In situ synthesis of thermo-responsive ACB triblock terpolymer nanoparticles through seeded RAFT polymerization

Abstract

Seeded RAFT polymerization is proposed to prepare thermo-responsive ACB triblock terpolymer nanoparticles of poly[N,N-(dimethylamino) ethyl methacrylate]-block-polystyrene-block-poly(N-isopropylacrylamide) (PDMAEMA-b-PS-b-PNIPAM) containing a middle hydrophobic C block and two thermo-responsive blocks A and B. Following this seeded RAFT polymerization, the nanoparticles of the AC diblock copolymer containing a thermo-responsive block of poly[N,N-(dimethylamino) ethyl methacrylate] (PDMAEMA) are prepared through the macro-RAFT agent mediated dispersion polymerization, and then the in situ synthesized AC diblock copolymer nanoparticles are used as seed in the RAFT polymerization, onto which the other thermo-responsive B block of poly(N-isopropylacrylamide) (PNIPAM) is introduced. The synthesized triblock terpolymer nanoparticles contain a hydrophobic core of the middle polystyrene (PS) block and a mixed corona of two thermo-responsive PNIPAM and PDMAEMA blocks. The thermo-response of the PDMAEMA-b-PS-b-PNIPAM nanoparticles is checked, and the morphology of the triblock terpolymer nanoparticles during the thermo-responsive phase transition is detected. The proposed seeded RAFT polymerization is believed to be a valid method to prepare ACB triblock terpolymer nanoparticles containing two thermo-responsive blocks.

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1 Introduction

Over the past decade, thermo-responsive block copolymer nano-objects have aroused great attention because of their potential use in drug delivery, biological separation and tissue engineering scaffolds.^1,2 Of all the thermo-responsive block copolymer nano-objects,^1–20 the doubly thermo-responsive ones containing two thermo-responsive blocks are interesting.^6–20 Up to now, some examples of doubly thermo-responsive nano-objects of ABC triblock terpolymers through their micellization in the selective solvent for A and B blocks have been reported.^17–20 However, this micellization of ABC triblock terpolymers in the block selective solvent suffers from the disadvantage of dilute polymer concentration usually below 1 wt% and sometimes poor control in the repetitive preparation of the block copolymer nano-objects.

Seeded polymerization has been demonstrated to a convenient method to synthesize polymeric nano-objects.^21–24 By employing the reversible addition–fragmentation chain transfer (RAFT) polymerization technology in seeded polymerization,^25–31 block copolymer nano-objects with much higher polymer concentration than those prepared by the micellization strategy can be prepared.^25–31 So far, three different kind of seeded RAFT polymerization have been reported to synthesize triblock copolymer nanoparticles. (1) The triblock copolymer nanoparticles with a CAB structure, in which A and B block are the solvophilic block, and C block is the solvophobic block. Herein, the CA diblock copolymer was used as the seed (macro-RAFT agent) in the seeded polymerization. In such a seeded RAFT polymerization, both the B monomer (or B block) and the second A block are solvophilic in the polymerization medium, and the further extension of the third B block just carries out in a homogeneous condition similar to the traditional solution RAFT polymerization;^31,32 (2) the triblock copolymers nanoparticles with a CAD structure, in which A block is the solvophilic block, and C block and D block are the solvophobic block, whereas the D monomer is soluble in the polymerization medium. The further extension of the third D block leads to a new polymerization-induced-self-assembly process;³³ (3) the triblock copolymer nanoparticles with an ACD structure, in which A block is the solvophilic block, and C block and D block are the solvophobic block, whereas the D monomer is soluble in the polymerization medium, the D monomer can diffuse into the core of the seed, and the third D block further extends within the core. However, triblock copolymer nanoparticles with an ACB structure have not been reported yet.^29,30 Based on this, by employing thermo-responsive diblock copolymer nanoparticles of AC diblock copolymer as seed, thermo-responsive nanoparticles of ACB triblock terpolymer containing two different thermo-responsive blocks of A and B could be possibly prepared through seeded RAFT polymerization.

In this study, thermo-responsive triblock terpolymer nanoparticles of poly[N,N-(dimethylamino) ethyl methacrylate]-block-polystyrene-block-poly(N-isopropylacrylamide) (PDMAEMA-b-PS-b-PNIPAM) containing two thermo-responsive blocks of poly(N-isopropylacrylamide) (PNIPAM) and poly[N,N-(dimethylamino) ethyl methacrylate] (PDMAEMA) are prepared through the RAFT polymerization of N-isopropylacrylamide (NIPAM) in ethanol in the presence of the seed nanoparticles of the poly[N,N-(dimethylamino) ethyl methacrylate]-block-polystyrene diblock copolymer (PDMAEMA-b-PS). It was found that, the second thermo-responsive block of PNIPAM could be introduced into the seed PDMAEMA-b-PS nanoparticles through RAFT polymerization to afford the PDMAEMA-b-PS-b-PNIPAM triblock terpolymer nanoparticles, which contain a hydrophobic core of the PS block and a mixed thermo-responsive corona of the PDMAEMA and PNIPAM blocks. The strategy of the seeded RAFT polymerization is believed to be another valid method to prepare ACB triblock terpolymer nanoparticles with two thermo-responsive blocks.

