genesis of the csiro polymer group and the discovery and significance of nitroxide-mediated living...

HIGHLIGHT

Genesis of the CSIRO Polymer Group and the Discoveryand Significance of Nitroxide-Mediated Living RadicalPolymerization

DAVID H. SOLOMONDepartment of Chemical & Biomolecular Engineering, University of Melbourne,Parkville, Vic 3010, Australia

Received 2 August 2005; accepted 5 August 2005DOI: 10.1002/pola.21067Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The background to

the formation of the Commonwealth

Scientific and Industrial Research

Organization (CSIRO) polymer

group is discussed. In particular,

the challenges of working with high-

conversion polymerization, as found

in commercial systems, and the need

to explain variations in polymer

properties led to important advan-

ces in the theory of radical poly-

merization and control over both

the initiation and termination steps.

Studies on the fate of the macro-

monomer, formed in termination

by disproportionation, led to an

early form of addition/fragmenta-

tion now known as reversible addi-

tion–fragmentation chain transfer,

whereas detailed studies on initia-

tion pathways using nitroxide trap-

ping led to nitroxide-mediated liv-

ing radical polymerization. These

studies contributed to the renais-

sance in free-radical polymerization

studies. VVC 2005 Wiley Periodicals, Inc. J

Polym Sci Part A: Polym Chem 43: 5748–

5764, 2005

Keywords: initiation; nitroxide

mediated living radical polymeriza-

tion (NMP); radical trapping and

kinetic control

David H. Solomon is currently a professorial fellow at the University

of Melbourne, where he was previously the head of the School of

Chemistry. Before that, he was the foundation chief of the Common-

wealth Scientific and Industrial Research Organization (CSIRO) Divi-

sion of Applied Organic Chemistry (now Molecular Science), where he

established and led the polymer research group. His original employ-

ment at Dulux Paints was the foundation and pathway for his interest

in polymer science.

In 1990, he accepted an invitation to a Chair at the University of

Melbourne, where he formed the polymer research group.DAVID H. SOLOMON

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 5748–5764 (2005)VVC 2005 Wiley Periodicals, Inc.

Correspondence to: D. H. Solomon (E-mail: [email protected])

5748

INTRODUCTION

In the 1960s and early 1970s, radical polymerization

was considered a mature science; the majority of poly-

mer scientists accepted that the definitive texts had

been written and that the detailed mechanisms for ini-

tiation, propagation, and termination were understood.

One leading researcher, Mayo,1 even went so far as to

suggest, in an article entitled Whither Free Radical Poly-merization, that

Those who want to pioneer on the borders of

polymer science will probably find better opportu-

nities than in free-radical polymerization.

Contrast this state of affairs with the renaissance in

radical polymerization that began in the late 1970s: today,

radical polymerization, particularly living polymeriza-

tion, is one of the most active areas in polymer science.

In this article, I will review the work carried out in

the Commonwealth Scientific and Industrial Research

(CSIRO) polymer group, which I formed in the Division of

Applied Organic Chemistry (later Chemicals and Polymers

and now Molecular Science) in 1974. In particular, I will

discuss how our work contributed to the significant change

in the approach to the theory of radical polymerization

(kinetic vs thermodynamic control), studies on macromo-

nomer copolymerization, which gave an early form of what

is now called reversible addition–fragmentation chain

transfer (RAFT), and the use of nitroxides as trapping

agents, which in turn led to nitroxide-mediated living

radical polymerization (NMP).

I will also indicate how the strong influence of

working with commercial polymer systems led to stud-

ies not normally undertaken in academic research.

BACKGROUND

I was fortunate to spend the early part of my career

(1959–1960) in the surface coating industry, and I was

actively researching new polymers at the time vinyl

and acrylic monomers became commercially available.

At that time, the choice of initiators for radical poly-

merizations was largely dictated by cost, critical tem-

perature, and availability. The different chemical end-

group structures expected from the individual initiators

were not considered significant; after all, they repre-

sented less than 0.2% of the polymer.2

He was appointed to the Order of Australia in 1990 for his services

to science and for his invention of the plastic banknote.

