Facile and highly efficient “living” radical polymerization of hydrophilic vinyl monomers in water

Abstract

In this work, well-defined polymerization of water soluble poly(ethylene glycol) monomethyl ether methacrylate (PEGMA), 2-hydroxyethyl methacrylate (HEMA), 2-(dimethylamino)ethyl methacrylate (DMAEMA) and N,N-dimethyl-acrylamide (DMA) were successfully conducted in water by a facile and efficient polymerization system, only including oxidatively stable copper(II) acetate and 1-cyano-1-methylethyl diethyldithiocarbamate (MANDC). The effects of temperature, copper concentration, and monomer concentration on polymerization of PEGMA were systematically investigated to optimize the polymerization conditions. The polymerization of PEGMA can be conveniently carried out with ppm levels of the copper catalyst at 30–70 °C. The linearity of the kinetic plots, linear increase of molecular weight with conversion, and narrow molecular weight distribution (M_w/M_n < 1.3) of the polymer showed the typical character of “living” radical polymerization (LRP). Chain-extension reactions further verify the “living” features of this polymerization system.

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1. Introduction

Poly(poly(ethylene glycol) monomethyl ether methacrylate) (PPEGMA),¹ poly(2-hydroxyethyl methacrylate) (PHEMA),² poly(2-(dimethylamino)ethyl methacrylate) (DMAEMA)³ and poly(N,N-dimethyl acrylamide) (PDMA)⁴ are biocompatible, hydrophilic polymers, which have received great attention in the area of biomaterials science because of their wide applications in drug delivery, gene transportation, etc. Different topologies of hydrophilic polymers including homopolymers, block copolymers, graft polymers, star polymers and dendrimers, have been synthesized and applied in biological areas⁵ and other areas.⁶

Well-defined hydrophilic polymers with predetermined molecular weights (MWs), narrow molecular weight distributions (MWDs) and designed constructions are very important for the properties of the materials. Transition metal-catalyzed “living” radical polymerization (LRP), namely atom transfer radical polymerization (ATRP), which was developed by Sawamoto⁷ and Matyjaszewski⁸ independently in 1995, is a robust and ideal technique for preparation of precise designed hydrophilic polymers. ATRP can be carried out under various polymerization conditions including bulk and solution polymerization systems.⁹ Generally speaking, volatile and noxious organic solvents are used for solution ATRP, so it's better to replace the organic solvents with “green” solvents like ionic liquid,¹⁰ supercritical fluid,¹¹ polyethylene glycol¹² or water.¹³ Among these solvents, water as a safe, low cost, environmentally friendly, and abundant solvent, is an ideal solvent for the homogenous polymerization of hydrophilic monomers. Inspired by these advantages, great efforts have been taken to conduct ATRP of hydrophilic monomers in water. However, in many cases, the controllability of the polymerizations were poor, resulting in polymers with broad MWDs and low initiator efficiency.¹³ Actually, just several successful examples were reported by aqueous ATRP.¹⁴ Recently, Matyjaszewski and co-workers reported the successful ARGET ATRP of PEGMA in water at 30 °C with large amount of expensive ligand tris(pyridin-2-ylmethyl)-amine (TPMA) and slow feeding of ascorbic acid in the presence of halide salts.^14b The molar ratio of [ligand]/[Cu], feeding rate of ascorbic acid and the amount of the halide salt had a great influence on the controllability of polymerization. Therefore, the construction of more convenient and economical well-controlled polymerization system for hydrophilic monomers in water is highly desirable.