2 Experimental

2.1 Materials

The monomer of N-isopropylacrylamide (NIPAM, >99%, Acros Organics) was purified by recrystallization in the acetone/n-hexane mixture (50 : 50 by volume). The monomer of N,N-(dimethylamino) ethyl methacrylate (DMAEMA, 98%, Alfa) was dried with CaH₂ overnight and distilled under reduced pressure prior to use. Styrene (St, >98%, Tianjin Chemical Company) was distilled under vacuum and stored at −5 °C prior to use. 2,2′-Azobis(2-methylpropionitrile) (AIBN, >99%, Tianjin Chemical Company) was recrystallized from ethanol before being used. The RAFT agent of 4-cyano-4-(dodecylsulfanylthiocarbonyl) sulfanyl pentanoic acid (CDTPA) was synthesized as discussed elsewhere.³⁴ Other chemical reagents were of analytic grade and were used as received. Deionized water was used in the present experiment.

2.2 Synthesis of PDMAEMA-TTC

The macro-RAFT agent of poly[N,N-(dimethylamino) ethyl methacrylate] trithiocarbonate (PDMAEMA-TTC, in which TTC represents the RAFT terminal of trithiocarbonate) was synthesized by RAFT polymerization in 1,4-dioxane using AIBN as initiator and CDTPA as RAFT agent. Into a 50 mL Schlenk flask with a magnetic bar, DMAEMA (15.0 g, 95.4 mmol), CDTPA (0.257 g, 0.64 mmol), and AIBN (13.1 mg, 0.08 mmol) dissolved in 1,4-dioxane (15.0 g) were added. The solution was degassed with nitrogen at 0 °C, and then the flask content was immersed into preheated oil bath at 70 °C for 5 h. The polymerization was quenched by rapid cooling upon immersion of the flask in iced water. To detect the monomer conversion, a drop of the polymerization solution was dropped into CDCl₃ and subjected to ¹H NMR analysis. The monomer conversion at 61% was calculated by comparing the integral areas of the protons of the C=C double-bond peaks at δ = 5.56 ppm to the protons peaks of the methylene at δ = 4.07 ppm. To collect the polymer, the flask content was purified by three precipitation/filtration cycles in n-hexane at 0 °C. The product was dried under vacuum at room temperature overnight to afford yellow powder of PDMAEMA-TTC (8.1 g, 53% yield).

2.3 Synthesis of the seed nanoparticles of PDMAEMA-b-PS-TTC

The PDMAEMA-b-PS-TTC seed nanoparticles were synthesized through the PDMAEMA-TTC macro-RAFT agent mediated dispersion polymerization in the ethanol/water mixture (80/20 by weight) at 70 °C under [St] : [PDMAEMA-TTC] : [AIBN] = 900 : 3 : 1 with a constant weight ratio of the fed styrene monomer to the solvent at 15%. Into a Schlenk flask with a magnetic bar, PDMAEMA-TTC (3.00 g, 0.193 mmol), St (6.022 g, 57.9 mmol), and AIBN (10.6 mg, 0.0645 mmol) dissolved in the 80/20 ethanol/water mixture (40.15 g) were added. The solution was degassed with nitrogen at 0 °C, and then the polymerization was performed at 70 °C for 13 h under vigorous stirring. After a given time, the polymerization was quenched by rapid cooling upon immersion of the flask in iced water to afford the seed nanoparticles of the PDMAEMA-b-PS-TTC diblock copolymer. The monomer conversion in the dispersion RAFT polymerization was detected by UV-vis analysis at 245 nm as discussed elsewhere.³⁵

To remove the residual St monomer and water in the colloidal dispersion, the colloidal dispersion was dialyzed against ethanol at room temperature (20–25 °C) for three days (molecular weight cutoff: 7000 Da) to afford the ethanol dispersion (10.5 wt%) of the PDMAEMA-b-PS-TTC seed nanoparticles. To collect the diblock copolymer for further gel permeation chromatography (GPC) analysis and ¹H NMR analysis, part of the ethanol dispersion of the seed nanoparticles was centrifuged (12 500 rpm, 30 min) and the precipitate was dried at room temperature under vacuum overnight to afford pale yellow powder of PDMAEMA-b-PS-TTC.

