polyethylene - chemistry and production processes

7/28/2019 Polyethylene - Chemistry and Production Processes

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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 catalysts 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 catalysts 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 106 g/mol can be synthesized by

either organometallic coordination or high pressure radical 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 [2429].

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 disproportionation or

recombination.

1

2

3

&RS\ULJKWE\0DUFHO'HNNHU$OO5LJKWV5HVHUYHG


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The rate constants for chain propagation and chain termination at 130 and180 MPa can be specified as follows [30]:

Mp 5:93 103L mol1 s1

Mt 2 108L mol1 s1

Intermolecular and intramolecular chain transfer 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:

4

5

Intramolecular chain transfer:

6

7

Radically created 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 simplified 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 [3541].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 JouleThompson effect. At 26 MPa, ethene separates from the 250 C hot

polymer melt. After further decompression down to normal pressure, the residual ethene

is removed [4246]. 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].



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Oxygen or peroxides are used as the initiators. Initiation is very similar to thatin 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 [4851]. 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)

(H3C)3-COOC(CH3)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



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as C36H74 [5557], strengthened the strictly logical view that macromolecules are a

multiplication of smaller elements joined by covalent bonds. LDPE crystallizes in single

lamellae with a thickness of 5.0 to 5.5 nm and a distance between lamellae of 7.0 nm whichis 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 105mol/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 ZieglerNatta catalysts with good thermalstability. The major problem to be solved is the reduction of the transition metal

(e.g., TiCl3) by the cocatalyst, which may be aluminum dialkyl halide, alkylsiloxyalanes

[60], or aluminoxane [59].

Luft and colleagues [61,62] investigated high-pressure polymerization 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 CatalystsEthene 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, AlEt3, AlEt2Cl, Al(i-Bu)3, GaEt3, ZnEt2, 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/cm3)

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.



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2. Catalysts based on chromium compounds supported by silica or aluminawithout 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 [6568]:

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 varyingbetween 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 hydrogen; the molecular weight distribution is regulated by

variation of the catalyst design or by polymerization in several steps under

varying conditions [6973]. 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, polymerizationin the gas phase has the advantage that no diluent is used which simplifies

the process [7476]. 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 bevaried between 0.7 and 7 MPa.

In contrast to high-pressure polyethene with long-chain branches, the polyethene

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].



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Chain propagation:

8

Chain termination:

(a) By b elimination with H transfer to monomer

9

(b) By hydrogenation

10

Figure 1 Comparison of various polyethenes.



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(c) By b elimination forming hydride

11

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 TiCl4 and Al(C2H5)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, TiCl4 is reduced by a very specific reduction process. This

reduction involves alkylation of TiCl4 with aluminum alkyl molecules followed by a

dealkylation reduction to a trivalent state:

Complexation: TiCl4AlEt3 TiCl4 AlEt3 12

Alkylation: TiCl4:AlEt3 EtTiCl3 AlEt2Cl 13

Reduction: 2EtTiCl3 2TiCl3 Et2 14

Under drastic conditions, TiCl3 can be reduced to TiCl2 in a similar way. The actual

TiCl3 product is a compound alloyed with small amounts of AlCl3 and probably some

chemisorbed AlEt2Cl. The mechanistic process is very complex and not well understood.

Instead of Al(C2H5)3, also Al(C2H5)2Cl, Al2(C2H5)3Cl3, 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 ofcatalyst with HCl and alcohols. This treatment is followed by washing the polyethene

with water and drying it with steam. Purification of the diluent recovered 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 TiCl3compounds or supported catalysts MgCl2/TiCl4/Al(C2H5)3 or CrO3(SiO2) (Phillips).



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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-TiCl3 AlEt3,

AlEt2Cl, Al(i-Bu)3, and Ti(III)alkanolate-chloride Al(i-hexyl)3 [85]. TiCl3 exists in four

crystalline modifications, the a, b, g, and d forms [86]. The composition of these TiCl 3s

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 TiCl3. The particular

method of reduction determines both composition and crystalline modification. a-TiCl3can be synthesized by reduction of TiCl4 with H2 at elevated temperatures (500 to 800

C)

or with aluminum powder at lower temperatures (about 250 C); in this case the a-TiCl3contains Al cations [87]. More active are g- and d-TiCl3 modifications. They are formed

by heating the a-TiCl3 to 100 or 200C. The preferred a-TiCl3 contains Al and is

synthesized by reducing TiCl4 with about 1/3 part AlEt3 or 1 part AlEt2Cl. A modem

TiCl3 catalyst has a density of 2.065 g/cm3, a bulk density of 0.82, a specific surface area

(BET) of 29 m2/g, and a particle size of 10 to 100 mm. The polymerization activity is in the

vicinity of 500 L mol1 s1 [88].

