polymerizations in sc-co2

8/2/2019 Polymerizations in SC-CO2

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Polymerizations in Supercritical Carbon Dioxide

Jonathan L. Kendall, Dorian A. Canelas, Jennifer L. Young, and Joseph M. DeSimone*

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290

Received June 30, 1998 (Revised Manuscript Received November 23, 1998

Contents I. Introduction 543II. Chain-Growth Polymerizations in CO2 546

A. Free-Radical Polymerizations 5461. Homogeneous Polymerizations 5472. Precipitation Polymerizations 5473. Dispersion and Emulsion Polymerizations 5484. Polymer Blend Synthesis 553

B. Cationic Polymerizations 553C. Transition Metal-Catalyzed Polymerizations 556D. Thermal Ring-Opening Polymerization 557

III. Step-Growth Polymerizations 557A. Melt-Phase Condensation Polymerizations 557B. Sol Gel Polymerizations 559C. Oxidative Coupling 559

IV. Conclusions 561V. References 561

I. Introduction In the search for new polymerization solvents,

scientists ha ve tur ned t o supercritical fluids. Super-critical carbon dioxide possesses many properties thathave a llowed it to emerge a s the most extensivelystudied supercritical fluid for polymerization reac-tions. The work present ed in th is review shows thatsupercritical CO2 is a viable and pr omising alterna -tive to traditional solvents used in polymer synt hesis.Much of this promise results from its fluid properties,effects on polymers, an d environment al a dvant ages.

Supercritical fluids have the best of two worlds:they can have gaslike diffusivities (which haveimportant implications for reaction kinetics) whilehaving liquidlike densities that allow for solvationof ma ny compoun ds. They exhibit changes in solventdensity with sma ll chan ges in t emperature or pr es-sure without altering solvent composition.1 Becauseof these advan tages, chemists ha ve explored r eactionmechanisms and solvent cage effects in supercriticalfluids.2,3 Chan ging solvent qu ality also has su bstan -tial effects on reaction workup: it can affect theseparation of the polymer from starting materialsand catalysts a nd the polymer molecular weightfractiona tion. In add ition, t he low viscosity of super-critical fluids and their ability to plasticize glassypolymers have implications on polymer processing

an d kinet ics. For example, condit ions t ha t give a loviscosity supercritical fluid result in diminishesolvent cage effects in free-radical initiator decompositions.4

When carbon dioxide is used as the supercriticsolvent, additional advantages can be realized. Thchemical industr y ha s become increasingly awar e environmental concerns over the use of volatiorganic solvent s an d chlorofluorocarbons in the manfacture and processing of commercial polymer proucts. The use of water alleviates these problemsomewhat, but still results in large amounts

hazar dous aqueous waste that require treat ment. Aa result of these en vironmenta l concerns, supercrical CO2 represent s a more environmenta lly friendlalternative to traditional solvents. CO2 is natur allyoccurring and a bundant : it exists in na tura l resevoirs of high pu rity locat ed th roughout th e world. addition, it is generated in large quantities as byproduct in ammonia, hydrogen, and ethanol planand in electrical power generation stations that bufossil fuels .5 CO2 has an easily accessible critical pointwith aT c of 31.1 C and aP c of 73.8 bar .6 Because itis an a mbient gas, CO2 can be easily recycled afteruse a s a solvent t o avoid any contribution to greehouse effects. Finally, it is inexpensive, nonflam

mable, and nontoxic, mak ing it an at tra ctive solvefor large-scale synthesis.There are several important issues, such as dryin

solubility, and polymer plasticization, that are involved when supercritical CO2 is used as a polymer-ization solvent. Because CO2 is an am bient gas, thepolymers can be isolated from the reaction media bsimple depressurization, resulting in a dry polymproduct. This feature eliminates energy-intensivdrying procedures r equired in polymer ma nu factuing to remove solvent and represents potential coan d energy sa vings for CO2-based system s.

Solubility plays a very important role in thsynthesis of polymers in supercritical CO2. While CO2is a good solvent for most nonpolar and some polmolecules of low molar ma ss,7 it is a poor solvent formost high molar mass polymers under mild condtions (< 100 C,< 350 bar ). For exam ple, poly(met hyacrylate) requires 2000 bar CO2 and 100 C for a 105g/mol polymer to dissolve in CO2.8 Such high pres-sures are not pra ctical a nd are too costly for thewidespread implementa tion in polymer m anu factuing. The only polymers that show good solubility CO2 under mild conditions are amorphous fluoropolmers an d silicones.1,4,9- 12

Present address: Lord Corp., Thomas Lord Research Center, P.O.Box 8012, Cary, NC 27512-8012.

543Chem. Rev.1999, 99, 543 563

10.1021/cr9700336 CCC: $35.00 1999 American Chemical SocietyPublished on Web 01/22/1999


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While the parameters that govern solubility of polymers in CO2 are not fully understood, several

studies have been performed to probe the nature of solute- solvent interactions between polymers andCO2. CO2 has a low dielectr ic constan t compar ed toalkanes, and its polarizability and polarizability/ volume are lower than ethane and propane.13 Al-though CO2 lacks a dipole moment, a substantialcontribution to its solubility parameter is due to alarge quadrupole moment. The quadrupole momentcoupled with its Lewis acidity allow CO2 to pa rtici-pate in intera ctions absent in h ydrocarbons. Earlywork in this ar ea suggested th at weak interactionsexist between CO2 and functional groups on thepolymer chains such as sulfones14 and carbonyls.15Later, systematic studies implied th at the interac-

tions between CO2 and the silicone portion of thepolymer ba ckbone govern the h igh solubility of th etypes of polymers.16 Work involving th e use of Four iertransform infrared spectroscopy reveals that CO2exhibits Lewis acid- base type interactions withelectron-dona ting functiona l groups of polymer chaisuch as th e carbonyl group of poly(meth yl metha crate) (PMMA).17 McHugh found th at in copolymers oethylene and m ethyl acrylate, increasing the methacrylate content increased polymer solubility.18 It was

Jonathan Kendall was born in Charlotte, NC, in 1969. He received hisB.S. in chemistry from the University of North Carolina at Chapel Hill in1991. He received his Ph.D. in chemistry from Stanford University in 1997,under the direction of Professor James P. Collman. During graduate school,he synthesized bis(porphyrin) sandwich complexes for three materialsapplications: nonlinear optics, liquid crystals, and semiconducting charge-transfer materials. He then returned to Chapel Hill for postdoctoral researchin Professor Joseph DeSimones lab. His work on polymer synthesis inliquid and supercritical carbon dioxide has been performed while servingas the Technical Coordinator for the Kenan Center for the Utilization ofCarbon Dioxide in Manufacturing, a CO2-based research center sponsoredby UNCs CH and North Carolina State University.

Dorian Canelas was born in 1970 in Honolulu, HI and raised in WheatRidge, CO. She received a B.S. in chemistry from Northeastern Universityin 1993. As an undergraduate, she worked on step-growth polymerizationsand membrane modification under the direction of L. F. Hancock at W.R.Grace. She earned a Ph.D. from the University of North Carolina at ChapelHill in 1997 under the direction of J. M. DeSimone. Her graduate workfocused on dispersion polymerizations of vinyl monomers in surfactant-modified supercritical carbon dioxide. She is currently working on newthermoset chemistry at Lord Corporation in Cary, NC.

Jennifer Young was born and raised in Timonium, MD. She earned hB.S. degree in chemistry from the University of Richmond in 1995. is currently working toward her Ph.D. under the direction of ProfessoDeSimone at the University of North Carolina at Chapel Hill. Her stuare focused on dispersion polymerizations in liquid and supercritical cardioxide.

Joseph DeSimone is Mary Ann Smith Professor of Chemistry at tUniversity of North Carolina at Chapel Hill and of Chemical Engineat North Carolina State University. He received his B.S. degree in chemin 1986 from Ursinus College in Collegeville, PA, and his Ph.D. in from the Department of Chemistry at Virginia Polytechnic Institute State University. At VPI he worked with Professor James McGrath inarea of living polymerizations to synthesize novel block and gcopolymers. At UNCs CH, his research efforts are focused on developingsynthetic pathways for polymer synthesis and processing in liquid asupercritical carbon dioxide. His accomplishments include homogeneoand heterogeneous polymerizations in CO2. Recently, he has investigatedthe design and synthesis of surfactants for use in CO2 as stabilizers indispersion polymerizations, delivery of materials, and extractions. In 19he cofounded MICELL Technologies, which has developed environmenfriendly CO2-based dry-cleaning and metal degreasing systems. In 1997he and Professor Ruben Carbonell at NCSU started the Kenan Centfor the Utilization of Carbon Dioxide in Manufacturing (at UNCs CH andNCSU) to facilitate research on CO2-based polymer synthesis and

processing. Professor DeSimone was a 1993 Presidential Faculty Felland winner of the 1997 Presidential Green Chemistry Challenge AwaHe is also the 1999 winner of the Carl S. Marvel Creative PolymChemistry Award.

544 Chemical Reviews, 1999, Vol. 99, No. 2 Kendall et


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concluded that dipole- quadrupole interactions be-tween CO2 and the a crylate u nit contributed to thepolymer solubility. Increa sing th e free volume of th epolymer also increased solubility at temperaturesabove 100 C, where the polar interactions aredecreased.

The solubility of fluorine-conta ining polymers h asbeen investigated. McHugh noted that while poly-ethylene is insoluble in CO2, even u p to 270 C and2750 bar, perfluorinated ethylene- propylene copoly-mer has greatly improved solubility (200 C, 1000bar).18 Other work revealed that the interactionsbetween CO2 and poly(trifluoropropyl methyl silox-ane) are anomalously high, and the authors at-tributed this finding to specific interactions betweenCO2 and the polar fluorine containing group.16,19More recently high-pressur e19F NMR stu dies haveindicated specific intera ctions between CO2 andfluorocarbons.20

These solubility requ irements dicta te t he t ypes of polymerization t echn iques employed in polymer syn-thesis. For example, amorphous fluoropolymers h ave

been synthesized homogeneously in supercriticalCO2. In contrast, many polymerization reactions inCO2 are conducted under heterogeneous processes,either as precipitation, dispersion, or emulsionpolymerizations, du e to t he inheren t insolubility of most polymers in CO2. Thus, importan t research inth is field involves the synt hesis of sur facta nt s, whichare a critical component of dispersion and emulsionpolymer izations, for use as st abilizers in CO2. Thesestabilizers have been successfully employed in theheterogeneous polymerizations of CO2-insoluble poly-mers. The n at ure of the sur factan t cont rols polymerproperties and morphology, and these factors needto be considered in surfactan t design.

