© 2016 Nicole Lyn Gibbons - University of Floridaufdcimages.uflib.ufl.edu/UF/E0/05/05/38/00001/GIBBONS_N.pdf4 ACKNOWLEDGMENTS I was told many years ago that graduate school would

LOW DENSITY POLYETHYLENE: USING CATALYSTS TO INFLUENCE BRANCHING AND MOLECULAR WEIGHT

By

NICOLE LYN GIBBONS

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2016

Experience is what you get when you did not get what you wanted

4

ACKNOWLEDGMENTS

I was told many years ago that graduate school would be the best years of my

life. Because of the many friends and mentors that I have had here at the University of

Florida, I agree with that statement. During these five years I have been able to learn

and grow in ways I could only hope, and I am honored to have had such an amazing

support group throughout this chapter of my life.

I thank my mother, for listening to my long phone calls and for her support in

moving so far away just to add three letters to my name. I thank my older sister Natasha

for showing me Alaska and reminding me how much I love to travel. I thank my younger

sister Jazmine, for keeping me humble and reminding me that being weird and nerdy is

a good thing. Finally, I would like to thank my grandma and late grandpa Gibbons who

helped me move to Florida and whom I have been able to grow closer to by living here.

I thank Katie Ames, whom I have been friends with since middle school, for

letting me share many wonderful moments with her. I also thank her for the countless

minutes on skype and for always offering support. I thank Stacy Sexton for showing me

Charleston and inspiring me to ask more of myself.

I have made many friends during my stay here. Robert (Trey) Powell III has been

my best friend throughout these years. I thank him for always pushing me to be my

best, for always encouraging me and motivating me every day to work hard and to set

my standards high. We have studied endless hours for exams, reviewed each other’s

writing and have had many wonderful Florida adventures. I am blessed to have him as a

friend.

I would like to thank Hipassia Moura for helping me complete my Gainesville

bucket list and for showing me how to make pão de queijos. Her research is referenced

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in this thesis and I would like to thank her for her help in the lab and for her friendship. I

would like to thank Mayra Rostagno for helping me be the person I want to be. Thank

you for your beauty tips and friendship. I thank Emma Bradic for her friendship and for

sharing in the crazy adventures of the “Thugs with Rugs”. (I still can’t believe everything

you fit in your pack). I also thank the previous Miller group members. I thank Dr.

Alexander Pemba for guiding me through all of the cornerstones in my degree. Thank

you for helping me with the GPC and for your invaluable advice. I thank Dr. Ersen

Gortuck, Amr Feteha, Matthew Burnstein, and Betsy Suda for making me feel

welcomed when I joined and for helping me get started in the lab and answering my

frequent questions. I thank John Garcia for showing me that you can do chemistry while

keeping a clean work space. I thank the Miller group members who came after me and

helped make the lab thee place to be. I thank Gabriel Short for letting me distract him

with countless jokes. I thank Olivier Nsengiyumva, Steven Shen, Florian Diot-Néant,

and Erik Price for their friendship. The Miller group members have been a tightly knit

group and I thank all of them for becoming my family. Thank you for making our lab the

cleanest, safest, and coolest lab to work in.

I would like to thank Jessica Cash, Tanner Lee, Dr. Soma Makherjee, Lauren

Douma, Ashton Bartley, Sarah Franz, Dan Dobbins, Dr. Donovan Thompson, Dr. Hillary

Lathrop, and Jenny Colding for their friendship and support. I would also like to thank

Megan Baucom, Sarah Klossner, Frank Farley, Larry Westra and Bob Johnson for

helping me throughout my years as a UF student.

I would like to thank all of my science mentors who have opened doors for me

and have shown me this fascinating world. I thank Mr. Hartley, my high school

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Chemistry and Physics teacher, who first taught me nomenclature and stoichiometry

and who let me come back to talk to the students and help out with the NI3 experiment. I

thank my organic professor at Grand Rapids Community College, Dr. Jennifer Batten,

who first told me about graduate school and how to survive it. I thank my undergraduate

research advisor Dr. Randy Winchester who taught me synthesis techniques one-on-

one and helped me put together my first poster presentation and research seminar. I

thank Dr. John Bender who first taught me polymers synthesis and characterization. I

thank Dr. Christopher Lawrence who gave me advice for choosing the right research

group for me.

I would like to thank my research committee (Professors Ronald Castellano, Ken

Wagener, John Stewart, and Helena Hagelin-Weaver) who have helped advised me on

my path and taking the time out of their schedules to help me. Thank you for your

advice after my oral qualifying examination and helping me reach my goals.

Finally, I would like to thank my research advisor Dr. Stephen Miller. Thank you

for guiding me in my research, for having an open door and an extra minute to hear my

concerns and for working with me through the hurdles of graduate school. Thank you for

pushing me to come up with my own ideas and valuing my opinion in group meeting. I

have learned a great deal about myself and about chemistry during my time here at the

University of Florida. Lastly, I would like to thank all those who are not mentioned who

have given me support, advice and encouragement along the way. Thank you.

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TABLE OF CONTENTS page

ACKNOWLEDGMENTS .................................................................................................. 4

LIST OF TABLES .......................................................................................................... 10

LIST OF FIGURES ........................................................................................................ 11

LIST OF OBJECTS ....................................................................................................... 20

LIST OF ABBREVIATIONS ........................................................................................... 21

ABSTRACT ................................................................................................................... 23

CHAPTER

1 INTRODUCTION .................................................................................................... 25

Alkenes/Olefins/Polyolefins ..................................................................................... 25 Types of Polyolefins ......................................................................................... 26 Methods of Polymerizations ............................................................................. 29

Ziegler-Natta Catalysts ........................................................................................... 30 History .............................................................................................................. 30 Types of Ziegler-Natta Catalysts ...................................................................... 31 Comparison of Homogeneous and Heterogeneous Catalysts .......................... 33 Mechanism ....................................................................................................... 34 Modern Day Uses and Development ................................................................ 38

Free Radical Polymerizations ................................................................................. 39 History .............................................................................................................. 39 Mechanism ....................................................................................................... 40 Modern Day Uses and Development ................................................................ 43

LDPE from Catalysts............................................................................................... 43 Octamido Catalyst .................................................................................................. 46 Overview of Dissertation ......................................................................................... 50

2 DIENES AS CROSSLINKERS ................................................................................ 51

Background ............................................................................................................. 51 Results and Discussion........................................................................................... 52

Diene Crosslinkers and Cognate Mono-enes ................................................... 52 Ethylene Homopolymerization and Copolymerization ...................................... 53 Molecular Weights and Thermal Data .............................................................. 56 Homopolymerizations of Olefins ....................................................................... 59 Additional Copolymerizations with 1/MAO ........................................................ 61

Conclusion .............................................................................................................. 64 Acknowledgements ................................................................................................. 65

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Experimental ........................................................................................................... 65 Materials ........................................................................................................... 65 Polymerizations ................................................................................................ 66 Polymer Analysis .............................................................................................. 67

3 SUPPORTING CATALYSTS ONTO INORGANIC SUPPORTS ............................. 69

Background ............................................................................................................. 69 Results and Discussion........................................................................................... 71

Ethylene Polymerizations from a Supported Catalyst ....................................... 73 Ethylene Polymerizations from a Supported Cocatalyst ................................... 77 Propylene Polymerizations ............................................................................... 79

Conclusion .............................................................................................................. 81 Experimental ........................................................................................................... 82

Materials ........................................................................................................... 82 Support Preparation ......................................................................................... 83 Supporting 1 ..................................................................................................... 85 Polymerizations ................................................................................................ 85 Polymer Analysis .............................................................................................. 86

4 CHAIN SHUTTLING POLYMERIZATION ............................................................... 87

Background ............................................................................................................. 87 Results and Discussion........................................................................................... 94 Conclusions .......................................................................................................... 103 Experimental ......................................................................................................... 105

Materials ......................................................................................................... 105 Polymerizations .............................................................................................. 105 Polymer Analysis ............................................................................................ 106

5 SUMMARY ........................................................................................................... 108

APPENDIX

A SUPPLEMENTARY INFORMATION FOR CHAPTER 2 ....................................... 110

TGA Spectra ......................................................................................................... 110 DSC Spectra ......................................................................................................... 110 Select Proton NMR ............................................................................................... 111 Select Carbon NMR .............................................................................................. 123 DOSY Spectra Information ................................................................................... 135 GPC Spectra ......................................................................................................... 137

