Synthesis of siloxane-crosslinked polysilylenemethylenes by chlorodephenylation

Synthesis of Siloxane-Crosslinked Polysilylenemethylenes byChlorodephenylation

TAKUYA OGAWA, SANG-DO LEE, MASASHI MURAKAMI

Research Center, Dow Corning Asia Limited, Yamakita, Kanagawa 258-0112, Japan

Received 27 July 2001; accepted 11 November 2001

ABSTRACT: Thermally stable polysilylenemethylenes (PSMs) with siloxane crosslink-ing moieties were successfully synthesized by chlorodephenylation of preformed poly-(methylphenylsilylenemethylene) (PMPSM) and subsequent in situ alcoholysis/hydro-lysis/condensation reactions. The simplified process and mild reaction conditions arequite advantageous. The crosslink density of these materials can be adjusted by thedegree of chlorodephenylation, although an alkoxysilyl group remains to some extent.The resulting crosslinked PSMs have well defined structures in which the backbone iscomposed of MePhSiCH2 and Me(MeO)SiCH2 as well as Me(O1/2)SiCH2 as a crosslink-ing moiety. The resulting crosslinked PSMs exhibited glass-transition temperaturesranging from 15 to 20 °C, whereas that of linear PMPSM was 22 °C. The crosslinkedPSMs remained unchanged in weight below 300 °C, suggesting that they are thermallystable up to that temperature. The good solvent resistance caused by crosslinking aswell as high thermal stability of these materials allow us to design new PSM-basedpolymer blends and preceramic polymers. © 2001 John Wiley & Sons, Inc. J Polym Sci PartA: Polym Chem 40: 416–422, 2002Keywords: polysilylene methylenes; polycarbosilanes; crosslinked polymers; chlo-rodephenylation; weight-loss temperature

INTRODUCTION

Polysilylenemethylenes (PSMs) in which thebackbone is composed of repeating Si–C units areone of the most well examined carbosilane poly-mers.1–21 Although several reports dealing withthe synthesis and pyrolytic behavior of PSMshave been published, the thermal and mechanicalproperties of these polymers have not been fullyclarified. Our recent articles have reported thesynthesis, fundamental physical properties,22

and thermal and mechanical properties23 of crys-talline poly(diarylsilylenemethylene)s as well asamorphous poly(methylphenylsilylenemethylene)

(PMPSM; [MePhSiCH2]n).24 These linear poly-mers are thermally stable with the 5% weight-loss temperatures of around 450 °C.

All of the studies described previously havefocused on linear polymers whereas few reportshave dealt with crosslinked and branch PSMs.There are basically two synthetic methodologiesto provide such nonlinear polymers. One is toform branch/network structures during the poly-merization process, whereas the other iscrosslinking/networking of preformed linear poly-mers by polymer reactions. Synthesis of Si–Hfunctional “hyperbranched” PSMs have been re-ported by Whitmarsh and coworkers25,26 andFroehling27 via a Grignard reaction and subse-quent reduction by using LiAlH4, whereas a pho-topatterning application of the Si–H functionalmaterials was reported by Fry and coworkers.28,29

The resulting materials are not real well defined

Correspondence to: T. Ogawa (E-mail: [email protected])Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 40, 416–422 (2002)© 2001 John Wiley & Sons, Inc.DOI 10.1002/pola.10118

416

“hyperbranched” polymers because they containmany structural defects including several cyclicstructures. The methodologies in the followingtwo articles30,31 can be classified into the lattercategory: Interrante reported network polycar-bosiloxane gels by sol–gel processing of an Si-OEtfunctional linear PSM and their pyrolytic conver-sion to silicon oxycarbide ceramics.29 Uhlig31 hasdeveloped triflic acid-catalyzed functionalizationmethods of preformed PSMs where polymeric si-lyl triflates derived from phenyl-substitutedPSMs were crosslinked by hydrosilylation or re-ductive coupling with potassium graphite. Thepolymer structures in Uhlig’s report are well de-fined, but it is surprising that the crosslinkedPSMs were soluble in chloroform and tetrahydro-furan despite their very high crosslink densities.

