Triple-Responsive Block Copolymer Micelles with Synergistic pH and

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Triple-Responsive Block Copolymer Micelles with Synergistic pH and Temperature Response Panagiotis G. Falireas†,‡ and Maria Vamvakaki*,†,‡ †

Institute of Electronic Structure and Laser, Foundation for Research and Technology - Hellas, 700 13 Heraklion, Crete, Greece Department of Materials Science and Technology, University of Crete, 700 13 Heraklion, Crete, Greece

‡

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S Supporting Information *

ABSTRACT: Multifunctional, stimuli-responsive block copolymers have been prepared via the sequential atom transfer radical polymerization (ATRP) of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and the in-house synthesized 1′(2-methacryloxyethyl)-3′,3′-dimethyl-6-nitrospiro-(2H-1-benzopyran-2,2′-indoline) (SPMA) monomer. Two PDMAEMAb-PSPMA diblock copolymers, containing 3 and 14 mol % SPMA, were synthesized. The amphiphilic nature of the PDMAEMA-b-PSPMA diblock copolymers led to the formation of well-defined spherical micelles, comprising a hydrophobic PSPMA core and a hydrophilic PDMAEMA shell, in water. The combination of the pH- and temperatureresponsive character of PDMAEMA with the pH-, temperature-, and light-sensitive properties of the PSPMA block has resulted in a complex responsive behavior of the copolymer micelles in aqueous solution, when applying three different external stimuli (i.e., light irradiation, pH, and temperature). More importantly, the synergistic response of the block copolymer micelles when varying simultaneously the solution pH and temperature is reported for the first time. Such multisensitive self-assembled nanostructures pave the way for the on-demand controlled capture and release of actives under complex environmental cues.

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INTRODUCTION The past decades have witnessed tremendous progress in the design of novel, stimuli-responsive block copolymers and the investigation of their self-assembly behavior into stable morphologies such as micelles, rods, or vesicles.1−4 These self-assembled structures undergo reversible or irreversible physical and/or chemical transformations upon exerting external stimuli, such as pH,5−10 temperature,11−15 ionic strength,16−20 and light.21−29 The latter in particular is very appealing since it can be manipulated remotely and allows spatiotemporal control without introducing chemical impurities. Because of these unique advantages of light, the design of photoresponsive block copolymers micelles has gained great attention for a variety of biomedical and technological applications. Light-responsive micelles are typically attained via the self-assembly of amphiphilic block copolymers which incorporate a photoresponsive hydrophobic block. When light is applied, the photoresponsive groups act as photoreceptors and undergo physical and/or chemical changes, which transform the hydrophobic block into hydrophilic, resulting in the disruption of the micelles.30 Photoresponsive organic molecules including azobenzenes,21,31−34 spiropyrans,23,24,35−37 pyrenylmethyl esters,38 dithienylethenes,39 coumarins,40−42 and pyrene43 have been vastly studied in the recent years for the design of lightresponsive (co)polymers and micelles. Among them, spiropyrans is a well-known class of photoresponsive molecules which can be easily, and reversibly, isomerized between the ring© XXXX American Chemical Society

closed, hydrophobic spiropyran (SP) and the open, hydrophilic merocyanine (MC) form under UV and visible light irradiation, respectively.44 Besides, SPs also exhibit unique multiresponsive properties being sensitive to several other external stimuli such as the solution pH and temperature and the solvent polarity. For example, SPs can be acid-induced isomerized to the protonated merocyanine (MCH+) form, which is transformed back to the SP isomer using base or visible light irradiation.45 Furthermore, SPs exhibit thermochromism with the colored MC form being isomerized to the colorless SP moieties upon heating. 46 Therefore, the incorporation of SP units along a polymer chain provides a convenient platform toward the design of multistimuli-sensitive polymers exhibiting complex responsive behavior. Recently, the design of multiresponsive polymeric materials that can recognize synergistically more than one stimuli, exhibiting collective responses similar to those found in nature, has attracted great attention. These materials can stimulate future developments in the fields of bio- and nanotechnology to produce, for example, advanced (bio)sensors, “smart” delivery systems, and complex electronic devices. Despite the multiresponsive character of SPs, copolymers triggered by independent stimuli have been only reported so far,23,36,47−49 whereas their cooperative response to multiple external stimuli Received: April 18, 2018 Revised: August 7, 2018

A

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Table 1. Molecular Characteristics of the PDMAEMA Macroinitiator and the Two PDMAEMA-b-PSPMA Diblock Copolymers GPC results sample

Mn (g/mol)

Mw (g/mol)

Mw/Mn

SPMA contenta (mol %)

PDMAEMA PDMAEMA-b-PSPMA3 PDMAEMA-b-PSPMA14

17100 20000 22200

18800 22400 25100

1.10 1.12 1.13

3.0 14.0

a

Calculated by 1H NMR. Synthesis of 1′-(2-Methacryloxyethyl)-3′,3′-dimethyl-6-nitrospiro-(2H-1-benzopyran-2,2′-indoline) (SPMA). The synthesis of the SPMA monomer was performed by a four-step reaction process according to previously reported work.35,52 The successful synthesis of the monomer was confirmed by proton nuclear magnetic resonance (1H NMR) spectroscopy (see Figure S1, Supporting Information) (CDCl3, 300 MHz): δ 1.16 (s, 3H), 1.28 (s, 3H), 1.91 (s, 3H), 3.38−3.60 (m, 2H), 4.28−4.32 (t, 2H) 5.57 (s, 1H), 5.86− 5.89 (d, 1H), 6.07 (s, 1H), 6.69−6.76 (q, 2H), 6.88−6.92 (t, 2H), 7.08−7.10 (d, 1H), 7.18−7.23 (t, 1H), 8.00−8.04 (m, 2H). Synthesis of the PDMAEMA Macroinitiator. A typical procedure for the synthesis of the PDMAEMA macroinitiator by ATRP, using EBiB as the initiator in a solvent-free polymerization, is described below. DMAEMA (11.48 mL, 0.068 mol), EBiB (0.1 μL, 0.68 mmol), and HMTETA (556 μL, 2.849 mmol) were added in a round-bottom flask under a nitrogen flow. The flask was sealed with a rubber septum, and the reaction mixture was stirred for 20 min at RT after being degassed and filled with nitrogen. Finally, Cu(I)Cl (101 mg, 1.02 mmol) was transferred in the flask under continuous stirring and a dry nitrogen flow. The mixture was degassed by five freeze− pump−thaw cycles, and the reaction was allowed to proceed at RT until almost complete monomer consumption. After completion of the polymerization, a sample from the reaction mixture was removed for 1H NMR analysis (DMAEMA monomer conversion = 87%), and next the PDMAEMA macroinitiator was purified by dissolution in THF and by passing the solution through a neutral Al2O3 column followed by precipitation into a 10-fold excess of cold hexane. The precipitation procedure was repeated twice. Finally, the purified macroinitiator was dried under vacuum. A sample was characterized by gel permeation chromatography (GPC) and 1H NMR spectroscopy (Mn = 17100 g/mol and Mw/Mn = 1.10) (Table 1). Synthesis of the PDMAEMA-b-PSPMA Diblock Copolymers. The synthesis of the PDMAEMA-b-PSPMA diblock copolymers was conducted using the above-synthesized PDMAEMA homopolymer as macroinitiator for the polymerization of SPMA. In a typical synthesis, anisole (1.4 mL), chloro-terminated PDMAEMA (PDMAEMA-Cl) (0.26 g, 13.99 μmol, 17100 g/mol), SPMA (0.13 g, 0.308 mmol), HMTETA (17.56 μL, 0.089 μmol), and Cu(I)Cl (1.4 mg, 0.014 mmol) were transferred in a dried round-bottom flask equipped with a stirrer bar under a dry nitrogen flow. Oxygen was removed by five freeze−pump−thaw cycles, and the flask was placed in a preheated oil bath at 40 °C for 6 h (SPMA monomer conversion = 87%). Next, the reaction mixture was cooled down to RT, opened to air, and was diluted (1:1) in THF. The crude product was purified by filtration through a column containing neutral alumina, followed by precipitation in petroleum ether. The purified product was obtained after two repeated precipitations, was dried under vacuum overnight, and was characterized by 1H NMR spectroscopy and GPC. This polymerization procedure yielded a PDMAEMA-b-PSPMA diblock copolymer containing 14 mol % PSPMA, with Mn = 22200 g/ mol and Mw/Mn = 1.13. Another diblock copolymer was synthesized using the same procedure as that described above by terminating the polymerization after 3 h reaction (SPMA monomer conversion = 16.5%) to yield a PDMAEMA-b-PSPMA diblock copolymer containing 3 mol % PSPMA, with Mn = 20000 g/mol and Mw/Mn = 1.12 (see Table 1). In the following, the diblock copolymers with SPMA content of 3 and 14 mol % will be referred as PDMAEMA-bPSPMA3 and PDMAEMA-b-PSPMA14, respectively. Preparation of the Diblock Copolymer Micelles. Polymeric micelles were obtained by dissolving 10 mg of PDMAEMA-b-

