PREPARATION OF HIGHER ALPHA OLEFIN OLIGOMERS
by
CHANTELLE CRAUSE
Dissertation
submitted in fulfilment of the requirements
for the degree
MASTER OF SCIENCE
in
CHEMISTRY
in the
FACULTY OF SCIENCE
at the
RAND AFRIKAANS UNIVERSITY
Supervisor: PROF. F.R. VAN HEERDEN
DECEMBER 1996 TABLE OF CONTENTS
Synopsis iv Ops omming vi List of Schemes viii List of Figures List of Tables r xi Abbreviations xii
Chapter 1 The oligomerization of higher alpha olefins : a literature study
1.1 Alpha olefin beneficiation: the key to value-adding and economic growth 1 1.1.1 The South African perspective 1 1.1.2 Focus on alpha olefins 2 1.1.3 Alpha olefin oligomers as base stocks for synthetic lubricants 3 1.1.4 Use and functionalization of poly(alpha olefins) 4 1.2 General principles of oligomerization 5 1.3 Olefin oligomerization catalysts 6 1.3.1 Friedel-Crafts catalysts 6 Polyalphaolefins 6 Cationic acid catalysed oligomerization 7 BF3-catalysed oligomerization 8 Mechanistic considerations 10 Correlation of paraffin structure with physical properties 16 Reaction variables 18 1.3.2 Free radical catalysts 19 1.3.3 Transition metal catalysts 20 Heterogeneous Ziegler-Natta catalysts 23 Homogeneous Ziegler-Natta catalysts: Metallocenes 26 (a) Nature and formation of the active species 27 11
Kinetics and mechanism of alpha olefin polymerization 29 Stereoregular polymers from chiral metallocene catalysts 32 Reaction control and product distribution 35 Future developments 36 (iii) Other important transition metal catalysts 36 Supported reduced transition metal oxide catalysts 36 Homogeneous nickel catalysts 37 1.4 Conclusion 38
Chapter 2 Discussion
2.1 Aim of research 39 2.2 Synthesis of model oligomers 40 2.2.1 Synthetic target and strategy 41 2.2.2 Synthesis of 1,2-epoxypentane 42 (i) Direct epoxidation 43 Magnesium monoperphthalate 44 Urea hydrogen peroxide 45 Dimethyldioxirane 45 (ii) Indirect epoxidation 46 Iodohydrin 47 Bromohydrin 48 1,2-Epoxypentane 49 2.2.3 Synthesis of the dimer analogue 50 2.2.4 Synthesis of the trimer analogue 57 (i) Bromination of the C 10 alcohol 57
(ii) Synthesis of the C15 alcohol 59 Formation of Grignard reagent 59 Carbon-carbon bond formation 61 (iii) Synthesis of a model trimer 62 2.2.5 Attempted synthesis of a model tetramer 63 111
2.3 Catalyst survey 65 2.3.1 Cationic oligomerization 65 2.3.2 Radical oligomerization 67 2.3.3 Metallocene catalysts 69 Metal 76 Metallocene trichloride 77 Substituted cyclopentadienyl rings 78 Temperature 79 Purity of the alpha olefin monomer 80 2.3.4 Comparison of different catalysts 81 Comparison of viscosity data 81 General comparison 81 2.4 Conclusion 82
Chapter 3 Experimental
3.1 General 84 3.2 Standard chemical methods 86 3.3 Preparation of model oligomer 88 3.4 Oligomerization reactions 95
References 102
Acknowledgments 109 SYNOPSIS
The economic growth of a country can be stimulated by value-adding to basic resources e.g., chemical intermediates. Alpha olefins are products of the Sasol Fischer-Tropsch process and oligomerization is one of the most significant beneficiation processes available for these alpha olefins.
Alpha olefin oligomers obtained from the cationic oligomerization with BF3/ROH, have been used mainly as base stocks in synthetic lubricants. These oligomers consist of a mixture of isomeric products that complicates the investigation and rational modification of this and other closely related oligomerization processes. A comparison of these isomeric mixtures to alkane standards could assist in the study of these processes, but almost no standards are available. This project was aimed, in part, at establishing a methodology for the synthesis of 1-pentene model oligomers. A model branched 1-pentene dimer and trimer as well as a precursor to the analogous tetramer were prepared. The syntheses of other model oligomers in conjunction with the comparison of the properties of model oligomers with different branching patterns will certainly be of value.
Some of the main classes of catalysts were evaluated for the oligomerization of higher alpha olefins and especially of 1-pentene. The BF3/n-BuOH cationic oligomerization of 1-pentene yielded dimers, trimer, tetramers and pentamers consisting of a large number of isomers and the product distribution peaked at the trimer. This is in agreement with previous results for other monomers. Radical oligomerization of 1-pentene with organic peroXide initiators proceeded in low yield. Dimers and trimers consisting of a mixture of isomers were mainly formed.
Metallocene catalysts offer a convenient route to single component, structurally well characterized alpha olefin oligomers. A variety of functional group transformations and consequent applications are possible for these oligomers. The general nature of the oligomerization of 1-pentene, 1-hexene and 1-octene with metallocene/methylaluminoxane catalysts was investigated. The influence of different metals, the number of cyclopentadienyl groups and substitution on the cyclopentadienyl groups in the metallocene moiety, the reaction temperature and the purity of the monomer were studied. By employing the appropriate metallocene in combination with methylaluminoxane, higher V
alpha olefin dimers and trimers could be formed in high yield at ambient temperatures. Cp2ZrC12 was the most active of the metallocene catalysts and single isomers were formed with high selectivity.
The metallocene-based oligomerization of higher alpha olefins offers great scope for further research and promises a new era in olefin oligomerization. OPSOMMING
Die ekonomiese groei van 'n land kan bevorder word deur waarde-toevoeging tot basiese hulpbronne, byvoorbeeld chemiese grondstowwe. Alfa-olefiene word in die Sasol Fischer- Tropsch proses vervaardig en oligomerisasie is een van die belangrikste prosesse vir waarde-toevoeging tot hierdie alfa-olefiene.
Alfa-olefienoligomere verkry deur die kationiese oligomerisasie met BF3/ROH, word hoofsaaklik as basisolies vir sintetiese smeermiddels aangewend. Hierdie oligomere bevat 'n mengsel van isomeriese produkte wat die ondersoek en rasionele modifikasie van hierdie en verwante oligomerisasieprosesse bemoeilik. Die vergelyking van hierdie isomeriese mengsels met alkaanstandaarde kan waardevol wees in die studie van hierdie prosesse, maar byna geen standaarde is beskikbaar nie. Hierdie projek was gedeeltelik daarop gemik om 'n metodologie daar te stel vir die sintese van 1-penteen modeloligomere. 'n Vertakte model 1-penteendimeer en - trimeer asook 'n voorloper tot die analoe tetrameer is berei. Die sintese van ander modeloligomere, tesame met die vergelyking van die eienskappe van modeloligomere met verskillende vertakkingspatrone, sal van verdere waarde wees.
Sommige van die hoofgroepe katalisatore is geevalueer in die oligomerisasie van hoer alfa- olefiene en spesifiek vir 1-penteen. Dimere, trimere, tetramere en pentamere, bestaande uit 'n groot aantal isomere, is gevorm in die BF3/n-BuOH oligomerisasie van 1-penteen en die produk verspreiding het gepiek by die trimeer. Dit is in ooreenstemming met resultate verkry vir ander monomere. 'n Lae opbrengs is verkry in die radikaal oligomerisasie van 1- penteen met organiese peroksiede as inisieerders. Hoofsaaklik dimere en trimere bestaande uit 'n aantal isomere, is gevorm.
