A Well-Defined Amphiphilic Polymer Conetwork from Sequence

Article pubs.acs.org/IECR

A Well-Defined Amphiphilic Polymer Conetwork from Sequence Control of the Cross-Linking in Polymer Chains Chao Zhou,† Linhong Deng,‡ Fang Yao,† Liqun Xu,§ Jian Zhou,∥ and Guo Dong Fu*,† †

School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu Province 211189, P. R. China Institute of Biomedical Engineering and Health Sciences, Changzhou University, Changzhou, Jiangsu Province 213164, P. R. China § Institute of Clean Energy & Advanced Materials, Southwest University, Chongqing, 400715, P. R. China ∥ School of Materials and Engineering, Jiangsu University of Technology, Changzhou, Jiangsu Province 213001, P. R. China ‡

S Supporting Information *

ABSTRACT: Well-defined amphiphilic polymer conetworks with precisely controlled number and position of cross-links were prepared by copper-catalyzed azide−alkyne cycloaddition (CuAAC) using linear polystyrene (PS) and poly(ethylene glycol) (PEG) as the building blocks. In this approach, linear polystyrene containing a specific number of bromo groups at a predetermined position of polymer chains was synthesized by multistep reversible addition−fragmentation chain transfer polymerization and chain extension using styrene and N-bromopropyl maleimide (PBMI) as the monomers. Subsequently, the bromo groups were transformed into the azido moieties via nucleophilic substitution. The well-defined linear multialkynyl PEG was prepared from PEG diglycidyl ether and propargylamine via epoxy-amine chain extension. The as-prepared PS−PEG amphiphilic polymer conetworks showed unique hydrophilic and hydrophobic phase separation with a variable swelling capacity and rheological behavior in both polar and nonpolar solvents and exhibited excellent mechanical properties with increased crosslinking density.

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INTRODUCTION The producing process of well-defined hydrogels by polymers of regular structures as well as biological reactions (gene expression, protein synthesis) have potential applications in tissue engineering such as hemodialyzers, artificial skin, and vascular prostheses.1−3 Amphiphilic conetworks (APCNs), composed of both hydrophilic and hydrophobic segments, can adsorb both water and nonpolar organic solvents4−8 and appear in the nanophase separation between the immiscible hydrophilic and hydrophobic polymer chains.9−11 The intriguing physical behavior of APCNs renders them appropriate materials for several applications, including soft contact lenses,12,13 membranes for pervaporation and filtration,14,15 biomaterials for tissue engineering,16−18 catalysis supports,19,20 and drug carriers.21−23 Various synthetic methodologies have been developed for the synthesis of APCNs, employing free-radical polymerization,24−26 anionic polymerization,27,28 and group transfer polymerization.29−31 However, less molecular structure regularity in APCNs is a serious defect, which leads to soft, weak, and brittle materials and limits their further applications in various fields. Recently, many scientists developed well-defined model APCNs using polymer chains with precise molecular weights, molecular weight distributions, and compositions.32,33 The quasiliving radical polymerization techniques,34 such as atom transfer radical polymerization (ATRP)35 and reversible addition−fragmentation chain transfer (RAFT) polymerization,36,37 have been proven to be effective and convenient tools for the synthesis of polymers with predetermined molecular weight and low molecular weight dispersity (Đ) from various monomers.38−40 Furthermore, RAFT polymerization has al© 2014 American Chemical Society

ready been successfully used for the preparation of various polymer architectures, especially block,41 multiblock copolymers,42,43 and polymer networks.36,44−46 Copper(I)-catalyzed azide−alkyne cycloaddition (CuAAC) “click chemistry”, because of its reaction specificity, quantitative yields, and good functional group tolerance, has been proven to be a powerful tool for the preparation of polymer networks with well-defined molecular structures, desired functionalities, and surprisingly enhanced mechanical properties.47−50 The click chemistry reactions have also been shown to be good candidates for the cross-linking reactions to prepare welldefined APCNs.51−54 Thus, the combination of CuAAC with RAFT polymerization provides a flexible approach toward the preparation of APCNs with well-defined molecular structure. Despite the aforementioned advances, the preparation of well-defined APCNs represents a challenging synthetic target. In this paper, a novel approach was developed for the preparation of a linear polystyrene (PS) building block with well-defined number and position of bromo side groups. The strategy relies on alternating RAFT polymerization of styrene and RAFT chain extension of N-bromopropyl maleimide (PBMI), which could not be homopolymerized but could effectively be copolymerized with styrene via the formation of a donor−acceptor comonomer pair.55−58 After removal of trithiocarbonate at a,ω ends of PS chains, the PS x Br y copolymers were transformed into PSx(N3)y by substitution Received: Revised: Accepted: Published: 19239

