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Synthesis and Characterization of Organic/Inorganic Polyrotaxanes from PolyhedralOligomeric Silsesquioxane and Poly(ethyleneoxide)/a-Cyclodextrin Polypseudorotaxanes viaClick Chemistry

Ke Zeng, Sixun Zheng*

Organic/inorganic polyrotaxanes were synthesized via Huisgen 1,3-dipolar cycloadditionbetween 3-azidapropylhepta(3,3,3-trifluoropropyl) POSS and dialkyne-terminated PEO/a-cyclodextrin polypseudorotaxanes. The organic/inorganic hybrid polyrotaxanes were charac-terized by means of 1H NMR spectroscopy andWAXRD. It was found that the nanosized POSSblocking agents significantly affected the crystalstructures of polyrotaxanes. Thermal gravimetricanalysis showed that the organic/inorganichybrid polyrotaxanes exhibited enhanced ther-mal stability compared to their parent polypseu-dorotaxanes, in terms of rate of thermaldegradation and the summation of char andceramic yields.

Introduction

Polyrotaxanes and polypseudorotaxanes are a class of

topologically interlocked supramolecules composed of

macrocyclic compounds threaded onto linear polymer

backbones without covalent bonds linking these two

species. Since Harada et al. first reported the inclusion

complexes of a-cyclodextrin (a-CD) with poly(ethylene

glycol) (PEG) in the 1990s,[1–8], this class of supramolecules

K. Zeng, S. ZhengCollege of Chemistry and Chemical Engineering, and State KeyLaboratory of Metal Matrix Composites, Shanghai Jiao TongUniversity, Shanghai 200240, ChinaFax: þ86 21 5474 1297; E-mail: [email protected]

Macromol. Chem. Phys. 2009, 210, 783–791

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

has been extensively investigated. Owing to their unique

structures, these supramolecular assemblies have been

utilized as stimuli-responsive hydrogels,[9] insulating

wires[10,11] and polyrotaxane networks.[12–14] For the

preparation of polyrotaxanes from the polypseudorotax-

anes, it is important to utilize efficient end-capping

reactions with the proper blocking agents. Harada et al.[15] used 2,4-dinitrofluorobenzene to react with amino-

terminated poly(ethylene oxide) (PEO)-polypseudorotax-

anes to obtain the corresponding polyrotaxanes. Kihara

et al. reported the preparation of polytetrahydrofuran-

polyrotaxanes via the reaction of 4-tritylphenyl isocyanate

with the hydroxyl terminals of the polypseudorotax-

anes.[16] Choi et al. reported the reaction of 9-anthralde-

hyde with amino-terminated poly(ethylene imine)-

DOI: 10.1002/macp.200800605 783

K. Zeng, S. Zheng

784

polypseudorotaxanes to obtain polyrotaxanes.[17] More

recently, Gecheler et al.[18] reported that fullerene-60 was

used as an end-capping agent to obtain fullerene-

terminated polyrotaxanes.

Polyhedral oligomeric silsesquioxanes (POSS) are a class

of important nanosized cage-like molecules, derived from

hydrolysis and condensation of trifunctional organosi-

lanes. POSS molecules possess a formula of [RSiO3/2]n,

n¼ 6–12, where R represents various types of organic

groups, one (ormore) of which is reactive or polymerizable.

A typical POSS molecule possesses the structure of a cube-

octameric framework, represented by the formula

(R8Si8O12), with an inorganic silica-like core (Si8O12)

(�0.53 nm in diameter) surrounded by eight organic

corner groups, one or more of which is reactive; the

distance between two adjacent organic groups is about

1–1.5 nm, depending on the length of the organic group

(Scheme 1).[19–21] It is noted that POSS macromers have

been recently incorporated into polypseudorotaxanes.[22–24]

Chang et al. [22] and He et al. [23] have investigated

supramolecular inclusion complexes of octa-armed star-

shaped poly(e-caprolactone) (PCL) and PEOwith a-CD. They

found that the efficiencies of inclusion complexation for

the organic/inorganic star-shaped polymers were lower

than those of their linear counterparts with CDs.

Furthermore, the presence of bulky POSS cages constituted

a steric hindrance to the formation of inclusion complexes

(ICs).[22,23] We recently examined the effect of the POSS

macromer at one end of the PCL chain on the efficiency of

inclusion complexation.[24] To the best of our knowledge,

there has been no previous report on organic/inorganic

polyrotaxanes with POSS macromers as the end-capping

agents. In this work, we explore the synthesis of the

organic/inorganic polyrotaxanes involving dialkyne-

terminated PEO/a-CD polypseudorotaxanes and POSS.

