Catalytic polymerization of acrylates and in supercritical carbon dioxide

Catalytic polymerization of acrylates and in supercritical

carbon dioxide

Dissertation

zur Erlangung des Akademischen Grades eines Doktors der Naturwissenschaften

(Dr. rer. nat.)

an der Universität Konstanz

Mathematisch-Naturwissenschaftliche Sektion Fachbereich Chemie

vorgelegt von

Damien Guironnet

aus Parnans

Konstanz 2009

Tag der mündlichen Prüfung : 14.09.2009

Referent: Prof. Dr. S. Mecking Koreferent:. Dr. M. Drescher Koreferent:. Prof. Dr. G. Müller

Vorsitzender des Prüfungsausschusses: Prof. Dr. S. Mecking

Fachbereichssprecher: Prof. Dr. G. Müller

Leiter der Arbeit: Prof. Dr. S. Mecking

Referent: Prof. Dr. S. Mecking

Koreferent: Dr. M. Drescher

Die vorliegende Arbeit entstand in der Zeit von Oktober 2005 bis August 2009 in der Arbeitsgruppe von Prof. Dr. Stefan Mecking im Fachbereich Chemie der Universität Konstanz.

Prof. Dr. Mecking danke ich sehr herzlich für sein fortwährendes Interesse an meiner Arbeit, die vielen Diskussionen und Anregungen und die wertvolle Unterstützung in den letzten drei Jahren.

Bei. Dr. M. Drescher bedanke ich mich für die Übernahme des Koreferats.

Dr. Lucia Caporaso von der Universität Salerno danke ich für die DFT Berechnungen.

Dr. Thierry Tassaing vom LPCM CNRS-UMR 5803 in Bordeaux danke ich für den Zugang zu CO2

Hochdruckzellen, die für die Löslichkeitsmessungen verwendet wurden.

Diese Arbeit wurde von dem BMBF (Projekt 03X5505) und der DFG (Me 1388/4) finanziell unterstützt.

Danksagung

Herzlicher Dank gebührt allen, die zum Gelingen dieser Arbeit beigetragen haben.

Besonderer Dank gilt Dr. Inigo Göttker-Schnetmann für sein immerwährendes Interesse, sowie seine intensive Unterstützung und Hilfe während meiner gesamten Promotion. Darüberhinaus bedanke ich mich bei ihm für die Durchführung von Einkristallstrukturbestimmungen.

Ich danke allen Kollegen der Arbeitsgruppe Mecking für fachliche Diskussionen, insbesondere Sze- Man Yu, Dr. Inigo Göttker-Schnetmann, Dr. Abderrahmane Amgoune, Dr. Juan Urbano, Johannes Huber, Stefan Matt, Dr. Peter Wehrmann und Dr. Andreas Berkefeld. Bei Dr. Werner Röll und Robin Kirsten bedanke ich mich für ihre Unterstützung in technischen Fragestellungen. Lars Bolk sei gedankt für die Durchführung von GPC- und DSC-Messungen. Bei Dr. Marina Krumova bedanke ich mich für TEM-Messungen. Anke Friemel und Ulrich Haunz sei gedankt für das Messen von NMR Spektren und für das Lösen NMR-gerätetechnischer Probleme.

Desweiteren möchte ich mich bei allen von mir betreuten Studenten für ihre engagierte Mitarbeit bedanken. Diese waren in chronologischer Reihenfolge: Anna Osichow, Philipp Roesle, Caroline Sugnaux, Thomas Rünzi, Philipp Wucher, Friederike Schütze, Fabian Geist und Tobias Friedberger. Ich hoffe, dass Sie von mir genau so viel lernen konnten wie ich von Ihnen.

Schließlich danke ich von ganzem Herzen meiner Familie, meinen Eltern, meiner Schwester, meiner Freundin und meinen Freunden für die fortwährende Unterstützung während meiner Promotion, obwohl ich während dieser Zeit zu wenig Zeit für Sie hatte.

Publications and communications

Parts of this work have been published:

D. Guironnet, T. Rünzi, I. Göttker-Schnetmann and S. Mecking: “Control of molecular weight in Ni(II)-catalyzed polymerization via the reaction medium” Chem. Commun. 2008, 4965-4967

D. Guironnet, P. Roesle, T. Rünzi, I. Göttker-Schnetmann and S. Mecking: “Insertion polymerization of acrylate” J. Am. Chem. Soc. 2009, 141, 422-423

D. Guironnet, I. Göttker-Schnetmann and S. Mecking: “Catalytic polymerization in dense CO2 to controlled microstructure polyethylenes”, Macromolecules, accepted.

D. Guironnet, T. Friedberger and S. Mecking: “Ethylene polymerization in supercritical carbon dioxide with binuclear Nickel (II) catalysts”, submitted

D. Guironnet and S. Mecking: “Copolymerization of ethylene and 1-olefins in supercritical CO2 by an electron poor Ni(II) complex” Polymer Preprints 2008, 49, 450-451

Publication related to this work:

D. Zhang, D. Guironnet, I. Göttker-Schnetmann and S. Mecking: “Water-soluble complexes [(κ²-P,O- Phosphinesulfonato)PdMe(L)] and their catalytic properties” Organometallics, 2009, 28, 4072-4078

Oral communications.

