Homogeneous and heterogeneous free radical polymerizations in environmentally responsible supercritical carbon dioxide

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 (SCFs). The homogeneous solution homo- and copolymerization of highly fluorinated acrylic, styrenic and olefinic monomers in supercritical carbon dioxide using free radical methods will be discussed [Science, 257 , 945 (1992)]. Detailed decomposition rates and efficiency factors will be presented for azobisisobutyronitrile (AIBN) in supercritical carbon dioxide and will be compared to conventional liquid solvents [Macromolecules, 26 , 2663 (1993)]. Additionally, viscosities of polymer solutions in supercritical CO₂ have been measured using a high pressure, falling cylinder viscometer. The results show that the polymer solution viscosities in supercritical CO₂ are an order of magnitude lower than with the same polymers in conventional organic solvents. The results from these homogeneous solution polymerization studies has allowed us to also consider heterogeneous polymerizations in a carbon dioxide continuous phase. Conventional emulsion polymerizations of unsaturated monomers are performed in either aqueous or organic dispersion media with addition of surface active agents (surfactants) to stabilize the colloidal dispersion that forms. With free radical initiators that are preferentially soluble in the continuous phase, high rates of polymerization and high molar mass polymers can be obtained simultaneously. Herein we describe an environmentally responsible alternative to aqueous and organic dispersing media for emulsion polymerizations which utilizes supercritical carbon dioxide, in conjunction with molecularly engineered free radical initiators and amphiphilic molecules that are specifically designed to be interfacially active in CO₂. Conventional lipophilic monomers, exemplified by methyl methacrylate and styrene, can be polymerized heterogeneously using a fluorinated azo-initiator in supercritical CO₂ in the presence of added surfactant to form stable emulsions that result in submicron size particles. Detailed surfactant and initiator syntheses and phase behavior will also be discussed.     

Emulsion copolymerization of D,L‐lactide and glycolide in supercritical carbon dioxide

Biodegradable polyesters were synthesized via an emulsion polymerization in supercritical carbon dioxide (SC‐CO₂). Copolymers of lactide and glycolide were synthesized in SC‐CO₂ with stannous octoate as the ring‐opening catalyst and a fluorocarbon polymer surfactant as an emulsifying agent. The conversion of lactide and glycolide was monitored with respect to the reaction time and temperature with ¹H NMR spectroscopy. The conversion of glycolide surpassed 99% within 72 h for an SC‐CO₂ phase maintained at 200 bar and 70 °C. Under the same conditions, lactide conversion reached 65% after 72 h of polymerization. Unpolymerized monomer was removed after the reaction by extraction with an SC‐CO₂ mobile phase. The molecular weights of all the copolymers were measured by gel permeation chromatography. Weight‐average molecular weights (M_w) ranged between 2500 and 30,200 g/mol and polydispersity indices ranged from 1.4 to 2.3 for polymerization times of 6 and 48 h, respectively. Although the molecular weight increased significantly during the first 48 h of reaction, there was no significant difference in the M_w for polymerization times of 48 and 72 h. Emulsion polymerization within the benign solvent SC‐CO₂ demonstrated improved conversion and molecular weight versus polymers synthesized without surfactant. The emulsion polymerization of lactide and glycolide copolymers in SC‐CO₂ is proposed as a novel production technique for high‐purity, biodegradable polymers. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 562–570, 2001     

Homogeneous phase polymerization of vinylidene fluoride in supercritical carbon dioxide

For the first time, stabilizer‐free vinylidene fluoride (VDF) homopolymerizations were carried out in homogenous phase with supercritical CO₂ using the conventional initiator di‐tert butyl peroxide (DTBP). In‐line FT‐NIR spectroscopy showed that complete monomer conversion may be obtained. Molecular weights were determined via size‐exclusion chromatography and polymer endgroup analysis by ¹H‐NMR spectroscopy. The number average molecular weights were below 10⁴ g mol^?1 and polydispersities ranged from 3.1 to 5.7 depending on DTBP and VDF concentration. For allowing isothermal reaction, high CO₂ contents ranging from 61 to 83 wt % were used. The high‐temperature and high‐pressure conditions required for homogeneous polymerization did not alter the amount of defects in VDF chaining. Scanning electron microscopy indicated that regular stack‐type particles are obtained upon expansion of the homogeneous polymerization mixture. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5626–5635, 2007     

