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Heterogeneous Atom Transfer Radical Polymerization of Methyl Methacrylate at Low Metal Salt Concentrations Santiago Faucher and Shiping Zhu* Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7
The heterogeneous atom transfer radical polymerization of methyl methacrylate using the catalyst complex CuBr/1,1,4,7,10,10-hexamethyltriethylene tetramine (CuBr/HMTETA) was undertaken under a number of conditions to elucidate the effects of key process variables. It was found that a 100-fold reduction in metal salt concentration, to 21 ppm in solution (metal to intiator ) 0.01:1), reduced polymerization rates 3-fold. PMMA produced at these low concentrations had high initiator efficiencies (Ieff ) 88%) and low polydispersisties (1.1). Decreasing metal salt concentrations 1000-fold, to 2 ppm (Mt:I ) 0.001:1), resulted in PMMA with controlled molecular weights (Ieff ) 96%) but high polydispersities (2.1). Polymerization rates above and below a metal concentration of 210 ppm were found to be proportional to the metal salt concentration to the power of 0.05 and 0.37, respectively. Rates were found to increase with ligand concentration to the power of 0.52. Stirring was found to increase polymerization rates by 40%. Introduction
Scheme 1
Free radical polymerization is the most widely used polymer production method. Its tolerance of impurities, wide range of polymerizable monomers, and mild reaction conditions account for its commercial success. Unfortunately, it lacks the ability to precisely control polymer molecular weight, a trait necessary for the design of tailored macromolecules with specific architectures and functionalities. Ionic polymerization has been used to fill this void but lacks many of the positive attributes of the free radical mechanism. Low reaction temperatures and careful purification of the reactants are often required. Furthermore, the ionic mechanism is less versatile than the free radical mechanism, polymerizing a smaller number of monomers. Great industrial and scientific interest therefore exists in the development of controlled/living radical polymerization (CRP) technologies. The discovery of transition metal-mediated atom transfer radical polymerization (ATRP) in 1995 was a major breakthrough in this field.1-3 ATRP is one of the most promising CRP technologies.1 It has been used to prepare functional polymers with a wide range of controlled compositions and topologies.4,5 ATRP relies, as do other CRPs, on a fast equilibrium between dormant and live radical species to control the polymerization. In the case of ATRP, the process is catalytic as shown in Scheme 1.1,4,5 A ligated metal salt (Mtn-Y/L) that abstracts a halide from the initiator, an alkyl halide (R-X), activates the polymerization. This catalyst is thereby oxidized (X-Mtn+1-Y/ L) and a radical (R•) is formed from the initiator. The radical can propagate with monomer (M), terminate with other radicals/impurities, or be deactivated by the oxidized metal catalyst. The deactivated polymer (RX) can be reversibly reactivated and deactivated following the same cycle. Termination is minimized by an equilibrium that favors the dormant alkyl halide and therefore a low radical concentration. The minimization of termination results in a quasi-living polymerization * To whom correspondence should be addressed. Tel.: 1(905) 525-9140 ext 24962. Fax: 1(905) 521-1350. E-mail: zhuship@ mcmaster.ca.
where molecular weight is a linear function of conversion. The polymers so produced have controlled molecular weight and narrow molecular weight distribution.2 Although ATRP is widely used in laboratories, it faces several challenges on the road to full commercialization at the industrial scale. One of the major challenges is the high catalyst loading required to achieve reasonable reaction rates. In addition to the high cost of the catalyst system, this high loading contaminates the final product, coloring it, and potentially accelerating polymer degradation. Post purification steps are therefore necessary to reduce catalyst concentrations to an acceptable level. Two general approaches have been studied to overcome this challenge. The first and most attractive is to improve the catalyst’s activity to a level at which postpurification becomes unnecessary, as in polyolefin production. The second is to support, recover, and recycle the catalyst. Our group has been focused primarily on this second, engineering-based approach. In this area we have studied soluble-recoverable catalysts, chemically grafted supported catalyst, and physically adsorbed supported catalyst for the ATRP of methyl methacrylate (MMA).6-12 Recently, however, we determined and reported that the location of the active catalyst sites using the silica supported and physically adsorbed recyclable catalyst we had developed was not on the support’s surface.13 Rather, we found it to be in solution. Given this, we suspected that the activity of the heterogeneous catalyst being used was high and would therefore allow for significant reductions in the concentration of catalyst used, ideally without affecting the polymerization rates and the control of molecular weight. Preferably, should the activity be found to be sufficiently high, no postpurification for metal salt removal would then be required.
