Simple Preparation of Various Nanostructures via in Situ

Article pubs.acs.org/Macromolecules

Simple Preparation of Various Nanostructures via in Situ Nanoparticlization of Polyacetylene Blocklike Copolymers by OneShot Polymerization Suyong Shin, Ki-Young Yoon, and Tae-Lim Choi* Department of Chemistry, Seoul National University, Seoul 151-747, Korea S Supporting Information *

ABSTRACT: Previously, we reported the one-pot synthesis of polyacetylene (PA) diblock copolymers which formed various nanostructures via the in situ nanoparticlization of conjugated polymers (INCP), using a two-step protocol based on sequential monomer addition. Herein, we report a much simpler one-shot method for nanostructure formation by the synthesis of PA blocklike copolymers. The blocklike copolymers could be prepared by the one-shot ROMP of comonomers with large differences in their reactivities because the monomers that formed the first block, namely norbornene (NB) derivatives or endo-tricyclo[4.2.2.0]deca-3,9-diene (TD) derivatives, polymerized much faster than the monomers that formed the second PA block, cyclooctatetraene (COT). Owing to their blocklike microstructures, the copolymers formed various nanostructures such as nanospheres, nanocaterpillars, and nanoaggregates depending on the chemical structures of the soluble shell polymers and feed ratio of COT, which formed the insoluble PA core. Using dynamic light scattering (DLS) and atomic force microscopy (AFM), it was observed that the nanostructures produced from the blocklike copolymers were essentially the same as those produced from the block copolymers synthesized by conventional sequential monomer addition. The blocklike microstructures of the copolymers formed by one-shot ROMP were further supported by an in situ 1H NMR kinetic experiment and UV/vis spectroscopy. From these results, we were able to confirm that the ROMP of TD and COT produced near-perfect block copolymers. Furthermore, the 1H NMR spectra of the one-shot copolymerization provided insights into the INCP process.

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INTRODUCTION Block copolymers play a key role in the development of various functional self-assembly materials,1−7 such as drug delivery materials based on amphiphilic block copolymers,2 catalytic reactors,3 and template materials for carbon nanoparticles used in dye-sensitized solar cells (DSSC).4 In addition, they form various nanostructures such as cylindrical or wormlike micelles,5 vesicles,6 and spherical or polygonal disk-shaped structures.7 However, the fabrication of these structures from block copolymers requires multistep synthetic procedures and postsynthetic treatments such as the addition of selective solvents,5a dialysis,5b aging,5c or addition of glue molecules5d to induce self-assembly (Scheme 1, conventional procedure). To simplify the procedure used to prepare self-assembled nanostructures from block copolymers, there are two viable strategies. One is to develop a simpler synthetic process such as a one-shot polymerization method where all comonomers in the reaction vessel are polymerized in a single step (Scheme 1, strategy 1). The one-shot block polymerization can be achieved by two different approaches. The first method is using heterofunctional initiators which could produce block copolymers by two different mechanisms simultaneously.8 For example, Grubbs’ group reported a heterofunctional initiator which could do atom transfer radical polymerization (ATRP) and ring-opening metathesis polymerization (ROMP) simultaneously.8a The heterofunctional initiator involving ring-opening © XXXX American Chemical Society

polymerization (ROP) and nitroxide-mediated polymerization (NMP) was also reported by Hawker’s group.8b Another approach to the one-shot copolymerization is using comonomers of the block copolymers which have large differences in their reactivities.9 Endo’s group9a,b and Bisht’s group9c synthesized one-shot block copolymers using comonomers which showed different ROP reactivities. Using this strategy, star-shaped polymers were also synthesized in one shot by various polymerization methods, namely by cationic polymerization reported by Sawamoto’s group10a and Aoshima’s group10b and radical polymerization such as ATRP,10c free radical polymerization,10d and reversible addition−fragmentation chain transfer (RAFT) polymerization.10e,f The second strategy toward a more facile self-assembly procedure from block copolymers is to simplify or even eliminate postsynthetic treatments via the direct, in situ formation of the nanostructures (Scheme 1, strategy 2). For example, novel strategies known as polymerization induced selfassembly (PISA)11 and in situ nanoparticlization of conjugated polymers (INCP)12 were recently developed for this purpose. These methods lead to the direct formation of self-assembled nanostructures in situ during polymerization by taking Received: December 16, 2014 Revised: February 11, 2015

