactic polypentene - 13C NMR
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Polymers and oligomers
Typical "polymer properties"
appear at  1000 monomer units
* more for small apolar monomers
(polyethene)
* less for large polar monomers
(polyester)
"Oligomers" consist of 5-50 units. This is the region where
separation of individual components is difficult.
Oligomers are mostly used as "performance chemicals"
(synthetic detergents, fuel additives)
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Questions
If you tear a plastic fiber, do you break individual polymer
chains?
And if you cut it?
What is the strongest polymer?
or
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Physical properties
are important








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Crystallinity
Transparency
Strength
Stiffness
Viscosity
Tg
"Melt flow"
Paintability
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Solid polymers
Amorphous
or
Partially crystalline:
small crystallites, with amorphous regions between them.
No sharp melting point, but a melting range of up to
10°C.
Also often a "glass transition temperature" (Tg) between a
brittle glass-like state and a rubber-like state.
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Chemistry
determines properties
Monomer
solubility, interaction between chains, surface reactivity
Chain regularity
crystallinity
Crosslinking
rubber-like properties
Molecular weight and distribution
viscosity, melt flow
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Chemical reactions
in polymer synthesis
Polymerization
 chain structure
Stereoselectivity
 chain regularity
Chemoselectivity
 branching
Initiation, termination, chain transfer
 mol wt and distribution
Chemical modification of polymers after their formation
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Molecular weight distribution
Mi = mol wt of polymer i
Ni = number of molecules of this polymer
i Ni M i
Number-average:
Mn 
N
i
i
Weight-average:
Mw
Polydispersity:
N M

N M
i
2
i
i
i
i
i
Q
Mw
Mn
Depends on polymerization kinetics,
but is often independent of molecular weight.
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Olefin polymerization
Exothermic reaction, but slow at RT:
"just add the right catalyst"
Anionic polymerization
RLi
+
R
Li
Insertion polymerization
Cl
Cl
Cl
Cl
Ti
Cl
Cl
R
Ti
Cl
Cl
R
R
Me2Si
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Zr
R
Me2Si
Zr
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Cationic polymerization
R+
R+
+
R
+
R
Radical polymerization
R
+
R
Ring-opening metathesis polymerization
M
M
M
+
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Metal-centered olefin
polymerization
Basic mechanism:
C
M
+
C
C
C
M
C
M
C
C
C
C
2+2 addition is normally "forbidden".
"Allowed" here because of asymmetry in M-C bond:
 Empty acceptor orbitals at M
 Polarity Md+-Cd- bond
 d-orbitals at M  easier to form small CMC angles
Insertion also happens at main-group metals,
but is much slower there
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
M
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*
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
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Requirements for an active
catalyst





M-C or M-H bond (can be formed in situ)
Empty site or labile ligand (anion)
Highly electrophilic metal center
No easily accessible side reactions
For stereoregular polymerization:
fairly rigid metal environment
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Insertion is a 2-site mechanism
2
M
C
1
Original Cossee mechanism
C
M
M
M
C
C
"backskip"
Modified mechanism
M
M
C
C
C
M
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C
M
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The carbene mechanism (Green)
H
H
M C
M C
H
H
M
M
C
C
Involves change in oxidation state of metal
 unlikely for LnIII, Ti/Zr/HfIV, Ni/PdII
Modified Green mechanism (a-agostic assistance)
H
H
C
M
M
C
M
C
H
M-H (or M-CH) interaction could facilitate
rotation of the C sp3 orbital.
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Agostic interactions occur frequently in electron-poor
transition metal complexes.
a- and b-agostic interactions are most the common types.
Examples of both have been found in crystal structures.
The interactions are usually weak (0-6 kcal/mol) and
fluxional. b-agostic interactions are strongest.
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Chain transfer
Main chain transfer mechanism: b-elimination
R
M
M
H
R
H
R
M
R'
H
R
R'
M
H
M
R'
H
R'
M
H
Balance between M-H and M-C bond
strengths very important for chain lengths.
