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STRUCTURAL CHARACTERIZATION OF COMPLEX SYSTEMS

BY DEGRADATION /

A Dissertation

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements of the Degree

Doctor of Philosophy

Panthida Thomya

December, 2006 STRUCTURAL CHARACTERIZATION OF COMPLEX POLYMER SYSTEMS

BY DEGRADATION / MASS SPECTROMETRY

Panthida Thomya

Dissertation

Approved Accepted

Advisor Department Chair Dr. Chrys Wesdemiotis Dr. Kim C. Calvo

Committee Member Dean of the College Dr. Matthew P. Espe Dr. Ronald F. Levant

Committee Member Dean of Graduate School Dr. Jun Hu Dr. George R. Newkome

Committee Member Date Dr. Wiley J. Youngs

Committee Member Dr. Frank W. Harris

ii ABSTRACT

The compositions and structures of large and complex macromolecular systems have been analyzed by combining low-temperature (<400 oC) with mass (MS) and tandem mass spectrometry (MS/MS) studies. The characterization of the pyrolysis products (pyrolyzates) was effected using primary matrix-assisted laser desorption/ ionization time-of-flight (MALDI-ToF) mass spectrometry. This approach was applied to amphiphilic membranes, composed of hydrophilic poly( glycol) (PEG) and hydrophobic polydimethylsiloxane (PDMS) domains, crosslinked by poly(pentamethyl- cyclopentasiloxane) (PD5) segments. The prepolymers, distyryl poly(ethylene glycol)

(St-PEG-St) and divinyl polydimethylsiloxane (V-PDMS-V), were also analyzed by the same method for comparison of their pyrolyzates to those from the more complicated amphiphilic networks. Pyrolyses were performed from as low as 75 C up to 300 C.

The structures of the pyrolyzates were confirmed by MALDI-ToF MS, electrospray ionization/quadrupole ion trap (ESI-QIT) tandem MS, and Fourier transform ion cyclotron resonance (FT-ICR) MS techniques. Combined application of these methods was necessary to distinguish isobaric pyrolysis products and verify the end groups of the pyrolyzates.

iii Increasing pyrolysis temperatures caused further fragmentation of the initially formed by pyrolyzates. Based on the PEG and PDMS identified in the pyrolyzates by

MALDI-ToF MS, ESI-QIT MS/MS, and FT-ICR MS, it was possible to determine the preferred degradation pathways of the prepolymers and the membrane. PEG chains and the PEG segments of the network were found to thermally degrade by free-radical decomposition mechanisms, initiated by C-C or C-O bond cleavages. PDMS chains and the PDMS segments of the network, on the other hand, formed cyclic oligomers upon pyrolysis. This study also compared the degradation efficiencies of the at different temperatures. The overall results conclusively show that the amphiphilic membrane synthesized contained the desired PEG and PDMS strands. These data constitute a unique direct spectroscopic characterization of partial features of a complex polymer assembly. The crosslinker or its oligomers could, however, not be detected in the pyrolysis products to ascertain that PD5 segments were indeed present in the membrane.

Neither did pyrolysis of genuine PD5 yield any detectable products characteristic of its structure or the crosslinker . Hence, the absence of PD5 signature products in the membrane pyrolyzates most likely resulted from the thermal stability of PD5 network in the probed temperature range.

iv ACKNOWLEDGEMENTS

I gratefully acknowledge my advisor Dr. Chrys Wesdemiotis for his wonderful encouragement and guidance in my mass spectrometry research, for always being there to listen, discuss, and give advice, and for support me to work on my desired research. I also wish to express my appreciation to all the current and former colleagues who have assisted and advised me, especially Dr. Kathleen Wollyung, Dr. Michael Polce,

Dr. Ping Wang, Edgardo Rivera, Alyson Leigh, and Sara Tomechko. They always made the mass spectrometry lab a wonderful workplace and home.

I am grateful to my committee members, Dr. Matthew Espe, Dr. Jun Hu,

Dr. Wiley Youngs, and Dr. Frank Harris, whose useful comments helped improve this dissertation.

I express my gratitude to my mom and dad for their love and support, my first brother Polkrit for helping me through the difficult times, my second brother Pornkrit for his understanding during my graduate school time even though we are all in different countries. I also want to thank my lovely friend, Diane Davis, for her love, heartening, understanding, and making me feel as warm as a family for the past four years.

v DEDICATION

This dissertation is dedicated to the love and memory of my wonderful grandmom, Aengsuan Saekhor, who is the light of my . She has encouraged me for over a number of years. Without her, I would not learn as much as I did about life. Her support and belief in me through the many years of the study were making me never give up. Her inspiration made me succeed this far. Her love and care gave me an appreciation for the meaning and importance of family.

vi TABLE OF CONTENTS

LIST OF TABLES ..……………………………………………………………………. xi

LIST OF FIGURES……………………………………..……………………………... xii

LIST OF SCHEMES …………………………………………………..………………xvii

CHAPTER

I. INTRODUCTION ………………………………………………….………….…….1

II. SCIENTIFIC BACKGROUND …………………………………………………….5

2.1. Complex polymer assemblies …………………………………………………..5

2.2. Polymer pyrolysis mass spectrometry ………………………………………….8

2.3. Mass Spectrometry instrumentation …………………………………………..11

2.4. Ionization methods ………………………………………………….…………13

2.4.1. Matrix-Assisted Laser Desorption Ionization (MALDI) ………….……..14

2.4.2. Electrospray Ionization (ESI) ………………………………………..…..17

2.5. Mass analyzers ………………………………………………………………..21

2.5.1. Time-of-Flight (ToF) mass analyzer ………………...... 22

2.5.2. Quadrupole mass analyzer ……………………………………………….26

2.5.3. Quadrupole-Time-of-Flight (Q-ToF) tandem mass analyzer ……………29

2.5.4. Quadrupole Ion Trap (QIT) mass analyzer ………………………………29

2.5.5. Quadrupole Ion Trap tandem mass analyzer ...... ….....…32

vii III. MATERIALS, METHODS, AND INSTRUMENTATION……….….…………….34

3.1. Complex polymer assemblies ………………………………………………...…34

3.2. Materials ………………………………………………………………………...35

3.3. Pyrolysis procedure ………………………………………………………….….36

3.4. Experimental preparation ………………………………………………………..38

3.4.1. Sample preparation for MALDI-ToF-MS, MALDI-Q-ToF-MS, and MALDI-Q-ToF-MS/MS experiments …………………………………..…39

3.4.1.1. Before pyrolysis ………………………………………………………39

3.4.1.2. After pyrolysis ………………………………………………………..39

3.4.2. Sample preparation for ESI-QIT-MS and ESI-QIT-MS/MS experiments ...40

3.4.3. Sample preparation for FT-ICR MS experiments ………………………….40

3.5. Experimental instrumentation ………………………………………………...…40

3.5.1. Bruker Daltonics Reflex III MALDI-ToF mass spectrometer ……………..40

3.5.2. Micromass MALDI-Q-Tof Ultima mass spectrometer …………....42

3.5.3. Bruker Daltonics Esquire-LC ESI-QIT mass spectrometer (ESI-QIT-MS)..45

3.5.4. Bruker APEX III 47e FT-ICR mass spectrometer ……………………..…..47

IV. PYROLYSIS MASS SPECTROMETRY ANALYSIS OF DIVINYL-POLYDIMETHYLSILOXANE ………………………………………...48

4.1. Unpyrolyzed divinyl-polydimethylsiloxane ……………………………….……48

4.2. Pyrolyzed divinyl-polydimethylsiloxane ……………………………...... …..52

4.3. Conclusion …………………………………………………………………...…55

V. PYROLYSIS MASS SPECTROMETRY ANALYSIS OF DISTYRYL-POLY(ETHYLENE GLYCOL) ……………………………………....57

5.1 Unpyrolyzed distyryl-poly(ethylene glycol) ………………………………….....57

viii 5.2 Pyrolyzed distyryl-poly(ethylene glycol) ………………………….……….……62

5.3. High resolution and mass accuracy experiment………………...………………..78

5.3.1 St-PEG-St pyrolyzates: Series A distinction ……………………………..…84

5.3.2 St-PEG-St pyrolyzates: Series B distinction ……………………………..…86

5.3.3 St-PEG-St pyrolyzates: Series C distinction ……………………………..…89

5.4 Conclusion ………………………………………………………………...... …..91

VI. PYROLYSIS MASS SPECTROMETRY ANALYSIS OF TRICOMPONENT AMPHIPHILIC NETWORKS ………………………….……..92

6.1. Pyrolyzed amphiphilic networks ………………………………………………..92

6.2. exchange reaction (H/D) ……………………………………...……102

6.3. Tandem mass spectrometry …………………………………………………….105

6.3.1. Tandem mass spectrometry on poly(ethylene glycol) standards …………105

6.3.2. Tandem mass spectrometry on amphiphilic networks …………………....121

6.3.2.1. Series A ……………………………………………………………...121

6.3.2.2. Series B ……………………………………………………………...125

6.3.2.3. Series C …………………………………………………………...…126

6.3.3. PDMS distribution ………………………………………..……………....133

6.3.4. Summary of MS/MS data …………….…………………..……………....133

6.4. Degradation efficiency …………………………………………………………137

6.4.1. Divinyl polydimethylsiloxane …………………………………………….137

6.4.2. Distyryl poly(ethylene glycol) ……………………………………………139

6.4.3. Amphiphilic network APN#1 ……………………………………...…..…141

6.4.4. Comparison of amphiphilic networks APN#1 and APN# .………...…..…144

ix 6.5. Conclusion ……………………………………………………………………..147

VII. SUMMARY ………………………………………………………………………148

REFERENCES ………………………………………………………………………...151

APPENDICES ...……………………………………………………………...…...….. 159

APPENDIX A ESI-QIT CAD mass spectra of select pyrolyzates from St-PEG-St…...160

APPENDIX B Additional ESI-QIT CAD data of PEG reference compounds and of pyrolyzates from APN#1 …………………………………….……..164

APPENDIX C Pyrolysis of genuine PD5, synthesized by G. Erdodi and J. P. Kennedy …………………………………………..173

x LIST OF TABLE

Table Page

3.1. List of pyrolysis temperatures for starting materials and crosslinking agent ……....37

3.2. List of pyrolysis temperatures for copolymer membranes ……………………..….37

3.3. List of the weight % compositions of the copolymer samples analyzed ……….….39

5.1. PEG pyrolysis products, identified based on MS and MS/MS experiments, and their end group masses ………………………………………………..……….74

5.2. Isobaric components of series A in the pyrolyzates of St-PEG-St …………………84

5.3. Observed mass-to-charge ratios versus calculated m/z values for A1-A4 oligomers with 5Na+ charges.………………….……………………..…….85

5.4. Isobaric components of series B in the pyrolyzates of St-PEG-St ………………….86

5.5. Observed mass-to-charge ratios versus calculated m/z values for B1-B3 oligomers with 5Na+ charges …………………...…………………………..87

5.6. Isobaric components of series C in the pyrolyzates of St-PEG-St .………...……… 89

5.7. Observed mass-to-charge ratios versus calculated m/z values for C1-C2 oligomers with 5Na+ charges …………………………...…………………..90

6.1. Signature ions of different PEG end groups …………………...……..………..….120

xi LIST OF FIGURES

Figure Page

2.1. The components of a mass spectrometer ……………………...……………………12

2.2. The matrix assisted laser desorption/ionization (MALDI) source …………..……..15

2.3. The MALDI process …………………………………………………………..……16

2.4. Detail of skimmers and ion guide sampling in an ESI source …………………...…18

2.5. The electrospray ionization (ESI) source …………………………………………...20

2.6. The ESI process …………………………………………………………….………20

2.7. The linear time of flight (ToF) mass analyzer …………………………………...…23

2.8. The reflectron time of flight (ToF) mass analyzer ………………………………….25

2.9. The quadrupole mass analyzer ……………………………………………………...27

2.10. Quadrupole ion trap mass analyzer ………………………………………………..30

3.1. Scheme of the Bruker Daltonics Reflex III MALDI-ToF-MS instrument ………....41

3.2. Scheme of the Micromass Ulrima MALDI-Q-ToF-MS instrument ………………..44

3.3. Scheme of the Bruker Daltonics Esquire-LC ESI-QIT-MS instrument ……………46

4.1. MALDI-ToF mass spectrum of non pyrolyzed V-PDMS-V with the insert shows on expanded view of the peaks generated by the 39-mer and 40-mer …………………………………………………………...... 49

4.2. MALDI-ToF mass spectrum of the products from V-PDMS-V pyrolysis at 150 C for 30 minutes …………………………………………………………....51

xii 4.3. MALDI-ToF mass spectra of the products from V-PDMS-V pyrolysis at 200 C (top) and 300 C (bottom) for 30 minutes ……………………………....53

4.4. Expanded view of the m/z 1250-1450 region in the MALDI-ToF mass spectrum of the products from V-PDMS-V pyrolysis at 300 C …………….54

5.1. MALDI-ToF mass spectrum of non pyrolyzed St-PEG-St ………………………...58

5.2. An expanded segment of the MALDI-ToF mass spectrum of non pyrolyzed St-PEG-St ……………………………………………………….….59

5.3. MALDI-ToF mass spectrum of non pyrolyzed St-PEG-St, measured over a narrower m/z region …………………………………………...…61

5.4. MALDI-ToF mass spectrum of the products from St-PEG-St pyrolysis at 75 C for 15 min …………………………………………………………………63

5.5. MALDI-ToF mass spectra of the products from St-PEG-St pyrolysis at 100 and 125C for 15 min ……………………………………………………….64

5.6. MALDI-Q-ToF mass spectrum of the products from St-PEG-St pyrolysis at 75 C, 15 min ……………………………………………………………………66

5.7. MALDI-ToF mass spectrum of the products from St-PEG-St pyrolysis at 150 C, 30 min ....…………………………………………………………..……67

5.8. MALDI-ToF mass spectrum of the products from St-PEG-St pyrolysis at 200 C, 30 min ……………………………………………………………….….68

5.9. MALDI-ToF mass spectrum of the products from St-PEG-St pyrolysis at 275 C, 30 min …………………………………………………………………..69

5.10. MALDI-ToF mass spectrum of the products from St-PEG-St pyrolysis at 300 C, 30 min …………………………………………………………………70

5.11. An expanded view of the MALDI-ToF mass spectrum of the PEG pyrolyzates generated at 275 C, 30 min ……………………………………72

5.12. An expanded view of the MALDI-ToF mass spectrum of the PEG oligomers generated upon St-PEG-St pyrolysis at 275 C, 30 min …………………………..73

5.13. Calculated masses and isotope patterns of PEG oligomers that can arise upon pyrolysis of St-PEG-St. The Na+ adducts of these oligomers appear at nominal m/z 1153 …………………………………………………………..……..79 xiii 5.14. A ESI/FT-ICR mass spectrum of the products from St-PEG-St pyrolysis at 100 C …………………………………………………………………..………80

5.15. Partial ESI/FT-ICR mass spectrum of the pyrolysis products of St-PEG-St (100 C), carrying 5+ charges ……………………………………………………..81

5.16. Partial ESI/FT-ICR mass spectrum of the pyrolysis products of St-PEG-St (100 C) with +4 Na+ charges …………………………………………………….82

5.17. Partial ESI/FT-ICR mass spectrum of the pyrolysis products of St-PEG-St (100 C) with +3 Na+ charges ………………………………………………..……83

6.1. MALDI-ToF mass spectrum of the pyrolysis products from APN#1 (275 C, 30 min) ………………………………………………………………..…..94

6.2. Expanded trace of the MALDI-ToF mass spectrum of APN#1 after pyrolysis at 275 C …………………………………………………..………..95

6.3. Expanded trace of the MALDI-ToF mass spectrum of APN#1 after pyrolysis at 275 C ……………………………………………..……………..96

6.4. MALDI-ToF mass spectrum of the products of APN#1 pyrolysis at 150 C for 30 min …………………………………………………………….….99

6.5. MALDI-ToF mass spectrum of the products of APN#1 pyrolysis at 350 C for 30 min …………………………………………………………....…100

6.6. MALDI-ToF mass spectrum of the products of APN#2 pyrolysis at 350 C for 30 min …………………………………………………………....…101

6.7. Partial MALDI-ToF mass spectrum of the deuterated pyrolyzates formed at 275 C from APN#1 ……….…………………………………………………....104

+ 6.8. ESI-QIT CAD mass spectrum of the Li adduct of H-[PEG]9-OH at m/z 421, generated from a PEG 600 standard with hydroxyl groups at both chain ends …...107

+ 6.9. ESI-QIT CAD mass spectrum of the Li adduct of H-[PEG]3-OEt at m/z 185, generated from a monoethyl PEG 200 standard (ethoxy and hydroxyl chain ends) …………………………………………………111

+ 6.10. ESI-QIT CAD mass spectrum of the Li adduct of MeO-[PEG]5-OMe at m/z 273, generated from a dimethyl PEG 400 standard (methoxy groups at both chain ends) ...……………………………………..……113

xiv + 6.11. ESI-QIT CAD mass spectrum of the Li adduct of H-[PEG]11-OMe at m/z 523, generated from a monomethyl PEG 550 standard (methoxy and hydroxyl chain ends) ………………………………………...……115

+ 6.12. ESI-QIT CAD mass spectrum of the Na adduct of St-[PEG]5-OSt at m/z 493, generated from the distyryl PEG ………………………………….....118

6.13. ESI-QIT CAD mass spectrum of the sodiated at m/z 405 in PEG series A of the APN#1 pyrolyzates obtained at 275 C ………………………….123

6.14. MS3 spectrum of the sodiated fragment at m/z 377 in the ESI-QIT CAD spectrum of the A series oligomer at m/z 405 ……………………………….…..124

6.15. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 465 in PEG series B of the APN#1 pyrolyzates obtained at 275 C……………………….….131

6.16. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 259 in PEG series C of the APN#1 pyrolyzates obtained at 300 C …………………...……..132

6.17. ESI-QIT CAD spectrum of the sodiated PDMS oligomer at m/z 1133.7 in the pyrolyzates from APN#1 at 275 C …………………………………….....135

6.18. MALDI-ToF mass spectra of V-PDMS-V after pyrolysis at 150, 200, and 300 C………………………………………………………………………..138

6.19. MALDI-ToF mass spectra of the products from St-PEG-St pyrolysis at 75, 200, and 250 C …………………………………………………...... ……140

6.20. MALDI-ToF mass spectra of the APN#1 pyrolyzates obtained at 200 and 250 C …………………………………………………………..…….142

6.21. MALDI-ToF mass spectra of the APN#1 pyrolyzates obtained at 300 and 350 C …………………………………………………………....…...143

6.22. MALDI-ToF mass spectra of the 350 C-pyrolyzates from amphiphilic networks #1 and #2 ……………………………………….………...145

6.23. Partial MALDI-ToF mass spectra of the 350 C-pyrolyzates from amphiphilic networks #1 and #2 …………………………………………….…...146

A.1. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 493 in PEG series A of the prepolymer St-PEG-St pyrolyzates obtained at 275 C …….161

xv A.2. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 333 in PEG series B of the prepolymer St-PEG-St pyrolyzates obtained at 275 C .……162

A.3. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 347 in PEG series C of the prepolymer St-PEG-St pyrolyzates obtained at 275 C ...... 163

B.1. ESI-QIT CAD mass spectrum of the Li+ adduct of D-[PEG]-OD at m/z 423, generated from a PEG 600 standard after H/D exchange with CH3OD …………………………………………………………………….…165

B.2. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 421 in PEG series B of the APN#1 pyrolyzates obtained at 275 C. Compare to Fig. 6.15., which includes the B oligomer with one more repeat unit …………………..…...166

B.3. ESI-QIT CAD mass spectrum of the same oligomer as in Fig. B.2., but using the Li+ instead of the Na+ adduct ………………………………..……..167

B.4. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 303 in PEG series C of the APN#1 pyrolyzates obtained at 300 C. Compared to Fig. 6.16., which includes the C oligomer with one less repeat unit …………………………168

B.5. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 243 in PEG series D of the APN#1 pyrolyzates obtained at 300 C ……………………...…...169

B.6. ESI-QIT CAD mass spectrum of the sodiated PDMS oligomer at m/z 541.3 in the pyrolyzates from APN#1 at 275 C ……………………………………..…170

C.1. MALDI-ToF mass spectrum of the pyrolyzates from a PD5 network that was heated at 300 C for 60 minutes …………………………………….…..174

C.2. Two regions of spectrum showm in Fig. C.1. ………………………………….....175

xvi LIST OF SCHEMES

Scheme Page

4.1. Thermal degradation pathways of V-PDMS-V at low temperatures …………….…56

5.1. St-PEG-St thermal degradation mechanism ………………………………………..75

6.1. Charge-induced fragmentation pathway of [H-(PEG)-OH + Li]+ to truncated linear fragments with HO/H and HO/vinyl end groups …………….………….….106

6.2. Charge-induced fragmentation pathway of [H-(PEG)-OEt + Li]+ to truncated linear fragments with HO/Et, HO/H, and vinyl-O/Et end groups …….……...... …112

6.3. Charge-induced fragmentation pathway of [Me-(PEG)-OMe + Li]+ to truncated linear fragments with HO/Me and MeO/ vinyl end groups ……………………….114

6.4. Charge-induced fragmentation pathway of [St-(PEG)-OSt + Na]+ to truncated linear fragments with St/OH and vinyl/St end groups ………………………….....119

+ 6.5. Charge-induced fragmentation pathway of [H-(PEG)-OCH2CH=O + Na] to truncated linear fragments with HO/vinyl and HO/aldehyde end groups ……...127

6.6. Charge-induced hydride transfer and consecutive charge-remote pericycle rearrangements ………………………………………………………….129

6.7. Charge-remote fragmentation of cyclic PDMS oligomers leading to smaller sodiated PDMS macrocycles by the expulsion of cyclic PDMS units via a four-membered ring transition state .………………………………..……….136

+ B.1. Charge-induced fragmentation pathway of [Et-(PEG)-O-CH2CH=O + Na] ….…171

+ B.2 Charge-induced fragmentation pathway of [H-(PEG)-O-CH=CH2 + Na] ……..…172

xvii CHAPTER I

INTRODUCTION

Complex polymer systems, such as hyperbranched polymers, star polymers, polymer brushes, block copolymers, and amphiphilic networks, are increasingly designed to create novel materials with improved physical, biological, and chemical properties.1-5

These systems combine the different properties of their constituent polymers. They range in size from nanometers to micrometers, depending on the selection of monomer and polymer components and polymerization conditions. Such multicomponent copolymers and networks can be produced with a variety of physicochemical properties for novel high technology and biomedical applications. Amphiphilic networks are two-component networks of covalently interconnected hydrophilic/hydrophobic phases of cocontinuous morphology; as such they swell both in and .2,4 An amphiphilic membrane recently synthesized by Joseph P. Kennedy and coworkers from glycol, polydimethylsiloxane, and a crosslinking agent6-9, has been studied in detail in this dissertation.

1 Polymer structures have been probed by mass spectrometry (MS) for years, as MS provides a beneficial way to analyze their chemical compositions and structures.10-12

Mass spectrometry requires that the sample be converted to gas ions. The advent of matrix-assisted laser desorption ionization (MALDI) has made this possible for many polymeric materials. Presently, the MALDI ionization technique is used regularly for polymer mass spectrometry analyses.13-15 Typically, MALDI is interfaced with a time-of-flight (ToF) mass analyzer, which affords the capability to determine both major and minor products in polymer compounds. The resolution of a MALDI-ToF mass spectrometer can be enhanced by the use of delayed extraction and a reflectron analyzer.

