STRUCTURAL STUDY OF EPOXIDIZED NATURAL RUBBER (ENR-50) AND ITS DERIVATIVES SYNTHESIZED VIA EPOXIDE RING-OPENING REACTIONS USING NMR

TECHNIQUES

ROSNIZA BINTI HAMZAH

UNIVERSITI SAINS MALAYSIA

2015

STRUCTURAL STUDY OF EPOXIDIZED NATURAL RUBBER (ENR-50) AND ITS DERIVATIVES SYNTHESIZED VIA EPOXIDE RING-OPENING REACTIONS USING NMR

TECHNIQUES

By

ROSNIZA BINTI HAMZAH

Thesis submitted in fulfilment of the requirements

for the degree of

Doctor of Philisophy

JUNE 2015

ii

ACKNOWLEDGEMENTS

First of all, Alhamdulillah. Thanks to Almighty Allah for giving me the strength,

patience and everything to complete this study.

First and foremost, I would like to dedicate my heartfelt appreciation to my

supervisor Prof. Dr. Mohamad Abu Bakar for his guidance, trust, consistent support and

advice throughout the completion of this thesis.

I would like to acknowledge financial support from Ministry of Higher Education

for Budget Mini Scheme and USM for Graduate Assistant Scheme and a research grant

USM-RU-PRGS (1001/PKIMIA/841023).

My gratitude goes to all staffs from School of Chemical Sciences mainly the NMR

and thermal analysis laboratory whom help me a lot in conducting analyses during this

study. My special and heartfelt thanks to all friends, too numerous to mention, for their

continued support since my M.Sc. study here.

Last but not least, I would like to dedicate this thesis to my wonderful mum Mrs.

Radziah Binti Haji Abdul Kadir and dad Mr. Haji Hamzah Bin Haji Sidek, my siblings, my

beloved husband and daughters for their love, support, patient and endless sacrifice that

gives me strength and energy to complete this Ph.D.

Rosniza Binti Hamzah

iii

Specially dedicated to

Mum, Dad, Dr. Nik Noriman Zulkepli

and my beloved daughters

Nik Izzara Iman Arissa Binti Nik Noriman

Nik Annisa Iman Azzahra Binti Nik Noriman

iv

TABLE OF CONTENTS

PAGE

ACKNOWLEDGEMENTS ii

DEDICATION iii

TABLE OF CONTENTS iv

LIST OF FIGURES x

LIST OF TABLES xiv

LIST OF SCHEMES xv

LIST OF EQUATIONS xvii

LIST OF ABBREVIATIONS xvii

ABSTRAK xix

ABSTRACT xxi

CHAPTER 1: RESEARCH BACKGROUND

1.1 Natural Rubber (NR) 1

1.2 Epoxidized Natural Rubber (ENR) 4

1.2.1 Preparation 4

1.2.2 Physical and Chemical Properties 5

1.2.3 Structure of ENR 8

1.3 Reactions of Epoxide 9

1.3.1 Crosslinking Reactions 9

1.3.2 Ring Opening Reactions (RORs) 9

1.3.2.1 ROR under Acidic Condition 10

1.3.2.2 ROR under Akaline Condition 12

1.3.2.3 ROR via Alcoholysis 13

1.3.3 Fixation or Insertion Reactions 14

v

1.3.3.1 Insertion of Carbon Disulfide

(Thiocarbonation)

14

1.3.3.2 Insertion of Carbon Dioxide

(Carbonation)

16

1.3.4 Epoxy/Metal Complex Formation 20

1.3.5 Epoxy/MO2 (M=Ti or Zr) Hybrids 25

1.3.5.1 Epoxy/Titania Hybrids 25

1.3.5.2 Epoxy/Zirconia Hybrids 29

1.3.5.3 ENR-based Hybrids 31

1.4 Problem Statements 34

1.5 Aim and Objectives of Research 36

1.6 Strategy and Scope of Research 37

CHAPTER 2: NOMENCLATURES

2.1 Epoxidized Natural Rubber 40

2.2 Ring Opening of Epoxidized Isoprene in ENR-50 42

2.3 Effect of Substituents 50

CHAPTER 3: EXPERIMENTAL

3.1 Chemicals 54

3.2 Measurements and Characterization Techniques 54

3.3 Theoretical Treatments 56

3.4 Synthesis and Characterization 58

3.4.1 Purification of ENR-50 58

3.4.2 Reaction of Purified ENR-50 with CS2 59

3.4.3 Acid Treated ENR-50 59

3.4.4 ENR/Titania Hybrid 60

vi

3.4.5 Base Treated ENR-50 60

3.4.6 ENR/Zirconia Hybrid 61

3.4.7 ENR/SnCl2.2H2O Composite 61

3.4.8 ENR/Tin(II) Chloride Complex Hybrids 62

CHAPTER 4: STRUCTURAL STUDY OF EPOXIDIZED NATURAL

RUBBER (ENR-50) AND ITS CYCLIC ITHIOCARBONATE

DERIVATIVE

4.1 Introduction 63

4.2 Epoxidized Natural Rubber 63

4.2.1 1H- and 13C-NMR Spectroscopy 63

4.2.2 HMQC 67

4.2.3 HMBC 70

4.2.4 COSY 70

4.3 Cyclic Dithiocarbonate Derivative of ENR-50 73

4.3.1 FTIR Spectroscopy 73

4.3.2 1H and 13C-NMR Spectroscopy 75

4.3.3 DEPT 78

4.3.4 Proposed Mechanism for the Formation of Cyclic

Dithiocarbonate 80

4.4 Summary 81

CHAPTER 5: THE STRUCTURAL STUDIES OF EPOXIDE RING

OPENING REACTIONS IN EPOXIDIZED NATURAL

RUBBER USING ACETIC ACID AND POTASSIUM

HYDROXIDE

5.1 Introduction 82

vii

5.2 Acid Treated ENR-50 84

5.2.1 1H-NMR 84

5.2.2 13C-NMR 88

5.2.3 HMQC 89

5.2.4 HMBC 92

5.2.5 COSY 94

5.3 Base Treated ENR-50 96

5.3.1 1H- and 13C-NMR 96

5.3.2 HMQC 100

5.3.3 HMBC 102

5.3.4 COSY 104

5.4 FTIR Spectroscopy of Acid and Base Treated ENR-50 107

5.5 Thermal Analysis of Acid and Base Treated ENR-50 109

5.5.1 TG/DTG Analyses 109

5.5.2 DSC 111

5.6 Summary 113

CHAPTER 6: SYNTHESIS AND STRUCTURAL STUDIES OF HYBRIDS

OF EPOXIDIZED NATURAL RUBBER/TITANIA (ENR/TiO2)

AND EPOXIDIZED NATURAL RUBBER/ZIRCONIA

(ENR/ZrO2) HYBRIDS

6.1 Introduction 114

6.2 Structural Studies of ENR/TiO2 hybrid 115

6.2.1 1H and 13C-NMR 115

6.2.2 HMQC 119

6.2.3 HMBC 120

viii

6.2.4 COSY 123

6.3 Structural Studies of ENR/Zr hybrid 126

6.3.1 1H and 13C-NMR 126

6.3.2 HMQC 130

6.3.3 HMBC 131

6.3.4 COSY 134

6.4 FTIR Spectroscopy of ENR/TiO2 and ENR/ZrO2 hybrids 135

6.5 TG/DTG Analyses of ENR/TiO2 and ENR/ZrO2 hybrids 136

6.6 DSC of ENR/TiO2 and ENR/ZrO2 hybrids 139

6.7 SEM-EDX of ENR/TiO2 and ENR/ZrO2 hybrids 141

6.8 XRD of ENR/TiO2 and ENR/ZrO2 hybrid 144

6.9 Summary 145

CHAPTER 7: FORMATION AND STRUCTURAL STUDIES OF

ENR/TIN COMPLEX HYBRIDS

7.1 Introduction 146

7.2 FTIR Spectroscopy 147

7.3 1H-NMR Spectroscopy 149

7.4 13C-NMR Spectroscopy 151

7.5 119Sn-NMR Spectroscopy 154

7.6 TG/DTG Analyses 156

7.7 DSC 159

7.8 TEM 161

7.9 SEM-EDX 162

7.10 Proposed Mechanism for the Formation of ENR/SnCl2.2H2O

Composite and ENR/Sn Complex Hybrid

165

ix

7.11 Summary 168

CHAPTER 8: CONCLUSION

8.1 Research Conclusion 169

8.2 Suggestion for Future Research 171

REFERENCES 174

APPENDICES

Appendix 1: Example calculation for percentage of epoxide 196

Appendix 2: Quantitative 13C NMR spectra of (a) purified ENR-50 and (b)

cyclic dithiocarbonate derivative of ENR-50 (in CDCl3).

