Water-Soluble âPoly(propylene oxide)â by Random Copolymerization

Article pubs.acs.org/Macromolecules

Water-Soluble “Poly(propylene oxide)” by Random Copolymerization of Propylene Oxide with a Protected Glycidol Monomer Martina Schömer and Holger Frey* Institute of Organic Chemistry, Johannes Gutenberg-University, Duesbergweg 10-14, D-55099 Mainz, Germany S Supporting Information *

ABSTRACT: Hydrophilic, functional poly(propylene oxide) (PPO) copolymers were prepared by anionic random copolymerization of propylene oxide with the protected glycidyl derivative ethoxy ethyl glycidyl ether (EEGE). The monobenzyl-protected ethylene glycol initiator 2-(benzyloxy)ethanol was used to initiate the polymerization because it allows for the introduction of hydroxyl groups at both ends of the polymer chain. Acidic deprotection permitted selective removal of the acetal protecting groups in the chain or alternatively orthogonal deprotection of the terminal hydroxyl group by catalytic hydrogenation. A series of narrowly distributed hydroxyl-functional PPO copolymers (Mw/Mn < 1.07−1.25 g mol−1) was obtained with varying composition between 2 and 75% glycerol units and molecular weights in the range of 3000 to 8000 g mol−1. Monomer consumption and compositional drift in monomer feed were studied via 1H NMR kinetics, revealing a slightly tapered structure, but confirmed a distribution of EEGE in the polar PPO-based copolymers. Cloud-point measurements showed temperature-dependent water solubility, with LCSTs in the range of 20 to 70 °C. The study demonstrates that an incorporation of 11% of the hydroxyl-functional, linear glycerol units suffices to obtain functional PPO copolymers that are water-soluble at room temperature.

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INTRODUCTION Polyether polyols are important materials that are widely used for the synthesis of polyurethanes (PUs) as a flexible, low glasstransition component. Their preparation via anionic polymerization strategies has been intensively investigated.1 Poly(propylene oxide) (PPO)-based structures2 play a key role among them and find application as functional oligomers, copolymers, or star polymer building blocks. Because of its apolar chemical structure, PPO is generally not water-soluble at ambient temperature. Only polymers with low molecular weights have been reported to exhibit solubility below 18 °C.3 The most widely used copolyethers based on propylene oxide (PO) are random and block copolymers with ethylene oxide (EO),4,5 also known under their trade names Poloxamer, Pluronics, or Jeffamine. These copolymers are more hydrophilic than PPO homopolymers and may show improved, but still limited solubility in water. Furthermore, copolymers of PO with tetrahydrofuran have been reported6 that exhibit reduced coldhardening and improved molding properties when used as a soft segment in PUs. To increase further polarity or achieve water solubility of PPO copolymers, incorporation of a more hydrophilic comonomer is necessary. This can be achieved via copolymerization of PO with glycidol, which adds one free hydroxyl group to the PPO chain per comonomer unit. However, because of the latent AB2 character of glycidol, this would lead to branched polymers, unless the hydroxyl group is reversibly © 2012 American Chemical Society

protected during the polymerization. Ethoxy ethyl glycidyl ether (EEGE) is an acetal-protected glycidyl ether structure that represents an ideal candidate for this purpose. EEGE can be polymerized in a living manner.7,8 The use of EEGE as a comonomer for the synthesis of random or block copolymers with other epoxides has attracted increasing attention recently and has been employed in several works.9−16 Polyether polyols that are suitable for further functionalization essentially require defined end groups at both termini to react with the surface or an isocyanate component. For example, when using polyols as soft segments in PUs, the presence of monofunctional alcohols in the mixture would lead to termination during the condensation step. Anionic ringopening polymerization offers the possibility to use a variety of initiators, for example, a protected hydroxyl-functional initiator to generate α,ω-hydroxy-polyether-polyols. Amphiphilic di- and triblock copolymers based on PPO-b-PEEGE, PPO-b-linPG, and PPO-b-hbPG were introduced by Tsvetanov et al.11a,12,17 as well as by our group,11b respectively, and were characterized with respect to association and solution properties in aqueous media. Carlotti et al. investigated the copolymerization properties of protected glycidols with PO via monomeractivated polymerization, resulting in copolymers with a Received: February 5, 2012 Revised: March 12, 2012 Published: March 28, 2012 3039