Prof. Solomon holds Ph.D. and D.Sc. degrees from the University

of New South Wales and a diploma in chemistry from the Sydney

Technical College.

He has worked across the field of polymer science and published

reference books on Step-Growth Polymerization, The Chemistry of

Organic Film Formers, The Chemistry of Pigments and Fillers, and The

Chemistry of Free-Radical Polymerization. The last was a significant

contribution to our present views on radical polymerization. In

addition, he has published 253 papers and holds 43 patents, includ-

ing that which describes the original nitroxide-mediated living

radical polymerization.

He has always been active in the Royal Australian Chemical Insti-

tute, being national president in 1979–1980. He was a prime mover

in the formation of the polymer division of the institute and was

foundation secretary and then chairman in 1966–1967.

Prof. Solomon is a Fellow of the Royal Society (London), the Australian

Academy of Science, the Australian Academy of Technological Sciences

and Engineering, and the Royal Australian Chemical Institute. He was

the sole recipient of the Australian Bicentennial Science Achievement

Award for his role in the invention and then commercialization

of the world’s first plastic banknote with greater security against

counterfeiting.

HIGHLIGHT 5749

Hence polymers were represented by general for-

mula 1:

This was satisfactory when physical properties were

evaluated but, as I will show, was not adequate when

chemical properties were considered.

From practical observations of poly(methyl meth-

acrylate) (PMMA) and polystyrene (PS), I knew that

polymers of the same molecular weight within each

series showed significant differences in their resistance

to weathering and that these differences could be

related to the polymerization conditions, including the

choice of initiator. Others had published similar obser-

vations.

There was growing evidence that anomalous ordefect structures could be formed at each of the initia-

tion, propagation, and termination steps. Examples arereported in our book2 and in the listed references.3,4 I

will illustrate the significant effect of one defect group

per polymer chain by using the example of PMMA.Radical-polymerized PMMA is less thermally stable

than a similar molecular weight polymer prepared byan anionic mechanism. Obvious differences between

the two polymers are the groups formed in the termi-

nation step: the head–head linkage and the unsaturatedend group in the radical polymer. We later showed that

such groupings or defects are the weak links at which

depolymerization is initiated in the radical polymer, as

reported by Cacioli et al.5 and Moad et al.6

There was also building evidence both from obser-

vations of polymers and from model systems that pro-vided strong grounds for questioning the exclusivity of

each of the steps in the traditional radical mechanism.

Of equal importance was the growing recognition thatstereoelectronic and polar effects were the dominant

factors in determining the outcome of radical reactions(kinetic control), not the product radical stability (ther-

modynamic control). I published on this as early as

1975.7 Our work on the structure of the cyclopolymersformed from 1,6-diallylamines was to prove the ex-

treme case demonstration that radical stability was notthe dominant factor.

POLYMERIZATION OF 1,6-DIALLYLAMINES

In the early 1970s, we were synthesizing polydiallyl-

amines for use in a water purification process. Diallyl-

amines had been shown to undergo a unique intramo-

lecular–intermolecular polymerization. The formation

of a cyclic structure had been established, and on the

basis of the existing theory that the thermodynamic

stability of the radical formed was the controlling fac-

tor, a six-membered ring was assumed to be formed.8

However, there was no direct experimental evidence to

support this ring size.

Mainly for scientific thoroughness, but also because of

some doubt we had concerning the prevailing theory, we

wanted to prove conclusively the ring size; structures

could be written for a five-, six- or seven-membered ring

(Scheme 1).

Scheme 1. Cyclopolymerization of 1,6-diallylamines (X ¼ N��R1 and R1 ¼ H or alkyl).

