Amphiphilic Polyesters Derived from Silylated and ... - ACS Publications

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Biomacromolecules 2009, 10, 850–857

Amphiphilic Polyesters Derived from Silylated and Germylated Fatty Compounds Nadia Katir, Abdelkrim El Kadib, Mohamed Dahrouch,* Annie Castel,* Nicolas Gatica, Zahra Benmaarouf, and Pierre Riviere Laboratoire d’He´te´rochimie Fondamentale et Appliqueé, UMR-CNRS 5069, Universite´ Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse cedex 9, France, Laboratoire des Substances Naturelles, Universite´ Ibn Zhor, Faculte´ des Sciences, Hay Dakhla, BP 8106, Agadir, Maroc, Departamento de Quı´mica Orgańica, Facultad de Ciencias Quı´micas, Universidad de Concepcioń, Casilla 160-C, Concepcioń, Chile, Departamento de Polimeros, Facultad de Ciencias Quı´micas, Universidad de Concepcioń, Casilla 160-C, Concepcioń, Chile Received November 21, 2008; Revised Manuscript Received February 3, 2009

New classes of amphiphilic polyesters were prepared from metallated (Si, Ge) fatty methyl ester (FAME) precursors and poly(tetramethylene oxide) glycol. Hydrosilylation of 10-undecenoic methyl ester by tetramethyldisiloxane occurred at 80 °C in the presence of Karstedt’s catalyst, and hydrogermylation of the same FAME derivative was obtained at the same temperature under radical AIBN initiation. These diester precursors, obtained in high yields (≈90%), reacted with poly(tetramethylene oxide) glycol under free solvent to give silicon polymers or germanium oligomers. These condensed materials display both the characteristic of organic-inorganic hybrid materials and those of amphiphilic polymers. The nature of organometallic fragment (hydrophobicity of tetramethyldisiloxy and sterical hindrance of diphenylgermyl) was shown to influence the chemical reactivity of the polymerizable monomers and the physical properties of the resulting copolymers. The amphiphilicity of these materials provides a driving force for the formation of small objects (∼1 nm), making them very attractive as hybrid nanocontainers.

Introduction One of the most important objectives of modern chemistry is the replacement of petroleum by inexpensive, biodegradable, and renewable starting materials.1-8 For this reason, the design of polymers from renewable sources has currently attracted increasing attention. Natural oils are one of the most important renewable raw materials having great advantages, including low cost and ready availability.9,10 They are generally triglyceride esters of fatty acids which offer several reactive sites for functionalization, such as, double bonds, allylic carbons, and ester groups. Polymers synthesized from these triglycerides or from fatty acids have been extensively investigated in recent years.11,12 Hitherto, the efforts have mainly concentrated on the unsaturation functionality, for example, copolymerization with ethylenic compounds such as divinylbenzene and styrene leading to thermoset copolymers,13 metathesis reactions,14 or direct polymerization of acrylate,15 glycidyl ether,16 and norbornyl17 derivatives of the oil. Hydrosilylation of unsaturated fatty acids derivatives has also been used to produce organic-inorganic hybrid materials.18 Moreover, polyesters are a very important class of organic polymers because of their numerous applications, including drug delivery,19-22 medical devices, tissue engineering,23 and thermoplastic elastomers.24,25 In this general context, the common raw biomaterials include poly(lactic acid), poly(glycolic acid), poly(caprolactone), and their copolymers.26-28 Most examples of these polymers have been prepared by ring opening polymerization (ROP) of derivatized lactones or lactides and include materials with amino, carboxyl, and hydroxyl groups.29-33 However, although spectacular improvements have * To whom correspondence should be addressed. Telephone: 56 41 207068 (M.D.); 33 5 61 55 84 12 (A.C.). Fax: 56 41 245974 (M.D.); 33 5 61 55 82 04 (A.C.). E-mail: [email protected] (M.D.); castel@ chimie.ups-tlse.fr (A.C.).

