Introduction to polymer science and chemistry a problem solving approach, second edition chanda, manas

See what’s new in the second edition: • Chapter on living/controlled radical polymerization, using a unique problem-solving approach • Chapter on polymer synthesis by “click” chemistry,

S e c o n d e d i t i o n

Manas Chanda

Introduction to Polymer Science and Chemistry

Industry and academia remain fascinated with the diverse properties and applications

of polymers However, most introductory books on this enormous and important field

do not stress practical problem solving or include recent advances, which are critical

for the modern polymer scientist-to-be Updating the popular first edition of “the polymer

book for the new millennium,” Introduction to Polymer Science and Chemistry: A

Problem-Solving Approach, Second Edition seamlessly integrates exploration of the

fundamentals of polymer science and polymer chemistry

See what’s new in the second edition:

• Chapter on living/controlled radical polymerization, using a unique

problem-solving approach

• Chapter on polymer synthesis by “click” chemistry, using a unique

problem-solving approach

• Relevant and practical work-out problems and case studies

• Examples of novel methods of synthesis of complex polymer molecules

by exciting new techniques

• Figures and schematics of the novel synthetic pathways described in

the new examples

Author Manas Chanda takes an innovative problem-solving approach in which the text

presents worked-out problems or questions with answers at every step of the development

of a new theory or concept, ensuring a better grasp of the subject and scope for

self study Containing 286 text-embedded solved problems and 277 end-of-chapter

home-study problems (fully answered separately in a Solutions Manual), the book

provides a comprehensive understanding of the subject These features and more set

this book apart from other currently available polymer chemistry texts

Introduction to Polymer Science and Chemistry

A Problem-Solving Approach

S e c o n d e d i t i o n

S e c o n d

e d i t i o n

Introduction to Polymer Science and Chemistry

A Problem-Solving Approach

CRC Press is an imprint of the

Taylor & Francis Group, an informa business

Boca Raton London New York

Manas Chanda

Introduction to Polymer Science and Chemistry

A Problem-Solving Approach

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of my beloved father and mentor