remains largely unexplored. Our group has reported the synthesis of a multiresponsive random copolymer, based on the pH- and temperature-responsive 2-(dimethylamino)ethyl methacrylate (DMAEMA) and SP methacrylate, which exhibited a cooperative response to light irradiation and the solution temperature.35 SP-based diblock copolymers have been shown to exhibit stimuli-tunable self-assembly behavior accompanied by reversible structural transformations. A schizophrenic poly(acrylic acid)-b-poly(spiropyran methacrylate) diblock copolymer undergoing reversible micellization in water regulated by light irradiation and the solution pH was prepared.50 In addition, a multiresponsive poly(N-isopropylacrylamide)-b-poly(N-acryloylglycine-co-N-acryloylglycine spiropyran ester) diblock copolymer with tunable micellar morphology when applying UV light or changing the solution pH or temperature was reported.51 However, the synergistic response of SP-based diblock copolymers to multiple external stimuli is unpresented. In the present work, we describe the synthesis of multiresponsive PDMAEMA-b-poly(1′-(2-methacryloxyethyl)-3′,3′-dimethyl-6-nitrospiro-(2H-1-benzopyran-2,2′-indoline)) (PDMAEMA-b-PSPMA) diblock copolymers by sequential atom transfer radical polymerization (ATRP). The copolymers self-assemble into micelles, comprising a hydrophobic PSPMA core and a hydrophilic PDMAEMA shell which respond to three independent stimulithe solution pH and temperature and UV-light irradiationdue to the combination of the pH- and thermoresponsive PDMAEMA block and the pH-, temperature-, and light-responsive PSPMA block. More importantly, the synergetic effect of the solution pH and temperature on the hydrophobic-to-hydrophilic transition of the PSPMA block is shown for the first time. This cooperative response to two external stimuli is particularly attractive in, for example, the selective release of active compounds from the micellar cores in acidic environments, only at elevated temperatures.

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EXPERIMENTAL SECTION

Materials and Methods. DMAEMA (Aldrich, 98%) was passed through a basic alumina column and then distilled from calcium hydride prior to polymerization. Copper(I) chloride (Cu(I)Cl) (Aldrich, 99.999%) was purified by washing with glacial acetic acid, followed by absolute ethanol and ethyl ether, and then dried under vacuum. Ethyl 2-bromoisobutyrate (EBiB) (Aldrich, 98%), 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) (Aldrich, 97%), 2-bromoethanol (Aldrich, 95%), 2,3,3-trimethylindolenine (Aldrich, 98%), 2-hydroxy-5-nitrobenzaldehyde (Aldrich, 98%), dicyclohexylcarbodiimide (DCC) (Aldrich, 99%), 4-(dimethylamino)pyridine (DMAP) (Aldrich, ≥99%), magnesium sulfate salt (MgSO4) (Fluka, 98%), potassium hydroxide, petroleum ether (Aldrich, 99%), Coumarin 102 (C102) (Fluka, 97%), anisole (Aldrich, 99%), triethylamine (TEA) (Aldrich, ≥99.5%), 2-propanol (Aldrich, ≥99.5%), and hexane (Aldrich, 95%) were used as received. Milli-Q water of specific resistivity of 18.2 ΜΩ cm at 25 °C was used in all experiments. B

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Macromolecules Scheme 1. Synthetic Route for the Preparation of the PDMAEMA-b-PSPMA Diblock Copolymers by ATRP

(50:1) and was injected to the system (20 μL) at a column temperature set to 40 °C. 1 H NMR Spectroscopy. The successful synthesis of the spiropyran monomer and the composition of the copolymers were determined by 1 H NMR spectroscopy using a 300 MHz Avance Bruker NMR spectrometer. CDCl3 was used as the solvent, and tetramethylsilane (TMS) served as the internal reference for the 1H NMR measurements. Light Sources. The UV light source was a Spectroline hand-held UV lamp (8 W) operating at 365 nm. The visible light source was a Variac Cermax 300 W xenon lamp (λ > 400 nm). The cuvettes containing the diblock copolymer solutions were aligned at a distance of 5 cm from the light source to ensure homogeneous irradiation of the sample. Dynamic Light Scattering (DLS). The size of the micelles as a function of solution temperature and pH was determined using a Zetasizer Nano ZS (Malvern Instruments Ltd., Malvern, UK) equipped with a 4 mW He−Ne laser operating at λ = 632.8 nm. The scattered light intensity was measured at a scattering angle of 90°. Data were collected over 2−10 min from 20 to 90 °C. The reported values are the intensity-average diameters which were calculated as average values of three repetitive measurements. Field Emission Scanning Electron Microscopy (FESEM). The morphology of the polymer, before and after irradiation with UV light, was studied by FESEM (JEOL JSM 7000F) at an accelerating voltage of 15 kV. Samples were prepared by depositing one drop of a dilute polymer solution on a silicon wafer followed by drying at room temperature. Another part of the dispersion was placed in a quartz cuvette and was irradiated with UV light for 5 min before being cast and dried on the silicon wafer for FESEM observation. Transmission Electron Microscopy (TEM). The morphology of the micelles was also studied by TEM (JEOL JEM-2100) by depositing a drop of a dilute micellar solution onto a carbon-coated copper grid followed by drying overnight at room temperature. UV/Vis Spectroscopy. The photoinduced SP-to-MC isomerization of the PDMAEMA-b-PSPMA diblock copolymers was monitored, as a function of irradiation time, using a Lambda 25 PerkinElmer UV/vis spectrophotometer in the wavelength range 250−700 nm. For this purpose, the absorption spectra of 0.1 wt % aqueous solutions of the PDMAEMA-b-PSPMA diblock copolymers were recorded following irradiation with UV light. The kinetic rate constants of the UVinduced SP-to-MC isomerization were determined by collecting the absorption spectra of a polymer solution at several UV irradiation time intervals at RT and were calculated using eq 3: ÅÄÅ ÑÉ Å A − A t ÑÑÑ lnÅÅÅÅ e ÑÑ = − kt ÅÅÇ Ae − A 0 ÑÑÑÖ (3)

