Synthesis and characterization of heptaphenyl polyhedral oligomeric silsesquioxane-capped poly(N-isopropylacrylamide)s

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<ul><li><p>ptapy</p><p>etal M</p><p>Received 3 November 2011Received in revised form 22 February 2012Accepted 4 March 2012Available online 11 March 2012</p><p>Keywords:Poly(N-isopropylacrylamide)Polyhedral oligomeric silsesquioxaneOrganicinorganic amphiphiles</p><p>(POSS) was synthesized via the copper-catalyzed Huisgen 1,3-cycloaddition (i.e., click</p><p>of which is reactive (Scheme 1). POSS-containing hybridnanocomposites have been becoming the focus of manystudies due to their excellent thermomechanical</p><p>chanical properties of POSS-containing nanocomposites.Poly(N-isopropylacrylamide) (PNIPAAm) is a kind of</p><p>interesting thermoresponsive polymer. In aqueous solu-tion PNIPAAm can display a lower critical solution temper-ature (LCST) behavior at ca. 32 C [1923]. Below thistemperature, individual PNIPAAm chains adopt a random</p><p>0014-3057/$ - see front matter 2012 Elsevier Ltd. All rights reserved.</p><p> Corresponding author. Tel.: +86 21 54743278; fax: +86 21 54741297.E-mail address: (S. Zheng).</p><p>European Polymer Journal 48 (2012) 945955</p><p>Contents lists available at SciVerse ScienceDirect</p><p>European Poly</p><p>else</p><p>MACR</p><p>OMOLECU</p><p>LARNANOTE</p><p>CHNOLO</p><p>GY Introduction</p><p>Polyhedral oligomeric silsesquioxanes (POSS) are a classof important nanosized cage-like molecules, derived fromhydrolysis and condensation of trifunctional organosilanes.In the past years, POSS reagents, monomers and polymershave been emerging as a new chemical technology for pre-paring the organicinorganic hybrids [18]. A typical POSSmolecule possesses a structure of cube-octameric frame-work represented by the formula (R8Si8O12) with an inor-ganic silica-like core (Si8O12) (0.53 nm in diameter)surrounded by eight organic corner groups, one (or more)</p><p>properties [18]. Recently, POSS cages have been incorpo-rated into organic polymers to optimize the functionalproperties of the materials via the formation of specicmorphological structures [9,10]. For instance, POSS cageswere incorporated into conjugated luminescent polymerssuch as polyuorene, polyphenylene and polythiopheneto reduce the formation of aggregates and to increase ther-mal stability [1113]. It was reported that the incorpora-tion of POSS can enhance deswelling and reswelling ratesof poly(N-isopropylacrylamide) hydrogels [1418]. How-ever, such an investigation remains largely unexploredvis--vis the studies on the improvement of thermome-Self-assemblychemistry). With this initiator, the atom transfer radical polymerization (ATRP) of N-iso-propylacrylamide (NIPAAm) was carried out to afford the POSS-capped PNIPAAm. Theorganicinorganic amphiphiles were characterized by means of nuclear magnetic reso-nance spectroscopy (NMR) and gel permeation chromatography (GPC). Atomic forcemicroscopy (AFM) showed that the POSS-capped PNIPAAm amphiphiles in bulk displayedmicrophase-separated morphologies. In aqueous solutions, the POSS-capped PNIPAAmamphiphiles were self-assembled into micelle-like aggregates as evidenced by dynamiclight scattering (DLS) and transmission election microscopy (TEM). It was found that thesizes of the self-organized nanoobjects decreased with increasing the lengths of PNIPAAmchains. By means of UVvis spectroscopy, the lower critical solution temperature (LCST)behavior of the organicinorganic amphiphiles in aqueous solution was investigated andthe LCSTs of the organicinorganic amphiphiles decreased with increasing the percentageof POSS termini. It is noted that the self-assembly behavior of the POSS-capped PNIPAAm inaqueous solutions exerted the signicant restriction on the macromolecular conformationalteration of PNIPAAm chains while the coil-to-globule collapse occurred.