Narayan Chandra Chanda

Preface xvii

1.1 Basic Definitions 1

1.1.1 Polymer 1

1.1.2 Monomer 2

1.1.3 Molecular Weight and Molar Mass 3

1.1.4 End Groups 3

1.1.5 Degree of Polymerization 4

1.1.6 Copolymers 4

1.2 Polymerization and Functionality 5

1.3 Polymerization Processes 7

1.3.1 Addition or Chain Polymerization 7

1.3.2 Step Polymerization 11

1.3.3 Supramolecular Polymerization 13

1.4 Molecular Architecture 18

1.5 Classification of Polymers 22

1.5.1 Thermoplastics and Thermosets 22

1.6 Plastics, Fibers, and Elastomers 26

1.7 Polymer Nomenclature 29

References 31

Exercises 31

2 Chain Dimensions, Structures, and Transitional Phenomena 35 2.1 Introduction 35

2.2 Polymer Chains: Structures and Dimensions 35

2.2.1 Conformational Changes 35

2.2.1.1 Polyethylene 38

2.2.1.2 Polyisobutylene 39

2.2.1.3 Polypropylene 40

2.2.2 Polymer Conformations in Crystals 40

2.2.3 Polymer Size in the Amorphous State 42

2.2.3.1 Freely Jointed Chains 43

2.2.3.2 Real Polymer Chains 44

vii

2.3 Constitutional and Configurational Isomerism 47

2.3.1 Constitutional Isomerism 47

2.3.2 Configurational Isomerism 50

2.3.2.1 Geometrical Isomerism 50

2.3.2.2 Stereoisomerism 50

2.4 Crystallinity in Polymers 57

2.4.1 Structure of Bulk Polymers 59

2.4.1.1 Spherulites 59

2.5 Thermal Transitions in Polymers 61

2.5.1 T g and T m 61

2.5.2 First- and Second-Order Transitions 63

2.6 Regions of Viscoelastic Behavior 64

2.7 Factors Affecting T g 66

2.8 Factors Affecting T m 68

2.9 Relation Between T m and T g 68

2.10 Theoretical Treatment of Glass Transition 69

2.10.1 Quantitative Effects of Factors on Tg 74

2.11 Chain Movements in Amorphous State 80

2.11.1 The Reptation Model 81

2.12 Thermodynamics of Rubber Elasticity 83

2.12.1 Stress-Strain Behavior of Crosslinked Elastomers 85

2.12.2 Nonideal Networks 91

2.12.2.1 Network Defects 91

2.12.2.2 Elastically Active Chain Sections 91

References 93

Exercises 94

3 Polymers in Solution 101 3.1 Introduction 101

3.2 Thermodynamics of Liquid Mixtures 101

3.2.1 Low-Molecular-Weight Mixtures: van Laar Model 104

3.2.2 Polymer-Solvent Mixtures: Flory-Huggins Model 107

3.2.2.1 Flory-Huggins Expressions for Thermodynamic Functions 111

3.2.2.2 Colligative Properties and Interaction ParameterΧ 113

3.2.2.3 Virial Coefficients 117

3.2.2.4 Modification of Flory-Huggins Theory 119

3.2.2.5 Flory-Krigbaum Theory 121

3.2.2.6 Excluded Volume Theory 122

3.3 Phase Equilibria in Poor Solvents 126

3.3.1 Upper and Lower Critical Solution Temperatures 129

3.4 Solubility Behavior of Polymers 131

3.5 Swelling of Crosslinked Polymers 138

3.5.1 Determination ofΧ from Swelling 142

3.6 Frictional Properties of Polymer Molecules in Dilute Solution 143

3.6.1 Viscosity of Dilute Polymer Solutions 145

3.6.1.1 Determination of Polymer Molecular Dimensions from Viscosity 148 References 151

Exercises 152

4 Polymer Molecular Weights 159 4.1 Introduction 159

4.2 Molecular Weight Averages 159

4.2.1 Arithmetic Mean 159

4.2.2 Number-Average Molecular Weight 160

4.2.3 Weight-Average Molecular Weight 162

4.3 Molecular Weights in Terms of Moments 163

4.3.1 Ratio of First and Zeroth Moments 164

4.3.2 Ratios of Higher Moments 165

4.4 Molecular Weight Determination 166

4.4.1 End-Group Analysis 167

4.4.2 Colligative Property Measurement 168

4.4.2.1 Ebulliometry (Boiling Point Elevation) 168

4.4.2.2 Cryoscopy (Freezing Point Depression) 169

4.4.2.3 Membrane Osmometry 169

4.4.2.4 Vapor-Phase Osmometry 176

4.4.3 Light-Scattering Method 180

4.4.3.1 Rayleigh Ratio 181

4.4.3.2 Turbidity and Rayleigh Ratio 183

4.4.3.3 Turbidity and Molecular Weight of Polymer 184

4.4.3.4 Dissymmetry of Scattering 188

4.4.3.5 Zimm Plots 191

4.4.4 Dilute Solution Viscometry 194

4.4.4.1 Calibration of the Mark-Houwink-Sakurada Equation 196

4.4.4.2 Measurement of Intrinsic Viscosity 197

4.4.5 Gel Permeation Chromatography 200

4.4.5.1 Data Interpretation and Calibration 202

References 208

Exercises 208

5 Condensation (Step-Growth) Polymerization 213 5.1 Introduction 213

5.2 Rates of Polycondensation Reactions 214

5.2.1 Irreversible Polycondensation Kinetics 216

5.2.2 Reversible Polycondensation Kinetics 222

5.3 Number-Average Degree of Polymerization 224

5.4 Control of Molecular Weight 227

5.4.1 Quantitative Effect of Stoichiometric Imbalance 228

5.5 Molecular Weight Distribution (MWD) 232

5.5.1 Breadth of MWD 234

5.6 Nonlinear Step Polymerization 241

5.6.1 Branching 241

5.6.2 Crosslinking and Gelation 243

5.6.2.1 Statistical Approach 246

5.6.2.2 Model for Gelation Process 255

5.6.2.3 Molecular Size Distribution 255

5.6.2.4 Post-Gel Relations 258

5.7 Recursive Approach for Average Properties 260

5.7.1 Linear Step-Growth Polymerization 260

5.7.2 Nonlinear Step-Growth Polymerization 264

5.7.2.1 Polymerization of Af  f > 2 264

5.7.2.2 Polymerization of Af ( f > 2) + B2 266

5.7.2.3 Polymerization ofAi  Bj 268

5.7.3 Post-Gel Properties 271

5.7.3.1 Polymerization of Af 272

5.7.3.2 Polymerization of Af  B2 276

5.8 Polycondensation of AxB Monomers 278

5.8.1 Dendritic and Hyperbranched Polymers 279

References 281

Exercises 283

6 Free Radical Polymerization 289 6.1 Introduction 289

6.2 Scheme of Radical Chain Polymerization 290

6.2.1 Overall Scheme 290

6.2.2 Chain Initiation 291

6.2.3 Chain Propagation 291

6.2.4 Chain Termination 292

6.2.5 Rate of Polymerization 293

6.2.6 Overall Extent of Polymerization 295

6.3 Experimental Determination of R p: Dilatometry 298

6.4 Methods of Initiation 300

6.4.1 Thermal Decomposition of Initiators 300

6.4.1.1 Initiator Efficiency 301

6.4.2 Redox Initiation 305

6.4.3 Photochemical Initiation 308

6.4.3.1 Direct Photoinitiation 308

6.4.3.2 Photosensitization 309

6.4.3.3 Rate of Photoinitiated Polymerization 310

6.4.4 Initiation by High-Energy Radiations 311

6.4.5 Thermal Initiation in Absence of Initiator 312

6.5 Dead-End Polymerization 313

6.6 Determination of Absolute Rate Constants 316

6.6.1 Nonsteady-State Kinetics 316

6.7 Chain Length and Degree of Polymerization 321

6.7.1 Kinetic Chain Length 321

6.7.2 Mode of Chain Termination 323

6.7.3 Average Lifetime of Kinetic Chains 325

6.8 Chain Transfer 325

6.8.1 Degree of Polymerization 328

6.8.2 Chain Transfer to Polymer 334

6.8.3 Allylic Transfer 335

6.9 Deviations from Ideal Kinetics 337

6.9.1 Primary Radical Termination 337

6.9.2 Initiator-Monomer Complex Formation 338

6.9.3 Degradative Initiator Transfer 339

6.9.4 Autoacceleration 341

6.10 Inhibition/Retardation of Polymerization 343

6.10.1 Inhibition/Retardation Kinetics 345

6.11 Effects of Temperature 347

6.11.1 Rate of Polymerization 347

6.11.2 Degree of Polymerization 350

6.11.3 Polymerization-Depolymerization Equilibrium 351

6.12 Molecular Weight Distribution 355

6.12.1 Low-Conversion Polymerization 355

6.12.1.1 Termination by Disproportionation and/or Transfer 356

6.12.1.2 Termination by Coupling 357

6.12.1.3 Termination by Coupling, Disproportionation, and Chain Transfer358 6.12.2 High-Conversion Polymerization 359

6.13 Polymerization Processes 359

