formation of elastomeric network polymers from ambient heterogeneous hydrosilations of...

Formation of Elastomeric Network Polymers from AmbientHeterogeneous Hydrosilations of Carboranylenesiloxaneand Branched Siloxane Monomers

MANOJ K. KOLEL-VEETIL, TEDDY M. KELLER

Advanced Materials Section, Materials Chemistry Branch, Chemistry Division, Naval Research Laboratory,Washington, DC 20375-5320

Received 17 June 2005; accepted 20 September 2005DOI: 10.1002/pola.21151Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The Karstedt catalyst-catalyzed ambient-condition hydrosilation reactionsin hexane of a monomeric vinyl-containing carboranylenesiloxane, 1, and three-branched siloxane crosslinker monomers were discovered to produce elastomeric net-work polymers at very rapid rates of formation. The flexible and transparent films ofthe saturated elastomeric network polymers were observed to possess low glass-tran-sition temperatures (below �35 8C). Similar hydrosilation reactions at two differentreactant ratios involving a diethynyl-containing carboranylenesiloxane, 2, and thesiloxane crosslinkers produced partially hydrosilated and completely hydrosilated poly-meric networked systems, which were transparent and elastomeric at room tempera-ture. The glass-transition temperatures of all the polymeric systems formulated from2 were below 0 8C. The elastomeric polymeric networks from 1 and 2 were found tohave degradation temperatures in the range of 500–550 8C. VVC 2005 Wiley Periodicals,

Inc.* J Polym Sci Part A: Polym Chem 44: 147–155, 2006

Keywords: ambient heterogeneous hydrosilation; colloidal; thermal, and thermooxi-datively stable carboranylenesiloxanes; crosslinking; high-temperature elastomers;Karstedt; polysiloxanes; silicones; siloxanes

INTRODUCTION

Materials with high-temperature thermooxida-tive stability and elasticity are needed for coat-ing and sealant applications in the aerospaceindustry. High-temperature elastomers are indemand for utilization in sealing assemblies oflanding gears and flight control systems and ascoating insulators for cables and capacitors. Theneed also exists in the electronic industry forhigh-temperature polymers for usage as resistlayers on computer chips. In this regard, linear

carboranylenesiloxane polymers stand out asexcellent candidates because of their exceptionalthermal, thermooxidative, and elastic proper-ties.1–4 The thermal and thermooxidative prop-erties of these linear polymers may be enhancedfurther by the modification of the chemistryfor their conversion into extended networkpolymers. The existing methods for the produc-tion of network polymers from a precursor car-boranylenesiloxane containing the respectiveside-chain unsaturated organic functionality areeither polymerization at the vinyl groups byorganic peroxides in air (at 315 8C for 300 h)4 orthermal crosslinking of the diacetylene groupsin an inert atmosphere in the range of 250–400 8C for several hours.5,6 These methods,being suitable for certain applications, involvehigh curing temperatures and protracted curing

Correspondence to: M. K. Kolel-Veetil (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 147–155 (2006)VVC 2005 Wiley Periodicals, Inc. *This article is a US Government work and, as

such, is in the public domain in the United States of America.

147

durations of the respective polymeric precursors.Under such conditions, a complete loss of theelasticity of the precursor polymeric product hasbeen observed to occur after the network forma-tion.5,6 Thus, the development of a less severeand more rapid curing protocol for the ambientformation of elastomeric crosslinked carborany-lenesiloxane network polymers is desirable.

In addition to the demonstrated free-radical-initiated high-temperature vulcanization (curing)of vinyls or thermal crosslinking of diacetylenes, acarboranylenesiloxane with an incorporated or-

ganic unsaturation such as a vinyl, ethynyl, ordiacetylene group can use the hydrosilationroute7–9 for its conversion to a network polymer.Under ambient conditions, chloroplatinic acid(Speier’s catalyst)-catalyzed hydrosilation re-actions (eq 1) at various vinyl-to-Si��H ratios(1.04, 0.53, or 0.27) involving a neat vinyl-con-taining carboranylenesiloxane, 1, and a reactivelinear polymeric siloxane crosslinker, poly(methylhydrosiloxane), have been reported10 toproduce plastic carboranylenesiloxane networkpolymers (eq 1):

These room-temperature network-forming reac-tions required up to 10 days for completion. Theplasticity of the reaction products and the slow-ness of these reactions make them unsuitable forapplications for which the rapid formation of elas-tomeric carboranylenesiloxane networks underambient conditions is desired.

