Polymerization of silacyclobutanes

Anionic polymerization of 1,3-disilacyclobutanes also is possible. Solid KOH and alkali metal silanolates were mentioned as being effective by Russian authors [18, 19. 20]. However, alkyllithiums, which can initiate polymerization of silacyclobutanes (eq. 8) [21], do not initiate polymerization of 1,3-disilacyclobutanes [18, 22]. The problem is one of steric hindrance. 

Though platinum and its complexes induce the polymerization of silacyclobutanes, palladium complexes catalyze the insertion of alkynes into the silacyclobutane ring. This reaction is very clean and provides a useful synthetic route to silacyclohexenes (Scheme... 

The anionic ring-opening polymerization of I is unexpected because under similar conditions, allyltrimethylsilane undergoes metallation to yield an a-trimethylsilyl-substituted allyl anion (14-16). Relief of ring strain may be an important factor in facilitating this ring-opening polymerization. For example, the polymerization of silacyclobutanes is catalyzed by various nu-... 

Table I. Polymerization of Silacyclobutanes by Transition Metal Catalysts ...

Ushakov NV, Finkel shtein ESh, Babich ED (1995) Polymerization of silacyclobutanes. Polym Sci SerA 37 470 92... 

Fig. 5. Mechanism of the transition metal-catalyzed polymerization of a silacyclobutane.

A more useful thermolytic polymerization which produces linear polysilmethylenes is that of 1,3-disilacyclobutanes carried out in the liquid phase. Such polymerization of l,l,3,3,-tetramethyl-l,3-disilacyclobutane was reported first by Knoth [17] (eq. 7). This process was studied in some detail by Russian workers [18]. l,l,3,3-Tetramethyl-l,3-disila-cyclobutane is more thermally stable than 1,1-dimethyl-l-silacyclobutane. 

Aryl and, more so, chlorine substituents on silicon enhance thermal stability of silacyclobutanes. The rate of the first-order thermal decomposition of silacyclobutanes varies inversely with the dielectric constant of the solvent used. Radical initiators have no effect on the thermal decomposition and a polar mechanism was suggested. Thermal polymerization of cyclo-[Ph2SiCH212 has been reported to occur at 180-200°C. The product was a crystalline white powder which was insoluble in benzene and other common organic solvents [19]. 

As Weber has shown, polymers bearing reactive Si-H bonds can be formed by ring-opening polymerization of 1-silacyclobutane <1992PB281>. These polymers can be further converted into Si-functionally-substituted polycarbo-silanes by Pt-catalyzed hydrosilylation reaction of Si-H functionality with alkenes (Scheme 18) <1993MM563>. 

The polycarbosilane [(CiyisSi -CsH ZrCy, 159, was prepared from the ring-opening polymerization of the spirocyclic silacyclobutane-bridged monomer (Ciy Si -CsH ZrCy 158, with Karstedt s catalyst (Scheme 2.42).229 The ring opening of the zirconocene ring was also attempted, but polymerization could not occur under the reaction conditions. Both 158 and 159 showed some catalytic activity for ethylene polymerization. 

We proposed the concept of a carbanion pump, where a silacyclobutane having high ring distortional energy was used to convert an oxyanion into a carbanion, which can further initiate polymerization of styrene as shown in Scheme 4.5. 

During the reaction of the silicon dimethylsubstituted silacyclobutanes la,b with MeLi/HMPA and subsequent methanolysis (Scheme 1), the head-to-head dimers 2a,b, which are separated from simultaneously formed oligomers 3a by distillation, are isolated as main products. The linear dimers 2a,b are fully characterized by the usual analytical methods ( H, C, Si NMR, GPC, and MS). Obviously, the formation of oligomers 3a results from anionic ring opening polymerization of la. 

Reacting la,b with MeLi in HMPA as active solvent and in the presence of MeOH as trapping agent, the attack of a methyl anion at the silicon atom of the silacyclobutane is the first reaction step and gives a pentacoordinated silicon anion (Scheme 2). Such five-coordinated species are discussed as intermediates during the ring opening polymerization of silacyclobutenes, -butanes, and -pentenes [2]. Furthermore, five-coordinated silicon species are well described to be stable compounds [3]. 

It is known from literature that the anionic polymerization of silaheterocycles [2] usually occurs very fast. From that it can he concluded that the formation of any primary carbanions, such as 5a,b and 8a,b, and its subsequent attack at a silacyclobutane should lead to chain propagation. In contrast, the formation of a secondary carbanion stops polymerization. From the poor yield of oligomers 3a we suppose that the formation of primary carbanions is disfavoured. The reason for that might result from the bulky neopentyl-substituent, which shields the silicon from the attack giving primary carbanions. In the case of educt lb the steric overcrowding at the silicon center is even more enlarged by the 1-methylvinyl-substituent. This obviously completely suppresses the formation of the primary carbanion 5b and consequently, no oligomeric and polymeric materials are formed. 

