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Abstract
Polymerization is the process of converting moeomer(s) to long chain molecules. It is a basic process to produce materials with “microstructural” features. The microstractural consequences of polymerization are reflected in the molecular weight, molecular weight distribution, chain end groups, repeat unit orientation and chain regularity (as in tacticity), monomer sequence distributions (as in copolymers), branching, or crosslinking. The chain microstructure influences the ultimate properties of polymers that find ever increasing use in our everyday life.
Polymers are long chain molecules that have become an indispensable part of the modem day living. They form the basis for materials of choice that are customized for a wide range of applications from baby diapers to medical devices, to computer boards. A variety of techniques are used in the synthesis, modification and processing of polymeric materials for a given end-use application.
In this study, the use of triblock (A–B–A) oligomers of ɛ-caprolactone (ɛ-CL) (A) and PEG400 (B) as stabilizers (SB) for the copolymerization of L-lactide (LLA) and ɛ-CL in supercritical carbon dioxide (scCO2) was investigated. To determine the effect of CO2-philic and polymer-philic segments on copolymerization, oligomers with three different average molecular weights (Mw=2000–6000 Da) were synthesized by changing the PEG400/ɛ-CL ratio. Copolymerizations were confirmed by 1H-nuclear magnetic resonance (NMR), 13C-NMR and differential scanning calorimeter data. It was possible to copolymerize LLA and ɛ-CL in scCO2 without any SB; however, the polymerization yields and average molecular weights were low, and significant aggregate formations were detected. Recipes featuring only 5% SB were successfully applied to reach high polymerization yields of ∼85% and polymers with average molecular weights greater than 20 kDa. When the polymer-philic segment of the SB increased, both the yield and molecular weight of the copolymer also increased significantly, resulting in white powdery products.
The use of supercritical carbon dioxide as a processing solvent for the physical processing of polymeric materials is reviewed. Fundamental properties of CO2/polymer systems are discussed with an emphasis on available data and measurement techniques, the development of theory or models for a particular property, and an evaluation of the current state of understanding for that property. Applications such as impregnation, particle formation, foaming, blending, and injection molding are described in detail including practical operating information for selected topics. The review concludes with some forward-looking discussion on the future of CO2 in polymer processing.
This paper is a review of polymer solutions at high pressures with a focus on miscibility, phase separation and morphological modifications in supercritical or compressible dense fluids. The review is aimed at presenting an account of advances made over the past two decades while highlighting the challenges and opportunities for future developments. Even though a broad review is attempted, specific examples are selected mostly from those published in the Journal of Supercritical Fluids or from research conducted in the author's laboratory. How the miscibility and phase separation considerations come into play in polymer synthesis, polymer modification, and polymer processing are emphasized. Recent advances made in morphological modifications and polymorphic transformations of polymers achieved by pressure or solvent tuning are discussed in more detail.
Free-radical copolymerization of acrylonitrile (AN) with methyl methacrylate (MMA) and 2-chlorostyrene (CS) in supercritical CO 2 was investigated. It was observed that homopolymerization of acrylonitrile led to powders, homopolymerization of 2-chlorostyrene resulted in a soft solid, and homopolymerization of methyl methacrylate resulted in a gel. The number-average molecular weight of the copolymers was found to range from 40 000 to 300 000, and the polydispersity index was between 1.4 and 4.8. It was found that the AN/MMA or AN/CS mole ratios in the copolymers were lower then the respective ratios in the feed mixtures.
