Living radical polymerization of polyacrylamide with submicrometer size by dispersion polymerization

2.1 Materials

Acrylamide and FeCl₂ were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Acrylamide was purified by recrystallization from acetone. Succinic acid (SA, AR grade) and isophthalic acid (IA, AR grade) were obtained from Chongqing Chuandong Chemical Group (Chongqin, China) and used without further purification. AIBN was purchased from Shanghai Chemical Holding Co. Ltd. (Shanghai, China) and was recrystallized twice from methanol before use. Polyvinylpyrrolidone (PVP-K30) was purchased from Shanghai Yunhong Pharmaceutical Aids and Technology Co. Ltd. (Shanghai, China). The other reagents were used as received.

2.2 Polymerization

Atom transfer radical dispersion polymerization of acrylamide was carried out in a 100-ml three-necked round-bottomed flask. The flask was equipped with a magnetic stirring bar. The molar ratio of [AM]₀/[AIBN]₀/[FeCl₂]₀/[SA]₀ was 300:1:1:2. A typical synthetic procedure is as follows: AM (0.003 mol, 0.213 g), AIBN (1×10^-5 mol, 0.0016 g), FeCl₂ (1×10^-5 mol, 0.002 g), SA (2×10^-5 mol, 0.0024 g), 10 ml ethyl acetate, 10 ml alcohol and 0.9 g PVP were added to the flask under stirring until a homogeneous solution was formed. The flask was degassed and backfilled with nitrogen three times, and then placed in an oil bath. The reactor was heated to the desired reaction temperature. The polymerization proceeded until a milky white liquid dispersion was obtained. The mixture was centrifuged. The obtained PAM was washed with alcohol several times and dried at 60°C in a vacuum oven.

2.3 Characterization

The monomer conversion was determined by gravimetric analysis. The number-average molecular weights (M_n,GPC) and molecular weight distribution (MWD) of the resulting PAM were determined by gel permeation chromatography (GPC). The measurement was performed at 30°C on a Waters 1515 system (Wyatt Technology Corporation, USA) with an Ultrahydrogel-2000 and an Ultrahydrogel-1000 column. An amount of 0.10 mol/l of NaNO₃ aqueous solution was used as the eluent with a flow rate of 1.00 ml/min. Poly(sodium acrylate) standards were used to calibrate the columns. ¹H NMR spectrum of the obtained PAM was recorded on a Bruker 400 MHz spectrometer instrument (Brucker Optics, Germany) using D₂O as the solvent. The particle sizes were examined on a Zetasizer 3000 HSA instrument (Malvern Instruments, Malvern, UK). A transmission electron microscopy (TEM) image was obtained from a JEM-3010 transmission electron microscope (JEOL Co., Ltd., Japan) operated at 200 kV by using diluted redispersions in acetone.

3.1 Atom transfer radical dispersion polymerization of acrylamide in ethyl acetate/ethanol

To further explore the feasibility of atom transfer radical dispersion polymerizations of acrylamide in ethyl acetate/ethanol, a series of experiments were conducted at 70°C, using AIBN as the initiator and FeCl₂/SA as the catalyst. The molar ratio between AM, AIBN, FeCl₂ and SA was set at 300:1:1:2. The result is shown in Figure 1. As can be seen in the figure, the pseudo-first-order kinetic plots were obtained with respect to monomer concentration, indicating that the number of growing chains was constant throughout the polymerization process. This indicates that the termination reactions were negligible. The apparent rate constant was equal to 1.54×10^-5 s^-1, which was obtained from the pseudo-first-order kinetic plots (Figure 1).

Figure 1

Kinetic plot for the atom transfer radical dispersion of AM in ethyl acetate/ethanol (1:1 v/v) at 70°C with AIBN as the initiator.

Figure 2 shows the dependence of molecular weight (M_n,GPC) and MWD on monomer conversion. As can be seen in the figure, the molecular weights increased linearly with the conversion and the M_n,GPC values agreed well with the theoretical values, and the resultant PAM exhibited a narrow molecular weight distribution as the value of MWD went below 1.5, as characterized by GPC. These experimental results suggest that the atom transfer radical dispersion of AM possessed the characteristics of living radical polymerization.

Figure 2

Dependence of molecular weight, M_n,GPC, and molecular weight distribution on monomer conversion. The experimental conditions were the same as in Figure 1.

