Handbook of Polymer Synthesis

HANDBOOK OF POLYMER SYNTHESIS

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Copyright 2005 by Marcel Dekker. All Rights Reserved.

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Marcel Dekker New York

Hans R. KricheldorfUniversitt HamburgHamburg, Germany

Oskar NuykenTechnical UniversityMnchen, Germany

Graham SwiftGS Polymer ConsultantsChapel Hill, North Carolina, U.S.A.

HANDBOOK OFPOLYMER SYNTHESISSecond Edition

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Preface

The purpose of the 1st edition of this handbook was to present a condensed butcomprehensive review of the methods used for syntheses and modifications of the mostimportant classes of polymers. The good acceptance of this handbook by the internationalscientific community has prompted the publisher to launch a second edition updating theliterature up to the year 2000 for the most widely studied groups of polymers. The editorshope that this 2nd edition will provide the chemists with an useful first hand informationon new preparative methods in the field of polymer science.


Contents

PrefaceList of Contributors

1. PolyolefinsWalter Kaminsky

2. Polystyrenes and Other Aromatic Poly(vinyl compound)sOskar Nuyken

3. Poly(vinyl ether)s, Poly(vinyl ester)s, and

Poly(vinyl halogenide)sOskar Nuyken, Harald Braun and James Crivello

4. Polymers of Acrylic Acid, Methacrylic Acid, Maleic Acid and

their DerivativesOskar Nuyken

5. Polymeric DienesWalter Kaminsky and B. Hinrichs

6. Metathesis Polymerization of CycloolefinsUlrich Frenzel, Bettina K. M. Muller and Oskar Nuyken

7. Aromatic PolyethersHans R. Kricheldorf


iiivii

1

151

241

333

427

v

73

381

8. PolyurethanesZoran S. Petrovic

9. PolyimidesJavier de Abajo and Jose G. de la Campa

10. Poly(vinyl aldehyde)s, Poly(vinyl ketone)s, andPhosphorus-Containing Vinyl PolymersOskar Nuyken

11. Metal-Containing MacromoleculesDieter Wohrle

12. Conducting PolymersHerbert Naarmann

13. Photoconductive PolymersP. Strohriegl and J. V. Grazulevicius

14. Polymers for Organic Light Emitting Devices/Diodes (OLEDs)O. Nuyken, E. Bacher, M. Rojahn, V. Wiederhirn,R. Weberskirch and K. Meerholz

15. Crosslinking and Polymer NetworksManfred L. Hallensleben

16. Biodegradable Polymers for Biomedical ApplicationsSamuel J. Huang

17. Controlled/Living Radical PolymerizationKrzysztof Matyjaszewski and James Spanswick


vi

503

541

737

779

943

Contents

603

659

811

841

881

895

Index

List of Contributors

E. Bacher, Technische Universitat Munchen, Garching, Germany

Harald Braun, Technische Universitat Munchen, Garching, Germany

James Crivello, Rensselaer Polytechnic Institute, Troy, New York

Javier de Abajo, Institute of Polymer Science and Technology, Madrid, Spain

Jose G. de la Campa, Institute of Polymer Science and Technology, Madrid, Spain

Ulrich Frenzel, Technische Universitat Munchen, Garching, Germany

J. V. Grazulevicius, Kaunas University of Technology, Kaunas, Lithuania

Manfred L. Hallensleben, Institut fur Makromolekulare Chemie, Universitat Hannover,

Hannover, Germany

B. Hinrichs, University of Hamburg, Hamburg, Germany

Samuel J. Huang, Institute of Materials Science, University of Connecticut, Storrs,

Connecticut

Walter Kaminsky, Institute of Technical and Macromolecular Chemistry, University of

Hamburg, Hamburg, Germany

Hans R. Kricheldorf, Institute of Technical and Macromolecular Chemistry, University of

Hamburg, Hamburg, Germany

Krzysztof Matyjaszewski, Center for Macromolecular Engineering, Carnegie Mellon

University, Pittsburgh, Pennsylvania

Bettina K. M. Muller, Technische Universitat Munchen, Garching, Germany

Herbert Naarmann, (emerit) BASF AG Ludwigshafen

Oskar Nuyken, Technische Universitat Munchen, Garching, Germany


Zoran S. Petrovic, Pittsburg State University, Kansas Polymer Research Center, Pittsburg,Kansas

M. Rojahn, Technische Universitat Munchen, Garching, Germany

James Spanswick, Center for Macromolecular Engineering, Carnegie Mellon University,Pittsburgh, Pennsylvania

P. Strohriegl, Universitat Bayreuth, Makromolekulare Chemie I, and BayreutherInstitut fur Makromolekulforschung (BIMF), Bayreuth, Germany

R. Weberskirch, Technische Universitat Munchen, Garching, Germany

V. Wiederhirn, Technische Universitat Munchen, Garching, Germany

Dieter Wohrle, University of Bremen, Bremen, Germany


1Polyolefins

Walter KaminskyUniversity of Hamburg, Hamburg, Germany

I. INTRODUCTION

The polyolens production has increased rapidly in the 40 years to make polyolens themajor tonnage plastics material worldwide. In 2003, 55 million tons of polyethene and 38million t/a polypropene were produced [1]. These products are used for packing material,receptacles, pipes, domestic articles, foils, and bers. Polyolens consist of carbon andhydrogen atoms only and the monomers are easily available. Considering environmentalaspects, clean disposal can be achieved by burning or by pyrolysis, for instance. Burninginvolves conversion to CO2 and H2O, exclusively.

By copolymerization of ethene and propene with higher n-olens, cyclic olens, orpolar monomers, product properties can be varied considerably, thus extending the eld ofpossible applications. For this reason terpolymers of the ethene/propene n-olen type arethe polymers with the greatest potential. Ethene can be polymerized radically or by meansof organometallic catalysts. In the case of polyisobutylene a cationic polymerizationmechanism takes place. All other olens (propene, 1-butene, 4-methylpentene) are poly-merized with organometallic catalysts. The existence of several types of polyethene as wellas blends of these polymers provides the designer with an unusual versatility in resinspecications. Thus polyethene technology has progressed from its dependence on onelow-density polymer to numerous linear polymers, copolymers, and blends that will extendthe use of polyethene to many previously unacceptable applications.

Polypropene also shows versatility and unusual growth potential. The mainadvantage is improved susceptibility to degradation by outdoor exposure. The increasein the mass of polypropene used for the production of bers and laments is inive of theversatility of this polymer.

Synthetic polyolens were rst synthetisized by decomposition of diazomethane [2].With the exception of polyisobutylene, these polymers were essentially laboratorycuriosities. They could not be produced economically. The situation changed with thediscovery of the high pressure process by Fawcett and Gibson (ICI) in 1930: ethenewas polymerized by radical compounds [3]. To achieve a sucient polymerization rate,a pressure of more than 100MPa is necessary. First produced in 1931, the low densitypolyethene (LDPE) was used as isolation material in cables.

Due to its low melting point of less than 100 C LDPE could not be appliedto the production of domestic articles that would be used in contact with hot water.