197

Appendix 3: Example calculation for epoxidation level, percentage of

cyclic dithiocarbonate and percentage of unreacted epoxide

ring

198

Appendix 4: List of Publications 199

Appendix 5: List of Conferences 200

x

LIST OF FIGURES

PAGE

Figure 1.1 Experimental flow chart for series of experiments

conducted in this research

37

Figure 2.1 The possible triad sequence of ENR-50 41

Figure 2.2 Notations for (a) CE(↑)ME and (b) CE(↓)ME triad sequences 43

Figure 2.3 Notations for multiple ring opening in a triad sequence 44

Figure 2.4 Possible attachment positions of ring opened epoxidized

isoprene of ENR-50 and its carbon numbering (reaction with

acetic acid)

44

Figure 2.5 The possible triad sequences of ENR-50 due to

modification at (↑) and (↓) positions

46

Figure 2.6 Possible carbon attachments of ring opened epoxidised

isoprene of ENR-50 and its carbon numbering (reaction with

isopropyl alcohol/KOH)

52

Figure 4.1 (a) 1H and (b) 13C-NMR spectra of purified ENR-50 (in CDCl3) 66

Figure 4.2 JRES spectra of purified ENR-50 (in CDCl3). 66

Figure 4.3 HMQC spectra of (a) purified ENR-50 and (b) enlargement of

the boxed region in (a)

68

Figure 4.4 HMBC spectra of (a) purified ENR-50 and (b) enlargement of

the boxed region in (a)

71

Figure 4.5 COSY spectra of purified ENR-50 72

Figure 4.6 FTIR spectra of (a) purified ENR-50 and (b) cyclic

dithiocarbonate derivative of ENR-50

74

Figure 4.7 (a) 1H and (b) 13C-NMR spectra of cyclic

xi

dithiocarbonate derivative of ENR-50 (in CDCl3) 76

Figure 4.8 (a) DEPT-135; (b) DEPT-90; (c) DEPT-45 and (d)

DEPT quartenary spectra of cyclic dithiocarbonate

derivative of ENR-50 (in CDCl3)

79

Figure 5.1 (a) 1H- and (b) 13C-NMR spectra of acid treated ENR-50 (in

CDCl3)

85

Figure 5.2 (a) HMQC spectra of acid treated ENR-50 and (b,c)

enlargement of the box region in (a)

90

Figure 5.3 (a) HMBC spectra of acid treated ENR-50 and (b,c,d,e)

enlargement of the box region in (a)

93

Figure 5.4 COSY spectra of acid treated ENR-50 94

Figure 5.5 (a) 1H- and (b) 13C-NMR spectra of base treated ENR-50 (in

CDCl3)

97

Figure 5.6 (a) HMQC spectra of base treated ENR-50 and (b)

enlargement of the box region in (a)

101

Figure 5.7 (a) HMBC spectra of base treated ENR-50 and (b,c)

enlargement of the box region in (a)

103

Figure 5.8 COSY spectra of base treated ENR-50 104

Figure 5.9 FTIR spectra of (a) acid and (b) base treated ENR-50 108

Figure 5.10 (a) TG and (b) DTG thermograms of (i) purified ENR-50, (ii)

acid and (iii) base treated ENR-50

110

Figure 5.11 DSC thermogram of (a) purified ENR-50, (b) acid and (c)

base treated ENR-50.

112

Figure 6.1 (a) 1H-and (b) 13C-NMR spectra of ENR/TiO2 hybrid

(in CDCl3)

116

xii

Figure 6.2 Proposed possible reactions of ENR/TiO2 hybrid from acid

treated ENR-50 and its carbon numbering.

117

Figure 6.3 HMQC spectra of (a) ENR/TiO2 hybrid and (b, c) enlargement

of the labelled boxed region in (a)

121

Figure 6.4 HMBC spectra of (a) ENR/TiO2 hybrid and (b, c,d)

enlargement of the boxed region in (a)

122

Figure 6.5 COSY spectra of ENR/TiO2 hybrid 124

Figure 6.6 (a) 1H- and (b) 13C-NMR spectra of ENR/ZrO2 hybrid (in

CDCl3)

127

Figure 6.7 Proposed formations of ENR/ZrO2 hybrid from base treated

ENR-50 and its carbon numbering

129

Figure 6.8 HMQC spectra of (a) ENR/ZrO2 hybrid and (b) enlargement

of the boxed region in (a)

132

Figure 6.9 HMBC spectra of (a) ENR/ZrO2 hybrid and (b, c, d)

enlargement of the boxed region in (a)

133

Figure 6.10 COSY spectra of ENR/ZrO2 hybrid 134

Figure 6.11 FTIR spectra of (a) ENR/TiO2 and (b) ENR/ZrO2 hybrids 136

Figure 6.12 (a) TG and (b) DTG thermograms of ENR/TiO2 and

ENR/ZrO2 hybrids

138

Figure 6.13 DSC thermograms of (a) ENR/TiO2 and (b) ENR/ZrO2 hybrids 140

Figure 6.14 SEM-EDX of ENR/TiO2 hybrid

142

Figure 6.15 SEM-EDX of ENR/ZrO2 hybrid 143

Figure 6.16 XRD spectra of (a) ENR/TiO2 and (b) ENR/ZrO2 hybrids 144

Figure 7.1 FTIR spectra of (a) ENR/SnCl2.2H2O composite, and ENR/Sn

complex hybrids at (b) 1 hour, (c) 3 hours, and (d) 5 hours of

xiii

CO2 treatment 148

Figure 7.2 1H-NMR spectra of (a) ENR/SnCl2.2H2O composite, and

ENR/Sn complex hybrid at (b) 1 hour, (c) 2 hours, (d) 3

hours, (e) 4 hours and (f) 5 hours of CO2 treatment

150

Figure 7.3 13C-NMR spectra of (a) ENR/SnCl2.2H2O composite, and

ENR/Sn complex hybrid at (b) 1 hour, (c) 2 hours, (d) 3

hours, (e) 4 hours and (f) 5 hours of CO2 treatment.

152

Figure 7.4 Enlarged 13C-NMR spectra (20-40 ppm region) of ENR/Sn

complex hybrid at various hours of CO2 treatments

154

Figure 7.5 119Sn-NMR spectra of ENR/Sn complex hybrids at various

hours of CO2 treatment

155

Figure 7.6 TG thermograms of ENR/SnCl2.2H2O composite and ENR/Sn

complex hybrids at various CO2 treatment period

157

Figure 7.7 DTG thermograms of ENR/SnCl2.2H2O composite and

ENR/Sn complex hybrids at various CO2 treatment period

157

Figure 7.8 DSC thermogram of ENR/SnCl2.2H2O composite and the

various ENR/Sn complex hybrid at (b) 1, (c) 2, (d) 3, (e) 4, (f)

5 hours of CO2 treatment

160

Figure 7.9 TEM micrographs of ENR/Sn complex hybrid at (a) 1, (b) 3,

and (c) 5 hours of CO2 treatment

163

Figure 7.10 SEM-EDX micrograph of ENR/Sn complex hybrid at 5 hrs of

CO2 gas treatment

164

xiv

LIST OF TABLES

PAGE

Table 1.1 Physical and chemical properties of NR 2

Table 1.2 Some physical properties of NR and the epoxidized (ENR)

derivatives

7

Table 4.1 1H-, 13C-NMR chemical shifts and HMQC, HMBC and COSY

spin coupling correlations of ENR-50

65

Table 4.2 1H- and 13C-NMR chemical shifts and triad assignments

for purified ENR-50 and its cyclic dithiocarbonate

derivative

77

Table 5.1 1H-, 13C-NMR chemical shifts and HMQC, HMBC and COSY

spin coupling correlations of acid treated ENR-50

86

Table 5.2 1H-, 13C-NMR chemical shifts and HMQC, HMBC and COSY

spin coupling correlations of base treated ENR-50

98

Table 6.1 1H-, 13C-NMR chemical shifts and HMQC, HMBC and COSY

spin coupling correlations of ENR/TiO2 hybrid.