Density and viscosity have been used as real-time probes to follow the progress of free-radical polymerization of methyl methacrylate in acetone at high pressures. Specifically, the polymerizations were conducted at 343 K and at 7, 14, 21, 28, 35, and 42 MPa in acetone with AIBN as initiator. Polymers with narrow molecular-weight distributions (Mw/Mn = 1.45–1.55) and without high-molecular-weight tail-ends (Mz/Mw = 1.26–1.33) were formed. The results show that the polymerization proceeds in two distinguishable regimes characterized by different rates of change in density or viscosity. The densities and viscosities of model mixtures of poly(methyl methacrylate) plus methyl methacrylate (the monomer) of known compositions were also measured and used to relate the change in viscosity during polymerization to change in the monomer concentration. The change in monomer concentration was then used in evaluating the rate constant and its pressure dependence. The overall apparent first-order rate constants were found to be in the range 0.002−0.004 min−1 at 7 MPa and 0.006−0.008 min−1 at 42 MPa. The reductions in the time rate of change in density and viscosity are discussed in terms of the polymer concentration and viscosity, reaching levels that can be associated with the chain-overlap concentrations; this leads to the onset of diffusional limitations for macroradicals in solution polymerizations even when the viscosities remain low, around 1 mPa s. The low polydispersities and the absence of high-molecular-weight tailing were interpreted in terms of termination by combination involving “short”−“long” coupling of macroradicals.
The kinetics of pressure-induced phase separation (PIPS) in solutions of polydimethylsiloxane (Mw=94 300; PDI=2.99) in supercritical carbon dioxide have been studied using time- and angle-resolved light scattering. Controlled pressure quench experiments were conducted at different polymer concentrations (0.38, 0.9, 1.9, 2.5, 3.9, 5.5% by mass) to determine both the binodal and spinodal envelopes, and the critical polymer concentration. At each concentration, a series of rapid pressure quenches with different depths of penetration into the region of immiscibility was imposed and the time evolutions of the scattered light intensities were followed to determine the pressure below which the mechanism changes from ‘nucleation and growth’ to ‘spinodal decomposition’.This crossover is identified from the characteristic fingerprint scattering patterns associated with each mechanism. The spinodal decomposition process is characterized by the formation and evolution of a spinodal ring during phase separation that leads to a maximum in the angular variation of the scattered light intensity. The nucleation and growth mechanism is characterized by the absence of such a maximum and the continual decrease of the scattered light intensities with increasing angles.The time scale of PIPS is shown to be relatively short. The late stage of phase separation is entered within seconds. For quenches leading to spinodal decomposition, the characteristic wave number qm corresponding to the scattered light intensity maximum Im is observed to be non-stationary, moving to lower wave numbers after a very short elapsed time. The growth of domain sizes is observed to follow power-law-type scaling with and with β≈2α.
Herein we report the synthesis of highly cross-linked polymers based on divinylbenzene by heterogeneous polymerization in supercritical CO2 (scCO(2)). The polymers were isolated in the form of discrete microspheres (diameter = 1.5-5 mu m) in good yields (greater than or equal to 90%), in the absence of any stabilizers. In the presence of a CO2-soluble polymeric stabilizer, much smaller particles (diameter < 0.5 mu m) were formed in high yields (greater than or equal to 95%) by emulsion polymerization in scCO(2).
Essential characteristics of chain molecules, their formation, properties and processing aspects are presented with a special focus on solvent dependent features. How supercritical fluids with their tunable characteristics can be put into use as the process or processing solvents in various stages of polymer formation, purification, modification, fabrication, recovery and recycling is discussed. Thermodynamic aspects of polymer solubility and the effects of various factors such as the polymer type, molecular weight and molecular weight distribution, concentration, solvent type, temperature and pressure on the solubility of polymers in supercritical fluids are presented. Modeling of polymer solutions at high pressures with lattice fluid (Sanchez-Lacombe) and perturbation (SAFT) methods is reviewed and their effectiveness is compared. The question of kinetics of phase separation is addressed and a new technique which permits repetitive penetration into the metastable and unstable regions of a polymer-solvent system by multiple slow- or rapid (at rates approaching 1000 MPa/s) pressure drops is described.
