Handbook of Polymer Synthesis Second Edition Episode 1 ppt

1
Polyolefins
Walter Kaminsky
University of Hamburg, Hamburg, Germany
I. INTRODUCTION
The polyolefins production has increased rapidly in the 40 years to make polyolefins the
major tonnage plastics material worldwide. In 2003, 55 million tons of polyethene and 38
million t/a polypropene were produced [1]. These products are used for packing material,
receptacles, pipes, domestic articles, foils, and fibers. Polyolefins consist of carbon and
hydrogen atoms only and the monomers are easily available. Considering environmental
aspects, clean disposal can be achieved by burning or by pyrolysis, for instance. Burning
involves conversion to CO
2
and H
2
O, exclusively.
By copolymerization of ethene and propene with higher n-olefins, cyclic olefins, or
polar monomers, product properties can be varied considerably, thus extending the field of
possible applications. For this reason terpolymers of the ethene/propene n-olefin type are
the polymers with the greatest potential. Ethene can be polymerized radically or by means
of organometallic catalysts. In the case of polyisobutylene a cationic polymerization
mechanism takes place. All other olefins (propene, 1-butene, 4-methylpentene) are poly-
merized wi th organometalli c catalysts. The existen ce of several types of polyethene as well
as blends of these polymers provides the designer with an unusual versatility in resin
specifications. Thus polyethene technology has progressed from its dependence on one
low-density polymer to numerous linear polymers, copolymers, and blends that will extend
the use of polyethene to many previously unacceptable applications.
Polypropene also shows versatility and unusual growth potential. The main
advantage is improved susceptibility to degradation by outdoor exposure. The increase
in the mass of polypropene used for the production of fibers and filaments is inive of the
versatility of this polymer.

Synthetic polyolefins were first synthetisized by decomposition of diazomethane [2].
With the exception of polyisobutylene, these polymers were essentially laboratory
curiosities. They could not be produced economically. The situation changed with the
discovery of the high pressure process by Fawcett and Gibson (ICI) in 1930: ethene
was polymerized by radical compounds [3]. To achieve a sufficient polymerization rate,
a pressure of more than 100 MPa is necessary. First produced in 1931, the low density
polyethene (LDPE) was used as isolation material in cables.
Due to its low melting point of less than 100

C LDPE could not be applied
to the production of domestic articles that would be used in contact with hot water.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Important progress for a broader application was made when Hogan and Banks [4]
(Phillips Petroleum) and Ziegler et al. [5] found that ethene can be polymerized by means
of activated transition metal catal yst systems. In this case the high density polyethene
(HDPE), a product consisting of highly linear polymer chains, softens above 100

C.
Hogan polymerized ethene using a nickel oxide catalyst and later a chromium salt on an
alumina-silica support. Zletz [6] used molybdenum oxide on alumina in 1951 (Standard
Oil); Fischer [7] used aluminum chloride along with titanium tetrafluoride (BASF 1953)
for the production of high-density polyethene. The latter catalyst has poor activity
and was never used commercially. Zieglers [5] use of transition metal halogenides and
aluminum organic compounds and the work of Natta [8] in applying this catalyst system
for the synthesis of stereoregular polyolefins were probably the two most important
achievements in the area of catalysis and polymer chemistry in the last 50 years.
They led to the development of a new branch of the chemical industry and to a large
production volume of such crystalline polyolefins as HDPE, isotactic polypropene,
ethane-propene rubbers, and isotactic poly(l-butene). For their works, Ziegler and Natta
were awarded the Nobel Prize in 1963. The initial research of Ziegler and Natta was

followed by an explosion of scientific papers and pa tents covering most aspects of
olefin polymerization, catalyst synthesis, and polymerization kinetics as well as the
structural, chemical, physical, and technological characteristics of stereoregular poly-
olefins and olefin copolymers. Since that first publication, more than 20 000 papers
and patents have been published on subjects related to that field. Several books and
reviews giving detailed information on the subjects of these papers have been published
[9–19].
The first generation of Ziegler–Natta catalysts, based on TiCl
3
/AlEt
2
Cl, was
characterized by low polymerization activity. Thus a large amount of catalyst was needed,
which contaminate d the raw polymer. A washing step that increased production costs was
necessary. A second generation of Ziegler–Natta catalysts followed, in which the transition
metal compound is attached to a support (MgCl
2
, SiO
2
,Al
2
O
3
). These supported catalysts
are of high activity. The product contains only traces of residues, which may remain in the
polymer. Most Ziegler–Natta catalysts are heterogeneous. More recent developments
show that homogeneous catalyst systems based on metallocene-alumoxane and other
single-site catalysts can also be applied to olefin polymerization [20–23]. These systems are
easy to handle by laboratory standards, and show highest activities and an extended range
of polymer products.

The mechanism of Ziegler–Natta catalysis is not known in detail. A two-step
mechanism is commonly accepted: First, the monomer is adsorbed (p-complex bonded) at
the transition metal. During this step the monomer may be activated by the configuration
established in the active complex. Second, the activated monomer is inserted into the
metal–carbon bond. In this sequence the metal-organic polymerization resembles what
nature accomplishes with enzymes.
Ziegler–Natta catalysts are highly sensitive, to oxygen, moisture, and a large number
of chemical compounds. Therefore, very stringent requirements of reagent purity and
utmost care in all manipulations of catalysts and polymerization reactions themselves are
mandatory for achieving experimental reproducibility an d reliability. Special care must be
taken to ensure that solvents and monomers are extremely pure. Alkanes and aromatic
compounds have no substantial effect on the polymerization and can therefore be used as
solvents. Secondary alkenes usually have a negative effect on polymerization rates, and
alkynes, allenes (1,2-butadiene), and conjugated dienes are known to act as catalyst
poisons, as they tend to form stable complexes.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Almost all polar substances exert a strong negative influence on the polymerization.
COS and hydrogen sulfide, particularly, are considered to be strong catalyst poisons, of
which traces of more than 0.2 vol ppm affect a catalyst’s activity. Neither the solvent nor
the gaseous monomer should contain water, carbon dioxide, alcohols, or other polar
substances in excess of 5 ppm. Purification may be carried out by means of molecular
sieves.
The termination of the polymerization reaction by the addition of carbon monoxide
is used to determine the active centers (sites) of the catalyst. Hydrogen is known to slightly
reduce the catalyst’s activity. Yet it is commonly used as an important regulator to lower
the molecular weights of the polyethene or polypropene produced.
II. POLYETHENE
The polymerization of ethene can be released by radical initiators at high pressures as
well as by organometallic coordination catalysts. The polymerization can be carried out
either in solution or in bulk. For pressures above 100 MPa, ethene itself acts as a solvent.

