supramolecular inclusion complexation of polyhedral oligomeric silsesquioxane capped...

Supramolecular Inclusion Complexation ofPolyhedral Oligomeric Silsesquioxane CappedPoly(e-caprolactone) with a-Cyclodextrin

YONG NI, SIXUN ZHENG

Department of Polymer Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240,People’s Republic of China

Received 8 September 2006; accepted 17 November 2006DOI: 10.1002/pola.21893Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Supramolecular inclusion complexes (ICs) involving polyhedral oligomericsilsesquioxane (POSS) capped poly(e-caprolactone) (PCL) and a-cyclodextrin (a-CD)were investigated. POSS-terminated PCLs with various molecular weights were pre-pared via the ring-opening polymerization of e-caprolactone (CL) with 3-hydroxypropyl-heptaphenyl POSS as an initiator. Because of the presence of the bulky silsesquioxaneterminal group, the inclusion complexation between a-CD and the POSS-capped PCLwas carried out only with a single end of a PCL chain threading inside the cavity of a-CD, which allowed the evaluation of the effect of the POSS terminal groups on the effi-ciency of the inclusion complexation. The X-ray diffraction results indicated that the or-ganic–inorganic ICs had a channel-type crystalline structure. The stoichiometry of theorganic–inorganic ICs was quite dependent on the molecular weights of the POSS-capped PCLs. With moderate molecular weights of the POSS-capped PCLs (e.g., Mn

¼ 3860 or 9880), the stoichiometry was 1:1 mol/mol (CL unit/a-CD), which was close tothe literature value based on the inclusion complexation of a-CD with normal linearPCL chains with comparable molecular weights. When the PCL chains were shorter(e.g., for the POSS-capped PCL of Mn ¼ 1720 or 2490), the efficiency of the inclusioncomplexation decreased. The decreased efficiency of the inclusion complexation could beattributed to the lower mobility of the bulky POSS group, which restricted the motion ofthe PCL chain attached to the silsesquioxane cage. This effect was pronounced with thedecreasing length of the PCL chains. VVC 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym

Chem 45: 1247–1259, 2007

Keywords: amphiphiles; biological applications of polymers; channel-type structure;host-guest systems; inclusion chemistry; inclusion complexation; nanocomposites;poly(e-caprolactone); polyhedral oligomeric silsesquioxane; polyrotaxanes

INTRODUCTION

In recent years, considerable interest has beenattracted to investigating the supramolecularchemistry involving organic macromolecules andcyclodextrins (CDs). CDs are a series of cyclic oli-

gosaccharides composed of six, seven, and eightglucose units linked by a-1,4-glucosidic bonds,which are called a-, b-, and c-CDs, respectively.The well-defined ring structures with cone-shaped cavities endow CDs with the ability to actas host molecules to afford inclusion complexes(ICs) with a great variety of guest molecules, in-cluding small molecules1 and long-chain poly-meric molecules. Since Harada et al.2 first re-ported supramolecular ICs of a-CD with poly(eth-

Correspondence to: S. Zheng (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 1247–1259 (2007)VVC 2007 Wiley Periodicals, Inc.

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ylene glycol), the ICs of polymers with CDs havebeen extensively investigated because of their uniquesupramolecular architectures, such as moleculartubes (or polyrotaxanes), and because they are goodmodels for molecular recognition in biological sys-tems.3 A variety of polymeric guests have been foundto be able to form ICswith CDmolecules.1(e),4 A chan-nel-type structure is characteristic for ICs of long-chain polymeric molecule guests with CDs. The driv-ing force for the formation of polyrotaxanes has beenattributed to the intermolecular hydrogen bondingbetween adjacent CDs as well as geometric compati-bility and hydrophobic interactions between CDs andpolymer chains. More recently, the investigation ofthe formation of ICs of CDs with polymeric guestmolecules with various topological structures hasprovoked considerable interest. Random copolymerscontaining nonincludable structural units,5(a) blockcopolymers,4(j),5(b,c) and star-shaped (or hyper-branched) polymers4(p),5(d–h) have been used to pre-pare ICs with CDs.5 One of the purposes of thesestudies is to understand the formation mechanismsof supramolecular ICs in depth.

Aliphatic polyesters such as poly(e-caprolactone)(PCL) and polylactide are a class of important poly-mer materials. Because of their degradability andbiocompatibility, these polymers represent a classof interesting candidates for environmentally be-nign packing materials and for biomedical applica-tions.6 It has been reported that some biodegrad-able polyesters can form crystalline supramolecu-lar ICs with CDs. Harada and coworkers7 reportedthat linear aliphatic polyesters, such as PCL, poly(ethylene adipate), and poly(1,4-butylene adipate),can form crystalline ICs with CDs. Tonelli and cow-orkers systematically investigated the inclusioncomplexation of PCL with a-CD,8(a) the formationof ICs between a PCL-b-PEO-b-PCL triblock copol-ymer [where PEO is poly(ethylene oxide)] and a-CD,8(b) and the effect of IC formation on the mor-phology of a poly(L-lactic acid)-b-PCL diblockcopolymer.4(b)

Polyhedral oligomeric silsesquioxanes (POSSs)are a class of important nanosized, cagelike mole-cules (Scheme 1) derived from the hydrolysis andcondensation of trifunctional organosilanes. POSSmolecules possess a formula of [RSiO3/2]n, wheren is 6–12 and R can be various types of organicgroups, one (or more) of which is reactive or poly-merizable. Polymers incorporated with well-de-fined nanosized POSS cages represent a class ofimportant organic–inorganic nanocomposites.9

Chang et al.4(p) and He et al.10 reported studieson supramolecular ICs of octa-armed star-shaped

