New organic–inorganic nanohybrids via ring opening polymerization of (di)lactones initiated by functionalized polyhedral oligomeric silsesquioxane

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<ul><li><p>Macromolecular N</p><p>New organicinorganic nanopolymerization of (di</p><p>by functionalized polyhedral oligomeric silsesquioxane</p><p>polyester chains onto the POSS nanocages was assessed by selective fractionation experiment, Fourier transform infra-red spectroscopy (FTIR) and further conrmed by proton nuclear magnetic resonance (1H NMR). The polymerization</p><p>ments has appeared to be a very ecient way tomodify polymer-based materials. By means of agood choice of the blended partners and an ecient</p><p>erved.</p><p>and Composite Materials, University of Mons-Hainaut, Place deParc 20, 7000 Mons, Belgium. Tel.: +32 65373483; fax: +3265373484.</p><p>E-mail address: philippe.dubois@umh.ac.be (P. Dubois).</p><p>European Polymer Journal 43 (</p><p>EUROPEANPOLYMER</p><p>MACROMOLECULAR</p><p>NANOTECHNOLOGY0014-3057/$ - see front matter 2007 Elsevier Ltd. All rights resproved to be well controlled as attested by the quite narrow polydispersity indices as determined by size exclusion chro-matography (SEC). Furthermore, well-dened semi-crystalline poly(e-caprolactone)-b-polylactide block copolymer carry-ing such POSS nanoparticle at one end was successfully synthesized attesting for the very ecient control over thepolymerization reaction. 2007 Elsevier Ltd. All rights reserved.</p><p>Keywords: Nanohybrids; Nanocomposites; POSS; Polyesters; Ring opening polymerization</p><p>1. Introduction</p><p>Since the emergence of nanotechnologies, astrong demand exists to continuously improve theproperties of the so-called nanomaterials. Thus,the introduction of nano-sized inorganic reinforce-</p><p>* Corresponding author. Address: Laboratory of PolymericAnne-Lise Gon a, Emmanuel Duquesne b, Sebastien Moins a,Michael Alexandre b, Philippe Dubois a,b,*</p><p>a Laboratory of Polymeric and Composite Materials, University of Mons-Hainaut, Place de Parc 20, 7000 Mons, Belgiumb Materia Nova Asbl, Parc Initialis, Avenue Copernic 1, 7000 Mons, Belgium</p><p>Received 4 June 2007; received in revised form 21 July 2007; accepted 22 July 2007Available online 10 August 2007</p><p>Abstract</p><p>The synthesis of new organicinorganic materials were investigated. Poly(e-caprolactone) and poly(L, L-lactide) cova-lently end-capped by a polyhedral oligomeric silsesquioxane (POSS) moiety, leading to new nanohybrid materials, weresuccessfully prepared by coordinationinsertion ring opening polymerization (ROP) of e-caprolactone (e-CL) and L,L-lac-tide (L,L-LA) respectively. The reaction was initiated from the primary amine available on aminopropylheptakis(isobu-tyl)POSS nanoparticles and catalyzed by tin(II) 2-ethylhexanoate (tin octoate, Sn(oct)2). The covalent grafting of thedoi:10.1016/j.eurpolymj.2007.07.041anotechnology</p><p>hybrids via ring opening)lactones initiated</p><p>2007) 41034113</p><p>www.elsevier.com/locate/europolj</p><p>JOURNAL</p></li><li><p>4104 A.-L. Gon et al. / European Polymer Journal 43 (2007) 41034113</p><p>MACROMOLECULAR</p><p>NANOTECHNOLOGYcompounding process, this technique can lead to theproduction of new materials with improved proper-ties. Indeed, it is now well known that the introduc-tion of rigid ller into thermoplastic or thermosetmatrices may lead to some improvement of theirmechanical, thermal, barrier, electrical or re resis-tance properties [14], depending on the nature ofthe chosen ller. Compared to micrometric size ll-ers for which a high amount of ller content isrequired for a real improvement of the compositeproperties, only a few percents of nanollers (15 wt%) are usually sucient to obtain a nanocom-posite showing the same behaviors, if not better [5].