Both low- and high-molecular-weight polymers up to 10
6
g/mol can be synthesized by
either organometallic coordination or high pressure radica l polymerization. The structure
of the polyethene differs with the two methods. Radical initiators give more-or-less
branched polymer chains, whereas organometallic coordination catalysts synthesize linear
molecules.
A. Radical Polymerization
Since the polymerization of ethene develops excess heat, radical polymerization on a
laboratory scale is best carried out in a discontinuous, stirred batch reactor. On a technical
scale, however, column reactors are widely used. The necessary pressure is generally
kept around 180 to 350 MPa and the temperature ranges from 180 to 350

C [24–29].
Solvent polymerization can be performed at substantial lower pressures and at tem-
peratures below 100

C. The high-pressure polymerization of ethene proceeds via a radical
chain mechanism. In this case chain propagation is regulated by dispropor tionation or
recombination.
ð1Þ
ð2Þ
ð3Þ
Copyright 2005 by Marcel Dekker. All Rights Reserved.
The rate constants for chain propagation and chain termination at 130

and
180 MPa can be specified as follows [30]:
M
p

¼ 5:93 Â 103 L Â mol
À1
s
À1
M
t
¼ 2 Â 108 L Â mol
À1
s
À1
Intermolecular and intramolecular chain trans fer take place simultaneously. This
determines the structure of the polyethene. Intermolecular chain transfer results in long
flexible side chains but is not as frequent as intramolecular chain transfer, from which
short side chains mainly of the butyl type arise [31,32].
Intermolecular chain transfer:
ð4Þ
ð5Þ
Intramolecular chain transfer:
ð6Þ
ð7Þ
Radically creat ed polyethene typically contains a total number of 10 to 50 branches
per 1000 C atoms. Of these, 10% are ethyl, 50% are butyl, and 40% are longer side
chains. With the simp lified formulars (6) and (7), not all branches observed could be
explained [33,34]. A high-pressure stainless steal autoclave (0.1 to 0.51 MPa) equipped
with an inlet and outlet valve, temperature conductor, stirrer, and bursting disk is used for
the synthesis. Best performance is obtained with an electrically heated autoclave [35–41].
To prevent self-degeneration, the temperature should not exceed 350

C. Ethene
and intitiator are introduced by a piston or membrane compressor. An in-built sapphire

window makes it possible to observe the phase relation. After the polymerization is
finished, the reaction mixture is released in two steps. Temperature increases are due to
a negative Joule–Thompson effect. At 26 MPa, ethene separates from the 250

C hot
polymer melt. After further decompression down to normal pressure, the residu al ethene
is removed [42–46]. Reaction pressure and temperature are of great importance for
the molecular weight average, molecular weight distribution, and structure of the
polymer. Generally, one can say that with increasing reaction pressure the weight average
increases, the distribution becomes narrower, and short- and long-chain branching both
decrease [47].
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Oxygen or peroxides are used as the initiators. Initiation is very similar to that
in many other free-radical polymerizations at different temperatures according to their
half-live times (Table 1). The pressure dependence is low. Ethene polymerization can also
be started by ion radiation [48–51]. The desired molecular weight is best adjusted by the
use of chain transfer reagents. In this case hydrocarbons, alcohols, aldehydes, ketones, and
esters are suitable [52,53].
Table 2 shows polymerization conditions for the high-pressure process and density,
molecular weight, and weight distribution of the polyethene (LDPE). Bunn [54] was
the first to study the structure of polyethene by x-ray. At a time when there was still
considerable debate about the character of macromolecules, the demonstration that
wholly synthetic and crystalline polyethene has a simple close-packed structure in which
the bond angles and bond lengths are identical to those found in small molecules such
Table 1 Peroxides as initiators for the high-pressure polymerization of ethene.
Peroxide Molecular weight Half-time period of 1 min
by a polymerization
temperature (

C)

(H
3
C)
3
-COOC(CH
3
)
3
146.2 190
174.2 110
146 115
216.3 130
286.4 120
230.3 160
246.4 100
194.2 120
194.2 170
234.3 90
Copyright 2005 by Marcel Dekker. All Rights Reserved.
as C
36
H
74
[55–57], strengthened the strictly logical view that macromolecules are a
multiplication of smaller elements joined by covalent bonds. LDPE crystallizes in single
lamellae with a thickne ss of 5.0 to 5.5 nm and a distance between lamellae of 7.0 nm which
is filled by an amorphous phase. The crystallinity ranges from 58 to 62%.
Recently, transition metals and organometallics have gained great interest as
catalysts for the polymerization of olefins [58,59] under high pressure. High pressure
changes the properties of polyethene in a wide range and increases the productivity

of the catalysts. Catalyst activity at temperatures higher than 150

C is controlled
primarily by polymerization and deactivation. This fact can be expressed by the practical
notion of catalyst life time, which is quite similar to that used with free-radical initiators.
The deactivation reaction at an aluminum alkyl concentration below 5 Â 10
À5
mol/l
seems to be first order reaction [60]. Thus for various catalyst-activator systems, the
approximate polymerization times needed in a continuous reactor to ensure the best use
of catalyst between 150 to 300

C are between several seconds and a few minutes.
Several studies have been conducted to obtain Ziegler–Natta catalysts with good thermal
stability. The major problem to be solved is the reduction of the transition metal
(e.g., TiCl
3
) by the cocatalyst, which may be aluminum dialkyl halide, alkylsiloxyalanes
[60], or aluminoxane [59].
Luft and colleagues [61,62] investigated high-pressure polymerizat ion in the presence
of heterogeneous catalysts consisting of titanium supported on magnesium dichloride
or with homogeneous metallocene catalysts. With homogeneous catalysts, a pressure of
150 MPa (80 to 210

C) results in a productivity of 700 to 1800 kg PE/cat, molecular
weights up to 110 000 g/mol, and a polydispersity of 5 to 10, with heterogeneous catalysts,
whereas the productivity is 3000 to 7000 kg PE/cat, molecular weight up to 70 000 g/mol,
and the polydispersity 2.
B. Coordination Catalysts
Ethene polymerization by the use of catalysts based on transition metals gives a polymer

exhibiting a greater density and crystallinity than the polymer obtained via radical
polymerization. Coordination catalysts for the polymerization of ethene can be of very
different nature. They all contain a transition metal that is soluble or insoluble in
hydrocarbons, supported by silica, alumina, or magnesium chloride [5,63]. In most cases
cocatalysts are used as activators. These are organometallic or hydride compounds of
group I to III elements; for example, AlEt
3
, AlEt
2
Cl, Al(i-Bu)
3
, GaEt
3
, ZnEt
2
, n-BuLi,
amyl Na [64]. Three groups are used for catalysis:
1. Catalysts based on titanium or zirconium halogenides or hydrides in connection
with aluminum organic compound (Ziegler catalysts)
Table 2 Polymerization conditions and product properties of high-pressure polyethene (LDPE).
Pressure
(MPa)
Temp.
(

C)
Regulator
(propane) (wt%)
Density
(g/cm

3
)
Molecular
weight MFI
Distribution
165 235 1.6 0.919 1.3 20
205 290 1.0 0.915 17.0 10
300 250 3.9 0.925 2.0 10
Source: Ref. 29.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
2. Catalysts based on chromium compounds supported by silica or alumina
without a coactivator (Phillips catalysts)
3. Homogeneous catalysts based on metallocenes in connection with aluminoxane
or other single site catalysts such as nickel ylid, nickel diimine, palladium, iron or
cobalt complexes.
Currently, mainly Ziegler and Phillips catalysts as well as some metallocene catalysts
[63] are generally used technically.
Three different processes are possible: the slurry process, the gas phase process, and
the solvent process [65–68]:
1. Slurry process. For the slurry process hydrocarbons such as isobutane,
hexane, n-alkane are used in which the polyethene is insoluble. The polymer-
ization temperature ranges from 70 to 90