September 17, 2014 November 14, 2014 November 20, 2014 November 20, 2014 dx.doi.org/10.1021/ie503649t | Ind. Eng. Chem. Res. 2014, 53, 19239−19248

Industrial & Engineering Chemistry Research

Article

Scheme 1. Schematic Illustration of the Synthesis of PSxBry Polymers Using Alternating RAFT Polymerization and RAFT Chain Extension and the Synthesis of PSx(N3)y by Nucleophilic Substitution

agent (RAFT CTA), ethane-1,2-diyl bis(2-((dodecylthio) carbonothioyl) thio-2-methylpropanoate) (EDBDCMP), were prepared according to a previous report.59 Synthesis of Linear PS50 by RAFT Polymerization. A mixture of EDBDCMP (0.1093g, 0.145 mmol), styrene (4.53g, 43.5 mmol), and AIBN (5.0 mg, 0.03 mmol) were charged in a 10 mL round-bottom flask. The reaction mixture was degassed by performing three successive freeze−pump−thaw cycles; the flask was then sealed and put into a preheated oil bath at 60 °C for 10 h. After the reaction, the resultant mixture was diluted in THF, and precipitated into an excess amount of methanol three times. The precipitates were collected by filtration and dried in vacuum at room temperature overnight. The linear PS was obtained with yield of 22.1%. Synthesis of PS50Br2 by RAFT Chain Extension. The prepared linear PS was used as macro RAFT CTA (PS CTA) (0.67 g, 0.15 mmol) and was dissolved in 2 mL of toluene with PBMI (0.327 g, 1.5 mmol) and AIBN (2.4 mg, 0.015 mmol). Then, the reaction mixture was degassed by performing three successive freeze−pump−thaw cycles; the flask was sealed and then put into a preheated oil bath at 60 °C. After 24 h, the resultant mixture was diluted in THF and precipitated into an excess amount of methanol three times. The precipitates were collected by filtration and dried in vacuum at room temperature overnight. Synthesis of PS126Br4 by RAFT Polymerization and the Following RAFT Chain Extension. The prepared PS50Br2

with sodium azide (Scheme 1). On the other hand, the linear PEG building block (PEGn(CCH))m was synthesized from PEG diglycidyl ether and propargylamine via epoxy-amine chain extension. Finally, the well-defined APCNs with accurate number of cross-linking points were prepared from PSx(N3)y and (PEGn(CCH))m via CuAAC.

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EXPERIMENTAL SECTION Materials. Styrene (St; Acros, 99%) was passed through a basic activated aluminum oxide (50−200 μm) column before use. Propargylamine (J&K Scientific, 80%) was distilled before use. 2,2-Azobis(isobutyronitrile) (AIBN; Sinopharm Chemical Reagent Co., 99%) was recrystallized in ethanol; cuprous bromide (CuBr; Acros, 98%) was washed with glacial acetic acid to remove any soluble oxidized species. Dichloromethane (CH2Cl2; Sinopharm Chemical Reagent Co., 99.9%) was dried with CaH2 and evaporative reflux. Tetrahydrofuran (THF; Sinopharm Chemical Reagent Co., 99.9%) was dried with sodium and distilled before use. PEG (Mn = 2000, Aldrich), N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA; J&K Scientific, 99.9%), epichlorohydrin (Chemical Reagent Plant, 99.9%), toluene (Sinopharm Chemical Reagent Co., 99.9%), N,N-dimethylformamide (DMF; Shanghai Chemical Reagent, China, 99.9%), sodium azide (NaN3; Sinopharm Chemical Reagent Co., 99.9%) and sodium hydride (NaH; Sinopharm Chemical Reagent Co., 90%) were used as received. NBromopropyl maleimide (PBMI) and RAFT chain transfer 19240

