Electron Spin Resonance (ESR) Observation of Radical Migration

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Chapter 4

Electron Spin Resonance (ESR) Observation of Radical Migration Reactions in the Polymerization of Alkyl Acrylates Atsushi Kajiwara Nara University of EducationTakabatake-cho, Nara 630-8528, JAPAN [email protected]

Electron Spin Resonance (ESR) spectroscopic studies directed at clarifying the fundamentals of radical polymerizations have been conducted. A combination of ESR and atom transfer radical polymerization (ATRP) provided significant new information on the reactivity and dynamics of propagating radicals in radical polymerizations. Moreover, chain lengths of the radicals can be differentiated by the combined method. Previously, it had been extremely difficult, even impossible, to obtain such information from ESR spectra during conventional radical polymerizations. To overcome this difficulty radical precursors of oligo- and poly(meth)acrylates were prepared by ATRP. The mechanisms of main chain transfer reactions have been investigated and the possibility of another chain transfer reaction, to ester side groups, was examined. Multi-step radical migration behavior in various alky acrylates was also investigated and experimental evidence for such chain transfer reactions will be shown in this paper.

Introduction Electron Spin Resonance (ESR) is the most powerful tool for investigation of radical species in radical polymerizations. When well-resolved spectra can be observed, the spectra provide information not only on the structure, properties, and concentration of radicals but also information on the initiating and © 2009 American Chemical Society In Controlled/Living Radical Polymerization: Progress in ATRP; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

49

50


propagating (oligomeric and polymeric) radicals in radical polymerizations [115]. ESR spectroscopy was successfully applied to quantify radical concentration in the polymerizations [4, 8-11]. However, the direct detection method of ESR did not reveal information on many additional points that are very significant in radical polymerization chemistry so far. For example, the length of propagating chain is not known, direct observation of the penultimate unit effect is almost impossible, and detailed mechanisms of radical reactions remain extremely difficult to examine. These problems have not yet been fully resolved but the development of controlled radical polymerization techniques, especially atom transfer radical polymerization (ATRP), enables us to resolve some of these problems. H CH2 = C

H CH3

C

C=O

Br

C=O

+

I

Cu Br/L

O

CH3

CH3 CH3

O CH2 CH3

CH3

C

ATRP

H

H

H

H

H

C

C

C

C

C

C=O

H

O CH2 CH3

Br

+

CuIBr/L

C=O n H C=O O O CH3 CH3 C C

CH3 CH3

CH3 CH3

Figure 1 Atom transfer radical polymerization (ATRP) of tert-butyl acrylate for preparation of model radical precursors. ATRP is one of the most widely applied polymerization techniques in the field of controlled/living radical polymerization. ATRP allows for preparation of a wide range of polymeric materials with controlled molecular weight and welldefined architecture [16-20]. The polymers formed in ATRP most frequently contain terminal carbon-halogen bonds. Giese et al. reported that these bonds can be homolytically cleaved by reaction with organotin compounds [21]. Accordingly, various radicals can be formed from the corresponding precursors prepared by atom transfer radical addition (ATRA) and ATRP (Fig. 1) and the generated radicals can be studied by ESR spectroscopy (Fig. 2). For example, polymeric (meth)acrylates with various chain lengths can be prepared as radical precursors by ATRP. Variation of the chain length of these polymeric radical precursors elucidates the effect of chain length of the formed radicals on the ESR spectra. The investigations of radical reactions are difficult without using model reactions. The method using model radical have applied to investigate chain transfer reactions for propagating acrylate radicals. Interpretation of ESR spectra observed in acrylate radical polymerizations has been very difficult [22-30]. However a clear interpretation of the ESR spectra of propagating and mid-chain radicals had been demonstrated with the aid of molecules prepared by ATRP[5,7]. A mechanism for the formation of mid-chain radicals was also clarified with the aid of uniform oligo model radicals generated from precursors prepared by ATRP[14,15]. Formation of a six-membered ring enables the radical to migrate to the monomer unit two units before the terminal unit. Since this shift needs at least three linked monomer units, it suggests that mid-chain radicals cannot be formed from a dimeric

In Controlled/Living Radical Polymerization: Progress in ATRP; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

51 radical. The ESR spectrum of the dimeric radical (H-ethyl acrylate (EA)tertbutyl acrylate (tBA)•) did not show any temperature dependent change in the ragne from -30°C to 150 °C. X CH3

C

X CH2

C=O O

O

R

R

H

X C

X Br

C

C

C=O n

H C=O O R


model radical precursor prepared by ATRP

(nBu3Sn)2

CH3

C

CH2

C=O hν

H

X

C

C

C

C=O n

H C=O O

X

O

O

R

R

R

model radical with given chain length

X = H (acrylate) or CH3 (methacrylate) R = methyl, ethyl, tert-butyl n = 0 (dimer), 1, 2, 3, 4, 15, 30, 50, 100, etc.

