Amide group-containing polar solvents as ligands for iron-catalyzed atom transfer radical polymerization of methyl methacrylate

Abstract

A series of amide group-containing polar solvents, formamide (Fo), N-methylformamide (MFo), N,N-dimethylformamide (DMF), acetamide (Ac), N-methylacetamide (MAc), N,N-dimethylacetamide (DMAc), urea, tetramethyl urea (TMU), 2-pyrrolidone (2-Py), N-methyl-2-pyrrolidone (NMP) and 5-methyl-2-pyrrolidone (MPy), were used as both solvents and ligands for iron(II)-catalyzed atom transfer radical polymerizations (ATRPs) of methyl methacrylate (MMA), with ethyl 2-bromo-2-phenylacetate (EBPA) as the initiator. Most of the polymerizations were well-controlled in character, and the structures of the polar solvents greatly affected the catalytic activity. In addition, the living features of the systems remained in the presence of limited amounts of polar solvents. Some of the polar solvents (MFo, TMU and 2-Py) were also employed for iron(III)-catalyzed activators generated by electron transfer (AGET) ATRPs of MMA, and the results were as good as those of the ATRPs.

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Introduction

Metal-catalyzed controlled radical polymerization (CRP), also known as atom transfer radical polymerization (ATRP), has been a versatile tool for synthesizing well-defined polymers^1–15 with controlled molecular weights (M_n) and narrow molecular weight distributions (MWDs, M_w/M_n) since it was first presented in 1995.^16–18 The catalyst used in ATRP is considered to be the most essential and important issue for controlling a polymerization,^6,7,14,15 and a great deal of effort has thus been made to research the effects of various complexes on polymerizations, such as those based on copper,^{10,11,19–27} ruthenium^2,6,28–35 and iron.^{14,15,36–57}

On account of the environmental and sustainability aspects, the iron-based catalysts are one of the most promising catalysts for ATRP among those catalysts. Firstly, iron exists widely in the Earth’s crust as an extremely abundant metal, which can be obtained easily at a low price. Secondly, iron has low toxicity and is biocompatible, properties which are significant for industrial applications.^2,6,7 What’s more, the oxidation states that usually exist, iron(II) and iron(III), have empty orbitals to coordinate to many ligands, and form active complexes used in CRP on a large scale. Compared with copper-based catalysts, iron complexes used in ATRP have generally been less active. However, the development of environmentally benign and sustainable iron-based systems is becoming a trend of “green” chemistry or “green” reactions in the applications of organic chemistry,^58–61 polymer chemistry^6,7,14,15,62 and electrochemistry.^63–68 Various ligands have been applied for complexing with iron salts to form iron-based catalysts,^14,15 such as the traditional phosphine (P), nitrogen (N) or P–N ligands. Notably, some organic acid- and salt-containing ligands have also been developed for iron-catalyzed ATRP.¹⁵ Although these catalysts are active for ATRP, more attention needs to be paid to the cost and separation of these iron complexes for potential practical applications of the ATRP technique.

Recently, FeBr₂-catalyzed ATRPs of methyl methacrylate (MMA) in polar solvents were reported by Matyjaszewski et al.^69,70 The resultant polymers had molecular weights which agreed with the theoretical values, and the MWDs remained low (M_w/M_n < 1.3) when conducted in N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF) and acetonitrile (MeCN), but the polymers obtained in dimethyl sulfoxide (DMSO) had poor controllability (M_w/M_n > 2.0). The experimental results suggested that certain polar solvents act as ligands for iron species due to the coordination ability of the solvents with FeBr₂. Actually, polar solvents which have lone pairs of electrons (nitrogen, oxygen, or phosphorus atoms) can also coordinate with organoiron complexes, and their coordination and catalytic properties have been studied in our previous work.^71,72 To study the function of polar solvents as ligands for conducting CRP, Bulgakova et al.⁷³ and Xue et al.⁷⁴ reported the activators generated by electron transfer (AGET) ATRP of MMA using polar solvents (DMF, NMP or MeCN) in the absence of additional ligands. Most of the polymerizations showed the typical characteristics of “living”/controlled radical polymerization. In addition, we have also used alcohols, such as methanol, ethanol, ethylene glycol (EG) and glycerol, as reducing agents for the iron-catalyzed AGET ATRP of MMA in the presence of polar solvents as ligands.⁷⁵ Very recently, Zhu and coworkers⁷⁶ also reported the iron-catalyzed AGET ATRP of MMA using polyethylene glycol 400 (PEG-400) as both a solvent and a ligand.