PSPMA14 in 3 mL of isopropanol, which is a good solvent for the two blocks. Under vigorous stirring, 7 mL of deionized water was added dropwise, and the solution was stirred slowly for another 12 h. Next, the solution was dialyzed against deionized water in a dialysis membrane (MWCO 3000 Da) for 24 h, to remove the isopropanol, with the water being replaced every 6 h. The final polymer concentration was adjusted to 0.1 wt %, and the solution was filtered through a 0.45 μm filter followed by irradiation with visible light for 5 min to isomerize any MC moieties to the initial SP state. Encapsulation and Release of C102 from the PDMAEMA-bPSPMA Micelles. The hydrophobic dye C102 was selected as a model compound to investigate its encapsulation and release behavior from the copolymer micelles. The dye molecules were encapsulated into the hydrophobic cores of the micelles using a similar procedure to that described above. Briefly, 1 mg of PDMAEMA-b-PSPMA14 diblock copolymer and 0.8 mg of C102 were dissolved in 3 mL of isopropanol, followed by the dropwise addition of 7 mL of water into the solution. Next, the dye-loaded micellar suspension was placed in a dialysis membrane (MWCO 3000 Da) and was dialyzed against 1 L of deionized water for 24 h and exchanged every 4 h in the dark at room temperature. The excess dye removed in the dialysate was quantified by UV/vis spectroscopy using an intensity−concentration linear standard calibration curve of C102 (Figure S2). The amount of C102 encapsulated within the micelles was determined as the difference of the mass of dye added initially in the solution to the mass of excess dye and was used to calculate the encapsulation efficiency (EL) and the C102 loading in the PDMAEMA-b-PSPMA14 micelles from eqs 1 and 2, respectively: encapsulation efficiency (%) =

loading (%) =

mass of C102 in micelles × 100 mass of C102 fed initially (1)

mass of C102 in micelles × 100 mass of micelles

(2)

Subsequently, the release of the dye from the micelle cores, following UV irradiation at λ = 365 nm, was investigated. The photocontrolled release of the encapsulated dye, as a function of the irradiation time, was followed by fluorescence spectroscopy. The amount of released dye was quantified following irradiation of a dialysis bag, which contained the C102 loaded micellar solution, for 2 h, dialysis of the micellar solution for 24 h in water, exchanged every 6 h, and analysis of the concentrated dialysate by UV/vis spectroscopy. Characterization. Gel Permeation Chromatography. The molecular weights and the molecular weight distributions of the diblock copolymers were determined by GPC utilizing a Thermo Finnigan TSP P1000 pump, two Mixed-D and Mixed-E columns (Polymer Laboratories), and a refractive index detector (model ERCRI 101). The software used for the analysis of the chromatograms was the Atlas Workstation and Cirrus GPC Reanalysis Software. The eluent was a THF/TEA (50:1) mixture at a 1 mL/min flow rate. The calibration curve was based on eight narrow molecular weight linear PMMA standards ranging from 850 to 342900 g/mol. In a typical measurement, a 1 wt % polymer solution was prepared in THF/TEA

where A0, At, and Ae are the MC absorbance values at time 0, t, and infinity, respectively.53 The acidochromic properties of the diblock copolymer were studied by monitoring the changes in the UV/vis absorption spectra of a 0.1 wt % aqueous solution of the copolymer upon the addition of aliquots of 0.1 and 1 M HCl or NaOH. Optical Transmittance Measurements. The temperature-responsive behavior of the diblock copolymer micelles was studied by turbidimetry measurements. For this, the optical transmittance of a 1 C

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Figure 1. FESEM images of the PDMAEMA-b-PSPMA3 and PDMAEMA-b-PSPMA14 diblock copolymers before (a and b, respectively) and after (c and d, respectively) UV irradiation of their aqueous copolymer solutions. wt % copolymer solution was measured at 750 nm, which is far from the absorption bands of the spiropyran and merocyanine chromophores, as the solution temperature was raised from 25 to 90 °C at a heating rate of 1 °C min−1. Fluorescence Spectroscopy. Fluorescence spectra were recorded on a Lumina fluorescence spectrometer (Thermo Scientific) equipped with a 150 W CW xenon-arc lamp. All samples were measured at RT. The fluorescence emission spectrum of the dye released from the copolymer micelles upon UV light irradiation was recorded from 400 to 650 nm at an excitation wavelength of 420 nm. Excitation and emission slit widths were both maintained at 5.0 nm, and spectra were accumulated at a scan speed of 100 nm min−1.

macroinitiator were found to be 17100 g/mol and 1.10, respectively. After chain extension with SPMA, the molecular weight by GPC increased to 20000 g/mol (Mw/Mn = 1.12) and 22200 g/mol (Mw/Mn = 1.13) for polymerization times 3 and 6 h (see Table 1), respectively, verifying the living “character” of the polymerization. This conclusion was further supported by the relatively low Mw/Mn values (typically below 1.15) of the obtained copolymers. The PDMAEMA-b-PSPMA copolymer terminated after 6 h reaction time exhibited a small shoulder at low elution times suggesting that a few coupling reactions might take place however, the Mw/Mn is below 1.15, which indicates an overall controlled polymerization. The absolute M n ’s and corresponding DP values for the PDMAEMA homopolymer and the PDMAEMA-b-PSPMA diblock copolymers were also calculated from the monomer conversions, determined by 1H NMR analysis of the reaction mixtures after the termination of the polymerization reactions. The Mn for the PDMAEMA macroinitiator was found to be 13700 g/mol (87% monomer conversion), which corresponds to a DP of 87, whereas the Mn values for PDMAEMA-bPSPMA3 and PDMAEMA-b-PSPMA14 were calculated at 14800 and 19900 g/mol corresponding to a DP of the SPMA block of 3 and 14, respectively. 1 H NMR analysis provided additional information for the PDMAEMA-b-PSPMA diblock copolymers (see Figures S4 and S5). Peaks attributed to both the PDMAEMA and the PSPMA blocks were clearly observed. The DMAEMA/SPMA mole ratio was determined by comparing the peak integrals of the signal at 7.88 ppm, which was assigned to two protons at the ortho position of the nitrobenzene ring of the PSPMA

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RESULTS AND DISCUSSION Synthesis of the PDMAEMA-b-PSPMA Diblock Copolymers. PDMAEMA-b-PSPMA diblock copolymers were synthesized via sequential ATRP (Scheme 1). First, the PDMAEMA-Cl macroinitiator was prepared using the Cu(I)Cl/HMTETA transition metal complex as the catalyst system and EBiB as the initiator in a solvent-free polymerization at room temperature. The synthesized macroinitiator was then employed for the polymerization of SPMA in anisole using the same transition metal complex as the catalyst. Two PDMAEMA-b-PSPMA diblock copolymers of different PSPMA block lengths were synthesized by varying the SPMA polymerization time at 3 and 6 h, respectively. The successful copolymerization was verified by GPC (see Figure S3). The unimodal GPC traces of the diblock copolymers, shifted toward lower elution times as the polymerization time was increased, without any trace of the PDMAEMA macroinitiator, indicated the controlled growth of the PSPMA block. Namely, the effective Mn and Mw/Mn of the PDMAEMA-Cl D