dx.doi.org/10.1021/ie503649t | Ind. Eng. Chem. Res. 2014, 53, 19239−19248


Article

Figure 1. 1H NMR spectra of (A) PS50, (B) PS50Br2, (C) PS126Br2, (D) PS126Br4, (E) PS240Br4, and (F) PS240Br6 polymers.

samples were collected by filtration and dried in vacuum at room temperature overnight. The resulting PS126Br2 was further chain extended using PBMI at the a,ω ends to obtain PS126Br4 sample. The copolymerization and chain extension procession of PS240Br4 and PS240Br6 were similar to that of PS50Br2 and PS126Br4 (Scheme 1).

(0.2 g, 0.03 mmol), Styrene (1.033 g, 9.92 mmol), and AIBN (1.1 mg, 0.007 mmol) were dissolved in 2 mL of toluene and degassed by performing three successive freeze−pump−thaw cycles. The vessel containing the reaction mixture was sealed and then put into a preheated oil bath at 60 °C. After 18 h, the resultant mixture was diluted in THF and precipitated into an excess amount of methanol three times. The obtained PS126Br2 19241



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Removal of Trithiocarbonate Groups at the a,ω Ends and Synthesis of PSx(N3)y Polymers via the Azidation of Respective PSxBry Polymers. A mixture of prepared PSxBry (0.14 mmol), AIBN (8.4 mmol), and toluene (8.0 mL) was added in an ampule, and the ampule was heated to 60 °C under nitrogen atmosphere for 12 h. Then, the solution was precipitated into methanol, yielding a white powder. Sodium azide (1.1 equiv) and as-prepared linear PS sample (1 equiv) were dissolved in DMF. The reaction mixture was stirred at room temperature for 12 h. Then, the polymer solution was passed through a neutral alumina column to remove excess sodium azide and was concentrated by reduced pressure distillation to remove DMF. The residue was diluted in THF and precipitated into excess methanol. PSx(N3)y was obtained after filtration and drying in vacuum at room temperature overnight. Preparation of APCN(PSx(PEG45)7) via Click Chemistry. The as-prepared (PEG45(CCH))7 (Mn= 14,800, Đ = 1.32, see Supporting Information; 0.50 g, 0.03 mmol) PS50(N3)2 (0.588 g, 0.105 mmol), CuBr (0.32 mg, 0.0022 mmol), and 1.5 mL of DMF were introduced into a small reaction tube with nitrogen and placed under ultrasonic agitation at room temperature until complete dissolution. Then, PMDETA (4.4 μL, 0.021 mmol) was quickly injected under ultrasonic agitation. The gelation of the APCN(PS50(PEG45)7) occurred within 5 min. The reaction mixture was further left to react at room temperature for 12 h to obtain a uniform solid structure. After complete gelation, the obtained APCN(PS50(PEG45)7) gel was immersed into an EDTA (5%) solution, followed by THF/ethanol (3/1 v/v) to remove the copper ions and PMDETA. APCN(PS126(PEG45)7) and APCN(PS240(PEG45)7) were prepared in the same way, with PS50(N3)2 being replaced by PS126(N3)4 and PS240(N3)6, respectively. Finally, all the APCNs were dried at 45 °C for 72 h to constant weight. The gel fraction could be obtained by following eq 1: gel fraction = [m0 /(mPS + mPEG)] × 100%

(JEOL) scanning electron microscopy (SEM) instrument at an accelerating voltage of 30 kV. Differential scanning calorimetry (DSC) measurements were conducted on a TA Instrument DSC Q-10 over the temperature range from −45 to 120 °C at a heating rate of 10 °C·min−1 under nitrogen environment. DSC was calibrated with metallic indium (99.9% purity). Swelling behavior of the prepared APCN(PSx(PEG45)7) networks was measured by a gravimetric method. Dry gels were immersed in THF, methanol, and deionized water separately at 25 °C, and the samples were taken out at certain time intervals, wiped by filter paper, and weighed. Swelling ratio (SR) of PS−PEG networks were determined gravimetrically using eq 2: SR = (mt − m0)/m0 × 100%

(2)

where mt is the mass of the swollen network at time t and m0 is the mass of the dry gel.

