Preparation of micron-size monodisperse poly(2-hydroxyethyl methacrylate) particles by dispersion polymerization

Dispersion polymerization of 2-hydroxyethyl methacrylate using four categories of polymeric stabilizers in a mixture of good and poor solvents was performed to produce polymeric particles. The stabilizers employed were methyl methacrylate and styrene homopolymers, methacryloyl-terminated poly(methyl methacrylate) and polystyrene macromonomers, an amphiphilic poly(methyl methacrylate-co-methacrylic acid-graft-styrene), and polybutadiene derivatives containing reactive vinyl groups. Dispersion copolymerization with a small amount of the macromonomer gave micron-size particles with relatively narrow size distribution. The amphiphilic graft copolymer and the polybutadiene derivatives also afforded monodisperse particles. The mixed ratio between good and poor solvents greatly affected the particle size and size distribution. © 1996 John Wiley & Sons, Inc.     

Preparation and characterization of microcellular polystyrene/polystyrene ionomer blends with supercritical carbon dioxide

The preparation of microcellular polystyrene (PS), lightly sulfonated polystyrene (SPS), zinc‐neutralized lightly sulfonated polystyrene (ZnSPS), and blends of PS/SPS and PS/ZnSPS via supercritical CO₂ was carried out with the pressure‐quench process. Both higher foaming temperature and lower pressure result in larger cell sizes, lower cell densities, and lower relative density for microcellular ionomers and blends as for microcellular PS. The difference among various microcellular samples is the change of cell size with the sample composition. The cell size decreases in the sequence from SPS, through PS/SPS blends, PS and PS/ZnSPS blends, to ZnSPS. The diffusivity of CO₂ in samples also decreases in the sequence from SPS, through PS/SPS blends, PS and PS/ZnSPS blends, to ZnSPS. For this series of samples with similar structure and identical solubility of CO₂, the varying diffusivity is responsible for the difference of cell sizes. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 368–377, 2003     

Preparation of silicone‐graft copolymers by homogeneous radical copolymerization in supercritical carbon dioxide

Copolymerizations of 1,1‐dihydroperfluorooctyl methacrylate (FOMA; M₁) and methacryloxypropyl‐terminated polydimethylsiloxane [M‐PDMS (M_n = 5.9 K); M₂] and homopolymerization of M‐PDMS in supercritical CO₂ are described. The homopolymerization of M‐PDMS proceeded homogeneously without difficulty to produce oligomers (M_n = 30 K). The copolymerizations of FOMA and M‐PDMS also proceeded homogeneously over a wide monomer feed ratio. The ratio of M‐PDMS incorporated into the copolymer obtained was almost equal to the monomer feed ratio even up to the high conversion. The reactivity ratio r₁ was determined to be 1.66. DSC examination of the copolymers indicated a microphase‐separated morphology consisting of poly‐FOMA (PFOMA) and PDMS domains for all copolymer compositions. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1139–1145, 2000     

Radical polymerizations of a silicone‐containing acrylic monomer in supercritical carbon dioxide

Radical homopolymerizations and copolymerizations of 3‐[tris(trimethylsilyloxy)silyl]propyl methacrylate (SiMA) in supercritical CO₂ were investigated. The homopolymer was obtained in CO₂ with a good yield. It was essentially insoluble in pure CO₂ at less than 500 bar at 65 °C but was soluble in a mixture of CO₂ and its monomer (10 w/v %) at 352 bar. The copolymerizations of SiMA with methyl methacrylate, 1,1‐dihydroperfluorooctyl methacrylate, and styrene with various monomer feed ratios were also examined in supercritical CO₂ and in bulk, and the reactivity ratios were determined. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3100–3105, 2000     

Dispersion polymerization of 2‐hydroxyethyl methacrylate in supercritical carbon dioxide

Herein we report a successful dispersion polymerization of 2‐hydroxyethyl methacrylate (HEMA) in a carbon dioxide continuous phase with a block copolymer consisting of polystyrene and poly(1,1‐dihydroperfluorooctyl acrylate) as a stabilizer. Poly(2‐hydroxyethyl methacrylate) was effectively emulsified in carbon dioxide with the amphiphilic diblock copolymer surfactant, and the successful stabilization of the polymerization simultaneously gave spherical particles in the submicrometer range with relatively narrow particle size distributions. The initial concentrations of HEMA and the stabilizer and the pressure had substantial effects on the size of the colloidal particles. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3783–3790, 2000     

Preparation of micron-size monodisperse polymer particles by dispersion copolymerization of styrene with poly(2-oxazoline) macromonomer

This article describes the preparation of micron-size monodisperse polymer particles by dispersion copolymerization of styrene with a poly(2-oxazoline) macromonomer in an aqueous ethanol solution. The macromonomer acted as a comonomer as well as a stabilizer. The diameter of the particles increased as the concentration of the macromonomer decreased. The higher the molecular weight of the macromonomer, the smaller the particle size. The copolymerization in the solvent containing higher water content gave smaller polymer particles. Under the condition giving the monodisperse particles, the particles volume increased linearly with the yield of the particles. From ESCA analysis of the particle surface, poly(2-oxazoline) chains were enriched on the surface. © 1993 John Wiley & Sons, Inc.     

