Alkene Polymerization Reactions with Transition Metal Catalysts

593 Pages·2007·5.4 MB·English

by  Yury V. Kissin (Eds.)

To Anna, James, Samuel, and Gabriel
P
REFACE
THAT SHOULD BE READ
During the past 30 years, the field of alkene polymerization with transition-
metal catalysts has undergone three major changes:
1. Themostvisiblechange,ofcourse,wasthevastexpansionofresearchdevotedto
metalloceneandothersolubletransition-metalcatalysts.ThediscoveryofMAO-
cocatalyzed metallocene systems for alkene polymerization in 1975 and the
subsequent establishment of the metallocenium mechanism of these reactions
transformed the field of metallocene polymerization catalysis from an obscure
academic research area into a vigorously pursued subject, both in academia and
in industry. Currently, the focus of this research has shifted further, toward the
studies of numerous multidentate complexes of early and late transition metals.
2. Most of the earlier researchers, starting with Natta himself, realized that solid
polymerization catalysts have active centers of different types in terms of their
isospecificity.However,relativelyrecentdevelopmentsinseveralanalyticalfields,
in particular, gel permeation chromatography, analytical temperature-rising
fractionation, and crystallization fractionation, provided the first reliable
information about differences between various active centers in terms of the
molecular weights of alkene polymers they produce, the stereoregularity of
homopolymers, compositions of copolymers, etc. These developments brought
one unexpected setback. They showed that various types of active centers are
formedanddecayatsignificantlydifferentrates.Thesefindingsmadealargepart
of earlier kinetic studies of solid and supported Ziegler-Natta catalysts (the
studies that essentially pre-supposed that the catalysts have one type of active
center) suspect or obsolete.
3. The rapid development of high-resolution 13C NMR spectroscopy resulted in a
greatlyexpandedunderstandingofchemicalandstericfeaturesofpolyolefinsand
alkene copolymers, such as the detailed structures of chemical and steric defects
in polymer chains, the structure of chain ends, etc. The NMR data were
complemented by GC studies of alkene oligomers, the fractions of polymers
withthelowestmolecularweights.Theseoligomerstudiesgavefor thefirsttime
an opportunity to examine each polymer molecule (albeit only a very short
polymer molecule),ineverydetail,includingthestructureofbothitschainends,
thechemicalandthestericstructureofthechain,etc.Themainoutcomeofthis
researchwasrealizationthatdeviationsfromregularchaininitiation,growth,and
transfer patterns, although often minor, can profoundly affect polymerization
kinetics and polymer properties.
These three developments call for a completely new review of all aspects of
alkene polymerization reactions over transition-metal catalysts, both solid and
soluble. Such a review should also be structured in a radically different way. In the
vii
viii Preface
traditional approach, the expositionusually startswith thepreparation methods and
compositions of solid catalysts or transition-metal complexes, then moves on to
polymerization reactions of particular alkenes (first, ethylene; then propylene, etc.),
and, finally, proceeds to the description of the polymerization kinetics,
stereoregularity, etc. This review structure has lost its rationale due to a much
better understanding of the true nature of active centers in the catalysts. Another
complication arises from the sheer volume of information. The number of
publications describing various aspects of catalytic alkene polymerization reactions
adds up to several thousands. In addition to research articles, the literature includes
several books and compilations of symposium proceedings [1-22].
Therefore, a new structure is adopted in this book. Chapter 1 is introductory;
it describes, in condensed form, the basics of polymerization reactions with
transition-metal catalysts: the types of catalysts and cocatalysts, the phenomenon
of stereoregularity, copolymerization reactions, etc. It also contains technical
information about such aspects of alkene polymerization reactions as compositions
ofthemostpopularcatalystsystems,structuresofmetallocenecomplexes,reactivity
of various alkenes, the helix types of isotactic and syndiotactic polyolefins, etc.
All researchers who are new the field of Ziegler-Natta catalysis should read this
chapter, while specialists may easily skip it.
