Carbocationic polymerization in supercritical carbon dioxide

Polymerization of isobutylene (IB) in supercritical carbon dioxide (SC·CO₂) at 32.5°C and 140 bar by the use of 2-chloro-2,4,4-trimethyl-pentane (TMPCl) initiator in conjunction with a mixture of TiCl₄/BCl₃ leads to well-defined polyisobutylenes (PIB) capped by a t-Bu head group and a t-Cl tail group (tBu-PIB-Cl^t) of M_n1800 g/mole and M_w/M_n=1.3. The TiCl₄/BCl₃ mixture may be viewed a new Friedel-Crafts Acid that effects rapid initiation, essentially chaintransferless propagation and reversible termination. The mechanism of IB polymerization of TiCl₄/BCl₃ mixtures is discussed.     

Dispersion polymerization in supercritical carbon dioxide using comb-like fluorinated polymer surfactants having different backbone structures

Comb-like fluorinated polymers with different backbone structures, poly(heptadecafluorodecyl acrylate) (PA-R_f), poly[oxy[(2-perfluorooctylethylene)thiomethyl]ethylene] (PEO-R_f), and poly[p-[[(perfluorooctylethylene)thio]methyl]styrene] (PS-R_f), were used as surfactants in dispersion polymerization to examine the effect of backbone structure on the formation of polymer particles. Dispersion polymerization of monomers with different polarities using these comb-like fluorinated polymer surfactants in CO₂ showed that PEO-R_f containing a polar oxyethylene backbone was an effective surfactant for the dispersion polymerization of a polar monomer, such as N-vinyl-2-pyrrolidone, whereas PA-R_f was effective for less polar monomers, such as methyl methacrylate and N-vinyl caprolactam.     

Dispersion polymerization of methyl methacrylate with three types of comblike fluorinated stabilizers in supercritical carbon dioxide

Dispersion polymerizations of methyl methacrylate in supercritical carbon dioxide were conducted with three types of comblike fluorinate polymer stabilizers: poly(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10‐heptadecafluorodecyl methacrylate) (PHDFDMA), poly(3,3,4,4,5,5,6,6,7,7,8,8,8‐tridecafluorooctyl methacrylate) (PTDFOMA), and poly(2,2,3,3,3‐pentafluoropropyl methacrylate) (PPFPMA). The effect of the polymerization pressure was not significant on the mean diameters of the poly(methyl methacrylate) (PMMA) particles from 20 to 40 MPa. However, the coefficients of variation of the particle diameters produced at 20 MPa ( , where is the number‐basis mean particle diameter), where the heterogeneous phase was found before polymerization, were larger than those produced at 30 and 40 MPa, where the homogeneous phase was found. The mean size of the PMMA obtained with PTDFOMA and PPFPMA strongly depended on the stabilizer concentration compared with that obtained with PHDFDMA. Moreover, the mean size decreased as the carbon dioxide‐philic side chain length increased. As shown by the results of this study, the best stabilizer among the three types of stabilizers for producing PMMA particles was PHDFDMA. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43813.     

Cross-linking polymerization of acrylic acid in supercritical carbon dioxide

Cross-linking polymerization of acrylic acid in supercritical carbon dioxide (scCO₂) was studied in a batch reactor at 50 °C and 207 bar with either triallyl pentaerythritol ether or tetraallyl pentaerythritol ether as the cross-linker and with 2,2′-azobis(2,4-dimethyl-valeronitrile) as the free radical initiator. All polymers were white, dry, fine powders. Scanning electron microscopy showed that the morphology of the polymer particles was not affected by cross-linking. As the cross-linker concentration was increased, the polymer glass transition temperature first decreased, then increased. Water-soluble and water-insoluble polymers were synthesized by adjusting the cross-linker concentration. Viscosity measurements showed that the polymer thickening effect strongly depended on the degree of cross-linking. Finally, cross-linking polymerization of acrylic acid in scCO₂ was carried out in a continuous stirred tank reactor. The use of cross-linker decreased the monomer conversion in this system.     