5750 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 43 (2005)

In a series of publications, we showed by the use

of models, by trapping with excess initiator, by elec-

tron spin resonance (ESR), and then by NMR of

the polymers that exclusive formation of the kinet-

ically preferred five-membered rings occurred.9,10

The NMR study included the use of a 13C recovery

technique to identify the end groups. Later, we

extended the use of NMR by using 13C-enriched ini-

tiators.11–16

This finding was a major step on the path to rewrit-

ing the theory of radical polymerization. Thermody-

namic control was not dominant: kinetic factors were

of greater significance. We had published our views on

kinetic control versus thermodynamic control and con-

tinued to do so over the years.17

In 1990, Graeme Moad and I wrote a review on this

fundamental change to our understanding of radical

polymerization,18 and this was a precursor to our

books, The Chemistry of Free Radical Polymeriza-tion19 and the revised edition, The Chemistry of Radi-cal Polymerization,20 due out this year.

This realization of the importance of kinetic factors

was an added reason for doubting the exclusive addi-

tion to the tail of the monomer and provided a theoret-

ical reason why we expected other pathways in the ini-

tiation step.

INDUSTRIAL CHALLENGES

In the commercial systems on which I had worked in

industry, we were concerned with a high conversion of

monomer to polymer. A high monomer conversion

raises questions not addressed in most academic stud-

ies, in which a low conversion is deliberately chosen

to simplify the analysis and interpretation of results.

For example, what is the initiator efficiency, does it

vary throughout the polymerization, and is the solvent

critical in determining polymer properties? In polymer-

izations that terminate by disproportionation, what is

the fate of the unsaturated terminated macromonomer?

Does it copolymerize and form grafts? If so, the struc-

ture of the polymer will change from being linear to a

mixture of linear and grafted chains with conversion.

This was to become a critical study leading to an early

example of what is now called RAFT.21,22 At high

conversions, the shortcomings of both the theory and

the published values for reactivity ratios in copolymer-

ization became apparent, and this also became a ques-

tion that we studied.

Another industrial observation made during my time

in the mineral industry, where I was concerned with

polymer/mineral composites, was to also play a pivotal

role in setting our polymer research program. The ren-

dering of clay (kaolin) particles organophilic for use as

a pigment or filler in polymer composites has been

pursued by many. In my own particular case, we were

using oleic acid to form an organophilic clay in which

the oleic acid, through its double bond, would provide

a reaction point for grafting or reacting into the poly-

mer matrix, the so-called graded seal structure for rein-

forcing composites.23

This was an exciting product to make; some batches

underwent spontaneous combustion! We devised a

process to avoid the spontaneous combustion, but I

needed an explanation for this unusual chemistry.

Some oxidation process seemed to be occurring, but its

exact nature was unclear. Oleic acid is the major fatty

acid of olive oil, and I knew from my experience in

the paint industry that this oil auto-oxidized only

slowly even in the presence of a transition-metal cata-

lyst; the mechanism is reported in my book entitled

The Chemistry of Organic Film Formers.24

We had been studying the effects of clay on poly-

merizations and on peroxide decomposition and had

shown that the strong acid surface that develops when

a clay is dried, even under mild conditions, catalyzes

cumyl peroxide decomposition by an ionic mechanism

(Scheme 2).

However, in the case of oleic acid, I had postulated,

in addition to the ionic decomposition, some acid-

induced homolysis of the aliphatic hydroperoxide

(from oleic acid) and subsequent free-radical chain

reaction to explain the exothermic and spontaneous

combustion.

The clay/oleic acid/air system is difficult to study

experimentally, so I set up a model system to study this

chemistry. Tertiary butyl hydroperoxide was chosen as a

model for the oleyl hydroperoxide postulated as the first

oxidation product of oleic acid, and sulfuric acid was

Scheme 2. Decomposition of cumyl peroxide via clay catalysis.

HIGHLIGHT 5751

used as the equivalent of the clay acidity.25 I chose

methyl methacrylate (MMA) as a solvent because we

expected the radical pathway to be the minor one and

a solvent that polymerized by a radical mechanism

would give a high yield of the polymer for even a few

radicals. Ezio Rizzardo joined the team at this time

and this was to be his introduction to polymer science.

