Aliphatic Hyperbranched Polycarbonates: Synthesis, Characterization

Article pubs.acs.org/Macromolecules

Aliphatic Hyperbranched Polycarbonates: Synthesis, Characterization, and Solubility in Supercritical Carbon Dioxide Mariusz Tryznowski,† Karolina Tomczyk,† Zbigniew Fraś,‡ Jacek. Gregorowicz,‡ Gabriel Rokicki,† Edyta Wawrzyńska,† and Paweł G. Parzuchowski*,† †

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

‡

S Supporting Information *

ABSTRACT: Recently much effort has been devoted to developing drug delivery systems based on macromolecules of three-dimensional structure. In addition to dendrimers which are widely studied, hyperbranched polymers are gaining more and more attention. Among numerous polymeric materials used in drug delivery systems, aliphatic polycarbonates are one of the most interesting ones due to biocompatibility, nontoxic degradation products, and the absence of autocatalytic effect during the degradation process. However, they show poor solubility in supercritical carbon dioxide. This paper describes the synthesis of 5-(4-hydroxybutyl)-1,3-dioxan-2-one and its application for preparation of hyperbranched aliphatic polycarbonates. Linear analogues of the poly(5-(4-hydroxybutyl)-1,3dioxan-2-one) were prepared, too, and the structures were compared by means of 13C NMR. Ring-opening polymerization of 5(4-hydroxybutyl)-1,3-dioxan-2-one led to polymers containing solely primary hydroxyl groups which were subsequently reacted with trifluoroacetic anhydride. The phase behavior of fluorinated polymer in supercritical carbon dioxide was explored as a function of concentration and temperature. Modified hyperbranched polycarbonate showed reasonably good solubility in carbon dioxide. It was shown that hyperbranched structure of a polymer facilitate solubility even though the carbonate structural units do not promote solubility in scCO2.

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INTRODUCTION Hyperbranched polymers (HBPs)highly branched, threedimensional moleculeshave gained widespread attention in the past decades.1−6 HBPs exhibit unique chemical and physical properties. In contrast to dendrimers, such polymers can be easily prepared in a one-pot procedure. The properties of hyperbranched polymers are often affected by the nature of the backbone and the chain-end functional groups, degree of branching, chain length between branching points, and the molecular weight distribution. Hyperbranched polymers can be easily modified to tailor their properties for a specialized purpose. To date, most of the hyperbranched polymers are polyols. Thus, the most popular and commercially available hyperbranched polymers terminated with OH groups are polyethers (based on glycidol: polyglycerol)7−9 and polyesters (based on 2,2-bis(hydroxymethyl)propionic acid: Boltorn).1,10,11 Hyperbranched polycarbonates (HBPC) have been known since the early works of Bolton and Wooley12,13 concerning synthesis and properties of aromatic analogues of linear, commercially available bisphenol A-based polycarbonates, but there is still a limited number of such polymers reported in the literature. Recently, similar aromatic HBP’s were reported by Nishikubo et al., who synthesized HBPC according to double monomer methodology (DMM) using di-tert-butyl tricarbonate © 2012 American Chemical Society

and 1,1,1-tris(4-hydroxyphenyl)ethane as A2 and B3 monomers.14 Synthesis and functionalization with methacrylic acid residues of aromatic hyperbranched polycarbonates based on bisphenol A bis(chloroformate) and 1,1,1-tris(4hydroxyphenyl)ethane performed in the presence of pyridine in THF were reported by the same authors.15 Aliphatic starshaped polycarbonates were reported by Hult and coworkers.16 They used hyperbranched polyesters based on 2,2bis(hydroxymethyl)propionic acid as cores for ROP of neopentyl carbonate. Endo and co-workers described the synthesis of HBPC based on glycidol and carbon dioxide in the presence of alkali metal halides or ammonium salts at atmospheric pressure at 100 °C.17 However, the authors did not take into consideration formation of five-membered cyclic carbonates which are thermodynamically stable at studied conditions and do not polymerize leading to aliphatic polycarbonates. Single monomer methodology (SMM) in synthesis of aliphatic HBPC is represented by two examples. One is ring-opening polymerization of glycerol-based monomer 5-{3-[(2-hydroxyethyl)thio]propoxy}-1,3-dioxan-2-one synthesized in our laboratory.18 The other one is polymerization of 5Received: June 1, 2012 Revised: August 13, 2012 Published: August 23, 2012 6819

