Hyperbranched Polycarbosiloxanes and Polysiloxanes with

May 14, 2012 - Tracy Zhang , Bob A. Howell , and Patrick B. Smith .... Jin Hu , Peter I. Carver , Dale J. Meier , Edmund J. Stark , Nita Xu , Tracy Zh...
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Hyperbranched Polycarbosiloxanes and Polysiloxanes with Octafunctional Polyhedral Oligomeric Silsesquioxane (POSS) Branch Points Nita Xu, Edmund J. Stark, Petar R. Dvornic, Dale J. Meier, Jin Hu,* and Claire Hartmann-Thompson* Michigan Molecular Institute, 1910 W. St. Andrews Road, Midland, Michigan 48640, United States ABSTRACT: Three series of hyperbranched polycarbosiloxanes (HB-PCSOX) and polysiloxanes (HB-PSOX) were prepared from octafunctional polyhedral oligomeric silsesquioxane (POSS) monomers and various difunctional silanes by bimolecular nonlinear A8-B2 polymerization (BMNLP) and characterized by IR, 1H NMR, 29Si NMR, SEC, DSC, and for refractive index. Series 1 was prepared from T8vinyl8 and disilane monomers, and series 2 was prepared from T8(OSiMe2H)8 and divinylsilane monomers (where T denotes an SiO3 unit), both via platinum-catalyzed hydrosilylation polymerization. Series 3 was prepared from T8(OSiMe2OH)8 and dichlorosilane monomers via hydrolysis−condensation polymerization. Structure−property relationships were determined as methyl vs phenyl content was varied. These HB POSS polymers were then functionalized with curable alkoxysilane end groups and formulated with linear polysiloxanes to fabricate transparent and robust nanostructured POSS-containing coatings for use in a range of high-performance space and solar applications.

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1)(y − 1), where r = [A]/[B]. A wide range of siliconcontaining HB polymers have been prepared using the Ax + By BMNLP method.18 Octafunctional POSS may be introduced into an HB architecture either as an AxBy monomer (where x + y = 8) or, alternatively, as an Ax monomer (where x = 8). Virtually all known POSS compounds synthesized to date either have eight identical functional groups (T8A8) or a seven− one configuration (T8A7B) where T denotes the SiO3 unit of the POSS cage. It is challenging to synthesize POSS-AxBy monomers of other configuations (e.g., T8A4B4, T8A6B2, and so on), and HB polymers of this form have not been reported. This study describes the preparation of HB polycarbosiloxanes (HB-PCSOX) and HB polysiloxanes (HB-PSOX) via the alternative route of BMNLP using various T8A8 and B2 monomers. Earlier studies20−23 reported the synthesis of insoluble resins with three-dimensional cross-linked network structures using a similar combination of T8 and other monomers. One resin was prepared by reacting vinylterminated carboranylenesiloxane (B2) with an octasilane POSS (A8) in a Karstedt-catalyzed hydrosilylation to give a network of POSS and carborane clusters,22 and another resin was prepared in a reaction between T8H8 and 1,3-divinyl1,1,3,3-tetramethyldisiloxane.20,21 However, to the best of our knowledge, this is the first report of free, soluble, and fully characterized HB polymers with octafunctional T8 branch points, although it should be noted that closely related soluble

INTRODUCTION Many studies of linear, dendrimeric, and hyperbranched polymers with polyhedral oligomeric silsesquioxanes (POSS) covalently bound into their structures have been carried out in the past 20 years.1,2 POSS has been used as the core of a dendrimer,3−8 or as the terminal group of a dendrimer,9 or all at once as the core, branch unit, and terminal group of a dendrimer.10 These dendritic structures should be distinguished from simpler star architectures that are also based upon POSS cores, e.g., poly(ε-caprolactone)11 or polyisobutylene.12 Hyperbranched (HB) polymers have been grown off of POSS moieties,13 HB polymers with pendant POSS groups have been prepared, and POSS has been introduced as the terminal group of a HB polymer.14,15 A HB polyethylene tethered to POSS was prepared by copolymerization of ethylene with acryloisobutyl-POSS,13 and a HB polysiloxysilane with terminal POSS was synthesized.15 In both these studies the POSS content was varied with the aim of controlling the degree of crystallinity. Other examples include amineterminated HB polyimides16a or HB polyureas16b functionalized with Si(OR)3 groups which were then hydrolyzed to introduce silsesquioxane into the system, a HB aromatic polyether functionalized with Si(OR)3 groups which was then condensed to give an organic−silsesquioxane nanocomposite,17 and a HB polyether-imide with terminal POSS groups.14 HB polymers may either be synthesized using one AxBy monomer or via a nonlinear bimolecular polymerization using an Ax monomer and a By monomer.18 The condition required to avoid gelation19 in an Ax + By bimolecular nonlinear polymerization (BMNLP) is 1/[(x − 1)(y − 1)] ≥ r ≥ (x − © XXXX American Chemical Society

Received: March 7, 2012 Revised: May 3, 2012

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dx.doi.org/10.1021/ma300470m | Macromolecules XXXX, XXX, XXX−XXX

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casting onto potassium bromide disks. 1H, 13C, and 29Si NMR spectra were recorded on a Varian Unity 400 MHz NMR spectrometer equipped with a 5 mm four nuclei probe. Quantitative 1H NMR was performed by adding a known mass of hexamethylcyclohexane reference material to the NMR sample. Solvent signals were used as internal standards, and chemical shifts are reported relative to tetramethylsilane (TMS). Size exclusion chromatography (SEC) was carried out using a Waters 510 pump, a Waters 717 autoinjector, a Waters CHM column heater, two Polymer Laboratory PLgel columns, and a Polymer Laboratory PL-ELS 1000 evaporative light scattering detector (ELSD). Detector conditions appropriate to the eluting solvent were used, and the column was calibrated with Dow 1683 polystyrene standards. MALDI-TOF mass spectra were measured by M-Scan, Inc. (West Chester, PA) using an Applied Biosystems Voyager DE-Pro instrument. A 2,5-dihydroxybenzoic acid (DHB) matrix was used, and samples were dissolved in chloroform. Differential scanning calorimetry (DSC) measurements were performed on a DuPont Instruments Model 912 unit. A Bausch and Lomb ABBE-3 L refractometer was used to make refractive index measurements at 23 °C. Elemental analysis was performed by Huffman Laboratories (Golden, CO). Synthesis of HB Polycarbosiloxanes from Octavinyl POSS (Series 1). HB Polycarbosiloxane (R1 = R2 = Me) 1. Octavinyl-POSS (4 g, 6.32 mmol, 1 equiv of vinyl) was weighed into a round-bottomed flask equipped with a cooling condenser. To the flask was added THF (40 mL) and Karstedt’s catalyst (0.04 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene). The solution was stirred for 10 min under nitrogen. 1,1,3,3-Tetramethyldisiloxane (23.2 g, 187 mmol, 7.4 equiv of SiH) was then added. The mixture was stirred at room temperature for 1 h and then heated in an oil bath at 50 °C for 16 h. The clear viscous oil was washed with anhydrous acetonitrile (5 × 25 mL) and dried using a rotary evaporator. It was further dried under vacuum overnight to yield 8 g of product. IR (thin film): ν (cm−1) 2959, 2922, 2897, 2872 (CH3 and CH2, sym and asym), 2119 (SiH), 1405, 1254 (SiCH3), 1117 (SiOSi), 1057 (SiOSi), 909, 839 (SiCH3 rock), 806 (SiOSi sym). 1H NMR (CDCl3): δ (ppm) 0.0 (s; SiCH3), 0.2 (s; CH3SiH), 0.6 (s, SiCH2), 4.7 (s; SiH), 3.6 mmol/g SiH. 29Si NMR (30 wt % in THF-d8): δ (ppm) −58.7 (O3SiCH2), −0.6 (OSiMe2H), 17.6 (OSiMe2CH2). SEC (toluene): Mw = 2670, Mn = 2300, polydispersity =1.2. DSC (10 °C min−1, nitrogen) −67 °C. Refractive index 1.4457 at 23 °C. HB Polycarbosiloxane (R1R2 = MePh) 2. Octavinyl-POSS (2.22 g, 3.51 mmol, 1 equiv of vinyl), 1,3-diphenyl-1,3-dimethyldisiloxane (12.00 g; 46.39 mmol, 7.5 equiv of SiH), and platinum− divinyltetramethyldisiloxane complex in xylene (0.02 g) were stirred in anhydrous THF under nitrogen at room temperature for 1 h and then heated and stirred at 50 °C for 16 h. The resulting product was washed with anhydrous acetonitrile (4 × 20 mL); all solvents were removed in vacuo, and the product was further dried overnight. 1H NMR (CDCl3): δ (ppm) 0.49−0.64 (m; SiCH3), 0.80−1.10 (m; SiCH2), 5.37 (m; SiH), 7.46−7.56 (m; ArH), 7.66−7.76 (m; ArH). 29 Si NMR (CDCl3): δ (ppm) −66.0 (SiO3), −31.4 (HSiOMePh), −12.4 (CH2SiOMePh). SEC (THF): Mw = 43 000, Mn = 33 000, polydispersity = 1.3. DSC (10 °C min−1, nitrogen) −79 °C. HB Polycarbosiloxane (R1 = R2 = Ph) 3. Octavinyl-POSS (1.50 g, 2.37 mmol, 1 equiv of vinyl), 1,1,3,3-tetraphenyldisiloxane (24.96 g, 65.25 mmol, 6.88 equiv of SiH), and platinum−divinyltetramethyldisiloxane complex in xylene (0.015 g) were stirred in anhydrous THF under nitrogen at room temperature for 1 h and then heated and stirred at 50 °C for 16 h. The resulting product was washed with anhydrous acetonitrile (4 × 20 mL); all solvents were removed in vacuo, and the product was further dried overnight. 1H NMR (CDCl3): δ (ppm) 0.80 (m; SiCH2), 5.92−6.16 (m; SiH), 7.17−7.65 (m; ArH). SEC (THF): Mw = 32 000, Mn = 22 000, polydispersity = 1.4. DSC (10 °C min−1, nitrogen) 10 °C. HB Polycarbosiloxane (R1 = R2 = Me, (OSiMe2)3 Spacer) 4. Octavinyl-POSS (1 g, 1.58 mmol, 1 equiv of vinyl) was weighed into a round-bottomed flask equipped with a cooling condenser. To the flask was added THF (10 mL) and Karstedt’s catalyst (0.01 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene). The