PCL and PEO with a-CD. They found that theefficiencies of inclusion complexation for the or-ganic–inorganic, star-shaped polymers werelower than those of their linear counterparts withCDs. It was proposed that the presence of bulkyPOSS cages constituted steric hindrance to theformation of ICs.4(p),10 Nonetheless, it is arguedthat the decreased efficiency of inclusion com-plexation should be ascribed to not only the bulkyPOSS cages but also the specific topological struc-tures of the star-shaped polymers. The decreasedefficiency of inclusion complexation could addi-tionally result from the interarm repulsion inter-actions because multiple arms are densely at-tached to one silsesquioxane core as in the forma-tion of ICs of other starlike, hyperbranched, andcomblike organic polymers with CDs.4(o)

In this work, we first synthesized POSS-terminated PCL with variable molecular weights(Scheme 1) and then investigated the supramolecu-lar inclusion complexation of the POSS-capped PCLwith a-CD. The POSS-capped PCL is a novel amphi-philic polymer because of the presence of organic(viz., PCL chain) and inorganic (viz., silsesquioxanecage) portions. It was expected that organic–inor-ganic, amphiphilic ICs could be prepared with theinclusion complexation of the organic portion (viz.,PCL chain) with a-CD. Because one of the two PCLchain ends was capped with a bulky silsesquioxanecage with a diameter of about 1.0 nm, whichexceeded the diameter of a-CD, the inclusion com-plexation was carried out by the threading of PCLchains inside the cavities of a-CD with only one ofthe two PCL ends. It is of interest to investigate theeffect of single-end threading on inclusion complexa-tion. The preparation of the POSS-terminated PCLprovides an ideal model system to examine theeffect of POSS steric hindrance on the formation ofICs without the interference of the interarm repul-sion interactions of star-shaped polymers. The goalof this work is to provide a solution to differentiatethe two effects. To this end, the stoichiometry andcrystal structure of ICs were investigated by meansof Fourier transform infrared (FTIR) spectroscopy,nuclear magnetic resonance (NMR) spectroscopy,X-ray diffraction (XRD), and thermal analyses.

EXPERIMENTAL

Materials

Phenyltrimethoxysilane [C6H5Si(OCH3)3; 98%]was supplied by Zhejiang Chem-Tech Ltd. Co.

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Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

(Zhejiang, China). 3-Chloropropyltrichlorosilane(ClC3H6SiCl3; 97%) was purchased from Gelest,Inc. (United States), and used as received. Themonomer, e-caprolactone (CL), was purchasedfrom Fluka Co. (Germany), and it was distilledover CaH2 under decreased pressure before use.A control PCL was obtained from Solvay Chemi-cal Co. (United Kingdom), and it had a quoted mo-lecular weight of Mn ¼ 50,000. Silver nitrate(AgNO3), sodium hydroxide (NaOH), and stan-nous(II) octanoate [Sn(Oct)2] were analyticallypure and were purchased from Shanghai ReagentCo. (Shanghai, China). Solvents such as tetrahy-drofuran (THF), dichloromethane (CH2Cl2), etha-nol, and petroleum ether (distillation range ¼ 60–90 8C) were obtained from Shanghai Reagent.Before use, THF was dried by distillation over Naand stored in a sealed vessel with a molecularsieve of 4 A.

Synthesis of HeptaphenyltricycloheptasiloxaneTrisodium Silanolate [Na3O12Si7(C6H5)7]

Heptaphenyltricycloheptasiloxane trisodium sila-nolate was synthesized with the method des-cribed by Fukuda et al.11 Typically, phenyltrime-thoxysilane (91.08 g, 0.46 mol), THF (500 mL),distilled water (10.5 g, 0.58 mol), and NaOH(7.9 g, 0.2 mol) were charged into a three-neckedflask equipped with a reflux condenser and a mag-netic stirrer. The mixture was refluxed in an oilbath at 70 8C for 5 h with vigorous stirring andthen cooled to room temperature. With continuousstirring for an additional 15 h at room tempera-ture, the volatile components were removed via ro-tary evaporation to obtain a white precipitate. Theprecipitate was collected via filtration with filterpaper with a pore diameter of 0.5 lm and washedwith THF at least three times. The white solids

Scheme 1

SUPRAMOLECULAR INCLUSION COMPLEXATION 1249


were then dried in vacuo at 60 8C for 24 h to afford65 g of the products with a yield of 98.9%.

FTIR (cm�1, KBr window): 3075, 1596, 1430,1135–1090 (Si��Ph) 1090–1000 (Si��O��Si). Solid29Si NMR (ppm): �77.64, �75.78, �71.77, �68.38,�66.22.

Synthesis of 3-Chloropropylheptaphenyl POSS[(C6H5)7(ClCH2CH2CH2)Si8O12]

Heptaphenyltricycloheptasiloxane trisodium silano-late (10.00 g, 10 mmol) was dissolved with 250 mL ofanhydrous THF in a three-necked flask, and 3-chlor-opropyltrichlorosilane (3.18 g, 15 mmol) was quicklyadded to the flask. With vigorous stirring, the cornercapping reaction was carried out at 0 8C for 3 h andat room temperature for 3 h. After the removal ofthe volatile components via rotary evaporation, thecrude product was obtained. The product was dis-solved with 100 mL of CHCl3 and washed with100 mL of deionized water three times. The organiclayer was separated out, dried with 5.0 g of anhy-drous MgSO4, and finally precipitated by beingdropped into 2000 mL of methanol. The white solidswere collected by filtration, redissolved in 20 mL ofTHF, and reprecipitated into 1000 mL of methanol.The resulting product (4.65 g) was dried in vacuo at60 8C for 48 h with a yield of 45%.