The utilization of POSS as the end-capping agent in this

work is based on the following considerations: i) POSS is a

nanosized molecule, which allows investigation of the

effect of the nanosized capping agent on the structures of

polyrotaxanes; and, ii) the end-capping agent used in this

Scheme 1. Structure of POSS.



work is a 3-azidopropylhepta(3,3,3-trifluoropropyl) POSS –

the semifluorinated ligands of the POSS possess fluor-

ophobic properties and, thus, endow the polyrotaxanes

with amphiphilicity.

In this communication, we report the synthesis of

organic/inorganic polyrotaxanes. The end-capping reac-

tion is the Huisgen 1,3-dipolar cycloaddition between 3-

azidapropylhepta(3,3,3-trifluoropropyl) POSS and dia-

lkyne-terminated PEO/a-CD polypseudorotaxane under

benign conditions, that is, click chemistry.[25–28] The effect

of the nanometer size of the terminal POSS groups on the

crystalline structures and thermal stability of polyrotax-

anes will be addressed on the basis of wide-angle X-ray

diffraction and thermal gravimetric analysis.

Experimental Part

Materials

3,3,3-Trifluoropropyltrimethoxysilane was obtained from Zhe-

jiang Chem-Tech Co., China. Both 3-bromopropyltrichlorosilane

and sodium azide (NaN3) were purchased from Gelest Co. USA and

used as received. Both propargyl bromide and a-CD were

purchased from Alfa Co., China. PEG with Mn ¼1 000, 2 000,

4 000 and 6000 were purchased from Fluka Co. Germany.

Pentamethyldiethylenetriamine (PMDETA) was purchased from

Aldrich Co., USA and used as received. Unless specifically

indicated, other reagents such as sodium, calcium hydride

(CaH2), sodium hydroxide and copper(I) bromide (CuBr) were of

chemically pure grade and obtained from Shanghai Reagent Co.,

China. Organic solvents, such as N,N0-dimethylformamide (DMF),

tetrahydrofuran (THF), dichloromethane triethylamine (TEA) and

petroleum ether (distillation range: 60–90 8C) were of chemically

pure grade, obtained from commercial sources. Before use, THF

was refluxed above sodium and then distilled and over a

molecular sieve of size 4 A. Triethylamine was refluxed over

calcium hydride and then purified with p-toluenesulfonyl

chloride, followed by distillation.

Synthesis of 3-Bromopropylhepta(3,3,3-

trifluoropropyl) POSS

Firstly, hepta(3,3,3-trifluoropropyl)tricycloheptasiloxane triso-

dium silanolate [Na3O12Si7(C3H4F3)7] was synthesized by follow-

ing themethod reported by Fukuda et al.[7] In a typical experiment,

(3,3,3-trifluoropropyl)trimethoxysilane (50.0 g, 0.23 mol), THF

(250mL), deionized water (5.25 g, 0.29mol) and sodium hydroxide

(3.95 g, 0.1mol) were charged to a flask equippedwith a condenser

and a magnetic stirrer. After refluxed for 5 h, the reactive system

was cooled down to room temperature and held at this

temperature for 15 h with vigorous stirring. Then, all the solvent

and other volatile were removed by rotary evaporation, leaving a

white solids. After drying at 40 8C in vacuo for 12 h, 37.3 g of

product were obtained at a yield of 98%. The as-prepared

hepta(3,3,3-trifluoropropyl)tricycloheptasiloxane trisodium sila-

nolate [Na3O12Si7(C3H4F3)7] (10.0 g, 8.8 mmol) and triethylamine

DOI: 10.1002/macp.200800605

Synthesis and Characterization of Organic/Inorganic Polyrotaxanes from . . .

(1.3 mL, 8.8 mmol) were charged to a flask equipped with a

magnetic stirrer and 200 mL anhydrous THF added with vigorous

stirring. The flask was immersed into an ice-water bath and

purged with high purity nitrogen for 1 h. Then, 3-bromopropyltri-

chlorosilane (2.47 g, 9.68mmol) dissolved in 20mL anhydrous THF

was slowly added, dropwise, within 30 min. The reaction was

carried out at 0 8C for 4 h and at room temperature for 20 h.