6.13.1 Emulsion Polymerization 361

6.13.1.1 Qualitative Picture 361

6.13.1.2 Kinetics of Emulsion Polymerization 364

6.13.1.3 Other Theories 371

6.13.2 Photoemulsion Polymerization 372

6.13.3 “Grafting-From” Polymerization 373

6.14 Living Radical Polymerization 376

References 376

Exercises 377

7 Chain Copolymerization 383 7.1 Introduction 383

7.2 Binary Copolymer Composition – Terminal Model 384

7.2.1 Significance of Monomer Reactivity Ratios 386

7.2.2 Types of Copolymerization 387

7.2.2.1 Alternating Copolymerization 388

7.2.2.2 Ideal (random) Copolymerization 388

7.2.2.3 Random-Alternating Copolymerization 388

7.2.2.4 Block Copolymerization 389

7.2.3 Instantaneous Copolymer Composition 389

7.2.4 Integrated Binary Copolymer Equation 392

7.2.5 Evaluation of Monomer Reactivity Ratios 396

7.2.5.1 Plot ofr1versusr2 396

7.2.5.2 Plot ofF1versus f1 397

7.2.5.3 Direct Curve Fitting 399

7.2.6 The Q  e Scheme 400

7.2.7 Sequence Length Distribution 402

7.2.8 Rate of Binary Free-Radical Copolymerization 405

7.3 Multicomponent Copolymerization: Terpolymerization 409

7.4 Deviations from Terminal Model 413

7.4.1 Penultimate Model 413

7.4.2 Complex-Participation Model 414

7.5 Copolymerization and Crosslinking 415

7.5.1 Vinyl and Divinyl Monomers of Equal Reactivity 415

7.5.2 Vinyl and Divinyl Monomers of Different Reactivities 418

7.5.3 One Group of Divinyl Monomer Having Lower Reactivity 419

7.6 Block and Graft Copolymerization 420

7.6.1 Block Copolymerization 421

7.6.1.1 Producing Internal Peroxide Linkages 421

7.6.1.2 Introducing Peroxide End Groups 421

7.6.1.3 Mechanical Cleaving of Polymer Chains 422

7.6.1.4 Controlled Radical Polymerization 423

7.6.2 Graft Copolymerization 423

7.6.2.1 Chain Transfer Methods 423

7.6.2.2 Irradiation with Ionizing Radiation 424

References 425

Exercises 426

8 Ionic Chain Polymerization 429 8.1 Introduction 429

8.2 Ionic Polymerizability of Monomers 430

8.3 Anionic Polymerization 433

8.3.1 Anionic Initiation 433

8.3.1.1 Nucleophilic Attack 433

8.3.1.2 Electron Transfer 434

8.3.2 Termination Reactions 436

8.3.2.1 Living Polymerization 436

8.3.2.2 Termination by Transfer Agents 436

8.3.2.3 Spontaneous Termination 438

8.3.3 Polymerization with Complete Dissociation of Initiator 438

8.3.3.1 Polymerization Kinetics 439

8.3.3.2 Experimental Methods 439

8.3.3.3 Average Kinetic Chain Length 441

8.3.3.4 Average Degree of Polymerization 441

8.3.3.5 Distribution of the Degree of Polymerization 443

8.3.3.6 Effects of Reaction Media 447

8.3.3.7 Effect of Excess Counterion 451

8.3.4 Polymerization with Incomplete Dissociation of Initiator 454

8.3.5 Polymerization with Simultaneous Propagation and Termination 455

8.4 Anionic Copolymerization 457

8.4.1 Reactivity Groups 458

8.4.2 Block Copolymers 459

8.4.2.1 Sequential Monomer Addition 460

8.4.2.2 Coupling Reactions 461

8.5 Cationic Polymerization 463

8.5.1 Cationic Initiation 463

8.5.1.1 Protonic Acids 464

8.5.1.2 Lewis Acids 464

8.5.2 Propagation of Cationic Chain 465

8.5.3 Chain Transfer and Termination 466

8.5.3.1 Chain Transfer to Monomer 466

8.5.3.2 Spontaneous Termination 467

8.5.3.3 Combination with Counterion 467

8.5.3.4 Transfer to Solvents/Reagents 469

8.5.3.5 Chain Transfer to Polymer 470

8.5.4 Kinetics 471

8.5.4.1 Ions and Ion Pairs 471

8.5.4.2 Simplified Kinetic Scheme 472

8.5.4.3 Degree of Polymerization 478

8.5.5 Molecular Weight Distribution 479

8.5.6 Cationic Copolymerization 482

References 482

Exercises 483

9 Coordination Addition Polymerization 487 9.1 Introduction 487

9.2 Ziegler-Natta Catalysts 488

9.2.1 Catalyst Composition 488

9.2.2 Nature of the Catalyst 488

9.2.3 Evolution of the Titanium-Aluminum System 489

9.3 Mechanism of Ziegler-Natta Polymerization 490

9.3.1 Mechanism of Stereospecific Placement 490

9.3.2 Bimetallic and Monometallic Mechanisms 491

9.3.2.1 Bimetallic Mechanism 492

9.3.2.2 Monometallic Mechanism 492

9.4 Kinetics of Ziegler-Natta Polymerization 495

9.4.1 Typical Shapes of Kinetic Curves 495

9.4.2 Effect of Catalyst Particle Size 497

9.4.3 Chain Termination 498

9.4.4 Kinetic Models 499

9.4.4.1 Early Models 499

9.4.4.2 Adsorption Models 502

9.4.4.3 Average Degree of Polymerization 514

9.5 Supported Metal Oxide Catalysts 515

9.5.1 Polymerization Mechanism 515

9.5.1.1 Bound-Ion-Radical Mechanism 516

9.5.1.2 Bound-Ion-Coordination Mechanism 520

9.6 Ziegler-Natta Copolymerization 521

9.7 Metallocene-Based Ziegler-Natta Catalysts 522

9.7.1 Catalyst Composition 523

9.7.2 The Active Center 525

9.7.3 Polymerization Mechanism 526

9.7.4 Kinetic Models 526

9.7.4.1 Ewen’s Model 526

9.7.4.2 Chien’s Model 527

9.7.4.3 Molecular Weight and Chain Transfer 530

9.8 Immobilized Metallocene Catalysts 531

9.9 Oscillating Metallocene Catalysts 534

References 536

Exercises 537

10 Ring-Opening Polymerization 541 10.1 Introduction 541

10.2 Polymerization Mechanism and Kinetics 543

10.2.1 Cyclic Ethers/Epoxides 544

10.2.1.1 Anionic Polymerization 544

10.2.1.2 Cationic Polymerization 548

10.2.2 Lactams 557

10.2.2.1 Hydrolytic Polymerization 557

10.2.2.2 Anionic Polymerization 560

10.2.3 Lactones 563

References 564

Exercises 565

11 Living/Controlled Radical Polymerization 567 11.1 Introduction 567

11.2 Stable Free Radical Polymerization 571

11.2.1 Monomers 573

11.2.2 Stable Nitroxide Radicals 573

11.2.3 Mechanism and Kinetics 573

11.2.4 Copolymerization 579

11.2.5 Aqueous Systems 589

11.3 Atom Transfer Radical Polymerization (ATRP) 593

11.3.1 ATRP Monomers 596

11.3.2 ATRP Initiators 599

11.3.3 ATRP Catalysts 600

11.3.4 ATRP Ligands 601

11.3.5 ATRP Solvents 601

11.3.6 ATRP Mechanism and Kinetics 602

11.3.7 Chain-End Functionality 607

11.3.8 Copolymerization 609

11.3.8.1 Block Copolymers 609

11.3.8.2 Graft Copolymers 618

11.3.8.3 Star and Hyperbranched Polymers 621

11.3.9 Aqueous Systems 624

11.4 Degenerative Chain Transfer 625

11.5 Reversible Addition-Fragmentation Chain Transfer 625

11.5.1 Mechanism and Kinetics 629

11.5.2 Theoretical Molecular Weight 635

11.5.3 Block Copolymers 636

11.5.3.1 Sequential Monomer Addition 636

11.5.3.2 Macro-CTA Method 641

11.5.4 Star (Co)polymers 641

11.5.5 Branched (Co)polymers 644

11.5.6 Surface Modification 644

11.5.7 Combination of RAFT and Other Polymerization Techniques 645

11.5.8 Transformation of RAFT Polymer End Groups 646

References 649

Exercises 653

12 Polymer Synthesis by Click Chemistry 661 12.1 Introduction 661

12.2 Copper-Catalyzed Azide-Alkyne Cycloaddition 665

12.2.1 Combination of ATRP and CuAAC Reactions 675

12.2.1.1 Macromonomer Synthesis 675

12.2.1.2 End-Functionalization of (Co)polymer Chains 680

12.2.1.3 Cyclization of Linear Polymers 681

12.2.1.4 Moldular Synthesis of Block Copolymers 682

12.2.1.5 Nonlinear Polymer Synthesis 683

12.2.2 Combination of RAFT Polymerization and CuAAC 690

12.3 Strain-Promoted Azide-Alkyne Coupling 694

12.4 Diels-Alder Click Reactions 696

12.4.1 Copolymer Synthesis 697

12.4.2 Thermoresponsive Systems, Dendrons, and Dendrimers 702

