European Polymer Journal 46 (2010) 465–471

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European Polymer Journal

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Synthesis, characterization, and thermal properties of newsilarylene–siloxane–acetylene polymers

Rui Wang a,b, Lei Fang a,b, Caihong Xu a,*

a Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, Chinab Graduate School of the Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o

Article history:Received 3 August 2009Received in revised form 16 October 2009Accepted 1 December 2009Available online 16 December 2009

Keywords:SynthesisThermal propertiesSilicon-containing polymerAcetyleneCoupling reaction

0014-3057/$ - see front matter � 2009 Elsevier Ltddoi:10.1016/j.eurpolymj.2009.12.010

* Corresponding author. Tel./fax: +86 10 6255448E-mail address: caihong@iccas.ac.cn (C. Xu).

a b s t r a c t

New silarylene–siloxane–acetylene polymers have been synthesized by coupling reactionsemploying 1,3-bis(p-ethynylphenyl)-1,1,3,3-tetraphenyldisiloxane (3) as the key mono-mer. Their thermal properties have been evaluated by thermogravimetric analysis (TGA)and differential scanning calorimetry (DSC). All of the new polymers showed good thermalstability, with their temperatures at 5% weight loss (Td5) being higher than 540 �C undernitrogen and higher than 460 �C in air. Their char yields at 1000 �C under N2 were above80%. Broad exothermic peaks, attributable to reaction of the acetylenic units, wereobserved by DSC analysis in the temperature range 270–450 �C.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Progress in modern technologies has created an intensedemand for new inorganic–organic hybrid polymers thatare lightweight, highly heat-resistant, and processable.These materials find use in the microelectronic and aero-space industries. A vast amount of research in the area ofinorganic–organic hybrid polymers has contributed to thedevelopment of silicon-containing polymers [1]. Varioussilicon-containing hybrid polymers have been synthesizedand investigated as functional materials for optoelectronics[2], precursors of Si-based ceramics [3], and new high-tem-perature-resistant materials [4]. Among them, two kinds ofsilarylene-containing polymers, silarylene–siloxane poly-mers (Scheme 1 a) and silarylene–acetylene polymers(Scheme 1 b), have attracted particular attention due totheir excellent thermal properties [5–7]. Silarylene–silox-ane polymers containing aromatic groups and siloxaneunits in the main chain are excellent high-temperatureelastomers with low Tg and high decomposition tempera-

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7.

tures [7], while silarylene–acetylene polymers composedof an alternating arrangement of organosilicon and acety-lene or diacetylene units display great potential as high-temperature-resistant thermosetting resins and SiC cera-mic precursors [4a,b,8]. By incorporating diacetylenegroups into the silarylene–siloxane main chain, Keller andco-workers synthesized silarylene–siloxane–diacetylenelinear polymers [9] (Scheme 1 c). Although these new poly-mers exhibited fascinating properties as high-temperature-resistant materials, to the best of our knowledge studies onsuch polymers are still very limited. The reported polymerswere generally synthesized by a polycondensation reactionof 1,4-dilithio-1,3-butadiyne (dilithiodiacetylene) with dic-hlorosiloxane [10], or by the aminosilane�disilanol poly-condensation method [11].

On the other hand, Sonogashira cross-coupling reactions[12], as well as oxidative coupling reactions [13], have beenwidely used in the preparation of p-conjugated polymerscontaining organosilicon and acetylene units [18]. How-ever, little attention has been paid to the synthesis of silar-ylene–siloxane polymers containing acetylene units bythese methods [14]. Herein, we report the synthesis ofnew silarylene–siloxane–acetylene polymers by coupling

Si SiMe

MeO

Me

Me

Si SiMe

MeO

Me

MeOSi

Me

MeSiMe

Me

SiSiPh

H

Ph

mnn

n

a b

c

Scheme 1. Representative silarylene-containing polymers.

466 R. Wang et al. / European Polymer Journal 46 (2010) 465–471

reactions employing a new ethynyl–terminated disiloxaneas the key monomer, as well as the thermal properties ofthe new polymers. We have found these coupling reactionsto offer a convenient means of preparing silarylene–silox-ane polymers containing acetylene or diacetylene units inthe main chain under mild conditions.