In a typical ATRP, Cu(I) species act as the catalyst, and Cu(II) species act as the deactivator. Recently, Matyjaszewski and co-worker reported a Cu(II) species catalyzed living radical polymerization, employ acetylacetonates as the catalyst, alkyl dithiocarbamates as the pseudohalogens, well-defined poly (methyl methacrylate) (PMMA) was produced.¹⁵ Moreover, a probable Cu(II)/Cu(III) catalytic cycle was proposed for the first time in LRP. Very recently, our group developed an economic and better controlled copper(II) acetate mediated LRP system.¹⁶ This simple system only contain monomer (MMA), catalyst (oxidatively stable and low cost copper(II) acetate (Cu(OAc)₂)), and pseudohalogens (dithiocarbamates), without any additives (free radical initiators, reducing agents, and toxic ligands), well-controlled PMMA was obtained even at ppm level of Cu(II) species. In addition, compared to the common used ATRP catalyst copper(II) halides, copper(II) acetate as an halogen-free catalyst, showed higher catalytic efficiency and lower toxicity. Therefore, it's inspiring us to promote this robust and convenient system to aqueous LRP of hydrophilic monomers.

In this paper, copper(II) acetate catalyzed polymerizations of a series of hydrophilic vinyl monomers were successfully conducted in water by using 1-cyano-1-methylethyl diethyl dithiocarbamate (MANDC) as pseudohalogens. The variables about polymerization of PEGMA in aqueous media were studied. Well-defined PEGMA can be conveniently prepared in water even with ppm level of copper(II) catalyst at 30–70 °C. Chain-extension reactions verified the “living” features of this polymerization system. This powerful and simple system may have a great prospect of application in biomaterial field.

2. Experimental section

2.1. Materials

Monomers, poly(ethylene glycol) monomethyl ether methacrylate (PEGMA, average molecular weight 475 g mol⁻¹, 99%, sigma-aldrich), styrene (St, 99%, Shanghai Chemical Reagents Co. Ltd (Shanghai, China)), were passed through a neutral alumina column before use; 2-hydroxyethyl methacrylate (HEMA, 98%, TCI (Shanghai)), 2-(dimethylamino)ethyl methacrylate (DMAEMA, 99%, Energy Chemical), N,N-dimethyl acrylamide (DMA, 99%, TCI (Shanghai)), were distilled under reduced pressure prior to use. Copper(II) acetate (Cu(OAc)₂) (>98%) was purchased from Alfa Aesar and used as received. Pure water (H₂O, Hangzhou Wahaha Group Co. Ltd) was purchased from supermarket. Tetrahydrofuran (THF) (analytical reagent), N,N-dimethylformamide (DMF) (analytical reagent), methanol (MeOH) (analytical reagent), azobisisobutyronitrile (AIBN) (98%), and all other chemicals were obtained from Shanghai Chemical Reagents Co. Ltd and used as received unless mentioned. 1-Cyano-1-methylethyl diethyldithiocarbamate (MANDC) (98%), was prepared according to a previously reported literature.¹⁷

2.2. General polymerization procedure of PEGMA

A typical aqueous polymerization procedure for the molar ratio of [PEGMA]₀/[MANDC]₀/[Cu(OAc)₂]₀ = 100/1/0.5 was as follows: a mixture was obtained by adding Cu(OAc)₂ (2.2 mg, 0.012 mmol), MANDC (5.1 mg, 0.024 mmol), PEGMA (1.0 mL, 2.28 mmol), H₂O (3 mL) to a dried ampoule with a stir bar. The mixture was thoroughly bubbled with argon for 20 min to eliminate the dissolved oxygen, and then flame-sealed and transferred into an oil bath held by a thermostat at the desired temperature (30 °C, 50 °C, 70 °C) to polymerize under stirring. After the desired polymerization time, the ampoule was cooled by immersing it into iced water. Afterwards, open it and take out 20 μL mixture to NMR tube, diluted by 0.5 mL deuterium oxide (D₂O). The monomer conversion was determined by ¹H NMR spectra. The residual mixture was frozen by liquid nitrogen and freeze-dried by freeze drier.

2.3. Chain extension polymerization or block copolymerization

A predetermined quantity of PPEGMA was added into a dried ampoule, and then the predetermined quantity of PEGMA or St, and Cu(OAc)₂ were added. The ampoule was bubbled with argon for 20 min to eliminate the dissolved oxygen in the solution, and then flame-sealed and transferred into an oil bath (70 °C) or water bath (25 °C, under UV radiation) to polymerize under stirring. The rest of the procedure was the same as that for the aqueous polymerization of PEGMA described above.

