Synthesis, Characterization, and Solubility in Supercritical Carbon Dioxide of Hyperbranched...

Synthesis, Characterization, and Solubility in Supercritical CarbonDioxide of Hyperbranched CopolyestersJacek Gregorowicz,‡ Edyta P. Wawrzynska,† Paweł G. Parzuchowski,*,† Zbigniew Fras,‡ Gabriel Rokicki,†

Kamil Wojciechowski,† Stefan A. Wieczorek,‡ Agnieszka Wisniewska,‡ Andrzej Plichta,†

Kazimierz Dąbrowski,† and Mariusz Tryznowski†

†Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland‡Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland

*S Supporting Information

ABSTRACT: Hyperbranched polymers (HBPs) have been knownand extensively investigated for over two decades. However, there arestill areas that need to be explored. Recently, much effort has beenplaced in developing drug delivery systems based on macromoleculesof three-dimensional structure. This paper describes the synthesis ofrandom hyperbranched copolyesters of 2,2-bis(hydroxymethyl)-propionic acid (bis-MPA) and ε-caprolactone (ε-CL) and their phasebehavior in supercritical carbon dioxide (scCO2) after modification with fluorinated anhydrides or chlorotrimethylsilane. Thestructure of the copolyesters was investigated by means of 1H and 13C NMR, FTIR spectroscopies, and MALDI-TOFspectrometry. The influence of the content of chain extender units of 6-hydroxyhexanoic acid (formed from ε-CL) and terminalgroup functionalities on the solubility of hyperbranched polymers was investigated. The phase behavior of the polymers insupercritical carbon dioxide was explored as a function of concentration and temperature. It was shown that introduction of smallamounts (5−10%) of chain extender units into the dense structure of branched poly(bis-MPA) facilitates solubility of a polymerin scCO2. Viscosity and contact angle measurements were performed for the synthesized polymers.

■ INTRODUCTION

Hyperbranched polymers (HBPs) have been known andextensively investigated for over two decades.1−5 However,there are still areas that need to be explored. HBPs exhibitunique chemical and physical properties. In contrast todendrimers, such polymers can be easily prepared in a one-pot procedure. The properties of hyperbranched polymers areoften affected by the nature of the backbone and the chain-endfunctional groups, degree of branching, chain length betweenbranching points, and the molecular weight distribution.Hyperbranched polymers can be easily modified to tailortheir properties for a specific purpose. Recently, much effort hasbeen placed in developing drug delivery systems based onmacromolecules of three-dimensional structure.6 One of theinvestigated topics is introduction of drug molecules into thepolymeric matrix. For this purpose an easily available, cheap,and biocompatible solventsupercritical carbon dioxide(scCO2)can be applied. Over the past 30 years there hasbeen intense interest in the use of scCO2 for laboratory andindustrial applications. Supercritical carbon dioxide is believedto be a good choice for polymer synthesis7,8 and processing.9

Unfortunately, carbon dioxide’s solvent power is low, especiallyfor polar and high molecular weight polymers. Design andsynthesis of CO2-soluble surfactants, ligands and phase transferagents demand a broad knowledge on solubility of polymers indense carbon dioxide. Identification of highly CO2-solublepolymers has been a subject of intense research for the past 20

years.10−15 In our previous paper,16 a more detailed descriptionof results on phase behavior of polymers in CO2 solutionspublished in the open literature up to 2009 was presented. Werefer a reader to this publication for more information on thesubject. In the recent years a number of research projects havebeen undertaken to explore the possibility to synthetize CO2-philic material based on poly(vinyl acetate) (PVAc). PVAc hasbeen recognized as one of the most CO2-philic hydrocarbonmaterial, and it was expected that their modifications wouldmake it possible to obtain inexpensive hydrocarbon material asCO2-philic as fluoro- and silicon-based polymers. Tan et al.17

have examined solubility of PVAc in CO2 by a high throughputgravimetric extraction (HTGE) screening method anddetermined cloud-point pressures for this system using avariable volume view cell (VVVC). They have shown that thesolubility of PVAc strongly depends on the molecular weightand end-groups functionalities. Recently, a random copolymerof vinyl acetate and vinyl butyrate (PVAc−PVB) and acopolymer of vinyl acetate and dibutyl maleate (PVAc−PDBM) have been shown to exhibit higher solubility thanPVAc.18,19 Girard et al. have also examined the structure−property relationship between PVAc-based copolymers andtheir solubility in supercritical CO2.

20,21

Received: May 24, 2013Revised: August 23, 2013Published: September 9, 2013

Article

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© 2013 American Chemical Society 7180 dx.doi.org/10.1021/ma401086m | Macromolecules 2013, 46, 7180−7195

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Our research group has undertaken efforts to explain therelationships between the structure and properties of hyper-branched polymers and their solubility in scCO2. The firstextensive experimental studies on hyperbranched polyestersand polyethers showed that the nature of the end-groups is animportant factor that influences phase transition parameters.16

The results have also shown that the nature of the interior ofthe hyperbranched macromolecules is also significant. Thegeneral observation was that hyperbranched polyesters dissolveat lower pressures than hyperbranched polyethers.16 Otherpolyesters−aliphatic hyperbranched polycarbonates modifiedwith fluorinated end-groups showed only moderate solubility inscCO2. It seemed that enhancement in solubility of thehyperbranched polymer in comparison to the linear poly-carbonates was caused mainly by the difference in the numberof the fluorinated end-groups.22

To our knowledge, there have not yet been any studiesconducted on the phase behavior of hyperbranched polymers inscCO2 with architectural variation and the effect of architecturalvariation on their solubility. In the present paper, hyper-branched poly(2,2-bis(hydroxymethyl)propionic acid)s, de-signed to incorporate the different lengths of the linearhomologous oligo(6-hydroxyhexanoate) [oligo(6-HH)] back-bone segments, were synthesized using ε-caprolactone as aprecursor of 6-HH units. The idea of incorporation of linearsegments of oligo(6-HH) postulated that increasing thedistances between branching points would make the structureof the HBP less dense and better accessible for the solvent(Figure 1).To date, the most popular and commercially available

hyperbranched polymers are hydroxyl-terminated polyethers(based on glycidolpolyglycerol)23 and polyesters (based on

Figure 1. Chemical structure of polyesters based on 2,2-bis(hydroxymethyl)propionic acid (Boltorn) (A) and the illustration of the idea ofincorporation of linear units increasing distances between branching points (B).

Scheme 1. Synthesis of Hyperbranched Poly((2,2-bis(hydroxymethyl)propionate)-co-(6-hydroxyhexanoate))a

aReagents and conditions: (i) polycondensation: TMP, H2SO4, 120 °C, 0.01 mmHg, 96−99%; (ii) modifications: trifluoroacetic anhydride (TFA)(or pentafluoropropionic anhydride (PFP)), dichloromethane, rt, 85−94% or chlorotrimethylsilane, imidazole, DMF, rt, 94%.