Metalloseenkatalisatore bied 'n eenvoudige metode om enkel-komponent, struktureel goed- gekarakteriseerde alfa-olefien oligomere te berei. 'n Verskeidenheid funksionele groep transformasies en gevolglike toepassings is moontlik vir hierdie oligomere. Die algemene eienskappe van die oligomerisasiereaksies van 1-penteen, 1-hekseen en 1-okteen met metalloseen/metielaluminoksaan katalisatore is ondersoek. Die invloed van verskillende metale, die aantal siklopentadienielgroepe en substitusie op die siklopentadienielgroepe in die metalloseeneenheid asook die reaksie temperatuur en suiwerheid van die monomeer is bestudeer. Hoer alfa-olefiendimere en - trimere is gevorm by kamertemperatuur deur die vii gepaste metalloseen saam met metielaluminoksaan as katalisator to gebruik. Cp2ZrC12 was die aktiefste metalloseenkatalisator en enkele isomere is gevorm met 'n hoe selektiwiteit.
Die metalloseengebaseerde oligomerisasie van hoer alfa-olefiene bied ruimte vir verdere navorsing en lei 'n nuwe era in olefienoligomerisasie in. vii'
LIST OF SCHEMES
Chapter 1 The oligomerization of higher alpha olefins : a literature study
Scheme 1.1. Kinetics of cationic chain growth oligomerization. 8 Scheme 1.2. Formation of catalyst complex. 9 Scheme 1.3. Conventional cationic mechanism. 11 Scheme 1.4. Shubkin's mechanism. 12 Scheme 1.5. Oligomerization of 1-butene and cis-2-butene. 15 Scheme 1.6. First step in oligomer formation. 16 Scheme 1.7. Free radical polymerization of vinyl monomers. 20 Scheme 1.8. Simplified hydride mechanism. 21 Scheme 1.9. Formation of active heterogeneous Ziegler-Natta catalyst. 24 Scheme 1.10. Monometallic Ziegler-Natta mechanism. 25 Scheme 1.11. Formation of active metallocene/ MAO species. 27 Scheme 1.12. Formation of active MAO-free catalysts. 29 Scheme 1.13. Basic steps in polymerization of alpha olefins using metallocene-based catalysts. 30 Scheme 1.14. Illustration of agostic interactions. 31 Scheme 1.15. An example of stereocontrol in metallocene/ MAO polymerization. 34
Chapter 2 Discussion
Scheme 2.1. General route to tetraalkylmethanes. 41 Scheme 2.2. Retrosynthetic strategy. 43 Scheme 2.3. MMPP epoxidation of 1-pentene. 44 Scheme 2.4. UHP epoxidation of 1-pentene. 45 Scheme 2.5. Dimethyldioxirane epoxidation of 1-pentene. 46 Scheme 2.6. Formation of iodohydrin. 47 Scheme 2.7. Formation of bromohydrin. 48 Scheme 2.8. Illustration of the preparation of an epoxide from a halohydrin. 49 ix
Scheme 2.9. Preparation of 1,2-epoxypentane. 50 Scheme 2.10. Formation of C10 alcohol. 53 Scheme 2.11. Formation of two stereogenic centres. 56 Scheme 2.12. Bromination of 2-heptanol. 58 Scheme 2.13. Bromination of alcohol 44. 58 Scheme 2.14. Generation of Clo Grignard reagent. 59 Scheme 2.15. Preparation of C10 iodide. 60 Scheme 2.16. Formation of C15 alcohol. 61 Scheme 2.17. Preparation of model trimer. 63 Scheme 2.18. Decomposition of a secondary organic peroxide. 69 Scheme 2.19. Formation of methylated oligomers. 73 Scheme 2.20. Formation of normal dimers and trimers. 74 LIST OF FIGURES
Chapter 1 The oligomerization of higher alpha olefins: a literature study
Figure 1.1. Possible functionalization and application of single component alpha olefin oligomers. 5 Figure 1.2. Gas chromatogram of typical oligomer. 9 Figure 1.3. Gas chromatogram of 1-decene trimer. 10 Figure 1.4. Bent metallocene. 26 Figure 1.5. Relationship of pro-catalyst symmetry to polypropylene stereostructure. 33 Figure 1.6. Two types of supported chromium catalysts used in ethylene polymerization. 37
Chapter 2 Discussion
Figure 2.1. 1-Pentene model trimer. 42 Figure 2.2. Gas chromatogram of two alcohol produdts. 54 Figure 2.3. Gas chromatogram of 1-pentene trimer. 66 Figure 2.4. Structure of oligomers. 75 xi
LIST OF TABLES
Chapter 2 Discussion
Table 2.1. Types of organocopper reagents. 51 Table 2.2. Summary of model C-C bond formation reactions. 52 Table 2.3. Influence of different organometallic nucleophiles on the addition of a Grignard reagent to an epoxide. 55 Table 2.4. Attempted functional group transformations. 64 Table 2.5. Product distribution of the BF3/n-BuOH oligomerization of 1-pentene. 66 Table 2.6. Radical oligomerization of 1-pentene with organic peroxide initiators. 68 Table 2.7. Oligomerization of alpha olefins with titanocene dichloride/MAO. 71 Table 2.8. Oligomerization of alpha olefins with Cp2ZrC12/ MAO. 77 Table 2.9. Oligomerization of alpha olefins with CpTiC13/ MAO. 78 Table 2.10. Room temperature oligomerization of alpha olefins with Cp2ZrC12/ MAO. 80 Table 2.11. Viscosity/temperature relationship of 1-pentene trimer fractions 81 xii
ABBREVIATIONS
by boiling point Bu butyl CI chemical ionization Cp cyclopentadienyl El electron ionization fg functional group GC gas chromatograph MAO methylaluminoxane
MCPBA meta - chloroperoxybenzoic acid Me methyl MMPP magnesium monoperphthalate MS mass spectrometer Ms mesylate NMR nuclear magnetic resonance PAO polyalphaolefin Pr propyl psi pounds per square inch rf radio frequency rt room temperature SHOP Shell Higher Olefin Process THE tetrahydrofuran Ts tosylate UHP urea hydrogen peroxide CHAPTER 1
THE OLIGOMERIZATION OF HIGHER ALPHA OLEFINS: A LITERATURE .STUDY
1.1 Alpha olefin beneficiation: the key to value-adding and economic growth
1.1.1 The South African perspective
The value-adding to manufactured goods is one of the pivotal factors governing the economic growth and competitiveness of a country.. South Africa is blessed with abundant natural resources. Gold, platinum, diamonds and coal are some examples of South Africa's mineral wealth. Coal is of particular importance to the local chemical industry since it can be converted into liquid hydrocarbons. The industrial process utilizing coal derived mixtures of carbon monoxide and hydrogen to produce hydrocarbons, is based on the work of Fischer and Tropsch. 1 South Africa, starting in the 1950's, developed an updated Fischer-Tropsch synthetic fuel oil plant (Sasol) using improved engineering and technology. These synfuels and synchemical plants are a lucrative means of adding substantial value to more than 40-million tons of bituminous coal. 2
The Sasol synfuel plants were originally designed to produce mainly fuels, ethylene and unavoidable byproducts such as ammonia. The Sasol/ Lurgi gasification process uses coal, water and oxygen to generate synthesis gas (syngas: CO + H2) and a range of hydrocarbon chemicals. The Fischer-Tropsch high-temperature synthesis reaction employs syngas to create liquid fuels and a broad spectrum of useful olefins, alcohols and ketones. Market growths in conjunction with the development of new separation and purification technologies has realized the opportunity to redirect synfuel feedstocks to petrochemical feedstocks. The 1994 Sasol alpha olefin plant, producing comonomer quality 1-hexene and 1-pentene, is a prime example of a profitable value-adding expansion. 2
Approximately two million tons of olefins (C2 - Cio range) are produced annually in the Fischer-Tropsch process, of which about a third is currently recovered for the petrochemical market. The balance is still being converted into liquid fuels. The new alpha olefin plant has the potential to yield 450 000 tons per year of high quality 1-pentene and 1-hexene. This beneficiation project forms part of Sasol's strategy to add greater value to its treasure chest of petrochemicals. Alpha olefins represent a major niche-market opportunity for Sasol, viz.: the 1-hexene market has been in tight supply, while the 1-pentene comonomer was not previously available in commercial quantities. The feasibility of the production of additional linear alpha olefins in the C2 to Cu range, is under consideration. 3,4
Sasol boasts a competitive advantage in petrochemicals, which stems from its unique one- step process of producing fuels and petrochemicals without the larger capital investment and higher operating costs of naphtha crackers. Seen in the light of this competitive edge, further attention should be focused on in-house alpha olefin beneficiation.