dx.doi.org/10.1021/ma300249c | Macromolecules 2012, 45, 3039−3046

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Figure 1. Synthesis of α,ω-hydroxy-poly(propylene oxide)-co-poly(glycerol) by copolymerization of PO with EEGE and subsequent cleavage of the acetal protecting groups to release the free hydroxyl groups and hydrogenation of the benzyl end group to generate α,ω-hydroxyl groups. 1 H NMR Kinetics.20 In a conventional NMR tube, a DMSO-d6 solution of the initiator and a mixture of PO and EEGE were separately frozen under an argon atmosphere. High vacuum was applied, and the tube was flame-sealed, whereas the solutions were kept frozen. To reduce the necessary time for locking and shimming of the polymerization mixture, we measured a sample of the pure monomer mixture in advance at the relevant temperature. Immediately after melting and mixing, the first spectrum was recorded. Sample spinning was turned off. Intervals between two measurements were 30 s. Synthesis of BnO-PPOn-co-PEEGEm−OH. Here an exemplary synthetic protocol is described for BnO-PPO49-co-PEEGE6−OH: A two-necked flask equipped with a septum, Teflon seal, and a magnetic stirrer was connected to a vacuum line. 2-(Benzyloxy)ethanol (152 mg, 1 mmol) was deprotonated with 0.9 equiv cesium hydroxide monohydrate in methanol and dried azeotropically with benzene to remove methanol together with formed water and other volatiles. PO (2.61 g, 45 mmol) was transferred to an ampule and subsequently to the reaction flask in vacuo. The flask was sealed, and 730 μL (5 mmol) of freshly distilled EEGE was introduced through the septum via syringe. The reaction mixture was immediately heated to 40 °C and stirred for 18 h. After the addition of an excess of methanol to quench the polymerization, the solution was extracted with sodium carbonate solution to remove remaining salts and dried in vacuo at 80 °C for 24 h to afford the BnO-PPOn-co-PEEGEm−OH copolymers in ca. 90% yield. Synthesis of BnO-PPOn-co-linPGm−OH. A 20% solution of the copolymer in ethanol was stirred with the anionic exchange resin DOWEX 50WX8 at 60 °C for 18 h to cleave the acetal protecting group. After filtration and removal of the solvents, the polymer was dried in vacuum at 80 °C for 24 h to afford the BnO-PPOn-co-linPGm− OH copolymers in 90−95% yield. Synthesis of HO-PPOn-co-PEEGEm−OH. A 20% solution of the copolymer in ethanol/dichloromethane (1:1) was stirred with palladium on activated charcoal under a hydrogen atmosphere at room temperature for 18 h to cleave the benzyl protecting group. After filtration and removal of the solvent, the polymer was dried in vacuum at 80 °C for 24 h to obtain α,ω-hydroxy-functional copolymers.

gradient composition manifest in a predominant incorporation of PO repeat units at the beginning of the chains and EEGE units at the end of the polymerization, however, without hydroxyl end groups.18 No data on the hydrophilicity and solubility of the respective deprotected polymers were given. To the best of our knowledge, the introduction of hydroxyl groups into PPO by random anionic copolymerization has not been reported to date. Herein, we describe the synthesis of multi-hydroxyl-functional PPOs via random anionic copolymerization of PO and the protected glycidol monomer EEGE and subsequent cleavage of the protecting groups. By using a suitable hydroxyl-initiator dihydroxy-terminated and in-chain hydroxyl-functional PPOs have been obtained in three steps (Figure 1). The materials were investigated with respect to their solubility in aqueous solution and their LCST behavior.

