Easy Access to Amphiphilic Heterografted Poly(2-oxazoline) Comb

Jun 18, 2013 - The reported data for P1 to P5 represent the average of three independent measurements. The refractive index increment (dn/dc) of P1 an...
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Easy Access to Amphiphilic Heterografted Poly(2-oxazoline) Comb Copolymers Christine Weber,†,‡ Michael Wagner,†,‡ Duygu Baykal,§ Stephanie Hoeppener,†,‡ Renzo M. Paulus,†,‡ Grit Festag,†,‡ Esra Altuntas,†,‡ Felix H. Schacher,†,‡ and Ulrich S. Schubert†,‡,⊥,* †

Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstrasse 10, 07743 Jena, Germany ‡ Jena Center for Soft Matter (JCSM), Friedrich Schiller University Jena, Philosophenweg 7, 07743 Jena, Germany § Chemistry Department, Bogazici University, Kare Blok Binası 3, 34342 Bebek/Istanbul, Turkey ⊥ Dutch Polymer Institute (DPI), John F. Kennedylaan 2, 5612 AB Eindhoven, The Netherlands S Supporting Information *

ABSTRACT: A series of heterografted polymers with a polymethacrylate backbone and varying composition of oligomeric 2-oxazoline side chains was synthesized by copolymerization of oligo(2-ethyl-2-oxazoline)methacrylate and oligo(2-n-nonyl-2-oxazoline)methacrylate (ONonOx) macromonomers using the reversible addition−fragmentation chain transfer (RAFT) technique. Kinetic studies revealed pseudofirst order kinetics, PDI values below 1.3, and a slight enrichment of ONonOx side chains toward the end of the reaction. The heterografted comb polymers were characterized by means of 1H NMR spectroscopy (0 to 100 mol % ONonOx) and size exclusion chromatography. Differential scanning calorimetry and wide-angle X-ray scattering revealed partial crystallinity of the lateral n-nonyl chains within the ONonOx domains. Depending on the composition of the amphiphilic copolymer, the comb polymers either formed unimers or aggregated (super)structures in water, methanol, and ethanol, as investigated by dynamic light scattering and cryo-transmission electron microscopy.

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phase separation.7 To the best of our knowledge, there is no direct experimental evidence of this prediction. Most likely, the reason for the few examples of this polymer type is the demanding synthesis that requires full exploitation of the opportunities provided by living and controlled polymerization techniques. The most commonly used strategy is the grafting of different side chains to one polymeric backbone.8−10 Alternatively, a combination of oligo(ethylene oxide) (PEO) macromonomers and the grafting-from approach,11,12 or a combination of both, grafting-from and grafting-onto,13 have been applied. For the latter type, a phase separation of the side chains could be successfully demonstrated in thin films.13 However, neither grafting-from nor grafting-onto can guarantee full functionalization with side chains, and a judgment about the distribution of the types of side chains along the backbone is difficult. In view of this background, it seems to be favorable to copolymerize two different macromonomers. Regardless of the disadvantage of the formation of shorter side chains, this simple approach would allow to extract a straightforward “picture” of the obtained structure by kinetic studies, and the targeted

INTRODUCTION

The advancements in the field of modern synthetic polymer chemistry has provided a useful toolbox for the synthesis of manifold polymeric architectures by utilization of living and controlled polymerization methods. In particular, comb polymers are of high fundamental interest because the steric demand of the side chains prevents the macromolecules from coiling and, thus, results in low viscosity, lowered glass transition temperatures, or unusual mechanical properties.1,2 Next to “simple” comb polymers, a range of more advanced architectures have been realized, e.g., brushes with diblock or even triblock copolymer side chains.3−5 On the other hand, there are only a few examples of comb polymers containing two different kinds of side chains (“heterografted” polymers1,6). These types of copolymers are highly interesting for both synthetic and physical polymer science. In particular, the combination of two types of side chains with different properties that are connected to the same polymer backbone might lead to new insights into the nature of macromolecules as well as materials with exceptional properties. As recently predicted by molecular simulations, such polymers (with appropriate composition and solvent environment) might exist as unimolecular cylinders of Janus-type or Janus dumbbell-type, if both side chain types are likely to undergo © 2013 American Chemical Society