„Copolymerization of ethylene and 1-olefins in supercritical CO₂ by an electron poor Ni(II) complex”

ACS meeting, New Orleans, 2008 Poster.

„Catalytic olefin polymerization in supercritical CO2 with Ni(II) complexes”

ICOMC, Rennes, 2008

„Catalytic olefin polymerization in supercritical CO2 with Ni(II) complexes”

Green Solvents: Progress in Science and Applications, Friedrichshafen, 2008

„Insertion polymerization of acrylate”

CaRLa winterschool, Heidelberg, 2009.

solvent for A

non-s olvent for A

branched oligomer M_n < 10³ g mol^-1

linear polymer M_n > 10⁵ g mol^-1

C₂H₄

> 10⁵ TO h^-1

> 10⁴ TO h^-1

Chapter 2: Control of molecular weight in Ni(II)- catalyzed polymerization via the reaction medium

A

n m

n m

n scCO₂

Ni L N O

Me

UHMWPE LLDPE

Poly(E-co-NB) Chapter 3: Catalytic polymerization in dense

CO₂ to controlled microstructure polyethylenes

P = 65 MPa

N R O CF₃ F₃C

F₃C

CF₃

CF₃ F₃C

Ni Me N

X

2

n

M_w ~ 10⁴ g mol^-1 M_w / M_n ~ 2 C₂H₄

+

scCO

P = 65 MPa T = 70 °C

Chapter 4: Ethylene polymerization in supercritical carbon dioxide with binuclear Nickel (II) catalysts

S P

O O

O O Pd

Me O

O O

+ dmso

O O O O

O O O O O O copolymerization

oligom

erization

up to 50 mol% MA

up to 9-units Chapter 5: Weakly coordinated complexes for the

(co)polymerization of ethylene and polar monomers

O O

+ S

P

O O

O O Pd

Me

O S O

Pd P

O O

Ar

O

OMe

O OMe

α β γ

*

* Ar

O O O O O O isolated intermediate Chapter 6: Mechanistic insight into

catalytic oligomerization of acrylate.

List of abbreviations

Equipment and methods

NMR nuclear magnetic resonance

GPC gel permeation chromatography

DSC differential scanning calorimetry

TEM transmission electron microscopy

FAB fast atom bombardment

HT high temperature

MALDI matrix assisted laser desorption ionization Compounds

acac acetylacetonate

Ar aryl

cod cyclooctadiene

Et ethyl

iPr iso-propyl

L ligand

M metal atom

MAO methylalumoxane

Me methyl

PE polyethylene

Ph phenyl

TCE-d2 1,1,2,2 tetrachloroethane-d2

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

X^Y κ² X-Y, coordinated ligand

dmso dimethylsulfoxide

MA methyl acrylate

THF tetrahydrofuran

scCO2 supercritical carbon dioxide

PMA poly(methyl acrylate)

BHT butylated hydroxytoluene

Spectroscopy

br broad

COSY correlation spectroscopy

δ chemical shift in ppm

d doublet

g-HMBC gradient - heteronuclear multiple bond correlation g-HSQC gradient- heteronuclear single bond correlation

nJXY coupling constant of atom X to atom Y over n bonds

m multiplet

ppm parts per million

q quartet

s singlet

t triplet

vt virtual triplet

Miscellaneous

M_n number average molecular weight

M_w weight average molecular weight

Mv viscosity average molecular weight

M_w / M_n molecular weight distribution

DP_n degree of polymerization

TON turnover (number) =

TOF = TO h^-1: turnover frequency =

RT room temperature

Tm melting temperature

vs. versus

Since the seminal reports by Ziegler and Natta, catalytic olefin polymerization has gained a tremendous impact. While the initial discoveries resulted from the “nickel effect”, today’s industrial catalysts are based on early transition metals. Later on, late transition metals were commercialized for ethylene oligomerization. More recently, olefin polymerization by late transition metals has received growing interest. This interest is mostly based on two major differences between late transition metal complexes and their early transition metals counterparts. First, due to their lower oxophilicity, late transition metal complexes are more tolerant to polar groups. And second, due to their propensity for β- hydride elimination, polymers with unique microstructure can be prepared.

A major advance in this area was the finding by Brookhart and co-workers that cationic diimine complexes of Ni(II) or Pd(II) can polymerize ethylene to a highly branched polymer, due to “chain walking” of the catalyst. This discovery motivates the interest of the chemical community to develop new more active late transition metal catalysts for olefin polymerization (chapter 2). A general issue is the suppression of chain transfer in order to form high molecular weight polymer.

Recent progress in catalyst design allows for the preparation of a broad scope of polyolefin microstructures, offering new opportunities in polymerization processes. In this context, carbon dioxide is an interesting reaction medium, due to its unique solvent properties. Obviously, the high oxophilicity of Ziegler catalysts and metallocenes (the catalysts used industrially) prevent their use in any oxygenated solvent. Thus, late transition metal catalysts appear to be particularly suited for this purpose. However this perspective requires the finding of catalysts well soluble in scCO2 and with increased polymerization productivity and a control of polymer microstructure and properties (chapters 3 and 4).

