Styrene-vinyl pyridine diblock copolymers: Achieving high molecular weights by the combination of anionic and reversible addition–fragmentation chain transfer polymerizations

Abstract

Anionic and reversible addition–fragmentation chain transfer (RAFT) polymerizations were combined for the preparation of high molecular weight (MW) amphiphilic diblock copolymers based on the hydrophobic styrene (Sty) and the more polar 2-vinyl pyridine (2VPy) or 4-vinyl pyridine (4VPy). In particular, four amphiphilic Sty-VPy diblock copolymers with MWs up to 271,000 g mol^–1 were prepared. For the polymer synthesis, first, living anionic polymerization of Sty using sec-butyl-lithium as initiator in tetrahydrofuran at −70 °C, followed by termination with ethylene oxide were employed for the preparation of OH-functionalized homopolyStys. Subsequently, a modification of the OH-terminal group was performed by the attachment of a 4-cyanopentanoic acid dithiobenzoate chain transfer agent (CTA) group, giving a polySty macroRAFT CTA, which was extended with 2VPy or 4VPy units using RAFT polymerization. Thus, the prepared diblock copolymers comprised a first block which was near-monodisperse in size, and a second more heterogeneous block. All diblock copolymers were characterized in terms of their MWs and compositions by gel permeation chromatography and ¹H NMR spectroscopy, respectively, giving results close to the theoretically expected values. Films cast from chloroform solutions of the diblock copolymers were investigated in terms of their bulk morphologies using transmission electron microscopy, which indicated that the minority block consistently formed the discontinuous microphase, spherical or cylindrical. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

INTRODUCTION

Amphiphilic block copolymers represent a very interesting class of polymeric materials with a rich phase behavior.1 These materials self-assemble from the nanometer through the micrometer length-scales, both in the bulk and in solution, and find applications in a broad range of areas, including microelectronics, colloids, and medicine.2

There are many synthetic techniques available for the preparation of block copolymers with well-defined composition, molecular weight (MW), and structure. Besides the classical synthetic routes of living anionic3 and quasi-living cationic4 polymerizations, and the new controlled radical polymerization methods such as nitroxide-mediated polymerization (NMP),5 atom transfer radical polymerization (ATRP),6 and reversible addition–fragmentation chain transfer (RAFT) polymerization,7 combination of two living/controlled polymerization methods has also been applied for the preparation of block copolymers, expanding the range of monomers that can be used.

Several examples on the combination of anionic polymerization with the different controlled radical polymerizations have been described in the literature.8 Usually, the first monomer is polymerized via anionic polymerization, followed by the modification of the chain ends into functional groups which can be used as initiating sites for the polymerization of the second monomer. In particular, anionic polymerization has been widely combined with ATRP6,9 and NMP6,10 for the preparation of block and graft copolymers, as well as for star copolymers.