12.4.3 Hetero-Diels-Alder (HDA) Cycloaddition 707

12.5 Thiol-Ene Reactions 710

12.5.1 Mechanisms of Thiol-Ene Reactions 714

12.5.2 Synthesis of Star Polymers and Dendrimers 716

References 719

Exercises 722

A question asked during discussion, or even ahead of it, excites a student’s mind and rouses hiseagerness to probe, thus making the process of learning more thorough During my teaching ofpolymer science and chemistry over a period of nearly four decades, I have thus always believedthat learning becomes much easier if problem solving with a question-and-answer approach is in-timately integrated with the text It was this belief that motivated me to embark on writing thisnew text on polymer science and polymer chemistry, even though I was fully aware that the fieldwas already crowded with more than a dozen well-written polymer texts Adopting a distinctlydifferent and innovative approach, the text in this new polymer book has been laced with ques-tions and answers at every step of the development of a theory or concept in each chapter Thebook thus features a significantly large number (286) of solved problems interspersed with thetext that is spread over 720 pages In addition, a large number (277) of problems are included as

end-of-chapter exercises and these are fully worked out in a separate Solutions Manual As my

experience in teaching has shown me the value of dealing with numbers to deepen one’s standing, most of the problems with which the text is studded are numerical The same is true forexercise problems appended at the end of each chapter and each such problem is provided withnumerical answers that the reader can compare with his own

under-To describe the present book briefly, it is a revised and enlarged second edition of Introduction

to Polymer Science and Chemistry: A Problem Solving Approach and it contains a total of 563

text-embedded solved problems and chapter-end exercise problems It has evolved from the first edition

by retaining all the latter’s ten chapters, which, however, have been fully relaid and restructured

to afford greater readability and understanding, and adding two new and large chapters to dealwith two recent topics that are said to have ushered in a renaissance in polymer chemistry, namely,living/controlled radical polymerization and application of “click” chemistry in polymer synthesis.The book thus has twelve chapters that fall into three distinct groups The first four chaptersintroduce the reader to polymers and their basic characteristics, both in solid state and in solution,and the next six chapters are concerned with various polymerization reactions, mechanisms, andkinetics, while the last two chapters are devoted to two recent topics, as cited above, of greatinterest and importance in polymer chemistry This division into groups is, however, notional and

is not made explicit by numbering these groups of chapters separately Instead, for convenience, asingle sequence of numbers is used throughout the book

Chapter 1 is devoted to introductory concepts and definitions, while Chapter 2 deals withphysical and molecular aspects of polymers, that is, those relating to molecular shape and size,distinctive characteristics, conformational and configurational behavior, structural features, mor-phology, thermal transitional phenomena, and relaxation properties Chapter 3 discusses polymersolution behavior, the emphasis being on thermodynamics, phase equilibria, solubility, swelling,frictional properties, and viscosity Molecular weight determination, which is one of the first steps

of polymer characterization and a centrally important topic of polymer science, mostly involves

xvii

analysis of polymers in solution The next chapter, Chapter 4, is therefore devoted to polymermolecular weights with focus on the fundamentals of molecular weight statistics and methods ofmeasurement, their origins, and significance.

The chemistry part of the book focusing on polymerization reactions, mechanisms, and ics starts with Chapter 5 Five main types of polymerization reactions — condensation (step), freeradical (chain), ionic (chain), coordination (chain), and ring-opening — are dealt with separately

kinet-in five essentially self-contakinet-ined chapters Copolymerization that may kinet-involve any of these merization mechanisms is included in respective chapters, an exception being free-radical chaincopolymerization which, in view of its great practical importance and considerable theoretical de-velopment that has taken place in this field, has been accorded the space of one full chapter Whilepolymerization reactions have been characterized on the basis of mechanisms and kinetic features,emphasis has been placed on understanding the reaction parameters which are important in con-trolling polymerization rates, degree of polymerization, and structural features, such as branchingand crosslinking

poly-The development of living/controlled radical polymerization (CRP) methods, which startedonly in 1985, has been a long-standing goal in polymer chemistry because a radical process ismore tolerant of functional groups and impurities and is the leading industrial method to pro-duce polymers, while the livingness of polymerization allows unprecedented control of polymertypes, architecture, end-functionalities, molecular weght, and distributions CRP is thus amongthe most rapidly developing areas of polymer chemistry, with the number of publications nearlydoubling each year in the initial phase of development Presently, the most popular CRP meth-ods are nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP),and reversible addition/fragmentation chain transfer (RAFT) techniques, all of which are describedelaborately in the newly added Chapter 11, using once again the unique problem solving approach,which is a hallmark of the book

Since being introduced only in 2001, click chemistry, which may thus be called a new 21stcentury technique, has made great advances in the realm of polymer chemistry over the last 10years, giving access to a wide range of complex polymers (dendrimers, dendronized linear poly-mers, block copolymers, graft copolymers, star polymers, etc.) and new classes of functionalizedmonomers in a controlled fashion, which would be inaccessible or difficult to synthesize via con-ventional chemistry Chapter 12, which is the last chapter of this new edition, is fully devoted toapplication of click chemistry in polymer synthesis, using the unique problem solving approach

In writing the first ten chapters, which deal with conventional polymer science and chemistry,

I have received much inspiration and valuable guidance from the many well-known polymer texts

that are currently available However, I should make particular mention of George Odian’s ples of Polymerization (McGraw-Hill, New York, 1970), the first edition of which appeared when

Princi-I was still a student and it made a marked impression on me Another book which influenced me

greatly was Rudin’s The Elements of Polymer Science and Engineering (Academic Press, Orlando,

