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POLYCARBOSILANES Introduction Polycarbosilanes (1) have a polymeric backbone composed of silicon atoms and difunctional organic groups that connect the silicon atoms (1–4).

The side groups R can be alkyl, aryl, or alkoxy groups, and hydrogen or halogen atoms. The bridging group R can vary widely from methylene to alkenylene, alkynylene, arylene, and a combination of these groups. Because of this structural variation, polycarbosilanes can exhibit various interesting properties that neither polysiloxanes nor the usual hydrocarbon polymers can achieve. Polycarbosilanes generally have a very flexible and chemically stable backbone derived from Si C bonds. Applications to ceramic precursors, conductive polymers, heat-resistant materials, and side-chain liquid crystalline polymers have been widely investigated. Methods commonly applied to prepare polycarbosilanes are ring-opening polymerization (ROP) of silacarbocycles, cross-condensation of dihalosilanes with difunctional organometallic compounds, and self-condensation of difunctional silicon compounds. Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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Table 1. T g ’s of Poly(silylenemethylene)s and Related Hydrocarbon Polymers Polymers [Si(CH3 )2 CH2 ]n [Si(CH3 )(CH2 CH3 )CH2 ]n [Si(CH3 )(n-C4 H9 )CH2 ]n [Si(CH3 )(n-C6 H13 )CH2 ]n [SiH(CH3 )CH2 ]n [SiH(n-C6 H13 )CH2 ]n [SiH(C6 H5 )CH2 ]n [CH(CH3 )CH2 ]n [CH(n-C6 H13 )CH2 ]n [CH(C6 H5 )CH2 ]n

Tg , ◦ C −87 −78 −63 −71 −112 −91 −37 −10 to −20 −65 100

Poly(silylenemethylene)s Poly(silylenemethylene)s, (SiR2 (CH2 ))n , polymers composed of alternating SiR2 and CH2 , are the simplest and the most extensively investigated polycarbosilanes. They can be regarded as the carbon analogues of polysiloxanes. They have been known for almost half a century, but keen interest has been paid since Yajima discovered that poly(silylenemethylene)s can be SiC ceramic precursors in 1975 (5–7). Poly(silylenemethylene)s have quite low glass-transition temperatures (T g ) owing to the relatively long Si C backbone bonds, as shown in Table 1. A direct comparison of T g ’s between asymmetrically substituted poly(silylenemethylene)s and the corresponding carbon analogues revealed the high flexibility of the poly(silylenemethylene)s (8,9). Poly(1,1-dimethylsilylenemethylene) (2) was first synthesized by reaction of ClCH2 Si(CH3 )2 Cl with sodium metal (eq. 1) (10), but the molecular weight of the polymer was relatively low (M w = 850).

(1) Higher molecular weight poly(silylenemethylene)s (3) can be prepared by ROP of 1,3-disilacyclobutanes (eq. 2).

(2)

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gave Heating of 1,1,3,3-tetramethyl-1,3-disilacyclobutane to 300◦ C poly(dimethylsilylenemethylene) (M w = 15,000) (11). On the other hand, platinum-catalyzed ROP of 1,1,3,3-tetramethyl-1,3-disilacyclobutane was reported in 1965 (12). Later, several researchers investigated other transition-metalcatalyzed ROP of 1,3-disilacyclobutane derivatives (13–19). Platinum-catalyzed ROP of 1,1,3,3-tetrachloro-1,3-disilacyclobutane (20,21) and 1,1,3,3-tetraalkoxy1,3-disilacyclobutane has been reported (22). The platinum-catalyzed ROP is thought to be the most efficient and convenient way so far to obtain high molecular weight poly(silylenemethylene)s. Successive transformations of preformed polymers containing Si X (X = H, Cl, OR) groups have been used to prepare a wide range of functionalized poly(silylenemethylene)s (eq. 3) (23–25).

(3) Polymers with mesogenic groups on silicon atoms have also been synthesized (26).

