Article pubs.acs.org/Macromolecules
Iodine Transfer Polymerization (ITP with CHI3) and Reversible Chain Transfer Catalyzed Polymerization (RTCP with Nitrogen Catalyst) of Methyl Methacrylate in Aqueous Microsuspension Systems: Comparison with Bulk System Yukiya Kitayama,† Mika Yorizane,† Hideto Minami,† and Masayoshi Okubo*,†,‡ †
Department of Chemical Science and Engineering, Graduate School of Engineering, Rokko, Nada, Kobe University, Kobe 657-8501, Japan ‡ Smart Spheres Workshop Co. Ltd., Koyo-cho Naka 2-1-214-122, Higashi-Nada, Kobe 658-0032, Japan ABSTRACT: Iodine transfer polymerization (ITP) with CHI3 as transfer agent and reversible chain transfer catalyzed polymerization (RTCP) with N-iodosuccinimide (NIS) as catalyst of methyl methacrylate (MMA) were successfully applied to aqueous microsuspension systems (respectively, microsuspension ITP and microsuspension RTCP). Both microsuspension ITP and RTCP proceeded smoothly without induction period and with a controlled/living manner. Polydispersity index (PDI, Mw/Mn) of the microsuspension RTCP was smaller than that of the microsuspension ITP, and it was larger than RTCP in a bulk system (bulk RTCP). The difference seems to be based on that NIS would partly react with water dissolved in the MMA phase, and formed succinimide, which is a low controllable catalyst in comparison with NIS. With decreasing particle (monomer droplet) size, the polymerization rate increased, and a reasonable control was maintained throughout the polymerization based on the segregation effect.
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INTRODUCTION Controlled/living radical polymerization (CLRP) leads to the preparation of well-defined vinyl polymers having predetermined molecular weight, narrow molecular weight distribution, and various complex architectures.1−9 One of the most frequently employed CLRP techniques is iodine transfer polymerization (ITP) (Scheme 1a).10−19 ITP is based on degenerative chain transfer (DT) process between dormant chains and propagating radicals, in which the iodide compounds such as cyanopropyl iodide and CHI3 were used as a control agent. ITP has attracted much attention as a simple and robust method which dose not use metal. Moreover, ITP is well-known as a only technique for the synthesis of the welldefined polymers based on the particular monomers such as fluoromonomers and phosphonic monomers.11,12,19,20 Therefore, the various kinds of techniques developed from the ITP system have been proposed until now. Lacroix-Desmazes and co-workers advanced the ITP technique to reverse iodine transfer polymerization (RITP), in which the iodide compounds were formed in situ from thermal initiator and iodine (I2).21,22 RITP is an important technique because the iodide compounds, which demand a lot of attention to storage, need not to be prepared before the polymerization. Until now, the various kinds of polymers were successfully synthesized by RITP including methacrylates, in which the polydispersity index © 2012 American Chemical Society
Scheme 1. Reversible Chain Transfer Catalyzed Polymerization (RTCP) (NIS = N-Iodosuccinimide)
Received: January 12, 2012 Revised: February 17, 2012 Published: February 28, 2012 2286
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In this article, the reason for the difference between bulk and microsuspension RTCPs with NIS of MMA will be clarified, in which the chain transfer agent was replaced from CP-I to CHI3 because CHI3 is a commercially available and inexpensive. The additional important point is a stability of CHI3 during storage. Moreover, the improving technique for the PDI control will be suggested.