</p><p>Among all the nanollers, layered silicates havebeen indubitably the most reported in the literature[68]. Even if they have drawn less attention thanlayered silicates, polyhedral oligomeric silsesquiox-ane (POSS) have been more and more studied inthe past decade [9,10]. Indeed, they have proved toinuence the thermal, mechanical [5] and particu-larly re resistance [2,3,11] properties of polymermaterials. Typically POSS nanoparticle is a 3D cagelike siloxane structure surrounded by eight organicR groups (RSiO1.5). These organic groups can behydrocarbon species, which allow compatibilizationand solubility of the particles into polymer matrices,or they can embody a large range of functionalgroups suitable for the preparation of nanohybridsby copolymerization or grafting [12]. The struc-tureproperty relationship of polymer incorporatingPOSS has been recently reviewed [9,13] and interest-ingly, several recent publications reported on theuse of these new nanomaterials in many dierentapplications [5,1421] including the potential ofPOSS-based nanocomposites in the biomedical eld[22,23]. However, as depicted in the literature, inmost of the cases, such nanollers tend to aggregatewhen they are simply melt blended within polymermatrices, mainly due to the existence of numerousnanollernanoller interactions [24,25]. In orderto improve the dispersion, the nanoller and thematrix need to be compatibilized. For instance,the nanoller can be organo-modied in order topromote more favorable nanollerpolymer interac-tions, leading to a reduction of the interfacialenergy. The creation of a strong chemical bondbetween the ller and the polymer matrix allowsfor a complementary enhancement of the materialproperties compared to the simple blends. Indeed,the presence of a covalent bond can improve theinterfacial adhesion between the organic and the</p><p>inorganic part of the composition, leading conse-quently to a material with better mechanical proper-ties for instance, as nicely reviewed by Pittman in2001 [26].</p><p>The synthesis of POSS-based nanohybrids, i.e.,polymer grafted POSS nanoparticles has drawn alot of attention over the last decade. For instance,Mather and et al. reported the preparation ofamphiphilic telechelic incorporating POSS copoly-mers. Indeed, a, x diol end-capped PEG homopoly-mers of various molecular weights were reacted withmonoisocyanate functionalized POSS nanoparticles[27]. They evidenced a strong modication of thethermal and morphological properties of the amphi-philic telechelics by controlling the hydrophilichydrophobic balance. Later, the group of Coughlinhas used the same monoisocyanatePOSS to synthe-size hemi-telechelic polystyrenePOSS copolymer[28]. Authors have developed a synthetic protocolthat can act as an experimental model to probethe ordering or aggregation behavior of inorganicnanoparticles within polymeric matrices.</p><p>In this contribution, we report on the synthesis ofwell-dened POSSpoly(e-caprolactone) (POSSPCL) and POSSpoly(L,L-lactide) (POSSPLA)nanohybrids prepared by ring opening polymeriza-tion (ROP) of e-caprolactone (CL) and L,L-lactide(LA) catalyzed by tin(II) 2-ethylhexanoate (tin octo-ate, Sn(oct)2). Purposely, the ROP is initiated by theprimary amine group made available onto anaminopropylheptakis(isobutyl)POSS (POSS-NH2).PCL and PLA have been chosen as the organiccounterpart of these novel nanohybrids owing totheir biodegradability and biocompatibility, whichcan be consequently useful as environmentallyfriendly material or for biomedical applications.Moreover, PCL is well known to be miscible or atleast mechanically compatible with a large varietyof polymers, e.g., SAN, PVC, chlorinated polyethyl-ene. . . [29]. Therefore, the as-synthesized POSSPCL nanohybrids could then be used as surface-compatibilized POSS for further dispersion in poly-meric matrices known to be miscible (or simplymechanically compatible) with the PCL segment.It is worth pointing out that very recently Zhenget al. have interestingly reported about the synthesisof octaarmed star PCL with a POSS core. Accord-ingly a octa(3-hydroxypropyl)POSS was consideredas an octafunctional initiator for promoting the CLring opening polymerization [32]. Even morerecently (while this contribution was submitted forpublication), Ni and Zheng extrapolated the CL</p><p>polymerization using 3-hydroxypropylheptaphenyl</p></li><li><p>A.