Poly(silyleneethylene)s Poly(silyleneethylene)s, composed of alternating SiR2 and (CH2 )2 , are synthesized by a catalytic hydrosilation of HSi(CH3 )2 CH CH2 (27,28). There are naturally copolymers of (Si(CH3 )2 CH2 CH2 ) and (Si(CH3 )2 CH(CH3 )) , owing to α- and β-addition. More efficient synthesis of this series of polymers is attained by hydrosilation of HSiCl2 CH CH2 (29–31). In this case, the obtained poly(dichlorosilyleneethylene) (4) can be transformed into poly(dihydrosilyleneethylene) (5) by treatment with LiAlH4 (eq. 4), which can be converted to SiC ceramics in high yield.

(4)

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Poly(silylenetrimethylene)s Poly(silylenetrimethylene)s (6), consisting of a Si (CH2 )3 easily synthesized by the ROP of silacyclobutanes (eq. 5).

backbone, are most

(5) Thermal ROP of 1-methylsilacyclobutane proceeds at 50◦ C (32), but those of 1,1-dimethylsilacyclobutane and 1-chloro-1-methylsilacyclobutane need high temperatures like 150–200◦ C (33) and 230◦ C (34), respectively. Platinum-catalyzed ROP of silacyclobutanes has also been reported (12,35). Other transition-metalcatalyzed ROPs have also been investigated (36). Very efficient Pt catalysts for the ROP of 1,1-dimethylsilacyclobutane has been recently reported (37). Polymers with oligooxyethylene pendant groups (38) or liquid crystalline mesogenic groups (26,39) are synthesized by utilizing the Pt-catalyzed polymerization. Anionic ROP of silacyclobutanes, which was first reported (40,41) in 1964, is also a powerful method for the preparation of poly(silylenetrimethylene)s (42,43). Recently, butyllithium-induced anionic polymerization of vinyl- (44) or hydride-substituted (45) silacyclobutanes was accomplished in THF at −78◦ C in the presence of hexamethylphosphoramide (HMPA). The produced poly(hydrosilylenetrimethylene) can be transformed to other poly(silylenetrimethylene)s having various pendant groups by hydrosilation with olefinic compounds such as allyl cyanide and allyl pentafluorobenzene (46). Organolithium-induced living anionic polymerization of 1,1-dialkylsilacyclobutanes was reported (47–51). This method is very attractive because it is applicable for synthesis of various block copolymers (eq. 6) (52,53).

(6) Self-assembly of amphiphilic block copolymers can be a new research area for applications of polycarbosilanes (54–56). An alternative route to prepare poly(silylenetrimethylene)s is hydrosilation of allylsilane derivatives bearing a Si H group. [C6 H5 SiCl(CH2 )3 ]n was synthesized by self-hydrosilation of allylchlorophenylsilane with Karstedt’s catalyst (57). The synthesis of

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poly(silylenetrimethylene) was reported, with a high degree of stereoregularity. Poly(methylphenylsilylenetrimethylene) rich in isotacticity was obtained by polyaddition of optically active allylmethylphenylsilane using platinum catalyst (58).

Other Polycarbosilanes with One Silicon and Three Carbons in the Repeating Unit. Various polymers consisting of one silicon and three carbons in the repeating backbone unit can be prepared by ROP of silacyclobutane derivatives. For instance, poly(silylenemethylene1,2-phenylene), (Si(CH3 )2 CH2 -o-C6 H4 )n , can be synthesized from 1,1dimethyl-2,3-benzosilacyclobutene (59,60). Poly(silylene-2-propenylene), (Si(CH3 )2 CH2 CH CH)n was synthesized from 1,1-dimethylsilacyclobutene (61). Poly(silylene-2-methylenetrimethylene), (Si(CH3 )2 CH2 (C CH2 )CH2 )n , was obtained from 1,1-dimethyl-3-methylenesilacyclobutane (62). Self-hydrosilation of propargylsilane also provides poly(silylene-2-propenylene). Optically active poly(methylphenylsilylene-2-propenylene) was synthesized by stereoselective polyaddition of (R)-methylphenylpropargylsilane using platinum catalyst (63).

Poly(silylene-2-butenylene)s Poly(silylene-2-butenylene)s, (SiR2 CH2 CH CHCH2 )n (7), can be readily prepared by butyllithium-induced anionic ROP of silacyclopent-3-ene derivatives in THF at −78◦ C in the presence of HMPA (eq. 7) (64).

