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Carbon dioxide as a continuous phase for polymer synthesis.

The purpose of SPE's New Technology Committee is to explore emerging topics in the field of plastics science and technology. The following article, which is part of a series of presentations organized by the Committee, shows that the improved processing and properties realized from the use of C[O.sub.2] in polymer synthesis result primarily from the lack of chain transfer and propensity for high plasticization.

The scientific community has recently developed considerable interest in using liquid and supercritical C[O.sub.2] as the continuous phase for polymerization reactions. The polymer industry, in particular, is under increasing scrutiny to reduce emission of volatile organic compounds (VOCs), to completely phase out the use of chlorofluorocarbons (CFCs), and to reduce the generation of aqueous waste streams. It is these environmental concerns that provide the principal driving force motivating the development of C[O.sub.2]-based polymerization technologies. Unique properties such as tunable density and the ability to significantly plasticize glassy polymers make supercritical fluids interesting - though relatively unstudied - solvents. Since 1990, we have made a concerted effort to explore the scope and limitations of C[O.sub.2] in polymer synthesis.

For numerous reasons, C[O.sub.2] represents an extremely attractive continuous phase in many applications. First, C[O.sub.2] is naturally occurring and readily available. Sources of C[O.sub.2] include both abundant natural reservoirs and recycled C[O.sub.2] recovered from the exhaust streams of power plants and industrial plants that produce ethanol, ammonia, hydrogen, and ethylene oxide.[3] Additionally, C[O.sub.2] has an easily accessible critical point with a critical temperature ([T.sub.c]) of 31.1 [degrees] C and a critical pressure ([P.sub.c]) of 73.8 bar.[2] Other benefits of the employment of C[O.sub.2] as a solvent include its low cost, nonflammability, nontoxicity, and easy recyclability; it also eliminates the need for energy-intensive drying processes since the products from a polymerization conducted in C[O.sub.2] are isolated completely dry upon venting to remove the C[O.sub.2]. For polymerization reactions, in particular, the solvency of C[O.sub.2] as a medium and the plasticization effects of C[O.sub.2] on the resulting polymeric products represent the properties of central importance. These significant properties of C[O.sub.2] are explored in detail below. When all these factors are combined with the fact that C[O.sub.2] may obviate the use of much more expensive and hazardous solvents, one cannot deny the importance of investigating its use as a continuous phase for polymer synthesis and processing.

When C[O.sub.2] is used as the continuous phase for a polymerization reaction, the solvency of the reagents and products is of primary importance. To begin with, C[O.sub.2] has a low dielectric constant; by varying temperature and density, Keyes and Kirkwood reported values ranging from 1.01 to 1.45 for gaseous C[O.sub.2] and 1.60 to 1.67 for liquid C[O.sub.2].[3] The solubility parameter of C[O.sub.2], which is strongly dependent on pressure, has been calculated by McFann and co-workers.[4] It has been noted that C[O.sub.2] behaves very much like a hydrocarbon solvent with respect to its capability of dissolving small molecules, and thus many monomers exhibit high solubility in C[O.sub.2].[5] Even though it is a relatively low dielectric medium, C[O.sub.2] is a Lewis acid, and it has a strong quadrupole moment that allows it to dissolve such polar molecules as methanol.[6] In contrast, other polar molecules, such as amides, ureas, urethanes and azo dyes, exhibit very poor solubility in C[O.sub.2].[5] Of more importance, the solubility of water was initially investigated by Lowry and Erickson, who determined that less than 0.05 wt% of water dissolves in liquid C[O.sub.2] over a range of temperatures.[7] King and co-workers have completed a more extensive study of the C[O.sub.2]/ water binary system at both liquid and supercritical conditions.[8] Bartle and co-workers have compiled a large table of the solubilities of compounds of low volatility in supercritical C[O.sub.2].[9] Indeed, the solubility characteristics of compressed C[O.sub.2] with respect to small molecules have been extensively studied.

