Improving the “Livingness” of ATRP by Reducing Cu Catalyst

Article pubs.acs.org/Macromolecules

Improving the “Livingness” of ATRP by Reducing Cu Catalyst Concentration Yu Wang,† Nicolai Soerensen,‡ Mingjiang Zhong,† Hendrik Schroeder,‡ Michael Buback,‡ and Krzysztof Matyjaszewski*,† †

Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States ‡ Institute of Physical Chemistry, University of Goettingen, Tammannstraße 6, D-37077 Goettingen, Germany S Supporting Information *

ABSTRACT: The mole percentage of dead chains (Tmol %) was quantified in a normal Cu-mediated atom transfer radical polymerization (ATRP) of methyl acrylate with tris(2-pyridylmethyl)amine and tris[2-(dimethylamino)ethyl]amine as the ligands in acetonitrile. The value for Tmol % significantly exceeded the values predicted for conventional termination between two radicals. The main reason for the additional loss of chain-end functionality was identified as a CuIinduced catalytic radical termination (CRT). The mechanism proposed for this CRT involves the formation of highly active R−CuII/L and/or H−CuII/L intermediates which react with radicals to form dead chains and regenerate the CuI activator. The contribution of CRT to the loss of terminal functionality can be significantly decreased by reducing [CuI] to 80%),17 the ESI-MS experiments revealed the terminated chains contained predominantly unsaturated and saturated end groups, as in disproportionation or chain transfer. The additional loss of CEF was also reported by Percec and co-workers, who analyzed the termination in SARA ATRP of MA in DMSO with DPT = 60 via 1H NMR and matrix-assisted laser desorption/ionization (MALDI).18 The polymerization was conducted at a slower rate due to small surface area of Cu wire used, conversion of

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RESULTS AND DISCUSSION Quantification of Tmol % in Normal ATRP. Three normal ATRP experiments of MA were performed in MA/MeCN = 1/ 1 (v/v) at 40 °C (Figure 1). Since the initial [CuI]0:[CuII]0 = 684

dx.doi.org/10.1021/ma3024393 | Macromolecules 2013, 46, 683−691

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Table 1. Rate Coefficients of CuI-Induced CRTa entry 1 2 3 4 5 6 7 8 9 10

M MA MA MA MA MA MA MA BA MA MA

ligand TPMA TPMA TPMA TPMA TPMA TPMA TPMA TPMA Me6TREN Me6TREN

M/S (v/v) 1/1 1/1 1/1 1/1 1/1 2/1 1/2 1/1 2/1 2/1

[EBrP]0:[CuIBr]0:[CuIIBr2]0

ktCu(I) (M−1 s−1) (25 °C)

1:0.05:0.05 1:0.1:0.1 1:0.2:0.2 1:0.2:0.1 1:0.4:0.2 1:0.08:0.08 1:0.1:0.1 1:0.1:0.1 1:0.1:0.1 1:0.67:0.67

× × × × × × ×

6.6 6.8 5.2 7.2 9.2 6.4 7.4

3

10 103 103 103 103 103 103

ktCu(I) (M−1 s−1) (40 °C) 6.6 4.6 4.8 6.8 6.0 6.2 7.8 1.1

× × × × × × × ×

103 103 103 103 103 103 103 104

4.4 × 104 4.4 × 104

a ATRP of MA or BA in MeCN with TPMA or Me6TREN as the ligand with the ratio [EBrP]0:[CuIBr]0:[CuIIBr2]0 as listed in the table, [EBrP]0 = 27.8 mM, M = monomer, and S = solvent. The ligand ratio to [CuI] + [CuII] is always 1.2.

coefficient ktCu(I). The rate law of CuI-induced CRT is expressed in eq 2.

