Synthesis and properties of carbosilane dendrimers

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Russian Chemical Bulletin, International Edition, Vol. 67, No. 8, pp. 1440—1444, August, 2018

Synthesis and properties of carbosilane dendrimers with perfluorohexyl groups in the outer layer of the molecular structure N. A. Sheremetyeva,a O. A. Serenko,b E. A. Tatarinova,a M. I. Buzin,b F. V. Drozdov,a I. V. Elmanovich,b,c M. O. Gallyamov,b,c and A. M. Muzafarova,b aN.

S. Enikolopov Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, 70 ul. Profsoyuznaya, 117393 Moscow, Russian Federation. Fax: +7 (495) 334 88 47 bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 ul. Vavilova, 119991 Moscow, Russian Federation. Fax: +7 (499) 135 5085. E-mail: aziz @ispm.ru cFaculty of Physics, M. V. Lomonosov Moscow State University, Build. 2, 1 Leninskie Gory, 119991 Moscow, Russian Federation. Fax: +7 (495) 932 8820 Branched perfluorohydrosiloxane with CF3CF2CF2C(CF3)2(CH2)3 groups at the silicon atom was synthesized by a sequence of chemical reactions. The resulting compound was used as a modifying agent for carbosilane dendrimers of the 3rd and 6th generations. Dendrimers with perfluorohexyl terminal groups in surface layer are characterized by a complex of physicochemical methods. It is demonstrated that due to the branching of perfluoroalkyl terminal groups, obtained carbosilane dendrimers are soluble in organic and inorganic media. Differences in the solubility of small and large dendrimers are caused by the formation of the outer fluoride shell of different densities. Key words: dendrimer, perfluoroalkyl terminal groups, solubility.

Dendrimers is an unique class of polymers that differ from other polymeric macromolecules by regular branched structure and monodisperse nature.1,2 High density of branching forces the molecules of the dendritic architecture to adopt three-dimensional globular conformations, which affects their characteristics. The composition of the terminal groups of dendrimers has a determining effect on such physical properties as solubility, glass transition temperature, packing density of monolayers, ability to form self-ordered structures, etc. The introduction of fluorine atoms into the surface layer of the dendrimer molecular structure promotes an increase in their hydrophobicity, a change in the friction coefficient, glass transition temperature, and thermal stability.3 Most of the work in the field of fluorine-containing carbosilane dendrimers involves the study of objects containing linear perfluoroalkyl substituents С6F13 in the surface layer.4,5 These first and second generation dendrimers are highly soluble in perfluorohexane. However, starting from third generation, they lose their solubility. According to the authors,6 the reason for this effect is the formation of an ordered layer of terminal linear perfluoroalkyl groups, which are rigid rods. The insolubility of high-generation carbosilane dendrimers with perfluorinated terminal groups not only

makes it difficult to study their structure and properties, but also hinders their potential practical application, for example, as highly effective surface active nanoscale macromolecule particles.7 In addition, the solubility of high generation dendrimers with perfluorinated terminal groups is one of the conditions for their use for encapsulating of catalysts and chemical reagents in a supercritical carbon dioxide.8,9 We assume that replacing the linear perfluoroalkyl terminal groups of carbosilane dendrimers by branched groups with the same number of fluorine atoms will create a "barrier" for the formation of an ordered interdendritic layer or lead to the formation of a layer with a less dense structure. This can promote the solubility of dendrimers of both low and high generations, at least in fluorinated solvents. The purpose of this work is to synthesize and study the properties of carbosilane dendrimers with branched fluorosilicon-organic groups in the surface layer of the molecular structure. A hydrosilylation method using a modifying agent with a fluorocarbon branched-chain substituent based on hexafluoropropylene dimers was chosen to produce them. Results and Discussion A modifying agent, a fluorosilicon compound having a branched f luorocarbon substituent С(СF 3) 2—

Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1440—1444, August, 2018. 1066-5285/18/6708-1440 © 2018 Springer Science+Business Media, Inc.