The fractionation of polymers is an other importan tarea affected by polymer solubility in supercriticalCO2. Car eful a nd contr olled lowering of the densityof a polymer solution allows precipitation of thehighest molecular weight polymer fraction.1 Forexample, CO2-soluble synthetic oils such as per-fluoroalkyl polyethers, chlorotr ifluoroethylene oligo-mers, and poly(dimethylsiloxane) (PDMS) werefra ctionated with supercritical CO2.10 In the case of PDMS, the starting sample with a n umber averagemolecular weight ( M n) of 4 105 g/mol was separ atedinto six fra ctions wit hM n ranging from 4 102 g/molto 1 106 g/mol.

Anoth er importan t featu re of supercritical CO2 inpolymerization is plasticization, which results in thelowering of the polymers glass tra nsition t empera-tu re (T g). Polymers have been shown to becomehighly plas ticized by CO2. This plas ticizat ion a llowsimportant effects such as the removal of residualmonomer from the polymer, incorporation of addi-tives, and formation of foams. Table 1 summarizessome of th e studies on th e effect of CO2 plasticizationon polymers. The effects of high-pressur e CO2 on theglass t ran sition t emperatur e an d mechanical proper-ties of polystyrene (PS) have been measured byYoungs m odulus a nd creep complian ce.21 This workdemonstrated the severe plasticization of PS as a

function of CO2 pressure. In work by Choiu, dif-ferential scanning calorimetry (DSC) was used study the plasticization of polymers by CO2 at pres-sures up t o 25 bar.22 Even at such modest pressuresas 25 bar, reductions in th e glass tr an sition tem pe

atures of up to 50 C were observed. Other studiincluded measu rements of polymer swelling ancreep compliance as a fun ction of time t o determinT gs of polycar bonat e (PC), PS, and PMMA in CO2.23,24Beckman examined the plasticization of PMMA bCO2 as a function of pressure using dielectric measurements.25 Johnston performed an in-depth studyof the solubility of CO2 in the polymer and its effecton the T g vs CO2 partial pressure behavior forPMMA, PS, and a random copolymer of methmethacrylate a nd styrene.26 J ohnston ha s also usedin situ measurements of creep compliance to detemine the T g depression of PMMA and poly(ethymethacrylate) in CO2.27 As evidenced by the above

Table 1. Summary of Li teratu re S tud ies on th eSwel l ing and Plas t ic izat ion o f Homopolymers ,Copolymers , and Po lymer Blends by CO 2

polymeric materia l t echnique ref(s)Homopolymers

poly(aryl ether ether ketone)(PEEK)

DSCa 170

polycarbonate (PC) DSC 22,32swell ing/sorp t ion 23

subst ituted polycarbonates DSC 171

poly(2,6-dimethylphenyleneoxide) (PP O) DSC 30,32poly(dimethylsiloxane)

(PDMS)sorpt ion 16,19,38

poly(ether imide) DSC 32polyethylene (PE) DSC 33poly(ethylene terephthalate)

(PET)DSC 22,32,33

p oly(e th yl met h acryl at e) sorpt ive d il at ion 2 9(PEMA) creep compliance 27

poly(iminoadipoylimino-hexamethylene) (nylon-6,6)

33

polyisoprene swelling/sorpt ion 172poly(methyl methacryla te) DSC 22,32

(PMMA) creep compliance 24,26,27swelling/sorption 23,172,173dielectric

measurements25

poly(oxymethylene) DSC 33poly(propylene) (PP) DSC 33polyst yr en e (P S) cr eep com plia nce 21,24,26

DSC 22swelling/sorption 23,172

polysulfone DSC 32polyu ret ha ne in fr ar ed spect roscopy 31

DSC 32p oly(vin yl ben zoa t e) s or pt ive d ila t ion 28poly(vin yl bu tyr al) sor pt ive dila tion 28poly(vinyl chloride) (PVC) DSC 22,32poly(vinylidene fluoride)

(PVF2)DSC 33

poly(vin ylp yr id in e) s wellin g/s or pt ion 172Copolymers

poly(methylmethacrylate-co-styrene)

creep compliance 26,172

acrylonitr ile butadienestyrene (ABS) DSC 32high impact polystyrene DSC 32

Polymer BlendsPMMA/PVF2 DSC 22PS/PC DSC 174P S/poly(vin ylm et hyl et her ) s or pt ion 175

a DSC ) differential scanning calorimetry.

Polymerizations in Supercritical Carbon Dioxide Chemical Reviews, 1999, Vol. 99, No. 545


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and other28- 33 work, th e plast icization of polymers byCO2 is well-established.

The highly plasticized state of the polymer canresult in increased polymerization rates by t heenhanced diffusion of monomer into the polymer.34- 36The effects of this plasticization can be studied bycontrolling the reaction pressure a nd temperatur e.The plas ticizat ion of polymers with SCFs can also beused to lower the melt and solution viscosities37- 39

and affect polymer morphology with supercriticaldrying40 or foaming.41- 43Polymer plasticization by supercritical CO2 can also

be used in t he removal of residual monomer.1 Th ehigh transport properties of supercritical CO2 suchas high diffusivity and low viscosity coupled with thepropensity of CO2 to plasticize polymers were em-ployed to removeN -vinylcarbazole from poly( N -vinylcarbazole).10 Supercritical CO2 treatment of polymer resulted in lowering m onomer levels from3.66% to 0.1%. In this system, traditional solventswere un suita ble for removal of residual m onomer.

Supercritical fluid CO2 has indeed been shown tobe a promising solvent in which to perform polym-erization r eactions. Cha in-growth routes su ch as free-radical polymerization of styrenics, acrylates, andmetha crylates, cationic polymer ization of isobutylene,vinyl ethers, an d styrene, and transition metal-catalyzed polymerization of norbornene and copo-lymerizat ion of epoxides and CO2 have been reported.Step-growth reactions in CO2 have produced poly-car bona tes, polyamides, polyesters, polypyrrole,polyphenoxides, and silica gels. In addition, thenumerous super critical CO2 processing techniques forpolymers th at ar e being developed become simplifiedwhen the synthesis of the polymer is performed insupercritical CO2. These examples illustrate theversatility and import ance of supercritical CO2 as asolvent for polymerization reactions.

II. Chain-Growth Polymerizations in CO 2 The major types of chain-growth polymerization

routes include free-radical, cat ionic, an ionic, an dmetal-catalyzed reactions. Most chain-growth poly-merizations in CO2 ha ve focused on free-radicalpolymerizations, but there are a number of reportsin th e ar eas of cat ionic and meta l-catalyzed r eactions.We are unaware of any reports of anionic polymer-izations in CO2: reactive anions would att ack theLewis acid CO2, and thus anionic polymerizations inCO2 ar e unlikely unless extremely weak n ucleophiles

are involved, such as siloxan olat es.Initial breakthroughs in the 1960s in the use of compressed CO2 as a continu ous pha se for polymer-izations, especially in cationic and free-radical pre-cipitation polymerizations, were followed by verylittle activity in the 1970s and 1980s. The 1990s,however, ha ve seen a n explosion of research in thisarea. The n ext breakthrough in t he u se of CO2 as apolymerizat ion medium was rea lized when siloxanesand amorphous fluoropolymers were identified aspolymeric materials which had high solubilities inCO2 at easily attainable temperatu res (T < 100 C)and pressures (P < 350 bar).1 This rea lizat ion openedup new areas of research in CO2, mainly homoge-

neous polymerizations as well as dispersion anemulsion polymerizations.

A. Free-Radical PolymerizationsFree-radical polymerizations can be classified a

either homogeneous or heterogeneous reactions. a homogeneous polymerization all components, icludin g monomer , initia tor, and polymer, ar e solubthroughout the duration of the reaction; a hetergeneous polymerization conta ins at least one isoluble componen t at some point du ring th e reactioBecau se t he term inology t o describe h eterogeneopolymerization processes h as been u sed inconsitently in the literature,44 a brief treatment of thissubject is necessa ry to avoid confusion. The four mowidely studied heterogeneous processes (precipittion, suspension, dispersion, and emulsion) can bclearly distinguished on t he basis of th e initial staof the polymerizat ion mixtur e, th e kinet ics of polyerization, t he m echan ism of par ticle formation, anthe shape and size of the final polymer particles45Other heterogeneous processes which are not pre

ently of indust rial importan ce an d which will not discussed here in detail include miniemulsion46,47polymerizat ion a nd m icroemulsion48 polymerization.In a precipitation polymerization, an initially homgeneous mixture of monomer, initiator, an d solvebecomes heterogeneous during the reaction as isoluble polymer chains aggregate t o form a sepa rapolymer phase. In a suspension polymerization, othe other ha nd, neither th e monomer nor the initiatare soluble in the continuous ph ase. The resultinpolymer is also insoluble in the continuous phaswhich simply acts as a dispersant and heat-dissiption a gent du ring th e polymerization. As a resu lt the high solubility of man y common monomers an

organic initiators in compressed CO2, suspensionpolymer izations in CO2 ar e not common an d will notbe present ed herein. Dispersion a nd emulsion polmerizations constitute the colloid-forming polymeization methods that have been recently exploreusing CO2. A brief discussion of the traditionadefinitions of these colloid-form ing p rocesses follow

A dispersion polymerization begins as a homgeneous mixtu re because of th e solubility of both tmonomer and the initiator in the continuous phasOnce th e growing oligomeric radicals r each a criticmolecular weight, the chains are no longer solubin the reaction medium and phase separation occuAt this point the surface active stabilizing molecu

adsorbs to or becomes chemically attached to thpolymer colloid and prevents coagulation or aglomera tion of the pa rt icles. In th is respect, a dispesion polymerization is often considered to be modified precipitat ion polymer ization.49 Polymeri-zation persists both in the continuous phase and the growing polymer par ticles. Since the initiator amonomer are n ot segregat ed or compar tmen ta lizedispersion polymerizations do n ot follow Sm ith-Ewart kinetics. However, enhanced rates of polymerization ar e often observed due to th e au toacceleation (Gel or Trommsdorf) effect within a growinpolymer particle. The product from a dispersiopolymerizat ion exists as sp her ical polymer par ticle



