Metal complexes with enolatoimine ligands for controlled olefin polymerizations

Metal complexes with

enolatoimine ligands for controlled olefin polymerizations

Dissertation

zur Erlangung des Akademischen Grades eines Doktors der Naturwissenschaften

(Dr. rer. nat.)

an der Universität Konstanz

Mathematisch-Naturwissenschaftliche Sektion Fachbereich Chemie

vorgelegt von

Sze-Man Yu

aus Hong Kong Konstanz 2009

Tag der mündlichen Prüfung: 21.09.2009

Referent: Prof. Dr. S. Mecking

Part of this work has been published

1. Polymerization of ethylene catalyzed by new titanium and zirconium complexes with fluorinated β-imineenolato ligands. S.-M. Yu, S. Mecking, Polymeric Materials Science and Engineering Preprint, 2008, 98, 885-886.

2. Extremely narrow-dispersed high molecular weight polyethylene from living polymerization at elevated temperatures with o-F substituted Ti enolatoimines. S.-M. Yu, S. Mecking, J. Am.

Chem. Soc. 2008, 130 (40), 13204-13205.

3. Variable crystallinity polyethylene nanoparticles. S.-M. Yu, S. Mecking, Macromolecules 2009, 42 (11), 3669–3673.

Conferences

1. Bayer AG, Summer school, August 20^th - August 25^th 2006, poster presentation.

2. Heidelberg Forum of Molecular Catalysis 2007, June 22^th 2007, poster presentation.

3. AM2NET-Meeting in Münster, December 7^th - December 8^th 2007, oral presentation.

4. ACS-Meeting in New Orleans, April 6^th - April 10^th 2008, oral presentation.

Acknowledgement

Die vorliegende Arbeit entstand in der Zeit von Juni 2006 bis Juni 2009 im Arbeitskreis von Prof. Dr. Stefan Mecking. Bei ihm möchte ich mich herzlichst für die jahrelange Betreuung, die hervorragenden Arbeitsbedingungen und seine Unterstützung bedanken.

Prof. Dr. Dr. h.c. Hans H. Brintzinger danke ich für die Übernahme des Zweitgutachtens. Auch möchte ich mich bei ihm für sein stetiges Interesse an meiner Arbeit, die zahlreichen Diskussionen im Common Center und seine Ermutigungen bedanken.

Prof. Dr. Heiko M. Möller danke ich für seine Unterstützung bei der Durchführung NMR- spektroskopischer Untersuchungen.

Besonderen Dank schulde ich Damien Guironnet und Dr. Inigo Göttker genannt Schnetmann für die Durchsicht der Arbeit, ansonsten auch für ihre Hilfsbereitschaft und ihre moralische Unterstützung. Dr. Inigo Göttker genannt Schnetmann sei außerdem gedankt für die Durchführung der Einkristallstrukturbestimmungen.

Lars Bolk danke ich ganz besonders für die Durchführung der GPC- und DSC-Messungen. Bei Dr. Marina Krumova möchte ich mich für das Anfertigen der TEM-Bilder bedanken. Dr.

Werner Röll schulde ich besonderen Dank für die schnellen Reparaturen der Glove-Box.

Bei Dr. Dieter Lilge (Basell, Frankfurt) möchte ich mich herzlichst für die Durchführung der GPC-Messung ausgewählter Proben bedanken.

Dr. Michael Burgert danke ich besonders für die Einführung in die Röntgendiffraktometrie und seine unermüdliche Hilfsbereitschaft.

Weiterer Dank geht an all meine Studenten, die gute Arbeit im Rahmen ihres Praktikums oder als wissenschaftliche Hilfskraft geleistet haben. (Uli Tritschler, Beate Stempfle, Moritz Baier &

Stefan Matt)

Allen meinen Freunden, die mich während meiner Promotion hier in Konstanz ganz oder teilweise begleitet haben, danke ich für die schöne, gemeinsame Zeit.

Philipp Roesle verdient hierbei einen extra Eintrag, da er immer für mich da war.

Zuletzt möchte ich mich besonders bei meinen Eltern und meinem Bruder für ihre Liebe, ihre Geduld und ihren Rückhalt bedanken und dafür daß sie stets an mich geglaubt haben.

Mein Dank gilt ansonsten auch all denen, die zum Gelingen dieser Arbeit beigetragen haben und nicht explizit erwähnt worden sind.

Table of contents

1. Introduction……….……12

1.1. Olefin polymerization catalyzed by late transition metal complexes………….………..13

1.1.1. Overview of development of late transition metal complexes………….………….14

1.1.2. Polymerizations with late transition metal complexes in aqueous system…….…...16

1.2. Living olefin polymerizations with transition metal complexes ……….…….18

1.2.1. General considerations ……….……….18

1.2.2. Living propylene polymerization……….………..20

1.2.2.1. Vanadium acetylacetonoate catalysts ………..21

1.2.2.2. Metallocene-based catalysts ………..22

1.2.2.3. Bis(phenoxyimine)titanium catalysts……….23

1.2.2.4. Bis(phenoxyketimine)titanium catalysts………28

1.2.3. Living ethylene polymerization……….………29

1.2.3.1. Non-group 4 metal polymerization catalysts………...29

1.2.3.1.1. Group 3 metal polymerization catalysts (Y, lanthanides)…………30

1.2.3.1.2. Group 5 metal polymerization catalysts (V, Nb, Ta)……….…31

1.2.3.1.3. Chromium polymerization catalysts ……….…...32

1.2.3.1.4. Late transition metal polymerization catalysts (Co, Ni, Pd)……...32

1.2.3.2. Bis(phenoxyimine)titanium catalysts………....34

1.2.3.3. Bis(phenoxyketimine)titanium catalysts……….…..………37

1.2.3.4.Titanium indolide-imine catalysts………..^………..^……..37

1.2.3.5. Bis(enaminoketonato)titanium catalysts………..38

1.2.4. Block copolymerization of ethylene/propylene……….………39

2. Scope of the thesis……….………..41

3. Variable crystallinity polyethylene nanoparticles……….…..43

3.1. Introduction……….………….………..44

3.2. Results and discussion………... …………..44

3.2.1. Synthesis and characterization of catalyst precursors………....44

3.2.2. Synthesis of aqueous dispersions……….………..47

3.3. Summary and conclusions……….………50

3.4.2. Synthesis and characterization of complexes………...…….53

3.4.3. Polymerization procedure……….………...………..56

4. Extremely narrow-dispersed high molecular weight polyethylene and block copolymers of ethylene/propylene from living polymerization at elevated temperatures with o-F substituted Ti enolatoimines……….……….58