Synthesis of hydrophilic/CO2‐philic poly(ethylene oxide)‐b‐poly(1,1,2,2‐tetrahydroperfluorodecyl acrylate) block copolymers via controlled/living radical polymerizations and their properties in liquid and supercritical CO2

Hydrophilic/CO₂‐philic poly(ethylene oxide)‐b‐poly(1,1,2,2‐tetrahydroperfluorodecyl acrylate) block copolymers were synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization, iodine transfer polymerization (ITP), and atom transfer radical polymerization (ATRP) in the presence of either degenerative transfer agents or a macroinitiator based on poly(ethylene oxide). In this work, both RAFT and ATRP showed higher efficiency than ITP for the preparation of the expected copolymers. More detailed research was carried out on RAFT, and the living character of the polymerization was confirmed by an ultraviolet (UV) analysis of the ? SC(S)Ph or ? SC(S)S? C₁₂H₂₅ end groups in the polymer chains. The quantitative UV analysis of the copolymers indicated a number‐average molecular weight in good agreement with the value determined by ¹H NMR analysis. The properties of the macromolecular surfactants were investigated through the determination of the cloud points in neat liquid and supercritical CO₂ and through the formation of water‐in‐CO₂ emulsions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2405–2415, 2004     

SYNTHESIS OF POLY(ACRYLIC ACID-CO-ITACONIC ACID) IN CARBON DIOXIDE–METHANOL MIXTURES

ABSTRACT

High fluidity solvents, such as supercritical fluids, have several advantages over traditional solvents as polymerization media, such as offering a more environmentally-friendly reaction media, providing increased reaction rates, and simplifying the separation and purification of polymers. In this study, a traditional glass-ionomer polymer, poly(acrylic acid-co-itaconic acid) (PAA/IA) was synthesized by using mixtures of CO₂ and methanol as the reaction solvent and was characterized by ¹H-NMR, FT-IR, GPC, and viscometry. The mechanical and working properties of the glass-ionomer cements, prepared by mixing aqueous solutions of the polymers with Fuji II glass powder, were evaluated for compressive strength (CS), diametral tensile strength (DTS) and flexural strength (FS), as well as setting time and working time. The results showed that the polymerization reaction in CO₂/methanol mixtures was faster and had higher conversion than the polymerization reaction in water. The glass-ionomer formulations made from the copolymer prepared under SC conditions showed higher CS, comparable FS and DTS compared with those made from the same polymer prepared in water. Both of the synthesized copolymers had significantly higher CS and FS than Fuji II. The working properties of PAA/IA made in CO₂/methanol met the requirement of ANSI/ADA No. 96.     

Formation of porous flat membrane by phase separation with supercritical CO2

Microporous polystyrene membranes were prepared by the phase separation process using the supercritical CO₂ as a nonsolvent for the polymer solution. The thin polymer solution in a laboratory dish was located inside a cell and the supercritical CO₂ was introduced to induce the phase separation. The dry flat microporous membranes were obtained without collapse of the structure after the CO₂ pressure was diminished. Effects of the experimental conditions such as the CO₂ pressure, the polymer concentration and the temperature on the average pore size and membrane porosity were investigated.     

Synthesis of cross-linked poly(methyl methacrylate) via dispersion polymerization in supercritical carbon dioxide

Cross-linked poly(methyl methacrylate) particles were prepared via dispersion polymerization in supercritical carbon dioxide (scCO₂) using poly(heptadecafluorodecyl methacrylate) (PHDFDMA) and 2,2′-azobisisobutyronitrile as the dispersant and the initiator, respectively. The following chemicals were used as cross-linking agents: ethylene glycol dimethacrylate (EGDMA), 1,4-buthanediol di(meth)acrylate (1,4-BD(M)A), and trimethylolpropane trimethacrylate. PHDFDMA was synthesized by solution polymerization in scCO₂. We investigated the effect of the chemical structure, concentration of the cross-linking agents, reaction pressure, and CO₂ density on the morphology, the polydispersity, and the cross-linking density of polymer particles. The resulting polymer particle was characterized by field emission SEM, differential scanning calorimetry, and thermal gravimetric analysis. The cross-linked PMMA particles is more agglomerate as the cross-linking agent concentration increased and as pressure decreased at constant temperature. Glass-transition temperature (T _g) of the resulting polymer increased as the cross-linking agent increased with temperature and pressure increasing at the same CO₂ density. Decomposition temperature is slightly increased as 1,4-BDA concentration increased. From these results, we can confirm that the thermal stability of the polymer increased as the cross-linking agent and EGDMA is the best cross-linking agent in term of the thermal stability.     

