- Email: [email protected]
Interaction of monomer radicals with nitroxides: A new access to the radical-radical combination rate constants
Available online at www.sciencedirect.com
Chemical Physics Letters 449 (2007) 231–235 www.elsevier.com/locate/cplett
Interaction of monomer radicals with nitroxides: A new access to the radical-radical combination rate constants Jacques Laleve´e a
a,*
, Didier Gigmes b,*, Denis Bertin b, Xavier Allonas a, Jean Pierre Fouassier a
De´partement de Photochimie Ge´ne´rale, Ecole Nationale Supe´rieure de Chimie de Mulhouse, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France b Universite´ de Provence, UMR 6517, Equipe CROPS. Case 542, Av.Escadrille Normandie Nie´men, 13397 Marseille Cedex 20, France Received 4 September 2007; in final form 10 October 2007 Available online 13 October 2007
Abstract Acrylate, methacrylate and styrene monomer radicals produced by the addition of an aminoalkyl radical to the corresponding monomer unit were directly observed by laser flash photolysis. These radicals are characterized by a strong absorption in the near UV/visible part of the spectrum allowing to directly follow their interaction with different nitroxides usable in nitroxide mediated polymerization (NMP). The recombination rate constants were measured: they are in agreement with the literature data when available; other values are new. This procedure is fast and reliable. 2007 Elsevier B.V. All rights reserved.
1. Introduction Acrylates, methacrylates and styrene represent widely used classes of monomers M for thermal [1a] and photoinduced [1b] polymerization reactions. The reactivity of the monomer radicals R–M produced after the initiation step (where R is the initiating radical) and the propagating radicals R–ðMÞn governs for a part the whole polymerization efficiency. Nitroxide mediated polymerization NMP – which is now recognized as a powerful method for the control of a polymerization reaction [2,3] – is based on a reversible cleavage of the C–O covalent bond of a dormant species R–(M)n–ONR 0 (R00 ) (formed between the propagating radical and a nitroxide R 0 (R00 )NO radical – Scheme 1) that further generates the growing polymer radical R–ðMÞn 0 00 and the persistent radical R (R )NO . The nitroxide is introduced into the monomer medium as either a radical (R 0 (R00 )NO such as TEMPO for bicomponent system) or advantageously with an alkoxyamine (R 0 (R00 )NOR that *
Corresponding authors. Fax: +33 3 89 33 68 95. E-mail addresses: [email protected] (J. Laleve´e), [email protected] (D. Gigmes). 0009-2614/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.10.040
thermally cleaves into an initiating radical R and a nitroxide R 0 (R00 )NO acting as the controlling agent) [3c]. The controlled character is strongly related to both the dissociation kd and the recombination kc rate constants of the dormant species [4]. The cleavage rates of alkoxyamines (kd) were investigated in some detail for both polymeric and low molecular model systems in wide temperature ranges, and predictive rationalization have been offered [5]. In comparison much less is known about kc between nitroxide and carbon centered radicals certainly because such studies require very specific devices but also the preparation of specific initiators [6–8]. However, a better knowledge of the recombination reaction of nitroxides with monomer radicals is particularly important for development of NMP processes. In the case of monomer type radicals, two main general procedures have been used previously for such determination: (i) the use of laser flash photolysis to determine kc between various nitroxides and model radicals [7,8] and (ii) pulsed laser polymerization (PLP) techniques with size exclusion chromatography (SEC) for the recombination between macroradicals and nitroxides [9]. Actually these methods require the preparation of the specific initiators
J. Laleve´e et al. / Chemical Physics Letters 449 (2007) 231–235
232
N OEt P OEt O
N
N
O
O
TEMPO
N
Molecular orbital calculations were carried out with the GAUSSIAN 03 suite of programs [13]. Using the time dependent density functional theory (TDDFT) at TD/ MPW1PW9/6-311++G** level, the absorption properties of the styrene radicals were calculated on the basis of the frequency checked geometries calculated at the UB3LYP/ 6-31G(d) level. 3. Results and discussion
O
SG1
DPAIO
3.1. Observation of the monomer radicals (methyl acrylate, methyl methacrylate, styrene)
2. Experimental section and methods
