Synthesis and self-assembly of tadpole-shaped organic/inorganic hybrid poly(N-isopropylacrylamide) containing polyhedral oligomeric silsesquioxane via RAFT polymerization
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Synthesis and Self-Assembly of Tadpole-Shaped Organic/Inorganic Hybrid Poly(N-isopropylacrylamide) ContainingPolyhedral Oligomeric Silsesquioxane via RAFTPolymerization
WEIAN ZHANG,1 LI LIU,2 XIAODONG ZHUANG,1 XIAOHUI LI,1 JINRUI BAI,1 YU CHEN1
1Lab for Advanced Materials, Department of Chemistry, East China University of Science and Technology,130 Meilong Road, Shanghai 200237, Peoples Republic of China
2School of Pharmacy, Shanghai Jiao Tong University, Shanghai 200240, Peoples Republic of China
Received 20 May 2008; accepted 8 August 2008DOI: 10.1002/pola.23010Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Aminopropylisobutyl polyhedral oligomeric silsesquioxane (POSS) wasused to prepare a POSS-containing reversible addition-fragmentation transfer(RAFT) agent. The POSS-containing RAFT agent was used in the RAFT polymeriza-tion of N-isopropylacrylamide (NIPAM) to produce tadpole-shaped organic/inorganichybrid Poly(N-isopropylacrylamide) (PNIPAM). The results show that the POSS-con-taining RAFT agent was an effective chain transfer agent in the RAFT polymeriza-tion of NIPAM, and the polymerization kinetics were found to be pseudo-rst-orderbehavior. The thermal properties of the organic/inorganic hybrid PNIPAM were alsocharacterized by differential scanning calorimetry. The glass transition temperature(Tg) of the tadpole-shaped inorganic/organic hybrid PNIPAM was enhanced by POSSmolecule. The self-assembly behavior of the tadpole-shaped inorganic/organic hybridPNIPAM was investigated by atomic force microscopy and dynamic light scattering.The results show the core-shell nanostructured micelles with a uniform diameter.The diameter of the micelle increases with the molecular weight of the hybrid PNI-PAM. Surprisingly, the micelle of the tadpole-shaped inorganic/organic hybrid PNI-PAM with low molecular weight has a much bigger and more compact core than thatwith high molecular weight. VVC 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem46: 70497061, 2008
Keywords: polyamides; polyhedral oligomeric silsesquioxane (POSS); poly(N-isopropylacrylamide); reversible addition-fragmentation chain transfer (RAFT); self-assembly; synthesis
Polymeric-inorganic nanocomposites with well-dened architectures have attracted much atten-
tion, because of their advantageous performancein mechanical and thermal properties.110 As aclass of unique inorganic agents, polyhedral oligo-meric silsesquioxanes (POSS) can be incorporatedinto polymer matrices to effectively enhance themechanical and thermal properties, oxidation re-sistance, and reduce ammability of the hybridpolymers.1125 A typical POSS molecule, repre-sented by the formula (R8Si8O12), consists of a
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 70497061 (2008)VVC 2008 Wiley Periodicals, Inc.