We have been pursuing thermally stable silicon-based thermoplastic materials with good solventresistance and toughness. We have demonstratedthe high thermal stability and good solvent resis-tance of crystalline poly(diphenylsilylenemethyl-ene),22,23 whereas improvement of toughness isnecessary to develop good materials. A polymerblending technology will be one of the candidatemethodologies to improve the brittleness ofPSMs. It is well known that viscosity matchingbetween the semicrystalline matrix and amor-phous-toughening domain components of polymerblends is critical to achieve a suitable morphologyresulting in good toughness of the materials.32

PSM-based polymer blends comprising linearPSMs, which will be published in a subsequentreport, lacked in solvent resistance and toughnessbecause of the presence of the amorphous compo-nent and insufficient viscosity matching betweenthe two components. The purpose of this study isto synthesize thermally stable crosslinked PSMswith well-defined structures and increased viscos-ities. Such material will be a suitable componentof polymer blends with good properties describedpreviously. In this article, we describe the conve-nient synthetic method, which can be conductedunder very mild conditions, of siloxane-cross-linked PSMs by chlorodephenylation of preformedPMPSM using iodine monochloride (ICl) and sub-sequent alcoholysis/hydrolysis/condensation reac-tions. PMPSM was chosen due to its very highthermal stability24 as compared with that of pol-ysiloxanes. The thermophysical properties of thecrosslinked materials were studied by differentialscanning calorimetry (DSC) and thermogravimet-ric differential thermal analysis (TG-DTA).

EXPERIMENTAL

Materials and Characterization Methods

All solvents and ICl were purchased from WakoPure Chemicals Ind., Ltd. and used without fur-ther purification. Synthesis of PMPSM is de-scribed elsewhere.24 Gel permeation chromatog-raphy (GPC) was performed using chloroform asan eluent with a Tosoh HLC-8020 GPC apparatusequipped with two TSKgel GMHHR-H columnsand a refractometer. The weight-average molecu-lar weight (Mw) and the polydispersity of PMPSMcalculated using polystyrene standards were 4.5� 105 and 3.5, respectively. Infrared (IR) spectrawere obtained with a Jasco FT/IR-5300 spectro-photometer. Proton NMR spectra of chloroform-d(CDCl3) solution and solid-state 29Si cross-polar-ization/magic-angle spinning (CP/MAS) NMRspectra were recorded with a Bruker ACP300spectrometer. Tetramethylsilane and 3-trimeth-ylsilyl-1-propanesulfonic acid sodium salt wereused as external standards for solution and solid-state measurements, respectively. DSC was car-ried out under a nitrogen atmosphere using aSeiko DSC 6200. The glass-transition tempera-tures (Tg’s) of the crosslinked samples were ob-tained from the second heating scan at a heatingrate of 20 °C/min. TG-DTA was performed using aRigaku TG8101D at a heating rate of 10 °C/minin air.

Synthesis of PSMs with SiOOOSi CrosslinkingMoieties

To the carbon tetrachloride (CCl4; 50 mL) solutionof PMPSM (3.0 g; 22.3 mmol as a repeating unit)was added a CCl4 solution of ICl (7.1 mL; 3.3mmol as ICl) at room temperature, and the re-sulting light orange solution was stirred for 90min, during which the solution turned light pink.The homogeneous solution thus obtained waspoured into 1000 mL of a methanol (MeOH)/watermixture (90/10 v/v) to yield a colorless solid. Theproduct was dried at 80 °C at ambient atmo-sphere for 1 h and then under reduced pressurefor 8 h. A colorless solid (2.72 g) was obtained in96% yield.

IR (thin film, cm�1): 2800–3100 (CH3 andCH2), 1427 (Si-phenyl), 1254 (Si-methyl), 1111(Si-phenyl), 1055 (SiOC and SiOOOSi), 941(SiOOMe), and 700–800 (phenyl). 29Si NMR (CP/MAS, solid state): �4.8 (ASiMePh), 8.3[ASiMe(O1/2)], and 15.9 [ASiMe(OMe)].