118

Table 6.2 1H-, 13C-NMR chemical shifts and HMQC, HMBC and COSY

spin coupling correlations of ENR/ZrO2 hybrid

128

Table 7.1 DSC data of ENR/SnCl2.2H2O composite and ENR/Sn

complex hybrids at various CO2 treatment period

160

xv

LIST OF SCHEME

PAGE

Scheme 1.1 The cis-1-4 polyisoprene. 1

Scheme 1.1 ) (a) The formation of conjugate acid of the epoxide and (b) the

attack of nucleophile on one of the epoxide carbon atoms

11

Scheme 1.3 The reaction of epoxide ring with carbon disulfide 15

Scheme 1.4 Ionic liquids used in the synthesis of cyclic carbonate

from CO2 and epoxide

17

Scheme 1.5 The proposed reaction mechanism for synthesis of cyclic

carbonates using TBAB and DMC

19

Scheme 1.6 Ring opening polymerization of the epoxide through

coordination of the cation with epoxide group

21

Scheme 1.7 Lewis acid-catalyzed anionic reaction mechanism proposed

for [Fe3O(O2CCF3)6(H2O)3] catalyzed epoxide polymerization

22

Scheme 1.8 Epoxide ring opening of cyclohexadiene monoepoxide using

titanium catalysts

24

Scheme 1.9 (a) Bidentate bis-titanium and (b) mono-titanium catalysts

and their respective chelation complex of cyclohexane

monoepoxide

24

Scheme 1.10 Coordination modes of zirconium with acetate group of acetic

acid

30

Scheme 2.1 The general structure and the numbering of carbon atom of C

and E units in ENR-50 employed in this work

41

Scheme 2.2 Reaction of RX with epoxidized isoprene 42

Scheme 4.1 Reaction of purified ENR-50 with carbon disulfide catalyzed

xvi

by 4-dimethylaminopyridine (DMAP). 64

Scheme 4.2 Proposed mechanism for the formation of cyclic

dithiocarbonate derivative of ENR-50 via oxirane ring

opening

80

Scheme 5.1 Reaction of acetic acid with epoxidized isoprene and its

carbon numbering

83

Scheme 5.2 Reaction of isopropyl alcohol/KOH with epoxidized

isoprene and its carbon numbering

83

Scheme 7.1 Proposed interactions of epoxidized isoprene with the

ENR/Sn complex hybrid

162

xvii

LIST OF EQUATIONS

PAGE

Equation 1.1 4

Equation 1.2 4

Equation 1.3 26

Equation 1.4 26

Equation 1.5 26

Equation 1.6 27

Equation 1.7 27

Equation 1.8 27

Equation 3.1 57

Equation 3.2 57

Equation 3.3 57

Equation 3.4 57

Equation 3.5 58

Equation 3.6 58

Equation 7.1 166

Equation 7.2 166

Equation 7.3 166

Equation 7.4 166

Equation 7.5 166

Equation 7.6 167

Equation 7.7 167

Equation 7.8 167

xviii

LIST OF ABBREVIATIONS

COSY Correlation Spectrosopy

DEPT Distortion Enhancement by Polarization Transfer

DSC Differential Scanning Calorimetry

DTG Differential Thermal Gravimetric

FTIR Fourier Transform Infra Red

HMBC Heteronuclear Multiple Bond Coherence

HMQC Heteronuclear Multiple Quantum Coherence

JRES J-Resolved Spectrosopy

NMR Nuclear Magnetic Resonance

SEM-EDX Scanning Electron Microscopy-Electron Diffraction X-ray

TEM Transmission Electron Miscroscopy

TG Thermal Gravimetric

XRD X-ray Diffractogram

xix

KAJIAN STRUKTUR GETAH ASLI TEREPOKSIDA (ENR-50) DAN TERBITANNYA

YANG DISINTESIS MELALUI TINDAK BALAS PEMBUKAAN GELANG EPOKSIDA

MENGGUNAKAN TEKNIK NMR

ABSTRAK

Satu terbitan siklik ditiokarbonat yang terdiri daripada getah asli terepoksida

(ENR-50) dan tiga jenis hibrid ENR-50/titania (TiO2), ENR-50/zirkonia (ZrO2) dan

kompleks ENR-50/stanum (Sn) telah disediakan. Teknik FTIR, 1D NMR; 1H-, 13C-, 119Sn-

NMR, 2D NMR; HMQC, HMBC, COSY, TGA, DSC, TEM, SEM-EDX dan XRD telah

digunakan untuk mencirikan terbitan dan hibrid ENR. Suatu susunan triad ENR-50

berjaya ditandakan menggunakan teknik NMR dan gabungan terbitan siklik ditiokarbonat

ENR-50. Suatu terbitan siklik ditiokarbonat ENR-50 telah disintesis daripada tindak balas

ENR-50 dan karbon disulfide (CS2) dengan kehadiran mangkin 4-dimetilaminopridina

(DMAP) pada suhu refluks (47 °C) selama 3 jam. Kedua-dua hibrid ENR/TiO2 dan

ENR/ZrO2 telah disediakan daripada tindak balas pembukaan gelang epoksida ENR-50

pada keadaan berasid lembut (ENR-50 terawat asid) dan berbes (ENR-50 terawat bes)

yang seterusnya ditindak balaskan dengan pemula masing-masing melalui teknik sol-gel

pada suhu refluks (60 °C) selama 24 jam. Sumber pemula TiO2 dan ZrO2 masing-

masing adalah Ti(OCH2CH2CH3)4 dan Zr(OCH2CH2CH3)4. Tindak balas pembukaan

gelang ENR-50 dengan terawat asid dan bes masing-masing menggunakan asid asetik

dan kalium hidroksida/isopropanol pada suhu refluks (110 °C) selama 3 jam. 1H-NMR

menunjukkan bahawa 19.56 dan 16.76% pembukaan gelang epoksida telah berlaku

daripada keseluruhan jumlah unit epoksida dalam ENR-50 bagi ENR-50 terawat asid

dan bes masing-masing dan disokong oleh spektroskopi FTIR kuantitatif. Pelekatan alkil

(R) berlaku pada kedua-dua karbon paling (↑) dan kurang (↓) berhalangan terhadap

xx

gelang epoksida. 13C-NMR menunjukkan lokasi R dalam rantai polimer melalui

pertambahan puncak asetat dan ester bagi ENR-50 terawat asid dan puncak eter bagi

ENR-50 terawat bes. Nilai Tg bagi ENR-50 terawat asid adalah lebih tinggi daripada

ENR-50 terawat bes kerana tahap pembukaan gelang dan lokasi pelekatan R.

Pembentukan hibrid ENR/TiO2 dan ENR/ZrO2 adalah melalui jaringan C-O-Ti dan C-O-

Zr masing-masing. Atom Ti terikat khusus pada karbon kuartet E6 manakala Zr pada

kedua-dua karbon (↑) dan (↓) (iaitu (E6 dan E7) pembukaan gelang epoksida. Kedudukan

R, Ti dan Zr telah dibuktikan melalui 13C-NMR dan 2D NMR. Nilai Tg hibrid ENR/TiO2

adalah lebih tinggi daripada ENR/ZrO2 kerana jenis moeiti takorganiknya. Kompleks

hibrid ENR/Sn disintesis daripada pelanjutan tindak balas refluks komposit

ENR/SnCl2.2H2O (110 °C) selama 3 jam dan dirawat dengan gas CO2 selama 1 hingga

5 jam semasa campuran tindak balas ini sedang disejukkan. Pembentukannya berlaku

melalui pelbagai tahap serangan elektrofilik dan akhirnya menyusun kepada kompleks

hibrid ENR/Sn. Pembentukan C-O-Sn berlaku pada karbon kuartet E6 pembukaan

gelang ENR-50.

xxi

STRUCTURAL STUDY OF EPOXIDIZED NATURAL RUBBER (ENR-50) AND ITS

DERIVATIVES SYNTHESIZED VIA EPOXIDE RING-OPENING REACTIONS USING

NMR TECHNIQUES

ABSTRACT

A cyclic dithiocarbonate derivatives of epoxidized natural rubber (ENR-50) and

three types of hybrids comprising ENR-50/titania (TiO2), ENR-50/zirconia (ZrO2) and

ENR-50/tin (Sn) complex were prepared. FTIR, 1D NMR; 1H-, 13C-, 119Sn-NMR, 2D

NMR; HMQC, HMBC, COSY, TGA, DSC, TEM, SEM-EDX and XRD techniques were

used to characterize the ENR derivative and hybrids. A triad sequence of ENR-50 was

successfully assigned using NMR techniques consolidated by the cyclic dithiocarbonate

derivative of ENR-50. A cyclic dithiocarbonate derivative of ENR-50 was synthesized

from the reaction of purified ENR-50 with carbon disulfide (CS2), in the presence of 4-

dimethylaminopyridine (DMAP) as catalyst at reflux temperature (47 °C) for 3 hours.

