Catalytic amounts of sodium hydroxide as additives for iron-mediated AGET ATRP of MMA

Abstract

The atom transfer radical polymerization using activators generated by electron transfer (AGET ATRP) of methyl methacrylate (MMA) in the presence of catalytic amounts of sodium hydroxide (NaOH) was studied to examine the function of NaOH in this system, using ethyl 2-bromoisobutyrate (EBiB) as the initiator, oxidative FeCl₃·6H₂O as the catalyst, tris-(3,6-dioxa-heptyl)amine (TDA-1) as the ligand, glucose as the reducing agent, and tetrahydrofuran (THF) as the solvent at 90 °C. The polymerization rate can be increased significantly while keeping good controllability over molecular weights and molecular weight distributions. For example, the polymerization rate of solution AGET ATRP with a molar ratio of [MMA]₀/[EBiB]₀/[FeCl_3·H₂O]₀/[TDA-1]₀/[glucose]₀/[NaOH]₀ = 500/1/1/3/1/1.5 in the presence of a limited amount of air with NaOH was 1.6 times faster than that without NaOH and kept low molecular weight distribution (M_w/M_n < 1.35).

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Introduction

Nowadays, people always endeavor in synthesizing of well-defined, narrow polydispersive, valuable end-functional polymers. “Living”/controlled radical polymerization (LRP) processes, such as stable free-radical polymerization or nitroxide-mediated polymerization (SFRP or NMP),^1,2 metal-catalyzed atom transfer radical polymerization (ATRP)^3–5 and reversible addition-fragmentation chain transfer (RAFT)^6,7polymerization, are the current effective and versatile ways to produce the desired polymers. ATRP, in particular, is the most widely applied because of its tolerance to a variety of functional groups.^8–10 Based on this point, ATRP has been rapidly developed in recent decades. One breakthrough is the proposal of activators generated by electron transfer (AGET) ATRP^11–15 first reported by Matyjaszewski's group, where the lower oxidation state catalyst (i.e. Cu(I) complex) in normal ATRP generated in situ by the reaction between the reducing agent and the higher oxidation state catalyst (i.e., Cu(II) complex). Thus, the most serious problems of catalyst for preservation and excessive use have been solved effectively. The key idea of AGET ATRP is similar with ATRP system of styrene/(meth)acrylates with zerovalent metal^16,17 (i.e.Cu(0)) as additives reported previously. The appropriate amount of zerovalent metal reduced the oxidized metal to generate the catalyst which can increase the rate of polymerization and the controllability both.

From the statements above, we can see that additives are sometimes essential for a successful ATRP. Most of the additives were committed to improving the rate of polymerization or/and obtaining well-defined polymer by affecting the reversible atom transfer between the lower oxidation state catalytic complex and the higher oxidation state catalytic complex. Some excellent work about additives, either directly reducing catalytic metal in higher oxidation states or competing with ligands for the coordination sites, have been reported. For example, addition of appropriate amount of reducing compound Cu(0),^16,17tin(II) 2-ethylhexanoate [Sn(EH)₂]¹⁸ascorbic acid or vitamin C (VC),^19,20glucose,^21,22etc. can reduce the concentration of copper(II) and simultaneously increase the concentration of copper(I) which can increase the rate of polymerization or/and the controllability; addition of the other kind of additives like phenols,^23,24benzoic acid salts,²⁵ carboxylate salts,²⁶aluminum isopropoxide [Al(Oi-Pr)₃],^27,28amine compounds,^29,30etc. to the metal-catalyzed ATRP led to a significant increase in the polymerization rate, presumably due to competition with the ligand for coordination sites and form more efficient catalysts. Another kind of additives for better controllable ATRP by external halogen delivery like molecular iodine (I₂),³¹triphenylmethyl chloride (Ph₃CCl)³² has also been widely used recently. Very recently, we reported a novel strategy with catalytic amount of inorganic base (NaOH or Fe(OH)₃) as the additives to enhance the polymerizaiton rate of the iron-mediated AGET ATRP of styrene using commercially available onium salt, tetrabutylammonium bromide (TBABr), as the ligand.³³

According to the information listed above, in this work, a novel catalyst system for the iron-mediated AGET ATRP of polar monomer methyl methacrylate was first developed in the presence of catalytic amounts of NaOH, using a simple, cheap and commercial chemicals tris-(3,6-dioxa-heptyl)amine (TDA-1) as the ligand and glucose as the reducing agent. The effect of the amount of base on the AGET ATRP and the corresponding polymerization kinetics were investigated in detail.

