- Email: [email protected]
The importance of surfactants for polymerizations in carbon dioxide
High Pressure Chemical Engineering Ph. Rudolf von Rohr and Ch. Trepp (Editors) 9 1996 Elsevier Science B.V. All rights reserved.
23
The I m p o r t a n c e of Surfactants for P o l y m e r i z a t i o n s in C a r b o n D i o x i d e D. E. Betts, J. B. McClain, J. M. DeSimone Department of Chemistry, CB#3290, Venable and Kenan Laboratories - The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3290 phone: (919) 962-2166; fax: (919) 962-5467; email" [email protected] 1.
INTRODUCTION
Over the past decade, carbon dioxide has become an attractive alternate solvent for a variety of polymer synthesis and processing applications due to its environmentally benign nature and chemical inertness 1-3. Properties of CO2, such as dielectric constant and density are sensitive to the temperature and pressure of the system. The fluid density and dielectric constant, can be fine tuned using temperature and pressure profiling. In addition, CO2 offers an environmentally sound medium with the potential to eliminate organic and aqueous waste streams in manufacturing facilities. Although CO2 dissolves many small molecules readily, it is a very poor solvent for most high molecular weight polymers. Currently, only amorphous or low melting fluoropolymers and silicone polymers are known to be very soluble in CO2 (T < 100 ~ P < 400 bar), or CO2-philic, while many industrially important polymers are relatively insoluble. In 1992, we reported the successful homogenous free radical polymerization of a CO2-philic fluorinated acrylate, 1,1-dihydroperfluorooctyl acrylate (FOA) 2. Homogenous polymer synthesis in CO2 is fundamentally limited however, by the extremely low solubility of most polymers at readily accessible conditions. In order for CO2 to be an effective continuos phase for polymerizations, heterogeneous reaction systems must necessarily be developed analogous to classical emulsion, inverse emulsion, dispersion, suspension and precipitation polymerization processes. With some highly reactive monomers such as acrylic acid 4 and tetrafluoroethylene 5, free-radical precipitation polymerization can afford polymers with high yields and molecular weight. However, many industrial monomers require the utilization of surfactant stabilized reaction conditions 2. We have reported the utilization of nonionic m homopolymer, block copolymer and reactive macromonomer m surfactants consisting of covalently bound CO2-philic and CO2-phobic segments in the dispersion polymerization of various CO2-insoluble polymers 2, 6-11 These reactions produce a stabilized polymer colloid in CO2 solution and dry, free flowing powders after isolation. In a study by Consani and Smith, over 140 commercial and commonly available surfactants have been screened for application in CO2 resulting in only a handful with, at best, minute CO2-solublility 12. Recent research in various laboratories, including our own, has developed more soluble surfactants active in CO2 based on the incorporation of CO2-philic fluorinated and silicone materials 3,7,13-15. Herein we describe the effects of various surfactant and stabilizer systems as they apply to polymerizations in CO2. 1.1 Mechanism of colloid stabilization: The role of surfactant The dispersion polymerization of lipophilic monomers in CO2 is initiated homogeneously with chain collapse into a discrete polymer particle at a critical molecular
24 weight as shown in Figure 1. The surfactant then stabilizes the polymer particle as a colloid and prevents flocculation through steric stabilization.
Figure 1.
The effectiveness of a given surfactant in the above scheme is governed by two factors; first there must be sufficiently strong anchoring to the polymer particle, second the soluble segment must be chain extended into the continuos phase facilitating a negative steric interaction between particles (e.g. of sufficient solvation and chain length). These factors are controlled in CO2 polymerizations by synthetic variation of both the composition and architecture of the surfactants in response to different polymer particles and conditions and by control of the density of CO2. 2. DESIGN AND APPLICATION POLYMERIZATIONS IN CO2
OF
SURFACTANTS
FOR
2.1 Methyl methacrylate polymerization For the stabilization of various insoluble hydrocarbon polymers in carbon dioxide, it has been found that no one surfactant works well for all systems. Therefore it has become necessary to tailor the surfactants to the specific polymerization reaction. Through variation of not only the composition of the surfactants, but also their architectures, surfactants have been molecularly-engineered to be surface activenpartitioning at the interface between the growing polymer particle and the CO2 continuous phase. The surfactants utilized to date include poly(FOA) homopolymer, poly(dimethylsiloxane) homopolymer with a polymerizable endgroup, poly(styrene-b-FOA), and poly(styrene-b-dimethylsiloxane). Through the utilization of these suffactants, the successful dispersion polymerization of methyl methacrylate (MMA), styrene, and 2,6-dimethylphenol in CO2 has been demonstrated. The polymerization of MMA was the first example of a successful dispersion polymerization conducted in CO2. The polymerization of MMA in CO2 was stabilized by the use of poly(FOA) homopolymer and the product was isolated in high yields as a free flowing powder 2. Scanning electron microscopy (SEM) of the product revealed the product morphology to consist of spherical particles. In contrast, the same reaction conducted in the absence of poly(FOA) stabilizer gives a nondescript morphology in low yields. In the polymerization of MMA, two different molecular weight samples of poly(FOA) were usedna low molecular sample with Mn = 1.0 x 105 g/mol and a high molecular sample with Mn = 1.4 x 106 g/mol. In both cases the successful dispersion polymerization of MMA was obtained. The low molecular weight poly(FOA) stabilizer consistently gave smaller sized PMMA particles than the high molecular weight poly(FOA) (Table 1).