Monomer radicals exhibit an absorption band close to 300 nm [14–18] and the overlap with the absorption of the starting compound, additives or other intermediates renders difficult their direct observation. As a consequence, despite the great interest concerned with their reactivity, very few direct studies have been devoted to these radicals. The procedure used below for the observation of monomer radicals was recently developed to characterize the reactivity of (meth) acrylate radicals [10]. Our original approach consisted into three consecutive reactions: generation of tert-butoxyl radicals through the photochemical decomposition of di-tert-butylperoxide, a(C–H) hydrogen abstraction reaction from triethylamine and addition of the aminoalkyl radical TEA to the monomer double bond. The aminoalkyl radical was easily observed [10]. A new transient appeared in the near UV visible wavelength range (whose the risetime corresponds to the decay time of the aminoalkyl radical): 480 nm and 440 nm for TEA–MA and TEA–MMA, respectively, [10]. The procedure is developed here for styrene: the transient absorption around 350 nm is ascribed to TEA–STY (Fig. 1) and the addition rate constant of TEA to styrene is 1.5 · 106 M1 s1. In Fig. 1, the transient spectrum obtained for TEA–STY was compared to that found for STY formed by a direct hydrogen abstraction between ethylbenzene and the tert-butoxyl radical: the absorption
Methyl acrylate (MA), methyl methacrylate (MMA), styrene (STY), 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO) were obtained from Aldrich. SG1 has been kindly provided by Arkema Company; DPAIO has been prepared according to a published procedure [11]. The stabilizer was removed from monomers by column purification (Aldrich AL-154). Di-tert-butylperoxide from Aldrich has been used directly without further purification. Triethylamine (TEA) was purified by distillation. The investigated nitroxide radicals are depicted Scheme 1. The nanosecond transient absorption setup has been previously described in detail [12]. The excitation source is a Q-switched nanosecond Nd/YAG laser (kexc = 355 nm; 9 ns pulses; energy reduced down to 10 mJ; Powerlite 9010 Continuum); the analyzing system consists in a pulsed xenon lamp, a monochromator, a fast photomultiplier and a transient digitizer.
Fig. 1. Transient absorption spectra of the STY (square) and TEA– STY(circle) radicals.
N H2C
H C CH3
STY
.
CH
TEA-STY
.
Scheme 1.
R–(M)n–ONR 0 (R00 ) to study the combination reaction. This corresponds to a strong limitation. As a consequence, the development of a method based on easy accessible materials, allowing temperature or solvent dependences studies is desired to understand clearly the different factors influencing kc but also to develop models for predicting these values as for the kd of alkoxyamines. In this Letter, such an approach is shown through the direct measurement of the kc recombination rate constants of various nitroxides with methyl acrylate, methyl methacrylate and styrene monomer radicals. This will be carried out from the direct observation of the monomer radicals by the LFP technique as already proposed in a recent study [10].
J. Laleve´e et al. / Chemical Physics Letters 449 (2007) 231–235
maximum is red-shifted by about 40 nm when going from STY to TEA–STY. The presence of TEA–STY is confirmed by quantum mechanical calculations using the time dependent density functional theory (TDDFT). The results are gathered in Table 1 for STY, TEA–STY and TEA–STY–STY. For STY, the absorption band observed at 315 nm is quite well reproduced by TDDFT calculations supporting the contention that these methods can describe electronic transitions of various radicals [19–21]. For TEA–STY, in addition to the usual UV absorption band of the styrene radicals, the intense transition experimentally observed at
233
350 nm is also quite well reproduced by quantum mechanical calculations (395 nm). As previously observed for MA and MMA [10], this new band is due to a hyperconjugation between the nitrogen lone pair and the HOMO– SOMO orbitals of the monomer radicals. This interaction is sensitive to the distance between the nitrogen and the radical centered, i.e., the addition of another monomer unit leads to a drastic decrease of the oscillator strength of this band and a blue shift of the absorption. The calculated absorption of TEA–STY–STY is similar to that of STY (see Fig. 2). 3.2. Reactivity of monomer radicals toward nitroxides
Table 1 Calculations of the absorption wavelengths and oscillator strengths (f) for STY, TEA–STY, TEA–STY-STY at TD/MPW1PW91/6-311++G** level and comparison with the experimental data Radical
STY TEA–STY TEA–STY–STY
TD/MPW1PW91/6-311++G**
Exp.
k (nm)
f
k (nm)
311.7 395.3 322.2
0.0298 0.067 0.022
315 350 –
The reactivity of these radicals toward the different nitroxides can now be easily probed. The change of kc with the amine structure was first investigated since the steric hindrance relative to this moiety of the monomer radical might affect this factor (Table 2). It can be noted that the amines used weakly affect kc. Therefore, we will now use TEA. The recombination rate constants kc of methyl acrylate, methyl methacrylate and styrene monomer radicals with TEMPO, SG1 and DPAIO have been measured (Table 3). The concentrations used for the monomers
Table 2 Effect of the amine structure on kc (system MA-SG1) Amine
kc M1 s1
Triethylamine Dibutylamine Diethylamine
4.4 · 107 6.2 · 107 6.0 · 107
Solvent: di-tert-butylperoxide.