Experimental part

Materials

The monomer, MMA (>99%), was purchased from Shanghai Chemical Reagents Co. (Shanghai, China). It was passed through a column filled with basic aluminum oxide before used then stored at −18 °C. Sodium hydroxide (NaOH) (>96%), iron(III) chloride hexahydrate (FeCl₃·6H₂O) (>99%), tris-(3,6-dioxa-heptyl)amine (TDA-1) (>99%), and glucose (>99%) were purchased from Shanghai Chemical Reagents Co. and used without treatment. Ethyl 2-bromoisobutyrate (EBiB) (98%) was purchased from Acros and used without treatment. Tetrahydrofuran (THF) (analytical reagent), and all other chemicals were obtained from Shanghai Chemical Reagents Co. and used as received unless mentioned.

General procedure for AGET ATRP of MMA

A typical polymerization procedure in the absence of NaOH with THF as the solvent for the ratio of [MMA]₀/[EBiB]₀/[FeCl₃·6H₂O]₀/[TDA-1]₀/[glucose]₀ = 500/1/1/3/1 was as follows: FeCl₃·6H₂O (15.4 mg, 0.057 mmol), TDA-1 (54.2 μL, 0.17 mmol), THF (1.0 mL), MMA (3.0 mL, 28.4 mmol), and EBiB initiator (8.3 μL, 0.057 mmol), glucose (11.4 mg, 0.057mmol) and THF (1.0 mL) were added to a dried ampoule to try to get a homogeneous mixture after ultrasonic waving for several seconds. The procedure in the presence of NaOH for the ratio of [MMA]₀/[EBiB]₀/[FeCl₃·6H₂O]₀/[TDA-1]₀/[glucose]₀/[NaOH]₀ = 500/1/1/3/1/1.5 was the same as mentioned above besides adding the predetermined quantity of NaOH (3.4 mg, 0.082 mmol). For the deoxygenated system (in the absence of air), the mixture thoroughly bubbled with argon for 20 min to eliminate the dissolved oxygen, and then flame-sealed. For the oxygenated system (in the presence of air), the ampoule was flame-sealed directly (no bubbling with argon) and then transferred into an oil bath held by a thermostat at the desired temperature (90 °C) to polymerize under stirring. The oxygen concentration from air ([O₂]₀ = 1.0 × 10⁻² mol L⁻¹, based on the reaction solution (4 mL)) was calculated from the residual volume (air volume, 4.3 mL) of ampoule after adding the reaction mixture.^34–37 After the desired polymerization time, the ampoule was cooled by immersing it into iced water. Afterwards, it was opened and the contents were dissolved in THF (∼2 mL), and precipitated into a large amount of methanol (∼200 mL). The polymer obtained by filtration was dried under vacuum until constant weight at 50 °C. The monomer conversion was determined gravimetrically.

Chain extension of PMMA

The predetermined quantity of PMMA sample (88.0 mg, M_n,GPC = 20200 g mol⁻¹, M_w/M_n = 1.18) obtained by polymerization of MMA in the presence of air and NaOH was dissolved in 3 mL of fresh MMA in a dried ampoule. Then the predetermined quantity of FeCl₃·6H₂O (15.4 mg, 0.057 mmol), TDA-1 (54.2 μL, 0.17 mmol), THF (1.0 mL) and glucose (11.4 mg, 0.057 mmol) was added in the presence of air. The rest of the procedure was the same as the oxygenated system described above. The chain extension experiment was carried out under stirring at 90 °C.

