organic–inorganic hybrid brushes consisting of macrocyclic oligomeric silsesquioxane and...

Organic–Inorganic Hybrid Brushes Consisting ofMacrocyclic Oligomeric Silsesquioxane andPoly(e-caprolactone): Synthesis, Characterization, andSupramolecular Inclusion Complexation with a-Cyclodextrin

JIN HAN, SIXUN ZHENG

Department of Polymer Science and Engineering and State Key Laboratory of Metal Matrix Composites,Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China

Received 26 June 2009; accepted 7 September 2009DOI: 10.1002/pola.23729Published online 5 November 2009 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Organic–inorganic hybrid brushes comprised of macrocyclic oligomericsilsesquioxane (MOSS) and poly(e-caprolactone) (PCL) were synthesized via the ring-opening polymerization of e-caprolactone (CL) with cis-hexa[(phenyl) (2-hydroxy-ethylthioethyldimethylsiloxy)]cyclohexasiloxane as the initiator. The MOSS macromerbearing hydroxyl groups was synthesized via the thiol-ene radical addition reactionbetween cis-hexa[(phenyl)(vinyldimethylsiloxy)]cyclohexasiloxane and b-mercaptoeth-anol. The organic–inorganic PCL cyclic brushes were characterized by means of nu-clear magnetic resonance spectroscopy (NMR) and gel permeation chromatography(GPC). These MOSS–PCL brushes were then used to prepare the supramolecularinclusion complexes with a-cyclodextrin (a-CD). The X-ray diffraction (XRD) indicatesthat the organic–inorganic inclusion complexes (ICs) have a channel-type crystallinestructure. It is noted that the molar ratios of CL unit to a-CD for the organic–inor-ganic ICs are quite dependent on the lengths of the PCL chains bonded to the silses-quioxane macrocycle. While the PCL chains were short, the efficiency of inclusioncomplexation was significantly decreased. The decreased efficiency could be attrib-uted to the repulsion of the adjacent PCL chains bonded to the silsesquioxane mac-rocycle and the restriction of the bulky silsesquioxane macrocycle on the motion ofPCL chains; this effect is pronounced with decreasing the length of the PCL chains.VVC 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6894–6907, 2009

Keywords: brush-like poly(e-caprolactone); channel-type structure; functionalizationof polymers; host-guest systems; inclusion complexation; macrocycles; macrocyclicoligomeric silsesquioxanes; nanocomposites; oligomers; polyesters

INTRODUCTION

During the past decade, inclusion complexes of or-ganic polymers with cyclodextrins have attracteda considerable interest owing to their uniquesupramolecular architectures such as molecular

tubes or polyrotaxanes, which could be good mod-els for macromolecular recognition in biologicalsystems.1–4 Cyclodextrins (CDs) are a series ofcyclic oligosaccharides composed of six, seven, andeight glucose units linked by a-1,4 glucosidicbonds, which are called a-, b-, and c-CD, respec-tively. The well-defined ring structures with cone-shaped cavities endow CDs with the ability to actas the host molecules to afford inclusion com-plexes (IC) with a great variety of guest molecules

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 6894–6907 (2009)VVC 2009 Wiley Periodicals, Inc.

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

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including small molecules and long-chain poly-meric molecules.1–4 The driving force for the for-mation of polyrotaxanes has been attributed to theintermolecular hydrogen bonding between adja-cent CDs as well as geometric compatibility andhydrophobic interactions between CDs and poly-mer chains. It is recognized that a channel-typestructure is characteristic for ICs of long-chainpolymeric molecule guest with CDs. Since Haradaet al. first reported the supramolecular inclusioncomplexes (IC) of a-CD with the poly(ethylene gly-col) in 1999,5 a variety of ICs have been founddepending on the size of inner cavities of CDs andpolymeric chains.6–10 Recently, inclusion complexa-tion of CDs with polymeric guests with various top-ologies has provoked considerable interest. It hasbeen reported that random copolymers containingnonincludable structural units,11 block copoly-mers,12–14 and star-shaped (or hyperbranched)polymers were used to prepare the inclusion com-plexes with CDs.15–20 One of the purposes for thesestudies is to examine the effect of topologies of or-ganic polymers on the structure and properties ofsupramolecular inclusion complexes.

Stereoregular oligomeric clusters containing sil-icon and oxygen have recently emerged as a classof new building blocks for network solids21–26 andinorganic–organic hybrid materials.27–40 Of them,polyhedral oligomeric silsesquioxanes (POSS)have been extensively investigated. The Si/O clus-ters can be used to access advanced materialsowing mainly to their elemental and structuralfeatures: (i) organic–inorganic hybrid assemblytogether with reactive functional groups, whichendows them with a good affinity with organic andinorganic materials and (ii) rigid stereoregularnanostructures, with which the Si/O clusters canbe used as the nanoreinforcement agent of compo-sites.27–41 Polymers incorporated with the well-defined nanosized POSS (and/or spherosilicates)cages represent a class of important organic–inor-ganic nanocomposites.29 It is worth noticing thatthe organic–inorganic ICs involving POSS-con-taining polymeric quests and CDs have beenrecently reported. Chang et al.42 and He et al.43

have reported the supramolecular inclusion com-plexes of a- and c-CDs with octa-armed star-shaped poly(e-caprolactone) (PCL) and poly(ethyl-ene oxide) (PEO) with POSS cores. It was foundthat the efficiencies of inclusion complexation forthe organic–inorganic star-shaped polymers werelower than those of their linear counterparts withCDs. It was proposed that the presence of bulkyPOSS cages constituted the steric hindrance to the

formation of ICs.42,43 Zheng et al.44 synthesized atadpole-like poly(e-caprolactone) with single POSSterminal group and noted that the presence of sin-gle POSS terminal groups did not significantlyaffect the efficiency of inclusion complexation.More recently, Zheng et al.45 prepared the or-ganic–inorganic polyrotaxanes involved with3,3,3-trifluropropyl POSS and the polypseudoro-taxanes of poly(ethylene oxide) with a-CD via cop-per-catalyzed azide-alkyne cycloaddition.