dx.doi.org/10.1021/ma3011153 | Macromolecules 2012, 45, 6819−6829

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ethyl-5-hydroxymethyl-1,3-dioxan-2-one, reported by Feng et al.19 Over the past 30 years there has been intense interest in the use of supercritical carbon dioxide (scCO2) as an environmentally friendly solvent for laboratory and industrial applications. Its popularity stems from the fact that it is nontoxic, nonflammable, readily available in vast amount, and that it is the second least expensive solvent after water. Supercritical carbon dioxide is believed to be a good choice as a versatile solvent for polymer synthesis and processing.20,21 Unfortunately, carbon dioxide’s solvent power is low, especially for polar and high molecular weight polymers. Design and synthesis of CO2-soluble surfactants, ligands, and phase transfer agents demand a broad knowledge on solubility of polymers in dense carbon dioxide. Identification of highly CO2-soluble polymers has been a subject of intense research for the past 20 years. Synthesized up to now CO2-philic polymeric materials are in principle linear or branched macromolecules. The first extensive experimental studies on solubility of hyperbranched polyesters and polyethers in supercritical fluids were performed in our research group.22 It was shown that the nature of the end groups is an important factor that influences phase equilibria of hyperbranched macromolecules in supercritical solvents. The results obtained have shown that the nature of the interior of the hyperbranched macromolecules is also significant. The general observation was that hyperbranched polyesters better dissolve in carbon dioxide than hyperbranched polyethers. It is generally accepted that carbon dioxide is a poor solvent for linear chain polycarbonates. However, as reported by Sarbu et al.,23 block poly(ether carbonates) with proper contents of both components have high solubility in scCO2. The main problem addressed in this research was to assess the effect of chemical structure of linear and hyperbranched polycarbonates on their solubility in scCO2.

Scheme 1. Synthesis of 5-(4-Hydroxybutyl)-1,3-dioxan-2-one (5)a

Reagents and conditions: (i) NaH, DEM, THF, 60 °C, 84%; (ii) LiAlH4, diethyl ether, rt, 85%; (iii) ClCOOEt, TEA, THF, −15 °C, 64%; (iv) H2, Pd/C, rt, 97%. a

Other dibromoalkanes (e.g., 1,6-dibromohexane) were also successfully used for spacer formation.27 In order to resolve the NMR spectra of the HBPC, it was necessary to synthesize three model compounds. Scheme 2

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RESULTS AND DISCUSSION 1. Syntheses. 1.1. Monomer and Model Compounds. Scheme 1 shows the synthetic pathway toward the new cyclic carbonate monomer: 5-(4-hydroxybutyl)-1,3-dioxan-2-one (5). Starting 1-benzyloxy-4-bromobutane (1) was obtained from 1,4-dibromobutane and benzyl alcohol. An excess of dibromobutane was used to promote formation of the product of monosubstitution. 1 was purified by vacuum distillation and obtained in a reasonable 67% yield. Alternatively, this product (1) can be obtained by PTC reaction with benzyltriethylammonium chloride as a catalyst.24,25 Next, 1 was reacted with sodium salt of diethyl malonate, giving diester 2 with 84% yield, which was then reduced with lithium aluminum hydride (LAH) to give diol 3. Six-membered cyclic carbonate monomer 4 was obtained in the reaction of 3 with ethyl chloroformate. The product 4 was purified by crystallization from cold THF, since cyclic carbonates are known to crystallize easily.26 Finally the benzyl group was removed using palladium catalyst and hydrogen under mild pressure. To reduce the possibility of polymerization of cyclic carbonate, the reaction was performed at room temperature. During this procedure the cyclic carbonate groups remained unaffected. The resulting compound 5 was stored in diluted ethyl acetate solution in a refrigerator, and the solvent was evaporated prior to use. The presented synthetic pathway is an universal way of preparation of a variety of 1,3-dioxan-2-ones substituted with an aliphatic spacer between carbonate ring and a hydroxyl group.

