Quinone Transfer Radical Polymerization of Styrene Catalyzed by

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Macromolecules 2005, 38, 6310-6315

Quinone Transfer Radical Polymerization of Styrene Catalyzed by Metal Complexes in the Presence of Various o-Quinones Antoine Debuigne,† Jean-Raphae1 l Caille,‡ and Robert Je´ roˆ me*,† Center for Education and Research on Macromolecules (CERM), University of Lie` ge, Sart-Tilman, B6, 4000 Lie` ge, Belgium, and Solvay Research and Technology, rue de Ransbeek 310, B-1120 Brussels, Belgium Received January 13, 2005; Revised Manuscript Received May 19, 2005

ABSTRACT: Several o-quinones have been tested as control agents for the quinone transfer radical polymerization (QTRP) of styrene in the presence of cobalt(II) acetylacetonate. In contrast to the commercially available 3,5-di-tert-butyl-o-benzoquinone (3,5-DtBBQ) and tetrachloro-1,2-benzoquinone (tetrachloro-1,2-BQ), 3,6-dimethoxy-9,10-phenanthrenequinone (3,6-DMPhQ) imparts a better control to the radical polymerization of styrene compared to 9,10-phenanthrenequinone (PhQ) used as a reference. Indeed, whenever 3,6-DMPhQ is added with a catalytic amount of metal complexes (Co(acac)2 or Al(acac)3), the evolution of the molar mass with the monomer conversion is linear, and low polydispersity (Mw/Mn ∼ 1.1-1.2) is observed until very high monomer conversion (∼90%). Finally, the methoxy protons of 3,6-DMPhQ have been detected by 1H NMR analysis of the polystyrene end groups, which is important mechanistic information.

Introduction

Scheme 1

Functionalized macromolecules with well-defined molecular structure and architecture have been actively synthesized by controlled radical polymerization (CRP) for the past few years. The interest for CRP lies in the relatively undemanding experimental conditions for implementation. The general strategy relies on the decrease of the instantaneous radical concentration, which is kinetically favorable to propagation with respect to termination. Three techniques of reversible end-capping of the growing macroradicals are known, i.e., nitroxide-mediated polymerization (NMP),1-4 atom transfer radical polymerization (ATRP),4-10 and reversible addition-fragmentation chain transfer (RAFT).11-14 In addition to these strategies, we reported recently on a new method able to control at least partly the radical polymerization of styrene, based on a complex of phenanthrenequinone (PhQ) (R ) H) and cobalt(II) acetylacetonate (Co(acac)2), according to the mechanism shown in Scheme 1.15 The experimental data suggested the formation of a persistent radical centered on an oxygen atom (I) by redox reaction of Co(acac)2 and PhQ. This radical species is able to trap radicals formed by the styrene autopolymerization,16 so forming dormant species (III) involved in an equilibrium with the active species. Interestingly enough, a catalytic amount of the metal complex compared to PhQ is enough to provide the styrene radical polymerization with control, at least at low monomer conversion. This system was called quinone transfer radical polymerization (QTRP). Because the polymerization control is lost at styrene conversion higher than 50%, there was a need for optimization. In a previous paper, several metal complexes were tested in combination with phenanthrenequinone (PhQ),17 including Co(acac)2, Al(acac)3, Ni(acac)2, and Mn(acac)2 or 3, without, however, any †

University of Lie`ge. Solvay Research and Technology. * To whom correspondence should be addressed: Tel (32)43663565; fax (32)4-3363497; e-mail [email protected]. ‡

significant improvement. This paper aims at reporting on the effect of a series of o-quinones of different structures on the extent of the polymerization control in the presence of catalytic amounts of Co(acac)2 and Al(acac)3. Experimental Section Materials. Styrene (Aldrich, >99%) was dried over calcium hydride, degassed by several freeze-thawing cycles, distilled under reduced pressure, and stored under nitrogen. Toluene (tolu) was dried over sodium and degassed by bubbling of nitrogen for 15 min. Phenanthrenequinone (PhQ) (>99%, Aldrich), cobalt(II) acetylacetonate (>98% Merck), aluminum(III) acetylacetonate (97%, Janssens Chimica), tetrachloro-1,2benzoquinone (tetrachloro-1,2-BQ) (Aldrich), 3,5-di-tert-butylo-benzoquinone (3,5-DtBBQ) (>98%, Fluka), diphenyl diselenide (98%, Aldrich), 4,4′-dimethoxybenzoin (95%, Aldrich), phenyl-

