Functional Open-Cage Silsesquioxanes As Specific Polymer Modifiers

polymers

Article Preparation of Tri(alkenyl)functional Open-Cage Silsesquioxanes as Speciﬁc Polymer Modiﬁers

Katarzyna Mituła 1,2, Michał Dutkiewicz 2,3 , Julia Duszczak 1,2, Monika Rzonsowska 1,2 and Beata Dudziec 1,2,*

1 Faculty of Chemistry, Adam Mickiewicz University in Poznan, Uniwersytetu Pozna´nskiego 8, 61-614 Poznan, Poland; [email protected] (K.M.); [email protected] (J.D.); [email protected] (M.R.) 2 Centre for Advanced Technologies, Adam Mickiewicz University in Poznan, Uniwersytetu Pozna´nskiego10, 61-614 Poznan, Poland; [email protected] 3 Adam Mickiewicz University Foundation, Rubiez 46, 61-612 Poznan, Poland * Correspondence: [email protected]; Tel.: +48-6-1823-1878

Received: 3 April 2020; Accepted: 30 April 2020; Published: 6 May 2020

Abstract: The scientific reports on polyhedral oligomeric silsesquioxanes are mostly focused on the formation of completely condensed T8 cubic type structures and recently so-called double-decker derivatives. Herein, we report on efficient synthetic routes leading to trifunctionalized, open-cage silsesquioxanes with alkenyl groups of varying chain lengths from -vinyl to -dec-9-enyl and two types of inert groups (iBu, Ph) at the silsesquioxane core. The presented methodology was focused on hydrolytic condensation reaction and it enabled obtaining titled compounds with high yields and purity. A parallel synthetic methodology that was based on the hydrosilylation reaction was also studied. Additionally, a thorough characterization of the obtained compounds was performed, also in terms of their thermal stability, melting and crystallization temperatures (TGA and DSC) in order to show the changes in the abovementioned parameters dependent on the type of reactive as well as inert groups at Si-O-Si core. The presence of unsaturated alkenyl groups has a profound impact on the application potential of these systems, i.e., as modifiers or comonomers for copolymerization reaction.

Keywords: open-cage silsesquioxanes; hydrolytic condensation; thermal analysis

1. Introduction Organosilicon compounds are of great interest and they have been investigated during the past few decades, which is reflected in the growing number of papers and patents published each year. These systems offer development of methodologies for the formation of advanced, multifunctional materials due to the combination of organic and inorganic segments. One of the fundamental examples of organosilicon derivatives of magnitude importance is silsesquioxane (SQ). They are a worldwide known family of nanosized hybrid compounds of the general [RSiO3/2]n formula, being represented by a versatile group of three-dimensional species where distinct structures may be formed, i.e., random, ladder, cage and incompletely condensed cage [1,2]. The most popular members of this family are cage silsesquioxanes, especially of cubic T8 type that may contain from one to eight functional/reactive groups (FG). The inorganic core is based on the Si-O-Si framework possessing a silicon atom at each vertex, being responsible for, i.e., the thermal stability of these systems. Reactive or inert moieties tetrahedrally coordinated at the silicon vertices affect SQs physical and chemical properties and result in their vast potential application. The studies on silsesquioxane based materials influenced almost every branch of science (i.e., medicine and biochemistry, material chemistry, optoelectronics, etc.), due to their structural diversity and interesting properties, i.e., thermal stability, oxidation resistance,

Polymers 2020, 12, 1063; doi:10.3390/polym12051063 www.mdpi.com/journal/polymers Polymers 2020, 12, 1063 2 of 15 dielectric coefficient, low toxicity, and possibility for their not complicated structural and functional modification [2,3]. As a consequence, these systems are promising candidates for building blocks for hybrid materials. Most studies concern the application of completely condensed silsesquioxanes, either mono- or octasubstituted T8 or, recently, the double-decker types [4–7]. On the other hand, open-cage silsesquioxanes, i.e., not-completely condensed systems have recently attracted much attention. The silsesquioxane trisilanol precursors enable the introduction of three reactive substituents to the Si-O-Si core by means of stoichiometric (hydrolytic condensation), but also catalytic (e.g., hydrosilylation, arylation, O-silylation), processes using functional groups that are already attached to the SQ core [8–16]. The symmetry of their trifunctional T7 open-cage derivatives is changed due to the lack of one vertex in the structure, which affects their physical properties (i.e., changes in molecular packing, melting points) [9,17–20]. However, they offer interesting synthetic possibilities due to the opportunity to introduce more than one functional group within the silsesquioxanes core bearing at the same time also inert ones. As a result, the perspectives for the further development of this group of compounds are vast, from nanostructured coating films [13], silica-grafted catalysts [21,22], tripodal ligands for transition metals [23], novel cage host molecules [24], aggregates in cosmetics and food manufacturing [25,26], or dentistry [27,28] to polymers modifiers as cross-linking agents [29–34]. Multi-functional SQs may additionally offer the possibility to obtain three-dimensional (3D) copolymeric networks, leading to substantial improvement in their physicochemical properties, i.e., increased thermal and oxidation resistance, mechanical properties, e.g., hardness, as well as the reduction of flammability, heat evolution, viscosity during processing, etc. [35–47]. This study was undertaken to design and elaborate efficient synthetic methodology for tri(alkenyl)functional silsesquioxane derivatives. Simple condensation reactions of chlorosilanes with silsesquioxane trisilanols allowed for us to obtain diverse types of incompletely condensed POSS compounds with three functional groups varying from -vinyl to -dec-9-enyl. The resulting tri(alkenyl)functional silsesquioxanes were obtained with high yields and were thoroughly analyzed spectroscopically. Their thermal stability, melting, and crystallization temperatures were evaluated at details (TGA and DSC) to tend to changes in thermal properties in relation with the reactive (FG) as well as inert (R) groups at Si-O-Si core. In parallel, we also studied a complementary synthetic protocol, while using hydrosilylation reaction of dienes with respective SQs bearing three Si-H reactive moieties. Unfortunately, the use of this method for the synthesis of a discussed group of unsaturated SQ derivatives was found to be unselective and it led to a mixture of products. As aforementioned, tri(alkenyl)functional silsesquioxane derivatives may be valuable multifunctional Si-O-Si based structural motifs for further modifications, e.g., as cross-linking agents for nanocomposite materials.

2. Materials and Methods

2.1. Materials The chemicals were purchased from the following sources: chlorodimethylsilane (Sigma-Aldrich, Saint Louis, MO, USA, CAS: 1066-35-9, assay 98%), chloro(dimethyl)vinylsilane (Sigma-Aldrich, Saint Louis, MO, USA CAS: 1719-58-0, assay 97%), allyl(chloro)dimethylsilane (Sigma-Aldrich, Saint Louis, MO, USA CAS: 4028-23-3, assay 97%), 1,5-hexadiene (CAS: 592-42-7, assay 97%), Karstedt’s catalyst (platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex [Pt2(dvs)3] solution in xylene with 2% of Pt, Sigma-Aldrich, Saint Louis, MO, USA CAS: 68478-92-2), triethylamine (CAS: 121-44-8, assay 99.5%), calcium hydride (CAS: 7789-78-8, assay 98%), sodium (Sigma-Aldrich, Saint Louis, MO, ≥ USA CAS: 7440-23-5, cubes, contains mineral oil, assay 99.9%), benzophenone (Acros Organics, Geel – Belgium CAS: 119-61-9, assay 99%), 1,9-decadiene (TCI, Portland, OR, USA, CAS: 1647-16-1, assay 98%), R7(Si7O9)(OH)3 (Hybrid Plastics, Hattiesburg, MS, USA, SQ-R-OH; R = iBu CAS: 307531-92-6, Ph CAS: 444315-26-8). Toluene, tetrahydrofuran (THF), methanol, n-hexane, dichloromethane (DCM), chloroform-d, molecular sieves type 4Å from P.O.Ch Gliwice, Poland. Silica gel-MN-Kieselgel 60 from Fluka Chemie AG, Buchs, Switzerland. Chloro(hex-5-enyl)silane and chloro(dec-9-enyl)silane were Polymers 2020, 12, 1063 3 of 15 obtained via the procedure described earlier [48]. SQ-iBu-OH was prepared prior to use, i.e., dissolved in DCM, precipitated in MeOH, and then dried under vacuum. All of the syntheses were conducted under argon atmosphere while using standard Schlenk-line and vacuum techniques. Toluene and n-hexane were dried over CaH2 and THF over Na with benzophenone before use and stored under argon over type 4Å molecular sieves. Amine was dried over CaH2 before use and then stored under argon in Schlenk.