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typically ranging in size from 100 nm to 10m. Thehistory and theory of dispersion polymerization inorganic media have been summarized in a book byBarrett44 an d a r ecent r eview by Sudol.50 Due to thegood solubility of many small organic molecules inCO2, dispersion polymerization constitu tes t he bestheterogeneous met hod tha t ha s been developed thu sfar for producing high molecular weight, CO2-insoluble, industr ially importa nt hydrocarbon poly-

mers.In contrast to dispersion polymerization, t he r eac-tion mixture in an emulsion polymerization isinitially heterogeneous due to the low solubility of the monomer in the continuous phase. Emulsionpolymerization is a very active area of research, andthe reader is referred to several recent books andreviews.20,49,51- 53 For a rea ction to ta ke advan tage of the desirable Smith- Ewart /Har kins kinetics,54 themonomer and initiator must be segregated with theinitiator preferentially dissolved in the continuouspha se and n ot in th e monomer ph ase. Traditionally,emulsion polymerizations employ oil-soluble mono-mers such a s acrylics or styrenics dispersed in an

aqueous medium containing a water-soluble initiatorsuch as sodium persulfate, while inverse emulsionpolymerizations employ water -soluble monomers,such as acrylamide, dispersed in a n organ ic mediumcont ain ing an oil-soluble initiator such as an orga nicazo or peroxide species. In either case, the insolublepolymer is stabilized as colloidal particles. Therepulsive forces, which result from the surfacecharges imparted by an ionic initiating species, anadded small molecule ionic surfactant, an addedamphiphilic macromolecular stabilizer, or a combina-tion of these to provide a charged surface, preventthe coagulat ion of the growing pa rt icles and lead toa stabilized colloid. As a result of the kinetics of anemulsion polymerizat ion, h igh molecular weight poly-mer can be produced at high rates, with th e rat e of polymerization depen dent upon t he n umber of poly-mer particles. The polymer which results from anemulsion polymerization exists a s spher ical par ticlestypically smaller th an 1m in diameter. Due to thehigh solubility of most vinyl monomers in CO2,emu lsion polymerizat ion in CO2 probably will not bea very useful process for the majority of commerciallyimporta nt monomers, alth ough t here a re exceptions.For example, inverse emulsion polymerization of acrylamide in a water/CO2 system has been re-ported.56

1. Homogeneous Polymerizations Amorphous or low-melting fluoropolymers can besynthesized homogeneously in CO2 by either free-radical or cationic methods. Poor solubility in mostcommon organic solvents represents an inherentproblem in the synthesis and processing of many highmolar mass amorphous fluoropolymers. In fact,chlorofluorocarbons (CFCs) and CO2 are the bestsolvents for these types of polymers.4 However, du eto the environment al problems a ssociated with CF Cs,they are no longer a viable solvent option. Carbondioxide presents an ideal inert solvent for the polym-erization of these t ypes of highly fluorinated mono-mers without the environmental concerns.

Using su percritical CO2 as t he solvent, DeSimoneused free-radical initiators to effect the synthesis high m olar mass amorphous fluoropolymers.4,11,57- 59Due to th e high solubility of th e polymers in t he C2continuous phase, the polymerizations remainehomogeneous th roughout t he cour se of th e reactionSeveral fluorinat ed acrylate m onomers, su ch as 1,dihydroperfluorooctyl acrylate (FOA), have beepolymerized using this methodology to give hig

yields of high molecular weight polymer. Styrenewith a perfluoroalkyl side chain in the para positiosuch asp -perfluoroethyleneoxymethylstyrene, constitute a second type of monomer polymerized vhomogeneous solution polymerization in supercriticCO2. The product obtained from this polymerizatioin CO2 was identical to the product of a solutionpolymer ization in 1,1,2-tr ichloro-1,2,2-tr ifluoroetha(Fr eon-113), indicating th at CO2 does indeed r epre-sent an exceptiona lly good replacement solvent fthis type of reaction. Moreover, this technique hproven to be valuable in the synthesis of statisticcopolymers of the appropriate fluorinated monomeand other hydrocarbon monomers such as meth

methacrylate (MMA), butyl acrylate, ethylene, anstyrene. Homogeneous solution polymerization halso been employed by DeSimone to prepar e CO2-soluble polymeric amines by copolymerizing FOwith 2-(dimethylamino)ethylacrylate or 4-vinylpyrdine.60

The initiation and propagation kinetics of freeradical polymerizations in supercritical CO2 havebeen studied. The earliest work in this area useultraviolet spectroscopy to explore the thermal dcomposition r ate and initiat or efficiency of 2,2 -azobis-(isobutyronitrile) (AIBN) in CO2.11 By comparing thedecomposition rate,k d, and th e initiat or efficiency,f ,in CO2 to that reported in the literature for othesolvents, the authors observed higher initiator eficiencies, relative to higher viscosity solvents, asresult of the decreased solvent cage effect in thsupercritical pha se. More recent work h as evaluatthe free-radical propagation kinetics of styrene anmethyl methacrylate in CO2 using the pulsed laserpolymerization (PLP) technique.61 Since the conver-sions were kept very low (< 5%), the polymer chainsremained in solution during these experiments astyrene monomer is a good (co)solvent for PS. Tpropagation rate,k p, was found to be very close tothe bulk values for both monomers, indicating ththe presen ce of CO2 does not interfere with th e cha in-growth process. Another study compa red th e kinetof the homogeneous polymerization of 1H,1H,5Hperfluoropent yl acrylate in super critical CO2 to thosein conven tional, liquid solvent s such as Fr eon-11362Indeed, these kinet ic stud ies have verified that CO2is an excellent medium in which to conduct frerad ical reactions.

2. Precipitation Polymerizations

A topic of man y early studies in CO2 was th e free-radical polymerization of indu stria lly importan t vinmonomers.63 Although many common vinyl monomers exhibit high solubility in CO2,7 most of thecorresponding polymers exhibit exceedingly po



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solubilities in CO2. As a r esult, all of the early stu diesin th is area focused on precipita tion polymerizations.

In 1968, Hagiwara and co-workers explored thefree-ra dical polymerizat ion of ethylene in CO2 usingeither radiation or AIBN initiation.64- 66 In thiswork, t he infrar ed spectra of the polymers revealedthat the pr esence of the CO2 continuous ph ase h adlittle effect on the polymer st ructure. They noted th atwhile the ethylene monomer wa s initially soluble inthe liquid CO2, the polyethylene produced existed ina powder form which could easily be removed fromthe reactor. Powder products typically result fromprecipitat ion polymerizations; the advan tage of usingCO2 stemmed from the dryness of the resultingpolymer. Also in 1968, a F rench P at ent issu ed to theSumitomo Ch emical Compa ny disclosed t he polym-erization of several vinyl monomers in CO2.67 Th eUnited States version of this patent was issued in1970, when F uku i and co-worker s published resu ltsfor the free-radical precipitation polymerization of several hydrocarbon monomers in liquid an d super -critical CO2.68 As examples of this meth odology, theydemons tra ted the preparat ion of the var ioushomopolymers including poly(vinyl chloride) (PVC),PS, poly(acrylonitrile), poly(acrylic acid), and poly-(vinyl acetate) (PVAc). In a ddition, they pr epar ed th eran dom copolymers poly(styrene-co-methyl meth-acrylate) and poly(vinyl chloride-co-vinyl acetate).Depending upon the monomer and the reactionconditions employed, these reactions resulted ingravimetric yields which varied from 15% to 97% andviscosity average molecular weights ( M v) rangingfrom 1.2 104 to 1.6 106 g/mol.

More recently, precipita tion polymerizations of semicrystalline fluoropolymers in CO2 have beenstudied by DeSimone. In particular, tetrafluoro-

ethylene is a monomer of interest since it wasdetermined th at t etra fluoroethylene ma y be ha ndledmore safely as a mixture with CO2.69 Tetrafluoro-ethylene copolymerizat ions70,71with perfluoro(propylvinyl ether) and with hexafluoropropylene orhomopolymerizations72 ha ve been perform ed in CO2,resulting in high yields of high molecular weight(> 106 g/mol) polymer. The two major advantages of this process, lack of chain transfer to solvent andabsence of undesirable endgroups, ha ve been recentlyreviewed.63

3. Dispersion and Emulsion Polymerizations

As was a lluded to earlier, one key to a successfuldispersion or emulsion polymerization is the surfac-tan t. Its role is to adsorb or chemically att ach to th esur face of the growing polymeric part icle and pr eventthe particles from aggregating by electrostatic, elec-trosteric, or st eric stabilization. Consa ni an d Sm ithhave studied the solubility of over 130 surfactantsin CO2 at 50 C and 100- 500 bar.73 They concludedthat microemulsions of commercial surfactants formmuch more readily in oth er low polar ity supercriticalfluids such as a lkanes an d xenon th an in CO2. Sincetraditional surfactants for emulsion and dispersionpolymerizat ions were designed for use in an a queousor organic continuous phase and are completely

insoluble in CO2, an exciting area of resear ch is th edesign an d synt hesis of novel surfactan ts specificafor CO2. Polymeric sur factant s for st eric stabilizat ioare effective in solvents with low dielectric constanThis is one reason steric stabilization, r ath er t haelectrostatic stabilization, provides the stabilizatiomechanism of choice for CO2-based systems. Thepolymeric stabilizer is a macromolecule that prefeentially exists a t t he polymer- solvent int erface an d

prevents aggregation of pa rticles by coating thsurface of each particle a nd imparting long-rangrepulsions between them. These long-range repusions must be great enough to compensate for thlong-range van der Waals attractions of the paticles.44 This complex phenomenon depends not onlon the amount and molecular weight of adsorbestabilizer, but also on its conformation, which affected by the nature of the solvent. When sterstabilization acts effectively in a heterogeneous sytem, th e sta bilizing molecule at taches t o the su rfaof the polymer particle by either chemical graftinor ph ysical adsorption. Amphiphilic ma terials sucas block and graft copolymers, which have oncomponent tha t is soluble in t he continuous ph aand another component, th e anchor, that prefers reside in th e polymer ph ase, offer t he h ighest proability for physical adsorption. The other route stabilization includes the chemical grafting of thstabilizer to the par ticle sur face, either th rough chatra nsfer to sta bilizer or by u sing a functiona l stablizer that can serve as an initiator, monomer, oterminating agent. In this case, th e chemicalgrafted stabilizer cannot physically desorb from thparticle surface, and as a result, grafted stabilizeimpart better colloidal st ability tha n physicalladsorbed sta bilizers. In CO2-based systems , the typesof sur factants that have been used for s terstabilization include CO2-philic (CO2-soluble) homo-polymers, copolymers (statistical, block, or grafcontaining a CO2-philic component a nd a CO2-phobic(CO2-insoluble) anchoring component, and CO2-philicreactive ma cromonomers. Stuctu res of th e polymersurfactan ts for CO2 described in this paper arenumbered an d located in F igures 1 a nd 2.

