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Journal of Thermal Analysis and Calorimetry

, Volume 132, Issue 3, pp 1545–1555 | Cite as

A library of monoalkenylsilsesquioxanes as potential comonomers for synthesis of hybrid materials

  • Katarzyna Mituła
  • Michał Dutkiewicz
  • Beata DudziecEmail author
  • Bogdan MarciniecEmail author
  • Krystyna Czaja
Open Access
Article

Abstract

In this paper we demonstrate a facile and efficient synthesis of monoalkenylsubstituted POSS compounds, i.e., R7(Si8O12)-alkenyl and R7(Si8O12)(OSiMe2)-alkenyl with a functional alkenyl group of chain length (from vinyl to dec-9-enyl, with or without additional –OSiMe2– spacer between the silsesquioxane core and alkenyl moiety) and with five types of inert groups (Et, iBu, iOc, Cy, Ph) at the POSS core. These compounds constitute a library of monosubstituted POSS systems, including thorough spectroscopic and thermal characteristics. These findings could be especially useful for material chemists applying the compounds as modifiers or comonomers for copolymerization reaction, e.g., with olefins.

Graphical Abstract

Keywords

Alkenylsubstituted silsesquioxanes Polyhedral oligomeric silsesquioxanes Thermal properties Thermal stability 

Introduction

The unique structures of polyhedral silsesquioxanes (POSS) of the general formula (RSiO3/2)n have attracted widespread attention as precursors and components of a variety of inorganic/organic hybrid materials, as well as their application in optics, catalysis, and electronics, etc. [1, 2]. They have also attracted the increasing attention of material scientists investigating the structure–property relationship of polymer/POSS composites. As nanosized building blocks, they have been widely studied because of their behavior at the nanoscale [3, 4]. There are reports indicating the great interest of material chemists in silsesquioxanes with one or more functionalities and their potential use in the modification of various types of polymers. As is known, there are a few possibilities to introduce functional silsesquioxane into the polymer matrix. This is related to the amount and type of the functional groups (FG) attached to the POSS core and whether they are reactive or inert [1, 2, 5]. When the POSS molecules contain only inert groups, it is possible to introduce them into the already existing polymer matrix by physical mixing or performing a polymerization of the comonomer in the presence of the abovementioned POSS compound (POSS is, in effect, “blended” into the polymer matrix). Another method is to perform copolymerization with the organic comonomer and silsesquioxane containing the same or similar FG. As far as copolymerization is concerned it also matters how many FG there are at the POSS core and if they are all capable of reacting with the comonomer. In this way, silsesquioxane may be incorporated as a pendant group or cross-linker, respectively. Finally, it is also feasible to perform direct cross-linking of POSS units possessing only reactive FG, without an additional copolymer, to form a three-dimensional network. As a result, POSS species can be dispersed or aggregated/agglomerated together as either crystalline or even amorphous phase. Taking this into consideration, the resulting materials of a hybrid, inorganic–organic nature may exhibit interesting and improved physicochemical parameters, i.e., thermal, mechanical and optical [3, 6, 7]. These options are visualized in Fig. 1.
Fig. 1

Possible organic polymer—POSS architectures

The monofunctionalized T8-silsesquioxanes are of specific importance as they can interact with the polymeric matrix in a grafting process or, alternatively, play the role of a comonomer in a copolymerization (or homopolymerization [8]) reaction with organic mers, resulting in a polymer with pendant POSS units [3, 9, 10, 11, 12, 13, 14]. However, the properties in resulting composites may significantly differ, due to the distinct amount of POSS pendant moieties in the organic matrix. As trisilanols are commercially available, it is feasible to introduce a great variety of FG to the POSS core. Resulting compounds may be used as modifiers of a wide range of organic polymers, e.g., polyolefins, polyethers, polyamides, polyimides, polystyrenes, polyacrylates, polyurethanes, epoxy resins or carbon-based materials. [3, 15]. Because of their advantageous properties, polyolefins, such as PE, PP and PS, are widely used in a number of areas (from industry to everyday life). However, the possibilities of their modification (especially PE and PP) with POSS units can be limited and concern either physical blending (POSS with inert groups) or their initial preparation, i.e., irradiation or temperature initiation to create active sites at the polymer matrix, which are able to be grafted with FG of modifiers (usually via radical mechanism) [12, 16]. For PS, it is also likely to functionalize their phenyl ring using, e.g., Friedel–Crafts reaction [17]. Unfortunately, for methods involving initial polymer preparation, partial degradation of the polymer may be observed. Hence, reports have been published on the preparation of polyolefin/POSS composites (containing mainly PE, PP and PS) via direct copolymerization of an olefin monomer with a functionalized POSS as a comonomer (in the presence of variety of catalytic systems including Ziegler–Natta, metallocene or post-metallocene complexes) [11, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29]. As is known, the resulting composites exhibit enhanced physicochemical properties (thermal, mechanical, morphological, rheological, etc.) to the parent polymers [3, 6, 12].