Both ENR/TiO2 and ENR/ZrO2 hybrids were prepared from epoxide ring opening

reaction of ENR-50 at mild acidic (acid treated ENR-50) and basic (base treated ENR-

50) conditions which later reacted with the respective precursors via sol-gel technique at

reflux temperature (60 °C) for 24 hours. The respective source of TiO2 and ZrO2

precursors used were Ti(OCH2CH2CH3)4 and Zr(OCH2CH2CH3)4. The ring opened

reaction of ENR-50 with acid and base treated ENR-50 were carried out using acetic

acid and potassium hydroxide/isopropanol respectively at reflux temperature (110 °C) for

3 hours. 1H-NMR revealed that 19.56 and 16.76% of epoxide were ring opened from the

total amount of the epoxide unit in ENR-50 for acid and base treated ENR-50

respectively and these were supported by quantitative FTIR spectroscopy. The alkyl (R)

attachment occured at both most (↑) and least (↓) hindered carbons of the epoxide. 13C-

xxii

NMR provides the location of R in the polymer chain via additional acetate and ester

peaks for acid treated ENR-50 and ether peaks for base treated ENR-50. The Tg value

of acid treated ENR-50 was higher than base treated ENR-50 due to extent of ring

opening and location of R attachment. The formation of ENR/TiO2 and ENR/ZrO2 hybrids

was via C-O-Ti and C-O-Zr networks respectively. Ti atom was specifically tailored at the

quaternary carbon E6 while Zr atom as at both (↑) and (↓) carbon (E6 and E7) of the ring

opened epoxide. The structural position of R, Ti and Zr were proven using 13C-NMR and

2D NMR. The Tg value of ENR/TiO2 was higher than ENR/ZrO2 hybrid due to types of

inorganic moiety. The ENR/Sn complex hybrid was synthesized upon further reaction of

ENR/SnCl2.2H2O composite at reflux temperature (110 °C) for 3 hours and upon cooling,

the reaction mixture was subject to 1 to 5 hours of CO2 gas treatment. The formation

was through various stages of electrophilic attack and finally rearranged to ENR/Sn

complex hybrid. The formation of C-O-Sn also occurred at quaternary carbon E6 of ring

opened ENR-50.

1

CHAPTER 1

RESEARCH BACKGROUND

1.1 Natural Rubber (NR)

Natural rubber (NR) is commercially available in the form of latex from tropical

tree Havea brasiliences. It is one of the main agricultural commodities of Malaysia

(Ismail and Abu Bakar, 2005). The NR comprises of 99% of cis-1,4-polyisoprene

(Scheme 1.1). The remaining 1% is a mixture of non-rubber components such as

protein, amino acid, sugar, fatty acids and other substances (Isayev and Sun, 2007).

The high content of cis-1,4-polyisoprene in NR, contributes to high degree of

crystallization as well as high mechanical properties for NR (Isayev and Sun, 2007).

C C

H2C

H3C

CH2

H

n Scheme 1.1: The cis-1-4 polyisoprene

Typically, the cis-1,4-isoprene unit sequence can be repeated up to 3000

times (Eng and Ong, 2001). Thus NR behaves like a rubber, resin and plastic

(Novesar, 2001). The physical and chemical properties of NR are given in Table 1.1.

Based on Table 1.1, NR has high tensile, elongation at break; tear strength and

compression properties (Menough, 1985; Novesar, 2001; Thakore 2014). NR is

suitable for application at room and medium operating temperature (less than 120

°C). NR has low melting (28 °C) and glass transition temperature, Tg (-75 °C)

(Novesar, 2001; Tantatherdtam, 2003).

2

Table 1.1: Physical and chemical properties of NR (Novesar, 2001)

*Range of resistance evaluation: excellent > good > moderate > poor

Typically NR is not comparable to synthetic polymers in term of oil and

solvent resistance as well as gas permeability (Baker, 2001). Therefore, the usage of

NR in automotive and manufacturing industry is restricted to certain products such as

tyres. While synthetic polymers like polysiloxane is resistant to oil, solvent and heat

thus applicable at wide range of operating temperature -100 to 316 °C.

NR tends to degrade when expose to oxygen, ozone, heat, dynamic stress

and radiation (Cibulkova et al., 2006). Oxygen molecule is able to diffuse into NR and

makes NR susceptible to thermal oxidation. Thermal oxidation involves scission and

cross-linking at C=C bond amongst the polymer chain (Bonfils et al., 2001;

Physical properties

Value

Density, ρ 0.92 g cm-3 Refractive index, n 1.52 Stretching coefficient 0.00062 °C-1 Thermal conductivity 0.00032 cal s-1 cm-3 °C-1 Dielectric constant, ε 2.37 Volume resistance 1015 ohms cc-1 Durometer range (Shore A) 30 – 90 Tensile strength 4000 psi or 28 Mpa Elongation at break 700 % Maximum operating temperature 75 – 120 °C Low temperature operation -60 °C Compression set, 24 hours/70 °C 10 – 15% Tear strength 35 – 44 KN m-1

Chemical properties

Value

Melting point, Tm 28 °C Glass transition temperature, Tg -75 °C Resistivity* – weather good - moderate Resistivity* – ozone poor Resistivity* – acid and alkaline good Resistivity* – oil and solvent poor Resistivity* – steam good Abrasion good - excellent Air permeability good

3

Ngolemasango et al., 2003). The poor permeability of NR to ozone gives opportunity

to ozone to react with C=C bond even at relatively low temperature. This caused

oxidation reaction to NR. Typically, ozonide and polyperoxide are formed at the main

chain of NR (Scott, 1995). Heating leads to NR polymer chains defect and

degradation through pyrolysis such as breaking of bonds of the main polymer chains

and side groups, elimination of organic group and depolymerization. However, the

extend of degradation is relative to the heat applied (Kind and Hull, 2012; Schanabel,

1981). Dynamic stress in NR is loads applied to the NR polymer chains and the

ability of NR to returns into its viscoelastic state after the load is relieved. The excess

amount of dynamic stress affects the molecular chains rearrangement and

reorientation (Eng and Ong, 2001). By radiation, radical is produced at C=C bond

and this radical abstracts hydrogen on the carbon atom in α-position to the double

bond (Cibulkova et al., 2006).

The prolonged storage of NR under ambient condition caused the progressive

increased in Mooney viscosity of NR. This is called ‘storage hardening’ and affects

processing behaviour of NR (Eng and Hong, 2001; Chaikumpollert et al., 2011;

Isayev and Sun, 2007). Storage hardening changes the molecular weight, molecular

weight distribution, molecular structure, and non-rubber components in NR

(Chaikumpollert et al., 2011). Generally this reaction involves crosslinking reaction

between C=C and the abnormal groups that exist in NR. These abnormal groups

include epoxide, ester, aldehyde and lactone that are formed during NR processing.

They are present as a very small amount of non-isoprene group and located on the

main-chain of NR molecules (Eng and Ong, 2001; Ngolemasango et al., 2003).

4

1.2 Epoxidized Natural Rubber (ENR)

1.2.1 Preparation

Epoxidized natural rubber (ENR) was developed to make NR comparable to

synthetic polymers (Ismail and Abu Bakar, 2005). ENR is a commercial polymer

produced from epoxidation process of NR with formic peroxide or acetic peroxide in a

reactor (Gelling, 1991). The chemical reactions between hydrogen peroxide, formic

acid and NR are consequently given in Equation (1.1) and (1.2) respectively.

H2O2 + + H2OC

O

H

O H

C

O

HOO

H

(1.1)

C CH

H3C

CH2 H2Cn

C CH

H3C

CH2 H2Cn

OC

O

HOO

H

+C

O

H

O H

(1.2)

The epoxidation process of NR is carried out in latex phase because it

provides relatively low cost route and the reagent can be recycled for a few times

(Gelling, 1991). NR latex is stabilized from its acid coagulation by adding anionic

surfactant. At the optimum reaction condition, the mole ratio between formic acid and

hydrogen peroxide ranges from 0.2 to 0.5 times (Baker and Gelling, 1987). While the

optimum temperature for epoxidation process is at 60 to 70 °C to avoid secondary

ring opening product of ENR or epoxidized latex such as diol and furan which occurs

5

at high temperature and low pH (Gelling, 1996). The ENR is treated with heat flow

before it was dried in hot air (Baker and Gelling, 1987).

