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Influence of the surface chemistry of activated carbons on the ATRP catalysis of methyl methacrylate polymerization
Applied Catalysis A: General 397 (2011) 225–233
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Influence of the surface chemistry of activated carbons on the ATRP catalysis of methyl methacrylate polymerization S. Barrientos-Ramírez a,b , G. Montes de Oca-Ramírez a , E.V. Ramos-Fernández a , A. Sepúlveda-Escribano a,∗ , M.M. Pastor-Blas a , A. González-Montiel b , F. Rodríguez-Reinoso a a
Laboratorio de Materiales Avanzados, Instituto Universitario de Materiales de Alicante, Departamento de Química Inorgánica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain CID, Research & Technological Development Centre, Av. de los Sauces No. 87, Mz. 6, Lerma 52000, Mexico
b
a r t i c l e
i n f o
Article history: Received 9 December 2010 Received in revised form 1 March 2011 Accepted 4 March 2011 Available online 11 March 2011 Keywords: ATRP Activated carbon Surface chemistry Catalyst PMMA Controlled polymerization
a b s t r a c t A parent activated carbon (C-0) was subjected to four different treatments: (i) heat treatment at 1273 K in Ar (C-1); (ii) heat treatment at 473 K in air (C-2); (iii) oxidation with H2 O2 (C-3) and, (iv) oxidation with HNO3 (C-4). These materials were evaluated as supports of CuBr–[1,1,4,7,10,10-hexamethyltriethylenetetramine] (CuI Br–HMTETA), a catalyst for the Atom Transfer Radical Polymerization (ATRP) of methyl methacrylate (MMA) using methyl-␣-bromophenylacetate (MBP) as radical initiator. The supported catalysts showed an adequate control of polymerization, evidenced by polydispersity indexes (PDI) falling in the range 1.13–1.55. The best performance was achieved when activated carbon was treated with nitric acid (C-4) and with air at 473 K (C-2). Some copper leaching was always detected. The catalysts were reused and an adequate polymerization control was obtained in subsequent runs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Controlled/“living” radical polymerization using transition metals based catalysts, also known as Atom Transfer Radical Polymerization (ATRP), is a leading industrial method to produce polymers with well-defined compositions, architectures and functionalities. Living polymerization techniques are used when the polymerization proceeds in absence of irreversible chain transfer and chain termination steps [1]. It is based on a dynamic equilibrium between the propagating polymeric radicals (Pn • ) and the dormant species (Pn –X, where X is an halogen atom), which is established through the reversible transition metal catalyzed cleavage of the covalent bond between the carbon of the polymeric chain and an halogen atom (e.g. Br) in the dormant Pn –Br species (Scheme 1). By using ATRP a good control of the polymerization reaction is achieved, because side reactions are suppressed as the dormant state of the polymer is favored in the equilibrium (the deactivation rate constant, kd , is higher than the activation rate constant, ka ). By lowering the concentration of radicals, termination is reduced and control is achieved.
∗ Corresponding author. Tel.: +34 965903974; fax: +34 9650903454. E-mail address: [email protected] (A. Sepúlveda-Escribano). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.03.004
The name Atom Transfer Radical Polymerization (ATRP) makes reference to the atom transfer step (Br in Scheme 1), which is the key elementary reaction responsible for the uniform growth of the polymeric chains and it is accomplished through fast initiation and rapid reversible deactivation. One of the keys of a successful ATRP process is the choice of a suitable catalyst, since it determines the position of the atom transfer equilibrium and the dynamics of exchange between the dormant and the active species. Some requisites are necessary for an efficient transition metal catalyst: (i) the metal center must have at least two readily accessible oxidation states separated by one electron (e.g. CuI and CuII ); (ii) the metal center should have a reasonable affinity towards an halogen (e.g. Br); (iii) the coordination sphere around the metal should be expandable upon oxidation to selectively accommodate an halogen, and (iv) the ligand (e.g. HMTETA) should relatively strongly complex the metal [1]. ATRP requires the design of an appropriate catalyst (transition metal and ligands), using an initiator with a suitable structure, and adjusting the polymerization conditions such that the polymer molecular weights increase linearly with conversion, and the polydispersities are typical of a living process (they must fall in the range 1.0–1.5). The ideal catalyst for ATRP should be highly selective for atom transfer and should not participate in other side reactions. It should deactivate extremely fast with diffusion-controlled rate constants and it should have easily tunable activation rate
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Scheme 1. Atom Transfer Radical Polymerization (ATRP) of methyl methacrylate (MMA) using a CuBr–[1,1,4,7,10,10-hexamethyltriethylenetetramine] (CuI Br/HMTETA) catalyst (Cat).
constants to meet particular requirements for specific monomers. Besides, facile rearrangement and expansion of the coordination sphere are required to accommodate the incoming halogen. Therefore, the equilibrium constant also depends on the affinity of the complex to halogens. ATRP of methyl methacrylate (MMA) has been reported on ruthenium, copper, nickel, iron, palladium and rhodium catalytic systems, but copper catalysts are superior in ATRP in terms of versatility and cost [1–17]. On the other hand, ATRP can be carried out in a homogeneous system (e.g. in bulk, solution). However, a considerable amount of catalyst is necessary to obtain considerable rates of polymerization, and the removal of the catalyst once the polymerization is completed increases the economical cost of the industrial process [4]. Thus, the use of a supported catalyst would improve the ATRP process, as it could be easily recovered and potentially recycled. Some earlier attempts with heterogeneous catalysts were unsuccessful [4,18,3,19]. Porous carbons have been widely used as catalyst supports due to their high surface area and large pore volume. However, the
applications of carbons as catalyst supports in ATRP have been scarcely studied. Carbon materials are strong radical scavengers due to the presence of polycondensed aromatic rings and quinonic and phenolic functionalities [20]. Thus, propagating polymeric radicals in ATRP systems could be easily trapped by carbon surface during polymerization (Scheme 2) [21]. Some previous works in the literature report grafting of polymers on carbon spheres [22], carbon nanofibers [23] or carbon nanoparticles [24,25] by surface-initiated Atom Transfer Radical Polymerization (ATRP). Modification of the surface chemistry of porous carbons can be an attractive route for the application of these materials as supports in ATRP, because radical polymerizations can tolerate many functional groups and can be performed under mild conditions. In a previous study [26], activated carbons exhibiting similar surface chemistry and different textural characteristics were considered as catalyst supports for CuBr–[1,1,4,7,10,10hexamethyltriethylenetetramine] (CuI Br–HMTETA) in the polymerization of methyl methacrylate. The pore distribution of the activated carbons played an important role in the control of the
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Scheme 2. Mechanism of polymeric radical trapping by a carbon surface.
ATRP reaction. A carbon with an adequate distribution of micro and mesoporosity provided a suitable balance between activation and deactivation processes involving Cu(I) and Cu(II) catalytic species, which resulted in the appropriate control of the molecular weight during the methyl methacrylate polymerization reaction. In this study, the surface functionalization of an activated carbon will be considered. Modification of the surface chemistry of a carbon exhibiting micro and mesoporosity will be performed by heat treatments in air and argon, and oxidation with hydrogen peroxide and nitric acid solutions. The effect of these surface modifications on the catalytic behavior of supported CuI Br–HMTETA in ATRP of methyl methacrylate will be established. 2. Experimental 2.1. Materials A commercial activated carbon (C-0, RGC-30, provided by Westvaco Co.) was subjected to four different treatments: (i) heat treatment at 1273 K in argon for 8 h (carbon C-1); (ii) heat treatment at 473 K in air for 8 h (carbon C-2); (iii) oxidation with an aqueous 5 M solution of H2 O2 at 298 K for 8 h (carbon C-3) and oxidation with an aqueous 5 M solution of HNO3 , at 298 K for 8 h (C-4). 2.2. Characterization techniques The textural characterization of the carbon samples was performed by N2 and CO2 adsorption. N2 adsorption at 77 K is commonly used for determining adsorption properties of porous materials. Although nitrogen gas can only access pores larger than more or less 0.4 nm, restricted diffusion could occur even for this small pore size due to the low temperature at which the measurement is carried out [27,28]. CO2 adsorption at 273 K is useful when carbons contain very narrow microporosity. This temperature is high enough to avoid diffusion problems, and the maximum relative pressure that can be reached in a conventional adsorption system is very low, p/po = 0.03. Besides, this technique does not require very complex equipment [29–31]. Therefore, it has been suggested as a complementary technique to N2 adsorption [28]. CO2 adsorption isotherms are usually evaluated by Dubinin–Radushkevich (D–R) method [32]. Nitrogen adsorption isotherms were determined in a Coulter Omnisorb-610 equipment at 77 K. Before the experiment, the samples were out-gassed at 523 K for 4 h under vacuum (10−6 kPa). The specific surface area (SBET ) was obtained using the BET method. The micropore volume (Vmic ) was deduced from the adsorption data by means of the ␣-method [32]. Finally, the mesopore volume (Vmeso )
was estimated by subtracting the micropore volume (Vmic ) from the total volume adsorbed at p/po = 0.95. CO2 adsorption isotherms were recorded in an Autosorb 6 (Quantachrome) apparatus at 273 K. The volume of narrow micropores of carbons was obtained by applying the D–R-equation to the adsorption data for CO2 [31]. Temperature-programmed desorption (TPD) measurements were carried out in a U-shaped quartz cell using a total glass flow of 50 cm3 min−1 and about 100 mg of sample, with a heating rate of 10 K min−1 . A mixture of 5% H2 /He gas flow was used. Gases evolved during the TPD experiment were followed-up by on-line mass spectroscopy.