This article reports the facile formation ofelastomeric networked polymers under ambientconditions from the Karstedt catalyst-catalyzedhydrosilation reactions of carboranylenesiloxanemonomers and monomeric branched siloxanecrosslinkers.

EXPERIMENTAL

The syntheses of vinyl-containing carboranylene-siloxane 1 and ethynyl-containing carboranylene-

siloxane 2 (Fig. 1) were performed under inertconditions with Schlenk line techniques accord-ing to published procedures.10,11 The reactionsoccurred in high yields. The clear and colorlessliquid products, 1 and 2, were purified by silicacolumn chromatography and characterized byFourier transform infrared (FTIR) and FT-NMRspectroscopies. They were reasonably stable inair. However, storage under argon in amber bot-tles was found to prolong their shelf lives.

Materials

Tetrahydrofuran (THF; anhydrous, 99.9%), vinyl-magnesium bromide (1.0 M in THF), and ethynyl-magnesium bromide (0.5 M in THF) were used asreceived from Aldrich. Tetrakis(dimethylsiloxyl)-silane (4 C-Ls), methyltris(dimethylsiloxyl)silane(3 C-Ls/Me), and phenyltris(dimethylsiloxyl)-silane (3 C-Ls/Ph) were used as received from

148 KOLEL-VEETIL AND KELLER

Gelest (Fig. 2). The Karstedt catalyst solution(platinum–divinyltetramethyldisiloxane complexin xylene, 2.1–2.4% platinum concentration) wasdiluted in xylene to half its concentration (�1.05–1.20%). Hexane was distilled over CaH2 and wasstored under argon. 1,7-Bis(chlorotetramethyldi-siloxyl)-m-carborane was purchased from DexsilCorp. and was used as received.

Instrumentation

Thermogravimetric analyses (TGAs) were per-formed on an SDT 2960 simultaneous DTA–TGA analyzer. Differential scanning calorimetry(DSC) studies were conducted on a DSC 2920modulated DSC instrument. All thermal experi-ments were carried out at a heating rate of 10 8C/min and at a nitrogen flow rate of 100 cc/min.Infrared spectra were obtained on thin filmsdeposited on NaCl disks with a Nicolet Magna750 FTIR spectrometer. Solution-state 1H NMRand 13C NMR spectra of 1 and 2 were acquired ona Bruker AC-300 spectrometer and referenced tothe internal solvent peak (CDCl3).

Experimental methods

Curing Reactions of 1 Involving Hydrosilation ofEach of Its Vinyl Units or of 2 Involving PartialHydrosilation of Each of Its Ethynyl Units

Compound 1 (0.2 g, 0.434 mmol) or compound 2(0.2 g, 0.438 mmol) was placed under ambientconditions in a reaction vial and was mixed vigo-rously for 2 min with the appropriate volume ofthe crosslinker 4 C-Ls (0.08 mL, 0.217 mmol), 3C-Ls/Me (0.09 mL, 0.289 mmol), or 3 C-Ls/Ph(0.10 mL, 0.289 mmol) with a mechanical stirrer.To this mixture, 0.5 mL of hexane was added andthoroughly mixed for 2 min. At this time, a drop(25 mL of a �1.1% Pt solution; 1.35 mmol of Pt) ofthe Karstedt catalyst solution was added to themixture with a 500-mL syringe and mixed byshaking. A transparent elastomeric product re-sulted upon the removal of the hexane in vacuo.

To produce well-formed transparent disks orfilms of all the products, each reaction solutionmixture was left under ambient conditions eitherin a vial or in a Teflon mold to yield a circular diskor a film, respectively, upon evaporation of thehexane.

Figure 1. Schematic representations of the reactive carboranylenesiloxanes, 1 and 2.

Figure 2. Schematic representations of the three-branched crosslinkers and theKarstedt hydrosilation catalyst.