2. Experimental

All reactions were carried out under nitrogen atmo-sphere unless otherwise noted. Anhydrous diethyl ether(Et2O) was freshly distilled over sodium/benzophenoneketyl before use. 1,3-Dichlorotetraphenyldisiloxane waspurchased from Meryer and used as received. 1-Bromo-4-trimethylsilylethynyl benzene was prepared accordingto the reported procedure [15]. All other chemicals werepurchased from Alfa Aesar or ACROS and used as received.1H and 13C NMR spectra were recorded on a BrukerAVANCE 400 spectrometer in CDCl3 or acetone-d6. 29SiNMR spectra were recorded with a Bruker DMX 300 spec-trometer in acetone-d6. Melting point (mp) determinationwas performed using a WRS-1A digital melting pointapparatus. Elemental analyses were obtained with theFLASH EA1112 Microanalytical analyzer. Thermogravi-metric analysis (TGA) and differential scanning calorimet-ric (DSC) were performed on a SII EXTRA 6300 TG/DTAand a SII EXTRA 6200 DSC at a heating rate of 10 �C/min in a nitrogen or air atmosphere, respectively. Molec-ular weights were determined by a gel permeation chro-matography (GPC) with polystyrene calibration using aWATERS 2690D Separation Module and WATERS 2410Refractive Index Detector equipped with TSK-GELGMHHR-M and TSK column at 30 �C in toluene.

2.1. Synthesis of 1,3-bis(p-trimethylsilylethynylphenyl)-1,1,3,3-tetraphenyl disiloxane (2)

To a solution of 1-bromo-4-trimethylsilylethynylben-zene (2.53 g, 10.0 mmol) in Et2O (100 mL) was added a hex-ane solution of n-butyllithium (n-BuLi, 2.5 M, 4.8 mL,12.0 mmol) dropwise slowly at �78 �C. After stirred for 2 hat �78 �C, a solution of 1,3-dichlorotetraphenyldisiloxane(2.26 g, 5.0 mmol) in Et2O (20 mL) was added at the temper-ature within 20 min. The reaction mixture was allowed towarm to room temperature and stirred for another 12 h.Then the reaction mixture was quenched with a saturatedNH4Cl aqueous solution, extracted with Et2O. The organiclayer was dried over MgSO4, filtered, and concentratedunder reduced pressure. The resulting yellow sticky mixture

was passed through a silica gel column (petroleum ether/ethyl acetate, 100/1, Rf = 0.32) and the obtained crude prod-uct was further recrystallized from a mixed solvent of petro-leum ether and ethyl acetate (100/1) at�16 �C to give 3.85 g(5.3 mmol) of compound 2 in 53% yield as a white solid. mp194–195 �C. 1H NMR (400 MHz, acetone-d6): d 0.23 (s, 18H),7.32–7.36 (m, 8H), 7.40–7.44 (m, 5H), 7.45–7.50 (m, 15H).13C NMR (400 MHz, acetone-d6): d 137.22, 135.89, 135.85,135.65, 131.85, 131.18, 128.87, 125.65, 105.75, 96.02,�0.05. 29Si NMR (300 MHz, acetone-d6): d �19.03, �20.02.Anal. Calcd. for C46H46Si4O: C, 75.98; H, 6.38. Found: C,75.79; H, 6.49.

2.2. Synthesis of 1,3-bis(p-ethynylphenyl)-1,1,3,3-tetraphenyldisiloxane (3)

A solution of 2 (0.81 g, 1.11 mmol) and K2CO3 (20 mg,0.15 mmol) in a 1/1 THF/n-propanol mixed solvent wasstirred for 36 h at room temperature. After evaporation ofsolvents under reduced pressure, a white solid was ob-tained and the following recrystallization from a mixedsolvent of petroleum ether and ethyl acetate (100/1) at�16 �C gave compound 3 in 83% yield as a white solid.mp 188–189 �C, 1H NMR (400 MHz, acetone-d6): d 3.72(s, 2H), 7.32–7.36 (m, 8H), 7.43–7.45 (m, 7H), 7.48–7.51(m, 13H). 13C NMR (400 MHz, acetone-d6): d 137.34,135.91, 135.84, 135.61, 132.06 131.19, 128.87, 124.93,84.01, 80.45. 29Si NMR (300 MHz, acetone-d6): d �18.19.Anal. Calcd. for C40H30Si2O: C, 82.43; H, 5.19. Found: C,82.24; H, 5.22.