There are also several other methods to carry out epoxidation process of NR

such as by using oxidizing agent such as acidic, alkaline and selected halogen. The

epoxidation of NR with acidic oxidizing agent is a direct process. NR latex is directly

reacted with the acidic medium i.e. perbenzoic acid, metachloroperbenzoic, peracetic

acid, trifluoroperacetic acid or perpropionic acid during coagulation process. While by

using alkaline oxidizing agent, the NR latex is reacted with a mixture of hydrogen

peroxide and NaOH aqueous solution. Other than that, NR latex is exposed to a

direct flow of oxygen gas in the presence of a catalyst i.e. Ag to produce ENR. The

selected epoxidation process of NR with halogen gas i.e. Cl2 and Br2 is carried out in

the presence of water to produce halohydrin. This is followed by an alkaline

treatment to produce ENR (Hogt et al., 1992).

1.2.2 Physical and Chemical Properties

The production of ENR from epoxidation process of NR is a stereospecific

reaction. Stereospesific reaction of ENR is the stereochemical outcome of ENR as

previously specified. This is due to the random epoxidation process of NR which

randomly distributes the epoxide group in the polymer chains while maintaining the

cis-NR configuration in ENR structure. NR with 4 isoprene units can accommodate 2

epoxides groups without affecting the tensile properties, tear strength and non-strain

crystallization (Baker and Geling, 1987).

NMR spectroscopy is well known and reliable method to determine the mole

percentage of epoxidation (Baker et al., 1984; Bradbury and Perera, 1985, Gemmer

and Golub, 1978, Saito et al., 2007) compared to the other methods such as

differential scanning calorimetry, elemental analysis, titration (Burfield et al., 1984)

and degradation (Burfield and Gan, 1977). Various epoxide content of ENR is

6

commercially available. However, there are three mole percentages of epoxidation

that are considered as standard i.e. ENR-10 (10 mole percentage of epoxidation),

ENR-25 (25 mol percentage of epoxidation) and ENR-50 (50 mole percentage of

epoxidation (Menough, 1985). Above 50 percent of epoxidation level lead to high

preparation cost as well as produced non-crystalline rubber with inferior tensile

properties. While less than 25 percentage of epoxidation level i.e. 10-15% produces

rubber with a slight different properties than NR itself (Baker and Gelling, 1985).

The physical properties of ENR are influenced by its mole percentage of

epoxidation as tabulated in Table 1.2. The increment in mole percentage of

epoxidation of ENR produces ENR with high mechanical strength such as tensile,

elongation, hardness and compression set. However, Mooney Scorch value shows a

decrement trend. ENR density increases with the increment in epoxide content. This

is consistent with additional oxygen atom in the ENR. Therefore the density and

viscosity of ENR-50 is higher than ENR-25. The refractive index of ENR-50 and 25

are lower than NR (Menough, 1985).

Higher level of epoxidation increases oil and solvent resistivity of ENR. ENR-

50 exhibits better oil resistance than ENR-25 (Menough, 1985). The oil resistivity of

ENR is related to its solubility parameter where one mole percentage of epoxide adds

0.131 (Jm-3)1/2 value of the solubility parameter. At high epoxidation level, ENR is

more resistant to non-polar solvent or hydrocarbon and decreases its resistivity in the

polar solvent (Baker et al., 1984; Baker and Gelling, 1987). The increment in mol

percentage epoxidation of ENR decreases the air (Ismail and Hashim, 1994) and

oxygen (Ismail and Ng, 1988) permeability across ENR membrane. Typically, the

permeability of air is generally lower than oxygen through ENR membrane. This is

due to easy diffusion of oxygen than air into the ENR membrane (Ismail and Hashim,

1994). The air permeability is an important parameter in tire liners industry.

7

Table 1.2: Some physical properties of NR and the epoxidised (ENR) derivatives

(Menough, 1985).

Property

NR

ENR-10

ENR-25

ENR-50

Density (mg m-3 or g cm-3) 0.92 - 0.97 1.03 Refractive index 1.52 1.52 1.52 1.51 Mooney Scorch, Mooney viscosity (min) 23 24 18 14 Tensile strength (psi) 3915 3683 3712 3954 Elongation at break (%) 550 515 580 565 300% Modulus (psi) 1117 1160 972 1262 Hardness, IRHD 59 54 52 58 Resillence (%) 78 73 59 74 Compression set (%) 17 14 15 17 Volume swell during immersion (70 hours & 70 °C)

ASTM D471; Oil 1 +15 - +12 +01 ASTM D471; Oil 3 (equivalent to IRM 903)

+78 - +40 +11

*All compounds were filled with 30 phr of N-220 black by using EV cure system.

The glass transition temperature increases by 1°C per mole percentage

epoxidation. Thus ENR-25 shows glass transition temperature, Tg at -48°C and ENR-

50 at -18°C. The Tg value affected the resilient properties of rubber at room

temperature. Rubber with low Tg value is more resilience than those with high Tg

value. This changes the purpose of rubber from general elastomer to a special

elastomer (Baker and Gelling, 1985; Menough, 1985). The thermal degradation of

ENR-50 under nitrogen gas flow is a single-step reaction. It displays Tonset at 348 °C,

maximum degradation temperature, Tmax deg at 392 °C and ends at, Tfinal 445 °C.

While thermooxidative degradation of ENR-50 under oxygen gas flow is a multi-step

reaction. It starts at Tonset 246 °C, maximum temperature, Tmax deg 378 °C and ends at,

Tfinal 584 °C. However the applied heating rate influences the thermooxidative

degradation of ENR (Novesar, 2001; Li et al., 1998).

8

1.2.3 Structure of ENR

NMR spectroscopy techniques are widely used to characterize various

polymers either in liquid or solid state. Characterization using 1D NMR gives

structural elucidation on reactive groups and monomer units in the polymer chain.

However, the 2D NMR extents detail structural arrangements of the monomer units

within the polymer chain. Either or both of these techniques have been applied to

ENR-25 (Jeerupun et al., 2004), ENR-50 (Lee et al., 2010; Gelling, 1991; Burfield et

al., 1984a; Gelling, 1996; Bhattacharjee et al., 1993; Gan and Ziana, 1997), and

ENR-75 (Burfield et al., 1984a), and other ENR related compounds (Derouet et al.,

2001a; 2001b). However, most of these works fall short of full structural assignments

due to the overlapping of signals arising from the randomly distributed isoprene (C)

and epoxidized isoprene (E) monomer units (Bradbury and Perera, 1985; Saito et al.,

2007). The arrangement of these monomer units in a group of three is called a triad

sequence (see Chapter 2).

Thus prior to the advent of the triad sequence, previous workers (Burfield et

al., 1984a; Gelling, 1996; Bhattacharjee et al., 1993; Gan and Ziana, 1997; Bradbury

and Perera, 1985; Thames and Gupta, 1997) were only able to ascertain the

assignments of 1H chemical shifts of methine proton of C and E units but sporadically

to either the methyl and or methylene protons of C and E units of the ENR. Saito et

al. (2007) have reported the use of computer simulation (also Furst and Pretsch,

1990; Pretsch et al., 1992) to predict chemical shifts of these protons and their triad

assignments. Gelling (1985), on the other hand, has assigned the triad sequence

based on 13C-NMR chemical shifts for ENR-20. However, the assignment deals with

certain triad sequences only and the position of carbon represented by the triad

sequences was found to be inconsistent. Consequently, Saito et al. (2007) have

successfully interpreted the 13C-NMR spectra for the ENR related compounds and

assigned their triad sequence.

9

1.3 Reactions of Epoxide

1.3.1 Crosslinking Reactions

ENR contains both epoxide and C=C bond (Gan and Ziana, 1997). Both offer

crosslinking reaction(s) with other polymers chains, fillers or other reactive

substances. Crosslinking is a process of forming a three-dimensional network

structure from a linear polymer by a physical or chemical method (Akiba and Hashim,

1997). Typically the physical methods are vulcanization (Akiba and Hashim, 1997)

and high energy electron irradiation (Ratnam et al., 2000a; 2000b; 2001). The

vulcanization involves the formation of 3-D polymer network at C=C bond in the

presence of sulfur (Akiba and Hashim, 1997). While high energy electron irradiation

produces free radical that reacts with the structure of the irradiated rubber. This

reaction is also known as irradiation-induced crosslinking and can take place either at

epoxide or C=C bond (Ratnam et al., 2001).