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DOI: 10.1021/ma502530x Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Conventional and New Strategies toward Self-Assembly of Block Copolymers

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advantage of the vastly different solubilities of the constituting blocks of the copolymers. PISA typically uses amphiphilic block copolymers, which are produced by aqueous dispersion/ emulsion polymerization. On the other hand, INCP takes advantage of the strong π−π interactions and insolubility of the conjugated polymer block as the driving force for self-assembly. As a result, nanostructure formation spontaneously occurs in any organic solvent, and the resulting nanostructures are highly stable under heat and mechanical forces. Among the various nanostructures obtained by INCP, nanocaterpillar, nanostar, and nanonetwork structures were prepared from fully conjugated diblock copolymers, which contained poly(3-(2ethylhexyl)thiophene) or poly(2,5-dihexyloxy-1,4-phenylene) as the soluble first block and polythiophene as the insoluble second block. These diblock copolymers were synthesized by Grignard metathesis (GRIM) polymerization via sequential addition of the monomers.12a,b Furthermore, nanocaterpillar structures were produced using sequential, two-step ROMP to prepare diblock copolymers composed of polynorbornene (PNB) as the soluble block and polyacetylene (PA)13,14 as the insoluble conjugated polymer block.12c,d In addition to 1D structures, higher-ordered 3D structures were fabricated by changing the soluble first block from PNB to the more rigid poly(endo-tricyclo[4.2.2.0]deca-3,9-diene) (PTD).12e,15 Although there have been many recent attempts to simplify the self-assembly process of block copolymers, the one-shot copolymerization of comonomers leading to direct nanostructure formation is the most simple and efficient approach. Fortunately, the viability of one-shot copolymerization was indicated previously in the INCP via the ROMP of strained monomers and cyclooctatetraene (COT). The ROMP of COT was much slower than that of the NB or TD derivatives.12c−e By taking advantage of the substantial difference in reactivity between the comonomers, we envisioned that an extremely simple method for the self-assembly of block copolymers would be feasible via a one-shot production of blocklike copolymers. Herein, we report an even simpler one-shot INCP method using ROMP to produce blocklike copolymers that spontaneously undergo various self-assembly processes (Scheme 1, new strategy). Blocklike copolymers containing a PA block were easily prepared by the one-shot ROMP of NB or TD and COT. The blocklike copolymers directly formed nanospheres, nanocaterpillars, and 3D aggregates that had the same nanostructures as the copolymers formed by conventional block copolymerization. Supporting data were acquired via AFM, DLS, 1H NMR, and UV/vis to verify that the blocklike microstructures were produced by a simple one-shot polymerization.

