Preparation masked disilenes

The masked disilene monomers 1-5 were prepared by reaction of dichlorodisilanes of the type ClSiR1R2SiMe2Cl with the biphenyl anion radical as described before. These are composed of two regio isomers, a and b, the predominant isomer being a, as determined by H NMR NOE difference spectra. 

Table 1. Preparation and Anionic Polymerization of Masked Disilene, 1-Phenyl-7,8-disila-bicyclo[2.2.2]octa-2,5-diene...

Scheme 23 Preparation of dialkylamino-substituted heteroatom polysilanes by masked disilene method.

Another interesting chiral chain end effect is exhibited by the helical polymer block co-polymer, poly(l,l-dimethyl-2,2-di-/z-hexylsilylene)- -poly(triphenylmethyl methacrylate), reported by Sanji and Sakurai (see Scheme 7) and prepared by the anionic polymerization of a masked disilene.333 The helical poly(triphenylmethyl methacrylate) block (PTrMA) is reported to induce a PSS of the same sign in the poly(di- -propylsilylene) block in THF below — 20 °C, and also in the solid state, by helicity transfer, as evidenced by the positive Cotton effect at 340 nm, coincident with a fairly narrow polysilane backbone UV absorption characteristic of an all-transoid-conformation. This phenomenon was termed helical programming. Above 20°C, the polysilane block loses its optical activity and the UV absorption shifts to 310 nm in a reversible, temperature-dependent effect, due to the disordering of the chain, as shown in Figure 45. 

Optimization of the latter reaction is an object of current study.26 Electrosynthesis of polysilanes has undergone a transformation from laboratory research experiments27-32 to industrial production of imaging polysilanes for microlithography.33 Anionic polymerization of masked disilenes was established as a new synthetic route to polysilanes of highly ordered structure.34 A functional polysilane with an ethereal group, poly[l-(6-methoxy-hexyl)-1,2,3-trimethyldisilanylene] (Mn = 7.2 X 103) was prepared by the mask disilene method.35... 

The anionic polymerization of masked disilenes proceeds via living anions, and therefore block copolymerization with a conventional vinyl monomer is possible. Recently, interesting hydrophobic block copolymer of PMHS with poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(methacrylic acid) (PMMA) have been prepared (Scheme 11). These polymers can be self-assembled and are transformed into polysilane micelles, shell cross-linked micelles (SCM), and nanometer-sized hollow particles. ... 

Polysilanes have been the first class of precursors used to prepare silicon carbide ceramics. In all cases, on pyrolysis, polysilanes are converted into polycarbosilanes by a Kumada rearrangement prior to the formation of an amorphous silicon carbide network. Several synthetic routes including dehydro-polymerization, ring-opening polymerization of strained cyclosilanes, polymerization of masked disilenes, or base catalyzed disproportionation reactions of disilanes have been described to prepare linear or branched polysilanes but despite its drawbacks the Wurtz-coupling route remains the method applied most, especially to prepare linear polysilanes. 

Anionic polymerization of masked disilenes presents exciting opportunities for the synthesis of polysilanes of well-defined structure and functionality [7, 8]. Indeed, we have found that amino-substituted masked disilenes could be prepared and polymerized successfully to unprecedented amino-substituted polysilanes with a completely head-to-tail structure, poly[ 1,1,2-trimethyl-2-(dibutylamino)disilene]s [9]. 

Fig. 1. Dimethylsilylene region of the C NMR of polysilane prepared a) by anionic polymerization, and b) by Wurtz coupling of of 6 4 mixture of masked disilene.

Butyllithium has also been used as the initiator for polymerizing masked disilenes. The latter are essentially disilane compounds which can be viewed as trapped or masked disilenes. If the disilene is liberated from this trap it has many choices. It can form a disilene. It can cyclize or polymerize. By a careful choice of substituents on silicon it is possible to use masked disilenes as monomers for polymerization. This method of polymerization has been shown to be quite effective for the preparation of a variety of polysilanes (see Eq. 1.28) [41]. 

Although many disilenes have now been prepared and characterized they are not very good monomers for the preparation of polysilanes. The very method of preparing these kinetically stabilized disilenes defeats their use as olefin-like monomers for addition polymerization. However, as will be shown in the next section trapped disilenes or masked disilenes that... 

Masked disilenes containing amino substituents on silicon can also be prepared (see Eq. 7.9) [18]. 

Fig. 7.15. Block copolymers prepared by the anionic initiation of masked disilenes...

Another way of preparing the ordered copolymer consists of using the masked-disilene method. If the monomer (masked disilene) is designed in such a manner that one of the silicon atoms has two methyl groups and the other silicon has two -hexyl substituents, it would be expected that the polymer obtained from it should have a stereo regular arrangement. This has been accomplished and in the Si NMR of this polymer two distinct... 

Anionic polymerization of masked disilenes proceeds via a living condition to give polymers having controlled molecular weight (33). Head-to-tail pol5uners can selectively obtained and polysilanes with fimctional groups such as diethylamino groups are prepared. However, the masked disilene method is only applicable to the synthesis of non-aromatic-substituted polysilanes. 

As the reaction conditions for the preparation of the masked disilenes are as harsh as those of the Wurtz-type reductive coupling reaction, the introduction of functional groups is not trivial. An elegant way to introduce at least some diversity is via the transformation of amine-functionalized polysilanes [39] to the chloro substituted derivatives. These can be converted to alkyl and aryl groups using the corresponding Grignard compounds (Fig. 9) [35]. 

Table 3 Examples of functionalized polysdanes prepared from masked disilenes...

Thus, it is possible to prepare structurally controlled polysilanes although the method is limited by the functionalities accessible with the masked disilene route. Surprisingly, to date, this has not been investigated any further. 

The chemical and physical properties of polysilanes are strongly influenced by substituents attached to the polymer backbone. In this respect, heteroatom-substituted polysilanes should be very much intriguing on their properties. However, heteroatom-functional substituents such as amino and alkoxy groups on silicon cannot survive under the vigorous synthetic conditions of polysilanes by the conventional Wurtz-type condensation of dichlorosilanes. Therefore, it is difficult to prepare heteroatom-functional polysilanes. We have recently found that amino-substituted masked disilenes can be prepared and polymerized successfully to unprecedented amino-substituted polysilane of the completely alternative structure, poly[l,l,2-trimethyl-2-(dialkylamino)disilene]. 

The requisite masked disilenes 14a-c were prepared as shown below. Rather surprisingly, high regiospecificity of the reaction was observed only one of the possible regioisomers was obtained in moderate yield. Indeed, no regioisomer was detected at all. 

Among a variety of new preparative methods for creating Ae Si-Si bond such as transition metal catalysed dehydrocoupling of hydrosilanes [12-14a], ring opening polymerization [11,12], polymerization of masked disilenes [14b], the most practice one is still the Wurtz type condensation of organomonochloro- or dichlorosilanes which requires a stoichiometric amount of an alkali metal and high reaction temperatures in the cases of Na and K [3-9, 12] (Equations 1 and 2). Lower temperature variants have yet recently been reported [14c]. 

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