In addition, late transition metal catalysts have attracted attention not only for their tolerance towards polar molecules as ideally unreactive reaction media but more importantly for the copolymerization of hydrocarbon monomer with readily available polar monomers as substrates. Up to now, from the large numbers of late transition metal catalysts reported, only two families of complexes have been found to

incorporate such polar monomers. Brookhart demonstrated the insertion copolymerization of methyl acrylate with ethylene by Pd(II) diimine catalysts. However, other common comonomers such as vinyl acetate, acrylonitrile and vinyl chloride could not be copolymerized by these cationic complexes. By contrast, neutral Pd(II) complexes with chelating phosphinosulfonate ligands copolymerize ethylene with a relatively broad scope of functional vinyl monomers. However, hindered dissociation of the catalyst precursor complexes renders a large part of the metal sites inactive. There is a need for more reactive complexes for utilization as catalyst precursors (chapter 5) and for stoichiometric studies directed towards a mechanistic understanding (chapter 6). This enables unprecedented reactions and polymer microstructures.

Scope of the thesis ______________________________________________________ 9 1 Insertion polymerization with late transition metals ________________________ 13

1-1. Introduction ____________________________________________________________________ 14 1-2. Polymerization of ethylene with late transition metal catalysts __________________________ 15 1-2-1. Cationic complexes __________________________________________________________________ 15 1-2-2. Neutral complexes ___________________________________________________________________ 19 1-3. Insertion polymerization of ethylene in unconventional solvents _________________________ 22 1-3-1. Aqueous medium ____________________________________________________________________ 22 1-3-2. Supercritical carbon dioxide __________________________________________________________ 25 1-4. Copolymerization of ethylene and polar monomers ___________________________________ 26 1-5. References ______________________________________________________________________ 30

2 Control of molecular weight in Ni(II)-catalyzed polymerization via the reaction medium _____________________________________________________________ 37

2-1. Introduction ____________________________________________________________________ 38 2-2. Results and discussion ____________________________________________________________ 39 2-2-1. Complex synthesis ___________________________________________________________________ 39 2-2-2. Polymerization of ethylene ____________________________________________________________ 40 2-3. Summary and conclusion _________________________________________________________ 45 2-4. Experimental section _____________________________________________________________ 45 2-5. References ______________________________________________________________________ 53

3 Catalytic polymerization in dense CO

to controlled microstructure polyethylenes 55

3-1. Introduction ____________________________________________________________________ 56 3-2. Results and discussion ____________________________________________________________ 57 3-2-1. Catalyst precursors __________________________________________________________________ 57 3-2-2. Polymerization of ethylene ____________________________________________________________ 62 3-2-3. Polyethylene microstructure __________________________________________________________ 64 3-2-4. Copolymerization of 1-olefins _________________________________________________________ 66 3-2-5. Polymer morphology ________________________________________________________________ 69 3-3. Summary and conclusion. _________________________________________________________ 70 3-4. Experimental section _____________________________________________________________ 71 3-5. References ______________________________________________________________________ 78

4 Ethylene polymerization in supercritical carbon dioxide with binuclear nickel (II) catalysts _____________________________________________________________ 83

4-1. Introduction ____________________________________________________________________ 84 4-2. Results and discussion ____________________________________________________________ 84 4-2-1. Synthesis of ligands and complexes _____________________________________________________ 84 4-2-2. Catalytic ethylene polymerization with binuclear Ni(II) methyl pyridine complexes ____________ 86 4-3. Summary and conclusion _________________________________________________________ 89

4-4. Experimental section _____________________________________________________________ 90 4-5. References ______________________________________________________________________ 94

5 Weakly coordinated complexes for the (co)polymerization of ethylene and polar monomers ___________________________________________________________ 97

5-1. Introduction ____________________________________________________________________ 98 5-2. Results and discussion ____________________________________________________________ 99 5-2-1. Complex synthesis ___________________________________________________________________ 99 5-2-2. Coordination chemistry _____________________________________________________________ 101 5-2-3. Ethylene polymerization _____________________________________________________________ 102 5-2-4. Ethylene acrylate copolymerization ___________________________________________________ 105 5-2-5. Dye copolymerization _______________________________________________________________ 108 5-2-6. Acrylate oligomerization ____________________________________________________________ 111 5-2-7. Copolymerization of ethylene with vinyl ether and acrylonitrile ____________________________ 116 5-3. Summary and conclusion ________________________________________________________ 117 5-4. Experimental section ____________________________________________________________ 119 5-5. References _____________________________________________________________________ 137

6 Mechanistic insight into catalytic oligomerization of acrylate _______________ 141

6-1. Introduction ___________________________________________________________________ 142 6-2. Results and discussion ___________________________________________________________ 143 6-2-1. Coordination chemistry of multinuclear weakly coordinated species ________________________ 143 6-2-2. Polymerization of ethylene with multinuclear weakly coordinated species ___________________ 146 6-2-3. Reactivity of multinuclear weakly coordinated species towards acrylate _____________________ 147 6-2-4. Coordination chemistry of double methyl acrylate insertion products _______________________ 149 6-2-5. DFT studies _______________________________________________________________________ 153 6-3. Summary and conclusion ________________________________________________________ 158 6-4. Experimental section ____________________________________________________________ 160 6-5. References _____________________________________________________________________ 170