The polymerizations were well controlled even when the concentration of PEG-400 was reduced to a catalytic amount. This system is much more environmentally friendly when compared with most traditional iron-catalyzed ATRP systems, due to the “green” nature of PEG-400. Compared to the conventional iron-based catalytic systems, it is particularly significant that the polymerizations were found to be more environmentally friendly ATRPs in the presence of polar solvents as ligands.

In considering the role of polar solvents, which act as ligands in the iron-catalyzed ATRPs mentioned above, they have electron donor groups, such as the oxygen atom in DMF or NMP, and all of them are able to coordinate to iron(II) ([Ar]3d⁶4s⁰) or iron(III) ([Ar]3d⁵4s⁰) which have empty orbitals. We expected that the coordination ability of polar solvents to iron salts and their universality would enable them to be applied in iron-catalyzed ATRP, and to be appropriate substitutes for traditional P ligands or N ligands. In this article, combining the various advantages of iron catalysts and polar solvents, the iron-catalyzed ATRP of MMA (Scheme 1) was investigated using polar solvents with amide groups as ligands. These amide group-containing ligands can be classified by their structures as polar solvents based on formamide, polar solvents based on urea or polar solvents based on pyrrolidone (see Scheme 2). They all have the ability to coordinate with iron salts which depend on their structures. The majority of them have been used for ATRP for the first time in this work. As expected, some of them resulted in good controllability of molecular weights and MWDs. In addition, the different structures of the polar solvents greatly affected their catalytic activities, which is significant for the optimization of polar solvents for ATRP applications. In order to ease the process of separating the solvents from the polymerization system for the purpose of environmental protection, we also reduced the concentrations of the polar solvents to catalytic amounts, and ATRP was still feasible. These amide group-containing polar solvents were also employed in iron(III)-catalyzed AGET ATRP (see Scheme 1), and most of the systems showed controlled features.

Scheme 1 Mechanism of iron-catalyzed ATRP using amide group-containing polar solvents as ligands.

Scheme 2 Structures of amide group-containing polar solvents.

Experimental

Materials

Methyl methacrylate (MMA, 98+%, Sinpharm) was passed through a column filled with neutral alumina, dried over calcium hydride (CaH₂), distilled under reduced pressure and stored in a freezer under argon. Formamide (Fo, 99+%, Sinpharm), N,N-dimethylformamide (DMF, 99.5+%, Sinpharm), N-methylacetamide (MAc, 99+%, Sinpharm), N,N-dimethylacetamide (DMAc, 99+%, Sinpharm), and N-methyl-2-pyrrolidone (NMP, 95+%, Sinpharm) were dried over CaH₂ and distilled under reduced pressure. N-Methylformamide (MFo, 99+%, Acros), acetamide (Ac, 98+%, Sinpharm), 2-pyrrolidone (2-Py, 99+%, Xiya), 5-methyl-2-pyrrolidone (MPy, 98+%, TCI), urea (99+%, Sinpharm), tetramethyl urea (TMU, 98+%, TCI), ethyl 2-bromo-2-phenylacetate (EBPA, 95%, Alfa Aesar), iron(II) bromide (FeBr₂, 98+%, Alfa Aesar), iron(III) bromide (FeBr₃, 98+%, Alfa Aesar), vitamin C (VC, 99+%, Sinpharm), sodium carbonate (Na₂CO₃, Sinpharm) and ethylene glycol (EG, 99+%, Sinpharm) were used without further purification.

