Journal of Polymer Research (2005) 12: 393–401 © Springer 2005DOI: 10.1007/s10965-005-1765-x

Synthetic Method Development and Molecular Weight Control for Homo- andCo-Polysilynes, Silicon-Based Network-Backbone Polymers

David A. Smith1, Scott J. Joray2 and Patricia A. Bianconi2,∗1Department of Chemistry, Pennsylvania State University, University Park, PA 16802, USA2Department of Chemistry, The University of Massachusetts at Amherst, Amherst, MA 01003, USA(∗Author for correspondence; Tel.: 413-665-3587; Fax: 413-545-4990; E-mail: bianconi@chem.umass.edu)

Received 19 October 2004; accepted in revised form 4 February 2005

Key words: network polymers, network polysilanes, polysilynes, silicon polymers, synthetic methods

Abstract

Optimized methods for the syntheses of polyaryl, polyalkyl, and co-polysilynes, using sonochemically-emulsified NaKalloy reductions of trichlorosilanes, are reported. The methods give oxygen and solvent-impurity free polymers, byFTIR and 1H NMR, for poly(phenylsilyne) 1, poly(n-butylsilyne) 2, poly(ethylsilyne) 3, poly(n-propylsilyne) 4, andpoly(cyclohexylsilyne) 5. Syntheses of the novel polysilynes poly(isobutylsilyne) 6, poly(amylsilyne) 7, and poly(phenyl-co-butylsilyne) 8 are also reported. The polymers were assessed for the photooxidation behavior that is characteristic of theseinorganic network-backbone polymers. A method of molecular weight control for polysilynes, by combining monomers thatbear substituents of differing steric size (n-butyltrichlorosilane and cyclohexyltrichlorosilane), is also reported.

Introduction

Polysilynes are silicon-based network backbone polymers,in which each silicon is bonded via three silicon–siliconbonds to the backbone, and bears one R substituent. Theirσ -conjugated three-dimensional network backbone confersinteresting electronic, optical, and preceramic properties onthese materials [1–4]. They have been studied as photopat-ternable thin-film waveguides [5, 6], as low band-gap energysemiconducting silicon-based materials [7–11], as modelsfor the luminescence properties of amorphous silicon [12],and as photoresists for 193 nm photolithography [13, 14].They have also been studied as precursors for silicon-basedceramics, such as amorphous silicon films [15], silicon car-bide [16, 17], silica, and mixtures of these three [18–20].Poly(arylsilynes) can also be functionalized by treatmentwith acid and reaction with a variety of nucleophiles to givenovel polymers that are not accessible via the ultrasonicreduction synthetic method [4, 21].

Different synthetic methods of variously-substitutedpolysilyne polymers have, however, not always been op-timized. Many reports have been made of synthesis ofthe polymers by condensing trichlorosilanes with sodiummetal in refluxing toluene, sometimes with the additionof crown ethers [8–10, 12, 15, 17]. These products canbe slightly contaminated with oxygen or chlorine or both,in the polymers’ network backbones. Electrochemical syn-theses of homopolymers and copolymers have also beenreported [22–24]; these polysilyne materials are more pure,but still contain trace amounts of oxygen as siloxanes andhydroxide and alkoxide substituents. Reduction of phenyl-

trichlorosilane with C8K also gives a product containingsome siloxanes [21].

We report here optimized syntheses for a variety ofhomo- and co-polysilynes, using the method of reduc-tive condensation of trichlorosilanes by an ultrasonically-generated emulsion of NaK alloy in organic solvents. Thesematerials are free from oxygen impurities, by FTIR, andfrom solvent-derived impurities, as assessed by 1H NMR.We also discuss a method of molecular weight control forthe copolysilynes.

Experimental

All polymers were synthesized in a Vac Atmospheres sys-tems dry-nitrogen purge glovebox. A Heat Systems Ultra-sonics Inc. ultrasonic immersion horn equipped with a 0.5′′tip was used within the glovebox, and connected througha power cord to an external W-385 ultrasonic processorunit (also Heat Systems Ultrasonics Inc.; 475 W, 20 kHz).All solvents (Baker Analyzed) were dried through distilla-tion over sodium/benzophenone ketyl prior to use. LiquidNaK alloy was synthesized in the glovebox by slowlyadding solid potassium (Aldrich) to an equimolar amountof molten sodium (Aldrich) in a 1 L beaker. All polysi-lyne monomers (n-butyltrichlorosilane, phenyltrichlorosi-lane, methyltrichlorosilane, n-propyltrichlorosilane, iso-butyltrichlorosilane, amyltrichlorosilane, n-hexyltrichloro-silane, tolyltrichlorosilane, and cyclohexyltrichlorosilane)were obtained from Huls America or Silar and were used asreceived. All Grignard and lithium reagents (Aldrich) werealso used as received.