PMMA was formed, and this led to a number of

related studies, including the acid-induced homolysis

of tertiary butyl hydroperoxide.26–28

We carried out traditional organic chemistry on the

PMMA formed. The t-butoxy end groups were cleaved

with boron trichloride to form t-butyl chloride. The

yield, however, was only about 60% of that necessary

for a stoichiometric balance. We then located the

remainder as t-butanol.29

Thus, we knew by 1979 that the initiation of MMA

by t-butoxy radicals was far from that shown in the text

books, in which exclusive addition to the tail of the

monomer was reported. What we did not know was the

extent of head and tail addition or where the hydrogen

abstraction was occurring. I will return to this shortly.

Other groups were also beginning to question the

exclusivity of the steps in radical polymerization. For

example, the ESR study by the Kochi group30 showed

that head addition was possible and that a mixture of

initiating radicals occurred with some monomers. Thus,

the scene was set to revisit radical polymerization.

The quote in the Mayo reference1 was an added

challenge to overcome in convincing CSIRO to fund

our research. They set up a panel of overseas experts

to review both our work on step-growth polymerization

(not reported here) and our free-radical studies. The

panel recommended that I be encouraged to continue

with this work and that extra resources be provided if

possible. They also suggested that given the magnitude

of the discoveries, the universities should be encour-

aged to undertake research in these areas. Thus, the

collaboration began, with funding from CSIRO, of a

group at Griffith University.

Thus, the aims of the polymer group included stud-

ies of each of the steps in radical polymerization. In

particular, because of our interest in defect groups and

their effect on the chemical properties of polymers, we

needed to elucidate all the pathways, not just the major

one in the initiation, propagation, and termination steps.

Additionally, and of critical importance, we aimed to

address the questions posed above for high-conversion

polymerization.

Summaries of these studies can be found in the pub-

lications of our group.19,20,31

In this review, I will briefly mention selected examples

of our work that had significant influence on the future

directions of radical chemistry, particularly early examples

of what was to become know as RAFT21,22 and the focus

on the initiation studies that were to lead to NMP.

COPOLYMERIZATION OFMACROMONOMERS

This study was initiated to address the question that I had

raised earlier, namely, in polymerizations that terminate

by disproportionation, does the unsaturated polymer, a

macromonomer, become incorporated into the structure at

a high conversion. I had asked Geoff Hawthorne to repeat

the work of Enikolopyan et al.32 with cobalt porphyrin,

and so within our laboratories, we were familiar with the

synthesis of low-molecular-weight models for the dispro-

portionation product. Using these models, we were able

to show that copolymerization took place with the PMMA

macromonomer but that the incorporation per polymer

chain was low with MMA and much higher with ethyl

acrylate, styrene, acrylonitrile, and vinyl acetate.33

The study showed that the macromonomer readily

added cyanoisopropyl radicals and did not homopoly-

merize and that the radical formed by addition did not

self-terminate by combination or by reaction with a

cyanoisopropyl radical. Importantly, the molecular

weight of the PMMA formed in the presence of the

methacrylate macromonomer was significantly lower

than that obtained in the absence of the macromono-

mer. This was explained by facile b scission of the

adduct radical, as shown in Scheme 3.

Thus, this was an early form of RAFT. Later, the

CSIRO group, particularly Graeme Moad, Ezio Riz-

zardo, and San Thang, developed the highly successful

thiocarbonylthio compounds as much more efficient

agents, and they coined the acronym RAFT to describe

such systems.22,34–36

STYRENE–BENZOYL PEROXIDE SYSTEM

Studies on the polymerization of styrene exemplify

clearly a number of the original aims of the research

group: the differences between initiator-derived radi-

cals in reactivity and selectivity, the formation of some

head addition radicals from the monomer, the change

in the initiator efficiency with the conversion, the

effect of a high conversion of the monomer on the

structure of the polymer formed, and the adverse effect

of a small number of unexpected (based on traditional

theories) end groups on the polymer properties.