Zusammenfassung ___________________________________________________ 172

1

Insertion polymerization with late transition metals

“In comparison to their early transition metal counterparts, d⁸ metals are much more tolerant towards functional groups in the substrates or reaction media”

1-1. Introduction

Polyolefins are of vast economic importance, which is reflected by an annual production of more than 70 million tons of polyethylene and polypropylene. While the major portion of these materials is produced with Ziegler- and chromium-based catalysts, the older free-radical process that affords low- density polyethylene (LDPE) has maintained its significance.¹ Despite the necessity of working at over 1500 bar and virtually a lack of any control of the polymer microstructure, 16 million tons of LDPE are currently consumed annually and new large plants continue to be built.^1d,e One attractive feature of the high-pressure process is the possibility of incorporating functionalized olefins, such as vinyl acetate or acrylates. Incorporation of even small amounts of polar moieties can increase adhesion properties and compatibility of polyolefins with other materials. Another attractive feature is the different property profile of LDPE compared to the linear ethylene homo- and copolymers produced by Ziegler catalysts.

In the free-radical polymerization of ethylene, short- as well as long-chain branches are formed without any added co-monomer. Short-chain branches affect polymer properties, such as crystallinity and melting temperature, and are important in controlling polyolefin application properties. Long-chain branches (typically containing 100 or more carbon atoms) particularly influence the rheology of polyolefin melts, and result in good processing properties of LDPE. These considerations exemplify existing challenges for transition metal catalyzed coordination polymerization in low-pressure processes.

In regard to the desirable incorporation of polar monomers, early transition metal based Ziegler catalysts and metallocenes are, unfortunately, highly sensitive to polar reagents. By comparison, late transition metal complexes are generally much more functional-group tolerant as a result of their less oxophilic nature. The presumed greater functional group tolerance of late transitions metals relative to early metals make them likely targets for the development of catalysts for the copolymerization of ethylene with polar comonomers. In addition, they can provide access to unique polyolefin branching structures.

major advances. However, an insertion polymerization of electron deficient polar-substituted vinyl monomers has remained elusive. E.g., polyacrylates are not accessible by insertion polymerization.²

1-2. Polymerization of ethylene with late transition metal catalysts

1-2-1. Cationic complexes 1-2-1-1. α-diimine ligands

In 1995, the report by Brookhart and co-workers that a family of new cationic Pd(II) and Ni(II) α- diimine catalyst precursors enables the polymerization of ethylene to high molecular weight polyethylene renewed the interest of developing new catalysts based on late transition metals.³

The approach of Brookhart was based involves the use of sterically bulky α-diimine ligands as fundamental feature. The sterically demanding o-aryl substituents are located in the axial position above and below the square planar coordination plan of the metal center. It enables polymerization to occur to high molecular weights by retardation of chain transfer. These catalysts are unique in polymerizing ethylene to highly branched, high molecular weight homopolymers at remarkable reaction rates.

Activities of up to 4 × 10⁶ TO h^-1 were reported for the cationic Ni(II) catalysts (Scheme 1-1).^3,,4

Scheme 1-1. Polymerization of ethylene by cationic Ni(II) and Pd(II) complexes with diimine ligands reported by Brookhart

N

R' N

R'

R R

R R

M Me L

+ Y^-

M = Pd, Ni

R' = Me, H, 0.5 napthtalene-1,8-diyl R = Me, iPr

L = Et₂O, R"CN Y^- = BArF^-, SbF₆^-

linear to highly branched ethylene homopolymer

1

1 (M = Ni, Pd) or [(N^N)NiBr₂] / MAO

-10 to 60 °C

N N

M P + Steric shielding

The polyethylenes obtained with these catalysts contain even- and also odd-number carbon branches (Me, Et, Pr and longer branches). Whereas the polymers obtained with Ni(II) catalysts exhibit predominantly methyl branches, the Pd(II) catalysts yield a large portion of longer branches (approximately 100 branches per 1000 carbon atoms) in the polymer. These microstructures arise from a

“chain running” process⁵, similar to the 2,ω− polymerization previously reported by Fink et al.⁶

The synthesis of α-diimines involves the condensation of a diketone with 2 equivalents of an aniline.

Using these synthetic routes, the modification of backbone and aryl substituents are readily achievable, enabling the preparation of a large number of ligands with independent control over the steric and the electronic effects at the metal center.⁷

Chart 1- 1

N N

M

X Y

Cyclophane diimine precursor (Guan)

N N

R' R

M

X Y

Terphenyl substituted diimine precursor (Rieger)

R R

R R

'

The steric hindrance of o-aryl substituents has a major impact on the polymer microstructure as well.

Many groups have focused their interest in the preparation of more bulky ligands (Chart 1-1).⁸ A model compound of the modifications possible on the diimine ligand is the cyclophane substituted diimine ligand reported by Guan. The unique cyclophane framework of the catalyst ligand prevents catalyst deactivation and provides more efficient steric blocking of the axial positions. This allows for the living polymerization of α-olefins at elevated temperature.⁹

1-2-1-2. Bis(imino)pyridine ligands

Bis(imino)pyridine iron (II) and cobalt (II) complexes, first introduced as catalysts for ethylene polymerization by Brookhart, Bennett and Gibson, have attracted substantial interest.¹⁰ These complexes show exceptionally high activities for ethylene polymerization producing strictly linear higher molecular weight polymer (Mn = 6 × 10⁵ g mol^-1, Mw/Mn ~ 9). In analogy to the previously mentioned diimine- substituted Ni(II) and Pd(II) catalysts, the steric bulk of the o-aryl substituents retards chain transfer.