been achieved in terms of activity and productivity,34-36 the relief of ring strain, which provides the driving force for the ROP, remains modest so that highly active promoters are required if the ROP of monomers is to occur under mild conditions. In addition, complex synthesis of the functionalized ring is required thus limiting the ease of tunability. Associated drawbacks are the typically high sensitivity of the promoters to these reactions, and significant amounts of undesirable transesterification reactions. By comparison, the direct polycondensation of diacid (or diester) with a diol was less developed.37-40 More surprisingly, there is, to the best of our knowledge, only one example concerning the use of dimerized fatty acid for the preparation of new thermoplastic multiblock elastomers by polycondensation reactions of polyesters (prepared from dimerized fatty acid and 1,4-butanediol) and oligoamide PA12.41 Regarding the application of these polyesters particularly in drug delivery and biomedical fields, the modification of material surface by controlled scaffolds hydrophilicity constitutes a versatile tool for tuning properties.23 These modifications include incorporation of hydrophobic segments and immobilization of bioactive compounds. Obviously, the introduction of fatty acid methyl esters (FAME) in these materials should be very promising to enhance their hydrophobicity and biocompatibility. Recently, we initiated a research program aimed at investigating various aspects of silylation of oleochemical compounds to obtain hydrophilic and hydrophobic biocompatible polysiloxanes. These studies have allowed silylation both at the terminal and at various internal positions of the FAME.42-46 Based on these results, we propose an original way to synthesize silylated and germylated fatty diesters using dihydrometalation reactions. Moreover, this incorporation of organometallic fragments represents a stimulating challenge because the organic-inorganic hybrid materials have unexpected properties. For example,

10.1021/bm8013457 CCC: $40.75  2009 American Chemical Society Published on Web 03/18/2009

Amphiphilic Polyesters Derived from Fatty Compounds

siloxane-containing polyurethanes have been extensively studied and often exhibit improved thermal stabilities and good mechanical properties.47-50 The second objective of this work is to prepare amphiphilic polyesters containing a hydrophobic fragment in the FAME chain, a hydrophilic segment from diol and an organometallic moiety by polytransesterification between aliphatic metallated (Si, Ge) diesters and poly(tetramethylene oxide) glycol. Their structural characteristics will be elucidated by various techniques including NMR and IR spectroscopies and gel permeation chromatography (GPC). Their physical properties will be investigated by intrinsic viscosity, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The amphiphilic character of these materials is highlighted by the formation of small nano-objects evidenced by transmission electronic microscopy analyses (TEM). This “green” and straightforward strategy bears opportunities that will further advance the potential application of oily compounds in various fields of nanochemistry and biomaterials.

Experimental Section Materials. All reactions were done under nitrogen using standard Schlenk tube techniques and dry solvents. Methyl 10-undecenoate, 1,1,3,3-tetramethyldisiloxane, and poly(tetramethylene oxide) glycol, with a molecular weight Mn of 650 and 1000 g/mol, were purchased from Aldrich and used without supplementary purification. Diphenylgermane was prepared according to the literature procedure.51 Karstedt’s catalyst [Pt0] [platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex, 0.100 M in poly(dimethylsiloxane)], AIBN (azo bis-isobutyronitrile (Me2C(CN)-Nd)2), and titanium-tetrabutoxide Ti(OBu)4 were used in a 1% concentration relative to organic reagents. Characterization Techniques. NMR spectra were recorded with the following spectrometers: 1H, Bruker Avance II 300 (300.13 MHz); 13C, Bruker Avance II 300 (75.47 MHz); 29Si, Bruker Avance II 300 MHz (59.62 MHz) and Avance II 400 MHz (79.49 MHz). Mass spectra were recorded with a Hewlett-Packard HP 5989 instrument in the electron impact mode (EI, 70 eV). Infrared spectra were recorded on a Perkin-Elmer 1600 FT spectrometer. Elemental analyses were done at the Centre de Microanalyses de l’Ecole Nationale Supe´rieure de Chimie de Toulouse. Gel permeation chromatography (GPC) was performed using a Perkin-Elmer Series 200 instrument at 20 °C with a refraction index detector. All runs were performed with tetrahydrofuran (THF) as the eluent at a flow rate of 0.5 mL/min. A molecular weight calibration curve was obtained with poly(styrene) standards in the range of molecular weight 580-3200000 g/mol. The number average (Mn) and weight average (Mw) molecular weights were evaluated from these measurements. The viscosity solution of the polymers in THF were measured at 25 °C using an Ubbelohde viscometer. The intrinsic viscosity, [η], was determined from measurements at five solution concentrations. Thermogravimetric (TG) measurements were conducted with STA 625 system in platinum pans at prescribed heating rates of 10 °C/min in a range of 25 to 550 °C under a steady flow of nitrogen (42 mL/ min). Differential scanning calorimetry (DSC) was performed with TA Q200 instruments in sealed aluminum pan under a nitrogen atmosphere (gas flow: 50 mL/min) in the temperature range from -60 to 200 °C at a heating and cooling rate of 5 °C/min. The polymers were heated from -60 to 200 °C (first scan), cooled to -60 °C (second scan), and then heated to 200 °C (third scan). The melting temperature (Tm) and the crystallization temperature (Tc) were determined as the peak value of the endothermal and the exothermal phenomena in the DSC curve, respectively. The heat of fusion (∆Hm) and the heat of crystallization (∆Hc) of the crystal phase were calculated from the areas of the DSC endotherm, respectively. Sample for transmission electron microscopy (TEM) was performed on a Jeol 1200EX II at an activation voltage of 120 kV.