10.1021/ma050076l CCC: $30.25 © 2005 American Chemical Society Published on Web 06/25/2005

Macromolecules, Vol. 38, No. 15, 2005

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Table 1. Styrene Radical Polymerization at 100 °C in the Presence of o-Quinones and Catalytic Amount of Co(acac)2a entry

o-quinone

1

PhQ

2

3,5-dtBBQ

3

tetrachloro-1,2-BQ

time (h)

conv (%)

Mn,SEC (g/mol)

Mn,theor (g/mol)

Mn,theor/Mn,SEC

Mw/Mn

28 45 52 73 165 26 51 67 43 67 92

6 26 32 44 77 6 16 24 8 20 27

3 300 11 700 13 700 17 700 18 500 42 000 57 000 68 000 46 000 56 000 63 000

2 700 11 800 14 600 20 000 35 000 2700 7300 10 900 3600 9100 12 300

0.82 1.01 1.07 1.13 1.89 0.06 0.13 0.16 0.08 0.16 0.20

1.30 1.28 1.29 1.31 1.50 2.16 2.55 2.53 2.38 2.40 2.47

a Styrene/toluene ) 5/1 (v/v); (1) [phenanthrenequinone]/[Co(acac) ]/[Sty]: 1/0.01/440; (2) [3,5-di-tert-butyl-o-benzoquinone]/[Co(acac) ]/ 2 2 [Sty]: 1/0.01/440; (3) [tetrachloro-1,2-benzoquinone]/[Co(acac)2]/[Sty]: 1/0.01/440.

Table 2. Styrene Radical Polymerization at 100 °C in the Presence of 3,6-Dimethoxyphenanthrenequinone (3,6-DMPhQ) and Catalytic Amount of Co(acac)2 or Al(acac)3 entry

[Sty]/[3,6-DMPhQ]

metal

time (h)

conv (%)

Mn,SEC (g/mol)

Mn,theor (g/mol)

Mn,theor/Mn,SEC

Mw/Mn

1

440

Co(acac)2

2 3

87 440

Co(acac)2 Al(acac)3

4

87

Al(acac)3

37 54 61 77 107 150 160 43 50 67 95 163 160

4 23 29 41 53 65 30 24 33 48 69 82 25

2 100 11 400 14 000 19 500 24 200 28 600 2 900 10 500 13 700 19 500 25 300 30 200 2 400

2 200 10 500 13 200 18 700 24 100 29 600 2 700 10 900 15 000 21 800 31 400 37 300 2 300

1.05 0.92 0.94 0.96 1.00 1.03 0.93 1.04 1.09 1.12 1.24 1.24 0.96

1.45 1.17 1.16 1.13 1.14 1.15 1.15 1.15 1.15 1.10 1.10 1.20 1.15

(1) [3,6-DMPhQ]/[Co(acac)2]/[Sty]: 1/0.01/440, styrene/toluene ) 5/1 (v/v); (2) [3,6-DMPhQ]/[Co(acac)2]/[Sty]: 1/0.01/87; (3) [3,6-DMPhQ]/ [Al(acac)3]/[Sty]: 1/0.01/440, styrene/toluene ) 5/1 (v/v); (4) [3,6-DMPhQ]/[Al(acac)3]/[Sty]: 1/0.01/87. a