Chapter 2 addresses one of the principal issues of alkene polymerization
catalysis: the existence of catalyst systems with only one type of active center
(mostly, hydrocarbon-soluble catalysts) and catalyst systems with several types of
active centers (all solid and supported catalysts). This chapter briefly describes the
modern analytical techniques of polymer characterization that revolutionized our
understanding of polymerization catalysis. It also presents both the experimental
manifestations of the single- and multi-center catalysis and their implications
for the studies of polymerization kinetics, polymer stereoregularity, copolymer
composition distribution, etc.
Chapter 3 is devoted to the chemistry and stereochemistry of polymerization
reactions. This field of research greatly benefited from the introduction of high-
resolution 13C NMR spectroscopy. Twenty five years ago such areas of research as
the chemistryand stereochemistryof chain initiation, growth and transfer reactions
were in a rudimentary state, but at the present time they provide a very detailed
picture of all aspects of polymerization reactions.
Chapter 4 describes three subjects: (a) the formal chemical composition of
various commercial polymerization catalysts(mostlybasedon thepatent literature),
(b) the true chemical composition of the catalysts (mostly based on spectroscopic
data), and, (c) the basic chemistry of the reactions between the catalysts and
cocatalysts.
Chapter 5 describes the polymerization kinetics. As far as our understanding
of alkene polymerization reactions with solid and supported Ziegler-Natta is
concerned, this research lost some of its value over the last two decades. The main
reason for this has been the development of modern analytical techniques for
polymercharacterization.Theapplicationofthesetechniqueshasshownthatallthe
solidandsupportedpolymerizationcatalysts(aswellasanumberofsolublecatalysts)
have several types of active centers which significantly differ in their kinetic and
Preface ix
stereochemical parameters. Therefore, the field of polymerization kinetics has
shifted its focus from attempts at describing reaction kinetics in terms of simple
kinetic schemes borrowed from the fields of radical and anionic polymerization
reactions to less over-reaching but more realistic tasks. They include the effects of
the catalyst type and the reaction parameters on the distribution of active centers,
main kinetic features of the most abundant active centers, etc.
Chapter 6 is devoted to the mechanistic aspects of polymerization reactions,
including relevant experimental data and theoretical analysis.
The texts of all the chapters contain numerous cross-references so that the
informationaboutparticularcatalystsystemsandreactionsthatisspreadoverseveral
chapters can be collected and examined separately.
Yury V. Kissin
Rutgers – The State University of New Jersey
USA
A D
BBREVIATIONS AND EFINITIONS
Alkyl groups: CH ¼ Me; C H ¼ Et; n-C H ¼ n-Pr; iso-C H ¼ i-Pr; n-
3 2 5 3 7 3 7
C H ¼ n-Bu; iso-C H ¼ i-Bu; tert-C H ¼ t-Bu; n-C H ¼ n-Hex; n-
4 9 4 9 4 9 6 13
C H ¼ n-Oct; Cyclopentyl ¼ Cpy; Cyclohexyl ¼ Cy.
8 17
Aryl groups: Phenyl (C H ) ¼ Ph; Benzyl (C H (cid:1)CH (cid:1)) ¼ Bz.
6 5 6 5 2
Z5 Ligands in metallocene complexes: Cyclopentadienyl (Z5-C H ) ¼ Cp; Penta-
5 5
methyl cyclopentadienyl (Z5-C Me ) ¼ Cp(cid:1); Indenyl (Z5-9-C H C H ) ¼ Ind;
5 5 6 4 5 3
1,2,3,4-Tetrahydroindenyl (Z5-9-C H C H ) ¼ Ind-H ; Z5-9-Fluorenyl ¼ Flu;
6 8 5 3 4
Z5-1,2,3,4-Tetrahydro-9-fluorenyl ¼ Flu-H ; Z5-Octahydro-fluorenyl ¼ Flu-H ;
4 8
Z5-Benz[e]indenyl ¼ Benz[e]Ind.
Monodentate and bidentate ligands: Tetrahydrofuran ¼ THF; Pyridine ¼ Py, 2,4-
pentanedionato ligand (derived from acetyl acetone) ¼ acac.
Bridge atoms or groups between two metal atoms: (m-Cl), (m-OR), (m-Me), etc. For
example, the dimer of AlEt ¼ Et Al(m-Et) AlEt .
