Handbook of free radical initiators 2003 denisov, denisova pokidova

HANDBOOK OF FREE
RADICAL INITIATORS


HANDBOOK OF FREE
RADICAL INITIATORS

E.T. DENISOV
T.G. DENISOVA
T.S. POKIDOVA
Institute of Problems of Chemical Physics
Russian Academy of Sciences
Moscow

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A JOHN WILEY & SONS, INC., PUBLICATION

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Copyright 0 2003 by John Wiley & Sons, Inc. All rights reserved.
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Library of Congress Cataloging-in-Publication Data:
Denisov, E. T. (Evgeny Timofeevich)
Handbook of free radical initiators / E.T. Denisov, T.G. Denisova,
T.S. Pokidova.
p. cm.
Includes bibliographical references and index.
ISBN 0-471-20753-5 (cloth : acid-free paper)
I . Radicals (Chemistry) -Handbooks, manuals, etc. 2. Free radicals
(Chemistry) -Handbooks, manuals, elc. 3. Free radicals
(Chemistry) -Mechanism of action -Handbooks, manuals, etc. I. Denisova,
Taisa G. 11. Pokidova, T. S. (Tamara S.) 111. Title.
QD471 .D47 2003
541.2’24-dc21
2002009951
Printed in the United States of America
109 8 7 6 5 4 3 2 1


To the memory of Grigorii Alekseevich Razwaev
a great scientist, teacher, and person
of extraordinary fate


CONTENTS

Preface

xv

Symbols and Abbreviations

PART I INITIATORS

xvii
1

1 Mechanisms of Decomposition of Initiators

1.1 Introduction
1.2 Nonconcerted Unimolecular Decomposition
1.3 Concerted Fragmentation of Initiators
1.4 Anchimerically Assisted Decomposition of Peroxides
1.5 Decay of Initiators to Free Radicals and Molecular
Products

10

1.6 Chain Decomposition of Initiators

12

References
2 Cage Effect

14

17

2.1 Introduction

17

2.2 Experimental Evidence for the Cage Effect

17

2.2.1

Quantum Yield

17
vii


viii

CONTENTS

2.2.2
2.2.3
2.2.4
2.2.5

Products of Radical Pair Combination
Crossover Experiments
Oxygen- 18 Scrambling and Racemization
Influence of the Viscosity and Pressure on
Decomposition of Initiators
2.2.6 Spin Multiplicity Effects

18

19
20
23
26

2.3 Mechanistic Schemes of Cage Effect

27

2.4 Cage Effect in Solid Polymers

29

2.4.1 Schemes of the Cage Effect with Translational
Motion of Particles
2.4.2 Schemes of the Cage Effect with Translational
and Rotational Motion of Particles
2.4.3 Role of the Cage Shape in the Polymer Matrix at
Initiator Decomposition
2.4.4 Concept of a Hard Cage of Polymer Matrix
References

3 Methods of Study of Initiator Decomposition and Free Radical
Generation

29
31
33
34
38

41

3.1 Kinetic Decay of Initiator (KDI)

41

3.2 Kinetic Product Formation (KPF)

42

3.3 Acceptors of Free Radicals (AFR)

42

3.4 Kinetic Chain Initiated Reaction (KIR)

50

3.5 Chemiluminescence (CL) Method

56

References
4 Dialkyl Peroxides and Hydroperoxides

4.1

Dialkyl Peroxides

4.1.1
4.1.2
4.1.3
4.1.4

Synthesis and Analysif
Structure of Dialkyl Peroxides
Thermochemistry of Dialkyl Peroxides
Decomposition of Dialkyl Peroxides

4.2 Hydroperoxides and Peracids
4.2.1 Synthesis and Analysis of Hydroperoxides
4.2.2 Structure of Hydroperoxides
4.2.3 Thermochemistry of Hydroperoxides and Peracids