The literature also includes some examples on the combination of anionic and RAFT polymerizations for the synthesis of block copolymers. In the first such study, Brouwer et al.11 described the chain extension of hydroxyl end-functional poly(ethylene-co-butylene) (number-average MW, M_n of 3800 g mol^–1) prepared by the anionic polymerization of butadiene followed by hydrogenation, with styrene (Sty)/maleic anhydride via RAFT polymerization (block copolymers with MWs < 23,000 g mol^–1). Following this first study, Xu and Huang12 synthesized a series of well-defined diblock terpolymers of poly(ethylene oxide)-b-poly(styrene-co-2-hydroxyethyl methacrylate) (PEO-b-P(Sty-co-HEMA)), with MWs ranging from 16,900 to 38,800 g mol^–1, using anionic and RAFT polymerizations for the first and second blocks, respectively. In addition, the same researchers also synthesized a PEO-b-P(Sty-co-HEMA)-g-poly(ε-caprolactone) (PEO-b-P(Sty-co-HEMA)-g-PCL) graft quaterpolymer by the grafting of CL units from the hydroxyl groups of the HEMA units using ring opening polymerization (ROP). Moreover, Hillmyer and coworkers published on the combination of anionic and RAFT polymerizations for the preparation of several copolymers.13–15 First, they reported13 the synthesis of three different poly(ethylene-alt-propylene)-b-PEO-b-poly(N-isopropylacrylamide) (PEP-b-PEO-b-PNiPAm) triblock terpolymers, with PEP and PEO MWs of 3000 and 25,000 g mol^–1, respectively, while the PNiPAm MW was varied at three values, 4000, 10 000, and 21,000 g mol^–1. The synthesis of the first block was accomplished by the anionic polymerization of isoprene, followed by end-capping with one unit of ethylene oxide, and completed by the hydrogenation of the double bonds in the isoprene units to yield hydroxyl-terminated PEP. The resulting polymer was further anionically polymerized using a counterion different from that used in the first step to give PEP-b-PEO, which was subsequently end-functionalized by the attachment of a RAFT chain transfer agent (CTA). This diblock macroRAFT CTA was extended with NiPAm via RAFT polymerization to give the final PEP-b-PEO-b-PNiPAm triblock terpolymers. In another work,14 these researchers presented the synthesis of polyethylene-b-poly(N, N-dimethylacrylamide) (PE-b-PDMA) and PEP-b-PDMA diblock copolymers, with PE and PEP MWs both of 3000 g mol^–1 and final diblock copolymer MWs of 12,000 and 15,000 g mol^–1, respectively, combining living anionic and RAFT polymerizations, following a similar procedure to that described in the previous example. More recently, the same workers15 prepared PEP-b-(PDMA-grad-2-(methacrylamido glycopyranose)) (PEP-b-P(DMA-grad-MAG)) diblock terpolymers using the same macroRAFT PEP, which was further polymerized with DMA and trimethylsilyl-protected MAG (TMS-MAG) through RAFT polymerization, while the final diblock terpolymers were obtained after the hydrolysis of the TMS-MAG unit.

Thus, in the previous studies involving the combination of anionic and RAFT polymerizations, low-to-medium MW copolymers were prepared. In the present study, we aim at expanding this MW limit to high-to-very-high values. The particular monomer pair was chosen to secure a large thermodynamic incompatibility between the blocks of the resulting copolymers, so that they microphase separate in the bulk and be studied by electron microscopy. Although both monomers are polymerizable by both anionic and RAFT polymerizations, this work provides a proof of concept that the particular combination of polymerization methods can lead to diblock copolymers of very high MW. Extension to other monomers, specific to each of the two polymerization methods, would be straightforward.

This study addresses another timely issue in polymer science, that of size polydispersity in block copolymers. There has recently been an understanding that block copolymers can still microphase separate in the bulk, even when they are heterogeneous in size, implying that it is not necessary that their synthesis be carried out using living anionic polymerization when morphology formation is desired.16 Thus, block copolymers prepared using other, less demanding and less expensive methods, and, in particular, controlled radical polymerization, can also be used when self-assembly in the bulk is required. Although it does not prevent microphase separation, polydispersity highly influences the type of morphology obtained. A particular literature example17 most dramatically demonstrated this by presenting three diblock copolymers with the same average composition but a polydisperse second block with different polydispersity indices, giving lamellae, gyroids, or cylinders, depending on the polydispersity index. The mechanism by which polydispersity impacts microphase separation is through the reduced stretching energy exhibited by the polydisperse blocks which can fill space more efficiently, leading to the formation of larger domains and also to the shift of order–order transitions toward compositions higher in the polydisperse component.18 Polydispersity also affects the onset of microphase separation, facilitating it when the polydisperse block forms the minority domains, but opposing it in the reverse case. This study exactly presents the development of a model diblock copolymer system comprising a low-polydispersity first block made by living anionic polymerization, and a less homogeneous second block prepared by RAFT polymerization.

EXPERIMENTAL

Materials

Sty (purity >99%), 2-vinyl pyridine (2VPy, >97%), 4-vinyl pyridine (4VPy, >95%), sec-butyl-lithium (sec-BuLi, 1.4 M in cyclohexane), ethylene oxide (≥99.5%), 4,4′-azobis(4-cyanopentanoic acid) (≥98%), bis(thiobenzyl) disulfide (>90%), 4-dimethyl(amino) pyridine (DMAP, ≥99%), dicyclohexylcarbodiimide (DCC, 99%), basic alumina (≥97%), calcium hydride (CaH₂, 95%), 2,2-diphenyl-1-picrylhydrazyl hydrate (DPPH, 95%), silica gel (60 Å, 70–230 mesh), n-hexane (96%), ethyl acetate (99.8%), dichloromethane (DCM, 99%), methanol (99.8%), N,N-dimethylacetamide (DMAc, ≥99.9%, HPLC grade), and N,N-dimethylformamide (DMF, 99.8%) were purchased from Aldrich, Germany. The CTA 4-cyanopentanoic acid dithiobenzoate (4-CPeDB) was synthesized in our laboratory as detailed in a following section. 2,2′-Azobis(isobutyronitrile) (AIBN, 95%), tetrahydrofuran (THF, ≥99.9%) and deuterated chloroform (CDCl₃) were purchased from Merck, Germany.