boronic acid (97%, Aldrich), and 3,3′-dimethoxybiphenyl (97%, Aldrich) were used as received. All polymerization experiments were performed using the classical Schlenck techniques under nitrogen. Liquids were transferred under nitrogen with syringes or stainless steel capillaries. Characterization. Molar mass and polydispersity of polystyrene were determined by size exclusion chromatography (SEC), with THF as an eluent at 40 °C, with a HewlettPackard 1090 liquid chromatograph (four columns HP PL gel 5 µm 105, 104 , 103, and 102 Å) connected to a Hewlett-Packard 1037 refractive index detector. Polystyrene standards, with a narrow molar mass distribution, were used for calibration. NMR spectra were recorded with a Bruker AM 400 spectrometer (400 MHz) in deuterated chloroform. Infrared spectra were recorded with a Perkin-Elmer FT-IR instrument. Elementary analyses (EA) were carried out with a Carlo-Erba elemental analyzer CHNS-O EA1108. General Recipe for the Styrene Radical Polymerization in the Presence of an o-Quinone and Catalytic Amount of the Metal Complex. The 3,6-dimethoxy-9,10phenanthrenequinone (3,6-DMPhQ) (0.026 g, 1.0 × 10-4 mol) was added into a 30 mL flask and degassed by three vacuumnitrogen cycles. Degassed styrene (5.0 mL, 44 × 10-3 mol) and a Co(acac)2 (1.0 × 10-6 mol) solution in toluene (1.0 mL) were added with a syringe under nitrogen. The reactive mixture was stirred and heated at 100 °C. No polymerization occurred for 36 h, followed by a substantial increase in the solution viscosity. Samples were withdrawn vs time, and the styrene conversion was calculated by gravimetry after elimination of toluene and the residual monomer in vacuo at 80 °C (conversion, Mn and Mw/Mn data are collected in Table 2, entry 1). The same recipe was used for the other o-quinones (PhQ, 3,5-DtBBQ, tetrachloro-1,2-BQ) and metal complexes (Al(acac)3). General Recipe for the Synthesis of Low Molar Mass Polystyrene Macroinitiator in the Presence of 3,6DMPhQ and Catalytic Amount of the Metal Complex. The 3,6-DMPhQ (0.108 g, 4.0 × 10-4 mol) was added into a 30

mL flask and degassed by three vacuum-nitrogen cycles. A Co(acac)2 (4.0 × 10-6 mol) solution in toluene (4.0 mL) was added with a syringe under nitrogen. Toluene was evaporated under reduced pressure at room temperature and degassed styrene (4.0 mL, 35 × 10-3 mol) was added. This solution was heated at 100 °C, under stirring. After a few hours, the color changed from orange to dark green, and no polymerization occurred for 6 days. After 160 h, a substantial increase in the solution viscosity was observed, and the initially colored reaction medium was discolored (monomer conversion ) 30%). After dilution by THF, the polymer was purified by repeated precipitation in methanol, filtered, and dried in vacuo at room temperature. The collected polystyrene (conversion ) 30%) was colorless and designated as PS macroinitiator 1 (Mn,SEC ) 2900 g/mol, Mw/Mn ) 1.15). The same procedure was repeated in the presence of Al(acac)3. After 160 h, the styrene conversion was 25%, and the PS macroinitiator 2 was collected with Mn,SEC ) 2400 g/mol and Mw/Mn ) 1.15. General Recipe for the Resumption of the Styrene Polymerization by a Polystyrene Macroinitiator and Catalytic Amount of the Metal Complex. The polystyrene macroinitiator end-capped by 3,6-DMPhQ (0.5 × 10-4 mol) was added into a 30 mL flask and degassed by three vacuumnitrogen cycles. A Co(acac)2 or Al(acac)3 (0.5 × 10-6 mol) solution in toluene (0.5 mL) was added with a syringe under nitrogen. Toluene was evaporated under reduced pressure at room temperature, degassed styrene (2.5 mL, 22 × 10-3 mol) was added, and the mixture was heated at 100 °C under stirring. Samples were regularly withdrawn from the polymerization medium, and the monomer conversion was calculated as before. The same reaction was also conducted without catalyst. Synthesis of 4,5-Bis(4-dimethoxyphenyl)-2-phenyl1,3,2-dioxaborole. 4,4′-Dimethoxybenzoin (9.25 g, 3.4 × 10-2 mol) and phenylboronic acid (4.12 g, 3.4 × 10-2 mol) were dissolved in dry toluene (200 mL) in a 250 mL flask equipped with a Dean-Stark. The reaction mixture was refluxed for 4 h. A dark brown residue was collected after evaporation of

