supramolecular inclusion complexes of star-shaped poly(ε-caprolactone) with α-cyclodextrin

Supramolecular Inclusion Complexes of Star-ShapedPoly(e-caprolactone) with a-Cyclodextrin

LU WANG, JING-LIANG WANG, CHANG-MING DONG

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

Received 20 May 2005; Revision accepted 15 June 2005; accepted 1 July 2005DOI: 10.1002/pola.20999Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Both star-shaped poly(e-caprolactone) (PCL) having 4 arms (4sPCL) and6 arms (6sPCL) and linear PCL having 1 arm (LPCL) and 2 arms (2LPCL) were syn-thesized and then investigated for inclusion complexation with a-cyclodextrin (a-CD).The supramolecular inclusion complexes (ICs) were in detail characterized by 1HNMR, differential scanning calorimetry, thermogravimetric analysis, wide angleX-ray diffraction, solid-state carbon nuclear magnetic resonance spectroscopy usingcross-polarization and magic-angle spinning, and Fourier transform infrared, respec-tively. The stoichiometry (CL:CD, mol:mol) of all ICs increased with the increasingbranch arm of PCL polymers, and it was in the order of a-CD-6sPCL1 ICs > a-CD-4sPCL ICs > a-CD-2LPCL ICs > a-CD-LPCL ICs. All analyses indicated that thebranch arms of star-shaped PCL polymers were included into the hydrophobic a-CDcavities and their original crystalline properties were completely suppressed. More-over, the ICs of star-shaped PCL with a-CD had a channel-type crystalline structuresimilar to that formed between the linear PCL and a-CD. Furthermore, the thermalstability of the free PCL polymers probably controlled that of the guest polymersincluded in the ICs. VVC 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43:

4721–4730, 2005

Keywords: cyclodextrin; star-shaped poly(e-caprolactone); stoichiometry; supramo-lecular inclusion complexes; thermal property

INTRODUCTION

Supramolecular inclusion complexes (ICs) formedbetween cyclodextrins and polymers haveattracted much attention because of the poten-tial biomedical applications in drug delivery sys-tems and tissue engineering.1–5 Cyclodextrins(CDs) constitute a series of cyclic oligosacchar-

ides composed of 6, 7, and 8 D-glucose units linkedby a-1,4 bonds and named a-, b-, and c-CD,respectively. The geometry of CDs is like a hol-low truncated cone forming a hydrophobic cav-ity, which have already been found to form ICswith both hydrophilic and hydrophobic poly-mers.1–17 For examples, Harada et al. reportedthe crystalline ICs of CDs with hydrophilic poly-mers such as poly(ethylene glycol), poly(pro-pylene glycol), poly(tetrahydrofuran), and poly(methyl vinyl ether).18–20 Li et al. preparedthe ICs of poly(propylene oxide)-b-poly(ethyleneoxide)-b-poly(propylene oxide) triblock copoly-mers with a-CD, and it was demonstrated thata-CD molecules could slide over the flankingbulky poly(propylene oxide) (PPO) block and

This article contains supplementary material availablevia the Internet at http://www.interscience.wiley.com/jpages/0887–624X/suppmat.

Correspondence to: C.-M. Dong (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 4721–4730 (2005)VVC 2005 Wiley Periodicals, Inc.

4721

then formed stable ICs with the middle poly(ethylene oxide) (PEO) block.10,11 Recently, a sti-muli-sensitive ICs formed between polyethyleni-mine-b-PEO-b-polyethylenimine and a-CD wasdesigned.21

Biodegradable aliphatic polyesters such as poly-lactides, poly(e-caprolactone) (PCL), and theircopolymers are increasingly investigated world-wide for pharmacological, biomedical, agricul-tural, and environmental purposes. Interestingly,different research groups have extensively inves-tigated the ICs of biodegradable polymers withCDs since Harada’s group first reported the com-plex formation of poly(e-caprolactone) with a-CD.22,23 Tonelli et al. thoroughly investigated theICs of PCL, poly(L-lactide) (PLLA), PCL/PLLAblend, PCL-b-PLLA, PCL-b-PEO-b-PCL, andPCL-b-PPO-b-PCL with CDs, respectively.24–28

