Supramolecular inclusion complexation of polyhedral oligomeric silsesquioxane capped poly(ε-caprolactone) with α-cyclodextrin

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<ul><li><p>Supramolecular Inclusion Complexation ofPolyhedral Oligomeric Silsesquioxane CappedPoly(e-caprolactone) with a-Cyclodextrin</p><p>YONG NI, SIXUN ZHENG</p><p>Department of Polymer Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240,Peoples Republic of China</p><p>Received 8 September 2006; accepted 17 November 2006DOI: 10.1002/pola.21893Published online in Wiley InterScience (</p><p>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 ef-ciency of the inclusion complexation. The X-ray diffraction results indicated that the or-ganicinorganic ICs had a channel-type crystalline structure. The stoichiometry of theorganicinorganic 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 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 1720 or 2490), the efciency of the inclusioncomplexation decreased. The decreased efciency 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: PolymChem 45: 12471259, 2007</p><p>Keywords: amphiphiles; biological applications of polymers; channel-type structure;host-guest systems; inclusion chemistry; inclusion complexation; nanocomposites;poly(e-caprolactone); polyhedral oligomeric silsesquioxane; polyrotaxanes</p><p>INTRODUCTION</p><p>In recent years, considerable interest has beenattracted to investigating the supramolecularchemistry involving organic macromolecules andcyclodextrins (CDs). CDs are a series of cyclic oli-</p><p>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-dened 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 rst re-ported supramolecular ICs of a-CD with poly(eth-</p><p>Correspondence to: S. Zheng (E-mail:</p><p>Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 12471259 (2007)VVC 2007 Wiley Periodicals, Inc.</p><p>1247</p></li><li><p>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(dh) 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.</p><p>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)</p><p>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 612 and R can be various types of organicgroups, one (or more) of which is reactive or poly-merizable. Polymers incorporated with well-de-ned nanosized POSS cages represent a class ofimportant organicinorganic nanocomposites.9</p><p>Chang et al.4(p) and He et al.10 reported studieson supramolecular ICs of octa-armed star-shaped</p><p>PCL and PEO with a-CD. They found that theefciencies of inclusion complexation for the or-ganicinorganic, 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 efciency of inclusion com-plexation should be ascribed to not only the bulkyPOSS cages but also the specic topological struc-tures of the star-shaped polymers. The decreasedefciency 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)</p><p>In this work, we rst 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 organicinor-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.</p><p>EXPERIMENTAL</p><p>Materials</p><p>Phenyltrimethoxysilane [C6H5Si(OCH3)3; 98%]was supplied by Zhejiang Chem-Tech Ltd. Co.</p><p>1248 NI AND ZHENG</p><p>Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola</p></li><li><p>(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 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 6090 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.</p><p>Synthesis of HeptaphenyltricycloheptasiloxaneTrisodium Silanolate [Na3O12Si7(C6H5)7]</p><p>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-neckedask equipped with a reux condenser and a mag-netic stirrer. The mixture was reuxed 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 ltration with lterpaper with a pore diameter of 0.5 lm and washedwith THF at least three times. The white solids</p><p>Scheme 1</p><p>SUPRAMOLECULAR INCLUSION COMPLEXATION 1249</p><p>Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola</p></li><li><p>were then dried in vacuo at 60 8C for 24 h to afford65 g of the products with a yield of 98.9%.</p><p>FTIR (cm1, KBr window): 3075, 1596, 1430,11351090 (SiPh) 10901000 (SiOSi). Solid29Si NMR (ppm): 77.64, 75.78, 71.77, 68.38,66.22.</p><p>Synthesis of 3-Chloropropylheptaphenyl POSS[(C6H5)7(ClCH2CH2CH2)Si8O12]</p><p>Heptaphenyltricycloheptasiloxane trisodium silano-late (10.00 g, 10 mmol) was dissolved with 250 mL ofanhydrous THF in a three-necked ask, and 3-chlor-opropyltrichlorosilane (3.18 g, 15 mmol) was quicklyadded to the ask. 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 nally precipitated by beingdropped into 2000 mL of methanol. The white solidswere collected by ltration, 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%.</p><p>FTIR (cm1, KBr window): 3075, 1596, 1430,11351090 (SiPh), 10901000 (SiOSi),2956 (CH3), 2934 (CH2). 1H NMR (ppm,CDCl3): 7.727.79 (m, 14.2H, protons of aromaticring), 7.337.51 (m, 21.4H, protons of aromaticring), 3.52 (t, SiCH2CH2CH2Cl, 2.0H), 1.99 (m,SiCH2CH2CH2Cl, 2.0H), 1.00 (m, SiCH2CH2CH2Cl, 2.0H).</p><p>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.</p><p>Synthesis of 3-Hydroxypropylheptaphenyl POSS[(C6H5)7(HOCH2CH2CH2)Si8O12]</p><p>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 rst 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</p><p>with deionized water three times. After that, toa round-bottom ask, 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 reuxed for 48 h withvigorous stirring in the dark. All the solids wereltered 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.</p><p>FTIR (cm1, KBr window): 3075, 1596, 1430,11351090 (SiPh), 10901000 (SiOSi), 2956,2934 (CH2), 3440 (OH). 1H NMR (ppm, CDCl3):7.727.79 (m, 14.2H, protons of aromatic rings),7.337.51 (m, 21.2H, protons of aromatic rings), 3.52(t, SiCH2CH2CH2OH, 2.0H), 1.99 (m, SiCH2CH2CH2OH, 2.0H), 1.00 (m, SiCH2CH2CH2OH, 2.0H).13C NMR (ppm, CDCl3): 9.75 (SiCH2CH2CH2OH),26.53 (SiCH2CH2CH2OH), 47.41 (SiCH2CH2CH2OH), 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.</p><p>To conrm 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 ask. Chlorotrime-thylsilane (1.08 g, 9.9 mmol) was added dropwiseto the ask 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%.</p><p>1H NMR (ppm, CDCl3): 7.727.79 (m, 14.2H,protons of aromatic rings), 7.337.51 (m, 21.2H,protons of aromatic rings), 3.52 [t, SiCH2CH2-CH2OSi(CH3)3, 2.0H], 1.99 [m, SiCH2CH2CH2O-</p><p>1250 NI AND ZHENG</p><p>Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola</p></li><li><p>Si(CH3)3, 2.0H], 1.00 [m, SiCH2CH2CH2OSi(CH3) 3,2.0H], 0.09 [s, SiCH2CH2CH2OSi(CH3)3, 8.8H].</p><p>Synthesis of POSS-Terminated PCL</p><p>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 l...</p></li></ul>


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