3. Supported Catalysts

MgCl2/TiCl4 catalysts. Good progress in increasing the polymerization activity was madewith the discovery of the MgCl2/TiCl4-based catalysts [89]. Instead of MgCl2, Mg(OH)Cl,

MgRCl, or MgR2 [9094] can be used. The polymerization activity goes up to

10000Lmol1 s1. At this high activity the catalyst can remain in the polyethene. For

example, the specific volume (BET) of the catalystis 60 m2/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 produced 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. While

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, blendingStabilization Stabilization

Source: Ref. 84.



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It is known that in the case of these supported catalysts the higher activity is linkedto 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/TiCl4/

AlEt3 catalyst and related systems including MgSO4, MgOSiO2, and MgO. It is not clear

whether all of the Ti centers in the supported catalysts are isolated. The high activity

suggests the incorporation of small TiCl3 crystallites into the Mg(OH)Cl. Fink and

Kinkelin [100] prepared a high-activity catalyst by combination of MgH2 and TiCl4.

The MgH2 has a much greater surface area (90 m2/g). It reacts with the TiCl4 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 [101103] or a silica support with an aqueous

solution of CrO3). High surface supports with about 400 to 600 g/m2 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 temperature is increased from 196 C (the melting point for

CrO3) to 400C. Interactions of chromium oxide with SiO2 and Al2O3 take place.

Hogan [103] calculated that for a silica support of 600 m2/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:

15

16



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It has been calculated that between 0.1 and 0.4 wt% of the total chromiumforms 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 catalysts 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 and tacticity by careful selection of the appropriate reactor condi-

tions. In addition, their catalytic activity is 10100 times higher than that of the classicalZieglerNatta 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 [9,112115]. It is necessary to differentiate between the soluble catalyst

system itself and the polymerization system. Unfortunately, the well-defined bis(cyclo-

pentadienyl)titanium system is soluble, but it becomes heterogeneous when polyethyleneis 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:

17



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18

19

20

Concomitant with continued olefin insertion into the metalcarbon 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:

TiIVCH2CH2TiIV ! CH2CH2 2TiIII 21

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 metalcarbon 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 (C5H5)2Ti-R, derived from



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complexing and dissociation,

C5H52TiRCl AlRCl2 C5H52TiRCl AlRCl2 22

C5H52TiRCl AlRCl2 C5H52TiRCl3 AlRCl3

23

could be the active species of polymerization. Sinn and Patat [137] drew attention to theelectron-deficient character of those main-group alkyls that afford complexes with the

titanium compound. Fink and co-workers [141] showed by 13C-NMR spectroscopy with13C-enriched ethene at low temperatures (where no alkyl exchange was observed) that

in higher halogenated systems, insertion 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 MW/Mn 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 polymerization temperature.

Using ally4Zr(allylZrBr3) at a polymerization temperature of 160C (80 C) yields poly-

ethene with a density of 0.966 g/cm, Mn of 10,500, (700), 3.0 CH3 groups per 1000C and 0.4

vinyl groups. The benzene- and allyl-containing transition metals are working without anycocatalyst 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 Al2O3 Or SiO2, the activity increases by a factor of more than 100 [124,143].