</p><p> 2012 Elsevier Ltd. All rights reserved.Article history: In this work, a novel initiator bearing heptaphenyl polyhedral oligomeric silsesquioxaneMacromolecular Nanotechnology</p><p>Synthesis and characterization of hesilsesquioxane-capped poly(N-isopro</p><p>Yaochen Zheng, Lei Wang, Sixun Zheng Department of Polymer Science and Engineering and State Key Laboratory of M</p><p>a r t i c l e i n f o a b s t r a c t</p><p>journal homepage: www.phenyl polyhedral oligomericlacrylamide)s</p><p>atrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China</p><p>mer Journal</p><p>vier .com/locate /europol j</p></li><li><p>946 Y. Zheng et al. / European Polymer Journal 48 (2012) 945955</p><p>MACR</p><p>OMOLECU</p><p>LARNANOTE</p><p>CHNOLO</p><p>GYOCH3coil conformation. While the solution is heated up to 32 Cor higher, the random coils will collapse into globules. It isproposed that in the former case, the intermolecularhydrogen bonding interactions between amide groups of</p><p>Si OCH3OCH3</p><p>NaN3</p><p>Si O SiO</p><p>SiO</p><p>Si O SiO</p><p>SiO</p><p>O</p><p>O</p><p>O OOO</p><p>Si</p><p>SiN3</p><p>Si O SiO</p><p>SiOSi O Si</p><p>OSiO</p><p>O</p><p>O</p><p>O</p><p>OO</p><p>OH</p><p>+ ClCl</p><p>O</p><p>Si O SiOSiO</p><p>Si O SiO</p><p>SiO</p><p>O</p><p>O</p><p>O OOO</p><p>Si</p><p>SiN3</p><p> CuBr / PMDETA</p><p>NIPAAm</p><p>CuCl / Me6TRENSi O Si</p><p>OSiO</p><p>Si O SiO</p><p>SiO</p><p>O</p><p>O</p><p>O OOO</p><p>S</p><p>S</p><p>Scheme 1. Synthesis of POSi O SiO</p><p>SiO</p><p>O-O- O-PNIPAAm and water promote the solubility of PNIPAAmwith water. In the latter cases, the intermolecular hydro-gen bonding interactions are interrupted owing to the con-formational changes of PNIPAAm chains. As a consequence,</p><p>Si O SiO</p><p>SiO O</p><p>OO</p><p>Si .</p><p>SiCl</p><p>ClCl Br</p><p>Si O SiO</p><p>SiO</p><p>Si O SiO</p><p>SiO</p><p>O</p><p>O</p><p>O OOO</p><p>Si</p><p>SiBr</p><p>3Na+</p><p>OSi</p><p>Si N</p><p>OCl</p><p>O</p><p>NN</p><p>O</p><p>OCl</p><p>i</p><p>i N NN</p><p>O</p><p>O</p><p>HNO</p><p>n</p><p>SS-capped PNIPAAm.</p></li><li><p>ylamine (1.3 ml, 8.8 mmol) were charged to a askequipped with a magnetic stirrer and then anhydrous</p><p>Y. Zheng et al. / European Polymer Journal 48 (2012) 945955 947</p><p>MACR</p><p>OMOLECU</p><p>LARNANOTE</p><p>CHNOLO</p><p>GYthe hydrophobic association among the collapsed PNI-PAAm chains takes place. During the past decades, theLCST behaviors of PNIPAAm in aqueous solutions havebeen extensively investigated [1930]. It is identied thatsome structural factors such as co-monomer, tacticity,crosslinking, grafting, topology of macromolecules, molec-ular weights and end groups all affect the LCST behavior ofPNIPAAm in aqueous solutions [19,3137]. Recently, thePNIPAAm amphiphiles containing hydrophobic end groupsor polymer chains are of interest since the specic archi-tectures can signicantly affect the LCST behaviors of PNI-PAAm in aqueous solutions [20,3841]. Winnik et al.investigated the effect of the hydrophobic n-octadecyl ter-mini on the merging of mesoglobules of PNIPAAm and pro-posed that the rigidity and partial vitrication ofmesoglobules may be the prevalent cause of their stabilityagainst aggregations [20]. Zhang et al. [40] found that theower-like aggregates resulting from PS-b-PNIPAAm-b-PStriblock copolymers are much less collapsed than thestar-like aggregates derived from the corresponding ow-er-liked aggregates.</p><p>POSS-capped telechelic polymers are a class of novel or-ganicinorganic hybrids owing to their specic topologiesand self-assembly behavior [16,4246]. Depending on thechemical strategies used, one (or two) end of a linear or-ganic polymer chain can be bonded with POSS cage to con-stitute so-called semi-telechelics (or telechelics). Recently,several POSS-capped polymer telechelics such as poly(eht-ylene oxide) [16,45], poly(e-caprolactone) [47,48], poly(acrylic acid) [49], polystyrene [46] and poly(hydroxyetherof bisphenol A) [50] have been reported by various invesit-gators; these POSS-capped telechelic polymers can displaysome interesting morphologies and properties. Wang et al.