C, with ethene pressure varying
between 0.7 and 3 MPa. The polymerization time is 1 to 3 h and the yield is
95 to 98%. The polyethene produced is obtained in the form of fine particles in
the diluent and can be separated by filtration. The molecular weight can be
controlled by hyd rogen; the molecular weight distribution is regulated by
variation of the catalyst design or by polymerization in several steps under
varying conditions [69–73]. The best preparation takes place in stirred vessels or

loop reactors.
In some processes the polymerization is carried out in a series of cascade
reactors to allow the variation of hydrogen concentration through the operating
steps in order to control the distribution of the molecular weights. The slurry
contains about 40% by weight polymer. In some processes the diluent is
recovered after centrifugation and recycled without purification.
2. Gas phase polymerization. Compared to the slurry process, polymerization
in the gas phase has the advantage that no diluent is used which simplifies
the process [74–76]. A fluidized bed that can be stirred is used with supported
catalysts. The polymerization is carried out at 2 to 2.5 MPa and 85 to 100

C.
The ethene monomer circulates, thus removing the heat of polymerization
and fluidizing the bed. To keep the temperature at values below 100

C, gas
conversion is maintained at 2 to 3 per pass. The polymer is withdrawn periodi-
cally from the reactor.
3. Solvent polymerization. For the synthesis of low-molecular-weight poly-
ethene, the solvent process can be used [77,78]. Cyclohexane or another
appropriate solvent is heated to 140 to 150

C. After addition of the catalyst,
very rapid polymerization starts. The vessel must be cooled indirectly by water.
Temperature control is also achieved via the ethene pressure, which can be
varied between 0.7 and 7 MPa.
In contrast to high-pressure polyethene with long-chain branches, the polyeth ene
produced with coordination catalysts has a more or less linear structure (Figure 1) [79].
A good characterization of high-molecular-weight-polyethenes gives the melt rheological
behaviour [80] (shear viscosity, shear compliance). The density of the homopolyethenes

is higher but it can be lowered by copolymerization. Polymers produced with unmodi-
fied Ziegler catalysts showed extremely high molecular weight and broad distribution
[81]. In fact, there is no reason for any termination step, except for consecutive
reaction. Equations (8) to (11) show simplified chain propagation and chain termination
steps [11].
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Chain propagation:
ð8Þ
Chain termination:
(a) By b elimination with H transfer to monomer
ð9Þ
(b) By hydrogenation
ð10Þ
Figure 1 Comparison of various polyethenes.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
(c) By b elimination forming hydride
ð11Þ
Termination via hydrogenation gives saturated polymer and metal hydride. The
termination of a growing molecule by an a-elimination step forms a polymer with an
olefinic end group and a metal hydride. In addition, an exchange reaction with ethene
forming a polymer with an olefinic end group and an ethyl metal is observed.
1. Titanium Chloride-Based Catalysts
The first catalyst used by Ziegler et al. [5,82] for the polymerization of ethene was a
mixture of TiCl
4
and Al(C
2
H
5
)

3
, each of which is soluble in hydrocarbons. In combination
they form an olive-colored insoluble complex that is very unstable. Its behavior is very
sensitive to a number of experimental parameters, such as Al/Ti ratio, temperature and
time of mixing of all components, and absolute and relative concentrations of reactants
[83]. After complexation, TiCl
4
is reduced by a very specific reduction process. This
reduction involves alkylation of TiCl
4
with aluminum alkyl molecules followed by a
dealkylation reduction to a trivalent state:
Complexation: TiCl
4
þAlEt
3
Ð TiCl
4
Á AlEt
3
ð12Þ
Alkylation: TiCl
4
:AlEt
3
Ð EtTiCl
3
Á AlEt
2
Cl ð13Þ

Reduction: 2EtTiCl
3
Ð 2TiCl
3
þ Et
2
ð14Þ
Under drastic conditions, TiCl
3
can be reduced to TiCl
2
in a similar way. The actual
TiCl
3
product is a compound alloyed with small amounts of AlCl
3
and probably some
chemisorbed AlEt
2
Cl. The mechanistic process is very complex and not well understood.
Instead of Al(C
2
H
5
)
3
, also Al(C
2
H
5

)
2
Cl, Al
2
(C
2
H
5
)
3
Cl
3
, or Al(i-Bu)
3
could be used.
These systems, called first-generation catalysts, are used for the classic process of olefin
polymerization. In practice, however, the low activity made it necessary to deactivate
the catalyst after polymerization, remove the diluent, and then remove the residues of
catalyst with HCl and alcohols. This treatment is followed by washing the polyethene
with water and drying it with steam. Purification of the diluent recover ed and feedback
of the monomer after a purification step involved further complications. The costs of
these steps reduced the advantage of the low-pressure polymerization process. Therefore,
it was one of the main tasks of polyolefin research to develop new catalysts (second
generation catalysts) that are more active, and can therefore remain in the polymer
without any disadvantage to the properties (Table 3) [84]. The process is just as sensitive
to perturbation, it is cheaper, and energy consumption as well as environmental loading
are lower. It is also possible to return to the polymerization vessel diluent containing
a high amount of the aluminum alkyl. The second generation is based on TiCl
3
compounds or supported catalysts MgCl

2
/TiCl
4
/Al(C
2
H
5
)
3
or CrO
3
(SiO
2
) (Phillips).
Copyright 2005 by Marcel Dekker. All Rights Reserved.
2. Unsupported Titanium Catalysts
There is a very large number of different combinations of aluminum alkyls and titanium
salts to make high mileage catalysts for ethene polymerization, such as a-TiCl
3
þ AlEt
3
,
AlEt
2
Cl, Al(i-Bu)
3
, and Ti(III)alkanolate-chloride þ Al(i-
hexyl
)
3

[85]. TiCl
3
exists in four
crystalline modifications, the a, b, g, and d forms [86]. The composition of these TiCl
3
s
can be as simple as one Ti for as many as three Cl, or they can have a more complex
structure whereby a second metal is cocrystallized as an alloy in the TiCl
3
. The particu lar
method of reduction determines both composition and crystalline modification. a-TiCl
3
can be synthesized by reduction of TiCl
4
with H
2
at elevated temperatures (500 to 800

C)
or with aluminum powder at lower temperatures (about 250

C); in this case the a-TiCl
3
contains Al cations [87]. More active are g- and d-TiCl
3
modifications. They are formed
by heating the a-TiCl
3
to 100 or 200


C. The preferred a-TiCl
3
contains Al and is
synthesized by reducing TiCl
4
with about 1/3 part AlEt
3
or 1 part AlEt
2
Cl. A modem
TiCl
3
catalyst has a density of 2.065 g/cm
3
, a bulk density of 0.82, a specific surface area
(BET) of 29 m
2
/g, and a particle size of 10 to 100 mm. The polymerization activity is in the
vicinity of 500 L mol
À1
 s
À1
[88].
3. Supported Catalysts
MgCl
2
/TiCl
4
catalysts. Good progress in increasing the polymerization activity was made
with the discovery of the MgCl

