Dispersion Reversible Chain Transfer Catalyzed Polymerization

Apr 15, 2015 - We demonstrated a dispersion reversible chain transfer catalyzed polymerization (dispersion RTCP) of methyl methacrylate (MMA) with 1-p...
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Dispersion Reversible Chain Transfer Catalyzed Polymerization (Dispersion RTCP) of Methyl Methacrylate in Supercritical Carbon Dioxide: Pushing the Limit of Selectivity of Chain Transfer Agent Taisuke Kuroda,† Tomoya Taniyama,† Yukiya Kitayama,† and Masayoshi Okubo*,†,‡ †

Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan Smart Spheres Workshop Co., Ltd., Koyo-Naka 2-1-214-122, Higashi-Nada, Kobe 658-0032, Japan

‡

S Supporting Information *

ABSTRACT: We demonstrated a dispersion reversible chain transfer catalyzed polymerization (dispersion RTCP) of methyl methacrylate (MMA) with 1-phenylethyl iodide (PEI) as chain transfer agent and GeI4 as catalyst in supercritical carbon dioxide (scCO2), where the PE-I was known as noneffective chain transfer agent for polymerization of MMA in RTCP. The dispersion RTCP in scCO2 proceeded with control/livingness. On the other hand, in bulk system (bulk RTCP) and in dispersion iodine transfer polymerization (dispersion ITP) in scCO2 under the same conditions except for GeI4, no livingness was maintained. From these results, it was assumed that the reason for the living character in the dispersion RTCP in scCO2 is based on an accelerated reversible chain transfer reaction in scCO2. Based on the insight, when the dispersion RTCP of MMA was carried out at higher scCO2 pressure, poly(MMA) (PMMA) having a narrower molecular weight distribution was obtained because of the higher degree of PMMA plasticization by scCO2. Moreover, we advanced the idea to synthesize polystyrene (PS)-b-PMMA, of which synthesis was difficult in homogeneous systems, by seeded dispersion RTCP of MMA with PS-I as macro-chain-transfer agent and GeI4 as catalyst in scCO2.

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INTRODUCTION CO2 exists as supercritical carbon dioxide (scCO2) in the region over the critical temperature (Tc) of 31.1 °C and the critical pressure (Pc) of 7.38 MPa. scCO2 is growing up as an important and environmentally friendly medium for chemical reactions.1−3 Especially in polymerization reaction, scCO2 has several advantages: nonflamable, nontoxic, easy removal of solvent after polymerization, negligible chain transfer, and tunable solubility power by changing the temperature and pressure.4−7 In the field of radical polymerization, the development of controlled/living radical polymerization (CLRP) opens the new era for precision polymer synthesis.8−20 CLRP enables to prepare well-defined vinyl polymers having predetermined molecular weight, narrow molecular weight distribution, and various complex molecular architectures.21−25 Among all CLRP techniques, iodide transfer polymerization (ITP) has attracted much attention as a simple and robust method because of its feature for cheap, safety, and nonuse of metal (Scheme 1a).26−31 Moreover, ITP is also an important technique from the viewpoint of the width of the polymerizable monomer species. Fluorinated, phosphonated, and unconjugated monomers are polymerized by ITP maintaining high livingness. However, the intrinsic problem of ITP is low control of © XXXX American Chemical Society

Scheme 1. Reversible Chain Transfer Catalyzed Polymerization (RTCP)34−36

molecular weight distribution due to its slow chain transfer reactions.32,33 Recently, Goto and co-workers updated ITP to reversible chain transfer catalyzed polymerization (RTCP) as a novel Received: January 18, 2015 Revised: March 10, 2015