2.2. Measurements Nuclear Magnetic Resonance (NMR) measurements were conducted using: for 1H NMR-Bruker 29 13 Ultrashield 300 MHz, for Si and C NMR—Brucker Ultrashield 400 MHz, using CDCl3 as a solvent. 1 Chemical shifts are reported in ppm with reference to the residual solvent (CHCl3) peaks for H and 13C and to TMS for 29Si. Fourier Transform-Infrared (FT-IR) spectra were recorded on a Nicolet iS5 (Thermo Scientific, Waltham, MA, USA) spectrophotometer that was equipped with a diamond ATR unit (iD7 ATR Optical 1 Base). In all cases, 16 scans at a resolution of 2 cm− were collected, in order to record the spectra in a range of 4000 400 cm 1. − − Gas Chromatography (GC) analysis was performed on Bruker 430 gas chromatograph that was equipped with a TCD detector and capillary column megabore HP-130 m (Hewlett Packard, Palo Alto, CA, USA). Thermogravimetric analysis (TGA) was performed on a TGA Q50 (TA Instruments, New Castle, DE, USA) apparatus. All experiments were carried out in air atmosphere (flow rate 60 mL/min.) in a temperature range from RT to 1000 ◦C and at the heating rate of 10 ◦C/min. Differential Scanning Calorimetry (DSC) analysis was carried out with the use of a DSC1 calorimeter (Mettler-Toledo). The samples of ca. 10 mg were heated in sealed aluminum crucibles in the temperature 1 1 range of 0 to 200 ◦C with a heating rate of 10 ◦C min.− in nitrogen (25 mL min.− ). The glass transition temperature (Tg) was designated as the inflection point in the curves that were obtained from the second run. Matrix-Assisted ultraviolet Laser Desorption/Ionization Time-Of-Flight Mass Spectroscopy (MALDI-TOF-MS) were recorded on an 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 matrix and it was prepared in TA30 solvent (30:70 v/v acetonitrile: 0.1% TFA in water) at a concentration of 20 mg/mL. The studied samples were dissolved in dichloromethane (2 mg/mL) and then mixed in a ratio 1:1 v/v with a matrix solution. Matrix/sample mixtures (1 µL) were spotted onto the MALDI target and then dried in air. Mass spectra were measured in reflection mode. The data were analyzed while using the software provided with the Ultraflex instrument—FlexAnalysis (version 3.4). Mass calibration (cubic calibration based on five to seven points) was performed using external standards (Peptide Calibration Standard).

2.3. Synthetic Procedures for Preparation of SQ-R-SiH R = iBu, Ph The exemplary procedure is presented for the SQ-Ph-SiH compound and it is analogous for SQ-iBu-SiH. Into a two-neck round-bottom ﬂask that was equipped with a magnetic stirrer, purged with argon and then placed in ice-water bath SQ-Ph-OH (1.03 g, 1.101 mmol), anhydrous THF (10 mL) and chloro(dimethyl)silane (0.39 mL, 3.4 mmol) were introduced and followed by dropping of Et3N (0.48 mL, 3.47 mmol) to the obtained solution. The reaction was carried out for 24 h at room temperature. Subsequently, insoluble solid of triethylammonium chloride was removed by ﬁltration on a glass frit, and then volatiles and THF were eliminated via rotary evaporation. The crude product was dissolved in DCM and crystallized in cold MeOH. After methanol removal, the solid was dried in a vacuum. For the spectroscopic analysis please see Supplementary Materials (Section 1.1). Polymers 2020, 12, 1063 4 of 15

2.4. General Procedure for Preparation of tri(alkenyl)functional SQs (SQ-R-FG; R = iBu, Ph; FG = Vi, All, Hex, Dec)

2.4.1. Procedure for Preparation of SQ-R-FG (R = iBu, Ph; FG = Vi, All, Hex, Dec) Compounds via a Hydrolytic Condensation Reaction The exemplary procedure is presented for SQ-iBu-Vi. Trisilanol SQ-iBu-OH (0.31 g, 0.39 mmol), anhydrous THF (4 mL), and chloro(dimethyl)vinylsilane (0.17 mL, 1.21 mmol) were introduced into a two-neck round-bottom ﬂask that was equipped with a magnetic stirrer and placed in an ice-water bath (the respective chloro(dimethyl)(hex-3-enyl)silane and chloro(dimethyl)(dec-5-enyl)silane were prepared according to the literature procedure [48]). This was followed by adding Et3N (0.17 mL, 1.23 mmol) to the reaction mixture. The reaction was carried out for 24 h at room temperature. Afterwards, insoluble solid of triethylammonium chloride was removed by ﬁltration on a glass frit, and then volatiles and THF were eliminated via rotary evaporation. The crude product was washed several times with cold MeOH. After methanol removal, the solid was dried in a vacuum. In the case of SQ-iBu-Hex and SQ-iBu-Dec, 0.25 g (0.317 mmol) of SQ-iBu-OH and 0.4 mL anhydrous THF was used.

2.4.2. Procedure for the Hydrosilylation of Dienes by SQ-R-SiH (R = iBu, Ph; diene = 1,5-hexadiene, 1,9-decadiene) The exemplary procedure is presented for SQ-iBu-Dec and it is analogous for SQ-R-FG (R = Ph; FG = Hex, Dec). SQ-iBu-SiH (0.16 g, 0.166 mmol), anhydrous toluene (4.15 mL), and 1,9-decadiene (0.11 mL, 0.60 mmol) were added into a two-neck round-bottom ﬂask equipped with a magnetic stirrer 8 and a reﬂux condenser. This was followed by adding Karstedt’s catalyst (0.57 µL, 4.99 x10− mol) to the reaction mixture. The reaction was heated to 95 ◦C and then carried out to the complete Si-H bond consumption controlled with the FT-IR technique based on changes in the surface areas of bands at 1 1 v = 895 cm− and v = 2130 cm− characteristic for stretching vibrations of Si-H bond (usually 20–24 h).

