NMR and mossbauer studies of copolymers

NMR AND MoSSBAUER STUDIES OF COPOLYMERS

by

GURPREET SINGH KAPUR

Department of Chemistry

THESIS SUBMITTED

IN FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF THE DEGREE OF

DOCTOR OF PHILOSOPHY

to the

INDIAN INSTITUTE OF TECHNOLOGY, DELHI

INDIA MARCH. 1990

dedicated to ..

My Mother

CERTIFICATE

This is to certify that the thesie entitled, uNMR AND OSSBAUER STUDIES OF COPOLYMERS", being submitted by Mr. Gurpreet Singh Kapur to the Indian Institute of Technology, Delhi, for the award of Degree of Doctor of Philosophy in Chemistry, is a record of bonafide research work carried out by him. Mr. Gurpreet Singh Kapur has worked under my supervision and fulfilled all the formalities as required under statutes before the submission of a Ph.D. thesis. This thesis has reached the requisite standard and is worthy of consideration for the award of Ph.D. degree.

The work embodied in this thesis has not been submitted, in part or full, to any other University or Institute for the award of any degree or diploma.

(A.S. BRAR)

Thesis Supervisor Assistant Professor Department of Chemistry

Indian Institute of Technology, New Delhi-110 016

INDIA

ACKNOWLEDGEMENT

I wish to place on record my sincere thanks to Dr. A.S. Brar, Assistant Professor, Department of Chemistry for his valuable guidance, .constant encouragement and unflinching help throughout the course of this project. It has been a great pleasure working with him.

I wish to express my sincere thanks to Professors B.L. Khandelwal and A.S.N. Murthy for providing me necessary facilities in the Department.

I owe a lot to my close friend Mr. Sunil Garg for his constant encouragement over the whole length of time. He has been a constant source of inspiration during hours of dispondency. My thanks are also due to Dr. Sanjiv Mazumdar and Dr. Nand Kishore, who, in their friendly manners were also a great source of help.

I am thankful to Mr. Bala Dutt Phuloria of NMR laboratory for his conscientious and interested efforts in helping me recording NMR spectra. Mr. L.C.Sharma, Mr. Durga Singh, Mrs. Shanta, and Mr. R.K. Singh of the Instrument laboratory also rendered valuable help during the course of my work.

It is a pleasure to record my appreciation for several of my friends and colleagues especially Mr. Naorem Homendra

and Ms. Sunita, and Messers Rameshawar Jha, Swarita Kapoor, T.S. Rao, Francis, Tarlok Singh, Akhlesh Gupta, Ashok Gupta, Vinay Karan, Ms. Amarjeet and Ms. Reshma for sharing problems and pleasures with me.

To my family, I owe a lot for their constant encouragement during the whole length of time.

Care has been taken to give proper credit for work of other authors in the literature. I regret any omissions which might have occurred by oversight.

I am grateful to CSIR, Delhi and IIT, Delhi for providing me the financial support and research facilities.

The arduous task of processing the text from often illegible manuscript was ably performed by Mr. Jagdish and Mr. Shamseer Singh Dagar.

(Gurpreet Singh Kapur)

07

ii

ABSTRACT

Monomer sequence distributions have a direct bearing on the chemical and physical properties of copolymers and a knowledge of the copolymer composition and microstructure is an important step in the evaluation of their utility.

Keeping in view the importance of polymer microstructure, a work was undertaken with a purpose to understand the theoretical and experimental basis for characterizing the structure of copolymer chains at the molecular level. The work described in this thesis comprised of preparation and characterisation of three series of copolymers of alkyl methacrylate / alkyl methacrylate, acrylonitrile / alkyl methacrylate, and vinyl acetate / alkyl methacrylate by nuclear magnetic resonance (NMR) spectroscopy. These copoly- mers were doped with ferric chloride and studied using Mossbauer spectroscopy.

The thesis consists of five chapters. The first introductory chapter briefly summarizes some aspects of copolymer microstructure and statistics. It also encompasses brief discussion on the basic principle of Mossbauer spectroscopy and the kind of information which can be obtained for iron containing homo/co polymers. Some land marks reached in earlier studies have been highlighted.

The chapter-II, describes the experimental procedures for the preparation of homopolymers and copolymers, which is mainly the free radical bulk polymerization except for the vinyl acetate copolymer series which were prepared by free radical initiated emulsion polymerization using a semicontinuous batch process. The above copolymer samples were also prepared by adding anhydrous ferric chloride during the course of polymerization. The molecular weights of the copolymers were determined by gel permeation chromatography and viscometry. 1

H-NMR spectra and C,H, and N analysis were used to obtain the copolymer composition wherever possible. 13

C[1

H]-NMR spectra were recorded and used to completely discern the microstructure of the copolymers. M6ssbauer investigations were carried out in order to understand the nature and environments of iron nucleus in the copolymers containing ferric chloride.

