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Polymer 55 (2014) 833e839

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Polymer

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Synthesis and self-assembly of amphiphilic polyphosphazene withcontrollable composition via two step thiol-ene click reaction

Chen Chen, Xiao-Jun Huang*, Yang Liu, Yue-Cheng Qian, Zhi-Kang XuMOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University,Hangzhou 310027, China

a r t i c l e i n f o

Article history:Received 10 September 2013Received in revised form20 December 2013Accepted 28 December 2013Available online 5 January 2014

Keywords:PolyphosphazeneThiol-eneSelf-assembly

* Corresponding author.E-mail address: hxjzxh@zju.edu.cn (X.-J. Huang).

0032-3861/$ e see front matter � 2014 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymer.2013.12.065

a b s t r a c t

Sequential thiol-ene click reaction is reported for amphiphilic glycosylated polyphosphazene. Poly[bis(allylamino)phosphazene] was used as precursor to go through UV irradiation with 2,3,4,6-tetra-O-acetyl-1-thiol-b-D-glucopyranose (SH-GlcAc4) and 1-dodecanethiol in sequence. Variation of the reac-tion conditions, including click reaction time and the dose of photoinitiator, led to different hydrophilic/hydrophobic ratios. As a result, glycosylated polyphosphazenes were synthesized with 53.3%, 77.7% and85.0% of glucose moieties. The different residual composition could give rise to different self-assemblybehaviors. Micelles of amphiphilic polyphosphazenes were formed in aqueous solution and the CMCvalue (0.79 � 10�3e4.00 � 10�3 mg/mL) as well as mean diameter (170e220 nm) varied along with thehydrophilic glucose moiety/hydrophobic dodecyl moiety ratio.

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1. Introduction

As well established, polyphosphazene possesses a backbonecontaining alternating nitrogen and phosphorus atoms, withorganic or inorganic groups on each side of each phosphorus atom[1e3]. Owing to the tunability of side groups on the poly-phosphazene platform, more than 700 different polyphosphazeneshave been synthesized, which have been widely applied in tissueengineering [4e8], fire retardation [9,10], arctic rubber [11,12] andion conductor materials [13,14]. Carbohydrate polyphosphazenes,with desirable hydrophilicity and biocompatibility, are of particularinterest in biological processes such as carbohydrateeproteininteraction [15e21]. Nonetheless, when it comes to practical use,the hydrophobic part is indispensable to control degradation rate,material strength and surface properties [22,23]. Moreover, self-assembly of glycopolymers can mimic cell surface of glycolipids,glycoproteins and glycans and participate in numerous biologicalevents such as cellular recognition, adhesion, cancer cell [24]. Thus,the construction of amphiphilic glycosylated polyphosphazenewith mutable hydrophilic/hydrophobic ratios should be high-lighted. Carbohydrate polyphosphazenes were first synthesized bynucleophilic reaction involving sodium alkoxide, glucose, andpoly(dichloro)phosphazene (PDCP) [25], which faced the unfavor-able steric effect of glucose and required an intricate protectione

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deprotection method to avoid crosslinking and degradation of thepolymer backbone.

Click chemistry is an emerging synthetic toolbox especiallyenergetic suitable for end group or side group functionalization[26e30], crosslinking of polymeric matrices [31,32] and highlybranched polymers [33e35]. In 2001, Sharpless et al. brought aboutthe essence of the click reaction by reporting a class of heteroatomlinkage processes conducted in various mild conditions [36].Typically, the Cu(I)-catalyzed azide/alkyne cycloaddition reaction(CuAAC) has been universally applied in macromolecular engi-neering with near-quantitative conversion in both aqueous andorganic media [35]. Other types of click reactions, such as thiol-eneand thiol-yne, are outstanding in terms of robustness, efficiency,and orthogonal conjugation [37e39]. These reactions could achievequantitative yields, requiring only small concentrations of relativelybenign catalysts, having rapid reaction rates with reactions occur-ring either in bulk or in environmentally benign solvents over alarge concentration range [40]. As claimed by Campos et al. [26], theuse of UV initiator in thiol-ene coupling reaction led to higheryields and shorter reaction time than thermal ones. Using clickchemistry, controllable glycosylation and multifunctional archi-tecture of polyphosphazene may be feasible.