B SUPPLEMENTARY INFORMATION FOR CHAPTER 3 ....................................... 139

TGA Spectra ......................................................................................................... 139 DSC Spectra ......................................................................................................... 139 Select Proton NMR ............................................................................................... 140

9

Select Carbon NMR .............................................................................................. 144

C SUPPLEMENTARY INFORMATION FOR CHAPTER 4 ....................................... 148

TGA Spectra ......................................................................................................... 148 DSC Spectra ......................................................................................................... 148 Select Proton NMR ............................................................................................... 149 Select Carbon NMR .............................................................................................. 155 DOSY Spectra Information ................................................................................... 161 GPC Spectra ......................................................................................................... 162

LIST OF REFERENCES ............................................................................................. 164

BIOGRAPHICAL SKETCH .......................................................................................... 171

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LIST OF TABLES

Table page 1-1 Comparison of polyolefin properties and applications ........................................ 27

2-1 Thermal, branching, and molecular weight results for ethylene homopolymerizations and copolymerizations ..................................................... 54

2-2 Molecular weight results for ethylene homopolymerizations and copolymerizations ............................................................................................... 56

2-3 Thermal and molecular weight analysis of homopolymers from catalysts 1/MAO, 2/MAO, and 3/MAO ............................................................................... 60

2-4 Thermal and branching analysis of ethylene/diene and ethylene/mono-ene copolymers from catalyst 1/MAO ........................................................................ 62

3-1 Ethylene polymerizations with supported 1/MAO ............................................... 74

3-2 Ethylene/1-octene copolymerizations with 1@magadiite/MAO .......................... 76

3-3 Ethylene polymerizations with 1/supported MAO ............................................... 78

3-4 Propylene polymerizations with 1/supported MAO ............................................. 81

4-1 Ethylene homopolymerizations with 1/MAO and 2/MAO with and without diethyl zinc .......................................................................................................... 95

4-2 Ethylene/1-hexene copolymerizations with 1/MAO ............................................. 96

4-3 1-hexene homopolymerizations with 1/MAO and diethyl zinc ............................. 97

4-4 Chain shuttling polymerization with 1/2 and increasing amounts of CSA ........... 97

4-5 Ethylene polymerization with differing ratios of 1/2 ............................................. 98

4-6 Ethylene polymerization with differing ratios of 1/4 ........................................... 100

A-1 DOSY NMR results for PS standards ............................................................... 135

A-2 Diffusion coefficient and calculated Mw for polymer samples in Chapter 2 ....... 136

C-1 Diffusion coeffecient and calculated Mw for polymer samples in Chapter 4 ...... 161

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LIST OF FIGURES

Figure page 1-1 The different types of plastics made globally in 2012 ......................................... 25

1-2 Chain-growth polymerization versus step-growth polymerization ....................... 26

1-3 Types of polyethylene branching ........................................................................ 29

1-4 Polypropylene tacticity ........................................................................................ 29

1-5 Common metallocene structures ........................................................................ 32

1-6 Proposed cage structures of MAO ...................................................................... 33

1-7 Catalyst activity for select alkene monomers ...................................................... 33

1-8 Generic structures of TiCl4 .................................................................................. 35

1-9 Homogeneous catalyst initiation ......................................................................... 35

1-10 Cossee-Arlman mechanism for olefin polymerization with heterogeneous Z/N catalyst ............................................................................................................... 36

1-11 The modified Green-Rooney mechanism for olefin polymerization .................... 37

1-12 Mechanisms for chain termination ...................................................................... 38

1-13 Initiation using di-t-butyl peroxide ....................................................................... 40

1-14 Propagation of a free radical polymerization ...................................................... 41

1-15 Intermolecular hydrogen transfer for long branches in polyethylene .................. 41

1-16 Intramolecular hydrogen transfer for making small braches in polyethylene ...... 42

1-17 Termination steps for free radicals ..................................................................... 42

1-18 Mechanism for by β-hydride elimination, dissociation, polymerization then reinsertion ........................................................................................................... 45

1-19 General structure for ansa-cyclopentadienyl-amido constrained geometry catalysts ............................................................................................................. 45

1-20 Example of a heterobinuclear catalyst ................................................................ 46

1-21 Octamethyloctahydrodibenzofluorene (Oct) synthesis ....................................... 46

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1-22 Oct-amido catalyst synthesis .............................................................................. 47

1-23 Crystal structure of Oct-amido catalyst ............................................................... 48

1-24 Chromium oligomerization catalyst used in a tandem catalyst system with Oct-amido ........................................................................................................... 49

2-1 Copolymerization of ethylene and dienes using 1/MAO to afford branched and crosslinked polyethylene ............................................................................. 51

2-2 Structure of Me2C(C29H36)(C5H4)ZrCl2 (2) and zirconocene dichloride (3) .......... 51

2-3 Dienes for copolymerization with ethylene and control group mono-enes for copolymerization and homopolymerization ......................................................... 53

2-4 GPC trace of crosslinked ethylene/1,9-decadiene copolymer (E/DD) from 1/MAO ................................................................................................................ 58

2-5 Copolymer melting temperatures decrease steadily with increasing diene feed equivalents vs. catalyst 1/MAO, 2/MAO, and 3/MAO ................................. 59

2-6 Ethylene copolymer melting temperatures respond to increasing comonomer feed equivalents for catalyst 1/MAO ................................................................... 63

2-7 13C NMR analysis of the ethylene/4-vinylcyclohexene copolymer from Table 2-4, entry 54 ....................................................................................................... 64

3-1 Anchoring of a metallocene on an inorganic silica support followed by the addition of MAO .................................................................................................. 70

3-2 General scheme for attaching 1 onto inorganic supports ................................... 71

3-3 Structures of the supports used with Oct-amido ................................................. 72

3-4 Solid state UV-Vis spectrum of 1 on the inorganic supports ............................... 73

3-5 13C NMR for polymers made from supported and unsupported Oct-amido ........ 75

3-6 Microscopy images of PE ................................................................................... 77

3-7 DSC thermograms of polymers made by 1/supported MAO ............................... 79

3-8 Sample NMR of atactic polypropylene and shorthand notation of polypropylene tacticity ........................................................................................ 80

4-1 Modified adaptation of figure from reference 45 ................................................. 87

4-2 Structure of catalysts and CSA used by Arriola et al. along with GPC trace of polymer made with and without the CSA ............................................................ 89

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4-3 Compressed molded samples of polymers made from chain shuttling polymerization .................................................................................................... 90

4-4 Melting temperatures of copolymers made from 1-octene and ethylene ............ 91

4-5 Chain shuttling polymerization catalysts for Xiao et al ........................................ 92

4-6 General scheme of chain walking polymerization ............................................... 93

4-7 General Scheme of chain shuttling polymerization system using Oct-amido and IsopropylideneOctCp catalysts .................................................................... 95

4-8 Relationship between NB and equivalents of 1-hexene ...................................... 96

4-9 Structure of diphenylmethylideneOctCp Catalyst (4) .......................................... 99

4-10 UV-Vis spectra of 1, 2, and a 1:1 ratio of both in toluene ................................. 101

4-11 UV-Vis spectra of 1, 2, and a 1:1 and 40:1 ratio of both in MAO and toluene .. 102

4-12 Expanded spectra of Figure 4-9 ....................................................................... 103

4-13 Expanded spectra of Figure 4-10 ..................................................................... 103

A-1 TGA Thermogram of entry 1, Table 2-1 ............................................................ 110

A-2 DSC Thermogram of entry 1, Table 2-1 ........................................................... 110

A-3 1H NMR spectrum of polyethylene from 1/MAO ............................................... 111



A-6 1H NMR spectrum of 5-vinyl-2-norbornene ....................................................... 112

A-7 1H NMR spectrum of polyethylene-co-5-vinyl-2-norbornene from 1/MAO ........ 112



A-10 1H NMR spectrum of 1,9-decadiene ................................................................. 113

A-11 1H NMR spectrum of polyethylene-co-1,9-decadiene from 1/MAO ................... 113



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A-14 1H NMR spectrum of 2,5-norbornadiene........................................................... 114