-L. Gon et al. / European Polymer Journal 43 (2007) 41034113 4105</p><p>MACROMOLECULAR</p><p>NANOTECHNOLOGYPOSS as a monofunctional intiator and allowing forsupramolecular inclusion complexes between theresulting POSS-b-PCL and alpha-cyclodextrin [33].</p><p>2. Experimental section</p><p>2.1. Materials</p><p>e-Caprolactone (CL) (99%, Acros) was dried 48 hover calcium hydride and distilled under reducedpressure. Toluene (p.a., Labscan) was dried overcalcium hydride and distilled. L,L-Lactide (LA)(99%, Galactic) was recristallized in toluene at0 C and dried under vacuum at ambient tempera-ture. Tin(II) ethylhexanoate (Sn(Oct)2) (95%,Aldrich) was used as received without further puri-cation. Aminopropylheptakis(isobutyl)POSS(POSS-NH2, 97%) was purchased from HybridPlastics. Note that the purity (i.e., amino functional-ization) of the received sample was actually esti-mated to 83% by multinuclear NMR, theimpurities proved to be open POSS deprived ofamino-functionalization. Since such impurities didnot participate in the polymerization reaction (andthe resulting polyester grafting) and furthermorethey were readily eliminated at the end of the poly-merization by simple selective precipitation step (seepolymerization procedure hereafter), the POSS-NH2 sample was used as received.</p><p>2.2. Polymerization procedure</p><p>Typically, 0.2 g (nNH2 = 2.26 104 mol) of</p><p>aminopropylheptakis(isobutyl)POSS (POSS-NH2)was introduced under nitrogen ow in a two-neckask equipped with a three-way stopcock and amagnetic stirring bar. Then, toluene (7 mL),Sn(Oct)2 (0.2 mL of a 0.09 M toluene solution)and e-caprolactone (VCL = 2 mL; mCL = 2.06 g;nCL = 0.018 mol) were added successively undernitrogen using previously ame-dried syringes. Themixture was then heated at 100 C under stirring.After the desired reaction time, the polymerizationwas stopped by adding of a few drops of dilutedhydrochloric acid (1 M). The POSS end-cappedPCL was recovered from heptane precipitation, l-tered and dried until constant weight at 40 C undervacuum. As far as L,L-lactide (LA) (co)polymeriza-tion was concerned, a determined amount of puri-ed LA (previously dissolved in hot dry toluene)was added to the reaction medium (after CL poly-</p><p>merization in the case of POSS-P(CL-b-LA) synthe-sis) and the (co)polymerization allowed to proceedat 100 C (80 C for POSS-b-PLA) before additionof a few drops of diluted hydrochloric acid(0.1 M) and further precipitation, ltration and dry-ing as reported above.</p><p>2.3. Characterization</p><p>Proton and carbon nuclear magnetic resonance(1H and 13C NMR) spectra were recorded on solu-tions prepared in CDCl3 using a Bruker AMX-300apparatus at a frequency of 300 MHz. Fouriertransform infrared (FTIR) spectra were recordedusing a BIO-RAD Excalibur spectrometer equippedwith an ATR Harrick Split PeaTM. Size exclusionchromatography (SEC) was performed in THF(sample concentration: 1 wt%) at 35 C using apolymer laboratories (PL) liquid chromatographequipped with a PL-DG802 degazer, an isocraticHPLC pump LC1120 (ow rate: 1 mL/min), aBasic-Marathon Autosampler, a PL-RI refractiveindex detector and four columns: a guard columnPLgel 10 lm (50 7.5 mm) and three columnsPLgel mixed-B 10 lm (300 7.5 mm). Molecularweight and molecular weight distribution were cal-culated by reference to a calibration curve built upby using polystyrene standards. The thermal behav-ior of the copolymer was analyzed by dierentialscanning calorimetry (DSC) using a DSC Q100from TA Instrument at a heating and cooling ratesof 10 K/min under nitrogen ow.