Characterization

The number-average molecular weight (M_n,GPC) values and molecular weight distribution (M_w/M_n) values of the polymers were determined using Waters 1515 gel permeation chromatograph (GPC) equipped with a refractive index detector (Waters 2414), using HR 1, HR 2 and HR 4 (7.8 × 300 mm², 5 μm beads' size) columns with measurable molecular weights ranged 10²–5 × 10⁵ g/mol. THF was used as an eluent at a flow rate of 1.0 mL min⁻¹ and 30 °C. The GPC samples were injected using a Waters 717 plus autosampler and calibrated with poly(methyl methacrylate) standards from Waters. ¹H NMR spectrum was recorded on an Inova 400 MHz nuclear magnetic resonance (NMR) instrument using CDCl₃ as the solvent and tetramethylsilane (TMS) as the internal standard at ambient temperature. Cyclic voltammetry (CV) experiments were performed at a scanning rate of 50 mV s⁻¹ with a RST5200 electrochemical system (Zhengzhou Shirui Si Technology Co. Ltd, China) at room temperature, using a Ag counter electrode, graphite working electrode, and a Ag/AgCl reference electrode with Bu₄NClO₄ (0.1 M) as an electrolyte (sweep rate: 50 mV s⁻¹). The ferrocene(II)/(III) couple (E_1/2 = 450 mV and ΔEp = 280 mV) was utilized as a benchmarked redox couple.

Results and discussion

Effect of concentration of NaOH on the AGET ATRP of MMA

AGET ATRPs of MMA catalyzed by Fe(III)/TDA-1 complex in different amounts of NaOH were carried out using glucose as the reducing agent. The results are listed in Table 1. From Table 1, the MMA polymerization rates increased with the concentration of NaOH (i.e., the polymerization time decreased from 694 min to 370 min in the case of [glucose]₀/[NaOH]₀ increasing from 1/0 to 1/6 meanwhile keeping the conversion between 50% and 60%). At the same time, it can also be seen that the molecular weight and molecular weight distribution became less controllable with increasing polymerization rates or concentration of NaOH (entries 1∼4: M_w/M_n values were 1.24, 1.35, 1.31 and 1.37 in the case of [glucose]₀/[NaOH]₀ being 1/0, 1/1, 1/1.5 and 1/3, respectively), and when [glucose]₀/[NaOH]₀ added to 1/6, the polymerization of MMA was out of control. (entry 5: M_w/M_n > 1.5), which indicated that a suitable catalytic amount of NaOH (i.e., [glucose]₀/[NaOH]₀ < 1/6 in this case) as additives was necessary to produce well-controlled polymers.

Table 1 Effect of the amount of NaOH on the solution AGET ATRP of MMA^a

Entry	[Glucose]0/[NaOH]0	Time (min)	Conv. (%)	M n,th (g mol^−1)	M n,GPC (g mol^−1)	M w/Mn
a Polymerization conditions: [MMA]0/[EBiB]0/[FeCl3·H2O]0/[TDA-1]0/[NaOH]0 = 500/1/1/3/x (x = 0.0, 1.0, 1.5, 3.0, 6.0), MMA = 3 mL, THF = 1 mL, T = 90 °C.
1	1/0	694	56.3	28160	39370	1.24
2	1/1	675	53.8	26900	43630	1.35
3	1/1.5	628	59.0	29500	43590	1.31
4	1/3	523	60.0	30000	50820	1.37
5	1/6	370	54.9	27500	62180	1.55

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Polymerization of MMA in the absence of oxygen