Sodium chloride was filtered out and the solvent, together with

other volatiles, were eliminated via rotary evaporation to afford

white solids. The solids were washed with 50 mL methanol three

times and dried in vacuo at 40 8C for 24 h; 8.2 g product was

obtained at a yield of 76%.

FTIR (KBr window): n¼1 090–1000 (Si�O�Si), 2 900–2850

(�CH2), 1 120–1300 (�CF3), 624 (C�Br) cm�1.1H NMR (acetone-d6): d¼ 3.52 (t, 2.0H, �CH2�Br), 2.32 (m,

14.0H, SiCH2CH2CF3), 1.96 (m, 2.0H, �CH2�CH2�Br), 1.03 (m,

14.0H, SiCH2CH2CF3), 0.96 (t, 2.0H, �CH2�CH2�CH2�Br).29Si NMR (acetone-d6): d¼�65.8, �66.8, �67.0.

Synthesis of 3-Azidopropylhepta(3,3,3-

trifluoropropyl) POSS

3-Azidopropylhepta(3,3,3-trifluoropropyl) POSS was synthesized

via the reaction between 3-bromopropylhepta(3,3,3-trifluoropro-

pyl) POSS and sodium azide (NaN3). In a typical experiment, 3-

bromopropylhepta(3,3,3-trifluoropropyl) POSS (3.0 g, 2.5 mmol)

and NaN3 (0.176 g, 2.75 mmol) were added to a flask equipped

with a magnetic stirrer and anhydrous DMF (10 mL) was added.

The reaction was carried out at room temperature for 24 h. Then,

the solution was concentrated and a large amount of deionized

water added to give a precipitate. The products was further dried

at 40 8C in a vacuumoven for 24 h and 2.6 g productwere obtained

at a yield of 90%.

FTIR (KBr window): n¼1 090–1000 (Si�O�Si), 2 900–2850

(�CH2), 1 120 �1300 (�CF3), 2 105 (�N3) cm�1.

1H NMR (acetone-d6): d¼ 3.35 (t, 2.0H, �CH2�N3), 2.32 (m,

14.0H, SiCH2CH2CF3), 1.76 (m, 2.0H, �CH2�CH2�N3), 1.03 (m,

14.0H, SiCH2CH2CF3), 0.86 (t, 2.0H, �CH2�CH2�CH2�N3).

Synthesis of Dialkyne-Terminated PEO

Dialkyne-terminated PEO was prepared via the reaction between

propargyl bromide and PEG in the presence of sodium hydride

(NaH). Prior to use, the PEG samples (Mn ¼ 1000, 2 000, 4 000 and

6000) were dried via azeotropic distillation with toluene. In a

typical experiment, to a 500 mL round bottom flask, NaH (0.96 g,

40 mmol) and anhydrous THF (50 mL), PEG (Mn ¼ 4 000) (20.0 g)

dissolved in 150mL anhydrous THFwas slowly added dropwise to

the system within 30 min. The mixture was maintained at this

temperature for 3 h and then propargyl bromide (4.76 g, 40 mmol)

dissolved in 50 mL anhydrous THF was added dropwise. The

reaction was carried out for a further 20 h with vigorous stirring,

to reach completion. The salt (i.e., NaBr) and the unreacted NaH

were removed by filtration. The filtrate was concentrated and

precipitated in a large amount of petroleumether. The precipitates

were dissolved in THF and re-precipitated with petroleum ether.



This procedure was repeated three times to purify the products.

The products were dried in vacuo at 30 8C for 24 h before use. The

dialkyne-terminated PEG (19.95 g) were obtained at a yield of 93%.

FTIR (KBr window): n¼ 3243 ( C�H), 2 106 (C C) cm�1.1H NMR (CDCl3, PEG, Mn ¼4 000): d¼2.42 (t, 2H, HC C�), 4.2

(d, 4H, �C C�CH2�), 3.6(m, 360H, �CH2CH2�O�).

Preparation of Polypseudorotaxanes

In a typical experiment, 0.2 g dialkyne-terminated PEG (Mn ¼4 000) was dissolved in deionized water to form a 20 wt.-%

solution. a-CD (2.20 g) was dissolved in 15 mL deionized water to

obtain a saturated solution. The saturated aqueous solution was

added dropwise to the aqueous solution of dialkyne-terminated

PEG at room temperature. The mixtures were ultrasonically

agitated for 10 min and then allowed to stand overnight at room

temperature. The precipitates were collected by filtration and

were further washed with water three times to remove

uncomplexed PEG and a-CD. The inclusion complexes were dried

in vacuo at 60 8C for 24 h before use.