Polyhedral oligoalkylmetallasiloxanes are aclass of novel organometallic compounds, whichconsist of stereoregular alkylsiloxane macrocyclescoordinated to alkaline and/or transition metals(e.g., Mn, Co, Ni, Cu, and trivalent lanthanidemetals).29,46–52 These metallasiloxane coordinatescan be conveniently converted into macrocyclic oli-gomeric silsesquioxanes (MOSS) with satisfactoryyields (Scheme 1). Functionalized MOSS macro-mers have potential to be used as a new family ofbuilding blocks for organic–inorganic hybrid com-posites due to their structural and elemental fea-tures similar to spherosilicates and POSS macro-mers. However, such a study remains largelyunexplored vis-a-vis POSS and POSS-containingorganic–inorganic hybrids. To the best of ourknowledge, there has been no precedent report onsupramolecular inclusion complexation of MOSS-containing organic–inorganic polymers with CDs.

In this contribution, we firstly report the syn-thesis of cis-hexa[(phenyl) (vinyldimethylsiloxy)]-cyclohexasiloxane, a novel MOSS macromer andits derivative bearing 2-hydroxyethylthioethylgroups via thiol-ene radical addition. The latterwill be used as the initiator of e-caprolactone (CL)to afford the MOSS–PCL macrocyclic brushes viaring-opening polymerization (Scheme 1). There-after, the inclusion complexation of the organic–inorganic MOSS–PCL macrocyclic brushes with a-CD is investigated. The purpose of this work is toexamine the effect of macrocyclic brush-like topo-logy of PCL on the inclusion complexation with a-CD. To this end, the organic–inorganic ICs wereinvestigated by means of nuclear magnetic reso-nance spectroscopy (NMR), X-ray diffraction(XRD), Fourier transform infrared spectroscopy(FTIR), and thermal gravimetric analysis (TGA).

EXPERIMENTAL

Materials

Organic silanes (i.e., phenyltrichlorosilane andvinlydimethylchlorosilane) were purchased from

ORGANIC–INORGANIC BRUSHES CONSISTING OF MOSS AND PCL 6895

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

Gelest Co., USA and used as received. Anhydrouscopper (II) chloride (CuCl2) was obtained fromRuanshi Chemical Co., Jiangsu, China. Sodiumhydroxide (NaOH), silver nitrate (AgNO3), pyri-dine and organic solvents such as anhydrousethanol, methanol, and toluene were purchasedfrom Shanghai Reagent Co., Shanghai, China.Before use, toluene and pyridine were distilledover calcium hydride (CaH2) and then stored in asealed vessel with the molecular sieve of 4 A. Themonomer, e-caprolactone (CL) was purchasedfrom Fluka Co. Germany and it was distilled overCaH2 under decreased pressure before use. Silver

nitrate (AgNO3), sodium hydroxide (NaOH), andstannous (II) octanoate [Sn(Oct)2] are of analyti-cally pure grade and were purchased from Shang-hai Reagent Co., China.

Synthesis of Cis-hexa[(phenyl)(vinyldimethylsiloxy)]cyclohexasiloxane (1)

The coordinate, {[(C6H5SiO2)6]2Ni4Na4(NaOH)2}�(n-C4H9OH)3�(H2O)3 was used to obtain hexa[(phenyl)(vinyldimethylsiloxy)]cyclohexasiloxane.This compound was prepared via the re-action among polyphenylsilsesquioxane, sodium

Scheme 1. Synthesis of macrocyclic oligomeric silsesquioxane (POSS)-PCL brushand its inclusion complex with a-cyclodextrin.

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hydroxide (NaOH), and [Ni(NH3)6]Cl2 by follow-ing the method of literature reported by Shchego-likhina et al.46 Typically, a solution of 100 mLphenyltrichlorosilane (0.624 mol) dissolved in100 mL toluene was added dropwise into the mix-ture of 400 mL water and 150 mL toluene within30 min with vigorous stirring and the systemwas held in an ice-water bath. The organic layerwas isolated and washed with deionized water toneutrality and then the solvent was eliminatedvia rotary evaporation and polyphenylsilses-quioxane was obtained. To a flask, the as-obtained polyphenylsilsesquioxane (13.66 g, 0.1mol with respect of silicon) and NaOH (2.0 g,0.05 mol dissolved in 300 mL n-butanol) werecharged and the mixture was refluxed with vigor-ous stirring until the homogenous solution wasobtained. After that, Na (1.15 g, 0.05 mol) wasadded at the temperature of 50–60 �C. The sys-tem was refluxed for 30 min and finally hexaam-moniate nickel (II) chloride [Ni(NH3)6]Cl2 (11.59g, 0.05 mol) were added. The reaction system wasrefluxed for additional 2 h and the hot solutionwas filtered to eliminate NaCl precipitates andthe n-butanol was removed from the filtrate byrotary evaporation. The mixture was kept at�5 �C overnight and the yellow crystals wereformed, which was isolated and dried underreduced pressure at 80–90 �C for 1 h. The yellowpowder (8.1 g) (i.e., nickel/sodium hexaphenylcy-clohexasiloxanolate) was obtained with the yieldof 84%.