Macromolecules Article

dx.doi.org/10.1021/ma401086m | Macromolecules 2013, 46, 7180−71957181

2,2-bis(hydroxymethyl)propionic acidBoltorn).24−27 Theyhave already been modified in numerous ways by end-cappingwith various functional groups28,29 or even outer or innerspheres differentiation.30

Because of the growing interest in biodegradable materials ofunique properties, new HBPs were synthesized utilizing 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) as a branchingmolecule. Trollsas et al. synthesized a series of ε-caprolactone-based AB2 macromonomers through a living ring-openingpolymerization, using aluminum benzyloxide as the initiator.The AB2 macromonomers were condensed into hyperbranchedpolymers through a room-temperature esterification usingDCC and 4-(dimethylamino)pyridinium 4-toluenesulfonate(DPTS).31 Very similar HBPs were synthesized from AB2macromonomers containing two oligo(ε-caprolactone) chainsterminated with hydroxyl groups grown from a bis-MPAinitiator by ROP by Choi and Kwak.32 The polymers weredesigned to incorporate different lengths of linear oligomericsegments consisting of 5, 10, and 20 ε-caprolactone monomerunits on the branched backbone chains. The dynamicviscoelastic relaxation behavior and the molecular mobility forthese polymers were investigated showing that the molecularmobility of three hyperbranched PCLs was higher than that oflinear one and was observed to enhance with decreasing lengthsof oligo(ε-caprolactone) segments and increasing relativedegree of branching (DB).33 Copolymers of bis-MPA withanother cyclic ester, δ-valerolactone, were obtained via lipase-mediated syntheses using compressed fluids: supercriticalcarbon dioxide or liquid 1,1,1,2-tetrafluoroethane.34

■ RESULTS AND DISCUSSION

Syntheses. Polycondensation. The synthetic pathwaytoward hyperbranched polyesters is given in Scheme 1. Theaim of this procedure was to obtain polymers containingvarious branching density and random distribution of 6-

hydroxyhexanoic acid units in the structure. In contrary toprocedures described in the literature, based on a tediousprotection/deprotection reaction pathways,31,32 for the syn-thesis of HB copolyesters we applied a direct one-potprocedure of polymerization/polycondensation. We assumedthat at applied reaction conditions (120 °C, H+) a ring-openingpolymerization (ROP), polycondensation, and transesterifica-tion reactions would proceed, leading to polymers of randommonomers distribution. The polymers were designed tomaintain a constant bis-MPA/TMP molar ratio (the samenumber of hydroxyl groups per average macromolecule) and avarying amount of 6-hydroxyhexanoate (6-HH) units (from 5to 60 mol %). The expected result of this procedure wasgradual increase of molecular weight of the polymers in theseries.A series of copolyesters consist ing of 4,4-bis-

(hydroxymethyl)propionic (bis-MPA) and 6-hydroxyhexanoicacid units were synthesized in a two-stage process.Trimethylolpropane (TMP) was used as a core and sulfuricacid as a catalyst of polycondensation reaction. Application of acore molecule (TMP) made it possible to control the molecularweight and helped to reduce the dispersity of the molecularweights of hyperbranched polymers.35,36 The presence of anethyl group in a core molecule also helped in determination ofan average molecular weight by 1H NMR spectroscopy. Actualamounts of reagents are given in the Supporting Information(Table 1s).In the first stage TMP, bis-MPA, ε-CL, and a catalyst

(H2SO4) were mixed together in one portion and heated at 120°C under an argon atmosphere. Temperature was raised slowlyto avoid distillation of ε-CL out of the reaction mixture. Theviscosity of the polymer gradually increased during the reaction.After approximately 10 h, the pressure in the flask was loweredto 0.01 mmHg, and the reaction was continued for anadditional 3 h. To remove any unreacted monomers, thepolymers were dissolved in acetone or acetone/methanol

Figure 2. Repeating units in the structure of hyperbranched copolymers P-1 to P-6.



solvent and precipitated into hexane. Polymers were thencarefully dried under reduced pressure. The yields varied from96 to 99% (Table 1s).Substitution of the End-Groups. Since polyols of Boltorn

type (Figure 1A) show very limited solubility in supercriticalcarbon dioxide, OH groups of the polymers P-1−P-6 weretransformed into fluorinated ester or siloxane ones, known toincrease the polymer solubility. Three different types ofresidues were introduced: trifluoroacetate (TFA), pentafluor-opropionate (PFP), and trimethylsiloxane. Exact amounts ofreagents and the reaction yields are given in the SupportingInformation in Tables 2s, 3s, and 4s.Briefly, all the parent copolymers P-1−P-6 were reacted with

an excess of TFA anhydride below 0 °C under an argonatmosphere. The polymers dissolved gradually in the reactionmixture and were left to stir overnight. Then the excess of TFAanhydride and the TFA acid formed during reaction wasremoved on a rotary evaporator protected with a trapcontaining KOH pellets. Any residues of the acid wereremoved by washing with bicarbonate solution and water. Asimilar procedure was applied for preparation of PFPderivatives. The siloxane derivatives were obtained in thereaction with chlorotrimethylsilane in the presence of imidazoleas a catalyst and HCl acceptor in a dry DMF solution. Thereaction mixture was left to stir at room temperature for 2weeks. Addition of dichloromethane to the reaction mixtureyielded precipitation of imidazole hydrochloride, which wasfiltrated off. The filtrate was then washed several times withwater, dried, and evaporated to dryness.Characterization of the Polymers. Parent Copolyesters.

The chemical structure of the synthesized polymers is quitecomplex. Because of the presence of a core molecule and twotypes of monomers, there are 11 possible substructures whichcan be divided into five different units: starting, dendritic,linear, terminal, and chain extending. They are shown in Figure2. 1H NMR spectra of all parent polyesters are given in Figure3. They are in good agreement with previously publishedspectra of hyperbranched polycaprolactones containing smallamount of bis-MPA units;31,32 however, in case of polymers P-1−P-6 the intensities of bis-MPA protons are much higher.Figure 4 shows an enlarged bis-MPA methyl proton range. Incase of unmodified poly(bis-MPA) this region consists of threepeaks which can be assigned to dendritic, linear, and terminalunits.Incorporation of 6-HH residues into the structure of

poly(bis-MPA) resulted in splitting of signals of methyl groups;however, it was still possible to estimate the degree ofbranching (DB) of obtained copolyesters. The signals weredeconvoluted with MestreNova (v. 6.2.0-7238) software, andthe DB was calculated according to the Frechet37 or Frey38

equation. Both the equations gave comparable results close tothe theoretical 0.5 value. Treating 6-HH chain extending unitsas linear units, one can calculate a relative number of branchingpoints per molecule. It was estimated taking into considerationcalculated DB values based on substitution of bis-MPA unitsand the composition of the polymer (bis-MPA/6-HH molarratio, Table 4). The results are collected in Table 1. It can beseen that introduction of up to 10% of 6-HH units slightlyreduces the relative number of branching points per molecule,while addition of 60% of 6-HH chain extending units canreduce it down to approximately 0.2.The complexity of the structure of the synthesized polymers

can be seen in the 13C NMR spectra. As an example the

quaternary carbon signals range of base poly(bis-MPA) andcopolymer containing 60% of 6-HH units is presented in Figure5. Spectrum P-1 shows the typical picture of three signals

Figure 3. 1H NMR (400 MHz, DMSO-d6) spectra of hyperbranchedcopolyesters P-1−P-6.