Recently, the traditional Napper theory for sterstabilization in colloidal systems has been evaluatfor a pplicability in h ighly compress ible supercrit icmedia. Peck and Johnston have developed a lattifluid s elf-consisten t field th eory to describe a sur fatant chain at an interface in a compressible flui

allowing tr aditional colloidal st abilizat ion theory74

tobe extended to supercrit ical fluid continuouphases.75,76 In th eir th eory, holes ha ve been int roduced into the lattice to account for the decreaseconcentr ation of molecules in th e less dense supecritical phase. In th is way, they ar e able to accoufor the compressibility of the continuous phase. Tmechan ism of ster ic stabilization was furth er stu diby the J ohnston group experimenta lly by t urbidiand t ensionmet ry measu rement s of emulsion sta bity, critical flocculation density, and reversibility flocculation.77 The system studied was an emulsionof poly(2-ethylhexyl acrylate) (PEHA) in CO2 in thepresence of a surfactant, either poly(1,1-dihydrope



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fluorooctyl acrylate) (PFOA) (1 ) homopolymer, P S-b-PFOA (2 ), or PFOA-b-poly(vinyl acetate)-b-PFOA. Adistinct change in the emulsion stability was ob-served at the critical flocculation density (CFD),

which corresponded with the point (where thesecond virial coefficient equ als zer o) for PF OA in buCO2. The CFD is the density at which particlefloccula te du e to collapse of th e soluble port ion of tsurfactant. While PFOA resulted in bridging floculation of PEHA particles below the CFD, PS-b-PF OA was m ore str ongly adsorbed a nd flocculat ibelow the CF D was slower. In the companion study78dynamic light scattering confirmed the correlatiobetween the CFD and the point density of PFOAin CO2 by monitoring the PEHA droplet size as

fun ction of time an d CO2 density in the presence of surfactant . F urth er theoretical developments havmore recently been made by Meredith an d J ohn stfor homopolymer sta bilizers79 and copolymer and end-graft st abilizers80 in a supercritical fluid. The adsorption of surfactant at a planar surface and the freenergies of interaction between two such surfacwere modeled. The effects of solvent density on tadsorbed amount of stabilizer and layer thicknesboth of which would in fluen ce colloid st ability, weexamined. Genera l design criteria wa s pr ovided fcopolymer and end-graft stabilizers. It is clear frothese studies on steric stabilization in CO2 that foreffective stabilization, the surfactant must anchor

Figure 1 . Stuctures of fluorinated polymeric surfactants for CO2.

Figure 2. Stuctur es of PDMS-based polymeric sur factan tsfor CO2.



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the particle surface and must not be in a collapsedstate a t t he reaction density.

The m ajority of the work in dispersion polymeriza-tions in supercritical CO2 has focused on methylmethacrylate (MMA). However, several other mono-mers h ave also been investigat ed as shown in Table2. In 1994, DeSimone r eported th e dispersion polym-erization of MMA in supercritical CO2.81 This workrepresent s t he first successful dispersion polymeri-zation of a lipophilic monomer in a supercritical fluidcontinuous phase. In th ese experiments, th e aut horstook advantage of the amphiphilic nature of thehomopolymer PFOA ( M n ) 1.1 104 or 2.0 105g/mol) to effect the polymerization of MMA to highconvers ions (> 90%) an d h igh degrees of polymeriza-tion (> 3000) in super critical CO2. These polymeriza-tions were condu cted in CO2 at 65 C and 207 bar,and AIBN or a fluorinated derivative of AIBN wasemployed as the initiator. The spherical polymerparticles that resulted from these dispersion poly-merizations were isolated following venting of CO2from th e reaction m ixture. Scann ing electron micros-copy showed that the product consisted of spheresin the 1- 3 m size range with a n arr ow part icle sizedistribut ion (see Figure 3). In cont rast, reactions tha twere performed in t he a bsence of PFOA resulted inrelatively low conversion (< 40%). Moreover, th epolymer t ha t resulted from th ese precipitation poly-merizations ha d an u nstr uctured, nonspherical mor-phology that contra sted sha rply with the sphericalpolymer particles produced in the dispersion poly-merizations. Without a doubt, the amphiphilic PFOAmacromolecule played a vital role in the sta bilizationof th e growing P MMA colloidal par ticles.

Recent work in this area has revealed that verylow amounts (0.24 wt % based on MMA) of PFOA( M w ) 1.0 106 g/mol) ar e needed to prepare a sta bledispersion of PMMA latex par ticles.34,82 Again, thePMMA particles were in the 1- 3 m size range. Inaddition, a large percentage (up to 83%) of t hestabilizer could be subsequently removed from thePMMA product by extraction with CO2. As a resultof the relatively high cost of the stabilizer and thepossible effects that residual stabilizer may have onproduct performance, the ability to remove andrecycle the P FOA constitut es an import ant aspect of this system. The effects of the reaction time andpressur e on t he resu lting conversion, molar ma sses,

and particle size of the polymer products werinvestigat ed. A gel effect occurs within the PMMparticles between 1 and 2 h of reaction time. Thresult parallels the gel effect within the polymepar ticles which is normally observed between 20 a80% conversion in a typical dispers ion polymerizatiin liquid organic media.44 More importantly, theability of CO2 to plasticize PMMA facilitates thediffusion of monomer into the growing polym

particles, allowing the reaction to proceed to higconversion. To complement the use of fluorinateacrylates as st abilizers, the ph ase beha vior of PF Oin CO2 was thoroughly investigated. These cloupoint experiments indicated lower critical solutiotemperature (LCST) phase behavior.

Fu rther st udy on MMA dispersion polymerizatiowith PFOA stabilization investigated the influenof helium concentra tion in CO2 on the resultingPMMA part icle sizes and distr ibution.83,84This studywas important since many tanks of CO2 ar e sold witha helium head pressure. I t was found that thpresence of 2.4 mol % helium in CO2 increased t hePMMA average particle diameters from 1.9 to 2.7m,

Table 2 . Summary of Dispers ion and Emulsion Po lymerizat ion S tud ie s in Liqu id and Supe rcr i t ical CarbonDioxide

monomer(s) stabilizers ref(s)acrylamide amide end-capped poly(hexafluoropropylene oxide) 562 ,6 -d imethylphenylene ox ide PFOA homopolymer, PFOA-based random copolymers ,

and PS- PFOA block copolymer60

d iviny lbenzene and e thylvinylbenzene fluor ina ted methacrylate-PMMA b lock copolymer 55methyl methacryla te PFOA homopolymer 34,81,84

PDMS-based macromonomer 90- 92fluor inated graft or block copolymers 94,176

styrene PFOA homopolymer; PS- PFOA block copolymers 95PDMS h omopolymer; PS- P DMS block copolym er s 96,177FVE-MVE block copolymer 135

vinyl aceta te PFOA and PDMS homopolymers, PDMS macromonomer ,PVAc- PDMS and PVAc- PFOA block copolymers

99,100

vin yl a cet at e a nd et hylen e P DMS h om opolym er ; P VAc- PFOA block copolymer 100

Figure 3 . Scann ing electron micrograph of PMMA paticles pr oduced by dispersion polymerization in CO2 usingPFOA as t he sta bilizer.



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while decreasing particle size distribution from 1.29to 1.03. Fur thermore, th e small percentage of heliumin the reaction mixture decreases the solvent strengthof the medium, as determined by solvatochromicinvestigations. Oth er scientists have noted the in-crease in retention times in SCF chromatography85- 87and t he reduced extra ction ra te in SCF extra ctions88when helium is present in the CO2. An acoustictechnique, used by Kordikowski and co-workers,

measured the composition of helium in CO2 tanksand showed the content to be unpredictable, presum-ably due to common lack of equilibration in thecylinder.89 Therefore, these diverse experimentsillustra te th at even seemingly benign component ssuch as an inert gas can effect the solvent quality of CO2, and the CO2 purity needs to be taken intoconsideration for process development and scale-upissues if the process, such as dispersion polymeriza-tion, is to be commercialized.

Another approach to pr eparing m onodispersePMMA particles in CO2 has been the use of graftcopolymers as stabilizers. While PFOA is an effectivesteric stabilizer for PMMA, fluoropolymers in generalare expensive. The other class of CO2-soluble poly-mers includes PDMS (12 ). PDMS-based su rfactant sare cheaper, but are less soluble than PFOA atcompa rable molecular weights in CO2. The first workin this area by DeSimone involved the use of amonomethacrylate-terminated PDMS macromono-mer (13 ), which reacts t o form th e stabilizer in situ .90However, these studies showed that only a smallportion of the a dded m acromonomer actua lly copo-lymerized; the m ajority of the u nreacted ma cromono-mer could be removed from the surface of th epar ticles by extra ction with h exane or CO2. Polymer-izations were conducted at both liquid and super-critical conditions, and by varying the reaction con-dit ions, part icles with a narrow particle s izedistribution were obtained in sizes ranging from 1.1to 5.8m. In contrast , PDMS homopolymer gave lowyield and unstable latexes, presumably due to itsinability to anchor and its lack of affinity for thePMMA particles. The dispersion polymerization of MMA stabilized by PDMS- monometha crylate h asrecently been investigated in detail by ONeill andco-workers, who stu died the par ticle form at ion91 andgrowth92 for this system using tu rbidimetry. In t heparticle formation regime, mechanisms of coagulativenucleation and controlled coagulation were identified.To prevent uncontrolled coagulation (precipitation),

a threshold pressure (207 bar) and stabilizer concen-tration (2 wt % based on monomer) were required.Latex stability was limited by the low solubility of surfactan t in t he a bsence of monomer in CO2. Thechronology of yield, part icle size, morphology, par ticlenum ber density, and molecular weight wa s studied.The results agreed with the experiments by Shaffer90and with predictions from the model of Paine93 forconventional dispersion polymerizations.

