Silacyclobutanes, ring-opening polymerization

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>. 

Weyenberg and Nelson (7) reported that the strained silacycles, 1,1-di-methyl-1-silacyclobutane (1) and l,l,3,3-tetramethyl-l,3-disilacyclobu-tane (2) undergo ring opening polymerization in the presence of platinum on carbon or chloroplatinic acid [Eqs. (11) and (12)]. 

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-... 

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. 

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]. 

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. 

The initial step in the reaction mechanism is formulated as an oxidative addition of the silacyclobutane to the transition-metal complex attaching Si to M (ring expansion). It is followed by a transfer of L2 from the metal to the silicon (ring opening) and polymer growth by insertion of further coordinated ring into the metal-carbon bond, similar to the mechanism proposed for olefin polymerization by Ziegler-type catalysts. 

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

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