Article pubs.acs.org/Macromolecules
Photoinitiated RAFT Dispersion Polymerization: A Straightforward Approach toward Highly Monodisperse Functional Microspheres Jianbo Tan, Xin Rao, Xionghao Wu, Hancheng Deng, Jianwen Yang, and Zhaohua Zeng* Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, and Key Laboratory of Designed Synthesis and Application of Polymer Material, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China S Supporting Information *
ABSTRACT: A straightforward dispersion polymerization procedure for the synthesis of monodisperse functional polymeric microspheres is proposed in this article. This method overcomes the problems deriving from the highly sensitive nucleation stage by introducing both photoinitiation and a RAFT chain transfer agent to the reaction. The process of the formation and growth of particles in the procedure was investigated and found to be quite different from that in a traditional dispersion polymerization. Various kinds of PMMA-based functional microspheres with high size uniformity were synthesized in a single step by this strategy. The microspheres remained uniform in size, even at concentrations of cross-linker or functional comonomer up to 10 wt %.
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particles although it is added after the end of nucleation stage.9 Employing the semicontinuous or multibatch method can increase the content of functional reagent to a more elevated level.9 But the procedure becomes complex, and the crosslinking of the microspheres obtained is inhomogeneous. In this paper, we propose a straightforward one-stage procedure that combines the photoinitiation and a reversible addition−fragmentation chain transfer (RAFT) process to synthesize monodisperse functional polymeric microspheres. Photoinitiated free radical polymerization is a very rapid procedure, in which photoinitiator decomposes and generates free radicals quickly under UV or visible light irradiation at room temperature. We have introduced this technique to dispersion polymerization and found that monomer conversion over 90% was achieved within 25 min.20,21 In these studies, the size distribution of the microspheres obtained was not very narrow in comparison with traditional dispersion polymerization. This can be attributed to the high rate of photoinitiated polymerization which induces such fast nucleation that there is not enough time for the stabilizer to capture and stabilize the nuclei effectively.20 To solve this problem, we tried to delay the nucleation by adding a reversible addition−fragmentation chain transfer (RAFT) agent to the system and successfully synthesized highly monodisperse microspheres by this procedure. We then used this procedure to synthesize crosslinked and functional microspheres by adding cross-linker or functional comonomer to the system at the beginning of reaction. It is surprising that the microspheres obtained were
INTRODUCTION Dispersion polymerization is an attractive method for preparing monodisperse microspheres with diameters in the range of 1− 15 μm, which have a wide variety of applications in fields such as biology, medical analysis, protein synthesis, flat panel displays, photonics, and chromatography.1−5 In dispersion polymerization, all reaction ingredients (monomer, initiator, and stabilizer) are dissolved in the reaction medium to form a homogeneous solution. Sterically stabilized polymer particles are formed by the precipitation of the polymer chains generated during polymerization. The reaction process can be separated into a nucleation stage and a particle growth stage. The nucleation stage is short but complex and sensitive and thus easily disturbed by functional reagents such as cross-linker, comonomer, and chain transfer agent (CTA).6 The addition of the functional reagents at the beginning of polymerization often results in poorly controlled particle size and broader size distribution and sometimes even causes coalescence.7−9 As a result, the synthesis of functional polymeric microspheres by dispersion polymerization is greatly limited. To overcome this problem, Winnik’s group developed an intelligent method, called two-stage dispersion polymerization, in which the functional reagents were added to the reaction after the nucleation stage was complete.6 This can avoid the disturbance of functional reagents to the nucleation, and various kinds of monodisperse functional microspheres have been synthesized by this two-stage method since it was proposed.2,3,9−15 This strategy is something like the seeded dispersion polymerization16−19 but more facile and more effective on keeping high monodispersity and controlling particle size. The two-stage procedure still has some limitations. Functional reagent at high levels may disturb the growth of © 2012 American Chemical Society
Received: August 27, 2012 Revised: October 3, 2012 Published: October 16, 2012 8790
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at 365 nm.24 Figure 1 shows that microspheres with very narrow size distribution were obtained by this procedure in the
highly uniform in size. The combination of photoinitiation and a RAFT process overcomes the problems deriving from the highly sensitive nucleation stage in traditional dispersion polymerization.