2.4. Characterization

The number-average molecular weight (M_n,GPC) values and molecular weight distribution (M_w/M_n) values of PPEGMA, PPEGMA-b-PSt were determined using a TOSOH HLC-8320 gel permeation chromatograph (GPC) equipped with a refractive-index detector (TOSOH), using 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. For PHEMA, PDMAEMA, and PDMA, DMF + 0.01 mol L⁻¹ LiBr was used as an eluent at a flow rate of 0.6 mL min⁻¹ operated at 40 °C. A TOSOH HLC-8320 gel permeation chromatograph (GPC) equipped with a refractive-index detector (TOSOH), using TSKgel guardcolumn SuperAW-H and TSKgel SuperAWM-H × 2 with measurable molecular weight ranging from 1 × 10³ to 1 × 10⁶ g mol⁻¹ was used. GPC samples were injected using a TOSOH plus autosampler and calibrated with PMMA standards purchased from TOSOH. ¹H NMR spectrum was recorded on a Bruker 300 MHz nuclear magnetic resonance (NMR) instrument using D₂O or DMSO as the solvent at ambient temperature. Matrix-assisted laser desorption ionization time-of-flight mass spectra (MALDI-TOF MS) were acquired on a Bruker Ultraflex-III TOF/TOF mass spectrometer (Bruker Daltonics, Inc., Billerica, MA) equipped with a Nd:YAG laser (355 nm). All spectra were measured in positive reflection mode.

3. Results and discussion

3.1. Polymerization for various types of hydrophilic vinyl monomers

In order to assess the generality of this simple polymerization system, four typical hydrophilic vinyl monomers (PEGMA, HEMA, DMAEMA, DMA) contain different functional groups were investigated using Cu(OAc)₂ as the catalyst, MANDC as the pseudohalogen in water with the molar ratio of [monomer]₀/[MANDC]₀/[Cu(OAc)₂]₀ = 100/1/0.5 at 70 °C. It should be noted that MANDC is an oil-soluble iniferter, which does not dissolve in water, while it can be dissolved in hydrophilic vinyl monomers very well. Encouragingly, MANDC dissolves in water and monomer mixtures well at 30–70 °C except PEGMA (Fig. 1), which need much higher temperature to obtain homogenous system (50–70 °C). The plots of UV-vis transmittance versus temperature of [monomer]₀/[MANDC]₀ = 100/1 in water were shown in Fig. 2. The transmittance of four mixtures were about 100%, which further verified the establishment of homogenous polymerization systems at 55–70 °C by this simple strategy for four hydrophilic vinyl monomers. As expected (Table 1), well-controlled polymerizations of all four monomers were successfully conducted. The molecular weights of PPEGMA and PHEMA were close to their theoretical ones and the molecular weight distributions (M_w/M_ns) kept narrow, while the values of M_w/M_ns of PDMAEMA and PDMA were 1.39 and 1.34, respectively. It probably caused by the amide groups of monomers. Among these polymers, PPEGMA-based polymer is one of the most ideal biomaterials because of its biocompatibility and nontoxicity. Therefore, we focused our attention on the polymerization of PEGMA in the following investigation.

Fig. 1 Photographs of [monomer]₀/[MANDC]₀ = 100/1 in water, V_monomer = 1.0 mL; (a) 0.023 mol MANDC in 3 mL water (b) V_PEGMA/V_water = 1/3 (v/v), 30 °C; (c) V_PEGMA/V_water = 1/3 (v/v), 50 °C; (d) V_PEGMA/V_water = 1/3 (v/v), 70 °C; (e) V_DMAEMA/V_methanol/V_water = 1/1.5/1.5 (v/v), 30 °C; (f) V_HEMA/V_methanol/V_water = 1/1.5/1.5 (v/v), 30 °C; (g) V_DMA/V_water = 1/3 (v/v), 30 °C.

Fig. 2 UV-vis transmittance at 550 nm versus temperature of [monomer]₀/[MANDC]₀ = 100/1 in water, V_monomer = 1.0 mL, V_water = 3.0 mL for PEGMA and DMA; V_monomer = 1.0 mL, V_water = 1.5 mL, V_methanol = 1.5 mL for HEMA and DMAEMA.