394 David A. Smith et al.

Fourier transform infrared (FTIR) spectra were obtainedon an IBM FTIR-32 spectrophotometer in percent transmit-tance. All samples were run for 80 scans and displayedbetween 4000 cm−1 to 450 cm−1. Polymer films for FTIRanalysis were cast from THF solutions onto KBr disks.1H and 13C nuclear magnetic resonance (NMR) spectra wereobtained on Bruker WP200, AM300, and/or WM360 spec-trometers. NMR spectra were run in either chloroform-d ordichloromethane-d2 solvents and referenced to an externaltetramethylsilane standard. Gel permeation chromatography(GPC) was performed by Scientific Polymer Products, Inc.of Ontario, New York. The instrument used was a PerkinElmer Series 10/LC25 R1. The system had a THF flowthrough Waters ultrastyragel columns (105 and 500 Å), andvalues obtained were based on linear polystyrene standards.Electronic spectra were obtained using a Hewlitt-Packard8450A electronic spectrophotometer and measured over200–800 nm in cylcohexane solvent.

FTIR peaks are reported as strong (s), medium (m), orweak (w) and are considered sharp and resolved unless notedotherwise (e.g., br = broad). All NMR peaks are singlets(unless otherwise noted) and reported as strong (s), medium(m), or weak (w). Relative broadness (br) or sharpness (sh)is also indicated.

Poly(phenylsilyne) 1

Tetrahydrofuran (250 mL) was added to a 600 mL beakerand sonicated at full power. NaK alloy (8.85 g, 142.5 mmol,285 reducing equivalents) was added to the sonicated sol-vent, generating a light blue emulsion. The mixture of reduc-ing agent and solvent was sonicated for three to five minutesto ensure optimum emulsification. Phenyltrichlorosilane(16.0 mL, 100 mmol) was diluted with ca. 20 mL pentane.The solution was added dropwise in a controlled mannerover eight minutes to the emulsion while sonication con-tinued at full power. The final reaction mixture was thensonicated for an additional five min to ensure complete re-action of the monomer. During the reaction, heat generatedfrom the high intensity ultrasonic irradiation caused an evo-lution of solvent vapor. Lost solvent was replaced with freshTHF throughout the reaction to maintain an approximatevolume of 200 mL. The glovebox was on a constant purgecycle of dry nitrogen to ensure the removal of THF vaporthroughout the reaction.

Once sonication was finished, an aliquot of theblue/black reaction mixture was removed from the glovebox,and hydrolyzed with 15 mL of distilled water. An acidicpH confirmed a total consumption of reducing agent, anda slight excess of unreacted silyl-chloride sites.

Phenylmagnesium chloride (2.0 M in THF) was added toa stirred polymer solution to substitute any remaining Si–Clsites with the appropriate phenyl substituent (“endcapping”).A neutral or slightly basic pH test from a hydrolyzed aliquotof the polymer solution confirmed the complete replacementof chloro sites with phenyl groups, and the mixture wassubsequently removed from the glovebox and quenched bythe slow addition of 200 mL of water, upon which the blue-black reaction mixture turned orange-yellow. The quenched

mixture was stirred for five to ten minutes to ensure sol-vation of NaCl and KCl byproduct in the aqueous phase.The clear, colorless aqueous phase was removed with a sep-aratory funnel, leaving behind the orange/yellow organicphase. An approximately equal volume of methanol wasadded to the THF solution, resulting in the precipitation ofyellow poly(phenylsilyne), 1. The precipitate was separatedby gravity filtration. The polymer was then redissolved inTHF, giving a clear orange solution which was reprecipitatedwith an equal volume of ethanol. The final, purified polymerprecipitate was once again removed through gravity filtra-tion and dried under vacuum. The sequential precipitationsremoved oligomers soluble in the respective alcohols. Theyield for the yellow soluble poly(phenylsilyne) solid 1 was3.95 g (39%). GPC Mw = 4160, polydispersity = 2.14.1H NMR δ = 7.0 (br). 13C {1H} NMR δ = 134.7 (br),137.6 (br). IR (neat film on KBr, cm−1) 3043 (s), 2924 (w),2088 (w), 1954 (w), 1887 (w), 1816 (w), 1483 (m), 1426 (s),1187 (w), 1156 (w), 1089 (s), 1067 (m), 1022 (m), 1000 (s),911 (w), 849 (w), 799 (w), 732 (vs), 693 (vs), 617 (w). Theelectronic spectrum showed a broad absorption band from200 nm, trailing down into the visible to 400 nm.