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Figure 2. UV/vis absorption spectra of 0.1 wt % aqueous solutions of the PDMAEMA-b-PSPMA3 (a) and PDMAEMA-b-PSPMA14 (b) diblock copolymers upon successive irradiation with UV light. Insets: absorption intensity of MC, at λmax, as a function of irradiation time (i) and rate constant plots for the first-order coloration reaction of the MC photochromic moieties of the copolymers (ii).

block, and the strong band at 2.56 ppm, which corresponds to the methylene protons (CH2−N) of PDMAEMA. The mole ratio for the PDMAEMA-b-PSPMA copolymer terminated after 6 h was calculated 100:16, which corresponds to a SPMA content of 14 mol %. Similarly, the SPMA content of the copolymer terminated after 3 h was found to be 3 mol %. However, given the much higher molecular weight of the SPMA comonomer compared to the DMAEMA units, the weight fraction of the SPMA block in the copolymers is significantly higher, 8 and 38% for PDMAEMA-b-PSPMA3 and PDMAEMA-b-PSPMA14, respectively. Photoresponsive Behavior of the PDMAEMA-bPSPMA Diblock Copolymers in Aqueous Media. The amphiphilic PDMAEMA-b-PSPMA diblock copolymers selfassembled into micelles in water with the hydrophobic PSPMA block in the micelle core and the hydrophilic PDMAEMA block forming the outer shell. The average hydrodynamic diameters (Dhs) of the micelles were measured by DLS at pH 8.5 and were found to be Dh = 125 ± 2 nm and Dh = 95 ± 2 nm for PDMAEMA-b-PSPMA3 and PDMAEMA-b-PSPMA14, respectively. The morphology of the micelles, before and after irradiation of the samples with UV light, was observed by FESEM. Figures 1a and 1b provide compelling evidence of spherical micelles for both samples before irradiation, with sizes D = 81 nm and D = 130 nm for PDMAEMA-b-PSPMA3 and PDMAEMA-b-PSPMA14, respectively. The spherical shape and the size of the micelles were also verified by TEM (Figure S6). After UV irradiation, of the copolymer solutions for 5 min, the absence of the spherical micellar structures and the formation of a polymeric film on the substrates were clearly observed (Figures 1c and 1d), indicating the dissociation of the micelles to their constituent polymeric chains, due to the isomerization of the hydrophobic SP moieties to the hydrophilic MC form which leads to an amphiphilic-todouble-hydrophilic transformation of the diblock copolymers. The photochemical isomerization of the spiropyran moieties of the PDMAEMA-b-PSPMA diblock copolymers in aqueous media at c = 0.1 wt % (see Scheme S1) was monitored by UV/ vis spectroscopy, and the results are illustrated in Figure 2. Before UV irradiation, the absorption at λ > 450 nm is very low, indicating that the photochromic moieties are in the SP form. Upon UV irradiation, the ring-opening isomerization of SP-to-MC was verified by a gradual increase of an absorption band from 450 to 650 nm, accompanied by a spontaneous coloration of the samples (see photographs in Figure 2).

Simultaneously, an increase of the absorption intensity between 350 and 450 nm was observed which was assigned to the nonplanar dipolar isomer X, which is commonly observed for nitro-substituted indolinospiropyrans.54 The absorption intensity, at λ = 534 nm, reached a plateau after 100 and 300 s of UV irradiation time for the PDMAEMA-bPSPMA3 and PDMAEMA-b-PSPMA14 copolymer, respectively (Figure 2, insets (i)), signifying the maximum conversion of SP to the MC isomer. The SP-to-MC isomerization rate constants, calculated using eq 3, obeyed first-order kinetics and were found to be 25.7 × 10−3 and 7.7 × 10−3 s−1 for the PDMAEMA-b-PSPMA3 and PDMAEMA-b-PSPMA14 copolymer, respectively, suggesting a small retardation of the ringopening reaction when increasing the hydrophobic SPMA block length, possibly due to steric hindrance. pH-Responsive Properties of the PDMAEMA-bPSPMA Diblock Copolymer Micelles. The effect of the solution pH on the spectral and morphological behavior and the size of the PDMAEMA-b-PSPMA diblock copolymer micelles was investigated. According to the literature, the SP form can be acid-induced isomerized to the protonated merocyanine (MCH + ) and the protonated spiropyran (SPH+) forms.55 On the other hand, the closed SP form can be accessed reversibly using base.35,52,55,56 The distinct absorption spectra of SP, MC, and MCH+ allow the spectroscopic monitoring of the above transformations. The spectral changes of a 0.1 wt % micellar solution of the PDMAEMA-b-PSPMA3 and PDMAEMA-b-PSPMA14 diblock copolymers in water were monitored by UV/vis spectroscopy as a function of solution pH, whereas the respective morphological changes were determined by FESEM measurements. Figure 3 shows the absorption spectra for an aqueous PDMAEMA-b-PSPMA14 diblock copolymer solution upon the sequential addition of acid, up to a 10-fold excess with respect to the ionizable DMAEMA and SPMA moieties of the copolymer. Surprisingly, the absorption spectra remained unchanged upon the addition of excess HCl, suggesting that the SP units were not isomerized to the MC species. This is in contrast to previous studies on random PDMAEMA-co-PSPMA copolymers, which have shown that the addition of acid first induces the isomerization of the SP chromophores to the open MC form, followed by the protonation of the MC isomer at lower pH values.35 Presumably, in our case, the self-assembly to the PDMAEMA-b-PSPMA diblock copolymers into micellar E

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Figure 3. UV/vis absorption spectra of a 0.1 wt % PDMAEMA-b-PSPMA14 diblock copolymer solution in water upon the addition of acid (HCl) in a 10-fold excess with respect to the ionizable copolymer groups at 25 °C (a) and FESEM image of a PDMAEMA-b-PSPMA14 sample at pH 1.4 (b).

