Hepta(3,3,3-trifluoropropyl) Polyhedral Oligomeric Silsesquioxane-capped Poly( N -isopropylacrylamide) Telechelics: Synthesis and Behavior of Physical Hydrogels

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<ul><li><p>Published: March 07, 2011</p><p>r 2011 American Chemical Society 898 dx.doi.org/10.1021/am101258k |ACS Appl. Mater. Interfaces 2011, 3, 898909</p><p>RESEARCH ARTICLE</p><p>www.acsami.org</p><p>Hepta(3,3,3-trifluoropropyl) Polyhedral OligomericSilsesquioxane-capped Poly(N-isopropylacrylamide) Telechelics:Synthesis and Behavior of Physical HydrogelsLei Wang, Ke Zeng, and Sixun Zheng*</p><p>Department of Polymer Science and Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University,Shanghai 200240, P. R. China</p><p> INTRODUCTION</p><p>Polyhedral oligomeric silsesquioxane (POSS) reagents, mono-mers, and polymers are emerging as a new chemical technologyfor preparing the organic-inorganic hybrids.1-3 During the pastdecade, POSS-containing organic-inorganic hybrids have becomethe focus of many studies because of the excellent comprehensiveproperties of this class of hybrid materials, such as mechanicalproperties, thermal stability and flame retardation etc.1-3 Recently,it has been identified that POSS could be employed to optimize theproperties of some functional materials via the formation of specificmorphological structures.4-24</p><p>Stimuli-responsive hydrogels have attracted considerable in-terests owing to their ability to change volume or shape in responseto environmental alteration. The environment-responsive proper-ties have potential applications in biomedical and biochemical fieldssuch as drug delivery and protein separation.25-28 Poly(N-al-kylacrylamide) hydrogels are among themost-investigated hydrogelsystems during the past decades.29-35 Of them, poly(N-iso-propylacrylamide) (PNIPAAm) hydrogels can exhibit a volumephase transition (VPT) behavior as the environmental tempera-ture changes,29-31,36 which stems from the lower criticalsolution temperature (LCST) behavior of PNIPAAm chainsin aqueous solution.37-41 However, PNIPAAm hydrogelsinherently displayed slow deswelling and reswelling ratesdue to the collective diffusion of water molecules in the cross-linked networks.42-46 Therefore, the modification of PNIPAAm</p><p>hydrogels is necessary to meet the requirement of application ofthe hydrogels. Recently, there have been several reports on themodification of PNIPAAm hydrogels with POSS macromers.7-10</p><p>Schiraldi et al.7 ever used octamethacryloxylpropyl POSS as across-linking agent of PNIPAAm to promote the interactionsbetween the cross-linking agent and silicate filler in the clay-containing poly(N-isopropylacrylamide) hydrogels. Mu et al.8</p><p>have reported themodification of PNIPAAmhydrogels via in situcross-linking with an octafunctional POSS macromer and it wasfound that the thermoresponsive properties of PNIPAAmhydro-gels were significantly improved compared to the control PNI-PAAm hydrogel. More recently, Zeng et al.9 prepared theorganic-inorganic PNIPAAm hydrogels via the copolymeriza-tion of NIPAAm with 3-acryloxypropylhepta(3,3,3-trifluoropropyl)POSS and found that the POSS-containing PNIPAAm hydrogelsdisplayed much faster temperature response rates than plainPNIPAM hydrogel.</p><p>POSS-capped telechelic polymers are a class of interestingorganic-inorganic hybrids owing to their specific topologies andself-assembly behaviavor.10,16-24,47-50 Mather et al.47 first reportedthe synthesis of heptacyclohexyl POSS-capped poy(ethyleneoxide) (PEO) telechelics via the direct reaction of isocyanato-propyldimethylsilylheptacyclohexyl POSS with poly(ethylene</p><p>Received: December 21, 2010Accepted: February 18, 2011</p><p>ABSTRACT:Hepta(3,3,3-trifluoropropyl) polyhedral oligomeric silses-quioxane (POSS)-capped poly(N-isopropylacrylamide) (PNIPAAm)telechelics with