The chemical methods are preferred due to their simplicity and low cost such

as the acid (Bhattacharjee et al., 1993; Brosse et al., 2000) or alkali treatments (Gan

and Ziana, 1997) which modify the epoxide or the C=C bond in the ENR chain.

These methods produce covalently bonded network in the polymer system.

1.3.2 Ring Opening Reactions (RORs)

Epoxide or oxirane is an ether in which the oxygen atom is incorporated into

the C=C bond to form a three-membered ring. Ethylene oxide is the simplest form of

epoxide. The carbons in an epoxide are very reactive electrophile thus able to react

with a variety of nucleophile. The ring strain in the three-membered ring of epoxide is

able to relieve through ring opening reaction (Bruice, 2001a). Majority of epoxide

reactions are studied in solution and most of the reaction involves the epoxide ring

opening and addition of a molecule of reagent (Brosse et al., 2000).

10

In practice, the oxirane ROR is a common reaction during preparation of

ENR. The ring opening products varies the properties and application of ENR (Yu et

al., 2008). Thus this reaction offers opportunities to functionalize ENR with other

reactive substances, fillers or polymers. The oxirane ring is reactive towards

nucleophilic reagents such as amine, carboxylic acid, alcohol (Derouet et al., 2001a;

2001b), dibutylphosphate and phosphoric acid derivatives (Derouet et al., 2005).

1.3.2.1 ROR under Acidic Condition

The mechanism of epoxide ring opening is considered under three conditions

(i) orientation of ring opening, (ii) stereochemistry of ring opening, and (iii) kinetics of

ring opening. The presence of acid accelerates the addition of most nucleophile to

the epoxide carbon atoms (Bruice, 2001a). By using propylene oxide to represent the

epoxide, Scheme 1.2 shows the protonation of epoxide by acid (H+). The formation of

conjugated acid of the epoxide is a reversible process. The protonated epoxide

(>+OH) is then attacked by a nucleophile (CH3OH) at the most substituted carbon.

Thus the carbon-oxygen bond breaks. A covalent bond between nucleophile and the

most substituted carbon i.e secondary carbon is formed. Thus the partial positive

charge is located at the most substituted carbon because this carbocation is more

stable than primary carbocation. The products are formed after elimination of a

proton (H+). The major product resulted from the addition of nucleophile at the most

substituted carbon while the minor product is from the addition of nucleophile at the

least substituted carbon. The pathway of the reaction is partially SN1 and SN2. This is

because SN1 reaction involves a carbocation as an intermediate and SN2 reaction

requires a departure of leaving group before the compound is able to attack by the

nucleophile (Bruice, 2001a).

11

(a)

(b)

Scheme 1.2: (a) The formation of conjugate acid of the epoxide and (b) the attack of

nucleophile on one of the epoxide carbon atoms (Bruice, 2001).

ENR is a pH sensitive polymer and favours neutral pH (Baker et al., 1984;

Menough, 1985). The exposure of ENR to pH below than 7 (acidic) is able to catalyst

the ring opening reaction of epoxide and provides crosslinking amongst the polymer

chains through ether groups (Baker et al., 1984; Baker and Gelling, 1985; Menough,

1985). The excessive reaction temperature and too low pH contributes to secondary

ring opening reaction of ENR-50 such as diol and furans (Gelling, 1996;

Tanrattanakul et al., 2003). Typically acidity and temperature are main factor for the

formation of ring opening products i.e hydroxyl-acetates, diols and intermolecular

ethers (Yu et al., 2008). The cleavage of epoxide ring under acidic condition involves

formation of oxonium ion. Then this oxonium ion is attacked by the nucleophile in a

SN2 displacement that produced β-alkoxy alcohols. However, the addition of

12

nucleophile onto epoxide ring is difficult to obtain under acidic condition due to

undesirable side reactions obtained (Derouet et al., 2001b).

The ring opening reaction of ENR-50 using periodic acid will produce

heterotelechelic NR. It involves chains scission at the epoxide structure and

degradation of ENR chains (Brosse et al., 2000; Phinyocheep et al., 2005). However,

periodic acid works as an oxidizing agent due to formation of carbonyl group after

prolonged reaction time (Phinyocheep et al., 2005).

A ring opening reaction in acid solution produces formyl ester and an alcohol

group (Bradbury and Perera, 1985). Acid catalysed the ring opening of the epoxide

and formed ether crosslinked (Gelling, 1991). The acid treatment is more preferred

because at neutral pH, the epoxide structure of the ENR is stable towards the

epoxide ring opening (Gan and Ziana, 1997). However, at low pH (not less than 3),

organic acids such as methacrylic acid (Derouet et al., 1990) and benzoic acid (Gan

and Burfield, 1989) and other carboxylic acids (Copeland and Thames, 1994) are

capable of inducing epoxide ring opening reactions. Typically the products are ester,

alcohol, hydroxyl and diol.

1.3.2.2 ROR under Alkaline Condition

In the common base, a nucleophile attacks the less substituted carbon of the

epoxide causing the β-opening at the epoxide ring (Grobelny et al., 2003). While in

the rubber processing via vulcanization reaction, the excess of base will cause

premature curing and accelerate the reaction. This premature curing is due to the

epoxide ring opening reaction under basic condition (Baker et al., 1984; Baker and

Gelling, 1985). Hydrolysis of epoxide ring of ENR-50 can occur by boiling ENR-50 in

THF for more than 30 minutes under alkaline conditions (Gan and Ziana, 1997).

13

1.3.2.3 ROR via Alcoholysis

The epoxide ring opening by alcohols are mainly conducted under mild and

neutral conditions (Derouet et al., 2001b). However this reaction requires metal

catalyst such as metal doped alumina, metal halides, organotin phosphate

derivatives and cerium ammonium nitrate (CAN) (Derouet et al., 2001b).

The alcoholysis using CAN catalyst shows the orientation of the reaction is

influenced by (i) the size and the nature of the alcohol substituents, (ii) polarity of the

solvent, and (iii) the complexing properties of the solvents (Derouet et al., 2001a;

2001b). The alcoholysis of epoxide using various alcohols produces β-alkoxy

alcohols. This reaction is regioselective and occurs exclusively at the most hindered

carbon. The small size alcohol is more reactive than large size alcohols. The electron

drawing substituents on the carbon that bear the hydroxyl group of the alcohols

decrease the reactivity of the alcohols. The solvents play an important role on

epoxide rearrangement during alcoholysis. In polar (i.e DMSO, DMF) and medium

polarity solvents (i.e THF, dichloromethane) epoxide rearranges into ketone and

allylic alcohol. While in non polar solvents (i.e benzene, toluene, dioxane) the

rearrangement product is allylic alcohol. The alcoholysis reaction is also influenced

by complexing power of the solvent. Solvents like dioxane and diglyme are able to

solvate cerium ion and deactivate CAN catalyst (Derouet et al., 2001b). The

alcoholysis reaction of ENR-20 in dichloromethane (DCM)-alcohol mixture is a slow

process. Thus the excess amount of alcohol (i.e. concentration) is necessary to

increase the reaction rate besides the reaction temperature. The products are

mixture of alkoxylated, ketone and allylic units (Derouet et al., 2001a). Alcoholysis

does not generate any crosslinking or cyclization reaction (Brosse et al., 2000).

14

1.3.3 Fixation or Insertion Reactions

1.3.3.1 Insertion of Carbon Disulfide (Thiocarbonation)

Carbon disulfide (CS2) is abundantly available and a low cost precursor. It is a

common sulphur source in organic chemistry and a solvent for various polymers. The

CS2 comprises both electron-donor and electron-acceptor moieties. As electron-

donor the CS2 is able to donate its electron and acts as reducing agent. While as

electron-acceptor, CS2 accepts the electron and acts as oxidizing agent (Ochiai and

Endo, 2005).

The reaction between epoxide and CS2 produces five- and six-membered

cyclic thiocarbonates. Typically five-membered cyclic thiocarbonates consists of

dithiocarbonates and trithiocarbonates. Dithiocarbonates and trithiocarbonates

contain 2 and 3 sulfur atoms in the cyclic structure respectively. There are three

regioisomers of five-membered cyclic thiocarbonates as given in Scheme 1.4. These

regioisomers present different order of 2 sulfur atoms, 1 oxygen atom and 2 carbon

atoms bonded together as a cyclic dithiocarbonates (Motokucho et al., 2001).