Figure 4. Variation of the hydrodynamic diameter of a 0.1 wt % PDMAEMA-b-PSPMA3 (a) and PDMAEMA-b-PSPMA14 (b) copolymer solution in water as a function of the solution pH.

extends toward the aqueous environment, and therefore at this pH value both samples exhibit a maximum Dh of 187 and 132 nm for PDMAEMA-b-PSPMA3 and PDMAEMA-b-PSPMA14, respectively. A monotonic decrease of the micelle size was observed for both samples upon increasing the solution pH, attributed to the decrease of the electrostatic repulsive interactions of the charged PDMAEMA blocks in the shell, which resulted in the decrease in the stretching of the polymer segments. At pH ∼8, the PDMAEMA chains in the shell become neutral, and thus the size of the micelles attains a minimum value (125 nm for PDMAEMA-b-PSPMA3 and 95 nm for PDMAEMA-b-PSPMA14) which remains constant at higher pH values. The above results reinforce our findings that the hydrophobic micelle cores formed by the PSPMA segments are not affected by the addition of acid in the solution, since no sharp structural changes were observed at low pH values. On the other hand, the shells confer stabilization to the micellar structures, regardless of the degree of protonation of the PDMAEMA segments, since no aggregation phenomena were observed. Thermoresponsive Behavior of the PDMAEMA-bPSPMA Diblock Copolymer Micelles. The temperatureresponsive properties of the PDMAEMA-b-PSPMA diblock copolymer micelles were assessed by monitoring the optical transmittance and the Dh of the copolymer solutions, while increasing the solution temperature from 20 to 90 °C (Figure 5). PDMAEMA is a well-known thermoresponsive polymer

structures hinders the effective protonation of the SP units, located in the hydrophobic micellar core, by the surrounding acidic medium (see Scheme S1). This hypothesis was further supported by FESEM, which showed that the spherical micellar morphology was retained for the diblock copolymer solution after being kept at pH 1.4 (10-fold excess of HCl) for 24 h (Figure 3b), and therefore the hydrophobic PSPMA block was not transformed into the hydrophilic MC-based segment. Similar results were obtained for the PDMAEMA-b-PSPMA3 diblock copolymer (see Figure S7), suggesting that the isomerization and subsequent protonation of the SP moieties in the hydrophobic micelle core, which would lead to the disassembly of the micelles, are not possible even with the shorter PSPMA block length. This unusual pH-responsive behavior of the diblock copolymer micelles was studied further by DLS. Potentiometric titration measurements of the micellar copolymer solution before the DLS studies suggested that the protonation/deprotonation process of the PDMAEMA units in the polymer shell takes place between pH 4 and 8 (see Figure S8). Therefore, the size measurements by DLS were performed in the pH 3−8.5 range. Figures 4a and 4b show the variation of the hydrodynamic diameter (Dh ) of the PDMAEMA-b-PSPMA3 and PDMAEMA-b-PSPMA14 diblock copolymer micelles, respectively, as a function of the solution pH. At pH 3, the micelles comprise a hydrophobic PSPMA core surrounded by a highly charged PDMAEMA shell, which F

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Figure 5. Hydrodynamic diameter at c = 0.1 wt % (○) and optical transmittance at λ = 750 nm and c = 1 wt % (□) of PDMAEMA-b-PSPMA3 (a) and PDMAEMA-b-PSPMA14 (b) diblock copolymer solutions in water as a function of the solution temperature.

which exhibits a cloud point at ca. 35−45 °C.57 In this study, it is noted that the pH of the solution was adjusted to 8.5, ensuring that all DMAEMA moieties are deprotonated and therefore neutral for all measurements.58 Figure 5 shows the optical transmittance of 1 wt % aqueous solutions of PDMAEMA-b-PSPMA3 and PDMAEMA-bPSPMA14, at λ = 750 nm, as a function of the solution temperature. The samples revealed an abrupt decrease of the transmittance at 48 and 71 °C for PDMAEMA-b-PSPMA3 and PDMAEMA-b-PSPMA14, respectively, signifying the aggregation of the copolymer. At this point it is noted that these transition temperatures are higher compared to the cloud point of the PDMAEMA-Cl macroinitiator, which was found at 44 °C (see Figure S9). This was attributed to the steric repulsion between the PDMAEMA segments in the shell of the micelles which hinders their phase transition. Similar results have been reported earlier by Zhang et al. for poly(N-isopropylacrylamide)-b-poly(N-acryloylglycine-co-N-acryloylglycine spiropyran ester) diblock copolymer micelles.51 To investigate further this behavior, DLS measurements of the samples were conducted as a function of the solution temperature. Figure 5 shows the Dh values of 0.1 wt % aqueous solutions of PDMAEMA-b-PSPMA3 (Figure 5a) and PDMAEMA-b-PSPMA14 (Figure 5b) at temperatures between 20 and 90 °C. In detail, the size of the PDMAEMA-b-PSPMA3 micelles remained constant from 20 to 35 °C, whereas from 35 to 50 °C a slight decrease of Dh from 125 to 110 nm (12% decrease) was found. Finally, above 55 °C (which is in agreement with the cloud point of the copolymer at 48 °C) the Dh increased gradually up to 85 °C, due to the formation of aggregates, and above this temperature an abrupt increase in size was observed, indicating macroscopic polymer precipitation. On the other hand, for the PDMAEMA-b-PSPMA14 sample similarly no significant change of the Dh was found between 20 and 35 °C; however, further increase of the solution temperature to 60 °C resulted in a significant decrease of the Dh from 93 to 53 nm (43% decrease), suggesting that the PDMAEMA chains collapse onto the micellar core due to the temperature-induced polymer−polymer interactions. Finally, above 65 °C (consistent with the higher cloud point of this copolymer) the size increased dramatically, signifying the formation of aggregates. The above results suggest that the characteristics of the micelles are different for the two samples. In particular, the small decrease in the micellar size between 35 and 50 °C found for the PDMAEMA-b-PSPMA3 copolymer denotes the presence of a highly dense outer shell which restricts the

chain collapse upon worsening the solvent quality. On the other hand, the increase of the PSPMA block length in the PDMAEMA-b-PSPMA14 copolymer leads to the formation of micelles comprising a less dense PDMAEMA shell, rendering it more susceptible to size changes, as it is clearly depicted by the larger temperature-induced decrease in the micellar diameter between 35 and 60 °C.59 Cooperative Temperature- and pH-Responsive Behavior of the PDMAEMA-b-PSPMA Diblock Copolymers. Next, the synergistic response of the copolymer micelles to a combination of external stimuli was investigated. A 0.1 wt % PDMAEMA-b-PSPMA14 micellar solution was subjected to a gradual decrease of the pH down to pH 1.4 at a temperature of 70 °C, and the spectroscopic changes of the solution were followed by UV/vis spectroscopy (Figure 6a). The UV/vis spectra show that upon the addition of a small amount of acid (1.5 × 10−5 mol) a gradual decrease of the absorption band at 270 nm, which corresponds to the closed SP form, is observed, while a new band at 568 nm, which corresponds to the open MC form, appears (Figure 6a, inset ai). Moreover, a small increase of the absorption band at 314 nm and at the same time a stronger increase of the peak at 424 nm are observed which are ascribed to the formation of the protonated SPH+ and MCH+ species, respectively (see Scheme S1). Further addition of acid results in the decrease of the intensity of the MC band (Figure 6a, inset ai) accompanied by the simultaneous gradual increase of the absorbance band of MCH+ (Figure 6a, inset bi). These results suggest that the SP conversion, first to the open MC isomer, followed by the protonation of the latter to form the MCH+ species upon further addition of acid, takes place upon the combined effect of acid addition and high solution temperature. This synergistic effect of the solution pH and temperature on the effective isomerization and protonation of the chromophore groups in the micellar cores is unpresented in the literature. A possible explanation could be related to a temperaturepromoted protonation of the SPMA monomer repeat units. To further elucidate these results, we have characterized the cooperative temperature and pH responsive behavior of the SPMA monomer in aqueous media. The UV/vis spectra of a 0.1 wt % SPMA solution in water at pH 7.4 and RT and at pH 1.4 at two different solution temperatures (RT and 70 °C) were measured (see Figure S10). As seen in Figure S10, the hydrophobic SP-based monomer does not exhibit any peaks above 300 nm and is essentially insoluble at pH 7.4 at RT as well as pH 1.4 at RT, whereas at 70 °C and pH 1.4 the monomer becomes soluble in the aqueous medium, and two G

