Polymerization of Olefins: An Outlook After 50 Years of Discovery. We probably could not imagine life in the 21 st 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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We probably could not imagine life in the 21 Discoveryst 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
1952 Natta reported: The multiple insertion of ethylene into
the Al-C bond. Growth reaction is called “Aufbaureaktion”.
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:
The Al-CH compounds occurs according to the following reactions:2CH3 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
Effect of temperature on ethylene compounds occurs according to the following reactions:
By the end of 1953, Zieglerdiscovered 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, Nattareported 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.
Natta’s Report compounds occurs according to the following reactions:
Natta compounds occurs according to the following reactions: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.
Fractions of Polyethylene compounds occurs according to the following reactions:
A: Acetone Insoluble-Ether Soluble
B: Ether Insoluble-Heptane Soluble
C: Heptane Insoluble
Group 4 component: Titanium tetrachloride, titanium
trichloride, vanadium tri chloride
Group 13 component: triethylaluminum, diethylaluminum
LB = Lewis Base (plays role concerning
stereoselectivity and activity)
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
The regularity in the configurations of successive stereocen-
ters is defined as the tacticity or overall order of the polymer
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-
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.
Polymerization processes that arise due to simple coordinat-
ion of monomer with catalyst (initiator) is called coordination
The terms isoselective and syndioselective are used to
describe catalysts (initiators) and polymerizations that give
isotactic and syndiotactic polymers respectively.
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.
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.
Vacant Coordination Site “fit” betw-
General Structure of Active Species
site on the octahedral
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
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.
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.
Syndiotacticity: coordinate VCl4 and [Et2AlCl]2
Polymer chain grows using two
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
[a] S: Polymerization in solvents G: polymerization in the gas phase; F: polymerization in the liquid monomer.
2. The time required to achieve the steady state is decreased by adding smaller
3. Settling period rise in rate to a maximadecay to a steady-state rateactive
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
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
1. β-Hydride transfer to the transition metal catalyst or the
β-hydride elimination leads to vinylidene and n-propyl end groups
Hydrolytic work up leads to a polymer with isopropyl end group
3. Chain transfer to an active hydrogen generator