Prepared by Precipitation Polymerization - ACS Publications


The microspheres were prepared by free-radical precipitation polymerization in a solvent mixture consisting of methyl ethyl ketone (favorable solvent)...
0 downloads 0 Views 3MB Size


Article pubs.acs.org/IECR

Thermostable Microspheres Consisting of Poly(N‑phenylmaleimideco-α-methyl styrene) Prepared by Precipitation Polymerization Yu Chen, Linyue Tong, Dongyue Zhang, Wantai Yang, and Jianping Deng* State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: General polymeric microspheres are not satisfactorily thermostable. This article reports on an unprecedented type of poly(N-phenylmaleimide-co-α-methyl styrene) [denoted as poly(N-PMI-co-AMS)] microspheres showing remarkable thermal stability. The microspheres were prepared by free-radical precipitation polymerization in a solvent mixture consisting of methyl ethyl ketone (favorable solvent) and heptane (unfavorable solvent). Microspheres of good morphology and narrow size distribution were obtained in high yield (>85%) under appropriate conditions. Growth of poly(N-PMI-co-AMS) microspheres was characterized by scanning electron microscopy. The microspheres, although without cross-linking, exhibited excellent thermal stability, and their decomposition temperature was up to about 370 °C. This feature cannot be achieved in typical polymeric microspheres. Also, notably, this is the first precipitation polymerization of maleimide and AMS and their derivatives for preparing microspheres. The present novel microspheres are expected to find practical applications as novel heat-resistant additives, solid carriers for catalysts, and so on.

1. INTRODUCTION

In our earlier work we successfully prepared heat-resistant polymeric microspheres, i.e. poly(N-(1-phenylethyl)maleimideco-styrene) microspheres by dispersion polymerization.25 However, therein we found it is difficult to obtain clean microspheres from dispersion polymerizations, due to the residual stabilizer [poly(vinyl pyrrolidone)] which is required for conducting the dispersion polymerizations. In contrast, clean spheres can be synthesized by precipitation polymerization, which does not require stabilizers.26,27 This advantage makes the method of precipitation polymerization much attractive. Accordingly, in the present study, we made an attempt to prepare clean heat-resistant microspheres derived from N-PMI and AMS by precipitation polymerization (Figure 1). It should be stressed that this is the first precipitation polymerization of N-substituted maleimides for preparing (co)polymeric microspheres. The advantages of the present microspheres, particularly in the simple preparation process and the exciting thermal stability, make them highly interesting. Also, notably, following the same concept, a series of novel microspheres can be further expected from maleimide derivatives.

Much interest has been focused on the field of polymer microspheres because of their significant potential applications in diverse areas, including analytical chemistry, biomedicine, and solid-phase supports.1−11 However, a majority of the polymeric microspheres reported so far exhibit low thermal stability. This disadvantage severely limits further extended practical applications of polymer-based microspheres, particularly in situations requiring high-temperature performance materials. Therefore extensive efforts have been made to improve the thermal stability of polymers for instance by crosslinking,12,13 by using heat-resistant additives14,15 and by introducing special rigid structures in polymer chains.16,17 In spite of these achievements, new heat-resistant polymer architectures are still highly required to satisfy a wide range of practical applications. This article reports a novel class of heat-resistant microspheres derived from N-substituted maleimide. Polymers from N-substituted maleimides have been widely recognized for their relatively high thermal stability.18,19 As a typical maleimide derivative, N-phenylmaleimide (NPMI),20−22 a strong dienophile containing 1,2-vinyl fivemembered ring, is desirable for increasing the heat resistance of (co)polymers thereof.23 On the other hand, α-methyl styrene (AMS) is an abundant chemical resource, but homopolymerizes with much difficulty by radical polymerization due to the highly steric hindrance.24 In the present study, we utilized N-PMI and AMS as comonomers for preparing the anticipated polymer microspheres, aiming to create a novel type of heat-resistant polymeric microspheres. Another important purpose for the present study is to open new uses of AMS, an abundant compound yet still awaiting more investigations to make full use of it. © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. N-Phenylmaleimide (N-PMI) was purchased from Tokyo Chemical Industry Co., Ltd., and used without further purification. α-Methyl styrene (AMS; Beijing Chemicals Co.) was distilled under reduced pressure before use. 2,2′-Azobisisobutyronitrile (AIBN) and benzoyl peroxide Received: Revised: Accepted: Published: 15610

July 21, 2012 November 2, 2012 November 14, 2012 November 14, 2012 dx.doi.org/10.1021/ie301941r | Ind. Eng. Chem. Res. 2012, 51, 15610−15617

Industrial & Engineering Chemistry Research

Article

Figure 1. Preparation of poly(N-PMI-co-AMS) microspheres by precipitation polymerization (MEK, methyl ethyl ketone).