Methods

The Sty, the 2VPy, and the 4VPy monomers were purified from inhibitors and acidic impurities by passing them through basic alumina columns. This was followed by stirring these compounds over CaH₂, in the presence of the free radical inhibitor DPPH, for 72 h to neutralize the last traces of moisture, and vacuum distillation. The AIBN radical initiator was recrystallized twice from ethanol. The polymerization solvent DMF was dried over CaH₂ and vacuum-distilled prior to use.

Syntheses

Synthesis of 4-Cyanopentanoic Acid Dithiobenzoate

The carboxylic acid-functionalized CTA CPeDB is not commercially available, and it was, therefore, synthesized in our laboratory. In a typical synthesis, 2.05 g of 4,4′-azobis(4-cyanopentanoic acid) (7.34 mmol) and 1.50 g of bis(thiobenzyl) disulfide (4.89 mmol) were added to 29 mL hot (at 85 °C) ethyl acetate and were left to react at 85 °C for 18 h, as described in the literature.19 Subsequently, the solvent was evaporated off, and CPeDB was purified by column chromatography using a solvent mixture composed of n-hexane and ethyl acetate at a volume ratio of 5:3. The purity of the CTA was confirmed by ¹H and ¹³C NMR spectroscopy. ¹H NMR (300 MHz, CDCl₃, 25 °C, δ): 7.88 (2H, d, Ph), 7.54 (1H, t, Ph), 7.39 (2H, t, Ph), 2.46–2.80 (4H, m, CH₂CH₂), 1.92 (3H, d, CH₃); ¹³C NMR (300 MHz, CDCl₃, 25 °C, δ): 177.4 (CS), 144.5 (COOH), 133.1 (Ph), 128.6–126.3 (Ph), 118.4 (CH₂CH₂COOH), 45.6 (SC(CH₃)(CN) CH₂), 33.0 (SC(CH₃)(CN) CH₂), 29.6 (CH₂COOH), 24.2 (CN). Yield: 70%.

Synthesis of the Hydroxyethyl-Terminated Polystyrenes

The Sty homopolymers were synthesized through living anionic polymerization according to the literature.8, 20 The polymerization of Sty (54 mL, 49 g, 0.47 mol) was initiated by sec-BuLi (2.6 mL of 1.4 M solution in cyclohexane, 3.64 mmol) at −70 °C in THF (800 mL). The reaction was left to complete for 2 h at this temperature. Subsequently, ethylene oxide (1.76 g, 40 mmol) was condensed in the reactor, after purification,21 and the solution was stirred for 30 min at −30 °C. The end-functionalization of the lithium salt of the living polymer with an alkyl hydroxyl group was achieved at this point by the reaction with ethylene oxide. Finally, the polymer solution was quenched with 1% solution of acetic acid in methanol to introduce the hydroxyl functional end-group. After removal of THF under reduced pressure, the polymer was precipitated in water and vacuum-dried.

Synthesis of the Polystyrenes MacroRAFT Chain Transfer Agents

The produced hydroxy-terminated polystyrenes (PSty-OH) were modified directly through an esterification reaction using a large excess of CPeDB in the presence of DCC activator and DMAP catalyst. In a typical reaction, 1 g of PS-OH of nominal MW of 11,600 g mol^–1 (86.2 μmol) (35% wt/wt in DCM), 0.140 g of CPeDB (502 μmol, ∼6-fold excess with respect to the hydroxyl end-groups), and 0.006 g of DMAP (49.1 μmol) were dissolved in 3.3 mL DCM. In a second vial, 0.114 g of DCC (552 μmol) was dissolved in 0.57 mL DCM (20% wt/wt), and the resulting solution was, subsequently, added drop-wise to the polymer solution. The reaction was left under stirring for 2 h at room temperature. Then, the reaction mixture was filtered to remove the produced urea side-product, and the resulting transparent solution was precipitated three times in methanol to separate the end-functionalized polymer (precipitate) from the excess of CPeDB which remained in solution. Finally, the hydroxy-terminated polySty was dried under vacuum at room temperature overnight.