[17] reported synthesis of POSS-capped PNIPAAm teleche-lics with a POSS-spreading strategy. In this approach, aPOSS-capped trithiocarbonate was synthesized and usedas a chain transfer agent, with which the reversible addi-tion-fragmentation chain transfer (RAFT) polymerizationof NIPAAm was carried out and thus two ends of PNIPAAmchain were capped with hepta(3,3,3-triuropropyl) POSS.Owing to the highly hydrophobic of the POSS end groups,the POSS-capped PNIPAAm telechelics can form physicalhydrogels in aqueous solutions. It was found that suchphysical hydrogels displayed fast deswelling and reswell-ing properties compared to traditional chemical hydrogels.</p><p>In this contribution, we reported the synthesis of hepta-phenyl POSS-capped PNIPAAm, a semi-telechelic PNIPAAmvia atom transfer radical polymerization (ATRP). Firstly, aninitiator bearing heptaphenyl POSS was synthesized via thecopper-catalyzed Huisgen 1,3-dipolar cycloaddition (i.e.,click chemistry). Thereafter, the atom transfer radical poly-merization (ATRP) of N-isopropylacrylamide (NIPAAm)was carried out with the initiator bearing POSS to affordthe POSS-capped semi-telechelic PNIPAAm. It should bepointed out that Zhang et al. [51] ever reported the synthe-sis of a semi-telechelic PNIPAAm (i.e., heptaisobutyl POSS-capped PNIPAAm) via RAFT polymerization. The RAFTagent was prepared via the direct reaction between3-aminopropylheptaisobutyl POSS and 3-benzylsulfanyl-thiocarbonylsufanylpropionic chloride. In this work, theself-assembly behavior of the organicinorganic amphi-THF (250 ml) were added with vigorous stirring. The askwas immersed into an ice-water bath and purged withhighly pure nitrogen for 1 h. After that, 3-bromopropyltri-chlorosilane (3.11 g, 12.12 mmol) dissolved in 20 ml anhy-drous THF were slowly dropped within 30 min. Thephiles was investigated by means of atomic force micros-copy (AFM), transmission electron microscopy (TEM) anddynamic laser scattering (DLS). The lower critical solutiontemperature behavior in aqueous solutions was addressedaccording to the results of cloud point analysis.</p><p>2. Experimental</p><p>2.1. Materials</p><p>Phenyltrimethoxysilane (98%) was supplied by Zhe-jiang Chem-Tech Ltd. Co., China and used as received. 3-Bromopropyltrichlorosilane (97%), sodium azide (NaN3)and N,N,N0,N0,N00-pentamethyldiethylenetriamine (PMDE-TA) were purchased from Aldrich Co., USA. Propargyl alco-hol was obtained from Alladin Reagent Co., China; it wasdried over anhydrous magnesium sulfate and distilled un-der reduced pressure before use. 2-Chloropropinoyl chlo-ride (99%) was purchased from Alfa Aesar Co., China.N-isopropylacrylamide (NIPAAm) was prepared in thislab via the reaction between isopropylamine and acryloylchloride. Both copper (I) bromide (CuBr) and copper (I)chloride (CuCl) were of chemically pure grade, suppliedby Shanghai Reagent Co., China. Tris(2-(dimethylamino)ethyl)amine (Me6TREN) was prepared by following themethods of literature by Matyjaszewski et al. [52]. Allthe solvents used in this work were obtained from com-mercial sources. Before use, tetrahydrofuran (THF) was re-uxed above sodium and distilled; triethylamine (TEA)and isopropyl alcohol (IPA) were dried over CaH2 and thendistilled; N,N-dimethylformamide (DMF) was dried overanhydrous magnesium sulfate and distilled under reducedpressure.</p><p>2.2. Synthesis of 3-bromopropylheptaphenyl POSS</p><p>Heptaphenyltricycloheptasiloxane trisodium silanolate[Na3O12Si7(C6H5)7] was synthesized by following themethod of literature reported by Fukuda et al. [53]. Typi-cally, phenyltrimethoxysilane [C6H5Si(OMe)3] (35.258 g,178.1 mmol), THF (195 ml), deionized water (4.064 g,225.8 mmol) and sodium hydroxide (3.080 g, 77.0 mmol)were charged to a ask equipped with a condenser and amagnetic stirrer. After reuxed for 5 h, the reactive systemwas cooled down to room temperature and held at thistemperature with vigorous stirring for additional 15 h. Allthe solvent and other volatile compounds were removedvia rotary evaporation and the white solids were obtained.After dried at 60 C in vacuo for 24 h, the product(12.262 g) was obtained with the yield of 98.5%.