Apparently, soluble catalysts are obtained by reaction of Ti(OR)4 with AlR3 [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(NR2)4 [145]. Soluble catalytic systems are also obtained by reaction of Ti(acac)3[146] and Cr(acac)3 [147] with AlEt3 as well as by reaction of Cr(acac)3 and VO(acac)2 with

AlEt2Cl in the presence of triethyl phosphite [121]. With vanadium catalysts the activity

reaches its maximum at Al/V ratio 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

Cp2TiCl2/AlMe2Cla 1:2.51:6 30 40200 117

Cp2TiCl2/AlMe2Cl/H2O 1:6:3 30 2000 117

Cp2TiCl2/AlEt2Cl 1:2 15 745 118

Cp2TiMe2/MAO 1:105.5 102 20 35 000 >15 0 00 110

Cp2TiMe2/MAO 1:100 20 200 >5 000 119

Cp2ZrCl2/MAO 1:1000 70 400 000 >10 0 00 120

VO(acac)2/Et2AlCl/activator 1:50 20 180 121

Cp2VCl2/Me2AlCl 1:5 50 13 122

Zr(allyl)4 80 2.0

Hf(allyl)4 160 0.6Cr(ally)3 80 0.3 123

Cr(acac)3/EtAlCl 1:300 20 150 121

Ti(benzyl)4 20(80) 8 103 (0.2) 124,125

Ti(benzyl)3Cl 20 0.4 124,125

Ti(benzyl)4



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6. Aluminoxane as CocatalystsThe 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]:

24

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] andBarron [150], it consists mainly of units of the basic structure [Al 4O3Me6], 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 [151153].

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 105 s only

(Table 6). A comparison with enzymes is not far-fetched.

As shown by Tait under these conditions every zirconium atom forms an activecomplex and produces about 20 000 polymer chains per hour. At temperatures above

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 systems that are totally free of halogen. Of the

cocatalysts, methylalumoxane is much more effective than the ethylaluminoxane or

isobutylalumoxane.

It is generally 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 polymerizationa with metallocene/methylaluminoxane catalysts.

Metalloceneb Structure Activity

[kg PE/(mol Zr.h.cmon]

Molecular weight

(g/mol)

Cp2ZrCl2 6 60 900 62 000

[Me2C(Ind)(Cp)]ZrCl2 8 3330 18 000

[En(IndH4)2]ZrCl2 9 22 200 1 000 000

[Em(Ind)2]ZrCl2 11 12 000 350 000

[En(Ind)2]HfCl2 12 2900 480 000

[Me2Si(Ind)2]ZrCl2 13 36 900 260 000

[Me2Si(2,4,7-Me3Ind)2]ZrCl2 15 111 900 250 000

[Me2C(Flu)(Cp)]ZrCl2 18 2000 500 000

aEthene pressure 2.5 bar. temp. 30 C. [metallocene] 6.25 106 M. Metaliocene/MAO 250. Solvent

toluene; bCp cyclopentadienyl; Ind indenyl; En C2H4; Flu fluorenyl.



14/26

and dimethyl compounds (Figure 3). In the further step, either Cl or CH3 is abstracted

from the metallocene compound by al Al-center 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 [(C6F5)4B], have been successfully applied to the

activation of metallocenes [158161].

Polyethenes synthesized by metallocene-alumoxane have a molecular weight dis-

tribution of Mw/Mn 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.



15/26

Table 6 Polymerization activity of bis(cyclopentadienyl)zirconium dichloride/methylalumoxane catalyst applied to ethene in 330 ml of toluene.

Activity (95 C), 8 bar 39.8 106 g PE/g Zr h

[Zirconocene] 6.2 108 mol/l

[Alumoxane] (M1200) 7.1 104 mol/l

Molecular weight of the polyethene obtained 78000

Degree of polymerization 2800

Macromolecules per Zr atom per hour 46 000

Rate of growth of one macromolecule 0.087 s

Turnover time 3.1 105 s

Figure 3 Reactions of zirconocenes with MAO.



16/26

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 weight is also lowered by

the addition of small amounts) (0.1 to 2 mol%) of hydrogen (e.g., without H2,

Mw 170 000; adding 0.5 mol% H2, Mw 42 000) [155].

7. Late Transition Metal Catalyst

Brookhart et al. [57,58] described square planar nickel and palladium-diimine systemswhich are capable of polymerizing ethene to high molecular weight polymers with activ-

ities comparable to the metallocene catalyst systems when activated with methyl-

aluminoxane.

25 26

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 provisionof 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.



17/26

27

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 chlorideor 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 50 C. These products are different to

all known conventionally 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 d8-system favors a square planar coordination sphere but the

iron d6-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].

28

Square planar



18/26

29

Tetrahedral

30

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 amajor 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.