Herein, we present a library of monofunctionalized silsesquioxanes with a different functional alkenyl group of diverse chain length (from vinyl to dec-9-enyl, with or without an additional –OSiMe2– spacer between the silsesquioxane core and alkenyl moiety) and with five types of inert groups (Et, iBu, iOc, Cy, Ph) at the POSS core. This paper reports on their synthetic procedures involving condensation and/or hydrosilylation reaction, as well as a thorough spectroscopic and thermal characterization. This provides the opportunity to discuss not only the synthetic aspects of preparation of this large group (40 compounds) of monoalkenylfunctionalized silsesquioxanes (most of which are not found in the existing literature), but also the issues connected with their thermal resistance and possible degradation paths. Elaboration of efficient synthetic methodology for the abovementioned POSS-based systems would be particularly important for further application of these compounds as polymers modifiers or comonomers for copolymerization reaction with olefins.

Experimental

All syntheses were conducted under argon atmosphere using standard Schlenk-line and vacuum techniques. 1H, 13C and 29Si NMR spectra were recorded using Brucker Ultrashield 300 and 400 MHz spectrometers, using CDCl3 as a solvent. Chemical shifts are reported in ppm with reference to the residual solvent (CHCl3) peaks for 1H and 13C and to TMS for 29Si. FT-IR spectra were recorded on a Bruker Tensor 27 Fourier transform spectrophotometer equipped with a SPECAC Golden Gate, diamond ATR unit with a resolution of 2 cm−1. GC analysis was performed on a Bruker 430 gas chromatograph equipped with a TCD detector and capillary column megabore HP-130m (Hewlett-Packard). TLC was conducted on TLC-sheets coated with 0.2 mm thick silica gel with fluorescent indicator UV254. Column chromatography was performed on silica gel 60 (70–230 mesh) using hexane/DCM. MALDI-TOF mass spectra were recorded on a UltrafleXtreme mass spectrometer (Bruker Daltonics), equipped with a SmartBeam II laser (355 nm) in 500–4000 m/z range. 2,5-Dihydroxybenzoic acid (DHB, Bruker Daltonics, Bremen, Germany) served as the matrix and was prepared in TA30 solvent (30:70 v/v acetonitrile: 0.1% TFA in water) at a concentration of 20 mg cm−3. High-resolution mass spectrometry analyses were performed using a Synapt G2-S mass spectrometer (Waters) equipped with an electrospray ion source and quadrupole-time-of-flight mass analyzer. Methanol was used as a solvent. The measurement was taken in positive ion mode with a desolvation gas flow of 600 L h−1 and capillary voltage set to 4500 V, with the flow rate 100 µL min−1. The EIMS analysis was performed using a Bruker 320 MS/450 GC. TGA analyses were performed using a TGA4000 (PerkinElmer) thermal gravimetric analyzer. The measurements were conducted in nitrogen atmosphere (flow of 20 cm−3 min−1), in a temperature range 30–1000 °C, and heating rate − 10 °C min−1.

Materials

The chemicals were purchased from the following sources: silicon tetrachloride, trichlorosilane, chlorodimethylsilane, trichlorovinylsilane, allyltrichlorosilane, chloro(dimethyl)vinylsilane, allyl(chloro)-dimethylsilane, 1,5-hexadiene, Karstedt’s catalyst (platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex [Pt(dvs)], solution in xylene with w2% of Pt), calcium hydride, sodium hydride, ammonium chloride, lithium aluminum hydride (LAH), magnesium sulfate, triethylamine, N,N-diisopropylethylamine, toluene, tetrahydrofuran (THF), diethyl ether, methanol, n-hexane, n-pentane, dichloromethane (DCM), chloroform-d, molecular sieves type 4 Å from Sigma-Aldrich. 1,9-decadiene from TCI. R7(Si7O9)(OH)3 (trisilanol POSS; R = Cy, Et, iBu, iOc, Ph) from Hybrid Plastic. Silica gel-MN-Kieselgel 60 from Fluka Chemie AG. TLC-sheets from Macherey–Nagel. Sartorius Minisart syringe filters (PTFE, 0.2 μm) from Sartorius AG.