A study by Lepilleur and Beckman employed apreformed graft copolymer dispersa nt which h ad a nanchoring backbone of poly(MMA-co-hydroxyethylmethacrylate) and CO2-soluble poly(perfluoropro-pylene oxide) side chains (3 ).94 Variations in the

molecular weight and architecture of the stabilizwere explored by examining the effect of factors suas graft chain length and graft density on thresu lting P MMA polymer colloid. At a given ba ckbolength, as the graft chain length was chan ged, thtrends in results varied, depending on the length the backbone. For a given gra ft chain length, increaing the graft density (number of grafts on thbackbone) resulted in bett er st abilization. At a co

stant backbone length, increasing the graft densiresulted in r educed particle size an d distribut ion, did more, shorter grafts compared to fewer, longgrafts. As the backbone length was increased, thpar ticle size and distr ibution decreased. It wa s showthat the backbone length must be long enough anchor to the particles, but there must be enougsoluble component to maintain the overall solubiliof the surfactan t in CO2 for a successful dispersionpolymerization.

Indeed, the dispersion polymerization of MMA compressed CO2 has been investigated in detail.Homopolymers, ra ndom copolymers, macromonmers, and graft copolymers have all been effectiveemployed as st abilizers to preven t coagu lation in thPMMA/CO2 system.

The dispersion polymerizat ion of styrene in su pecritical CO2 using amphiph ilic diblock copolymers h aalso been studied in detail. The surfactant s whicwere investigated contained a PS anchoring blocand either a PFOA95 or PDMS (14 )96 soluble block.These reactions yield spherical PS particles whicwere isolated in the form of a dry, free-flowinpowder. The resulting high yield (> 90%) of PS wasobtained in the form of a stable polymer colloicomprised of submicron-sized particles. For the blocopolymer ic sta bilizers , the a nchor-to-soluble balan(ASB), or ratio of the two block lengths, of thstabilizer was foun d t o be a crucial factor affectinboth the stability of the resulting latex in CO2 andthe particle morphology. The affinity of these amphiphilic diblock copolymers for the PS particsurface was confirmed by interfacial tension mesurements in a CO2 continuous phase.97

In the studies of PFOA-based surfactants, theffects of sur facta nt block cha in lengths, sta bilizconcentration, and anchor block length on PS produwere stu died.95 A series of PS-b-PFOA copolymerswere stu died in which th e length of both blocks wincreased. Increasing the anchor and soluble bloclengths resu lted in decreased pa rt icle size (from 0. m to 0.24m) an d pa rticle size distribut ion (from8.3 to 1.1), pr esuma bly due to more effective sta blization of PS particles. With a constan t ancholength of 3.7 103 g/mol PS, var ying the PF OA blockfrom 1.4 104 to 2.5 104 g/mol did not produceany trends in PS molecular weight or particle sizIn this series, the anchor block may have been tshort for st rong adsorption to th e par ticles, reducithe efficiency of stabilization. The effect of varyinthe anchor length at constant soluble block lengwas not studied with PS-b-PFOA. The PF OA homo-polymer wa s not an efficient stabilizer for P S, resuing in low polymer yields of 44%. In contr ast , PFOhomopolymer was an efficient stabilizer for PMM



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as a lready illust ra ted. The specific affinity tha t P FOAhas for P MMA is not extended t o PS. However, PF OAhomopolymer with unst able thiura m endgroups wasshown to stabilize micron-sized PS particles viachemical grafting.98

PS-b-PDMS polymeric stabilizers were stud ied todetermine the effects of anchor block length andsoluble block length, CO2 pressure, an d concentra -tions of surfactant, monomer, and initiator.96 Incompa ring two sta bilizers with a P DMS block lengthof 2.5 104 g/mol, the longer PS an chor block lengthyielded par ticles with a la rger diameter (0.22 vs 0.46 m) with a na rrower pa rticle size distribution (1.31vs 1.08). The sta bilizers with a longer PDMS of 6.5 104 g/mol resulted in collapsed latexes and coagu-lated particles. The authors speculate that this maybe due to the lower ratio of anchor-to-soluble blockor due to lower solubility of th e sur factan t in CO2. Akinetic study of the dispersion polymerization showedthat the molecular weight and the conversion in-crease as a function of time. As expected, both theconcentration of monomer and the concentration of

stabilizer affected the morphology of the resu lting PSparticles. Additionally, the temperature and pressureof the reaction mixture were foun d t o effect results,such as average particle diameter and molecularweight, of the PS product. The importance of polymercollection procedures was shown by the aggregationof PS particles collected from the reactor aftercomplete d epressur izat ion. In contra st, t he polymerpart icles released from the r eactor du ring the depres-surization did not show any signs of flocculation inthe SEM images. It was noted that none of the PS-b-PDMS copolymers were soluble in CO2 in theabsence of monomer. PDMS homopolymer was notcapable of sta bilizing styren e dispersion polymer iza-tions in CO2, as yield and molecular weights of PSwere compar able to reactions in which no sur factan twas used.

The pr epar at ion of sta ble dispersions of poly(vinylacetate) (PVAc) and ethylene- vinyl acetate copoly-mers in liquid and supercritical CO2 has recentlybeen investigated.99,100Both fluorinated a nd siloxane-based sta bilizers includin g homopolymers , block co-polymers, and reactive macromonomers were em-ployed. The influence of the concentration of stabilizer,stabilizer anchor-soluble balance, and pressur e on theresu lting colloidal product wa s explored. In addit ion,turbidimetry was used successfully to monitor dis-

persed phase volume fractions, particle sizes, andnumber densities during the polymerization. Thevinyl acetate polymerizations stabilized by PDMShomopolymer, vinyl-terminat ed PDMS macromono-mer (15 ), or PVAc-b-PDMS (16 ) all produced col-lapsed latexes of high yield and high molecularweight polymer, wherea s t he polymerizations sta bi-lized by PVAc-b-PFOA (4 ) remained stable latexes.Turbidity showed that P FOA and PVAc-b-PFOA witha sh ort an choring group (PVAcM n ) 4 103 g/mol)had inefficient anchoring to the PVAc particles. ThePVAc-b-PFOA with the longest blocks (PVAcM n )3.1 104 g/mol; PFOAM n ) 5.4 104 g/mol)produced the smallest diameter polymer particles.

Cooper and coauthors recently reported the firsynthesis of well-defined cross-linked polymer micrspheres in supercritical CO2.55 Divinylbenzene an dethylvinylbenzene were copolymerized at 65 C an310 ( 15 bar u sing AIBN as a n initiat or. The r eactiowas performed both in the presence and in thabsence of a surfactant . Without surfactant , 1.5- 5 m part icles were isolated. When t he polymer izatioemployed a 3 wt % (based on monomer) of a blo

copolymer su rfactan t cont aining PMMA and a fluonated methacrylate (5), much smaller particles (e 0.41 m) with a relatively narrow particle size distributiowere observed. Yields greater than 90% were otained both with and without surfactant. This studemonst ra ted the form at ion of various sizes of croslinked microspheres in su percritical CO2.

Adamsky an d Beckma n h ave explored the possibity of car rying out an inverse emu lsion polymeriztion of acrylam ide in super critical CO2.56,101In thesereactions, acrylamide was polymerized in the preence of water , a cosolvent for t he monomer, in a C2continuous pha se at 345 bar an d 60 C with AIBinitiation. Reactions were condu cted both with anwithout the stabilizer, a n amide end-capped pol(hexafluoropropylene oxide) (6). In t he absence of thestabilizer, the precipitation polymerization of acryamide resulted in a high conversion of high moleculweight polymer which formed a single solid mass the bottom of the r eaction vessel. In th e presencestabilizer, the reaction solution was reported to haa milky-white appeara nce tha t was indicat ive of latformation; however, the conversion and moleculweights were comparable to those obtained in thun stabilized r eactions.

More studies in the field of inverse emulsiopolymerizations in CO2 are expected with t he recent

discoveries of surfactants which have the ability form microemulsions of water and water solubmolecules in CO2. In 1993, DeSimone reported theuse of th e macromonomer technique to synth esiand characterize an amphiphilic graft copolymer wia CO2-philic PFOA backbone an d hydrophilic pol(ethylene oxide) (PEO) grafts (8 ).102 In this study,solvatochromic chara cterization was employed demonstrate that the PEO grafts enabled the solubity of the hydrophilic, CO2-insoluble dye methylorange in su percritical CO2. This graft copolymer wa sfurth er chara cterized by small-angle X-ra y scat terin(SAXS) and shown to form spherical micelles in tpresen ce of wat er in a CO2 cont inuous pha se.103 This

work represented the first direct confirmation thmicelles can form in a CO2 cont inuous ph ase. Beck-man an d co-workers ha ve studied the pha se behaviof both silicone-based an d fluoro eth er-fun ctionamphiphiles in supercritical CO2.12,104A fluoro ether -functional amphiphile (7 ) was shown to permit theextra ction of th ymol blue from aqueous solut ion inCO2. The effect of pressur e on the em ulsion beha viof fluoro ethers terminated with sorbitol estersulfates, and sulfona tes in m ixtures of CO2 and waterwere investigated.105 Another advance in this areawas made when Johnston and co-workers demostra ted t he formation of a one-pha se microemulsiconsisting of the hybrid fluorocarbon/hydrocarbo



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surfactant C7F15CH(OSO3- Na+ )C7H15 and water inCO2.106 In th is work, th e water-to-surfactan t ra tio ina single-phase microemulsion was as high as 32 at25 C and 231 bar . It was shown th at with 1.9 wt %surfactant, 2 wt % water was solubilized in CO2,which is 10 times the a mount of water soluble in pureCO2. In more recent work, Johnston demonstratedthe formation of aqueous microemulsion droplets ina CO2 continuous phase using an ammonium car-

boxylate perfluoropolyether surfactant, [(OCF2CF -(CF3))n(OCF2)m ]OCF2COO- NH4+ .107 Several spectro-scopic techniques were emp loyed to investigate th eproperties of these aqueous microemulsions. Theseapproaches to the formation of microemulsions innonpolar super critical fluids ha s been th e focus of tworecent r eviews.108,109Since evidence was shown in thestudies by DeSimone, Beckman , an d J ohnston thattheir su rfacta nts form microemulsions of water an d,in some cases wa ter soluble molecules, in CO2, thesesurfactan ts could potent ially be used t o form m icro-emulsions of wat er a nd a water soluble monomer inCO2 for inverse emulsion polymerizations.

The fields of dispersion and emulsion polymeriza-tions h ave been developed over th e past 30 years tobecome convenient methods for the preparation of spherical polymer p art icles. Advances in th e designan d synt hesis of am phiph ilic block and gra ft copoly-mers for use as stabilizers in supercritical fluids hasallowed tremendous latitude in the composition of thepolymeric microspheres. Breakth roughs in the designand synt hesis of nonionic surfactant s tha t ar e inter-facially active in a CO2 medium h ave been pivotal tothe development of th e dispersion and emulsionpolymerizations technique in th is fluid.

