Handbook of Polymer Synthesis Second Edition Episode 1 ppt

The polymerization of ethene can be released by radical initiators at high pressures aswell as by organometallic coordination catalysts.. Coordination Catalysts Ethene polymerization by

By copolymerization of ethene and propene with higher n-olefins, cyclic olefins, orpolar monomers, product properties can be varied considerably, thus extending the field ofpossible applications For this reason terpolymers of the ethene/propene n-olefin type arethe polymers with the greatest potential Ethene can be polymerized radically or by means

of organometallic catalysts In the case of polyisobutylene a cationic polymerizationmechanism takes place All other olefins (propene, 1-butene, 4-methylpentene) are poly-merized with organometallic catalysts The existence of several types of polyethene as well

as blends of these polymers provides the designer with an unusual versatility in resinspecifications 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 increase

in the mass of polypropene used for the production of fibers and filaments is inive of theversatility of this polymer

Synthetic polyolefins were first 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 sufficient polymerization rate,

a pressure of more than 100 MPa 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 applied

to the production of domestic articles that would be used in contact with hot water

Important progress for a broader application was made when Hogan and Banks [4](Phillips Petroleum) and Ziegler et al [5] found that ethene can be polymerized by means

of activated transition metal 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 tetrafluoride (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 polyolefins 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 polyolefins 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 scientific papers and patents covering most aspects ofolefin polymerization, catalyst synthesis, and polymerization kinetics as well as thestructural, chemical, physical, and technological characteristics of stereoregular poly-olefins and olefin copolymers Since that first publication, more than 20 000 papersand patents have been published on subjects related to that field Several books andreviews giving detailed information on the subjects of these papers have been published[9–19]

The first generation of Ziegler–Natta 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 Ziegler–Natta 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 Ziegler–Natta catalysts are heterogeneous More recent developmentsshow that homogeneous catalyst systems based on metallocene-alumoxane and othersingle-site catalysts can also be applied to olefin polymerization [20–23] These systems areeasy to handle by laboratory standards, and show highest activities and an extended range

of polymer products

The mechanism of Ziegler–Natta catalysis is not known in detail A two-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 configurationestablished in the active complex Second, the activated monomer is inserted into themetal–carbon bond In this sequence the metal-organic polymerization resembles whatnature accomplishes with enzymes

Ziegler–Natta catalysts are highly sensitive, to oxygen, moisture, and a large number

of chemical compounds Therefore, very stringent requirements of reagent purity 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 effect on the polymerization and can therefore be used assolvents Secondary alkenes usually have a negative effect on polymerization rates, andalkynes, allenes (1,2-butadiene), and conjugated dienes are known to act as catalystpoisons, as they tend to form stable complexes

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

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 slightlyreduce the catalyst’s activity Yet it is commonly used as an important regulator to lowerthe molecular weights of the polyethene or polypropene produced

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 100 MPa, ethene itself acts as a solvent.Both low- and high-molecular-weight polymers up to 106g/mol can be synthesized byeither organometallic coordination or high pressure radical polymerization The structure

of the polyethene differs 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 350 MPa and the temperature ranges from 180 to 350
C [24–29].Solvent polymerization can be performed at substantial lower pressures and at tem-peratures below 100
C The high-pressure polymerization of ethene proceeds via a radicalchain mechanism In this case chain propagation is regulated by disproportionation orrecombination

ð1Þð2Þ

ð3Þ

The rate constants for chain propagation and chain termination at 130 and

180 MPa can be specified as follows [30]:

Mp¼5:93  103 L  mol1s1

Mt¼2  108 L  mol1s1Intermolecular and intramolecular chain transfer take place simultaneously Thisdetermines the structure of the polyethene Intermolecular chain transfer results in longflexible 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:

ð4Þ

ð5ÞIntramolecular chain transfer:

ð6Þ

ð7ÞRadically 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 simplified formulars (6) and (7), not all branches observed could beexplained [33,34] A high-pressure stainless steal autoclave (0.1 to 0.51 MPa) 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 [35–41]