This procedure was also followed for the preparation of the two other macroRAFT polyStys with MWs of 48,100 (macroRAFT2) and 225,000 g mol^–1 (macroRAFT3). However, larger excesses of CPeDB were used in these cases to face the greater steric hindrances resulting from the larger polymer sizes. In particular, 24-fold and 113-fold excesses of CPeDB with respect to the hydroxyl end-groups of macroRAFT2 and macroRAFT3, respectively, were employed.

Synthesis of the Sty-VPy Diblock Copolymers

This paragraph details the procedure followed for the synthesis of one Sty-2VPy diblock copolymer (Sty₁₁₁-b-2VPy₃₉₅) based on macroRAFT1. A 10-mL Schlenk-tube containing a magnetic stirring bar was loaded with 0.5 g macroRAFT1 (43.1 μmol), 4 mg of AIBN (24.4 μmol), and 1.8 mL of freshly distilled 2VPy (1.75 g, 16.6 mmol). The contents of the tube were, subsequently, degassed by three freeze-vacuum-thaw cycles, were placed under an inert argon atmosphere, and were heated in an oil bath to 65 °C where they were kept for 72 h. After the polymerization, samples of the produced Sty-2VPy diblock copolymer were characterized by gel permeation chromatography (GPC) and ¹H NMR spectroscopy analyses (monomer conversion by ¹H NMR spectroscopy = 98.9%; GPC number-average molecular weight = M_n = 42,300 g mol^–1 compared to the theoretically expected of 42,800 g mol^–1; M_w/M_n = polydispersity index (PDI) = 1.61; (M_w is the weight-average molecular weight). Finally, the diblock copolymer was precipitated in n-hexane three times, and was dried in a vacuum oven at room temperature for 72 h.

Characterization of the Sty-VPy Diblock Copolymers

Gel Permeation Chromatography

Samples of the macroRAFT CTAs and the Sty-VPy diblock copolymers were characterized in terms of their MWs and molecular weight distributions (MWD) using GPC. The measurements were performed on a chromatograph equipped with a Knauer refractive index (RI) detector and a set of two PSS 10 µ GRAM-Gel columns (3000 and 1000 Å, 8 × 300 mm each). The mobile phase was DMAc containing LiCl as an additive (0.05 mol L^–1) at 50 °C, at a flow rate of 1 mL min^–1. Samples of 20 μL of polymer solutions (0.2 wt % in the mobile phase) were injected using a Rheodyne 7725i manual injection valve. Eluograms were flow-rate corrected using the method of internal standard (250 ppm diethylene glycol). Apparent MW averages, M_n and M_w, were based on PSty calibration and were calculated using the WinGPC software package (PSS GmbH, Mainz, Germany).

Nuclear Magnetic Resonance Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy was performed on either a 300 MHz or a 500 MHz Avance Bruker NMR spectrometers equipped with Ultrashield magnets. CDCl₃ was used as the NMR solvent, whereas calibration was performed using the signal from residual protonated (CHCl₃) solvent (peak at 7.26 ppm). ¹H NMR spectroscopy was used to characterize the starting polySty-OHs, the macroRAFT CTAs, and the final diblock copolymers. ¹H NMR spectroscopy was also used to confirm the structure and purity of the synthesized CPeDB. The complete end-functionalization of polySty-OHs with CPeDB to the macroRAFT CTAs was confirmed by the downfield shift of the ¹H NMR signal from the oxymethylenes of polySty-OH upon esterification. The copolymer composition was determined from the relative areas of the peak due to the one or the two aromatic proton(s) next to the nitrogen atom at 8.3 ppm for poly2VPy or poly4VPy, respectively, divided by that due to the five aromatic protons of polySty at 6.0–7.2 ppm. Furthermore, the conversion of 2VPy and 4VPy to polymer was also calculated using ¹H NMR spectroscopy as described below. For 2VPy, monomer conversion was calculated from the ratio of the area of the peak due to the one aromatic proton of 2VPy at 8.45 ppm divided by the area of the peak due to the corresponding aromatic proton of poly2VPy at 8.3 ppm, while for 4VPy, monomer conversion was calculated from the ratio of the areas of the peaks at 8.50 ppm due to 4VPy (two protons) and that at 8.3 ppm due to poly4VPy.