Delayed extraction (also called time-lag focusing) is accomplished by extraction pulse modification and narrows the spatial and kinetic energy distributions of ions with the same mass, thereby improving the resolution.16 Electrospray ionization is another increasingly used method for polymer characterization, especially for end group analysis of small oligomers via tandem mass spectrometry experiments (MS/MS and MSn).17

This dissertation concerns the characterization of the pyrolysis products (pyrolyzates) from the functionalized and polydimethylsiloxane starting materials as well as the amphiphilic network prepared from these prepolymers with the crosslinking agent. MALDI-ToF MS, which is the major technique used in this research, reveals chemical composition, repeat unit, and end group information; it has been used to characterize the types of polymers formed upon mild pyrolysis.11 Because this method is very sensitive and has a low detection limit, it is effective for the detection of minor products.18-20 Furthermore, additional structural information has been obtained by

2 MS/MS experiments employing electrospray ionization quadrupole-ion trap (ESI-QIT) instrumentation.17

Polyethylene glycol (PEG) is incorporated in a variety of industrial products because of its properties. Its flexible chains make it a common raw material for elastomer, fiber, and rubber production.21-24 Polydimethylsiloxane (PDMS), on the other hand, is viscoelastic and, due to this property, present in a large number of industrial products, for instance, adhesives, lubricants, cosmetics, and others.23-24

These two polymers have been used in J. P. Kennedy’s group to synthesize novel amphiphilic networks. An advantage of using PDMS in copolymer synthesis is that it can be cross-linked to form networks. Those investigated in this dissertation were synthesized by Kennedy and coworkers, by cocrosslinking PEG and PDMS prepolymers with

6 pentamethylpentacyclosiloxane (D5H) crosslinker.

Large and complex are the focus of many research projects, but they are difficult to study directly by spectroscopic techniques.25-28 Since they are extensively crosslinked during synthesis, they can rarely be dissolved and, hence, cannot be analyzed by methods requiring formation of solutions, such as MS. Biological macromolecules, for example proteins and carbohydrates, are made analyzable by digestion, which is a degradation method. This dissertation examines a similar approach for synthetic macromolecules, involving mild thermal degradation. Mild pyrolysis and MALDI-ToF,

ESI-QIT MS/MS, and FT-ICR MS methods are combined to characterize the compositions and structures of the starting materials, their pyrolyzates, and the pyrolysis products of the polymer networks. The prepolymers, distyryl-poly(ethylene glycol)

3 (St-PEG-St) and divinyl-polydimethylsiloxane (V-PDMS-V), could be investigated before and after performing pyrolysis. In the MALDI-ToF mass spectra of the original materials (no pyrolysis), peaks corresponding to the intact St-PEG-St and V-PDMS-V prepolymers could be observed with significant abundances. No mass spectra could be obtained for the amphiphilic networks without prior pyrolysis. After pyrolysis, the

MALDI-ToF mass spectra of the prepolymers contain additional products arising from thermal degradation. More importantly, the pyrolysis mass spectra of the networks display PEG and PDMS distributions confirming that the amphiphilic networks contained

PEG and PDMS chains. ESI-QIT and FT-ICR MS data were used for additional structural verification of the pyrolysis products. The concept presented in this dissertation could be expanded to include other degradation methods, for example selective or selective oxidative cleavages. Future research in the Akron mass spectrometry group will investigate this issue.

4 CHAPTER II

SCIENTIFIC BACKGROUND

2.1. Complex polymer assemblies

Complex polymer assemblies have experienced increased biomedical use because of their desirable chemical, physical, and biological properties.1-5 Examples of such assemblies include hyperbranched polymers, star-branched polymers, polymer brushes, and amphiphilic networks. Since complex polymer systems can be prepared using a variety of materials, they have a wide range of usages. Polymer networks may consist of flexible polymer chains and strong orientational linear polymer chains that improve the dynamic properties of the network.1-2

An amphiphilic network, which is the complex polymer analyzed in the research presented in this dissertation, is a random cocontinuous polymer assembly, composed of hydrophilic and hydrophobic sequences that can be swollen in both water and solvents.6-9 Because of this, the morphology or conformation of the amphiphilic networks can be altered based upon the in which they are immersed.

The selection of component and polymerization conditions can control the structure and dimension of a polymer network. The goal of amphiphilic network synthesis is primarily to produce new expertise in polymer materials and improve materials properties, so that they can have useful new applications, for example,

5 as macroimmunoisolatory membranes for diabetes treatment. Significant efforts have been devoted to create such membranes with properly sized pores, allowing the hydrophilic particles to move and possessing enhanced permeability and improved mechanical properties.9 Novel applications of such multicomponent copolymers and networks are likely to result from their combination of different properties of incompatible polymers. These macromolecules bring together otherwise incompatible functionalities and, hence, properties for innovative high technology applications.

Polyethylene glycol (PEG) is non toxic and flexible. Because of these characteristics PEG appears in a variety of products and is also used as a substrate to produce other polymers. In household products, PEG is used, for instance, in skin creams, lubricants, toothpaste, and softeners. PEG can be connected to proteins for a longer acting medicine effect, increased healing, or decreased toxicity in medical goods. Additionally,

PEG is also used in liquid diabetes-related body armor. Moreover, the blend of PEG with other polymers grants high-quality rubberlike properties which are needed in elastomers, fibers, rubbers, etc.21-24

Polydimethylsiloxane (PDMS) has a number of extraordinary features: it is transparent, non toxic, non flammable (inert), and possesses an especially unique rheological property, called viscoelastic (viscous liquid-like) property. PDMS can be used to enhance surface defects or coat a surface in order to create a rubberlike phase.

Since it contains viscoelasticity and is rubbery, PDMS has widespread use in toys, adhesives, lubricants, aquarium sealants, cosmetics, hair conditioners, and other applications.23-24

6 Another well-known usage of PDMS is in breast implants. Furthermore, PDMS can be cross-linked to form networks and is a commonly used system for studying the elasticity of polymer networks. For the reason that both PEG and PDMS have unique and useful properties, they are suitable for the production of copolymers for tailored applications.

This makes it necessary to design proper methods for the structural analysis of such products, which is the goal of this dissertation.

The compositions and structures of such large and complex molecules cannot be probed by routine spectroscopic techniques directly, but structural information about such macromolecules could be obtained by combining mild pyrolysis with mass and tandem mass spectrometry analyses. The research presented in this dissertation concerns the characterization of novel tricomponent amphiphilic membranes containing well-defined hydrophilic PEG domains for water transport, pentamethylpentacyclosiloxane (PD5) domains for enhanced O2 diffusion, and hydrophobic silica-reinforced PDMS domains for increased O2 transmission and strength. The membranes were synthesized by

Kennedy and coworkers, by cocrosslinking hydrophilic distyryl-polyethylene glycol

(St-PEG-St) and hydrophobic divinyl-polydimethylsiloxane (V-PDMS-V) domains with

6,9 pentamethylpentacyclosiloxane (D5H) crosslinker. The ultimate objective of this research is to determine whether the components of the amphiphilic network mentioned above can be characterized by combining mild thermal degradation with mass and tandem mass spectrometry. Validating the presence of the desired PDMS, PEG, and PD5 domains in the network would constitute the first spectroscopic characterization of a complete, complex polymer assembly.

7 2.2. Pyrolysis mass spectrometry of polymers

Pyrolysis is the chemical decomposition of organic compounds by heat in the absence of oxygen. Normally, pyrolysis is operated at temperatures above 430°C.29-31

A common pyrolysis technique, called „flash pyrolysis‟, utilizes a high heating rate in the temperature range 350-500 °C and a short process time. „Thermal depolymerization‟ (TDP) is another pyrolysis method; it decreases the length of polymer chains under pressure and heat.32-42

Pyrolysis mass spectrometry has been applied to macromolecules for over fifty years. The very first study was reported by Madorsky and Straus and concerned the thermal degradation of poly(ethylene glycol) (PEG) and polypropylene oxide (PPO).43

They investigated the pyrolysis behavior of a number of such polymers and obtained the same results: C-O rather than C-C bond cleavages took place. Based on this observation, they concluded that regular hydrocarbon polymers, which only contain C-C bonds, have higher thermal stability than poly(ethylene glycol) (PEG), which possesses both C-C and

C-O bonds. They proposed two major degradation mechanisms from the pyrolysis products of PEG. These paths involved homolytic C-O bond cleavage or intramolecular transfer of a atom in order to yield radical molecules and molecules with saturated and unsaturated end groups, respectively. Eighteen years later, another mechanism was proposed by Bortel and Lamot to explain the products arising from C-O bond cleavage.44 They presented two pathways; H abstraction and - scission, leading to radical species and molecules with aldehyde or ethyl ends, respectively.

Seven years later, Grassie and Mendoza used thermal volatilization analysis to decompose polyether-urethanes and PEG polymers and IR and GC-MS to analyze the

8 pyrolyzates.45 Polymer pyrolysis was a subject of immense interest at that time, especially for PEGs. Within the next ten years, Fares and coworkers studied the degradation of high molecular weight PEG by employing direct pyrolysis mass spectrometry (Py-MS) and utilizing electron impact mass spectrometry to observe the resulting high mass oligomers.46 They still rationalized all products from the pyrolysis process with the previously proposed C-O and C-C homolysis mechanisms. At the same time, Voorhees and coworkers also investigated the thermal degradation of PEG by performing pyrolyses under gas at temperatures as high as 450 and 550C.47

Pyrolyzates could be identified by the combination of GC-MS and GC-FTIR. Three years later, there was another PEG pyrolysis study by Arisawa and Brill which used a different method to pyrolyze the polymer at 370–550°C.48-49 Their particular technique, called

T-jump-FTIR , used a resistant heat filament to pyrolyze the sample and

FTIR to identify the pyrolyzates, respectively.49 They concluded that homolytic C-O and

C-C bond cleavages were taking place evenly under the used pyrolysis conditions. After a year, Lattimer and coworkers investigated the pyrolysis of PEG and a

PEG-containing /.50 They used Matrix-Assisted Laser Desorption

Ionization for mass spectrometry (MALDI-MS) to characterize the pyrolyzates and discovered that this technique was superior compared to the several previous ones.

It allowed for better characterization of larger pyrolyzate components.50-56,61,70

The research presented in this dissertation used a similar pyrolysis procedure, which will be described in chapter 3.

9 The thermal degradation of polydimethylsiloxane (PDMS) was studied nearly sixty years ago by Lewis.71 Thirty years later, Lewis, Grassie, and Macfarlane proposed thermal depolymerization mechanisms of PDMS that gave rise to trimethyl-siloxy

72-73 [(CH3)Si-] chain ends. About the same time, Lewis, Thomas, and Kendrick reported the formation of PDMS cyclic oligomers upon the degradation of PDMS under inert circumstances that initiated depolymerization, and their degradation mechanism could still be used about ten years later to account for PDMS decomposition products.74-76

Nielsen and Zeldin looked into the PDMS products resulting upon heating up glassware.77-78 Clarson and Semlyen pyrolyzed seven years later high molecular weight

PDMS at 420 C. They proposed that depolymerization led to cyclic oligomers.79-81

Camino and coworkers studied kinetic aspects and degradation mechanisms at the thermal degradation of PDMS a few years ago.82-83 They employed a flash pyrolysis method that combined programmed heating control. Their results verified that cyclic species were created after thermal energy was applied. Furthermore, they reported that the transition state to the cyclics was the rate determination step.

10 2.3. Mass spectrometry instrumentation

Mass spectrometry, one of the most widely-used analytical tools, separates substances according to atomic and molecular mass and is used to determine the structure of chemical compounds and identify the components of mixtures.10,12 The mass spectrometer is an instrument which was invented to separate gas phase ions based on their mass to charge ratio (m/z). In the first mass spectrometers, a compound was bombarded with an electron beam that was strong enough to fragment the molecules.10,12,19,91 For the purpose of determining the structural characteristics of molecules, a mass spectrometer transfers them to ions so that they can be moved and manipulated by external electric and/or magnetic fields. Mass spectrometry has applications in an enormous range of fields in basic research and industry. Mass spectrometers are used in environmental chemistry, to study organic and inorganic compounds, in material science, in biomedical applications, polymer analysis, chemistry and biochemistry. All mass spectrometers are composed of four major components, which are the sample introduction system, ionization source, mass analyzer, and detector.12 Figure. 2.1. shows the main sections in every mass spectrometer. Because ions are very reactive and short-lived, the mass spectrometer must operate under high vacuum, so that the ions can travel inside the instrument undestructed. Firstly, the prepared analyte is inserted into the mass spectrometer via the sample inlet. Secondly, the analyte is ionized in the ion source using a suitable ionization method. Subsequently, the ions move to the mass analyzer, which is viewed as a "heart" of the mass spectrometer; its function is to sort the gas phase ions based on their mass and such a separation is achieved by

11

in every mass spectrometer. every mass in

components components

2.1. The basic The 2.1.

ure Fig

12 accelerating and focusing the ions in a beam, whose components are dispersed according to their mass and charge.The analyzer utilizes electrical and/or magnetic fields to transfer the ions from the ion source, where they are created, to the detector, where the ions are then detected. The detector observes the separated ions and provides the result, which is accumulated and analyzed in a computer data system and finally displayed as a mass spectrum, which is plotted as mass-to-charge ratio (m/z) versus ion abundances.

The following sections explain the fundamental principles and concepts of operation of the ionization methods and mass analyzers used in the research presented in this dissertation.

2.4. Ionization methods

The ionization process generates the analyte ions in the gas phase. This is considered as a very significant function in mass spectrometry and is achieved by adding or removing electrons or ions from the analyte. Selection of the appropriate ionization technique is essential for the proper characterization of the analyte. Ionization techniques can be classified as hard and soft.10,19,91 The difference between those two types is that hard ionization can break chemical bonds and generate fragment ions, whereas soft ionization keeps the of interest fully intact, creating intense molecular ions and causing the least fragmentation. An example of hard ionization is electron impact (EI), whereas chemical ionization (CI), matrix-assisted laser desorption ionization (MALDI), and electrospray (ESI) are considered as soft ionization methods. MALDI and ESI were used for the research in this dissertation and, therefore, are described in more detail in the following subsections.

13 2.4.1. Matrix-Assisted Laser Desorption Ionization (MALDI)

MALDI is a soft desorption ionization technique which applies a short pulsed laser beam to ionize an analyte.10-12,91 MALDI provides a fast, reliable, and easy way of mass analysis using a minimal amount of sample. This method needs a medium, called

“matrix”, to dilute and suspend the analyte molecules; this suspension enhances the probability of ionization without fragmentation. The matrix adsorbs the laser irradiation and enhances analyte desorption. The matrix contains a chromophore for absorbing the laser light. The matrix is usually a small organic molecule, such as -cyano-4- hydroxycinnamic (CHCA), sinapinic acid (SA), 2,5-dihydroxybenzoic acid (DHB), and 1,8,9-trihydroxyanthracene (dithranol). The sample is dispersed in a large excess of matrix which is able to strongly absorb the incident laser light, as well as to prevent interactions between analyte molecules.14,16 Instead, this excess promotes interactions between the analyte and matrix molecules. Schemes of the MALDI source and MALDI process are shown in Figures 2.2. and 2.3., respectively.

Solutions of the analyte, matrix, and, if needed, a cationizing agent are combined, and a small volume (L) of the mixture is spotted onto the sample holder, allowed to dry in air, and inserted into the ionization source of the mass spectrometer via the sample introduction inlet. Under vacuum, the solvent is lastly totally removed, leaving crystallized analyte molecules (and cationizing agent) homogeneously dispersed within the matrix molecules.

14

Sample target Laser

h

AH+

Variable Ground Grid Grid +20 kV

Figure 2.2. The matrix assisted laser desorption/ionization (MALDI) source.92

15 Figure 2.3. The MALDI process.11

16 Subsequently, a short laser pulse is focused onto the surface of the analyte mixture on the sample target and volatilizes the analyte and matrix. The excess energy from the high intensity laser pulse of short duration is absorbed by the matrix and excites the matrix molecules; the absorbed energy initiates desorption and ionization of the matrix molecules which, as they expand into the gas phase, also carry intact analyte molecules into the gaseous state, where the analyte is charged by ion/molecule reactions with the matrix ions. The generated ions are then directed to the mass analyzer. The more laser shots are collected, the larger the signal-to-noise ratio, and the better the peak shapes, which eventually increases the accuracy of the determination. MALDI mass spectrometry commonly produces singly charged ions and occasionally causes slight fragmentation.

2.4.2. Electrospray Ionization (ESI)

ESI is a soft ionization technique and also one of the most recently developed ionization methods. ESI is a very sensitive and versatile technique, which is appropriate for the analysis of small amounts of large and labile molecules such as peptides, proteins, organometallics, and polymers.10-12,17,19

This technique involves spraying a solution of the sample, which is the analyte dissolved in a large amount of a volatile solvent, through a needle which is kept at a potential. As the solution is passed through the very fine needle and sprayed into the spray chamber, charged droplets of solution are generated. A nebulizing gas, usually N2, is added inside the ionization chamber (coaxially to the spraying needle) in order to obtain a good spray and steady ion current. Furthermore, the spray condition can further

17

Figure 2.4. Detail of skimmers and ion guide sampling in an ESI source.17

18 be optimized by controlling the pressure of the nebulizing gas. The droplets are desolvated by the drying gas, usually N2, which is heated and flows around the glass capillary that transports the droplets to the mass snalyzer. While the drying gas helps to desolve the droplets, it theoretically does not thermally decompose the analyte molecules.

Since the droplets are formed at a high-voltage gradient, they are charged and are forced toward the glass capillary by an electrostatic gradient. A further factor that induces charged species to enter the capillary is the pressure difference between the spray chamber and the ion focusing and transport region. As the solvent on droplets evaporates, ions can desorb form their surface. The ions travel through the glass capillary into octopole ion guides which focus the ions as they are transported from high pressure through the skimmers to exit lenses. The skimmers and ion guide sampling layout are shown as Figure. 2.4. The desolvated ions form a beam that is centered and carried by the skimmers, octopoles, and exit lenses, as the nebulizing and drying gases are pumped away. Ultimately, the ions are transferred into the following component, which is the mass analyzer of the mass spectrometer. Schemes of ESI source and ESI process are shown in Figures 2.5. and 2.6., respectively.

In ESI, the ions observed are generally quasimolecular ions formed by the addition of a proton, the addition of another cation such as sodium ion, or the removal of a proton. Additionally, multiply charged ions are frequently observed, especially from such large molecules as polymers. ESI is very useful for the detection of relatively large molecules, which are observed as multiply charged ions in lower mass-to-charge ratio regions, which are easily scanned with common commercial mass spectrometers. ESI regularly produces little, if any, fragmentation.10-12

19

Figure 2.5. The electrospray ionization (ESI) source.17

Figure 2.6. The ESI process.17

20 2.5. Mass analyzers

It has been stated at the beginning of section 2.3. that the vital part of every mass spectrometer is the mass analyzer, where sorting and separating of ions take place according to their masses and charges. Extensively used analyzers are time of flight

(ToF), quadrupole, quadrupole ion trap (QIT), quadrupole time of flight (Q-ToF), magnetic/electric sector, and ion cyclotron resonance (ICR) mass analyzers; only the first three types, which were utilized for the research presented in this dissertation, are discussed in this section. The ions created in the ion source are pushed toward the mass analyzer in order to separate them and measure their masses. It is increasingly widespread to combine a number of mass analyzers in a spatial or time sequence, with the first one used to collect a mass spectrum of the generated ions and the other one(s) to observe the tandem mass spectra of a selected ion. Each type of mass analyzer has its own properties; all contain three main features, which are the measurable upper mass limit, which is the highest m/z value that can be detected, the transmission, which is the ratio between the number of ions reaching the detector and the ions generated in the source, and the resolution, which is the ability to resolve two peaks having a small mass difference.

21 2.5.1. Time-of-Flight (ToF) mass analyzer

There is no doubt that the ToF mass analyzer is the simplest device for mass measurement, even if, there are complexities with higher resolution applications. The

ToF mass analyzer has many favorable characteristics, which are exceptionally high sensitivity, with all ions being actually detected, theoretically unlimited mass range, complete mass spectrum measurement for each ionization event, high transmission, minimal sample amount requirement, reasonably low cost, and rapid analysis; these all make ToF mass spectrometry one of the most desirable methods of mass analysis.10-12,15,19,21,92 Principally, the produced ions are separated by their mass to charge ratio (m/z). A ToF analyzer measures the times it takes for the ions created within the ion source to pass through the field-free drifting flight tube and reach the detector; ions are separated by their flight times which are related with their mass to charge ratios.10 Different m/z travel at different speeds.16 There are linear and reflectron modes for ToF analyzers, as illustrated in Figures 2.7. and 2.8., respectively. Ions are accelerated to a given kinetic energy upon exiting the source that is determined by the applied acceleration potential. Because all ions are applied the same, very strong electric field, they can be presumed as beginning at the same position. There are correlations between kinetic energy and other variables as shown in the following equation, where each parameter is defined as follows: kinetic energy (KE), mass (m), velocity (v), charge (ze), acceleration potential (V).10

KE = mv2/2 = zeV

22

Figure 2.7. The linear time of flight (ToF) mass analyzer.16

23 The relationship between the required time for ion travel (t) along the field-free drift region (distance d) is given by

t = d/v

If v is replaced by the variables correlated in the earlier equation, the following equation is obtained, which relates the mass to charge ratio (m/z) of an ion to its travel time (t)

t2 = (m/z)(d2/2Ve)

Ions with lower mass move faster and arrive at the detector ahead of ions of higher mass.

The mass spectrum, which yields structural and chemical details, is plotted as m/z, x axis, versus abundance, y axis. ToF mass spectra suffer from weak resolution, because the ions may be formed at slightly different locations in the source. This broadens the kinetic energies of ions with the same m/z, which causes signal broadening. A reflectron ToF mass spectrometer can be utilized to compensate this problem and enhance the resolution; the layout of such an instrument is shown in Figure 2.8. In the reflectron mode, the generated ions that exit the source have the same kinetic energy as in the linear mode and move in the same direction. These ions are not allowed to reach the linear detector, which is located at the very end of the flight tube, but are reflected inside the reflector and reach the reflectron detector instead. The reflector is composed of a series of lenses, which are floated at voltages which can be either equal or slightly higher than the voltages applied to the ions when they exit the ion source.

24

Figure 2.8. The reflectron time of flight (ToF) mass analyzer. A detector is usually located at the end of the ion mirror (linear detector).16

25 Ions of the same mass (m/z) may have different kinetic energies, as explained above. Ions having smaller energy, slower ions, spend less time in the reflector and depart quicker than the faster ions, having larger energy and thus penetrating deeper inside the reflector; as a result, fast and slow ions arrive at the detector at the same time. The reflector improves dramatically the resolution. Furthermore, increasing the length of the flight distance traversed by the ions minimizes the effect of laser pulse widths, detector response, and digitizing rate; these improvements enhance additionally the resolution. In principle, the ToF mass analyzer provides unlimited mass range and very high transmission, which make it ideally suitable for biopolymer and synthetic polymer analysis.

2.5.2. Quadrupole mass analyzer

The quadrupole mass analyzer, shown in Figure 2.9, is a very fast and efficient mass separating device. The principle of mass separation for this analyzer is based on how the stability of ion trajectories depends on m/z ratios.10-12,17,19,91,94 The quadrupole mass analyzer consists of four, usually cylindrical, rods which are applied electric fields with alternating (RF) and direct (DC) components that control the ions‟ movement.

Opposite rods are connected. Ions begin to fluctuate as soon as they enter the quadrupole in both X and Y planes. Ions whose trajectories are unstable are quickly discarded and do not reach the detector. Ions move along the z axis. Positive ions are repelled by the positive and attracted by the negative rods and vice versa.