4.1. Introduction……….………..……….59

4.2. Results and discussion……….……….…….60

4.2.1. Synthesis of Ti complexes………...………60

4.2.2. Ethylene homopolymerization……….………..61

4.2.3. Block copolymer synthesis……….…………...……63

4.3. Summary and conclusions……….………68

4.4. Experimental section……….……….68

4.4.1. Materials and general considerations……….………68

4.4.2. Synthesis of ketoenamines……….………70

4.4.3. Synthesis of complexes……….……….71

4.4.4. Ethylene or propylene homopolymerization……….…….74

4.4.5. Synthesis of PE-PP block copolymer……….………...75

4.4.6. Synthesis of PE-PP-PE block copolymer……….………….75

4.4.7. Synthesis of PE-EPR block copolymer……….……….75

4.4.8. X-ray crystal structure determination……….………...76

5. Dimethylamido titanium enolatoimine complexes……….……..78

5.1. Introduction……….………...79

5.2. Results and discussion……….……..81

5.2.1. Synthesis and characterization of ketoenamine ligands……….………81

5.2.2. Synthesis and characterization of complexes……….………...83

5.2.3. Ethylene polymerization studies………...………...94

5.3. Summary and conclusions………95

5.4. Experimental section……….96

5.4.1. Materials and general considerations……….……96

5.4.2. Synthesis of ketoenamines……….………97

5.4.3. Synthesis of complexes……….……...100

5.4.4. Ethylene polymerization……….….108

5.4.5. X-ray crystal structure determinations………...109

6. Olefin polymerization by zirconium enolatoimine complexes……….115

6.1. Introduction……….………116

6.2. Results and discussion……….………117

6.2.1. Synthesis and characterization of ketoenamine ligands………...117

6.2.2. Synthesis and characterization of complexes……….………….118

6.2.3. Polymerization studies of ethylene……….……….122

6.2.4. Polymerization of 1-olefins……….124

6.3. Summary and conclusions………..125

6.4. Experimental section………...126

6.4.1. Materials and general considerations……….……..126

6.4.2. Synthesis of the ketoenamines……….………127

6.4.3. Synthesis of dichloro complexes………...…129

6.4.4. Synthesis of dibenzyl complexes……….……131

6.4.5. Ethylene or propylene polymerization……….……....131

6.4.6. 1-Hexene polymerization……….………132

6.4.7. X-ray crystal structure determinations……….…132

7. Synthesis and characterization of copolymers from ethylene and cyclopentene by an ortho-fluorinated enaminoketonato Ti complex………..……...137

7.1. Introduction……….……….138

7.2. Results and discussion……….………139

7.2.1. Cyclopentene/ethylene copolymerization……….…...139

7.3. Summary and conclusions……….………..144

7.4. Experimental section……….……….…………..145

7.4.1. Materials and general considerations……….………..………145

7.4.2. Synthesis of poly(ethylene-co-cyclopentene)……….….146

8. Zusammenfassung..…….……….………147

9. Addendum………...……….……….149

10. References…..………..………210

Graphical abstracts

Chapter 3: Variable crystallinity polyethylene nanoparticles

F3C O N

Ni CH₃ R R

F3C WSL O

water-soluble electron-poor catalyst precursor

high molecular weight controlled crystallinity

nanoparticles (11 nm, M_w 10⁶ g mol^-1,

25-50 % crystallinity) stable for hours

Chapter 4: Extremely narrow-dispersed high molecular weight polyethylene and block copolymers of ethylene/propylene from living polymerization at elevated temperatures with o-F substituted Ti enolatoimines

TiCl₂ 2 N

O F₃C

F

F n

25 °C, 1 atm ethylene, 2 000 eq. MAO

M_n= 271 000 M_w/M_n= 1.01 15 min

0 3 6 9 12 15

0 10 20 30 40

1,0 1,2 1,4 1,6 1,8

Mn [104 g/mol]

Polymerization time [min]

Polymerization at 75°C Linear Fit

Mw / Mn

Chapter 5: Dimethylamido titanium enolatoimine complexes

M(NMe₂)X 2

N O F₃C

R¹

R¹ R⁴

R³ R²

R²

M= Ti, Zr X = NMe₂, Cl

Backbone 1: R⁴= CH₃ Backbone 2: R⁴= CF₃

R¹= F, CH₃, H R²= F, H R³= F, H

Chapter 6: Olefin polymerization by zirconium enolatoimine complexes

Chapter 7: Synthesis and characterization of copolymers from ethylene and cyclopentene by an ortho-fluorinated enaminoketonato Ti complex

Abbreviations

I. Methods

DLS dynamic light scattering

DSC differential scanning calorimetry GPC gel permeation chromatography IR infrared

MS mass spectrometry

NMR nuclear magnetic resonance TEM transmission electron microscopy

II. Compounds & molecular fragments acac acetylacetonate

aPP atactic polypropylene Ar aryl

cod cyclooctadiene Cp cyclopentadiene DMF dimethylformamide EPR ethylene propylene rubber Et ethyl

iBu isobutyl

iPP isotactic polypropylene iPr isopropyl

L ligand M metal

MAO methylaluminoxane

Me methyl

MMAO modified methylaluminoxane N^O κ²-N,O coordinate ligand PDI polydispersity

PE polyethylene Ph phenyl

PP polypropylene PS polystryol

SDS sodium dodecyl sulfate sPP syndiotactic polypropylene tBu tert-butyl

THF tetrahydrofuran TIBA triisobutylaluminium

tmeda N,N,N’,N’-tetramethylethylene-1,2- diamine

tol toluene

TPPDS sodium di(p-sulfonatophenyl)phenyl phosphine

TPPTS sodium tris(m-

sulfonatophenyl)phenyl phosphine

III. Spectroscopy br broad

δ chemical shift in ppm

d doublet

dd double doublet

gCOSY gradient correlation spectroscopy gHMBC gradient heteronuclear multiple

bond correlation

gHMQC gradient heteronuclear multiple quantum correlation

nJAB J-coupling between nuclei A and B via n bonds

m multiplet

ppm parts per million q quartet

s singlet t triplet

vt virtual triplet

IV. Miscellaneous calc calculated cat. catalyst cf. compare cryst. crystallinity equiv equivalent

M_n number average molecular weight Mw weight average molecular weight

ref. reference

RT room temperature temp. temperature

TON turnover number, mol (substrate) mol^-1 (catalyst)

TOF turnover frequency, mol (substrate) mol^-1 (catalyst) h^-1

T_g glass transition temperature Tm melting temperature

vs versus

wt.-% weight percent

1. Introduction

Polymeric materials are indispensable to our daily life, which is reflected by an annual worldwide consumption of more than 260 million tons by the year 2007.¹ Based on annual production volume, polyolefins are by far the most important commercial class of synthetic polymers. As food packing, coatings, building materials or household items, polyolefins are used on a vast scale. While a great part of these materials is produced by catalytic polymerization employing Ziegler or Phillips catalysts based on early transition metals (Ti, Zr, Cr, V), the older free radical process which affords low-density polyethylene (LDPE) has maintained its importance.² In spite of the necessity of working at over 1500 bar, the demand for LDPE is still immense. 16 million tons are currently produced annually.^2d,e An advantage of the high-pressure process is the possibility of incorporating functionalized olefins, such as vinylacetate or acrylates, which leads, amongst others, to improved adhesion properties and a better compatibility of polyolefins with other materials. Another important criterion for the use of the free radical polymerization is the ability to form short- as well as long-chain branches.