Table 3 Recombination rate constants between monomer radicals and nitroxides (solvent: di-tert-butylperoxide) Monomer
SG1 (M1 s1)
DPAIO (M1 s1)
MA
5.5 · 108 (4.4 · 108)a (6.4 · 108)e
4.4 · 107 (4.0 · 107)a
2.5 · 108
MMA
2.5 · 108 (1.5 · 108)a (6.3 · 108)d
1.2 · 106 (6.4 · 105)a (9.9 · 105)b (2.6 · 106)d
<3 · 107f
1.2 · 108 (9.5 · 107)a (1.75 · 108)c (3 · 108)d
2.2 · 106 (1.8 · 106)a (3.0 · 105)c (3.0 · 106)d
1.0 · 108
STY
a
Solvent (15% di-tert-butyl peroxide/85% acetonitrile). Solvent (85% di-tert-butyl peroxide/15% ethanol). c Literature data from PLP experiments: Ref. [9]. d Literature data from model radical quenching: Ref. [7]. e Literature data from SEMF CIDNP experiments: Ref. [23] in benzene. f The intrinsic absorption of DPAIO at 355 nm prevents the use of higher concentrations in quenching experiments leading to this upper value. b
Fig. 2. Interaction of TEA–MA with SG1: (a) effect of [SG1] on the TEA–MA kinetics; an increase of the concentration (in the series 0.078; 0.134; 0.248; 0.475 M) leads to a decrease of the radical lifetime time; (b) Stern–Volmer plot for the determination of kc.
TEMPO (M1 s1)
234
J. Laleve´e et al. / Chemical Physics Letters 449 (2007) 231–235
were high enough to neglect the recombination of TEA with the nitroxides. When available, the literature LFP data [7] are found in good agreement with the values determined here (for MMA/TEMPO, STY/TEMPO, MMA/SG1 and STY/SG1). The PLP results [9] satisfactorily match the present values (STY/TEMPO and STY/ SG1). The value for MA/TEMPO derived from SEMFCIDNP experiments is also in agreement. This gives confidence to the proposed procedure. Values for MA (or MMA, STY)/DPAIO are new. For a given monomer, kc decreases in the series TEMPO > DPAIO SG1. This trend agrees with previous experiments which have shown that kc decreases when the steric hindrance of the nitroxide moiety increases [6b,7,9,22]. For the DPAIO structure, more studies are needed to evaluate the influence of the single electron delocalization on the kc value. As already reported for a given nitroxide [6b–8], kc decreases when the steric hindrance of the alkyl radical increases (TEA–MA > TEA–MMA) and when the stabilization radical increases (TEA–MA > TEA–STY). It has to be pointed out that the TEA–MA/ TEA–MMA, TEA–MA/TEA–STY and TEA–MMA/ TEA–STY ratios are different for TEMPO (2.2; 4.6; 2.1, respectively) and SG1 (36.6; 20; 0.5, respectively). These results suggest that for the kc values, both steric and polar effects are more important with SG1 than with TEMPO. DPAIO is as selective as SG1 to the MMA type radical: for example, when going from the recombination of TEA–MA to TEA–MMA, a decrease of at least one order of magnitude is observed in the case of DPAIO, 35 for SG1 and only 2 for TEMPO. However, compared to SG1, DPAIO is less sensitive to secondary stabilized alkyl radicals. Indeed for DPAIO, kc decreases by a factor of 2.5 from TEA–MA to TEA–STY, this factor being equal to 4.6 for TEMPO and 20 for SG1. The high recombination rate constants and the low dissociation rate constants explain quite well why DPAIO is inefficient [3e] for the controlled radical polymerization of acrylate and styrene monomers. The effect of the polarity and the hydrogen bonding ability of the medium on kc was also investigated. An increase of the solvent polarity upon the addition of acetonitrile in di-tert-butylperoxide leads to a slight decrease of kc (about a two fold factor). This can be explained by the resonance zwitterionic structure of the nitroxide which is weakly favored in polar media [6a]. The addition of ethanol (15%) in di-tert-butylperoxide leads to a very weak change of kc which decreases from 1.2 · 106 to 9.9 105 M1 s1 for TEA–MMA/SG1. This result evidences the weak influence of the hydrogen bonding on the recombination rate constant for this system.