31

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



19/26

improved and polarity is increased. The architecture of the copolymer can be controlledexperimentally 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

CatM1polymer M1 !k11

CatM1M1polymer 32


CatM2M1polymer 33


CatM1M2polymer 34


CatM2M2polymer 35

r1 k11

k12r2 k

22

k2136

where k11 and k22 are the homopolymerization propagation rates for monomers M1 and M2and k12 and k21 are cross-polymerization rate constants. The definition of reactivity ratios is

dM1

dM2

M1r1M1 M2

M2M1 r2M237

The product r1 r2 usually ranges from zero to 1. When r1 r2 1, the copoly-

merization is random. As r1 r2 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 [174180] (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 polymers 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 variation 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



20/26

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 Polyethene (LLDPE)

In contrast to LDPE produced with the high-pressure process, the tensile strength in

LLDPE is much higher. Therefore, there has been a considerable boost in the production

of LLDPE [186]. All Ziegler catalysts listed earlier are suitable for the copolymerization ofethene 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.

38

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 (M1) with various comonomers (M2).

Comonomer r1 r2 Pressure (MPa) Temp. (C)

Propene 3.2 0.62 102170 120220

1-Butene 3.2 0.64 102170 130220

Isobutylene 2.1 0.49 102170 130220

Styrene 0.7 1 150250 100280

Vinyl acetate 1 1 110190 200240

Vinyl chloride 0.16 1.85 30 70

Acrylic acid 0.09 196204 140226

Acrylic acid methylester 0.12 13 82 150

Acrylnitrile 0.018 4 265 150

Methacrylic acid 0.1 204 160200

Methacrylic acid methylester 0.2 17 82 150



21/26

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, adhesives, 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 r1, 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 r1 r2 is important for the distribution of the comonomer and

is close to one when using C2-symmetric catalysts [190] (Table 9).Under the same conditions, syndiospecific (Cs-symmetric) metallocenes are more

effective in inserting a-olefins into an ethene copolymer than isospecific working

(C2-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 TiCl3 catalystby 70 C.

Comonomer Cocatalyst r1 r2 Ref.

Propene Al(C6H13)3 15.7 0.11 174

Propene AlEt3 9.0 0.10 174

1-Butene AlEt3 60 0.025 178

4-Methyl-1-pentene AlEt2Cl 195 0.0025 177

Styrene AlEt3 81 0.012 179

Table 9 Results of ethene reactivity ratio determinations with soluble catalystsa.

Metallocene Temp. (

C) a-Olefin r1 r2 r1 r2

Cp2ZrMe2 20 Propene 31 0.005 0.25

[En(Ind)2]ZrCl2 50 Propene 6.61 0.06 0.40

[En(Ind)2]ZrCl2 25 Propene 1.3 0.20 0.26

Cp2ZrCl2 40 Butene 55 0.017 0.93

Cp2ZrCl2 60 Butene 65 0.013 0.85

Cp2ZrCl2 80 Butene 85 0.010 0.85

[En(Ind)2]ZrCl2 30 Butene 8.5 0.07 0.59

[En(Ind)2]ZrCl2 50 Butene 23.6 0.03 0.71

Cp2ZrMe2 60 Hexene 69 0.02 1.38

[Me2Si(Ind)2]ZrCl2 60 Hexene 25 0.016 0.40



22/26

Studies of ethene copolymerization with 1-butene using the Cp2ZrCl2/MAO catalystindicated 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 elastic properties and, together with 25 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) [192195]. 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)) [198200].

39

Table 10 Results of ethene reactivity ratio determinations with soluble catalysts

a

.Catalyst Cocatalyst Temp. (C) r1(Ml) r2(M2) r1 r2 Ref.

VCl4 AlEt2Cl 21 3.0 0.073 0.23 192

VCl4 Al-i-Bu2Cl 20.0 0.023 0.46 193

VOCl3 Al-i-Bu2Cl 30 16.8 0.052 0.87 192

V(acac)3 Al-i-Bu2Cl 20 16.0 0.04 0.64 193

VOCl2(OEt) Al-i-Bu2Cl 30 16.8 0.055 0.93 194

VOCl2 Al-i-Bu2Cl 30 18.9 0.069 1.06 194

VO(OBu)3 Al-i-Bu2Cl 30 22.0 0.046 1.01 194

VO(OEt)3 Al-i-Bu2Cl 30 15.0 0.070 1.04 194

VO(OEt)3 AlEt2Cl 30 26.0 0.039 1.02 195

aMonomer 1 ethene, monomer 2 propene.