Figure 2 Radicals observed by ESR. Generation of model radicals from radical precursors prepared by ATRP In the case of the model trimeric (H-EA-tBA-tBA•) and tetrameric radicals (H-EA-tBA-tBA-tBA•), their ESR spectra showed a clear temperature dependent change. The spectrum observed at -30 °C was very similar to that of the dimeric radical whereas the spectra observed at higher temperatures were totally different from that measured at lower temperatures. Interpretation of the higher temperature spectra suggested that a 1,5-hydrogen shift occurred in the uniform oligomeric model radicals. No other radicals except for the specific radicals at low and high temperatures were detected. These findings strongly suggest that the mechanism of the chain transfer reaction in an acrylate radical polymerization is a 1,5-hydrogen shift that ocurrs through formation of a six-membered ring structure. The transfer is possible because formation of a six-membered ring is a kinetically favored process and the transfer occurred from a secondary radical to form a thermodynamically more stable tertiary radical. One further piece of information can be obtained from the result of the pentamer radical since the pentamer provides one more chance for the radical to migrate from a mid-chain radical to the other chain end. However, this migration was not observed for the pentameric model radical which has H-EA-tBA-tBA-tBA-tBA• structure. At that time, the reason for this was unresolved. Although there may be some minor contribution of intermolecular chain transfer, these systematic studies have provided a clearer perspective of the mechanism of the intramolecular chain transfer reaction of propagating acrylate radicals. Nevertheless further investigation was required to provide decisive proof of the mechanism. Two topics were exameind in this research work: One is the possibility of 1,5-hydrogen shift reaction to form radicals on branched ester side groups; the other is a two-step radical migration reaction via sequential 1,5-hydrongen shift mechanism reactions. The results from these studies may be relevant to chain transfer reactions of alkyl radicals.


52

Results and Discussion Possibility of radical migration to the side group of 2-ethylhexyl acrylate In the investigation of tBA oligomers, 1,5-hydrogen shift reaction mechanism was explained by a combination of formation of a kinetically favorable six-membered ring structure and thermodynamically favorable transfer from secondary to tertiary radical (Fig. 3a) [15]. a)

b)

RO CH2 H

C


RO CH2

CH2

C

H OR

H

CH2

OR

b

H

H

C

CH2X CH2X CH2

CH

H

O=C

a

CH2 C

C

CH2 C=O

C O=C

C

O

O

OR

Figure 3 Schematic diagram of potential 1,5-hydrogen shift for propagating acrylate radicals (a) and potential 1,5-hydrogen shift for propagating radicals of acrylates with branched ester side groups (b). This raises the possibility that we might observe a similar radical migration in any compound which has a chance to form a six-membered ring structure and form a tertiary radical from a secondary radical. Suitable compounds for investigation of another route for radical migration are isobutyl acrylate (iBA) and 2-ethylhexyl acrylate (2EHA). Both acrylates have branched ester side chains and they can form a six-membered ring structure with a propagating radical and a side branched methine proton as shown in Figure 3b. If the 1,5hydrogen shift occurs, the resulting radicals should be a tertiary radicals, just as in the case of a 1,5-hydrogen shift to the main chain. This question can be examined by ESR spectroscopy of dimeric model radicals of iBA or 2EHA. As mentioned above, dimeric model radical of acylates do not udergo the 1,5-hydrogen shift reaction to the main chain. On the other hand, a radical generated from the precursor still has a possibility to migrate to the ester side group through formation of a six-membered ring structure. A model dimeric radical precursor of 2EHA (H-EA-2EHA-Br) was prepared by ATRP. Initially a mixture of oligomers with dimeric, trimeric, tetrameric, and so on was obtained. The model dimeric radical precursor was separated and purified. ESR spectra of the radicals generated from the precursor at various temperatures were recorded. At 30 °C, the spectrum showed typical features associated with the propagating radical of acrylates. The spectra showed similar features as those measured at 30 °C (Fig. 4) even at higher temperatures of 90 and 120 °C, which indicates that no relocation of the initially formed rdical center took place.