hyperbranched polycarbosiloxanes with non-T8 silsesquioxane branch points have also been prepared via bimolecular hydrosilylation polymerization24 using either a difunctional “double decker” silsesquioxane A2 monomer or a trifunctional R7T4D3 open-cage B3 monomer. While HB-PCSOX are commonplace and easily synthesized via hydrosilylation,23 far fewer examples of HB-PSOX18f appear in the literature. Those reported include HB polysiloxanes from siloxane disilanoles and tri- or tetrachlorosilane,18f cyclotrisiloxanes,26 HB polyethoxysiloxane from a condensation reaction between tetraetoxysilane (TEOS) and acetic anhydride in the presence of an organotitanium catalyst,27 and carbazolylfunctional HB-PSOX cross-linked to form light-emitting diode (LED) compositions.28 In this study, the first examples of HB polymers with octafunctional POSS branching units29 were prepared in simple one-pot syntheses and fully characterized, and the structure− property relationships for HB-PCSOX and HB-PSOX architectures with varying phenyl contents (Figure 1) were

Figure 1. HB-PCSOX prepared from T8Vinyl8 monomer (series 1, top, where R1R2 = Me2, MePh, or Ph2), HB-PCSOX prepared from T8(OSiMe2H)8 monomer (series 2, middle, where R1R2 = Me2, MePh, Ph2, or (OEt)2), and HB-PSOX prepared from T8(OSiMe2OH)8 monomer (series 3, bottom, where R1R2 = Me2, MePh, or Ph2).

determined. HB polymer variants with different reactive end groups were prepared and used to fabricate transparent and robust nanostructured POSS-based coatings. Such coatings, films, elastomers, and adhesives have great potential for a range of high-performance space and solar applications; multijunction space solar photovltaic cells require the high transmission across an increasing range of UV−vis wavelengths conferred by siloxane and POSS compositions; in addition, POSS compounds have excellent resistance to UV, protons, electrons, atomic oxygen, and the extremes of temperature encountered in the aggressive space environment.30

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EXPERIMENTAL SECTION

Materials and Characterization. POSS reagents were purchased from Hybrid Plastics (Hattiesburg, MS), silane reagents and silanolterminated polysiloxanes were purchased from Gelest, Inc. (Tullytown, PA), and other reagents and solvents were purchased from SigmaAldrich, Inc. (Milwaukee, WI) and used in this work as received unless otherwise stated. IR spectra were recorded on a Nicolet 20DXB FTIR spectrometer, and samples were prepared for analysis by solutionB

dx.doi.org/10.1021/ma300470m | Macromolecules XXXX, XXX, XXX−XXX

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The reaction was then continued at 80 °C for 48 h. The final product showed a negligible SiH peak in its IR spectrum. The THF was removed using a rotary evaporator, and the product was washed with anhydrous acetonitrile (5 × 5 mL) and dried under vacuum overnight to yield 1.155 g of a slightly opaque viscous oil. IR (thin film): ν (cm−1) 3069, 3051, 3011 (PhH and CH2CH), 2957, 2910, 2876 (CH3 and CH2, sym and asym), 1955, 1883, 1819, 1591, 1487, 1428, 1406, 1254 (SiCH3), 1170, 1090, 1052 (SiOSi), 842 (SiCH3 rock), 793 (SiOSi sym). 1H NMR (CDCl3): δ (ppm) 0.0−0.2 (s; SiCH3), 0.3 (m; Me2SiCH2), 0.4 (m; MePhSiCH2) 5.8 (d; CHCH2), 6.0 (d; CHCH2), 6.4 (d; CHCH2), 7.4 (m; ArH); 7.7 (m; ArH). SEC (toluene): Mw = 5500, Mn = 2600, polydispersity = 2.1. HB Polycarbosiloxane (R1 = R2 = Ph) 8. Divinyltetraphenyldisiloxane (3.84 g, 8.83 mmol, 7.5 equiv of vinyl) was weighed into a roundbottomed flask equipped with a cooling condenser, and THF (5 mL) was added. Karstedt’s catalyst (0.03 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene) was added, the solution was stirred for 10 min under nitrogen, and then octasilane POSS (0.3 g, 0.294 mmol, 1 equiv of SiH) in THF (2 mL) was added. The mixture was stirred at room temperature for 1 h and then heated in an oil bath at 80 °C for 3 days. The IR spectrum showed a negligible SiH peak. THF was removed using a rotary evaporator, and the resulting product was washed with warm anhydrous acetonitrile (5 × 5 mL) to remove the excess divinyltetraphenyldisiloxane. The product was dried under vacuum overnight to yield a white powder (0.61 g). IR (KBr disk): ν (cm−1) 3088, 3069, 3049, 3022 (PhH and CH2CH), 2957, 2913, 2876 (CH3 and CH2, sym and asym), 1958, 1887, 1822, 1775, 1712, 1590, 1487, 1428, 1254 (SiCH3), 1135, 1112, 1090 (SiOSi), 901, 842 (SiCH3 rock), 788 (SiOSi sym). 1H NMR (CDCl3): δ (ppm) 0.0 (s; SiCH3), 0.4 (m; MePhSiCH2) 5.8 (d; CHCH2), 6.0 (d; CH CH2), 6.4 (d; CHCH2), 7.4 (m; ArH); 7.7 (m; ArH). SEC (toluene): Mw = 3000, Mn = 1800, polydispersity = 1.7. HB Polycarbosiloxane (R1 = R2 = OEt) 9. 1,3-Divinyltetraethoxydisiloxane (18.02 g, 58.8 mmol, 7.5 equiv of vinyl) was weighed into a round-bottomed flask equipped with a cooling condenser. Karstedt’s catalyst (0.02 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene) was added. The solution was stirred for 10 min under nitrogen. Octasilane POSS (2.0 g, 1.96 mmol, 1 equiv of SiH) in THF (20 mL) was added into the flask. The mixture was initially stirred at room temperature for 1 h and then heated in an oil bath at 50 °C for 16 h. The reaction was heated at 80 °C for 24 h and 100 °C for an additional 24 h. THF was removed using a rotary evaporator, and the resulting viscous oil was washed with anhydrous acetonitrile (5 × 20 mL) and dried under vacuum overnight to yield 4 g of colorless oil. IR (thin film): ν (cm−1) 3040 (CH2CH), 2974, 2926, 2886, 2736 (CH3 and CH2, sym and asym), 1600,1483, 1443, 1407, 1391,1366, 1295,1254 (SiCH3), 1168, 1103 (SiOSi), 1009, 961, 844 (SiCH3), 789 (SiOSi). 1H NMR (CDCl3): δ (ppm) 0.14 (SiCH3), 0.63−0.66 (SiCH2), 1.21−1.24 (OCH2CH3), 3.82−3.86 (OCH2), 5.93−6.11 (CHCH2). 29Si NMR (CDCl3): δ (ppm) −104.6 (SiO4), −61.7 (OSi(OEt)2CHCH2), −47.8(OSi(OEt)2CH2), 17.8 (OSiMe2CH2). SEC (THF): Mw = 133 000, Mn = 16 000, polydispersity = 8.3. DSC (10 °C min−1, nitrogen) −51 °C. TGA (10 °C min−1, nitrogen), 2% mass loss at 350 °C. Refractive index 1.4464 at 23 °C. Synthesis of HBP Polysiloxanes from Octa(SiOH)POSS (Series 3). T8(OSiMe2OH)8 Monomer. A solution of octasilanePOSS (0.36 g, 0.35 mmol) in 1,4-dioxane (25 mL) was added dropwise to a suspension of 10% palladium on charcoal (0.1 g) in a mixture of dioxanes (5 mL) and a buffer solution (0.7 g, 4.7 mmol/L NaH2PO4.H2O and 4 mmol/L NaOH). After the addition, the solution was stirred at room temperature for 16 h. The solution was filtered, and the solvent was removed using a rotary evaporator. The resulting white solid was dissolved in ethyl acetate and filtered again. The product was dried in the rotary evaporator and then further dried for 12 h under vacuum to yield 0.3 g of a white powder. Melting point >300 °C. IR (KBr disk): ν (cm−1) 3385 (SiOH), 2966 (CH3), 1264 (SiCH2), 1092 (SiOSi asym), 888, 849 (SiCH3 rock), 799 (SiOSi sym). 1H NMR (CD3COCD3): δ (ppm) 0.0 (s; SiCH3). 29Si NMR (30 wt % in THF-d8): δ (ppm) −109.6 (SiO4), −11.1 (OSiMe2OH).

