Experimental Evidence and Beneficial Use of Confined Space Effect in

Sep 19, 2012 - of the confined space effect in the microemulsion NMP of BA, ... n-hexadecane (Nacalai Tesque Inc., Kyoto, Japan) were used as received...
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Experimental Evidence and Beneficial Use of Confined Space Effect in Nitroxide-Mediated Radical Microemulsion Polymerization (Microemulsion NMP) of n‑Butyl Acrylate† Yukiya Kitayama,‡,⊥ Seita Tomoeda,‡ and Masayoshi Okubo*,‡,§ ‡

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

§

S Supporting Information *

ABSTRACT: The confined space effect, which was found by the authors, in nitroxide-mediated radical polymerization (NMP) in a microemulsion system (microemulsion NMP) of n-butyl acrylate (BA) was investigated, where the diameter of micelles (monomer droplets) was 5−10 nm and that of poly(BA) (PBA) particles at the completion of the polymerization was ∼60 nm. To clarify the importance of diameter of monomer droplets (dm) in the initial stage of the microemulsion NMP, NMP in a miniemulsion system (miniemulsion NMP) (dm: ∼60 nm) was carried out as a comparative experiment. The miniemulsion NMP proceeded without molecular weight distribution (MWD) control; on the other hand, in the microemulsion NMP the MWD shifted to higher molecular weight with increasing conversion. The livingnesses of PBAs obtained in the initial stages of the miniemulsion and microemulsion NMPs, which were determined by chain extension test, were 0.01% and 64%, respectively. From these results, it is concluded that the confined space effect in the initial stage of the microemulsion NMP effectively operated and resulted in PBA with predetermined molecular weight and good control of MWD even if the diameter of polymerizing particles increased with conversion.

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INTRODUCTION As one of the most developing areas in radical polymerization, controlled living radical polymerization (CLRP) has been studied actively in the past two decades. The development of CLRP becomes possible to prepare vinyl polymers having predetermined molecular weight, narrow molecular weight distribution, and complex molecular architecture (i.e., block, graft, and star polymers).1 The most well-known CLRPs were nitroxidemediated radical polymerization (NMP),2 atom transfer radical polymerization (ATRP),3 and reversible addition−fragmentation chain transfer radical polymerization (RAFT).4 Until now, the many fundamental researches about CLRPs such as kinetics and designing catalyst have been reported in homogeneous systems.5 Recently, a lot of efforts have been directed toward CLRP techniques in aqueous dispersed systems (i.e., suspension, miniemulsion, and microemulsion) for industrial applications.6 In the aqueous dispersed systems, the polymerization proceeds in each polymer particle, in which the compartmentalization works in kinetics of radical polymerization.7 In a dispersed system, polymerization rate (Rp) is faster than that in homogeneous systems because radicals are isolated in each particle and termination reaction between particles is inhibited (segregation effect). In addition, especially in nanometer-sized particles in NMP and ATRP, deactivation rate between propagation radical and control agent accelerates. In the dispersed systems, the number of droplets/particles (reactors) is increased with decreasing the © 2012 American Chemical Society

droplets/particles size at the same solids content. When the size of droplets/particles reaches nanometer scale, the number of them becomes larger than the number of control agents, where two kinds of droplets/particles (which contain a control agent or not) exist. In that case, the concentration of control agent in droplet/particle containing control agent (polymerization loci) is locally higher than that of corresponding bulk system because the droplets/particles containing no control agent exist as a monomer tank and decrease the total volume of polymerization loci, resulting in the acceleration of deactivation rate between propagation radical and control agent. We have applied the word “confined space effect” to the phenomenon and clarified by computer simulation that it works in the nanometer-sized particles.8 Microemulsion polymerization is one of the useful methods for the preparation of nanometer-sized polymer particles in aqueous dispersed systems.9 A monomer microemulsion is macroscopically homogeneous mixtures of two immiscible liquids (i.e., monomer and water) and a surfactant. The emulsion contains thermodynamically stable nanometer-sized droplets and consequently forms spontaneously without external shear forces. We conducted NMP in microemulsion (microemulsion NMP) of styrene using tetradecyltrimethylammonium bromide (TTAB) as a cationic surfactant and Received: June 9, 2012 Revised: September 8, 2012 Published: September 19, 2012 7884

