Opening Kinetics of Oxazine Ring and Hydrogen Bonding Effects on Fast Polymerization Of

Home , Phloroglucinol

UNDERSTANDING THE VIBRATIONAL STRUCTURE, RING-

OPENING KINETICS OF OXAZINE RING AND HYDROGEN

BONDING EFFECTS ON FAST POLYMERIZATION OF 1,3-

BENZOXAZINES

LU HAN

Submitted in partial fulfillment of the requirements

for the degree of Doctor of Philosophy

Dissertation Advisor: Dr. Hatsuo Ishida

Department of Macromolecular Science and Engineering

CASE WESTERN RESERVE UNIVERSITY

May, 2018

CASE WESTERN RESERVE UNIVERSITY

SCHOOL OF GRADUATE STUDIES

We hereby approve the thesis/dissertation of

Lu Han

Candidate for the degree of Doctor of Philosophy. *

Committee Chair

Dr. Hatsuo Ishida

Dr. Gary Wnek

Dr. Lei Zhu

Dr. Daniel Lacks

Date of Defense

Nov. 2nd, 2017

*We also certify that written approval has been obtained

for any proprietary material contained therein.

DEDICATION

To my parents, Jianjun Han and Jinyan Zhang

TABLES OF CONTENTS

TABLE OF CONTENTS i

LIST OF TABLES iv

LIST OF SCHEMES v

LIST OF FIGURES vi

ACKNOWLEDGEMENTS xiii

ABSTRACT xv

CHAPTER 1: Introduction 1

1.1 Review of benzoxazine 2

1.2 Oxazine ring related modes 3

1.3 Intrinsic ring opening of oxazine ring 3

1.4 Hydrogen bonding in amide-containing benzoxazine 4

1.5 References 4

CHAPTER 2: Study of oxazine ring-related vibrational modes of benzoxazine monomers

2.1 Introduction 8

2.2 Experimental 13

2.3 Results and Discussion 25

2.4 Conclusions 51

2.5 References 51

CHAPTER 3: Investigation of intrinsic self-initiating thermal ring-opening polymerization of 1,3-benzoxazines 55

3.1 Introduction 56

3.2 Experimental 60

3.3 Results and Discussion 65

3.4 Conclusions 91

3.5 References 92

CHAPTER 4: Study of the effects of intramolecular and intermolecular hydrogen- bonding of amide-containing benzoxazines 96

4.1 Introduction 97

4.2 Experimental 99

4.3 Results and Discussion 102

4.4 Conclusions 120

4.5 References 120

CHAPTER 5: Conclusion 125

5.1 Oxazine ring related modes 126

5.2 Intrinsic ring opening of oxazine ring 126

5.3 Hydrogen bonding in amide-containing benzoxazine 126

SUPPORTING INFORMATION 128

iii

LIST OF TABLES

Table 2.1 Potential energy distribution for the oxazine ring-related modes of PH-a which

is calculated using Gaussian 09 with B3LYP/6-31G** theory and basis and VEDA4

software. 43

Table 2.2 Frequency of the characteristic band in experimental FT-IR and Raman and

theoretical model for PH-a, PH-a-phenol-d4, PH-a-2oxazine-d2, PH-a-4oxazine-d2, PH-

15 a-oxazine-d4, PH-d-a and PH-a- N. 46

Table 3.1 Elemental Analysis Results of PH-a. 71

Table 3.2 Weight of 4-methoxyphenol and PH-a of different phenolic content mixture

sample. 72

Table 3.3 Phenolic content in each PH-a/4-metoxyphenol mixture and the specific weight

of every sample studied by DSC. 73

Table 3.4 Polymerization temperature of mixture samples measured by DSC. 76

Table 4.1 Maxima of the N-H stretching absorption bands observed by FT-IR. Spectra were recorded in solution using CHCl3 as solvent. 107

Table 4.2 Examples of reported results on secondary amide N-H stretching band

positions determined by FT-IR. 111

Table 4.3 Chemical shifts of the –O–CH2–N= and =N–CH2–Ar in CDCl3 and DMSO-d6

as solvent. 113

LIST OF SCHEMES

Scheme 2.1 Synthesis scheme of all substituted benzoxazine. 26

Scheme 2.2 Synthesis scheme of isotopes of PH-x, PH-x-phenol-d4, PH-x-oxazine-d4 and

PH-d-a. 30

Scheme 2.3 Synthesis scheme of isotopes of PH-x-2oxazine-d2 and PH-x-4oxazine-d2.

Scheme 3.1 Solvent-less PH-a synthesis. 65

Scheme 4.1. Synthesis of benzoxazine monomers PH-a, pHBA-a, and oHBA-a. 104

LIST OF FIGURES

Figure 2.1 Chemical structure of all-substituted benzoxazines. 10

Figure 2.2 Chemical structure of PH-a series. 12

Figure 2.3 Chemical structure of PH-t series. 12

Figure 2.4 Chemical structure of methyl group substituted benzoxazine series. 13

1 Figure 2.5 H-NMR spectra for (a)25DMR-h, (b)PG-a and (c)PG-anis in CDCl3 at room

temperature. 27

Figure 2.6 Comparison of FT-IR spectra of all-substituted benzoxazine monomer (top)

and its corresponding polymer obtained by heating the monomer at 180oC for 2 h (bottom):

(a) 25DMR-h and its polymer; (b) PG-a and its polymer; (c) PG-anis and its polymer.

Figure 2.7 1H-NMR spectra for PH-a isotopes (left) and PH-t isotopes (right). (a) PH-a,

(b) PH-a-phenol-d4, (c) PH-a-2oxazine-d2, (d) PH-a-4oxazine-d2, (e) PH-a-oxazine-d4,

(f) PH-d-a, (g) PH-t, (h) PH-t-phenol-d4, (i) PH-t-2oxazine-d2, (j) PH-t-oxazine-d4 in

CDCl3 at room temperature. 33

Figure 2.8 FT-IR spectra of PH-a isotopes (left) and PH-t isotopes (right). (a) PH-a, (b)

PH-a-phenol-d4, (c) PH-a-2oxazine-d2, (d) PH-a-4oxazine-d2, (e) PH-a-oxazine-d4, (f)

PH-d-a, (g) PH-t, (h) PH-t-phenol-d4, (i) PH-t-2oxazine-d2, (j) PH-t-oxazine-d4.

Figure 2.9 FT-IR spectra of (a) PH-a, (b) PH-a-15N. 36

1 Figure 2.10 H-NMR spectra for PH-a-4oxazine and PH-a-2oxazine in CDCl3 at room

temperature. 38

Figure 2.11 FT-IR spectra of oxazine ring substituted PH-a. (a) PH-a-4oxazine, (b) PH-

a-2oxazine. 39

Figure 2.12 The crystal structure and optimized structure of PH-a. 40

Figure 2.13 Experimental and theoretical FT-IR results for PH-a. 41

Figure 2.14 Comparison of experimental FT-IR results and theoretical results for FT-IR

spectra of PH-a isotopes (a) PH-a, (b) PH-a-phenol-d4, (c) PH-a-2oxazine-d2, (d) PH-a-

4oxazine-d2, (e) PH-a-oxazine-d4, (f) PH-d-a. 43

Figure 2.15 Characteristic band fitting of FT-IR spectrum of PH-a. 48

Figure 3.1 FT-IR spectrum of benzoxazine monomer, PH-a. 67

1 Figure 3.2 H NMR spectrum of PH-a. Satellites of each –CH2– peak corresponding to H2

and H4 can be better observed in the inset of the figure, which shows a close look near the

baseline within the region between 4 and 6 ppm. Spectrum recorded at 25 °C, using DMSO- d6 as solvent. 68

Figure 3.3 a) DSC thermograms showing the change in the polymerization temperature of

PH-a after successive recrystallization processes. b) Crystals of PH-a after all recrystallization processes. 70

Figure 3.4 DSC thermograms, region between 240 and 190 oC showing the exothermic

polymerization peaks of different mixturesamples with increasing percentages of 4-

methoxyphenol: (a) 0.00% of 4-methoxyphenol (pure PH-a), (b) 0.23 mol% of 4-methoxy-

vii

phenol, (c) 0.44 mol% of 4-methoxyphenol, (d) 0.56 mol% of 4-methoxyphenol, (e) 0.88 mol% of 4-methoxyphenol, and (f) 1.05mol% of 4-methoxyphenol. 75

Figure 3.5 Polymerization temperature as a function of the concentration of a phenol- containing compound (4-methoxypheonol). 77

Figure 3.6 DSC thermogram of PH-a from 30 to 300 °C. The endothermic peak at 59 °C

is assigned to the melting while the exothermic one at 272 °C to the polymerization of PH-

a. 79

Figure 3.7 Resonance structures of the unsubstituted benzoxazine PH-a. Structures in

involving the aromatic ring that was adjacent to the oxazine ring (a) and the aniline moiety

(b). 81

Figure 3.8 Simplified mechanisms for the polymerization of benzoxazines. a) The

classically accepted thermally accelerated (or activated) polymerization mechanism,

where in addition to heat impurities must participate in the initiation step; and b), thermal

polymerization mechanism, induced only by heat. 83

Figure 3.9 Ring-chain tautomeric equilibrium of the unsubstituted benzoxazine PH-a.

Figure 3.10 Resonance structures of the chain-like tautomer of the unsubstituted

benzoxazine PH-a involving the aromatic ring that was adjacent to the oxazine ring.

Figure 3.11 Plots to graphically calculate the activation energy (Ea) of the polymerization

reaction of PH-a applying the Kissinger and Ozawa methods. In the plots, β is the constant

heating rate and Tp is the maximum value of the exothermic polymerization peak. Linear viii

regression obtained for each method are: R2 = 0.996 for the Kissinger method and R2 =

0.997 for the Ozawa method. 87

Figure 3.12 TGA thermogram of the highly purified poly(PH-a). 89

Figure 4.1 (a) 1H and (b) 13C NMR spectra of pHBA-a, (c) 1H and (d) 13C NMR spectra

of oHBA-a. 105

Figure 4.2 FT-IR spectra showing the region between 3500 and 3380 cm-1 for (a) pHBA- a, and (b) oHBA-a. Spectra were recorded in solution at different concentrations using

CHCl3 as solvent. 107

Figure 4.3 pHBA-a, and oHBA-a N-H stretching wavenumber vs. concentration. 109

Figure 4.4 1H NMR spectra of pHBA-a at different concentrations. All spectra were recorded at 25 °C using CDCl3 as solvent. 112

Figure 4.5 DSC thermograms comparing oHBA-a, pHBA-a with PH-a. 115

Figure 4.6 DSC thermograms comparing poly(PH-a) (blue), poly(pHBA-a) (red), and

poly(oHBA-a) (black). 119

Figure S.1 NMR Spectrum of 25DMR-h 129

Figure S.2 NMR Spectrum of PG-a 130

Figure S.3 NMR Spectrum of PG-anis 131

Figure S.4 NMR Spectrum of PH-a 132

Figure S.5 NMR Spectrum of PH-a-phenol-d4 133

Figure S.6 NMR Spectrum of PH-a-2oxazine-d2 134

Figure S.7 NMR Spectrum of PH-a-4oxazine-d2 135

Figure S.8 NMR Spectrum of PH-a-oxazine-d4 136

Figure S.9 NMR Spectrum of PH-d-a 137

Figure S.10 NMR Spectrum of PH-t 138

Figure S.11 NMR Spectrum of PH-t-phenol-d4 139

Figure S.12 NMR Spectrum of PH-t-2oxazine-d2 140

Figure S.13 NMR Spectrum of PH-t-oxazine-d4 141

Figure S.14 NMR Spectrum of PH-a-4oxazine 142

Figure S.15 NMR Spectrum of PH-a-2oxazine 143

Figure S.16 FT-IR Spectrum of 25DMR-h 144

Figure S.17 FT-IR Spectrum of PG-a 145

Figure S.18 FT-IR Spectrum of PG-anis 146

Figure S.19 FT-IR Spectrum of PH-a 147

Figure S.20 FT-IR Spectrum of PH-a-phenol-d4 148

Figure S.21 FT-IR Spectrum of PH-a-2oxazine-d2 149

Figure S.22 FT-IR Spectrum of PH-a-4oxazine-d2 150

Figure S.23 FT-IR Spectrum of PH-a-oxazine-d4 151

Figure S.24 FT-IR Spectrum of PH-d-a 152

Figure S.25 FT-IR Spectrum of PH-t 153

Figure S.26 FT-IR Spectrum of PH-t-phenol-d4 154

Figure S.27 FT-IR Spectrum of PH-t-2oxazine-d2 155

Figure S.28 FT-IR Spectrum of PH-t-oxazine-d4 156

Figure S.29 FT-IR Spectrum of PH-a-4oxazine 157

Figure S.30 FT-IR Spectrum of PH-a-2oxazine 158

Figure S.31 FT-IR Spectrum of PH-a-15N 159

Figure S.32 Raman Spectrum of PH-a 160

Figure S.33 Raman Spectrum of PH-a-phenol-d4 161

Figure S.34 Raman Spectrum of PH-a-2oxazine-d2 162

Figure S.35 Raman Spectrum of PH-a-4oxazine-d2 163

Figure S.36 Raman Spectrum of PH-a-oxazine-d4 164

Figure S.37 Raman Spectrum of PH-d-a 165

Figure S.38 Raman Spectrum of PH-t 166

Figure S.39 Raman Spectrum of PH-t-phenol-d4 167

Figure S.40 Raman Spectrum of PH-t-2oxazine-d2 168

Figure S.41 Raman Spectrum of PH-t-oxazine-d4 169

Figure S.42 Raman Spectrum of PH-a-15N 170

Figure S.43 Theoretical Calculation of PH-a 171

Figure S.44 Theoretical Calculation of PH-a-phenol-d4 172

Figure S.45 Theoretical Calculation of PH-a-2oxazine-d2 173

Figure S.46 Theoretical Calculation of PH-a-4oxazine-d2 174

Figure S.47 Theoretical Calculation of PH-a-oxazine-d4 175

Figure S.48 Theoretical Calculation of PH-d-a 176

Figure S.49 Theoretical Calculation of PH-a-15N 177

xii

ACKNOWLEDGEMENTS

First of all, I would like to express my sincere gratitude to Prof. Dr. Ishida, for his guidance,

help, encouragement. His devotion and enthusiasm to research, teaching, mentoring not

only impart to me of how to do good research, but also how to be a great mentor and even

a good person. Next, I appreciate it that all my committee members, Prof. Lei Zhu, Prof.

Gary Wnek and Prof. Daniel Lacks, spend their previous time on reading my dissertation

and giving me valuable advice and suggestion.

I also extend my great thanks to my parents, who support me all the way, financially and

spiritually. I really appreciate the educational concept my parents shared with each other, which they encourage me to do things that I have passion on, which I show a lot of interest in, which I would like to pursue.

Also, I would like to thank all my co-workers, past and present Ishida group members: Dr.

Seishi Ohashi, Dr. Pablo Froimowicz, Dr. Kan Zhang, Dr. Daniela Iguchi, Dr. Carlos R.

Arza, Dr. Xianze Yin, Dr. Qichao Ran, Dr. Jia Liu, Dr. Wenfei Zhang, Dr. Mariana Fonseca,

Dr. María L. Salum, Phwey Gil, Tyler R. Heyl, Victoria M. Sedwick, Matthew L. Szigeti,

Poroshat Taheri, Chen Ma, Fei Shan, Jeffrey George, Eric Rachita, Vivek Pandey for their guidance and cooperation on the projects we worked together.

Friendly Faculty, staff and students in the department of Macromolecular Science and

Engineering in Case Western Reserve University also help me a lot and make my life much more fun to be with you. I would like to express my thanks to Prof. David Schiraldi, Prof.

João Maia, Prof. Alex Jamieson, Prof. Michael Hore, Prof. Liming Dai, Prof. Ica Manas-

Zloczower, Prof. Jon Pokorski, Dr. Anna Akkus, Lisa Hodges, Charlotte Foster, Dr.

xiii

Jaqueline Wallat, Dr. Parker Lee, Dr. Saide Tang, Dr. Guoqiang Zhang, Dr. Yufeng Zhu,

Zhongbo Zhang, Dr. Longhe Zhang, Malanie Hutnick, Dr. Qiong Wu, Qiyi Chen, William

Lenart, Symone Alexander, Machelle Leslie, Min Wang, Xinyue Chen, Dr. Huadong

Huang, Zhenpeng Li, Ci Zhang, Xinting Wang, Kimberly DeGrace, Qiong Li, Hua Sun,

Ammar Patel, Cong Zhang, Jun Zhang, Mingzhe Sun, Sunsheng Zhu, Yi Zheng, Dr.

Taneisha Deans, Dr. Imre Treufeld.

Last but not least, I want to extend my gratitude to all the supporting and nice friends that

I met in Cleveland/Akron: Prof. Stephen Z. D. Cheng, Qihuan Song, Jing Jiang, Orange

Som, Dan Huang, Yu Zhang, Steven Mankoci, Jacob Alan Hill, Carl Pfromm, Wenpeng

Shan, Kan Wu, Fei Li, Mary Swanzy, Xiaozhou Shen, Jie Yu, Qiyao Li, Qianhui Liu,

Ximing Li, Suzi Mason. They made my life so much more colorful and thanks for keeping me company for an unforgettable journey.

xiv

Understanding the Vibrational Structure, Ring-opening Kinetics of Oxazine

Ring and Hydrogen Bonding Effects on Fast Polymerization of 1,3-

Benzoxazines

Abstract

LU HAN

Polybenzoxazines are relatively new but already commercialized thermosetting resin. The material has already gathered much attention from not only academia but also industry field due to its superior properties than conventional phenolic, epoxy and bismaleimide resins. However, many people focus on working towards high performance material, much less work has been done on the fundamental study on understanding the vibrational structure, ring-opening kinetics of oxazine ring, which will be a great guidance on future molecular design and higher performance material, resins, composites.

Chapter 1: In this chapter, the previous work and current research interest are covered,

which leads to my research motivation for understanding the vibrational structure, ring-

opening kinetics of oxazine ring and hydrogen bonding effects on fast polymerization of

1,3-benzoxazines.

Chapter 2: In this chapter, the characterization band of benzoxazines and

polybenzoxazines in FT-IR spectra is reassigned. Polymerization of benzoxazine resins is

indicated by the disappearance of a 960-900 cm-1 band in infrared spectroscopy (IR).

Historically, this band was assigned to the C-H out-of-plane bending of the benzene to which the oxazine ring is attached. This study shows that this band is a mixture of the O-

C2 stretching of the oxazine ring and the phenolic ring vibrational modes. Vibrational

frequencies of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine (PH-a) and 3-(tert-butyl)-

3,4-dihydro-2H-benzo[e][1,3]oxazine (PH-t) are compared with isotope-exchanged and all-substituted compounds. Deuterated benzoxazine monomers, 15N-isotope exchanged

benzoxazine monomers, and all-substituted benzoxazine monomers without aromatic C-H

groups are synthesized and studied meticulously. The various isotopic-exchanges involved

deuteration around the benzene ring of phenol, selective deuteration of each CH2 in the O-

CH2-N (2) and N-CH2-Ar (4) positions on the oxazine ring, or simultaneous deuteration of

both positions. The chemical structures were confirmed by 1H nuclear magnetic resonance

spectroscopy (1H NMR). The IR and Raman spectra of each compound are compared.

Further analysis of 15N isotope-exchanged PH-a indicates the influence of the nitrogen

isotope on the band position, both experimentally and theoretically. This finding is

important for polymerization studies of benzoxazines that utilize vibrational spectroscopy. xvi

Chapter 3: In this chapter, the intrinsic ring opening of oxazine ring mechanism is proposed.

A phenol/aniline type monofunctional benzoxazine monomer, PH-a, is synthesized and

highly purified into single crystals to study the intrinsic thermal ring-opening

polymerization of benzoxazines without the influence of any impurity. The successful

synthesis of the monomer and its corresponding chemical structure are confirmed by

Fourier transform infrared spectroscopy (FT-IR) and 1H nuclear magnetic resonance (1H

NMR) spectroscopy. Purity of the compound is evaluated through differential scanning

calorimetry (DSC) as well as elemental analysis (EA). Moreover, the thermal behavior of

benzoxazine monomer toward polymerization is also studied by DSC, indicating that the

highly purified benzoxazine monomer actually polymerize upon heating. The results

present evidence of an intrinsic tendency for 1,3-benzoxazines to undergo thermally

induced ring-opening polymerization upon heating only without any impurity participating

during the reaction. This reveals that polybenzoxazines can be obtained by both the

traditional thermally accelerated (or activated) polymerization, where impurities or

purposefully added initiators are involved in the reaction; or, by the classic thermal

polymerization, where only heat is enough to initiate the reaction.

Chapter 4: Base on previous understanding the vibrational structure, ring-opening kinetics of oxazine ring, smart molecular design of ortho-structure containing hydrogen bonding benzoxazine monomer is proposed. Pure para and ortho position amide benzoxazines, pHBA-a and oHBA-a, are synthesized. The hydrogen bonding interactions occurring in

xvii

pHBA-a, oHBA-a are studied by Fourier transform infrared (FT-IR) spectroscopy and

proton nuclear magnetic resonance spectroscopy (1H NMR). FT-IR results show that, while pHBA-a presents intermolecular hydrogen bonding interactions, oHBA-a exhibits an intramolecular five-membered-ring hydrogen-bonding system between the NH in the amide group and the oxygen in the oxazine ring. Differential scanning calorimetry (DSC) is used to study the thermal properties of the resins and their respective polymers. A deeper understanding of the hydrogen bond interactions in this family of resins is attempted to have better insights on how these systems influence the polymerization behavior not only with respect to the polymerization temperature but also with respect to the propagation step.

Chapter 5: This chapter focuses on the conclusion that obtained from the previous study. It also demonstrates the great guidance for future study.

xviii

CHAPTER 1

Introduction

1.1 Review of benzoxazine

Polybenzoxazine is a relatively new but already commercialized thermosetting resin that

has attracted a lot of attention, not only from academia but also from industry field, due to

its unique and excellent properties, such as zero shrinkage upon polymerization,1 high

thermal stability,2 low water absorption,3 low flammability,4 excellent chemical

resistances,5 low surface free energy.6 Polybenzoxazines can be abtained by polymerizing

1,3- benzoxazine precursor monomers. 1,3-benzoxazine monomers are synthesized by the

Mannich condensation from phenol, amine, and formaldehyde.7 Precursor resins can be

either monomeric or polymeric with the benzoxazine moiety in the main chain,8 side chain,9 or chain ends.10 The most interesting and exciting characteristic of this resin is the

extraordinarily rich molecular design flexibility, which allows designing and synthesizing

a variety of molecular structures, catering different desired properties and difference

usage.11,12

Benzoxazines can be polymerized as matrices in composite materials in combination with

glass fibers,13 carbon fibers,14 cellulose fibers15 or inorganic powders, for example, calcium carbonate,16 boron nitride,17 silicon nitride,18 and silicon carbide.19 Also, fillers and

nanofillers are also studied including carbon nanotubes,20 graphene,21 graphene oxide,22.

Therefore, the wide and deep scientific study of benzoxazine and polybenzoxazine shows

a great potential of a was variety of application, such as materials for aerospace and

spacecraft, electronic packaging materials, ballistics.