A

DOI: 10.1021/acs.macromol.5b00104 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules technique (Scheme 1).34−39 RTCP is controlled by adding very small amount of catalyst (A-I) such as germanium (Ge) or nitrogen (N) compound, e.g., N-iodosuccinimide (NIS) to the ITP system. ITP is based on degenerative chain transfer (DT) process between dormant chains and propagating radicals. In RTCP, A-I works as deactivator of P•, in situ producing A•, and A• works as activator of P−I, producing P• and A-I. Because this reversible chain transfer (RT) cycle (Scheme 1b) occurs frequently compared to the DT cycle, the catalyst works as an effective radical career, resulting in dramatical improvement of the controllability of polydispersity relative to ITP system. One of the most challenging topics in CLRP is a breaking limit of the selectivity of appropriate chain transfer agents for the monomer species.40 For example, for the RTCP of methyl methacrylate (MMA), the chain transfer agent such as 1phenylethyl iodide (PE-I) generating a secondary radical species cannot be adopted.34 In that case, the polymerization did not proceed with living nature because the initiation reaction of 1-phenylethyl radical with MMA is much slow. The similar problems were reported by the other CLRP techniques such as reversible addition−fragmentation chain transfer (RAFT) and ITP systems.10,41 Moreover, the limit expands to the preparation of the block copolymer. The poly(MMA) (PMMA)-b-polystyrene (PS) could be synthesized from PMMA-I macro-chain-transfer agent, but PS-b-PMMA was not successfully prepared from PS-I macro-chain-transfer agent due to the same reason. In this article, we report an important discovery for breaking the selectivity of chain transfer agent in RTCP of MMA with germanium(IV) iodide (GeI4) in scCO2, where PE-I was selected as noneffective chain transfer agent for the polymerization system in bulk system. Moreover, the discovery will be developed to synthesize PS-b-PMMA from PS-I macro-chaintransfer agent, leading to more flexible block copolymer synthesis.

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at room temperature. The polymer was obtained as a powder above 65% conversion. The polymer particles were observed with a scanning electron microscope (SEM, JSM-6510, JEOL Ltd., Tokyo, Japan) after centrifugal washing three times with hexane. Effect of scCO2 Pressure on Molecular Weight Distribution Control in Dispersion RTCP. The polymerizations were carried out with same polymerization recipes and procedures as described above. The effect of scCO2 pressure was investigated for dispersion RTCP under 10, 30, and 50 MPa of scCO2 pressures at 80 °C, where the polymerization time was fixed for 6 h. Dispersion ITP of MMA in scCO2. Dispersion ITP of MMA in scCO2 was conducted with the same conditions as the dispersion RTCP except for without GeI4. Solution RTCP of MMA in Toluene. MMA (5.0 g, 50 mmol), VPS-0501 (125 mg, 25 μmol), AIBN (72 mg, 440 μmol), PE-I (116 mg, 500 μmol), GeI4, (7.5 mg, 12 μmol), and toluene (20 mL) were added into a round-bottom Schlenk flask in the dark, which was sealed off with a silicon rubber septum and then degassed using several N2/ vacuum cycles. The flask was then placed in a water bath at 80 °C (taken to be the start of the polymerization, t = 0). Dispersion RTCP of MMA in Hexane. Dispersion RTCP of MMA in hexane was conducted in the same conditions as the solution RTCP, where the hexane was used in place of toluene. Chain Extension Test. Styrene (2.5 g, 24 mmol), AIBN (16.5 mg, 100 μmol), PMMA-I (160 μmol), and GeI4 (3.8 mg, 6 μmol) were mixed, and the bulk RTCP was carried out at 80 °C in a N2 atmosphere in sealed glass tubes. The obtained polymers at low conversions were corrected by reprecipitation with hexane. The chainend functionality was calculated according to the previous reported procedure using GPC with RI and UV detectors at low conversion due to the negligibility of new chain generation during the chain extension test.37,44 Preparation PS-I seed particles by dispersion RTCP in scCO2 and PS-b-PMMA by seeded dispersion RTCP in scCO2. Dispersion RTCP of styrene in scCO2 was conducted in a 25 mL stainless steel reactor. Styrene (5.0 g, 48 mmol), VPS-0501 (125 mg, 25 μmol azo content of whole VPS-0501), BPO (206 mg, 850 μmol), CHI3 (189 mg, 480 μmol), and GeI4 (7.5 mg, 12 μmol) were added to therein and degassed at several N2/vacuum cycles, and subsequently the reactor was pressurized it with liquid CO2 to 9 MPa at room temperature using a high-pressure pump. The reactor temperature was subsequently raised to 80 °C by immersing it in a temperaturecontrolled water bath using magnetic stirring at 200 rpm, during which the pressure was increased to approximately 30 MPa. The polymerization was stopped by cooling the reactor in a water bath to rt, and the CO2 was vented slowly. When seeded dispersion RTCP of MMA with PS-I in scCO2 was carried out, the polymerization was sequentially demonstrated from the preparation of PS-I seed particles by dispersion RTCP in scCO2 in a 25 mL stainless steel reactor. MMA (2.0 g, 20 mmol), AIBN (8 mg, 50 μmol), and GeI4 (2.9 mg, 5 μmol) were further added to the reactor at rt, and reactor was heated to 80 °C, where the pressure was increased to 40 MPa from 30 MPa due to further injection of further monomer species. Molecular Weight Measurement. Weight- and number-average molecular weights (Mw and Mn, respectively) and molecular weight distribution (MWD) were measured by GPC with two styrene/ divinylbenzene gel columns (TOSOH corporation, TSKgel GMHHRH, 7.8 mm i.d. × 30 cm) using THF as eluent at 40 °C at a flow rate of 1.0 mL/min employing a refractive index (RI) detector (TOSOH RI8020/21) and ultraviolet (UV) detector (TOSOH UV-8II). The columns were calibrated with six standard PS samples (1.05 × 103− 5.48 × 106, Mw/Mn = 1.01−1.15), where the PS standard was selected because of our final purpose in this article to prepare PS-b-PMMA from PS-I by dispersion RTCP in scCO2. Theoretical molecular weight (Mn,th) was predicted by eq 1

