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338 Chapter 8

properties of the polymer. An typical catalyst contains a bridged fluorenyl-

cyclopentadienyl framework, with a methyl group substituted in the 3-position

of the cyclopentadiene ring. The bridging group can consist of a -CMe

- or

-SiMe

- group.

8.6.3. Industrial Operation

The initial reluctance to introduce single-site catalysts was due to the high costs

of the catalysts and the methyl aluminoxane activator, as well as difficulties in

the processing of the new polymer with existing equipment. By 1995, the ratio

of the aluminoxanes required to activate the catalyst had fallen from about 200

to 20, thus offering a significant reduction in costs, and new activators based on

non-coordinating anions (NCA) were being developed.

From 1995 the new catalysts, now known as single-site catalysts, were de-

veloped and many new structures were introduced. This was particularly true

with catalysts used to produce a range of polypropylenes with new and different

properties and to introduce long-chain co-monomers. A feature of the new pol-

ymers was a narrow range in the molecular weight distribution. By combining

different single-site catalysts, a wider range of molecular weight distribution

could be provided. The synthesis of single-site catalysts has been simplified and

is now the domain of the specialist chemical companies. Catalyst costs have

been reduced from about two cents per pound of polymer in 1993/1994 to 0.4

cents in 1998, compared with 0.2 cents for the older Ziegler–Natta types.

This

reduction in overall catalyst cost is a result of both a reduction in the costs of

making the catalysts and increases in the productivities of these catalysts. By

1998 it was estimated that 2–4 tonnes of the organometallic component of sin-

gle-site catalysts could supply about 500,000 tonnes of polymer.

8.6.4. Catalyst Activators

Methyl aluminoxanes activate metallocene catalysts by alkylation of the metal-

locene precursor, forming a cationic complex of the type, Cp

ZrR(olefin)

When these catalysts were first used, a very large excess of the aluminoxanes

was required, but the need for such a large excess has now been removed. MAO

is formed by the controlled reaction of a limited quantity of water with tri-

methylaluminium. Normally, water reacts with explosive violence with Me

Al,

to give methane and a hydroxylated form of aluminium oxide or aluminium hy-

droxide when an excess of water is used. However, by careful control of the

reaction conditions, an intermediate degree of hydrolysis can be achieved. The

product of reaction is a polymeric molecule, with a spherical shape, and has the

formula (Al

(Me)

)

.Al(Me)

. The MAO molecule contains a single unit of

trimethylaluminium, and further reaction with methyl groups produces the com-

Olefin Polymerization Catalysts 339

plex anion, (Al

(Me

)

−

. Any excess trimethylaluminium can be extracted

with diethyl ether.

There is a tendency for the equilibrium between the complexes to be re-

versed, in polymerizations which use MAO as the activator. So-called ion equi-

librium deactivation led to a reduction in the productivity of the catalyst, and a

large excess of activator was required to maintain performance. It is now known

that the active form of the catalyst is a cation formed in an equilibrium reaction

during the activation stage, and more stable activators have since been devel-

oped. These can react irreversibly with the metallocene by the abstraction of

anionic ligands during the alkylation step and thus generate a large non-

coordinating anion, (NCA), that stabilizes the cationic catalytic species formed.

The NCA must be large, inert to the catalytic cation, and have its negative

charge localized within the structure. A most important feature is that it is labile

and easily displaced by ethylene or propylene during the polymerization pro-

cess.

The first NCA precursor to be identified was tetrafluoroboric acid, although

this was not sufficiently labile to generate an active catalyst.

More efficient

NCA precursors were later developed and included tri-n-butylammonium

tetrakispentafluorophenyl borate and N,N-dimethylanilinium tetrakis-

pentafluoro-phenyl borate., each used in a 1:3 molar ratio with the metallocene.

Compounds such as these were of critical importance in the optimization of the

catalyst.

As the demand for new polymers increased, catalysts with two active sites

per molecule were developed. It was found that tris-pentafluorophenyl-

aluminium, (C

)

Al, could be used to replace the borate derivatives, and when

used in conjunction with the typical transition metallocene catalyst precursors, a

series of di-cationic derivatives of titanium and zirconium could be produced.

Work in Dow showed that these di-cationic catalysts were up to 30 times more

reactive than boron activated catalysts, while still producing polymers with the

same molecular weight and melting point.