1.1.2 Focus on alpha olefins
"Alpha olefins" is a generic term referring to linear alkenes with the double bond in the first or alpha position. These olefins are important feedstocks in the petrochemical industry for they are versatile chemical intermediates. Higher linear alpha olefins (C6 - Go) can be transformed into biodegradable surfactants, new types of polymers, lubricants and several industrially useful chemicals.
The widespread commercial usage of alpha olefins can be attributed to their ready availability, low cost, high reactivity and ease of transformation into a range of valuable products. Higher linear olefins have been available in commercial quantities since the 1960's. The processes used for the preparation of alpha olefins comprise: thermal and catalytic cracking of paraffins, oligomerization of ethylene, dehydrogenation of paraffins, dimerization and metathesis of olefins, dehydration of alcohols and electrolysis of C3 - C30 straight-chain carboxylic acids. The first process is used for commercial production of C2 - C5 olefins and the second for the large scale manufacturing of C9 - C30 alpha olefins. 3 • The high reactivity of the alpha olefins can be explained in terms of the basic structure of alkenes. The carbon-carbon double bond is best described by using an sp 2 hybridized orbital model which predicts a strong c bond and a weaker it bond. The overlapping 7C-TC bond behaves as a high-electron-density site and can be accessible to electrophilic reagents. In alpha olefins the double bond is in the terminal position and is more available than internal alkenes that is more sterically hindered. Therefore, alpha olefins are generally more reactive than internal olefins. Additionally, functional group modifications often result in products that are also linear, and this may be advantageous to the industrial user, e.g. the linear surfactants obtained from alpha olefins are biodegradable. 4,5
1.1.3 Alpha olefin oligomers as base stocks for synthetic lubricants
Oligomerization of alpha olefins to produce feedstocks for synthetic lubricants is one of an array of processes available for the beneficiation of alpha olefins. In the most fundamental sense, a lubricant may be defined as a substance that has the ability to reduce friction between two solid surfaces rubbed against each other. Animal fats, vegetable oils as well as mineral oils have been used throughout history to achieve this effect. For more than a millennium petroleum products have been employed in a primitive fashion and to this day, petroleum based lubricants play a dominant role in the world lubricant market.
The first synthetic hydrocarbon oils were produced in 1877 by Charles Friedel and James Mason Crafts. Synthetic lubricants are not obtained directly from crude oil and may be described as blends of base stocks made from low-molecular weight (chiefly petrochemical) building blocks. These base stocks consist of a limited number of components with uniform molecular structures and, therefore, well defined properties that can be tailored to specific applications.
Initial attempts to commercialize synthetic hydrocarbon oils failed due to insufficient demand. However, this situation is changing. Technological advances in modern machines and engines have been placing a greater pressure on the performance of existing functional fluids. Today's lubricants are facing increasingly stringent demands. Some of these challenges are cost effectiveness, lessened dependance on crude oil reserves, biodegradability, specialized performance requirements and operation under more severe 4 conditions. These limitations led to the development of synthetic lubricants such as polyalphaolefins, organic esters, silicones and polyglycols. Although the replacement of mineral oil by synthetic lubricants seems farfetched, it should be noted that current estimates predict that we may run out of crude oil in the next century.
Hydrogenated polyalphaolefins form a class of high-performance fluids considered to be the most versatile of the synthetic lubricants. The term polyalphaolefin (PAO) is commonly used to designate such fluids derived from the catalytic oligomerization of linear alpha olefins having six or more (usually ten) carbon atom s. Owing to inherent and highly sought after characteristics of PAO's these fluids are rapidly gaining ground as high-performance lubricants. PAO's are currently being used as base stocks in, for example, automatic crankcase lubricants, hydraulic oils and turbine oils. Synthetic lubricants manufactured from alpha olefins can improve motor fuel efficiency and prolong oil lifetime. 6,7
1.1.4 Use and functionalization of poly(alpha olefins)
Alpha olefin oligomers and their derivatives are central to modem life, not only as synthetic lubricants but in a variety of end-uses. Attention will henceforth be focused on the general oligomerization process, use and functionalization of oligomers and the possible catalyst systems active in this reaction.
Isomeric mixtures of olefin oligomers accessible via cationic catalysts like the BF3/ROH system are commonly used in lubricant applications. Metallocene catalysts now offer a convenient route to single component, structurally well characterized alpha olefin oligomers. The possible functionalization and potential application of these oligomers are depicted in Figure 1.1.8 5
fuel additives additives for lacquers, / paper and leather industry
NH2 OH
R OH (Co-)Polymer R'..,. .OH -I. adhesives
lubricants ointment --PI.. fragrances white oil R' CH3
Figure 1.1. Possible functionalization and application of single component alpha olefin
oligomers.8
1.2 General principles of oligomerization
According to the IUPAC nomenclature, oligomers 9 are molecules containing a few constitutional units repetitively linked to each other. No specific molecular weight or degree of oligomerization is defined. It states, however, that the physical properties of oligomers vary with the addition or removal of one or several units. Polyolefin chain formation, depending on a number of n reacting molecules, can in most cases be named as follows: dimerization (n = 2), oligomerization (2 < n < 100) and polymerization (n > 100). 5
Alpha olefins may be oligomerized thermally or catalytically; the latter having industrial application. A variety of oligomerization catalysts are known and reactions can be classified accordingly as acid-, base-, radical- or metal catalysed.
The typical catalytic cycle of these catalysts consists of initiation, propagation and termination steps. The relative propagation (r e) and termination (1-.1 rates determine the molecular weight of the obtained product. If r p > rt many propagation steps take place before termination and a high molecular weight polymer is formed, when rt > r p dimers are produced, and finally when rt rp oligomers are obtained.5 6
The main oligomerization catalysts will be discussed in the following section, with specific reference to higher alpha olefin monomers, reaction mechanisms and the factors governing the product distribution.
1.3 Olefin oligomerization catalysts
1.3.1 Friedel-Crafts catalysts -
As early as 1877 Charles Friedel and James Mason Crafts found that anhydrous aluminium chloride could be used to catalyse the polymerization of unsaturated hydrocarbons. During the ensuing years aluminium chloride was shown to be a most versatile catalyst effecting an almost unlimited number of reactions. Today any substitution, isomerization, elimination, cracking, polymerization, or addition reaction that takes place under the catalytic effect of Lewis acid-type acidic halides (with or without cocatalysts) or proton acids, are regarded as Friedel-Crafts-type reactions. 10
i) Polyalphaolefins
Friedel-Crafts type catalysts are currently the only ones used commercially for producing PAO base stocks for synthetic lubricants. A few aspects concerning polyalphaolefins should be highlighted and expanded. Polyalphaolefins are the largest volume synthetic lubricant base stock worldwide and are considered to have the largest potential for growth. The combination of their relative low cost and desirable properties make the PAO's the preferred base stock for a wide variety of synthetic lubricant applications. Among these favorable properties are:
A wide operational temperature range High viscosity index Thermal stability
Oxidative stability
Hydroly tic stability
• Shear stability
Low corrosivity 7
Compatibility with mineral oils Compatibility with various materials of construction Low toxicity Manufacturing flexibility that allows the "tailoring" of products to specific end-use application requirements.
However, PAO's lack the ability to swell the elastomers used in the seals of many lubricant systems, and they have limited ability to dissolve the additives needed to enhance critical properties. As a result, they are often used in combination with petroleum-based oils and/or esters (primarily dibasic acid esters) to overcome these deficiencies. The synthetic lubricants compete with petroleum lubricants on a cost-competitive basis, but some key environmental and quality policy changes could affect future growth positively. 6,7 ii) Cationic acid catalysed oligomerization
Lewis - and Bnansted acids, that is, the common Friedel-Crafts catalysts, may effect the oligomerization of alkenes. 11 Cationic oligomerization is distinguished by the nature of the carrier ions on the growing chains: in the process of growth, an ion pair is formed in which the chains carry carbocations. Initiation occurs by electrophilic attack of the initiators on the monomers. A general scheme for cationic oligomerization is presented in Scheme 1.1. The coinitiator is presented as A, the initiator as RH, and the monomer as M.