57

61
61
62
66
66
66
73
73
96
98


CONTENTS

4.2.4 Hydrogen Bonding and Acidity of
Hydroperoxides and Peracids
4.2.5 Unimolecular Decomposition of Hydroperoxides
4.2.6 Chain Decomposition of Hydroperoxides
4.2.7 Interaction of Hydroperoxides with Ketones
References

5 Diacyl Peroxides, Peroxy Esters, Polyatomic, and
Organometallic Peroxides
5.1 Diacyl Peroxides
5.1.1
5.1.2
5.1.3
5.1.4

Synthesis and Analysis
Structure of Diacyl Peroxides
Thermochemistry of Diacyl Peroxides
Decomposition of Diacyl Peroxides

5.2 Peroxy Esters
5.2.1 Synthesis and Analysis
5.2.2 Thermochemistry of Peroxy Esters
5.2.3 Decomposition of Peroxy Esters

ix

101
102

102
117
122

129
129
129
134
135
137
143
143
193
194

5.3 Decomposition of Polyatomic Peroxides

275

5.4 Organometallic Peroxides

275

References

6 Organic Polyoxides
6.1 Dialkyl Trioxides
6.1.1 Synthesis
6.1.2 Thermochemistry of Trioxides
6.1.3 Decay of Trioxides

6.2 Hydrotrioxides
6.2.1
6.2.2
6.2.3
6.2.4

Synthesis
Structure and Spectrum of Hydrotrioxides
Thermochemistry
Decomposition of Hydrotrioxides

6.3 Tetroxides
6.3.1 Peroxyl Radical-Tetroxide Equilibrium
6.3.2 Decay of Tetroxides
6.3.3 Thermochemistry of Tetroxides
References

275

283
283
283
284
284
286
286
286
287
288
298

298
299
299
300


X

CONTENTS

7 Azo Compounds

303

7.1 Synthesis and Structure of Azo Compounds

303

7.2 Thermochemistry of Azo Compounds

306

7.3 Decomposition of Azo Compounds

308

References

8 Compounds with weak C-C, N-N, C-N, and N - 0 Bonds

353
359

8.1 Polyphenylhy drocarbons

359

8.2 Substituted Hydrazines

360

8.3 Alkoxyamines

372

8.4 Nitro Compounds

372

8.5 Nitrates and Nitrites

384

8.6 Disulfides and Polysulfides

387

8.7 Organometallic Compounds

413

References

PART I1 BIMOLECULAR REACTIONS OF FREE RADICAL
GENERATION
9 Parabolic Model of Bimolecular Homolytic Reaction

414

421
423

9.1 Introduction

423

9.2 Principles for the Parabolic Model of Bimolecular
Homolytic Reaction

425

9.2.1 Main Equations of IPM
9.2.2 Calculation of E and k for Bimolecular Reactions
9.3 Parameters of Bimolecular Homolytic Reaction in the
Parabolic Model
References