Polymerization procedures

A typical polymerization procedure with the molar ratio of [MMA]₀/[EBPA]₀/[FeBr₂]₀/[solvent]₀/[Na₂CO₃]₀ = 200 : 1 : 1 : 20 : 2 is as follows. A Schlenk flask (25 mL) was charged with FeBr₂ (61 mg, 0.28 mmol) and Na₂CO₃ (60 mg, 0.57 mmol). The flask was sealed with a rubber septum and was cycled three times between vacuum and argon (Ar) to remove oxygen. Degassed polar solvents and MMA (6 mL, 56.3 mmol) were then added to the flask through degassed syringes. The solution was stirred for 30 min at room temperature. After three additional freeze–pump–thaw cycles, the initiator, EBPA (49.5 μL, 28.2 mmol) was added, and the flask was immersed in a thermostated oil bath at 60 °C. At timed intervals, samples were withdrawn from the flask with a degassed syringe. The monomer conversion was determined gravimetrically after removal of the unconverted monomer and solvent under reduced pressure, and the resulting residue was diluted with tetrahydrofuran (THF) and then filtered through a column filled with neutral aluminum oxide to remove the iron catalyst. The poly(methyl methacrylate) (PMMA) solution was then precipitated using an excess of n-hexane, and these polymers were dried under vacuum overnight at 80 °C for gel permeation chromatography (GPC) characterization. The same experimental procedures were carried out for the iron(III)-catalyzed AGET ATRP.

Chain extension experiment

A predetermined quantity of PMMA macroinitiator (PMMA–Br) obtained by ATRP of MMA with a molar ratio of [MMA]₀/[PMMA]₀/[FeBr₂]₀/[TMU]₀/[Na₂CO₃]₀ = 500 : 1 : 1 : 50 : 5 was added to a Schlenk flask, and then a predetermined quantity of MMA, FeBr₂, TMU and Na₂CO₃ was added. The rest of the procedure was the same as that described above. The chain extension polymerization was carried out under stirring at 60 °C.

Measurements

¹H NMR spectroscopy was performed using a Bruker AV400 NMR spectrometer, with deuterated chloroform (CDCl₃) as the solvent and tetramethylsilane (TMS) as the standard. The number-average molecular weight (M_n,GPC) and M_w/M_n of the polymers were determined with GPC using an Agilent 1100 gel permeation chromatograph, with a PLgel 79911GP-104 (7.5 × 300 mm, 10 μm bead size) column. THF was used as the eluent at a flow rate of 1 mL min⁻¹ at 35 °C. Linear polystyrene standards were used for calibration.

Results and discussion

Some previous works^74–76 have demonstrated that EBPA is the optimal initiator for the iron-catalyzed ATRP of MMA, because both the phenyl and ester groups in EBPA contribute to the stabilization of the generated radicals. Therefore, all polymerizations were conducted with EBPA as the initiator in this study. General amide group-containing polar solvents were chosen for our study as far as possible, and we classified them as polar solvents based on formamide, polar solvents based on urea or as polar solvents based on pyrrolidone by their structures.

Polar solvents based on formamide as ligands

As mentioned in the Introduction section, polar solvents (NMP, DMF, MeCN and DMSO) can coordinate to iron salts, and can be used as ligands for ATRP. As far as we know, the oxygen atom in the amide group can act as an electron donor to coordinate to an iron ion.^77,78 To probe the role of the amide group as a ligand, various polar solvents based on formamide that have linear structures (Fo, MFo, DMF, Ac, MAc, and DMAc) were used as ligands for the ATRP of MMA at 60 °C, with a molar ratio of [MMA]₀/[EBPA]₀/[FeBr₂]₀ = 200 : 1 : 1 and [MMA]₀/[polar solvent]₀ = 2 : 1 (v/v) without additional ligands. As shown in Table 1, no reaction occurred in the absence of Na₂CO₃ for almost all of the solvents (entries 1, 3, 6, 9, and 11), except for DMF (entry 13), which was consistent with work previously reported.⁶⁹ Zhu’s work^48,79–81 suggested that a catalytic amount of base can significantly accelerate iron-catalyzed ATRP, and that the controllability of the polymerizations can be maintained as well. They considered that an increased pH could reduce the redox potential (E_1/2) of polymerizations. The lower redox potential induced a faster reversible cleavage (activation) of a carbon–halogen terminal and, in turn, generated more radical species, which resulted in the enhancement of rate and controllability of the polymerization. Moreover, a base (Na₂CO₃ or NaOH) was also applied for the iron(III)-catalyzed AGET ATRP of MMA with FeX₃ (X = Br or Cl)/polar solvent catalytic systems using different alcohols as reducing agents in our previous work,⁷⁵ and the results revealed the exciting rate-enhancement effect of base on polymerization. Likewise, Bai et al.⁸² reported Cu-catalyzed AGET ATRP using alcohol as a reducing agent. Cu(II) was reduced to Cu(I), and the alcohol was oxidized to an aldehyde or ketone in the presence of base. The polar solvents studied in this article have lower coordination abilities than the traditional P or N ligands. The reactions may proceed very slowly. No polymerization conversion was even obtained at all after a long time in the absence of base (entries 1, 3, 6, 9 and 11 in Table 1). In light of the above results, base was added to our systems. As expected, most of them showed good controllability and relatively fast polymerization rates when the ratio of Na₂CO₃ to FeBr₂ was 2. What interested us was that the resulting PMMA obtained from the DMAc system (entry 7) had a number-average molecular weight (M_n) consistent with the predicted value and a low MWD value (M_w/M_n = 1.23). In the case of the MAc system, the M_n matched the theoretical value well (entry 4). However, the MWD was a little bit high. When the polymerization was carried out in Ac or Fo (entries 2 and 10), the molecular weights were greatly bigger than the theoretical values and the polydispersities (PDIs) were high. A possible reason was the poor dissolubility of Ac or Fo in MMA.