Biomacromolecules, Vol. 10, No. 4, 2009

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Preparation of Silylated Diester 1. A solution of methyl 10undecenoate (2.00 g, 10.01 mmol) and tetramethyldisiloxane (0.68 g, 5.07 mmol) was heated in toluene (4 mL) in the presence of Karstedt’s catalyst (Pt0) at 80 °C for 4 h. The solvent was evaporated and the residue distilled to give 2.35 g of 1.52 Yield: 88%; bp 180 °C/0.03 mmHg. 1H NMR (CDCl3) (300.13 MHz): δ 0.03 (s, 12H, CH3-Si), 0.46 (t, 3J ) 7.1 Hz, 4H, CH2-Si), 1.24 (sbr, 28H, (CH2)7), 1.58 (m, 4H, CH2-CH2-CO), 2.26 (t, 3J ) 7.5 Hz, 4H, CH2-CO), 3.62 (s, 6H, OCH3). 13C NMR (CDCl3; 75.48 MHz): δ 0.40 (CH3-Si), 18.41 (CH2Si), 23.29, 24.97, 29.18, 29.29, 29.40, 29.55, 33.46 (CH2)8), 34.10 (CH2CO), 51.41 (OCH3), 174.30 (CO). 29Si NMR (CDCl3; 59.62 MHz): δ 7.21. IR (pure): ν ) 1739 (CdO) cm-1. MS (EI): m/z ) 515 [M CH3, 5%], 499 [M - OCH3, 11%]. Anal. Found: C, 62.79; H, 10.69. Calcd for C28H58O5Si2: C, 63.34; H, 11.01. Preparation of Germylated Diester 2. A solution of methyl 10undecenoate (1.00 g, 5.05 mmol) and diphenylgermane (0.58 g, 2.52 mmol) was heated in toluene (1.5 mL) in the presence of AIBN at 80 °C for 2 h. The solvent was evaporated and the residue distilled to give 1.38 g of 2. Yield: 87%; bp 245 °C/0.03 mmHg. 1H NMR (CDCl3; 300.13 MHz): δ 1.16 (sbr, 28H, (CH2)7), 1.33 (m, 4H, CH2-Ge), 1.52 (m, 4H, CH2-CH2-CO), 2.21 (t, 3J ) 6.6 Hz, 4H, CH2-CO), 3.57 (s, 6H, OCH3), 7.24-7.38 (m, 10 H, C6H5). 13C NMR (CDCl3; 75.48 MHz): δ 13.32 (CH2-Ge), 25.03, 25.54, 27.58, 29.24, 29.33, 29.53, 29.58 ((CH2)7), 32.65 (CH2-CH2-CO), 34.14 (CH2-CO), 51.49 (OCH3), 128.41 (Cm), 128.51 (Cp), 134.50 (Co), 138.87 (Cipso), 174.30 (CO). IR (pure): ν ) 1740 (CdO) cm-1. MS (EI): m/z ) 626 [M+, 1%], 595 [M - OCH3, 5%], 549 [M - Ph, 11%]. Anal. Found: C, 69.14; H, 9.02. Calcd for C36H56O5Ge2: C, 69.40; H, 9.36. Preparation of Polymer 3a. Poly(tetramethylene oxide) glycol with Mn of 650 g/mol (PTMO-650; 0.61 g, 0.94 mmol) and Ti(OBu)4 were added to 1 (0.50 g, 0.94 mmol) under stirring. The mixture was heated to 140 °C and the pressure was reduced to 0.5 mmHg. When the temperature reached 190 °C, the polycondensation in the melt was stirred for 1 h. After cooling the mixture, 1.06 g of a sticky solid was obtained. Yield: 98%. 