The above nickel/sodium hexaphenylcyclohexa-siloxanolate was then used to generate cis-hexa[(phenyl)(vinyldimethylsiloxy)]cyclohexasiloxane. Aflask was charged with anhydrous benzene(35 mL), pyridine (5.5 mL), and vinyldimethyl-chlorosilane (10.32 g, 85.6 mmol). With vigorousstirring, nickel/sodium phenylsiloxanolate (1.30 g,0.56 mmol) was added. The reaction mixture wasrefluxed for 24 h. After cooling to room tempera-ture, the precipitate was filtered off. The filtratewas washed free of chloride by water and thendried over sodium sulfate. Petroleum ether wasremoved via rotary evaporation to afford 1.47 gcis-hexa [(phenyl)(vinyldimethylsiloxy)]cyclohex-asiloxane with the yield of 97%. 1H NMR (ppm,CDCl3): 7.17 (t, 1H, p-Ph), 7.15 (d, 2H, o-Ph), 6.90(t, 2H, m-Ph), 6.22–5.70 (m, 3H, SiCH¼¼CH2),0.22 [s, 6H, Si(CH3)2]. 13C NMR (ppm, CDCl3):139.19 and 132.31 (vinyl), 134.43, 133.50, 129.51,127.32 (phenyl), 0.71 (methyl). 29Si NMR (ppm,CDCl3): �1.18 [s, OSi(CH3)2(CH¼¼CH2)], �80.85(s, C6H5SiO). Electrospray ionization mass spec-

troscopy (ES-MS) (product þ Naþ): 1372.4 Da(calculated: 1373.3 Da).

Synthesis of Cis-hexa[(phenyl)(2-hydroxyethyl-thioethyldimethylsiloxy)]cyclohexasiloxane

The above macrocyclic oligomeric silsesquioxanesbearing vinyl groups (1.33 g, 1.0 mmol), b-mercap-toethanol (0.94 g, 12 mmol) and AIBN (0.10 g, 0.6mmol) were dissolved in toluene (6 mL) andhighly pure nitrogen (N2) was purged into the sol-utions for 30 min, to eliminate oxygen. The reac-tion mixtures were heated upto 70 �C with vigor-ous stirring and maintained at this temperaturefor 10 h. After that 1,4-dioxane and the excessiveb-mercaptoethanol were removed via rotary evap-oration. The crude products were dissolved inTHF and precipitated with water. This procedurewas repeated for three times to purify the sample.Then, the precipitates were dissolved in THF again,and the solution was dried over anhydrous sodiumsulfate and the salt was filtrated off. The resultingproduct (1.58 g) was obtained via rotary evapora-tion with the yield of 88%. 1H NMR (ppm, CDCl3):7.40–6.80 (m, 5H, Ph), 3.60 (t, 2H, CH2OH), 2.43(broad, 1H, OH), 2.59 (t, 2H, HOCH2CH2S), 2.44(m, 2H, SCH2CH2Si), 0.92 (m, 2H, SCH2CH2Si),0.21 [s, 6H, Si(CH3)2].

13C NMR (ppm, CDCl3):134.17, 132.87, 130.01, 127.65 (phenyl); 60.5(HOCH2CH2S); 35.3 (HOCH2CH2S); 26.7 (SCH2-CH2Si); 19.3 (SCH2CH2Si); 0.76 Si(CH3)2.

Synthesis of Organic–Inorganic MOSS–PCL Brushes

The ring-opening polymerization (ROP) of e-capro-lactone (CL) catalyzed by stannous (II) octanoate[Sn(Oct)2] was used to prepare MOSS–PCLbrushes. The ROP was carried out on a standardSchlenk line system. Typically, hexa[(phenyl)(2-hydroxyethylthioethyldimethylsiloxy)]cyclotetra-siloxane (0.18 g, 0.10 mmol) and anhydrous tolu-ene (15.0 mL) were charged to a three-neckedflask and the system was dried via azotropic dis-tillation with anhydrous toluene. Cooled to roomtemperature, e-caprolactone (0.62 g, 5.44 mmol)and Sn(Oct)2 dissolved in anhydrous toluene(0.1 wt % with respect of e-caprolactone) wereadded and used as the catalyst. The reactive mix-ture was degassed via three pump-thaw-freezecycles and then immersed in a thermostated oilbath at 120 �C for 24 h. The crude products weredissolved with THF and precipitated using anexcessive amount of petroleum ether. This



procedure was repeated for three times. The pre-cipitates were filtered and dried in vacuo at 30 �Cfor 24 h to give 0.79 g polymer with the yield of98%. 1H NMR(400 MHz, CDCl3): 7.30–6.80 (broad,5H, phenyl), 4.13–3.98 [t, 18.7H, COOCH2CH2Sand OCO(CH2)4CH2], 3.63 (t, 2H, CH2OH, termi-nal hydroxymethylene group), 2.56 (broad, 2H,COOCH2CH2S), 2.43 (broad, 2H, SCH2CH2Si),2.36–2.20 [t, 18.7H, OCOCH2(CH2)4], 1.70–1.5 (m,37.5H, OCOCH2CH2CH2CH2CH2), 1.45–1.28 (m,18.7H, OCOCH2CH2CH2CH2CH2), 0.88 (broad,2H, SCH2CH2Si), 0.19 [broad, 6H, Si(CH3)2].