4. Conclusions Our approach appears as a very convenient way to gather a lot of experimental values for the recombination
of the monomer radical models R–M with nitroxides. Taking into account the agreement with the PLP experiments, this could be an indication for the recombination of nitrox ides with propagating radicals R–ðMÞn . The main advantages of the present method lie on the direct observation of the radicals as well as the reliability of the measurements. This approach appears powerful and promising to study and compare the reactivity of different NMP mediators for a wide range of monomers. The improvement of the controlled polymerization reactions through a careful selection of nitroxide/monomer couples appears as a fascinating task. References [1] (a) K. Matyjaszewski, in: K. Matyjaszewski, Y. Gnanou, L. Leibler (Eds.), Macromolecular Engineering, vol. 1, Wiley VCH, Weinheim, 2007, p. 161; (b) J.P. Fouassier, Photoinitiation Photopolymerization and Photocuring, Hanser Publishers, Munich, New York, 1995. [2] (a) K. Matyjaszewski, in: ACS Symposium Series Advances in Controlled/Living Radical Polymerization, American Chemical Society, 2002; (b) K. Matyjaszewski, in: ACS Symposium Series Controlled Radical Polymerization, American Chemical Society, 1998. [3] (a) D.H. Solomon, E. Rizzardo, P. Cacioli, US Patent 4(581) (1986) 429; (b) M.K. Georges, R.P.N. Veregin, P.M. Kazmaier, Macromolecules 26 (1993) 2987; (c) C.J. Hawker, A.W. Bosman, E. Harth, Chem. Rev. 101 (2001) 3661; (d) G. Moad, D.H. Solomon, The Chemistry of Radical Polymerization, second edn., Elsevier, Amsterdam, 2006; (e) Y. Guillaneuf, D. Gigmes, S.R.A. Marque, P. Astolfi, L. Greci, P. Tordo, D. Bertin, Macromolecules 40 (2007) 3108. [4] (a) H. Fischer, Chem. Rev. 101 (2000) 3581; (b) H. Fischer, ACS Symp. Ser. 854 (2003) 10; (c) A. Goto, T. Fukuda, Prog. Polym. Sci. 29 (2004) 329. [5] (a) S. Marque, J. Org. Chem. 68 (2003) 7582; (b) D. Bertin, D. Gigmes, S. Marque, Recent Res. Devel. Organic Chem. 10 (2006) 63. [6] (a) A.L.J. Beckwith, V.W. Bowry, K.U. Ingold, J. Am. Chem. Soc. 114 (1992) 4983; (b) V.W. Bowry, K.U. Ingold, J. Am. Chem. Soc. 114 (1992) 4992. [7] J. Sobeck, R. Martschke, H. Fischer, J. Am. Chem. Soc. 123 (2001) 2849. [8] W.G. Skene, J.C. Scaiano, N.A. Listigovers, P.M. Kazmaier, M.K. Georges, Macromolecules 33 (2000) 5065. [9] Y. Guillaneuf, D. Bertin, P. Castignolles, B. Charleux, Macromolecules 38 (2005) 4638. [10] J. Lalevee, X. Allonas, J.P. Fouassier, Chem. Phys. Lett. 415 (2005) 287. [11] C. Berti, M. Colonna, L. Greci, L. Marchetti, Tetrahedron 31 (1975) 1745. [12] J. Laleve´e, X. Allonas, J.P. Fouassier, J. Am. Chem. Soc. 124 (2002) 9613. [13] M.J. Frisch et al., GAUSSIAN 98, Revision A.11, Gaussian, Inc., Pittsburgh PA, 2001. [14] W. Knolle, U. Muller, R. Mehnert, Phys. Chem. Chem. Phys. 2 (2000) 1425. [15] W. Knolle, R. Mehnert, Nucl. Instrum. Methods Phys. Res., Sect. B 105 (1995) 154. [16] E. Takacs, L. Wojnarovits, Nucl. Instrum. Methods Phys. Res., Sec. B 105 (1995) 282.
J. Laleve´e et al. / Chemical Physics Letters 449 (2007) 231–235 [17] P. Kujawa, N. Mohid, K. Zaman, W. Manshol, P. Ulanski, J.M. Rosiak, Rad. Phys. Chem. 53 (1998) 403. [18] R. Martschke, R.D. Farley, H. Fischer, Helv. Chim. Acta 80 (1997) 1363. [19] J.L. Weisman, M. Head-Gordon, J. Am. Chem. Soc. 123 (2001) 11686.
235
[20] S. Hirata, M. Head-Gordon, Chem. Phys. Lett. 302 (1999) 375. [21] G.A. DiLabio, G. Litwinienko, S. Lin, D.A. Pratt, K.U. Ingold, J. Phys. Chem. A 106 (2002) 11719. [22] K.O. Siegenthaler, A. Studer Macromolecules 39 (2006) 1347. [23] N.V. Lebedeva et al., Phys. Chem. Chem. Phys. 6 (2004) 2254.