Due to the short-chain branches, the LDPE possesses certain polymer properties, such as crystallinity and melting temperature which are important for the polyolefin application properties. The long-chain branches (typically containing 100 or more carbon atoms) principally affect the rheology of polyolefin melts, and result in good processing properties of LDPE.³ On the other hand, free radical processes do not allow for controlling the microstructure, as is the case with catalytic polymerization.

Since the initial discoveries of Ziegler⁴ and Natta⁵, remarkable advances have been made in the catalysis of olefin polymerization, especially the discovery of metallocene.⁶ Metallocene- based catalysts reveal several differences compared to previous generations of catalysts. For example, their homogeneous nature leads to lower polydispersities and more uniform incorporation of 1-olefin comonomers than obtained with Ziegler catalysts.

A limitation of the early transition metal catalysts (e.g. titanium, zirconium or chromium) is their high oxophilicity which hampers their application in polymerization with most functionalized olefins, particularly the commercially available polar comonomers. However, examples of copolymerizations with special substrates⁷ or with very high levels of a Lewis acid incorporated into the polymerization system to protect the polar functionality through complexation⁸ have been published. Alternative routes to polar copolymers involving metathesis of cyclic olefins and functionalization of the resulting unsaturated polymer or metathesis of polar cycloolefins followed by hydrogenation to remove the resulting unsaturation are known as well. ⁹ However, these multi-step approaches are cost-intensive. Hence, copolymers of functionalized olefins with ethylene are still produced commercially by free-radical polymerizations.¹⁰ The lower oxophilicity and presumed greater functional-group tolerance of late transition metals relative to early metals make them likely targets for the development of catalysts for the copolymerization of ethylene with polar comonomers under mild conditions.¹¹ In addition, they can provide access to unique polyolefin branching structures as a consequence of the preferred β-hydride elimination. Recent discoveries of novel olefin-polymerization catalysts based on late transition metals represent major advances.¹²

1.1. Olefin polymerization catalyzed by late transition metal complexes

Polymerization of ethylene or 1-olefins by Ziegler-Natta or metallocene catalysis or by late transition metal complexes rely on the same basic types of reactions, namely chain growth by migratory insertion in alkyl – olefin complexes and chain transfer by various mechanisms (Scheme 1-1).¹³ Since the ß-hydride elimination step is relatively facile for late transition metals such as Co, Rh, Ni and Pd, these metals preferentially oligomerize ethylene to butenes or hexenes or higher olefins.¹⁴

Scheme 1-1. Simplified schematic representation of ethylene oligomerization and polymerization by transition metal (M) complexes (R= growing polymer/ oligomer chain).

L_nM

R

L_nM

R

chain growth

L_nM

H

R R

L_nM

H

chain transfer by ß-hydride elimination

1.1.1. Overview of development of late transition metal complexes

An industrial application of the ability of late transition metals to convert ethylene into oligomers is the so-called Shell Higher Olefin Process (SHOP), developed by Keim et al. in the 1960s. These catalysts are based on formally anionic phosphino-enolate ligands. In this reaction, linear 1-olefins are obtained by Ni-catalyzed oligomerization of ethylene.¹⁵ Klabunde et al.

showed that by the addition of triphenylphosphane as scavenger, a SHOP-type catalyst 1 produces linear polyethylene (Scheme 1-2).¹⁶

Scheme 1-2. Effect of a monodentate triphenylphosphane ligand on the molecular weight of the product.

H n

1

toluene

oligomeric olefins

1 / phosphane scavenger

toluene linear polyethylene

P

Ni

Ph

Ph H

Ph Ph

O PPh₃

1

By modification of the bidentate ligand structure (Figure 1-1), linear polyethylenes of very high molecular weight are obtained.¹⁷ However, in general, such systems exhibit modest activities and afford broad molecular weight distributions with a low M_n < 10⁴ g/mol.

In the development of the SHOP and in subsequent related academic research, a number of ethylene polymerization catalysts were discovered. Amongst the most outstanding examples of catalyst versatility are the α-diimine complex systems 3 developed by Brookhart and co- workers.¹⁸ These cationic catalysts serve as oligomerization or polymerization catalysts according to the steric properties of the ligand. The groups of Brookhart, Benett and Gibson have used the Fe (II) and Co (II) pyridinediimine complexes 4 for the polymerization and the oligomerization of ethylene.^12,14,19 Fe (II) pyridinediimine systems were first reported by the tire and rubber company Goodyear for the codimerization of butadiene and ethylene.^{12, 20} Approximately at the same time as the discovery of cationic iron and cobalt polymerization catalysts, a new class of neutral nickel catalysts based on salicylaldimine ligands was reported independently by Johnson et al. and Grubbs and co-workers.^11b,21

P Ni

Ph

Ph NaO₃S

Ph Ph

O L

2a L = CR'₂PR₃ or

pyridine

P Ni

Ph Ph Ph

O L

2b L = CR'₂PR₃ or

O=PPh₃ HO

P Ni

Ph

Ph H

Ph Ph

O PPh₃

1960s 1 (SHOP)

1980s

N N

M

R¹ R¹

X X

M = Ni, Pd X = Cl R¹ = 2,6- ⁱPr; 2,6- ^tBu

Brookhart 1995

N

N M N

X X

R¹ R¹

M = Fe, Co X = Cl R¹ = 2,6- ⁱPr; 2,6- ^tBu, CH3, H

Brookhart, Gibson, Benett 1998

R

X O

N Ni

R' L

R' = Ph, CH3

R = H, ^tBu, 9-phenanthrenyl X = H, NO2

L = PPh₃, CH₃CN Grubbs, Johnson

1998 3

4 5

Figure 1-1. Overview of complexes for the catalytic oligomerization or polymerization of

1.1.2. Polymerizations with late transition metal complexes in aqueous system

The functional group tolerance of late transition metal catalysts allows for polymerization in polar reaction media, as illustrated by the SHOP. In polymer chemistry, the most prominent polar medium is water. It is employed on a large scale in emulsion polymerization, affording polymer dispersions. Water as the dispersing medium offers a unique combination of advantages:²²

- A high heat capacity, which facilitates an effective transfer of the heat of polymerization.