3 2 2 2
Silicon compounds: The nomenclature of silicon compounds in the literature on
Ziegler–Natta catalysts is different from the common nomenclature: the symbols of
alkyl/aryl substituents are placed before the Si symbol. For example, diphenyldi-
methoxy silane is depicted as Ph Si(OMe) rather than SiPh (OMe) .
2 2 2 2
Organic compounds added to components of heterogeneous catalyst systems: Modern
supportedcatalystsystemscontain,inadditiontoTi(orV)andMgspecies,twogroups
of organic compounds that are added to catalyst components with the main goal of
improving the yield of the crystalline fraction of the polymers. In the literature, these
organic compounds are usually called ‘‘electron donors,’’they supposedly modify the
structure of solid catalysts and cocatalysts. Two types of donor compounds are often
used. The chemical compound added to a solid catalyst component is named ‘‘an
internaldonor’’andachemicalcompoundaddedtoacocatalystisnamed‘‘anexternal
donor.’’Numerousexperimentsshowthatwhencomponentsofthesecatalystsystems
areproduced,byreactingthesolidcatalystandthecocatalysts,thesedonorcompounds
areusuallyconvertedintocompletelydifferentcompounds,andthelattercompounds,
inturn,playdifferentrolesinthecatalysts,whichonlyrarelydependontheirelectron–
donating properties. For this reason, the organic ‘‘donor’’ compounds are called here
organic modifiers:
An organic compound used in the preparation of a solid catalyst component,
‘‘internal electron donor’’ ¼ Modifier I.
Anorganiccompoundaddedtoacocatalyst,‘‘externalelectrondonor’’¼ ModifierII.
Active centers: For uniformity, the same symbols for active centers are used
throughout the book:
Active centers in heterogeneous catalysts: WTi–R, WV–R, WCr–R, etc.
Active centers in metallocene catalysts: Cp Ti+–R, Cp Zr+–R, Cp Hf+–R, etc.
2 2 2
Active centers in non-metallocene homogeneous catalysts: (L)Ti–R, (L)Ni–R,
(L)Fe–R, etc.
xi
CHAPTER 1
The Beginner’s Course: General
Description of Transition Metal Catalysts
and Catalytic Polymerization Reactions
Contents
1.1. ClassificationsofTransition MetalCatalysts 2
1.1.1. Components oftransition metal catalysts 3
1.1.2. Catalyst classificationbased on solubility 4
1.2. Composition andStructure ofZiegler–Natta Catalysts 6
1.2.1. Organoaluminum cocatalysts 6
1.2.2. Transition metal catalyst componentsof Ziegler–Natta catalysts 6
1.2.3. Examplesof Ziegler–Natta catalysts 7
1.3. Metallocene Catalysts 11
1.4. HomogeneousCatalystsContainingNon-MetalloceneComplexesofEarly-and
Late-Period Transition Metals 14
1.5. ChromiumOxideCatalysts 15
1.6. Main Features of Alkene PolymerizationReactions 17
1.6.1. Basicprinciples of polymerization kinetics 18
1.6.2. Copolymerization reactions ofalkenes 20
1.6.3. Auto-copolymerization reactions and formation of polymer chains with
long-chainbranches 21
1.6.4. Oligomerization reactions 22
1.6.5. Stereospecific alkene polymerizationand stereoregular polyolefins 22
1.6.6. Nonuniformity of activecenters intransition metal catalysts 27
1.7. Classes ofPolymers Producedwith TransitionMetal Catalysts 28
1.7.1. Linearpolyethylene and semi-crystallineethylene copolymers 29
1.7.2. Ethylene/propylene elastomers 31
1.7.3. Poly(olefins) 32
Themainpurposeofthisintroductorychapter istoprovideaconcisedescriptionof
a large family of transition metal catalysts used for the polymerization of various
alkenes.Thechapter isintendedfor scientistsandengineerswhoarejustbeginning
their work in the field of alkene polymerization catalysis.