Scheme 2. Synthesis of Model Compounds 6, 7, and 8a

a

Reagents and conditions: (i) H2, Pd/C, rt, 98%; (ii) DMC, MeONa, 85 °C, 7: 46.2%; 8: 22.8%.

shows their structures. Compound 6 was obtained by a reduction of benzyl group with hydrogen in analogy to compound 5. Compounds 7 and 8 were obtained in a onepot reaction with an excess of dimethyl carbonate (DMC) and separated by a column chromatography. 1.2. Polymerization. The ring-opening polymerization (ROP) of 5 was carried out in bulk, in the presence of a catalytic amount (1 mol %) of Sn(Oct)2a coordination 6820


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Scheme 3. Polymerization of 5 in the Presence of TMP (p3), Catalyzed by Sn(Oct)2: (a) Initiation and Polymerization; (b) Chain Transfer Reaction

polymerization and transesterification catalyst28,29 or DBU or DMAPtypical anionic polymerization initiators.30 Scheme 3 shows a mechanism of coordination polymerization of 5 in the presence of stannous octanoate. An example of anionic polymerization mechanism can be found in our earlier work.18 The ROP of monomer 5 was initially performed without a core molecule using initiating alcohol groups present in the monomer molecule. The polymerization was performed at 60 °C for anionic polymerization or at 80 °C for coordination polymerization mechanism. In the latter case the molar ratio of monomer to Sn(Oct)2 ([M]/[Cat]) was equal 100:1. It allowed chain transfer reactions to be effective (Scheme 3b) and promoted formation of both dendritic and linear units of the hyperbranched polymer. In the case of higher

molar ratios (e.g., 400:1) a polymer containing majority of linear units would be formed.18 The polymerization reaction temperature was always kept below 100 °C to avoid the crosslinking reaction at high conversion. The cross-linking phenomenon is related to transesterification reaction. Polycarbonates are ”carbonic acid” derivatives, which is twofunctional. At higher conversion this two-functional moiety can chemically bind macromolecules leading to the crosslinking in the reaction with three-functional alcohol. Polymers for scCO2 solubility experiments were obtained using trimethylolpropane (TMP) as a core molecule. The monomer solution was added slowly to the reaction mixture by means of an infusion pump. Application of a core molecule and slow 6821


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Scheme 4. Synthesis of a Linear Polycarbonate