Most polymerization reactions proceed at a faster rate under high pressure. Higher molecular weights are obtained at higher pressures. High pressure polymerizations of a wide variety of vinyl type monomers and copolymerizations of two or more monomers have been investigated by many researchers around the world.1–8 Such studies have been conducted with free radicals derived from oxygen and organic peroxides, photoinitiation,9,10 and high energy radiation.11 High pressure polymerizations have also been conducted with anionic12 and cationic13 initiators (catalysts) and with transition metal catalysts.14
Particle growth rates were analyzed for the dispersion polymerization of methyl methacrylate (MMA) in supercritical carbon dioxide at 65°C stabilized with a poly(dimethyl siloxane)-methyl methacrylate (PDMS-mMA) macromonomer. Although pure CO2 is a mediocre solvent for PDMS even at 4000 psia, the monomer behaves as a cosolvent to prevent flocculation. As pressure is decreased, the dispersion flocculates sooner, as expected due to the reduced solvent quality of CO2. Final particle size is only mildly dependent on pressure as a result of the solvation from the high monomer concentration during the particle formation stage, however particle coagulation increases with decreasing pressure. There exists both a minimum pressure (∼3000 psia) and stabilizer concentration (∼2 wt % stabilizer/ monomer) below which particles are highly coagulated due to insufficient steric stabilization. Here polymerization rates are reduced due to diffusional restrictions. This threshold pressure and stabilizer concentration are required to change the mechanism from precipitation polymerization to dispersion polymerization, as indicated by product morphology, molecular weight, and molecular weight polydispersity. Final particle size and number density determined from the model of Paine {Macromolecules 1990, 23, 3109} agree with the measured values.
It was the aim of this work to examine and understand the phenomena occurring in a SCF precipitation polymerization, and contrast them to corresponding effects in a liquid-phase reaction system. Precipitation polymerizations of styrene were conducted in the presence of supercritical ethane with AIBN as a free-radical initiator. The feasibility of this synthesis scheme to produce polystyrenes of a low average molecular weight and a relatively narrow molecular weight distribution was explored work. A statistical-mechanics based lattice equation of state was developed to correlate the data on the solubility of polystyrene chains into SCF. This model can quantitatively reproduce all the experimental trends for the polystyrene-ethane system, across changes in state variables, with one apparently constant binary interaction parameter. The authors illustrate the quality of fits obtained by the use of the kinetic model to correlate the data obtained from the precipitation polymerization reactions. Use of this model, along with activated state theories, illustrates the fact that the use of supercritical ethane selectively slows the kinetic rate of the propagation step relative to corresponding values in a liquid solvent.
Soluble metallocene catalysts can be easily metered into high pressure polymerization reactors. The performance of a homogeneous zirconocene catalyst for the polymerization of ethylene under high pressures was tested at 1500 bar and temperatures between 80 and 260°C in the presence of methyl aluminoxane as cocatalyst. Besides the productivity of the catalyst and the overall rate of polymerization, the most important properties of the prepared polymers were determined. The results were compared with those found earlier with a heterogeneous titanium catalyst supported on magnesium dichloride.Lösliche Metallocen-Katalysatoren können ohne Schwierigkeiten in Hochdruck-Polymerisationsreaktoren dosiert werden. Die Eignung eines solchen zirkonhaltigen Katalysators für die Hochdruckpolymerisation von Ethylen wurde in Gegenwart von Methylaluminoxan als Cokatalysator bei 1 500 bar und Temperaturen von 80 bis 260°C geprüft. Außer der Produktivität des Katalysators und der Polymerisationsgeschwindigkeit wurden die wichtigsten Eigenschaften der erhaltenen Polymeren bestimmt. Die Ergebnisse wurden mit Daten verglichen, die vorher mit einem heterogenen Titankatalysator auf Magnesiumdichlorid erhalten wurden.
Extended solids made of covalently bonded low-Z elements are a new class of high energy density materials (HEDM) with dramatically enhanced energy density and great potential for use as a new energy source. The existence of such an energetic HEDM has previously been demonstrated by high-pressure synthesis of polymeric carbon monoxide (CO) in diamond-anvil cells (DACs). The detailed characterization of the structure and energetics of polymeric CO has, however, remained a challenge because of the minute DAC sample and its high metastability. Therefore, the emphasis of our recent studies has been focused on (i) understanding the metastability of CO-derived extended solids synthesized at various thermal and photo-chemical conditions and (ii) scale-up synthesis for characterization of structure and energetics. In this talk, we will summarize these results with emphasis on the structural relationship with metastability and energetics. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.