53

a)

H

H

H

CH3 C

C

C C O

O

2 mT

C

O H O CH2 CH2 H CH3 C CH2 CH2 CH3 CH2 CH2 CH3

b)

RO CH2 C


CH3

b

H

H

C

H

CH2X CH2X CH2

CH CH2 C

C O=C

a

O

O

OR

Figure 4 ESR spectra of dimeric model radical of 2EHA at - 30 °C in toluene and 150 °C in mesitylene (measurement condition; modulation: 100 kHz, 0.05 mT, time constant: 0.03 sec, sweep time: 15 mT/4 min). ESR spectra of radicals due to organo-tin compounds are shown in grey.

In the next step, ESR measurements were conducted for trimeric model radicals of 2EHA (H-EA-2EHA-2EHA•) as shown in Figure 5. Similar to the case of tBA [15], the ESR spectrum of this trimeric model radical showed that the radical migrated to the other end of the molecule at 120 °C. From these results, we can discard the possibility of radical migration to the ester side groups since no other kind of radicals were observed throughout the measurements. Lovell et al. reported that they observed no clear evidence for branching from side group of poly(2EHA) [31]. They investigated the structures of the resulting poly(2EHA) by 13C NMR spectroscopy. In that case, evidence for the presence of the radicals at the ester side groups was indistinct and some possibility for the migration of the radical center to the side groups remained in the NMR measurements. The present ESR results clearly show that the radical center did not migrate to the ester side group of poly(2EHA). Raise of the temperature caused remarkable spectroscopic change from -30 °C to + 120 °C The ester side group of 2-ethylhexyl chain could be imagined to form a sixmembered ring structure and the resulting radical should have a tertiary radical structure but the results indicate that the 1,5-hydrogen shift reaction did not occur indicating that there should be some electronic interactions for a formation of six-membered ring structure in polyacrylates or the stability of the resulting radical in “route a” and “route b” might be different. In “route a”, the resulting radical is located next to a carbonyl group and the presence of the carbonyl group would stabilize the radical to a greater degree than in the case of alkyl radical without a neighboring carbonyl group.


54

a)

2.24 mT

0.08 mT H

H

H

H

H

CH3 C

C

C

C

C

2.02 mT

X

C OH C O C OH O O O CH2 CH2 H CH2 H CH3 C C CH2 CH2 CH2 CH2 CH3 CH2 0.17 mT CH3 CH2 CH2 CH2 CH3 CH3


b) 2.19 mT

1.38 mT 1.58 mT H

H

H

H

C

C

C

C

CH3 C

0.10 mT

H

C OH C O C OH O O O CH2 CH2 H CH2 H CH3 C C 0.11 mT CH2 CH2 CH2 CH2 CH3 CH2 CH3 CH2 CH2 CH2 CH3 CH3

Figure 5 Temperature dependent change in ESR spectra of trimeric model radical of 2EHA (measurement condition at -30 °C; solvent: toluene, modulation: 100 kHz, 0.1 mT, time constant: 0.03 sec, sweep time: 15 mT/4 min, at +120 °C; solvent: mesitylene, modulation: 100 kHz, 0.05 mT, time constant: 0.03 sec, sweep time: 15 mT/4 min). Multi-step radical migration in propagating radicals of Octadecyl Acrylate As mentioned above, in the previous study examinig the pentameric oligomer of tBA a second radical migration step was not observed [15]. At that time, the reason was not clear. When model radicals, exemplary of radicals present in the polymerizations of various kinds of alkyl acrylates, were observed by ESR some alkyl acrylates showed faster radical migration to form mid-chain radicals even at low temperatures. For example, a remarkable acceleration of the apparent rate of the radical migration was observed in the case of dodecyl- and 2-ethylhexyl-acrylates. These results suggested that acrylates with longer alkyl groups at their ester side groups showed faster radical migration than that observed with tBA radicals. Therefore the possibility of a second radical migration step was studied again using ethyl-, hexyl-, 2-ethylhexyl, dodecyl- and octadecyl-acrylates. Among these acylates, octadecyl acrylate (OdA) demonstrated clear results. Uniform dimeric, trimeric, tetrameric, and pentameric oligomeric model radical precursors of OdA were prepared by ATRP then separated and purified by repeated column chromatography. In the case of the radical prepared from the pentamer there are two chances of 1,5-hydrogen shift reaction from the initially generated radical. The initial radical center migrated to form mid-chain radical through six-membered ring structure and then the mid-chain radical moved to form a radical located at the other end of the pentamer. Since expected spectroscopic features of radicals at