25
Figure 2. Surfactants for polymerizations in CO2
(~H3 (~H3 H'-)-(- ~i --O -)-~i -- OH3 CH3 CH3
C4H9'-('-OH2 C ----~O
I
(a)
OCH2C7F15
(b)
'-'(-CH2
(~
H3
(C)
CH3
CH3
CH2--C~H-)"C~--.O I
O CH3
..)_~i__CSa6__ O II
I
OCH2C7F15*
OH3
(d)
(a) Poly(FOA) h o m o p o l y m e r - * the fluorinated alkyl side chain contains ca. 25% CF3 branches per chain (b) Polystyrene-b-PDMS diblock copolymer (c) PDMS macromonomer (d) Polystyrene-b-Poly(FOA) diblock copolymer Table 1. Results of MMA polymerizations with AIBN as the initiator in CO2 at 204 bar and 65 ~ stabilizer is either LMW or HMW poly(FOA).
a
Stabilizer (w/v %)
Yield (%)
(kg/mol)
PDI a
Particle size (Bm)
0% 4% LMW 4% HMW
39 92 95
149 220 321
2.8 2.6 2.2
-1.3 (+0.4) 2.5 (+0.2)
Polydispersity index of the molecular weight distribution, Mw/Mn.
In addition to studying the effect of the stabilizer molecular weight on the MMA dispersion polymerization, the effect of the stabilizer concentration was also analyzed 8. The concentration of poly(FOA) stabilizer was systematically varied while the amount of MMA was held constant at 21 w/v % in CO2 (Table 2). It was found that as little as 0.24 wt. % (based on monomer) is needed to stabilize the polymerization and give spherical particles. Additionally, excess surfactant could be washed from the finished particle surface with CO2, resulting in
26 residual surfactant levels of less than 0.5 wt. %. The very low amount of polymeric stabilizer needed for a successful dispersion polymerization attests to the high amphiphilicity and strong anchoring effect of poly(FOA) in the PMMA-CO2 system. Table 2. Effect of the concentration of poly(FOA) (21 w/v % MMA, 4 h) poly(FOA) (wt %)
Yield (%)
Mn (kg/mol)
PDIa
Particle size (gm)
Particle size distribution
0 0.24 1.2 4.5 16
47 86 89 92 90
85 255 252 316 293
3.72 2.40 2.56 2.09 2.33
m 2.86 2.08 2.44 1.55, 0.93 b
1.17 1.01 1.02 1.08
Polydispersity index of the molecular weight, Mw/Mn. b PMMA particles exhibit bimodal particle size distribution. Both primary and secondary particle diameters are reported. a
Furthermore, it has been found that a commercially available poly(dimethylsiloxane) macromonomer terminated with a polymerizable endgroup can also be used for the successful dispersion polymerization of MMA in CO2 (Figure 2) 8. The macromonomer had a number average molecular weight of 1.13 x 104 and a polydispersity index (PDI) of 1.1. Although using a very small amount of PDMS macromonomer in the reaction gave a considerable increase in yield and molecular weight over the reaction done without stabilizer, powdery products comprised of relatively monodisperse particles were only obtained when greater than 3.5 wt. % of PDMS macromonomer was used. In addition, a bimodal particle size distribution was obtained for reactions conducted with less than 3.5 wt. % PDMS macromonomer (Table 3). Again, excess surfactant could be washed from the particle surface with CO2 and the residual surface coating was less than 0.3 wt. %. In comparison, unfunctionalized PDMS homopolymer (lacking the polymerizable endgroup) was explored as a stabilizer and found to be ineffective. Using 6.8 wt. % PDMS homopolymer with a Mn of 1.33 x 104 and a PDI of 1.1, gave results similar to those obtained when only 0.05 wt. % PDMS macromonomer was used. Table 3. Dispersion polymerization of MMA in CO2 a Stabilizer
PDMS PDMS PDMS PDMS
none PDMS macromonomer macromonomer macromonomer macromonomer
Wt. % 0 6.8 0.05 0.26 1.7 3.5
Yield (%) 24 51 56 67 70 80
(kg/mol) 65 210 271 280 200 383
PDI b 3.7 2.5 2.4 2.1 2.6 2.1
Particle size (~tm)
2.8 2.8 2.8
Reactions were conducted at 65 ~ and 340 bar for 4 h with 2.1 g of MMA and 0.0070 g AIBN. b Polydispersity index of the molecular weight distribution, Mw/Mn.
a
27
2.2 Styrene polymerization Although it has been shown that poly(FOA) is an effective stabilizer for the polymerization of MMA in CO2, it was not an effective stabilizer for the polymerization of styrene in CO2. This is believed to be due to ineffective anchoring of the stabilizer on the growing polymer particle. To increase the anchoring ability of the surfactants, amphiphilic block copolymers were designed which contained a polystyrene segment to serve as the anchoring moiety and a poly(FOA) or PDMS segment to serve as the steric stabilizing moiety (Figure 2)10,11 The solution behavior of these amphiphilic block copolymers was examined in CO2 (Table 4). It was found that while many of the polystyrene-b-poly(FOA) block copolymers were soluble in CO2, giving a clear solution, the polystyrene-b-PDMS block copolymers were insoluble, giving cloudy dispersions in solution. This is due to the much higher solubility of poly(FOA) in CO2 than PDMS and hence the ability of poly(FOA) to solubilize larger polystyrene segments. But at the start of a typical dispersion polymerization, with 20 % styrene monomer present, the initial solution is clear and homogeneous since the monomer can act as a cosolvent to help solvate the stabilizer.