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Macromolecules, Vol. 38, No. 15, 2005 Scheme 2

toluene under reduced pressure. Yellow crystals of 4,5-bis(4dimethoxyphenyl)-2-phenyl-1,3,2-dioxaborole (8.5 g) were collected after crystallization in hexane and kept in a sealed tube at -20 °C (yield ) 70%, mp ) 113 °C). 1H NMR (CDCl3): 8.05 ppm (2 Harom, d, J ) 6.50 Hz); 7.60 ppm (4 Harom, d, J ) 8.75 Hz); 7.53-7.46 ppm (3 Harom, m); 6.92 ppm (4 Harom, d, J ) 8.75 Hz); 3.85 ppm (6 H, -OCH3, s). Synthesis of 3,6-Dimethoxy-9,10-phenanthrenequinone (3,6-DMPhQ). 4,5-Bis(4-dimethoxyphenyl)-2-phenyl-1,3,2dioxaborole (7.83 g, 2.2 × 10-2 mol) and diphenyl diselenide (6.86 g, 2.2 × 10-2 mol) were dissolved in benzene (300 mL). The reaction mixture was degassed by bubbling of argon for 30 min and irradiated with a mercury lamp (HPK 125 W, peak at 260 nm) for 4 days. A 1 M NaOH solution (200 mL) was then added. This aqueous phase, which turned red, was taken off with a cannula and extracted with chlorofom (500 mL). The organic phase was then washed four times with a 1 M NaOH solution (4 × 100 mL) and two times with a 5% HCl solution (2 × 100 mL). The final yellow solution was dried over magnesium sulfate, filtered, and evaporated under reduced pressure. The collected orange powder was washed several times with hexane (50 mL) to remove the residual diphenyl diselenide. Finally, an orange powder of 3,6-DMPhQ (2.4 g) was collected (yield ) 41%, mp ) 235 °C). 1H NMR (CDCl3): 8.18 ppm (2 Harom, d, J ) 8.9 Hz); 7.36 ppm (2 Harom, d, J ) 2.1 Hz); 6.95 ppm (2 Harom, dd, J ) 8.7 and 2.3 Hz); 3.97 ppm (6 H, -OCH3, s). Elementary analysis: Calcd for C16H12O2 : C, 71.63%; H, 4.51%. Found: C, 70.96%; H, 4.68%.

Results and Discussion Combination of Various o-Quinones and Metal Complexes. Recently, we showed that the styrene autopolymerization falled under partial control when conducted in the presence of phenanthrenequinone and a catalytic amount of cobalt(II) acetylacetonate (Table 1, entry 1).15 Indeed, polystyrene of low molar mass and narrow molar mass distribution was formed under these conditions. Moreover, the molar mass of polystyrene increased with the monomer conversion until 50%, beyond which deviation from linearity was observed. Then, the Mn,theor/Mn,SEC ratio exceeded 1 as shown in Table 1 (entry 1). With the purpose to improve the control of the styrene radical polymerization, o-quinones of different structures have been tested, such as the commercially available 3,5-di-tert-butyl-o-benzoquinone (3,5-DtBBQ) and tetrachloro-1,2-benzoquinone (tetrachloro-1,2-BQ) shown in Scheme 2. Polystyrene with an exceedingly high molecular weight and broadly dispersed is formed in the presence of these two o-quinones and a catalytic

Figure 1. Dependence of molar mass and polydispersity on monomer conversion for the styrene polymerization at 100 °C in the presence of phenanthrenequinone (PhQ) and 3,6dimethoxy-9,10-phenanthrenequinone (3,6-DMPhQ), added with a catalytic amount of Co(acac)2 (entries 1 in Tables 1 and 2, respectively). ([Quinone]/[Co(acac)2]/[Sty]: 1/0.01/437, styrene/ toluene ) 5/1 (v/v)). The dotted line is the theoretical dependence.