Table 1 Polymerization of hydrophilic vinyl monomers with Cu(OAc)₂ and MANDC in water^a

Entry	Time (h)	Monomer	Con.^c (%)	Mn,th^d (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn
a Polymerization conditions: R = [monomer]0/[MANDC]0/[Cu(OAc)2]0 = 100/1/0.5; Vmonomer = 1.0 mL; Vmonomer/Vwater = 1/3 (v/v); T = 70 °C.b Vmonomer/Vmethanol/Vwater = 1/1.5/1.5 (v/v/v).c Calculated by ^1H NMR spectra (Fig. S1, ESI).d Mn,th = ([M]0/[MANDC]0) × Mw,monomer × conversion%.
1	2	PEGMA	39.3	18 700	19 700	1.08
2^b	2	HEMA	62.9	8200	10 000	1.12
3^b	4	DMAEMA	24.3	3800	5000	1.39
4	24	DMA	13.8	1400	2200	1.34

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3.2. Polymerization of PEGMA at 70 °C

The effect of monomer amount was firstly studied to identify a suitable molar ratio of [PEGMA]₀/[MANDC]₀/[Cu(OAc)₂]₀. From Table 2, well-controlled PPEGMAs with narrow MWDs were prepared with all the molar ratios of [PEGMA]₀/[MANDC]₀ from 50/1 to 1000/1, showing a wide range of designed molecular weights of PPEGMAs.

Table 2 Effect of monomer amount on polymerization of PEGMA^a

Entry	R^b	Time (h)	Conv.^c (%)	Mn,th^d (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn
a Polymerization conditions: VPEGMA = 1.0 mL; VPEGMA/Vwater = 1/3 (v/v); T = 70 °C.b R = [PEGMA]0/[MANDC]0/[Cu(OAc)2]0.c Calculated by ^1H NMR spectra.d Mn,th = ([M]0/[MANDC]0) × Mw,PEGMA × conversion%.
1	50/1/0.5	3	73.1	17 400	20 100	1.14
2	100/1/0.5	3	61.8	29 300	25 900	1.08
3	300/1/0.5	12	59.2	84 300	70 400	1.11
4	500/1/0.5	12	41.2	97 900	97 000	1.16
5	1000/1/0.5	12	21.1	100 300	105 100	1.15

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In order to further investigate the polymerization behavior of this system, the polymerization kinetics of PEGMA were first studied with different amounts of Cu(OAc)₂ at 70 °C. Fig. 3(a) shows the kinetics for polymerization of PEGMA with the molar ratio of [PEGMA]₀/[MANDC]₀/[Cu(OAc)₂]₀ = 100/1/x (x = 0.5, 0.1, 0.05). The linearity of the kinetic plots indicated that the concentration of propagation radicals kept constant during the polymerization process. The apparent rate constant of the polymerization, k^app_p (R_p = −d[M]/dt = k_p[P_n˙][M] = k^app_p[M]) could be calculated from the slopes in Fig. 3(a). The k^app_p values were 10.32 × 10⁻⁵ s⁻¹, 3.46 × 10⁻⁵ s⁻¹, and 3.28 × 10⁻⁵ s⁻¹ corresponding to x = 0.5, 0.1, 0.05, respectively, which demonstrated that the polymerization rate decreased with decreasing the amount of Cu(OAc)₂. About 15 min induction period were observed when x = 0.5 and 0.1, indicating a fast establishment of the activation-deactivation equilibrium. However, when the amount of Cu(OAc)₂ further decreased to x = 0.05 (500 ppm catalyst), the induction period was about 3 h, indicated that the equilibrium was built up much slower with small amount of catalyst. In addition, as show in Fig. 3(b), M_n,GPC values of resultant PPEGMAs increased linearly with monomer convension and were consistent with their corresponding theoretical ones. Meanwhile, the M_w/M_n values of the polymers were narrow (M_w/M_n < 1.22), indicating a well-controlled polymerization process. GPC traces of PPEGMAs were illustrated in Fig. 3(c) and (d), the distributions were unimodal and normal, showing typical character of “living” radical polymerization.