Received: May 6, 2013 Revised: June 2, 2013 Published: June 18, 2013 5107

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nitrogen. Dichloromethane (Aldrich) was dried in a solvent purification system (Pure Solv EN, InnovativeTechnology) before use as a polymerization solvent. Methyl tosylate (98%, Aldrich, MeTos) was distilled under reduced pressure and stored under nitrogen. Methacrylic acid (99%, Aldrich, MAA) and acetic acid (99.7%, Sigma-Aldrich) were used as received. Triethylamine was dried over potassium hydroxide and distilled under nitrogen. 2,2′Azobis(2-methylpropionitrile) (98%, Acros, AIBN) was recrystallized from methanol, and the chain transfer agent (CTA) 2-cyanopropyl dithiobenzoate (CPDB, 97%) was obtained from Aldrich. Preparative size exclusion chromatography was performed using BioBeads-SX1 from BioRad. The fluorescent probe pyrene was purchased from Sigma. All other chemicals were obtained from standard suppliers and were used without further purification. Instrumentation. The polymerization of EtOx and NonOx was performed in a Biotage Initiator Sixty microwave synthesizer. 1H NMR spectra were recorded in CDCl3 on a Bruker Avance 250 or 300 MHz using the residual solvent resonance as an internal standard. UV−vis emission spectra were recorded on a FP-6500 spectrofluorometer from Jasco. Size exclusion chromatography (SEC) was measured on a Shimadzu system equipped with a SCL-10A VP system controller, a LC-10AD VP pump, and a RID-10A refractive index (RI) detector using a solvent mixture containing chloroform, triethylamine, and isopropanol (94:4:2) at a flow rate of 1 mL min−1 on a PSS-SDVlinear S 5 μm column at 40 °C. The system was calibrated with polystyrene (PS; Mp = 374 to 128 000 g mol−1) standards. A second SEC system (Agilent 1200 series) was equipped with a G1362A refractive index detector, and both a PSS Gram30 and a PSS Gram1000 column in series, whereby N,N-dimethylacetamide (DMAc) with 2.1 g L−1 of LiCl was applied as an eluent at 1 mL min−1 flow rate and the column oven was set to 40 °C. For the determination of the absolute molar masses, a third SEC system from Shimadzu was used. This system was equipped with a SCL-10A VP system controller, a LC-10AD VP pump, a RID-10A refractive index (RI) detector, and a multiangle laser light scattering (MALLS) detector from PSS (SLD 7000, 660 nm) using THF with 1% diethylamino ethylamine as eluent (flow rate 1 mL min−1, PSSSDV-linear M column, 40 °C). The reported data for P1 to P5 represent the average of three independent measurements. The refractive index increment (dn/dc) of P1 and P5 was determined on an Optilab rEX from Wyatt using the eluent of the SEC system at 40 °C. The dn/dc values of P2−P4 were extrapolated from these according to the copolymer composition (molar ratios of M1 and M2). The corresponding values are as follows: dn/dc(P1) = 0.102 mL mg−1; dn/dc(P2) = 0.097 mL mg−1; dn/dc(P3) = 0.094 mL mg−1; dn/ dc(P4) = 0.090 mL mg−1; dn/dc(P5) = 0.087 mL mg−1. Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectra (MALDI TOF MS). These spectra were measured on a Voyager-DE STR Biospectrometry Workstation (Applied Biosystems) in the positive reflector mode using sodium iodide as ionization salt and trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) or dithranol as matrix. The instrument was calibrated prior to each measurement with an external PMMA standard from PSS Polymer Standards Services GmbH (Mainz, Germany). Electrospray Ionization (ESI) Q-TOF MS. These measurements were performed with a microTOF (Bruker Daltonics) mass spectrometer equipped with an automatic syringe pump, which was supplied by KD Scientific for sample injection. The mass spectrometer was operating in the positive ion mode. The standard electrospray ion (ESI) source was used to generate the ions. The ESI Q-TOF MS instrument was calibrated in the m/z range 50−3000 using an internal calibration standard (Tunemix solution), which was supplied from Agilent. Data were processed via Bruker Data Analysis software version 4.0. Differential Scanning Calorimetry (DSC). This was performed on a Netzsch DSC F1 Phoenix under nitrogen atmosphere with a heating rate of 20 K min−1 from −100 to +200 °C. Wide Angle X-ray Scattering (WAXS). These measurements were performed on a Bruker AXS Nanostar (Bruker, Karlsruhe, Germany),

macromolecule could be obtained in three independent steps. Despite this charming approach, its application mostly relies on very simple macromonomers without further substituents, such as monomers with long hydro- or fluoro-carbon chains,14,15 their combination with PEO macromonomers,16 or two PEO macromonomers with different end groups.17 The sterically less demanding PEO macromonomers have also been copolymerized with polystyrene (PS)18−20 and poly(lactic acid)21 macromonomers. The only reported copolymerization of two macromonomers that both carry substituents is the ringopening metathesis polymerization of norbonene-functional PS and polybutadiene.18 Compared to those, a less well-known class of polymers are poly(2-oxazoline)s (POx).22,23 They can be obtained by living cationic ring-opening polymerization (CROP), and their properties can be adjusted by variation of the substituent in 2-position of the monomer ring. The living nature of the CROP has been exploited in order to synthesize simple comb polymers with POx side chains via the commonly used synthetic approaches: macromonomer method,24,25 graftingonto26 and grafting-from.27 In addition, the latter approach was recently applied to obtain more complex comb polymers with block- or random copolymer side chains.28 Conversely, Puts and Sogah reported the synthesis of a comb polymer with a POx backbone and PS side chains using the macromonomer method already in 1997.29 We have demonstrated that POx macromonomers with a methacrylate end-group can be polymerized via reversible addition−fragmentation chain-transfer (RAFT) polymerization as long as the substituent is rather small, i.e., for ethyl-24 or cyclopropyloxazoline.30 In order to induce phase separation of the side chains in a heterografted comb polymer, the respective macromonomers should be highly incompatible, as would be the case for a hydrophilic and a highly hydrophobic polymer. Poly(2-n-nonyl-2-oxazoline) (PNonOx, hydrophobic) and poly(2-ethyl-2-oxazoline) (PEtOx, hydrophilic with lower critical solution temperature in water) fulfill this criterion.31,32 The synthetic strategy toward the resulting amphiphilic comb polymers is summarized in Scheme 1. Herein, we present the synthesis and careful Scheme 1. Schematic Representation of the Synthesis Route toward Heterografted Poly(2-oxazoline)-Based Comb Polymers

structural characterization of these heterografted POx based comb polymers at each step. In addition, the polymer properties in the bulk as well as in solution are studied by means of differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), dynamic light scattering (DLS), and cryo-transmission electron microscopy (cryo-TEM).