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RESULT AND DISCUSSION Polymer Synthesis. As described in the Introduction, the present study relies on the precise control of the number and position of alkynyl and azido groups in the polymer side chains. Such polymers can be simply prepared by two strategies: (i) well-defined (PEG45(CCH))7 macromolecules with precisely controlled number and position of alkynyl groups were prepared via chain extension of poly(ethylene glycol) diglycidyl ether (DEP45, Mn = 2 000) and propargylamine (Scheme S1 in Supporting Information); (ii) PSx(N3)y containing identified bromo groups in the side chains was synthesized using alternating RAFT polymerization and RAFT chain extension to form PSxBry, followed by nucleophilic substitution of the bromo moieties with sodium azide (Scheme 1). The PSxBry copolymers were prepared using styrene and PBMI as the monomers by alternating RAFT polymerization and RAFT chain extension. Initial polymerization of linear PS using EDBDCMP as RAFT CTA was found to proceed reasonably with 22% monomer conversion after 10 h. After purification and isolation, the corresponding 1H NMR spectrum in Figure 1A displayed characteristic signals of the CTA at 0.80, 1.26, and 1.44 ppm and aromatic proton signals of PS at 6.3−7.2 ppm. The degree of polymerization (DP) of prepared PS macro CTA, calculated from the integral area ratio of the methylene protons at the a,ω chain ends at 1.26 ppm and the aromatic protons in the range of 6.3−7.2 ppm, is about 50. The successful preparation of PS was further confirmed by GPC. The GPC curves of PS macro CTA shows Mn of 5.1 × 103 with Đ of 1.11 in Figure 2A. The DP of PS macro CTA is about 42 calculated from GPC, which is close to the value obtained from 1H NMR. The DP values of PS macro CTA and the other linear PS samples were determined by 1H NMR. Subsequent RAFT chain extension of PBMI monomer from PS50 gave rise to the PS50Br2 polymer. The chemical shifts at 3.68 and 3.36 ppm (m and k in Figure 1B) were assigned to methylene protons in the PBMI located at a,ω ends of PS50Br2 polymer. The integration of the methylene protons (m, k) and aromatic protons at 6.3−7.2 ppm indicated the presence of about 2.0 bromo groups at a,ω ends of PS50Br2. The end functionalized efficiency of PS50 by bromo moieties was higher than 99% (Table 1). The Mn of PS50Br2 moieties increased slightly to 5.9 × 103, while the Đ remained at 1.11, suggesting that no cross-linking or side reaction took place during the RAFT chain extension. The synthesis of PS126Br2 was carried out by RAFT polymerization starting from PS50Br2 macro-CTA using styrene

(1)

where m0 is the mass of the dry gel, mPS the mass of PSx(N3)y (x = 50, 126, 240; y = 2, 4, 6), and mPEG the mass of (PEG45(CCH))7. Characterization. 1H NMR spectra were recorded in CDCl3 with a 400 MHz Bruker Avance instruments. IR spectra were obtained using Fourier-transform infrared (FT-IR) spectrometer (Avatar 370, Thermo Fisher Nicolet, U.S.). Ultraviole−visible (UV−vis) spectra were recorded on a UV1750 UV−vis spectrometer (Shimadzu) in the wavelength range from 210 to 600 nm. Measurements were carried out in dry CH2Cl2 . Molecular weights and molecular weight distributions were determined by gel permeation chromatography (GPC) performed at 40 °C in anhydrous THF (flow rate, 1 mL·min−1), using Waters Styragel column (HR 5E). The detection was performed with a Model 2414 differential refractometer detector. For calibration, linear polystyrene standards were used as the references. The rheological studies on the APCNs (PSx(PEG45)7) were performed on a Kinexus Modular Compact Rheometer (Malvern, U.K.) equipped with parallel plate geometry with a diameter of 10 mm at 25 °C. The gap distance between two plates was fixed at 1 mm. A frequency sweep test was conducted on each sample to determine values of storage modulus (G′) and loss modulus (G″) over a frequency range of 0.1−10 Hz. The steady-flow studies were performed over a shearing rate range of 0.01−65 s−1. The morphology of the APCNs was studied on a JSM-6510 19242