Fig. 3 ln([M]₀/[M]) as a function of time (a); average-number molecular weight (M_n,GPC) and molecular weight distribution (M_w/M_n) versus monomer conversion (b); GPC traces with conversion while x = 0.5 (c) and x = 0.05 (d) for the polymerization of PEGMA with different amount of catalyst in water. Polymerization conditions: [PEGMA]₀/[MANDC]₀/[Cu(OAc)₂]₀ = 100/1/x (x = 0.5, 0.1, 0.05); V_PEGMA = 1.0 mL, V_PEGMA/V_water = 1/3 (v/v); T = 70 °C.

3.3. Polymerization of PEGMA at 50 °C and 30 °C

It is energy-efficient and highly desirable to synthesize well-defined water-soluble polymers at lower temperature; thus, the polymerization of PEGMA was carried out with the molar ratio of [PEGMA]₀/[MANDC]₀/[Cu(OAc)₂]₀ = 100/1/0.5 at 50 °C and 30 °C. It was found that the homogenous systems could be obtained after 6 h and 12 h at 50 °C and 30 °C, respectively. As shown in Fig. 4(a), first-order kinetics can be observed, indicating the constant concentration of propagation radicals after the induction period. The k^app_p values were 0.79 × 10⁻⁵ s⁻¹ and 1.83 × 10⁻⁵ s⁻¹ at 30 °C and 50 °C, which demonstrated that the polymerization rate slowed down at lower temperature. From Fig. 4(b), M_n,GPC values increased linearly with monomer conversion and were consistent with their theoretical ones. In addition, the M_w/M_ns of the obtained PEGMAs were narrow (M_w/M_n < 1.2), revealing “living” features of the polymerization. Therefore, well-controlled polymerization of PEGMA mediated by MANDC and Cu(OAc)₂ can be conducted at 50 °C and 30 °C successfully in spite of lower polymerization rate.

Fig. 4 ln([M]₀/[M]) as a function of time (a); average-number molecular weight (M_n,GPC) and molecular weight distribution (M_w/M_n) versus monomer conversion (b) for the polymerization of PEGMA at different temperatures in water. Polymerization conditions: [PEGMA]₀/[MANDC]₀/[Cu(OAc)₂]₀ = 100/1/0.5; V_PEGMA = 1.0 mL, V_PEGMA/V_water = 1/3 (v/v).

3.4. Effect of the amount of copper(II) acetate on polymerization of PEGMA

Because of the potential toxicity and color of Cu catalyst, the obtained polymer may be polluted by the residue of Cu catalyst if a large amount of catalyst was used. Therefore, it is beneficial to biomaterial and industrial application with lower amount of metal catalyst. Table 3 shows the effect of catalyst amount on polymerization of PEGMA with different target degree of polymerization (DP). When target DP is 100, well-controlled PEGMAs can be prepared with 100 ppm of Cu(II) catalyst (Entries 1–2 in Table 3); narrow M_w/M_ns were still maintained while the M_n,GPC values were higher than the corresponding theoretical ones with 50 ppm of Cu(II) catalyst (Entries 3–4 in Table 3). Since the dispersity in ATRP is a function of the ratio of [MANDC]/[Cu(OAc)₂] in solution, increasing the target DP from 100 to 500 could improve the controllability of polymerization.^14b As expected, while the target DP increased to 500 (Entry 7 in Table 3), well-defined PEGMAs can be obtained even with 40 ppm Cu(II) catalyst. The molecular weights from GPC results were close to their corresponding theoretical ones and the M_w/M_ns were relatively narrow (M_w/M_n < 1.35), indicating a well-controlled polymerization process.