Transmission Electron Microscopy

Transmission electron microscopy (TEM) measurements were carried out using a FEI Tecnai F20 transmission electron microscope operated at 200 kV in bright-field mode. All samples were cast from 5 wt % solutions in chloroform, following a procedure described below. The diblock copolymers were dissolved in CHCl₃ and transferred into a Teflon dish inside a desiccator rich in solvent vapor. The overall evaporation (casting) procedure lasted for a period of three weeks. The resultant films were taken out of the Teflon dish and were then thermally treated for another week in a vacuum oven from room temperature up to 130 °C to remove potentially remaining solvent. Approximately 50 nm thick sections were obtained by microtoming the samples with a Leica Ultracut UCT microtome equipped with a diamond knife. Finally, the sections were stained by iodine vapor.

RESULTS AND DISCUSSION

In this work, four amphiphilic diblock copolymers of high MW based on Sty and 2VPy or 4VPy were synthesized, combining anionic and RAFT polymerizations. First, Sty homopolymers with one hydroxyethyl-terminal group were prepared using anionic polymerization. Subsequently, a modification of the hydroxyethyl-terminal group was performed by the attachment of a CPeDB CTA group, giving a polySty macroRAFT CTA which was extended with VPy units using RAFT polymerization in the presence of AIBN as a radical source. In the following sections, we present and discuss all synthesis results, starting from the OH-functionalized polyStys, their conversion to the macroRAFT polyStys, and their extension for the preparation of the amphiphilic diblock copolymers.

Synthesis of the OH-Functionalized polyStys

First, Sty was polymerized via living anionic polymerization using sec-BuLi as initiator in THF at −70 °C. The produced polystyrene was terminated using ethylene oxide, leading to the incorporation of precisely one hydroxyethyl group at the polymer terminus (Fig. 1).

Figure 1. Synthetic route followed for the preparation of the hydroxyethyl-terminated polystyrene: living anionic polymerization of Sty, followed by termination with ethylene oxide.

Three hydroxyl-terminated Sty homopolymers, polySty1, polySty2, and polySty3, of different MWs were prepared. Their successful synthesis was confirmed by GPC, while their MWDs are presented in Figure 2. The calculated MW averages and the PDIs are given in Table 1. The M_n values of the three polymers were 11,600, 48,100, and 225,000 g mol^–1, corresponding to degrees of polymerization (DP) of 111, 462, and 2160, respectively, while the PDIs ranged between 1.05 and 1.32, with polySty1 presenting the highest PDI of 1.32, and the other two presenting very low values.

Figure 2. Molecular weight distributions (calculated from the GPC traces) of the three hydroxyl end-functionalized polyStys prepared by living anionic polymerization.

Table 1. Molecular Weights and Polydispersity Indices of the Three PolySty-OH Prepared

Synthesis of the MacroRAFT PolyStys

Subsequently, the hydroxyethyl-terminated polyStys were modified to macroRAFT CTAs using CPeDB, a carboxylic acid-functionalized RAFT CTA, through an esterification reaction. CPeDB is not commercially available, and it was, therefore, synthesized from the reaction of 4,4′-azobis(4-cyanopentanoic acid) with bis(thiobenzyl) disulfide, as explained in the Experimental section. The esterification reaction for the modification of the hydroxyethyl-terminated polyStys with CPeDB was accomplished directly, using large excesses of CPeDB, DCC activator, and DMAP catalyst (Fig. 3), with the CPeDB excess being larger for the larger polymers to more efficiently address the stronger steric hindrance. In particular, the CPeDB:polySty-OH molar ratio was 5.8, 24, and 113 for polySty1, polySty2, and polySty3, respectively.

Figure 3. Esterification of hydroxyethyl-polySty and its conversion to a RAFT macroCTA.

