Polymerization anionic, 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.

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. 

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... 

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. 

The overall procedures of the methodology are shown in Scheme 11.1. The first reaction depicts the synthesis of masked disilenes of representative structures and the second is a conventional anionic polymerization such as might be applied in the synthesis of polystyrene. Masked disilenes are so-called as they are to all intents and purposes disilenes stabilized by a biphenyl bridge. During the polymerization reaction the bridge is released. Product polymers have been reported to have molecular weights up to 50,000 with... 

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). 

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.

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. 

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. 

Table 3. Anionic polymerization of dibutylamino-substituted masked disilene 1 at various temperatures.

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... 

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