EXPERIMENTAL SECTION

Materials. MMA (Tokyo Kasei Kogyo Co. Ltd., Tokyo, Japan, >99.8%) and styrene (Mitsubishi Chemical Co. Ltd., Tokyo, Japan, >99.6%) were purified by distillation under reduced pressure in a nitrogen atmosphere. Reagent grade of 2,2′-azobis(isobutyronitrile) (AIBN; Wako Pure Chemicals) was purified by recrystallization using methanol. An initiator having a polydimethylsiloxane unit, of which molecular weight is ca. 5000 (VPS-0501) (Wako Pure Chemicals), GeI4 (Aldrich, Japan) was used as received. PE-I was prepared according to Matyjaszewski and co-workers’ report.42 Industrial grade CO2, with a purity of 99.5% or more was used (Kobe Sanso Co., Japan). Dispersion RTCP of MMA in ScCO2. Dispersion RTCP of MMA in scCO2 was conducted in a 25 mL stainless steel reactor. MMA (5.0 g, 50 mmol), VPS-0501 (125 mg, 25 μmol azo content of whole VPS0501), AIBN (72 mg, 438 μmol), PE-I (116 mg, 500 μmol), GeI4 (7.5 mg, 12 μmol) were added to therein and degassed at several N2/ vacuum cycles, and subsequently the reactor was pressurized it with liquid CO2 to 9 MPa at room temperature using a high-pressure pump. The relatively high concentration of AIBN compared to PE-I was selected due to slow polymerization rate in scCO2 as reported in elsewhere.43 The reactor temperature was subsequently raised to 80 °C by immersing it in a temperature-controlled water bath using magnetic stirring at 200 rpm, during which the pressure was increased to approximately 30 MPa. The polymerization was stopped by cooling the reactor in a water bath to rt, and the CO2 was vented slowly. The conversion was measured by gravimetry. At conversion lower than 65%, the polymer was obtained as a highly viscous solution, which was dissolved in toluene, and the obtained polymer was collected by filtration after precipitation in an excess of methanol and dried in vacuo

M n,th = B

[M]0 MWMα [PE‐I]0

(1) DOI: 10.1021/acs.macromol.5b00104 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules where [M]0 and [PE-I]0 are the initial concentrations of monomer and transfer agent, respectively. MWM is the molecular weight of monomer and α is monomer conversion.