Preparation of Inclusion Complexes (ICs)

The inclusion complexes of MOSS–PCL brusheswith a-CD were prepared as follows. Typically,MOSS–PCL1K cyclic brushes (0.24 g, 1.62 mmolCL unit) was dissolved in 18 mL acetone and2.18 g a-CD was dissolved in 8 mL of deionizedwater. The PCL solution was added dropwise intothe aqueous a-CD solution at 50 �C with vigorousstirring. The mixture was kept at 60 �C for 6 hand then was cooled to room temperature withcontinuous stirring overnight. The white precipi-tate was collected and washed with 5 mL acetoneand 20 mL distilled water for three times respec-tively, to remove uncomplexed polymer and free a-CD. The resulting product (i.e., inclusion complex)was dried in vacuo at 30 �C for 24 h before use.

Measurement and Techniques

Nuclear Magnetic Resonance Spectroscopy (NMR)

The 1H NMR, 13C NMR measurements were car-ried out on a Varian Mercury Plus 400 MHz NMRspectrometer at 25 �C and 29Si NMR measure-ments were carried out on an Avance Brucker400 MHz NMR spectrometer. The samples weredissolved with deuterated CDCl3 and the solu-tions were measured with tetramethylsilane(TMS) as an internal reference.

Electronic Spray Ionization MassSpectroscopy (ES-MS)

Electrospray ionization mass spectra (ES-MS)were recorded on a Finnigan LCQ Mat LC/MSMass Spectrometer System using dichloro-methane-methanol as mobile phase. Sodium wasused as the cationizing agent and all the datashown are for positive ions.

Fourier Transform Infrared Spectroscopy (FTIR)

The FTIR measurements were conducted on a Per-kin–Elmer Paragon 1000 Fourier transform spec-trometer at room temperature (25 �C). The samplefilms were prepared by dissolving the polymerswith CHCl3 (5 wt %) and the solutions were castonto KBr windows. The residual solvent wasremoved in a vacuum oven at 60 �C for 2 h. For theinclusion complexes (ICs), the powder sampleswere mixed with the pellet of KBr and were pressedinto disk flakes. All the specimens were sufficientlythin to be within a range where the Beer-Lambertlaw is obeyed. In all cases, 64 scans at a resolutionof 2 cm�1 were used to record the spectra.

Gel Permeation Chromatography (GPC)

The molecular weights of polymers were meas-ured on a Perkin–Elmer 200 gel permeation chro-matography (GPC) instrument with a PL mixed-B10m column. Polystyrene was used as the stand-ard, and tetrahydrofuran (THF) was used as theeluent at a flow rate of 1 mL/min.

Wide Angle X-Ray Diffraction

The wide-angle X-ray diffraction (XRD) experi-ments were carried out on a Shimadzu XRD-6000X-ray diffractometer with CuKa (k ¼ 0.154 nm)irradiation at 40 kV and 30 mA using a Ni filter.Data were recorded in the range of 2h ¼ 5–40� atthe scanning rate and step size of 4.0�/min and0.02�, respectively.

Thermal Gravimetric Analysis (TGA)

A Perkin–Elmer thermal gravimetric analyzer(TGA-7) was used to investigate the thermal sta-bility of samples. The samples (about 10 mg) wereheated in nitrogen atmosphere from ambient tem-perature to 800 �C at the heating rate of 20 �C/min in all cases.

RESULTS AND DISCUSSION

Synthesis of MOSS Macromer Bearing2-Hydroxyethylthioethyl Groups

The macrocyclic oligomeric silsesquioxane (MOSS)bearing 2-hydroxyethylthioethyl groups wasused as the initiators of e-caprolactone (CL)to obtain the MOSS–PCL brushes via ring-open-ing polymerization. The synthetic route of the

6898 HAN AND ZHENG


MOSS macromer is summarized in Scheme 1. Inthe first step, the MOSS macromer bearing vinylgroups was prepared via the silylation reactions ofthe metallasilsesquioxane (i.e., nickel/sodiumhexaphenylcyclohexasiloxanolate) with mono-chlorosilane with excellent yield. The metallasil-sesquioxanes used in this work is nickel/sodium-hexaphenylcyclohexasiloxanolate {[(C6H5SiO2)6]2Ni6(NaCl)}�(n-BuOH)5(H2O)6, which was pre-pared via the metal-mediated self-assembly of tri-functional alkyoxylsilane by following the litera-ture method.38 The above metallacyclosiloxanolatewas allowed to react with the vinyldimethylchloro-silane to afford the macrocyclic oligomeric silses-quioxane bearing vinyl groups with the excellentyields ([94%). In the 1H NMR spectrum (Fig. 1),the resonance assignable to protons of aromaticrings, vinyl groups, and methyl groups connectedto silicon atom was detected at 6.2–6.9, 6.2–5.7,and 0.2 ppm, respectively. The resonancesignals of carbon nuclei assignable to aromaticring, vinyl, and methyl groups are indicated in the13C NMR spectrum of the MOSS macromer asshown in Figure 2. The 1H and 13C NMR spectros-