Poly(n-butylsilyne) 2The preparation of poly(n-butylsilyne) was first publishedby Bianconi et al. [2]. An optimized synthetic procedureindicating sonication times and addition rates is given here.In the glovebox, 17.0 mL (100 mmol) n-butyltrichlorosilanewas combined with 200 mL pentane in a 600 mL beakerand sonicated at full power (475 W) for three minutes. NaKalloy (8.85 g, 142.5 mmol, 285 reducing equivalents) ofwas added dropwise over five to seven minutes to the soni-cated solution. Vaporized solvent was replenished with freshpentane to maintain a net volume of at least 150 mL. Theglovebox was on a constant slow purge cycle throughoutthe reaction to ensure the removal of evolved solvent vapor.The mixture was sonicated for an additional eight min, then200 mL of THF was slowly added as sonication continued.Once THF addition was complete, sonication again contin-ued for an additional eight min. Endcapping, as describedabove, was performed with n-butyllithium (n-BuLi, 2.5 Min hexane), which was added until the pH of a hydrolyzedaliquot tested seven.

The reaction mixture was removed from the gloveboxand quenched with water. Workup and purification was iden-tical to 1. Final yield of the dark yellow polymer powder 2was 4.45 g (50%). GPC Mw = 22,000, polydispersity =2.0. 1H NMR δ = 1.34 (m, br), 0.88 (s, br). 13C NMRδ = 32.3 (w, br), 27.2 (m, br), 13.8 (s, br). IR (neat filmon KBr, cm−1) 2952 (s), 2925 (s), 2869 (m), 1464 (m),1375 (m), 1174 (w), 1073 (m), 1012 (w), 866 (m), 672 (s).The electronic spectrum showed a broad absorption bandfrom 200 nm, trailing down into the visible to 450 nm.

Poly(ethylsilyne) 3Anhydrous pentane (100 mL) and 4.1 g (25 mmol) of eth-yltrichlorosilane were added to a dried 600 mL beaker. Thesolvent and ethyltrichlorosilane were irradiated at full power

Synthetic Methods for Polysilynes 395

by immersion of the horn for three minutes. NaK alloy(2.23 g, 37.5 mmol, 75 reducing equivalents) was addeddrop-wise over a period of 8 minutes. A few milliliters ofTHF were added drop-wise, slowly enough to control thevigorous reaction, and then THF (100 mL) was added. Son-ication was continued for a further eight minutes duringwhich methyllithium (4 mL, 1.4 M in ether) was added.The reaction was then sonicated for a further eight minutes.Water (100 mL) was then slowly added to the reaction mix-ture while stirring, upon which the mixture turned yellow.The aqueous and organic layers were separated, and solventwas removed from the yellow organic layer under vacuum.Yields = 1.03 g (72%). GPC Mw = 7233, polydispersity =2.5. IR (neat film on KBr, cm−1): 2973, 2862 (vs), 1460,1245 (vs), 1069 (vs), 903 (m), 861 (m). 1H NMR: 0.37,very broad. 13C NMR δ = 32.3 (w, br), 13.8 (s, br). Theelectronic spectrum showed a broad absorption band from200 nm, trailing down into the visible to 400 nm.

Poly(n-propylsilyne) 4The preparation of poly(n-propylsilyne) was first publishedby Bianconi et al. [2]. An optimized synthetic proce-dure indicating sonication times and addition rates is givenhere. The procedure for poly(n-propylsilyne) synthesis wasidentical to that of poly(n-butylsilyne) (2) using the samemolar amounts of monomer (n-propyltrichlorosilane) andreducing agent (NaK). Endcapping was performed with n-butyllithium (n-BuLi, 2.5 M in hexane), which was addeduntil the pH of a hydrolyzed aliquot tested seven. The reac-tion mixture was removed from the glovebox and quenchedwith 200 mL of water, and the THF layer was then removedwith a separatory funnel. After precipitation and isolationof the polymer from a THF/methanol solution, the solidpolymer was dissolved with 400 mL hexane. A high molec-ular weight fraction of poly(n-propylsilyne) was insolubleand was separated from the soluble portion with gravityfiltration. The remaining poly(n-propylsilyne) solution wasrotary evaporated to dryness and redissolved in THF. Theyellow THF solution was then reprecipitated sequentiallywith methanol and ethanol as described for 1. The polymerwas dried in a vacuum chamber, giving a yellow solid with ayield of 4.05 g (57%). GPC Mw = 6942, polydispersity =2.3. 1H NMR δ = 1.41 (m, br), 0.99 (s, br). 13C NMRδ = 19.16 (br, w) 24.15. The FTIR and electronic spectrawere identical to those of 2.

