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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 everythingtoday 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 intothe 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 ethyleneoligomerization

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Ziegler and NattaBy 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 affordedfour very different fractions: The first one was an oilyproduct soluble in acetone; the second was a rubber-likeproduct soluble in diethyl ether; the third was a partially crystalline solid soluble in boiling heptane; and finally a whitehighly crystalline powder was obtained, which had a meltingpoint 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 PolyethyleneA: Acetone Insoluble-Ether SolubleB: 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 polymerchain.

If the R groups on the successive stereocenters are randomlydistributed 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 eachrepeating 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 tothe next with the R groups located alternately on the oppositesides of the plane of the polymer chain.

Atactic polymers are noncrystalline, soft materials with lowerphysical strength while isotactic and syndiotactic polymersare crystalline materials.

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Polypropylene Tacticity

H2CHC

CH3

Propylene

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Isotactic

Atactic

13C NMR

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Polystyrene Tacticity

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Isotactic Syndiotactic Atactic

C

C

HH

C

C

H

HH

H

C HH

CH

C

C

H H

H

CH H

CH

C

C

H H

C

H

H H

C

C

HH

C

C

H

HH

H

C

C

HH

C

H

H H

CH

C

C

C

H

H

C HH

H H

CH H

C

C

H H

CH H

CH

CH H

C H

CH H

C

H

H

C

C

C

C

C

C

H H

H

H H

H

H H

H

Polystyrene with diff.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 coordinationpolymerization.

The terms isoselective and syndioselective are used to describe catalysts (initiators) and polymerizations that giveisotactic and syndiotactic polymers respectively.

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Syndiotactic placement should be preferred over isotactic placement as a result of steric and/or electronic repulsionsbetween 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 ofthe propagation step (and also in the final polymer) when they are located in the alternating arrangement of syndiotacticplacement. 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 toapproach 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

Transition State Active Species

General Structure of Active Species

Vacant coordination site on the octahedralcomplex

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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 oftitanium. The monomer subsequently inserts into the polymer-titanium bond.

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The polymer migrates from its original site to that occupiedby the monomer. This is called migratory insertion.Isoselective propagation requires the migration of the polymerchain to its original position with regeneration of original configuration of the vacant site. This is called back-skip orback-flip. The chain migrates twice for each monomer insertionand 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 interactionsbetween the polymer chain end and the incoming monomer.Syndiotacticity decreases with increase in temperature!

Soluble Ziegler-Natta systems only yield atactic polymersand syndiotactic polymers. The later is possible only in thecases where there is intrinsic stereochemistry associted with the catalyst (metallocene or Ziegler-Natta type) alongwith polymer chain end control.

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Syndiotacticity: VCl4 and [Et2AlCl]2

Polymer chain grows using twosites!

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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 nextpropagation step. Syndiotactic propagation occurs alternatelyat the two ligand positions.Isotactic placement occurs against this inherent tendency when chiral active sites force monomer to coordinate withthe same enantioface at each propagating step. Syndioselecti-ve placement occurs because of the repulsive interactionsbetween the methyl groups from the polymer chain end andthe incoming monomer.Some metallocenes yield syndioselectivity through catalystsite 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 thegrowing 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 isreached. At this state, the smallest sizedparticles 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 maximadecay to a steady-state rateactivesites 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 ProcessesActive sites may have a lifetime of several hours whereas thepropagating chains may last for few seconds or minutes. Themajor chain termination mechanisms for the propagating chain are:

1. β-Hydride transfer to the transition metal catalyst or themonomer

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