solution was stirred for 10 min under nitrogen. 1,1,3,3,5,5Hexamethyltrisiloxane (9.22 g, 44.23 mmol, 7 equiv SiH) was then added. The mixture was stirred at room temperature for 10 min and then heated in an oil bath at 50 °C for 16 h. The clear viscous oil was washed with anhydrous acetonitrile (5 × 25 mL) and dried using a rotary evaporator. It was further dried under vacuum overnight to yield 3 g of product. IR (thin film): ν (cm−1) 2960, 2924, 2901 (CH3 and CH2, sym and asym), 2126 (SiH), 1408, 1258 (SiCH3), 1112 (SiOSi), 1049 (SiOSi, asym), 912, 840 (SiCH3 rock), 796 (SiOSi sym). 1H NMR (CDCl3): δ (ppm) 0.07, 0.10 (2s; SiCH3), 0.22 (s; CH3SiH), 0.60 (s, SiCH2), 4.74 (s; SiH), 2.2 mmol/g SiH. 29Si NMR (30 wt % in THF-d8): δ (ppm) −63.0 (O3SiCH2), −16.5 (O2SiMe2), −5.4, −2.8 (OSiMe2H), 11.5 (OSiMe2CH2). SEC (THF): Mw = 3100, Mn = 2300, polydispersity = 1.3. Refractive index 1.4346 at 23 °C. HB Polycarbosiloxane (R1 = R2 = Me, (OSiMe2)4 Spacer) 5. Octavinyl-POSS (1 g, 1.58 mmol, 1 equiv of vinyl) was weighed into a round-bottomed flask equipped with a cooling condenser. To the flask was added THF (10 mL) and Karstedt’s catalyst (0.01 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene). The solution was stirred for 10 min under nitrogen. 1,1,3,3,5,5,7,7Octamethyltrisiloxane (12.5 g, 44.23 mmol, 7 equiv SiH) was then added. The mixture was stirred at room temperature for 10 min and then heated in an oil bath at 50 °C for 16 h. The clear viscous oil was washed with anhydrous acetonitrile (5 × 25 mL) and dried using a rotary evaporator. It was further dried under vacuum overnight to yield 3.1 g of product. IR (thin film): ν (cm−1) 2960, 2913, 2901 (CH3 and CH2, sym and asym), 2127 (SiH), 1409, 1259 (SiCH3), 1110 (SiOSi), 1084, 1041 (SiOSi, asym), 913, 839 (SiCH3 rock), 798 (SiOSi sym). 1 H NMR (CDCl3): δ (ppm) 0.01 (s; SiCH3), 0.20 (s; CH3SiH), 0.58 (s, SiCH2), 4.74 (s; SiH), 1.55 mmol/g SiH. 29Si NMR (30 wt % in THF-d8): δ (ppm) −64.9 (O3SiCH2), −20.3, −18.9 (O2SiMe2), −7.1, −4.5 (OSiMe2H), 9.5 (OSiMe2CH2). SEC (THF): Mw = 3800, Mn = 2700, polydispersity = 1.4. Refractive index 1.4316 at 23 °C. Synthesis of HBP Polycarbosiloxanes from Octasilane POSS (Series 2). HB Polycarbosiloxane (R1 = R2 = Me) 6. 1,3Divinyltetramethyldisiloxane (5.49 g, 29.4 mmol, 7.5 equiv of vinyl) was weighed into a round-bottomed flask equipped with a cooling condenser. Karstedt’s catalyst (0.02 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene) was added. The solution was stirred for 10 min under nitrogen. Octasilane POSS (1.0 g, 0.98 mmol, 1 equiv SiH) in THF (10 mL) was added into the flask. The reaction was monitored by IR to follow the disappearance of the SiH band. The mixture was initially stirred at room temperature for 1 h and then heated in an oil bath at 50 °C for 16 h. Karstedt’s catalyst (0.0272 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene) was added, and the reaction was heated at 80 °C for 24 h. Karstedt’s catalyst (0.024 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene) was added again, and the reaction was run at 80 °C for an additional 24 h. The final product did not show an SiH peak in its IR spectra. THF was removed using a rotary evaporator, and the resulting hazy viscous oil was washed with anhydrous acetonitrile (5 × 10 mL) and dried under vacuum overnight to yield 1.95 g. IR (thin film): ν (cm−1) 3051 (CH2CH), 2957, 2909, 2877, 2791 (CH3 and CH2, sym and asym), 1595, 1407, 1254 (SiCH3), 1136, 1094, 1053 (SiOSi), 955, 836 (SiCH3 rock), 789 (SiOSi sym). 1H NMR (CDCl3): δ (ppm) 0.0−0.2 (s; SiCH3), 0.5 (s; SiCH2), 5.7 (d; CHCH2), 5.9 (d; CH CH2), 6.2 (d; CHCH2). 13C NMR (CDCl3): δ (ppm) 0 (SiCH3), 10 (SiCH2), 132 (CHCH2), 140 (CHCH2). SEC (Toluene): Mw = 15 000, Mn = 3400, polydispersity = 4.4. HB Polycarbosiloxane (R1R2 = MePh) 7. 1,3-Divinyl-1,3-diphenyl1,3-dimethyldisiloxane (4.57 g, 14.7 mmol, 7.4 equiv of vinyl) was weighed into a round-bottomed flask equipped with a cooling condenser. Karstedt’s catalyst (0.04 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene) was added. The solution was stirred for 10 min under nitrogen. Octasilane POSS (0.5 g, 0.49 mmol, 1 equiv SiH) in THF (5 mL) was added to the flask. The reaction was monitored by IR to follow the disappearance of the SiH band. The mixture was initially stirred at room temperature for 1 h and then heated in an oil bath at 80 °C for 48 h. Karstedt’s catalyst (0.02 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene) was added. C

