4.09Olefin Polymerizations with Group IV MetalCatalysts

L Resconi, Basell Polyolefins, Ferrara, Italy

J C Chadwick, Eindhoven University of Technology, Eindhoven, The Netherlands

L Cavallo, University of Salerno, Salerno, Italy

2007 Elsevier Ltd. All rights reserved.

4.09.1 Introduction 1006

4.09.2 Pre-catalysts by Chemical Type and Reaction Principles 1007

4.09.2.1 MC as Propagating Species/Activation 1008

4.09.2.2 Monomer Coordination and Insertion Reactions 1010

4.09.2.3 Concepts of Stereo-, Regio-, and Enantioselectivity 1015

4.09.2.3.1 Regio- and stereochemistry of monomer insertion 10154.09.2.3.2 Definition of stereoregular polymers 10164.09.2.3.3 Elements of chirality 10164.09.2.3.4 Mechanism of stereocontrol 10184.09.2.3.5 Symmetry rules for stereocontrol 1020

4.09.2.4 Mechanism of Regiocontrol and Stereochemistry of Regioirregular Insertions 1023

4.09.2.5 Chain-release and Isomerization Reactions 1023

4.09.2.6 Kinetics 1028

4.09.3 ZieglerNatta Polymerizations with Heterogeneous Catalysts 1031

4.09.3.1 Catalyst Structure and Characterization 1031

4.09.3.2 Polymer Particle Growth 1033

4.09.3.3 Mechanistic Studies of ZieglerNatta Catalysts 1034

4.09.3.3.1 Oxidation state 10344.09.3.3.2 Number of active centers 10354.09.3.3.3 Internal/external donor effects and the nature of the active species 10354.09.3.3.4 Effects of hydrogen 10374.09.3.3.5 Effects of temperature 1038

4.09.3.4 Polyolefins Accessible from ZieglerNatta Catalysts 1038

4.09.3.5 Polymerization of Acyclic Internal Olefins 1040

4.09.3.6 Major Industrial Processes 1040

4.09.4 Polymerizations with Metallocene Catalysts 1041

4.09.4.1 Ethylene Polymers 1041

4.09.4.1.1 Polyethylene 10414.09.4.1.2 Ethylene/-olefin co-polymers 10434.09.4.1.3 Ethylene/propylene co-polymers and ethylene/propylene/diene terpolymers 10454.09.4.1.4 Ethylene co-polymerization with ,0-disubstituted and internal olefins 10474.09.4.1.5 Ethylene co-polymers with cycloolefins 10474.09.4.1.6 Ethylene/styrene co-polymers 1049

4.09.4.2 Propylene Polymers 1051

4.09.4.2.1 Amorphous polypropylene 10524.09.4.2.2 Isotactic polypropylene 10564.09.4.2.3 Low isotacticity: from flexible to elastomeric isotactic polypropylene 10644.09.4.2.4 Syndiotactic crystalline and elastomeric polypropylene 10704.09.4.2.5 Semicrystalline propylene/ethylene co-polymers 10734.09.4.2.6 Propylene/butene co-polymers 1075

1005

4.09.4.2.7 Propylene/higher -olefin co-polymers 10764.09.4.2.8 Propylene co-polymerization with macromonomers 1077

4.09.4.3 Polybutene 1078

4.09.4.4 Poly(-olefins) from Monomers Higher than Butene 1080

4.09.4.5 Polystyrene 1081

4.09.4.6 Cyclopolymers 1084

4.09.4.7 Polymers of Cyclic Olefins 1084

4.09.4.8 Polymerization of Conjugated Dienes 1084

4.09.5 Polymerization of Ethylene, Propylene, and Higher -Olefins with other

Single-Center Catalysts 1086

4.09.5.1 Complexes with Coordination Number 4 1086

4.09.5.1.1 Ligands with coordinating OO atoms 10864.09.5.1.2 Ligands with coordinating NN atoms 10874.09.5.1.3 Other ligands 1090

4.09.5.2 Complexes with Coordination Number 5 1091

4.09.5.2.1 Ligands with coordinating OO atoms 10914.09.5.2.2 Ligands with coordinating NO atoms 10914.09.5.2.3 Ligands with coordinating NN atoms 10924.09.5.2.4 Other ligands 1095

4.09.5.3 Complexes with Coordination Number 6 1095

4.09.5.3.1 Ligands with coordinating OO atoms 10954.09.5.3.2 Ligands with coordinating NO atoms: phenoxyimine-catalysts for polyethylene 10964.09.5.3.3 Ligands with coordinating NO atoms: phenoxyimine catalysts for syndiotactic polypropylene 11154.09.5.3.4 Ligands with coordinating NO atoms: phenoxyimine catalysts for isotactic polypropylene 11264.09.5.3.5 Other ligands with coordinating NO atoms 11274.09.5.3.6 Complexes with NN chelate ligands 11384.09.5.3.7 Other ligands 11424.09.5.3.8 Olefin co-polymerizations with post-metallocene catalysts 11434.09.5.3.9 Polystyrene and olefinstyrene co-polymerization with post-metallocene catalysts 1145

References 1146

4.09.1 Introduction

This chapter covers the polymerization of alkenes with homogeneous and heterogeneous catalysts based on group 4

metals, including the underlying reaction principles and the relationship between catalyst structure and polymer

properties. Applications of related complexes in CC bond-forming reactions in organic synthesis are covered in

Chapter 00125. The use of transition metal catalysts in polymer synthesis is more widely discussed in chapter 11.06.

Catalytic olefin polymerization by means of groups 4 and 5 (ZieglerNatta) or group 6 (Phillips) metal catalysts is

one of the major chemical industries in the world. Polyethylene (PE) (both high density (HDPE) and linear low

density (LLDPE) and polypropylene ((PP); including propylene-rich co-polymers and heterophasic co-polymers) are

the two major thermoplastic polymers, with world productions of about 40 and 36 million tons/year, respectively (2003

figures). Titanium-based, heterogeneous ZieglerNatta catalysts dominate PP production and also play a leading role

in the manufacture of HDPE and LLDPE. Chromium-based Phillips catalysts are also widely used in HDPE

production, while metallocene and related single-site catalysts are making significant inroads in LLDPE produc-

tion. The total market for industrial polyolefin catalysts is estimated to exceed 6000 tons/year.

In the last 20 years or so, thanks to the development of the metallocene and single-site organometallic catalysts,

catalytic olefin polymerization has further evolved into one of the most actively studied branches of catalysis. (The

term single-site catalyst is widely used; however, in order to avoid confusion with coordination sites, and to

underline the chemical uniformity of the active species in metallocene catalysts, we prefer the term single-center

catalyst.) While characterized as mature about 10 years ago, and despite its cyclic nature, the polyolefin business is

recognized today as a healthy and growing business, thanks to continuing technology innovations, and significant

1006 Olefin Polymerizations with Group IV Metal Catalysts

expansions in the Asian market. The huge commercial success of polyolefin materials has, in turn, fueled research

activities in academia and industrial R&D institutions. In addition to the continuing expansion of established

technologies, such as the Spheripol and Unipol processes, several new processes have been developed, and new

plants built, in order to fulfill the ever-growing market request for new polyolefin-based materials. Most recent

examples are those of Basells new Spherizone gas-phase process for PP, Basells new two-reactor polybutene plant,

and Dows and Exxons solution processes for the production of propylene-based plastomers and elastomers. Without

diminishing the importance of process and material design, polymer science, and obviously market economics, the

success of these new technologies is to a great extent due to catalyst development.

Despite the heterogeneous and multi-component nature of the industrial MgCl2- or silica-supported ZieglerNatta

catalysts, which hampers the understanding of the elementary steps and kinetics of monomer insertion, chain growth,

and termination mechanisms, significant progress has been made, especially in the elucidation of fundamental

aspects of stereoregulation and molecular mass control. New and more efficient catalyst modifiers (donors) that

enable the tuning of chain stereoregularity, molecular mass distribution, and co-monomer incorporation in isotactic

polypropylene (iPP) have been found.

On the other hand, in order to simplify the nature of the active species and better unravel the many elementary

steps simultaneously operating during catalytic polymerization, group 4 bis(cyclopentadienyl) complexes1 were

studied by Natta and Breslow as early as 1957 as soluble and structurally well-defined models for TiCl3-based

heterogeneous ZieglerNatta catalysts.2,3 However, for many years, these complexes remained just models due to

their uncompetitively low catalytic activities. At the end of the 1970s, the pioneering work of Brintzinger on the

synthesis of chiral metallocenes,413 combined with Sinn and Kaminskys seminal discovery of methylalumoxane

(MAO) as a superior activator for metallocene catalysts,14 suddenly turned zirconocenes from model catalysts into

highly effective ethylene polymerization systems, endowed also with an unprecedented co-monomer incorporation

ability. These discoveries, and Ewens subsequent groundbreaking work on ligand effects in stereoselective poly-

merization, marked the birth of a new era in catalytic olefin polymerization: that of well-defined, purposely designed,

single-center organometallic catalysts.

Organometallic chemists have played a key role in designing new ligands, organometallic complexes, and catalyst

systems, understanding their activation chemistry, and determining the mechanisms of olefin interaction with

transition metals and the stereochemical implications of chain growth. In addition to a much clearer understanding

of the chemistry involved in polymerization catalysis, detailed mechanistic investigations have also generated a

wealth of new polyolefin materials, new applications, and ultimately markets, that were inaccessible with the

heterogeneous ZieglerNatta catalysts.

Many extensive reviews and books have been recently dedicated to the field of catalytic olefin polymerization,

both for ZieglerNatta catalysts1517 and for metallocene and other single-center catalysts.1820 Nevertheless, the

pace of development is so quick that a new, comprehensive review appears timely. In the following, we describe the

evolution of ZieglerNatta catalysts, the revolution of single-center catalysts, and their application most at

laboratory level only to the synthesis of novel or improved polyolefins in the last 10 years.