Figure 4. Bis-MPA methyl group range of the 1H NMR (400 MHz,DMSO-d6) spectra of hyperbranched copolyesters P-1−P-6.



assigned to terminal, linear, and dendritic units. In thecopolyester P-6 containing 60% of 6-HH units the signals aresplit, indicating that the quaternary carbon atom chemical shiftdepends not only on the number and type of substituents ofhydroxyl groups but also the type of unit linked to the carboxylgroup (bis-MPA-dendritic, bis-MPA-linear, or 6-HH). Intheory, there are three signals possible for terminal bis-MPAunit and nine for linear and dendritic units. The signals markedwith asterisk in Figure 5 most probably come from carboxyl-terminated macromolecules. Since they are not present in thepolymer P-1 spectrum, they may be a result of hydrolysiscaused by increased hydrophilicity of P-6.In the MALDI-TOF spectra the majority of signals were

observed as sodium adducts; however, there were also smallones of potassium adducts. Since the molar mass oftrimethylolpropane (molecular peak 134.094) is very close tothe molar mass of bis(methylol)propionic acid (molecular peak134.058), it was not possible to judge from the MALDI-TOFwhich of those molecules worked as a starting unit for thepolymerization process. The repeating unit mass difference ofbis-MPA (116.047) and 6-HH (114.068) is only 2 Da, whichmakes the analysis of the spectra complicated. As it can be seenin Figure 6, the apparent mass difference changes from 116 forpolymer P-1 (not containing 6-HH units) to 114 for polymer

P-6 containing 60% of 6-HH units. This phenomenon can beexplained when we take a closer look at the spectra.What is seen in the spectra in Figure 6 as a single peak is

actually a set of signals whose molecular peaks differ by 2 Da.Figure 7A shows enlarged area for the 1202 m/z peak. Thehomopolyester P-1 shows only one molecular peak at 1202 m/z which can be assigned to a sodium adduct of a moleculecontaining a TMP core and nine bis-MPA repeating units. Forcopolymers containing 6-HH, new signals appear at lower m/zvalues, and in each case the mass difference is equal 2 Da.Those are peaks of macromolecules containing a TMP core andnine repeating units: bis-MPA or 6-HH. The more 6-HH units,the lower molecular mass observed. The signals assignment forFigure 7A is given in Table 2. The amount of macromoleculesof exact combination of bis-MPA and 6-HH units is visualizedin Figure 7B. The dark areas show the combinations of thegreatest intensity. It is seen that polymer P-2 contains mainlymacromolecules with one 6-HH unit per molecule, whilepolymer P-6 is built in the majority of 6-HH units but alsocontains a small fraction of poly(bis-MPA).The results obtained by MALDI-TOF refer only to a fraction

of the polymer that was ionized during the MALDI-TOFexperiments. As it will be explained later, the molecular weightsof the copolyesters are higher than those observed by MALDI-TOF.As far as MALDI-TOF experiments are concerned, it is also

worth mentioning that certain amount of cyclic structures wereobserved (Figure 7A, a′ signal), especially for the P-1homopolyester. For the copolyesters P-2−P-6 no significantamounts of cyclic structures were detected.

Modified Copolyesters. The parent polymers P-1−P-6 weremodified with TFA, PFP, or siloxane residues. The presence ofthese groups in the polymer structure can be observed by FTIRspectroscopy. Figure 8 shows a set of spectra of derivatives ofpolymer P-3. This picture is representative for all thesynthesized materials since there are only minor changesrelated to a varying amount of 6-HH units which can be

Table 1. Degree of Branching of Synthesized HyperbranchedPolymers

DBa

(D + T)/(D + T + L)DBb

(2D)/(2D + L)

rel no. of branchingpointsc (D + T)/

(D + T + L + CE6‑HH)

P-1 0.46 0.46 0.46P-2 0.47 0.46 0.45P-3 0.49 0.49 0.44P-4 0.49 0.49 0.39P-5 0.49 0.50 0.29P-6 0.50 0.50 0.20

aAccording to Frechet.37 bAccording to Frey.38 cCalculated based onthe DB of bis-MPA units and the amount of 6-HH units.

Figure 5. Bis-MPA methyl group range of the 13C NMR (100 MHz,DMSO-d6) spectra of hyperbranched copolyesters P-1−P-6.

Figure 6. MALDI-TOF (DHB, linear mode) spectra of unsubstitutedhyperbranched copolyesters.



observed in the polymer FTIR spectrum. Introduction offluorinated residues is represented by an appearance of a newtrifluoroacetic or pentafluoropropionic ester absorption band at1790 cm−1. Siloxane derivatives are characterized by anappearance of new 1258, 1054, and 842 cm−1 bands.The FTIR spectra also revealed small absorption bands at

3600 cm−1 related to unsubstituted hydroxyl groups of the

copolyester. The presence of these bands is in agreement withthe results of other analyses.Figure 9 shows a proton NMR spectrum of a fluorinated

polymer P-3f. Substitution of hydroxyl groups led toappearance of new methylene group peaks in the range of4.75−4.3 ppm. There are also signals of methylene groups nextto unreacted hydroxyl groups (3.8−3.6 ppm). The ratio ofintegrals of those peaks allowed to calculate the degree ofsubstitution of hydroxyl groups (see Table 4). Because of apartial substitution of hydroxyl groups, the pattern of methylgroups signals of bis-MPA units at 1.5−1.0 ppm was quitecomplicated and could not be used for confirmation of DB ofinvestigated polymers. However, due to well-separated signalsof methyl group of TMP (a, Figure 9) and methylene group of6-HH unit (b, Figure 9), the spectra allowed to calculate themolecular weight (MW) and composition of the modifiedcopolyesters. The calculations were more precise than thosemade for parent polymers P-1−P-6 due to a better signalsresolution (especially the methyl group of TMP; compareFigures 3 and 9). MW and composition calculations were madefor all the fluorinated polymers in a similar way. In case ofsiloxane residues siloxane methyl groups were integrated andused for MW and composition determination. The results aregiven in Table 4. On the basis of NMR spectra of fluorinatedpolymers, it was also possible to calculate back the MW ofparent copolyesters (Table 4).

Figure 7. Changes in the MALDI-TOF spectra of polymers P-1−P-6 observed for growing amount of 6-HH units (A). Distribution of monomers incopolymers P-2−P-6 observed by the MALDI-TOF technique (B). Polymerix software; numbers on the axis refer to the actual number of repeatingunits in the copolymer chains. Signal assignments (a−j) are explained in Table 2. High-resolution picture of (B) is given in the SupportingInformation.

Table 2. Signal Assignment for MALDI-TOF Spectra ofUnsubstituted Copolyesters Shown in Figure 7Aa

number of units

peak m/z TMP bis-MPA 6-HH adduct

a 1202 1 9 0 Na+

b 1200 1 8 1 Na+

c 1198 1 7 2 Na+

d 1196 1 6 3 Na+

e 1194 1 5 4 Na+

f 1192 1 4 5 Na+

g 1190 1 3 6 Na+

h 1188 1 2 7 Na+

i 1186 1 1 8 Na+

j 1184 1 0 9 Na+

aTMP: C6H14O3 (134 Da); bis-MPA: C5H8O3 (116 Da); 6-HH:C6H10O2 (114 Da).