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RESULTS AND DISCUSSION Dispersion RTCP of MMA with PE-I and GeI4 was carried out in scCO2, compared with bulk RTCP. Both polymerizations proceeded smoothly and reached approximately 80% conversions in 6 and 1.5 h for the dispersion RTCP in scCO2 and the bulk RTCP, respectively (Figure S1 in Supporting Information). In the bulk RTCP, the MWD shift with increasing conversion was not observed, and Mn reached much high value relative to Mn,th value from the initial stage of the polymerization (Figure 1a,b) because of the slow initiation

Figure 2. Photograph of product (a) obtained by the dispersion RTCP of MMA in scCO2 at 80 °C for 6 h at 30 MPa with PE-I and GeI4 and SEM photograph of PMMA particles after centrifugal washing with hexane.

extension test was carried out by utilizing bulk RTCP of styrene with the PMMA-I and GeI4. The MWD shifted to higher molecular weight in the chain extension test and Mn increased linearly with conversion although they deviated downward from corresponding Mn.th values, which would be caused by generation of new polymer chains derived from the initiators (Figure 3). Moreover, we compared MWDs obtained by GPC

Figure 1. MWD (by GPC with RI detector) (a, c) and Mn and Mw/Mn (b, d) at various conversions for bulk RTCP (a, b) and dispersion RTCP in scCO2 at 30 MPa (c, d) of MMA at 80 °C with PE-I as chain transfer agent and GeI4 as catalyst.

of the PE-I. The phenomenon was in accordance with Goto group’s report34 described in the Introduction. On the other hand, in the dispersion RTCP in scCO2, the MWD shifted to higher molecular weight with increasing conversion. Moreover, the Mn value increased linearly with maintaining close value with corresponding Mn.th, and the Mw/Mn values were kept at approximately 1.5 throughout the polymerization, where the effect of VPS-0501 used as inistab on the Mw/Mn values seems to be negligible due to its low molar contents as well as low initiation efficiency. (Figure 1c,d). The particles prepared by the dispersion RTCP in scCO2, which were obtained as a dry state by vending (depressurizing the reactor) of CO2, had broad micrometer-size distribution with nonspherical morphology. The morphology seems to be caused by deformation due to dead weight of sedimentary particles after stopping agitation because of the high plasticization effect of scCO2 for PMMA with short chain length (Figure 2). The spherical morphology would be obtained by preparing the high molecular weight polymers reported previous study.45 In order to confirm the livingness of the PMMA-I prepared by the dispersion RTCP in scCO2 at 80 °C for 6 h, a chain

Figure 3. MWD (by GPC with RI (a) and UV (b) detectors) (a) and Mn and Mw/Mn (c) at various conversions for chain extension test (bulk RTCP) of styrene at 80 °C with PMMA-I as macro-chaintransfer agent, which was prepared by dispersion RTCP of MMA in hexane with GeI4 as catalyst.

with refractive index (RI) and ultraviolet (UV) detectors, in which RI detector detects all polymer chains, on the other hand, UV detector with 254 nm wavelength detects only polymer chains containing styrene units. The MWDs obtained using RI and UV detectors were practically identical (Figure 3). This chain extension test indicates that the PMMA-I prepared with PE-I had a sufficient livingness. The chain end functionality of the PMMA-I calculated using these MWDs, was 64% of total PMMA chains (Figure S2 in Supporting Information). On the other hand, the PMMA prepared by the bulk RTCP did not have livingness. C