In contrast to its behavior with respect to small molecules, C[O.sub.2] acts as an exceedingly poor solvent for most high molar mass polymers. As an illustration, at 80 [degrees] C a pressure exceeding 2000 bar is required to obtain a homogeneous solution of poly(methyl acrylate) (PMA; ([M.sub.n]) = 10,600 g/mol) in C[O.sub.2].[10] In fact, the only classes of polymers that have demonstrated good solubility ([greater than]10 wt%) in C[O.sub.2] at mild conditions (T[less than]100 [degrees] C, P [less than] 350 bar) are amorphous or low-melting fluoropolymers and silicones.[11-16] While the parameters that govern the solubility of polymeric materials in C[O.sub.2] are not yet completely understood, numerous studies have explored the possible specific solute-solvent interactions between various polymers and C[O.sub.2]. Early work in this area suggested that weak dipole-dipole interactions exist between C[O.sub.2] and functional groups on polymer chains, such as sulfones[17] and carbonyls.[18] Later, systematic studies of the interactions between C[O.sub.2] and silicones indicated that specific interactions between C[O.sub.2] and the silicone of the polymer backbone govern the high solubility of C[O.sub.2] in these types of polymers.[19,20] Finally, recent work that involves the use of Fourier transform infrared spectroscopy reveals that C[O.sub.2] exhibits Lewis add-base type interactions with electron-donating functional groups of polymer chains, such as the carbonyl group of poly (methyl methacrylate) (PMMA).[21] Regardless of the nature of these interactions, the poor solubility of most polymers in C[O.sub.2] demands that the polymerization of most industrially important hydrocarbon monomers at reasonably accessible temperatures and pressures be conducted either by heterogeneous polymerization techniques or by C[O.sub.2]-modified melt processes.

The plasticization of polymers by C[O.sub.2] constitutes another important parameter that affects polymerizations. This plasticization of polymers plays a key role in both the diffusion of monomer into the polymer phase[22-24] and the incorporation of additives into a polymer matrix.[25-28] Researchers have studied the effects of high pressure C[O.sub.2] on the glass transition temperature ([T.sub.g]) and mechanical properties of a variety of materials, including both semicrystalline and amorphous polymers, such as polystyrene (PS), poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), polycarbonate (PC), poly(vinyl chloride) (PVC), poly(ethylene terephthalate) (PET), poly(2,6 dimethyl phenylene oxide) (PPO), polyurethanes, poly(vinyl benzoate), and poly(vinyl butyral).[29-41] In most cases, reductions in [T.sub.g]s of the polymers were observed in liquid and supercritical C[O.sub.2]. By systematically examining both thermal and mechanical properties of twenty different polymers over a wide range of conditions, Shieh and co-workers concluded that both the morphology and polarity of the polymer play key roles in determining the absorption of C[O.sub.2] and subsequent plasticization.[40,41] Models have also been developed to predict the depression of the [T.sub.g] of a polymer in the presence of compressed fluids.[42,43] The predictions of [T.sub.g] by these models are in reasonable agreement with the experimental data.

The well-developed processing methods for polymers in supercritical fluids can be exploited most effectively when the polymer is actually synthesized in the supercritical medium. For example, supercritical C[O.sub.2] can be used to efficiently extract residual monomer, solvent, or catalyst from a solid polymer.(11) In addition, because the density of a supercritical fluid can be changed dramatically by simply altering the pressure, a mixture of polymers of different molecular weights can be efficiently fractionated.[18,44-46] Since supercritical C[O.sub.2] sorption by a polymer reduces its melt and solution viscosities,[47-49] the morphology of the polymer can be controlled with supercritical drying[50] or foaming.[51-53] Other processing advantages afforded by supercritical C[O.sub.2] derive from its easily accessible critical conditions. Because its [T.sub.c] is very close to room temperature, supercritical C[O.sub.2] can be used in applications involving polymers and heat sensitive materials such as enzymes,[54,55] flavors,[56] pharmaceuticals,[57] and highly reactive monomers.[58]

Polymer Synthesis

Step Growth Polymerizations

Many polymers produced by step growth processes are synthesized in the melt phase. One advantage of traditional melt phase polymerizations is the ability to produce high molecular weight polymer without the need for organic solvents. Even though melt polymerizations avoid the expense and environmental hazards associated with many solvents, a disadvantage to such methods is the high viscosity associated with the attainment of high molecular weight product toward the end of the polymerization. Furthermore, in a reaction system driven by the removal of a small molecule condensate, any enhancement of condensate removal would be expected to produce a corresponding increase in reaction rate. Conventional processing methods rely on high vacuum to remove the condensate, but such methods are capital intensive and require much maintenance in order to be operational for long periods of time on a commercial scale. A constant flow of liquid or supercritical C[O.sub.2] through the reaction vessel could improve on the present approach to condensate removal by solubilizing the condensate for subsequent removal from the fluid phase downstream from the reactor, prior to recycling and recovery of the C[O.sub.2] mobile phase. In fact, a low pressure approach has been suggested for the synthesis of polyesters and polyamides,[59] but prior work does not take into account the benefits derived from introducing a swelling or plasticizing agent into the polymer melt.