Figure 10 shows a 13C NMR spectrum of fluorinatedpolymer P-5f. Because of a complex structure, all the signalsappear as multiplets. The presence of TFA residues is

confirmed by sets of quadruplets in the ranges of 160−155ppm for TFA carbonyl carbon atom and 120−110 ppm forfluorine-substituted carbon atom. The characteristic pattern ofquaternary carbon atom signals observed for unsubstitutedpolymers is changed. There are only two groups of peaks in therange of 50−45 ppm. The most intensive group at 47 ppmcomes from quaternary carbon atoms of dendritic and fullyesterified units (D, Figure 10), while the small one at 48 ppmfrom linear or partly esterified ones (L, Figure 10).The fluorinated polymers were also investigated by means of

MALDI-TOF spectrometry. Figure 11 shows a sample MALDI-TOF spectrum of a P-2f polymer. The spectrum consists ofseveral groups of peaks of residual mass equal 134 Da (TMP orbis-MPA). Each of the observed m/z value is a combination ofmolecular masses of a core molecule, bis-MPA, and 6-HHrepeating units as well as TFA terminal units. Since polymer P-2f contains only 4.4% of 6-HH units, the majority (highestintensity) of signals can be assigned to poly(bis-MPA)containing partly fluorinated terminal groups, and the spectrumseems relatively clear. The signals’ assignments are given inFigure 11. The polymer sample consisted of polyestermolecules containing both hydroxyl and trifluoroacetate end-groups. Actually, the number of observed TFA residues wasquite low (up to 4 per molecule for mass range presented inFigure 11). It is not in agreement with the 1H NMR analysis.However, this phenomenon was already seen earlier forfluorinated polycarbonate samples22 and is most probablycaused by hydrolysis of a polymer during sample preparation(ambient moisture, acidic matrix).Figure 12 shows a comparison of MALDI-TOF spectra of

fluorinated polymers containing various amounts of 6-HHunits. The result of growing amount of 6-hydroxyhexanoate ishigher “multiplicity” of the signals. The same was observed forunsubstituted polymers (see Figure 7A). The signals assign-ment for spectrum shown in Figure 12 is given in Table 3.

Determination of Molecular Weight. The knowledge ofmolecular weights of the obtained copolyesters and theirderivatives was crucial for the interpretation of phase equilibria.For this reason a thorough characterization of the synthetizedpolymers was performed.The investigated polymers were of the Boltorn type having

hyperbranched structures with TMP as a central unit.Characterization of polymers of this type (Boltorns H20,H30, and H40 with pentaerythritol as a central unit) was

Figure 8. FTIR spectra (thin film, KBr) of P-3 polymer and itsderivatives.

Figure 9. 1H NMR (400 MHz, CDCl3) spectrum of derivative P-3f.

Figure 10. 13C NMR (100 MHz, CDCl3) spectrum of a polymer P-5f.



described by Zagar et al. [Macromolecules 2002, 35, 9913 andPolymer 2006, 47, 166]. They have shown that the number-averaged molecular mass obtained from 1H NMR spectra wasvery close to those obtained from SEC-MALS measurements.Our investigations [Paczesny et al. Polymer 2013, 54, 174] of amodified Boltorn H30 polymer confirmed this result. Thus, itseems that NMR spectroscopy is a good enough method forthe characterization of the Boltorn-type hyperbranched

polymers. On the basis of our experience and the literaturereports, we have applied NMR spectroscopy for the character-ization of the polymers investigated in this work. We have alsoperformed SEC-RI measurements with polystyrene as acalibration standard. Despite the fact that as expected themolecular masses obtained were lower than those obtainedfrom NMR spectra, the SEC-RI measurements made it possibleto estimate the molecular mass polydispersities of the polymers.We have attempted to do SEC-MALS investigations of thepolymers. The results were not good enough for thepublication, since the molecular masses obtained were muchhigher than the theoretical molecular masses. There were twopossible reasons for the effect. First, it is plausible that traceimpurities present in the samples cause absorption of laser lightand as a result disturb the light scattering experiments. Second,the static light scattering (SLS) method, which is a basis for theSEC-MALS, works properly for scattering species of a sizeequal to or greater than 10 nm. The size of our polymers wasabout 2 nm or lower. Thus, it seems that in our case the SEC-MALS worked on the edge of its applicability.Because of the overlapping of the TMP methyl group signal

with bis-MPA methyl groups signals, the molecular weights ofthe base TMP/bis-MPA/6-HH copolyesters (P-1−P-6) weredifficult to calculate. However, in the case of OH-substitutedpolymers, the signals were well resolved (see Figure 9), and theMn values obtained from 1H NMR experiments werecomparable with theoretical ones (Mn3 and Mn4; Table 4).To calculate the Mn, three groups of signals were integrated.The integrals of signals of TMP methyl group (0.75−0.87ppm) and methylene group of 6-HH units (2.35−2.2 ppm)

Figure 11. MALDI-TOF (DHB, linear mode) spectra of fluorinated polymer P-2f.

Figure 12. MALDI-TOF spectra of selected fluorinated polymersshowing signal shifts caused by incorporation of growing amount of 6-HH units; the signals’ assignments are given in Table 3.



were obtained directly. The area of signal of methyl groups ofbis-MPA was obtained indirectly by integrating the region from1.7 to 0.92 ppm and subtracting calculated integrals related tothree methylene groups from CL units and one methylenegroup of TMP. Taking into consideration the degree ofsubstitution of OH groups, it was also possible to calculate backthe Mn of unsubstituted polyesters P-1−P-6 (Mn2; Table 4).The GPC traces of samples P-1f−P-6f in dichloromethane

recorded with a RI detector are shown in Figure 13. The results

of the measurements cannot be used for the determination themolecular weights of the polymers mainly due to lack of anappropriate standards. However, the shapes of the retentioncurves are very interesting and can provide qualitativeinformation on the structures of the synthesized polymers.Apart from the fact that the difference between the refractiveindexes of the solutions and the pure solvent showed positiveand negative values, it is clearly seen that, as it was expected, themolecular weight of the polymers increased and all of them hadreasonable polydispersity. As a first approximation at very lowconcentration of the solute the difference between the refractiveindexes of the polymer solution and the pure solvent can beexpressed as (n2 − ns

2) ∼ NMPA(αMPA − αs) + NHH (αHH − αs),where n2 is the square of the refractive index of the polymerssolution, ns

2 the square of the refractive index of pure solvent,NMPA the amount of the bis-MPA units in unit volume, αMPAthe average polarizability of the bis-MPA unit, NHH the amountof the 6-HH units in unit volume, αHH the average polarizabilityof the 6-HH unit, and αs the average polarizability of thesolvent molecules. Addition of the chain extender to thepolyester significantly increased the polarizability of themacromolecule, since it contained alkyl chains. From theresults presented in Figure 13 it is clear that for the polyestersP-2f and P-3f the distribution of the 6-HH units between thepolymers molecules is not uniform. It is particularly striking forthe polymer P-3f for which bimodal distribution was observed.Moreover, apparently macromolecules with the highermolecular weight contained more 6-HH units. It seems thatthere was a significant difference in the kinetics of the additionof bis-MPA and ε-CL units during the polymerization process.Only the amount of about 20 mol % of ε-CL in the reactionmixture assured a uniform distribution of 6-HH units in themacromolecules.