FORMATION OF ELASTOMERIC NETWORK POLYMERS 149

Curing Reactions of 2 Involving CompleteHydrosilation of Each of Its Ethynyl Units

Compound 2 (0.2 g, 0.438 mmol) was placedunder ambient conditions in a reaction vial andwas mixed vigorously for 2 min with the appropri-ate volume of the crosslinker 4 C-Ls (0.16 mL,0.438 mmol), 3 C-Ls/Me (0.18 mL, 0.578 mmol),or 3 C-Ls/Ph (0.20 mL, 0.578 mmol) with a me-chanical stirrer. To this mixture, 0.5 mL of hex-ane was added and mixed as before for 2 min.This was followed by the addition of a drop (25 mLof a �1.1% Pt solution; 1.35 mmol of Pt) of theKarstedt catalyst solution with a 500-mL syringe,and the mixing was repeated. The workups toobtain the product and to fabricate well-formeddisks or films were performed as described in theprevious section.

RESULTS AND DISCUSSION

The selection of the reactant and catalyst speciesin the hydrosilation reactions for the ambient for-mation of elastomeric networks in this study wasmade by the careful evaluation of similar factorsin the reported hydrosilation reactions that hadyielded plastic products (eq 1). The production ofcolorless, hard plastic networks in the threereported10 reactions involving 1 and linear poly-(methylhydrosiloxane) at different vinyl to linearpolymer Si��H unit ratios (1.04, 0.53, and 0.27)suggested that the crosslinking densities in theproducts were very high, thereby thwarting anyelasticity in the products. For example, in theproduct formed with a 1.04 ratio, any pair of adja-cent crosslinked Si��C bonds in the backbone ofthe linear polymeric crosslinker was separatedonly by a single oxygen atom. The product fromthe reaction with the lowest ratio (0.27) of 1 andlinear poly(methylhydrosiloxane) also produced ahard, colorless solid. This was rather surprisingas the product should have had about 75% ofunreacted Si��H centers in the linear polymericcrosslinker backbone, which would have made itmore flexible than the products formed with thehigher ratios. If adjacent crosslinking Si��C bondshad formed uniformly, each pair of such bondsshould have been separated by as many as sevenatoms, which should have rendered the productmore flexible and therefore elastomeric. Becausethe product was plastic in nature, one is led to theassumption that this product and the other twonetworks formed at the higher ratios with poly-

(methylhydrosiloxane) preferentially formed net-work polymers that packed into highly orderedstructures. The results also imply that the re-action of 1 with any other polysiloxane such aspoly(dimethylmethylhydrosiloxane), which con-tains a lower number of Si��H units in the back-bone, would still result in plastic products. Hence,it was deduced that monomeric, rather than poly-meric, branched siloxane crosslinkers would bemore conducive to the retention of elasticity insuch hydrosilated products.

In the interest of producing elastomeric carbor-anylenesiloxane networks, monomeric branchedsiloxane crosslinkers such as 4 C-Ls, 3 C-Ls/Me,and 3 C-Ls/Ph were chosen as siloxane cross-linkers in this study. Among the crosslinkers, 4C-Ls has four dimethylsiloxyl branches, whereas3 C-Ls/Me and 3 C-Ls/Ph have three dimethylsi-loxyl branches, each with the fourth valence onthe central silicon atom being satisfied by either amethyl group or a phenyl group, respectively. Inthe three crosslinkers, a pair of consecutive reac-tive Si��H bonds was separated by three atoms(a silicon and two oxygens) as opposed to a singleoxygen atom in poly(methylhydrosiloxane). Thecarboranylenesiloxane unit in eq 1 was main-tained except for the additional utilization of itsethynyl variant. The introduction of either avinyl or ethynyl group into a carboranylenesilox-ane was achieved by the reaction of the appropri-ate Grignard reagent, vinylmagnesium bromideor ethynylmagnesium bromide, with 1,7-bis-(chlorotetramethyldisiloxyl)-m-carborane. In com-parison with the dark and opaque diacetylenecar-boranylenesiloxane-containing polymer precursorliquids used in network formations by thermal cur-ing,5,6 the facile Grignard reactions yielded clearprecursors 1 and 2 for the hydrosilation curing.