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EXPERIMENTAL SECTION

Materials. The monomers 2-ethyl-2-oxazoline (99%, Acros, EtOx) and 2-n-nonyl-2-oxazoline (NonOx, kind gift from Henkel, Germany) were dried over barium oxide and distilled under nitrogen prior to use. Acetonitrile (extra dry) was purchased from Acros and stored under 5108

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equipped with a microfocus X-ray source (Incoatec IμSCu E025, Incoatec, Geesthacht, Germany), operating at λ = 1.54 Å. A pinhole setup with 750, 400, and 1000 μm (in the order from source to sample) was used and the sample-to-detector distance was 12 cm. The samples were mounted on a metal rack with temperature control and fixed using tape. The scattering patterns were corrected for the beam stop and the background (Scotch tape) prior to evaluations. All samples were annealed for 4 h at 40 °C under vacuum before measurement. Data collection was performed within 2 h. Dynamic Light Scattering (DLS). This was performed on an ALVCGS-3 system (ALV, Langen, Germany) equipped with a He−Ne laser operating at a wavelength of λ = 633 nm. All measurements were carried out at an angle of 90° and at 25 °C after an equilibration time of 120 s. For analyzing the autocorrelation function (ACF), the CONTIN algorithm33 was applied. Apparent hydrodynamic radii were calculated from translational diffusion coefficient according to the Stokes−Einstein equation, eq 1: rh =

kT 6πηD

BioBeads-SX-1 column using THF as eluent. Subsequent to removal of the solvent under reduced pressure, the samples were dried in vacuum overnight and analyzed by 1H NMR spectroscopy (250 MHz, CDCl3). Comb Polymers. For each of the comb polymers P1−P5, the ratio of the monomers M1 and M2 was varied, but the [M]total/[CPDB]/ [AIBN] ratio was kept constant at 60/1/0.25. The overall monomer concentration was kept at 0.5 mol L−1. In a representative example for P2, 1.13 g (1.9 mmol) of M1, 1 g (0.63 mmol) of M2, 1.7 mg (0.01 mmol) of AIBN, 9.3 mg (0.04 mmol) of CPDB were dissolved in 1.3 mL of THF. Subsequent to the removal of oxygen from the solution, the RAFT polymerization was performed in an oil bath at 70 °C for 22 h. The comb polymer was purified from residual monomers by preparative SEC on a BioBeads-SX-1 column using THF as eluent. Subsequent to the removal of the solvent under reduced pressure, the polymer was dried in vacuum overnight. Oligo(2-n-nonyl-2-oxazoline) Acetate (ONonOxOAc, P6). P6 was obtained in a similar fashion as described for M2. Instead of MAA and NEt3, acetic acid (0.78 mL, 13.5 mmol) and NEt3 (2.5 mL, 18 mmol) were used as end-capping agents. SEC (CHCl3, RI detection, PS calibration): Mn = 1 710 g mol−1, PDI = 1.13. 1H NMR (300 MHz, CDCl3): Quantitative degree of functionalization, degree of polymerization (DP) = 5 (see Supporting Information for further discussion and figures). Sample Preparation for DLS and Cryo-TEM. The solutions of P2− P4 in ethanol and of P2 in methanol were prepared by dissolving each polymer directly in the solvent at a concentration of 1 mg mL−1. The samples of P3−P4 in methanol and of P2−P4 in water were prepared as follows:34 1 mg of the polymer was dissolved in 100 μL THF, and the solution was dropped into 1 mL water. Subsequently, the THF was evaporated from the aqueous suspensions. Determination of the Critical Micelle Concentration (Cmc). The cmc of P2 in water was determined using pyrene as fluorescent probe following a procedure reported in literature.12 An aqueous solution of pyrene (c = 6 × 10−7 mol L−1) was prepared by adding 76 μL of a 7.9 mM solution of pyrene in acetone to 1 L of water. Varying amounts of solutions of P2 in THF were added to 3 mL aliquots of the aqueous solution to reach the final concentrations of P2 given in Figure SI-9, Supporting Information. All UV−vis emission spectra were recorded at room temperature using an excitation wavelength λex of 339 nm.

(1)

Here, rH is the hydrodynamic radius, k the Boltzmann constant, T the absolute temperature, η the viscosity of the sample, and D the translational diffusion coefficient. Cryo-Transmission Electron Microscopy (Cryo-TEM). Investigations were performed on a Philips CM-120 equipped with a 1 × 1 k CCD camera system. Quantifoil grids (R2/2 Quantifoil, Germany) were cleaned by plasma treatment prior to use. Sample preparation was carried out in a home-build, customized blotting chamber at saturated water atmosphere to avoid evaporation. Then, 6 μL of the sample solution was deposited onto the grid, equilibrated shortly, and blotted for 2 s. The grids were immediately plunched into liquid ethane to obtain vitrification, and samples were stored at liquid nitrogen temperature until transferred to the TEM utilizing a Gatan cryo transfer system. Synthesis. Macromonomers. Oligo(2-ethyl-2-oxazoline) methacrylate (OEtOxMA, M1) was synthesized as reported previously,24 and oligo(2-n-nonyl-2-oxazoline) methacrylate (ONonOxMA, M2) was obtained in a similar fashion: MeTos (1.67 g, 9 mmol), NonOx (8.88 g, 45 mmol), and CH2Cl2 (8.2 g) were weighed in a predried microwave vial under inert conditions. The CROP was performed at 140 °C for 1 min by microwave heating, the end-capping agents MAA (1.2 mL, 13.5 mmol) and NEt3 (2.5 mL, 18 mmol) were added through the septum of the vial, and the end-capping reaction was performed by heating the solution at 50 °C in an oil bath overnight. The reaction mixture was dissolved in chloroform and washed with aqueous sodium bicarbonate solution and brine. The organic phase was dried over sodium sulfate, and the volatiles were removed under reduced pressure. SEC (CHCl3, RI detection, PS calibration): Mn = 1 790 g mol−1, PDI = 1.17. 1H NMR (250 MHz, CDCl3): Degree of functionalization = 93%; degree of polymerization = 5.3 (see Figure SI-3, Supporting Information). Kinetic Studies of the RAFT Polymerization of M1 and M2. A stock solution containing 1.49 g (2.51 mmol) of M1, 3.28 g (2.06 mmol) of M2, 3.4 mg (0.021 mmol) of AIBN, 18.6 mg (0.084 mmol) of CPDB, and 5 mL of THF was gently purged with nitrogen for 30 min. Aliquots of 0.7 mL from this stock solution were transferred to separate vials, and each vial was shortly purged with nitrogen once more to remove residual oxygen. All vials were immersed in an oil bath at 70 °C and taken out after varying polymerization times. In order to obtain the overall monomer conversion, Mn and PDI values SEC (CHCl3) was measured directly from the reaction solutions. The conversion is calculated from the peak areas of comb polymer and macromonomers in the SEC traces (RI detection) according to conversion = (area comb)/(area comb + area macromonomers). The corresponding dn/dc values (CHCl3/NEt3/iPrOH, 40 °C) are as follows: dn/dc(M1) = 0.056 mL g−1; dn/dc(M2) = 0.058 mL g−1; dn/ dc(P3) = 0.061 mL g−1. To determine the copolymer composition, three samples were purified from unreacted M1 and M2 by preparative SEC on a

