Silylenes polymerization

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

Recently, a variety of silylenes were generated and characterized by matrix isolation techniques. The observed loose donor adducts between silylenes and the matrix molecules (THF, CO) are only stable at very low temperatures. Melting of the matrix induces polymerization of the silylenes which proceeds through disilenes. However, 0->Si transfer reactions do not occur only in the case of 1-methyl-THF has an insertion of the silylene into the C —O bond been observed [155-158],... 

The polymerization reaction of silanes with Cp2ZrMe2 as catalyst has also been investigated by several research groups. Some evidence for a reaction mechanism proceeding through silylene complexes as intermediates has been given... 

The discussion about the mechanism of the dehydrogenative polymerization reaction has not yet been completed. However, the reaction mechanism seems to be strongly influenced by the specific random conditions that apply for each particular system. Presumably with late transition metals a silylene mechanism is more appropriate. It may be a matter of the steric constraints of the system to shift the reaction towards a-bond metathesis. 

The first step in the polymerization is the electron transfer from sodium to dichlorosilane and the formation of the corresponding radical anion. The latter upon elimination of the chloride anion is transformed to the silyl radical. To fit the chain growth mechanism, the reactivities of the macromolecular radicals must be higher than the reactivities of the monomeric radicals. The latter after electron transfer and elimination of chloride anion could be transformed to the reactive silylenes. Thus, in principle, two or more mechanisms of chain growth are possible ... 

Functionalization of polysilanes by chemical modification (post-polymerization) was covered in COMC II (1995) (chapter Organopolysilanes, p 101), where the formation of precursor polysilanes with potentially functionalizable side groups such as chloride, type 34 (via HCI/AICI3 chlorodephenylation of PMPS), 6 triflate, type 35 (via triflate replacement of phenyl groups)135,137 or alkyl halide (via chloromethylation of phenyl groups,138,139 type 36, or addition of HC1 or HBr to double bonds140) was discussed. Four other precursor polysilanes, which utilize the reactivity of the Si-Cl or Si-H bond, have been successfully applied in functionalization since COMC (1995) perchloropolysilane, 17 (see Section 3.11.4.2.2.(i) for synthesis),103 poly[methyl(H)silylene-f >-methylphenylsilylene],... 

Related crown ether-pendant polysilanes 45 were recently prepared by hydrosilylation post-polymerization functionalization of poly[methyl(H)silylene- -methylphenylsilylene], 37, although due to the low molecular weight of 37, the product 45 is also of low molecular weight.153... 

Scheme 14.4 Synthesis of poly(phenylene-silylene-ethylene)s via hydrosilylation polymerization process.

Transport polymerization has also been studied with other monomers, including methylene and other carbenes (phenylcarbene, 1,4-phenylenecarbene), silylenes such as Si2 and germy-lenes such as GeCl2 and Ge2 [Lee, 1977-1978 lee and Wunderlich, 1978]. 

At the same time, advances in the field of organosilicon chemistry have also been incredibly large2. Characterization of unstable species such as divalent silylenes or compounds with silicon-containing double bonds were successfully achieved for many compounds. Advances in the field of siloxanes and other polymeric materials are also remarkable. 

Besides the cr-bond metathesis mechanism proposed by Tilley23 for the dehydrogenative coupling of silanes, a Zr(II) pathway25 and a silylene mechanism26 have been proposed based on the nature of the products. The dehydrogenative polymerization of 1,2,3-trimethyltrisilane or of a mixture of diastereomers of 1,2,3,4-tetramethyltetrasilane showed evidence that, besides Tilley s mechanism, a further mechanism is present. The product formation can be explained by a silylene mechanism where the silylenes are formed by a-elimination from the silyl complexes by a new type of /J-elimination which involves Si—Si bond cleavage (/F-bond elimination) as described in Scheme 727. 

There is a dramatic kinetic stabilization by bulky substituents of the three-membered ring products from silylene addition to jr-bonds. Addition of t-Bu2Si to ethylene led to the first silirane with no substituents on its ring carbon atoms as a distillable liquid 164. Interestingly, l,l-di(terf-butyl)silirane does not undergo photochemical or thermal silylene extrusion, but instead polymerizes. A distillable silirane was also reported from addition of t-Bu2Si to 2-methylstyrene165. 

The silylene 79 abstracts oxygen or sulfur from isocyanates and isothiocyanates413. Cyclic siloxanes are the products with isocyanates (equation 129) these probably arise by polymerization of the silanone, Ar2Si=0 (83). 

The stable silylenes 83-85 do not react with conventional C=C double bonds however, diazasilole 83 is an efficient catalyst for the polymerization of alkenes, terminal alkynes, and 1,3-butadienes <2000ACR704, 2002USP028920, 2004JOM4165>. The stable bisaminosilylene 85 reacts with the activated double bond in 177-phosphirenes 134. The heterobicyclobutane 135 is however only a transient species and after addition of a second silylene 85 phosphasiletes 136 were isolated. Use of more sterically demanding substituted phosphirenes hampered the attack of the second silylene and the phosphasiletes 137 and 138, which are valence isomers of bicyclobutane 135, were obtained (Scheme 14) <2004AGE3474>. 

The existence and the relatively high stability of the silylene can be explained as a result of the singlet state. If the silylene were in a triplet state, polymerization would proceed faster and the formation of addition products would not be preferred. Herzberg postulated a singlet state, Nefedov433) found poor reactivity in all electrophilic reactions, and the difference in the reactivity in comparison to organic alkylcarbenes makes the triplet state unlikely. 

Polymeric Si-Si compounds can be formed by thermal or chemical polymerization of monomeric Si compounds, as already described. Some investigations indicated the appearance of intermediate radicals such as silylenes (see the review of Burger and Eujen673 and cf. p. 58) or others, which were sometimes isolated or identified spectroscopically. Recombination of these reactive units results in more or less irregular polymeric compounds. 

Polymeric silicon fluoride Silicon fluorides in the polymeric state are products of the recombination of monomeric SiF2 radicals. These SiF2 radicals are described in detail in Chapter 7 Silylenes (see p. 58). 

Polymeric silicon alkyl compounds Polymeric compounds are often observed in the Wurtz synthesis of dihalodialkyl(aiyl)silanes with metals. The intermediate product is probably a dialkyl(aryl)silylene (see Chapter 7). A polymeric dimethylsilane was found by Burkhard68). A yellow polymeric (SiCH3) was isolated after pyrolysis of SiH4 with ethylene156). 

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