Transcript Polymerization of Olefins: An Outlook After 50 Years of Discovery

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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Natta’s 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 stereocenters 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 configuration.
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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 configuration 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
H
H2C C
CH3
Propylene
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Isotactic
Atactic
13C
NMR
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Polystyrene Tacticity
Polystyrene with diff.
Tacticity
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Syndiotactic
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Stereoregulation in Alkene
Polymerization
Polymerization processes that arise due to simple coordination 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 between 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 syndioselective 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” between the catalyst and monomer that over rules the natural tendency 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 propagation 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
Transition State
Active Species
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 coordination 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 monomer. This results in either a syndiotactic or atactic polymer.
Syndiotactic polymer formation dominates over atacticity
when the monomer catalyst coordination is strongly favoured 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 [Et2AlCl]2
Polymer chain grows using two
sites!
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Isotacticity Vs Syndiotacticity: Summary
Isotactic placement occurs since only configuration is fovoured for coordination and addition of the monomer to the
propagating chain. It proceeds with the migration of the polymer 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. Syndioselective 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
1. 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. β-Hydride transfer to the transition metal catalyst or the
monomer
β-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
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