Correspondence to: W. Zhang (E-mail: firstname.lastname@example.org) or Y. Chen (E-mail: email@example.com)
rigid and cubic silica core with a 0.53-nm sidelength, and the core is surrounded by eightorganic corner groups, which endow the POSSmolecule with higher reactivity and solubilityin organic solvent. POSS molecules have beenextensively incorporated into polymer matricesby chemical reaction or physical blending to pre-pare many POSS-containing hybrids with goodproperties.2632
Recently, much attention has focused on usingPOSS molecules to construct hybrid polymerswith novel architectures.3336 Lichtenhan et al.,described the use of POSS molecules to preparePOSS-containing homopolymers or copolymers.33
Atom transfer radical polymerization (ATRP)has been applied to the preparation of POSS-containing polymer hybrids.37 The homopolymers,triblock copolymers, and star-shaped block copoly-mers based on POSS monomers have been syn-thesized using ATRP. POSS molecules were alsomodied into ATRP initiators to prepare POSS-containing hybrid polymers with novel architec-tures such as star and tadpole-shaped hybridpolymers.3843
Compared with ATRP and other living poly-merization techniques, reversible addition-frag-mentation chain transfer (RAFT) polymerization,as a novel living radical polymerization technique,was found later. RAFT polymerization is widelyused to design the architecture of many novelpolymeric materials with well-dened structures,since it can be applied to a wide range of mono-mers containing carboxyl, amino and ionic groupsand used under a broad range of experimentalconditions including aqueous solutions. In RAFTpolymerization, the molecular weight can be effec-tively controlled by a dynamic equilibriumbetween the propagating radical and the RAFTterminated chain transfer agents (CTAs).4447
Poly(N-isopropylacrylamide) (PNIPAM) hasalso been intensely studied. Since it possesses aphase transition at the critical solution tempera-ture (LCST) in water of 32 C, it may offer drugdelivery.48,49 Among the several living radicalpolymerization methods, RAFT polymerizationoffers a very effective method of controlling thepolymerization of NIPAM.5054
Here, we combined RAFT polymerization withPOSS molecules to prepare novel POSS-contain-ing hybrid polymers. The POSS molecule wasmodied into POSS-containing RAFT agent,which was further used to produce tadpole-shapedorganic/inorganic hybrid PNIPAMs. The thermalproperties of the POSS-containing organic/inor-
ganic hybrid PNIPAMs were characterized by dif-ferential scanning calorimetry (DSC). The self-as-sembly behavior of the resulting novel amphi-philic organic/inorganic hybrid polymer was alsostudied.
Aminoisobutyl POSS was purchased from HybridPlastics Company. Benzyl bromide and 3-mercap-topropionic were purchased from Aldrich andused without further purication. Other reagentsand solutions in analytical grade were obtainedfrom Shanghai Chemical Reagents Company. Thi-onyl chloride and carbon disulde were distilledbefore use. Tetrahydrofuran (THF) was distilledfrom a purple sodium ketyl solution. Azobisisobu-tyronitrile (AIBN) was recrystallized from etha-nol. Dichloromethane was dried over CaH2 anddistilled under nitrogen before use. N-Isopropyla-crylamide (NIPAM) was prepared according to theliterature method and recrystallized from hexanethree times.55
Synthesis of 3-Benzylsulfanylthiocarbonyl-sufanylpropionic Acid I (BSPA, RAFT Acid)
3-Benzylsulfanylthiocarbonylsufanylpropionic acidI (BSPA) was synthesized according to the reportof Stenzel et al.56 Mercapto propionic acid(10 mL, 0.12 mol) was slowly added into a solu-tion of potassium hydroxide (13.0 g, 0.23 mol) in125 mL of water. And carbon disulde (15 mL)was added dropwise into the solution for morethan 30 min with vigorous stirring, and the solu-tion was further stirred for 8 h. After that, benzylbromide (18.8 g, 0.12 mol) was added into the so-lution, and then the reaction mixture was heatedat reux for 12 h. After the reaction was cooledto room temperature, 150 mL of chloroform wasadded. The reaction mixture was acidied withconcentrated hydrochloric acid until the color ofthe organic layer became yellow. The waterphase was extracted with 100 mL of chloroformtwice. The combined organiclayers were washedwith 100 mL of saturated sodium chloride andthen with 100 mL of water twice and nally driedover anhydrous magnesium sulfate overnight.After evaporation of the solvent, the remainingproduct was crystallized in dichloromethanethree times, and 25 g of yellow solid with a yieldof 80% was obtained.
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Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola
1H NMR (CDCl3, ppm): 7.347.25 (m, 5H,APh), 4.61 (s, 2H, ACH2Ph), 3.63 (t, 2H,ASCH2CH2COOH), 2.84 (t, 2H, ASCH2CH2COOH).