(7) The polymerization proceeds with high stereoselectivity to provide polymers having Z geometry around C C double bonds with relatively narrow molecular weight distributions. Vinyl (65) groups and hydride (66) on the silicon atom are tolerant to the polymerization conditions and functional polymers bearing various pendant groups such as oligooxyethylene (67) or liquid crystalline mesogenic group (68) can be prepared. Another method to prepare poly(silylene-2butenylene)s is acyclic diene metathesis (ADMET) polymerization of diallylsilanes, (CH2 CHCH2 )2 SiRR (where R, R = CH3 or Cl) catalyzed by tungsten carbene complex, [{(CF3 )2 CH3 CO}2 (N-2,6-C6 H3 -(i-(C3 H7 )2 )Pr2 )W CHC(CH3 )2 R] (where R = CH3 or C6 H5 ) (69,70).

Poly(silylenevinylene)s In 1962, it was reported that platinum-catalyzed condensation of methylphenylsilane and diethynylmethylphenylsilane gave low molecular weight

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poly(methylphenylsilylenevinylene) (SiCH3 C6 H5 CH CH2 )n (71). High molecular weight poly(silylenevinylene)s, (SiR2 CH CH2 )n (8), can be prepared by platinumcatalyzed hydrosilation of dialkyl-substituted ethynylsilane. For instance, poly(dimethylsilylenevinylene) (M w = 30,380), poly(diethylsilylenevinylene) (M w = 110,700), and poly(methylphenylsilylenevinylene) (M w = 11,400) can be synthesized by hydrosilation of the corresponding dialkylethynylsilane, HSiR1 R2 C CH (eq. 8) (72).

(8)

Melt-spun fibers from these polymers are thermally converted to SiC fibers in good ceramic yield. Another way to synthesize poly(silylenevinylene) is ruthenium- or rhodium-catalyzed polycondensation of dimethyldivinylsilane (73).

Poly(silylenealkynylene)s Because silicon is able to conjugate electronically with adjacent alkyne π bonds, poly(silylenealkynylene)s, (SiR2 C C)n (9), can be expected to exhibit conducting or nonlinear optical properties. A very efficient method to synthesize poly(silyleneethynylene) has been reported. Dilithium acetylide prepared from trichloroethylene and butyllithium was treated with Cl2 SiR2 to give high molecular weight polymers (M w = 30,000, when R = C6 H5 ; M w = 20,000, when R = C6 H5 , CH3 ) (eq. 9) (74).

(9)

However, these polymers were found to be very poor conductors, even after doping with iodine. Another type of acetylenic polymer, poly(silylenediethynylene), (SiR2 C C C C)n (10), can be synthesized by condensation of Cl2 SiR1 R2 (R1 , R2 = CH3 , C2 H5 , or C6 H5 ) and 1,4-dilithio-1,3-butadiyne prepared from 1,4bistrimethylsilylbut-1,3-diyne and methyllithium (eq. 10) (75).

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(10) These polymers exhibit conductivity (10 − 5 – 10 − 3 S·cm − 1 ) after oxidation with FeCl3 , which is in the range of semiconductors. Application of polymer 10 as SiC ceramic precursors has also been investigated (76).

Poly(silylenearylene)s Poly(dimethylsilylene-p-phenylene), (Si(CH3 )2 -p-C6 H4 )n (11), is successfully synthesized via condensation of p-chlorophenyldimethylchlorosilane and sodium (eq. 11).

(11) The polymer showed significantly high thermal stability and was not decomposed even at 400◦ C (77). It was reported that poly(silylenephenylene)s can be synthesized from the reaction of dichloromethylsilane or dichlorophenylsilane with p- and m-dilithiobenzene (78). An efficient method to prepare poly(silylene-mphenylene) (12)(M w = 4000–20,000), that is, a ruthenium-catalyzed desilylative condensation of m-bis(trihydrosilyl)benzene (eq. 12), was reported (79).

(12) The synthesis and the high heat-resistant property of poly{[bis (ethynylphenyl)silylene]phenylene} have also been reported (80). Poly [(diethoxysilylene)phenylene] was first synthesized by a reaction of (3bromophenyl)triethoxysilane with magnesium. Then the polymer was treated with (trimethylsilylethynyl)phenyllithium, followed by deprotection of the trimethylsilyl groups.