dx.doi.org/10.1021/ma300470m | Macromolecules XXXX, XXX, XXX−XXX

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reaction was cooled down again to −78 °C in a dry ice/acetone bath, and dimethyldichlorosilane (3 mL, 24.72 mmol, 7.1 equiv of SiCl) was added through a syringe. This was followed by the addition of distilled pyridine (4 mL, 49.46 mmol). The reaction was then warmed up to room temperature and stirred overnight for 16 h. The volatiles were removed using a rotary evaporator, and hexane (20 mL) was added. Pyridine (4.5 mL) was added dropwise via a syringe, followed by the addition of distilled water (5 mL). The solution was stirred at room temperature for 2 h and transferred to a separation funnel. The water layer was washed with hexane three times. All hexane washings were combined and evaporated to give a viscous oil. The mixture was redissolved in hexane (20 mL) and washed three times with water. Sodium sulfate was used to dry the moisture in hexanes solution and was filtered out. The hexane phase yielded 1.04 g of sticky pale yellow oil. IR (KBr disk): ν (cm−1) 3312 (SiOH), 3072 (PhH), 2963, 2905 (CH3), 1593, 1487, 1430, 1411, 1262 (SiCH2), 1068 (SiOSi asym), 848 (SiCH3 rock), 805 (SiOSi sym). 1H NMR (THF-d8): δ (ppm) 0.0 (s; SiCH3) 7.37 (m; ArH), 7.63 (m; ArH). 29Si NMR (30 wt % in THF-d8) −107.6 (SiO4), −45.4 (O2SiPh2), −19.2 (O2SiMe2), −16.9 (O2SiMe2), −11.2 (OSiMe2OH). SEC (THF): Mw = 3100, Mn = 2000, polydispersity = 1.6. DSC (10 °C min−1, nitrogen) −31 °C. Refractive index 1.4616 at 23 °C. Hyperbranched Polymers with Alkoxysilyl End Groups. Methoxysilane-Terminated HB Polycarbosiloxane (Series 1, R1 = R2 = Me) 13. A 50 mL round-bottomed flask equipped with a vertical cooling condenser was charged with HB polycarbosiloxane (series 1, R1 = R2 = Me) 1 (3.0 g) and vinyltrimethoxylsilane (3.2 g). It was flushed with N2 and stirred for 5 min. Karstedt’s catalyst (0.0121 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene) was added to the mixture. It was stirred at room temperature for 1 h and then heated in an oil bath at 50 °C for 16 h. The clear viscous oil was washed with anhydrous acetonitrile (5 × 15 mL) and dried in a rotary evaporator. The product designated as POSS-HBP-(SiMe2C2H2Si(OMe)3)m was further dried under vacuum for 5 h. The yield was 2.35 g. IR (thin film): ν (cm−1) no SiH at 2119. Ethoxysilane-Terminated HB Polycarbosiloxane (Series 1, R1 = R2 = Me) 14. A 50 mL round-bottomed flask equipped with a vertical cooling condenser was charged with HB polycarbosiloxane (series 1, R1 = R2 = Me) 1 (3.0 g) and vinyltriethoxylsilane (4.1 g). It was flushed with N2 and stirred for 5 min. Karstedt’s catalyst (0.0121 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene) was added to the mixture. It was stirred at room temperature for 1 h and then heated in an oil bath at 50 °C for 16 h. The clear viscous oil was washed with anhydrous acetonitrile (5 × 15 mL) and dried in a rotary evaporator. The product designated as POSS-HBP-(SiMe2C2H2Si(OEt)3)m was further dried under vacuum for 5 h. The yield was 4.5 g. IR (thin film): ν (cm−1) 2973, 2924, 2887 (CH3 and CH2, sym and asym), 1442, 1407, 1389, 1253 (SiCH3), 1105 (SiOSi), 1080 (SiOSi), 956, 840 (SiCH3 rock), 783 (SiOSi sym), no SiH at 2119. Refractive index 1.4438 at 23 °C. Ethoxysilane-Terminated HB Polycarbosiloxane (R1 = R2 = Me, (OSiMe2)3 Spacer) 15. A dried 25 mL round-bottom flask equipped with a magnetically controlled stirrer bar and a nitrogen inlet atop a water-cooled condenser was charged with HB polycarbosiloxane (R1 = R2 = Me, (OSiMe2)3 spacer) 4 (1.509 g, 2.2 mmol/g SiH, 3.3 mmol SiH) and triethoxyvinylsilane (0.840 g, 4.41 mmol). The mixture was stirred under nitrogen for 5 min, and then Karstedt’s catalyst (8.5 mg, Pt−tetramethyldivinyldisiloxane complex in xylenes, 2.1−2.4% Pt, 0.30 μmol Pt) was added. The mixture was stirred under nitrogen at room temperature for 1 h, allowing for an initial exotherm, and then heated at 50 °C for 16 h. The pale yellow reaction mixture was washed with anhydrous acetonitrile (25 mL, then 4 × 15 mL) under nitrogen, concentrated by rotary evaporation, and further concentrated under vacuum for 16 h to yield 1.96 g of a clear pale yellow oil. IR (film): 2960 (m), 2924 (w), 2887 (w) (CH3 and CH2, sym and asym), 1440 (w), 1408 (w), 1390 (w), 1257 (s,s) (SiCH3), 1141 (s), 1105 (s) (SiOSi), 1081 (s), 1046 (s) (SiOSi asym), 957 (m), 840 (s) (SiCH3 rock), 793 (s) (SiOSi sym), 700 (w), 471 (w). 1H NMR (CDCl3): δH (ppm) 3.84 (q, J = 7.0 Hz; OCH2CH3), 1.25 (t, J = 7.1 Hz;

