Redox-Responsive Block Copolymers: Poly(vinylferrocene)-b-poly

Article pubs.acs.org/Organometallics

Redox-Responsive Block Copolymers: Poly(vinylferrocene)‑b‑poly(lactide) Diblock and Miktoarm Star Polymers and Their Behavior in Solution Jan Morsbach,† Adrian Natalello,†,‡ Johannes Elbert,§ Svenja Winzen,∥ Anja Kroeger,∥ Holger Frey,*,† and Markus Gallei*,§ †

Institute of Organic Chemistry, Organic and Macromolecular Chemistry, Johannes Gutenberg-University (JGU), Duesbergweg 10-14, D-55099 Mainz, Germany ‡ Graduate School Materials Science, University of Mainz, Staudinger Weg 9, D-55128 Mainz, Germany § Ernst-Berl Institut für Technische und Makromolekulare Chemie, Technische Universität Darmstadt, Petersenstraße 22, D-64287 Darmstadt, Germany ∥ Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany S Supporting Information *

ABSTRACT: The synthesis of diblock and miktoarm star polymers containing poly(vinylferrocene) (PVFc) and poly(L-lactide) (PLA) blocks is introduced. End functionalization of PVFc was carried out via end capping of living carbanionic PVFc chains with benzyl glycidyl ether (BGE). By hydrogenolysis of the benzyl protecting group a dihydroxyl end-functionalized PVFc was obtained. Both monohydroxyland dihydroxyl-functionalized PVFcs have been utilized as macroinitiators for the subsequent polymerization of L-lactide via catalytic ring-opening polymerization. A series of block copolymers and AB2 miktoarm star polymers was synthesized with varied PLA chain lengths. All polymers were characterized in detail, using 1H NMR spectroscopy, size exclusion chromatography (SEC), and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDIToF). The molecular weight of the block copolymers and AB2 miktoarm star polymers are in the range of 8000−15000, containing a PVFc block of weight 7800. In addition, the self-assembly behavior of the polymers in dichloromethane (CH2Cl2) was investigated by using dynamic light scattering (DLS) and transmission electron microscopy (TEM). In a selective solvent for PLA the block copolymers and miktoarm star polymers formed vesicle-like structures with different diameters.

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INTRODUCTION

PFS for many applications has motivated various groups to develop pathways for a variety of PFS-containing copolymers.22−25 Recently, in-chain functionalized water-soluble PFSPEO block copolymers were synthesized via a novel epoxide termination strategy.26 In contrast to PFS with ferrocene units in the main chain, the PVFc system carries the fc groups laterally. PVFc revealed remarkable stability after many oxidation/reduction cycles.11 In the past decade the anionic polymerization of VFc was established to provide well-defined PVFc with good molecular weight control for the first time.21,27 The living character of the polymerization allows for subsequent carbanionic polymerization. For example, PVFc-bPMMA was synthesized to prepare redox-active patchy nanocapsules.28 The recently disclosed direct termination of the carbanionic polymerization with epoxide derivatives opens

The synthesis of block copolymers is a widely used strategy to combine different polymer characteristics.1−4 Following this concept, considerable effort has been taken to combine the unique properties of organic polymers with the manifold advantages of metal-containing structures.5−7 In the field of metal-containing polymers especially the incorporation of ferrocene (fc) moieties has raised enormous attention.8−10 For example, the fc/ferrocenium redox couple can be used as an efficient electron-transfer relay system for biosensors.11−13 In addition to this characteristic feature also the catalytic,14 semiconductive, mechanical, photophysical, optoelectronic, and magnetic properties are of great interest.15−18 The first fccontaining polymer was synthesized in the 1950s by radical polymerization of vinylferrocene (VFc) with low molecular weights and broad size distributions.19 Meanwhile Manners et al. introduced the ring-opening polymerization of strained ferrocenophanes (FS) to obtain well-defined fc-containing polymers and various block copolymers.20,21 The potential of © XXXX American Chemical Society