The completion of the esterification reaction was confirmed using ¹H NMR spectroscopy to probe the protons of the end-groups. Figure 4 shows the ¹H NMR spectra of one of the three polyStys before and after end-group modification. In particular, Figure 4(a) displays the ¹H NMR spectrum of the original polySty1, whose two oxymethylene protons exhibited a resonance at 3.75 ppm (in black circle), which was shifted to 4.20 ppm (also in black circle) upon esterification in the ¹H NMR spectrum of macroRAFT1 in Figure 4(b).

Figure 4. ¹H NMR spectra of (a) polySty1 and (b) macroRAFT1.

Synthesis of the Sty-VPy Diblock Copolymers

Four amphiphilic diblock copolymers of high MWs based on Sty and 2VPy or 4VPy were synthesized by the extension of the macroRAFT polyStys with 2VPy or 4VPy, via RAFT polymerization using AIBN as the initiator (radical source) in the bulk (without added solvent). Figure 5 presents schematically the procedure and the chemical reaction followed for the preparation of a Sty-b-2VPy copolymer. In the figure, the Sty units are shown in black, and the 2VPy units are shown in gray. The black asterisks at one end of the chains represent the active sites of the polymerization. In the present work, three diblock copolymers based on 2VPy and one based on 4VPy were prepared.

Figure 5. Chemical reaction and schematic representation of the synthetic procedure followed for the preparation of a Sty-b-2VPy diblock copolymer. The Sty units are shown in black, while the 2VPy units are in gray. The black asterisks at one end of the polymer chains denote the active polymerization sites.

After the polymerizations, all the produced Sty-VPy diblock copolymers were characterized in terms of their MWs and compositions using GPC and ¹H NMR spectroscopy, respectively. Figure 6 presents the MWDs of all the diblock copolymers together with those of their homopolymer precursors, whereas Table 2 lists the calculated MW and composition characteristics of the diblock copolymers. The diblock copolymers presented relatively narrow MWD and PDIs greater than those of their homopolymer precursors. However, the two largest diblocks, Sty₂₁₆₀-b-2VPy₈₃₆ and Sty₂₁₆₀-b-4VPy₄₂₄ [Fig. 6(c, d)], displayed small tails on the high MW side of their MWDs, probably resulting from radical recombination and partial chain dimerization. Nonetheless, to the best of our knowledge, these are the largest block copolymers prepared by the combination of anionic and RAFT polymerizations, with MWs reaching almost 300,000 g mol^–1. Thus, the final diblock copolymers were much less homogeneous in size compared to their homopolymer precursors due to the rather high polydispersity of their second blocks introduced by RAFT polymerization.

Figure 6. Molecular weight distributions of the four Sty-VPy diblock copolymers together with those of their homopolymer precursors. (a) Sty₁₁₁-b-2VPy₃₉₅, (b) Sty₄₆₂-b-2VPy₁₈₇, (c) Sty₂₁₆₀-b-2VPy₈₃₆, and (d) Sty₂₁₆₀-b-2VPy₄₂₄.

Table 2. Molecular Weight and Composition Characteristics of the Sty-VPy Diblock Copolymers Prepared

The successful formation of the diblock copolymers was also confirmed using ¹H NMR spectroscopy. Figure 7 displays the ¹H NMR spectrum of the diblock copolymer Sty₄₆₂-b-2VPy₁₈₇ [Fig. 7(b)] and that of its Sty homopolymer precursor [Fig. 7(a)]. The spectrum of the diblock copolymer presented the characteristic peak “f” at 8.3 ppm of the proton attached to the carbon next to the nitrogen atom in the pyridine ring, indicating the incorporation of 2VPy units in the polySty macroRAFT CTA. The VPy contents in the diblock copolymers, listed in Table 2, ranged from 16 to 78 mol %.

Figure 7. ¹H NMR spectra of (a) macroRAFT2 and (b) Sty₄₆₂-b-2VPy₁₈₇.