DOI: 10.1021/acs.macromol.5b00104 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules In order to rationalize the reason why the dispersion RTCP of MMA with PE-I in scCO2 proceeded with livingness, first, we examined the effect of viscosity in the polymerization media. The chain transfer reaction in RTCP is occurred between longer polymer chains except for initial stage, and the reaction rate slows down in a viscous medium because of low diffusivity of polymer chains, which is similar to termination reaction. In dispersion polymerizations in scCO2, it is known that scCO2 works as a plasticizing agent, resulting in comparatively low viscosity states even in the polymerizing particles.46,47 On the other hand, in bulk polymerization the viscosity is high even at a low conversion and markedly increases with conversion. When the solution RTCP of MMA with PE-I and GeI4 in toluene was carried out, in which the viscosity is maintained at low values until a middle stage of conversion, the polymerization proceeded without maintaining the control/livingness similar to the bulk system. The polymerization rate became slower compared to in the bulk RTCP because of decrease in the concentrations of initiator and monomer, and the conversion reached ca. 80% in 4 h (Figure S3 in Supporting Information). The MWD shift with increasing conversion was not observed, and the Mw/Mn values were in the range of 1.8− 2.1 (Figure 4). These results indicate that the viscosity of the medium is not a decisive subject for livingness in the dispersion RTCP of MMA in scCO2 with PE-I and GeI4.

Figure 5. MWD (by GPC with RI detector) (a) and Mn and Mw/Mn (b) at various conversions for seeded dispersion RTCP of MMA in hexane with PE-I as chain transfer agent and GeI4 as catalyst at 80 °C.

Figure 6. MWD (by GPC with RI detector) (a) and Mn and Mw/Mn (b) at various conversions for chain extension test (bulk RTCP) of styrene at 80 °C with PMMA-I as macro-chain-transfer agent, which was prepared by dispersion RTCP of MMA in hexane, with GeI4 as catalyst.

RT reaction in dispersion RTCP in scCO2 (Figure S5 in Supporting Information). As shown in Figure 7, the MWD

Figure 4. MWD (by GPC with RI detector) (a) and Mn and Mw/Mn (b) at various conversions for solution RTCP of MMA in toluene at 80 °C with PE-I as transfer agent and GeI4 as catalyst.

In order to confirm the importance of dispersed system for maintaining the livingness, dispersion RTCP of MMA with PEI and GeI4 was carried out in hexane, which is hydrophobic and is miscible with MMA but a nonsolvent for PMMA like CO2. The polymerization proceeded smoothly, and the spherical particles were obtained (see Figure S4 in Supporting Information), but high Mn values were obtained even in the initial stage of the polymerization and Mw/Mn values were also comparatively high (1.7−1.9), as shown in Figure 5. In the chain extension test for PMMA-I prepared by the dispersion RTCP in hexane, the living character was not observed (Figure 6). The results indicate that the dispersion RTCP of MMA in hexane with PE-I and GeI4 did not proceed with living nature, which was a similar to the bulk RTCP described above. Therefore, a factor of that means the dispersed system is not also a decisive subject for the livingness of the dispersion RTCP in scCO2. As shown in Scheme 1, RTCP consists of DT and RT. In dispersion ITP of MMA with PE-I in scCO2 only DT mechanism works. The polymerization rate in dispersion RTCP in scCO2 was lower than that for dispersion ITP in scCO2 due to the decreasing the propagating radical species via

Figure 7. MWD (by GPC with RI detector) (a) and Mn and Mw/Mn (b) at various conversions for dispersion ITP of MMA in scCO2 at 80 °C and 30 MPa with PE-I as transfer agent.