1

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 meansof activated transition metal catalyst 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 analumina-silica support. Zletz [6] used molybdenum oxide on alumina in 1951 (StandardOil); Fischer [7] used aluminum chloride along with titanium tetrauoride (BASF 1953)for the production of high-density polyethene. The latter catalyst has poor activityand was never used commercially. Zieglers [5] use of transition metal halogenides andaluminum organic compounds and the work of Natta [8] in applying this catalyst systemfor the synthesis of stereoregular polyolens were probably the two most importantachievements 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 largeproduction volume of such crystalline polyolens as HDPE, isotactic polypropene,ethane-propene rubbers, and isotactic poly(l-butene). For their works, Ziegler and Nattawere awarded the Nobel Prize in 1963. The initial research of Ziegler and Natta wasfollowed by an explosion of scientic papers and patents covering most aspects ofolen polymerization, catalyst synthesis, and polymerization kinetics as well as thestructural, chemical, physical, and technological characteristics of stereoregular poly-olens and olen copolymers. Since that rst publication, more than 20 000 papersand patents have been published on subjects related to that eld. Several books andreviews giving detailed information on the subjects of these papers have been published[919].

The rst generation of ZieglerNatta catalysts, based on TiCl3/AlEt2Cl, wascharacterized by low polymerization activity. Thus a large amount of catalyst was needed,which contaminated the raw polymer. A washing step that increased production costs wasnecessary. A second generation of ZieglerNatta catalysts followed, in which the transitionmetal compound is attached to a support (MgCl2, SiO2, Al2O3). These supported catalystsare of high activity. The product contains only traces of residues, which may remain in thepolymer. Most ZieglerNatta catalysts are heterogeneous. More recent developmentsshow that homogeneous catalyst systems based on metallocene-alumoxane and othersingle-site catalysts can also be applied to olen polymerization [2023]. These systems areeasy to handle by laboratory standards, and show highest activities and an extended rangeof polymer products.

The mechanism of ZieglerNatta catalysis is not known in detail. A two-stepmechanism is commonly accepted: First, the monomer is adsorbed (p-complex bonded) atthe transition metal. During this step the monomer may be activated by the congurationestablished in the active complex. Second, the activated monomer is inserted into themetalcarbon bond. In this sequence the metal-organic polymerization resembles whatnature accomplishes with enzymes.

ZieglerNatta catalysts are highly sensitive, to oxygen, moisture, and a large numberof chemical compounds. Therefore, very stringent requirements of reagent purity andutmost care in all manipulations of catalysts and polymerization reactions themselves aremandatory for achieving experimental reproducibility and reliability. Special care must betaken to ensure that solvents and monomers are extremely pure. Alkanes and aromaticcompounds have no substantial eect on the polymerization and can therefore be used assolvents. Secondary alkenes usually have a negative eect on polymerization rates, andalkynes, allenes (1,2-butadiene), and conjugated dienes are known to act as catalystpoisons, as they tend to form stable complexes.


2

Almost all polar substances exert a strong negative inuence on the polymerization.COS and hydrogen sulde, particularly, are considered to be strong catalyst poisons, ofwhich traces of more than 0.2 vol ppm aect a catalysts activity. Neither the solvent northe gaseous monomer should contain water, carbon dioxide, alcohols, or other polarsubstances in excess of 5 ppm. Purication may be carried out by means of molecularsieves.

The termination of the polymerization reaction by the addition of carbon monoxideis used to determine the active centers (sites) of the catalyst. Hydrogen is known to slightlyreduce the catalysts activity. Yet it is commonly used as an important regulator to lowerthe molecular weights of the polyethene or polypropene produced.

II. POLYETHENE

The polymerization of ethene can be released by radical initiators at high pressures aswell as by organometallic coordination catalysts. The polymerization can be carried outeither in solution or in bulk. For pressures above 100MPa, ethene itself acts as a solvent.Both low- and high-molecular-weight polymers up to 106 g/mol can be synthesized byeither organometallic coordination or high pressure radical polymerization. The structureof the polyethene diers with the two methods. Radical initiators give more-or-lessbranched polymer chains, whereas organometallic coordination catalysts synthesize linearmolecules.

A. Radical Polymerization

Since the polymerization of ethene develops excess heat, radical polymerization on alaboratory scale is best carried out in a discontinuous, stirred batch reactor. On a technicalscale, however, column reactors are widely used. The necessary pressure is generallykept around 180 to 350MPa 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 radicalchain mechanism. In this case chain propagation is regulated by disproportionation orrecombination.

1

2

3


3

The rate constants for chain propagation and chain termination at 130 and180MPa can be specied as follows [30]:

Mp 5:93 103Lmol1 s1Mt 2 108Lmol1 s1

Intermolecular and intramolecular chain transfer take place simultaneously. Thisdetermines the structure of the polyethene. Intermolecular chain transfer results in longexible side chains but is not as frequent as intramolecular chain transfer, from whichshort 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 branchesper 1000 C atoms. Of these, 10% are ethyl, 50% are butyl, and 40% are longer sidechains. With the simplied formulars (6) and (7), not all branches observed could beexplained [33,34]. A high-pressure stainless steal autoclave (0.1 to 0.51MPa) equippedwith an inlet and outlet valve, temperature conductor, stirrer, and bursting disk is used forthe synthesis. Best performance is obtained with an electrically heated autoclave [3541].

To prevent self-degeneration, the temperature should not exceed 350 C. Etheneand intitiator are introduced by a piston or membrane compressor. An in-built sapphirewindow makes it possible to observe the phase relation. After the polymerization isnished, the reaction mixture is released in two steps. Temperature increases are due toa negative JouleThompson eect. At 26MPa, ethene separates from the 250 C hotpolymer melt. After further decompression down to normal pressure, the residual etheneis removed [4246]. Reaction pressure and temperature are of great importance forthe molecular weight average, molecular weight distribution, and structure of thepolymer. Generally, one can say that with increasing reaction pressure the weight averageincreases, the distribution becomes narrower, and short- and long-chain branching bothdecrease [47].


4

Oxygen or peroxides are used as the initiators. Initiation is very similar to thatin many other free-radical polymerizations at dierent temperatures according to theirhalf-live times (Table 1). The pressure dependence is low. Ethene polymerization can alsobe started by ion radiation [4851]. The desired molecular weight is best adjusted by theuse of chain transfer reagents. In this case hydrocarbons, alcohols, aldehydes, ketones, andesters 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] wasthe rst to study the structure of polyethene by x-ray. At a time when there was stillconsiderable debate about the character of macromolecules, the demonstration thatwholly synthetic and crystalline polyethene has a simple close-packed structure in whichthe 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 1minby 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


5

as C36H74 [5557], strengthened the strictly logical view that macromolecules are amultiplication of smaller elements joined by covalent bonds. LDPE crystallizes in singlelamellae with a thickness of 5.0 to 5.5 nm and a distance between lamellae of 7.0 nm whichis lled by an amorphous phase. The crystallinity ranges from 58 to 62%.