In addition to the plasticization effects noted above, high pressure C[O.sub.2] has the capability of swelling the polymer melt phase,[47] thus providing more surface area for condensate removal by increasing free volume, and also reducing chain entanglements by essentially diluting the polymer melt. The increase in free volume of the melt serves to decrease melt viscosity, as shown previously, and viscosity reduction of the melt phase may translate into a more easily processed material. Although a swelling agent can prove beneficial from a processing standpoint, it is not advantageous from an end use perspective if it introduces into the product a substance that is toxic or difficult to separate from the reaction mix; as mentioned previously, C[O.sub.2] is both nontoxic and easily separable from the polymeric product at the end of the reaction. These characteristics make C[O.sub.2] particularly well suited for use as a swelling agent in the polymerization of polyesters or polycarbonares that have high volume usage as food and beverage packaging.

One of the first reactions investigated for use in the constant C[O.sub.2] flow system is the melt condensation of bis(2-hydroxyethyl terephthalate) (BHET) to PET (Scheme 1).

Extension of the polymer chain is by the removal of one molecule of ethylene glycol per polymer linkage formed. Rate of removal of the ethylene glycol by-product is directly related to rate of polymerization. Ethylene glycol is soluble in supercritical C[O.sub.2] at 2 to 3 wt%, and thus an increased flow rate of C[O.sub.2] through the reaction vessel should enhance ethylene glycol removal and therefore increase polymerization rate. Prior investigation of swollen state polymerizations of PET[60] using organic solvents has shown that a rate increase is indeed achieved by increasing the surface area in the melt available for condensate removal; the extent to which supercritical C[O.sub.2] is able to swell the polymer melt in this synthesis is currently under investigation.

The replacement of traditional vacuum synthesis methods with supercritical fluid extraction methods has also been illustrated for the melt phase synthesis of a polycarbonate, shown in Scheme 2.

Unlike our synthesis of PET from BHET, which has an inherent 1:1 stoichiometry, the polycarbonate synthesis shown here requires a precise stoichiometric ratio of bisphenol-A (BPA) and diphenyl carbonate (DPC). Since DPC is soluble in C[O.sub.2], it was necessary to employ a slight molar excess (1.005 equivalents of BPA) to maintain the desired stoichiometric ratio. After a degree of polymerization of two is reached, which is attained quite early in the polymerization, extraction of DPC by C[O.sub.2] is no longer a concern.

The desired reaction temperature is reached in two steps: the reactants are brought to an initial temperature at which all components are molten, and then temperature is gradually raised to the desired reaction temperature. As in all step growth polymerizations, the reaction must be driven to completion in order to obtain a high molecular weight product; here we exploit the solubility of phenol in supercritical C[O.sub.2] to enhance phenol removal from the reaction mix and also to monitor the progress of the reaction by collecting phenol from the reaction effluent upon decompression.

The use of C[O.sub.2] to enhance step growth polymerizations is not restricted to condensate extraction. For example, the C[O.sub.2]-assisted preparation of polyamides is demonstrated in the synthesis of nylon 6,6 from nylon salt (Scheme 3). Use of the nylon salt route is essential because primary amines are known to react with C[O.sub.2]. A 1:1 mixture of adipic acid and hexamethylene diamine in methanol allowed for the formation of the ammonium salt. Although the nylon 6,6 salt malts at 190 [degrees] C, in the presence of C[O.sub.2] we found that the melting point was lowered more than 40 [degrees] to 150 [degrees] C. Upon further heating in the presence of C[O.sub.2], high molecular weight nylon 6,6 was obtained.

Chain Growth Polymerizations

Carbon dioxide presents an ideal inert solvent to effect the polymerization of highly fluorinated monomers and obviates the use of solvents that are being phased out because of environmental concerns. Using supercritical C[O.sub.2] as the solvent, we have used free radical initiators to effect the synthesis of high molar mass amorphous fluoropolymers via homogeneous solution polymerization.[14,61-63] For example, the polymerization of 1,1-dihydroperfluorooctyl acrylate (FOA) is shown in Scheme 4.