obtained from the dispersion ITP of MMA in scCO2 did not shift to higher molecular weight side and Mn value was much high from an initial stage of the polymerization. The results indicate that the DT mechanism based on the reaction between propagating macroradical and macro-chain-transfer agent was not enhanced by scCO2, which was a similar to the bulk ITP system (Figure 8). In other words, this may suggest that RT mechanism is enhanced by in the scCO2 system, resulting in maintaining the livingness in dispersion RTCP of MMA in the scCO2 with PE-I and GeI4. In a previous article,48 the rate of a precipitation nitroxidemediated polymerization (precipitation NMP) of styrene in scCO2 was lower in the corresponding solution NMP, most D

DOI: 10.1021/acs.macromol.5b00104 Macromolecules XXXX, XXX, XXX−XXX

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Figure 8. MWD (by GPC with RI detector) (a) and Mn and Mw/Mn (b) at various conversions for bulk ITP of MMA at 80 °C with PE-I as transfer agent.

likely as a result of monomer partitioning between the particles (polymerization locus) and the continuous phase. The level of control was significantly higher in scCO2 compared to the corresponding solution NMP. Monomer partitioning in the scCO2 system would be one reason for thisa lower monomer concentration at the locus of polymerization results in an increase in the rate of deactivation relative to the rate of propagation; thus, fewer monomer units are added per activation−deactivation cycle, and each chain experiences a greater number of activation−deactivation cycles during its growth. In addition, propagation rate constant (kp) of MMA in scCO2 is reduced by 40%.49 These ideas must be applied to the dispersion RTCP of MMA in scCO2 and positively affect the molecular weight distribution in the dispersion RTCP. As well as the dispersion RTCP, these phenomena also influenced to dispersion ITP; however, the living character was not obtained as discussed above. Therefore, monomer partitioning and decreasing kp were not direct answer toward successful demonstration of dispersion RTCP of MMA. On the other hand, reasons previously given for improved control of the precipitation NMP in scCO2,50 such as greater GeI4 mobility in particles highly plasticized by scCO2, still stand. The diffusivity of small molecules such as GeI4 in scCO2 is 1−2 orders of magnitude higher than in organic solvents, resulting in an increase in the activation and deactivation rates in RT mechanism.51 We consider this may be also one of the reasons why the dispersion RTCP of MMA in scCO2 using GeI4 could be successfully carried out in this article, although Goto et al. reported that bulk RTCP of MMA with PE-I and GeI4 could not be successfully carried out.52 It has been clarified that the degree of plasticization depended on the various parameters such as temperature and scCO2 pressure.53 If the activation−deactivation cycle for RT mechanism is accelerated by the polymer plasticization, the MWD control should depend on the scCO2 pressure. We next demonstrated the dispersion RTCP of MMA with PE-I and GeI4 in different pressures of scCO2 (10, 30, and 50 MPa). As a result, the Mw/Mn value was decreased with increasing scCO2 pressure, i.e., the degree of polymer plasticization, where the conversions were 92%, 80%, and 82% at 10, 30, and 50 MPa, respectively (Figure 9). The capability of the block copolymer synthesis is one of another important point for CLRP. Until now, Howdle et al. extensively developed the dispersion reversible addition− fragmentation chain transfer (dispersion RAFT) polymerization in scCO2, and they successfully prepared PMMA-b-PS particles from PMMA macro-chain-transfer agent.54,55

Figure 9. Mw/Mn values at various scCO2 pressures for dispersion RTCP of MMA in scCO2 at 80 °C for 6 h with PE-I as chain transfer agent and GeI4 as catalyst.