2.4 Seeded RAFT polymerization and synthesis of the triblock terpolymer nanoparticles

The seeded RAFT polymerization of NIPAM in ethanol at 70 °C was performed with the molar ratio of [NIPAM]₀ : [PDMAEMA-b-PS-TTC]₀ : [AIBN]₀ = 900 : 3 : 1 and with the weight percent of the fed monomer plus the PDMAEMA-b-PS-TTC seed nanoparticles at 15%. Typically, into the freshly prepared dispersion of the PDMAEMA-b-PS-TTC seed nanoparticles (5.06 mL containing 0.421 g or 0.00974 mmol of PDMAEMA-b-PS-TTC), NIPAM (0.331 g, 2.92 mmol), AIBN (0.533 mg, 0.0032 mmol) dissolved in ethanol (3.58 g), and the internal standard of 1,3,5-trioxane (0.0263 g, 0.292 mmol) dissolved in ethanol (0.679 g) were added. The flask content was degassed with nitrogen, and then the polymerization was initiated by immersing the flask into preheated oil bath at 70 °C. After a given time, the polymerization was quenched by immersing the flask in iced water. The monomer conversion in the polymerization was detected by ¹H NMR analysis, in which a drop of the polymerization solution (about 0.1 mL) was diluted with CDCl₃ (0.5 mL) and subjected to ¹H NMR analysis to determine the monomer conversion following eqn (S1).†

To remove the residual NIPAM monomer in the colloidal dispersion, the colloidal dispersion was dialyzed against water at room temperature (20–25 °C) for three days (molecular weight cutoff: 7000 Da) to afford the aqueous dispersion of the triblock terpolymer nanoparticles of PDMAEMA-b-PS-b-PNIPAM, in which the polymer concentration was 1–10 wt% dependent on the monomer conversion. To collect the polymer for further gel permeation chromatography (GPC) analysis and ¹H NMR analysis, part of the aqueous colloidal dispersion was extracted with dichloromethane, and then the organic phase was collected and dried over anhydrous magnesium sulfate overnight. After filtration of magnesium sulfate and removal of the solvent, the polymer was collected and dried under vacuum at room temperature overnight to afford pale yellow powder of the PDMAEMA-b-PS-b-PNIPAM triblock terpolymer.

2.5 Characterization

The GPC analysis was performed on a Waters 600E GPC system equipped with three TSK-GEL columns and a Waters 2414 refractive index detector, where THF containing 3 wt% triethylamine was used as eluent at flow rate of 0.5 mL min⁻¹ at 30.0 °C and the narrow-polydispersity polystyrene (molecular weight: 500–280500 Da) was used as calibration standard. The ¹H NMR analysis was performed on a Bruker Avance III 400 MHz NMR spectrometer. The phase transition temperature (PTT) of the thermo-responsive polymers was determined by turbidity analysis at 500 nm on a Varian 100 UV-vis spectrophotometer equipped with a thermo-regulator (±0.1 °C) with the heating rate at 1 °C min⁻¹. The PTT was determined at the middle point of the transmittance change. The transmission electron microscopy (TEM) observation was performed using a Tecnai G² F20 electron microscope at an acceleration of 200 kV. To detect the morphology of the block copolymer nano-objects dispersed in ethanol, a small drop of the diluted dispersion of the block copolymer nanoparticles was deposited onto a piece of copper grid covered with thin film of carbon, dried at room temperature, and last observed by TEM. To detect the morphology of the block copolymer nano-objects dispersed in water, into the diluted aqueous dispersion of the PDMAEMA-b-PS-b-PNIPAM nano-objects preheated at a given temperature (1 mL), a drop of 1.5 wt% phosphotungstic acid (PTA) aqueous solution (∼0.01 mL) was added, kept at the given temperature for 2 min, and then a small drop of the colloids was dripped onto a piece of copper grid, dried at the given temperature till the solvent being removed, and finally observed by TEM. Dynamic light scattering (DLS) analysis was performed on Nano-ZS90 (Malvern) laser light scattering spectrometer with He–Ne laser at the wavelength of 633 nm at 90° angle, in which the hydrodynamic diameter D_h was determined by intensity following the CONTIN method.