Shin et al. compared the formation of ICsbetween biodegradable polyesters (such as poly(3-hydroxypropionate), poly(4-hydroxybutyrate),and PCL) and a-CD. They showed that the stoi-chiometry of monomeric unit to CD increased asthe monomeric unit length of polymer becameshorter.14 Choi et al. reported the preparation ofICs between PLLA-b-PEO-b-PLLA triblock copoly-mer and a-CD, where a-CD could thread onto boththe PLLA and PEO blocks.29

As an extension of these efforts, increasingattention was paid to the preparation of ICsbetween star-shaped and/or hyperbranched poly-mers with CDs. Huh et al. prepared supramolec-ular hydrogels based on the inclusion complexa-tion between PEO grafted dextran and a-CD.30

Jiao et al. and Sabadini et al. reported that CDscould form ICs with star-shaped PEO havingdifferent branch arms, respectively,.31,32 Simi-larly, He et al. reported the ICs of comb-likePEO grafted polymers with a-CD.33,34 Zhu et al.obtained lamellar ICs of a hyperbranched poly-ether with a-CD.35 Recently, Chan et al. synthe-sized an organic/inorganic star-shaped PCLhaving 8 arms and studied its ICs with CDs.36

However, the studies on the inclusion complexa-tion of hydrophobic star-shaped polymers withCDs are rare in addition to the hydrophilicPEO-based polymers, and the ICs of star-shaped PCL having 4 and 6 arms with a-CDhave not been reported. Significantly, we havecompared the ICs of the star-shaped PCL hav-ing 4 and 6 arms with a-CD to those of the lin-ear PCL having 1 and 2 arms with a-CD,respectively. This showed that the branch armsof PCL polymer mainly controlled both the stoi-

chiometry (CL:CD, mol:mol) and the thermalproperties of the ICs.

EXPERIMENTAL

Materials

Stannous octoate (SnOct2, Aldrich, Milwaukee,WI) was used as received. e-Caprolactone (CL,Aldrich), toluene, and benzyl alcohol were dis-tilled from CaH2, respectively. 1,6-hexanediol(Aldrich) was dried in vacuo at room tempera-ture for 24 h. Pentaerythritol, dipentaerythritol,and a-CD were purchased from Aldrich and thendried in vacuo for 24 h at 100 8C. The otherreagents and solvents were local commercialproducts and used without further purification.

Methods

Molecular weights and molecular weight distri-butions of the polymers were determined on aWaters 717 plus autosampler gel permeationchromatograph equipped with Waters RH col-umns and the DAWN EOS (Wyatt Technology)multiangle laser light scattering detector at30 8C, THF as the eluent (1.0 mL/min). The dif-ferential scanning calorimetry (DSC) analysiswas carried out using a PerkinElmer Pyris 1instrument under nitrogen flow (10 mL/min). Allsamples were first heated from 0 to 100 8C at10 8C/min and held for 2 min to erase the ther-mal history, then cooled to 0 8C at 10 8C/min,and finally heated to 100 8C at 10 8C/min. Ther-mogravimetric analysis (TGA) was performedfrom room temperature to 600 8C at a heatingrate of 20 8C/min under nitrogen flow (10 mL/min), using a PerkinElmer TGA 7 instrument.Wide angle X-ray diffraction (WAXD) patterns ofpowder samples were obtained at room tempera-ture on a Shimrdzu XRD-6000 X-ray diffractom-eter with a Cu Ka radiation source (wavelength¼ 1.54 A). The supplied voltage and currentwere set to 40 kV and 30 mA, respectively. Sam-ples were exposed at a scan rate of 2h ¼ 48min�1 between 2h ¼ 58 and 408. Fourier trans-form infrared (FTIR) spectra were recorded on aPerkinElmer Paragon 1000 spectrometer atfrequencies ranging from 400 to 4000 cm�1.Samples were thoroughly mixed with KBr andpressed into pellet form. 1H NMR spectra wererecorded at room temperature on a Varian Mer-cury-400 spectrometer. CDCl3 and DMSO-d6