FTIR (cm�1, KBr window): 3075, 1596, 1430,1135–1090 (Si��Ph), 1090–1000 (Si��O��Si),2956 (��CH3), 2934 (��CH2).

1H NMR (ppm,CDCl3): 7.72–7.79 (m, 14.2H, protons of aromaticring), 7.33–7.51 (m, 21.4H, protons of aromaticring), 3.52 (t, SiCH2CH2CH2Cl, 2.0H), 1.99 (m,SiCH2CH2CH2Cl, 2.0H), 1.00 (m, SiCH2CH2

CH2Cl, 2.0H). 13C NMR (CDCl3, ppm): 9.77 (SiCH2CH2CH2Cl); 26.54 (SiCH2CH2CH2Cl); 47.40(SiCH2CH2CH2Cl); 128.14, 128.20, 130.38, 130.49,131.06, 131.13, 134.40, 134.45 (carbons of aromaticrings). 29Si NMR (ppm):�64.06,�77.02.

Synthesis of 3-Hydroxypropylheptaphenyl POSS[(C6H5)7(HOCH2CH2CH2)Si8O12]

3-Hydroxypropylheptaphenyl POSS was synthe-sized by the hydrolysis of 3-chloropropylhepta-phenyl POSS in the presence of fresh silver oxide(Ag2O; Scheme 1). In the first step, Ag2O was pre-pared. AgNO3 (0.2468 g, 1.45 mmol) was dis-solved in 10 mL of deionized water, and 10 wt %aqueous NaOH (0.0581 g, 1.45 mmol) was slowlydropped into the solution with vigorous stirring.The Ag2O precipitates were isolated and washed

with deionized water three times. After that, toa round-bottom flask, 3-chloropropylhepta-phenyl POSS (1.0 g, 0.97 mmol), ethanol (25mL), and THF (25 mL) were charged to afford atransparent solution, and then the aforemen-tioned newly prepared Ag2O together with 0.5mL of deionized water was added to the system.The reactive system was refluxed for 48 h withvigorous stirring in the dark. All the solids werefiltered out, and aqueous nitric acid (20 wt %)was added to remove any unreacted Ag2O. Theremaining insolubility (viz., AgCl) was driedand collected. The procedures were repeatedthree times to access a complete substitution ofchlorine atoms of 3-chloropropylheptaphenylPOSS by hydroxyl groups. All the insolubility(i.e., AgCl) was recovered together and weighedto be 0.1338 g, and thus the conversion of thesubstitution reaction was estimated to be96.3%. The portion of the solution was subjectedto rotary evaporation, and 0.87 g (88.4% yield)of a white solid was obtained after drying at60 8C in a vacuum oven for 24 h.

FTIR (cm�1, KBr window): 3075, 1596, 1430,1135–1090 (Si��Ph), 1090–1000 (Si��O��Si), 2956,2934 (��CH2), 3440 (O��H). 1H NMR (ppm, CDCl3):7.72–7.79 (m, 14.2H, protons of aromatic rings),7.33–7.51 (m, 21.2H, protons of aromatic rings), 3.52(t, SiCH2CH2CH2OH, 2.0H), 1.99 (m, SiCH2CH2

CH2OH, 2.0H), 1.00 (m, SiCH2CH2CH2OH, 2.0H).13C NMR (ppm, CDCl3): 9.75 (SiCH2CH2CH2OH),26.53 (SiCH2CH2CH2OH), 47.41 (SiCH2CH2

CH2OH), 128.12, 128.20, 130.38, 130.50, 131.06,131.14, 134.38, 134.44 (carbons of aromatic rings).29Si NMR (ppm):�64.06,�77.02.

To confirm the formation of 3-hydroxypropyl-heptaphenyl POSS, the derivative of the afore-mentioned product was prepared and subjected to1H NMR analysis. The as-obtained product (1.0 g,0.99 mmol), anhydrous THF (10 mL), and tri-ethylamine (1.0 g, 9.9 mmol) were charged into athree-necked, round-bottom flask. Chlorotrime-thylsilane (1.08 g, 9.9 mmol) was added dropwiseto the flask with a dropping funnel. The mixturewas reacted at 0 8C for 3 h and at room tempera-ture for 24 h with vigorous stirring. After theinsolubility was removed, the volatile componentswere removed via rotary evaporation to obtain asolid product, which was further dried in a vac-uum oven at 40 8C for 24 h with a yield of 92%.

1H NMR (ppm, CDCl3): 7.72–7.79 (m, 14.2H,protons of aromatic rings), 7.33–7.51 (m, 21.2H,protons of aromatic rings), 3.52 [t, SiCH2CH2-

CH2OSi(CH3)3, 2.0H], 1.99 [m, SiCH2CH2CH2O-

1250 NI AND ZHENG


Si(CH3)3, 2.0H], 1.00 [m, SiCH2CH2CH2OSi(CH3) 3,2.0H], 0.09 [s, SiCH2CH2CH2OSi(CH3)3, 8.8H].

Synthesis of POSS-Terminated PCL

The ring-opening polymerization (ROP) of CL cat-alyzed by Sn(Oct)2 was used to prepare the PCLterminated with POSS. The ROP was carried outon a standard Schlenk line system. Typically, toa predried flask connected to a Schlenk line, 3-hy-droxypropylheptaphenyl POSS (2 g, 1.97 mmol)and CL (2.92 g, 25.64 mmol) were charged, and acatalytic amount of Sn(Oct)2 was added. The reac-tive mixture was degassed via three pump–thaw–freeze cycles and then immersed in a thermo-stated oil bath at 110 8C for 24 h. The crude prod-ucts were dissolved with CH2Cl2 and precipitatedwith an excessive amount of petroleum ether.This procedure was repeated three times. Theprecipitates were filtered and dried in vacuo at40 8C for 48 h to give 4.797 g of the polymer witha yield of 97.5 wt %.