copy indicates that this compound possesses aro-matic ring, vinyl, and methyl groups connected tosilicon atom. To confirm the formation of macrocy-clic oligomeric silsesquioxane bearing vinylgroups, the above product of silylation reactionwere subjected to 29Si NMR spectroscopy, and the29Si NMR spectrum is shown in Figure 3. The tworesonance signals of silicon nuclei were clearly dis-played at �1.24 and �80.9 ppm, respectively. Thesignal at high field is ascribed to the resonance ofsilicon of macrocyclic backbones, whereas that atthe low field to the resonance of silicon from vinyl-dimethylsiloxyl group. In the 29Si NMR spectrum,two single peaks of 29Si resonance indicate thatthe silsesquioxane takes a cis- macrocyclic struc-ture. To measure the molecular weight of theMOSS macromer, the sample was subjected toelectrospray ionization mass spectroscopy and themass spectrum was presented in Figure 4. It isseen that apart from the peaks of the fragments inthe m/z range of 0–800, the intense molecule-ionpeaks centered at the position of m/z to be 1372.4were detected. In terms of the ratio of mass tocharge, the molecular weight of the MOSS bearingvinyl groups was determined, which was in a goodagreement with the value calculated in terms oftheir structural formula. The NMR and mass

Figure 1. 1H NMR spectra of cis-hexa[(phenyl)(vinyldimethylsiloxy)] cyclohexasiloxane and cis-hexa[(phenyl) (2-hydroxyethylthioethyldimethylsiloxy)]cyclohexasiloxane.

Figure 2. 13C NMR spectra of cis-hexa[(phenyl)(vinyldimethylsiloxy)] cyclohexasiloxane and cis-hexa[(phenyl) (2-hydroxyethylthioethyldimethylsiloxy)]cyclo-hexasiloxane.



spectroscopy indicates that the MOSS macromerbearing vinyl groups was successfully obtained.

Thiol-ene radical addition is an important reac-tion, which proceeds via the reaction of a thiylradical with a terminal ene to form a carbon-cen-tered radical. By a hydrogen abstraction processfrom another thiol group, the carbon-centeredradical is transferred into a new thiyl radical andthe addition product was thus formed.52–57 Withthiol-ene radical addition, vinylsilsesquioxanescan undergo a series of useful transformations tobear a variety of functional groups, depending onthe uses of various mercapto compounds. In thiswork, the MOSS macromer bearing vinyl groupswere used to prepare the MOSS macromer bear-

ing 2-hydroxyethylthioethyl groups via thiol-eneradical addition by the use of b-mercaptoethanol(Scheme 1). The thiol-ene radical addition wascarried out with 2,20-azobisisobutylnitrile (AIBN)as the initiator under a mild condition. To pro-mote the reaction to completion, b-mercaptoetha-nol was kept excessive. The unreacted mercaptocompound can be easily eliminated via the precip-itation of the reacted solution with water and theresulting product was obtained with the satisfiedyield ([80%). The 1H and 13C NMR spectra ofthis compound are incorporated into Figures 1and 2. It is noted that with the thiol-ene radicaladdition, the resonance assignable to protons ofvinyl groups at 6.2–5.7 ppm disappeared, suggest-ing that the addition reaction has been performedto completion. Concurrently, the several new sig-nals of resonance appeared at 0.9–4.0 ppm. Thepeaks of resonance at 0.95 and 2.44 ppm areascribed to methylene protons connected to siliconand sulfur atoms in the moiety of ASCH2CH2SiA.The signals at 3.60 and 2.59 ppm are assigned tothe methylene protons of HOCH2CH2S moiety.According to the ratio of integral intensity of theprotons of methylene groups connected to 2-hydroxyethylthioethyl groups to those of methyl(or phenyl) protons, it is judged that the all thevinyl groups of the MOSS macromer bearing vinylgroups were successfully functionalized into 2-hydroxyethylthioethyl groups. In the 13C NMRspectrum of the thiol-ene addition product(Fig. 2), these new signals at 60.5, 35.3, 26.7, and19.3 ppm are ascribed to four types of carbon nu-cleus in the HOCH2CH2SCH2CH2SiA structuralunit, which is in a good agreement with the resultof 1H NMR spectrocopy. It should be pointed outthat the 13C NMR spectroscopy indicates that the

Figure 3. 29Si NMR spectrum of cis-hexa[(phenyl)(vinyldimethylsiloxy)] cyclohexasiloxane.

Figure 4. Mass spectrum of cis-hexa[(phenyl)(vinyldimethylsiloxy)] cyclohexasilox-ane.

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thiol-ene radical addition was carried out in ananti-Markovnikov fashion, which is identical withthe results of literature.54 The 1H and 13C NMRspectroscopy shows that with the thiol-ene radicaladdition, the MOSS macromers bearing vinylgroups have been successfully functionalized intothe derivatives bearing 2-hydroxyethylthioethylgroups, respectively.