3 Results and discussion

3.1 Synthesis of PDMAEMA-TTC and the PDMAEMA-b-PS-TTC seed nanoparticles

The PDMAEMA-TTC macro-RAFT agent was synthesized by the RAFT polymerization of DMAEMA in 1,4-dioxane using CDTPA as RAFT agent and AIBN as initiator (Scheme 1). The synthesized PDMAEMA-TTC macro-RAFT agent was characterized by ¹H NMR analysis (Fig. 1A) and GPC analysis (Fig. 2A). The molecular weight M_n,NMR of the PDMAEMA-TTC macro-RAFT agent, 14.3 kg mol⁻¹, is calculated by comparing the proton resonance signals at δ = 4.07 ppm (c) and δ = 1.26 ppm (q). The molecular weight M_n,GPC of PDMAEMA-TTC by GPC analysis is 19.6 kg mol⁻¹ with the molecular weight distribution index of PDI = 1.16. It is found that, the molecular weight M_n,NMR of PDMAEMA-TTC by ¹H NMR analysis is close to the theoretical molecular weight M_n,th at 15.5 kg mol⁻¹, which is calculated by the monomer conversion following eqn (1),³⁶ and M_n,NMR is smaller than M_n,GPC. The overestimation of M_n,GPC could be possibly due to the non-polar polystyrene standard employed in the GPC analysis. In the next discussion, PDMAEMA-TTC is labelled as PDMAEMA₉₆-TTC, in which the DP is determined by M_n,th.

Scheme 1 Synthesis of PDMAEMA-TTC and PDMAEMA-b-PS-TTC.

Fig. 1 The ¹H NMR spectra of PDMAEMA₉₆-TTC (A) and PDMAEMA₉₆-b-PS₂₆₆-TTC (B).

Fig. 2 The GPC traces of PDMAEMA₉₆-TTC (A), and PDMAEMA₉₆-b-PS₂₆₆-TTC (B).

The PDMAEMA-b-PS-TTC seed nanoparticles was synthesized through the PDMAEMA-TTC macro-RAFT agent mediated dispersion polymerization in the 80/20 ethanol/water mixture at 70 °C under [St] : [PDMAEMA-TTC] : [AIBN] = 900 : 3 : 1 (Scheme 1) with a constant weight ratio of the fed styrene monomer to the solvent at 15%. The strategy of the macro-RAFT agent mediated dispersion polymerization is chosen, since this strategy affords the in situ synthesis of concentrated block copolymer nano-objects following the polymerization-induced self-assembly (PISA) as discussed elsewhere.^37–44 After 13 h of polymerization, 88.9% monomer conversion was achieved and the seed nanoparticles of PDMAEMA-b-PS-TTC were obtained. The PDMAEMA-b-PS-TTC diblock copolymer was characterized by ¹H NMR analysis (Fig. 1B) and GPC analysis (Fig. 2B). The molecular weight M_n,NMR of PDMAEMA-b-PS-TTC, 41.3 kg mol⁻¹, is calculated by comparing the proton resonance signals at δ = 4.07 ppm (c) and δ = 7.22–6.26 ppm (f, g, h). The molecular weight M_n,GPC of PDMAEMA-b-PS-TTC by GPC analysis is 49.9 kg mol⁻¹, and the molecular weight is narrowly dispersed with PDI = 1.16. It is found that, the molecular weight M_n,NMR of PDMAEMA-b-PS-TTC by ¹H NMR analysis is close to M_n,th, which is 43.2 kg mol⁻¹ calculated by the monomer conversion following eqn (1), and M_n,GPC is larger than M_n,NMR by ¹H NMR analysis. The overestimation of M_n,GPC is also possibly due to the non-polar polystyrene standard employed in the GPC analysis. In the following discussion, the diblock copolymer is termed PDMAEMA₉₆-b-PS₂₆₆-TTC.

The synthesized PDMAEMA₉₆-b-PS₂₆₆-TTC seed nanoparticles can be uniformly dispersed in ethanol at room temperature, which is reflected by the nanoparticle dispersion with polymer concentration at 10.5 wt% shown in Fig. 3A. The TEM images shown in Fig. 3B and C suggest that the particle diameter of the seed nanoparticles with the average value at 30 nm is narrowly distributed, which is essential to prepare the nanoparticles of the PDMAEMA-b-PS-b-PNIPAM triblock terpolymer with narrowly distributed molecular weight. The PDMAEMA₉₆-b-PS₂₆₆-TTC seed nanoparticles are expected to contain a solvophobic PS core and a solvophilic PDMAEMA corona as shown in Fig. 3D due to the PS block being insoluble and the PDMAEMA block being soluble in ethanol. Based on the structure of the PDMAEMA₉₆-b-PS₂₆₆-TTC diblock copolymer, the Z-group RAFT terminal is located at the inside of the PS core as shown in Fig. 3D.