amount of Co(acac)2 (Table 1, entries 2 and 3, respectively). These disappointing results confirm the strong influence of the structure of the quinone on the polymerization control and are incentives for testing other quinones. Therefore, the better results noted in Table 1 prompted us to maintain the phenanthrenequinone core unchanged and to modulate its reactivity by substitution with methoxy groups in positions 3 and 6. Synthesis of 3,6-dimethoxy-9,10-phenanthrenequinone (3,6-DMPhQ) was reported by Machiori et al. and consists of two steps: formation of 4,5-bis(4-dimethoxyphenyl)-2-phenyl-1,3,2-dioxaborole by condensation of phenylboronic acid and 4,4′-dimethoxybenzoin, followed by the oxidative photocyclization of the dioxaborole by irradiation with a mercury lamp in the presence of diphenyl diselenide (Scheme 3).18 The radical polymerization of styrene has been conducted in the presence of 3,6-DMPhQ and a catalytic amount of Co(acac)2 (1% with respect to the quinone) at 100 °C. The experimental data listed in Table 2 (entry 1) show that the control imparted to the styrene polymerization by 3,6-DMPhQ is improved compared to PhQ, as assessed by a narrower molar mass distribution and a Mn,theor/Mn,SEC ratio closer to 1. Figure 1 confirms the linearity of the molar mass of polystyrene with the monomer conversion up to 65% conversion when Co(acac)2 is combined with 3,6-DMPhQ instead of phenanthrenequinone. It must be recalled that the theoretical molar masses are calculated on the assumption that the quinone is the precursor of the control agent. The very good fit between experiment and prediction gives credit to this proposal. Figure 2 gives insight into the polymerization control. Indeed, the SEC chromatograms are clearly shifted as a whole to smaller elution volumes when the styrene conversion is increased.

Scheme 3



Figure 2. Size exclusion chromatograms (SEC) of polystyrene formed at 100 °C in the presence of 3,6-dimethoxy-9,10phenanthrenequinone (3,6-DMPhQ) and a catalytic amount of Co(acac)2 (Table 2, entry 1). [3,6-DMPhQ]/[Co(acac)2]/[Sty]: 1/0.01/440, styrene/toluene ) 5/1 (v/v).

Figure 5. (a) Plot of ln([M]0/[M]) vs time and (b) dependence of Mn and Mw/Mn on monomer conversion for the styrene polymerization in the presence of 3,6-dimethoxy-9,10-phenanthrenequinone (3,6-DMPhQ) and a catalytic amount of Co(acac)2 at 100 and 120 °C ([3,6-DMPhQ]/[Co(acac)2]/[Sty]: 1/0.01/440, sty/tolu ) 5/1 (v/v)). The dotted line is the theoretical dependence. Figure 3. Plot of ln([M]0/[M]) vs time for the styrene polymerization at 100 °C in the presence of phenanthrenequinone (PhQ) and 3,6-dimethoxy-9,10-phenanthrenequinone (3,6DMPhQ), added with a catalytic amount of Co(acac)2 (entries 1 in Tables 1 and 2, respectively). [o-Quinone]/[Co(acac)2]/[Sty]: 1/0.01/440, styrene/toluene ) 5/1 (v/v).

Figure 4. Dependence of molar mass and polydispersity on styrene conversion for the styrene polymerization at 100 °C in the presence of phenanthrenequinone (PhQ) and 3,6dimethoxy-9,10-phenanthrenequinone (3,6-DMPhQ), added with a catalytic amount of Al(acac)3 (Table 2, entry 3) ([oquinone]/[Al(acac)3]/[Sty]: 1/0.01/440, styrene/toluene ) 5/1 (v/ v)). The dotted line is the theoretical dependence.

Kinetics of the styrene polymerization at 100 °C, in the presence of a catalytic amount of Co(acac)2 and 3,6DMPhQ, is first order in monomer as assessed by the linear dependence of ln([M]0/[M]) on time (Figure 3). An induction period of time is observed, as was the case with phenanthrenequinone, the tentative explanation