Special Issue: Ferrocene - Beauty and Function Received: June 10, 2013

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dx.doi.org/10.1021/om400536q | Organometallics XXXX, XXX, XXX−XXX

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up pathways for a variety of well-defined polymer architectures.29,30 Tonhauser et al. employed epoxide termination to synthesize water-soluble PVFc-b-poly(ethylene oxide) block copolymers and AB2 miktoarm stars. They reported quantitative termination for two epoxide derivatives with rather short PVFc blocks in the molecular weight range of 1000−3600 (4− 15 repeating units of VFc, respectively).31 Benzyl glycidyl ether (BGE) was utilized as an end-capping reagent to obtain either monohydroxyl- or dihydroxyl-functionalized macroinitiators. Although using hydroxyl-terminated macroinitiators to synthesize polylactide (PLA)-containing block copolymers is a widely used strategy,32 the combination of PVFc and PLA has not been reported in the literature to date. PLA blocks are important with respect to microphase separation and degradability under mild conditions.33−36 The combination of these characteristic features renders PLA-containing polymers interesting materials for the formation of porous 3D networks. For related polystyrene (PS)-based block copolymers, Wang et al. studied the microphase separation of PS-b-PLA and obtained nanoporous PS with a continuous nanochannel network after hydrolysis of PLA.37,38 In this contribution we describe a synthesis strategy for both PVFc-b-PLA block copolymers and PVFc-(PLA)2 miktoarm star polymers (Scheme 1). The carbanionic polymerization of

Article

EXPERIMENTAL SECTION

Reagents. All solvents and reagents were purchased from Acros Organics or Sigma-Aldrich. Unless further mentioned, all compounds were used as received. Dichloromethane-d2 was purchased from Deutero GmbH. Tetrahydrofuran (THF) was distilled from sodium/ benzophenone under reduced pressure (cryo-transfer) before use. Benzyl glycidyl ether was dried over calcium hydride (CaH2). VFc was purified as published previously.21 L-Lactide was recrystallized from toluene before use. Instrumentation. All syntheses were carried out under an argon atmosphere, using either Schlenk techniques or a glovebox equipped with a Coldwell apparatus. 1H NMR spectra were recorded at 300 or 400 MHz on a Bruker AC300 or Bruker AMX400 spectrometer. All spectra are referenced internally to the residual proton signals of the deuterated solvents used. For size exclusion chromatography (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 (106/105/104 g mol−1), a UV source (275 nm), and a RI detector. All SEC diagrams show the UV detector signal, and the molecular weight refers to linear polystyrene (PS) standards provided by Polymer Standards Service (PSS). In addition, SEC was performed with THF as the mobile phase (flow rate 1 mL min−1) on a Mixed Gel column set from PL (PL Mixed Gel B, PL Mixed Gel C, PL Mixed Gel D) or an SDV column set from PSS (SDV 1000, SDV 100000, SDV 1000000) at 30 °C. Here, calibration was achieved using PS (provided by PSS) and PVFc (self-made) calibration standards.21 For the SEC-MALLS experiments a system composed of a Waters 515 pump (Waters, Milford, CT), a TSP AS100 auto sampler, a Waters column oven, a Waters 486 UV detector operating at 254 nm, a Waters 410 RI detector, and a DAWN DSP light scattering detector (Wyatt Technology, Santa Barbara, CA) was used. The data acquisition and evaluation of the static lightscattering experiments were carried out with Astra version 4.73 (Wyatt Technology, Santa Barbara, CA). The instrument was calibrated using pure toluene, assuming a Rayleigh ratio of 9.78 × 10−6 cm−1 at 690 nm. An injection volume of 118 μL, a sample concentration of 1−2 g L−1, a column temperature of 35 °C, and a THF flow rate of 1 mL min−1 were used, and SEC analysis was performed on a highresolution column set from PSS (SDV 5 μm 106 Å, SDV 5 μm 105 Å, SDV 5 μm 1000 Å). MALDI-ToF mass spectroscopy experiments were performed on an Axima-TOF2 spectrometer (Shimadzu Biotech, Manchester, U.K.) equipped with a nitrogen laser (337 nm). Data acquisition, evaluation, and generation of the spectra were performed with the software MALDI-MS Shimadzu Biotech Launchpad. Samples for MALDI-ToF mass spectrometry investigations were prepared by dissolving the polymer in dioxane (c = 4 mg mL−1). A 15 μL portion of the solution and 15 μL of a matrix solution of 1,8,9trihydroxyanthracene (dithranol) dissolved in dioxane (10 mg mL−1) were mixed and placed on a steel target using the dried droplet method.39,40 DSC curves were recorded on a Perkin-Elmer DSC 8500 instrument. TGA measurements were conducted with a Perkin-Elmer Pyris 6 TGA instrument. Dynamic light scattering (DLS) experiments were performed on an ALV instrument consisting of a goniometer and an ALV-5000 multiple-τ full-digital correlator with 320 channels. The light source comprised a helium−neon laser from JDS Uniphase (25 mW, wavelength of 632.8 nm). All measurements were carried out at T = 20 °C. Dust-free solutions were obtained by filtration through hydrophobic fluoropore (PTFE) membrane filters with a pore size of 0.45 μm (Millipore, Millex-FH). DLS data evaluation was performed by using the CONTIN algorithm, which can be applied to the analysis of multiple decay processes as described in the literature.41,42 A Philips EM420 transmission electron microscope (TEM) using a LaB6 cathode at an acceleration voltage of 120 kV was used to obtain TEM images. TEM grids (carbon film on copper, 300 mesh) were purchased from Electron Microscopy Sciences, Hatfield, PA. Staining was unnecessary because of the iron atoms incorporated. As autoclave a high-pressure laboratory autoclave Model II from Carl Roth GmbH + Co. KG was used.