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became transparent (microemulsion). Approximately 20 mL of the microemulsion was charged in glass tubes and degassed using N2/ vacuum cycles, where we quickly vacuumed to avoid the loss of monomer species as well as the other polymerizations described below, and then the tubes were sealed off. Microemulsion polymerization was conducted at 100 °C. The glass tubes were rapidly cooled in cool water at appropriate time interval, and conversion was measured by gas chromatography (GC-18A, Shimadzu Co., Kyoto, Japan) with helium as carrier gas, using N,N-dimethylformamide (DMF) and toluene as solvent and internal standard, respectively. After a microemulsion polymerization, the aqueous phase was removed under reduced pressure, and the obtained residue was washed with water for removing TTAB. Bulk Polymerization. Bulk polymerization of BA (6 g, 46.9 mmol) with solution of SG1 (61 mg, 0.207 mmol) and AIBN (20 mg, 0.122 mmol) was conducted at 100 °C in the glass tube after degassing using N2/vacuum cycles. The conversion was measured in the same way as the microemulsion polymerization. Miniemulsion Polymerization. A solution of SG1 (61 mg, 0.207 mmol), AIBN (20 mg, 0.122 mmol), BA (6 g, 46.9 mmol), and n-hexadecane (480 mg, 2.12 mmol) was added to SDBS (600 mg, in 99 g of water) aqueous solution. Emulsification was carrying out with an ultrasonic homogenizer (Nihonseiki Kaisha Ltd., Tokyo, Japan, US600T, 12 mm diameter tip, set at “Power 10”). Approximately 20 mL of the microemulsion was charged in glass tubes and degassed using N2/ vacuum cycles, and then the tubes were sealed off. Miniemulsion polymerization was conducted at 100 °C. The glass tubes were rapidly cooled in cool water at appropriate time intervals, and conversion was measured by gas chromatography with helium as carrier gas, using DMF and toluene as solvent and internal standard, respectively. Chain Extension. Chain extension tests of PBA macroinitiators prepared at 10% conversion of miniemulsion and 13% conversion of microemulsion NMPs were carried out, respectively, by bulk (styrene, 1 g (10.2 mmol); PBA, 660 mg (0.12 mmol)) and solution (styrene 1 g (10.2 mmol); PBA, 16.2 g (0.12 mmol); toluene, 40 g) NMPs of styrene as second monomer at 120 °C for 1.5 h in the sealed glass tube after degassing using N2/vacuum cycles, in which the polymerizations stopped at low conversions. Mn and Mw/Mn of PBA macroinitiator were respectively 5.5 × 103 and 1.7 for miniemulsion NMP and 1.3 × 105 and 3.1 for microemulsion NMP. Measurements. Molecular weight (MW) and its distribution (MWD) were analyzed by gel permeation chromatography (GPC) using two styrene/divinylbenzene gel columns [TSKgel GMHHR-H, 7.8 mm id × 30 cm, Tosoh Corporation, Yamaguchi, Japan] using THF as eluent at 40 °C at a flow rate of 1.0 mL min−1 with refractive index (RI) and ultraviolet (UV) detectors (respectively RI-8020/21 and UV-8II, Tosoh Corporation). Dry polymer (10 mg) was dissolved in THF (5 g) for GPC sample. After the polymer was completely dissolved in THF, the solution was filtered, and then MWD was measured by GPC. The columns were calibrated with PS standards (1.05 × 103−5.48 × 106 g/mol). The livingness of the macroinitiator was estimated by transforming from the MWDs to the number distributions (w(log M)/M2) vs M. The weight distributions (w(log M)/M) vs M were normalized before the transformation. The number of chains could be calculated as the integral of the number distributions. Therefore, the number of polymer chain vs M was obtained from RI and UV detectors, and livingness was calculated from these graph areas. Size distributions of droplets/micelles/particles were measured by dynamic light scattering (DLS-7000, Otsuka Electronics, Osaka, Japan) at the light scattering angle of 90° at 25 °C in concentration mode of DLS. Those of particles were measured in same condition after dilution with deionized water. Number-average diameters (dn) of droplets and particles were obtained using the Marquadt analysis routine.