8.6.5. Molecular Weight Control

An early objective in the development of single-site catalysts was an increase in

the molecular weight of the polymers formed. The polymerization process can

be thought of as happening in three discrete steps, namely initiation, propagation

and termination. The final molecular weight of the polymer is determined by the

relative rates of the propagation and termination stages. The rate of propagation

could be increased by the substitution of appropriate organic groups into the 2-

position of the ligand rings, and by the introduction of bridging atoms or groups

between the ligands. Typical processes of chain termination include abstraction

of hydrogen from the ß-position of the growing chain to produce a polymer

340 Chapter 8

molecule with a double bond at the end of the chain, and chain transfer with

monomer to start a new chain.

The original metallocene catalyst, Cp

ZrCl

/MAO, produced a polymer

with a molecular weight of less than 5000 g mol

-1

. At 50

C, by using a single-

site catalyst, ethylene (1-indenyl)

ZrCl

/MAO, the molecular weight of the pol-

ymer could be increased to about 50,000 g mol

−1

The ability to produce a wide range of polymers with different molecular

weights is useful and ethylene oligomers are, of course, produced by the Aufbau

reaction and SHOP process. Single-site catalysts are now able to provide a range

of stereospecific oligomers of propylene to replace or extend the use of current

petrochemical processes (Chapter 7). They are used as α-olefins or as intermedi-

ates in the manufacture of surfactants.

8.6.6. New Catalyst Developments

New catalysts have been introduced since 1995 as a result of cooperation be-

tween industry and academia. DuPont and Professor Brookhart from the Univer-

sity of North Carolina discovered active nickel and palladium cationic complex-

es based on bulky di-imine ligands that formed catalysts in conjugation with a

suitable activator.

Chain migration can be encouraged or restricted to produce

extensive chain branching or chain growth, by variations in the operating tem-

perature and pressure. This can lead to a wide range of polymer densities. Nickel

catalysts are produced more easily than single-site catalysts and provide the ba-

sis for DuPont’s Versipol technology.

Later, in 1998, the same group, and, independently, BP Chemicals working

with Imperial College, London, discovered tridentate iron(II) and cobalt(II)

complexes with 2,6-bis(imino)pyridyl ligands that could be activated with

MAO. The activity of the catalysts was as high as that of the single site polyeth-

ylene catalysts available at that time. Polypropylene and a wide range of other

α-olefin polymers could also be produced with appropriate catalyst modifica-

tions.

In a flashback to the early Ziegler experiments, a group at the University of

California at Santa Barbara prepared organo-nickel catalysts that, when activat-

ed with the tetrakis-pentafluorophenyl borate anion, can produce butene-1 by the

dimerization of ethylene, selectively at low temperatures and pressures. When

the same nickel complex was combined with a conventional Dow-type single-

site catalyst and the same NCA activator, a range of branched polyethylenes

could be produced which varied according to the nickel/titanium ratio.

Other

developments introduced by Professor Grubbs from the California Institute of

Technology include neutral, single component nickel catalysts that do not re-

quire co-catalysts. These were based on the Shell SHOP catalysts, in which the

phosphine ligand was replaced by the bulky salicylaldiamine molecule. Poly-

Olefin Polymerization Catalysts 341

mers with an average molecular weight greater than 250,000, few chain branch-

es, and a narrow molecular weight distribution could be produced.

8.7. THE MOLECULAR STRUCTURE OF POLYOLEFINS

The polymerization of olefins proceeds by the regular insertion of successive

monomer units into the growing head to tail chain, and the stereochemical con-

figuration of the growth reaction can be controlled by asymmetric carbon cen-

ters. This is particularly important for polypropylene, because the crystalline

isotactic form is required, and each tertiary carbon (methyl group) must have the

same orientation along the chain. Each propylene molecule must be adsorbed by

the catalyst in an identical position on the active site with respect to the growing

chain. This requires the same alignment of each secondary carbon atom in the

double bonds during the cis insertion. The presence of uniformly active catalyst

sites is desirable for both ethylene and propylene polymerization reactions but is

not easy to achieve. The quantity of active sites that formed on the lateral crystal

faces of α- and γ-titanium trichloride in the early catalysts was increased by

grinding to give δ-titanium trichloride, and later by supporting titanium tetra-

chloride on magnesium chloride together with a suitable electron donor. The use

of electron donors led to fewer inactive sites and greater stereoregularity in the

polymer. The active sites on the catalysts still displayed differences, but in gen-

eral, the polymers showed much less variations in the molecular weights. The

application of metallocene compounds has resulted in the development of single

site catalysts which can achieve excellent control over the average molecular

weight, the molecular weight distribution, and the stereoregularity of the poly-

mer.

8.7.1. Formation of Polymer Chains

The Cossee–Arlman mechanism for the polymerization of olefins is the most

widely accepted theory but as yet it is not complete.