For many years of manufacturing or inventing novel plastics, industrial chemists have been at the mercy of the available chemical tools. Today, however, a new category of catalysts, called metallocenes, has come to their rescue. The molecular machines allow more effective control over the growth of polymer chains. The metallocene catalyst have also revolutionalized the industrial synthesis of valuable plastics.
Polypyrrole is synthesisedvia thermal decarboxylation of a precursor monomer, pyrrole-2-carboxylic acid, using ferric salts in both supercritical carbon dioxide and supercritical fluoroform; pressed pellet conductivities were as high as 2×10–2 S cm–1 and scanning electron microscopy studies revealed an unusual non-spherical morphology.
The recycling post-consumer polymers into useful feedstocks is important for both environmental and economic reasons; however, the processing of highly crosslinked polymers such as tire rubber constitutes a significant technical challenge. In this work, supercritical H2O and CO2 were used to controllably depolymerize tire and natural rubber. Molecular weight analysis of the degraded material indicates that reaction time can be used to control the degree of breakdown; materials in the molecular weight range of 103–104 were obtained. Also, both material composition and supercritical fluid affect the rate at which material is depolymerized. Functional analysis of the processed material shows carbonyl and aromatic groups.
The heterogeneous (solid/supercritical fluid solution) free-radical polymerization of styrene in supercritical carbon dioxide-swollen poly(chlorotrifluoroethylene) (PCTFE) polymer film (63 mil) has been studied as an approach to polymer blend preparation. Decompression followed by thermal initiation using AIBN or tert-butyl perbenzoate yields polymer blends with the polystyrene trapped inside the matrix polymer. Polymerization prior to decompression yields more extensively modified products. Polystyrene content and the distribution of polystyrene in the blend can be controlled by adjusting the concentration of styrene in the supercritical fluid or by controlling the time that the PCTFE film is in contact with the fluid. The control imparted is consistent with the phase behavior and absorption kinetics of the styrene-CO2-PCTFE system. Transmission electron microscopy and energy dispersive X-ray analysis (EDX) indicate that the polystyrene exists as discrete phase-segregated regions throughout the thickness of the PCTFE film. Thermal analysis of blends and infrared data from extraction experiments indicate that radical grafting reactions do not occur to any significant extent. EDX data indicate that blends with composition gradients of adjustable degrees of severity can be produced by using soaking periods of shorter duration than is required to achieve equilibrium solubility of the monomer in the substrate.
The grafting of carboxylated and oxygenated functions on low density and high density polyethylenes is carried out by gamma irradiation in the presence of carbon dioxide. Both crosslinking and grafting of oxidised groups occur. The extent of these molecular modifications depends on the physical conditions of the gas phase and on the nature of the polymer matrix.
Free radical polymerization of styrene in supercriticaln-butane (Tc = 152°C,Pc = 3.87MPa) has been studied usingt-butyl peroxide as the initiator. The effect of pressure (in the range from 6 to 35 MPa), temperature (in the range 160 to 180°C) and polymerization time (from 1 to 9 hours) have been evaluated in terms of the molecular weight and molecular weight distributions, and the yields of the resulting polymers. Polymerizations were carried out both in a batch and a continuous mode. It has been found that at a given pressure and polymerization time, increasing the temperature reduces the molecular weight and broadens the molecular weight distributions. All experiments resulted in relatively narrow molecular weight distributions with polydispersities below 2.4. Higher pressures were found to favor formation of polymers with higher molecular weights.
Critical points of systems of ethylene with different polyethylene wax mixtures have been measured in the temperature range 413 to 513 K and at pressures up to 93 MPa. The results show a distinct influence of the polydispersity of the polyethylene wax on the critical point. A methodology for evaluating critical points of mixtures of ethylene and polydisperse polyethylene wax with a cubic equation of state and the Wong–Sandler mixing rule is proposed. We find that this method is able to predict critical points with good accuracy over a wide range of polydispersity. For industrial calculations it is advantageous to have a simple model with a limited number of parameters. This method applicates simple cubic equations of state to mixtures under high pressure.