Episulfide is a sulfur analogue of epoxide. The position of oxygen atom in the

epoxide structure (Scheme 1.3) is replaced by a sulfur atom. The five-membered

cyclic trithiocarbonates and episulfide are selectively obtained using high

temperature experiment i.e. 120 °C (Kihara et al., 1995; Motokucho et al., 2001).

Synthesis of five-membered cyclic dithiocarbonates depends on the catalyst

and reaction conditions i.e. temperature and pressure. It is important to produce a

selective intermediate which later produce a selective product (Ochiai and Endo,

2005). The reaction requires a strong nucleophile as catalyst (Shamsuzzaman and

Salim, 1997). A 4-dimethylaminopyridine (DMAP), triethylamine (Et3N) and lithium

bromide (LiBr) are common catalysts in this system because they posses lone pair

electron to attack the carbon at CS2. The usage of DMAP or Et3N as catalyst and

15

water as solvent accelerates Diels-Alder reaction and produces high yield of cyclic

dithiocarbonates (Halimehjani et al., 2009). The usage of LiBr as catalyst produces a

selective product of cyclic dithiocarbonates which is 5-substituted 1,3-oxathiolane-2-

thiones (Ochiai and Endo, 2005; Shamsuzzaman and Salim, 1997). Typically the

catalyst prefers to attack at the less substituted carbon of the epoxide. This is due to

the regioselectivity, steric and electronic effect (Azizi et al., 2010).

O

R

+ CS2

O S

S

R

S O

S

R

S S

O

R

S S

S

R

epoxide ring

regioisomers of f ive-membered cyclic dithiocarbonates

f ive-memberedcyclic trithiocarbonates

S

R

episulf ide

Scheme 1.3: The reaction of epoxide ring with carbon disulfide (Motokuchu et al.,

2001).

The single SN2 is a well known reaction route between epoxide and CS2. The

CS2 is inserted into the epoxide structure by the ring opening reaction of epoxide by

catalyst, nucleophile addition of catalysts to carbon disulfide to form xanthate salts,

ring opening of epoxide by xanthate salts, and cyclization in dithiocarbonates

synthesis (Ochiai and Endo, 2005). Xanthate salts is a formula of ROCS2-M+ (R =

16

alkyl; M+ = Na+, K+). Other typical simple epoxide employed in the reaction with CS2

such as glycidyl methacrylate and propargyl alcohols (Ochiai and Endo, 2005).

Other reaction involving sulfur nucleophile for the ring opening of epoxide is

thiolysis. This reaction produces β-hydroxy sulphide. Various catalysts such as water

and Rongalite ® (sodium formaldehyde sulfoxylate, NaHSO2.CH2O.2H2O) have been

employed (Azizi et al., 2010; Guo et al., 2009).

1.3.3.2 Insertion of Carbon Dioxide (Carbonation)

The coupling reaction of epoxide with carbon dioxide (CO2) is similar to the

synthesis of cyclic dithiocarbonate from the coupling reaction of the epoxide with CS2

(Kim et al., 2008). The fixation of carbon dioxide (CO2) to the epoxide group is a way

to protect the environment because CO2 is a primary greenhouse gas. The emission

of CO2 to the atmosphere contributes to global warming (Xiong et al., 2013). As a

chemical reactant however, the CO2 presents numerous advantages such as highly

abundant, inexpensive, renewable resources, non-toxic, non-flammable, low

chemical reactivity, ease of attainment of its critical conditions, and high diffusion

(Supercritical fluids, 2013; Xiong et al., 2013). The cyclic carbonate is widely used as

electrolyte solution for lithium ion battery, aprotic polar solvent, monomer for various

polymeric material products i.e polycarbonates, chemical ingredients in medicine and

agricultural product, alkylation agents, engineering plastics, and biomedical devices

(Sako et al., 2002; Xiong et al., 2013).

The synthesis of cyclic carbonate via insertion or cycloaddition of CO2 to

epoxide requires the presence of a catalyst in the reaction. Two types of catalyst

commonly used in the reaction are homogeneous and heterogeneous catalysts.

Homogeneous catalyst possesses good solubility of catalyst, reactant and product

because these are in the same phase. The disadvantage of this catalyst is the time

consumed during catalyst-product separation. While heterogeneous catalyst provides

17

easy catalyst-product separation and its high thermostability (Kawanami et al., 2003).

However at the optimum conditions, both of the catalyst gives high yield, good

selectivity and high recyclability. While the examples for heterogeneous catalysts are

ionic liquids, alkali metal halides (i.e. LiBr), metal oxides, transition metal complexes,

salen complex (Wu et al., 2012), and ion exchange resin (Xiong et al., 2013).

Ionic liquid is widely used in the synthesis of cyclic carbonate from CO2 and

epoxide. The ionic liquid can be used as a prominent acid-base catalyst and provides

easy penetration of CO2 gas molecules into the catalyst (Kawanami et al., 2003).

This will enhance the reaction rate as well as the product yield. Various types of ionic

liquid salts such as quartenary ammonium, phosphonium, imidazolium, and

pyridinium cations with inorganic anions are as shown in Scheme 1.4 (Darensbourg

and Holtcamp, 1996; Sun et al., 2005).

Cations: N+

R1R2

R3

R4

P+

R1 R2

R3

R4

NN

R1 R2 N+

R

Ammonium Phosphonium Imidazolium Pyridinium

Anions: BF4-, PF6

-, X- (X = Cl, Br, I), NO3-, CF3SO3

-, PhSO3-

Scheme 1.4: Ionic liquids used in the synthesis of cyclic carbonate from CO2 and

epoxide (Sun et al., 2005).

Typically, alkali metal halide is solely used or with support i.e crown ether or

inorganic support. The use of alkali metal salt with crown ether shows the catalytic

18

activity of alkaline metal salt increases with an increased in the anion nucleophilicity

and cation diameter (Darensbourg and Holtcamp, 1996). The combination of ionic

liquid with other alkali metal halide i.e 1-butyl-3-imidazolium tetrafluoroborate/ZnBr2

as co-catalysts is necessary for conversion of styrene oxide to styrene carbonate.

This is due to less reactivity of β-carbon atom as compared to propylene oxide and

ethylene oxide. Both of the catalyst phases can be separated after the reaction and

reused without significant loss in activity (Sun et al., 2005). Basic metal oxides such

as magnesium oxide (MgO) are highly active and selective to produce cyclic

carbonate from ethylene oxide or propylene oxide and CO2. Using the same catalyst,

cyclic carbonates is an important precursor for transesterification reaction to produce

glycol or dimethyl carbonate (DMC) from alcohols (Bhanage et al., 2001).

Polymer supported ionic liquid i.e highly crosslinked chloromethylated

polystyrene supported quartenary phosphonium salt produces high yield and

excellent selectivity of cyclic carbonates. The anchored catalysts on the polymer

remain active and produce consistent performances of the catalyst activity (Xiong et

al., 2013).

The utilization of non-toxic and non-corrosive solvents such as DMC as a

reaction media provides fast and quantitative conversion of cyclic carbonate without

side-products. Tetrabutylammonium bromide (TBAB) acts as catalyst. Both can be

recovered and reused for subsequent experiment up to six runs. The known reaction

mechanism is shown in Scheme 1.5. The reaction involves nucleophilic attack of

bromide ion of quaternary salt to the epoxide ring. The ring opening reaction of the

epoxide is facilitated by the promoting effect of DMC by non-bonding interaction with

the oxygen of the epoxide. An oxy anion species is formed and reacts with CO2 and

subsequent cyclization produces the corresponding carbonate (Kumar et al., 2011).

19

O

R

Bu4N+Br- MeO

MeO

O

Br R

O-N+Bu4 CO2 O-

O

Br

RO

N+Bu4

O

O

O

R

+ Bu4N+Br-

nucleophilic attack

formation of oxy anion species

cyclization cyclic carbonate species

Scheme 1.5: The proposed reaction mechanism for synthesis of cyclic carbonates

using TBAB and DMC (Kumar et al., 2011).