3. Results and Discussion This work aimed to examine the possibility of synthesizing a series of tri(alkenyl)substituted open-cage SQ derivatives varying in the length of the alkenyl chain and also in inert substituents (iBu and Ph) at silsesquioxane core. The type of inert and functional groups affects the synthetic procedure and also the physicochemical properties of these systems. As a result of this goal, a library of eight compounds was set and their thermal properties were also analyzed. It was meant to classify them as potential polymer modifiers and be chosen as those in terms of the polymer matrix thermal stability. A starting point was the design of the synthetic route for our aim. The hydrolytic condensation of trisilanol SQ’s with respective chlorosilane seemed to be the most probable way to achieve our aim. The first step of our work was to prepare substrates that are not commercially available, i.e., alkenylchlorosilanes with longer alkenyl chains, i.e., -hex-5-enyl and -dec-9-enyl. These compounds were efficiently obtained via hydrosilylation of a specific diene with chlorodimethylsilane while using Karstedt’s catalyst that we described earlier [48]. The next stage was the hydrolytic condensation reactions of heptaisobutyl- and heptaphenyltrisilanol (SQ-R-OH,R = iBu, Ph) with respective alkenylchloro(dimethyl)silane, as summarized in Figure1. All of these condensation reactions (Figure1) where conducted using a concentrated THF solution of reagents. In the case of alkenyl(chloro)silane with longer alkenyl chain and heptaisobutyltrisilanol SQ, preliminary tests revealed that a reduction of the solvent amount to a minimum (just to dissolve the SQ-iBu-OH, estimated at 0.63 M) was crucial for obtaining fully substituted SQ-iBu-FG products. It was especially important for the SQ-iBu-Hex derivative (see Supplementary Materials, Figure S1b, in 29Si NMR spectrum the absence of 58 ppm resonance line derived from not reacted Si-OH moiety − might be noted). The presence of not reacted Si-OH group in the SQ framework of SQ-iBu-Hex results in the difference in the chemical surrounding of other Si atoms and the appearance of an additional Polymers 2020, 12, 1063 5 of 15 signal at 9.94 ppm and at ca. 68 ppm for T type Si atoms (see Supplementary Materials, Figure S1b). − It is in accordance with the data of compounds described by Naka et al. [11]. Additionally, the important aspect of the synthetic procedure to ensure effective product formation was the proper cooling of the reaction mixture in an ice bath and the sequence of chlorosilane and amine addition. The reverse order of first chloro(alkenyl)silane and then dropping of amine, known in hydrolytic condensations methodology, allowed for obtaining of SQ-R-FG in the most efficient mean. Subsequently, the reaction was warmed up to RT and left for 24 h being stirred. The crude product was isolated from the reaction mixture by simple filtration from ammonium chloride and purified by precipitation in cold methanol. The basicity of amine was also verified and NEt3 was found to be more effective in comparison with (iPr)2NEt, which led to the inferior yield of SQ-R-FG. The stoichiometry of reagents was also tested and it was dependent on the length of the alkenyl group in the chlorosilane. The longer alkenyl chain was, the higher excess of reagents per 1 mol of SQ-R-OH were exploited (from 3.09, 3.15 equivalent for FG = Vi, 3.3, 3.9 for FG = All to 3.6, 4.5 equivalent for FG = Hex, Dec in chlorosilane and amine, respectively). All of the compounds obtained via the condensation reaction were isolated with high yields (84–96%). They are air-stable solids, waxy solids, or colorless oils, and can be synthesized on a multigram scale. They are soluble in organic solvents like DCM, CHCl3, THF, and toluene but not in, e.g., methanol, MeCN, and hexane (only for R = Ph). They were isolated and characterized by employing spectroscopic methods (1H, 13C, and 29Si NMR, as well as FT-IR, see Supplementary Materials). Figure1 summarizes the information on their isolation yields. Polymers 2019, 11, x FOR PEER REVIEW 5 of 16

Figure 1. Synthesis of of tri(alkenyl)functional tri(alkenyl)functional silsesquioxanes silsesquioxanesSQ-R-FG SQ-R-FGvia a hydrolyticvia a hydrolytic condensation condensation reaction. reaction. In the case of -hex-3-enyl (FG =Hex) and -dec-5-enyl (FG = Dec) functionalities, a parallel synthetic methodologyAll of these that condensation was based on reactions the hydrosilylation (Figure 1Error! reaction Reference was also source veriﬁed. not For found. this purpose,) where weconducted prepared using silsesquioxane a concentrated derivatives THF solution with threeof reag Si-Hents. reactive In the case groups of alkenyl(chloro)silane (SQ-R-SiH,R = iBu, with Ph), longervia hydrolytic alkenyl condensationchain and heptaisobutyltrisilanol of trisilanol SQ-iBu-OH SQ, preliminary(R = iBu, Ph) tests form revealed with ClSiMe that a2 reductionH basing onof the solvent procedure amount known to froma minimum the literature (just to [dissolve13] (see paragraphthe SQ-iBu-OH 2.3). The, estimated resulting at SQ-R-SiH0.63 M) was(R crucial= iBu, forPh) obtaining were reagents fully forsubstituted tests of hydrosilylation SQ-iBu-FG products. reaction It with was 1,5-hexadieneespecially important and 1,9-decadiene for the SQ-iBu-Hex that was derivativeproceededPolymers 2019 (see, while 11, x Supplementary FOR using PEER Karstedt’s REVIEW Materials, catalyst Figure (Figure S1b,2). in 29Si NMR spectrum the absence of −586 ppmof 16 resonance line derived from not reacted Si-OH moiety might be noted). The presence of not reacted

Si-OH group in the SQ framework ofSi SQ-iBu-HexH results in the differencem in the chemical OH Si surroundingR ofOH other Si atoms and the appearanceO Si H of an additional signal at 9.94 ppmm and at ca. −68 Si R R O Si OH R O Si O O Si m R O ppm for T type Si atoms (see SupplementaryO O Si Materials,O Si H Figure S1b). ItSi is Rin accordance with the data Si O R O Si R Si O Si m R ClMe2Si H Si R [Pt (dvds) ] O O of compounds OdescribedO by Naka SietO al.R [11].O Additionally,2 the3 important aspect of the synthetic Si R Si O R O Si R O O Si Si O O Et N, THF R O procedure to ensureO effective3 productO OformationSi was toluene,the proper 65/95oC cooling Siof theO reaction mixture in an Si R O O Si Si O R ice bath andR the sequence of chlorosilane and amine addition. SiTheO reverseR order of first R chloro(alkenyl)silaneSQ-R-OH and then droppingSQ-R-SiH of amine, known in hydrolyticR condensationsSQ-R-FG-HS* methodology, allowed for obtaining of SQ-R-FG in the most efficient mean. Subsequently,R the = iBu, reaction Ph was warmed up toFigure RT and 2. SynthesisSynthesisleft for 24 of of h tri(alkenyl)functional tri(alkenyl)functionalbeing stirred. The crudesilsesquioxa silsesquioxanes productnes via was a sequenceisolated sequence offrom of hydrolytic hydrolytic the reaction condensation condensation mixture by simplefollowed filtration by a from hydrosilylation ammonium reaction reaction chloride of of dienes and pu by byrified SQ-R-SiH by precipitation..* *SQ-R-FG-HS in coldaccompanied methanol. by The basicitycompounds of amine of of wasalkenyl alkenyl also chains chains verified with with and the the isomerizedNEt isomerized3 was foundC=C C= Cgroup. group.to be For moreFor respective respectiveeffective structure, structure,in comparison please please see with (iPr)2SupplementaryseeNEt, Supplementary which led Materi to Materials, theals, inferior Section Section yield3a. 3a. of SQ-R-FG. The stoichiometry of reagents was also tested and it was dependent on the length of the alkenyl group in the chlorosilane. The longer alkenyl chain was, Inthe the higher experiments, excess of the reagents hydrosilylation per 1 mol reactionof SQ-R-OH resulted were in exploited>99% conversion (from 3.09, of Si-H 3.15 bonds.equivalent The forloading FG = of Vi, [Pt 3.3,2(dvds) 3.9 for3] was FG strictly= All to controll 3.6, 4.5ed equivalent and estimated for FG at = 10 Hex,−4 mol Dec of in per chlorosilane one Si-H group, and amine, along respectively).with the excess All of ofdiene, the compoundsusually 3.6 equivalent obtained via The th reactione condensation was conducted reaction at were 65 °C isolated in the case with of high 1,5- yieldshexadiene (84–96%). and 95They °C arefor air-stable 1,9-decadiene solids, until waxy the solids, total or conversion colorless oils, of andSi-H can moiety. be synthesized The reaction on a −1 multigramcompletion scale. was monitored They are soluble using FT-IRin organic analysis solvents (changes like DCM,in the areaCHCl of3, theTHF, bands and attoluene ῡ = 895 but cm not and in, e.g.,ῡ = 2130 methanol, cm−1, ascribed MeCN, to and the hexanestretching (only vibrations for R =of Ph). Si–H They bond) were and isolatedwas usually and performed characterized for 20–by employing24 h (exemplary spectroscopic spectra for methods the SQ-Ph (1H, derivatives 13C, and in29Si Figure NMR, 3). as well as FT-IR, see Supplementary Materials). Figure 1 summarizes the information on their isolation yields. In the case of -hex-3-enyl (FG =Hex) and -dec-5-enyl (FG = Dec) functionalities, a parallel synthetic methodology that was based on the hydrosilylation reaction was also verified. For this purpose, we prepared silsesquioxane derivatives with three Si-H reactive groups (SQ-R-SiH, R = iBu, Ph), via hydrolytic condensation of trisilanol SQ-iBu-OH (R = iBu, Ph) form with ClSiMe2H basing on the procedure known from the literature [13] (see paragraph 2.3). The resulting SQ-R-SiH (R = iBu, Ph) were reagents for tests of hydrosilylation reaction with 1,5-hexadiene and 1,9-decadiene that was proceeded while using Karstedt’s catalyst (Figure 2).