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Poly(silylenealkenylenearylene)s. Poly(dimethylsilyenephenyleneviny lene)s were synthesized by chloroplatinic acid catalyzed self-hydrosilation of o-, m-, and p-(dimethylsilyl)phenylacetylene (81). These polymers are interesting as not only ceramic precursors but also blue-light-emitting materials. Poly(methylphenylsilyelenephenylenevinylene) and poly (diphenylsilyelenephenylenevinylene) were also prepared by chloroplatinic acid-catalyzed hydrosilation of methylphenylsilane or diphenylsilane with 1,4-diethynylbenzene (82). These polymers had both β- and α-hydrosilation addition structures in the vinylene moieties. Poly(silylenealkynylenearylene)s. The first poly(silylenealkynylene arylene) to be reported was poly(dimethylsilyleneethynylene-pphenyleneethynylene), (Si(CH3 )2 C CC6 H4 C C)n , which was prepared by polycondensation of the disodium salts of p-diethynylbenzene with dimethyldichlorosilane (83). A more efficient method was reported, which is a palladium-catalyzed polycondensation of dihaloaromatics and diethynyldiphenylsilanes (eq. 13).

(13) This polymerization can be applied to a wide range of condensations between diethynyldiphenylsilane and aromatic halides including pyridine and thiophene derivatives (13) (84). It was reported that dehydrogenative coupling of phenylsilane with m-diethynylbenzene in the presence of MgO to provide poly(silylene-mdiethynylenephenylene) (14) (eq. 14) (85,86).

(14)

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This polymer exhibited significantly high heat-resistant and burn-resistant properties, which is due to cross-linking of the polymer backbone through Si H and C C bonds (85,86).

Poly(methylene(silacylohexanylene))s Polycarbosilanes with silicon-containing six-membered rings in the backbone can be synthesized by cyclopolymerization of organosilicon monomers bearing two olefinic substituents. Poly(methylene-(1-sila-3,5-cyclohexanylene))s (15) are obtained by cyclopolymerization of dialkyldiallylsilanes. Coordination polymerization by Ziegler-type catalysts (87–89), cationic polymerization by AlBr3 (90–92), and radical polymerization induced by AIBN (93) have been reported (eq. 15).

(15) Cyclopolymerization of another type of organosilicon monomer has been reported. Poly(methylene-4,6-disila-1,3-cyclohexanylene) (16) was prepared by an anionic polymerization of bis(dimethylvinylsilyl)methane in the presence of tetramethylethylenediamine (TMEDA) (eq. 16).

(16) The polymerization can be controlled well to provide a sufficiently high molecular weight polymer (M w = 12,400) having a six-membered ring structure in a very high yield (94,95).

Carbosilane Dendrimers In 1992, the synthesis of a carbosilane dendrimer was reported (96). Starting from tetrachlorosilane, allylations of Si Cl groups by allyl Grignard reagents and hydrosilations of the allyl groups with trichlorosilane were alternately repeated to give a dendrimer composed of silylenetrimethylene units up to the fifth generation (G5) (eq. 17) (96).

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(17) Other carbosilane dendrimers composed of silyleneethylene units, such as 18 and 19, can be synthesized analogously by applying vinylations of Si Cl groups instead of the allylations (97,98).

Many researchers employed these dendrimers as frameworks to attach other functional groups. For instance, dendrimers with mesogenic cholesteryl (99), perfluoroalkyl (100,101), hydroxyl, dimethylamino, sodium sulfonate,

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trimethylammonium groups (102), palladium complexes (103–105), iron/gold clusters (106), and cyclopentadienyltitanium complexes (107) were synthesized.

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KOZO MATSUMOTO Kyoto University

POLYCHLOROPRENE.

See CHLOROPRENE POLYMERS.

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See Volume 3.

POLY(CYCLOHEXYLENE TEREPHTHALATE). See CYCLOHEXANEDIMETHANOL POLYESTERS.

POLYDIACETYLENE.

See DIACETYLENE AND TRIACETYLENE POLYMERS.