EXPERIMENTAL SECTION

Materials and Characterization. All reagents which are commercially available from Alfa Aesar, Tokyo Chemical Industry, and Sigma-Aldrich were used without further purification. All reactions were carried out under dry argon atmospheres using standard Schlenkline techniques. All anhydrous deuterium solvents (≥99.95%) were purchased from Euriso-Top and were used degassed for 10 min before polymerization. 4,4-Dimethylbiphenyl was used as an internal standard for 1H NMR analysis. NMR spectra were recorded by Varian/Oxford As-500 (500 MHz for 1H/125 MHz for 13C) spectrometer. Gel permeation chromatography (GPC) for polymer molecular weight analysis was carried out with a Waters system (1515 pump, 2414 refractive index detector) and Shodex GPC LF-804 column eluted with THF (GPC grade, Honeywell Burdick & Jackson). Flow rate was 1.0 mL/min, and temperature of column was maintained at 35 °C. Samples in 0.5−1.0 mg/mL THF were filtered by a 0.45 μm PTFE filter before injection. UV/vis spectra were obtained by a Jasco Inc. UV/vis spectrometer V-630, and dynamic light scattering (DLS) data were obtained by a Malvern Zetasizer Nano ZS. Multimode 8 and Nanoscope V controller (Veeco Instrument) were used for AFM imaging. All images were obtained on tapping mode using noncontact mode tip from Nanoworld (Pointprobe tip, NCHR type) with spring constant of 42 N m−1 and tip radius of ≤8 nm. General Procedure for the One-Shot Synthesis of Blocklike Copolymers. Both comonomers (0.063 mmol for NB or TD and 0.013−0.063 mmol for cyclooctatetraene (COT)) were weighed in a 2 mL sized screw-cap vial with septum and purged with argon. Anhydrous and degassed dichloromethane was added (0.06−0.42 mL) to the vial. The solution of the third-generation Grubbs catalyst (1.0 mg, 0.0013 mmol) was added (0.03 mL) to the monomer solution at once under vigorous stirring, and the vial was tightly sealed by using parafilm and Teflon tape. The mixture was stirred for 2−13 h at the room temperature. The reaction was quenched by excess ethyl vinyl ether. The crude mixture was precipitated into methanol, and the obtained dark powder was dried in vacuo. Poly(NB)-PA Blocklike Copolymers. Yield: 87−97% (see Table 1, entries 1−5). 1H NMR (500 MHz, CDCl3): δ 6.08−6.82 (m), 5.76 (s, 1H), 5.45−5.50 (d, 1H), 3.88 (s, 1H), 3.28−2.67 (m, 4H), 2.16 (m, 3H), 1.81 (s, 2H), 1.65−1.55 (m, 4H), 1.30−1.20 (m, 3H). 13C NMR (125 MHz, CDCl3): δ 178.5, 132.1, 52.6, 51.5, 50.9, 46.7−46.2, 43.0− 41.0, 29.0, 26.1, 25.3 (all the PNB−PA one-shot copolymers have identical 1H NMR and 13C NMR spectra, except the intensity of δ 6.08−6.82 signal at 1H NMR). Poly(TD)-PA Blocklike Copolymers. Yield: 63−95% (see Table 1, entries 6−10). 1H NMR (500 MHz, CDCl3): δ 6.32 (s, 1H), 5.09 (s, 1H), 3.20−2.70 (br, 7H), 1.68 (s, 1H), 1.59 (s, 1H), 1.25−1.16 (br, 8H) 1.00−0.81 (br, 6H). 13C NMR (125 MHz, CDCl3): δ 178.7, 132.4, 131.3 44.0 (br), 42.7, 40.8, 38.6 (br), 37.5, 30.6, 28.7, 23.7, 23.3, 14.3, 10.4 (all the PTD−PA one-shot copolymers have identical 1H NMR and 13C NMR).

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RESULTS AND DISCUSSION Even though ROMP is an efficient living polymerization method to construct well-controlled block copolymers, oneshot block copolymerization by ROMP has been rarely B


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Macromolecules Table 1. Synthesis of PNB−PA and PTD−PA Blocklike Copolymers with Various Feed Ratios of COT

entry

comonomera

Xb

concc (M)

reaction time (h)

COT convd (%)

PhHd,e (%)

yield (%)

Dhf (nm)

1 2 3 4 5 6 7 8 9 10

NB NB NB NB NB TD TD TD TD TD

10 20 30 30 50 10 20 30 30 50

0.14 0.14 0.14 0.33 0.14 0.14 0.14 0.14 0.33 0.14

2 3 7 6.5 5.5 5 9 13 13 13

>99 >99 >99 >99 86 66 88 >99 96 95

22 22 19 13 15 8 24 19 18 23

97 94 92 92 87 63 95 97 96 91

37.8 50.8 106 109 122 83.6 295 576 625 2670

a

Use 50 equiv. bCOT equivalent. cBased on [COT]. dMeasured by 1H NMR. eYield of benzene formation calculated on the basis of the total number of double bonds from converted COTs. fMeasured by DLS.

