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Book Review The Chemistry of Free Radical Polymerization. By G. Moad and D. H. Solomon

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The fifth and longcst chapter in the
book is titled “Supramolecular Inorganic
Chemistry.” In spitc of this cxciting title,
I found this to be the low point of thc
book. It was difficult to read, and filled
with verbose and lengthy discussions on
terminology and scniantics. I found thc
conccpts that were introduced, such as
“domains of molecules,” to be of no value. The illustrations were dreadful and the
organization was hard to follow. Several
scctions had littlc to d o with crystalline
systcms. It is unfortunatc that this chapter
consumes one-third of the entirc book.
Chapter 6 was an interesting and informative description of thc (beta-alpha)8
bar]-el protein architecture. The author
worked from the basics to build a wonderful picture of the marvels of protein sclfassembly and self-organization. Even
though the subject did not fit pcrfectly into the main t h e m of crystals as supramolecular entitics, therc are many valuable lessons to be lcarned from these
biomolecular systems that could be applied to more traditional molecular solid
state chemistry. The chaptcr was well
writtcn and easy to understand, even for a
non-specialist in this area.
On the whole, the book is consistent
with the philosophy of thc “Perspectives”
series conccntrating on goal-oriented
supramolccular chemistry. Although I
would not call this a treasure-trove of
crystalline suprainolceular chemistry,
thcre are a fcw contributions which may
be worthwhile to thosc with an intcrcst in
the field.
Jeffic~yS. h.1oorc
Dcpartments of Chemistry and
Materials Science and Engineering
Roger Adams Laboratory
University of Illinois
Urbana, IL (USA)
Energetics of Organic Free Radicals.
(Series: Search, Vol. 4). Edited by
J A . M . Simcies, A . Greenberg and
J F: Liehmun. Chapman and Hall,
L o n d o n , 1996. 301 pp., hardcover
E 39.95.-ISBN 0-7514-0378-4
I have often wished, while working with
an intercsting textbook or monograph,
that it had been availablc to me some
years carlicr, for cxamplc in my time as a
final year degree student or during postgraduate study. In that way I might have
gained a deeper insight into some of the
topics studicd. espccially if, for example,
the concepts of thcrmochcmistry and the
rcactivc propertics of frec radicals had
been explained so clearly as thcy are in
this fourth volumc of the Seurcli series. In
this book thc editors havc set out to
provide a broadly based introduction to
the structurcs, enei-gctics, and reactivities
of free radicals, by including multidisciplinary contributions from a number of
recognized expcrts.
Frcc radical rcactions occur much more
commonly in thc biosphere and atmosphere around us than onc might at first
suppose in view of the short-lived nature
of these intermediates. Chevcs Walling,
author of the article “Frcc Radical Reactions”, has several decades of expcrience
in this ficld of research. He bcgins with thc
basic principlcs of free radical chemistry,
and cxplains thc meanings of thc various
thermochemical quantitics involved. The
concept of an elemcntary reaction is then
dcfined, with appropriate examples, and
on this basis some important rcaction
mechanisms are identified. This approach
enables thc reader to gain a sound undcrstanding of encrgy barriers and thc factors
that contribute to thcm, and of the rate
constants for some important processes.
Since reactions involving free radicals
mostly take place under neutral conditions, thc thermochemistry of the educts,
intermediates, and products is of great importance for predicting thc behavior of
proccsses with applications in synthesis.
The various mcthods for determining
standard enthalpies of formation, for example from kinetic measurements, from
calculations based on thermodynamic
cyclic processes, o r cvcn directly from
mass-spectronietric data, arc describcd
hcrc in two articles, by Wing Tsang and by
John C. Traeger and Barbara M. Kompe.
But how does onc obtain meaningful values for heats of formation in systems for
which it is difficuit to sct up precise expcrimcnts. such as those involving ethynyl,
vinyl or chloroxyl radicals? In fact, for
many such systems theoretical methods
have been developcd to such an cxtent
that, after first carefully cvaluating the
available methods and choosing the one
best suited to the problem, it is possible to
calculate thc required thermodynamic
quantities with reasonable accuracy. In
some cases this gives vcry good results indeed, as exemplificd in thc article “Theoretical Studies of the Energetics of Radicals”, by Joseph S. Francisco and John A.