The slowness of the product formation in eq 1was an issue of concern in the development of afacile reaction system for the ambient formationof network polymers by a hydrosilation route.The sluggishness of the polymerization reactioncould be attributed to the reaction conditions,such as the choice of the reaction medium (neatreagents) and the catalyst that was used (Speier’scatalyst). It has been well established by Lewisand Lewis12 that the Karstedt catalyst, whichoperates by a heterogeneous mechanism similar tothat of Speier’s catalyst, is a more potent catalystas it forms much finer catalytically active colloidalPt particles during the hydrosilation initiationstep.12 Hence, the Karstedt catalyst was chosen asthe hydrosilation catalyst for this study. For the


hydrosilation reactions, the Karstedt catalyst solu-tion (platinum–divinyltetramethyldisiloxane com-plex in xylene, 2.1–2.4% platinum concentration),obtained from Gelest, was diluted in xylene tohalve its Pt concentration. Thus, the platinumconcentration in the diluted Karstedt catalyst sol-ution was approximately the same as that of theplatinum concentration (�1%) in the 0.5 MH2PtCl6 (Speier’s) catalyst THF solution used ineq 1 hydrosilations.

The hydrosilation reactions of 1 or 2 with theKarstedt catalyst and the branched crosslinkingsiloxanes were performed in air as is typical withall heterogeneous hydrosilation catalysts.13 Ini-tial trials of the reactions with neat reagentswere found to be instantaneous and extremelyexothermic. The observed rapidity in the reactionrate was in sharp contrast to the extremely slug-gish reactions (requiring 10 days) in eq 1, provid-ing yet another example of the superior reactivityof the Karstedt catalyst in comparison withSpeier’s catalyst. To control the exothermicityof the reaction and to maintain uniform proc-essing conditions of the products, the hydrosila-tion reactions were carried out in hexane. Be-

fore use, the hexane was distilled over CaH2,as it has been well established that residualmoisture reduces the efficiency of hydrosilationreactions.14 The presence of moisture in the sol-vent has been observed to result in the forma-tion of siloxane products, along with H2, by thehydrolysis of the metal–silicon bond during thehydrosilation propagation step. Additionally,deleterious side effects such as the poisoning ofthe catalyst and introduction of voids in theproducts are common.14

The hydrosilation reactions with equimolarratios of the reactants in hexane proceeded tocompletion within the initial hour. The reactionwas monitored by FTIR to observe the completedisappearance of the respective Si��H absorp-tion of the branched crosslinkers in the 2100–2300 cm�1 range. The extra unit of unsatura-tion in the ethynyl functionality in 2, in com-parison with the vinyl functionality in 1,afforded the possibility of an additional set ofhydrosilated products. The reaction of 1 witheach of the three crosslinkers produced a prod-uct that contained only saturated hydrosilatedorganic moieties (eq 2):

In the case of 2, the hydrosilation reactioninvolving only one of the two reactive bonds (orpartial hydrosilation) of each ethynyl unit yieldeda product that had a silylated unsaturated al-

kenyl moiety. This alkenyl moiety was then avail-able for further hydrosilation with another equiv-alent of the crosslinker, thereby yielding a moreextensively crosslinked product (eq 3):


The formation of the alkenyl units in the initialreaction of 2 with the crosslinker and the subse-quent conversion of this unsaturated materialinto a saturated crosslinked polymer upon fur-ther reaction were monitored by FTIR spectro-scopy. After the reaction with the initial equiva-lent of the crosslinker, the stretches at 2138(CBC) and 3290 cm�1 (C��H), attributed to theethynyl groups in 2, were completely absent. TheC¼¼C and C��H absorptions of the resulting unsa-turated alkenyl moieties were observed around1600 and 3050 cm�1, respectively. These absorp-tions disappeared upon the further reaction ofthe alkenyl product with another equivalent ofthe crosslinker. The observed hydrosilation of theinternal alkenyl units is rather uncommon asthere are very few reports of hydrosilations ofinternal unsaturations.15–17 The occurrence of aninternal hydrosilation during the production ofthe partially hydrosilated products of 2 (eq 3; 0.5-equiv 4 C-Ls reaction) was not observed as theFTIR spectra of the products do not containabsorptions either of the unreacted 2 or of the

completely hydrosilated product, both of whichwere expected to form in the event of an internalhydrosilation. The partially hydrosilated prod-ucts of 2 provided an important group of networkpolymers, as they contained uniformly dispersedunsaturations (alkenyl units) in an elastomericnetwork matrix. Thus, in addition to their utilityas substrates for conversion into the completelyhydrosilated products, they could be potentiallyused as elastomeric network precursors for thereactions involving additions across the unsatu-rations (alkenyl units).