Florida, 1982), a prescribed text at the University of Waterloo, Canada, where I taught during asabbatical year (1985–1986)

For writing Chapters 11 and 12 on the two recent topics, living/controlled radical tion and polymer synthesis by click chemistry, which have not yet made a significant appearance inpolymer chemistry textbooks, I have depended exclusively on original articles that appeared, espe-cially in the last ten years, in many reputed journals Most of the articles have, however, appeared

polymeriza-in journals published by the American Chemical Society and John Wiley & Sons I am grateful tothem for granting permission to reproduce some material in the book from these journals.While SI units are being used increasingly in all branches of science, non-SI units like the oldercgs system are still in common use This is particularly true of polymer science and chemistry

In this book, therefore, both SI and non-SI units have been used However, in most places wherenon-SI units have been used, equivalent values are given in SI terms A suitable conversion table

is also provided as an appendix

Synthesized polymers are utilized increasingly in our daily life, and a myriad of industrialapplications have contributed to their phenomenal growth and expansion As this requires poly-mer chemists and specialists in polymers, many universities throughout the world have set upteaching programs in polymer chemistry, science, and engineering Their students are drawn fromvarious disciplines in science and engineering The present book is designed primarily for bothundergraduate and graduate students and is intended to serve specially as a classroom text for aone-year course in polymer science and chemistry Moreover, as two chapters have been added inthe new edition focusing on recent advances in polymer chemistry over the last two decades, thebook will also be useful to students doing research in the area of polymers

Polymer industry is the single largest field of employment for students of both science andengineering However, most workers entering the field have little background in polymer scienceand chemistry and are forced to educate themselves in its basic principles This book, with itseasy style and a large number of illustrative, worked-out problems, will be useful to them as aself-contained text that guides a beginner in the subject to a fairly advanced level of proficiency.The manuscript of the book originated from a course in polymers that I offered to graduatestudents of chemistry and chemical engineering during my sabbatical year (1985-1986) at theUniversity of Waterloo, Ontario, Canada, where I have also been a summer-term visiting facultyspanning over two decades (1980-2000) The manuscript has been tested since then and improvedyear after year to its present state as the course has been offered every year to a mixed class ofstudents from various disciplines including chemistry, chemical engineering, metallurgy, civil en-gineering, electrical engineering, electronics, and aerospace engineering at the Indian Institute ofScience, Bangalore, where I have served as a permanent faculty A basic knowledge of mathemat-ics, chemistry, and physics is assumed on the part of the reader, while the book has been written to

be self-contained, as far as possible, with most equations fully derived and any assumptions stated

In the interest of time, I took up the onerous task of preparing the entire book electronically.While I did all the (LaTex) typesetting, formatting, and page designing, I received valuable helpfrom two colleagues, Dr Ajay Karmarkar and Ms B G Girija, who prepared computer graphicsfor all diagrams, chemical structures, and chemical formula-based equations I thank both ofthem I am deeply indebted to Dr P Sunthar, an acknowledged software expert on the campus,for guiding me patiently in the use of word processing softwares during this difficult ventureand to Shashi Kumar of Cenveo Publisher Services, Noida, India, for performing the necessaryconversions to font-embedded PDF for printing

Several academicians have contributed, directly or indirectly, to the preparation of this book.Among them I would like to mention Prof K F O’Driscoll, Prof G L Rempel, and Prof AlfredRudin, all of the University of Waterloo, Waterloo, Ontario (Canada), Prof Kenneth J Wynne ofthe Virginia Commonwealth University, Richmond, Virginia (USA), Prof Harm-Anton Klok ofEcole Polytechnique F´ed´erale de Lausanne, Lausanne (Switzerland), Prof Premamoy Ghosh ofthe Univeristy of Calcutta, Calcutta (India), and Prof S Ramakrishnan and Prof M Giridhar,both of the Indian Institute of Science, Bangalore (India) I express my gratitude to all of them.Interaction with students whom I met over the years during my long academic career, both

in India and abroad, contributed greatly to the evolution of the book to its present form featuring

a unique problem solving approach It is not possible to thank them individually as the number

is too large However, I should mention, gratefully, two of my erstwhile students, Dr AmitavaSarkar and Dr Ajay Karmarkar, who were closely associated with me during the last few years

of my service at the Indian Institute of Science, Bangalore, and provided help in many ways in

the making of this book Finally, a word of appreciation and gratitude is due to three persons veryclose to me, namely, my wife Mridula, daughter Amrita, and little granddaughter Mallika, whoshowed remarkable understanding and patience, and gladly sacrificed their share of my time tofacilitate my work.

Manas Chanda

Manas Chanda has been a professor and is presently an emeritus professor in the Department of

Chemical Engineering, Indian Institute of Science, Bangalore, India He also worked as a term visiting professor at the University of Waterloo, Ontario, Canada with regular summer visitsfrom 1980 to 2000 A five-time recipient of the International Scientific Exchange Award from theNatural Sciences and Engineering Research Council, Canada, Professor Chanda is the author or

summer-coauthor of more than 100 scientific papers, articles, and books, including Plastics Technology Handbook, 4th Edition (CRC Press, Boca Raton, Florida) His biographical sketch is listed in Marquis’ Who’s Who in the World Millennium Edition (2000) by the American Biographical So-

ciety A Fellow of the Indian National Academy of Engineers and a member of the Indian PlasticsInstitute, he received B.S (1959) and M.Sc (1962) degrees from Calcutta University, and a Ph.D.(1966) from the Indian Institute of Science, Bangalore

xxi

Introductory Concepts

1.1 Basic Definitions

Many of the terms, definitions, and concepts used in polymer science are not encountered in otherbranches of science and must be understood in order to fully discuss the synthesis, characterization,structure, and properties of polymers While most of these are discussed in detail in subsequentchapters, some are of such fundamental importance that they must be introduced at the beginning

1.1.1 Polymer

The term polymer stems from the Greek roots poly (many) and meros (part) The word thus means

“many parts” and designates a molecule made up by the repetition of some simpler unit called

a mer Polymers contain thousands to millions of atoms in a molecule that is large; they are also called macromolecules Polymers are prepared by joining a large number of small molecules called monomers.

