Iron-mediated AGET ATRP with crown ether as both ligand and solvent

Abstract

It is well known that crown ethers can selectively complex with metal ions due to their unique structure, and they have been widely used as an ion carrier and in phase-transfer catalysis in organic synthesis. In this work, crown ethers such as 18-crown-6 or 15-crown-5 were introduced into an iron-mediated AGET ATRP (Activators Generated by Electron Transfer for Atom Transfer Radical Polymerization) system as both ligand and solvent for the first time. Herein, FeCl₃·6H₂O was used as the catalyst, ethyl alpha-bromophenylacetate (EBPA) as the initiator, Na₂S₂O₄ as the reducing agent, and methyl methacrylate (MMA) as the model monomer, and then the method was extended to styrene (St), acrylonitrile (AN) and tert-butyl acrylate (t-BA), without any additional ligands. The effect of various factors, such as the type of reducing agents, the amounts of 18-crown-6, and polymerization temperatures (60–90 °C) on the polymerization were investigated. Furthermore, the polymerization kinetics revealed the typical “living” features of this polymerization system. For example, the molecular weights of PMMA increased linearly with the monomer conversion while maintaining a relatively low molecular weight distribution (M_w/M_n < 1.31). As well as this, the “living” feature of this polymerization system was further confirmed by chain-end analysis (¹H NMR, MALDI-TOF) and chain extension experiments.

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1. Introduction

Compared to living anionic polymerization¹ or living cationic polymerization² and even the normal atom transfer radical polymerization (ATRP),³ activators generated by electron transfer (AGET) ATRP⁴ have unique advantages such as a very facile operation since relatively stable higher oxidation state transition metal catalysts are used. Herein, the activators which mediate the propagation of the polymerization are generated in situ by reducing the higher state transition metal complexes with a reducing agent such as glucose, tin(II) 2-ethylhexanoate, ascorbic acid, or zero-valent metals.

For a transition metal catalyzed “living”/controlled radical polymerization, transition metal catalysts play a vital role in the ATRP process.⁵ Iron catalysts, considered as “green” catalysts, have recently attracted extensive attention in view of increasing environmental awareness, because of their abundance, low toxicity, low cost and biocompatibility.⁶ On the other hand, it is well known that ligands are usually necessary for a successful ATRP since they not only improve the solubility of the transition metal salts in the organic media but also adjust the redox potential of the metal center for the atom transfer.⁷ The most common ligands that have been employed in the iron-catalyzed ATRP system are phosphines⁸ and simple amines.⁹ Simultaneously, some polar solvents¹⁰ (e.g., N,N-dimethyl formamide (DMF)) and ionic liquids¹¹ have been confirmed as both ligand and solvent for iron-mediated ATRP systems. However, these ligands, including the solvent DMF, are either relatively expensive or relatively toxic, which partly minimizes the advantage of using iron as a “green” catalytic center in the ATRP catalysts. Therefore, it’s urgent to find more low toxic or non-toxic ligands for extending the application of iron catalysts. According to published reports, succinic acid,¹² isophthalic acid,¹³ and iminodiacetic acid (IDA)¹⁴ can be used as ligands for iron-catalyzed ATRP. These ligands are relatively cheap and less toxic. Furthermore, low molecular weight poly(ethylene glycol) (PEG) can also play a dual function as both ligands and solvents, as reported by our group and others.¹⁵ PEG is an inexpensive non-volatile green solvent, and it has low toxicity to both human beings and the environment. Since crown ether and PEG have similar structures that consist of –CH₂CH₂O– units, and several references relate to the interaction of Fe(III) ion with crown ethers,¹⁶ in this work we tried to use crown ether as both a ligand and solvent for the iron-mediated ATRP systems.