General trends:
early
late
M-C weaker
lower MW
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Other chain transfer reactions
Alkyl transfer to cocatalyst
M R
+ Al
R'
M R'
+ Al
R
b-Me elimination
R
M
Me
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M
R
Me
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R
M
Me
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Termination mechanisms
• Allyl formation
M R
+
M
+
RH
• M-C or M-H bond homolysis (reduction of metal)
M R
M
+R
• Reactions with impurities in monomer
• "Burning"
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Fast chain transfer
M H
Isomerization
M
+
M
M H
+
• primary alkyls more stable and more reactive
• equilibrium favours internal olefins
C4H8
Dimerization
fast
C2H4
M H
+
C2H4
M Et
M Bu
slower
fast but
non-productive
Insertion rates:
ethene > a-olefin >> internal olefins
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Slow chain transfer
M-H (relatively) unstable
Polymers up to 106-107 D possible (but often not desirable)
If:
 there is only a single "site"
 kCT is independent of chain length
 kinit is not too small
 tpol >> tchain
then
Q = Mw/Mn  2
Any deviation from the first 3 conditions increases Q.
Determination of Mn by NMR (for up to 1000 units):
detectable vinyl and vinylidene end groups
Main MWD analysis method: GPC
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No chain transfer
"living" polymerization
If:
 there is only a single "site"
 kCT "zero" (on time scale of experiment)
 kinit is large relative to kprop
then
Q = Mw/Mn  1
Any deviation increases Q.
Living polymerization can occur if the M-H bond is very
weak.
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Block copolymers
If kterm is "zero" at the laboratory time scale, one can make
block copolymers, e.g.
M(C2H4)nR
M
M(C3H6)m (C2H4)nR
Not often used for polyolefins, except (Doi):
C3H6
V
MMA
M(C3H6)nMe
(MMA)m (C3H6)nMe
Also used for SBS rubbers:
Butadiene
Styrene
RLi
R(Styr)nLi
R(Styr)n(But)m Li
BrCH2CH2Br
R(Styr)n(But)m CH2CH2(But)m (Styr)nR
S
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S
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Controlling the molecular weight
Very high MW is not always desirable
 add an MW "control agent"
 H2
M R
+
H2
M H
+
RH
saturated end groups (NMR)
 other olefin
M R
M H
M
R
+
R
MW control agents increase kct and lower the MW, but do not
necessarily affect Mw/Mn
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MW distributions
Schulz-Flory distribution (polymerization with chain transfer)
characterized by
g = kprop/(kprop+kct)
mole fraction of n-mer = gn(1-g)
Poisson distribution (living polymerization)
characterized by
a = ratio monomer:"catalyst"
mole fraction of n-mer = ane-a/n!
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Schulz- Flory distribution
g max of MWD mol frac at max wt frac at max
0.70
0.90
0.95
0.99
0.22523
0.04542
0.02038
0.00375
0.14573
0.03919
0.01892
0.00370
4.33
11.00
21.00
101.00
Poisson distribution
a max of MWD mol frac at max wt frac at max
Mn/M
1.00
3.00
5.00
10.00
30.00
100.0
0
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0.80
7.49
17.50
97.50
Mn/M
1.00
3.00
5.00
10.00
30.00
100.00
0.36788
0.22404
0.17547
0.12511
0.07263
0.03986
0.36788
0.22404
0.17547
0.12511
0.07263
0.03986
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3.00
5.00
7.00
12.00
32.00
102.00
Mw/M Mw/Mn
4.13
17.18
37.10
197.02
0.95
1.56
1.77
1.95
Mw/M Mw/Mn
3.33
5.60
7.71
12.83
32.94
102.98
1.11
1.12
1.10
1.07
1.03
1.01
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Typical polymerization
conditions
(Pre)catalyst on a support
Active species generated in situ (e.g., by addition of
"cocatalyst")
Al alkyls or similar added as "scavengers"
Very pure monomer used
70 - 150°C
 Gas phase (ethene, ethene/propene)
 Liquid olefin (propene, higher olefins)
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Classical Ethene
Polymerization Catalysts
Ziegler catalysts
MCl n + Et2AlCl
"ClxMEt"
Role of Al alkyl
 alkylation
 reduction of M (via M-C or M-H bond homolysis?)
 scavenger
 weakly coordination anion?
P
M Cl
M
P
+
alCl
al
"Almost any metal can polymerize ethene"
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Commercial Ziegler catalysts
are always heterogeneous
 High-surface TiCl3 formed in situ from TiCl4 and Al alkyl
 TiCl3 on support (e.g. MgCl2), prepared in a complicated
process starting e.g. from TiCl4 and Mg(OAr)2.
Nature of active site not very clear.
Chain transfer is slow  high MW
H2 or temperature used to control MW.