26

Figure 2.9. The quadrupole mass analyzer.96

27 As the rod polarities change, the ions oscillate while moving in the z direction. By proper adjustment of RF and DC potentials only ions of a given m/z value pass the quadrupole and reach the detector.10,12,94 All other ions are discharged on the rods or pumped out.

Usually, the DC potential and the RF amplitude are kept at a constant ratio and their amounts are scanned to successively allow ions of increasing (or decreasing) m/z to pass the quadrupole. Even if the analyzer itself includes focusing lenses for keeping ions in the center of all poles and directing them toward the detector as smoothly as possible, there still are some ions that cannot be focused because of their momentum. Low mass ions reach the detector more easily than high mass ions because they are affected less by the changing AC field. On the contrary, high mass ions happen to contain more momentum that can force them out of the middle path. The quadrupole is a real mass to charge analyzer which does not rely on how much energy the ions contain when they leave the source. Moreover, the combination of a quadrupole mass analyzer and other analyzers, or even with themselves, allows one to perform tandem mass spectrometry analysis.10 The quadrupole has high transmission, an upper mass limit of approximately 4000 Da, and ~

3000 resolution.19,91 Because of its relatively low upper mass limit, the quadrupole has limited effectiveness for large molecules such as synthetic polymers.

.

28 2.5.3. Quadrupole-Time-of-Flight (Q-ToF) tandem mass analyzer

The combination of quadrupole and time of flight mass analyzers make a hybrid mass spectrometer with tandem in-space mass spectrometry capability.10,97 The quadrupole functions as an ion conductor in regular mass spectrometry mode and as mass selection tool in tandem mass spectrometry mode.12 The ToF analyzer is connected to the quadrupole and performs as a mass resolving device in both mass and tandem mass spectrometry modes. A collision cell, which is placed between the two analyzers, is used to stimulate fragmentation in tandem mass spectrometry experiments. Ions created in the source are forced to move into the quadrupole, which selects only a specific m/z; these ions are focused into a collision cell where product ions are generated, which are continuously moved to the ToF analyzer where the tandem mass spectrum is acquired.

2.5.4. Quadrupole Ion Trap (QIT) mass analyzer

The quadrupole ion trap mass analyzer is a very sensitive and versatile analytical apparatus for the characterization of large and small molecules.94-95 A schematic of a quadrupole ion trap (QIT) mass analyzer is shown in Figure 2.10. The basic advantages of the QIT mass analyzer are simple, low cost instrumentation, that can be applied to a wide range of analytical problems. One outstanding feature of the QIT mass analyzer is that it is capable of multiple stage tandem mass spectrometry experiments which are very useful in investigations of the physical properties of isolated ions.10,12

29

Figure 2.10. Quadrupole ion trap mass analyzer; (top) ring electrode (1) and end cap electrodes (2 incoming, 3 outcoming) (bottom) quadrupole ion trap analyzer.17

30 The QIT mass analyzer contains three hyperbolic electrodes which consist of a ring and two end caps; there are holes in the two end-cap electrodes so that ions can enter and exit the ion trap mass analyzer. Ion storage and passage are controlled by a three-dimensional quadrupole field generated by the voltages applied to three electrodes, as illustrated in

Figure 2.10., bottom; in addition, helium buffer gas is used to collisionally cool the ions and keep them in the middle of the trap until the detection event. The ring electrode of the quadrupole ion trap is supplied with the primary radio frequency (RF) field, one end cap is grounded, and the other end cap is provided a direct current (DC) and/or alternating current (AC) voltages to allow for stable trajectories of the desired m/z values in the ion trap and to control the motion of the ions. A three dimensional quadrupolar field is established by the overlapping potentials of ring and end cap electrodes which form a cavity that traps ions of a desired mass range on a three dimensional 8-shaped trajectory, while ions outside this mass range have unstable trajectories and are driven out of the trap. The ions generated in the ion source enter the trap through the incoming end cap and then are trapped by the electrodes, until they are ejected through the outcoming end cap.

The ions are detected by proper scanning of the electrode potentials, which allows ions of increasing m/z to exit the outcoming end cap and reach the detector for the acquisition of a mass spectrum. Ion motion inside the trap depends on the trap size (r is the half distance between the two end caps) and is described by the following equation10,12

2 2 qz= 4eV/mr ω

31 where qz is a dimensionless parameter, V is the amplitude of the voltage on the ring electrode, ω is the oscillating RF frequency, e is the charge of the ion, and m is the mass of the ion. After entering the trap, some ions are moving toward the middle of the trap, while some other ions move outward; as the AC field changes polarity, these motions reverse and the ions stay trapped. Generally, small ions tend to oscillate near the center of the trap, while large ions move to more outward positions. The upper mass limit is about

6000.

2.5.5. Quadrupole Ion Trap tandem mass analyzer

The QIT mass analyzer is ideally suitable for in-time tandem mass spectrometry experiments.10-12,17,94-95 Ions of a particular m/z are isolated in the trap, while all other ions are driven out. Afterward, the isolated ions are fragmented by collisionally activated dissociation (CAD) experiments. Tandem mass spectrometry trials, i.e. isolation and fragmentation, can be repeated a number of times which is controlled by the trap efficiency. The simplest tandem mass spectrometry process, MS2, begins with the isolation step, in which only ions of a given m/z are allowed to stay trapped, while ions below or above the selected m/z window are ejected from the trap by raising the RF amplitude on the ring electrode and by rapidly sweeping the frequency of the AC current on the end caps, respectively. Very narrow mass to charge ratio ranges can be chosen effectively by appropriate choice of these potentials. In the next step, known as collisionally activated dissociation, the isolated ions are excited by raising their kinetic energy, so that they undergo energetic collisions with the buffer collision helium gas. The energy deposited this way causes fragmentation. Subsequently, ion detection is attained

32 by ramping the RF frequency on the ring electrode and axial modulation. Alternatively, a select fragment ion can be isolated once more, excited, and fragmented for an MS3 experiment. All those three processes can be repeated over again for multistage mass spectrometry analysis, which provides additional structural information for compound identification, fragmentation mechanism elucidation, and substance sequencing studies.

33 CHAPTER III

MATERIALS, METHODS, AND INSTRUMENTATION

3.1. Complex polymer assemblies

The complex polymer analyte studied in this dissertation is an amphiphilic membrane whose putative composition was discussed in chapter II and whose characterization is discussed in detail later. The synthetic pathway will be described briefly in this section. All the reagents and solvents used for the synthesis were of high purity. Reagents: distyryl telechelic poly(ethylene glycol) (St-PEG-St), Mn = 4800, pentamethylcyclopentasiloxane (D5H), divinyl telechelic polydimethylsiloxane reinforced with 18–20% fumed silica (V-PDMS-V), Mn = 28000, and Karstedt’s catalyst (Pt with

1,3-divinyl tetramethyldisiloxane in xylene solution) were from Gelest Chemicals and used as received. Solvents: hexane, , and toluene, were purchased from Fischer

Scientific.

The synthetic pathway6-9 to the amphiphilic network started with dissolving the proper amounts of V-PDMS-V and St-PEG-St in toluene using a round-bottom flask that held a magnetic stirring bar. After a period of time with steady stirring rate, the appropriate amounts of D5H and Karstedt’s catalyst were added and the reaction was

34 allowed to proceed for 36 hours at 100 °C. The reaction progress was monitored by the rate of bubbling (H2 evolution). After H2 evolution diminished, a small amount of water was added to the charge to stop network growth, and stirring was continued for 12 hours.

A vastly viscous fluid was formed, which was poured into open 15x8.5 cm Teflon or glass casts. After toluene evaporation overnight at room temperature, the polymer settled.

The crosslinking was completed during that time. Later, the polymer was removed from the mold and dried in a vacuum oven until weight consistency was achieved in approximately 2 days. Multiple sequential extractions with hexane and methanol were consecutively performed to eliminate unreacted materials.

3.2. Materials

Amphiphilic copolymer networks were synthesized in the research group of

Professor Joseph P. Kennedy5-9, The University of Akron, from distyryl-poly(ethylene glycol) (St-PEG-St) (Mn = 4800), divinyl-polydimethylsiloxane (V-PDMS-V)

5 (Mn = 28000), pentamethylcyclopentasiloxane (D5H) , and Platinum based catalyst

(Karstedt)98-100. The three significant membrane components distyryl poly(ethylene glycol), divinyl polydimethylsiloxane, and poly(pentamethylcyclopentasiloxane) (PD5) were also investigated separately. They were probed by mass spectrometry both directly as well as after mild pyrolysis. The mass spectrometry (MS) experiments employed solvents (tetrahydrofuran, chloroform, toluene; HPLC grade), sodium trifluoroacetate

(NaTFA), (NaI), and/or lithium trifluoroacetate (LiTFA), all of which were purchased from Sigma-Aldrich. Two matrices were mainly used in the MALDI experiments, which were 1,8,9-trihydroxyanthracene (dithranol), 97%, purchased from

35 Alfa Aesar; and trans-2-[3-(4-tert.-butylphenyl)-2-methyl-2-propenylidene]-malononitrile

(DCTB),  99%, HPLC grade, purchased from Fluka. Thin-walled Pyrex pyrolysis tubes

(5 mm O.D. x 25 mm length), were purchased from Wilmad. gas and propane gas fuel were purchased from ACE in a and tank, respectively.

3.3. Pyrolysis procedure

The mild pyrolysis50-56 was performed by weighing a 5-mg sample, placing it in a

Pyrex tube, flushing the tube with argon gas, sealing it at one end with a propane torch, placing it vertically in a metal holder inside a gas chromatography (GC) oven, which was used as the heating device, and heating it to a preset temperature. It took comparatively 2-

5 min to attain a preset temperature. The pyrolyses were carried out at the various temperatures listed in Tables 3.1. and 3.2.

36 Table 3.1. List of pyrolysis temperatures for starting materials and crosslinking agent.

Pyrolysis Starting materials Crosslinkage

Temp (C) St-PEG-St V-PDMS-V PD5 75    100   - 125   - 150    175   - 200   - 225   - 250    275   - 300    350 - - 

Table 3.2. List of pyrolysis temperatures for copolymer membranes.

Pyrolysis Copolymer membranes Temp (C) APN#1 APN#2 75  - 100  - 125 - - 150   175  - 200   225  - 250   275  - 300   350  

37 The pyrolyses progressed for either 15 (at 75, 100, and 125 C) or 30 min

(heat-up time was excluded). Afterwards, the samples were allowed to cool down. Two replicates were performed at each temperature. Subsequently, the pyrolyzates were analyzed by mass spectrometry, as described in the following sections.

3.4. Experimental preparation

The prepolymers distyryl-poly(ethylene glycol) (St-PEG-St) with number average molecular weight (Mn) of 4800 and divinyl-polydimethylsiloxane

(V-PDMS-V) with Mn = 28000, as well as a poly(pentamethylcyclopentasiloxane) (PD5) prepared from pentamethylcyclopentasiloxane (D5H) crosslinking agent were initially characterized without further alteration by matrix-assisted laser desorption/ionization time-of flight (MALDI-ToF) and matrix-assisted laser desorption/ionization quadrupole- time-of flight (MALDI-Q-ToF) mass spectrometry. Another crosslinking agent, (triethyl pentamethylcyclopentasiloxane disilanol), having a molecular weight of 374 was analyzed by electrospray ionization quadrupole ion trap mass spectrometry

(ESI-QIT-MS). Furthermore, some pyrolyzates were investigated by Bruker APEX III

47e Fourier Transform ion cyclotron resonance (FT-ICR) mass spectrometry which has exceptionally high resolution and mass accuracy.10-12,91,101

The two amphiphilic networks (APN) investigated contained 10% PD5 crosslinking agent. Table 3.3. summarizes the corresponding compositions.

38 Table 3.3. List of the weight % compositions of the copolymer samples analyzed.

Samples St-PEG-St (Mn 4800) V-PDMS-V (Mn 28K) PD5

APN#1 30% 60% 10%

APN#2 35% 55% 10%

3.4.1. Sample preparation for MALDI-ToF-MS, MALDI-Q-ToF-MS, and MALDI-Q-

ToF-MS/MS experiments

3.4.1.1. Before pyrolysis

Solutions of the matrices, dithranol or DCTB, and the cationizing agents, NaTFA or LiTFA, were prepared in tetrahydrofuran at 20 mg/ml and 10 mg/ml, respectively. The samples were dissolved at 10 mg/ml in a suitable solvent (tetrahydrofuran for prepolymers, mixed 7:3 tetrahydrofuran:chloroform for copolymer samples, and toluene for linkage substance). Subsequently, matrix, cationizing agent, and sample solutions were mixed in the ratio 10:1:2 or 10:1:4 to obtain the best spectra.

3.4.1.2. After pyrolysis

Solutions of the matrices, dithranol or DCTB, and the cationizing agents, NaTFA or LiTFA, were prepared in tetrahydrofuran at 2 mg/ml and 1 mg/ml, respectively.

Pyrolyzates and pyrolyzed compounds were dissolved at approximately 1 mg/ml in a suitable solvent, as referred to in the previous section. Matrix, cationizing agent, and sample were blended in the ratio 10:1:2, 10:1:4, 7:1:4, or 7:1:5 to acquire the best spectra.

39 3.4.2. Sample preparation for ESI-QIT-MS and ESI-QIT-MS/MS experiments

The cationizing agent, NaTFA or LiTFA, was dissolved at 1 mg/ml in tetrahydrofuran. Pyrolyzates were dissolved at approximately 1 mg/ml in an appropriate solvent as, again, mentioned previously. Cationizing agent and sample were then mixed in the ratio 1:1 (v/v) before acquiring the spectra.

3.4.3. Sample preparation for FT-ICR MS experiments

The cationizing agent, NaI, was dissolved at 1 mg/ml in tetrahydrofuran.

Pyrolyzates were dissolved at approximately 1 mg/ml in a proper solvent as stated before.

Afterwards, cationizing agent and sample were mixed in the ratio 1:1 (v/v) before performing the mass spectrometry analysis.

3.5. Experimental instrumentation

All experiments in this dissertation were conducted on the four mass spectrometers described in the following sections.

3.5.1. Bruker Daltonics Reflex III MALDI-ToF mass spectrometer

The Reflex III MALDI-ToF16, manufactured by Bruker Daltonics (Billerica, MA), is a mass spectrometer with a MALDI source and time-of-flight mass analyzer.10-12,15-16,19,91-93 The schematic arrangement of the components of this instrument is shown in Fig. 3.1. and the principle of how each part functions have been described in chapter II. Different mixtures of matrix, cationizing agent, and analyte were analyzed, as explained above, in order to obtain a spectrum with good signal/noise ratio.

40

Figure 3.1. Scheme of the Bruker Daltonics Reflex III MALDI-ToF-MS instrument.16

41 From each mixture, ~0.4 L were applied onto the polished stainless steel SCOUT-26 sample holder, the sample was allowed to dry in air, and introduced into the MALDI source of the mass spectrometer. In the MALDI source, ions were generated from the analyte using a N2 laser, which emits at 337 nm. Upon laser irradiation, the matrix molecules absorb energy from laser beam, which causes desorption of the analyte together with matrix and matrix ions, and then the analyte molecules undergo collisions with desorbed matrix ions to form analyte ions. The created ions are passed through into the time-of-flight mass analyzer. The time-of-flight mass analyzer measures the time it takes for the ions produced in the MALDI source across reach the detector. There are two

ToF operating modes, linear and reflectron. In the linear mode, all ions are accelerated from the MALDI source to the same electric field and for that reason, ions with the same charge carry the same kinetic energy. Principally, ions of lower mass arrive at the linear detector ahead of ions of higher mass. In reflectron mode, the ions are reflected back to the reflectron detector. The reflector enhances resolution and mass accuracy; hence, it has been applied mostly in this dissertation. The spectra from MALDI-ToF-MS were obtained with delayed extraction that employed laser powers slightly above the threshold power for detection of sample ions.

3.5.2. Waters Micromass MALDI-Q-Tof Ultima mass spectrometer

The MALDI-Q-ToF Ultima, manufactured by Micromass (Manchester, UK), is a mass spectrometer fitted with a MALDI source and a hybrid quadrupole/orthogonal acceleration time-of-flight (Q/oa-ToF-MS) mass analyzer which allows exact mass measurement in both mass and tandem mass spectrometry analysis.97

42 The schematic arrangement of the components of this instrument is shown in Fig. 3.2. and the principle of how each part functions have been described in chapter II. Sample preparation is as same as for regular MALDI mass spectra, which has been explained in section 3.1. The blend of matrix, analyte, and cationizing agent (if needed) is introduced onto the ninety-six-well target plate (~0.5 L), dried out in air, and placed in the source of the apparatus. This instrument also utilizes a N2 laser for MALDI. The ions formed during MALDI first pass the quadrupole mass analyzer. This quadrupole can function as ion focusing lens in MS mode or ion selection device in MS/MS mode. In the later case, the ions undergo fragmentation by CAD in the subsequent collision cell.

43

Figure 3.2. Scheme of the Micromass Ulrima MALDI-Q-ToF-MS instrument.97

44 There is a second mass analyzer present in this equipment, which is a ToF tube, positioned right after the collision cell. Ions exiting the collision cell are pushed down to the ToF tube, which is in orthogonal direction and travel along the reflectron until the microchannel plate detector (MCP). Afterwards, the data system creates the corresponding mass spectrum.

The advantages of MALDI-Q-ToF-MS are the capability to perform tandem mass spectrometry experiments on macromolecules, as well as higher mass accuracy and resolution than with regular MALDI-ToF-MS.

3.5.3. Bruker Daltonics Esquire-LC ESI-QIT mass spectrometer (ESI-QIT-MS).

The Esquire-LC ESI-QIT mass spectrometer is manufactured by Bruker Daltonics

(Billerica, MA); a schematic arrangement of the components of this instrument is shown in Fig. 3.3. It is an ion trap mass spectrometer with electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) sources.10-12,17,91,94 This apparatus has mainly been used in ESI mode and has multi-stage MS capabilities, which can be obtained up to MS6 with intense precursor ions. A solution of the analyte is inserted to the ion source via a syringe pump system at a rate that is adjustable within 150-250

L/hr. Nitrogen nebulizing gas is employed at a steady 1-10 psi pressure to control the flow rate. How the ESI source functions has been described in detail in chapter 2.4.2.

45

17

MS instrument. MS

-

IT

Q

-

LC ESI LC

-

erEsquire Daltonics

Scheme of the Bruk Schemethe of

. . Figure3.3

46 The needle is grounded while the capillary entry voltage is set at -4 kV for positive ions.

The neutral particles in the spray are removed by having the sample injection at a right angle to the capillary. Injection of the analyte solution through a very narrow grounded needle and having a constant flow of nebulizing gas facilitates formation of very small droplets. The drying gas temperature is held in the range of 100-300 C and flow around the capillary is kept at 1-10 L/min, which assists desolvation of the droplets without decaying the analyte. The ion transportation and focusing region, which includes the capillary, skimmers, octapole, and focusing lense, is enclosed in vacuum, which helps to remove the solvent and drying gas. Fragmentation can be avoided by proper setting of the potentials of the end of capillary and the skimmers. The skimmers center the ions into the ion trap mass analyzer. The RF only octopole ion guide further focuses the ions as they move from the skimmers to the focusing lenses and enter the trap.

How the ion trap mass analyzer functions was discussed earlier in the chapter

2.5.4. In the research presented in this dissertation, this instrument was mostly used for tandem mass spectrometry experiments.

3.5.4. Bruker APEX III 47e FT-ICR mass spectrometer

The APEX III 47e FT-ICR, manufactured by Bruker Daltonics (Billerica, MA), is a mass spectrometer with an ESI source and FT-ICR mass analyzer.101 This instrument was used for very high mass accuracy measurement and high resolution experiments needed to differentiate isobarics. These experiments have been done in collaboration with

Dr. David J. Aaserud, at Lubrizol Corporation (Wickliffe, OH), where the instrument is located.

47 CHAPTER IV

PYROLYSIS MASS SPECTROMETRY ANALYSIS OF

DIVINYL-POLYDIMETHYLSILOXANE

4.1. Unpyrolyzed divinyl-polydimethylsiloxane

The raw material, divinyl-polydimethylsiloxane (V-PDMS-V) prepolymer,

Mn=28,000 Da, was examined by MALDI-ToF mass spectrometry to obtain data that could be compared with those from the pyrolysis products. The MALDI-ToF mass spectrum of non pyrolyzed V-PDMS-V, ionized by Na+, is shown in Fig. 4.1. The spectrum shows a series of peaks at the expected intervals of 74 Da, consistent with the

C2H6OSi dimethylsiloxane (DMS) repeat unit of the polymer. Ions were collected up to m/z ~17000 but detectable signals were observed only up to ~7000. Higher mass products were not observed in the spectra of V-PDMS-V, even when a high ion deflection cutoff was used in order to eliminate low masses from the beam path and improve the detection limit of the high masses. The MALDI mass spectrum is dominated by low-mass oligomers about ~1,000 Da and shows a Schulz molecular weight distribution (MWD). These characteristics are generally observed for polymers of high polydispersity (Mw/Mn >1.4). Low mass oligomers are more volatile and, thus, are preferentially ionized; moreover, low-mass ions are transmitted and detected more

48

3027.2 3101.3 A A n = 39 n = 40 74 Da

3020 3040 3060 3080 3100 3120

1000 3000 5000 7000 9000 11000 13000 15000 m/z

Figure 4.1. MALDI-ToF mass spectrum of non pyrolyzed V-PDMS-V; the insert shows on expanded view of the peaks generated by the 39-mer and 40-mer.

49 efficiently than high-mass ions because they move with a higher velocity in the ToF analyzer. All these factors lead to the mentioned discrimination against high masses when

Mw/Mn exceeds ~ 1.4.

The m/z values of the ions observed in the MALDI mass spectrum agree well with linear, vinyl-terminated PDMS chains, ionized by Na+ addition as shown below.

Na+

C H3 C H3 H2 C =C H S i O S i C H =C H2 C H3 C H3 n

The only ion series observed appears at m/z 74n + 112 + 23 where 74, 112, and 23 Da are the masses of the dimethylsiloxane repeating unit (-C2H6OSi-)n , end groups (C6H12Si), and Na+, respectively, and n is the degree of polymerization. The agreement between observed and calculated m/z (for above structure) is excellent. For example, the peak at

+ 3027.2 m/z (average mass) matches the mass calculated for [(DMS)39-C6H12Si + Na ] within experimental error (<0.03%): 39 x 74.155 (39 x DMS) + 112.247 (C6H12Si) +

22.989 (Na+) = 3027.3. Similarly, the peak at 3101.3 matches the mass calculated for the

V-PDMS-V 40-mer (3101.4).

50

1000 3000 5000 7000 9000 11000 13000 15000 m/z

Figure 4.2. MALDI-ToF mass spectrum of the products from V-PDMS-V pyrolysis at 150 C for 30 minutes.

51 4.2. Pyrolyzed divinyl-polydimethylsiloxane

Pyrolysis of V-PDMS-V was achieved under inert atmosphere and mild temperatures. Pyrolyzates were introduced into MALDI-ToF and ESI-QIT mass spectrometers for mass and tandem mass spectrometry experiments, respectively.

The ESI-QIT MS/MS spectra (not shown) show only consecutive 74-Da losses, which are diagnostic for the DMS repeat unit.