10.1021/ie0494317 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/14/2005
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Upon reviewing the literature, it became clear that no highly active catalyst system had been identified for the polymerization of MMA at low catalyst concentrations ([Mt] < 200 ppm) and that CuBr/HMTETA might be such a catalyst.14-25 Matyjaszewski first reported the successful ATRP of MMA using CuBr/HMTETA, but the study did not investigate the activity of the heterogeneous catalyst system and the factors affecting it.18 To fill this void we studied several factors that were likely to affect the catalyst activity: metal salt concentration, ligand concentration, and stirring. Through an optimization of these parameters we finally yielded ATRPs with controlled molecular weights and relatively fast rates given the low metal salt concentration used (21 ppm in solution). Significantly our work demonstrates that one catalyst site can mediate the growth of more than 100 PMMA chains. Finally we have identified, to our knowledge, the most active catalyst for the ATRP of MMA. Experimental Section Materials. MMA (Aldrich, 99.9%) was distilled under vacuum and stored at 4 °C before use. 1,1,4,7,10,10Hexamethyltriethylenetetramine (HMTETA, 99%), CuBr (98%), and methyl R-bromophenylacetate (MBP, 97%) were used as received from Aldrich. Toluene was distilled from CaH2. NMR Measurements. 1H NMR spectra were recorded on a Bruker ARX-200 spectrometer at 200 MHz. The chemical shifts in CDCl3 were reported downfield from 0.00 ppm using the residual CHCl3 signal at 7.26 ppm as an internal reference. Samples were diluted in CDCl3. Monomer conversion was calculated from the intensity ratio of OCH3 signals from the polymer (3.60 ppm) and monomer (3.75 ppm). GPC Measurements. The number- and weightaverage molecular weights (Mn and Mw, respectively) were determined by gel permeation chromatography relative to narrow polystyrene standards. A Waters 710 sample autoinjector, three linear columns in series (Waters Styragel HR 5E, 2× Shodex KF-804L), a Waters 600 pump system, and a 410 RI detector were used for the assays. The eluent, THF, was pumped through the system at a fixed flowrate of 1 mL/min. The columns and detector were heated to 35 °C and 40 °C, respectively. Data were recorded and manipulated using the Waters Millennium software package. Polymerizations. Glass vessels were loaded with varying quantities of CuBr to give ratios of metal salt to initiator (Mt:I) ranging from 1:1 to 0.001:1. Test tubes (20 mm inner diameter, 150 mm length) were used for Mt:I ratios from 1:1 to 0.1:1 and were each loaded with 8 mL of reaction solution. Reactions with Mt:I ratios between 0.1:1 and 0.01:1 were done in 100 mL Schlenk flasks that held 80 mL of reaction solution. Those at a Mt:I ratio of 0.001:1 used 250 mL Schlenk flasks holding 250 mL of reaction solution. The larger flasks and reaction volumes were used to offset measurement errors at the low metal salt concentrations. A magnetic stir bar was added to the vessel if the polymerization was to be stirred, and the vessel was then sealed with a rubber septum. Five vacuum-nitrogen cycles were then applied to the vessel that contained copper salt. A typical polymerization run for Mt:I ratios ranging from 1:1 to 0.1:1 proceeded as follows: MMA (2.4 g, 24 mmol), toluene (4.7 g), and HMTETA (65.2 µL, 0.24 mmol) were added to the CuBr bearing flask. In all of