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RESULTS AND DISCUSSION Synthesis. The first step of the comb polymer synthesis comprised the preparation of two oligomeric POx macromonomers with a polymerizable methacrylate end group. In order to induce phase separation between both types of side chains, we chose hydrophobic PNonOx and hydrophilic PEtOx.31 To simplify the subsequent RAFT polymerization of the macromonomers and the purification of the comb polymer, the degree of polymerization (DP) was set to 5 by using the corresponding ratio of [monomer] to [initiator] for the CROP of EtOx and NonOx, respectively. The previously established synthesis route for POEtOxMA, M1, could be transferred to oligo(2-n-nonyl-2-oxazoline)methacrylate (ONonOxMA, M2). Both macromonomers were readily obtained by end-functionalization of the living oxazolinium species resulting from the CROP of EtOx and NonOx, respectively, with in situ deprotonated methacrylic acid. The macromonomers were characterized by SEC (PDI < 1.2), 1H NMR spectroscopy, MALDI and ESI Q TOF MS. For a detailed discussion of the characterization results of M1, the reader is referred to literature.24 As depicted in Figure SI-1, Supporting Information, M2 had a degree of functionalization (DF) with the polymerizable methacrylate end group of 93%, as estimated from the peak integrals derived from the POx backbone and both end groups in the 1H NMR spectrum. 5109

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a phase separation within the comb polymer would presumably take place at different “sides” of the polymethacrylate backbone in the random copolymer. In case of a preferential incorporation of one macromonomer, the resulting gradient distribution of the two types of side chains would probably result in properties comparable to linear gradient (block-like) copolymers. Thus, kinetic studies of the statistical RAFT copolymerization of M1 and M2 were performed using AIBN as initiator and CPDB as chain transfer agent (CTA). For this purpose, a stock solution containing M1 and M2 in an almost equal molar ratio ([M1]/[M2]/[CTA] = 30/25/1) was prepared in THF. In order to enable detailed characterization of the forming comb polymer, a number of vials containing aliquots of the stock solution were reacted for varying polymerization times at 70 °C, and were subsequently analyzed. The overlapping signals of M1 and M2 in SEC and 1H NMR spectra prevented an accurate determination of the independent conversions of each monomer, but an integration of the peaks derived from both macromonomers and the formed comb polymer in the SEC traces gave access to the overall monomer conversion throughout the kinetic studies due to the similar dn/dc values of M1, M2, and P3, respectively (Figure 2, left). The resulting semilogarithmic kinetic plot (Figure 3) revealed an induction period as often observed for RAFT

Figure 1 shows the MALDI TOF mass spectrum of M2, clearly revealing the presence of the desired methacrylate end

Figure 1. MALDI TOF mass spectrum of M2 (NaI, dithranol, laser intensity 1141). The insets show the measured and calculated isotopic patterns for the representative structures of both series that are depicted on the right-hand side (pentamers).

group. The distance between two neighboring peaks (Δm/z = 197.2) corresponds to the mass of one NonOx repeating unit, and the macromonomer is ionized with a sodium cation. However, the methacrylate end group is prone to undergo fragmentation during the MALDI process. Upon cleavage of the ester-based end group an oxazolinium species is formed, which is intrinsically positively charged and, thus, results in rather intense peaks in the spectrum (series B in Figure 1). Variation of the matrix could not completely suppress this fragmentation. When DCTB was applied as matrix instead of dithranol (Figure SI-2, Supporting Information), a higher laser intensity had to be applied to ionize M2 resulting in an even increased intensity of the series B. Together with the fact that unfunctionalized oxazolinium species from the CROP will be transformed to POx with terminal −OH groups during the aqueous purification procedure this indicates that species “B” indeed is a result from an in-source fragmentation during the MALDI process. Surprisingly, similar observations were made when M2 was analyzed by ESI TOF MS despite of the softer ionization technique, although the fragmentation occurred to a less pronounced extent compared to MALDI TOF MS (Figure SI-3, Supporting Information). With respect to the properties of the comb polymers with mixed side chains, it is of major importance whether the two types of macromonomer copolymerize in a random or gradient fashion. If both monomers are incorporated with an equal rate,

Figure 3. Kinetic studies of the statistical copolymerization of M1 and M2 (M1/M2/CTA = 30/25/1, [M] = 0.5 mol L−1 in THF, T = 70 °C). All data include both monomers. Left: Semilogarithmic kinetic plot. The linear fit corresponds to ln([M]0/[M]t) = −0.24 + 0.105 h−1 × t. The data points within the induction period (2.3 h) were excluded from the fit. Right: Evolution of the molar mass and the PDI value with conversion (determined by SEC, RI detection, PS calibration).