Previous work in our lab accomplished the glycosylation ofpolyphosphazene by CuAAC using poly[di(propargylamine)phos-phazene] as a precursor [41]. However, the residual copper at ppmlevels in the product after tedious purification will place re-strictions on biological application [42,43]. As a possible alterna-tive, the thiol-yne reaction was conducted to synthesize

C. Chen et al. / Polymer 55 (2014) 833e839834

carbohydrate polyphosphazene [44]. Owing to the fact that at mosttwo bulky glucosyl groups might “click” to every alkyne group, notall C^C bonds went through the two-step thiol-yne click reaction.For instance, Ren et al. [44] found that approximately 55% of thealkyne group and 8% of the alkene group did not participated in theclick reaction even after 4 h of UV irradiation with excess 2,3,4,6-tetra-O-acetyl-1-thiol-b-D-glucopyranose (SH-GlcAc4). At themean time, the final glycosylated density was nearly 67.7%. With anaim to ease the steric repulsion among the induced groups andavoid the requirement for toxic copper cations, the thiol-ene re-action, in which poly[bis(allylamino)phosphazene] (PBAAP) wasused as precursor, can be a potential route to design multigrouppolyphosphazene by clicking two or more thiol agents in a row.

One-step click reaction was explored to synthesis amphiphilicpolyphosphazene, however, weak controllability took up owing tothe different thiol-ene reaction rates of SH-GlcAc4 and 1-dodecanethiol. In this work, amphiphilic polyphosphazenes con-taining hydrophilic glucosyl and hydrophobic long alkyl groupswith mutable ratio were synthesized by a two-step thiol-ene re-action. In the first step, a glucose moieties derived from SH-GlcAc4was immobilized on PBAAP. Controllable glycosylation could bereached by varying the reaction time or the dose of 2,2-dimethoxy-2-phenylacetophenone (DMPA) during UV irradiation. Then excessactive 1-dodecanethiol was clicked to carbohydrate poly-phosphazene to form the hydrophobic part. The ultimate poly(b-D-glucose-co-1-dodecyl)phosphazene with different density ofglycosylation, P-37.1%, P-53.3%, P-77.7% and P-85.0%, were synthe-sized and studied. The resulting polyphosphazenes were amphi-philic and could self-assemble in water when the ratio of glucosegroup to dodecyl group was up to 50%. A larger proportion ofglucose moieties caused a lower critical micelle concentration(CMC) value and micelle size expansion, according to the resultsfrom pyrene fluorescence spectrometry and dynamic light scat-tering (DLS). The morphology of micelles was intuitively observedby transmission electron microscopy (TEM).

2. Experimental section

2.1. Materials

Hexachlorocyclotriphosphazene (Bo Yuan New Materials &Technique, Ningbo, China) was purified by recrystallization fromheptane and subsequent vacuum sublimation at 60 �C. Poly(di-chlorophosphazene) was synthesized via the thermal ring-openingpolymerization of the purified hexachlorocyclotriphosphazene inan evacuated Pyrex tubes at 250 �C. The polymer was treated withpetroleum ether (Sinopharm Chemical Reagent, China) to removeunpolymerized hexachlorocyclotriphosphazene before use under adry nitrogen atmosphere. Allylamine (Zoupin Mingxing ChemicalCo., China) was purified by vacuum distillation. Tetrahydrofuran(THF) was dried by distillation from a NaeK alloy with benzophe-none until a blue color was obvious. Triethylamine (TEA) was driedover CaH2 prior to use. 2,2-Dimethoxy-2-phenylacetophenone(DMPA, Aladdin Reagent, China), trifluoroethanol (TFE, AladdinReagent, China, 99.5%), and 1-dodecanethiol were used withoutfurther purification. 2,3,4,6-Tetra-O-acetyl-1-thiol-b-D-glucopyr-anose (SH-GlcAc4) was synthesized as previously reported [45].