Fig. 3 The PDMAEMA₉₆-b-PS₂₆₆-TTC seed nanoparticles dispersed in ethanol with polymer concentration at 10.5 wt% (A) and the TEM images (B and C) and the schematic structure (D) of the PDMAEMA₉₆-b-PS₂₆₆-TTC seed nanoparticles.

3.2 Seeded RAFT polymerization and synthesis of the triblock terpolymer nanoparticles

The seeded RAFT polymerization of NIPAM was carried out in ethanol under [NIPAM]₀ : [PDMAEMA₉₆-b-PS₂₆₆-TTC]₀ : [AIBN]₀ = 900 : 3 : 1. Since the Z-group RAFT terminal is located at the inside of the PS core as shown in Fig. 3D and both the fed NIPAM and the newly formed PNIPAM block are soluble in the solvent of ethanol, therefore the NIPAM monomer dissolved in the solvent of ethanol will swell somewhat the PS core and the initial RAFT polymerization takes place within the seed nanoparticles. With the proceeding of the monomer conversion during the RAFT polymerization, the newly formed PNIPAM block extends and it is dragged out of the PS core to be dissolved in the solvent, since the PNIPAM chains are incompatible with the PS core,¹⁶ and therefore core-corona triblock terpolymer nanoparticles containing a core of the middle solvophobic PS block and a mixed corona of two solvophilic PDMAEMA and PNIPAM blocks as shown in Scheme 2 are formed.

Scheme 2 The seeded RAFT polymerization of NIPAM in the presence of the PDMAEMA₉₆-b-PS₂₆₆-TTC seed nanoparticles.

The kinetics of the seeded RAFT polymerization is summarized in Fig. 4A and B. The monomer conversion increases with the polymerization time and it finally reaches to 83% in 13 h, and the polymerization time further increasing just leads to a very slight increase in the monomer conversion. From the linear ln([M]₀/[M])-time plot shown in Fig. 4B, the pseudo-first-order kinetics of the seeded RAFT polymerization just like a general homogeneous RAFT polymerization is concluded.^31–33 Fig. 4C shows the GPC traces of the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM triblock terpolymer synthesized at different polymerization time (or monomer conversion), from which the molecular weight M_n,GPC and PDI of the triblock terpolymer are obtained. As summarized in Fig. 4D, the molecular weight M_n,GPC of the triblock terpolymer is narrowly distributed as indicated by the relatively low PDI values at about 1.2, and M_n,GPC of the triblock terpolymer increases with the monomer conversion. As the molecular weight analyzed by GPC is the relative molecular weight, the interaction between the nitrogen-containing triblock terpolymer and the GPC columns cannot be ignored, although triethylamine is added into the THF eluent to decrease the interaction as discussed elsewhere.^16,45 To detect the exact molecular weight of the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM triblock terpolymer, ¹H NMR analysis is made (Fig. 5). The M_n,NMR of the triblock terpolymer is calculated by comparing the proton resonance signals at δ = 2.57 ppm (b) and δ = 4.22–3.88 ppm (n, c) following eqn (S2),† and the M_n,NMR values are summarized in Fig. 4D. From Fig. 4D, it is concluded that M_n,NMR of the triblock copolymer is close to the theoretical molecular weight M_n,th calculated by the monomer conversion following eqn (1). Besides, the linear increase in M_n,NMR with the monomer conversion is also found. These results confirm the good control in the molecular weight and the molecular weight distribution of the synthesized PDMAEMA-b-PS-b-PNIPAM triblock terpolymers.

Fig. 4 The monomer conversion-time plots (A) and the ln([M]₀/[M])-time plots (B) for the seeded RAFT polymerization of NIPAM in the presence of PDMAEMA₉₆-b-PS₂₆₆-TTC seed nanoparticles, the GPC traces (C) and the evolution of the molecular weight and the PDI (M_w/M_n) values (D) of the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM triblock terpolymers.

Fig. 5 The ¹H NMR spectra of the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM triblock terpolymers synthesized at different polymerization times.