425
427
44 1

476

10 Bimolecular and Trimolecular Reactions of Free Radical
Generation by Dioxygen

479

10.1 Reaction of Dioxygen with C-H Bonds of Organic
Compounds

479

10.2 Reaction of Dioxygen with the Double Bond of Olefins

480


CONTENTS

10.3 Trimolecular Reaction of Radical Initiation by Dioxygen
References

11 Bimolecular Reactions of Free Radical Generation by Ozone

xi

493
499

503

11.1 Initiation of Radicals by Ozone Reactions

503

11.2 Chain Reactions of Ozone Decomposition

506

References

12 Bimolecular Reactions of Hydroperoxides with Free Radical
Generation

521

525

12.1 Bimolecular Decomposition of Hydroperoxides

525

12.2 Bimolecular Reactions of Hydroperoxides with a x-Bond
to Olefins

526

12.3 Bimolecular Reactions of Hydroperoxides with C-H,
N-H, and 0 - H Bonds of Organic Compounds

53 1

12.3.1 Hydrocarbons
12.3.2 Alcohols and Acids

532
532

12.4 Acid Catalysis for Homolytic Reactions of
Hydroperoxides

538

12.5 Reaction of Peroxides with Amines

540

References

13 Free Radical Generation by Olefins

544

547

13.1 Reactions of Retrodisproportionation

547

13.2 Chain Initiation in Thermal Radical Polymerization

561

References

14 Initiation by Haloid Molecules and Nitrogen Dioxide

563

565

14.1 Reactions of Fluorine Compounds

565

14.2 Reactions of Dichlorine and Other Chlorine Compounds

573

14.3 Initiation by Nitrogen Dioxide

58 1

References

588

15 Free Radical Generation by Reactions of Ions with Molecules

591

15.1 Decomposition of Hydrogen Peroxide Catalyzed by
Transition Metal Ions

591


xii

CONTENTS

15.2 Catalysis by Ions and Complexes of Transition Metals in
Liquid-Phase Oxidation of Organic Compounds

595

15.3 Reactions of Free Radicals with Transition Metal Ions

604

15.4 Oxidation of Transition Metal Ions by Dioxygen

618

15.5 Oxidation of Organic Compounds by Transition Metal
Ions

620

15.5.1

15.5.2
15.5.3
15.5.4
15.5.5
15.5.6

Oxidation
Oxidation
Oxidation
Oxidation
Oxidation
Oxidation

by
by
by
by
by
by

Tetravalent Cerium
Trivalent Cobalt
Copper Ions
Trivalent Iron
Trivalent Manganese
Pentavalent Vanadium

15.6 Reduction of Peroxides by Radical Anions
References

624
624
633
633
633
636
644
644

PART I11 REACTIONS OF FREE RADICALS

655

16 Isomerization and Decomposition of Free Radicals

657

16.1 Intramolecular Abstraction of Hydrogen Atom

657

16.1.1 Alkyl Radicals
16.1.2 Alkoxyl Radicals
16.1.3 Peroxyl Radicals

657
659
659

16.2 Cyclization of Free Radicals

663

16.3 Decyclization of Cyclic Radicals

683

16.4 Fragmentation of Free Radicals

69 1

16.4.1
16.4.2
16.4.3
16.4.4
16.4.5

Alkyl Radicals
Acetyl Radicals
Alkoxyl Radicals
Carboxyl Radicals
Peroxyl Radicals

References

17 Free Radical Abstraction Reactions

69 1
704
704

704
712
712
719

17.1 Classification of Radical Abstraction Reactions

719

17.2 Enthalpy of Reaction

720


CONTENTS

xiii

17.3 Force Constants of Reacting Bonds

737

17.4 Triplet Repulsion

737

17.5 Electron Affinity of Atoms in Reaction Center

74 1

17.6 Repulsion of Atoms Forming the Reaction Center

742

17.7 Influence of n-Bonds in the Vicinity of the Reaction
Center

745

17.8 Steric Effect

747

17.9 Polar Effect in Radical Reactions

749

17.10 Effect of Multidipole Interaction

750

17.11 Solvating Effect

752

References

18 Free Radical Reactions for Hydrogen Transfer and
Substitution

754

757

18.1 Reactions of Hydrogen Atom Transfer from a Free
Radical to a Molecule

757

18.2 Free Radical Substitution Reactions

762

18.3 Reaction of Peroxides with Ketyl Radicals

772

References

19 Free Radical Addition

777

781

19.1 Enthalpy and Entropy of Free Radical Addition

78 1

19.2 Empirical Correlation Equations

782

19.2.1
19.2.2
19.2.3
19.2.4

Activation Energy and Heat of Reaction
Q - e Scheme
The Bamford and Jenkins a - 8 Scheme
The Ito and Matsuda K - P Scheme