Due to the flexible molecular design and wide range of applications, many people focused

on novel material design, new structures, leading to specific high performance properties.23-

25 However, less work has been done regarding fundamental understanding the vibrational structure, ring-opening kinetics of oxazine ring and hydrogen bonding effects on fast

polymerization of 1,3-benzoxazines, which in turn will have a great benefit for molecular

design and higher performance material without many aimless trial and error.

1.2 Oxazine ring related modes

Polybenzoxazines are characterized by the band between 960-900 cm-1 in FT-IR spectra.

Benzoxazine monomer shows a very broad band in this region. When it starts to

polymerize, the band gradually disappear. A totally polymerized benzoxazine does not

show the band. Therefore, it is a very important band to characterize benzoxazine and

observe the polymerization process. In 1995, Dunkers and Ishida reported that the band does not exactly coincide, but that there is a time lag between the intensity change of the

1948 cm-1 (which is directly associated with the polymerization) and the band of current

interest near 960-900 cm-1.26 They showed in the paper the band is strong related to the C-

H out-of-plane bending vibration until the evidence show that even totally substituted

benzoxazine on benzene ring show the characterization band and the band disappeared

after polymerization. Therefore, oxazine ring-related vibrational modes of benzoxazine

monomers is studied by using fully aromatically substituted, deuterated, 15N isotope

exchanged and oxazine-ring-substituted compounds and theoretical calculation.

1.3 Intrinsic ring opening of oxazine ring

Polybenzoxazines are obtained by polymerizing benzoxazine monomers, during which the

second position of the oxazine ring opens and polymerized.27 Multifuntional benzoxazine

can crosslink during this polymerization process.28 Therefore, it is very important to study

the ring opening mechanism and kinetics. Previously, people has very blurry concepts on

intrinsic polymerization and thermally accelerated polymerization. The main purpose of

this project is to obtain strong experimental evidence of benzoxazine polymerization by an

intrinsic-ring opening mechanism, rather than the initiation by the external initiators or

impurities. To achieve this goal, meticulous effort for monomer purification was carried

out. Thus, only single crystals of the monomers that were prepared under carefully

controlled environment were used in this work. The detailed synthetic strategy,

purifications, and polymerization mechanisms are discussed in this article.

1.4 Hydrogen bonding in amide-containing benzoxazine

Based on the previous study, the oxazine ring can open by itself without any impurities,

initiators, catalyst. Therefore, it is practical to design molecules within the structure to help

the ring opening. Hydrogen bonding was reported to be very useful in molecule structure

formation. By designing the molecules, forming five or six member rings within the

molecules, it can lead to a very stable ring structure resulting from hydrogen bonding.

Previous studies demonstrated the unusual phenomena that ortho-substituted benzoxazines

shows enhanced properties when compared to para position counterpart, which is quite

unique compared to other molecules.29 However, the reason behind it is not clearly studied.

Pure para and ortho position amide benzoxazines, pHBA-a and oHBA-a, are synthesized.

The hydrogen bonding interactions occurring in pHBA-a, oHBA-a are studied by Fourier

transform infrared (FT-IR) spectroscopy and proton nuclear magnetic resonance

spectroscopy (1H NMR).

1.5 References

(1) Ishida, H.; Allen, D. J. J. Polym. Sci. Part B: Polym. Phys. 1996, 34, 1019-1030.

(2) Agag, T.; Takeichi, T. Macromolecules 2003, 36, 6010-6017.

(3) Rimdusit, S.; Tanthapanichakoon, W.; Jubsilp, C. J. Appl. Polym. Sci, 2006, 99, 1240–

1253.

(4) Espinosa, M. A.; Galia, M.; Cadiz, V. Polymer 2004, 45, 6103-6109.

(5) Kim, H. D.; Ishida, H. J. Appl. Polym. Sci. 2001, 79, 1207-1219.

(6) Wang, C. F.; Su, Y. C.; Kuo, S. W.; Huang, C. F.; Shen, Y. C.; Chang, F. C. Angew.

Chem.-Int. Edit. 2006, 45, 2248-2251.

(7) Ishida, H.; Froimowicz, P. Advanced and Emerging Polybenzoxazine Science and

Technology; Elsevier: Amsterdam, 2017.

(8) Chernykh, A.; Agag, T.; Ishida, H. Polymer 2009, 50, 382-390.

(9) Agag, T.; Vietmeier, K.; Chernykh, A. Ishida, H. J. Appl. Polym. Sci., 2012, 125, 1346–

1351.

(10) Nakamura, M.; Ishida, H. Polymer 2009, 50, 2688–2695.

(11) Tuzun, A.; Lligadas, G.; Ronda, J. C.; Galià, M.; Cádiz, V. In Advanced and

Emerging Polybenzoxazine Science and Technology; Ishida, H.; Froimowicz, P. Eds.;

Elsevier: Amsterdam, 2017, 65-74.

(12) Verge, P.; Puchot, L.; Vancaeyzeele, C.; Vidal, F.; Habibi, Y. In Advanced and

Emerging Polybenzoxazine Science and Technology; Ishida, H.; Froimowicz, P. Eds.;

Elsevier: Amsterdam, 2017, 89-107.

(13) Wang, X. Y.; Gu,Y. J. Macromol. Sci. Phys. 2011, 50, 2214–2226.

(14) Ishida, H.; Chaisuwan,T. Polym. Compos. 2003, 24, 597–607.

(15) Jubsilp, C.; Takeichi, T.; Hiziroglu, S.; Rimdusit, S. Bioresour. Technol. 2008, 99,

8880–8886.

(16) Suprapakorn, N.; Dhamrongvaraporn, S.; Ishida, H. Polym. Compos. 1998, 19, 126–

132.

(17) Ishida, H.; Rimdusit,S. Thermochim. Acta. 1998, 320, 177–186.

(18) Ramdani, N.; Wang, J.; Wang, H.; Feng, T. T.; Derradji, M.; Liu, W. B. Compos. Sci.

Technol. 2014, 105, 73–79.

(19) Kasemsiri, P.; Hiziroglu, S.; Rimdusit, S. Composites Part A 2011, 42, 1454–1462.

(20) Chen, Q.; Xu, R. W.; Yu, D. S. Polymer 2006, 47, 7711–7719.

(21) Ho, K. K.; Hsiao, M. C.; Chou, T. Y.; Ma, C. C. M.; Xie,.X F.; Chiang, J. C.; Yang, S.

H.;

Chang, L. H. Polym. Int. 2013, 62, 966–973.

(22) Arza, C. R.; Ishida, H.; Maurer, F. H. J. Macromolecules 2014, 47, 3685–3692.

(23) Shen, X.; Dai, J.; Liu, Y.; Liu, X.; Zhu, J. Polymer 2017, 122, 258-269.

(24) Shen, S. B.; Ishida, H. Polym. Compos. 1996, 17, 710-719.

(25) Kiskan, B.; Ghosh, N.N.; Yagci, Y. Polym. Intern

(26) Dunkers, J.; Ishida, H. Spectrochim. Acta. A-M. 1995, 51, 1061-1074.

(27) Wang, M. W.; Jeng, R. J, Lin, C. H. Macromolecules 2015, 48, 530-535. 6

(28) Soto M.; Hiller, M.; Oschkinat, H.; Koschek, K. Polymers 2016, 8, 278.

(29) Liu, J.; Ishida, H. Macromolecules 2014, 47, 5682-5690.

CHAPTER 2

Study of oxazine ring-related vibrational modes of benzoxazine

monomers

This work has been published as “Oxazine Ring-related Vibrational Modes of Benzoxazine

Monomers Using Fully Aromatically Substituted, Deuterated, 15N Isotope Exchanged and

Oxazine-ring-substituted Compounds and Theoretical Calculation”, L. Han, D. Iguchi, P.

Gil, T. R. Heyl, V. M. Sedwick, C. R. Arza, S. Ohashi, D. J. Lacks, H. Ishida, J. Phys.

Chem. A 2017, 121, 6269−6282.

Characteristic Band

972 953 941 928

999 987

947cm-1

Wavenumber(cm-1)

2.1 Introduction

Polybenzoxazine is a relatively new and commercialized thermoset resin obtained by polymerizing 1,3-benzoxazine. Recently, polybenzoxazine received much attention because of their many unique and desirable properties, such as near-zero shrinkage upon polymerization,1,2 low flammability,3 lower surface free energy than

polytetrafluoroethylene without having fluorine atoms,4,5 extremely flexible molecular

design capability,6-11 which even allows the use of phenols and amines from natural

renewable resources,12 and other excellent mechanical13-15 and thermal properties.16-17

Benzoxazines are characterized by a broad and intense band at 960-900 cm-1 in the FT-IR

spectrum. This band gradually disappears when benzoxazine is heated to the

polymerization temperature. For fully polymerized samples, this band completely

disappears.7,18 It has been reported by Dunkers and Ishida that the disappearance of this

band does not exactly coincide, but that there is a time lag between the intensity change of

the 1498 cm-1 band (which is directly associated with the polymerization) and the band of

current interest near 960-900 cm-1.19,20 While the circumstantial evidence clearly points to

the strong association of the 960-900 cm-1 band with benzoxazine polymerization, the

nature of this band has not been well understood. Historically, this band of 1,3-benzoxazine

was assigned to the C-H out-of-plane bending vibrations of benzene to which the oxazine is attached. Since the band assignment of this mode in early days of benzoxazine research many years ago, no rigorous effort was made to understand this mode in detail despite its very important role in polymerization studies of benzoxazine resin. Therefore, the goal of this study is to understand the nature of this band by systematically studying the FT-IR and

Raman spectra of the compounds specifically designed to answer questions related to the group that is involved and nature of the vibration.

Benzoxazines are usually synthesized by one-pot Mannich condensation of phenol, amine and formaldehyde.21 Various benzoxazine monomers can be synthesized by using different

phenolic derivatives and primary amines, which are commercially available. To selectively

substitute the oxazine ring, however, there is a limitation for one-pot synthesis. In order to 9

selectively substitute or deuterate the 2-position of the oxazine ring, 2-step synthesis should

be conducted by starting with ortho-hydroxyl substituted benzaldehyde.22-23 By using

different benzaldehyde or formaldehyde, different oxazine-substituted benzoxazine

monomers can be synthesized. On the other hand, 4 position deuterated benzoxazine can

be obtained by hydrolyzing the all deuterated benzoxazine,24 followed by closing the ring

using normal paraformaldehyde.

The approaches taken in the current paper can be largely classified into two steps. First, in

order to test if the band is directly from the aromatic C-H groups, a series of fully

aromatically substituted benzoxazines were designed and synthesized. Two benzoxazine

monomers, bis- and tris-oxazine as shown below in Figure 1.1, are designed to evaluate

the role of the aromatic C-H on the benzoxazine modes. If the band of interest is due to the

aromatic C-H, these monomers should not show this characteristic band. Although PG-a

and PG-anis are the same with respect to the substitution structure, it was nonetheless

synthesized to perturb the overlapped vibrational bands.

Figure 2.1 Chemical structure of all-substituted benzoxazines.

Isotopic substitution is a useful technique to assign FT-IR or Raman bands because isotopically substituted molecules are different from the unsubstituted molecule only by 10

weight but not the electronic structure, leading to different vibrational frequencies from the

25 same vibrational modes. PH-a-phenol-d4, shown in Figure 2.2, was designed to further evaluate if the characteristic mode is aromatic related.

The second goal of the current paper is to examine, if the band of current interest is not aromatic C-H related, then which group of the oxazine it is most strongly related to.

Compounds PH-a-2oxazine-d2, PH-a-4oxazine-d2 and PH-a-oxazine-d4, are designed to evaluate if the band is related to oxazine 2, 4 position CH2. By using symmetric and asymmetric deuteration, the specific position of the CH2 that is related to the band can be examined. Furthermore, if the band is related to the skeletal oxazine ring mode rather than oxazine CH2 modes, the position of the vibration, either O-C-N or N-C-Ar can be evaluated.

PH-d-a was designed to study whether the modes are associated with aniline part.

Figure 2.2. Chemical structure of PH-a series.

The following four compounds (shown in Figure 2.3) are the corresponding tert-amine versions of the monomers shown above. By using these compounds, the overlapped aromatic amine proton information can be further eliminated. Although some information obtained from the aniline-series and tert-amine series are redundant, it is nonetheless useful to make sure that the information obtained is reproducible.

Figure 2.3. Chemical structure of PH-t series.

15N-exchanged PH-a monomer, PH-a-15N, will further allow if the characteristic mode is related

to the N-C group or Ar-O/Ar-C groups.

Finally, by changing the substitution position of methyl group on the oxazine ring (shown in

Figure 2.4), further strong evidence can be obtained if the band belongs either to the N-CH2-Ar or

15 O-CH2-N group and the conclusion derived from the N-isotope compound can be verified.

Figure 2.4. Chemical structure of methyl group substituted benzoxazine series.

2.2 Experimental

2.2.1. Materials

2,5-dimethylresorcinol (95%), hexylamine (99%), phloroglucinol (≥99%), formaldehyde solution (37 wt. % in H2O, contains 10-15% ethanol as stabilizer), aniline (99%), glacial

acetic acid, p-anisidine (≥99%), salicylaldehyde (98%), paraformaldehyde-d2 (98 atom %

D), phenol (≥99%), phenol-d6 (99 atom % D), hydrochloric acid (37.5%), distilled water, ammonium hydroxide (25% solution), tert-butyl amine (98%), aniline-15N (98 atom %

15 N), 2-acetylphenol (≥98%), acetaldehyde(≥99%), and aniline-2,3,4,5,6-d5 (98 atom % D)

were purchased from Sigma-Aldrich. Chloroform-d was obtained from Cambridge Isotope

Laboratories, Inc. Paraformaldehyde (96%) was obtained from Acros Organics.

Magnesium sulfate anhydrous, sodium hydroxide, toluene (99.9%), ethanol (200 proof),

hexane (4.2% various methylpentanes), ethyl acetate, acetone, formic acid (88%),

chloroform (99.9%), dichoromethane (99.9%), and diethyl ether (99.9%) were purchased

from Fisher Scientific.

2.2.2 Characterization

1H nuclear magnetic resonance (NMR) spectra were acquired on a Varian Oxford AS600

at a proton frequency of 600 MHz. The average number of transients for 1H measurement

was 64. A relaxation time of 10 s was used for the integrated intensity determination of 1H

NMR spectra. Fourier transform infrared (FT-IR) spectra were obtained using a Bomem

Michelson MB100 FTIR spectrometer, which was equipped with a deuterated triglycine

sulfate (DTGS) detector and a dry air purge unit. Coaddition of 64 scans was recorded at a

resolution of 4 cm-1. The intensity of the strongest absorption band was controlled to 0.7-1

for accuracy, by changing the thickness of the film casted on the KBr plate.

2.2.3. Synthesis

2.2.3.1. Synthesis of 3,7-dihexyl-5,10-dimethyl-3,4,7,8-tetrahydro-2H,6H-benzo[1,2-

e:5,4-e']bis([1,3]oxazine) (abbreviated as 25DMR-h)

Toluene (5 mL) was added to a mixture of 2, 5-dimethylresorcinol (0.100 g, 0.72 mmol), paraformaldeyde (0.109 g, 3.62 mmol) and hexylamine (0.147 g, 1.45 mmol) in a 15 mL round bottom flask. The mixture was magnetically stirred for 6 h under reflux. The completed reaction product was washed with 1N NaOH three times and water three times.

The product was dried over magnesium sulfate anhydrous overnight. The solution was filtered to remove the salt. After drying the filtrate, the product was dried in a vacuum oven

1 to obtain a light brown powder. Yield: 73%. H-NMR (600 MHz, CDCl3, ppm): δ 0.81-

0.87 (m, 6H, -CH3), 1.23-1.32 (m, 16H, CH2-(CH2)4- CH3), 1.89 (s, 3H, -CH3), 1.98 (s, 3H,

-CH3), 2.69 (t, 4H, N-CH2-(CH2)4), 3.83 (s, 4H, Ar-CH2-N), 4.76 (s, 4H, N-CH2-O).

2.2.3.2. Synthesis of 3,7,11-triphenyl-3,4,7,8,11,12-hexahydro-2H,6H,10H-benzo[1,2- e:3,4-e':5,6-e'']tris([1,3]oxazine) (abbreviated as PG-a)

Ethanol (5 mL) was added to a mixture of phloroglucinol (0.100 g, 0.79 mmol), formaldehyde (0.483 g, 5.95 mmol), aniline (0.222 g, 2.38 mmol) and a catalytic amount of glacial acetic acid in a 15 mL round bottom flask. The mixture was magnetically stirred for 10 h at room temperature. The product precipitated from the reaction solvent and was collected by filtration. The product was dried in a vacuum oven to obtain a white powder.

1 Yield: 74%. H-NMR (600 MHz, CDCl3, ppm): δ 4.43 (s, 6H, Ar-CH2-N), 5.27 (s, 6H, N-

CH2-O), 6.91 (d, 3H, ArH), 7.08 (d, 6H, ArH), 7.24 (t, 6H, ArH).

2.2.3.3. Synthesis of 3,7,11-tris(4-methoxyphenyl)-3,4,7,8,11,12-hexahydro-

2H,6H,10H-benzo[1,2-e:3,4-e':5,6-e'']tris([1,3]oxazine) (abbreviated as PG-anis)

Ethanol (5 mL) was added to a mixture of phloroglucinol (0.100 g, 0.79 mmol), formaldehyde (0.483 g, 5.95 mmol), p-anisidine (0.293 g, 2.38 mmol) and a catalytic amount of glacial acetic acid in a 15 mL round bottom flask. The mixture was magnetically stirred for 8 h at room temperature. The product precipitated from the reaction solvent and collected by filtration. The product was dried in a vacuum oven to obtain white powder.

1 Yield: 92%. HNMR (600 MHz, CDCl3, ppm): δ 3.73 (s, 9H, CH3), 4.34 (s, 6H, Ar-CH2-

N), 5.18 (s, 6H, N-CH2-O), 6.78 (d, 6H, ArH), 7.02 (d, 6H, ArH).

2.2.3.4. Synthesis of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine (abbreviated as

PH-a)

Toluene (3 mL), phenol (0.100 g, 1.06 mmol), aniline (0.094 g, 1.01 mmol) and

paraformaldehyde (0.076 g, 2.53 mmol) were added to a 10 mL round bottom flask. The

mixture was magnetically stirred and heated under reflux for 6 h. The completed reaction

was washed with 1N NaOH three times and water three times. The product was dried over

magnesium sulfate anhydrous. The material was purified in an alumina neutral column

with hexane and ethyl acetate (9:1) as the eluent. The material was recrystallized from

1 hexane. The product was a white crystal. Yield: 85%. H-NMR (600MHz, CDCl3, ppm): δ

4.64 (s, 2H, Ar-CH2-N), 5.38 (s, 2H, N-CH2-O), 6.84-6.85 (d,1H, ArH), 6.90-6.92 (t, 1H,

ArH), 6.94-6.97 (t, 1H, ArH), 7.02-7.04 (d, 1H, ArH) 7.13-7.14 (m, 3H, ArH), 7.27-7.30

(m, 2H, ArH).

2.2.3.5. Synthesis of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine-5,6,7,8-d4

(abbreviated as PH-a-phenol-d4)

Toluene (3 mL), phenol-d6 (0.100 g, 1.01 mmol), paraformaldehyde (0.072 g, 2.40 mmol), and aniline (0.089 g, 0.96 mmol) were added to a 10 mL round bottom flask. The mixture was magnetically stirred and heated under reflux for 6 h. The completed reaction was washed with 1N NaOH three times and water three times. The product was dried over magnesium sulfate anhydrous. The material was purified in an alumina neutral column with hexane and ethyl acetate (9:1) as the eluent. The material was recrystallized from

1 hexane. The product was a white crystal. Yield: 88%. H-NMR (600 MHz, CDCl3, ppm):

4.62 (s, 2H, Ar-CH2-N), 5.34 (s, 2H, N-CH2-O), 6.90-6.92 (t, 1H, ArH) 7.08-7.10 (d, 2H,

ArH), 7.23-7.26 (t, 2H, ArH).

2.2.3.6. Synthesis of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine-2,2-d2

(abbreviated as PH-a-2oxazine-d2)

Ethanol (40 mL) was added to a mixture of salicylaldehyde (1.000 g, 8.18 mmol) and aniline (0.762 g, 8.18 mmol) in a 100 mL round bottom flask. The mixture was magnetically stirred for 2 h under reflux. After cooling to room temperature, NaBH4 (1.549 g, 40.8 mmol) was added once an hour, three times. The solution was precipitated in water and dried to yield a white powder 2-((phenylamino)methyl)phenol (2-pamp). The powder was dried in vacuum oven to eliminate the remaining water. Yield: 94%.

Toluene (3 mL), 2-pamp (0.467 g, 2.34 mmol), and paraformaldehyde-d2 (0.093 g, 2.93

mmol) were added to a 10 mL round bottom flask. The mixture was magnetically stirred

and heated under reflux for 6 h. The completed reaction was washed with 1N NaOH three

times and water three times. The product was dried over magnesium sulfate anhydrous.

The material was purified in an alumina neutral column with hexane and ethyl acetate (9:1)

as the eluent. The material was recrystallized from hexane. The product produced was a

1 white crystal. Yield: 83%. H-NMR (600 MHz, CDCl3, ppm): δ 4.62 (s, 2H, Ar-CH2-N),

6.79-6.80 (d, 1H, ArH), 6.86-6.88 (t, 1H, ArH), 6.91-6.93 (t, 1H, ArH), 6.99-7.01 (d, 1H,

ArH), 7.09-7.11 (t, 3H, ArH), 7.23-7.26 (t, 2H, ArH).

2.2.3.7. Synthesis of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine-2,2,4,4-d4

(abbreviated as PH-a-oxazine-d4)

Toluene (15 mL) was added to a mixture of phenol (0.500 g, 5.32 mmol), paraformaldehyde-d2 (0.405 g, 5.00 mmol), and aniline (0.472 g, 5.10 mmol) in a 50 mL round bottom flask. The mixture was magnetically stirred and heated under reflux for 6 h.

The completed reaction was washed with 1N NaOH three times and water three times. The

product was dried over magnesium sulfate anhydrous. The material was purified in an

alumina neutral column with hexane and ethyl acetate (9:1) as the eluent. The material was

recrystallized from hexane. The product produced was a white crystal. Yield: 85%. 1H-

NMR (600 MHz, CDCl3, ppm): δ 6.73-6.750 (d, 1H, ArH), 6.81-6.83 (t, 1H, ArH), 6.94-

6.96 (d, 1H, ArH), 7.03-7.06 (t, 3H, ArH), 7.18-7.21 (t, 2H, ArH).

2.2.3.8. Synthesis of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine-4,4-d2

(abbreviated as PH-a-4oxazine-d2)

PH-a-oxazine-d4 (0.500 g, 2.32 mmol) was dissolved in 25 mL of n-propanol,

hydrochloric acid (37.5% w/w, 7.14 mL) and distilled water (3.57 mL) was added to the

solution. The mixture was magnetically stirred and heated under reflux for 2 h. After

cooling to 0oC, 25% ammonium hydroxide (17.87 mL) was added to the solution. The

mixture was magnetically stirred for 1 hour at room temperature. After extracting with

ethyl acetate 3 times, a white powder 2-((phenylamino)methyl-d2)phenol (2-pamd2p) was

obtained after drying the solvent in the vacuum oven. Yield: 95%.