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Figure 2. Gel permeation chromatography (GPC) curves of (A) PS50, (B) PS50Br2, (C) PS126Br2, (D) PS126Br4, (E) PS240Br4, and (F) PS240Br6 polymers.

Figure 3. FT-IR spectra of (A) PS50Br2, (B) PS126Br4, (C) PS240Br6, (D) PS240(N3)6, and (E) APCN(PS240(PEG45)7).

as the monomer. After 18 h of polymerization, the monomer conversion is 35.6%. Figure 1C shows the 1H NMR spectrum of PS126Br2 with enhanced aromatic proton signal intensities at 6.3−7.2 ppm. However, the chemical shifts of PBMI at 3.36 and 3.68 ppm appeared broader because of the adjacent styrene repeating units. The as-synthesized linear PS sample is 126, calculated by the integration of the methylene protons at the a,ω chain ends at 1.26 ppm and the aromatic protons in the range of 6.3−7.2 ppm. The GPC curve provided in Figure 2C corresponds to the molecular weight of PS126Br2 in comparison to that of PS50Br2, with a Mn of 10.6 × 103 (Đ = 1.12). PS126Br4, PS240Br4, and PS240Br6 copolymers were prepared from previous adducts by RAFT chain extension, RAFT polymerization, and another step of RAFT chain extension, respectively. The molecular weight of the obtained polymers increased from 11.7 × 103 to 21.8 × 103 (Table 1), in line with the increase of the aromatic infrared absorption signal appearing at 1600 cm−1 in Figure 3. As calculated by integration of the methylene protons at 3.68 and 3.36 ppm and aromatic protons at 6.3−7.2 ppm, the end functionalized efficiency of PBMI decreased with the increase in the alternating times of RAFT polymerization and RAFT chain extension. Furthermore, the Đ of PS50, PS50Br2, PS126Br4, and PS240Br6 also increased from 1.11 to 1.30. These results in Table 1 suggest the inactivation of CTA groups in multisteps of RAFT polymerization.60 Figure 4 shows the UV−vis absorption

Figure 4. UV−vis absorption curves of (A) PS50 CTA, (B) PS50Br2, (C) PS126Br4, (D) PS240Br6, and (E) PS240Br6 after the removal of BTTC groups at a,ω ends.

spectra of PSxBry in dichloromethane. The styrene repeating unit in linear polymer has a maximum absorption (λmax) at 262 nm, and the butyl trithiocarbonate (BTTC) group λmax is 320 nm. The λmax intensity of styrene repeating units increased gradually with the increase in the alternating times of RAFT polymerization and RAFT chain extension, while the λmax intensity of BTTC decreased. The BTTC absorption intensity of PS240Br6 decreased by 65.7% in comparison to that of PS50, which accounts for the increase in the styrene repeat units in

Table 1. Characteristics of Polymers end functionalized efficiency (mol %)

GPC resultse

[St]:[PBMI]

polymer structure

reaction time (h)

Sta

PBMIb

theoryc

PS50 PS50Br2 PS126Br2 PS126Br4 PS240Br4 PS240Br6

10 24 18 24 26 24

22.1 − 35.6 − 41.3 −

− 99.8 − 98.5 − 95.3

50:0 50:2.0 126:2.0 126:4.0 240:4.0 240:6.0

H NMRd

Mn

Mp

Đ

50:0 50:2.0 126:1.95 126:3.94 240:3.85 240:5.72

5 100 5 900 10 600 11 700 20 100 21 800

5 600 6 100 11 400 12 600 24 900 24 500

1.11 1.11 1.12 1.12 1.30 1.30

1

a The St conversion was calculated by gravimetric method. bThe PBMI conversion was calculated by 1H NMR. cThe theoretic values were assumed by monomer conversion in each step. dCalculated from the integrated area ratio corresponding 1H NMR spectra. eObtained using linear PS standards, Đ = Mw/Mn.