Thermal gravimetric analysis (TGA) was carried out to study the thermal stability of the samples. Infrared spectroscopy has been used for the identification of species formed during the thermal decomposition of copolymers.

Comonomer reactivity ratios and the theoretical comonomer sequence distributions were obtained by using various computer programs written in FORTRAN on PC-AT. All the calculations regarding the fractional area measurements in NMR and M6ssbauer spectra were performed using a

deconvolution computer program on ICL-2960 main frame computer.

The first part of the chapter III explains the copolymerization behaviour of methyl methacrylate / ethyl methacrylate(M/E), ethyl methacrylate/nbutyl methacrylate (E/B), and methyl methacrylate/nbutyl methacrylate (M/B) 'copolymers. Copolymer composition was determined by 1 H-NMR

spectroscopy and comonomer reactivity ratios hitherto unestablished have been found to have the values: rm=1.04, rE=0.98; rE=1.27, rB=1.00; and rm=0.96, rB=1.04. 13

C[1 111- NMR spectra have been analyzed in order to determine triad comonomer sequence distribution. The carbonyl carbon resonance 6177 (ppm downfield TMS) multiplicity has been attributed to various monomer centered triad sequences in M/B, M/E copolymers and to only configurational sequences in E/B copolymers. Triad sequence distributions have obtained over whole range of feed in monomer concentration. Harwood's computer program was used for estimating sequence distribution up to pentad for these copolymer systems.

Second part of the chapter III contains results of M6.ssbauer studies of above copolymers doped with ferric chloride. No changes in the oxidation state of iron have been observed during the polymerization. Thermal stability of these doped copolymers increased by =50°C after the inclusion of ferric chloride. M6ssbauer spectra of the doped

copolymers heated at different temperatures revealed the presence of more asymmetric environments around Fe3+ and in certain cases its reduction to Fe2+ was observed. The oxidation-reduction phenomenon was found to be more pronounced in those copolymers where ethyl methacrylate was one of the comonomer. Heating the doped polymers at 500°C resulted in the formation of a-Fe203. Oxidation-reduction, and substitution reactions have been attributed to the increased thermal stability of doped copolymers.

The first part of chapter IV, includes details on the copolymerization of acrylonitrile/methyl methacrylate (A/M), acrylonitrile/ethyl methacrylate (A/E), and acrylonitrile/

nbutyl methacrylate (A/B) copolymers. Comonomer reactivity ratios have been obtained by various methods, the reactivity ratio values being r A = 0.21, r M = 1.42; r A = 0.18, r E 1.41; and r A = 0.15, r E = 0.93, and are in good agreement with those reported in the literature. Both compositional and configurational effects have been observed in the 13C[1

11]-NMR spectra of these copolymers. Carbonyl resonance at 6176 ppm and nitrile resonance at 6120 ppm are each split into three main groups and are assigned to the six possible triad sequences with the help of variable composition carbon-13 NMR. Within a group further assignments have been made to various configurational arrangements. Penultimate group effect was found to be

vii

operative in all the cases, and the penultimate reactivity ratios obtained are in good agreement with the reported values. The configurational probabilities calculated from 13C-NMR were indicative of random cotactic placements. A good agreement between the expected values of compositional and configurational sequence distribution and those obtained from 13

C-NMR spectroscopy was obtained.

Second part of chapter IV includes results of the M6ssbauer studies of the above copolymers doped with ferric chloride which showed no reduction of the Fe3+

species during the polymerization. Acrylonitrile/alkyl methacrylate copolymers doped with ferric chloride showed improved thermal stability at higher temperatures despite of the early weight loss. TGA also showed a multistep degradation in doped copolymers. Heated copolymers did not show the formation of a species other than Fe3+

. IR and Mbssbauer studies of the samples heated at 500°C showed the formation of -Fe2O3.a

The purpose of the work described in chapter V, is to prepare vinyl acetate/methyl methacrylate (V/M), vinyl acetate/ethyl methacrylate (V/E), and vinyl acetate/nbutyl methacrylate (V/B) copolymers by free radical initiated emulsion copolymerization using a semicontinuous batch process. Copolymer composition determined from 1

H-NMR spectroscopy was found to be in good agreement with those

expected theoretically. The different resonances in 13C-NMR spectra like carbonyl and quaternary carbon have been assigned to comonomer triad sequences with the help of variable composition NMR and were used for calculating alkyl methacrylate centered triad fractions. The methylene carbon resonance of vinyl acetate unit gave information about V centered triad distribution in V/M copolymers, whereas in other copolymers, the resonances giving information regarding the V-centered triads were not properly resolved.

As a different procedure was adopted for the preparation of above samples, they could not be doped with ferric chloride.