Toluene (3 mL), 2-pamd2p (0.259 g, 1.29 mmol), and paraformaldehyde-d2 (0.048 g, 1.61

mmol) were added to a 10 mL round bottom flask. The mixture was magnetically stirred

and heated under reflux for 6 h. The completed reaction product was washed with 1N

NaOH three times and water three times. The product was dried over magnesium sulfate

anhydrous. The material was purified in an alumina neutral column with hexane and ethyl

acetate (9:1) as the eluent. The material was recrystallized from hexane. The product

1 produced was a white crystal. Yield: 82%. H-NMR (600 MHz, CDCl3, ppm): δ 5.34 (s,

2H, N-CH2-O), 6.78-6.80 (d, 1H, ArH), 6.86-6.88 (t, 1H, ArH), 6.91-6.93 (t, 1H, ArH),

6.99-7.00 (d, 1H, ArH), 7.08-7.11 (t, 3H, ArH), 7.23-7.26 (t, 2H, ArH).

2.2.3.9. Synthesis of 3-(phenyl-d5)-3,4-dihydro-2H-benzo[e][1,3]oxazine (abbreviated as PH-d-a)

Toluene (3 mL), phenol (0.100 g, 1.06 mmol), paraformaldehyde (0.076 g, 2.53 mmol) and aniline-2,3,4,5,6-d5 (0.099 g, 1.01 mmol) were added to a 10 mL round bottom flask. The mixture was magnetically stirred and heated under reflux for 6 h. The completed reaction

was washed with 1N NaOH three times and water three times. The product was dried over

magnesium sulfate anhydrous. The material was purified in an alumina neutral column

with hexane and ethyl acetate (9:1) as the eluent. The material was recrystallized from

hexane. The product produced was a white crystal. Yield: 90%. 1H-NMR (600 MHz,

CDCl3, ppm): 4.66 (s, 2H, Ar-CH2-N), 5.39 (s, 2H, N-CH2-O), 6.83-6.85 (d, 1H, ArH)

6.90-6.93 (t, 1H, ArH), 7.04-7.05 (d, 1H, ArH), 7.13-7.16 (t, 1H, ArH).

2.2.3.10. Synthesis of 3-(tert-butyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine (abbreviated

as Ph-t)

Toluene (3 mL) was added to a mixture of phenol (0.100g, 1.06 mmol), tert-butyl amine

(0.074 g, 1.01 mmol) and paraformaldehyde (0.076 g, 2.40 mmol) in a 10 mL round bottom

flask. The mixture was magnetically stirred for 2 h under reflux. The yellowish solution

was washed with 1N NaOH three times and water three times. The solution was dried over

magnesium sulfate anhydrous overnight. The material was purified in an alumina neutral

column with hexane as the eluent. The purified compound was a transparent liquid. Yield:

1 77%. H-NMR (600 MHz, CDCl3, ppm): δ 1.22 (s, 9H, CH3), 4.130 (s, 2H, Ar-CH2-N),

5.00 (s, 2H, N-CH2-O), 6.76-6.77 (d, 1H, ArH), 6.85-6.87 (t, 1H, ArH), 6.99-7.00 (d, 1H,

ArH), 7.08-7.10 (t, 1H, ArH).

2.2.3.11. Synthesis of 3-(tert-butyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine-5,6,7,8-d4

(abbreviated as PH-t-phenol-d4)

Toluene (3 mL) was added to a mixture of phenol-d6 (0.100 g, 1.00 mmol), tert-butyl amine

(0.070 g, 0.95 mmol) and paraformaldehyde (0.071 g, 2.40 mmol) in a 10 mL round bottom flask. The mixture was magnetically stirred for 2 h under reflux. The yellowish solution was washed with 1N NaOH three times and water three times. The solution was dried over

magnesium sulfate anhydrous overnight. The material was purified in an alumina neutral

column with hexane as the eluent. The purified compound was a transparent liquid. Yield:

1 78%. H-NMR (600 MHz, CDCl3, ppm): δ 1.184 (s, 9H, CH3), 4.097 (s, 2H, Ar-CH2-N),

4.962 (s, 2H, N-CH2-O).

2.2.3.12. Synthesis of 3-(tert-butyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine-2,2-d2

(abbreviated as PH-t-2oxazine-d2)

Ethanol (20 mL) was added to a mixture of salicylaldehyde (1.000 g, 8.19 mmol) and

aniline (0.599 g, 8.19 mmol) in a 50 mL round bottom flask. The mixture was magnetically

stirred for 2 h under reflux. After cooling to room temperature, NaBD4 (1.549 g, 40.95

mmol) was added once an hour for three times. The solution was precipitated in 100 mL

water by adjusting the pH to 8. Dichloromethane was used to extract the intermediate 2-

((tert-butylamino)methyl)phenol (2-tbamp) from the aqueous layer three times. The solution was dried over magnesium sulfate anhydrous overnight. After filtering the salt, the filtrate was dried in a vacuum oven. A white crystal was obtained. Yield: 62%.

Toluene (3 mL), 2-tbamp (0.200 g, 1.12 mmol), and paraformaldehyde-d2 (0.045 g, 1.40

mmol) were added to a 10 mL round bottom flask. The mixture was magnetically stirred

and heated under reflux for 2 h. The yellowish solution was washed with 1N NaOH three

times and water three times. The solution was dried over magnesium sulfate anhydrous

overnight. The material was purified in an alumina neutral column with hexane as the

eluent. The purified compound was a transparent liquid. Yield: 77%. 1H-NMR (600 MHz,

CDCl3, ppm): δ 1.20 (s, 9H, CH3), 4.11 (s, 2H, Ar-CH2-N), 6.75 (d, 1H, ArH), 6.83-6.85

(t, 1H, ArH), 6.95-6.97 (d, 1H, ArH), 7.05-7.08 (t, 1H, ArH).

2.2.3.13. Synthesis of 3-(tert-butyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine-2,2,4,4-d4

(abbreviated as PH-t-oxazine-d4)

Toluene (3 mL) was added to a mixture of phenol (0.100g, 1.06 mmol), tert-butyl amine

(0.074 g, 1.01 mmol) and paraformaldehyde-d2 (0.081 g, 2.53 mmol) in a 10 mL round

bottom flask. The mixture was magnetically stirred for 2 h under reflux. The yellowish

solution was washed with 1N NaOH three times and water three times. The solution was

dried over magnesium sulfate anhydrous overnight. The material was purified in an

alumina neutral column with hexane as the eluent. The purified compound was a

1 transparent liquid. Yield: 76%. H-NMR (600 MHz, CDCl3, ppm): δ 1.174 (s, 9H, CH3),

6.72-6.73 (d, 1H, ArH), 6.81-6.83 (t, 1H, ArH), 6.95-6.96 (d, 1H, ArH), 7.04-7.07 (t, 1H,

ArH).

2.2.3.14. Synthesis of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine-3-15N

(abbreviated as 15N-PH-a)

Toluene (3 mL), phenol (0.110g, 1.12 mmol), paraformaldehyde (0.080 g, 2.66 mmol) and

aniline-15N (0.100 g, 1.06 mmol) were added to a 10 mL round bottom flask. The mixture

was magnetically stirred and heated under reflux for 6 h. The completed reaction product

was washed with 1N NaOH three times and water three times. The product was dried over

magnesium sulfate anhydrous. The material was purified in an alumina neutral column

with hexane and ethyl acetate (9:1) as the eluent. The material was recrystallized from

hexane. The product produced was a white crystal. Yield: 90%. 1H-NMR (600 MHz,

CDCl3, ppm): 4.67 (s, 2H, Ar-CH2-N), 5.40 (s, 2H, N-CH2-O), 6.84-6.86 (d, 1H, ArH)

6.91-6.94 (t, 1H, ArH), 6.95-6.98 (t, 1H, ArH), 7.05-7.06(d, 1H, ArH), 7.14-7.16 (d, 3H,

ArH), 7.29-7.32 (t, 2H, ArH).

2.2.3.15. Synthesis of 4-methyl-3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine

(abbreviated as Ph-a-4oxazine)

Ethanol (20 mL) was added to a mixture of 2-acetylphenol (0.500 g, 3.67 mmol) and aniline

(0.342 g, 3.67 mmol) in a 50 mL round bottom flask. The mixture was magnetically stirred

for 2 h under reflux. After cooling to room temperature, NaBH4 (0.695 g, 18.36 mmol) was added once an hour for three times. The solution was precipitated in water and dried to yield a white powder (2-(1-(phenylamino)ethyl)phenol (abbreviated as 2-1-pamep). The powder was dried in a vacuum oven to get rid of the remaining water. Yield: 89%.

Toluene (3 mL), 2-1-pamep (0.400g, 1.88 mmol), and paraformaldehyde (0.070 g, 2.34 mmol) were added to a 10 mL round bottom flask. The mixture was magnetically stirred and heated under reflux for 6 h. The completed reaction was washed with 1N NaOH three times and water three times. The product was dried over magnesium sulfate anhydrous.

The material was purified in an alumina neutral column with hexane and ethyl acetate (9:1)

as the eluent. The material was recrystallized from hexane. The product produced was a

1 white crystal. Yield: 83%. H-NMR (600 MHz, CDCl3, ppm): δ 1.73-1.74 (d, 3H, CH-

CH3), 4.68-4.71 (m, 1H, CH-CH3), 5.24-5.26 (d, 1H, N-CH2-O), 5.43-5.45 (d, 1H, N-CH2-

O), 6.88-6.97 (m, 3H, ArH), 7.05-7.06 (d, 1H, ArH), 7.13-7.15 (m, 3H, ArH), 7.27-7.30

(m, 2H, ArH).

2.2.3.16. Synthesis of 2-methyl-3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine

(abbreviated as Ph-a-2oxazine)

Toluene (3 mL), 2-pamp (0.400 g, 2.08 mmol), and acetaldehyde (0.093 g, 2.11 mmol) were added to a 10 mL round bottom flask. The mixture was magnetically stirred and heated under reflux for 6 h. The completed reaction was washed with 1N NaOH three times and water three times. The product was dried over magnesium sulfate anhydrous. The material was purified in an alumina neutral column with hexane and ethyl acetate (9:1) as the eluent. The product produced was a viscous liquid. Yield: 83%. 1H-NMR (600 MHz,

CDCl3, ppm): δ 1.54-1.55 (d, 3H, CH-CH3), 4.49-4.55 (q, 2H, Ar-CH2-N), 5.62-5.65 (q,

1H, N-CH-Ar), 6.80-6.82 (d, 1H, ArH), 6.86-6.88 (t, 1H, ArH), 6.96-7.00 (m, 2H, ArH),

7.08-7.10 (d, 2H, ArH), 7.12-7.14 (t, 1H, ArH), 7.22-7.25 (m, 2H, ArH).

2.2.4 Computational Calculation

The Kohn-Sham formulation of density functional theory (DFT)26,27 with the B3LYP

functional28-31 and the 6-31G** basis set32-33 is used. The 6-31G** basis set is used because

the inclusion of polarization functions on hydrogen atom orbitals significantly improved

the agreement between computed and experimental FT-IR spectra.

To carry out the vibrational analysis, the geometry of the molecule must first be optimized

with the same level of theory and basis set. The crystal structure of 3-phenyl-6,8-dichloro-

3,4-dihydrogen-1,3-benzoxazine (dichlorinated PH-a) has been reported by Liu and Gu using x-ray diffraction (XRD) experiments.34 This molecule was reported to take a semi-

chair conformation, where the benzene-oxazine rings and the aniline rings have an

interplanar angle of nearly 90°. This structure was used as the starting structure of PH-a,

the monomer of current interest, in the geometry optimization calculations. The same initial

structure was used for simulations of molecular isotopes, since molecular isotopes have

nearly identical geometries.35 Avogadro, a molecule editing software, was used to draw the

initial structure of all simulated molecules.36

Geometry optimization and vibrational analysis were performed with Gaussian 09

software.37,38 GaussSum339 was used to extract the IR and Raman spectra from the

Gaussian output. Computed vibrational frequencies are often multiplied by a constant

factor. The scaling partially compensates for the errors associated with the assumption of

harmonic vibrational modes (whereas real vibrational modes are anharmonic) and with the

electron correlation approximations of the theory. The scaling factors are empirical; they

are determined by comparing experimental and calculated frequencies. To achieve this 15

strong bands from each PH-a and 6 PH-a isotopes experimental FT-IR spectra were

correlated to the theoretical results obtaining the average scaling factor of 0.970 (See Table

S1 in SI). This methodology is similar to how Irikura et al. determine scaling factors.40

Vibrational Energy Distribution Analysis 4 software (VEDA4)41 was utilized to perform

the potential energy distribution (PED) calculations. VEDA4 optimization procedure is

designed so that combinations of linearly independent internal coordinates are as close as 24

possible to the normal coordinates. VEDA4 was used to process the Gaussian output,

yielding the full PED matrix for analysis.

2.2.5 Deconvolution of Heavily Overlapped Bands

Peakfit V4.12 (Systat Software Inc.) was used for deconvolution of heavily overlapped IR

bands, in particular the band in question around 960-900 cm-1. The range for curve

resolving is set based on the two minimum data points close to characteristic band in FT-

IR spectrum. AutoFit Peaks II Second Derivative method (Baseline: Linear, D2, total%=3.0, Smoothing%=1.00, Peak type: Spectroscopy, Gaussian * Lorentz, AutoScan:

Amp%=1.50, Vary Widths, Vary shape) gave the initial peak positions. Repeated the

Numerical Fitting routine until iteration is equal to 7.42,43

2.3 Results and Discussion

In this paper, we have taken two different approaches to understand the nature of the

prominent IR band around 960-900 cm-1 which was previously assigned as the C-H out-

of-plane mode of benzene ring to which oxazine is attached.

2.3.1 Results

2.3.1.1. All-substituted benzoxazine monomers

Three all-substituted benzoxazine monomers, have been synthesized according to Scheme

2.1. The first monomer is a bisoxazine which is abbreviated as 25DMR-h and is derived

from 2, 5-dimethylresorcinol and hexylamine. Others are trisoxazines which are

abbreviated as PG-a or PG-anis. These are derived from phloroglucinol with aniline.

Scheme 2.1. Synthesis scheme of all substituted benzoxazine.

In order to verify if the synthesis was successful, 1H NMR spectra of these three compounds

were obtained as shown in Figure 2.5.

1 Figure 2.5. H-NMR spectra for (a)25DMR-h, (b)PG-a and (c)PG-anis in CDCl3 at

room temperature.

The characteristic oxazine CH2 resonances are observed at 3.83 and 4.76 ppm for 25DMR- h, 4.43 and 5.27 ppm for PG-a, and 4.34 and 5.18 ppm for PG-anis, indicating that the intended synthesis was successful.

The FT-IR spectra of those three fully substituted benzoxazine monomer and its heated samples are shown in Figure 2.6. All three monomers, 25DMR-h, PG-a and PG-anis, show the characteristic band at 941, 948, and 936 cm-1, respectively, suggesting that this

mode is not exclusively the C-H out-of-plane bending mode of the benzene ring to which

the oxazine ring is attached, as historically assigned, since all three compounds lack the

aromatic C-H group. The possible contribution of C-H out-of-plane bending to this band, however, cannot be excluded by this experiment. These bands completely disappear upon heat treatment at 180 oC for 2 h, strongly supporting that this characteristic mode relates to

the oxazine group. Hence, it is now possible to reassign this band as oxazine ring related.

It is interesting to note that this characteristic band for the all-substituted benzoxazine

monomers is much shaper in comparison to the monomers with C-H groups in the ring (for example Figure 2.8. Left. (a)). The absence of C-H groups eliminates their contribution to the characteristic band. Thus, it is possible that this band still correlates to the C-H out-of-

plane bending vibration.

Figure 2.6. Comparison of FT-IR spectra of all-substituted benzoxazine monomer (top)

and its corresponding polymer obtained by heating the monomer at 180oC for 2 h

(bottom): (a) 25DMR-h and its polymer; (b) PG-a and its polymer; (c) PG-anis and its

polymer.

In order to further verify the correctness of this new reassignment, various deuterated benzoxazine monomers have been synthesized as follows.

2.3.1.2 Deuterium exchanged benzoxazine monomers.

Benzoxazines are often synthesized via one-pot Mannich condensation reaction by simply mixing phenol, amine and formaldehyde. However, this method can only be used to synthesize benzoxazines where no distinction of 2 and 4 positions of oxazine ring is required, if substitution on the oxazine ring is desired. Thus, one-pot synthesis is used for the synthesis of PH-x, PH-x-phenol-d4, PH-x-oxazine-d4 and PH-d-a, as shown in

Scheme 2.2 where x can be either a phenyl or t-butyl group.

Scheme 2.2. Synthesis scheme of isotopes of PH-x, PH-x-phenol-d4, PH-x-oxazine-d4

and PH-d-a.

D D R OH OH OH O N R-NH2 NaBH CD2O CHO R 4 R n N N (1) R.T. 3hrs H toluene, reflux

2-tbamp PH-x-2oxazine-d2

D D CH O HCl, H2O 2 n O N OH HN O N (2) n-propanol D D toluene, reflux D D D D

PH-a-4oxazine-d2

(a) (b)

Scheme 2.3. Synthesis scheme of isotopes of PH-x-2oxazine-d2 and PH-x-4oxazine-d2.

In the case of PH-x-2oxazine-d2 and PH-x-4oxazine-d2 where the CH2 groups of the oxazine group at the 2 and 4 positions, respectively, are selectively deuterated, multiple- step preparation methods as shown in Scheme 2.3 were adopted. For the synthesis of PH- x-2oxazine-d2 monomer, the amine was reacted with 2-hydroxybenzaldehyde, and the obtained compound was reduced by sodium borohydride. Deuterated paraformaldehyde is used to close the ring, selectively deuterating the CH2 group at the 2 position. Total yields of 78% and 48% for PH-a-oxazine-d2 and PH-t-oxazine-d2 were obtained, respectively.

The lower yield for PH-t-oxazine-d2 is due to the difficulty in precipitation of the intermediate 2-tbamp from water. The compound was extracted from the aqueous layer.

When the PH-a and PH-t series are compared, it is noted that the PH-t series require less reaction time because of the higher basicity of alkyl chain amine than aromatic amine. The reaction time decreases from 6 h to 2 h. Additionally, the PH-t series eliminates completely the aromatic C-H signals that arise from the aromatic amine of the PH-a series.

For the case of PH-a-4oxazine-d2 monomer, the reaction starts from the PH-a-oxazine-

d4, opening the all deuterated oxazine ring by hydrolysis, then closing the ring by normal

paraformaldehyde.

In addition to the all-substituted benzoxazine monomers, the deuterium-exchanged

benzene structure will also help verify if the band in question is aromatic C-H origin as

previously assigned. In case the prominent band in the range of 960-900 cm-1 is suspected

to arise from the oxazine rather than the benzene, the deuterated oxazine ring in whole or

in part will help identify specific mode of the oxazine ring. The synthesis procedures

adopted for each benzoxazine monomer are shown in Scheme 2.2.

The successful synthesis of all these monomers with high purity are supported by the 1H

NMR spectra shown in Figure 2.7. The characteristic CH2 resonances of the oxazine ring

are clearly seen at 5.38 and 4.64 ppm for PH-a, 5.35 and 4.62 ppm for PH-a phenol-d4

and 5.39 and 4.66ppm for PH-d-a. These two singlets became only one resonance at 4.62 ppm for PH-a-2oxazine-d2, at 5.34 ppm for PH-a-4oxazine-d2, and as expected, no CH2- related resonances are seen for PH-a-oxazine-d4. Exactly the same trend is observed for

PH-t series, also strongly supporting that the expected synthesis had been successfully

achieved.

Figure 2.7. 1H-NMR spectra for PH-a isotopes (left) and PH-t isotopes (right). (a) PH-a,

(b) PH-a-phenol-d4, (c) PH-a-2oxazine-d2, (d) PH-a-4oxazine-d2, (e) PH-a-oxazine-d4,

(f) PH-d-a, (g) PH-t, (h) PH-t-phenol-d4, (i) PH-t-2oxazine-d2, (j) PH-t-oxazine-d4 in

CDCl3 at room temperature.

The FT-IR spectra of all six compounds of the PH-a series and four of the PH-t series are shown in Figure 2.8. The characteristic band in question did not significantly change its position for any compound. Therefore, it is concluded that the band of interest should not be exclusively assigned to either the aromatic C-H or the oxazine CH2 groups. It is

interesting to note that the rather large frequency shift, from around 940 cm-1 to 915 cm-1,

is observed when the substituent on the oxazine ring changed from a heavy phenyl group

to a lighter t-butyl group. Therefore, it is now hypothesized that this mode in question may

be of the oxazine skeletal mode.

Special attention should be paid to the band shift from around 753 cm-1 to 582 cm-1 in PH- t series. The band can be assigned to the C-H out-of-plane bending of the aromatic group due to the substitution of C-H by C-D.44

Figure 2.8. FT-IR spectra of PH-a isotopes (left) and PH-t isotopes (right). (a) PH-a, (b)

PH-a-phenol-d4, (c) PH-a-2oxazine-d2, (d) PH-a-4oxazine-d2, (e) PH-a-oxazine-d4, (f)

PH-d-a, (g) PH-t, (h) PH-t-phenol-d4, (i) PH-t-2oxazine-d2, (j) PH-t-oxazine-d4.

2.3.1.3. 15N-exchanged benzoxazine

Support that this characteristic mode is indeed the oxazine skeletal mode can also be

obtained by synthesizing 15N-exchanged monomer, PH-a-15N. The synthetic procedure of

PH-a-15N is shown in Scheme 4.

Scheme 4. Synthesis scheme of isotopes of PH-a-15N.

The FT-IR spectra of PH-a and PH-a-15N are shown in Figure 2.9. The characteristic

mode in question for PH-a is observed at 946 cm-1 whereas that of the 15N isotope

exchanged PH-a is at 929 cm-1. The shift of the characteristic band from 946 cm-1 to 929

cm-1 demonstrates that 15N isotope exchange influences the vibration significantly. The band can be either from direct nitrogen atom participation or atoms that are very close to the nitrogen.

Figure 2.9. FT-IR spectra of (a) PH-a, (b) PH-a-15N.

However, at this stage, it is not clear if the vibration is the CN mode related to the O-C-N group or N-C-C group in the oxazine. Therefore, the final compounds synthesized are the oxazine ring with heavier substituent than deuterium at the 2 and 4 positions of the oxazine ring as shown in the synthetic Scheme 5.

2.3.1.4. Oxazine ring-substituted benzoxazine

Up to now, it has been demonstrated that the characteristic band in question is related to the vibration related to the nitrogen atom and is a skeletal mode. However, it is still not

clear whether the band is from N-C-Ar vibration or O-C-N vibration of the oxazine ring.

Two compounds of oxazine ring substituted PH-a were used to verify this question.

Two different oxazine ring substituted benzoxazines, PH-a-4oxazine and PH-a-2oxazine, were obtained as shown in Scheme 5 by multiple step preparation following the PH-x-

1 2oxazine-d2 synthesis method. The structure of all the compounds were confirmed by H-

NMR spectra in CDCl3 at room temperature. Both of the compounds show rather high purity. The band for O-CH2-N split into two doublet resonances due to the difference of

the two hydrogen atom in PH-a-4oxazine. For PH-a-2oxazine, the appearance of two quartet resonance at 4.49-4.55 ppm and 5.62-5.65ppm are attributed to O-CH-N and Ar-

CH2-N, as shown in Figure 2.10.