Soluble polyfluorinated carbosilane dendrimer

Russ. Chem. Bull., Int. Ed., Vol. 67, No. 8, August, 2018

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Scheme 1

(СF2)2—CF3 was prepared at the first stage of the work (Scheme1). We used known data on the synthesis of fluorinecontaining alkylsilanes and siloxanes10 in the preparation of hydrosiloxane with a perfluorocarbon substituent (compound 3). The synthesis consisted of three subsequent reactions. The reaction of the original fluorocarbon reagent, perfluoro-2-methylpent-2-ene, with allyl bromide in the presence of cesium fluoride was carried out according to a known procedure.11 The product, 5,5,6,6,7,7,7-heptafluoro-4,4-bis(trifluoromethyl)hept-1-ene (1), was isolated by distillation at 120 С under atmospheric pressure. The purity of the main fraction was 98% by GC. The structure of the obtained product was confirmed by 1Н NMR spectroscopy In the next step, the hydrosilylation of polyfluoroalkene 1 with chlorodimethylsilane in the presence of Karstedt´s catalyst was carried out to form the chlorosilyl derivative (compound 2) in quantitative yield. The entire reaction mass was used in the next step of cohydrolysis. Cohydrolysis of compound 2 with chlorodimethylsilane was carried out in the presence of pyridine and traces of water. The resulting hydrosiloxane was isolated by distillation in vacuo at a temperature of 85 С (35 Torr). The purity and structure of (5,5,6,6,7,7,7-heptafluoro-4,4-bis (trifluoromethyl)heptyl)tetramethylsiloxanehydride (3) was confirmed by GC, 1Н NMR spectroscopy, and elemental analysis. The product was a colorless transparent liquid. Synthesized perfluorohydrosiloxane was used as a modifying agent for carbosilane dendrimers of the third and sixth generations with terminal allylic groups. In all cases, the modification of the outer layer of the molecular structure of the dendrimers was carried out by hydrosilylation of allyl groups of macromolecules with an excess of compound 3 in the presence of Karstedt´s catalyst in methyl tert-butyl ether medium according to the previously described method.12 The reaction was followed by the disappearance of proton signals at a double bond in 1Н NMR spectra. After complete conversion of the allyl groups, the products were separated from the reaction mixture by reprecipitation. Carbosilane dendrimers of generations 4.5 (G-4.5F) and 7.5 (G-7.5F) with branched perfluorohexyl groups in the surface layer of the molecular structure were obtained as a result.

The fluorine-containing carbosilane dendrimer G-4.5F is a colorless viscous liquid highly soluble in various organic solvents, such as THF, hexane, diethyl ether, hexafluorobenzene, and freons. Dendrimer G-7.5F is a transparent colorless resinous substance soluble in fluorine-containing solvents, for example in hexafluorobenzene and fluoroxylol. The composition and structure of fluorine-containing carbosilane dendrimers are proved by elemental analysis and 1H NMR spectroscopy. In view of the specific features associated with the solubility of the resulting fluorine-containing dendrimers, the GPC method was not possible for analysis of their purity. In the case of G-4.5F generation dendrimer, the analysis was complicated by close refractive indices of the system under study (nd20 = 1.415) and THF eluent (nd20 = 1.405). As already noted larger generation dendrimer (G-7.5F) is soluble only in fluorine-containing solvents and is insoluble in non-fluorinated organic solvents used as eluent in the GPC method. Figure 1 shows the distribution curves for the hydrodynamic radii R of the synthesized dendrimers, and Table 1 shows the values of R. Obviously, the R value for the dendrimer G-7.5F is larger than for the dendrimer of smaller generation. As it can be seen from Fig. 1, the samples are

f(R) 1.0 1

2

0.5

0 1

10

100 R/nm

Fig. 1. Distribution functions for hydrodynamic radii of dendrimers G-4.5F (1) and G-7.5F (2).

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Dendrimer G-7.5F

monodisperse and are detected as unassociated objects. The absence of G-7.5F associates in the hexafluorobenzene solution is indirectly indicated by the results of viscosimetry. The values of the intrinsic viscosity of dendrimers of two generations are similar (see Table 1).

Table 1. Properties of carbosilane dendrimers G-4.5 and G-7.5 with perfluorinated terminal groups Characteristics Molecular weight, calculated Intrinsic viscosity, []/dL g–1 Hydrodynamic radius, R/nm Glass transition temperature, Tg/С Thermostability, Т5%/С

G-4.5F

G-7.5F

19550 0.035 1.3 –76 400/200*

158590 0.036 3.6 –75 400/210*

* Values of Т5% are given in the numerator under thermal destruction (argon atmosphere), and in the denominator under thermaloxidative destruction (air atmosphere).