Scheme 1. Synthetic Strategy to Ferrocene-Containing Block Copolymers (PVFc-b-PLA, 3) and AB2 Miktoarm Star Polymers (PVFc-(PLA)2, 4)a

a

Abbreviations: Sn(Oct)2, tin(II) ethyl hexanoate; BGE, benzyl glycidyl ether.

VFc is terminated with an epoxide derivative (BGE) to obtain either monohydroxyl- or dihydroxyl-functionalized macroinitiators. In the following step catalytic ring-opening polymerization of L-lactide is carried out to obtain the desired polymer architectures. The copolymers are characterized in detail with SEC, 1H NMR, and MALDI-ToF, and their self-assembly behavior in solution (CH2Cl2) is studied by using DLS and TEM. B


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Poly(vinylferrocene)benzyl Glycidyl Ether (PVFc-BGE, 1): Exemplary Synthesis Procedure for PVFc36-BGE. In an ampule equipped with a stirring bar, 2.5 g (11.8 mmol) of purified VFc was dissolved in 60 mL of dry THF and the solution was cooled to −12 °C. A 205 μL portion (0.33 mmol) of a 1.6 M solution of n-BuLi in hexane was added quickly with a syringe. After 12 h a sample of the reaction mixture was taken to ensure complete conversion. Subsequently 2.5 mL of freshly distilled BGE (1.65 mmol, 5 equiv compared to living chain ends) was added. The reaction mixture was warmed to room temperature and stirred for an additional 3 h. To terminate the reaction, a small amount of degassed methanol was added. For precipitation the reaction mixture was poured into a 10fold excess of methanol. The resulting end-functionalized PVFc was collected by filtration, washed with methanol, and dried in vacuo (yield: quantitative). 1 H NMR (300 MHz, CD2Cl2, δ in ppm): 7.50−7.25 (m, BGE, aromatic), 4.60−3.60 (br, PVFc cyclopentadiene, aromatic −CH−), 2.90−1.60 (br, PVFc backbone −CH2CH−). Poly(vinylferrocene)-(OH)2 (PVFc-(OH)2, 2). The reaction was carried out in a laboratory autoclave. The flask was filled with 450 mg (0.06 mmol) of 1, 30 mL of dichloromethane (DCM), 15 mL of EtOH, and 80 mg of palladium catalyst on activated carbon (Pd/C, 10%). The flask was flushed with hydrogen (80−120 bar) and stirred at 30 °C for 72 h. Subsequently, the catalyst was removed by filtration over Celite, and the solvent was removed in vacuo. PVFc-(OH)2 was obtained by precipitation in methanol and drying in vacuo. Due to the strong interaction between polymer and catalyst, reduced yields of 60−80% were obtained. 