2.3. Reaction experiments The surface modified carbon materials were evaluated as supports of CuBr–[1,1,4,7,10,10-hexamethyltriethylenetetramine] (CuI Br–HMTETA), a catalyst for the Atom Transfer Radical Polymerization (ATRP) of methyl methacrylate (MMA) using methyl␣-bromophenylacetate (MBP) as radical initiator. To conduct the ATRP polymerization reaction, methyl methacrylate (99.9%) from Aldrich was distilled under vacuum and stored at 269 K. Toluene (99%), CuBr (99.99%), methyl ␣-bromophenylacetate (MBP, 99%) and 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA, 99%) were also obtained from Aldrich and used without prior treatment. Polymerization with CuI Br/HMTETA physisorbed on activated carbon was carried out as follows: a typical ratio between methyl methacrylate (MMA), copper and initiator (MBP) was set, [MMA]/[Cu]/[MBP] = 100/1/1. Therefore, CuBr (1.35 mmol), toluene (40 ml), and activated carbon (3 g) were added to a 200 ml 4560 mini bench top reactor (Parr Instrument Co.). HMTETA ligand (1.35 mmol) and toluene (20 ml) were added to the cylinder. Description of the experimental setup is detailed elsewhere [33]. Three helium purge cycles were applied to the reactor and cylinder. The HMTETA ligand solution was transferred from the cylinder to the reactor vessel and stirred overnight to support the catalyst on the activated carbon. MMA (135 mmol), initiator (MBP, 1.35 mmol) and toluene (25 ml) were added to the cylinder and purged under three helium purge cycles. The mixture was then added to the reactor vessel. The reactor was heated to 358 K under stirring. Samples (1 ml) were taken out using a helium-purged sampler to determine conversion by 1 H NMR from the ratio between the OCH3 signal intensity in the polymer (3.60 ppm) and in the monomer (3.75 ppm). 1 H NMR spectra were recorded on a Bruker AC-300 spectrometer (Bruker BioSpin, Billerica, MA, USA) at 300 MHz. 1 H NMR chemical shifts in CDCl3 were reported from 0.00 ppm using tetramethyl silane (TMS) as an internal reference.
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The molecular weight and the polydispersity of the polymer were measured by GPC. Number- and weight-average molecular weights (Mn and Mw , respectively) were determined by gel permeation chromatography (GPC: SHK polystyrene gel column; flow rate 1.0 ml min−1 ) relative to polystyrene using tetrahydrofurane (THF) as solvent at 298 K with refractive index detector. Data were recorded and manipulated using the Waters Millennium software package provided with the instrument. The residual copper content in the poly(methylmethacrylate) final polymer (RCu ) was measured by inductively coupled plasma atomic emission spectroscopy (ICP-OES). The ICP-OES analyses were performed on the Inductively Coupled Plasma-Optical Emission Spectrometer Perkin-Elmer Optima 3000TM (Norwalk, CT, USA). The sample was quantified with great accuracy and adequately dissolved with requisite hydrogen fluoride. Then, the solution was vaporized to remove silicon fluoride. The remaining residue was dissolved by 0.2 mol/L hydrogen chloride and the solution formed was adjusted to weak acidity by ammonia. The resulting solution was diluted and it was then ready for ICP-OES measurement.
3. Results and discussion 3.1. Chemical and textural characterization of activated carbons The nature and concentration of oxygen surface groups on activated carbons can be modified by thermal or chemical treatments. Thermal treatments under inert atmosphere produce the selective decomposition of the functional groups on the carbon surface. Oxidizing treatments in gaseous (i.e. air) or liquid phases (i.e. oxidizing aqueous solutions) result in an increase of surface oxygen without substantially modifying the porous texture of the carbon. Gaseous oxidation of carbons produces an increase of phenol and carbonyl functionalities, while oxidation in the liquid phase enhances creation of carboxylic moieties on the carbon surface [34]. Fig. 1 shows the TPD profiles corresponding to the different activated carbons used in this study. TPD allows to quantify the amount of oxygen surface groups in activated carbons by monitoring CO2 and CO evolved upon the decomposition of the different oxygen groups by heat treatment, under an inert atmosphere, at temperatures between 300 and 1300 K [35]. CO2 is produced by the more acidic and less thermally stable oxygenated groups, while CO comes from the more stable and less acidic oxygenated groups on the carbon surface. The amount of CO2 and CO evolved from all the
Table 1 Amounts of CO and CO2 evolved during TPD experiments. Carbon
CO (mol/g)
CO2 (mol/g)
CO/CO2
C-0 C-1 C-2 C-3 C-4
1880 660 1700 3150 3800
170 70 120 600 1400
11.0 9.3 14.2 5.2 2.7
samples upon TPD experiments was determined by integration of the area under the curves of the profiles, after proper calibration, and is listed in Table 1. With regard to the CO2 evolution profiles (Fig. 1a), it can be seen that the parent activated carbon (C-0) contains a very low amount of acidic oxygen surface groups (carboxylic, lactone) and that, as expected, this amount is even decreased by the heat treatment under an argon flow (carbon C-1). Carbon obtained by thermal treatment of C-0 in air (C-2) shows no CO2 evolution below 500 K (it has been heat treated at 473 K), and only a slightly larger amount of CO2 is evolved at higher temperatures than in the case of carbons C-0 and C-1. Oxidation of C-0 with H2 O2 (carbon C-3) increases the amount of CO2 evolving oxygen surface groups, mainly carboxylic groups (maximum evolution rate at about 550 K). However, the larger oxidation effect is achieved with HNO3 . For carbon C-4, the CO2 -TPD profile shows several peaks, corresponding to different surface groups in different locations on the surface of carbon. Thus, peaks with maxima at 420 K and 650 K can be assigned to the decomposition of carboxylic acid groups (low temperature CO2 groups), whereas the peak at 710 K can be related to the decomposition of lactones and carboxylic anhydride (high temperature CO2 groups). Carboxylic anhydrides, that can be formed from two adjacent carboxylic groups, are more stable than carboxylic acid groups and, therefore, will decompose evolving CO2 at a higher temperature [36]. Fig. 1b shows the CO evolution profiles from the carbons in TPD experiments. CO evolution from the starting carbon (C-0) shows a peak at around 1000 K. Treatment of carbon C-0 in Ar at 1273 K (carbon C-1) produces a decrease in CO evolution and the displacement of the peak to higher temperatures, which corresponds to the decomposition of quinone groups. Thus, most of the surface oxygen groups present in the starting carbon (C-0) are removed by the heat treatment in Ar (carbon C-1) [37–40]. CO-TPD of C-2 carbon, shows a peak centered at 1000 K. Thus, heat treatment in air modifies the nature of the oxygen groups present in the starting carbon material, producing more stable groups [41].
Fig. 1. CO2 and CO evolution profiles (TPD) of as-received carbon (C-0) and carbons treated in argon (C-1), air (C-2), H2 O2 (C-3) and HNO3 (C-4).
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range (around 48–44% microporosity and 52–56% mesoporosity). Consequently, four activated carbons, with relatively similar textural properties but very different surface chemistry have been prepared, and they will be used as support for ATRP polymerization of methyl methacrylate. 3.2. Heterogeneous polymerization of MMA using active carbon as support for CuBr/HMTETA catalyst
Fig. 2. Adsorption isotherms of N2 at 77 K of as-received carbon (C-0) and carbons treated in argon (C-1), air (C-2), H2 O2 (C-3) and HNO3 (C-4).
Oxidation of carbon C-0 with H2 O2 (carbon C-3) produces an increase of the amount of CO-evolving oxygen groups. Table 1 shows an increase of evolved CO from 1882 mol/g in the parent C-0 carbon to 3150 mol/g in H2 O2 treated C-3 carbon. The CO evolution profile shows two partially overlapped peaks, at 1000 K and 1050 K, which may be ascribed to the decomposition of phenolic, quinone and/or pyrone groups [42]. On the other hand, the CO evolution profile of carbon prepared by treatment with HNO3 (carbon C-4) shows two peaks at 950 and 1100 K, which are ascribed to the decomposition of phenolic groups, carboxylic anhydrides, ethers and carbonyl groups of quinones and pyrones. The intensity of these peaks (larger amount of CO-evolving surface groups, Table 1) is an evidence of the higher oxidation ability of HNO3 compared to H2 O2 (carbon C-3). Fig. 2 shows the N2 adsorption–desorption isotherms at 77 K for the different carbons. Additionally, Table 2 reports the textural parameters obtained for the parent and the treated carbons. All carbons show modified Type I isotherms, characteristic of microporous solids where the pore filling occurs at low relative pressures, but with non-coincident adsorption and desorption branches. This hysteresis loop indicates capillary condensation associated to mesoporosity. The thermal treatment at high temperature under inert atmosphere (C-1) gives rise to a slight decrease of the specific surface area (SBET ) and the mesopore volume (Vmeso ). The thermal treatment in air at 473 K (C-2) only slightly modifies the porous structure of activated carbon. On the other hand, oxidation with H2 O2 solution (C-3), despite introducing a considerable amount of surface oxygenated moieties, does not greatly affect the carbon porous texture. The most important modification in porous texture was produced after treatment with HNO3 solution (carbon C-4). The BET surface area and the micropore and mesopore volumes decrease, and there is also a decrease of the narrowest porosity. However, the relative contribution of micro and mesopores (% meso) remains similar in all carbons, always within an intermediate