Generally, cationic oligomerization depends on the initial formation of positive and negative ions in an organic environment. Initiating systems include: Lewis acid/Bronsted acid combinations, Brensted acids by themselves, stable cationic salts, some organometallic compounds, and some cation forming substance's. Propagation takes place through successive addition of monomeric units to the charged or reactive groups of the propagating chains. Terminations are the result of unimolecular reactions, or transfers to other molecules (such as monomers, solvents or impurities like moisture). They can also result from quenching by deliberate addition of reactive terminating species.'2 8
Cationic Initiation Process : Rate of initiation : K A + RH [H+] [AR- ] Ri = KKi[A][RH][M] Ki [H+][AR- ] + M [HM+][AR- ]
Cationic Propagation Process : Rate of propagation : Kp [HMn+] [AR- ] + M [HMnM+][Ak- Rp = KpRHM+) (AR - )] [M]
Cationic Termination : By transfer : Rate of termination : Kt [HMnM+] [AR- + M _ Mn+1 + [HM+][AR- Rt = Kt [ (HMnM+)(ARli
By spontaneous termination where Kt is a combination of Ktr, Kst and Kr [HMnM+] [AR] Kst Mn+1 [HI [AR-
By rearrangement of the kinetic chain :
[HMnM+][AR- ] Kr HMnMAR
Scheme 1.1. Kinetics of cationic chain growth oligomerization. 12 iii) BF3-catalysed oligomerization of alkenes
Cationic catalysts, such as AlCb, have been reported for the oligomerization of alkenes as early as 1931. 13 In 1951 a patent was issued to Gulf oil company describing the use of AlCb for the oligomerization of 1-octene. 14 Brennan (Mobil Oil) . showed in 1968 that oligomerization of alpha olefins could be carried out in a reproducible fashion with boron trifluoride in the presence of a protic cocatalyst. 15 Commercial PAO's are made from 1- decene. The low viscosity products are made using BF3 catalysts, while the higher viscosity PAO's are produced with catalysts like a trialkylaluminium or alkylaluminiumhalide in conjunction with a halogen or halide source. 716
BF3 is the catalyst of choice for the most common grades of PAO's. While incapable of catalyzing the oligomerization of alpha olefins on its own, BF3 is active when used in conjunction with a proton-donating cocatalyst (typically a low molecular weight alcohol
9 such as n-butanol). BF3 complexes with proton-donating compounds by one of the following general reactions:
BF3 (g) + ROH (1) --> BF3.ROH (1) or BF3 (g) + 2 ROH (1) ---> BF3.2ROH (1)
Scheme 1.2. Formation of catalyst complex!
Since BF3 is normally used in excess, the BF3 and alcohol cocatalyst react in a 1:1 molar ratio to form the catalyst complex BF3.ROH. The BF3.ROH system is unique for two reasons: the oligomer peaks at the trimer (Figure 1.2) and the products exhibit unexpectedly good low-temperature viscosity characteristics due to the presence of excess branching in the molecular skeleton.
TA ImEF,S
TC7.7AmEA S
?ENT AmEqS CMEPS
HEXAMEAS
10' 1Sr 2d 23. T al. fain) Figure 1.2. Gas chromatogram of typical oligomer.16
Figure 1.2 is a high resolution GC trace of a typical oligomerization product resulting from the reaction of 1-decene in the presence of the BF 3.ROH complex. Figure 1.3 shows clearly that the single oligomer fractions are composed of a very large number of branched isomers. 10
These isomer mixtures form even under tightly controlled reaction conditions. Notwithstanding, this has the advantage, especially in lubricant applications, of lowering the melting point of the trimer mixture relative to that of pure compounds of comparable molecular weight and structure. 17
2522 a lOr 12. 14' le 18" 20' 22' 24' 19" 19' Ti.. (ain) Figure 1.3. Gas chromatogram of 1-decene trimer.' 6
a) Mechanistic considerations
1 H NMR studies of the hydrogenated oligomers indicated about one additional methyl group compared to that predicted on the basis of the conventional cationic mechanism (Scheme 1.3). As yet no mechanism that explains this branching has been proven conclusively in view of the complexity of the products. Various studies of the hydrogenated and distilled oligomer fractions indicate anomalies with the theoretically expected results. Skeletal rearrangement at the dimer stage, 18 rearrangement of the monomer 19 as well as positional isomerization of the monomer 20 have been proposed in order to explain these discrepancies. 11
- H-F H+ Isomers R— CH-2 CH= CH2 R- CH2 CH—CH3 - H+ H-F MONOMER R—CH-2 CH=CH2
CH3 CH H+ I + R— CH- CH— CH= CH—CH- R R— CH- CH— CH- CH—CH— R 2 2 - H+ 2 2 2 (+ isomers) R— CH- CH= CH DIMER 2 2
CH3 CH3 H+ R— CH— CI H— CH—C H— CH- R R— CH- CH— CH- CH— CH-2 R 2 2 2 - H+ 2 2 CH CH2 I I (+ isomers) CH +CH CH H2 2 R R
TRIMER
Scheme 1.3. Conventional cationic mechanism.
Experimental work of Shubkin et al.18 indicated that the conventional mechanism did not explain the composition of the oligomerization products adequately. Firstly, the unique product distribution could not be clarified: at lower temperature (30 °C) mainly trimer was formed, while the product peaked at the dimer at elevated temperature (85 °C). Arguments based on steric crowding were unconvincing. Secondly, the physical properties of the oligomerization products differed from the expected: lower viscosities and significantly lower pour points than would have been predicted from the expected molecular structures, were found. Further investigation revealed that on average two-thirds of the oligomers had one more methyl group (in other words one more branch) than expected.
The observed nature of the BF3-catalysed oligomerization reaction, as well as the physical and chemical properties of the products, led Shubkin 18 to suggest a mechanism that includes skeletal rearrangement (Scheme 1.4). According to Shubkin, rearrangement occurs only at the dimer stage, proceeding via a protonated cyclopropyl intermediate.
12
H+ Double bond R—CH=CH—CH3 migration 3
H-F 2 R—CH-2 CH=CH2 R—CH-2 CH— CH3 - 1 + 1
CH CH H I 3 \ 3/ R—CH Cx, / CH —CH2—R R—CH— C 2 2 '■ CH2 2 R H+ CH2 6 4 1
CH3 CH3 H+ Double bond R—CH-2 C—CH—CH-2 R R—CH-2 CH—CH=CH—CH-2 R migration- H+ 7 CH3 + 5
H+1[ -H+ CH3 R—CH- C—CH —CH—R 2 1 \ 2 CH CH 2 CH3 I 3 + CH R—CH=C—CH—CH—R 9 I 2 I CH3 8 CH I 2 + R
C H\14+ I H3 + 1 R—CH—— C= C —CHR 2 I 2 Tetramer CH CH3 3 11 R—CH —C—CH—CH—R 2 1 \ 2 CH CH3 CH 10
CH2 R
Scheme 1.4. Shubkin's mechanism's 13
Protonation of the olefin (1 ---> 2) is followed by nucleophilic attack by another olefin molecule to give a protonated dimeric species (4). This species may then deprotonate to form dimeric product (5) and/or isomerize by double bond migration. Shubkin states, however, that there is a unique feature about intermediate 4 which is the key to this reaction: there is a labile, tertiary hydrogen p to the carbonium ion. Skeletal rearrangement through a protonated cyclopropyl intermediate (6) to give the more stable tertiary carbocation 7 is thermodynamically favored. Intermediate 7 may deprotonate to 8 and undergo isomerization by double-bond migration. But the inherent stability of a tertiary carbocation makes it available for attack by another olefin to produce the trimeric carbocation 9. This is a secondary carbocation and therefore less stable than 7; furthermore, it cannot rearrange the way that 4 does. The carbon atoms p to the carbocation either contain no hydrogen atom or only secondary hydrogen atoms which are not particularly labile. Thus deprotonation to 10, the trimeric olefin, is favored over reaction with an additional alpha olefin to give 11, the tetramer.
