Masked disilenes to polysilanes

Anionic Polymerization of Masked Disilenes to Polysilanes Mechanism and Applications... 

This method has also been used for the synthesis of defined polysilanes in some applications. For example, Li et al. applied the polymerization of masked disilenes to synthesize an aryl substituted diazene chromophore functionalized polysilane with nonlinear optical properties [40]. 

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. 

In another elegant application of the masked disilene method, the anionic polysilanyl chain ends were chemically bound to substrate surfaces to form end-graft polysilanes. These systems are discussed in Section 3.11.4.2.3. 

The masked disilene strategy was also successfully applied to the synthesis of dialkylamino-substituted polysilanes,60 61 63 by the -butyllithium-initiated polymerization of dialkylamino-substituted phenyldisilabicycloocta-dienes, as shown in Scheme 23. 

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. 

Another innovative route to polysilanes involves the anionic polymerisation of disilabicyclooctadienes 10.9, which function as sources of masked disilenes, e.. Me2Si=SiMe Bu [eqn (10.10)]. ... 

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... 

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... 

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...

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... 

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). 

Scheme 11.2. The masked-disilene methodology applied to the synthesis of dialkylamine-substituted polysilanes.

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]. 

Very recently, we have disclosed the stereoselective anionic polymerization of dibutylamino-substituted masked disilenes 1 [12]. The stereochemistry of the polymers is analyzed based on diad and triad sequences. Under the appropriate conditions, the polymerization produced a polymer rich in syndiotacticity up to 89 % in diad. Typical examples of the analysis are shown in Table 2. A high r content in the diad tacticity as well as a high rr fraction in the triad tacticity indicates clearly that the polysilane is rich in syndiotacticity. 

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]. 

ROP of the masked disilene) with an alkyl bromide that contains the tripo-dal sulfur ligand leads to an interesting polysilane which has three coordinating sulfixr units at its terminus. This polymer could be utilized for chemisorption on gold surface (Fig. 7.22) [67]. 

Another route to polysilanes that involves the anionic polymerization of dis-ilabicyclooctadienes, which function as sources of masked disilenes (eq. 12), has been described (39,40). Amphiphilic block copolymers formed by this anionic route, such as poly(l,l-dimethyl-2,2-dihexyldisilene)-6-(2-hydroxyethyl methacrylate), undergo self-assembly to form micelles (41). 

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]. 

This route towards polysilanes seems to be of high potential concerning PDI and molecular weight. Few recent publications concerning polymerization of masked disilenes can be found containing scientific results with respect to further improvement of synthesis [35, 38, 39],... 

As the ring opening polymerization of masked disilenes seems to occur in a strict head to tail fashion, highly ordered polysilanes have been obtained showing two sharp signals in the Si-NMR spectrum indicating a completely syndiotactic polymer [34, 35]. 

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. 

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