Sonochemical Synthesis of Polysilylenes by Reductive Coupling of Disubstituted Dichlorosilanes with Alkali Metals

Abstract

Monomodal polysilylenes with relatively narrow molecular weight distributions (MJMn < 1.5) have been prepared by the reductive coupling of methylphenyldichlorosilane and di-n-hexyldichlorosilane with alkali metals in toluene in the presence of ultrasound. The optimum conditions for the sonochemical synthesis of well-defined polysilylenes in the presence of an immersion-type probe were studied. Sonochemical synthesis is accompanied by selective degradation which decreases the molecular weight to the limiting value of Mn « 50 000 and also reduces polydispersities (MJMn < 1.2). Polymerization depends strongly on the substituents on the silicon atoms, solvents, alkali metals, and temperature. For example, the polymerization of methylphenyldichlorosilane in toluene with potassium, in contrast to that with sodium, does not produce a polysilylene because the reactivity of potassium is too high, leading to side reaction.

Organosilicon Polymers?Synthesis, Architecture, Reactivity and Applications

“…22 Usually a trimodal weight distribution is observed using alkali-metal (sodium) dehalocoupling: the low-molecularweight fraction consists of cyclosilanes mainly with four-, five-and six-membered rings, the second fraction contains a mixture of lowermolecular-weight ( < 10 000) and the highest fraction consists of higher-molecular-weight ( > 100 000) polymers. 14,23 Cyclosilanes are probably formed either by intramolecular cyclization reactions of oligomeric tetra-, penta-or hexasilanes containing both Si-Cl and silylsodium ends (end-biting), or by polymer degradation, when active sites attack through back-biting. 18,23 The simultaneous formation of lower-and higher-molecular-weight fractions is not well understood so far.…”

“…14,23 Cyclosilanes are probably formed either by intramolecular cyclization reactions of oligomeric tetra-, penta-or hexasilanes containing both Si-Cl and silylsodium ends (end-biting), or by polymer degradation, when active sites attack through back-biting. 18,23 The simultaneous formation of lower-and higher-molecular-weight fractions is not well understood so far. It seems that as the degree of polymerization approaches ≈ 50, the probability of the chain leaving the metal surface increases.…”

“…This process may be strongly affected by solvent and temperature. 23 Indeed, polar solvents and additives, e.g. crown ethers, cryptands or glymes, increase polymer yields but usually decrease molecular weights.…”

“…crown ethers, cryptands or glymes, increase polymer yields but usually decrease molecular weights. 14,23,24 Polar solvents appear to promote faster initiation leading to lower molecular weights since more chains form simultaneously, and thus competition for monomers is higher. It is also evident that polar entities solvate -SiR 2 Na end groups more efficiently, favoring their departure from the metal surface to the solution.…”

Electroreductive Synthesis of Polysilanes with Ordered Sequences

“…This method, however, requires drastic reaction conditions and hence, is very much limited in the type of substituent that is allowed to be located on the monomer. Although several modified or alternative methods such as sonochemical coupling of dichlorosilane promoted by alkali metal [9][10][11], transition metal catalyzed reaction of hydrosilane [12,13], anionic polymerization of masked disilene [14,15], and ring opening polymerization of cyclic organosilane [16,17] have been proposed, they are not always extensively effective as preparative methods.…”