Recently, transition metals and organometallics have gained great interest ascatalysts for the polymerization of olens [58,59] under high pressure. High pressurechanges the properties of polyethene in a wide range and increases the productivityof the catalysts. Catalyst activity at temperatures higher than 150 C is controlledprimarily by polymerization and deactivation. This fact can be expressed by the practicalnotion 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/lseems to be rst order reaction [60]. Thus for various catalyst-activator systems, theapproximate polymerization times needed in a continuous reactor to ensure the best useof 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 presenceof heterogeneous catalysts consisting of titanium supported on magnesium dichlorideor with homogeneous metallocene catalysts. With homogeneous catalysts, a pressure of150MPa (80 to 210 C) results in a productivity of 700 to 1800 kgPE/cat, molecularweights up to 110 000 g/mol, and a polydispersity of 5 to 10, with heterogeneous catalysts,whereas the productivity is 3000 to 7000 kgPE/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 polymerexhibiting a greater density and crystallinity than the polymer obtained via radicalpolymerization. Coordination catalysts for the polymerization of ethene can be of verydierent nature. They all contain a transition metal that is soluble or insoluble inhydrocarbons, supported by silica, alumina, or magnesium chloride [5,63]. In most casescocatalysts are used as activators. These are organometallic or hydride compounds ofgroup 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 connectionwith 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)

Molecularweight MFI

Distribution

165 235 1.6 0.919 1.3 20205 290 1.0 0.915 17.0 10

300 250 3.9 0.925 2.0 10

Source: Ref. 29.


6

2. Catalysts based on chromium compounds supported by silica or aluminawithout a coactivator (Phillips catalysts)

3. Homogeneous catalysts based on metallocenes in connection with aluminoxaneor other single site catalysts such as nickel ylid, nickel diimine, palladium, iron orcobalt complexes.

Currently, mainly Ziegler and Phillips catalysts as well as some metallocene catalysts[63] are generally used technically.

Three dierent processes are possible: the slurry process, the gas phase process, andthe 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 3MPa. The polymerization time is 1 to 3 h and the yield is95 to 98%. The polyethene produced is obtained in the form of fine particles inthe diluent and can be separated by filtration. The molecular weight can becontrolled by hydrogen; the molecular weight distribution is regulated byvariation of the catalyst design or by polymerization in several steps undervarying conditions [6973]. The best preparation takes place in stirred vessels orloop reactors.

In some processes the polymerization is carried out in a series of cascadereactors to allow the variation of hydrogen concentration through the operatingsteps in order to control the distribution of the molecular weights. The slurrycontains about 40% by weight polymer. In some processes the diluent isrecovered 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 simplifiesthe process [7476]. A fluidized bed that can be stirred is used with supportedcatalysts. The polymerization is carried out at 2 to 2.5MPa and 85 to 100 C.The ethene monomer circulates, thus removing the heat of polymerizationand fluidizing the bed. To keep the temperature at values below 100 C, gasconversion 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 anotherappropriate 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 7MPa.

In contrast to high-pressure polyethene with long-chain branches, the polyetheneproduced with coordination catalysts has a more or less linear structure (Figure 1) [79].A good characterization of high-molecular-weight-polyethenes gives the melt rheologicalbehaviour [80] (shear viscosity, shear compliance). The density of the homopolyethenesis higher but it can be lowered by copolymerization. Polymers produced with unmodi-ed Ziegler catalysts showed extremely high molecular weight and broad distribution[81]. In fact, there is no reason for any termination step, except for consecutivereaction. Equations (8) to (11) show simplied chain propagation and chain terminationsteps [11].


7

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.


8

(c) By b elimination forming hydride

11

Termination via hydrogenation gives saturated polymer and metal hydride. Thetermination of a growing molecule by an a-elimination step forms a polymer with anolenic end group and a metal hydride. In addition, an exchange reaction with etheneforming a polymer with an olenic end group and an ethyl metal is observed.

1. Titanium Chloride-Based Catalysts

The rst catalyst used by Ziegler et al. [5,82] for the polymerization of ethene was amixture of TiCl4 and Al(C2H5)3, each of which is soluble in hydrocarbons. In combinationthey form an olive-colored insoluble complex that is very unstable. Its behavior is verysensitive to a number of experimental parameters, such as Al/Ti ratio, temperature andtime of mixing of all components, and absolute and relative concentrations of reactants[83]. After complexation, TiCl4 is reduced by a very specic reduction process. Thisreduction involves alkylation of TiCl4 with aluminum alkyl molecules followed by adealkylation 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 actualTiCl3 product is a compound alloyed with small amounts of AlCl3 and probably somechemisorbed 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 rst-generation catalysts, are used for the classic process of olenpolymerization. In practice, however, the low activity made it necessary to deactivatethe catalyst after polymerization, remove the diluent, and then remove the residues ofcatalyst with HCl and alcohols. This treatment is followed by washing the polyethenewith water and drying it with steam. Purication of the diluent recovered and feedbackof the monomer after a purication step involved further complications. The costs ofthese steps reduced the advantage of the low-pressure polymerization process. Therefore,it was one of the main tasks of polyolen research to develop new catalysts (secondgeneration catalysts) that are more active, and can therefore remain in the polymerwithout any disadvantage to the properties (Table 3) [84]. The process is just as sensitiveto perturbation, it is cheaper, and energy consumption as well as environmental loadingare lower. It is also possible to return to the polymerization vessel diluent containinga 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).


9

2. Unsupported Titanium Catalysts

There is a very large number of dierent combinations of aluminum alkyls and titaniumsalts to make high mileage catalysts for ethene polymerization, such as a-TiCl3AlEt3,AlEt2Cl, Al(i-Bu)3, and Ti(III)alkanolate-chlorideAl(i-hexyl)3 [85]. TiCl3 exists in fourcrystalline modications, the a, b, g, and d forms [86]. The composition of these TiCl3scan be as simple as one Ti for as many as three Cl, or they can have a more complexstructure whereby a second metal is cocrystallized as an alloy in the TiCl3. The particularmethod of reduction determines both composition and crystalline modication. 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 modications. They are formedby heating the a-TiCl3 to 100 or 200 C. The preferred a-TiCl3 contains Al and issynthesized by reducing TiCl4 with about 1/3 part AlEt3 or 1 part AlEt2Cl. A modemTiCl3 catalyst has a density of 2.065 g/cm

3, a bulk density of 0.82, a specic surface area(BET) of 29m2/g, and a particle size of 10 to 100 mm. The polymerization activity is in thevicinity of 500Lmol1 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 to10 000Lmol1 s1. At this high activity the catalyst can remain in the polyethene. Forexample, the specic volume (BET) of the catalystis 60m2/g [95]. The high activity isaccomplished by increasing the ethene pressure. The dependence is not linear as it wasfor rst-generation catalysts, and the morphology is also dierent. The polyethene has acobweb-like structure, whereas rst generation catalysts produced a worm-like structure[90,91]. The cobweb structure is caused by the fact that polymerization begins at thesurface of the catalyst particle. The particle is held together by the polymer. Whilepolymerization is in progress, the particle grows rapidly and parts of it break. Cobwebstructures 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 preparationPolymerization PolymerizationLimited inuence to molecular weight and

weight distribution

Great variation of molecular weight and

weight distributionCatalyst deactivation with alcoholFiltration Filtration

Washing with water (HCl), wastewater treatment,purication, and drying of diluent

Feedback of diluent

Drying of PE Drying of PE

Finishing FinishingThermal degradation of molecular weight, blendingStabilization Stabilization

Source: Ref. 84.


