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Block copolymers 2003 hadjichristidis et all

Block Copolymers: Synthetic Strategies, Physical Properties, and Applications.
Nikos Hadjichristidis, Stergios Pispas and George Floudas
Copyright  2003 John Wiley & Sons, Inc.
ISBN: 0-471-39436-X

BLOCK COPOLYMERS


BLOCK COPOLYMERS
Synthetic Strategies, Physical
Properties, and Applications

NIKOS HADJICHRISTIDIS
STERGIOS PISPAS
GEORGE FLOUDAS

A John Wiley & Sons, Inc., Publication


Copyright # 2003 by John Wiley & Sons, Inc. All rights reserved.
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Library of Congress Cataloging-in-Publication Data:
Hadjichristidis, Nikos, 1943Block copolymers : synthetic strategies, physical properties, and
applications / Nikos Hadjichristidis, Stergios Pispas, George Floudas.
p. cm.
Includes index.
ISBN 0-471-39436-X (cloth : acid-free paper)
1. Block copolymers. I. Pispas, Stergios, 1967- II. Floudas, George,
1961- III. Title.
QD382.B5 H33 2003
5470 .84–dc21
Printed in the United States of America.
10 9 8 7 6 5 4 3 2 1

2002014989


To Our Wives


Dina, Hara, and Maria


CONTENTS

Preface

xiii

Abbreviations and Symbols

xvii

III BLOCK COPOLYMER SYNTHESIS
1

BLOCK COPOLYMERS BY ANIONIC POLYMERIZATION

1
3

1. Synthesis of AB Diblock Copolymers / 4
2. Synthesis of Triblock Copolymers / 11
3. Linear Block Copolymers With More Than Three Blocks / 23
2

BLOCK COPOLYMERS BY CATIONIC POLYMERIZATION

28

1. Synthesis of AB Diblock Copolymers / 29
2. Synthesis of Triblock Copolymers / 40
3

BLOCK COPOLYMERS BY LIVING FREE
RADICAL POLYMERIZATION

47

1. Synthesis of AB Diblock Copolymers / 48
2. Synthesis of ABA Triblock Copolymers / 58
vii


viii

CONTENTS

3. Synthesis of ABC Triblock Terpolymers and
ABCD Tetrablock Quarterpolymers / 61
4

BLOCK COPOLYMERS BY GROUP TRANSFER
POLYMERIZATION

65

1. Synthesis of AB Diblock Copolymers / 66
2. Synthesis of ABA Triblock Copolymers / 72
3. Synthesis of ABC Triblock Terpolymers / 75
5

BLOCK COPOLYMERS BY RING OPENING
METATHESIS POLYMERIZATION

80

1. Synthesis of AB Diblock Copolymers / 82
2. Synthesis of ABA Triblock Copolymers / 88
6

SYNTHESIS OF BLOCK COPOLYMERS BY A
COMBINATION OF DIFFERENT POLYMERIZATION
METHODS
1. Synthesis of Block Copolymers by Anionic to
Cationic Mechanism Transformation / 92
2. Synthesis of Block Copolymers by Anionic to
Living Free Radical Mechanism Transformation / 94
3. Synthesis of Block Copolymers by Cationic to
Anionic Mechanism Transformation / 97
4. Synthesis of Block Copolymers by Cationic to
Onium Mechanism Transformation / 98
5. Synthesis of Block Copolymers by Cationic to
Living Free Radical Mechanism Transformation / 100
6. Synthesis of Block Copolymers by Living Free
Radical to Cationic Mechanism Transformation / 102
7. Synthesis of Block Copolymers by Ring Opening Metathesis
to Living Free Radical Mechanism Transformation / 103
8. Synthesis of Block Copolymers by Ring Opening
Metathesis to Group Transfer Mechanism Transformation / 104
9. Other Combinations / 105
10. Bifunctional (DUAL) Initiators / 107
11. Synthesis of Block Copolymers by Direct Coupling
of Preformed Living Blocks / 107
12. Synthesis of Block Copolymers by Coupling of
End-functionalized Prepolymers / 110

91


ix

CONTENTS

7

SYNTHESIS OF BLOCK COPOLYMERS BY
CHEMICAL MODIFICATION
1.
2.
3.
4.
5.
6.
7.
8.