Figure 3. Fourier Transform-Infrared (FT-IR) spectra of SQ-Ph-SiH, SQ-Ph-Hex-HS, and SQ-Ph-Dec- HS after completion of hydrosilylation reaction.

The stacked spectra that are presented in Figure 3 exhibit band characteristics for SQ-Ph-SiH (marked on the spectra a). Additionally, there are new bands on the spectra, characteristics of -hex- 3-enyl and -dec-5-enyl chains attached to the SQ core (SQ-Ph-Hex-HS and SQ-Ph-Dec-HS). These new bands may be attributed to stretching vibrations of C-H methylene groups in the aliphatic chain at ca. 2855 cm−1 and 2920 cm−1, C=C at 1640 cm−1 (weak intensity), and also arise of new bands resulting from rocking vibrations of C-H methylene groups at ca. 780 cm−1 and 840 cm−1. Additionally, one might note two bands denoted as A and B in Figure 3. Band A, at 920 cm−1, might be attributed to

Polymers 2019, 11, x FOR PEER REVIEW 6 of 16

Si H m OH Si R OH O Si H m Si R R O Si OH R O Si O O Si m R O O O Si O Si H Si R Si O R O Si R Si O Si m R ClMe2Si H Si R [Pt (dvds) ] O O O O Si O R O 2 3 Si R Si O R O Si R O O Si Si O O Et N, THF R O O 3 O O Si toluene, 65/95oC Si O Si R O O Si Si O R R Si O R R SQ-R-OH SQ-R-SiH R SQ-R-FG-HS* R = iBu, Ph Figure 2. Synthesis of tri(alkenyl)functional silsesquioxanes via a sequence of hydrolytic condensation followed by a hydrosilylation reaction of dienes by SQ-R-SiH. *SQ-R-FG-HS accompanied by compounds of alkenyl chains with the isomerized C=C group. For respective structure, please see Polymers 2020, 12, 1063 6 of 15 Supplementary Materials, Section 3a.

InIn thethe experiments, the the hydrosilylation hydrosilylation reaction reaction resulted resulted in in>99%>99% conversion conversion of Si-H of Si-H bonds. bonds. The Theloading loading of [Pt of2(dvds) [Pt (dvds)3] was] strictly was strictly controll controlleded and estimated and estimated at 10− at4 mol 10 4 of mol per of one per Si-H one group, Si-H group, along 2 3 − alongwith the with excess the excess of diene, of diene, usually usually 3.6 equivalent 3.6 equivalent The reaction The reaction was conducted was conducted at 65 °C at in 65 the◦C incase the of case 1,5- ofhexadiene 1,5-hexadiene and 95 and °C 95 for◦C for1,9-decadiene 1,9-decadiene until until the the total total conversion conversion of of Si-H Si-H moiety. moiety. The reactionreaction −1 1 completioncompletion waswas monitored using FT-IR FT-IR analysis analysis (changes (changes in in the the area area of of the the bands bands at atῡ =v 895= 895 cm cm and− −1 1 andῡ = 2130v = 2130 cm cm, ascribed− , ascribed to the to stretching the stretching vibrations vibrations of Si–H of Si–H bond) bond) and andwas wasusually usually performed performed for 20– for 20–2424 h (exemplary h (exemplary spectra spectra for forthe the SQ-Ph SQ-Ph derivatives derivatives in Figure in Figure 3).3 ).

FigureFigure 3.3.Fourier Fourier Transform-Infrared Transform-Infrared(FT-IR) (FT-IR) spectra spectra of SQ-Ph-SiHof SQ-Ph-SiH, SQ-Ph-Hex-HS,, SQ-Ph-Hex-HS,and andSQ-Ph-Dec-HS SQ-Ph-Dec- afterHS after completion completion of hydrosilylation of hydrosilylation reaction. reaction.

TheThe stackedstacked spectraspectra thatthat areare presentedpresented inin FigureFigure3 3exhibit exhibit band band characteristics characteristics for for SQ-Ph-SiHSQ-Ph-SiH (marked(marked onon the spectra spectra a). a). Additionally, Additionally, there there are are new new bands bands on the on spectra, the spectra, characteristics characteristics of -hex- of -hex-3-enyl3-enyl and and-dec-5-enyl -dec-5-enyl chains chains attached attached to tothe the SQ SQ core core ( (SQ-Ph-Hex-HSSQ-Ph-Hex-HS andand SQ-Ph-Dec-HS). TheseThese newnew bandsbands maymay bebe attributedattributed toto stretchingstretching vibrationsvibrations ofof C-HC-H methylenemethylene groupsgroups inin thethe aliphaticaliphatic chainchain 1 1 1 atat ca.ca. 2855 2855 cm cm−1− andand 2920 2920 cm cm−1, C=C− ,C at= C1640 at 1640cm−1 (weak cm− (weakintensity), intensity), and also and arise also of new arise bands of new resulting bands 1 1 resultingfrom rocking from rockingvibrations vibrations of C-H ofmethylene C-H methylene groups groups at ca. at780 ca. cm 780−1 cmand− 840and cm 840−1 cm. Additionally,− . Additionally, one 1 onemight might note note two two bands bands denoted denoted as asA Aandand BB inin Figure Figure 3.3. Band Band A, atat 920920 cmcm−1, ,might might be attributed toto 1 out-of-plane bending vibrations of the terminal = C-H moiety, and B at ca. 980 cm− is overlapped and probably resulting from out-of-plane bending vibrations of new (trans) = C-H group from diene isomerization that we also observed in the NMR spectra (see Figures 5 and 6). As it was abovementioned, an isomerization of C=C in dienes and respective products of their hydrosilylation reaction was observed. It should be highlighted that the hydrosilylation process might not be selective and side reactions may occur, e.g., the isomerization of olefin, α- and/or β-addition products or dehydrogenative silylation products, etc. [49]. For the hydrosilylation of 1,5-hexadiene and 1,9-decadiene, despite >99% consumption of SQ-R-SiH reagent the crude products were accompanied by traces (for 1,9-decadiene) or notable, up to 30% amount (for 1,5-hexadiene) of olefin isomerization products (for possible structures, see Supplementary Materials, Section 3a). It was previously observed for the hydrosilylation of 1, 5-hexadiene by chlorosilane by Saiki et al. [50] and might be evaluated not only by FT-IR (Figure3), but also by 1H, 13C, and 29Si NMR analysis. For SQ-Ph-Dec-HS, the evaluation Polymers 2019, 11, x FOR PEER REVIEW 7 of 16 out-of-plane bending vibrations of the terminal = C-H moiety, and B at ca. 980 cm−1 is overlapped and probably resulting from out-of-plane bending vibrations of new (trans) = C-H group from diene isomerization that we also observed in the NMR spectra (see Figures 5 and 6). As it was abovementioned, an isomerization of C=C in dienes and respective products of their hydrosilylation reaction was observed. It should be highlighted that the hydrosilylation process might not be selective and side reactions may occur, e.g., the isomerization of olefin, α- and/or β- addition products or dehydrogenative silylation products, etc. [49]. For the hydrosilylation of 1,5- hexadiene and 1,9-decadiene, despite >99% consumption of SQ-R-SiH reagent the crude products were accompanied by traces (for 1,9-decadiene) or notable, up to 30% amount (for 1,5-hexadiene) of olefin isomerization products (for possible structures, see Supplementary Materials, Section 3a). It was previously observed for the hydrosilylation of 1, 5-hexadiene by chlorosilane by Saiki et al. [50] Polymers 2020, 12, 1063 7 of 15 and might be evaluated not only by FT-IR (Figure 3), but also by 1H, 13C, and 29Si NMR analysis. For SQ-Ph-Dec-HS, the evaluation of NMR data resulted in the identification of a by-product of double bondof NMR diene data isomerization resulted in thethat identification was reflected ofin athe by-product additional of resonance double bond line at diene 11.94 isomerization ppm in 29Si NMR that spectrumwas reflected attributed in the additional to Si-Dec resonancemoiety (Figure line at 4). 11.94 What ppm should in 29 Sibe NMR underlined, spectrum this attributed product towas Si-Dec not obtainedmoiety (Figure when4 SQ-Ph-Dec). What should wasbe obtained underlined, via hydrolytic this product condensation was not obtained and the when same SQ-Ph-Decwas in the casewas ofobtained SQ-Ph-Hex via hydrolytic. condensation and the same was in the case of SQ-Ph-Hex.