3.2 Effect of the ligand

The main role of the ligand was to dissolve the catalyst and tune the catalyst activity. The effect of the ligand on the atom transfer radical dispersion of AM was examined in ethyl acetate/ethanol using PVP as the steric stabilizer (see Table 1), including SA, IA and bpy, as presented in Runs 1–3 of Table 1. As can be seen in the table, SA used as the ligand (Run 1) provided the fastest rate among the three different ligands. The monomer conversion reached 42.88% within 10 h. However, the monomer conversion reached 41.32% within 16 h and 13.12% within 18 h, respectively, when IA (Run 2) and bpy (Run 3) were used as the ligand. It indicates that a higher ratio of activation/deactivation rate was achieved when the atom transfer radical dispersion of AM was mediated by SA compared to that mediated by IA and bpy. It is possible that SA, IA and bpy had different molecular structures. SA had higher complex flexibility and faster transformation from Fe(II) to Fe(III). Nevertheless, IA and bpy increased the steric effect of the catalyst, deactivating the catalyst or lowering the effective amount in this system. The particle diameters varied from 206 to 215 nm in the order of SA, IA and bpy (Runs 1–3, respectively, in Table 1). The TEM images of the PAM microspheres are shown in Figure 3 according to Run 8. The diameter was about 214 nm.

Figure 3

TEM images of PAM microspheres according to Run 8.

3.3 Effect of FeCl₂/SA

The effect of FeCl₂/SA on the atom transfer radical dispersion of AM was investigated. A higher polymerization rate was achieved by increasing the amount of the catalyst (Runs 1, 4 and 5 in Table 1). With R=300:1:3:6, after 7 h of reaction, a 47.45% conversion rate was obtained compared with R=300:1:2:4 (46.57% after 8 h of reaction) and R=300:1:1:2 (42.88% after 10 h of reaction), respectively. However, a greater deviation between M_n,th and M_n,GPC was observed in comparison with R=300:1:1:2. Furthermore, a broader MWD value was observed (MWD=1.42). It indicates that less equilibration took place between the chains. The particle sizes increased from 206 to 250 nm (Runs 1, 4 and 5 in Table 1). It is possible that a faster polymerization rate was achieved by increasing the amount of FeCl₂/SA, resulting in a bigger particle size.

3.4 Effect of concentration of the initiator

Based on the mechanism of ATRP, the amount of initiator played a vital role in the atom transfer radical dispersion polymerization of AM. The effect of FeCl₂/SA on the atom transfer radical dispersion of AM was investigated (Runs 1, 6 and 7 in Table 1). The polymerization rate increased with the increase in AIBN concentration from 300:1:1:2 to 300:2:1:2. As expected, the M_n,GPC values decreased with increasing amount of AIBN. However, the MWD and the diameters of the resulting PAM increased from 206 to 212 nm with increasing amount of AIBN. It is possible that the molecular weight of the resulting PAM decreased with increasing concentration of AIBN, leading to the increase in the solubility of the PAM in solvent and the particle size.

3.5 Effect of amount of monomer

To investigate the effect of AM concentration on the atom transfer radical dispersion polymerization of AM, a series of experiments were performed at 70°C. The results (Runs 1, 8, 9 and 10) are listed in Table 1. As can be seen in the table, the molecular weights, the MWDs and the diameter of the obtained PAM increased from 206 to 238 nm with increasing concentration of AM. However, the MWD values remained almost constant (<1.5) and were gradually close to their corresponding M_n,th values. The polymerization rate increased with increasing amount of AM, leading to the bigger particle size.

On the basis of the aforementioned discussion, the particle sizes of the obtained PAM were affected by the experimental conditions, e.g., the concentration of the monomer and the initiator. An analogous phenomenon was observed by Ni et al. (19, 20) in preparing hydrogel microspheres of poly(acrylamide-co-methacrylic acid). The diameters of the particles were in the range of 600–1600 nm.

3.6 Analysis of the chain end and chain extension

The chain end of the PAM was analyzed by ¹H NMR spectroscopy, as shown in Figure 4. The signals in the range 1.30–1.80 ppm were assigned to the protons of the methylene groups (–CH₂-). The signals in the range 1.80–2.65 ppm were assigned to the protons of the methine groups (–CH-).

Figure 4

¹H NMR spectrum of PAM in D₂O.

As reported in the literature (17), the chain extension experiments were carried out by a two-step method. First, acrylamide was polymerized using the method as described previously. Then isolated PAM (M_n,GPC=4200 and MWD=1.33) was used as a macroinitiator for the chain extension with AM as the monomer. Polymerization conducted in the presence of the FeCl₂/SA catalytic system at 70°C in ethyl acetate/ethanol at a conversion rate of 31% yielded a PAM with M_n,GPC=13,500 and MWD=1.36. Figure 5 shows the GPC curves of the macroinitiator and the PAM after the chain extension process.

Figure 5

GPC traces for the chain extension experiment of PAM.

As can be seen from Figure 5, the GPC traces show the absence of the macroinitiator and a clean shift of the GPC trace occurs. It indicates that a successful chain extension to PAM was achieved using the PAM macroinitiator.