Table 1. Effects of Solvent Mixture on Microspheresa

(BPO) were purchased from Beijing Yili Fine Chemical Co. and recrystallized from ethanol in advance. Methyl ethyl ketone (MEK), heptane, and ethanol were purchased from Beijing Modern Eastern Fine Chemical Co. and used without further purification. 2.2. Measurements. Because the microspheres were not cross-linked, they could be dissolved in suitable solvents. 1H NMR spectra were recorded in CDCl3 at room temperature. FTIR spectra were recorded with a Nicolet NEXUS 870 infrared spectrometer (KBr tablet). Molecular weights and polydispersity indexes of the (co)polymers were determined by gel permeation chromatography (GPC; Waters 150C) calibrated using polystyrenes as standards and tetrahydrofuran as the eluent. The surface morphology of the particles was observed with a Hitachi S-4700 scanning electron microscope. Thermal gravimetric analysis (TGA) was conducted with a STA 449C thermal analyzer (Netzsch) under N2 at a heating rate of 10 °C min−1. Differential scanning calorimetry (DSC) measurements were performed using a Netzsch DSC204F1 instrument under a flow of nitrogen at a heating rate of 20 °C min−1. Elemental analysis data were obtained on a Vario EL cube (Elementar Analysensysteme GmbH). 2.3. Synthesis of Microspheres. A typical procedure for the preparation of poly(N-PMI-co-AMS) microspheres was as follows: Predetermined amounts of monomers (N-PMI and AMS) and the initiator (AIBN or BPO) were added to the solvent mixture in a reaction tube, and then nitrogen was bubbled for 5 min to exclude air. Subsequently, the tube was sealed and kept at 75 °C for 6 h without stirring. After polymerization, the product was filtered, collected, washed, and dried.

MEK/heptane ratio (v/v)

Mnb

Mw/Mnb

yieldc (%)

1:9 2:8 3:7 4:6 5:5 6:4

3800 4000 4100 4500 4700 4900

3.70 3.95 4.16 5.39 5.11 4.62

35.1 68.4 71.5 85.4 86.1 86.2

Polymerization conditions: [N-PMI] = [AMS] = 0.2 mol L−1, [AIBN] = 0.02 mol L−1, 75 °C for 6 h. bBy GPC. cYield of precipitate, determined by gravimetric method. a

mixture exerts a significant influence on the microsphere morphology. In the present polymerization systems, MEK served as a favorable solvent, whereas heptane was an unfavorable solvent. When the unfavorable solvent heptane was in an excess amount, namely, over 60 vol %, only irregular solid architectures were formed (Figure 2a,b). This is because the poly(N-PMI-co-AMS) chains were forced to precipitate quickly from the system before growing long enough. As the amount of favorable solvent MEK increased to 30 vol %, the morphology of the microspheres became much more regular (Figure 2c). At an appropriate ratio of heptane to MEK, namely, 4:6 (v/v), the polymerization medium led to microspheres with a much better morphology (Figure 2d). Nevertheless, a further increase in the favorable solvent MEK resulted in microspheres with an irregular morphology again (Figure 2e), indicating that more favorable solvent was undesirable for the spheres to grow regularly. This conclusion is further supported by Figure 2f. When the MEK content reached 60 vol %, a portion of much larger microspheres (Figure 2f, average diameter of approximately 2.1 μm) formed, together with a coagulum. According to the morphology of the spheres, a solvent mixture consisting of MEK/heptane = 4:6 (v/v) was the most suitable solvent medium and was used for preparing microspheres in the subsequent experiments. Here, it should be pointed out that we also characterized the microspheres by TEM (transmission electronic microscopy); however, only solid spheres in black were observed. In other words, TEM images could not provide more detailed information. Therefore, the microspheres were observed only by SEM in subsequent experiments. To acquire more information about the microspheres, we characterized them by FTIR spectroscopy. The FTIR spectra of poly(N-PMI-co-AMS) microspheres and of the two monomers, N-PMI and AMS, are shown in Figure 3 for a clear comparison. In spectrum a, the signal at 1710 cm−1 indicates the stretching vibration of CO in the imide group, which existed only in monomer N-PMI. Moreover, in spectrum c, the peaks around 1630 and 890 cm−1 indicate CCH2 groups (CC and CCH, respectively) in AMS. These peaks disappeared in spectrum a. The HCCH group in monomer N-PMI should appear around 1680 cm−1 (CC) and 830 cm−1 (C