addition of the monomer to the reaction mixture allowed to reduce the dispersity of the hyperbranched polymer.31,32 The presented synthetic pathway toward aliphatic polycarbonates allows the preparation of HBPC and their linear analogues (shown in Scheme 4). The linear polycarbonate was obtained by anionic ROP of compound 4 using DBU as a catalyst. Deprotection and further modification of a linear polycarbonate bearing hydroxyl groups can possibly lead to grafted polymers, interpenetrating polymer networks and other more sophisticated polymeric systems. 1.3. Polymer Modification. Results reported in the literature clearly show that fluorinated polymers like fluorinated polyacrylates and polyethers are the most CO2-philic polymers discovered to date.33 As a result, the synthesis of fluorinated polymers may be carried out efficiently in supercritical carbon dioxide at mild conditions. These polymers may be used as “CO2-philic” amphiphiles that efficiently transport insoluble or poorly soluble materials into scCO2 solvent, resulting in the development of CO2-based processes including homogeneous and heterogeneous polymerization, extraction of proteins and heavy metals, and homogeneous catalysis. Since polyols like the polymer p3 (Scheme 3) are not soluble in scCO2, OH groups were transformed into trifluoroacetate ones by the reaction with an excess of trifluoroacetic anhydride. Our earlier experience showed that substitution of hydroxyl groups with trifluoroacetic residues significantly reduces the temperatures and pressures of scCO2 needed to dissolve the polymer. To avoid the hydrolysis of carbonate backbone, the reaction was performed under anhydrous conditions. The work-up of the product was also performed without the use of water. The excess of anhydride and TFA formed as a side product was removed under vacuum, and the polymer was purified by reprecipitation. 2. Structure Analysis. 2.1. Monomer. Figures 1 and 2 show the NMR spectra of the cyclic carbonate substituted with an aliphatic chain containing a hydroxyl group at the end. In the 1 H NMR spectrum there are characteristic signals a of the cyclic carbonate moiety methylene groups present in the range of 4.5−4.0 ppm. They appear as two sets of double doublets showing splitting between axial and equatorial protons and the methine proton. The presence of two stereoisomers possible for the cyclic structure, where the alkyl substituent can take the axial or equatorial position, is not seen in this case. Probably during the reaction of diol 2 (Scheme 1) with ethyl chloroformate the sterical hindrance of the alkyl substituent promoted formation of only one isomer. The b CH proton signal is a multiplet at around 2.2 pmm and overlaps with the signal of the OH group. The rest of the signals belong to the side chain of the molecule. The f CH2 protons appear as a triplet. The 13C NMR and FT-IR measurements confirmed the

Figure 1. 1H NMR (CDCl3, 400 MHz) spectrum of 5.

Figure 2. 13C NMR (CDCl3, 100 MHz) spectrum of 5.

number of carbon atoms and presence of carbonyl and hydroxyl groups in the molecule. 2.2. General Structure of the Polymer p3. The theoretical chemical structure of the obtained hyperbranched polycarbonate p3 is shown in Figure 3. The structure of polymer p2 without a core is similar, the only difference in that case is the presence of a monomer molecule with reacted OH group and unreacted cyclic carbonate ring in place of the starting unit A. There are four main types of subunits characteristic for the hyperbranched polycarbonate. The polymerization starts at the core unit A. The completely substituted subunit B contains the branching points. The partly substituted linear subunit C contains one hydroxyl group reacted and one unreacted. The terminal subunit D contains two hydroxyl groups. In case of polymers obtained in this work there is a possibility of formation of two kinds of linear units, C and C′, and two kinds of terminal units, D and D′. However, the spectral analysis confirmed the presence of two types of linear units and only 6822


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changes from nearly 43.0 ppm for the structure with two hydroxyl groups to 36.8 ppm for the structure with two carbonate groups. Small differences can be also observed for other signals. 2.4. Linear Polycarbonates. Figures 6 and 7 show the NMR spectra of the poly[5-(4-benzyloxybutyl)-1,3-dioxan-2-one] p1. It is clear that the polymerization of 4 yielded a polymer of linear structure. The 13C spectrum shows a set of single peaks, while the proton spectrum contains methine protons of only one type. Because of a relatively high molecular weight of the polymer (Mw = 34 100 Da, Mn = 14 800 Da, PDI = 2.3) the end hydroxymethyl group’s signals are not visible, and it is not possible to calculate the Mn value from the NMR. The spectra of HBPC (p3) shown in Figures 8 and 9 are more complicated. Figure 8 shows the signal assignment for the HBPC structure in the proton spectrum. The signals of the starting subunit (Figure 3, A) are hardly visible. The methyl group of TMP unit is present at 0.85 ppm but due to a low signal-to-noise ratio cannot be used for calculation of the molecular weight. The signals of the methine protons are clearly seen at approximately 1.7 and 2.0 ppm and correspond to the presence of units containing one (C′, D, Figure 3) or two (B, C, Figure 3) carbonate groups next to the methine group. The protons of the subunit D′ are not present, indicating that no hydrolysis or intramolecular transesterification with hydroxybutyl group took place during the synthesis of the polymer. It is difficult to judge about the degree of branching of the polymer which is typically calculated from the 1H NMR spectrum. The fully substituted 1,3-propanediol substructures (peak at 2.0 ppm in Figure 8) are present in both branching structure B (Figure 3) and linear structure C (Figure 3). The same situation is for the monosubstituted 1,3-propanediol substructures. The 1.7 ppm signal can be assigned to terminal unit D (Figure 3) and linear unit C′ (Figure 3). However if one would take into consideration only the ratio of the integrals of the methine protons of fully substituted 1,3-propanediol