variable lengths of PNIPAAm midblocks were synthe-sized by the combination of reversible addition-fragmentation chaintransfer polymerization (RAFT) and the copper-catalyzed Huisgen1,3-cycloaddition (i.e., click chemistry). The POSS-capped trithiocar-bonate was synthesized and used as the chain transfer agent for theRAFT polymerization of N-isopropylacrylamide. The organic-inor-ganic amphiphilic telechelics were characterized by means of nuclearmagnetic resonance spectroscopy (NMR) and gel permeation chro-matography (GPC). Atomic force microscopy (AFM) shows that allthe POSS-capped PNIPAAm telechelics exhibited microphase-separated morphologies, in which the POSS terminal groups wereself-assembled into the microdomains and dispersed into the continuous PNIPAAm matrices. The POSS nanodomains couldbehave as the physical cross-linking sites and as a result the physical hydrogels were formed while these POSS-capped PNIPAAmtelechelics were subjected to the solubility tests with water. These physical hydrogels possessed well-defined volume phase transitionphenomena and displayed rapid reswelling and deswelling thermoresponsive behavior compared to control PNIPAAm hydrogel.</p><p>KEYWORDS: polyhedral oligomeric silsesquioxane, poly(N-isopropylacrylamide), telechelics and physical hydrogels</p></li><li><p>899 dx.doi.org/10.1021/am101258k |ACS Appl. Mater. Interfaces 2011, 3, 898909</p><p>ACS Applied Materials &amp; Interfaces RESEARCH ARTICLE</p><p>glycol) (PEG) and the behavior of crystallization and rehologywas invesitgated.48,49 Zeng et al.10 synthesized hepta(3,3,3-trifluoropropyl) POSS-capped PEO telechelics via copper-cata-lyzedHuisgen cycloaddition reaction (i.e., click chemistry) between3-azidopropylhepta(3,3,3-trifluoropropyl) POSS and alkyne-termi-nated PEO.More recently, Mueller et al.50 reported the synthesisof POSS-capped polystyrene telechelics via the combination ofatom transfer radical polymerization (ATRP) and the copper-catalyzed Huisgen 1,3-cycloaddition. All these POSS-cappedtelechelic polymers were obtained via the reaction of the endgroups of existing polymers with POSS, i.e., the so-called POSS-terminating approach was employed; this approach is effectivefor the preparation of the POSS-capped telechelic polymers withrelatively low molecular weights. Nonetheless, the efficiency ofthis route is open to question for the preparation of high-molecular-weight samples because it is not easy to ensure thatall the ends of the polymers are capped with POSS because of toolow concentration of end groups. To the best of our knowledge,however, there have been no precedent reports on the POSS-capped PNIPAAm telechelics.</p><p>In this contribution, we report a synthesis of POSS-cappedPNIPAAm telechelics with a POSS-spreading strategy. In thisapproach, a POSS-capped trithiocarbonate was synthesized andused as a chain transfer agent, with which the reversible addi-tion-fragmentation chain transfer polymerization (RAFT) ofNIPAAm was carried out. With the insertion of NIPAAm, thetwo POSS end groups were spread and the POSS-cappedPNIPAAm telechelics with variable length of PNIPAAm wereafforded. With this approach, the ends of PNIPAAm chains werecapped with two hepta(3,3,3-trifluoropropyl) POSS groups,which are quite hydrophobic.9,10,51,52 The POSS-capped PNI-PAAm telechelics could display some interesting propertiesowing to the combination of the thermoresponsive midblock(viz. PNIPAAm) and bulky and hydrophilic end groups (viz.POSS). It is expected that the microphase-separatedmorphologyin bulk could be exhibited in the organic-inorganic telechelicpolymers because of the immiscibility between PNIPAAm andthe POSS, in which the POSS microdomains are dispersed incontinuous PNIPAAmmatrix. In water, the POSSmicrodomainscould behave as physically cross-linking sites and thus thePNIPAAm physical hydrogels could be afforded. In this work,the POSS-capped PNIPAAm telechelics were characterized bymeans of nuclear magnetic resonance spectroscopy (NMR) andgel permeation chromatography (GPC). The morphology andsurface properties of the organic-inorganic telechelics wereinvestigated by means of atomic force microscopy (AFM) andstatic contact angle analysis. The thermoresponsive behavior ofthe physical hydrogels was addressed on the basis of swelling,deswelling, and reswelling tests.