Head Addition

Although the cyanoisopropyl and the t-butoxy radicals

added exclusively tail to styrene, we were able to show

that benzoyl peroxide derived radicals gave an interest-

ing mixture of products, as shown in Scheme 4.37–39


The 6% head addition radical (2) is highly signifi-cant. It is powerful supporting evidence that kinetic

factors dominate over thermodynamic considerationsbecause this radical (2) lacks the resonance stabiliza-

tion of the tail addition radical (3) and would not be

expected if thermodynamic stability were the control-ling factor in radical addition.

Effect of Conversion

On Initiator Efficiency

Many researchers still assume initiator efficiencies of

100%. In our studies on styrene, the azobisisobutyroni-

trile (AIBN) efficiency was shown to vary from 76%

Scheme 4. Products from the reaction of benzoyl peroxide and styrene.

Scheme 3. b scission of the cyanoisopropyl–MMA macromonomer radical adduct.

HIGHLIGHT 5753

at a low conversion down to <20% at a 90–95% con-

version (Fig. 1).31

On Polymer Structure

At high conversions, we were able to show that addi-

tional secondary benzoates, similar to those formed by

head addition in initiation, arise in the termination step

by either primary radical termination or chain transfer

to initiator (Scheme 5).

Thus, the balance between the different end groups

in benzoyl peroxide initiated PS changes with conver-

sion. At a high conversion, PS contains many more

chains terminated by secondary benzoates.37,40

On Properties

We were able to show that secondary benzoate esters

are less thermally stable than the primary ester formed

by tail addition. The primary benzoates persist (>50%

at 300 8C under nitrogen), whereas the secondary ben-

zoates are extremely labile under these conditions

(half-life at 300 8C < 5 min).41,42

These studies highlight the need to specify in detail

the history of polymer samples when properties are

reported. All PSs prepared from benzoyl peroxide are

not the same and will vary with the polymerization

conditions, solvents used, conversion, and so forth.

Thus, the styrene studies clearly showed the need

for careful selection of the initiator, the change in the

thermal stability with conversion in the benzoyl perox-

ide system, and the effect of the conversion on the ini-

tiator efficiency.

MMA–t-BUTOXY SYSTEM

Effect of the Solvent on End Groups

The variety of products formed in the reactions of

t-butoxy radicals with MMA in the absence of a solvent

Figure 1. AIBN initiator efficiency versus the conversion percentage of styrenemonomer with toluene as the polymerization solvent.

Scheme 5. Termination by primary radical termination or chain transfer to initia-tor (BPO ¼ benzoyl peroxide).


(monomer as the solvent) is discussed in Scheme 9. In

commercial polymerization systems, a solvent is a ne-

cessity, and toluene and 2-butanone, commonly called

MEK, are often used. Residues of these solvents can

be incorporated as the end groups, and this is conver-

sion-dependent; at high conversions, more solvent-

derived groups are found. However, by the careful

selection of the solvent and initiator, it is possible to

form the majority of end groups from solvent-derived

radicals formed by abstraction. For example, when tet-

rahydrofuran (THF) is used as solvent in a t-butoxy-initiated MMA polymerization, virtually no end groups

result from the direct interaction of the t-butoxy radi-

cals with the monomer.43

The use of solvents that compete with the monomer

in a reaction with t-butoxy radicals (e.g., cyclohexane

and THF) is a way of minimizing initiator-derived end

groups, and the use of functional solvents offers the

possibility of end-functional polymers (Scheme 6).43

NITROXIDES AS TRAPPING AGENTS

Our aim in studying initiation was to determine all

pathways, not just the major one, which in most cases

we expected to be tail addition. So the challenge was

to find a technique that would establish all pathways in

the initiation step.

Thus, we considered that 13C NMR and ESR tech-

niques being developed by ourselves and others lacked

the sensitivity necessary to detect these minor pathways.