Table 4. Polystyrene block copolymers and solubility in CO2 as a function of composition Solubility (4% wt/vol) Diblock copolymer (kg/mol) PS 3.7k -b- PFOA 14k PS 3.7k -b- PFOA 17k PS 3.7k -b- PFOA 28k PS 3.7k -b- PFOA 40k PS 3.7k -b- PFOA 6 lk PS 4.5k -b- PFOA 25k PS 6.6k -b- PFOA 35k PS 4.3k PS 4.3k PS 9-6k PS 9-6k
-b-b-b-b-
PDMS 25k PDMS 65k PDMS 24k PDMS 64k
mol % Styrene 54.0 49.4 37.0 28.9 20.9 44.5 45.1
40 ~ 340 bar insoluble cloudy translucent clear clear translucent cloudy
65 ~ 340 bar insoluble cloudy cloudy clear clear cloudy cloudy
10.9 4.5 22.2 9.7
insoluble insoluble insoluble insoluble
insoluble insoluble insoluble insoluble
In this system using a polystyrene containing block copolymer, the polystyrene segment should readily partition into the lipophilic polystyrene particle core while the poly(FOA) or PDMS block is solubilized in the CO2 continuous phase to provide steric stabilization and prevent coagulation. In comparison of the polystyrene-b-poly(FOA) diblock copolymers to the polystyrene-b-PDMS diblock copolymers, it was found that the use of a polystyrene-b-PDMS stabilizer gives much more monodisperse particles. This most likely arises from the synthetic technique employed in the surfactant synthesis. The blocks in the polystyrene-b-PDMS block copolymers have a much narrower polydispersity than the blocks in the polystyrene-b-poly(FOA) block copolymers. It was noted that the particles obtained in
28 this system are much smaller (< 1 ~tm) than the particles obtained in the PMMA-CO2 system employing poly(FOA) homopolymer as stabilizer (> 1 lam) (Table 5).
Table 5. Results of styrene polymerizations in CO2 Stabilizer anone apoly(FOA) aps3"7k -b- PFOA 17k aps4"5k -b- PFOA 25k aps6"6k-b- PFOA 35k bps4.3k-bbps9.6k-bbps4.3k-bbps9.6k-b-
PDMS 25k PDMS 24k PDMS 65k PDMS 64k
Stabilizer conch. (w/v %) 0 4 4 4 4
yield (%)
Mn (kg/mol)
PDI c
Particle size (~tm)
22.1 43.5 72.1 97.7 93.6
3.8 12.8 19.2 22.5 23.4
2.3 2.8 3.6 3.1 3.0
0.40 0.24 0.24
2 2 2 2
90.6 91.7 52.2 90.9
65 56 21 39
3.6 4.3 9.2 8.1
0.22 0.46 coagulated coagulated
Polymerization in CO2 at 204 bar and 65 ~ with 2.0 g styrene and 2.4 x 10-2 M AIBN. Polymerization in CO2 at 204 bar and 65 ~ with 2.0 g styrene and 1.2 x 10-2 M AIBN c Polydispersity index of the molecular weight distribution, Mw/Mn.
a
b
It was also found that the polystyrene-b-PDMS block copolymers were not only effective at stabilizing styrene polymerizations in CO2, but also in stabilizing MMA polymerizations. When using a polystyrene-b-PDMS block copolymer as the stabilizer the resulting PMMA was recovered in 94.1% yield with a Mn = 1.8 x 105 g/mol and a PDI = 2.8. The particles obtained are much smaller and more polydisperse than the particles obtained when using poly(FOA) homopolymer as the stabilizer (particle size = 1.55 - 2.86 lam vs. 0.23 lam and particle size distribution = 1.05 vs. 1.46).
2.3
Polymerization of 2,6-dimethylphenol In addition to being a good stabilizer for the polymerization of styrene in CO2, polystyrene-b-poly(FOA) block copolymers were also found to be successful stabilizers of poly(2,6-dimethylphenylene oxide), (PPO), in CO212. Polystyrene and PPO are miscible with each other and so the polystyrene block is anchored to the polymer particle by blending with the PPO while the poly(FOA) block is solvated and provides steric stabilization. It can be seen in comparison to other stabilizers, that the use of polystyrene-b-poly(FOA) leads to a higher yield and a higher molecular weight. Although polystyrene-co-poly(FOA) random copolymer gave higher yields, the product obtained is of a very low molecular weight. Conversely, the molecular weight increased while the yield decreased upon the use of poly(FOA) homopolymer as the stabilizer. The polystyrene-b-poly(FOA) diblock copolymer was the most effective stabilizer for the synthesis of poly(phenylene oxide) in CO2 as it gave both high yields and high molecular weights (Table 6).