The structure of polystyrene, for example, can be written as

(I)

or, more conveniently, as (II), which depicts the mer or repeating unit of the molecule within

parentheses with a subscript, such as n, to represent the number of repeating units in the polymer

molecule

(II)

The value of n usually ranges from a few hundred to several thousand, depending on the

molecular weight of the polymer The polymer molecular weight may extend, on the higher side,

1

to several millions Often the term high polymer is also used to emphasize that the polymer under

consideration is of very high molecular weight

1.1.2 Monomer

Monomers are generally simple organic molecules from which the polymer molecule is made.The structure of the repeating unit of a polymer is essentially that or closely related to that of

the monomer molecule(s) The formula of the polystyrene repeating unit (II) is thus seen to be

The repeating unit of a linear polymer is a small portion of the macromolecule such that linkingtogether these units one after another gives rise to the formula of the whole molecule A repeating

unit may be a single component such as (II) for the polymer (I), or it may consist of the residues

of several components, as in poly(ethylene terephthalate), which has the structure :

(III)

The repeating unit in (III) may be written as

(IV)

Thus, the whole molecule of (III) can be built by linking the left-hand atom shown in (IV) to

the right-hand atom, and so on

Though it has been stated above that structures of repeating units are essentially those of themonomers from which the polymers are made, this is not always the case Considering, for exam-ple, poly(vinyl alcohol) :

(V)

unsta-ble tautomer of acetaldehyde and does not exist Poly(vinyl alcohol) is instead made by alcoholysis

of poly(vinyl acetate),

OCCH3O

OCCH3O

OCCH3O

OCCH3O

OCCH3O

CH2

(VI)

where n is a few thousand The structure is

O O H H H H OH

H OH

CH2OH

O

H H H H OH

CH2OH

O O H H H

CH2OH

1 4

(VII)

Complete hydrolysis of cellulose by boiling with concentrated hydrochloric acid yields D-glucose,

though it cannot be synthesized from glucose The concept of the repeating unit is most useful forlinear homopolymers (see later)

1.1.3 Molecular Weight and Molar Mass

The term ‘molecular weight’ is frequently used in practice instead of the term ‘molar mass’, thoughthe former can be somewhat misleading Molecular weight is really a dimensionless quantitygiven by the sum of the atomic weights in the molecular formula The atomic weights, in turn,

assigned (AMU, a unit used for expressing the atomic masses of individual isotopes of elements,

of 1 mol of the substance and usually is quoted in units of g/mol or kg/mol The numerical value ofmolecular weight is multiplied by the specific units, such as g/mol, to convert it into an equivalentvalue of molar mass in dimensions of g/mol Thus, a molecular weight of 100,000 is equivalent to

a molar mass of 100,000 g/mol or 100 kg/mol

In this book, we shall retain the term ‘molecular weight’ because of its widespread use in thepolymer literature However, when using in numerical calculations we shall substitute the molec-ular weight by its numerically equivalent molar mass (g/mol) to facilitate dimensional balancing

1.1.4 End Groups

In none of the above examples of the structural representation of polymers have the end groupsbeen shown This is partly because the exact nature of the end groups of polymer molecules isoften not known and partly because end groups constitute an insignificant fraction of the mass

of high molecular weight polymer and so usually have negligible effect on polymer properties ofmajor interest

Problem 1.1 Calculate the end group content (weight fraction) of polystyrene of molecular weight 150,000,assuming that phenyl (C6H5-) groups constitute both the end groups of an average polymer molecule

Answer:

Molar mass of phenyl group 612  51  77 g mol1

Wt fraction of end groups 277 g mol1

1.5 105 g mol1 0.001

1.1.5 Degree of Polymerization

This term refers to the number of repeating units that constitute a polymer molecule We shall use

the abbreviation DP for the degree of polymerization defined in this way The subscript n used on the parentheses in the foregoing structural formulas for polymers represents this DP The relation between degree of polymerization and molecular weight M of the same macromolecule is given

by

1.1.6 Copolymers

If a macromolecule is made from only one species of monomer, the product is a homopolymer,

referred to simply as a polymer The word homopolymer often is used more broadly to describepolymers whose structure can be represented by repetition of a single type of repeating unit con-

also a homopolymer, e.g., poly(ethylene terephthalate) (III).

The formal definition of a copolymer is a polymer derived from more than one species of

monomer The copolymer with a relatively random distribution of the different mers or repeating

units in its structure is commonly referred to as a random copolymer Representing two different

mers by A and B, a random copolymer can be depicted as

A block copolymer is a linear polymer with one or more long uninterrupted sequences of each

mer in the chain :

—AAAAAAAAAABBBBBBBBBB—

Block copolymers may have a different number of blocks in the molecule Thus, AxBy, AxByAx,

multiblock copolymers, respectively Since there is a distribution of block lengths and number of blocks along the copolymer chain, x and y as well as n represent average values.

A graft copolymer, on the other hand, is a branched copolymer with a backbone of one type of

mer and one or more side chains of another mer :

Figure 1.1 Copolymer arrangements : (a) Two different types of mers (denoted by open and filled circles)

are randomly placed (b) The mers are alternately arranged (c) A block copolymer (d) A graft copolymer

AAAAAAAAAAAAAAAAAAA

BBBBB

Copolymerization, which, in its objective, may be compared to alloying in metallurgy, is veryuseful for synthesizing polymer with the required combination of properties

1.2 Polymerization and Functionality

Polymerization may occur only if the monomers involved in the reaction have the proper

function-alities Functionality is a very useful concept in polymer science The functionality of a molecule

is the number of sites it has for bonding to other molecules under the given conditions of the polymerization reaction (Rudin, 1982) Thus, a bifunctional monomer, i.e., a monomer with func-

for example, has functionality 2 because of the presence of a carbon-carbon double bond Theminimum functionality required for polymerization is 2

A polyfunctional monomer is one that can react with more than two molecules under the

con-ditions of the polymerization reaction Thus, divinyl benzene (VIII) is tetrafunctional in reactions involving additions across carbon-carbon double bonds, while glycerol (IX) is trifunctional and

Problem 1.2 What is the functionality of the following monomers in reactions with (i) styrene, C6H5CH=CH2and (ii) adipic acid, HOOC(CH2)4COOH ?