Table 1 FeBr₂-catalyzed ATRPs of MMA using polar solvents based on formamide as ligands^a

Entry	Solvent	[M]0/[solvent]0^b	Base	Time (h)	Conv. (%)	Mn,th^c (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn
a [MMA]0/[EBPA]0/[FeBr2]0/[Na2CO3]0 = 200 : 1 : 1 : 2 or 0, 60 °C.b The ratio of [M]0 to [solvent]0 is the molar ratio, except where marked with “(v/v)”, which stands for the volume ratio.c Mn,th = ([MMA]0/[initiator]0) × MMMA × conversion + MEBPA, MMMA and MEBPA represent the molecular weights of MMA and EBPA, respectively.
1	Ac (solid)	4 : 1		30	0	NA	NA	NA
2	Ac (solid)	100 : 1	Na2CO3	17	70	14 250	23 600	1.87
3	MAc	2 : 1 (v/v)		68	0	NA	NA	NA
4	MAc	2 : 1 (v/v)	Na2CO3	29	34	7050	6900	2.04
5	MAc	10 : 1	Na2CO3	1.5	56	11 500	12 600	1.28
6	DMAc	2 : 1 (v/v)		25	0	NA	NA	NA
7	DMAc	2 : 1 (v/v)	Na2CO3	3.5	57	11 600	13 600	1.23
8	DMAc	10 : 1	Na2CO3	1.5	42	8700	9360	1.31
9	Fo	2 : 1 (v/v)		30	0	NA	NA	NA
10	Fo	10 : 1	Na2CO3	1.3	25	5250	120 000	3.5
11	MFo	10 : 1		43	0	NA	NA	NA
12	MFo	10 : 1	Na2CO3	3.2	50	10 250	14 200	1.43
13	DMF	2 : 1 (v/v)		7	15	3250	2300	1.27
14	DMF	10 : 1	Na2CO3	25	40	8250	10 800	1.56

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In addition, when the amount of solvent was reduced such that the molar ratio of [MMA]₀/[solvent]₀ was 10 : 1, almost all of the reactions still expressed controlled characters (entries 5, 8, 12 and 14). This finding is very significant for the application of ATRP in environmentally-friendly industrial processes. Kinetic plots for the ATRP of MMA with a molar ratio of [MMA]₀/[EBPA]₀/[FeBr₂]₀/[solvent]₀/[Na₂CO₃]₀ = 200 : 1 : 1 : 20 : 2 at 60 °C are depicted in Fig. 1a. The polymerizations proceeded with approximately first-order kinetics in all cases, indicating a constant concentration of growing radicals during polymerizations. The dependence of M_n and M_w/M_n on the monomer conversion within different polar solvents based on formamide is shown in Fig. 1b. The molecular weights of all PMMA increased linearly with the conversion. It is noted that all the experimental molecular weights were slightly higher than the corresponding theoretical ones, which may be attributed to the low initiator efficiency. Other explanations need further study. The MWD values from polymerizations with DMAc and MAc were relatively low (M_w/M_n = 1.18–1.28), but those for MFo were a little higher (M_w/M_n = 1.42–1.52). Even so, this could still be regarded as a “living”/controlled radical polymerization.