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the polymer chain and the decrease in the relative content of BTTC. Furthermore, a slow addition−fragmentation process in the RAFT polymerization would result in the loss of the trithioester end group.61−63 Synthesis of PSx(N3)y. The BTTC groups at a,ω ends were removed by addition of a large amount of AIBN (60-fold excess). The successful removal of BTTC groups at the ends of PS240Br6 was confirmed by the disappearance of UV absorbance at 320 nm (Figure 4E). Nucleophilic substitution was performed at the side chains of PS50Br2, PS126Br4, and PS240Br6 after the removal of BTTC groups. In the FT-IR spectrum of PS240(N3)6 (Figure 3D), an intense absorption band at 2090 cm−1, corresponding to the asymmetric stretching vibration of the azido function, was observed, confirming the successful preparation of PS240(N3)6. Synthesis of APCN(PSx(PEG45)7) Networks by CuAAC. Well-defined amphiphlic polymer conetworks of PS and PEG were prepared from linear (PEG45(CCH))7 and PSx(N3)y with precisely controlled alkynyl and azido groups, respectively, in the side chains by CuAAC. The gel fraction of APCN(PS 50 (PEG 45 ) 7 ), APCN(PS 126 (PEG 45 ) 7 ), and APCN(PS240(PEG45)7) from eq 1 are 92.3%, 94.2%, and 95.7%, respectively, which shows high reaction yield by CuAAC. Figure 3E shows the FT-IR spectrum of APCN(PS240(PEG45)7). The complete disappearance of azido vibration peak at 2090 cm−1 reveals that the CuAAC between (PEG45(CCH))7 and PSx(N3)y has fully taken place. Swelling of APCN(PSx(PEG45)7). The APCNs has unique swelling behavior in different solvents due to the combination of hydrophilic and hydrophobic polymer segments within their structure. Figure 5 provides photographs of the APCN-

Figure 6. Swelling ratios of (A, a, a′) APCN(PS50(PEG45)7), (B, b, b′) APCN(PS126(PEG45)7), and (C, c, c′) APCN(PS240(PEG45)7) in tetrahydrofuran (THF), methanol, and water, respectively.

cross-linking density, the larger the extension capabilities of the polymer network lattice. Figure 6 also shows the SRs of APCN(PS50(PEG45)7), APCN(PS126(PEG45)7), and APCN(PS240(PEG45)7) in methanol and water. The SRs of APCN(PS50(PEG45)7) are 517% and 427% in methanol and water, respectively. The higher SR of APCN(PS50(PEG45)7) in methanol than in water may account for the lower polarity of the former (δ = 6.6) than that of the later (δ = 10.2). APCN(PS126(PEG45)7) has a SR of 403% in methanol and 357% in water, while APCN(PS240(PEG45)7) has a SR of 232% in methanol and 155% in water. All APCNs exhibit reduced SR values in methanol and water, which is consistent with the fact that only the PEG segments can be extended in methanol or water. Rheological Behavior of APCN(PSx(PEG45)7). The dynamic rheological behavior represents the mechanical properties of polymeric networks. The storage modulus (G′) of all the APCNs exhibited an essential elastic response and was greater than the loss modulus (G″) over the entire range of testing frequencies. Figure 7 shows the dynamic rheological behavior of APCN(PS50(PEG45)7) swelled in THF and water at the frequency range from 0.1 to 10 Hz. The G′ values of APCN(PS50(PEG45)7) in water are higher than those in THF, whereas the G″ values of APCN(PS50(PEG45)7) in water are

Figure 5. Photograph of APCN(PS240(PEG45)7) swelled in THF, methanol, and H2O.