- A high polarity that results in a perceptibly different miscibility with many monomers and polymers compared to organic solvents.

- The viscosity of an aqueous latex is essentially lower than of a solution of the polymer in an organic solvent. Thus, high polymer yields can be obtained during emulsion polymerization in a given amount of reaction volume.

- Dispersions of hydrophobic polymer particles can be effectively stabilized by surfactants in water towards aggregation.

- Water is non-flammable and nontoxic.

- Water is cheap.

- Water is particularly environmentally friendly.

About 7% of the annual plastics production is obtained as aqueous polymer dispersions.

Until today, aqueous polymer dispersions for coatings and paints are produced industrially by free radical routes exclusively.²³ By contrast to radical polymerization, catalytic polymerization allows for a control of microstructure, and correspondingly materials properties, over a wide range. Therefore, catalytic polymerizations in emulsion have been studied recently.^{24 , 25} By polymerization of ethylene with Ni (II) salicylaldiminato complexes, high molecular weight polyethylene dispersions can be prepared.^24b By appropriately tailoring the catalyst via remote substituents of N-terphenyl moieties in the salicylaldimine (Figure 1-2), linear, semicrystalline or highly branched, amorphous polymer can be obtained. The latter goes at the expense of

molecular weight, however.^24c

R¹

O Ni

N Me

R³ R⁴ L

R¹ R¹ R¹

R² R²

Figure 1-2. Salicylaldiminato complexes.

Recently, particles with sizes <10 nm were obtained using water-soluble salicyaldiminato Ni(II) complexes (Figure 1-3).^{24e, 29} Particles sizes in this range are difficult to access by other polymerization mechanisms, e.g. industrial free radical emulsion polymerization affords particles with sizes between 50 nm and 1 µm.²⁶ Otherwise, very small polymer particles of 10-30 nm size have been prepared by means of catalyst microemulsions.²⁷ Microemulsion as an approach is, indeed, not per se general since microemulsions often only form under specific conditions that require large quantities of alcohols, or other organic solvents, and surfactants. An aqueous solution of a hydrophilic catalyst clearly represents the highest possible initial degree of dispersion of the catalyst precursor, and is, therefore, of great interest in view of the synthesis of small particles. Dispersions of nanocrystals are of interest for applications such as, for example, delivery of poorly soluble drugs, controlled release of active molecules, and homogeneous incorporation of functional molecules such as dyes into solid materials or as carriers in aqueous multiphase catalysis.²⁸ Also, semicrystalline particles in this size regime can contribute to fundamental understanding of crystallization in confinement,²⁹ crystallinity can be a source of anisotropy, and such particles can serve as crystalline mesoscopic building blocks for ultrathin films.^30,31

Figure 1-3. Cryo-TEM micrograph of nanoscale polyethylene crystals in aqueous dispersion.

(from ref. 29)

1.2. Living polymerizations with transition metal complexes 1.2.1. General considerations

Living catalytic polymerizations are a topic of current interest, and the development has recently been reviewed comprehensively.⁵⁶ In contrast to late transition metal complexes, catalysts based on early transition metals are less prone to β-hydride eliminations. Thus, living polymerizations can be carried out with various catalyst systems (albeit some late transition metal catalysts also polymerize in a living fashion). Living polymerizations³² are desirable due to their capability to enchain monomer units without termination. Precise molecular weight control as well as the synthesis of a wide array of polymer architectures are possible.³³ Living methods also allow the synthesis of end-functional polymers if special initiation and/or quenching methods are employed. Nevertheless, living polymerizations have the limitation that each catalyst only forms one chain, in contrast to common alkene polymerization catalysts that can produce thousands of chains each as a result of chain transfer. In terms of material properties, living polymerization methods allow for the synthesis of new materials from a basic

set of available monomers.

Indeed, olefin insertion catalysts have always been inferior to other chain-growth polymerizations in one respect. While tremendous advances in living/controlled polymerization have been reported by using anionic,³⁴ cationic³⁵ and radical-based³⁶ polymerization, until recently there existed a comparative lack of living olefin polymerization systems. A significant number of advances have been discovered not until the last half decade. Seven generally accepted criteria for a living polymerization are:³⁷

1) Polymerization proceeds to complete monomer conversion, and chain growth continues upon further monomer addition.

2) Number average molecular weight (Mn) of the polymer increases linearly as a function of conversion.

3) The number of active centers remains constant for the duration of the polymerization.

4) Molecular weight can be precisely controlled through stoichiometry.

5) Polymers display narrow molecular weight distributions, described quantitatively by the ratio of the weight average molecular weight to the number average molecular weight (Mw/Mn = 1).

6) Block copolymers can be prepared by sequential monomer addition.

7) End-functionalized polymers can be synthesized.

Only few polymerization systems, whether ionic-, radical-, or metal-mediated, that are claimed to proceed by a living mechanism have been shown to meet all of these criteria.

Common features of alkene polymerization catalyst systems are chain transfer and elimination reactions that terminate the growth of a polymer chain and result in the initiation of a new polymer chain by the catalyst. For example, in metallocene catalysts, consecutive alkene insertion into the metal carbon bond connecting the catalyst and polymer chain³⁸ proceeds until β-hydrogen elimination and/or transfer to monomer occurs.³⁹ When alkylaluminum cocatalysts

monomer addition methods for block copolymer synthesis futile. Several strategies have been devised to decrease the rate of chain termination relative to that of propagation such that living systems can be formed. The first consideration in many cases is simply lowering the polymerization temperature of an ordinary nonliving catalyst system to achieve living or at least controlled behavior. Since β-hydrogen elimination process is unimolecular while propagation can be bimolecular, a lowering in temperature can more adversely affects elimination processes relative to enchainment. However, particularly for semicrystalline polymers, precipitation of polymers from solution at low temperatures can hinder the controlled nature of a living polymerization. Therefore, a second strategy for discovering living systems is to design new catalysts through empirical modification and/or rational methods.⁴⁰ By creating species that are unreactive for common termination reactions even at ambient temperature, living catalysts have been devised. A specific consideration is to eliminate the use of alkyl aluminum cocatalysts that give the potential for chain-transfer reactions. In this regard, the development of weakly coordinating anions has made significant advances in living olefin polymerization possible.⁴¹

1.2.2. Living propylene polymerization

syndiotactic polypropylene sPP

isotactic polypropylene iPP

atactic polypropylene aPP

regioirregular polypropylene rir-PP

Figure 1-4. Microstructures of propylene homopolymers.