4722 WANG, WANG, AND DONG

were used as the deuterated solvents for thePCL polymers and the ICs, respectively. Solid-state carbon nuclear magnetic resonancespectroscopy using cross-polarization and magic-angle spinning (13C CP/MAS NMR) was per-formed on a Varian Mercury-400 spectrometerwith a sample spinning rate of 6 KHz at roomtemperature. The spectra were acquired with acontact time of 5 ms, a reception time of 10 ms,and 2000 accumulations.

Synthesis of Six-Arm Star-ShapedPoly(e-caprolactone) (6sPCL)

The polymerization tubes were kept at 110 8Cfor 24 h. CL, dipentaerythritol, and a dry stir-ring bar were put into the warm tube quickly.The tube was then connected to a Schlenk line,where an exhausting–refilling process wasrepeated three times. The tube was put into anoil bath at 120 8C with vigorous stirring forabout 5 min. A certain amount of SnOct2 ([CL]/[SnOct2] ¼ 1000/1, mol:mol) in dry toluene wasadded to the melt mixture and the exhausting–refilling process was carried out again forremoval of the toluene. The polymerization reac-tion was stopped after 24 h. The resulting prod-uct was dissolved in CH2Cl2 and poured drop-wise into an excess of cold methanol under vigo-rous stirring, and then the precipitate wasfiltered. The purified PCL polymer was driedin vacuo until a constant weight was obtained,and the polymer yield was determined gravimet-rically. A typical example follows: 1.44 mg(3.61 lmol) of the SnOct2 catalyst was added tothe melt mixture of the dipentaerythritol in-itiator (7.7 mg, 0.03 mmol) and CL monomer(412 mg, 3.61 mmol). The polymerization wascarried out in bulk at 120 8C for 24 h. Then, theresulting product was dissolved in 5 mL ofCH2Cl2 and poured dropwise into 50 mL of coldmethanol under vigorous stirring at room tem-perature. The precipitate was filtered and driedin vacuo at 40 8C to give 368.3 mg of the 6sPCL2sample (89.4 wt % yield).

1H NMR (CDCl3) of 6sPCL2 sample: d (ppm)¼ 1.10–1.40 (d), 1.40–1.70 (c), 2.10–2.40 (b),3.90–4.10 (a), 3.25–3.40 (g), 3.60 (f), 4.12–4.25(e), as shown in supporting information (S1).

Preparation of the ICs of 6sPCL with a-CD

As a representative protocol, the ICs of PCLpolymers with a-CD were prepared as follows.

6sPCL2 polymer (70.4 mg, 5.42 lmol) was dis-solved in 7 mL of acetone at 50 8C and a-CD(300.1 mg, 308.4 lmol) was dissolved in 2.8 mLof distilled water at 60 8C. Then the 6sPCL2 solu-tion was added dropwise to the a-CD solution at60 8C with vigorous stirring. After stirring at60 8C for 6 h, the mixture was cooled to roomtemperature and stirred vigorously for 35 h. Theprecipitated products were collected by filtra-tion, twice washed with acetone (15 mL) toremove free polymers, and then twice washedwith distilled water (15 mL) to remove uncom-plexed a-CD. The white powder was then driedovernight in vacuo at 60 8C until a constantweight was obtained. The yield (wt %) for allICs varied from 50 to 70%.