Poly(cyclohexylsilyne) 5Poly(cyclohexylsilyne) was synthesized by the sameprocedure as outlined in the optimized synthesis ofpoly(phenylsilyne) 1, with cyclohexyltrichlorosilane as themonomer. Endcapping was performed with n-butyllithium(n-BuLi, 2.5 M in hexane), which was added until the pHof a hydrolyzed aliquot tested seven. Quenching and repre-cipitation procedure followed that for (1). The final productyield was 7.85 g (71%) of an orange/yellow polymer pow-der. GPC Mw = 1210, polydispersity = 1.23. 1H NMRδ = 1.73 (s, br), 1.18 (s, br). 13C {1H} NMR δ = 34 (m, br),29 (s, br), 27 (s, br). FTIR (neat film on KBr, cm−1) 2925 (s),

2750 (s), 1455 (m), 1174 (w), 1073 (w), 990 (m), 872 (w),795 (w), 672 (w). The electronic spectrum showed a broadabsorption band from 200 nm, trailing down into the visibleto 450 nm.

Poly(iso-butylsilyne) 6The preparation of poly(iso-butylsilyne) was based uponthe optimized synthesis of poly(n-butylsilyne), 2. Molaramounts of the reactants were the same, using the monomeriso-butyltrichlorosilane. Endcapping was performed with n-butyllithium (n-BuLi, 2.5 M in hexane), which was addeduntil the pH of a hydrolyzed aliquot tested seven. Quench-ing, reprecipitation, and purification was identical to theprocedure outlined for 1. The poly(iso-butylsilyne) yieldwas 1.81 g (21%). GPC Mw = 2645, polydispersity = 2.8.1H NMR δ = 1.82 (w, br), 1.32 (w, br), 1.0 (s, br). 13CNMR δ = 33.1 (w, br), 28.3 (m, br), 13.8 (s, br). The FTIRspectrum was identical to that of 2 with an additional peakat 1390 cm−1 (m). The electronic spectrum showed a broadabsorption band from 200 nm, trailing down into the visibleto 400 nm.

Poly(amylsilyne) 7The synthesis of poly(amylsilyne) was based upon the opti-mized procedure for 2, again with the same molar amountsof NaK and amyltrichlorosilane as the monomer. Endcap-ping was performed with n-butyllithium (n-BuLi, 2.5 Min hexane), which was added until the pH of a hydrolyzedaliquot tested seven. Quenching and purification procedurewas identical to that reported for 1. The final product was2.38 g (24%) of a yellow solid polymer. GPC Mw =2240, polydispersity = 2.2. 1H NMR δ = 1.41 (m, br),0.87 (s, br). 13C {1H} NMR δ = 31.2 (w, br), 28.8 (w, shoul-der), 26.4 (br), 13.8 (sh). The electronic spectrum showed abroad absorption band from 200 nm, trailing down into thevisible to 450 nm. The FTIR spectrum was identical to thatof 6.

Poly(phenyl-co-n-butylsilyne) 8This copolymer was synthesized using the optimizedpolysilyne synthesis procedure for poly(phenylsilyne) 1.A 30 : 70 monomer ratio of phenyltrichlorosilane : n-butyltrichlorosilane was added simultaneously from separatepentane solutions to a sonicated emulsion of NaK/THF. Thefinal product substituent ratio of phenyl : n-butyl as deter-mined through 1H NMR integration was 22 : 78. Polymeryield was 44%, GPC Mw = 16615, polydispersity = 2.75.1H NMR δ = 7.07 (s, br), 1.18 (s, br), 0.83 (s, br),aryl : alkyl = 1H : 0.32H. FTIR (neat film on KBr, cm−1)3043 (m), 2952 (m), 2925 (s), 2869 (m), 1426 (s), 1259 (w),1084 (s), 1067 (w), 1022 (m), 1000 (w), 799 (w), 732 (s),693 (s). The electronic spectrum showed a broad absorptionband from 200 nm, trailing down into the visible to 400 nm.