Synthetic procedures

Preparation of monosilanolPOSS (1-3OH)- R7(Si8O12)(OH) (R = Et, Ph)

An example for alkyl inert group (R = Et)

Et7(Si7O9)(OH)3 (1-Et-3OH) (5.25 g, 8.8 × 103 mol), anhydrous THF (100 cm−3) and Et3N (1.85 cm−3, 13.0 × 103 mol) were added to a two-neck, round-bottom flask equipped with a magnetic stirrer. The reaction mixture was placed in an ice-water bath and purged with argon, after which silicon tetrachloride (1.06 cm−3, 9.3 × 103 mol) was added. The reaction was carried out for 24 h at room temperature. Next, the insoluble solid of triethylammonium chloride was removed by filtration on a glass frit. The precipitate was washed with THF (3 × 5 mL), and the solvent was evaporated. The residue left after evaporation was dissolved in a mixture of THF (50 cm−3) and water (10 cm−3) and heated at 65 °C for 24 h. After this time, the THF was evaporated and the remains were extracted with n-hexane (100 cm−3), water and brine. The organic phase was collected and dried with MgSO4. Evaporation gave the analytically pure product 1-Et-OH in the form of a white powder (5.45 g, 97% yield) [30].

An example for phenyl inert group (R = Ph)

Ph7(Si7O9)(OH)3 (1-Ph-3OH) (1.14 g, 1.23 × 103 mol), anhydrous THF (80 cm−3) and EtN(iPr)2 (1.07 cm−3, 6.14 × 103 mol) were added to a two-neck round-bottom flask equipped with a magnetic stirrer. The reaction mixture was placed in an ice-water bath and purged with highly pure argon. Next, silicon tetrachloride (0.14 cm−3, 1.24 × 103 mol) was added. The reaction was carried out for 24 h at room temperature. The THF was evaporated from the reaction mixture. After that, diethyl ether (50 cm−3) was added. The mixture was then extracted with water and brine. The organic phase was dried with MgSO4. Evaporation gave the analytically pure product as a white powder: 1.12 g of 1-Ph-OH with 94% yield.

General procedure for preparation of alkenyl POSS compounds (3 and 3-OSi) (R7POSS-alkenyl and R7POSS-OMe2Si-alkenyl) via condensation reaction

An example for synthesis of iBu7POSS-Vi (3-iBu-Vi)

iBu7(Si7O9)(OH)3 (1-iBu-3OH) (1.00 g, 1.27 × 103 mol), anhydrous THF (15 cm−3) and Et3N (0.64 cm−3, 4.56 × 103 mol) were added to a two-neck round-bottom flask equipped with a magnetic stirrer. The reaction mixture was placed in an ice-water bath and purged with highly pure argon. Next, trichlorovinylsilane (0.16 cm−3, 1.28 × 103 mol) was added. The reaction was carried out for 24 h at room temperature. After this, the insoluble solid of triethylammonium chloride was removed by filtration on a glass frit. The organic phase was evaporated to obtain the crude product, which was precipitated in cold MeOH. After methanol removal, the solid was dried in vacuo. (see Supplementary Material for FT-IR analysis).

An example for synthesis of Ph7POSS-OMe2SiVi (3-Ph-OSi-Vi)

Ph7(Si8O12)(OH) (1-Ph-OH) (0.50 g, 0.51 × 103 mol), anhydrous THF (10 cm−3) and Et3N (0.22 cm−3, 1.54 × 103 mol) were added to a two-neck round-bottom flask equipped with a magnetic stirrer. The reaction mixture was placed in an ice-water bath and purged with highly pure argon. Chloro(dimethyl)vinylsilane (0.10 cm−3, 0.77 × 103 mol) was then added. The reaction was carried out for 24 h at room temperature. Next, the insoluble solid of triethylammonium chloride was removed by filtration on a glass frit. The organic phase was evaporated to obtain the crude product that was precipitated in cold MeOH. After methanol removal, the solid was dried in vacuo. 