783
783
783
784

19.3 Quantum Chemical Calculations for the Activation Energy

785

19.4 Parabolic Model of Radical Addition

786

19.5 Contribution of Enthalpy for an Addition Reaction to Its
Activation Energy

787

19.6 Force Constants of Reacting Bonds

789

19.7 Triplet Repulsion in the Transition State of Addition
Reactions

790


xiv

CONTENTS

19.8 Influence of Neighboring n-Bonds on the Activation
Energy of Radical Addition

79 1

19.9 Role of the Radius of the Atom Bearing the Free Valence

793

19.10 Interaction of Two Polar Groups

794

19.11 Multidipole Interaction in Addition Reactions

795

19.12 Steric Hindrance

797

19.13 Addition of Alkyl Radicals to Dioxygen

797

References

20 Recombination and Disproportionation of Free Radicals

Index

81 1

817

20.1 Alkyl Radicals

817

20.2 Macroradicals

820

20.3 Peroxyl Radicals

829

References

844

849


PREFACE

The aim of this Handbook is to present an up-to-date account of the physicochemical data on radical initiators and radical generation reactions. Initiators
are used in technological processes, for example, polymerization, oligomerization, network formation, and modification of polymers. They are widely used
in organic synthesis to initiate chain reactions. Initiators are one of the important elements of the mechanistic study of different chain reactions. In addition
to initiators, we offer comprehensive information about bimolecular reactions
of free radical generation. The chemistry of initiators and reactions of radical
generation were intensively studied during the last 40 years and are very complex. A few mechanisms of homolytic splitting of molecules into free radicals
were found. Researchers were faced with the simultaneous occurrence of unimolecular and chain reaction mechanisms of initiator decay. Both homolytic
and heterolytic decomposition of some initiators exist. Homolytic decay of an
initiator is accompanied by the cage effect in solvents and polymers. All data
concerning the peculiarities of initiator decay were collected and discussed in
this Handbook. We attempted to write a comprehensive physicochemical encyclopedia on initiators and their initiating reactions. Comprehensive information
concerning initiators was collected. Readers will find the data and bibliography
on the synthesis, structure, and thermochemistry of initiators, as well as detailed
information on the rate constants and activation energies of the decomposition
of initiators and bimolecular reactions of free radical generation.
This Handbook is divided into three parts.
Part I is devoted to initiators of free radicals and contains eight chapters. In
Chapter 7, the different mechanisms of initiator decomposition are discussed.
Chapter 2 is devoted to the cage effect that accompanies the decomposition of

initiators in liquids and solid polymers. Chapter 3 presents a short description of
xv


xvi

PREFACE

the methods of study of initiator decomposition. Chapters 4-8 include complex
scientific information about initiators: peroxides, polyoxides, azo compounds,
polyphenylbutanes, phenylhydrazins, nitrites, nitro compounds, and so on.
Part I1 is devoted to bimolecular reactions of free radical generation. A wide
variety of such reactions was observed. The reader will find the data on free
radical generation by reactions of retrodisproportionation, reactions of paraffins
and olefins with haloid molecules (F2, Clz, etc.), bimolecular and trimolecular
reactions of RH with 0 2 , reactions of ozone and N 0 2 , bimolecular reactions
of hydroperoxides, reactions of thermal chain initiation in polymerization, and
reactions of transition metal ions.
In Part 111 the data on the rate constants of reactions of free radicals formed
from initiators are collected: decay and isomerization of free radicals, reactions of
radicals with solvents and monomers, recombination, and disproportionation of
free radicals. It is anticipated that the majority of the users of this Handbook will
be researchers and technologists, as well as undergraduate students, postgraduate
students, and professors who will find it a unique and helpful reference book.
Symbols and units used in this Handbook are in accordance with UPAC recommendations as written in the manual “Quantities, Units and Symbols in Physical
Chemistry”, Blackwell Scientific Publications, London, 1988.
We are grateful to Elena Batova for attentive English editing of this book and
to Ludmila Pilipetskaya and Lidiya Abramova for rapid and accurate typing of
the manuscript.
All comments, critical notes, and suggestions are welcomed by the authors.