In order to investigate the NaOH as additives further, the kinetics of AGET ATRP of MMA in the presence/absence of NaOH were carried out in THF solution, using EBiB as the initiator, FeCl₃·6H₂O as the catalyst, TDA-1 as the ligand and glucose as the reducing agent. Fig. 1(a) shows the kinetics of solution AGET ATRP of MMA with a molar ratio of [MMA]₀/[EBiB]₀/[FeCl₃·6H₂O]₀/[TDA-1]₀/[glucose]₀/[NaOH]₀ = 500/1/1/3/1/x (x = 0, 1.5) in the absence of oxygen at 90 °C. It can be seen that both the kinetics showed linear plots, which indicate that the polymerizations were approximately first order with respect to the monomer concentration and the number of active species remained constant during the polymerization process. It is obviously observed that the polymerization rate for the solution AGET ATRP using catalytic amount of NaOH as the additives was much faster than that without NaOH and the induction period of the polymerization in the presence of NaOH was significantly shorter. By calculating the apparent rate constant of polymerization, k_p^app (R_p = −d[M]/dt = k_p[P_n·][M] = k_p^app [M]), as determined from the kinetic slopes, a k_p^app of 1.11 × 10⁻⁴s⁻¹for the solution polymerization in the presence of catalytic amount of NaOH and k_p^app of 8.19 × 10⁻⁵s⁻¹ in the absence of NaOH were obtained. The k_p^app of polymerization with NaOH was 1.35 times than that without NaOH. Fig. 1(b) shows that number-average molecular weight (M_n,GPC) and molecular weight distribution (M_w/M_n) versus conversion for solution AGET ATRP of MMA in the absence of oxygen with/without NaOH at 90 °C. As shown in Fig. 1(b), the M_n;GPC values of the polymers with/without NaOH increased linearly with monomer conversion while keeping polymerization controllable (M_w/M_n < 1.5) and the M_n;GPC values were close to the corresponding theoretical molecular weights. The results in Fig. 1 indicate that the solution AGET ATRP of MMA in the absence of oxygen with the catalytic amount of NaOH ([FeCl₃·6H₂O]₀/[NaOH]₀ = 1/1.5) was a faster, shorter induction and well-controlled radical polymerization process than that without NaOH.

Fig. 1 ln([M]₀/[M]) as a function of time (a) and number-average molecular weight (M_n,GPC) and molecular weight distribution (M_w/M_n) versus conversion (b) for solution AGET ATRP of MMA in the absence of oxygen with/without NaOH. Polymerization conditions: [MMA]₀/[EBiB]₀/[FeCl₃·6H₂O]₀/[TDA-1]₀/[glucose]₀/[NaOH]₀ = 500/1/1/3/1/1.5, MMA = 3 mL, THF = 1 mL, T = 90 °C.

Polymerization of MMA in the presence of a limited amount of air

To further demonstrate the polymerization of MMA catalyzed by Fe(III)/TDA-1 in the presence of NaOH can accelerate the polymerization rate, the kinetics of AGET ATRP of MMA in the presence of a limited amount of air was carried out in THF solution. Fig. 2(a) shows the kinetic plots of AGET ATRP of MMA in THF solution with the molar ratios of [MMA]₀/[EBiB]₀/[FeCl₃·6H₂O]₀/[TDA-1]₀/[glucose]₀/[NaOH]₀ = 500/1/1/3/1/x (x = 0, 1.5) in the presence of oxygen at 90 °C. The first-order kinetics was also observed, indicating that the propagating radical concentration still kept constant in the presence of a limited amount of oxygen. Comparing the data between Fig. 1(a) and Fig. 2(a), it can be seen, as expected, the polymerization rate in the presence of oxygen is slower than that in the absence of oxygen and the induction period is longer. It is contributed to that much more Fe(III) complexes formed from the oxidation reaction between Fe(II) complexes with oxygen, which decreased the polymerization rate and delayed the dynamic equilibrium between the active Fe(II) complexes and Fe(III) species. It can also be seen that the slope of the kinetic plot with NaOH is relatively larger than that without NaOH. The corresponding k_p^app was 2.54 × 10⁻⁵s⁻¹ in the case of without NaOH, while 4.14 × 10⁻⁵s⁻¹ in the case of with NaOH. The latter was 1.6 times of the former. Fig. 2(b) shows that M_n,GPC increased linearly with the monomer conversion and the M_w/M_n of the obtained PMMA remained low (M_w/M_n < 1.35) during the polymerization process and the M_n,GPC values were close to the corresponding theoretical ones under both polymerization conditions, These results about AGET ATRP of MMA in the presence of oxygen with catalytic amount of NaOH in Fig. 2 demonstrated that the polymerization can be conducted successfully while keeping the features of the controlled/living radical polymerization even if in the presence of a limited amount of oxygen (air).

Fig. 2 ln([M]₀/[M]) as a function of time (a) and number-average molecular weight (M_n,GPC) and molecular weight distribution (M_w/M_n) versus conversion (b) for solution AGET ATRP of MMA in the presence of oxygen with/without NaOH. Polymerization conditions: [MMA]₀/[EBiB]₀/[FeCl₃·6H₂O]₀/[TDA-1]₀/[glucose]₀/[NaOH]₀ = 500/1/1/3/1/1.5, MMA = 3 mL, THF = 1 mL, [O₂]₀ = 1.0 × 10⁻² mol L⁻¹, T = 90 °C.