Detailed experimental procedures for obtaining of hex-5-enyl- and dec-9-enylchlorosilane derivatives (2m and 2m-OSi) as well as analytical data of all isolated compounds (3 and 3-OSi) and TG and DTG analysis are presented in Supplementary Material.

A general suggestion for the synthesis of both 3 and 3-OSi is that after solvent evaporation there are two possible ways to purify the remaining crude products. For inert groups R = iBu, Cy, Ph it is best to purify them by a precipitation from DCM in cold methanol (R = iBu, Cy, Ph) or n-hexane (R = Ph). Because of better solubility of products with inert groups R = Et, iOc in the abovementioned solvents, it is better to purify them by column chromatography followed by washing with cold methanol. It should be noted that presence of –OSiMe2– spacer in 3-OSi affects the inconsiderably higher solubility of these compounds in comparison with 3. Additionally, the presence of longer alkenyl chain also improves their solubility. Further spectroscopic characterization of the pure products confirmed their identity and purity.

Results and discussion

Our main interest was to synthesize a series of monoalkenylsubstituted POSS compounds varying not only in the aspect of the length of the alkenyl group at the silsesquioxane core but also the type of inert groups at seven Si vertexes. These two modifications not only affect differences in the synthetic procedure of certain silsesquioxanes, but also their physical properties, i.e., solubility, the state of matter, and their thermal behavior. As far as we are concerned, there is no existing scientific research of this kind. Thus, collecting the abovementioned aspects of monoalkenylPOSS compounds could be very useful for material scientists, e.g., use as comonomers for diverse applications.

We divided our experimental studies into two parallel paths which concern the synthesis of monoalkenylPOSS compounds without a –OSiMe2– spacer (3) (Scheme 1) between alkenyl moiety and the POSS core with this spacer (3-OSi) (Scheme 2). It involved using either incompletely condensed silsesquioxane trisilanols R7Si7O9(OH)3 (1-3OH) or its cubic monosilanol form R7Si8O12(OH) (1-OH). Naturally, the first step was to perform the synthesis of title compounds via the hydrolytic condensation of trisilanol (1-3OH) or monosilanol (1-OH) with respective alkenyltrichlorosilane or alkenylchlorodimethylsilane (2). The general synthetic concept for condensation reaction of silanols with chlorosilanes is documented, but we performed additional studies on the impact of type of solvent, base and the stoichiometry of the reagents, etc., on the overall efficiency of the process [31, 32, 33].
Scheme 1

The synthetic route for obtaining monoalkenylPOSS (3) compounds

Scheme 2

The synthetic route for obtaining monoalkenylsiloxyPOSS (3-OSi) compounds

Synthesis of product type 3

The “corner capping” process was optimized to enable total conversion of trisilanol 1-3OH (R = Et, iBu, iOc, Cy, Ph). The general reaction path for synthesis of 3 is presented on Scheme 1.

The optimized condensation reaction conditions involved a series of parameters.
  • Type of solvent and its amount THF, pentane or hexane were selected for tests and all of them gave similar, positive results. For 1-Ph-3OH, only THF could be used because of the solubility of the trisilanol (for 1-Ph-3OH THF was used for all further studies). The reaction time was dependent on the silanol concentration in a solvent (best results for 0.03–0.05 M) and was monitored by FT-IR, allowing observation of disappearance of the bands assigned to Si–OH moieties, usually 24 h (see Supplementary Material. Par. 1.2).

  • Silane reagent For vinyl- and allylPOSS compounds the silane was commercially available, but for hex-5-enyl- and dec-9-enylPOSS we used pre-functionalized trichlorosilanes (2m) obtained via [Pt2(dvs)3] catalyzed hydrosilylation of 1,5-hexadiene and 1,9-decadiene, respectively, by Cl3SiH [34]. The reactions were performed at room temperature and in the excess of the corresponding diene that played the role of the reagent and reaction medium (due to ease of isolation procedure). The reaction conditions and catalyst enable high selectivity and yields of the desired β-hydrosilylation product (see Supplementary Material. Paragraph 1.1, Scheme S1). For the effective product formation, the respective trichlorosilane (2n or 2m) was used in 1.01 equiv. per 1 mol of 1-3OH.