Chemical Physics Letters 449 (2007) 231–235 www.elsevier.com/locate/cplett
Interaction of monomer radicals with nitroxides: A new access to the radical-radical combination rate constants Jacques Laleve´e a
a,*
, Didier Gigmes b,*, Denis Bertin b, Xavier Allonas a, Jean Pierre Fouassier a
De´partement de Photochimie Ge´ne´rale, Ecole Nationale Supe´rieure de Chimie de Mulhouse, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France b Universite´ de Provence, UMR 6517, Equipe CROPS. Case 542, Av.Escadrille Normandie Nie´men, 13397 Marseille Cedex 20, France Received 4 September 2007; in final form 10 October 2007 Available online 13 October 2007
Abstract Acrylate, methacrylate and styrene monomer radicals produced by the addition of an aminoalkyl radical to the corresponding monomer unit were directly observed by laser flash photolysis. These radicals are characterized by a strong absorption in the near UV/visible part of the spectrum allowing to directly follow their interaction with different nitroxides usable in nitroxide mediated polymerization (NMP). The recombination rate constants were measured: they are in agreement with the literature data when available; other values are new. This procedure is fast and reliable. 2007 Elsevier B.V. All rights reserved.
1. Introduction Acrylates, methacrylates and styrene represent widely used classes of monomers M for thermal [1a] and photoinduced [1b] polymerization reactions. The reactivity of the monomer radicals R–M produced after the initiation step (where R is the initiating radical) and the propagating radicals R–ðMÞn governs for a part the whole polymerization efficiency. Nitroxide mediated polymerization NMP – which is now recognized as a powerful method for the control of a polymerization reaction [2,3] – is based on a reversible cleavage of the C–O covalent bond of a dormant species R–(M)n–ONR 0 (R00 ) (formed between the propagating radical and a nitroxide R 0 (R00 )NO radical – Scheme 1) that further generates the growing polymer radical R–ðMÞn 0 00 and the persistent radical R (R )NO . The nitroxide is introduced into the monomer medium as either a radical (R 0 (R00 )NO such as TEMPO for bicomponent system) or advantageously with an alkoxyamine (R 0 (R00 )NOR that *
Corresponding authors. Fax: +33 3 89 33 68 95. E-mail addresses: [email protected] (J. Laleve´e), [email protected] (D. Gigmes). 0009-2614/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.10.040
thermally cleaves into an initiating radical R and a nitroxide R 0 (R00 )NO acting as the controlling agent) [3c]. The controlled character is strongly related to both the dissociation kd and the recombination kc rate constants of the dormant species [4]. The cleavage rates of alkoxyamines (kd) were investigated in some detail for both polymeric and low molecular model systems in wide temperature ranges, and predictive rationalization have been offered [5]. In comparison much less is known about kc between nitroxide and carbon centered radicals certainly because such studies require very specific devices but also the preparation of specific initiators [6–8]. However, a better knowledge of the recombination reaction of nitroxides with monomer radicals is particularly important for development of NMP processes. In the case of monomer type radicals, two main general procedures have been used previously for such determination: (i) the use of laser flash photolysis to determine kc between various nitroxides and model radicals [7,8] and (ii) pulsed laser polymerization (PLP) techniques with size exclusion chromatography (SEC) for the recombination between macroradicals and nitroxides [9]. Actually these methods require the preparation of the specific initiators
J. Laleve´e et al. / Chemical Physics Letters 449 (2007) 231–235
232
N OEt P OEt O
N
N
O
O
TEMPO
N
Molecular orbital calculations were carried out with the GAUSSIAN 03 suite of programs [13]. Using the time dependent density functional theory (TDDFT) at TD/ MPW1PW9/6-311++G** level, the absorption properties of the styrene radicals were calculated on the basis of the frequency checked geometries calculated at the UB3LYP/ 6-31G(d) level. 3. Results and discussion
O
SG1
DPAIO
3.1. Observation of the monomer radicals (methyl acrylate, methyl methacrylate, styrene)
2. Experimental section and methods