Synthesis of Polyrotaxanes

Dialkyne-terminated polypseudorotaxanes were used to react 3-

azidopropylhepta(3,3,3-trifluoropropyl) POSS; that is, ‘‘click chem-

istry’’ was carried out to afford organic/inorganic polyrotaxanes.

In a typical experiment, the inclusion complexes of PEG

(Mn ¼ 4000) with a-CD (1.0 g) and 3-azidopropylhepta(3,3,3-

trifluoropropyl) POSS (0.10 g) were charged to a 25 mL flask

equipped with a magnetic stirrer. 0.10 g a-CD and 5 mL DMF were

added to the flask. The system was purged with high purity

nitrogen for 30 min and then CuBr (5 mg) was added as a catalyst.

After purging with high purity nitrogen for an additional 10 min,

PMDETA (10 mL) was added to the system using a syringe. The

reaction was carried out at room temperature for 24 h and then

the solvent was removed via rotary evaporation. The crude

products were purify by washing with water and THF several

times to remove a-CD and any unreacted 3-azidopropyl-

hepta(3,3,3-trifluoropropyl) POSS, respectively. After drying

in vacuo at 60 8C for 24 h, 1.04 g of product was obtained at a

yield of 95%.1H NMR (DMSO-d6): d¼7.97 (s, Hc, 2H), 5.53 (d, H7, 230H), 5.48

(d, H8, 230H), 4.86 (d, H1, 230H), 4.54 (s, triazole�CH2�O�, 4H),

4.43 (t,H9, 230H), 3.84 (t,H3, 230H), 3.78–3.64 (m,H6,5, 690H), 3.53

(s, CH2 of PEO, 360H), 3.44 (t, H2, 230H), 3.38–3.32 (m, H4, 230H),

2.26(m, CF3�CH2�, 28H), 0.92 (m, CF3�CH2�CH2�, 28H).

Techniques and Measurement

NMR Spectroscopy

The 1H NMRmeasurements were carried out on a Varian Mercury

Plus 400 MHz NMR spectrometer. The samples were dissolved

with deuterated acetone (acetone-d6) or chloroform (CDCl3) and

the solutions were measured with tetramethylsilane (TMS) as the

internal reference.

www.mcp-journal.de 785

K. Zeng, S. Zheng

Scheme 2. Synthesis of 3-azidopropylhepta(3,3,3-trifluoropropyl) POSS.

786

Fourier-Transform Infrared (FTIR)Spectroscopy

The FTIR measurements were conducted on a

Perkin Elmer Paragon 1000 Fourier-transform

spectrometer at room temperature (25 8C). Thesupramolecular inclusion complexes were

mixed with a powder of KBr and then pressed

into small flakes. All the specimens were

sufficiently thin to be within a range where

the Beer-Lambert law is obeyed. In all cases, 64

scans at a resolution of 2 cm�1 were used to

record the spectra.

Thermal Gravimetric Analysis (TGA)

A Perkin Elmer TGA-7 thermal gravimetric

analyzer was used to investigate the thermal

stability of the inclusion complexes. All the

thermal analysis was conducted in nitrogen

atmosphere from ambient temperature to

800 8C at a heating rate of 20 8C �min�1.

The thermal degradation temperature was

taken as the onset temperature at which 5 wt.-% of weight loss

occurred.

Wide-Angle X-Ray Diffraction (WAXRD)

The WAXRDmeasurements were carried out on a Shimadzu XRD-

6000 X-ray diffractometer with Cu Ka (l¼0.154 nm) irradiation at

40 kV and 30 mA with a Ni filter. Data were recorded in the

range 2u¼ 4–408 at a scanning rate and step size of 4.08 �min�1

and 0.028, respectively.

Results and Discussion

Synthesis of Organic/Inorganic Polyrotaxanes

Scheme 3. Synthesis of polyrotaxanes.