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EXPERIMENTAL SECTION

Reagents. All reagents and solvents were used as received, if not otherwise mentioned. Deuterated DMSO-d6 and CDCl3 were purchased from Deutero. PO was dried over CaH2 and distilled under vacuum prior to use. Ethoxyethyl glycidyl ether (EEGE) was prepared as described by Fitton et al.,19 dried over CaH2, and freshly distilled before use. Instrumentation. 1H NMR and 13C NMR spectra were recorded at 300 and 75 MHz, respectively, on a Bruker AC300 and were referenced internally to residual proton signals of the deuterated solvent. For SEC measurements in DMF (containing 0.25 g L−1 of lithium bromide as an additive), an Agilent 1100 series was used as an integrated instrument including a PSS HEMA column (106/104/102 Å porosity) and both a UV and RI detector. Calibration was achieved with poly(ethylene oxide) (PEO) standards provided by Polymer Standards Service (PSS). Matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF) measurements were performed on a Shimadzu Axima CFR MALDI-TOF mass spectrometer, using dithranol (1,8,9-trishydroxyanthracene) as matrix. DSC curves were recorded with a Perkin-Elmer DSC 7. Samples were dried for 24 h at 80 °C in vacuum before measurements. Cloud-points were determined in deionized water at varying concentration and observed by optical transmittance of a light beam (λ = 632 nm) through a 1 cm sample quartz cell. The measurements were performed in a Jasco V630 photospectrometer with a Jasco ETC-717 Peltier element. The intensities of the transmitted light were recorded versus the temperature of the sample cell. The heating/cooling rate was 1 K min−1, and values were recorded every 0.1 K.

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RESULTS AND DISCUSSION Synthesis of Multifunctional Poly(propylene oxide)s. The synthetic strategy developed to obtain the multihydroxyfunctional PPO structures with two terminal hydroxyl functionalities is shown in Figure 1. The monobenzyl-protected ethylene glycol initiator 2-(benzyloxy)ethanol (BnO−) was 3040


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Table 1. Characterization Data for BnO-PPOn-co-PEEGEm−OH and BnO-PPOn-co-linPGm−OH Copolymers with Varying Monomer Composition

a

no.

sample

EEGE/mol %a

Mn,th/g mol−1

Mn/g mol−1a

Mn/g mol−1b

Mw/Mnb

Tg/°C

LCST/°C

1a 2a 3a 4a 5a 6a 7a 8a 9a

BnO-PPO50−OH BnO-PPO62-co-PEEGE1−OH BnO-PPO49-co-PEEGE6−OH BnO-PPO45-co-PEEGE9−OH BnO-PPO38-co-PEEGE11−OH BnO-PPO36-co-PEEGE12−OH BnO-PPO17-co-PEEGE20−OH BnO-PPO16-co-PEEGE43−OH BnO-PEEGE53−OH

0 2 11 16 22 25 54 74 100

3060 3280 3500 3720 3940 4160 5260 6360 7460

3010 3970 3960 4000 3900 4000 4100 7400 7940

1960 2450 2040 2220 2330 1440 1880 3710 2730

1.10 1.07 1.12 1.09 1.16 1.14 1.13 1.09 1.15

−73 −71 −69 −70 −69 −72 −67 −66 −67

20.0 13.3 13.5 11.4 8.5 n.d. 10.9 4.7 6.7

2b 3b 4b 5b 6b 7b 8b 9b

BnO-PPO62-co-linPG1−OH BnO-PPO49-co-linPG6−OH BnO-PPO45-co-linPG9−OH BnO-PPO38-co-linPG11−OH BnO-PPO36-co-linPG12−OH BnO-PPO17-co-linPG20−OH BnO-PPO16-co-linPG43−OH BnO-linPG53−OH

2 11 16 22 25 54 74 100

3100 3140 3180 3220 3260 3460 3660 3860

3890 3500 3390 3140 3020 2650 4270 4100

2370 2080 1950 2190 1350 1610 2920 1910

1.07 1.14 1.12 1.18 1.12 1.21 1.22 1.25

−69 −63 −61 −63 −55 −31 −29 −27

21.2 32.6 35.9 39.3 79.2 − − −

Calculated from 1H NMR. bCalculated from SEC (PEO standards).