Fig. 1 (a) Kinetic plots of ln([M]₀/[M]) versus time and (b) plots of M_n (filled symbols) and M_w/M_n (open symbols) values versus conversion for FeBr₂-catalyzed ATRP of MMA using polar solvents based formamide as ligands. [MMA]₀/[EBPA]₀/[FeBr₂]₀/[solvent]₀/[Na₂CO₃]₀ = 200 : 1 : 1 : 20 : 2, 60 °C. ■ = MAc; ▲ = DMAc; ● = MFo.

Polar solvents based on urea as ligands

Urea is generally used as a nitrogenous fertilizer in agricultural production and exists extensively in nature. It contains the amide group, like the polar solvents based on formamide discussed above. Moreover, urea has one more amidogen than the latter. This may endow the urea with a stronger coordination ability to form catalyst complexes with iron salts. However, there was no reaction when urea was added into the system with a ratio of [MMA]₀/[urea]₀ = 4 : 1, as shown in entry 1 of Table 2. In comparison with the results obtained with polar solvents based on formamide, it took a long time for the urea to coordinate with FeBr₂ to express activity. Thus, Na₂CO₃ was used to accelerate the polymerization. It can be seen from Table 2 that the resulting polymer had a high experimental molecular weight and polydispersity (entry 2). The result indicated that the base’s rate-enhancing effect had occurred. Another reason for the poor controllability may be the weak solubility of urea in MMA. The viscosity of the polymerization system increased along with the reaction, and lots of catalyst complex formed at the bottom of the flask, resulting in nonuniformity of the system.

Table 2 FeBr₂-catalyzed ATRPs of MMA using polar solvents based on urea as ligands^a

Entry	Solvent	[M]0/[solvent]0^b	Base	Time (h)	Conv. (%)	Mn,th (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn
a [MMA]0/[EBPA]0/[FeBr2]0/[Na2CO3]0 = 200 : 1 : 1 : 2 or 0, 60 °C.b The molecular weight was too small and out of the range of the instrument.
1	Urea (solid)	4 : 1		30	0	NA	NA	NA
2	Urea (solid)	100 : 1	Na2CO3	5	35.2	7290	92 100	2.7
3	TMU	2 : 1 (v/v)		56	11	2400	NA^b	NA^b
4	TMU	2 : 1 (v/v)	Na2CO3	2	64	13 000	12 000	1.24
5	TMU	2 : 1 (v/v)	Na2CO3	3	78	16 000	14 000	1.24
6	TMU	10 : 1	Na2CO3	3	41	8450	7400	1.27

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The results from TMU-containing systems were quite different from those with urea (entries 3–6 in Table 2). When the initial volume ratio of MMA to TMU was 2, the reaction could be carried out in the absence of base (entry 3). However, the polymerization reached a low conversion of 11% in 56 h, which is extremely low compared with those for the polymerizations shown in entries 4 and 5 ([catalyst]₀/[Na₂CO₃]₀ = 1 : 2). It can be seen that the polymerization reached a very high conversion of 64%, with good values of molecular weight and MWD (M_w/M_n = 1.24) of the resultant polymers, in just 2 h when using Na₂CO₃. The catalytic amount of base not only improved the rate of polymerization, but also kept the “living” feature of the system (M_w/M_n = 1.24–1.27). Some differences can be seen when comparing the results for TMU with those for polar solvents based on formamide, as shown in Table 1. The polymerizations had higher rates and better controllability when using TMU as the ligand, especially when the ratio of [MMA]₀/[solvent]₀ was 2 : 1 (v/v). For example, when MAc or DMAc were used as ligands, the conversions of monomers reached 34% and 57% in 29 h and 3.5 h, respectively. But for TMU, the conversion reached 78% in just 3 h. Bearing in mind the structures of these solvents, the four methyl groups on TMU could enhance its coordination ability because the methyl group is a better electron donor (electron-donating substituent) than the hydrogen group, and can improve the electron cloud density of nitrogen or oxygen.⁸³ Therefore, the FeBr₂/TMU complex showed a better catalytic activity. The polymerization was still well controlled when the molar ratio of the [TMU]₀ to [MMA]₀ was reduced to 1 : 10, although the polymerization rate decreased a little (entry 6). The linear increase observed for the TMU in Fig. 2a indicated that the concentration of growing radicals remained constant during the polymerization process. A relatively long induction period existed. The evolution of the M_n and MWD values with monomer conversion is shown in Fig. 2b. The molecular weights of all PMMA increased linearly with the conversion, and deviated little from the theoretical molecular weights. In addition, the values of M_w/M_n remained relatively small (<1.3). All of these observations suggest that TMU is suitable for carrying out the ATRP of MMA, acting as a ligand to form an active complex with FeBr₂.