As early as 1967, Charles J. Pedersen¹⁷ synthesized a kind of macrocycle polyether by chance, and it was named crown ether because it has a structure like a crown. Crown ether can selectively complex with metal ions due to its unique structure,¹⁷ so it is widely used as an ion carrier and in phase-transfer catalysis. The two most commonly used crown ethers, 18-crown-6 and 15-crown-5, are commercially available from some chemical suppliers (Fig. 1). 18-Crown-6 is a white crystal at room temperature, and it starts to melt at about 40 °C; so the reaction system can remain homogeneous at polymerization temperature (above 60 °C). 15-Crown-5 is a colorless, transparent, and sticky liquid with a boiling point about 94 °C. Inspired by the above-mentioned results and the possibility of crown ether acting as the ligand as well as the solvent, in this work we reported for the first time a facile and highly efficient iron-mediated AGET ATRP in crown ether in the absence of any additional ligands using methyl methacrylate (MMA) as the model monomer, and then the method was extended to styrene (St), acrylonitrile (AN) and tert-butyl acrylate (t-BA). Herein, FeCl₃·6H₂O was used as the catalyst, ethyl α-bromophenylacetate (EBPA) as the initiator and Na₂S₂O₄ as the reducing agent.

Fig. 1 The structures of 18-crown-6 (a) and 15-crown-5 (b).

2. Experimental section

2.1. Materials

Methyl methacrylate (MMA, +99%), acrylonitrile (AN, 99%), tert-butyl methacrylate (t-BA, 99%), and styrene (St, 99%) were purchased from Sinopharm Chemical Reagent Co. Ltd. and purified by passing through a short alumina column to remove the inhibitor before use. Ethyl α-bromophenylacetate (EBPA, 97%) was purchased from Alfa Aesar. Azobisisobutyronitrile (AIBN, +97%) was obtained from Sigma-Aldrich and purified by recrystallizing twice from methanol. Iron(III) chloride hexahydrate (FeCl₃·6H₂O, +99%), sodium dithionite (Na₂S₂O₄, analytical reagent) and 18-crown-6 (98%) were purchased from Sinopharm Chemical Reagent Co. Ltd. and used as received. 15-Crown-5 (97%) was purchased from Energy Chemical and used as received. Sodium metabisulfite (Na₂S₂O₅, 97%) purchased from J&K Chemical and used as received. Triethylamine (TEA, 99%), glucose (99%), ascorbic acid (AsAc, +99.7%), sodium ascorbate (AsAc-Na, +99.7%) and hydrazine monohydrate (N₂H₄·H₂O, 85%) were purchased from Shanghai Chemical Reagents Co. Ltd. and used as received. All other chemical reagents were obtained from Sinopharm Chemical Reagent Co. Ltd. and were used as received unless mentioned.

2.2. Typical procedure for AGET ATRP of MMA

The typical AGET ATRP of MMA was performed with the molar ratio of [MMA]₀ : [EBPA]₀ : [FeCl₃·6H₂O]₀ : [Na₂S₂O₄]₀ = 200 : 1 : 1 : 1, and the typical polymerization procedures are as follows: FeCl₃·6H₂O (12.7 mg, 0.047 mmol), Na₂S₂O₄ (8.2 mg, 0.047 mol), EBPA (8.25 μL, 0.047 mmol), MMA (1.0 mL, 9.44 mmol) and 18-crown-6 (desired amount) were added to a clean ampule. The obtained reaction mixture was degassed by three freeze–pump–thaw cycles to remove the dissolved oxygen. Then, the ampule was sealed and transferred into a stirring hot plate equipped with a thermostat at the desired temperatures (70 °C, 80 °C or 90 °C) to conduct the polymerization of MMA. After the desired polymerization time, the mixture was cooled with ice water to stop the reaction. Afterwards, the ampule was opened and the contents were dissolved in tetrahydrofuran (THF) and then were poured into a large amount of methanol (∼200 mL) for precipitation. The product was obtained after filtration and drying in vacuum at 35 °C to a constant weight. The monomer conversion was determined by gravimetry. Similar experimental procedures were carried out for the iron-catalyzed AGET ATRP of St, AN and t-BA.