Montomery, Jr. One method for the direct
mcasuremcnt of reaction enthalpies in solution is photoacoustic calorimetry, described in the article by Joshua L. Goodman, with examples of applications to
photochemical reactions and to the reactions of 1,2- or 1,3-biradicals. The data
thus obtained can be used to cstimate thc
behavior of allowed or forbidden processes. By applying such Considerations to
free radical 1,2-rcarrangcincnts, which arc
common in thc chemistry ofcarbocations,
it has becn shown that thcrc is a high cncrgy barrier to such processes if a paramagnetic intermediate is involved. However,
in the articlc “Resonance and 1,2-Rearrangement Enthalpies in Radicals: From
Alkyl Radicals to Alkylcobalamins”,
Arthur Grccnberg and Joel F. Liebinan
describe how the lattcr reactions can be
carried o u t remarkably easily with the
help of the coenzymc B I Z . Against this
background onc can well understand why
the editors have included in this book on
free radicals the articlc “A Short and 11lustrated Guide to Mctal-Alkyl Bonding
Energetics”, by J. A. Martinho SoniBcs
and M. E. Minas D a Piedadc. This article
and the last one, which deals with a topic
that will undoubtedly remain vcry important in the future: “Solvent Effects of
Neutral Frec Radicals”, by James M.
Tanko and N. Kamrudin Sulenian, are the
two inost impressive in the book.
The book is intended mainly for advanced and postgraduate students whose
particular interests include organic and
physical Chemistry. It offers a thoroughly
up-to-date trcatmcnt of the methods, with
many refercnces to the relevant litcrature.
The introductory sections on basic concepts are seldom longer than necessary,
while adequate space is devoted to dcscribing the mcthods and critically cvaluating the extensive (and sometimes contradictory) literature data on the
thermodynamics of frce radical reactions.
Such an evaluation is an important aid for
anyonc involved in mechanistic studies or
synthetic work using free radicals. The
tables of thcrniochemical data and the dcscriptions of the methods used to determine them will cnablc the reader to quickly gain an appreciation of the factors
affecting thc quality of such data.
Jens Hurtzing
Institut fur Organischc Chemie
dcr Universitiit Wiirzburg (Germany)
The Chemistry of Free Radical Polymerization. By G. Moad and D . H .
Solomon. Pergamon/EIsevier, Oxford, 1995. 408 pp., hardcover
E 75.00. - ISBN 0-08-42078-8
Interest in free radical polymcrization
has increased greatly in the last fcw years.
This is due mainly to a combination of
two factors. Firstly, thc conventional thcories d o not give a coinplctely satisfactory
description of free radical polymerization; in particular, thcy d o not cnable one
to predict the dcpendcnce of the polymer-
ization rate and the polymer microstructure on the reaction conditions, the proportion of monomer converted, and the
molecular mass distribution. Secondly,
new techniques can now yield detailed information about the polymer microstructure and the kinetics and mechanisms
of polymerization, leading to a more
thorough and practically useful knowledge of the relationships between mechanism, structure. and properties. This now
makes i t possible to obtain homopolymers and block copolymers with a narrow
range of molecular variation by the free
radical polymerization route.
These recent developments were the
main reason that motivated the authors to
write this book. In it they review and evaluate the most recent published work, and
take a fresh look at some older literature,
occasionally reinterpreting it when necessary. The main purpose is to arrive at a
better understanding of free radical polymerization, and to clarify the relationship
between reaction conditions and the
structurc and properties of the resulting
material. so that one can produce polymers with predictable and reproducible
properties. This does not, of course, mean
throwing out all our previous knowledge
about free radical polymerization or
standing it on its head, and therefore the
structure of the book essentially follows
the conventional sequence: initiation,
then growth, then chain termination.
The first chapter is an introduction, in
which the authors begin by explaining the
reasons for the technological success of
free radical polymerization. The most important relationships governing the process were discovered as early as the 1940s
and 1950s, but studies of the role of defect
structures in determining the properties of
the polymers are more recent. This latter
topic recurs constantly through all the
chapters, with particular emphasis on the
mechanisms that cause such defects and
on techniques for studying them.