Address comments to E. T. Denisov, T. G. Denisova, and T. S. Pokidova, Institute of Problems of Chemical Physics, Chernogolovka, Moscow Region, 142432,
Russia. Email:
Chernogolovka, Moscow Region
June 19. 2002

EVCENY
T. DENISOV
TAISAG. DENISOVA
TAMARA
S. POKIDOVA


SYMBOLS AND ABBREVIATIONS

PHYSICOCHEMICAL SYMBOLS
Symbol
A

Meaning
Preexponential factor in Arrhenius
equation of reaction rate constant
k = A x exp(-EIRT)

Preexponential factor of reaction rate
constant per attacked atom among
bonds with equireactivity

b

D

DY-x
e
e

2b2 is the force constant of chemical bond
Diffusion coefficient
Dissociation energy of Y-X bond
Probability of formed free radical pair to
escape the cage of solvent or polymer
Base of natural logarithms

Unit
s-l (unimolecular
reaction),
Lrno1-l s-l
(bimolecular),
L~ rnoI-2 s - ~
(trimolecular)
s-l (unimolecular
reaction),
L mol-' s-l
(bimolecular),
L~ mol-2 s-2
(trimolecular)
kJ'/2 mol-1/2

,-I

m2 spl
kJ mol-'

2.7183

xvii


xviii

E

SYMBOLS AND ABBREVIATIONS

Activation energy for the reaction of the
Arrhenius equation for reaction rate
constant
Activation energy for the reaction of the
parabolic model of the bimolecular
reaction; E, = E O.5hLv - 0.5 RT
Stoichiometric coefficient for free radical
acceptance by an acceptor of free
radicals
Gibbs energy of a reaction under standard
conditions (298 K, 1 atm)
Enthalpy of reaction (298 K, 1 atm)
Enthalpy of transition state
Enthalpy of reaction that includes the
difference of zero vibrational energies
for reacting bonds
Enthalpy of a molecule formation under
standard conditions (298 K, 1 atm)

Enthalpy of a molecule evaporation under
standard conditions (298 K, 1 atm)
Planck constant, h = 6.626075 x
Equilibrium constant, RT In K c = -AGO
Reaction rate constant

kJ mol-'

kJ mol-'

+

f
AGO

AH;

AH:
h
KC

k

Rate constant for an acceptor reaction with
free radicals
Rate constant for decomposition of an
initiator to free radicals
Rate constant for a diffusion controlled
reaction
Rate constant for an initiation (free

radicals formation)

kind

Rate constant for the induced
decomposition of initiator

kJ mol-'
kJ mol-'
klmol-'
kJ mol-'
kJ mol-'

Js
(moY1)'"
s-l (unimolecular
reaction),
L mol-' s-l
(bimolecular),
L~ mol-2 sp2
(trimolecular)
Lmol-' s-'
S-'

s-l (unimolecular

~

reaction),
Lmo1-ls-l

(bimolecular)
' / mol-'/2
2
s-1


xix

SYMBOLS AND ABBREVIATIONS

kis
kl

Rate constant for the initiator
isomerization
Reaction rate constant in the liquid phase
Rate constant for the initiator
decomposition to molecular products
Rate constant for chain propagation
Rate constant for free radical rotation in
the cage
Reaction rate constant in the solid phase

kt
L
nD

An
P
R

YR

SO(RH)

AVf

Rate constant for the scrambling reaction
in the cage
Rate constant for the chain termination
Avogadro's number,
L = 6.0221367 x
Refractive index
Mol change in a reaction
Pressure
Gas constant, R = 8.314510
Radius of radical R
Entropy of formation for RH in the gas
phase under standard conditions
Entropy of activation
Time
Absolute temperature
Molecular partition function of RH
Molecular partition function of transition
state
Reaction rate
Rate for the initiation reaction
Rate for the thermal initiation reaction
Rate for the induced decomposition of the
initiator
Change of molecular volume due to

formation of the transition state,
A V f = Vf (transition state) - V
(reactants)

Pa
Jmol-' K-'
m
J mol-' K-'
J mol-' K-'
S

K

cm3 mol-'


xx

SYMBOLS AND ABBREVIATIONS

Ratio b,/bf for the attacked (b,) and
forming (bf) bonds
Degree of stretching of a polymer film

Q!