29
Scheme 2. Polymerization of PPO in CO2
OH
+
n/2 02
\
CO2 CuBr / amine
nil2|
Stabilizer
\CH 3
CH3
Table 6. Results of phenylene oxide polymerizations in CO2 a Stabilizer none PFOA ps4.5k _b_PFOA25k PS_co_PFOA b
Stabilizer concn. (w/v %) 0 2.8 2.8 2.8
Yield (%)
Mn (kg/mol)
PDI
67 50 74 83
6.1 8.7 17.2 3.0
1.6 1.6 5.8 2.8
a Polymerization conditions: 345 bar, 40 ~ 20 h, using a copper:amine:monomer ratio of 1:22:75 with pyridine as the amine, b 46.3% styrene : 53.7% FOA
3.
C H A R A C T E R I Z A T I O N OF SURFACTANTS IN CO2
Small angle neutron scattering (SANS) has been employed in the characterization of both CO2-philic homopolymers and self assembled polystyrene-b-poly(FOA) surfactant molecules in CO2.14, 15 Experiments were preformed at the W. C. Koehler 30 m SANS spectrometer at the Oak Ridge National Laboratory. 16 Utilizing SANS we are able to determine the weight average molecular weight (Mw), the radius of gyration (Rg), and the second virial coefficient (A2) of solvated macromolecules. We also determined the degree of aggregation and characteristic dimensions of surfactant molecules in a micelle. Dilute concentration series of two different molecular weight samples of PFOA were analyzed over a range of temperatures and pressures (40 ~ < T < 65 ~ and 340 bar < P < 395 bar). A2 values were found to be positive, indicating that supercritical CO2 is a thermodynamically good solvent for PFOA over these conditions. The radius of gyration was found to be 110/k for the high molecular weight sample (Mw --- 1.4 x 106 g/mol) and 35/k for the low molecular weight sample (Mw -- 1.1 x 105 g/mol). Analysis was preformed on 1.3 x 104 g/mol PDMS and 1.6 x 104 g/mol poly(hexafluoro propylene oxide), Krytox | at similar conditions finding that CO2 is a poor solvent for PDMS and approximately a Theta (| solvent for Krytox @. A series of block copolymers were studied by SANS in which the CO2-philic PFOA block length was varied (16,600 < Mn (g/mol) < 61,100) and the CO2-phobic polystyrene block was held constant (Mn = 3,700 g/mol). Fitting of the scattering curves to a core shell model predicts spherical structures with an average degree of association of 7 surfactant molecules per micelle. This value was constant for the series of materials as would be expected
30 for surfactants with the same insoluble or 'core' segment. The total radius, however, increases smoothly (80 to 100 ,~) with the increasing soluble or 'shell' segment length. 4.
CONCLUSIONS
Polymerization of industrially important monomers can be realized in CO2 with the application of logically designed surfactant technology. The binding segment of the surfactants has been tailored to specific growing polymer particles. Both chemical binding and physical absorption have been displayed as mechanisms for stabilization of polymer colloids during synthesis. The synthesis of commodity and engineering plastics in environmentally benign CO2 could facilitate the removal of tremendous amounts of aqueous and organic waste in the chemical process industry. REFERENCES
(1) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction-Principles and Practice; 2nd ed.; Butterworths-Heineman: Stoneham, Boston, 1993. (2) DeSimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J. B.; Romack, T. J.; Combes, J. R. Science 1994, 265, 356. (3) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271,624. (4) Romack, T. J.; Maury, E. E.; DeSimone, J. M. Macromolecules 1995, 28, 912. (5) Romack, T. J.; Treat, T. A.; DeSimone, J. M. Macromolecules 1995, 28, 8429. (6) DeSimone, J. M.; Maury, E. E.; Combes, Y. Z.; Menceloglu, Y. Z. Polym. Prepr. (Am. Chem. Soc. Div. Polym. Mat. Sci. Eng.) 1993, 68, 41. (7) Guan, Z.; DeSimone, J. M. Macromolecules 1994, 27, 5527. (8) Hsiao, Y.; Maury, E. E.; Johnston, K. P.; Mawson, S.; DeSimone, J. M. Macromolecules 1995, 28, 8159. (9) Shaffer, K. A.; Jones, T. A.; Canelas, D. A.; Wilkinson, S. P.; DeSimone, J. M. Macromolecules 1996, 29, 2704. (10) Canelas, D. A.; Betts, D. E.; DeSimone, J. M. Macromolecules 1996, 29, 2818. (11) Canelas, D. A.; Betts, D. E.; DeSimone, J. M. Polym. Prep. (Am. Chem. Soc. Div. Polym. Mat.Sci. Eng.) 1996, 74, 400. (12) Kapellen, K. K.; Mistele, C. D.; DeSimone, J. M. Macromolecules 1996, 29, 495. (13) Consani, K A.; Smith, R. D. J. Supercrit. Fluids 1990, 5, 51. (14) Fulton, J. L.; Pfund, D. M.; McClain, J. B.; Romack, T. J.; Maury, E. E.; Combes, J. R.; Samulski, E. T.; DeSimone, J. M. Langmuir, 1995, ! 1, 4241. (15) McClain, J. B.; Londono, J. D.; Combes, J. R.; Romack, T. J.; Canelas, D. A.; Betts, D. E.; Wignall, G. D.; Samulski, E. T.; DeSimone, J. M. J. Am. Chem. Soc. 1996, 118,917. (16) Koehler, W. C. Physica (Utrecht) 1986, 138B, 320. (17) Barrett, K.E.J.; Dispersion Polymerization in Organic Media, Wiley: London, 1975. (18) Napper, D. H.; Polymer Stabilization of Colloidal Dispersions, Academic Press: London, 1983.
