COMMUNICATION www.rsc.org/polymers | Polymer Chemistry

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Use of spin traps to measure the addition and fragmentation rate coefficientsof small molecule RAFT-adduct radicals†

Elena Chernikova,*a Vladimir Golubev,a Anatoly Filippov,a Ching Yeh Linb and Michelle L. Coote*b

Received 2nd August 2010, Accepted 16th August 2010

DOI: 10.1039/c0py00245c

Spin traps have been used to obtain the first direct experimental

determination of the addition (5 � 106 L mol�1 s�1), fragmentation

(8 � 10�3 s�1) and self-termination (6.5 � 102 L mol�1 s�1) rate

coefficients in RAFT polymerization using the interaction of a tert-

butyl radical with tert-butyl dithiobenzoate at ambient temperature

as a model reaction.

Since the invention of the addition fragmentation chain-transfer

(RAFT) polymerization process,1 there has been an ongoing debate

about the origin of retardation and inhibition effects in polymeriza-

tions mediated by cumyl dithiobenzoate and related dithiobenzoate

agents.2,3 On the one hand, a kinetic model that assumes that inter-

mediate radical cross- and self-termination reactions (IRT) occur

with diffusion-controlled rate coefficients, similar to those for the

bimolecular termination of conventional propagating polymer radi-

cals (i.e. <kt> < ca. 107–108 L mol�1 s�1), can be fitted successfully to

experimentally determined overall reaction rates and ESR-derived

radical concentrations.4 However, this so-called IRT model predicts

that significant concentrations of the IRT products should be

produced even under standard RAFT conditions and these are not

observable in significant quantities in the resulting polymer except

under forcing conditions.5 Furthermore, the fragmentation rate

coefficient (kfr¼ 104 s�1) and equilibrium constants (K¼ kad/kfr¼ 55

mol L�1, where kad is the rate coefficient for addition) that are

obtained under the IRT model are incompatible with those predicted

from quantum chemistry,6 with radical storage experiments,7 and

with the kinetics of thioketone mediated polymerization.8 Many of

these problems can be addressed if one instead assumes that IRT is

not kinetically significant under normal polymerization conditions

(i.e., <kt> < ca. 104 L mol�1 s�1). In this case, model fitting to

experimental kinetic data indicates that the intermediate radicals are

much more stable (K¼ 1.06� 107 L mol�1) and there is no significant

IRT product formation.6c,9 However, this so-called slow fragmenta-

tion model also predicts intermediate radical concentrations that are

completely incompatible with the available ESR data, measured for

polymerizing systems. Resolving these inconsistencies and finding the

correct kinetic model for RAFT polymerization would improve our

aLenin Hills, 1, bld. 3, Polymer Department, Faculty of Chemistry,Lomonosov Moscow State University, Moscow, 119991, Russia. E-mail:chernikova_elena@mail.ru; Fax: +7 495 9390174; Tel: +7 495 9395409bARC Centre of Excellence for Free-radical Chemistry and Biotechnology,Research School of Chemistry, Australian National University, Canberra,ACT 0200, Australia. E-mail: mcoote@rsc.anu.edu.au; Fax: +61 2 61250750

† Electronic supplementary information (ESI) available: Additionalkinetic and computational data, and full descriptions of theexperimental and theoretical procedures. See DOI: 10.1039/c0py00245c

This journal is ª The Royal Society of Chemistry 2010

understanding of the process and assist in process optimization and

control.