was >1.46 because of the low frequency of iodine compounds by the exchange constant.21,23,24 Percec and co-workers discovered that vinyl chloride could be polymerixed with CHI3 as a control agent via single electron transfer and DT mechanisms with copper(0) or Na2S2O4 as catalyst (SETDTLRP).25,26 The SET-DTLRP enabled to synthesize various vinyl polymers with higher molecular weight in short polymerization time.27,28 Montairo and co-workers simulated the evolution of PDI with conversion in reversible addition−fragmentation chain transfer (RAFT) polymerization,29,30 which is based on the DT mechanism as well as the ITP system, using a numerical simulation.15,31 The simulation indicates that the ratio of the chain transfer constant to propagation rate constant (Cex) is very important for obtaining a low PDI value. When Cex value is 5.0, the PDI value is small (1.3). However, when the Cex value is decreased to 1.0, the PDI value is maintained at a constant value (2.0). The trend was also clarified by the experimental work.12,32 Cex value in the ITP system is normally below 5.0; therefore, the obtaining PDI value normally was approximately >1.5. Recently, Goto and co-workers developed a new CLRP named reversible chain transfer catalyzed polymerization (RTCP) (Scheme 1).33,34 This technique is controlled by adding very small amount of catalyst (A-I) such as germanium (Ge)35,36 or nitrogen (N)33,37 compound, e.g., N-iodosuccinimide (NIS) to a ITP system. A-I works as deactivator of P•, in situ producing A•. A• works as activator of P−I, producing P• and A-I. Because this cycle (Scheme 1b) occurs frequently, the controllability of polydispersity is improved dramatically (∼1.2). Attractive features of RTCP include low cost, environmentally friendliness, and simplicity. In order for CLRP to be useful for industrial applications, it must be developed from homogeneous systems to heterogeneous systems, and significant progress in this area has been made in recent years.38−44 Several CLRP techniques, which are nitroxide-mediated radical polymerization (NMP),38,45,46 atom transfer radical polymerization (ATRP),47−49 RAFT polymerization,29,50,51 and organotellurium-mediated radical polymerization (TERP),52−56 were successfully applied to the aqueous dispersed systems. ITP was also successfully applied to the aqueous dispersed systems. Charleux and co-workers carried out ITP with C6F13I as a control agent in the miniemulsion systems (miniemulsion ITP) and successfully synthesized polystyrene (PS) and PS-bpoly(n-butyl acrylate).18 Lacroix-Desmazes and co-workers synthesized triblock copolymers by the miniemulsion ITP using an end-iodefunctionalized dimethylsiloxane macrotransfer agent.57 Moreover, they successfully applied RITP to the miniemulsion systems, in which well-defined PS was synthesized.58 The Lacroix-Desmazes group is also a pioneer for the RITP in the emulsion polymerization system (emulsion RITP) using water-soluble and nonhazardous NaI as a precursor to the control agent.32,59,60 In our previous work, it was reported that ITP in a microsuspension system (microsuspension ITP) of methyl methacrylate (MMA) with 2-cyanopropyl iodide (CP-I) was not carried out with a control/living manner. On the other hand, microsuspension RTCP with NIS was successfully conducted with a good control/living manner for the first time;61 however, the molecular weight distribution was broader than RTCP in a bulk system (bulk RTCP). The reason for the difference was unclear.
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EXPERIMENTAL PART
Materials. MMA (Tokyo Kasei Kogyo Co. Ltd., Tokyo, Japan, >99.8%) was purified by distillation under reduced pressure in a nitrogen atmosphere. Water used in all experiments was obtained using an Elix UV (Millipore, Japan) purification system and had a resistivity of 18.2 MΩ·cm. Reagent grade 2,2′-azobis(isobutyronitrile) (AIBN) (Nacalai Tesque, Kyoto, Japan) was purified by recrystallization. Iodoform (CHI3), n-hexadecane (Nacalai Tesque), N-iodosuccinimide (NIS) (Aldrich Chem Co. Ltd., 95%), n-tetradecyltrimethylammonium bromide (TTAB) (Tokyo Kasei Kogyo), sodium dodecyl sulfate (SDS) (Wako Chemical Industries, Ltd., Japan), and poly(vinyl alcohol) (PVA) (Nippon Synthetic Chemical Ind. Co., Ltd., Japan; Gohsenol GH-17: degree of polymerization, 1700; degree of saponification, 88%) were used as received. ITP in Microsuspension System (Microsuspension ITP). A monomer solution (MMA 10 g, 100 mmol; AIBN 20 mg, 0.13 mmol; CHI3 0.39 g, 1.0 mmol) was mixed with an aqueous (90 g) solution of TTAB (0.45 g) and stirred vigorously at 5000 rpm using a homogenizer (NISSEI ABM-2) for 2 min. The monomer emulsion was transferred to glass ampules and degassed using several vacuum/ N2 cycles, and