1HNMR (ppm, CDCl3): 7.72–7.79 (m, protons of ar-omatic rings); 7.33–7.51 (m, protons of aromatic rings);3.52 (SiCH2CH2CH2��PCL); 1.99 (SiCH2CH2 CH2

��PCL); 1.00 (SiCH2CH2CH2��PCL); 4.02–4.10[��OCO(CH2)4CH2]; 3.61–3.68 (CH2OH, terminalhydroxymethylene group); 2.26–2.35 [��OCO CH2

(CH2)4]; 1.58–1.70 (��OCOCH2CH2CH2CH2CH2);1.32–1.44, (��OCOCH2CH2CH2CH2CH2).

Preparation of ICs

The ICs of POSS-capped PCL with a-CD wereprepared as follows. Typically, POSSPCL1 (0.249g, 1.27 mmol in CL unit) was dissolved in 15 mLof acetone at 50 8C, and the solution was addeddropwise to a saturated aqueous solution of a-CDat 60 8C with vigorous stirring. With stirring at60 8C for an additional 6 h, the mixture wascooled to room temperature and left to stand over-night with vigorous stirring. The precipitateswere collected by filtration. The crude productswere washed with acetone three times to removefree polymers and then washed with distilledwater three times to remove uncomplexed a-CD.The white powder was dried in vacuo at 40 8C fortwo days.

Measurement and Techniques

NMR Spectroscopy

The 1H and 13C NMR measurements were carriedout on a Varian Mercury Plus 400-MHz NMR

spectrometer at 25 8C. The samples were dis-solved with deuterated CDCl3, and the solutionswere measured with tetramethylsilane (TMS) asan internal reference. The high-resolution solid-state 29Si NMR spectra were obtained with crosspolarization and magic angle spinning (CPMAS)together with the high-power dipolar decoupling(DD) technique. The 908-pulse width of 4.1 ls wasemployed for free induction decay signal accumu-lation, and the cross polarization (CP) Hart-mann–Hahn contact time was set at 3.5 ms for allthe measurements. The rate of MAS was 4.0 kHzfor measuring the spectra. The Hartmann–HahnCP matching and DD field was 57 kHz. The chem-ical shifts of all 13C CPMAS NMR spectra weredetermined from the carbons of adamantine withrespect to TMS, whereas those of all 29Si spectrawere determined from the silicon of solid Q8M8

with respect to TMS as an external referencestandard.

FTIR Spectroscopy

The FTIR measurements were conducted on aPerkinElmer Paragon 1000 Fourier transformspectrometer at room temperature (25 8C). Thesample films were prepared by the dissolution ofthe polymers with CHCl3 (5 wt %), and the solu-tions were cast onto KBr windows. The residualsolvent was removed in a vacuum oven at 60 8Cfor 2 h. For the ICs, the powder samples weremixed with a pellet of KBr and were pressed intodisk flakes. All the specimens were sufficientlythin to be within a range in which the Beer–Lambert law is obeyed. In all cases, 64 scans at aresolution of 2 cm�1 were used to record the spectra.

Gel Permeation Chromatography (GPC)

The molecular weights of the polymers weremeasured on a PerkinElmer 200 GPC instrumentwith a PL mixed-B10m column. Polystyrene wasused as the standard, and THF (THF) was usedas the eluent at a flow rate of 1 mL/min.

Wide-Angle XRD

The wide-angle XRD experiments were carriedout on a Shimadzu XRD-6000 X-ray diffractome-ter with Cu Ka (k ¼ 0.154 nm) irradiation at40 kV and 30 mA with a Ni filter. Data wererecorded in the range of 2h ¼ 5–408 at the scan-ning rate and step size of 4.0 8/min and 0.028,respectively.



Thermogravimetric Analysis (TGA)

A PerkinElmer TGA-7 thermogravimetric ana-lyzer was used to investigate the thermal stabilityof the samples. The samples (ca. 10 mg) wereheated in a nitrogen atmosphere from the ambi-ent temperature to 800 8C at a heating rate of20 8C/min in all cases.

RESULTS AND DISCUSSION

Synthesis of the Polymer

The synthetic route of the organic–inorganic PCLend-capped with POSS is described in Scheme 1.In this work, the initiator for preparing mono-POSS-capped PCL was 3-hydroxypropylhepta-phenyl POSS. To prepare 3-hydroxypropylhepta-phenyl POSS, the starting compound heptaphe-nyltricycloheptasiloxane trisodium silanolate wasused, and it was prepared with the literaturemethod described by Fukuda et al.11 Readilyavailable heptaphenyltricycloheptasiloxane triso-dium silanolate was corner-capped with 3-chloro-propyltrichlorosilane to afford 3-chloropropylhep-taphenyl POSS. After that, the nucleophilic sub-stitution of chlorine atoms by hydroxyl groups in3-chloropropylheptaphenyl POSS was employed

to achieve the transformation of 3-chloropropyl-heptaphenyl POSS into 3-hydroxypropylhepta-phenyl POSS. To promote the nucleophilic substi-tution to completion., the reaction was carried outin the presence of freshly prepared Ag2O. Shownin Figure 1(C) is the FTIR spectrum of 3-hydrox-ylpropylheptaphenyl POSS. The appearance ofstretching vibration bands at 3640 and 3440 cm�1