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EXPERIMENTAL SECTION
Materials. Methyl methacrylate (MMA, Tianjin Kermel Chemical Reagents Development Center) was used after removing inhibitors. Dipropylene glycol diacrylate (DPGDA, Sartomer), trimethylolpropane triacrylate (TMPTA, Sartomer), poly(ethylene glycol) diacrylate (PEGDA200, Sartomer), 2-hydroxypropyl methacrylate (HPMA, Aladdin), methacrylic acid (MAA, Tianjin Kermel Chemical Reagents Development Center), acrylic acid (AA, Tianjin Kermel Chemical Reagents Development Center), glycol methacrylate (GMA, Aladdin), 4-acryloylmorpholine (ACMO, Aladdin), 2,2-dimethyl-2-phenylacetophenone (Darocur 1173, Ciba), and poly(N-vinylpyrrolidone) (PVP K30, Shanghai BoAo Biologic Technology) were used as received. The RAFT agents including S-1-dodecyl-S′-(α,α′-dimethyl-α″-acetic acid) trithiocarbonate (DDMAT), S,S′-bis(α,α′-dimethyl-α″-acetic acid) trithiocarbonate (BDMAT), and S-1-ethyl-S′-(α,α′-dimethylα″-acetic acid) trithiocarbonate (EDMAT) were synthesized using the published procedures.22,23 Dispersion Polymerization. In a typical experiment, an ethanol/ water mixture with weight ratio of 40/60 was introduced into the reactor as the reaction medium, and 10 wt % of monomer (MMA) relative to the system, 15 wt % of stabilizer (PVP) relative to MMA, 0.5 wt % of RAFT agent relative to MMA, and 2 wt % of photoinitiator (Darocur 1173) relative to MMA were dissolved into the medium. Other recipes used in this research are listed in Table S1. The mixture was purged with nitrogen for 15 min and sealed and then irradiated by a 3 W 365 nm LED lamp (light intensity 0.8 mW/cm2) from the top of the reaction cell for 3 h or other time points if needed. The reaction mixture was centrifuged at a high rpm. The precipitate was rinsed with ethanol/water mixture and centrifuged repeatedly, dried in a vacuum oven for 24 h to obtain fine powder, and then weighed for calculating the reaction conversion. The supernatant was concentrated by rotary evaporation, rinsed with water to remove the residual PVP, and then filtrated. The precipitate obtained was dried and weighed for calculating the concentration of soluble chains in the medium. Characterization. Morphology analysis was carried out on a JSM6330F field emission scanning electron microscope (FMSEM) at 10 kV. Samples were gold-coated after dispersed in water and dropped onto glass slides. The diameters and size distribution of microsphers were determined by analysis of micrographs obtained by SEM using the software named Image-Pro Plus 5.1 (Media Cybernetics). Transmission electron microscope (TEM) observations were carried out on a JEM-2010HR instrument operated at 120 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Electron Corporation ESCALAB 250 spectrometer. An Al Kα X-ray source was used. The determination was operated at 20 eV pass energy. The molecular weight and polydispersity were determined by a Waters 1515 GPC instrument with tetrahydrofuran as the mobile phase. The flow rate of tetrahydrofuran was 1 mL/min. Narrow distribution linear polystyrenes were used as the standard to calibrate the apparatus; the molecular weights of the samples were measured using universal calibration.
Figure 1. SEM images of PMMA particles prepared by photoinitiated dispersion polymerization with 0.5 wt % of (a) BDMAT, (b) DDMAT, and (c) EDMAT as RAFT agent and (d) without RAFT agent.
presence of various kinds of RAFT agents. Meanwhile, the procedure without adding a RAFT agent gave wider size distribution (Figure 1d). The reaction process was monitored by GPC and conversion (Figures 2 and 3). The sharp increase in both the molecular
Figure 2. GPC traces for photoinitiated RAFT dispersion polymerization of MMA in the presence of 0.5 wt % BDMAT.