Effect of concentration of catalyst on polymerization of MMA in the presence of NaOH

To investigate the effect of Fe(III) concentration on the AGET ATRP of MMA, the solution polymerizations of MMA in the presence of catalytic amount of NaOH were investigated using 50–200 mol % catalyst relative to the initiator with a molar ratio of [MMA]₀/[EBiB]₀/[NaOH]₀ = 500/1/1. The results are listed in Table 2. From Table 2, it can be seen that molecular weight and molecular weight distribution became less controllable with decreasing amount of FeCl₃·6H₂O (entries 1–3: M_w/M_n values were 1.23, 1.25 and 1.30 in the case of [FeCl₃·6H₂O]₀/[EBiB]₀ being 2/1, 1.5/1, and 1/1, respectively), and when [FeCl₃·6H₂O]₀/[EBiB]₀ decreased to 0.5/1, the polymerization of MMA was out of control. (entry 4: M_w/M_n > 1.5), which indicated that the polymerization lost controllability. Therefore, a suitable amount of iron catalyst (i.e., [FeCl₃·6H₂O]₀/[EBiB]₀ ≥ 1)should be used in order to obtain a well-controlled polymerization process in this case.

Table 2 Effect of the amount of FeCl₃·6H₂O on the solution AGET ATRP of MMA in the presence of NaOH^a

Entry	[FeCl3·6H2O]0/[NaOH]0	Time (min)	Conv.(%)	M n,th (g mol^−1)	M n,GPC (g mol^−1)	M w/Mn	^aeff
a Polymerization conditions: [MMA]0/[EBiB]0/[FeCl3·6H2O]0/[TDA-1]0/[glucose]0/[NaOH]0 = 500/1/x/3x/1/1 (x = 2.0, 1.5, 1.0, 0.5), MMA = 3 mL, THF = 1 mL, T = 90 °C. ^aeff means initiator efficiency, eff = Mn,th/Mn,GPC.
1	2/1	184	63.6	31800	40100	1.23	0.79
2	1.5/1	301	66.2	33100	41700	1.25	0.79
3	1/1	336	64.9	32400	40000	1.30	0.81
4	0.5/1	464	58.0	29000	43800	1.51	0.66

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Analysis of chain end and chain extension

The chain end of PMMA obtained in the presence of NaOH was analyzed by ¹H NMR spectroscopy, as shown in Fig. 3. The chemical shifts at δ = 4.09 ppm (a in Fig. 3) was corresponded to the methylene protons of the ethyl ester unit in the initiator EBiB, which confirmed that EBiB initiated the polymerization of MMA successfully and attached to the polymer PMMA chain ends. The peak at δ = 3.78 ppm (c in Fig. 3) was attributed to the methyl ester group at the chain end, which has little deviation with the chemical shift (3.60, b in Fig. 3) of other methyl ester groups in PMMA because of the electron-attracting function of ω-Cl atom.^27,28 Furthermore, the integral value of peaks a to c was about 2 : 3, being consitent with the theoretical molar ratio of methylene to methyl protons. These results indicated that the chain ends in PMMA obtained in the presence of NaOH were attached by EBiB moieties, being consistent with the mechanism of ATRP. As a result, the obtained PMMA should be used as macroinitiator to conduct chain-extension reaction. The PMMA (M_n,GPC = 20200 g mol⁻¹, M_w/M_n = 1.18) obtained in the presence of NaOH was used as the predecessor in chain extension experiment. There was a peak shift from the original PMMA to the chain extended PMMA with M_n,GPC = 25400 g mol⁻¹ and M_w/M_n = 1.15 (Fig. 4). The successful chain extension reaction further confirmed the living features of AGET ATRP of MMA in the presence of NaOH.