Thus reducing the steric bulk of R in the iron complexes yields catalysts for ethylene oligomerization (Scheme 1-2). Unprecedented activities of up to 10⁸ TO h^-1 for formation of > 99% linear α-olefins were observed, far exceeding the activities reported for the shell higher olefin process (SHOP) for oligomerization of ethylene to α-olefins and even of most early transition metal polymerization catalysts.¹¹

Scheme 1-2 Polymerization of ethylene by cationic Fe(II) and Co(II) complexes with bis(imino)pyridine ligands.

N N R

R

N R

R M

Cl Cl

2

+ (M = Fe, Co)

MAO

R = iPr R = Me

n

n

HDPE

α-olefin

Despite their impressive activity, these iron and cobalt complexes exhibit a major drawback, since they require aluminum alkyl co-catalysts for activation.¹² Consequently, the use of aluminum alkyls makes these catalyst systems incompatible with polar media or even polar monomers. As a remarkable feature, the iron catalysts do not incorporate 1-olefins, even under conditions were most other polymerization catalyst generate ethylene-1-olefin copolymers.

1-2-2. Neutral complexes 1-2-2-1. [P^O] ligand

Monoanionic [P^O] ligands on group 10 metals have been extensively studied in the context of the Shell higher olefin process (oligomerization of ethylene to linear α-olefins, complex 3).¹³ Depending on the catalyst and on the reaction conditions, higher molecular weight oligomers and polymer can be afforded. Nevertheless, number average molecular weights obtained by phosphinoenolate nickel complexes remain limited.¹⁴

Chart 1-2

P O

Ni Ph L Ph Ph R

R HO P

O Ni Ph Ph

L Ph '

L = iPr₃P=CH₂, O=PPh₃ or pyridine

P O

O Ni Ph Ph

(C₆F₅)₃B

3 4 5

Gibson and coworkers have shown that very bulky groups positioned adjacent to the oxygen donor afford dramatic increases in activity. However, the polymer properties were not affected. Linear PE with Mn in the range of 5 × 10³ - 1 × 10⁴ g mol^-1 is obtained.¹⁵ An approach to transform a neutral Ni catalyst into a more active catalyst has been reported by Bazan and co-workers. Addition of B(C6F5)3 to the phosphinocarboxylate Ni complex results in the formation of the electron poor cationic species via coordination of the carboxylate unit (complex 5). The cationic derivatives were highly active catalyst for the insertion polymerization of ethylene but only low molecular weight oligomers are formed.¹⁶

More recently Pugh and co-workers reported an in situ catalyst prepared by reaction of o-sulfonated phosphine (Ar2PC6H4SO3H, complex 10), with [Pd⁰(dibenzylideneacetone)] for the preparation of linear ethylene acrylate copolymer (vide infra). In ethylene homopolymerization, linear polymer was obtained.¹⁷ Further optimization of the structure of the catalyst precursors resulted in substantially increased ethylene polymerization activities and polymer molecular weight.¹⁸ With bulky substituted

aryl groups on the phosphine donor (Ar = 2-(2-methoxyphenyl)phenyl) ethylene homopolymerization proceeds with an average activity of 7 × 10⁵ TO h^-1 at 100 °C in a one hour polymerization experiment.

A linear polyethylene with M_n = 1.4 × 10⁵ g mol^-1 (M_w/M_n ~ 3) was obtained.

1-2-2-2. [N^O] ligand

The perhaps most prominent class of neutral Ni(II) polymerization catalysts are salicylaldiminato complexes. These are versatile catalysts that afford high molecular weight polyethylene, and are capable of homoligomerization and of copolymerization of α-olefins. In addition, the polymer microstructure can be varied over a wide range by remote susbstituents.¹⁹

Chart 1-3

F₃C O N

Ni Me R¹ R¹

F₃C O

L N

O Ni

Me L N

R¹ R¹

R²

R² O

Ni Me L N

O Ni

Me L

L = PPh_3, NCR, pyridine, tmeda R¹ = iPr, 3,5-substituted aryl R² = I, 3,5-substituted aryl

6 7 8 9

Salicylaldiminato nickel complexes were introduced as polymerization catalysts by Grubbs and co- workers and by Johnson et al. (complex 6).^19a,b Alike the diimine-based late-metal polymerization catalysts, the N-aryl ring is roughly perpendicular to the square plane defined by the N-, O-, and nickel atom. On the one hand, the bulky susbstituents in the ortho position of the aniline moiety are believed to retard chain transfer and on the other hand the steric bulk of the anthryl group is believed to promote ligand (L) dissociation and to protect the catalytically active nickel center towards deactivation routes, which ultimately afford bischelate complexes [(N^O)₂Ni], thus facilitating long-lasting high activity

Interestingly, replacement of the 2,6-diisopropylphenyl moiety by different terphenyls generally leads to much more active precatalysts, that in addition, produced material ranging from high molecular weight semicrystallline polyethylene with a low degree of branching to low molecular weight amorphous material depending on the 3’,5’-substitution of the terphenyl under otherwise identical conditions (complex 7). ^19h-m

A neutral Ni(II) catalyst based on an anilitropone ligand, forming a five membered chelate, has been reported by Brookhart and co-workers (complex 8).²⁰ No phosphine scavenger is required and 8 produces high molecular weight PE with high activity 6.1 × 10⁴ TO h^-1. The morphology of the obtained polymer was dependent on the ethylene concentration, branching number decreases and the molecular weight increases with the ethylene pressure.