10

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 rst-generation catalysts inwhich only 0.1 to 1% of all titanium atoms form active sites, in supported catalysts 20 to80% 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 clearwhether all of the Ti centers in the supported catalysts are isolated. The high activitysuggests the incorporation of small TiCl3 crystallites into the Mg(OH)Cl. Fink andKinkelin [100] prepared a high-activity catalyst by combination of MgH2 and TiCl4.The MgH2 has a much greater surface area (90m

2/g). It reacts with the TiCl4 underthe evolution of hydrogene. By 30 C and 2 bar ethene pressure, 110 kg of PE per gram ofTi 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 aqueoussolution of CrO3). High surface supports with about 400 to 600 g/m

2 are used [104]. Afterthe water is removed, the powdery catalyst is uidized and activated by a stream of dry airat temperatures of 400 to 800 C to remove the bound water. The impregnated catalystscontain 1 to 5wt% chromium oxides. When this catalyst is heated in the presence ofcarbon monoxide, a more active catalyst is obtained [105]. The Phillips catalyst specicallycatalyzes the polymerization of ethene to high-density polyethene. To obtain polyethene oflower crystallinity, copolymers with known amounts of an a-olen, usually several percentof 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 temperaturesbut becomes active as the temperature is increased from 196 C (the melting point forCrO3) to 400

C. Interactions of chromium oxide with SiO2 and Al2O3 take place.Hogan [103] calculated that for a silica support of 600m2/g and about 5% Cr(VI),

the average distance between adjacent Cr atoms is 10 A. This corresponds to the acceptedpopulation of silanol groups on silica after calcination. The structures (15) and (16) areproposed:

15

16


11

It has been calculated that between 0.1 and 0.4wt% of the total chromiumforms active centers [105]. A dicult question relates to the valences of chromium in theactive sites. Valences of II, III, IV, V, and VI have been established [106]. Because of thesmall number of total chromium atoms that are active centers, it has not been possible tounequivocally assign the active valence [107,108]. Krauss and Hums [109] concluded thatthe reduction of hexavalent chromium centers linked to support produced coordinatelyunsaturated Cr(II) surface compounds. A speciality of the Phillips catalyst is that there isno inuence of hydrogen to control the molecular weight of the polyethylene. Only byhigher activation temperatures can the molecular weight be lowered.

5. Homogeneous (Single Site) Catalysts

Among the great number of Ziegler catalysts, homogeneous systems have beenpreferentially studied in order to understand the elementary steps of the polymerizationwhich is simpler in soluble systems than in heterogeneous systems. The situationhas changed since in recent years homogeneous catalyst based on metallocene andaluminoxane [12,110], nickel and palladium diimin complexes [111], and iron and cobaltcompounds were discovered which are also very interesting for industrial and laboratorysynthesis. 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 theirchemical structure can be easily changed. These properties allow one to predict accuratelythe properties of the resulting polyolens by knowing the structure of the catalyst usedduring 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 usedin Ziegler systems, are indeed capable of polymerising ethene, but only at a very lowactivity. Only with the discovery and application of methylaluminoxane (MAO) it waspossible to enhance the activity, surprisingly, by a factor of 10 000 [113]. Therefore, MAOplays a crucial part in the catalysis with metallocenes.

Kinetic studies and the application of various methods have helped to dene thenature of the active centers, to explain the aging eects of Ziegler catalysts, to establishthe mechanism of interaction with olens, and to obtain quantitative evidence of someelementary steps [9,112115]. It is necessary to dierentiate between the soluble catalystsystem itself and the polymerization system. Unfortunately, the well-dened bis(cyclo-pentadienyl)titanium system is soluble, but it becomes heterogeneous when polyethyleneis formed [116].

The polymerization of olens, promoted by homogeneous Ziegler catalysts basedon 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


12

18

19

20

Concomitant with continued olen insertion into the metalcarbon bond of thetransition metal aluminum complex, alkyl exchange and hydrogen-transfer reactionsare observed. Whereas the normal reduction mechanism for transition metal organiccomplexes is initiated by release of olens with formation of a hydride followed by hydridetransfer to an alkyl group, a reverse reaction takes place in the case of some titaniumand zirconium acompounds. A dimetalloalkane is formed by the release of ethane. Insecond step, ethene is evolved from the dimetalloalkane:

TiIVCH2CH2TiIV ! CH2CH2 2TiIII 21leaving 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 sidereactions.

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 withtrialkylaluminum, alkylaluminum halides, or aluminoxanes. Breslow [126] discovered thatbis(cyclopentadienyl)titanium(IV) compounds, which are easily soluble in aromatichydrocarbons, could be used instead of titanium tetrachloride as the transition metalcompound together with aluminum alkyls for ethene polymerization. Subsequent researchon 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 ofpolymerization and side reactions, this soluble system is probably the one that is bestunderstood. It is found that the polymerization takes place primarily if the titanium existsas titanium(IV) [136,137]. According to Henrici-Olive and Olive [138], the speedof polymerization decreases with increasing intensity of ESR signals of the developingtitanium(III) compound.

The increase in length of the polymer chain occurs by insertion of the monomer in toa metalcarbon bond of the active complex. Dyachkovskii et al. [139] and Eisch et al. [140]were the rst to believe, based on kinetic measurements and synthesis, that the insertiontakes place on a titanium cation. An ion of the type (C5H5)2Ti

-R, derived from


13

complexing and dissociation,

C5H52TiRClAlRCl2 C5H52TiRCl AlRCl2 22C5H52TiRCl AlRCl2 C5H52TiRCl3 AlRCl3 23

could be the active species of polymerization. Sinn and Patat [137] drew attention to theelectron-decient character of those main-group alkyls that aord complexes with thetitanium compound. Fink and co-workers [141] showed by 13C-NMR spectroscopy with13C-enriched ethene at low temperatures (where no alkyl exchange was observed) thatin higher halogenated systems, insertion of the ethene takes place only into a titaniumcarbon 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 usingthe 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 160

C (80 C) yields poly-ethene with a density of 0.966 g/cm,Mn of 10,500, (700), 3.0 CH3 groups per 1000

C and 0.4vinyl groups. The benzene- and allyl-containing transition metals are working without anycocatalyst and therefore are alkyl free. If transition metal organometallic compounds suchas Cr(allyl)3, Zr(allyl)4, Zr(benzyl)4, Ti(benzyl)4, and Cr(cyclopentadienyl)2 are supportedon 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 ratiosranging between 10 and 50. Similar results are attained by replacing titanium alkoxideby 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 withAlEt2Cl in the presence of triethyl phosphite [121]. With vanadium catalysts the activityreaches its maximum at Al/V ratio 50. Under these conditions up to 67% vanadium is inthe bivalent oxidation state. Bivalent and trivalent compounds will be active.