</p><p>EXPERIMENTAL SECTION</p><p>Materials. 3,3,3-Trifluoropropyltrimethoxysilane (99%) was obtainedfrom Zhejiang Chem-Technology Co., China. 3-Bromopropyltrichlorosi-lane (BrCH2CH2CH2SiCl3), sodium azide (NaN3) and pentamethyl-diethylene triamine (PMDETA) were purchased from Aldrich Co, USAand used as received. N-Isopropylacrylamide (NIPAAm) was preparedin this lab by following the literature method.53 Propargyl alcohol(98%) was purchased from Aladdin Reagent Co., Shanghai, China.Other reagents such as metal sodium, calcium hydride (CaH2), sodiumhydroxide (NaOH) 2,2-azobisisobutylnitrile (AIBN), thionyl chlorideand carbon disulfide (CS2) were of chemically pure grade, purchased</p><p>from Shanghai Reagent Co., China. The solvents such as N,N-dimethyl-formamide (DMF), tetrahydrofuran (THF), dichloromethane, chloro-form, acetone, pyridine, petroleum ether (distillation range: 60-90 C),and triethylamine (TEA) were of chemically pure grade, also obtained fromcommercial sources. Before use, THFwas refluxed above sodium and thendistilled and stored in the presence of the molecular sieve of 4 . TEAwas refluxed over CaH2 and then was treated with p-toluenesulfonylchloride, followed by distillation.Synthesis of 3-Bromopropylhepta(3,3,3-trifluoropropyl)</p><p>POSS. Hepta(3,3,3-trifluoropropyl)tricycloheptasiloxane trisodium si-lanolate [Na3O12Si7(C3H4F3)7] was synthesized by following themethod of literature reported by Fukuda et al.54 In a typical experiment,(3,3,3-trifluoropropyl)trimethoxysilane [CF3CH2CH2Si(OMe)3] (50.0 g,0.23 mol), THF (250 mL), deionized water (5.25 g, 0.29 mol) andsodium hydroxide (3.95 g, 0.1 mol) were charged to a flask equippedwith a condenser and a magnetic stirrer. After refluxed for 5 h, thereactive system was cooled down to room temperature and held at thistemperature with vigorous stirring for additional 15 h. All the solvent andother volatile were removed via rotary evaporation and the white solidswere obtained. After dried at 40 C in vacuo for 12 h, 37.3 g productswere obtained with the yield of 98%. 3-Bromopropylhepta(3,3,3-trifluoropropyl) POSS was prepared via the corner capping reactionbetween Na3O12Si7(C3H4F3)7 and 3-bromopropyl trichlorosilane. Ty-pically, Na3O12Si7(C3H4F3)7 (10.0 g, 8.8 mmol) and triethylamine(1.3 mL, 8.8 mmol) were charged to a flask equipped with a magneticstirrer, 200 mL anhydrous THF were added with vigorous stirring. Theflask was immersed into an ice-water bath and purged with highly purenitrogen for 1 h. After that, 3-bromopropyltrichlorosilane (2.47 g,9.68 mmol) dissolved in 20 mL anhydrous THF were slowly droppedwithin 30 min. The reaction was carried out at 0 C for 4 h and at roomtemperature for 20 h. The insoluable solids (i.e., sodium chloride) wasfiltered out and the solvent together with other volatile was removed viarotary evaporation to afford the white solids. The solids were washedwith 50mLmethanol for three times and dried in vacuo at 40 C for 24 hand 8.2 g of product was obtained with the yield of 76%. FTIR (cm-1,KBr window): 1090-1000 (Si-O-Si), 2900-2850 (-CH2), 1120-1300(-CF3), 540 (C-Br).