More recently, Brookhart et al. reported that enolatoimine Ni(II) phenyl complexes [κ²-N,O - {ArN=CH-C(COCF3)=C(CF3)O}NiPh(L)] (9) (Ar = 2,6-iPr2C6H3) are very active for the polymerization of ethylene, activities exceeding those of all previously reported neutral κ²-N,O Ni(II) complexes.²¹ By contrast to phosphinoenolate complexes, these κ²-N,O Ni(II) complexes afforded higher molecular weight polymer with moderate degree of branching. Alike the other catalysts containing an aniline-based moiety, complexes with different o-substituents were prepared, resulting in even higher catalyst activity up to 1.7 × 10⁵ TO h^-1.²²

1-3. Insertion polymerization of ethylene in unconventional solvents

The tolerance to polar groups of the aforementioned catalyst precursors was studied by running ethylene polymerization in the presence of small molecule additives.^{19c, 20a} On the basis of polymer molecular weights and catalyst activities a picture of the tolerance of the catalysts was made. Such polymerizations in the presence of added polar reagents are of interest with regard to polymerization with less rigorously purified solvents and monomers. On a different level, utilization of polar solvent as a reaction medium for the olefin polymerization offers new perspectives. Carrying out transition metal catalyzed polymerization in aqueous medium or in supercritical fluid is of strong interest, as they can afford a variety of new materials not accessible by any other methods.²³

1-3-1. Aqueous medium

Whereas early transition metal centers generally strongly bind to protic solvents (and in most cases react further), coordination to relatively less oxophilic late transition metal complexes can be comparably weak and does not pose a severe problem in many cases. In addition to the catalytically active metal centers, a reactivity of the added co-catalyst towards water obviously can also be detrimental. For polymerization in protic solvents, catalysts which do not require water sensitive co- catalysts are desirable.

Figure 1-1. Cryo TEM micrograph of polyethylene nanoparticles obtained with water soluble salicyladiminato based catalysts in water. Picture (a) amorphous polymer, picture (b) semi-crystalline polymer.²⁴

Consecutive insertions of ethylene in protic solvent have been employed for a long time. The SHOP process runs in a biphasic butandiol/hydrocarbon system, the catalyst being located in the polar medium while the products are continuously extracted into the apolar phase. Nevertheless, it was only in 1993 that Flood and co-workers reported ethylene polymerization in pure water using a rhodium complex as a catalyst precursor. After 90 days of reaction at 60 bar ethylene pressure and room temperature, some low-molecular weight polyethylene was obtained. The amount of polymer obtained corresponded to 1 TO per day.²⁵ The polyethylene was reported to precipitate during the reaction as a solid.

However, in the presence of an emulsifier, with appropriate catalysts and under otherwise suitable conditions, stable polymer latices can be obtained. Such polymer dispersion can be obtained with water soluble catalysts or by using (mini/micro/macro)emulsion techniques. In the case of a non-water soluble catalyst, the metal complex is finely dispersed in the reaction medium in the form of an aqueous miniemulsion (or microemulsion) of a solution of the precatalyst in an organic phase, that is a small amount of organic solvent or liquid monomer.²⁶

Preliminary investigation on the polymerization of ethylene with palladium diimine complexes in water showed that the reaction can occur in a suspension-type polymerization with substantial activities.²⁷ However detailed investigation by Mecking et al. revealed that this high activity is due to an encapsulation of the water insoluble catalyst in the growing hydrophobic polymer, which protects the catalyst from access of water.²⁸ Aqueous solutions of water-soluble diimine catalyst precursor are inactive for ethylene polymerization. Mechanistic studies revealed that the catalyst precursor is stable in pure water but decomposition occurs instantaneously upon addition of ethylene monomer.²⁹

Scheme 1-3 Preparation of water soluble catalyst precursor

N OH

Ni N

Me Me N

+

N O

Ni L - CH₄ Me

-tmeda

0.95 equiv. L ⁽²⁾

L: hydrophilic ligand

e.g.: TPPTS or PEG-NH₂ TPPTS: P

SO₃Na

3

Mecking et al. and Spitz et al. independently reported nickel (II) catalyzed polymerization of ethylene to linear material in aqueous emulsion.^30,31 [P^O] nickel complexes based on known ligands were found to be suited as catalyst precursors. By comparison to traditional polymerization in non-aqueous organic media such as toluene, catalyst activities (and in some cases polymer molecular weight) are reduced in the aqueous polymerization.

Most recently, high molecular weight polyethylene with various polymer microstructures has been prepared in aqueous polymerizations with two different classes of water soluble catalysts. The hydrophilicity of the catalyst precursor was provided by means of a water soluble labile ligand (Scheme 1-3). Salicylaldimine based catalysts display high activities but a rather limited stability in this extremely disperse system.³² By comparison, electron poor enolatoimine based complexes show a remarkable

extremely small particles of high molecular weight polyethylene under organic solvent-free aqueous conditions. Such particular morphology could not be achieved by any other process (Figure 1-1).