FigureFigure 4. 29SiSi NMR NMR spectra spectra of a) (a )SQ-Ph-Dec-HSSQ-Ph-Dec-HS obtainedobtained in a hydrosilylation and (b)b) SQ-Ph-DecSQ-Ph-Dec obtainedobtained via a condensation reaction.

AnotherAnother aspect aspect is is that hydrosilylation of of dienes dienes should should be be also conducted in in a proper dilution ofof SQ-R-SiH inin toluene toluene ( (ca. 0.040.04 M) M) to to reduce reduce possible possible hydrosilylation hydrosilylation of of one one diene diene molecule molecule by two separateseparate particles particles of of SQ-R-SiHSQ-R-SiH.. It It was observed, especially especially in in th thee case of 1,5-hexadiene (probable structurestructure presented in Supplementary Materials, Se Sectionction 3b). The The formation formation of of specific specific aggregates containingcontaining one oror twotwo SQsSQs moieties moieties may may occur occur in in a reactiona reaction performed performed in toluenein toluene athigher at higherSQ-R-SiH SQ-R- SiHconcentration, concentration, i.e., i.e., 0.22–0.25 0.22–0.25 M. M. In In the the case case of ofSQ-R-Hex-HS SQ-R-Hex-HSderivatives, derivatives, when when thethe reaction was conductedconducted at at the more concentrated system, despite >99,9%>99,9% conversion conversion of of Si-H Si-H bonds, bonds, the the crude product contained a notable amount of the by-product of 1,5-hexadiene isomerization, but but also the aggregatesaggregates ifif two two or or more more molecules molecules of SQ-R of openSQ-R cages. openNMR cages. spectra NMR may spectra confirm may these confirm assumptions these assumptions(Figures5 and (Figures6). 5 and 6). In the 1H NMR spectrum, there is a multiplication of signals at ca. 0.11ppm derived from methyl groups at Si (-O-SiMe2-Hex) due to the presence of saturated aliphatic hexane chains linking different SQs open cages, and, what is more important, the ratio of methyl protons (-O-SiMe2-Hex) to vinyl protons (-HC = CH2) in one molecule of SQ-iBu-Hex should be fixed at 2:1. In this case, the value ratio is underestimated, i.e., 8.8:1, which might indirectly suggest the decrease of vinyl moieties in the system, i.e., their consumption in the intermolecular hydrosilylation of 1,5-hexadiene by two distinct SQ-iBu-SiH molecules. This assumption might be confirmed by the GPC analysis (see. Supplementary Materials, Figures S3 and S4). As for the 29Si NMR spectrum of crude SQ-iBu-Hex-HS, there are also visible changes (Figure6). Due to the presence of saturated aliphatic hexane chains linking di fferent SQs open cages, there is a multiplication of signals at ca. 9.02 ppm derived from functional -O-Si-Hex, i.e., M-type Si atoms and there are also additional resonance lines originated from SQs core at ca. 68 ppm, i.e., t-type Si atoms. − The hydrolytic condensation and hydrosilylation reaction should not be considered as alternative procedures for obtaining tri(hex-3-enyl)- and tri(dec-5-enyl)- functional SQ’s compounds. Both of the methods require additional reaction step, i.e., for condensation, the synthesis of alkenylchlorosilanes Polymers 2020, 12, 1063 8 of 15 with longer chains and hydrosilylation, and the synthesis of SQ-R-SiH (R = iBu, Ph). However, the formation of undesired by-products might not be circumvented in the case of hydrosilylation reactionPolymers 2019 and,, 11 as, x a FOR result, PEER the REVIEW application of hydrolytic condensation should be the preferred methodology8 of 16 for the efficient synthesis of final products. Polymers 2019, 11, x FOR PEER REVIEW 8 of 16

Figure 5. 1H NMR spectrum of crude SQ-iBu-Hex-HS obtained via hydrosilylation of 1,5-hexadiene Figure 5. 1H NMR spectrum of crude SQ-iBu-Hex-HS obtained via hydrosilylation of 1,5-hexadiene Figure 5. 1H NMR spectrum of crude SQ-iBu-Hex-HS obtained via hydrosilylation of 1,5-hexadiene byby SQ-iBu-SiH SQ-iBu-SiH performed performed at at higher higher concentrations, concentrations, i.e., 0.22–0.250.22–0.25 M. M. by SQ-iBu-SiH performed at higher concentrations, i.e., 0.22–0.25 M.

Figure 6. 29Si NMR spectra of crude SQ-iBu-Hex-HS obtained via hydrosilylation of 1,5-hexadiene by 29 FigureSQ-iBu-SiH 6. 29SiSi NMR NMR performed spectra spectra at of ofhigher crude crude concentration, SQ-iBu-Hex-HSSQ-iBu-Hex-HS i.e., 0.22–0.25obtainedobtained M.via via hydrosilylation hydrosilylation of 1,5-hexadiene by SQ-iBu-SiH performed at higher concentration, i.e.,i.e., 0.22–0.250.22–0.25 M.M. In the 1H NMR spectrum, there is a multiplication of signals at ca. 0.11ppm derived from methyl groupsIn the at 1 HSi (-O-SiMeNMR spectrum,2-Hex) due there to the is a presence multiplication of saturated of signals aliphatic at ca. hexane 0.11ppm chains derived linking from different methyl groupsSQs open at Si (-O-SiMecages, and,2-Hex) what due is more to the import presenceant, theof saturated ratio of methyl aliphatic protons hexane (-O-SiMe chains2 linking-Hex) to different vinyl SQsprotons open cages,(-HC = and,CH2 )what in one is moleculemore import of SQ-iBu-Hexant, the ratio should of methyl be fixed protons at 2:1. (-O-SiMeIn this case,2-Hex) the tovalue vinyl protonsratio is (-HC underestimated, = CH2) in one i.e., molecule 8.8:1, which of SQ-iBu-Hexmight indirectly should suggest be fixed the decrease at 2:1. In of thisvinyl case, moieties the value in ratio is underestimated, i.e., 8.8:1, which might indirectly suggest the decrease of vinyl moieties in

Polymers 2019, 11, x FOR PEER REVIEW 9 of 16

the system, i.e., their consumption in the intermolecular hydrosilylation of 1,5-hexadiene by two distinct SQ-iBu-SiH molecules. This assumption might be confirmed by the GPC analysis (see. Supplementary Materials, Figures S3 and S4). As for the 29Si NMR spectrum of crude SQ-iBu-Hex- HS, there are also visible changes (Figure 6). Due to the presence of saturated aliphatic hexane chains linking different SQs open cages, there is a multiplication of signals at ca. 9.02 ppm derived from functional -O-Si-Hex, i.e., M-type Si atoms and there are also additional resonance lines originated from SQs core at ca. −68 ppm, i.e., t-type Si atoms. The hydrolytic condensation and hydrosilylation reaction should not be considered as alternative procedures for obtaining tri(hex-3-enyl)- and tri(dec-5-enyl)- functional SQ’s compounds. Both of the methods require additional reaction step, i.e., for condensation, the synthesis of alkenylchlorosilanes with longer chains and hydrosilylation, and the synthesis of SQ-R-SiH (R = iBu, Ph). However, the formation of undesired by-products might not be circumvented in the case of hydrosilylation reaction and, as a result, the application of hydrolytic condensation should be the Polymerspreferred2020 methodology, 12, 1063 for the efficient synthesis of final products. 9 of 15