Fig. 2 (a) Kinetic plots of ln([M]₀/[M]) versus time and (b) plots of M_n (filled symbols) and M_w/M_n (open symbols) values versus conversion for the FeBr_2/TMU-catalyzed ATRP of MMA. [MMA]₀/[EBPA]₀/[FeBr₂]₀/[TMU]₀/[Na₂CO₃]₀ = 200 : 1 : 1 : 20 : 2, 60 °C.

Polar solvents based on pyrrolidone as ligands

NMP has been reported to coordinate well to iron salts in general, and the generated catalyst complex has expressed dramatic living characteristics when used in CRP, both in ATRP⁶⁹ and in AGET ATRP.^74,75 To identify the activity for the ATRP of MMA catalyzed by FeBr₂/NMP, some experiments were run under the same conditions as above. From entries 9 and 10 in Table 3, it can be seen that the conversion was up to 33% in 7 h when the ratio of [MMA]₀/[NMP]₀ was 2 : 1 (v/v). What’s more, the polymerizations were well-behaved (M_w/M_n = 1.39). This result confirmed the excellent activity of NMP in ATRP of MMA as a ligand, especially compared with the other amide group-containing polar solvents mentioned in this article. Polymerizations also proceeded in some other polar solvents in the absence of base, such as DMF, TMU and MPy, with conversions of monomers reaching 15%, 11% and 19% in 7 h, 56 h and 6 h, respectively; these polymerizations were relatively slower than when NMP was used as a ligand. In order to further probe the ability of NMP as a ligand in ATRP, the same experiments were conducted in the presence of a catalytic amount of NMP ([MMA]₀/[NMP]₀ = 10 : 1), and base was also added into the reaction to compare with the other systems. From Table 3, it can be seen that the resulting polymer reached a conversion of 33% in just 3 h, with a value of M_n,GPC which was pretty similar to M_n,th, and also had a narrow MWD (M_w/M_n = 1.21) (entry 11). Thereafter, the conversion reached 68% in 6.7 h, with a well-controlled result (entry 12).

Table 3 FeBr₂-catalyzed ATRPs of MMA using polar solvents based on pyrrolidone as ligands^a

Entry	Solvent	[M]0/[solvent]0	Base	Time (h)	Conv. (%)	Mn,th (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn
a [MMA]0/[EBPA]0/[FeBr2]0/[Na2CO3]0 = 200 : 1 : 1 : 2 or 0, 60 °C.
1	2-Py	2 : 1 (v/v)		27	0	NA	NA	NA
2	2-Py	2 : 1 (v/v)	Na2CO3	1.5	48	9800	19 500	1.38
3	2-Py	2 : 1 (v/v)	Na2CO3	2	74	15 000	20 900	1.35
4	2-Py	10 : 1	Na2CO3	4.4	17	3650	4650	2.23
5	MPy	2 : 1 (v/v)		6	19	4050	7200	1.33
6	MPy	2 : 1 (v/v)		27	67	13 650	16 800	1.54
7	MPy	2 : 1 (v/v)	Na2CO3	2.3	54	11 000	10 500	1.93
8	MPy	10 : 1	Na2CO3	2	40	8250	10 700	1.8
9	NMP	2 : 1 (v/v)		7	33	6850	4300	1.39
10	NMP	2 : 1 (v/v)		23	45	9250	5700	1.42
11	NMP	10 : 1	Na2CO3	3	33	6850	6700	1.21
12	NMP	10 : 1	Na2CO3	6.7	68	13 850	12 300	1.22