4.09.2 Pre-catalysts by Chemical Type and Reaction Principles

The most common geometries adopted by group 4 catalysts are depicted in Scheme 1. In all practical cases, the active

center is a cationic, strongly electrophilic metal complex capable of activating the CTC double bond of the insertingmonomer. This positive charge of the complex cation is counterbalanced by a weakly (or non-) coordinating

Coordination 4Tetrahedral

Coordination 5Trigonal bipyramid

PP

M

P = growing polymer chain;

M M

L1

L1 ++ +L1

L2L2 L

2

L4L3 L3

L = generic ligand

P

XX

X = counterion;

X

Coordination 6Octahedral

Scheme 1

Olefin Polymerizations with Group IV Metal Catalysts 1007

counteranion. The active center must have two coordination sites in mutually cis-positions in order to enable the

transfer of the growing polymeryl chain to the coordinated monomer. In the absence of the monomer, one of these

cis-coordination positions is usually saturated by the counterion. The ligand(s) must confer the required steric and

electronic properties, which control the microstructure and the molecular mass of the produced polymers. The

generic ligands L can be based on anionic aromatic groups such as the cyclopentadienyl (Cp, 5-C5H5) ring and its

derivatives, as well as on anionic or neutral ,-donors usually based on heteroatoms, such as O, N, S, and P. Overall,

the set of coordinating L ligands is usually dianionic. Finally, the metal atom most often is a d 0-metal in the oxidation

state IV.A brief listing of the most typical pre-catalysts used in catalytic olefin polymerizations is represented in Figure 1.

These are the systems that will be discussed in much more detail in the following sections. Examples of catalysts

based on 1 are dialkoxide- and diamide-based tetrahedral systems. Introduction of an extra neutral donor ligand as in

2 results in pentacoordinate catalysts. Structures with a piano-stool geometry such as 3 are usually denominated half-

sandwich complexes. Pre-catalysts such as 4 include the remarkably interesting class of ansa-monocyclopentadienyl

amido complexes (also known as constrained-geometry catalysts or CGC), while the well-known bis-cyclopentadienyl

metallocenes correspond to pre-catalysts of generic formula 5. Systems 68 present an octahedral coordination

geometry at the metal atom, and include systems with two unbridged chelating ligands as in 6, and the most well-

known complexes of this kind are the bis(phenoxyimine) complexes of titanium. Pre-catalysts with tetradentate

ligands as in 7 include the bridged bis(phenoxyamine)-based catalysts, while pre-catalysts such as 8 are character-

ized by a tridentate ligand with an extra donor arm. Systems such as 35, which contain at least one Cp ligand, are

discussed in Section 4.09.4, while systems 1, 2, 6, 7, and 8, which can be broadly defined as non-metallocene

catalysts, are discussed in Section 4.09.5

4.09.2.1 MC as Propagating Species/Activation

The propagating active site in olefin polymerizations mediated by group 4 catalysts is the MC(polymer) bond of a

metalalkyl complex.2133 Although a few neutral group 4 catalysts, such as complexes 9 (MZr, Hf34) and 10,3539have been synthesized, almost all effective group 4 complexes are inactive in polymerization if not activated by a

suitable co-catalyst.

9 10

M

5-C2B9H

11

Me M+ R

B(C6F5)3

L

MX2

L1

L1

MX2

L1

L2

2

XMX

X

R5

3

MX2L

R5

4

MX2

R5

R5

5

L2

L1

MX2

2

6

MX

L1

L2L2

L1X

7

L1 M L1

L2L3

XX

8

Figure 1 Structure of the most typical pre-catalysts used in catalytic olefin polymerizations.

1008 Olefin Polymerizations with Group IV Metal Catalysts

Activation and formation of the cationic species are accomplished through a suitable activating species, the co-

catalyst, and thus the importance of the co-catalyst in olefin polymerizations with group 4 systems is fundamental.

The activator becomes an anion after the activation process, forming a cationanion pair, which is now accepted to be

the real catalytically active polymerization species. With different activators, dramatic differences in activity are

possible for a given pre-catalyst structure.40,41,86,86a Furthermore, the counteranion was demonstrated to influence

the stability and activity of the catalyst, as well as the molecular masses and even stereoregularity of the polymers

produced. It was the discovery of MAO by Sinn and Kaminsky14 that started the metallocene revolution, although the

complexity of the catalytic system did not allow conclusions about the structure of the active species. After the

cationic nature of the active catalytic species was established,2232 several other activators were designed, most of

them based on non-coordinating borates and aluminates. Excellent reviews on the subject have appeared.40,41

Selected examples of activators are shown in Figure 2.

In order to produce an active catalyst upon reaction with the activator, the pre-catalyst has to be alkylated

either during its synthesis or in situ by an aluminum alkyl compound. Al-alkyls and Al-alkyl chlorides are

important components of heterogeneous ZieglerNatta systems. However, their inability to efficiently activate

group 4 metallocenes has for a long time limited developments in this field, until the arrival of MAO, which is

now the most widely used activator. The structure of MAO is still rather undefined. In solution, MAO exists as an

equilibrium of species with different aggregation numbers and structures.4244 Proposed structures include linear

chains, cyclic rings, three-dimensional clusters, and cage structures.40,4556 MAO as co-catalyst has some draw-

backs: low solubility in aliphatic solvents, poor long-term stability in solution, the high content of MAO residues

(alumina) in the final product, and the relatively high cost, not least in view of the rather large amount needed for

effective activation (the typical Al/M molar ratio needed for homogeneous systems is 103 : 1104 : 1, although in

supported systems, ratios around 100 : 1 are sufficient). This is especially true for systems of not very high activity.

F B

FF 3

11

F

FFFF

FF

FF

B

F FF F 2

C6F5F

F

12

F

B

FF3

FF

F

F

13

(C6F5)2B B(C6F5)2

FF14

F

F

F

F

B(C6F5)2

B(C6F5)2

15(C6F5)2

(C6F5)2 F

F

F

F

F

F

F

F

16

F BH

FF 2 2

17

F

FF

FF

Ph3C+ B

4

18

F

FF

FF FF FF

R3NH+ B

4

19

F

FF

Ph3C+ Al

4

20

SiiPr3

FF

Ph3C+ B

4

21

F

F

F

Ph3C+ BF

F

F

F

F

2

22

F

FF

O

F F

Ph3C+ Al

4

23

(C6F5)3B C N B(C6F5)3

Ph3C+

24

B(C6F5)3(C6F5)3BR3NH+

25

N B(C6

+ F5)3

26

Figure 2 Selected examples of activators.

Olefin Polymerizations with Group IV Metal Catalysts 1009

Finally, the danger inherent in the use of extremely pyrophoric AlMe3 has to be taken into account. Surrogates of

MAO include ethylalumoxane and isobutylalumoxane synthesized from AlEt3 and AlBui3, although they do not

perform as well as MAO.5763 To solve the above problems, modified MAOs have been investigated. The patent

literature reports the use of MAO/AlBui3 mixtures,64 or the hydrolysis products of AlBui3 and other branched

Al-alkyls.61,65,66 The presence of residual AlMe3 is another problem associated with MAO. Several authors

showed that increasing the AlMe3/MAO ratio or replacing AlMe3 with AlEt3 or AlBui3 results in a decrease of

both activity and molecular masses.44,6772 MAOs which contain much less residual AlMe3 have been developed

and are claimed to exhibit better performances than conventional MAO.73,74 Several other approaches have been

proposed to reduce the amount of AlMe3 in MAO.7577

A different strategy toward stoichiometric co-catalysts has been the use of perfluoroaryl boranes such as 1115 and

17. Ewen and Marks independently introduced the already known strongly Lewis-acidic borane B(C6F5)3 11 as

activator for olefin polymerizations with group 4 metallocenes.7881 Reaction of B(C6F5)3 with group 4 dimethyl

metallocenes (Figure 3) is rapid and quantitative at room temperature in non-coordinating solvents. Crystal structures

of the products show that the methyl group of the [MeB(C6F5)3] moiety remains coordinated to the cationic

metallocene.78,79 Other perfluoroaryl borane activators were developed, such as the bifunctional borane

[HB(C6F5)2]2 17,82 and the sterically encumbered perfluorobiphenyl and perfluoronaphthyl boranes.8385 Trityl

and ammonium borates such as 18, 19, 21, and 22 and aluminate salts such as 20 and 23 are other classes of widely

used activators.40,83,86,86a90 Different approaches include the cyano-bridged 24, the weakly coordinating 25, and the

pyrrole-based 26.86,86a,91,92 Although the [B(C6F5)4]-based activators are highly effective in olefin polymerization, 9398

they have some drawbacks. They are poorly soluble in many hydrocarbon solvents and can have limited thermal

stability, which results in short catalytic lifetimes.89 On the other hand, whereas MAO and related co-catalysts are

used in large stoichiometric excess, for borane, borate, and similar co-catalysts, a 1 : 1 molar ratio of activator and

dialkyl pre-catalyst is sufficient. In some cases, [Ph3C][B(C6F5)4] used in excess over the metallocene can

significantly increase the productivity of some propylene polymerization catalysts, in particular, those with high

activity systems such as constrained-geometry titanium complexes.99,100

Since the catalyst activator has been shown to exert a remarkable influence on the performance of olefin

polymerization catalysts,41,99,101106 the search for new co-catalysts is an active field of research. This, however, is

beyond the scope of this review.86,86a,107113

The products of activation with the three main classes of activators described above are shown in Figure 3. We only

add that in order to have more reproducible results, and to reduce the amount of catalyst needed for optimum activity,

adding small amounts of AlR3 (such as AlBui3 and AlEt3) to the reaction system is a common practice to scavenge

impurities and, with metallocene dihalide precursors, to alkylate the metal.114,115 It is worth noting that small

aluminum alkyls such as AlMe3 and AlEt3 form heterobinuclear complexes with metallocene alkyl cations, of the

type [L2M(-Me)2AlMe2], such that high concentrations of these aluminum alkyls reduce the catalyst activity.31,33

However, there is no evidence that bulky aluminum alkyls such as AlBui3 form similar adducts with group 4

metallocene catalysts.