Figure 3. Theoretical chemical structure of the hyperbranched poly(5) p3.

one type of the terminal unit. All the hydroxyl groups in the molecule are primary groups of similar reactivity. This is an advantage over structures like polyglicydol7−9 or poly(6HDON)34 where there are also secondary OH groups present of lower reactivity. 2.3. Model Compounds. Figures 4 and 5 show the chemical shifts and multiplicity (in the case of 1H NMR) of signals depending on the chemical surrounding. In case of a proton spectrum the most significant differences are observed for methine proton. Its chemical shift changes depending on the degree of substitution of the neighboring hydroxymethyl groups. The chemical shift changes from 1.4 ppm for the structure with two hydroxyl groups to 2.0 ppm for the structure with two carbonate groups. More differences in chemical shifts can be observed in the 13 C NMR spectra measured in d6-DMSO. It is possible to differentiate not only methylene groups next to carbonate from methylene groups next to hydroxyl groups but also methylene groups next to methine group from the methylene groups at the end of aliphatic chain no matter if they are substituted with carbonate or not. The bigger changes in chemical shifts are observed for the methine proton signal. The chemical shift

Figure 4. A compilation of 1H NMR spectra of compounds 3 and 6−8 (DMSO-d6, 400 MHz) showing chemical shifts and multiplicity of the signals depending on the chemical surrounding. 6823


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Figure 5. 13C NMR spectra of compounds 3 and 6−8 (DMSO-d6, 100 MHz) showing chemical shifts of the signals depending on the chemical surrounding.

Figure 6. 1H NMR (CDCl3, 400 MHz) spectrum of linear polycarbonate p1.

Figure 7. The methylene groups region of MHz) spectrum of the polymer p1.

13

C NMR (CDCl3, 100

ether groups present, which proves that no decarboxylation took place during polymerization. The 13C NMR spectra of all synthesized polymers look the same. The peaks of the core TMP molecule are not visible. One could expect differences in the polymer structure for polymers obtained according to the anionic polymerization mechanism and coordination mechanism. Such differences would be visible for shorter reaction times (lower conversion) and lower catalyst-to-monomer molar ratio (in case of stannous octanoate catalyst).18

substructure to a monosubstituted one, the DB calculated according to a Frey definition,35 DB = 2D/(2D + L), would be equal 0.48 for p2 and 0.47 for the p3 polymer. The lack of the 1,3-propanediol structures in the polymer was also confirmed by 13C NMR spectroscopy. The spectrum (Figure 9) contains only signals at 40 and 37 ppm for the methine carbons while the signal at 43 ppm is not present. Figure 9 shows the chemical shifts and signal assignment. There are no signals present in the spectrum other than those predicted with model compounds. It means that there are no 6824


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Figure 10. Structure of fluorinated polycarbonate p4 used for the solubility studies in scCO2.

Figure 8. 1H NMR (400 MHz, DMSO-d6) spectrum of poly(5) p3. The upper 3.2−3.8 ppm region taken from a spectrum of other sample, where the signals of water and methylene protons did not overlap.

Figure 11. 1H NMR spectra of HBPC before (a) (400 MHz, DMSOd6) and after (b) (400 MHz, CDCl3) reaction with trifluoroacetic anhydride.

Figure 9. 13C NMR (100 MHz, DMSO-d6) spectrum of poly(5) p3.