Bulk Morphologies

Films cast from chloroform solutions of all four diblock copolymers were investigated in terms of their bulk morphologies using TEM, and the recorded images are displayed in Figure 8. In the Figure, the light areas correspond to the polySty domains, while the dark areas belong to the poly2VPy or the poly4VPy domains. With the exception of Sty₄₆₂-b-2VPy₁₈₇, all the diblock copolymers present spherical structures. Sty₄₆₂-b-2VPy₁₈₇ [Fig. 8(b)] forms hexagonally packed cylinders of poly2VPy in a polySty matrix, as indicated by the hexagonal pattern of the cross-sections of the cylindrical domains in some areas and lying cylinders in other areas of the TEM image.22–24 The diameter of the cylinder in the TEM image was found to be 40 nm. This value is in between the theoretical value of two fully stretched poly2VPy blocks of 94 nm (each monomer repeating unit contributes 0.252 nm25) and the same blocks under the assumption of randomly coiled conformation, leading to a radius of gyration of approximately 4.5 nm (in this latter calculation, the characteristic ratio, C_∞, of poly2VPy was assumed to be the same as that of polySty, and equal to 1026), that is, a domain diameter of approximately 9 nm. These domain sizes are summarized in Table 3, which also lists the experimental (TEM) and theoretical (upper and lower limits) domain sizes for the other three diblock copolymers. Sty₁₁₁-b-2VPy₃₉₅ forms polySty spheres, while Sty₂₁₆₀-b-2VPy₈₃₆ and Sty₂₁₆₀-b-4VPy₄₂₄ present polyVPy spheres which indicate that the minority block of each polymer consistently builds up the discontinuous spherical microphase.

Figure 8. TEM images of the diblock copolymers cast from chloroform solutions. (a) Sty₁₁₁-b-2VPy₃₉₅, scale-bar = 100 nm; (b) Sty₄₆₂-b-2VPy₁₈₇, scale-bar = 100 nm; (c) Sty₂₁₆₀-b-2VPy₈₃₆, scale-bar = 200 nm; and (d) Sty₂₁₆₀-b-4VPy₄₂₄, scale-bar = 200 nm.

Table 3. Experimental Domain Sizes of the Discontinuous Nanophase as Determined from TEM on Diblock Copolymer Films Cast from Chloroform

It is interesting to note that, from composition considerations, sample Sty₂₁₆₀-b-2VPy₈₃₆ was expected to form cylinders, just as sample Sty₄₆₂-b-2VPy₁₈₇. The discrepancy might be due to the rather high PDI of the minority block of the former diblock copolymer, which would lead to a shift of the phase boundaries to compositions richer in the polydisperse (minority, here) component, and replacing cylinders with spheres.

CONCLUSIONS

Three high MW amphiphilic Sty-2VPy and one Sty-4VPy diblock copolymers were prepared via the combination of living anionic and RAFT polymerizations. First, living anionic polymerization was employed for the preparation of OH-functionalized homopolyStys, where their OH-terminal groups were converted to polySty macroRAFT CTAs by the attachment of a 4-CPeDB CTA group. The macroRAFT CTAs were, subsequently, extended with 2VPy or 4VPy using RAFT polymerization in the presence of AIBN as initiator. The MWs of the diblock copolymers ranged between 42,000 and 271,000 g mol^–1, which are the highest reported to date for the particular combination of polymerization methods. Films cast from chloroform solutions of the diblock copolymers were investigated in terms of their bulk morphologies using TEM. Sty₄₆₂-b-2VPy₁₈₇ formed hexagonally packed cylinders of P2VPy in a PSty matrix, while Sty₁₁₁-b-2VPy₃₉₅, Sty₂₁₆₀-b-2VPy₈₃₆, and Sty₂₁₆₀-b-4VPy₄₂₄ formed Sty or VPy spheres, confirming that the minority block consistently formed the discontinuous spherical microphase. It is possible that one of the samples, Sty₂₁₆₀-b-2VPy₈₃₆, did not self-assemble to the expected cylindrical morphology but formed spheres as a result of the increased polydispersity of its second block, in line with previous findings regarding the effect of size heterogeneity on microphase separation in the bulk.

Acknowledgements

The authors wish to thank the European Commission for funding this work within the FP7 project SELFMEM (grant agreement no. NMP3-SL-2009-228652). Authors are also grateful to the A. G. Leventis Foundation, and the Cyprus Research Promotion Foundation and the EU Structural and Cohesion Funds for Cyprus (project NEKYP/0308/02) for the establishment of the NMR infrastructure at the University of Cyprus.