is indicative of the presence of hydroxyl groups,which is in marked contrast to the FTIR spectrumof 3-chloropropylheptaphenyl POSS, as shown inFigure 1(B), in this range of frequencies. Theband at 3640 cm�1 is ascribed to the stretchingvibration of free hydroxyl groups, whereas theband at 3440 cm�1 is attributed to that of self-associated hydroxyl groups. The 1H spectra of 3-chloropropylheptaphenyl POSS and 3-hydroxyl-propylheptaphenyl POSS together with POSS-terminated PCL are shown in Figure 2. In Figure2(A,B), it can be seen that the spectra of 3-chloro-propylheptaphenyl POSS and 3-hydroxylpropyl-heptaphenyl POSS are composed of the resonanceof the protons from aromatic rings and aliphatichydrocarbon groups (viz., CH2). In terms of theintegration intensity of protons in the aliphaticmethylene moiety to protons of aromatic rings, itis judged that the POSS compounds possess one3-chloropropyl (or 3-hydroxypropyl) group andseven phenyl groups. In the 1H NMR spectrum[Fig. 2(B)], a ramp resonance centered at 1.54ppm is discernible for 3-hydroxylpropylhepta-phenyl POSS, which can be ascribed to the pro-tons of the hydroxyl group. The ramp resonance

Figure 1. FTIR spectra: (A) heptaphenyltricycloheptasi-loxane trisodium silanolate, (B) 3-chloropropylheptaphenylPOSS, (C) 3-hydroxypropylheptaphenyl POSS, and (D)POSSPCL1.

Figure 2. 1H NMR spectra: (A) 3-chloropropylhepta-phenyl POSS, (B) 3-hydroxypropylheptaphenyl POSS,(C) 3-chlorotrimethylsilyether propylheptaphenyl POSS,and (D) POSSPCL1.

1252 NI AND ZHENG


could be responsible for the proton exchangebetween the hydroxyl groups and the traceamount of deuterium water in the deuterated sol-vents (CDCl3). It is not easy to determine the con-tent of 3-hydroypropyl groups according to theintegration intensity of this resonance because ofthe presence of the trace amount of deuteriumwater. To confirm the formation of 3-hydroxypro-pylheptaphenyl POSS, the derivative of the afore-mentioned product was prepared. 3-Hydroxypro-pylheptaphenyl POSS, obtained via the nucleo-philic substitution of chlorine atoms by hydroxylgroups, was allowed to react with chlorotrime-thylsilane, and the derivative was subjected to 1HNMR analysis [Fig. 2(C)]. The appearance of theresonance at 0.09 ppm indicates the presence ofthe trimethylsiloxyl group in the product. Interms of the ratios of the integration intensity ofprotons of the trimethylsiloxyl group to those ofthe aromatic ring, it is judged that the hydroxylgroups of 3-hydroxypropylheptaphenyl POSSwere almost totally capped with the trimethylsi-loxyl group. This result further confirms the for-mation of 3-hydroxypropylheptaphenyl POSS,which is in good agreement with the estimationaccording to the quantity of the insoluble AgClsolids produced because of the nucleophilic sub-stitution.

Shown in Figure 3 are the 29Si CPMAS NMRspectra of heptaphenyltricycloheptasiloxane triso-dium silanolate, 3-chloropropylheptaphenyl POSS,and 3-hydroxylpropylheptaphenyl POSS. Multi-spectral lines of resonance were observed for silicon

nuclei in heptaphenyltricycloheptasiloxane triso-dium silanolate. Only two lines of resonance weredisplayed with the occurrence of a corner cappingreaction. The resonances at �77.05 and �64.05ppm were assigned to the nucleus of silicon con-nected to phenyl and 3-chloropropyl (or 3-hydroxy-propy) groups, respectively. The fact that no signifi-cant change in the chemical-shift spectra was foundin the 29Si NMR spectra after and before the nucle-ophilic substitution indicates that the cage arrange-ment of silsesquioxane did not occur under thisreaction condition. On the basis of these results, itis judged that the target compound (viz., 3-hydrox-ylpropylheptaphenyl POSS) was successfullyobtained.

To obtain the POSS-terminated PCL via theROP of CL, 3-hydroxypropylheptaphenyl POSSwas used as an initiator, and Sn(Oct)2 was usedas a catalyst to carry out the polymerization ofCL. The polymerization in bulk was carried out at110 8C for 24 h. Through the control of the molarratios of 3-hydroxypropylheptaphenyl POSS toCL, a series of POSS-terminated PCLs with vari-ous molecular weights were obtained. Figure 1(D)presents the FTIR spectrum of POSSPCL1. Thestretching vibration bands of hydroxyl groups inthe range of 3200–3500 cm�1 virtually disap-peared, and this implied that the number of ter-minal hydroxyl groups of POSS-terminated PCLwas too small to be detected. The appearance of acarbonyl stretching vibration band at 1725 cm�1

together with the detection of Si��O��Si bands inthe range of 1090–1000 cm�1 indicated that thesyntheses of POSS-terminated PCLs were suc-cessfully conducted. Shown in Figure 2(D) is the1H NMR spectrum of the POSSPCL1 sample. Theresonance of the phenyl moiety in POSS is dis-cernible in the NMR spectrum, indicating thepresence of the POSS moiety. The results of thepolymerization are summarized in Table 1, andthe plots of the molecular weights and molecularweight distributions as functions of the molar ra-tio of CL to 3-hydroxypropylheptaphenyl POSSare presented in Figure 4. Under this condition ofpolymerization, the conversion of the monomerwas performed to completion, and this suggeststhat 3-hydroxypropylheptaphenyl POSS is anefficient initiator. The observation that the exper-imental molecular weights were in good agree-ment with the theoretical values calculated ac-cording to the molar ratios of 3-hydroxypropyl-heptaphenyl POSS to CL indicates that themolecular weight of the POSS-ended PCL can becontrolled; that is, the polymerization is living.