Table 4 Homogeneous catalysts for ethene polymerization.

System Transition metal(M) compound

Polymerizationtemperature (C)

Normalizedactivity

Catalystyield

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 118Cp2TiMe2/MAO 1:10

5.5 102 20 35 000 >15 000 110Cp2TiMe2/MAO 1:100 20 200 >5 000 119Cp2ZrCl2/MAO 1:1000 70 400 000 >10 000 120VO(acac)2/Et2AlCl/activator 1:50 20 180 121Cp2VCl2/Me2AlCl 1:5 50 13 122

Zr(allyl)4 80 2.0Hf(allyl)4 160 0.6Cr(ally)3 80 0.3 123

Cr(acac)3/EtAlCl 1:300 20 150 121Ti(benzyl)4 20(80) 8 103 (0.2) 124,125Ti(benzyl)3Cl 20 0.4 124,125Ti(benzyl)4


14

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 laboratoryscale. 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 alternatelyand free valences are saturated by methyl substituents. It is gained by careful partialhydrolysis of trimethylaluminum and, according to investigations by Sinn [149] andBarron [150], it consists mainly of units of the basic structure [Al4O3Me6], which containsfour aluminum, three oxygen atoms and six methyl groups. As the aluminum atoms inthis structure are co-ordinatively unsaturated, the basic units (mostly four) join togetherforming clusters and cages. These have molecular weights from 1200 to 1600 and aresoluble in hydrocarbons.

If metallocenes, especially zirconocenes but also titanocenes, hafnocenes and othertransition metal compounds (Figure 2) are treated with MAO, then catalysts are acquiredthat 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 theinsertion 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 above50 C, the zirconium catalyst is more active than the hafnium or titanium system; the latteris decomposed by such temperatures. Transition metal compounds containing somehalogene show a higher activity than systems that are totally free of halogen. Of thecocatalysts, methylalumoxane is much more eective than the ethylaluminoxane orisobutylalumoxane.

It is generally assumed that the function of MAO is rstly to undergo a fast ligandexchange 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 106M. Metaliocene/MAO 250. Solventtoluene; bCp cyclopentadienyl; Ind indenyl; EnC2H4; Flu uorenyl.


15

and dimethyl compounds (Figure 3). In the further step, either Cl or CH3 is abstractedfrom the metallocene compound by al Al-center in MAO, thus forming a metallocenecation and a MAO anion [156,157]. The alkylated metallocene cation represents the activecenter (Figure 4). Meanwhile, other weakly coordinating cocatalysts, such astetra(peruorophenyl)borate anions [(C6F5)4B]

, have been successfully applied to theactivation 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 vinylgroups, and 0.02 trans vinyl group per 100 C atoms. The molecular weight can easily belowered by increasing the temperature, increasing the metallocene concentration, or

Figure 2 Some classes of metallocene catalysts used for olefin polymerization.


16

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

Activity (95 C), 8 bar 39.8 106 g PE/g Zr h[Zirconocene] 6.2 108mol/l[Alumoxane] (M 1200) 7.1 104mol/lMolecular weight of the polyethene obtained 78 000

Degree of polymerization 2800Macromolecules per Zr atom per hour 46 000Rate of growth of one macromolecule 0.087 s

Turnover time 3.1 105 s

Figure 3 Reactions of zirconocenes with MAO.


17

decreasing the ethene concentration. The molecular weight distribution can be decreasedup 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 bythe addition of small amounts) (0.1 to 2mol%) of hydrogen (e.g., without H2,Mw 170 000; adding 0.5mol% 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 bulkyaryl group. The task of this substituent is to ll up the coordination spheres below andabove the square plane of the complex and thus enable the growing polymer chain to staycoordinated to the metal center. This is one of the main dierences to the well-knownSHOP catalysts invented by Keim et al. [164] and Ostoja-Starzewski and Witte [165] whichproduces 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 afree 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 (about2000 propene molecules per catalyst molecule per second), thus rendering a polymer chain.


18

27

The use bis(ylid)nickel catalysts by reaction of nickel oxygen complexes andphosphines [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 000mol ofreacted ethene per mole of nickel [167].

A very interesting feature of this new catalyst generation is that chain isomerizationprocesses can take place during the polymerization cycles. This results in more or lessbranched polymers with varying product properties depending on polymerizationconditions and catalyst type. The number of isomerization cycles which are carried outdirectly one after another determines the nature of the branching formed. Branchesranging from methyl to hexyl and longer can be formed.

The extent of branching can be tailored precisely by tuning the polymerizationconditions and products, from highly crystalline HDPE to completely amorphous poly-mers with glass transition temperatures of about 50 C. These products are dierent toall known conventionally produced copolymers due to their content and distributionpattern of short chain branching [168].

Another new catalyst generation based on iron and cobalt. The direct iron analogsof the nickel-diimine catalysts derived from structures (25) and (26) did not seem to bevery active in olen polymerization at all. The electronic and steric structure analysisshows why: the nickel d8-system favors a square planar coordination sphere but theiron d6-system favors a tetrahedral one. It is very likely that these tetrahedral coordinationsites are not available for olen insertion, and hence no polymerization can take place.

The next logical step was the employment of another electron donating atom inthe ligand structure in order to obtain a trigonal-bipyramidal coordination sphere.Gibson and Brookhart both succeeded with a catalyst system based on an ironbisiminopyridyl complex. The structures (28)(30) illustrate the three types of catalysts[169,170].

28

Square planar


19

29

Tetrahedral

30

Trigonal-bipyramidal

The ethene polymerization activity of these new family of catalysts is comparablewith the one obtained with the most productive metallocenes under similar conditionsif 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 thusonly linear HDPE is formed.

In 1998, Grubbs [171,172] reported on a new type of neutral nickelII-complexes withsalicylaldimin ligands (structure (31)). With these catalysts low branched polyethyleneswere obtained with a narrow molecular weight distribution. The copolymerization ofethene and norbornene is possible.

31

C. Copolymers of Ethene

The properties of polyethene could be varied in a wide range by copolymerization ofethene with other comonomers. Most commercial products contain at least small amountsof other monomers. In general, adding comonomers to the polymerization reduces thepolyethenes crystallinity, thereby reducing the melting point, the freezing point, and inmany cases the tensile strength and modulus. At the same time, optical properties are


20

improved and polarity is increased. The architecture of the copolymer can be controlledexperimentally by the following factors: operating conditions, chemical composition andphysical state of used catalyst, physical state of the copolymer being formed, and structureof the comonomers.

The practically most important copolymer is made from ethene and propene.Titanium- and vanadium-based catalysts have been used to synthesize copolymers thathave a prevailingly random, block, or alternating structure. Only with Ziegler or singlesite catalyst, longer-chain a-olens 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 alsopossible to incorporate functional monomers (e.g., carbon monoxide, vinyl acetate). Thepolymerization 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 polymerizationreaction is by comparison of their reactivity ratios in copolymerization reactions.