1:1 was the same in all three experiments, the initial rates of polymerization were essentially identical. All of the experiments showed a linear increase of molecular weights with conversion, and the Mw/Mn values were 20%. The evolution of CuII species, measured by UV−vis, is shown in Figure 1c. The Tmol % values were calculated according to the principle of halogen conservation15 (Figure 1d). In all cases, Tmol % was 5, while CuI-induced CRT would dominate when [RX]/[CuII] < 5. Since the measurements of KATRP via modified PRE were typically performed at [RX]0:[CuI]0 = 10:1 and limited to relatively low conversion of CuI, the determined values should be accurate because the fraction of CRT was low. However, values measured at [RX]0: [CuI]0 = 1:1 could be less precise. Two experiments were performed at [RX]0:[CuIBr]0: [TPMA]0 = 10:1:1.2 and 1:1:1.2, [CuIBr]0 = 5 mM, in MeCN at 25 °C. Plots of F(Y)29 (eq 8) vs time are shown in Figure 2. F (Y ) =

∫0

Y

Y2 dY = 2k tKATRP 2t (I0 − Y ) (C0 − Y )2 2

Y

Y dY = k tCu(I)KATRPt (I0 − Y )(C0 − Y )2

(9)

The plot was slightly negatively curved during the first 2 × 104 s because termination between two radicals still contributed substantially to overall termination: Rt/RtCu(I) > 0.39 (Figure 2c). However, after 2 × 104 s an essentially linear plot was observed with slope = 2.15 × 10−3 s−1, giving ktCu(I) = 5.8 × 103 M−1 s−1. This value agreed with the measurement from polymerization (ktCu(I) = 7.0 × 103 M−1 s−1) which confirmed that solvent had no significant effect on the rate of CRT. The G(Y) function was also applied to the model study with Me6TREN as the ligand, and ktCu(I) = 2.2 × 104 M−1 s−1 was calculated (see Figure S7). This value can be compared with ktCu(I) = 4.4 × 104 M−1 s−1 determined with polymerization (Table 1, entries 9 and 10). The G(Y) function could be also applied to a polymerization system: in an ATRP of BA with [BA]0:[EBrP]0:[CuIBr]0:[TPMA]0 = 1000:1:1:1.2, in BA/ MeCN = 1/1 (v/v), at 25 °C, the ktCu(I) = 1.1 × 104 M−1 s−1 value was calculated (see Figure S8). It may be compared to ktCu(I) = 1.1 × 104 M−1 s−1 at 40 °C (Table 1, entry 8). The good agreement of ktCu(I) values deduced from two distinctly different procedures, from polymerization systems as shown in Table 1 and via the G(Y) function in Figure 2, confirms the validity of the kinetic model in eq 2. Mechanism of CuI-Induced CRT. CuI can be oxidized by electrophilic acrylate radicals via two pathways: CuI can either

I

Rt [RX] = 0.22 R tCu(I) [Cu II]

∫0

(8)

I

At a ratio [RX]0:[Cu BrL]0 = 10, a linear plot was obtained with slope = 6.87 × 10−4 s−1, giving a value for KATRP = 3.7 × 686

dx.doi.org/10.1021/ma3024393 | Macromolecules 2013, 46, 683−691

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Scheme 1. (a) Possible Mechanism of CuI-Induced CRT; (b) ATRP Equilibrium

Figure 3. (a) EPR spectra at −40 °C of R−CuII/L and/or H−CuII/L complex with Br− counterions after different irradiation times and after heating up to 20 °C. (b) EPR spectra of R−CuII/L and/or H−CuII/L complexes with either Br− or PF6− counterions and of the Br−CuII/L ATRP complex at −40 °C. (c) NIR measurement after reaction of 3.4 mM CuIBr/TPMA with radicals generated by pulsed laser application on a solution of BA/ MeCN with MMMP as the initiator. The absorption is attributed to R−CuII/TPMA and/or H−CuII/TPMA complex which is stable at −40 °C but readily decomposes upon heating to ambient temperature.