Onopchenko and coworkers 19 studied the oligomerization of 4-methyl-1-pentene by the BF3 catalyst system to test Shubkin's proposed mechanism. 1H NMR of the dimer and trimer product fractions showed 5.0 and 6.8 methyl groups, respectively, instead of the expected 6 and 8 predicted by Shubkin's mechanism. According to Onopchenko, this indicated that: (a) no methyl group rearrangement occurred at the dimer stage or, (b) that the initial secondary 4-methylpentyl carbocation had rearranged at the monomer stage and was behaving as the tertiary carbocation derived from the vinylidene alkenes.
In order to provide evidence that methyl group migrations can occur at the monomer stage (and not only at the dimer stage as claimed by . Shubkin et al.18), the composition of unreacted decenes were determined under varying conditions. Onopchenko et al.19
confirmed the results of Brennan: 17 At high 1-decene conversion (90%), the unreacted alkene contained only 60% of 1-decene, 35% of internal alkenes and 5% of methyl nonenes. Conversely, at low 1-decene conversion (30%), less than 0.5% of methylnonenes was detected. This data suggested that the BF3 complex is a mild isomerization catalyst, but an effective oligomerization catalyst.
The complexity of the oligomers formed from 1-decene necessitated the examination of a simpler system. The structures of the dimers from the oligomerization of 1-butene and 14 cis-2-butene using the BF3/mannitol complex were investigated (theoretically only 18 isomers are possible for C8F112paraffins).
The data obtained showed that all of the isomers had either three, four or five methyl groups. The similarity of the product isomers from the two butenes suggested that the same intermediate had to be involved in both cases. The common intermediate is the sec- butyl carbocation. The only significant difference in the oligomerization products from the two butenes was the formation of substantially more 3-methylheptane from 1-butene and 3,4-dimethylhexane from 2-butene.
Onopchenko et al.19 assumed that two combinations (sec-butyl 12 and tert-butyl 13 in Scheme 1.5) were involved in the dimerization of the two butenes. This assumption followed from the postulated mechanism for the Koch reaction, 21 where a protonated cyclopropane is the key-intermediate in the formation of tertiary alkyl carboxylic acids. Onopchenko and coworkers used this reaction to predict the nature of the major isomers.
Following the protonation of either 1- or 2-butene, the secondary butyl carbocation rearranges to the tertiary butyl carbocation, and both then add to an isomerized mixture of alkenes. The intermediate ions can then undergo deprotonation to afford the corresponding olefinic dimers and their isomers. Eventually six paraffins (14 - 19) would result from each butene, but these accounted for less than 70% of the experimentally determined number of isomers. Eight additional isomers were present and this suggested that some of the expected intermediates had rearranged further and thus the model used by Onopchenko et al. was only an approximation. The total percentage of isomers 16 - 19, nevertheless, could be used to estimate the extent of methyl group migration at the monomer stage - it was in the range of 14 - 22% for the two butenes considered.
Driscoll and Linkletter 20 used NMR to analyse 1-decene dirner fractions. Their conclusion was that the identified structures were formed by the attack of secondary carbocations on secondary olefin. Further studies, incorporating 1-decene selectively enriched in position 2, suggested that in most cases oligomerizations took place internally and not at the terminal double bond.
15
- C—C—C=C C—C=C—C rearrangement C—c—C - 12 13
C C C I I I C—C—C—C C—C—C—C C—C—C—C C—C—C C—C—C C—C—C + + + + + + 1 C=C—C—C 1C—C=C—C 1C —LC 1C=C—C—C 1C—C=C—C 1C—LC c c c I I I C—C—C—C c—c—c—c C—C—C—C C—C—C C—C—C C—C—C I I I I I C C—C C C C—C C I I I I I +C -EC C—C+ +C +C C—C + l I I I I C c c C C C I C
1- H+ 1- H+ H+ 1- H+ H+
C C C I I I c—c—c—c c—c—c—c c—c—c—c c —C—C C—C—C C—C—C I I I I I I C c—c c II II CII CII C—CI I II C C C—C C C C—C I I I I I I c c C C C C 1 i C c
H2 1 H2 "1 H2 I H2 1 H2 1 H2
C C C I I I C—C—C—C C—C—C—C C—C—C—C C—C—C C—C—C C—C—C I I I I C C—C C C C—C 1 1 1 1 i 14 C 15 c 16 c—c 17 c 18 c 19 c—c I I I I I C c c C c C C C
3-methyl- 3,4-dimethyl- 2,4-dimethyl- 2,2-dimethyl- 2,2,3-trimethyl- 2,2,4-trimethyl- heptane hexane hexane hexane pentane pentane Total: 1-C4= : (34.4%) (8.6%) (14.3%) (2.2%) (1.8%) (3.8%) 64.1% 2-C4= : (14.2%) (41.2%) (8.7%) (1.5%) (1.1%) (3.0%) 69.7%
Scheme 1.5. Oligomerization of 1-butene and cis-2-butene."
16
Thus, double bond isomerization took place appreciably faster than oligomerization and the first step in the mechanism can be depicted as follows:
H2C=CH(CH2)6h-I2CH3 H3c(042)7&1=cH2 H3C(CH2)mCH=CH(CH2)nCH3
Scheme 1.6. First step in oligomer formation 2°
This step embodies the formation of the dimer through the reaction of a secondary carbocation with a primary or secondary olefin. According to Driscoll and Linkletter 2° linear olefins such as decene, in all probability have limited thermodynamic preference for the internal olefin isomers and as a result, carbocation formation will proceed at random along the chain. The attack on the primary olefin will be preferred sterically, but not statistically. This led Driscoll and Linkletter to claim that random bonding, primarily by secondary carbon atoms, would take place to afford a large number of isomers containing the labelled carbon next to the methyl group. This proposed mechanism explains the presence of more methyl groups than predicted by the conventional cationic mechanism (attack of a secondary carbocation on a primary olefin)..
b) Correlation of paraffin structure with physical properties
Before the synthesis of novel fluids with improved physical and thermal properties can be considered, some understanding of the structure-property relationship is required. Cupples, Kresge, Onopchenko and Pelligrini 22 related the structure of paraffins to physical properties such as viscosity, pour point and in some cases thermal stability. The pour point of a lubricant may be described as the temperature just before the fluid solidifies during cooling, in other words it is the freezing point.
As the carbon number of the feed decreased, the low temperature viscosity increased; thus the viscosity-temperature relationship worsened. Likewise, as more branching was introduced into the molecule, the viscosity of the oligomers deteriorated. Introduction of a short chain (methyl) in the 2-position of n-alkanes practically had no effect on the viscosity, but moving the methyl group toward the centre of the alkane backbone showed some 17 deviation. The introduction of a single methyl branch into a linear chain lowered the melting point significantly, but the effect was most pronounced when the branch was near the centre of the chain. Lengthening the branch to C4 or longer reduced the melting point even further. When considering the low temperature performance of fluids, including both viscosity and pour point, it should be noted that the structural requirements for good low temperature viscosity and low pour point are conflicting. The low degree of branching of the linear alpha olefin oligomers which imparts their superior viscosity properties is precisely what will cause these oligomers to be the first to fail a given pour point. In short, the preferred structure would appear to be a molecule having linear characteristics but with sufficient branching to provide the needed pour points.
Thermal stability seemed to be favored by less branched, more linear structures; as the number of tertiary hydrogens increased, the thermal stability decreased. This, of course, is consistent with the data of Benson, 23 who reported that the tertiary carbon-hydrogen bonds have the lowest bond dissociation energies and are therefore considered the weak points to thermal stability.