The solid CuBr/HMTETA catalyst used for the heterogeneous polymerization of methyl methacrylate was supported on the four previously characterized activated carbons. Table 3 shows the data obtained in the polymerization reaction using the fresh catalyst, and also during an ulterior use of it, as supported CuBr/HMTETA catalysts were reused in a second polymerization reaction without any regeneration procedure in order to demonstrate that they are still active after a polymerization process. Leaching of catalyst once the polymerization reaction was over was detected in all cases, the residual copper (RCu ) present in the final poly(methylmethacrylate) being determined by ICP-OES. The evolution of conversion as a function of time on reaction is plotted in linear and in semilogarithmic coordinates, for the different catalysts. Conversion is represented by the ratio [M]0 /[M]t ; where [M]0 is the initial concentration of monomer and [M]t is the concentration of monomer at a given time (Figs. 3a–7a). On the other hand, the evolution of the experimental molecular weight of the obtained polymer (Mn ) and of its polydispersity index (Mw /Mn ) with conversion is plotted in Figs. 3b–7b. CuBr/HMTETA catalyst supported on the heat treated carbon in Ar (C-1) produced conversions (determined after 24 h reaction) of 55% (first use of catalyst) and 49% (second use of catalyst), which are lower than those obtained for the as-received C-0 carbon exp (Table 3, Figs. 3a and 4a). Experimental molecular weights (Mn ) theor ), represented are higher than theoretical molecular weights (Mn by a dashed line in Fig. 4b, but with both the fresh and the re-used catalysts supported in carbon C-1, molecular weights show a lineal tendency versus conversion (Fig. 4b). The lower conversion degree achieved with the CuBr/HMTETA catalyst supported on carbon C-1 can be attributed to the surface chemical properties of the support. Treatment of carbon in argon at 1273 K removed oxygenated moieties from the carbon surface, as shown by TPD experiments. Thus, surface chemistry of C-1 carbon is similar to that of a carbon black, with a high amount of unsaturated carbon atoms resulting from the removal of oxygen groups. Therefore, the ability of this support to act as a polymeric radical scavenger (Scheme 2) may be responsible for the low catalytic activity of CuBr/HMTETA supported on C-1 carbon in the ATRP polymerization of methyl methacrylate, by lowering deactivation rate (kd ) in the ATRP reaction (Scheme 1). As shown in Table 2, the textural characteristics of carbons C-0 and C-2 are quite similar, so a comparative study of catalytic activity of CuBr/HMTETA catalyst supported on both activated carbons will be based just on their surface chemistry. When CuBr/HMTETA catalyst was supported on the carbon thermally treated in air (C-2), conversions of 73% (first catalyst use) and 55% (second cat-
Table 2 Specific surface area (SBET ), total pore volume (Vt ), mesopore volume (Vmeso ), micropore volume (Vmic ) and narrow microporosity (Vn ) for the as-received (C-0) and the treated activated carbons. Carbon
SBET (m2 g−1 )
Vt (cm3 g−1 )
Vmeso (cm3 g−1 )
Vmic (cm3 g−1 )
Vn (cm3 g−1 )
% meso
C-0 C-1 C-2 C-3 C-4
1530 1410 1520 1510 1200
1.16 1.07 1.13 1.11 0.87
0.65 0.56 0.61 0.60 0.46
0.51 0.49 0.52 0.51 0.41
0.32 0.32 0.32 0.32 0.27
56 52 54 54 52
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Table 3 Conversion, experimental and theoretical number-average molecular weights (Mn ), polydispersity index (Mw /Mn ) and residual copper (RCu ) as a result of the ATRP polymerization of MMA. Support C-0 First use Second use C-1 First use Second use C-2 First use Second use C-3 First use Second use C-4 First use Second use
exp
Mn
(g mol−1 )
Time (h)
Conversion (%)
24 24
72 83
12,652 12,912
24 24
55 49
24 24
Mntheor (g mol−1 )
Mw /Mn
RCu (ppm)
7,200 8,300
1.13 1.14
45 47
23,006 21,935
5,500 4,900
1.55 1.44
34 43
73 55
15,530 14,608
7,300 5,500
1.27 1.31
6 39
24 24
75 60
15,420 14,597
7,500 6,000
1.43 1.46
5 4
24 24
82 70
11,246 12,329
8,200 7,000
1.19 1.27
6 14
Reaction conditions: [MMA]/[Cu]/[MBP] = 100/1/1.
Fig. 3. (a) Kinetic plots and (b) number-average molecular weight (Mn ) and polydispersity index (Mw /Mn ) versus conversion (%) plots in the ATRP polymerization of methyl methacrylate using CuBr/HMTETA catalyst supported in as-received (C-0) carbon. Dashed line represents the theoretical Mn versus conversion, () first use and (䊉) second use of catalyst. Reaction conditions: [MMA]/[Cu]/[MBP] = 100/1/1.
alyst use) were obtained 24 h after the beginning of the methyl methacrylate (MMA) polymerization reaction. Fig. 5a shows a first-order kinetics when carbon C-2 is used as support for the polymerization catalyst, this evidencing chain termination reactions
by formation of Pn • –X in the Atom Transfer Radical Polymerization Reaction (Scheme 1). The obtained experimental molecular weight is higher than the predicted theoretical molecular weight after the first and the second uses of catalyst (Fig. 5b), but a
Fig. 4. (a) Kinetic plots and (b) number-average molecular weight (Mn ) and polydispersity index (Mw /Mn ) versus conversion (%) plots in the ATRP polymerization of methyl methacrylate using CuBr/HMTETA catalyst supported in carbon treated in argon (C-1). Dashed line represents the theoretical Mn versus conversion, first use () and second use (䊉) of catalyst. Reaction conditions: [MMA]/[Cu]/[MBP] = 100/1/1.
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Fig. 5. (a) Kinetic plots and (b) number-average molecular weight (Mn ) and polydispersity index (Mw /Mn ) versus conversion (%) plots in the ATRP polymerization of methyl methacrylate using CuBr/HMTETA catalyst supported in carbon treated in air (C-2). Dashed line represents the theoretical Mn versus conversion, first use () and second use (䊉) of catalysts. Reaction conditions: [MMA]/[Cu]/[MBP] = 100/1/1.
linear tendency in the average-number molecular weight versus conversion plot is obtained. However, control of polymerization is not improved when comparing C-2 carbon to C-0 carbon as catalyst supports, this being reflected by the fact that the polydispersity index of the poly(methylmethacrylate) obtained after polymerization using the CuBr/HMTETA catalyst supported on carbon C-2 (Mw /Mn = 1.27–1.31) is higher than that obtained when the as-received activated carbon (C-0) was used as catalyst support (Mw /Mn = 1.13–1.14) (Fig. 5b (right) and Table 3). Therefore, although C-0 and C-2 carbons behave similarly from a kinetic point of view, polymerization control is decreased by the air treatment of the carbon support. This can be explained in terms of loss of oxygenated surface moieties during thermal treatment, as shown by TPD analysis in Fig. 1. The use of carbon C-3, obtained from as-received carbon treated with a 5 M H2 O2 solution, as catalyst support provided a final conversion of 75% (first use) and 60% (second use of the catalyst) (Fig. 6a) in the polymerization. However, polydispersity index increased from 1.13 (C-0 carbon as catalyst support) to 1.46 (C-3 carbon as catalyst support), which reflects a poorer control of the polymerization reaction when C-3 carbon is used as catalyst support. A relatively good control of polydispersity should fall in the range 1.0 < Mw /Mn < 1.5. However, a well-controlled polymerization, with a narrow molecular weight distribution, may show a polydispersity index (Mw /Mn ) below 1.1, which is not achieved with C-3 carbon supported catalyst. Although carbon C-3 contains an increased amount of oxygenated surface groups, as a consequence of the treatment with H2 O2 solution, the decrease of mesopore volume (Vmeso in Table 2) produced in this activated carbon could also affect polymerization control. As stated previously [26], the texture of the activated carbon plays a definitive role in the polymerization rate and the control of polymerization reaction, evaluated by the polydispersity index of the final poly(methylmethacrylate). The accessibility of the catalyst to pores in the activated carbon is limited by pore size and growing polymer size. ATRP polymerization reaction is successfully achieved when an activated carbon exhibits an adequate range of both micro and mesoporosity. The mesoporosity of the activated carbons used as support of the CuI -Br/HMTETA catalyst enhances the activation/deactivation process, as it allows the polymeric chains to bond to the catalyst complex. The catalyst undergoes a one-electron oxidation producing Br–CuII -Br/HMTETA, and then returns to Cu(I) Br/HMTETA (Scheme 1). However, in the micropores the catalyst is isolated from the growing polymeric chains located
in the mesopores, or even it could also be blocked in the micropores themselves when is bonded to an incipient growing polymeric chain whose size would allow it to enter a micropore. Thus, transport limitations inside micropores produced by a slow diffusion of the growing polymeric radicals towards the metal center (Cu) of the catalyst complex result in a low polymerization rate. On the other hand, the access of growing polymer chains is favored in mesopores and deactivation process (which implies reduction of catalyst to CuI -Br/HMTETA) is enhanced. Consequently, the decrease of mesopore volume of carbon C-3 could also negatively affect the control of polymerization. Nevertheless, a lack of an adequate surface chemistry seems to be determining over textural properties. Consequently, a relatively high polydispersity index is obtained (1.46). Finally, carbon treated with nitric acid (C-4) showed the highest polymerization control, compared to carbons C-1, C-2 and C-3. A polydispersity index of 1.19 was achieved when supporting the catalyst on C-4 carbon. Although this polydispersity index is similar to that of the as-received C-0 carbon, the advantage of using the nitric acid treated C-4 carbon instead of the as-received C-0 carbon is that copper leaching (RCu ) is highly decreased (from 45 ppm in C-0 to 6 ppm in C-4) and conversion is improved (Table 3) in the first use. This is due to the surface functionalization (creation of carboxylic moieties) produced by the treatment with nitric acid. The CuBr/HMTETA catalyst used in this study exhibited copper leaching during the polymerization process, irrespective of the activated carbon used as catalyst support. A possible cause of leaching may be structural rearrangements of the surface copper species during reaction. The structural changes required upon copper oxidation create unstable species that favor leaching [43]. The residual copper content in the polymethyl methacrylate (PMMA) final polymer (RCu ) (Table 3) was measured using inductively coupled plasma atomic emission spectroscopy (ICP-OES). The theoretical copper content, assuming that all the copper in the catalyst suffers leaching and that a 100% conversion of methyl methacrylate (MMA) into poly(methylmethacrylate) (PMMA) is reached, would be 1329 ppm. When as-received C-0 carbon was used as support, the residual copper content in solution (RCu ) fell in the range 45–47 ppm. Heat or chemical treatment of carbon contributed to reduce copper leaching. The functionalization of the carbon surface by introduction of oxygenated groups contributes to enhance adsorption of the CuBr/HMTETA catalyst. Therefore, C-4 carbon shows an interesting behavior in the MMA polymerization reaction control. Leaching
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Fig. 6. (a) Kinetic plots and (b) number-average molecular weight (Mn ) and polydispersity index (Mw /Mn ) versus conversion (%) plots in the ATRP polymerization of methyl methacrylate using CuBr/HMTETA catalyst supported in carbon treated in H2 O2 (C-3). Dashed line represents the theoretical Mn versus conversion, first use () and second use (䊉) of catalysts. Reaction conditions: [MMA]/[Cu]/[MBP] = 100/1/1.