4. Polymer Blend Synthesis

The p lasticizat ion of polymers by su percritical CO2has been exploited for the synthesis of polymerblends. In general, CO2 is used to swell a CO2-insoluble polymer substrate and to infuse a CO2-soluble monomer and initiator into the substrate.Subsequent polymerization of the monomer gener-ates t he polymer blend. Watson a nd McCar th y usedsupercritical CO2 to swell solid polymer substr at es,including poly(chlorotrifluoroethylene), poly(4-meth-yl-1-penten e), high-density polyethylene (HDPE),nylon-6,6, poly(oxymeth ylene), and bisphen ol A car -bonat e, and to infuse styrene m onomer an d a free-ra dical initia tor into the swollen polymer.35,36,110Th epolymerization reaction was run either before de-

compression of the CO2 or after decompression andrecompression with N2. Mass upt akes of up to 118%based on the original polymer were reported. Asexpected, diffusion rates in CO2-swollen polymerswere dramatically increased over nonswollen poly-mers . Using ethylbenzene as a model for styrene, theequilibrium for monomer uptake was 25 times fasterwith CO2 und er the conditions stu died than with outCO2. X-ray analysis showed that the polystyreneexisted in phase-segregated regions throughout thematr ix polymer, an d th ermal a nalysis showed thatradical grafting r eactions were n ot significan t.

In a relat ed study, styrene (with an d without cross-linker) was infused into three fluoropolymer sub-

strates, polymerized, and subsequently sulfonated order t o provide surface modification to the polymesubstrates.111 Polystyrene was shown to be presenthroughout the blends and semi-interpenetratinnetworks of each of the three polymer substratePTFE , poly(TFE-co-hexafluoropropylene), an d poly-(chlorotrifluoroethylene). The modified surfaces wecharacterized by X-ray photoelectron spectroscopand by water contact angle measurements. Th

wettability of all modified fluoropolymer substratwas increased by t he surface modificat ion.In a separate study, the morphology and mecha

ical per forman ce of PS/HDPE composites were idetified.112 As in the previous experimen ts, th e polymesubstrate, H DPE, was swollen with CO2 in thepresence of styrene and initiator. The subsequenpolymerization produced the blend, with PS locatin th e am orphous interlam ellar regions of the polethylene spherulite and in th e spherulite centers. TPS/HDPE blends exhibited a modulus enh an cemenstrength improvement with increasing PS contenloss in fracture toughness, a nd increase in brittlness.

B. Cationic PolymerizationsCationic polymerizat ions represent a challengin

field in polymer science, and their extension tsupercritical fluids has been equally difficult. Thhigh reactivity of carbocations results in fast polmerizat ion rea ctions, but a lso leads to unwan ted sireactions such as chain transfer and terminatioThese side r eactions limit th e u sefulness of cationpolymerizations. The inherent basicity of monomethat ar e capa ble of being polymer ized cat ionically athe acidity of the protons to the car bocation on t hepolymer ma ke proton abstr action by th e monomer

built-in side reaction that is difficult to suppresThese side reactions are often reduced by lowerinthe reaction temper at ur e. Upon cooling, because sireactions typically have higher activation energiethe propagation rate decreases less relative t o thsecondary reactions, and a higher degree of polmerization is achieved.

Living cat ionic polymerizat ion meth ods have beedeveloped to produce well-defined polymers.113 Theseliving methods allow for control of molecular weigmolecular weight distribution, end group functionaity, polymer microstructur e, and reactivity. They alpermit the synthesis of block copolymers of preciblock length and compositions. Polymerization co

trol is gained in living cationic systems by stabiliztion of the active carbocation through nucleophilinteractions.114 This stabilizat ion is generally achievedby association with a suitable nucleophilic counterior, if the counterion is only weakly nucleophilic, bassociation with an added Lewis base. Both methoreduce the positive charge on the carbocation anthe acidity of the-hydrogens, which results inessentially no chain tran sfer to monomer. The ncleophilic interaction between th e counter ion and tcarbocation is key in stabilizing the polymerizatioand prevent ing side reactions. Winstein developed ion pair spectrum to describe the active site icationic polymerizations.115 Classical nonliving car-



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bocationic polymerization is found to have solvent-separated ion pairs, while living systems have contaction pairs. Solvent choice plays a n importa nt role incationic polymerizations because it affects the equi-librium between contact pairs and solvent-separatedion pairs an d t he a ctivation energy of tran sfer a ndterm inat ion r eactions. Nonpolar solvents a re gener-ally desirable for cationic polymerizations becau sethey suppress ion separation.

Becau se of the t un ability of the solvent propertiessuch as dielectric constant, supercritical CO2 shouldmake for an interesting medium for stu dying cat ionicreactions. One disadvan ta ge is th e critical temper a-tur e of CO2 (31.1 C). Since cationic polymerizationsare usually performed at low temperatures (often- 70 to - 30 C) to diminish side rea ctions, cationicpolymerizations in supercritical CO2 are inherentlyproblemat ic. In fact, mu ch of th e ear ly experiment susing CO2 as a solvent for cationic reactions wereperformed in liquid CO2 at low temperatures. How-ever, it has been shown that good results can beobtained in liquid and supercritical CO2. Further,CO2 ha s been sh own t o be inert to cationic polymer-ization conditions.

The earliest work in cationic polymerizations inCO2 was aimed a t preparing industrially importan thydrocar bon polymers in CO2. These initial experi-ment s u tilized chain-growth polymerization mecha-nisms to produce polymers which were relativelyinsoluble in th e CO2 continuous phase at the reactionconditions employed. While these early experimentsoften resu lted in a low yield of low molecula r weightproducts, this work was funda menta l in demonstra t-ing th e compa tibility of cat ionic cha in-growth m ech-anisms with CO2.

A 1960 report by Biddulph a nd P lesch explored theheterogeneous polymerization of isobutylene in liquidCO2 at - 50 C.116 Two cata lyst systems were shownto be effective: AlBr3 an d TiCl4 (using ethyl bromideand isopropyl chloride, respectively, as cosolvents).The AlBr3-catalyzed rea ctions proceeded very fast ,but were incomplete a nd gave molecular weight s of about 5 105 g/mol. The low conversion was at-tributed to catalyst becoming embedded in the whitepolymer precipitate. The TiCl4 reaction was slower,but proceeded to completion and gave m olecularweight s of about 3 104 g/mol.

A 1970 patent covering the precipitation polymer-izat ion of vinyl compounds in liquid CO2 included thepolymerization of ethyl vinyl ether at room temper-ature.68 This heterogeneous reaction was catalyzedby either SnCl4 or BF3OE t2. A yield of 57% wasreported, but no other characterization was given.

The first systemat ic study of cat ionic polymeriza-tions in compressed liquid was a series of papers inthe late 1960s, report ing the p recipitation polymer-ization of formaldehyde in liquid and supercriticalCO2. A carboxylic acid, such as acetic or trifluoro-acetic acid, was added to catalyze the polymeriza-tion.117- 119 The polymerizations were performed at20- 50 C and gave conversions of 50- 60%. Byinfrared spectroscopy, it was shown th at CO2 was notbeing incorpora ted into th e polymer backbone. This

spectroscopic measurement confirmed that CO2 is

inert to th e propagating cationic species. It was alnoted tha t some polymer was produced in the absenof added catalyst. The authors speculated that aimpurity was causing th e polymerization in thabsence of added catalyst because the degree polymerization increased linearly with conversion. 1969, the authors elucidated their impurity as formacid (formed from t he r eaction of form aldeh yde wiwater) by careful control of monomer synthesis either r epress or increase a cid forma tion.120

Kennedy, building on earlier work by P lesch,116reported the polymerization of isobutylene (IB) supercritical CO2 using 2-(2,4,4-tr imethylpentyl)chloride (TMPCl) as an initiator and a Lewis ac

catalyst such as BCl3, TiCl4, or SnCl4 as the co-initiator.121 Polymerizat ions wer e condu cted at 32.5-36 C an d 75- 135 bar. Methyl chloride was addedas a cosolvent (3%) (presum ably to solubilize the ionspecies), and its presence gave higher conversioand narrower polydispersity indices (PDI) M w / M n).Conversions of up to 30- 35% and polymers withM nof 1 103 to 2.5 103 g/mol and PDIs of 1.5- 3.1were produced. Because1H NMR results showedsignifican t am ounts of unsaturated end groups, chatransfer to monomer likely limited molecular weighas expected from such a high reaction temperaturThe mixed initiating system of TMPCl/(TiCl4 /BCl3)was used to form well-defined polyisobutylene wi

term inal chloride (- Clt) end groups (see Scheme 1).

122

Reaction conditions similar to those used previouswere employed: 32.5 C, 140 bar CO2, and 5- 10%MeCl. After 3 h, conversions of 40- 45%, molecula rweight s of 1.8 103 to 2.4 103 g/mol, and relat ivelylow PDIs of 1.3- 1.5 were obta ined. The same experiments performed in hexane gave essentially npolymer and a conversion of only about 0.5%. Thualthough low molecular weight material was prduced in CO2, the molecular weights were higherthan in hexane.1H NMR spectr a sh owed no evidencof olefinic end groups (in contrast to the TiCl4 orSnCl4 initiated polymers). Dehydrochlorination folowed by1H NMR spectroscopy determined each

polymer chain wa s term inat ed by a chloride, indicaing an absence of chain-tran sfer side reactions.Kennedy studied the temperature effects of I

polymerization in CO2.123 Over t he r ange of 32- 48C, conversion dropped from 40% to 4% and thmolecular weights dropped from 2 103 to 7 102g/mol. In addition, although polymers syn th esized32 C had greater than 99% term inal chloride grouptha t value fell to 60% for 38 C an d 44% for 48 consistent with a higher p robability of chain tr an sfto monomer at the higher temperatures. For threaction, a ceiling temper at ur e of 88( 9 C, calcu-lated by linear extrapolation of molecular weight 56 g/mol (mass of IB) as a function of temper atu r

S c h e m e 1 . S y n t h e s is o f - Cl-Terminate d P IB



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was consistent with those determined for conven-tiona l solvent systems.124- 128

An isobutylene- styrene copolymer was also syn-thesized in CO2 using MeCl as a cosolvent and theTMPCl/TiCl4 initiating system.129 A conversion of 15% was pr oduced, but m olecular weight s an d PDIswere not reported. The au thors only noted th at longerreaction times produced higher molecular weights.