Scheme 5. Synthesis scheme of isotopes of oxazine ring substituted benzoxazine.

1 Figure 2.10. H-NMR spectra for PH-a-4oxazine and PH-a-2oxazine in CDCl3 at room

temperature.

Figure 2.11 shows the comparison of FT-IR spectra of PH-a-4oxazine and PH-a-

2oxazine. Both of them show the characteristic band. When the methyl group substituent is between the nitrogen atom and aromatic ring, the band is at 951cm-1 that is + 5 cm-1 shift

from 946 cm-1 of unsabstituted PH-a, while the counterpart compound, with the substitution between the oxygen and nitrogen, the characteristic band shifts to 903 cm-1 by

-43 cm-1 and it overlaps with other bands. The frequency shift of this characteristic band is

much more significant when the substitution is at the 2-position of the oxazine ring than the 4-position, suggesting that the characteristic band is closely related to O-C-N part

compared to C-N-Ar.

Figure 2.11. FT-IR spectra of oxazine ring substituted PH-a. (a) PH-a-4oxazine, (b)

PH-a-2oxazine.

2.3.2 Theoretical Calculation

Figure 2.12. The crystal structure and optimized structure of PH-a.

In order to understand the experimental results obtained during the FT-IR analysis of PH-

a and its isotopes, the corresponding molecules were also studied through a computational

point of view. Figure 2.12 shows the crystal structure and optimized molecular structure of

PH-a. Compared to the crystalline molecular structure of an analogous compound 3- phenyl-6, 8-dichloro-3, 4-dihydrogen-1, 3-benzoxazine, the aniline ring is rotated ~60°.

All other internal coordinates are very similar, all angles were within 5°, and all other torsions are within 15°, showing only slight distortions overall. (See Table S2 in SI).

Figure 2.13. Experimental and theoretical FT-IR results for PH-a.

The IR spectra obtained via ab initio calculation using the Gaussian program and FT-IR measurement of PH-a are shown in Figure 2.13, showing good agreement between the two after scaling by a factor of 0.970. Since the band assignment of benzoxazine monomer has

already been reported and the majority of the band assignment is not controversial, this

paper focuses only on the modes heavily related to the oxazine ring. The strong band at

1227 cm-1 in the FT-IR spectrum corresponds to the band at 1225 cm-1 in the theoretical

model. This band has been previously known as the (C-O-C) antisymmetric stretching. The potential energy distribution (PED) shows that the main contributor to this mode is the stretching between the oxygen and the carbon in the phenol ring with a smaller contribution

-1 from the torsion of H-C2-O-C9. The band that appears overlaid at 1199 cm in the

experimental FT-IR spectrum, corresponds to the band at 1188 cm-1 in the theoretical 41

model. This band is typically assigned to the (C-N-C) antisymmetric stretching. The PED in this case is given partially by the N-C2 stretching with smaller contributions from N-Ca

and N-C4.

Lastly, the band at 946 cm-1 in the experimental results also appears in the theoretical model

at 947 cm-1. Although this band was assigned to the C-H out-of-plane bending of the benzene ring,18 the experimental results presented in this paper suggests that that is not the only case and it is also proposed to be caused by skeletal O-C2 stretches. The PED in Table

2.1 indicates that the main contributor to this mode is the O-C2 stretching with significant

participation of the H-C-C-C torsion (out-of-plain-bending) and C-C-C-C torsion of the phenolic ring and minor contribution of the N-C2 stretch and H-C4-C-C torsion.

The complete correlation between the experimental FT-IR and Raman spectra and the

theoretical results for PH-a and its isotopes are shown in Table S3 in the Supporting

Information. All of them show satisfactory agreement as it can be observed in Figure 2.14,

which presents a closer look at the experimental FT-IR and theoretical results for PH-a and

isotopes at the fingerprint region.

The corresponding symmetric (C-N-C) stretching can be observed in the Raman spectrum

at 737 cm-1 as a strong line. The computational model shows the corresponding band at

731 cm-1.

Figure 2.14. Comparison of experimental FT-IR results and theoretical results for FT-IR spectra of PH-a isotopes (a) PH-a, (b) PH-a-phenol-d4, (c) PH-a-2oxazine-d2, (d) PH-

a-4oxazine-d2, (e) PH-a-oxazine-d4, (f) PH-d-a.

Table 2.1. Potential energy distribution for the oxazine ring-related modes of PH-a

which is calculated using Gaussian 09 with B3LYP/6-31G** theory and basis and

VEDA4 software.

8 1 O 7 9 2

3 b 6 N 10 a c 5 4

FT-IR Raman Theoretical Potential energy distribution (%) (cm-1) (cm-1) model (cm-1)

2956 2977 2946 C2-H stretching (92)

2923 2912 2923 C4-H stretching (95)

1479 1471 H-C2-H bend (59)

1440 1447 H-C4-H bend (40)

1385 1389 1388 H-C2-O-C torsion (54)

1370 1367 1373 H-C2-O-C torsion (19), N-Ca stretch (17), C2-N-C4 bend (14)

1339 1341 1339 H-C4-C-C torsion (30), C-C stretch of phenol ring (28)

1254 1255 1246 H-C2-O bend (41), H-C2-O-C torsion (13)

O-C9 stretch (33), C-C stretch of the phenol ring (14), H-C-C 1227 1224 1225 bend of phenol ring (13)

1199 1196 1201 C9-C4-H stretch (23), H-C4-C-C torsion (11)

C-C-C bend of phenol ring (18), C-C stretch of phenol ring 1193 1188 (15), N-C2 stretch (11)

N-C4 stretch (25), H-C2-O-C torsion (14), H-C2-C-C torsion 972 972 975 (12)

962 N-C4 stretch (19), N-C2 stretch (10), O-C2 stretch (13)

948 H-C-C-C torsion od phenol ring (63)

946 947 947 O-C2 stretch (32), H-C-C-C torsion of phenol ring (17)

H-C-C bend of phenol ring (39), C-C stretch of phenol ring 737 731 (13), O-C9 stretch (10)

H-C-C bend of aniline ring (24), N-C2-O bend (14), N-C2 580 stretch (10)

H-C-C bend of phenol ring (30), C2-N-Ca bend (11), O-C9 556 stretch (8)

N-C2 stretch (20), Ca-N-C2-O torsion (12), H-C2-O-C torsion 384 362 (10)

C-C9-O bend (34), Ca-N-bend (18), H-C-C bend of phenol 323 310 ring (9)

293 286 C-C-C-C torsion of phenol ring (35), C2-O-C-C torsion (12)

Table 2.2. Frequency of the characteristic band in experimental FT-IR and Raman and

theoretical model for PH-a, PH-a-phenol-d4, PH-a-2oxazine-d2, PH-a-4oxazine-d2,

15 PH-a-oxazine-d4, PH-d-a and PH-a- N.

FT-IR Raman Theoretical PH-a isotopes (cm-1) (cm-1) model (cm-1)

PH-a 946 947 947

PH-a-phenol-d4 936 934 939

PH-a-2oxazine-d2 941 949 945

PH-a-4oxazine-d2 956 962 954

PH-a-oxazine-d4 946 944 940

PH-d-a 943 961 951

PH-a-15N 929 932 946

The comparison between the experimental FT-IR and Raman results and the predicted value for the characteristic band are presented in Table 2.2. All the results show that the theoretical model accurately predicts the trend in the direction of frequency shift of the characteristic band with small differences in the magnitude. It is also clear that this particular mode is not a pure one, which is why the isotopic substitution in the phenol portion of the benzoxazine or the oxazine portion both causes an impact in the frequency of the characteristic band. This is also substantiated by the potential energy distribution of 46

PH-a (Table S4). PED indicates that 48% of this mode is caused by different vibrations in

the oxazine ring and 30% is caused by the phenol portion of the molecule, which is why

the selective deuteration of different parts of PH-a presents a shifting in this particular

mode. This heavily mixed mode is in part the reason why previously this mode was

assigned to the C-H mode of benzene to which oxazine is attached. However, this mode

completely disappears rather than shift its position upon ring opening of the oxazine

group.7,18 Thus, it is more appropriate to assign this mode to the oxazine group related and, more specifically, to O-C-N group rather than N-C-C group of the oxazine as the N-C-C

group still exists even in the ring opened compound.

2.3.3 Deconvolution of Vibrational Bands

The previous discussion reveals that the characteristic band is not a pure band in the

following three aspects: (1) In the experimental results, the characteristic band always

appears regardless of the position of the deuterium substitution; (2) In the theoretical

calculation, three main vibrational modes contribute to the characteristic band in the

Potential Energy Distribution (3) By visual inspection, the band clearly does not conform

to a single Gaussian-Lorentz contour and, indeed, even a shoulder can be observed which

then disappears upon polymerization of the monomer.

Figure 2.15. Characteristic band fitting of FT-IR spectrum of PH-a.

Figure 2.15 shows an example deconvolution of this characteristic band can be fit into three

Gaussian* Lorenz bands. The peak position of these three bands are 953, 941 and 928 cm-

1. The three bands can be corresponded to 948, 947 and 939 cm-1 in the theoretical

calculation. Therefore, H-C-C-C torsion of phenol ring (948), O-C2 stretch and H-C-C-C

torsion of phenol ring (947), and H-C-C-C torsion of aniline ring (939) all contribute to the characteristic band. It is thus safe to conclude this band is not a pure band.

2.3.4 Band Assignment

Comparing the region between 3200 cm-1 and 2800 cm-1 in FT-IR spectra, which represents

the C-H stretching in aromatic (greater than 3000 cm-1) and in aliphatic (3000 cm-1 to 2800

cm-1) part, the band positions are almost the same.

In the range of 2800cm-1 to 1600cm-1, the C-D stretching vibration is very obvious from the comparison of the PH-a series FT-IR spectra (Figure 2.8, left and Figure 2.9).

Expectedly, PH-a does not show any band in the range of 3000cm-1 to 2000cm-1. PH-a-

-1 phenol-d4 shows the C-D stretching band at 2278 cm , which corresponds to the aromatic

-1 C-D stretching. PH-a-2oxazine-d2 shows the C-D stretching band at 2150 cm , PH-a-

-1 -1 -1 4oxazine-d2 at 2121 cm , and PH-a-oxazine-d4 at 2146 cm , and PH-d-a at 2275 cm .

The three isotopes of oxazine deuteration at 2, 4 positions and both, shows smaller

wavenumbers compared to the aromatic deuteration. It shows the same behavior of the

corresponding C-H stretching.

In Table S3, the region between 1600 cm-1 to 1440 cm-1 is the aromatic skeletal mode. Four main modes are shown in this region. 1601(s), 1584(s), 1491(vs), 1456(s) in the FT-IR spectrum and 1602 (m), 1583(w), 1492(vw), 1453(m) cm-1 in the Raman spectrum.

Comparing these regions, the most obvious difference comes from PH-a-phenol-d4, a very strong band appears at 1562 cm-1, which might come from the deuteration of the benzene

ring on the phenol part. The other difference comes from the PH-a-15N. PH-a-15N shows

one less strong band at the positon of around 1455cm-1, instead, it becomes a weak band.

It is probably due to the isotope 15N influence on the skeletal mode.

-1 -1 In the range from 1440 cm to 1210 cm , the range represents the CH2 vibration of the

oxazine ring. The relatively strong band in the FT-IR spectra from 1227 cm-1 to 1218 cm-1

is the asymmetric C-O-C vibration. For PH-a, PH-a-phenol-d4, PH-a-4oxazine-d2 and

PH-d-a the isotopes do not influence the asymmetric C-O-C vibration, therefore, the bands

appears at 1227, 1218, 1225, 1226 cm-1 with a shoulder on the right side of the bands,

respectively. On the other hand, the bands of asymmetric C-O-C vibration will be significantly influenced due to the substitution on 2 positon, both 2 and 4 position isotope of PH-a-2oxazine-d2 and PH-a-oxazine-d4. Therefore, the band for these three isotope compounds disappeared in this region and shifts to lower wavenumbers, from 1210 cm-1

to 1200 cm-1, respectively. One skeletal vibration appears at 1158, 1149, 1146, 1154, 1157,

-1 1158 and 1159 cm for PH-a, PH-a-phenol-d4, PH-a-2oxazine-d2, PH-a-4oxazine-d2,

15 PH-a-oxazine-d4, PH-a- N and PH-d-a respectively. The skeletal vibration is influenced

by the isotope substitution on benzene ring and oxazine ring. Therefore, without isotope

substitution on benzene ring and oxazine ring, PH-a and PH-a-15N show the original band at 1158 cm-1. The band shifts to lower wavenumbers with the deuteration. For symmetric

C-O-C vibration, the shift based on the isotope position is quite obvious. The band appears

-1 at 1033, 1031, 1029, 1034, 1034 cm for PH-a, PH-a-phenol-d4, PH-a-4oxazine-d2, PH-

a-15N and PH-d-a when no isotope substitution near oxygen atom. The band for PH-a-

-1 -1 2oxazine-d2 and PH-a-oxazine-d4 shift to 1019 cm and 1018 cm .

In the range of characteristic band, broad bands appear for all the PH-a isotopes.

Incidentally, the C-H out-of-plane bending and O-C2-N stretching overlaps. PH-a, broad

-1 -1 band at 946 cm , shoulder on right, PH-a-phenol-d4, broad band at 936 cm , shoulder on

-1 right, PH-a-4oxazine-d2 shows a sharp band at 935 cm , PH-a-2oxazine-d2 shows a

-1 -1 15 broad band at 941 cm , PH-a-oxazine-d4 shows a broad band at 946 cm , PH-a- N is at

929 cm-1 a sharp band, PH-d-a shows a broad band at 943 cm-1. The band position was not 50

influenced significantly by the isotopes since the deuterium does not participate in the

stretching vibration. Integrated band assignment information can be found in Table S3 in

SI.

2.4 Conclusions

All substituted benzoxazines, PH-a and PH-t isotope series, and oxazine ring substituted benzoxazine were successfully synthesized and the structures were confirmed by 1H NMR.

FT-IR results of all substituted benzoxazine results show that the previously assigned

characteristic band for benzoxazine at 960-900 cm-1 is not only related to C-H out-of-plane bending, but also oxazine skeletal vibrations significantly contribute to this band. 15N

isotope compound further demonstrated the band is related to a vibration in which the

oxazine ring is involved. C2 and C4 substituted oxazine ring benzoxazine results show that,

when the substitution is between the oxygen and nitrogen atoms, the characteristic band is

influenced the most. The results of the theoretical calculation are consistent with the

experimental results. The deconvolution of the characteristic FT-IR band in question shows

that this band is not a pure band, instead, it is a contribution of three main vibrations.

Therefore, we conclude the characteristic band near 930 cm-1 is mainly related to the

oxazine ring, the vibration of O-C2 vibration with a minor contribution from the phenolic

ring.

2.5 References

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(2) Ishida, H.; Allen, D. J. J. Appl. Polym. Sci. 2001, 79, 406-417. 51

(3) Low, H. Y.; Ishida, H. J. Polym. Sci. Part B: Polym. Phys. 1998, 36, 1935-1946.

(4) Wang, C.-F.; Su, Y.-C.; Kuo, S.-W.; Huang, C.-F.; Sheen, Y.-C.; Chang, F.-C. Angew.

Chem., Int. Ed. 2006, 45, 2248-2251.

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CHAPTER 3

Investigation of intrinsic self-initiating thermal ring-opening polymerization of 1,3-benzoxazines

This work has been published as “Intrinsic Self-Initiating Thermal Ring-Opening

Polymerization of 1,3-Benzoxazines without the Influence of Impurities Using Single-

Crystal Resin”, L. Han, M. Salum, K. Zhang, P. Froimowicz, H. Ishida, J. Polym. Sci.,

Part A: Polym. Chem. 2017, 55, 3434–3445.

3.1 Introduction

Benzoxazine is a newly developed thermosetting resin that can be synthesized by the

Mannich condensation from phenol, amine, and formaldehyde. Upon polymerization, these

resins form polybenzoxazines, which have been extensively reviewed due to their

outstanding features.1 Among the major advantages of polybenzoxazines are the low

flammability,2 high thermal resistance3 even for fully bio-based polybenzoxazines,4 near-

zero shrinkage upon polymerization,5 low surface free energy,6 and excellent chemical

resistance.7 One of the most interesting characteristics of this class of polymer is the

extraordinarily rich molecular design flexibility that allows designing a vast variety of

molecular structures to tailor the desired properties.8

Polybenzoxazines are in general produced via cationic ring-opening polymerization by

heating benzoxazine monomers without added initiators and/or catalysts in the typical

temperature range of 150-230 °C; however, some applications desire lower polymerization

temperatures than this typical range. Many compounds have been reported to influence the

polymerization temperature of benzoxazines, including (1) strong and weak carboxylic

acids and even phenols;9 (2) basic compounds such as amines and imidazoles;10 and (3)

metal-containing compounds such as metal halides.11

Although various promoting effects caused by catalysts on the ring-opening

polymerization have been studied, the intramolecular catalyzing effect based on the units

or within the structures of benzoxazine molecule itself has rarely been reported. In this

regard, Andreu et al. evaluated the influence of the electronic effects caused by different substituent groups in the benzoxazine nuclei on the polymerization temperature.12 The

study particularly focused on several 3-phenyl-3,4-dihydro-2-H-1,3-benzoxazine monomers with electron-withdrawing or electron-donating groups in the 6 and 4’ positions only. Complementing the prior study, Wang et al. reported the influence of electronic effects but in this case from the bridging groups of bisphenols on the ring-opening polymerization of benzoxazines.13 They found that electron-withdrawing groups promoted

the thermally activated polymerization, as detected from a polymerization temperature

decrease by increasing the bond length and lowering the bond energy of C-O in the oxazine rings.

Recently, polybenzoxazines derived from benzoxazines containing ortho-functional

phenolic components have been shown to possess significant advantages over their para-

phenolic counterparts.14 Among these ortho-functional benzoxazines, ortho-amide

benzoxazine, which can act as a precursor for polybenzoxazoles,15 has been shown to

polymerize at much lower temperatures than any other known pure benzoxazine without

added initiators and/or catalysts. In fact, it has long been hypothesized16 and recently

demonstrated,17 the existence of an intramolecular 5-membered ring hydrogen bonding

between the NH of the amide and the oxygen in the oxazine ring acting as an internal

incentive to stimulate the ring-opening polymerization in a smart way, like a self-

complementary initiator.

It is also known that existing impurities generated during the synthesis of the benzoxazine

monomers can act as very efficient initiators and/or catalysts in the polymerization. For

instance, free phenolic –OH groups in the resin, coming from unreacted phenols or small

oligomers formed during the synthesis of the monomers, have been reported to effectively

lower the polymerization temperature.9 These impurities have played contrasting roles 57

during the development of the benzoxazine field. While they were helpful for lowering the polymerization temperatures and broadening the range of applications, they interfered with undertaking reliable mechanistic studies strictly on pure benzoxazines. As a consequence, the most accepted mechanisms assume the participation of a very small amount of a cationic initiator or initiators. This is the main reason for which the use of the term, thermal polymerization, has been discouraged for benzoxazine polymerizations since the term implies a thermal event like a bond cleavage, molecular rearrangement, or spontaneous reaction induced only by heat in the absence of any other participating substance. Hence, as the mechanisms always assumed not only the existence but also the involvement of those cationic species intermolecularly, the more appropriate denomination is thermally accelerated (or activated) polymerizations.16b However, the close terminology which fundamentally differs very much in meaning has unfortunately continued to be misused without evidence of its existence in the literature introducing a conceptual error. To date, no paper that specifically aims at studying the possibility of ring opening via thermally induced mechanism has been reported.

In the last few years, as a consequence of the increasing demands for this exciting and growing new polymer field, much more emphasis has been put on the purity of the monomeric benzoxazines used to produce the resulting polybenzoxazines. Since then, although still with comprehensible minute differences, high reproducibility and consistency were experimentally observed not only in the polymerization itself but also in the polymerizations’ parameters for similar systems carried out by different researchers at different places. However, if impurities were indeed the promoters for these polymerizations to happen, it would be extremely difficult to expect such a high and 58

consistent reproducibility. The vast variety of procedures reported in the literature to obtain

those resins used as raw materials, the actual different background and skills of every

researcher performing the syntheses, and the unlikely possibility of having equal quality

for every single chemical compound and solvents utilized, make impossible to originate

the exact same impurity at the exact same ratio in every synthesis carried out in all

laboratories. Thus, the existence of an uncatalyzed, common, and intrinsic mechanism for

the polymerization of benzoxazine resins induced only by heat seems to be possible. It

must be emphasized at this stage, that we do not disagree in any manner with the well- studied, supported, and established self-catalyzed mechanisms accepted hitherto. For example, it is well known that the polymerization of a given monomer could be achieved by different mechanisms. It is also possible for a polymerization to follow the same general mechanism, after having undergone different initiation processes. A good example here is styrene, which can undergo anionic and radical polymerization, two different mechanisms.

Furthermore, the radical polymerization of styrene has also been reported by different mechanisms, such as ATRP,18 and RAFT polymerization19 in addition to the uncontrolled

free radical polymerization.20 Even this last uncontrolled free radical polymerization,

which follows one mechanism, can still be initiated in different ways, thus establishing

thermally accelerated (or activated) polymerizations when radical initiators are used, or thermal polymerizations when no radicals are added to the system. The latter has also been called spontaneous, self-initiated, or simply thermal polymerization, and has been extensively studied.21

Thus, based on these facts, we have developed a particular interest in studying the possible

intrinsic self-initiating ring-opening polymerization of 1,3-benzoxazines induced only by 59

heat and without the influence of impurities. The results will then reveal that

polybenzoxazines might be obtained by thermally accelerated (or activated)

polymerizations when impurities, or any added catalyst or initiator, are indeed participating

in the process; or, by thermal polymerizations when only heat promotes the reaction in the

absence of the initiating impurities or added initiators.

It is, therefore, one of the purposes of the current paper to obtain strong experimental

evidence of benzoxazine polymerization by an intrinsic-ring opening mechanism, rather

than the initiation by the external initiators or impurities. To achieve this goal, meticulous

effort for monomer purification was carried out. Thus, only single crystals of the

monomers that were prepared under carefully controlled environment were used in this

work. The detailed synthetic strategy, purifications, and polymerization mechanisms are

discussed in this article.

3.2 Experimental

3.2.1 Materials

Phenol (98%) and paraformaldehyde (96%) were used as received from Sigma-Aldrich.

Aniline (99%) was purchased from Sigma-Aldrich and purified by distillation. Ethyl acetate (≥99.9%), toluene (≥99.5%), hexane (≥98.5 %), chloroform (≥99.8%), 1,4-dioxane

(≥99 %), sodium hydroxide (≥97.0 %), and sodium sulfate (≥99 %) were obtained from

Fisher Scientific and used as received. 4-Methoxyphenol (99%) from Sigma-Aldrich was recrystallized several times in ethyl acetate before use. Anhydrous n-hexane (≥99.9%) was

purchased from Fisher Scientific, and kept over molecular sieves and packaged under argon

in resealable ChemSeal™ bottles.