Surprisingly, as it was earlier mentioned the 1,3-propanediol structures (D′, Figure 3) were not formed. The presence of such structures was reported in our earlier work.18 The lack of 1,3-propanediol terminal units (D′) in the HBPC suggests that the transesterification reactions are suppressed and only chain transfer reaction proceeds leading to formation of linear units C′ (Figure 3) and branching units B (Figure 3) (see Scheme 3). In the case of transesterification a cross-linked polymer would be formed. It was the case when the reaction was carried out at 100 °C and a rubber like cross-linked product was obtained. 2.5. Fluoropolymers. Figure 10 shows the general structure of the fluorinated polycarbonate used for the solubility studies in supercritical carbon dioxide. As it was mentioned before, the polymer was obtained in the reaction of HBPC p3 with trifluoroacetic anhydride. Over 95% of hydroxyl groups were esterified, leaving only a little free hydroxyl groups as indicated on 1H NMR spectrum shown in Figure 11. A similar degree of substitution was obtained in our earlier studies for polyesters and polyethers.22 The FTIR spectrum shown in Figure 12 confirms the disappearance of free hydroxyl group band at 3350 cm−1 and the appearance of a new 1787 cm−1 band which can

Figure 12. FTIR spectrum of the fluorinated polycarbonate p4.

be assigned to the carbonyl group of the trifluoroacetic acid ester. For the comparison purposes for the measurements of solubility of polycarbonates in scCO2 three polymers of a simple linear structure were synthesized. The poly(hexamethylene carbonate) (p5, Figure 13) was obtained by a two-step method using hexamethylenediol bis(methoxycarbonate) of Mn = 1750 Da described in detail in the literature.26 Two poly(trimethylene carbonate)s p6 and p7 of average molecular weights approximately 2000 and 5600 Da 6825


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p8−p10. Similar changes in 1H NMR and FTIR spectra were seen for those polymers after modification. Figure 13 shows the structures of synthesized polymers and their derivatives. Structures of other polymers used in scCO2 solubility tests are also given. 2.6. Mass Spectrometry. The fluorinated polymer p4 was investigated by means of MALDI-TOF spectrometry. Figure 14 shows a MALDI-TOF spectrum of a polymer sample prepared at ambient conditions using dihydroxybenzoic acid (DHB) as a matrix. The spectrum consists of two series of peaks which can be assigned to macromolecules containing as a core: trimethylolpropaneof residual mass 134 Da (TMP, Figure 14)and 2-hydroxymethyl-1,6-hexanediolof residual mass 148 Da (T, Figure 14). To each of the core molecules several carbonate repeating units (174 Da, C, Figure 14) are attached in which part of the hydroxyl groups are substituted with trifluoroacetates (additional mass −96 Da, F, Figure 14). The number of carbonate units (n) grows from 2 in the series, while the number of trifluoroacetate residues depends on the n number and varies from 0 to n + 3, where 3 is the number of hydroxyl groups in the core. It means that the polymer sample consisted of polycarbonate molecules containing both hydroxyl and trifluoroacetate endgroups. For example, the molecular weight of 985 Da can be assigned to a TMP molecule with two carbonate units attached, fully substituted with five F residues (TMP + 2C + 5F), while the molecular weights of 903 and 807 Da refer to molecules containing respectively one and two trifluoroacetate residues less (TMP + 2C + 4F and TMP + 2C + 3F). The presence of macromolecules with T core (Figure 14) suggests that the hydrolysis of carbonate bond took place. In case of such chain cleavage two molecules are produced: one containing TMP unit and one containing a hydrolyzed monomer unit T. Since NMR and FTIR data confirmed that over 95% of hydroxyl groups were substituted with TFA residues, the reason for

Figure 13. Structures of polymers used for comparison in scCO2 solubility studies.

were synthesized by a ROP of trimethylene carbonate using 1,3-propanediol as an initiator and DMAP as a catalyst. Similarly linear polycarbonates p5−p7 were treated with a trifluoroacetic anhydride resulting in fluorinated derivatives