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2-Py and MPy contain an amide group (see Scheme 2), and have ringed structures, similar to NMP. They were chosen to test the activity as ligands as well. As seen from entries 1–8 in Table 3, when the polymerization was carried out with a ratio of [MMA]₀/[polar solvent]₀ of 2 : 1 (v/v) in the absence of Na₂CO₃, there was no polymerization in 27 h for 2-Py, but for MPy, the conversion reached up to 19% in 6 h. A possible explanation for this is that the methyl group of MPy increases the ability of the amide group to coordinate to the iron salt, and makes it express higher “living” features. This explanation is also applicable to NMP. The polymerizations using 2-Py showed poor controllability, presumably caused by its poor dissolution in MMA. In the absence of base, the resulting polymers when MPy was used showed higher M_n,GPC values than the theoretical molecular weights, which may have arisen from low initiator efficiency. When the base was added into the polymerizations, the experimental molecular weights behaved well, but the values of M_w/M_n were a little higher. A suitable amount of base needs to be chosen for the ATRP of MMA using MPy as a ligand.

The obtained results above demonstrated the potential of using amide group-containing polar solvents for Fe(II)-catalyzed ATRP of MMA. The addition of base was essential for most of the systems. The role of the base has been stated above. The selected polar solvents act as ligands that increase the dissolubility of FeBr₂ in MMA, and also adjust the catalytic activity, which results in the good living features of the polymerizations. It’s particularly interesting that polymerization can be carried out in the absence of base when using DMF, TMU, NMP and MPy. As mentioned above, all of them have an electron donor (methyl group) near or on the amide group, and this can increase the activity of the solvents by enhancing the solvents’ coordination to iron salts. For TMU in particular, the four methyl groups around the oxygen atom greatly heighten the activity. As a result, the polymerizations have higher rates and the resultant polymers are well-controlled in the presence of TMU. This conclusion has potential application in the choice of suitable polar solvents for CRP. Moreover, the configurations of the different solvent molecules may also influence their activities, which needs further investigation.

Iron(III)-catalyzed AGET ATRP

AGET ATRP is easier to operate than normal ATRP on account of using a high oxidation state catalyst, and has similar “living” characteristics to ATRP when suitable reducing agents (RAs) are added. The iron(III)-catalyzed AGET ATRP of MMA using different reducing agents in the presence of some polar solvents (DMF, NMP and MeCN) has been reported in our previous studies,^74,75 and most of the polymerizations showed well-controlled features. To identify the activity of some other polar solvents based on the amide group from this article, three solvents (MFo, TMU and 2-Py) were selected for the FeBr₃-catalyzed AGET ATRP of MMA, using ethylene glycol (EG) or vitamin C (VC) as the reducing agent. It can be seen from Table 4 that all of the polymerizations were conducted successfully with an additional limited amount of base. The polymerization rate with VC was obviously faster than that with EG, because of the stronger reducing ability of VC compared with EG. Some of the resulting polymers showed well-controlled features, such as those in entries 2, 3 and 4 with MWD values of 1.38, 1.23 and 1.37, respectively, and with little deviation of M_n,GPC with respect to M_n,th at the same time. As for the other instances shown in Table 4 (entries 1, 5 and 6), the differences between the molecular weights from experiment and from theory were a little big. All of them had the phenomenon that some precipitates formed during these reactions, which caused the nonuniformity of the systems. In spite of this, the chosen amide group-containing polar solvents have potential for conducting iron(III)-catalyzed AGET ATRP. The results here further express the universality of the selected polar solvents used for CRP.