Thus, the above suggestion that replacing the linear perfluoroalkyl terminal groups of siloxane dendrimers by branched groups with the same empirical formula will promote the solubility of these dendrimers turned out to be correct. Carbosilane dendrimer G-4.5F with branched perfluoroalkyl groups is soluble not only in fluorinated solvents, but also in ordinary organic solvents. Dendrimer G-7.5F is soluble in fluorine-containing solvents. Differences in the solubility of the "small" and "large" dendrimers, given almost complete identity of their chemical composition, indicate that in the latter case a dense shell of fluoridecontaining fragments is formed. This isolates the inner sphere of the dendrimer from the solvent. To disorder the outer layer of the dendrimer molecule, stronger (fluorinecontaining) solvating agent is required. The resulting dendrimers are readily soluble in liquid СО2. Figure 2 shows the phase diagrams of G-4.5F—СО2 and G-7.5F—СО2 systems for the supercritical region of the solvent. Regardless of the generation number of dendri-

Soluble polyfluorinated carbosilane dendrimer


P/atm

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Experimental 2

Homogenous area 200

160

1

120 Heterogeneous area

40

50

60

T/С

Fig. 2. Phase diagrams for systems G-4.5F—СО 2 (1) and G-7.5F—СО2 (2).

mers, they are soluble in supercritical СО2, with the phase diagram for the G-7.5F—СО2 system shifted to higher pressures relative to the G-4.5F—СО2 diagram. Consequently, the solubility of dendrimer G-7.5F in supercritical СО2 is somewhat lower than solubility of G-4.5F, since at the same temperature, a change in the quality of the solvent by increasing the pressure is required to convert the G-7.5F—СО2 system to a homophase state. Thus, even in supercritical СО2, the differences due to changes in the density of the shell are also clearly manifested. The solubility results of dendrimers with branched perfluoroalkyl groups suggest that the G-7.5F dendrimer with a dense outer layer is capable of performing the function of a nanoscale container. Table 1 shows thermal characteristics of dendrimers and the surface properties of films based on them. According to the results of the calorimetric study, the modified carbosilane dendrimers G-4.5F and G-7.5F are amorphous systems with a low glass transition temperature, which is independent of the generation number. The limit of thermal stability of dendrimers for both G-4.5F and G-7.5F is 400 С in an inert medium and 200 С in an oxidizing atmosphere. Thus, we have obtained and characterized with the help of variety of physicochemical methods dendrimers of two generations (4.5 and 7.5), containing perfluoroalkyl groups in the surface layer of the molecular structure and soluble in organic and inorganic media. The branching of perfluoroalkyl terminal groups of carbosilane dendrimers is an effective tool for controlling the solubility of these dendrimers, both low and high generations. The proposed approach to the synthesis of high-generation dendrimers with fluorine-containing terminal groups, soluble in supercritical carbon dioxide, expands the prospects for their use, in particular, in the solution of synthetic problems by the methods of "green chemistry".