POLYSILYLENEMETHYLENES 417

RESULTS AND DISCUSSION

Synthesis of PSMs with SiOOOSi CrosslinkingMoieties

Chlorodephenylation of PMPSM To Yield Si–ClFunctional PSM

It has been well known that an Si-aryl linkage iseasily converted to an Si-X (X: an anionic compo-nent of acids) group by reacting with strong proticacid such as HCl/AlCl3 and CF3SO3H. Becausethese reagents are not very tractable because oftheir hygroscopic properties, we examined the useof ICl as a dephenylation reagent. Synthesis ofaryl iodides by halodesilylation of trimethylsilyl-substituted aromatic compounds has been re-ported by Jacob et al.33 and Yamaguchi et al.34

They used a 2 mol of AgBF4 in addition to a 2 molof ICl below 0 °C to activate the electrophilicity ofICl. In this study, only ICl was used for chlorode-phenylation, not halodesilylation,35 of preformedPMPSM. An overall reaction scheme is shown inScheme 1.

One can monitor the progress of the reaction bythe increase of a new resonance at 7.7 ppm in the1H NMR spectrum. This resonance, assignable tothe orthoprotons of iodobenzene, was separatelyobserved from the remaining Si-phenyl protons asshown in Figure 1.

In addition, new resonances at around 0.1–0.5ppm assignable to methyl protons on chlorine-sub-stituted silicons were observed, whereas only oneresonance for methyl protons was recorded at �0.1ppm for unreacted PMPSM (not shown in Fig. 1).The intensity of the resonance assignable to theSi-phenyl protons decreased as expected. The NMRresults indicate that the intended reaction quanti-tatively proceeded within 2 h at room temperatureto yield Si–Cl functional PSM. Furthermore, it wasalso demonstrated by a GPC analysis that the mo-lecular weight of the Si–Cl intermediate remainedessentially unchanged by this reaction.36 The de-gree of chlorodephenylation proved to be controlledby the amount of ICl. The Si–Cl functional PSMs, asintermediate materials obtained by various degreesof chlorodephenylation, were used without isolation

for the subsequent hydrolysis/alcoholysis/condensa-tion reactions.

Crosslinking of Si–Cl Functional PMPSM

Various procedures for hydrolysis/alcoholysis andsubsequent crosslinking were examined to obtaincrosslinked PSMs. Because iodobenzene as a by-product is soluble in alcohol, it seems very conve-nient that a solution of Si–Cl functional PSMcontaining iodobenzene can be poured into alcoholto separate an insoluble polymeric material fromthe byproduct. The synthetic procedures are sum-marized in Table I.

Formation of Crosslinked PSM by Simple Precip-itation into Solvents. Pouring the CCl4 solution ofSi–Cl functional PSM into water as run #1 re-sulted in phase separation, and therefore a solidproduct was not obtained. An insoluble materialwas obtained in good yield by just pouring thereaction solution into MeOH (run #2) and n-hex-ane (run #6). The IR spectra of the products at thewave-number range between 800 and 1200 cm�1

for runs #2 and #6 are depicted in Figure 2 alongwith that of unreacted PMPSM.

Scheme 1. Synthesis of PSMs with siloxane crosslinking moieties.

Figure 1. 1H NMR spectrum in CDCl3 of the reactionmixture after 1.5 h stirring at room temperature. Theamounts of substrates are described in the experimen-tal section. A resonance at 1.4 ppm is assignable toSi-OH formed by hydrolysis of Si-Cl during preparationof an NMR sample in an ambient atmosphere.

418 OGAWA, LEE, AND MURAKAMI

One can find a discernible difference at thecharacteristic absorption at 1055 cm�1, the ab-sorption for the SiOCOSi backbone. The absorp-tion bands in Figures 2(a,b) are broader than thatin Figure 2(c) because of an absorption ofSiOOOSi linkages overlapped on the SiOCOSiabsorption. The IR spectrum of the product de-picted in Figure 2(a) showed a sharp absorptionat 941 cm�1, which is indicated with an arrow inthe figure, assignable to an Si-OMe functional-

ity37 derived by alcoholysis of Si-Cl with MeOH.This assignment was supported by an experimentusing n-hexane as a solvent for precipitation (run#6). An absorption was observed at 938 cm�1 inFigure 2(b), but the intensity was very weak,nearly background level. This is because no alco-holysis took place in this run. No characteristicband assignable to Si-OH was observed for the IRspectra of these products, although they are notshown in this text. Resonances centered ataround 8 and 16 ppm in the 29Si NMR spectrumdepicted in Figure 3(a) indicate that the productof run #2 has'SiOOOSi' and'SiOOMe func-tionalities. According to these spectrometric anal-yses, the product of run #2 is PSM with SiOOOSicrosslinking moieties and residual SiOOMegroups. In this case, condensation or crosslinkingreactions of SiOOMe groups would have takenplace in the drying process after alcoholysis ofSiOCl groups by the precipitation process.