Molar absorption coefficient
Viscosity
Quantum yield of chemiluminescence
Ionic strength, K = 0.5Cc1z~.

c, and z1 are
concentration and charge of ith ion in
solution
Length of light wave
Dipole moment of molecule
Frequency of valence vibration for the
reacting bond
Frequency of valence vibration for the
forming bond
Frequency of free radical R' rotation
Ratio of circumference to diameter of a
circle, JI = 3.141592
Density
Induction period
Photochemical yield

Vf

L mo1-I cm-'
Pa s
mol L-'

m
D
S-'

S-'

2Jc s-'

kg mP3
S

SYMBOLS DESIGNATING PHYSICOCHEMICAL METHODS
AFR
BEBO
BEBL
bP
CL
DNP
EG
EPR
GLC
IPM
IR
KDI
KPF

Acceptors of free radicals method
Bond energy-bond order
Bond energy-bond length
Boiling point, K
Chemiluminescence method
Dynamic nuclear polarization
Electronography
Electron paramagnetic resonance
Gas-liquid chromatography
Intersecting parabola model of reaction
Infrared spectroscopy
Kinetics of decay of initiator

Kinetics of end product formation


SYMBOLS AND ABBREVIATIONS

KIR

*P
MS
MW
NMR
QCH
RRKM
RSA

uv

Kinetics of chain reaction in the presence of decomposing
initiator
Melting point, K
Mass spectrometry
Molecular weight, g mol-'
Nuclear magnetic resonance spectroscopy
Quantum chemical calculation
Rice-Ramsperger-Kassel-Marcus theory of unimolecular
reactions
Rentgen structural analysis
Ultraviolet

CHEMICAL SYMBOLS AND ABBREVIATIONS

acac
AIBN
Amp
ArOH
Ar20H
Ar2 0
'
BDE
DBPO
DBP
DCP
EDTA
I
InH
PE
p'

PH
P02'
PP

Q
RH
RIH

R~H

Acetylacetonate
Azoisobutyronitrile
Nitroxyl radical

Phenol
Sterically hindered phenol
Sterically hindered phenoxyl radical
Bond dissociation energy
Di-tert-butyl peroxalate
Peroxide, bis( 1,l-dimethylethyl)Peroxide, bis( 1-methyl- 1-phenylethyl)Ethylenediaminetetraacetic acid
Initiator
Acceptor reacting with alkoxyl and peroxyl radicals
Polyethylene
Macroradical
Polymer
Peroxyl macroradical
Polypropylene
Acceptor reacting with alkyl radicals
Organic substance reacting with its C-H bond
Aliphatic or alicyclic hydrocarbon
Olefin hydrocarbon

xxi


xxii

R3H
RN2R
RO, H
R0.X R
RO'
R02'
RS'

TMS

SYMBOLS AND ABBREVIATIONS

Alkylaromatic hydrocarbon
Azo compound
Hydroperoxide (x = 2), hydrotrioxide (x = 3)
Peroxide (x = 2), trioxide (x = 3), tetroxide (x = 4)
Alkoxyl radical
Peroxyl radical
Thiyl radical
Tetramethyl silane


Handbook of Free Radical Initiators. E.T. Denisov, T.G. Denisova, T.S. Pokidova
Copyright 0 2003 John Wiley & Sons, Inc.
ISBN: 0-471-20753-5