8

III

Hydrogenation / 115
Hydrolysis / 116
Quaternization / 117
Sulfonation / 118
Hydroboration/Oxidation / 119
Epoxidation / 121
Chloro/BromoMethylation / 121
Hydrosilylation / 123

NONLINEAR BLOCK COPOLYMERS
1.
2.
3.
4.

126

Star Block Copolymers / 126
Graft Copolymers / 134
Miktoarm Star Copolymers / 142
Other Complex Architectures / 156

MOLECULAR CHARACTERIZATION OF
BLOCK COPOLYMERS
9

114

MOLECULAR CHARACTERIZATION OF
BLOCK COPOLYMERS

173
175

1. Purification of Block Copolymers by Fractionation / 175
2. Molecular Characterization / 177

III

SOLUTION PROPERTIES OF
BLOCK COPOLYMERS
10

11

195

DILUTE SOLUTIONS OF BLOCK COPOLYMERS IN
NONSELECTIVE SOLVENTS

197

DILUTE SOLUTIONS OF BLOCK COPOLYMERS IN
SELECTIVE SOLVENTS

203

1.
2.
3.
4.

Thermodynamics of Micellization / 203
Phenomenology of Block Copolymer Micellar Structure / 206
Experimental Techniques for Studying Micelle Formation / 207
Equilibrium Structure of Block Copolymer Micelles / 215


x

CONTENTS

5. Effect of Architecture / 219
6. Kinetics of Micellization / 222
7. Solubilization of Low Molecular Weight Substances in
Block Copolymer Micelles / 224
8. Ionic Block Copolymer Micelles / 225
12

ADSORPTION OF BLOCK COPOLYMERS AT
SOLID-LIQUID INTERFACES

232

1. Phenomenology of Block Copolymer Adsorption / 232
2. Experimental Techniques for Studying Block
Copolymer Adsorption / 235
3. Theories of Block Copolymer Adsorption / 242
4. Experiments on Block Copolymer Adsorption / 246

IV

PHYSICAL PROPERTIES OF
BLOCK COPOLYMERS
13

THEORY
1.
2.
3.
4.

14

15

268

Graft Copolymers / 269
AnBn Star Block Copolymers / 274
(AB)n STAR COPOLYMERS / 277
ABA Triblock Copolymers / 280
Tapered Block Copolymers / 281
Multiblock Copolymers / 282

BLOCK COPOLYMER PHASE STATE
1.
2.
3.
4.
5.

257

Strong Segregation Limit (SSL) / 257
Weak Segregation Limit (WSL) / 259
Structure Factor / 261
Intermediate Segregation Limit (ISL) and
Self-consistent Field Theory (SCFT) / 263

STRUCTURE FACTOR AND CHAIN ARCHITECTURE
1.
2.
3.
4.
5.
6.

255

Fluctuation Effects / 287
Conformational Asymmetry / 290
The Known Phase Diagrams / 292
The PEO-PI Phase Diagram / 294
The PS-PI-PEO Phase Diagram / 295

286


xi

CONTENTS

16

VISCOELASTIC PROPERTIES OF
BLOCK COPOLYMERS
1.
2.
3.
4.

17

18

Localization of the (Apparent) Order-to-Disorder Transition / 300
Viscoelastic Spectrum of Block Copolymers / 301
Viscoelastic Response of Ordered Phases / 303
Flow-induced Alignment of Block Copolymer Melts / 305

PHASE TRANSFORMATION KINETICS
1.
2.
3.
4.
5.
6.

298

313

Detection and Analysis of the Ordering Kinetics / 314
The Equilibrium Order-to-disorder Transition Temperature / 318
Effect of Fluctuations / 320
Grain Growth / 322
Effect of Block Copolymer Architecture / 325
Transitions Between Different Ordered States / 327

BLOCK COPOLYMERS WITH STRONGLY
INTERACTING GROUPS

335

1. Cylinder-forming Functionalized SI Diblock Copolymers / 337
2. Lamellar-forming Functionalized Diblock and
Triblock Copolymers / 340
3. ABC Block Copolymers With a Short but Strongly
Interacting Middle Block / 342
4. Effect of Salt on the Lamellar Spacing and
Microdomain Morphology / 344
19

BLOCK COPOLYMER MORPHOLOGY
1.
2.
3.
4.
5.
6.