2.3. Chain extension

The resultant PMMA was used as a macro-initiator for chain extension by the AGET ATRP of MMA with the molar ratio of [MMA]₀ : [PMMA]₀ : [FeCl₃·6H₂O]₀ : [Na₂S₂O₄]₀ = 500 : 1 : 2 : 2 in 18-crown-6 at 90 °C. The specific procedure is as follows: a predetermined quantity of PMMA was added to a clean ampule, and then a predetermined quantity of MMA, FeCl₃·6H₂O, Na₂S₂O₄, and 18-crown-6 was added. The other steps were the same as that described above.

2.4. Characterization

The number-average molecular weight (M_n,GPC) and molecular weight distribution (M_w/M_n) values of the resultant polymers were determined by a TOSOH HLC-8320 gel permeation chromatograph (GPC) equipped with a refractive-index detector (TOSOH), using a TSKgel guardcolumn SuperMP-N (4.6 × 20 mm) and two TSKgel SupermultiporeHZ-N (4.6 × 150 mm) with measurable molecular weight ranging from 5 × 10² to 5 × 10⁵ g mol⁻¹. THF was used as the eluent at a flow rate of 0.35 mL min⁻¹ and 40 °C. GPC samples were injected using a TOSOH plus autosampler and calibrated with PMMA standards purchased from TOSOH. The ¹H NMR spectrum of the obtained polymer was recorded on an INOVA 400 MHz nuclear magnetic resonance (NMR) instrument using DMSO-d₆ as the solvent and tetramethylsilane (TMS) as an internal standard.

3. Results and discussion

3.1. Effect of reducing agent

The reducing agent plays an important role in an AGET ATRP process because the presence of the reducing agents make the oxidatively stable iron(III) complex be reduced to form a lower oxidation state active iron(II) catalyst. In addition, excess reducing agents can eliminate the dissolved oxygen in the solution; and suitable reducing agents can even reduce the amount of transition metal catalyst or start a polymerization at room temperature. Therefore, we firstly investigated the effect of various reducing agents on the AGET ATRP of MMA. As shown in Table 1 (entries 1–8), although 8 reducing agents (AsAc-Na, AsAc, Na₂S₂O₅, glucose, trimethylamine, AIBN, N₂H₄·H₂O and Na₂S₂O₄) were used, only AIBN, N₂H₄·H₂O and Na₂S₂O₄ could result in a well-controlled polymerization under the designed polymerization conditions. Just as reported by the Matyjaszewski group,¹⁸ Na₂S₂O₄ was considered to be the most efficient reducing agent. It acted not only as a powerful reducing agent but also as a supplemental activator for ATRP. By contrast, seven other reducing agents had a relatively lower reducibility for the iron(III)-catalyst in the polymerization initiated by the EBPA/FeCl₃·6H₂O/18-crown-6 system. In particular, no polymerization was observed with the first five reducing agents used, which may be caused by their poor reducing capacity and insufficient polymerization time. Judging by the polymerization rate and controllability, we chose Na₂S₂O₄ as the reducing agent for further investigation.

Table 1 Effect of each kind of reducing agent (RA) on the polymerization of MMA^a

Entry	RA	Time (h)	Conv. (%)	Mn,th^b (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn
a Polymerization conditions: [MMA]0 : [EBPA]0 : [FeCl3·6H2O]0 : [RA]0 = 200 : 1 : 1 : 1, VMMA = 1.0 mL, m18-crown-6 = 500.0 mg, T = 90 °C.b Mn,th = ([M]0/[EBAP]0) × Mw,MMA × conv.%.c NA represents no polymers were obtained.
1	AsAc-Na	24	NA^c	NA	NA	NA
2	AsAc	24	NA	NA	NA	NA
3	Na2S2O5	24	NA	NA	NA	NA
4	Glucose	24	NA	NA	NA	NA
5	TEA	24	NA	NA	NA	NA
6	AIBN	48	30.5	6100	11 400	1.28
7	N2H4·H2O	72	24.1	4800	12 600	1.31
8	Na2S2O4	1.25	79.5	15 900	23 900	1.20

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3.2. Effect of the amount of 18-crown-6