The simplest kinetic scheme of binary copolymerization in the case of olen insertionreaction is

CatM1polymerM1 !k11 CatM1M1polymer 32




r1 k11k12

r2 k22k21

36

where k11 and k22 are the homopolymerization propagation rates for monomersM1 andM2and k12 and k21 are cross-polymerization rate constants. The denition of reactivity ratios is

dM1dM2

M1r1M1 M2M2M1 r2M2 37

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 towardalternation.

1. Radical Copolymerization

At elevated temperatures, ethene can be copolymerized with a number of unsaturatedcompounds by radical polymerization [174180] (Table 7). The commercially mostimportant comonomers are vinyl acetate [181], acrylic acid, and methacrylic acid as well astheir esters. Next to these carbon monoxide is employed as a comonomer, as it promotesthe polymers degradability in the presence of light [182].

As a consequence of the diversied nature of the comonomers, a large number ofvariants of copolymer composition can be realized, thus achieving a broad variation ofproperties. 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 chaintransfer constants (e.g., tert-butanol, benzene, 1,4-dioxane) are preferred. Solution


21

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 40MPa [183].

For the polymerization of ethene with vinyl acetate and vinyl chloride, emulsionpolymerization in water is particularly suitable. The polymerizates have gained someimportance 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 inLLDPE is much higher. Therefore, there has been a considerable boost in the productionof LLDPE [186]. All Ziegler catalysts listed earlier are suitable for the copolymerization ofethene with other monomers. Monomers that decrease the melting point and crystallinityof a polymer at low concentrations are of great interest. Portions of 2 to 5mol% areused. Longer-chained monomers such as 1-hexene are more eective at the same weightconcentration than smaller units such as propene. It results in a branched polyethene withmethyl branching (R) if propene is used, ethyl if butene is used, and so on.

38

Important for the copolymerization are the dierent ractivities of the olens. Theprincipal order of monomer reactivities is well known [187]; ethene> propene>1-butene>linear a-olens > branched a-olens. Normally propene reacts 5 to 100 times slower thanethene, and 1-butene 3 to 10 times slower than propene. Table 8 shows the reactivity ratiosfor the copolymerization of ethene with other olens. The data imply that the reactivity ofthe polymerization center is not constant for a given transition metal compound butdepends on the structure of the innermost monomer unit of the growing polymer chainand on the cocatalyst.

On a laboratory scale, single site catalysts based on metallocene/MAO are highlyuseful for the copolymerization of ethene with other olens. 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 potentialand 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 1202201-Butene 3.2 0.64 102170 130220Isobutylene 2.1 0.49 102170 130220

Styrene 0.7 1 150250 100280Vinyl acetate 1 1 110190 200240Vinyl chloride 0.16 1.85 30 70

Acrylic acid 0.09 196204 140226Acrylic acid methylester 0.12 13 82 150Acrylnitrile 0.018 4 265 150

Methacrylic acid 0.1 204 160200Methacrylic acid methylester 0.2 17 82 150


22

the incorporated a-olen, the copolymers show lower melting points, lower crystallinities,and lower densities, making lms formed from these materials more exible and betterprocessible. Applications of the copolymers can be found in packaging, in shrink lmswith a low steam permeation, in elastic lms, which incorporate a high comonomerconcentration, in cable coatings in the medical eld because of the low part of extractables,and in foams, elastic bers, adhesives, etc. The main part of the comonomers is randomlydistributed over the polymer chain. The amount of extractables is much lower than inpolymers synthesized with Ziegler catalysts.

The copolymerization parameter r1, which says how much faster an ethene unit isincorporated into the growing polymer chain than an a-olen, if the last inserted monomerwas an ethene unit, lies between 1 and 60 depending on the kind of comonomerand catalyst. The product r1 r2 is important for the distribution of the comonomer andis close to one when using C2-symmetric catalysts [190] (Table 9).

Under the same conditions, syndiospecic (Cs-symmetric) metallocenes are moreeective in inserting a-olens into an ethene copolymer than isospecic working(C2-symmetric) metallocenes or unbridged metallocenes. In this particular case,hafnocenes are more ecient than zirconocenes, too.

An interesting eect 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 olens is eected by the variation ofthe Al/Zr ratio, temperature and catalyst concentration. These variations change themolecular weight and the ethene content. Higher temperatures increase the ethene contentand 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 174Propene AlEt3 9.0 0.10 174

1-Butene AlEt3 60 0.025 1784-Methyl-1-pentene AlEt2Cl 195 0.0025 177Styrene AlEt3 81 0.012 179

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

Metallocene Temp. (C) a-Olen r1 r2 r1 r2Cp2ZrMe2 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 0.26Cp2ZrCl2 40 Butene 55 0.017 0.93

Cp2ZrCl2 60 Butene 65 0.013 0.85Cp2ZrCl2 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 0.71Cp2ZrMe2 60 Hexene 69 0.02 1.38[Me2Si(Ind)2]ZrCl2 60 Hexene 25 0.016 0.40


23

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 greatindustrial interest. These EP-polymers show elastic properties and, together with 25wt%of dienes as third monomers, they are used as elastomers (EPDM). Since there areno 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 mosttechnical processes for the production of EP and EPDM rubber, soluble or highly disposedvanadium components have been used in the past (Table 10) [192195]. Similar elastomerswhich are less coloured can be obtained with metallocene/MAO catalyst at a muchhigher activity [196]. The regiospecicity of the metallocene catalysts towards propeneleads exclusively to the formation of head-to-tail enchainments. Ethylidenenorbornenepolymerizes via vinyl polymerization of the cyclic double bond and the tendency ofbranching is low. The molecular weight distribution of about 2 is narrow [197].

At low temperatures the polymerization time to form one polymer chain is longenough to consume one monomer and then to add another one. So, it becomes possibleto 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 olens into thegrowing polymer chain, is obtained with silyl bridged amidocyclopentadienyltitaniumcompounds (structure (39)) [198200].

39

Table 10 Results of ethene reactivity ratio determinations with soluble catalystsa.

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

VCl4 AlEt2Cl 21 3.0 0.073 0.23 192VCl4 Al-i-Bu2Cl 20.0 0.023 0.46 193

VOCl3 Al-i-Bu2Cl 30 16.8 0.052 0.87 192V(acac)3 Al-i-Bu2Cl 20 16.0 0.04 0.64 193VOCl2(OEt) Al-i-Bu2Cl 30 16.8 0.055 0.93 194

VOCl2 Al-i-Bu2Cl 30 18.9 0.069 1.06 194VO(OBu)3 Al-i-Bu2Cl 30 22.0 0.046 1.01 194VO(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.


24

These catalysts, in combination with MAO or borates, incorporate oligomers withvinyl endgroups which are formed during polymerization by b-hydrogen transfer resultingin long chain abranched polyolens. In contrast, structurally linear polymers are obtainedwhen catalysed by other metallocenes. Copolymers of ethylene with 1-octene are veryexible materials as long as the comonomer content is less than 10%.