Distribution of different sites on surface
 broad MWD (Q = 5-30)
Catalyst productivity > 106 g/g Ti/hr
 catalyst residue removal not needed.
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Types of polyethylene
different application areas
High-density (HDPE)
produced by Z-N or metallocene catalysis
Low-density (LDPE)
produced by high-temperature radical polymerization
Linear low-density (LLDPE)
produced by Z-N or metallocene catalysis with some aolefin comonomer
Long-chain-branched
produced by Brookhart-type catalysts or by metallocene
catalysis with long-chain a-olefin comonomer
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Other classical PE catalysts (1)
VCl4/EtAlCl2 and related systems
"Possibly the most active ethene polymerization catalysts"
(Shell, 1972).
But: V catalyzes PE autoxidation and most be removed
Homogeneous? VIII?
Halogenated compounds added to improve catalyst stability
(reoxidation of VII to VIII?)
Fairly narrow MWD (Q = 2-4)
VIII is paramagnetic
 this is a difficult system to study!
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Other classical PE catalysts (2)
CrO3 or Cp2Cr on silica (Phillips)
Cr oxides need to be "activated" with a reductant (H2,
ethylene)
Metallocenes are currently replacing Z-N catalysts for
commercial production of specific types of PE.
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Generalization
In simple, hard-donor ligand environments, early first-row
metals are the best catalysts.




M-H vs M-C
easier to form "naked" ions
accessibility of different oxidation states
cost
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Polypropene
Propene polymerization is more difficult than ethene
polymerization:
 Insertion of a-olefins is slower than of ethene
 Termination by b-elimination is easier
 Generally, some degree of stereocontrol is necessary to
make an interesting product
But (isotactic) PP is attractive because it has a higher melting
point than PE.
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PP stereoregularity
atactic
(Post-It)
isotactic
(Most common PP)
syndiotactic
(Not a commercial
product yet)
hemi-isotactic
Laboratory
curiosities
block-isotactic
PP is not chiral!!!
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Commercial PP production
Ziegler-Natta technology
The process to make an iso-specific catalyst is complex. An
example:
 Pre-treatment of support with an organic "donor" (ether or
ester)
 Absorption of TiCl4 on surface
 Addition of Al alkyl mixed with a second "donor" (ester or
di-ester)
 Some surface sites are better for isospecificity than others
 Aspecific sites can be converted into isospecific ones
Fair syndiotacticity is also possible, but much more difficult.
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The active site
of Z-N catalysts?
There have been many proposals for the active site in Z-N
catalysts and the reason for its isospecificity.
These are probably all incorrect or very incomplete. Our basic
understanding of the system is still poor, and this is the
reason metallocenes have had such a dramatic impact
recently.
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The active site
of Z-N catalysts?
There is consensus about the direct Ti environment on the surface; and it is
Cl
probably TiIII.
P
Cl
Cl Ti
Cl
Insertion occurs primarily in a 1,2-fashion (determined from end-group
analysis).
P
Ti
+
Me
Ti
P
Me
Steric bulk near the Ti center must be responsible for the isospecificity (site
control).
The Cossee mechanism was originally proposed to explain the isospecificity. If the site has
approximate C2 symmetry, the modified Cossee mechanism would also explain the
results.
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Errors in Z-N PP
 Stereo-errors
 Regio-errors
 "1,3-insertion"
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Mechanism of
1,3-insertion
H
Ti
P
Ti
Ti
+
P
P
Ti
Ti
H
+
P
P
(has been demonstrated by labelling)
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Effect of addition of H2
 Reduction of MW
 Increase in activity
 Increase in iso-specificity
Explanation:
"Dormant sites", formed by an insertion error, have lower
propagation rate but still react rapidly with H2.
The H2 effect allows a better determination of the number of active
sites. From this, we can conclude that in the best Z-N catalysts,
high isospecificity is obtained not just by blocking aspecific
insertion, but by "optimizing" for isospecific insertion!
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PP tacticity
Analysis of errors by 13C NMR.
Stereochemistry usually expressed in "linkages":
m ("meso") linkage
r ("racemic") linkage
NMR can be used to distinguish different linkages, e.g. mmrm
vs mrrm "pentads".
Formal view: site vs chain-end control
Assume stereochemistry of next insertion is determined
completely by:
Geometry of metal site ("site control")
Stereochemistry of last inserted unit ("chain-end control")
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These extremes can be distinguished by the insertion errors.
site control:
m
m
m
r
r
m
chain-end control:
m
m
m
r
m
m
Always observe a distribution of different linkages.