The polymerization and oligomerization of fluoroolefins in C[O.sub.2] has particular advantages over other solvents because of the lack of chain transfer to C[O.sub.2]. The radicals derived from fluoroolefin monomers, such as tetrafluoroethylene (TFE), are highly electrophilic and will chain transfer to almost any hydrocarbon present. In addition, highly reactive monomers, such as TFE, may be handled more safely as a mixture with C[O.sub.2].[58] Early work examining the free-radical telomerization of TFE in supercritical C[O.sub.2] took advantage of the high solubility of the low molecular weight perfluoroalkyl iodide products to prepare these materials homogeneously (Scheme 5).[64]

We have also polymerized several fluorinated cyclic and vinyl ethers via homogeneous cationic polymerzation in liquid and supercritical C[O.sub.2] (Scheme 6).[65] The result for the oxetane polymerization in C[O.sub.2] was comparable to a control experiment, conducted in Freon-113 (DuPont), another indication that C[O.sub.2] provides an excellent alternative to CFCs in solvent applications. These results also indicate that cationic chain growth can effectively be used in the homogeneous polymerization of fluorinated monomers in C[O.sub.2].

Although the fluoropolymers outlined above can be prepared via solution polymerization, most polymers are insoluble in C[O.sub.2] and heterogeneous polymerization techniques must be employed. Techniques we have explored to date include precipitation, emulsion, and dispersion polymerization.

Precipitation polymerizations dominated the early work, which aimed at preparing industrially important hydrocarbon polymers, such as polyethylene, PVC, PS, poly(acrylonitrile) (PAN), poly(acrylic acid) (PAA), and poly(vinyl acetate) (PVAc) in C[O.sub.2].[66-73] The precipitation polymerization of acrylic acid has been particularly well studied. An advantage of this process lies in the extremely fast propagation rate of this reaction, which allows the synthesis of high molecular weight PAA even though the polymer demonstrates poor solubility in the continuous phase.[72] We have explored the synthesis of PAA in supercritical C[O.sub.2] and have expanded this study to include effective molecular weight control through the use of ethyl mercaptan as a chain transfer agent.[73] Other work in this area by Dada and co-workers has shown that the molecular weight of the PAA product can be controlled by manipulation of the temperature and pressure.[74]

Emerging chain growth mechanisms have also been explored in C[O.sub.2]. For example, the use of stable free radical polymerization techniques in supercritical C[O.sub.2] represents an exciting new topic of research.[75] Another area that has been studied includes metal catalyzed polymerizations, such as ring opening metathesis polymerization.[76-77] Advances in these areas have demonstrated the stability of C[O.sub.2] to a wide variety of chemistries.

Other heterogeneous chain growth polymerization techniques that have been effectively used in a C[O.sub.2] continuous phase include emulsion[78] and dispersion[24,79,80] polymerizations. Because the insoluble polymer is stabilized as a colloid, these techniques offer the advantage of allowing high molecular weights and high yields to be simultaneously achieved. A representative scanning electron micrograph of polystyrene particles produced by dispersion polymerization in C[O.sub.2] is shown here. These particles were prepared using an amphiphilic diblock copolymer stabilizer containing a polystyrene anchoring segment and a poly(dimethylsiloxane) soluble segment; parameters such as temperature, pressure, and ratio of reagents play a pivotal role in the size and size distribution of the resulting particles.[81]

The need for effective surfactants for these systems has led to design criteria for amphiphilic materials in C[O.sub.2]. For example, block copolymers that are amphiphilic in C[O.sub.2] have been shown via small angle neutron scattering to form micelles.[82] The breakthroughs in the design and synthesis of surfactants for C[O.sub.2] have been the key to successful emulsion and dispersion polymerizations.

Conclusions

The use of liquid and supercritical C[O.sub.2] clearly offers significant advantages beyond the simple elimination of the use of organic solvents or water. Advantages in the properties and processing of materials can be realized through the use of this medium principally as a result of the lack of chain transfer and high plasticization propensity.

Acknowledgement

The authors gratefully acknowledge financial support from the National Science Foundation (NSF) through a Presidential Faculty Fellowship (JMD: 1993-1997), the Environmentally Benign Chemical Synthesis and Processing Program sponsored by the NSF and the Environmental Protection Agency, and the Consortium for Polymeric Materials Synthesis and Processing in Carbon Dioxide sponsored by Air Products and Chemicals, Bayer, BFGoodrich, DuPont, Eastman Chemical, Hoechst Celanese, and Xerox. In addition, the authors thank Isco for the use of its syringe pump.

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Date:Dec 1, 1997
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