Synthesis of MOSS–PCL Brushes

The MOSS macromers bearing 2-hydroxyethylth-ioethyl groups were used as the initiator to per-form the ring-opening polymerization of e-capro-lactone with stannous (II) octanoate [Sn(Oct)2] asthe catalysts to afford the organic–inorganicMOSS–PCL brushes. The polymerization was car-ried out at 120 �C for 24 h to obtain the organic–inorganic polymers. Representatively shown inFigure 5 is the 1H NMR spectrum of the hexa-armed MOSS–PCL brush with the arm length to

be Mn ¼ 1100. It is seen that the resonance peakscharacteristic of PCL are detected at 4.0, 3.6, 2.3,1.6, and 1.4 ppm, respectively. In addition, the sig-nals of proton resonance at 6.8–7.4, 2.5, 2.4, 0.9,and 0.2 ppm assignable to the MOSS moieties arediscernible. The 1H NMR spectroscopy indicatesthat the as-polymerized product combined thestructural features from the MOSS moiety andPCL. The molecular weight of the MOSS–PCLbrushes can be estimated by means of 1H NMRspectroscopy according to the ratios of integrationintensity of methylene protons (viz. at 3.6 ppm) atPCL chain ends to methylene protons in the mid-chain of PCL. By controlling the molar ratios ofthe MOSS macromer to e-caprolactone (CL), theMOSS–PCL brushes with various lengths of PCLarms were obtained. All the MOSS–PCL brusheswere subjected to gel permeation chromatography(GPC) measurements and the GPC curves areshown in Figure 6. The molecular weights of allthe MOSS–PCL brushes together with thelengths of PCL arms are summarized in Table 1.It is seen that each GPC curve displayed a unimo-dal peak, suggesting that no MOSS macromer

Figure 5. 1H NMR spectrum of MOSS–PCL1K.

Figure 6. GPC curves of MOSS–PCL hybridbrushes.

Table 1. Molecular Weight and Polydispersity of MOSS–PCL Brushes

Samples Larma Mn,th Mn,NMR Mn,GPC Mw/Mn

MOSS-PCL1k 1,100 7,620 8,000 6,600 1.46MOSS-PCL 2k 2,100 13,620 13,900 12,400 1.13MOSS-PCL 3k 3,000 19,620 19,700 13,700 1.19MOSS-PCL 5k 5,100 31,620 32,000 26,300 1.26

Mn,th, molecular weights calculated according to the molar ratios of the initiator to CLmonomer.

aLarm stands for the length of PCL chain.



and PCL homopolymer were detected. Figure 7shows the plots of molecular weight and polydis-persity as functions of the molar ratio of [CL] to[MOSS] for the MOSS–PCL brushes. It is seenthat the average molecular weight (Mn) of the or-ganic–inorganic MOSS–PCL brushes linearlyincreased with increasing the molar ratio ofmonomer to initiator, suggesting that the molecu-lar weights of the organic–inorganic brushes canbe controlled in terms of the molar ratios of mono-mer to initiator. All the MOSS–PCL brushes pos-sess the quite narrow polydispersity of molecularweights ranged from 1.10 to 1.46, implying thatthe polymerization was carried out in a living po-lymerization fashion. It should be pointed thatthe values by GPC are the molecular weights rela-tive to the linear polystyrene standard. It is pro-posed that 1H NMR spectroscopy is the moreaccurate molecular weight determination method.The 1H NMR spectroscopy and the results of GPCindicates that the organic–inorganic MOSS–PCLbrushes were successfully obtained.

Supramolecular Inclusion Complexationof MOSS–PCL Brushes with a-CD

The MOSS–PCL brushes were employed to pre-pare the inclusion complexes (ICs) with a-CD.While the acetone solution of MOSS–PCL brusheswas added to the saturated aqueous solution of a-CD, the system immediately became turbid and

the precipitates were then formed, indicating thatthe inclusion complexation occurred betweenMOSS–PCL brushes and a-CD. The precipitateswere collected and washed with acetone and dis-tilled water to remove any uncomplexed compo-nents and all the yields of ICs were obtained to beat �50 wt %.

Structure of Crystals

The inclusion complexes of MOSS–PCL brusheswith a-CD were subjected to wide-angle X-ray dif-fraction (XRD) to investigate the structure of crys-tals for the inclusion complexes. The diffractionpatterns of a-CD, plain PCL, MOSS–PCLbrushes, and the inclusion complexes are shownin Figure 8. The positions of diffraction peaks forthe plain 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).58

The intense diffraction peaks at 2h ¼ 21.21�,21.81�, 23.52�, were observed for plain PCL corre-sponding to (110), (111), and (200) reflections ofPCL.59 For the MOSS–PCL brushes, the diffrac-tion peaks are found to be 2h ¼ 21.40�, 22.03�,

Figure 7. Plots of number-average molecularweights and polydispersity of molecular weight asfunctions of molar ratio of e-caprolactone to cis-hexa[(phenyl)(2-hydroxyethylthioethyldimethylsiloxy)]cyc-lohexasiloxane.

Figure 8. X-ray diffraction profiles of PCL1K,MOSS–PCL2K, PCL1K-IC, and the ICs of MOSS–PCL with a-CD.