By comparison to ethylene, the prochirality of this monomer gives rise to significantly greater complexity in the resultant polymers by introducing the variables of stereo- and regioregularity. The tacticity of the polymer is intimately related to its bulk properties, with

atactic polypropylene (aPP; Figure 1-4) being an amorphous material with limited industrial uses (e.g. adhesives, sealants, and caulks) and syndiotactic polypropylene (sPP) and isotactic polypropylene (iPP) being semicrystalline materials with relatively high T_m values of 150 and 165 °C respectively. Only a very few syndiospecific propylene polymerization catalysts have been discovered to date; this fact coupled with its lower crystallinity have limited the commercial impact of sPP. On the other hand, numerous catalysts, both heterogeneous and homogeneous, are capable of isospecific propylene polymerization. iPP possesses highly desirable mechanical properties (durability, chemical resistance and stiffness).

1.2.2.1. Vanadium acetylacetonoate catalysts

Vanadium compounds have been found to promote living olefin polymerization. In 1960s, Natta and coworkers discovered that vanadium tetrachloride with Et2AlCl at -78 °C in the presence of propylene furnished syndio-enriched polypropylene.⁴² More than a decade later, Doi reported [V(acac)3]/Et2AlCl as a living catalyst at -65 °C for the synthesis of syndio- enriched PP ([r] 0.81). At temperatures even slightly above -65 °C, the living behavior was greatly diminished as evidenced by an increase in molecular weight distribution (M_w/M_n = 1.37–

1.45 at -48 °C). Moreover, the activity was extremely low, only 4 % of the catalyst was active.

By the addition of anisole or b_y replacing the acetylacetonoate ligands with 2-methyl-1,3- butanedionato ligands, the number of active centers could be increased considerably.

Although the non-group 4 early metal complexes exhibit some characteristics of a living polymerization, a great part of the aforementioned criterion is still not fulfilled. Additionally, the polymerizations have to be carried out at very low temperature so that the productivity was very low. High molecular weight polymers with narrow molecular weight distribution could not be obtained in many cases.

1.2.2.2. Metallocene-based catalysts

Some of the metallocene-based catalysts which promote living propylene polymerization are given in Figure 1-5.

Figure 1-5. Metallocene catalyst precursors for living propylene polymerization.

Bochmann and co-workers reported that 6 activated with B(C6F5)3 at -20 °C produces atactic, high molecular weight PP (M_n = 789 000 g/mol, M_w/M_n = 1.4) that exhibits elastomeric properties.⁴³ The polymerization showed a linear increase in molecular weight with time. Living propylene polymerization has also been observed for bis- Cp type Group IV metallocenes at low temperatures. Fukui and co-workers have reported that [Cp₂ZrMe₂] (7) activated with B(C₆F₅)₃ at -78 °C produces PP with Mw/Mn = 1.15 (Mn = 9 400–27 300 g/mol).⁴⁴ The polymerization shows a linear increase in M_n with time. While at -50 °C, the molecular weight distribution broadens (Mw/Mn = 1.55) for the PP produced by 7/B(C6F5)3, the hafnium analogue (8) produces polymer with Mw/Mn = 1.05–1.08 (Mn = 6 000–34 300 g/mol). Under similar conditions, a titanium complex bearing a linked monocyclopentadienyl-amido ligand, 9/B(C₆F₅)₃ produces

low molecular weight (M_n = 9 800–19 900 g/mol) syndio-enriched PP ([rr] 0.49) with a narrow molecular weight distribution (Mw/Mn = 1.15–1.17).⁴⁵ Also, the polymerization shows a linear increase in M_n with polymer yield. Complex 10, when activated with “dried” MMAO (dMAO) (free of trimethylaluminium) at 0 °C, produced polymer with even higher tacticity ([rr] 0.93) and the molecular weight distribution was 1.45.⁴⁶ Employing an indenyl-based ligand 11 and dMAO at 0 °C in toluene affords quasi-living propylene polymerization.⁴⁷ The PP produced is iso-enriched ([mm] 0.40), has Mw/Mn = 1.38 (Mn = 48 900 g/mol) and shows a linear increase in M_n with time.

In summary, the living character of the metallocene-based catalysts is only retained at low temperature. In general, the molecular weight is relatively low or a broader molecular weight distribution is observed.

1.2.2.3. Bis(phenoxyimine)titanium catalysts

In 1999, Fujita and co-workers reported on a class of group IV complexes bearing chelating phenoxyimine ligands (Figure 1-6). When activated with MAO, these complexes showed extremely high activity for ethylene polymerization.⁴⁸

In contrast to the non-fluorinated analogous 13,⁴⁹ the incorporation of fluorinated N-aryl moieties into the bis(phenoxyimine) ligand framework could provide catalyst precursors for the syndiotactic and living polymerization of propylene. When activated with MAO at 0 °C, 14 (Figure 1-6) produced highly syndiotactic PP ([rrrr] 0.96).⁵⁰ The polymerization exhibited a linear increase in Mn with PP yield and while this trend deviated somewhat at higher molecular weights, polydispersities remained low (M_w/M_n = 1.11) for M_n up to 100 000 g/mol. Living behavior was further exemplified by a good correlation between measured and theoretical Mn

values and by the ability to make block copolymers (see 1.2.4). Remarkably, living behavior was also exhibited at 20 °C (Mn = 100 000 g/mol, Mw/Mn = 1.13).

Fujita and co-workers reported that 15/MAO was also living for propylene polymerization at room temperature, producing polymer with M_n = 28 500–108 000 g/mol and Mw/Mn = 1.07–1.14.⁵¹ While low molecular weight oligomers (Mn = 2000 g/mol, Mw/Mn = 1.08) exhibited 98% rr triads, higher molecular weight samples exhibited lower tacticites ([rr] 87%).

A catalyst using 15 activated with a supported cocatalyst has also been prepared and screened for propylene polymerization. Polypropylene formed using 15/MgCl2/i-BunAl(OR)3-n had narrow PDIs (M_w/M_n = 1.09–1.17, M_n = 53 000–132 000 g/mol) and the polymerization exhibited a linear increase in Mn with reaction time.⁵²

In the last few years, many new bis(phenoxyimine)-based titanium complexes have been synthesized and screened for propylene polymerization activity. Attempts to improve upon early complexes led to new catalysts with both independent and simultaneous variation on the N-aryl and phenoxide moieties. Of note, a complex with a trimethylsilyl group ortho to the oxygen of the phenoxide moiety, 16 (Figure 1-7), has been shown to produce sPP with very high melting temperatures (T_m up to 156 °C).⁵³ Polymerizations with 16/MAO conducted at 0 and 25 °C produced polymer with very narrow molecular weight distributions (Mw/Mn as low as 1.05, Mn = 24 700–47 000 g/mol) while raising the reaction temperature to 50 °C increased the PDI (Mw/Mn

= 1.18–1.23, M_n = 25 500–35 100 g/mol).