A-15 1H NMR spectrum of polyethylene-co-2,5-norbornadiene from 1/MAO ............ 115



A-18 1H NMR spectrum of 1-octene .......................................................................... 116

A-19 1H NMR spectrum of poly-1-octene from 1/MAO .............................................. 116



A-22 1H NMR spectrum of 1-decene ......................................................................... 117

A-23 1H NMR spectrum of poly-1-decene from 1/MAO ............................................. 117



A-26 1H NMR spectrum of 1,7-octadiene .................................................................. 118

A-27 1H NMR spectrum of poly-1,7-octadiene from 1/MAO ...................................... 119



A-30 1H NMR spectrum of polyethylene-co-12-butyl-1,22-tricosadiene from 1/MAO 120

A-31 1H NMR spectrum of 4-vinylcyclohexene .......................................................... 120

A-32 1H NMR spectrum of polyethylene-co-4-vinylcyclohexene from 1/MAO ........... 120

A-33 1H NMR spectrum of vinylcyclohexane ............................................................. 121

A-34 1H NMR spectrum of polyethylene-co-vinylcyclohexane from 1/MAO .............. 121

A-35 1H NMR spectrum of cyclohexene .................................................................... 121

A-36 1H NMR spectrum of polyethylene-co-cyclohexene from 1/MAO ..................... 122

A-37 13C NMR spectrum of polyethylene from 1/MAO .............................................. 123


15


A-40 13C NMR spectrum of 5-vinyl-2-norbornene ..................................................... 124

A-41 13C NMR spectrum of polyethylene-co-5-vinyl-2-norbornene from 1/MAO ....... 124



A-44 13C NMR spectrum of 1,9-decadiene ................................................................ 125

A-45 13C NMR spectrum of polyethylene-co-1,9-decadiene from 1/MAO ................. 125

A-46 13C NMR spectrum of polyethylene-co-1,9-decadiene from 3/MAO ................. 126

A-47 13C NMR spectrum of 2,5-norbornadiene ......................................................... 126

A-48 13C NMR spectrum of polyethylene-co-2,5-norbornadiene from 1/MAO ........... 126



A-51 13C NMR spectrum of 1-octene ........................................................................ 127

A-52 13C NMR spectrum of poly-1-octene from 1/MAO ............................................ 128



A-55 13C NMR spectrum of 1-decene ....................................................................... 129

A-56 13C NMR spectrum of poly-1-decene from 1/MAO ........................................... 129



A-59 13C NMR spectrum of 1,7-octadiene ................................................................. 130

A-60 13C NMR spectrum of poly-1,7-octadiene from 1/MAO ..................................... 130



A-63 13C NMR spectrum of polyethylene-co-12-butyl-1,22-tricosadiene from 1/MAO .............................................................................................................. 132

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A-64 13C NMR spectrum of 4-vinylcyclohexene ........................................................ 132

A-65 13C NMR spectrum of polyethylene-co-4-vinylcyclohexene from 1/MAO .......... 132

A-66 13C NMR spectrum of vinylcyclohexane ........................................................... 133

A-67 13C NMR spectrum of polyethylene-co-vinylcyclohexane from 1/MAO ............. 133

A-68 13C NMR spectrum of cyclohexene................................................................... 133

A-69 13C NMR spectrum of polyethylene-co-cyclohexene from 1/MAO .................... 134

A-70 PS calibration curve .......................................................................................... 135

A-71 GPC spectrum of polyethylene-co-1,9-decadiene from 1/MAO ........................ 137

A-72 GPC spectrum of polyethylene-co-1,9-decadiene from 1/MAO ........................ 137

A-73 GPC spectrum of poly-1-octene from 1/MAO ................................................... 138

B-1 TGA Thermogram of entry 1, Table 3-1 ............................................................ 139

B-2 DSC Thermogram of entry 1, Table 3-1 ........................................................... 139

B-3 1H NMR spectrum of polyethylene from 1/MAO ............................................... 140

B-4 1H NMR spectrum of polyethylene from 1@magadiite/MAO ............................ 140

B-5 1H NMR spectrum of polyethylene from 1@AlPO-kanemite/MAO .................... 140

B-6 1H NMR spectrum of polyethylene-co-1-octene from 1@magadiite/MAO ........ 141

B-7 1H NMR spectrum of polyethylene from 1/MAO@magadiite ............................ 141

B-8 1H NMR spectrum of polyethylene from 1/MAO@CTMA-magadiite ................. 141

B-9 1H NMR spectrum of polyethylene from 1/MAO@AlPO-kanemite .................... 142

B-10 1H NMR spectrum of polyethylene from 1/MAO@MCM-41 .............................. 142

B-11 1H NMR spectrum of polyethylene from 1/MAO@MCM-48 .............................. 142

B-12 1H NMR spectrum of polypropylene from 1/MAO ............................................. 143

B-13 1H NMR spectrum of polypropylene from 1/MAO@magadiite .......................... 143

B-14 1H NMR spectrum of polypropylene from 1/MAO@AlPO-kanemite .................. 143

B-15 13C NMR spectrum of polyethylene from 1/MAO .............................................. 144

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B-16 13C NMR spectrum of polyethylene from 1@magadiite/MAO ........................... 144

B-17 13C NMR spectrum of polyethylene from 1@AlPO-kanemite/MAO .................. 144

B-18 13C NMR spectrum of polyethylene-co-1-octene from 1@magadiite/MAO ....... 145

B-19 13C NMR spectrum of polyethylene from 1/MAO@magadiite ........................... 145

B-20 13C NMR spectrum of polyethylene from 1/MAO@CTMA-magadiite ................ 145

B-21 13C NMR spectrum of polyethylene from 1/MAO@AlPO-kanemite .................. 146

B-22 13C NMR spectrum of polyethylene from 1/MAO@MCM-41 ............................. 146

B-23 13C NMR spectrum of polyethylene from 1/MAO@MCM-48 ............................. 146

B-24 13C NMR spectrum of polypropylene from 1/MAO ............................................ 147

B-25 13C NMR spectrum of polypropylene from 1/MAO@magadiite ......................... 147

B-26 13C NMR spectrum of polypropylene from 1/MAO@AlPO-kanemite ................ 147

C-1 TGA Thermogram of entry 1, Table 4-1 ............................................................ 148

C-2 DSC Thermogram of entry 1, Table 4-1 ........................................................... 148

C-3 1H NMR spectrum of polyethylene from 1/MAO ............................................... 149

C-4 1H NMR spectrum of polyethylene from 1/MAO with diethyl zinc ..................... 149

C-5 1H NMR spectrum of polyethylene from 2/MAO ............................................... 149

C-6 1H NMR spectrum of polyethylene from 2/MAO with diethyl zinc ..................... 150

C-7 1H NMR spectrum of polyethylene-co-1-hexene from 1/MAO .......................... 150

C-8 1H NMR spectrum of poly-1-hexene from 1/MAO ............................................. 150

C-9 1H NMR spectrum of polyethylene from 1/2/MAO and no diethyl zinc .............. 151

C-10 1H NMR spectrum of polyethylene from 1/2/MAO and diethyl zinc ................... 151

C-11 1H NMR spectrum of polyethylene from 1/2/MAO in a 4:1 ratio ........................ 151

C-12 1H NMR spectrum of polyethylene from 1/2/MAO in a 10:1 ratio ...................... 152



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C-15 1H NMR spectrum of polyethylene from 1/2/MAO in a 10:1 ratio under reduced pressure .............................................................................................. 153

C-16 1H NMR spectrum of polyethylene from 1/2/MAO in a 1:1 ratio with 6,000 equivalents MAO .............................................................................................. 153

C-17 1H NMR spectrum of polyethylene from 1/4/MAO in a 1:1 ratio ........................ 153




C-21 13C NMR spectrum of polyethylene from 1/MAO .............................................. 155

C-22 13C NMR spectrum of polyethylene from 1/MAO with diethyl zinc .................... 155

C-23 13C NMR spectrum of polyethylene from 2/MAO .............................................. 155

C-24 13C NMR spectrum of polyethylene from 2/MAO with diethyl zinc .................... 156

C-25 13C NMR spectrum of polyethylene-co-1-hexene from 1/MAO ......................... 156

C-26 13C NMR spectrum of poly-1-hexene from 1/MAO ........................................... 156

