Active, effective, and “green” iron(III)/polar solvent catalysts for AGET ATRP of methyl methacrylate with various morphologies of elemental silver as a reducing agent

Abstract

Different morphologies of elemental silver were employed as the reducing agent and polar solvents were used as both the solvent and ligand for the iron-catalyzed activators generated by electron transfer atom transfer radical polymerization (AGET ATRP) of methyl methacrylate. The polymerizations showed a high reaction rate and good control when silver nanowires with high surface area were used. The iron-catalyzed polymerizations of MMA were also processed at room temperature or even at 0 °C. Additionally, silver can act as a supplemental activator in the generation of radicals from the initiator, ethyl 2-bromo-2-phenylacetate, which may be the non-negligible reason for the deviated molecular weights.

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1. Introduction

Atom transfer radical polymerization (ATRP) is the most commonly utilized reversible deactivation radical polymerization (RDRP) method, which has been used to achieve control over polymer chain growth and architecture with predetermined molecular weight (M_n), narrow molecular weight distribution (MWD), and useful end-functionalities.^1–9 ATRP is typically mediated by intermittent and repeated activation/deactivation cycles to ensure the growth of the polymer chains at a same rate.

In the past two decades, various catalytic systems for ATRP based on copper,^10–13 ruthenium,^14–21 iron,^8,9,22–28 other transition metals,^29–32 and metal-free catalysts^33–36 have been developed. Among them, iron catalysts have attracted extensive attention due to their particular and irreplaceable features in environmental and sustainable aspects, although iron complexes were generally considered to be inferior to copper complexes for polymerization control. Iron is an extremely abundant metal with low toxicity in the earth's crust, and has an important effect in living process, for example, as a cofactor for metalloproteins.^37–40 Most importantly, iron can be easily obtained with its low price, resulting in a wide range of applications of iron in inorganic chemistry, organic chemistry, and polymer chemistry.^8,41 In addition, as a chemical element belongs to group 8, iron exists in a wide range of oxidation states from −2 to 6. Iron(0) often coordinates 5 or 6 ligands with trigonal bipyramidal and octahedral geometry. In the iron-catalyzed ATRP, the catalyst activity can be tuned by a variety of ligands. Since the first example of iron complex for ATRP,⁴² various ligands have been developed for these complexes.⁹ Phosphorus-containing ligands are the most extensively used ligands for the formation of iron complexes. Some polar solvents containing a lone pair electron on atoms such as N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), acetonitrile (MeCN), and dimethyl sulfoxide (DMSO) can be used as ligand candidates to coordinate with iron salts.^43–49 Low molecular weight polyethylene glycol (PEG) has also been used as a new kind of environmentally friendly and less expensive solvent in organic synthesis, biphasic catalysis and polymerization. PEG can coordinate with some metal centers to form complexes,⁵⁰ and thus the highly efficient ATRP using PEG as both the solvent and ligand was performed successfully.^51,52

Generally, a high concentration of lower oxidation metal catalyst is required in ATRP to achieve good control over polymerization. This necessity is due to the unavoidable radical–radical termination, which results in a buildup of deactivator species and therefore a decrease of low-valent metal catalyst for activation. Besides that, the handling or preservation of the lower oxidation state transition metal is also a difficulty. In an effort to develop more sustainable, “greener” catalytic systems, several improved ATRP methods using higher oxidation state iron catalysts were developed to facilitate their industrial process, including A(R)GET (activators (re)generated by electron transfer) ATRP,^53–55 ICAR (initiators for continuous activators regeneration) ATRP,^56–59 SARA (supplemental activators and reducing agents) ATRP,^56,60 photo-induced ATRP,⁴⁹ as well as electrochemically mediated ATRP (eATRP).^61,62 Among these, AGET ATRP utilizes reducing agents such as ascorbic acid (AsAc), tin^II 2-ethylhexanoate (Sn(EH)₂), alcohol or zerovalent metals to react with the higher oxidation state catalyst and generate the lower oxidation state catalyst in situ. It should be noted that the catalyst concentration can be reduced to the ppm level without sacrificing the controllability with careful and delicate selection of polymerization conditions and components.