RESULTS AND DISCUSSION

It is known that biodegradable aliphatic poly-esters with well-defined architecture can besynthesized by using very dry systems with acontrolled amount of a hydroxy-containing com-pound initiator and SnOct2 catalyst.37–39 To fac-ilely synthesize 6sPCL, we chose commercialdipentaerythritol compound as a multifunctionalinitiator for the ring-opening polymerization ofe-CL monomer. The molecular weights of 6sPCLpolymers can be controlled by the molar ratio ofCL monomer to multifunctional initiator, andthe molecular weight distribution (Mw/Mn) wasrather narrow (Table 1). Based on the analysesof GPC and NMR (see supporting information,S1), the 6sPCL polymers with well-definedarchitecture have been successfully synthesiz-ed under the experimental conditions used(Scheme 1). Meanwhile, both linear PCL having1 arm (LPCL) and 2 arms (2LPCL) and star-shaped PCL having 4 arms (4sPCL) were syn-thesized using benzyl alcohol, 1,6-hexanediol,and pentaerythritol as initiators, respectively.These PCL polymers had similar molecularweight coupled with narrow polydispersities,and were used for the comparison studies.

ICs of linear or star-shaped PCL polymerswith a-CD were successfully prepared by mixinga solution of a-CD with that of the PCL, followedby rigorous stirring (Scheme 1). As a representa-tive example, the 1H NMR spectrum of a-CD-6sPCL1 ICs is given in Figure 1. It can beclearly seen that both a-CD and PCL compo-nents existed in the ICs. Comparing the integralof peak for a-CD (1H) with that of the PCL

STAR-SHAPED POLY("-CAPROLACTONE) 4723

methylene groups, the host–guest stoichiometryof ICs was calculated by the molar ratio of themonomeric repeating unit of PCL to a-CD(Table 2). It can be seen that the stoichiometryis 1.08 for a-CD-LPCL ICs, which is consistentwith that for a-CD-PCL ICs reported in the liter-

atures.8,36 However, the stoichiometry of otherPCL polymers having 2, 4, and 6 arms is a littlehigher than that of the linear PCL having 1arm. Moreover, the stoichiometry of all ICsincreased with the increasing branch arm ofPCL polymers, and it was in the order of a-CD-

Scheme 1. Preparation of supramolecular ICs of 6sPCL with a-CD.

Table 1. Synthesis of LPCL and Star-Shaped PCL Using Hydroxy-ContainingCompound Initiator and SnOct2 Catalyst in Bulk at 120 8C

Entry [M]/[I]a Mn,GPC Mn,thb Mn,NMR

c Mw/Mnd Yield (%)

LPCLe 60/1 5,980 6,450 5,980 1.57 94.32LPCLf 60/1 6,460 5,180 – 1.10 94.54sPCLg 58/1 6,060 5,870 – 1.10 88.86sPCL1h 58/1 7,160 6,170 7,650 1.05 92.66sPCL2h 122/1 11,820 12,430 12,980 1.08 89.4

[CL]/[SnOct2] ¼ 1000/1 (mol:mol); polymerization time, 24 h.a M ¼ CL, I ¼ initiator.b Mn,th ¼ [M]/[I] � MCL � Yield, Mn,th denotes the theoretical number–average molecular

weight of PCL polymers.c Mn,NMR was determined from the integral ratio of the signal on the main chain of poly-

mer (��CH2, dHb ¼ 2.10–2.40 ppm) and the signal on the primary hydroxy methylene endgroup (HOCH2, dH

f ¼ 3.60 ppm).d Weight–average molecular weight (Mw) and number–average molecular weight (Mn) are

determined by GPC.e LPCL was the linear PCL having 1 arm synthesized with benzyl alcohol as an initiator.f 2LPCL was the linear PCL having 2 arms synthesized with 1,6-hexanediol as an initiator.g 4sPCL was the star-shaped PCL having 4 arms synthesized with pentaerythritol as an

initiator.h Both 6sPCL1 and 6sPCL2 having 6 arms were synthesized with dipentaerythritol as an

initiator.