The seeded RAFT polymerization of NIPAM affords the in situ synthesis of the PDMAEMA-b-PS-b-PNIPAM triblock terpolymer nanoparticles. As indicated by the TEM images shown in Fig. 6, uniform spherical nanoparticles of the triblock terpolymer are formed. The average diameter (D) of the triblock terpolymer nanoparticles is evaluated by statistical analysis of above 100 particles, and it is found that the average diameter of the PDMAEMA-b-PS-b-PNIPAM triblock terpolymer nanoparticles are at 31–33 nm, and the DP of the PNIPAM block seems to have no influence on D of the PDMAEMA-b-PS-b-PNIPAM nanoparticles. As discussed elsewhere,^46–51 for the general core-corona nanoparticles of block copolymers, just the insoluble core can be observed in the TEM images and the soluble corona of the solvophilic block is usually hard to be detected. In the synthesized PDMAEMA-b-PS-b-PNIPAM nanoparticles as shown in Scheme 2, the insoluble PS chains form the core and the soluble PDMAEMA and PNIPAM chains form the corona. Thus, the TEM images shown in Fig. 6 just reflect the PS core in the PDMAEMA-b-PS-b-PNIPAM core-corona nanoparticles. The almost constant D of the PDMAEMA-b-PS-b-PNIPAM nanoparticles suggests that the aggregation number of the PDMAEMA-b-PS-b-PNIPAM nanoparticles, N_agg, which is calculated by eqn (2) as discussed elsewhere,⁵² keeps constant during the seeded RAFT polymerization, and the reason is due to the PS core of the triblock copolymer nanoparticles being frozen in the solvent of ethanol at the polymerization temperature below the glass transition temperature T_g of the PS block, which is about 100 °C as discussed elsewhere.¹⁶

Fig. 6 The TEM images of the triblock terpolymer nanoparticles of PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₄₅ (A), PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₁₂₉ (B), PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₁₆ (C), and PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ (D).

To evaluate the PDMAEMA-b-PS-b-PNIPAM triblock terpolymer nanoparticles, the triblock terpolymer nanoparticles with different DP of the PNIPAM block are characterized by DLS analysis. As shown in Fig. 7, the apparent hydrodynamic diameter D_h of the PDMAEMA-b-PS-b-PNIPAM nanoparticles increases from 105 to 141 nm with the DP of the PNIPAM block increasing from 45 to 249. Based on the TEM images in Fig. 6 indicating the almost constant D of the core and the DLS analysis showing the increasing D_h of the nanoparticles with the DP of the solvophilic PNIPAM block, the core-corona structure of the PDMAEMA-b-PS-b-PNIPAM nanoparticles as shown in Scheme 2 is concluded.

Fig. 7 The hydrodynamic diameter distribution f(D_h) of the seed nanoparticles of PDMAEMA₉₆-b-PS₂₆₆-TTC (A), the triblock terpolymer nanoparticles of PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₄₅ (B), PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₁₂₉ (C), PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₁₆ (D), and PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ (E) formed at different polymerization times.

3.3 Double thermo-response of the triblock terpolymer nanoparticles

Since the PDMAEMA-b-PS-b-PNIPAM triblock terpolymer nanoparticles contain two thermo-responsive blocks of PNIPAM and PDMAEMA, their thermo-response in water is expected. To check the thermo-response of the triblock terpolymer nanoparticles, these triblock terpolymer nanoparticles prepared through the seeded RAFT polymerization in ethanol are transferred into water by dialysis against water at room temperature for three days, diluted with water to form 0.1 wt% aqueous dispersion of the triblock terpolymer nanoparticles, and then the transmittance of the aqueous colloidal dispersion at a given temperature is checked. Note: transferring the triblock terpolymer nanoparticles from ethanol into water does not change the triblock terpolymer nanoparticle morphology, since these triblock terpolymer nanoparticles containing a long hydrophobic PS block are frozen in the solvent of ethanol or neat water at room temperature; and the aqueous nanoparticle dispersions was set at pH = 8 to dismiss the pH effect on the PTT of the PDMAEMA block, since the PDMAEMA block is pH-responsive in water and pH can exert somewhat influence on PTT as discussed elsewhere.⁵³