2.2. Synthesis of poly[bis(allylamino)phosphazene] (PBAAP)

Poly(dichlorophosphazene) (2.00 g, 17.3 mmol) was dissolved indry THF (250 mL) and the resulting solution was added in a drop-wise manner to a stirred solution of allylamine (5.2 mL,69.6 mmol) and TEA (9.7 mL, 69.6 mmol) in dry THF (50 mL) undernitrogen. The mixture was stirred at 25 �C for 24 h and then heated

to 40 �C and stirred for a further 18 h at that temperature. Themixture was filtered to remove the resulting triethylamine hydro-chloride and the filtrate was concentrated by evaporation. Theconcentrated solution was added in a drop-wise manner to anexcess of ethanolewater (1:1 by volume) to precipitate the poly-mer. The solid was subsequently filtered and the filter-cake washedseveral times with ethanol and dried under a vacuum to give thedesired polymer product as a white fibrous solid (1.63 g, 60%).

2.3. Synthesis of poly(b-D-glucose-co-allyamine)phosphazene

PBAAP (150 mg) was dissolved in TFE at a concentration of15 mg/mL and the mixturewas then transferred to a quartz reactor.SH-GlcAc4 (2 equiv. with respect to the double bonds) and DMPA(0.1 or 0.02 equiv. with respect to the double bonds) were addedsequentially to the reactor and N2 was then gently bubbled throughthemixture for 10min to eliminate dissolved oxygen. The thiol-enereaction was initiated by UV irradiation (max ¼ 365 nm, 0.6 mW/cm2) and conducted for the described time (40 min, 2 h and 4 h)with stirring at ambient temperature. After the first click reaction,the acetyl protecting group was removed by the addition of a 1 Msolution of CH3ONa in CH3OH to the polymer solution. The mixturewas dialyzed against water for 2 days (molecular weight cut-off:3.5 kDa) and purification of glycosylated phosphazene was con-ducted by freeze drying. The ratio of glucose moieties was calcu-lated by integration for the signal at 3.66 ppm with 5.90 ppm.

2.4. Synthesis of poly(b-D-glucose-co-1-dodecyl)phosphazene

Glycosylated polyphosphazenes were dissolved in 2.25 mL DMFand 1-dodecanethiol (5 equiv. with respect to the double bonds)and DMPA (0.01 equiv.) were added to the solution. The mixturewas under UV irradiation for 1 h and subsequently dialyzed againstalcohol for 2 days and water for 1 day with the final product wasobtained by freeze-drying. Take account for different proportion ofglucose moieties, these amphiphilic polyphosphazenes can benamed as P-53.3%, P-77.7%, P-85.0% and P-37.1% for short.

2.5. Self-assembly of poly(b-D-glucose-co-1-dodecyl)phosphazeneby crew-cut method

Poly(b-D-glucose-co-1-dodecyl)phosphazenes with differentglucose to dodecyl ratios were first dissolved in DMF (0.1 mg/mL),and then deionized water was gradually added at a constant rate(25 mL/min). The magnitude of water jump was 1 wt% per time. Thesolution was subjected to under moderate stirring at room tem-perature during water addition. The morphology of the micelleswas quenched by adding a large amount of water after reaching thedesired water content. DMF was removed by dialysis against water(molecular weight cut-off: 3.5 kDa).

2.6. Characterization

1H NMR spectra were recorded on a Bruker Advance DMX500 inDMSO-d6. A 2e5 wt % polymer solution was prepared in DMSO-d6for each analysis. H3PO4 in D2O was used as an external referencefor the 31P NMR measurements. Number-average and weight-average molecular weights and molecular weight distributionswere determined using gel permeation chromatography (GPC).FTIR spectra were recorded using a Bruker Vector 22 FourierTransform Infrared Spectrometer, using samples pressed into po-tassium bromide pellets. Pyrene for fluorescence measurements(FLM) was dissolved in acetone solution (concentration6.0� 10�7 mol/L), the whole mixture was then added to volumetricflask before acetone was separated from pyrene by evaporation.

Scheme 1. Synthesis of amphiphilic polyphosphazene via thiol-ene coupling [Polymer 1 PDCP; Polymer 2 PBAAP; Polymer 3 poly(b-D-glucose-co-allylamine)phosphazene;Polymer 4 poly(b-D-glucose-co-1-dodecyl) phosphazenes].