Figure 1-7. Bis(phenoxyimine) titanium complexes with various phenoxide substituents.

Changing the aforementioned ortho position to a larger triethylsilyl group (17) did not significantly affect the peak melting temperature or molecular weight distribution of the PP formed relative to 16.⁵⁴ However, employment of a methyl (18) or isopropyl (19) group in the ortho position resulted in a substantial loss of stereocontrol with 18/MAO and 19/MAO both producing amorphous PP which exhibited fairly narrow polydispersities (M_w/M_n = 1.2).

Complex 20 which has a tert-butyl group at the ortho position of the phenoxide moiety and a methyl group para to that position, has also been synthesized. At 25 °C, 20/MAO behaved similarly to the related catalyst 15/MAO (Figure 1-6) yielding sPP with a peak melting temperature of 140 °C and a narrow molecular weight distribution (Mw/Mn = 1.11, Mn = 16 500 g/mol).

Studies on the effect of the fluorination pattern of the N-aryl ring have been conducted and it has been revealed that complexes bearing the 2,4-di-tert-butyl phenoxide moiety require at least one ortho fluorine on the N-aryl ring to exhibit living propylene polymerization behavior.⁵⁵ As the amount of fluorination of the N-aryl moiety is decreased from the perfluoro complex 14 (Figure 1-6) to the monofluoro complex 21 (Figure 1-8), activities for propylene polymerization with the corresponding MAO-activated catalysts decreased while the

Figure 1-8. Bis(phenoxyimine) titanium complexes with varying N-aryl fluorine substitution patterns.

The PP produced by 23/MAO is highly tactic ([rrrr] 0.95), while 22/MAO produces polymer that is somewhat less stereoregular ([rrrr] 0.83). Notably, tacticity drops off significantly upon moving to the monofluorinated complex 21 which affords polypropylene with [rrrr] 0.52. Lastly, increasing the level of fluorination beyond that of 14 by introducing a trifluromethyl group at the para-position of the N-aryl moiety led to an increase in activity. It was reported that 25/MAO was 1.5 times more active than 14/MAO for propylene polymerization but was still able to provide highly syndiotactic polymer ([rrrr] 0.91) with a narrow molecular weight distribution (Mw/Mn = 1.13).⁵⁶

Simultaneous changes to both the phenoxide and N-aryl moieties, relative to 14 (Figure 1-6) have also been made. For example, 26 (Figure 1-9) exhibits ortho fluorination on the N-aryl moiety and iodine substituents on the phenoxide moiety. When activated with MAO at 25 °C, 26 was reported to produce amorphous PP which exhibited a narrow molecular weight distribution (Mw/Mn = 1.17, Mn = 200 000 g/mol).⁵⁷

Figure 1-9. Bis(phenoxyimine) titanium complexes.

Complexes 27-28 employ 3,5-difluorophenyl N-aryl groups and substituents smaller than tert-butyl in the ortho position of the phenoxide moiety. Both 27-28/MAO were shown to polymerize propylene at 0 °C to produce high molecular weight polymer (M_n up to 240 000 g/mol) that is amorphous ([rrrr] 0.48).⁵⁸ For both of these catalysts, there was no evidence of β- H transfer and the polymer produced in each case exhibited a narrow molecular weight distribution (Mw/Mn = 1.13–1.16). This finding was remarkable at the time in that both complexes lack ortho fluorines on the N-aryl moiety.

One recent variation to the bis(phenoxyimine) complexes involves the coordination of two different phenoxyimine ligands to one titanium center. Using pooled combinatorial approach, several heteroligated complexes employing both a ‘‘living’’ and ‘‘non-living’’ ligand were independently synthesized and screened for propylene polymerization. While the molecular weight distribution of the resulting polymers formed from the mixed catalysts fell between those of the corresponding homoligated catalysts, the activity was significantly higher in some cases. For example, PP produced with 13/MAO (Figure 1-6) exhibited a broad PDI (Mw/Mn = 1.41) and a turnover frequency of 42 h^-1 while 14/MAO exhibited a narrow PDI

turnover frequency of 760 h^-1. Syndiotactic polymer ([rrrr] 0.91] was formed with this heteroligated catalyst.

Figure 1-10. Heteroligated bis(phenoxyimine) titanium complex.

1.2.2.4. Bis(phenoxyketimine)titanium catalysts

Bis(phenoxyimine) titanium complexes produce sPP through a chain-end control mechanism despite the fact that the catalyst precursors are C2-symmetric in the solid state and in solution. The reason for this phenomenon stems from a proposed ligand isomerization event that provides enantiomeric coordination sites. It had been proposed that placing a substituent at the imine carbon of the phenoxyimine ligand could prevent this isomerization and lead to the formation of iPP.⁵⁹ Ketimine complexes 30-32 (Figure 1-11) were synthesized and while they were shown to polymerize ethylene in a living fashion upon activation, they proved to be sparingly active for propylene polymerization.^59-60 It was reasoned that reduction of the size of the ortho substituent of the phenoxide moiety could provide a sterically less encumbered active site potentially enabling higher activities for propylene polymerization. Complexes 33-37 were synthesized and screened for propylene polymerization.⁵⁹ Upon activation with MAO at 0 °C, each complex produced PP with a narrow molecular weight distribution (Mw/Mn = 1.12–1.17, Mn = 2 700–35 400 g/mol) and 35/MAO was shown to exhibit a linear increase in PP molecular

weight as a function of yield. The tacticities of the resulting polymers differed. Amorphous, moderately isotactic PP was produced by 33/MAO and 34/MAO ([mmmm] 0.45) while 35/MAO produced PP with higher tacticity ([mmmm] 0.53) and a peak melting temperature of 69.5 °C. This level of tacticity was increased ([mmmm] 0.61, Tm 96.4 °C) when the polymerization was run at 20 °C. Having R1= H, 36/MAO exhibited a significant loss of stereocontrol ([mmmm] 0.08). The aldimine analogue of 35 (37) in which R₃= H, furnishes atactic PP with Mn = 123 100 g/mol and Mw/Mn = 1.13.

Figure 1-11. Bis(phenoxyimine) titanium complexes.