Perfluoro-2-methylpent-2-ene (98%, CAS 1584-03-8, PiMinvest, Russia) was used for synthesis. 1H NMR spectra were registered using a Bruker WP-250SY (250.13 MHz) spectrometer, solvent was CDCl3 with addition of hexafluorobenzene, internal standard was tetramethylsilane. Investigation of dendrimers by dynamic light scattering was carried out using an ALV/DLS/SLS-5022F Compact Goniometer System (ALV-GmbH, Germany) with a He—Ne-laser ( = 632.8 nm, power 22 mW). All measurements were carried out at scattering angle of 90 at 25 С using hexafluorobenzene as a solvent. Glass transition temperature (Tg) of samples was determined using differential scanning calorimetry with a DSC-822e thermoanalyzer (Mettler-Toledo, Swiss) at heating rate of 20 deg min–1. A temperature corresponding to the inflection point at the stage of its devitrification was assumed to be Tg of the sample. Thermostability of the samples was evaluated from the results of thermogravimetric analysis. The studies were conducted using a Derivatograf_К device (МОМ, Hungary) at heating rate of 10 deg min–1 in air and in argon. Phase diagrams for dendrimer—СО2 systems were obtained by measuring the cloud points of a solution in a variable-volume reactor.13 The device for measuring cloud points was a highpressure flow reactor equipped with a piston, one outlet of which was connected to an optical cell manufactured by SITEC, and the other output was used for regulation and pressure control. The initial pressure in the optical cell was 400 atm, while the system for both dendrimers was a homophase. After equilibrium was established, the pressure was reduced at a rate of 0.5 atm s–1 for 30 min. The cloud point was determined visually through the sapphire windows of the optical cell. The measurements were carried out at various temperatures, the phase diagram was obtained in the "pressure-temperature" coordinates. 5,5,6,6,7,7,7-Heptafluoro-4,4-bis(trifluoromethyl)hept-1-ene (1). Perfluoro-2-methylpent-2-ene (98%, 169.9 g, 0.6 mol) was added dropwise in argon atmosphere to the mixture of anhydrous CsF (86.0 g, 0.6 mol) and freshly distilled diglyme (161.5 g, 1.0 mol). The reaction mixture was heated to 30 С and allyl bromide (68.5 g, 0.6 mol) was added at 40 С and intensive stirring. After storing at this temperature for 40 h diethyl ether (250 mL) was added and the mixture was washed with NaCl solution to remove the main part of diglyme. After storing of the reaction mixture over Na2SO4 for 8 h and evaporation of diethyl ether, the reaction mixture was distilled at atmospheric pressure into several fractions. Product (88 g, 85% yield) was obtained. The purity was 98% (by GC). 1Н NMR (CDCl3), : 2.95 (m, 2 Н, CH2CH=CH2); 5.24 (m, CH2CH=CH2); 5.85 (m, 1 Н, CH2CH=CH2). Chloro(5,5,6,6,7,7,7-heptafluoro-4,4-bis(trifluoromethyl)heptyl)dimethylsilane (2). 5,5,6,6,7,7,7-Heptafluoro-4,4-bis(trifluoromethyl)gept-1-ene (40.06 g, 0.111 mol) (1) was added to Pt-catalyst РС-072 (Karstedt´s catalyst, 80 μL) in inert atmosphere. Then chlorodimethylsilane (33.22 g, 0.351 mol) was added dropwise to the mixture. The reaction mixture was stored for 48 h at 40 С. The signals of СН2—СН=СН2 bond practically disappeared in 1Н NMR spectra for this time. Complete reaction mixture was used in the synthesis of hydrosiloxane 3. (5,5,6,6,7,7,7-Heptafluoro-4,4-bis(trifluoromethyl)heptyl)tetramethylsiloxanehydride (3). The water (2 mL, 0.111 mol) in

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400 mL of THF was added dropwise to the mixture of chloro(5,5,6,6,7,7,7-heptafl uoro-4,4-bis(trifluoromethyl)heptyl)dimethylsilane (50.58 g, 0.111 mol) (2), chlorodimethylsilane (84.11 g, 0.889 mol), pyridine (136 g, 1.732 mol), and dried THF (100 mL). Formed precipitate was removed by extraction with diethyl ether. Then 35.23 g of product (72%) was obtained by distillation in vacuo of water jet pump. B.p. 85 С (35—40 Torr), nD20 = 1.3549. Found (%): C, 31.26; H, 3.78; F, 50.25; Si, 11.04. Calculated (%): C, 31.58; H, 3.87; F, 49.95; Si, 11.36. 1Н NMR (CDCl3), : 0.18 (m, 12 Н, Si(CH3)2); 0.55 (m, 2 Н, CH2CH2CH2Si); 1.7 (m, 2 Н, CH2CH2CH2Si); 2.09 (m, 2 Н, CH2CH2CH2Si); 4.65 (m, 1 Н, SiН). Carbosilane dendrimer with perfluorohexyl groups Si6132F. Dendrimer Si2932(All) (1.13 g, 0.3 mmol) in dry tert-butyl methyl ether (8 mL) was added to Pt-catalyst РС-072 (15 μL) in inert atmosphere. (5,5,6,6,7,7,7-Heptafluoro-4,4-bis(trifluoromethyl)heptyl)tetramethylsiloxanehydride (7.2 g, 0.015 mol) (3) was added to the reaction mixture under stirring. Reaction mixture was kept for 48 h at 40 С and for 72 h at 20 С. Conversion was 98% according to 1Н NMR data. An isomerization of ~2% allyl groups occurred during hydrosilylation. The resulting dendrimer was fractionated by reprecipitation, nD20 = 1.4152. Yield 4.6 g (78%). Found (%): C, 38.44; H, 5.18; F, 38.54; Si, 13.40. C 624H 1020F 416O 32Si 93. Calculated (%): C, 38.33; H, 5.22; F, 40.40; Si, 13.36. 1Н NMR (CDCl3), : –0.9 (m, 84 Н, SiСН3); 0.05 (m, 384 Н, Si(СН3)2); 0.55 (m, 304 Н, SiCH2); 1.35 (m, 120 H, CH2); 1.70 (m, 64 Н, CH2CH2CН2Si); 2.15 (m, 64 Н, CH2CH2CH2Si). Carbosilane dendrimer with perfluorohexyl groups Si509256F. Pt-catalyst РС-072 (13 μL) was added in inert atmosphere to dendrimer Si253256(All) (1.085 g, 0.0033 mmol) solution in dry tert-butyl methyl ether (8 mL). Excess of (5,5,6,6,7,7,7-heptafluoro-4,4-bis(trifluoromethyl)heptyl)tetramethylsiloxanehydride (6.86 g, 0.014 mol) (3) was added to the reaction mixture under stirring. Reaction mixture was kept for 48 h at 40 С in sealed flask. After achieving 88% conversion of allyl groups (according to 1Н NMR) solution was double concentrated and the catalyst РС-072 (8 μL) was added. The mixture was kept 48 h at 45 С. After achieving of total conversion of allyl groups of dendrimer (1Н NMR spectra), the product was reprecipated four time from C6F6 into C2H5OH. 4.3 g (80%) was obtained. Found (%): C, 38.73; H, 5.48; F, 38.23; Si, 13.48. C5104H8412F3328O256Si765. Calculated (%): C, 38.65; H, 5.30; F, 39.85; Si, 13.47. 1Н NMR (CDCl3), : 0.05 (m, 756 Н, SiСН3); 0.20 (m, 3072 Н, Si(СН3)2); 0.70 (m, 2544 Н, SiCH2); 1.45 (m, 1016 H, CH2); 1.80 (m, 512 Н, CH2CH2CН2Si); 2.30 (m, 512 Н, CH2CH2CH2Si).