2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO), N-tert-butyl-N[1-diethlphosphono-(2,2-dimethylpropyl)] nitroxide (SG1), or 2,2,5-trimethyl-4-phenyl-3-azahexane-3-oxy (TIPNO) as nitroxide at 125 °C.9i,j The rates (Rp) of the microemulsion NMPs were slower than those of corresponding bulk NMPs, and Mw/Mn values were smaller than those of the bulk NMPs in the initial stage of polymerization. It was assumed that the reason was based on the contribution of confined space effect to the microemulsion. Gnanou and co-workers reported successful bulk NMP of BA using macroinitiator (PBA-SG1), which was synthesized separately with BA, AIBN, and SG1 (5 times as much as AIBN).10 LacroixDesmazes and co-workers investigated the kinetics of bulk NMP of BA with PS-SG1 and excess of free SG1.11 Charleux and coworkers reported the successful miniemulsion NMP of BA in submicrometer-sized droplets/particles (410−640 nm), where the NMP was started from SG1-based alkoxyamine as an initiator with 2.5 mol % excess of free SG1.12 On the other hand, we have reported that bulk NMP of BA using free SG1 and thermal initiator did not proceed with good control/livingness even in the precense of the 20 mol % excess of SG1 relative to radical species from thermal initiator (see in Table 1).13 These results Table 1. Polymerization Results of Bulk and Microemulsion NMPs of BA Using SG1 at 100 °C

bulk microemulsion

time (min)

conv (%)

Mn,th

Mn

Mw/Mn

∼10 840

65 59

2.0 × 104 1.8 × 104

2.3 × 103 2.6 × 104

21.0 1.64

indicate that the formation of alkoxyamine in the initial stage of the polymerization should be difficult in the bulk NMP system from SG1 and thermal initiator. We also showed that Rp was slow and Mw/Mn was relatively low in microemulsion NMP using SG1 and thermal initiator with TTAB under the same experimental conditions of the bulk NMP expect for water and TTAB. The microemulsion NMP, where the particle/droplet works as a nanoreactor, was thus successfully controlled even under the conditions of being uncontrolled in the bulk NMP. However, the diameter (d) of poly(BA) (PBA) particles prepared by the microemulsion NMP at final conversion was relatively large (approximately 60−90 nm), and therefore it was not clear whether the confined space effect worked or not. In this work, in order to obtain some clear experimental evidence of the confined space effect in the microemulsion NMP of BA, we compared experimental and simulation results especially in initial stages of the miniemulsion and microemulsion NMPs (BA/SG1/100 °C).

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EXPERIMENTAL PART

Materials. BA and styrene were purified by distillation under reduced pressure in a nitrogen atmosphere prior to use. 2,2′-Azoisobutyronitrile (AIBN; Wako Pure Chemicals, Osaka Japan) was purified by recrystallization. SG1 provided by Arkema Japan (Tokyo, Japan), n-tetradecyltrimethylammonium bromide (TTAB; Tokyo Kasei Kogyo Co. Ltd., Tokyo, Japan), sodium dodecyl benzensulfonate, and n-hexadecane (Nacalai Tesque Inc., Kyoto, Japan) were used as received. Deionized water with a specific resistance of 18.2 MΩ·cm−1 was obtained by Elix (Nihon Millipore K. K., Tokyo, Japan) purification system. Microemulsion Polymerization. A solution of SG1 (61 mg, 0.207 mmol), AIBN (20 mg, 0.122 mmol), and BA (6 g, 46.9 mmol) was added to TTAB (15 g, in 99 g of water) aqueous solution. After stirring for 30 min at room temperature using a magnetic stirrer, the mixture

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SIMULATION MODEL DEVELOPMENT Homogeneous System. Bulk NMP was modeled using eqs 1−4

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d[M] = −k p[P•][M] dt

(1)

d[PT] = kdeact[P•][T•] − kact[PT] dt

(2)

static. (This model does not include secondary nucleation or Ostwald ripening.23) Also, it is assumed that phase transfer events such as exit and subsequently entry of nitroxide, which may be negligible relative to the other reaction rates coefficient, do not occur to make the problem more tractable. Moreover, the simulation started from alkoxyamine (not free SG1 and AIBN) for the same reason. The equations were implemented and solved using software VisSim (version 6.0A11, Visual Solutions Inc., Jacksonville, FL) employing numerical integration (Backward Eular integration algorithm).