Statistics can be used to analyze the results in terms of site
control, chain-end control, mixed control, or mixture of
different sites
with different specificities.
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The Doi system for propene polymerization
V(acac)3 + EtAlCl2





Mainly syndiotactic
Living (Q = 1.0-1.2) at low temperature (up to -40 °C)
Activity in the order of kg/g V/hr
Homogeneous
Large ligand effects:
O
ca 6% of V active
V
O
3
O
V
ca 95% of V active
O
3
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Doi system
Possible explanation:
 active site relatively open:
O
P
V X
O
 2,1-insertion (at least for the syndiotactic blocks)
P
 chain-end control P
P
V
Me
H
Me
V
H
Me
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Me
Me
Me
Olefin Polymerization
V
r linkage
50
Metallocenes
"the next generation"
M R
X
M = TiIV, ZrIV, HfIV
X = weakly coordinating anion
(MAO, borate, carborate)
Kaminsky, Ewen, Marks
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Cp2TiR+ polymerizes propene with slight and temperaturedependent isospecificity
 mainly 1,2-insertion
 chain-end control
H
H
P
Me
Me
Ti
H
Ti
H
jkk
Me
Me
Ti
Me
m linkage
Me
Ti
vs
H
P
P
H
Me
P
Me
Ti
P
Me
Me
P
Me
Me
r linkage
Ti
Stereopreference depends on difference in steric interaction
with Cp of P and Me groups.
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The "trick" of metallocenes
Discriminating between chain positions ("site control")
P
M
vs
P
M
Preferred chain position independent of previous insertion
stereochemistry
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One "real" catalyst
Si
Zr
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Preferred alkyl chain orientation
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"Wrong" chain orientation
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Preferred olefin orientation
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"Wrong" olefin orientation
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After every insertion, the chain moves to the other site at Zr.
 Chain preferences the same (i.e. related by 2-fold axis)
 isospecific polymerization
 Chain preferences opposite (i.e. related by mirror plane)
 syndiospecific polymerization
 One site with a strong preference, the other without preference
 hemi-isotactic polymerization
Double stereodifferentiation: the absolute configuration of the last
inserted monomer affects the preference, so the tacticity of a
unit after an insertion error is atypical. This complicates
interpretation of NMR data.
In an extreme case, one could (theoretically)make a new polymer:
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Typical metallocenes
Zr
Me2Si
isospecific
Zr
Zr
Zr
Zr
syndiospecific
see e.g. Brintzinger,
Angew. Chem. IE 34(1995)1143
block-isotactic
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Characteristics of metallocene
polymerization
 Difficult to get MW as high as Z-N catalysts
 H2 has same effects as in Z-N catalysis (dormant sites)
 Narrow MWD (Q  2)
 Rates can be extremely high (> 107 g/g Zr/hr)
 Chain transfer by b-H and b-Me elimination
Errors same as in Z-N catalysis, plus:
stereo-errors after insertion
(demonstrated by labelling studies and by dependence of
isospecificity on [monomer])
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Mechanism of epimerization
Zr
Zr
H
H
Zr
Zr
vinylene end-groups
Zr
H
H
Zr
Zr
Zr
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Back-skip
Inversion at the metal
M R
M R
Does not affect isospecific polymerization but results in a
stereo-error in syndiospecific polymerization
Back-skip more difficult in highly bent systems (short bridges
between the rings)
Back-skip after each insertion will usually result in
isospecific polymerization
(cf Z-N systems)
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Non-metallocene ETM
polymerisation catalysts
(just a selection)
Ar
N
Ti R
Me 2Si
Ti
N
N
Cr
THF
R
Ar
McConville
R
Theopold
"Constrained-Geometry"
SiMe 3
Ti
R
N
Me 2Si
R
R
"LovaCat"
O
N
X
Zr
Me 2Si
R
R
Zr
R
O
N
SiMe 3
Horton
Schaverien
Mostly non-stereoregular. Differences in rate of termination (MW)
and incorporation of higher olefins.
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General conclusion
An M-C bond, an "empty site" and electrophilicity are enough
for polymerization. Most unsaturated ETM compounds can
in principle polymerize olefins.
The ligand environment is needed for suppressing side
reactions and for "tuning" MW, chemospecificity and
stereospecificity
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Polymerisation
at LTM centers
Mostly low MW, but with "hard"
donors MW can be higher.