6sPCL1 ICs > a-CD-4sPCL ICs > a-CD-2LPCLICs > a-CD-LPCL ICs. This could be attributedto the increasing steric hindrance effect with theincreasing branch arm of PCL polymers, whichpossibly induced a few CL units near the linkingpoints in the star-shaped PCL polymers not tobe included by a-CD molecules.31,34,36 Mean-while, the stoichiometry of a-CD-6sPCL1 ICs isa little higher than that of a-CD-6sPCL2 ICs.This shows that the polymer molecular weightor the increasing branch arm length of 6sPCLalso has some effect on the stoichiometry. Theformation of ICs was then investigated by DSC,TGA, WAXD, 13C CP/MAS NMR, and FTIR tech-niques, respectively.

The DSC technique was employed to deter-mine the formation of ICs and to determinewhether the ICs contain free polymers. Themelting and cold crystallization behavior of purepolymers and the ICs are shown in Figures 2–4.The melting peaks of the pure PCL polymers

were observed at 58.0 8C for LPCL, 48.6 8C for6sPCL1, and 54.5 8C for 6sPCL2 in the firstheating run, respectively, and no melting peakwas observed for a-CD. However, no meltingpeak was observed for a-CD-LPCL ICs, a-CD-6sPCL1 ICs, and a-CD-6sPCL2 ICs in the firstheating run, respectively. Similarly, the cold crys-tallization peak and the second melting peakwere not observed for the ICs in the cooling runand in the second heating run, respectively,although both LPCL and 6sPCL polymers gavethe related cold crystallization and the secondmelting peaks in these runs, respectively, (seeFigs. 3 and 4). This indicates that the crystalliza-tion of LPCL and 6sPCL polymers was com-pletely suppressed in the a-CD cavities, and thatthe ICs contain negligible free guest polymers.

The thermal properties of the ICs were in-vestigated by the TGA technique, as shown inFigure 5. Compared with a-CD and the free PCLpolymers, the ICs presented a two-step thermal

Table 2. The Synthesis of a-CD-PCL ICs and the Thermal Properties of a-CD, theFree PCL Polymers and the ICs

EntryYield(wt %)

CL:CD(mol:mol)

Td,free (8C)a Td,ICs (8C)

b

a-CD PCL a-CD PCL

a-CD-LPCL ICs 69.2 1.08 306.7 354.6 366.2 408.9a-CD-2LPCL ICs 60.6 1.27 306.7 284.5 332.7 378.0a-CD-4sPCL ICs 66.0 1.56 306.7 313.9 330.9 388.5a-CD-6sPCL1 ICs 52.5 1.66 306.7 332.5 329.6 390.9a-CD-6sPCL2 ICs 67.6 1.39 306.7 331.5 363.7 419.1

a Td,free denotes the initial decomposition temperature of free a-CD and free PCL polymers, res-pectively.

b Td,ICs denotes the initial decomposition temperature of a-CD and the guest PCL polymersincluded in the ICs, respectively.

Figure 1. 1H NMR spectrum (DMSO-d6) of a-CD-6sPCL1 ICs formed between6sPCL1 and a-CD.


degradation. The first step can be mainly attrib-uted to the decomposition of a-CD, while the sec-ond step is mainly that of the guest PCL poly-mers, which have already been observed in CD-PCL ICs.31,33,36 The initial decomposition tem-peratures of all ICs, the free PCL polymers, anda-CD are compiled in Table 2. As an example,

the initial decomposition temperature for both a-CD and the guest 6sPCL1 in a-CD-6sPCL1 ICsis 329.6 and 390.9 8C, while both the free a-CDand the free 6sPCL1 decomposed at the initialtemperature of 306.7 and 332.5 8C, respectively.This indicates that the ICs are more thermallystable. Moreover, both the a-CD and the guest

Figure 3. The cooling DSC curves of (a) a-CD, (b) a-CD-LPCL ICs, (c) pure LPCL,(d) a-CD-6sPCL1 ICs, (e) pure 6sPCL1, (f) a-CD-6sPCL2 ICs, and (g) pure 6sPCL2.