1H NMR (CDCl3; 300.13 MHz): δ -0.06 (s, 12H, CH3-Si), 0.42 (t, 3J ) 7.2 Hz, 4H, CH2-Si), 1.20 (sbr, 28H, (CH2)7), 1.55 (sbr, (CH2)2-CH2-O and CH2-CH2-CO), 2.22 (t, 3J ) 7.6 Hz, 4H, CH2-CO); 3.35 (sbr, CH2-O-CH2), 4.02 (t, 3J ) 6.3 Hz, 4H, COOCH2). 13C NMR (CDCl3; 75.48 MHz): δ 0.41 (CH3-Si), 18.44 (CH2-Si), 23.31, 25.02, 25.56, 26.29, 29.22, 29.32, 29.40, 29.55, 29.58, 33.47 ((CH2)7-CH2), 26.53 ((CH2)2-CH2-O), 34.38 (CH2-CO), 64.09 (COOCH2), 70.63 (CH2-O-CH2), 173.96 (CO). 29Si NMR (CDCl3; 79.49 MHz): δ 7.24. IR (pure): ν ) 1734 (CdO) cm-1. Preparation of Polymer 3b. A similar procedure to that described for 3a was used. Diester 1 (0.50 g, 0.94 mmol), poly(tetramethylene oxide) glycol with Mn of 1000 g/mol (PTMO-1000; 0.94 g, 0.94 mmol), and Ti(OBu)4 gave 3b: 1.33 g as a sticky solid. Yield: 94%. 1H NMR (CDCl3) (300.13 MHz): δ -0.05 (s, 12H, (CH3-Si), 0.41 (t, 3J ) 7.3 Hz, 4H, CH2-Si), 1.19 (s.br, 28H, (CH2)7), 1.54 (s.br, (CH2)2-CH2-O and CH2-CH2-CO), 2.20 (t, 3J ) 7.6 Hz, 4H, CH2-CO), 3.34 (s.br, CH2O-CH2), 4.00 (t, 3J ) 6.3 Hz, 4H, COOCH2). 13C NMR (CDCl3; 75.48 MHz): δ 0.40 (CH3-Si), 18.42 (CH2-Si), 23.30, 25.01, 25.55, 26.28, 29.20, 29.30, 29.39, 29.53, 29.56, 33.45 ((CH2)7-CH2), 26.52 ((CH2)2CH2-O), 34.37 (CH2-CO), 64.08 (COOCH2), 70.61 (CH2-O-CH2), 173.93 (CO). 29Si NMR (CDCl3; 79.49 MHz): δ 7.23. IR (pure): ν ) 1729 (CdO) cm-1. Preparation of Polymer 4a. A similar procedure to that described for 3a was used. Diester 2 (0.50 g, 0.80 mmol), PTMO-650 (0.51 g, 0.80 mmol), and Ti(OBu)4 gave 4a: 0.93 g as a sticky solid. Yield: 96%. 1H NMR (CDCl3; 300.13 MHz): δ 1.26 (sbr, 28H, (CH2)7), 1.64 (sbr, ((CH2)2-CH2-O and CH2-CH2-CO), 2.29 (t, 3J ) 7.5 Hz, 4H, CH2CO), 3.45 (sbr, CH2-O-CH2), 4.09 (t, 3J ) 6.3 Hz, 4H, COOCH2), 7.30-7.47 (m, 10H, C6H5). 13C NMR (CDCl3) (75.48 MHz): δ 13.29 (CH2-Ge), 25.00, 25.56, 26.28, 26.89, 29.15, 29.17, 29.27, 29.45, 29.52, 30.31 (CH2-(CH2)7-CH2-Ge), 26.52 ((CH2)2)-CH2-O), 34.36 (CH2-CO),