Figure 2. Left: Normalized SEC traces (CHCl3, RI detection) obtained during kinetic studies of the statistical copolymerization of M1 and M2. Right: Molar fractions of both monomers incorporated in the comb polymer (determined by 1H NMR spectroscopy of the purified kinetic samples). Lines are added as a guide to the eye. 5110

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Table 1. Characterization Results of the Comb Polymers P1−P5 with Varying Molar Fractions of M1 and M2 (M/CTA = 60) SEC (CHCl3) P1e P2 P3 P4 P5f

SEC (DMAc)

M1/M2 (% in feed)

M1/M2 (% 1H NMR)a

convn [%]

Mnb [g mol−1]

PDIb

Mnc [g mol−1]

PDIc

Mwd SECMALLS

100/0 75/25 56/44 28/72 0/100

100/0 63/37 44/56 16/84 0/100

69 85 71 74 66

8 400 21 000 (27 200) (35 600) 20 800

1.14 1.15 (1.47) (1.35) 1.14

15 800 30 400 40 200 38 300 (19 600)

1.24 1.15 1.24 1.19 (1.10)

25 700 59 000 93 700 106 000 86 000

a

Determined by 1H NMR (250 MHz, CDCl3). bDetermined by SEC (CHCl3, RI detection, PS calibration). P3 and P4 are very close to the exclusion limit. cDetermined by SEC (DMAc, RI detection, PS calibration). P5 not completely solubilized. dDetermined by SEC (THF, MALLS detection). dn/dc(P1) = 0.102 mL mg−1; dn/dc(P5) = 0.087 mL mg−1. The dn/dc values for P2−P4 were calculated from these according to the composition of the copolymers. eMn,theo = [M]/[CTA] × Mn(M1) × convn = 25 000 g mol−1. fMn,theo = [M]/[CTA] × Mn(M2) × convn = 63 000 g mol−1.

polymerizations; subsequently the polymerization proceeds with pseudofirst order kinetics up to conversions of >60%. From the intersection of the linear fit of the kinetic data with the t-axis, the induction period was estimated to be 2.3 h. In addition, the molar mass of the comb copolymer increases linearly with the overall monomer conversion. Together with the fact that the polydispersity index (PDI) values remained below 1.25, a controlled polymerization can be assumed. However, these data alone do not allow any judgment about the random or gradient structure of the comb copolymer. Thus, three samples representative of the beginning, the middle and the end of the kinetic studies were purified by preparative SEC on a BioBeads column. Subsequently, the composition of the purified comb copolymers was determined by 1H NMR spectroscopy to determine the ratio of both monomers incorporated (Figure 2, right, a discussion of the 1H NMR spectra can be found below). At the beginning, the molar fraction of both monomers is close to that in the feed, so that a rather random structure is expected. However, after 6 h (conversions of around 30%), the NonOxMA M2 seemed to be incorporated with a slight preference. Thus, it is likely to assume that the copolymer will have a random distribution at the beginning but may become slightly enriched with PNonOx side chains toward the end of the reaction. Subsequently, a series of comb polymers P1−P5 was synthesized under similar conditions by varying the feed ratio of M1 and M2, but keeping the overall monomer to CTA ratio constant at 60. Thus, all copolymers would have a comparable length of the methacrylate backbone but varying molar fractions of OEtOx and ONonOx side chains. All polymers were purified from the residual MMs by preparative SEC on a BioBeads column and characterized by means of 1H NMR spectroscopy and SEC (Table 1). Despite the steric demand of the n-nonyl substituents of the ONonOxMA, M2 could even be homopolymerized resulting in a comb polymer with a polymethacrylate backbone and exclusively ONonOx side chains (PONonOxMA, P5). Figure 4 shows an overlay of the 1H NMR spectra of both homopolymers (POEtOxMA P1 and PONonOxMA P5) and of a comb polymer with mixed side chains P2 together with the structural assignment of the peaks. Obviously, the signals derived from the methyl end groups, the POx backbone and the methylene group next to the amide bonds overlap for both types of side chains (labeled with numbers). On the other hand, it is possible to distinguish these by the peaks derived from the different aliphatic substituents on the POx side chains (labeled with letters: capital letters for ONonOx and small letters for OEtOx side chains) and, thus, to calculate the copolymer

Figure 4. 1H NMR spectra (250 MHz, CDCl3) of P1, P2 and P5 and structural assignment of the observed peaks. The integrals of the signals “a” and “C” were used to calculate the copolymer composition of P2−P4.