1-3-2. Supercritical carbon dioxide

Dense carbon dioxide, that is liquid or supercritical CO2 (scCO2), possesses unique properties, such as the possibility of variation of its density and solvent properties over a wide range (Figure 1-2).³⁴ For example the preparation of elastomers,complicated by difficult solvent or monomer removal, could be principally improved by polymerization in supercritical carbon dioxide. In polymerization processes and polymer processing, dense carbon dioxide can be useful as a solvent or suspension medium. It can be removed conveniently by variation of the pressure, resulting in a dry polymer powder. While free-radical polymerization in dense CO2 has been studied most intensively, various examples of coordination polymerization have also been reported. ^35,36

Figure 1-2 Schematic phase diagram for carbon dioxide (left) and density of CO2 as a function of pressure at different temperatures (right)

Ring opening metathesis polymerization (ROMP) of norbornene was described.³⁷ Fürstner et al. have shown that ruthenium-carbene Grubbs catalyst and a molybdenum-carbene Schrock catalyst are active in supercritical carbon dioxide. Both catalysts produced high molecular weight polymers (10⁵ – 10⁶ g mol^-

1) in good yield (up to 94%), highlighting that precipitation of the polymer does not limit the polymer molecular weight. Leitner and co-workers reported the synthesis of polyphenylacetylene (PPA) in dense

carbon dioxide using a soluble rhodium catalyst precursor.³⁸ The catalyst was rendered miscible in the reaction mixture by coordination of a perfluoroalkyl-substituted triphenylphosphine ligand. There was some evidence to suggest that the stereochemistry of the PPA was different to that of PPA synthesized in common organic solvents. Epoxide-carbon dioxide³⁹ and ethylene-carbon monoxide copolymerization⁴⁰ were reported as well. Ethylene polymerization has been studied in scCO2 with cationic Pd(II)-diimine catalysts.^27,41 Highly branched amorphous polyethylene was obtained invariably.

An activity of 2 × 10³ TO h^-1 was reported under the conditions studied. Mecking and Leitner have reported ethylene homopolymerization with CO2-soluble neutral Ni(II) salicylaldiminato complexes.⁴² Activities of up to 5 × 10³ TO h^-1 were observed under the conditions studied. Linear semicrystalline polyethylenes with molecular weights up to Mn = 2.4 × 10⁴ g mol^-1 were obtained. Polymer crystallinities could be varied in a limited range via the choice of catalyst, by introduction of methyl branches as a result of “chain walking”. However, with increased branching molecular weights decrease, as β-hydride elimination is a key step both for chain walking and chain transfer.

1-4. Copolymerization of ethylene and polar monomers

The incorporation of polar moities into otherwise apolar polymer like polyolefin is of high interest.

Polar groups exercise control over important properties such as toughness, adhesion, stability or surface properties. Currently, commercial procedures for the copolymerization of ethylene with polar monomers such as acrylates, methacrylates and vinyl acetate employ free radical processes similar to those used for LDPE production. The incorporation of the comonomer is random, and the control of the polymer microstructure is limited.

Scheme 1-4. Copolymerization of ethylene with acrylate yielding branched copolymer

N N

M

R

C(O)OMe +

N N

M

R O

O +

2,1 insertion rearrangement N

N M

+

O O R

+

-

N N

M

O O insertion = chain R

growth

COOMe COOMe

COOMe

In recent years, late transition metal catalysts have attracted attention not only for the polymerization of α-olefins, but more importantly for the copolymerization of hydrocarbon monomers with readily available polar monomers such as acrylates.⁴³ Brookhart was the first to demonstrate the insertion copolymerization of methyl acrylate with ethylene using a well-defined catalyst.⁴⁴ Owing to the propensity of diimine Pd(II) complexes employed for chain walking on the growing chain during the polymerization, highly branched amorphous polyethylene with the acrylate units located predominantly at the ends of branches are obtained (Scheme 1-4). In-depth NMR spectroscopy studies revealed that acrylate insert into the palladium-carbon bond in a 2,1 fashion leads to a 4-membered chelate. This product rapidly rearranges to form a 6-membered chelate complex. A corresponding rearrangement is thought to occur in the copolymerization prior to the next insertion of monomer and thus accounts for the incorporation of the ester functionality at the ends of branches in the copolymers. Formation of analogous chelate compounds during the copolymerization is believed to hinder monomer coordination and thus to be responsible for the lower rates in acrylate copolymerization reactions by comparison to ethylene and α-olefins polymerizations.

However in contrast to acrylates, other common comonomers such as vinyl acetate, acrylonitrile and vinyl chloride could not be copolymerized by these cationic complexes.^45,46 Mechanistic studies show

that insertion of these comonomers is possible but it invariably leads either to decomposition (β-X elimination for vinyl acetate and vinyl chloride) or to the formation of oligomeric species (in the case of acrylonitrile), which are rather stable owing to the strongly coordinating nitrile groups.