(i) In reaction with styrene, the functionalities of the monomers are :

(a) 2 (one reactive carbon-carbon double bond)

(b) 0 (-OH groups do not take part in addition reactions)

(c) 2 (one reactive carbon-carbon double bond; –OH group

nonreactive)

(d) 4 (two reactive carbon-carbon double bonds)

(e) 2 (one reactive carbon-carbon double bond)

(ii) In reaction with adipic acid, the functionalities of the monomers are :

(a) 0 (carbon-carbon double bond nonreactive)

(b) 2 (two reactive -OH groups)

(c) 1 (one reactive -OH group; carbon-carbon double bond

nonreactive)

(d) 0 (carbon-carbon double bond nonreactive)

(e) 2 (one reactive anhydride group, which is equivalent to

two carboxylic acid groups, each of functionality 1)

1.3 Polymerization Processes

There are two fundamental polymerization mechanisms Classically, they have been

differenti-ated as addition polymerization and condensation polymerization In the addition process, no

by-product is evolved, as in the polymerization of vinyl chloride (see below); whereas in thecondensation process, just as in various condensation reactions (e.g., esterification, etherification,

is evolved Polymers formed by addition polymerization do so by the successive addition of

un-saturated monomer units in a chain reaction promoted by the active center Therefore, addition polymerization is called chain polymerization Similarly, condensation polymerization is referred

to as step polymerization since the polymers in this case are formed by stepwise, intermolecular

condensation of reactive groups Another polymerization process that has now appeared as a new

research area of considerable interest is supramolecular polymerization (see Section 1.3.3).

1.3.1 Addition or Chain Polymerization

In chain polymerization, a simple, low-molecular-weight molecule possessing a double bond, ferred to in this context as a monomer, is treated so that the double bond opens up and the resultingfree valences join with those of other molecules to form a polymer chain For example, vinyl chlo-ride polymerizes to poly(vinyl chloride) :

It is evident that no side products are formed; consequently the composition of the mer or repeating

composition of the repeating unit of a polymer and its monomer(s) is, in most cases, an indicationthat the polymer is an addition polymer formed by chain polymerization process The commonaddition polymers and the monomers from which they are produced are shown in Table 1.1

Chain polymerization involves three processes: chain initiation, chain propagation, and chain termination (A fourth process, chain transfer, may also be involved, but it may be regarded as a

combination of chain termination and chain initiation.) Chain initiation occurs by an attack on themonomer molecule by a free radical, a cation, or an anion; accordingly, the chain polymerization

processes are called free-radical polymerization, cationic polymerization, or anionic tion A free radical is a reactive substance having an unpaired electron and is usually formed by the

polymeriza-decomposition of a relatively unstable material called an initiator Benzoyl peroxide is a common

free-radical initiator and can produce free radicals by thermal decomposition as

Free radicals are, in general, very active because of the presence of unpaired electrons (denoted

by dot) A free-radical species can thus react to open the double bond of a vinyl monomer and add

to one side of the broken bond, with the reactive center (unpaired electron) being transferred to theother side of the broken bond :

R C

O

H X

The new species, which is also a free radical, is able to attack a second monomer molecule in

a similar way, transferring its reactive center to the attacked molecule The process is repeated,and the chain continues to grow as a large number of monomer molecules are successively added

to propagate the reactive center :

Table 1.1 Typical Addition Polymers (Homopolymers)

Monomer Polymer Comments

CH3O

n

Water pipes, bottles, gramophone records, plasticized

to make PVC film, leather cloth, raincoats, flexible pipe, tube, hose, toys, electrical cable sheathing , e.g., Benvic, Darvic, Geon, Hostalit, Solvic, Vinoflex, Welvic.

CF2 CF2

Polytetrafluoroethylene (PTFE)

CF2 CF2

High temperature resistance, chemically inert, excellent electrical insulator, very low coefficient of friction, expensive; moldings, films, coatings; used for non-stick surfaces, insulation, gaskets; e.g., Teflon, Fluon.

CH2 C C

CH3

OCH3O

n

Transparent sheets and moldings; more expensive than

PS; known as organic glass, used for aeroplane

windows; e.g., Perspex, Plexiglass, Lucite, Diakon, Vedril.

A second termination mechanism is disproportionation, shown by the following equation :

X

ORH

X

CHCH

of the chain initiation mechanism—free radical, cationic, or anionic—once a reactive center isproduced it adds many more molecules in a chain reaction and grows quite large extremely rapidly,usually within a few seconds or less (However, the relative slowness of the initiation stage causesthe overall rate of reaction to be slow and the conversion of all monomers to polymers in mostpolymerizations requires at least 30 minutes, sometimes hours.) Evidently, at any time during

a chain polymerization process the reaction mixture will consist only of unreacted monomers,high polymers and unreacted initiator species, but no intermediate sized molecules The chainpolymerization will thus show the presence of high-molecular-weight polymer molecules at allextents of conversion (see Fig 1.2)

Problem 1.3 Styrene monomer containing 0.02% (by wt.) benzoyl peroxide initiator was reacted until allthe initiator was consumed If at this stage 22% of the monomer remained unreacted, calculate the averagedegree of polymerization of the polymer formed Assume 100% efficiency of the initiator (i.e., all initiatormolecules are actually consumed in polymer formation) and termination of chain radicals by coupling alone

Answer:

Basis: 1000 g styrene

Molar mass of styrene (C8H8)  812  81  104 g/mol

Molar mass of initiator, (C6H5CO)2O2  242 g/mol

Amount of initiator  0.20 g

 (0.20 g) / (242 g mol1) or 8.26104mol

Initiator fragment forming

polymer end groups  2 (8.26104mol)  1.652103mol

Polymer formed by termination

by coupling  (1.652103mol) / 2  8.26104mol

Figure 1.2 Variation of molecular weight withconversion in (a) step polymerization, (b) free-radical chain polymerization, and (c) ionic chainpolymerization (Adapted from Odian, 1991.)

1.3.2 Step Polymerization

Step polymerization occurs by stepwise reaction between functional groups of reactants Thereaction leads successively from monomer to dimer, trimer, tetramer, pentamer, and so on, until

finally a polymer molecule with large DP is formed Note, however, that reactions occur at random

between the intermediates (e.g., dimers, trimers, etc.) and the monomer as well as among theintermediates themselves In other words, reactions of both types, namely,

occur equally Thus, at any stage the product consists of molecules of varying sizes, giving a range

of molecular weights The average molecular weight builds up slowly in the step polymerizationprocess, and a high-molecular-weight product is formed only after a sufficiently long reactiontime when the conversion is more than 98% [see Fig 1.2(a)] In contrast, polymerization by chainmechanism proceeds very fast, a full-sized polymer molecule being formed almost instantaneouslyafter a chain is initiated; the polymer size is thus independent of reaction time [Fig 1.2(b)]

In certain ionic chain polymerizations, which feature a fast initiation process coupled with theabsence of reactions that terminate the propagating reactive centers, molecular weight increaseslinearly with conversion [Fig 1.2(c)]

Since most (though not all) of the step polymerization processes involve polycondensation(repeated condensation) reactions, the terms “step polymerization” and “condensation polymer-ization” are often used synonymously Consider, for example, the synthesis of a polyamide, i.e., apolymer with amide (–CONH–) as the characteristic linkage If we start with, say, hexamethylene-diamine and adipic acid as reactants, the first step in the formation of the polymer (nylon) is thefollowing reaction producing a monoamide :