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 isfinished, the reaction mixture is released in two steps Temperature increases are due to

a negative Joule–Thompson effect At 26 MPa, ethene separates from the 250
C hotpolymer melt After further decompression down to normal pressure, the residual ethene

is removed [42–46] 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]

Oxygen or peroxides are used as the initiators Initiation is very similar to that

in many other free-radical polymerizations at different temperatures according to theirhalf-live times (Table 1) The pressure dependence is low Ethene polymerization can also

be started by ion radiation [48–51] The desired molecular weight is best adjusted by 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 first 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

by a polymerizationtemperature (
C)

as C36H74 [55–57], 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 which

is filled 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 olefins [58,59] under high pressure High pressurechanges the properties of polyethene in a wide range and increases the productivity

of 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 first order reaction [60] Thus for various catalyst-activator systems, theapproximate polymerization times needed in a continuous reactor to ensure the best use

of catalyst between 150 to 300
C are between several seconds and a few minutes.Several studies have been conducted to obtain Ziegler–Natta catalysts with good 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, molecularweights up to 110 000 g/mol, and a polydispersity of 5 to 10, with heterogeneous catalysts,whereas the productivity is 3000 to 7000 kg PE/cat, molecular weight up to 70 000 g/mol,and the polydispersity 2

B Coordination Catalysts

Ethene polymerization by the use of catalysts based on transition metals gives a polymerexhibiting a greater density and crystallinity than the polymer obtained via radicalpolymerization Coordination catalysts for the polymerization of ethene can be of verydifferent 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

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 orcobalt 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, andthe solvent process [65–68]:

1 Slurry process For the slurry process hydrocarbons such as isobutane,hexane, n-alkane are used in which the polyethene is insoluble The polymer-ization temperature ranges from 70 to 90
C, with ethene pressure 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 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 [69–73] 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, polymerization

in the gas phase has the advantage that no diluent is used which simplifiesthe process [74–76] A fluidized bed that can be stirred is used with supportedcatalysts 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 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 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 7 MPa

poly-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 homopolyethenes

is higher but it can be lowered by copolymerization Polymers produced with fied 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 simplified chain propagation and chain terminationsteps [11]

(c) By b elimination forming hydride

ð11Þ

Termination via hydrogenation gives saturated polymer and metal hydride Thetermination of a growing molecule by an a-elimination step forms a polymer with anolefinic end group and a metal hydride In addition, an exchange reaction with etheneforming 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 amixture of TiCl4and 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 specific reduction process Thisreduction involves alkylation of TiCl4 with aluminum alkyl molecules followed by adealkylation reduction to a trivalent state:

Under drastic conditions, TiCl3can be reduced to TiCl2in 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)3could be used.These systems, called first-generation catalysts, are used for the classic process of olefinpolymerization 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 Purification of the diluent recovered and feedback

of the monomer after a purification 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 polyolefin 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 sensitive

to perturbation, it is cheaper, and energy consumption as well as environmental loadingare 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 TiCl3

compounds or supported catalysts MgCl2/TiCl4/Al(C2H5)3or CrO3(SiO2) (Phillips)

2 Unsupported Titanium Catalysts

There is a very large number of different combinations of aluminum alkyls and titaniumsalts to make high mileage catalysts for ethene polymerization, such as a-TiCl3þAlEt3,AlEt2Cl, Al(i-Bu)3, and Ti(III)alkanolate-chloride þ Al(i-hexyl)3 [85] TiCl3exists in fourcrystalline modifications, 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 modification a-TiCl3

can be synthesized by reduction of TiCl4with H2at elevated temperatures (500 to 800
C)

or with aluminum powder at lower temperatures (about 250
C); in this case the a-TiCl3

contains Al cations [87] More active are g- and d-TiCl3modifications They are formed

by 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 modemTiCl3catalyst 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 thevicinity of 500 L mol1s1[88]

3 Supported Catalysts

MgCl2/TiCl4catalysts 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 [90–94] can be used The polymerization activity goes up to

10 000 L mol1s1 At this high activity the catalyst can remain in the polyethene Forexample, the specific volume (BET) of the catalystis 60 m2/g [95] The high activity isaccomplished by increasing the ethene pressure The dependence is not linear as it wasfor first-generation catalysts, and the morphology is also different The polyethene has acobweb-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 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