The activation of metallocene catalyst by a cationic activator in the presence of triisobutylaluminium was examined in the copolymerization of ethylene and 1-butene under high pressure of 150 MPa and temperatures of 463 to 473 K. A continuously operated micro-pilot plant unit equipped with a stirred autoclave was used. The catalyst productivity determined from the amount of polymer and the catalyst feed was in the range of 100 to 300 kg polymer/g zirconium. The resulting copolymers were analyzed for their physical properties and the amount of 1-butene incorporated. The results of the polymerization with the metallocene/cationic activator system were compared to the data obtained earlier using methyl aluminoxane as the activator.
Supercritical carbon dioxide was found to be a suitable substitute for halogenated organic solvents in its copolymerization with propylene oxide. The system was studied utilizing a heterogenous catalyst, zinc glutarate. The polymers were analyzed for selectivity in polycarbonate linkages by 1H NMR spectroscopy and for molecular weight by gel permeation chromatography.
Modification reactions are reported for fully-hydrogenated butadiene-acrylonitrile (35.8 mol-% AN) copolymer, ethylene(E)-butyl acrylate (4.7 mol-% BA) copolymer, and ethylene(E)-methyl acrylate (45 mol-% MA) copolymer in dense, near-critical water at 250°C and 300°C and at 300 bar and 1500 bar. Nitrile, amide, and ester moieties can be converted into COOH groups, Kinetic analysis of the ester to acid transformations suggests autocatalytic activity of the acid groups.
This paper describes pyrolysis of polyethylene (PE) and n-hexadecane (nC16) in supercritical water (SCW). Batch reactions were conducted at temperatures ranging from 673 to 723K, at a reaction time of 30min, and water density between 0 and 0.42g/cm3. The pyrolysis rate of nC16 in SCW was almost the same as that in 0.1MPa argon (Ar) atmosphere. Product distribution was also more or less the same for both cases. PE pyrolysis results were clearly different from pyrolysis results in Ar. In SCW, higher yields of shorter chain hydrocarbons, higher 1-alkene/n-alkane ratio, and higher conversion were obtained. This difference of PE pyrolysis in SCW and in Ar could be explained by considering the difference in the reaction phase.
Pressure is an element in the control of natural phenomena, as is temperature. The investigation of the role of pressure must be understood so that we may control and use pressure effectively.
The use of supercritical fluids as media for several types of polymer modification is demonstrated. Treatment of mixtures of chitosan with glucose or malto-oligosaccharides in supercritical carbon dioxide afforded the corresponding water soluble imine-linked, branched chitosan derivatives with high degrees of conversion. These transformations were substantially more facile and complete than previously reported equivalent reactions in conventional media. Phosphorylations of amylose and poly(vinyl alcohol) were conducted in supercritical carbon dioxide, yielding phosphate ester derivatives with phosphorus contents of 3.70% and 0.88%, respectively. Treatment of poly(vinyl alcohol), starch, maltodextrins, cellulose acetate and paper with mixtures of supercritical carbon dioxide and oxygen (19:1 v/v) led to the corresponding oxidized materials.
The copolymerization of carbon dioxide and propylene oxide using rare-earth-metal catalyst systems was investigated in this study. It was found that the ternary rare-earth-metal coordination catalyst consisting of Y(CF3CO2)3 (I), diethylzinc (II), and glycerine (III) was the most effective catalyst system to generate an alternating polycarbonate. The effects of the molar ratio of catalyst components, the solvent, and the operating temperature and pressure on the yield and the molecular weight of polycarbonate were systematically studied. At an appropriate combination of all variables, the yield could be as high as 4200 (g/(mol of Y))/h and the molecular weight as high as 1.0 × 105 in a 12 h reaction period. The carbonate content in the resulting polycarbonate was found to be 95.6%.