The route of synthesis for the organic/

inorganic polyrotaxanes is depicted in

Scheme 2 and 3. The starting compound

for 3-azidopropylhepta(3,3,3-trifluoropropyl)

POSS is hepta(3,3,3-trifluoropropyl) tricy-

cloheptasiloxane trisodium silanolate

[Na3O12Si7(C3H4F3)7], which was pre-

pared via the condensation and arrange-

ment of 3,3,3-trifluoropropyltrimethoxy-

silane in the presence of sodium

hydroxide.[29] The corner-capping reac-

tion between hepta(3,3,3-trifluoropropyl)

tricycloheptasiloxane trisodium silano-

late and 3-bromopropyltrichlorosilane

was employed to obtain 3-bromopropyl-

hepta(3,3,3-trifluoropropyl) POSS. Figure 1

shows the 29Si NMR spectrum of

3-bromopropylhepta(3,3,3-trifluoropropyl)

POSS. The resonances at d¼�65.8, �66.8,



�67.0 are assignable to the silicon nucleus of the

silsesquioxane cage. In terms of the ratio of integration

intensity for these silicon resonance peaks, it was judged

that octameric silsesquioxane was successfully obtained.

The 3-bromopropylhepta(3,3,3-trifluoropropyl) POSS was

used to react with sodium azide (NaN3) to afford 3-

azidopropylhepta(3,3,3-trifluoropropyl) POSS. The substi-

tution reaction of bromine atomswith azido groups can be

traced by means of FTIR spectroscopy. The appearance of

the stretching vibration band of azido groups at 2 015 cm�1

indicates 3-azidopropylhepta(3,3,3-trifluoropropyl) POSS

was obtained from the substitution reaction. The complete

substitution of bromine atoms by azido groups was

confirmed by 1H NMR spectroscopy. Figure 2 shows the

DOI: 10.1002/macp.200800605


Figure 1. 29Si NMR spectrum of 3-bromopropylhepta(3,3,3-fluoro-propyl) POSS. Figure 3. FTIR spectra of: a) PEO, and, b) dialkyne-terminated PEO

(Mn ¼4000).

1H NMR spectra of 3-bromopropylhepta(3,3,3-trifluoropro-

pyl) and 3-azidopropylhepta(3,3,3-trifluoropropyl) POSS. It

can be seen that, with the occurrence of the substitution

reaction, the resonance of methylene protons connected to

bromine atoms shifted to higher field – from d¼ 3.53 to

3.36. The observation that no remnant resonance at

d¼ 3.53 was detected suggests that the substitution

reaction occurred to completion under the reaction

condition. The FTIR, 1H and 29Si NMR spectra indicate

that 3-azidopropylhepta(3,3,3-trifluoropropyl) POSS was

successfully obtained.

Dialkyne-terminated PEO was prepared via the reac-

tions between propargyl bromide and PEG. To promote the

Figure 2. 1H NMR spectra of: a) 3-bromopropylhepta(3,3,3-trifluor-opropyl) POSS, and, b) 3-azidopropylhepta(3,3,3-trifluoropropyl)POSS.



reaction to completion, PEG were reacted with excessive

propargyl bromide in the presence of sodium hydride.

Representatively shown in Figure 3 are the FTIR spectra of

PEG (Mn ¼ 4000) and its derivative with dialkyne term-

inals. It can be seen that the stretching vibration band of

the terminal hydroxyl groups at 3 395 cm�1 virtually

disappeared with the occurrence of the capping reaction;

concurrently a new band appeared at 3 243 cm�1, which is

ascribed to the stretching vibration of the C�H bond of

alkyne groups. This result can be further confirmed by1H NMR spectroscopy. According to the ratio of the

integration intensity of alkyne to ethylene protons in the1H NMR spectra (Figure 4), it was judged that the capping

Figure 4. 1H NMR spectrum of dialkyne-terminated PEO(Mn ¼4000).


K. Zeng, S. Zheng

Figure 5. 1H NMR spectra of polypseudorotaxane and polyrotaxane from PEO(Mn ¼4000) and a-CD.

788

reaction of PEG with alkyne groups was

carried out to completion. The FTIR and1H NMR spectra indicated that dialkyne-

terminated PEOs were successfully

obtained.

Dialkyne-terminated PEOs with var-

ious molecular weights were used to

prepare polypseudorotaxaneswith a-CD.