The reaction continues step-by-step to give the polyamide nylon-6,6 The overall reaction maythus be represented as

1.9

We see that the composition of the repeating unit (enclosed in square brackets) equals that oftwo monomer molecules minus two molecules of water Thus a condensation polymer may bedefined as one whose synthesis involves elimination of small molecules or whose repeating unitlacks certain atoms present in the monomer(s)

Problem 1.4 Poly(hexamethylene adipamide) (Nylon-6,6) was synthesized by condensation tion of hexamethylenediamine and adipic acid in 1:1 mole ratio Calculate the acid equivalent of the polymer

Molar mass of repeating unit  226 g/mol

Average molar mass  440226  18 or 99,458 g/mol

Wt of polymer (in grams) containing

one acid equivalent  99,458

With the development of polymer science and the synthesis of new polymers, the previousdefinition of condensation polymer is inadequate For example, in polyurethanes (Table 1.2),which are classified as condensation polymers, the repeating unit has the same net composition

as the two monomers (i.e., a diol and a diisocyanate), which react without eliminating any smallmolecule To overcome such problems, chemists have introduced a definition which describescondensation polymers as consisting of structural units joined by internal functional groups such

as :

A polymer satisfying either or both of the above definitions is classified as a condensationpolymer Phenol-formaldehyde, for example, satisfies the first definition but not the second Somecondensation polymers along with their repeating units and condensation reactions by which theycan be synthesized are shown in Table 1.2 Some high-performance polymers prepared by poly-condensation are listed in Table 1.3.

The ring-opening polymerizations of cyclic monomers, such as propylene oxide,

1.3.3 Supramolecular Polymerization

Supramolecular polymers are a relatively new class of polymers in which mono-meric repeating

units are held together with directional and reversible (noncovalent) secondary interactions (Lehn,2000), unlike conventional macromolecular species in which repetition of monomeric units ismainly governed by covalent bonding A schematic comparison of a covalent polymer and asupramolecular polymer is shown in Fig 1.3

The directionality and strength of the supramolecular bonding, such as hydrogen bonding,

in dilute and concentrated solutions, as well as in the bulk It should be noted that supramolecularinteractions are not new to polymer science, where hydrogen bonding and other weak reversibleinteractions are important in determining polymer properties and architectures (Sherrington andTaskinen, 2001) However, for linear supramolecular polymers to form, it is a prerequisite to havestrong and highly directional interactions as a reversible alternative for the covalent bond Hydro-gen bonds between neutral organic molecules, though they hold a prominent place in supramolec-ular chemistry because of their directionality and versatility, are not among the strongest noncova-lent interactions Hence, either multiple hydrogen bonds with cooperativity must be used or hydro-gen bonds should be supported by additional forces like excluded volume interactions (Brunsveld

et al., 2001) Though the concept has been known for years, it was not known how to incorporatesuch sufficiently strong but still reversible interactions However, in the past decade followingthe development of strong hydrogen-bonding dimers, several research groups have applied thesedimers for the formation of hydrogen-bonded supramolecular polymers Thus the finding by Si-

jbesma et al (1997) that derivatives of 2-ureido-4[1H]-pyrimidinone (UPy, 1 in Fig 1.4) are easy

self-complementary quadrupole (array of four) hydrogen bonding (2 in Fig 1.4) prompted them to use

this functionality as the associating end group in reversible self-assembling polymer systems

O R R

Beetle, Resolite, Cibanoid.

Melamine-formaldehyde

(MF)

C N N C

C N N

C N

HCl (2 n -1 )

n

Moldings and sheets; transparent and tough: used for safety glasses, screens and glazings, electrical and electronics, appliances, compact discs, e.g Merlon, Baylon, Jupilon.

Polyethersulfone (PES)

O O

O O

KCl (n -1 )

n

Moldings, coatings, membranes; rigid, transparent, self-extinguishing, resistant to heat deformation: used for electrical components, molded circuit boards, appliances operating at high temperatures, e.g., Victrex PES.

Moldings, composites, bearings, coatings; very

in coatings and insulation for high performance wiring, composite prepregs with carbon fibers, e.g., Victrex PEEK.

Cl O

C Cl

one-fifth of weight, ideally suited as tire cord materials and for ballistic vests, e.g., Kevlar, Twaron.

O

O

Films, coatings, adhesives, laminates;

outstanding in heat resistance, flame resistance, abrasion resistance, electrical insulation resistance, resistance to oxidative degradation, high energy radiation and most chemicals (except strong bases): used in specialist applications, e.g., Kapton, Vespel.

Figure 1.3 Schematic representation of (a) a covalent polymer and (b) a supramolecular polymer (After

Brunsveld et al., 2001.)

Figure 1.4 Synthesis of a monofunctional 2-ureido-4[1H]-pyrimidinone (UPy) (1) and dimerization of 1 in

solution forming a quadrupole hydrogen-bonded unit (After Sijbesma et al., 1997.)

A difunctional UPy compound, 4 in Fig 1.5, possessing two UPy units can be easily made in

Fig 1.5) in solution as well as in the bulk (Brunsveld et al., 2001) Dissolving a small amount

of the compound in chloroform gives solutions with high viscosities, while calculations show that

amounts of monofunctional compounds (1 in Fig 1.4) results in a sharp drop in viscosity, proving

that linkages between the building blocks are reversible and unidirectional and that the tional compounds act as chain stoppers For the same reason, the supramolecular polymers showpolymer-like viscoelastic behavior in bulk and solution, whereas at elevated temperatures theyexhibit liquid-like properties (Brunsveld et al., 2001)

monofunc-The quadrupole hydrogen-bonded unit can be employed in the chain extension of telechelicoligomers such as polysiloxanes, polyethers, polyesters, and polyacrbonates (Folmer et al., 2000)

Thus the electrophilic isocyanate group (–NCO) of ‘synthon’ (3 in Fig 1.5) can be reacted with

supramolec-ular polymers by chain extension (Fig 1.6) Thus the material properties of telechelic polymershave been shown to improve dramatically upon functionalization with synthon, and materials havebeen obtained that combine many of the mechanical properties of conventional macromolecules

Figure 1.5 Preparation of (a) UPy possessing an isocyanate functional group (3) and (b) a difunctional UPy

compound (4) which forms a supramolecular polymer by hydrogen bonding (cf Fig 1.4) (After Folmer et

al., 2000; Brunsveld et al., 2001.)