Modified Procedures To Increase the Contents ofCrosslinked Moieties. Because reactive SiOOMegroups remained in the crosslinked product after

Table I. Synthetic Procedures forHydrolysis/Alcoholysis and Subsequent Crosslinkinga

Run Procedures

1 Pour directly into water.2 Pour directly into MeOH.3 Heat with KOH/water (reflux), wash with

water, and pour into MeOH.4 Heat with water (reflux), and pour into

MeOH.5 Pour directly into an MeOH/water

mixture (90/10 v/v).6 Pour directly into n-hexane.

a The degree of chlorodephenylation was 15% for all runs.Each product was dried under reduced pressure to removesolvents after procedures in this table.

Figure 2. IR spectra of (a) the product of run #2, (b)the product of run #6, and (c) unreacted PMPSM.

Figure 3. 29Si CP/MAS NMR spectra of (a) the prod-uct of run #2 and (b) the product of run #5.


the simple precipitation procedure as describedpreviously, some modified procedures were exam-ined to increase the degree of crosslinking. Inruns #3 and #4, the CCl4 solution was heatedunder reflux with excess water in the presence(#3) and absence (#4) of base catalyst, expectingthat the SiOCl groups could be hydrolyzed tosilanol groups. In run #3, a washing process wasused to neutralize the reaction medium. The IRspectra of the two products were almost identicalto that of the product of run #2 indicating that aheterogeneous hydrolysis reaction did not pro-ceed. Taking these results into account, the Si–Clfunctional PSM solution was poured into aMeOH/water mixture in run #5. One can expecthydrolysis of SiOCl to SiOOH as well as alcohol-ysis of SiOCl to SiOOMe. In addition, the forma-tion of siloxane bonds by condensation betweenthe SiOOH thus formed and SiOCl is possibleduring the precipitation process. The product af-ter the drying process was an insoluble solid withSiOOOSi crosslinks and residual SiOOMe func-tionalities according to the NMR and IR spectra.The [SiOOOSi]/[SiOOMe] molar ratio estimatedby the 29Si NMR spectrum was much larger thanthat for the product of run #2 as shown in Figure3(b). This is definitely due to the presence of wa-ter used in the precipitation solvent. Condensa-tion or crosslinking reactions of SiOOH/SiOOMegroups should have proceeded in both the precip-itation and drying processes. We may be able toincrease the water content in the precipitationsolvent, but the increase would result in de-creased solubility of iodobenzene in the solvent.Although the MeOH/water mixing ratio of theprecipitation solvent was not optimized, this ratiowas used in the following experiments to synthe-size PSMs with different crosslinking densities.The 29Si NMR spectrum of crosslinked PSM withthe highest crosslinking density (sample CL-5 in

Table II) is depicted in Figure 4. The [SiOOOSi]/[SiOOMe] molar ratio is approximately 4.2, indi-cating that more than 80% of the SiOCl groupswere converted to siloxane crosslinks. In otherwords, more than 40% of the phenyl groups ofPMPSM were converted into crosslinking moi-eties.

Although several procedures for hydrolysis/con-densation reactions have been examined, we couldnot obtain completely crosslinked PSM in which allSiOCl groups formed by chlorodephenylation con-verted into crosslinks. This is probably due to thesterically hindered location of the functional groupson a high molecular weight polymer. On the otherhand, the remaining SiOOMe groups can be re-garded as reactive groups for reactions under moresevere conditions.

Tg’s of Crosslinked PSMs

The Tg’s of the crosslinked PSMs thus obtainedwere measured by DSC and are listed in Table IIalong with that of linear PMPSM. The Tg’s werein the range between 15 and 20 °C, whereas thatof PMPSM was 22 °C. They tend to decrease withincreasing crosslink densities of the samples. Thedecrease in the Tg can be explained by the re-duced number of phenyl groups that function as

Table II. Tg’s of Crosslinked PSMs with VariousDegrees of Chlorodephenylation

Sample Number Degree of CD (%)a Tg (°C)

CL-1 5 20CL-2 10 19CL-3 15 17CL-4 30 16CL-5 50 15PMPSM 0 22

a Degree of chlorodephenylation.