MALDI-TOF MS (DHB, THF): 1169 (calc 1169, M + Na). Elemental analysis: 4.97% H (calc 4.93%), 14.35% C (calc 16.77%). HB Polysilxane (R1 = R2 = Me) 10. Step 1. SiMe2Cl-Terminated HB Polycarbosiloxane. A triple-neck round-bottomed flask and a condenser were dried in an oven at 120 °C for 12 h and then cooled to room temperature under nitrogen. T8(OSiMe2OH)8 monomer (0.5 g, 0.44 mmol, 1 equiv SiOH) was weighed into the flask equipped with the cooling condenser, and THF (10 mL) was added. The solution was cooled to −78 °C in a dry ice/acetone bath, and dimethyldichlorosilane (1.58 mL, 13 mmol, 7.5 equiv of SiCl, freshly distilled prior to use) was added through a syringe. This was followed by the addition of freshly distilled pyridine (0.30 mL, 3.71 mmol). The reaction was allowed to warm up to room temperature and stirred overnight for 16 h. Step 2. Hydrolysis of SiMe2Cl-Terminated HB Polycarbosiloxane in the Presence of Pyridine. Solvent was evaporated from the reaction solution of step 1 using a rotary evaporator, and hexane (20 mL) was added. Pyridine (2.2 mL) was added dropwise using a syringe, and then distilled water (1.2 mL) was added. The solution was stirred at room temperature for 1.5 h and washed three times with water. The hexane solvent was evaporated to yield 0.42 g of a clear viscous oil. IR (KBr disk): ν (cm−1) 3317 (SiOH), 2906 (CH3), 1448, 1411, 1262 (SiCH2), 1112, 1067 (SiOSi asym), 849 (SiCH3 rock), 803 (SiOSi sym). 1H NMR (CD3COCD3): δ (ppm) 0.0 (s; SiCH3). 29Si NMR (30 wt % in THF-d8): δ (ppm) −109.3 (SiO4), −20.9 (O2SiMe2), −18.7 (O2SiMe2), −12.9 (OSiMe2OH). SEC (THF): Mw = 4300, Mn = 2300, polydispersity = 1.9. DSC (10 °C min−1, nitrogen) −42 °C. Refractive index 1.4215 at 23 °C. HB Polysiloxane (R1R2 = MePh) 11. A triple-neck round-bottomed flask and a condenser were dried in an oven at 120 °C for 12 h and then cooled to room temperature under nitrogen. T8(OSiMe2OH)8 monomer (1.0 g, 0.87 mmol, 1 equiv SiOH) was weighed into the flask equipped with the cooling condenser, and THF (20 mL) was added. The solution was cooled to −78 °C in a dry ice/acetone bath, and methylphenyldichlorosilane (0.112 mL, 0.7 mmol) was added through a syringe. This was followed by the addition of freshly distilled pyridine (0.114 mL, 1.41 mmol). The reaction was allowed to warm up to room temperature and stirred for 16 h. The reaction was cooled down again to −78 °C in a dry ice/acetone bath, and dimethyldichlorosilane (3 mL, 24.72 mmol, 7.1 equiv of SiCl) was added through a syringe. This was followed by the addition of distilled pyridine (4 mL, 49.46 mmol). The reaction was then warmed up to room temperature and stirred overnight for 16 h. The volatiles were removed using a rotary evaporator, and hexane (20 mL) was added. Pyridine (4.5 mL) was added dropwise via a syringe, followed by the addition of distilled water (5 mL). The solution was stirred at room temperature for 2 h and transferred to a separation funnel. The water layer was washed with hexane three times. All hexane washings were combined and evaporated to give a viscous oil. The mixture was redissolved in hexane (20 mL) and washed three times with water. Sodium sulfate was used to dry the moisture in hexanes solution and was filtered out. The hexane phase yielded 0.82 g of sticky pale yellow oil. IR (KBr disk): ν (cm−1) 3301 (SiOH), 3072 (PhH), 2964, 2906 (CH3), 1593, 1447, 1430, 1411, 1262 (SiCH2), 1076 (SiOSi asym), 891, 849 (SiCH3 rock), 804 (SiOSi sym). 1H NMR (CD3COCD3): δ (ppm) 0.0 (s; SiCH3) 7.4 (m; ArH), 7.6 (m; ArH). 29Si NMR (30 wt % in THF-d8): δ (ppm) −109.3 (SiO4), −33.7 (O2SiMePh), −20.9 (O2SiMe2), −18.5 (O2SiMe2), −12.8 (OSiMe2OH). SEC (THF): Mw = 8500, Mn = 3200, polydispersity = 2.7. DSC (10 °C min−1, nitrogen) −52 °C. Refractive index 1.4330 at 23 °C. HB Polysiloxane (R1 = R2 = Ph) 12. A triple-neck round-bottomed flask and a condenser were dried in an oven at 120 °C for 12 h. They were cooled to room temperature with a nitrogen stream. T8(OSiMe2OH)8 monomer (1.0 g, 0.87 mmol, 1 equiv of SiOH) was weighed into the flask equipped with the cooling condenser, and THF (20 mL) was added. The solution was cooled to −78 °C in a dry ice/acetone bath, and diphenyldichlorosilane (0.144 mL, 0.7 mmol) was added through a syringe. This was followed by the addition of freshly distilled pyridine (0.114 mL, 1.41 mmol). The reaction was allowed to warm up to room temperature and stirred for 16 h. The D

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Table 1. Monomers, Terminal Groups, Molecular Weight Data, and Glass Transition Temperatures for HB-PCSOX 1−5 (Series 1) and 6−9 (Series 2)a monomers

terminal group

1

T8Vinyl8 HSiMe2OSiMe2H series 1 T8Vinyl8 HSiMePhOSiMePhH series 1 T8Vinyl8 HSiPh2OSiPh2H series 1 T8Vinyl8 H(SiMe2O)2SiMe2H series 1 T8Vinyl8 H(SiMe2O)3SiMe2H series 1 T8(OSiMe2H)8 Vinyl-SiMe2OSiMe2-vinyl series 2 T8(OSiMe2H)8 Vinyl-SiMePhOSiMePh-vinyl series 2 T8(OSiMe2H)8 Vinyl-SiPh2OSiPh2-vinyl series 2 T8(OSiMe2H)8 Vinyl-Si(OEt)2OSi(OEt)2-vinyl series 2

SiH

Mw 2670* Mn 2300

liquid

−67

SiH

Mw 43 000 Mn 33 000

liquid

−79

SiH

Mw 32 000 Mn 22 000

liquid

+10

SiH

Mw 3100 Mn 2300

liquid

ND

SiH

Mw 3800 Mn 2700

liquid

ND

vinyl

Mw 15 000* Mn 3400

hazy liquid

NA

vinyl

Mw 5500* Mn 2600

hazy liquid

NA

vinyl

Mw 3000* Mn 1800

solid

ND

vinyl

Mw 133 000 Mn 16 000

liquid

−51

2

3

4

5

6

7

8

9

mass data

state at RT

Tg (°C)

HBP

a Molecular weights are from SEC in THF or in toluene if marked by an asterisk. For both series 1 and 2 the monomer Ax contributing the terminal group A was used in molar excess relative to the second monomer By such that [A]/[B] ≥ 7. ND (“not discernible”) denotes materials that showed no discernible Tg. NA (“not applicable”) denotes materials that were not characterized for Tg because of haziness and inhomogeneity.

viscous oil was obtained and washed with anhydrous acetonitrile (5 × 15 mL) and dried in a rotary evaporator, but the product cross-linked during the drying process. Ethoxysilane-Terminated HB Polycarbosiloxane (Series 2, R1 = R2 = Me) 17. A 50 mL round-bottomed flask equipped with a vertical cooling condenser was charged with HB polycarbosiloxane (series 2, R1 = R2 = Me) 6 (1.5 g), THF (5 mL), and triethoxysilane (2.21 g, 13.5 mmol). It was flushed with N2 and stirred for 5 min. Karstedt’s catalyst (0.01 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene) was added to the mixture. It was stirred at room temperature for 1 h and then heated in an oil bath at 50 °C for 16 h. THF was removed using a rotary evaporator. A clear viscous oil was obtained, washed with anhydrous acetonitrile (5 × 15 mL), and dried under vacuum overnight to yield 1.65 g. IR (thin film): ν (cm−1) 2957, 2909, 2882 (CH3 and CH2, sym and asym), 1442, 1406, 1390, 1253 (SiCH3), 1167, 1136, 1086, 1052 (SiOSi), 956, 835 (SiCH3 rock), 788 (SiOSi sym). 1H NMR (CDCl3): δ (ppm) 0.0−0.2 (s; SiCH3), 0.4 (m; SiCH2), 1.2 (t; OCH2CH3), 3.8 (q; OCH2CH3). SEC (toluene): Mw = 16 700, Mn = 3900, polydispersity = 4.3. Ethoxysilane-Terminated HB Polycarbosiloxane (Series 2, R1R2 = MePh) 18. A 50 mL round-bottomed flask equipped with a vertical cooling condenser was charged with HB polycarbosiloxane (series 2, R1R2 = MePh) 7 (1.0 g), triethoxysilane (2.6 mL, 2.28 g, 13.8 mmol), and THF (4 mL). It was flushed with N2 and stirred for 5 min. Karstedt’s catalyst (0.01 g, ∼2% platinum−divinyltetramethyldisiloxane complex in xylene) was added to the mixture. It was stirred at room temperature for 1 h and then heated in an oil bath at 50 °C for 48 h. THF was removed using a rotary evaporator. A clear viscous oil was obtained, washed with anhydrous acetonitrile (5 × 15 mL), and dried under vacuum overnight to yield 0.98 g of an opaque sticky product. IR (thin film): ν (cm−1) 3069, 3049 (PhH), 2959, 2910 (CH3 and CH2, sym and asym), 1957, 1883, 1818, 1767, 1590, 1428, 1406,