2
/TiCl
4
-based catalysts [89]. Instead of MgCl
2
, Mg(OH)Cl,
MgRCl, or MgR
2
[90–94] can be used. The polymerization activity goes up to
10 000 L mol
À1
s
À1
. At this high activity the catalyst can remain in the polyethene. For
example, the specific volume (BET) of the catalystis 60 m
2
/g [95]. The high activity is
accomplished by increasing the ethene pressure. The dependence is not linear as it was
for first-generation catalysts, and the morphology is also different. The polyethene has a
cobweb-like structure, whereas first generation catalysts pro duced a worm-like structure
[90,91]. The cobweb structure is caused by the fact that polymerization begins at the
surface of the catalyst particle. The particle is held together by the polymer. W hile
polymerization is in progress, the particle grows rapidly and parts of it break. Cobweb
structures are formed by this fast stretching process of the polyethene.
Table 3 Comparison of various catalyst processes for ethene polymerization.
First generation Second generation
Catalyst preparation Catalyst preparation
Polymerization Polymerization
Limited influence to molecular weight and
weight distribution

Great variation of molecular weight and
weight distribution
Catalyst deactivation with alcohol
Filtration Filtration
Washing with water (HCl), wastewater treatment,
purification, and drying of diluent
Feedback of diluent
Drying of PE Drying of PE
Finishing Finishing
Thermal degradation of molecular weight, blending
Stabilization Stabilization
Source: Ref. 84.
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It is known that in the case of these supported catalysts the higher activity is linked
to a higher concentration of active titanium. In contrast to first-generation catalysts in
which only 0.1 to 1% of all titanium atoms form active sites, in supported catalysts 20 to
80% of them are involved in the formation of active sites [97,98].
Solvay workers [99] have investigated extensively the supported Mg(OH)Cl/TiCl
4
/
AlEt
3
catalyst and related systems including MgSO
4
, MgOSiO
2
, and MgO. It is not clear
whether all of the Ti centers in the supported catalysts are isolated. The high activity
suggests the incorpo ration of small TiCl
3

crystallites into the Mg(OH)Cl. Fink and
Kinkelin [100] prepared a high-activity catalyst by combination of MgH
2
and TiCl
4
.
The MgH
2
has a much greater surfa ce area (90 m
2
/g). It reacts with the TiCl
4
under
the evolution of hydrogene. By 30

C and 2 bar ethene pressure, 110 kg of PE per gram of
Ti could be obtained.
4. Phillips Catalyst
The widely investigated Phillips catalyst, which is alkyl free, can be prepared by impreg-
nating a silica-alumina (87:13 composition [101–103] or a silica support with an aqueous
solution of CrO
3
). High surface supports with about 400 to 600 g/m
2
are used [104]. After
the water is removed, the powdery catalyst is fluidized and activated by a stream of dry air
at temperatures of 400 to 800

C to remove the bound water. The impregnated catalysts
contain 1 to 5 wt% chromium oxides. When this catalyst is heated in the presence of

carbon monoxide, a more active catalyst is obtained [105]. The Phillips catalyst specifically
catalyzes the polymerization of ethene to high-density polyethene. To obtain polyethene of
lower crystallinity, copolymers with known amounts of an a-olefin, usually several percent
of 1-butene can be synthesized. The polymerization can be carried out by a solution,
slurry, or gas-phase (vapor phase) process.
The chromium oxide-silica is inactive for polymerizing ethylene at low temperatures
but becomes active as the temperatur e is increased from 196

C (the melting point for
CrO
3
) to 400

C. Interactions of chromium oxide with SiO
2
and Al
2
O
3
take place.
Hogan [103] calculated that for a silica support of 600 m
2
/g and about 5% Cr(VI),
the average distance between adjacent Cr atoms is 10 A
˚
. This corresponds to the accepted
population of silanol groups on silica after calcination. The structures (15) and (16) are
proposed:
ð15Þ
ð16Þ

Copyright 2005 by Marcel Dekker. All Rights Reserved.
It has been calculated that between 0.1 and 0.4 wt% of the total chromium
forms active centers [105]. A difficult question relates to the valences of chromium in the
active sites. Valences of II, III, IV, V, and VI have been established [106]. Because of the
small number of total chromium atoms that are active centers, it has not been possible to
unequivocally assign the active valence [107,108]. Krauss and Hums [109] concluded that
the reduction of hexavalent chromium centers linked to support produced coordinately
unsaturated Cr(II) surface compounds. A speciality of the Phillips catalyst is that there is
no influence of hydrogen to control the molecular weight of the polyethylene. Only by
higher activation temperatures can the molecular weight be lowered.
5. Homogeneous (Single Site) Catalysts
Among the great number of Ziegler catalysts, homogeneous systems have been
preferentially studied in order to understand the elementary steps of the polymerization
which is simpler in soluble systems than in heterogeneous systems. The situation
has changed since in recent years homogeneous catalyst based on metallocene and
aluminoxane [12,110], nickel and palladium diimin complexes [111], and iron and cobalt
compounds were discovered which are also very interesting for industrial and laboratory
synthesis. Some special polymers can only be synthesized with these catalysts.
In comparison to Ziegler systems, metallocene catal ysts represent a great develop-
ment: they are soluble in hydrocarbons, show only one type of active site and their
chemical structure can be easily changed. These properties allow one to predict accurately
the properties of the resulting polyolefins by knowing the structure of the catalyst used
during their manufacture and to control the resulting molecular weight and distribution,
comonomer content an d tacticity by careful selection of the appropriate reactor condi-
tions. In addition, their catalytic activity is 10–100 times higher than that of the classical
Ziegler–Natta systems.
Metallocenes, in combination with the conventional aluminum alkyl cocatalysts used
in Ziegler systems, are indeed capable of polymerising ethene, but only at a very low
activity. Only with the discovery and application of methylaluminoxane (MAO) it was
possible to enhance the activity, surprisingly, by a factor of 10 000 [113]. Therefore, MAO

plays a crucial part in the catalysis with metallocenes.
Kinetic studies and the application of various methods have helped to define the
nature of the active centers, to explain the aging effects of Ziegler catalysts, to establish
the mechanism of interaction with olefins, and to obtain quantitative evidence of some
elementary steps [9,112–115]. It is necessary to differentiate between the soluble catalyst
system itself and the polymerization system. Unfortunate ly, the well-defined bis(cyclo-
pentadienyl)titanium system is soluble, but it becomes heterogeneous when polyethylene
is formed [116].
The polymerization of olefins, promoted by homogeneous Ziegler catalysts based
on biscyclopentadienyltitanium(IV) or analogous compounds and aluminum alkyls,
is accompanied by a series of other reactions that greatly complicate the kinetic inter-
pretation of the polymerization process:
ð17Þ
Copyright 2005 by Marcel Dekker. All Rights Reserved.
ð18Þ
ð19Þ
ð20Þ
Concomitant with continued olefin insertion into the metal–carbon bond of the
transition metal aluminum complex, alkyl exchange and hydrogen-transfer reactions
are observed. Whereas the normal reduction mechanism for transition metal organic
complexes is initiated by release of olefins with formation of a hydride followed by hydride
transfer to an alkyl group, a reverse reaction takes place in the case of some titanium
and zirconium acompounds. A dimetalloalkane is formed by the release of ethane. In
second step, ethene is evolved from the dimetalloalkane:
TiðIVÞÀCH
2
ÀCH
2
ÀTiðIVÞ!CH
2