Figure 14. MALDI-TOF spectrum of the HBPC p4. Abbreviations: TMP = trimethylolpropane, T = 2-hydroxymethyl-1,6-hexanediol, C = carbonate repeating unit, F = trifluoroacetate group. 6826


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observing a large population of molecules containing free hydroxyl groups may be hydrolysis during the MALDI sample preparation (water presence, acidic matrix). 2.7. Phase Equilibria. The design of inexpensive and environmentally benign CO2-philes is still an open problem of CO2-based technology. In principle, oxygen-containing polymers have a potential for a good solubility in carbon dioxide. Nevertheless, the presence of carbonyl and alkoxy structural units alone is not a sufficient condition for a good solubility of a polymer in scCO2.36 It is a well-known fact that small differences in a polymer structure (e.g., poly(methyl acrylate) versus poly(vinyl acetate)) may have a huge impact on the solubility in CO2. Homopolymers containing ether groups or carbonyl groups in the backbone, like poly(ethylene oxide) or polylactide, are better CO2-soluble than hydrocarbons but still are far less soluble than fluorinated polymers. Sarbu et al. have reported that solubility of a copolymer of propylene oxide and CO2 (poly(ether carbonates)) is similar to this of fluorinated polymers.23 They have shown that a proper balance of the amount of carbonyl and alkoxy structural units in the backbone of the copolymer significantly lowers the solubility pressure. To understand this intriguing effect, more information on the solubility of polycarbonates is needed. In this contribution the results of the study on the solubility of polycarbonates in scCO2 are presented. In principle, carbonyl structural units should promote solubility in scCO2. However, it is well-known that polycarbonates synthesized from biphenol A and CO2 are completely insoluble in carbon dioxide. On the other hand, permeation of carbon dioxide through the bisphenol A-based polycarbonates is very high, indicating strong interactions of the gas molecules with the polymer. In our study we have focused on copolymers of aliphatic alcohols and CO2. Both chain and hyperbranched polycarbonates were investigated. Polycarbonates p1 and p3 were not soluble in carbon dioxide up to 150 MPa and 423 K. Low solubility of the polymer p3 is a result of large number of terminal hydroxyl groups, while in the case of p1 both hydroxyl terminal groups and cohesion of the polymers chains are responsible for the effect. The hyperbranched polycarbonate p4 obtained by esterification of the hydroxyl terminal groups in p3 with trifluoroacetic acid had significantly higher solubility in scCO2. In Figure 15 phase behavior for the system polymer p4 + carbon dioxide is presented. As can be seen in Figure 15, temperature had minor influence on the phase transition for the system polymer p4 + carbon dioxide. The temperature effect can be particularly well observed in the P−w phase diagram presented in Figure 15b. The phase boundaries for different temperatures lie close to each other. Analysis of the pressure (P)−temperature (T) phase diagram in Figure 15a revealed that the temperature effect was composition dependent. For the solutions with the solute content up to about 3.5 wt % the system exhibited lower critical solution temperature (LCST) type behavior, i.e., increase of temperature at constant pressure induced phase separation. This type of phase behavior is characteristic for supercritical systems containing fluorinated polymers. Unexpectedly, for the solutions containing from about 6 to 12 wt % of the polycarbonate p4 the upper critical solution temperature was observed (UCST), i.e., increase of temperature at constant pressure induced homogenization of the system. For the mixture containing 6 wt % of the polymer both UCST and LCST behaviors were observed. This observation

Figure 15. Phase diagram for the system polycarbonate p4 + carbon dioxide: (a) Pressure (P)−temperature (T) diagram at constant composition; curves are labeled with weight fraction of the polymer. (b) Pressure (P)−weight fraction (w) diagram at constant temperature; curves are labeled with temperature.

may indicate that the system polycarbonate p4 + CO2 exhibited in fact an hourglass-type phase behavior in the temperature (T)−weight fraction (w) coordinates. In Figure 16, cloud points for the polycarbonates p4, p8, and p9 in carbon dioxide are presented. For comparison, the literature data for hyperbranched polyesters PE1 and PE2 (Figure 13) and poly(lactic acid) (PLA) in carbon dioxide are