•

d[P ] = kact[PT] − kdeact[P•][T•] − 2k t[P•]2 dt

(3)

•

d[T ] = kact[PT] − kdeact[P•][T•] dt

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(4)

RESULTS AND DISCUSSION Miniemulsion NMP vs Microemulsion NMP. In order to clarify the importance of the confined space effect in the initial stage of the microemulsion NMP of BA using SG1 and AIBN, we carried out miniemulsion NMP, in which droplet/particle size is ideally almost the same throughout the polymerization, at the same concentrations of BA, AIBN, and SG1, except for surfactant and hydrophobe (n-hexadecane) for the comparison with the microemulsion NMP. We selected 100 °C as a polymerization temperature because the temperature was proved for the good control/livingness of the microemulsion NMP in our previous report.13 Figure 1 shows conversion−time plots for the miniemulsion and microemulsion NMPs. In the miniemulsion NMP system, dn

where M donates the monomer, PT donates the alkoxyamine (i.e., dormant species), T• is the nitroxide, and P• is the propagation radical. kp is the propagation rate coefficient, kact is the activation rate coefficient, kdeact is the deactivation rate coefficient, and kt is the termination coefficient. All rate coefficients were assumed to be conversion-independent (Table 2)14 Table 2. Rate Coefficients for the NMP System nBA/ Alkoxyamine at 100 °C rate parameter

value

kact (activation)19 kdeact (deactivation)19 kp (propagation)20 kt (termination)17 kd (chain transfer)21

1.05 × 10−3 (s−1) 377 × 105 (M−1 s−1) 46.1 × 103 (M−1 s−1) 4.27 × 107 (M−1 s−1) 1.25 × 10−3 (s−1)

(calculated from kact and kdeact (both at 100 °C)), and chain-length dependence of rate coefficient15 was not included in the model. Radical polymerization of butyl acrylate is complicated by interand intramolecular chain transfer generating midchain radicals with considerably lower reactivity than chain-end radicals.16 kp for BA obtained by pulsed laser polymerization17 corresponds to propagation of chain-end radicals only and is thus higher than that is observed in experimentally radical polymerizations. Dispersed System. We have reported the details of modeling approach used to investigate compartmentalization effects in NMP in dispersed systems.7a,e Here, the number of particles Nji (particles containing iP• and jT•) described based on the modified Smith−Ewart equations18 dN ji dt

= NAvpkact[PT][Nij−−11 − Nij] + × Nij+ 2 − (i)(i − 1)Nij] + × Nij++11 − (i)(j)Nij]

Figure 1. Conversion−time plots for miniemulsion (open circles) and microemulsion (closed circles) NMPs of BA using SG1 at 100 °C; [AIBN] = 18.6 mmol/L-monomer, [SG1]/2f [AIBN] = 1.05 (f = 0.8). The numbers mean the number-average particle size at those conversions.

kt [(i + 2)(i + 1) NAvp

of the droplets before the polymerization was ∼50 nm, and that of the particles after the polymerization was ∼60 nm, indicating that almost ideal miniemulsion polymerization proceeded. The assumption was supported by particles (droplets) size distributions obtained from DLS (see Figure S1). In the microemulsion NMP system, the obtained micelle (droplet) size from DLS was 5−10 nm, and the particle size after the polymerization became significantly large (60−90 nm), which was on a similar scale with the miniemulsion NMP system. However, the rate (Rp) of the miniemulsion NMP was much faster than that of the microemulsion NMP and close to that of the bulk NMP shown in Table 1. Figures 2 and 3 respectively show MWDs, Mn, and Mw/Mw of PBA obtained from the microemulsion and miniemulsion NMPs. The MWD of the PBA obtained from the miniemulsion NMP did not shift to higher MW with increasing conversion. The Mn did not increase linearly, and the Mw/Mn was high. On the other hand, the MWD of PBA obtained from the microemulsion