Various proposals have been made to reconcile these conflicting

observations. Buback et al.10 have suggested that the absence of IRT

products could be explained if the IRT product reacts with another

propagating radical to form a terminated chain and re-form the

intermediate radical; however, direct evidence for the existence of this

reaction has yet to be provided. Moreover, such a revised IRT model

still predicts fragmentation rates and equilibrium constants that are

incompatible with the results from quantum chemistry, with radical

storage experiments and thioketone-mediated polymerization. In

a further alternative, Konkolewicz et al.11 recently suggested that all

incompatible observations might be reconciled by allowing for

a strongly chain length dependent version of the IRT model with

concomitant slow fragmentation; however, the physical basis for such

strong chain length effects has been questioned in the literature.3 This

is not to say that either of these proposals are incorrect, rather more

evidence is required, preferably based on direct experimental

measurements of model compounds.3

To date, experimental measurements of the individual rate

coefficients have taken place within the context of a polymerizing

system and have necessarily involved the fitting of some type of

assumed kinetic model to the experimental data. As illustrated

above, these kinetic assumptions can have a large influence on the

resulting estimated parameters. In that regard, the use of quantum

chemistry is particularly attractive as, unlike experiments, such

calculations depend only on the laws of quantum mechanics and

the numerical approximations that are introduced to solve the

resulting equations. However, since the accuracy of such calcula-

tions can vary considerably with the level of theory chosen, external

experimental benchmarking is important. Experimentally, one can

reduce the dependence on model-based assumptions by carrying

out experiments in situations where the kinetic effects of these

assumptions are minimal, or by studying the reactions in isolation,

usually on much simpler model compounds. A promising example

of the former approach is a laser flash photolysis technique recently

introduced by Buback et al. and used to measure the fragmentation

rate and equilibrium constants for S–S0-bis(methyl-2-propionate)-

trithiocarbonate mediated polymerization of butyl acrylate in

toluene at�30 �C.12 It is worth noting that results generated in this

study, which were shown to be insensitive to the assumed rate of

IRT termination, are in very good agreement with the quantum-

chemical calculations.13 However, this experimental technique has

not as yet been applied to the more controversial dithiobenzoate

mediated polymerizations as the dithiobenzoate chromophore

absorbs in the UV region and undergoes photo-induced decom-

position. To study these systems an alternative approach is

required.

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To this end, in the present work we use a spin trapping technique

based on electron spin resonance (ESR) spectrometry to study the

kinetics and mechanism of the elementary steps of RAFT polymeri-

zation. Spin traps are strong inhibitors of radical reactions; hence

they are useful when the need arises to examine processes involving

unstable radicals in a wide range of temperatures. Once added to the

reaction mixture, the spin trap starts to capture active radicals

forming relatively inactive radical species, i.e. spin adducts, which

have characteristic ESR-spectra. It is important to note in this context

that spin traps do not react with stable or relatively stable radicals.14

These spin adducts are stable enough to be observed by ESR-spec-

troscopy; by integration of each of these signals, one can measure the

concentrations of each species and thereby study the kinetics of the

process. At the same time, the spin traps themselves act as a constant

‘‘radical vacuum cleaner’’, mopping up the released radicals and

preventing side reactions, such as termination processes or re-addi-

tion of radicals to the RAFT agent. This implies that the reaction

scheme can be greatly simplified. Using the model compounds

employed herein – rather than a full polymerizing system – the rate

and equilibrium constants can thus be measured more directly, with

significantly reduced dependence on model-based assumptions than

the previous ESR studies.4,12

A key challenge to applying this approach to dithioester RAFT

agents is the need to avoid the problem of photo-induced decom-

position. To this end, in contrast to experiments described before,12 all

of the experiments were carried out at room temperature using a spin

trap, 2-methyl-2-nitrosopropane (MNP) that functions simulta-

neously as a visible-light photoinitiator.

When irradiated by visible light, MNP decomposes and forms an

active tert-butyl radical (Scheme 1, reaction 1); in solvents like

benzene the tert-butyl radical rapidly interacts with another MNP

species and forms the stable nitroxide radical di-tert-butylnitroxide

(DTBN), which has its own specific ESR-spectrum (reaction 2).

When a RAFT agent such as tert-butyl dithiobenzoate (TB) is added

to the system, the tert-butyl radical can interact with it (reaction 3). It

is important to note that the presence of RAFT agent does not

influence the MNP decomposition under visible light; the use of

visible light (instead of UV) also avoids the problems of RAFT agent

absorbance and – importantly – its decomposition.

Scheme 1 The reactions taking place upon irradiation of the MNP–TB–

benzene system at 20 �C.

1438 | Polym. Chem., 2010, 1, 1437–1440

The observed ESR-spectrum for the system TB–MNP–benzene at

20 �C is given in Fig. 1, and is evidently a result of the superposition

of the signals attributed to the spin adduct DTBN (triplet with AN¼15.1 G) and multiplet (g ¼ 2.0041 G, AdH

¼ 0.42 G, AoH ¼ 3.65 G,

AmH ¼ 1.34 G, ApH ¼ 3.99 G) corresponding to the intermediate

radical (Int) formed in reaction 3. We have observed analogous

spectra before upon irradiation of MNP in benzene solution and

upon heating TB with AIBN, benzene and styrene;15 examples are

provided in Fig. S1 and S2 of the ESI.† As both of the signals are very

intense, we can use them to evaluate the concentrations of the

respective radicals: for DTBN the end component of the spectrum

was used for integration; for Int the third component was used.14b

During irradiation, the DTBN and the Int concentrations both

increase as a result of the trapping of tert-butyl radicals by MNP and

TB, respectively. After the irradiation is switched off the DTBN

concentration increases, while the Int concentration slowly decreases

(Fig. 2). This is because the intermediate radical has a finite lifetime

and it decomposes releasing tert-butyl radicals (reverse of reaction 3),

which are subsequently trapped by MNP forming DTBN (reaction

2).