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CONTENTS

Certificate Acknowledgement Abstract

List of figures

ii iv

CHAPTER - I Introduction

I.1 Microstructure of Macromolecules 1 1.2 Nuclear Magnetic Resonance Spectroscopy (NMR) 2 1.3 Homopolymer Composition 4

1.4 Copolymer Structure 10

1.5 Copolymerization Models 11 1.6 Determination' of Reactivity Ratios 19 1.7 Alkyl Methacrylate/Alkyl Methacrylate Copolymers 22 1.8 Acrylonitrile/Alkyl Methacrylate Copolymers 23 1.9 Vinyl Acetate/Alkyl Methacrylate Copolymers 24 1.10 Mdssbauer Spectroscopy 26

References 34

CHAPTER-II Experimental Details

II.1 Purification of Monomers 44 11.2 Preparation of Homopolymers 44 11.3 Preparation of Copolymers 47 11.4 Molecular Weight Determination 50

11.5 NMR Analysis 52

11.6 11.7 11.8 11.9

11.1

M6ssbauer Spectroscopy Studies Thermogravimetric Analysis Elemental Analysis

Infrared Analysis

Harwood Computer Program References

53 55 55 56 56 57 CHAPTER-III

Illa NMR Studies of Alkyl Methacrylate Homo/Co 58 Polymers

IIIa.1 Introduction 58

IIIa.2 Methyl Methacrylate/Ethyl Methacrylate 59 (M/E) Copolymer

111a.2.1 Proton-NMR Studies 59 111a.2.2 Carbon-13 NMR Studies 64 IIIa.3 Methyl Methacrylate/nButyl Methacrylate 70

(M/B) Copolymer

IIIa.3.1 Proton-NMR Studies 70 IIIa.3.2 Carbon-13 NMR Studies 74 IIIa.4 Ethyl Methacrylate/nButyl Methacrylate 78

(E/M) Copolymers

IIIa.4.1 Proton-NMR Studies 78 IIIa.4.2 Carbon-13 NMR Studies 82

Conclusions 85

IIIb MOssbauer Studies of Alkyl Methacrylate 87 Homo/Co Polymers Doped with Ferric

Chloride

IIIb.1 Introduction 87

IIIb.2 Polyalkyl Methacrylates 87

IIIb.3

IIIb.4

IIIb.5

IIIb.6

Methyl Methacrylate/Ethyl Methacrylate 95 (M/E) Copolymers

Ethyl Methacrylate/nButyl Methacrylate 97 (E/B) Copolymers

Methyl Methacrylate/nButyl Methacrylate (M/B) Copolymers

Mechanism of Thermal Stabilization Conclusion

Tables Figures References

101

102 105 106 121 158 CHAPTER-IV

IVa NMR Studies of Acrylonitrile/Alkyl Meth- 161 acrylate Copolymers

IVa.1 Introduction 161

IVa.2 Acrylonitrile/Methyl Methacrylate (A/M) 162 Copolymers

IVa.2.1 Copolymer Composition 162 IVa.2.2 Proton-NMR Studies 163 IVa.2.3 Carbon-13 NMR Studies 165 IVa.2.4 Cotacticities of Acrylonitrile/Methyl 176

Methacrylate Copolymers

IVa.3 Acrylonitrile/Ethyl Methacrylate (A/E) 184 Copolymers

IVa.3.1 Copolymer Composition IVa.3.2 Proton-NMR Studies IVa.3.3 Carbon-13 NMR Studies

184 185 186

IVa.3.4 Cotacticities of Acrylonitrile/Ethyl 192 Methacrylate Copolymers

IVa.4 Acrylonitrile/nButyl Methacrylate (A/B) 196 Copolymers

IVa.4.1 Copolymer Composition 196 IVa.4.2 Proton-NMR Studies 197 IVa.4.3 Carbon-13 NMR Studies 198

Conclusion 201

IVb Acrylonitrile/Alkyl Methacrylate Copoly- 203 mers Doped with Ferric Chliride

IVb.1 Introduction 203

IVb.2 Results and Discussion 204

Conclusion 210

Tables 211

Figures 226

References

252 CHAPTER -V

V.1

NMR Studies of Vinyl Acetate/Alkyl 255 Methacrylate Copolymers

Introduction 255

V.2 Vinyl Acetate/Methayl Methacrylate 256 Copolymers

V.2.1 Proton-NMR Studies 256 V.2.2 Carbon-13 NMR Studies

V.3 Vinyl Acetate/Ethyl Methacrylate Copolymers V.3.1 Proton-NMR Studies

V.3.2 Carbon-13 NMR Studies

258 262 262 264

V.4 Vinyl Acetate/nButyl Methacrylate Copolymers 266 V.4.1 Proton-NMR Studies 266 V.4.2 Carbon-13 NMR Studies 268

Conclusion 272

Tables 273

Figures 281

References 294

Curriculum Vitae