In the synthesis of phenol-terminat ed polyiso-butylene, the first example of electrophilic aromaticalkylation in supercritical CO2 was reported.130 TMPCl

and polyisobutylene-

Clt

( M n)

2000 g/mol) werealkylated by phenol at 32.5 C and 140 bar in thepresence of BF3OE t2 for 24 h. Yields of 75% an d 60%respectively were observed.

The isomerization polymerization of 3-meth yl-1-buten e and 4-methyl-1-pentene has also been stud iedin supercritical CO2.131,132 The reactions were per-formed at 140 bar CO2 and 32.5 C with residualwater and AlCl3 as the catalyst system. As with theIB polymerization, a cosolvent (5% methyl chlorideor 10% ethyl chloride) was used to obtain goodresults. For 3-methyl-1-butene, a 40% conversion, amolecular weight of 1000 g/mol, a nd a PDI of 1.41were observed; for 4-methyl-1-pentene the results

were 70%, 1700 g/mol, and 2.16.DeSimone reported a study of cationic polymeri-zation of vinyl ethers in supercritical CO2.133,134Both precipitat ion and homogeneous polymerizationswere reported. The initiation system was based onHigash imur as living cationic polymerization methoddeveloped for hydrocarbon solvent s. This method usesthe Lewis acid ethyl aluminum dichloride an d t heacetic acid a dduct of isobut yl vinyl ether (IBVE) asthe initiator in the pr esence of a Lewis base deactiva-tor such as ethyl acetate (see Scheme 2).113 Th epolymerization of IBVE began homogeneously, butbecame heterogeneous as the polymer precipitated.Yields of polymer synthesized in CO2 were similar

to results obtained in cyclohexan e, but with broaderPDIs (for example, 1.2 for cyclohexane and 1.8 forCO2 at 40 C, 345 bar). At 60 C, the polydispersityof polymers produced in CO2 increased to greaterthan 9, indicating no molecular weight control,perha ps due to increased chain t ra nsfer to monomerand lower CO2 dens ity, which would allow for fasterprecipitation of polymer.

The homogeneous polymerization of 2-( N -propyl- N -perfluorooctylsulfonamido)ethyl vinyl ether (FVE)was also performed (see Scheme 3).133,134The poly-merizations were homogeneous throughout the reac-tion an d gave molecula r weight s of, for example, 4103 g/mol with a PDI of 1.6. The narrow PDIs

achieved with t he CO2-soluble fluorinated polymercompared to the br oad P DIs obtained with t he CO2-insoluble poly(IBVE) suggest tha t solubility of thresulting polymer plays an important role in detemining polydispersity in th ese cationic polymeriztions in su percritical CO2.

Ring-openin g polymerizat ion of cyclic ether s weinitiated by BF3 in liquid CO2 and compared toreactions performed in methylene chloride (see Schem4).134 Bis(ethoxymethyl)oxetan e (BEMO) was polymerized at- 10 C in CO2 (290 bar). As expected frominsolubility of the resulting polymer, the CO2 reactionwas heterogeneous, but the same reaction performin methylene chloride was homogeneous. The yielwere comparable (about 70%), but the PDI was 1for CH2Cl2 an d 2.7 for CO2. A fluorina ted cyclic eth er,3-methyl-3 -[(1,1-dihydroheptafluorobutoxy)methyl]oxeta ne (FOx-7) was polymerized homogeneously both CO2 (0 C an d 289 bar) an d Fr eon-113 (- 10 C).The polymer synthesized in Freon-113 had a moleular weight of 3.9 104 g/mol, a PDI of 1.7 and ayield of 70%; th e resu lts for t he polymer syn th esizin CO2 were 2.0 104 g/mol, 2.0 and 77% yield. Thu s,when homogeneous conditions were used, similresults were obtained for the BEMO polymerized methylene chloride and the FOx-7 polymerized either CO2 or Freon-113.

Initial attempts to perform the first cationic dipersion polymerization in CO2 were with BE MO andIB.134 The polymerizations were catalyzed by BF3THF (for BEMO) or SnCl4 (for IB) and were con-ducted in the presence of CO2-soluble surfactantssuch as poly(FOx-7), poly(FOA), and poly(styrenb-PDMS. In the absence of surfactant, the polymprecipitated. Yields and molecular weights were nimproved by the presence of surfactants, but thpolymer products were stable dispersed colloids CO2 which could be redispersed in Fr eon-113 afteremoval of CO2. Scann ing electron microscopy (SEM)which examined polymer following removal of CO2,detected slight part icle formation despite t he sem

Scheme 2 . Ca t ion i c Syn thes i s o f P( IBVE) inSupercr i t ica l CO 2

Scheme 3 . Syn thes i s o f Po ly (FVE)

Scheme 4 . R ing-Open ing Po lymer i za t ion o f Oxe tanes i n CO 2



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crystalline n at ur e of poly(BEMO) and lowT g of poly-(IB). H owever, since an in situ technique was notused to measure the particles, the absence of well-defined par ticles in SEM images does not m ean th eparticles were not stable and dispersed in the CO2.

To overcome the problems associated with SEManalysis of poly(BEMO) and poly(IB), a more conclu-sive example of cationic dispersion polymerizationsin CO2 was reported with styrene.135 First, a suita blesurfactant for polystyrene was synthesized in CO2,using th e vinyl ether system pr eviously reported.134A block copolymer was synthesized from FVE andmethyl vinyl ether (MVE) (9 ) using the E tAlCl2 /FVEacetic acid addu ct/EtOAc initiat ing system. The poly-(FVE) block serves as the soluble block for stericstabilization, and the poly(MVE) block serves as the

anchoring un it du e t o its miscibility with PS. Thesurfactan ts wer e employed in t he cationic polymer-izat ion of styrene initiated by TiCl4 at 330 bar CO2in the temperatur e ran ge of 0 to 25 C. The resultsof these reactions were qu ite sensitive to temperatureeffects. No improvement in yields or molecular weightsand no stable colloids were observed for reactionsperformed at 0 C. At 15 C, the presence of 4 wt %stabilizer leads to increased yields (from about 50%to 95- 97%), increased molecular weights, and de-creased PDIs. The reaction had a milky-white ap-pear an ce, indicative of a sta ble polymer colloid. SEManalysis showed well-defined PS particles with abroad distribution of sizes th at ran ged from several

hun dred na nometers to one micrometer in diameter.At 25 C, yields for the reaction in the presence orabsence of surfactan t a re lower th an at 15 C, andparticles appear coagulated by SEM for reactionsperformed in the presence of surfactant. The loweryields are probably due to higher rate of chaintransfer at the higher temperature.

C. Transition Metal-Catalyzed Polymerizations

Metal-catalyzed polymerizations have been per-formed in supercritical CO2. The ring-openingmetathesis polymerization (ROMP) of bicyclo[2.2.1]-

hept-2-ene (norbornene) in CO2 was catalyzed by [Ru-(H2O)6(tos)2] (tos ) p-toluenesulfonate) (see Scheme5).136,137The reaction was performed at 65 C withpressur es ra nging from 60 t o 345 bar. The insolublepolymer precipitated, and there was no obviouscorrelation between pressure and molecular weight(which ran ged between 104 and 105 g/mol), yield (30-76%), or PDI (2.0- 3.6).

The [Ru(H2O)6(tos)2] cata lyst is insoluble in CO2,but can be solubilized by th e a ddition of meth anol.When th e polymerization was performed with up t o16 wt % methanol as a cosolvent, th eM n and PDIwere in the same range as the polynorborneneproduced in t he a bsence of metha nol, but th e yields

increased with increasing meth anol content. Foexample, the rea ction with 16 wt % metha nol gavesimilar yield in 5 h as the reaction without methangave in 16 h. Therefore th e reaction was much fastin the presence of methanol. A profound effect opolymer microstructure was found with increasinmethanol content. The presence of methanol dcreased thecis -vinylene content in the resultingpolymer (83% cis for no met ha nol an d 33% cis for wt % meth an ol). Presu mably th e addition of a pol

cosolvent favors the trans-propagating species at tmetal center and allows for control of polymer micrstructure by control of cosolvent content. This hpothesis could be confirmed by observing the effeof other polar cosolvents on the polymer trancontent. H owever, these experiments have n ot beconducted by the authors.

Higher a ctivities for the ROMP of norbornene weobserved with ruthenium and molybdenum carbecata lysts reported by Grubbs138- 140 and Schrock,141respectively (see Figure 4).142 While th e Ru cata lystappeared insoluble in CO2, the Mo catalyst waspar tia lly soluble. These catalysts ga ve up to 94% yieof precipitated polynorbornene in CO2 (97% using

toluene as a cosolvent) and molecular weights in trange 105- 106 g/mol at much milder reaction conditions of 25- 45 C and a bout 100 bar. The Ru cata lysgave about 25% cis conten t with no appar ent depedence on dens ity while th e Mo cat alyst ga ve 66% content at a reaction density of 0.57 g/mL and 82cis at 0.72 g/mL. The ru thenium carbene catalyst walso used to polymerizecis -cycloocten e in 50% yieldand a molecular weight of 105 g/mol. The polymersproduced in CO2 were similar in molecular weightand microstructure to those produced by convent ionmeans in dichloromethane.

There have also been a number of reports polycar bona te synth esis from the copolymerizatio

of CO2 and epoxides. The precipitation copolymerzation of CO2 and propylene oxide in supercriticaCO2 has been reported using zinc(II) glutarate as heterogeneous catalyst143 (Scheme 6). The polycar-bona te, with a m olecular weight of about 104 g/mol,was formed at 60 or 85 C with both sub- ansupercritical pressures (21- 83 bar ). Polymerizationsperformed in CO2 above the critical pressur e had a nincreased percentage of carbonat e linkages r elativto the ether linkages (over 90% vs less than 75%Propylene carbonate was formed as a byproduct, aits pr oduction was increased with increasing temperature. Yields were generally around 10- 20%, butthe addition of acetonitrile or hexane as a cosolve

Scheme 5 . ROMP of Norbornene in Supe rc ri t i c a lCO 2

Figure 4 . Catalysts for ROMP of norbornene in supecritical CO2.



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decreased the polymer yield while increasing thefra ction of car bona te linka ges relative to eth er link-ages. In contrast, dichloromethane increases carbon-ate linkages with out significantly decreasing yield.