and their sizes increased with increasing feed ratios of COT (Figure 1g−k). The resulting nanostructures from PNB−PA and PTD−PA copolymers showed the same structural evolution as those from the authentic block copolymers (Figure 1f,l and Figure S1). Furthermore, DLS analysis of the nanostructures synthesized by the one-shot method revealed that the average hydrodynamic diameter (Dh) of nanostructures for both PNB−PA and PTD−PA in chloroform increased with increasing feed ratios of COT (Table1 and Figure 2). Additionally, the measured data matched well with the sizes of the nanostructures observed in the AFM images (Figure 1) and also with the increase of Dh in the previous nanostructures formed by block copolymers synthesized by sequential monomer addition (Figure S2). These macroscopic analysis suggested that the newly synthesized copolymers produced by one-shot copolymerization had blocklike microstructures, which were similar to those of conventional block copolymers produced by the sequential method. To verify that the one-shot copolymerization method could produce copolymers with blocklike microstructures, the conversion of each comonomer, NB or TD and COT, during one-shot copolymerization was monitored via in situ 1H NMR (Figure 3). The kinetics experiment using NB and COT as the comonomers was conducted at 5 °C owing to the rapid polymerization of NB with the third-generation Grubbs catalyst at room temperature (the detailed studies at early conversion can be found in Table S1 and Figure S3), whereas the conversion of TD and COT was slow enough to be monitored at room temperature. Since the ROMP of NB and TD was much faster than the ROMP of COT, only a small portion of, or almost no, COT was consumed until the NB or TD monomer was fully consumed (Figure 3a,b). By calculating the propagation rate of each monomer during one-shot ROMP from the logarithmic conversion vs time plot, we observed that the propagation rate of NB was 24 times faster than that of COT (0.066 vs 0.0028 for the rate constants, Figure 3c), and

observed because most of the commonly used ROMP monomers have similar polymerization reactivities.13a To our delight, we observed a vastly different rate between the ROMP of NB or TD monomers (typically 10−30 min for degree of polymerization (DP) = 50) and the ROMP of COT (6−18 h for DP = 50) while investigating nanostructure formation via INCP using PNB-b-PA and PTD-b-PA block copolymers.12c,e On the basis of this rate difference, we envisioned that block copolymers could be prepared by the simple copolymerization of NB or TD and COT in one shot because NB or TD would be consumed prior to the ROMP of COT. Then, the blocklike copolymers with an insoluble PA block would undergo direct self-assembly via INCP to produce nanostructures similar to those of the block copolymers obtained by the conventional sequential addition method. To determine whether or not one-shot copolymerization would produce blocklike copolymers, we synthesized various copolymers using the method summarized in Table 1. An initiator, the highly active and fast-initiating third-generation Grubbs catalyst, was added to the reaction mixture of two comonomers, NB or TD and COT, in dichloromethane (DCM). Using 50 equiv of NB and TD, we varied the feed ratio of COT (Table 1) to determine whether a similar structural evolution from 0D micelles to 1D nanocaterpillars from PNBb-PA12c and from 0D micelles to 3D nano and microaggregates from PTD-b-PA12e would be possible via one-shot copolymerization. In all cases, soluble polymers were isolated in high yields after simple precipitation in methanol. The resulting polymers were analyzed by AFM and DLS (Figures 1 and 2); both techniques confirmed that regardless of the synthetic method, the copolymers synthesized by the one-shot method behaved like the block copolymers synthesized by the sequential addition method. AFM clearly showed the formation of nanospheres and 1D undulated nanocaterpillars from PNB− PA (Figure 1a−e). Moreover, the formation of 3D nanoaggregates from PTD−PA copolymers was visualized by AFM, C