the solution polymerization runs, the weight ratio of monomer to toluene remained constant at 1:2. The mixture was bubbled with nitrogen for the desired time (3 or 60 min). Initiator, MBP (37 µL, 0.24 mmol), was then added dropwise to the flask with shaking or stirring. The flask holding 8 mL of reaction solution was placed within an oil bath operated at 90 °C; stirring (if any) was provided by a magnetic stir bar. Kinetic samples were withdrawn at timed intervals with a nitrogen-purged syringe. The samples were oxygenated with air, placed in hermetic vials, and stored in a freezer for future assay. Monomer conversion and polymer molecular weight were assayed by NMR and GPC, respectively. Polymerizations at Mt:I ratios between 0.1:1 and 0.01:1 followed a similar procedure but used a larger reaction volume (80 mL). Polymerizations at a Mt:I ratio of 0.001:1 used larger reaction volumes (250 mL) to allow for the practical measurement of the metal salt required. The metal salt was placed in the Schlenk flask with the stir bar and then sealed with a rubber septum, Parafilm, and PVC tape to eliminate the potential for air ingress. Five vacuum-nitrogen cycles were then applied to the flask. The solvent and monomer in separate sealed graduated cylinders were also subjected to five vacuum-nitrogen cycles and then purged with nitrogen for 60 min. The solvent and monomer were then transferred to the copper bearing Schlenk flask through stainless steel needles using nitrogen pressure. HMTETA and MBP were nitrogen purged for 10 and 15 min, respectively, prior to being transferred to the Schlenk flask. Polymerizations with varying ligand concentrations were carried out as follows: test tubes holding 8 mL of reaction solution were loaded with CuBr to give ratios of metal salt to initiator (Mt:I) of 1:1 or 0.1:1. MMA (2.4 g, 24 mmol) and toluene (4.7 g) were then added to the tube along with a magnetic stir bar if required. The appropriate quantity of HMTETA was then added to the flask to achieve the desired ligand to initiator (L:I) molar ratio, which ranged from 10:1 to 0:1. The mixture was bubbled with nitrogen for the desired time (3 or 30 min). Degassed initiator, MBP (37 µL, 0.24 mmol), was then added dropwise to the flask with shaking or stirring. The flask was placed within an oil bath operated at 90 °C, stirring (if any) was provided by the magnetic stir bar. Kinetic samples were taken, stored, and analyzed as described above. Results and Discussion Effect of Metal Salt Concentration. Metal salts added to ATRP systems as catalyst remain within the final polymer product coloring it and potentially accelerating degradation. This impurity must later be removed, usually at great costs, for the polymer to be marketable. Minimizing its use in polymerizations, given acceptable rates, should therefore be a priority in ATRP’s development. While many studies have examined homogeneous catalyst systems and their modification for improved rates and reduced use, little attention has been paid in this area to their heterogeneous counterparts. In part, this is because heterogeneous catalysts generally yield less controlled molecular weights and broader polydispersities than homogeneous catalyst.2,15,16 Heterogeneous systems do however offer great potential for reductions in metal salt concentrations. In these systems, much of the metal salt remains undissolved and therefore may not contribute to the
Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 679 Table 1. Effect of Mt:I Ratio and Stirring on the ATRP of MMA entry
stirred
MMA
1a 2 3 4a 5 6b 7b 8 9c 10c 11c,d
no no no no no no yes yes yes yes yes
100 100 100 100 100 100 100 100 100 100 100
a
molar ratios MBP CuBr 1 1 1 1 1 1 1 1 1 1 1
1 0.5 0.2 0.1 0.05 0.01 1 0.5 0.1 0.001 0.001
HMTETA
[Cu] (ppm)
conversion (%)
time (min)
kapp (1/s)
Mnsec (g/mol)
Mntheo (g/mol)
Ieff (%)
PD
1 1 1 1 1 1 1 1 1 1 1
2130 1070 430 210 110 21 2130 1070 210 2 6
72 66 68 68 54 39 87 81 78 23 52
375 375 375 375 375 375 375 375 375 375 280
6.2 × 10-5 5.3 × 10-5 5.6 × 10-5 5.4 × 10-5 4.2 × 10-5 2.3 × 10-5 8.9 × 10-5 7.4 × 10-5 6.7 × 10-5 1.1 × 10-5 4.4 × 10-5
9170 8110 7420 7420 5730 4460 15880 8780 9500 2350 4742
7190 6590 6810 6850 5430 3910 8660 8110 7800 2260 5207
79 81 92 93 95 88 57 92 82 96 110
1.09 1.09 1.09 1.10 1.08 1.13 1.23 1.11 1.11 2.10 6.31
Average of three experiments. b Average of four experiments. c Average of two experiments.
