Polymerization of Olefins: An Outlook After 50 Years of Discovery

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Polymerization of Olefins An Outlook After 50
Years of Discovery
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We probably could not imagine life in the 21st
century without polymers. Almost everything today
can be, and is, made from plastic. But this is
an inaccurate term, since plastics are only a
sub-set of the world of polymers
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Growth Reaction Genesis of Polyolefin Synthesis
1952 Natta reported The multiple insertion of
ethylene into the Al-C bond. Growth reaction is
called Aufbaureaktion.
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Ethylene oligomerization in the presence of alkyl
aluminum compounds occurs according to the
following reactions
Thermal decomposition of the aluminum-alkyl bond
yields the Al-H bond and ?-olefin. At the end
of the process the hydridoaluminum compound
reacts very fast with ethylene, as follows
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The Al-CH2CH3 bond can initiate the oligomer
chain growth by inserting the next ethylene
molecule, and thus beginning a cycle of ethylene
oligomers production. The chain growth occurs
through a four-center intermediate
Maximum Chain length 200
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Effect of temperature on ethylene oligomerization
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Ziegler and Natta
By the end of 1953, Ziegler discovered that high
polymers of ethylene can be obtained on the
addition of a transition metal salt (e.g. TiCl4)
to the alkyl aluminum species.
In 1955, Natta reported the properties of highly
crystalline polypropylene and other
poly-?-olefins which possess, at least in long
sections of the main chain, asymmetric carbon
atoms of the same absolute configuration
(isotactic poly-?-olefins). The discovery of
the new crystalline polymers was judged at that
time revolutionary in its significance and
heralded a new era in polymer science and
technology.
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Nattas Report
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Natta and his co-workers obtained a rubber-like
polymer of propylene in the very first
experiments. However, the product was not
homogeneous and contained some white solid
particles. Fractionation by solvent extraction
surprisingly afforded four very different
fractions The first one was an oily product
soluble in acetone the second was a
rubber-like product soluble in diethyl ether the
third was a partially crystalline solid soluble
in boiling heptane and finally a white highly
crystalline powder was obtained, which had a
melting point higher than 160 ºC, was insoluble
in boiling heptane, and represented 30-40 of the
total polymer. The series of solvents and the
extraction conditions chosen effected a
fractionation which was very efficient indeed, as
was recently shown by 13C-NMR spectroscopy.
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Fractions of Polyethylene A Acetone
Insoluble-Ether Soluble B Ether
Insoluble-Heptane Soluble C Heptane Insoluble
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1963 Nobel Prize
Karl Ziegler
Giulio Natta
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Traditional Ziegler-Natta Systems
Group 4 component Titanium tetrachloride,
titanium trichloride, vanadium tri
chloride Group 13 component triethylaluminum,
diethylaluminum chloride, diethylzinc
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Commercial Technologies based on Ziegler-Natta
Discovery
LB Lewis Base (plays role concerning
stereoselectivity and activity)
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Over the years, these catalysts have evolved from
simple TiCl3 crystals into the current systems
based on MgCl2 as a support for TiCl4. Different
routes have been developed for the preparation of
the supported catalysts.
Catalyst is incorporated in the lateral cuts in
the planes (110) and (100) of MgCl2
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Tacticity
The regularity in the configurations of
successive stereocen- ters is defined as the
tacticity or overall order of the
polymer chain. If the R groups on the successive
stereocenters are randomly distributed on the two
sides of the planar zigzag polymer chain, the
polymer does not have order and is called
atactic. An isotactic structure occurs when the
stereocenter in each repeating unit in the
polymer chain has the same configura- tion.
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All the R groups will be located on one side of
the plane of the C-C polymer chain. These may be
all above or all below. A syndiotactic polymer
structure occurs when the configura- tion of the
stereocenters alternate from one repeating unit
to the next with the R groups located alternately
on the opposite sides of the plane of the polymer
chain. Atactic polymers are noncrystalline, soft
materials with lower physical strength while
isotactic and syndiotactic polymers are
crystalline materials.
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Polypropylene Tacticity
Propylene
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Isotactic Atactic
13C NMR
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Polystyrene Tacticity
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Stereoregulation in Alkene Polymerization
Polymerization processes that arise due to simple
coordinat- ion of monomer with catalyst
(initiator) is called coordination polymerization.
The terms isoselective and syndioselective are
used to describe catalysts (initiators) and
polymerizations that give isotactic and
syndiotactic polymers respectively.
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Syndiotactic placement should be preferred over