3. RESULTS AND DISCUSSION 3.1. Selection of Suitable Polymerization Media. The key point for smoothly preparing microspheres by the precipitation polymerization approach might be the selection of a suitable solvent or solvent mixture. Therefore, we first attempted to determine the appropriate solvent systems. We tried several kinds of reaction media, such as ethyl acetate, butyl acetate, hexane, ethanol, and their mixtures, but finally found methyl ethyl ketone (MEK)/heptane could offer a suitable solvent mixture. We further investigated the effects of the ratio of MEK to heptane on the formation of regular microspheres. The results from relevant investigations are presented in Table 1. We noticed that both the number-average molecular weight (Mn) and the product yield increased as the proportion of MEK (a good solvent for the microspheres) increased. It seems that a greater amount of the good solvent was favorable for the copolymerization to occur. Nevertheless, only a suitable MEK/heptane ratio led to regular microspheres, as described below. The polymeric spheres formed in the polymerization systems were observed by scanning electron microscopy (SEM), as presented in Figure 2. The SEM images show that the solvent 15611

dx.doi.org/10.1021/ie301941r | Ind. Eng. Chem. Res. 2012, 51, 15610−15617

Industrial & Engineering Chemistry Research

Article

Figure 2. SEM images of poly(N-PMI-co-AMS) microspheres prepared at various MEK/heptane ratios: (a) 1:9, (b) 2:8, (c) 3:7, (d) 4:6, (e) 5:5, (f) 6:4 (v/v). Polymerization conditions: [N-PMI] = [AMS] = 0.2 mol L−1, [AIBN] = 0.02 mol L−1, 75 °C, 6 h.

3.2. Growth of Microspheres. It is important to note that, in the elegant work28,29 dealing with precipitation polymerization of vinyl monomers for the preparation of polymeric microspheres, cross-linking was frequently required. However, in our study, a cross-linking reaction seems not to be necessary. To acquire deep insight into the formation of the microspheres, we observed the growth of the microspheres with MEK/ heptane (4:6 in v/v) as the solvent for conducting the precipitation polymerizations. The obtained SEM images are presented in Figure 4, demonstrating clearly and vividly the growth of poly(N-PMI-co-AMS) microspheres. Initially, pronounced bulk instead of microsphere was observed (Figure 4a−d). With increased polymerization time, the bulk disappeared gradually, and more microspheres formed until all of the bulk was consumed (Figure 4e,f). A further prolongation of the polymerization time led to more regular and larger microspheres (Figure 4g,h). After a polymerization of 6 h, the microspheres changed little (SEM images omitted). Accordingly, we considered the microspheres to have stopped growing when the polymerization lasted for 6 h, because the microsphere size and morphology changed little thereafter. It is worth pointing out that, the eventually obtained microspheres (Figure 4h, average diameter of approximately 1.24 μm) were just slightly larger than the ones isolated from the bulk (Figure 4b, approximately 1 μm); however, the number of the microspheres was much larger for the former (Figure 4h). According to the SEM images in Figure 4, we consider that the mechanism for the formation of the present microspheres is similar to that for microspheres derived from vinyl chloride or acrylonitrile formed by precipitation polymerization.28 Briefly, the formation of the present microspheres consisted of two stages: nucleation and growth. In nucleation stage, the propagation chains grew beyond their solubility limit in the polymerization system, and then precipitation occurred. The precipitated polymer chains entangled together to form the particle nuclei. In growth stage, the obtained nuclei grew larger

Figure 3. FTIR spectra of (a) poly(N-PMI-co-AMS) microspheres, (b) monomer N-PMI, and (c) monomer AMS (KBr tablet). The microspheres were obtained under the following polymerization conditions: [N-PMI] = [AMS] = 0.2 mol L−1, [AIBN] = 0.02 mol L−1, 75 °C, 6 h; solvent, MEK/heptane = 4:6 (v/v).