3.1.2 Sample Preparation

3.1.2.1 Preparation of 3-Phenyl-3,4-hidhydro-2H-benzo[e][1,3]-oxazine (Abbreviated as PH-a)

Phenol (50g, 0.53mol), aniline (47.12g, 0.51mol) and paraformaldehyde (37.99g,

1.265mol) were added to a 250mL round bottom flask. The solvent-less synthesis method was adopted by magnetically stirring the mixture at 90 °C for 2 hours in order to minimize the presence of the solvent from the final product. The product was then dissolved in toluene and washed by NaOH (1M) three times, followed by distilled water three times.

The organic layer was collected after washing. Toluene was eliminated by rotary evaporator. The product was dissolved in methylene chloride for column chromatographic purification using hexane as the eluent. The differential scanning calorimetric (DSC) thermogram carried out for this product is designated as sample 0. Further purification was carried out and the results of these processes are presented in the section. 1H NMR (DMSO-

d6), δ (ppm): 4.63 (s, 2H, Ar-CH2-N, oxazine), 5.42 (s, 2H, O-CH2-N, oxazine), 6.69-7.22

(mult., 9H, Ar). FT-IR (KBr), cm-1: 1225 (C-O-C asymmetric stretching), 937

(benzoxazine related band). Elemental analysis: Detailed data and discussion about the elemental analysis results will be shown in the Results and Discussion section.

3.1.2.2 Recrystallization of PH-a.

First recrystallization of PH-a:

Column chromatographically purified PH-a (20 g) was mixed with normal hexane (80 mL) and the system was slowly heated to 40 °C until all PH-a was dissolved. After 10 minutes

at room temperature, needle-like white crystals formed. The crystals were dried in a

vacuum oven overnight at 30 °C. The DSC thermogram obtained for this sample is

designated as sample 1.

Second recrystallization of PH-a:

PH-a (16 g) obtained from the first recrystallization was dissolved in normal hexane (100

mL) with stirring. After 30 minutes at room temperature, needle-like white crystals formed.

The crystals were dried in a vacuum oven overnight at 30 °C. DSC thermogram obtained

for this sample is designated as sample 2.

Third recrystallization of PH-a:

PH-a (5 g) obtained from the second recrystallization was dissolved in anhydrous n-hexane

(100 mL). The solution was placed in a 250 mL flask and covered with aluminum foil with

small holes. This flask was covered by an upside-down 1 L flask to reduce the rate of solvent evaporation. After 10 days, transparent square-rectangular cross-section columnar crystals formed. The crystal was dried in a vacuum oven overnight at 30 °C. Thermogram

obtained for these crystals is designated as sample 3.

Fourth recrystallization of PH-a:

This fourth recrystallization was carried out in a glove box. PH-a (2 g) that were

recrystallized three times as before described were dissolved in anhydrous n-hexanes (8

mL) in a 20 mL glass vial. Additional small amounts of anhydrous n-hexanes (2 mL) were

added every hour. After the fourth time, all PH-a totally dissolved. The vials were covered 62

with aluminum foil with small holes. The vials were then covered by an upside-down flask

to ensure slow evaporation of the solvent. The dryness of the glove box was monitored by

placing a piece of Na metal and checking the metallic shiny surface of the metal. After 8

days, transparent square-rectangular cross-section columnar crystals formed. The crystals

were dried in a vacuum oven overnight at 30 °C. DSC thermogram carried out for of these

samples are designated as sample 4.

Fifth recrystallization of PH-a:

Sample 4 was used to carry out a fifth recrystallization. The procedure followed at this

instance was a repetition of the fourth recrystallization of PH-a. The obtained sample was

named sample 5.

3.1.2.3 Sample Preparation by the Addition of a Known Concentration of 4- methoxyphenol

Phenol is widely used as an initiator and catalyst for benzoxazine polymerization, yet not easy to work with especially at low and quantitative conditions. Given the close chemical

structure, 4-methoxyphenol was chosen as a model compound for this purpose, in additions

to the advantage of being crystalline and having the melting temperature of 57 °C, which

is very close to the melting temperature of PH-a (59 °C). As a result, it makes the mixing

easier and even more as the two-phase will melt together at the same time. Homogeneity of the two compounds is essential to evaluate the effect of the added initiator and/or catalyst

(4-methoxyphenol) to the monomer (PH-a). The PH-a and 4-methoxyphenol were weighed using a quartz balance with a sensitivity of ± 1 μg. The two compounds were mixed in a glass vial and heated to 65 °C, followed by keeping it at the melt state for 2

minutes. The mixture was fully mixed by shaking in the glass vial. After cooling to room

temperature, the mixture was kept at room temperature overnight to recrystallize. The

sample was grounded by a mortar and pestle to fully and evenly mix before recording the

DSC thermograms. The programed ramp rate was set as 10 °C/min until 65 °C, keeping at

65 °C for 2 minutes to fully melt the crystals and homogenize and stabilize the system, and

further heated to 300 °C at a ramp rate of 10 °C/min.

3.1.2.4 Preparation of Poly (PH-a)

Polymerization of PH-a was done by heating at a rate of 10oC/min until onset temperature

(260oC), isothermal heating for 1 h at 260oC. Next, PH-a was heat at 10oC/min rate until exothermic peak temperature, isothermal heating for another 1 h. Poly (PH-a) was

obtained.

3.1.3 Characterization

1H and 13C nuclear magnetic resonance (NMR) spectra were acquired on a Varian Oxford

AS600 at a proton frequency of 600 MHz. The average number of transients for 1H and

13C NMR measurement was 64 and 1024, respectively. A relaxation time of 10 s was used

for the integrated intensity determination of 1H NMR spectra. Fourier transform infrared

(FT-IR) spectra were obtained using a Bomem Michelson MB100 FT-IR spectrometer,

which was equipped with a deuterated triglycine sulfate (DTGS) detector and a dry air

purge unit. Coaddition of 64 scans were recorded at a resolution of 4 cm-1. A TA

Instruments Differential Scanning Calorimeter (DSC) Model 2920 was used with a heating

rate of 10 °C/min and a nitrogen flow rate of 60 mL/min. In the analysis to determine the

activation energy of benzoxazine polymerization, the samples (2.0 ± 0.2 mg) were scanned

at the different heating rates of 2, 5, 10, 15, 20 °C/min. All samples were sealed in hermetic

aluminum pans. Elemental analysis (carbon, hydrogen, and nitrogen) was performed at

Galbraith Laboratories, Inc. The samples were shipped under argon. PerkinElmer 2400

Series II CHNS/O Analyzer was used for the elemental analysis. Samples were dried before the measurement. Elemental analysis was performed under argon atmosphere.

Thermogravimetric analysis (TGA) was carried out on a TA Instruments Q500 TGA with a heating rate of 10 °C/min under nitrogen at a flow rate of 60 mL/min.

3.3 Results and Discussion

3.3.1 Synthesis and exhaustive purification of PH-a

Synthesis of PH-a was carried out in a solvent-less method following a modified reported procedure,21 as seen in Scheme 3.1.

90oC O OH H2N + + CH O 2 n N

Scheme 3.1 Solvent-less PH-a synthesis.

The workout of the reaction crude was performed as usual. The crude was dissolved in

toluene, washed three times with NaOH (1M), and then three more times with distilled

water. Toluene was evaporated from the organic layer, thus generating the crude product.

Purification of a synthesized compound is typically a routine operation, which often does

not require fully detailed descriptions if known procedures, such as column

chromatography and recrystallization, are followed. However, the nature of impurities and purity of the benzoxazine used in this work are essential parameters for the current study.

Therefore, the full detailed description of the purification procedure, followed by the characterization of the materials obtained after each step to evaluate its successfulness throughout the entire purification process is reported in the experimental section. Briefly, the product was first re-dissolved in methylene chloride and then purified by column chromatography using hexane as eluent. An aliquot of this sample was kept apart as well as studied by DSC. Next, the column chromatographed product was further purified by successive recrystallization steps following different conditions, every time more exhaustive than before, as fully described in the Experimental section. After each recrystallization performed, aliquots were separated and evaluated by DSC. All results will be discussed in the next section.

However, before going forward, a deeper spectroscopic characterization of the obtained

PH-a was necessary. In this regard, FT-IR allowed us to monitor the success of the syntheses. The FT-IR spectra of the benzoxazine monomer, PH-a, using the KBr pellet method is shown in Figure 3.1. The presence of the aromatic oxazine ring in the PH-a monomer is indicated by bands centered at 1225 cm-1, which is due to the C-O-C antisymmetric stretching modes, and the characteristic oxazine mode is observed at 937 cm-1 for PH-a.22

benzoxazine related band

Figure 3.1. FT-IR spectrum of benzoxazine monomer, PH-a.

Figure 3.2 shows the 1H NMR spectra of ultrapure PH-a. The 1H NMR spectrum is

consistent with the ultra-high purity of the compound.

1 Figure 3.2. H NMR spectrum of PH-a. Satellites of each –CH2– peak corresponding to

H2 and H4 can be better observed in the inset of the figure, which shows a close look near

the baseline within the region between 4 and 6 ppm. Spectrum recorded at 25 °C, using

DMSO-d6 as solvent.

It can be seen in Figure 3.2 the characteristic spectrum of PH-a where no signals other than those belonging to PH-a are observed. It is worth noticing the two groups of signals symmetrically located at each side of the peaks associated to H2 and H4 from the oxazine

ring of PH-a. These signals are called satellites and are important because often they are

an observable characteristic on the spectra of highly pure samples even though they are in

no way proof of purity.

3.3.2 Evaluation of purity 68

From the 1H NMR spectra, one can clearly see that no other signals other than those strictly

belonging to the compounds are observed in the spectra, except for the solvent peak

(DMSO-d6) and H2O contained in that solvent. However, NMR spectroscopy is not

sensitive enough to be considered as a purity criterion. Therefore, it was important to use

complementary techniques to evaluate the purity of the resin synthesized in this work. Thus,

elemental analysis and DSC studies were carried out for each crystalline benzoxazine resin

system, and the results are presented in Table 3.1 and Figure 3.4, respectively.

After the first two recrystallizations, needle-like crystals of PH-a were obtained due

possibly to the small amount of impurities influencing the molecular packing. The third

and fourth time, the recrystallizations were carried out with anhydrous n-hexane. As the purity of the crystals improved, the form of the crystals changed from needle-like to a square-rectangular cross-section column nature (Figure 3.3.b). The fourth recrystallization was carried out in a glove box under dried argon atmosphere, with copper catalyst to capture oxygen, which was a more stable environment. In those conditions, the crystals of

PH-a grew without the influence of oxygen and moisture. As shown in Figure 3.3.a, the polymerization temperature of PH-a increased with the increasing number of recrystallizations performed, indicating that the purity of PH-a increased with each recrystallization step. The polymerization temperature 272 °C of the final sample was the highest since further purifications did not increase the polymerization temperature (sample

5).

Figure 3.3． a) DSC thermograms showing the change in the polymerization temperature

of PH-a after successive recrystallization processes. b) Crystals of PH-a after all recrystallization processes.

Table 3.1 shows the elemental analysis results. Samples were dried before analysis for 24 hours at 30 °C under vacuum; however, tiny amounts of water, moisture, or even humidity

will influence the results. Furthermore, the precaution of working under argon as inert atmosphere instead of molecular nitrogen (N2) was taken into consideration thus minimizing the influence of atmospheric N2 on the nitrogen content of our samples. The results are summarized as follows.

Table 3.1. Elemental Analysis Results of PH-a.

Element Calculated Found

C 79.59 79.46

N 6.63 6.64

H 6.20 6.34

O 7.58a 7,56a

aThese values are obtained by difference.

The elemental analysis results summarized in Table 3.1 show the excellent agreement between the calculated and found percentage values for each element, thus evidencing that the targeted compound, PH-a, was obtained in high purity.

3.3.3 Polymerization study of PH-a with known amounts of an added initiator/catalyst

Working with a benzoxazine monomer which have demonstrated to have a very high purity, allows us now to examine the effect of a phenol-containing compound at precisely known amounts on the polymerization temperature. By systematically varying the amount of a phenol-containing compound, one might examine the mechanism of oxazine ring opening, whether or not the ring opening is intermolecularly induced by the presence of such a 71

phenolic compound or intramolecularly even in the absence of an external substance. If in

fact the external phenolic compound (in many cases impurities from the synthesis) is

required, the polymerization exotherm peak temperature should increase as the amount of

the added phenolic compound approaches zero mole %. On the other hand, if the

intramolecular mechanism based on an intrinsic ring-opening dominates without the effect of the external initiator, the exotherm peak temperature should reach an asymptotic value.

In order to maintain the homogeneity of the samples and reproducibility of the DSC analyses results, two strategies were adopted:

1. 4-methoxyphenol was chosen for this experiment because it has a melting point of

57 °C, which is very close to PH-a (59 °C), shows similar solubility as well as good

miscibility with PH-a, and does not bear any polymerizable group.

2. The different phenolic content mixture sample was prepared as shown in Table 3.2.

The mixture samples were all weighed at similar weights, approximately 1.5 ± 0.2

mg. The specific weight of each sample is shown in Table 3.3. Controlling the

weight will maintain similar mass/heat transfer balance, thus minimizing the

influence on the polymerization temperature triggered by any small difference in

the amount of material in each specific sample.

Table 3.2. Weight of 4-methoxyphenol and PH-a of different phenolic content

mixture sample.

4- PH-a Phenol methoxyphenol (mg)

content (mg)

(mol%)

0.23 0.045 32.745

0.44 0.057 21.770

0.56 0.079 23.843

0.88 0.088 16.865

1.05 0.071 11.400

Table 3.3. Phenolic content in each PH-a/4-metoxyphenol mixture and the

specific weight of every sample studied by DSC.

Phenol 1 2 3

content (mol%) (mg) (mg) (mg)

0 1.529 1.398 1.420

0.23 1.621 1.559 1.505

0.44 1.574 1.580 1.360

0.56 1.365 1.300 1.577

0.88 1.508 1.371 1.717

1.05 1.364 1.376 1.359

3. The temperature program was set as follow: ramp rate at 10 °C/min to 65 °C, kept

at 65 °C for 2 minutes, and new ramp rate at 10 °C/min to 300 °C. The stabilization

temperature (65 °C) was chosen safely right above the melting temperature of both

compounds and way below the onset of the exothermic peak, thus avoiding

premature polymerization.

Figure 3.3 presents the results obtained by DSC showing the polymerization temperature of all studied mixture samples. In order to evaluate the reproducibility of the results, each system was measured three times. The average polymerization temperature and standard deviation were calculated and are shown in Table 3.4 and Figure 3.4.

Figure 3.4. DSC thermograms, region between 240 and 190 oC showing the exothermic

polymerization peaks of different mixturesamples with increasing percentages of 4-

methoxyphenol: (a) 0.00% of 4-methoxyphenol (pure PH-a), (b) 0.23 mol% of 4-

methoxy-phenol, (c) 0.44 mol% of 4-methoxyphenol, (d) 0.56 mol% of 4-

methoxyphenol, (e) 0.88 mol% of 4-methoxyphenol, and (f) 1.05mol% of 4-

methoxyphenol.

Table 3.4. Polymerization temperature of mixture samples measured by DSC.

Phenol 1 2 3 Average Standard Content (mole (°C) (°C) (°C) (°C) deviation %)

0 272 272 272 272.00 0.00

0.23 272 271 271 271.33 0.58

0.44 272 270 271 271.00 1.00

0.56 269 271 271 270.33 1.15

0.88 266 269 268 267.67 1.53

1.05 268 266 266 266.67 1.15

Figure 3.5. Polymerization temperature as a function of the concentration of a phenol-

containing compound (4-methoxypheonol).

Figure 3.5 shows the change in the polymerization temperature of PH-a as a function of

the increasing molar concentrations of 4-methoxypheonol. The fitted data is represented

by the polynomial equation described by Equation 1.

2 Tp = 271.96 – 0.78 [C] – 4.21 [C] (1)

where [C] is the molar concentration of 4-methoxyphenol.

This correlation indicates an extrapolated polymerization temperature of 271.96 °C at zero

concentration of the phenolic compound. This temperature is therefore considered as the

intrinsic ring-opening polymerization temperature of the oxazine ring of PH-a without the

influence of any residual impurity behaving as initiator or catalyst.

3.3.5 Thermal Behavior of PH-a 77

As briefly mentioned in the Introduction section, Andreu et al. as well as Wang et al.

presented extensive studies on how the nature and strength of different substituents in the

benzoxazine monomers affect the polymerization temperature.12,13 To enhance the

reliability of the influence of those substituents on the polymerization mechanism, which

should not be affected by impurities, they have particularly paid great attention to the purity

of each one of the monomers used in such elegant publications. They have demonstrated

in their articles such good purity through melting point and elemental analysis. What needs

to be mentioned is that they have achieved such a high purity of the monomers, and yet

these benzoxazine monomers polymerized upon heating. The controversy arises in the fact that if we take for granted the accepted mechanism of benzoxazine polymerization where only the impurities are the promoters for initiating the polymerization, these polymerizations should not have occurred using monomers with such a purity. These results are in accordance with our interest in elucidating a possible intrinsic ring-opening

polymerization of benzoxazines. In other words, the capacity of benzoxazine to polymerize

only upon heating without any other substance participating in the reaction. This would

scientifically establish for the first time the concept of thermal polymerization of

benzoxazines, in addition-but not instead-to the general accepted thermally accelerated (or activated) polymerization of benzoxazines mechanism. Thus, the polymerization studies of the high purity PH-a is presented next.

The thermal behavior of PH-a toward polymerization was then studied by DSC as depicted

in Figure 3.6.

Figure 3.6. DSC thermogram of PH-a from 30 to 300 °C. The endothermic peak at 59 °C

is assigned to the melting while the exothermic one at 272 °C to the polymerization of

PH-a.

The thermogram of PH-a shows that this compound does polymerize despite its high purity.

The exothermic peak assigned to polymerization has its maximum centered at 272 °C. It

must be highlighted, however, that PH-a in this condition has polymerized at a noticeably

much higher temperature than the usual temperatures reported for this compound, usually

around 260 °C.12, 23 What is shown in this study, however, is that even though the polymerization temperatures is indeed affected, increased by the absence of impurities, there is real polymerization of PH-a.

3.3.7 Polymerizations

In recent years, extensive works on polybenzoxazine chemistry have been reported by

benzoxazine researchers.1a After intensive studies of various cationic, anionic and radical

initiators, and mechanistic understanding, it is accepted that the ring-opening polymerization of benzoxazines proceeds through a cationic mechanism. As any other polymerization mechanism, the polymerization of benzoxazine proceeds through the initiation, propagation, and finally termination steps.

For many years, polymerization of benzoxazine resins have been thought to be initiated

without added initiators because of the presence of cationic impurities remaining from the

monomer synthesis. Nevertheless, there was no detailed study reported on this subject.

From a different perspective and despite achieving high purity and using only benzoxazine

single crystals in the current study, benzoxazine reins still polymerized with similar rates,

albeit somewhat at higher temperatures, as seen in Figure 3.6.

It is known that for a reaction between two species to happen both species should be

sufficiently activated. The cases presented in this study evaluate a single compound

reacting with itself. Under these conditions, the compound should then present at least two

different positions appropriately and sufficiently activated to be able to react between them.

In this regard, compound PH-a can be analyzed in detail as follows.

Figure 3.7 shows the positions in the benzoxazines that are actually rich in a negative

density, induced by electronic delocalization along the structure. It can be clearly seen that

all these positions are not only activated but also reactive, although perhaps at different and

specific temperatures in each case, to some chemical reactions. Thus, while a given

position may be responsible for the initiation process, another one less favored might be

the cause for crosslinking and even side reactions.

Figure 3.7. Resonance structures of the unsubstituted benzoxazine PH-a. Structures in

involving the aromatic ring that was adjacent to the oxazine ring (a) and the aniline

moiety (b).

As has been mentioned earlier, the main interest in this study is in evaluating the possibility of an intrinsic ring-opening polymerization of benzoxazines. This will then set the bases for establishing a complementary form of initiation for polymerizing benzoxazines. It is important at this point to remember that the accepted mechanism is a thermally accelerated

(or activated) polymerization since there must exist an impurity able to react with the benzoxazine to only then be capable of undergoing propagation. A simplified

representation of this mechanism is depicted in Figure 3.8.a, where the H+ provided by the impurity promotes the polymerization at a certain temperature. It is worth remembering that benzoxazines polymerize via a cationic polymerization, where the cation in the chain- like tautomer is attacked by a nucleophile present in the reaction medium. Nevertheless, and in agreement with reported data,12-13 the benzoxazine single crystals synthesized in this

work does undergo polymerization induced only by heat without any impurity participating

in or during the reaction. Therefore, under this condition, benzoxazines are polymerized

through a strictly thermal polymerization. This conceptually new, but somehow expected,

mechanism is illustrated in Figure 3.8.b, where no impurities are present and the

polymerization occurs at a certain temperature, which is understandably higher than the

temperature needed for the previous mechanism including an impurity. It must be

highlighted that both initiation processes are, in a way, similar since both proceed

establishing the ring-chain tautomeric equilibrium induced by thermal treatment. One

important difference between these two mechanisms is that in Mechanism A the presence

of impurities will cause a significant shifting in the equilibrium toward the chain-like

tautomers as well as induce this equilibrium to happen at lower temperature, thus

promoting eventually a lower polymerization temperature. A similar equilibrium is present

in Mechanism B, although this time the equilibrium is much less favored than in the

previous case, thus needing higher temperatures to happen. Thus, the absence of such

impurities might be the simple reason for the resins to need a higher temperature to yield

the same ring-chain tautomerism. Once the tautomeric pair is formed, the carbocation will

react through an electrophilic substitution on any activated position (preferably on the

ortho-position since is the most activated one) of other benzoxazine monomer, thus starting

and establishing the propagation step. From this stage forward both mechanisms are very likely to proceed in the same manner. The proposed mechanisms are consistent with the previous concentration dependent experiments where not only the presence of the impurity influences the polymerization temperature but also permits to see that increasing the concentration of the impurity shifts the ring-chain tautomeric equilibrium to the chain-like form thus inducing the polymerization to happen at lower temperatures.

Figure 3.8. Simplified mechanisms for the polymerization of benzoxazines. a) The

classically accepted thermally accelerated (or activated) polymerization mechanism,

where in addition to heat impurities must participate in the initiation step; and b), thermal

polymerization mechanism, induced only by heat.

The ring-chain tautomeric equilibrium of benzoxazine resins is where the cation responsible for the cationic polymerization of benzoxazines is formed. This equilibrium is often simplified as depicted in Figure 3.9.

Figure 3.9. Ring-chain tautomeric equilibrium of the unsubstituted benzoxazine PH-a.

However, a closer look to the chain-like tautomer shows that not only the two zwitterionic

structures often described are possible, but instead there are at least six (6) different

structures resonating, as shown in Figure 3.10.

Figure 3.10. Resonance structures of the chain-like tautomer of the unsubstituted

benzoxazine PH-a involving the aromatic ring that was adjacent to the oxazine ring.

Resonance structures of the chain-like tautomer of PH-a involving the charge delocalization in the aniline moiety are much less favored and therefore unlikely to form.

One reason for this less favored charge delocalization on the aniline moiety is that two positive charges will have to be far too close, in fact right next to each other and competing for the same nitrogen atom, thus making the system very unstable. It must be emphasized, however, that the aniline moiety does have the ability to delocalize those charges, although not in the chain-like tautomer form.