23
The I m p o r t a n c e of Surfactants for P o l y m e r i z a t i o n s in C a r b o n D i o x i d e D. E. Betts, J. B. McClain, J. M. DeSimone Department of Chemistry, CB#3290, Venable and Kenan Laboratories - The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3290 phone: (919) 962-2166; fax: (919) 962-5467; email" [email protected] 1.
INTRODUCTION
Over the past decade, carbon dioxide has become an attractive alternate solvent for a variety of polymer synthesis and processing applications due to its environmentally benign nature and chemical inertness 1-3. Properties of CO2, such as dielectric constant and density are sensitive to the temperature and pressure of the system. The fluid density and dielectric constant, can be fine tuned using temperature and pressure profiling. In addition, CO2 offers an environmentally sound medium with the potential to eliminate organic and aqueous waste streams in manufacturing facilities. Although CO2 dissolves many small molecules readily, it is a very poor solvent for most high molecular weight polymers. Currently, only amorphous or low melting fluoropolymers and silicone polymers are known to be very soluble in CO2 (T < 100 ~ P < 400 bar), or CO2-philic, while many industrially important polymers are relatively insoluble. In 1992, we reported the successful homogenous free radical polymerization of a CO2-philic fluorinated acrylate, 1,1-dihydroperfluorooctyl acrylate (FOA) 2. Homogenous polymer synthesis in CO2 is fundamentally limited however, by the extremely low solubility of most polymers at readily accessible conditions. In order for CO2 to be an effective continuos phase for polymerizations, heterogeneous reaction systems must necessarily be developed analogous to classical emulsion, inverse emulsion, dispersion, suspension and precipitation polymerization processes. With some highly reactive monomers such as acrylic acid 4 and tetrafluoroethylene 5, free-radical precipitation polymerization can afford polymers with high yields and molecular weight. However, many industrial monomers require the utilization of surfactant stabilized reaction conditions 2. We have reported the utilization of nonionic m homopolymer, block copolymer and reactive macromonomer m surfactants consisting of covalently bound CO2-philic and CO2-phobic segments in the dispersion polymerization of various CO2-insoluble polymers 2, 6-11 These reactions produce a stabilized polymer colloid in CO2 solution and dry, free flowing powders after isolation. In a study by Consani and Smith, over 140 commercial and commonly available surfactants have been screened for application in CO2 resulting in only a handful with, at best, minute CO2-solublility 12. Recent research in various laboratories, including our own, has developed more soluble surfactants active in CO2 based on the incorporation of CO2-philic fluorinated and silicone materials 3,7,13-15. Herein we describe the effects of various surfactant and stabilizer systems as they apply to polymerizations in CO2. 1.1 Mechanism of colloid stabilization: The role of surfactant The dispersion polymerization of lipophilic monomers in CO2 is initiated homogeneously with chain collapse into a discrete polymer particle at a critical molecular
24 weight as shown in Figure 1. The surfactant then stabilizes the polymer particle as a colloid and prevents flocculation through steric stabilization.
Figure 1.
The effectiveness of a given surfactant in the above scheme is governed by two factors; first there must be sufficiently strong anchoring to the polymer particle, second the soluble segment must be chain extended into the continuos phase facilitating a negative steric interaction between particles (e.g. of sufficient solvation and chain length). These factors are controlled in CO2 polymerizations by synthetic variation of both the composition and architecture of the surfactants in response to different polymer particles and conditions and by control of the density of CO2. 2. DESIGN AND APPLICATION POLYMERIZATIONS IN CO2
OF
SURFACTANTS
FOR
2.1 Methyl methacrylate polymerization For the stabilization of various insoluble hydrocarbon polymers in carbon dioxide, it has been found that no one surfactant works well for all systems. Therefore it has become necessary to tailor the surfactants to the specific polymerization reaction. Through variation of not only the composition of the surfactants, but also their architectures, surfactants have been molecularly-engineered to be surface activenpartitioning at the interface between the growing polymer particle and the CO2 continuous phase. The surfactants utilized to date include poly(FOA) homopolymer, poly(dimethylsiloxane) homopolymer with a polymerizable endgroup, poly(styrene-b-FOA), and poly(styrene-b-dimethylsiloxane). Through the utilization of these suffactants, the successful dispersion polymerization of methyl methacrylate (MMA), styrene, and 2,6-dimethylphenol in CO2 has been demonstrated. The polymerization of MMA was the first example of a successful dispersion polymerization conducted in CO2. The polymerization of MMA in CO2 was stabilized by the use of poly(FOA) homopolymer and the product was isolated in high yields as a free flowing powder 2. Scanning electron microscopy (SEM) of the product revealed the product morphology to consist of spherical particles. In contrast, the same reaction conducted in the absence of poly(FOA) stabilizer gives a nondescript morphology in low yields. In the polymerization of MMA, two different molecular weight samples of poly(FOA) were usedna low molecular sample with Mn = 1.0 x 105 g/mol and a high molecular sample with Mn = 1.4 x 106 g/mol. In both cases the successful dispersion polymerization of MMA was obtained. The low molecular weight poly(FOA) stabilizer consistently gave smaller sized PMMA particles than the high molecular weight poly(FOA) (Table 1).