  • Type of HCl scavenger A number of compounds were tested, e.g., K2CO3, N(iPr)2Et (DIPEA) and NEt3 (TEA) and the latter was found to be the most efficient. The amount of amine was calculated for 1.5 equiv. per one Si–OH moiety. However, in the case of phenyl trisilanol (1-Ph-3OH) the TEA was not sufficient to form the desired 3-Ph. It is probable that the basicity of amine is important and affects its possibility to react with the evolving HCl. The more basic amines, i.e., N(iPr)2Et (DIPEA pKa = 11.4, TEA pKa = S10.7) enable acquisition of high yields of final products (3).

As a result, we developed a procedure for effective corner capping of 1-Ph-3OH with respective silane, which was performed in THF and with the use of DIPEA. The inert groups did not affect changes in the synthetic methodology.

In the above-optimized conditions, we investigated the scope of this reaction, using various trisilanols (1-3OH, R = Et, iBu, iOc, Cy, Ph) and trichlorosilanes (2n and 2m), respectively. These results are summarized in Table 1.
Table 1

Results of condensation reactiona to obtain monoalkenylPOSS (3) compounds and their thermal properties (measured in N2)

Prod. No

Isolated yield/%

Mass loss temperature/°C

Decomposition temperature Tonset/°C

Residue at 1000°C/%

T d 5%

T d 10%

3-Et-Vi

90

205

219

223

13

3-iBu-Vi

98

239

272

217

42

3-iOc-Vi

97

343

366

368

24

3-Cy-Vi

97

350

382

326

49

3-Ph-Vi

81b

437

462

421

55

3-Et-All

96

203

214

207

16

3-iBu-All

96

242

256

254

16

3-iOc-All

96

319

351

359

15

3-Cy-All

98

359

409

332

65

3-Ph-All

81b

439

460

441

63

3-Et-Hex

89

207

224

226

20

3-iBu-Hex

96

247

265

259

19

3-iOc-Hex

91

294

331

328

26

3-Cy-Hex

93

432

463

451

40

3-Ph-Hex

76b

453

476

440

54

3-Et-Dec

94

219

237

237

12

3-iBu-Dec

96

271

292

285

17

3-iOc-Dec

92

319

349

335

20

3-Cy-Dec

96

353

414

297

42

3-Ph-Dec

75b

391

441

370

52

aReaction conditions: [1-3OH]:[2]:[TEA] = 1:1.01:3.6, THF, RT, 24h, Ar

b[1-3OH]:[2]:[DIPEA] = 1:1.01:5

Synthesis of product type 3-OSi

The condensation reaction for obtaining monoalkenylsiloxyPOSS compounds (3-OSi), i.e., with the –OSiMe2– was verified and optimized analogously to the procedure described above for 3. The general reaction path for the synthesis of 3-OSi is presented in Scheme 2.

The presence of the siloxy linkage required using respective monosilanol 1-OH with five different inert substituents (R = Et, iBu, iOc, Cy, Ph). Again, the vinyl- and allylsiloxyPOSS compounds were synthesized using commercially available chlorodimethylsilanes (2n-OSi), but for hex-5-enyl- and dec-9-enylPOSS we used pre-functionalized chlorodimethylsilanes (2m-OSi) obtained via [Pt2(dvs)3] catalyzed hydrosilylation of 1,5-hexadiene and 1,9-decadiene by ClMe2SiH, respectively (see Supplementary Material. Par. 1.1). The first step was a synthesis of respective monosilanol (1-OH), which precedes via standard condensation reaction conditions with SiCl4, followed by hydrolysis of the Si–Cl bond, described earlier [31, 32, 33]. However, during synthesis of 1-Ph-OH, we encountered some difficulties employing known hydrolysis conditions [35] (the acidic conditions seemed to favor intercondensation of two 1-Ph-OH molecules, which was verified by 29Si NMR: − 68 ppm). Instead, we applied modified condensation procedure using DIPEA instead of TEA and, interestingly here, the condensation of 1-Ph-3OH with SiCl4 was followed by subsequent hydrolysis of the resulting Si–Cl bond only during extraction, in a one-pot procedure. In the next step, the condensation of 1-OH with the respective chlorosilane proceeded smoothly, applying TEA as amine, without any differences with regard to the inert group type present (R = Et, iBu, iOc, Cy, Ph). It should be noted that synthesis of hex-5-enyl- and dec-9-enylPOSS derivatives (both types: 3 and 3-OSi) could be also performed in another way. The silsesquioxanes with Si–H reactive bond in the POSS core or are attached via –OSiMe2– moiety could be a hydrosilylating agent for 1,5-hexadiene and 1,9-decadiene. We have tested this option and when the Si–H is embedded in the core, the steric hindrance of the reagent unfavors the hydrosilylation and the final products yields are low (the reaction mixture contains unreacted POSS substrate). Additionally, to avoid diene hydrosilylation from both sides, a substantial excess of diene should be applied and this solves problems concerning product isolation. As a result, the condensation procedure is more efficient for obtaining the desired products.