Washing with water (HCl), wastewater treatment,

purification, and drying of diluent

It is known that in the case of these supported catalysts the higher activity is linked

to a higher concentration of active titanium In contrast to first-generation catalysts 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/AlEt3catalyst 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 (90 m2/g) It reacts with the TiCl4 underthe 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 nating a silica-alumina (87:13 composition [101–103] or a silica support with an aqueoussolution of CrO3) High surface supports with about 400 to 600 g/m2are used [104] Afterthe water is removed, the powdery catalyst is fluidized and activated by a stream of dry air

impreg-at temperimpreg-atures of 400 to 800
C to remove the bound water The impregnated catalystscontain 1 to 5 wt% chromium oxides When this catalyst is heated in the presence ofcarbon monoxide, a more active catalyst is obtained [105] The Phillips catalyst specificallycatalyzes the polymerization of ethene to high-density polyethene To obtain polyethene oflower 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 temperaturesbut becomes active as the temperature is increased from 196
C (the melting point forCrO3) to 400
C Interactions of chromium oxide with SiO2and Al2O3take 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 acceptedpopulation of silanol groups on silica after calcination The structures (15) and (16) areproposed:

ð15Þ

ð16Þ

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 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 is

no influence 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 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 polyolefins 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 10–100 times higher than that of the classicalZiegler–Natta systems

develop-Metallocenes, in combination with the conventional aluminum alkyl cocatalysts used

in 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 define thenature of the active centers, to explain the aging effects of Ziegler catalysts, to establishthe mechanism of interaction with olefins, and to obtain quantitative evidence of someelementary steps [9,112–115] It is necessary to differentiate between the soluble catalystsystem itself and the polymerization system Unfortunately, the well-defined bis(cyclo-pentadienyl)titanium system is soluble, but it becomes heterogeneous when polyethylene

is formed [116]

The polymerization of olefins, promoted by homogeneous Ziegler catalysts based

on biscyclopentadienyltitanium(IV) or analogous compounds and aluminum alkyls,

is accompanied by a series of other reactions that greatly complicate the kinetic pretation of the polymerization process:

inter-ð17Þ

leaving two reduced metal atoms Some of the aging processes occurring with geneous and heterogeneous Ziegler catalysts can be explained with the aid of these sidereactions.

homo-Table 4summarizes important homogeneous Ziegler catalysts The best known 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 research

sys-on this and other systems with various alkyl groups has been csys-onducted 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 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 developingtitanium(III) compound

The increase in length of the polymer chain occurs by insertion of the monomer in to

a metal–carbon bond of the active complex Dyachkovskii et al [139] and Eisch et al [140]were the first to believe, based on kinetic measurements and synthesis, that the insertiontakes place on a titanium cation An ion of the type (C5H5)2Tiþ-R, derived from

complexing and dissociation,

ðC5H5Þ2TiRCl  AlRCl2Ð ½ðC5H5Þ2TiRCl3þþ ½AlRCl3 ð23Þcould 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 thetitanium compound Fink and co-workers [141] showed by 13C-NMR spectroscopy with

13

C-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/Mn¼1.07 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, Mnof 10,500, (700), 3.0 CH3groups 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 such

as Cr(allyl)3, Zr(allyl)4, Zr(benzyl)4, Ti(benzyl)4, and Cr(cyclopentadienyl)2are supported

on Al2O3Or SiO2, the activity increases by a factor of more than 100 [124,143]

Apparently, soluble catalysts are obtained by reaction of Ti(OR)4with 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 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 AlEt3as well as by reaction of Cr(acac)3and VO(acac)2withAlEt2Cl in the presence of triethyl phosphite [121] With vanadium catalysts the activityreaches its maximum at Al/V ratio ¼ 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

(M) compound

Polymerizationtemperature (
C)

Normalizedactivity

Catalystyield

6 Aluminoxane as Cocatalysts

The use of metallocenes and alumoxane as cocatalyst results in extremely high 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]:

poly-ð24Þ

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 [151–153]