(PS240(PEG45)7) swelled in three different solvents. The APCN(PS240(PEG45)7) was transparent in THF, while it turned nontransparent in methanol and water. THF is a good solvent for both PEG and PS segments; thus, the polymer chains in the conetwork were highly extended. Methanol and water are good solvents for PEG, but poor for PS. The PS segment in APCN(PS240(PEG45)7) underwent phase separation in water and methanol. Thus, APCN(PS240(PEG45)7) was partially swelled and turned white. Figure 6 shows the swelling ratios of APCN(PS50(PEG45)7), APCN(PS126(PEG45)7), and APCN(PS240(PEG45)7) in THF, methanol, and water within 1800 min. APCN(PS50(PEG45)7) has a maximum SR of 850% in THF, which is higher than that (780%) of APCN(PS126(PEG45)7) and that (580%) of APCN(PS240(PEG45)7) in THF. The SRs are in line with the fact that the lower the

Figure 7. Storage modulus, G′ (solid symbols), and loss modulus, G″ (open symbols), for APCN(PS50(PEG45)7) swelled in THF (circles) and water (pentagons) as a function of frequency. 19244



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lower than those in THF. An explanation for the observed mechanical response might be that only the PEG chains of APCN(PS50(PEG45)7) were swelled in water, while the PS chains were shrunk and entangled in water, which can be treated as cross-linking segments. Thus, the cross-linking density of APCN(PS50(PEG45)7) swelled in water is considered to be higher than that swelled in THF, resulting in higher G′ values. Figure 8 shows the G′ and G″ curves of APCN(PS 50 (PEG 45 ) 7 ), APCN(PS 126 (PEG 45 ) 7 ), and APCN-

Figure 9. Steady-state shear viscosity as a function of stress for APCN(PS240(PEG45)7) (squares), APCN(PS126(PEG45)7) (triangles), and APCN(PS50(PEG45)7) (circles) as a function of shear rate.

shear rates exceeded 12.6 s−1, because APCN(PS240(PEG45)7) was broken at high shear rate. That is attributed to the fact that the PS chain is more rigid and less stretchable compared to PEG. The longer PS chain in the APCNs is, more brittle the structure of the APCNs will be. Network Morphology. The morphology of APCN(PSx(PEG45)7 was further investigated by scanning electron microscopy. Figure 10 shows the SEM cross-sectional view of freeze-dried APCN(PS50(PEG45)7), APCN(PS126(PEG45)7), and APCN(PS240(PEG45)7). The APCN(PS50(PEG45)7) exhibits highly open, loose, and porous structure, with the pore size of about 100 μm. The APCN(PS126(PEG45)7) and APCN(PS240(PEG45)7) exhibit smaller and fewer pores. This is consistent with the fact that the increase in azido groups in the PS side chains results in the increase in the cross-linking density. Thermal Behavior of APCN(PSx(PEG45)7). The thermal property is very important for materials destined for use in biomedical applications. If the materials were crystallized around human body temperature, it would limit their application as biomaterials. Here, the thermal properties of APCN(PSx(PEG45)7) were studied by differential scanning calorimetry. Figure 11 shows the DSC results of APCN(PS 50 (PEG 45 ) 7 ), APCN(PS 126 (PEG 45 ) 7 ), and APCN(PS240(PEG45)7). The melting behavior of the APCNs is largely affected by the content of PEG and PS blocks. There is one endothermic peak at 40.8 °C in the heating process of APCN(PS50(PEG45)7), which is assigned to melting temperature (Tm) of PEG segments. However, Tm of APCN(PS126(PEG45)7) and APCN(PS240(PEG45)7) shifted to 39.6 and 38.4 °C, respectively. This phenomenon is due to the increased hydrophobic elastic PS segments content in APCN(PS126(PEG45)7) and APCN(PS240(PEG45)7).32 The PEG crystallization temperature (Tc) of APCN(PS50(PEG45)7) was 22.6 °C, while the Tc of APCN(PS126(PEG45)7) and APCN(PS240(PEG45)7) were decreased to 14.9 and 12.6 °C, respectively. The presence of PS segments could destroy PEG crystallization capacity. Thus, the PEG crystallization peak intensity and Tc in APCNs decreased with the increase in the content of PS chains.