Upon qualitative visual evaluations of thecrosslinked films or disks formulated from 1 and2, they appeared flexible and elastomeric at theambient temperature. The glass-transition tem-peratures (Tg’s) of the networks were determinedfrom their DSC thermograms (Fig. 3). Two dis-tinct regions of transitions were apparent inthe temperature ranges of �60 to �20 and 5 to30 8C, respectively. The transitions presumablyoriginated from two distinct domains in the net-works that were produced upon crosslinking, as


observed previously in the diacetylene-crosslinkedcarboranylenesiloxane networks.18,19 The promi-nent transitions below 0 8C were attributed to thedomain that contained the interconnecting unitsbetween the crosslinking sites. The transitionsabove 0 8C were assigned to the domain thatincorporated the crosslinking sites. In the net-works formed from 1 with the three respectivecrosslinkers, the products obtained from 3 C-Ls/Me and 3 C-Ls/Ph exhibited lower Tg values thanthe product formed from 4 C-Ls. Between the twothree-branched crosslinkers, the product obtainedfrom 3 C-Ls/Me possessed a lower Tg than the oneformed from 3 C-Ls/Ph, presumably because ofthe lesser detrimental impact of the methyl groupon the elasticity of the networked polymer than ofthe phenyl group. In siloxane chemistry, it is cus-tomary to replace methyl groups with planar aro-matic rings in siloxanes to increase Tg and thecrystallinity of the polymer.20 A similar trend wasobserved in the Tg values of the products formedfrom 2. For the products from the partial hydrosi-lation of the ethynyl groups in 2, the Tg values ofthe transitions of the interconnecting units in theregion below 0 8C were higher than those of simi-lar transitions of the products obtained from 1.The reason for this increase is unclear. Isolateddouble bonds in the backbone in polymers such asnatural, polyethylene, polypropylene, polybuta-diene, polyisobutylene, and poly(methyl metha-crylate) rubbers are known to increase the mobi-lity of vicinal single bonds, thereby making theregions away from the double bonds in the back-bone more flexible.21–24 Hence, the Tg values ofthe transitions in the region below 0 8C of theproducts from the partial hydrosilation of theethynyl groups in 2 yielding an alkenyl moietywithin the backbone should have been lowered. Asexpected, the products formed by the completehydrosilation of both ethynyl groups in 2 had thehighest Tg values among all of the networks gener-ated in this study.

The thermal and oxidative stabilities of thenetworks produced from the reaction of 1 and 2with the three crosslinkers were determined inboth N2 and air with TGA. The onset tempera-tures for the weight loss of the networks producedfrom 1 and 2 (both the partially and completelyhydrosilated networks) were in the ranges of300–325, 340–375, and 315–345 8C, respectively.The degradation temperature, defined as the tem-perature for 10% weight loss, for each of the threeproducts of 1 in both N2 and air was found to bebetween 510 and 570 8C (Fig. 4). The correspond-

Figure 3. DSC thermograms depicting the Tg val-ues of the networks from (a) reactions of 1 and cross-linkers, (b) reactions of a single reactive bond of eachethynyl unit of 2 and crosslinkers, and (c) reactions ofboth reactive bonds of each ethynyl unit of 2 andcrosslinkers.


ing degradation temperature ranges for the parti-ally and completely hydrosilated networks from 2were 530–590 and 520–570 8C, respectively. Thethermal stability up to 1000 8C was greater in airby at least 10% than in N2, as has been previou-sly observed with the diacetylene-crosslinkedcarboranylenesiloxane networks.5,6,18,19 The en-hanced weight retention in air could be attributedto the oxidation of the borons within the carbor-ane units. The thermooxidative stability wasfound to depend on the nature of the crosslinkerused in the hydrosilation reaction. Among thethree crosslinkers, the four-branched and methylthree-branched crosslinkers produced the mostand least stable networks, respectively. Thisobservation was not surprising. The greaterextent of crosslinking density resulting from thefour-branched crosslinker was expected to renderthe product more thermally stable. Between thetwo three-branched crosslinkers, the thermalstability of the generated networks followed the