A series of graft copolymers, poly(methyl methacrylate-co-hydroxyethyl methacrylate)-g-poly(perfluoropropylene oxide), was synthesized for application as stabilizers in dispersion polymerization of methyl methacrylate in supercritical carbon dioxide. The backbone, poly(methyl methacrylate-co-hydroxyethyl methacrylate), is effectively insoluble in carbon dioxide and the grafted chains, poly(perfluoropropylene oxide), are completely miscible in carbon dioxide at moderate pressures. The effect of molecular architecture on polymerization rate and PMMA particle size was evaluated by varying the molecular weight of the anchor group (backbone of the copolymer), molecular weight of the CO2-soluble graft chain, and graft chain density. The efficiency of the graft copolymers as dispersants was demonstrated as micron-size polymer beads of molecular weight greater than 100 000 were produced. The results showed that a careful balance between anchor group size (backbone length) and amount of soluble component (either graft chain length or graft chain density) is necessary but not sufficient to achieve adequate stabilization and that the distribution of the soluble component along the anchor group was also important. Furthermore, the backbone molecular weight was shown as the key component for stabilization, provided that enough CO2-philic component has been included to ensure solubility.
The copolymerization of 1,2-epoxycyclohexane (cyclohexene oxide, CHO) and carbon dioxide was performed using no other solvent than carbon dioxide itself. A CO2-soluble, ZnO-based catalyst was synthesized and used to catalyze the polymerization. Polymerizations were conducted at different temperatures, pressures, and mole fractions of CHO giving rise to polymer yields as high as 69% with catalyst activities as high as 400 g of polymer/g of Zn. The best results were obtained at temperatures of 100−110 °C and at mole fractions of CHO above XCHO = 0.15. This polymer was >90% polycarbonate with weight average molecular weights (Mw) ranging from 50 000 to 180 000.
Vinyl ethers and oxetanes was successfully polymerized cationically in carbon dioxide. These processes become heterogeneous using hydrocarbon vinyl ethers and oxytanes but high molar mass and high conversions can still be achieved. Homogeneous cationic polymerization are also possible with fluorocarbon-based alkyl vinyl ether and oxetane monomers. Polymerization occurs without incorporation of carbon dioxide into the polymer backbone for the systems used.
This is a review of the phase equilibria in supercritical monomer solutions of ethylene homopolymers and copolymers, such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), poly(ethylene-co-methacrylate) (EMA), poly(ethylene-co-vinyl acetate) (EVA), poly(ethylene-co-methacrylic acid) (EMAA), and poly(ethylene-co-acrylic acid)(EAA). The knowledge of such phase equilibria underlies the high-pressure polyethylene (HPPE) technology. The ability to estimate such phase equilibria allows for smooth and robust process optimization during grade transitions. This is important because the HPPE technology makes it possible to minimize the product cross-contamination and, hence, to make higher-value, fluctuating-demand speciality polymers. Experimental data, phase diagrams, and patterns of phase disengagement presented in this paper are related to the reactor system, the high-pressure separator (HPS), and the high-pressure recycle system. These data and diagrams are used to characterize the monomer-polymer miscibility defined as a cloud point transition. The cloud point pressures in such systems are found to depend on thermodynamic parameters, such as temperature and composition, and on the dissimilarity between the polymer and the monomer. This dissimilarity is characterized in terms of the differences in molecular weight and density (e.g., for LDPE and LLDPE), in polarity (e.g., for EVA and EMA), and in association (e.g., for EAA and EMAA).
The steady-state and transient behaviors of a high-pressure ethylene polymerization reactor have been analyzed for a continuous stirred autoclave reactor consisting of two reaction compartments. The nonlinear temperature dependence of the specific initiator consumption rate is modeled using the initiator efficiency factor which varies with reaction temperature. It is shown that there exists a critical reaction temperature at which the steady-state process gain (reactor temperature change/initiator feed rate change) changes its sign. When only the first reaction zone temperature is controlled by regulating the initiator injection rate, the second zone temperature is strongly affected by the first zone temperature for certain initiator types. It has also been shown that the volume ratio of the two reaction zones is an important reactor design parameter that affects the temperature rise in the second reaction zone. Dynamic closed loop reactor simulations have also been carried out to illustrate potential control problems when the reactor operating condition is changed from one steady state to another.