OCH2CH3), 0.65 (br s; SiCH2), 0.57 (br s; SiCH2), 0.08 (s; SiCH3), 0.03 (s; SiCH3). Refractive index 1.4393 at 25 °C. Ethoxysilane-Terminated HB Polycarbosiloxane (R1 = R2 = Me, (OSiMe2)4 Spacer) 16. A dried 25 mL round-bottom flask equipped with a magnetically controlled stirrer bar and a nitrogen inlet atop a water-cooled condenser was charged with HB polycarbosiloxane (R1 = R2 = Me, (OSiMe2)4 spacer) 5 (1.921 g, 1.55 mmol/g SiH, 2.98 mmol SiH) and triethoxyvinylsilane (0.749 g, 3.94 mmol). The mixture was stirred under nitrogen for 5 min, and then Karstedt’s catalyst (6.8 mg, Pt−tetramethyldivinyldisiloxane complex in xylenes, 2.1−2.4% Pt, 0.23 μmol Pt) was added. The mixture was stirred under nitrogen at room temperature for 1 h, allowing for an initial exotherm, and then heated at 50 °C for 16 h. The yellow reaction mixture was washed with anhydrous acetonitrile (25 mL, then 4 × 15 mL) under nitrogen, concentrated by rotary evaporation, and further concentrated under vacuum for 16 h to yield 2.43 g of a clear dark yellow oil. IR (film): 2961 (m), 2923 (w), 2889 (w) (CH3 and CH2, sym and asym), 1441 (w), 1408 (w), 1390 (w), 1259 (s, s) (SiCH3), 1142 (s), 1104 (s) (SiOSi), 1082 (s), 1039 (s) (SiOSi asym), 957 (w), 840 (s) (SiCH3 rock), 796 (s) (SiOSi sym), 699 (w), 471 (w). 1H NMR (CDCl3): δH (ppm) 3.84 (q, J = 7.1 Hz; OCH2CH3), 1.25 (t, J = 7.1 Hz; OCH2CH3), 0.65 (br s; SiCH2), 0.57 (br s; SiCH2), 0.09 (s; SiCH3), 0.03 (s; SiCH3). Refractive index 1.4351 at 25 °C. Attempted Preparation of Methoxysilane-Terminated HB Polycarbosiloxane (Series 2, R1 = R2 = Me). A 50 mL round-bottomed flask equipped with a vertical cooling condenser was charged with HB polycarbosiloxane (series 2, R1 = R2 = Me) 6 (3.57 g), THF (5 mL), and trimethoxysilane (2.62 g, 21.4 mmol). It was flushed with N2 and stirred for 5 min. Karstedt’s catalyst (0.01 g, ∼2% platinum− divinyltetramethyldisiloxane complex in xylene) was then added. It was stirred at room temperature for 1 h and then heated in an oil bath at 50 °C for 16 h. THF was removed using a rotary evaporator. A E

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Figure 2. Synthesis of HB-PSOX from T8(OSiMe2OH)8 monomer.

Si NMR band at −3 to −10 ppm. Terminal vinyl functionality was indicated by IR bands at ∼3000, 2000, and 1600 cm−1 and two 1H NMR bands in the 5−7 ppm region. Internal silicon atoms had chemical shifts dictated by the nature and number of their substituents (O, Me, Ph, or OEt).31 In earlier studies of HB-PCSOX synthesized from small silane (non-POSS) monomers,32,33 quantitative 1H NMR indicated compositions of approximately 4 mmol/g SiH. This study gave comparable quantitative 1H NMR end-group data: HBP 1 had 3.6 mmol/g SiH, HBP 4 had 2.2 mmol/g SiH, and HBP 5 had 1.55 mmol/g SiH (with values decreasing with increasing n in silane monomer HSiMe2(OSiMe2)nH). While the series 1 HB polymer products were transparent oils, in series 2, HB polymers 6 and 7 were hazy liquids (i.e., the dimethyl and methylphenyl variants) and polymer 8 (the diphenyl variant) was a white solid. In an attempt to investigate whether the haziness was caused by a portion of very high molecular weight polymer that was incompatible with lower molecular weight species, the molar ratio of divinyltetramethylsiloxane to T8(OSiMe2H)8 monomer in the preparation of HBP-(8SiH)POSS-SiMe2Vi was varied in an attempt to create a slightly different distribution of molecular masses in a more compatible and less hazy mixture. However, when the molar ratio of divinyltetramethylsiloxane to T8(OSiMe2H)8 monomer was increased from ∼30:1 to 40:1, the reaction went to completion (according to the disappearance of the SiH IR band), but the polymer was still hazy. Since HB polymers 6 and 7 were hazy and apparently inhomogeneous in composition, DSC characterization for Tg was not undertaken. HB polymers 1, 2, 3, and 9 (Table 1) showed a general increase in Tg with the increasing mass of the repeat unit as expected. HB polymers 4, 5, and 8 had the lowest molecular masses and showed no discernible Tg transition. Molecular Weights. It can be seen from Table 1 that high molecular weight polymers were obtained in fast-reacting hydrosilylation systems, and lower molecular weight polymers were obtained in slower-reacting hydrosilylation systems. Since IR and NMR spectroscopic data show that the reactions go to completion, this observation is best explained in terms of the more reactive monomers being more able to overcome the steric obstacles involved in coupling with the larger entities 29

1391, 1254 (SiCH3), 1167, 1135, 1087, 1053 (SiOSi asym), 957, 843 (SiCH3 rock), 790 (SiOSi sym). 1H NMR (CDCl3): δ (ppm) 0.0−0.2 (s; SiCH3), 0.4 (m; Me2SiCH2), 0.6 (m; MePhSiCH2), 1.2 (t; OCH2CH3), 3.8 (q; OCH2CH3) 7.4 (m; ArH), 7.6 (m; ArH). Preparation of HBP-POSS Nanostructured Materials. The selected HB polymer and linear components (Gelest silanolterminated polydimethylsiloxane DMS-S15, Mw 2000 or Gelest silanol-terminated polydimethylsiloxane−polydiphenylsiloxane random copolymer PDS-1615, Mw 1000) in the desired w/w ratio were mixed in hexane. Bis(2-ethylhexanonate)tin (Gelest, in 10% w/w hexane solution, Sn(II) catalyst) was added at 2 wt % based on total mass of polymers used. The homogeneous solution was cast on a glass microscope slide and cured at 120 °C for 24 h.

■

RESULTS AND DISCUSSION Synthesis of HB Polycarbosiloxanes. HB-PCSOX with octafunctional POSS branch points were synthesized using either T8vinyl8 (as A8 POSS monomer) and a disiloxane of the form HSiR1R2OSiR1R2H (a B2 monomer where R1R2 = Me2, MePh or Ph2) or using T8(OSiMe2H)8 (as A8 POSS monomer) with a disiloxane of the form vinyl-SiR1R2OSiR1R2-vinyl (as B2 monomer where R1R2 = Me2, MePh, Ph2 or (OEt)2); see series 1 and 2 of Figure 1 and Table 1. In order to avoid gelation and to satisfy the requirement for Ax + By BMNLP, (i.e., 1/[(x − 1)(y − 1)] ≥ r ≥ (x − 1)(y − 1) where r = [A]/[B]), the monomer ratio was set such that the desired terminal groups were present at a molar excess of 7 or greater. Hydrosilylation reactions were carried out in THF in the presence of Karstedt’s catalyst (i.e., platinum−divinyltetramethyldisiloxane complex in xylene) and with 0.5−1.0 g of reagents per mL of THF. Series 1 reactions where T8vinyl8 was used were heated at 50 °C for 16 h, by which time IR indicated that the reaction was complete and that the vinyl band at 3000−3100 cm−1 was no longer visible. Series 2 reactions where T8(OSiMe2H)8 was used were heated for stretches of 24 h at 80 °C with additional catalyst addition until IR showed that the reaction was complete and that the SiH IR band at 2100 cm−1 was no longer visible. Products were isolated by removing THF in vacuo, washing with acetonitrile in order to remove unreacted monomers and low mass species, and then drying in a vacuum oven at 50 °C. The HB-PCSOX were characterized by IR, 1H NMR, 29Si NMR, and SEC. Terminal SiH functionality was indicated by an IR band at ∼2100 cm−1, a 1H NMR peak at ∼4.7 ppm, and a F