Synthesis of 3-Benzylsulfanylthiocarbonyl-sufanylpropionic Chloride
BSPA (5.0 g, 0.18 mol) and 15 mL of anhydrousdichloromethane were introduced into a dried50-mL schlenk ask with a magnetic stirringbar. This schlenk ask was connected to astandard Schlenkline system and was degassedand then relled with highly pure nitrogen.Freshly distilled thionyl chloride (5 mL) wasadded dropwise into the BSPA mixture solutionover 30 mins. And then the reaction mixturewas heated at reux for about 1 h. The solventwas removed by rst by distilling at atmos-pheric pressure and then under vacuo (103
Torr), and a yellow and highly viscous liquidwas obtained, that was further dried under ahigh vacuum (103 Torr) overnight at room tem-perature. The 3-benzylsulfanylthiocarbonylsufa-nylpropionic chloride (BSPCl) was not charac-terized before use in the next step.
3-Benzylsulfanylthiocarbonylsufanyl-N-(3-(isobutylpolyhedral oligomeric silsesquioxane)propyl)propanamide (POSS-BSAP, POSS-Containing RAFTAgent)
Aminoisobutyl-POSS (2.5 g, 2.86 mmol) was dis-solved in 15 mL of absolute THF, and 0.5 mL ofdistilled pyridine was added. A solution of freshlyprepared BSPCl (1.17 g, 4.28 mmol) in 10 mL ofabsolute THF was added slowly into the abovePOSS solution at 0 C. After stirring for 12 h atroom temperature, the mixture was ltered, andthe ltrate was concentrated under reduced pres-sure by heating evaporation. The residue waspuried by a silica gel chromatography with ethylacetate/petroleum ether (1:4, v/v) as the eluent. Ayellow solid was obtained (2.74 g, yield 85%).
1H NMR (CDCl3, ppm): 7.277.34 (m, 5H,APh), 4.61(s, 2H, ACH2Ph), 3.67 (t, 2H, ASiACH2CH2CH2NHA), 3.25 (q, 2H, AHNCOCH2CH2SA),2.60 (t, 2H, AHNCOCH2CH2SA), 1.85 (m, 7H,ASiACH2CH(CH3)2), 1.60 (m, 2H, ASiACH2CH2CH2NHA), 0.951 (d, 42H, ASiACH2CH(CH3)2),0.599 (q, 16H, ASiACH2CH(CH3)2, ASiACH2CH(CH3)2).
A typical polymerization procedure for the synthe-sis of POSS-containing inorganic/organic hybridPNIPAM as follows. A 10-mL glass ask contain-ing a stir bar was charged with NIPAM (0.5 g,4.42 mmol), POSS-containing RAFT agent(25.0 mg, 0.022 mmol), AIBN (1.2 mg, 0.0074mmol), and 1.0 mL of 1,4-dioxane. The glass askwas connected to a standard Schlenkline systemwith highly pure nitrogen. The solution wasdegassed by three freeze-evacuate-thaw circles,and then the glass tube was sealed under a vac-uum. The polymerization was carried out in athermostated oil bath at 65 C for 6 h. The poly-merization was stopped by plunging the tube intoice water. The polymerization tube was openedand 3 mL of THF was added and then precipitatedinto 250 mL of diethyl ether. The precipitate wasltered, and the product was dried at 50 C in avacuum oven for 24 h to give a yield of 24.6%.
1H NMR (CDCl3) ppm, 4.02 (s, ANHCH(CH3)2),1.42.7 (m, polymer backbone protons, ASiACH2CH(CH3)2), 1.14 (s, ANHCH(CH3)2), 0.96 (d,ASiACH2CH(CH3)2)), 0.60 (d, ASiACH2CH(CH3)2, ASiACH2CH(CH3)2). Mn(GPC) 9, 558,Mw/Mn 1.13.
Nuclear Magnetic Resonance Spectroscopy
The 1H NMR measurements were carried out on aVarian Mercury Plus 400 MHz nuclear magneticresonance (NMR) spectrometer at 25 C. The sam-ples were dissolved with deuterated CDCl3(10 mg/mL), and the solutions were measuredwith tetramethylsilane as an internal reference.