Table 3. Signal Assignments for MALDI-TOF Spectra of Fluorinated Copolyesters Shown in Figure 12a

peak m/z TMP bis-MPA 6-HH TFA adduct peak m/z TMP bis-MPA 6-HH TFA adduct

a′ 1161 1 7 0 2 Na+ a 1181 1 8 0 1 Na+

b′ 1159 1 6 1 2 Na+ b 1179 1 7 1 1 Na+

c′ 1157 1 5 2 2 Na+ c 1177 1 6 2 1 Na+

d′ 1155 1 4 3 2 Na+ d 1175 1 5 3 1 Na+

e′ 1153 1 3 4 2 Na+ e 1173 1 4 4 1 Na+

f′ 1151 1 2 5 2 Na+ f 1171 1 3 5 1 Na+

g′ 1149 1 1 6 2 Na+ g 1169 1 2 6 1 Na+

h′ 1147 1 0 7 2 Na+ h 1167 1 1 7 1 Na+

i 1165 1 0 8 1 Na+

aTMP: C6H14O3 (134 Da); bis-MPA: C5H8O3 (116 Da); 6-HH: C6H10O2 (114 Da); TFA: C2F3O (−H) (96 Da).

Table 4. Composition of Copolymers Substituted with TFA, PFPA, or Silane Groupsa

TMP/bis-MPA/ε-CL ε-CL (%) terminal residue Mn1 Mn2 Mn3 % subst Mn4 Mn (GPC) Mw (GPC) PDI (GPC)

P-1f 1/28.4/0 0 O(CO)CF3 3614 3428 6782 80 5840P-2f 1/29.1/1.34 4.4 O(CO)CF3 3797 3658 6965 81 6151 980 1590 1.62P-3f 1/28.6/3.03 9.6 O(CO)CF3 4001 3791 7169 78 6154P-4f 1/29.0/7.28 20.1 O(CO)CF3 4469 4331 7637 80 6791 2900 4200 1.45P-5f 1/28.8/19.1 39.8 O(CO)CF3 5894 5658 9062 79 8073 5560 15100 2.71P-6f 1/29.9/44.06 59.6 O(CO)CF3 8744 8619 11912 83 11234P-2pf 1/29.1/1.34 4.4 O(CO)C2F5 3797 3658 8615 78 7309 1060 2470 2.33P-3pf 1/28.6/3.03 9.6 O(CO)C2F5 4001 3791 8820 79 7431 1050 3050 2.90P-2s 1/29.1/1.34 4.4 OSi(CH3)3 3797 3658 6173 94 5828 1372 2496 1.819P-3s 1/28.6/3.03 9.6 OSi(CH3)3 4001 3791 6378 88 5791 1886 3583 1.899

aMn1: theoretical value for OH terminated polymer; Mn2: NMR determined value for OH-terminated polymer; Mn3: theoretical value for 100%substituted polymer, Mn4: NMR determined value for substituted polymer.

Figure 13. GPC traces for polymers P-1f−P-6f from the RI detector.



Contact Angle. The contact angles (Θ) for polyestersmodified both with TFA and siloxane residues showed hugetime drifts. These drifts were least pronounced for P-1f and P-2f polymers, as evidenced by the lowest difference of contactangle values at t = 0 and t = 5 min. For the remaining polymersof this series, even though the surface appeared hydrophobicimmediately after the drop deposition, the water dropscontinuously flattened with time. It is worth noting that thedecrease of contact angle continued even after t = 5 min, andconsequently no equilibrium values of Θ could be obtained. Forthis reason, estimation of the so-called most stable contactangle as an average of cosines from the advancing and recedingangles was not possible for these polymers. Farris et al.39

assigned such behavior to a combination of spreading and waterabsorption by a polymer. In addition, most of the polymers inthe present study showed low viscosity. Therefore, besides thewater absorption and spreading, expulsion of the polymer layerby water drops should also be considered. In that case, thecontact angle would eventually tend to the value characteristicfor the support used to spin-coat the polymer samples.However, the PET foil used as the support for all the polymerswas rather hydrophobic, with Θ ≈ 80°, while the contact anglesafter 5 min (see Table 5) were much lower. Therefore, wesuggest that the continuous spreading of the water droplets(lowering Θ values) with time could be a consequence ofhydrolysis of the polymers in contact with water.Viscosity Measurements. Viscosity of the synthesized

polymers was measured with Malvern Kinexus Pro rheometerin the parallel plate geometry (details in SupportingInformation). Zero shear viscosities obtained from flowmeasurements in the linear regime (i.e., at low shear rate) attemperatures 25, 40, 60, and 70 °C are reported in Table 5.The polyester P-1f had the highest viscosity of all the

synthesized polymers. Temperature had a significant effect onviscosity: increase of temperature by about 35 K decreasedviscosity by 2 orders of magnitude. The temperature effect andthe compact structure of this polymer suggested that theinteraction of the trifluoroacetic terminal groups determinedthe viscosity of the material. Addition of the 6-HH units madethe macromolecules more flexible and resulted in loweringviscosity. 4.4% content of the chain extender in the hyper-branched structure (polymers P-2f) decreased viscosity by anorder of magnitude. Increase of the amount of 6-HH units to9.6 mol % did not change viscosity significantly (polymers P-3f), and in fact, a small increase of viscosity was observed.Increase of the amount of 6-HH beyond 10% resulted in a largedecrease in viscosity. The influence of the amount of the chain

extender on viscosity was less pronounced at higher temper-atures.The terminal group functionalities could have also significant

influence on viscosity. The polymers P-3f and P-3s had thesame internal structure and differed in the terminal groupfunctionalities (trifluoroacetate, P-3f; trimethylsilane, P-3s).Viscosities of these polymers differed by 2 orders of magnitude(see Table 5). The effect was almost insignificant for polymersP-3f and P-3pf which terminal functionalities were made offluorinated moieties differing only in the amount of fluorineatoms.Introductory investigations of the viscoelastic properties of

two polymeric materials P-1f and P-2f were also performed.The frequency sweep experiments were performed at temper-ature 40 °C in the frequency range from 0.5 to 200 rad/s. Themeasurements were carried out within the linear viscoelasticregime where the storage G′ and loss G″ moduli areindependent of the shear stress. The results are presented inFigure 14. With increasing frequency, the storage modulus G′

always remains below the corresponding loss modulus G″ andboth rise in very nearly parallel fashion for more than 2 decades.Such behavior is characteristic of unentangled polymers. Thecorresponding power-law exponents are 1.02 (G′) and 0.83(G″) for P-1f and 1.09 (G′) and 1.0 (G″) for P-2f. Bothpolymers did not reach the terminal slope region. Theviscoelastic response of the polyesters P-1f and P-2f at lowfrequencies not reaching a terminal slope is believed to relate tothe exponentially slow terminal relaxation of the innerbranches.

Table 5. Properties of Modified Copolyesters

viscositya η [Pa·s] contact angleb [deg]

terminal residue 25 °C 40 °C 60 °C 70 °C t0 t5 min

P-1f O(CO)CF3 46781.95 309.16 107.28 95.2 ± 0.5 89.1 ± 7.0P-2f O(CO)CF3 1170.15 13.38 5.57 104.9 ± 0.7 66.6 ± 4.3P-3f O(CO)CF3 1598.18 20.55 8.46 101.8 ± 0.9 48.2 ± 1.8P-4f O(CO)CF3 101.16 2.91 1.42 71.6 ± 6.8 29.5 ± 2.2P-5f O(CO)CF3 96.37 4.90 2.66 72.6 ± 3.3 49.5 ± 1.4P-3pf O(CO)C2F5 1175.8 13.18 5.73P-2s OSi(CH3)3 54.02 9.56 4.56 79.9 ± 3.6 29.6 ± 0.9P-3s OSi(CH3)3 21.78 4.18 0.99 0.44 82.6 ± 8.4 36.8 ± 2.1

aSamples measured with Malvern Kinexus Pro rheometer in the parallel plate geometry (the plate diameter 20 and 40 mm with the spacing gap 0.5and 0.2 mm, respectively). bInitial (t = 0) and final (t = 5 min) values of the water contact angle [deg] on thin layers of polymers spin-coated onPET foils. The values represent averages of three measurements and the respective standard deviation.