20

346

Rod-Coil Copolymers / 346
ABC Triblock Terpolymers / 352
More Complexity With ABCs / 353
ABC Miktoarm Star Terpolymers With Amorphous Blocks / 355
ABC Star Terpolymers With Crystallizable Blocks / 357
Architecture-induced Phase Transformations / 358

BLOCK COPOLYMER DYNAMICS

362

1. Dynamic Structure Factor of Disordered Diblock Copolymers / 362
2. Dynamic Structure Factor of Ordered Diblock Copolymers / 366
3. Dielectric Relaxation in Diblock Copolymers in
the Disordered and Ordered Phases / 370


xii

CONTENTS

4. Dynamic Interfacial Width in Block Copolymers / 373
5. Dielectric Relaxation in Block Copolymer/
Homopolymer Blends / 376

V APPLICATIONS
21

BLOCK COPOLYMER APPLICATIONS

383
385

1. Commercialized Applications / 386
2. Potential Applications / 397

Index

409


PREFACE

Block copolymers are a fascinating class of polymeric materials belonging to a big
family knwon as ‘‘soft materials.’’ This class of polymers is made by the covalent
bonding of two or more polymeric chains that, in most cases, are thermodynamically incompatible giving rise to a rich variety of microstructures in bulk and in
solution. The length scale of these microstructures is comparable to the size of the
block copolymers molecules (typically 5–50 nm); therefore, the microstructures are
highly coupled to the physical and chemical characteristics of the molecules. The
variety of microstructures gives rise to materials with applications ranging from
thermoplastic elastomers and high-impact plastics to pressure-sensitive adhesives,
additives, foams, etc. In addition, block copolymers are very strong candidates for
potential applications in advanced technologies such as information storage, drug
delivery, and photonic crystals.
There is a rapid development of research activity on the synthesis, physical
properties, and applications of block copolymers with an enormous number of
scientific papers published every year in leading journals in the fields of polymer
and material science. Excellent reviews on the physical properties of block
copolymers by F. S. Bates and G. H. Fredrickson and a book by Ian Hamley
have served many in the scientific community.
This book deals with the synthesis, characterization methods, physical properties, and applications of block copolymers. It is meant to serve as an advanced
introductory book for scientists who will be working with block copolymers. The
book provides guidelines to strategies that can be employed in the synthesis of a
variety of block copolymer architectures with well-defined molecular characteristics; an overview of some fundamental physical properties; and the description of

xiii


xiv

PREFACE

how the information on chemical structure and physical properties can be utilized in
designing block copolymers with particular functions.
In Chapter 1 the possible ways for synthesizing block copolymers by anionic
polymerization are presented. This is one of the oldest and more versatile
polymerization mechanisms to date to synthesize block copolymers with a plethora
of complex architectures.
The possibilities for block copolymer preparation by cationic polymerization are
given in Chapter 2. This polymerization methodology is considered as the second
more advanced route for obtaining well-defined block copolymers from monomers
that cannot be polymerized by anionic means.
The synthetic capabilities of controlled radical polymerization are outlined in
Chapter 3. This newly invented technique has emerged as a very valuable tool for
obtaining block copolymers using simple experimental protocols and a large variety
of the available monomers.
The synthesis of block copolymers, derived mainly from (meth)acrylic monomers, by group transfer polymerization is described in Chapter 4. Some examples
of the applicability of catalysts and coordination chemistry in the synthesis of block
copolymers are given in Chapter 5, in the form of ring opening metathesis
polymerization of multicyclic olefins.
The possibilities of obtaining block copolymers by combining more than one
polymerization mechanisms are outlined in Chapter 6. These combination methods
are invaluable in cases where block copolymers comprised of monomers that
cannot be polymerized by the same polymerization mechanism, have to be
synthesized.
Some examples of obtaining new block copolymers from preformed precursor
block copolymers are presented in Chapter 7. By the use of post polymerization
derivatization reactions, adapted to polymers from classical organic chemistry, new
materials with substantially different properties can be obtained, sometimes even
from commercially available copolymers.
Synthetic strategies for the preparation of nonlinear block copolymers with a variety
of complex architectures are presented in Chapter 8. Macromolecular architecture
has recently emerged as another factor of controlling macromolecular properties in
the molecular level. The imagination of polymer chemists, together with new
emerging synthetic methodologies, led to the synthesis of complex macromolecules
that aided in the establishment of the fundamental structure-properties relationships
in these systems.
The experimental protocols and available methods for the molecular characterization of all classes of block copolymers are given in Chapter 9. Primary chemical
structure identification, molecular weight, composition, and size determination are
essential elements in understanding the properties of complex materials like block
copolymers.
A brief description of the behavior and conformation of simple and complex
block copolymers in their isolated molecular state, i.e., in dilute solutions, is given
in Chapter 10.