CH) in spectrum b; however, the former was overlapped by a peak at 1710 cm−1, whereas the latter could be observed clearly. In spectrum a, no pronounced peak appeared at about 830 cm−1. In addition, the broad peak around 1710 cm−1 attributed to the stretching vibrations of CO and CC before copolymerization became very sharp after polymerization, because of the disappearance of the CC signal at about 1680 cm−1. From these observations, we preliminarily concluded that the microspheres consisted of N-PMI and AMS. 1 H NMR spectra of the microspheres were also recorded, but quantitative information could not be obtained because of the complexity and significant overlapping of the signals. (A typical 1 H NMR spectrum is presented in Figure S1 in the Supporting Information.) To acquire more insight into the copolymer microspheres, we characterized them by elemental analysis (a detailed discussion of which is presented in section 3.6.) 15612

dx.doi.org/10.1021/ie301941r | Ind. Eng. Chem. Res. 2012, 51, 15610−15617

Industrial & Engineering Chemistry Research

Article

Figure 4. SEM images of poly(N-PMI-co-AMS) microspheres prepared as a function of polymerization time: (a) 10, (b) 20, (c) 30, (d) 60, (e) 120, (f) 180, (g) 240, and (h) 360 min. Conditions: [N-PMI] = [AMS] = 0.2 mol L−1, [AIBN] = 0.02 mol L−1, 75 °C; solvent, MEK/heptane = 4:6 (v/ v).

Table 2. Effects of Monomer Concentrationa

into microspheres by capturing other particles and polymer chains or by adsorbing monomers that polymerized therein. To optimize the preparation conditions and the morphology of the microspheres and to understand the precipitation polymerization more deeply, we further investigated the effects of some major affecting factors, as discussed in the following sections. 3.3. Effects of Monomer Concentration. Monomer concentrations strongly affected the copolymerization and the formation of microspheres, as indicated by the data in Table 2 and the SEM images in Figure 5. Table 2 demonstrates that an increase in monomer concentration led to an increase in both the molecular weight of the polymer chains and the yield of the product. Figure 5 shows that the poly(N-PMI-co-AMS) microspheres changed greatly with the variation in monomer concentrations. The concentration of both N-PMI and AMS in run 1 was 0.1 mol L−1 (Table 2), lower than that in run 2 (0.2 mol L−1), but the microspheres obtained in run 1 (average diameter of 1.84 μm, Figure 5) were larger than those obtained in run 2 (average diameter of 1.24 μm), whereas the morphology of the microspheres in run 1 was much less regular. The relevant explanation is as follows: When the monomer concentrations were low, the relatively shorter polymer chains and the low

run

[N-PMI] = [AMS] (mol L−1)

Mnb (×103)

Mw/ Mnb

yieldc (%)

average diameterd (μm)

CVd (%)

1 2 3

0.1 0.2 0.4

3.5 4.5 12.0

3.98 5.39 3.25

73.8 85.4 87.0

1.84e 1.24 1.62

7.01e 6.49 25.96

Polymerization conditions: [AIBN] = 0.02 mol L−1, MEK/heptane = 4:6 (v/v), 75 °C for 6 h. bBy GPC. cYield of precipitate, determined by gravimetric method. dAverage diameter and CV (coefficient of variance) determined according to SEM images. eOnly spherical polymer particles were calculated. a

monomer amount could not ensure that all of the polymer chains formed microspheres and also could not ensure that all of the microspheres grew regularly. Some of the polymer chains were forced to coagulate into nonspherical particles. When the monomer concentrations were both increased to 0.4 mol L−1 (run 3 in Table 2), the average diameter of the microspheres (1.62 μm) was greater than that from run 2. In run 3 in Table 2, the Mn value of the copolymer chains was 12.0 × 103. The polymer chains were considered as being too long to move freely, which led to the formation of larger particles together with much smaller ones. We assumed that the cause for the 15613

dx.doi.org/10.1021/ie301941r | Ind. Eng. Chem. Res. 2012, 51, 15610−15617

Industrial & Engineering Chemistry Research

Article

Figure 5. SEM images of poly(N-PMI-co-AMS) microspheres prepared at various monomer concentrations: [N-PMI] = [AMS] = (a) 0.1, (b) 0.2, and (c) 0.4 mol L−1. For other conditions, see Table 2.

Table 3. Effects of Initiatora run

initiator

concentration (mol L−1)

Mnb (×103)

Mw/Mnb

yieldc (%)

average diameterd (μm)

CVd (%)

4 2 5

AIBN AIBN BPO

0.004 0.02 0.02

13.5 4.5 7.4

2.45 5.39 2.72

61.7 85.4 93.2

1.38 1.24 1.42

29.04 6.49 28.28

Polymerization conditions: [N-PMI] = [AMS] = 0.2 mol L−1, MEK/heptane = 4:6 (v/v), 75 °C for 6 h. bBy GPC. cYield of precipitate, determined by gravimetric method. dAverage diameter and CV determined according to SEM images. a

Figure 6. SEM images of poly(N-PMI-co-AMS) microspheres prepared with different initiators: (a) AIBN, 0.004 mol L−1; (b) AIBN, 0.02 mol L−1; and (c) BPO, 0.02 mol L−1. For other conditions, see Table 3.