55 these specific sites are totally different, the resulting spectra can be differentiated by ESR, as shown in Figure 6. 1000

a) H

H

H

H

H

H

H

H

H

H

H

C

C

C

C

C

C

C

C

C

C

H

C=O H

O

C=O H

C=O H

O

O

O R

C=O H

0

O R

R

R

500

C=O R

-500

-1000 0 400

b)

H

H

H

H

H

H

C

C

C

C

C


H

C=O H

C=O H

O

C

H

H

H

C

C

C

C

C=O H

C=O H O

O

O R

H

0.04

0.06

0.08

0.1

0.12

200

H

C=O

0

O R

R

R

R

0.02

-200

2 mT -400

3200

3250

3300

c) H H

H

H

H

H

H

H

H

H

C

C

C

C

C

C

C

C

C

C

H

C=O H

O

C=O H R

C=O H

O

O

O R

C=O H R

H

C=O

O R

R

Figure 6 Expected ESR spectra for initially generated radical (a), mid-chain radical (b), and radical formed by two-step migration to locate at the other end (c) along with their expected structures.

a)

H

H

H

H

H

H

H

H

H

H

H

C

C

C

C

C

C

C

C

C

C

H C H C H C H C O OO O O O O O O CH3 CH2 CH2 CH2 CH2 C CH3 CH CH2 CH2 CH2 CH H

C

O

3

2

CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2

2 mT

CH2 CH2

CH2 CH2 CH2

CH2

CH2

CH2 CH2

CH2 CH2

CH2

CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2

CH2

CH2

CH2

CH2

CH2

CH2 CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2 CH2 CH2

H

CH2

H

H

H

H

H

C

C

C

C

C

C

CH2

CH2

CH2 CH2 CH2 CH2 CH 2 CH 2 CH 2 CH 2 CH2

H

C H

b)

C

H

H

H

H

H

H

H

H

C

C

C

C

C

C

C

C H

H C H C H C H C O OO O O O O O O CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH 2 CH2 CH2 CH2 CH 2 CH2

C

O CH 3

C

CH2

CH2

CH 2 CH2 CH 2 CH2 CH2 CH2 CH2 CH2

CH2 CH2

CH2 CH2

CH2

CH2

CH2

CH2 CH2

CH 2

CH 2

H

H

H

C

C

C

C H

C

O

CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3 CH3 CH3 CH3

H

H

H C H C H C H C O OO O O O O O O CH 3 CH2 CH2 CH2 CH 2 C CH3 CH CH2 CH2 CH2 3CH2 CH2 CH2 CH2 CH 2 CH 2 CH2 CH2 CH2 CH2 CH 2 CH2 CH2 H

CH2 CH2

CH2 CH 2 CH2

CH2 CH 2

CH2 CH 2 CH2 CH2 CH2

CH2

CH 2

CH 2 CH2

CH2 CH 2

CH2 CH2 CH2 CH2 CH2 CH2

CH2 CH2 CH2 CH 2 CH2 CH2

CH 2 CH2 CH2 CH2 CH 2 CH2 CH2 CH2 CH2 CH 2 CH2 CH2 CH 3 CH3 CH3 CH3

CH2 CH2 CH2 CH2

CH2 CH2 CH2 CH2 CH2 CH 2 CH2 CH2 CH2 CH2 CH 2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH3 CH3 CH3 CH3

Figure 7 Temperature dependent spectroscopic change in ESR spectra of pentameric model radical of OdA. (measurement condition at 30 °C; solvent: hexane, modulation: 100 kHz, 0.1 mT, time constant: 0.03 sec, sweep time: 15 mT/4 min, at +120 °C; solvent: decane, modulation: 100 kHz, 0.05 mT, time constant: 0.03 sec, sweep time: 15 mT/4 min).