PREFACE

xv

The interesting property, both from academic as well as from the application
point of view, of block copolymers to form micelles in solvents selective for one of
the blocks, like low molecular weight surfactants, is discussed in Chapter 11.
The equally important phenomenon of block copolymer adsorption onto solid
surfaces from solutions is presented in Chapter 12, together with some of the
available techniques for investigating this phenomenon, as well as results from
theoretical and experimental studies.
The theoretical aspects related to the structure factor and the phase state of bulk
block copolymers in the weak and strong segregation limits are discussed in
Chapter 13.
In Chapter 14 the effects of block copolymer architecture on the compatibility of
individual blocks in complex copolymers are presented. The phase diagrams of
families of block copolymers are discussed in Chapter 15. Emphasis is given
to the effects of conformational asymmetry of the blocks comprising the block
copolymers.
The viscoelastic properties of block copolymers are discussed in Chapter 16.
The viscoelastic behavior of ordered block copolymers, the possibility of shearinduced orientational order, and the determination of the order-to-disorder transition temperature are included in the discussion.
The equilibrium order-to-disorder transition temperature, together with the phase
transformation kinetics (i.e., disorder-to-order kinetics, kinetics of the transition
between different ordered phases, epitaxy, activation barriers, etc.) are investigated
in Chapter 17. Chapter 18 is devoted to the phase state, viscoelastic properties, and
dynamics of block copolymers containing strongly interacting groups in their
chains. The discussion is focused on the effects and changes in the phase state of
block copolymers due to the presence of these groups.
The experimentally determined morphologies of a variety of block copolymer
architectures (linear diblock and triblock copolymers, star block and miktoarm star
copolymers, etc.) are presented in Chapter 19. The effects of block sequence and
overall block copolymer architecture are also described. Chapter 20 deals with the
dynamics of block copolymer chains in concentrated solution and in the melt state.
Finally, Chapter 21 is devoted to the commercialized and potential applications
of block copolymers as thermoplastic elastomers, structural materials: in encapsulation technologies, in nanotechnology, and in other areas.
NIKOS HADJICHRISTIDIS
STERGIOS PISPAS
GEORGE FLOUDAS


ABBREVIATIONS AND SYMBOLS

A2, A3
AcOVE
AGTR
AIBN
AN
AnBm
ATRP
9-BN
BPO
Bpy
Cmc
CMC
Cmt
D
d
D(f,x)
DLS
DNbpy
DPE
DPMK
F
f
F(x,f)
fH
G*

Second and third virial coefficient
Acetoxyvinylether
Aldol group transfer polymerization
a,a0 -azo-diisobutylnitrile
Acetonitrile
Star copolymer with n A arms and m B arms
Atom transfer radical polymerization
9-borabicyclo[3.3.1]nonane
Benzoylperoxide
2,20 -bipyridyl
Critical micelle concentration
Constant mean curvature interfaces
Critical micelle temperature
Diffusion coefficient
Microdomain period
Debye function
Dynamic light scattering
4,40 -di-(5-nonyl)-2,20 -bipyridyl
1,1-Diphenylethylene
Diphenylmethylpotassium
Free energy
Volume fraction
Combination of Debye functions
Free energy density (Hartree approximation)
Complex shear modulus
xvii


xviii

ABBREVIATIONS AND SYMBOLS

G0
G00
GTP
IB
Is
ISL
L, l
LAOS
M
Mapp
Me
MeVE
MFT
Miktoarm star
Mn
Mw
n
"
N
N
P
PaMeS
P2VP
PBd
PCL
PCPPHMA
PDI
PDMS
PEB
Pentablock copolymer
Pentablock terpolymer
PEO
PI
PIB
PMMA
PpMeS
PS
PS-PI or PS-b-PI
PtBOS
PtBu(M)A
PtBuA
PTHF
q, Q
Ry
R, Rg
Rh