kdeact [(i + 1)(j + 1) NAvp (5)

where NA is Avogardo’s number and vp is the particle volume. Equation 5 accounts for compartmentalization of both P• and T•. In this model chain-length dependence of rate coefficients and nitroxide partitioning were not included although these effects are likely to exert some influence especially for highly watersoluble nitroxide.7g,9j Equation 5 is employed to calculate the distributions of P• and T• in the particles, whereas a separate set of equations are used to calculate the monomer consumption rate and other related quantities.7a,c,e The model corresponds to an ideal miniemulsion polymerization,22 where the system initially comprises monomer droplets that subsequently convert to polymer particles. The total number of monomer droplets/polymer particles is assumed to remain 7886

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Figure 2. MWD (using GPC, RI detector) at various conversions for the micronemusion (a) and miniemulsion (b) NMPs of BA using SG1 at 100 °C ([AIBN] = 18.6 mmol/L-monomer, [SG1]/2f [AIBN] = 1.05).

Figure 4. Simulated (using the software VisSim) conversion−time curves for NMP of BA at 100 °C in bulk (dotted line) and dispersed (full lines) systems for various droplet/particle diameters and [PBA-SG1 macroinitiator]0 = 0.03 mol/L-monomer.

NMP shifted to higher MW with increasing conversion. The Mn increased linearly with conversion maintaining relatively small PDI, which was consistent well with Mn.th. These results indicate that the microemuslion NMP proceeded under good control/ livingness; meanwhile, the miniemulsion NMP did not. A great difference between the miniemulsion and microemulsion NMP systems was the size of the polymerization loci in the initial stages of the polymerizations, which were respectively 60 and 5−10 nm. It seems that the good control/livingness in the microemulsion NMP is derived from the confined space effect due to nanosize of the micelles (droplets) in the initial stage. Simulation about Compartmentalization in NMP of BA. On the basis of simulation results, we have reported that the confined space effect, which accelerates deactivation reaction and leads the better MWD control, works in nanometer-sized polymerizing particles. Figure 4 shows simulated conversion−time curves of NMP of BA in bulk and dispersed systems with various droplet/particle sizes. Because the simulation was carried out in the system that the polymerization is started from alkoxyamine (not free SG1 and AIBN) without excess of free SG1, the simulated values cannot be directly compared with the experimental data shown in Figure 1, where the polymerization started using SG1 and AIBN in the presence of excess of free SG1. In the dispersed system, the more d increases, the more (Rp)s increases. (Rp)s in d = 60 nm was similar to that in the bulk system. Actually, the bulk and miniemulsion NMPs with 60 nm diameter proceeded with a similar experimental Rp, which was accordance with the (Rp)s. Moreover, for d < 60 nm, (Rp)s in dispersed systems was slower than that in the bulk system.

Figure 5 shows log[P•] vs log d, showing the concentration of propagation radical in dispersed systems. When d is 60 nm, [P•]

Figure 5. Simulated propagating radical concentrations [P•] vs droplet/ particle diameter (d) at 10% conversion for NMP of BA in dispersed system at 100 °C ([PT]0 = 0.03 M). The dotted line denotes the simulated [P•] under the bulk system.

equals that of the bulk. When d is 80 nm, [P•] reached a maximum value. When d < 60 nm, [P•] in the dispersed system is smaller than that in the bulk system, in which the confined space effect on deactivation is more effective than the segregation effect on termination (see Figure 6), resulting in higher control (narrower MWD) and better livingness than that of bulk system.7a,c,e The effects of compartmentalization on the individual reactions of deactivation and termination are able to consider

Figure 3. Mn and Mw/Mn vs conversions for microemulsion (a) and miniemulsion (b) NMPs of BA using SG1 at 100 °C ([AIBN] = 18.6 mmol/Lmonomer, [SG1]/2f [AIBN] = 1.05). The lines are the theoretical Mn (Mn,th) based on f = 0.8. 7887

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Table 3. Results of Chain Extension Tests of Styrene Using PBA Macroinitiator, Which Were Obtained in Low Conversion of Miniemulsion and Microemulsion NMPs of BA, at 120°C for 1.5 h PBA miniemulsion microemulsion

using our previously reported approach,8a−c where the compartmentalized deactivation and termination rates (Rc) are compared with the corresponding noncompartmentalized rates (Rnc) based on eqs 6−9.