composition (Table 1). Comparison of the peak integrals of peaks “a” and “C” revealed that the amount of ONonOx side chains in the comb polymer is increased with respect to the feed ratio of M1 and M2 for all copolymers P2−P4. This observation is in agreement with the kinetic data presented above that showed a preferential incorporation of M2 toward the end of the copolymerization. The SEC characterization of P1−P5 was performed on three systems with different eluents. First, the comb polymers were eluted over a SEC system with a CHCl3-based mobile phase, which represents a good solvent for both, PEtOx as well as PNonOx and, thus, should be suitable to provide results that enable a straightforward comparison of the hydrodynamic volumes within the comb polymer series P1−P5. As depicted in Figure SI-4, Supporting Information, the hydrodynamic volume of the comb polymers increases with increasing molar fraction of M2, the more sterically demanding ONonOxMA (which has a higher Mn, too). The fact that PONonOxMA (P5) is an exception from this trend may be a result of the lower monomer conversion compared to P3 and P4, respectively (compare Table 1). On the other hand, the comb polymers with mixed side chains may be more stretched in the used eluent and, thus, result in lower elution volumes Vel (higher apparent molar masses). In fact, P3 and P4 have an increased PDI value in comparison to the other comb polymers and, in addition, elute close to the exclusion limit of the CHCl3 SEC system. Consequently, the polymers were characterized by a SEC system suitable for polymers with higher molar masses (Figure 5). It should be stated that, due to the fact that the present eluent DMAc represents a rather poor solvent for the 5111

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peak with a maximum around 56 °C during the first heating run. In the second heating run, the DSC thermograms only displayed a glass transition temperature (Tg), except for P3 where a slight indication of the original peak can be observed. As exemplarily shown for P5, the endothermal peak can be partially recovered by annealing of the sample at 40 °C (above the Tg, Figure SI-5, Supporting Information). In fact, a storage of the polymers at room temperature corresponds to a longterm annealing between the Tg and the temperature of the endothermal peak, explaining the difference of the thermograms from the first and the second heating run, respectively. Since it is known that linear PNonOx displays a melting temperature (T m ≈ 155 °C), which is attributed to crystallization of the lateral n-nonyl chains,35 it is reasonable to assume that the endothermal peak observed for P2 to P5 originates from the same effect. The significantly lowered Tm for the comb shaped polymers might be explained by the fact that the crystallization is disturbed by the constraint induced through the copolymer architecture, as has been observed for linear P(EtOx-ran-NonOx).32 Unlike for these polymers, where a decreasing NonOx content resulted in lowered Tm, the Tm stays roughly constant within the series P2 to P5. This observation correlates to the thermal properties of a series of PEtOx-b-PNonOx block copolymers,32 where the crystallization of the PNonOx block remains unaffected by the PEtOx block. In fact, if phase separation of the ONonOx and OEtOx side chains took place within the comb polymers with mixed side chains, the side chains might be regarded as parallel aligned short block copolymers covalently attached to a polymeric backbone. The enthalpy of fusion ΔHf of the melting peak increases with increasing content of M2, which is obvious because the amount of n-nonyl side chains increases in the same direction within the series of P2 to P5. However, the trend remains unchanged when the ΔHf is calculated in kJ/mol of M2 (i.e., ONonOx side chains present in each sample), hinting toward the fact that the crystallization of the lateral n-nonyl chains of the comb polymer side chains occurs to a lesser extent in the polymers with mixed side chains. In addition, the value of ΔHf allows a statement about the effect of the architecture of PNonOx on the extent of crystallinity, if it is calculated per mol of NonOx (i.e., per mol of n-nonyl chains). For linear PNonOx, Litt et al. reported a value of ΔHf = 43 J g−1, which corresponds to 8.5 kJ per mol NonOx.35 As shown in Table 2, this value is almost reduced by half for the comb shaped PNonOx P5. This appears reasonable because of the tight steric situation within the comb polymer, where the “side chains of the side chains” have to crystallize. The lowered Tg of comb shaped polymers in comparison to their linear analogues is well-known, and has been reported previously for POEtOxMA P1 as well.36 The disturbed crystallization in the comb shaped PNonOxMA P5 resulted in an additional Tg, which did not occur for the linear analogue PNonOx. However, for linear POx the Tg is reported to decrease with increasing length of the aliphatic lateral chains;37 therefore an even lowered Tg value would be expected for P5. Indeed, this behavior was observed. Interestingly, not all Tg values of the copolymers P2 to P4 are in between the values of the homopolymers P1 and P5, as would be typical for onephase systems of statistical copolymers. The Tg of P2 (the polymer with the highest content of OEtOx side chains) still matches quite well with the value calculated via the Fox and Wood equations,38 respectively (Table 2). However, in P3 and

Figure 5. Normalized SEC traces (DMAc, RI detection) of the comb polymers P1−P5.

hydrophobic PNonOx, P5 could not be completely dissolved in the eluent. Thus, one might expect that the polymers would become more expanded with increasing PEtOx content, which would result in a reverse trend in the elution behavior of the comb polymer series in comparison to the CHCl3 system. However, this was not the case, and the same trend of the apparent molar masses was observed (Table 1). It is clear that, due to the comb architecture, the hydrodynamic volume of P1 to P5 is not comparable to that of linear standard polymers and, thus, the true molar masses are not represented by the obtained values with simple RI detection. In order to provide a full understanding, P1−P5 were analyzed by a third SEC system equipped with a MALLS detector (THF-based eluent). Unfortunately, the polymers revealed an unfavorable elution behavior on this system (noisy LS signals due to the low dn/dc values and possibly incomplete elution). Thus, the absolute molar masses (Mw) provided in Table 1 should be considered with care. However, they follow the same trend as described above: Except for P5 (lower monomer conversion), increasing molar fraction of M2 increases the molar mass of the comb polymer, as expected from the molar masses of both macromonomers (Mn(M1) = 600 g mol−1; Mn(M2) = 1590 g mol−1). In addition, the obtained values for the homopolymers P1 and P5 seem to be in the expected range, when compared to the theoretically expected molar masses Mn,theo. Properties in the Bulk. To gain first insights into the properties of these materials, differential scanning calorimetry (DSC) experiments were performed (Figure 6, Table 2). All NonOx-containing polymers P2 to P5 revealed an endothermal