Upon addition of the aqueous solution of

a-CD, white solids were produced in the

aqueous solution of dialkyne-terminated

PEGs, suggesting that supramolecular

inclusion complexation occurred bet-

ween the dialkyne-terminated PEO and

a-CD. 1H NMR spectroscopy was used

to estimate the molar ratio of a-CD to

ethylene oxide (EO) units in the poly-

pseudorotaxanes. As expected, the ratio

was 2:1, which is quite close to those re-

ported in other supramolecular inclusion

complexes involving PEO to a-CD.[30–34]

The above dialkyne-terminated polypseu-

dorotaxanes were reacted with 3-azido-

propylhepta(3,3,3-trifluoropropyl) POSS to

obtain organic/inorganic polyrotaxanes.

At room temperature, the Huisgen 1,3-

Table 1. Molar ratios of ethylene oxide (EO) unit to a-CD inpolyrotaxanes.

Polyrotaxanes Mn (PEG) EO/a-CDa)

1 1000 2.24:1

2 2000 2.26:1

3 4000 2.38:1

4 6000 2.33:1

a)Determined by 1H NMR spectroscopy.

dipolar cycloaddition was carried out in DMF solution with

copper(I) bromide as the catalyst. To suppress the de-

association of a-CD from the polypseudorotaxanes during

the click reaction, a small amount of free a-CD (10 wt.-%

with respect to the polypseudorotaxanes) was added to

the reactive system, since the free a-CD and excessive

3-azidopropylhepta(3,3,3-trifluoropropyl) POSS are easily

eliminated due to the difference in their solubilities with

that of the polyrotaxanes. It was noted that the resulting

products became no longer soluble in hot water with

the occurrence of click chemistry, in marked contrast to

the case of the polypseudorotaxanes obtained from the

dialkyne-terminated PEO and a-CD. It has been proposed

that the solubility of polypseudorotaxanes in hot water

results from the significant deassociation of polypseudor-

otaxanes at elevated temperature.[5,35,36] Here, the inso-

lubility of the products in hot water suggests that the

de-association (or dethreading) of a-CD from polypseudor-

otaxanes could not be achieved due to the presence of

the bulky terminal POSS groups at the ends of the

polyrotaxanes.

Figure 5 shows representative 1H NMR spectra of the

polypseudorotaxane and the polyrotaxane from the PEO

with Mn ¼ 4 000. The resonance assignable to PEO and

a-CD can be clearly detected in the 1HNMR spectrum of the

polypseudorotaxane. It is noted that the signals at 0.92 and

2.25 ppm were discernible, which are ascribed to the

resonance of methylene protons connected to silicon



atoms and 3,3,3-trifluoropropyl groups in the POSS cage.

The resonance at 7.97 ppm is attributed to the protons of

triazole structures resulting from the click reaction. The1H NMR spectra indicate that organic/inorganic polyro-

taxanes were successfully obtained. The molar ratios of

ethylene oxide units to a-CD in the organic/inorganic

polyrotaxanes were estimated in terms of the ratios of

integration intensity of a-CD protons (e.g., H1 at d¼ 4.80) to

methylene protons of PEO at d¼ 3.55; the results are

summarized in Table 1. It can be seen that the molar ratios

of a-CD to ethylene oxide (EO) in the organic/inorganic

polyrotaxanes are slightly higher than 2:1, suggesting that

slight de-association of a-CD occurred during the click

chemistry between the dialkyne-terminated polypseudor-

otaxanes and 3-azidopropylhepta(3,3,3-trifluoropropyl)

POSS.

DOI: 10.1002/macp.200800605


Figure 6. XRD profiles of: a) a-cyclodextrin, b) PEG (Mn ¼ 4000),c) polypseudorotaxanes with Mn (PEO)¼4000, d) polyrotaxanewith Mn (PEO)¼ 1 000, e) polyrotaxane with Mn (PEO)¼ 2 000, f)polyrotaxane with Mn (PEO)¼ 4000, g) polyrotaxane with Mn

(PEO)¼6000.

Figure 7. TGA curves of polypseudorotaxanes.

Crystal Structures of Organic/InorganicPolyrotaxanes

Figure 6 shows X-ray diffraction profiles of a-CD, PEO,

a-CD/dialkyne-terminated PEO inclusion complexes (e.g.,

polypseudorotaxanes) and organic/inorganic polyrotaxanes.