Recently, Matyjaszewski and coworkers reported copper-catalyzed ARGET ATRP of various acrylates and methacrylates using elemental silver (Ag⁰) wire as the reducing agent.^63,64 Very good control over polymerization was observed and highly regular block copolymers were synthesized. The reduction with Ag⁰ is like one electron process because Ag⁰ only has two common oxidation states (0 and +1). Both Ag⁰ and the oxidized species Ag^IBr are insoluble in the reaction media, which diminish the side reactions. The inert Ag⁰ can minimize or eliminate undesirable radical generation or termination events that are commonly observed in SARA ATRP. Considering the advantages of AGET ATRP, iron complex, “green” solvent, and Ag⁰ reducing agent, in the present work, we employed Ag⁰ with various morphologies as reducing agent for the iron-catalyzed ATRP of methyl methacrylate (MMA) in polar solvents for the first time (Scheme 1). The polymerizations showed the features of controlled/“living” radical polymerization in PEG-400 and DMF using ethyl 2-bromo-2-phenylacetate (EBPA) as an initiator, FeBr₃ as the catalyst without any additional ligand. Importantly, the reaction could be achieved at room temperature, or even at 0 °C.

Scheme 1 Proposed mechanism of iron-catalyzed ATRP in the presence of Ag⁰.

2. Experimental

2.1 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 atmosphere. 1,3-Dimethyl-2-imidazolidinone (DMI, 99%, J&K), N-methylpyrrolidone (NMP, 95+%, Sinpharm), N,N-dimethylformamide (DMF, 99.5+%, Sinpharm), and acetonitrile (CH₃CN, 99+%, Sinpharm) were dried over CaH₂ and distilled under reduced pressure. Polyethylene glycol 400 (PEG-400) was purchased from TCI. Ethyl 2-bromo-2-phenylacetate (EBPA, 95%, Alfa Aesar), iron(III) bromide (FeBr₃, 98+%, Alfa Aesar), sodium carbonate (Na₂CO₃, Sinpharm), Ag wire (AgW, 99%, Aladdin), and Ag powder (AgP, 99.9%, Aladdin) were used without further purification. Silver nanowire (AgNW) was synthesized according to reported literature.^65,66

2.2 Characterization

¹H NMR spectroscopy was performed by a Bruker AV400 NMR spectrometer with tetramethylsilane (TMS) as the standard. The number-average molecular weight (M_n,GPC) and molecular weight distribution (MWD, M_w/M_n) of the polymers were determined using an Agilent 1100 gel permeation chromatograph (GPC) using 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. UV-Vis/NIR spectroscopy was performed on an Evolution 220 UV-Visible spectrophotometer using DMF as the solvent at ambient temperature.

2.3 Polymerization procedures

A typical AGET ATRP procedure with the molar ratio of [MMA]₀ : [EBPA]₀ : [FeBr₃]₀ : [solvent]₀ : [Ag⁰]₀ = 200 : 1 : 1 : 20 : 2.5 in the absence of air is as follows: a mixture was obtained by adding FeBr₃ (83.6 mg, 0.283 mmol), Ag⁰ (75 mg, 0.707 mmol) to a dried ampule. The flask was sealed with a rubber septum and was cycled three times between vacuum and argon (Ar) to remove oxygen. Degassed polar solvents, 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, 0.283 mmol) was added, and the flask was immersed in a thermostated oil bath. After the desired polymerization time, samples were withdrawn from the flask with a degassed syringe. The monomer conversion was determined gravimetrically after the removal of 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 GPC characterization.

2.4 Chain extension

A predetermined quantity of PMMA (obtained by AGET ATRP of MMA) with a molar ratio of [MMA]₀ : [PMMA]₀ : [FeBr₃]₀ : [Ag⁰]₀ : [solvent]₀ = 500 : 1 : 1 : 2.5 : 50 was added to a dried ampule, and then a predetermined quantity of MMA, FeBr₃, Ag⁰, and solvent was added. The rest of the procedure was the same as is described above. The chain extension polymerization was carried out under stirring at 60 °C.