As explained in the previous chapter, V-PDMS-V was pyrolyzed for 30 minutes at 75, 100, 125, 150, 175, 200, 225, 250, 275, and 300 C. The distribution of pyrolyzates generated at 150 C is shown in Fig. 4.2. (all ionized by Na+). In comparison to the non pyrolyzed material, the distribution is shifted slightly toward lower masses; this trend becomes more pronounced, if the sample is pyrolyzed at higher temperatures, as attested by the spectra obtained after pyrolysis at 200 C and 300 C in Fig 4.3. Ions were collected up to m/z ~17000, but measurable signals were detected only up to ~6500 and

~4500 at 200 C and 300 C, respectively. Two PDMS distributions are observed in the mass spectra of the pyrolyzates, as attested by the expanded trace of Fig. 4.4. The major distribution (A) appears at m/z 74n + 112 + 23 and corresponds to truncated V-PDMS-V chains. The minor distribution (B) appears at m/z 74n + 23 and corresponds to PDMS oligomers with no end groups, most likely PDMS macrocycles with the structure shown below. C H 3 O S i

C H 3 n

52

200 C, 30 min

1000 3000 5000 7000 9000 11000 13000 15000 m/z

300 C, 30 min

1000 3000 5000 7000 9000 11000 13000 15000 m/z Figure 4.3. MALDI-ToF mass spectra of the products from V-PDMS-V pyrolysis at 200 C (top) and 300 C (bottom) for 30 minutes; both spectra are plotted in the same scale.

53

1245.6 n = 15 1319.7 n = 16 1393.7 A n = 17 74 A A 74

1281.1 m = 17

B 1355.7 m = 18

B

1280 1330 1380 m/z

Figure 4.4. Expanded view of the m/z 1250-1450 region in the MALDI-ToF mass spectrum of the products from V-PDMS-V pyrolysis at 300 C.

54

These assignments are supported by the masses of the members of the A and B peak series. For example, the peak at m/z 1245.6 of major series A in Fig. 4.4. (monoisotopic

+ mass) agrees well with the composition [(DMS)15-C6H12Si + Na ]; the calculated monoisotopic mass of this ion, (15 x 74.019) + 112.070 + 22.989 = 1245.3, matches within experimental error (<0.03 %) the measured m/z value; such agreement is also observed for the other oligomers in the same series. Similarly, the mass of series B peak at 1281.1 m/z agrees within experimental error with the calculated monoisotopic mass for

+ [(DMS)17 + Na ]: (17 x 74.019) + 22.989 = 1281.3.

4.3. Conclusion

Two significant distributions are observed in all pyrolyzates from V-PDMS-V, namely truncated linear V-PDMS-V and cyclic PDMS oligomers. The pyrolysis products can be explained via the mechanism shown in Scheme 4.1., which involves Si-O bond rearrangements via a cyclic transition state.81,82 When cyclic oligomers are formed this way, the linear V-PDMS-V chains are shortened. This reaction occurs repeatedly depending on the thermal energy available and until the polymer chain is too small to form a cyclic structure. The PDMS distribution does not shift dramatically when the pyrolysis temperature is increased, pointing out that the activation energy of the process shown in Scheme 4.1. is quite high.

55

Me Me Me Me S i Me O S i O PDMS chain O S i Me O Me Me S i O S i Me Me S i O O Me n Me

Me Me Me Me Me O S i S i S i O O O Me + Me Me O S i S i Me Me S i O O Me n Me

Linear chains (A) cyclic pyrolyzates (B)

Scheme 4.1. Thermal degradation pathways of V-PDMS-V at low temperatures (< 400 C).83

56 CHAPTER V

PYROLYSIS MASS SPECTROMETRY ANALYSIS OF

DISTYRYL-POLY(ETHYLENE GLYCOL)

5.1 Unpyrolyzed distyryl-poly(ethylene glycol)

Pyrolysis of distyryl-poly(ethylene glycol) (St-PEG-St) prepolymer was conducted under inert atmosphere and mild temperatures. The pyrolyzates were characterized by MALDI-ToF, MALDI-Q-ToF, ESI-QIT, and ESI-FT-ICR mass and tandem mass spectrometry studies. Firstly, the mass spectrum of non pyrolyzed

St-PEG-St (Mn 4800) was acquired by MALDI-ToF MS and the result is shown in

Fig. 5.1. From the various matrices examined (dithranol, indolacrylic acid, retinoic acid) for the best signal to noise ratio, dithranol offered the best result. The spectrum presents a narrow molecular weight distribution, centered near the nominal Mn molecular weight and a low-mass tail. All ions observed are Na+ adducts of PEG oligomers. The mass spectrum contains a series of peaks at the expected intervals of 44 Da, corresponding to the C2H4O poly(ethylene glycol) repeat unit of the polymer. Ions are observed up to m/z

~6000.

57

500 1500 2500 3500 4500 5500 6500 7500 8500 m/z

Figure 5.1. MALDI-ToF mass spectrum of non pyrolyzed St-PEG-St.

58

n = 98 n = 100 n = 102 n = 104

4633.9 4633.9 4678.1 4678.1 4766.1 4766.1

4722.1 4722.1

4810.1 4810.1

4590.0 4590.0

4546.0 4546.0

4854.1 4854.1

4898.1 4898.1

44 44 44 44 44 44 44 44

4600 4700 4800 4900

Figure 5.2. An expanded segment of the MALDI-ToF mass spectrum of non pyrolyzed St-PEG-St.

59 There is one dominant product distribution, arising from Na+ cationized PEG having the expected C9H9O and C9H9 terminal groups (as shown below) and a varying number of repeating units.

Na+

C H =C H C H O C H C H O C H C H = C H 2 2 2 2 2 2 n

For instance, as seen in Fig. 5.2., the peak at m/z 4678.1 has the exact (average) mass of a

PEG oligomers with 100 (OC2H4) units and two styryl end groups with the combined composition C18H18O, the calculated (average) mass for this peak is

+ 100 x 44.054 ([OC2H4]100) + 250.342 (C18H18O) + 22.989 (Na ) = 4678.7, which matches

+ the observed value [(PEG)100-C18H18O + Na ] within experimental error (<0.03 %).

There were no additional Na+ adducts detected above noise level, even when the spectrum was acquired in linear detection mode (spectrum not shown), indicating no byproducts or fragments. For the best comparison of the spectra before and after pyrolysis, a MALDI-ToF mass spectrum was measured over a narrower m/z region, as shown in Fig. 5.3. The original spectrum was accumulated over the range 500-8500 m/z, but the range 1300-6000 m/z was selected for this spectrum to remove matrix cluster interference.

60

Doubly charged PEG distribution

1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 m/z

Figure 5.3. MALDI-ToF mass spectrum of non pyrolyzed St-PEG-St, measured over a narrower m/z region (compare to Fig. 5.1.).

61 The spectrum reproduces the original one (Fig. 5.1.) and, additionally it shows an extra series of peaks with a repeating unit mass of 22 Da (half of the PEG repeat unit mass :

44 Da) which originates from doubly sodiated poly(ethylene glycol), [St-PEG-St +

2Na]2+. Doubly charged ions give narrower peaks, necessitating the detection of more points per m/z unit than singly charged ions. This is achieved by narrowing the scanned m/z range.

5.2. Pyrolyzed distyryl-poly(ethylene glycol)

The St-PEG-St prepolymer was pyrolyzed at 75, 100, 125, 150, 175, 200, 225,

250, 275, and 300 C according to the procedure explained in the chapter 3. The pyrolysis residues were analyzed using MALDI-ToF, MALDI-Q-ToF, ESI-QIT, and ESI-

FT-ICR mass spectrometers. Since the PEG was found to degrade easily, its pyrolysis was conducted under very mild temperatures, starting as low as 75 C, and the pyrolysis time was kept short (15 minutes). The MALDI-ToF mass spectrum of the products formed under mildest conditions is shown in Fig. 5.4.

62

500 1500 2500 3500 4500 5500 6500 7500 m/z

Figure 5.4. MALDI-ToF mass spectrum of the products from St-PEG-St pyrolysis at 75 C for 15 min.

63 100 C, 15 min

125 C, 15 min

Figure 5.5. MALDI-ToF mass spectra of the products from St-PEG-St pyrolysis at 100 and 125C for 15 min.

64 The spectra in Figs. 5.4 and 5.5 demonstrate the changes observed as the pyrolysis temperature is increased from 75 to 125 C. At the lowest temperature (75 C), two PEG distributions of different molecular weight (MW) are clearly observed, one narrow at the original prepolymers MW range (centering at ~m/z 4800) and one boarder, appearing as a low-mass tail in the ~m/z 1000-3000 region. These distribution are observed both sodiated and potassiated; the K+ ions presumably originate from the glass tubes in which the pyrolysis was performed , as no K+ was added to the samples. The data of Fig. 5.4. indicates that the polymer is partially broken down into shorter chains, which create small

Na+ cationized species that are detectable together with the intact polymer. As the pyrolysis temperature increases, the distribution of intact St-PEG-St decreases markedly

(Fig. 5.5.); nevertheless, the MALDI spectrum of the pyrolyzates generated at 125 C still includes a minute amount of unpyrolyzated product. There are several products (different end groups) generated upon this degradation, which will be described later in the peak assignment section. The degraded materials were also analyzed by MALDI-Q-ToF MS.

One such spectrum is shown in Fig. 5.6. Qualitatively, the spectra obtained via ToF and

Q/ToF MS (Figs. 5.4. and 5.6., respectively) are similar. However, the proportion of low- mass oligomers appears to be higher with the Q/ToF instrumentation. This could be due to differences in the ion optics of these mass spectrometers or to the higher laser power of the Q/ToF mass spectrometer, in which the laser light cannot be attenuated.

Pyrolyses were also performed at greater temperatures and for longer times

(30 min) in order to enhance the concentration of the pyrolyzates for more reliable peak assignments and to study the dependence of the degree of depolymerization on temperature. 65

100

%

0 m/z 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000

Figure 5.6. MALDI-Q-ToF mass spectrum of the products from St-PEG-St pyrolysis at 75 C, 15 min.

66

Figure 5.7. MALDI-ToF mass spectrum of the products from St-PEG-St pyrolysis at 150 C, 30 min.

67

Figure 5.8. MALDI-ToF mass spectrum of the products from St-PEG-St pyrolysis at 200 C, 30 min.

68

Figure 5.9. MALDI-ToF mass spectrum of the products from St-PEG-St pyrolysis at 275 C, 30 min.

69

Figure 5.10. MALDI-ToF mass spectrum of the products from St-PEG-St pyrolysis at 300 C, 30 min.

70 Higher pyrolyzate concentrations are also necessary for tandem mass spectrometry experiments. St-PEG-St was pyrolyzed at 150-300 C, with a 25 C gradient. The mass spectra of the pyrolyzates obtained at 150, 200, 275, and 300C, which showed noticeable differences, are depicted in Figs. 5.7, 5.8, 5.9, and 5.10, respectively. All ions observed are Na+ adducts. When the pyrolysis temperature is raised, the PEG product distributions observed shift to lower masses as a result of additional fragmentation. The abundances of low mass ions can be enhanced by using more laser power, but this will cause further photo-induced fragmentation at the same time. Therefore, the MALDI-ToF mass spectra were measured only near the threshold power needed for production and detection of ions. Inspection of Figs. 5.7.-5.10. reveals that the raw material distribution is not present even in the pyrolyzates obtained at 150 C, signifying that St-PEG-St degrades completely already at mild conditions.

At least eight series of PEG pyrolysis products can be identified in the pyrolyzates. These are labeled by A, B, C, D, E, F, G, and H, in Figs. 5.11. and 5.12., which display a narrow mass range of the spectrum measured after pyrolysis at 275 C.

The two most significant series are A and B. Isomeric and isobaric PEG oligomers overlap in series A, B, C, D, and G. Possible end groups derived from the mass spectra and select MS/MS experiments are summarized in Table 5.1. MS/MS data could only be obtained for the more abundant oligomers A, B, and C. They are shown in the appendix and are fairly similar to those obtained from membrane pyrolyzates with the compositions A, B, and C, which will be discussed in detail in chapter VI.

71

A 44 A 44 44

A

1153.4 1153.4 1197.4 1197.4

B B A 1241.4

1241.4 B

1169.4 1169.4 1285.5 1285.5

1213.4 1213.4

1257.5 1257.5

G G F G F F C C C G F

D D D

1271.5 1271.5

1227.5 1227.5

1183.4 1183.4

Figure 5.11. An expanded view of the MALDI-ToF mass spectrum of the PEG pyrolyzates generated at 275 C, 30 min.

72

44 Da * + isobars A* B* A* E F C* F D H G G

800 810 820 830 840 850 m/z

Figure 5.12. An expanded view of the MALDI-ToF mass spectrum of the PEG oligomers generated upon St-PEG-St pyrolysis at 275 C, 30 min. This spectrum displays a different m/z region from that shown in Fig. 5.11.

73 Table 5.1. PEG pyrolysis products, identified based on MS and MS/MS experiments, and their end group masses

Series Pyrolyzates structures  End groups (Da) St-[PEG]-O-St 250 = 74 + (4 x 44) A St-[PEG]-O-Et 162 = 74 + (2 x 44) Et-[PEG]-O-Et 74

Me-[PEG]-O-CH2CHO 74

H-[PEG]-O-St 134 = 46 + (2 x 44) B H-[PEG]-O-Et 46 Me-[PEG]-O-Me 46

C St-[PEG]-O-Me 148 = 60 + (2 x 44) Me-[PEG]-O-Et 60

H-[PEG]-O-CH2CHO 60

D St-[PEG]-O-CH2CHO 176 = 4 x 44

Et-[PEG]-O-CH2CHO 88 = 2 x 44

H-[PEG]-O-CH=CH2 44

E H-[PEG]-OH 18

F H-[PEG]-O-Me 32

G St-[PEG]-O-CH=CH2 160 = 72 + (2 x 44)

Et-[PEG]-O-CH=CH2 72

H HCOCH2-[PEG]-O-CH2CHO 102 = 58 + 44

Me-[PEG]-O-CH=CH2 58

74 Scheme 5.1. St-PEG-St thermal degradation mechanism

H H H C O 2 2 2 St St-(PEG)-O-St C O CH2 + O C C St C C A C O H2 H2 H2

+H +H - H

- H H H H2 2 2 C O CH3 HO C C St St C C C O H H2 H2 2 A B Et chain ends - H2O HO chain ends

H2 H2 H O C C St 2 C O C O CH H St C C 2 H2 H D G aldehyde chain ends

CH=CH2 chain ends

= PEG chain

H2 H2 St-(PEG)-O-St C C C O H C C St + 2 C CH2 O C St A H2 H2

+ H + H

H2 St C O C CH3 H2

C Me chain ends

75 Series G and F are observed 2 Da below and 2 Da above the A series, respectively.

The D series is 2 mass lighter than the B series. Series H and E have 2 Da lower or higher masses than series C, respectively. The E and H series are detectable only at the lowest masses and are minor pyrolysis products. The PEG degradation mechanism proposed in

Scheme 5.1. is adapted from the free radical degradation pathways found by Lattimer to take place upon mild pyrolysis of HO-PEG-OH.51 This mechanism can account for the pyrolyzates observed from St-PEG-St.

All series labeled in Fig. 5.11. contain C2H4O (44 Da) repeat unit. Many of these series can have isobaric or isomeric structures, whose identification required the use of more mass spectrometry techniques. For example, the peak at m/z 1153.4 of series A, arises from a PEG oligomers with the mass 1153.4-22.99 (Na+) = 1130.41 Da. Dividing this mass by the repeat unit mass (44.0262 Da) gives 25.68, indicating that the oligomers contains  25 repeat units. The remainder, 0.68 x 44.0262 = 30 Da, gives a lower limit for the end group masses. Thus, the 1130.41 Da oligomer could have 25 repeat units and end groups adding 30 Da, 24 repeat units and end groups adding (30 + 44) = 74 Da, 23 repeat units and end groups adding [30 + (2 x 44)] = 118 Da, etc. In other words, the possible end group masses are 30 + 44n. The mechanism outlined in Scheme 5.1. narrows down the number of plausible end group masses to 3, namely 74, 162, and 250 (see also Table

5.1). The predominant composition of the series A oligomers can be determined by the high resolution MS analyses discussed later in this chapter.

Since series A is the major PEG distribution at the lowest pyrolysis temperatures, it is reasonable to assume that it mainly originates, at least at low pyrolysis temperatures, from unpyrolyzed St-PEG-St, which has a combined end group mass of 250 Da

76 (Table 5.1). As shown in Scheme 5.1., thermal degradation can occur via hemolytic C-O or C-C bond cleavages. Homolysis at the C-O bond generates two types of polymer radicals; an ethyl (CH2-CH2 ) and an oxy radical (CH2-CH2-O ). Ethyl radicals can abstract a hydrogen atom (from surroundings chains) to produce ethyl ether ends

(A-Ethyl chain end) or lose a hydrogen to create vinyl ether ends (G). Analogously, oxy radicals can abstract a hydrogen atom to form hydroxyl ends (B) or lose a hydrogen atom to create aldehyde ends (D). Alternatively, C-C homolytic cleavage generates two radicals (CH2 ) that can abstract hydrogen atoms to produce methyl ether ends (C).

77 5.3. High resolution and mass accuracy experiments

Several isobaric compositions are possible for most of the pyrolyzates from St-PEG-St

(Table 5.1.). Fig 5.13. shows the calculated masses and isotope patterns for the products that can contribute to the peak of distribution A observed at the nominal m/z value of

1153. In order to differentiate such isobaric structures, enhanced resolution and mass accuracy experiments were required. These experiments were carried out with a

Bruker APEX III 47e FT-ICR mass spectrometer, equipped with an ESI source, by

David J. Aaserud from Lubrizol Corporation. The pyrolyzates were ionized by Na+ attachment. The full spectrum obtained by ESI is shown in Fig. 5.14. It includes multiply charged oligomer distributions (+3, +4, and +5). The monoisotopic mass of the PEG

+ + repeat unit, C2H4O, is 44.0262 Da. Adding C2H4O to an oligomers with +5Na , +4Na , and +3Na+ charges increases m/z by 8.8052, 11.0066, and 14.6754, respectively, as shown in Fig. 5.15-5.17. Because of the high resolution and high mass accuracy of

FT-ICR MS, m/z ratios with six decimal numbers could be measured, based on which isobaric structures can be distinguished. Isobaric oligomers are possible in series A, B, C,

D, and G. Only those in the major series A, B, and C could be determined, but not those in the minor series D and G because of poor signal to noise ratio. The ions with +5Na+ charges (appearing at ~ 800-1000 m/z), which have the highest abundances, were used in the isobarics analysis.

78 Figure 5.13. Calculated masses and isotope patterns of PEG oligomers that can arise upon pyrolysis of St-PEG-St. The Na+ adducts of these oligomers appear at nominal m/z 1153.

St-[PEG]n-O-St

n = 20

St-[PEG]n-O-Et

n = 22

Et-[PEG]n-O-Et

n = 24

Me-[PEG]n-O-CH2CHO

n = 24

79

+ 5 Na

+ 4 Na

+ 3 Na

500 750 1000 1250 1500 1750 2000 2250 m/z

Figure 5.14. A ESI/FT-ICR mass spectrum of the products from St-PEG-St pyrolysis at 100 C.

80

8.8 8.8

944.740563944.740563

962.347913962.347913

953.538547953.538547

Figure 5.15. Partial ESI/FT-ICR mass spectrum of the pyrolysis products of St-PEG-St (100 C). The displayed m/z range contains PEG oligomers (series A) carrying 5+ charges (i.e. 5Na+).

81

n=97

n=100 n=102 11.0 Da

n=108

Figure 5.16. Partial ESI/FT-ICR mass spectrum of the pyrolysis products of St-PEG-St (100 C). The diaplayed m/z range contains PEG oligomers (series A) with +4 Na+ charges.

82

n=96

n=98

n=100

14.7 n=102

14.7 14.7 n=104 14.7 14.7

14.7 14.7 14.7

1520 1540 1560 1580 1600 1620 m/z

Figure 5.17. Partial ESI/FT-ICR mass spectrum of the pyrolysis products of St-PEG-St (100 C). The diaplayed m/z range contains PEG oligomers (series A) with +3 Na+ charges.

83 5.3.1. St-PEG-St pyrolyzates: Oligomers in series A

Series ‘A’ can include four isobaric structures, shown in Table 5.2. below.

Table 5.2. Isobaric components of series A in the pyrolyzates of St-PEG-St.

Series A Symbol EG Formula Calculated EG mass (Da) # mer

St-[PEG]-O-St A1 C18H18O 250. 135 765 1980 n

St-[PEG]-O-Et A2 C11H14O 162. 104 465 0700 n+2

Et-[PEG]-O-Et A3 C4H10O 74. 073 164 9420 n+4

Me-[PEG]-O-CH2CHO A4 C3H6O2 74. 036 779 4360 n+4

These four structures are symbolized by A1-A4. In the MALDI-ToF mass spectra, isobaric oligomers are detected at the same nominal mass (for example, m/z 918.3 for n = 96). In order to identify the major structure, deuterium (H/D) exchange, tandem mass spectrometry, and high resolution and mass accuracy FT-MS experiments were performed. In this chapter, the FT-MS results will be described. The H/D and MS/MS data are discussed in the next chapter.

High resolution/high mass accuracy experiments can distinguish A1, A2, A3, and

A4 because their exact masses differ. This is shown in Table 5.3. which lists the calculated monoisotopic m/z values of ten distinct A1-A4 oligomers with 5Na+ charges

(top, columns 2-5). Note that because these ions carry 5+ charges, the distance between n- and (n+1)-mer is 44.0262/5 = 8.8 m/z units. Also note that the A1-A4 oligomers appearing at the same nominal m/z (e.g., 918.3) have different number of repeat units because the corresponding end groups have different masses (see Table 5.2.); for example, for the peak at m/z 918.3, n = 96 for A1, 98 for A2, and 100 for A3 and A4.

Table 5.3. also lists the experimentally measured m/z ratios (top, column 1).

84

Table 5.3. Observed mass-to-charge ratios versus calculated m/z values for A1-A4 oligomers with 5Na+ charges.

Observed Calculated m/z m/z A1 A2 A3 A4 918.324813 918.319393 918.323613 918.327833 918.320556 927.130200 927.124633 927.128853 927.133073 927.125796 935.935516 935.929873 935.934093 935.938313 935.931036 944.740563 944.735113 944.739333 944.743553 944.736276 962.347913 962.345593 962.349813 962.354033 962.346756 979.961774 979.956073 979.960293 979.964513 979.957236 988.764133 988.761313 988.765533 988.769753 988.762476 997.569373 997.566553 997.570773 997.574993 997.567716 1006.374513 1006.371793 1006.376013 1006.380233 1006.372956 1015.179453 1015.183653 1015.181253 1015.185473 1015.178196

(Observed-Calculated) A1 A2 A3 A4 -0.0054 0.0012 -0.0030 0.0043 0.0056 0.0013 -0.0029 0.0044 0.0056 0.0014 -0.0028 0.0045 0.0054 0.0012 -0.0030 0.0043 -0.0023 -0.0019 -0.0061 0.0012 0.0057 0.0015 -0.0027 0.0045 0.0028 -0.0014 -0.0056 0.0027 0.0028 -0.0014 -0.0056 0.0027 0.0027 -0.0015 -0.0057 0.0061 -0.0042 -0.0018 -0.0060 0.0031 Average || 0.0043 0.0014 0.0044 0.0038

85 The bottom part of Table 5.3. summarizes the differences between experimental result and theoretical m/z of each oligomer. The average differences for oligomers A1-A4 ( || ) are given in the bottom row of Table 5.3. The smallest individual and average differences are observed for A2, for which calculated and experimental mass-to-charge ratio match within 1.4 ppm. Consequently, the main component of series A is structure A2 which includes styryl and ethyl terminating groups; St-[PEG]-O-Et. This result will be compared later to those obtained via H/D exchange and MS/MS on pyrolyzates of the amphiphilic membrane. (chapter VI)

5.3.2. St-PEG-St pyrolyzates: Oligomers in series B

The isobaric structures shown in Table 5.4. may overlap within series B.

Table 5.4. Isobaric components of series B in the pyrolyzates of St-PEG-St.