Figure 2. The first heating DSC curves of (a) a-CD, (b) a-CD-LPCL ICs, (c) pureLPCL, (d) a-CD-6sPCL1 ICs, (e) pure 6sPCL1, (f) a-CD-6sPCL2 ICs, and (g) pure6sPCL2.


PCL polymers in all other ICs also have ahigher initial decomposition temperature thanthat of the free a-CD or the free PCL polymers.It is demonstrated that the inclusion complexa-tion between the linear or the star-shaped PCLpolymers and a-CD not only enhances the ther-mal stability of the guest PCL polymers but alsoimproves that of a-CD.

Furthermore, the thermal stability of the freePCL polymers (Td,free for PCL) is in the order ofLPCL > 6sPCL > 4sPCL > 2LPCL. This indi-cates that the free LPCL has better thermalstability than other free PCL polymers having 2,4, and 6 arms. It is probably attributed to theless crystalline defects, as LPCL has the highestdegree of crystallinity (data not shown). How-

Figure 5. The TGA scans of (a) a-CD, (b) a-CD-LPCL ICs, (c) pure LPCL, (d) a-CD-6sPCL1 ICs, (e) pure 6sPCL1, (f) a-CD-6sPCL2 ICs, and (g) pure 6sPCL2.

Figure 4. The second heating DSC curves of (a) a-CD, (b) a-CD-LPCL ICs, (c) pureLPCL, (d) a-CD-6sPCL1 ICs, (e) pure 6sPCL1, (f) a-CD-6sPCL2 ICs, and (g) pure6sPCL2.


ever, the thermal stability of the PCL polymershaving 2, 4, and 6 arms increases with theincreasing branch arm of polymer, which maybe attributed to the increasing inter- and intra-molecular hydrogen-bond interactions amongthe branch arms of the PCL polymers. Similarly,the guest PCL polymers included in the ICshave the same trend of variation in thermalstability as the free PCL polymers, and theirthermal stability (Td,ICs for PCL) is in the orderof a-CD-LPCL ICs > a-CD-6sPCL1 ICs > a-CD-4sPCL ICs > a-CD-2LPCL ICs. This suggeststhat the thermal stability of the free PCL poly-

mers probably controlled that of the guest poly-mers included in the ICs.

The wide angle X-ray diffractograms of a-CD,the pure PCL polymers (LPCL, 6sPCL1, and6sPCL2), and the respective ICs are shown inFigure 6. The pure PCL polymers showed promi-nent peaks at 21.4 and 23.7 for LPCL, at 21.3and 23.6 for 6sPCL1, and at 21.2 and 23.6 for6sPCL2, respectively. This indicates that thestar-shaped PCL polymers have crystallinestructure similar to that of linear LPCL, whichis consistent with that for PCL crystals locatedat 21.4 and 23.8.40 However, all the IC samples

Figure 6. WAXD patterns of (a) a-CD, (b) a-CD-LPCL ICs, (c) pure LPCL, (d) a-CD-6sPCL1 ICs, (e) pure 6sPCL1, (f) a-CD-6sPCL2 ICs, and (g) pure 6sPCL2.

Figure 7. Expanded 13C CP/MAS NMR of a-CD (a) and a-CD-6sPCL2 ICs (b).


showed prominent peaks at 20.0 and 23.0 for a-CD-LPCL ICs, at 19.9 and 23.0 for a-CD-6sPCL1ICs, and at 20.0 and 23.0 for a-CD-6sPCL2 ICs,respectively, while the major crystalline peaksfor LPCL and 6sPCL polymers disappeared.Moreover, the diffraction patterns of the ICs aresimilar and all have two strong peaks at 20.0and 23.0, which are well-known to be the char-acteristic of a-CD-polymer ICs adopting a chan-nel structure.14 These suggest that the brancharms of 6sPCL polymers were included into thehydrophobic cavities of a-CD, and they formed achannel-type crystalline structure similar tothat formed between the linear PCL and a-CD.