Cossee developed his ear-

ly ideas of polyethylene growth at a titanium–carbon bond and supported the

theory by molecular orbital calculations.

The role of the alkyl aluminium co-

catalyst was in the generation of the active species, via the alkylation of the tita-

nium chloride bonds, and to remove impurities in both the gas stream and cata-

lyst preparative procedure. There was also the suggestion that it might be in-

volved in the insertion of each monomer molecule, and also in the regeneration

of dormant sites or the formation of new active sites.

Titanium atoms on the lateral faces or edges of the α-TiCl

crystals may be

adjacent to vacant chlorine sites within the crystal structure. The active site is

then formed when the titanium chloride bond is alkylated by the reaction with

the alkylaluminium co-catalyst. This results in the formation of a titanium-

342 Chapter 8

carbon bond, adjacent to the vacancy. Olefin molecules, such as ethylene and

propylene, can then co-ordinate with the vacant site, forming a γ-complex,

which leads to a weakening of both the double bond of the olefin and the titani-

um-carbon bond itself. The olefin can then insert into the titanium-carbon bond,

creating a new alkyl group of higher molecular weight, and thus initiating the

polymerization reaction, but also regenerating the vacancy at which the olefin

had originally been coordinated. Successive olefin molecules could then com-

plex in turn with the vacancy, and thus propagate the growth reaction.

It has been shown by experiments using deuterium labeling of the propyl-

ene, that stereospecific polymerization of α-olefins to the isotactic form requires

that the α-olefin is complexed to the active center in a cis- configuration.

The

orientation of the olefin monomer can thus be arranged in the same relative posi-

tion as the previous monomers in the growing chain. It is possible that other

exposed titanium atoms may be converted to give active isotactic sites by the

migration of the polymer chain, caused by steric overcrowding of surrounding

chlorine atoms.

The different structural forms of polypropylene are described

in Table 8.8.

8.7.2. Polymer Chain Termination

The growth of the polymer chain can be terminated by various reactions, and

some procedures have been developed to stop the chain growth as a means of

controlling the molecular weight of the polymer.

Some of these reactions are

shown below:

• ß-hydrogen abstraction:

[Cat]−CH

−CH

−P

→ [Cat]−H + CH

=CH−P

(8.6)

TABLE 8.8. Forms of Polypropylene.

Polymer Structure

Head to tail Crystallinity controlled by branching from the

asymmetric carbon. Crystalline forms have

helical conformation but, in practice, always

contain amorphous regions.

Isotactic (typical stereospeciﬁc form) Crystalline with three monomers per turn of

helix. All methyl branches on same side of

helix. Ideal melting point 176

Syndiotactic Crystalline with four monomers per turn of

helix. Methyl branches alternate on opposite

side of helix. Ideal melting point 183

Atactic Amorphous, does not crystallize. Methyl

branches randomly placed along the chain.

Olefin Polymerization Catalysts 343

Spontaneous termination by ß-hydrogen abstraction is less typical with

Ziegler–Natta catalysts than with Phillips catalysts, because of the low

temperature of operation at 60°–80°C. It can be induced at a higher tem-

perature, however, and results in the formation of vinyl and vinylidene

end groups. The polymer chain can also exchange with the monomer to

form a new active center, with the release of free polymer, in a process

known as chain transfer.

• Reaction with molecular hydrogen:

[Cat]−P

+ H

→ [Cat]−H + H−P

(8.7)

Both Hercules

and Montecatini

have shown that hydrogen can act as a

transfer agent for Ziegler–Natta catalysts and it can restore the active

site. It is necessary to be able to control the magnitude of the molecular

weight so that the resulting polymers can be processed more easily. The

use of molecular hydrogen can be applied to all types of catalyst. The mo-

lecular weight of the polymer is approximately proportional to the square

root of the partial pressure of the added hydrogen.

A typical example of

the effect of the hydrogen concentration on the molecular weight of poly-

propylene is shown in Table 8.9. The results are taken from a stirred bed

gas phase process at 70°C using a TiCl

/MgCl

/di-isobutyl phthalate, sili-

ca-supported catalyst with an AlEt

and dialkyldimethoxysilane co-

catalyst.