and/or H−CuII/L species should be very reactive and may quickly terminate with another radical to regenerate CuI/L and form dead chains. Direct reaction between R−CuII/L and polymeric radical could be sterically hindered. Thus, it is plausible that R−CuII/L undergoes a β-H elimination to form an unsaturated polymer chain and H−CuII/L. Alternatively, the polymeric radical may react with CuI/L to form an unsaturated dead chain (R=) and H−CuII/L directly without R−CuII/L intermediates, as in Co-mediated catalytic chain transfer (CCT). However, the generated paramagnetic H−CuII/L species does not react with monomer (as in CCT), but rather reacts rapidly with another radical to reform CuI/L and a saturated dead chain (H−R). The low proportion of termination products formed by coupling (ref 16 and Figure S5), which are favored in conventional termination between two radicals for polyacrylates,17 supports the formation of H−CuII/L. It should be also noted that CRT contribution is much larger in polymerization of acrylates than in polymer-

associate with a radical via inner-sphere electron transfer (ISET) and form organometallic species or may undergo an outer-sphere electron transfer (OSET) process to generate R− and a CuII cation (eq 10). R− may then react with adventitious moisture or other protic impurities in the reaction system to generate a saturated polymer dead chain (eq 11).3,21,31 However, in this case, each loss of CEF would result in oxidization of two CuI atoms to CuII (one CuI is consumed to reduce RX to R• and the second one to reduce R• to R−), which does not agree with the experimental observations (see Figure S1). R• + Cu IX/L → R− + [Cu IIX/L]+

(10)

R− + H 2O → RH + OH−

(11) I

Thus, the most probable reaction between Cu /L and a radical should be the formation of either R−CuII/L and/or H− CuII/L species (Scheme 1a). The paramagnetic (d9) R−CuII/L 687

dx.doi.org/10.1021/ma3024393 | Macromolecules 2013, 46, 683−691

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Figure 4. Predicted values of Tmol % vs time needed to reach 60% conversion for the ATRP of MA in MA/MeCN = 1/1 (v/v) at 40 °C with various adopted values of DPT and with CuIBr/TPMA as the catalyst, considering both termination between two radicals and CuI-induced CRT, while [CuI] is constant. (a) Ideal RDRP without CuI; (b) [CuI] = 0.1 mM; (c) [CuI] = 1 mM; (d) [CuI] = 10 mM.

also used to avoid the presence of halogen and thus formation of any Br−CuII/L ATRP-type complexes. Figure 3a illustrates EPR signals at −40 °C measured in the presence of CuIBr/TPMA prior to irradiation (blue) and after laser pulsing at 0.2 Hz for 1000 s (purple) and 2100 s (black). During reaction, a broad signal between 2900 and 3500 G emerges, which is assigned to a CuII complex because of the similarity with the separately measured spectrum of the Br−CuII/TPMA complex (Figure 3b, blue). The other two spectra in Figure 3b have been measured for the butyl acrylate system in the presence of MMMP and two CuI salts. The black curve in Figure 3b is identical to the black one in Figure 3a. The red spectrum was measured when CuIPF6/L was used instead of CuIBr/L. The field positions in Figure 3b exhibit the typical signature of CuII complexes. The conversion of CuI to R−CuII/L and/or H−CuII/L complexes at −40 °C has an efficiency ca. 30% with CuIBr and ca. 45% with CuIPF6, according to double integration. After laser pulsing, EPR signal intensity remains constant at −40 °C. Upon heating to 20 °C, the signals disappear (see Figure 3a), which demonstrates the poor stability of the intermediate CuII species. The complex may dissociate to CuI and a radical or react with a radical by CuI-induced CRT via an H−CuII/L intermediate. It should however be noted that no more radicals are photochemically produced during heating to 20 °C. CuIBr/TPMA was also reacted with primary radicals from photodissociation of MMMP at −40 °C without BA being present. Monomer was replaced by butyl propionate, the saturated analogue of BA, acting as the solvent. In the absence