1 H NMR (300 MHz, CD2Cl2, δ in ppm): 4.60−3.60 (br, PVFc cyclopentadiene, aromatic −CH−), 2.90−1.60 (br, PVFc backbone −CH2CH−). Poly(vinylferrocene)-b-poly( L -lactide) (PVFc-b-PLA, 3): Exemplary Synthesis Procedure for PVFc36-b-PLA15. A 202 mg portion of macroinitiator 1 (0.025 mmol) and 47 mg of freshly recrystallized L-lactide (0.36 mmol) were dried in vacuo at 60 °C for 2 h. Afterward the mixture was dissolved in 2.4 mL of absolute toluene. The reaction flask was flushed with argon, and the mixture was heated to 100 °C. A 1 mL portion of a Sn(Oct)2 solution in absolute toluene (c = 10 mg mL−1) was added via syringe, and the reaction mixture was stirred for 15 h. Precipitation in cold methanol and drying in vacuo gave PVFc-b-PLA as a bright orange powder (yield: 90%). 1 H NMR (300 MHz, CDCl3, δ in ppm): 5.20−5.00 (m, PLA −CH−), 4.60−3.60 (br, PVFc cyclopentadiene, aromatic −CH−), 2.90−1.60 (br, PVFc backbone −CH2CH−), 1.50−1.20 (m, PLA −CH3−). Poly(vinylferrocene)-b-(poly(L-lactide))2 (PVFc-(PLA)2, 4): Exemplary Synthesis Procedure for PVFc36-(PLA15)2. The reaction was carried out analogously to the synthesis of PVFc-bPLA. A 198 mg portion of the dihydroxy-functionalized macroinitiator 2, 98 mg of L-lactide and 10 mg of Sn(Oct)2 were stirred at 100 °C for 14 h under an argon atmosphere. Precipitation in cold methanol and drying in vacuo led to an orange powder (yield: 89%). 1 H NMR (300 MHz, CDCl3, δ in ppm): 5.20−5.00 (m, PLA −CH−), 4.60−3.60 (br, PVFc cyclopentadiene, aromatic −CH−), 2.90−1.60 (br, PVFc backbone −CH2CH−), 1.50−1.20 (m, PLA −CH3−).

demonstrated that the polymerization of PVFc is slowed down as soon as a molecular weight range between 3000 and 5000 is reached.21 Although the still “living” chain ends are in a “sleeping state” with respect to further propagation, quantitative terminal functionalization with epoxide derivatives is possible.31 The end-capping reaction with BGE leads to a terminal monohydroxyl functionality of PVFc within short reaction times and without side reactions. The generated PVFc-BGE has been used as a macroinitiator for the synthesis of PVFc-b-PLA block copolymers. Following a hydrogenolysis protocol, the benzyl protecting group of the PVFc-BGE can be removed. This results in a dihydroxylfunctionalized PVFc, which again can be utilized as a macroinitiator for the synthesis of AB2 miktoarm stars. The synthesis strategy is summarized in Scheme 1. Table 1 summarizes the synthesized PVFc-BGE samples (Mn 3200 and 7800), which have been characterized by SECTable 1. Characterization Data of PVFc-BGE no. 1 2

polymer d

PVFc15-BGE PVFc36-BGEd

Mna

Mnb

Mnc

Mw/Mnc

1700 3200

3100 7500

3200 7800

1.09 1.22

a

Molecular weight determined by SEC (PS standards, DMF). Molecular weight obtained by 1H NMR (CD2Cl2). cMolecular weight and molecular weight distribution by SEC-MALLS in THF (PVFc standards21). dDegree of polymerization calculated by SECMALLS. b