Figure 1. Conversion vs time for the polymerization of MMA catalyzed by CuBr-HMTETA; 90 °C, toluene/MMA ) 2 (w/w), [MMA]/[initiator]/[CuBr]/[HMTETA] ) 100:1:1:1 (molar). Control experiment (2, 4) and aliquot (filtered) experiment ([, ]).
polymerization kinetics. Thus, by reducing metal salt concentrations, the kinetics may not be affected but the downstream efforts required to remove the metal salt are minimized. The role of the undissolved metal salt in these polymerizations can be easily determined by undertaking simple filtering experiments.13 In these polymerizations, an aliquot of the reacting solution (control) is hotfiltered partway through the polymerization and then reacted further in parallel to the control solution to contrast its activity. The difference in activity between the control and aliquot solution therefore represents the contribution of the undissolved solids to the polymerization. We undertook this test in the ATRP of MMA using the heterogeneous catalyst CuBr/HMTETA and show the results in Figures 1 and 2. No significant difference is observed between the polymerization kinetics of the control and aliquot solution as seen in Figure 1. Similarly, no great difference is observed in the polymer characteristics (Mn and PD) between polymerizations as seen in Figure 2. From this, we conclude that the undissolved metal salt does not significantly contribute to the polymerization kinetics. Given this, large reductions in the metal salt used should be viable without affecting reaction rates and product characteristics.
d
Bulk polymerization.
Figure 2. PMMA molecular weight and polydispersity as a function of conversion in the MMA polymerization described in Figure 1. Control experiment (2, 4) and aliquot (filtered) experiment ([, ]). Continuous line represents theoretical molecular weight.
Reductions in metal salt concentration were explored through a series of batch polymerizations that are summarized in Table 1. Our goal was to determine the effect of a reduction in metal salt concentration on the polymerization kinetics and identify any external causes for reductions in rates with decreased metal salt concentrations. In these polymerizations, the monomer, initiator, and ligand concentrations were kept constant, while that of the metal salt was varied. In a first series of experiments (Table 1 entries 1 through 6), we examined the effect of 2-, 5-, 10-, 20-, and 100-fold reductions in the metal salt concentration on the ATRP kinetics. Figure 3 shows the conversion versus time and the firstorder rate plots for some of these polymerizations. Figure 4 shows the corresponding molecular weight (Mn) and molecular weight distribution (PD) development. The kinetic data in Table 1 and Figure 3 show that a 10-fold reduction in the catalyst concentration from Mt:I ) 1:1 down to 0.1:1 does not significantly affect the polymerization rate (Table 1, entries 1 through 4). Conversions averaged 72% at a Mt:I ratio of 1:1 (Table 1, entry 1), while those at a ratio of 0.1:1 averaged 68% over the same reaction time. Equally, the apparent rate constants (kapp), defined by eq 1, were not greatly affected by a change in metal salt concentration (kapp ) 6.2 × 10-5 s-1 vs 5.4 × 10-5 s-1).
RP ) kP[R•][M] ) kapp[M]
(1)
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Figure 3. Conversion vs time and first-order rate plot for the polymerization of MMA catalyzed by CuBr-HMTETA at varying metal salt to initiator molar ratios: 1:1 ([, ]); 0.1:1 (b, O); 0.05:1 (2, 4) and 0.01:1 (9, 0); toluene/MMA ) 2 (w/w), [MMA]/[initiator]/ [HMTETA] ) 100:1:1 (molar), 90 °C.
Figure 5. Apparent rate constants (kapp) vs metal salt concentration for stirred (2, 4) and unstirred ([, ]) ATRPs of MMA, toluene/MMA ) 2 (w/w), [MMA]/[initiator]/[HMTETA] ) 100:1:1 (molar), 90 °C. Error bars show 95% confidence intervals.
polymerization rate is reflected by the low order (0.05) of the law. The second rate law (eq 3) describes the rate behavior at Mt:I ratios below 0.1:1. In this range, rates decrease with reduced metal salt concentration as reflected in the higher order (0.37) of the rate law. The discontinuous behavior in the polymerization kinetics observed at Mt:I ) 0.1:1 is presumably a result of the limited solubility of the metal salt in this reaction system. Thus, at Mt:I g 0.1:1, adding metal salt to the system does not increase the concentration of catalyst in solution and rates remain largely unchanged. In contrast, at Mt:I < 0.1:1, solubility limits have not yet been reached and catalyst concentrations in solution, and therefore rates, increase with metal salt additions. Visual observations support the solubility theory. At Mt:I g 0.1:1, final polymerization solutions are close in color irrespective of the metal salt loaded. This is not the case at Mt:I < 0.1:1.