2.3. Synthesis of bis(4-phenylethynyl-p-phenyl)-tetraphenyldisiloxane (4)

To a mixture of 3 (117 mg, 0.20 mmol), PdCl2(PPh3)2

(2.8 mg, 4.0 lmol), CuI (1.5 mg, 8.0 lmol) in a 3/1 tolu-ene/Et3N mixed solvent was added iodobenzene(81.6 mg, 0.40 mmol) at room temperature. After stirredfor 24 h at the same temperature, the reaction mixturewas filtered and concentrated. The obtained yellow solidwas passed through a silica gel column (petroleumether/ethyl acetate, 50/1, Rf = 0.40) and then recrystallizedfrom a mixed solvent of petroleum ether and ethyl ace-tate (50/1) at �16 �C to give 111 mg (0.15 mmol) of com-pound 4 in 75% yield as a white solid. mp 210–211 �C, 1HNMR (400 MHz, CDCl3): d 7.29–7.33 (m, 8H), 7.35–7.37(m, 6H), 7.40–7.42 (m, 4H), 7.44–7.46 (m, 4H), 7.48–7.51 (m, 8H), 7.54–7.56 (m, 8H). 13C NMR (400 MHz,CDCl3): d 135.92, 135.29, 135.21, 135.08, 131.81, 130.87,

R. Wang et al. / European Polymer Journal 46 (2010) 465–471 467

130.17, 128.49, 127.98, 124.83, 123.30, 90.55, 89.54. 29SiNMR (300 MHz, CDCl3): d �17.88. Anal. Calcd. forC40H30Si2O: C, 84.97; H, 5.21. Found: C, 84.54; H, 5.20.

2.4. A typical procedure for the synthesis of the polymers:polymer P1

A mixture of 3 (243 mg, 0.42 mmol), 1,4-diiodobenzene(138 mg, 0.42 mmol), Pd(PPh3)2Cl2 (6.0 mg, 8.5 lmol), CuI(3.2 mg, 17 lmol) in a 3/1 toluene/Et3N mixed solvent(60 mL) was stirred at room temperature for 24 h. Afteraddition of water, the reaction mixture was extracted withchloroform. The organic layer was dried over MgSO4, fil-tered, and concentrated under reduced pressure. The re-sulted mixture was dissolved into a small amount oftoluene and poured into 90 mL of vigorously stirred n-hex-ane to form a grayish yellow precipitation. After filtration,the solid was dried in vacuo to give 180 mg of polymer P1in 66% yield as a grayish yellow solid. 1H NMR (400 MHz,CDCl3): d 7.28–7.30(m, 8H), 7.38–7.49(m, 24H). 13C NMR(400 MHz, CDCl3): d 137.67, 136.23, 135.28, 135.00,133.27, 131.73, 130.88, 130.20, 128.00, 124.57, 123.22,91.47, 90.27.

2.5. Polymer P2

This polymer was synthesized essentially in the samemanner as described for P1 by the reaction of 3 and 1,3-diiodobenzene in 75% yield as a light yellow solid. 1H NMR(400 MHz, CDCl3): d 7.28–7.30 (m, 8H), 7.38–7.47 (m, 24H).13C NMR (400 MHz, CDCl3): d 137.70, 136.18, 135.28,135.24, 135.00, 133.22, 131.73, 131.64, 130.88, 130.19,128.00, 124.55, 123.66, 90.17, 89.68.

2.6. Polymer 3

A mixture of 3 (168 mg, 0.29 mmol), CuCl (8.5 mg,0.09 mmol) in a 3/1 toluene/pyridine mixed solvent(32 mL) was vigorously stirred at room temperature for72 h under oxygen atmosphere. The reaction mixturewas washed with water, extracted with chloroform, andthe organic layer was dried over MgSO4, filtered, andconcentrated under reduced pressure. The mixture wasdissolved into a small amount of toluene and poured into90 mL of vigorously stirred n-hexane to form a whiteprecipitation. After filtration, the solid was dried in vacuoto give 134 mg of polymer P3 in 80% yield as a white so-lid. 1H NMR (400 MHz, CDCl3): d 3.10 (s, 2H), 7.28–7.29(m, 45H), 7.41–7.45 (m, 144H). 13C NMR (400 MHz,CDCl3): d 137.27, 136.56, 135.23, 135.17, 134.82,134.69, 131.69, 131.39, 130.28, 128.02, 123.23, 123.22,83.73, 82.08, 78.30, 74.97.