C-27 13C NMR spectrum of polyethylene from 1/2/MAO and no diethyl zinc............. 157

C-28 13C NMR spectrum of polyethylene from 1/2/MAO and diethyl zinc ................. 157

C-29 13C NMR spectrum of polyethylene from 1/2/MAO in a 4:1 ratio ...................... 157

C-30 13C NMR spectrum of polyethylene from 1/2/MAO in a 10:1 ratio .................... 158



C-33 13C NMR spectrum of polyethylene from 1/2/MAO in a 10:1 ratio under reduced pressure .............................................................................................. 159

C-34 13C NMR spectrum of polyethylene from 1/2/MAO in a 1:1 ratio with 6,000 equivalents MAO .............................................................................................. 159

C-35 13C NMR spectrum of polyethylene from 1/4/MAO in a 1:1 ratio ...................... 159



19


C-37 GPC spectrum of poly-1-hexene from 1/MAO .................................................. 162




20

LIST OF OBJECTS

Object page A-1 TGA thermograms for Chapter 2 ...................................................................... 110

A-2 DSC thermograms for Chapter 2 ...................................................................... 110

A-3 GPC spectra for Chapter 2 ............................................................................... 138

B-1 TGA thermograms for Chapter 3 ...................................................................... 139

B-2 DSC thermograms for Chapter 3 ...................................................................... 139

C-1 TGA thermograms for Chapter 4 ...................................................................... 148

C-2 DSC thermograms for Chapter 4 ...................................................................... 148

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LIST OF ABBREVIATIONS

CGC Constrained geometry catalyst

CH Cyclohexene

Cp Cyclopentadienyl ligand

DD 1,9-decadiene

DOSY Diffusion-ordered spectroscopy (NMR)

DSC Differential scanning calorimetry

EPDM Ethylene-propylene-diene copolymer

EPM Ethylene-propylene copolymer

GPC Gel permeation chromatography

HDPE High density polyethylene

LDPE Low density polyethylene

LLDPE Linear low density polyethylene

LMCT Ligand to metal charge transfer

MAO Methylaluminoxane

Mn Number average molecular weight

Mw Weight average molecular weight

NBD 2,5-norbonadiene

NMR Nuclear magnetic resonance

Nx Number of branches per 1,000 carbons in a polyethylene chain. X represents the length of the branches where M= methyl, E= ethyl, P= n-propyl, B= n-butyl, A= n-pentyl, L≥ n-hexyl, and T= total branching.

Oct Octamethyloctahydrodibenzofluorene

OD 1,7-octadiene

PDI Polydispersity index

22

PE Polyethylene

PP Polypropylene

PVC Polyvinylchloride

RCP Random copolymer

TCD 12-butyl-1,22-tricosadiene

Tg Glass transition temperature

TGA Thermogravimetric analysis

Tm Melting temperature

UHMWPE Ultra high molecular weight polyethylene

ULDPE Ultra low density polyethylene

VCH 4-vinylcyclohexene

VCHA Vinylcyclohexane

VNB 5-vinyl-2-norbornene

Z/N Ziegler-Natta

23

Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

LOW DENSITY POLYETHYLENE: USING CATALYSTS TO INFLUENCE BRANCHING

AND MOLECULAR WEIGHT

By

Nicole Lyn Gibbons

December 2016

Chair: Stephen A. Miller Major: Chemistry

Plastics have become a vital part of our lives due to their improved properties

over metal, wood and cement. The most common plastic produced today is

polyethylene and all polyethylene is made through catalysts except for low density

polyethylene (LDPE). The branching content that is required for its flexibility is not easily

replicated by catalysts, and thus it is made through free radical polymerization.

Making LDPE with catalysts would greatly reduce the cost of synthesis due to the

high temperatures and pressures needed for free radical polymerization. Our group has

synthesized a constrained geometry catalyst and has been studying its properties for

the last decade. It is able to produce a high branching content under mild conditions but

the polymers possess low molecular weights. The projects in this dissertation explore

ways to increase the molecular weights while maintaining the high branching content.

Using dienes as crosslinkers improved the molecular weights of the polymers

upwards of 500 fold. These polymers also possess large PDIs which help in industrial

processing. This came at a cost, however, as the melting temperatures of the polymers

greatly decreased (from 112 °C with no additives down to 60 °C).

24

Supporting the catalyst on inorganic supports helped to heterogenize the catalyst

so it can be used in industrial reactors. These supports, however, hindered the active

site, forcing the catalyst to only produce linear polyethylene. It also was not able to

produce any polypropylene. In contrast, supporting the cocatalyst (MAO) allowed the

catalyst more room to create branched polyethylene and to polymerize propylene.

Lastly, this catalyst did not work well in a dual-catalyst system. When

polymerizing in the presence of another zirconium catalyst, it no longer produced its

high branching content. It is still unclear if it is inactive in solution or only producing

linear polyethylene. However, because of the cost ratio between it and the monomer,

this catalyst would not be suitable for industrial scales.

These studies add to our knowledge of constrained geometry catalysts and will

help future chemists design catalysts to replace the current harsh conditions used to

produce LDPE globally.

25

CHAPTER 1 INTRODUCTION

Alkenes/Olefins/Polyolefins

Polyolefins are a class of polymers that is comprised of commonly produced

plastics such as polypropylene (PP) and polyethylene (PE). These are the two most

highly synthesized plastics in the world.1 Of the 211 million metric tons of plastics

produced globally, nearly 62% (130 million metric tons) are polyolefins.2 Figure 1-1

shows the percentages of the different classes of polymers made globally in 2012.3 The

different types of polyolefins include: high density polyethylene (HDPE), linear low

density polyethylene (LLDPE), low density polyethylene (LDPE), and polypropylene

(PP). These polymers can have a broad range of different properties and thus are used

for different applications. This is why plastics have become so ubiquitous in our daily

lives. They make up everything from bottles and bags to hip replacements and films.

Figure 1-1. The different types of plastics made globally in 2012.

Polyolefins are polymers made from monofunctional alkenes (olefins) such as

ethylene, propylene, 1-hexene, etc. Often, they are made from terminal alkenes (α-

olefins) because these are less sterically hindered than internal alkenes. Products that

Polyolefins

Polyvinylchloride

PET

ABS

Polystyrene

Polycarbonate

HDPE

LLDPELDPE

PP

26

contain alkenes are highly desirable because their functionality can be altered easily.

Because of this, they are often vital intermediates in the synthesis of many

pharmaceuticals and plastics. Industrially, most ethylene is converted into polymers.

The rest is functionalized to make other chemicals such as ethanol, acetic acid,

ethylene glycol and vinyl chloride (which is later polymerized into poly vinyl chloride—

PVC).4 Alkenes can be polymerized through cationic, anionic or via free radical

polymerization. Polymerization of alkenes generally proceeds via addition (chain-

growth) polymerization, in which only one monomer is added to the chain end at a

time.4 This is different from step-growth polymerization where monomers first combine

to form dimers, which combine to form tetramers and continues on until the

polymerization terminates. (Figure 1-2).

Figure 1-2. Chain-growth polymerization versus step-growth polymerization.

Types of Polyolefins

There are numerous different types of polyolefins, many of which have recently

become commercially available. Table 1-1 compares the properties of several common

classes of polyolefins, such as their melting temperatures, densities, and applications.

27

Data compiled from several different sources.1,5–13

The most common polyolefins are high density polyethylene (HDPE), low density

polyethylene (LDPE), linear low density polyethylene (LLDPE) and isotactic

polypropylene (PP).2 The more novel classes of polyolefins include ultra-low density

polyethylene (ULDPE), ultra-high molecular weight polyethylene (UHMWPE) and a

class of elastomers made from ethylene and propylene copolymers.1 Although these

make up a smaller percentage of the polyolefins produced in industry, they are still used

for important applications. UHMWPE is used for many biomedical applications such as

hip replacement parts. The most common of the elastomers are ethylene-propylene

Table 1-1. Comparison of polyolefin properties and applications.