Table 3 Polymerization of PEGMA with ppm level of catalyst^a

Entry	R^b	Cu(II) (ppm)	Time (h)	Con.^c (%)	Mn,th^d (g mol^−1)	Mn,GPC (g mol^−1)	Mw/Mn
a Polymerization conditions: VPEGMA = 1.0 mL; VPEGMA/Vwater = 1/4 (v/v); T = 70 °C.b R = [PEGMA]0/[MANDC]0/[Cu(OAc)2]0.c Calculated by ^1H NMR spectra.d Mn,th = ([M]0/[MANDC]0) × Mw,PEGMA × conversion%.
1	100/1/0.01	100	12	33.5	16 000	22 700	1.09
2	100/1/0.01	100	22	88.4	42 000	42 200	1.11
3	100/1/0.005	50	29	45.5	21 700	54 500	1.17
4	100/1/0.005	50	40	62.7	29 800	72 400	1.22
5	500/1/0.1	200	24	37.9	90 000	93 500	1.16
6	500/1/0.05	100	48	31.5	74 800	68 900	1.25
7	500/1/0.03	60	48	25.7	61 000	52 600	1.32
8	500/1/0.03	60	72	58.7	139 400	177 100	1.35
9	500/1/0.02	40	72	35.9	85 300	82 700	1.35

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3.5. Chain end analysis and chain extension

The resultant four polymers were analyzed by ¹H NMR spectroscopy. From Fig. S1 (ESI†), for the obtained four polymers, the peaks of the methylene protons (at 3.76–4.12 ppm) of end group (–SC(S)N(CH₂CH₃)₂) were overlapped by the proton peaks of polymer chains except PDMA. Thus, the chain end of the PDMA (M_n,GPC = 2200 g mol⁻¹, M_w/M_n = 1.34) using MANDC as the pseudohalogen and Cu(OAc)₂ as the catalyst without any other additives was further characterized, as shown in Fig. 5(a). The chemical shifts at 2.32–3.83 ppm assigned to the methenyl protons (b in Fig. 5(a)) of PDMA chains. The chemical shifts at 2.83–3.40 ppm correspond to the methyl protons (c in Fig. 5(a)). The chemical shifts at 0.96–1.90 ppm (d in Fig. 5(a)) are attributed to methylene protons and methyl protons of PDMA chains and segments of MANDC. The peaks at 3.76–4.12 ppm are assigned to the methylene protons of end group (–SC(S)N(CH₂CH₃)₂) (a in Fig. 5(a)). Besides, the molecular weight (M_n,NMR) calculated by integral of a and c was 1900 g mol⁻¹, which was very close to the M_n,GPC value (2200 g mol⁻¹), indicating that the obtained PDMA was end-capped by (–SC(S)N(CH₂CH₃)₂) groups with high fidelity. In addition, the structure of obtained PPGEMA (M_n,GPC = 10 100 g mol⁻¹, M_w/M_n = 1.18) was also analyzed by ¹H NMR spectrum, as shown in Fig. 5(b). The chemical shifts at 3.62–4.33 ppm and 3.42 ppm correspond to the methylene protons (b in Fig. 5(b)) and methoxyl groups (c in Fig. 5(b)) of the pendant PEG brushes of PPEGMA. The chemical shifts at 0.81–2.16 ppm (d in Fig. 5(b)) are assigned to methylene protons and methyl protons of PPEGMA chains and segments of MANDC. However, the peaks of the methylene protons (a in Fig. 5(b)) of the end group (–SC(S)N(CH₂CH₃)₂) are overlapped by peak b, make them hard to be identified. Moreover, MALDI-TOF MS of PPEGMA (M_n,GPC = 12 200 g mol⁻¹, M_w/M_n = 1.12) was investigated. From Fig. 6, several kinds of fragments were observed in the spectrometry. It is probably attributed to that the PEG side chains in PPEGMA were damaged partly during the analysis, and that the PEGMA monomer is a mixture with different PEG side chain length (m = 8, 9). However, the dominant presence of cyanoisopropyl groups were found in the polymer chains (with Na cations) (Fig. 6), which indicated that the cyanoisopropyl groups of MANDC were attached to the PPEGMA chain ends. Chain-extension reactions were carried out to verify the “living” characters of the obtained PPEGMA.