Poly(cyclohexyl-co-n-butylsilyne) 9, 10, 11, 12The following copolysilynes were synthesized by the pen-tane/THF two-step cosolvent procedure used for poly(n-butylsilyne) (2). The monomer percentages (of 100 mmol

396 David A. Smith et al.

Table 1. Monomer percentages for poly(cyclohexyl-co-n-butylsilynes)

Copolymer 9 10 11 12

n-butyltrichlorosilane (%) 95 90 80 70

Cyclohexyltrichlorosilane (%) 5 10 20 30

Table 2. Yields and molecular weights for poly(cyclo-hexyl-co-n-butylsilynes)

Yield(g)

Yield(%) GPC Mw/P a

9 3.90 45 18, 300/2.26

10 3.80 43 18, 600/3.57

11 3.63 40 11, 800/3.02

12 5.00 54 5650/1.94

aP = polydispersity.

total) used are shown in Table 1. Endcapping was performedwith n-butyllithium (n-BuLi, 2.5 M in hexane), which wasadded until the pH of a hydrolyzed aliquot tested seven.Quenching, workup, and polymer purification proceduresfollowed those for 1.

Mass yields, percent yields, and GPC molecular weightsare listed in Table 2. 9: 1H NMR δ = 1.41 (s, sh),1.36 (s, br), 0.90 (s, br). 10: 1H NMR δ = 1.7 (w, shoulder),1.35 (s, br), 0.89 (s, br). 11: 1H NMR δ = 1.7 (w, shoulder),1.36 (s, br), 0.91 (s, br). 13C {1H} NMR δ = 32.39 (w, br),28.8 (w, shoulder), 27.08 (m, br), 13.77 (s, sh). 12: 1H NMRδ = 1.72 (m, shoulder), 1.36 (s, br), 0.89 (s, br). FTIR spec-tra for 9–12 were identical. FTIR (neat film on KBr, cm−1)2952 (s), 2925 (s), 2750 (s), 1464 (m), 1455 (m), 1375 (w),1147 (w), 1073 (m), 1012 (m), 872 (w), 866 (w), 795 (w),672 (m).

Results and Discussion

The reaction conditions for the synthesis of known and novelpolysilynes were developed and refined so optimum polymeryield and quality could be obtained. Reductive condensationreactions of chlorosilanes are typically carried out in hydro-carbon solvents in the presence of a polar ethereal solventsuch as THF. The presence of a polar solvent apparentlystabilizes the formation of silyl intermediates and aids thepolymerization process. However, alkyltrichlorosilanes arereactive with THF, since they can be initiators for its ring-opening polymerization. Therefore, for poly(alkylsilynes), atwo-step cosolvent system is employed [2]. The monomeris first reacted with an ultrasonic emulsion of NaK alloy innonpolar pentane. The monomer becomes oligomerized toa point where the reaction of silyl chlorides with THF issterically improbable, and a polar solvent, THF, can thenbe introduced. The silyl intermediate stabilizing effect ofTHF allows polymerization to efficiently continue until allthe reducing agent has been consumed.

Another advantage of the use of THF is its solubiliz-ing powers. Polysilynes, particularly poly(phenylsilyne), are

less soluble in nonpolar solvent such as pentane. Therefore,as the monomer polymerizes in pentane, it becomes increas-ingly less soluble and the reaction more heterogeneous. Theintroduction of THF resolubilizes the forming polymer andallows for a more homogeneous reaction environment inwhich to complete the reaction. For poly(phenylsilyne), theuse of an initial nonpolar solvent is unnecessary as phenyl-trichlorosilane is relatively unreactive with THF. The resultis minimal side reactions between monomer and solvent,giving a pure polysilyne product.

Because of the polysilynes’ tendency to decompose torefractory silicon carbide-based ceramics upon heating [16–20], chemical analyses can unpredictably underreport thepercentages of carbon and silicon; also, conventional com-bustion analysis cannot detect oxygen, the most commonimpurity incorporated into polysilyne backbones. Spectro-scopic evidence therefore is the most reliable assay ofthe polysilynes’ purity. For poly(phenylsilyne) synthesizedby this method, no solvent-based impurities are seen inNMR studies (Figure 1), and the polymer product has rel-atively narrow polydispersities (<2.2). FTIR spectroscopyof poly(phenylsilyne) 1, (Figure 2), shows the presence ofexpected poly(phenylsilyne) bands, but also the absence ofimpurities. The inefficient synthesis of a polysilyne can re-sult in the formation of broad Si–O–Si and/or Si–H bandswhich can be seen in IR spectra at 1050 and 2088 cm−1,respectively.