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present during the later stages of the polymerization. Series 2 reactions of T8(OSiMe2H)8 required additional heating and catalyst, whereas series 1 reactions of T8vinyl8 did not. In platinum-catalyzed hydrosilylations, both electronic and steric effects must be taken into account. Electron-rich vinyl groups react faster than electron-poor vinyl groups, and SiH compounds substituted with electron-withdrawing groups react faster than SiH compounds substituted with electrondonating groups. Steric hindrance decreases reaction rate, but SiH groups are more sensitive to steric hindrance than vinyl groups.34 In series 1, HSiMe2O (where SiMe2O is defined as an M unit) is reacting with vinyl-functionalized POSS (comprised of SiO3, defined as a T unit). The HSiMe2O (M unit) is expected to react fast (since it is electron-poor and unhindered) while the vinyl-SiO3 (T vinyl unit) is expected to react slowly (since it is electron-poor and hindered). For the M SiH unit, the substitutent groups become increasingly electron-withdrawing in the following order: Ph > MePh > Me. Hence, the highest molecular masses were found for methylphenyl polymer 2 and diphenyl polymer 3. Increasing the length of the siloxane unit in the HSiMe2(OSiMe2)nH monomer (n = 1 for HB polymer 1, n = 2 for HB polymer 4, and n = 3 for HB polymer 5) had no discernible effect on measured molecular masses. In series 2, HSiMe2OSiO3 (an MQ unit, where M is defined as above and SiO4 is defined as a Q unit) is reacting with vinylSiMe2O (an M unit). The MQ SiH unit is expected to react moderately slowly (since it carries a moderately electron-poor and slightly hindered SiH group more sensitive to hindrance than a vinyl group) while the M vinyl unit is expected to react fast (since it is electron rich and unhindered). For the M vinyl unit, the order of reactivity is OEt ≫ Me > MePh > Ph, as groups become increasingly electron-donating, and hence the highest molecular weights are seen for methyl polymer 6 and ethoxy polymer 9. Synthesis of HB Polysiloxanes. In contrast to the preparation of HB-PCSOX via hydrosilylation described above, HB-PSOX were prepared from the octasilanol monomer T 8 (OSiMe 2 OH) 8 (Figure 2). The preparation of T8(OSiMe2OH)8 monomer from T8(OSiMe2H)8 in the presence of water and palladium on charcoal has been described in the literature,35 and its synthesis plus a more extensive set of characterization data is also included here. The product had good solubility in THF and acetone and could be stored in solution under nitrogen over long periods. In the absence of trace amounts of either acid or base, it was hydrolytically stable and did not cross-link with itself. The molecular ion for T8(OSiMe2OH)8 was observed in the MALDI TOF mass spectrum, and two peaks were observed in its 29 Si NMR spectrum at −110 and −11 ppm, corresponding to SiO4 cage silicon atoms and to silanol silicon atoms respectively (Figure 3). The octafunctional A8 monomer T8(OSiMe2OH)8 was used to prepare a series of HB-PSOX (Table 2) with variable phenyl contents, attained by varying the phenyl contents of the difunctional B2 dichlorosilane monomers used (Figure 2). The monomer ratio was set at a molar excess of SiOH groups of 7 or greater, and the reactions were performed in THF under nitrogen with liquid nitrogen cooling to −78 °C. The hydrogen chloride side product was removed using pyridine, and the resulting pyridinium hydrochloride salt was removed during the water washing work-up step. It was initially intended to use an excess of T8(OSiMe2OH)8 in a reaction with a dichlorosilane to generate an HBP polysiloxane with silanol-POSS chain ends,

Figure 3. 29Si NMR spectrum of T8(OSiMe2OH)8 monomer showing OSiMe2OH at −11 ppm and SiO4 at −110 ppm.

Table 2. Monomers, Terminal Groups, Molecular Weight Data, and Glass Transition Temperatures for HB-PSOX 10 to 12 (Series 3)a HBP

monomers

10

T8(SiMe2OH)8 Cl2SiMe2 series 3 T8(SiMe2OH)8 Cl2SiMePh series 3 T8(SiMe2OH)8 Cl2SiPh2 series 3

11

12

terminal group

mass data

state at RT

Tg (°C)

SiOH

Mw 4300 Mn 2300

liquid

−42

SiOH

Mw 8500 Mn 3200

liquid

−52

SiOH

Mw 3100 Mn 2000

liquid

−31

a

All molecular weights are from SEC in THF. The monomer Ax contributing the terminal group A was used in molar excess relative to the second monomer By such that [A]/[B] ≥ 7.

but these polar silanol-terminated HB polysiloxanes had poor hydrocarbon solubility and were thus unsuitable for later use in fabricating films and coatings for the space and solar applications of interest. Hence, the synthetic route was modified by using an excess of dichlorosilane in order to form a POSS-HBP with chlorosilane chain ends and then hydrolyzing these groups to create silanol chain ends (left-hand side, Figure 2). In this way, HB-PSOX 10 was obtained, and was found to have excellent solubility in hexane. An attempt was also made to react T8(OSiMe 2OH) 8 with excess Cl2SiPhMe in an analogous reaction, but the excess Cl2SiPhMe was not sufficiently volatile to be stripped off in vacuum after completion of the condensation reaction prior to the hydrolysis of SiCl to SiOH, and this resulted in an intractable product after hydrolysis. HB-PSOX 11 and 12 were instead prepared by reacting an excess of T8(OSiMe2OH)8 with Cl2SiPhMe and then end-capped with excess Cl2SiMe2 (right-hand side, Figure 2). After completion of the condensation reaction, the excess volatile Cl2SiMe2 was readily stripped off under vacuum prior to the hydrolysis of the SiMe2Cl-terminated HB intermediate. The HB-PSOX were characterized by IR, 1H NMR, 29Si NMR, SEC (in THF), and DSC. In the 29Si NMR spectra, SiO4 cage silicon atoms appeared at −110 ppm (as in the octasilanol monomer), terminal OSiMe2OH groups appeared at −13 ppm, O2SiMe2 units appeared at −20 ppm, O2SiMePh units appeared at −34 ppm, and O2SiPh2 units appeared at −45 G

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terminal groups had no significant effect upon refractive index (HB polymers 1, 4, and 5), but increasing phenyl content resulted in an increase in refractive index (HB polymers 10− 12). In an earlier study24 it was shown that the use of silsesquioxane monomers (versus smaller silane or cyclic siloxane monomers) resulted in polymeric products with increased refractive index, and this was attributed to a decrease in density and an increase in free volume. Transparent Nanostructured HB-POSS Materials. HB POSS polymers 9−12, 14, and 15 were formulated with silanolterminated linear siloxanes, bis(2-ethylhexanoate) tin(II) catalyst (also known as tin(II) octoate), and hexane at various HBP/linear w/w compositions (Table 4). The HBPs in this