TG and DSC Analysis TG and DSC Analysis The discussed SQ-R-FG (R = iBu, Ph; FG = Hex, Dec) derivatives can be considered as i.e., valuableThe discussedtripodal SQ-R-FGcrosslinking(R =agentsiBu, Ph; fo FGr polymer= Hex, Dec) systems derivatives processed can be via considered TM-catalyzed as i.e., valuabletransformations tripodal (e.g., crosslinking hydrosilylation) agents for polymeras well as systems UV or processed free radical via TM-catalyzedinitiated ones transformations (e.g., thiolene (e.g.,addition), hydrosilylation) as mentioned as well above. as UV The or free performance radical initiated of the ones materials (e.g., thiolene obtained addition), in this as mentionedmanner is above.significantly The performance affected by ofboth the materialsthe properties obtained of the in thisstarting manner polymers is significantly and the a ffcrosslinkersected by both used, the propertiesamong others, of the their starting thermal polymers properties. and the As crosslinkers pointed by used,Naka, among they are others, determined their thermal not only properties. by the Astype pointed of seven by chemically Naka, they inert are determinedorganic substituents not only (isobutyl by the type or ph ofenyl) seven present chemically in the inert structure organic of substituentssilsesquioxane, (isobutyl but also or by phenyl) the chemical present structure in the structure of the three of silsesquioxane, substituents at butthe SQ also opening by the chemical moieties structure[19]. In order of the evaluate three substituents the influence at theof hydrocar SQ openingbon moietieschain length [19]. of In the order alkenyl evaluate group the anchored influence the of hydrocarbonSi-O-Si core of chain the discussed length of thegroup alkenyl of compounds group anchored on their the thermal Si-O-Si properties, core of the they discussed were subjected group of compoundsto the TG and on DSC their analyses. thermal properties, they were subjected to the TG and DSC analyses. Figure7 7presents presents the the TG andTG DTGand curvesDTG curves acquired acquired for SQ-iBu for derivatives SQ-iBu derivatives and Table1 summarizesand Table 1 thesummarizes results of the their results analysis. of their It cananalysis. be observed It can be that observed the length that ofthe the length alkyl of chain the alkyl visibly chain influences visibly theinfluences thermal stabilitythe thermal of prepared stability compounds. of prepared The incorporation compounds. of threeThe dimethylvinylsilylincorporation of groupsthree intodimethylvinylsilyl the structure of groups the SQ-iBu-Vi into the structurederivative of resulted the SQ-iBu-Vi in a significant derivative (by resulted more than in a 70 significant◦C) decrease (by inmore the than decomposition 70 °C) decrease onset in temperature the decomposition (Tonset )onset as compared temperature to the (Tonset starting) as comparedSQ-iBu-OH to the, while starting the presenceSQ-iBu-OH of dimethylsilyl, while the presence moieties ofof dimethylsilyl a longer alkyl moieties chain (-allyl, of a longer -hex-3-enyl, alkyl chain and -dec-5-enyl(-allyl, -hex-3-enyl, ) in the SQ-iBu-Alland -dec-5-enyl, SQ-iBu-Hex ) in the SQ-iBu-All, and SQ-iBu-Dec, SQ-iBu-Hexderivatives, and SQ-iBu-Dec caused an increase derivatives in their caused Tonset anvalues increase to 236, in 250,their and Tonset 246 values◦C, respectively. to 236, 250, and 246 °C, respectively.

Figure 7. TG and DTG curves of SQ-iBu-FG silsesquioxane derivatives.

Based on the coursecourse of TGTG andand DTGDTG curves,curves, itit cancan bebe alsoalso observedobserved thatthat thethe decompositiondecomposition processprocess ofof the the dimethylalkenylsilyl dimethylalkenylsilyl functionalized functionalized silsesquioxanes silsesquioxanes is much moreis much complex more (multimodal), complex significantly spread in time, and shifted to the higher temperatures as compared to SQ-iBu-OH derivative. The char yield values at 1000 ◦C measured for starting SQ-iBu-OH and functionalized SQ-iBu-All, SQ-iBu-Hex, and SQ-iBu-Dec samples were very similar and ranged from 36% to 40%, regardless of the alkenyl chain length. Interestingly, the char yield value that was measured at 1000 ◦C for the SQ-iBu-Vi sample was over 14% higher and reached 54%. It should be noted that the thermal behavior of the SQ-iBu-Vi sample is quite different from the rest of the SQ-iBu derivatives. The decrease in Tonset temperature and increase in char yield value observed for SQ-iBu-Vi derivative can be explained by the relatively higher reactivity and lower thermal stability of dimethylvinylsilyl groups. They may undergo either free-radical polymerization or thermal cleavage, followed by hydrolytic polycondensation, leading to the formation of more thermally stable, crosslinked products, and finally a higher amount of char at elevated temperatures. Polymers 2020, 12, 1063 10 of 15

Table 1. Results of TG and DTG analysis of SQ-iBu-FG and SQ-Ph-FG silsesquioxane derivatives.

Mass Loss Temperature [ C] ◦ a Sample Tonset [◦C] CHAR Yield [%] 1% 5% 10% SQ-iBu-OH 206.6 220 240.4 218.2 37.7 SQ-iBu-Vi 165.9 223.2 392 147.5 54.2 SQ-iBu-All 218.1 245.2 288.2 236.4 39.5 SQ-iBu-Hex 220 262.8 374 250.2 36.3 SQ-iBu-Dec 210.8 259 339.2 245.9 38.9 SQ-Ph-OH 223.4 451.8 590.2 426.3 42.5 SQ-Ph-Vi 281.4 327.9 442.6 300.9 38.3 SQ-Ph-All 267.7 334.9 435.7 307.2 39.3 SQ-Ph-Hex 267.6 343.5 454.1 327.2 28.8 SQ-Ph-Dec 198.6 299.9 424.1 305.8 27.7 a @1000 ◦C.

Opposite to the SQ-iBu-FG derivatives discussed above, the incorporation of the dimethylalkenylsilyl moieties into the heptaphenyltrisilanol silsesquioxane, regardless of the alkenyl chain length, resulted in the formation of derivatives of lower thermal stability when compared to the starting material (SQ-Ph-OH). Their Tonset temperatures ranged from 300 to 327 ◦C, while the Tonset temperature of the main decomposition step measured for SQ-Ph-OH derivative was at least 100 ◦C higher, as shown in Figure8 (see Table1). The decomposition process of the SQ-Ph functionalized silsesquioxanes also became more complex, spread in time and was shifted to the lower temperatures when compared to SQ-Ph-OH derivative. The values of the char yield at 1000 ◦C obtained for all functionalized SQ-Ph-Vi, SQ-Ph-All, SQ-Ph-Hex, and SQ-Ph-Dec silsesquioxanes (38, 39, 29, and 28%, respectively) were lower than those that were measured for starting SQ-Ph-OH derivative (43%) unlike in the case of SQ-iBu-OH derivative. Moreover, it can be observed that char yields of the SQ-Ph-Vi and SQ-Ph-All samples were notably higher than those of SQ-Ph-Hex and SQ-Ph-Dec ones, which result from the lower carbon content in their structure. The decrease in Tonset temperatures and char yields observed for all functionalized SQ-Ph-FG derivatives when compared to the SQ-Ph-OH could be explained by the occurrence of its thermally initiated dehydration, followed by the hydrolytic polycondensation leading to the formation of the higher amount of thermally stable char. The presence of the characteristic step transition observed in the course of the SQ-Ph-OH TG curve at 250 C could be attributed to the mentioned dehydration process and justify the above hypothesis. Polymers◦ 2019, 11, x FOR PEER REVIEW 11 of 16

Figure 8. TG and DTG curves of SQ-Ph-FG silsesquioxane derivatives.