RAFT polymerization in supercritical carbon dioxide based on an induced precipitation approach: Synthesis of 2‐ethoxyethyl methacrylate/acrylamide block copolymers

A new controlled/living heterogeneous polymerization technique using RAFT in benign supercritical CO₂ is described involving the formation of monomer‐swollen seed particles by precipitation of macroRAFT agent prior to polymerization. Controlled/living character of the induced precipitation is compared with the equivalent solution polymerization. The large scale synthesis of poly(2‐ethoxyethyl methacrylate)‐b‐poly(acrylamides) useful for biomedical applications is made possible with the polymer isolated as powders at high conversions, thus circumventing the requirement for volatile organic solvents. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2351–2356     

Lipase‐Catalyzed Ring‐Opening Polymerization of β‐Butyrolactone: End‐Group Analysis of Poly(3‐hydroxybutanoate) Using Supercritical Fluid Chromatography

The ring‐opening polymerization of (R,S)‐β‐butyrolactone (BL) in bulk was analyzed with respect to the polymer structure of the resulting poly[(R,S)‐3‐hydroxybutanoate)] [P(3HB)] by isolation of the pure form using preparative supercritical CO₂ fluid chromatography. It was confirmed that the four‐membered BL was polymerized in bulk by lipase to yield the corresponding cyclic, hydroxy‐ and crotonate‐terminated P(3HB)s. The relative ratios of the three types of polymers depended on the lipase concentration as well as on the monomer conversion. It was also confirmed that both cyclic and linear P(3HB) polymer species were subject to hydrolysis, and inter‐ and intramolecular transesterification by lipase to produce two series of polymers having linear and cyclic structures with higher and lower molecular weight. The formation of the cyclic P(3HB) iss regarded as the characteristic feature of the lipase‐catalyzed polymerization of BL.     

Propagation Rate Coefficients for Homogeneous Phase VDF–HFP Copolymerization in Supercritical CO2

For the first time, propagation rate coefficients, k_p,COPO, for the copolymerizations of vinylidene fluoride and hexafluoropropene have been determined. The kinetic data was determined via pulsed‐laser polymerization in conjunction with polymer analysis via size‐exclusion chromatography, the PLP‐SEC technique. The experiments were carried out in homogeneous phase with supercritical CO₂ as solvent for temperatures ranging from 45 to 90 °C. Absolute polymer molecular weights were calculated on the basis of experimentally determined Mark–Houwink constants. The Arrhenius parameters of k_p,COPO vary significantly compared with ethene, which is explained by the high electronegativity of fluorine and less intra‐ and intermolecular interactions between the partially fluorinated macroradicals.     

Oxygen‐Triggered Switchable Polymerization for the One‐Pot Synthesis of CO2‐Based Block Copolymers from Monomer Mixtures

Switchable polymerization provides the opportunity to regulate polymer sequence and structure in a one‐pot process from mixtures of monomers. Herein we report the use of O₂ as an external stimulus to switch the polymerization mechanism from the radical polymerization of vinyl monomers mediated by (Salen)Co^III?R [Salen=N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexanediamine; R=alkyl] to the ring‐opening copolymerization (ROCOP) of CO₂/epoxides. Critical to this process is unprecedented monooxygen insertion into the Co?C bond, as rationalized by DFT calculations, leading to the formation of (Salen)Co^III?O?R as an active species to initiate ROCOP. Diblock poly(vinyl acetate)‐b‐polycarbonate could be obtained by ROCOP of CO₂/epoxides with preactivation of (Salen)Co end‐capped poly(vinyl acetate). Furthermore, a poly(vinyl acetate)‐b‐poly(methyl acrylate)‐b‐polycarbonate triblock copolymer was successfully synthesized by a (Salen)cobalt‐mediated sequential polymerization with an O₂‐triggered switch in a one‐pot process.     