25
Figure 2. Surfactants for polymerizations in CO2
(~H3 (~H3 H'-)-(- ~i --O -)-~i -- OH3 CH3 CH3
C4H9'-('-OH2 C ----~O
I
(a)
OCH2C7F15
(b)
'-'(-CH2
(~
H3
(C)
CH3
CH3
CH2--C~H-)"C~--.O I
O CH3
..)_~i__CSa6__ O II
I
OCH2C7F15*
OH3
(d)
(a) Poly(FOA) h o m o p o l y m e r - * the fluorinated alkyl side chain contains ca. 25% CF3 branches per chain (b) Polystyrene-b-PDMS diblock copolymer (c) PDMS macromonomer (d) Polystyrene-b-Poly(FOA) diblock copolymer Table 1. Results of MMA polymerizations with AIBN as the initiator in CO2 at 204 bar and 65 ~ stabilizer is either LMW or HMW poly(FOA).
a
Stabilizer (w/v %)
Yield (%)
PDI a
Particle size (Bm)
0% 4% LMW 4% HMW
39 92 95
149 220 321
2.8 2.6 2.2
-1.3 (+0.4) 2.5 (+0.2)
Polydispersity index of the molecular weight distribution, Mw/Mn.
In addition to studying the effect of the stabilizer molecular weight on the MMA dispersion polymerization, the effect of the stabilizer concentration was also analyzed 8. The concentration of poly(FOA) stabilizer was systematically varied while the amount of MMA was held constant at 21 w/v % in CO2 (Table 2). It was found that as little as 0.24 wt. % (based on monomer) is needed to stabilize the polymerization and give spherical particles. Additionally, excess surfactant could be washed from the finished particle surface with CO2, resulting in
26 residual surfactant levels of less than 0.5 wt. %. The very low amount of polymeric stabilizer needed for a successful dispersion polymerization attests to the high amphiphilicity and strong anchoring effect of poly(FOA) in the PMMA-CO2 system. Table 2. Effect of the concentration of poly(FOA) (21 w/v % MMA, 4 h) poly(FOA) (wt %)
Yield (%)
Mn (kg/mol)
PDIa
Particle size (gm)
Particle size distribution
0 0.24 1.2 4.5 16
47 86 89 92 90
85 255 252 316 293
3.72 2.40 2.56 2.09 2.33
m 2.86 2.08 2.44 1.55, 0.93 b
1.17 1.01 1.02 1.08
Polydispersity index of the molecular weight, Mw/Mn. b PMMA particles exhibit bimodal particle size distribution. Both primary and secondary particle diameters are reported. a
Furthermore, it has been found that a commercially available poly(dimethylsiloxane) macromonomer terminated with a polymerizable endgroup can also be used for the successful dispersion polymerization of MMA in CO2 (Figure 2) 8. The macromonomer had a number average molecular weight of 1.13 x 104 and a polydispersity index (PDI) of 1.1. Although using a very small amount of PDMS macromonomer in the reaction gave a considerable increase in yield and molecular weight over the reaction done without stabilizer, powdery products comprised of relatively monodisperse particles were only obtained when greater than 3.5 wt. % of PDMS macromonomer was used. In addition, a bimodal particle size distribution was obtained for reactions conducted with less than 3.5 wt. % PDMS macromonomer (Table 3). Again, excess surfactant could be washed from the particle surface with CO2 and the residual surface coating was less than 0.3 wt. %. In comparison, unfunctionalized PDMS homopolymer (lacking the polymerizable endgroup) was explored as a stabilizer and found to be ineffective. Using 6.8 wt. % PDMS homopolymer with a Mn of 1.33 x 104 and a PDI of 1.1, gave results similar to those obtained when only 0.05 wt. % PDMS macromonomer was used. Table 3. Dispersion polymerization of MMA in CO2 a Stabilizer
PDMS PDMS PDMS PDMS
none PDMS macromonomer macromonomer macromonomer macromonomer
Wt. % 0 6.8 0.05 0.26 1.7 3.5
Yield (%) 24 51 56 67 70 80
PDI b 3.7 2.5 2.4 2.1 2.6 2.1
Particle size (~tm)
2.8 2.8 2.8
Reactions were conducted at 65 ~ and 340 bar for 4 h with 2.1 g of MMA and 0.0070 g AIBN. b Polydispersity index of the molecular weight distribution, Mw/Mn.
a
27
2.2 Styrene polymerization Although it has been shown that poly(FOA) is an effective stabilizer for the polymerization of MMA in CO2, it was not an effective stabilizer for the polymerization of styrene in CO2. This is believed to be due to ineffective anchoring of the stabilizer on the growing polymer particle. To increase the anchoring ability of the surfactants, amphiphilic block copolymers were designed which contained a polystyrene segment to serve as the anchoring moiety and a poly(FOA) or PDMS segment to serve as the steric stabilizing moiety (Figure 2)10,11 The solution behavior of these amphiphilic block copolymers was examined in CO2 (Table 4). It was found that while many of the polystyrene-b-poly(FOA) block copolymers were soluble in CO2, giving a clear solution, the polystyrene-b-PDMS block copolymers were insoluble, giving cloudy dispersions in solution. This is due to the much higher solubility of poly(FOA) in CO2 than PDMS and hence the ability of poly(FOA) to solubilize larger polystyrene segments. But at the start of a typical dispersion polymerization, with 20 % styrene monomer present, the initial solution is clear and homogeneous since the monomer can act as a cosolvent to help solvate the stabilizer.