With higher 1-octene content they show that elastic properties polyolen elastomers(POE) are formed [201]. EPDM is a commercially important synthetic rubber. The dienesas terpolymers are curable with sulfur. This rubber shows a higher growth rate than theother synthetic rubbers [202]. The outstanding property of ethene-propene rubber is itsweather resistance since it has no double bonds in the backbone of the polymer chain andthus is less sensitive to oxygen and ozone. Other excellent properties of this rubber are itsresistance to acids and alkalis, its electrical properties, and its low-temperatureperformance [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 inthe housing industry, for roong as well as for many other purposes, replacing specialrubbers [204].

For technical uses, the molecular weight (Mw) is in the range 100 000 to 200 000.EPDM rubber, synthesized with vanadium catalyst, show a molecular weight distributionbetween 3 and 10, indicating that two and more active centers are present.

The properties of the copolymers depend to a great extent on several structuralfeatures of the copolymer chains as the relative content of comonomer units, the way thecomonomer units are distributed in the chain, the molecular weight and molecular weightdistribution, 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 olens such as cyclobutene, cyclopentene, norbornene, DMON andother sterically hindered olens [205210]. While polymerization of cyclic olens byZieglerNatta catalysts is accompanied by ring opening [10], homogeneous metallocene[211], nickel [212,213], or palladium [214,215], catalysts achieve exclusive double bondopening polymerization.

40 41 42

Copolymerization of these cyclic olens with ethylene or a-olens cycloolencopolymers (COC) can be produced, representing a new class of thermoplastic amorphousmaterials [217220]. Early attempts to produce such copolymers were made usingheterogeneous TiCl4/VAlEt2Cl or vanadium catalysts, but rst signicant progress was


25

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 comonomerdistribution may be varied over a wide range by selection of the appropriate cycloolenand its degree of incorporation into the polymer chain. Statistical copolymers becomeamorphous at comonomer incorporations beyond 1015mol% cycloolen.

COCs are characterized by excellent transparency and very high, long-life servicetemperatures. They are soluble, chemically resistant and can be melt-processed. Due totheir high carbon/hydrogen ratio, these polymers feature a high refractive index, e.g. 1.53for ethene-norbornene copolymer at 50mol% norbornene incorporation. Their stabilityagainst hydrolysis and chemical degradation, in combination with their stiness lets thembecome desirable materials for optical applications, e.g. for compact disks, lenses, opticalbers and lms. The rst commercial COC plant run by Ticona GmbH with a capacityof 30 000 tons a year commerced production in September 2000 and is located inOberhausen, Germany.

The rst metallocene-based COC material was synthesized from ethene and cyclo-pentene [218]. While homopolymerization of cyclopentene results in 1,3-enchainment ofthe monomer units [219], isolated cyclopentene units are incorporated into the ethene-cyclopentene copolymer chain by 1,2-insertion. Ethylene is able to compensate the sterichindrance at the a-carbon of the growing chain after and before the insertion ofcyclopentene [220].

Ethene-norbornene copolymers are most interesting for technical applicationsas they can be made from easily available monomers and provide glass transitiontemperatures up to 200 C. Table l1 presents the activities and comonomer ratios for theseveral applied catalysts of C2- and Cs-symmetry. Cs-symmetric zirconocenes are moreactive in the copolymerization than for the homopolymerization of ethene. Under thechosen conditions, [En(Ind)2]ZrCl2 develops the highest activity while the highestcomonomer incorporation is achieved by [Ph2C(Ind)(Cp)]ZrCl2.

Due to dierent incorporation ratios of the cyclic olen into the copolymer, the glasstransition temperature can vary over a wide range which is basically independent of theapplied catalyst. A copolymer containing 50mol% of norbornene yields a material witha glass transition point of 145 C. Considering COCs of dierent comonomers with equalcomonomer ratios, increased Tg values can be observed for the bulkier comonomer, forinstance 72 C for ethene-norbornene and 105 C for ethene-DMON at comonomer moleratio 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 parametersrl and r2 and of the inuence of the catalyst systems. Metallocene catalysts show low r1values, which increases with the temperature and allows the easy incorporation of bulkycycloolens into the growing polymer chain. Surprisingly, the copolymerization parameterr1 1.83.1 for cyclopentene and norbornene is surprisingly low. The r1 value of 2 meansthat 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 dierent catalysts produce copolymers with structures thatare between statistical and alternating.

Due to dierent incorporation values of the cyclic olen in the copolymer, the glasstransition temperature can vary over a wide range that is independent of most of the usedcatalysts (Figure 5). A copolymer with 50mol% of norbornene yields a material with a glasstransition point of 145 C. A Tg of 205 C can be reached by higher incorporation rates.


26

The Tg values are raised with a bulkier cycloolen, regarding the same incorporationrate of 30% (norbornene: Tg 72 C; DMON: Tg 105 C). The highest glass transitiontemperature 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 amelting 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 asmetallocenes and amido (see structure (33)) [222,223]. Amounts of more than 50mol%of styrene could be incorporated into the copolymer. In dependence of the styrenecontent the copolymers show elastic to sti properties. The polymerization happens byboth 1,2- and 2,1-insertion of the styrene unit; the regioselectivity is low.

While it is dicult 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 iscommercialized to use free radical polymerization by high ethene pressure.

Vinyl acetate and acrylate esters used as comonomers containing sucient stabilizerto prevent the homopolymerization. The eect of the copolymerization with polar

Table 11 Copolymerization of norbornene (N) and ethene (E) by different metallocene/MAOcatalysts at 30 C. Conditions: MAO/Zr 200, c(Zr) 5 106 mol/l; p(E) 2.00 bar,c(N) 0.05mol/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 withdifferent metallocene/MAO catalysts.

Cycloolen Catalyst Temp. in C r1 r2 r1 r2Cyclopentene [En(IndH4)2]ZrCl2 0 1.9

monomers is to reduce the crystallinity and to receive materials for blending. Acrylateesters such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate formexible copolymers. They provide enhanced adhesion, particularly in coextruded lms orlaminates.

Late transition metal complexes are more ecient in the copolymerization of etheneand polar monomers. Nickel or palladium complexes (see strtuctures (30)(32) arefunctional-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 propenewith concentrated sulfuric acid leads to oily or waxy amorphous polymers of lowmolecular weight [228]. Next to strong acids, catalysts such as complex Lewis acidsmay serve as initiators in the cationic polymerization of propene. The polymerizationis conducted at temperatures between 100 and 80 C. Chlorinated hydrocarbons arecommonly used as solvents.

Under cationic conditions, migration of the CC double bond is observed. Like allother a-olens, propene cannot be polymerized via an anionic route. The same applies tofree-radical polymerization. In polymerization with ZieglerNatta catalysts, propene orlonger-chained a-olens are inserted into the growing chain in a head-to-tail fashionwith 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 differentzirconocenes.


28

a tertiary carbon atom bearing a methyl or even larger alkyl group:

43

This construction principle is mandatory for the stereoregular structure of thepolypropene molecule. In addition, head to head

44

and tail-to-tail

45

arrangements occur. These links can be detected by IR and 13C NMR spectroscopy.Exclusive head-to-tail bonding is a mandatory but not a sucient condition for stereo-regularity. Another important detail is the sterical orientation of the pendant methylgroups with respect to the main CC axis of the polymer molecule.