Figure 4. PMMA molecular weight and polydispersity as a function of conversion in the MMA polymerizations described in Figure 3 for varying metal-salt to initiator molar ratios: 1:1 ([, ]); 0.1:1 (b, O); 0.05:1 (2, 4) and 0.01:1 (9, 0). Continuous line represents theoretical molecular weight.
Reductions in catalyst concentrations below Mt:I ratios of 0.1:1 were also studied. A significant decrease in the polymerization rate (Table 1 entries 5 and 6) is observed below this concentration of metal salt. The average conversion drops from 68% at a Mt:I ratio of 0.1:1 to 54% and 39% at Mt:I ratios of 0.05:1 and 0.01: 1, respectively, for the same reaction time. The apparent rate constants for the runs in Table 1 are plotted in Figure 5 as a function of the metal salt concentration. Two rate laws (eqs 2 and 3) are derived from Figure 5 to model the rate behavior at Mt:I ratios above and below 0.1:1. The first rate law (eq 2) describes the plateau in kapp observed at Mt:I ratios above 0.1:1. Here the weak effect of metal salt concentration on the
RP ∝ [Mt]0.05, for Mt:I g 0.1
(2)
RP ∝ [Mt]0.37, for Mt:I e 0.1
(3)
While the limited solubility of the metal salt provides an explanation for the low order of the rate law at or above Mt:I ratios of 0.1:1, the reason for the fractional order (0.37) in the second rate law (eq 3) for Mt:I ratios below 0.1:1 remains unclear to us. Matyjaszewski et al. reported a similar order (RP ∝ [Mt]0.40) for the heterogeneous polymerization of styrene with CuCl/Bpy but at higher metal salt concentrations.15 In contrast, polymerization rates in homogeneous systems have been found to be almost directly proportional to the metal salt concentration used.16,19,26-30 We believe this difference and the fractional order may be related to another phenomenon we have observed at low metal concentrations (Mt:I ) 0.001). In these experiments, the reaction solution becomes opaque and white 30 min into the polymerization as a white polymer-like precipitate forms. Presumably this polymer precipitate is nucleated by metal salt particles. The metal salt is thereby
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izations were approximately 40% faster than their still counterparts. Apparent rate constants as a function of metal salt concentration for the stirred polymerizations are shown in Figure 5. Rates were found to increase slightly with metal salt concentration. At Mt:I ratios of 1:1 and 0.1:1, apparent rate constants were 8.9 × 10-5 s-1 and 6.7 × 10-5 s-1, respectively. The rate law derived from these data is shown as eq 6 and reflects the small increase in rate with metal salt concentration (order of 0.11). This is presumably explained by the limited solubility of the metal salt in solution, as is thought to be the case for the unstirred polymerizations. The large increase in rate with stirring (40%) illustrates that the system is diffusion controlled. This control and the increase in rate may be a result of the polymer film over the metal salt and its thinning with stirring.
RP ∝ [Mt]0.11, for Mt:I g 0.1 and stirred
Figure 6. ATRP kinetics for stirred (2, 4) and unstirred ([, ]) polymerizations of MMA, toluene/MMA ) 2 (w/w), [MMA]/[initiator]/[CuBr]/[HMTETA] ) 100:1:1:1 (molar), 90 °C.
entrapped by a polymer film. This entrapment may have a leveling effect on the polymerization kinetics making the rates less dependent on the metal salt concentration and explaining the fractional order. This theory is supported by a report from Matyjaszewski et al. of polymer buildup on a heterogeneous recyclable catalyst.31 Here the buildup was proposed to account in part for the loss in catalyst activity observed with recycling. Further investigations are under way within our group to clarify the effects of solubility and the polymer precipitate on the observed rate behavior. The polymer collected from the reaction solution was characterized by SEC. Polymerizations at lower metal salt concentrations (Table 1) generally yielded polymers with molecular weights that were closer to those theoretically calculated by eq 4. As seen in Figure 4, the measured molecular weights (Mn) versus conversion are high at a Mt:I ratio of 1:1 when compared to the theoretically calculated ones. In contrast, a 10-fold reduction in metal salt concentration results in an improved agreement between the theoretical and measured values. Initiator efficiencies, defined by eq 5, are summarized in Table 1 for the polymer collected at the end of each set of experiments. These values reflect the improvement in molecular weight control with reductions in metal salt concentration. Polydispersities were generally low for all experiments irrespective of the metal salt concentration. On average, polydispersities of 1.1 or less were achieved.