超临界CO2在聚合反应中的应用

对超临界CO2在溶液聚合、悬浮聚合、乳液聚合、沉淀聚合、分散聚合等聚合反应中的应用进行了综述。超临界CO2已广泛应用于萃取和生物工程等方面,由于其作为聚合反应介质具有无毒、无污染的特性．在聚合物制备中已得到越来越广泛的重视。     

Dispersion polymerization of methyl methacrylate in supercritical carbon dioxide using a silicone‐containing fluoroacrylate stabilizer

Free radical dispersion polymerization of methyl methacrylate (MMA) was carried out in supercritical carbon dioxide (scCO₂) using poly{(heptadecafluorodecyl acrylate)‐co‐3‐[tris(trimethylsilyloxy)silyl]propyl methacrylate} (p(HDFDA‐co‐SiMA)) as stabilizer. Dry, fine powdered spherical poly(methyl methacrylate) (pMMA) particles with well‐defined sizes were produced. The resulting high yield of spherical and relatively uniform micron‐size pMMA particles was formed utilizing various amounts of p(HDFDA‐co‐SiMA) random copolymer. The particle diameter was shown to be dependent on the weight percent of the stabilizer added to the system. The effects of varying the concentration of stabilizer (1–7 wt%), reaction time (4–12 h) and pressure (15–35 MPa) upon the polymerization yield, molar mass and morphology of pMMA were investigated. Copyright © 2005 Society of Chemical Industry     

Detailed modelling of MMA dispersion polymerization in supercritical carbon dioxide

A comprehensive model of dispersion polymerization in supercritical media is presented. With respect to previous ones reported in the literature, this model accounts in detail for the interphase mass transport of the active polymer chains and specifically for its dependence on the chain length. All the model parameters have been evaluated from independent literature sources in the case of poly(methyl methacrylate) dispersion polymerization in supercritical carbon dioxide. The corresponding a priori predictions of the model have been validated with various sets of experimental data, covering a rather wide range of operating conditions, in terms of conversion, pressure and molecular weight distribution as a function of time. It has been found that the interphase mass transport of active chains plays a significant role in determining the number of reaction loci and consequently the final polymer properties and, in particular, the molecular weight distribution.     

Dispersion copolymerization of acrylonitrile‐vinyl acetate in supercritical carbon dioxide

Dispersion copolymerization of acrylonitrile‐vinyl acetate (AN‐VAc) had been successfully performed in supercritical carbon dioxide (ScCO₂) with 2,2‐azobisisobutyronitrile (AIBN) as a initiator and a series of lipophilic/CO₂‐philic diblock copolymers, such as poly(styrene‐r‐acrylonitrile)‐b‐poly(1,1,2,2‐tetrahydroperfluorooctyl methacrylate) (PSAN‐b‐PFOMA), as steric stabilizers. In dispersion copolymerization, poly(acrylonitrile‐r‐vinyl acetate) (PAVAc) was emulsified in ScCO₂ effectively using PSAN‐b‐PFOMA as a stabilizer. Compared with the precipitation polymerization (absence of stabilizer), the products prepared by dispersion polymerization possessed of higher yield and higher molecular weight. In addition, the particle morphology of precipitation polymerization was irregular, but the particle morphology of dispersion polymerization was uniform spherical particles. In this study, the effects of the initial concentrations of monomer and the stabilizer and the initiator, and the reaction pressure on the yield and the molecular weight and the resulting size and particle morphology of the colloidal particles were investigated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5640–5648, 2006     

Dispersion polymerization of <Emphasis Type=Italic>N</Emphasis>-vinyl caprolactam in supercritical carbon dioxide

Dispersion polymerization of N-vinyl caprolactam (NVCL) was carried out in supercritical carbon dioxide (scCO₂) using three surfactants. The polymerization was performed in the presence of fluorine-based poly(heptadecafluorodecyl acrylate) (PHDFDA), poly(heptadecafluorodecyl methacrylate) (PHDFDMA) or siloxane-based PDMS-g-pyrrolidonecarboxylic acid (Monasil PCA) as a surfactant. FE-SEM and image analyzer were used to characterize particle morphology, size, and size distribution. When fluorine-based surfactants were used, spherical PVCL particles were obtained. Using Monasil PCA resulted in agglomerated and irregular polymer particles. The effect of concentration of surfactants, initiators, and monomer, and reaction pressure on the particle morphology, average particle size and particle size distribution (PSD) was also investigated with fluorine-based surfactant, PHDFDA or PHDFDMA.     