6902 HAN AND ZHENG


and 23.75�, respectively, which are close to thoseof plain PCL. However, the diffraction peaks ofthe MOSS–PCL brushes were significantly shiftedto the higher values of 2y, suggesting that thecrystal structures of the MOSS–PCL brusheswere changed to some extent. It is proposed thatduring crystallization, the MOSS cycles cannot berejected from the front of crystal growths andthus the presence of the bulky MOSS moietycould alter the structure of PCL crystals. It isnoted that the XRD pattern of each IC was not asimple superposition of those of MOSS–PCLbrushes and a-CD. The characteristic reflectionscorresponding to PCL crystalline phase and a-CDsignificantly disappeared while some new strongdiffraction peaks appeared at 20.0� and 22.5�,respectively. In terms of the appearance of thesharp diffraction peaks, it is judged that the inclu-sion complexes of MOSS–PCL brushes with a-CDare crystalline. The new diffraction peaks at 20.0�

and 22.5� are characteristics of the crystals of a-CD/PCL IC adopting the channel-type structure,indicating that the ICs of MOSS–PCL brusheswith a-CD were formed with the PCL chainthreading inside the cavities of a-CD.60–62 It is ofinterest to note that the presence of macrocyclicsilsesquioxanes did not change the columnarstructures of the IC. It should be pointed out thatfor the IC of the MOSS–PCL brushes with a-CD,a new reflection was discernible and centered at7.4�, which could be associated with MOSS moietyin the organic–inorganic inclusion complexes.

Stoichiometry of SupramolecularInclusion Complexation

The molar ratios of e-caprolactone (CL) unit to a-CD in the organic–inorganic supramolecularinclusion complexes were determined by means of1H NMR spectroscopy. Figure 9 representativelyshows the 1H NMR spectrum of the IC of MOSS–PCL1K with a-CD. The molar ratios of CL unit toa-CD in the organic–inorganic inclusion com-plexes with MOSS–PCL brushes of various molec-ular weights were summarized in Table 2. Themolar ratio of CL to a-CD for the IC of plain PCL(viz. PCL1K) with a-CD is 1:1, which is in a goodagreement with the values reported in litera-ture.63,64 For the inclusion complexes of a-CDwith MOSS–PCL brushes, the molar ratios of CLunit to a-CD are significantly lower than that ofplain PCL/a-CD IC. For the IC of MOSS–PCL1Kwith a-CD, the molar ratio was decreased to about1:2, implying that about 50 wt % of CL unitsremained uncomplexed with a-CD. It is noted thatwith increasing the length of PCL chains, themolar ratios increased. While the length of PCLchain is 5100, the molar ratio of CL unit to a-CD(viz. 1:1.1) was quite close to that of the IC ofplain PCL with a-CD. This observation suggeststhat the presence of macrocyclic silsesquioxanesignificantly decreased the efficiency of inclusioncomplexation. It is proposed that in the inclusioncomplexation of a-CD with MOSS–PCL brushes,the stoichiometry of inclusion complexation couldbe affected by the following two factors. First, theinclusion complexation was carried out by thePCL threading inside a-CD only with one chainend owing to the presence of the bulky 12-mem-bered silsesquioxane cycles. However, it has beendemonstrated that the inclusion complexation ofPCL (or PEO) with a-CD with PCL (or PEO)threading inside a-CD only with one chain enddoes not affect the ratio of inclusion complexationin POSS-capped PCL (or fullerene-capped PEO)IC with a-CD.44,65 Therefore, it is proposed that in

Figure 9. 1H NMR spectrum of the supramolecularinclusion complex of MOSS–PCL1K with a-CD.

Table 2. Molar Ratios of a-CD to CL Unit inOrganic–Inorganic Inclusion Complexes

Inclusion Complexes Molar Ratio of a-CD to CL

PCL1K IC 1:1.0MOSS-PCL1K IC 1:1.9MOSS-PCL2K IC 1:1.5MOSS-PCL3K IC 1:1.2MOSS-PCL5K IC 1:1.1



the present case this factor is not dominant. Sec-ond, the efficiency of inclusion complexation couldbe decreased by the presence of adjacent arms,which are densely attached to the macrocyclic sil-sesquioxanes containing six ASiAOA units. Theadjacent PCL arms could constitute the steric hin-drance to inclusion complexation. This case is justlike that inclusion complexation of comb-like PEOwith short PEO chain with a-CD.67 In addition,the decreased efficiency of inclusion complexationcould be attributed to the lower mobility of thebulky MOSS, which restricted the motion of PCLchain. This effect is pronounced when the PCLchains are short.

Hydrogen Bonding Interactions

Figure 10 shows the FTIR spectra of the a-CD,MOSS–PCL, and their inclusion complexes in therange of 1680–1840 cm�1. At room temperature,plain PCL was characterized by the bands at 1725and 1736 cm�1, respectively, (see curve A). Thesetwo components are the conformation-sensitivebands. The former is assigned to the stretchingvibration of the carbonyl group of PCL chain inthe crystalline region, whereas the latter to thatof the carbonyl groups of PCL in amorphousregion.58 Upon heating the PCL to elevated tem-

perature (e.g., 100 �C), the stretching band of car-bonyl groups in the crystalline region disappeareddue to the melting of the PCL crystals. For theFTIR spectra of the ICs of MOSS–PCL1 with a-CD, the band at 1736 cm�1 was displayed,whereas the stretching vibration (at 1725 cm�1) ofcarbonyl groups in the crystalline region of PCLwas not detectable. This observation indicatesthat no crystals of PCL were found in the inclu-sion complexes, i.e., the PCL chains have beenthreaded inside the cavities of a-CD. The fact thatno crystalline stretching vibration of carbonylgroups was detected in the IC could be ascribed tothe confinement of a-CD internal cavities to thecrystallization of PCL chains. It is worth noticingthat there were new shoulder bands at 1715 cm�1

