Polysilane types

Silenes derived from aromatic di- or polysilanes have been characterized in particular by the ene-type reactions they undergo when treated with isobutene or acetone. Recently, Leigh80 observed the first reported case of one of these silenes undergoing [2+2] cycloaddition (21% yield) with acetone. The ene product, the only product previously detected from the reaction of such silenes, was formed in 41% yield, as shown in Eq. (33). 

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)... 

In a continuing study of substituent effects on the spectra of polysilane derivatives, we have succeeded in the preparation of the first soluble poly(diarylsilane) homopolymers. Materials of this type have traditionally proved to be insoluble and intractable. Very recently, West and co-workers have reported the preparation of some soluble copolymers which contain diphenylsilylene units (48,49). 

We have observed a dependence of the yield, polymerization degree, and polydispersity of polysilanes on temperature and also on the power of ultrasonication. In the ultrasonication bath the simplest test of the efficiency of cavitation is the stability of the formed dispersion. It must be remembered that the ultrasonic energy received in the reaction flask placed in the bath depends on the position of the flask in the bath (it is not the same in each bath), on the level of liquid in the bath, on temperature, on the amount of solvent, etc. When an immersion probe is used the cavitation depends on the level of the meniscus in the flask as well. The power is usually adjusted close to 50% of the output level but it varies with the reaction volume, flask shape, and other rection conditions. The immersion-type probe is especially convenient at lower temperatures. 

Tanaka, H. et al. 2007. Spin-on n-type silicon films using phosphorous-doped polysilane. Jpn. J. Appl. Phys. 46 L886-L888. 

Although main-chain chirality refers to both polymers with stereogenic centers in the main chain (configurational main-chain chirality) and polymers with main chains consisting of helical stereogenic bonds induced by chiral side groups or end termini (conformational main-chain chirality), in this chapter we mainly focus on polysilanes exhibiting the latter type of chirality. 

Chart 4.2 Wurtz-type reduction preparation of optically active polysilanes. 

The most common route to polysilane-high polymers is by the Wurtz-type alkali metal-mediated polycondensation of dichlorosilanes. However, , u-dihalooligosi lanes can also be coupled in a similar manner. Wurtz-type polymerization of BrSiMe2SiR2SiMe2Br led to bimodal molecular weight distributions of alternating co-polymers of the type (SiMe2SiR2SiMe2) (R = hexyl or Bu),17 as in Equation (1). 

It is noted here that the related poly(l,l-silole)s, polysilanes, in which each CRLJ is a silole ring catenating through the silicon atom, have been prepared by Wurtz-type coupling of dichlorosilole monomer54,55 and are described above in the section on Wurtz-type coupling (Section 3.11.4.1.1). 

Functionalization of polysilanes by chemical modification (post-polymerization) was covered in COMC II (1995) (chapter Organopolysilanes, p 101), where the formation of precursor polysilanes with potentially functionalizable side groups such as chloride, type 34 (via HCI/AICI3 chlorodephenylation of PMPS), 6 triflate, type 35 (via triflate replacement of phenyl groups)135,137 or alkyl halide (via chloromethylation of phenyl groups,138,139 type 36, or addition of HC1 or HBr to double bonds140) was discussed. Four other precursor polysilanes, which utilize the reactivity of the Si-Cl or Si-H bond, have been successfully applied in functionalization since COMC (1995) perchloropolysilane, 17 (see Section 3.11.4.2.2.(i) for synthesis),103 poly[methyl(H)silylene-f >-methylphenylsilylene],... 

Polysilanes with functionalities Z directly attached to the silicon main chain are generally more difficult to synthesize and less stable, due to the reactivity of both the Si-Z and Si-Si linkages. Such polymers are usually not accessible via the Wurtz-type coupling reaction, and alternative synthetic strategies (see Sections 3.11.4.1 and subsequent) or postpolymerization techniques have to be employed. 

Harrod-type catalytic dehydrocoupling method using early transition metal catalysts (see COMC II (1995), chapter Organopolysilanes, p 99, and earlier in this review, Section 3.11.4.1.3. (i)).69,75 Si-H bonds are susceptible to free radical attack, and use of this was made in the free radical substitution of 38 to prepare a number of oxy-functionalized polysilanes, as shown in Scheme 25.185,186... 

Silole incorporation was noted above in Section 3.11.4.1.l.(iii) where poly( 1,1 -silole)s synthesized by Tamao and co-workers via Wurtz-type coupling were described. The related silole-incorporated polysilanes bearing silole units on every fifth silicon in a regular sequence prepared by Sakurai and co-workers using anionic ROP were described in Section 3.11.4.1.4. 

Most recently, a simple, mild, one-pot immobilization method was developed to attach the rigid rod-like helical polysilane, poly( -decyl-/-butylsilylene), via a siloxy linkage to hydrophilic quartz or mica substrate surfaces.28 195 Triethylamine was used as a catalyst to couple the Si-H and/or Si-OR termini of the dialkylpolysilane chains (which are generated during the course of Wurtz-type synthesis and workup)51,195 with the surface -OH groups. AFM, UV, and IR data were used to analyze the reactions. 

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