Figure 14. Comparison of dynamic moduli for P-1f and P-2f.



Phase Equilibria. Hyperbranched polymers are highlybranched, polydisperse, three-dimensional macromoleculeswhich, due to their unique structures and properties, haveattracted great scientific attention in recent years. In particular,the large number of functional terminal groups allows for thetailoring of their rheological, surface, and solution propertiesand thus provides a powerful tool to design hyperbranchedpolymers for a wide variety of applications. In our previousinvestigations we have concentrated on hyperbranchedpolyesters with bis-MPA as monomeric units, polyglycerol,and aliphatic polycarbonates. It was shown that the number andtype of the functional end-groups of the polymers wereessential to control their solubility in supercritical solvents andsurface behavior. Understanding of underlying physics andchemistry of supercritical fluid (SCF)−polymer solutions hasbeen an active research area for over four decades. Althoughduring this period of time a detailed understanding of thephysics and chemistry of linear polymers−SCF mixtures hasbeen achieved, challenges for developing the same level ofunderstanding of branched and dendritic polymers−SCFsolutions have remained. In the previous study we haveshown that hydroxyl-terminated hyperbranched polyesters andpolyethers are not soluble in supercritical carbon dioxide up toabout 150 MPa and 423 K. End-capping of these polymers withaliphatic carboxylic acids, trifluoroacetic acid, or trimethyl-chlorosilane significantly improved solubility in supercriticalcarbon dioxide. The functionalization with trifluoroacetic acidwas shown to be the most effective. The terminal groups’functionalities of the hyperbranched polymers were proven to

be a significant factor determining the solubility in supercriticalcarbon dioxide. Nevertheless, it was also shown that the sizeand chemical nature of the hyperbranched polymer’s interioralso played some role. The conclusion from our previous studywas that in general hyperbranched polyesters were moresoluble in supercritical carbon dioxide than hyperbranchedpolyethers or polycarbonates. However, it should be stressedthat interplay between the structure of the polymer’s interiorand the end-groups functionality has to be taken intoconsideration when analyzing phase equilibria of hyper-branched polymers in SCF.The phase behavior of linear polymers in SCF is well

understood. It is a well-known experimental fact that linearsilicone polymers exhibit upper critical solution temperature(UCST) behavior, while fluorinated linear polymers generallyexhibit lower critical solution temperature (LCST) behavior.For the LCST phase behavior increasing temperature tends toraise the pressure required to generate single-phase solution,while for UCST phase behavior lowers the miscibility pressure.In our previous study we have shown that linear polycarbonatesend-capped with trifluoroacetic acid showed UCST behavior,while hyperbranched polycarbonates with the same terminalfunctionality exhibited LCST behavior. It was an example of awell-known fact that branching of a linear polymer induces thetransition from the UCST to the LCST phase behavior. Thiseffect is a result of the change of the type of phase behaviorform the type III to the type IV, where the types of phasebehavior are termed according to the classification of Scott andvan Konynenburg.40 The effect is not purely entropic since it

Figure 15. Phase behavior for the system P-3f + CO2: (A) pressure−temperature cloud point curves at constant composition; (B) pressure−weightfraction phase diagram at constant temperature; (C) temperature−weight fraction phase diagram at constant pressure.



should be remembered that the formation of the branched typearchitecture is always coupled with the formation of a largenumber of terminal groups. The influence of these two effects,branching (entropic) and number of end-groups functionalities(energetic), on the phase behavior of polymers’ solutionscannot be separated. The end-group functionalities effect wasexpressed in our study by comparison of phase behavior ofhyperbranched polyesters functionalized with trifluoroaceticacid and the same polyester functionalized with the silanegroups. The hyperbranched polyesters functionalized withtrifluoroacetic acid exibited LCST behavior, and thosefunctionalized with silane groups exhibited both LCST andUCST phase behavior; i.e., the transition from the type IV tothe type III was observed.In this study the copolymers of bis-MPA and 6-HH with

TMP as a central unit were investigated. The polymers differ inmolecular weight, ratio of the amount of bis-MPA and 6-HHmonomeric units, and the type of terminal groups. Thesystematic changes of the interior structure of the investigatedhyperbranched polyesters make it possible to assess theirinfluence on the phase behavior of the polymers in supercriticalcarbon dioxide.The phase behavior for a series of the systems composed of

hyperbranched polyesters P-1f−P-6f, P-2pf−P-3pf, P-2s−P-3s,and carbon dioxide was investigated in a temperature rangefrom 300 to 355 K. From the results obtained it would bedifficult to assign a type of phase behavior to the investigated

systems, i.e., discriminate between the type IV and type IIIphase behavior. For all systems the LCST phase behavior wasobserved in the temperature widow in which measurementswere performed. It seems that the UCST part of the phaseboundary was shifted to lower temperatures. As examples,phase diagrams for three systems P-3f + CO2, P-3pf + CO2,and P-3s + CO2 are described below in more detail. The resultsof the cloud pressure measurements for the system P-3f + CO2are presented in Figure 15. The polymer P-3f was fairly goodsoluble in supercritical carbon dioxide. The cloud pointpressures spanned from about 50 MPa at 355 K to about 27MPa at 303 K. The maximum cloud point pressure wasobserved at about 0.05 weight fraction of the polymer. This isclearly seen in Figure 15b where miscibility gaps in thepressure−weight fraction coordinates at four temperatures arepresented. These results also clearly showed that temperaturehad a significant impact on the phase transition: increase oftemperature by 50 K increased the cloud point pressure byabout 20 MPa. At higher polymer concentrations the cloudpoint pressure was less sensitive to the polymer concentrationin the system. In Figure 15c, miscibility gaps in thetemperature−weight fraction coordinates at constant pressureare presented. At 35 MPa the phase separation began to occurat temperature close to the critical temperature of carbondioxide.The results of the cloud pressure measurements for the

system P-3pf + CO2 are presented in Figure 16. The polymer

Figure 16. Phase behavior for the system P-3pf + CO2: (A) pressure−temperature cloud point curves at constant composition; (B) pressure−weightfraction diagram at constant temperature.

Figure 17. Phase behavior for the system P-3s + CO2: (A) pressure−temperature cloud point curves at constant composition; (B) pressure−weightfraction diagram at constant temperature.