All these frequently overlooked chemical structures, most likely present in very little amounts, bear their own reactivity and are reactants for side reactions to happen during the entire polymerization process, including the initiation, propagation, and termination steps.

Thus, the combination of all these systems reacting simultaneously makes difficult a fair comparison on how an “initiator” acts in a conventional controlled/living (radical or ionic)

polymerization or in benzoxazine polymerization. This is, for instance, why there is not

clear relationship between the impurity concentration and the molecular weight reached

after polymerization of benzoxazines.

As the herein uncovered self-initiation process occurs without disrupting the rest of the

mechanism for this resin, the entire polymerization should proceed through a single

mechanism. Therefore, the graphical manner to calculate the activation energy (Ea) of this system can easily be applied to corroborate this behavior. Specifically, a single mechanism

2 can be assumed to happen if the plots of ln (β/Tp ) and ln β as a function of 1/Tp are linear

(being β the constant heating rate and Tp the maximum value of the exothermic

24 polymerization peak). Therefore, based on this concept, we graphically calculated the Ea

of the polymerization reaction of PH-a applying the Kissinger and Ozawa methods, and

the results are presented in Figure 3.11.

Figure 3.11. Plots to graphically calculate the activation energy (Ea) of the polymerization reaction of PH-a applying the Kissinger and Ozawa methods. In the plots,

β is the constant heating rate and Tp is the maximum value of the exothermic polymerization peak. Linear regression obtained for each method are: R2 = 0.996 for the

Kissinger method and R2 = 0.997 for the Ozawa method.

As can be seen in Figure 3.11, the Ea is easily calculated from the slope of the resulting straight lines for the polymerization reaction of PH-a, being 92.9 and 96.7 kJ/mol for the

Kissinger and Ozawa methods, respectively.

The most meaningful result at this point, however, is the fact that in both cases the plots resulted in perfect straight lines. This result indicates that the polymerization of the highly purified PH-a follows a single mechanism, same as the accepted mechanisms where initiation is promoted by impurities. 87

Having observed an efficient self-initiated polymerization occurring through a single mechanism for this benzoxazine monomer, we developed an interest in comparing the thermal properties of the resulting polymer against the ones commonly reported in the literature which polymerizations are based on impurities acting as initiators. This comparison serves as an indication on the similarity between the possible chemical structures of the polybenzoxazines obtained in each process. In this regard, thermogravimetric analysis (TGA) is a very useful tool to evaluate, although indirectly, the possible similarities between those polymeric structures. This might be achieved by studying the degradation profile and measuring the typical thermal properties of the corresponding polymers.

Thermal analysis and degradation studies of PH-a have been reported and are often used for comparison, although the resin never presented the level of purity reported herein. As most polybenzoxazines, poly(PH-a) presents a degradation profile consisting of three main steps, being the evaporation of amine moieties at the chain ends and branches the first one, followed by loss of amines from the main chain, to finally undergo simultaneous breaking of phenolic bonds and decomposition of remaining Mannich bases.25 Similar results have

been obtained by TGA and pyrolysis when comparing the respective first derivative mass

loss with the total ion current, TIC, curves.26

The first step of this study was the actual polymerization of the highly purified PH-a

sample, which was carried out reproducing a reported method to enhance the validity of

the comparison.27 Then, TGA was performed as usual and the results obtained for the

highly purified poly(PH-a) are shown in Figure 3.12.

Figure 3.12. TGA thermogram of the highly purified poly(PH-a).

As can be seen from Figure 3.12, the TGA thermogram of the poly(PH-a) synthesized in this work presents the typical 3-step degradation profile. Specific data and thermal properties are summarized in TABLE 5.

TABLE 5. Summary of thermal properties for the highly purified poly(PH-a) compared with others from the literature.

Char degradation step (°C) Tg Td5 Td10

Yield from - from - from - references (°C) (°C) (°C) (%) to to to

180 - 300 - 450 - poly(PH-a) 120 259 341 44 This work 300 450 650

190 - 315 - 460 - poly(PH-a) 163 288 343 40 27(b) 315 460 650

poly(PH- 190 - 330 - 440 – n.r.c 326 353 35 25(a), 26 a)a 330 440 550

poly(PH- 190 - 350 - 460 - n.r.c 260d 340d n.s.e 26 a)b 350 460 650 aData obtained by TGA. bData obtained by pyrolysis. cn.r.: not reported. dData estimated from the figure shown in original paper. en.s.: not shown.

The data shown in TABLE 5 shows good agreement for each property between the different poly(PH-a) polymers synthesized using different PH-a resins, thus suggesting that polymeric structure should in consequence be reasonably similar.

This last result complements our hypothesis where the initiation step of the intrinsically self-initiated thermal polymerization does not affect in any way the propagation and termination steps since the polymer structure should, in principle, be the same.

Combining the previous results, it can be demonstrated that benzoxazines are capable of undergoing self-initiation upon heating and without the influence of any impurity, thus initiating a strictly and intrinsic thermal polymerization. Then, this self-initiation step is followed by the same propagation and termination steps as the already proposed and accepted thermally accelerated (or activated) polymerization promoted by impurities. In other words, this self-initiating thermal polymerization of benzoxazines is in full agreement with the well-studied and established accelerated (or activated) polymerization

mechanisms initiated/catalyzed by impurities accepted hitherto.

3.4 Conclusions

A phenol-aniline based benzoxazine single crystal was successfully prepared and highly

purified. A novel initiation step for an intrinsic ring-opening polymerization of

benzoxazines is herein proposed. We have demonstrated in this study that temperature

alone is sufficient to initiate the polymerization of benzoxazines.

The results presented in this work demonstrating the intrinsic thermal polymerization of

benzoxazines are in no way in disagreement with any of the thermally accelerated (or

activated) polymerization studies well-known and accepted in the past. Instead, they are

complementary. The relevance of the herein presented results is that it has now been shown

that impurities are not necessarily needed to initiate the polymerization. However, they do

help to lower the polymerization temperature. The implications of this result are not only

in proving the possibility of this inherent new property of benzoxazines, namely “intrinsic

ring-opening polymerization”, but also in the purity in which new benzoxazines and

polybenzoxazines might be generated.

3.5 References

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1428-1435.

CHAPTER 4

Study of the effects of intramolecular and intermolecular

hydrogen-bonding of amide-containing benzoxazines

This work has been published as “Study of the effects of intramolecular and intermolecular hydrogen-bonding systems on the polymerization of amide-containing benzoxazines”, L.

Han, K. Zhang, H. Ishida, P. Froimowicz, Macromol. Chem. Phys. 2017, 218, 1600562.

4.1 Introduction

Polybenzoxazine is a new category of thermoset materials that has been extensively studied

in recent years due to the easy synthesis and outstanding properties. 1-3 Benzoxazines can

be synthesized by Mannich condensation from phenol, amine and formaldehyde.4 There are no reaction by-products formed during the cationic ring-opening polymerization.

Benzoxazines possess an extremely rich molecular design flexibility making possible for benzoxazines to satisfy different applications due to the multiple choices of phenols, amines and aldehyde.5-7 The resulting polybenzoxazines show many unique properties

such as near-zero volume changes upon polymerization,8,9 low water absorption,9 variable

range of glass transition temperature,10,11 fast physical and mechanical properties

development at low conversion, and high char yield. 12,13 One additional advantage of

polybenzoxazines is its good and easy compatibility with other polymers thus enhancing

the final properties of the obtained materials.14-16

Benzoxazine resins exhibit various unusual and often advantageous properties. One

example is that most ortho-substituted benzoxazines shows enhanced properties when

compared to para position counterpart. In 2014, Liu and coworkers first demonstrated that

the double ortho-substituted bisphenol F-based polybenzoxazines showed the glass

transition temperature (Tg ) in the following order o-, o-;> o-,p-; > p-,p- isomers. ΔTg =Tg,

o-, o- – Tg, p-,p- is 60 , which is completely opposite trend of all the literature examples

of the ortho and para℃ isomer studies reported. Apart from this, the ortho-substituted

benzoxazines show the best thermal stability among all the isomers. 17 Later, Zhang and coworkers found that the ortho-amide-imide functional benzoxazine and ortho-norbornene functional benzoxazine monomers showed milder synthesis condition (shorter reaction 97

time with high yield) as compared to para counterpart. Furthermore, the corresponding

1,18,19 ortho-functionalized polymer showed higher Tg.

Although a large number of benzoxazines have been synthesized to improve the physical

and mechanical properties and the ortho-substituted functional benzoxazines shows highly

desirable properties, the fundamental reasons for the appearance of unexpected results are

poorly understood. In our group’s previous work, the synthesis of the pHBA-a and oHBA-

a have been reported and a ring-opening and propagation mechanism for the ortho substituted isomer involving a five-membered ring hydrogen bonding was proposed, which reveals how the intramolecular hydrogen bonding is influencing ring opening and chain propagation. Froimowicz et al. studied the isomeric benzoxazine system by solution NMR

and demonstrated that the intramolecular hydrogen bonding does exist and that the 7-

position on the benzene ring becomes more acidic and reactive than without hydrogen

bonding.20

Para position benzoxazines are far less investigated in comparison to ortho position

benzoxazines in their capability of forming hydrogen bonds. In this study, the hydrogen

bonding behavior of two isomeric amide-containing benzoxazines, substituted at the para and ortho position, are studied by FT-IR. These results complement not only the long hypothesized existence of an intramolecular five-membered-ring hydrogen bond in oHBA- a proposed in previous papers18,20 but also those more recently reported using NMR techniques.3 It must be highlighted that to make this spectroscopic study fully reliable much emphasis has been paid on obtaining highly purified benzoxazine resins.

4.2 Experimental

4.2.1 Materials o-Aminophenol (98%), p-aminophenol (98%), phenol (98%), benzoyl chloride and

paraformaldehyde (96%), were used as received from Sigma-Aldrich. Aniline was

purchased from Aldrich and purified by distillation. PH-a was synthesized following reported methods.21 Ethyl acetate, toluene, hexane, chloroform, 1,4-dioxane, sodium

hydroxide (NaOH), lithium chloride (LiCl) and sodium sulfate were obtained from Fisher

Scientific and used as received.

4.2.2 Sample Preparation

4.2.2.1 Synthesis of N-(2-hydroxyphenyl) Benzamide

o-Aminophenol (3.00 g, 27.5 mmol) and LiCl (1.15g, 27.5 mmol) were dissolved in NMP

(25 mL). The solution was cooled to 0°C, and benzoyl chloride (3.88 g, 27.5 mmol) was

added dropwise with a syringe. Then, the solution was kept at room temperature overnight.

Next, the reaction mixture was poured into cold water. The precipitate was filtered and

afterward dried under vacuum. Light pink crystals were obtained (yield 85%). 1H NMR

(600 MHz, DMSO-d6, δ): 6.77-7.95 (m, 9H, ArH), 9.51 (s, 1H, NH), 9.74 (s, 1H, OH).

FT-IR (KBr, cm-1): 3408 (N-H stretching), 3029 (O-H stretching), 1645 (amide I), 747

(C=O bending).

4.2.2.2 Synthesis of N-(4-hydroxyphenyl) Benzamide

Exactly the same amounts and procedures as N-(2-hydroxyphenl) benzamide were used to synthesize N-(4-hydroxyphenl) benzamide, except that p-aminophenol was used instead of

o-aminophenol. The product was dried under vacuum to obtain a white powder (yield 99

82%). 1H NMR (600 MHz, DMSO-d6, δ): 6.70-7.92 (m, 9H, ArH), 9.25 (s, 1H, OH), 10.02

(s, 1H, NH). FT-IR (KBr, cm-1): 3390 (O-H stretching), 3327 (N-H stretching), 1649

(amide I), 720 (C=O bending).

4.2.2.3 Preparation of N-(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-

yl)benzamide (Abbreviated as pHBA-a)

Into a 50 mL round flask were added 20 mL of 1,4-dioxane, aniline (0.44 g, 4.70 mmol),

N-(4-hydroxyphenyl) benzamide (1.00 g, 4.70 mmol), and paraformaldehyde (0.23 g, 9.40 mmol). The mixture was stirred at 100 °C for 24 h, and subsequently cooled to room temperature. Then the reaction mixture was poured into cold water (200 mL) to give a powder like precipitate. The product was dissolved in ethyl acetate and washed with 0.5 N

NaOH solution and then water to eliminate the unreacted residual starting materials. The crude product was dried over anhydrous sodium sulfate followed by fractionating with column chromatography (eluent: hexanes and ethyl acetate, in a volume ratio of 3:1) to obtain the final product (yield 73%). 1H NMR (600 MHz, DMSO-d6, δ): 4.64 (s, 2H, Ar-

CH2-N), 5.42 (s, 2H, O-CH2-N), 6.70-7.91 (m, 13H, ArH), 10.07 (s, 1H, NH). FT-IR (KBr), cm-1: 1646 (amide I), 1227 (C-O-C asymmetric stretching), 940 (oxazine related).

Elemental analysis: calcd. for C21H18N2O2: C, 76.34%; H, 5.49%; N, 8.48%; found:

75.44%; H, 5.23%; N, 8.33%.

4.2.2.4 Preparation of N-(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-8-

yl)benzamide (Abbreviated as oHBA-a)

100

Into a 50 mL round flask were added 20 mL of chloroform, aniline (0.44 g, 4.70 mmol),

N-(2-hydroxyphenyl) benzamide (1.00 g, 4.70 mmol), and paraformaldehyde (0.23 g, 9.40

mmol). The mixture was stirred at 80 °C for 24 h, and subsequently cooled to room

temperature. Then, the solution was washed with cold water. The chloroform solution was

dried over anhydrous sodium sulfate to obtain crude product. The crude product was

fractionated by the column chromatography (eluent: hexanes and ethyl acetate, in a volume

ratio of 3:1) to obtain pure final product (yield 80%). 1H NMR (600 MHz, DMSO-d6, δ):

4.67 (s, 2H, Ar-CH2-N), 5.51 (s, 2H, O-CH2-N), 6.87-7.94 (m, 13H, ArH), 9.35 (s, 1H,

NH). IR spectra (KBr), cm-1: 1666 (amide I), 1237 (C-O-C asymmetric stretching), 921

(oxazine related). Elemental analysis: calcd. for C21H18N2O2: C, 76.34%; H, 5.49%; N,

8.48%; found: 75.96%; H, 5.34%; N, 8.41%.

4.2.3 FT-IR Characterization

PH-a, pHBA-a, and oHBA were studied by solution FT-IR in a liquid cell. The optical

crystals are made of thallium bromo iodide (KRS-5). The average pathlength is 0.539 mm,

which is the actual cell pathlength calculated from the interference fringe of the empty cell.

To minimize possible interference caused by water, moisture, and humidity, chloroform

was kept in activated molecular sieves overnight and distilled at boiling temperature. The

distilled chloroform was collected and ready for preparing the different solution at the

proper concentrations for each compound, PH-a, pHBA-a, and oHBA. The compounds

were also dried and kept in a vacuum oven for two days to eliminate water.

101

pHBA-a, and oHBA-a were dissolved in chloroform at the following concentrations: 1.0,

1.9, 3.0, 3.8, 10.0, 19.0, 30.0, and 38.0 mM. During the process of dissolving the

compounds, it was found that oHBA-a was the easiest to dissolve, whereas pHBA-a took

longer to dissolve in chloroform. The compounds were dissolved completely without any

heating procedure which might potentially disrupt the natural arrange or hydrogen bonding

system. Such care is important as hydrogen bonding structure is the focus of this study.

A Bomem Michelson MB100 FTIR spectrometer equipped with a deuterated triglycine

sulfate (DTGS) detector and a dry air purge unit was used to carry out this study. Sixty-

four scans were coadded at the resolution of 4 cm−1. The frequency of a band was determined after 11th polynomial curve fitting and interpolation of the data points.

4.2.4 NMR Characterization

1H and 13C NMR spectra were recorded on a Varian Oxford AS600 NMR spectrometer at the proton frequency of 600 MHz and its corresponding carbon frequency of 150.864 MHz

using trimethylsilane as an internal standard. The parameters for the NMR acquisitions are:

64 scans for 1H NMR and 1024 scans for 13C NMR, and room temperature data collection.

For quantitative intensity data collection, a relaxation time of 10 s was used for 1H NMR

spectral measurements. Concentration of the samples were 10 mg of compound/ 0.8 mL

of solvent unless otherwise mentioned.

4.3 Results and Discussion

4.3.1 Synthesis

102

The successful synthesis of amide monofunctional benzoxazine monomers was achieved using primary amine (aniline), paraformaldehyde, and amide functional phenols (para- and ortho- isomers) as well as phenol, as indicated in Scheme 4.1. The monomers are designated as pHBA-a and oHBA-a in which the small letters in italic represent the position of the amide group either at the para or ortho position with respect to the oxygen atom in the oxazine ring. The reaction condition for pHBA-a is harsher than oHBA-a: 1,4- dioxane as a solvent allows a higher reaction temperature, which increases the solubility of the reactants. The reason that a polar solvent is favored for pHBA-a is probably because the hydroxyl group on the para position of the amide group is exposed to interact favorably with the hydrophilic solvent. There is no possibility of the formation of the intramolecular hydrogen bonding between oxygen atom and NH in amide group when the NH group is in the para position with respect to the oxygen atom of the oxazine ring. PH-a represents a phenol/aniline type monofunctional benzoxazine monomer synthesized with no amide group in the structure. Its synthesis is also presented in Scheme 4.1.

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Scheme 4.1. Synthesis of benzoxazine monomers PH-a, pHBA-a, and oHBA-a.

Previous study on pHBA-a and oHBA-a was focused on the monofunctional benzoxazines functioned as precursors when synthesizing polybenzoxazoles. The samples used for the study were not of the sufficient purity to minimize the interference from the impurities.

Column chromatography and recrystallization after common synthesis were carried out to obtain highly pure material for the current study.

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Figure 4.1. (a) 1H and (b) 13C NMR spectra of pHBA-a, (c) 1H and (d) 13C NMR spectra

of oHBA-a.

4.3.2 FT-IR Study

Intra- and intermolecular hydrogen bonding studies are most often carried out using carbon tetrachloride as the solvent of choice. One of the main reasons for this selection is that carbon tetrachloride is neither a hydrogen boning acceptor nor a donor. Additionally, this

105

solvent presents a rather low polarity.22,23 Therefore, extra interactions with, or caused by, the solvent are eliminated and all spectroscopic information can be assumed to be from system under study. Also, this molecule exhibits no interfering band in the spectral region of interest between 4000 and 2000 cm-1. Taking into consideration that if two or more molecules are to be studied and compared, the conditions for the study must ideally be kept identical. Solubility tests of the two isomeric compounds in carbon tetrachloride (CCl4) revealed that one of the isomers, oHBA-a, exhibited a good solubility, whereas the other one, pHBA-a, a very poor one in the same solvent within the desired concentration ranges.

The main reason for this difference is related to the solvation capability of the solvent

(CCl4) toward each isomeric molecule. oHBA-a bears an intramolecular hydrogen bond within its molecular structure and no intermolecular ones. This favors its solvation and solubilization in CCl4 since there seems to be no important connectivity between different molecules of the same compound. On the contrary, pHBA-a forms intermolecular hydrogen bonds, thus establishing interactions between different molecules of the same pHBA-a. As these interactions between molecules of pHBA-a are stronger than those existing between the solvent (CCl4) and pHBA-a, pHBA-a cannot be easily solvated nor totally dissolved in this solvent. Formation of aggregate via hydrogen bonding might also occur in these conditions,24 thus interfering with the measurements. These experimental results do not permit to prepare a set of solutions covering the very same concentration range for each isomer, thus making impossible to have identical conditions for the study and losing therefore comparability. In consequence, carbon tetrachloride was replaced by chloroform as the solvent due to the better solubility of the two compounds in it albeit the

106

possibility of an interaction between the solvent and solute. This change in the solvent has been reported for other systems.25

FT-IR spectra at different concentrations of the two isomeric compounds were collected covering the concentration range between 1 and 38 mM. The shift observed for the maximum of the N-H stretching band was used for the hydrogen bonding study.

Figure 4.2. FT-IR spectra showing the region between 3500 and 3380 cm-1 for (a) pHBA-a, and (b) oHBA-a. Spectra were recorded in solution at different concentrations

using CHCl3 as solvent.

Table 4.1. Maxima of the N-H stretching absorption bands observed by FT-IR. Spectra

were recorded in solution using CHCl3 as solvent.

Wavenumber (cm-1) Concentration (mM) pHBA-a oHBA-a

107

1.0 3436.46 3427.08

1.9 3437.45 3426.96

3.0 3437.81 3426.70

3.8 3438.36 3426.71

10.0 3438.76 3426.23

19.0 3438.77 3426.79

30.0 3438.84 3426.79

38.0 3438.80 3426.54

4.3.2.1 PH-a

This compound was not studied in solution since there is no hydrogen bonding to be analyzed.

4.3.2.2 pHBA-a

The N-H stretching mode responded to concentration change in an unexpected way.

The pHBA-a N-H stretching frequency changes with concentration change. However, increasing the concentration, the band position increases. This blue shifting caused by increasing the concentration is known as improper hydrogen bonding. The change indicates 108

that, in the case of pHBA-a, there might be another hydrogen bonding, most likely weak

and influenced by the concentration. The half width at half height of the N-H stretching

mode shown in Figure 4.2 is broad and the positions are typical for hydrogen bonded N-

H stretching region, indicating the absence of free N-H stretching mode.

4.3.2.3 oHBA-a

oHBA-a shows intramolecular hydrogen bonding behavior which can be concluded from

the relatively constant peak position within the concentration range studied. More

importantly, the N-H position red-shifted to around 3426.7 cm-1 compare to pHBA-a, the

-1 Δwavenumber is 12.2 cm , which is consistent with intramolecular hydrogen bonding behavior

of oHBA-a since intramolecular hydrogen bonding is typically stronger than the

intermolecular hydrogen bonding.

Figure 4.3. pHBA-a, and oHBA-a N-H stretching wavenumber vs. concentration. 109

Figure 4.3 indicates that, with the increase on the concentration, the maximum of the N-H

stretching band of pHBA-a shifts to higher wavenumbers. The change, although in a small

magnitude, perfectly follows a well-defined tendency. This behavior is related to

intermolecular hydrogen bonding. In the case of oHBA-a, however, the maximum of the

same N-H stretching band remains at the very same value with the concentration changes,

which is characteristic for intramolecular hydrogen bonding. Intramolecular hydrogen

bonding is formed between the N-H in amide group and the O in oxazine ring. No matter

how low the concentration is, the frequency stays unaltered thus suggesting the presence

of the mentioned intramolecular hydrogen bonding.

Extensive previous studies have been done on the hydrogen bonding of secondary amides.

FT-IR is a powerful tool to study hydrogen bonding as the frequency is very sensitive to

the strength of hydrogen bonds and pattern of the shift representative to the mode of

hydrogen bonding.26,27 Free N-H hydrogen bonding is characterized by the wavenumber in the range of 3500-3400 cm-1. The free N-H stretching mode of N-methylacetamide, the

simplest secondary amide model compound, in the vapor phase, was reported to be of 3472

cm-1. The position of the N-H stretching band is in the range of intermolecular hydrogen

bonding region. The standard intermolecular hydrogen bonded amide-amide (C=O···H-N),

appears in the range between 3370 to 3250 cm-1.28 A specific frequency of the N-H

stretching mode depends on the molecular structure of amide compound, type of solvent

used, and temperature. Thus, weak interactions will also influence the position. The peak

position can shift due to interaction with the solvent from 3452 cm-1 in chloroform to 3442

110

-1 cm in C6H6, the latter one is influenced by N-H···π hydrogen bonding interactions. The steric hindrance also affects the N-H stretching band position.