Figure 8. Storage modulus, G′ (solid symbols), and loss modulus, G″ (open symbols), for APCN(PS240(PEG45)7) (squares), APCN(PS126(PEG45)7) (triangles), and APCN(PS50(PEG45)7) (circles) swelled in THF as a function of frequency.

(PS240(PEG45)7) swelled in THF. The G′ value of APCN(PS240(PEG45)7) was much higher than those of APCN(PS126(PEG45)7) and APCN(PS50(PEG45)7), suggesting that APCN(PS240(PEG45)7) has a higher cross-linking density between PS240(N3)6 and (PEG45(CCH))7 after CuAAC and lowest swelling degree in THF compare to that of the other two APCNs. The structural regularity of APCN(PS 50 (PEG 45 ) 7 ), APCN(PS 126 (PEG 45 ) 7 ), and APCN(PS240(PEG45)7) can be calculated according to eq 3: APCN(PSx (PEG45)7 ) = [T /(m + 1)] × Convclick (m = 7) (3)

where T is the total amount of cross-linking points and m is the number of alkynyl groups. Under the same number of alkynyl groups in the (PEG45(CCH))7 and almost the same CuAAC conversion (the intriguing nature of CuAAC), the value of the structural regularity of APCN(PS240(PEG45)7 is larger than that of APCN(PS50(PEG45)7) and APCN(PS126(PEG45)7). Furthermore, the well-defined molecular structure and high content of cross-linking can largely suppress the formation of macroscopic network defects. Therefore, the precise control of the number and position of cross-linking points is an effective approach for preparing APCNs exhibiting high mechanical toughness. Figure 9 depicts the steady rheological behavior of APCN(PS50(PEG45)7), APCN(PS126(PEG45)7), and APCN(PS 240 (PEG45) 7 ). The viscosity of the APCNs greatly diminished with the increase in the shearing rate. In comparison to APCN(PS 1 2 6 (PEG 4 5 ) 7 ) and APCN(PS50(PEG45)7), the low shear ratio viscosity (LSRV) of APCN(PS240(PEG45)7) was higher at the shear rates from 0.25 to 12.6 s−1. However, the shear rates decreased rapidly when 19245



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Figure 10. Scanning electron microscopy (SEM) cross-sectional views of the freeze-dried samples of (A) APCN(PS50(PEG45)7), (B) APCN(PS126(PEG45)7), and (C) APCN(PS240(PEG45)7).

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21274020 and 21304019).

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Figure 11. Differential scanning calorimetry (DSC) curves of (A) APCN(PS50(PEG45)7), (B) APCN(PS126(PEG45)7), and (C) APCN(PS240(PEG45)7).

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CONCLUSIONS We have successfully developed a flexible approach for controlling the number of bromo groups in the side chains of linear PS via alternating RAFT polymerization and RAFT chain extension using styrene and PBMI. Well-defined APCN(PSx(PEG45)7) samples were obtained via CuAAC of the linear PSx(N3)y (y = 2, 4, 6) with linear multialkynyl PEG ((PEG 45 (CCH)) 7 ). APCN(PS 50 (PEG 45 ) 7 ), APCN(PS126(PEG45)7), and APCN(PS240(PEG45)7) exhibit variable SR and rheological properties in different solvents. The mechanical and thermal properties of APCNs can be regulated by the control of cross-linking density and composition of APCNs. The precise synthesis of linear polymers with a predetermined number of functional groups can provide a novel strategy for the preparation of well-defined polymer networks with controlled molecular structure, good structural integrity, and desired functionality.

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ASSOCIATED CONTENT

S Supporting Information *

Details regrading the synthetic route of alkynyl-pendant linear (PEG45(CCH))7 and 1H NMR, FT-IR, and GPC data of the as-prepared (PEG45(CCH))7. This material is available free of charge via the Internet at http://pubs.acs.org.

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A Well-Defined Amphiphilic Polymer Conetwork from Sequence

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