The following chapter is devoted to free
radical reactions, and begins by considering how radicals react with carbon-carbon double bonds. The authors explain
concisely and clearly how these reactions
are affected by steric and polar factors
and by the conditions under which they
occur, such as the temperature and the
solvent, and they discuss the theory of
these effects. A separate section explains
the importance of the transfer of hydrogen atoms from the substrate to the radicals, and describes the steric, polar, and
stereoelectronic factors that control this
transfer. The role of the competition between addition and transfer processes is
described, and it is shown that this compeA n g w . Clicm In[. E d Enxl. 1997, 36. No. 5
tition can be controlled by altering the nucleophilicity of the radicals. This chapter
also discusses radical-radical reactions
and the factors that control the competition between recombination and disproportionation. The chapter contains a
comprehensive bibliography, including
both recent publications and references to
important earlier work.
Chapter 3 (about 100 pp.) is concerned
with initiation, and includes critical appraisals of both recent and earlier studies.
It is shown how the initiation stage affects
not only the kinetics of the reaction but
also the properties of the polymer obtained. Particular attention is given to
methods for characterizing the endgroups and polymer defects caused during
initiation. First the various types of possible reactions are described with examples; these include reactions with the
monomer, with solvents, with oxygen,
with radicals, and with impurities, and also fragmentation reactions. The types of
compounds that can be used for generating free radicals are described in detail.
This chapter contains much interesting information about the many facets of free
radical chemistry. The chemistry involved
in the initiation of a polymerization reaction is remarkably complex and absorbing, and can certainly not be reduced
to a straightforward and rather uninteresting process in which a radical is formed
and simply adds on to a carbon-carbon
double bond. The authors thoroughly and
conscientiously summarize the various
methods that have been or can be used for
analyzing the products and thereby investigating the different reaction pathways.
The text is backed up by 480 literature
references. This excellent chapter is a mine
of information for polymer chemists and
organic chemists. However, in a future
edition of the book, which I hope will appear in due course, the authors should
devote more space to the numerous studies on asymmetrical azo compounds that
have been reported by our group and by
that of Ruchardt.
Chapter 4, dealing with growth reactions, is much more interesting than even
the specialist in the field might at first expect to find. The many different reactions
that the growing radicals can undergo are
described very competently. The question
of tacticity is discussed concisely but in an
understandable way; the authors explain
why this is not such a n important problem
in the case of free radical polymerization.
Considerable space is devoted to the analysis of defect structures, such as head-tohead linkages. The general principles are
reinforced by practical examples and by
well-planned tables and diagrams. The
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section on the polymerization of cyclic
monomers with vinyl substituents is especially well worth reading. Some space is
also devoted to the polymerization of
double-ring compounds such as spiro-orthocarbonates. However, the comment on
page 182 concerning the volume expansion of monomers of this type during
polymerization is not supported by the
latest knowledge.
Chapter 5 is concerned with chain termination. One’s first reaction on finding
70 pages devoted to a reaction that textbooks often dismiss in only a few lines is
that the authors have overdone this. However, that impression changes as one reads
this very important chapter. Every kind of
event that can end the growth of an active
chain is covered here, for example a reaction between two polymeric radicals, termination of the chain by addition of a
primary radical, chain transfer reactions,
and inhibition processes. After discussing
these in a general way, the authors describe in detail how they relate to several
actual polymers. It is especially pleasing
that the discussions are reinforced by descriptions of experiments. To enable the
reader to study growth reactions in more
depth, nearly 300 relevant literature references are given at the end of this chapter.
Chapter 6, on copolymerization, is
much shorter than the previous ones, but
is nevertheless a lucid treatment, covering
all the most important aspects of the terminal model, the penultimate unit model,
and the monomer complexes model. It is
pleasing to find that methods for preparing block copolymers and graft copolymers are also included, although the treatment of these is too brief.
Chapter 7, on controlled polymerizations, is excellent. The authors describe
methods for the control of microstructure,
functionality, stereochemistry, molecular
size, and molecular mass distribution. The
most recent published work is reviewed
and expertly evaluated. They even include
several recent publications on the possibility of “living free radical polymerization”.
I can unhesitatingly recommend this
highly informative and comprehensive account of the present state of knowledge on
free radical polymerization. Scientists in
universities and industry will find it extremely useful. It should also be of considerable help to advanced degree and postgraduate students who include polymer
chemistry in their interests. 1 value the
book as an important addition to my personal library.
Oskar Nuyken
Institut fur Technische Chemie
der Universitat Munchen, Garching
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