超临界二氧化碳介质中过渡金融催化反应研究进展

超临界介质中的化学反应研究是目前热点研究领域之一，对超临界CO2介质中 过渡金属催化反应研究进行了总结和述评，同时讨论了有关提高过渡金属催化剂在 超临界CO2中溶液度的方法。     

Anionic dispersion polymerization of styrene. I. Investigation of parameters for preparation of uniform micron-size polystyrene particles with narrow molecular weight distribution

Anionic dispersion polymerization in a hexane medium has been applied to the synthesis of monodisperse polystyrene particles in the size range of 1.41–6.16 μm, and having narrow molecular weight distributions M_w/M_n of 1.02–1.28. sec-Butyllithium was used as the initiator. Polystyrene-block-polybutadiene diblock copolymer containing 23% polystyrene block, (i.e., Stereon 730A) with a molecular weight of 147,000 g/mol and a polydispersity of 1.05, was found to be a suitable steric stabilizer for the preparation of micron-size polystyrene particles with narrow size distribution. Tetrahydrofuran (THF) was used as a promoter for obtaining narrow molecular weight distributions. However, this study revealed that the addition of small amounts of THF as promoter broadened the particle size distribution. High solids content polystyrene dispersions were also prepared without using any promoter by both batch and/or multi-addition monomer processes. © 1996 John Wiley & Sons, Inc.     

Copolymerization of vinylidene difluoride with hexafluoropropene in supercritical carbon dioxide

Vinylidene difluoride and hexafluoropropene are copolymerized in supercritical carbon dioxide at 280 bar and 50 °C by means of free radical copolymerization, initiated by diethyl peroxydicarbonate. The first stages of the reaction were monitored by turbidity measurements and the time/conversion curve was followed gravimetrically to measure the initial rates of polymerization. The obtained copolymers possessed bimodal molecular weight distributions, their average comonomer composition was well described by the Lewis‐Mayo equation with the copolymerization parameters r_VDF = 4.8 and r_HFP = 0. The glass transition and melting temperatures of the copolymers are similar to that of the materials resulting from aqueous emulsion polymerization. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1299–1316, 2006     

Conformational changes of poly(ethylene oxide) in poly(ethylene oxide)/poly(methyl methacrylate) blends by supercritical carbon dioxide

The effects of supercritical carbon dioxide (SC CO₂) fluids on the morphology and/or conformation of poly(ethylene oxide) (PEO) in PEO/poly(methyl methacrylate) (PMMA) blends were investigated by means of differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and Fourier transform infrared (FTIR). According to DSC data for a given blend, the melting enthalpy and, therefore, degree of crystallinity of PEO were increased, whereas the melting temperature of PEO was decreased, with SC CO₂ treatment. The enhancement of PEO crystallization with SC CO₂ treatment, as demonstrated by DSC data, was supported by WAXD data. According to FTIR quantitative analyses, before SC CO₂ treatments, the conformation of PEO was transformed from helix to trans planar zigzag via blending with PMMA. This helix‐to‐trans transformation of PEO increased proportionally with increasing PMMA content, with around 0.7% helix‐to‐trans transformation per 1% PMMA incorporation into the blend. For a given blend upon SC CO₂ treatments, the conformation of PEO was transformed from trans to helix. This trans‐to‐helix transformation of PEO decreased with increasing PMMA contents in the blends because of the presence of interactions between the two polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2479–2489, 2004     

Synthesis of polyhedral particles by dispersion polymerization in supercritical carbon dioxide

The novel synthesis of polyhedral particles was attained by the dispersion polymerization of styrene in supercritical carbon dioxide using a polydimethylsiloxane-based macroazoinitiator as a precursor of the surfactant. The macroazoinitiator, VPS-1001, composed of poly(dimethylsiloxane) and 6-8 molecules of the azo groups served as a precursor of the surfactant for the dispersion polymerization by azobisisobutylonitrile as an initiator to produce 0.8-4 μm polyhedral particles. The size of the particles decreased as a result of increasing the VPS-1001 concentration. Too high a concentration of VPS-1001 caused coagulation of the particles. A decrease in the temperature increased the particle size and size distribution, while a decrease in the pressure produced particles with nonspecific shapes. An increase in the stirring rotation speed tended to increase the size and size distribution. However, too high a speed of rotation also caused coagulation of the particles.     