23/26

These catalysts, in combination with MAO or borates, incorporate oligomers withvinyl endgroups which are formed during polymerization 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 alkalis, 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 (Mw) is in the range 100 000 to 200000.

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 [205210]. While polymerization of cyclic olefins by

ZieglerNatta catalysts is accompanied by ring opening [10], homogeneous metallocene

[211], nickel [212,213], or palladium [214,215], catalysts achieve exclusive double bond

opening polymerization.

40 41 42

Copolymerization of these cyclic olefins with ethylene or a-olefins cycloolefin

copolymers (COC) can be produced, representing a new class of thermoplastic amorphous

materials [217220]. Early attempts to produce such copolymers were made using

heterogeneous TiCl4/VAlEt2Cl or vanadium catalysts, but first significant progress was



24/26

made by utilizing metallocene catalysts for this purpose. They are about ten times moreactive than vanadium systems and by careful choice of the metallocene, the comonomer

distribution may be varied over a wide range by selection of the appropriate cycloolefin

and its degree of incorporation into the polymer chain. Statistical copolymers become

amorphous at comonomer incorporations beyond 1015 mol% cycloolefin.

COCs are characterized by excellent transparency and very high, long-life service

temperatures. They are soluble, chemically resistant and can be melt-processed. Due to

their high carbon/hydrogen ratio, these polymers feature a high refractive index, e.g. 1.53

for ethene-norbornene copolymer at 50 mol% norbornene incorporation. Their stability

against hydrolysis and chemical degradation, in combination with their stiffness lets them

become desirable materials for optical applications, e.g. for compact disks, lenses, optical

fibers and films. The first commercial COC plant run by Ticona GmbH with a capacity

of 30 000 tons a year commerced production in September 2000 and is located inOberhausen, Germany.

The first metallocene-based COC material was synthesized from ethene and cyclo-

pentene [218]. While homopolymerization of cyclopentene results in 1,3-enchainment of

the monomer units [219], isolated cyclopentene units are incorporated into the ethene-

cyclopentene copolymer chain by 1,2-insertion. Ethylene is able to compensate the steric

hindrance at the a-carbon of the growing chain after and before the insertion of

cyclopentene [220].

Ethene-norbornene copolymers are most interesting for technical applications

as they can be made from easily available monomers and provide glass transition

temperatures up to 200 C. Table l1 presents the activities and comonomer ratios for the

several applied catalysts of C2- and Cs-symmetry. Cs-symmetric zirconocenes are more

active in the copolymerization than for the homopolymerization of ethene. Under the

chosen conditions, [En(Ind)2]ZrCl2 develops the highest activity while the highestcomonomer incorporation is achieved by [Ph2C(Ind)(Cp)]ZrCl2.

Due to different incorporation ratios of the cyclic olefin into the copolymer, the glass

transition temperature can vary over a wide range which is basically independent of the

applied catalyst. A copolymer containing 50 mol% of norbornene yields a material with

a glass transition point of 145 C. Considering COCs of different comonomers with equal

comonomer ratios, increased Tg values can be observed for the bulkier comonomer, for

instance 72 C for ethene-norbornene and 105 C for ethene-DMON at comonomer mole

ratio XCo 0.30 each.

The copolymerization parameters r1 and r2 were calculated from the rates of incor-

poration, determined by 13C NMR spectroscopy, dependent on the reaction temperature.

Table 12 shows the temperature dependence of the copolymerization parameters

rl and r2 and of the influence of the catalyst systems. Metallocene catalysts show low r1values, which increases with the temperature and allows the easy incorporation of bulky

cycloolefins into the growing polymer chain. Surprisingly, the copolymerization parameter

r1 1.83.1 for cyclopentene and norbornene is surprisingly low. The r1 value of 2 means

that ethylene is inserted only twice as fast as norbornene.

The product r1 r2 shows whether statistical insertion (r1 r2) or alternating one

(r1 r2 0) has occurred. The different catalysts produce copolymers with structures that

are between statistical and alternating.