The stable free radical diphenyl picrylhydrazyl (DPPH)

had been used as a diagnostic test for free-radical poly-

merizations. It was claimed to form DPPH–H by the

abstraction of a hydrogen from the initiating or propa-

gating radical species; that is, the interaction was one

of disproportionation between the two radicals. How-

ever, with Geoff Hawthorne, I had previously used

DPPH in mechanistic studies and shown that the chem-

istry was not straightforward, and a complex mixture

of products could form.44 Scheme 7 shows our results

for its reaction with AIBN.

Moreover, DPPH reacts with minerals and with

acids.45,46

Thus, we needed a trap that would preferably add to

the initiating radicals (i.e., the first radical formed from

the monomer) to form stable products that we could

isolate, separate, and characterize and thereby delineate

the initiation pathways.

A trap that abstracted hydrogen from the initiating

radical would, for example, give another monomer that

could undergo further reaction and so complicate prod-

uct analysis (Scheme 8).

Alkoxyamines were, and still are, used as stabil-

izers in polymer composites, and nitroxides were

known to be inhibitors of radical polymerization. So

we eventually chose as our trapping agent 2,2,6,6-tet-

ramethylpiperidinyl-1-oxy (TEMPO or 4), a nitroxide

that combines with carbon-centered radicals at or near

diffusion-controlled rates to yield stable alkoxyamines.

Later, we greatly extended the range of nitroxides, includ-

ing the new isoindoline nitroxide [1,1,3,3-tetramethylisoin-

doline-2-oxyl (5)]; this proved invaluable because the

aromatic ring allowed for easy detection and quantifi-

cation of the separated alkoxyamine by ultraviolet

spectroscopy (Scheme 9):47–49

Scheme 6. Solvent-derived end groups arising from t-butoxy-initiated radicals inMMA polymerization.

HIGHLIGHT 5755

Because the nitroxides did not react with hetero-

atom-centered radicals, they were ideal for studying the

reactivities of monomers with the common initiators

formed from oxygen-centered radicals, that is, benzoyl

peroxide or t-butoxy sources such as t-butyl hydroper-oxide and di-t-butyl peroxide. In these systems, we

trapped the first monomer-derived radical (Scheme 9).

Some radicals form by unimolecular rearrangement

or fragmentation of the primary radicals to form second-

ary radicals. In this case (Scheme 9), the b scission of

the t-butoxy radicals to methyl radicals that form 10 or

11 and acetone had been studied previously, and the

absolute rate constant was known.48 Thus, the extent of

these b-scission reactions can be used as a radical clock

to calibrate the absolute rate constant of the bimolecular

reactions between the primary radicals (i.e., t-butoxy)and monomer.

Our extensive studies on the initiation processes

using nitroxides as trapping agents are reviewed in a

number of references from our group.19,20,50

I was fortunate to have recruited scientists who

were not only excellent theorists but also gifted experi-

Scheme 8. Generation of monomer via hydrogen ab-straction from an initiating radical.

Scheme 7. Some of the products from the reaction between DPPH and AIBN.


mentalists. They isolated compounds present in minor

amounts, often less than 1%, and noted properties of

the alkoxyamines that collectively led to NMP. Fol-

lowing is a summary of these observations:

1. Compounds with more than one monomer resi-

due were isolated. Thus, with MMA, compound

949 could be theoretically formed from the first

alkoxyamine (8) by thermal dissociation, the

addition of the monomer, and then trapping

(Scheme 10). Similarly, styrene compounds 12and 1338 could be formed by the thermolysis of

14 and 15, respectively, and the insertion of a

monomer unit (Scheme 11). (We did not favor

these mechanisms, and our publications give alter-

native pathways.)

Scheme 10. Possible mechanism for the formation of MMA dimer from the ther-molysis of alkoxyamine.

Scheme 9. Products from the reaction of t-butoxy radicals with MMA in the pres-ence of 5.