Type Density (g/mL)

Melting Temperature

(°C)

Molecular Weight (g/mol)

Glass Transition

Temperature (°C)

Applications Branching (per 1,000 carbons)

ULDPE 0.86-0.90

~80-85 NA -45 Stretch wrap for food packaging

~70

LLDPE 0.91-0.94

99-108 50,000-200,000

-110 Packaging films

20-60

LDPE 0.915-

0.94 100-129

Typically less than

50,000 -110 to -120

Flexible Bottles

~2-7

HDPE 0.94-0.97

108-129 50,000- 250,000

-110 Bottles, pipes, films

1-20

UHMWPE 0.93-0.94

125-138 2-6 million -160 Hip Replacements

0 unless copolymerized

with another monomer

PP 0.89-0.92

147-170 220,000-700,000

-35-26 Rope, carpeting, auto parts

—

EPM 0.86-0.99

93-110 NA -55 Car bumper guards

—

EPDM 0.86-0.87

NA 300,000-500,000

-45 to -60 Hose, seals —

28

monomer (EPM) and ethylene-propylene-diene monomer (EPDM). These elastomers

are impervious to oxidation and hydrolysis and are normally combined with either PP or

polyamides to form impact-resistant plastics.1

PE is categorized by its density, which is partially determined by the branching of

the polymer.14 PE is a straight chain of saturated carbon atoms. The branching content

of PE significantly influences its crystallinity and consequently its properties. Figure 1-3

shows the different types of branching that can extend off the PE chains and the

different polymers in which these occur. HDPE has linear chains with little to no

branching, which makes it more crystalline. Often, the branching content is so low that

the end plastic product becomes brittle. To compensate for this, α-olefins such as 1-

hexene or 1-octene are added to introduce branching and make HDPE less brittle and

more durable.15 LDPE has a high branching content and can have branches upon

branches. This makes LDPE a more flexible plastic that can be used for things like

squeezable bottles. Crosslinkers, such as dienes, can also be introduced to obtain a

network polymer. This increases the molecular weight of the polymer as well as the

polydispersity index. Finally, LLDPE has many short chain branches which improve its

properties and it behaves similar to LDPE.15 The short chains also increase LLDPEs

puncture strength, which is why it is the plastic used for bags that can hold jagged

objects like rocks or gravel.

Polypropylene (PP) is unique in that it has regularly spaced methyl branching.

Thus, stereochemistry of the branching can be considered. The arrangement of the

branching along the PP backbone is called tacticity. Tacticity in PP can be syndiotactic,

atactic, or isotactic. Figure 1-4 shows the different kinds of tacticity. Atactic PP has a

29

random stereochemical orientation which makes the polymer amorphous. Syndiotactic

PP has alternating stereochemistry and isotactic PP has the same stereochemistry

along the backbone.1,6,14,15 From this point on, however, the focus will be on

polyethylene almost exclusively.

Figure 1-3. Types of polyethylene branching.

Figure 1-4. Polypropylene tacticity.

Methods of Polymerizations

As stated before, alkenes can undergo cationic, anionic, and radical

polymerizations. In industry though PP, and LLDPE are exclusively made via Ziegler-

30

Natta catalysts, HDPE is made via Phillip’s catalysts, while LDPE is produced through

free radical polymerization.6,14 Both catalytic and free radical polymerization methods

will be discussed extensively as they are both crucial to the work done in this

dissertation.

Ziegler-Natta catalysts have been used to a great extent because they easily

control the molecular weight. They mainly produce low branching material and thus,

additives are used to introduce further branching and improve the flexibility of the

polymers (for both HDPE and LLDPE).15 Since they cannot produce highly branched

material, free radical polymerization is industrially used for producing LDPE. By using a

peroxide initiator, high temperatures and high pressures, industry can force the free

radical to make the high branching content needed for LDPE.14 These harsher reaction

conditions are required due to the less stable intermediates,4 which will be discussed in

more detail later in the introduction.

Ziegler-Natta Catalysts

History

The commercial history of most polyolefins started in the early 1950s at the Max

Plank Institute in Mulheim. Here, Karl Ziegler discovered that when certain transition

metals were combined with organometallic compounds they could polymerize ethylene

using low reaction temperatures and low ethylene pressures.14–16 This gave a linear

structure of polyethylene and is known today as HDPE. Not long after, at the Milan

Polytechnic Institute in Italy, Giulio Natta used a catalyst that was considered “Ziegler-

type” and showed that it can make stereoregular polyolefins such as syndiotactic and

isotactic polybutadienes.15,17

31

Three years after Ziegler and Natta published on these catalytic systems, they

shared the Nobel Prize in Chemistry in 1963.6 Commercial production of HDPE also

started around this time.15 Many people thought that this would be the end for LDPE

since HDPE could be made under more mild conditions. However, due to the vast

difference in their properties, there continues to be a large market for both polymers.15

Since the 1950s and 1960s, many scientists have extended the range of catalysts and

the range of the stereoregular structures made from these catalysts.6

Types of Ziegler-Natta Catalysts

Ziegler-Natta (Z/N) catalysts can be broken up into two categories:

Homogeneous and heterogeneous catalysts. The distinction is determined by the

catalyst’s solubility in the reaction medium. Heterogeneous catalysts are insoluble while

homogeneous catalysts are soluble in the reaction media. Heterogeneous catalysts

were the first type discovered by Ziegler and used by Natta. They contain a transition

metal compound from groups IV to VIII (which is the active catalyst system) combined

with an organometallic compound of a metal from groups I to III (the cocatalyst).6,15

Ziegler for example, used TiCl4 with Al(Et)3 in a hydrocarbon solvent for making HDPE.

Overall these heterogeneous catalysts gave low yields of polymers. It was later

discovered that using solid supports such as MgCl2 or MgO greatly improved yields by

maximizing the number of active sites on the catalyst.15

Homogeneous Z/N catalysts include the broad class of metallocene catalysts.

These are “sandwich” catalysts where the metal is coordinated to either two

cyclopentadienyl (Cp) or some other ring ligands. Figure 1-5 shows the general

structure for metallocenes and some modern day metallocene catalysts. These are

considered Z/N catalysts based on the mechanism which is shown later. Other

32

homogeneous Z/N catalysts have metal centers connected to ligands via a non-metallic

heteroatom such as nitrogen, sulfur, or phosphorus.14

Figure 1-5. Common metallocene structures.

The first metallocene was a simple Cp2ZrCl2 which used (CH3)2AlCl as the

cocatalyst. Unfortunately, these had low activity towards ethylene and would not

polymerize propylene. Interestingly, when water was added to the system the activity

significantly increased.18 The water in the system reacted with the methylaluminum

complex to form methylaluminoxane (MAO). MAO has a complex oligomeric structure of

MW ~ 1,000-1,500. There is still debate today on the actual structure of MAO. Since a

crystalline sample cannot be obtained of MAO, x-ray diffraction cannot elucidate the

structure. Calculations have shown that the most likely structure involves cages of

aluminum/oxygen/methyl.19 The lowest energy cage structure from Zurek and Ziegler

are shown in Figure 1-6. The discovery of hydrolyzed trimethylaluminum (MAO) as a

cocatalyst in 1980 was a crucial step in projecting the area of homogeneous Z/N

catalysts into the realm of competing with other olefin polymerizing catalysts.18 The

mechanisms for these cocatalysts will be discussed later in this section.

33

Figure 1-6. Proposed cage structures of MAO. Methyl groups are omitted for clarity.

Comparison of Homogeneous and Heterogeneous Catalysts

Similarities for these kinds of catalysts lie within the monomer selection. Overall,

Z/N catalysts work better with non-polar monomers, which include α-olefins, alkynes,

dienes and cycloalkenes. The efficiency of the catalyst decreases with increasing steric

hindrance around the double bond, as shown in Figure 1-7.15

Figure 1-7. Catalyst activity for select alkene monomers.

In general, homogeneous catalysts produce more narrow Poly Dispersity Indexes

(PDIs) than heterogeneous catalysts. Having a narrow PDI gives the polymer better

mechanical properties. On the other hand, larger PDIs are better industrially because

these polymers can be better processed.20 This difference in PDI is thought to be due to

either the decay of catalyst activity during heterogeneous polymerization or the

presence of variable activity sites.15 Homogeneous catalysts are “single-site catalysts”

whereas heterogeneous catalysts in solution have active pockets on the surface.

Homogeneous metallocene catalysts are also able to polymerize strained cycloalkenes,

such as norbornene, whereas heterogeneous Z/N catalysts cannot.15 Ring opening

34

polymerization is more common with heterogeneous catalysts and these catalysts are

used to produce the majority of polyolefins in the US.15,21 Lastly, homogeneous catalysts

tend to be easier to modify and to study.