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Figure 6. UV/vis absorption spectra of a 0.1 wt % PDMAEMA-b-PSPMA14 diblock copolymer solution in water upon the addition of acid (HCl) (a) and base (NaOH) (b) at 70 °C. Insets: ai and bi show the variation of the absorption intensity for MC and MCH+, respectively, at λmax as a function of added HCl; aii and bii show the variation of the absorption intensity for MC and MCH+, respectively, at λmax as a function of added NaOH.

of C102 in the PDMAEMA-b-PSPMA14 copolymer micelles is depicted schematically in Scheme S2. The amount of C102 encapsulated with the micellar cores was calculated using UV/vis measurements. The encapsulation efficiency was found to be 9.7%, while the C102 loading was calculated at 7.6%. The light-controlled release of the encapsulated C102 from the micellar cores was investigated using fluorescence spectroscopy as reported previously.23,29 A freshly prepared aqueous solution of the PDMAEMA-bPSPMA14 micelles, loaded with C102, was irradiated at 365 nm, and the fluorescence intensity was recorded as a function of UV light exposure time (Figure 7). Before irradiation, a strong emission peak at 495 nm was observed, signifying the successful loading of the dye within the hydrophobic environment of the core. Following irradiation at 365 nm, a gradual decrease of the fluorescence intensity with irradiation time was found accompanied by a shift of the peak to lower wavelengths. It is noted that upon UV irradiation the SP moieties undergo isomerization to the polar MC isomers which results in the transformation of the PSPMA block from hydrophobic to hydrophilic, leading to the disintegration of the micelles, as discussed above. The observed decrease of the fluorescence intensity signifies the release of the C102 molecules from the micellar cores into the aqueous environment in which the dye is insoluble and its fluorescence is quenched. Similar results have been reported by Matyjaszewski et al.23 for the release of C102 from PEO-b-PSPMA diblock copolymer micelles. The percent release of C102 was calculated by comparing the fluorescence intensity before and after 60 min UV irradiation and was found to be ≈87%. The above results confirmed the successful loading and release of C102 within the core of the PDMAEMA-b-PSPMA14 micelles, rendering them promising candidates for use as smart nanocarriers of active compounds.

new peaks at 314 and 424 nm are observed in the UV/vis spectrum, which are ascribed to the formation of the protonated SPH+ and MCH+ species, respectively. At the same time the color of the solution becomes yellow (see the inset in Figure S10), suggesting that the isomerization of SP to MC and the protonation of the latter to form the MCH+ species occur only upon the combined effect of high solution temperature and low pH. This cooperative temperature and pH responsive behavior of the SPMA monomer was unexpected and supports our assumption of a temperaturepromoted protonation of the SPMA units in the micellar core. On the other hand, this acid-induced protonation process was fully reversible; after the addition of base the deprotonation of the SPH+ and MCH+ isomers was observed, as a gradual decrease of the absorbance bands at 314 and 424 nm (Figure 6b, inset bii), respectively, and the reappearance of the peak attributed to the MC isomer at 568 nm (Figure 6b, inset aii) was found. Similar results were obtained for the PDMAEMAb-PSPMA3 diblock copolymer micelles, suggesting that the synergistic effect of solution pH and temperature on the protonation and deprotonation of the SP groups upon the addition of acid and base, respectively, is not dependent on the hydrophobic block length (see Figure S11). Encapsulation and Photoinduced Release of C102 from the Copolymer Micelles. The ability of the PDMAEMA-b-PSPMA14 diblock copolymer micelles to encapsulate hydrophobic molecules within their core and release them, in response to light irradiation, was investigated using C102 as a model hydrophobic molecule. C102 is a solvatochromic molecule which has been extensively employed as a fluorescence probe because its fluorescence intensity increases in the hydrophobic environment of the micellar cores, while it decreases in aqueous solutions, in which the dye is insoluble and the fluorescence is quenched.60,61 Another advantage of C102 is that its excitation wavelength is in the visible region (420 nm), and therefore it does not interfere with the UV-induced (365 nm) isomerization of SP to MC. Finally, its fluorescence emission spectrum does not overlap with that of the MC isomer at 600−780 nm. Herein, we explore the encapsulation efficiency of C102 within the PDMAEMA-b-PSPMA14 diblock copolymer micelles by UV/vis spectroscopy and its release profile as a function of time by fluorescence spectroscopy. The process followed for the quantification of the loading and encapsulation efficiency

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CONCLUSIONS In this study, we report the synthesis of novel multiresponsive amphiphilic diblock copolymers which comprise a thermo- and pH-responsive PDMAEMA and a temperature-, pH-, and lightsensitive PSPMA block. The amphiphilic PDMAEMA-bPSPMA diblock copolymers self-assemble into well-defined spherical micelles in aqueous solutions comprising a hydrophobic PSPMA core and a hydrophilic PDMAEMA shell. When exposed to UV irradiation, the micellar structures were H

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been cofinanced by the European Union (European Social Fund − ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF) - Research Funding Program: Heraclitus II. Investing in knowledge society through the European Social Fund.

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(1) Riess, G. Micellization of block copolymers. Prog. Polym. Sci. 2003, 28 (7), 1107−1170. (2) Mai, Y.; Eisenberg, A. Self-assembly of block copolymers. Chem. Soc. Rev. 2012, 41 (18), 5969−5985. (3) Discher, D. E.; Eisenberg, A. Polymer Vesicles. Science 2002, 297 (5583), 967−973. (4) Blanazs, A.; Armes, S. P.; Ryan, A. J. Self-Assembled Block Copolymer Aggregates: From Micelles to Vesicles and their Biological Applications. Macromol. Rapid Commun. 2009, 30 (4−5), 267−277. (5) Gillies, E. R.; Fréchet, J. M. J. pH-responsive copolymer assemblies for controlled release of doxorubicin. Bioconjugate Chem. 2005, 16 (2), 361−368. (6) Schilli, C. M.; Zhang, M.; Rizzardo, E.; Thang, S. H.; Chong, Y. K.; Edwards, K.; Karlsson, G.; Müller, A. H. E. A new doubleresponsive block copolymer synthesized via RAFT polymerization: Poly(N-isopropylacrylamide)-block-poly(acrylic acid). Macromolecules 2004, 37 (21), 7861−7866. (7) Baines, F. L.; Billingham, N. C.; Armes, S. P. Synthesis and Solution Properties of Water-Soluble Hydrophilic−Hydrophobic Block Copolymers. Macromolecules 1996, 29 (10), 3416−3420. (8) Gohy, J.-F.; Creutz, S.; Garcia, M.; Mahltig, B.; Stamm, M.; Jérôme, R. Aggregates Formed by Amphoteric Diblock Copolymers in Water. Macromolecules 2000, 33 (17), 6378−6387. (9) Liu, S.; Armes, S. P. Polymeric Surfactants for the New Millennium: A pH-Responsive, Zwitterionic, Schizophrenic Diblock Copolymer. Angew. Chem., Int. Ed. 2002, 41 (8), 1413−1416. (10) Bae, Y.; Fukushima, S.; Harada, A.; Kataoka, K. Design of environment-sensitive supramolecular assemblies for intracellular drug delivery: Polymeric micelles that are responsive to intracellular pH change. Angew. Chem., Int. Ed. 2003, 42 (38), 4640−4643. (11) Chung, J. E.; Yokoyama, M.; Suzuki, K.; Aoyagi, T.; Sakurai, Y.; Okano, T. Reversibly thermo-responsive alkyl-terminated poly(Nisopropylacrylamide) core-shell micellar structures. Colloids Surf., B 1997, 9 (1−2), 37−48. (12) Chung, J. E.; Yokoyama, M.; Yamato, M.; Aoyagi, T.; Sakurai, Y.; Okano, T. Thermo-responsive drug delivery from polymeric micelles constructed using block copolymers of poly(N-isopropylacrylamide) and poly(butylmethacrylate). J. Controlled Release 1999, 62 (1−2), 115−127. (13) Chung, J. E.; Yokoyama, M.; Okano, T. Inner core segment design for drug delivery control of thermo-responsive polymeric micelles. J. Controlled Release 2000, 65 (1−2), 93−103. (14) Rapoport, N. Physical stimuli-responsive polymeric micelles for anti-cancer drug delivery. Prog. Polym. Sci. 2007, 32 (8−9), 962−990. (15) Schmaljohann, D. Thermo- and pH-responsive polymers in drug delivery. Adv. Drug Delivery Rev. 2006, 58 (15), 1655−1670. (16) Bütün, V.; Billingham, N. C.; Armes, S. P. Unusual Aggregation Behavior of a Novel Tertiary Amine Methacrylate-Based Diblock Copolymer: Formation of Micelles and Reverse Micelles in Aqueous Solution. J. Am. Chem. Soc. 1998, 120 (45), 11818−11819. (17) Liu, S.; Armes, S. P. Synthesis and aqueous solution behavior of a pH-responsive schizophrenic diblock copolymer. Langmuir 2003, 19 (10), 4432−4438. (18) Babin, J.; Rodriguez-Hernandez, J.; Lecommandoux, S.; Klok, H. A.; Achard, M. F. Self-assembled nanostructures from peptide-