3.5. Effects of Polymerization Temperature. In principle, polymerization temperature directly affects the formation of radicals and the propagation of polymer chains. The effects of polymerization temperature were investigated in detail, and the results are presented in Table 4 and Figure 7. When the copolymerization was carried out at 65 °C (run 6 in Table 4), the number of radicals was lower than the number obtained when the copolymerization was carried out at 75 °C (run 2). Thus, in the former case, higher Mn (11700), narrower polydispersity (2.26), and lower yield of the product (76.4%)

formation of the much smaller microspheres was secondary nucleation. 3.4. Effects of Initiator. Table 3 and Figure 6 demonstrate the effects of initiator on the copolymerization and the formation of microspheres. Figure 6a,b compares the microspheres formed with different amounts of the initiator AIBN. When AIBN was the effluent (run 2 in Table 3), more radicals were generated and initiated polymerization, resulting in not-as-long polymer chains (Mn = 4500). This situation is favorable for the formation of regular microspheres with a relatively narrow distribution (CV = 6.49%), small size (average diameter = 1.24 μm), and a high yield. In contrast, a low amount of AIBN (run 4 in Table 3) resulted in a much higher Mn (13500) and fewer nucleation sites, finally leading to microspheres with a larger size (average diameter = 1.38 μm) and broader distribution (CV = 29.04%). Runs 2 and 5 (Table 3) and Figure 6b,c compare two cases, one with AIBN and the other with BPO as initiator. In comparison with the microspheres prepared by using AIBN (run 2), the microspheres prepared using BPO (run 5) were larger (average diameter = 1.42 μm) and had a broader distribution (CV = 28.28%). The differences are considered to be due to the different thermal decomposition temperatures of AIBN and BPO.

Table 4. Effects of Reaction Temperaturea run

temperature (°C)

Mnb (×103)

Mw/ Mnb

yieldc (%)

average diameterd (μm)

CVd (%)

6 2 7

65 75 80

11.7 4.5 4.2

2.26 5.39 2.60

76.4 85.4 85.3

−e 1.24 1.18

−e 6.49 7.66

Polymerization conditions: [N-PMI] = [AMS] = 0.2 mol L−1, [AIBN] = 0.02 mol L−1, MEK/heptane = 4:6 (v/v), 6 h. bBy GPC. c Yield of precipitate, determined by gravimetric method. dAverage diameter and CV determined according to SEM images. eMorphology of polymer particles so poor that average particle size could not be calculated. a

15614

dx.doi.org/10.1021/ie301941r | Ind. Eng. Chem. Res. 2012, 51, 15610−15617

Industrial & Engineering Chemistry Research

Article

Figure 7. SEM images of poly(N-PMI-co-AMS) microspheres prepared at various temperatures: (a) 65, (b) 75, and (c) 80 °C. For other conditions, see Table 4.

Table 5. Effects of Monomer Ratioa run

[N-PMI]/[AMS]

Mnbvol %(×103)

Mw/Mnb

yieldc (%)

Nd (%)

N-PMI/AMSe (mol/mol)

average diameterf (μm)

CVf (%)

8 2 9

0.3:0.1 0.2:0.2 0.1:0.3

8.8 4.5 2.0

2.39 5.39 2.52

68.5 85.4 38.6

6.095 4.949 4.609

2.08 1.07 0.90

1.23 1.24 1.54

21.42g 6.49 11.25g

Polymerization conditions: [N-PMI] + [AMS] = 0.4 mol L−1, [AIBN] = 0.02 mol L−1, MEK/heptane = 4:6 (v/v), 75 °C for 6 h. bBy GPC. cYield of precipitate, determined by gravimetric method. dNitrogen percentage in the products, measured by elemental analysis. eComposition of the products, calculated based on elemental analysis. fAverage diameter and CV determined according to SEM images. gOnly spherical polymer particles were calculated. a

Figure 8. SEM images of poly(N-PMI-co-AMS) microspheres prepared at various monomer ratios: [N-PMI]/[AMS] (in mol L−1) = (a) 0.3:0.1, (b) 0.2:0.2, and (c) 0.1:0.3. For other polymerization conditions, see Table 5.