Brennanl7 found that the following theoretical molecular structure, in which the concentration of atoms is very close to the centre of the chain of atoms, is associated with superior low temperature fluidity: R1 R— C— R2 R3 where R and R2 are nearly identical in size, R is generally a hydrogen but may be an alkyl group, and R3 can vary from a methyl group to a group larger than R and R 2.. The carbon atoms comprising R, R2 and R3 may be arranged in a chain or a ring. Compounds with carbon numbers up to 36 in which Ri is a hydrogen and R, R2 and R3 are alkyl groups and nearly identical in size, exhibited very good wide temperature range fluidity. Long chain linear alpha olefins should yield trimer fractions structurally similar to this ideal and thus the trimerization of long-chain olefins is a one-step synthesis of a superior lubricant molecule.17 18 c) Reaction variables
Even though the mechanism of the BF3.ROH catalysed oligomerization remains to be fully elucidated, researchers have learned how to advantageously control the composition of the final PAO products in order to tailor the oligomer distribution to fit the requirements of specialized end-use applications. 16 This customizing may be done by manipulation of the reaction variables, which include: chain length of olefin raw material, temperature, reaction time and pressure, catalyst concentration, cocatalyst type and concentration, cocatalyst feed rate, olefin feed rate, reaction quench and recovery procedures, hydrogenation catalyst and conditions, and distillation.6
It is now clear why BF3/decene was adopted as the basis of manufacture by every major producer of low viscosity PAO's. It is, however, normally not possible to manufacture higher viscosity products using BF3.ROH technology, but other catalyst systems are known that can give the desired products. These include alkylaluminium compounds in conjunction with TiC4 or allcylhalides, peroxides and A1Cb catalysts. 6,16
The function of the cocatalyst is obviously complex. Shubkin and Kerkemeyer 16 showed empirically that the choice of cocatalyst and the concentration at which it is used plays an important role in determining the composition of the product PAO. Priola et al.24 observed that an increased temperature decreased the molecular weight of the oligomers as well as increased the presence of fragmentation products. Shubkin et al.16 also established that higher temperatures increased the branching in the oligomers as well as promoted some depolymerization. Relative to the reaction time, both the composition and the isomeric distribution of the product change. This represents a useful opportunity for tailor making PAO's with special physical properties. 6
In addition to controlling the relative distribution of oligomers by manipulation of the reaction parameters, major alterations can be made to the product properties by the choice of starting olefin to better satisfy the requirements for a particular end-use application. Fluids prepared from short-chain monomers tend to have low pour points and moderately low viscosity indices, whereas fluids prepared from long chain monomers have moderately low pour points and high viscosities. 22 A final consideration is the position of the double bond in the starting olefin. Shubkin et al.16 established that increased levels of internal 19 olefins in the starting mixture resulted in slower reactions and more dimer in the product. In addition the low temperature properties of the hinter were influenced: the pour point decreased and the viscosity increased.
The cationic oligomerization of unrefined C 10 cuts obtained from the Fischer-Tropsch process have also been researched. 28,26 The raw Cio Fischer-Tropsch products are suitable for oligomerization and the included alcohols serve as promoters. Oxygenated compounds, aromatics as well as branched, internal and cyclic olefins were found to be more problematic. The presence of ketones led to early termination of the oligomerization reaction and as a result the oligomer distribution was shifted to the lower end. Additionally, the aromatics caused a lowering of the viscosity indices. Preliminary purification of the feed enabled better reaction control and product quality could be improved.
Much of the recent research activity is centred on the development of either new catalysts that would alleviate the environmental problems associated with BF3, or alternate feedstocks that are less expensive than 1-decene. 7
1.3.2 Free radical catalysts
The use of organic peroxides as free-radical initiators for polymerization became important during World War II as a result of the increased demand for synthetic rubber and plastics. 27 It has been reported in a Mobil European patent application that high viscosity PAO's may be produced by dimerizing lower oligomers with peroxides. 28 In reality this is not currently economically feasible, since stoichiometric quantities of di-tert-butyl peroxide are used.
The steps in free radical polymerization are illustrated below. The formation of initiating radicals is the rate-determining step in the initiation reaction. Numerous reactions lead to the formation of free radicals: thermal decompositions with azo and peroxy groups are common sources, but "redox" reactions, light induced decompositions and ionizing radiation can also be used to form initiating radicals. 12 These initiating radicals must be energetic enough to react with the vinyl compounds. Once the initiating radical is formed, there is competition between the addition to the monomer and all other secondary reactions. After the initiating radical has diffused into the proximity of the monomer, the capture of 20 the free radical by the monomer is a straightforward addition reaction and completes the step of initiation. By comparison to other steps in the polymerization, initiation is slow and requires high activation energy. The rate of propagation depends upon both the reactivity of the monomer and the growing radical chain. Steric hindrance, polar effects and resonance are important factors in this reaction. The termination processes in free-radical polymerization are: combination, disproportionation or chain transfer.
Initiation :
R—R --I,- 2 R".
R* + CH2= CHX R—CHT CHX •
Propagation :
R— CH- CHX " + n CH =---- CHX ---0- R— (—CH- CHX—)n— CH- CHX • 2 2 2 2
Termination :
By combination
R— (— CH— CHX--)n—CHT CHX • + • XHC— CHT(—XHC— CH2)n—R
R— (—CHTCHX—)n— CHTCHX— XHC— CHT (— XHC— CHT-) n— R
By disproportionation
R— (— CHT CHX — CHT CHX • + • XHC— CHT (— XHC— CHHn—R R— (— CHT CHX—)n— CHT CH2X + XHC:= CH— (— XHC— CHT-)n— R
By transfer
R— (— CHT CHX--)n— CHT CHX + R'H
-111' R— (— CH2— CHX —)n — CHT CH 2X + R''
Scheme 1.7. Free radical polymerization of vinyl monomers. 12
1.3.3 Transition metal catalysts
In the last two decades, there has been as surge of interest in the chemistry and application of catalysis by certain transition metal complexes. Polymerization catalysts have been the focus and examples of industrial application abound. The oligomerization of alkenes by 21 transition metal catalysts is however of considerable academic and industrial interest for the synthesis of linear and branched higher mono-olefins. The oligomerization of ethylene and propylene are the most well known, e.g. in the formation of higher linear alpha olefins in the Shell Higher Olefin Process (SHOP). 29,30
A number of catalysts are known for the oligomerization of higher alpha olefins. Catalysts similar to those used for ethylene and propylene oligomerization and polymerization can be used for higher alpha olefin oligomerization. It is assumed that the polymerization mechanisms of these catalysts can be applied to the corresponding oligomerizations of higher linear alpha olefins. Among the most popular catalysts are titanium, zirconium and nickel complexes; homogeneous and heterogeneous Lewis and Bronsted acids; and inorganic oxides. 5 The reactivity of the olefinic substrates can be correlated to their relative coordination abilities. Olefin reactivity decreases in the following order : ethylene >> propylene > 1-butene > 1-pentene > 1-hexene. 31,32
In general, two different reaction mechanisms are considered relevant: 29,30 a hydride mechanism and a mechanism based on metallacyclic intermediates. Keim 29 states that the `hydride insertion-alkyl migration mechanism is the most generally accepted one, but proof is still questionable. It is, however, the accepted one for the majority of oligomerization reactions. Considerable attention has also been attracted by the catalytic sequence involving metallacyclopentane intermediates. A simplified scheme for the metal-hydride mechanism is portrayed in Scheme 1.8.
H LnIvi—R' LnM—C—C—R
Kp propagation n C=C—R
Scheme 1.8 Simplified hydride mechanism.29,3° 22
The first step comprises the insertion of an olefin into a metal-hydrogen bond. Further insertion of olefins into the metal-alkyl bond leads to growth products (propagation step). The elimination of the thus formed oligomers occurs by a n-hydrogen elimination returning to Li,M-H which re-enters in the catalytic cycle.
Analyzing the sequence given in Scheme 1.8, the relative propagation and elimination rates again determine whether polymerization, oligomerization or dimerization will be observed. Which of these reactions prevail is dependent on the nature of the metal and its oxidation state, the nature of the ligand and the reaction parameters. These parameters critically influence the electronic structure of the complex.