PART I
INITIATORS

1


Handbook of Free Radical Initiators. E.T. Denisov, T.G. Denisova, T.S. Pokidova
Copyright 0 2003 John Wiley & Sons, Inc.
ISBN: 0-471-20753-5

MECHANISMS OF DECOMPOSITION
OF INITIATORS

1.1 INTRODUCTION
A lot of organic molecules, dealing with technique, technological processes, and
organic synthesis, are stable at moderate (-300-400 K) and elevated (>400 K)
temperatures. Atoms of these compounds are connected by sufficiently strong
chemical bonds with bond dissociation energy (BDE) -350-500 kJ mol-' .
Radical initiators are molecules bearing one or several weak bonds with BDE
100-200 kJ mol-' . When the temperature of the reaction is sufficiently high,
the initiator decomposes with homolysis of the weakest bond and produces free
radicals. These free radicals initiate a chain or nonchain free radical reaction.
What are the factors that influence the BDE of any chemical bond? First, there
are atoms forming the bond. Here are a few examples of the types of bonds in
various compounds:',2

-

Compound
Bond
D (Hmol-')

CH4
C-H
440

Et
C-C
378

MeNH2
C-N

358

MeOH

c-0
388

Me1
c-I
240

MeOOMe
0-0
161

The following bonds have sufficiently low values of BDE:
Compound
Bond
D (Hmol-')

MeOOMe MeONO HON02 Me3CN02 MeNO PrN2CH2CH=CH2
0-0
0-N
0-N
C-N
C-N
N-C
157
175
207

245
1 67
141

3


4

MECHANISMS OF DECOMPOSITION OF INITIATORS

Organometallic compounds have weak metal-carbon bonds:*
Compound
Bond
D (kJmol-')

SnMe4
Sn-C
294

PbMe4
Pb-C
239

HgMe,
Hg-C
255

SbMe5
Sb-C

255

BiMe4
Bi-C
218

TiMe4
Ti-C
167

Atoms surrounding the atom with the bond being split also influence the BDE.
Here are a few example^:^-^
Peroxide3
DoPo (kJmol-')

MeOOMe
158.1

EtOOEt
153.1

(Me3CO),
166.5

(MeC(0)0)2
131.2

(PhC(O)O),
124.4

Hydrazine'
DNPN(kJ mol-')

H2NNH2
275.3

H2NNHMe
268.2

H2NNMe2
246.9

HzNNHPh
218.8

Ph2N-NPh2
125.0

Polysulfide4
Ds-s (kJmol-')

EtSSEt
285.0

PhSSPh
223.0

EtS-SSH
213.0

PhS-SSH
182.0

EtSS-SSEt
142.0

A n-bond in the a-position has a strong influence on the dissociating bond.
This influence is clearly seen for several alkylaromatic hydrocarbon^:',^
Hydrocarbon
DcPc (kJmol-')

Et-Et
364

Et-CH'Ph
318

PhPr,C-CPr*Ph
150.6

Ph2MeC-CMePh2
125.5

This dependence is the result of stabilization of the formed radical due to the
interaction of an unpaired electron with n-electrons of the benzene ring.
Different mechanisms of free radicals formation as a result of initiator decomposition are known. Most initiators decompose with dissociation of the weakest
bond, for example,
R'O-OR* + R'O- R ~ O -

+

Initiators that decompose with simultaneous dissociation of two or more bonds
are known, for e ~ a m p l e , ~ . ~

R-N=N-R

+ R' + N2 + R'

RC(0)O-OR' + R'

+ C02 + R'O'

This decay is known as concerted fragmentation (see Section 1.3).
Some ortho-substituted benzoyl peroxides decompose with formation of unstable intermediates. These intermediates are the result of formation of an additional


5

NONCONCERTED UNIMOLECULAR DECOMPOSITION

bond between the oxygen atom of the benzoyloxy radical and ortho substituent,
for examp~e,~Jj