the formation of a charge-transfer complex (CTC) in the system, with N-PMI as the electron acceptor and AMS as the electron donor.30−32 When compared to run 2, the copolymers from run 8 were of higher Mn (8800) and narrower polydispersity (2.39), in which the amount of N-PMI was 3 times the amount of AMS; more nonspherical polymer particles were also formed in this case (Figure 8a, CV = 21.42%). We attributed the differences discussed above to the higher thermal stability and steric hindrance provided by the high content of N-PMI units in the resulting copolymer chains. This consideration can be supported by the nitrogen percentage and the copolymer compositions in Table 5. On the other hand, when the concentration of N-PMI was in a 1:3 ratio with that of AMS (run 9, Figure 8c), the Mn value of the produced copolymer was very low (Mn = 2000) and the yield was also only 38.6%. Additionally, some nonspherical particles can be observed in Figure 8c. The nitrogen percentage of run 9 in Table 5 (N-PMI/AMS = 0.90) gave the reason for the low yield. The data demonstrate that the monomer ratio exerted a significant influence not only on the copolymerizations but also on the morphology of the microspheres. The

were obtained. Nevertheless, the morphology of the microspheres was poor because the low temperature decreased the mobility of the polymer chains (Figure 7a). In contrast, when the reaction temperature was 80 °C (run 7), the relatively rapid thermal decomposition of AIBN resulted in a lower Mn (4200) and narrower polydispersity (2.60). Although the microsphere morphology remained good (Figure 7c), the average diameter (1.18 μm) was smaller, and the size distribution was slightly broader when compared to those at 75 °C (run 2, Figure 7b). Accordingly, 75 °C was selected as a favorable temperature for preparing satisfactory microspheres under the investigated conditions. 3.6. Effects of Monomer Ratio. Taking into account the fact that AMS is relatively hard to homopolymerize, we believe that the monomer ratio should exert a large influence on the copolymerization and formation of microspheres. We investigated three cases, [N-PMI]/[AMS] = 3:1, 1:1, and 1:3 ( mole ratios). Table 5 and Figure 8 show information on the copolymerization and how the poly(N-PMI-co-AMS) microspheres varied with monomer ratio. In Table 5, run 2 shows that the copolymer microspheres contained nearly 1:1 (in mol) of the two monomers, suggesting 15615

dx.doi.org/10.1021/ie301941r | Ind. Eng. Chem. Res. 2012, 51, 15610−15617

Industrial & Engineering Chemistry Research



thermal properties of the microspheres were also dependent on their composition, as discussed next. 3.7. Thermal Properties of the Microspheres. We further investigated the thermal stability of the microspheres prepared in Table 5 as representative microspheres. The TGA thermograms are shown in Figure 9.

Article

ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectrum and DSC thermograms of the microspheres. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author

*Fax: +86-10-6443-5128. Tel.: +86-10-6443-5128. E-mail: [email protected] Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was generously supported by the National Natural Science Foundation of China (Nos. 21274008, 21174010), the Funds for Creative Research Groups of China (No. 51221002), and the Fundamental Research Funds for the Central Universities (No. ZZ1117).

Figure 9. TGA thermograms of poly(N-PMI-co-AMS) microspheres measured in N2 at a heating rate of 10 °C min−1: (a) run 8, (b) run 2, and (c) run 9 in Table 5.