Storage modulus
Loss modulus
Group transfer polymerization
Isobutylene
Isoprene
Intermediate segregation limit
Grain size
Large amplitude oscillatory shear
Molecular weight
Apparent molecular weight
Entanglement molecular weight
Methylninylether
Mean-field theory
Star polymer with different arms
Number-average molecular weight
Weight-average molecular weight
Avrami exponent
Fluctuation corrected N
Degree of polymerization
Total scattering power
Poly(a-methylstyrene)
Poly(2-vinyl pyridine)
Polybutadiene
Poly(e-caprolactone)
Poly{6[4-(40 -cyanophenyl)phenoxy]hexyl methacrylate}
Polydispersity index
Poly(dimethylsiloxane)
1,3(or 1,4)bis(1-phenylethenyl)benzene
Five blocks, two of them different (f.e. ABABA)
Five blocks, three of them different (f.e. ABCBA)
Poly(ethylene oxide)
Poly(isoprene)
Polyisobutylene
Poly(methyl methacrylate)
Poly(p-methylstyrene)
Polystyrene
Diblock copolymer of styrene and Isoprene
Poly(p-tert-butoxystyrene)
Poly(tert-butyl methacrylate)
Poly(tert-butyl acrylate)
Poly(tetrahydrofurane)
Scattering wavevector
Rayleigh ratio
Radius of gyration
Hydrodynamic radius


ABBREVIATIONS AND SYMBOLS

RIE
ROMP
RPA
S(q)
SANS
SAXS
s-BuLi
SCFT
SEC
SFRP
SLS
SSL
St
T
t1/2
tand
TASHF2
TBABB
t-BOS
TEM
TEMPO
Tetrablock copolymer
Tetrablock
quaterpolymer
Tetrablock terpolymer
Tg
THF
Tm
TMEDA
T mo
TODT
TODTo
TPA
TPE
TPES
TPU
Triblock copolymer
Triblock terpolymer
WSL
x
z
a-MeSt
a
Á
d

xix

Reactive ion etching
Ring opening metathesis polymerization
Random phase approximation
Structure factor
Small angle neutron scattering
Small angle x-ray scattering
sec-Butyllithium
Self-consistent field theory
Size exclusion chromatography
Stable free radical polymerization
Static light scattering
Strong segregation limit
Styrene
Temperature
Half-time
Loss tangent
Tris(dimethylamino)sulfonium difluoride
Tetra-n-butylammonium bibenzoate
p-tert-butoxystyrene
Transmission electron microscopy
2,2,6,6-tetramethylpiperidinoxy
Four blocks, two of them different (f.e. ABAB)
Four different blocks (ABCD)
Four different blocks, three of them different (f.e. ABCA)
Glass transition temperature
Tetrahydrofurane
Melting temperature
N,N,N0 ,N0 -Tetrametylethylenediamine
Equilibrium melting temperature
Order-to-disorder transition temperature
Equilibrium order-to-disorder transition temperature
Thermoplastic polyamide
Thermoplastic elastomer
Thermoplastic polyesters
Thermoplastic polyurethane
Three blocks, two of them different (ABA)
Three different blocks (ABC)
Weak segregation limit
q2R2g
Rate constant
a-methylstyrene
Statistical segment length
Interfacial thickness
Undercooling parameter


xx

ÁH
Áe
e0
e00

 eff
[Z]
ko
s
se
j(r)
w
c(r)
o
oc, od

ABBREVIATIONS AND SYMBOLS

Heat of fusion
Dielectric strength
Dielectric permittivity
Dielectric loss
Conformational asymmetry parameter
Effective friction coefficient
Intrinsic viscosity
Composition polydispersity
Interfacial tension
Fold-surface free energy
Local volume fraction
Interaction parameter
Order parameter
Frequency
Characteristic frequencies in the viscoelastic response


Block Copolymers: Synthetic Strategies, Physical Properties, and Applications.
Nikos Hadjichristidis, Stergios Pispas and George Floudas
Copyright  2003 John Wiley & Sons, Inc.
ISBN: 0-471-39436-X

COLOR PLATES

Figure 13.8. Interfacial surfaces associated with elementary units of the
hexagonal (C), double gyroid (G), perforated layer (PL), and double diamond
(D) structures calculated at the C/G
phase boundary (␹N = 20, f = 0.3378).
For each structure, the distribution of
mean curvature H over the surface is
indicated using the color scale, and the
area-average ͗H͘ and standard deviation
␴H of H is provided. Notice the large
values of ␴H in the PL and G phases,
which imply a large degree of packing
frustration perturbing the interface
away from CMC (Matsen 1996).

Figure 15.8. PEO-PI/inorganic phase diagram (Simon 2001).


COLOR PLATES

Intensity (a.u.)