Mechanism

There are three parts to olefin polymerization: initiation, propagation and

termination. The mechanism for the propagation step is still under debate for both type

of catalysts. There are several mechanisms that have been proposed and each has

results that support their hypothesis. Each of these mechanisms and steps will be

covered in detail, as well as the differences between the two catalysts with respect to

their structure.

There are subtle differences between the mechanisms of homogeneous and

heterogeneous catalysts which arise from the differences in physical structure.

Heterogeneous catalysts are crystalline structures where there are active “pockets” on

the surface as previously stated. Homogeneous catalyst are often called “single-site

catalysts” because each metal center is an active site. Figure 1-8A shows a generic

structure of a heterogeneous catalyst crystal with active pockets.22 The boxes represent

empty orbitals extending off of the titanium. Not all titaniums in the crystallite have an

empty orbital, and the metal centers within the structure do not take part in the

polymerization. Supports such as MgCl2 or MgO help to increase the surface area of the

crystallites. This is why heterogeneous catalysts using supports have increased activity

compared to unsupported solid catalysts.23 A generic structure of supported

heterogeneous Z/N catalysts is shown in Figure 1-8B. Because there are many different

environments for these metal species, they are harder to study than their homogeneous

counterpart.

35

Figure 1-8. Generic structures of TiCl4. A) Along a crystal surface, B) along an MgCl2

support. Empty boxes indicate empty orbitals.

The active species of the Z/N catalysts is a coordinately unsaturated metal-alkyl

complex. To get to this complex, an alkylaluminum reagent abstracts an alkyl group

from the metal center.14 For heterogeneous catalysts, this is the triethylaluminum that

was mentioned earlier. For homogeneous catalysts, MAO was found to be a better

alternative. MAO serves three purposes: initiator, cocatalyst, and scavenger. MAO acts

as a cocatalyst because it replaces one or both of the chlorides on the metal centers

with alkyl groups. It then acts as an initiator because it abstracts the last chloride or an

alkyl group, leaving a cationic monoalkyl species. And finally, it acts as a scavenger of

all oxygen and water in the system.24,25 A scheme of this cocatalyst and initiation from

MAO is shown in Figure 1-9. MAO starts the polymerization and helps to keep it going.

To obtain the best results, a ratio for Al:Zr of 1,000:1 is common.24

Figure 1-9. Homogeneous catalyst initiation.

36

Once the metal complex has been initiated by the opening of an orbital, the next

step is for the donated alkyl group to propagate into a polymer chain. There are several

proposals for the mechanism of these catalysts. For heterogeneous catalysts, most

people agree that the monometallic mechanism, or Cossee-Arlman mechanism, is most

likely.14,15,21 A scheme of this mechanism is shown in Figure 1-10. First, ethylene is

coordinated to the metal center and takes up space in the open orbital represented by a

box (step a), then an insertion of this ethylene group occurs with a 4 member transition

state (step b and c). Finally, since this is a heterogeneous catalyst, the polymer chain

migrates allowing the open orbital to be in the same active pocket on the surface.15,26

This cycle repeats itself until termination.

Figure 1-10. Cossee-Arlman mechanism for olefin polymerization with heterogeneous

Z/N catalyst. A box represents an open orbital and P represents the polymer chain.

The modified Green-Rooney is a second mechanism that can occur for

homogeneous catalysts.14 Both the Cossee-Arlman and the modified Green-Rooney

37

have been observed for homogeneous catalysts, and depends on the catalyst system.

The modified Green-Rooney mechanism is shown in Figure 1-11. Although this looks

very similar to the Cossee-Arlman mechanism shown in Figure 1-10, there was a

previous mechanism (the unmodified Green-Rooney), that involved an α-hydrogen

migration to the metal center that resulted in an alkylidene between the alkyl group and

the metal.27 This was later shown to be much less likely. It has thus been modified to

include an “α-agostic” complex where there is a partial cleavage of the α-hydrogen from

the carbon to help with the transition state.28,29 Lastly, some have modified this

mechanism even further to only include this α-agostic interaction in only the transition

state (only after step b and c).

Figure 1-11. The modified Green-Rooney mechanism for olefin polymerization. A box

represents and empty orbital and P represents the polymer chain.

There are several ways in which the polymer chain can be terminated. These

include β-hydrogen elimination, β-hydrogen transfer to monomer, and chain transfer to

aluminum. Each of these is shown in Figure 1-12. β-hydrogen elimination is a common

method of chain termination and is essential to the research presented in this

dissertation. The β-hydrogen of the polymer chain is transferred to the metal center

which results in an α-olefin chain end that is coordinated to the center. The α-olefin

chain end will eventually leave, giving an open site on the metal center which can be

occupied by another ethylene monomer to restart the polymerization.14,30 For part B of

Figure 1-12, the β-hydrogen of the chain connected to the metal center is transferred to

38

a coordinated monomer resulting in another α-olefin terminated polymer chain, and a

metal center with an open site to continue propagation with a new polymer chain.14

Lastly, chain transfer to aluminum happens when there is low ethylene pressure in the

system. Since MAO has residual trialkylaluminum within its mixture, this has been

shown to pick up polymer chain ends by a chain transfer mechanism shown in part C of

Figure 1-12.31,32

Figure 1-12. Mechanisms for chain termination. A) β-hydrogen elimination, B) β-

hydrogen transfer to monomer, C) chain transfer to aluminum.

Additionally, industry terminates the polymerization by adding hydrogen to the

system. This also allows the molecular weight to remain in the range where it can be

easily processed.15 This method is a cheap and clean way to terminate the

polymerization while controlling the molecular weights of the polymers made.

Modern Day Uses and Development

Progress has occurred in leaps and bounds since the first Z/N catalyst was made

back in the 1950s. We now have more control over the polymer structures. Molecular

weight (Mw), catalyst activity, and polymer branching can all be tuned and altered. This

39

can be done by either changing ligands on the catalysts, the transition metal of the

catalyst or by changing the cocatalyst.33 For example, chemists can now attach the

MAO cocatalyst to a carbon nanofiber to create polypropylene with increased tensile

strength,34 homopolymerize larger α-olefins stereospecifically to a high molecular

weight,35 or control the molecular weight distribution by using a living polymerization

catalyst.36

Free Radical Polymerizations

Another way to produce polyethylene is through free radical polymerization. This

is in fact the only way to make LDPE industrially. Since the bulk of the polymers

researched in this dissertation is LDPE, it is necessary to cover the different aspects of

free radical polymerization.

History

Free radical polymerization was first discovered through the storage of styrene

monomers, around 1839. This liquid monomer would become more and more viscous

during storage because it self-initiated.37 Later Blyth and Hofmann observed that this

viscous material could be made by exposing the liquid to light. This observation was

probably the first light-induced polymerization reported.38

It was not until much later that chemists started adding initiators to monomers to

get these products. In 1912, Fritz Klatte used peroxy compounds to start vinyl

polymerizations.39 Over the next decade chemists started to agree that the free radicals

generated from the peroxides were initiating the polymerizations.40 From here free

radical polymerization became more and more popular.

In the early 1930s, employees at Imperial Chemical Industries in England

discovered LDPE using free radical polymerization.41,42 In 1938, larger quantities were

40

made and the plastic was successfully used to insulate cables. The following year, a

commercial plant began producing polyethylene plastic full time. Up through 1945, all

polyethylene made was used for insulation of radar cables. After World War II, the

plastic found its way to more commercial uses.42 Today free radical polymerization is

used to produce about 45% of all plastic materials and about 40% of synthetic

rubbers.40 Poly(vinyl chloride), polystyrene, acrylonitrile-butadiene-styrene copolymer

(ABS), and poly(methyl methacrylate) are all made via free radical polymerization.15

Mechanism

Though the mechanism for free radical polymerization is straightforward, there

are extra considerations when applying this method to the production of LDPE. The

steps for free radical polymerization are the same as the Z/N mechanism: initiation,

propagation and termination. First, initiators are added to start the polymerization. For

LDPE, these are normally organic peroxide initiators such as benzoyl peroxide or di-t-

butyl peroxide. These are thermally unstable and when heated at high temperatures will

form the radical initiator. Figure 1-13 shows the two steps involved with initiation using

di-t-butyl peroxide: forming the radical initiator, and reacting this with the first monomer.