3. Results and discussion

3.1. Synthesis and characterization

1,3-Bis(p-ethynylphenyl)-1,1,3,3-tetraphenyldisiloxane(3) was synthesized in two steps as indicated in Scheme 2.Lithium–halogen exchange of 1-bromo-4-trimethylsilyle-

thynylbenzene (1) with n-BuLi followed by treatment with1,3-dichlorotetraphenyldisiloxane produced compound 2.Subsequent alkaline desilylation of the terminal trimethyl-silyl groups produced the disiloxane 3 bearing terminalethynyl groups. For the desilylation of 2, the use of n-pro-panol/THF mixed solvent proved crucial. When the reac-tion was carried out in either methanol/THF or ethanol/THF, the Si–O bonds were also broken and disiloxane 3was not obtained.

To test the Sonogashira coupling reaction of bis-(p-ethynylphenyl)tetraphenyl disiloxane 3, the model com-pound 4 was designed. The reaction of 3 and iodobenzene ina mixed solvent of toluene/Et3N using Pd(PPh3)2Cl2 and CuIasco-catalystsproduced4ingoodyield(Scheme2).Thiscom-pound was found to be highly soluble in commonly used or-ganic solvents, such as tetrahydrofuran, toluene, andchloroform.

Based on the model reaction, the Sonogashira cross-coupling reactions between 3 and the requisite diiodoaryl-ene compounds were carried out (Scheme 3). The polymer-ization reactions proceeded well under the sameconditions as those applied for the synthesis of model com-pound 4. Thus, the reactions of 3 with 1,4-diiodobenzeneand 1,3-diiodobenzene produced polymer P1 containingpara-phenylene units and polymer P2 containing meta-phenylene units, respectively. In addition, polymer P3was synthesized by the oxidative homo-coupling reactionof 3 using CuCl as catalyst in toluene/pyridine under oxy-gen atmosphere (Scheme 3), referring to a literature meth-od [16]. The homo-coupling of 3 was highly sensitive to thereaction time and the dosage of CuCl. Products with higheraverage molecular weight (Mw P 2.4 � 104) but inferiorsolubility were formed when a longer reaction time or anincremental dosage of catalyst was applied. All of the poly-mers were obtained in moderate to good yields. PolymerP2 was found to be readily soluble in common organic sol-vents, such as chloroform, toluene, and tetrahydrofuran.Though the solubilities of polymers P1 and P3 were some-what lower than that of P2 due to their more rigid molec-ular structures, they could also be dissolved in hot tolueneand chloroform.

The molecular weights of the polymers were estimatedby GPC measurements using polystyrene standards and arelisted in Table 1. The number average molecular weights(Mn) of P1�P3 range from 3.60 � 103 to 5.30 � 103, withthe associated polydispersity distribution indices (PDI)ranging from 1.21 to 1.57. Among the polymers, P2 has aslightly higher Mn and a narrower PDI, probably due tothe better solubility of its incipient polymer in the solventused. The structures of the polymers were characterized by1H and 13C NMR spectroscopy. In the 1H NMR spectra, theresonances between 7.28 and 7.49 ppm were originatedfrom the phenyl protons. An obvious weak signal at3.10 ppm for polymer P3 was attributed to the terminalacetylenic proton, but no such signal was observed in the1H NMR spectra of P1 and P2. In the 13C NMR spectra, be-sides signals of phenyl carbons, only a pair of resonancesattributed to internal acetylenic carbons appeared at90.27, 91.47 ppm for P1, and 89.68, 90.17 ppm for P2,respectively. However, in the 13C NMR spectra of P3, thereare distinguishable two pairs of signals at 74.97, 82.08, and

Br SiMe3SiMe3SiPh

PhO Si

Ph

PhSiMe3

SiPh

PhO Si

Ph

PhSiPh

PhO Si

Ph

Ph

i, ii iii

iv

1 2 53%

3 83% 4 75%

Scheme 2. Synthetic routes to monomer 3 and model compound 4. Reaction conditions: (i) n-BuLi, Et2O, �78 �C, 2 h; (ii) 1,3-dichloro-1,1,3,3-tetraphenyldisiloxane, �78 �C to r.t., 12 h; (iii) K2CO3, n-PrOH/THF, r.t., 36 h; (iv) PdCl2(PPh3)2, CuI, toluene, Et3N, r.t., 24 h.