¼CH
2
þ 2TiðIIIÞð21Þ
leaving two reduced metal atoms. Some of the aging processes occurring with homo-
geneous and heterogeneous Ziegler catalysts can be explained with the aid of these side
reactions.
Table 4 summarizes important homogeneous Ziegler catalysts. The best known sys-
tems are based on bis(cyclopentadienyl)titanium(IV), bis(cyclopentadienyl)zirconium(IV),
terabenzyltitanium, vanadium chloride, allyl metal, or chromium acetylacetonate with
trialkylaluminum, alkylaluminum halides, or aluminoxanes. Breslow [126] discovered that
bis(cyclopentadienyl)titanium(IV) compounds, which are easily soluble in aromatic
hydrocarbons, could be used instead of titanium tetrachloride as the transition metal
compound together with aluminum alkyls for ethene polymerization. Subsequent research
on this and other systems with various alkyl groups has been conducted by Natta [127],
Belov et al. [128,129] , Patat and Sinn [130], Shilov [131], Henrici-Olive and Olive [132],
Reichert and Schoetter [133], and Fink et al. [134,135]. With respect to the kinetics of
polymerization and side reactions, this soluble system is probably the one that is best
understood. It is found that the polymerization takes place primarily if the titanium exists
as titanium(IV) [136,137]. According to Henrici-Olive and Olive [138], the speed
of polymerization decreases with increasing intensity of ESR signals of the developing
titanium(III) compound.
The increase in length of the polymer chain occurs by insertion of the monomer in to
a metal–carbon bond of the active complex. Dyachkovskii et al. [139] and Eisch et al. [140]
were the first to believe, based on kinetic measurements and synthesis, that the insertion
takes place on a titanium cation. An ion of the type (C
5
H
5
)
2

Ti
þ
-R, derived from
Copyright 2005 by Marcel Dekker. All Rights Reserved.
complexing and dissociation,
ðC
5
H
5
Þ
2
TiRCl þ AlRCl
2
ÐðC
5
H
5
Þ
2
TiRCl Á AlRCl
2
ð22Þ
ðC
5
H
5
Þ
2
TiRCl Á AlRCl
2

нðC
5
H
5
Þ
2
TiRCl
3

þ
þ½AlRCl
3

À
ð23Þ
could be the active species of polymerization. Sinn and Patat [137] drew attention to the
electron-deficient character of those main-group alkyls that afford complexes with the
titanium compound. Fink and co-workers [141] showed by
13
C-NMR spectroscopy with
13
C-enriched ethene at low temperatures (where no alkyl exchange was observed) that
in higher halogenated systems, inser tion of the ethene takes place only into a titanium–
carbon bond.
At low polymerization temperatures with benzene as a solvent, Hocker and Saeki
[142] could prepare polyethene with a molecular weight distribution M
W
/M
n
¼ 1.07 using

the bis(cyclopentadienyl)titanium dichloride/diethylaluminum chloride system. The mole-
cular weight could be varied in a wide range by changing the polyme rization temperature.
Using ally
4
Zr(allylZrBr
3
) at a polymerization temperatur e of 160

C (80

C) yields poly-
ethene with a density of 0.966 g/cm, M
n
of 10,500, (700), 3.0 CH
3
groups per 1000

C and 0.4
vinyl groups. The benzene- and allyl-containing transition metals are working without any
cocatalyst and therefore are alkyl free. If transition metal organometallic compounds such
as Cr(allyl)
3
, Zr(allyl)
4
, Zr(benzyl)
4
, Ti(benzyl)
4
, and Cr(cyclopentadienyl)
2

are supported
on Al
2
O
3
Or SiO
2
, the activity increases by a factor of more than 100 [124,143].
Apparently, soluble catalysts are obtained by reaction of Ti(OR)
4
with AlR
3
[144].
High-molecular-weight polyethene is obtained in variable amounts, with Al/Ti ratios
ranging between 10 and 50. Similar results are attained by replacing titanium alkoxide
by Ti(NR
2
)
4
[145]. Soluble catalytic systems are also obtained by reaction of Ti(acac)
3
[146] and Cr(acac)
3
[147] with AlEt
3
as well as by reaction of Cr(acac)
3
and VO(acac)
2
with

AlEt
2
Cl in the presence of triethyl phosphite [121]. With vanadium catalysts the activity
reaches its maximum at Al/V ratio ¼ 50. Under these conditions up to 67% vanadium is in
the bivalent oxidation state. Bivalent and trivalent compounds will be active.
Table 4 Homogeneous catalysts for ethene polymerization.
System Transition metal
(M) compound
Polymerization
temperature (

C)
Normalized
activity
Catalyst
yield
Refs
Cp
2
TiCl
2
/AlMe
2
Cl
a
1:2.5–1:6 30 40–200 117
Cp
2
TiCl
2

/AlMe
2
Cl/H
2
O 1:6:3 30 2000 117
Cp
2
TiCl
2
/AlEt
2
Cl 1:2 15 7–45 118
Cp
2
TiMe
2
/MAO 1:10
5
.5 Â 10
2
20 35 000 >15 000 110
Cp
2
TiMe
2
/MAO 1:100 20 200 >5 000 119
Cp
2
ZrCl
2

/MAO 1:1000 70 400 000 >10 000 120
VO(acac)
2
/Et
2
AlCl/activator 1:50 20 180 121
Cp
2
VCl
2
/Me
2
AlCl 1:5 50 13 122
Zr(allyl)
4
80 2.0
Hf(allyl)
4
160 0.6
Cr(ally)
3
80 0.3 123
Cr(acac)
3
/EtAlCl 1:300 20 150 121
Ti(benzyl)
4
20(80) 8 Â 10
À3
(0.2) 124,125

Ti(benzyl)
3
Cl 20 0.4 124,125
Ti(benzyl)
4
Copyright 2005 by Marcel Dekker. All Rights Reserved.
6. Aluminoxane as Cocatalysts
The use of metallocenes and alumoxane as cocatalyst results in extremely high poly-
merization activities (see Tables 4 and 5). This system can easily be used on a laboratory
scale. The methylalumoxane (MAO) is prepared by careful treatment of trimethylalumi-
num with water [148]:
ð24Þ
MAO is a compound in which aluminum and oxygen atoms are arranged alternately
and free valences are saturated by methyl substituents. It is gained by careful partial
hydrolysis of trimethylaluminum and, according to investigations by Sinn [149] and
Barron [150], it consists mainl y of units of the basic structure [Al
4
O
3
Me
6
], which contains
four aluminum, three oxygen atoms and six methyl groups. As the aluminum atoms in
this structure are co-ordinatively unsaturated, the basic units (mostly four) join together
forming clusters and cages. These have molecular weights from 1200 to 1600 and are
soluble in hydrocarbons.
If metallocenes, especially zirconocenes but also titanocenes, hafnocenes and other
transition metal compounds (Figure 2) are treated with MAO, then catalysts are acquired
that allow the polymerization of up to 100 tons of ethene per g of zirconium [151–153].
At such high activities the catalyst can remain in the product. The insertion time (for the

insertion of one molecule of ethene into the growing chain) amounts to some 10
À5
s only
(Table 6). A comparison with enzymes is not far-fetched.
As shown by Tait under these conditions every zirconium atom forms an active
complex and produces about 20 000 polymer chains per hour. At temperatures ab ove
50

C, the zirconium catalyst is more active than the hafnium or titanium system; the latter
is decomposed by such temperatures. Transition metal compounds containing some
halogene show a higher activity than syst ems that are totally free of halogen. Of the
cocatalysts, methylalumoxane is much more effective than the ethylaluminoxane or
isobutylalumoxane.
It is general ly assumed that the function of MAO is firstly to undergo a fast ligand
exchange reaction with the metallocene dichloride, thus rendering the metallocene methyl
Table 5 Ethene polymerization
a
with metallocene/methylaluminoxane catalysts.
Metallocene
b
Structure Activity
[kg PE/(mol Zr.h.c
mon
]
Molecular weight
(g/mol)
Cp
2
ZrCl
2