Figure 7. Fluorescence emission spectra of the PDMAEMA-bPSPMA14 diblock copolymer micelles with encapsulated C102 during irradiation at 365 nm for 60 min. Inset: variation of the maximum intensity of C102 as a function of irradiation time.

dissembled owing to the photoisomerization of the hydrophobic SP isomer to the hydrophilic MC form. UV/vis absorption spectra verified the photochemical isomerization of the spiropyran moieties induced by UV light irradiation and allowed the calculation of the coloration constants. The thermoresponsive behavior of the block copolymer micelles was shown to depend on the micellar size, which in turn is determined by the block copolymer composition. More importantly, we show that the PDMAEMA-b-PSPMA diblock copolymer micelles are stable at pH values as low as 1.4 due to the suppression of the acidochromic properties of the PSPMA block in the micellar core and require the synergistic effect of two external stimuli, namely an acidic solution pH and high solution temperature, to dissociate the micelles into their constituent diblock copolymer chains. UV/vis and fluorescence emission studies verified the successful encapsulation of hydrophobic guest molecules, such as C102, within the cores of the PDMAEMA-b-PSPMA14 diblock copolymer micelles as well as the UV-induced release of the dye molecules in the surrounding solution. These multiresponsive block copolymer micelles, which allow to tune their physicochemical and selfassembly properties upon application of cooperative external stimuli, create new perspectives for the development of smart nanocarriers for use in a range of environmental and biological applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00810. Polymer characterization data (GPC traces, 1NMR and UV/vis spectra, FESEM and TEM images) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +30 2810 545019 (M.V.). ORCID

Maria Vamvakaki: 0000-0003-2150-9415 I

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(39) Pasparakis, G.; Manouras, T.; Argitis, P.; Vamvakaki, M. Photodegradable Polymers for Biotechnological Applications. Macromol. Rapid Commun. 2012, 33 (3), 183−198. (40) Jiang, J.; Qi, B.; Lepage, M.; Zhao, Y. Polymer Micelles Stabilization on Demand through Reversible Photo-Cross-Linking. Macromolecules 2007, 40 (4), 790−792. (41) Babin, J.; Lepage, M.; Zhao, Y. Decoration” of Shell CrossLinked Reverse Polymer Micelles Using ATRP: A New Route to Stimuli-Responsive Nanoparticles. Macromolecules 2008, 41 (4), 1246−1253. (42) He, J.; Tong, X.; Zhao, Y. Photoresponsive Nanogels Based on Photocontrollable Cross-Links. Macromolecules 2009, 42 (13), 4845− 4852. (43) Dong, J.; Wang, Y.; Zhang, J.; Zhan, X.; Zhu, S.; Yang, H.; Wang, G. Multiple stimuli-responsive polymeric micelles for controlled release. Soft Matter 2013, 9 (2), 370−373. (44) Tyer, N. W.; Becker, R. S. Photochromic spiropyrans. I. Absorption spectra and evaluation of the.pi.-electron orthogonality of the constituent halves. J. Am. Chem. Soc. 1970, 92 (5), 1289−1294. (45) Minkin, V. I. Photo-, Thermo-, Solvato-, and Electrochromic Spiroheterocyclic Compounds. Chem. Rev. 2004, 104 (5), 2751− 2776. (46) Knott, E. B. 673. The colour of organic compounds. Part V. Thermochromic spirans. J. Chem. Soc. 1951, No. 0, 3038−3047. (47) Zhang, J.; Zhang, Y.; Chen, F.; Zhang, W.; Zhao, H. Selfassembly of photoswitchable diblock copolymers: salt-induced micellization and the influence of UV irradiation. Phys. Chem. Chem. Phys. 2015, 17 (18), 12215−12221. (48) Jiang, F.; Chen, S.; Cao, Z.; Wang, G. A photo, temperature, and pH responsive spiropyran-functionalized polymer: Synthesis, selfassembly and controlled release. Polymer 2016, 83, 85−91. (49) Chen, S.; Jiang, F.; Cao, Z.; Wang, G.; Dang, Z.-M. Photo, pH, and thermo triple-responsive spiropyran-based copolymer nanoparticles for controlled release. Chem. Commun. 2015, 51 (63), 12633−12636. (50) Zhou, Y.-N.; Zhang, Q.; Luo, Z.-H. A Light and pH DualStimuli-Responsive Block Copolymer Synthesized by Copper(0)Mediated Living Radical Polymerization: Solvatochromic, Isomerization, and “Schizophrenic” Behaviors. Langmuir 2014, 30 (6), 1489−1499. (51) Zhang, Y.; Chen, S.; Pang, M.; Zhang, W. Synthesis and micellization of a multi-stimuli responsive block copolymer based on spiropyran. Polym. Chem. 2016, 7 (45), 6880−6884. (52) Raymo, F. M.; Giordani, S. Signal Processing at the Molecular Level. J. Am. Chem. Soc. 2001, 123 (19), 4651−4652. (53) Shiraishi, Y.; Itoh, M.; Hirai, T. Thermal isomerization of spiropyran to merocyanine in aqueous media and its application to colorimetric temperature indication. Phys. Chem. Chem. Phys. 2010, 12 (41), 13737−13745. (54) Krysanov, S. A.; Alfimov, M. V. Ultrafast formation of transients in spiropyran photochromism. Chem. Phys. Lett. 1982, 91 (1), 77−80. (55) Wojtyk, J. T. C.; Wasey, A.; Xiao, N.-N.; Kazmaier, P. M.; Hoz, S.; Yu, C.; Lemieux, R. P.; Buncel, E. Elucidating the Mechanisms of Acidochromic Spiropyran-Merocyanine Interconversion. J. Phys. Chem. A 2007, 111 (13), 2511−2516. (56) Raymo, F. M.; Giordani, S.; White, A. J. P.; Williams, D. J. Digital Processing with a Three-State Molecular Switch. J. Org. Chem. 2003, 68 (11), 4158−4169. (57) Pietrasik, J.; Sumerlin, B. S.; Lee, R. Y.; Matyjaszewski, K. Solution Behavior of Temperature-Responsive Molecular Brushes Prepared by ATRP. Macromol. Chem. Phys. 2007, 208 (1), 30−36. (58) Plamper, F. A.; Ruppel, M.; Schmalz, A.; Borisov, O.; Ballauff, M.; Müller, A. H. E. Tuning the Thermoresponsive Properties of Weak Polyelectrolytes: Aqueous Solutions of Star-Shaped and Linear Poly(N,N-dimethylaminoethyl Methacrylate). Macromolecules 2007, 40 (23), 8361−8366. (59) Zhulina, E. B.; Borisov, O. V.; Pryamitsyn, V. A.; Birshtein, T. M. Coil-globule type transitions in polymers. 1. Collapse of layers of grafted polymer chains. Macromolecules 1991, 24 (1), 140−149.