SiPh

PhO Si

Ph

Ph

SiPh

PhO Si

Ph

Ph

P1 66%

P2 75%

n

SiPh

PhO Si

Ph

Phn

n

I I

SiPh

PhO Si

Ph

PhP3 80%

I

I

3 +

3 +

i

i

ii

Scheme 3. Synthesis of polymers P1–P3. Reaction conditions: (i) PdCl2(PPh3)2, CuI, toluene, Et3N, r.t., 24 h; (ii) CuCl, O2, toluene, pyridine, r.t., 72 h.

Table 1Average molecular weights and polydispersity indices of polymers.

Polymer Yielda (%) Mnb Mw

b Mw/Mn

P1 66 4900 7700 1.57P2 75 5300 6400 1.21P3 80 3600 4600 1.28

a After reprecipitation from n-hexane.b Estimated by GPC in toluene on the basis of a polystyrene calibration.

Fig. 1. DSC traces of compounds 3 and 4.

468 R. Wang et al. / European Polymer Journal 46 (2010) 465–471

78.30, 83.73 ppm, which were assigned to internal and ter-minal acetylenic carbons, respectively [17].

3.2. Thermal properties

The thermal properties of the monomer 3, model com-pound 4, and the three new polymers were studied by DSCand TGA.

The DSC traces (in nitrogen) for 3 and 4 are shown inFig. 1, while those of the polymers are shown in Fig. 2,respectively. The DSC curves of both 3 and 4 feature asharp endothermic peak and a strong exothermic peak,which are centered at 189 and 225 �C for 3, and at 210and 367 �C for 4. The endothermic peaks are due to meltingof the compounds, while the exothermic peaks may beattributed to thermal cross-linking reactions of the ethynylgroups [10b,18]. Due to the presence of terminal phenylgroups, the activity of the ethynyl groups in compound 4is lower than that in the case of 3, as indicated by a broaderexothermic peak at higher temperature and a lower

exothermic energy. Melting or glass transitions (Tg) werenot observed for any of the polymers; rather, only broadexothermic peaks appeared in their DSC curves. There areseveral notable points. P1 and P2, containing p- and m-phenylene units, respectively, display similar complicatedexothermic behaviors over a broad temperature range.Strong exothermic peaks in the range 270–450 �C may beattributed to cross-linking reactions of internal ethynylgroups, but the assignments of weak exothermic signalspeaking at 236 �C for P1 and 195 �C for P2 are as yet un-clear. A cross-linking reaction between primary ethynyl

Fig. 2. DSC traces of polymers P1–P3.Fig. 3. TGA traces of compounds 3, 4, and polymers P1–P3 in nitrogen.

Fig. 4. TGA traces of polymers P1–P3 in air.

R. Wang et al. / European Polymer Journal 46 (2010) 465–471 469

groups may be responsible for the exothermic peak at225 �C, as indicated by the thermal behavior of 3, and asimilar thermal reaction can also be produced by internaldiacetylene units [18]. The presence of these two kinds ofacetylene groups in the polymers prepared by Sonogashirareaction of a,x-diethynyl-terminated monomers has beenreported [19]; however, there are no signals in the 1H and13C NMR spectra of P1 and P2 that indicate the presence ofthese groups. The exothermic maximum of P1 is centeredat a temperature of 380 �C, which is higher than that ofP2 by 60 �C, probably due to the greater rigidity of its back-bone incorporating p-phenylene units. In addition, com-pared with that of model compound 4, the exothermicpeaks of polymers P1 and P2 in the range 270–450 �C aresplit, which may be attributed to the existence of productswith different molecular weights and terminal groups. Incontrast to those of P1 and P2, the DSC curve of polymerP3 displays only one strong exothermic peak centered at238 �C, indicative of a cross-linking reaction induced byterminal acetylene or internal diacetylene groups, or both[10,18]. Because there are clearly signals derived from ter-minal acetylene groups in the NMR spectra of P3, we as-sume that thermal reaction of both terminal acetyleneand internal diacetylene units is responsible for the exo-thermic behavior.