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Figure 1. Evolution of nanostructures depending on COT feed ratio. AFM height images of nanostructures from the PNB−PA blocklike copolymers prepared by monomer ratios [NB]:[COT] = (a) 50:10, (b) 50:20, (c) 50:30, and (d) 50:50, and (e) phase image of [NB]:[COT] = 50:30. (f) Comparison to the AFM height image of nanostructures from the PNB-b-PA block copolymer of [NB]:[COT] = 50:30.12c Analogous AFM height images from PTD−PA blocklike copolymers prepared by monomer ratio [TD]:[COT] = (g) 50:10, (h) 50:20, (i) 50:30, and (j) 50:50, and (k) phase image of [TD]:[COT] = 50:30. (l) Comparison to the AFM height image of nanostructures from PTD-b-PA block copolymer of [TD]: [COT] = 50:30.12e

same method because COT was not consumed at all at the early stage due to the too large different reactivities between the TD and COT monomers. UV/vis spectroscopy also supported the blocklike microstructure of the polymers prepared by one-shot copolymerization (Figure 4). The PTD−PA blocklike copolymer showed almost the same UV/vis spectrum as that of the PTD-b-PA block copolymer (Figure 4a), suggesting that the PTD−PA copolymer was a near-perfect blocklike copolymer as implied by the kinetic studies (Figure 3b,d). However, when the PNB− PA blocklike copolymer was prepared at 330 mM, based on the concentration of COT, its spectrum contained some additional signals near 330−390 nm (Figure 4b, black line with red arrow), which were absent from the spectrum of the PNB-b-PA block copolymer synthesized by sequential monomer addition (Figure 4b, blue line). According to the Fieser−Kuhn rule (Table S2),20 which estimates the λmax values of oligoenes and the experimental spectra of oligoenes,13,14 the new signals from the PNB−PA blocklike copolymers could be attributed to oligoenes containing five to seven conjugated double bonds. This result implies that oligoene domains were inserted into the PNB-rich domain, as indicated by the kinetic analysis (Figure 3a,c). To achieve more blocklike microstructures via the ROMP of NB and COT by the one-shot method, the polymerization was conducted at a lower concentration, specifically 140 mM based on the concentration of COT. As expected, the absorption

the propagation rate of the TD was 170 times faster than that of COT (0.12 vs 0.00071 for the rate constants, Figure 3d). This large difference in the reaction rates between NB or TD and COT arises from the difference in the ring-opening rate of each monomer. Since the driving force of the ring-opening of cyclic monomers is the relief of ring strain, the ring-opening of NB and TD is very fast and irreversible due to the large ringstrain energy of the olefin on the bridged bicyclic six-membered ring of NB (27 kcal/mol) and cyclobutene of TD (30 kcal/ mol). However, the ring-opening of the less-strained COT (2.5 kcal/mol) is very slow and reversible.13b,17 Therefore, when the initiator is added to the reaction mixture containing NB or TD and COT, the ring-opening of NB or TD preferentially occurs to produce PNB or PTD polymers, and subsequently the ringopening of COT occurs slowly and reversibly to form PA, which eventually forms the core block. Hence, blocklike copolymers are produced, and they successfully form various nanostructures via INCP. Upon comparing the relative reaction rate between TD and COT (kp,TD/kp,COT) and that of NB and COT (kp,NB/kp,COT), we observed that the former was 7 times larger than the latter (kp,TD/kp,COT = 170 vs kp,NB/kp,COT = 24, Figure 3c,d). This phenomenon could be explained by the difference in the rate of catalyst coordination to the NB and TD monomers. Previously, we reported that the initiator could bind faster to the TD derivatives than to the NB derivatives because the cyclobutene moiety on the TD monomers has less local steric hindrance around its active olefin, whereas the olefin on the NB monomers is more hindered due to a neighboring bridging methylene group (Figure S4).18 Thus, one can expect that the preference toward the TD monomer over COT would lead to a near-perfect, blocklike microstructure for the PTD−PA copolymer, even via the one-shot copolymerization method. To confirm the reactivity differences of the comonomers, the reactivity ratios for each pair of the comonomers were measured using the Fineman−Ross equation (Supporting Information).19 As a result, the reactivity ratios of NB and COT were calculated to be r1 = 12.6 for NB and r2 = 0.605 for COT (Figure S5). These values matched well with the values for gradient copolymerization (r1 ≫ 1 ≫ r2). However, we could not estimate the reactivity ratios of TD and COT by the

Figure 2. DLS profiles of nanostructures formed from (a) PNB−PA and (b) PTD−PA blocklike copolymers with various COT ratios. D


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Figure 3. Conversion vs time and logarithmic conversion vs time plot for ROMP of (a , c) NB and COT at 5 °C and (b, d) TD and COT at room temperature with feed ratio [NB] (or [TD]):[COT] = 50:30 at 140 mM based on the concentration of COT.