MALLS, MALDI-ToF MS, and 1H NMR spectroscopy. The molecular weights obtained by SEC (PS standards) in DMF are smaller in comparison to the SEC values obtained with PVFc standards, due to the considerably smaller hydrodynamic volumes in comparison to PS samples with similar molar masses. The bulky ferrocenyl units at every second carbon atom of the backbone lead to a highly stretched polymer, and thus the polymer chain can be thought to have a very compact chain conformation.21 The molecular weights calculated by 1H NMR are in accordance with the SEC (PVFc standards21) results. The molecular weight distributions of the PVFc samples are narrow and monomodal. However, with increasing molecular weights and simultaneously decreasing rates of propagation a higher PDI is observed, as soon as the molecular weight reaches values around 7000. The polymerization of VFc was terminated with a 5-fold excess of BGE, leading to a terminal hydroxyl group and also a second benzyl-protected hydroxyl group. MALDI-ToF analysis verified end functionalization of PVFc in very high yields. The main signals can be assigned to BGE-terminated PVFc, and the peak distance of 212 mirrors the VFc repeating unit. Small subdistributions can be assigned to H-terminated PVFc and PVFc with two terminal BGE units (Supporting Information, Figure S1).43 Tonhauser et al. demonstrated that short PVFc chains are stable under hydrogenolysis conditions required to remove the benzyl protecting group.31 By using 1H NMR spectroscopy and MALDI-ToF mass spectrometry the removal of the protection group was monitored. Figure S2 (Supporting Information) shows an overlay of two 1H NMR spectra, one obtained before the hydrogenolysis and one afterward. The removal of the signal in the aromatic region δ 7.50−7.25 ppm confirms successful elimination of the protection group. The signals obtained by MALDI-ToF mass spectrometry can be assigned to

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RESULTS AND DISCUSSION Synthesis of Linear and Star-Shaped PVFc-block-PLA Copolymers. The main objective of this work was to establish an epoxide termination strategy to combine the carbanionic polymerization of vinylferrocene (VFc) with the catalytic ringopening polymerization (ROP) of L-lactide. A systematic series of AB block copolymers and AB2 miktoarm stars with varied PLA block lengths has been prepared by catalytic ROP of Llactide with stannous octoate (Sn(Oct)2). For this purpose VFc was synthesized as described in the literature,21 and the polymerization was initiated with n-BuLi. Previous studies C