If the irradiation time is noticeably less than the intermediate

lifetime (e.g., in the present work the irradiation time is 5 s whilst

the intermediate decays over several hundred seconds), we can

separately consider the processes of intermediate accumulation and

consumption; for DTBN all side reactions (Scheme 2) may be also

excluded due to short period of irradiation. Thus, the ratio of the

DTBN to Int formed during irradiation provides a measure of the

relative rates of tert-butyl radical addition to MNP versus to TB: that

is, [DTBN]0/[Int]0 ¼ krT[MNP]0/kad[TB]0. In this equation, [DTBN]0and [Int]0 are obtained by the extrapolation of the kinetic curves up

to zero time, [MNP]0 and [TB]0 are preset, and krT ¼ 3.3 � 106 L

mol�1 s�1,16 and hence kad can be obtained. Over the range of TB

concentrations 10�2 – 10�1 mol L�1 and at [MNP]¼ 10�2 mol L�1 the

average value of rate coefficient of tert-butyl radical addition to TB

was found to be equal to (5 � 1)�106 L mol�1 s�1; a collation of kad

values obtained under various TB concentrations is provided in Table

S1 of the ESI.† These results are in good qualitative agreement with

both experimental4 and theoretical6d values reported in the literature

for the addition of oligomeric and polymeric radicals to various

RAFT agents.

Provided the only pathway affecting the concentration of the Int

after switch-off is the fragmentation reaction (i.e. reverse of reaction

3), its rate coefficient kfr can be measured directly from the slope of

the kinetic curves of intermediate consumption after switch-off, when

Fig. 1 An ESR-spectrum observed upon irradiation by visible light

(through red light filter RL-15 with l > 650 nm) of the TB–MNP–benzene

system at 20 �C.

This journal is ª The Royal Society of Chemistry 2010

Fig. 2 Kinetic curves of accumulation and consumption of intermediate

radical Int (1) and DTBN (2) in the system TB–MNP–benzene after

switch-off of MNP photolysis: [TB] � 102 ¼ 1 (a) and 9 (b) and [MNP] ¼10�2 mol L�1 at 20 �C. Curves for other concentrations and full experi-

mental details are provided in the ESI†.

Scheme 2 The reactions taking place after switch-off irradiation of the

MNP–TB–benzene system at 20 �C.

Fig. 3 Semi-logarithmic plot of the kinetic curves 1 presented in Fig. 2a

and 2b, [TB] � 102 ¼ 1 mol L�1 (1) and 9 mol L�1 (2).

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plotted in semi-logarithmic form (Fig. 3): kfr ¼ (5 � 1)�10�3 s�1.

However, for this to be a valid direct measure of this rate coefficient,

it is important to consider the likelihood and effect of any additional

side reactions that might affect the Int concentration during switch

off. These possible reactions include re-addition of the tert-butyl

radical to the RAFT (reaction 3), and various possible termination

reactions (see Scheme 2, reactions 4, 6, 8 and 9).

The addition reaction (3) would contribute to an increase in the

intermediate concentration and cause the decay to occur over a

This journal is ª The Royal Society of Chemistry 2010

longer time period, leading us to underestimate the decay rate.

However, the experimental design of our current study largely

excludes the re-addition of tert-butyl radicals to TB, as the released

radicals are immediately trapped by the high spin trap concentration.

This is one of the key differences between the current case and the

earlier laser flash photolysis experiments of RAFT polymerization; in

the latter experiment there is no spin trap present and thus the released

radical can undergo re-addition, along with other reactions such as

propagation, termination, transfer, etc. However, for the sake of

rigor, we can take the addition reaction into account explicitly using

the correction factor p ¼ Rad/(RrT + Rad) ¼ kad[TB]/(krT[MNP] +

kad[TB]), where RrT and Rad are the rates of tert-butyl radical

addition to MNP and TB, correspondingly. For [TB] ¼ [MNP] ¼10�2 mol L�1 the p value is 0.6; i.e. the real kfr is slightly higher: kfr¼kfr

exp/p ¼ (8 � 2)�10�3 s�1.

The various termination reactions are more difficult to rule out

completely but it is important to note that if any of these do occur,

they would contribute to a decrease in the intermediate concentration

and cause the decay to occur over a shorter time period than in their

absence. In other words, in this experimental set-up, if Int termination

processes are important, our fragmentation rate coefficient provides an

upperbound to the true value. In fact the linear dependence of Fig. 3

indicates that at the chosen conditions all side reactions (4, 6, 8, 9)

involving intermediate radical can be largely excluded, and are

unlikely to be affecting the results. However, there is evidence that

termination processes are occurring to a small extent, and this is now

examined in detail.

After switching off the irradiation, the intermediate radical very

slowly vanishes over a 10–15 min period depending on the MNP and

TB concentrations; simultaneously the DTBN concentration

increases (Fig. 2). This latter effect is due to the production of tert-

butyl radicals during intermediate fragmentation. Thus, MNP should

quantitatively register the intermediate fragmentation events. In this

case the number of additional DTBN adducts formed should be

equal to the number of intermediate radicals consumed; indeed, this is

observed when equal amounts of MNP and TB are present (Fig. 2a).