Table 4. Polystyrene block copolymers and solubility in CO2 as a function of composition Solubility (4% wt/vol) Diblock copolymer
-b-b-b-b-
PDMS 25k PDMS 65k PDMS 24k PDMS 64k
mol % Styrene 54.0 49.4 37.0 28.9 20.9 44.5 45.1
40 ~ 340 bar insoluble cloudy translucent clear clear translucent cloudy
65 ~ 340 bar insoluble cloudy cloudy clear clear cloudy cloudy
10.9 4.5 22.2 9.7
insoluble insoluble insoluble insoluble
insoluble insoluble insoluble insoluble
In this system using a polystyrene containing block copolymer, the polystyrene segment should readily partition into the lipophilic polystyrene particle core while the poly(FOA) or PDMS block is solubilized in the CO2 continuous phase to provide steric stabilization and prevent coagulation. In comparison of the polystyrene-b-poly(FOA) diblock copolymers to the polystyrene-b-PDMS diblock copolymers, it was found that the use of a polystyrene-b-PDMS stabilizer gives much more monodisperse particles. This most likely arises from the synthetic technique employed in the surfactant synthesis. The blocks in the polystyrene-b-PDMS block copolymers have a much narrower polydispersity than the blocks in the polystyrene-b-poly(FOA) block copolymers. It was noted that the particles obtained in
28 this system are much smaller (< 1 ~tm) than the particles obtained in the PMMA-CO2 system employing poly(FOA) homopolymer as stabilizer (> 1 lam) (Table 5).
Table 5. Results of styrene polymerizations in CO2 Stabilizer anone apoly(FOA) aps3"7k -b- PFOA 17k aps4"5k -b- PFOA 25k aps6"6k-b- PFOA 35k bps4.3k-bbps9.6k-bbps4.3k-bbps9.6k-b-
PDMS 25k PDMS 24k PDMS 65k PDMS 64k
Stabilizer conch. (w/v %) 0 4 4 4 4
yield (%)
Mn (kg/mol)
PDI c
Particle size (~tm)
22.1 43.5 72.1 97.7 93.6
3.8 12.8 19.2 22.5 23.4
2.3 2.8 3.6 3.1 3.0
0.40 0.24 0.24
2 2 2 2
90.6 91.7 52.2 90.9
65 56 21 39
3.6 4.3 9.2 8.1
0.22 0.46 coagulated coagulated
Polymerization in CO2 at 204 bar and 65 ~ with 2.0 g styrene and 2.4 x 10-2 M AIBN. Polymerization in CO2 at 204 bar and 65 ~ with 2.0 g styrene and 1.2 x 10-2 M AIBN c Polydispersity index of the molecular weight distribution, Mw/Mn.
a
b
It was also found that the polystyrene-b-PDMS block copolymers were not only effective at stabilizing styrene polymerizations in CO2, but also in stabilizing MMA polymerizations. When using a polystyrene-b-PDMS block copolymer as the stabilizer the resulting PMMA was recovered in 94.1% yield with a Mn = 1.8 x 105 g/mol and a PDI = 2.8. The particles obtained are much smaller and more polydisperse than the particles obtained when using poly(FOA) homopolymer as the stabilizer (particle size = 1.55 - 2.86 lam vs. 0.23 lam and particle size distribution = 1.05 vs. 1.46).
2.3
Polymerization of 2,6-dimethylphenol In addition to being a good stabilizer for the polymerization of styrene in CO2, polystyrene-b-poly(FOA) block copolymers were also found to be successful stabilizers of poly(2,6-dimethylphenylene oxide), (PPO), in CO212. Polystyrene and PPO are miscible with each other and so the polystyrene block is anchored to the polymer particle by blending with the PPO while the poly(FOA) block is solvated and provides steric stabilization. It can be seen in comparison to other stabilizers, that the use of polystyrene-b-poly(FOA) leads to a higher yield and a higher molecular weight. Although polystyrene-co-poly(FOA) random copolymer gave higher yields, the product obtained is of a very low molecular weight. Conversely, the molecular weight increased while the yield decreased upon the use of poly(FOA) homopolymer as the stabilizer. The polystyrene-b-poly(FOA) diblock copolymer was the most effective stabilizer for the synthesis of poly(phenylene oxide) in CO2 as it gave both high yields and high molecular weights (Table 6).
29
Scheme 2. Polymerization of PPO in CO2
OH
+
n/2 02
\
CO2 CuBr / amine
nil2|
Stabilizer
\CH 3
CH3
Table 6. Results of phenylene oxide polymerizations in CO2 a Stabilizer none PFOA ps4.5k _b_PFOA25k PS_co_PFOA b
Stabilizer concn. (w/v %) 0 2.8 2.8 2.8
Yield (%)
Mn (kg/mol)
PDI
67 50 74 83
6.1 8.7 17.2 3.0
1.6 1.6 5.8 2.8
a Polymerization conditions: 345 bar, 40 ~ 20 h, using a copper:amine:monomer ratio of 1:22:75 with pyridine as the amine, b 46.3% styrene : 53.7% FOA
3.