R tc = nc R deact

kdeact (NAvp)2 2k t 2

(NAvp)

∑ ∑ ijNij i

j

(6)

∑ ∑ i(i − 1)Nij i

j

•

•

= kdeact[P ][T ]

R tnc = 2k t[P•]2

Mn

Mw/Mn

Mn

1.3 × 105 5.5 × 103

3.1 1.7

2.6 × 104 9.0 × 103

Mw/Mn livingness (%) 9.5 1.4

0.01 64

than that of Mn of the original PBA macroinitiator generated during the chain extension test, where the low molecular weight polymer chains would be caused by thermal initiation of styrene. The livingness of the PBA macroinitiator was only 0.01%. On the other hand, in the chain extension test with the macroinitiaor prepared at 13% conversion of the microemulsion NMP system, the Mn and Mw/Mn after the chain extension respectively became larger and smaller than those before it. The livingness of the product was 64%. These results indicate that the macroinitiator was synthesized not in the miniemulsion NMP system but successfully in the microemulsion NMP system. From the above results, it was concluded that the confined space effect works strongly in the initial stage of the microemulsion NMP of BA, resulting in PBA macroinitiator with high livingness. In other words, the confined space effect in the dispersed system, which cannot work in the homogeneous system, is a powerful tool for the CLRP with good control/livingness.

Figure 6. Simulated ratios of “compartmentalized” (Rc) and “noncompartmentalized” (Rnc) deactivation (circle) and termination (triangle) rates for NMP of BA in dispersed systems at 100 °C ([PT]0 = 0.03 M) at 10% conversion.

c R deact =

PBA-b-PS

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(7) (8)

ASSOCIATED CONTENT

S Supporting Information *

Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

(9)

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Figure 6 shows simulation results on the deactivation and termination rates in a compartmentalized system compared to those in the corresponding noncompartmentalized system. In case of deactivation rate, Rc was larger than Rnc over the all particle size range (d < 95 nm), and Rc became larger with decreasing d. On the other hand, in case of termination rate, Rc was smaller than Rnc and Rc kept similar values over the all particle size range (d < 95 nm). In much smaller particle size (1−2 nm), which was the micelle (droplet) size in the microemulsion NMP, Rdeact was much larger than Rt because of the confined space effect. The confined space effect of the miniemulsion and microemlsion NMPs can be considered as follows. In the miniemulsion NMP at d = 60 nm because the confined space effect works weakly, the deactivation between propagation radical and control agent does not occur well, resulting in a bad control. The miniemulsion NMP proceeded thus in similar to the bulk NMP. On the other hand, in the microemulsion NMP at d = 1−2 nm, the micelle size in the initial stage because the confined space effect works strongly and the deactivation occurs well, PBA-SG1 oligomer would be successfully formed in the initial stage, resulting in a good MWD control. Chain Extension. In order to confirm above idea, chain extension of PBA macroinitiator obtained at low conversions of the miniemulsion and microemulsion NMPs was conducted using styrene, at 120 °C for 1.5 h. Table 3 shows the results. In the chain extension test with the PBA macroinitiaor prepared at 10% conversion of the miniemulsion NMP, the Mn and Mw/Mn after the chain extension were respectively lower and much larger than those of the original PBA macroinitiator. Those indicate that new PS chains with lower Mn

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Part CCCLVIII of the series “Studies on Suspension and Emulsion”. ⊥ Materials Research Laboratory (MRL), University of California, Santa Barbara (UCSB), CA 93106. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grant-in-Aid for Scientific Research (A) (Grant 21245050) from the Japan Society for the Promotion of Science (JSPS) and by Research Fellowships of the JSPS for Young Scientists (given to Y.K.). We appreciate to Arkema, Japan, for donating the nitroxide SG1.

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dx.doi.org/10.1021/ma3011763 | Macromolecules 2012, 45, 7884−7889