6 60 900 62 000
[Me
2
C(Ind)(Cp)]ZrCl
2
8 3330 18 000
[En(IndH
4
)
2
]ZrCl
2
9 22 200 1 000 000
[Em(Ind)
2
]ZrCl
2
11 12 000 350 000
[En(Ind)
2
]HfCl
2
12 2900 480 000
[Me
2
Si(Ind)
2
]ZrCl
2
13 36 900 260 000

[Me
2
Si(2,4,7-Me
3
Ind)
2
]ZrCl
2
15 111 900 250 000
[Me
2
C(Flu)(Cp)]ZrCl
2
18 2000 500 000
a
Ethene pressure ¼ 2.5 bar. temp. ¼30

C. [metallocene] ¼ 6.25 Â 10
À6
M. Metaliocene/MAO ¼ 250. Solvent ¼
toluene;
b
Cp ¼ cyclopentadienyl; Ind ¼ indenyl; En ¼ C
2
H
4
; Flu ¼ fluorenyl.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
and dimethyl compounds (Figure 3). In the further step, either Cl
À

or CH
À
3
is abstracted
from the metallocene compound by al Al-cen ter in MAO, thus forming a metallocene
cation and a MAO anion [156,157]. The alkylated metallocene cation represents the active
center (Figure 4). Meanwhile, other weakly coordinating cocatalysts, such as
tetra(perfluorophenyl)borate anions [(C
6
F
5
)
4
B]
À
, have been successfully applied to the
activation of metallocenes [158–161].
Polyethenes synthesized by metallocene-alumoxane have a molecular weight dis-
tribution of M
w
/M
n
¼ 2, 0.9 to 1.2 methyl groups per 1000 C atoms, 0.11 to 0.18 vinyl
groups, and 0.02 trans vinyl group per 100 C atoms. The molecular weight can easily be
lowered by increasing the temperature, increasing the metallocene concentration, or
Figure 2 Some classes of metallocene catalysts used for olefin polymerization.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Table 6 Polymerization activity of bis(cyclopentadienyl)zirconium dichloride/
methylalumoxane catalyst applied to ethene in 330 ml of toluene.
Activity (95


C), 8 bar 39.8 Â 10
6
g PE/g Zr Á h
[Zirconocene] 6.2 Â 10
8
mol/l
[Alumoxane] (M ¼ 1200) 7.1 Â 10
À4
mol/l
Molecular weight of the polyethene obtained 78 000
Degree of polymerization 2800
Macromolecules per Zr atom per hour 46 000
Rate of growth of one macromolecule 0.087 s
Turnover time 3.1 Â 10
À5
s
Figure 3 Reactions of zirconocenes with MAO.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
decreasing the ethene concentration. The molecular weight distribution can be decreased
up to 1.1 (living polymerization) by bis(phenoxy-imine)titanium complexes [161].
Molecular weights of 170 000 were obtained. The molecular weigh t is also lowered by
the addition of small amounts) (0.1 to 2 mol%) of hydrogen (e.g., without H
2
,
M
w
¼ 170 000; adding 0.5 mol% H
2
, M

w
¼ 42 000) [155].
7. Late Transition Metal Catalyst
Brookhart et al. [57,58] described square planar nickel and palladium-diimine systems
which are capable of polymerizing ethene to high molecular weight polymers with activ-
ities comparable to the metallocene catalyst systems when activated with methyl-
aluminoxane.
ð25Þð26Þ
Important for the polymerization activity is the substituent 1 which has to be a bulky
aryl group. The task of this substituent is to fill up the coordination spheres below and
above the square plane of the complex and thus enable the growing polymer chain to stay
coordinated to the metal center. This is one of the main differences to the well-known
SHOP catalysts invented by Keim et al. [164] and Ostoja-Starzewski and Witte [165] which
produces mainly ethene oligomers.
Figure 4 Mechanism of the polymerization of olefins by zirconocenes. Step 1: The cocatalyst
(MAO: methylalumoxane) converst the catalyst after complexation into the active species that has a
free coordination position for the monomer and stabilizes the latter. Step 2: The monomer (alkene) is
allocated to the complex. Step 3: Insertion of the alkene into the zirconium alkyl bond and provision
of a new free coordination position. Step 4: Repetition of Step 3 in a very short period of time (about
2000 propene molecules per catalyst molecule per second), thus rendering a polymer chain.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
ð27Þ
The use bis(ylid)nickel catalysts by reaction of nickel oxygen complexes and
phosphines [166].
For the one-component catalyst, it is possible to use solvents of various polarities.
Even in THF or acetone there is good activity. The best solvents are methylene chloride
or hexane. If the hydrogen next to the oxygen in the ylid is replaced by larger groups,
the activity increases and reaches at 10-bar ethene pressure and 100

C about 50 000 mol of

reacted ethene per mole of nickel [167].
A very interesting feature of this new catalyst generation is that chain isomerization
processes can take place during the polymerization cycles. This results in more or less
branched polymers with varying product properties depending on polymerization
conditions and catalyst type. The number of isomerization cycles which are carried out
directly one after another determines the nature of the branching formed. Branches
ranging from methyl to hexyl and longer can be formed.
The extent of branching can be tailored precisely by tuning the polymerization
conditions and products, from highly crystalline HDPE to completely amorphous poly-
mers with glass transition temperatures of about À50

C. These products are different to
all known conventi onally produced copolymers due to their content and distribution
pattern of short chain branching [168].
Another new catalyst generation based on iron and cobalt. The direct iron analogs
of the nickel-diimine catalysts derived from structures (25) and (26) did not seem to be
very active in olefin polymerization at all. The electronic and steric structure analysis
shows why: the nickel d
8
-system favors a square planar coordination sphere but the
iron d
6
-system favors a tetrahedral one. It is very likely that these tetrahedral coordination
sites are not available for olefin insertion, and hence no polymerization can take place.
The next logical step was the employment of another electron donating atom in
the ligand structure in order to obtain a trigonal-bipyramidal coordination sphere.
Gibson and Brookhart both succeeded with a catalyst system based on an iron–
bisiminopyridyl complex. The structures (28)–(30) illustrate the three types of catalysts
[169,170].
ð28Þ