Fig. 3 ¹H NMR spectrum of PMMA (M_n,GPC = 14300 g mol⁻¹, M_w/M_n = 1.41) obtained by AGET ATRP of MMA in the absence of oxygen using CDCl₃ as solvent and tetramethylsilane (TMS) as internal standard. Polymerization conditions: [MMA]₀/[EBiB]₀/[FeCl₃·6H₂O]₀/[TDA-1]₀/[glucose]₀/[NaOH]₀ = 500/1/0.5/1.5/0.5/1, MMA = 3 mL, THF = 1 mL, T = 90 °C.

Fig. 4 GPC traces of before and after chain extension using PMMA prepared by AGET ATRP of MMA as the macroinitiator. Polymerization conditions: [MMA]₀/[EBiB]₀/[FeCl₃·6H₂O]₀/[TDA-1]₀/[glucose]₀ = 500/0.08/1/3/1, MMA = 3 mL; THF = 1 mL; T = 90 °C.

Cyclic voltammetry (CV)

The value of redox potential of catalyst always shows the catalytic ability in iron-mediated ATRP because it undergoes one electron redox reversibly between Fe(II) and Fe(III). Therefore, CV for the FeCl₃/TDA-1 complexes in the presence/absence of NaOH was measured in DMF solution. The redox peaks and CV data are shown in Fig. 5. From Table 3, it can be seen that the redox potential (E_1/2) of the FeCl₃/TDA-1 complex in the absence of NaOH was 0.0235 V, correspondingly −0.118 V in the presence of NaOH, which was consistent with the polymerization results shown in Fig. 1 and Fig. 2 where the FeCl₃/TDA-1 complexes with additives NaOH accelerate the rate of polymerization significantly. This is contributed to that pH of the reaction system increased in the presence of base, and that E_1/2 usually decreases with the increase of pH as reported by the document,³⁸ which was consistent with the polymerization results where the FeCl₃/TDA-1 complexes with additives NaOH lead to a fast “living” free-radical polymerization. It is concluded that the polymerization rate enhancement resulted from the increase of basicity of the reaction system in the presence of NaOH. The plausible polymerization mechanism is shown in Scheme 1.

Fig. 5 Cyclic voltammograms (50 mV s⁻¹) of FeCl₃·6H₂O/TDA-1 complexes (10 mM) under different additives in DMF at room temperature. [Bu₄ClO₄] = 100 mM (supporting electrolyte). [FeCl₃·6H₂O]₀/[TDA-1]₀/[additives]₀ = 1/3/x (x = 0, 1.5), additives = NaOH.

Table 3 Redox potentials of Fe complexes measured in DMF

Additives/x^a	E pc (V)	E pa (V)	ΔEP (V)	E 1/2 (V)
a [FeCl3·6H2O]0/[TDA-1]0/[additives]0 = 1/3/x. Epa and Epc are the peak potentials of the oxidation and reduction waves, respectively. ΔEp = Epa − Epc. E1/2 = (Epa + Epc)/2.
None/0	−0.036	0.083	0.119	0.0235
NaOH/1.5	−0.199	0.081	0.2	−0.118

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Scheme 1 Plausible mechanism for iron-mediated AGET ATRP in the presence of catalytic amounts of NaOH.

Conclusions

Catalytic amounts of NaOH can be successfully used as an efficient rate-enhancement additives for the iron-mediated AGET ATRP of MMA, using EBiB as the initiator, FeCl₃·6H₂O as the catalyst, TDA-1 as the ligand, and glucose as the reducing agent. The polymerization can also be successfully carried out in the presence of a limited amount of air but don't sacrifice the features of living/controlled radical polymerization. The polymerization rate increased with the amount of NaOH but control the reaction was reduced or lost with higher polymerization rate. Therefore, a suitable amount of NaOH (i.e., [glucose]₀/[NaOH]₀ < 1/6) was necessary to avoid the lost of controllability over molecular weight and molecular weight distribution.

Acknowledgements

The financial support from the National Natural Science Foundation of China (Nos. 20974071 and 20904036), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20103201110005), the Project of Science and Technology Development Planning of Suzhou (No. SYG201026), the Project of International Cooperation of the Ministry of Science and Technology of China (No. 2011DFA50530), the Qing Lan Project, the Program of Innovative Research Team of Soochow University, and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) is gratefully acknowledged.

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