for all samples (Figure 2). Molecular weights can be determined from SEC and, more accurately, by end group

used as cesium salt to initiate the copolymerization of PO and EEGE. This initiator was chosen to generate α,ω-hydroxypoly(propylene oxide)-co-poly(glycerol) (HO-PPO n -colinPGm−OH) copolymers in a stepwise deprotection approach (Figure 1). This strategy also permits to only deprotect the functional polymers at the chain end, if required. Cesium alkoxides are known to be efficient initiators for the anionic polymerization of epoxides that also lower the tendency of PO to isomerize to allyl alcohol, which would lead to a substantial amount of terminal double bonds.11a,12 The ring-opening anionic polymerization of EEGE does not involve any side reactions, and thus copolymers with narrow molecular weight distribution can be obtained in good yields. The polymerization was quenched with methanol after 18 h, and the cesium salts were extracted with sodium carbonate solution from the polymer solution in chloroform. Subsequent acidic hydrolysis of the acetal protecting groups releases the primary hydroxyl groups and generates the linear glycerol units. The hydrolysis can be conducted without further purification of the polymer obtained. To regenerate the hydroxyl functionality at the α-position, the benzyl protective group was removed by catalytic hydrogenation with palladium on activated charcoal. The EEGE content in the copolymer was varied between 2 and 74% to adjust systematically the hydrophilicity of the materials. Characterization data for the series of copolymers obtained from the copolymerization of PO and EEGE are given in Table 1. The ratio of monomer to initiator (50:1) was kept constant. Because EEGE exhibits a higher molecular weight than PO, the theoretical molecular weights of the polymers increase with increasing amount of EEGE incorporated, referring to a molecular weight of 3000 g mol−1 for the PPO homopolymer and up to 7500 g mol−1 for the PEEGE homopolymer. In general, polymers with a slight deviation in molecular weight with respect to the theoretical values based on the ratio of monomers and initiator were obtained. This may be explained by experimental inaccuracies when measuring the monomers volumetrically. Size exclusion chromatography (SEC) shows narrow molecular weight distributions with Mw/Mn values below 1.15

Figure 2. SEC results of several BnO-PPOn-co-PEEGEm−OH copolymers.

analysis of the corresponding 1H NMR spectra of the copolymers (Table 1), provided that chain transfer to the monomer does not take place (which would lead to allylinitiated chains, as will be discussed below). A deviation of the molecular weights calculated from NMR and the values obtained from SEC characterization becomes progressively obvious with increasing amount of EEGE. NMR and MALDI TOF investigations show (see below) that the initiator is incorporated in each polymer chain. Therefore, the molecular weights calculated from NMR are taken as the correct values. The deviation can most likely be explained by the different chemical structure of the polymers investigated in comparison with the calibration standard applied (PEO): The more EEGE units are incorporated, the more side chains are introduced, and the hydrodynamic radius that determines the elution volume changes compared with the PEO standards employed. Therefore, an increasing difference between the values is observed, which was previously reported in one case for other EEGE copolymers.21 3041


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Figure 3. 1H NMR spectrum (CDCl3, 300 MHz) and structural assignment of BnO-PPO20-co-PEEGE30−OH.

Figure 4. Typical MALDI-TOF mass spectra of BnO-PPO49-co-PEEGE6−OH (sample 3a): (a) full spectrum and (b) zoom-in with peak assignment.

copolymer with 2-(benzyloxy)ethanol as initiator, 47 units of PO, and 3 units of EEGE + Cs+ (calculated isotopic mass = 3453.4 g mol−1). The multitude of mass peaks results from the different combinations of the two comonomers, but the expected mass difference of both comonomers (PO: 58.1 g mol−1, EEGE: 146.2 g mol−1) can be found between different signals. Furthermore, it has to be emphasized that peaks corresponding to allyloxy-initiated chains that would result from deprotonated, rearranged PO molecules are absent in the mass spectrum, confirming once again that chain transfer to the monomer does not take place. Copolymerization Kinetics Characterization via 1H NMR. From a comparison with the copolymerization of EO and PO,22 a higher reactivity of EEGE compared with PO can be expected. A drastic difference of reactivity would lead to gradient or block-like copolymers rather than random copolymers. In a previous study with copolymers of PO and EEGE generated by monomer-activated polymerization, higher reactivity of PO compared with EEGE was reported.18

Furthermore, NMR analysis permits the calculation of the ratio of comonomers incorporated by determination of the fraction of EEGE units from the intensity of the acetal proton signal (Figure 3). (A detailed description of the calculation is given in the Supporting Information.) The targeted fractions were 5, 10, 15, 20, 25, 50, and 75% for samples 2a to 8a. The incorporated monomer ratios determined by NMR are summarized in Table 1. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) as a crucial characterization method for the detailed investigation of end-functional polymers was used to confirm that both comonomers are incorporated in the polymer chain and to demonstrate the absence of homopolymer as well as polymer chains initiated by allyloxy groups resulting from rearranged PO. The MALDI-TOF spectrum (Figure 4) reveals only one distribution mode, which is unambiguously assigned to 2(benzyloxy)ethanol-initiated copolymer of PO and EEGE. One representative mass peak at m/z 3453.2 corresponds to the 3042