Figure 3. 29Si NMR spectra: (A) heptaphenyltricyclo-heptasiloxane trisodium silanolate, (B) 3-chloropropyl-heptaphenyl POSS, and (C) 3-hydroxypropylheptaphenylPOSS.



Supramolecular Inclusion Complexationof POSS-Terminated PCL with a-CD

In this work, POSS-capped PCL samples withvarious molecular weights were used to preparethe ICs. The lengths of the PCL chains togetherwith the overall molecular weights of the POSS-terminated PCLs are summarized in Table 2.With an acetone solution of POSS-terminatedPCL being added to a saturated aqueous solutionof a-CD, the system immediately became turbid,and the precipitates were then produced; thisindicated that the inclusion complexationoccurred between POSS-terminated PCLs and a-CD. The precipitates were washed with acetoneand distilled water to remove any uncomplexedcomponents, and all the yields of the ICs were inthe neighborhood of 50 wt %.

The crystal structures of the ICs of POSS-capped PCL with a-CD were investigated bymeans of XRD. The diffraction patterns of a-CD,POSS-terminated PCL (e.g., POSSPCL1), andtheir IC are shown in Figure 5. For comparison,the XRD curve of a control PCL was also incorpo-rated; it was measured under identical condi-tions. The positions of the diffraction peaks forthe control PCL agree well with those reported inthe literature with unit cell parameters (ortho-rhombic, a ¼ 7.47 A, b ¼ 4.98 A, c ¼ 17.05 A).12

Intense diffraction peaks at 2h values of 21.21,21.81, and 23.528 were observed for the controlPCL, corresponding to (110), (111), and (200)reflections of PCL.13 For the POSS-terminated

PCL, the diffraction peaks were found at 2h val-ues of 21.40, 22.03, and 23.758, respectively,which were close to those of the control PCL.However, the diffraction peaks of the POSS-ter-minated PCL were significantly shifted to highervalues of 2h, and this suggested that the crystalstructures of the POSS-terminated PCL werechanged to some extent. It is plausible to proposethat during crystallization, the bulky POSSblocks cannot be rejected from the front of crystalgrowths, and thus the presence of the bulkyPOSS cages could alter the structure of the PCLcrystals. It should be pointed out that a new dif-fraction peak at 2h ¼ 7.288 was found for thePOSS-terminated PCL. This reflection could beascribed to the POSS portion of the sample.Because of the immiscibility between the PCLchains and POSS groups, the terminal POSSgroups could be self-assembled into microdo-mains, which displayed a diffraction profile simi-lar to that of octaphenyl POSS.14

Returning to Figure 5, we note that the XRDpattern of the IC is not merely a simple superposi-tion of those of POSS-terminated PCL and a-CD.The characteristic reflections corresponding tothe PCL crystalline phase and a-CD significantlydisappeared, whereas some new strong diffrac-tion peaks appeared at 20.0 and 22.58, respec-tively. In terms of the appearance of the sharp

Figure 4. Plots of the molecular weight and molecu-lar weight distribution as functions of the molar ratio ofCL to 3-hydroxypropylheptaphenyl POSS. The opensymbols represent the data of the molecular weightsdetermined by 1H NMR spectroscopy, whereas the filledsymbols stand for the data determined by GPC.

Table 1. Results of the ROP of CL with3-Hydroxypropylheptaphenyl POSS as theInitiator and Sn(Oct)2 as the Catalyst

Polymer [CL]/[I]a Mn,theob Mn Mw/Mn

dYield(%)

POSSPCL0 4/1 1,410 1,720c — 96.0POSSPCL1 13/1 2,430 2,490c — 97.5POSSPCL2 27/1 3,970 3,860c — 97.1POSSPCL3 80/1 9,760 9,880c — 96.3POSSPCL4 135/1 15,880 16,400d 1.30 96.8POSSPCL5 350/1 39,440 40,940d 1.24 96.4

a I¼ initiator (3-hydroxypropylheptaphenyl POSS).bMn,theo is the theoretical number-average molecular

weight of the PCL polymers: Mn,theo ¼ ([M]/[I] � MCL þ MI) �Yield, where MCL is the molecular weight of CL and MI is themolecular weight of the initiator.

cMn,NMR was determined on the basis of the integral ratioof the methylene signal on the polymer backbone (��CH2,2.10–2.40 ppm) to the signal on the primary hydroxy methyl-ene end group (HOCH2, 3.60 ppm).

d Themolecular weights (Mn andMw) were determined byGPC.

1254 NI AND ZHENG


diffraction peaks, it is judged that the ICs ofPOSS-capped PCL with a-CD are crystalline. Thenew diffraction peaks at 20.0 and 22.58 are char-acteristic of the crystals of the a-CD/PCL ICadopting the channel-type structure, indicatingthat the ICs of POSS-terminated PCL with a-CDwere formed with the PCL chain threading insidethe cavities of a-CD.15 It is of interest that thepresence of the silsesquioxane cage at the PCLchain end did not change the columnar structuresof the IC. It should be pointed out that for the ICof the POSS-terminated PCL with a-CD, thereflection of POSS was still discernible and wascentered at 7.48, indicating the presence of themicrodomains of POSS in the ICs.