Series B Symbol EG Formula Calculated EG mass (Da) # mer

H-[PEG]-O-St B1 C9H10O 134. 073 164 9420 n

H-[PEG]-O-Et B2 C2H6O 46. 041 864 8140 n+2

Me-[PEG]-O-Me B3 C2H6O 46. 041 864 8140 n+2

B1-B3 are detected at the same nominal peak positions for all degrees of polymerization.

B2 and B3 also are isomers, i.e. they have the same chemical compositions and masses and, thus, cannot be differentiated by high resolution MS. B2 and B3 are grouped together in Table 5.5., which compares calculated and observed m/z values.

86

Table 5.5. Observed mass-to-charge ratios versus calculated m/z values for B1-B3 oligomers with 5Na+ charges.

Observed Calculated m/z (Observed-Calculated) m/z B1 B2 or B3 B1 B2 or B3 921.522938 921.522593 921.526813 0.0003 -0.0039 939.132773 939.133073 939.137293 -0.0003 -0.0045 947.937913 947.938313 947.942533 -0.0004 -0.0046 956.742653 956.743553 956.747773 -0.0009 -0.0051 965.547893 965.548793 965.553013 -0.0009 -0.0051 974.353233 974.354033 974.358253 -0.0008 -0.0050 983.160373 983.159273 983.163493 0.0011 -0.0031 991.965313 991.964513 991.968733 0.0008 -0.0034 1000.770253 1000.769753 1000.773973 0.0005 -0.0037 1009.573693 1009.574993 1009.579213 -0.0013 -0.0055 Average || 0.0007 0.0044

87 Isobaric oligomers of B1 and B2 (or B3) include different amounts of repeat units, with

B2 (or B3) having two more units than B1 to counterbalance the lower end group mass.

For example, B1 and B2 (or B3) oligomers detected at m/z ~921, with +5 Na+ attached, have 99 and 101 of PEG repeat units (n), respectively, and total end group masses of 134

Da (B1) and 46 Da (B2 or B3). The measured m/z ratios (Table 5.5., left column) agree much better with those calculated for B1, for which the average difference between observed and theoretical data is only 0.7 ppm. In contrast, the m/z values calculated for

B2 (or B3) are significantly smaller (by 4.5 ppm on average). Accordingly, series B mainly contains structure B1 which has styryl and hydroxyl terminating groups,

H-[PEG]-O-St.

88 5.3.3. St-PEG-St pyrolyzates: Oligomers in series C

There are two likely isobaric structures for series C, as shown in Table 5.6.

Table 5.6. Isobaric components of series C in the pyrolyzates of St-PEG-St.

Series C Symbol EG Formula Theoretical mass (Da) # mer

St-[PEG]-O-Me C1 C10H12O 148. 088 815 0060 n

Me-[PEG]-O-Et C2 C3H8O 60. 057 514 8780 n+2

H-[PEG]-O-CH2CHO C3 C2H4O2 60. 051 843 0210 n+2

The calculated monoisotopic m/z ratios for C1-C3 with 5Na+ charges are given in Table

5.7. C1 (n) oligomers contain two less repeat units but heavier end groups than isobaric

C2 or C3 (n+2) oligomers. For example, the C1 member observed at m/z 924 has 99 repeat units (n) and end groups with a mass of 148 Da, while the C2 or C3 member appearing at the same nominal mass, m/z 924, has 101 repeat units (n+2) and end groups of 60 Da. The measured m/z ratios (Table 5.7., left column) lie closer to those calculated for C1 than C2 or C3, the corresponding average differences between measured and calculated m/z values being 0.0012 and 0.0044 or 0.0042, respectively. Hence, C1 carries styryl and methyl teriminating chain ends, St-[PEG]-O-Me, appears to be the major constituent of peak series C.

89

Table 5.7. Observed mass-to-charge ratios versus calculated m/z values for C1-C3 oligomers with 5Na+ charges.

Observed Calculate masses  (Observe-Calculate) masses C1 C2 C3 C1 C2 C3 924.326923 924.325723 924.329943 924.330722 0.0012 -0.0030 -0.0038 933.130363 933.130963 933.135183 933.134932 -0.0006 -0.0048 -0.0046 950.740343 950.741443 950.745663 950.744825 -0.0011 -0.0053 -0.0045 959.547983 959.546683 959.550903 959.551064 0.0013 -0.0029 -0.0031 977.155663 977.157163 977.161383 977.160833 -0.0015 -0.0057 -0.0052 985.961003 985.962403 985.966623 985.967015 -0.0014 -0.0056 -0.0060 994.766443 994.767643 994.771863 994.770436 -0.0012 -0.0054 -0.0040 1003.57398 1003.572883 1003.577103 1003.577092 0.0011 -0.0031 -0.0031 1012.37722 1012.378123 1012.382343 1012.382343 -0.0009 -0.0051 -0.0051 1021.18506 1021.183363 1021.187583 1021.187583 0.0017 -0.0025 -0.0025 Average || 0.0012 0.0044 0.0042

90 5.4. Conclusion

The mass spectrum of non pyrolyzed distyryl-poly(ethylene glycol) (St-PEG-St) prepolymer shows only one distribution, arising from St-PEG-St itself and extending up to m/z ~7000. This indicates that the prepolymer is not altered during mass spectrometry experiments. After the PEG prepolymer is pyrolyzed, several additional products are observed in the PEG pyrolyzates (distributions A-H; some consisting of isobarics):

A, St-[PEG]-OSt, St-[PEG]-OEt, Et-[PEG]-OEt, and Me-[PEG]-OCH2CHO;

B, H-[PEG]-OSt, H-[PEG]-OEt, and Me-[PEG]-OMe; C, St-[PEG]-OMe,

Me-[PEG]-OEt, and H-[PEG]-O-CH2CHO; D, St-[PEG]-OCH2CHO, H-[PEG]-

OCH=CH2; E, H-[PEG]-OH; F, H-[PEG]-OMe; G, St-[PEG]-OCH=CH2, Et-[PEG]-

OCH=CH2; H, Me-[PEG]-OCH=CH2. When the pyrolysis temperature is raised, the PEG distributions observed shift to lower masses as a result of additional fragmentation.

Mass analysis of the PEG pyrolyzates at high mass accuracy and high resolution could be completed for series A, B, and C but not for the other composite series (D and G) due to their small abundances. The results indicated that the major components of series A, B, and C were: A, St-[PEG]-OEt; B, H-[PEG]-OSt; C, St-[PEG]-OMe. The pyrolyzates probably arise by free-radical degradation mechanisms; initially, C-O and C-C bond cleavages take place in the PEG chains, preferentially C-O bond cleavages, and the radicals emerging this way undergo subsequent H atom transfer reactions to yield the observed products.

91 CHAPTER VI

PYROLYSIS MASS SPECTROMETRY ANALYSIS OF

TRICOMPONENT AMPHIPHILIC NETWORKS

6.1. Pyrolyzed amphiphilic networks

The amphiphilic copolymer networks were composed of hydrophilic distyryl- poly(ethylene glycol) (St-PEG-St) and hydrophobic divinyl-polydimethylsiloxane

(V-PDMS-V), which were crosslinked with pentamethylpentacyclosiloxane (D5H).

MS analysis of the networks before pyrolysis gave no detectable spectra. Evidently, the networks are greatly crosslinked and of very large molecular weight, which makes their volatilization and analysis by MALDI MS impossible. Therefore, the networks were subjected to pyrolysis mass spectrometry analysis. Pyrolyses were conducted at several temperatures (150-350 C) for 30 minutes, as described in the chapter III. Two different amphiphilic copolymer membranes were studied in the research presented in this dissertation, as listed in Table 3.3. Both membranes contain the same raw substances, which are St-PEG-St (Mn 4800) and V-PDMS-V (Mn 26K) prepolymers and D5H crosslinking agent, and different weight ratios of starting polymers but consistent crosslinker content.

92 Pyrolyses were performed under inert atmosphere at relatively mild temperatures.

The resulting pyrolyzates were analyzed by MALDI-ToF MS and ESI-QIT MS/MS experiments. Several matrices were examined, and dithranol offered the best results.

The network sample labeled as APN#1 and prepared using 30% PEG, 60% PDMS, and

10% D5H and will be discussed first. Thermal degradation of this membrane started at

150 C, which was the mildest pyrolysis temperature that led to observable pyrolyzates.

The MALDI-ToF mass spectra of the pyrolyzates contain Na+ cationized oligomers. Ions were observed up to m/z ~5000 and all spectra were plotted with the same Y scale

(intensity) to facilitate comparison of the products formed at different temperatures.

The MALDI-ToF mass spectrum of APN#1 after pyrolysis at 275 C is shown in

Fig. 6.1. Two types of polymer distributions are observed. These distributions encompass peaks at intervals of 44 and 74 Da, corresponding to polymers with C2H4O poly(ethylene glycol) (PEG) repeat units and C2H6OSi dimethylsiloxane (DMS) repeat units, respectively. This mass spectral data confirms that the amphiphilic network contains the desired PDMS and PEG strands. Expanded traces of the MALDI-ToF mass spectrum of the pyrolyzates generated from APN#1 at 275 C are shown in Figs. 6.2. and Fig. 6.3.

93

PEG  44 Da PDMS  74 Da

Figure 6.1. MALDI-ToF mass spectrum of the pyrolysis products from APN#1 (275 C, 30 min).

94

Cyclo-PDMS Cyclo-PDMS

mer-17 mer-18

1281.0360 74 Da 1355.0721

B A B C D E G F D H

Figure 6.2. Expanded trace of the MALDI-ToF mass spectrum of APN#1 after pyrolysis at 275 C.

95

B A B 1301 C 1329 1345

1315

D E F G 1299 1317 1331 D 1327 H 1343

1313

Figure 6.3. Expanded trace of the MALDI-ToF mass spectrum of APN#1 after pyrolysis at 275 C.

96 As is evident from Fig. 6.2., the mass spectrum contains only one PDMS distribution which corresponds to cyclic oligomers (74n). The peaks observed at m/z 1281.04 and

1355.07 match within experimental error (<0.03%) the calculated monoisotopic masses

+ + of [(DMS)17 + Na ] (1281.3) and [(DMS)18 + Na ] (1355.3), respectively. Further, the mass spectrum also presents eight series of PEG pyrolyzates, marked by A-H in Fig. 6.2. and having the nominal monoisotopic masses labeled in Fig. 6.3. The same PEG distributions were also detected in the pyrolyzates of the St-PEG-St prepolymers. The m/z values of the series A-H oligomers are consistent with the compositions listed in

Table 5.1. No high resolution FT-ICR spectra could be obtained from the membrane pyrolysis products, presumably because of their low concentrations. H/D exchange and

MS/MS experiments could, however, be performed and verify the presence of most structures listed in Table 5.1., but exclude a few of them (see following section).

Pyrolysis products were observed from the network starting at 150 C; the highest temperature examined in this investigation was 350 C. All network pyrolyses lasted for

30 min. The MALDI mass spectra of the soluble residue after pyrolysis at 150 C and

350 C are shown in Figs. 6.4. and 6.5., respectively. Already at 150 C PEG and PDMS distributions are detected. The PEG pyrolyzates at this low temperature extend to higher masses as compared to the pyrolyzates at higher temperatures. More importantly, distribution A is dominant and all other distributions are either negligible or below noise level. A major component of distribution A is St-PEG-St (see MS/MS data later). Thus, at the mildest temperatures, intact prepolymers detach from the network, presumably because the Si-C bonds connecting the PEG chains to the crosslinker are the weakest bonds in the membrane. The PEG prepolymers had an average MW of ~4800 (Mn), while 97 the PEG chains observed in Fig. 6.4. only extent to ~m/z 1,000. Apparently, short chains are detached more easily in intact form than longer chains.

At the highest pyrolysis temperature of 350 C (Fig. 6.5.) both PEG and PDMS distributions have drifted to the lower masses, and extra PEG products appear due to further degradation. The changes with pyrolysis temperature are much more dramatic for the PEG than the PDMS products. It is noteworthy that St-PEG-St yielded detectable pyrolyzates at 75, 100, and 125 C, whereas the membrane necessitated heating to at least

150 C in order to thermally degrade. Hence, crosslinking increased substantially the thermal stability.

The pyrolysis products from APN#1, which was synthesized from 30%

St-PEG-St, 60% V-PDMS-V, and 10% D5H, are also observed upon pyrolysis of APN#2, which was synthesized from 35% St-PEG-St, 55% V-PDMS-V, and 10% D5H.

The MALDI-ToF mass spectrum of the pyrolyzates from APN#2 at 350 C is shown in

Fig. 6.6. The PEG and PDMS distributions observed are the same as those from APN#1.

98

PEG  44 Da

PDMS  74 Da

Figure 6.4. MALDI-ToF mass spectrum of the products of APN#1 pyrolysis at 150 C for 30 min.

99

PEG  44 Da

PDMS  74 Da

Figure 6.5. MALDI-ToF mass spectrum of the products of APN#1 pyrolysis at 350 C for 30 min.

100

PEG  44 Da

PDMS  74 Da

500 1000 1500 2000 2500 3000 3500 4000 m/z

Figure 6.6. MALDI-ToF mass spectrum of the products of APN#2 pyrolysis at 350 C for 30 min.

101 Note, however, that the relative proportion of PEG products is higher from APN#2, which contains more PEG. Hence, compositional changes in the synthesis are reflected by the MALDI’ mass spectra of the pyrolyzates and, thus, can be monitored by such spectra.

6.2. H/D exchange experiments

Some pyrolyzate components contain one or two –OH end groups, which can be identified by H/D exchange. The conversion –OH  –OD increases the oligomer mass by +1Da; hence, mass shifts by 0, +1, and +2Da after H/D exchange indicate no OH, one

OH, or two OH chain ends, respectively. The H/D exchange was performed as explained in chapter III. The deuterated samples were introduced into the mass spectrometer as soon as possible to minimize back-exchange with water vapor from the environment.

A MALDI-ToF mass spectrum of the deuterated pyrolyzates obtained at 275 C is shown in Fig. 6.6. Ions are observed up to 4500 Da, all being Na+ adducts. The extent of exchange observed for series A-H is indicated by d0, d1, and d2. Some major PEG pyrolyzates (Table 5.1.) can be identified based on the mass shifts resulting after deuterium exchange. The A series remains largely d0. This result agrees with the likely structures of A given in Table 5.1., none of which contains terminal OH groups. Since series D also largely remains d0, its composition must predominantly be

St-[PEG]-O-CH2CHO and/or Et-[PEG]-O- CH2CHO, not H-[PEG]-O-CH=CH2.

On the other hand, the peaks of series B shift, to a significant extent, one mass unit higher

(d1), indicating that most of the corresponding oligomers have one OH chain end.

This feature is present in two of the structures possible for B, H-[PEG]-O-St and 102 H-[PEG]-O-Et, but not in the isobaric/isomeric B structure Me-[PEG]-O-Me; the latter structure is, thus, a minor pyrolyzate constituent. The extent of deuterium incorporation in series C and F is more difficult to assess because of the severely overlapping isotope clusters. Still, the observed peak patterns suggest that substantial fractions of both series shift one m/z unit higher (d1); such a pattern is consistent the structure H-[PEG]-O-Me for F and suggests that C mainly contains H-[PEG]-O-CH2CHO, which possess one free

OH chain end. Series G and H are too weak with the deuterated sample to make any meaningful conclusions. Finally, series E becomes partly d2-E, corroborating the proposed structure that contains two hydroxyl termini.

103

B-d1 1302 B-d1 1346

E-d1 1318 E-d2 1319 A-d0 1329 F-d1 C-d1 1332 1316

D-d0 D-d 0 H-d0 G-d0 1343 1299 1313 1327

1300 1310 1320 1330 1340 1350 m/z

Figure 6.7. Partial MALDI-ToF mass spectrum of the deuterated pyrolyzates formed at 275 C from APN#1.

104 6.3. Tandem mass spectrometry studies

The CAD spectra of select pyrolyzates from APN#1 were examined by ESI-QIT tandem mass spectrometry to further ascertain their end groups. A few PEG standards with known end groups were also investigated to facilitate structural assignments.

6.3.1. Tandem mass spectrometry of poly(ethylene glycol) standards

Before analyzing the CAD tandem mass spectra of network pyrolyzates, ESI-QIT

CAD mass spectra of four PEG standards were acquired. The reference compounds were

H-[PEG]-OH (Mn 600), H-[PEG]-OEt (Mn 200), H-[PEG]-OMe (Mn 550), and

Me-[PEG]-OMe (Mn 400). The CAD spectra of these compounds unveil which fragmentation pathways are diagnostic of the corresponding end groups, and this information can be used to characterize the end groups of the degradation products from the amphiphilic networks.

+ The ESI-QIT CAD mass spectrum of the Li adduct of H-[PEG]9-OH at m/z 421

(9-mer), is shown in Fig. 6.8. There are two homologous series of fragments. The primary series arises by losses of 44n Da from the precursor ion (m/z 157, 201, 245, 289,

333, 377). The less abundant series arises by loss of 44n + 18 Da (m/z 139, 183, 227,

271, 315, 359) from the precursor ion. A plausible mechanism is shown in Scheme 6.1. and involves charge-directed dissociation, promoted by the Li+ Lewis acid. Li+-catalyzed cleavage of a C-O bond, followed by a facile 1,2-hydride shift, leads to an alkoxyethyl cation interacting with a lithium alkoxylate. Proton transfer within this complex, from the ethyl cation to the basic alkoxylate group, followed by dissociation, gives rise to the major fragment ion series, composed of truncated PEG oligomers with HO/H chain ends

105 (m/z 157, 201, 245, 289, 333, 377). Alternatively, proton transfer may be accompanied by Li+ migration to form a Li+-bound complex between two truncated PEG chains with

HO/vinyl and HO/H end groups. The latter chains compete for the Li+ charge when the dimmer dissociates. The chains with HO/H end groups (-44n) contain more O atom per repeat unit and, thus, should have higher Li+ affinities than the chains with HO/vinyl end groups [-(44n +18)], which would explain the significantly higher abundances of the former series (i.e. m/z 157, 201, 245, 289, 333, 377). The fragmentation pattern in Fig.

6.8. reveals that the signature ions for PEG hydroxyl chain ends are an abundant 44n loss series and a significant (albeit less abundant) 44n + H2O loss series.

106

421

-44

201 245 289 333 377 -44 -44 -44 -44 -18

-18 -44

157 -18 -18 -18 -18

100 200 300 400 m/z

+ Figure 6.8. ESI-QIT CAD mass spectrum of the Li adduct of H-[PEG]9-OH at m/z 421, generated from a PEG 600 standard with hydroxyl groups at both chain ends.

107

H L i

H O C H C H O C C H O C H C H O H 2 2 x H 2 2 2 y

r H -

L i C H 2 C H 2 O H y C H C H O H + 4 4 n L i 2 2 y O rH + O

H O C H 2 C H 2 O C H C H 3 x H

m /z 1 5 7 , 2 0 1 , 2 4 5 , 2 8 9 , 3 3 3 , 3 7 7 (y = 3 -8 ) r H + rL i +

C H C H O H H 2 2 y O

L i L i

H O C H C H O CH C H H O C H 2 C H 2 O C H C H 2 + (4 4 n + 1 8 ) 2 2 x 2 x

m /z 1 8 3 , 2 2 7 , 2 7 1 , 3 1 5 , 3 5 9 , 4 0 3 (x = 3 -8 )

Scheme 6.1. Charge-induced fragmentation pathway of [H-(PEG)-OH + Li]+ to truncated linear fragments with HO/H and HO/vinyl end groups.

108 CAD experiments were performed on H-[PEG]-OEt to unveil the fragments characteristic for an ethyl ether chain end. The ESI-QIT CAD mass spectrum of lithiated

H-[PEG]3-OEt at m/z 185 (3-mer) is shown in Fig. 6.9. There are peaks at m/z 113, 123

(trace), 141, 157, and 167, corresponding to the losses of 72, 62, 44, 28, and 18 Da, respectively. A mechanism analogous to that shown in Scheme 6.1. and outlined in

Scheme 6.2. can account for the observed fragments. Since the molecule is asymmetric, the C-O bonds adjacent to the lithiated O site are not equivalent and their cleavages lead to different products, depending on whether the bond cleaved is on the side of the HO or the side of the OCH2CH3 chain end. The losses of 44n and 44n + 18 (Scheme 6.2.) include the HO chain end and, as explained above, diagnose this end group. Conversely, the losses of 44n + 28 provide signature ions for the Et chain end. Note that within the series originating from nominal 44n and 44n + 18 eliminations, only the first members

(n = 1 and 0, respectively) are observed with considerable abundances.

Mono- as well as dimethyl PEG derivatives were investigated to determine the signature ions of MeO chain ends. The ESI-QIT CAD spectrum of lithiated

Me-[PEG]5-OMe at m/z 273 (5-mer) is shown in Fig. 6.10. Two fragment ion series are obtained, arising by nominal losses of 44n + 58 and 44n + 32 Da. These oligomer series are readily rationalized by the pathway presented in Scheme 6.3., which is completely analogous to that taking place with dihydroxy PEG. It is noteworthy again that the largest fragment within the 44n + 32 loss series (i.e. m/z 241, formed by loss of 32 Da) is quite abundant, while the other losses within this series proceed with significantly lower yields.

CAD of lithiated Me-[PEG]5-OMe also causes the elimination of 88 Da (but not 44 Da).

109 This characteristic has been reported previously; it was proposed to involve the elimination of two internal ethylene oxide units in form of dioxane.102

The fourth PEG reference was H-[PEG]-OMe and the ESI-QIT CAD mass spectrum of its lithiated 11-mer at m/z 523 is shown in Fig. 6.11. The fragments observed in this spectrum are a convolution of those formed from the dihydroxy and dimethoxy derivatives. Thus, the series arising by nominal losses of 44 Da (m/z 215, 259, 303, 347,

391, 435, 479) and the sizable H2O (18 Da) loss are indicative of the HO chain end

(vide supra), while the series arising by nominal losses of 44n + 58 Da (m/z 289, 333,

377, 421, 465) and the sizable CH3OH (32 Da) loss are indicative of the OCH3 chain end

(vide supra). The intensities of the fragments formed by 44n + 18 and 44n + 32 Da losses are only significant for n = 0 and at or below noise level for n  1, in analogy with the trends described so far the other PEG samples analyzed.

110

185

-72

-44

-28

-18

97 113 123 141 157 167

60 80 100 120 140 160 180 m/z

+ Figure 6.9. ESI-QIT CAD mass spectrum of the Li adduct of H-[PEG]3-OEt at m/z 185, generated from a monoethyl PEG 200 standard (ethoxy and hydroxyl chain ends).

111

H L i

H O C H C H O C C H O C H C H O C H C H 2 2 x H 2 2 2 y 2 3

rH - C H C H O C H C H L i 2 2 y 2 3 O

H O C H 2 C H 2 O C H C H 3 a s in S c h e m e 6 .1 . x (rH - fro m th e s id e w ith + th e C H 2 C H 3 c h a in e n d ) r H

C H C H O C H C H + 4 4 n L i 2 2 y 2 3

O (O = C H C H 3 fo r n = 1 ; H O -C H 2 C H 2 -O -C H = C H 2 fo r n = 2 ) H m /z 9 7 , 1 4 1 (y = 1 -2 )

L i

H O C H C H O H + (4 4 n + 2 8 ) 2 2 x m /z 1 1 3 , 1 5 7 (x = 2 -3 )

L i

C H C H O C H C H O C H C H 2 2 2 y 2 3 + (4 4 n + 1 8 )

m /z 1 2 3 , 1 6 7 (y = 1 -2 )

Scheme 6.2. Charge-induced fragmentation pathway of [H-(PEG)-OEt + Li]+ to truncated linear fragments with HO/Et, HO/H, and vinyl-O/Et end groups. Interestingly, the series + [HO-(CH2CH2-O)-CH=CH2 + Li ] (HO/vinyl end groups; losses of 44n + 46 Da) is not observed.