Figure 1-13. Initiation using di-t-butyl peroxide.

Propagation for free radical polymerization is simple and will continue until a

termination step occurs. This step is shown in Figure 1-14.

41

Figure 1-14. Propagation of a free radical polymerization. R stands for initiator.

Two types of branching can occur during propagation for LDPE: long branches

(longer than six carbons in length) and short branches. For LDPE, these are made by

intermolecular and intramolecular hydrogen transfer respectively. For intermolecular

hydrogen transfers to make long branches, a free radical at the end of the chain

abstracts a hydrogen from a secondary carbon along another chain. This new radical

can then keep propagating into a new long branch. This happens often when using high

temperatures and is common for LDPE because 2° radicals are more stable than the 1°

ones found at the chain ends.4 The mechanism for long branching is shown in Figure 1-

15.

Figure 1-15. Intermolecular hydrogen transfer for long branches in polyethylene.

Intramolecular hydrogen transfers make the short branching in LDPE. In this

method, the radical chain end abstracts a hydrogen from a carbon along its own chain.

The most common short branch is the n-butyl branch because it forms a 6-member

intermediate. After this new 2° radical is formed, a monomer can be added and undergo

a second intramolecular hydrogen transfer to create ethyl branches.8 These pathways

are shown in Figure 1-16.

42

Figure 1-16. Intramolecular hydrogen transfer for making small braches in polyethylene.

Termination can be classified into one of two pathways: combination and

disproportionation. Combination is simply combining two radicals together, whether it is

two chain ends, a chain end with a radical initiator, etc (Figure 1-17A).

Disproportionation is the abstraction of hydrogen from one chain end. This leaves a

terminal alkene at the end of one of the polymer chains (Figure 1-17B).

Figure 1-17. Termination steps for free radicals. A) Combination and B)

disproportionation.

Lastly, one thing to consider is the energy used to make LDPE. Since the

intermediates of these free radical polymerizations are less stable, high pressures and

temperatures are used.4 LDPE is made in bulk industrially, which is the simplest form

but it can be difficult to control heating transfer which can lead to fouling.43 Traditionally

it takes 200 °C and 44,000 psi to induce the correct branching and therefore result in

43

the mechanical properties expected for LDPE.4 HDPE on the other hand is made in

solution which allows good heat transfer but does require the use of organic solvents

which can be hard to separate from the final product.15,43 These can be done at lower

temperatures and pressures since it is done with catalysts.

Modern Day Uses and Development

Recent developments in this field include exerting better control of the

polymerization through the process of “Reversible Addition Fragmentation Chain

Transfer” (RAFT) and “Atom Transfer Radical Polymerization” (ATRP). Both of these

systems use a “dormant” or inactive species to transfer the radical to. By making this

interconversion between the polymer chain and the chain transfer agent fast, the growth

of all chains have similar probability. This is a way to specifically control the

polydispersity index of the polymers.40,44 Also since these additives also minimize the

rate of termination, the molecular weight can be controlled and block copolymers can be

synthesized.40

LDPE from Catalysts

There is a small niche of Z/N catalysts that make LDPE. Although the method for

industrially making LDPE has been around for many decades, the thought of using

catalysts to make LDPE is still enticing. Using a catalyst that can perform under mild

conditions instead of the traditional extreme temperatures and pressures would greatly

reduce the energy and resources of manufacturing.45 Also, catalysts have more control

over the branching content and molecular weight compared to free radical

polymerization.14

The discovery of LLDPE was actually the result of chemists trying to reproduce

LDPE from catalysts. They added higher amounts of α-olefins to the PE polymerizations

44

to increase the branching and they also lowered the molecular weight. This resulted in a

material with high puncture resistance (due to the short chain branching). However,

LLDPE was still not able to replace LDPE in the market because of LDPE’s mechanical

properties that stem from the long branching content. LLDPE also has increased cost of

synthesis because it is not from a pure ethylene feed.

Another system chemists have tried to produce LDPE from a pure ethylene feed

is a dual-catalyst system. This kind of system would normally have one catalyst that can

make α-olefins from ethylene and then a second catalyst that can incorporate these

chains (usually 1-hexene or 1-octene in length) into the polymer backbone.46 The only

drawback is that these are actually not LDPE but rather LLDPE because you cannot

produce the long branches. However, it differs from LLDPE in that it is produced from a

pure ethylene feed.

Also, some late transition metal catalysts (containing nickel and palladium) have

been known to produce hyperbranched polyethylene.14 The polymers made have a

much larger degree of branching and can have multiple branching points along the

individual branches. Because of this hyperbranched content, the materials made from

these catalysts are usually oils.47

Lastly, a way to produce LDPE from catalysts is by using a constrained geometry

catalyst (CGC). This is a new category of catalysts that can produce large branches

from a pure ethylene feed. It does it by first terminating the polymer chain via a β-

hydride elimination, which is common for many Z/N catalysts. What sets these catalysts

apart from the rest is that after disassociation of the olefin, the catalyst can later

coordinate to the olefin and reinsert it into the polymer chain as a branch (Figure 1-18).

45

The reason for this coordination and reinsertion is the large exposed active center. The

constrained geometry of the ligand allows the metal center to be open and accept large

olefins, whereas other catalysts do not have as much room to accommodate this. The

companies at the forefront of this research are Dow and Exxon, using ansa-

cyclopentadienyl-amido constrained geometry catalysts.14,45 A general structure of

these catalyst are shown in Figure 1-19.

Figure 1-18. Mechanism for by β-hydride elimination, dissociation, polymerization then

reinsertion.

Where M= Ti, Zr, or Hf, R= akyl or aryl, and R’= H or Me

Figure 1-19. General structure for ansa-cyclopentadienyl-amido constrained geometry catalysts.

Interestingly, heterobinuclear catalysts that have been covalently linked together

have also been synthesized and have the ability to produce more long-chain branches

(Figure 1-20).14 These contain two constrained geometry catalyst analogues.

46

Figure 1-20. Example of a heterobinuclear catalyst.

Octamido Catalyst

The main catalyst used in these studies is the “Oct-amido” catalyst. This

zirconium fluorenyl-amido system is a CGC and its synthesis48 is shown in Figure 1-21

and Figure 1-22. It starts with the synthesis of the octamethyloctahydrodibenzofluorene

(Oct), which is made by first reacting 2,5-dimethyl-2,5-hexanediol with concentrated

hydrochloric acid to isolate 2,5-dichloro-2,5-dimethylhexane. This is then added onto

fluorene via a double cyclo Friedel-Crafts alkylation.

Figure 1-21. Octamethyloctahydrodibenzofluorene (Oct) synthesis.

Once the Oct moiety is isolated, it is then deprotonated and added to an excess

of dimethyldichlorosilane which is then reacted with premade lithium t-butyl amide,

47

without isolation; this is reacted with two equivalents of n-butyl lithium and the Zirconium

metal center (Figure 1-22). This results in an overall 30% yield from Oct to catalyst,

however it is done on a large scale and about 14 grams of the yellow catalyst was

obtained.

Figure 1-22. Oct-amido catalyst synthesis.

This catalyst was first published back in 2004 from the Miller group and has been

shown to have unique properties.48–51 In 2004, it was shown to give exceptional activity

towards bulky α-olefins. It gave activities in the 100,000s kg polymer/(mol Zr·hr) for

copolymers of ethylene with either 1-octene or 4-methyl-1-pentene or copolymers of 1-

octene with 4-methyl-1-pentene.48

48

It was later shown to also give the most syndiotactic polypropylene to date ([rrrr]>

99%) which resulted in high melting temperatures of the polymers (165 °C for the

unannealed, and 174 °C for the annealed polymers).49 Following this, the Miller group

studied the copolymerization of α-olefins with propylene.50 The results showed a linear

relationship between the melting temperature (Tm) and the incorporation of the α-olefins;

however the size of the comonomer did not matter. Also, out of all the catalysts tested,

the Oct-amido produced the fewest misinsertions which resulted in the highest melting

polymers.