Two sets of definitions related to alkene polymerization reactions are used
interchangeably in academia and in industry:
(cid:1) The historic name for alkenes, olefins, is nearly universally used in industry
whereas researchers in academia usually prefer the chemically correct term,
1
2 AlkenePolymerizationReactionswithTransitionMetalCatalysts
alkenes.Inthisbook,theterm‘‘alkene’’isusedtodefineahydrocarbonwithone
doublebond.Thedoublebondisnearlyalwaysthevinylbond,CH QCH–,and
2
the alkene is denoted as CHQCH–R, where RQH for ethylene, CH for
2 3
propylene, cyclo-C H for vinylcyclohexane, etc. Three other classes of unsatu-
6 11
rated hydrocarbons are mentioned in the book, (a) dienes, both conjugated,
such as butadiene and isoprene, and non-conjugated, such as 1,4-pentadiene,
1,9-decadiene, etc., (b) cycloalkenes such as cyclopentene and norbornene, and
(c) styrenes.
(cid:1) Alkene polymers are nearly always called polyolefins. Terms ‘‘polymers’’
or ‘‘copolymers’’ are used for the description of these materials in academia
(e.g., propylene polymers, ethylene/1-alkene copolymers) whereas the same
materials arecalled ‘‘resins’’ in industry: polypropylene resins, LLDPE resins, etc.
1.1. Classifications of Transition Metal Catalysts
Inthepast,theterm‘‘Ziegler–Nattacatalysts’’wasusedasagenericexpression
thatdescribesavarietyofcatalystsbasedontransitionmetalcompoundsandcapable
of polymerizing and copolymerizing alkenes and dienes. The products of these
polymerization reactions, poly(alkenes), alkene copolymers, poly(dienes), and
poly(cycloalkenes),aremanufacturedcommerciallyinaverylargevolumeandhave
numerous applications as general-purpose and engineering plastics, elastomers, and
synthetic rubbers. Polymers produced with Ziegler–Natta catalysts include many
widely known commercial materials: high-density polyethylene; linear low-density
polyethylene; ethylene-based plastomers; crystalline isotactic polyolefins such as
polypropylene, poly(1-butene), and poly(4-methyl-1-pentene); crystalline syndio-
tactic polypropylene and polystyrene; ethylene–propylene elastomers; ethylene–
cycloalkene engineering plastics, and synthetic rubbers based on polybutadiene and
polyisoprene.
However,thedevelopmentofnumerousnewcatalystsforalkenepolymerization
in the last 20 years called for separation of all transition metal-based polymerization
catalysts into several groups. The following general terminology is commonly
adopted.
The first group, which includes mostly titanium and vanadium-based catalysts,
hasretainedthename‘‘Ziegler–Nattacatalysts.’’ThesecatalystsarenamedafterKarl
Ziegler (Germany) and Giulio Natta (Italy). In the early 1950s, these chemists
discovered the first catalytically active compositions for alkene polymerization,
determined principles of their action, and investigated the structures and properties
of polymers produced with the catalysts [1,23,24]. The monumental contributions
of Ziegler and Natta received universal recognition and these scientistswere jointly
awarded the Nobel Prize in chemistry in 1963. Ziegler–Natta catalysts have been
used in the commercial manufacture of various polymeric materials since 1956.
Today, the total volume of plastics, elastomers, and rubbers produced from alkenes
with these catalysts worldwide exceeds 100 million metric tons. Together, these
TransitionMetalCatalystsandCatalyticPolymerizationReactions 3
polymers represent the largest-volume commodity plastics as well as the largest-
volume commodity chemicals in the world.
The members of the second catalyst group are commonly called ‘‘metallocene
polymerization catalysts.’’ D. Breslow (USA) and G. Natta discovered first metall-
ocene catalysts for alkene polymerization soon after the original discovery of the
Ziegler–Natta catalysts [25,26]. The early metallocene catalysts had relatively low
activity and were regarded as most suitable for academic research. However,
German scientists W. Kaminsky and H. Sinn in 1976 discovered a new class of
metallocene catalyst systems that exhibit extremely high activity [27–29]. Nowa-
days,twotypesofmetallocenecomplexesarewidelyusedascomponentsofcatalyst
systems (see Scheme 1.1). The first type of the metallocene complex contains two
cyclopentadienyl rings attached to a transition metal atom (usually Zr, Ti, or Hf)
and the second type contains one cyclopentadienyl ring. Both types of metallocene
complexes were the subjects of an enormous volume of research, both in academia
and in industry. These catalysts and their subsequent modifications presently
compete with Ziegler–Natta catalysts for many applications.