Monomer radicals exhibit an absorption band close to 300 nm [14–18] and the overlap with the absorption of the starting compound, additives or other intermediates renders difficult their direct observation. As a consequence, despite the great interest concerned with their reactivity, very few direct studies have been devoted to these radicals. The procedure used below for the observation of monomer radicals was recently developed to characterize the reactivity of (meth) acrylate radicals [10]. Our original approach consisted into three consecutive reactions: generation of tert-butoxyl radicals through the photochemical decomposition of di-tert-butylperoxide, a(C–H) hydrogen abstraction reaction from triethylamine and addition of the aminoalkyl radical TEA to the monomer double bond. The aminoalkyl radical was easily observed [10]. A new transient appeared in the near UV visible wavelength range (whose the risetime corresponds to the decay time of the aminoalkyl radical): 480 nm and 440 nm for TEA–MA and TEA–MMA, respectively, [10]. The procedure is developed here for styrene: the transient absorption around 350 nm is ascribed to TEA–STY (Fig. 1) and the addition rate constant of TEA to styrene is 1.5 · 106 M1 s1. In Fig. 1, the transient spectrum obtained for TEA–STY was compared to that found for STY formed by a direct hydrogen abstraction between ethylbenzene and the tert-butoxyl radical: the absorption
Methyl acrylate (MA), methyl methacrylate (MMA), styrene (STY), 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO) were obtained from Aldrich. SG1 has been kindly provided by Arkema Company; DPAIO has been prepared according to a published procedure [11]. The stabilizer was removed from monomers by column purification (Aldrich AL-154). Di-tert-butylperoxide from Aldrich has been used directly without further purification. Triethylamine (TEA) was purified by distillation. The investigated nitroxide radicals are depicted Scheme 1. The nanosecond transient absorption setup has been previously described in detail [12]. The excitation source is a Q-switched nanosecond Nd/YAG laser (kexc = 355 nm; 9 ns pulses; energy reduced down to 10 mJ; Powerlite 9010 Continuum); the analyzing system consists in a pulsed xenon lamp, a monochromator, a fast photomultiplier and a transient digitizer.
Fig. 1. Transient absorption spectra of the STY (square) and TEA– STY(circle) radicals.
N H2C
H C CH3
STY
.
CH
TEA-STY
.
Scheme 1.
R–(M)n–ONR 0 (R00 ) to study the combination reaction. This corresponds to a strong limitation. As a consequence, the development of a method based on easy accessible materials, allowing temperature or solvent dependences studies is desired to understand clearly the different factors influencing kc but also to develop models for predicting these values as for the kd of alkoxyamines. In this Letter, such an approach is shown through the direct measurement of the kc recombination rate constants of various nitroxides with methyl acrylate, methyl methacrylate and styrene monomer radicals. This will be carried out from the direct observation of the monomer radicals by the LFP technique as already proposed in a recent study [10].
J. Laleve´e et al. / Chemical Physics Letters 449 (2007) 231–235
maximum is red-shifted by about 40 nm when going from STY to TEA–STY. The presence of TEA–STY is confirmed by quantum mechanical calculations using the time dependent density functional theory (TDDFT). The results are gathered in Table 1 for STY, TEA–STY and TEA–STY–STY. For STY, the absorption band observed at 315 nm is quite well reproduced by TDDFT calculations supporting the contention that these methods can describe electronic transitions of various radicals [19–21]. For TEA–STY, in addition to the usual UV absorption band of the styrene radicals, the intense transition experimentally observed at
233
350 nm is also quite well reproduced by quantum mechanical calculations (395 nm). As previously observed for MA and MMA [10], this new band is due to a hyperconjugation between the nitrogen lone pair and the HOMO– SOMO orbitals of the monomer radicals. This interaction is sensitive to the distance between the nitrogen and the radical centered, i.e., the addition of another monomer unit leads to a drastic decrease of the oscillator strength of this band and a blue shift of the absorption. The calculated absorption of TEA–STY–STY is similar to that of STY (see Fig. 2). 3.2. Reactivity of monomer radicals toward nitroxides
Table 1 Calculations of the absorption wavelengths and oscillator strengths (f) for STY, TEA–STY, TEA–STY-STY at TD/MPW1PW91/6-311++G** level and comparison with the experimental data Radical
STY TEA–STY TEA–STY–STY
TD/MPW1PW91/6-311++G**
Exp.
k (nm)
f
k (nm)
311.7 395.3 322.2
0.0298 0.067 0.022
315 350 –
The reactivity of these radicals toward the different nitroxides can now be easily probed. The change of kc with the amine structure was first investigated since the steric hindrance relative to this moiety of the monomer radical might affect this factor (Table 2). It can be noted that the amines used weakly affect kc. Therefore, we will now use TEA. The recombination rate constants kc of methyl acrylate, methyl methacrylate and styrene monomer radicals with TEMPO, SG1 and DPAIO have been measured (Table 3). The concentrations used for the monomers
Table 2 Effect of the amine structure on kc (system MA-SG1) Amine
kc M1 s1
Triethylamine Dibutylamine Diethylamine
4.4 · 107 6.2 · 107 6.0 · 107
Solvent: di-tert-butylperoxide.