Fig. 7. (a) Kinetic plots and (b) number-average molecular weight (Mn ) and polydispersity index (Mw /Mn ) versus conversion (%) plots in the ATRP polymerization of methyl methacrylate using CuBr/HMTETA catalyst supported in carbon treated in HNO3 (C-4). Dashed line represents the theoretical Mn versus conversion, first use () and second use (䊉) of catalysts. Reaction conditions: [MMA]/[Cu]/[MBP] = 100/1/1.
H
N
N
O N
HO
N
OH O
N
Cu Br
N
N
Cu
N
Br
H
Scheme 3. Cleavage of CuBr/HMTETA catalyst favored by carboxylic acid groups in activated carbon.
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is reduced and conversion is slightly improved (compared to asreceived C-0 carbon) (Fig. 7a). Furthermore, experimental number average molecular weight (Mn ) is closer to theoretical number average molecular weight (dashed line in Fig. 7b) with nitric acid treated (C-4) carbon. Carboxylic –COOH groups on the C-4 surface (detected by TPD) favored Atomic Transfer Polymerization (ATRP) of MMA. Oxygenated acidic groups act as primary adsorption centers that modify the hydrophobic character and the acid–base nature of the carbon surface, thus enhancing adsorption of mild basic compounds like CuBr/HMTETA catalyst (Scheme 3), and minimizes radicals trapping. Consequently, the control of surface chemistry of carbon used as support of CuBr/HMTETA catalyst is of paramount importance in the Atomic Transfer Polymerization methyl methacrylate. 4. Conclusions It can be concluded from the obtained results that activated carbon is a suitable support of CuBr/HMTETA for ATRP polymerization of methyl methacrylate. The control of functionality of the activated carbon by means of heat or chemical treatments plays a definitive role in reducing copper leaching and controlling polymerization reaction to achieve a narrow molecular weight distribution (i.e. a polydispersity index of the final poly(methylmethacrylate) close to 1). Carbon treated with nitric acid (C-4) showed the highest polymerization control, with a polydispersity index of 1.19, and decreased copper leaching. This has been attributed not only to its optimal porous structure (exhibiting micro and mesoporosity) but also to the adequate compatibility of C-4 carbon with the CuBr/HMTETA catalyst due to the high functionalization of its surface with carboxylic groups obtained by oxidation with nitric acid. Treatment with H2 O2 (carbon C-3) was not as effective as nitric acid in functionalizing carbon surface. Carbons heat treated in argon (C-1) or in air (C-2) exhibited poor polymerization control. Absence of functional groups as a consequence of the thermal treatment decreases the effectiveness of these carbons as catalyst supports for CuBr/HMTETA mediated Atom Transfer Radical Polymerization of methyl methacrylate. Besides, the surface of carbon C-1 may act as a polymeric radical scavenger, lowering deactivation step in the Atom Transfer Radical Polymerization and lowering activity of CuBr/HMTETA catalyst. Although the textural properties of carbon C-2 are similar to those of as-received C-0 carbon, and both carbons behave similarly from a kinetic point of view, the decrease of polymerization control in the air treated C-2 carbon can be explained in terms of loss of acidic oxygenated surface moieties during thermal treatment. This reflects that the lack of surface functionalization plays a crucial role. These results suggest that the accessibility of the growing polymer chains to the supported catalyst together with an adequate surface chemistry of the carbon support is a crucial parameter to obtain a good polymerization control with a narrow molecular weight distribution and polydispersities close to 1. Acknowledgements The Mexican Council of Science and Technology (CONACYT) is gratefully acknowledged for the Pre-Doctoral fellowship No. 187157 to S. Barrientos-Ramírez. Financial support from Generali-
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tat Valenciana (Project PROMETEO/2009/002 – FEDER) is gratefully acknowledged. We thank M. Dolores Baeza for her help with GPC experiments. Supply of activated carbon by MeadWestvaco Co. is gratefully acknowledged. References [1] K. Matyjaszewski, J. Xia, Chem. Rev. 101 (9) (2001) 2921–2990. [2] Y. Shen, S. Zhu, R. Pelton, Macromolecules 34 (17) (2001) 5812–5818. [3] G. Kickelbick, H.J. Paik, K. Matyjaszewski, Macromolecules 32 (9) (1999) 2941–2947. [4] Y. Shen, S. Zhu, F. Zeng, R.H. Pelton, Macromolecules 33 (15) (2000) 5427–5431. [5] S.C. Hong, K. Matyjaszewski, Macromolecules 35 (20) (2002) 7592–7605. [6] S.C. Hong, H.J. Paik, K. Matyjaszewski, Macromolecules 34 (15) (2001) 5099–5102. [7] D.M. Haddleton, C. Waterson, P.J. Derrick, C.B. Jasieczek, A.J. Shooter, Chem. Commun. 7 (1997) 683–684. [8] D.M. Haddleton, D.J. Duncalf, D. Kukulj, A.P. Radigue, Macromolecules 32 (15) (1999) 4769–4775. [9] S. Liou, J.T. Rademacher, D. Malaba, M.E. Pallack, W.J. Brittain, Macromolecules 33 (12) (2000) 4295–4296. [10] M.E. Honigfort, W.J. Brittain, Macromolecules 36 (9) (2003) 3111–3114. [11] Y. Shen, S. Zhu, F. Zeng, R. Pelton, J. Polym. Sci. A: Polym. Chem. 39 (7) (2001) 1051–1059. [12] Y. Shen, S. Zhu, F. Zeng, R. Pelton, Macromol. Chem. Phys. 201 (13) (2000) 1387–1394. [13] Y. Shen, S. Zhu, R. Pelton, Macromolecules 34 (10) (2001) 3182–3185. [14] Y. Shen, S. Zhu, Macromolecules 34 (25) (2001) 8603–8609. [15] Y. Shen, S. Zhu, R. Pelton, Macromol. Rapid Commun. 21 (14) (2000) 956–959. [16] T. Opstal, K. Melis, F. Verpoort, Catal. Lett. 74 (3–4) (2001) 155–159. [17] J.V. Nguyen, C.W. Jones, J. Polym. Sci. A: Polym. Chem. 42 (6) (2004) 1384–1399. [18] T. Otsu, T. Tazaki, M. Yoshioka, Chem. Express. 5 (10) (1990) 801–804. [19] D.M. Haddleton, D. Kukulj, A.P. Radigue, Chem. Commun. 1 (1999) 99–100. [20] T. Liu, R. Casado-Portilla, J. Belmont, K. Matyjaszewski, J. Polym. Sci. A: Polym. Chem. 43 (2005) 4695–4709. [21] Q. Yang, L. Wang, W. Xiang, J. Zhou, J. Li, Polymer 48 (2007) 2866–2873. [22] Y.Z. Jin, C. Gao, H.W. Kroto, Macromol. Rapid Commun. 26 (2005) 1133–1139. [23] L. Li, C.M. Lukehart, Chem. Mater. 18 (2006) 94–99. [24] Q. Yang, L. Wang, W. Xiang, J. Zhou, G.J. Tan, J. Polym. Sci. A: Polym. Chem. 45 (2007) 3451–3459. [25] Q. Yang, L. Wang, W. Xiang, J. Zhou, G. Tan, Polymer 48 (2007) 3444–3451. [26] S. Barrientos-Ramírez, G. Montes de Oca-Ramírez, M.M. Pastor-Blas, A. Sepúlveda-Escribano, A. González-Montiel, Catal. Today 150 (2010) 42–48. [27] F. Rodríguez-Reinoso, J. López-Gonzalez, C. Berenguer, Carbon 20 (3) (1982) 185–189. [28] R. Ríos, J. Silvestre-Albero, A. Sepúlveda-Escribano, M. Molina-Sabio, F. Rodríguez-Reinoso, J. Phys. Chem. C 111 (10) (2007) 3803–3805. [29] J.W. Patrick, in: E. Arnold (Ed.), Porosity in Carbons: Characterization and Applications, John Wiley & Sons, New York, 1995. [30] D.H. Everett, J.C. Powl, J. Chem. Soc. Faraday Trans. 1: Phys. Chem. Condens. Phases 72 (3) (1976) 619–636. [31] H. Marsh, F. Rodríguez-Reinoso, Activated Carbon, Elsevier, Oxford, 2006. [32] M.M. Dubinin, in: P.L. Walker (Ed.), Chemistry and Physics of Carbon, vol. 2, Dekker, New York, 1966. [33] S. Barrientos-Ramírez, E.V. Ramos-Fernández, J. Silvestre-Albero, A. SepúlvedaEscribano, M.M. Pastor-Blas, A. González-Montiel, Micropor. Mesopor. Mater. 120 (2009) 132–140. [34] F. Rodríguez-Reinoso, in: E.A. de Patrick (Ed.), Porosity in Carbons: Characterization and Applications, Arnold, London, 1998, p. 253. [35] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. Orfao, Ind. Eng. Chem. Res. 46 (2007) 4110–4115. [36] C. Moreno-Castilla, M.A. Ferro-García, J.P. Joly, I. Bautista-Toledo, F. CarrascoMarín, J. Rivera-Utrilla, Langmuir 11 (11) (1995) 4386–4392. [37] S. Shin, J. Jang, S.H. Yoon, I. Mochida, Carbon 35 (1997) 1739–1743. [38] A.D. Seyed, T.J. Karanfil, J. Colloid Interface Sci. 274 (2004) 1–8. [39] C. Moreno-Castilla, F. Carrasco-Marın, F.J. Maldonado-Hodar, J. Rivera-Utrilla, Carbon 36 (1998) 145–151. [40] F. Rodríguez-Reinoso, in: H. Marsh, E.A. Heintz, F. Rodríguez-Reinoso (Eds.), Introduction to Carbon Technologies, Secretariado de Publicaciones, University of Alicante, Alicante, Spain, 1997, p. 35. [41] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. Orfao, Carbon 37 (1999) 1379–1389. [42] T.E. Patten, K. Matyjaszewski, Adv. Mater. 10 (12) (1998) 901–915. [43] J.V. Nguyen, C.W. Jone, J. Polym. Sci. A: Polym. Chem. 42 (6) (2004) 1367–1383.