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 105s 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 somehalogene show a higher activity than systems that are totally free of halogen Of thecocatalysts, methylalumoxane is much more effective than the ethylaluminoxane orisobutylalumoxane

It is generally assumed that the function of MAO is firstly to undergo a fast ligandexchange reaction with the metallocene dichloride, thus rendering the metallocene methylTable 5 Ethene polymerizationawith metallocene/methylaluminoxane catalysts

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

Molecular weight(g/mol)

and dimethyl compounds (Figure 3).In the further step, either Clor 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(perfluorophenyl)borate anions [(C6F5)4B]

, have been successfully applied to theactivation of metallocenes [158–161]

Polyethenes synthesized by metallocene-alumoxane have a molecular weight tribution of Mw/Mn¼2, 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, orFigure 2 Some classes of metallocene catalysts used for olefin polymerization

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

Molecular weight of the polyethene obtained 78 000

Figure 3 Reactions of zirconocenes with MAO

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

Mw¼170 000; adding 0.5 mol% H2, Mw¼42 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

Important for the polymerization activity is the substituent 1 which has to be a bulkyaryl group The task of this substituent is to fill 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 differences 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) isallocated to the complex Step 3: Insertion of the alkene into the zirconium alkyl bond and provision

of a new free coordination position Step 4: Repetition of Step 3 in a very short period of time (about

2000 propene molecules per catalyst molecule per second), thus rendering a polymer chain

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 different 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 analogs

of the nickel-diimine catalysts derived from structures (25) and (26) did not seem to bevery active in olefin 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 olefin 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 iron–bisiminopyridyl complex The structures (28)–(30) illustrate the three types of catalysts[169,170]

ð28Þ

Square planar

Tetrahedral

ð30Þ

Trigonal-bipyramidalThe ethene polymerization activity of these new family of catalysts is comparablewith 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 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

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 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 thathave a prevailingly random, block, or alternating structure Only with Ziegler or singlesite 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 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 olefin insertionreaction is

where k11and k22are the homopolymerization propagation rates for monomers M1and M2

and k12and k21are cross-polymerization rate constants The definition of reactivity ratios isd½M1

copoly-1 Radical Copolymerization

At elevated temperatures, ethene can be copolymerized with a number of unsaturatedcompounds by radical polymerization [174–180] (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 polymer’s degradability in the presence of light [182]

As a consequence of the diversified 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

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, 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 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% areused Longer-chained monomers such as 1-hexene are more effective 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

ð38Þ

Important for the copolymerization are the different ractivities of the olefins Theprincipal 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 thanethene, and 1-butene 3 to 10 times slower than propene.Table 8shows the reactivity ratiosfor the copolymerization of ethene with other olefins 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 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 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)

the incorporated a-olefin, the copolymers show lower melting points, lower crystallinities,and lower densities, making films formed from these materials more flexible and betterprocessible Applications of the copolymers can be found in packaging, in shrink filmswith a low steam permeation, in elastic films, which incorporate a high comonomerconcentration, 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 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-olefin, if the last inserted monomerwas an ethene unit, lies between 1 and 60 depending on the kind of comonomerand catalyst The product r1r2is 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 moreeffective 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 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 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

ethylene(bisindenyl)-Table 8 Reactivity ratios of ethene with various comonomers and heterogeneous TiCl3catalyst

Table 9 Results of ethene reactivity ratio determinations with soluble catalystsa

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 2–5 wt%

of dienes as third monomers, they are used as elastomers (EPDM) Since there are

no double bonds in the backbone of the polymer, it is less sensitive to oxidation reaction.Ethylidenenorbornene, 1,4-hexadiene and dicyclopentadiene are used as dienes In mosttechnical processes for the production of EP and EPDM rubber, soluble or highly disposedvanadium components have been used in the past (Table 10)[192–195] Similar elastomerswhich are less coloured can be obtained with metallocene/MAO catalyst at a muchhigher activity [196] The regiospecificity 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 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 thegrowing polymer chain, is obtained with silyl bridged amidocyclopentadienyltitaniumcompounds (structure (39)) [198–200]