being the time required for the in-situ formation of the control agent.15 Al(acac)3 is slightly less efficient that Co(acac)2 in controlling the styrene polymerization in the presence of 3,6-DMPhQ. Although the polymer polydispersity remains very low, the Mn,theor/Mn,SEC ratio exceeds 1 at lower conversion as shown in Table 2 (entry 3) and Figure 4. Effect of Temperature. Although 3,6-DMPhQ allows control of the radical polymerization of styrene at 100 °C, this polymerization is slow (65% conversion after 150 h). Therefore, the polymerization temperature has been increased, and expectedly, the polymerization rate is much higher and the induction period is shortened at 120 °C compared to 100 °C (Figure 5a). However, Figure 5b clearly shows that Mn strongly deviates from the ideal behavior and that Mw/Mn is increased at higher temperature. Polymerization of Styrene at Low [Sty]/[3,6DMPhQ] Ratio. Styrene has also been polymerized with a lower styrene/3,6-DMPhQ molar ratio in the presence of Co(acac)2 or Al(acac)3 to prepare polystyrene oligomers with a low polydispersity (Table 2, entries 2 and 4). Polymers were precipitated several times in methanol in order to eliminate the catalyst. 1H NMR analysis of the purified low molar mass polystyrene, prepared with Co(acac)2, shows the characteristic resonances of the methoxy group, consistent with the endcapping of the chains by the quinone (Figure 6b). Moreover, the two NMR signals of the methoxy protons at 3.7 and 3.9 ppm suggest a nonsymmetrical structure, in contrast to the original 3,6-DMPhQ (Figure 6a). This observation agrees with the proposed mechanism (Scheme 1). The molar mass calculated from the ratio

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Figure 6. 1H NMR spectra for 3,6-dimethoxy-9,10-phenanthrenequinone (3,6-DMPhQ) (a) and a polystyrene macroinitiator end-capped by 3,6-DMPhQ (Mn,NMR ) 3200 g/mol) (b) (Table 2, entry 2).


Figure 8. Evolution of the size exclusion chromatograms (SEC) for the resumption of the styrene polymerization by a polystyrene macroinitiator 1 (a) with and (b) without catalytic amounts of Co(acac)2 at 100 °C. [PS macroinitiator 1]/[Co(acac)2]/[Sty] ) 1/0.01/440 and [PS macroinitiator 1]/[Sty] ) 1/440.

Figure 7. Dependence of molar mass and polydispersity on styrene conversion for resumption of the styrene polymerization at 100 °C by a polystyrene macroinitiator 1 end-capped by 3,6-dimethoxy-9,10-phenanthrenequinone (3,6-DMPhQ) (Mn ) 2900 g/mol, Mw/Mn ) 1.15) in the presence of a catalytic amount of Co(acac)2 ([PS macroinitiator 1]/[Co(acac)2]/[Sty] ) 1/0.01/440). The dotted line is the theoretical dependence.

Figure 9. Plot of ln([M]0/[M]) vs time for the resumption of the styrene polymerization at 100 °C by a polystyrene macroinitiator 1 in the presence of a catalytic amount of Co(acac)2 ([PS macroinitiator 1]/[Co(acac)2]/[Sty] ) 1/0.01/440).

of the aromatic polystyrene protons and the methoxy protons is in agreement with the experimental value determined by SEC (Mn,NMR ) 3200 g/mol, Mn,SEC ) 2900 g/mol). The same observations are reported when Al(acac)3 is substituted for Co(acac)2. Resumption of the Styrene Polymerization by a PS Macroinitiator. Polystyrene oligomers endcapped by 3,6-DMPhQ have been used as macroinitiators for styrene polymerization in the presence of Co(acac)2 at 100 °C. As shown in Figure 8, the depen-

dence of the molar mass on the monomer conversion is linear up to 85% of conversion. Moreover, the close agreement between experimental and theoretical molar masses (Figure 7) and the regular shift of the SEC chromatograms with the monomer conversion (Figure 8a) indicate that the resumption of the styrene polymerization is close to 100%. The need of adding a catalytic amount of Co(acac)2 is illustrated by the high percentage of unreacted macroinitiator chains, the broad molar mass distribution of polystyrene, and no significant shift



effect by the methoxy substituents of the phenanthrenequinone. This new system not only allows the radical polymerization of styrene to be controlled, but in contrast to ATRP, 1% of metal compared to the endchains may be used. Whenever polystyrene chains are collected at low monomer conversion, they are effective macroinitiators that have been characterized and used to resume the styrene polymerization successfully, which is additional evidence of control.