Upon activation, the metalalkyl cation and the counteranion form an ion pair. In the low-polarity solvents used in

olefin polymerizations, the interaction between the cation and the anion is rather strong. Methyl borates derived from

11 (activation reaction (b) in Figure 3) represent an example of a tight ion pair, with a bridging Me

group.40,116Conversely, ion pairs with a tetrakis(perfluoroaryl) borate counterion (Figure 3, reactions(c) and (d))

represent examples of less tightly bound ion pairs and the anion in an outer-sphere position.40,101,116 The exact

mechanism and energetics of ion pair formation (pre-catalyst activation) have been widely investigated by several

groups.40,117120,122,126,127 The structure and dynamics of ion pairs is conveniently investigated by spectroscopic

NMR techniques.116 Finally, the possible aggregation of ion pairs to form species such as ion quadruples, hextuples,

and higher-order aggregates has also been investigated.101,105,121125 The main conclusion seems to be that, at the

concentrations typically used in olefin polymerizations, catalyst ion pairs are unlikely to be present as higher

aggregates.105,123,124 These aspects have been summarized in pertinent reviews.41,116

4.09.2.2 Monomer Coordination and Insertion Reactions

The fundamental reaction in catalytic olefin polymerizations is monomer insertion into an MC bond, schematically

described in Scheme 2. The general mechanistic features are well covered in two reviews.126,127

The mechanism generally accepted for the chain-growth reaction of Scheme 2 is reported in Figure 4. Cossee

originally proposed this mechanism, now known as the ArlmanCossee mechanism.128,129 It substantially occurs in

1010 Olefin Polymerizations with Group IV Metal Catalysts

two steps: (i) olefin coordination to the metal, (ii) alkyl migration of the -coordinated growing chain to the

-coordinated olefin. Green, Rooney, and Brookhart slightly modified this mechanism with the introduction of

a stabilizing -agostic interaction,130 which would facilitate the insertion reaction.131133 The role of -agostic

interactions in olefin insertion has been rationalized by Grubbs and Coates.134

MMe2Si

MAO-X

MMe2Me2Si

MMe2Me2Si

+Me

MX2Me2Si

MMe2Si-MeB(C6F5)3

+Me

MMe2Si[B(C6F5)4]

+Me

+ MAO

+ B(C6F5)3

+ [Ph3C][B(C 6F5)4]Ph3CMe

MMe2Me2Si MMe2Si+

Me+

N B(C6F5)3

N B(C6

F5)3+

CH4

MMe2Me2Si MMe2Si[B(C6F5)4]

+Me

+ [R3NH][B(C 6F5)4]NR3 ,CH4

(a)

(b)

(c)

(g)

(d)

MMe2Si MMe2Si+

+ B(C6F5)3

(C6F5)3B

MMe2Me2Si MMe2Si-MeAl(C6F5)3

+-MeAl(C6F5)3

+ 2Al(C6F5)3

(e)

(f)

Figure 3 Selected examples of metallocene activation processes.

Olefin Polymerizations with Group IV Metal Catalysts 1011

The first step of the insertion reaction requires that the active metal center has an available coordination site for the

incoming monomer. For many years, it was commonly accepted that olefin coordination to the cationic metal was an

easy process, with a low activation energy possibly connected to the displacement of a weakly coordinated solvent

molecule or of a weakly agostic interaction between the metal and a CH bond of the growing polymer chain. In

recent years, this view has changed. Certainly, with coordinating anions like [MeB(C6F5)3], olefin coordination

requires anion displacement, and it has even been suggested that olefin coordination could represent the rate-limiting

step.135,136

The second step of the chain-growth reaction, the insertion step, occurs via chain migration to the closest carbon of

the olefin double bond, which undergoes cis-opening with formation of the new metalcarbon and carboncarbon

bonds.137 Consequently, at the end of the reaction, the new Mchain -bond is on the site previously occupied by the

coordinated monomer molecule (chain-migratory mechanism). At the end of the reaction, the coordination position

previously occupied by the growing chain is then occupied by the counteranion. This mechanism is schematically

represented in Figure 4(b). It is important to note that the inclusion of the anionic counterion does not pertain to

heterogeneous ZieglerNatta catalytic systems since no anionic co-catalysts are used in this case.

The overall activation energy of the reaction is the result of different contributions, from counterion displacement

to the breaking and forming of the MC bonds. Of course, the strength of the ion pair interaction contributes

sensitively to the overall activation barrier, and it explains why catalysts with tightly bound counteranions such as

[MeB(C6F5)3] show lower activities relative to catalysts with weakly bound counteranions such as [B(C6F5)4]

. Insome cases, it has been suggested that the nature of the monomer influences the position of the transition state for

monomer insertion, with anion displacement being important in propene polymerization, while with 1-hexene alkyl

transfer to the coordinated monomer was found to be rate determining, independent of the anion.138 Further details

on this topic can be found in a critical review.41

After insertion, the growing chain can swing back to the coordination position occupied before insertion. This

isomerization mechanism, represented in Scheme 3, is usually referred to as site isomerization or backskip of the

M P Pn + Monomer M n + 1

Scheme 2

M

H

PnH

M

H

PnH

M

H

PnH

MH

PnH

M

Pn

+C2H4

M

H

PnH

M

H

PnH

M

H

PnH

MH

PnH

M

Pn

+C2H4A

+ ++++

A A

AA

(a)

(b)

+ + + + +

Figure 4 (a) Modified Cossee mechanism for olefin polymerizations with group 4 transition metals; (b) modified mechanism inthe presence of an anionic counterion.

MPn

MPnchainbackskip

Scheme 3

1012 Olefin Polymerizations with Group IV Metal Catalysts

growing chain.139 The backskip of the growing chain can have an effect on the sequence of enantioselective

steps which determine the microstructure of the resulting polymer in the case of prochiral olefins.104,139 While the

chain-migratory mechanism is commonly accepted, there are cases in which regular (or predominant) chain migration

at each insertion step is not operative. In this case, the growing chain returns to the original coordination position at

the end of each insertion reaction, and olefin coordination occurs predominantly at one coordination site. This last

mechanism was shown to occur in some particular cases, and its occurrence is highly dependent on the nature of the

counteranion.104 We refer to it as chain-retention mechanism.

Detailed quantum mechanics calculations have indicated that agostic interactions occur between the growing chain

and the metal atom. The most typical are shown in Figure 5. Calculations on gas-phase metal alkyl cations indicate

that the -agostic interaction is the most stable, with the -agostic interaction roughly 25 kcal mol1 higher inenergy, and the less stable -agostic interaction about 10 kcal mol1 higher in energy.140,141

Quantum mechanics calculations indicated that olefin coordination to the naked cationic catalyst is a barrierless

and exothermic process that leads to the olefin coordination intermediate of Figure 6. The coordination intermediate

eventually evolves to the four-center Cossee-like transition state of Figure 6, and then collapses into the products that

resemble the agostically bound alkyl species of Figure 5.140146 Interestingly, these quantum mechanics calculations

confirmed that the transition state is assisted by -agostic interactions, as proposed by Green, Rooney and

Brookhart.130 Quite a small energy barrier (15 kcal mol1) has been calculated for the insertion step in the case ofthe naked cationic catalyst.141,144146

While the naked cation could be a model of a catalyst with a completely non-coordinating counteranion, the energy

profile in the presence of a tightly coordinating counterion such as [MeB(C6F5)3] is remarkably different. The first

issue is how the olefin enters the metal coordination sphere. The three different olefin approaches shown in

Scheme 4 have been investigated with quantum mechanics approaches.

-Agostic -Agostic -Agostic

2.36 2.17 2.31

Zr ZrZr

Figure 5 Agostic interactions between an isobutyl group (simulating a growing chain) and the Zr atom in [Me2Si(1-Ind)2Zr-Bui];

distances in A.

Olefincoordination

Insertiontransition state

OlefinOlefin

2.262.36

2.38 Zr 2.10

1.421.35 3.01

2.762.73 Zr

2.29

Growingchain

Growingchain

Figure 6 Olefin coordination intermediate and transition state for insertion of propylene into the ZrBui bond of [Me2Si(1-Ind)2-Zr-Bui]; the Bui group simulates the growing chain; distances in A.