• Metal alkyls:

[Cat]−P

+ ZnEt

→ [Cat]−Et + EtZnP

(8.8)

This method was also introduces by Montecatini,

and the rate of chain

termination was dependent upon the concentration of the alkyl. The al-

kylated catalyst was still active, but the polymer chain contained zinc res-

idues and the addition of further qualities of diethyl zinc was constantly

required. The same effect could be achieved by the addition of more co-

catalyst but this was not as effective, possibly because triethyl aluminium

exists as a dimer, and can form a complex with the catalyst. In the case of

chain termination with the Phillips catalyst, the addition of cyclopentadi-

ene was effective.

TABLE 8.9. Polypropylene Molecular Weight Control with Hydrogen.

Hydrogen (mol %) Molecular weight (g mol

-1

) Productivity (g g

-1

catalyst)

0.1

6 × 10

1.5 × 10

1.0 4 × 10

2.5 × 10

10 2 × 10

1 × 10

344 Chapter 8

• Proton donors: Chain transfer and chain termination are also possible us-

ing proton donors. However, the use of this method is limited by the high

probability that any proton donor would deactivate the catalyst complete-

ly and that further chain initiation would not be possible.

The use of molecular hydrogen to control the molecular weight of the pol-

ymer has been the most important method to date, particularly in the polymeri-

zation of propylene. The polymer chains growing on catalysts derived from me-

tallocenes, however, appear to be terminated by β-hydrogen abstraction and the

molecular weight can be controlled by the design of the ligand framework.

8.7.3. Molecular Weight

The variations in the chain length of the polymers produced by different types of

catalysts arise because of several different reasons, as shown below:

• Different active sites on the catalyst operate at different rates of reaction.

• The sites on the catalyst are not necessarily all active at the same time.

• The ratio of the rate of propagation to the rate of termination changes as

the length of the polymer chain increases.

• The chain growth reaction is retarded as the active center is covered by

growing polymer.

• The chain termination reaction can be retarded if the hydrogen or metal

alkyl used to control the molecular weight of the polymer is unable to ac-

cess the active site for chain transfer.

To define a polymer structure completely, the following properties are im-

portant:

• Average molecular weight.

• Molecular weight distribution.

• Distribution and type of branching within each polymer chain.

A broad molecular weight distribution gives a high polymer density with

better alignment of chains in the crystals, whereas a narrower molecular weight

distribution gives lower density with more tangling of the chain. The insertion of

co-monomers can provide some control over density by disrupting the crystalli-

zation of the polymer. When co-monomers are introduced, they may be more

easily inserted at different sites. Insertion is retarded as the chain length of the

polymer increases.

The average molecular weight of a given polymer has a different value ac-

cording to the method by which the measurement was made. The weight aver-

age molecular weight (M

), is measured by light scattering, or by gel permeation

chromatography, whereas the number average molecular weight (M

), can be

measured relatively easily by ebullioscopy, or by measurement of the osmotic

Olefin Polymerization Catalysts 345

TABLE 8.10. Properties of Commercial Polyolefins.

Polymer Structure

(branches per 1000

carbons)

Density Melting point M

LDPE 20–30 0.92 110–120 20–50

HDPE (Ziegler) 7 0.95 125–130 3–7

HDPE (Phillips) Linear 0.96 7–15

PE (metallocene) Linear ~2

PP (conventional) Linear isotactic 0.91 160–165 5–12

PP (metallocene) Linear isotactic 140–160 ~2

Ratio of weight average molecular weight and number (molecules) average molecular weight gives

an approximate measure of polymer chain length variation.

pressure of the solution. The ratio, M

is used to define the molecular

weight distribution. The melt index, a measure of the ease of extruding heated

polymer through a standard die under controlled conditions, also gives an indi-

cation of the range of the molecular weight.

In practical terms, the useful range of molecular weights for a commercial

polymer is from 30,000 to 1,000,000 g mol

−1

. Below 30,000, the softening point

of the polymer tends to be too low, and the properties of polymers with a molec-

ular weight above the higher limit are beyond the capacity of most powder ex-

trusion equipment. The range of properties of some typical polyolefins is shown

in Table 8.10.

Since the introduction of Ziegler–Natta and Phillips catalysts, polyolefin

processes are now used to produce a broad range of commercial polymers with

controlled molecular weight. The use of a variety of catalysts can control the

density of the polyolefin and its processing properties. All typical operating

modes—slurry, solution, and gas phase—are used to satisfy market demands

while allowing rapid changeover from one polymer grade to another.

“Dr Ziegler himself explained that his discovery was not the direct out-

come of attempts to solve a set problem. He set out simply to follow a

broad course of study in which ‘my only guide was initially just the de-

sire to do something which gave me pleasure’. This course threw up

many interesting conclusions, some of them of highly practical value,

and one of these led ultimately to a method of making polyethylene.”

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