A new PDMS macromonomer stabilizer for dispersion polymerization of styrene in supercritical carbon dioxide

Free radical polymerization of styrene in supercritical CO₂ requires addition of a surfactant to produce polystyrene (PS) in high conversion and molecular weight with well‐defined particle sizes. In this work, we examined a new stabilizer that can provide effective stabilization for the polymerization of styrene. A commercially available poly(dimethylsiloxane) macromonomer has been employed as a stabilizer for dispersion polymerization of PS in scCO₂. The reactions were conducted in a 225‐mL stainless steel autoclave over the temperature range 60–80°C and under pressures of 1,500 to 3,000 psi. After 2–12 h of polymerization, the conversion determined by gravimetrical method was between 20 and 80%. These preliminary results suggest that this macromonomer offers satisfactory stabilization for the styrene system. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 545–549, 2004     

Fluoropolymer synthesis in supercritical carbon dioxide

Herein we review the environmentally friendly synthesis of fluorinated polymers in supercritical carbon dioxide (scCO₂). Historically, many high-performance fluorinated materials are commercially synthesized in aqueous media using fluorinated surfactants or in non-aqueous conditions using fluorinated solvents. Our group has pioneered both the homogeneous and heterogeneous polymerization of fluorinated monomers in scCO₂. This review includes discussions on the synthesis of main-chain and side-chain fluoropolymers conducted via a chain-growth or continuous process. Specific materials consist of acrylate- and styrene-based systems, poly(vinyl ether)s, tetrafluoroethylene- and vinylidenefluoride-based, as well as novel fluorinated elastomers and thermoplastics.     

Carbocationic polymerizations in supercritical carbon dioxide

The first carbocationic polymerization of isobutylene (IB) in supercritical carbon dioxide (SC·CO₂) has been accomplished. It was demonstrated that in CO₂ at 32.5°C and 120 bar the 2-chloro-2,4,4-trimethyl-pentane (TMPCl)/SnCl₄ and TMPCl/TiCl₄ initiating systems lead to 30% IB conversion, and gave polyisobutylenes (PIB) with M_n2000 and M_w/M_n2.0. This is the highest temperature IB was ever polymerized to reasonably high molecular weight products. Polymerizations at 32.5 °C under similar but conventional (non-living) conditions in the absence of SC·CO₂ would yield only very low molecular weight oligomers (tetramers). The structure of the PIBs obtained in SC·CO₂ is virtually identical to those obtained at much lower temperatures in conventional liquid-phase systems indicating the presence of chain transfer to monomer in both systems. In contrast to TMPCl initiated polymerizations, the 1,3-bis-(2-hydroxy-2-propyl)-5-tert-butylbenzene (HPBB) initiator in conjunction with BCl₃ and SnCl₄ yields only oligomers (M_n500) in SC·CO₂.     

Reversible addition-fragmentation chain transfer polymerization of vinyl acetate and vinyl pivalate in supercritical carbon dioxide

Two highly supercritical CO₂-soluble, poly(vinyl acetate) (PVAc)-based macro-reversible addition-fragmentation chain transfer (RAFT) agents were synthesized. The RAFT agents were used for the first time in RAFT/macromolecular design via the interchange of xanthates (MADIX) and polymerization of vinyl acetate (VAc) and vinyl pivalate (VPi) in supercritical carbon dioxide (scCO₂). A homopolymer PVAc and a block copolymer PVAc-b-PVPi made by RAFT/MADIX polymerization were characterized, and the effects of time and RAFT agents on polymerization were examined. For the 8.4 wt% RAFT agent in VAc, the molecular mass (M _n ) of homopolymer PVAc was 26,000 g mol^?1 and PDI was 1.35. For the copolymerization of VPi using 9.8 wt% PVAc-RAFT agent in VPi for 24 h, the M _n and PDI of PVAc-b-PVPi reached 32,400 g mol^?1 and 1.42, respectively. These results suggest that the polydispersity can be controlled during the clean production of PVAc and PVPi by RAFT/MADIX polymerization in scCO₂.     

Synthesis of biodiesel in supercritical alcohols and supercritical carbon dioxide

Biodiesel was synthesized in supercritical fluids by two routes: non-catalytically in supercritical alcohols and by enzyme catalysis in supercritical carbon dioxide. Two oils, sesame oil and mustard oil, and two alcohols, methanol and ethanol, were used for the synthesis. Complete conversion was observed for synthesis in supercritical alcohols whereas only a maximum of 70% conversion was observed for the enzymatic synthesis in supercritical carbon dioxide. For the synthesis in supercritical alcohols, the activation energies and pseudo-first order rate constants were determined. For the reactions in supercritical carbon dioxide, a mechanism based on ping pong bi-bi was proposed and the kinetic parameters were determined.     