600

400

200
101

102

103

104

Time(s)
Figure 17.3. Evolution of the peak scattering intensity I(q*)(t) following a temperature jump
from the disordered to the ordered state of a block copolymer. The line is a fit to the Avrami
equation (eq. 17.3). The evolution of the scattering profiles is shown in the inset (Floudas
1994b).

Publisher's Note:
Permission to reproduce this image
online was not granted by the
copyright holder. Readers are kindly
asked to refer to the printed version
of this chapter.

Figure 19.4. (Left) Molecular structure of rod-coil diblock copolymers and a highly schematic
illustration of its hierarchical self-assembly into ordered microporous materials. (Right)
Fluorescence photomicrographs of solution-cast micellar films revealing a two-dimensional
hcp structure composed of air holes (Jenekhe 1999).


COLOR PLATES

Publisher's Note:
Permission to reproduce this image
online was not granted by the
copyright holder. Readers are kindly
asked to refer to the printed version
of this chapter.

Figure 19.6. (Left) Molecular model of a supramolecular unit composed of 100 triblock
copolymers with a rod-coil structure giving rise to mushroom-like nanostructures. (Right)
Schematic representation of how these nanostructures of the triblock copolymers could organize to form a macroscopic film (Stupp 1997).

A

B

C

a

b

c

d

e

f

g

h

i

j

k

l

Figure 19.7. ABC linear triblock copolymer morphologies. Microdomains are colored following the code of the triblock molecule in the top (Stadler 1995, Zheng 1995, Bates 1999).


COLOR PLATES

Figure 19.8. (Left) Bright field TEM of the SEM (35/27/38) triblock copolymer stained with
RuO4 showing the celebrated “knitting” pattern. (Right) Schematic description of the knitting
pattern morphology (Breiner 1998).

Figure 19.9. Schematic representation of the core-in-shell gyroid morphology found in the
poly(isoprene-b-styrene-b-dimethylsiloxane) (ISD) triblock copolymer (fPI = 0.40, fPS = 0.41,
fPDMS = 0.19). Blue, red, and green regions correspond to I, S, and D domains, respectively
(adapted by Shefelbine 1999).


Block Copolymers: Synthetic Strategies, Physical Properties, and Applications.
Nikos Hadjichristidis, Stergios Pispas and George Floudas
Copyright  2003 John Wiley & Sons, Inc.
ISBN: 0-471-39436-X

PART I

BLOCK COPOLYMER SYNTHESIS


Block Copolymers: Synthetic Strategies, Physical Properties, and Applications.
Nikos Hadjichristidis, Stergios Pispas and George Floudas
Copyright  2002 John Wiley & Sons, Inc.
ISBN: 0-471-39436-X

CHAPTER 1

BLOCK COPOLYMERS BY ANIONIC
POLYMERIZATION

Anionic living polymerization has been known for almost fifty years. Since
its discovery in the 1950s, it has emerged as the most powerful synthetic tool
for the preparation of well-defined polymers, i.e., narrow molecular weight
distribution polymers with controlled molecular characteristics including
molecular weight, composition, microstructure, and architecture. Its ability to
form well-defined macromolecules is mainly due to the absence of termination
and chain transfer reactions, under appropriate conditions (Young 1984,
Hsieh 1996).
Anionic polymerization proceeds via organometallic sites, carbanions (or
oxanions) with metallic counterions. Carbanions are nucleophiles; consequently,
the monomers that can be polymerized by anionic polymerization are those bearing
an electroattractive substituent on the polymerizable double bond. Initiation of
polymerization is accomplished by analogous low molecular weight organometallic
compounds (initiators). A wide variety of initiators has been used so far in order to
produce living polymers. Among them, the most widely used are organolithiums
(Hadjichristidis 2000). The main requirement for the employment of an organometallic compound as an anionic initiator is its rapid reaction with the monomer at
the initiation step of the polymerization reaction and, specifically, with a reaction
rate larger than that of the propagation step. This leads to the formation of polymers
with narrow molecular weight distributions because all active sites start polymerizing the monomer almost at the same time. Propagation proceeds through nucleophilic attack of a carbanionic site onto a monomer molecule with reformation of the
first anionic active center. The situation is similar in the case of the ring opening
polymerization of cyclic monomers containing heteroatoms (oxiranes, lactones,
thiiranes, siloxanes, etc). The role of the solvent and additives in the polymerization
3


4

BLOCK COPOLYMERS BY ANIONIC POLYMERIZATION

mechanism is important and has been studied extensively in several cases (Young
1984, Hsieh 1996).
Under appropriate experimental conditions (Hadjichristidis 2000), due to the
absence of termination and chain transfer reactions, carbanions (or, in general,
anionic sites) remain active after complete consumption of monomer, giving the
possibility of block copolymer formation, in the simplest case, by introduction of a
second monomer into the polymerization mixture. However, a variety of different
synthetic strategies have been reported for the preparation of linear block copolymers by anionic polymerization.