Square planar
Copyright 2005 by Marcel Dekker. All Rights Reserved.
ð29Þ
Tetrahedral
ð30Þ
Trigonal-bipyramidal
The ethene polymerization activity of these new family of catalysts is comparable
with the one obtained with the most productive metallocenes under similar conditions
if activated with methylaluminoxane. Again, the nature of the aryl substituents R1 plays a
major role in controlling the molecular weight of the polymers.
In contrast to nickel-diimine catalysts no chain isomerization takes place and thus
only linear HDPE is formed.
In 1998, Grubbs [171,172] reported on a new type of neutral nickelII-complexes with
salicylaldimin ligands (structure (31)). With these catalysts low branched polyethylenes
were obtained with a narrow molecular weight distribution. The copolymerization of
ethene and norbornene is possible.
ð31Þ
C. Copolymers of Ethene
The properties of polyethene could be varied in a wide range by copolymerization of
ethene with other comonomers. Most commercial products contain at least small amounts
of other monomers. In general, adding comonomers to the polymerization reduces the
polyethenes crystallinity, thereby reducing the melting point, the freezing point, and in
many cases the tensile strength and modulus. At the same time, optical properties are
Copyright 2005 by Marcel Dekker. All Rights Reserved.
improved and polarity is increased. The architecture of the copolymer can be controlled
experimentally by the following factors: operating conditions, chemical composition and
physical state of used catalyst, physical state of the copolymer being formed, and structure
of the comonomers.
The practically most important copolymer is made from ethene and propene.
Titanium- and vanadium-based catalysts have been used to synthesize copolymers that

have a prevailingly random, block, or alternating structure. Only with Ziegler or single
site catalyst, longer-chain a-olefins can be used as comonomer (e.g., propene, 1-butene,
1-hexene, 1-octene). In contrast to this, by radical high-pressure polymerization it is also
possible to incorporate functional monomers (e.g., carbon monoxide, vinyl acetate). The
polymerization could be carried out in solution, slurry, or gas phase. It is generally accepted
[173] that the best way to compare monomer reactivities in a particular polymerization
reaction is by comparison of their reactivity ratios in copolymerization reactions.
The simplest kinetic scheme of binary copolymerization in the case of olefin insertion
reaction is
CatÀM
1
Àpolymer þ M
1
À!
k
11
CatÀM
1
ÀM
1
Àpolymer ð32Þ
CatÀM
1
Àpolymer þ M
2
À!
k
12
CatÀM
2

ÀM
1
Àpolymer ð33Þ
CatÀM
2
Àpolymer þ M
1
À!
k
21
CatÀM
1
ÀM
2
Àpolymer ð34Þ
CatÀM
2
Àpolymer þ M
2
À!
k
22
CatÀM
2
ÀM
2
Àpolymer ð35Þ
r
1
¼

k
11
k
12
r
2
¼
k
22
k
21
ð36Þ
where k
11
and k
22
are the homopolymerization propagation rates for monomers M
1
and M
2
and k
12
and k
21
are cross-polymerization rate constants. The definition of reactivity ratios is
d½M
1

d½M
2


¼
½M
1
r
1
½M
1
þ½M
2

½M
2
½M
1
þr
2
½M
2

ð37Þ
The product r
1
 r
2
usually ranges from zero to 1. When r
1
 r
2
¼ 1, the copoly-

merization is random. As r
1
 r
2
approaches zero, there is an increasing tendency toward
alternation.
1. Radical Copolymerization
At elevated temperatures, ethene can be copolymerized with a number of unsaturated
compounds by radical polymerization [174–180] (Table 7). The commercially most
important comonomers are vinyl acetate [181], acrylic acid, and methacrylic acid as well as
their esters. Next to these carbon monoxide is employed as a comonomer, as it promotes
the polymer’s degradability in the presence of light [182].
As a consequence of the diversified nature of the comonomers, a large number of
variants of copolymer composition can be realized, thus achieving a broad varia tion of
properties. The copolymerization can be carried out in the liquid monomer, in a solvent,
or in aqueous emulsion. When high molecular mass is desired, solvents with low chain
transfer constants (e.g., tert-butanol, benzene, 1,4-dioxane) are preferred. Solution
Copyright 2005 by Marcel Dekker. All Rights Reserved.
polymerization permits the use of low polymerization temperatures and pressures.
Poly(ethylene-co-vinyl acetate, for instance, is produced at 100

C and 14 to 40 MPa [183].
For the polymerization of ethene with vinyl acetate and vinyl chloride, emulsion
polymerization in water is particularly suitable. The polymerizates have gained some
importance as adhesives, binding materials for pigments, and coating materials [184,185].
2. Linear Low-Density Pol yethene (LLDPE)
In contrast to LDPE produced with the high-pressure process, the tensile strength in
LLDPE is much higher. Therefor e, there has been a considerab le boost in the production
of LLDPE [186]. All Ziegler catalysts listed earlier are suitable for the copolymerization of
ethene with other monomers. Monomers that decrease the melting point and crystallinity

of a polymer at low concentrations are of great interest. Portions of 2 to 5 mol% are
used. Longer-chained monomers such as 1-hexene are more effective at the same weight
concentration than smaller units such as propene. It results in a branched polyethene with
methyl branching (R) if propene is used, ethyl if butene is used, and so on.
ð38Þ
Important for the copolymerization are the different ractivities of the olefins. The
principal order of monomer reactivities is well known [187]; ethene > propene >1-butene >
linear a-olefins > branched a -olefins. Normally propene reacts 5 to 100 times slower than
ethene, and 1-butene 3 to 10 times slower than propene. Table 8 shows the reactivity ratios
for the copolymerization of ethene with other olefins. The data imply that the reactivity of
the polymerization center is not constant for a given transition metal compound but
depends on the structure of the innermost monomer unit of the growing polymer chain
and on the cocatalyst.
On a laboratory scale, single site catalysts based on metallocene/MAO are highly
useful for the copolymerization of ethene with other olefins. Propene, 1-butene, 1-pentene,
1-hexene, and 1-octene have been studied in their use as comonomers, forming linear low-
density polyethene (LLDPE) [188,189]. These copolymers have a great industrial potential
and show a higher growth rate than the homopolymer. Due to thee short branching from
Table 7 Copolymerization of ethene (M
1
) with various comonomers (M
2
).
Comonomer r
1
r
2
Pressure (MPa) Temp. (

C)

Propene 3.2 0.62 102–170 120–220
1-Butene 3.2 0.64 102–170 130–220
Isobutylene 2.1 0.49 102–170 130–220
Styrene 0.7 1 150–250 100–280
Vinyl acetate 1 1 110–190 200–240
Vinyl chloride 0.16 1.85 30 70
Acrylic acid 0.09 196–204 140–226
Acrylic acid methylester 0.12 13 82 150
Acrylnitrile 0.018 4 265 150
Methacrylic acid 0.1 204 160–200
Methacrylic acid methylester 0.2 17 82 150
Copyright 2005 by Marcel Dekker. All Rights Reserved.
the incorporated a-olefin, the copolymers show lower melting points, lower crystallinities,
and lower densities, making films formed from these materials more flexible and better
processible. Applications of the copolymers can be found in packaging, in shrink films
with a low steam permeation, in elastic films, which incorporate a high comonomer
concentration, in cable coatings in the medical field because of the low part of extractables,
and in foams, elastic fibers, ad hesives, etc. The main part of the comonomers is randomly
distributed over the polymer chain. The amount of extractables is much lower than in
polymers synthesized with Ziegler catalysts.
The copolymerization parameter r
1
, which says how much faster an ethene unit is
incorporated into the growing polymer chain than an a-olefin, if the last inserted monomer
was an ethene unit, lies between 1 and 60 depending on the kind of comonomer
and catalyst. The product r
1
Á r
2
is important for the distribution of the como nomer and

is close to one when using C
2
-symmetric catalysts [190] (Table 9).
Under the same conditions, syndiospecific (C
s
-symmetric) metallocenes are more
effective in inserting a-olefins into an ethene copolymer than isospecific working
(C
2
-symmetric) metallocenes or unbridged metallocenes. In this particular case,
hafnocenes are more efficient than zirconocenes, too.
An interesting effect is observed for the polymerization with ethylene(bisindenyl)-
zirconium dichloride and some other metallocenes. Although the activity of the homo-
polymerization of ethene is very high, it increases when copolymerizing with propene [191].
The copolymerization of ethene with other olefins is effected by the variation of
the Al/Zr ratio, temperature and catalyst concentration. These variations change the
molecular weight and the ethene content. Higher temperatures increase the ethene content
and lower the molecular weight.
Table 8 Reactivity ratios of ethene with various comonomers and heterogeneous TiCl
3
catalyst
by 70