weight and monomer conversion (particles yield) at 45 min irradiation is attributed to the gel effect at the growth stage at which the polymerization takes place primarily in the monomer swollen particles.25 The results suggest a nucleation time length of about 45 min. This is supported by the fact that the particle number became constant basically after 45 min irradiation (Figure S1 in Supporting Information). This time length is rather long in comparison to the process without adding a RAFT agent (Figure S2), suggesting that the nucleation was effectively prolonged by adding a RAFT agent. Meanwhile, it is still greatly shorter than that in the thermal initiated RAFT dispersion polymerization, in which the presence of RAFT agent greatly prolongs the nucleation period.11 Choe’s group
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RESULTS AND DISCUSSION Photoinitiated RAFT Dispersion Polymerization. The photoinitiated RAFT dispersion polymerization of methyl methacrylate (MMA) proceeded in a single step under the irradiation of a 365 nm LED lamp at room temperature. Darocur 1173 (2,2-dimethyl-2-phenylacetophenone) was selected as photoinitiator in consideration of its suitable solvent affinity and the high photolysis rate under UV irradiation. Three kinds of trithiocarbonates were employed as RAFT agents, chosen for their low UV absorbance of trithiocarbonate 8791
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Figure 3. Plots of number-average molecular weight (a) and conversion (b) versus time for photoinitiated RAFT dispersion polymerization of MMA in the presence of 0.5 wt % BDMAT (I: nucleation stage; II: growth stage; [M]: monomer concentration).
Figure 4. GPC curves for PMMA produced in the two-step process. The first step is the typical one-stage photoinitiated RAFT dispersion polymerization with 2 g of MMA. After 3 h irradiation, 1 g of MMA was added to the mixture and then the reaction continued for 3 h (the second step). Mcalcd: molecular weight calculated from the initial monomer and RAFT agent concentrations.
reported that no turbidity was observed at 4 h when a RAFT agent was present at the beginning of dispersion polymerization of styrene.7 The nucleation period is too long to generate uniform particles in this case. They also tried to initiate the polymerization via the photolysis of the RAFT agent in RAFT dispersion polymerization.26 The polymerization was carried out for up to 16 h at 50−70 °C, and the size distribution of the particles obtained was very broad. A similar result was reported by Youk’s group.27 In fact, the preparation of monodisperse microspheres by controlled/living dispersion polymerization remains a challenge unless the two-stage procedure is employed.11,28,29 Although controlled/living dispersion polymerization has recently received increased attention,30−41 the studies primarily focused on the aspects such as the synthesis of nanogels, nanolatexes, and vesicles instead of obtaining uniform microspheres. The present study is the first case to synthesize highly monodisperse “living” polymeric microspheres by onestage RAFT dispersion polymerization. The controlled/livingness characteristic of the polymerization to form microspheres is supported by a two-step process, in which a new batch of monomer was added at the end of the typical one-stage procedure, and then the reaction was continued by turning on the LED lamp again. We found that the GPC curve shifted to higher molecular weight without obvious profile change, and the increment of molecular weight basically matched the amount of monomer added at the second step (Figure 4). The inset plot in Figure 3a shows linear growth of molecular weight with conversion in the growth stage, which is a characteristic of controlled/living radical polymerization. The linear relationship between ln([M]0/[M]) and reaction time (inset plot in Figure 3b) suggests a pseudo-first-order kinetics of polymerization. Formation and Growth of particles. The irregular change of molecular weight with time or conversion at the nucleation stage (Figure 3a) implies that the process at this stage may be complicated. We have monitored the morphology of the particles formed at different times (Figure 5) and found that a lot of particles with extra-large size were produced at the beginning of reaction and disappeared at the end of the nucleation stage. This interesting and surprising phenomenon always occurred in our one-stage procedures in the presence of a RAFT agent (Figures S3−S5) but was not found in the absence of a RAFT agent (Figure S6). We presumed that the big spheres were primary composed of polymer chains with a shorter chain length. This idea was supported by an experiment
Figure 5. SEM images of PMMA particles prepared by photoinitiated RAFT dispersion polymerization of MMA with 0.5 wt % BDMAT at irradiation time marked on the images.
in which the centrifuged solid obtained at 15 min irradiation was rinsed with an ethanol/water mixture that contained a little more ethanol than that of the reaction medium for compensating for the solvency contribution of the monomer. We found that the big spheres were dissolved and thus disappeared in this process (Figure 6). The existence of the big spheres at early stage is related to the characteristic of RAFT process. Figure 7 illustrates the process of the one-stage photoinitiated RAFT dispersion polymerization. In this procedure, the content of RAFT agent was set to a low level (0.5 wt %) so as to avoid excessive retardation of nucleation. Photoinitiator with higher concentration generated large amount of free radicals rapidly under irradiation, resulting in initiator-derived polymer chains during the polymerization. These uncontrolled chains propagated very fast, precipitated after reaching the critical chain length, and then aggregated to form nuclei. We call these nuclei formed at very early period “pseudo-nuclei”. At the same time, a large 8792
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Figure 6. SEM images of PMMA particles prepared by photoinitiated RAFT dispersion polymerization of MMA in ethanol/water mixture (40/60 weight ratio) with 0.5 wt % BDMAT at irradiation time of 15 min: (a) original product, (b) rinsed with ethanol/water mixture (45/ 55 weight ratio), and (c) rinsed with ethanol/water mixture (50/50 weight ratio).