The choice of ligand is a pretty important research direction in ATRP, because an appropriate ligand can not only improve the activity of the catalyst but can also lower the redox potential of the metal center. As already mentioned, the crown ether can selectively complex with metal ions owing to its ring structure composed of –CH₂CH₂O– units. We tested the two most commonly used crown ethers, 18-crown-6 and 15-crown-5, as both ligand and solvent and investigated the effect of the amount used on the polymerization of MMA. The obtained results are listed in Table 2. It can be seen from Table 2 (entries 2–10) that the rate of polymerization in 18-crown-6 increased to a plateau and then decreased with the increasing of the amount of crown ether. This is because 18-crown-6 played a dual function as both a ligand and a solvent in this polymerization system. These results reflected the comprehensive influence of the high complexing ability between FeCl₃·6H₂O and 18-crown-6 as a ligand, and the better solubility and decreased radical concentration with 18-crown-6 as a solvent. Also, even catalytic amounts of 18-crown-6 (entries 2–4 in Table 2) could still maintain good controllability, but it should be noted that no polymers were obtained in the absence of 18-crown-6 (entry 1 in Table 2). When using 15-crown-5 as both ligand and solvent (entries 11–14), good controllability was still maintained over the polymerization. Therefore, in order to obtain controlled polymerization, crown ether is a necessary component in this polymerization system due to its function as a ligand, which complexes with the iron catalyst.¹⁹

Table 2 Effect of the amount of crown ether on the polymerization of MMA^a

Entry	Crown ether (mg or μL)	Time (h)	Conv. (%)	Mn,th^d (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn
a Polymerization conditions: [MMA]0 : [EBPA]0 : [FeCl3·6H2O]0 : [Na2S2O4]0 = 200 : 1 : 1 : 1, VMMA = 1.0 mL.b 18-crown-6 (mg), T = 90 °C.c 15-crown-5 (μL), T = 80 °C.d Mn,th = ([M]0/[EBAP]0) × Mw,MMA × conv.%.e NA represents no polymers were obtained.
1^b	0.0	24	NA	NA^e	NA	NA
2^b	10.0	2	51.1	10 200	16 200	1.18
3^b	30.0	1.17	52.9	10 600	17 000	1.17
4^b	50.0	1.83	52.6	10 500	17 700	1.15
5^b	100.0	0.83	53.3	10 700	18 300	1.16
6^b	300.0	0.67	58.9	11 800	20 000	1.18
7^b	500.0	1.25	79.5	15 900	23 900	1.20
8^b	1000.0	0.83	61.2	12 200	24 700	1.34
9^b	1500.0	0.83	58.5	11 700	24 800	1.29
10^b	2000.0	0.83	54.8	11 000	21 500	1.31
11^c	100.0	3	33.3	6700	14 600	1.19
12^c	100.0	4	47.9	9600	19 000	1.17
13^c	200.0	2	35.2	7000	15 800	1.19
14^c	200.0	3	53.0	10 600	19 300	1.17

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3.3. Effect of temperature

In an AGET ATRP system, the activator is generated by reducing the higher oxidation state catalyst with a reducing agent. A higher temperature can promote this process and the polymerization rate, and therefore shorten the induction period. Therefore, the higher the temperature is, the less time needed to establish a dynamic equilibrium between the iron(II) species and iron(III) species. As shown in Table 3, it can be seen that the polymerization rate slowed down as the temperature decreased from 90 °C to 60 °C. Fig. 2 illustrates the Arrhenius plot of ln k^app_p versus 1000/T for the iron-mediated AGET ATRP of MMA with crown ether as both the ligand and solvent. Based on the equation of ln k^app_p = ln A_app − ΔE^≠_app/(R × T), the apparent activation energy for this polymerization system was calculated as 63.9 kJ mol⁻¹.