A possible complication during the synthesis of polysi-lynes is overreduction of the polymer or underreduction ofthe trichlorosilane monomer. In these optimized syntheses,the monomer is purposely underreduced, and residual silyl-chlorides are removed through an endcapping procedurewith an appropriate alkyllithium or Grignard reagent. In allpolysilyne syntheses, a 2.85 equivalent of reducing agentis used with a 1.0 equivalent of trichlorosilane monomer.The use of a stoichiometric amount (or higher) of reducingagent, i.e. greater than 3.0 equivalents to 1.0 equivalent ofan RSiCl3 monomer, results in overreduction and the for-mation of silyl anions (Scheme 1). Conversely, if less than2.85 equivalents of reducing agent is used, the growing poly-mer retains so many chloride groups that capping procedurescannot efficiently remove them (Scheme 2). In either case,when the polymer is quenched with water, impurities suchas Si–H or Si–O–Si bonds are observed. Schemes 1 and 2illustrate the formation of these impurities which result fromoverreduction and underreduction, respectively [2].

Overreduction of polysilyne monomers produces silylanions, which occurs when NaK alloy attacks already-formed silicon–silicon bonds. When water is added toquench the reaction products, the silyl radical reacts to forma silyl hydride and sodium/potassium hydroxides. The silylhydrides have a stretching frequency in the IR region at2088 cm−1, thereby allowing the qualitative evaluation ofoverreduction products in polysilyne syntheses. Conversely,in severe underreduction (Scheme 2), excessive residualchlorides, the result of using less than 2.85 equivalents NaKalloy, are difficult to remove through a capping procedure.When the polymer reaction mixture is quenched, hydroly-

Synthetic Methods for Polysilynes 397

Figure 1. 1H NMR spectrum of poly(phenylsilyne) 1 in CDCl3.

Figure 2. IR of poly(phenylsilyne) 1 (neat film on KBr).

sis of the remaining silyl-chlorides occurs, forming silanolgroups and hydrochloric acid. Neighboring silanol groupsundergo a condensation reaction which forms an Si–O–Silinkage and water. This condensation can be intramolecularor intermolecular. If two silanol groups exist on neighboringsites of the same polysilyne fragment, they may intramolec-ularly condense. However, if two silanol groups on separatepolysilyne fragments condense, a crosslink is formed be-tween the two fragments. This intermolecular crosslinkingwill form high molecular weight polymer which become in-soluble as the degree of crosslinking increases. The results ofthis process can be viewed by the formation of a large, broadIR absorption band at 1050 cm−1, which allows qualitativeevaluation of this impurity.

Although the synthesis of 1 through the THF/NaK emul-sion route seemed sufficient, modifications in the procedurewere performed in an attempt to develop additional generalprocedures to further optimize the product yield. One syn-thetic variable which was manipulated was the power of theultrasound used. In one variation, a 2.5 x ultrasonic boosterhorn was used to ascertain whether a significant increase insonication power would give better polymer yields. Total ul-

Scheme 1. Overreduction products from a polysilyne synthesis.

Scheme 2. Underreduction products from a polysilyne synthesis.

trasonic outputs of 712.5 W and 950 W was used to generatethe NaK alloy/THF emulsion. The high ultrasonic powerwas applied throughout the monomer addition process as

398 David A. Smith et al.

well as during additional sonication times. The polymeryields from high ultrasound immersion were reasonable, butspectroscopy showed undesirable impurities.

An intense Si–H stretch at 2088 cm−1 was evident in theFTIR spectra of poly(phenylsilyne) produced by high powerultrasound irradiation. Also, sharp resonances were preva-lent in these materials’ 1H NMR spectra in the region from1.4 to 1.15 ppm. As stated above, a large Si–H peak in the IRis indicative of overreduction. Sharp resonances seen in the1H NMR spectra alkyl region also indicate the possibilityof monomer and solvent side reactions and/or decomposi-tion products. The high intensity of the ultrasonic irradiationused is the most likely source of the undesired reactivity,since these resonances do not appear in poly(phenylsilyne)synthesized using the normal 475 W sonication power. Highpower irradiation thus appears to be destructive of polymerproduct quality.

Other attempted modifications of the poly(phenylsilyne)syntheses included variations in solvent systems. A THF/di-methoxyethane (glyme) one-step cosolvent system was at-tempted. A small amount of glyme (20 mL) combined with180 mL THF was treated with NaK as described in thesynthesis of 1. As stated earlier, the use of more polarsolvents in polysilane polymerization reactions tends to in-crease the polymer yield and molecular weight. Therefore,the addition of a more polar solvent (glyme) may have im-proved the reduction efficiency and polymer yield. However,the yield of 1 obtained using this solvent system was low(9%) and showed spectroscopic impurities similar to poly-mers obtained from high-power ultrasound methods. Sharpresonances were seen in the alkyl region of the 1H NMRspectrum, and a reasonably intense Si–H stretch appeared inthe IR at 2087 cm−1. It was concluded that an increase insolvent system polarity caused overreduction of the polymerand formation of impurities, probably by reducing the size ofparticles of the NaK emulsion by the greater solvation powerof glyme.