ization of methacrylates and styrene (St) (cf. Table S3). This observation, contrasting classical CCT behavior, also supports organometallic intermediates rather than a pathway via direct formation of unsaturated chains and H−CuII/L species. The rate-limiting step should be the formation of CuII intermediate species via the reaction between CuI and a radical, which explains first-order kinetics with respect to both [CuI] and [R•]. The net effect of CuI-induced CRT consists of formation of dead chains from two radicals, whereas the [CuI] remains unchanged, as it is a catalytic process. However, because concentration of paramagnetic H−CuII or R−CuII is much higher than concentration of propagating radicals, the termination process is accelerated. Nevertheless, loss of Br from each dormant species corresponds to irreversible oxidation of one CuIBr/L to one CuIIBr2/L. EPR/NIR Study of a CuI-Induced CRT Process. The EPR/NIR investigations illustrated in Figure 3 were carried out in order to directly observe R−CuII/L and/or H−CuII/L species under ATRP-relevant conditions. A solution of 3.4 mM CuIBr/TPMA and 40 mM of the photoinitiator, α-methyl-4(methylmercapto)-α-morpholinopropiophenone (MMMP), in MeCN/BA was prepared. Thus, the mixture contained the CuIactive ATRP catalyst; however, due to the absence of ATRP initiator, RBr, formation of the Br−CuII/L ATRP species via halogen transfer did not occur. Therefore, radicals were generated not by ATRP activation, but by laser-induced decomposition of the photoinitiator. Such conditions could enable the reaction of CuIBr/TPMA with radicals to form R−CuII and/or H−CuII species. Instead of CuIBr, CuIPF6 was 688

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of monomer, formation of H−CuII/L should not occur. Only a very weak signal was observed in the EPR range which is characteristic of CuII complexes. Under otherwise identical photoinitiation and thus radical-production conditions, this signal intensity is only ca. 5% of the one seen in the presence of BA. The weak signal suggests that at −40 °C reaction of MMMP-derived carbon-centered radicals with the CuI complex occurs only to a minor extent. Moreover, the observed weak component is not stable at −40 °C. In addition to EPR analysis, the same reaction was carried out in conjunction with subsequent NIR measurement (see the absorption spectra in Figure 3c). The absorption around 885 nm exhibits the typical signature of d−d transition of a CuII complex. A similar band shape, with however distinctly different peak absorption, has been measured for the Br−CuII/TPMA ATRP complex (cf. Figure 1c). As observed during EPR measurement, the intermediate was stable at −40 °C but decomposed upon heating to ambient temperature. Improving the Livingness of ATRP. The above analysis of termination indicates that Cu catalyst should be used at lower concentrations to improve the livingness of acrylate ATRP. However, the amount of Cu catalyst cannot be significantly reduced in a normal ATRP; otherwise, polymerization would stop at relatively low conversion as a consequence of increased levels of CuII complex and loss of CuI activator. Thus, new ATRP methods, namely activators regeneration by electron transfer (ARGET),32,33 initiators for continuous activator regeneration (ICAR), 34 SARA ATRP,31,35−39 eATRP,40 and photochemically mediated ATRP,41−44 all of which require only ppm level of Cu catalyst, are recommended. The effect of reaction time (rate) on Tmol % for various DPT and different ratios of [CuIBr]/[TPMA] for ATRP of MA in MA/MeCN = 1/1 (v/v) to reach 60% conversion at 40 °C is shown in Figure 4. The systems with CuIBr/TPMA show the combined effect of termination between two radicals and of CuI-induced CRT for the simplified case, with assumed constant CuI concentration (see eq 12). Tmol% =

2k t ln 2

The reaction between CuI/L and a radical should be accompanied by larger steric hindrances than the activation of RX (Scheme 1b). Therefore, a CuI complex with a more bulky or perhaps stronger bonded ligand could enhance the selectivity of ATRP activation over CRT process. A similar process, involving organometallic R−Cu II species, was recently proposed in polymerization of acrylates in the presence of the highly active CuI/L complex,45 with tris([(4-methoxy-2,5dimethyl)-2-pyridyl] methyl)amine (TPMA*) as the ligand.22

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CONCLUSIONS In order to improve the livingness of ATRP, one needs to select reaction conditions that suppress/reduce the loss of CEF. Recently developed procedures focused on diminishing termination between two radicals which was identified as the main cause for the loss of CEF. However, the quantification of termination in ATRP and the kinetic study of CuI-induced CRT process provide better understanding and facilitate selection of appropriate reaction conditions for ATRP. One can optimize reaction conditions to limit Tmol % to 10 mM, significant amounts of termination may occur. In order to suppress CuI-induced radical termination, Cu-mediated ATRP should be carried out at low catalyst concentrations.