Table 3 Effect of temperature on the polymerization of MMA^a

Entry	T (°C)	Time (h)	Conv. (%)	Mn,th^b (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn	k^appp × 10^3^c (min^−1)
a Polymerization conditions: [MMA]0 : [EBPA]0 : [FeCl3·6H2O]0 : [Na2S2O4]0 = 400 : 1 : 1 : 1, VMMA = 1.0 mL, m18-crown-6 = 100.0 mg.b Mn,th = ([M]0/[EBAP]0) × Mw,MMA × conv.%.c k^appp (Rp = −d[M]/dt = kp[Pn·][M] = k^appp [M]) was calculated from the slope of the polymerization kinetics as shown in Fig. 3(a).
1	60	7	53.7	21 500	36 200	1.20	1.95
2	70	3	48.3	19 300	36 000	1.17	3.98
3	80	2.33	54.7	21 900	33 600	1.19	5.82
4	90	0.83	52.6	21 000	34 800	1.18	14.21

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Fig. 2 Arrhenius plot of ln k^app_p versus 1000/T for the iron-mediated AGET ATRP of MMA with crown ether as both ligand and solvent.

3.4. Effect of solvent

As reported by Matyjaszewski et al.,^10a some polar solvents can be used as both ligands and solvents for iron-catalyzed ATRP. In this work, we also investigated the effect of some typical polar solvents on the polymerizations of MMA without any additional ligands. The results in Table 5 show that when conventional polar solvents such as DMF, DMSO and MeCN are used as both ligands and solvents, the polymerization rate decreased significantly compared with that in the case of 18-crown-6. For example, the monomer conversion could reach up to 79.5% within 1.25 h in the case of 18-crown-6 (entry 3 in Table 3), while reaching 55.1% within 5 h (entry 1 in Table 4), 18.1% within 72 h (entry 2 in Table 4) and 13.0% within 72 h (entry 3 in Table 4) for DMF, DMSO and MeCN, respectively, further indicating the advantages of 18-crown-6 as both ligand and solvent over these conventional polar solvents.

Table 4 Effect of solvent on the polymerization of MMA^a

Entry	Solvent	Time (h)	Conv. (%)	Mn,th^b (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn
a Polymerization conditions: [MMA]0 : [EBPA]0 : [FeCl3·6H2O]0 : [Na2S2O4]0 = 200 : 1 : 1 : 1, VMMA = Vsolvent = 1.0 mL, T = 90 °C.b Mn,th = ([M]0/[EBAP]0) × Mw,MMA × conv.%.
1	DMF	5	55.1	11 000	16 100	1.33
2	DMSO	72	18.1	3620	12 300	1.35
3	MeCN	72	13.0	2600	12 500	1.31

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Table 5 Polymerization for different monomers^a

Entry	Monomer	Time (h)	Conv. (%)	Mn,th^b (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn
a Polymerization conditions: [monomer]0 : [EBPA]0 : [FeCl3·6H2O]0 : [Na2S2O4]0 = 200 : 1 : 1 : 1, Vmonomer = 1.0 mL, m18-crown-6 = 100.0 mg, T = 90 °C.b Mn,th = ([M]0/[EBAP]0) × Mw,MMA × conv.%.
1	St	66	64.1	13 400	9900	1.22
2	AN	18	21.9	2300	5000	1.26
3	t-BA	2.5	88.1	22 600	34 800	1.15

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3.5. Polymerization kinetics

In order to further investigate the polymerization behaviors using 18-crown-6 as both a ligand and solvent, the polymerization kinetics with the molar ratio of [MMA]₀ : [EBPA]₀ :[FeCl₃·6H₂O]₀ : [Na₂S₂O₄]₀ = 400 : 1 : 1 : 1 was studied at 60 °C, 70 °C, 80 °C, and 90 °C, respectively. As depicted in Fig. 3(a), first-order polymerization kinetics was observed in all cases, which means a constant concentration of propagating radicals during the polymerization processes. It should be noted that an induction period (the regression curves do not pass the diagram origin) was also observed in the cases of lower polymerization temperatures (e.g., 70 °C and 60 °C). This may be attributed to the fact that some time was needed to establish a dynamic equilibrium between the dormant Fe(III) and active Fe(II) species due to the presence of the initial dormant Fe(III) species at lower temperatures.^8c In addition, the apparent rate constants (k^app_p) (R_p = −d[M]/dt = k_p[P_n·][M] = k^app_p [M]) could be calculated according to the slope of polymerization kinetics as 14.21 × 10⁻³ min⁻¹ in the case of 90 °C, 5.82 × 10⁻³ min⁻¹ in the case of 80 °C, 3.98 × 10⁻³ min⁻¹ in the case of 70 °C and 1.95 × 10⁻³ min⁻¹ in the case of 60 °C. Fig. 3(b) shows the evolution of M_n,GPC and M_w/M_n versus monomer conversion. The molecular weight of the resultant polymers increased linearly with the monomer conversion and the molecular weight distribution remains narrow (M_w/M_n < 1.31). However, the experimental molecular weights were 50% bigger than the theoretical ones in almost all of the polymerizations. It may be mainly a result of the low initiator efficiency caused by the “cage effect” of crown ether.