Figure 4. UV/vis absorbance spectra of the blocklike copolymers polymerized at different concentrations and comparison to authentic block copolymers using (a) TD and COT and (b) NB and COT as comonomers. (c) PNB−PA blocklike copolymers polymerized with various monomer feed ratios.

However, the signals corresponding to the oligoenes disappeared from the spectra of the copolymers with COT feed ratios lower than 30 (Figure 4c). The effects of the comonomers and synthetic conditions on the blocklike microstructures were further investigated by liquid 1 H NMR spectroscopy. Our previous results regarding INCP revealed that the NMR signals for the second insoluble polymer blocks were undetectable by liquid NMR, owing to the core− shell formation of the block copolymers via INCP.12 For example, peaks corresponding to the conjugated PAs, whose chemical shifts ranged from 6 to 7 ppm, were absent from the spectra of the PNB-b-PA and the PTD-b-PA block copolymers. Upon comparison of the NMR spectra of the conventional block copolymers with the spectra of the blocklike copolymers synthesized by the one-shot method, the PTD−PA blocklike copolymers showed a 1H spectrum identical to that of the authentic PTD-b-PA block copolymers (Figure S7a). This further supported the near-perfect block microstructure of the PTD−PA copolymer prepared by the one-shot approach. In

signals of the oligoenes almost disappeared from the UV/vis spectrum (Figure 4b, red line), indicating that the resulting copolymers contained fewer oligoene defects in the PNB-rich domain. Therefore, more blocklike structures were produced at a lower concentration than at higher concentrations. Presumably, copolymerization at the lower concentration increased the benzene generation as a result of backbiting, thereby shortening the length of the oligoene defects on the shell block. More amount of benzene generated at the lower concentration, as observed by 1H NMR, supports this assumption (Table 1, entries 3 and 4). Alternatively, the lower concentration could favor the ROMP of NB over COT because the ring-opening of COT is reversible process due to its low ring strain. Additionally, we observed that the amount of inserted oligoenes increased with higher feed ratios of COT. The UV/vis spectrum of the copolymer with the feed ratio of [NB]: [COT] = 50:50 had stronger signals attributed to the oligoenes, even though the polymerization was carried out at 140 mM based on [COT] (Figure 4c, green line with red arrow). E


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Figure 5. In situ monitoring of changes in 1H NMR integration of blocklike copolymers. The copolymerization was conducted at 140 mM based on the concentration of COT at room temperature. (a) Overlay of in situ 1H NMR spectra of PNB−PA blocklike copolymer over time. (b) Integration vs time plot for PNB, PA, and benzene signals and conversion vs time plot for each monomer. (c) Overlay of in situ 1H NMR spectrum and (d) integration and conversion vs time plot for PTD−PA blocklike copolymer. (e) Schematic illustration of the nanostructure formation and solvent penetration into the micelles.

ratio of [NB]:[COT] = 50:30; for details, see Supporting Information, Figure S8). During the in situ monitoring of the one-shot copolymerization using 1H NMR, we observed an interesting reduction in the integration of the polymer signals for both PNB−PA and PTD−PA blocklike copolymers (Figure 5a−d). As the polymerization proceeded, the intensity of the exposed PA signals at δ = 6−7 ppm decreased gradually. More surprisingly, not only did the intensity of the PA signals decrease, but those of the PNB or PTD olefin signals (δ = 5.7 and 5.5 ppm for PNB and 6.35 ppm for PTD) decreased as well (Figure 5a,c). The plot of the relative integration changes in the polymer signals vs time showed these changes more clearly (Figure 5b,d). We believe that these phenomena may give further insight into the INCP process. First, in the case of the PNB−PA blocklike copolymerization, every part of the polymer chain, including the oligoenes, can be detected by liquid NMR during the early stage of the polymerization because the PNB chain