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Influence of the surface chemistry of activated carbons on the ATRP catalysis of methyl methacrylate polymerization S. Barrientos-Ramírez a,b , G. Montes de Oca-Ramírez a , E.V. Ramos-Fernández a , A. Sepúlveda-Escribano a,∗ , M.M. Pastor-Blas a , A. González-Montiel b , F. Rodríguez-Reinoso a a
Laboratorio de Materiales Avanzados, Instituto Universitario de Materiales de Alicante, Departamento de Química Inorgánica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain CID, Research & Technological Development Centre, Av. de los Sauces No. 87, Mz. 6, Lerma 52000, Mexico
b
a r t i c l e
i n f o
Article history: Received 9 December 2010 Received in revised form 1 March 2011 Accepted 4 March 2011 Available online 11 March 2011 Keywords: ATRP Activated carbon Surface chemistry Catalyst PMMA Controlled polymerization
a b s t r a c t A parent activated carbon (C-0) was subjected to four different treatments: (i) heat treatment at 1273 K in Ar (C-1); (ii) heat treatment at 473 K in air (C-2); (iii) oxidation with H2 O2 (C-3) and, (iv) oxidation with HNO3 (C-4). These materials were evaluated as supports of CuBr–[1,1,4,7,10,10-hexamethyltriethylenetetramine] (CuI Br–HMTETA), a catalyst for the Atom Transfer Radical Polymerization (ATRP) of methyl methacrylate (MMA) using methyl-␣-bromophenylacetate (MBP) as radical initiator. The supported catalysts showed an adequate control of polymerization, evidenced by polydispersity indexes (PDI) falling in the range 1.13–1.55. The best performance was achieved when activated carbon was treated with nitric acid (C-4) and with air at 473 K (C-2). Some copper leaching was always detected. The catalysts were reused and an adequate polymerization control was obtained in subsequent runs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Controlled/“living” radical polymerization using transition metals based catalysts, also known as Atom Transfer Radical Polymerization (ATRP), is a leading industrial method to produce polymers with well-defined compositions, architectures and functionalities. Living polymerization techniques are used when the polymerization proceeds in absence of irreversible chain transfer and chain termination steps [1]. It is based on a dynamic equilibrium between the propagating polymeric radicals (Pn • ) and the dormant species (Pn –X, where X is an halogen atom), which is established through the reversible transition metal catalyzed cleavage of the covalent bond between the carbon of the polymeric chain and an halogen atom (e.g. Br) in the dormant Pn –Br species (Scheme 1). By using ATRP a good control of the polymerization reaction is achieved, because side reactions are suppressed as the dormant state of the polymer is favored in the equilibrium (the deactivation rate constant, kd , is higher than the activation rate constant, ka ). By lowering the concentration of radicals, termination is reduced and control is achieved.
∗ Corresponding author. Tel.: +34 965903974; fax: +34 9650903454. E-mail address: [email protected] (A. Sepúlveda-Escribano). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.03.004
The name Atom Transfer Radical Polymerization (ATRP) makes reference to the atom transfer step (Br in Scheme 1), which is the key elementary reaction responsible for the uniform growth of the polymeric chains and it is accomplished through fast initiation and rapid reversible deactivation. One of the keys of a successful ATRP process is the choice of a suitable catalyst, since it determines the position of the atom transfer equilibrium and the dynamics of exchange between the dormant and the active species. Some requisites are necessary for an efficient transition metal catalyst: (i) the metal center must have at least two readily accessible oxidation states separated by one electron (e.g. CuI and CuII ); (ii) the metal center should have a reasonable affinity towards an halogen (e.g. Br); (iii) the coordination sphere around the metal should be expandable upon oxidation to selectively accommodate an halogen, and (iv) the ligand (e.g. HMTETA) should relatively strongly complex the metal [1]. ATRP requires the design of an appropriate catalyst (transition metal and ligands), using an initiator with a suitable structure, and adjusting the polymerization conditions such that the polymer molecular weights increase linearly with conversion, and the polydispersities are typical of a living process (they must fall in the range 1.0–1.5). The ideal catalyst for ATRP should be highly selective for atom transfer and should not participate in other side reactions. It should deactivate extremely fast with diffusion-controlled rate constants and it should have easily tunable activation rate
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Scheme 1. Atom Transfer Radical Polymerization (ATRP) of methyl methacrylate (MMA) using a CuBr–[1,1,4,7,10,10-hexamethyltriethylenetetramine] (CuI Br/HMTETA) catalyst (Cat).
constants to meet particular requirements for specific monomers. Besides, facile rearrangement and expansion of the coordination sphere are required to accommodate the incoming halogen. Therefore, the equilibrium constant also depends on the affinity of the complex to halogens. ATRP of methyl methacrylate (MMA) has been reported on ruthenium, copper, nickel, iron, palladium and rhodium catalytic systems, but copper catalysts are superior in ATRP in terms of versatility and cost [1–17]. On the other hand, ATRP can be carried out in a homogeneous system (e.g. in bulk, solution). However, a considerable amount of catalyst is necessary to obtain considerable rates of polymerization, and the removal of the catalyst once the polymerization is completed increases the economical cost of the industrial process [4]. Thus, the use of a supported catalyst would improve the ATRP process, as it could be easily recovered and potentially recycled. Some earlier attempts with heterogeneous catalysts were unsuccessful [4,18,3,19]. Porous carbons have been widely used as catalyst supports due to their high surface area and large pore volume. However, the
applications of carbons as catalyst supports in ATRP have been scarcely studied. Carbon materials are strong radical scavengers due to the presence of polycondensed aromatic rings and quinonic and phenolic functionalities [20]. Thus, propagating polymeric radicals in ATRP systems could be easily trapped by carbon surface during polymerization (Scheme 2) [21]. Some previous works in the literature report grafting of polymers on carbon spheres [22], carbon nanofibers [23] or carbon nanoparticles [24,25] by surface-initiated Atom Transfer Radical Polymerization (ATRP). Modification of the surface chemistry of porous carbons can be an attractive route for the application of these materials as supports in ATRP, because radical polymerizations can tolerate many functional groups and can be performed under mild conditions. In a previous study [26], activated carbons exhibiting similar surface chemistry and different textural characteristics were considered as catalyst supports for CuBr–[1,1,4,7,10,10hexamethyltriethylenetetramine] (CuI Br–HMTETA) in the polymerization of methyl methacrylate. The pore distribution of the activated carbons played an important role in the control of the
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Scheme 2. Mechanism of polymeric radical trapping by a carbon surface.
ATRP reaction. A carbon with an adequate distribution of micro and mesoporosity provided a suitable balance between activation and deactivation processes involving Cu(I) and Cu(II) catalytic species, which resulted in the appropriate control of the molecular weight during the methyl methacrylate polymerization reaction. In this study, the surface functionalization of an activated carbon will be considered. Modification of the surface chemistry of a carbon exhibiting micro and mesoporosity will be performed by heat treatments in air and argon, and oxidation with hydrogen peroxide and nitric acid solutions. The effect of these surface modifications on the catalytic behavior of supported CuI Br–HMTETA in ATRP of methyl methacrylate will be established. 2. Experimental 2.1. Materials A commercial activated carbon (C-0, RGC-30, provided by Westvaco Co.) was subjected to four different treatments: (i) heat treatment at 1273 K in argon for 8 h (carbon C-1); (ii) heat treatment at 473 K in air for 8 h (carbon C-2); (iii) oxidation with an aqueous 5 M solution of H2 O2 at 298 K for 8 h (carbon C-3) and oxidation with an aqueous 5 M solution of HNO3 , at 298 K for 8 h (C-4). 2.2. Characterization techniques The textural characterization of the carbon samples was performed by N2 and CO2 adsorption. N2 adsorption at 77 K is commonly used for determining adsorption properties of porous materials. Although nitrogen gas can only access pores larger than more or less 0.4 nm, restricted diffusion could occur even for this small pore size due to the low temperature at which the measurement is carried out [27,28]. CO2 adsorption at 273 K is useful when carbons contain very narrow microporosity. This temperature is high enough to avoid diffusion problems, and the maximum relative pressure that can be reached in a conventional adsorption system is very low, p/po = 0.03. Besides, this technique does not require very complex equipment [29–31]. Therefore, it has been suggested as a complementary technique to N2 adsorption [28]. CO2 adsorption isotherms are usually evaluated by Dubinin–Radushkevich (D–R) method [32]. Nitrogen adsorption isotherms were determined in a Coulter Omnisorb-610 equipment at 77 K. Before the experiment, the samples were out-gassed at 523 K for 4 h under vacuum (10−6 kPa). The specific surface area (SBET ) was obtained using the BET method. The micropore volume (Vmic ) was deduced from the adsorption data by means of the ␣-method [32]. Finally, the mesopore volume (Vmeso )
was estimated by subtracting the micropore volume (Vmic ) from the total volume adsorbed at p/po = 0.95. CO2 adsorption isotherms were recorded in an Autosorb 6 (Quantachrome) apparatus at 273 K. The volume of narrow micropores of carbons was obtained by applying the D–R-equation to the adsorption data for CO2 [31]. Temperature-programmed desorption (TPD) measurements were carried out in a U-shaped quartz cell using a total glass flow of 50 cm3 min−1 and about 100 mg of sample, with a heating rate of 10 K min−1 . A mixture of 5% H2 /He gas flow was used. Gases evolved during the TPD experiment were followed-up by on-line mass spectroscopy.