order of the strength of the silicon bond to therespective group at the fourth valence on the cen-tral silicon atom. The established greater stabil-ity of a silicon–phenyl bond compared with thatof a silicon–methyl bond was thus reflected in theenhanced thermal performance of the correspond-ing network.20 Among the networks generated inthis study (Table 1), the products obtained fromthe partial hydrosilation of the ethynyl groups of2 with the three crosslinkers exhibited the great-est thermal stability upon heat treatment in bothN2 and air. The weight retentions were evenhigher than that for the networks formed by thecomplete hydrosilation of the ethynyl groups of 2.This unexpected result might be due to thermalcrosslinking of the alkenyl groups upon exposureto elevated temperatures. The degradation tem-peratures of the networked polymers producedfrom 2 were around 550 8C, affording them a mar-ginally larger range of application temperaturesthan that for the networked polymers of 1.

Figure 4. TGA thermograms to 1000 8C in N2 and in air depicting the weightretention of the networks obtained from 1 with the three crosslinkers.

Table 1. Thermal Data for the Three Sets of Networks Produced in This Study

1 þ2 (Single Bond perEthynyl Unit) þ

2 (Both Bonds ofEthynyl Unit) þ

4 C-Ls 3 C-Ls/Me 3 C-Ls/Ph 4 C-Ls 3 C-Ls/Me 3 C-Ls/Ph 4 C-Ls 3 C-Ls/Me 3 C-Ls/Ph

Subzero Tg (8C) �46 �59 �48 �28 �37 �36 �6 �23 �21Char yield (%)

N2 72 51 68 81 70 78 72 42 56Air 84 71 77 88 82 83 86 77 80

Onset temperature(8C; N2/air)

323/345 303/320 305/325 371/385 341/359 358/373 342/358 317/333 321/339

Degradationtemperature(8C; N2/air)

566/556 509/511 532/534 587/582 534/531 545/551 568/561 523/528 543/548


The stereochemistry of the products from theaddition of the crosslinkers, 4 C-Ls, 3 C-Ls/Me,and 3 C-Ls/Ph, across the unsaturated centers of1 and 2 could affect the thermooxidative proper-ties of the polymeric networks. In hydrosilationreactions, the stereochemistry of the additionproduct depends mainly on the nature of the cata-lytic metal.25 Because Pt-mediated hydrosilationsof alkynes and alkenes are well known26 to pro-ceed with the initial formation of a b-trans prod-uct, a similar mechanism could be in operation inthese systems.

CONCLUSIONS

This study has demonstrated that the elasticity ofthe networks formed by hydrosilation reactions ofcarboranylenesiloxanes can be controlled by acareful choice of the crosslinkers that are used insuch network-forming reactions. It has been wellestablished that a reduction in the crosslinkingdensity will improve the elasticity of a networkpolymeric system. In this study, the dilution of thecrosslinking density has been achieved by agreater spatial separation of neighboring cross-linking sites in the used crosslinker. This hasresulted in the discovery of potentially importantnetworked hydrosilated polymers that are formedfrom monomeric rather than polymeric precur-sors. The study has also successfully developed amethod for the rapid production of elastomericnetworks of carboranylenesiloxanes under ambi-ent conditions. By the optimization of the condi-tions, such as the choice of the catalyst for the het-erogeneous hydrosilation reaction, elastomerichigh-temperature networks have been realizedfrom carboranylenesiloxanes 1 and 2. The highonset temperatures for the weight loss (>300 8C)of all of the networks suggest that these networkscan be effectively used as high-temperature ther-mally and thermooxidatively stable elastomericmaterials up to 300 8C. In addition, the high deg-radation temperatures in the range of 500–550 8Cof these elastomeric networks bode well for theirmaterial properties and utility in high-tempera-ture applications.

The authors acknowledge the Office of NavalResearch for its financial support of this work. M. K.Kolel-Veetil thanks the American Society of Engineer-ing Education/Naval Research Laboratory Postdoc-toral Fellowship Program for its support.

REFERENCES AND NOTES

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