Isomerization ("chain running").
Lower sensitivity to functionalized
olefins.
R2
P
R2
P
R
Ni
N
R
O
Ni
N
N
Rh
O
X
O
Flood
SHOP (Keim)
SiMe3
Me3Si
N
N
Co
(MeO)3P
R
Brookhart
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R
+
P
Me3Si
Ni(COD)2
N
SiMe3
Fink
66
The SHOP process (1)
Basic reaction:
"
"
O
P
Ni
Main byproducts:
Special properties of SHOP catalyst:
isomerization rate low
very little insertion of a-olefin
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The SHOP process (2)
Depending on market,
desired products are:
 Hexene and Octene,
as comonomers for polyethene and polypropene
 C10-C18 olefins
for detergents
(hydroformylation or sulphonation)
Butene is "waste"
Schulz-Flory distribution is broad
 relatively large amount of undesired products
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The SHOP process (3)
separation
"Ni"
C6, C8
C10-C18
isomerization
Na/K/Al2O3
metathesis
useful
products
C4
C20 and up
"Co/Mo"
broad range of
internal olefins
separation
C6-C18 internal olefins
Oligomerisation carried out in polar solvent (diol) in which
products are insoluble.
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LTM oligomerization
and polymerization
Problem:
easy chain transfer by b-elimination because M-H bond
stable (relative to
H
M-C bond).
H
M
M
 Low MW (dimers or oligomers instead of polymers)
 Isomerization
But there are exceptions ! Why ?
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Chain transfer
via b-elimination?
H
H
H
M
M
M
M
M
H
+
P
Propagation
Chain transfer
According to theoretical studies,
termination could never compete with propagation!
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Explanation
Chain transfer does not involve a free hydride
Associative displacement:
R'
M
M
R
R'
H
R
+
R'
H
M
H
+X
M
R'
+
H
R
X
R
R
M
"chain transfer to monomer"
Anion or solvent assistance:
M
H
R
X
H
+
M
H
R
Monomer, anion and solvent can occur in chain transfer rate
equations.
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The "famous”
Brookhart system
M = Ni or Pd
Ar
Ar = hindered aryl
N
R
M
N
Ar
Enough space for insertion but not for associative
displacement
 high MW.
Enough space for (reversible) b-elimination and olefin
rotation
 chain running
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Chain running
1,2-insn
M
M
P
P
M
M
2,1-insn
M
M
P
P
P
P
Product more linear than expected
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2,1- vs 1,2-insertion
1,2-insertion is the most common reaction
2,1-insertion can be favoured if:
 The metal is relatively open
 Insertion is in a M-H bond (as opposed to M-C)
 The olefin has a functional group in
a-position
MeO
O
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LTM systems are more
compatible with functionalized
olefins
COOMe
N
COOMe
R
M
N
O
MeO
COOMe
Low incorporation of acrylate because M-acrylate
coordination weaker than M-ethene coordination (why?).
But after coordination, acrylate is more reactive.
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Acrylate inserts
in a 2,1-fashion
P
P
O
M
M
OMe
O
M
OMe
OMe
P
O
chain running
O
OMe
M
P
The 5-membered chelate ring is relatively stable.
Copolymerisation is much slower than ethene polymerisation
(factor 30-300).
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Characteristics of LTM
polymerization catalysis




Rates can be comparable to ETM catalysts
Anion coordination is not a major problem
Can be compatible with protic solvents and functionalized monomers
MW tends to be (much) lower, unless associative displacement is
blocked
 Chain running gives rise to unusual branching
 No convenient general "ligand skeleton" for easy tuning
Hard ligands destabilize hydride
 higher MW.
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d+
M (n)
dH
Olefin Polymerization
dM (n-2)
d+
H
78
Comparison of ETM and LTM
catalysts
H
H
M
M
M
M
H
can be a significant
barrier for ETM
barrier significant
systems (Ziegler)
for LTM systems
but much lower for
ETM systems.
a-agostic assistance
mainly for ETM systems.
H
M
faster for
LTM systems
H
M
Anion displacement may be
a significant factor in
the rate of polymerization.
chain
running
chain
transfer
H
M
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Challenges in olefin
polymerization






Comonomer incorporation
Stereocontrol
Control over branching (long chain branching)
Morphology (reactor fouling, space-time yield)
"True gas-phase" processes
New monomers
 New block copolymers
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