REFERENCES

(1) Franc, G.; Kakkar, A. K. “Click” methodologies: Efficient, simple and greener routes to design dendrimers. Chem. Soc. Rev. 2010, 39, 1536. (2) Kelly, T. L.; Wolf, M. O. Template approaches to conjugated polymer micro- and nanoparticles. Chem. Soc. Rev. 2010, 39, 1526. (3) Kawaguchi, H. Functional polymer microspheres. Prog. Polym. Sci. 2000, 25, 1171. (4) Á lvarez-Diaz, A.; Salinas-Castillo, A.; Camprubí-Robles, M.; Costa-Fernández, J. M.; Pereiro, R.; Mallayia, R.; Sanz-Medel, A. Conjugated polymer microspheres for “turn-off”/“turn-on” fluorescence optosensing of inorganic ions in aqueous media. Anal. Chem. 2011, 83, 2712. (5) Go, D. P.; Gras, S. L.; Mitra, D.; Nguyen, T. H.; Stevens, G. W.; Cooper-White, J. J.; O’Connor, A. J. Multilayered microspheres for the controlled release of growth factors in tissue engineering. Biomacromolecules 2011, 12, 1494. (6) Cerroni, B.; Chiessi, E.; Margheritelli, S.; Oddo, L.; Paradossi, G. Polymer shelled microparticles for a targeted doxorubicin delivery in cancer therapy. Biomacromolecules 2011, 12, 593. (7) Abdelrahman, A. I.; Dai, S.; Thickett, S. C.; Ornatsky, O.; Bandura, D.; Baranov, V.; Winnik, M. A. Lanthanide-containing polymer microspheres by multiple-stage dispersion polymerization for highly multiplexed bioassays. J. Am. Chem. Soc. 2009, 131, 15276. (8) Bratkowska, D.; Marcé, R. M.; Cormack, P. A. G.; Sherrington, D. C.; Borrull, F.; Fontanals, N. Synthesis and application of hypercrosslinked polymers with weak cation-exchange character for the selective extraction of basic pharmaceuticals from complex environmental water samples. J. Chromatogr. A 2010, 1217, 1575. (9) Walmsley, R. S.; Ogunlaja, A. S.; Coombes, M. J.; Chidawanyika, W.; Litwinski, C.; Torto, N.; Nyokong, T.; Tshentu, Z. R. Imidazolefunctionalized polymer microspheres and fibersUseful materials for immobilization of oxovanadium(IV) catalysts. J. Mater. Chem. 2012, 22, 5792. (10) Meyer, R. F.; Rogers, W. B.; McClendon, M. T.; Crocker, J. C. Producing monodisperse drug-loaded polymer microspheres via crossflow membrane emulsification: The effects of polymers and surfactants. Langmuir 2010, 26, 14479. (11) Xiong, X. P.; Zou, W. W.; Yu, Z. J.; Duan, J. J.; Liu, X. J.; Fan, S. H.; Zhou, H. Microsphere Pattern Prepared by a “Reverse” Breath Figure Method. Macromolecules 2009, 42, 9351. (12) Krishnamoorthy, S.; Haria, M.; Fortier-Mcgill, B. E.; Mazumder, M. A. J.; Robinson, E. I.; Xia, Y.; Burke, N. A. D.; Stöver, H. D. H. High Tg microspheres by dispersion copolymerization of N-phenylmaleimide with styrenic or alkyl vinyl ether monomers. J. Polym. Sci. A: Polym. Chem. 2011, 49, 192.

The results indicate that more N-PMI units in the copolymer chains provided the microspheres with higher heat resistance. The microspheres with the best morphology obtained from run 2 began to decompose at about 340 °C. The highest thermal decomposition temperature (run 8) among the three samples was over 370 °C, much higher than those of most typical polymeric microspheres. It is thus confirmed that the present poly(N-PMI-co-AMS) microspheres have outstanding heat resistance. Additionally, such microspheres were also subjected to DSC analysis. However, no obvious glass transition temperature (Tg) was observed (Figure S2 in the Supporting Information), likely because of the alternating sequence structure of the polymer chains. This phenomenon was also observed in other alternating copolymers.33 We are convinced that, after optimization of the microsphere recipe and structure, their heat resistance will be further markedly improved. Such novel microspheres are expected to find a wide range of practical applications, and our studies are currently ongoing along this direction.

4. CONCLUSIONS We successfully prepared a novel class of heat-resistant poly(NPMI-co-AMS) microspheres by precipitation polymerization. Microspheres of satisfactory morphology and narrow size distribution could be obtained under optimized conditions. The optimal microspheres (average diameter = 1.24 μm, CV = 6.49%) of regular morphology were obtained with [N-PMI] = [AMS] = 0.2 mol L−1, [AIBN] = 0.02 mol L−1 in mixed reaction medium of MEK and heptane [4:6 (v/v)] at 75 °C for 6 h. We found that when, more N-PMI was charged, the microspheres showed higher heat resistance. The remarkable and controllable heat resistance of poly(N-PMI-co-AMS) microspheres with different monomer concentrations was demonstrated by TGA. The highest thermal decomposition temperature can be over 370 °C, indicating that the microspheres have significant potential to be applied in some high-temperature environments. 15616

dx.doi.org/10.1021/ie301941r | Ind. Eng. Chem. Res. 2012, 51, 15610−15617

Industrial & Engineering Chemistry Research

Article

styrene and N-substituted maleimides. Macromol. Rapid Commun. 2011, 32, 127. (33) Li., Y.; Turner, R. Free radical copolymerization of methyl substituted stilbenes with maleic anhydride. Eur. Polym. J. 2010, 46, 821.