3. Results and discussion

3.1 AGET ATRP of MMA in different polar solvents

In iron-mediated ATRP, the coordinating ligand allows starting an ATRP with a better solubility of the transition-metal salt in the organic media, which is realized by the in situ generation of metal complexes in the process appropriate reactivity and dynamics for the atom transfer; appropriate ligands can even reduce the transition metal catalyst amounts or allow a polymerization to be conducted at ambient temperature. Among the various ligands, polar solvent provides a low cost and environmentally friendly ATRP process. The influence of different polar solvents on the polymerization was thus investigated. As shown in Table 1, when PEG-400 was utilized, a controlled polymerization could be obtained (entry 5 in Table 1). The polymerizations were carried out at 60 °C with the ratio [MMA]₀ : [EBPA]₀ : [FeBr₃]₀ : [Ag wire]₀ = 200 : 1 : 1 : 2.5, MMA/solvent = 10/1, without any additional ligand. After 4 h and 9 h, the conversions had reached 78.7% and 77.3% in DMF and PEG-400, respectively. The molecular weights of the obtained PMMA agreed with the theoretical values, and the M_w/M_n values remained low, particularly for the results in PEG-400 (M_w/M_n = 1.10–1.19). However, when MeCN, NMP and DMI were used as the solvent, the polymerizations were not controlled well. The conversions reached 45%, 62.8%, 84% with a molecular weight of 17 600, 29 700 and 26 000, respectively, which are higher than the theoretical values, whereas the M_w/M_n values were also as high as 1.67, 1.69 and 1.46. Taking the polymerization rate and controllability into consideration, DMF and PEG-400 were selected as the ligands for our following polymerizations.

Table 1 AGET ATRP of MMA in different polar solvents without any additional ligands^a

Entry	Polar solvent	Reducing agent	Time (h)	Conv. (%)	Mn,th	Mn,GPC	Mw/Mn
a [MMA]0 : [EBPA]0 : [FeBr3]0 : [AgW]0 : [solvent]0 = 200 : 1 : 1 : 2.5 : 20, T = 60 °C.
1	MeCN	AgW	16.0	45.0	9300	17 600	1.67
2	NMP	AgW	20.0	62.8	12 800	29 700	1.69
3	DMI	AgW	4.0	84.0	17 000	26 000	1.46
4	DMF	AgW	4.0	78.7	16 000	20 000	1.48
5	PEG-400	AgW	9.0	77.3	15 800	18 800	1.11

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3.2 AGET ATRP of MMA with different morphologies of Ag⁰

It has been reported that the reaction rate in Ag AGET ATRP was dependent on the silver surface area-to-solution volume ratio (SA/V) and not on the total amount of silver used.^63,64 Thus, the reaction rate could feasibly be increased by either increasing the surface area of silver or decreasing the total reaction volume. Because of this, we suppose that using smaller sizes of Ag⁰ play an active role in the reduction efficiency and reaction rate. Accordingly, three separate experiments were conducted with varying morphologies of silver: Ag wire (AgW), Ag powder (AgP) and Ag nanowire (AgNW). The morphologies of the AgP and AgNW were examined by TEM and SEM, and typical results are shown in Fig. 1. It can be seen from Fig. 1b that the synthesized AgNWs have lengths between 5 and 10 μm and diameters between 80 and 150 nm.

Fig. 1 (a) SEM image of AgP. (b) SEM image of AgNW. Insert is TEM image of AgNW.

The polymerization kinetic plots for the iron-based AGET ATRP of MMA with a molar ratio of [MMA]₀ : [EBPA]₀ : [FeBr₃]₀ : [Ag⁰]₀ : [solvent]₀ = 200 : 1 : 1 : 2.5 : 20 are depicted in Fig. 2a. It can be seen from Fig. 2a that polymerizations proceeded with approximately first-order kinetics in both cases, indicating a constant concentration of growing radicals during polymerization. As anticipated, as the surface area of silver was increased, the induction period evidently declined, and the rate of polymerization similarly increased, suggesting the involvement of silver in the rate-determining step of the reaction. Evolution of M_n,GPC and M_w/M_n versus conversion (Fig. 2b) shows that M_n,GPC values of the resulting polymers increased linearly with monomer conversion and polydispersity remained relatively low (M_w/M_n < 1.30), except for AgW in DMF (M_w/M_n < 1.5). As the surface area increased, the controllability also improved accordingly. It was observed that M_n,GPC values slightly deviated from the corresponding theoretical ones. This phenomenon could be attributed to a high radical concentration at the beginning of polymerization and the to the fact that it needs time to establish a dynamic equilibrium between the Fe(II) and Fe(III) species, which results in a radical termination. In addition, a faster polymerization rate was observed in DMF than in PEG-400, which was independent of the silver morphologies, and yet, the controllability in PEG-400 was better than in DMF with a lower polymerization index and closer molecular weight. Fig. 3 shows GPC curves of the resulting PMMAs under the two different polar solvents mentioned above using AgP as the reducing agent. The GPC peaks are both monomodal, but only with a small shoulder on the low molecular weights side.