Olefin Polymerizations with Group IV Metal Catalysts 1013

For the [H2Si(Cp)(NBut)TiCH3][MeB(C6F5)3] system, olefin coordination/insertion along path A is slightly

favored over paths B and C because it requires less cationanion separation. In any case, olefin coordination in the

presence of the counterion requires that a sizeable energy barrier must be overcome.147149 Modeling ethylene

insertion on the [Me2Si(Cp)(NBut)TiCH3][MeB(C6F5)3] system confirmed that ethylene approach pathways A and B

are of very similar energy, but they also indicated that for insertion into longer Tialkyl bonds, such as insertion into

the TiPrn bond, path B is favored.136 More interestingly, they also suggested that the rate-limiting step could be

olefin coordination and not olefin insertion.135,136 Ethylene insertion into the [Cp2ZrC2H5] cation with both the

[MeB(C6F5)3] and [B(C6F5)4]

counterions has been modeled; for these systems too, the approach along path B wasfound to be favored.117

Many experimental mechanistic studies have been devoted to clarify the role of the counterion in monomer

insertion (and thus on catalyst activity).41,55,99,101106,117,119,135,136,147,148,150156 Based on the results of studies on

the competitive coordination to the metal atom of the counterion versus an added Lewis base, it has been proposed

that the tight ion pair is unable to insert the monomer, and that displacement of the counterion has to occur. After

dissociation, one (or possibly more) olefin molecules may insert into the Mchain bond before the counterion

recoordinates to the metal, and chain growth is stopped until the counterion dissociates.157 This mechanism closely

resembles that proposed by Fink based on early studies on the kinetics of ethylene oligomerization, which led to a

mechanistic scheme where the polymer chain-growth process could be interrupted at any stage by the reversible

formation of a resting state, the so-called intermittent mechanism.158

On the other hand, studies on the polymerization of 1-hexene polymerization catalyzed by rac-C2H4(Ind)2ZrMe-

(-Me)B(C6F5)3 showed that monomer insertion, anion displacement, and anion recoordination are part of a

concerted process, for which the term continuous mechanism was suggested.154 NMR studies on metallocene

ion pairs bearing a longer-chain alkyl ligand as polymeryl model, [rac-Me2Si(Ind)2ZrCH2SiMe3 X], indicated that

a mechanism of the continuous type is operative for the tightly bound counteranion [MeB(C6F5)3]. By contrast, if

the cation is paired with the very weakly coordinating [B(C6F5)4], the counteranion does not enter the inner

coordination sphere of the metallocenium cation, and as a result the inserting monomer does not have to compete

with the counteranion for coordination to the metal. The catalysts differ therefore structurally, with [MeB(C6F5)3]

forming an inner-sphere ion pair (ISIP), while [B(C6F5)4] gives an outer-sphere ion pair (OSIP). Nevertheless,

although the degree of anion coordination and the catalyst structures are strongly anion dependent, both insertion

mechanisms are similar in the sense that both involve an exchange of the alkyl ligand and anion positions after each

insertion step, following the principle outlined in Figure 4(b).101 This mechanistic model is in agreement with Finks

original concept of an intermittent process.158 On the basis of combined X-ray and NMR studies, it has been

suggested that the different binding capability of the two counterions results in different resting states, involving an

-agostic methyl interaction with tightly bound [MeB(C6F5)3], and a -agostically bonded alkyl chain in OSIPs with

non-bonded counterions such as [B(C6F5)4] (see Scheme 5).41,101

PnM

Cp Cp

X

+

A

B

C

Scheme 4

M

Cp Cp

C

+

Tightly bound ion pair

M

Cp Cp

X

+

PnMe

Weakly bound ion pair(a) (b)

Pn

Me

HH

B(C6F5)3H

Scheme 5

1014 Olefin Polymerizations with Group IV Metal Catalysts

4.09.2.3 Concepts of Stereo-, Regio-, and Enantioselectivity

While ethylene insertion can occur in a single mode, insertion of -olefins can occur in the four geometrically

different modes represented in Scheme 6. Thus, polymerization of prochiral monomers requires the definition of a

few terms.

4.09.2.3.1 Regio- and stereochemistry of monomer insertionThe regiochemistry of insertion (the catalyst regioselectivity) defines whether olefin insertion is primary or secondary

(also called 1,2 or 2,1 insertions, respectively). Any catalyst will insert some olefin molecules with the wrong

regiochemistry. Regioirregular insertions (regioerrors or regiomistakes) mean occasional secondary (primary) insertion

if propagation is prevailingly primary (secondary). Monomer insertion is mostly primary for metallocene catalysts (the

amount of regiomistakes being usually

-ligands framework. The most typical situations are shown in Figure 7. If the pre-catalyst has an overall

C1-symmetry, the two coordination positions are sterically and electronically different. The activity and catalytic

properties of the two possible sites can be completely different. If the pre-catalyst has an overall C2-symmetry, the

two coordination positions are homotopic and thus the activity and catalytic properties of the two sites are identical. If

the pre-catalyst has an overall CS-symmetry, two cases are possible. If the local mirror plane contains the coordination

positions available to the growing chain and the monomer, the two coordination positions are not symmetry related,

and, by symmetry, they are both non enantioselective. More interesting is the case in which the local mirror plane

relates the coordination positions available to the growing chain and to the monomer. In fact, in this case the two

coordination positions are enantiotopic, and thus the possible asymmetric induction (in the framework of the chain-

migration mechanism) is opposite at each insertion step.

4.09.2.3.2 Definition of stereoregular polymersThe structure of perfectly regular isotactic and syndiotactic PP(iPP and sPP) is shown in Figure 8. Isotactic PP is

characterized by a sequence of tertiary C atoms with the same local spatial arrangement (a single configuration) in the

polymer chain. Syndiotactic PP is characterized by a sequence of tertiary C atoms with alternate configuration in the

polymer chain. Atactic PP(aPP) has no regularity in the sequence of the configuration of the tertiary C atoms. While

the iPP is of major industrial interest, and sPP- and aPP have found some applications, hemiisotactic PP, in which one

in every two tertiary C atoms is isotactic while the other is atactic, also shown in Figure 8, is an academic curiosity. If

the relative configuration of two successive tertiary C atoms (a diad) is considered, an isotactic polymer can be

considered as composed by a sequence of m (meso) diads, while a syndiotactic polymer can be considered to be

composed by a sequence of r (racemic) diads. Finally, in the case of diolefins, stereoselective polymerization can lead

to diisotactic and disyndiotactic polymers, whose structure is also reported in Figure 8.166

4.09.2.3.3 Elements of chiralityStereoselective -olefin polymerization is the result of a sequence of asymmetric reactions (-olefin coordination/

insertion). The main elements of chirality are as follows.

(i) Monomer coordination. Coordination of the two enantiofaces of a prochiral -olefin gives rise to chiral si and re -

olefin coordinations.167 Isotactic polymers are generated by multiple insertions of -olefin molecules with the

same enantioface (either re or si), while syndiotactic polymers are generated by a regular alternation of insertions

of re- and si-coordinated monomers.

(ii) Chirality of the active site. Different cases are present here. (a) The chirality can arise from coordination of

prochiral ligands. In this case, the notation (R) or (S), in parenthesis, according to the CahnIngoldPrelog rules as

extended by Schlogl can be used.168,169 As an example, the (R,R) chirality of coordination of the H2C(1-Ind)2ligand, labeled according to the absolute configurations of the bridgehead carbon atoms marked by arrows, is

shown in Figure 9(a). In the case of complexes with two bidentate ligands, the relative orientations of the two

bidentate ligands can be chiral and generate chirality at the metal. This chirality can be labeled with the notation

or , defined for octahedral coordination compounds (Figure 9(b)).170 (c) An intrinsic chirality at the central

metal atom, which for tetrahedral or pseudo-tetrahedral situations can be labeled with the notation R or S of

C1-symmetricpre-catalyst

M M M M

C2-symmetricpre-catalyst

Cs-symmetricpre-catalysts

Figure 7 Schematic representation of the most common symmetries of group 4 olefin polymerization catalysts. Grayrectangles define the space occupied by the organic ligand. Hollow squares represent the coordination positions available to thegrowing chain and to the monomer. Dashed lines represent the local mirror plane of the two CS-symmetric catalysts.

1016 Olefin Polymerizations with Group IV Metal Catalysts

A A A A A A A A A A

B B B B B B B B B

A A A A A A A A A ABBBBBBBB B

A B

A BA B

A B

A B

A B

A

A B A

BA B

(a)

(b)

(c)

(d)

(e)

(f)

(g)

m m m m m m m m m

r r r r r r r r r

m r r r r r m r m

Figure 8 Segments of isotactic (a), syndiotactic (b), atactic (c), and hemiisotactic polypropylene (d) chains. Segments oferythro-diisotactic (e), threo-diisotactic (f), and disyndiotactic (g) poly-diolefin chains. The modified Fischer projection is shown.For parts, (a)(c) a zigzag representation is also reported.

Monomer

Monomer

Growingchain Growing

chain

Mt Mt Mt(R )

(a) (b) (c)

Growingchain

Monomer

R(R )

Figure 9 Schematic representation of the chirality at the active site in the case (a) of a C2-symmetric pseudo-tetrahedralmetallocene, (b) of a C2-symmetric octahedral model for heterogeneous catalysts, and (c) of a syndiospecific CS-symmetricpseudo-tetrahedral metallocene.

Olefin Polymerizations with Group IV Metal Catalysts 1017

CahnIngoldPrelog rules as extended by Stanley and Baird.168,171 For instance, the diastereoisomer with

intrinsic R configuration at the central metal atom is shown in Figure 9(c), for the case of a metallocene with a

H2C(Cp)(9-Flu) ligand. It is important to note that chirality of type (c) requires that the two coordination

positions available for the growing chain and the monomer are occupied by different ligands. This implies that

compounds such as the dichloride pre-catalysts are not chiral.

One or more of these kinds of chirality of the site can be present in the active site. However, for the case of

catalytic complexes in which the two ligands are tightly connected through chemical bonds and which are called

hereafter as stereorigid, only the chirality of kind (c) can change during the polymerization reaction.

(iii) Chirality of the growing chain. The last tertiary C atom of the growing chain is chiral, and its configuration is

determined by the chirality of monomer coordination in the last insertion step. The R/S CahnIngoldPrelog

nomenclature can be used. However, it is common to label the two configurations as si- or re-ending growing

chains, according to the configuration of the monomer during coordination/insertion.168,172

4.09.2.3.4 Mechanism of stereocontrolIt is well accepted that two mechanisms of stereocontrol (the chiral induction responsible for selecting the monomer

enantioface) are operative in stereoselective -olefin polymerizations. In the simpler cases, the discrimination

between the two faces of the prochiral monomer may be dictated either by the configuration of the asymmetric

tertiary C atom of the last inserted monomer unit or by the chirality of the catalytic site. These two different

mechanisms of stereocontrol are named chain-end stereocontrol and enantiomorphic-site or site stereocontrol. In the

case of chain-end stereocontrol, the selection between the two enantiofaces of the incoming monomer is operated by

the chiral environment provided by the last inserted tertiary C atom of the growing chain, whereas in the case of site

stereocontrol this selection is operated by the chirality of the catalytic site. The origin of stereocontrol in olefin

polymerization has been reviewed extensively.162,172178

The distribution of steric defects along the polymer chain may be indicative of which kind of stereocontrol is

operative. The type and amount of stereomistakes (enantioface insertion errors) is measured by solution 13C NMR

spectroscopy, a sensitive technique that is able to see the steric environment of a given propylene unit up to

undecads (five propylene units on each side of the observed monomeric unit). Routine analysis is usually performed

at the pentad level (two propylene units on each side of the observed monomeric unit).162,179 The microstructures

which result from stereomistakes are shown in Scheme 7.