(13) Shim, S. E.; Yang, S.; Choi, H. H.; Choe, S. Fully crosslinked poly(styrene-co-divinylbenzene) microspheres by precipitation polymerization and their superior thermal properties. J. Polym. Sci. A: Polym. Chem. 2004, 42, 835. (14) Wang, X. L.; Wu, L. H.; Li, J. Influence of nanozirconia on the thermal stability of poly(methyl methacrylate) prepared by in situ bulk polymerization. J. Appl. Polym. Sci. 2010, 117, 163. (15) Watanabe, S.; Ueno, K.; Kudoh, K.; Murata, M.; Masuda, Y. Preparation of core−shell polystyrene−polyimide particles by dispersion polymerization of styrene using poly(amic acid) as a stabilizer. Macromol. Rapid Commun. 2000, 21, 1323. (16) Wakabayashi, K.; Uchida, T.; Yamazaki, S.; Kimura, K. Preparation of poly(p-phenylenepyromelliteimide) microspheres with rugged surfaces using crystallization during isothermal polymerization. Macromol. Chem. Phys. 2011, 212, 159. (17) Kawaguchi, Y.; Itamura, Y.; Onimura, K.; Oishi, T. Effects of the chemical structure on the heat resistance of thermoplastic expandable microspheres. J. Appl. Polym. Sci. 2005, 96, 1306. (18) Ahokas, M.; Wilén, C. E. Synthesis of poly(styrene-comaleimide) and poly(octadecene-co-maleimide) nanoparticles and their utilization in paper coating. Prog. Org. Coat. 2009, 66, 377. (19) Wu, H. H.; Chu, P. P. Structure characteristics contributing to flame retardancy in diazo modified novolac resins. Polym. Degrad. Stab. 2009, 94, 987. (20) Takeda, K.; Matsumoto, A. Thermosetting maleimide/isobutene alternating copolymer as a new class of transparent materials. Macromol. Chem. Phys. 2010, 211, 782. (21) Senel, S.; Rzaev, Z. M. O.; Piskin, E. Copolymerization of Nphenylmaleimide with 2-hydroxyethyl and ethyl methacrylates. Polym. Int. 2003, 52, 713. (22) Ren, Y.; Zhu, Z. G.; Huang, J. L. Radical copolymerization of maleimide with ethyl α-ethylacrylate and α-ethylacrylic acid via RAFT. J. Polym. Sci. A: Polym. Chem. 2004, 42, 3828. (23) Howell, B. A.; Zhang, J. Thermal degradation of vinylidene chloride/vinyl chloride copolymers in the presence of N-substituted maleimides. J. Therm. Anal. Cal. 2006, 83, 83. (24) Barner-Kowollik, C.; Davis, T. P. Using kinetics and thermodynamics in the controlled synthesis of low molecular weight polymers in free-radical polymerization. Macromol. Theory Simul. 2001, 10, 255. (25) Tong, L. Y.; Cui, X.; Yang, W. T.; Deng, J. P. Heat-resistant poly(N-(1-phenylethyl)maleimide-co-styrene) microspheres prepared by dispersion polymerization. J. Mater. Chem. 2012, 22, 6697. (26) Downey, J. S.; McIsaac, G.; Frank, R. S.; Stöver, H. D. H. Poly(divinylbenzene) microspheres as an intermediate morphology between microgel, macrogel, and coagulum in cross-linking precipitation polymerization. Macromolecules 2001, 34, 4534. (27) Macková, H.; Proks, V.; Horák, D.; Kučka, J.; Trchová, M. Magnetic poly(N-propargylacrylamide) microspheres: Preparation by precipitation polymerization and use in model click reactions. J. Polym. Sci. A: Polym. Chem. 2011, 49, 4820. (28) Downey, J. S.; Frank, R. S.; Li, W. H.; Stöver, H. D. H. Growth mechanism of poly(divinylbenzene) microspheres in precipitation polymerization. Macromolecules 1999, 32, 2838. (29) Medina-Castillo, A. L.; Fernandez-Sanchez, J. F.; SeguraCarretero, A.; Fernandez-Gutierrez, A. Micrometer and submicrometer particles prepared by precipitation polymerization: Thermodynamic model and experimental evidence of the relation between Flory’s parameter and particle size. Macromolecules 2010, 43, 5804. (30) Boyer, C.; Otazaghine, B.; Boutevin, B.; Joly-Duhamel, C.; Robin, J.-J. Synthesis of maleimide-terminated n-butyl acrylate oligomers by atom transfer radical polymerization: Study of their copolymerization with vinyl ethers. J. Polym. Sci. A: Polym. Chem. 2005, 43, 4303. (31) Lazzara, T. D.; van de Ven, T. G. M.; Whitehead, M. A. Nanotube self-assembly of a styrene and maleimide alternating copolymer. Macromolecules 2008, 41, 6747. (32) Lutz, J. -F.; Schmidt, B. V. K . J.; Pfeifer, S. Tailored polymer microstructures prepared by atom transfer radical copolymerization of 15617

dx.doi.org/10.1021/ie301941r | Ind. Eng. Chem. Res. 2012, 51, 15610−15617