in the FTIR spectra of the organic–inorganic ICs.The shoulder band could be attributed to the for-mation of the hydrogen bonding interactionsbetween the hydroxyl groups of a-CD and the car-bonyl groups of PCL chain.66 From the determina-tion of the molar ratios of CL unit to a-CD in theICs by means of 1H NMR spectroscopy, it wasidentified that a great number of CL unitsremained uncomplexed with a-CD owing to theconfinement of the bulky silsesquioxane cycles,especially for the ICs with the lower molecularweights of MOSS–PCL brushes. It is proposedthat the formation of hydrogen bonding interac-tions resulted from the association between thehydroxyl groups of a-CD with the uncomplexedCL units. It is seen that the fraction of H-bondedcarbonyl groups increased with decreasing thelength of PCL chains. In another word, the morehydrogen-bonded carbonyl groups exist in the ICsfor the MOSS–PCL with a-CD when the PCLarms of MOSS–PCL are shorter. This observationis consistent with the stoichiometry of inclusioncomplexation determined by means of 1H NMRspectroscopy. It should be pointed out that it isnot easy quantitatively to obtain the fraction ofH-bonded carbonyl groups due to the lack of theratio of the molar absorption coefficients (ec/ea),where ec and ea are the molar absorption coeffi-cients of the carbonyl groups of PCL chainthreaded into a-CD and the H-bonded carbonylgroups of PCL chains, respectively.66 Nonetheless,the determination of the molar absorption coeffi-cient of the carbonyl groups of PCL chainthreaded into a-CD will be an interesting work. Itis noted that for the ICs, the width of spectral lineat the half height for the carbonyl groups of PCLchains threaded into the cavity of a-CD were sig-nificantly lower than that of PCL in the melt.

Figure 10. FTIR spectra of PCL and the ICs ofMOSS–PCL with a-CD.

6904 HAN AND ZHENG


This observation is attributed to the fact thatthere were not the dipole–dipole interactions ofcarbonyl groups for the carbonyl groups of PCL inthe cavity of a-CD.

Thermal Stability

Thermogravimetric analysis (TGA) was applied toevaluate the thermal stability of MOSS–PCLbrushes, a-CD, and their inclusion complexes.Shown in Figure 11 are the TGA curves ofMOSS–PCL1K, a-CD, and the ICs recorded innitrogen atmosphere. For a-CD, the initial decom-position temperature that was defined as 5%mass loss temperature is about 300 �C and thechar yield at 800 �C is about 9.4 wt %. ForMOSS–PCL1K brush, the initial decompositionoccurred at 298 �C, which was close to that of a-CD. According to the fraction of degradation resi-due at 800 �C, it is judged that the residue of deg-radation is mainly composed of inorganic compo-nent (e.g., ceramic). All the ICs displayed two-stepprofiles of degradation, which are in contrast toMOSS–PCL1K, implying that the supramolecularinclusion complexation changed the mechanism ofdecomposition. For the inclusion complexes, theinitial decomposition temperatures and the resi-due yield are intermediate between those of thetwo pure components, suggesting that the ICs arefairly stable. It is seen that MOSS–PCL brushesdisplayed much lower residue of decomposition at800 �C from char and ceramic yields than a-CD.

The yields of residue increased with increasingthe length of MOSS–PCL arms. This observationis in an agreement with the content of a-CD inthe ICs. It is proposed that lower the content of a-CD in the ICs, lower the char yields.

CONCLUSIONS

Organic–inorganic hybrid brushes consisting ofmacrocyclic oligomeric silsesquioxane (MOSS)and poly(e-caprolactone) (PCL) were synthesizedvia ring-opening polymerization of e-caprolactone(CL) with cis-hexa[(phenyl) (2-hydroxyethylthioe-thyldimethylsiloxy)]cyclohexasiloxane as the ini-tiator. The hydroxyl-functionalized MOSS macro-mer was synthesized via the thiol-ene radicaladdition reaction between cis-hexa[(phenyl)(vinyl-dimethylsiloxy)]cyclohexasiloxane and b-mercap-toethanol. The organic–inorganic PCL cyclicbrushes with various molecular weights werecharacterized by means of nuclear magnetic reso-nance spectroscopy (NMR) and gel permeationchromatography (GPC). The supramolecularinclusion complexation of MOSS–PCL hybridbrushes with a-cyclodextrin (a-CD) was investi-gated. The X-ray diffraction (XRD) indicated thatthe organic–inorganic inclusion complexes (ICs)have a channel-type crystalline structure. It isnoted that the stoichiometry of CL to a-CD in theorganic–inorganic ICs is quite dependent on themolecular weights of the MOSS–PCL brushes.When the PCL arms are short, the efficiencyof inclusion complexation was significantlydecreased. The decreased efficiency of inclusioncomplexation could be attributed to the repulsionbetween the adjacent PCL arms and the restric-tion of the bulky silsesquioxane cycles on themotion of PCL arms.

The financial supports from Natural Science Founda-tion of China (No. 20474038 and 50873059) andNational Basic Research Program of China (No.2009CB930400) are acknowledged. The authors thankShanghai Leading Academic Discipline Project (ProjectNumber: B202) for the partial support.

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