ð39Þ

Table 10 Results of ethene reactivity ratio determinations with soluble catalystsa

Catalyst Cocatalyst Temp (
C) r1(Ml) r2(M2) r1r2 Ref

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 obtainedwhen catalysed by other metallocenes Copolymers of ethylene with 1-octene are veryflexible 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 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 roofing 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 merize strained cyclic olefins such as cyclobutene, cyclopentene, norbornene, DMON andother sterically hindered olefins [205–210] While polymerization of cyclic olefins byZiegler–Natta catalysts is accompanied by ring opening [10], homogeneous metallocene[211], nickel [212,213], or palladium [214,215], catalysts achieve exclusive double bondopening polymerization

Copolymerization of these cyclic olefins with ethylene or a-olefins cycloolefincopolymers (COC) can be produced, representing a new class of thermoplastic amorphousmaterials [217–220] Early attempts to produce such copolymers were made usingheterogeneous TiCl4/VAlEt2Cl or vanadium catalysts, but first significant progress was

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 cycloolefinand its degree of incorporation into the polymer chain Statistical copolymers becomeamorphous at comonomer incorporations beyond 10–15 mol% cycloolefin.

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 50 mol% norbornene incorporation Their stabilityagainst hydrolysis and chemical degradation, in combination with their stiffness lets thembecome desirable materials for optical applications, e.g for compact disks, lenses, opticalfibers 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 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]

cyclo-Ethene-norbornene copolymers are most interesting for technical applications

as they can be made from easily available monomers and provide glass transitiontemperatures up to 200
C.Table l1presents 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 different incorporation ratios of the cyclic olefin into the copolymer, the glasstransition temperature can vary over a wide range which is basically independent of theapplied 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 equalcomonomer ratios, increased Tgvalues can be observed for the bulkier comonomer, forinstance 72
C for ethene-norbornene and 105
C for ethene-DMON at comonomer moleratio XCo¼0.30 each

The copolymerization parameters r1and r2were calculated from the rates of poration, determined by13C NMR spectroscopy, dependent on the reaction temperature.Table 12 shows the temperature dependence of the copolymerization parameters

incor-rland r2and of the influence of the catalyst systems Metallocene catalysts show low r1

values, which increases with the temperature and allows the easy incorporation of bulkycycloolefins into the growing polymer chain Surprisingly, the copolymerization parameter

r1¼1.8–3.1 for cyclopentene and norbornene is surprisingly low The r1value of 2 meansthat ethylene is inserted only twice as fast as norbornene

The product r1r2 shows whether statistical insertion (r1r2) or alternating one(r1r2¼0) has occurred The different catalysts produce copolymers with structures thatare between statistical and alternating

Due to different incorporation values of the cyclic olefin 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 50 mol% of norbornene yields a material with a glasstransition point of 145
C A Tgof 205
C can be reached by higher incorporation rates

The Tgvalues are raised with a bulkier cycloolefin, regarding the same incorporationrate of 30% (norbornene: Tg¼72
C; DMON: Tg¼105
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)]ZrCl2

leads 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 50 mol%

of styrene could be incorporated into the copolymer In dependence of the styrenecontent the copolymers show elastic to stiff properties The polymerization happens byboth 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 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

single-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/Zr ¼ 200, c(Zr) ¼ 5  106mol/l; p(E) ¼ 2.00 bar,

c(N) ¼ 0.05 mol/l

[kg/mol h]

Incorp of norbornene[weight %]

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

Late transition metal complexes are more efficient 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 [224–227]

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 polymerization

is conducted at temperatures between 100 and 80
C Chlorinated hydrocarbons arecommonly used as solvents

Under cationic conditions, migration of the C–C double bond is observed Like allother a-olefins, propene cannot be polymerized via an anionic route The same applies tofree-radical polymerization In polymerization with Ziegler–Natta catalysts, propene orlonger-chained a-olefins 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) withFigure 5 Glass transition temperatures of norbornene/ethene copolymers catalyzed with differentzirconocenes

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

Natta formulated three different structures [229]

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 metric carbon atoms is explicit, there are no similarly accurate rules for the description ofpolymers Tertiary carbon atoms in polyolefin 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 differences between the three forms ofpolypropene with identical molecular weight distribution and branching percentage areconsiderable (Table 13)

asym-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 stereospecific polymerization of propene still consists

of 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

a LB 1

¼ Lewis base 1, methylmethacrylate; LB 2

¼ Lewis base 2, diisoanyl ether.