To examine the stoichiometry of the ICs ofPOSS-capped PCL with a-CD, the ICs were dis-solved in DMSO-d6 and subjected to 1H NMRmeasurements. Figure 6 representatively shows

the 1H NMR spectrum of the ICs of POSSPCL0with a-CD. The molar ratios of CL structuralunits to a-CD in the ICs of a-CD with POSS-capped PCLs of various molecular weights weredetermined according to the ratios of the integra-tion intensity of the proton resonance of the a-CDmoiety to that of the PCL portion, and the resultsare summarized in Table 2. For the ICs of a-CDwith POSS-capped PCLs with longer PCL chains(i.e., POSSPCL2 and POSSPCL3), the molarratios of CL units to a-CD are 1.06 and 1.08,respectively. The two values are quite close to 1:1and are significantly consistent with the valuesreported in the literature for a-CD with linearPCL without any bulky end groups.7(c),8(a) The ex-perimental results indicate that the presence ofthe bulky silsesquioxane cage at one PCL chainend does not significantly change the stoichiome-try of inclusion complexation. In other words, thestoichiometric values of the ICs were not affectedeven when the inclusion complexation was car-ried out by PCL chain threading inside cavities ofa-CD with only one chain end. This result is im-portant for understanding the mechanism of theinclusion complexation of PCL with a-CD. It isworth pointing out that the aforementioned judg-ment on the basis of POSS-capped PCL with a-CD

Figure 5. XRD curves: (A) control PCL, (B) POSSPCL1,(C) a-CD, and (D) IC of a-CDwith POSSPCL1.

Figure 6. 1H NMR spectrum of the ICs betweenPOSSPCL0 and a-CD.

Table 2. Stoichiometry (CL Unit/CD) of the ICs

Polymer Mn,NMR

Length of thePCL Chain Yield (wt %)

CL/CD(mol/mol)

POSSPCL0 1720 706 42.3 1.21:1POSSPCL1 2490 1476 55.6 1.12:1POSSPCL2 3860 2846 52.1 1.08:1POSSPCL3 9880 8866 61.8 1.06:1



is different from those based on the inclusion com-plexation of star-shaped (or hyperbranched) PCLwith a-CD.4(p,q) In the inclusion complexation ofa-CD with star-shaped (or hyperbranched) PCL,the stoichiometry of inclusion complexation couldbe affected by the following two factors. On theone hand, the inclusion complexation is carriedout by the PCL threading inside a-CD with onlyone chain end. On the other hand, the efficiencyof inclusion complexation could be reduced by thepresence of adjacent arms, which are denselyattached to one core. The design of this POSS-capped PCL is helpful for segregating the effect ofthe terminal group of PCL on inclusion complexa-tion. It should be pointed out that the stoichiome-tries of the ICs for a-CD with POSS-terminatedPCL with the shorter PCL chains (viz., POSSPCL0and POSSPCL1) are 1.21:1 and 1.12:1, respectively,which are much higher than the value of linearPCL without the bulky terminal groups and a-CDICs (viz., 1:1 CL unit/a-CD).2 The decreased effi-ciency of the inclusion complexation of the twosamples with a-CD could result from the followingfactors. First, the number of CDs allowing thread-ing is too small, and the ICs cannot be stabilized ifthe PCL chains are too short. This case is just likethe inclusion complexation of comblike PEO withshort PEO chains with a-CD.4(o) Second, the de-creased efficiency of inclusion complexation can beattributed to the lower mobility of the bulky POSSgroup, which restricts the motion of the PCL chain.This effect is pronounced when the PCL chains areshort.

Solid-state NMR spectroscopy is a powerfultool for investigating the formation of ICs. Shownin Figure 7 are the solid-state 13C CPMAS/high-power decoupling NMR spectra of POSS-termi-nated PCL, a-CD, and their ICs. The 13C CPMASNMR spectrum of IC0 cannot be taken as a simplesuperposition of those of POSSPCL0 and a-CD.The spectral lines of the 13C CPMAS NMR spec-tra of the ICs seem to be unresolved. For a-CD, its13C CPMAS NMR spectrum exhibits multiplespectral lines due to the C1 and C4 resonancefrom each of the six a-1,4-linked glucose resi-dues.16 Nonetheless, the resonance of C1 and C4nuclei of the a-CD portion in the IC presentssharp and symmetric signals, which suggest thatthe chemical shielding environments of C1 andC4 nuclei in each glucose unit are identical.4(f),17

Therefore, it is judged that the a-CD portionadopts a symmetric cyclic conformation in theIC.15(a) For POSSPCL0, the sharp signal at 175ppm is assigned to the resonance of the carbon

nucleus of PCL carbonyl groups. It is noted thatthis resonance was split into multiple componentswith the formation of the IC [see Fig. 7(C)]. Thesplitting of the resonance of the carbon nucleusfor PCL carbonyl groups could be interpreted onthe basis of the change in the shielding environ-ments of the nucleus with the formation of the IC.From the value of the stoichiometry (Table 2), it isproposed that there are at least two types of PCLstructural units in the IC. The first are the struc-tural units in the PCL chains that thread insidethe cavities of a-CD, whereas the others are thePCL segments that remained uncomplexed witha-CD. It is plausible to propose that the two typesof PCL structural units have quite different mobi-lities, and thus the shielding environments of theircarbon nuclei are different. For the ICs of a-CDwith the POSS-terminated PCL having longer PCLchains (e.g., POSSPCL2 and POSSPCL3), the split-ting of the carbon resonance for the carbonylgroups became less pronounced [see Fig. 7(D,E)]because all the PCL structural units were almost

Figure 7. Solid-state 13C CPMAS NMR spectra: (A)POSSPCL0, (B) a-CD, (C) IC of POSSPCL0 with a-CD,(D) IC of POSSPCL2 with a-CD, and (E) IC of POSSPCL3with a-CD.