</p><p>3. Results and discussion</p><p>3.1. Synthesis of POSSPCL nanohybrids</p><p>In a rst series of experiments, the polymeriza-tion/grafting of polyesters chains has been per-formed using commercially available POSS-NH2nanollers (from Hybrid Plastics) in toluene in a rel-atively diluted medium ([CL]0 = 2 M) at high tem-perature (T = 100 C). The monomer conversionhas been recorded in function of time and themolecular parameters of the as-synthesized POSSend-capped PCL chains characterized by 1H NMRand SEC. The experimental conditions and resultsare reported in Table 1. The mechanism of theROP of cyclic esters initiated by tin octoate and pri-mary amines has been deeply studied and recentlyreported in the literature [30]. Accordingly, thepolymerization reaction proved to take place</p><p>through a coordinationinsertion mechanism</p></li><li><p>involving the O-acyl cleavage of the cyclicmonomer.</p><p>After one hour of polymerization time, no poly-</p><p>are soluble enough to lead to a partial fractionationof the sample. After 2 h, the grafted PCL chain getssuciently long to promote the POSSPCL nano-</p><p>Table 1Characterization of the POSSPCL and POSSPLA nanohybrids as synthesized by ROP of CL and L,L-LA, respectively</p><p>Entry Monomer Time (h) Conv.a (%) DPthb DPNMR</p><p>c Mn SECd (g/mol) Mw/Mn</p><p>d</p><p>1 CL 2 24 23 40 6500 1.222 CL 3 42 40 50 9400 1.173 CL 4 86 83 85 14,300 1.274 CL 5 81 78 85 14,800 1.315 CL 6 89 85 75 13,700 1.336 CL 7 99 95 100 18,600 1.38</p><p>7 LA 1 53 48 42 5000 1.078 LA 2 71 64 57 6700 1.089 LA 3 79 71 66 7900 1.0810 LA 4 86 78 72 8200 1.0911 LA 5 90 82 75 8300 1.09</p><p>([CL]0[NH2]0 = 95; [NH2]0/[Sn(oct)2]0 = 2.5; [CL]0 = 2 M; T = 100 C) ([LA]0/[NH2]0 = 90; [NH2]0/[Sn(oct)2]0 = 2.5; [LA]0 = 1 M;T = 80 C).a As measured by gravimetry.b DPth = ([Monomer]0/[NH2]0) conv.c As measured by 1H NMR in CDCl3; DP(PCL)NMR = [(Id/2)/(Ih,f+g/58)] (cf. Fig. 1). DP(PLA)NMR = [(Ia)/(Ih,f+g/58)] (cf. Fig. 5).d As measured by SEC (relative to polystyrene standards).</p><p>4106 A.-L. Gon et al. / European Polymer Journal 43 (2007) 41034113</p><p>MACROMOLECULAR</p><p>NANOTECHNOLOGYmer is recovered from tentative precipitation in hep-tane. This observation can be explained by thesolubility of POSS-NH2 in this solvent. Indeed,the POSS grafted with short polyester oligomerFig. 1. 1H NMR spectrum of the POSS-NH2 and a POhybrid precipitation allowing the monomer conver-sion measurement. A typical 1H NMR spectrum isdepicted in Fig. 1. This spectrum attests for thecovalent grafting of the PCL chains onto the POSSSSPCL nanohybrid in CDCl3 (entry 2, Table 1).</p></li><li><p>cages by the quantitative shift of the signal CH2NH2 from 2.6 ppm (signal i in Fig. 1) to a new signalCH2NHC(O) at 3.2 ppm (signal i</p><p>0 in Fig. 1).Moreover, one can note the presence of a signalcharacteristic of an amide proton around 5.5 ppm.These observations have been conrmed by FTIRanalysis. Indeed, the FTIR spectrum in Fig. 2. evi-dences the presence of both POSS and PCL by theappearance of the siloxane and carbonyl character-istic absorption bands at 1099 and 1720 cm1,respectively. Moreover, the presence of a weak bandaround 1648 cm1 characteristic of an amide link-</p><p>age is again in favor for the covalent grafting ofthe polyester chains.</p><p>In Table 1, one can note a gradual increase of themonomer conversion in function of reaction time.This evolution goes with a continuous growth ofthe grafted polyester chains as attested by the poly-merization degree (DP) measured by 1H NMR.Interestingly enough, this chain growth is clearlyevidenced by SEC (Fig. 3). Fig. 3 displays the evo-lution of the SEC signal towards lower elution vol-umes, i.e., higher molecular weights, with increasingmonomer conversion. Moreover, a good control</p><p>70</p><p>75</p><p>80</p><p>85</p><p>90</p><p>95</p><p>100</p><p>700120017...</p></li></ul>

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