HIGHLIGHT 5757

2. In the separation of alkoxyamines, their instabil-

ity in a gas–liquid chromatography column was

noted.51

3. In the recrystallization of some alkoxyamines, the

use of solvents with high boiling points resulted in

the formation of the characteristic color of the ni-

troxide. We therefore routinely used solvents with

low boiling points or solvent/nonsolvent mixtures

to recrystallize alkoxyamines; this provided com-

pelling evidence for the reversibility of alkoxy-

amine formation.52

4. Other observations on the reversibility of the

alkoxyamines were as follows:

a. Graeme Moad and I noted that the cis–trans

ratio of styrene dimers changed on heating

(Scheme 12).53

b. The allylic alkoxyamine from hydrocarbons

reverted to an equilibrium mixture of products

on heating (Scheme 13).54

5. Solvent effects showed that highly polar solvents

(methanol and acetic acid) caused alkoxyamine

mixtures to rearrange.43

Scheme 12. Interconversion of cis–trans isomers of styrene dimers.

Scheme 11. Possible mechanism for the formation of styrene dimer from the ther-molysis of alkoxyamine.


Thus, from these observations, the idea developed of

forming low-molecular-weight oligomers from alkoxy-

amines by the variation of reaction conditions such as

the temperature, time, and solvent.

NMP

Compound 16 was readily synthesized and heated with

methyl acrylate (17) under a variety of conditions to

yield oligomers of general formula 18 (Scheme 14).

Thus, NMP was born.

In the 1984 patent,55 we described nitroxides and

alkoxyamines as a route to controlling the growth of

the radical polymerization of a range of monomers,

including acrylates, methacrylates, styrenes, and vinyl

acetate. The terms living, controlled, and step-growthwere used in the patent. The last was in reference to

the then used terminology of Paul Flory to describe

polymer formation. Although strictly not correct, it

was meant to indicate that each polymer chain grew in

a series of discrete monomer insertions and not by a

single chain polymerization.

This patent had a nondescript title, PolymerizationProcesses and Polymers Produced Thereby, and was

issued in April 1986. For commercial reasons, we were

not permitted to publish these data in the open scien-

tific literature. We did, however, reference the patent

in a number of articles published in conventional jour-

nals. Unfortunately, recent reviews and texts on NMP

clearly indicate that this original disclosure has not

been studied, and significant errors of fact have been,

and continue to be, propagated in the literature.

The original 1986 patent reports 50 examples that

detail nitroxide synthesis, alkoxyamine preparation and

properties, and various polymerizations. I have sum-

marized the disclosures of the patent as follows:55

Scheme 14. Oligomers of general formula 18. n rangedfrom 1 to 70.

Scheme 13. Equilibrium mixture of allylic alkoxyamine from hydrocarbons.

HIGHLIGHT 5759

Table 1. Structures: Patent Examples55

1.1. Preparation of Nitroxides 1.3. Controlled Homopolymer

1.2. Various Alkoxyamines 1.4. Controlled Block Copolymer


1. The synthesis, characterization, and use of a

range of nitroxides (19–23; Table 1.1).

2. The determination of nitroxide interchange, evi-

dence for the free-radical nature of the NMP process.

3. The synthesis and characterization of alkoxy-

amines (Table 1.2):a. With monomer unit insertion (24–26). The

presence of functional groups in

b. The alkyl portion (27–30).c. The nitroxide entity (31).

4. Stability of alkoxyamine (16, 24–26, 28, and 31–33; Table 2):

a. Effect of solvent: More polar solvents increased

the rate of dissociation of alkoxyamine.

b. Nitroxide structure: In general, steric crowding

enhanced the rate of dissociation.

c. Alkyl structure: The alkyl stability was in the order

of methacrylonitrile (MAN) < a-methylstryrene

(MeS)<MMA with tetraethyl isoindoline (20).