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To confirm or dispute the random nature of the copolymerization, an investigation of the relative reactivity of EEGE and PO in anionic polymerization with respect to copolymerization kinetics and a detailed analysis of the evolution of the copolymer structure in the course of the polymerization are crucial. An experimental procedure that has recently been developed in our group20 was used to investigate the copolymerization behavior of EEGE and PO: A DMSO-d6 solution of the initiator and a mixture of PO and EEGE were separately transferred to an NMR tube under an argon atmosphere and cooled with liquid nitrogen. The cold NMR tube was evacuated and flame-sealed, and the polymerization was subsequently initiated by warming the tube to the respective temperature of the kinetics experiment. Immediately after melting and mixing, the first NMR spectrum was recorded. For experimental reasons, a minimum amount of DMSO-d6 was used instead of polymerizing the comonomer mixture in bulk. To examine the copolymerization kinetics, three different temperatures, that is, 25, 40, and 60 °C, with molar fractions of EEGE of 15% were applied. The molecular weight distributions of all samples obtained in these NMR experiments were narrow, with polydispersities around 1.07 (Supporting Information, Figure S2). Incorporation of the two comonomers and the growth of the polyether chain as well as the compositional drift in monomer feed were followed by the decrease in the epoxide signals located at 2.91 ppm for the methine proton of PO and at 3.07 ppm for the methine proton of EEGE, respectively. All signals are referenced to the peak of the methylene group of the initiator at 4.54 ppm, which was set to 2 and remains constant during the reaction. The temperature dependence of the monomer conversion for 25, 40, and 60 °C is illustrated in Figure 5. Whereas at 60

Figure 6. Percentage of monomer conversion of propylene oxide and ethoxy ethyl glycidyl ether (ca. 15% EEGE) versus total monomer conversion for copolymerization at 25, 40, and 60 °C, measured in DMSO-d6.

illustrates the evolution of monomer feed composition during polymerization, revealing a somewhat faster incorporation of EEGE compared with PO into the growing polymer chain in the initial stage of the polymerization. Throughout the polymerization, the molar ratio of EEGE units in the polymer chain is higher than the initial ratio of the monomer feed. However, toward the end of the copolymerization, the total composition drops to the ratio of the initial feed. This indicates the formation of mainly random copolymers of PO and EEGE by anionic copolymerization, with a tapered structure constituted of more EEGE units at the beginning of the chains and PO units at the end, in contrast with the results found for the monomer-activated mechanism. Most importantly, a strong gradient or even block formation is clearly not observed. Plotting the polymer molecular weight versus total monomer conversion resulted in a linear graph (Supporting Information, Figure S3). 1 H NMR measurements show furthermore that the wellknown proton abstraction mechanism in the polymerization of PO can be avoided by adjusting the reaction conditions. First experiments at 60 °C in DMSO solution had shown the occurrence of up to 30% allyl ether initiated polymer chains.23 This can be completely suppressed by lowering the polymerization temperature to 40 °C and carrying out the polymerization in bulk (Supporting Information, Figure S4). Upon cleavage of the ethoxy ethyl protecting groups from the glycidol repeat units of the PPO-co-PEEGE copolymers by acidic hydrolysis, linear multifunctional PPO copolymers (deprotected samples 2b−9b, Table 1) with free hydroxyl groups at the backbone were obtained. Deprotection was carried out using a strongly acidic ion-exchange resin, such as Dowex 50WX8. SEC measurements show a shift of the distribution mode of the polymer to higher elution volumes that reflects the polarity change of the copolymer upon cleavage of the hydrophobic ethoxy ethyl protective group to the very polar hydroxyl group. The molecular weight distributions remained monomodal and narrow for all samples, confirming the stability of the polyether backbone under the acidic conditions chosen for the removal of the protective groups (Figure 7). The disappearance of the acetal proton signal (4.75 ppm) can also be monitored by 1H and 13C NMR spectroscopy (see Supporting Information, Figure S5 for the 13C NMR

Figure 5. Monomer conversion versus time plots for copolymerization of propylene oxide with ethoxy ethyl glycidyl ether (15%) at 25, 40, and 60 °C, measured in DMSO-d6.