It was not until 2008 that the results of ethylene homopolymerization using Oct-

amido was published.51 The Oct-amido catalyst was shown at this time to be able to

create both small (ethyl) and large (≥ hexyl) branching without the need of α-olefin

comonomers when polymerizing ethylene. In short, this catalyst can make LDPE from a

pure ethylene source. Not only this, but the polymers possessed high amounts of long

branches—more than any other early transition metal catalyst to date (between 10-50

branches per 1,000 carbon).51 The crystal structure published in 2008 (Figure 1-23)

gives insight into why this catalyst likes bulky α-olefins.

Figure 1-23. Crystal structure of Oct-amido catalyst. Reprinted with permission from

reference 48.

49

The Oct-amido catalyst is a constrained geometry catalyst, similar in structure to

the ansa-cyclopentadienyl-amido catalyst shown earlier. The side view of Figure 1-23

shows that there is room around the zirconium center for macromonomers to coordinate

to. The diethyl ether that is coordinated to the zirconium center would dissociate during

polymerization and the chlorines show where the polymer chain eventually grows from.

This largely open metal center gives the catalyst the ability to produce branching from

ethylene alone.

Figure 1-24. Chromium oligomerization catalyst used in a tandem catalyst system with

Oct-amido.

There is one drawback to this catalyst. As shown in the 2008 paper, this catalyst

produces polymer with low molecular weights, ~1,000 g/mol. This is far too low for any

industrial applications. Ideally, LDPE has ~2-7 large branches per 1,000 carbon and a

molecular weight of ~50,000-500,000 g/mol.1,13 There have been previous attempts to

increase the molecular weights of these polyethylene samples created by Oct-amido

including tandem catalyst systems.52 This dual-catalyst system involved using the Oct-

amido catalyst in tandem with a Chromium oligomerization catalyst (Figure 1-24) along

with the cocatalyst MAO. The chromium catalyst produced 1-hexene in situ which was

incorporated by the Oct-amido catalyst into the polymer backbone chain. The

incorporation of 1-hexene could be easily measured by carbon NMR since Oct-amido

does not produce n-butyl branching. This resulted in the production of LLDPE from a

50

pure ethylene feed but it did not help to improve the molecular weights of the polymers

produced.

Overview of Dissertation

The following chapters in this dissertation include ways of improving the

molecular weights of the polyethylene formed by the Oct-amido catalyst and looking at

the properties of the polymers made. The main goal is to increase these weights without

having to sacrifice the melting temperatures, or branching of the polymer or the activity

of the catalyst. Chapter 2 discusses using various dienes as crosslinkers to link polymer

chains together. Chapter 3 explores supports for the Oct-amido catalyst as well as other

metallocene catalysts. This work was done in collaboration with Hippasia M. Moura from

Dr. Heloise Pastore’s group at the University of Campinas in Brazil. Chapter 4 explains

the use of a dual-catalyst system in conjunction with a chain shuttling agent. This would

result in polymers that contain properties given by both catalysts. Finally, Chapter 5 is a

summary of the findings throughout the projects and future outlook for the catalyst.

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CHAPTER 2 DIENES AS CROSSLINKERS

Background†

This chapter describes increasing the molecular weight of polymers through

chain crosslinking by copolymerization of ethylene with various non-conjugated dienes.

The branching and thermal properties of these polymers were investigated, as well as

diene homopolymerization for comparison. The copolymerizations follow the general

reaction of Figure 2-1.

Figure 2-1. Copolymerization of ethylene and dienes using 1/MAO to afford branched

and crosslinked polyethylene.

Two other catalyst systems were investigated for comparison: 2/MAO, the

“IsopropylideneOctCp” metallocene Me2C(C29H36)(C5H4)ZrCl2 previously synthesized

and published by our lab;53 and 3/MAO, the commercially available zirconocene

dichloride (Figure 2-2).

Figure 2-2. Structure of Me2C(C29H36)(C5H4)ZrCl2 (2) and zirconocene dichloride (3).

†This chapter is based on a manuscript in preparation.

52

Catalyst 2 has been shown to afford rather high molecular weights (upwards of

500,000 g/mol) and highly stereoregular syndiotactic polypropylene.53 This high degree

of stereoregularity translates into high polymer melting temperatures near 154 °C

without annealing. Olefin polymerizations with 3 have been investigated for decades

and this was the first metallocene to be activated by MAO for the polymerization of

ethylene and α-olefins.12,54,55 3/MAO generally provides low molecular weight atactic

polypropylene but affords polyethylene with good molecular weights (100,000–200,000

g/mol).12,56,57

Results and Discussion

Diene Crosslinkers and Cognate Mono-enes

Figure 2-3 depicts the non-conjugated diene crosslinkers and cognate mono-

enes employed; the latter are included as a control group to assay the inherent

polymerizability of an alkene type. These compounds can be divided into two groups:

cyclic and acyclic. The first group, the cyclic dienes, includes 5-vinyl-2-norbornene

(VNB), 2,5-norbornadiene (NBD), and 4-vinylcyclohexene (VCH). These dienes are

used academically and commercially in copolymerizations with ethylene and are fairly

inexpensive.1,58 Also in this group are the cognate cyclic mono-enes vinylcyclohexane

(VCHA) and cyclohexene (CH). These were used to investigate the reactivity of the

component alkene functional groups of VCH—particularly for catalyst 1/MAO. The

second group includes acyclic dienes. 1,7-octadiene (OD) and 1,9-decadiene (DD) are

obvious choices for non-conjugated diene crosslinkers, as they are similar in structure

to the simple α-olefins that 1/MAO incorporates very well with ethylene.59 Another

acyclic diene investigated was 12-butyl-1,22-tricosadiene (TCD), an ADMET (Acyclic

Diene METathesis) monomer studied by the Wagener laboratory and readily available

53

to us.60 The incorporation of this butyl-branched diene is observed by 13C NMR. 1-

octene and 1-decene are simple α-olefin mono-enes used for comparison in

homopolymerizations.

Figure 2-3. Dienes for copolymerization with ethylene and control group mono-enes for

copolymerization and homopolymerization.

Ethylene Homopolymerization and Copolymerization

First, all three catalysts were directly compared in the homopolymerization of

ethylene. Entries 1-3 in Table 2-1 show that 1 gives the highest activity under the same

reaction conditions—twice the amount of polymer compared to 2 or 3. It also gives the

lowest melting temperature, which comports with the high branching content of about 40

long branches per 1,000 carbons. Catalysts 2 and 3 give linear polyethylene under

these conditions whereas 1 produces the aforementioned ethyl and long branching.

Molecular weights for these catalysts are compared later in Table 2-2.

The copolymerization of ethylene with dienes is designed to crosslink the

polymer chains—resulting in higher molecular weight polymers. 5-vinyl-2-norbornene

(VNB), 1,9-decadiene (DD) and 2,5-norbornadiene (NBD) were chosen because they

are relatively inexpensive dienes and have been used previously to prepare crosslinked

polyethylene with other catalysts.61,62

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Table 2-1. Thermal, branching, and molecular weight results for ethylene homopolymerizations and copolymerizations.a

Entry Catalyst

Comonomerb Yield (g)

Activity (kg/mol Zr·hr)

Mole % comonomer incorporation

Tm (°C)

Branchingc

Type Conc.

(mmol/L)

Equiv. vs.

Catalyst NE NL NT

1 1 none - - 1.41 2,120 - 112 6.3 39.5 45.8 2 2 none - - 0.51 870 - 132 Linear PE 3 3 none - - 0.67 1,020 - 133 Linear PE 4

1 VNB 28 220 1.13 2,040 0.5 d 96 8.9 28.8 37.7

5 VNB 56 440 1.18 2,130 1.0 d 86 7.9 25.6 33.5 6 VNB 140 1,090 1.70 3,060 3.2 d 60 8.3 13.2 21.5 7

2 VNB 28 170 0.54 790 1.3 d 112 -

8 VNB 56 340 0.59 850 1.0 d 106 - 9 VNB 140 850 0.79 1,140 13.8 d 89 -

10 3

VNB 28 190 0.53 840 0.3 d 121 -

11 VNB 56 370 0.47 750 1.3 d 120 - 12 VNB 140 92

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© 2016 Nicole Lyn Gibbons - University of Floridaufdcimages.uflib.ufl.edu/UF/E0/05/05/38/00001/GIBBONS_N.pdf4 ACKNOWLEDGMENTS I was told many years ago that graduate school would