The TGA traces of the new molecules under nitrogenand oxidative atmospheres are shown in Fig. 3 and Fig. 4,respectively. The detailed thermal data obtained from theTGA are summarized in Table 2. All of the moleculesshowed good thermal stability and high char yields undera nitrogen atmosphere. The char yield of 4 was 69%, whileall of the polymers gave char yields above 80% at 1000 �C.The TGA trace of 4 shows a slight weight loss between 200and 500 �C, the region in which melting, cross-linking, andnetwork formation also commence. Most of the weight lossoccurred in the range 550–700 �C, and almost no furtherweight change occurred above 700 �C. Compared with itsmethyl-substituted analogue, bis(4-phenylethynyl-p-phenyl)tetramethyldisiloxane [14], 4 displays improvedthermal stability due to the introduction of bulky phenylgroups. P1, P2, and P3 show almost identical TG curves,

even though they have different molecular structures.The decomposition temperatures at 5% weight loss (Td5)under nitrogen for all of the samples were above 550 �C,and no obvious weight losses were observed below500 �C. The major weight losses occurred in the range550–650 �C, and the weight had stabilized by 700 �C withresulting char yields of 83%, 82%, and 81% for P1, P2, andP3, respectively, at 1000 �C. Compared with silarylene–siloxane polymers [5a,7], these new polymers exhibitedhigher thermal stability and char yield at high tempera-ture. It can be attributed to the formation of more stabledense network structures due to existence of thermal cros-slinkable acetylene or diacetylene groups in the main chainof the new polymers [4]. Though the thermal stability ofthe new polymers were some lower than that of silaryl-ene–acetylene polymers containing Si–H units in the mainchain [4,6], which are higher than that of many other silar-ylene–acetylene polymers [19].

Under oxidative atmosphere (air), all of the polymersremained stable with no weight loss up to 400 �C. Thedecomposition temperatures at 5% weight loss of P1,P2, and P3 are 466 �C, 503 �C, and 533 �C, respectively

Table 2TGA data of compound 3, 4, and polymers P1–P3.

Compounds/polymers In nitrogen In air

Td5 (�C) Weight% at 1000 �C Td5 (�C) Weight% at 1000 �C

3 224 52.8 – –4 472 68.7 – –P1 550 83.3 466 16.0P2 558 81.6 501 17.0P3 561 81.2 553 20.3

470 R. Wang et al. / European Polymer Journal 46 (2010) 465–471

(Fig. 4). P1 and P2 showed abrupt onsets of weight lossat around 560 �C; this weight loss had stabilized by660 �C, and their retentions were less than 20% at1000 �C. In comparison, P3 showed better thermo-oxida-tive stability, with its major weight loss occurring atabout 610 �C and a char yield of 20% at 1000 �C. Com-pared with the other two polymers, P3 has a higher ratioof reactive C„C units relative to other moieties in itsmain chain. Thus, during the process of TG analysis, adenser network may be formed in a sample of P3 priorto its decomposition, which results in a slight improve-ment in thermo-oxidative stability.

4. Conclusion

Based on the development of a new monomer bearingterminal acetylene groups, three new silarylene–siloxane–acetylene polymers have been synthesized under mild con-ditions by Sonogashira cross-coupling or oxidative homo-coupling reactions. Their thermal properties have beenstudied by TGA and DSC. All of the polymers showed highthermal decomposition temperatures and high ceramicyields at 1000 �C in a nitrogen atmosphere. These polymersalso showed high 5% weight loss temperatures, even in air,although most of their weight loss occurred before 800 �C.Thermal cross-linking of acetylene or diacetylene groupsin their main chains accounts for their outstanding thermalstability. Further studies concerning the thermal curebehavior and mechanical properties of the synthesized poly-mers are underway in our laboratory. The presence of termi-nal ethynyl groups in the monomer also holds promise forchemical manipulations enabling the synthesis of otherethynyl-containing organosiloxane polymers.

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