According to Keim 29 the following generalizations can be made: Group VIII transition metals favor 13-hydrogen transfer (elimination) . and thus olefin dimerization and oligomerization are observed preferentially. In contrast, with Group IV to VI transition metals propagation prevails and predominately polymerization catalysts are found in this class. With the same metal, chain propagation decreases with increasing oxidation state. Furthermore, the metal character can be altered by adding ligands. Donor ligands tend to increase and acceptor ligands to decrease the chain propagation.
There should be distinguished between the two possible modes of addition of a metal- carbon or metal-hydrogen bond to an unsymmetrically substituted olefin, namely the Markovnikov and anti-Markovnikov addition. The direction of addition determines whether linear or branched products are formed. Higher alpha olefins can form a greater number of isomers, as a result the selectivity of oligomerization is lower. Highly selective formation of certain isomers can be achieved by the selection of appropriate ligands.
Homogeneous, transition metal based catalysis has received attention over the past number of years and a variety of industrial processes eXiSt. 33,34,35 It is broadly assumed that the true catalysts consist of a metal surrounded by a ligand field. The ligands are essential in controlling the activity, selectivity and catalyst life. 36 The theoretical interest in homogeneous catalysts, as opposed to heterogeneous catalysts, stems from the unique insight into the mechanism of catalytic action. The nature and surfaces of solids are not well known or understood, but the nature of molecular and ionic species existing in solution has been relatively well established. 23
For catalyst preparation one of three routes are generally followed. Transition metal salts in a higher oxidation state are reduced to the catalyst, low valent transition metal salts are oxidized to the catalyst or organometallic complexes are used as catalyst precursors. 36 Organometallic complex catalyst precursors are favored, because they tend to be highly active, specific and selective thus minimizing unwanted reactions. 37
i) Heterogeneous Ziegler-Nana catalysts
The Ziegler-Natta catalysts received their initial attention when Ziegler showed that some transition metal halides, upon reaction with aluminium alkyls, can initiate the polymerization of ethylene. 38 Simultaneously, Natta demonstrated that similar catalysts can polymerize various other olefins like propylene, butylene and higher alpha olefins. 39 Higher molecular weight linear polymers formed as well, and more importantly, stereospecific ones. These reactions require much lower pressures than do free-radical polymerizations and form the foundation of today's polyalkene industry. Despite the consequent development of a wide range of highly reactive and selective catalysts, the complex multisited nature of heterogeneous Ziegler-Natta catalysts has hindered their detailed characterization and rational modification. As a result, fundamental questions concerning the mechanism of monomer addition and the origin of stereochemical control in heterogeneous Ziegler-Natta catalysts remain unanswered.
0
Ziegler-Natta catalysts are products from reactions of metal alkyls or hydrides of Group I to III of the Periodic Table with metals, salts or complexes of Group IV to VIII. The catalysts that originally gained commercial importance were combinations of TiC14 or TiC13 with R 3A1 or R2A1C1. These so-called first generation heterogeneous Ziegler-Natta catalysts exhibited rather low activity, water-sensitivity and were susceptible to catalyst poisoning. High- activity-high-yield (second generation) catalysts were developed incorporating trivalent alkoxy chlorides of transition metals with certain halogen-free organoaluminium compounds. The third generation comprises supported Ziegler-Natta catalysts prepared by reacting titanium compounds with hydroxylated magnesium derivatives, magnesium alkoxides, or magnesium organometallics followed by activation with aluminium alkyls.11,12
24
The Ziegler-Natta catalysts can be subdivided into two groups: (1) heterogeneous insoluble catalysts, and (2) homogeneous or soluble, ones. Soon after the initial discoveries of Ziegler and Natta, efforts were made to devise homogeneous models of the heterogeneous catalysts that would prove more amenable to study. In 1957 Nattao and Breslow 41 reported that Cp2TiC12 could be activated for alkene polymerization by Et3A1 or Et2A1C1.
Several mechanisms have been proposed to explain the action of heterogeneous Ziegler- Natta catalysts. All agree that the polymerization takes place at localized active sites on the catalyst surfaces by coordinated anionic mechanisms. The organometallic component is believed to activate the site on the surface by alkylating the transition metal. Some controversy still exists about the exact mechanism of catalytic action, whether it is monometallic or bimetallic. The majority of opinion leans to the former. In addition, it is well accepted that the monomer insertion into the polymer chain is between the transition metal atom and the terminal carbon of the growing polymeric chain. 42
The role of the metal alkyl in forming the active catalyst is illustrated in Scheme 1.9. After complexation with TiC14 (or in general, with the transition metal compound), the transition- metal alkyl is generated through halogen-alkyl exchange. This complex is then reduced in a subsequent intricate reduction step including dealkylation to yield low-valence titanium. These reduced metal species are believed to constitute the active catalytic species. 11
TiC14 + AlEt3 TiC14.AlEt3
TiC14.AlEt3 EtTiC13.AlEt2C1
2 EtTiC13.A1Et2C1 2 TiC13.A1Et2C1 C2H4 C2H6
Scheme 1.9 Formation of active heterogeneous Ziegler-Natta catalyst. 11
All evidence to date indicates that chain growth occurs through repeated four-centred
insertion reactions of the monomer into the transition metal-carbon cy-bond. It is still not established whether the basic metal alkyls serve only to produce the active centres and have no additional function. This would imply that the mechanism is monometallic. If the active centres must be stabilized by coordination with base metals, then the mechanism would be bimetallic. 25
An example of the monometallic mechanism is one proposed by Cossee and Arlman (Scheme 1.10).43 This mechanism assumes that the reaction occurs at a transition metal ion on the surface layer of the metal trichloride (or perhaps dichloride) lattice. Here, the halide is replaced by an alkyl (R) group. The adjacent chloride site is vacant and accommodates the incoming monomer molecule. The newly formed transition metal-alkyl bond becomes the active centre and a new vacant site form in place of the previous transition metal alkyl bond.
R' a R R' ss Cl cl 1 I ...-• CH—R + CH=CH2 —110- Cl—Ti--H al I al 1 CH2 CI CH—R R' R' /CH— R / a ' CH2 ,CH, al - Cl al I Cl
four-centre intermediate
Scheme 1.10 Monometallic Ziegler-Natta mechanism. 11,12
Terminations in coordination polymers usually occur by chain transfer either by an internal hydride transfer or by transfer to a monomer. Hydrogen or protonic acids also terminates the reaction and is often used to control the molecular weights of the products. 12
Ziegler catalysts yield outstanding wide-temperature range hydrocarbon fluids when used in the oligomerization of alpha olefins. Compared to similar cationic catalysed reaction products, the Ziegler derived materials exhibit higher oxidative and thermal stability as well as a slightly higher viscosity index. The pour point is lower due to the presence of highly linear chain segments in the oligomer. The high quality of these Ziegler prepared fluids can be ascribed to the little or no isomerization that takes place during oligomerization. The BF3 cationic oligomerization still holds some advantages: while BF3 uses none, the Ziegler process uses an unspecified amount of solvent; the Ziegler oligomerization takes much longer; and the conversion for the BF3 process as well as the yield of the trimer are higher.17 26 ii) Homogeneous Ziegler-Natta catalysts: Metallocenes
Forty years after the discoveries of Ziegler and Natta we are witnessing the evolution of new generations of catalysts and polyolefin materials which originate from studies on homogeneous, metallocene-based polymerization catalysts. Research on metallocene-catalysed olefin polymerization derived much of its impetus from the desire to model the reaction mechanisms of the -heterogeneous polymerization catalysts.
The first soluble catalysts (Cp2TiC12 activated by Et3A1 or Et2A1C1) exhibited lower activities in ethylene polymerization than their heterogeneous counterparts and were inactive for propylene.40,41 This situation changed dramatically in the early 1980's when Sinn and Kaminsky44• 5 discovered that partially hydrolyzed Me3A1 activated Group 4 metallocenes catalysed the polymerization of ethylene and other alpha olefins.