In the above-optimized conditions, we investigated the scope of this reaction, using various monosilanols (1-OH, R = Et, iBu, iOc, Cy, Ph) with monochlorosilanes (2n-OSi and 2m-OSi), respectively. These results are summarized in Table 2.
Table 2

Results of condensation reactiona to obtain monoalkenylsiloxyPOSS (3-OSi) compounds and their thermal properties

Prod. No

Isolated yield/%

Mass loss temperature/°C

Decomposition temperature Tonset/°C

Residue at 1000 °C/%

T d 5%

T d 10%

3-Et-OSi-Vi

94

194

213

205

42

3-iBu-OSi-Vi

93

246

267

225

48

3-iOc-OSi-Vi

91

300

331

307

30

3-Cy-OSi-Vi

95

394

440

442

53

3-Ph-OSi-Vi

94b

414

513

506

60

3-Et-OSi-All

92

188

205

207

14

3-iBu-OSi-All

95

234

252

255

20

3-iOc-OSi-All

91

290

327

310

31

3-Cy-OSi- All

96

404

451

452

49

3-Ph-OSi-All

94b

390

424

371

47

3-Et-OSi-Hex

89

246

264

243

32

3-iBu-OSi-Hex

95

226

260

259

24

3-iOc-OSi-Hex

94

306

354

380

16

3-Cy-OSi-Hex

95

314

367

441

40

3-Ph-OSi-Hex

96b

271

395

410

44

3-Et-OSi-Dec

89

225

261

255

22

3-iBu-OSi-Dec

95

275

297

293

18

3-iOc-OSi-Dec

93

285

356

397

16

3-Cy-OSi-Dec

94

311

399

448

35

3-Ph-OSi-Dec

95b

213

335

409

28

aReaction conditions: [1-OH]:[2]:[TEA] = 1:1.01:1.5, THF, RT, 24h, Ar

b[1-OH]:[2]:[TEA] = 1:1.01:3

All obtained products are air-stable solids (only for the iOc inert group they are viscous liquids) and can be synthesized on a multigram scale. For alkyl inert groups, they are soluble in organic solvents like DCM, CHCl3, THF, hexane, and toluene, but insoluble in, e.g., methanol, MeCN. The phenyl inert groups affect the solubility; not only the silanols (1-Ph-3OH and 1-Ph-3OH), but also resulting products (3-Ph and 3-OSi-Ph) and they are insoluble in methanol, MeCN, and hexane.

Thermal analysis of monoalkenyl-(3) and monoalkenylsiloxyPOSS (3-OSi) compounds

Subsequently, all prepared samples of 3 and 3-OSi were analyzed in terms of the influence of their structure (the type of inert functional groups as well as the chain length of the alkenyl group and the presence of –OSi(Me2)– spacer) on their thermal stability. The influence of the abovementioned structural parameters on thermal properties of measured samples were evaluated on the basis of the 5% (T d 5% ) and 10% (T d 10% ) mass loss temperatures, the amount of residue at 995 °C and the temperature of the main decomposition step (Tonset). The details of the thermogravimetric analysis evaluated on the basis of TG curves are summarized in Tables 1 and 2. All collected thermograms are available in the Supplementary Material.

Despite the alkenyl group chain length and the presence or absence of –OSi(Me2)– moiety in the structure, the most significant influence on the thermal stability of the measured samples had the type of seven inert groups present in the silsesquioxane cage corners. It was observed that the thermal stability of the compounds increased in a row: Et < iBu < iOc < Cy < Ph. It was abbreviated by the increase of the 5 and 10% mass loss temperatures as well as the temperatures of start and maximum degradation rate. Furthermore, the observed amount of residue at 995 °C increased in a similar way, with the exception of iOc derivatives. This dependency can be easily observed in the exemplary set of TG curves of hex-5-enyl-derived silsesquioxane series, presented in Fig. 2.
Fig. 2

TG curves of hex-5-enyl-derived silsesquioxanes bearing different inert organic groups

In all cases, the value of residue observed for iOc derivatives was smaller than those observed for corresponding iBu derivatives. This can be attributed to the higher molecular weight of iOc group in comparison with iBu moiety and their similar thermal stability arising from the similar (alkyl) structure. Obviously, the predominant influence of the type of inert functional groups on the thermal stability of considered compounds family is related to the stoichiometric ratio between inert and reactive groups of 7 to only 1.