amount of controlled polymer chains generated via RAFTmediated polymerization and propagated gradually. Some of these chains were absorbed by the pseudo-nuclei, resulting in the formation of the big spheres. The rest stayed in the medium. Because the control efficiency of the RAFT agents used was not high for MMA (PDI 1.6 typically22 and 1.5−2.2 in present work), a fraction of chains initially reached the critical chain length and precipitated from the solution, leading to the nucleation. Then nuclei and stabilized particles with very small size were formed. As a result, the particles formed at early stages of the reaction displayed a bimodal size distribution (Figure S7). Figure 8 shows that the size of the big particles, as well as the concentration of soluble chains, increased quickly at early period, reached a maximum at 20 min irradiation, and then decreased. And the small particles began to grow rapidly after 20 min irradiation. These changes may be related to the equilibrium between the adsorption of short chains to the particles and desorption from the particles (or in other words, dissolution in the medium). At the beginning, the controlled polymerization produced soluble short chains, and the pseudo-nuclei absorbed the chains from solution. With the progress of the controlled polymerization, the soluble chains concentration increased, and the adsorption of the chains to the particles dominated the equilibrium correspondingly, resulting in the increase in particle size. Afterward, a great quantity of real nuclei generated and absorbed the chains from the solution, leading to the formation and growth of small particles as well as the decrease of soluble chains concentration. Then the desorption of short chains from the big particles gradually dominated the equilibrium, leading to the decrease in the size of big particles. At the end of nucleation
Figure 8. Plots of concentration of the chains in solution and weightaverage diameters (Dw) of the big and small particles versus time for photoinitiated RAFT dispersion polymerization of MMA in the presence of 0.5 wt % BDMAT.
stage, the main reaction loci moved gradually from the medium to the growing particles, and the soluble chains content in the medium trended to a very low level. Accordingly, the big particles finally regressed to the normal size due to the lack of short chains. The temporary big spheres act as a reservoir for the short polymer chains at the nucleation stage, which may provide a buffering effect to the nucleation. It must be one of the key factors making the nucleation stage insensitive to the functional reagents and contributing to the formation of highly monodisperse microspheres even at longer nucleation stage. The higher time length of nucleation stage is beneficial to the absorption of stabilizer to nuclei or growing particles, leading to higher stabilizer coverage on the microspheres. The content of PVP on the surface of PMMA microspheres can be tested by Xray photoelectron spectroscopy (XPS). As shown in Table 1, the presence of RAFT agent really enhanced the content of PVP on the surface of PMMA microspheres. But the value fell back at the content of RAFT agent up to 1.0 wt %. At higher RAFT concentration, the PMMA chains grafted on PVP would become shorter. This is unfavorable to the absorption of stabilizer to the particles. Cross-Linked Microspheres. The synthesis of cross-linked microspheres is a big challenge for dispersion polymerization
Figure 7. Illustration of the process of photoinitiated RAFT dispersion polymerization. 8793
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is supported by the fact that the big spheres still appeared and then disappeared at the nucleation stage even in the present of 5 wt % of DPGDA cross-linker (Figure S5). Therefore, the cross-linking reaction caused little disturbance to the nucleation stage in the RAFT system. It also resulted in more homogeneous networks and higher swelling in the growth stage.44 As a consequence monomers and propagating chains are able to enter the growing particles, which is in favor of forming uniform particles. Copolymeric Functional Microspheres. Another important application of the one-stage procedure besides preparing cross-linked microspheres is the synthesis of functional copolymeric microspheres via adding comonomer(s) to the system at the beginning. We have tried various kinds of comonomer, including MAA, AA, GMA, HPMA, and ACMO. These monomers have different reactivity and solvent affinity and contain different kinds of functional groups. For all comonomers tried, microspheres with highly narrow size distribution (CV ∼ 1%) were obtained at 6 wt % of comonomer content (Figure 10). Comonomer content up to
Table 1. Atomic Concentration of Nitrogen (Based on the Area Fraction of N 1s Peak in XPS) and Corresponding Surface Content of PVP on MMA Microspheres Prepared by Photoinitiated RAFT Dispersion Polymerization with Different Content of RAFT Agent RAFT agent
atomic concn of nitrogen (wt %)
surf. content of PVP (wt %)
without RAFT 0.25 wt % BDMAT 0.5 wt % BDMAT 1.0 wt % BDMAT 0.5 wt % DDMAT
1.89 2.21 2.65 2.01 2.57
13.0 15.2 18.2 13.8 17.8
because the presence of cross-linker not only affects the nucleation stage but also disturbs the particle growth.9 After the successful synthesis of the highly monodisperse microspheres by photoinitiated RAFT dispersion polymerization, we exammed this procedure to synthesize cross-linked microspheres (Figure 9 and Table S1). The bifunctional cross-linkers
Figure 10. SEM images of PMMA copolymeric microspheres prepared by photoinitiated RAFT dispersion polymerization with (a) 6 wt % AA, (b) 6 wt % GMA, (c) 6 wt % HPMA, (d) 6 wt % ACMO, (e) 6 wt % GMA + 6 wt % MAA, and (f) 2 wt % PEGDA + 6 wt % MAA as comonomer(s).