synthetic hybrid block copolymers: Complex, stimuli-responsive rodcoil architectures. Faraday Discuss. 2005, 128, 179−192. (19) Colombani, O.; Ruppel, M.; Schubert, F.; Zettl, H.; Pergushov, D. V.; Müller, A. H. E. Synthesis of poly(n-butyl acrylate)-blockpoly(acrylic acid) diblock copolymers by ATRP and their micellization in water. Macromolecules 2007, 40 (12), 4338−4350. (20) Zhang, L.; Nguyen, T. L. U.; Bernard, J.; Davis, T. P.; BarnerKowollik, C.; Stenzel, M. H. Shell-cross-linked micelles containing cationic polymers synthesized via the RAFT process: Toward a more biocompatible gene delivery system. Biomacromolecules 2007, 8 (9), 2890−2901. (21) Wang, G.; Tong, X.; Zhao, Y. Preparation of azobenzenecontaining amphiphilic diblock copolymers for light-responsive micellar aggregates. Macromolecules 2004, 37 (24), 8911−8917. (22) Zhao, Y. Light-responsive block copolymer micelles. Macromolecules 2012, 45 (9), 3647−3657. (23) Lee, H.-i.; Wu, W.; Oh, J. K.; Mueller, L.; Sherwood, G.; Peteanu, L.; Kowalewski, T.; Matyjaszewski, K. Light-Induced Reversible Formation of Polymeric Micelles. Angew. Chem., Int. Ed. 2007, 46 (14), 2453−2457. (24) Kotharangannagari, V. K.; Sánchez-Ferrer, A.; Ruokolainen, J.; Mezzenga, R. Photoresponsive Reversible Aggregation and Dissolution of Rod−Coil Polypeptide Diblock Copolymers. Macromolecules 2011, 44 (12), 4569−4573. (25) Menon, S.; Ongungal, R. M.; Das, S. Photocleavable glycopolymer aggregates. Polym. Chem. 2013, 4 (3), 623−628. (26) Niu, Y.; Li, Y.; Lu, Y.; Xu, W. Spiropyran-decorated lightresponsive amphiphilic poly([small alpha]-hydroxy acids) micelles constructed via a CuAAC reaction. RSC Adv. 2014, 4 (102), 58432− 58439. (27) Nakahara, Y.; Nakamura, J.; Shirotani, N.; Kimura, K. Synthesis of Amphiphilic Copolymers Bearing a Spirobenzopyran Moiety at the End Group and Their Photoresponsive Micellar Behaviors in Water. Chem. Lett. 2012, 41 (10), 1142−1144. (28) Shen, H.; Zhou, M.; Zhang, Q.; Keller, A.; Shen, Y. Zwitterionic light-responsive polymeric micelles for controlled drug delivery. Colloid Polym. Sci. 2015, 293 (6), 1685−1694. (29) Jin, Q.; Liu, G.; Ji, J. Micelles and reverse micelles with a photo and thermo double-responsive block copolymer. J. Polym. Sci., Part A: Polym. Chem. 2010, 48 (13), 2855−2861. (30) Zhao, Y. Photocontrollable block copolymer micelles: what can we control? J. Mater. Chem. 2009, 19 (28), 4887−4895. (31) Tong, X.; Wang, G.; Soldera, A.; Zhao, Y. How Can Azobenzene Block Copolymer Vesicles Be Dissociated and Reformed by Light? J. Phys. Chem. B 2005, 109 (43), 20281−20287. (32) Jochum, F. D.; Theato, P. Thermo- and light responsive micellation of azobenzene containing block copolymers. Chem. Commun. 2010, 46 (36), 6717−6719. (33) Yan, Q.; Xin, Y.; Zhou, R.; Yin, Y.; Yuan, J. Light-controlled smart nanotubes based on the orthogonal assembly of two homopolymers. Chem. Commun. 2011, 47 (34), 9594−9596. (34) Han, K.; Su, W.; Zhong, M.; Yan, Q.; Luo, Y.; Zhang, Q.; Li, Y. Reversible Photocontrolled Swelling-Shrinking Behavior of Micron Vesicles Self-Assembled from Azopyridine-Containing Diblock Copolymer. Macromol. Rapid Commun. 2008, 29 (23), 1866−1870. (35) Achilleos, D. S.; Vamvakaki, M. Multiresponsive SpiropyranBased Copolymers Synthesized by Atom Transfer Radical Polymerization. Macromolecules 2010, 43 (17), 7073−7081. (36) Shiraishi, Y.; Miyamoto, R.; Hirai, T. Spiropyran-Conjugated Thermoresponsive Copolymer as a Colorimetric Thermometer with Linear and Reversible Color Change. Org. Lett. 2009, 11 (7), 1571− 1574. (37) Krongauz, V. A.; Goldburt, E. S. Crystallization of poly(spiropyran methacrylate) with cooperative spiropyran-merocyanine conversion. Macromolecules 1981, 14 (5), 1382−1386. (38) Jiang, J.; Tong, X.; Zhao, Y. A New Design for Light-Breakable Polymer Micelles. J. Am. Chem. Soc. 2005, 127 (23), 8290−8291. J

DOI: 10.1021/acs.macromol.8b00810 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (60) Prazeres, T. J. V.; Beija, M.; Fernandes, F. V.; Marcelino, P. G. A.; Farinha, J. P. S.; Martinho, J. M. G. Determination of the critical micelle concentration of surfactants and amphiphilic block copolymers using coumarin 153. Inorg. Chim. Acta 2012, 381, 181−187. (61) Jones, G.; Jackson, W. R.; Choi, C. Y.; Bergmark, W. R. Solvent effects on emission yield and lifetime for coumarin laser dyes. Requirements for a rotatory decay mechanism. J. Phys. Chem. 1985, 89 (2), 294−300.

K

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