Figure 6. DSC thermograms of P1−P5 (heating rate 20 K min−1). Left: first heating run showing a melting peak. Right: Second heating run showing a glass transition. The traces are shifted vertically for clarity. 5112

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Table 2. Thermal Characteristics of P1−P5 Obtained by DSC Measurements P1 P2 P3 P4 P5

Tm [°C]

ΔHf [J/g]

m % M2

ΔHf [kJ/mol M2]

ΔHf [kJ/mol NonOx]

Tg [°C]

ΔCp [J g−1 K−1]

Tg [°C] Foxa

Tg [°C] Woodb

− 56.5 58.6 54.0 55.6

− 6.2 8.2 11.0 14.3

0 60.9 80 93.3 100

− 16.2 16.3 18.9 22.7

− 3.2 3.2 3.7 4.5

31.9 15.4 −13.2 −4.7 5.6

0.54 0.38 0.35 0.47 0.28

15.3 10.5 7.2

20.1 14.2 8.8

a

Calculated according to 1/Tg = M1/Tg1 + M2/Tg2, where M1 and M2 are the weight fractions of M1 and M2, respectively. bCalculated according to Tg = (M1ΔCp1Tg1 + M2ΔCp2Tg2)/(M1ΔCp1 + M2ΔCp2).

Figure 7. Wide angle X-ray diffraction curves: (A) reference sample P6; (B) comb polymer with exclusively ONonOx side chains P5; (C) comb polymer with mixed side chains P3. Each sample was annealed for 4 h inside the instrument before the measurement. The table presents the angles 2Φ and distances d corresponding to the peaks in the diffraction curves.

P4, the OEtOx side chains seem to act as “internal plasticizers” for the PONonOxMA, resulting in a further depression of the Tg. Alternatively, this thermal behavior may indicate phase separation occurring. Because of the immiscibility of PEtOx and PNonOx, smaller domains of ONonOx side chains could form that undergo the phase transition at lower temperature than the corresponding bulk material. Similar observations have, for instance, been reported for graft polymers with a PS backbone and isotactic polypropylene side chains.39 In addition, all polymers were studied by WAXS. In a first measurement at room temperature, all NonOx-containing polymers revealed similar diffraction patterns with the most intense reflections at 3.5°, corresponding to a d-spacing of 2.6 nm (Figure SI-6, Supporting Information). The peak intensity increases with increasing content of M2, hinting toward crystalline domains formed by the lateral n-nonyl substituents on the side chains. In order to rule out that the observed peaks arise solely from side chain crystallinity or that the WAXS data could support the idea of phase separation in the bulk, P6 was synthesized as reference system that would display the properties of a single side chain without interconnection by the comb polymer backbone. To prevent an autopolymeriza-

tion during WAXS measurements at elevated temperatures, the MA end group of M2 was replaced by an acetyl end group (see Supporting Information for details). Figure 7 depicts the WAXS diffraction patterns of this reference sample P6, the pure PONonOxMA P5 and a comb polymer with mixed sides chains P3 measured at varying temperatures between −20 and 100 °C. The peaks “a”, “b”, and “c” in Figure 7 correspond to (010), (020), and (030) reflexes of PNonOx;35 the length of a C9 chain would correspond to 0.84 nm (peak “c”). This signal vanishes at 60 °C (above the Tm), which is in good agreement with the DSC measurements described above. Compared to the “pure side chains” P6, in case of the PONonOxMA comb polymer P5 or the mixed comb P3 all three signals become less pronounced. In particular, peak “b” is overlaid by the amorphous background and therefore not being clearly visible for P3. This indicates that it becomes more and more difficult for the n-nonyl chains to crystallize when the polymer structure is altered from linear to comb-like to mixed-comb-like. Detailed X-ray studies by Litt et al. demonstrated that peak “d” corresponds to the distance between two linear PNonOx chains.40 The fact that this peak occurs at the same angles in the diffraction curves of P6, P5, and P3 shows that the ONonOx 5113

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Figure 8. DLS CONTIN plots for P2−P4 in water, methanol, and ethanol (c = 1 mg mL−1, straight lines: intensity size distribution, dotted lines: number size distributions). The corresponding hydrodynamic radii rh are provided in Table SI-1 (Supporting Information).

Figure 9. Cryo-TEM images of aggregated structures formed by P2−P4 in water.

side chains) in methanol are comparable to those observed for P3 in water, whereas in case of P4 the shrinkage was more pronounced. According to DLS, two types of somewhat welldefined nano-objects with rh of around 30 and 150 nm are formed. To provide clear insight into the nature of the formed aggregates in water, the aqueous solutions/suspensions were investigated via Cryo-TEM. These samples were prepared from aged solutions at room temperature. As depicted in Figure 9, the comb polymers represent very different structures in the solution equivalent state. P2 forms diffuse sheet-like or large hollow structures. This is supposed to be triggered by the large amount of hydrophilic OEtOx side chains that can effectively shield the few hydrophobic ONonOx side chains of this comb polymer. These remaining small differences in the solubility are apparently sufficient to result in the formation of extended sheet-like aggregates with a high degree of mechanical flexibility and overall small stability. As a consequence, this results in the formation of the aggregated and ill-defined structures that could also be detected in the intensity size distribution of the DLS. To further provide evidence for the amphiphilc properties of P2 its critical micelle concentration (cmc) in water was determined using pyrene as fluorescent probe (see Supporting Information for details; cmc = 4 × 10−4 g L−1). The situation changes for P3. The roughly 50 mol % of hydrophilic OEtOx side chains forms the base to allow hydrophobic interactions of the ONonOx side chains and leads to a rod-like aggregation. As a main result, the rod-like appearance might be seen as an indication for an internal phase separation. Apparently the interaction of the individual rods is still strong enough to trigger an aggregation of the rods into larger aggregates which tend to form spherical-like objects, e.g., due to the minimization of the contact between the ONonOx with the surrounding water. This results in the formation of the superstructures which show a rather broad size distribution