112

273

-146

-102

-88

-58

-32

171 (-120) 127 215 241 153 185 (-76)

80 120 160 200 240 m/z

+ Figure 6.10. ESI-QIT CAD mass spectrum of the Li adduct of MeO-[PEG]5-OMe at m/z 273, generated from a dimethyl PEG 400 standard (methoxy groups at both chain ends).

113

H L i

H C O C H C H O C C H O C H C H O C H 3 2 2 x H 2 2 2 y 3

a s in S c h e m e 6 .1 .

L i

C H C H O C H H 3 C O C H 2 C H 2 O C H C H 2 L i 2 2 y 3 x O m /z 1 5 3 , 1 9 7 , 2 4 1 (x = 2 -4 ) H + m /z 1 2 7 , 1 7 1 , 2 1 5 (y = 2 -4 ) (4 4 n + 3 2 ) + (4 4 n + 5 8 )

Scheme 6.3. Charge-induced fragmentation pathway of [Me-(PEG)-OMe + Li]+ to truncated linear fragments with HO/Me and MeO/ vinyl end groups.

114

523

44n losses

-58

-32

-18

-234 -190 -146 -102

200 250 300 350 400 450 m/z

+ Figure 6.11. ESI-QIT CAD mass spectrum of the Li adduct of H-[PEG]11-OMe at m/z 523, generated from a monomethyl PEG 550 standard (methoxy and hydroxyl chain ends).

115 In addition, ESI-QIT CAD experiments were carried out on the starting polymer, distyryl poly(ethylene glycol) (St-PEG-OSt). Its fragmentation pattern is characteristic for styryl end groups, which can be present in the amphiphilic network pyrolyzates. The

ESI-QIT CAD mass spectrum of the St-[PEG]-OSt 5-mer (at m/z 493) is shown in Fig.

6.12. There are two major fragment series which are symbolized by  and  in the spectrum. The resulting fragments can be rationalized by the Na+-induced fragmentation pathway depicted in Scheme 6.4., which involves charge-catalyzed C-O bond cleavages at different positions of the PEG chain. Again, this mechanism is completely analogous to that operating with all other PEG oligomers studied and which was presented in detail in Scheme 6.1. The major series () appears at m/z 359, 315, 271, 227, and 183, and nominally results after (44n + 134)-Da losses from the precursor ion. The other series

() appears at m/z 377, 333, 289, and 245, and formally arises by (44n + 116)-Da losses.

Within both series, the smallest neutral losses (134 and 116 Da, respectively; n = 0) give the most abundant fragment ions. Such fragments (-116, -134 Da), thus, represent appropriate signature ions for styryl chain ends. Note that Na+ was used to ionize

St-[PEG]-St, because it led to more abundant precursor ions, compared to Li+ ionization, presumably because the prepolymer was contaminated with sodium salt during its synthesis and/or isolation.

Table 6.1. summarizes the signature ions resulting from the different PEG end groups discussed so far. All RO- groups except CH3CH2O- lead to sizable fragments via

HOR loss upon CAD. Ethoxy termini undergo C2H4 loss to yield –OH groups (Fig. 6.9. and Scheme 6.2.), which could lose H2O sequentially (Fig. 6.8. and Scheme 6.1.), for an overall apparent loss of 46 Da (isobaric with CH3CH2OH). Such a fragment was not

116 + present in the CAD spectrum of [HO-( CH2CH2O)3- CH2CH3 + Li] , but it appears in the

CAD spectra of other oligomers with Et end groups (vide infra). Internal energy effects can account for such discrepancies.

117

493 Na+

 359 -134

 245 315 377 227  271 289  -116 183  333     200 250 300 350 400 450 m/z

+ Figure 6.12. ESI-QIT CAD mass spectrum of the Na adduct of St-[PEG]5-OSt at m/z 493, generated from the distyryl PEG used in the synthesis of the membranes analyzed in this study.

118

N a

S t O C H C H O C H C H O C H C H O S t 2 2 x 2 2 2 2 y

r H -

N a

S t O C H C H O 2 2 x C H O C H C H O S t 2 2 y

C H 3

H r H + rN a +  S t O C H C H O + (4 4 n + 1 1 6 ) 2 2 x

N a H m /z 2 4 5 , 2 8 9 , 3 3 3 , 3 7 7 (x = 2 -5 ) S t O C H C H O 2 2 x N a  C H C H O C H C H O S t + (4 4 n + 1 3 4 ) C H C H O C H C H O S t 2 2 2 y 2 2 2 y m /z 1 8 3 , 2 2 7 , 2 7 1 , 3 1 5 , 3 5 9 (y = 0 -4 )

Scheme 6.4. Charge-induced fragmentation pathway of [St-(PEG)-OSt + Na]+ to truncated linear fragments with St/OH and vinyl/St end groups.

119

Table 6.1. Signature ions of different PEG end groups

End group Diagnostic losses '-OH 44n 18a

'-OCH2CH3 44n + 28 46b

'-OCH3 44n + 58 32c

'-OCH2-C6H4-CH=CH2 116 134

a Losses of 44n + 18 Da (n > 1) may also occur, but have generally much lower abundances b 3 Loss of C2H4 (28 Da) followed by loss of H2O (18 Da), as determined by MS

(vide infra). Losses of 44n + 46 (n > 1) may also be observed. c Losses of 44n + 32 (n > 1) may also be observed.

120 6.3.2. Tandem mass spectrometry of amphiphilic network pyrolyzates

ESI-QIT CAD experiments could be performed on pyrolyzates oligomers belonging to the three major PEG series A, B, and C. The goal of these experiments was to further characterize the distribution of end groups generated upon thermal degradation of the amphiphilic network and to verify the compositions suggested by the H/D exchange experiments. ESI-QIT CAD spectra of PDMS oligomers were also obtained.

Based on the m/z values of its oligomers, the sole PDMS distribution observed has no end groups and, thus, is most likely composed of macrocycles. In this case, the composition is evident from the mass-to-charge ratio, but CAD spectra were also measured to ascertain that no isobaric PEG (or PDMS) distributions are hidden under the rather intense macrocycle series.

The membrane pyrolyzates contain considerable amounts of Na+, originating mainly from the glass tubes, in which the pyrolyzates were performed. Therefore, Na+ adducts were used in the CAD experiments. Although Li+ adducts usually yield spectra with a better signal/noise ratio, the fragmentation patterns of lithiated and sodiated ions of the same oligomer are generally very similar.

6.3.2.1. Series A

The ESI-QIT CAD spectrum of the PEG oligomer at m/z 405 in series A of the amphiphic network pyrolyzates is shown in Fig. 6.13; this oligomer is sodiated and was produced from APN#1 after pyrolysis at 275 C. The sizable fragments at m/z 271

(loss of 134 Da) and 289 (loss of 116 Da) provide evidence that a substantial fraction of this oligomer (and series A) contains styryl-terminated PEG chains (Table 6.1.).

121 The fragments at m/z 333 and 377 (losses of 44n + 28), on the other hand, are indicative of the presence of ethyl chain ends. The dominant dissociation channel involves expulsion of 46 Da to generate the fragment at m/z 359. This fragment can be formed through the sequence m/z 405  m/z 377 (loss of C2H4)  m/z 359

(loss of C2H4 + H2O) or in one step, via elimination of CH3CH2OH. The occurrence of the consecutive process is corroborated by the MS3 spectrum of the m/z 377 fragment, which does contain a m/z 359 product (Fig. 6.14.). It cannot be excluded, however, that m/z 359 is also generated in one step (-CH3CH2OH). Either pathway is characteristic of an ethyl end group. CAD of the A oligomer at m/z 405 also causes an internal loss of

88 Da, which is particularly abundant in PEG ions with alkyl groups at both chain ends

(see spectrum of Me-[PEG]-OMe in Fig. 6.10.). Finally, fragment ions are observed at m/z 375 (-30 Da) and 345 (-60 Da), which are ascribed to –CH2CH=O (aldehyde) end groups, as will be explained later. Based on the CAD spectra in Figs. 6.13. and 6.14., m/z 405 and, hence, series A, is mainly composed of PEG oligomers with styryl and ethyl chain ends; the major constituents of m/z 405 appear to be St-O-(CH2CH2O)3-St ,

St-O-(CH2CH2O)5-Et and Et-O-(CH2CH2O)7-Et.

122

359

Na+

-46

-18 405

-30 (-72) (-134) (-116) 333 377 271 (-88) (-60) 289 317 345 -28

240 260 280 300 320 340 360 380 m/z

Figure 6.13. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 405 in PEG series A of the APN#1 pyrolyzates obtained at 275 C.

123

333

44n losses Na+

377

-44

-18 289 201 245 157 359 113

150 200 250 300 350 m/z

Figure 6.14. MS3 spectrum of the sodiated fragment at m/z 377 in the ESI-QIT CAD spectrum of the A series oligomer at m/z 405.

124 6.3.2.2. Series B

The ESI-QIT CAD spectrum of a PEG series B product (m/z 465) from APN # 1 pyrolysis at 275 C is shown in Fig. 6.15. The fragment at m/z 331 (-134 Da) and 349

(-116 Da) provide evidence for the presence of PEG chains with styryl end groups in this series (Table 6.1.). Conversely, the losses of H2O (18 Da) and C2H4O (44 Da) unveil the presence of hydroxyl end groups (Table 6.1.). The remaining fragments, viz. m/z 393 and

437 (-44n + 28 Da), and m/z 419 (-46 Da), are characteristic for ethyl chain ends

(Table 6.1.). Overall, the CAD characteristic of the m/z 465 precursor ion agree well with a mixture of H-(O-CH2CH2)7-O-St and H-(O-CH2CH2)9-O-Et. No fragment diagnostic of the isobaric/isomeric oligomer Me-(O-CH2CH2)9-O-Me (Table 5.1.) is detected. The

CAD results substantiate the H/D exchange finding that the constituents of series B must possess one exchangeable proton, because their masses increase by +1 Da after deuteration.

It is noticed that the elimination of water is significantly more abundant than the elimination of C2H4O. The reverse is true for the reference compounds investigated, as, for example, attested by the CAD spectra in Figs. 6.9. and 6.11. This difference in branching ratio is attributed to internal energy effects, which depend on the size and substituents of the oligomer under study.

125 6.3.2.3. Series C

The ESI-QIT CAD spectrum of the sodiated series C oligomer at m/z 259 is shown in Fig. 6.16. The 300 C-pyrolyzates of APN#1 were used in this experiment, because they appeared to contain higher concentrations of C than pyrolyzates at other temperatures. Three isobaric compositions are possible for series C, St-[PEG]-O-Me,

Me-[PEG]-O-Et, and H-[PEG]-O-CH2CH=O (Table 5.1.). The signature ions for styryl end groups (see Table 6.1.) are minuscule, pointing out that the amount of

St-[PEG]-O-Me oligomers in distribution C is very small. The fragments at m/z 113, 157, and 201 arise by nominal losses of 44n + 58 Da, which are characteristic for Me chain ends (Table 6.1.). Me end groups also lead to 44n + 32 Da losses; indeed, such losses can be detected in the CAD spectrum at m/z 227 (trace), 183, and 139 (trace). The presence of ethyl chain ends is indicated, on the other hand, by the fragments at m/z 143, 187, and

231 (losses of 44n + 28 Da) and their dehydration products at m/z 125 (trace), 169, and

213 (nominal losses of 44n + 46).

The H/D exchange experiments revealed that a significant fraction of C oligomers is deuterated, which agrees well with the composition H-[PEG]-O-CH2CH=O.

A reference spectrum for HO/aldehyde end groups is not available. Using a mechanism analogous to the ones operating with the other end groups encountered so far, the likely fragments from a combination of HO/aldehyde end groups can be predicted, as depicted in Scheme 6.5. Carbonyl end groups have higher metal ion binding affinities than or ether groups.103

126

O C H H N a C H 2 H O C H C H O C H C H O C H C H O 2 2 x 2 2 2 y

(fro m s id e re m o te (fro m s id e p ro x im a te - - rH r H to C H 2 C H O ) to C H 2 C H O ) O C H N a C H O C H 2 O C H 2 C H 2 O C H 2 y

C H 3 C H O C H 2 C H 2 O H O C H C H O C H C H y 2 2 x 3

H O C H 2 C H 2 O N a x rH + r N a + r H + r N a +

H O C H C H O C H C H 2 2 x 2 O C H N a N a C H 2

H O C H C H O H O C H 2 C H 2 O 2 2 x y H + (4 4 n + 8 6 ) m /z 1 2 9 , 1 7 3 (x = 2 -3 )

O C H O C H N a N a C H C H 2 2

C H C H O C H C H O H O C H 2 C H 2 O C H C H 2 O C H 2 C H 2 O 2 2 2 y x y H m /z 1 5 3 , 1 9 7 (y = 1 -2 ) N a m /z 1 2 7 , 1 7 1 , 2 1 5 (y = 1 -3 ) + (4 4 n + 6 0 ) m /z 1 5 5 , 1 9 9 (x = 2 -3 )

+ Scheme 6.5. Charge-induced fragmentation pathway of [H-(PEG)-OCH2CH=O + Na] to truncated linear fragments with HO/vinyl and HO/aldehyde end groups.

127 Therefore, the aldehyde chain end is probably involved in the coordination of the Na+ ion, facilitating the breakup of C-O bonds at the sides remote to the C=O group, as shown at the right part of Scheme 6.5. These cleavages give rise to losses of 44n Da (m/z 127,

171, 215) as well as 44n + 60 Da (m/z 155, 199), which diagnose OH (Table 6.1.) and

OCH2CH=O end groups, respectively. The losses expected from cleavage of the C-O bonds at the sides proximate to the C=O group, viz. 44n + 86 Da (m/z 129, 173) and

44n + 18 Da (m/z 153, 197), see left side of Scheme 6.5. are discriminated against due to the mentioned involvement of the C=O terminus in the metal ion complexation.

The two most abundant CAD products from the C oligomer at m/z 259 are observed at m/z 229 and 185 and correspond to losses of 30 and 44+30 = 74 Da, respectively. These fragments are rationalized via pericyclic reactions in Scheme 6.6.

Activation of the carbonyl carbon by Na+ is proposed to facilitate hydride transfer to this atom with simultaneous C-C and C-O bond cleavages to yield a trimeric complex, in

+ which Na is bound to two O=CH2 ligands and a truncated, vinyl-terminated PEG ligand.

Elimination of a O=CH2 ligand gives rise to the major fragment at m/z 229. Subsequent loss of O=CHCH3, via a pericyclic reaction at the vinyl chain end, yields the second most abundant fragment at m/z 185.

128

N a O C H H R O C H C H O C H C H 2 2 2 2 C H 2 O r H - C H 2 O C H 2 N a O

-2 O C H 2 R O C H 2 C H 2 O 2 (6 0 D a ) m /z 1 9 9

- O C H 2 (3 0 D a )

R O R O O -O C H C H 3 C H 2 C H 2 C H 2 C H 2 C H (4 4 D a ) C H 2 C H C H 2 C H C H 2 O O H N a N a O C H 2 O C H 2 m /z 1 8 5 m /z 2 2 9

-O C H C H 3 (4 4 D a )

m /z 1 4 1

Scheme 6.6. Charge-induced hydride transfer and consecutive charge-remote pericycle rearrangements, giving rise to the losses of 30 Da (O=CH2) and 44n + 30 Da + (n x O=CHCH3 + O=CH2) from [H-(PEG)-OCH2CH=O + Na] oligomers.

129 The pericyclic reaction regenerates a vinyl end group which can undergo anew this dissociation if enough internal energy is still available. For the C precursor ion at m/z 259, up to two consecutive O=CHCH3 losses are observed (m/z 185 and 141).

The trimeric intermediate may alternatively lose both O=CH2 ligands, providing an additional route to m/z 199 (see Scheme 6.5. and 6.6.). The fragmentation pattern of m/z 259, which is duplicated with other oligomers of series C, is consistent with a mixture of mainly Me-[PEG]-OEt and H-[PEG]-OCH2CH=O chains. For the oligomer at m/z 259, the corresponding compositions are CH3-(O-CH2CH2)4-OCH2CH3 and

H-(O-CH2CH2)4-OCH2CH=O, respectively.

130

465 Na+

(-18) (-46) 447 419

(-44) (-134) (-72) 421 (-116) 393 331 349 (-28)

275 300 325 350 375 400 425 450 m/z

Figure 6.15. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 465 in PEG series B of the APN#1 pyrolyzates obtained at 275 C.

131

259  44n + 58 losses (Me) +  44n + 32 losses (Me) Na  44n + 28 losses (Et)  44n + 46 losses (Et)  44n losses (OH) 229 44n + 60 losses (aldehyde) 44n + 30 losses (aldehyde)

  143 171     185  227 231  125  157 199 213 113     

100 120 140 160 180 200 220 240 m/z

Figure 6.16. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 259 in PEG series C of the APN#1 pyrolyzates obtained at 300 C.

132 6.3.3. PDMS distribution

+ The ESI-QIT CAD spectrum of [(C6H2OSi)15 + Na] (m/z 1133.3), depicted in

Fig. 6.17., shows a series of 74-Da losses only. Such fragmentation behavior is consistent with the elimination of cyclic (DMS)n units from cyclic PDMS precursor ions to form smaller sodiated macrocycles, as rationalized in Scheme 6.7. Based on the CAD spectrum, the cyclic PDMS distribution generated upon pyrolysis does not overlap measurably with any of the coproduced PEG distributions. Note that the fragmentation mechanism proposed in Scheme 6.7. is very similar to the one postulated to explain the pyrolysis products of PDMS (Scheme 4.1.).

6.3.4. Summary of MS/MS data

The ESI-QIT experiments described confirm the cyclic structure of the PDMS distribution and help to identify the major PEG distribution generated upon mild pyrolysis of the amphiphilic membrane. The major constituents of PEG distribution A are

St-[PEG]-OEt, St-[PEG]-OSt and Et-[PEG]-OEt oligomers; the major constituents of distribution B are H-[PEG]-OSt and H-[PEG]-OEt oligomers; and the major constituents of distribution C are Me-[PEG]-OEt and H-[PEG]-OCH2CH=O oligomers. The PEG strands of the membrane are found to preferentially dissociate at their C-O and crosslinking Si-C bonds to produce PEG chains with styryl/ethyl, styryl/OH, ethyl/ethyl, and ethyl/OH end groups.

The PDMS distribution detected in the pyrolyzates suggests that the favored dissociation channel of the PDMS strands of the membrane proceeds through a four-membered ring transition state, similar to that shown in Scheme 4.1. and 6.7.

133 This degradation reaction leads to the observed PDMS macrocycles but, unfortunately, it does not provide any information on the vinyl end groups, with which the V-PDMS-V prepolymer was attached to the crosslinker.

134

1133 Na+

(-74) 1059

(-2 x74) (-3 x74) (-6 x74) (-5 x74) (-4 x74) (-7 x74) 985 (-8 x74) 689 763 837 911 541 615

600 700 800 900 1000 1100 m/z

Figure 6.17. ESI-QIT CAD spectrum of the sodiated PDMS oligomer at m/z 1133.7 in the pyrolyzates from APN#1 at 275 C.

135

N a

N a S i O x

S i O O S i x

O S i S i O

S i O + O S i

S i O O S i

O S i y S i O (7 4 n ) O S i y

Scheme 6.7. Charge-remote fragmentation of cyclic PDMS oligomers, ionized by Na+ attachment, leading to smaller sodiated PDMS macrocycles by the expulsion of cyclic PDMS units via a four-membered ring transition state.

136 6.3.2.2. Series B

The ESI-QIT CAD spectrum of a PEG series B product (m/z 465) from APN # 1 pyrolysis at 275 C is shown in Fig. 6.15. The fragment at m/z 331 (-134 Da) and 349

(-116 Da) provide evidence for the presence of PEG chains with styryl end groups in this series (Table 6.1.). Conversely, the losses of H2O (18 Da) and C2H4O (44 Da) unveil the presence of hydroxyl end groups (Table 6.1.). The remaining fragments, viz. m/z 393 and

437 (-44n + 28 Da), and m/z 419 (-46 Da), are characteristic for ethyl chain ends

(Table 6.1.). Overall, the CAD characteristic of the m/z 465 precursor ion agree well with a mixture of H-(O-CH2CH2)7-O-St and H-(O-CH2CH2)9-O-Et. No fragment diagnostic of the isobaric/isomeric oligomer Me-(O-CH2CH2)9-O-Me (Table 5.1.) is detected. The

CAD results substantiate the H/D exchange finding that the constituents of series B must possess one exchangeable proton, because their masses increase by +1 Da after deuteration.

It is noticed that the elimination of water is significantly more abundant than the elimination of C2H4O. The reverse is true for the reference compounds investigated, as, for example, attested by the CAD spectra in Figs. 6.9. and 6.11. This difference in branching ratio is attributed to internal energy effects, which depend on the size and substituents of the oligomer under study.

125 6.3.2.3. Series C

The ESI-QIT CAD spectrum of the sodiated series C oligomer at m/z 259 is shown in Fig. 6.16. The 300 C-pyrolyzates of APN#1 were used in this experiment, because they appeared to contain higher concentrations of C than pyrolyzates at other temperatures. Three isobaric compositions are possible for series C, St-[PEG]-O-Me,

Me-[PEG]-O-Et, and H-[PEG]-O-CH2CH=O (Table 5.1.). The signature ions for styryl end groups (see Table 6.1.) are minuscule, pointing out that the amount of

St-[PEG]-O-Me oligomers in distribution C is very small. The fragments at m/z 113, 157, and 201 arise by nominal losses of 44n + 58 Da, which are characteristic for Me chain ends (Table 6.1.). Me end groups also lead to 44n + 32 Da losses; indeed, such losses can be detected in the CAD spectrum at m/z 227 (trace), 183, and 139 (trace). The presence of ethyl chain ends is indicated, on the other hand, by the fragments at m/z 143, 187, and

231 (losses of 44n + 28 Da) and their dehydration products at m/z 125 (trace), 169, and

213 (nominal losses of 44n + 46).

The H/D exchange experiments revealed that a significant fraction of C oligomers is deuterated, which agrees well with the composition H-[PEG]-O-CH2CH=O.

A reference spectrum for HO/aldehyde end groups is not available. Using a mechanism analogous to the ones operating with the other end groups encountered so far, the likely fragments from a combination of HO/aldehyde end groups can be predicted, as depicted in Scheme 6.5. Carbonyl end groups have higher metal ion binding affinities than alcohol or ether groups.103

126

O C H H N a C H 2 H O C H C H O C H C H O C H C H O 2 2 x 2 2 2 y

(fro m s id e re m o te (fro m s id e p ro x im a te - - rH r H to C H 2 C H O ) to C H 2 C H O ) O C H N a C H O C H 2 O C H 2 C H 2 O C H 2 y

C H 3 C H O C H 2 C H 2 O H O C H C H O C H C H y 2 2 x 3

H O C H 2 C H 2 O N a x rH + r N a + r H + r N a +

H O C H C H O C H C H 2 2 x 2 O C H N a N a C H 2

H O C H C H O H O C H 2 C H 2 O 2 2 x y H + (4 4 n + 8 6 ) m /z 1 2 9 , 1 7 3 (x = 2 -3 )

O C H O C H N a N a C H C H 2 2

C H C H O C H C H O H O C H 2 C H 2 O C H C H 2 O C H 2 C H 2 O 2 2 2 y x y H m /z 1 5 3 , 1 9 7 (y = 1 -2 ) N a m /z 1 2 7 , 1 7 1 , 2 1 5 (y = 1 -3 ) + (4 4 n + 6 0 ) m /z 1 5 5 , 1 9 9 (x = 2 -3 )

+ Scheme 6.5. Charge-induced fragmentation pathway of [H-(PEG)-OCH2CH=O + Na] to truncated linear fragments with HO/vinyl and HO/aldehyde end groups.