Any catalyst will make some enantioface insertion mistakes (stereoerrors or stereomistakes). In the case of

stereomistakes in propylene polymerization, the chemical shift of the methyl groups is highly sensitive to the relative

stereochemistry of neighboring monomer units. The degree of tacticity can be given as the pentad, triad, or diad

content (% mmmm, % mm, % m for isospecific polymerization, and % rrrr, % rr, % r for syndiospecific polymerization,

respectively). In the case of low stereoregularity, the diad excess, % m-r (% r-m) better represents the degree of iso-

(syndio-) tacticity.180 Isolated insertion errors unambiguously identify the polymerization mechanism.

Stereomistakes are easily and quantitatively detected by NMR spectroscopy, and useful relationships for the four

stereospecific polymerization mechanisms discussed above are reported in Table 1. Additionally, the relationships

2[rr]/[mr] 1 and 2[mm]/[mr] 1 identify iso- and syndiospecific site control, respectively, whereas the relationship4[mm][rr]/[mr]2 1 identifies chain-end control. The average stereoregular block lengths are 2[m]/[r] 1 and 2[r]/[m] 1 for isotactic and syndiotactic polymers, respectively.

It is worth noting that in the case of syndiospecific propagation, the microstructure of the polymer is also affected

by other secondary reactions such as the backskip of the growing chain, or backside attack of the olefin. For example,

a backskip of the growing chain followed by correct enantioface selection and regular chain migration would originate

a microstructure identical to that generated from a stereomistake in the case of chain-end stereocontrol.104,179,181,182

The origin of stereocontrol in -olefin polymerizations has been clarified in detail, and it was shown to be

essentially driven by steric effects.172,174176,178 The commonly accepted mechanism was developed by the

Corradini school, and it is called the mechanism of the chiral orientation of the growing chain. It was proposed

at the beginning of the 1980s to explain the stereospecificity of heterogeneous catalysts.183188 In the case of primary

propagation, the chiral environment provided by the chirality of the complex (in the case of site stereocontrol) or by

the chirality of the tertiary C atom of the last inserted monomeric unit (in the case of chain-end stereocontrol)

imposes a chiral orientation to the growing chain in order to minimize the steric interaction between the ligand

skeleton and the growing chain. It is this chiral orientation of the growing chain that selects between the two

enantiofaces of the incoming monomer molecule. The preferred enantioface is the one that minimizes steric

1018 Olefin Polymerizations with Group IV Metal Catalysts

interaction with the chirally oriented growing chain. Usually, this is the enantioface that places the methyl group of

the propylene trans to (i.e., away from) the chirally oriented growing chain (Figure 10).172,174,175,183,185,187,189192

In the case of secondary propagation, the mechanism of the chiral orientation of the growing chain is still operative

for stereoflexible compounds. For example, Cavallo and Guerra suggested that for the bis(phenoxyimine)Ti

catalysts, the secondary growing chain assumes a chiral orientation in order to establish an -agostic interaction

with the metal.172,193 This chiral orientation imposes a configuration to the stereoflexible complex, and it is the

chirality of the complex (imposed by the growing chain) that selects between the two enantiofaces of the secondary

inserting propylene. This mechanism was originally proposed for the V-based systems.194 Finally, in the case of

secondary propagation and stereorigid complexes, stereoselectivity is simply determined by steric interactions

between the ligand and the monomer molecule. The growing chain plays no role here.

The mechanism of the chiral orientation of the growing chain has strong experimental support. The 13C NMR

analysis of the PP end groups performed by Zambelli and co-workers showed that propylene insertion is essentially

non-enantioselective in the first polymerization step (when the alkyl group bonded to the metal is a methyl group),

whereas it is enantioselective in successive insertion steps when an isobutyl group is bonded to the metal. This holds

good for both heterogeneous195 and homogeneous196 ZieglerNatta catalysts. The same mechanism predicts that re

insertion of the monomer is favored in case of (R,R) chirality of coordination of the C2H4(1-Ind)2 ligand. This is in

agreement with optical activity measurements by Pino197,198 on saturated propylene oligomers obtained with this

MPn

Isolated stereomistake

Stereomistake propagated

Isolated stereomistake

Stereomistake propagated

isospecific

syndiospecific

isospecific

syndiospecific

site control

chain-end control

primaryinsertion

Scheme 7

Table 1 Relationship between the microstructures which result from the stereomistakesshown in Scheme 7

Mechanism Misinsertions (triads) Misinsertions (pentads)

Isospecific chiral-site control [mr] 2[rr] [mmmr] [mmrr] 2[mrrm]Syndiospecific chiral-site control [mr] 2[mm] [rrrm] [mmrr] 2[rmmr]Isospecific chain-end control mr only [mmmr] [mmrm]Syndiospecific chain-end control mr only [rrrm] [rrmr]

Olefin Polymerizations with Group IV Metal Catalysts 1019

kind of catalyst, proving that re insertion of the monomer is indeed favored in case of (R,R) chirality of coordination of

the C2H4(1-Ind)2 ligand. Moreover, deuteration and deuterio-oligomerization studies of -olefins (propylene,

1-pentene, 4-methyl-pentene) using catalysts based on (R,R) C2H4(H4-1-Ind)2 zirconium derivatives showed that the

R enantioface of the olefin is predominantly involved in dimerizations and oligomerizations whereas the S enantioface is

favored in the deuterations.197 These results confirm that the growing chain plays a primary role in enantioface

discrimination.199 Results relative to deuteration and deuterodimerization experiments on isotopically chiral 1-pentene,

as well as on propylene insertion with betaine derivatives of classical metallocenes, also agree with a mechanism

involving a chiral orientation of the growing chain.200,201

In conclusion, in site-controlled stereoselective polymerizations, it is accepted and proved that the site chirality is

unable to select directly between the two enantiofaces of the inserting monomer. Instead, it is accepted and proved

that the site chirality can force a chiral orientation of the growing chain, which in turn is able to select between the

two enantiofaces of the inserting monomer. Thus, the growing chain acts as a messenger to transfer the chiral

information from the catalytic site to the monomer.172

4.09.2.3.5 Symmetry rules for stereocontrolThe key to understanding the variety of different tacticities obtained in -olefin polymerizations is the chain-

migratory mechanism with the site-switching mechanism of Figure 4, and the fact that insertion of the olefin can

actually occur on two different active sites. It is the symmetry relationship between the two situations corresponding

to an exchange of the positions of the growing chain and of the monomer that determines the tacticity of the resulting

poly--olefin. Considering the space around the metal center divided in four quadrants, the steric bulkiness of the

ligands shapes a chiral pocket that can result in asymmetric insertion. The relationship between metallocene

symmetry and polymer stereochemistry has been fully understood. In the quadrants representation, a gray quadrant

means that space in this quadrant is occupied by the ligand, and thus is scarcely accessible to either the monomer or

the growing chain. In this representation, it does not matter if the geometry of coordination around the metal atom is

tetrahedral or octahedral, since space occupation is relevant. The most typical symmetries and the (possibly

expected) microstructures of the resulting poly--olefins are shown in Scheme 8. These rules apply to stereorigid

catalysts operating under site-control mechanism, and for primary monomer insertion.

Representative examples of aspecific C2v-symmetric pre-catalysts are Me2Si(Cp)2MCl2 and Me2Si(9-Flu)2MCl227. The ligands of these two catalysts have very different space occupation, and in the quadrants representation can

be considered to correspond to systems with all white quadrants or all gray quadrants, respectively. Another example

of an aspecific catalyst is based on the Cs-symmetric meso-C2H4(1-Ind)2MCl2 complex 28.202 In the quadrant

representation, this catalyst can be considered a combination of the two C2v-symmetric catalysts just described, and

thus it is characterized by two white quadrants on the side of the Cp rings and by two gray quadrants on the side of the

six-membered rings of 28.

Growingchain

Growingchain

Indenyl

Zr Zr

FluorenylCyclopentadienyl

Transition state for isotactic propagation

re-Propene re-Propene

Indenyl

(a) Transition state for syndiotactic propagation(b)

Figure 10 Transition states for primary insertion of propylene (a) with the isospecific Me2Si(1-Ind)2Zr system and (b) with thesyndiospecific Me2C(Cp)(9-Flu)Zr systems.

1020 Olefin Polymerizations with Group IV Metal Catalysts

ZrCl2Si

27

ZrCl2

28

Examples of isospecific C2-symmetric catalysts are the pseudo-tetrahedral rac-C2H4(1-Ind)2MCl2 complex

296,202,203 and the rac-H2C(3-But-1-Ind)2MCl2 complex 30.

204 In the quadrants representation of Scheme 8,

sterically encumbered gray quadrants correspond to the six-membered rings in 29, and to the But groups in 30.