Natta formulated three dierent structures [229]

Isotactic structure

46

Syndiotactic structure

47

Atactic structure

48

1. In the structure all pendant methyl groups are located on one side of the zigzagplane; these polymers are called isotactic


29

2. For polymers in which the position of the pendant methyl groups is alternatinglyabove and below the backbone plane, the term syndiotactic is used.

3. When the pendant methyl groups are randomly positioned above and below theplane, the polymer is said to be atactic.

While the structural description of low molecular weight compounds with asym-metric carbon atoms is explicit, there are no similarly accurate rules for the description ofpolymers. Tertiary carbon atoms in polyolen chains are not asymmetric in a generalchemical sense. Even with one of the substituents bearing a double bond at its end andthe other terminated by an ethyl group, they are very similar. Therefore, these carbonatoms are often called pseudo asymmetric. The dierences between the three forms ofpolypropene with identical molecular weight distribution and branching percentage areconsiderable (Table 13).

B. Isotactic Polypropene

In view of the stereochemistry, Natta managed to synthesize isotactic crystallinepolypropene with the combination catalysts that have previously been discovered byZiegler [230]. He thus achieved a breakthrough for a technical application of polypropene.The most widely used catalyst for the stereospecic polymerization of propene still consistsof titanium halogenides and alkylaluminum compounds. In addition to this catalyst, alarge number of other systems have been tested. Table 14 lists important heterogeneoussystems.

Table 13 Some characteristics of polypropene.

Characteristic Isotactic Syndiotactic Atactic

Melting point (C) 160171 130160 Crystallinity (%) 5565 5075 0Tensile strength (kP/cm2) 320350 0

Table 14 Heterogeneous catalysts for the propene polymerization.

Catalysta Activity(g PP/gTi h atm)

Part of isotacticPP (%)

Refs

TiCl4/Al(C2H5)3(1:3) 30 27 231a-TiCl3/Al(C2H5)2Cl 25 87 232a-TiCl3/AlCl3Al/(C2H5)2Cl 120 80 233b-TiCl3/Al(C2H5)3/H2 15 234TiCl3/LiAlH4 235TiCl3/LiAl2H7/NaF 70 90 236

b-TiCl3/Al(C2H5)2Cl/LB1 99 95 237

b-TiCl3/AlCl3/Al(C2H5)2Cl/LB2 520 98 238

TiCl3/TiCl3CH3 Low Low 239

Ti/I2 Low Low 240

aLB1Lewis base 1, methylmethacrylate; LB2Lewis base 2, diisoanyl ether.


30

The nature of the ligands and the valency of the transition metal atoms essentiallygovern activity, productivity, and stereospecity. Another strong inuence is exerted bythe nature of the cocatalyst. It consists of organometallic compounds of the main groups 1and 3 of the periodic table. For the propene polymerization, alkyls of lithium [64],beryllium [240], magnesium [240], zinc [241], aluminum [64], and gallium [10] have beenused. Aluminum alkyls have been proven to be particularly suitable. Nowadays theyare used exclusively as cocatalysts since they are superior to all other organometalliccompounds as far as activity, stereospecity, accessibility, and availability are concerned.Only lithium alanate is an exception to this. It possesses higher thermostability and istherefore preferred for solution polymerization at temperatures between 150 and 200 C.

Heterogeneous catalysts are suspended in the solvent. The Ziegler catalyst TiCl4/Al(C2H5)3 aords polypropene with very low stereospecity (compare Table 14). Onecriterion for the determination of stereospecity is the isotacticity index, which is denedas the percentage of polymer that is insoluble in boiling heptane [10]. Natta achieveda substantial increase in stereoselectivity by using TiCl3 instead of TiCl4 [240].

3TiCl4 Al! 3TiCl3 AlCl3 49

The aluminum halogenide content of TiCl3 leads to the formation of defects inthe crystal lattice, thereby eecting an increase in activity. At temperatures up to 100 C,b-TiCl3 is formed, which upon tempering assumes the layered structure of -TiCl3. Above200 C, a-TiCl3 is formed. Today, TiCl3 in combination with Al(C2H5)2Cl is still used as acatalyst for the polymerization of propene. It is referred to as a rst-generation catalyst.The use of Al(C2H5)3 decreases the stereospecity and Al(C2H5)Cl2 drastically lowers thecatalytic activity [242]. Table 15 gives the inuence of various ligands on stereoregularityfor the system TiCl3/Al(C2H5)2X [243].

The preferred metal alkyls possess ethyl and isobutyl ligands. Typical examplesare AlEt3, AliBu3, AlEt2Cl, and Al-(i-Bu)2Cl. The stereoregularity of the polypropenedecreases with increasing size of R in AlR3 [244]. Vanadium salts attracted much attentionbecause they led predominantly to statistical copolymers, as opposed to block copolymersproduced with titanium salts.

Depending on reaction temperatures, rst-generation catalysts produce increasingamounts of atactic polypropene (8 to 20%) next to the isotactic main product. Bymodication with electron donors (Lewis bases; see also Table 14) of the desiredcomplexation tendency, the atactic polymerization sites can be largely deactivated, thusraising the isotaxy index to 94 to 98% [245]. It is obvious that atactic polymerization

Table 15 Varying X in Al(C2H5)2X/TiCl3 catalysts polymerizing.

X Rate of polymerization

(relative to XC2H5)Stereoregularity

I.I. (%)

C2H5 100 85

F 30 83Cl 33 93Br 33 95

I 9 98

Source: Ref. 243.


31

centers have a greater tendency towards complexation than do isotactic ones. Catalyststhat are modied in this manner are also known as second-generation catalysts.

The partial blocking of active sites leads to a decrease of catalytic activity. Due toa tremendous increase in surface area of the TiCl3 the activity of the modied catalyst canbe increased by a factor of 2 to 5.

1. Kinetic Aspects

To date, numerous papers dealing with the kinetics of propene polymerizations have beenpublished [246263]. Since in the course of the polymerization the TiCl3 crystallites breakinto smaller pieces, thereby exposing new active centers, the kinetical investigation of thereaction is made more dicult. For the majority of systems, however, it was found that thepolymerization rates are proportional to the concentrations of catalyst and monomer butdo not depend on the aluminum organic component as long as a threshold concentrationis maintained.

r kp TiCl31 C3H61 AlC2H52Cl 50

This means that there is practically no dependence of the propene polymerizationrate on the Al(C2H5)3/TiCl3 ratio over a wide range. However, a dependence of thereaction rate on the metal component ratio was observed by Tait [264] and Zakharov et al.[265] in the presence of AlR3/VCl3 and Al(C2H5)3/AlCl3/TiCl3 systems. It must beremarked that extremely high aluminum alkyl concentrations of 0.3mol/l were used,whereas these are normally 0.005mol/l. The authors introduced kinetic models of theLangmuirHinshelwood type with reversible adsorption of aluminum alkyl on thetransition metal halogenide surface.

Other dierences in behaviour between the investigated catalytic systems concern thedependence of the polymerization rate on time. In the rst minutes the activity increasesuntil it reaches a maximum value, which

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