Fig. 2 (a) Kinetics (b) evolution of M_n and M_w/M_n with conversion in the ATRP of MMA with various morphologies of silver in DMF or PEG-400. Reaction conditions: [MMA]₀ : [EBPA]₀ : [FeBr₃]₀ : [Ag⁰]₀ = 200 : 1 : 1 : 2.5, [MMA]/[solvent] = 10/1 at 60 °C. ▲ AgNW/DMF; ★ AgNW/PEG-400; ● AgP/DMF; ◆ AgP/PEG-400; ■ AgW/DMF; ▼ AgW/PEG-400.

Fig. 3 GPC curves for AGET ATRP of MMA using AgP as the reducing agent with molar ratio of [MMA]₀ : [EBPA]₀ : [FeBr₃]₀ : [Ag⁰]₀ : [solvent]₀ = 200 : 1 : 1 : 2.5 : 20 (a) DMF (b) PEG-400 without any additional ligand. T = 60 °C.

It has been reported by Zhu et al. that a suitable amount of inorganic bases can have a dual enhancement of polymerization rate and controllability over molecular weights of the obtained polymers.^54,67

In this work, the catalytic amount of Na₂CO₃ was used as the additives. The results are listed in Table 2. It can be seen that the polymerization rate in the presence of Na₂CO₃ was enhanced when compared with that without base. In all cases, the molecular weight distributions were narrow (M_w/M_n < 1.30).

Table 2 Effect of Na₂CO₃ on AGET ATRP of MMA with AgP or AgNW in DMF or PEG-400^a

Entry	Polar solvent	Reducing agent	Base	Time (h)	Conv. (%)	Mn,th	Mn,GPC	Mw/Mn
a [MMA]0 : [EBPA]0 : [FeBr3]0 : [Ag^0]0 : [Na2CO3]0 : [solvent]0 = 200 : 1 : 1 : 2.5 : 2 : 20, T = 60 °C.
1	DMF	AgP	Na2CO3	1.0	79.7	16 200	22 400	1.27
2	DMF	AgP	NA	3.0	70.7	14 400	19 100	1.24
3	DMF	AgNW	Na2CO3	1.5	73.6	15 000	22 200	1.25
4	DMF	AgNW	NA	4.0	54.0	11 100	12 900	1.20
5	PEG-400	AgP	Na2CO3	1.5	74.9	15 200	19 300	1.12
6	PEG-400	AgP	NA	2.5	66.7	13 600	18 300	1.14
7	PEG-400	AgNW	Na2CO3	1.3	87.0	18 500	27 300	1.19
8	PEG-400	AgNW	NA	1.7	84.5	17 200	21 900	1.17

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3.3 AGET ATRP of MMA at ambient temperature

For an AGET ATRP process, the regeneration of low oxidation state metal catalyst depends on the redox reaction between higher oxidation state catalyst and reducing agent. In the polymerization mentioned earlier, however, at low conversions the obtained molecular weight values were a little bit greater than the theoretically predicted values for a living polymerization. One possible explanation for this observation could be the rapid regeneration of Fe(II) and radical in the very early stages of polymerization at a higher temperature, which would result in larger than expected molecular weight values but still allow for linear evolution of molecular weight. In addition, compared to copper-catalyzed ATRP, the iron-catalyzed ATRP have received less attention due to the lower activity of iron(II) or iron(III) complex. The polymerizations catalyzed by iron need to be carried out at high temperatures for obtaining a rapid polymerization rate. Considering the environmentally benign, active, and effective catalytic systems for controlled radical polymerizations, the improvement in the activity of less toxic and ubiquitous iron complexes captured our attention. Thus, the polymerization temperature was lowered to 25 °C for modulating the Fe/Ag reduction equilibrium. It can be seen from Fig. 4 that the polymerization kinetics showed linear plots, indicating that the polymerizations were approximately first order with respect to the monomer concentration and the number of active species remained nearly constant during the polymerization process. It was also observed that the polymerization rate at 25 °C was slower than that at 60 °C. Notably, the conversion arrived to 71.5% at 25 °C in 1.92 h in the case of polymerization with AgNW as the reducing agent and PEG-400 as the ligand. M_n,GPC values of the resulting polymers with AgP and AgNW increased nearly linearly with monomer conversion and relatively low molecular weight distributions (M_w/M_n < 1.38) were also obtained. However, the M_n,GPC values largely deviated from the corresponding theoretical ones. The poor solubility of the catalyst at low temperature resulted in a decreased initiation efficiency. Moreover, some zero valent metals can be directly involved in radical generation.^51,55 To verify the direct activation by Ag⁰, the polymerization of MMA with an EBPA initiator was therefore carried out in the absence of iron salt. As shown in Table 3, the polymerization in DMF was quite fast, with 12.4% and 22.7% conversion obtained after 1 h using AgP and AgNW as the reducing agent, respectively. Molecular weight values were quite high relative to the predicted values for standard controlled radical polymerization, and MWD values ranged from 1.98 to 3.23, indicative of radical generation by Ag⁰ followed by standard free radical polymerization in the absence of a deactivator complex. The same result was obtained in the case of polar solvents not being added. The use of Ag⁰ with a larger surface area may improve the surface activity greatly. Based on these reasons, the polymerization at 25 °C showed a relatively poor controllability.