The free-radical polymerization of styrene has been studied in the homogeneous phase of supercritical (sc) CO2 at 80°C and pressures between 200 and 1 500 bar. 2,2'-Azobisisobutyronitrile is used as initiator and CBr4 as chain-transfer agent. The polymerization is monitored by means of online FT-IR/NIR spectroscopy. In the presence of CO2 a solution polymerization may be carried out up to a considerable degree of monomer conversion. At 500 bar, for example, maximum styrene conversions of 34.4 and 11.9% may be reached in homogeneous phase at CO2 contents of 16.8 and 44.5 wt.-%, respectively. Analysis of the measured conversion-time profiles yields termination rate coefficients, kt, which are by one order of magnitude larger than kt for styrene bulk polymerizations at identical temperature and pressure. The enhanced termination rate in fluid CO2 is assigned to the poor solvent quality of scCO2 for polystyrene.
Dispersion polymerizations of methyl methacrylate utilizing poly(1,1,-dihydroperfluorooctyl acrylate) as a steric stabilizer in supercritical carbon dioxide (CO2) were carried out in the presence of helium. Particle size and particle size distribution were found to be dependent on the amount of inert helium present. Particle sizes ranging from 1.64 to 2.66 μm were obtained with various amounts of helium. Solvatochromic investigations using 9-(α-perfluoroheptyl-β,β-dicyanovinyl)julolidine indicated that the solvent strength of CO2 decreases with increasing helium concentration. This effect was confirmed by calculations of Hildebrand solubility parameters. Dispersion polymerization results indicate that PMMA particle size can be attenuated by the amount of helium present in supercritical CO2. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 2009–2013, 1997
The thermal decomposition of low-density polyethylene, isotactic polypropylene, and polyisobutylene has been studied in helium at a heating rate of 20°C/min using an experimental system which consists of a programmable pyrolyzer, a thermal conductivity cell, and a mass chromatograph.
For low-density polyethylene, the formation of a homologous series of volatile products corresponding to alkanes and alkanes is interpreted in terms of an intramolecular radical transfer process in the primary macroradicals to the 5th, 9th, 13th, and 17th carbon atoms of the chain.
For isotactic polypropylene, the formation of a homologous series of volatile products corresponding to monomer, dimers, trimers, and higher oligomers is explained also in terms of intramolecular radical transfer processes. Transfers to the 5th, 9th, and 13th carbon atoms in the secondary macroradicals (indexing from the secondary carbon atom at the chain end) and transfers to the 6th, 10th, and 12th carbon atoms in the primary macroradicals are shown to account for the major products of pyrolysis.
For polyisobutylene, in addition to the depolymerization process which accounts for the extensive formation of monomer, intramolecular radical transfer processes in the primary and tertiary macroradicals (the processes proceeding predominantly in the primary macroradicals) are shown to account for the formation of the dimers, trimers, and higher oligomers that occur in the volatile products of decomposition.
The first carbocationic polymerization of isobutylene (IB) in supercritical carbon dioxide (SCCO2) has been accomplished. It was demonstrated that in CO2 at 32.5C 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 Mn2000 and Mw/Mn2.0. 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 SCCO2 would yield only very low molecular weight oligomers (tetramers). The structure of the PIBs obtained in SCCO2 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 (Mn500) in SCCO2.
In this paper we propose a new method to hydrolyze cellulose rapidly in supercritical water (SCW) to recover glucose, fructose and oligomers (cellobiose, cellotriose, cellotetraose, etc.). Cellulose decomposition experiments were conducted with a flow type reactor in the range of temperature from 290 to 400°C at 25 MPa. A high pressure slurry feeder was developed to feed the cellulose–water slurries. Hydrolysis product yields (around 75%) in supercritical water were much higher than those in subcritical water. At a low temperature region, the glucose or oligomer conversion rate was much faster than the hydrolysis rate of cellulose. Thus, even if the hydrolysis products, such as glucose or oligomers, are formed, their further decomposition rapidly takes place and thus high yields of hydrolysis products cannot be obtained. However, around the critical point, the hydrolysis rate jumps to more than an order of magnitude higher level and becomes faster than the glucose or oligomer decomposition rate. This is the reason why we obtained a high yield of hydrolysis products in supercritical water.