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Materials. Methyl acrylate (MA, 99+%, Aldrich), butyl acrylate (BA, 99+%, Aldrich), methyl methacrylate (MMA, 99+%, Aldrich), and styrene (St, 99+%, Aldrich) were passed through a column filled with basic alumina prior to use. Methyl 2-bromopropionate (MBrP, 98%, Aldrich), ethyl 2-bromopropionate (EBrP, 99%, Aldrich), ethyl α-bromoisobutyrate (EBiB, 98%, Aldrich), CuIBr (99.999%, Aldrich), tetrakis(acetonitrile)copper(I) hexafluorophosphate (97%, Aldrich), CuIIBr2 (99.999%, Aldrich), copper(II) trifluoromethanesulfonate (98%, Aldrich), tris(2-pyridylmethyl)amine (TPMA, 99%, ATRP solutions), tris[2-(dimethylamino)ethyl]amine (Me6TREN 99%, ATRP solutions), α-methyl-4-(methylmercapto)-α-morpholinopropiophenone (MMMP, 98%, Aldrich), dimethyl sulfoxide (DMSO, 99+%, Aldrich), toluene (99.8%, Aldrich), acetonitrile (MeCN, 99.8%, Aldrich), and butyl propionate (99%, Aldrich) were used as received. 2,2′-Azobis(2-methylpropionitrile) (AIBN 98%, Aldrich) was recrystallized from ethanol before use. Poly(methyl acrylate) (PMA-Br, Mn = 4200, Mw/Mn = 1.20) and polystyrene (PSt-Br, Mn = 12 000, Mw/Mn = 1.10) macroinitiator with Br CEF were prepared via normal ATRP. Characterization. Spectroscopic measurements were performed on a Varian Cary 5000 UV/vis/NIR spectrometer. The concentration of CuII species was followed during ATRP via the absorbance at 960 nm. The conversion of MA was calculated via integration between 1610 and 1625 nm.15,25 Molecular weight and Mw/Mn values were determined by GPC using polystyrene (PSt) and poly(methyl methacrylate) (PMMA) standards for calibration, conducted with a Waters 515 pump and Waters 2414 differential refractometer using PSS columns (Styrogel 105, 103, and 102 Å) with THF as an eluent at 35 °C and at a flow rate of 1 mL/min. EPR spectra were recorded on a Bruker Elexsys E 500 series cw EPR spectrometer. NIR measurements were carried out on a FT-IR spectrometer (Bruker IFS 88). Samples were irradiated by a UV laser (LPX200i, Lambda Physik, and LPXPro 240, Coherent) operated at 351 nm with laser energies per pulse between 50 and 100 mJ. Temperature control was achieved either via an ER 4131VT unit

( 1 − 1conv ) + ktCu(I)t[CuI] ln( 1 − 1conv ) k p2t[RX]0

EXPERIMENTAL SECTION

(12)

Lines in Figure 4a were calculated based on eq 1 with exclusive termination between two radicals. Lines in Figures 4b−d were calculated based on eq 12 which also includes CRT. In ideal RDRP with exclusive termination between two radicals, Tmol % decreases significantly for slower reactions (lower radical concentrations), i.e., longer reaction times needed to reach the same conversion. Tmol % values are smaller for lower DPT (higher [RX]0), because the fraction of terminated chains with respect to the overall amount of dormant chains is smaller. Similar trends are observed for low [CuI] = 0.1 mM (see Figure 4b). Interestingly, with [CuI] = 1 mM, especially for large DPT of 500 and 1000, the extension of polymerization time to >10 000 s does not improve “livingness” because of the dominant contribution of CRT (see Figure 4c). Even more dramatic effects are observed with [CuI] = 10 mM shown in Figure 4d. Regardless of how slow the polymerization is, it is not possible to prepare polymers with Tmol % < 20% for DPT = 500 and