Table 3 Recombination rate constants between monomer radicals and nitroxides (solvent: di-tert-butylperoxide) Monomer
SG1 (M1 s1)
DPAIO (M1 s1)
MA
5.5 · 108 (4.4 · 108)a (6.4 · 108)e
4.4 · 107 (4.0 · 107)a
2.5 · 108
MMA
2.5 · 108 (1.5 · 108)a (6.3 · 108)d
1.2 · 106 (6.4 · 105)a (9.9 · 105)b (2.6 · 106)d
<3 · 107f
1.2 · 108 (9.5 · 107)a (1.75 · 108)c (3 · 108)d
2.2 · 106 (1.8 · 106)a (3.0 · 105)c (3.0 · 106)d
1.0 · 108
STY
a
Solvent (15% di-tert-butyl peroxide/85% acetonitrile). Solvent (85% di-tert-butyl peroxide/15% ethanol). c Literature data from PLP experiments: Ref. [9]. d Literature data from model radical quenching: Ref. [7]. e Literature data from SEMF CIDNP experiments: Ref. [23] in benzene. f The intrinsic absorption of DPAIO at 355 nm prevents the use of higher concentrations in quenching experiments leading to this upper value. b
Fig. 2. Interaction of TEA–MA with SG1: (a) effect of [SG1] on the TEA–MA kinetics; an increase of the concentration (in the series 0.078; 0.134; 0.248; 0.475 M) leads to a decrease of the radical lifetime time; (b) Stern–Volmer plot for the determination of kc.
TEMPO (M1 s1)
234
J. Laleve´e et al. / Chemical Physics Letters 449 (2007) 231–235
were high enough to neglect the recombination of TEA with the nitroxides. When available, the literature LFP data [7] are found in good agreement with the values determined here (for MMA/TEMPO, STY/TEMPO, MMA/SG1 and STY/SG1). The PLP results [9] satisfactorily match the present values (STY/TEMPO and STY/ SG1). The value for MA/TEMPO derived from SEMFCIDNP experiments is also in agreement. This gives confidence to the proposed procedure. Values for MA (or MMA, STY)/DPAIO are new. For a given monomer, kc decreases in the series TEMPO > DPAIO SG1. This trend agrees with previous experiments which have shown that kc decreases when the steric hindrance of the nitroxide moiety increases [6b,7,9,22]. For the DPAIO structure, more studies are needed to evaluate the influence of the single electron delocalization on the kc value. As already reported for a given nitroxide [6b–8], kc decreases when the steric hindrance of the alkyl radical increases (TEA–MA > TEA–MMA) and when the stabilization radical increases (TEA–MA > TEA–STY). It has to be pointed out that the TEA–MA/ TEA–MMA, TEA–MA/TEA–STY and TEA–MMA/ TEA–STY ratios are different for TEMPO (2.2; 4.6; 2.1, respectively) and SG1 (36.6; 20; 0.5, respectively). These results suggest that for the kc values, both steric and polar effects are more important with SG1 than with TEMPO. DPAIO is as selective as SG1 to the MMA type radical: for example, when going from the recombination of TEA–MA to TEA–MMA, a decrease of at least one order of magnitude is observed in the case of DPAIO, 35 for SG1 and only 2 for TEMPO. However, compared to SG1, DPAIO is less sensitive to secondary stabilized alkyl radicals. Indeed for DPAIO, kc decreases by a factor of 2.5 from TEA–MA to TEA–STY, this factor being equal to 4.6 for TEMPO and 20 for SG1. The high recombination rate constants and the low dissociation rate constants explain quite well why DPAIO is inefficient [3e] for the controlled radical polymerization of acrylate and styrene monomers. The effect of the polarity and the hydrogen bonding ability of the medium on kc was also investigated. An increase of the solvent polarity upon the addition of acetonitrile in di-tert-butylperoxide leads to a slight decrease of kc (about a two fold factor). This can be explained by the resonance zwitterionic structure of the nitroxide which is weakly favored in polar media [6a]. The addition of ethanol (15%) in di-tert-butylperoxide leads to a very weak change of kc which decreases from 1.2 · 106 to 9.9 105 M1 s1 for TEA–MMA/SG1. This result evidences the weak influence of the hydrogen bonding on the recombination rate constant for this system.