</p><p>1H NMR (ppm, acetone-d6): 3.52 (t, 2.0H, -CH2-Br), 2.32 (m, 14.0H, SiCH2CH2CF3), 1.96 (m, 2.0H, -CH2-CH2-Br),1.03 (m, 14.0H, SiCH2CH2CF3), 0.96 (t, 2.0H, -CH2-CH2-CH2-Br).29Si NMR (ppm, acetone-d6):-65.8,-66.8, and-67.0. MALDI-TOF-MS (product Na): 1240.1 Da.Synthesis of 3-Azidopropylhepta(3,3,3-trifluoropropyl)</p><p>POSS. 3-Azidopropylhepta(3,3,3-trifluoropropyl) POSS was synthesizedvia the reaction between 3-bromopropylhepta(3,3,3-trifluoropropyl) POSSand sodium azide (NaN3). Typically, 3-bromopropylhepta(3,3,3-trifluoropropyl) POSS (3.0 g, 2.5 mmol) and NaN3 (0.1760 g, 2.75mmol) were charged to a flask equipped with a magnetic stirrer and10 mL of anhydrous DMF was added to the flask. The reaction wascarried out at the room temperature for 24 h. After that, a large amountof deionized water was used to perticipate the product. The products wasfurther dried at 40 C in a vacuum oven for 24 h. The product (2.6 g) wasobtained with the yield of 90%. FTIR (cm-1, KBr window): 1090-1000(Si-O-Si), 2900-2850 (-CH2), 1120-1300 (-CF3), 2105 (azido).1H NMR (ppm, acetone-d6): 3.35 (t, 2.0H, -CH2-N3), 2.32 (m, 14.0H,SiCH2CH2CF3), 1.76 (m, 2.0H, -CH2-CH2-N3), 1.03 (m, 14.0H,SiCH2CH2CF3), 0.86 (t, 2.0H, -CH2-CH2-CH2-N3).Synthesis of S,S0-Bis(r, r0-dimethyl-r00-propargyl acet-</p><p>ate)-trithiocarbonate. S,S0-Bis(R,R0-dimethyl-R0 0-acetic acid)-trithiocarbonate (BDATC) was synthesized by following the methodof literature.55-58 Typically, carbon disulfide (6.85 g, 0.09 mol),chloroform (26.875 g, 0.225 mol), acetone (13.075 g, 0.225 mol),tetrabutylammonium bromide (0.572 g, 1.775 mmol), and 30 mL ofmineral spirits were charged into a three-necked flask equipped with amechanical stirrer. The flask was immersed into an ice-water bath</p></li><li><p>900 dx.doi.org/10.1021/am101258k |ACS Appl. Mater. Interfaces 2011, 3, 898909</p><p>ACS Applied Materials &amp; Interfaces RESEARCH ARTICLE</p><p>and purged with highly pure nitrogen. Sodium hydroxide solution(50%) (50.5 g, 0.63 mol) were slowly dropped within 30 min. Thereaction was carried out at room temperature overnight and then 225 mLwater was then added to dissolve the solid, followed by adding 30 mL ofconcentrated HCl to acidify the aqueous layer. After vigorous stirring foradditional 30 min, the earth-colored insoluable solids was filtered out andwashed with water thrice. After being dried in vacuo at 40 C for 24 h, thecrude product was collected. The resulting product was obtained byrecrystallization in the mixture of toluene with actone (4/1 v/v) to affordyellow crystalline solids. 1H NMR (ppm, CDCl3): 1.69 (s, -CSSC-(CH3)2COO).</p><p>13C NMR (ppm, DMSO-d6): 25.6 [-CSSC(CH3)2COO],56.8 [-CSSC(CH3)2COO], 173.7 [-CSSC(CH3)2COO], 219.8 [-CSSC(CH3)2COO].Synthesis of S, S0-Bis(r,r0-dimethyl-r00-propargyl acet-</p><p>ate)-trithiocarbonate. S,S0-Bis(R,R0-dimethyl-R0 0-propargyl acet-ate)-trithiocarbonate was synthesized via the esterification of S,S0-bis(R,R0-dimethyl-R0 0-acetic acid)-trithiocarbonate with propargyl alcohol.59-62</p><p>First, 3.10 g of propargyl alcohol (27.60 mmol), 1 mL of pyridine, and15 mL of dichloromethane were charged to a flask immersed into an ice-water bath. BDATC (0.78 g, 2.762 mmol) and thionyl chloride (20 mLSOCl2) were charged to another flask with vigorous stirring for 3 h at60 C. After excess thionyl chloride (SOCl2) was distilled out, 5 mL ofanhydrous dichloromethane was added to system and the solution wasdropwise added to the above flask containing propargyl alcohol. Withcontinuous stirring for 24 h at room temperature, the reacted solution waswashed free of propargyl alcohol and pyridine chloride by water and thendried over sodium sulfate. The solvent, dichloromethane, was removed viarotary evaporation to afford 0.85 g alkyne-capped trithiocarbonates withthe yield of 86%. 1H NMR (ppm, CDCl3): 1.67 (s, 6.0H, -CSSC-(CH3)2COOCH2CCH), 2.46 (t, 1.0H, -CSSC(CH3)2COOCH2CCH),4.67 (d, 2.0H, -CSSC(CH3)2COOCH2CCH).Synthesis of POSS-capped Trithiocarbonate. The above</p><p>alkyne-capped trithiocarbonate (BDPT) was used to reacted with the3-azidopropyl...</p></li></ul>

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