The platinum-catalyzed or thermal ring opening polymerization of the substituted silacyclobutane monomers yields linear polycarbosilanes. The platinum-catalyzed hydrosilations of the AB (allyl) and AB2 (diallyl) monomers yield linear and hyperbranched polycarbosilanes, respectively. A wide range of random copolymers are readily available from polymerization of mixtures of the silacyclobutane or AB and AB2 monomers. The preparation of random copolymers is often of interest for achieving desired physical properties. 

Polysilaethylenes that contain alkyl groups on the silicon can also be prepared by the H2ptCl6 catalyzed ring-opening polymerization of the corresponding silacyclobutanes, [RRSiCH2]2 (see Eq. 1.30) [42]. 

Abstract The presented paper is a summary of our results on synthesis and polymerization of silyl-containing norbomenes and norbomadienes via ring-opening metathesis polymerization (ROMP) and addition processes as well as ring-opening polymerization (ROP) of silacyclobutanes and disilacyclobutanes. The synthesis of heterochain and carbochain polymer families with regularly varied substituents at Si atom and various number and location Si(CH3)3-substituents has been realized. Systematic study of gas transport parameters of polycarbosilanes series of different classes allowed us to find out real eorrelations between features of chemical polymer structure and its gas separation eharacteristics. 

Molecular design and precision synthesis of silicon-containing polymers are described. Polymerizations of substituted silacyclobutanes by phenyllithium and platinum complexes gave poly(carbosilane)s of head to tail regular structure. However, extensive chain transfer seems to have occurred in the polymerization by platinum complexes. 

By Transition Metal Complex. The results of polymerization of dimethyl-silacyclobutane derivatives using transition metal complexes are summarized in Table I. 

To prepare amphiphilic siltrimethylene polymers, the authors of [47] synthesized two symmetrically substituted MSCB monomers carrying oxyethylene groups in substituents - l,l-bis[4-(tert-butyldimethylsilyloxy-monooxyethylene)phenyl]-l-silacyclobutane and 1,1 -bis[4-(o)-/ert-butyldimethylsilyloxy-trioxyethylene)phe-nyl]-1-silacyclobutane. The polymerization of these monomers occurred at —78°C or —48°C in the presence of n-BuLi in THF without any HMPA additives. The use... 

In the case of the latter monomer (with the triple bond in a substituent), the molecular weight of polysitrimethylenes produced by the Pt-catalyzed ROP of MSCBs was controlled by the dosed addition of EtsSiH. The specific features of this process were examined by example of polymerization of 1,1-dimethyl-1-silacyclobutane carried out in the presence of Pt(l,5-cod)2. This gave a chance to develop the general mechanism of the ROP of MSCBs catalyzed by Pt-compounds... 

Ushakov NV, Fedorova GK, Finkelshtein ESh (1995) Synthesis and polymerization of Si-substituted silacyclobutanes. Izv Acad Nauk Ser Khim 2 2475-2477... 

Nametkin NS, Vdovin VM, Zav yalov VI (1965) Polymerization of 1,1-substituted silacyclobutanes. Izv Acad Nauk Ser Khim 8 1448-1453... 

Ushakov NV, Pritula NA, Rebrov A1 (1993) Synthesis and polymerization of 9-carbazolyl-containing 1-silacyclobutane derivatives. Russ Chem Bull 42 1372-1376 [translated from (1993) Izv Acad Nauk Ser Khim 8 1437-1441]... 

Finkelshtein ESh, Ushakov NV, Portnykh EB et al. (1993) Ring-opening metathesis and ther-moinitiated polymerization of 1-methyl-l-norbornenylmethyl-l-silacyclobutane. Vysoko-... 

Matsumoto K, Miyagawa K, Yamaoka H (1997) Anionic polymerization of 3-methylene-silacyclobutanes and reactivity of poly(3-methylenesilabutane)s. Macromolecules 30 2524-2526... 

Alekhin NN (1975) Investigation of polymerization of 1,1-dimethyl-1-silacyclobutane in the presence of alkali-metal silanolates, alkoholates, and alkatis. Ph D Thesis Academy of Sciences of the SSSR Topchiev Institute of Petrochemical Synthesis, Moscow... 

Sheikh MRK, Tharanikkarasu K, Imae I, Kawakami YY (1999) Application of sUacy-clobutanes as Carboanion Pump . Anionic polymerization of styrene using potassium-tert-butoxide and silacyclobutanes. 1st International Workshop on Silicon Chemistry-Polymers (May 29-31 1999, Tatsunokuchi, Japan) [Poster presentation]... 

Sheikh Md RK, Imae I, Tharanikkarasu K et al. (2000) Silacyclobutanes as Carban-ion pump in anionic polymerization. I. Anionic polymerization of styrene by potassium t-butoxide in the presense of silacyclobutanes. Polym J 32 527-530... 

Yamashita H, Tanaka M, Honda K (1995) Oxidative addition of the Si-C bonds of silacyclobutanes to Pt(PEt3)3 and highly selective platinum (O)-catalyzed di- or polymerization of 1,1-dimethyl-l-silacyclobutane. J Am Chem Soc 117 8873-8874... 

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