Applying above aspect, we tried to synthesize PS-b-PMMA particles by seeded dispersion RTCP of MMA in scCO2 with PS-I seed particles (Mn: 3900; Mw/Mn: 1.41) as macro-chaintransfer agent and GeI4 as catalyst, in which PS-I generates the secondary radicals as well as PE-I, where, first, the PS-I particles were in advance synthesized by dispersion RTCP in scCO2 with PE-I and GeI4. MWD of polymer prepared by the seeded dispersion RTCP shifted clearly to higher molecular weight from the PS-I macro-chain-transfer agent (Figure 10). More-

Figure 10. MWD (by GPC with RI (red line) and UV (blue line) detectors) for PS-b-PMMA prepared by seeded dispersion RTCP of MMA in scCO2 at 80 °C for 12 h and 40 MPa with PS-I seed particles as macro-chain-transfer agent, which were prepared by dispersion RTCP of styrene in scCO2 at 80 °C and 30 MPa with PE-I and GeI4 as catalyst. The black line is a MWD for PS-I seed particles.

over, the MWD obtained with RI detector shows good accordance with that with UV detector, indicating that the PMMA homopolymer content among all polymer chains (PS, PS-b-PMMA, and PMMA) was small (ca. 25% among all polymer chains). Such a result could not be obtained from solution RTCP of MMA with the PS-I and GeI4, where the MWD from RI was bimodal with PS-I peak did not shift to higher molecular weight side. In addition, the MWDs from both detectors were significantly different, indicating that the high molecular weight peak was derived from PMMA homopolymer. (Figures S6 and S7 in Supporting Information). From these results, the chain extension from PS-I to PS-bE

DOI: 10.1021/acs.macromol.5b00104 Macromolecules XXXX, XXX, XXX−XXX

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PMMA was successfully demonstrated only in scCO2. The PSb-PMMA particles (conversion: 70%; Mn: 12 500; Mw/Mn: 2.05) were obtained as dry powder state after venting scCO2 in the reactor, which is another important character of the polymerization system in scCO2 (Figure S8), where the Mw/Mn value was high, and it will be further improved under the optimization of the polymerization conditions in the future. The particle morphology was not spherical because of the high plasticization by scCO2 and low molecular weights of both chains of PS and PMMA in the particles. The latter suppressed the phase separation in the particle. Actually, phase-separated polymer particles were obtained in PMMA-b-PS particles with long chain length in each block by seeded dispersion RAFT polymerization.45,56 In this way, we have successfully synthesized PS-b-PMMA particles by the seeded dispersion RTCP of MMA in scCO2 with PS-I seed particles and GeI4.

CONCLUSIONS We discovered that scCO2-assisted dispersion RTCP of MMA with noneffective chain transfer agent and catalyst, i.e., with PEI as chain transfer agent and GeI4 as catalyst in a dispersed system could be successful carried out with keeping narrow molecular weight distribution as well as high livingness of polymer chain end. Such a high control could not be attained in the dispersion RTCP in hexane that is also a nonpolar solvent as well as scCO2 In the dispersion ITP of MMA in scCO2 with PE-I as chain transfer agent, the chain transfer reaction (DT mechanism) was not crucially enhanced in scCO2. These results imply the reversible chain transfer reaction (RT mechanism) may be enhanced in scCO2 system due to its plasticizing effect for polymerizing particles, which means increases in rate constants of activation and deactivation, leading to the successful demonstration of dispersion RTCP of MMA with PE-I as noneffective chain transfer agent. Actually, the scCO2 pressure greatly affected the molecular weight distribution control of the obtained polymers. Moreover, PS-bPMMA was also successfully synthesized by applying the insight into the seeded dispersion RTCP of MMA in scCO2 with PS-I as noneffective macro-chain-transfer agent and GeI4 as catalyst. The discovery of the expanding selectivity of the (macro)chain transfer agents will be of great assistance for flexible molecular design. The catalyst importance in scCO2 will be investigated in the future. ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S9. This material is available free of charge via the Internet at http://pubs.acs.org.

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*(M.O.): Tel/Fax +81-78-803-6594, e-mail [email protected]. jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research (B) (Grant 25288054; given M.O.) from the Japan Society for the Promotion of Science (JSPS). F

DOI: 10.1021/acs.macromol.5b00104 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b00104 Macromolecules XXXX, XXX, XXX−XXX