Figure 4. 29Si CP/MAS NMR spectrum of CL-5.


bulky substituents to suppress the movement ofthe polymer main chains. It is intriguing that amaterial with a higher crosslink density exhibitslower Tg.

Thermal Weight-Loss Behavior of Crosslinked PSMs

The thermal weight-loss behavior is one of theimportant properties when one considers the ma-terials for applications where thermal stability isvery critical. The TG traces of crosslinked PSMs(CL-3 and CL-5) are depicted in Figure 5(A) alongwith the data for linear PMPSM. All samplesremained unchanged in weight below about 300°C, suggesting that the crosslinked PSMs are alsothermally stable up to that temperature. CL-3starts its weight loss at a temperature somewhatlower than PMPSM, resulting in a lower 5%weight-loss temperature (Td5) of 420 °C, whereasthat of PMPSM was 467 °C.24 The Td5 valuesdecreased with an increasing degree of crosslink-

ing. In fact, the Td5 of CL-5 was 360 °C. Thislowered thermal stability of the crosslinked PSMscan be explained by an exothermic reaction be-tween 300 and 550 °C as depicted in the DTAtraces [see Fig. 5(B)]. Because CL-5 exhibited alarger heat flow than CL-3 in that temperaturerange, decomposition reactions involving SiOOMegroups such as further crosslinking via condensa-tion are likely to be the main cause of the exother-mic reaction.

Despite the lower temperatures for an initialweight loss of the crosslinked PSMs, these poly-mers exhibited larger residual weights thanPMPSM after heating to 800 °C as shown in Fig-ure 5(A). The crosslinked PSMs exhibited slight(CL-3) or small (CL-5) weight loss above 650 °C,whereas the weight loss for PMPSM above 600 °Cwas quite large. These are advantageous proper-ties when one designs preceramic polymers. Wehave reported that a second stage of thermalweight loss of PMPSM above 600 °C in air isattributable to oxidative decomposition that ischaracterized by a very large exothermic peak inFigure 5(B).24 These TG and DTA data indicatethat the crosslinked PSMs do not tend to undergooxidative decomposition. In fact, the residue fromeach crosslinked PSM after heating to 800 °C wasa black solid consisting of amorphous SiC plusfree carbon. On the other hand, the residue fromPMPSM after a similar thermal treatment was anoff-white solid forming a silica-like material.24

This rather intriguing phenomenon also suggeststhat the crosslinked PSMs undergo thermal deg-radation by a mechanism that is different fromthat of PMPSM. Thus, we expect to be able todesign PSM-based polymer blends with good ther-mal and solvent-resistant properties. Improve-ment in the mechanical properties of the blendscan also be expected because the viscoelasticproperties, such as the melt viscosities, of thecrosslinked PSMs were significantly altered bythe introduction of crosslinking.33 These studieswill be reported in a subsequent article.

CONCLUSIONS

A chlorodephenylation reaction of a preformedPSM and subsequent in situ alcoholysis/hydroly-sis/condensation reactions were used to synthe-size a thermally stable PSM with SiOOOSicrosslinking moieties. The high conversion andmild reaction conditions are notable. The result-ing crosslinked PSMs have well defined struc-

Figure 5. (A) TGA and (B) DTA traces of crosslinkedPSMs (CL-3 and CL-5) and unreacted PMPSM.


tures in which the backbone is composed of MePh-SiCH2 and Me(MeO)SiCH2, with Me(O1/2)SiCH2as a crosslinking moiety. Crosslinked PSMs withwide structural variety can be synthesized by al-tering the molecular weights of PMPSM and thedegree of chlorodephenylation. Although thereare some residual alkoxysilyl groups on thecrosslinked PSM, it is possible to use them asreactive sites at elevated temperatures.38 Thischlorodephenylation reaction can be applied tovarious carbosilane-based polymers with Si-arylgroups to impart additional functional groups. Wecan also design PSM-based polymer blends withgood thermal and solvent-resistant properties byusing this crosslinking technology.

The authors appreciate the New Energy and IndustrialTechnology Development Organization for financial sup-port of this study that was conducted as a part of theIndustrial Science and Technology Frontier Program.

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Synthesis of siloxane-crosslinked polysilylenemethylenes by chlorodephenylation