P-3pf was more soluble in supercritical carbon dioxide than P-3f. It is not a surprise since both polymers have the samestructure and size of the hyperbranched interior and differ onlyin solvophilicity of the terminal groups. The polymer P-3pf wasend-capped with pentafluoropropionic ester, giving rise to thestronger interaction with carbon dioxide molecules. For P-3pfthe cloud point pressures spanned from about 40 MPa at 355 Kto about 20 MPa at 303 K. There was no maximum cloud pointpressure observed for the system P-3pf + CO2 up to at about0.02 weight fraction of the polymer. This is clearly seen inFigure 17b where miscibility gaps in the pressure−weightfraction coordinates at four temperatures are presented. Theresults also clearly showed that temperature had significantimpact on the position of the boundary of the phase transition:increase of temperature by 50 K increased the cloud pointpressure by about 18 MPa.The results of the cloud pressure measurements for the

system P-3s + CO2 are presented in Figure 17. The polymer P-3s had similar solubility in supercritical carbon dioxide as P-3pf.Both polymers have the same structure and size of thehyperbranched interior and differ only in the terminal groupsfunctionalities. Apparently energies of interaction of penta-fluoropropionic and trimethylsilane groups with carbon dioxidemolecules were similar. For P-3s the cloud point pressuresspanned from about 47 MPa at 355 K to about 22 MPa at 303K. The maximum cloud point pressure was observed at about0.10 weight fraction of the polymer. The maximum in themiscibility gap for the silane-substituted polyester was observedat higher concentration of the polymer in comparison to thefluoro-substituted polyesters. This is clearly seen in Figure 17bwhere miscibility gaps in the pressure−weight fractioncoordinates at three temperatures are presented. The resultsalso clearly showed that temperature had significant impact onthe position of the boundary of the phase transition: increase oftemperature by 40 K increased the cloud point pressure byabout 15 MPa.In Figure 18a, cloud point curves for the polyesters P-1f−P-

6f in carbon dioxide at constant composition are presented. Allsystems showed LCST behavior with appreciable temperatureeffect on the cloud point pressures. The results presented inFigure 18 demonstrate how cloud point pressure was affectedas 6-HH spacers were added to the hyperbranched poly(bis-MPA) structure. The addition of the spacer molecules does notchange the shape of the cloud point curve; however, theincreasing amount of 6-HH had a significant impact on thepressure of the phase transition. The polymer P-1f, containing

only bis-MPA as monomeric units, had the cloud point pressure50 MPa at 303 K. Addition of about 4.5 mol % of 6-HH to thehyperbranched poly(bis-MPA) structure decreased the cloudpoint pressure by about 30 MPa. Further increase of theamount of 6-HH resulted in the increase of the cloud pointpressure. The addition of the chain extender of about 40 mol %for the polymer P-5f made the transition pressure equal tothose for the polymers P-1f. The dependence of the cloudpoint pressures on the caprolactone content at two temper-atures is presented in Figure 18b. It is clearly seen thattemperature did not influence the effect. According to ourknowledge, the synergetic effect of solubility was observed forthe first time for hyperbranched polymers. However, effects ofsimilar type were observer by Sarbu et al.41 and Byun et al.42 forlinear polymers. Sarbu et al. have shown that the copolymer ofethylene oxide and carbon dioxide (34 mol % of carbonatecontent) was highly soluble in supercritical carbon dioxide,while homopolymers (polycarbonates and poly(ethyleneoxide)s) of the same molecular weight were essentiallyinsoluble in CO2. Byun et al. have observed the same effectwhen methyl acrylate units were added to the backbone ofpolyethylene and the copolymer was dissolved in ethylene. Inboth cases addition of the new monomeric unit (carbonate ormethyl acrylate) both increased the degree of flexibility of themacromolecules (entropic contribution) and enhanced theenergy of interaction between the copolymers and the solvents(enthalpic contribution), giving rise to the synergetic effect.The same phenomena stayed behind the increase of solubilityobserved for hyperbranched copolymers of bis-MPA and 6-HH.The polymers P-1f, composed of bis-MPA monomeric units,have compact structure which hamper the swelling of thepolymer interior by the solvent. Introduction of the chainspacer increased the degree of flexibility and made the wholestructure looser and as result facilitated interaction of carbondioxide molecules with the polymer’s carbonyl groups. Furtherincrease of the amount of 6-HH had a negative effect onsolubility. The looser structure facilitated direct interaction ofcapolactone monomeric units mainly through the aliphaticchains. Moreover, increased amount of aliphatic units made thesolvation of the macromolecules with the carbon dioxidedifficult, and in effect the pressure needed to form the onephase solution increased. It is worth noting that the synergeticeffect in the solubility, as depicted in Figure 18b for thepolymers P-1f−P-3f, was also observed for viscosity of purepolymers (see Table 5). It seems that to some extent the

Figure 18. Cloud point curves for polyesters P-1f−P-6f in carbon dioxide.



comparison of viscosity of hyperbranched polymers may beregarded as an indication of the solubility.In Figure 19, the cloud point pressure curves for the

polymers P-1f, P-2f, P-2pf, and P-2s are presented together

with the literature data for polymers 2e, 3e, and 4e from ref 16.The structures of polymers 2−4e are given in Figure 1s oftheSupporting Information. The polymers 2e, 3e, and 4e arebuilt of bis-MPA monomers with pentaerythritol as a core. It isinteresting to compare phase behavior of the polymers P-1f and4e in CO2. Both polymers were built on average of similaramount of monomeric units; i.e., they had similar molecularweights, and their terminal groups were functionalized withtrifluoroacetic acid. P-1f and 4e differed in the structure of thecore: in the case of P-1f it was trifunctional TMP, while in thecase of polymer 4e it was four-functional pentaerythritol. Thepolymers differed slightly in the amount of the end-groups.Polymer 4e had about 29 terminal groups, and according to ourNMR study all of them were functionalized with trifluoroaceticacid. The polymer P-1f had about 31 terminal groups, out ofwhich only 80% (i.e., 25) were substituted with trifluoroaceticacid and 6 remained unreacted hydroxyl groups. As it can beseen in Figure 19, the apparent differences in the internalstructure and the amount of the terminal groups had significantimpact on the phase behavior. The cloud point curve for thesystem P-1f + CO2 lay 20 MPa higher than those for the system4e + CO2.

In Figure 20a, the cloud point pressure curves for solutionsof the polymers P-2f, P-3f, P-2p, P-3pf, P-2s, and P-3s incarbon dioxide are presented together with the literature datafor polymers 2g, 3g, and 4g from ref 16 (Figure 1s). As can beseen in the figure, the solubility of the polymer P-2s in carbondioxide was similar to those of the polymer P-3f. It is also clearthat the position and the shape of the cloud point pressurecurve for the system P-2s + CO2 were similar to those obtainedfor the system 4g + CO2. Comparison of the results for P-2sand 4g made it possible to assess the influence of the internalstructure of siloxane terminated hyperbranched polyesters ontheir phase behavior in supercritical carbon dioxide. Thepolymers P-2s and 4g had similar amounts (about 30 and 29respectively) of terminal siloxane groups. Thus, the internalstructure of P-2s containing 5 mol % of 6-HH was to the sameextent accessible for the solvent molecules as was for thepolymer 4g. The most interesting observation for the siloxaneterminated polymers P-2s and P-3s was the influence of theamount of 6-HH on the solubility in carbon dioxide. Theincrease of chain extender content from 5 to 10 mol %decreased the cloud point pressure. It is worth noting that thesame changes of the internal structure of the polymers pairs P-2f−P-3f and P-2pf−P-3pf gave the opposite result. The cloudpoint pressures of polymers P-2f, P-3f, P-2pf, P-3pf, P-2s, andP-3s in carbon dioxide at 303 K shown in Figure 20b presentthe effect. It is also interesting to note that viscosity of thesepolymers follow the changes in solubility (see Table 5). Itseems that the interplay between interactions of the solventmolecules with internal carbonyl groups and terminal groupsstay behind the effect. The energetic interplay was apparentlydifferent for fluorinated and siloxane substituted polymers.However, the difference in the extent of the substitution of theterminal groups between the fluorinated and siloxanesubstituted polymers (see Table 4) might also contribute tothe final effect.In Figure 21, the cloud point curves of polymers P-2f, P-2pf,