side chains of both comb polymers can still be packed in a similar fashion as reported for the linear polymer. The fact that peak “d” corresponds to a distance of 0.48 nm explains the lower crystallinity in the comb polymers in comparison to linear PNonOx, since the comb polymer side chains are attached to every second atom in the backbone chain, which should prevent crystallization of the side chains directly at the backbone due to steric constraint. Additional X-ray measurements at lower angles (SAXS) did not reveal any long-range order for the samples P2−P4. Thus, the assumption of phase separation occurring between ONonOx and OEtOx side chains is so far merely based on DSC results. Solution Properties. PEtOx is soluble in water and methanol under ambient conditions, both nonsolvents for PNonOx, which should result in phase separation. In fact, linear PEtOx-b-PNonOx block copolymers form micelles or other nanostructures in water41 and water/ethanol mixtures.31 Consequently, the properties of P2−P4 were investigated in water, methanol and ethanol by means of DLS (Figure 8, Table SI-1, Supporting Information) and cryo-TEM (Figure 9). All values obtained for the hydrodynamic radii of P2−P4 in ethanol are comparable to those of pure PEtOx comb polymers in water with rh of 3 to 5 nm:36 In all cases, the major fraction resembles unimolecular macromolecules, with small amounts of aggregates being present, which are only visible in the intensity weighted distributions (Figure 8, solid lines). Similar observations were made for P2 (the comb polymer with the highest molar fraction of hydrophilic OEtOx side chains) in methanol and water, respectively. As indicated by the smaller hydrodynamic radius rh the macromolecules are contracted in water if compared to alcohols. For P3, aggregates were found both in methanol and water. The aggregates were rather welldefined in methanol (rh ≈ 250 nm) but ill-defined and, again, shrunken in water. The size distributions of P4 (the comb polymer with the highest fraction of hydrophobic ONonOx 5114

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comb polymer molecules and to judge the presence of individual Janus-type macromolecules, it is necessary to increase the DP of the backbone as well. In addition, the copolymerization of short hydrophobic macromonomers with slightly longer hydrophilic or oligo-2-methyl-2-oxazoline macromonomers might favor the presence of unimolecular structures in aqueous media.

which is in agreement with the corresponding DLS investigations. In contrast, P4, the comb polymer with the highest content of hydrophobic ONonOx chains, aggregates to smooth round objects with varying diameters. The apparently reduced amphiphilic character prevents the formation of small rod-like aggregates but results in the formation of large round shaped objects of varying size. Two main morphologies are found in accordance with the corresponding DLS results. Being less welldefined in size, the spherical objects might be considered as large compound micelles (LCMs).42,43 Indeed, the content of hydrophobic ONonOx in P4 is high (84 mol %), and the interconnection by the comb polymer backbone restricts a free self-organization of immiscible side chains.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR, MALDI TOF, and ESI TOF spectra of M2, SEC traces of P1 to P5, DSC thermograms after annealing, WAXS diffraction curves of P2 to P5 without annealing, additional discussion and characterization data (ESI TOF MS, 1H NMR, SEC) related to the synthesis of P6, hydrodynamic radii obtained from DLS measurements, and determination of the cmc of P2. This material is available free of charge via the Internet at http://pubs.acs.org/.

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CONCLUSION A series of amphiphilic comb polymers that are composed of heterografted ONonOx and OEtOx side chains and a polymethacrylate backbone could be obtained by application of solely the macromonomer method, using a combination of CROP and RAFT polymerization. Kinetic studies during the formation of the comb polymer backbone revealed a controlled copolymerization, but a slight enrichment of ONonOx side chains toward the ω-end of the backbone. The lateral n-nonyl chains of the ONonOx can crystallize despite the sterically hindered arrangement in the comb (co)polymer structures, but apparently this is less pronounced if compared to the linear analogue. Although DSC and WAXS investigations do not provide other evidence for phase separation of the different interconnected side chain types in bulk than the crystallinity of ONonOx domains, cryo-TEM investigation indicated its presence in an aqueous medium due to the very different aggregation behavior of the investigated comb polymers. The origin of the observed effects in both, bulk and aqueous environment, may be seen in a possible (yet not proven) molecular structure of the heterografted comb polymers of Janus-dumbell type as is illustrated graphically in Figure 10. Future research will concentrate on the application of the surprisingly easy synthetic route for the preparation of polymers with increased DP values. Increasing the length of the side chains should enable an investigation of the possible phase separation in bulk (thin films) by application of atomic force microscopy. However, in order to visualize individual

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +49 (0) 3641 9482 00 (Secretary: 01). Fax: +49 (0) 3641 9482 02. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work forms part of the research program of the Dutch Polymer Institute (DPI, technology area high-throughput experimentation). The authors thank the Freistaat Thüringen (Grant No. B715-07011) for the financial support of the study.

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REFERENCES

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Figure 10. Illustration of a possible conformation of the heterografted comb polymer P4 as single molecule, in bulk and in aqueous environment based on internal phase separation of ONonOx and OEtOx side chains. 5115

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