Fig. 5 ¹H NMR spectra of PDMA (M_n,GPC = 2200 g mol⁻¹, M_w/M_n = 1.34) (a) and PPEGMA (b) (M_n,GPC = 10 100 g mol⁻¹, M_w/M_n = 1.18) obtained with D₂O as solvent.

Fig. 6 Matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) of PPEGMA (M_n,GPC = 12 200 g mol⁻¹, M_w/M_n = 1.12). Polymerization conditions: [PEGMA]₀/[MANDC]₀/[Cu(OAc)₂]₀ = 100/1/0.5, T = 70 °C, time = 1 h. m = 8, n = 6, MW_cal. = 68.05 + (n − 1) × 452.24 + 451.24 + 22.99.

Firstly, the chain-extension reaction was conducted at 70 °C with Cu(OAc)₂ by employing PPEGMA as the macroinitiator (M_n,GPC = 5300 g mol⁻¹, M_w/M_n = 1.22), PEGMA as the monomer. As shown in Fig. 7(a), there was an obvious increasing of M_n,GPC of PPEGMA with narrow M_w/M_n (M_n,GPC = 59 800 g mol⁻¹, M_w/M_n = 1.23) just after 5 min chain-extension reaction. In view of the ultrafast chain-extension reaction at elevated temperature, another chain-extension reaction was carried out at 30 °C. Similarly, a peak shift from the macroinitiator (M_n,GPC = 10 100 g mol⁻¹, M_w/M_n = 1.18) to the chain-extended PPEGMA (M_n,GPC = 17 800 g mol⁻¹, M_w/M_n = 1.06) was observed in Fig. 7(b). It is noted that the resultant PPEGMAs should be end-capped with (–SC(S)N(CH₂CH₃)₂) group due to iniferter agent MANDC as the pseudohalogen initiator. To further verify that, chain-extension reaction was carried out via iniferter mechanism by using the obtained PPEGMA as the macro-iniferter agent (M_n,GPC = 10 100 g mol⁻¹, M_w/M_n = 1.18), and styrene as the monomer under UV light irradiation at 30 °C. The GPC curve shifted from macro-iniferter agent to the block copolymers (PEGMA-b-PSt) (M_n,GPC = 15 800 g mol⁻¹, M_w/M_n = 1.27) (Fig. 7(c)). All these results demonstrated the “living” features of PPEGMA catalyzed by Cu(OAc)₂ using MANDC as pseudohalogen.

Fig. 7 GPC curves before and after chain-extension using PPGEMA as the macroinitiator catalyzed by Cu(OAc)₂ at 70 °C (a) and 30 °C (b) in water, and GPC curves before and after block copolymerization using PPGEMA as the macroiniferter via iniferter mechanism under UV light irradiation at 30 °C (c). Polymerization conditions: (a) [PEGMA]₀/[PPEGMA]₀/[Cu(OAc)₂]₀ = 100/1/0.5, V_PEGMA = 0.5 mL, V_water = 2.0 mL, time = 5 min, conversion = 99.4%; (b) [PEGMA]₀/[PPEGMA]₀/[Cu(OAc)₂]₀ = 100/1/0.5, V_PEGMA = 0.2 mL, V_water = 1.0 mL, time = 2 h, conversion = 18.7%; (c) [St]₀/[PPEGMA]₀ = 100/1, V_St = 0.2 mL, V_THF = 1.0 mL, time = 15 h, conversion = 72.1%.

4. Conclusions

A facile and robust living radical polymerization method for hydrophilic vinyl monomers was developed in water using oxidatively stable Cu(OAc)₂ as the catalyst and MANDC as the initiator. The results of polymerization kinetics and chain-extension reactions showed high chain-end functionality and the “living” character of the obtained polymers. The polymerization of PEGMA could be conducted at 30–70 °C even with ppm level of catalyst, which makes this convenient and robust system more attractive for actual applications, especially for the preparation of well-defined biomaterials.

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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Footnote

† Electronic supplementary information (ESI) available: Experimental section giving the polymerization conversion calculation method. See DOI: 10.1039/c4ra09439e

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