The nature of the ligands and the valency of the transition metal atoms essentiallygovern activity, productivity, and stereospecifity Another strong influence 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, stereospecifity, 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 affords polypropene with very low stereospecifity (compare Table 14) Onecriterion for the determination of stereospecifity is the isotacticity index, which is defined

as the percentage of polymer that is insoluble in boiling heptane [10] Natta achieved

a substantial increase in stereoselectivity by using TiCl3instead of TiCl4[240]

The aluminum halogenide content of TiCl3 leads to the formation of defects inthe crystal lattice, thereby effecting an increase in activity At temperatures up to 100
C,b-TiCl3is formed, which upon tempering assumes the layered structure of
-TiCl3 Above

200
C, a-TiCl3is formed Today, TiCl3in combination with Al(C2H5)2Cl is still used as acatalyst for the polymerization of propene It is referred to as a first-generation catalyst.The use of Al(C2H5)3decreases the stereospecifity and Al(C2H5)Cl2drastically lowers thecatalytic activity [242] Table 15 gives the influence 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, first-generation catalysts produce increasingamounts of atactic polypropene (8 to 20%) next to the isotactic main product Bymodification 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/TiCl3catalysts polymerizing

(relative to X ¼ C2H5)

StereoregularityI.I (%)

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

The partial blocking of active sites leads to a decrease of catalytic activity Due to

a tremendous increase in surface area of the TiCl3the activity of the modified catalyst can

be increased by a factor of 2 to 5

1 Kinetic Aspects

To date, numerous papers dealing with the kinetics of propene polymerizations have beenpublished [246–263] Since in the course of the polymerization the TiCl3crystallites breakinto smaller pieces, thereby exposing new active centers, the kinetical investigation of thereaction is made more difficult For the majority of systems, however, it was found that thepolymerization rates are proportional to the concentrations of catalyst and monomer but

do not depend on the aluminum organic component as long as a threshold concentration

is maintained

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 0.3 mol/l were used,whereas these are normally 0.005 mol/l The authors introduced kinetic models of theLangmuir–Hinshelwood type with reversible adsorption of aluminum alkyl on thetransition metal halogenide surface

Other differences in behaviour between the investigated catalytic systems concern thedependence of the polymerization rate on time In the first minutes the activity increasesuntil it reaches a maximum value, which keeps constant for several hours [266,267]

2 Active Sites

To measure the activity of the catalyst, it is necessary to know something about theportion of titanium atoms that form active sites A lot of studies have been carried out toevaluate the concentration of active sites and their location [268] Such studies arefacilitated by the fact that the first polymer chains forming on active sites preserve only atrace of their origin as well as by the fact that the variation of their molecular weight withpolymerization time depends on the number of active sites On the other hand, a correctdetermination of the active sites is made complex by the nature of the phenomenaoccurring during the polymerization

There are two parameters linked to the concentration of active sites, the merization rate (propagation rate) and the growing time (average lifetime) of the polymerchains The various methods are summarized as follows:

poly-1 Variation of the molecular weight as a function of polymerization time [268–278](kinetic method) This could only be obtained at low temperatures and lowmonomer conversions

2 Determination of the number of labeled alkyl groups bound to the polymerchains (end groups) of polymers obtained with catalysts prepared in the presence

of14C-labeled aluminum alkyls [268,279]

3 Inhibition of the active sites with compounds such as methanol, iodine, orallenes [280] Since it is most unlikely for these compounds to react with activesites only, the method gives too high values.