4. Conclusions Our approach appears as a very convenient way to gather a lot of experimental values for the recombination
of the monomer radical models R–M with nitroxides. Taking into account the agreement with the PLP experiments, this could be an indication for the recombination of nitrox ides with propagating radicals R–ðMÞn . The main advantages of the present method lie on the direct observation of the radicals as well as the reliability of the measurements. This approach appears powerful and promising to study and compare the reactivity of different NMP mediators for a wide range of monomers. The improvement of the controlled polymerization reactions through a careful selection of nitroxide/monomer couples appears as a fascinating task. References [1] (a) K. Matyjaszewski, in: K. Matyjaszewski, Y. Gnanou, L. Leibler (Eds.), Macromolecular Engineering, vol. 1, Wiley VCH, Weinheim, 2007, p. 161; (b) J.P. Fouassier, Photoinitiation Photopolymerization and Photocuring, Hanser Publishers, Munich, New York, 1995. [2] (a) K. Matyjaszewski, in: ACS Symposium Series Advances in Controlled/Living Radical Polymerization, American Chemical Society, 2002; (b) K. Matyjaszewski, in: ACS Symposium Series Controlled Radical Polymerization, American Chemical Society, 1998. [3] (a) D.H. Solomon, E. Rizzardo, P. Cacioli, US Patent 4(581) (1986) 429; (b) M.K. Georges, R.P.N. Veregin, P.M. Kazmaier, Macromolecules 26 (1993) 2987; (c) C.J. Hawker, A.W. Bosman, E. Harth, Chem. Rev. 101 (2001) 3661; (d) G. Moad, D.H. Solomon, The Chemistry of Radical Polymerization, second edn., Elsevier, Amsterdam, 2006; (e) Y. Guillaneuf, D. Gigmes, S.R.A. Marque, P. Astolfi, L. Greci, P. Tordo, D. Bertin, Macromolecules 40 (2007) 3108. [4] (a) H. Fischer, Chem. Rev. 101 (2000) 3581; (b) H. Fischer, ACS Symp. Ser. 854 (2003) 10; (c) A. Goto, T. Fukuda, Prog. Polym. Sci. 29 (2004) 329. [5] (a) S. Marque, J. Org. Chem. 68 (2003) 7582; (b) D. Bertin, D. Gigmes, S. Marque, Recent Res. Devel. Organic Chem. 10 (2006) 63. [6] (a) A.L.J. Beckwith, V.W. Bowry, K.U. Ingold, J. Am. Chem. Soc. 114 (1992) 4983; (b) V.W. Bowry, K.U. Ingold, J. Am. Chem. Soc. 114 (1992) 4992. [7] J. Sobeck, R. Martschke, H. Fischer, J. Am. Chem. Soc. 123 (2001) 2849. [8] W.G. Skene, J.C. Scaiano, N.A. Listigovers, P.M. Kazmaier, M.K. Georges, Macromolecules 33 (2000) 5065. [9] Y. Guillaneuf, D. Bertin, P. Castignolles, B. Charleux, Macromolecules 38 (2005) 4638. [10] J. Lalevee, X. Allonas, J.P. Fouassier, Chem. Phys. Lett. 415 (2005) 287. [11] C. Berti, M. Colonna, L. Greci, L. Marchetti, Tetrahedron 31 (1975) 1745. [12] J. Laleve´e, X. Allonas, J.P. Fouassier, J. Am. Chem. Soc. 124 (2002) 9613. [13] M.J. Frisch et al., GAUSSIAN 98, Revision A.11, Gaussian, Inc., Pittsburgh PA, 2001. [14] W. Knolle, U. Muller, R. Mehnert, Phys. Chem. Chem. Phys. 2 (2000) 1425. [15] W. Knolle, R. Mehnert, Nucl. Instrum. Methods Phys. Res., Sect. B 105 (1995) 154. [16] E. Takacs, L. Wojnarovits, Nucl. Instrum. Methods Phys. Res., Sec. B 105 (1995) 282.
J. Laleve´e et al. / Chemical Physics Letters 449 (2007) 231–235 [17] P. Kujawa, N. Mohid, K. Zaman, W. Manshol, P. Ulanski, J.M. Rosiak, Rad. Phys. Chem. 53 (1998) 403. [18] R. Martschke, R.D. Farley, H. Fischer, Helv. Chim. Acta 80 (1997) 1363. [19] J.L. Weisman, M. Head-Gordon, J. Am. Chem. Soc. 123 (2001) 11686.
235
[20] S. Hirata, M. Head-Gordon, Chem. Phys. Lett. 302 (1999) 375. [21] G.A. DiLabio, G. Litwinienko, S. Lin, D.A. Pratt, K.U. Ingold, J. Phys. Chem. A 106 (2002) 11719. [22] K.O. Siegenthaler, A. Studer Macromolecules 39 (2006) 1347. [23] N.V. Lebedeva et al., Phys. Chem. Chem. Phys. 6 (2004) 2254.