Preparation of micron-size monodisperse polymer microspheres having chloromethyl group

Micron-size monodisperse polymer microspheres having chloromethyl groups thereon were prepared by two-step polymerization process as follows. First, micron-size monodisperse polystyrene particles were prepared by dispersion polymerization with 2,2-azobisisobutyronitrile as initiator in ethanol-water medium in the presence of poly(acrylic acid) as stabilizer under various conditions. Secondly, in the presence of the 1.9-m monodisperse polystyrene particles produced under the optimum conditions, seeded copolymerization for styrene and chloromethyl styrene was carried out. The seeded copolymerization proceeded smoothly without producing new particles, and it was confirmed by x-ray photoelectron spectroscopy that the chloromethyl group existed more at the surface of the produced microsphere than at that of film cast from the benzene solution in which the microspheres were dissolved.Part CVI of the series Studies on Suspension and Emulsion.     

Precipitation polymerization of 2‐hydroxyethyl methacrylate in supercritical carbon dioxide

We report the successful precipitation polymerization of 2‐hydroxyethyl methacrylate (HEMA) in supercritical carbon dioxide (scCO₂) at pressures ranging from 15 to 27 MPa utilizing 2, 2′‐azobisisobutyronitrile (AIBN) as a free radical initiator. The effects of the reaction pressure, initiator concentration, monomer concentration, reaction temperature, and reaction time were investigated. Analyses by scanning electron microscopy (SEM) indicated that in all reaction conditions, polymerization in the absence of stabilizer led to the formation of large aggregates of partially coalesced particles, with diameters of approximate 1–10 µm. Analyses by gel permeation chromatography (GPC) indicated that for the reaction pressure, initiator concentration, monomer concentration, reaction temperature, and reaction time studied there are appreciable effect on product molecular weight. Copyright © 2011 John Wiley & Sons, Ltd.     

Controlled/living heterogeneous radical polymerization in supercritical carbon dioxide

Supercritical carbon dioxide (scCO₂) is an inexpensive and environmentally friendly medium for radical polymerizations. ScCO₂ is suited for heterogeneous controlled/living radical polymerizations (CLRPs), since the monomer, initiator, and control reagents (nitroxide, etc.) are soluble, but the polymer formed is insoluble beyond a critical degree of polymerization (J_crit). The precipitated polymer can continue growing in (only) the particle phase giving living polymer of controlled well‐defined microstructure. The addition of a colloidal stabilizer gives a dispersion polymerization with well‐defined colloidal particles being formed. In recent years, nitroxide‐mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer (RAFT) polymerization have all been conducted as heterogeneous polymerizations in scCO₂. This Highlight reviews this recent body of work, and describes the unique characteristics of scCO₂ that allows composite particle formation of unique morphology to be achieved. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3711–3728, 2009     

Preparation of poly(acrylic acid)‐b‐polystyrene by two‐step atom transfer radical polymerization in supercritical carbon dioxide

It is known that it is difficult to polymerize carboxylic acid‐based monomer by atom transfer radical polymerization (ATRP) in polar solvents due to the protonation of ligand caused by acidic dissociation of the monomer. In this study, precipitation reverse ATRP of acrylic acid (AA) was carried out in supercritical carbon dioxide (scCO₂), which is a nonpolar solvent to dissolve transition metal complexes, at 30 MPa and 45 °C. The polymerization proceeded smoothly and the conversion reached 86% for 3 h. After vending of scCO₂, a dry poly(acrylic acid) (PAA) powder was obtained. Weight‐average molecular weight and polydispersity of the methylated PAA, which were measured by gel‐permeation chromatography after methyl esterification, were 3.5 × 10⁴ and 2.07, respectively, indicating that the precipitation reverse ATRP proceeded with a bad control manner. However, chain extension of the methylated PAA with styrene was possible by ATRP in a bulk system. Moreover, PAA‐b‐polystyrene was successfully prepared in scCO₂ directly by two‐step ATRP, although its molecular weight distribution was broad. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Synthesis of biocompatible and biodegradable polymer particles in supercritical carbon dioxide

The free radical copolymerization of N-vinyl-2-pyrrolidone and 2-methylene-1,3-dioxepane was carried out in supercritical carbon dioxide (scCO₂) using three kinds of dispersants and 2,2′-azobisisobutyronitrile as the initiator. Polymerization was performed with fluorinated polymeric dispersants synthesized in scCO₂ using the solution polymerization method and commercially available siloxane-based surfactant. Spherical biocompatible and biodegradable polymeric particles were prepared within the sub-micron size range. The effect of various ratios of the comonomer, reaction temperature, and concentration of initiator, in addition to the types and concentrations of the dispersants, on the particle size and morphology was investigated. The particle size and particle size distribution of copolymer particles were controlled using the above mentioned experimental parameters. Glass transition temperatures of copolymers were varied according to the comonomer ratios used.     