Another attempted synthetic method utilized a two-stepcosolvent system (similar to the procedure for polyalkylsi-lynes, as in that for 2 above) with toluene as the preliminarysolvent. A solution of phenyltrichlorosilane and toluene wassonicated at full power, and used in the reduction of themonomer with added THF as outlined in the synthetic pro-cedure for 2. The polymer yield was extremely low (2%),and spectral data showed impurities like those obtained pre-viously, as well as siloxane (Si–O–Si) formation. The FTIRspectrum had a reasonably large Si–H peak at 2087 cm−1

and a broad, obscuring Si–O–Si peak centered at 1050 cm−1.The 1H NMR spectrum showed sharp resonances in thealkyl region. This data was not only indicative of overre-duction, but underreduction as well, as indicated by thestrong, broad Si–O–Si IR peak. Using toluene in this mannerwith glyme as the secondary added solvent also producedonly impure polymer. Toluene therefore appears to be aninappropriate solvent for poly(phenylsilyne)-NaK emulsionpolymerization methods.

The most efficient synthetic method for obtaining purepoly(phenylsilyne) is described in the above synthesis for 1.

Table 3. Sonication and reagent addition times for poly(n-butylsilyne) 2

Monomer/pentanesonication

NaKaddition

Pre-THFsonicationinterval

Sonicationafter THFaddition

Yield(%)

3 min 5–7 min 8 min 8 min 50

3 min 5 min 5 min 8 min 4.9

A one-solvent system of THF in an ultrasonic emulsion withNaK alloy, sonicated at 475 W, was the optimum reductionenvironment. Monomer was added to the emulsion whilesonicating, which reacted to form poly(phenylsilyne) withreasonable yield and good quality.

Poly(alkylsilynes)

The synthetic methods used to produce poly(alkylsilynes)were based upon those published by Bianconi et al. [2]. Son-ication times for each stage of the reaction was optimizedfor poly(n-butylsilyne), 2. Table 3 shows a comparison ofaddition rates and sonication times for each preparation.Method 1 for the synthesis of 2 gave a substantial improve-ment in yield, showing that a longer sonication time prior tothe addition of THF is critical to polymer formation.

Poly(ethylsilyne) 3, poly(n-propylsilyne) 4, poly(iso-butylsilyne) 6, and poly(amylsilyne) 7 were all synthesizedbased on the procedure developed for 2. Reactant additionrates and sonication time intervals were relatively consis-tent and each synthesis generated reasonable yields of highquality polymer. 3, 6 and 7 are novel polysilynes that havenot been reported previously. The basic synthetic method,however, for poly(alkylsilynes) seems applicable to thesesystems as well. Modifications were needed only in thecase of 3, poly(ethylsilyne). The growing polymer is fairlyinsoluble in pentane, so three reducing equivalents (as op-posed to 2.85) must be used to achieve enough reductivecondensation for endcapping to efficiently remove chlorine.However, the reduction of the ethyltrichlorosilane monomerin the presence of THF is so vigorous that the reductionmust be started in pentane, and THF must at first be slowlyadded in small aliquots to control the reaction. In addition,the methyllithium endcapping reagent must be sonicatedwith the reaction mixture in order to remove the final tracesof chloride. Only with these modifications could soluble,impurity-free poly(ethylsilyne) be obtained.

The synthesis of a copolysilyne from a mix of aryl/alkylmonomers, poly(phenyl-co-n-butylsilyne) 8 was also at-tempted, and the procedure optimized for 1 was foundto be successful; these sorts of aryl/alkyl polymers werereported to be unobtainable with C8K as the reducingagent [21]. The procedure optimized for 1 also producespoly(cyclohexylsilyne) 5 in good purity and yields.