0
II

- do
+

a C ' O CH=CH2

' O R

ROO

This decay was called anchimerically assisted peroxide cleavage (see Section 1.4).
The decomposition of initiators very often proceeds homolytically with dissociation of the molecule to free radicals only. However, there are compounds
that decay simultaneously to free radicals and molecular products. For example,
peresters decay to free radicals with dissociation on the 0-0 bond and their
isomerization to aryloxyester proceeds in parallel:6
PhMe2COOC(O)Ph + PhMe2CO'

+ PhC(0)O'

PhMezCOOC(0)Ph + PhOCMezOC(0)Ph
Free radicals formed from the initiator react with the reactant or recombine.
Free radicals formed from the initiator or reactant also can react with the initiator.6
If this reaction proceeds intensely, the initiator is decomposed by free radicals,
and this decreases its effectiveness as an initiator (see Section 1.5).

1.2 NONCONCERTED UNIMOLECULAR DECOMPOSITION
Before decomposition, a molecule of the initiator (I) should be activated. Its
activation is the result of collisions of the initiator molecule with other molecules
M in the gas or liquid phase. The energized molecule may undergo deenergization
by collision with a normal molecule, or it may undergo a unimolecular reaction
to form products. These three processes are quite distinct and the situation may
be represented as:
I+M+I*+M
I*+M+I+M*
I* + 1' -+ R1'

+ Rz'

where I* is the activated molecule and 1' is the activated complex. The
activated molecule I* passes through the top of the activation barrier. The
energized molecule I* has acquired all the energy it needs to become the activated
molecule I#. The full description of the activation process and reaction is
given by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory of unimolecular
reaction^.^-'^ This theory describes the dependence of the rate constant for


6

MECHANISMS OF DECOMPOSITION OF INITIATORS

unimolecular decay of molecules on gas pressure. When the pressure is growing,
the rate constant of decomposition kd --+ k,, and

where k# is the rate constant of I# decay. The expression for initiator decay
in solution, where the frequency of molecular collisions is extremely great, is
the same. According to transition state theory, the rate constant of unimolecular
reactions at high pressure is the following: l 1
kd

= k,

= e-eRT A S # / R ~ - E / R T

Lh

When a polyatomic molecule, for example, peroxide ROOR, decomposes

to two free radicals RO', the following changes in the energy distribution are
observed: l3
1. One stretching vibration along the 0-0 bond disappears,
2. One inner rotation of the 0-0 bond disappears,
3. Two C-0-0 angles vibrations disappear.
As a result, the activation entropy of unimolecular decomposition AS' > 0 and
the preexponential factor [A = e RT(Lh)-' exp(AS'/R)] is sufficiently higher
than eRT(Lh)-' RZ l o L 3SKI.For many unimolecular reactions AS' RZ 20-80
Jmol-l
K-1 13
Due to the elongation of the dissociating bond (e.g. 0-0 in peroxide), the
volume of the transition state I' > I. As a result, the difference in the volumes
V(I#) - V(1) = AV' is positive. The study of decomposition of initiators with
one bond dissociation under pressure gives evidence that AV' is positive and
helps us to evaluate A V' according to the following dependence of k on pressure
D : 3 , 14

Ink =Ink"

-

AV'
p
RT l + b p

~~

where ko = k at p = 0 and b = 9.2 x lop9 Pa-'. The value of AV' depends on
the pressure p :
AVf = AV,f(l bp)-2

(1.4)

+

1.3 CONCERTED FRAGMENTATION OF INITIATORS
Peroxides have the weak 0-0 bond and usually decompose with dissociation of
this bond. The rate constants of this decomposition of ROOR into RO' radicals
demonstrates a low successibility of the BDE of the 0-0 bond to the structure
of the R fragment (see Chapter 4). Bartlett and Hiatl5 studied the decay of many