Thethirdgroupincludespolymerizationcatalystsbasedonhydrocarbon-soluble
non-metallocene transition metal complexes. M. Brookhart (USA) in 1995
discovered the first catalysts of this type [30]. In the past several years this field
underwent a rapid development and now encompasses well-defined complexes
of many early-period and late-period transition metals in the Periodic Table
(Schemes 1.2 and 1.3). Some of these catalysts are relatively stable in a polar
environment.Theyalsoprovidethebestroutetothesynthesisofalkenecopolymers
with polar vinyl compounds.
The fourth group constitutes chromium-based catalysts. Historically, chromium
oxide catalysts were the first transition metal catalysts used for alkene polymeriza-
tion;J.P.HoganandR.L.Banks(USA)discoveredthemintheearly1950s[31,32].
PhillipsPetroleum Companyextensivelyused these catalystsfor the polymerization
of ethylene to high molecular, highly crystalline ethylene homopolymers. Later,
researchers at Phillips Petroleum Company have found that the same type of
catalyst, after modification, is suitable for the polymerization of other alkenes and
for alkene copolymerization reactions [33].
1.1.1. Components of transition metal catalysts
The majority of transition metal catalyst systems, except for chromium oxide
catalysts, consist of two components. The first component is a derivative of a
transition metal, such as titanium, vanadium, zirconium, nickel, palladium, iron,
cobalt, etc. For example, typical transition metal compounds that were used in the
earlyZiegler–Nattacatalystsandwhicharestilluniversallyusedforthemanufacture
of modern catalysts are TiCl , TiCl , VCl , and VOCl ; the majority of
4 3 4 3
metallocene catalysts are based on complexes of zirconium and titanium, etc.
Thesecondcomponentsofthecatalyst systems,whicharecalledcocatalysts,are
organometallic compounds, mostly organoaluminum compounds. Typical orga-
noaluminum cocatalysts are Al(CH ) , Al(C H ) , Al(i-C H ) , Al(C H ) Cl,
3 3 2 5 3 4 9 3 2 5 2
Al(i-C H ) Cl, Al (C H ) Cl , etc.; and methylalumoxane, [Al(CH )O] . Some
4 9 2 2 2 5 3 3 3 n
4 AlkenePolymerizationReactionswithTransitionMetalCatalysts
metallocene and late-period transition metal catalysts use second components of a
completely different type, perfluorinated boronaromatic compounds. Although
these catalyst components areemployed for different reasons than organoaluminum
cocatalysts, they are also usually called ‘‘cocatalysts.’’
Neitherofthesetwocatalystcomponents,ifusedalone,canpolymerizealkenes.
However, when the two components of the catalyst systems are mixed, a series of
chemical reactions takes place and some of the products of these reactions, called
active centers, readily polymerize alkenes and dienes. Although transition metal
catalysts have been known for more than 50 years, the exact chemical structure of
active centers in Ziegler–Natta and chromium oxide catalysts is still unknown. On
theotherhand,amuchhigher levelofunderstandingofthetruestructureof active
centers was achieved for metallocene and non-metallocene soluble catalysts.
Chapter 6 discusses possible structures of the true catalytic species, mostly based on
detailed spectroscopic studies and kinetic analysis.
Inadditiontothetwoprincipalcatalystcomponents,manymoderncommercial
transition metal catalysts also contain three other components: supports, inert
carriers, and modifiers. Catalyst supports, although inactive by themselves, have a
significant influence on the catalyst performance; they increase catalyst activity or
change properties of polymers produced with the catalysts. In terms of their effect
on active centers, supports can be viewed as ligands in homogeneous catalysis. The
mostwidelyusedsupportsincludeMgCl andsilica[4,6,34,35].Incontrast,carriers
2
do not affect catalyst performance to any noticeable degree and their use is
warranted by various technological requirements. For instance, carriers dilute very
active solid catalysts, make them more easily transportable, and agglomerate catalyst
species into particles of a specific desirable shape, usually spheres. Often, the same
material, e.g., MgCl and silica, serves both purposes; it acts as a true support and,
2
simultaneously, forms catalyst particles of a required shape. Soluble metallocene
and non-metallocene transition metal catalysts are also often produced in the
supported form or packaged inside carrier particles.