Fig. 4 (a) Kinetics (b) evolution of M_n and M_w/M_n with conversion in the ATRP of MMA with various sizes of silver in DMF or PEG-400. Reaction conditions: [MMA]₀ : [EBPA]₀ : [FeBr₃]₀ : [Ag⁰]₀ = 200 : 1 : 1 : 2.5, [MMA]/[solvent] = 10/1 at 25 °C. ▼ AgNW/PEG-400; ▲ AgP/PEG-400; ● AgNW/DMF; ■ AgW/DMF.

Table 3 Polymerization of MMA with Ag⁰-mediated AGET ATRP^a

Entry	Polar solvent	Reducing agent	Time (h)	Conv. (%)	Mn,GPC	Mw/Mn
a [MMA]0 : [EBPA]0 : [Ag^0]0 : [solvent]0 = 200 : 1 : 1 : 2.5 : 20, T = 60 °C.
1	DMF	AgP	1.0	12.4	322 500	1.98
2	DMF	AgNW	1.0	22.7	618 800	3.19
3	NA	AgP	6.0	28.8	336 300	3.23
4	NA	AgNW	6.0	62.7	929 900	2.37

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To indicate the activity of polymerization with Ag⁰ at a low temperature, different reducing agents were also used to proceed the AGET ATRP polymerization of MMA. The results are listed in Table 4. The polymerizations were quite slow when common reducing agents such as VC and Sn(EH)₂ were used, with only 13.8% and 18.3% conversion obtained in over ten hours. In addition, the polymerizations of MMA with AgP were also run at 0 °C in DMF or PEG-400 (entries 8 and 9). The polymerizations were slow, with only 7.8% and 16.8% conversions obtained after 6 h in DMF and PEG-400, respectively. The molecular weights were also quite high relative to the predicted values. However, the resulting polymer had relatively low dispersity (M_w/M_n < 1.2). This result is consistent with a controlled polymerization, although the controllability is far from ideal when compared with that of the system for 60 °C or 25 °C.

Table 4 Effect of different reducing agents on the polymerization of MMA at ambient temperature^a

Entry	Polar solvent	Reducing agent	Temp (°C)	Time (h)	Conv. (%)	Mn,th	Mn,GPC	Mw/Mn
a [MMA]0 : [EBPA]0 : [FeBr3]0 : [reducing agent]0 : [Na2CO3]0 : [solvent]0 = 200 : 1 : 1 : 2.5 : 2 : 20.
1	DMF	AgP	25	3.0	67.1	13 700	45 300	1.47
2	DMF	AgNW	25	3.0	77.4	15 800	63 500	1.31
3	PEG-400	AgP	25	2.4	74.5	15 200	31 900	1.46
4	PEG-400	AgNW	25	1.5	67.5	13 800	28 300	1.32
5	DMF	VC	25	17.0	13.8	3000	8700	1.07
6	DMF	Sn(EH)2	25	3.5	11.9	2700	11 100	1.13
7	DMF	Sn(EH)2	25	18.5	18.3	3900	13 600	1.15
8	DMF	AgP	0	6.0	7.8	1800	18 600	1.17
9	PEG-400	AgP	0	6.0	16.8	3600	25 200	1.15

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3.4 Reducing ability of different morphologies of Ag⁰

As mentioned above, as the surface area of Ag⁰ increased, the polymerization rate also increased accordingly, which indicates an enhanced reducing ability in the case of the high surface area of Ag⁰. Thus, UV-Vis/NIR spectroscopy was used to analyze the reduction progress in the presence of different morphologies of Ag⁰ as the reducing agents at room temperature. The absorption bands of Fe(III) complex between 680 and 735 nm decreased strongly after addition of Ag⁰ (Fig. 5a, c and e). The absorbance dropped off over time, indicating the reduction process.