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the respective polymer species and therefore confirm quantitative removal of the protection group (Supporting Information, Figure S3). SEC analysis showed no change in size distribution and no significant change of molecular weight. Therefore, we conclude that the polymer backbone was unaltered under the hydrogenation conditions. Consequently, two macroinitiators with either one or two terminal hydroxyl groups have been successfully synthesized. This opens up a pathway to various block copolymer architectures. To demonstrate the potential of this approach, some exemplary PVFc-b-PLA block copolymers and AB2 miktoarm star polymers have been prepared. In the following section the results will be discussed in detail. Linear PVFc-b-PLA Diblock Copolymer 3. For the synthesis of linear block copolymers a monohydroxyl-functionalized PVFc has been used for the catalytic ring-opening polymerization of L-lactide. A common strategy for the ROP of lactide with Sn(Oct)2 as a catalyst, toluene as a solvent, and a temperature of 90 °C was used to ensure a reasonable reaction time of 15 h with a minimal level of side reactions. The product can be obtained in high yields after precipitation in cold methanol (90%). After drying, an orange powder was obtained. Block copolymers with varying block lengths of PLA were used to study the aggregation behavior in solution. The results are discussed in detail below. The incorporation of both blocks in the block copolymer structures can be verified by 1H NMR, MALDI-ToF, and SEC. The 1H NMR spectra show two additional signals in comparison to the macroinitiator. The signals typical for PLA in the regions between δ 5.20−5.00 and 1.50−1.20 ppm were obtained (Supporting Information, Figure S4). The CH group shows no overlap with the cyclopentadienyl signals. Thus, comparison of the integrals allows determination of the PVFc:PLA ratio and the total molar mass by 1H NMR. In addition, SEC data confirm the block formation, because on the one hand the SEC traces show a significant increase of molecular weight in comparison to the macroinitiator and on the other hand the comparison of the RI and UV signals supports block formation as PLA is not UV active (Supporting Information, Figure S5). In combination the 1H NMR and SEC data give strong evidence of successful block formation. In addition, MALDI-ToF studies were carried out. The molecular weight shift of 212 (Figure 1, red curved arrows) can be assigned to the VFc repeating unit. In addition, the incorporation of lactide can be confirmed from the molecular weight shifts of 144. Additional shifts of 72 (green straight arrows) occur due to transesterification reactions of the polyester structures of different polymer chains. The copolymers showed good stability over several months when they were stored in a fridge. No molecular weight shift by SEC was detectable, permitting us to conclude that degradation of the PLA blocks was absent. The corresponding characterization data for all copolymers (nos. 3−6) are given in Table 2, and the respective MALDI-ToF data are depicted in Figure 1. The molecular weights obtained by SEC are smaller in comparison to the values determined by 1H NMR and the theoretical values. As described earlier, the PVFc block possesses a smaller hydrodynamic radius than the PS standards. The molecular weight distributions slightly increase with increasing molecular weights. In summary, BGE as a capping reagent provides access to hydroxyl-terminated macroinitiators for catalytic ring-opening polymerizations. Block copolymers can be synthesized with

Figure 1. Overview (A) and zoom-in (B) of a MALDI-ToF spectrum of PVFc36-b-PLA2 (no. 3, Table 2; matrix dithranol).

Table 2. Characterization Data for PVFc-b-PLA and AB2 Miktoarm Star Polymers PVFc-(PLA)2 no.

polymer

Mnb

Mna

Mw/Mna

3 4 5 6 7 8

PVFc36-b-PLA2c PVFc36-b-PLA15c PVFc36-b-PLA95c PVFc36-b-PLA110c PVFc36-(PLA15)2c PVFc36-(PLA45)2c

7900 8800 14 600 19700 9400 14300

6000 7400 9600 13700 8300 10900

1.27 1.28 1.34 1.36 1.25 1.21

a

Molecular weight determined by SEC and molecular weight distribution (PS standards, DMF). bMolecular weight obtained by 1 H NMR (CD2Cl2). cDegree of polymerization calculated by 1H NMR.

remarkable control over molecular weight and narrow molecular weight distributions. The PLA content can be varied over a wide range, which will allow detailed micro phase separation studies in the future. Star-Shaped PVFc-(PLA)2 Miktoarm Star Polymers 4. After the successful release of the benzyl group, PVFc-(OH)2 has also been used as a bifunctional macroinitiator for the ROP of lactide. The procedure was carried out in analogy to the linear block copolymer synthesis. With the Sn(Oct)2 catalyst system both terminal hydroxyl groups were transferred into active species for the ROP of L-lactide. PVFc-(PLA)2 miktoarm star polymers were obtained as an orange powder. Two exemplary miktoarm star polymers with molecular weights of 9 400 and 14 300 g mol−1 were synthesized with narrow size distributions (Mw/Mn of 1.21− 1.25). The characterization data are summarized in Table 2 (nos. 7 and 8). D


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Figure 2 shows an overlay of SEC traces from the macroinitiator and the AB2 miktoarm stars. Again there is a

Figure 3. Relaxation functions C(q,t) (empty circles) for the concentration fluctuations along with the corresponding distributions of relaxation times H(ln τ) (straight line) at q = 0.0108 nm−1 of (A) PVFc36-b-PLA15, (B) PVFc36-b-PLA90 and (C) PVFc36-b-PLA110 block copolymers in CH2Cl2 with c = 0.05 g L−1 at T = 20 °C.