Polymerization of acrylonitrile in supercritical carbon dioxide

A series of fluorinated diblock copolymers, consisting of styrene (St)-acrylonitrile (AN) copolymer [poly(St-co-AN)] and poly-2-[(perfluorononenyl)oxy]ethyl methacrylate, with various compositions as well as with different molecular weights were synthesized by atom transfer radical polymerization and characterized. Dispersion polymerization of acrylonitrile in supercritical carbon dioxide (scCO₂) at 30 MPa and at 65 °C with this kind of amphiphilic block copolymer as a stabilizer and 2,2′-azobisisobutyronitrile as an initiator was investigated. The experimental results indicated that, in the presence of a small amount of poly(St-co-AN) (5 wt% to AN), spherical particles of polyacrylonitrile (PAN) were prepared with small diameter and narrow polydispersity (d_n = 153 nm, d_w/d_n = 1.12), resulting from the high stabilizing efficiency of this fluorinated block copolymer. Furthermore, the polymerization of AN in scCO₂ under different initial pressures especially under low pressure (<14 MPa) was studied. When the polymerization was carried out around the critical pressure of CO₂ (7.7-7.8 MPa), the PANs with high molecular weight (M_ν ≈ 130,000-194,000) were synthesized at high monomer conversion (>90%) no matter whether the stabilizer was added, compared to those synthesized by dispersion polymerization at 30 MPa. It was also found that the crystallinity of PAN synthesized at 7.7-7.8 MPa was somewhat higher than that synthesized at 30 MPa, while its crystallite size did not change.     

Solubility of astaxanthin in supercritical carbon dioxide

The solubility of astaxanthin in carbon dioxide was measured under supercritical conditions of a pressure range from 80 to 300 bar, and temperature range from 303 to 333 K, by using a dynamic flow-type. The solubility of astaxanthin increasing from 0.42×10⁻⁵ to 4.89×10⁻⁵ with higher temperature and pressure maintains certain density of supercritical carbon dioxide. The solubility data obtained were applied to the Chrastil model, based on the density of carbon dioxide. The data fitted well with the Chrastil model at most experimental conditions.     

Solubility of gabapentin in supercritical carbon dioxide

The purpose of this study was to measure the solubility of gabapentin in supercritical carbon dioxide at different pressures and temperatures of 16–40 MPa and 308–338 K, respectively. The measured solubility data revealed that solubility of gabapentin was 8.97 × 10⁻⁵–7.36 × 10⁻³ based on the mole fraction in supercritical carbon dioxide at the aforementioned operational conditions. At last, the obtained results were correlated using four density based semi-empirical correlations namely Bartle et al., Mendez-Santiago and Teja (MST), Chrastil and Kumar and Johnston (K–J). The results revealed that according to the obtained average absolute relative deviations (AARD) for the used density based correlations of MST (AARD % = 9.88%), Chrastil (AARD % = 9.47%), K–J (AARD % = 11.5%) and Bartle et al. (AARD % = 9.29%), none of the correlations can be considered as the most accurate one. In other words, all the examined semi-empirical correlations are in the same level of accuracy.     

Polar attributes of supercritical carbon dioxide

Supercritical carbon dioxide (scCO(2)) is increasingly promoted as an environmentally benign alternative to conventional organic solvents. The supercritical state bridges the gap between liquid and gaseous states by offering gaslike diffusion rates and liquidlike solvent densities, thereby enabling potential opportunities as a reaction and separation medium in chemical industry. Understanding the solvent behavior of liquid and scCO(2) is of critical importance to enable the design of CO(2)-philic molecular systems and to expand the use of these solvent systems to a wider range of chemical processes. Historically CO(2) was treated as a nonpolar solvent, primarily because of its low dielectric constant and zero molecular dipole moment. CO(2) has also been described as a quadrupolar solvent because of its significant quadrupole moment. Recent studies suggest that, as far as the microscopic solvent behavior of CO(2) is concerned, CO(2) has the potential to act as both a weak Lewis acid and Lewis base. Also, strong theoretical and experimental evidence indicates that CO(2) can participate in conventional or nonconventional hydrogen-bonding interactions. All of these site-specific solute-solvent interactions are important to understand the fundamental nature of CO(2) as a solvent. In this Account, we discuss these polar attributes of CO(2) and their relation to solvation.