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64.09 (COOCH2), 70.61 (CH2-O-CH2), 127.91 (Cm), 128.12 (Cp), 134.42 (Co), 138.86 (Cipso), 173.93 (CO). IR (pure): ν ) 1729 (CdO) cm-1. Preparation of Polymer 4b. A similar procedure to that described for 3a was used. Diester 2 (0.50 g, 0.80 mmol), PTMO-1000 (0.80 g, 0.80 mmol)), and Ti(OBu)4 gave 4b: 1.20 g as a sticky solid. Yield: 96%. 1H NMR (CDCl3) (300.13 MHz): δ 1.25 (sbr, 28H, (CH2)7), 1.63 (sbr, (CH2)2-CH2-O and CH2-CH2-CO), 2.29 (t,3J ) 7.5 Hz, 4H, CH2CO), 3.42 (sbr, CH2-O-CH2), 4.09 (t, 3J ) 6.3 Hz, 4H, COOCH2), 7.30-7.49 (m, 10H, C6H5). 13C NMR (CDCl3; 75.477 MHz): δ 13.28 (CH2-Ge), 24.99, 25.55, 26.28, 26.89, 29.14, 29.17, 29.26, 29.45, 29.52, 30.31 (CH2-(CH2)7-CH2-Ge), 26.52 ((CH2)2-CH2-O), 34.36 (CH2-CO), 63.97 (COOCH2), 70.49 (CH2-O-CH2), 127.95 (Cm), 128.27 (Cp), 134.42 (Co), 138.86 (Cipso), 173.92 (CO). IR (pure): ν ) 1729 (CdO) cm-1.

Results and Discussion Hydrometalation of 10-Undecenoic Methyl Ester. The fatty acid derivatives exhibit an almost endless variety of structures which differ in their chain lengths, the position and number of unsaturations. The monounsaturated ones usually represent a non-negligible fraction, about which there is great interest due to their functional potential. Among this family of compounds, the undecenoic methyl ester is usually used as representative of fatty acid methyl ester (FAME), we thus performed hydrometalation of the terminal unsaturation of this ester. Although the hydrosilylation and hydrogermylation of olefins have been widely studied,51,53-57 only a few about fatty compounds and oils have been reported.42,43 In addition, all of these reports describe the hydrometalation by a hydrometal bearing only one M-H bond. Depending on the nature of the metal (Si, Ge) and its surrounding substituants, we did these reactions with transition metal catalysts or by radical pathway initiation. Thus, the synthesis of bis(silylated fatty acid methyl ester)oxide 1 was achieved with Karstedt’s catalyst (Pt0)57,58 by using tetramethyldihydrosiloxane as the hydrometalating agent. The reaction was monitored by 1H NMR and the complete consumption of starting materials was observed after 4 h of reaction at 80 °C (eq 1).

Starting from diphenylgermane as hydrometalating agent, hydrogermylation was not observed under transition metal catalysis. By contrast, under radical pathway initiation (AIBN, 80 °C, 2 h), diphenylgermane added easily two molecules of fatty acid methyl ester to yield germylated diester 2 (eq 2).

The metallated diesters 1 and 2 were isolated as analytically pure liquids by distillation. NMR analyses (1H, 13C, and 29Si NMR) revealed the formation of one isomer: the anti-Markovnikov adduct. The 1H NMR spectra showed only one signal at 3.62 and 3.57 ppm assigned to the MeO group of 1 and 2,

Katir et al. Table 1. Weight Average Molecular Weight (Mw), Number Average Molecular Weight (Mn), Weight Average Polymerization Degree (xw), Number Average Polymerization Degree (xn), and Intrinsic Viscosity ([η]) for 3a, 3b, 4a, and 4b Mw Mn [η] polyester (g/mol) (g/mol) xw ) Mw/M0*a xn ) Mn/M0*a (dL/g) 3a 3b 4a 4b a

63400 34200 6700 11300

29600 17900 3600 4800

56.8 23.3 5.5 7.2

26.5 12.2 3.0 3.1

0.44 0.31 0.26

M0*: molecular weight of repeating unit.

respectively. This result was further confirmed by 29Si NMR since the spectrum of 1 exhibits a unique signal at 7.2 ppm, characteristic of siloxane derivatives.59 On the other hand, the 13 C and IR analyses indicated the presence of the ester function, as evident by the resonance of the carbonyl group at 174.30 ppm and the strong CdO absorptions at 1739 and 1740 cm-1 (1 and 2). Preparation of Polyesters. Amphiphilic polyesters were prepared by direct transesterification60,61 between the metallated diesters 1 and 2 and the poly(tetramethylene oxide) glycol (PTMO-650 or PTMO-1000) using Ti(OBu)4 as catalyst. A general procedure was used: in a first step, equimolecular amounts of the two reactants were gradually heated to 140 °C. Then, the reaction was carried out under vacuum (0.5 mmHg) while increasing temperature to 190 °C. The process was considered to be complete after one supplementary hour under stirring. In this way, four polyesters were obtained as yellow or pale orange sticky solids in almost quantitative yields (eq 3).