The temperature dependent transmittance of the aqueous dispersion of the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ triblock terpolymer nanoparticles as well as the references of the PNIPAM₉₂-TTC and PDMAEMA₉₆-TTC homopolymers, the PNIPAM₉₂-b-PS₂₄₇ nanoparticles, and the PDMAEMA₉₆-b-PS₂₆₆ nanoparticles (see the synthesis and characterization of the references in ESI†) is summarized in Fig. 8. As shown in Fig. 8A and B, the transmittance of the reference homopolymers keeps high at low temperature below PTT and it follows a drastic decrease in the transmittance when temperature increases to PTT. Both the PTTs of the reference homopolymers, 29.7 °C for PNIPAM₉₂-TTC and 34.0 °C for PDMAEMA₉₆-TTC, are slightly lower than those reported elsewhere,^54–57 possibly due to the hydrophobic terminal of the dodecyl group in the present homopolymers. Fig. 8C and D show the temperature dependent transmittance of the diblock copolymer nanoparticles of PNIPAM₉₂-b-PS₂₄₇ and PDMAEMA₉₆-b-PS₂₆₆, respectively, from which PTT at 39.2 °C for the PNIPAM₉₂ block and 43.5 °C for the PDMAEMA₉₆ block, respectively, is indicated. Compared with the homopolymers of the PNIPAM₉₂-TTC and PDMAEMA₉₆-TTC, both the PTT values of the diblock copolymer nanoparticles are much higher, and furthermore the phase transition of the diblock copolymer nanoparticles occurs within a wide temperature range, e.g. 34.0 to 44.1 °C for the PNIPAM₉₂-b-PS₂₄₇ nanoparticles and 37.7 to 55.0 °C for the PDMAEMA₉₆-b-PS₂₆₆ nanoparticles, respectively. The reason for the increase in the PTTs of the diblock copolymer nanoparticles is possibly due to the steric repulsion among the crowded thermo-responsive chains tethered on the PS core, which retards the soluble-to-insoluble phase transition of the PNIPAM or PDMAEMA chains as discussed elsewhere.⁵⁸ Fig. 8E shows the temperature dependent transmittance of the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ triblock terpolymer nanoparticles, in which the transmittance initially decreases fast when temperature increases from 34 to 40 °C and then a slow decrease in transmittance follows when temperature further increases from 40 °C to 55 °C. The phase transition of the triblock terpolymer nanoparticles occurs within a broader PTT (34.0 to 55.0 °C) than that of the PNIPAM₉₂-b-PS₂₄₇ diblock copolymer nanoparticles (34.0 to 44.1 °C) or the PDMAEMA₉₆-b-PS₂₆₆ diblock copolymer nanoparticles (37.7 to 55.0 °C), confirming the phase transition of the two thermo-responsive blocks of PNIPAM and PDMAEMA. Since the PTT of PNIPAM is lower than that of PDMAEMA, it is expected that the phase transition of the PNIPAM block in the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ triblock terpolymer nanoparticles occurs initially at the temperature within the broad PTT range and then the phase transition of the PDMAEMA block occurs when the temperature further increases.

Fig. 8 The temperature dependent transmittance of the 0.1 wt% aqueous solution (pH = 8) of the homopolymers of PNIPAM₉₂-TTC (A) and the PDMAEMA₉₆-TTC (B), and the 0.1 wt% aqueous dispersion (pH = 8) of the PNIPAM₉₂-b-PS₂₄₇ nanoparticles (C) and the PDMAEMA₉₆-b-PS₂₆₆ nanoparticles (D), and the triblock terpolymer nanoparticles of PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ (E).

The thermo-response of the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ triblock terpolymer nanoparticles is also checked by DLS analysis at different temperatures. Fig. 9 shows the typical results at the three temperatures of 30 °C, 40 °C and 60 °C, and the apparent hydrodynamic diameter D_h of the triblock terpolymer nanoparticles at the three temperatures is 141 nm, 133 nm, and 94 nm, respectively. At 30 °C, both the PNIPAM and PDMAEMA blocks are soluble, and therefore the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ nanoparticles have a core-corona morphology as shown by the inset, in which the PS₂₆₆ block forms the core and the PDMAEMA₉₆ and PNIPAM₂₄₉ blocks form the mixed corona; at 40 °C the PNIPAM₂₄₉ block undergoes the soluble-to-insoluble phase transition and deposits on the PS core, which leads to the D_h of the triblock terpolymer nanoparticles decreasing from 141 to 133 nm as shown in Fig. 9. As to the PDMAEMA₉₆ block, it is soluble and keeps the triblock terpolymer nanoparticles suspending in water; and at 60 °C, the PDMAEMA₉₆ block further undergoes the soluble-to-insoluble phase transition and both the PDMAEMA₉₆ and PNIPAM₂₄₉ blocks become insoluble and deposit on the PS core, which leads to the D_h decrease from 133 to 94 nm, and the core-corona nanoparticles convert into core–shell ones as shown by the inset in Fig. 9. The triblock terpolymer nanoparticles are also checked at other temperatures and the results are shown in Fig. S4.† It is concluded that the hydrodynamic diameter gradually decreases upon the increasing of the temperature and the phase transition does occur during the heating process.

Fig. 9 The hydrodynamic diameter distribution f(D_h) of the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ nanoparticles dispersed in water at 30 °C (A), 40 °C (B) and 60 °C (C). Insets: the schematic morphology of the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ nanoparticles at a given temperature.