Mn,theoretical ) mwmonomer[M]0x/[I]0
(4)
Ieff ) [I]eff/[I]0 ) Mn,theoretical/Mn,SEC
(5)
Effect of Stirring. Given the heterogeneous nature of the catalyst, we chose to investigate the effect of stirring on polymerization kinetics. Polymerizations at Mt:I ratios ranging from 1:1 down to 0.1:1 were repeated in the presence of stirring and were found to have increased rates, Table 1 entries 7 through 9. Figure 6 compares the kinetic data for stirred and unstirred polymerizations at a Mt:I ratio of 1:1. Stirred polymer-
(6)
Molecular weight control and polydispersities in the stirred and unstirred polymerization runs were similar for low metal salt concentrations (Mt:I < 1:1). Initiator efficiencies were in the range 80-90%, while PDs were ∼1.1 (Table 1). At higher metal salt concentrations (Mt:I ) 1:1), molecular weight control and polydispersities deteriorated with stirring. Initiator efficiencies averaged 57% with stirring compared to 79% without. PDs averaged 1.2 for stirred polymerizations and 1.1 for their unstirred counterparts. Polymerization of MMA at Low Metal Salt Concentrations (Mt:I ) 0.01 to 0.001). In part, the focus of this work has been to find the lowest concentration of catalyst that leads to controlled polymerization while still maintaining acceptable rates. To this end we ran polymerization ranging from Mt:I ratios of 1:1 down to 0.001:1 as summarized in Table 1. As outlined in Table 1, Figures 3 and 4, polymerizations at a Mt:I ratio of 0.01:1 yield an average conversion of 39% over 375 min of polymerization time. The polymer molecular weights are well controlled (Ieff ≈ 88%), and polydispersities are low (PD ≈ 1.1). Furthermore, the first-order rate plot and molecular weight development plot are linear (Figures 3 and 4) indicating that the system is living in nature throughout the polymerization. To our knowledge this is the first report of the ATRP of MMA at such low metal salt concentrations (21 ppm) that have yielded controlled molecular weights and low polydispersities. The catalyst complex CuBr/HMTETA appears to be the most active catalyst system for the ATRP of MMA reported to date. Table 2 compares this catalyst system to others reported in the literature for the ATRP of MMA. Apparent rate constants and metal concentrations were calculated from the available data.14-25 In general, reported ATRPs have used an Mt:I ratio of 1:1 with metal concentrations typically over 1000 ppm. The lowest effective Mt:I ratios reported are ∼0.25:1.16,20 Haddleton et al. reported moderate polymerization rates (kapp ≈ 2.8 × 10-6 s-1) for the ATRP of MMA using CuBr complexed with a homogeneous Schiff based ligand at a metal concentration of 270 ppm.20 This rate, however, is 1 order of magnitude lower than that of CuBr/ HMTETA (kapp ) 2.2 × 10-5 s-1) at a metal concentration of 21 ppm as reported in this work. Zhu et al. reported faster rates (kapp ) 7.6 × 10-5 s-1) at a metal concentration of 220 ppm using microwave irradiation and the heterogeneous catalyst complex CuCl/PM-
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Table 2. Comparison of Catalyst Activity for the ATRP of MMAa catalyst ligand
entry
system
Mt
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
het. het. het. het. homo. homo. homo. homo. homo. het. homo. homo. het. het. homo. homo. het. het. het. homo.