C. Chen et al. / Polymer 55 (2014) 833e839 835

Subsequently, amphiphilic polyphosphazene solutions withdifferent concentrations (2.0 � 10�1, 2.0 � 10�2, 2.0 � 10�3,2.0 � 10�4, 2.0 � 10�5, 2.0 � 10�6, 2.0 � 10�7 mg/mL) were allowedto mix with pyrene. The resultant solutions were kept under at40 �C for 4 h and then 25 �C for 6 h. The investigation of criticalmicelle concentration was carried out by a fluorescence spectro-photometer (RF-3510PC; Shimadzu, Japan) equipped with a pyreneprobe. The solution of amphiphilic polyphosphazenes (P-53.3%, P-77.7%, and P-85.0%) was prepared by the crew-cut method. Nofiltration was carried out before dynamic light scattering (DLS)measurement. A 90 plus DLS instrument (Brookhaven Germany)was employed to measure the hydrodynamical diameter and thedistribution of the diameter (PDI). Transmission electron micro-scopy (TEM) was used to observe the microstructure of the self-assembled polyphosphazene and was performed on a JEM-1200EX (NEC, Japan) microscope with the accelerating rate up to120 kV.

Fig. 1. 31P NMR (a) and 1H NMR (b) spectrum

3. Results and discussion

3.1. Synthesis of poly[bis(allylamino)phosphazene] (PBAAP)

Poly(dichloro)phosphazene (Polymer 1) was synthesized byring opening polymerization of hexachlorocyclophosphazene at250 �C under vacuum. Then PBAAP (Polymer 2) was obtained bynucleophilic substitution reactions between poly(dichloro)phos-phazene (PDCP) and allylamine (Scheme 1). 1H and 31P NMRspectroscopy (Fig. 1) confirmed the exact structure of PBAAP. Twomain peaks corresponding to vinyl protons at 4.97e5.17 ppm (eCH]CH2) and 5.85e5.92 ppm (eCH]CH2) were present in the 1HNMR spectrum. A single peak was detected at 2.52 ppm by 31PNMR. FTIR spectra (Fig. 2) showed that the C]C bonds appeared at1640 cm-1. Similar to other amino organic polyphosphazenes,PBAAP is chemically stable under atmospheric conditions and couldbe dissolved in acidic organic media, which makes PBAAP a

of poly[bis(allylamino)phosphazene].

Fig. 2. FTIR spectra for (a) PBAAP, (b) poly(b-D-glucose-co-allyamino)phosphazene,and (c) poly(b-D-glucose-co-1-dodecyl) phosphazenes.

Fig. 3. 1H NMR for glycosylated polyphosphazene with different click reaction time. (a)4 h, (b) 2 h and (c) 40 min.

C. Chen et al. / Polymer 55 (2014) 833e839836

desirable precursor leading to multifunctional polyphosphazene.Taking advantage of the thiol-ene click reaction, multiple sidegroups could be efficiently introduced along the phosphorus-nitrogen backbone comparedwith nucleophilic reactionwith PDCP.

Table 1Parameters for synthesis of polyphosphazenes with different hydrophilic/hydrophobic r

Sample name Reaction time in first Stepa (h) DMPA (equiv.) Glub

P-85.0% 4 0.1 85.0P-77.7% 2 0.1 77.7P-53.3% 0.67 0.1 53.3P-37.1% 2 0.02 37.1PBAAP e e 0

a The reaction time in the first thiol-ene reaction between PBAAP and SH-GlcAc4.b The proportion of glucose moieties in the final poly(b-D-glucose-co-1-dodecyl)phosc The results from DLS.d The results from pyrene excitation fluorescence spectrum.e The results from GPC.

3.2. Synthesis of glycosylated polyphosphazene

After the click reaction between PBAAP and SH-GlcAc4, new mul-tiplets appeared in the 1H NMR spectrum at 5.13e3.67 (protons ofglucosyl ring), 3.26e2.97 (OH protons of sugar), 2.83(NHCH2CH2CH2SR), 2.66 (NHCH2CH2CH2SR), 1.70 (NHCH2CH2CH2SR),and 1.24 [NHCH(CH)3SR] (Fig. 3). The FTIR spectra illustrated suc-cessful introduction of a hydroxyl groupbymeans of the absorption at3400 cm�1 (Fig. 2). According to the ratio of the integrals for the signalat 3.66ppmwith5.90ppm, theproportionof glucosyl group increasedas the click reaction time under UV increased. As shown in Table 1,53.3% C]C bonds were converted to a glucosyl group when the irra-diation time was 40 min. Longer reaction times enhanced the intro-ductionof glucosewith2h corresponding to 77.7%and4h to85.0%.Asmore and more glucose was introduced along the backbone, stericrepulsion played a crucial role, and began to dominate the reactionrate. Thus, further lengthening the reaction time tomore than4honlygave rise to a negligible increase of glucosylation. Moreover, the doseof photoinitiator, DMPA, also determined the final composition ofcarbohydrate. When the amount of DMPA dropped from 0.1 equiv. to0.02 equiv., the proportion of glucose dropped from 77.7% to 37.1%.