High‐pressure rheology of polystyrene melts plasticized with CO2: Experimental measurement and predictive scaling relationships

A high‐pressure extrusion slit die rheometer was constructed to measure the viscosity of polymer melts plasticized by liquid and supercritical CO₂. A novel gas injection system was devised to accurately meter the follow of CO₂ into the extruder barrel. Measurements of pressure drop, within the die, confirm the presence of a one‐phase mixture and a fully developed flow during viscosity measurements. Experimental measurements of viscosity as a function of shear rate, pressure, temperature, and CO₂ concentration were conducted for three commercial polystyrene melts. The CO₂ was shown to be an effective plasticizer for polystyrene, lowering the viscosity of the polymer melt by as much as 80%, depending of the process conditions and CO₂ concentration. Existing theories for viscoelastic scaling of polymer melts and the prediction of T_g depression by a diluent were used to develop a free volume model for predicting the effects of CO₂ concentration and pressure on polymer melt rheology. The free volume model, dependent only on material parameters of the polymer melt and pure CO₂, was shown to accurately collapse the experimental data onto a single master curve independent of pressure and CO₂ concentration for each of the three polystyrene samples. This model constitutes a simple predictive set of equations to quantify the effects of gas‐induced plasticization on molten polymer systems. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3168–3180, 2000     

Homogeneous free‐radical polymerization of styrene in supercritical CO2

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, k_t, which are by one order of magnitude larger than k_t 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.     

Neutron reflectivity study of glassy polymer brushes in density fluctuating supercritical carbon dioxide

By using in situ neutron reflectivity, we measured the swelling behavior of two types of polymer brushes, deuterated polystyrene with a trichlorosilane end group and deuterated polystyrene-block-poly(4-vinylpyridine) block copolymer, in supercritical carbon dioxide (scCO₂). The measurements were conducted in the pressure range of 0.1–20 MPa at 36 °C. The pressure dependence of the brush height clearly showed an anomalous peak at the density fluctuation ridge (pressure = 8.2 MPa) that defined the maximum long-range density fluctuation amplitude in the pressure–temperature phase diagram of carbon dioxide (CO₂). The density profile of the brush, which could be approximated by a simple step function, and the magnitude of the brush height both indicated that the solvent quality of scCO₂ for the deuterated polystyrene brushes was still poor even at the density fluctuation ridge. In addition, atomic force microscopy images for the frozen polystyrene brush prepared by the rapid drying of CO₂ showed a phase-separated structure, as predicted from the numerical calculations of Grest and Murat, as a function of the variable Nσ, where N is the polymerization index and σ is the grafting density. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3282–3289, 2004     

Utilization of N,N‐Dialkylcarbamic Acid Derived from Secondary Amines and Supercritical Carbon Dioxide: Stereoselective Synthesis of Z Alkenyl Carbamates with a CO2‐Soluble Ruthenium–P(OC2H5)3 Catalyst

Reversible transformation of diethylamine ( 1 ) and supercritical carbon dioxide (scCO₂) into N,N‐diethylcarbamic acid ( 2 ) was confirmed by direct acquisition of ¹H and ¹³C{¹H} NMR spectra. The equilibrium between 1 +CO₂ and 2 is strongly influenced by conditions of the supercritical state. Low temperature favors formation of carbamic acid, whereas high temperature causes decarboxylation. On the basis of the spectroscopic results of carbamic acid formation under scCO₂ conditions, the ruthenium‐catalyzed formation of alkenyl carbamates from terminal alkynes, 1 , and carbon dioxide was investigated to demonstrate the useful transformation of elusive carbamic acids. Selectivity toward the CO₂‐fixation products over enynes obtained by dimerization of the alkyne substrates was improved by the use of scCO₂ as a reaction medium. In particular, a CO₂‐soluble ruthenium complex, trans‐[RuCl₂{P(OC₂H₅)₃}₄], was found to be effective in affording Z alkenyl carbamates with high stereoselectivity.     