CuBr CuBr CuBr CuBr CuCl +MI CuBr CuBr CuCl CuBr CuCl CuBr FeCl2 CuBr CuBr CuBr CuCl CuBr CuBr CuCl CuBr CuBr
temp (°C)
stirred
Ieff (%)
PD
Mt (ppm)
kapp (1/s)
[MMA]
90 90 90 90 69 100 90 69 90 90 90 90 90 90 90 60 75 90 130 90 90
yes yes no yes no no no no? no yes no yes no no no no? no yes no no yes
96 110 88 82 168 50 NA 92 108 75 NA 82 94 79 79 35 75 57 NA 80 NA
2.10 6.31 1.13 1.11 1.75 1.2 NA 1.2 1.1 1.3 NA 1.4 1.38 1.15 1.12 2.2 1.2 1.23 NA 1.2 NA
2 6 21 210 220 270 340 540 660 690 1120 1120 1510 1510 1510 1590 2120 2130 2580 3300 6830
1.1 × 10-5 4.4 × 10-5 2.3 × 10-5 6.7 × 10-5 7.6 × 10-5 2.8 × 10-6 5.0 × 10-5 9.3 × 10-6 8.3 × 10-5 1.2 × 10-4 4.5 × 10-5 6.3 × 10-5 1.4 × 10-4 5.6 × 10-5 6.9 × 10-5 3.8 × 10-5 1.1 × 10-4 8.9 × 10-5 5.6 × 10-4 1.7 × 10-4 1.3 × 10-4
100 100 100 100 3000 200 200 2400 222 800 100 500 200 200 200 200 300 100 246 100 30
HMTETA HMTETA HMTETA HMTETA PMDETA 3 dNBpy PMDETA dNBpy PMDETA nBu-1 I.Acid TMEDA PMDETA HMTETA Me6TREN HHTETA HMTETA Bpy Et-1 NHPMI
molar ratios [I] [Mt] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
0.001 0.001 0.010 0.100 1.000 0.250 0.230 2.000 0.500 1.000 0.360 2.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000
[L]
reference
1 1 1 1 2 0.75 0.46 2 1 2 1.07 4 2 1 1 1 1 1 3 2 3
this work this work this work this work 21 20 16 21 17 22 19 23 18 18 18 14 24 this work 15 19 25
a Abbreviations: PMDETA, N,N,N′,N′′,N′′-pentamethyldiethylenetriamine; 3, 2-pyridinecarbaldehyde n-propylimine; Et-1, N-ethyl-2pyridylmethanimine; n-Bu-1, N-(n-butyl)-2-pyridylmethanimine; HHTETA, hexahexyl triethylenetetramine; Me6TREN, tris[2-(dimethylamino)ethyl]amine; Bpy, 2,2′-bipyridine; dNBpy, 4,4′-di(5-nonyl)-2,2′-bipyridine; TMEDA, tetramethylethylenediamine; NHPMI, N-(nhexyl)pyridylmethanimine (NHPMI); I.Acid, isophthalic acid.
Table 3. Effect of L:I Ratio on the ATRP of MMA entry
stirred
MMA
1 2 3 4 5 6 7 8
no no yes yes yes yes yes yes
100 100 100 100 100 100 100 100
molar ratios MBP CuBr 1 1 1 1 1 1 1 1
0.1 0.1 1 1 1 1 1 1
HMTETA
conversion (%) 135 min 375 min
1 0.1 10 5 1 0.5 0.1 0
DETA.21 This rate, however, is comparable to that found here for CuBr/HMTETA at similar metal concentrations (210 ppm, kapp ) 6.7 × 10-5 s-1). Furthermore, unlike the PMMA produced in this work with CuBr/HMTETA, the PMMA produced with microwave irradiation was poorly controlled (Ieff ≈ 170% and PD ) 1.75). While low metal salt concentrations ( 90%) and polydispersities were low (∼1.1) for both polymerizations. To develop a rate law based on ligand concentration, a second set of batch polymerizations were run (Table 3 entries 3 through 8). Here metal salt concentrations were fixed at a Mt:I ratio of 1:1, while L:I ratios were varied from 10:1 to 0:1. Two samples were taken and later assayed for each polymerization, the first sample was withdrawn 135 min into the polymerization, and the second, at its end (375 min). An increase in polymerization rate with ligand concentration was observed in the first sample set. Apparent rate constants were calculated for each polymerization based on these data and are shown in Table 3. It was found that a 10-fold increase in the ligand concentration from an L:I ratio of 1:1 to 10:1 results in a 3-fold increase in the apparent rate constant from 5.0 × 10-5 s-1 to 1.6 × 10-4 s-1. Apparent rate constants are plotted against ligand concentration in Figure 11 and are found to fit the rate law shown in eq 8 well (R2 ) 0.979).