In the 1H and 13C NMR spectra, we observed the disappearance of the characteristic signals of the starting reactants: the methyl ester groups of 1 and 2 and the CH2OH group of the diols, confirming a complete transesterification reaction. New signals appeared, for example, in the case of 4a, at 4.02 ppm (1H NMR) and 64.09 ppm (13C NMR) assignable to the CH2 group neighboring the ester fragment (OCO). Concerning the organometallic fragments (Me2Si and Ph2Ge), no significant variation was observed, suggesting the absence of interaction between the chains and, in particular, complexation between the oxygen of the carbonyl group and silicon or germanium center. This is corroborated by the IR spectra which showed a moderate increase in the absorption of the carbonyl group ∆ν ) 5-10 cm-1. Molecular weights Mw and Mn of the polyesters were determined by gel permeation chromatography (Table 1). Molecular weights of silicon products are higher than those of germylated oligomers. Moreover, the values indicate that a higher molecular weight is obtained for the silylated polymer



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Table 2. T50 Values for Studied Monomers, Oligomers, and Polymers

monomer

polymer

Figure 1. Thermal decomposition profiles of 3a (red), 3b (blue), 4a (green), 4b (black).

3a containing the poly(tetramethylene oxide) with Mn of 650 g/mol (PTMO-650). By contrast, the use of poly(tetramethylene oxide) with Mn of 1000 g/mol (PTMO-1000) lead to polymer 3b having lower molecular weight. Assuming that the difference between PTMO-650 and PTMO-1000 consists in the size of hydrophilic polyether chain; therefore, the difference in molecular weight in resulting polyesters should probably be related to the miscibility of the dimeric metallated fatty acid with different polyetherglycols, since an increase in miscibility favors polymerization.62 It has been reported that the yield of transesterification of methyl oleate with polyethylene glycols PEGs was influenced by the molecular size of PEG. According to Corma et al., the decrease of hydrophilic character of PEG by a shortening of the chain (as in the case of PTMO-650) will increase the solubility in the fatty acid methyl ester and both reactants will be better mixed, thereby enhancing the rate of transesterification.63 Therefore, the hydrophobic siloxydiester 1 has more affinity with PTMO-650, the less hydrophilic polyether, and lead to the higher molecular weight for 3a. The corresponding values of weight average polymerization degree xw of the silylated polymers vary in the same way as the molecular weight (Table 1). Surprisingly, the germylated derivatives 4a and 4b do not follow this tendency since the higher molecular value is obtained with PTMO-1000. In this case, other factors such as phenyl effects of the germylated diester should be taken into account. In this case, the polymerization degree xw varies from 5.5 in 4a to 7.2 in 4b, showing an increase of Mw beyond the molecular weight of PTMO-1000. The PTMO-1000 could allow a larger separation of the phenyl groups in the macromolecule and a lower steric hindrance would allow a higher molecular weight. Thus, in the case of silicon, the polymerization is mainly directed by the molecular weight (or hydrophilicity) of the polyetherglycol. On the contrary, for the germylated compound, 4a and 4b, the polycondensation degree would be more dependent on the phenyl effects (steric or miscibility) than on the diol used. It is important to underline that the polycondensation reactions were done without solvent. The intrinsic viscosity for 3b, 4a, and 4b was investigated (Table 1). The [η] values are relatively low confirming that the obtained products have moderate molecular weights as observed in the GPC analysis. Thermal Properties of Polyesters. The thermal properties of the polyesters were investigated by TGA under nitrogen. Figure 1 shows the thermal decomposition profiles of 3a, 3b, 4a, and 4b and Table 2 summarizes the thermogravimetric data. All silylated or germylated polyesters have similar high thermal stability, up to 400 °C (considering, as reference, the T50 temperature) and comparable thermal decomposition curves. This indicates that all the polymerization processes produce thermally stable products, regardless of the starting compounds.

compound

T50 (°C)