Table 4.2. Examples of reported results on secondary amide N-H stretching band

positions determined by FT-IR.29

Chemical Structure Solvent Concentration N-H Stretching

(mM) Peak Position

(cm-1)

CHCl3 10 3424 O

C N 3 H DMSO 10 3298 (major)

3426 (minor)

CHCl3 10 3449 O CH C N 3 3 H DMSO 10 3315 (major)

3449 (minor)

It can be observed in Figure 4.3 that pHBA-a showed an unexpected hydrogen bonding behavior. With the increase of concentration, pHBA-a N-H hydrogen bonding maxima position blue shifted to higher wavenumbers. It is worth noticing that the profile observed

111

with the increase in concentration is asymptotic, whose maximum value at the plateau

frequency of 3439 cm-1 is reached at concentration of approximately 10 mM.

4.3.3 NMR Study

A detailed work on oHBA-a by different NMR techniques has recently been reported by

our group.20 However, much less attention has been paid to the pHBA-a isomer. In this section, we focus on this isomer complementing not only the reported work on NMR but also the FT-IR studies presented in the previous section. Figure 4.4 shows 1H NMR spectra

of pHBA-a at different concentrations.

112

Figure 4.4. 1H NMR spectra of pHBA-a at different concentrations. All spectra were

recorded at 25 °C using CDCl3 as solvent.

It is known that for a reaction between two species to happen both species should be

sufficiently activated. The cases presented in this study evaluate a single compound

reacting with itself. Under these conditions, the compound should then present at least two

different positions appropriately and sufficiently activated to be able to react. In this regard,

compounds oHBA-a and pHBA-a were recently studied. This article reported on how the

different positions on the anilide ring were activated.20 Indeed, it was shown that the

activation was more effective in oHBA-a, followed by pHBA-a, and then PH-a. At this

stage the 1H NMR spectra of each benzoxazine were analyzed to estimate the reactivity but now at the 2-position. Table 4.3 shows the chemical shifts of those –O–CH2–N=, and the

respective =N–CH2–Ar to be taken as internal references, using DMSO-d6 and CDCl3 as

solvents.

Table 4.3. Chemical shifts of the –O–CH2–N= and =N–CH2–Ar in CDCl3 and DMSO-d6

as solvent.

a Benzoxazine -O-CH2-N= (ppm) =N-CH2-Ar (ppm) Δδ

resin CDCl3 DMSO-d6 CDCl3 DMSO-d6 in CDCl3 in DMSO-

113

PH-a 5.35 5.42 4.63 4.63 0.72 0.79

pHBA-a 5.37 5.42 4.66 4.64 0.71 0.78

oHBA-a 5.47 5.51 4.65 4.67 0.82 0.84

a Δδ = (δ-O-CH2-N=) - (δ=N-CH2-Ar).

In both solvents the Δδ is greater for oHBA-a than for the other two resins. As has been

reported, however, DMSO-d6 has a high polarity and a strong capability to act as a hydrogen-bond acceptor. Thus, DMSO-d6 compete and disrupt the intramolecular

hydrogen bonds in oHBA-a, possibly forming intermolecular ones with the solvent. This

may be the very reason for this decreased difference between the Δδ in CDCl3 compared to the Δδ in DMSO-d6. In contrast, as CDCl3 is a low polar solvent, behavior in this solvent

might be closer to what is experienced by benzoxazines when reacting, except by the

understandable dilution. As can be seen in Table 4.3, the reference protons from the =N-

CH2-Ar of the three resins show frequencies practically at the same chemical shifts.

Nevertheless, the results are different for the frequency corresponding to –O–CH2–N=.

While resins PH-a and pHBA-a present the peaks at very similar frequencies, in oHBA-a

the peak is shifted toward greater δ (downfield). Thus, the –O–CH2–N= of oHBA-a are a little more acidic than those of the other two resins. This could in turn be interpreted as oHBA-a having this particular position a little more activated. Once again, oHBA-a shows a higher reactivity not only in its anilide positions but also in its 2-position. This is in agreement to its higher reactivity toward polymerization as experimentally observed by

114

DSC. Thus, given the thermal conditions, positions from the anilide could directly attack

the more activated 2-position in oHBA-a, thus initiating the polymerization, as has been

suggested in previous studies.20 Finally, it must be mentioned that, in terms of reactivity,

pHBA-a is in an intermediate situation between PH-a and oHBA-a. Consequently, the

initiation mechanism(s) that pHBA-a could undergo will depend on the specific conditions.

4.3.4 Thermal Behavior Study

DSC was used to study the thermal behavior of each resin. Thus, Figure 4.5 shows the thermograms of each benzoxazine monomer, oHBA-a and pHBA-a, as well as the unsubstituted PH-a.

Figure 4.5. DSC thermograms comparing oHBA-a, pHBA-a with PH-a.

115

As can be seen in the figure, PH-a shows a sharp melting endotherm centered at 58 °C, which at the same time is highly symmetric. This aspect ratio for the melting of crystals is characteristic for compounds exhibiting high purity. In fact, melting point is often used in organic chemistry as a simple although efficient purity criterion.

Similarly, pHBA-a shows a sharp and highly symmetric endothermic peak at 151 °C, also indicating high purity for this benzoxazine resin. In the case of oHBA-a a more complex situation is observed, where a nicely defined endothermic peak centered at 116 °C is first observed, followed by a very small, highly overlapped exothermic peak at 122 °C, which is then followed by a new endothermic peak centered at 130 °C. This behavior is consistent with a previously reported crystal-to-crystal phase transition.30,31 The crystal of oHBA-a

might be recrystallized into another more stable crystal form during the first melting stage.

However, the meaningful observation to pay attention here is that both melting endotherms,

and especially the second one, which is a cleaner transition, are once again sharp and highly

symmetric. This result also strongly supports the high purity achieved for oHBA-a.

As can be seen, PH-a exhibits the lowest melting point, followed by oHBA-a (including

both melting points), and finally pHBA-a showing the highest one. The order in the melting temperatures of analogous compounds, or even isomers as is the cases for pHBA-a and oHBA-a, can also uncover molecular interactions. For instance, the intramolecular hydrogen bonds in oHBA-a interact within the molecule, whereas the intermolecular hydrogen bonds in pHBA-a interact between molecules. Thus, intermolecular hydrogen bonds result in a stronger driving force for crystal formation, generating higher melting temperature. As a result, oHBA-a exhibits a lower melting temperature than pHBA-a.

116

A similar rationalization can be applied based on the same hydrogen-bonding interaction,

but now addressing this issue with the solubility exhibited by these two compounds,

pHBA-a and oHBA-a. Understandably, oHBA-a has better and higher solubility than

pHBA-a in most of the organic solvents.20

The thermal behavior of PH-a, pHBA-a, and oHBA-a toward polymerization was then studied by DSC as depicted in Figure 4.5. The thermogram of PH-a shows that this compound does polymerize despite its high purity. The exothermic peak assigned to polymerization has its maximum centered at 272 °C. It must be highlighted, however, that

PH-a in this condition has polymerized at a noticeably much higher temperature than the usual temperatures reported for this compound, usually around 260 °C,21 reflecting the

higher purity of the PH-a used in this study as compared to those used for other reported

papers. Similar to what has been observed for PH-a, pHBA-a and oHBA-a also showed

exothermic peak associated with their polymerizations. Specifically, the peak maximum is

centered at 251 and 215 °C for pHBA-a and oHBA-a, respectively. Summarizing, PH-a

exhibits the highest polymerization temperature, followed by pHBA-a, and finally oHBA-

a showing the lowest one of the three studied compounds. This behavior has in fact already

been reported.20 What is shown in this study, however, is that their polymerization

temperatures are indeed affected, but the order in which they do even though their high

purity is not altered.

A close look at Figure 4.5 evidences that compared to the exotherm seen for PH-a, oHBA-

a is broader, and pHBA-a is the broadest one. Interestingly, the results obtained by DSC

show that while both inter- and intramolecular hydrogen-bonding systems have an

excellent acceleration effect to initiate the polymerization, they seem to somehow lower 117

the rate of the propagation step on the crosslinking polymerization. The previous result

strongly suggests that regardless of the polymerization temperature of these benzoxazines,

the propagation step for each resin face different conditions, although not necessarily

different mechanisms. It has been reported that benzoxazine resin based on bisphenol A

and aniline (abbreviated as BA-a) that is polymerized in hydrogen bond forming solvent

(or blend partner) shows higher Tg than the values predicted by the Fox equation for the

blend composition of these two materials.32-39 A possible mechanism for this has also been

reported which is the competition between the propagating species and hydrogen bonding

environment.40,41 A similar situation might exists in the current comparison of those three

monomers, PH-a, oHBA-a and pHBA-a. It can be seen that PH-a do not have hydrogen

bond forming structures at the onset of polymerization, and oHBA-a to some extent of

hydrogen forming species created by the weakening of the intramolecular hydrogen

bonding at elevated temperature, and finally a large number of available hydrogen bonding

species in pHBA-a. Thus, based on simply hydrogen bonding environment point of view,

the extensive network formation improves in the order of PH-a, oHBA-a and pHBA-a,

which is different from the polymerization rate order of PH-a, pHBA-a and oHBA-a.

Figure 4.6 shows the comparison of the DSC thermograms of three polymers obtained by

polymerizing, for 2 hours, the three resins at the onset temperature of their respective

exotherm (according to figure 4.5). The onset temperatures are 260, 240 and 210 °C for

PH-a, pHBA-a, and oHBA-a, respectively.

118

Figure 4.6. DSC thermograms comparing poly(PH-a) (blue), poly(pHBA-a) (red), and

poly(oHBA-a) (black).

The result can be explained by the electronic effects we reported previously. The glass transition temperature (Tg) order for the obtained thermoset is as follows: poly(oHBA-a) > poly(pHBA-a) > poly(PH-a), which is in fact a reverse sequence of the polymerization temperature order of their precursors: PH-a > pHBA-a > oHBA-a. Thus, PH-a shows the highest polymerization temperature but the lowest Tg when compared to pHBA-a and oHBA-a. On the contrary, oHBA-a presents the lowest polymerization temperature and the highest Tg. Finally, pHBA-a exhibits temperatures between the other two compounds in both cases. It is interesting to note that the Tg difference between poly(pHBA-a) and poly(oHBA-a) is not as dramatic as the polymerization exotherm temperature difference

119

might indicate. This might be due to the competing effect of the rate of polymerization and

network formation as discussed above.

The previous results might be rationalized as follows: on the one hand, oHBA-a presents

more reactive positions that are more activated than in the other two compounds in the

aromatic ring adjacent to the oxazine moiety. Thus, initiation occurs at lower temperature.

Moreover, the other activated positions in the same aromatic ring might simultaneously act

as crosslinking points during the propagation step, thus generating the most cross-linked network. On the other hand, PH-a shows the highest polymerization temperature since this compound does not have any activating substituent in its structure. The only reactive positions in the aromatic ring adjacent to the oxazine moiety (the 6- and 8-positions, also called para and ortho positions) are intrinsically and slightly activated by the oxygen from the oxazine moiety inducing low crosslinking, thus obtaining the thermoset with the lowest

Tg of the three studied systems. Finally, pHBA-a bears more reactive positions which are

also more activated than in PH-a. However, when compared to oHBA-a, pHBA-a presents the same number of reactive positions but less activated. The intermediate reactivity of pHBA-a makes it more reactive than PH-a but less than oHBA-a in terms of initiation as well as in its crosslinking ability.

4.4 Conclusions

The hydrogen bond structure and its influence on the amide-containing benzoxazines pHBA-a and oHBA-a have been studied by FT-IR and 1H NMR. The results have offered clear evidence for the existence of two different types of hydrogen-bonding systems, inter-

120

and intramolecular hydrogen bonds. While pHBA-a exhibits an unexpectedly strong intermolecular hydrogen bonding behavior, which has made difficult its analysis in the past, results obtained for oHBA-a are in agreement with those supporting the intramolecular five-membered-ring hydrogen bonding.

Finally, it has been observed that the hydrogen bonding systems influence the benzoxazine polymerization behavior not only with respect to the polymerization temperature, but also and importantly, with respect to the propagation rates as well as crosslinking ability.

4.5 References

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CHAPTER 5

Conclusion

125

5.1 Oxazine ring related modes

The results reveals that the characteristic band in 960-900 cm-1 is mainly related to the

oxazine ring, the vibration of O-C2 vibration with a minor contribution from the phenolic ring. The solid and sound experiment including PH-a, PH-t isotopes, 15N isotope compound, C2 and C4 substituted oxazine ring benzoxazine molecules, the FT-IR and Raman spectra of all the compounds correspond with theoretical calculation. The study is essential for the understanding on oxazine ring related vibrational modes. It is a great guidance for understanding the ring opening of oxazine ring, which is an important step in polymerizing benzoxazine monomer.

5.2 Intrinsic ring opening of oxazine ring

The study demonstrates that when oxazine opens, temperature alone is sufficient to

initiate the polymerization of benzoxazines, no initiator or impurity is needed. Initiator

and impurities can lower the polymerization temperature. Therefore, polybenzoxazine

can be obtained by both the traditional thermally accelerated (or activated)

polymerization, where impurities or initiators are needed in the reaction; or, by the classic

thermal polymerization, where only heat is enough to initiate the reaction. It is a

significant result which can guide future molecular design to obtain lower

polymerization, easy polymerized benzoxazine monomers.

5.3 Hydrogen bonding in amide-containing benzoxazine

FT-IR and 1H NMR results of pHBA-a and oHBA-a, amide-containing benzoxazine, give clear evidence for the existence of two different types of hydrogen-bonding systems. pHBA-a exhibits an unexpectedly strong intermolecular hydrogen bonding behavior,

126

which has made difficult its analysis in the past, results obtained for oHBA-a are in agreement with those supporting the intramolecular five-membered-ring hydrogen bonding.

Hydrogen bonding system influences the polymerization behavior, thermal behavior as well as crosslinking ability.

127

SUPPORTING INFORMATION

128

NMR Spectra

O O

N N

Figure S1. NMR Spectrum of 25DMR-h

129

O N

O O

Figure S2. NMR Spectrum of PG-a

130

O O N

O O

Figure S3. NMR Spectrum of PG-anis

131

O N

Figure S4. NMR Spectrum of PH-a

132

O N D

D D D

Figure S5. NMR Spectrum of PH-a-phenol-d4

133

D D O N

Figure S6. NMR Spectrum of PH-a-2oxazine-d2

134

O N D D

Figure S7. NMR Spectrum of PH-a-4oxazine-d2

135

D D O N D D

Figure S8. NMR Spectrum of PH-a-oxazine-d4

136

D D D

O N D D

Figure S9. NMR Spectrum of PH-d-a

137

O N

Figure S10. NMR Spectrum of PH-t

138

O N D

D D D

Figure S11. NMR Spectrum of PH-t-phenol-d4

139

D D O N

Figure S12. NMR Spectrum of PH-t-2oxazine-d2

140

D D O N D D

Figure S13. NMR Spectrum of PH-t-oxazine-d4

141

O N

Figure S14. NMR Spectrum of PH-a-4oxazine

142

O N

Figure S15. NMR Spectrum of PH-a-2oxazine

143

FT-IR Spectra

O O

N N

Figure S16. FT-IR Spectrum of 25DMR-h

144

O N

O O

Figure S17. FT-IR Spectrum of PG-a

145

O O N

O O

Figure S18. FT-IR Spectrum of PG-anis

146

O N

Figure S19. FT-IR Spectrum of PH-a

147

O N D

D D D

Figure S20. FT-IR Spectrum of PH-a-phenol-d4

148

D D O N

Figure S21. FT-IR Spectrum of PH-a-2oxazine-d2

149

O N D D

Figure S22. FT-IR Spectrum of PH-a-4oxazine-d2

150

D D O N D D

Figure S23. FT-IR Spectrum of PH-a-oxazine-d4

151

D D D

O N D D

Figure S24. FT-IR Spectrum of PH-d-a

152

O N

Figure S25. FT-IR Spectrum of PH-t

153

O N D

D D D

Figure S26. FT-IR Spectrum of PH-t-phenol-d4

154

D D O N

Figure S27. FT-IR Spectrum of PH-t-2oxazine-d2

155

D D O N D D

Figure S28. FT-IR Spectrum of PH-t-oxazine-d4

156

O N

Figure S29. FT-IR Spectrum of PH-a-4oxazine

157

O N

Figure S30. FT-IR Spectrum of PH-a-2oxazine

158

O 15N

Figure S31. FT-IR Spectrum of PH-a-15N

159

Raman Spectra

O N

Figure S32. Raman Spectrum of PH-a

160

O N D

D D D

Figure S33. Raman Spectrum of PH-a-phenol-d4

161

D D O N

Figure S34. Raman Spectrum of PH-a-2oxazine-d2

162

O N D D

Figure S35. Raman Spectrum of PH-a-4oxazine-d2

163

D D O N D D

Figure S36. Raman Spectrum of PH-a-oxazine-d4

164

D D D

O N D D

Figure S37. Raman Spectrum of PH-d-a

165

O N

Figure S38. Raman Spectrum of PH-t

166

O N D

D D D

Figure S39. Raman Spectrum of PH-t-phenol-d4

167

D D O N

Figure S40. Raman Spectrum of PH-t-2oxazine-d2

168

D D O N D D

Figure S41. Raman Spectrum of PH-t-oxazine-d4

169

O 15N

Figure S42. Raman Spectrum of PH-a-15N

170

Theoretical Calculations

Using Gaussian 09 with B3LYP/6-31G** theory and basis

O N

Figure S43. Theoretical Calculation of PH-a

171

O N D

D D D

Figure S44. Theoretical Calculation of PH-a-phenol-d4

172

D D O N

Figure S45. Theoretical Calculation of PH-a-2oxazine-d2

173

O N D D

Figure S46. Theoretical Calculation of PH-a-4oxazine-d2

174

D D O N D D

Figure S47. Theoretical Calculation of PH-a-oxazine-d4

175

D D D

O N D D

Figure S48. Theoretical Calculation of PH-d-a

176

O 15N

Figure S49. Theoretical Calculation of PH-a-15N

177

Table S1. Determination of scaling factor by comparison of experimental results against calculated frequencies using B3LYP/6-31G** for PH-a, PH-a-phenol-d4, PH-

15 a-2oxazine-d2, PH-a-4oxazine-d2, PH-a-oxazine-d4, PH-d-a and PH-a- N

Experimental FTIR Theoretical Ratio Average (cm-1) Frequencies (cm-1)

3012 3196 0.942

2898 3037 0.954

1601 1660 0.964

1584 1638 0.967

1491 1536 0.971

1456 1500 0.970 PH-a 0.969 1370 1415 0.968

1339 1380 0.970

1254 1284 0.976

1227 1263 0.972

1158 1185 0.977

1033 1060 0.974

178

946 977 0.969

753 768 0.981

695 714 0.974

3028 3037 0.997

2924 3013 0.970

2278 2370 0.961

1600 1661 0.963

1562 1613 0.968

1496 1544 0.969

1415 1429 0.990

PH-a-phenol-d4 0.975 1259 1288 0.977

1218 1246 0.978

1149 1182 0.972

1031 1045 0.987

936 969 0.966

754 779 0.968

694 712 0.974

179

586 597 0.982

180

3041 3196 0.951

2856 3014 0.948

2150 2215 0.970

1600 1660 0.964

1485 1533 0.969

1456 1501 0.970

1270 1303 0.975

PH-a-2oxazine-d2 1250 1283 0.974 0.969

1104 1133 0.975

1052 1082 0.972

1019 1046 0.974

941 968 0.973

876 891 0.983

754 777 0.971

694 713 0.974

3061 3154 0.970

PH-a-4oxazine-d2 0.973 3038 3036 1.001

181

1600 1660 0.964

1582 1635 0.968

1488 1533 0.971

1453 1494 0.972

1349 1391 0.970

1296 1339 0.968

1285 1314 0.978

1253 1282 0.977

1225 1262 0.971

1154 1192 0.968

1029 1055 0.976

951 984 0.967

752 772 0.974

3037 3207 0.947

2925 3203 0.913

PH-a-oxazine-d4 0.965 2853 3196 0.893

2146 2216 0.968

182

1600 1660 0.964

1495 1542 0.969

1485 1529 0.971

1454 1496 0.972

1249 1286 0.971

1032 1062 0.972

1018 1044 0.975

946 969 0.976

894 866 1.033

754 775 0.973

694 713 0.974

183

3066 3138 0.977

2895 2975 0.973

1568 1631 0.961

1489 1589 0.937

1406 1467 0.959

1387 1421 0.976

1328 1401 0.948

PH-d-a 1226 1236 0.992 0.975

1175 1203 0.977

1159 1183 0.980

1138 1162 0.979

1111 1113 0.998

1034 1042 0.992

943 960 0.982

754 755 0.999

3092 3196 0.967 PH-a-15N 0.965 3066 3154 0.972

184

1598 1660 0.962

1495 1543 0.969

1380 1431 0.965

1350 1401 0.964

1199 1235 0.971

1158 1221 0.948

1143 1176 0.972

1127 1137 0.991

1034 1058 0.977

929 989 0.939

913 976 0.936

751 776 0.968

691 713 0.969

Scaling factor 0.970

185

Table S2. Bond Length, Angle and Torsion Comparison Between Crystal Structure and Gaussian Optimized Structure for PH-a