One characteristic common to polysilynes is the pho-tooxidation of a film when it is exposed to UV light andoxygen. To verify this behavior in previously-unreportedpolymers, photooxidation trend studies were performed oneach of the new polysilynes synthesized. Thin films of thepolymer on KBr disks were exposed to UV irradiation in

Synthetic Methods for Polysilynes 399

Figure 3. Photooxidation of poly(isobutylsilyne) 6. FTIR progression showing growth of the broad Si–O–Si band at 1050 cm−1 with exposure of 6 to UVirradiation in air: (A) unexposed, (B) five minutes exposure, (C) ten minutes exposure, (D) fifteen minutes exposure.

a five mercury lamp open air exposure chamber. Figure 3is a spectroscopic illustration of the photooxidation processfor a film of poly(iso-butylsilyne) 6. FTIR data demon-strate the growth of an Si–O–Si stretch near 1050 cm−1 asa film of 6 is irradiated with UV light in the presence ofair for progressively longer periods of time. All the newpolysilynes display similar photooxidation behavior, fur-ther demonstrating that this reactivity is characteristic ofpolysilynes.

Poly(cyclohexyl-co-n-butylsilyne) 9–12 and MolecularWeight ControlThe synthesis of homopolymer poly(cyclohexylsilyne) 5was successfully accomplished through the synthetic pro-cedure developed for poly(phenylsilyne) 1. The proce-dure gave polymer in high yield (71%) and high quality,verified by FTIR and NMR analysis. The success of apoly(alkylsilyne) synthesis with the poly(arylsilyne) methodwas attributed to the bulk of a cyclohexyl substituent, which

400 David A. Smith et al.

may have sterically hindered the side reaction of the cyclo-hexyltrichlorosilane with the THF solvent system. Photoox-idation studies of the final product through exposure to UVand air once again verified an inherent property of polysi-lynes by showing the growth of a Si–O–Si stretch similarto that of Figure 3. The GPC molecular weight of 5 pro-vided the first data point in a study of molecular weighttrends based upon copolymers with different sized organicsubstituents in varied ratios.

The molecular weights of polysilynes are often prob-lematic: very high molecular weights lead to insolubility,very low ones to oils. Even soluble polymers of high mole-cular weight can present problems: for example, poly(n-butylsilyne), with GPC Mw of 22,000, cannot pass througha 0.25 micron frit, as can a linear polymer of this size range.The rigidity of the network backbone does not allow anychange in conformation in solution in order to pass throughthe frit pores. Filtering on such a level is necessary to removedust or other impurities for many photonic and electronicapplications [5, 6, 11, 13, 14]. A general trend in polysilynesis that the more sterically bulky the sidechain, the lowerthe molecular weight. Therefore the synthesis of copoly-mers of trichloroalkylsilynes was performed in order to see ifthis synthetic method could give a desired molecular weightrange.

The synthesis of copolyalkylsilynes was straightforward,as the procedure developed for homopoly(alkylsilynes)was directly applicable. Table 1 shows synthesis ra-tios of the comonomers cyclohexyltrichlorosilane andn-butyltrichlorosilane, which were confirmed in the finalcopolymers by integration of the cyclohexyl and butylgroups in the copolymers’ 1H NMR spectra. The FTIRand NMR data showed consistently high quality copoly-mer products, and all copolymer yields (Table 2) werereasonable.

The predicted trend in GPC molecular weight was basedupon the differing steric sizes of the cyclohexyl and n-butylgroups. When copolymers with varied ratios of these twosubstituents were synthesized, a trend should be apparent,where copolymers with a higher degree of n-butyl sub-stituents would have a higher GPC molecular weight thancopolymers with a higher degree of cyclohexyl substituents.Figure 4 illustrates that this was the case. This was the firstanalysis of substituent effects upon copolysilyne molecularweight. It suggests that polysilyne molecular weights andthus properties can be tailored by judicious incorporation ofsterically differing sidechains into the same polymer back-bone. Although these GPC molecular weights, relative topolystyrene standards, are estimates only, the results foundby GPC are internally consistent, showing that the increasingtrend with sterically smaller sidechains seen in copolymermolecular weights is an accurate one.

Conclusions

This research describes repeatable syntheses of high qual-ity polysilyne polymers, which can be used in polysilynefunctionalization, electronic, and photooxidation studies.

Figure 4. GPC molecular weight trend for poly(cyclohexyl-co-n-butyl-silynes).

Though several methods of polysilyne synthesis are known,this work describes optimization of the sonochemically-generated NaK emulsion method. Poly(phenylsilyne) is theprimary starting material for functionalization experiments,and consistently high quality product is necessary in order toeffectively and systematically probe the optimum function-alization reaction conditions for that polymer. The syntheticmethod optimized for poly(n-butylsilyne) has been shownto be generally useful in the synthesis of a variety of otherpoly(alkylsilynes), and the synthesis of a novel aryl/alkylcopolymer was performed. Molecular weight control of thepolymers can be achieved by polymerizing two monomerswith sterically differently-sized R groups.

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