However, as the ratio of TB to MNP increases, one observes that

there is a greater loss of intermediate radicals than the corresponding

increase in DTBN concentration (Fig. 2b). This could either be as

a result of the additional consumption of Int by termination reactions

such as 6, 8 or 9 in Scheme 2, or the consumption of the DTBN itself

via by termination reactions 5 and 7. The remaining process in

Scheme 2, termination reaction 4, consumes both Int and DTBN and

does not affect their concentration ratio.

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The reactions of DTBN and Int with NO radicals (reactions 5 and

6) can be ruled out as they require NO to be produced in significant

quantities through the decomposition of MNP; this will only occur

over a long duration of irradiation and at high MNP concentrations.

The reaction of DTBN with tert-butyl radicals (reaction 7) is well

known, and when it plays a visible role the DTBN kinetic curve

reaches its saturation limit. However, we have designed our experi-

mental conditions so that this reaction can also be neglected due to

the low concentration of tert-butyl radicals compared with MNP and

TB. This was confirmed by showing in an independent experiment

that the DTBN concentration increased linearly with growth of

irradiation time, and hence reaction 7 was not consuming DTBN.

This leaves the termination of the Int via coupling either with tert-

butyl radical (reaction 8) or with itself (reaction 9). Whilst both

reactions are plausible from a chemical perspective, under the present

reaction conditions reaction 8 can also be excluded. Even if its rate

coefficient is assumed to be of a similar magnitude to the rate coef-

ficient of tert-butyl radical reaction with DTBN, TB and MNP (106–

107 L mol�1 s�1); as the intermediate concentration is 3–4 orders of

magnitude lower than that of MNP and TB, reaction 4 cannot

compete with reactions of tert-butyl radical with TB and MNP under

these conditions.

Thus, the only plausible pathway for the consumption of inter-

mediate radicals in these experiments is their self-termination (reaction

9). Since this reaction becomes more likely as the Int concentration

increases, we examine the system with highest concentration of TB

(Fig. 2b). It is obvious that the termination rate kt00 can be calculated

as the difference between the rate of intermediate radical consump-

tion and the rate of DTBN accumulation. This leads to eqn (10):

k00¼��

d½Int�=dt

d½DTBN�=dtþ 1

�kfrkrT ½MNP�

½Int�ðkrT ½MNP� þ kad ½TB�Þ (10)

From the measured concentrations (Fig. 2b) and the measured rate

coefficients, this is equal to (6.5 � 3.0) � 102 L mol�1 s�1.

Finally, having measured the addition and fragmentation rate

coefficients, and verified that other reactions are not substantially

affecting the results, we can obtain the first directly measured

experimental equilibrium constant K¼ kad/kfr for reaction (3) as (6�4)�108 L mol�1. This can then be compared with quantum-chemical

calculations on the same system under the same conditions (i.e. at

20 �C in benzene solution). To this end, we used the same method-

ology as in our recent study of S–S0-bis(methyl-2-propionate)-tri-

thiocarbonate mediated polymerization of methyl acrylate;13 full

computational details are provided in the ESI.† The equilibrium

constant we obtain for the present system is 4.5�108 L mol�1, which

is in excellent agreement with the experimental results. This result

therefore further reinforces the accuracy and applicability of our

quantum-chemical methodology for model RAFT reactions.

Conclusions

In conclusion, we have used an ESR-based spin trapping

technique to provide the first direct experimental measurements

for the rate coefficients of the addition and fragmentation reactions of

tert-butyl dithiobenzoate. The results indicate that the fragmentation

processes of dithiobenzoate derived intermediate RAFT radicals can

be relatively slow, in agreement with the original predictions of the so-

called slow fragmentation model.6,9 The equilibrium constant of (6�

1440 | Polym. Chem., 2010, 1, 1437–1440

4) � 108 L mol�1 at 20 �C in benzene is in excellent agreement with

the corresponding quantum-chemical calculations for the same

reaction 4.5 � 108 L mol�1, mutually verifying the accuracy of the

experimental and theoretical results. Our results also confirm that

intermediate self-termination can occur, albeit at a relatively low

reaction rate (kt0 0¼ (6.5� 3.0)� 102 L mol�1 s�1), at least for the low

molecular weight model compound studied.

Acknowledgements

MLC gratefully acknowledges financial support from the Australian

Research Council under their Centres of Excellence program and

generous allocations of supercomputing time on the National Facility

of the National Computational Infrastructure. EC gratefully

acknowledges financial support from the Russian Foundation for

Basic Research (project No 08-03-00269).

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