1.2.3. Living ethylene polymerization

1.2.3.1. Non-group 4 metal polymerization catalysts

Apart from the group 4 metal complexes, examples with group 3, group 5 transition metals and also with late transition metals have been reported relating to the living ethylene

1.2.3.1.1. Group 3 metal polymerization catalysts (Y, lanthanides)

The potential of f-block metal complexes to serve as living olefin polymerization catalysts was demonstrated by Marks and co-workers in 1985.⁶¹ The dimeric bis(Cp*) hydride complexes of lanthanum, neodymium, and lutetium (38-40, Figure 1-12) were shown to polymerize ethylene with extremely high activities (up to 3040 g PE/mmol^-1 min^-1 atm^-1). The PEs exhibited high molecular weights (M_n = 96 000-648 000 g/mol) with M_w/M_n = 1.37–4.46.

For the most part, the PDIs were lower than 2.0 (e.g. 61c exhibited Mw/Mn = 1.37–1.68), with higher values being due to inhomogeneities brought about by precipitation of the polymer, possible mass-transport effects and rate-limiting dissociation of the dimer. Further evidence in support of the living nature of ethylene polymerization catalyzed by 38-40 includes the observations that catalytic activity is maintained at room temperature for up to 2 weeks, M_n increases with increasing reaction time, and the number of polymer chains per metal center is consistently less than one which argues against chain-transfer followed by reinitiation.

Figure 1-12. Lanthanide and group 3 metal catalysts for living olefin polymerization.

Hessen and coworkers reported on the synthesis and ethylene polymerization activity of dialkyl(benzamidinate)yttrium complexes which displayed some characteristics of living behavior.⁶² When treated with [PhNMe2H][B(C6F5)4], 41 (Figure 1-12) catalyzed the formation of PE. The resultant polymers possessed narrow molecular weight distributions (M_w/M_n = 1.1–

1.2) and high molecular weights (Mw = 430 000–1 269 000 g/mol). It was shown that about 1.1

polymer chains per metal center were produced and this number was constant over the course of the 30 min. Taken together, these observations suggest that 41 [PhNMe2H][B(C6F5)4] is living for ethylene polymerization.

1.2.3.1.2. Group 5 metal polymerization catalysts (V, Nb, Ta)

Figure 1-13. Niobium and tantalum complexes for living ethylene polymerization.

Nomura and co-workers reported that arylimido(aryloxo)vanadium dichloride complexes activated with Et2AlCl exhibited characteristics of living ethylene polymerization.⁶³ In particular, 42/Et₂AlCl (Figure 1-13) furnished PE with a narrow molecular weight distribution (M_w/M_n = 1.42) and high molecular weight (Mn = 2 570 000 g/mol) at 0 °C. Examples of living ethylene polymerization by Group 5 metal complexes are not limited to those based on vanadium. In 1993, Nakamura and coworkers reported on the synthesis and ethylene polymerization behavior of cyclopentadienyl(η⁴-diene)tantalum complexes.⁶⁴ Upon activation with MAO at temperatures of 20 °C or below, compounds 43-45 (Figure 1-13) furnished PEs with narrow molecular weight distributions (Mw/Mn = 1.4) with Mn = 8 600–42 900 g/mol. In a subsequent report, the analogous niobium complexes were shown to behave as living ethylene polymerization catalysts up to 20 °C.⁶⁵ Compounds 46-49 (Figure 1-13) when activated with MAO in the presence of ethylene furnished polymers with narrow polydispersities (Mw/Mn = 1.05–1.30) and Mn = 5 100–

1.2.3.1.3. Chromium polymerization catalysts

Theopold and co-workers investigated ethylene polymerization catalyzed by chromium complexes bearing 2,4-pentane-N,N’-bis(aryl)ketiminato ((Ar)₂nacnac) ligands as a potential model for ethylene polymerization by the heterogeneous Philips catalyst.⁶⁶ When exposed to ethylene at room temperature, 50 (Figure 1-14) formed polymer with Mn = 10 000–110 000 g/mol and narrow molecular weight distributions (M_w/M_n = 1.17–1.4). The M_n was shown to increase as a linear function of polymer yield. These results represented the first report of living ethylene polymerization with a chromium-based catalyst.

Figure 1-14. β-Diiminate chromium complex for living ethylene polymerization.

1.2.3.1.4. Late transition metal polymerization catalysts (Co, Ni, Pd)

Brookhart and coworkers have used varied Cp* cobalt complexes for the synthesis of end-functionalized PEs.⁶⁷ Reaction of 51-55 (Figure 1-15) with 1 atm of ethylene in chlorobenzene for 3 h led to the formation of low molecular weight, aryl-substituted PEs with quite narrow molecular weight distributions (Mn up to 21 200 g/mol, Mw/Mn = 1.11–1.16).

Significant advances both in terms of increasing molecular weights and decreasing polydispersities of PE were achieved with palladium catalysts 56-57 (Figure 1-15).⁶⁸ At 5 °C, highly branched (100 branches/1000 carbons), amorphous PEs with very narrow molecular weight distributions (M_w/M_n = 1.1) were produced. Molecular weight distributions broadened

somewhat at 27 °C for long reaction times. Pd diimine catalysts were further extended to generate both mono- and di-end functionalized, monodisperse, highly branched polyethylenes.⁶⁹ Use of ester-functionalized catalyst 56 allowed for preparation of branched polyethylenes with a methyl ester end group at the beginning of the chain. Further, a telechelic polymer could be produced by addition of alkyl acrylates before the silane quench. Acrylates undergo one insertion into the growing chain, forming stable chelates, but do not insert further, allowing for clean end-functionalization without block formation. By this method it was possible to generate polymers with two distinct ester end groups.

Figure 1-15. Late transition metal catalysts for living ethylene polymerization.

Guan and co-workers have extended the study of hindered diimine catalysts with cyclophane complex 58 (Figure 1-16).⁷⁰ When activated with MMAO, 58 is highly active for production of branched PEs (66–97 branches/1000 carbons) with relatively narrow polydispersities (M /M as low as 1.23 at 50 °C). Most significantly, these catalysts exhibit

polydispersities increase, and the activities decrease somewhat at higher temperatures.

In order to circumvent the above-mentioned instability of conventional Ni α-diimine catalysts Brookhart and co-workers have investigated a series of anilinotropone-based nickel catalysts 59-61.⁷¹ With activation by Ni(COD)2, high activities and long lifetimes were observed, particularly in the aryl-substituted cases, 60-61 (Figure 1-16). The Mn was shown to increase in nearly linear fashion with time, suggesting minimal chain transfer. PDIs were relatively narrow (as low as 1.2 at room temperature), but increased at higher temperatures and with longer reaction times.

Figure 1-16. Ni complexes for living polymerization.