In addition, nearly all modern titanium- and vanadium-based Ziegler–Natta
catalystsystemsincludeoneorseveralorganiccomponentswhichare,intermittingly,
caller catalyst modifiers, catalyst activators, external or internal organic donors, etc.
Their true role in the catalysis is discussed in detail in Chapter 4.
1.1.2. Catalyst classification based on solubility
Until the 1970s, nearly all polymerization reactions with transition metal catalysts
werecarriedoutininerthydrocarbonsolventssuchashexane,heptane,or toluene.
These solvents readily dissolve all alkenes and all above-listed organoaluminum
cocatalysts. Historically, the catalyst systems were classified based on solubility of
their transition metal components in a polymerization medium. This classification,
although initially arbitrary, turned out to be sufficiently meaningful in practice and
is still widely used (see examples in Table 1.1).
Homogeneous transition metal catalysts: The catalyst systems in which both the
starting transition metal compounds and the products of their interaction with
organometallic cocatalysts, including active centers of polymerization reactions, are
TransitionMetalCatalystsandCatalyticPolymerizationReactions 5
Table1.1 Classification of Ziegler–Nattacatalysts
Catalystsforethylenepolymerization Catalystsforpolymerizationandcopolymerization
of1-alkenes
Homogeneous catalyst systems
V(acetylacetonate) -AlEt Cl (at low VCl -AlEt Cl
3 2 4 2
temperatures) VOCl -Al Et Cl
3 2 3 3
VCl -AlEt Cl
4 2
Pseudo-homogeneous catalyst systems
TiCl -AlEt Cl TiCl -AlEt
4 2 4 3
VCl -AlEt Cl VOCl -AlEt
4 2 3 3
VOCl -Al Et Cl
3 2 3 3
Heterogeneous catalyst systems
d-TiCl -AlEt a-TiCl -AlEt
3 3 3 3
TiCl /silica-AlEt d-TiCl -AlEt
4 3 3 3
TiCl /MgCl /silica-AlEt VCl -Ali-Bu
4 2 3 3 3
VOCl /MgCl /silica-AlEt TiCl /MgCl /modifier – AlEt /modifier
3 2 3 4 2 3
soluble in the reaction medium. Table 1.1 lists some homogeneous Ziegler–Natta
catalysts that found wide laboratory applications. Several catalysts of this type
containing vanadium compounds, VCl and VOCl , have industrial significance in
4 3
the manufacture of elastomeric ethylene–propylene copolymers and cross-linkable
ethylene-propylene–diene terpolymers. All unsupported catalyst systems based on
metallocene complexes (Section 1.3) and soluble non-metallocene transition metal
complexes (Section 1.4) also belong to the group of homogeneous polymerization
catalysts,althoughinthiscasetheimpliedsolventisnotaliphaticbutaromatic,usually
toluene.
Pseudo-homogeneous transition metal catalysts: The group of catalyst systems in
which the starting transition metal compound is also soluble in the hydrocarbons.
However, when its solution is combined with solution of an organoaluminum
cocatalyst, a rapid reaction ensues with the formation of solid products (Table 1.1).
The most important example of such a transition metal compound is TiCl . The
4
first industrial catalysts for ethylene polymerization discovered by Ziegler in 1953,
TiCl -Al(C H ) Cl and TiCl -Al(C H ) systems, belong to this class [23].
4 2 5 2 4 2 5 3
A number of pseudo-homogeneous titanium-based Ziegler–Natta systems are still
important in commercial polymerization reactions of ethylene and its copolymer-
ization with alkenes.
Heterogeneous transition metal catalysts: Catalyst systems in which both the starting
catalyst containing a transition metal component and the products of its interaction
withanorganometalliccocatalystareinsolubleinthepolymerizationmedium.This
largeclassincludesthecatalystscurrentlywidelyusedinindustryfor polymerization
and copolymerization of alkenes. All supported Ziegler–Natta catalysts, supported
metallocene and non-metallocene transition metal catalysts, as well as chromium
oxide catalysts belong to this class.