Another relevant system is the rac-C2H4(3-Me-1-Ind)2MCl2 complex 31, which is substantially aspecific although

it is C2-symmetric.205 The effect of the increase of the bulkiness in different quadrants is best understood

comparing the three C2-symmetric systems of Scheme 9. While the parent bis(indenyl)-based catalyst is iso-

specific, the presence of a methyl group in position 3 of the Cp ring substantially counterbalances the bulkiness

of the six-membered rings. In the quadrant representation, all quadrants are gray, and the catalyst is substantially

aspecific because it is unable to impose a chiral orientation on the growing chain. Further increase of the steric

bulkiness by introduction of But groups in position 3 of the Cp rings again corresponds to an isospecific catalyst,

but the quadrants occupied by the But groups are substantially forbidden to the growing chain (black quadrants).

Theoretical calculations have in fact indicated that the But group is able to induce a stronger steric pressure on

the growing chain relative to the flat indenyl group, and that for the same configuration of the complexes,

catalysts based on 29 and 30 impose opposite chiral orientations to the growing chain, which results in the

insertion of opposite propylene enantiofaces.206,207

C2v-symmetric, achiral

C2-symmetric, chiral

C1-symmetric, chiral

Cs-symmetric, achiral

Cs-symmetric, prochiral

Diastereotopic sites

Enantiotopic sites

X M XX M X

X M X X M X X M X

X M X X M X

Homotopic sites

Homotopic sites

Isotactic polymer

From hemiiso- to isotactic polymer

From hemiiso- to isotactic

Atactic polymer

Syndiotactic polymer

Scheme 8

Olefin Polymerizations with Group IV Metal Catalysts 1021

ZrCl2

29

ZrCl2

30

ZrCl2

31

Returning to Scheme 8, an example of syndiospecific Cs-symmetric catalysts is the Me2C(Cp)(9-Flu)ZrCl2system 32,208 characterized by enantiotopic coordination sites that favor opposite chiral orientations of the

growing chain, which results in the insertion of opposite propylene enantiofaces. It is the (almost) regular chain

migration between the two coordination sites that rationalizes the syndiospecificity of this kind of catalysts.

Finally, C1-symmetric catalysts can range from hemiisospecific, as for the Me2C(3-MeCp)(9-Flu)ZrCl2 system

33,209211 to isospecific as for the Me2C(3-But-Cp)(9-Flu)ZrCl2 structure, 34.

212 The effect of the bulkiness of

the substituents on the parent Me2C(Cp)(9-Flu)ZrCl2 complex is summarized in Scheme 10. Introduction of a

single methyl substituent on position 3 of the Cp ring counterbalances the steric effect of the Flu group. In the

quadrants representation, there are three gray quadrants, and when the growing chain is located in the coordina-

tion position between the Flu and the Me group, insertion is non-stereoselective because the catalyst is unable to

impose a chiral orientation to the growing chain. In the framework of the chain-migratory mechanism, this C1-

symmetric catalyst is hemiisospecific, because there is an (almost) regular alternance between non-selective and

stereoselective propylene insertions. Instead, a single But substituent on position 3 of the Cp ring sterically

dominates the Flu ligand, and the quadrant occupied by the But group is forbidden to the growing chain. Thus, at

each insertion step, the catalyst imposes the same chiral orientation to the growing chain, and the same propylene

enantioface is selected. This results in an isotactic polymer. Detailed theoretical calculations have rationalized

these effects at molecular level.175,213,214

ZrCl2

32

ZrCl2

33

ZrCl2

34

XX M XX

Zr Zr

Isotactic polymerIsotactic polymer Atactic polymer

Zr

ZrCl2 ZrCl2 ZrCl2

M

C2-symmetric, chiral homotopic sites

XX M

Scheme 9

1022 Olefin Polymerizations with Group IV Metal Catalysts

These rules, now referred to as Ewens symmetry rules, are the result of milestone papers by Brintzinger,6

Ewen,181,202,205,208,215,216 Kaminsky,203 and co-workers. Although they have been developed for primary propylene

insertion with pseudo-tetrahedral metallocenes, they can be extended to octahedral catalysts as well. On the other

hand, in the case of secondary monomer insertion with unbridged octahedral systems, the tacticity of the polymer

produced is the result of a delicate balance between the chiralities of the chain end and of the fluxional active species.

This particular case has been discussed in a review.172

4.09.2.4 Mechanism of Regiocontrol and Stereochemistry of Regioirregular Insertions

The origin of regiochemistry in propylene polymerization by group 4 catalysts has been investigated in great detail.

While the preference for primary insertion in metallocenes is mainly steric in nature,163,164 for most post-metallocene

catalysts it is a subtle interplay of steric and electronic effects.165 In the case of the prototype-based catalysts 29 and 32,

the most favored transition states leading to secondary propylene insertion are shown in Figures 11(a) and 11(b),

respectively. These transition states are disfavored with respect to the corresponding transition states for primary

propylene insertion due to some steric interaction between the methyl group of the inserting propylene molecule and

the nearby Cp ring of the metallocene ligand. These steric interactions are at the origin of the preference for primary

propagation with metallocene-based catalysts. Transition states of Figures 11(c) and 11(d) correspond to secondary

insertion of the other propylene enantioface, and are much higher in energy due to strong steric interactions between

the methyl group of the inserting propylene molecule and the closer and bulkier (relative to the Cp ring) six-membered

ring of the metallocene ligand. This implies that insertion of the secondary propylene is enantioselective, and that for

C2-symmetric metallocenes opposite enantiofaces are favored for primary and secondary insertion (compare Figures

10(a) and 11(a)), whereas in the case of CS -symmetric metallocenes the same propylene enantioface is favored in

primary and secondary insertion (compare Figures 10(b) and 11(b)). This fact has been used to develop a kinetic model

that links the regioselectivity of a given metallocene to its stereoselectivity, and that rationalizes the higher regioselec-

tivity usually exhibited by syndiospecific CS -symmetric metallocenes relative to C2-symmetric metallocenes.164

4.09.2.5 Chain-release and Isomerization Reactions

The average degree of polymerization P

n of a polyolefin (its molecular mass), produced under steady-state conditions,

with a non-living process, is determined by the ratio between propagation rates and chain release rates (Equation (1)).

Syndiotactic polymer Hemiisotactic polymer Isotactic polymer

Zr Zr

X XMX XM

C1-symmetric C1-symmetricCs-symmetric

X XM

ZrCl2 ZrCl2 ZrCl2

Zr

Scheme 10

Olefin Polymerizations with Group IV Metal Catalysts 1023

Pn Rp

Rr1

The structure and the mechanisms of formation of end groups have been reviewed.217 The most important chain-

release reactions are -H transfer after primary or secondary insertion (either to the metal or to a coordinated

monomer molecule), -Me transfer, and chain transfer to the aluminum co-catalyst or scavenger, when present.

For a given set of polymerization conditions, the rates of these chain-release reactions are obviously inherent to the

steric and electronic nature of the active species. In other words, the molecular mass, as is the case with stereo-

regularity, strongly depends on the structure of the catalyst. Since propagation and chain-release reaction rates often

have different dependencies on temperature and monomer concentration, the molecular mass of polyolefins is also

strongly affected by the polymerization conditions. This important point is further discussed in Section 4.09.4.2.2.

In cases in which the catalyst produces too high molecular mass polymers for a given process or application,

molecular masses can be controlled more effectively by hydrogenolysis (chain transfer to hydrogen),218221 or, in

specific cases, by chain transfer to ethylene after a primary insertion.222,223 It is worth noting that the activity of most

propylene polymerization catalysts is increased by both hydrogen and ethylene. Thus, designing catalysts able to

produce polyolefins with much higher molecular masses than needed, then requiring hydrogen (or ethylene) for

molecular mass control, provides the additional advantage of increasing the catalyst activity.224

The chain-transfer and -release reactions occurring with Ti-based heterogeneous ZieglerNatta catalysts are discussed

in Section 4.09.3. In the following, the most important chain-release reactions occurring at metallocene and other single-

center group IV catalysts are summarized. Chain transfer to ethylene is also addressed in Sections 4.09.4.1 and 4.09.4.2.

For ethylene polymerization, the picture is fairly simple, including bimolecular -hydride transfer to a coordinated

ethylene monomer225227 and transfer to the aluminum co-catalyst.42,228 Formation of internal unsaturations has

been reported,227,229,230 often connected to the formation of hydrogen (see below). With some catalysts, isomeriza-

tion and formation of ethyl and longer branches have also been observed, as a consequence of this chain-release

reaction. This aspect is described in Section 4.09.4.1.

Growingchain

C2-symmetricisospecific catalyst

Cs-symmetricsyndiospecific catalyst

Growingchain

Growingchain

Growingchain

Stericinteraction

Severe stericinteraction

Severe stericinteraction

Stericinteraction

si-Propylene

si-Propylene

re-Propylene

re-Propylene

(a)

(c) (d)

(b)

Figure 11 Favored transition states for the secondary insertion of propylene with (a) the isospecific Me2Si(1-Ind)2Zr system andwith (b) the syndiospecific Me2C(Cp)(9-Flu)Zr system. High-energy transition states for the secondary insertion of propylene with(c) the isospecific Me2Si(1-Ind)2Zr system and (d) the syndiospecific Me2C(Cp)(9-Flu)Zr system.

1024 Olefin Polymerizations with Group IV Metal Catalysts

The most ubiquitous chain-release reactions occurring in -olefin polymerizations are the unimolecular and

bimolecular -hydride transfers after primary insertion.217,231236 These are shown in Scheme 11.

Unimolecular -H transfer to the metal in propylene polymerization is key to understanding growing-chain-end

isomerization237 and formation of internal vinylidenes.217

Detailed studies have unambiguously shown that in the case of zirconocenes,237246 and other single-center

catalysts,247,248 isotacticity decreases at lower propylene concentrations due to unimolecular primary-growing-

chain-end epimerization, which scrambles the chirality of the last chirotopic methine of the growing chain.