°C full conversion (>99%) was reached within 100 min, the copolymerization at 40 °C takes about 7 h to complete. At the lowest investigated temperature (25 °C), even after 12 h the overall conversion is only 70%. Despite the reactivity differences at the temperatures studied that lead to different polymerization rates, our NMR results demonstrate that the relative reactivities of the comonomers remain constant at all investigated temperatures. Figure 6 3043


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Thermal Behavior. The thermal behavior of the random copolymers is of interest with respect to the effect of hydroxyl groups on the glass-transition temperature of PPO (Table 1). To the best of our knowledge, the thermal properties of random copolymers of the type PPO-co-PEEGE and PPO-colinPG have not been studied to date. Differential scanning calorimetry (DSC) has been used to quantifiy the thermal properties of the materials. Whereas the glass-transition temperature (Tg) of the copolymers PPO-co-PEEGE shows only slight variation reflecting the very similar Tg values of the corresponding homopolymers (−73 °C for PPO and −65 °C for PEEGE), the glass-transition temperature increases from −69 °C for BnO-PPO62-co-linPG1−OH to −31 °C for the copolymer BnO-PPO20-co-linPG30−OH after deprotection, as can also be seen in the Supporting Information (Figure S6). We ascribe this to the additional interaction of the hydroxyl groups via hydrogen-bonding in the copolymers. As both homopolymers are not crystalline, no melting peak is observed for all copolymer samples. Aqueous Solution Properties and LCST Behavior. From other aliphatic polyether copolymers, temperaturedependent solubility behavior in aqueous solution is known.24 Therefore, the cloud-points of aqueous copolymer solutions of BnO-PPOn-co-linPGm−OH have been investigated by monitoring the transmittance of a laser beam through a sample cell at a heating rate of 1 K min−1. The transition from transparent to opaque solutions was taken as cloud-point and defined as the temperature corresponding to 50% transmission. In the following, we will refer to the LCST of the copolymers in a general sense, although the term LCST in a strict sense corresponds to the lowest cloud-point temperature possible, which is observed only at a certain concentration of a polymer. Whereas the LCSTs of the BnO-PPOn-co-PEEGEm−OH copolymer samples are generally below room temperature (5− 13 °C, Table 1), which is in agreement with the PPO-PEEGE block copolymers investigated by Dimitrov et al.,11a the

Figure 7. SEC traces showing the molecular weight distribution shift upon cleavage of the acetal protecting groups.

DEPT spectrum). In this manner, full deprotection was confirmed (Figure 8). A broad signal corresponding to the proton of the newly generated primary hydroxyl groups arises between 2.7 and 3.3 ppm (depending on the interaction of the hydrogen bonds in solution; in Figure 8 this resonance can be seen at 3.2 ppm). To obtain the α-deprotected PPO copolymers with hydroxyl functionalities at both termini and several OH-groups randomly distributed along the PPO chain, we carried out catalytic hydrogenation with palladium on activated charcoal at 60 °C for 18 h with the BnO-PPOn-co-PEEGEm−OH copolymer. Deprotection was followed by 1H NMR, monitoring the disappearance of the aromatic protons and the methylene group in benzyl position, as can be seen from Figure 9 from the disappearance of the signals at 4.48 and 7.15 to 7.50 ppm. Subsequent acidic deprotection of the acetal groups (as previously described) yields HO-PPOn-co-linPGm−OH.

Figure 8. 1H NMR spectra (CDCl3, 300 MHz) showing the cleavage of the acetal protecting groups: (a) BnO-PPOn-co-PEEGEm−OH and (b) BnOPPOn-co-linPGm−OH. 3044


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Figure 9. 1H NMR spectra (CDCl3, 300 MHz) of BnO-PPOn-co-PEEGEm−OH: (a) and HO-PPOn-co-PEEGEm−OH and (b) showing complete cleavage of the benzyl protecting groups.

deprotected BnO-PPOn-co-linPGm−OH copolymer samples show considerably higher cloud-points. Samples 2b−5b with 2−22% free glycerol units in the PPO backbone show the typical clouding behavior presented by sigmoidal curves (Figure 10), corresponding to a thermally induced phase separation.

