Disilene monomers

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. 

Before we consider the synthesis of polysilanes in the next section, let us consider if addition polymerization involving disilene monomers can be used for the preparation of polysilanes. Robert West from the University of Wisconsin prepared the first stable disilene in 1981. His synthetic route consisted of photolyzing an acyclic trisilane. Loss of McsSi-SiMcs leads to a silylene intermediate that dimerizes to the stable disilene (Fig. 7.4) [12-13]. 

The masked disilene approach, while offering major improvements in polymer structural control, has a few drawbacks monomers bearing bulky groups (e.g., 1,1-di-f-Bu, aryl) and Si-OR substituents cannot be polymerized the overall synthetic scheme from commonly available reagents is rather involved. Despite these, however, the route remains attractive, and further developments, such as in the control of stereochemistry, will likely be explored. 

Anionic polymerization of masked disilenes has been used for the synthesis of polysilanes. The monomers are bridged disilabicyclooctadienes such as (8). Treatment of these with an organohthium compound converts them to living, anionically terminated polysilanes (equation 37). The... 

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. ... 

Digermenes, " the heavier analogs of disilenes, cannot exist as monomers under normal conditions owing to... 

Anionic polymerization of masked disilenes has opened up a novel route to polysilanes (95). I-Phenyl-7,8-disilabicyclo[2.2.2]octa-2,5-dienes can be used as masked disilenes. n-BuLi works as an initiator. The polymerization may involve the attack of the polysilanyl anions on a silicon atom of the monomer, resulting in the formation of the new propagating polymer anion and biphenyl. This method is applicable to aminopolysilane synthesis (Scheme 28). 

Enantiotopic polymerization of a prochiral vinyl monomer leads to either isotactic or syndiotactic polymer. Stereocontrol of the polymerization is a main issue of the polymer chemistry. Contrary to the situation for the vinyl monomers, a masked disilene such as 1 is chiral. Monomers are obtained as a racemic mixture, which can be separated into each enatiomer by using a chiral column on liquid chromatography. Herein we report the first highly enatioselective polymerization of the racemic masked monomer. The stereochemical course of the propagation step in the anionic polymerization of 1 should be extremely interesting. 

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... 

On the other hand, compounds that can be described as trapped or masked disilenes can be viewed as potential monomers for polymer synthesis. The trapped or masked disilene can be liberated from its adduct by chemical or photochemical process. However, even if moderately buUq substituents are present on silicon in such compounds, the liberated disilene combines to afford the thermodynamically favorable cyclized products. Thus, the masked disilene containing isopropyl substituents on silicon affords the cyclotetrasilane [zPr2Si]4 (see Eq. 7.7) [49]. 

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... 

Unquenched product of masked disilene polymerization is used as the initiator of carhon monomers (78) (eq. 2). 

This anionic polymerization is evidently of a living type as the unterminated polymer can be co-polymerized with either other masked disilenes bearing different functional groups or methylmethacrylate (MMA) to yield a block co-polymer [35]. Furthermore, it was found that the relationship between molecular weight and the degree of monomer conversion is linear, which is a necessary condition for a living polymerization [36]. 

This (RRSi-SiRR )n substitution pattern is characteristic of polysilanes synthesized by the anionic living polymerization of masked disilenes. For synthetic reasons, the substituents only vary on one of the two silicon atoms in the masked monomer. 

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