Based onon thethe resultsresults of of thermogravimetric thermogravimetric analysis, analysis it, seemsit seems that that the the length length of alkenyl of alkenyl chains chains only marginallyonly marginally aﬀects affects the thermal the thermal stability stability of the tested of the group tested of group compounds of compounds and the type and of the the type remaining of the remaining seven functional groups (iBu or Ph) or rather the superimpose of both parameters has a profound impact on silsesquioxanes thermal stability. In order to further explain the above-discussed relationships and evaluate the impact of the alkenyl chain length on the melting and crystallization characteristics of both series of compounds, they were subjected to DSC analysis. First, the starting SQ-iBu-OH and SQ-Ph-OH derivatives were analyzed and Figure 9 presents their DSC thermograms. It can be seen that SQ-iBu-OH derivative melts at 196 °C and crystalize during cooling at 169 °C. In the case of SQ-Ph-OH derivative, an endothermic signal with the maximum at 260 °C on a heating curve and a characteristic step transition at 123 °C (midpoint) on a cooling curve can be observed. This means that the SQ-Ph-OH derivative melts at 260 °C with the decomposition and formation of an amorphous polymeric phase of glass transition at 123 °C, which is consistent with the results of TG analysis that were obtained for this compound and suggested the explanation of its thermal stability. It should be noted that melting and crystallization temperatures measured for SQ-iBu-OH derivatives are quite different from those provided by suppliers in the product technical documentation (SDS). A possible explanation of this difference is that used for the syntheses and subjected to the TG and DSC analysis derivative was recrystallized before use, as described earlier.

Figure 9. DSC curves of the heating and cooling run of a) SQ-iBu-OH and b) SQ-Ph-OH derivative.

Subsequently, a series of functionalized derivatives SQ-iBu-FG and SQ-Ph-FG were subjected to DSC analysis, and their thermograms are presented in Figure 10a,b respectively. Table 2 summarizes all of the DSC analysis results.

Polymers 2019, 11, x FOR PEER REVIEW 11 of 16

Figure 8. TG and DTG curves of SQ-Ph-FG silsesquioxane derivatives. Polymers 2020, 12, 1063 11 of 15 Based on the results of thermogravimetric analysis, it seems that the length of alkenyl chains only marginally affects the thermal stability of the tested group of compounds and the type of the sevenremaining functional seven groupsfunctional (iBu groups or Ph) (iBu or rather or Ph) the or superimposerather the superimpose of both parameters of both parameters has a profound has a impactprofound on silsesquioxanesimpact on silsesquioxanes thermal stability. thermal stability. InIn orderorder toto furtherfurther explainexplain thethe above-discussedabove-discussed relationshipsrelationships andand evaluateevaluate thethe impactimpact ofof thethe alkenylalkenyl chainchain lengthlength onon thethe meltingmelting andand crystallizationcrystallization characteristicscharacteristics of bothboth seriesseries ofof compounds,compounds, theythey werewere subjectedsubjected toto DSCDSC analysis.analysis. First,First, thethe startingstarting SQ-iBu-OHSQ-iBu-OH andand SQ-Ph-OHSQ-Ph-OH derivativesderivatives werewere analyzedanalyzed andand FigureFigure9 9 presents presents their their DSC DSC thermograms. thermograms. ItIt cancan be be seen seen that that SQ-iBu-OHSQ-iBu-OH derivativederivative meltsmelts atat 196196 ◦°CC andand crystalizecrystalize duringduring coolingcooling atat 169169 ◦°C.C. InIn thethe casecase ofof SQ-Ph-OHSQ-Ph-OH derivative,derivative, anan endothermicendothermic signalsignal with with the the maximum maximum at 260 at ◦260C on °C a heatingon a heating curve and curve a characteristic and a characteristic step transition step attransition 123 ◦C (midpoint) at 123 °C (midpoint) on a cooling on curve a cooling can becurve observed. can be Thisobserved. means This that means the SQ-Ph-OH that the SQ-Ph-OHderivative meltsderivative at 260 melts◦C with at 260 the °C decomposition with the decomposition and formation and formation of an amorphous of an amorphous polymeric polymeric phase of phase glass transitionof glass transition at 123 ◦ C,at 123 which °C, iswhich consistent is consistent with the wi resultsth the results of TG of analysis TG analysis that were that were obtained obtained for this for compoundthis compound and and suggested suggested the the explanation explanation of itsof its thermal thermal stability. stability. It It should should be be noted noted that that meltingmelting andand crystallizationcrystallization temperaturestemperatures measuredmeasured forfor SQ-iBu-OHSQ-iBu-OH derivativesderivatives areare quitequite didifferentﬀerent fromfrom thosethose providedprovided byby supplierssuppliers inin thethe productproduct technicaltechnical documentationdocumentation (SDS).(SDS). AA possiblepossible explanationexplanation ofof thisthis didifferenceﬀerence isis thatthat usedused forfor thethe synthesessyntheses andand subjectedsubjected toto thethe TGTG andand DSCDSC analysisanalysis derivativederivative waswas recrystallizedrecrystallized beforebefore use,use, asas describeddescribed earlier.earlier.

FigureFigure 9.9.DSC DSC curvescurves ofof the the heating heating and and cooling cooling run run of of (a a)) SQ-iBu-OH SQ-iBu-OHand and ( bb)) SQ-Ph-OH derivative.

Subsequently,Subsequently, a a series series of of functionalized functionalized derivatives derivativesSQ-iBu-FG SQ-iBu-FGand andSQ-Ph-FG SQ-Ph-FGwere were subjected subjected to DSCto DSC analysis, analysis, and theirand thermogramstheir thermograms are presented are pres inented Figure in10 a,bFigure respectively. 10a,b respectively. Table2 summarizes Table 2 allsummarizes of the DSC all analysis of the DSC results. analysis results.

Table 2. Results of DSC analysis of SQ-iBu-FG and SQ-Ph-FGs silsesquioxane derivatives.

Melting Reaction Crystallization Glass Sample Transition [ C] Temp. [ C] ∆H [Jg 1] Temp. [ C] ∆H [Jg 1] Temp. [ C] ∆H [Jg 1] ◦ ◦ − ◦ − ◦ − SQ-iBu-OH 196 32.3 - - 169.1 26 - − SQ-iBu-Vi - - 159.5 189.8 275.9 180.3 - - - SQ-iBu-All - - 164.7 114.9 - - - SQ-iBu-Hex - - 203.7 15.1 - - - SQ-iBu-Dec - - 195.6 8.8 - - - SQ-Ph-OH 259.7 64.8 - - - - 123 − SQ-Ph-Vi 132.1 139.5 8.2 17.1 - - 107.3 18 - − − SQ-Ph-All 63.7 26.8 - - 1.1 6.2 - − − SQ-Ph-Hex 76.7 22.2 - - 20.2 15.5 - − SQ-Ph-Dec 18.6 44.1 73.3 1.3 9.8 0.4 - - - - - * − − − * Possible glass transition below 40 C. − ◦

DSC analysis of the SQ-iBu-FG derivatives series revealed the presence of only exothermic signals on the heating threads of DSC curves of all the measured samples in the range from 100 to 200 ◦C Polymers 2019, 11, x FOR PEER REVIEW 12 of 16

Table 2. Results of DSC analysis of SQ-iBu-FG and SQ-Ph-FGs silsesquioxane derivatives.