P O S O

O Pd

R L Ar Ar R'

Ar = 2-MeOC₆H₄; 2-(2,6-(MeO)₂C₆H₃)C₆H₄; Ph

R = Me; R' = H, Me; L = py, lutidine or 1/2 Me₂NCH₂CH₂NMe₂ R, L = 2η¹, 6,7-tricyclo[5.2.1.0]-deca-6-ene-3-ethoxy-2-yl 10

More recently, the incorporation of acrylate monomers into linear polyethylene was reported by Pugh and co-workers who described the use of a neutral palladium catalyst with a chelating [P^O] ligand to generate linear copolymer with random incorporation of acrylate monomers. A linear copolymer with an acrylate incorporation of 13 mol% and a number average molecular weight of Mn = 1.3 × 10⁴ g mol^-1 (Mw/Mn ~ 1.6) was obtained. The reaction conditions correspond to roughly equal concentrations of the two monomers in the reaction mixture, that is, the aforementioned copolymer composition corresponds to incorporation of ethylene being somewhat preferred over incorporation of methyl acrylate.17, 18b, 47

Recent reports have revealed that complexes 10 even enable catalytic copolymerization of the challenging monomer acrylonitrile. Thus, exposure of 10 to ethylene and acrylonitrile results in copolymer formation.⁴⁸ With an average activity of about 10 TO h^-1 under typical reaction conditions (100 °C, 30 atm ethylene pressure, 120 h), the reaction is slow but occurs in a catalytic fashion. The rather slow reaction is very likely due to κ-N coordination of acrylonitrile. The polymer formed contained up to 9 mol % acrylonitrile. The acrylonitrile is distributed in approximately equal amounts in the polymer backbone and in end groups, apparently chain transfer occurs preferably after an acrylonitrile insertion. (Table 1-1).

Catalyst 10 is also capable of copolymerizing ethylene with other polar vinyl monomers. N-vinyl pyrrolidinone and N-isopropylacrylamide were successfully inserted into the polyethylene backbone.⁴⁹

other aforementioned polar vinyl compounds, and ethylene could be formed with a degree of incorporation close to 8 mol%.⁵⁰. Copolymers of ethylene and vinyl fluoride could be prepared as well.

The comonomer is randomly distributed in the polymer chain but rather low degree of incorporation is observed (0.45 mol%, 350 TO h^-1).⁵¹

Finally coordination polymerization of vinyl acetate by 10 was realized by copolymerization with carbon monoxide. Perfectly alternating copolymer was obtained.⁵² The reaction is relatively slow, typically proceeding with 20 TO h^-1 (70°C, 60 atm CO, 20 h). In addition to this finding, an alternating copolymer of methyl acrylate with carbon monoxide has been synthesized for the first time via coordination polymerization using 10 as catalysts. The head-to-tail structure of the copolymer was confirmed by NMR spectroscopy.⁵³

Table 1-1. Overview of the copolymerizations reported with phosphinosulfonate palladium catalyst (10).

Monomer Monomer

Typical activity (TO h^-1)^[a]

Degree of incorporation

(mol%)

Typical polymer Copolymer microstructure Mn (g mol^-1) Mw/Mn

C₂H₄ - 1 × 10⁵ - 2 × 10⁴ 2.0 Linear polyethylene

C₂H₄ acrylate 320 17 7 × 10³ 1.8 Random copolymer

C2H4 acrylonitrile 10 7 3 × 10³ 1.7 Random copolymer

C2H4 vinyl ether 40 7 5 × 10³ 2.0 Random copolymer

C2H4 vinyl fluorine 350 0.5 7 × 10³ 2.5 Random copolymer

C₂H₄ ^N-vinyl

pyrrolidinone 34 2.6 4 × 10³ - Random copolymer

C2H4 N-isopropyl-

acrylamide 23 4.1 1 × 10³ - Random copolymer

CO methyl

acrylate 20 - 25 × 10³ 1.5 Alternated

copolymer

CO Vinyl acetate 25 - 30 1.4 Alternated

copolymer

[a] The activities have been determined under strongly varying conditions, thus they should only be taken as a measure of the order of magnitude. TO (turnovers): substrate converted [mol] per metal used [mol]

However, no homopolymerization of the aforementioned monomers has been reported to date. This would require multiple insertions observed thus far only for the electron rich monomer vinyl ether.⁵⁴

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2

Control of molecular weight in Ni(II)-catalyzed polymerization via the reaction medium

Abstract. A dramatic switch of polymer molecular weight selectivity is observed by appropriate choice of reaction solvents. The novel (P^O)-phosphinesulfonato nickel methyl pyridine complex 4 with P^O = κ²-P,O-2-(2-MeOC₆H₄)₂PC₆H₄SO₃)) is a single component precursor to a highly active catalyst, affording branched low molecular weight (Mn) material or very high molecular weight linear polyethylene depending on the reaction solvent. The polymerization behavior appears to correlate with the solubility of the catalyst precursor in the reaction medium, at the concentrations employed for polymerization studies. Insolubility (heptane, 1-octene) correlates with formation of linear high molecular weight polymer. This hypothesis is underlined by polymerization with the respective nonyl(pyridine) substituted complex (5), which due to the additional alkyl-chain becomes soluble in heptane. Low molecular weight branched polymer is formed with very high activities. This is also the first example of formation of high number average molecular weights with this important class of catalysts, i.e. formation of polymer samples in which the majority of chains are large.