Preparation and characterization of new polycarbosilanes : part 1, synthesis, characterization and thermal properties of substituted poly(silylenemethylenes); part 2, stoichiometric polycarbosilane precursors to SiC

Abstract

A mixture of the cis and trans isomers of 1,3-dialkyl-1,3-dimethyl-1,3-disilacyclobutanes was prepared from the reactions of DCDMDSCB and the alkyl Grignard reagents (RMgX). The alkyls, ranged from ethyl to n-hexyl, and phenyl substituted [SiMe(R)CH2]n polymers were obtained in high molecular weights via the ROP of the corresponding 1,3-disilacyclobutane monomers under the H2PtCl6.catalyst. The Si-H containing polymers, [SiH(Ph)CH2]n, an analog of poly(styrene), and [SiH(n-Hex)CH2]n, were successfully prepared. from Cl3SiCH2Cl in six steps, which include: a Grignard coupling reaction of Cl3SiCH2Cl with RMgCl (R = Ph and n-Hex), a substitution of one Cl of the resulting ChSi(R)CH2Cl by an EtO group, a ring closure reaction of the Cl(EtO)Si(R)CH2Cl compound, a chlorination of the ethoxy groups in the ring compound R(EtO)Si[CH2]2(0Et)R, a ROP of the final monomer R(Cl)Si[CH2hCCl)R, and finally, a reduction of the [SiR(Cl)CH2Jn polymer by using LiAlH4. All of the [SiMe(R)CH2Jn (R = Et, n-Pr, n-Bu, n-Pe, n-Hex, and Ph) and [SiH(R)CH2Jn (R = n-Hex and Ph) polymers were found to occur in an atactic configuration by NMR spectroscopy. For the polymers with relatively long and flexible R groups, lower glass transition temperatures were observed; the polymers with rigid side groups showed higher glass transition temperatures. The glass transition temperatures of the [SiR'(R)CH2]n polymers are generally lower than those of the corresponding carbon polymers, [CR'(R)CH2]n. A TGA study indicated that the [SiR'(R)CH2Jn polymers with short alkyl side chains and aromatic groups are more thermally stable than those with longer flexible side chains. Those polymers with Si-H groups, such as [SiH(R)CH2]n system, decompose at lower temperatures and give higher ceramic yields, due to cross-linking reactions from Si-H groups.

A high molecular weight poly(silaethylene) (PSE) was obtained from the ROP of TCDSCB , while direct ROP of TEDSCB gave a linear PSE with a relatively lower molecular weight. The CH3 and SiH3 end groups, the CH2SiH2 repeat unit, and CH3 side groups were identified in the linear PSE by the aid of a model compound 1,3-disila-n-butane and PSE with deliberately added CH2SiH3 or CH3 side groups. In addition, the small number of the methyl side groups in PSE was found to originate from a by-product dimethyldichlorosilane in the starting material methyltrichlorosilane.

In the "reverse addition" Grignard reactions for Cl(R0)2SiCH2Cl (where R = Me, Et, and i-Pr), a ring compound, 1,1,3,3-tetrethoxy1,3-disilacyclobutane (TEDSCB), was isolated in a 40% yield. Low yields were obtained for the 1, 1,3,3-tetramethoxy and iso-propoxy substituted four-membered rings. The trimers from the "reverse addition" reaction of Cl(Et0)2SiCH2Cl were isolated and characterized. TEDSCB was converted into another important monomer TCDSCB, by usmg acetyl chloride under the catalysis of FeCl3. 1 ,3,5-trisilacyclohexane, [SiH2CH2]3, was prepared from a ring closure reaction of (Me0)3SiCH2Cl, followed by chlorination of the obtained 1,1 ,3 ,3,5 ,5-hexamethoxy-1 ,3 ,5-trisilacyclohexane and reduction of the 1,1 ,3 ,3 ,5 ,5-hexachloro-1 ,3 ,5-trisilacyclohexane.

Part 2: Three dialkoxy-substituted compounds, Cl(ORhSiCH2Cl (R=Me, Et, and i-Pr) were made directly from the reaction of alcohol with ChSiCH2Cl. A branched hydridopolycarbosilane (HPCS), [SiHxCH2]n (where X=O, 1, 2, or 3, the average value for X is 2.), was obtained via a direct Grignard coupling of ClOMehSiCH2Cl in THF, followed by reduction of the methoxy-polymer [Si(OMeh-xClxCH2]n by using LiAlH4. The side reactions between ether and chlorosilanes were avoided in this approach, leading to a purer version of HPCS without significant amount of alkyl groups from reactions with solvent. An allyl-substituted (AHPCS) was successfully prepared by adding allyl chloride to the methoxy-polymer before reduction. The excess magnesium and allyl chloride formed a Grignard reagent (allylmagnesium chloride) in situ. These Grignard reagents have sufficient time to be homogeneously distributed and bonded to the polymer structures. The reaction process for this AHPCS containing 5% allyl group was scaled up to 12 L.

Part 1: Three alkoxysilanes, Cl(RO)Si(Me)CH2Cl (R = Me, Et, and i-Pr), were prepared in a convenient procedure, in which alcohol was directly added to the chlorosilane CL2Si(Me)CH2Cl at 0°C; the by product HCl was removed by flushing with nitrogen gas; no separation of solid by-product or solvent was required. The yields of the formation of 1,3-dialkoxy-1,3-dimethyl-1,3-disilacyclobutanes made from Cl(RO)Si(Me)CH2Cl were found to increase with the size of the alkoxy groups, i.e., MeO < EtO < i-PrO. The ring closure Grignard reaction originally introduced by Kriner was improved to provide a 50% yield in the case of the largest group, isopropoxy. 1,3-dichloro-,3-dimethyl-1,3-disilacyclobutane (DCDMDSCB), a frequently used monomer throughout this study, was obtained in a much improved yield from 1,3-diisopropoxy-1,3-dimethyl-1,3-disilacyclobutane by using acetyl chloride as the chlorinating reagent. Separation of the products by distillation was simplified due to the lower boiling points of the by-product (isopropoxy acetate) and the excess starting material (acetyl chloride).

1,1 ,3,3-tetraalkyl-1 ,3-disilacyclobutanes were prepared from the reaction of 1,1,3,3-tetrachloro-1,3-disilacyclobutane (TCDSCB) with various RMgX reagents. ROP of these monomers gave the symmetrically substituted polymers, [SiR2CH2]n in which R ranges from ethyl to n-hexyl groups. In comparison with the asymmetrically substituted polymers [SiMe(R)CH2]n, the molecular weight of the [SiR2CH2]n polymers decreased with the increase of the length of the alkyl groups, owing, presumably, to the steric hindrance from the alkyl groups during the ROP. DSC analysis showed that two endothermic transitions were found in all of the dialkyl substituted polymers [SiR2CH2]n, but the intermediate phases in these polymers existed in a very narrow temperature range in comparison with those phases in the corresponding poly(siloxanes) [SiR20]n and poly(silanes) [SiR2O]n. Lower thermal stabilities were observed for the [SiR2CH2]n polymers substituted with the longer side chains. All of the [SiR2CH2]n polymers gave low ceramic yields due to the lack of cross-linking groups.