addition, the PNB−PA blocklike copolymer with a lower feed ratio (50:20) exhibited a spectrum almost identical to that of the independently prepared block copolymer (Figure S7b). On the other hand, as the feed ratio of COT increased to 50, the spectrum of the PNB−PA blocklike copolymer showed weak but stronger oligoene signals in the range of δ = 6−7 ppm (Figure S7b). These more oligoene signals suggested the presence of short PA defects on the PNB-rich domain because the well-solvated PNB-rich shell could be detected by liquid NMR, whereas most long PAs forming the core were not detected. In short, the NMR results supported the conclusions obtained by kinetic and UV/vis analyses (Figures 3a,c and 4c). This gradient composition for the PNB−PA blocklike copolymer could slightly shorten the length of the second PA block when compared to the PNB-b-PA block copolymer. However, the influence to the resulting nanostructure formation was not that significant as the ratios of worm/ sphere were similar for each case (2.3 by one-shot method vs 2.6 by sequential-addition method12c from the monomer feed F


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with very short PAs is fully solvated (A in Figure 5e). As the ROMP of COT proceeds to form elongated solvophobic PA, the polymer chains self-assemble into spherical micelles, so that the soluble PNB shell covers the insoluble PA core and the intensity of the signal attributed to PA decreases (B in Figure 5e). During this process, a small part of the PNB-rich domain near the PA core may be buried together with the PA core, resulting in a reduction in the integration of the signals corresponding to PNB. With the further elongation of the PA core, the spherical micelle structure is not sufficient to cover the extended PA core, and the micelles spontaneously cling to each other to form 1D nanocaterpillar structures.12c This process leads to tightening of the shell as the polymer chains condense, and more of the chains near the core become less mobile (C in Figure 5e). Thus, the signal intensity of the polymers decreases because this tightly packed or hardly solvated corona takes a solidlike state, similar to the PA core, and it becomes harder for NMR solvents to penetrate deep into the shell (Figure 5e, green arrows).21 Second, a larger decrease in the PTD intensity was observed during the one-shot polymerization of TD and COT (Figure 5b,d). This was because the PTD−PA blocklike copolymer self-assembled into 3D-aggregates or higher dimensional nanostructures with denser packing than the 1D assembly of PNB−PA. Therefore, a larger portion of the PTD shell becomes concealed and undetectable by 1H NMR. In short, the changes in the NMR intensities during polymerization may indirectly shed light on how various INCP processes actually occur in solution.

K.-Y.Y.: Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, TX 78712. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from Basic Science Research and Nano-Material Technology Development through NRF of Korea.

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CONCLUSION In summary, a new and simple strategy to form various nanostructures from simple monomers was demonstrated. We successfully synthesized PNB−PA and PTD−PA blocklike copolymers via a one-shot ROMP approach by taking advantage of the different reactivities of the comonomers, NB or TD and COT. On the basis of AFM and DLS analyses, we observed that the resulting blocklike copolymers containing an insoluble conjugated PA block spontaneously self-assembled into various nanostructures such as nanospheres, 1D nanocaterpillars, and 3D nanoaggregates during polymerization. The nanostructures formed from the blocklike microstructures of the copolymers were essentially the same as those obtained from the block copolymers synthesized by sequential monomer addition. Further analyses by UV/vis and NMR spectroscopy strongly supported that the blocklike microstructures were indeed possible via the one-shot method. Notably, near-perfect, blocklike copolymers could be produced by the one-shot copolymerization of TD and COT. Additionally, monitoring the 1H NMR spectra during one-shot polymerization provided some useful insights into the INCP mechanism and the production of nanostructures during polymerization.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, information about block copolymers, calculated λmax values of polyenes, and NMR spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

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