Table 1. Continued

1.5. Graft Copolymers

HIGHLIGHT 5761

5. Control of polymer architectures:

a. Homopolymers (34 and 35; Table 1.3).

b. Block copolymers (36–38; Table 1.4).

c. Graft copolymers (40; Table 1.5).

i. By forming alkoxyamine on the backbone

by copolymerizing the unsaturated alkoxy-

amines.

ii. By forming polymeric alkoxyamine by ab-

straction from a preformed polymer.

6. Location of specific end groups. By selection of

monomer mixtures, one terminal alkoxyamine is

stable under polymerization conditions.

7. Formation of macromonomers that are not avail-

able by other means.

8. Removal of alkoxyamine end groups (Table 3).

9. Formation of low-molecular-weight functional

polymers for further use in, for example conden-

sation polymerization, that is, diol.

NARROW-POLYDISPERSITY POLYMERSAND COPOLYMERS BY NMP

The molecular weight distributions (MWDs) of most of

the examples in this original patent were broad. However,

Graeme Moad recognized the potential of this living sys-

tem for preparing narrow-polydispersity polymers by

NMP. We published the theoretical requirements for nar-

row MWDs,56,57 and these have since been demon-

strated in a variety of systems by many workers.58

NMP received scant attention until Georges et al.58

used NMP to prepare narrow-polydispersity PS. There

followed a period of intense interest in NMP; for

example, the Hawker group59,60 added to the range of

suitable nitroxides and polymer structures. Thus, NMP

enabled the synthesis of a range of polymer structures

not previously attainable by radical chemistry, including

low-molecular-weight, narrow-polydispersity homopoly-

mers and copolymers (block and graft copolymers), as

well as many other intriguing structures not accessible by

other chemistries.

FUTURE

The original CSIRO alkoxyamine/nitroxide patent

expired this year and is no longer a restriction in the

commercialization of NMP by others. We can confi-

dently expect increased activity as systems described

in scientific conferences become commercial realities.

Other forms of controlled living radical polymerization

have been developed. In 1995, the groups led by Sawa-

moto61 and Matyjaszewski62 reported atom transfer poly-

merization, and more effective catalysts for these systems

were developed in the same year by the group led by

Percec.63 A major advance in 1998 was the discovery of

RAFT35,36 using thiocarbonylthio compounds by the

CSIRO team now led by Ezio Rizzardo. Although each

technique has its virtues, combinations of these methodol-

ogies and the interconversion of one form to another are

opening up exciting possibilities for the synthesis of

highly controlled polymer structures by radical polymer-

ization. At the time of the discovery of NMP living ionic

systems were the standard we aimed to emulate; the sit-

uation has now changed drastically. Radical polymeriza-

tions now offer the versatility of radical chemistry, a

greater tolerance of impurities, and the ability to form

structures not accessible with ionic systems.

Thus, of the three classical steps in radical polymer-

ization, we now have control over initiation and termina-

tion. Control over propagation remains the holy grail,

Table 2. Solvent Effects: Patent Examples55

Solvent Alkoxyamine Half-Life at 60 8C (min)

Ethyl acetate 16 3324 7525 1026 18a

28 8.5a

31 13032 3133 280

Light petroleum 16 38Acetonitrile 16 22Dimethylformamide 16 20Methanol 16 17Methanol/water (9:1) 16 16Methanol/acetic acid (9:1) 16 15

a Half-life at 40 8C.


and when this is achieved, radical polymerizations will

offer unlimited control over polymer architecture using

easily achievable reaction conditions.

The author is pleased to acknowledge Geoff Hawthorne,

Graeme Moad, Ezio Rizzardo, Algi Serelis, and San Thang. All

joined his group with strong backgrounds in physical organic

chemistry but with no previous polymer experience, and it has

been most satisfying for him to work with them and see them

develop. More recently, at the University of Melbourne, Greg

Qiao has undergone a similar transition, and his valuable contri-

butions are also a pleasure to acknowledge. Tiziana Russo’s

help in preparing this article is also very much appreciated.

Duncan McAllister’s input is also acknowledged.

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genesis of the csiro polymer group and the discovery and significance of nitroxide-mediated living...

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