The synthesis of cyclic carbonate under solvent free condition is typically

conducted using supercritical CO2 as both reactant and solvent. Hence it is an

alternative route to replace conventional solvents. Supercritical CO2 enhances the

reaction rate because the boundary of vapour-liquid changes to supercritical uniform

phase. The supercritical CO2 property is easily tuned by the applied pressure and

temperature. The miscible CO2 is easy to penetrate and reacts with the epoxide

(Kawanami and Ikushima, 2000; Sako et al., 2002).

Various types of simple epoxide were used in the cycloaddition reaction such

as 2-methyloxirane (Sako et al., 2002), propylene oxide and epichlorohydrin (Xiong

et al., 2013). The cycloaddition with industrial epoxide i.e diglycidylether of bisphenol

A (DER 331) is obtained using ruthenium chloride (RuCl3) as alkali metal halide

catalyst supported on co-catalyst of tetraethylammonium bromide (TEAB). At both

normal pressure and supercritical CO2 condition i.e pressurized CO2/Ar gas mixture

produces high product conversion and selectivity. While using RuCl3 without any co-

catalyst or supported on SiO2 produces good conversion respectively as compared to

TEAB. The use of polyphosphostungstic acid (HPW) as co-catalyst enabled DER 331

to be converted to polycarbonate at similar experimental conditions (Gomes et al.,

2008).

20

The cycloaddition reaction of liquid epoxidized deproteinized natural rubber

(LEDPNR) in super critical CO2 condition in the presence of LiBr catalyst has been

reported. The reaction produces novel, natural, organic polymer with the isoprene

unit and polar carbonate group in the polymer chains. The removal of catalyst is not

necessary because it can be a component in electrolyte media for lithium ion battery

(Kawahara and Saito, 2006).

1.3.4 Epoxy/Metal Complex Formation

The coordination of aluminium with the epoxide group is obtained from the

reaction of triethylaluminium and diastereomeric 2-methyl-3,4-epoxy alcohol. The

cleavage of epoxide is obtained from a formation of tetracoordinated aluminium-

oxygen. A stereochemical disposition of the substituents proceeds through a

bidentate aluminium complex by promoting the attack at the most substituted epoxide

carbon (Torres et al., 2012).

The epoxide group of diglycidyl ether of bisphenol-A (DGEBA) is able to

coordinate with the copper cation of copper acetate. Copper acetate works as

hardener and later cures DGEBA. It reacts with epoxide of DGEBA through the

coordination of the copper cation with the epoxide group and later formed a transition

complex. The transition complex acts as initiator of ionic mechanism for the ring

opening polymerization of the epoxide as shown in Scheme 1.6. The chelation

mechanism depends on the equilibrium process of dissociation of the chelate at a

certain particular temperature and a concentration of the copper acetate (Ghaemy et

al., 1999).

21

Cu2+(OAc-)2 H2C CHR

O

(O-Ac)2Cu2+CH

CHRO AcO

H2C CHR

O-Cu2+(O-Ac)

+

coordination of the cation with the epoxide group

transition complex

AcOH2C CHR

O-Cu2+(O-Ac)

H2C CHR

O

+ AcOH2C CHR

OH2C CHR

O-Cu2+(O-Ac)

(A)

(A) H2C CHR

O

+ AcOH2C

HC

O-Cu2+(O-Ac)

H2C

R

CHRn+1

polymerized DGEBA

ionic polymerization

R = alkyl groupn = whole number

Scheme 1.6: Ring opening polymerization of the epoxide through coordination of the

cation with epoxide group (Ghaemy et al., 1999).

A series of copper(II) coordination polymers (CPs) was derived from bis-

pyridyl-bis-urea ligands. These CPs use oxalate, succinate and 2,6-napthalene

dicarboxylate as co-ligands in the systems. The CP was used as catalyst for ring

opening methanolysis of various simple epoxides such as cyclopentene-oxide,

cyclohexene-oxide, styrene oxide, trans-stilbene oxide and cis-stilbene oxide. The

reaction is facilitated by hydrogen bonding interaction between epoxide and

functionality of urea as co-ligand. This interaction brings epoxide in close proximity of

the catalytic Cu(II) in the CPs structure (Banerjee et al., 2012).

The epoxide group of glycidyl esters of phosphoric acids (GEPs) is cured

using various types of rare-earth elements (REE). The cured polymer is optically

transparent with required refractive index and high concentration of REE. Anhydrous

or hydrated REE chlorides, nitrates and sulphates were used in GEPs production.

22

The rate of curing process depends on salt content, cation, anion, GEP structure,

curing temperature and moisture content. For example, the incorporation of

gadolinium salts in GEPs will increase the Tg of the cured polymer while maintaining

its hardness (Amirova et al., 2003).

In small amount, mixed-valent iron trifluoroacetate complex [Fe2IIIFeII(μ3-

O)(O2CCF3)6(H2O)3] is an effective catalyst for a ring opening polymerization (ROP)

of epoxide. The employed epoxide monomers are cyclohexane oxide, cyclopentene

oxide and epichlorohydrin. The reaction mechanism involves Lewis acid catalyzed

anionic polymerization in the presence of a nucleophilic anion. A coordination of

[FexO(O2CCF3)y]+ cation with oxygen of the epoxide activates the epoxide ring

opening reaction towards nucleophilic addition. While the counter anion [CF3CO2]-

functions as a nucleophilic initiator. This reaction is followed by some chain transfer

reactions to produce polymerized epoxide as given in Scheme 1.7 (Ertürk et al.,

2012).

O Fe3O(O2CCF3)6(H2O)3O

[FexO(O2CCF3)y]

CF3CO2

O

chain transfer reaction

OF3CCO2 LA

LA = [FexO(O2CCF3)y]

n

n polymerized epoxide

Scheme 1.7: Lewis acid-catalyzed anionic reaction mechanism proposed for

[Fe3O(O2CCF3)6(H2O)3] catalyzed epoxide polymerization (Ertürk et al., 2012).

23

The ROP is carried out using organotin phosphate (Bu2SnO-Bu3PO4)

condensate using two types of monomer i.e monofunctional and bifunctional epoxide.

The steric effect of the monofunctional monomer was studied using monosubstituted

and disubstituted epoxide monomers. The ROP results show that the

monosubstituted epoxide monomer give high product conversion while disubstituted

epoxide monomer was considerably low and produce mixture of products in

moderate yield. The ROP reaction using bifunctional epoxide produces polyether with

the epoxide ring in the side chain (Iwasa et al., 2008).

The epoxide ring opening of cyclohexadiene monoepoxide with an amine

base is obtained using bidentate bis-titanium or mono-titanium catalyst as shown in

Scheme 1.8. The reaction with bidentate bis-titanium catalyst produces 73% of amino

alcohol while mono-titanium produces 3% alcohol under identical reaction conditions.

The high catalytic activity of bis-titanium catalyst is due to two titanium metal centers

in its structure and hence two sites of Lewis acidity as compared to the mono-

titanium catalyst. Thus, bidentate bis-titanium is able to form chelation complex with

cyclohexane monoepoxide through double coordination. While mono-titanium is

through single coordination as shown in Scheme 1.9 (Asao et al., 1998).

The ring opening of epoxide from tetra(2,3-epoxypropoxysilane) with

dicyclopentadiene titanium dichloride (Cp2TiCl2) is obtained through free radical

polymerization combined with coordination polymerization. The free radical species

Ti(III) is end-capped at the epoxide ring of the polymer chains. This locates a non-

pair electron at most hindered epoxide carbon which later reacts with copolymer

through coordination polymerization to produce the 4-armed hydroxyl-functionalized

copolymer (Sheng et al., 2014).

24

O +

NBn2

OHcyclohexadiene monoepoxide

Bn2NH

dibenzylamine amino alcohol

bidentate bis-titanium or mono-titanium

CH2Cl20 oC, 9 hours

Scheme 1.8: Epoxide ring opening of cyclohexadiene monoepoxide using titanium

catalysts (Asao et al., 1998).

O

O

O O

(OPr)3Ti Ti(OPr)3

O

O

O

Ti(OPr)3

OPr = isopropoxy

O

O

2

(a)

(b)

O

O

O O

(PrO)3Ti (OPr)3

Ti

bidentate bis-titanium

O

O

O

(OPr)3Ti

mono-titanium

2

mono-titanium chelation complex

bidentate bis-titaniumchelation complex

cyclohexanemonoepoxide

cyclohexanemonoepoxide

Scheme 1.9: (a) Bidentate bis-titanium and (b) mono-titanium catalysts and their

respective chelation complex of cyclohexane monoepoxide (Asao et al., 1998).