Fig. 3 Kinetic plots (a) and evolution of number-average molecular weight (M_n,GPC) and molecular weight distribution (M_w/M_n) versus conversion (b) for the AGET ATRP of MMA with various polymerization temperatures in 18-crown-6 without any additional ligands. Polymerization conditions: [MMA]₀ : [EBPA]₀ : [FeCl₃·6H₂O]₀ : [Na₂S₂O₄]₀ = 400 : 1 : 1 : 1, V_MMA = 1.0 mL, m_18-crown-6 = 100.0 mg.

As shown in Table 2, the amount of 18-crown-6 had a significant effect on the polymerization rate. Fig. 4(a) shows the polymerization kinetics with different amounts of 18-crown-6 (100.0 mg and 500.0 mg, respectively) at 80 °C. It can be seen that the apparent rate constants (k^app_p) were 7.77 × 10⁻³ min⁻¹ in the case of 500.0 mg and 5.82 × 10⁻³ min⁻¹ in the case of 100.0 mg. The former is obviously larger than the latter as discussed above. In addition, all the M_n,GPC values of the resultant polymers in both cases increased linearly with the monomer conversion while maintaining a relatively low molecular weight distribution (M_w/M_n < 1.31) (Fig. 4(b)). All of these results from Fig. 3 and 4 confirmed the controlled/“living” features of the polymerizations under various temperatures and the amounts of 18-crown-6 discussed above.

Fig. 4 Kinetic plot (a) and evolution of number-average molecular weight (M_n,GPC) and molecular weight distribution (M_w/M_n) versus conversion (b) for the AGET ATRP of MMA with different amounts of 18-crown-6 without any additional ligands. Polymerization conditions: [MMA]₀ : [EBPA]₀ : [FeCl₃·6H₂O]₀ : [Na₂S₂O₄]₀ = 400 : 1 : 1 : 1, V_MMA = 1.0 mL, T = 80 °C.

3.6. Analysis of chain-end and chain extension

The chain-end of the obtained PMMA (M_n,GPC = 12 800 g mol⁻¹, M_w/M_n = 1.18) was analyzed by ¹H NMR spectroscopy, as shown in Fig. 3. The chemical shift at 3.8 ppm (a in Fig. 5) belonged to the protons of the methyl ester group at the chain-end, and the chemical shift at 3.60 ppm (b in Fig. 5) was attributed to other methyl ester groups in the PMMA. The chemical shifts at 3.4 ppm (c in Fig. 5), 7.1–7.4 ppm (d in Fig. 5) and 3.9–4.2 ppm (e in Fig. 5) were assigned to the protons of methane, phenyl, and methylene of the initiator EBPA moieties, respectively. Therefore, the resultant PMMA was end-capped by the EBPA moieties. Furthermore, mass spectrometry was used to further confirm the structure of the obtained polymers. The MALDI-TOF-MS of PMMA ranging from 3000 to 18 000 is shown in Fig. 6(a), and an enlargement of the 10 100–10 900 range is shown in Fig. 6(b). These major peaks are attributed to a series of polymer chains [R–(MMA)_n–Cl + K]⁺, and the calculated molecular weights were in full agreement with the values obtained by MS analysis (e.g., n = 100, M_n,cal = 10 249.80 g mol⁻¹, 10 249.80 = 163.2 + 100 × 100.12 + 35.5 + 39.1, where 163.2, 100.12, 35.5 and 39.1 correspond to the molar mass of EBPA without a bromine atom, MMA, Cl and K⁺, respectively).