8 1 O 7 9 2

3 b 6 N 10 a c 5 4

Crystal Structure Optimized Structure Difference

C8-C7 1.378 1.392 0.014

C4-C10 1.508 1.520 0.012

C2-O 1.452 1.445 0.007

N-C2 1.431 1.429 0.002

Ca-N 1.425 1.423 0.002

Cf-Ca 1.392 1.406 0.014 Bond Length (Å) Length Bond

C9-C8 1.380 1.398 0.019

Cb-Ca 1.394 1.405 0.010

Ce-Cf 1.379 1.391 0.012

Cc-Cb 1.372 1.396 0.024

Cd-Cc 1.373 1.394 0.021

186

C10-C9 1.390 1.406 0.016

C5-C10 1.390 1.399 0.009

C6-C5 1.372 1.393 0.021

O-C9 1.368 1.373 0.004

187

Crystal Structure Optimized Structure Difference

C4-C10-C9 118.6 119.9 1.3

H-C5-C10 120.3 118.8 1.5

C2-O-C9 116.1 114.0 2.2

N-C2-O 113.6 114.2 0.6

H-C4-C10 109.3 110.1 0.8

H-C4-C10 110.1 110.6 0.5

O-C2-H 108.9 107.8 1.1

Angle (degrees) O-C2-H 108.8 105.6 3.3

Ca-N-C2 117.0 118.9 1.9

N-Ca-Cf 122.4 118.8 3.6

C9-C8-C7 120.9 119.8 1.1

Cb-Ca-N 120.0 122.5 2.5

Ce-Cf-Ca 120.7 120.7 0.0

Cc-Cb-Ca 121.2 120.3 0.9

188

Cd-Cc-Cb 120.9 120.8 0.0

H-Cc-Cb 119.5 119.1 0.5

H-Cb-Ca 119.5 120.0 0.5

H-Cf-Ca 119.6 119.3 0.2

f H-Ce-C 119.6 119.3 0.2

H-Cd-Cc 120.6 120.5 0.1

C10-C9-C8 119.7 120.7 1.0

C5-C10-C9 119.5 118.4 1.2

C6-C5-C10 119.6 121.4 1.7

H-C6-C5 119.9 120.1 0.2

H-C7-C8 120.5 119.5 1.0

H-C8-C7 119.3 121.6 2.3

O-C9-C8 118.1 117.1 1.0

189

Crystal Structure Optimized Structure Difference

C4-C10- 179.0 178.3 0.7 C9-C8

H-C5-C10- 181.7 179.8 1.9 C9

C2-O-C9- 177.2 167.3 9.9 C8

N-C2-O- 38.1 46.4 8.3 C9

H-C4-C10- Torsion (degrees) Torsion 210.3 223.5 13.3 C9

H-C4-C10- 91.7 105.7 14.0 C9

H-C2-O- 276.6 285.8 9.2 C9

H-C2-O- 159.5 168.6 9.0 C9

190

Ca-N-C2- 72.4 77.7 5.2 O

Cf-Ca-N- 220.7 163.3 57.4 C2

Cb-Ca-N- 43.9 345.9 58.1 C2

Ce-Cf-Ca- 182.0 182.1 0.0 N

Cc-Cb-Ca- 179.2 176.7 2.5 N

Cd-Cc-Cb- 358.5 1.2 2.8 Ca

H-Cc-Cb- 178.5 180.0 1.5 Ca

H-Cb-Ca- 359.1 354.4 4.7 N

H-Cf-Ca- 2.1 0.4 1.7 N

191

H-Ce-Cf- 179.3 180.0 0.7 Ca

H-Cd-Cc- 179.5 179.4 0.1 Cb

C10-C9- 0.2 0.9 0.7 C8-C7

C5-C10- 359.0 359.0 0.0 C9-C8

C6-C5- 1.3 0.3 1.0 C10-C9

H-C6-C5- 178.9 179.8 0.9 C10

H-C7-C8- 180.1 179.7 0.4 C9

H-C8-C7- 182.5 180.3 2.2 C9

O-C9-C8- 175.4 180.0 4.6 C7

192

193

Table S3. Band Assignment of FT-IR (red), Raman (blue) and Theoretical

Calculations (Black) Spectra

PH-a PH-a-phenol-d4 Band Assignment

FT-IR Raman Theo FT-IR Raman Theo

3159 sh

3122 3122

3117

3110 3110

3107

3100 3100

3092 sh 3091 3088 w 3092 vw

3089 3089

3078 sh 3082 3078 sh 3082

3070 sh 3071 3067 m

3064 m

3058 vs 3059 3061 m 3054 sh 3059 CH of phenol

3040 m 3036 sh 3040 vw

194

3030 3028 m 3027 vw 3030

3012 s 3014 w 3014 w

3009 sh 3007 w 3009 sh

2989 m 2989 m Ar-CH2-N

2977 sh 2981 sh

2956 m

2946 2945

2923 m 2923 2923

2960 m 2965 vw

2924 s

2912 w 2912 w Ar-CH2-N

2888 m

2868 vw

195

2862 vw

2856 m 2853 m 2853 vw

2310

2294 sh 2298 Ar CD stretch

2289 m Ar CD stretch

2278 w 2278 w 2282 Ar CD stretch

196

2274 sh Ar CD stretch

2267 w 2266 Ar CD stretch

alph CD stretch

2255 w 2259 sh Ar CD stretch

2242 w Ar CD stretch

1617

1601 s 1602 m 1611 1600 s 1600 m 1610 skeletal mode

1592

1584 s 1583 w 1589 1583 s 1581 w skeletal mode

1586 1562 vs 1560 w 1586

1539 vw 1564

1498

1491 vs 1492 vw 1489 1496 vs 1492 vw 1497 skeletal mode

1479 vw

1471 1470 sh 1473 vw 1470

1456 s 1453 m 1455 1454 m 1453 vw 1453 skeletal mode

1452 1451

197

1447 1449 sh

1440 vw 1440 w H-C4-H bend

1415 s 1416 w 1417

1385 m 1389 vw 1388

1381 s 1379 vw 1381

1370 m 1367 w 1373 1370 s 1368 w 1373 H-C2-O-C torsion

1361 w 1367

1339 m 1341 vw 1339 1337 vw 1342 vw H-C4-C-C torsion

1333 vw 1334 vw skeletal mode

1327 1327

1312 w 1310 vw 1318

1305 1304

1297 w 1298 w 1301

198

1286 w 1287 w 1289

1259 s

1264

1254 s 1255 m 1253 w 1249 C2H2 of ozazine

1246

1227 s 1224 m 1225

1218 s

1210 m 1208 sh 1208 O-C9 stretching

199

1201

1199 sh 1196?

1193 m 1188 1188 sh 1190 m 1190 skeletal mode

1177 1177

1158 m 1159 sh 1157 w skeletal mode

1152 1152

1147 m 1151 w 1149

1145 1149 m 1145 m 1147

1131 sh 1130 w

1114 w 1115 w 1106 1117

1096 w 1098 vw

1088 w 1090 vw 1082 1089

1075 1079 w 1078

1040 sh

1033 m 1031 vs 1031 1031 s 1030 s

1027 1027 sh 1025 sh 1028

200

1013

998 w 999 vs 997 vw 997 vs skeletal mode

990 w 983 982 vw 987 w 983 skeletal mode

986 sh

972 m 972 sh 975 975 sh 972 oxazine ring

962 963 s 962 w 960

960 958 sh 954

948

946 s 947 vw 947 O-C2 stretch

939 936 s 934 vw 940 oxazine ring

938

911

894 w oxazine ring

886 886 oxazine ring

870 vw 872 w

865 w 860

201

852 w 854 w 853 vs

845 w 842 846

830 827 sh 826 w

823 823

820

804 s 806 w

801 s 797 w

790

777 s 768 774

202

763 sh 764 m

753 vs 759 m 753 756

754 s 754 m

749

745 744 sh 743

737 s phenolic CH

731

725 w 725 vw

709 sh 714 m 713 phenolic CH

695 m 695 m 698 699 m 694

692 694 s 693 w 691

680 684 m

671

636 w

628 w

620

617 vw 616 m 612 617 vw 614 m 612

203

590 w 592 w

586 s

580 573 m 570 vw 579

565

556 555 s 553 w

548

538 w 539 vw 531 537 vw

528 527

519 w 519 vw 517 w 514 w

511 508

483 vw

475

445

439 w

432 vw 428 431

414 vw

410 410

204

399 401

384 vw 379 w 384 N-C2 stretch

362 360

323 vw

310 309 vw C-C9-O bend

300

293 vw

286 286 w skeletal mode

278

205

262 vw

252 250 w 246

211 m

203 m

182 179

138 136

94 89

54 54

31 30

206

PH-a-2oxazine-d2 PH-a-4oxazine-d2 Band Assignment

FT-IR Raman Theo FT-IR Raman Theo

3163 vw

3122 3122

3117 3117

3110 3110

3107 3107

3100 3100

3091 3091

3089 3089

3078 m 3082 3077 s 3082

3068 m 3071 3068 s 3071

3061 m 3057 m 3061 m 3061 w 3059 CH of phenol

3057 s

3041 m 3041 w 3038 m 3041 w

3027 m 3027 vw 3030 3028 vw

3013 w

207

3009 vw

2989 m Ar-CH2-N

2963 w 2968 w

2945

2923

2920 w 2928 vw

2914 w Ar-CH2-N

2907 w

2893 m

2862 vw

2856 m

2802 sh

2268 w 2267 w 2270 alph CD stretch

2260 sh alph CD stretch

2240 alph CD stretch

2159 w

2154 sh

208

2150 m 2149

alph CD stretch

2140 vw 2135 alph CD stretch

2121 vw alph CD stretch

1616 1617 skeletal mode

1600 s 1598 m 1610 1600 s 1598 s 1610

skeletal mode

1584 s 1581 w 1588 1582 s 1581 m 1587

209

1587 1586

1492 vw skeletal mode

1495 vs 1492 vw 1496 1488 vs 1497

1485 vs 1487 vw 1487

1472 w 1469 skeletal mode

1456 s 1456 1453 s 1452 vw

1452 1454

1450 1449 H-C4-H bend

1442 w

1385 w 1385

H-C2-O-C torsion

1368 m

1348 m 1346 w 1349 1349 m 1347 s 1350 H-C4-C-C torsion

skeletal mode

1338 m 1338 w 1333

1325 m 1326 1327

1312

210

1305

1298 w 1298 vw 1302

1292 vw 1296 m 1299

1285 m

1270 m 1273 vw 1275

1264 C2H2 of ozazine

1250 s 1250 vw 1253 m

1245 sh 1245 1243 m 1244

1225 s 1223 m 1224

O-C9 stretching

1209 m

1201 w 1201

skeletal mode

1192 w 1186 1190 m 1195

1177 1177

skeletal mode

1158 w 1157 w 1153 1154 m 1158 m 1156

211

1151 1152 sh 1151

1146 w 1146 1146

1123 vw 1123 vw

1115 vw 1117 m

1110 1110

212

1104 w 1104 w 1104 w

1099 1098

1092 sh

1083 w 1080

1076 sh 1073 vw 1072 1073 w 1072 m

1068

1052 m 1052 w 1050 1047 m

1042

1031 w 1030 s 1027 vs

1030 1029 m

1028 1026

1023

1019 m 1106 sh 1014

skeletal mode

996 sh 998 vs 998 vs

skeletal mode

986 vw 988 vw 983 987 w 983

213

oxazine ring

978 w 978 m

962 vw 960 962 w 960

954 m 956 w 956 m 954

O-C2 stretch

941 s 947 949 w 947 oxazine ring

945

939 939

931 935 m 931 w

927

925 w 919

oxazine ring

908 oxazine ring

886 886 w 887 w 888

880 w

876 m 872 m 876 w 876

864 862

214

850 w

840 839 w

830 w 828 w 824 w 824 w 829

823 822

815 811

796 vw

770 s

766 m 770 sh

753 760

215

754 vs 752 vs 750 s

748 m 746 749

744 743

737 s phenolic CH

730 m

724 phenolic CH

714 w 713 w 718 w 716 s

710

698 698 m

694 s 689 w 691 694 m 694 sh 692

683 m 683

674 673

617 vw 615 m 613 614 m 612

585 w

576 w 576 579 w 578 m

570

553 w

216

546 w 542 549

530 w

530 w 533 w 529 532 m 526

522 523

510 w 506 m

503 500

455 vw 452 w

442

439

428 w 424 m

421 420

410 vw 409 410 m 409

394 393 N-C2 stretch

370 m

355

351 m

321 vw 323

217

C-C9-O bend

310

304

skeletal mode

283 vw 281

272 m

268

260 w

218

259 m

249 249

210 w

204 m

181 174

136 136

94 94

54 54

30 30

219

Band 15 PH-a-oxazine -d4 PH-a- N PH-d-a Assignment

FT-IR Raman Theo FT-IR Raman Theo FT-IR Raman Theo

3150 w 3149

3139

3122 3122 3123

3117 3117

3110 3110 3103

3107 3107

3100 3100 3091

3091 3091

3089 3089

3078 m 3082 3092 w 3074 m 3082

3067 m 3071 3071

3066 w 3064 sh 3066 w 3061 m

3061 m 3055 s 3059 3061 CH of phenol

3038 m 3040 m 3034 sh 3034 w 3042 w

220

3025 m 3026 vw 3030 3022 w

3017 w

3009 sh 3009 vw Ar-CH2-N

3000 vw

2991 w

2980 vw

2970 w 2976

2958 m 2953 w 2953

2941 w 2945 vw 2945

2923

2965 vw

2925 s Ar-CH2-N

2915 w

2895 w

2873 vw 2868 vw

2854 m 2856 w

221

2847 v Ar CD stretch

2335 Ar CD stretch

2328 Ar CD stretch

2317 Ar CD stretch

2304 Ar CD stretch

2298 m 2295 alph CD stretch

2288 sh

2283 sh Ar CD stretch

222

2275 w 2268 w Ar CD stretch

2260 w 2264 m 2270 alph CD stretch

2253 vw

2240 alph CD stretch

2235 w

2215 vw

2196 vw

2158 w alph CD stretch

alph CD stretch

2146 m 2150 alph CD stretch

alph CD stretch

2140 w 2135 alph CD stretch

2124 m 2122 w alph CD stretch

2102 w alph CD stretch

2072 m alph CD stretch

2060 sh alph CD stretch

1633

223

1616 1617

1600 s 1597 m 1610 1598 s 1594 m 1610 1609 m 1608 vw 1605 skeletal mode

1592

1582 s 1580 m 1587 1577 m 1578 w 1589 1583 m skeletal mode

1586 1586

1568 s 1571 m 1568

1559 sh

1505

1495 vs 1490 vw 1496 1495 s 1493 w 1497 1487 skeletal mode

1485 vs 1483 w 1484 1489 1489 vs 1481 w

1443 w

1476 w 1470 1469

1454 s 1453 w 1453 1455 w 1455 w 1456 m 1464 skeletal mode

1451 1455

1448 vw 1452

1442 w 1441 w 1447 H-C4-H bend

1422

224

1406 s 1403 m 1401

1387 w 1388 1387 m 1391 sh

H-C2-O-C 1376 w 1380 w torsion

1371 w 1373 w

1359 1358

H-C4-C-C 1350 w 1353 w 1357 torsion

1338 1342 w 1345 sh 1344

skeletal mode

225

1336 vw 1334 w

1316 sh 1329 1320 m 1319 m 1326 1328 m 1318

1312

1305 m 1301 sh

1316

1308 1303

1300

1297 s 1292 vw 1294 1296 vw 1293 w

1286 s

1281 w

1275 1275 vw 1275

1262 1267 sh

1253 w 1256

1249 vs 1245 w 1248 1245 1242 m 1237 C2H2 of ozazine

1224 w 1221 1226 vs 1227 m

1210 w 1208 vw 1210 w 1205

226

1199 s 1198 w 1182 O-C9 stretching

1190 w 1194 1190 w 1198 1194 sh

1184 skeletal mode

1178 w 1177 1176 1175 m 1173 m

1163 1160

1158 w 1158 s 1158 w 1159 m 1157 vw 1157

1151 1151 skeletal mode

1151 w

1141 w 1147 1143 s 1146

1141 1138 m

1127 w 1127 m 1124 m

1116 m

1109 w 1111 m

1120 1114

1084 w

1104 1103

1088 vw 1083 1079 1079 m 1076

227

1074 1076 w

1067 w 1063 m 1065

1056 w 1052 w 1053

1039 sh 1038 vs 1034 m 1037 s 1042

1032 m 1027 vs 1034 1034 m 1034 sh 1031 1036

1030

1026

1018 s 1016 sh 1013

1007 vw

228

997 vs 996 993 sh 995 vs

985 vw 982 990 sh 983 984

981 m 981 vs

977 vw 977 w 970 m 972 sh 973 970 sh 974 m skeletal mode

961 vw 960 965 vw 960 961 vs 968

957 sh 955 vw 959 951 sh 959 skeletal mode

957

946 vs 944 vw 947 947 951 oxazine ring

940 929 s 946 943 vs

939 939

932 vw 935

O-C2 stretch

919 920 oxazine ring

913 s 912 w 911

895 vw

888 w 887 vw 888 881 m 884 w 886

877 870 w 871 vw 877 m 871

229

865 vw oxazine ring

849 s 851 m 854 w 855 m 852 oxazine ring

846 842 846 sh 847

840 839 w 841

825 w 831 w 830 m 829 s 829 833 vw 831

820 m 823

823 825

817

812 w 808 w 814 vw

800 804

775 sh

766 m 765 769 m 765

764 m 759 w 760

754 vs 752 751 vs 750 w 752 754 s 754 s 754

750

744 745

745 w

230

740 s 739 735 m

731 731 sh 731

714 w 711 s 713 w

706 708 w 704 phenolic CH

698 m 698

694 s 693 sh 691 691 vs 692 phenolic CH

684 w

231

678 w 682 677 w 676

668 675

662 m 659 m 654 w

636 w 644 w 646

641

617 vw 614 m 612 617 w 613

593 w 590 m 592 w 595 w 594

584 w 583

576 w 573 m 580

566 561 vw 558

558 vw 554 553 m 551

543 vw

530 w 532 w 534 sh 535

537 sh 535 530

533 m

526 sh 525 525

232

519 513 w 518 w 511

506 m 508

496 m 493 491 vw

479 w 473 vw 478

446 w 444

434 427 435 vw 430

419 w 414

409 vw 409 405 w 410

398 w 401 sh

395

387 389 w 385

362 362 w 370

356

327 w 332 w

322 vw N-C2 stretch

315 vw 314

310 306

233

301

293 vw

286 286

270 m 271 w C-C9-O bend

267

260 w

skeletal mode

255 w

234

247 249 w 252 246

207 s 212 w

203 m

173 182 180

134 138 135

93 94 93

53 54 51

30 30 30

235

Table S4. Potential Energy Distribution of PH-a

Frequency Molecular Vibration Model Potential Energy Distribution (%) (cm-1)

3122 C-H stretching of aniline ring (100)

3117 C-H stretching of phenol ring (100)

3110 C-H stretching of aniline ring (100)

236

3107 C-H stretching of phenol ring (99)

3100 C-H stretching of aniline ring (100)

237

3091 C-H stretching of phenol ring (99)

3089 C-H stretching of aniline ring (100)

3082 C-H stretching of aniline ring (100)

3071 C-H stretching of phenol ring (100)

238

3059 C2-H stretching (94)

239

3030 C4-H stretching (96)

2946 C2-H stretching (92)

2923 C4-H stretching (95)

1617 C-C stretching of phenol ring (67)

240

1611 C-C stretching of aniline ring (55)

241

1589 C-C stretching of phenol ring (45)

1586 C-C stretching of aniline ring (56)

1498 H-C-C bend of aniline ring (58)

1489 H-C-C bend of phenol ring (48)

242

1471 H-C2-H bend (59)

C-C stretching of phenol ring (27),

1455 H-C-C bend of phenol ring (24), H-

C-C bend of aniline ring (19)

243

H-C4-H bend (22), H-C-C bend of 1452 aniline ring (20)

1447 H-C4-H bend (40)

1388 H-C2-O-C torsion (54)

H-C2-O-C torsion (19), N-Ca stretch 1373 (17), C2-N-C4 bend (14)

244

H-C4-C-C torsion (30), C-C stretch of 1339 phenol ring (28)

1327 H-C-C bend of aniline ring (68)

245

1305 C-C stretch of aniline ring (58)

1301 C-C stretch of phenol ring (47)

H-C-C stretch of phenol ring (37), C- 1264 C stretch of phenol ring (18)

H-C2-O bend (41), H-C2-O-C torsion 1246 (13)

246

O-C9 stretch (33), C-C stretch of the

1225 phenol ring (14), H-C-C bend of

phenol ring (13)

C9-C4-H stretch (23), H-C4-C-C 1201 torsion (11)

247

C-C-C bend of phenol ring (18), C-C

1188 stretch of phenol ring (15), N-C2

stretch (11)

H-C-C bend of aniline ring (74), C-C 1177 stretch of aniline ring (19)

H-C-C bend of aniline ring (81), C-C 1152 stretch of aniline ring (15)

H-C-C bend of phenol ring (28), C-C 1149 stretch of phenol ring (17)

248

1145 H-C-C bend of phenol ring (62)

H-C-C bend of phenol ring (27), C-C- 1106 C bend of phenol ring (17)

249

H-C-C bend of aniline ring (25), C- 1082 C stretch of aniline ring (19)

C-C stretch of aniline ring (29), H- 1075 C-C bend of aniline ring (19)

C-C stretch of phenol ring (60), H- 1031 C-C bend of phenol ring (29)

C-C stretch of aniline ring (62), H- 1027 C-C bend of aniline ring (24)

250

C-C-C bend of aniline ring (75), C- 983 C stretch of aniline ring (19)

251

N-C4 stretch (25), H-C2-O-C torsion 975 (14), H-C2-C-C torsion (12)

N-C4 stretch (19), N-C2 stretch (10), 962 O-C2 stretch (13)

960 H-C-C-C torsion of aniline ring (73)

948 H-C-C-C torsion od phenol ring (63)

252

O-C2 stretch (32), H-C-C-C torsion 947 of phenol ring (17)

253

939 H-C-C-C torsion of aniline ring (90)

911 H-C-C-C torsion of phenol ring (82)

886 H-C-C-C torsion of aniline ring (86)

842 H-C-C-C torsion of phenol ring (83)

254

C-C-C bend of phenol ring (29), C2- 830 O-C bend (15)

823 H-C-C-C torsion of aniline ring (99)

255

C-C-C bend of aniline ring (36), N-Ca 768 stretch (10), N-C2-O bend (9)

H-C-C-C torsion of aniline ring (45), 753 C-C-C-C torsion of aniline ring (38)

745 H-C-C-C torsion of phenol ring (78)

H-C-C bend of phenol ring (39), C-C

731 stretch of phenol ring (13), O-C9

stretch (10)

256

C-C-C-C torsion of phenol ring (59), 698 H-C-C-C torsion of phenol ring (19)

C-C-C-C torsion of aniline ring (62), 692 H-C-C-C torsion of aniline ring (31)

257

H-C-C bend of phenol ring (20), N- 680 C2-O bend (16)

613 C-C-C bend of aniline ring (78)

H-C-C bend of aniline ring (24), N- 580 C2-O bend (14), N-C2 stretch (10)

H-C-C bend of phenol ring (30), C2- 556 N-Ca bend (11), O-C9 stretch (8)

258

C-C-C-C torsion of phenol ring (17),

531 H-C-C-C torsion of phenol ring (12),

C2-O-C-C torsion (9)

C-C-C-C torsion of aniline ring (17),

528 H-C-C-C torsion of aniline ring (14),

Ca-N torsion (11)

259

C-C-C bend of aniline ring (20), H- 511 C-C bend of phenol ring (17)

C-C-C-C torsion of phenol ring (34),

445 O-C9-C10-C5 torsion (13), H-C-C-C

torsion of phenol ring (7)

428 C-C-C-C torsion of phenol ring (40)

C-C-C-C torsion of aniline ring (71), 410 H-C-C-C torsion of aniline ring (19)

260

C-C-C-C torsion of aniline ring (29), 399 C2-O-C bend (14), C-C9-O bend (9)

N-C2 stretch (20), Ca-N-C2-O torsion 362 (12), H-C2-O-C torsion (10)

261

C-C9-O bend (34), Ca-N bend (18), 310 H-C-C bend of phenol ring (9)

C-C-C-C torsion of phenol ring (35), 286 C2-O-C-C torsion (12)

Ca-N bend (19), C-C-C-C torsion of

252 phenol ring (15), C-C-C-C torsion of

aniline ring (11)

C2-O-C-C torsion (22), Ca-N torsion 182 (17), C2-N-Ca bend (12)

262

Ca-N torsion (38), C2-O-C-C torsion 138 (23), N-C2-O-C9 torsion (7)

C2-N-Ca bend (32), C-C-C-C torsion

94 of phenol ring (19), N-C2-O-C9

torsion (18)

263

54 Ca-N-C2 torsion (84)

Ca-N-C2-O torsion (33), N-C2-O-C9 31 (30)

264

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