Active homogeneous olefin polymerization catalysts are obtained on mixing bent metallocenes Cp'2MX2 (Cp' = general cyclopentadienyl ligand; M = Zr, Hf, Ti; X = Cl, Me), with excess methylaluminoxane (MAO) in inert solvents such as toluene (Figure 1.4). Such uniform site catalysts generally contain a single kind of active site and offer potential advantages over traditional multi-site Ziegler-Natta polymerization catalysts. The most important is the ability to control polymer structure and properties by variation of the catalyst structure, allowing production of a wide range of new polymers, unavailable using the classic catalysts. 46
M x
where M = Hf, Zr, or Ti and X= Me or Cl
Figure 1.4 Bent metallocene. 27 a) Nature and formation of the active species
Initial metallocene research was focused on establishing the catalytically active species of the metallocene/MAO system. Both a cationic monometallic species and a neutral bimetallic species have been proposed. In 1960, Shilov 47 proposed that the active species was a 14- electron cationic metal alkyl of the form [Cp2MR] 4- .• The isolation of the cationic insertion product from the reaction of Cp2TiC12 -/MeA1C12 with an alkyne by Eich48 supported Shilov's proposal. This result suggested that the metal alkyl cation [Cp2TiMe]+ was the initially formed catalytically active species for this alkene polymerization system. In addition, the catalytic nature of discrete do metallocenes (n = 0, 1) substantiated the existence of a cationic active site. Strong evidence for the involvement of cationic intermediates came in 1986 when Jordan49 isolated and characterized the cationic complex [Cp2ZrMe(THF)]+[BPh4] , which polymerized ethylene in the absence of a cocatalyst.
Controlled hydrolysis of Me3A1 produces a mixture of methylaluminoxanes (MAO) thought to have the general formula [Al(Me)O]n (5 < n , 20). 50 MAO plays at least three roles in polymerization: as a alkylating reagent for the generation of transition metal-alkyl adducts; as a Lewis acid for anion abstraction from the complex generating an electrophilic species; and as a scavenger for the removal of impurities, particularly water in the olefin and solvent.46
Highly active alkene polymerization catalysts are formed when a large excess of MAO is mixed with Group 4 metallocenes (Scheme 1.11). It is thought that the excess of MAO is required to shift an equilibrium involving a metal alkyl (20) and MAO to the active cationic species (21). 51 Initially a fast ligand exchange takes place between Cp2MC12 and MAO to generate primarily the monomethyl complex Cp2M(Me)Cl; excess MAO leads to Cp2M(Me)2.52
Cp2MC12 + ---(- jid—C) --IP- Cp2M(Me)C1 + MAO' [Cp2MMer[(MA0 1)Clr CH3 n 20 21.
Scheme 1.11 Formation of active metallocene/MAO species. 28
It is generally assumed that some of the Al centres in MAO have an exceptionally high propensity to abstract a CH3 ion from Cp2M(Me)2 and to sequester it in a weakly coordinating ion CHs—MAO -. High catalyst activity suggests that this MAO derived anion is labile and easily displaced by the monomer prior to insertion. This weak association gives way in the presence of substituted olefins, to olefin-separated ion pairs
[Cp2MR(olefin)]+CH3—MAO-, the presumed prerequisite for olefin insertion into the M-R bond (where R is the growing polymeric chain). This hypothesis - that the unusually low coordinating capability of the anion AT in [Cp2M(Me)]+A - is crucial for catalytic activity 53 - led to the discovery of a series of highly active cationic metallocene catalysts for the polymerization of propylene and higher alpha olefins.
The key to the synthesis and activity of such active cationic metallocenes is the utilization of robust and poorly coordinating anions showing good solubility in inert aromatic solvents.
Early attempts to use the traditional 'non-coordinating anions', such as BF3 and PF6, were unsuccessful as fluorine transfer to the cation led to deactivation. 54 Even for large, weakly coordinating anions such as (C6H5)413 - and C2B9Hu- fairly strong interactions have been observed with cationic (alkyl)zirconocene species and as a result propylene was polymerized only at low rates, if at all.55'56'57 A breakthrough in this regard was the introduction of perfluorinated tetraphenylborate as counterion by Hlatsky and Turner 58 and by Marks et al.56 These was the first well-defined metallocene catalysts capable of polymerizing propylene and higher olefins at high rates without addition of further activators. Similar activities for propylene polymerization were subsequently observed with other base-free or weakly stabilized (alkyl)metallocene cations. Tetraphenyl borates, alkyl triphenylborates and, to a lesser extent, carborane anions have found wide use in cationic metallocene chemistry. 54,55,56,59,60
The formation of these highly active, base-free cationic metallocene alkyls is represented in
Scheme 1.12. Usually an ion pair such as [Cp'2ZrMe]f(C6F5)13 - (where Cp' is some substituted Cp or indenyl ligand) is formed in which residual coordinative contacts between the cationic Zr centre and its counterion exists. This contact appears to resemble those yet unidentified interactions that stabilize an (alkyl)metallocene cation in contact with a
CH3—MAO- counterion. In both cases, a fast ligand exchange occurs between the cationic and anionic complex moieties; most importantly, bo. th types of contact appear to be weak
29 enough to allow an alpha olefin to displace the anion from its coordination site at the metal centre.59,61
ZrMe2 + [Bu3NFIRB(C6F5)4]
- Bu3N Zr+ - B(C6F5)4
` Me
ZrMe2 + [Ph3C][B(C6F5)4] HIGH ACTIVITY
Me / B(C6F5)3 ZrMe2 Zr (C6F5)3
` Me
MODERATE ACTIVITY
Scheme 1.12 Formation of active MAO free catalysts. 46
Of even greater simplicity are catalysts based on an (alkyl)metallocene complex containing a Scm, Yifi or a trivalent lanthanide centre. Both Ballard 62 and Watson63 found that neutral complexes of the type Cp2MEHR acted as single component catalysts for the oligomerization of alpha olefins. 64,65 These catalysts are however, more difficult to prepare than the Group 4
metallocene systems.
b) Kinetics and mechanism of alpha olefin polymerization
The availability of MAO-free polymerization catalysts has allowed the study of the olefin insertion and termination (chain transfer) steps in the catalytic cycle. Important aspects of
30 alpha olefin insertion are the regioselectivity (1,2-insertion is normally found for simple olefins), the stereospecificity and the mode of stabilization of the insertion product
(presumed to be via weak secondary bonding interactions). In Scheme 1.13 the basic steps believed to be involved in polymerization using metallocene-based catalysts are shown.
n R
Scheme 1.13. Basic steps in polymerization of alpha olefins using metallocene-based catalysts.66
The reactive complex, [Cp2MR(olefin)]+, is firstly formed by the displacement of an anion
CH3—MAO- from its [Cp2MR]+ counterion by an olefin molecule. Coordination of the alkene followed by migratory insertion generates a coordinatively unsaturated species (22) that can repeat this propagation sequence. For alpha olefins the insertion step is typically highly regioselective, with greater than 98% of the monomer units undergoing primarily 1,2-cis insertion.67
Kinetic isotope studies by Bercaw 68 and Brintzinger, 69 as well as molecular modeling" suggested that the insertion process can be accompanied by an agostic interaction between the metal centre and one of the a-hydrogen atoms of the growing polymer chain. According to Morokuma et al.71 this transition state for alkene insertion into [M]—R+ exhibits an interaction of an a-H atom of the growing chain with the electron-poor metal, such that the electron pair in the a-C-H a-bond is partly donated to the metal; this is the so-called
31 a-agostic interaction (Scheme 1.14). The stabilization of the alkyl product of insertion by an p-agostic interaction (of a (3-C-H bond with the metal) is also suggested by Morokuma and coworkers, although others support either a y-agostic interaction" or no agostic interaction at all.72
+ M —CH-2 R' M CHR
CH2 CH 2 2 1
,,H + ,,CH R' ' transition state / CSR 2 M• „CHR \ CH2 H2C