and P-3s in CO2 together with the cloud points curves for thepolyesters 4e and 4g,16 PDMS,43 fluorooctyl poly(p-phenyl-ene)) (FPP),44 and PVAc-PDBM18 are presented. It is clearthat the solubility of the polymer P-2pf in compressed carbondioxide was at the same level as PDMS and FPP. PDMS andFPP are considered to be the best CO2-soluble polymericmaterials synthesized up to now. Our results suggest thathyperbranched polyesters with properly designed terminalgroups have equally good CO2-philicity. Although the cloudpoints curves for the systems P-2f + CO2 and P-3s + CO2 were

Figure 19. Cloud point pressure curves for polymers P-1f, P-2f, P-2pf,and P-2s and the literature data for polymers 2e, 3e, and 4e from ref16.

Figure 20. (A) Cloud point pressure curves for solutions of polymers P-2f, P-3f, P-2pf, P-3pf, P-2s, and P-3s in carbon dioxide and the literaturedata for the cloud point pressure curves for polymers 2g, 3g, and 4g in carbon dioxide from ref 16. (B) Cloud point pressures of polymers P-2f, P-3f,P-2pf, P-3pf, P-2s, and P-3s in carbon dioxide at 303 K.



shifted to higher pressures by about 15 MPa, they lay still atpressures lower than for the systems 4e + CO2 and 4g + CO2.The polymers P-2f and 4e had the same end-groupfunctionalities, and both contained bis-MPA as monomericunits. It is clearly seen that P-2f had significantly bettersolubility in CO2 than 4e. It was interesting since two oppositestructural factors stayed behind the effect. First, P-2f containedalso 6-HH monomeric units, making the internal structure ofthe polymer looser than the structure of 4e what certainlycontributed to the increase of the solubility in CO2. Second, P-2f contained a three-functional central unit (TMP) in contrastto 4e and as a result had smaller amount of end-groupfunctionalities which certainly results in decrease of thesolubility.In Figure 21, cloud point curve for the polymer PVAc-

PDBM18 (open diamonds) is also presented. PVAc has beenrecognized as one of the most CO2-philic hydrocarbonmaterial. In the recent years a number of research projectshave been undertaken to explore the possibility to synthetizeCO2-philic material based on PVAc. One of the mostinteresting result was reported by Howdle et al.18 They havereported that copolymerization of vinyl acetate (VAc) anddibutyl maleate (DBM) gave material of significantly bettersolubility in CO2 than PVAc. Transition pressures for PVAc-PDBM were close to the transitions pressure of polymers P-2fand P-3s but still was much higher than those observed forpolymers containing fluorinated units. However, It should bealso stressed that the cloud point curve for the system PVAc-PDBM + CO2 presented in Figure 21 was obtained for amixture with very low polymer content, and as a result directcomparison with our results was difficult. Increase of thepolymers concentration of about 1 order of magnitude (from0.006 to 0.05 weight fraction) would certainly shift significantlythe cloud point curve to higher pressures.

■ SUMMARY AND CONCLUSIONSThe copolyesters of 2,2-bis(hydroxymethyl)propionic acid andε-caprolactone were synthesized using a one-step procedureyielding hyperbranched aliphatic polyesters of varying amount

of linear units. Incorporation of 6-hydroxyhexanoic acid unitsinto the hyperbranched polymer structure caused increase ofdistances between branching points and a change in chemicalcharacter of the polymer.Parent copolyesters containing hydroxyl terminal groups

were reacted with trifluoroacetic, pentafluoroacetic anhydride,or chlorotrimethylsilane. The phase behavior of those modifiedpolymers in supercritical carbon dioxide was explored as afunction of concentration and temperature. Modified polyestersshowed fairly good solubility in carbon dioxide. The synergeticeffect of solubility was observed for a series of the hyper-branched polyesters differing in the amount of 6-hydroxyhex-anoic acid units in the internal structure. The gradual additionof the chain extender to the polyester built of bis-MPA unitsinitially significantly lowered the cloud point pressure and thencaused the decrease of solubility of the polymers in carbondioxide. Apparently, the initial gain in flexibility of thepolyesters (entropic effect) was balanced with unfavorableinteractions of aliphatic fragments when the amount of the 6-hydroxyhexanoic acid units increased. The synergetic effect ofsolubility was also observed for viscosity of pure polymers. Thechanges of the chemical structure of the terminal groupsinfluenced coherently both solubility and viscosity. Thus, itseems that the relationship between the chemical structure andviscosity of the hyperbranched synthesized polymers may be asimple indication of their solubility in scCO2.

■ ASSOCIATED CONTENT*S Supporting InformationExperimental details. This material is available free of charge viathe Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] (P.G.P.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis paper is based upon work supported by the PolishNational Science Centre research grant (N N209 028440) andthe Polish Foundation of Science International PhD program(MPD/2010/4).

■ REFERENCES(1) Gao, C.; Yan, D. Hyperbranched polymers: from synthesis toapplications. Prog. Polym. Sci. 2004, 29 (3), 183−275.(2) Voit, B. Hyperbranched polymers - All problems solved after 15years of research? J. Polym. Sci., Part A: Polym. Chem. 2005, 43 (13),2679−2699.(3) McKee, M. G.; Unal, S.; Wilkes, G. L.; Long, T. E. Branchedpolyesters: recent advances in synthesis and performance. Prog. Polym.Sci. 2005, 30 (5), 507−539.(4) Yates, C. R.; Hayes, W. Synthesis and applications ofhyperbranched polymers. Eur. Polym. J. 2004, 40 (7), 1257−1281.(5) Jikei, M.; Kakimoto, M. Hyperbranched polymers: a promisingnew class of materials. Prog. Polym. Sci. 2001, 26 (8), 1233−1285.(6) Suttiruengwong, S.; Rolker, J.; Smirnova, I.; Arlt, W.; Seiler, M.;Luderitz, L.; de Diego, Y. P.; Jansens, P. J. Hyperbranched polymers asdrug carriers: Microencapsulation and release kinetics. Pharm. Dev.Technol. 2006, 11 (1), 55−70.(7) Canelas, D. A.; DeSimone, J. M. Polymerizations in liquid andsupercritical carbon dioxide. In Metal Complex Catalysts Supercritical

Figure 21. Cloud point pressures as a function of temperature atconstant compositions for solutions of the polyesters P-2f, P-2pf, P-3s,4e, and 4g16 in carbon dioxide. Comparison with the literature data forPDMS,43 fluorooctylpoly(p-phenylene) (FPP),44 and PVAc-PDBM.18



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Synthesis, Characterization, and Solubility in Supercritical Carbon Dioxide of Hyperbranched...

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