1 2 PTMO-650 PTMO-1000 3a 3b 4a 4b

320 380 298 312 428 412 425 415

Although the T50 values for the polymers (Table 2) contemplate experimental error, it is possible to observe a trend: the T50 values decrease as PTMO length increases, both for the 3a/3b and the 4a/4b polymers pairs. This trend can be interpreted in terms of the lower molar mass of polyetherglycol (PTMO-650 vs PTMO-1000), which allows the formation of a well compacted structure in the solid state. The densely packed crosslinked network does not release easily the gaseous matter that results from thermal decomposition, thus enhancing the initial temperature of decomposition. Additionally, both polymers have similar structures and consequently the same bonds are broken at a set temperature. Having a similar thermal stability, the weight loss observed in the TGA experiments can differ considerably according to the weight percentage of the thermally labile PTMO included in the polymer. The higher the percentage weight, the faster should be the weight loss. So the lowest T50 value is observed for polymer 3b. In the case of polyesters (3a and 3b), the main degradation stage was achieved around 450 °C with a residual mass of about 10%. For organic-inorganic hybrid materials with covalent bonds between the siloxane moiety and organic molecules, it has been reported that the degradation process starts by the cleavage of Si-CH2, Si-CH3, and C-C bonds.64,65 Then, the rearrangement of the main network structure and the occurrence of the distribution reactions among the different silicon sites lead to inorganic silica (SiO2) or silicon oxycarbides (SiCxO4-x) as the major product.66,67 For the germylated compounds (4a and 4b), the higher amount of residues can be due not only to the higher molecular mass of the corresponding oxides but also to the fact that complete mineralization of germanium compounds is often difficult.68 To get more insight, the prepared materials were subjected to differential scanning calorimetry (DSC) analysis and compared with their respective monomers. Figure 2 shows the thermograms of PTMO-1000 and 2 and Figure 3 shows the thermograms of 4a and 4b as representative examples of the studied monomers and polymers respectively. The melting and crystallization temperatures (Tm and Tc) and the melting and crystallization heats (∆Hm and ∆Hc) were evaluated and are listed in Table 3. Interestingly, all the monomers and polymers (except the germylated monomer 2) exhibit some degree of crystallinity, as it can be concluded from the values in Table 3. The absence of peaks of either melting or crystallization for the germylated diester 2 can be related to the effect of the phenyl groups bonded to germanium. They are bulky side chains that hinder the intermolecular packing process. When 2 is reacted with PTMO-650, the corresponding polymer 4a has a little melting peak (Figure 3, left), corresponding to 4.5 kJ per mol of monomer unit. In contrast, the polymer 4b, resulted from the condensation of 2 with PTMO1000, has two clear melting and crystallization peaks (Figure 3, right) corresponding to 60.3 and -67.2 kJ per mol of

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Katir et al.

Figure 2. DSC thermograms of PTMO-1000 and 2.

Figure 3. DSC thermograms of 4a and 4b. Table 3. DSC Data of Monomers and Polymers compound

Tm (°C)

∆Hm (kJ/mol)

Tc (°C)

∆Hc(kJ/mol)

PTMO-650 PTMO-1000 1 2 3a 3b 4a 4b

18.1 21.9 7.9

36.9 86.3 8.3

-4.9 1.1 -14.0

-46.2 -96.5 -8.6

6.4 17.2 0.54 9.1

48.2 69.6 4.5 60.3

-21.9 -13.9

-58.9 -86.7

-28.2

-67.2

monomer unit. This notable difference between 4a and 4b can be rationalized by two factors: first, ∆Hm values obtained for PTMO-1000 and PTMO-650 (86.3 and 36.9, respectively) reveal a higher molecular ordering degree for the former, which contributes to the higher crystallinity observed for 4b. Second, the higher molecular weight of PTMO-1000 could allow a larger separation of the phenyl groups in the obtained macromolecule; the same effect observed and previously discussed for the different molecular weights obtained for the studied polymers. The increased separation of side chain polymer is of considerable importance for polymer crystallinity. Actually, regular chain structure is necessary to allow the chains to pack into ordered and regular three-dimensional crystal lattice.69 In contrast, irregularly spaced and protruding side groups hinder crystallinity.69 For instance, syndiotactic polystyrene exhibit higher crystallinity, whereas the irregular atactic one is totally amor-