Bazan and co-workers have observed quasi-living ethylene polymerization behavior with nickel diimine variant 62 (Figure 1-16).⁷² With Ni(COD)2 as activator, Mn increased linearly with time up to 30 min at 20 °C, producing a PE with low branching (12–19 methyl branches/1000 carbons). Molecular weight distributions were as low as 1.3, somewhat larger than expected for a truly living system, which the authors attribute to a slow initiation, or precipitation of the product.

1.2.3.2. Bis(phenoxyimine)titanium catalysts

It has been shown that incorporation of ortho-fluorinated N-aryl moieties into the bis(phenoxyimine) framework is typically required to obtain catalysts that are living for

propylene polymerization. The living polymerization of ethylene has been reported with many of the same catalysts. For example, two of the earliest bis(phenoxyimine) titanium complexes reported for living syndiospecific propylene polymerization, 14 and 15 (Figure 1-6, have also been shown to polymerize ethylene in a living fashion. In 2001, Fujita and co-workers reported that at 25 °C, 15/MAO polymerized ethylene to produce linear PE with a high molecular weight and narrow molecular weight distribution (M_n = 412 000 g/mol, M_w/M_n = 1.13).⁷³ Furthermore, polymerizations at 25, 50 and 75 °C exhibited a linear increase in Mn with reaction time although, at 75 °C, molecular weight distributions broadened with longer reaction times (M_w/M_n

= 2.05 at 15 min). As previously discussed, some of the earliest group IV bis(phenoxyimine) complexes (including 12 (Figure 1-6 and its zirconium analogue) have been shown to be precursors for highly active ethylene polymerization catalysts.⁴⁸ In 2003, Coates and co-workers reported that 13/MAO polymerized ethylene at 50 °C to produce polyethylene with Mn = 44 500 g/mol and M_w/M_n = 1.10; the measured molecular weight of the polymer matching well with the theoretical value.⁶⁰ The ability of 12 to polymerize ethylene in a living fashion was reported later. Fujita and co-workers reported on ethylene polymerization behavior with the same system and found that while molecular weight distributions were low at a reaction time of 1 min (Mw/Mn = 1.12, Mn = 52 000 g/mol), the molecular weight distribution broadened significantly at reaction times of just 5 min (M_w/M_n = 1.61, M_n = 170 000 g/mol).⁷⁴

Figure 1-17. Bis(phenoxyimine) titanium catalyst precursors for living ethylene polymerization.

While 63-64/MAO (Figure 1-17) produced amorphous polypropylene with bimodal GPC traces, each of these catalysts has been shown to be well-behaved for ethylene polymerization at 50 °C and 63/MAO and 64/MAO produced polymer with narrow molecular weight distributions at reaction times between 1 and 5 min (Mw/Mn = 1.05, Mn = 13 000–64 000 g/mol).⁷⁵ Lastly, Fujita and co-workers reported that ZnEt2 could be used as a chain-transfer agent in the living ethylene polymerization employing 63/MAO.⁷⁶ In this system, a living PE chain-end reacts with ZnEt2 only after all ethylene has been consumed. This leads to a zinc endfunctionalized polymer chain and a titanium species that is able to grow another living chain upon addition of monomer.

When 63 is employed, residual ZnEt2 does not appear to interfere with the second stage of the polymerization as PDI values for the final polymer are low. However, the authors found that when 64 is used in this system, the second stage of the polymerization was no longer living as the ZnEt2 appeared to react with the living chain-end despite the presence of monomer. It was concluded that the nature of the ortho phenoxide substituent was crucial in dictating reactivity between the polymer chain-end and ZnEt2.

1.2.3.3. Bis(phenoxyketimine)titanium catalysts

While bis(phenoxyketimine) titanium complexes that have a bulky substituent ortho to the phenoxide-bearing carbon show very low activity for propylene polymerization, such complexes, upon activation, can polymerize ethylene in a living fashion. For example, Coates and co-workers reported that at 0 and 20 °C, 30-32/MAO (Figure 1-11) all produced PE that exhibited a narrow molecular weight distribution (M_w/M_n = 1.08) and had number average molecular weights (Mn = 15 000–47 000 g/mol) that coincided with theoretical values.

Furthermore, additional experiments demonstrated a linear increase in M_n with polymer yield for the polymerization catalyzed by 32/MAO at 0 °C and for 31/MAO at 50 °C.⁶⁰ It was also shown that a related complex (65, Figure 1-18), when activated with MAO at 50 °C, produced PE with M_w/M_n = 1.08 (M_n = 9 000 g/mol).⁷⁴

Figure 1-18. Bis(phenoxyketimine) titanium catalyst precursor for living ethylene polymerization.

1.2.3.4. Titanium indolide-imine catalysts

As the bis(phenoxyimine)titanium complexes exhibited good living propylene and ethylene polymerization properties, Fujita and co-workers synthesized structurally similar bis(indolide-imine)- titanium complexes and evaluated their potential as ethylene polymerization catalysts.⁷⁷ When activated with MAO in the presence of ethylene at room

distributions (M_w/M_n = 1.11–1.23) with M_n = 11 000–90 000 g/mol. Ethylene polymerization by 68/MAO exhibited a linear increase of Mn with increasing polymer yield. The catalyst system comprised of 68/MAO also showed impressive thermal stability in that the PE obtained at 50 °C retained a narrow PDI value of 1.24. In a subsequent report, it was shown that exhaustive fluorination of the N-aryl moiety (69) had deleterious effects on the living character of ethylene polymerization.^77b The PE produced by 69/MAO at 25 °C possessed a broadened molecular weight distribution relative to the polymers produced by 66-68/MAO under identical conditions (69: M_w/M_n = 1.93). The polymerization activity of 69/MAO was much higher than those for 66- 68/MAO. Upon lowering the temperature to 10 °C, ‘‘quasi-living behavior’’ was observed; the Mn was shown to increase linearly with time over the course of 6 min, and the molecular weight distribution decreased to give M_w/M_n = 1.12–1.15. No evidence for chain transfer to aluminum or β-H transfer was observed.

Figure 1-19. Bis(indolide-imine) titanium catalysts.

1.2.3.5. Bis(enaminoketonato)titanium catalysts

There are myriad examples of homogeneous olefin polymerization catalysts based on titanium, and a significant portion of these exhibits living behavior. The most prolific classes of living, titanium-based olefin polymerization catalysts bear ligands containing nitrogen and oxygen donors. In line with this observation, Li and co-workers reported on the synthesis and ethylene polymerization activity of bis(enaminoketonato)titanium complexes.⁷⁸ Upon activation with MMAO in the presence of ethylene at 25 °C, 70-71 (Figure 1-20) furnish linear PEs with