Figure 9. SEM images of cross-linked PMMA microspheres prepared by photoinitiated RAFT dispersion polymerization with (a) 5 wt % DPGDA, (b) 5 wt % PEGDA, (c) 10 wt % DPGDA, and (d) 1 wt % TMPTA as cross-linker.
10 wt % still gave narrow size distribution (CV < 3%; see Table S1 and Figure S9). This comonomer level is very high for preparing monodisperse microspheres by dispersion copolymerization and hard to achieve even employing the two-stage strategy. Herein, photoinitiated RAFT dispersion polymerization exhibited again its insensitivity to functional reagents. This may profit from the controlled/“living” process in addition to the buffering effect of the big spheres mentioned above. In the RAFT process, the majority of chains grow gradually throughout the polymerization. Compositional drift associated with difference in reactivity ratios or partitions into the particles occurs along the polymer chains and not among the various chains.45,46 Thus, the problems resulting from the difference between monomer and comonomer on reactivity and polarity can be avoided. One-stage photoinitiated RAFT dispersion polymerization provides a facile platform to prepare various kinds of functional monodisperse poly(methyl methacrylate) microspheres, for example, microspheres containing two different kinds of functional groups (carboxyl and epoxy, Figure 10e) and carboxyl-containing cross-linked microspheres (Figure 10f).
DPGDA or PEGDA were added to the reaction mixture together with the other reactants at the beginning. When 5 wt % (relative to total monomer) of DPGDA was added, highly uniform microspheres having a coefficient of variation (CV) of 0.89% were achieved. Cross-linker PEGDA gave a similar result. DPGDA content up to 10 wt % still gave narrow size distribution, but the particle surfaces became rough which is characteristic of highly cross-linked microspheres.29,42 We have even used the trifunctional monomer TMPTA as cross-linker and successfully synthesized monodisperse microspheres with CV of 1.62% at 1 wt % TMPTA (Figure 9d) and 1.87% at 2 wt % TMPTA (Figure S8c). These results indicate that the synthesis of cross-linked monodisperse microspheres becomes simple in our strategy. The success of this reaction strategy can be attributed to the characteristics of the RAFT process. In nonliving cross-linking copolymerization, intramolecular cross-linking dominates the early stage of reaction due to the great primary chain length, resulting in large heterogeneities to the solution.43 In the RAFT system, the cross-linking reaction occurs more randomly because of the lower length and higher concentration of primary chains, leading to a more homogeneous solution. This 8794
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CONCLUSION We described a novel one-stage dispersion polymerization procedure that combines the photoinitiation and RAFT polymerization. With this strategy, one can synthesize “living”, cross-linked, and functional microspheres in a single batch mode. The concentration of cross-linker or functional comonomer can reach to 10 wt %, which is much higher than that in the two-stage dispersion polymerization. Moreover, based on the feature of photoinitiation (low reaction temperature, low energy cost, and high reaction rate), this is a promising alternative to synthesize biofunctional microspheres.
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ASSOCIATED CONTENT
S Supporting Information *
Charts for the structures of some chemical materials used, recipes and particle size data, supporting figures, and some SEM images of the microspheres. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 20974126 and 21174165) and Guangdong Science and Technology Department (Grant 2011B090400310), which is gratefully acknowledged.
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REFERENCES
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