The thermo-response of the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ nanoparticles is further checked by TEM. To clearly detect the morphology of the triblock terpolymer nanoparticles during the phase transition of the thermo-responsive PNIPAM and PDMAEMA blocks, these triblock terpolymer nanoparticles were preheated at a given temperature, e.g. 30 °C, 40 °C, or 60 °C and then stained with PTA, and then the morphology of the nanoparticles was checked by TEM. PTA is usually used in the TEM sampling,^50,59,60 which is a negatively stainer and it can also selectively stain the PDMAEMA chains due to the appending amino groups, and therefore the PDMAEMA morphology on the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ nanoparticles can be detected through this TEM sampling. Under this TEM sampling, the central grey region in the TEM images represents the PS core, and the black region represents the PDMAEMA phase, and the PNIPAM chains are either invisible when they are soluble at temperature below PTT or observed as the pale region on the periphery of the PS core when they become insoluble to deposit on the PS core at temperature above PTT. As shown by the TEM images shown in Fig. 10, nanoparticles with the diameters at 40–45 nm have been observed at three cases of temperature (note: the nanoparticle diameter herein is slightly larger than those shown in Fig. 6, since the PDMAEMA chains or the desolvated PNIPAM layer is detected). By carefully checked the high-resolution TEM images (seeing at the lower left corner), the difference among the nanoparticles at three temperature cases has been found. At the case of 30 °C, both the PNIPAM₉₆ and PDMAEMA₂₄₉ blocks are soluble, and therefore a watery black corona of PDMAEMA on the grey core of the PS core is observed (Fig. 10A), and therefore the core-corona structure of the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ nanoparticles as schematically shown at the lower left corner in Fig. 10A is concluded. At the case of 40 °C, the PNIPAM₂₄₉ block becomes insoluble and deposit on the PS core. During the depositing of the long PNIPAM₂₄₉ block, some segment of the PDMAEMA₉₆ chains are embedded or enclosed in the deposited PNIPAM₂₄₉ block and the residual segment of the PDMAEMA₉₆ chains are soluble in water as schematically shown at the lower left corner, and therefore a PDMAEMA (black) dotted PNIPAM shell (pale) on the grey core of the PS core is observed (Fig. 10B); at the case of 60 °C, the PDMAEMA₉₆ block further undergoes soluble-to-insoluble phase transition, and the unenclosed segment of the PDMAEMA₉₆ block is deposited on the collapsed PNIPAM layer on the PS core as schematically shown at the lower left corner in Fig. 10C, and therefore a compact PDMAEMA shell (black) on the core–shell particles is observed (Fig. 10C).

Fig. 10 TEM images and schematic structure of the PDMAEMA₉₆-b-PS₂₆₆-b-PNIPAM₂₄₉ nanoparticles dispersed in water at the temperature of 30 °C (A), 40 °C (B) and 60 °C (C).

4 Conclusions

Seeded RAFT polymerization is proposed to prepare thermo-responsive ACB triblock terpolymer nanoparticles containing a middle hydrophobic B block and two thermo-responsive blocks of A and C. Following this seeded RAFT polymerization, the nanoparticles of the AB diblock copolymer of PDMAEMA-b-PS containing a PDMAEMA block are prepared through the PDMAEMA-TTC macro-RAFT agent mediated dispersion polymerization, and then the synthesized AB diblock copolymer nanoparticles are used as seed in RAFT polymerization, onto which the third B block of PNIPAM is introduced, and the ACB triblock terpolymer nanoparticles of PDMAEMA-b-PS-b-PNIPAM containing two thermo-responsive blocks are prepared. The seeded RAFT polymerization undergoes a pseudo-first-order kinetics, and the molecular weight of the triblock terpolymer with relatively low PDI increases linearly with the monomer conversion. The synthesized triblock terpolymer nanoparticles are characterized by TEM and DLS, and the core-corona structure containing a PS core and a mixed corona of two thermo-responsive PNIPAM and PDMAEMA blocks is confirmed. The thermo-response of the PDMAEMA-b-PS-b-PNIPAM nanoparticles is checked, and the phase transition of the PDMAEMA-b-PS-b-PNIPAM nanoparticles with a broad PTT range is found, and the morphology of the triblock terpolymer nanoparticles during the thermo-responsive phase transition is demonstrated. Our strategy of the seeded RAFT polymerization is believed to be a valid method to prepare ACB triblock terpolymer nanoparticles.

Acknowledgements

The financial support by 973 Program of China, under the contract (No. 2015CB655105), National Science Fund for Distinguished Young Scholars (51225801), National Nature Science Foundation (51408275) and the Provincial Science and Technology Cooperation Project of Jiangsu-Guangxi cooperation project (BM2014050) is gratefully acknowledged.

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Footnotes

† Electronic supplementary information (ESI) available: Text showing the equations, the synthesis and characterizations of the PNIPAM₉₂-TTC and PNIPAM₉₂-b-PS₂₄₇. See DOI: 10.1039/c6ra08725f

‡ These authors contribute equally to this work.

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