1256 NI AND ZHENG


included into the cavities of a-CD. It is worth notic-ing that the resonance of the carbonyl carbonnuclei for C1 and C2 became quite feeble in com-parison with the resonance of other carbon nucleiof the PCL portion in the ICs, and this implies thequite low efficiency of CP at these carbon-13 nuclei.It is proposed that with the formation of ICs, thestructural units of PCL are trapped inside the cav-ities of a-CD and the CP between adjacent protonsand the unprotonated carbons of carbonyl groupsbecomes difficult.

Figure 8 shows the FTIR spectra of a-CD,POSSPCL1, and their ICs. The POSS-terminatedPCL was characterized by a band at 1725 cm�1,which was assigned to the stretching vibration ofthe carbonyl group of the PCL chain [see Fig. 8(B)].The expanded spectrum in the range of 1680–1800cm�1 shows that the asymmetrical band was in real-ity composed of two components at 1735 and 1725cm�1, which were sensitive to the conformation ofthe PCL chain. The former was ascribed to thestretching vibration of the carbonyl groups in theamorphous region, whereas the latter was attrib-uted to that of the carbonyl groups in the crystallineregion.18 The a-CDwas characterized by the stretch-ing vibration of its hydroxyl groups at 3369 cm�1. Incomparison with POSSPCL1 and a-CD, the FTIRspectrum of IC1 shows a feature that is indicative ofthe formation of inclusion complexation. By compar-ing the stretching vibration bands of the carbonylgroup in the range of 1800–1680 cm�1 [see Fig.8(B,C)], we found that the stretching vibration (at1725 cm�1) of the carbonyl groups in the crystalline

region of PCL disappeared, whereas the intensity ofthe band at 1735 cm�1 was significantly increased.This observation indicates that no crystals of PCLwere found in the ICs; that is, the PCL chains werethreaded inside the cavities of a-CD. It is proposedthat the fact that no crystals of PCL were present inthe IC resulted from the confinement of a-CD to thecrystallization of PCL chains. It should be pointedout that there was a weak shoulder band at 1710cm�1 in the FTIR spectrum of the IC. The appear-ance of the shoulder band could be attributed to theformation of the hydrogen-bonding interactionsbetween the hydroxyl groups of a-CD and the car-bonyl groups of uncomplexed PCL segments. Thisresult is identical to the observation of the FTIRspectrum in the stretching vibration range ofthe hydroxyl groups. The stretching vibration of a-CD hydroxyl groups [viz., centered at 3369 cm�1;Fig. 8(A)] shifted to the higher frequency [at 3378cm�1; Fig. 8(C)], and this implied the association ofthe hydroxyl groups with carbonyl groups.

TGA was applied to evaluate the thermal sta-bility of POSS-terminated PCL, a-CD, and theirICs. Shown in Figure 9 are the TGA curves ofPOSSPCL1, a-CD, and their IC recorded in anitrogen atmosphere. For a-CD, the initial decom-position temperature, which was defined as the5% mass loss temperature, was 310 8C, and thechar yield at 800 8C was approximately 9.4 wt %.For POSSPCL1, the initial decomposition oc-curred at 348 8C, which was much higher than

Figure 8. FTIR spectra: (A) a-CD, (B) POSSPCL1,and (C) ICs of a-CD and POSSPCL1. The inset presentsan expansion of the carbonyl absorption region.

Figure 9. TGA curves: (A) a-CD, (B) ICs of a-CD andPOSSPCL1, and (C) POSSPCL1.



that of a-CD. POSSPCL1 displayed a much higherresidue (i.e., 28 wt %) of decomposition at 800 8Cfrom char and ceramic yields. For the IC, the ini-tial decomposition temperatures and the residueyield were intermediate between those of the twopure components, and this suggested that the ICswere fairly stable.

CONCLUSIONS

The supramolecular inclusion complexation ofPOSS-capped PCL with a-CD was investigated.The POSS-terminated PCL was prepared via theROP of CL with 3-hydroxypropylheptaphenylPOSS as an initiator, which was prepared via thehydrolysis of 3-chloropropylheptaphenyl POSS inthe presence of fresh Ag2O. The POSS-terminatedPCLs with Mn values of 1720, 2490, 3860, and9880 were used to form the ICs with a-CD. XRDresults indicated that the POSS-containing ICshad a channel-type crystalline structure. Bymeans of 1H NMR spectroscopy, the stoichiometryof the ICs was determined. The stoichiometryof the ICs with moderate molecular weights (Mn

¼ 3860 or 9880) was close to 1:1 mol/mol (CL unit/a-CD), although the inclusion complexation wascarried out by the PCL threading inside a-CDwith only one chain end. Nonetheless, the effi-ciency of the inclusion complexation decreasedwhen the PCL chains were shorter (e.g., for thePOSS-capped PCLs with Mn ¼ 1720 or 2490). It isproposed that the lower efficiency of the inclusioncomplexation results from the short PCL chainand the lower mobility of the bulky silsesquioxanegroup, which restricts the motion of the PCLchain attached to the POSS cage.

The authors are grateful to Akira Harada of OsakaUniversity for his helpful and enlightening discussionduring his visit to Shanghai Jiao Tong University inNovember 2005. The financial support of the ShanghaiScience and Technology Commission (China) under akey project (no. 02DJ14048) is acknowledged. Theauthors also acknowledge the Natural Science Founda-tion of China (project nos. 20474038, 50503013 and50390090) and the Shanghai Educational DevelopmentFoundation (China) for partial support under a Shu-guang Scholar award (2004-SG-18).

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