1. SYNTHESIS OF AB DIBLOCK COPOLYMERS
Linear AB block copolymers are the simplest block copolymer structures where
two blocks of different chemical structures are linked together through a common
junction point.
The most general method for the preparation of AB block copolymers is
sequential monomer addition. In this method one of the monomers is polymerized
first. After its complete consumption, the second monomer is added, and the
polymerization is again allowed to proceed to completion. At this point an
appropriate terminating agent is added, and the diblock copolymer can then be
isolated (usually by precipitation in a nonsolvent).
The most important conditions (mechanistic and experimental features) that
must be fulfilled in order to synthesize well-defined block copolymers are:
iii) The carbanion formed by the second monomer must be more, or at least
equally, stable than the one derived from the first monomer. In other words
the first monomer carbanion must be able to initiate the polymerization of
the second monomer, i.e., the first monomer carbanion must be a stronger
nucleophile than the second one.
iii) The rate of the crossover reaction, i.e., the initiation of polymerization of the
second monomer by the anion of the first, must be higher then the rate of
propagation of monomer B. This ensures narrow molecular weight distribution for block B and absence of A homopolymer in the final block
copolymer that can arise from incomplete initiation.
iii) The purity of the second monomer must be high. Otherwise, partial termination of the living A anions can take place leading to the presence of A
homopolymer in the final product. Additionally, loss of molecular weight
and composition control of the second block and of the whole copolymer
will occur because the concentration of the active centers will be decreased.
Taking into account these mechanistic and experimental features, a large number
of AB diblock copolymers have been synthesized by sequential addition of
monomers. Some representative examples are given in Table 1.1. The list is by


SYNTHESIS OF AB DIBLOCK COPOLYMERS

5

TABLE 1.1. AB Diblock Copolymers Formed by Sequential Addition of
Monomers Using Anionic Polymerization
1st Monomer

2nd Monomer

Reference

Styrene

Isoprene
Butadiene
Cyclohexadiene
Methylmethacrylate
tert-Butyl methacrylate
tert-Butyl acrylate
2,3-glycidyl methacrylate
Stearyl methacrylate
2-Vinylpyridine
4-Vinylpyridine
Ethylene Oxide
e-Caprolactone
Hexamethylcyclotrisiloxane
Ferrocenyldimethylsilane
Hexyl isocyanate
Butadiene
2-Vinyl pyridine
Ethylene oxide
Hexamethylcyclotrisiloxane
e-Caprolactone
Ethylene oxide
tert-Butyl methacrylate
tert-Butyl methacrylate
e-Caprolactone
Ethylene oxide

Corbin 1976
Quirk 1992
Hong 2001
Varshney 1990
Varshney 1990
Hautekeer 1990
Hild 1998
Pitsikalis 1999
Schindler 1969
Grosius 1970
Finaz 1962
Paul 1980
Zilliox 1975
Ni 1996
Chen 1995
Elgert 1973
Matsushita 1986
Forster 1999
Almdal 1996
Balsamo 1998
Forster 1999
Allen 1986
Yin 1994
Diuvenroorde 2000
Martin 1996

a-Methyl styrene
Isoprene

Butadiene
Methyl methacrylate
2-Vinyl pyridine

no means exhaustive. More examples are given in the general literature on anionic
polymerization (Hsieh 1996, Morton 1983).
A wide variety of diblock copolymers of styrene and isoprene or butadiene,
having predictable molecular weight and composition as well as narrow molecular
weight and compositional distribution, have been synthesized by sequential addition of monomers (Morton 1983, Hsieh 1996). Synthesis of these diblocks starts
with styrene, and then the diene is added to the reaction mixture because it is well
established that PSLi active centers can initiate efficiently the polymerization of
dienes in hydrocarbon solvents (Scheme 1.1) and not vice versa. The use of

CH3OH

Benzene

+ s-BuLi

PS Li

Scheme 1.1

PS-PI

Li

PS-PI


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