Table 4 FeBr₃-catalyzed AGET ATRPs of MMA using amide group-containing polar solvents as ligands^a

Entry	Solvent	RA^b	Time (h)	Conv. (%)	Mn,th (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn
a [MMA]0/[EBPA]0/[FeBr3]0/[Na2CO3]0 = 200 : 1 : 1 : 2, 60 °C.b The ratio of RA to catalyst is 3 : 1 for EG, and 1 : 1 for VC.
1	MFo	EG	6.1	80	16 300	27 580	1.59
2	MFo	VC	2.7	70	14 250	16 500	1.38
3	TMU	VC	15	53	10 850	8600	1.23
4	2-Py	EG	8.5	17	3650	3550	1.37
5	2-Py	EG	12	77	15 650	21 850	1.46
6	2-Py	VC	1.1	62	12 650	24 250	1.55

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Chain end analysis and chain extension experiment

The chain end of the PMMA prepared from the FeBr₂-catalyzed ATRP in the presence of a limited amount of TMU was analyzed by ¹H NMR spectroscopy, as shown in Fig. 3. The signal labelled ‘a’ (3.77 ppm) was attributed to the methyl ester group at the chain end, and the signal ‘b’ (3.60 ppm) came from the other methyl ester groups in PMMA. The signals ‘c’ (3.38 ppm), ‘d’ (7.17–7.37 ppm) and ‘e’ (3.97–4.17 ppm) corresponded to the protons derived from EBPA in methine, phenyl and methylene groups. The molecular weight (M_n,NMR) can be determined by the integrals in the ¹H NMR spectrum based on eqn (1):

M_n,NMR (g mol⁻¹) = (I_a,b/3) × 100.12/(I_e/2) + 243.1 (1)

Fig. 3 ¹H NMR spectrum of PMMA (M_n,GPC = 7200, M_w/M_n = 1.45) with CDCl₃ as the solvent. [MMA]₀/[EBPA]₀/[FeBr₂]₀/[TMU]₀/[Na₂CO₃]₀ = 200 : 1 : 1 : 20 : 2, 60 °C.

The calculated molecular weight of PMMA from the ¹H NMR spectrum (M_n,NMR = 6300) is in agreement with the GPC result (M_n,GPC = 7200). The result suggested that the end of the obtained PMMA was end-capped by the EBPA moieties.

In order to further confirm the mechanism of ATRP carried out using polar solvents as ligands, a chain extension experiment was done using one of the resulting polymers as a macroinitiator. The macroinitiator (PMMA–Br, M_n,GPC = 10 400, M_w/M_n = 1.29) came from the ATRP with a ratio of [MMA]₀/[EBPA]₀/[FeBr₂]₀/[NMP]₀/[Na₂CO₃]₀ = 200 : 1 : 1 : 20 : 2 at 60 °C with a conversion of 61%, and the chain-extended polymer (PMMA-b-PMMA–Br) was obtained from an ATRP with a ratio of [MMA]₀/[PMMA–Br]₀/[FeBr₂]₀/[NMP]₀/[Na₂CO₃]₀ = 500 : 1 : 1 : 50 : 5 at 60 °C in 1 h. As shown in Fig. 4, a peak shift can be seen from the macroinitiator to the chain-extended PMMA with M_n,GPC = 30 100 and M_w/M_n = 1.40. The increased value of M_w/M_n may be due to a little of the polymer being out of activity in the macroinitiator. The successful chain extension reaction confirms the controlled features of the polymerizations.

Fig. 4 GPC curves for the chain extension experiment.

Conclusions

In summary, iron(II)-catalyzed ATRPs of MMA were carried out in a series of amide group-containing polar solvents without additional ligands. Most of the systems showed characteristics of “living”/controlled radical polymerization in the presence of a catalytic amount of base. The results here confirmed the coordination potential of polar solvents containing the amide group to iron salts, and the formed catalyst complexes showed good activity. The structures of the polar solvents had a great effect on their coordination abilities, which is very important for the polymerization activity. The addition of an electron donor (methyl group) in the structures improved the solvents’ activity significantly, which can be seen from the results when DMF, TMU, MPy and NMP were used. It is particularly interesting that the polymerizations remained controlled even when the polar solvents were reduced to a limited amount (the initial molar ratio of catalyst to solvent was 1 : 20), which is of great significance for green applications. Some polar solvents were also employed for iron(III)-catalyzed AGET ATRP of MMA; as expected, they behaved well, similarly as for ATRP. More details about the mechanism of the coordination between polar solvents and the metal salt, and work expanding the scope of polar solvents applied for CRP, still need to be investigated in future studies.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (21304036, 51210004, 51433002, and 51473056), and the Specialized Research Fund for the Doctoral Program of Higher Education (20120142120024) for support of this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05460e

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