</p><p>The corner capping reaction between Na3O12Si7(C6H5)7and 3-bromopropyltrichlorosilane was carried out. Typi-cally, Na3O12Si7(C6H5)7 (10.030 g, 10.04 mmol) and trieth-</p></li><li><p>948 Y. Zheng et al. / European Polymer Journal 48 (2012) 945955</p><p>MACR</p><p>OMOLECU</p><p>LARNANOTE</p><p>CHNOLO</p><p>GYreaction was carried out at 0 C for 3 h and at room tem-perature for 24 h. The insoluble solids (i.e., sodium chlo-ride) were ltered out and the solvents together withother volatile compounds were removed via rotary evapo-ration to obtain the white solids. The solids were washedwith 50 ml methanol for three times and dried in vacuoat 30 C for 24 h and the product (6.997 g) was obtainedwith the yield of 62.3%.1H NMR (CDCl3, ppm): 7.307.55,7.707.87 (35H, C6H5), 3.40 (2H, SiCH2CH2CH2Br),2.06 (2H, SiCH2CH2CH2Br), 1.00 (2H, SiCH2CH2CH2Br). 29Si NMR (CDCl3, ppm): 65.37, 77.76, 78.15.</p><p>2.3. Synthesis of 3-azidopropylheptaphenyl POSS</p><p>3-Azidopropylheptaphenyl POSS was synthesized viathe reaction between 3-bromopropylheptaphenyl POSSand sodium azide (NaN3). Typically, 3-bromopropylhepta-phenyl POSS (3.11 g, 2.78 mmol) and NaN3 (0.21 g,3.23 mmol) were added into a ask equipped with a mag-netic stirrer and then anhydrous THF (5 ml) and DMF(6 ml) was added. The reaction was carried out at roomtemperature for 24 h. After that, the solution was concen-trated and dropped a great amount of deionized water toafford the precipitates. The precipitates were further driedat 40 C in a vacuum oven for 24 h and the product (2.24 g)was obtained with the yield of 76.6%.1H NMR (CDCl3,ppm): 7.297.54, 7.727.85 (m, 35H, C6H5), 3.24 (t, 2H,SiCH2CH2CH2N3), 1.81 (m, 2H, SiCH2CH2CH2N3),0.92 (t, 2H, SiCH2CH2CH2N3).</p><p>2.4. Synthesis of propargyl 2-chloropropionate</p><p>To a 150 ml ask equipped with a magnetic stirrer,propargyl alcohol (4.60 g, 82 mmol) dissolved in 50 ml ofanhydrous dichloromethane and TEA (8.10 g, 80 mmol)were charged. Cooled to 0 C, 2-chloropropinoyl chloride(9.52 g, 75 mmol) dissolved in 10 ml of dichloromethanewas added by dropping funnel within 40 min with vigor-ous stirring in a highly pure nitrogen atmosphere. Thereaf-ter, the reaction was performed at room temperature for24 h. After the solids were removed via ltration, the l-trate was diluted by 50 ml of dichloromethane and washedwith 40 ml of distilled water (40 ml 3). The organic layerwas dried over anhydrous magnesium sulfate. The ltratewas concentrated via rotary evaporator and then passedthrough a neutral aluminum oxide column with petroleumether as an eluent. All the solvents were removed by rotaryevaporation, a colorless liquid was obtained with the yieldof (9.03 g, 65.7%). 1H NMR (CDCl3, ppm): 4.77(2H, CHCCH2), 4.414.45 (1H, CH3CHCl), 2.52(1H, CHCCH2),1.681.70 (3H, CH3CHCl).</p><p>2.5. Synthesis of initiator bearing POSS</p><p>The copper-catalyzed Huisgen 1,3-dipolar cycloadditionreaction between 3-azidopropylheptaphenyl POSS andpropargyl 2-chloropropionate was used to prepare the ini-tiator bearing POSS. Typically, 3-azidopropylheptaphenylPOSS (1.0740 g, 1.00 mmol), propargyl 2-chloropropionate(0.1378 g, 0.95 mmol) and 10 ml tetrahydrofuran werecharged to a 25 ml round-bottom ask equipped withmagnetic stirrer. The reactive system was purged withhighly pure nitrogen for 40 min and then Cu(I)Br(14.3 mg, 0.1 mmol) and PMDETA (20.8 ll, 0.1 mmol) wereadded. The system was degassed via three pumpfreezethaw cycles. The reaction was performed at room temper-ature for 24 h and the reacted mixture was dropped into agreat amount of the mixture of methanol with water (50/50, v/v) to afford the precipitates; this procedure was re-peated three times to remove the catalyst. To removePMDETA, the precipitat...</p></li></ul>