For pure PEO, intense diffraction peaks were observed

at 2u¼ 19.08, 23.21 and 26.208, assignable to (120), (200)

and (240) reflections of PEO[37] (see curve B). The positions

of the diffraction peaks agree well with those reported in

the literature with unit cell parameters (monoclinic,

a¼ 8.05A, b¼ 13.04 A, c¼ 19.48 A). For the polypseudor-

otaxanes, it can be seen that the reflections characteristic

of PEO crystals and a-CD had virtually disappeared, while

new diffraction peaks appeared at 11.9, 13.0, 19.9 and

22.68, suggesting the formation of crystalline supramole-

cular inclusion complexes (see curve C). The crystalline

structures of the inclusion complexes of a-CDs with low-

molecular-weight compounds can be classified as ‘‘cage

type’’ or ‘‘channel type’’.[38–40] The diffraction peaks at

19.87 and 22.618 are characteristics of the crystals of a-CD/PEO ICs adopting a channel-type structure.[35,40] It is noted

that the effect of the terminal POSS groups on the crystal

structures of the organic/inorganic polyrotaxanes is quite

dependent on the molecular weight of PEO used for the

synthesis of the polyrotaxanes. When the molecular



weights of PEO is lower than 4 000, the intensity of

diffraction peaks was significantly diminished, suggesting

the degree of crystallinity was reduced (see curve D and E).

When the molecular weight of PEO is 4 000 or higher, the

polyrotaxanes displayed intense diffraction peaks at 19.95

and 22.68, indicating that the crystals of polyrotaxanes stillfollows a channel-like structure. However, the diffraction

peaks at 2u¼ 11.9 and 19.98 were seen to shift to lower

diffraction angles compared to those of the polypseudor-

otaxanes (see curve E–G). This observation indicates that

the terminal POSS groups have a significant effect on the

structures of the polyrotaxane crystals; the nanosized POSS

terminals can either reduce the crystallinity of the

polyrotaxanes or hinder the dense packing of polyrotaxane

chains, depending on the content of POSS in the organic/

inorganic polyrotaxanes.

Effect of POSS Terminals on Thermal Stability

TGA was applied to evaluate the thermal stability of the

polypseudorotaxanes and the organic/inorganic polyro-

taxanes; their respective TGA curves are presented in

Figure 7 and 8. Within the experimental temperature

range, the thermal degradation of the polypseudorotax-

anes and the hybrid polyrotaxanes displayed similar

multi-step degradation profiles, implying that the exis-

tence of the terminal POSS does not significantly change

the degradation mechanism of the ICs. It can be seen that

the temperatures of initial decomposition for the poly-

pseudorotaxanes and the hybrid polyrotaxanes are all

approximately 310 8C. Nonetheless, the polyrotaxanes


K. Zeng, S. Zheng

Figure 8. TGA curves of polyrotaxanes.

790

exhibited lower rates of degradation at higher tempera-

tures than did the polypseudorotaxanes. This observation

can be ascribed to the formation of organic/inorganic

hybrid nanocomposites. It is plausible to propose that the

segmental decomposition via gaseous fragments of the

polyrotaxanes was significantly suppressed by the nano-

sized blocking agent (i.e., POSS). In addition, it is noted that

the polyrotaxanes showed an increased residue of thermal

degradation, compared to the polypseudorotaxanes, due to

the formation of ceramics.

Conclusion

Organic/inorganic polyrotaxanes were successfully

synthesized from 3-azidopropylhepta(3,3,3-trifluoropro-

pyl) POSS and dialkyne-terminated PEO/a-CD polypseu-

dorotaxanes. The polyrotaxanes were characterized by

means of 1H NMR spectroscopy and WAXRD. It is found

that the existence of the terminal bulky POSS significantly

altered the crystal structure of polyrotaxanes, compared to

their parent polypseudorotaxanes. TGA shows that the

organic/inorganic hybrid nanocomposites exhibited an

increased thermal stability, compared to the correspond-

ing polypseudorotaxanes, in terms of initial temperature

of decomposition and summation of char and ceramic

yields

Acknowledgements: Financial support from the Natural ScienceFoundation of China (nos. 20474038 and 50873059) and the



National Basic Research Program of China (no. 2009CB930400) areacknowledged. The authors thank Shanghai Leading AcademicDiscipline Project (project number B202) for partial support.

Received: December 9, 2008; Revised: February 23, 2009;Accepted: February 24, 2009; DOI: 10.1002/macp.200800605

Keywords: click chemistry; polyhedral oligomeric silsesquiox-anes; polyrotaxanes

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