56 ESR spectra of the pentameric model radical of OdA observed at 30 (a) and 150 °C (b) are shown in Figure 7. At 30 °C, a complicated ESR spectrum was observed while at 150 °C an ESR spectrum due to the radical formed by two radical migrations-steps was exclusively observed. The spectrum is very similar to that observed in trimer model radical of various kinds of acrylates such as the spectrum of 2EHA shown in Figure 5b. In the ESR spectra observed at 30 °C, the spectra of three different radicals overlap. The observation of three different spectra indicates that radicals formed by two radical migration steps are detected. The reason for the observation of the second migration step can be considered to result from the formation of a helix structure for the propagating radicals (Fig. 8). The presence of the long alkyl ester groups may support the formation of the helix structure and facilitate the second migration step. ESR spectra for radicals formed from a uniform heptamer radical were also observed and the presence of a radical at the other chain end was observed. The successive radical migration reaction may be called as “multi-step” migration. In the previous study, a second migration step was not observed for the model radical formed from the tBA pentamer. If it occurred at all, the second migration step was extremely slow. The reason for the retardation of the second migration step is still unresolved.

CH2

H C

H

C

H C

CH2

CH2 CH2 CH

C

CH2

CH2

CH

H

CH2 CH

Figure 8 Potential helical structure of propagating radical of alkyl acrylates Evidence of intermolecular radical migration of alkyl radicals There was no evidence in the ESR study of radicals formed from 2EHA precursors for the formation of tertiary alkyl radicals in the 2-ethylhexyl ester side group via a 1,5-hyodrogen shift. On the other hand, 1,5-hydrogen shift reactions have been considered in the radical polymerizations of ethylene using various kinds of alkyl bromides as radical precursors to model propagating ethylene radicals of various chain lengths [32]. Model terminal propagating and mid-chain radicals can be generated using alkyl bromides that contain bromine atom at the chain end or mid-chain. The ESR spectra of the model propagating and mid-chain radicals are different and we can clearly differentiate one from the other. The ESR spectra of a butyl radical generated from butyl bromide can be observed. The butyl radical is a model of a dimeric ethylene radical and it does not show any trace of a 1,5-hydrogen shift reaction. The ESR spectrum of the butyl radical in benzene showed no change even at 80 °C. On the other hand,


57 the ESR spectrum of radicals formed at temperatures higher than 80 °C in decane indicate the presence of a mid-chain radical. These results suggest the occurrence of intermolecular chain transfer rather than intramolecular backbiting reaction in the case of alkyl radicals.


Conclusions The development of ATRP provides an opportunity to prepare precursor molecules that can be used to investigate reaction mechanism of radicals in radical polymerizations. Previous studies have clearly showed that radical migrations occur through a 1,5-hydrogen shift mechanism. Model radicals of oligo(OdA) and oligo(2EHA) clearly showed faster 1,5-hydrogen shift reaction through their main chains but the dimeric model 2EHA radical exhibited no evidence for a 1,5-hydrogen shift to its ester side group. On the other hand the model OdA pentamer radical clearly showed the results of two radical step migrations through the 1,5-hydrogen shift mechanism. A butyl radical, a model dimeric ethylene radical, demonstrated intermolecular chain transfer in decane solution. These results indicate that there should be some tendency for oligoand poly-acrylates to form six-membered ring structures, like a helix structure through some kinds of internal electronic interactions. Experimental Monomers, initiators, copper salts, and ligands were purified in the usual manner [33]. The presence of carbon-bromine terminal groups and the chain lengths of the radical precursors were checked by Electron Spray Ionization (ESI) mass spectra. Generation of model radicals from the model precursors was conducted by a reaction with an organotin compound under irradiation [21]. ESR spectra of radicals were recorded on a JEOL JES RE-2X spectrometer operating in the X-band, utilizing a 100 kHz field modulation, and a microwave power of 1 mW. A TE011 mode universal cavity was used. Measurement temperature was controlled by JEOL DVT2 variable-temperature accessory. Spectroscopic simulations were carried out by JEOL IPRIT Data Analysis System. Molecular weights and molecular weight distributions were roughly estimated using a TOSOH CCP&8020 series GPC (SEC) system with TSK-gel columns. Polystyrene standards were used to calibrate the columns. Acknowledgements The author is grateful to Professor Emeritus Mikiharu Kamachi, Osaka University, for his kind advice and discussions on ESR study of radical polymerizations. Sincere thanks are due to Professor Krzysztof Matyjaszewski, Carnegie Mellon University, for his kind suggestions and continuous encouragements. The author wishes to thank Professor Bernadette Charleux, Université Pierre et Marie Curie -Paris 6, for her suggestions for investigation of 2-ethylhexyl acrylate radicals.


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Electron Spin Resonance (ESR) Observation of Radical Migration

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