ppm. Methylphenyl HB-PSOX 11 had a higher measured SEC molecular weight than diphenyl HB-PSOX 12, reflecting the relative reactivity of chlorosilanes in the formation of siloxanes, where alkylchlorosilanes are more reactive than phenylchlorosilanes, and steric factors are significant.36 Since HBPSOX 10 was prepared via a different route to 11 and 12 (see above and Figure 2), inclusion of the molecular weight data for 10 in this comparison would not be meaningful. HB-PSOX 11 and 12, synthesized via the same route, showed Tg’s that increased with increasing molecular weight of the repeat unit, as expected. HB-PSOX 10 had the lowest molecular weight repeat unit but was synthesized via a different route using fewer flexible siloxane spacers between T8 cages, and these two opposing factors are reflected in its Tg value (intermediate between those of HB polymers 11 and 12). HB Polymers with Reactive End Groups for CrossLinking into Nanostructured Materials. In a desire to produce components for nanostructured films or coatings that cure via the alkoxysilane hydrolysis−condensation route (either slowly in the presence of ambient moisture or more rapidly in the presence of catalysts such as tin), SiH-terminated HBPCSOX 1, 4, and 5 (series 1) were reacted with vinyltrialkoxysilane CH2CHSi(OEt)3, and vinyl-terminated HBPCSOX 6 and 7 (series 2) were reacted with trialkoxysilane HSi(OEt)3 via hydrosilylation. The series 3 HB-PSOX already carried curable terminal silanol groups and thus required no further modification. In addition, HB polymers 1 and 6 were also functionalized with faster-reacting methoxy groups, but HB polymer 6 cross-linked immediately under ambient conditions. Most of the resulting oils were transparent and colorless, as desired for space solar cell applications37 where high transmission across a wide range of wavelengths is required. HBPCSOX 9 is notable because it is the only HB polymer from series 1, 2, and 3 (Tables 1 and 2) that carries internal curable SiOEt groups (as opposed to terminal curable SiOR or SiOH groups as discussed above). Thus, it has the potential to form a nanocomposite comprised of discrete cured HB domains.38 HB polymer 9 was also characterized by TGA for thermal stability and only lost 2% of its weight at 350 °C at a heating rate of 10 °C/min under nitrogen. Refractive indices for a selection of starting and alkoxysilyl-modified HB polymers are listed in Table 3. It can be seen from the data that modification of

Table 4. Qualitative Transparency of HB Polymer/Linear Polysiloxane Formulationsa linear HB HB-PCSOX 14 (OSiMe2)2 spacer HB-PCSOX 15 (OSiMe2)3 spacer HB-PCSOX 9 internal SiOEt HB-PSOX 10 OSiMe2 spacer HB-PSOX 11 OSiMePh spacer HB-PSOX 12 OSiPh2 spacer

1 14 4 15 5 16 9

10 11 12

monomers T8Vinyl8/HSiMe2OSiMe2H series 1 T8Vinyl8/H(SiMe2O)2SiMe2H series 1 T8Vinyl8/H(SiMe2O)3SiMe2H series 1 T8(OSiMe2H)8/VinylSi(OEt)2OSi(OEt)2-vinyl series 2 T8(SiMe2OH)8/Cl2SiMe2 series 3 T8(SiMe2OH)8/Cl2SiMePh series 3 T8(SiMe2OH)8/Cl2SiPh2 series 3

terminal group

refractive index

SiH SiOEt SiH SiOEt SiH SiOEt Vinyl

1.4457 1.4438 1.4346 1.4393 1.4316 1.4351 1.4464

SiOH

1.4215

SiOH

1.4330

SiOH

1.4616

PDS-1615 Mw 1000, 14% SiPh2

25/75 50/50 75/25 25/75 50/50 75/25 25/75

clear clear hazy clear clear clear clear

25/75 50/50 75/25 25/75 50/50 75/25 NA

25/75 50/50 75/25 25/75 50/50 75/25 25/75

clear clear cracked clear hazy hazy hazy

25/75 clear

25/75 50/50 75/25 25/75 50/50 75/25

clear clear cracked clear clear clear

clear clear clear clear clear clear

a x/y denotes a formulation comprised of x wt% HB polymer/y wt% linear polysiloxane.

group all carried moisture-curable terminal groups (SiOEt or SiOH) with the exception of HBP 9, which had vinyl terminal groups but moisture-curable internal SiOEt functionality. The formulations were cured at 120 °C for 24 h to give a range of robust and predominantly transparent coatings on glass slides. Two linear silanol-terminated polysiloxanes of similar molecular weights (Mw = 1000−2000) were used: one dimethylsiloxane homopolymer (Gelest DMS-S15) and one polydimethylsiloxane−polydiphenylsiloxane random copolymer (Gelest PDS1615, 14 mol % diphenyl repeat units). It can be seen from these data that increased haziness and cracking were associated with higher HB polymer content in a formulation, and with HB polymers carrying Si(OEt)3 (vs SiOH) terminal groups, factors that would both be expected to cause an increase in cross-link density in the cured formulations. For the formulations containing the linear PDMS DMS-S15, haziness also increased with increasing phenyl content of the HB polymer component, indicating increasing incompatibility between the phenyl- and methylbearing segments. The transparency of the 75/25 formulations based upon HB polymer 15 versus the failure of the 75/25 formulations based upon HB polymer 14 may reflect the extra length and flexibility of the spacers in the former.

Table 3. Refractive Indices of Selected Starting and Alkoxysilyl-Modified HB Polymers HBP

DMS-S15 SiOH Mw 2000 PDMS

H

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The preparation and structure−property relationships for nanostructured materials based upon these HB-POSS polymers and a more extensive range of linear polysiloxanes with varying phenyl content and architecture (pendant diphenyl or methylphenyl in polyphenylsiloxanes vs backbone phenyl in polysilarylene siloxanes39), their characterization for optical transmission, coefficient of thermal expansion, permeability and resistance to extreme temperature cycling, outgassing and ultraviolet, proton, and electron radiation (properties of key importance in space solar cell applications), and the tailoring of their physical properties to fabricate both rigid and flexible coatings will be reported in a separate publication.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Air Force Research Laboratory Space Vehicles Directorate, 3550 Aberdeen Ave SE, Kirtland Air Force Base, Albuquerque NM 87117-5776, under Contracts FA9453-08-M-0010, FA9453-09-C-0015, and FA9453-10-M0179.

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REFERENCES

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CONCLUSIONS

The first synthesis of two varieties of hyperbranched polymers (polycarbosiloxanes, HB-PCSOX, and polysiloxanes, HBPSOX) with polyhedral oligosilsesquioxane, POSS, branch junctures, and their characterization by FTIR, 1H and 29Si NMR, SEC, TGA, and DSC is described. The polymers were prepared by the bimolecular nonlinear polymerization, BMNLP, utilizing either platinum-catalyzed hydrosilylation or silanol condensation reactions. For the former, commercially available −CHCH2 and −OSiMe2H octafunctional POSS and complementary Me, MePh, Ph, and OEt functionalized disiloxanes were used as monomers and the progress of polymerization was monitored by FTIR, while for the latter, the OSiMe2OH-containing octasilanol POSS was prepared, characterized, and used in the BMNLP with dimethyl-, methylphenyl-, and diphenyldichlorosilanes in the presence of pyridine as the hydrochloric acid acceptor. In the HB-PCSOX series, all polymers except the diphenyl polymer from octasilane-POSS (T8(OSiMe2H)8) and Vi-SiPh2O-SiPh2-Vi were liquids, including the closely related counterpart from octavinyl POSS and H-SiPh2-O-SiPh2-H. Their glass temperatures ranged to as low as −67 to −79 °C for polymers from octavinyl-POSS and dimethyl- and methylphenyldisiloxane, respectively. It was found that polymers with silanol-POSS end groups had poor hydrocarbon solubility and were unsuitable for later use in fabricating films and coatings but that this desirable property was readily achievable by making the chlorosilane-terminated analogues via BMNLP and subsequent hydrolysis of the end groups of the resulting intermediate. The polymer molecular weights (Mw by light scattering) ranged from about 3000 to about 43 000 in most cases, with the notable exception of the polymer from octasilane-POSS (T8(OSiMe2H)8) and tetraethoxydivinyldisiloxane which had an Mw as high as 133 000. The polymers prepared were used in various formulations with α,ω-telechelic silanol-terminated linear polysiloxanes to obtain cross-linked films and coatings with tailored cross-link densities either by induced silanol condensation of the silanolterminated HB-PSOX or by hydrolysis of the alkoxysilylterminated HB-PCSOX derivatives and subsequent condensation of the resulting silanol end groups. For the latter approach, the ≡Si−Vi- or ≡Si−H-terminated HB-PCSOX were first converted into their alkoxysilyl derivatives via end-group hydrosilylartion with appropriately functionalized silanes. Most of the obtained cross-linked products were clear, colorless, and transparent nanostructured materials with mechanical properties suitable for various optical applications in the space and solar areas. I

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