Preparation cyclic polysilanes

In any event, between 1951 and 1975, no papers appeared on polysilane high polymers. However, linear permethylpolysilanes of the type MelSiMezhiMe were prepared and studied, especially by Kumada and his students,(5) and cyclic polysilanes were being investigated in several laboratories.(6,7) Studies of the permethyl-cyclosilanes, (Me2Si)n where n = 4 to 7, showed that these compounds exhibit remarkable delocalization of the ring sigma electrons, and so have electronic properties somewhat like those of aromatic hydrocarbons.(6)... 

The di-Grignard reagent 164, formed from l,2-diethynyl-l,l,2,2-tetraalkyldisilane, has been prepared by reaction with either ethyl- or isopropylmagnesium bromide [Eq. (73) 135,136]. Reactions of di-Grignard reagent 164 with 1,2-dichlorotetraalkyldisilanes and 1,3-dichlorohexamethyltrisilane produced cyclic polysilanes 165 and 166, respectively, in yields that were typically 37-73% [135-137]. 

Excess metal used except for cyclic polysilanes Potassium derivatives prepared from Na-K alloy. 

The preparation of cyclic polysilanes from dialkylchlorosilanes and lithium metal in tetrahydrofiiran presumably involves a silicon—lithium intermediate (125). 

Cyclic polysilanes are more stable than chain polymers. Attempts to produce high polymers of polysilanes by ring opening of cyclic compounds are unsuccessful, but conversions from linear polymers or chain oligomers to cyclic compounds are feasible. Most of the limited work in this area is done with permethylpolysilanes. As mentioned in 15.2.4.1.2, the preparation of methylcyclosilanes from Me2SiCl2 and alkali metals proceeds through polymer formation followed by depolymerization, at least in some cases . ... 

In recent years, considerable attention has been directed toward a study of the more highly catenated linear and cyclic polysilanes (I, 2, 14, 18). An interesting facet of this recent work involves the preparation of branched polysilanes such as those illustrated below. 

The preparation of soluble polysilylenes by the condensation of dialkyldichlorosilanes with sodium was first described by West et al. (1). The polymer was found to have many interesting properties, and to be of potential commercial interest (2-4). This method of preparation is of general utility, although it has some shortcomings particularly in regard to the variety of polymeric species produced. There have been several studies on the reaction (5-11), with several suggestions to improve the yield. Alternate routes to polysilylenes have also been described (12,13), but these will not be considered here, except for one. This exception is the polymerization of some cyclic polysilanes, which was found to occur in the course of this work. 

GPC traces of poly(phenylmethylsilylenes) prepared in the ultrasonication bath are shown in Fig. 1. In contrast to thermal condensation, monomodal high molecular weight polymer is formed. Oligomeric cycles (mostly cyclic pentamer), formed usually in high yield (cf. Table 1), can be very easily separated from the reaction mixture by precipitation with isopropanol. The molecular weight of polysilanes decreases and polydispersity increases with temperature. 

Kipping experiment of the coupling of diphenyldichlorosilane had to be very low because in addition to cyclic tetramer, large quantities of cyclic pentamer and hexamer were formed. Only very recently was existance of polysilanes with two aryl substituents proved by Miller who prepared... 

In recent years, dehydrocoupling reactions catalysed by early transition metal complexes have become an increasingly important method for generating catenated species of the p-block elements. In addition to producing cyclic oligomers, this approach is used to prepare linear oligomers and polymers such as polysilanes and polystannanes of the type H(MR2) H (M = Si, Sn) (see Section 10.1.4). ... 

The proton NMR spectral data of organopolysilanes have often been published incidental to preparative studies (51, 54, 62, 74, 108, 119, 177, 187, 190). Only recently has a systematic investigation to determine the chemical shifts and coupling constants in linear and cyclic permethylated polysilanes, and to study the effects of substituents on the NMR properties of methyl derivatives of disilane and trisilane been reported (see Table V-VII) (206). 

C-O bond cleavage is most efficiently set up in strained systems, e.g. in oxiranes [26] and oxetanes [27], whereas 0-0 bond cleavage in cyclic peroxides [28] and C-S [29] and C-Se [30] bond rupture occur efficiently even in unstrained compounds. Si(Ge)-Hal bond cleavage [31] has become a valuable reaction, e.g. in the preparation of network polysilanes and polygermanes [32]. 

Various linear and cyclic fully methylated polysilanes can be made by catalytic retyrangement of Si6(CH3)i4, and the cyclic compounds [Si(CH3)2] ( = 5-7) have been prepared by the action of Li on Si(CH3)2Cl2 in tetrahydrofuran. Structural data for some of these compounds are included in Table 21.1 (P-727). 

Polysilanes are compounds which contain silicon-silicon bonds. Although the disilane Et3SiSiEt3 was reported as early as 18691, and the cyclic perphenyl compounds (Ph2Si) (n = 4-6) were prepared over 70 years ago by Kipping and Sands2, the belief that silicon had limited capability for catenation persisted until quite recently. The isolation within recent years of cyclic dialkylpolysilanes containing up to 40 silicon atoms in a ring, and of linear polymers with more than 40,000 silicon atoms in the chain, have however effectively dispelled this myth. 

More recently, stable silylenes which are structurally the repeating unit of a polysilane chain have been prepared and characterized [14-15]. Their synthesis involves the reduction of silicon dichlorides. Two methods of synthesis are shown in the Fig. 7.7. Both of these procedures involve the preparation of cyclic compounds containing Si(II). Although these compounds are highly interesting from the point of view of their chemistry, they have not been shown to serve as precursors for polymers. 

Synthesis. The first report of a soluble polysilane appeared in 1978 and the material was prepared by the treatment of a mixture of organodichlorosilanes with sodium metal (33). Instead of only the expected cyclic oligomers, a polymeric product, termed polysilastyrene (10), was formed. Poly(dimethylsilane) had been previously prepared as a highly crystalline insoluble material (1,27,28). The introduction of phenyl groups in the random copolymer reduces the crystallinity and allows the material to be soluble and processible. 

Planned syntheses have been devised for the preparation of specific linear polysilanes. This is illustrated by the following example starting from the cyclic hexamer. 

The main method of preparing both linear and cyclic di- and polysilanes is a coupling of a chloroorganosilane by means of an alkali metal (Eq. 3.262) ... 

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