then the ampules were sealed off under vacuum. Microsuspension ITP of MMA was carried out at 80 °C with shaking of the ampules horizontally at 80 cycles min−1 (3 cm strokes). All of these processes were performed under a safe light because iodo compounds are light-sensitive. RTCP in Microsuspension System (Microsuspension RTCP). Microsuspension RTCP of MMA was carried out in the same way as the microsuspension ITP, except for containing NIS (3.0 mg, 13 μmol) in the monomer phase. Conversion Measurement. Conversion was determined by gas chromatography (GC-18A, Shimadzu Co., Kyoto, Japan) employing helium as carrier gas, N,N-dimethylformamide as solvent, and p-xylene as internal standard. Molecular Weight Measurement. Number-average molecular weight (Mn), weight-average molecular weight (Mw), and molecular weight distribution (MWD) were measured by gel permeation chromatography (GPC) with two S/divinylbenzene gel columns (TOSOH Corp., TSKgel GMHHR-H, 7.8 mm i.d. × 30 cm; bead size = 5 μm) using tetrahydrofuran (THF) as eluent at 40 °C at a flow rate of 1.0 mL/min employing refractive index (RI) (TOSOH RI-8020/21) and ultraviolet (UV) detectors (TOSOH UV-8II). The columns were calibrated with six standard PS samples (1.05 × 103−5.48 × 106, Mw/ Mn = 1.01−1.15). Theoretical molecular weight (Mn,th) was obtained using Mn,th = αMM[M]0/[R-I]0, where α is the fractional conversion of monomer, MM is the molecular weight of monomer, and [M]0 and [RI]0 are the initial concentrations of monomer and transfer agent (alkyl iodide), respectively. It was reported that CHI3 works as a bifunctional chain transfer agent;25 however, the Mn,th is not dependent on monoor bifunctional. Particle Size Measurement. The number-average diameter (Dn) and coefficient of variation (Cv) of the particles were measured the diameters of more than 200 particles using transmission electron microscopy (TEM) (JEM-1230, JEOL Ltd., Tokyo, Japan) using image analysis software (WinROOF, Mitani Co. Ltd., Fukui, Japan) for a Macintosh computer. Partitioning of NIS to Aqueous Phase. A MMA/NIS/water mixture, where NIS was used a larger amount (90 mg), which dissolves in an aqueous phase (i.e., the solubility of NIS in water is >1.0 mg/gwater), to facilitate the detection of absorbance, consisting of the same composition as the polymerization recipe except for NIS was stirred at 80 °C for 1 h in Schlenk flask protected with an aluminum film. In this 2287
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experiment, NIS was initially dissolved in MMA. The absorbance of NIS in water was determined by UV−vis (Shimazdu UV-2500). The concentration of NIS in the aqueous phase was calculated from the difference in absorbencies (at 463 nm) before and after partitioning.
the NIS concentration. As a result, the reaction between NIS and radical species, which leads to a retardation of the polymerization, gradually decreased. This would be the reason why the polymerization did not follow the first-order kinetics in the microsuspension RTCP (Figure 1b). Figure 2 shows MWD, Mn and Mw/Mn at different conversions of the microsuspension ITP (a, b) and RTCP (c,
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RESULTS AND DISCUSSION Select of Proper Emulsifier. In this article, we tried to use CHI3 as transfer agent, which is a commercially available, for microsuspension ITP and RTCP of MMA. Matyjaszewski and co-workers have already reported that bulk ITP of styrene using CHI3 was successfully controlled.14 To our knowledge, ITP and RTCP using CHI3 as a transfer agent in aqueous dispersed systems has not been reported. First of all, in order to choose an appropriate emulsifier for microsuspension RTCP of MMA with NIS as catalyst, the preliminary polymerizations with various kinds of emulsifiers (SDS, TTAB, and PVA) were carried out. Coagulation occurred in the microsuspension RTCP systems with SDS. Using PVA as a surfactant, the emulsion has a small amount of coagulation. On the other hand, the microsuspension RTCP system with TTAB proceeded without coagulation. Therefore, TTAB was chosen as an appropriate emulsifier in the microsuspension RTCP of MMA with NIS. The reason is unclear in this work. We will clarify the reason in a following paper. Microsuspension ITP and RTCP of MMA. Figure 1 shows conversion−time plots and first-order plots of microsuspension
Figure 2. Molecular weight distribution (with RI detector) at different conversions (a, c) and number-average molecular weight (Mn) (open circles) and polydispersity index (Mw/Mn) (closed circles) vs conversion plots (b, d) for the microsuspension ITP (a, b) and microsuspension RTCP (c, d) of MMA at 80 °C. [CHI3]0/[AIBN]0/ [NIS]0 for ITP and RTCP were respectively 80/10/0 and 80/10/1 (molar ratios).