2.3. Reaction experiments The surface modified carbon materials were evaluated as supports of CuBr–[1,1,4,7,10,10-hexamethyltriethylenetetramine] (CuI Br–HMTETA), a catalyst for the Atom Transfer Radical Polymerization (ATRP) of methyl methacrylate (MMA) using methyl␣-bromophenylacetate (MBP) as radical initiator. To conduct the ATRP polymerization reaction, methyl methacrylate (99.9%) from Aldrich was distilled under vacuum and stored at 269 K. Toluene (99%), CuBr (99.99%), methyl ␣-bromophenylacetate (MBP, 99%) and 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA, 99%) were also obtained from Aldrich and used without prior treatment. Polymerization with CuI Br/HMTETA physisorbed on activated carbon was carried out as follows: a typical ratio between methyl methacrylate (MMA), copper and initiator (MBP) was set, [MMA]/[Cu]/[MBP] = 100/1/1. Therefore, CuBr (1.35 mmol), toluene (40 ml), and activated carbon (3 g) were added to a 200 ml 4560 mini bench top reactor (Parr Instrument Co.). HMTETA ligand (1.35 mmol) and toluene (20 ml) were added to the cylinder. Description of the experimental setup is detailed elsewhere [33]. Three helium purge cycles were applied to the reactor and cylinder. The HMTETA ligand solution was transferred from the cylinder to the reactor vessel and stirred overnight to support the catalyst on the activated carbon. MMA (135 mmol), initiator (MBP, 1.35 mmol) and toluene (25 ml) were added to the cylinder and purged under three helium purge cycles. The mixture was then added to the reactor vessel. The reactor was heated to 358 K under stirring. Samples (1 ml) were taken out using a helium-purged sampler to determine conversion by 1 H NMR from the ratio between the OCH3 signal intensity in the polymer (3.60 ppm) and in the monomer (3.75 ppm). 1 H NMR spectra were recorded on a Bruker AC-300 spectrometer (Bruker BioSpin, Billerica, MA, USA) at 300 MHz. 1 H NMR chemical shifts in CDCl3 were reported from 0.00 ppm using tetramethyl silane (TMS) as an internal reference.
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The molecular weight and the polydispersity of the polymer were measured by GPC. Number- and weight-average molecular weights (Mn and Mw , respectively) were determined by gel permeation chromatography (GPC: SHK polystyrene gel column; flow rate 1.0 ml min−1 ) relative to polystyrene using tetrahydrofurane (THF) as solvent at 298 K with refractive index detector. Data were recorded and manipulated using the Waters Millennium software package provided with the instrument. The residual copper content in the poly(methylmethacrylate) final polymer (RCu ) was measured by inductively coupled plasma atomic emission spectroscopy (ICP-OES). The ICP-OES analyses were performed on the Inductively Coupled Plasma-Optical Emission Spectrometer Perkin-Elmer Optima 3000TM (Norwalk, CT, USA). The sample was quantified with great accuracy and adequately dissolved with requisite hydrogen fluoride. Then, the solution was vaporized to remove silicon fluoride. The remaining residue was dissolved by 0.2 mol/L hydrogen chloride and the solution formed was adjusted to weak acidity by ammonia. The resulting solution was diluted and it was then ready for ICP-OES measurement.
3. Results and discussion 3.1. Chemical and textural characterization of activated carbons The nature and concentration of oxygen surface groups on activated carbons can be modified by thermal or chemical treatments. Thermal treatments under inert atmosphere produce the selective decomposition of the functional groups on the carbon surface. Oxidizing treatments in gaseous (i.e. air) or liquid phases (i.e. oxidizing aqueous solutions) result in an increase of surface oxygen without substantially modifying the porous texture of the carbon. Gaseous oxidation of carbons produces an increase of phenol and carbonyl functionalities, while oxidation in the liquid phase enhances creation of carboxylic moieties on the carbon surface [34]. Fig. 1 shows the TPD profiles corresponding to the different activated carbons used in this study. TPD allows to quantify the amount of oxygen surface groups in activated carbons by monitoring CO2 and CO evolved upon the decomposition of the different oxygen groups by heat treatment, under an inert atmosphere, at temperatures between 300 and 1300 K [35]. CO2 is produced by the more acidic and less thermally stable oxygenated groups, while CO comes from the more stable and less acidic oxygenated groups on the carbon surface. The amount of CO2 and CO evolved from all the
Table 1 Amounts of CO and CO2 evolved during TPD experiments. Carbon
CO (mol/g)
CO2 (mol/g)
CO/CO2
C-0 C-1 C-2 C-3 C-4
1880 660 1700 3150 3800
170 70 120 600 1400
11.0 9.3 14.2 5.2 2.7
samples upon TPD experiments was determined by integration of the area under the curves of the profiles, after proper calibration, and is listed in Table 1. With regard to the CO2 evolution profiles (Fig. 1a), it can be seen that the parent activated carbon (C-0) contains a very low amount of acidic oxygen surface groups (carboxylic, lactone) and that, as expected, this amount is even decreased by the heat treatment under an argon flow (carbon C-1). Carbon obtained by thermal treatment of C-0 in air (C-2) shows no CO2 evolution below 500 K (it has been heat treated at 473 K), and only a slightly larger amount of CO2 is evolved at higher temperatures than in the case of carbons C-0 and C-1. Oxidation of C-0 with H2 O2 (carbon C-3) increases the amount of CO2 evolving oxygen surface groups, mainly carboxylic groups (maximum evolution rate at about 550 K). However, the larger oxidation effect is achieved with HNO3 . For carbon C-4, the CO2 -TPD profile shows several peaks, corresponding to different surface groups in different locations on the surface of carbon. Thus, peaks with maxima at 420 K and 650 K can be assigned to the decomposition of carboxylic acid groups (low temperature CO2 groups), whereas the peak at 710 K can be related to the decomposition of lactones and carboxylic anhydride (high temperature CO2 groups). Carboxylic anhydrides, that can be formed from two adjacent carboxylic groups, are more stable than carboxylic acid groups and, therefore, will decompose evolving CO2 at a higher temperature [36]. Fig. 1b shows the CO evolution profiles from the carbons in TPD experiments. CO evolution from the starting carbon (C-0) shows a peak at around 1000 K. Treatment of carbon C-0 in Ar at 1273 K (carbon C-1) produces a decrease in CO evolution and the displacement of the peak to higher temperatures, which corresponds to the decomposition of quinone groups. Thus, most of the surface oxygen groups present in the starting carbon (C-0) are removed by the heat treatment in Ar (carbon C-1) [37–40]. CO-TPD of C-2 carbon, shows a peak centered at 1000 K. Thus, heat treatment in air modifies the nature of the oxygen groups present in the starting carbon material, producing more stable groups [41].
Fig. 1. CO2 and CO evolution profiles (TPD) of as-received carbon (C-0) and carbons treated in argon (C-1), air (C-2), H2 O2 (C-3) and HNO3 (C-4).
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range (around 48–44% microporosity and 52–56% mesoporosity). Consequently, four activated carbons, with relatively similar textural properties but very different surface chemistry have been prepared, and they will be used as support for ATRP polymerization of methyl methacrylate. 3.2. Heterogeneous polymerization of MMA using active carbon as support for CuBr/HMTETA catalyst
Fig. 2. Adsorption isotherms of N2 at 77 K of as-received carbon (C-0) and carbons treated in argon (C-1), air (C-2), H2 O2 (C-3) and HNO3 (C-4).
Oxidation of carbon C-0 with H2 O2 (carbon C-3) produces an increase of the amount of CO-evolving oxygen groups. Table 1 shows an increase of evolved CO from 1882 mol/g in the parent C-0 carbon to 3150 mol/g in H2 O2 treated C-3 carbon. The CO evolution profile shows two partially overlapped peaks, at 1000 K and 1050 K, which may be ascribed to the decomposition of phenolic, quinone and/or pyrone groups [42]. On the other hand, the CO evolution profile of carbon prepared by treatment with HNO3 (carbon C-4) shows two peaks at 950 and 1100 K, which are ascribed to the decomposition of phenolic groups, carboxylic anhydrides, ethers and carbonyl groups of quinones and pyrones. The intensity of these peaks (larger amount of CO-evolving surface groups, Table 1) is an evidence of the higher oxidation ability of HNO3 compared to H2 O2 (carbon C-3). Fig. 2 shows the N2 adsorption–desorption isotherms at 77 K for the different carbons. Additionally, Table 2 reports the textural parameters obtained for the parent and the treated carbons. All carbons show modified Type I isotherms, characteristic of microporous solids where the pore filling occurs at low relative pressures, but with non-coincident adsorption and desorption branches. This hysteresis loop indicates capillary condensation associated to mesoporosity. The thermal treatment at high temperature under inert atmosphere (C-1) gives rise to a slight decrease of the specific surface area (SBET ) and the mesopore volume (Vmeso ). The thermal treatment in air at 473 K (C-2) only slightly modifies the porous structure of activated carbon. On the other hand, oxidation with H2 O2 solution (C-3), despite introducing a considerable amount of surface oxygenated moieties, does not greatly affect the carbon porous texture. The most important modification in porous texture was produced after treatment with HNO3 solution (carbon C-4). The BET surface area and the micropore and mesopore volumes decrease, and there is also a decrease of the narrowest porosity. However, the relative contribution of micro and mesopores (% meso) remains similar in all carbons, always within an intermediate
The solid CuBr/HMTETA catalyst used for the heterogeneous polymerization of methyl methacrylate was supported on the four previously characterized activated carbons. Table 3 shows the data obtained in the polymerization reaction using the fresh catalyst, and also during an ulterior use of it, as supported CuBr/HMTETA catalysts were reused in a second polymerization reaction without any regeneration procedure in order to demonstrate that they are still active after a polymerization process. Leaching of catalyst once the polymerization reaction was over was detected in all cases, the residual copper (RCu ) present in the final poly(methylmethacrylate) being determined by ICP-OES. The evolution of conversion as a function of time on reaction is plotted in linear and in semilogarithmic coordinates, for the different catalysts. Conversion is represented by the ratio [M]0 /[M]t ; where [M]0 is the initial concentration of monomer and [M]t is the concentration of monomer at a given time (Figs. 3a–7a). On the other hand, the evolution of the experimental molecular weight of the obtained polymer (Mn ) and of its polydispersity index (Mw /Mn ) with conversion is plotted in Figs. 3b–7b. CuBr/HMTETA catalyst supported on the heat treated carbon in Ar (C-1) produced conversions (determined after 24 h reaction) of 55% (first use of catalyst) and 49% (second use of catalyst), which are lower than those obtained for the as-received C-0 carbon exp (Table 3, Figs. 3a and 4a). Experimental molecular weights (Mn ) theor ), represented are higher than theoretical molecular weights (Mn by a dashed line in Fig. 4b, but with both the fresh and the re-used catalysts supported in carbon C-1, molecular weights show a lineal tendency versus conversion (Fig. 4b). The lower conversion degree achieved with the CuBr/HMTETA catalyst supported on carbon C-1 can be attributed to the surface chemical properties of the support. Treatment of carbon in argon at 1273 K removed oxygenated moieties from the carbon surface, as shown by TPD experiments. Thus, surface chemistry of C-1 carbon is similar to that of a carbon black, with a high amount of unsaturated carbon atoms resulting from the removal of oxygen groups. Therefore, the ability of this support to act as a polymeric radical scavenger (Scheme 2) may be responsible for the low catalytic activity of CuBr/HMTETA supported on C-1 carbon in the ATRP polymerization of methyl methacrylate, by lowering deactivation rate (kd ) in the ATRP reaction (Scheme 1). As shown in Table 2, the textural characteristics of carbons C-0 and C-2 are quite similar, so a comparative study of catalytic activity of CuBr/HMTETA catalyst supported on both activated carbons will be based just on their surface chemistry. When CuBr/HMTETA catalyst was supported on the carbon thermally treated in air (C-2), conversions of 73% (first catalyst use) and 55% (second cat-
Table 2 Specific surface area (SBET ), total pore volume (Vt ), mesopore volume (Vmeso ), micropore volume (Vmic ) and narrow microporosity (Vn ) for the as-received (C-0) and the treated activated carbons. Carbon
SBET (m2 g−1 )
Vt (cm3 g−1 )
Vmeso (cm3 g−1 )
Vmic (cm3 g−1 )
Vn (cm3 g−1 )
% meso
C-0 C-1 C-2 C-3 C-4
1530 1410 1520 1510 1200
1.16 1.07 1.13 1.11 0.87
0.65 0.56 0.61 0.60 0.46
0.51 0.49 0.52 0.51 0.41
0.32 0.32 0.32 0.32 0.27
56 52 54 54 52
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Table 3 Conversion, experimental and theoretical number-average molecular weights (Mn ), polydispersity index (Mw /Mn ) and residual copper (RCu ) as a result of the ATRP polymerization of MMA. Support C-0 First use Second use C-1 First use Second use C-2 First use Second use C-3 First use Second use C-4 First use Second use
exp
Mn
(g mol−1 )
Time (h)
Conversion (%)
24 24
72 83
12,652 12,912
24 24
55 49
24 24
Mntheor (g mol−1 )
Mw /Mn
RCu (ppm)
7,200 8,300
1.13 1.14
45 47
23,006 21,935
5,500 4,900
1.55 1.44
34 43
73 55
15,530 14,608
7,300 5,500
1.27 1.31
6 39
24 24
75 60
15,420 14,597
7,500 6,000
1.43 1.46
5 4
24 24
82 70
11,246 12,329
8,200 7,000
1.19 1.27
6 14
Reaction conditions: [MMA]/[Cu]/[MBP] = 100/1/1.