Figure 2. SEC traces of PVFc-BGE and PVFc-PLA2 samples (DMF, UV signal, PS standard).

significant shift in molecular weight from 3200 to 14300, and the obtained polymers show strong UV absorption. Analogously to the block copolymer structures, all 1H NMR signals can be assigned. However, solely on the basis of 1H NMR spectra it is not possible to determine a difference between the linear and the starlike structures because the hydrogens at the junction point are overlaid by backbone signals. Properties of Linear and Star-Shaped PVFc-b-PLA Copolymers. The main objective of this work was to combine carbanionic and catalytic ring-opening polymerization techniques via an epoxide termination step to obtain novel PVFc-bPLA block and miktoarm star polymers. The synthesized structures show unexpected self-assembly behavior in a PLAselective solvent, CH2Cl2, which will be discussed in the following. In contrast to PLA, the PVFc block is poorly soluble in CH2Cl2, which represents the driving force for the selfassembled structure formation. In all scattering experiments, ensemble averages were determined. Thus, the DLS experiments permit certain statistical analyses of the self-assembled objects whose structures can be inferred from direct imaging via TEM. Furthermore, structural analysis of the different types of formed objects that coexist in solution or dispersion and correspond to different levels of aggregation can be performed by DLS.44 The contributions of the different supramolecular structures to the total scattering can be separated in the time domain.45,46 To analyze the computed correlation functions, C(q,t), inverse Laplace transformations using the constrained regularization CONTIN method42,47 were applied (see the Supporting Information). The CONTIN method assumes that C(q,t) can be written as a superposition of exponentials, where H(ln τ) results as the distribution of relaxation times. The characteristic relaxation times correspond to the peak positions of H(ln τ), which are labeled with k = a, b, ...45,48 We carried out DLS measurements to verify if the self-assembly leads to well-defined structures. Figure 3 displays the correlation functions C(q,t) along with the corresponding distributions of

relaxation times H(ln τ) at a scattering vector q = 0.0108 nm−1 for three representative PVFc-b-PLA block copolymers. All samples exhibit one well-defined main diffusive process shifting to higher relaxation times t with increasing PLA block length. This phenomenon results in an increase of the hydrodynamic radius Rh from 152 to 347 nm. Sample A (PVFc36-b-PLA15) containing the lowest PLA contentshows a faster diffusive process (a) beyond the main process (b). As confirmed by the q2 dependence of the relaxation rates, this mode also corresponds to a distinct diffusing entity, resulting in Rh,a = 25 nm. This dimension of diffusing objects suggests micelle-like structures. DLS measurements were also carried out for the miktoarm star polymers. The results have been compared with linear block copolymer samples with similar PLA content. Similar to the results of the linear block copolymers, one main diffusion process was observed. The star-shaped structures also show self-assembly behavior in CH2Cl2 with resulting hydrodynamic radii in similar dimensions. In comparison to the linear block copolymers, the star-shaped polymers should form smaller supramolecular structures because the same PLA content is divided onto two arms. As assumed, a significant decrease of Rh (from 215 to 132 nm) was observed for PVFc36-(PLA45)2 in comparison to PVFc36-b-PLA 95 (Figure S6, Supporting Information). This finding can be seen as verification for the successful synthesis of star-shaped copolymers. In order to support the results of the DLS measurements, additional transmission electron microscopy (TEM) measurements were carried out to obtain information on the shape of the structures formed. To this end, the polymers were dissolved in CH2Cl2, and 1 drop of the solution (c = 1.0 mg mL−1) was deposited on a carbon-coated copper TEM grid. Subsequently the organic solvent was evaporated under vacuum. No further sample treatment was necessary due to the high electron density of the iron-containing PVFc segments. E


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preferred. Further studies on the solution structures are in progress.

