Disilenes, masked

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... 

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. 

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 6 Masked disilene route to regularly alternating co-polymers.

Scheme 7 Helical sense programming from masked disilene.

Similarly, use of the chiral anionic initiator (+) or (—) potassium menthoxide with the same masked disilene afforded a polysilane with a PSS helical conformation. Optically active polysilanes are discussed further in Section 3.11.6. 

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 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. 

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. 

Scheme 23 Preparation of dialkylamino-substituted heteroatom polysilanes by masked disilene method.

The first examples employed a lithium-terminated polymer chain, utilizing the primary product of the -butyl-lithium-catalyzed polymerization of a masked disilene (see Section 3.11.4.1.2) and a reactive siloxy alkylbromide anchor-derivatized quartz surface, affording the end-grafted polysilane 79, as shown in Scheme 27.191... 

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

Most polysilanes are synthesized by the Wurtz-type coupling reaction, which is non-stereospecific, and hence the configuration is usually undefined, although in a few cases, stereochemically defined polysilanes have been synthesized (ROP of all-anti l,2,3,4-tetramethyl-l,2,3,4-tetraphenylcyclotetrasilane99 101 and masked disilene polymerization).61... 

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. 

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. 

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)]. ... 

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. 

Alkoxy- and amino-substituted disilenes (17, 18 and 19) are produced by photolysis of the corresponding masked disilenes (20, 21 and 22)29. Thus, when a degassed 3-methylpentane (3-MP) matrix of 20 was irradiated at 77 K, the corresponding alkoxy-substituted disilene 17 was produced (equation 5) as indicated by its broad band at 373 nm. 

Generation of various phenyl-substituted disilenes by the photolysis of the masked disilenes, 7,8-disilabicyclo[2.2.2]octadiene derivatives, is quite useful, especially for unsymmetrically substituted disilenes. Investigation of the regiochemistry as well as the diastereochemistry of alcohol addition to phenyltrimethyldisilene was made possible for the first time by using this method27. 

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... 

Masked disilenes, into polysilane high polymers, 3, 561 Mass spectrometry... 

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...

The masked disilene procednre was nsed by Saknrai and co-workers to synthesize two samples of diblock copolymers of 1,1-dimethyl-2,2-dihexylsilane (MHS) and 2-(trimethylsilyloxy)ethyl methacrylate, which differed only in the relative lengths of their blocks. Hydrolysis of the trimethylsilyl protecting groups gave the corresponding amphiphilic diblock copolymers, poly(l,l-dimethyl-2,2-dihexyldisilene)-fe-poly(2-hydroxyethyl methacrylate) (PMH S-fc-PHEMA), depicted in Pig. 22 [48]. 

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