RP ∝ [L]0.52, for Mt:I ) 1 and stirred
(8)
Molecular weights are generally well controlled in the slower polymerizations, at low ligand concentrations, but this control deteriorates as reaction rates increase. For example, at an L:I ratio of 1:1 (kapp ) 5 × 10-5 s-1), initiator efficiencies are 78%, while, at an L:I ratio of 10:1 (kapp ) 1.6 × 10-4 s-1), efficiencies drop to 46%. This deteriorated control presumably results from higher active radical concentrations and therefore higher termination rates. Polydispersities also increase with reaction rate as seen in Table 3. While Figure 11 shows no apparent limit to the improvement in rate with ligand concentration, it can be expected that complete solubilization of the salt would provide such a limit. Conclusions The effects of metal salt concentration, stirring, and ligand concentration on the solution ATRP of MMA
using the heterogeneous catalyst complex CuBr/HMTETA were studied. A 10-fold reduction in metal salt concentration from Mt:I ratios of 1:1 down to 0.1:1 did not significantly affect the polymerization kinetics. The rate law Rp ∝ [Mt]0.05 was developed from the data to describe this behavior. At Mt:I ratios below 0.1:1, rates decreased with reductions in metal salt concentration. The rate law Rp ∝ [Mt]0.37 describes this decrease in rate. Mt:I ratios down to 0.01:1 were found to yield controlled polymerizations with high initiator efficiencies (88%) and low polydispersities (1.1). This is the first report of well controlled polymerizations of MMA at such low metal concentrations in solution (21 ppm). Further reductions in metal salt concentrations to 2 ppm in solution (Mt:I ) 0.001:1) yielded polymers with controlled molecular weights but high polydispersities (2.1). Stirring was applied to polymerizations from Mt:I ratios ranging from 1:1 down to 0.1:1 and increased polymerization rates by approximately 40%. A slightly higher order was observed in the rate law for these polymerizations, Rp ∝ [Mt]0.11 than for the unstirred runs in the same Mt:I range. It is believed that this increase in rate results from a reduction in the thickness of polymer-like films that entrap the undissolved metal salt. The effect of ligand concentration was also studied through a series of ATRPs where all other parameters were fixed. Increasing ligand concentration resulted in an increase in the polymerization rate. This is thought to result from an increased concentration of catalyst in solution via solubilization and complexation of the undissolved metal salt by the ligand. The rate law Rp ∝ [L]0.52 was developed to describe the observed rate behavior. Acknowledgment We thank the Natural Science and Engineering Research Council of Canada (NSERC) for supporting this research and the Canada Foundation of Innovation who support our research facilities. Literature Cited (1) Matyjaszewski, K. Controlled Radical Polymerization; Matyjaszewski, K., Ed.; ACS Symposium Series 685; American Chemical Society: Washington, DC, 1998; preface. (2) Wang, J.-S.; Matyjaszewski, K. Controlled living radical polymerization atom-transfer radical polymerization in the presence of transition-metal complexes. J. Am. Chem. Soc. 1995, 117 (20), 5614. (3) Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Polymeriztion of Methyl Methacrylate with the Carbon-Tetrachloride dichlorotris(triphenylphosphine)ruthenium(II) methylaluminum bis(2,6-di-tert-butylphenoxide) Initiating System - Possibility of Living Radical Polymerization. Macromolecules 1995, 28 (5), 1721. (4) Matyjaszewski, K.; Xia J. Atom transfer radical polymerization. Chem. Rev. 2001, 101, 2921. (5) Kamigaito, M.; Ando, T.; Sawamoto, M. Metal-catalyzed living radical polymerization. Chem. Rev. 2001, 101, 3689. (6) Shen, Y.; Zhu, S.; Zeng, F.; Pelton, R. Atom Transfer Radical Polymerization of Methyl Methacrylate by Silica Gel Supported Copper Bromide/Multidentate Amine. Macromolecules 2000, 33, 5427. (7) Shen Y.; Zhu S.; Pelton R. Packed column reactor for continuous atom transfer radical polymerization: Methyl methacrylate polymerization using silica gel supported catalyst. Macromol. Rapid Commun. 2000, 21, 956. (8) Shen, Y.; Zhu, S.; Zeng, F.; Pelton, R. Synthesis of methacrylate macromonomers using silica gel supported atom transfer radical polymerization. Macromol. Chem. Phys. 2000, 201, 1387.
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Received for review June 29, 2004 Revised manuscript received November 3, 2004 Accepted November 23, 2004 IE0494317