127 Therefore, the aldehyde chain end is probably involved in the coordination of the Na+ ion, facilitating the breakup of C-O bonds at the sides remote to the C=O group, as shown at the right part of Scheme 6.5. These cleavages give rise to losses of 44n Da (m/z 127,

171, 215) as well as 44n + 60 Da (m/z 155, 199), which diagnose OH (Table 6.1.) and

OCH2CH=O end groups, respectively. The losses expected from cleavage of the C-O bonds at the sides proximate to the C=O group, viz. 44n + 86 Da (m/z 129, 173) and

44n + 18 Da (m/z 153, 197), see left side of Scheme 6.5. are discriminated against due to the mentioned involvement of the C=O terminus in the metal ion complexation.

The two most abundant CAD products from the C oligomer at m/z 259 are observed at m/z 229 and 185 and correspond to losses of 30 and 44+30 = 74 Da, respectively. These fragments are rationalized via pericyclic reactions in Scheme 6.6.

Activation of the carbonyl carbon by Na+ is proposed to facilitate hydride transfer to this atom with simultaneous C-C and C-O bond cleavages to yield a trimeric complex, in

+ which Na is bound to two O=CH2 ligands and a truncated, vinyl-terminated PEG ligand.

Elimination of a O=CH2 ligand gives rise to the major fragment at m/z 229. Subsequent loss of O=CHCH3, via a pericyclic reaction at the vinyl chain end, yields the second most abundant fragment at m/z 185.

128

N a O C H H R O C H C H O C H C H 2 2 2 2 C H 2 O r H - C H 2 O C H 2 N a O

-2 O C H 2 R O C H 2 C H 2 O 2 (6 0 D a ) m /z 1 9 9

- O C H 2 (3 0 D a )

R O R O O -O C H C H 3 C H 2 C H 2 C H 2 C H 2 C H (4 4 D a ) C H 2 C H C H 2 C H C H 2 O O H N a N a O C H 2 O C H 2 m /z 1 8 5 m /z 2 2 9

-O C H C H 3 (4 4 D a )

m /z 1 4 1

Scheme 6.6. Charge-induced hydride transfer and consecutive charge-remote pericycle rearrangements, giving rise to the losses of 30 Da (O=CH2) and 44n + 30 Da + (n x O=CHCH3 + O=CH2) from [H-(PEG)-OCH2CH=O + Na] oligomers.

129 The pericyclic reaction regenerates a vinyl end group which can undergo anew this dissociation if enough internal energy is still available. For the C precursor ion at m/z 259, up to two consecutive O=CHCH3 losses are observed (m/z 185 and 141).

The trimeric intermediate may alternatively lose both O=CH2 ligands, providing an additional route to m/z 199 (see Scheme 6.5. and 6.6.). The fragmentation pattern of m/z 259, which is duplicated with other oligomers of series C, is consistent with a mixture of mainly Me-[PEG]-OEt and H-[PEG]-OCH2CH=O chains. For the oligomer at m/z 259, the corresponding compositions are CH3-(O-CH2CH2)4-OCH2CH3 and

H-(O-CH2CH2)4-OCH2CH=O, respectively.

130

465 Na+

(-18) (-46) 447 419

(-44) (-134) (-72) 421 (-116) 393 331 349 (-28)

275 300 325 350 375 400 425 450 m/z

Figure 6.15. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 465 in PEG series B of the APN#1 pyrolyzates obtained at 275 C.

131

259  44n + 58 losses (Me) +  44n + 32 losses (Me) Na  44n + 28 losses (Et)  44n + 46 losses (Et)  44n losses (OH) 229 44n + 60 losses (aldehyde) 44n + 30 losses (aldehyde)

  143 171     185  227 231  125  157 199 213 113     

100 120 140 160 180 200 220 240 m/z

Figure 6.16. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 259 in PEG series C of the APN#1 pyrolyzates obtained at 300 C.

132 6.3.3. PDMS distribution

+ The ESI-QIT CAD spectrum of [(C6H2OSi)15 + Na] (m/z 1133.3), depicted in

Fig. 6.17., shows a series of 74-Da losses only. Such fragmentation behavior is consistent with the elimination of cyclic (DMS)n units from cyclic PDMS precursor ions to form smaller sodiated macrocycles, as rationalized in Scheme 6.7. Based on the CAD spectrum, the cyclic PDMS distribution generated upon pyrolysis does not overlap measurably with any of the coproduced PEG distributions. Note that the fragmentation mechanism proposed in Scheme 6.7. is very similar to the one postulated to explain the pyrolysis products of PDMS (Scheme 4.1.).

6.3.4. Summary of MS/MS data

The ESI-QIT experiments described confirm the cyclic structure of the PDMS distribution and help to identify the major PEG distribution generated upon mild pyrolysis of the amphiphilic membrane. The major constituents of PEG distribution A are

St-[PEG]-OEt, St-[PEG]-OSt and Et-[PEG]-OEt oligomers; the major constituents of distribution B are H-[PEG]-OSt and H-[PEG]-OEt oligomers; and the major constituents of distribution C are Me-[PEG]-OEt and H-[PEG]-OCH2CH=O oligomers. The PEG strands of the membrane are found to preferentially dissociate at their C-O and crosslinking Si-C bonds to produce PEG chains with styryl/ethyl, styryl/OH, ethyl/ethyl, and ethyl/OH end groups.

The PDMS distribution detected in the pyrolyzates suggests that the favored dissociation channel of the PDMS strands of the membrane proceeds through a four-membered ring transition state, similar to that shown in Scheme 4.1. and 6.7.

133 This degradation reaction leads to the observed PDMS macrocycles but, unfortunately, it does not provide any information on the vinyl end groups, with which the V-PDMS-V prepolymer was attached to the crosslinker.

134

1133 Na+

(-74) 1059

(-2 x74) (-3 x74) (-6 x74) (-5 x74) (-4 x74) (-7 x74) 985 (-8 x74) 689 763 837 911 541 615

600 700 800 900 1000 1100 m/z

Figure 6.17. ESI-QIT CAD spectrum of the sodiated PDMS oligomer at m/z 1133.7 in the pyrolyzates from APN#1 at 275 C.

135

N a

N a S i O x

S i O O S i x

O S i S i O

S i O + O S i

S i O O S i

O S i y S i O (7 4 n ) O S i y

Scheme 6.7. Charge-remote fragmentation of cyclic PDMS oligomers, ionized by Na+ attachment, leading to smaller sodiated PDMS macrocycles by the expulsion of cyclic PDMS units via a four-membered ring transition state.

136 6.4. Degradation efficiencies

The pyrolysis results at various temperatures provide information about the degradation efficiencies of the PDMS and PEG starting substances as well as the amphiphilic networks. This topic is discussed in this section.

6.4.1. Divinyl polydimethylsiloxane

The MALDI mass spectra of divinyl-PDMS pyrolyzed at 150, 200, and 300 C are shown in Fig. 6.18. These three temperatures were selected to illustrate the changing of the PDMS distribution after the polymer was heated up. All spectra are plotted over the same mass range (600-15,000 m/z) and in the same abundance scale for easy comparison. At the low temperature (150C), the pyrolyzates produced PDMS ions up to

7000 Da. When the temperature was increased to 300 C, PDMS ions were detected up to nearly 5500 Da. In addition, the PDMS distributions observed shift to lower masses. The

PDMS pyrolysis contain two products which have truncated divinyl PDMS and cyclic structures. The changes with temperature are caused by further fragmentation as the quantity of heat supplied to the system is increased. The expanded traces of the spectra of the pyrolyzates reveal how the relative proportions of the two distributions vary with temperature.

137 150 C A A A A A A A

B B B B B B

2100 2200 2300 2400 m/z

A 200 C A A A A A A

B B B B B B

2100 2200 2300 2400 m/z

m/z

300 C A

A A A A A A B B B B B B

2100 2200 2300 2400 m/z

1000 5000 10000 15000 m/z

Figure 6.18. MALDI-ToF mass spectra of V-PDMS -V after pyrolysis at 150, 200, and 300 C. The inserts show an expanded view of the m/z 2000 -2500 region. A and B denote V-PDMS-V and cyclic PDMS oligomers, respectively. All ions are Na+ adducts. 138 6.4.2. Distyryl poly(ethylene glycol)

Full MALDI-ToF mass spectra of the distyryl-PEG pyrolysis products, series

A-H, were shown and discussed in detail in chapter V. The expanded traces

(m/z 1192-1296 region) of the spectra, depicted in Fig. 6.19., illustrate how the relative proportion of distributions A-H change when the pyrolysis temperature is varied from

75 to 250 C. Substantial variations in the relative peak intensities are observed. Overall, the fraction of series A, which includes the initial, intact distyryl-PEG oligomers, is reduced because of homolytic degradation processes, as described in Scheme 5.1. These processes enhance the concentrations of the other distributions at the expense of distribution A. Nevertheless, the fraction of A does not decline dramatically, because thermal degradation leads partly to pyrolyzates that are isobaric with the starting material

(see Table 5.1.). The high-resolution data presented in chapter V identified

St-[PEG]-O-Et, St-[PEG]-OH, and St-[PEG]-OMe as the major components of series A,

B, and C, respectively, in the pyrolyzates obtained from distyryl-PEG. The former two result from C-O bond cleavages, and the latter from C-C bond cleavages. At the lowest temperature (75 C), B is the major product, but the yield of C increases dramatically at the higher temperatures. These trends suggest that C-O bond cleavages in the PEG chain are kinetically favored over C-C bond cleavages, as also found by Lattimer.51

139 75 C A A A

B B

F F F G D G D C G C

200 C A A A

F B F B F

G G C C D G D

250 C A B

A B A F C D C F D G G F G

1200 1216 1232 1248 1264 1280 m/z

Figure 6.19. MALDI-ToF mass spectra of the products from St-PEG-St pyrolysis at 75, 200, and 250 C (top to bottom). The relative abundances of series A-G are (top to bottom): A (43%, 28%, 26%); B (23%, 21%, 24%); C (5%, 11%, 13%); D (6%, 10%, 12%); F (15%, 21%, 15%); G (8%, 10%, 10%). The abundances of series F were not corrected for the contamination by the double-13C isotope of series A and, thus, are upper limits.

140 6.4.3. Amphiphilic network APN#1

The MALDI-ToF mass spectra of APN#1 pyrolyzates are shown in Figs. 6.20 and

6.21. All spectra were plotted over the same mass range (up to 4000 Da, approximately) and with the same intensity scale for easy comparison. In Fig. 6.20. the PDMS distribution does not change considerably, but the PEG distribution does; it shifts to low masses (from 200 to 250 C), and additional pyrolysis products are observed with increased intensities due to further fragmentation. In Fig. 6.21. (300 and 350 C), the relative abundances of low-mass PEG oligomers increase significantly and the tail of

PEG chains with higher masses is substantially diminished because of a more extensive thermal degradation of the copolymer. At the highest temperature, even the PDMS distribution shifts slightly to lower masses. Apparently, the cyclic oligomers can condense to smaller macrocycles via a dissociation mechanism similar to the one presented in Scheme 6.7. (without Na+). Since the PDMS pyrolyzates from APN#1, which are cyclic, are less sensitive to temperature variations than the V-PDMS-V prepolymers, the activation energy for their degradation via Scheme 6.7. must be associated with a higher activation energy than the degradation of linear PDMS chains via Scheme 4.1.

141 200 C, 30 min

800 900 1000 1100 m/z

400 800 1200 1600 2000 2400 2800 3200 3600 m/z

250 C, 30 min

800 900 1000 1100 m/z

600 1000 1400 1800 2200 2600 3000 3400 m/z

Figure 6.20. MALDI-ToF mass spectra of the APN#1 pyrolyzates obtained at 200 and 250 C.

142 300 C, 30 min

800 900 1000 1100 m/z

200 600 1000 1400 1800 2200 2600 3000 3400 3800 m/z

350 C, 30 min

800 900 1000 1100 m/z

500 1000 1500 2000 2500 3000 3500 4000 m/z

Figure 6.21. MALDI-ToF mass spectra of the APN#1 pyrolyzates obtained at 300 and 350 C.

143 6.4.4. Comparison of amphiphilic networks APN#1 and APN#2

APN#1 and APN#2 were synthesized from the same prepolymers, using the same amount of crosslinker, but different weight ratios of St-PEG-St and V-PDMS-V. The

PDMS/PEG weight ratio was 2.00 for APN#1 and 1.57 for APN#2 (Table 3.3.). The

MALDI-ToF mass spectra of the 350C-pyrolyzates from APN#1 and APN#2 are depicted in Fig. 6.22. The proportion of PDMS oligomers is detectably larger in the

MALDI-ToF spectrum of APN#1 which was synthesized using more V-PDMS-V.

Hence, pyrolysis MS can be used to distinguish amphiphilic membranes prepared from slightly different prepolymers compositions. Note, however, that mass spectral abundances reflect molar, not weight ratios. Based on the Mn values of the prepolymers

(Table 3.3.), the PDMS/PEG molar ratios are approximately 0.4 and 0.3 for APN#1 and

APN#2, respectively. The latter ratios are quite similar with the proportions of PDMS vs.

PEG oligomers detected in Fig. 6.22. Nevertheless, it must be cautioned that a reliable quantitative MALDI-MS analysis presupposes calibration of the abundance scale using

PDMS/PEG mixtures of known composition, as ionization and detection efficiencies depend on the type and size of polymer analyzed. Fig. 6.23. shows expanded traces of

Fig. 6.22., covering part of the PEG distributions observed after pyrolysis of APN#1 and

APN#2. These spectra are essentially indistinguishable, as expected, since the same

St-PEG-St was incorporated in both membranes.

144

APN#1: 350 C, 30 min

500 1000 1500 2000 2500 3000 3500 4000 m/z

APN#2: 350 C, 30 min

500 1000 1500 2000 2500 3000 3500 4000 m/z

Figure 6.22. MALDI-ToF mass spectra of the 350 C-pyrolyzates from amphiphilic networks #1 and #2. See Table 3.3. for the network compositions.

145 a.i. APN#1: 350 C, 30 min 22000

20000 A A

18000 B B 16000 C F C 14000

12000 E F E D 10000

8000 D H G 6000 H G 4000

2000

8 820 840 860 m/z 0 a.i. A APN#2: 350 C, 30 min 22000 B A 20000 C F C 18000 B 16000 D

14000 E F E 12000 D 10000 H 8000 G H G 6000

4000

2000

800 820 840 860 m/z

Figure 6.23. Partial MALDI-ToF mass spectra of the 350 C-pyrolyzates from amphiphilic networks #1 and #2. The displayed m/z region shows the different PEG distributions formed after pyrolysis. 146 6.5. Conclusion

The mass spectra of the residue from pyrolysis of the amphiphilic network were complex, containing eight series of PEG pyrolyzates, A-H, and one PDMS series, cyclo-[PDMS], which confirmed that the amphiphilic network contained the desired PEG and PDMS strands. Ions were observed up to m/z ~7000. ESI-QIT-MSn spectra of select, low-mass amphiphilic network pyrolyzates reveal the following major PEG products;

A, St-[PEG]-OSt, St-[PEG]-OEt and Et-[PEG]-OEt; B, H-[PEG]-OSt and H-[PEG]-OEt;

C, Me-[PEG]-OEt and H-[PEG]-OCH2CH=O. The pyrolyzates possibly arise by free-radical degradation mechanisms initiated by Si-C, C-O, and C-C bond cleavages in the PEG strands of the network, preferentially Si-C and C-O bond cleavages. PD5 oligomers (crosslinker) were not detected, even when pyrolysis was done at 350C and the polymer network turned fairly dark. It is proposed that D5H preferably crosslinks with the PEG and PDMS prepolymers than with itself and/or that PD5 is difficult to degrade.

The degree of degradation of pure PEG increases substantially with temperature, but not that of PDMS. Obviously, free PDMS polymers or embedded PDMS strands are thermally much more stable than PEG polymers or PEG stands. The thermal stability of

PEG increases markedly after incorporation into the membrane.

147 CHAPTER VII

SUMMARY

The prepolymers used to synthesize an amphiphilic membrane, St-PEG-St and

V-PDMS-V, have been pyrolyzed at mild temperatures ( 350 C). The dominant products observed at the lowest temperature are the linear St-[PEG]-St and V-[PDMS]-V precursors. In the PDMS pyrolyzates, only two products were observed at all temperatures, short linear V-[PDMS]-V (majority) and cyclo-[PDMS]. The distribution of PDMS pyrolyzates did not shift to lower masses significantly when temperature increased. The cyclic oligomers and short linear V-[PDMS]-V chains are produced by siloxane bond rearrangement. The MALDI mass spectrum of non pyrolyzed St-PEG-St contains only one product which is St-PEG-St itself. This certifies that the prepolymers was pure and that no fragments are created during the mass spectrometry experiments.

In the PEG pyrolyzates, several products were identified (series A-H):A, St-[PEG]-OSt,

St-[PEG]-OEt, Et-[PEG]-OEt, and Me-[PEG]-OCH2CHO; B, H-[PEG]-OSt,

H-[PEG]-OEt, and Me-[PEG]-OMe; C, St-[PEG]-OMe, Me-[PEG]-OEt, and

H-[PEG]-O-CH2CHO; D, St-[PEG]-OCH2CHO, Et-[PEG]-O-CH2CHO, and

H-[PEG]-OCH=CH2; E, H-[PEG]-OH; F, H-[PEG]-OMe; G, St-[PEG]-OCH=CH2 and

Et-[PEG]-OCH=CH2; H, HCOCH2-[PEG]-O-CH2CHO and Me-[PEG]-OCH=CH2.

148 PEG distributions shift to lower masses when temperatures rise as a result of increased thermal degradation. Some of the PEG pyrolyzates could be eliminated by deuterium exchange and high mass accuracy/high resolution experiments carried out on a FT-ICR mass spectrometer, as well as on the basis of tandem mass spectrometry studies conducted on an ESI-QIT mass spectrometer.

From the high resolution/high mass accuracy experiments on the St-PEG-St pyrolyzates, series A from this prepolymer contains styryl and ethyl terminating groups,

St-[PEG]-O-Et; series B is mainly composed of PEG chains with styryl and hydroxyl terminating groups, H-[PEG]-O-St; and series C is mainly St-[PEG]-O-Me which has styryl and methyl terminating ends.

CAD mass spectra of several PEG reference compounds were acquired using

ESI-QIT mass spectrometry in order to determine the decompositions characteristics of ethyl-, methyl-, hydroxyl-, and styryl- chain ends. With the help of such reference CAD spectra and H/D exchange experiments, the PEG components of the amphiphilic network pyrolyzates are mainly: A, St-[PEG]-OSt, St-[PEG]-OEt and Et-[PEG]-OEt;

B, H-[PEG]-OSt and H-[PEG]-OEt; C, Me-[PEG]-OEt and H-[PEG]-OCH2CH=O; D,

Et-[PEG]-OCH2CHO; E, H-[PEG]-OH; and F, H-[PEG]-OMe. Series G and H had too small abundances to allow for MS/MS experiments or meaningful conclusions from H/D exchange experiments; based on their m/z values, they could be: G, St-[PEG]-OCH=CH2 and Et-[PEG]-OCH=CH2; H, HCOCH2-[PEG]-O-CH2CHO and Me-[PEG]-OCH=CH2.

The MALDI mass spectra of the amphiphilic network pyrolysis products contain eight series of PEG pyrolyzates and a cyclic PDMS series. This result confirms that amphiphilic networks contained PEG and PDMS chains.

149 The relative intensities of the PEG and PDMS distributions are proportional to the relative amounts of PEG and PDMS prepolymers used in the network synthesis; hence, the spectra can also serve as a probe of the feed from which the membranes were prepared.

Attempts to pyrolyze an authentic PD5 polymer (polymeric D5H) under the conditions employed for the amphiphilic membranes were unsuccessful. Detectable products were obtained only if pyrolysis was conducted at high temperature (300 C) for a prolonged time (45-60 minutes). The resulting MALDI-ToF mass spectra, shown in the

Appendix, contain two PDMS distributions, 14 Da apart from each other. The minor distribution corresponds to PDMS macrocycles (no end groups), and the major to PDMS chains with end groups of 74n + 14 Da or macrocycles with an additional CH2 moiety. It is unclear whether these products were formed from PD5 units or whether they originate from an impurity in the PD5 polymer. Since PD5 gave no observable products under the membrane pyrolysis conditions, such domains are probably quite stable thermally. It is also conceivable that D5H crosslinks quickly with St-PEG-St and V-PDMS-V, leaving no free Si-H for crosslinking with itself.

Overall, this study accomplished the first direct spectroscopic characterization of the chemical composition of a complex copolymer assembly. At the same time it provided insight about the thermal stability and degradation products of the assembly, which should be valuable in investigations of its applications.

150 REFERENCES

1. Polymer Network, Structure and Mechanical Properties, edited by A. J. Chompff and S. Newman, 1971, Plenum press.

2. Polymer Networks : Principles of Their Formation, Structure, and Properties, edited by R.F.T Stepto, 1998, Blackie Academic & Professional.

3. Experimental and Theoretical Investigation of Complex Polymer Structures, 2000, The European Science Foundation

4. Amphiphilic Block Copolymers, Self-Assembly and Applications, edited by Paschalis Alexandria and Bjorn Lindman, 2000, Elsevier.

5. Cationic Polymerization, Fundamentals and Applications, edited by Rudolf Faust and Timothy D. Shaffer, 1997, American Chemical Society

6. Kurian. P.; Kennedy. J. P. “Novel Tricomponent Membranes Containing Poly(ethyleneglycol)/Poly(pentamethylcyclopentasiloxane)/Poly(dimethylsiloxane) Domains” J. Appl. Polym. Sci. 2002, 40, 3093-3102.

7. Isayeva, I. S.; Kennedy. J. P. “Amphiphilic Membranes Crosslinked and Reinforced by POSS” J. Polym. Sci. 2004, 42, 4337-4352.

8. Erdodi. G.; Kennedy. J. P. “Ideal Tetrafunctional Amphiphilic PEG/PDMS Conetworks by a Dual-Purpose Extender/Crosslinker. I. Synthesis” J. Appl. Polym. Sci. 2005, 43, 4953-4964.

9. Erdodi. G.; Kennedy. J. P. “Amphiphilic Conetworks: Definition, Synthesis, Applications” Prog. Polym. Sci. 2006, 31, 1-18.

10. Mass Spectrometry: Principles and Applications, Edmond Hoffmann, Jean Charette, and Vincent Stroobant, 1996, John Wiley & Sons, Inc.

11. Mass Spectrometry of Polymer, edited by by Giorgio Montaudo and Robert P. Lattimer, 2001, CRC press.

12. Mass Spectrometry, Jurgen H. Gross, 2004, Springer.

151 13. Cotter, R. J.; Honovich, J. P.; Olthoff, J. K.; Lattimer, R.P. “Laser Desorption Time-of-Flight Mass Spectrometry of Low-Molecular-Weight Polymers” Macromolecules, 1986, 19, 2996-3001.

14. Laser Ionization Mass Analysis, edited by Akos Vertes, Renaat Gijbels, and Fred Adams, 1993, John Wiley & Sons, Inc.

15. Nielen, M. W. F. “MALDI time-of-flight mass spectrometry of synthetic polymers” Mass Spectrom. Reviews. 1999, 18, 309-344.