Figure 10. Dependence of Mn and Mw/Mn on monomer conversion for the resumption of the styrene polymerization at 100 °C by a PS macroinitiator end-capped by 3,6-dimethoxy9,10-phenanthrenequinone (Mn ) 2400 g/mol, Mw/Mn ) 1.15) in the presence of a catalytic amount of Al(acac)3 ([PS macroinitiator 2]/[Al(acac)3]/[Sty] ) 1/0.01/440). The dotted line is the theoretical dependence.

of the SEC chromatograms in the absence of the metal complex (Figure 8b). So, when the initiator is preformed, the radical polymerization of styrene is under control. The first order in monomer is maintained under these conditions, as assessed by the linear dependence of ln([M]0/[M]) on time (Figure 9). In contrast to polymerization of styrene in the presence of 3,6-DMPhQ, no induction period of time is observed, which confirms that the polystyrene oligomers end-capped by 3,6-DMPhQ are the actual dormant species and that the induction period of time is the time required for their formation. Once again, the use of Al(acac)3 instead of Co(acac)2 does not change the situation (Figure 10). Conclusions Polystyrene with predetermined molecular characteristics (Mn, Mw/Mn) can be prepared by radical polymerization with the o-quinone/Co(acac)2 (or Al(acac)3) system. Although no control of this polymerization is observed in the presence of 3,5-di-tert-butyl-o-benzoquinone and tetrachloro-1,2-benzoquinone, 3,6-dimethoxy9,10-phenanthrenequinone imparts a very good control, much better than the unsubstituted phenanthrenequinone. Indeed, in the presence of 3,6-DMPhQ, polystyrene is formed with a low polydispersity and experimental molar masses close to the theoretical values, even at high monomer conversion. The improvement in the control more likely originates from an electronic

Acknowledgment. The authors gratefully acknowledge Solvay for financial support and fellowship to A. Debuigne. They are also grateful to F. Declercq, V. Bodart, A. Momtaz (Solvay), and Ch. Detrembleur (FNRS) for fruitful discussions. The authors are also indebted to the “Belgian Science Policy” for financial support to CERM in the frame of the “Interuniversity Attraction Poles Programme (PAI V/03)-Supramolecular Chemistry and Supramolecular Catalysis”. References and Notes (1) Moad, G.; Rizzardo, E.; Solomon, D. H. Macromolecules 1982, 15, 909-914. (2) Hawker, C. J. J. Am. Chem. Soc. 1994, 116, 11185-11186. (3) Hawker, C. J. Acc. Chem. Res. 1997, 30, 373-382. (4) Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661-3688. (5) Patten, T. E.; Matyjaszewski, K. J. Adv. Mater. 1998, 10, 901-915. (6) Matyjaszewski, K. J. Chem.sEur. J. 1999, 5, 3095-3102. (7) Sawamoto, M.; Kamigaito, M. Chem. Technol. 1999, 30-38. (8) Coessens, V.; Pintauer, T.; Matyjaszewski, K. Prog. Polym. Sci. 2001, 26, 337-377. (9) Matyjaszewski, K.; Xia, J. Chem. Rev. 2001, 101, 2921-2990. (10) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689-3745. (11) Chiefari, J.; Chong, Y. K.; Ercole, F.; Kristina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559-5562. (12) Moad, G.; Chiefari, J.; Chong, Y. K.; Krstina, J.; Mayadunne, R. T. A.; Postma, A.; Rizzardo, E.; Thang, S. H. Polym. Int. 2000, 49, 993-1001. (13) Rizzardo, E.; Chiefari, J.; Mayadunne, R. T. A.; Moad, G.; Thang, S. H. ACS Symp. Ser. 2000, 768, 278-296. (14) Moad, G.; Mayadunne, R. T. A.; Rizzardo, E.; Skidmore, M.; Thang, S. H. Macromol. Symp. 2003, 192, 1-12. (15) Caille, J.-R.; Debuigne, A.; Je´roˆme, R. Macromolecules 2005, 38, 27-32. (16) Mayo, F. R. J. Am. Chem. Soc. 1968, 90, 1289-1295. (17) Caille, J.-R.; Debuigne, A.; Je´roˆme, R. J. Polym. Sci., Polym. Chem. Ed., in press. (18) Togashi, D. M.; Nicodem, D. E.; Machiori, R.; Machiori, M. L. P. F. C. Synth. Commun. 1998, 28, 1051-1063.

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Quinone Transfer Radical Polymerization of Styrene Catalyzed by

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