4 Reaction between the transition metal–carbon bonds present in the tion system and a labeled quenching agent such as 131I2, tritiated alcohols orwater [276], deuterated methanol or water, and14CO or14CO2[281,282].Table 16 summarizes some results of the propagation rate kp and the number of activesites for various TiCl3catalysts It can be seen that 0.3–3.6% of the total titanium atomsform active sites

polymeriza-Polymerization does not seem to involve proper kinetic chain terminationphenomena, but polymer chain transfer processes, making the active site available toinitiate a new polymer chain These can be formulated as a chain transfer process withthe monomer M:

or as a transfer process with the organometallic compound:

There is evidence that many polymer chains are bound to aluminum at the end ofpolymerization [283] Very important is a spontaneous termination process by b-hydrideextraction:

When operating in the presence of H2as molecular weight regulator, saturated polymerchains are formed:

Table 16 Percentage of active sites (C*) and propagation constants (kp) for propene

polymerization with TiCl3catalysts

(
C)

kp(L mol1s1)

a MW, molecular weight variation method; K, kinetic method; I, inhibitor method; 14 C, ratoactive alkyl method;

T, tritiated quenching agent method; 14 CO, radioactive carbon monoxide method.

Dividing the chain propagation rate constant kpby the average transfer rate constant kt

gives the average molecular weight Mnof the polymer chain Mw/Mnwas assumed to be

in the range 5 to 10 [284]

Various models of catalytic centers and of monomeric unit addition mechanism have beenproposed to interpret the isospecific polymerization of a-olefins with Ziegler–Nattacatalytic systems [285–293] For the a-olefins the combination of x-ray diffraction and IRanalysis showed very early that the polymers obtained with the Ziegler catalytic system[294] are substantially linear polymers with head-to-tail enchainments The regioselectivity

of the amorphous product is slightly lower than that of the crystalline polymer a-Olefinpolymerization is shown to occur through a cis-insertion reaction by using deuteratedpropene The cis addition to the double bond was proven when Miyazawa and Ideguchi[295] and Natta et al [296] established that the polymer of cis-ld1-propene is erythro-diisotactic, whereas the polymer from trans-ld1-propene is threo-diisotactic The metalatom of the catalyst bearing the growing chain and the growing chain end are addedsimultaneously to the double bond of the incoming monomer:

ð55Þ

To synthesize isotactic polypropene, the catalytic center must sharply discriminatebetween the two prochiral faces of the a-olefin To do this, the catalytic system mustpossess one or more chirality centers

Considering the simplest model of a monometallic catalytic center (55), there tainly is a chiral carbon atom in the growing chain in a-position with respect to the metalatom; furthermore, the metal atom itself can be a center of chirality [297], which beingbound to a solid surface could maintain its absolute configuration during the insertionreaction Therefore, stereoselectivity is caused by the chirality of the catalytically activecenter and not by chiral atoms in the growing chain

cer-One model for the active center, proposed by Arlmann and Cossee [298,299], is based

on monometallic catalytic centers (56) with hexacoordinated transition metal:

ð56ÞThe Ti atoms close to the TiCl3surface have a vacant octahedral site, one chlorineligand singly bonded and four chlorine ligands bridge bonded with neighboring Ti atoms

By reaction with the aluminum alkyl, the singly bonded chlorine atom is substituted by

an alkyl group with the formation of a Ti–C bond The olefin is complexed on the vacant

site with the double bond parallel to an octahedral axis Two orientations are thereforepossible, giving rise to the stereospecifity After monomeric unit insertion, the Ti–R bondenters the vacant site and another olefin molecule is coordinated.

This model is modified by Pino [300,301], Corradini [302], Kissin [303], Keii [304],Terano [305], Cecchin [306] to other titanium complexes Bimetallic models between thetitanium compound and the cocatalyst were discussed by Sinn and Patat [137], Pino [301],and Zakharov [307] Others suggest that the growing polymer chain is bound to thetransition metal through a double bond (carbene complex) and that the insertion reactionoccurs through formation of a metal-cyclobutane intermediate [308,309]