phous. Hence, in 4b, the higher distance between the phenyl rings allows a higher level of intermolecular packing due to a higher number of permitted macromolecular conformations. In 4a, however, the presence of the bulky and protruding phenyl groups originates a polymer product with a low crystallinity degree, which is reflected in a melting heat of 4.5 kJ/mol, that is, the lower heat amount needed to produces the fusion of the polymer (Table 3). Self-Organization of Polyesters. The interesting features of our polymers in term of their chemical structure (PTMO, fatty hydrophobic chains, organometallic fragments) prompted us to investigate their tensioactive properties. Actually, the design of nanoscaffolds from biological resources will be very relevant for drug delivery and medical applications.70,71 To assess this, the four materials were dissolved in ethanol/water mixture (1: 1) and sonicated for 1 h; after that, a drop was deposited on a gate of carbon and analyzed by transmission electronic microscopy (TEM). Interestingly, the four samples show various small objects as evidenced by the formation of homogeneous nanoparticles of 1 nm of size (Figure 4). In the case of siloxane based polymers (3a and 3b), these nanoparticles assemble to lead to microspheres of 0.12-0.14 µm of diameter. The TEM analyses of germylated materials (4a and 4b) showed a fibrous network in which the small nanoparticles were highly dispersed (Figure 4). This result is in line with selected area electron diffraction (SAED) analyses, which evidenced that the two



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Figure 4. TEM micrographs of polymers 3a, 3b, 4a, and 4b.

Figure 5. SAED of polymers 4a and 4b.

germylated polymers 4a and 4b exhibit continuous order in nanoscal domain (Figure 5). By comparison of the behavior of the four samples, we can conclude that the variation in molecular weight of the polymers does not disturb the formation of nanoparticles since the similar tendency was observed for the two siloxanes (3a and 3b) and for the two germylated materials (4a and 4b). The self-assembling of the nonionic surfactants is an active research area because this characteristic opens up a new field of structured materials,72,73 drug delivery,74,75 and biocatalysis.76,77 For instance, the commercially available Brij35 (dodecyl-poly ethyelene-oxide-ether) forms micelles in

water.78 If an additional solvent was added (alcohol for example), the partition of the latter between the aqueous pseudophase and the surfactant aggregation pseudophase leads to new nanoarchitectures.76,77,79 In our case, although the tetramethylenate is less hydrophilic than the commonly used polyoxoethylene, the segregation tendency of the hydrophobic fatty chain fragment from water and the affinity of tetramethylenate ester to the aqueous media are suspected to be the driving force for the formation of these small objects. In addition, the presence of ethanol as ternary system80 and the oxophilicity of the silicon or germanium cannot be completely

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ruled out. The nature of the organometallic fragment (dimethyldisiloxy or diphenylgermyl) seems to play a pivotal role during the interaction “polymer-solvent”. Phenyl groups that prevent the crystallization of 4a in the solid state bring additional π-π stacking interactions in the solution. These interactions are known to be involved in the approaches to hierarchically dimensioned materials.46,77 While additional research has to be done on the nature of these aggregation phenomena (and the effect of the dispersing media), these results constitute an outstanding advance to design new and biocompatible nanocontainers. The acidic Lewis character of silicon and germanium (which have to be taken in consideration especially when nucleophile drug is present) can enhance their selective encapsulation.

Conclusions New silylated and germylated diesters were prepared by hydrometalation of FAME. These biomaterial precursors allow the direct synthesis of amphiphilic polyesters through a onestep transesterification reaction. These new silicon and germanium based materials exhibit three distinguishable characteristics that should be mentioned: (i) their biocompatibility and availability because they are derived from renewable resources, (ii) their robustness and thermal stability, and (iii) their selfassembling properties stemmed from their amphiphilic character. These easily accessible materials can have emergence application in various fields of chemistry and more especially in drug delivery and release of active molecules where self-assembling of biocompatible materials is of the utmost importance. By yielding easy transesterification reactions, the new and various metallated FAME diesters open an easy route to a great variety of new biopolyesters. This work is currently in progress. Acknowledgment. The authors thank the ECOS-CONYCIT Programme C04E05 and C08E01, “Le comite´ mixte interuniversitaire franco-marocain” (PHC: No. MA/05/123) and the Investigation Direction of the Concepcioń University (Chile) for partial financial support. Supporting Information Available. The 1H and 13C NMR and the FTIR spectra of the polymers are presented. This material is available free of charge via the Internet at http:// pubs.acs.org.

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