Melting Reaction Crystallization Glass transition Sample Temp. ΔH Temp. ΔH Temp. ΔH [°C] [°C] [Jg−1] [°C] [Jg−1] [°C] [Jg−1] SQ-iBu-OH 196 −32.3 - - 169.1 26 - 275.9 SQ-iBu-Vi - - 159.5 189.8 - - - 180.3 SQ-iBu-All - - 164.7 114.9 - - - SQ-iBu-Hex - - 203.7 15.1 - - - SQ-iBu-Dec - - 195.6 8.8 - - - SQ-Ph-OH 259.7 −64.8 - - - - 123 −8.2 SQ-Ph-Vi 132.1 139.5 - - 107.3 18 - −17.1 SQ-Ph-All 63.7 −26.8 - - −1.1 6.2 - SQ-Ph-Hex 76.7 −22.2 - - 20.2 15.5 - 18.6 44.1 −1.3 −9.8 SQ-Ph-Dec - - - - -* 73.3 −0.4 * Possible glass transition below −40 °C. Polymers 2020, 12, 1063 12 of 15 DSC analysis of the SQ-iBu-FG derivatives series revealed the presence of only exothermic signals on the heating threads of DSC curves of all the measured samples in the range from 100 to and200 °C no and signals no weresignals observed were observed on their on cooling their cooling threads threads (Figure (Figure10a). This 10a). indicates This indicates that the that tested the derivativestested derivatives are not are crystalline not crystalline and undergo and undergo exothermic exothermic chemical chemical reactions reactions in the mentionedin the mentioned range ofrange temperatures, of temperatures, leading leading to the formation to the formatio of amorphousn of amorphous products. products. Moreover, Moreover, the observed the observed gradual decrease in the enthalpy (∆Hr) of reaction from 456 to 115, 15, and 9 Jg 1 for SQ-iBu-Vi, SQ-iBu-All, gradual decrease in the enthalpy (ΔHr) of reaction from 456 to 115, 15,− and 9 Jg−1 for SQ-iBu-Vi, SQ- SQ-iBu-HexiBu-All, SQ-iBu-Hex, and SQ-iBu-Dec, and SQ-iBu-Dec, respectively,, respectively, indicates that indicates the reactivity that the of reactivity alkenyl groups of alkenyl is inversely groups proportionalis inversely proportional to their hydrocarbon to their chainhydroc length.arbon The chain decrease length. in The the ∆decreaseHr values in staysthe Δ inH goodr values agreement stays in withgood the agreement char yields with of SQ-iBu-FGthe char yieldsderivatives of SQ-iBu-FG established derivatives based on established TG analysis based results on (seeTG Tableanalysis1). Nevertheless,results (see Table to additionally 1). Nevertheless, conﬁrm to the additionally occurrence ofconfirm a chemical the occurrence process involving of a chemical the reactivity process of unsaturatedinvolving the CH reactivity= CH2 groups of unsaturated of the SQ-iBu-Vi CH = sample,CH2 groups the one of of the potentially SQ-iBu-Vi highest sample, reactivity, the one it was of 1 heatedpotentially in an highest air atmosphere reactivity, at 200 it ◦wasC for heated 30 min. in and an subsequentlyair atmosphere subjected at 200 to °CH for NMR 30 analysismin. and to evaluatesubsequently changes subjected in its structure. to 1H NMR The resultsanalysis of to this evaluate analysis changes presented in inits Figurestructure. S2 (see The Supplementary results of this Materials)analysis presented revealed in the Figure complete S2 (see disappearance Supplementar ofy signals Materials) of chemical revealed shift the complete in the range disappearance from 5.7 to 6.2of signals ppm characteristic of chemical forshift vinyl in the protons range andfrom the 5.7 formation to 6.2 ppm of characteristic new signals at for 1.25, vinyl 1.33, protons and 2.18 and ppm the asformation the result of ofnew the signals formation at 1.25, ofnew 1.33, methylene and 2.18 ppm (CH as2) the units. result This of might the formation conﬁrm of the new hypothesis methylene of the(CH occurrence2) units. This of thermallymight confirm initiated the hypothesis vinyl group of polymerization. the occurrence of However, thermally it shouldinitiated also vinyl be notedgroup 1 thatpolymerization. the intensity However, of the signal it should present also in thebe notedH NMR that at the 0.2 intensity ppm attributed of the signal to the present protons in present the 1H inNMR dimethylsilyl at 0.2 ppm groups attributed was to also the decreased. protons present This means in dimethylsilyl that part of groups the dimethylsilyl was also decreased. groups might This havemeans been that thermally part of the cleaved dimethylsilyl and formed groups active migh sitest have on silsesquioxane been thermally core cleaved that could and formed subsequently active undergosites on silsesquioxane polycondensation core process that could typical subseque for organosiliconntly undergo derivatives, polycondensation also leading process to the typical formation for oforganosilicon high molecular, derivatives, random also structures. leading to the formation of high molecular, random structures.

Figure 10. DSC curves of (a) the ﬁrst heating run of SQ-iBu-FG derivatives, and (b) the second heating and cooling run of SQ-Ph-FG derivatives.

In contrast to the SQ-iBu-FG derivatives, the incorporation of three dimethylalkenylsilyl groups into the structure of heptaphenylsilsequioxane resulted in the formation of crystalline products and no thermal degradation process was observed below their melting points, as the characteristic exothermic crystallization signals were observed on the cooling threads of their DSC thermograms (Figure 10b) for all of the measured samples, except the SQ-Ph-Dec one. In this case, the crystallization temperature was observed on its cooling DSC curve in the measured temperature range. Nevertheless, the curve inflection at the end of the temperature program might indicate the occurrence of a glass transition. It can be also observed that melting and crystallization temperatures of SQ-Ph-All, SQ-Ph-Hex, and SQ-Ph-Dec silsesquioxanes of longer alkenyl chains were significantly lower as compared to the SQ-Ph-Vi derivative. The incorporation of dimethylsilyl groups into the SQ-Ph-OH structure effectively inhibited its hydrolytic polycondensation at elevated temperatures what explain the decrease in Tonset temperatures of functionalized SQ-Ph-FG derivatives (see Figure8 and Table1). Polymers 2020, 12, 1063 13 of 15

4. Conclusions In summary, we designed and synthesized novel tri(alkenyl)functional silsesquioxanes SQ-R-FG (R = iBu, Ph; FG = Vi, All, Hex, Dec), with various chain lengths and with two types of inert groups. We revealed simple and efficient routes of their synthesis and isolation with high purity using a hydrolytic condensation reaction. Additionally, a parallel synthetic procedure was tested for the synthesis of tri(hex-3-enyl)- and tri(dec-5-enyl )functional SQs, based on the hydrosilylation of 1,5-hexadiene and 1,9-decadiene with SQ-R-SiH with three reactive Si–H moieties. However, this procedure appeared to be of lower selectivity of desired products due to possible side reactions and it should not be applied for obtaining the unsaturated SQ-based open-cage compounds. All of the obtained compounds were analyzed with TG and DSC techniques in terms of the evaluation of the influence of the alkenyl chain length in the dimethylsilyl groups on their thermal stability, melting and crystallization temperatures. The obtained results of the thermal analysis revealed that the length of alkenyl chains do affect the thermal stability as well as the melting and crystallization temperatures of the tested group of compounds. Additionally, it was proved that the type of the remaining seven functional groups (iBu or Ph) has a crucial impact on the considered parameters. It was also found that the trisubstituted derivatives of SQ-iBu can undergo thermally initiated polymerization and polycondensation processes with the formation of homogeneous, transparent polymeric materials. These observations will be the starting point for further research while using the discussed group of compounds as substrates for protective coating materials synthesis and assessing the impact of the structure of reactive groups introduced on their physicochemical properties.

Supplementary Materials: Detailed analytical data with NMR spectra of all isolated products along with the additional NMR spectra and GPC chromatograms are available online at http://www.mdpi.com/2073-4360/12/5/ 1063/s1. Author Contributions: B.D. contributed to conceptualization of paper and designed the experiments; K.M. performed most experiments and analyzed the data, M.D. performed and interpreted T.G. and D.S.C. analyzes; J.D. and M.R. prepared substrates; K.M., M.D. and B.D. contributed to manuscript preparation. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by [National Science Centre of Poland (NCN)] grant number UMO 2017/27/N/ST5/00175] and by [European Union through the European Social Fund under the Operational Program− Knowledge Education Development] grant number [POWR.03.02.00 00-I023/17]. − Conﬂicts of Interest: The authors declare no conﬂict of interest.

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