4 Supported Catalysts

The traditional Ziegler–Natta catalyst, based on TiCl3 and aluminum alkyls (firstgeneration), is not active enough to do without the removal of catalyst residues from thepolymer This is why only a small part of the titanium present on the side surface ofTiCl3 crystallites is deemed to be active in propene polymerization Researchers haveendeavored to obtain better utilization of the titanium halogenide by trying to attach it tothe surface of proper supports Great industrial interest is evidenced by the numerouspatent applications following the initial Shell patent [310–319] Commonly used supportsare MgCl2,CoCl2, SiO2, Mg(OH)2, Mg(OH)Cl, MgR(Cl), MgO, MgCO3, SiO2, and SiO2/

Al2O3[320–333] The preferred halides are those having the same layered lattice structure

as d-TiCl3 The dimensions of MgCl2and COCl2(ionic radii of Mg2þand Co2þare 0.066and 0.072 nm) make them particularly suitable carriers for TiCl4 (ionic radius of Ti4þ,0.068 nm) These catalysts were demonstrated to substantially increase the activity inpropene polymerizations using AlEt3 or AlEt2Cl as cocatalysts (second generation).Furthermore, the use of electron donors, notably esters of carbocylic acids such asethylbenzoate, was demonstrated to increase stereoselectivity (third generation) [334–347].With TiCl4 supported on SiO2, the activity is low but the crystallinity of the resultingpolypropylene is high [348] The addition of NaCl, CaCl2, or BaCl2increases the activity

by a factor of up to 5

Soga studied propene polymerizations with catalytic systems based on Mg(OH)2,Mg(OH)Cl, or MgCl2/TiCl4/AlEt3[94,322] Unlike the TiCl4/AlEt3system, these catalystsexhibit an almost constant overall rate of polymerization (4.1 g PP/g Cat h atm) for at least

2 h Catalysts obtained by reaction of Ti benzyl and cyclopentadienyl derivates withMg(OH)Cl have been investigated as well as Grignard reagents together with TiCl4 atvarying ratios have been investigated [317]

The most important catalysts are obtained by supporting titanium halides onactivated MgCl2 By combination with the cocatalyst AlR3, a very high activity is given,although the stereospecifity is low (Table 17) [321] The discovery of catalysts sup-ported on activated MgCl2 and modified Lewis bases has solved the problems of lowstereospecifity

5 Role of Lewis Base Esters

The catalyst can be prepared on different routes such as ball milling, vibration milling, orchemical conversions [349] First, commercially available anhydrous MgCl2is ball milledwith ethyl bonzoate over 20 h to afford active MgCl2 By this process the dimensions of theagglomerated primary MgCl2crystallites (60  30 nm) are broken (3  2 nm) and stabilized

by ethyl benzoate The support develops a surface area of 50 to 300 m2/g [334] Second, theball milled support is mixed with TiCl4by further ball milling of the catalyst support in

the presence of TiCl4or by suspending the MgCl2/EB in hot undiluted TiCl4 The resultingsolid is washed to remove soluble titanium complexes The catalyst contains 1 to 5 wt% Tiand 5 to 20 wt% ethyl benzoate or diisobutylphthalate The Lewis base used in thisprocedure is called an internal Lewis base.

Therefore the function of the internal donor in MgCl2-supported catalysts istwofold One function is to stabilize small primary crystallites of magnesium chloride; theother is to control the amount and distribution of TiCl4in the final catalyst Activatedmagnesium chloride has a disordered structure comprising very small lamellae

An essential part of every Ziegler catalyst is the cocatalyst Supported MgCl2/TiCl4

or MgCl2/EB/TiCl4 are combined with AlEt3or AlEt3/EB/ to give high polymerizationactivities The donor used for this procedure is called an external Lewis base Carboxylicacid esters or aromatic silanes, preferably alkoxisilane or derivatives such as para-ethylanisate, are described as external Lewis bases [338,345] Silyl ethers RnSi(OR0)4nsuch as

Ph3SiOCH3, Ph2Si(OCH3)2, PhSi(OCH3)3, and (C2H5)Si(OCH3) have also been found to

be highly active promoters in stereospecific olefin polymerization [350,351]

Both internal and external Lewis bases react with aluminum alkyls forming a 1:1complex in the first step (57) [352,353] The second step is an alkylation reaction affording

a new alkoxyaluminum species (58)

Table 17 Polymerization of propene with supported MgCl2/TiCl4catalysts by 70 C