isotactic placement as a result of steric and/or
electronic repulsions between substituents in the
polymer chain. Repulsion betw- een the R groups
on the terminal and penultimate units of the
propagating chain are minimized in the transition
state of the propagation step (and also in the
final polymer) when they are located in the
alternating arrangement of syndiotactic placement.
The mechanism and driving force for
syndioselec- tive polymerization is called
polymer chain end control. Steric and electronic
repulsions between R groups is maxm for
isotactic placement! If the catalyst
(initiator) fragment forces each monomer unit
to approach the propagating center with the same
face (re or si) then isotactic polymerization
occurs. This is called catalyst (initiator)
control or enantiomorphic site control mechanism.
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One can conclude that there exists a
stereochemical fit betw- een the catalyst and
monomer that over rules the natural ten- dency
towards a syndiospecific process. The catalyst
in an isotactic polymerization process is
mandatorily a mixture of two enantiomers
(racemic mixture). The two stereo components act
forces independent propagat- ion using the re and
si faces of the monomer. The resultant polymer
obtained from each of the racemic catalyst
components are super imposable i.e. the polymer
is all isotactic.
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Titanium Chloride Organoaluminum Components?
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Vacant Coordination Site
Active Species
Transition State
General Structure of Active Species
Vacant coordination site on the
octahedral complex
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Mechanism for Isoselective Propagation
A four-center transition state is obtained as a
result of coor- dination of the monomer into the
vacant coordination site of titanium. The monomer
subsequently inserts into the polymer -titanium
bond.
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The polymer migrates from its original site to
that occupied by the monomer. This is called
migratory insertion. Isoselective propagation
requires the migration of the polymer chain to
its original position with regeneration of
original configuration of the vacant site. This
is called back-skip or back-flip. The chain
migrates twice for each monomer insertion and the
overall process is called site epimerization.
This is Cossee-Ariman mechanism.
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When the catalyst is achiral, the active sites
can coordinate more or less equally with either
face of the incoming mono- mer. This results in
either a syndiotactic or atactic
polymer. Syndiotactic polymer formation dominates
over atacticity when the monomer catalyst
coordination is strongly favou- red which in turn
compensates the repulsive interactions between
the polymer chain end and the incoming
monomer. Syndiotacticity decreases with increase
in temperature! Soluble Ziegler-Natta systems
only yield atactic polymers and syndiotactic
polymers. The later is possible only in the cases
where there is intrinsic stereochemistry
associted with the catalyst (metallocene or
Ziegler-Natta type) along with polymer chain end
control.
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Syndiotacticity VCl4 and Et2AlCl2
Polymer chain grows using two sites!
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Isotacticity Vs Syndiotacticity Summary
Isotactic placement occurs since only
configuration is fovou- red for coordination and
addition of the monomer to the propagating
chain. It proceeds with the migration of the
poly- mer chain to its original ligand position
prior to the next propagation step. Syndiotactic
propagation occurs alternately at the two ligand
positions. Isotactic placement occurs against
this inherent tendency when chiral active sites
force monomer to coordinate with the same
enantioface at each propagating step.
Syndioselecti- ve placement occurs because of the
repulsive interactions between the methyl groups
from the polymer chain end and the incoming
monomer. Some metallocenes yield
syndioselectivity through catalyst site control!
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Industry
a S Polymerization in solvents G
polymerization in the gas phase F
polymerization in the liquid monomer.
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Kinetics of Heterogeneous Ziegler-Natta Systems

• The mechanical pressure exerted by the
• growing polymer chain on the catalyst
• surface tends to cleave the later. As a
• result the number of catalyst particles
• increase ? surface area of catalyst
• increases. Hence rate enhances. After a
• buildup or settling period, a steady-state is
• reached. At this state, the smallest sized
• particles are present.

2. The time required to achieve the steady state
is decreased by adding smaller particles
initially. 3. Settling period ?rise in rate to a
maxima?decay to a steady-state rate?active sites
with differing activities with some decaying with
time. 4. With either of the above the active
sites may decay and there can be a fall in
activity.
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Chain Termination Processes
Active sites may have a lifetime of several hours
whereas the propagating chains may last for few
seconds or minutes. The major chain termination
mechanisms for the propagating chain are
1. ß-Hydride transfer to the transition metal
catalyst or the monomer
ß-hydride elimination leads to vinylidene and
n-propyl end groups
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2. Chain transfer to the group 13 metal component
Hydrolytic work up leads to a polymer with
isopropyl end group
3. Chain transfer to an active hydrogen generator