3.3. Synthesis of poly(b-D-glucose-co-1-dodecyl)phosphazene

Taking into consideration the highmodification efficiency of alkylsulfhydrate, the second step of the thiol-ene click reaction couldachieve higher efficiency than SH-GlcAc4, and indeed just 1 h wasneeded to reach complete conversion of C]C bonds. The ultimatepoly(b-D-glucose-co-1-dodecyl)phosphazene, P-53.3%, P-77.7%, P-85.0% andP-37.1%,wasobtainedafter the second stepof the thiol-enereaction. The peak for vinyl protons vanished (Fig. 4), suggesting acomplete conversion of C]C bonds into aliphatic groups. Charac-teristic peaks of dodecyl group including 0.86 [CH2CH2(CH2)8CH3],1.23 [CH2CH2(CH2)8CH3], 1.60 [CH2CH2(CH2)8CH3] and 2.47[CH2CH2(CH2)8CH3] also appeared. Disappearance of vinyl CeHbonds (n ¼ 2800 cm�1) and C]C bonds (n ¼ 1640 cm�1) in the IRspectrum (Fig. 2) confirmed that the final polyphosphazene con-tained little residual vinyl groups. The Mw/Mn (PDI) before click re-action was 1.54 for PBAAP. After postfunctionalization by thiol-enereactions, PDI varied from 1.47 to 1.52.

3.4. Self-assembly of the amphiphilic polymer

The amphiphilic polyphosphazenes (P-85.0%, P-77.7% and P-53.3%) were white powders that could be dissolved in commonorganic solvents including DMF, CHCl3, DMSO, DMAc, and alcohol.Depending on the extent of glucosylation, P-85.0%, P-77.7% and P-53.3% possessed varying solubility in water. In fact, P-37.1% hardlydissolved in water because of its relatively less hydrophilic glucosemoieties. According to pyrene excitation fluorescence spectra [46],the symmetry-forbidden (0,0) band shifted from 334 nm to 338 nmwith increasing polymer concentration. The CMC values can be

atios.

(%) Dhc (nm) CMCd (mg/mL) Mne (g/mol) Mw

e (g/mol)

171.0 4.00 � 10�3 1.82 � 104 2.76 � 104

194.2 1.78 � 10�3 2.69 � 104 3.97 � 104

227.5 1.32 � 10�3 2.19 � 104 3.33 � 104

e e 2.28 � 104 3.45 � 104

e e 1.62 � 104 2.50 � 104

phazene.

Fig. 4. 1H NMR for poly(b-D-glucose-co-1-dodecyl)phosphazenes. The mole ratio ofglucosyl group is (a) P-85.0%, (b) P-77.7% and (c) P-53.3%.

C. Chen et al. / Polymer 55 (2014) 833e839 837

determined by the ratios of the peak intensities at 334 nm and338 nm. Fig. 5 shows the intensity ratios (I334/I338) of pyrene exci-tation spectra versus the logarithm of concentrations of amphi-philic polyphosphazenes (log C). At low concentrations, the changein the intensity ratio (I334/I338) was negligible as no aggregatesexisted. But at a critical concentration, the intensity ratios began toshow a substantial decrease, which reflects the shift of the pyreneprobe from an aqueous phase to a hydrophobic one. The concen-tration at which the intensity ratio starts to fall illustrates theminimum concentration of the amphiphilic polyphosphazeneneeded for the formation of micelles, in which the most hydro-phobic chains aggregated to form the inner core and the othermade up the rest of micelle with decreasing compactivity. Ac-cording to Fig. 5, CMC values can be determined by the turningpoint of the curve and range from 1.32 to 4.00 mg/L. These CMCvalues are an order of magnitude lower compared with poly-phosphazenes substituted with glucose and n-butyl group [44].Therefore, the introduction of longer alkyl group greatly enhancesthe hydrophobicity of the corresponding polyphosphazene.