16. Bruker Reflex III MALDI-ToF Mass Spectrometer User Guide, Bruker Analytical System, Inc., 1995.

17. Esquire-LC Operations Manual ver. 3.1, Bruker Daltonics, Inc., Germany, 1999.

18. Mark, A.; Polce, M. J.; Quirk, R. P., Wesdemiotis, C. “Probing Chain-end Functionalization Reactions in Living Anionic Polymerization via Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry” Int. J. Mass Spec, 2004, 238, 245-255.

19. Principles of Instrumental Analysis, 5th edition, Douglas A. Skoog, F. James Holler, and Timothy A. Nieman, 1998, Thomson Learning, Inc.

20. Analytical Chemistry, 7th edition, Douglas A. Skoog, Donald M. West, F. James Holler, and Stanley R. Crouch, 1999, Saunders college publishing.

21. Rotational Moulding of Plastics, edited by R. J. Crawford, 1992, John Wiley & Sons, Inc.

22. Emerging Technologies in Plastics Recycling, edited by Gerald D. Andrews and Pallatheri M. Subramanian, 1992, American Chemical Society.

23. The Chemistry of Polymers, John W. Nicholson, 1997, Royal society of chemistry.

24. Principles of Polymerization, 4 Edition, George Odian, 2004, Wiley-Interscience.

25. Matsubara, H.; Yoshida, A.; Kondo, Y.; Tsuge, S.; Ohtani, H. “Characterization of Network Structures in UV-Cured Acrylic Ester by Pyrolysis-Gas Chromatography in the Presence of Organic Alkali” Macromolecules, 2003, 36, 4750-4755.

26. Kim, S. J.; Lee, C. K.; Lee, Y. M.; Lee, K. B.; Park, Y. D. “Thermal Characterizations of Semi-Interpenetrating Polymer Networks Composed of Poly(ethylene oxide) and Poly(N-isopropylacrylamide)” J. Appl. Polym. Sci. 2003, 90, 3922-3927.

152 27. Sung, Y.; Park, K.; Park, S. M.; Anikumar, G. M. “Formation of Nanoporous and Nanocrystalline Anatase Films by Pyrolysis of PEO–TiO2 Hybrid Films” J. Crystal Growth. 2006, 286, 173-177.

28. Mothe, C. G.; Drumond, W. S.; Wang, S. H. “Phase Behavior of Biodegradable Amphiphilic Poly(l,l-lactide)-b-poly(ethylene glycol)-b-poly(l,l-lactide)” Thermochimica Acta, 2006, 445, 61-66.

29. Thermal Degradation of Organic Polymers, Samuel L. Madorsky, 1964, John Wiley & Sons, Inc.

30. Pyrolysis and GC in Polymer Analysis, edited by S.A. Liebman and E.J. Levy, 1985, Marcel Dekker, Inc.

31. Applied Pyrolysis Handbook, edited by Thomas P. Wampler, 1995, Marcel Dekker, Inc.

32. Chemistry of High Polymer Degradation Processes, Norman Grassie, 1956, Interscience Publishers, Inc.

33. Developments in Polymer Degradation, Vol. 3, edited by Norman Grassie, 1981, Applied Science Publishers, Ltd.

34. Developments in Polymer Degradation, Vol. 4, edited by Norman Grassie, 1982, Elsevier Applied Science Publishers.

35. Polymer Degradation, Tibor Kelen, 1983, Van Nostrand Reinhold Co.

36. Polymer Degradation and Stabilization, W. Lincoln Hawkins, 1984, Springer-Verlag.

37. Developments in Polymer Degradation, Vol. 6, edited by Norman Grassie, 1985, Elsevier Applied Science Publishers.

38. Polymer Degradation and Stabilization, Norman Grassie and Gerald Scott, 1985, Cambridge University Press.

39. Degradation and Stabilization of Polymers: theory and practice, edited by G.E. Zaikov, 1995, Nova Science Publishers, Inc.

40. Degradable Polymers: Principles and applications, edited by Gerald Scott and Dan Gilead, 1995, Chapman and Hall.

41. Handbook of Polymer Degradation, 2nd Edition, Revised and Expanded, edited by S. Halim Hamid, 2000, Marcel Dekker, Inc.

153

42. Emerging Themes in Polymer Science, edited by Anthony J. Ryan, 2001, Royal society of chemistry.

43. Madorsky, S. L. “Thermal Degradation of Poly(ethylene oxide) and Poly(propylene oxide)” J. Polym. Sci. 1959, 36, 183-194.

44. Bortel, E.; Lamot, R. “Study on the Degradation of High-molecular-weight Poly(ethylene oxide) in Solid State” Makromol. Chem. 1977, 178, 2617-2628.

45. Grassie, N.; Mendoza, P.; Gilberto, A.;Madorsky, S. “Thermal Degradation of Polyether-urethanes: Part 1. Thermal Degradation of Poly(ethylene glycols) used in the Preparation of ” Polym. Deg. Stab. 1984, 9, 155-165.

46. Fares, M. M.; Hacaloglu, J.; Suzer, S. “Characterization of Degradation Products of Polyethylene Oxide by Pyrolysis Mass Spectrometry” Eur. Polym. J. 1994, 30, 845-850.

47. Voorhees, K. J.; Baugh, S. F.; Stevenson, D. N. “An Investigation of the Thermal Degradation of Poly(ethylene glycol)” J. Anal. Appl. Pyrolysis. 1994, 30, 47-57.

48. Arisawa, H.; Brill, T. B. “Flash Pyrolysis of Polyethyleneglycol Part I: Chemometric Resolution of FTIR Spectra of the Volatile Products at 370-550°C” Combust. Flame. 1997, 109, 87-104.

49. Arisawa, H.; Brill, T. B. “Flash Pyrolysis of Polyethyleneglycol II: Kinetics Determined by T-Jump / FTIR Spectroscopy” Combust. Flame. 1997, 109, 105-112.

50. Lattimer, R. P.; Polce, M. J.; Wesdemiotis, C. “MALDI-MS Analysis of Pyrolysis Products from a Segmented Polyurethane” J. Anal. Appl. Pyrolysis. 1998, 48, 1-15.

51. Lattimer, R. P. “Mass Spectral Analysis of Low-Temperature Pyrolysis Products From Poly(ethylene glycol)” J. Anal. Appl. Pyrolysis. 2000, 56, 61-78.

52. Lattimer, R. P. “Tandem Mass Spectrometry of Poly(ethylene glycol) Proton- and Deuteron-Attachment Ions” Int. J. Mass Spectrom. Ion Proc. 1992, 116, 23-36.

53. Barton, Z.; Kemp, T. J.; Buzy, A.; Jennings, K. R. “Mass Spectral Characterization of the Thermal Degradation of Poly(propylene oxide) by Electrospray and Matrix-Assisted Laser Desorption Ionization” Polymer. 1995, 36, 4927-33.

54. Lattimer, R.P. “Pyrolysis Mass Spectrometry of Acrylic Acid Polymers” J. Anal. Appl. Pyrolysis. 2003, 68, 3-14.

154 55. Lattimer, R.P. “Pyrolysis Field Ionization Mass Spectrometry of Hydrocarbon Polymers” J. Anal. Appl. Pyrolysis. 1997, 39, 115-127.

56. Lattimer, R.P.; Williams. R. C. “Low-Temperature Pyrolysis Products from a Polyether-based Urethane” J. Anal. Appl. Pyrolysis. 2002, 63, 85-104.

57. Han, S.; Kim, C.; Kwon, D. “Thermal Degradation of Poly(ethylene glycol)” Polym. Deg. Stab. 1995, 47, 203-208.

58. Han, S.; Kim, C.; Kwon, D. “Thermal/Oxidative Degradation and Stabilization of Polyethylene glycol” Polymer, 1997, 38, 317-323.

59. Wang, F.; Ma, C. M.; Wu, W. “Thermal Degradation of Polyethylene Oxide Blended with Novolac Type Phenolic Resin” J. Materials Sci. 2001, 36, 943-947.

60. Bhattarai, N.; Kim, H. Y.; Lee, D. R. “Thermogravimetric Study of Copolymers Derived from p-dioxanone, L-lactide and Poly(ethylene glycol)” Polym. Deg. Stab. 2002, 78, 423-433.

61. Gallet, G.; Carroccio, S.; Rizzarelli, P.; Karlsson, S. “Thermal Degradation of Poly(ethylene oxide-propylene oxide-ethylene oxide) Triblock Copolymer: Comparative Study by SEC/NMR, SEC/MALDI-ToF-MS, and SPME-GC-MS” Polymer. 2002, 43, 1081-1094.

62. Chen, B.; Evans, J. R. G.; Holding, S. “Decomposition of Poly(ethylene glycol) in Nanocomposites” J. Appl. Polym. Sci. 2004, 94, 548-552.

63. Qian, Z.; Li, S.; He, Y.; Liu, X. “Thermal and Hydrolytic Degradation Behavior of Degradable Poly(etheresteramide) Copolymers based on E-caprolactone, 11-aminoundecanoic acid, and Poly(ethylene glycol)” Polym. Deg. Stab. 2004, 84, 41-47.

64. Mkhatresh, O. M.; Heatley, F. “A Study of the Products and Mechanism of the Thermal Oxidative Degradation of Poly(ethylene oxide) Using 1H and 13C 1-D and 2-D NMR” Polym. Int. 2004, 53, 1336-1342.

65. Pielichowski, K.; Flejtuch, K. “Non-oxidative Thermal Degradation of Poly(ethylene oxide): Kinetic and Thermoanalytical Study” J. Anal. Appl. Pyrolysis. 2005, 73, 131-138.

66. Mohan, Y. M.; Raju, M. P.; Raju, K. M. “Synthesis and Characterization of GAP-PEG Copolymers” Int. J. Polymeric Materials. 2005, 54, 651-666.

67. Schulten, H.; Simmleit, N.; Muller, R. “High-Temperature, High-Sensitivity Pyrolysis Field Ionization Mass Spectrometry” J. Anal. Chem. 1987, 59, 2903-2908.

155

68. Ford, T.; Sacco, E.; Black, J.; Kelley, T.; Goodacre, R.; Berkeley, R. C. W.; Mitchell, R. “Characterization of Exopolymers of Aquatic Bacteria by Pyrolysis Mass Spectrometry” Appl. Environ. Mocrobiology. 1991, 57(6), 1595-1601. 69. Ertas, M.; Hacaloglu, J.; Toppare, L. “Characterization of the Polymer of a Dipyrrolyl Monomer by Pyrolysis Mass Spectrometry” Polym. Int. 2004, 53, 1198-1204.

70. Meetani, M. A.; Basile, F.; Voorhees, K. J. “Investigation of Pyrolysis Residues of Poly(amino ) Using Matrix Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry” J. Anal. Appl. Pyrolysis. 2003, 68, 101-113.

71. Lewis, R. N. “Methylphenylpolysiloxanes” J. Am. Chem. Soc. 1948, 70, 1115-1117.

72. Lewis, C. W. “Pyrolysis of dimethylpolysiloxanes I” J. Polym. Sci. 1958, 33, 153-159.

73. Grassie, N.; Macfarlane, I. G. “The Thermal Degradation of Polysiloxanes I Poly(dimethylsiloxane)” Eur. Polym. J. 1978, 14, 875-884.

74. Lewis, C. W. “The Pyrolysis of Dimethylpolysiloxanes II” J. Polym. Sci. 1959, 37, 425-429.

75. Thomas, T. H.; Kendrick, T. C. “Thermal Analysis of Polydimethylsiloxanes. I. Thermal Degradation in Controlled Atmospheres” J. Polym. Sci. Part A-2, 1969, 7, 537-549.

76. Thomas H.; Kendrick, T. C. “Thermal Analysis of Polysiloxanes II Thermal Vacuum Degradation of Polysiloxanes with Different Substituents on Silicon and in the Main Siloxane Chain” J. Polm. Sci. 1970, 8, 1823-30.

77. Nielsen, J. M. “Degradation of Methyl Silicone Fluids under Nitrogen Atmosphere at 370 C” J. Appl. Polm. Sci. 1979, 35, 223-234.

78. Zeldin, M.; Qian, B. R.; Choi, S. J. “Mechanism of Thermal Depolymerization of Trimethylsiloxy-terminated Polydimethylsiloxane” J. Polym. Sci. 1983, 21, 1361-1369

79. Bannister, D. J.; Semlyen, J. A. “Studies of Cyclic and Linear Poly(dimethyl siloxanes) 6. Effect of heat” Polymer 1981, 22, 377-381.

80. Clarson, S. J.; Semlyen, J. A. “Cyclic Polysiloxanes: Preparation and Characterization of Poly(phenylmethylsiloxane)” Polymer 1986, 27, 1633-1636.

81. Clarson, S. J.; Semlyen, J. A. “Studies of Cyclic and Linear Poly(dimethylsiloxanes) High-temperature Tthermal Behavior” Polymer 1986, 27, 91-95.

156

82. Camino, G.; Lomakin, S. M. “Polydimethylsiloxane Thermal Degradation Part 1. Kinetic Aspects” Polymer. 2001, 42, 2395-2402.

83. Camino, G.; Lomakin, S. M. “Thermal Polydimethylsiloxane Degradation. Part 2. The Degradation Mechanisms” Polymer. 2002, 43, 2011-2015.

84. Ceccato, S.; Ceccato, R. “Structural and Microstructural Evolution During Pyrolysis of Hybrid Polydimethylsiloxane-Titania Nanocomposites” J. Sol. Gel. Sci. 2005, 36, 4927.

85. Chenoweth, K.; Cheung. S.; van Duin, A. C. T.; Goddard III, W. A.; Kober, E. M. “Simulations on the Thermal Decomposition of a Poly(dimethylsiloxane) Polymer Using the ReaxFF Reactive Force Field” J. Am. Chem. Soc. 2005, 127, 7192-7202.

86. Lomakin, S. M.; Koverzanova, E. V.; Shilkina, N. G.; Usachev, S. V.; Zaikov, G. E. “Thermal Degradation of Polystyrene Polydimethylsiloxane Blends” J. App. Chem. 2003, 76(3), 472-482.

87. Deshpande1, G.; Rezac, M. E. “Kinetic Aspects of the Thermal Degradation of Poly(dimethylsiloxane) and Poly(dimethyl diphenyl siloxane)” Polym. Deg. Stab. 2002, 76, 17-24.

88. Deshpande1, G.; Rezac, M. E. “The Effect of Phenyl Content on the Degradation of Poly(dimethyl diphenyl) Siloxane Copolymers” Polym. Deg. Stab. 2001, 74, 363-370.

89. Zulfiqar, S.; Ahmad, S. “Thermal Degradation of Blends of PVAC with Polysiloxane-II” Polym. Deg. Stab. 2001, 71, 299-304.

90. Radhakrishnan, T. S. “New Method for Evaluation of Kinetic Parameters and Mechanism of Degradation from Pyrolysis–GC Studies: Thermal Degradation of Polydimethylsiloxanes” J. Appl. Polym. Sci. 1999, 73, 441-450.

91. Spectrometric Identification of Organic Compounds, Robert M. Silverstein and Francis X. Webster, 1998, John Wiley & Sons, Inc.

92. Introduction to MALDI-ToF MS, Jennifer Krone, 2002, Applied Biosystems.

93. Time-of-Flight Mass Spectrometry: Instrumentation and Applications in Biological Research, Robert J. Cotter, 1997, ACS.

94. Quadrupole Ion Trap Mass Spectrometry, Raymond E. March and John F. J. Todd, 2005, John Wiley & Sons, Inc.

157 95. Practical Aspects of Ion Trap Mass Spectrometry, volume 1: Fundamentals of Ion Trap Mass Spectrometry, edited by Raymond E. March and John F. J. Todd, 1995, CRC press, Inc.

96. Characteristics of different mass analyzers, Ludwig Gruber, 2000, Fraunhofer Institute for Process Engineering and Packaging. 97. QToF-UltimaTM MALDI User guide, ver. 4.0, Micromass Limited, 2002.

98. Lewis, L. N.; Stein, J.; Gao, Y.; Colborn, R. E. “Technical Information Series” GE Research and Development Center, 1996, 1-13.

99. Zhou, H.; Venumbaka, S. R.; Fitch III, J. W.; Cassidy, P. E. “New Poly(-siloxanes) via Hydrosilation in Supercritical CO2 and Subsequent Crosslinking” Macromol. Symposium, 2003, 192, 115-121.

100. Jain, R.; Choi, H.; Lalancette, R. A.; Sheridan, J. B. “Poly{ferrocene(phenylene) bis(silylenevinylene)}s via Platinum- and Rhodium-Catalyzed Hydrosilylation of Diethynylbenzenes with 1,1'-Bis(dimethylsilyl)ferrocene” Organometallics, 2005, 24, 1458-1467.

101. Plasma Source Mass Spectrometry, New Developments and Applications, edited by Grenville Holland and Scott D. Tanner, 1999, Royal society of chemistry.

102. Lattimer, R. P. “Tandem Mass Spectrometry of Poly(Ethylene Glycol) Lithium-Attachment Ions” J. Am. Soc. Mass Spectrom. 1994, 5, 1072-1080.

103. Hoyau, S.; Norrman, K.; McMahon, T. B.; Ohanessian, G. “A Quantitative Basis for a Scale of Na+ Affinities of Organic and Small Biological Molecules in the Gas Phase” J. Am. Soc. Mass Spectrom. 1999, 121, 8864-8875.

158

APPENDICES

159

APPENDIX A

160

447 493 Na+

421 465 359 377 405 433

300 350 400 450 m/z

Figure A.1. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 493 in PEG series A of the prepolymer St-PEG-St pyrolyzates obtained at 275 C.

161

287

Na+ 333

315 261 199 217 305

200 220 240 260 280 300 320 m/z

Figure A.2. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 333 in PEG series B of the prepolymer St-PEG-St pyrolyzates obtained at 275 C.

162

347 Na+

317

273 287 201 245 259 301

190 210 230 250 270 290 310 330 m/

z Figure A.3. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 347 in PEG series C of the prepolymer St-PEG-St pyrolyzates obtained at 275 C.

163

APPENDIX B

164 335 379 Li+

246 202 291 158 184 228 272 316 360

150 200 250 300 350 m/z

Figure B.1. ESI-QIT CAD mass spectrum of the Li+ adduct of D-[PEG]-OD at m/z 423, generated from a PEG 600 standard after H/D exchange with CH3OD. The ESI solvent was CH3OD. The mass shifts in this spectrum, compared to Fig. 6.8., corroborated the mechanism shown in Scheme 6.1. and provide evidence that the 44n losses are O=CHCH3 molecules if n = 1 and HO-(CH2CH2-O-)nCH=CH2 molecules if n > 1.

165 421 Na+

403

375 287 305 349 393

200 250 300 350 400 m/z

Figure B.2. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 421 in PEG series B of the APN#1 pyrolyzates obtained at 275 C. Compare to Fig. 6.15., which includes the B oligomer with one more repeat unit.

166

405

Li +

-46

-28

(-134) -18 271 (-116) (-72) 289 333 -18

200 250 300 350 m/z

Figure B.3. ESI-QIT CAD mass spectrum of the same oligomer as in Fig. B.2., but using the Li+ instead of the Na+ adduct.

167

303  44n + 58 losses (Me) +  44n + 28 losses (Et) Na  44n losses (OH) 44n + 60 losses (aldehyde) 44n + 30 losses (aldehyde) 273  201 243

 157   171 229      288 113 185 215 

100 125 150 175 200 225 250 275 m/z

Figure B.4. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 303 in PEG series C of the APN#1 pyrolyzates obtained at 300 C. Compared to Fig. 6.16., which includes the C oligomer with one less repeat unit.

168

243  Ethyl chain end +  Aldehyde chain end Na

 153    215    113 139 157  197 171 183

100 120 140 160 180 200 220 m/z

Figure B.5. ESI-QIT CAD mass spectrum of the sodiated oligomer at m/z 243 in PEG series D of the APN#1 pyrolyzates obtained at 300 C. The fragmentation pattern is consistent with structure Et-[PEG]-O-CH2CH=O as the main D component. The isomeric structure H-[PEG]-O-CH2CH=O does not reconcile satisfactorily the observed fragments. See attached fragmentation schemes for the major dissociation pathways expected for these isomers.

169

541

74n + losses Na

467 393 319 245 171 97

100 150 200 250 300 350 400 450 500 m/z

Figure B.6. ESI-QIT CAD mass spectrum of the sodiated PDMS oligomer at m/z 541.3 in the pyrolyzates from APN#1 at 275 C.

170 H N a

H C H C O C H C H O C H C H O C H C H O C H C H O 3 2 2 2 x 2 2 2 y 2

r H - a s in S c h e m e 6 .2 . (r H - fro m s id e w ith H 3 C H 2 C O C H 2 C H 2 O C H C H 3 C H 2 C H = O c h a in e n d ) x N a O C H C H O C H C H O 2 2 y 2

H 3 C H 2 C O C H 2 C H 2 O N a x r H + rN a + C H C H O C H C H O C H C H O 3 2 2 y 2

r H + r N a + H C H C O C H C H O C H C H 3 2 2 2 x 2 N a N a

H O C H C H O C H C H O H 3 C H 2 C O C H 2 C H 2 O H 2 2 y 2 x + (4 4 n + 8 6 ) m /z 1 1 3 , 1 5 7 , 2 0 1 (x = 1 -3 ) N a

H O C H C H O C H C H O 2 2 y 2

C H C H O C H C H O C H C H O + (4 4 n + 2 8 ) 2 2 2 y 2 + (4 4 n + 4 6 ) m /z 1 7 1 , 2 1 5 (y = 2 -3 ) m /z 1 5 3 , 1 9 7 (y = 1 -2 ) H C H C O C H C H O C H C H 3 2 2 2 x 2 N a + (4 4 n + 6 0 ) m /z 1 3 9 , 1 5 3 (x = 1 -2 )

+ Scheme B.1. Charge-induced fragmentation pathway of [Et-(PEG)-O-CH2CH=O + Na] .

171

H N a

H O C H C H O C H C H O C H C H O C H C H 2 2 x 2 2 2 y 2

r H -

a s in S c h e m e 6 .1 . (r H - fro m th e sid e w ith H O C H 2 C H 2 O C H C H 3 th e C H = C H 2 c h a in e n d ) x

N a O C H 2 C H 2 O C H C H 2 y

r H + r N a +

H O C H C H O C H C H 2 2 x 2 N a N a H O C H C H O H 2 2 x H O C H 2 C H 2 O C H C H 2 + (4 4 n + 7 0 ) y

m /z 1 7 3 , 2 1 7

H O C H C H O C H C H + 4 4 n 2 2 x 2 N a

m /z 1 1 1 , 1 5 5 , 1 9 9

+ Scheme B.2. Charge-induced fragmentation pathway of [H-(PEG)-O-CH=CH2 + Na] .

172

APPENDIX C

173

2000 6000 10000 14000 18000 m/z

Figure C.1. MALDI-ToF mass spectrum of the pyrolyzates from a PD5 network that was heated at 300 C for 60 minutes.

174

3595.63595.63595.6 3669.73669.73669.7

3818.03818.03818.0 3892.13892.13892.1

3744.33744.33744.3

3655.73655.73655.7

3581.53581.53581.5

3729.63729.63729.6

3803.53803.53803.5

3877.93877.93877.9

3600 3700 3800 m/z

6855.96855.96855.9

6930.26930.26930.2

7004.47004.47004.4

7078.67078.67078.6 7226.57226.57226.5

7152.87152.87152.8

7300.77300.77300.7

7448.87448.87448.8

7524.47524.47524.4

7374.67374.67374.6

7597.27597.27597.2

7671.37671.37671.3

7892.77892.77892.7

7745.97745.97745.9

7819.57819.57819.5

7967.37967.37967.3

8041.58041.58041.5

7000 7200 7400 7600 7800 m/z Figure C.2. Two regions of spectrum shown in Fig. C.1. 175