The now-accepted mechanism of epimerization, first proposed by Busico and Cipullo,238 and recently confirmed

by Yoder and Bercaw by means of an elegant double-labeling study,237 involves the reaction product of -H transfer

to the metal, a metal cationpolymeryl olefin complex, and is shown in Scheme 12. Allylic activation on the same

metal cationolefin complex has been proposed to generate internal double bonds in PE229 and vinylidene unsatura-

tions in PP (Scheme 13).249

An important phenomenon occurs in ethylenepropylene co-polymerization, especially in liquid propylene poly-

merization: addition of small amounts (< 30 mol%) of ethylene causes a strong decrease in PP molecular masses. This

LnMH

HCH3H

P+

LnM+ H

P

HLnM

CH3

H CH3H P

+LnM+ P

Unimolecular-H transfer

Bimolecular-H transfer

P

Insertion(initiation)

Insertion(chain growth)

Scheme 11 Primary -H transfer reactions.

BA

+(R,R )Zr(R,R )Zr

H

P1

3+ H

P

1

3

Primaryinsertion

Primaryinsertion

In-plane olefinrotation

Me

P

H

Secondaryinsertion

-H from C(3) Secondaryinsertion

-H from C(1)

PMe

In-plane olefinrotation

H

-H

P

Me

H

-H

Me

P

H3

1

1 3

P

Me3

1

Scheme 12 Epimerization via unimolecular -hydride transferdouble bond in-plane rotationreinsertion.

Olefin Polymerizations with Group IV Metal Catalysts 1025

has been observed with isospecific and aspecific metallocenes,222,223,250253 and has limited the development of

industrial metallocene catalysts for isotactic polypropylene (iPP).

In order to avoid this limitation, new (and more complex) bis-indenyl ligands have been developed, and are

described in Section 4.09.4.2.5.

When the catalyst is not fully regioselective, chain release by a -H transfer after a secondary insertion with

formation of internal double bonds is often observed. This has been reported for ethylene/-olefin co-poly-

mers,229,254 PP,255 and other polyolefins,232 as well as for 1-hexene polymerization with dialkoxide catalysts.256

The reaction is shown in Scheme 14 for the case of propylene, where kinetic studies have shown it to be a bimolecular

process, following the rate law sR-Hsk-H[sZr][m].217,257 [sZr] refers to the concentration of active Zr centersbearing a growing chain having a secondary propylene unit linked to the metal.

Differently, in the case of secondary chain growth with bis(phenoxy-imine)titanium catalysts, Coates and co-

workers reported that chain release occurs exclusively by -H hydride transfer from the terminal methyl. This

generates an allylic end group as shown in Scheme 15,160 and it has been utilized to produce functionalized

syndiotactic propylene oligomers.258

Chain release by -CH3 transfer to the metal is unimolecular (Scheme 16)32,207,259 and obviously limited to the

presence of propylene or other 2-methyl-substituted -olefins, such as isobutene,260 and 2-methyl-1,5-hexadiene.261

No -alkyl transfer has been reported for higher alkyls, except for -trimethylsilyl transfer262 and cases in which a

strained ring is formed.263265 -Methyl transfer is an important and sometimes prevalent cause of molecular mass

depression in the case of propylene polymerization with sterically hindered metallocenes262,266,267 or high polymer-

ization temperatures combined with low propylene concentrations.268 Eisen reported this mechanism to be the

P P

L2Zr+

PP

L2Zr+

L2Zr + P P

L2Zr+H Allylic

activationP

AllylicactivationC3H8

H2

nL2Zr+A

B

Scheme 13 Formation of internal vinylidene unsaturations. A: via bimolecular -H transfer; B: via unimolecular -H transfer.

LnMCH3

HHH

P+ -CH3

transferLnM++P CH3

Scheme 16

LnTi

CH3

Pn+ LnTiPn+ H +-H

transfer

Scheme 15 -H transfer to the metal from terminal methyl of a secondary chain.

HLnZr

CH3

H HH3C P

+ -Htransfer

LnZr++P

Scheme 14 -H transfer to a coordinated propylene monomer after a secondary propylene insertion.

1026 Olefin Polymerizations with Group IV Metal Catalysts

exclusive chain-release reaction with bis(benzamidinate)-based catalysts.247,248 -Methyl transfer has been used for

the production of allyl-terminated propylene oligomers, which are themselves polymerizable monomers and have

been used for the production of long-chain-branched (LCB) polyolefins.269,270

Figure 12 shows the most common terminal unsaturations of iPP, generated by chain-release reactions after

primary insertion. The internal vinylidene, likely produced by allylic activation, is also shown.217,246

Figure 13 shows the terminal unsaturations of iPP, generated by chain-release reactions after a secondary insertion.246,257

In addition to chain transfer to the coordinated monomer, there are three other relevant chain-transfer reactions:

(i) Chain transfer to the aluminum alkyl species (usually the co-catalyst or the AlR3 species used as scavenger of

impurities) or other organometallic species, usually zinc alkyls.271,272 Chain transfer to aluminum usually occurs

4.64.85.05.25.45.65.8 ppm

Figure 13 Olefinic region of a 1H NMR spectrum (120 C, solvent C2D2Cl4 at 5.95 ppm) showing terminal and internalunsaturations in iPP (catalyst rac-C2H4(4,7-Me2Ind)2ZrCl2/MAO).

4.64.85.05.25.45.65.8 ppm

Figure 12 Olefinic region of a 1H NMR spectrum (120 C, solvent C2D2Cl4 at 5.95 ppm) showing terminal and internalunsaturations in iPP (catalyst rac-Me2C(3-Bu

tInd)2ZrCl2/MAO).

Olefin Polymerizations with Group IV Metal Catalysts 1027

with the residual AlMe3 present in MAO, and has been used to prepare hydroxo-terminated PP.273,274 Using

borate activators in combination with higher Al alkyls such as AlEt3 or AlBui3 effectively reduces chain transfer

to Al (Scheme 17).

Transfer to Al was reported to be operative with several non-metallocene catalysts. It is the only chain-release

mechanism operative with the diamido complexes MCl2{ArN(CH2)nNAr} catalysts, as well as with the mono-

and tris(benzamidinate) catalysts, since no olefinic resonances were observed in the 1H or 13C NMR spectra of

these polymers.275,276 This chain-release reaction is also dominant with bis(phenoxyimine)zirconium cat-

alysts,277 as well as with tris(pyrazolyl)borate-based catalysts.278,279

(ii) Chain transfer to hydrogen is the most widely used means of molecular mass control, in both classic Ziegler

Natta catalysis224,280,281 and metallocenes.218221,282 Although molecular masses can be reduced by either

reducing monomer concentration (in propylene and higher olefin polymerizations) or increasing polymerization

temperature, these two experimental parameters can be varied only within a narrow range in a given polymer-

ization process. Hence, a good hydrogen response is a must for an industrial polymerization catalyst.

(iii) Chain transfer to organosilanes, introduced by Marks for lanthanides, corresponds to the silanolytic Mchain

reaction shown in Scheme 18. This chain-transfer process efficiently produces silyl end-capped PEs as well as

ethylene/1-hexene and ethylene/styrene co-polymers.283,284

4.09.2.6 Kinetics

The kinetics of olefin polymerization are the subject of several studies,104,153156,162,182,221,226,240,241,246,252,255,266,285312

and of an excellent book by Keii.17 The most relevant studies will be discussed below. However, we first note

that the precise description of the kinetics of catalytic olefin polymerization under industrially relevant poly-

merization conditions has proved to be very difficult. For a given catalytic system, one has to identify all possible

insertion, chain-release, and chain-isomerization reactions, and their dependence on the polymerization para-

meters (most importantly, temperature and monomer concentration). Once the kinetic laws for each elementary

step have been determined, they have to be combined in one model in order to be able to predict the catalyst

performance. This has been attempted for both ethylene226,285 and propylene polymerizations. The case of

propylene polymerization with a chiral, isospecific zirconocene is shown in Figure 14.162

Both polymerization temperature and monomer concentration usually have a strong influence on catalyst activity

and polyolefin molecular mass. With respect to monomer concentration, the rate of monomer insertion must obey the

simple first-order law

Rp kpCm 2

LnM+Al CH3

R2Al P LnM+ CH3

H+

HO

O2

+

H2O2/NaOH

n n

n

Scheme 17 Chain transfer to aluminum (the case of methylated aluminum species, such as Al(CH3)3, is shown).

LnMPn + R3SiH LnM H + R3SiPn

R = Bun , Bn, Ph

Scheme 18

1028 Olefin Polymerizations with Group IV Metal Catalysts

Here, [C] is the concentration of active catalyst and [m] is the concentration of the monomer. In practice, polymer

production rates have instead been found to deviate substantially from first-order behavior, and follow the non-linear

dependence on monomer concentration

Rp kp9mn 3

for both ethylene286 and propylene,241,246,255,288 with 1< n< 2. This behavior has been rationalized by the coex-

istence of two different active species differing in monomer insertion rates.289 In this model, the two species are in

equilibrium with each other and the position of the equilibrium is determined by the concentration of the monomer,

with each active species obeying the rate law of Equation (2). This results in the law expressed by Equation (4).

Rp k1Cm k2Cm2

k3 k4m 4

Chain-releasereactions

Chain-isomerization Insertionreactions

reactions

Primary growing chain

+

P

epimerization Re = ke[C]

P-HR-H = k-H[C][M]

(R,R )Zr

(R,R )Zr

(R,R )Zr

(R,R )Zr

(R,R )Zr

+

P

1,2 re

2,1 si

1,2 si

+

P

+

P

+

P

P

-CH3

or R-H = k-H[C]

Secondary growing chain

threo (t ) Regioinversion

P

3,1-Insertion

P P

erythro (e) Regioinversion

P

12

3

12

3

1

2

(R,R)Zr+P

1,2 re

1,2 si

2 1

3

3

3,1-isomerization

1,2 re

sRis = skis[sC]

P

-H

sR -H = sk -H[sC][M]

R -Me = k-Me[C]

Figure 14 The most relevant elementary steps observed at the (R,R)-enantiomer of a chiral, C2-symmetric, isospecificzirconium center with a primary growing chain end (top) and a secondary growing chain end (bottom). The (S,S)-enantiomerproduces the opposite stereochemistry of each single event, but overall the same polymer chains and the same insertionmistakes. In practice, in the case of C2-symmetric metallocenes, the racemic mixture (R,RS,S) is always used. P growingpolypropylene chain; [C] concentration of active primary centers; [sC] concentration of active secondary centers.