C.
Comonomer Cocatalyst r
1
r
2
Ref.
Propene Al(C

6
H
13
)
3
15.7 0.11 174
Propene AlEt
3
9.0 0.10 174
1-Butene AlEt
3
60 0.025 178
4-Methyl-1-pentene AlEt
2
Cl 195 0.0025 177
Styrene AlEt
3
81 0.012 179
Table 9 Results of ethene reactivity ratio determinations with soluble catalysts
a
.
Metallocene Temp. (

C) a-Olefin r
1
r
2
r
1
Á r

2
Cp
2
ZrMe
2
20 Propene 31 0.005 0.25
[En(Ind)
2
]ZrCl
2
50 Propene 6.61 0.06 0.40
[En(Ind)
2
]ZrCl
2
25 Propene 1.3 0.20 0.26
Cp
2
ZrCl
2
40 Butene 55 0.017 0.93
Cp
2
ZrCl
2
60 Butene 65 0.013 0.85
Cp
2
ZrCl
2

80 Butene 85 0.010 0.85
[En(Ind)
2
]ZrCl
2
30 Butene 8.5 0.07 0.59
[En(Ind)
2
]ZrCl
2
50 Butene 23.6 0.03 0.71
Cp
2
ZrMe
2
60 Hexene 69 0.02 1.38
[Me
2
Si(Ind)
2
]ZrCl
2
60 Hexene 25 0.016 0.40
Copyright 2005 by Marcel Dekker. All Rights Reserved.
Studies of ethene copolymerization with 1-butene using the Cp
2
ZrCl
2
/MAO catalyst
indicated a decrease in the rate of polymerization with increasing comonomer concen-

tration.
3. Ethene-Propene Copolymers
The copolymers of ethene and propene, with a molar ratio of 1:0. 5 up to 1:2, are of great
industrial interest. These EP-polymers show elast ic properties and, together with 2–5 wt%
of dienes as third monomers, they are used as elastomers (EPDM). Since there are
no double bonds in the backbone of the polymer, it is less sensitive to oxidation reaction.
Ethylidenenorbornene, 1,4-hexadiene and dicyclopentadiene are used as dienes. In most
technical processes for the production of EP and EPDM rubber, soluble or highly disposed
vanadium components have been used in the past (Table 10) [192–195]. Similar elastomers
which are less coloured can be obtained with metallocene/MAO catalyst at a much
higher activity [196]. The regiospecificity of the metallocene catalysts towards propene
leads exclusively to the formation of head-to-tail enchainments. Ethylidenenorbornene
polymerizes via vinyl polymerization of the cyclic double bond and the tendency of
branching is low. The molecular weight distribution of about 2 is narrow [197].
At low temperatures the polymerization time to form one polymer chain is long
enough to consume one monomer and then to add another one. So, it becomes possible
to synthesize block copolymers if the polymerization, catalyzed especially by hafnocenes,
starts with propene and, after the propene is nearly consumed, continues with ethene.
High branching, which is caused by the incorporation of long chain olefins into the
growing polymer chain, is obtained with silyl bridged amidocyclopentadienyltitanium
compounds (structure (39)) [198–200].
ð39Þ
Table 10 Results of ethene reactivity ratio determinations with soluble catalysts
a
.
Catalyst Cocatalyst Temp. (

C) r
1
(M

l
) r
2
(M
2
) r
1
Á r
2
Ref.
VCl
4
AlEt
2
Cl 21 3.0 0.073 0.23 192
VCl
4
Al-i-Bu
2
Cl 20.0 0.023 0.46 193
VOCl
3
Al-i-Bu
2
Cl 30 16.8 0.052 0.87 192
V(acac)
3
Al-i-Bu
2
Cl 20 16.0 0.04 0.64 193

VOCl
2
(OEt) Al-i-Bu
2
Cl 30 16.8 0.055 0.93 194
VOCl
2
Al-i-Bu
2
Cl 30 18.9 0.069 1.06 194
VO(OBu)
3
Al-i-Bu
2
Cl 30 22.0 0.046 1.01 194
VO(OEt)
3
Al-i-Bu
2
Cl 30 15.0 0.070 1.04 194
VO(OEt)
3
AlEt
2
Cl 30 26.0 0.039 1.02 195
a
Monomer 1 ¼ ethene, monomer 2 ¼ propene.
Copyright 2005 by Marcel Dekker. All Rights Reserved.
These catalysts, in combination with MAO or borates, incorporate oligomers with
vinyl endgroups which are formed during polyme rization by b-hydrogen transfer resulting

in long chain abranched polyolefins. In contrast, structurally linear polymers are obtained
when catalysed by other metallocenes. Copolymers of ethylene with 1-octene are very
flexible materials as long as the comonomer content is less than 10%.
With higher 1-octene content they show that elastic properties polyolefin elastomers
(POE) are formed [201]. EPDM is a commercially important synthetic rubber. The dienes
as terpolymers are curable with sulfur. This rubber shows a higher growth rate than the
other synthetic rubbers [202]. The outstanding property of ethene-propene rubber is its
weather resistance since it has no double bonds in the backbone of the polymer chain and
thus is less sensitive to oxygen and ozone. Other excellent properties of this rubber are its
resistance to acids and alkal is, its electrical properties, and its low-temperature
performance [203].
EPDM rubber is used in the automotive industry for gaskets, wipers, bumpers,
and belts. In the tire industry, EPM and EPDM play a role as a blending component,
especially for sidewalls. Furthermore, EPDM is used for cable insulation and in
the housing industry, for roofing as well as for many other purposes, replacing special
rubbers [204].
For technical uses, the molecular weight (M
w
) is in the range 100 000 to 200 000.
EPDM rubber, synthesized with vanadium catalyst, show a molecular weight distribution
between 3 and 10, indicating that two and more active centers are present.
The properties of the copolymers depend to a great extent on several structural
features of the copolymer chains as the relative content of comonomer units, the way the
comonomer units are distributed in the chain, the molecular weight and molecular weight
distribution, and the relative content of normal head-to-tail addition or head-to-head/
tail-to-tail addition.
4. Ethene-Cycloolefin Copolymers
Metallocene/methylaluminoxane (MAO) catalysts can be used to polymerize and copoly-
merize strained cyclic olefins such as cyclobutene, cyclopentene, norbornene, DMON and
other sterically hindered olefins [205–210]. While polymerization of cyclic olefins by

Ziegler–Natta catalysts is accompanied by ring opening [10], homogeneous metallocene
[211], nickel [212,213], or pallad ium [214,215], catalysts achieve exclusive double bond
opening polymerization.
ð40Þð41Þð42Þ
Copolymerization of these cyclic olefins with ethylene or a-olefins cycloolefin
copolymers (COC) can be produced, representing a new class of thermoplastic amorphous
materials [217–220]. Early attempts to produce such copolymers were made using
heterogeneous TiCl
4
/VAlEt
2
Cl or vanadium catalysts, but first significant progress was
Copyright 2005 by Marcel Dekker. All Rights Reserved.