Figure 16. Comparison of fluid phase behavior of polycarbonates p4, p8, and p9 in carbon dioxide with the literature data. Filled symbols and dashed lines represent the literature data on phase boundary curves for polyesters PE1, PE2, and poly(lactic acid) (PLA). 6827


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also shown in the figure. The polymer PE1 is the most CO2philic material among hyperbranched polymers investigated in our laboratory. The solubility of hyperbranched polycarbonate p4 in carbon dioxide was quite good, however not as good as the polyester PE1. The polymers PE1 and p4 had the same terminal groups but differ in the internal structure. Apparently, the polyester structure of the inside of the hyperbranched polymer was more CO2-philic than polycarbonate structure. Polycarbonate p4 had much higher solubility in scCO2 than hyperbranched polyester with the hydroxyl end-groups substituted with linear polycarbonates end-capped with acetic acid (PE2, Figure 13). It seems that the short chains of oligocarbonates attached to the terminal groups were responsible for a high pressure needed to homogenize the system. Chain poly(lactic acid) with a carboxylic end group estherified with dodecanoic acid had slightly lower solubility in scCO2 than polymer p4. Moreover, below 360 K the solid PLA precipitated from the solution. This observation shows that hyperbranched structure of a polymer facilitate solubility in scCO2 even though the carbonate structural units do not promote the solubility. It is interesting to compare phase behavior of linear and hyperbranched polycarbonates. Linear polycarbonates p5−p7 with terminal hydroxyl groups were solids or viscous liquids. The polycarbonates were not soluble in scCO2 up to 150 MPa and 423 K. The hydroxyl end groups of the polymers p5−p7 were esterified with trifluoroacetic acid. The modified polymer p10 of molecular weight of about 5800 Da was also not soluble in scCO2 up to 150 MPa and 423 K. However, the polymers p8 of 1950 Da and p9 of 2200 Da showed enhanced solubility in scCO2. Pressure needed to homogenize the systems was about 120 MPa. Both systems p8 + CO2 and p9 + CO2 exhibited UCST-type phase behavior. From the experiments performed for linear polycarbonates it is evident that the length of the polycarbonate chain for a given structure of the CO2-philic end groups was the main factor influencing the solubility. Thus, it would be difficult to assign the difference in solubility between polymers p4 and p8 or p9 only to the difference in the structures of the polymers. It seems that enhancement in solubility of the polymer p4 in comparison to the linear polycarbonates is caused mainly by the difference in the number of the fluorinated end-groups.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is based upon work supported by the Polish National Science Centre research grant (N N209 028440) and the Polish Foundation of Science International PhD program (MPD/2010/4).

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REFERENCES

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CONCLUSIONS The monomer 5-(4-hydroxybutyl)-1,3-dioxan-2-one was synthesized and used for the ring-opening polymerization, yielding hyperbranched aliphatic polycarbonates. The chemical structures of the monomer and the polymer were analyzed by means of 1H and 13C NMR as well as MALDI-TOF spectrometries and GPC chromatography. Linear analogues of the poly(5-(4hydroxybutyl)-1,3-dioxan-2-one) were prepared, too, and the structures were compared by means of 13C NMR. Ring-opening polymerization of 5-(4-hydroxybutyl)-1,3-dioxan-2-one led to polymers containing solely primary hydroxyl groups which were subsequently reacted with trifluoroacetic anhydride. The phase behavior of fluorinated polymer in supercritical carbon dioxide was explored as a function of concentration and temperature. Modified polycarbonate shows reasonably good solubility in carbon dioxide. It seems that enhancement in solubility of the hyperbranched polymer in comparison to the linear polycarbonates is caused mainly by the difference in the number of the fluorinated end-groups. 6828


Macromolecules

Article

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Aliphatic Hyperbranched Polycarbonates: Synthesis, Characterization

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