The observed impact of the type of alkenyl group of different C–C chain length directly bonded to the Si–O–Si skeleton on the thermal stability of investigated derivatives was rather weak and more complex. There was no direct correlation between the T d 5% , T d 10% and Tonset temperatures and the alkenyl group length. However, it is worth noting that in all cases the residue observed at 995 °C was the highest for vinyl-derived silsesquioxanes and decreased with the increment of the number of carbon atoms in the alkenyl group. This can be clearly observed in the series of heptacyclohexyl silsesquioxane derivatives (see Fig. 3).
Fig. 3

TG curves of heptacyclohexyl silsesquioxanes bearing different alkenyl functional groups

It was also observed that the incorporation of dimethylsiloxy moiety into silsesquioxane structure significantly affect their thermal properties. The onset temperatures of main decomposition step observed for derivatives with alkenyl group bonded to the silsesquioxane core through the –SiO(Me2)– bridge were, in general, higher than those observed for corresponding derivatives without this bridge. Moreover, for ethyl, isobutyl and isooctyl derivatives the values of onset temperature significantly increased with the growing alkenyl chain length while for more thermally stable derivatives (cyclohexyl and phenyl ones) this trend was reversed and the decrease in onset temperatures was observed along with the increment of the C–C chain length of the reactive group.

DSC analysis of monoalkenyl-(3) and monoalkenylsiloxyPOSS (3-OSi) compounds

The thermogravimetric studies of a described group of compounds were followed by the DSC experiments for the determination of their melting and crystallization temperatures. The measurements were taken in a temperature range from − 80 °C to the temperature of the beginning of sample decomposition established on the basis of TG analysis. Unfortunately, due to the technical limitations and samples nature, it was not possible to measure mentioned parameters for all the samples. In many cases, either the melting temperatures (Tm) of the measured samples were above their decomposition temperatures, or they melted with decomposition (e.g., phenyl derivatives), which made it impossible to get insight into any correlation between their structure and thermal properties. Nevertheless, on the basis of the partial results presented in Table 3, it can be observed that the number of carbon atoms present in the alkenyl group chain influences the melting and crystallization temperatures (Tc) of monoalkenyl-derived silsesquioxanes despite the type of inert organic groups present in their structure and the presence or absence of –SiO(Me2)– bridge. The obtained results suggest that melting and crystallization temperatures decrease with the increment of alkenyl chain length. Unfortunately, the obtained data were not sufficient to determine the impact of the—SiO(Me2)—linkage on the thermal behavior of measured sampled.
Table 3

Melting and crystallization temperatures of selected monoalkenyl-derived silsesquioxanes

Prod. No

Tm/°C

Tc/°C

3-Et-Hex

110.87

86.40

3-Et-Dec

104.26

74.60

3-Et-OSi-All

88.99

53.73

3-iBu-Hex

180.51

117.67

3-iBu-Dec

113.82

76.49

3-iBu-OSi-All

191.98

186.67

3-iBu-OSi-Hex

117.29

98.71

3-iBu-OSi-Dec

83.56

51.26

3-Cy-Dec

259.19

244.80

3-Cy-OSi-Hex

290.87

281.74

3-Cy-OSi-Dec

271.12

228.13

Although we were not able to establish the melting and crystallization temperatures for isooctyl silsesquioxane derivatives, the performed experiment enabled us to determine the glass transition temperatures for this group of compounds. As shown in Table 4, Tg temperatures decrease with an increasing amount of carbon atoms in the alkenyl group from − 32 to almost − 45 °C for vinyl and decenyl derivative, respectively. In this case, it was also possible to observe the influence of the presence of –SiO(Me2)– bridge in the heptaisooctylsilsesquioxane series structure on their thermal behavior. In comparison with the “regular” silsesquioxanes, those bearing alkenyl group bonded to the Si–O–Si core through the –SiO(Me2)– moiety were characterized with lower glass transition temperatures of more than 3 °C.
Table 4