Cationic equilibrium ring-opening polymerization of bicyclic monomers and its application to chemical recycling

Spiro orthoesters give poly(cyclic orthoester)s by single ring-opening polymerization in the presence of acid catalysts, and this process undergoes the equilibrium polymerization. We have applied the function of equilibrium polymerization to chemical recycling of polymeric materials. Crosslinked poly(cyclic orthoester)s, prepared by radical additions of poly(cyclic orthoester)s possessing exomethylene groups and dithiols, efficiently decrosslinked to bifunctional spiro orthoesters in the presence of CF₃CO₂H in CH₂Cl₂. The dithiol-linked bifunctional spiro orthoester monomers, prepared by the radical additions of spiro orthoester possessing exomethylene group and dithiols, afforded the corresponding crosslinked polymers in the presence of CF₃CO₂H as a catalyst in bulk. The decrosslinking of the obtained crosslinked polymer proceeded quantitatively to obtain the corresponding bifunctional monomer at room temperature in CH₂Cl₂. Further, an acid-catalyzed reversible crosslinking-decrosslinking of a polymer having a spiro orthoester group in the side chain was carried out. The copolymer obtained by the radical copolymerization of 2-methylene-1,4,6-trioxaspiro[4.6]undecane with acrylonitrile was treated with CF₃CO₂H at −10 °C in CH₂Cl₂ to afford the crosslinked polymer quantitatively. The crosslinked polymer was then treated with CF₃CO₂H at room temperature at a low concentration in CH₂Cl₂ to recover the original polymer.     

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     

Thermal expansion of polymers submitted to supercritical CO2 as a function of pressure

Interactions of polymers with compressed supercritical CO₂ has been studied by using pressure‐controlled scanning calorimetry (PCSC). Global cubic thermal expansion coefficients (α_{pol‐g‐int}) for medium density polyethylene (MDPE) and poly(vinylidene fluoride) (PVDF) saturated with supercritical CO₂ have been determined at 352.4 K over the pressure range from 0.1 MPa to 100 MPa. In both cases, the isotherms of global α_{pol‐g‐int} exhibit minima near 20 MPa. At pressures below the minimum, α_{pol‐g‐int} for the PVDF–CO₂ system are higher than for the MDPE–CO₂ system, while at pressures above the minimum the opposite was observed. This proves that incorporation of CO₂ in PVDF is stronger than in MDPE. The appearance of the minimum is attributed to the action of compressed CO₂ molecules, which at higher pressures are forced to enter deep inside the interstitial or other voids in the polymer and cause their mechanical distension, which must be associated with an endothermic effect. The measurements have been performed on polymers used for fabrication of pipelines. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44:185–194, 2006     

Structure of poly(maleic anhydride)

The bulk polymerization of maleic anhydride initiated with acylperoxides, di-tert-butyl peroxide, AIBN, or pyridine proceeds with evolution of CO₂. The amount of CO₂ generated depends on the nature and the concentration of the initiator. With peroxide initiators, less than 5% of the polymerized maleic anhydride is decarboxylated. ¹H-NMR spectra, obtained on the benzoyl peroxide-initiated polymer and its methyl ester, are consistent with the unrearranged poly(maleic anhydride) structure and rule out the polycyclopentanone structure proposed by Braun and co-workers. Base-initiated polymaleic anhydride is substantially decarboxylated, and the resulting polymer has anhydride and carboxyl groups. Elemental analyses and ¹H-NMR spectra obtained on the pyridine-initiated polymer and its methyl ester refute both the cis-poly(vinylene ketoanhydride) structure suggested by Schopov and the polycylopentanone structure proposed by Braun and co-workers.     

Dispersion polymerization of MMA in supercritical CO2 stabilized by random copolymers of 1H,1H–perfluorooctyl methacrylate and 2‐(dimethylaminoethyl methacrylate)

A series of random copolymers, composed of 1H,1H‐perfluorooctyl methacrylate (FOMA) and 2‐dimethylaminoethyl methacrylate (DMAEMA) were prepared as stabilizers for the dispersion polymerization of methyl methacrylate in supercritical CO₂ (scCO₂). Free‐flowing, spherical poly(methyl methacrylate) (PMMA) particles were produced in high yield by the effective stabilization of poly(FOMA‐co‐DMAEMA) containing 34–67 w/w % (15–41 m/m %) FOMA structural units. Less stabilized but micron‐sized discrete particles could be obtained even with 25 w/w % (10 m/m %) FOMA stabilizer. The result showed that the composition of copolymeric stabilizers had a dramatic effect on the size and morphology of PMMA. The particle size was controllable with the surfactant concentration. The effect of the monomer concentration and the initial pressure on the polymerization was also investigated. The dry polymer powder obtained from dispersion polymerization could be redispersed to form stable aqueous latexes in an acidic buffered solution (pH = 2.1) by an electrostatic stabilization mechanism due to the ionization of DMAEMA units in the stabilizer. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1365–1375, 2008