Fig. 3. (a) Kinetic plots and (b) number-average molecular weight (Mn ) and polydispersity index (Mw /Mn ) versus conversion (%) plots in the ATRP polymerization of methyl methacrylate using CuBr/HMTETA catalyst supported in as-received (C-0) carbon. Dashed line represents the theoretical Mn versus conversion, () first use and (䊉) second use of catalyst. Reaction conditions: [MMA]/[Cu]/[MBP] = 100/1/1.
alyst use) were obtained 24 h after the beginning of the methyl methacrylate (MMA) polymerization reaction. Fig. 5a shows a first-order kinetics when carbon C-2 is used as support for the polymerization catalyst, this evidencing chain termination reactions
by formation of Pn • –X in the Atom Transfer Radical Polymerization Reaction (Scheme 1). The obtained experimental molecular weight is higher than the predicted theoretical molecular weight after the first and the second uses of catalyst (Fig. 5b), but a
Fig. 4. (a) Kinetic plots and (b) number-average molecular weight (Mn ) and polydispersity index (Mw /Mn ) versus conversion (%) plots in the ATRP polymerization of methyl methacrylate using CuBr/HMTETA catalyst supported in carbon treated in argon (C-1). Dashed line represents the theoretical Mn versus conversion, first use () and second use (䊉) of catalyst. Reaction conditions: [MMA]/[Cu]/[MBP] = 100/1/1.
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231
Fig. 5. (a) Kinetic plots and (b) number-average molecular weight (Mn ) and polydispersity index (Mw /Mn ) versus conversion (%) plots in the ATRP polymerization of methyl methacrylate using CuBr/HMTETA catalyst supported in carbon treated in air (C-2). Dashed line represents the theoretical Mn versus conversion, first use () and second use (䊉) of catalysts. Reaction conditions: [MMA]/[Cu]/[MBP] = 100/1/1.
linear tendency in the average-number molecular weight versus conversion plot is obtained. However, control of polymerization is not improved when comparing C-2 carbon to C-0 carbon as catalyst supports, this being reflected by the fact that the polydispersity index of the poly(methylmethacrylate) obtained after polymerization using the CuBr/HMTETA catalyst supported on carbon C-2 (Mw /Mn = 1.27–1.31) is higher than that obtained when the as-received activated carbon (C-0) was used as catalyst support (Mw /Mn = 1.13–1.14) (Fig. 5b (right) and Table 3). Therefore, although C-0 and C-2 carbons behave similarly from a kinetic point of view, polymerization control is decreased by the air treatment of the carbon support. This can be explained in terms of loss of oxygenated surface moieties during thermal treatment, as shown by TPD analysis in Fig. 1. The use of carbon C-3, obtained from as-received carbon treated with a 5 M H2 O2 solution, as catalyst support provided a final conversion of 75% (first use) and 60% (second use of the catalyst) (Fig. 6a) in the polymerization. However, polydispersity index increased from 1.13 (C-0 carbon as catalyst support) to 1.46 (C-3 carbon as catalyst support), which reflects a poorer control of the polymerization reaction when C-3 carbon is used as catalyst support. A relatively good control of polydispersity should fall in the range 1.0 < Mw /Mn < 1.5. However, a well-controlled polymerization, with a narrow molecular weight distribution, may show a polydispersity index (Mw /Mn ) below 1.1, which is not achieved with C-3 carbon supported catalyst. Although carbon C-3 contains an increased amount of oxygenated surface groups, as a consequence of the treatment with H2 O2 solution, the decrease of mesopore volume (Vmeso in Table 2) produced in this activated carbon could also affect polymerization control. As stated previously [26], the texture of the activated carbon plays a definitive role in the polymerization rate and the control of polymerization reaction, evaluated by the polydispersity index of the final poly(methylmethacrylate). The accessibility of the catalyst to pores in the activated carbon is limited by pore size and growing polymer size. ATRP polymerization reaction is successfully achieved when an activated carbon exhibits an adequate range of both micro and mesoporosity. The mesoporosity of the activated carbons used as support of the CuI -Br/HMTETA catalyst enhances the activation/deactivation process, as it allows the polymeric chains to bond to the catalyst complex. The catalyst undergoes a one-electron oxidation producing Br–CuII -Br/HMTETA, and then returns to Cu(I) Br/HMTETA (Scheme 1). However, in the micropores the catalyst is isolated from the growing polymeric chains located
in the mesopores, or even it could also be blocked in the micropores themselves when is bonded to an incipient growing polymeric chain whose size would allow it to enter a micropore. Thus, transport limitations inside micropores produced by a slow diffusion of the growing polymeric radicals towards the metal center (Cu) of the catalyst complex result in a low polymerization rate. On the other hand, the access of growing polymer chains is favored in mesopores and deactivation process (which implies reduction of catalyst to CuI -Br/HMTETA) is enhanced. Consequently, the decrease of mesopore volume of carbon C-3 could also negatively affect the control of polymerization. Nevertheless, a lack of an adequate surface chemistry seems to be determining over textural properties. Consequently, a relatively high polydispersity index is obtained (1.46). Finally, carbon treated with nitric acid (C-4) showed the highest polymerization control, compared to carbons C-1, C-2 and C-3. A polydispersity index of 1.19 was achieved when supporting the catalyst on C-4 carbon. Although this polydispersity index is similar to that of the as-received C-0 carbon, the advantage of using the nitric acid treated C-4 carbon instead of the as-received C-0 carbon is that copper leaching (RCu ) is highly decreased (from 45 ppm in C-0 to 6 ppm in C-4) and conversion is improved (Table 3) in the first use. This is due to the surface functionalization (creation of carboxylic moieties) produced by the treatment with nitric acid. The CuBr/HMTETA catalyst used in this study exhibited copper leaching during the polymerization process, irrespective of the activated carbon used as catalyst support. A possible cause of leaching may be structural rearrangements of the surface copper species during reaction. The structural changes required upon copper oxidation create unstable species that favor leaching [43]. The residual copper content in the polymethyl methacrylate (PMMA) final polymer (RCu ) (Table 3) was measured using inductively coupled plasma atomic emission spectroscopy (ICP-OES). The theoretical copper content, assuming that all the copper in the catalyst suffers leaching and that a 100% conversion of methyl methacrylate (MMA) into poly(methylmethacrylate) (PMMA) is reached, would be 1329 ppm. When as-received C-0 carbon was used as support, the residual copper content in solution (RCu ) fell in the range 45–47 ppm. Heat or chemical treatment of carbon contributed to reduce copper leaching. The functionalization of the carbon surface by introduction of oxygenated groups contributes to enhance adsorption of the CuBr/HMTETA catalyst. Therefore, C-4 carbon shows an interesting behavior in the MMA polymerization reaction control. Leaching
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Fig. 6. (a) Kinetic plots and (b) number-average molecular weight (Mn ) and polydispersity index (Mw /Mn ) versus conversion (%) plots in the ATRP polymerization of methyl methacrylate using CuBr/HMTETA catalyst supported in carbon treated in H2 O2 (C-3). Dashed line represents the theoretical Mn versus conversion, first use () and second use (䊉) of catalysts. Reaction conditions: [MMA]/[Cu]/[MBP] = 100/1/1.