Fourier Transform Infrared Spectroscopy
The Fourier transform infrared spectroscopy(FTIR) measurements were conducted on a Nico-let Nagma-IR 550 Fourier transform spectrometerat room temperature (25 C). The samples mixedwith KBr (about 0.5 wt %) were granulated intopowder. The mixture powder were put intobetween two stainless steel disks out of the desic-cator and pressed into akes for IR measurementsby a hydraulic pump.
Gel Permeation Chromatography
The molecular weights and molecular weightdistribution were measured on Waters 150C gel
POSS-CONTAINING RAFT AGENT 7051
Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola
permeation chromatography (GPC) equipped withUltrastyragel columns of 100, 10,000 A porosities.A series of monodisperse polystyrene (PS) stand-ards were used for calibration, and THF was usedas the eluent at a ow rate of 1 mL/min.
Differential Scanning Calorimetry
The calorimetric measurements were performedon a PerkinElmer Diamond differential scanningcalorimeter in a dry nitrogen atmosphere. Theinstrument was calibrated using a standard In-dium. The samples (about 8 mg) were rst heatedup to 180 C at 20 C/min and held at this temper-ature for 5 min to remove any thermal history, fol-lowed by quenching to 30 C. Second heatingscans were carried out from 30 to 160 C at 20 C/min and the thermograms were recorded.
Atomic Force Microscopy
Atomic force microscopy (AFM) images wereobtained using Tapping Mode on a Nanoscope IVof Digital Instruments equipped with a siliconcantilever 125 lm and E-type vertical engage pie-zoelectric scanner. The AFM samples were pre-pared by spin-coating the samples on the freshlycleaved mica and then dried naturally at roomtemperature for at least 24 h.
Dynamic Light Scattering
Dynamic light scattering (DLS) was used to inves-tigate the micellar behavior using a MalvernNano_S instrument (Malvern, U.K.). In all cases1 mg/mL micellar solution were analyzed at ascattering angle of 90 and at 25 C. All the mea-surements were repeated three times and aver-aged value were used to determine the meandiameter (standard deviation).
RESULTS AND DISCUSSION
Synthesis of POSS-Containing RAFT Agent
In RAFT polymerization, the RAFT agent is themost crucial factor, and the transfer capability ofthe CTA determines the process of the controllingpolymerization and the molecular weight disper-sion. Nevertheless, for RAFT polymerization ofNIPAM, previous reports demonstrated thatdithioester and trithioester were very effective inthe polymerization process.53,5760 Even for thetrithioester, RAFT polymerization can still be well
controlled in aqueous solution at ambient temper-ature.51 However, a dithioester is more effectivethan trithioester in many RAFT systems.4447,6163
In our study, we chose trithioester BSPA as theprecursor to prepare POSS-containing CTA.BSPA has been shown to be an effective CTA inmany RAFT polymerization systems. Since itcontains a carbonyl group, it can be organicallymodied to prepare novel CTAs.64,65 For exam-ple, BSPA was used to modify hyperbranchedpolyesters to prepare a hyperbranched polymercontaining a hyperbranched polyester core.56 Inthis work, aminopropylisobutyl POSS was modi-ed to a RAFT agent by BSPA via the reactionbetween the POSS amino group and BSPCl. ThePOSS-containing RAFT agent was then used toprepare the POSS-containing inorganic/organichybrid PNIPAMs (Scheme 1). Figure 1 shows thetypical 1H NMR spectrum of the POSS-contain-ing RAFT agent. The signals at d 0.951 ppm(a), 1.85 ppm (b), 0.599 ppm (c, d), 1.60 ppm (e)and 3.67 ppm (f) are respectively, assigned to themethyl protons, methine protons, and the pro-tons of the methylene of POSS moiety. The sig-nals at d 2.60 ppm (g), 3.25 ppm (h), 4.61 ppm(i) and 7.277.34 ppm (j) are respectively,ascribed to the resonance of the protons of themethylene (AHNCOCH2CH2SA, AHNCOCH2CH2SA, ASCH2C6H5) and the aromatic protons.Compared with the 1H NMR spectrum of BSPA,the methylene protons peak (g) shifts from 2.84ppm in BSPA to 2.60 ppm in the POSS-contain-ing CTA. In addition...