To investigate further the properties of the copolymers, the concentration dependence of the LCST was investigated by comparing solutions with concentrations between 0.1 and 10 mg mL−1 of one representative sample. The critical temperatures were found to decrease with increasing copolymer concentration, indicating that the solutions become less translucent at higher concentrations, as also reported for the corresponding block copolymers.12

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CONCLUSIONS PPO is generally known as a nonpolar polyether that possesses no in-chain functionalities. In the current work, we have described a convenient synthesis for hydrophilic, functional PPO-based copolymers. The materials were obtained by random copolymerization of PO and EEGE, followed by subsequent cleavage of the protecting groups to release an adjustable number of primary hydroxyl groups randomly distributed in the PPO chain and two hydroxyl functionalities at both chain ends. Selective deprotection of the end group to generate an in-chain protected diol as a telechelic structure has also been demonstrated. It is important to emphasize that carrying out the polymerization at low temperature with Cs counterions permitted to avoid the commonly problematic proton abstraction from PO. 1H NMR kinetic measurements show gradient monomer consumption and incorporation with a faster incorporation of EEGE compared with PO. Temperature-dependent aqueous solubility can be achieved by adjusting the comonomer composition. A glycerol content as low as 11% is sufficient to render the PPO copolymers completely water-soluble at ambient temperature. Precise design of polarity, functionality, and LCST offers intriguing options for the α,ω-hydroxy-poly(propylene oxide)-co-poly(glycerol) structures as functional soft segment in PUs or for biomedical applications, for example, in hydrophilic PUs for wound management as well as generally for functional PUs.25 Furthermore, a usage as protein repellent surface coating is imaginable. Currently, we are studying toxicity and cell uptake of the copolymers as a potential alternative for poly(ethylene glycol)26 in biomedical applications. It is obvious that the presented copolymers also offer opportunities for the grafting of other polymers onto the PPO copolymer backbone. We are

Figure 10. Intensity of transmitted laser light versus the temperature for BnO-PPOn-co-linPGm−OH copolymers of varied composition at a concentration of 1 mg mL−1 in aqueous solution.

Samples with glycidol content exceeding 54% show no temperature-dependent transmittance and are thus soluble in water over the whole temperature range. Sample 6b with 25% glycidol content is unique for this series of copolymers in that it shows no sharp decrease but only rather slow clouding that does not reach 0% within the investigated temperature range (up to 95 °C). A comparison of the LCSTs of all copolymers with varying comonomer content reveals that with increasing amount of incorporated hydrophilic comonomer (linear glycerol units with a free hydroxyl group), the LCST increases progressively. Remarkably, a glycerol content as low as 11% renders the PPO copolymer completely soluble in water at ambient temperature. 3045


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(22) Heatley, F.; Yu, G.-E.; Booth, C.; Blease, T. G. Eur. Polym. J. 1991, 27 (7), 573−579. (23) Yu, G.-E.; Masters, A. J.; Heatley, F.; Booth, C.; Blease, T. G. Macromol. Chem. Phys. 1994, 195 (5), 1517−1538. (24) Mangold, C.; Obermeier, B.; Wurm, F.; Frey, H. Macromol. Rapid Commun. 2011, 32 (23), 1930−1934. (25) Basko, M.; Bednarek, M.; Billiet, L.; Kubisa, P.; Goethals, E.; Du Prez, F. J. Polym. Sci., Part A: Polym. Chem. 2011, 49 (7), 1597−1604. (26) Dingels, C.; Schömer, M.; Frey, H. Chem. Unserer Zeit 2011, 45 (5), 338−349.

convinced that our results will inspire further development of other PPO copolymers with functional side chains.

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ASSOCIATED CONTENT

* Supporting Information S

Additional spectra and characterization data. 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 We thank Dr. Mihail Mondeshki for NMR measurements, Dr. Elena Berger-Nicoletti for MALDI-TOF measurements, and Sebastian Jäger for technical assistance. M.S. acknowledges financial support by the Max Planck Graduate Center (MPGC) with the Johannes Gutenberg-University.

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REFERENCES

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