Polymerization of olefins an outlook after 50 years of discovery
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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


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Growth Reaction: Genesis of Polyolefin Synthesis Discovery

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-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


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Effect of temperature on ethylene compounds occurs according to the following reactions:

oligomerization


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Ziegler and Natta 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.


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Natta’s Report compounds occurs according to the following reactions:


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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.


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Fractions of Polyethylene compounds occurs according to the following reactions:

A: Acetone Insoluble-Ether Soluble

B: Ether Insoluble-Heptane Soluble

C: Heptane Insoluble


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1963 Nobel Prize compounds occurs according to the following reactions:

Karl Ziegler

Giulio Natta


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Traditional Ziegler-Natta Systems compounds occurs according to the following reactions:

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 compounds occurs according to the following reactions:

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 TiCl

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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Isotactic

Atactic

13C NMR



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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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Vacant Coordination Site “fit” betw-

Active Species

Transition State

General Structure of Active Species

Vacant coordination

site on the octahedral

complex


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Mechanism for Isoselective Propagation “fit” betw-

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 “fit” betw-

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: coordinate VCl4 and [Et2AlCl]2

Polymer chain grows using two

sites!


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Isotacticity Vs Syndiotacticity: Summary coordinate

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 coordinate

[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 coordinate

  • 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 coordinate

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 coordinate

Hydrolytic work up leads to a polymer with isopropyl end group

3. Chain transfer to an active hydrogen generator


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