Olefin Polymerizations with Group IV Metal Catalysts 1029

Propagation rates of first order in monomer concentration have been reported for ethylene226,287 and for propylene

in the case of aspecific metallocenes266 as well as for propylene polymerization with isospecific metallocenes

activated with MAO, B(C6F5)3, and [Ph3C][B(C6F5)4].290,297 Moreover, first-order kinetics were also observed for

1-hexene polymerization with the [rac-C2H4(1-Ind)2ZrMe][MeB(C6F5)3].156

Higher orders affect the laws determining the degree of polymerization as a function of propylene concentration.

The dependence of molecular mass on propylene concentration is given by

Pn am bm

2

c dm 5

If the propagation rate is first order in propylene concentration, then Equation (5) reduces to Equation (6), which

can be linearized (Equation (7)).266

Pn kpm

kt0 kt1 m6

1

Pn

kt1kp

kt0kpm 7

Models able to rationalize and predict catalyst performances for propylene polymerization with C1- and

Cs-symmetric metallocenes such as Me2C(Cp)(1-Ind)ZrMe2 and Me2C(Cp)(9-Flu)ZrCl2 have been proposed.291,292

A kinetic model has been proposed based on microstructural analysis, including both chain-epimerization and site-

epimerization reactions in both C2- and Cs-symmetric metallocenes, and rationalizing the observed pseudo-second-

order kinetics of propylene polymerization promoted by C2-symmetric metallocene catalysts.182,293 This point has

been extended to co-polymers.298 A thorough study of propylene polymerization with the Me2C(Cp)(9-Flu)ZrCl2system in the presence of a large series of different counterions that rationalized the correlation between the nature of

ion pair and the microstructure of the resulting PPs has been performed.104

As regards a comparison between initiation (i.e., insertion of the monomer into the MCH3 bond) and propagation

(i.e., insertion of 1-hexene into the Mchain bond), in the case of 1-hexene polymerization with the [rac-C2H4(1-

Ind)2ZrMe][MeB(C6F5)3] catalyst, initiation is about 70 times slower than propagation.156 An even more pronounced

effect was found in the polymerization of propylene with the rac-Me2Si(1-Ind)2 zirconocene; in the presence of MAO,

initiation is about 800 times slower than propagation, and this value increases to 6000 when polymerizing in the

presence of the [B(C6F5)4] counterion.290

The amount of metal centers actually active during polymerization has been investigated using quenched-flow

kinetic techniques. The concentration of active sites in MAO-activated systems is significantly lower (about 10%)

than the analytical concentration of the metal, and a large fraction of metal species are in a dormant state.290,297 In

the case of propylene polymerization, it was suggested that the resting state after secondary propylene insertion was a

major contributor to the dormant-state concentration, since insertion of a new propylene molecule into the M

(secondary alkyl) is greatly slowed by steric effects.240,290,294,295,299301 However, any other situation that slows chain

propagation may contribute to the dormant state. These conclusions have been extended also to [Ph3C][B(C6F5)4]-

activated systems, and the higher activity exhibited by borate-activated systems relative to the MAO-activated

systems has been ascribed to the weaker coordination ability of borate relative to MAO, rather than to the differences

in the active site concentrations.290 Finally, the mole fraction of active species is about 80% when the

[Ph3C][B(C6F5)4]-activated bis(phenoxyamine)Zr catalyst is considered.296

Using deuterium-labeling experiments, about 100% of the metal was shown to be active in 1-hexene polymeriza-

tions with the [rac-C2H4(1-Ind)2ZrMe][MeB(C6F5)3] catalyst,922 and the reactivity of M(secondary alkyl) bonds at

80 C was comparable to that of primary alkyl metallocenes.302 These relative monomer insertion rates appearstrongly ligand specific. However, when these comparisons are made, it must be borne in mind that different authors

use very different catalysts, as well as different definitions of the term active center: Landis defined the active

species as the product of the first monomer insertion, whereas Bochmann290 and Busico297 used the term to describe

the proportion of total [Zr] actively engaged in chain growth based on kinetic measurements. Similarly important is

the fact that very different activities and polymerization mechanisms are operative with the weakly coordinating

1030 Olefin Polymerizations with Group IV Metal Catalysts

MAO and [B(C6F5)4] counterions on one hand,290,297 and the tighly bound [MeB(C6F5)3]

counterion on theother.101,156,302

4.09.3 ZieglerNatta Polymerizations with Heterogeneous Catalysts

4.09.3.1 Catalyst Structure and Characterization

As indicated above, ZieglerNatta catalysts occupy a dominant position in polyolefins manufacture, and have

evolved from first- and second-generation titanium trichloride catalysts, developed up until the 1970s313315 and

described in COMC (1982),316 to the high-activity MgCl2-supported catalysts used in modern industrial processes.317

PP catalysts, often termed third-, fourth-, and fifth-generation, according to the catalyst performance and catalyst

composition, comprise magnesium chloride as support material, a titanium component (usually TiCl4), and an

electron donor (Lewis base). The basis for the development of the high-activity supported catalysts lay in the

discovery, in the late 1960s, of activated MgCl2 able to support TiCl4 and give high catalyst activity, and the

subsequent discovery, in the mid-1970s, of electron donors (Lewis bases) capable of increasing the stereospecificity

of the catalyst so that (highly) isotactic PP could be obtained.318321 A further feature which contributed greatly to the

commercial success of MgCl2-supported catalysts was the development of spherical catalysts with controlled particle

size and porosity.317

High-activity ZieglerNatta catalysts comprising MgCl2, TiCl4, and an internal electron donor are typically

used in combination with an aluminum alkyl co-catalyst such as AlEt3 and an external electron donor added in

polymerization. The third-generation catalyst systems contained ethyl benzoate as internal donor and a second

aromatic ester as external donor, whereas the now widely used fourth-generation catalysts contain a diester (e.g.,

diisobutyl phthalate) as internal donor and are used in combination with an alkoxysilane external donor of type

RR1Si(OMe)2 or RSi(OMe)3. The most effective alkoxysilane donors for high stereospecificity are methoxysilanes

containing relatively bulky groups to the silicon atom.322325 In the early stages of MgCl2-supported catalyst

development, activated magnesium chloride was prepared by ball milling in the presence of ethyl benzoate,

leading to the formation of very small ( 3 nm thick) primary crystallites within each particle.315 High resolutiontransmission electron microscopy has shown that the MgCl2 crystal structure is severely distorted by ball milling,

changing the structure from crystalline to amorphous.326 Nowadays, the activated support is prepared by chemical

means, such as via complex formation of MgCl2 and an alcohol, or by reaction of a magnesium alkyl or alkoxide

with a chlorinating agent or TiCl4. A chemical rather than a physical route to catalyst preparation can also lead to

a more uniform titanium distribution.327 Many of these approaches are also effective for the preparation of

catalysts having controlled particle size and morphology. For example, the cooling of emulsions of molten

MgCl2?nEtOH in paraffin oil gives almost perfectly spherical supports, which are then converted into the

catalysts.317 Characterization of a number of adducts of magnesium chloride and ethanol has been described by

Bart and Roovers328, while Sozzani et al.329 have recently reported the use of advanced solid-state NMR

techniques to determine the various components present in MgCl2?nEtOH adducts. It was found that n values

of 1.5 and 2.8 represented stable and well-defined complexes.

A typical catalyst preparation involves reaction of the MgCl2?nEtOH support with excess TiCl4 in the presence of

an internal electron donor. Temperatures of at least 80 C and at least two TiCl4 treatment steps are normally used,in order to obtain high-performance catalysts in which the titanium is mainly present as TiCl4 rather than the

TiCl3OEt generated in the initial reaction with the support.

The function of the internal donor in MgCl2-supported catalysts is twofold. One function is to stabilize small

primary crystallites of magnesium chloride; the other is to control the amount and distribution of TiCl4 in the final

catalyst, affecting stereoselectivity. Activated magnesium chloride has a disordered structure comprising very small

lamellae. X-ray diffraction studies have revealed rotational disorder in the stacking of the ClMgCl triple

layers.330,331 Small MgCl2 crystallite size and large rotational disorder appear to give high catalyst activity.332

Giannini319 has indicated that, on preferential lateral cleavage surfaces, the magnesium atoms are coordinated with

four or five chlorine atoms, as opposed to six chlorine atoms in the bulk of the crystal. These lateral cuts correspond to

the (110) and (100) faces of MgCl2, as shown in Figure 15. It has been proposed that bridged, binuclear Ti2Cl8 species

can coordinate to the (100) face of MgCl2 and give rise to the formation of chiral, isospecific active species (Figure 16),

it being pointed out that Ti2Cl6 species formed by reduction on contact with AlEt3 would resemble analogous species

in TiCl3 catalysts.333,334 An extended X-ray absorption fine structure (EXAFS) investigation of a MgCl2/TiCl4

catalyst has indicated the presence of dimeric TiCl4 complexes on the (100) face of MgCl2.335 It has been

Olefin Polymerizations with Group IV Metal Catalysts 1031

suggested317 that a possible function of the internal donor is preferential coordination on the more acidic (110) face of

MgCl2, such that this face is prevailingly occupied by donor and the (100) face is prevailingly occupied by Ti2Cl8dimers. However, as outlined in Section 4.09.3.3.3, this is by no means the only likely mo