Polymerization of masked disilenes

Polymerization ofiVIasked Disilenes. A novel approach, namely, the anionic polymerization of masked disilenes, has been used to synthesize a number of poly(dialkylsilanes) as well as the first dialkylamino substituted polysilanes (eq. 13) (111,112). The route is capable of providing monodisperse polymers with relatively high molecular weight M = lO" — 10 ), and holds promise of being a good method for the synthesis of alternating and block copolymers. 

Anionic Polymerization of Masked Disilenes Mechanism, Scope and Applications... 

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

Table 2. Anionic Polymerization of Masked Disilene 7 initiated by Silyl Anions in the Presence of Cryptand in Benzene at Room Temperature...

Scheme 27 End-graft polysilanes via anionic polymerization of masked disilene.

Sakurai, H. Yoshida, M. Synthesis of Polysilanes by New Procedures Part 1 Ring-opening Polymerizations and the Polymerization of Masked Disilenes. In Silicon-based Polymers The Science and Technology of their Synthesis and Application-, Jones, R. G., Ando, W., Chojnowski, J., Eds. Kluwer Dordrecht, 2000 pp 375-399. 

Synthesis of Polysilanes. The most commonly utilized method is based on the Wurt/ type alkali metal coupling or dichlorosilanes. Other synthesis methods include dehydrogenative coupling, ring-opening polymerization, polymerization of masked disilenes. electrochemical synthesis, and polymer modification. 

Polyphenylsilane, (PhSiH) , can be derivatized by free-radical hydrosilylation in the presence of a radical initiator. Alkenes, ketones and aldehydes react readily, to replace up to 93% of the Si-H bonds. This route can be employed to make polysilanes with hydrophilic groups, such as hydroxy, amino and carboxylic acid functions.43 Dialkylamino substituted polysilanes, made by the anionic polymerization of masked disilenes (see equation (17)), when treated with acetyl chloride give chloro-substituted poly silanes. The chlorine can then be displaced by other nucleophiles.27... 

A nearly fully ordered polysilane copolymer has been synthesized by careful condensation of a 1,3-dibromotrisilane, yielding the AABAAB-type copolymer shown in equation (30).118 The best method for obtaining ordered polysilane copolymers is however by the anionic polymerization of masked disilenes (Section 5.3.3). Formation of an ABAB-type copolymer is illustrated in equation (31).119 These polymers show only two narrow lines in their 29Si NMR spectra, consistent with their ordered structure. In a similar reaction, the Sakurai group synthesized an exactly alternating polysilastyrene polymer, (PhMeSi-SiMe2) .120... 

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... 

H. Sakurai and M. Yoshida, Synthesis of polysilanes by new synthetic procedures Ring opening polymerizations and the polymerization of masked disilenes , in Reference 4, pp. 375 -400. 

Fig. 4 Approaches to the synthesis of polysilane block copolymers by living polymerization techniques (a) via anionic polymerization of masked disilenes (b) via anionic ring-opening polymerization of cyclotetrasilanes...

In addition to organohthium compounds, other reagents such as organomagnesium compounds and potassium alkoxides have also been used as initiator for anionic polymerization of masked disilenes. ... 

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. ... 

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). 

The anionic polymerization of masked disilenes, that is, l-phenyh7,8-disilabicyclo[2.2.2]octa-2,5-diene derivatives, is... 

The first of these new synthetic methodologies, the anionic polymerization of masked disilenes developed by Sakurai and co-workers,is elegant and its concept is unique. Almost uniquely, it offers access to a range of polyaUcylsilane structures with narrow molecular weight distributions but involves some procedures that are so painstaking as to cause the principal researcher to caution the uninitiated who might wish to follow a similar path. 

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]. 

Amino-substituted polysilanes were synthesized in high yield by the anionic polymerization of masked disilenes in THF with a catalytic amount of n-butyllithium (Scheme 1). 

Anionic polymerization of masked disilenes and strained cyclosilanes [18]... 

The mechanism of polymerization of masked disilenes by anionic initiators has been suggested as involving a propagating silylanion with concomitant expulsion of the trap viz., biphenyl. The synthesis of polysilanes... 

Sakuiai H, Yoshida M (2000). Synthesis of polysilanes by new procedures Part 1 Ring-opening polymerizations and polymerization of masked disilenes. In Jones RG, Ando W, Chojnowski J (eds) Silicon containing polymers. Kluwer Academic, Dordrecht, pp 375-399... 

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. 

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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