Fig. 5 UV/Vis/NIR spectroscopic study of 30 mM FeBr₃ in DMF with different morphologies of Ag⁰: (a) AgW (c) AgP (e) AgNW as the reducing agent at room temperature. (b–f) Change of absorbance corresponding to Fe^III and Fe^II.

The reduction rate increased with decreasing size of Ag⁰. The full transition of Fe(III) to Fe(II) took 100 min, 15 min and 10 min for AgW, AgP, and AgNW, respectively, which was consistent with the polymerization rate. The broad absorbance of Fe(II), which could be observed at 2000 nm, appeared immediately after the addition of reducing agent and continued to increase in the entire process, and reached a maximum at the end. Fig. 5b, d and f vividly display the reduction process with the decreasing Fe(III) and increasing Fe(II).

3.5 Chain end analysis and chain extension experiment

The chain end of the PMMA prepared using EBPA as the initiator, FeBr₃ as the catalyst and Ag⁰ as the reducing agent without any additional ligand was analyzed by ¹H NMR spectroscopy, and is shown in Fig. 6. The signal of a (3.80 ppm) was attributed to the methyl ester group, and the signal of b (3.61 ppm) indicated the other methyl ester groups. The signals of c (3.38 ppm), d (7.17–7.37 ppm) and e (3.97–4.17 ppm) corresponded to the protons derived from EBPA with methine, phenyl, and methylene. The molecular weight (M_n,NMR) can be determined by the integrals in the ¹H NMR spectrum based on eqn (1) as follows:

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

Fig. 6 ¹H NMR spectrum of PMMA (M_n,GPC = 11 000, M_w/M_n = 1.17) with CDCl₃ as the solvent. [MMA]₀/[EBPA]₀/[FeBr₃]₀/[PEG-400]₀/[AgP]₀ = 200 : 1 : 1 : 20 : 2.5, 60 °C.

The calculated molecular weight of PMMA from the ¹H NMR spectrum (M_n,NMR = 10 100),which was in agreement with GPC result (M_n,GPC = 11 300). This result suggests that the end of the obtained PMMA was end-capped by the EBPA moieties.

To determine whether acceptable retention of chain-end functionality was achieved, the chain extension experiment was carried out using the resulting polymers as the macroinitiator. The macroinitiator (PMMA-Br, M_n,GPC = 17 800, M_w/M_n = 1.16; M_n,GPC = 20 400, M_w/M_n = 1.23) came from the ATRP with a ratio of [MMA]₀/[EBPA]₀/[FeBr₃]₀/[AgP]₀/[solvent]₀ (PEG-400 or DMF) = 200 : 1 : 1 : 2.5 : 20 at 60 °C, and the chain extended polymer (PMMA-b-PMMA) was obtained from the ATRP with a ratio of [MMA]₀/[PMMA-Br]₀/[FeBr₃]₀/[AgP]₀/[solvent]₀ = 500 : 1 : 1 : 2.5 : 50 at 60 °C. As shown in Fig. 7, a peak shift can be seen from the macroinitiator to the chain extended PMMA with increased M_n,GPC. Certainly, the chain extension polymerization still had good activity. The successful chain extension reaction confirms the controlled features of the polymerizations using Ag⁰ as the reducing agent.

Fig. 7 GPC curves for chain extension experiment using AgP as the reducing agent in (a) PEG-400 (b) DMF without any additional ligand.

4. Conclusion

A facile and highly efficient iron-catalyzed AGET ATRP of MMA in the absence of any additional ligands was developed using FeBr₃ as the catalyst, Ag⁰ as the reducing agent and polar solvent for both the solvent and ligand. The use of silver as a reducing agent in ATRP represented a significant advance and the effect of different morphologies of Ag⁰ were tested. As the surface area of silver was increased, the polymerization rate and controllability improved. In addition, this novel iron-catalyzed AGET ATRP system can also be processed at room temperature, even at 0 °C, although the M_n,GPC was relatively larger than the ideal controlled polymerization. These facts fully embody the advantage of using Ag⁰ as the reducing agent and make it possible to carry out low temperature reaction of iron catalyst.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (Grant No. 21304036, 51622303, 51210004, and 51473056) for support of this work.

Notes and references

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