The solid-state 13C CP/MAS NMR spectroscopywas also used to investigate the formation of theICs, and the spectra of a-CD and the a-CD-6sPCL2ICs are shown in Figure 7 and S2 in supportinginformation. It can be concluded that a-CD in a-CD-6sPCL2 ICs adopts a more symmetrical cyclicconformation, while a-CD in its pure crystalpresents a less symmetrical conformation, which issimilar to that in a-CD-PCL ICs.7,23,27,29,36 Thisresult is consistent with the above analyses, and itimplies that the ICs of 6sPCL polymer with a-CDformed through a-CD threading onto the brancharms of 6sPCL polymer.

FTIR is a very useful tool to verify the pres-ence of both host and guest components in ICcrystals and it can also give more informationabout the formation of ICs, as shown in Figure 8.

The free PCL polymers are characterized by dis-tinct carbonyl stretching bands at 1720 cm�1 forLPCL, at 1724 cm�1 for 6sPCL1, and at 1720 cm�1

for 6sPCL2, respectively. The spectrum of a-CDshows a broad band at 3387 cm�1due to the sym-metric and antisymmetric O��H stretchingmode, and other three intense bands at1151 cm�1 (C��O��C glycosidic bridge) coupledwith 1072 cm�1 (C��C) and 1031 cm�1(C��O).Notably, the spectra of all ICs confirm the pres-ence of both host and guest components in theircrystals. Moreover, it can be seen that the C¼¼Oband of LPCL at 1720 cm�1 is shifted to higherfrequency at 1729 cm�1 in a-CD-LPCL ICs,meanwhile the broad O��H band of a-CD at3387 cm�1 is also up shifted at 3401 cm�1. Simi-larly, the C¼¼O band at 1723 cm�1 is shifted tohigher frequency at 1730 cm�1 in a-CD-6sPCL1ICs, with the O��H band of a-CD upshifted at3415 cm�1, and the same trend existed in a-CD-6sPCL2 ICs. These further indicate that the ICsof 6sPCL with a-CD successfully formed througha-CD threading onto the branch arms of thestar-shaped PCL polymers. Moreover, the char-acteristic shifts in all a-CD-PCL ICs are prob-ably due to the formation of hydrogen bonds,which mainly occur between the hydroxylgroups of a-CDs and the carbonyl groups of theguest polymers, as well as between the hydroxylgroups of a-CDs and those of a-CDs in adjacentbranch arms of star-shaped PCL polymers.14

Figure 8. FTIR spectra of (a) a-CD, (b) a-CD-LPCL ICs, (c) pure LPCL, (d) a-CD-6sPCL1 ICs, (e) pure 6sPCL1, (f) a-CD-6sPCL2 ICs, and (g) pure 6sPCL2.


CONCLUSIONS

Supramolecular ICs of PCL having differentbranch arms with a-CD were successfully pre-pared by mixing a solution of a-CD with that ofthe PCL polymer. The stoichiometry (CL:CD,mol:mol) of the ICs formed between star-shapedPCL polymers and a-CD is a little higher thanthat of the ICs formed between linear PCL anda-CD. The ICs of star-shaped PCL with a-CDformed through a-CD threading onto the brancharms of star-shaped PCL polymers, whose origi-nal crystalline properties were completely sup-pressed in the hydrophobic a-CD cavities. More-over, both star-shaped PCL and a-CD formedthe crystalline ICs with a channel-type structureas is formed between linear PCL and a-CD.Furthermore, the thermal stability of the freePCL polymers probably controlled that of theguest polymers included in the ICs.

The authors are grateful for the financial support ofthe National Natural Science Foundation of China(20404007).

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