d). In the microsuspension ITP, the Mn increased linearly with conversion, and the values were always higher than corresponding Mn,th and the Mw/Mn values were relatively high (∼1.84). These results suggest that CHI3 worked as transfer agent in an aqueous dispersed system, but the control was not enough like bulk ITP in comparison with the other CLRP because of the low chain transfer rate constant of poly(MMA) (PMMA) radical to CHI3 relative to propagation rate constant. In the microsuspension RTCP, the Mn increased linearly with conversion and the values agreed with the corresponding Mn,th values. The Mw/Mn values were lower (1.3−1.6) than those in the microsuspension ITP. These results indicate that NIS works effectively as catalyst in the microsuspension RTCP. Comparison between Bulk and Microsuspension RTCP Systems. Figure 3 shows conversion−time plots of bulk and microsuspension RTCPs of MMA with NIS and CHI3. Both polymerizations proceeded smoothly, but the polymerization rate of the microsuspension RTCP was slightly lower than that of the bulk RTCP conducted under the same conditions except for without TTAB aqueous solution as medium. Figure 4 shows Mn and Mw/Mn at different conversions of the bulk and microsuspension RTCPs of MMA with NIS as catalyst. The Mw/Mn of the microsuspension RTCP was ∼1.48
Figure 1. Conversion vs time plots (a) and first-order plots (b) for iodine transfer polymerization (ITP) (open circles) with CHI3 and reversible chain transfer catalyzed polymerization (RTCP) (closed circles) with N-iodosuccinimide (NIS) of methyl methacrylate (MMA) in microsuspension systems with 2,2′-azobis(isobutyronitrile) (AIBN) at 80 °C. [CHI3]0/[AIBN]0/[NIS]0 molar ratios for ITP and RTCP were respectively 80/10/0 and 80/10/1.
ITP with CHI3 as transfer agent (closed circles) and microsuspension RTCP with NIS as catalyst (open circles) of MMA in aqueous dispersed systems. Both polymerizations smoothly proceeded without an induction period and almost finished within a couple of hours. Size distributions of obtained particles were almost the same as those of original droplets, which were measured with an optical microscope. The rate of the microsuspension RTCP was somewhat slower than that of the microsuspension ITP. This must be based on that the nitrogen compound radical (A•), which was generated from the reaction between NIS and radical species (reversible chain transfer reaction) in the RTCP system, could not add with MMA, leading to the retardation of the polymerization relative to the ITP system, in which NIS did not exist. However, A• derived from the NIS terminates with propagating radical species during the polymerization, leading to gradual decreasing 2288
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Figure 5. FT-IR spectra of (a) NIS, (b) succinimide, and (c) NIS reacted with water previously.
Figure 3. Conversion vs time plots for the bulk (triangles) and microsuspension (circles) RTCPs of MMA at 80 °C. [CHI3]0/ [AIBN]0/[NIS]0 = 80/10/1 (molar ratio).
Figure 6. Mn (open circles) and Mw/Mn (closed circles) vs conversion plots for bulk RTCPs of MMA at 80 °C. Catalyst: (a) treated NIS in water for 3 h at 80 °C; (b) succinimide. [CHI3]0/[AIBN]0/[NIS]0 = 80/10/1 (molar ratio).
RTCP with the NIS. Mw/Mn values were larger than those of the bulk RTCP with NIS, but similar to those in the bulk RTCP with succinimide and in the microsuspension RTCP with NIS (Figure 4). These results indicate that NIS in the microeuspension RTCP with NIS reacted with water, resulting in succinimide. Improving PDI Control in Microsuspension RTCP with NIS. In order to improve PDI control by suppressing hydrolysis of NIS in the microsuspension RTCP, the polymerization time should be shortened. In CLRP in aqueous dispersed systems based on the DT mechanism, the particle size was an important parameter for the polymerization rate.62,63 Radicals located in separated particles do not react each other (segregation effect).39 When the particle size is small (