Glass transition temperatures of heptaisooctylalkenyl silsesquioxane derivatives

Prod. No

Tg/°C

3-iOct-Vi

− 31.88

3-iOct-All

− 34.07

3-iOct-Hex

− 43.01

3-iOct-Dec

− 46.70

3-iOct-OSi-Vi

− 37.57

3-iOct-OSi-All

− 41.24

3-iOct-OSi-Hex

− 44.05

3-iOct-OSi-Dec

− 51.64

Solubility tests of monoalkenyl-(3) and monoalkenylsiloxyPOSS (3-OSi) compounds with iBu and Ph inert groups

Finally, the THF, DCM and toluene solubility of selected derivatives with iBu and Ph inert groups was investigated. The resulted data are presented in Table 5.
Table 5

Solubility of 3-iBu- and 3-Ph- compounds with different alkenyl functional groups

Prod. No

Solubility/g cm−3

DCM

THF

Toluene

3-iBu-Vi

0.50

0.63

0.42

3-iBu-All

0.80

0.87

0.90

3-iBu-Hex

1.47

1.15

0.95

3-iBu-Dec

2.22

1.61

1.35

3-iBu-OSi-Vi

0.61

0.73

0.49

3-iBu-OSi-All

1.22

1.00

1.00

3-iBu-OSi-Hex

2.00

1.33

1.05

3-iBu-OSi-Dec

1.82

2.00

1.67

3-Ph-Vi

0.13

0.11

0.02a

3-Ph-All

0.36

0.31

0.04a

3-Ph-Hex

0.35

0.31

0.05

3-Ph-Dec

0.43

0.41

0.10

3-Ph-OSi-Vi

0.16

0.13

0.03a

3-Ph-OSi-All

0.42

0.37

0.05a

3-Ph-OSi-Hex

0.40

0.33

0.06

3-Ph-OSi-Dec

0.50

0.59

0.15

aAt 50 °C

Besides the fundamental difference in the solubility of isobutyl and phenyl silsesquioxane derivatives, a correlation between the alkenyl group chain length, the presence of the –SiO(Me2)–, and the compound’s solubility were all also observed. The solubility of silsesquioxanes increases despite of solvent type with a growing number of carbon atoms in the alkenyl chain. Also, the presence of the –SiO(Me2)– bridge increases the solubility of iBu and Ph derived silsesquioxanes by approximately 20 and 10%, respectively.

Conclusions

Herein, we have reported the design and synthesis of a library of monoalkenylsubstituted silsesquioxanes characterized by a rigid Si–O–Si structure and the presence of one alkenyl functional moiety with a different chain length, from vinyl to dec-9-enyl-, and five inert groups, i.e., Et, iBu, iOc, Cy, Ph. Efficient procedures for their synthesis were described, which are based on a condensation, “corner capping” type reaction, as well as a hydrosilylation process using respective chlorosilanes which are not all commercially available. As a result of our studies, we obtained 40 compounds, more than a half of which have not previously been described. They were thoroughly characterized by spectroscopic methods. The results of thermal analysis exhibited a strong structure–thermal properties relation. It was shown that even a subtle difference in the structure of only one of eight functional groups present in the POSS molecule can strongly affect its thermal stability, melting and crystallization behavior, and change the glass transition point. The same structural parameters impact the solubility of a described group of compounds. All of these physicochemical parameters, together with the modulated reactivity of functional groups and their accessibility (caused by different steric hindrance), can influence the properties of the final materials prepared with their use, as well as the conditions for their synthesis.

Notes

Acknowledgements

Financial support from the National Science Centre (Poland) Project OPUS No. DEC-2012/07/B/ST5/03042 and National Centre for Research and Development in Poland PBS3/A1/16/2015 is acknowledged.

Supplementary material

10973_2018_7121_MOESM1_ESM.doc (9.5 mb)
Supplementary material 1 (DOC 9754 kb)

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Authors and Affiliations

  1. 1.Faculty of ChemistryAdam Mickiewicz University in PoznanPoznanPoland
  2. 2.Centre for Advanced TechnologiesAdam Mickiewicz University in PoznanPoznanPoland
  3. 3.Faculty of ChemistryOpole UniversityOpolePoland

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