Fig. 5 ¹H NMR spectrum of PMMA (M_n,GPC = 12 800 g mol⁻¹, M_w/M_n = 1.18) with DMSO-d₆ as the solvent and tetramethylsilane (TMS) as the internal standard. Polymerization conditions: [MMA]₀ : [EBPA]₀ :[FeCl₃·6H₂O]₀ : [Na₂S₂O₄]₀ = 100 : 1 : 1 : 1, V_MMA = 1.0 mL, m_18-crown-6 = 100.0 mg, t = 40 min, T = 90 °C, conversion = 59.5%.

Fig. 6 MALDI-TOF-MS in the linear mode of PMMA (a) (M_n,GPC = 12 700 g mol⁻¹, M_w/M_n = 1.19) and an enlargement of the MALDI-TOF-MS from m/z 10 100 to 10 900 of PMMA–Cl (b).

In order to further prove the “living” feature of the resultant polymer from the AGET ATRP of MMA in 18-crown-6 without any additional ligands, a chain extension experiment was carried out using the resultant PMMA as a macroinitiator. Fig. 7 shows both the GPC curves of the macroinitiator and the chain-extended PMMA. The molecular weight increased from 17 000 to 31 100 g mol⁻¹ after the chain extension with fresh MMA monomer while the molecular weight distribution also stayed narrow (M_w/M_n = 1.18), demonstrating the “living” feature of this polymerization system.

Fig. 7 GPC traces before and after chain extension using PMMA as the macro-initiator. Original PMMA: [MMA]₀ : [EBPA]₀ : [FeCl₃·6H₂O]₀ : [Na₂S₂O₄]₀ = 200 : 1 : 1 : 1, V_MMA = 1.0 mL, m_18-crown-6 = 30.0 mg, T = 90 °C, t = 1.17 h, conversion = 52.9%. Chain extended PMMA: [MMA]₀ : [PMMA]₀ : [FeCl₃·6H₂O]₀ : [Na₂S₂O₄]₀ = 500 : 1 : 2 : 2, V_MMA = 1.0 mL, m_18-crown-6 = 100.0 mg, T = 90 °C, t = 0.5 h, conversion = 27.6%.

3.7. Monomer generality

As discussed above, this polymerization system showed a “living” feature for the polymerization of MMA. How about for other monomers? We selected other typical monomers such as styrene (St), acrylonitrile (AN) and tert-butyl acrylate (t-BA) instead of MMA for similar polymerization in 18-crown-6 without any additional ligands. From Table 5, all the three monomers could be polymerized smoothly while maintaining relatively narrow molecular weight distributions (M_w/M_n < 1.26), indicating that this catalyst system consisting of FeCl₃·6H₂O, 18-crown-6, EBPA and Na₂S₂O₄ is suitable for a wide range of polymerizable monomers.

4. Conclusions

A facile iron-catalyzed AGET ATRP system was successfully achieved using 18-crown-6 or 15-crown-5 as both ligand and solvent with EBPA as the initiator, FeCl₃·6H₂O as the catalyst, and Na₂S₂O₄ as the reducing agent. This polymerization system showed all the characteristics of controlled/“living” radical polymerization with a higher polymerization rate (k^app_p was up to 14.21 × 10⁻³ min⁻¹), and was a universal polymerization system suitable for a wide range of monomers.

Acknowledgements

The financial support from the National Natural Science Foundation of China (No. 21174096, 21274100, 21234005), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20123201130001), the Project of Science and Technology Development Planning of Suzhou (No. ZXG201413, SYG201430), the Project of Science and Technology Development Planning of Jiangsu Province (No. BK20141192) and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) is gratefully acknowledged.

Notes and references

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