Fig. 7. (a) Kinetic plots and (b) number-average molecular weight (Mn ) and polydispersity index (Mw /Mn ) versus conversion (%) plots in the ATRP polymerization of methyl methacrylate using CuBr/HMTETA catalyst supported in carbon treated in HNO3 (C-4). Dashed line represents the theoretical Mn versus conversion, first use () and second use (䊉) of catalysts. Reaction conditions: [MMA]/[Cu]/[MBP] = 100/1/1.
H
N
N
O N
HO
N
OH O
N
Cu Br
N
N
Cu
N
Br
H
Scheme 3. Cleavage of CuBr/HMTETA catalyst favored by carboxylic acid groups in activated carbon.
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is reduced and conversion is slightly improved (compared to asreceived C-0 carbon) (Fig. 7a). Furthermore, experimental number average molecular weight (Mn ) is closer to theoretical number average molecular weight (dashed line in Fig. 7b) with nitric acid treated (C-4) carbon. Carboxylic –COOH groups on the C-4 surface (detected by TPD) favored Atomic Transfer Polymerization (ATRP) of MMA. Oxygenated acidic groups act as primary adsorption centers that modify the hydrophobic character and the acid–base nature of the carbon surface, thus enhancing adsorption of mild basic compounds like CuBr/HMTETA catalyst (Scheme 3), and minimizes radicals trapping. Consequently, the control of surface chemistry of carbon used as support of CuBr/HMTETA catalyst is of paramount importance in the Atomic Transfer Polymerization methyl methacrylate. 4. Conclusions It can be concluded from the obtained results that activated carbon is a suitable support of CuBr/HMTETA for ATRP polymerization of methyl methacrylate. The control of functionality of the activated carbon by means of heat or chemical treatments plays a definitive role in reducing copper leaching and controlling polymerization reaction to achieve a narrow molecular weight distribution (i.e. a polydispersity index of the final poly(methylmethacrylate) close to 1). Carbon treated with nitric acid (C-4) showed the highest polymerization control, with a polydispersity index of 1.19, and decreased copper leaching. This has been attributed not only to its optimal porous structure (exhibiting micro and mesoporosity) but also to the adequate compatibility of C-4 carbon with the CuBr/HMTETA catalyst due to the high functionalization of its surface with carboxylic groups obtained by oxidation with nitric acid. Treatment with H2 O2 (carbon C-3) was not as effective as nitric acid in functionalizing carbon surface. Carbons heat treated in argon (C-1) or in air (C-2) exhibited poor polymerization control. Absence of functional groups as a consequence of the thermal treatment decreases the effectiveness of these carbons as catalyst supports for CuBr/HMTETA mediated Atom Transfer Radical Polymerization of methyl methacrylate. Besides, the surface of carbon C-1 may act as a polymeric radical scavenger, lowering deactivation step in the Atom Transfer Radical Polymerization and lowering activity of CuBr/HMTETA catalyst. Although the textural properties of carbon C-2 are similar to those of as-received C-0 carbon, and both carbons behave similarly from a kinetic point of view, the decrease of polymerization control in the air treated C-2 carbon can be explained in terms of loss of acidic oxygenated surface moieties during thermal treatment. This reflects that the lack of surface functionalization plays a crucial role. These results suggest that the accessibility of the growing polymer chains to the supported catalyst together with an adequate surface chemistry of the carbon support is a crucial parameter to obtain a good polymerization control with a narrow molecular weight distribution and polydispersities close to 1. Acknowledgements The Mexican Council of Science and Technology (CONACYT) is gratefully acknowledged for the Pre-Doctoral fellowship No. 187157 to S. Barrientos-Ramírez. Financial support from Generali-
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tat Valenciana (Project PROMETEO/2009/002 – FEDER) is gratefully acknowledged. We thank M. Dolores Baeza for her help with GPC experiments. Supply of activated carbon by MeadWestvaco Co. is gratefully acknowledged. References [1] K. Matyjaszewski, J. Xia, Chem. Rev. 101 (9) (2001) 2921–2990. [2] Y. Shen, S. Zhu, R. Pelton, Macromolecules 34 (17) (2001) 5812–5818. [3] G. Kickelbick, H.J. Paik, K. Matyjaszewski, Macromolecules 32 (9) (1999) 2941–2947. [4] Y. Shen, S. Zhu, F. Zeng, R.H. Pelton, Macromolecules 33 (15) (2000) 5427–5431. [5] S.C. Hong, K. Matyjaszewski, Macromolecules 35 (20) (2002) 7592–7605. [6] S.C. Hong, H.J. Paik, K. Matyjaszewski, Macromolecules 34 (15) (2001) 5099–5102. [7] D.M. Haddleton, C. Waterson, P.J. Derrick, C.B. Jasieczek, A.J. Shooter, Chem. Commun. 7 (1997) 683–684. [8] D.M. Haddleton, D.J. Duncalf, D. Kukulj, A.P. Radigue, Macromolecules 32 (15) (1999) 4769–4775. [9] S. Liou, J.T. Rademacher, D. Malaba, M.E. Pallack, W.J. Brittain, Macromolecules 33 (12) (2000) 4295–4296. [10] M.E. Honigfort, W.J. Brittain, Macromolecules 36 (9) (2003) 3111–3114. [11] Y. Shen, S. Zhu, F. Zeng, R. Pelton, J. Polym. Sci. A: Polym. Chem. 39 (7) (2001) 1051–1059. [12] Y. Shen, S. Zhu, F. Zeng, R. Pelton, Macromol. Chem. Phys. 201 (13) (2000) 1387–1394. [13] Y. Shen, S. Zhu, R. Pelton, Macromolecules 34 (10) (2001) 3182–3185. [14] Y. Shen, S. Zhu, Macromolecules 34 (25) (2001) 8603–8609. [15] Y. Shen, S. Zhu, R. Pelton, Macromol. Rapid Commun. 21 (14) (2000) 956–959. [16] T. Opstal, K. Melis, F. Verpoort, Catal. Lett. 74 (3–4) (2001) 155–159. [17] J.V. Nguyen, C.W. Jones, J. Polym. Sci. A: Polym. Chem. 42 (6) (2004) 1384–1399. [18] T. Otsu, T. Tazaki, M. Yoshioka, Chem. Express. 5 (10) (1990) 801–804. [19] D.M. Haddleton, D. Kukulj, A.P. Radigue, Chem. Commun. 1 (1999) 99–100. [20] T. Liu, R. Casado-Portilla, J. Belmont, K. Matyjaszewski, J. Polym. Sci. A: Polym. Chem. 43 (2005) 4695–4709. [21] Q. Yang, L. Wang, W. Xiang, J. Zhou, J. Li, Polymer 48 (2007) 2866–2873. [22] Y.Z. Jin, C. Gao, H.W. Kroto, Macromol. Rapid Commun. 26 (2005) 1133–1139. [23] L. Li, C.M. Lukehart, Chem. Mater. 18 (2006) 94–99. [24] Q. Yang, L. Wang, W. Xiang, J. Zhou, G.J. Tan, J. Polym. Sci. A: Polym. Chem. 45 (2007) 3451–3459. [25] Q. Yang, L. Wang, W. Xiang, J. Zhou, G. Tan, Polymer 48 (2007) 3444–3451. [26] S. Barrientos-Ramírez, G. Montes de Oca-Ramírez, M.M. Pastor-Blas, A. Sepúlveda-Escribano, A. González-Montiel, Catal. Today 150 (2010) 42–48. [27] F. Rodríguez-Reinoso, J. López-Gonzalez, C. Berenguer, Carbon 20 (3) (1982) 185–189. [28] R. Ríos, J. Silvestre-Albero, A. Sepúlveda-Escribano, M. Molina-Sabio, F. Rodríguez-Reinoso, J. Phys. Chem. C 111 (10) (2007) 3803–3805. [29] J.W. Patrick, in: E. Arnold (Ed.), Porosity in Carbons: Characterization and Applications, John Wiley & Sons, New York, 1995. [30] D.H. Everett, J.C. Powl, J. Chem. Soc. Faraday Trans. 1: Phys. Chem. Condens. Phases 72 (3) (1976) 619–636. [31] H. Marsh, F. Rodríguez-Reinoso, Activated Carbon, Elsevier, Oxford, 2006. [32] M.M. Dubinin, in: P.L. Walker (Ed.), Chemistry and Physics of Carbon, vol. 2, Dekker, New York, 1966. [33] S. Barrientos-Ramírez, E.V. Ramos-Fernández, J. Silvestre-Albero, A. SepúlvedaEscribano, M.M. Pastor-Blas, A. González-Montiel, Micropor. Mesopor. Mater. 120 (2009) 132–140. [34] F. Rodríguez-Reinoso, in: E.A. de Patrick (Ed.), Porosity in Carbons: Characterization and Applications, Arnold, London, 1998, p. 253. [35] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. Orfao, Ind. Eng. Chem. Res. 46 (2007) 4110–4115. [36] C. Moreno-Castilla, M.A. Ferro-García, J.P. Joly, I. Bautista-Toledo, F. CarrascoMarín, J. Rivera-Utrilla, Langmuir 11 (11) (1995) 4386–4392. [37] S. Shin, J. Jang, S.H. Yoon, I. Mochida, Carbon 35 (1997) 1739–1743. [38] A.D. Seyed, T.J. Karanfil, J. Colloid Interface Sci. 274 (2004) 1–8. [39] C. Moreno-Castilla, F. Carrasco-Marın, F.J. Maldonado-Hodar, J. Rivera-Utrilla, Carbon 36 (1998) 145–151. [40] F. Rodríguez-Reinoso, in: H. Marsh, E.A. Heintz, F. Rodríguez-Reinoso (Eds.), Introduction to Carbon Technologies, Secretariado de Publicaciones, University of Alicante, Alicante, Spain, 1997, p. 35. [41] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. Orfao, Carbon 37 (1999) 1379–1389. [42] T.E. Patten, K. Matyjaszewski, Adv. Mater. 10 (12) (1998) 901–915. [43] J.V. Nguyen, C.W. Jone, J. Polym. Sci. A: Polym. Chem. 42 (6) (2004) 1367–1383.