C H A R A C T E R I Z A T I O N OF SURFACTANTS IN CO2
Small angle neutron scattering (SANS) has been employed in the characterization of both CO2-philic homopolymers and self assembled polystyrene-b-poly(FOA) surfactant molecules in CO2.14, 15 Experiments were preformed at the W. C. Koehler 30 m SANS spectrometer at the Oak Ridge National Laboratory. 16 Utilizing SANS we are able to determine the weight average molecular weight (Mw), the radius of gyration (Rg), and the second virial coefficient (A2) of solvated macromolecules. We also determined the degree of aggregation and characteristic dimensions of surfactant molecules in a micelle. Dilute concentration series of two different molecular weight samples of PFOA were analyzed over a range of temperatures and pressures (40 ~ < T < 65 ~ and 340 bar < P < 395 bar). A2 values were found to be positive, indicating that supercritical CO2 is a thermodynamically good solvent for PFOA over these conditions. The radius of gyration was found to be 110/k for the high molecular weight sample (Mw --- 1.4 x 106 g/mol) and 35/k for the low molecular weight sample (Mw -- 1.1 x 105 g/mol). Analysis was preformed on 1.3 x 104 g/mol PDMS and 1.6 x 104 g/mol poly(hexafluoro propylene oxide), Krytox | at similar conditions finding that CO2 is a poor solvent for PDMS and approximately a Theta (| solvent for Krytox @. A series of block copolymers were studied by SANS in which the CO2-philic PFOA block length was varied (16,600 < Mn (g/mol) < 61,100) and the CO2-phobic polystyrene block was held constant (Mn = 3,700 g/mol). Fitting of the scattering curves to a core shell model predicts spherical structures with an average degree of association of 7 surfactant molecules per micelle. This value was constant for the series of materials as would be expected
30 for surfactants with the same insoluble or 'core' segment. The total radius, however, increases smoothly (80 to 100 ,~) with the increasing soluble or 'shell' segment length. 4.
CONCLUSIONS
Polymerization of industrially important monomers can be realized in CO2 with the application of logically designed surfactant technology. The binding segment of the surfactants has been tailored to specific growing polymer particles. Both chemical binding and physical absorption have been displayed as mechanisms for stabilization of polymer colloids during synthesis. The synthesis of commodity and engineering plastics in environmentally benign CO2 could facilitate the removal of tremendous amounts of aqueous and organic waste in the chemical process industry. REFERENCES
(1) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction-Principles and Practice; 2nd ed.; Butterworths-Heineman: Stoneham, Boston, 1993. (2) DeSimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J. B.; Romack, T. J.; Combes, J. R. Science 1994, 265, 356. (3) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271,624. (4) Romack, T. J.; Maury, E. E.; DeSimone, J. M. Macromolecules 1995, 28, 912. (5) Romack, T. J.; Treat, T. A.; DeSimone, J. M. Macromolecules 1995, 28, 8429. (6) DeSimone, J. M.; Maury, E. E.; Combes, Y. Z.; Menceloglu, Y. Z. Polym. Prepr. (Am. Chem. Soc. Div. Polym. Mat. Sci. Eng.) 1993, 68, 41. (7) Guan, Z.; DeSimone, J. M. Macromolecules 1994, 27, 5527. (8) Hsiao, Y.; Maury, E. E.; Johnston, K. P.; Mawson, S.; DeSimone, J. M. Macromolecules 1995, 28, 8159. (9) Shaffer, K. A.; Jones, T. A.; Canelas, D. A.; Wilkinson, S. P.; DeSimone, J. M. Macromolecules 1996, 29, 2704. (10) Canelas, D. A.; Betts, D. E.; DeSimone, J. M. Macromolecules 1996, 29, 2818. (11) Canelas, D. A.; Betts, D. E.; DeSimone, J. M. Polym. Prep. (Am. Chem. Soc. Div. Polym. Mat.Sci. Eng.) 1996, 74, 400. (12) Kapellen, K. K.; Mistele, C. D.; DeSimone, J. M. Macromolecules 1996, 29, 495. (13) Consani, K A.; Smith, R. D. J. Supercrit. Fluids 1990, 5, 51. (14) Fulton, J. L.; Pfund, D. M.; McClain, J. B.; Romack, T. J.; Maury, E. E.; Combes, J. R.; Samulski, E. T.; DeSimone, J. M. Langmuir, 1995, ! 1, 4241. (15) McClain, J. B.; Londono, J. D.; Combes, J. R.; Romack, T. J.; Canelas, D. A.; Betts, D. E.; Wignall, G. D.; Samulski, E. T.; DeSimone, J. M. J. Am. Chem. Soc. 1996, 118,917. (16) Koehler, W. C. Physica (Utrecht) 1986, 138B, 320. (17) Barrett, K.E.J.; Dispersion Polymerization in Organic Media, Wiley: London, 1975. (18) Napper, D. H.; Polymer Stabilization of Colloidal Dispersions, Academic Press: London, 1983.