In Figure 4 some representative images of the obtained vesicle-like structures are shown. Figure 4A,B shows the self-

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CONCLUSION To the best of our knowledge, PVFc has not been combined with degradable polyester structures to date. In this work, the carbanionic polymerization of PVFc and the catalytic ringopening polymerization of lactide have been combined via an epoxide termination strategy to generate PVFc-PLA copolymers. Using BGE as a termination reagent for the polymerization of VFc, either one or two terminal, functional hydroxyl groups are generated for a subsequent polymerization step. Characterization of the obtained AB block and AB2 miktoarm star copolymers was carried out by SEC, 1H NMR, and MALDI-ToF to confirm successful block formation. The selfassembly behavior in CH2Cl2 was studied with DLS and TEM. Both the linear and the star-shaped block copolymers selfassemble into spherical structures with well-defined diameters. The wide range of PLA block lengths permits detailed microphase separation studies, which are underway. Subsequent degradation of the PLA block aiming at nanoporous morphologies is an interesting option for the block copolymers introduced here.

Figure 4. TEM images of (A + B) PVFc36-b-PLA110 and (C + D) PVFc-(PLA45)2 (solvent CH2Cl2, c = 1.0 g L−1).

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assembly of PVFc36-b-PLA110 (no. 4, Table 2) into spherical objects. In Figure 4C,D an overview and a zoom-in for the selfassembly of star-shaped PVFc36-(PLA45)2 is shown. There is no significant difference between the PVFc36-b-PLA110 and PVFc36(PLA45)2 copolymers in size or shape. Both polymers selfassemble to spherical objects. The radius of the formed structures varies between 55 and 80 nm. The sizes measured with TEM are substantially smaller than those determined by DLS (Table 3). It should be noted that

Figures giving an overlay of 1H NMR of PVFc36-BGE and PVFc36-(OH)2, 1H NMR of PVFc36-b-PLA15, MALDI-ToF spectra of PVFc36-BGE and PVFc36-(OH)2, an overlay of SEC curves of PVFc36-BGE, PVFc36-b-PLA80, and PVFc36-b-PLA110, and additional DLS data for PVFc36-(PLA45)2. This material is available free of charge via the Internet at http://pubs.acs.org.

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Table 3. Radii of Self-Assembled Structures Obtained by DLS and TEM no.

polymer

Rh(DLS)/nm

R(TEM)/nm

5 8

PVFc36-b-PLA95c PVFc36-(PLA45)2c

215 ± 22 132 ± 13

75 ± 15 60 ± 12

ASSOCIATED CONTENT

S Supporting Information *

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.G.); [email protected] (H.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Julia Beer and Karsten Rode for MALDI-ToF mass spectrometry measurements. A.N. thanks the Graduate School of Excellence MAINZ for financial support. M.G. and J.E. thank the Landesoffensive zur Entwicklung Wissenschaftlich-ökonomischer Exzellenz (LOEWE Soft Control) for financial support of this work.

the averages calculated from TEM micrographs are n averages in comparison to z average radii determined by DLS. Furthermore, in DLS, structural analyses are carried out in solution. Consequently, the PLA-containing corona of the formed structures is swollen. In contrast, TEM measurements are performed on dried samples, where the PLA corona of the self-assembled objects is collapsed. Therefore, the size difference between the TEM and DLS results is explained by drying effects. The PLA shell appears to collapse as soon as the solvent is removed, which is indicated by the brighter surrounding of the darker PVFc-containing core. The radius of the core varies between 40 and 55 nm and clearly exceeds the theoretical chain length of the PVFc block. Therefore, the whole core can not only be composed of PVFc. It is also visible that the core seems to be surrounded by a layer with higher contrast then the carbon grid. We assume that the structures exhibit a double layer shell or membrane like vesicles. In our current understanding the extremely stretched and inflexible PVFc blocks prefer to assemble in parallel, and consequently vesicles with a lower surface curvature compared to micelles are

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REFERENCES

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