Due to different incorporation values of the cyclic olefin in the copolymer, the glass

transition temperature can vary over a wide range that is independent of most of the used

catalysts (Figure 5). A copolymer with 50 mol% of norbornene yields a material with a glass

transition point of 145 C. A Tg of 205C can be reached by higher incorporation rates.



25/26

The Tg values are raised with a bulkier cycloolefin, regarding the same incorporation

rate of 30% (norbornene: Tg 72C; DMON: Tg 105

C). The highest glass transition

temperature with 229 C was reached by a copolymer of ethene and 5-phenylnorbornene.

Copolymerization of ethene and norbornene with [Me2C(Flu)(tert-BuCp)]ZrCl2leads to a strong alternating structure [221]. This copolymer is crystalline and shows a

melting point of 295 C, good heat resistance and resistance against unpolar solvents.

5. Ethene-Copolymerization by Styrene or Polar Monomers

The copolymerization of ethene and styrene is possible by single site catalysts such as

metallocenes and amido (see structure (33)) [222,223]. Amounts of more than 50 mol%

of styrene could be incorporated into the copolymer. In dependence of the styrene

content the copolymers show elastic to stiff properties. The polymerization happens by

both 1,2- and 2,1-insertion of the styrene unit; the regioselectivity is low.

While it is difficult to copolymerize ethene and polar monomers by Ziegler- or single-

site catalysts because of the great reactivity of the active sites to polar groups, it is

commercialized to use free radical polymerization by high ethene pressure.

Vinyl acetate and acrylate esters used as comonomers containing sufficient stabilizer

to prevent the homopolymerization. The effect of the copolymerization with polar

Table 11 Copolymerization of norbornene (N) and ethene (E) by different metallocene/MAOcatalysts at 30 C. Conditions: MAO/Zr200, c(Zr) 5 106 mol/l; p(E)2.00bar,

c(N) 0.05 mol/l.

Catalyst t [min] Activity

[kg/mol h]

Incorp. of norbornene

[weight %]

Cp2ZrCl2 30 1200 21.4

[En(Ind)2]ZrCl2 10 9120 26.1

[Me2Si(Ind)2]ZrCl2 15 2320 28.4

[En(IndH4)2]ZrCl2 40 480 28.1

[Me2C(Flu)(Cp)]ZrCl2 10 7200 28.9

[Ph2C(Flu)(Cp)]ZrCl2 10 6000 27.3

[Ph2C(Ind)(Cp)]ZrCl2 15 2950 33.3

Table 12 Copolymerization parameters r1 and r2 of ethene/cycloolefin copolymerization with

different metallocene/MAO catalysts.

Cycloolefin Catalyst Temp. in C r1 r2 r1 r2

Cyclopentene [En(IndH4)2]ZrCl2 0 1.9


26/26

monomers is to reduce the crystallinity and to receive materials for blending. Acrylate

esters such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate form

flexible copolymers. They provide enhanced adhesion, particularly in coextruded films orlaminates.

Late transition metal complexes are more efficient in the copolymerization of ethene

and polar monomers. Nickel or palladium complexes (see strtuctures (30)(32) are

functional-group tolerant allowing the copolymerization of ethene and methyl methacry-

late or CO [224227].

III. POLYPROPENE

A. Homopolymerization

In contrast to the polymerization of ethene, only coordination catalysts are successful inpolymerizing propene to a crystalline polymer. The cationic polymerization of propene

with concentrated sulfuric acid leads to oily or waxy amorphous polymers of low

molecular weight [228]. Next to strong acids, catalysts such as complex Lewis acids

may serve as initiators in the cationic polymerization of propene. The polymerization

is conducted at temperatures between 100 and 80 C. Chlorinated hydrocarbons are

commonly used as solvents.

Under cationic conditions, migration of the CC double bond is observed. Like all

other a-olefins, propene cannot be polymerized via an anionic route. The same applies to

free-radical polymerization. In polymerization with ZieglerNatta catalysts, propene or

longer-chained a-olefins are inserted into the growing chain in a head-to-tail fashion

with high selectivity. Every CH2-group (head) is followed by a CH(R)-group (tail) with

Figure 5 Glass transition temperatures of norbornene/ethene copolymers catalyzed with different

zirconocenes.


polyethylene - chemistry and production processes

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