Partitioning effect of nitrogen catalyst into polymerizing particles on dispersion reversible chain transfer catalyzed polymerization (dispersion RTCP) of methyl methacrylate in supercritical carbon dioxide and organic solvents

Reversible chain transfer catalyzed polymerization (RTCP) in dispersion polymerization system (dispersion RTCP) of methyl methacrylate (MMA) was performed with N‐iodosuccimide (NIS) as a nitrogen catalyst in supercritical carbon dioxide (scCO₂). The solubility of NIS in scCO₂ can be controlled by tuning the pressure, and this led to promote NIS partitioning into polymerizing particles. As a result, the molecular weight distribution control was successfully improved by decreasing the NIS solubility in the medium by tuning the scCO₂ at a low pressure of 20 MPa. On the other hand, at the same NIS concentration, a solution RTCP of MMA in toluene as a homogeneous polymerization system did not proceed with a controlled/living manner. The importance of NIS partitioning into the polymerizing particles was also confirmed in hexane as well as scCO₂ medium. From these results, it was clarified that the NIS catalyst partitioning into the polymerizing particles as main polymerization loci is a key factor to control the molecular weight distribution in the dispersion RTCP of MMA in scCO₂. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 613–620     

Competitive influence of atactic polystyrene and supercritical carbon dioxide on the conformation of syndiotactic polystyrene

The crystallization behavior of miscible syndiotactic polystyrene (sPS) and atactic polystyrene (aPS) blends with different sPS/aPS weight ratios was investigated in supercritical CO₂ by using Fourier‐transform infrared spectroscopy, differential scanning calorimetry, and wide‐angle X‐ray diffraction. Supercritical CO₂ and aPS exhibited different effects on the conformational change of sPS and competed with each other. Increasing the content of amorphous aPS in the blends made its effect on the conformational change of sPS gradually surpass that of supercritical CO₂. Supercritical CO₂ favored the formation of the helical conformation of sPS in lower temperature range and the all trans planar conformation in higher temperature range, instead of forming the latter one only in higher temperature range in ambient atmosphere. However, increasing aPS content in the blends pushed the range for forming the helical conformation to lower temperature and made the all trans planar conformation dominant in aPS/sPS 25/75 blend after treating in supercritical CO₂ above 60 °C. The all trans planar zigzag conformation was more favorable than the helical conformation after mixing aPS in sPS in supercritical CO₂. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1755–1764, 2007     

在超临界CO2流体中的化学反应

超临界CO2流体中的化学反应是继超临界流体应用于萃取分离过程之后进一步将其应用于化学反应的新尝试。本文重点综述了超临界CO2流体中的化学反应研究进展, 并对其发展前景作了展望。     

Role of interfacial interactions on the anomalous swelling of polymer thin films in supercritical carbon dioxide

It has recently been shown that thin polymer films in the nanometer thickness range exhibit anomalous swelling maxima in supercritical CO₂ (Sc‐Co₂) in the vicinity of the critical point of CO₂. The adsorption isotherm of CO₂ on carbon black, silica surfaces, porous zeolites, and other surfaces, is known to exhibit anomalous maxima under similar CO₂ conditions. It is believed that because CO₂ possesses a low cohesive energy density, there would be an excess amount of CO₂ at the surfaces of these materials and hence the CO₂/polymer interface. This might cause excess CO₂ in the polymer films near the free surface, and hence the swelling anomaly. In addition, an excess of CO₂ would reside at the polymer/substrate and polymer/CO₂ interfaces for entropic reasons. These interfacial effects, as have been suggested, should account for an overall excess of CO₂ in a thin polymer film compared to the bulk, and would be responsible for the anomalous swelling. In this study, we use in situ spectroscopic ellipsometry to investigate the role of interfaces on the anomalous swelling of polymer thin films of varying initial thicknesses, h₀, exposed to Sc‐CO₂. We examined three homopolymers, poly(1,1′‐dihydroperflurooctyl methacrylate) (PFOMA), polystyrene (PS), poly(ethylene oxide) (PEO), that exhibit very different interactions with Sc‐CO₂, and the diblock copolymer of PS‐b‐PFOMA. We show that the anomalous swelling cannot be solely explained by the excess adsorption of CO₂ at interfaces. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1313–1324, 2007