In recent years, considerable interest has developed in the use of supercritical fluids (SCFs) for processing of synthetic and naturally-occurring polymers. The use of SCFs as polymerization media, however, has remained a relatively unexplored area. A review of the literature indicates that supercritical polymerization reactions have been conducted on an industrial scale for almost 60 years. While the advantages of performing polymerization reactions at high pressure are well known, the benefits of synthesizing polymers in the supercritical region have been largely overlooked. This manuscript reviews the various polymerization reactions which have been performed in the supercritical region and the advantages SCFs afford over conventional reaction media. Areas which might warrant further investigation will also be discussed.
Fluoropolymers are used in many technologically demanding applications because of their balance of high-performance properties.
A significant impediment to the synthesis of variants of commercially available amorphous fluoropolymers is their general
insolubility in most solvents except chlorofluorocarbons (CFCs). The environmental concerns about CFCs can be circumvented
by preparing these technologically important materials in supercritical fluids. The homogeneous solution polymerization of
highly fluorinated acrylic monomers can be achieved in supercritical carbon dioxide by using free radical methods. In addition,
detailed decomposition rates and efficiency factors were measured for azobisisobutyronitrile in supercritical carbon dioxide
and were compared to those obtained with conventional liquid solvents.
The extent of dissolutions and the ease of precipitations of the dissolved fragments from a variety of lignocellulosic model compounds and wood species in single-component and multicomponent supercritical fluids have been studied. The behavior of D-glucose, D-xylose, xylan, arabinogalactan, α-cellulose, kraft lignin, red spruce, sugar maple, and white pine in supercritical carbon dioxide, ethylene, nitrous oxide, n-butane, ammonia, and methylamine has been examined. The extent of dissolution of red spruce has been further investigated in binary mixtures of carbon dioxide-ethanol, carbon dioxide-water, carbon dioxide-sulfur dioxide, nitrous oxide-methylamine, ethylene-ammonia, ammonia-water, and ethanol-water and in a ternary mixture of carbon dioxide-water-ethanol. The residues and dissolved fractions following extractions have been characterized by chemical, spectroscopic, and thermal techniques.
Conventional heterogeneous dispersion polymerizations of unsaturated monomers are performed in either aqueous or organic dispersing
media with the addition of interfacially active agents to stabilize the colloidal dispersion that forms. Successful stabilization
of the polymer colloid during polymerization results in the formation of high molar mass polymers with high rates of polymerization.
An environmentally responsible alternative to aqueous and organic dispersing media for heterogeneous dispersion polymerizations
is described in which supercritical carbon dioxide (CO2) is used in conjunction with molecularly engineered free radical initiators and amphipathic molecules that are specifically
designed to be interfacially active in CO2. Conventional lipophilic monomers, exemplified by methyl methacrylate, can be quantitatively (>90 percent) polymerized heterogeneously
to very high degrees of polymerization (>3000) in supercritical CO2 in the presence of an added stabilizer to form kinetically stable dispersions that result in micrometer-sized particles with
a narrow size distribution.
(I997) Polymerization in liquid and supercritical carbon dioxide Dispersion polymerization in supercritical carbon dioxide
- D A Canelas
- J M Desimone
- J M De Simone
- E E Maury
- Y Z Menceloglu
- J B Mclain
- T J Romack
Fundamentals of the free radical polymerization of ethylene High-pressure free-radical copolymerization of ethene and methyl acrylate
- P Ehrlich
- G A Mortimer
Copolymerization of carbon dioxide andepoxide
- S Inoue
- H Koinuma
- T Tsurata
- SKoinuma Inoue
The crosslinking polymerization of styrene and methylmethacrylate in supercritical carbon dixoide
- J C Launch
- JC Launch
Potential utilization of supercritical fluids-Reflections forthermoset systems
- E Kiran
Critical points of mixtures of ethylene and polyethylene wax under high pressure, Fluid Phase Equilibria
- D Huekelbach
- G Luft
High pressure extraction and delignification of red spruce with near and supercritical fluids
- H Balkan