A phenoxide zinc cata lyst, [(2,6-diphenylphenoxide)2-

Zn(Et2O)2], produced polycarbonate from CO2 andcyclohexene oxide in CO2.144 The reaction was per-formed at 80 C and 55 ba r t o yield a polymer witha molecular weight of 3.8 105 g/mol an d only 9%ether linkages. The terpolymerization of propyleneoxide, cyclohexene oxide, an d CO2 gave a good yieldof polymer with about 20% propylene carbonatelinkages.

A CO2-soluble Zn catalyst was developed for copoly-mer izat ion of cyclohexene oxide and CO2 (see Scheme7).145 The catalyst was synthesized from ZnO and themono(1H,1H ,2H,2H-perfluorooctyl) ester of maleicacid and was soluble in CO2 over a wide pressureran ge an d u p t o 90 C. Above 90 C, an irr eversible

phase sepa rat ion occurred. The tur nover n umbers of up to 400 g of polymer/g of Zn obtained for thiscatalyst are among the highest reported for thecopolymer ization of epoxides a nd CO2.

D. Thermal Ring-Opening Polymerization

Ring-opening polymerizations represent an acces-sible r oute to organometa llic polymers from highlystrained precursors . Manners obtained a highmolecular weight poly(ferrocenylsilane) by the ther-ma l ring-opening polymerizat ion of a silicon-bridged[1]ferrocenophane in the melt.146,147This melt-pha se

route pr oduced h igh molecular weight polymer ( M w) 5.2 105 g/mol). Other synthetic methods, suchas condensat ion polymerization,148 do not pr oduce thepolymer in sufficiently high molecular weight forcommercial a pplications. Exten ding th eir work, th eauthors investigated this reaction in supercriticalCO2.149 Performing a ther mal ring-opening polymer-ization reaction in a solvent would normally requirehigh boiling compounds that are difficult to remove.However, using supercritical CO2 avoids this prob-lem. The thermal ring-opening polymerization of a[1]silaferroceneophane was performed in the presenceof supercritical CO2 at 207 bar an d at th ree differenttemperatures: 75 C, 100 C, and 130 C (see Scheme

8). The molecular weights obtained in CO2 were thesame order of magnitude as the solvent-free methobut the values were uniformly lower. The highemolecular weight sample, withM w of 2.87 105g/mol, was produced at 130 C. In addition to thlower molecular weights, the PDIs of the CO2 sampleswere broader (3.0- 5.1 vs 1.5). However, when theconcentration of monomer was increased from 50mg/60 mL CO2 to 2 g/60 mL for t he 130 C rea ctionthe molecular weight increased to 5.9 105 g/mol andthe P DI decreased to 2.0. Furt her experiments arequ ired to deter mine t he efficacy of performing threaction in su percritical CO

2.

III. Step-Growth Polymerizations

A. Melt-Phase Condensation Polymerizations

There are two features that make melt-phascondensation reactions performed in the presence supercritical CO2 advanta geous: easier processingand high molecular weight m ater ials. Many condesation polymerizations are performed in the mephase to produce high molecular weight materiwithout the need for organic solvents. A disadvanta

to th is route is th e high viscosity of the high m oleular weight polymer produced. Because CO2 is ca-pable of plasticizing t he polymer melt phase, increases the free volume of the melt an d lowers thmelt viscosity, which translates into a more easiprocessed ma terial. Because CO2 is a swelling agentthat is a nontoxic ambient gas, it is a particularattractive swelling agent for polymers, such as polesters and polycarbonates, whose end use includpackaging for food or beverage applications.

In addition to greater processability, polymersynth esized in supercritical CO2 can achieve highermolecular weights. In conden sat ion polymerizationthe reaction is driven by the removal of a sma

molecule condensa te. En hancement of the condensaremoval results in higher reaction rates and highmolecular weights. Conventional m ethods u se higvacuum t o remove the condensat e,150 but th is methodrequires high capital costs and n ecessitates ma intnance to remain operational for long periods of timon a commercial scale. Because CO2 can highlyplasticize the polymer a nd solubilize th e sma ll moecule condensat e, it can ser ve to assist in t he rem ovof condensate by carrying it out of the reactor. Thincrease in free volume a s a result of th e plasticiztion should provide more polymer surface area fcondensate removal and also result in greater mobity of chain ends to allow better reaction kinetic

Scheme 6 . Scheme fo r Copo lymer i za t i on o f Propy lene Oxide and CO 2

Scheme 7 . Syn thes i s o f CO 2 -Soluble Zinc Cata lystfor Cyclohexene Oxide/CO 2 Copolymeriza t ion

Scheme 8 . R ing-Open ing Po lymer i za t ion o f Si l icon-Bridged [1]Ferroceneophane inSupercr i t ica l CO 2



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This stra tegy has been employed in th e synthesis of polycarbonat es, polyesters, and polyamides.

There are two industrially important routes topolycar bona tes: interfacial reactions in met hylenechloride using phosgene, and melt tr an sesterificat ionof bisphenol and diphenyl carbonate.151 The latteravoids the u se of phosgene and methylene chloride,152

but the high viscosity of the melt limits the molecularweight attained.151 In fact, th e cha in st iffness, whichadds to the commercial value of polycarbonates,causes t he h igh viscosity.153 The utility of supercriti-cal CO2 in producing high m olecular weight polycar -bonates by melt polymerization is 2-fold: CO2 solu-bilizes the ph enol (about 12 wt % at 272 bar a nd 100C)154 to extract t he byproduct, driving th e reactionto higher conversion, and plast icizes the polycar bon-ate155 to lower its viscosity, facilitating the process-ing.151

Odell studied the melt polymerization of bisphenols(such as bisphenol A, bisphenol P, bisphen ol AF, and

bisphenol Z) with diphenyl carbonate in CO2 (seeScheme 9).151,156The reactor was first heated t o 70C to melt the r eactant s. The system was t hen filledwith CO2 and heated to the target temperature.Temperatures between 180 and 250 C and pressuresof 207- 241 bar wer e used to obtain n umber a veragemolecular weights ran ging from 2.2 103 to 1.1 104 g/mol ( M w ) 4.5 103 to 2.7 104 g/mol). Whilemolecular weight increased with increasing tem per-ature, the temperature and pressure parameterswere selected to provide sufficient extraction of phenol condensate while minimizing removal of diphenyl carbonate starting material. The plasticiza-tion of the polymer by CO2 allows for much easier

s t irr ing versus the vacuum system. Like in thevacuum system , the ra te did not depend on th e choiceof bisphenol used. In the vacuum system, tempera-ture is used to lower melt viscosity and drive thereaction, but in th e CO2 case the high temperatureis used to increase solubility of byproducts withoutextracting the reactants and drive the reaction. Inaddition, to improve polymer molecular weight, adispersant was employed.151 (Polycar bonat e A)-b-poly(dimeth ylsiloxan e) was used t o produce a micro-cellular foam in the synthesis of polycarbonate A inCO2. The authors speculated that a dispersed poly-mer may allow for more effective removal of phenolbecau se of higher polymer su rface area.

Polycarbonate synthesis from bisphenol A andiphenyl carbonate catalyzed by tetraph enylphophonium tetraphenyl borate was performed in thpresen ce of supercritical CO2 by DeSimone.157,158Th eauthors noted that although diphenyl carbonate soluble in CO2, only a slight excess (1.005 equivwas required for the reaction because, once a degrof polymerization of two is achieved, removal diphenyl carbona te is not significant. The system w

heated to 150 C to melt the reactants and theheated at 160 C under a slow flow of argon, followby pressurization with CO2 an d hea ting to 270 C for1 h. High m olecular weight polymer (up to 1.3 104g/mol) was a chieved at 270 C and 296 bar CO2. Thehigh solubility of phenol in CO2 allows for its efficientremoval, and its recovery was used t o monitor threaction progress.

Beckman showed that exposure of thin films bisphenol A polycarbonat e t o CO2 at 50 C to 87 Cand up to 600 bar for 12 h, resulted in crystallinpolymer.155 The crystallinity was observed as anendotherm at a bout 210- 230 C in th e hea ting cycleof th e differen tia l scan ning calorimet ry (DSC) of tsample. Usually the crystallization of polycarbonais indu ced by organic solvents. DeSimone obtainsimilar CO2-induced crystallization results on amorphous polycarbonate chips157 and demonstra ted thatthe CO2-crystallized chips could be chain extendeto high molecular weight using solid-state polymeization methods. The use of CO2 could potentiallyallow the molecular weight of polycarbonate to bincreased by solid-state polymerization in supecritical CO2.

Melt-phase polycondensation reactions are commonly used to prepare poly(ethylene terephthalat(PET). PET is a n importan t plastic that sees widspread use in fiber, film, and food packaging applictions for mat erials. Bis(hydroxyethyl) terepht hala(BHET) was converted to poly(ethylene terephthlate) (PE T) using an Sb2O3 cata lyst and temperatu resof 250- 280 C under a va riable flow (2- 10 mL/min)of CO2 at 207 bar.157,159Molecular weights producedun der t hese conditions va ried from 3 103 to 6.3103 g/mol. Polymer molecular weight, determinefrom intrinsic viscosity measurements, increasesignificantly with flow rate and/or reaction timHowever, th e m olecular weights reported here aless than those normally produced by vacuu m m ephase polymerization (about 2 104 g/mol).160 Be-cause the ethylene glycol condensa te is soluble in C2

up to 2-

3 wt %, it is expected that the condensatcould be effectively r emoved from t he swollen pomer product to result in a higher molecular weighHowever, the CO2 solubility of ethylene glycol islower than that of phenol (vide supra) and mapartially account for the lower effectiveness of spercritical CO2 in PET synthesis compared to polycarbonate synthesis.

Polyamides have also been synthesized in the mephase in the presence of supercritical CO2.157,158Because primary amines react with CO2 to formcarbamates, the nylon salt route was used (seScheme 10). A 1:1 salt of hexameth ylenediamine aadipic acid was h eated a t 220 C for 2 h a nd t hen

Scheme 9 . Syn thes i s o f Po lyca rbona te f romBispheno l A and Diphen y l Ca rbona te i n t hePre sence o f Supe rc r i t i c a l CO 2



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280 C for 3 h in the presence of 207 bar CO2,producing a molecular weight of up to 2.45 104g/mol of nylon 6,6. Int erestingly, the melting pointof th e salt in th e presence of CO2 was depr essed from190 to 150 C. This melting point depression isbeneficial because the polymerizations can be run atlower temper

polymerizations in sc-co2

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