The work was financially supported by the Russian Foundation for Basic Research (Project No. 17-03-

Sheremetyeva et al.

01037 А) and Russian Science Foundation (Project No. 16-13-10521, synthesis of allyl derivatives of carbosilane dendrimers). References 1. A. M. Muzafarov, E. A. Rebrov, Polym. Sci., Ser. C (Engl. Transl.), 2000, 55. 2. D. Astruc, E. Boisselier, C. Ornelas, Chem. Rev., 2010, 110, 1857. 3. A.-M. Caminade, C.-O. Turrin, P. Sutra, J.-P. Majoral, Curr. Opin. Colloid Interface Sci., 2003, 8, 282. 4. B. A. Omotowa, J. M. Shreeve, Macromolecules, 2003, 36, 8336. 5. K. Lorenz, H. Frey, B. Stuhn, R. Mulhaupt, Macromolecules, 1997, 30, 6860. 6. B. Trahasch, B. Stuhn, H. Frey, K. Lorenz, Macromolecules, 1999, 32, 1962. 7. B. A. Omotowa, J. M. Shreeve, Macromolecules, 2003, 36, 8336. 8. I. Cooper, J. D. Londono, G. Wignall, J. B. McClain, E. T. Samulski, J. S. Lin, A. Dobrynin, M. Rubinstein, A. L. C. Burke, J. M. J. Frechet, J. M. DeSimone, Nature, 1997, 389, 368. 9. N. A. Shumilkina, V. D. Myakushev, E. A. Tatarinova, M. I. Buzin, N. V. Voronina, T. V. Laptinskaya, M. O. Gallyamov, A. R. Khokhlov, A. M. Muzafarov, Polym. Sci., Ser. A (Engl. Transl.), 2006, 48, 1240. 10. A. E. Shamaev, A. V. Ignatenko, S. P. Krukovsky, Russ. Chem. Bull., 2004, 53, 2229. 11. T. Konakahara, S. Okada, J. Furuhashi, J. Sugaya, T. Monde, N. Nakayama, K. Y. F. Nemoto, T. Kamiusuki, J. Fluorine Chem., 2000, 101, 39. 12. N. A. Sheremetyeva, N. V. Voronina, A. V. Bystrova, V. D. Miakushev, M. I. Buzin, A. M. Muzafarov, Advances in Silicones and Silicone-Modified Materials, ACS Symposium Series, American Chemical Society, Washington, DC, 2010, p. 111—134. 13. S. Mawson, K. P. Johnston, J. R. Combes, J. M. DeSimone, Macromolecules, 1995, 28, 3182.

Received February 19, 2018; accepted May 23, 2018