Fig. 5. Plot of I334/I338 (from excitation spectra of pyrene) versus the logarithm of theconcentration for aqueous solutions of the sample P-85.0%, P-77.7% and P-53.3%.

Dynamic light scattering (DLS) was engaged to determine the di-ameters of the micelles. As indicated by Fig. 6, the diameter wasapproximately 171 nm for P-85.0%, 194 nm for P-77.7% and 227 nmfor P-53.3% in aqueous solution at 25 �C. The size distribution (PDI)varied from 0.44 to 0.62. At the mean time, the Dh value is larger

Fig. 6. The mean diameter of (a) P-85.0% (b) P-77.7%, and (c) P-53.3%.

C. Chen et al. / Polymer 55 (2014) 833e839838

than previous work concerning poly(b-D-glucose-co-butylamine)phosphazene (55 nm-90 nm) [44], which probably because longeralkyl group induced increasing aggregation number per micelle.Moreover, increased molecular weight could also contribute to thesize expansion of micelles. The data indicates that the increase ofthe hydrophobic dodecyl density gave rise to relative higher degreeof hydrophobic accumulation. On the contrary, the increase of stericglucose moieties caused intermolecular hydrophilic repulsion andthen lowered the aggregation number.

The size and morphology of micelles were also observed byTEM. Self-assembled amphiphilic poly(b-D-glucose-co-1-dodecyl)phosphazene was prepared by the way [47e49] similar to crew-cutmethod for block copolymer owing to the poor solubility in water.

Fig. 7. TEM micrographs of (a) P-85

Samples of P-85.0%, P-77.7% and P-53.3% were first dissolved inDMF with the concentration 0.1 mg/mL and then water was grad-ually added (25 mL/min). As presented in Fig. 7, when the watercontent was relatively low (1e5 wt%), micelles were formed andthemean diameters observed by TEMwere smaller than the resultsfrom DLS suggested, which was ascribed to the volatilization ofsolvent. It should be noted that the particles were not fully well-rounded. One explanation could be gradual chain aggregationmechanism suggested by Li et al. [48], in which polymeric chainsorganize themselves from cores to the shells in the hydrophobicitydecrease order. Moreover, owing to the relative high molecularweight distribution (PDI) and irregular structure of randomcopolymer, the shape of micelles was not perfectly global.

.0% (b) P-77.7% and (c) P-53.3%.

C. Chen et al. / Polymer 55 (2014) 833e839 839

Interestingly, globular micelle went throughmorphology transitionas the water content increased (Table S1).

4. Conclusions

The amphiphilic poly(b-D-glucose-co-1-dodecyl)phosphazenewas successfully synthesized via a two step thiol-ene reaction,which involved the original PBAAP and the correspondingmercaptan reagents. This synthetic route is characterized by highefficiency, good controllability, and mild reaction conditions. Thedegree of glycosylation could be generally controlled by the UVexposure time during the thiol-ene reaction, and the carbon doublebonds remaining reacted quantitatively with 1-dodecanethiol. As aresult, amphiphilic polyphosphazenes P-85.0%, P-77.7% and P-53.3%and P-37.1% were prepared and the relative ratio of glucose versusdodecyl group greatly affected the self-assembly behavior of thepolyphosphazenes above. DLS results showed that the meandiameter increased with enhanced proportion of hydrophobicdodecyl moieties. CMC values, measured by pyrene fluorescencespectra, ranged from 0.79� 10�3 to 4.00� 10�3 mg/mL. Lower CMCvalues were caused by an increasing proportion of hydrophobicgroup. The size of the micelles, which was 170e220 nm, alsoincreased with higher dodecyl group composition. Globular mi-celles were also observed by TEM with similar size. These self-assembled amphiphilic polyphosphazenes have potential uses inhydrophilic modification, protein adsorption, and drug loading.

Acknowledgments

The authors would like to thank the Financial support from theFundamental Research Funds for the Central Universities (Grant no.2013QNA4049), the National Natural Science Foundation of China(Grant no. 21274126), National “Twelfth Five-Year” Plan for Science& Technology Support of China (No. 2012BAI08B01) and the OpenFoundation of Key Laboratory of Advanced Textile Materials andManufacturing Technology (No. 2012001) are gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.polymer.2013.12.065.

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