Silicon ADMET polymerization

ADMET polymerization has been used to integrate silicon into linear and network hydrocarbon polymers in an attempt to produce novel materials with enhanced thermal and mechanical stability. While ADMET has been used to produce copolymeric architectures unattainable through conventional methods, application of this polymerization to synthesis is feasible only if the silicon-based functionality does not inhibit metathesis. This research, initiated in the early 1990s by Wagener and colleagues, has shown that the silane and siloxane... 

ADMET polymerization represents a versatile technique for the synthesis of unique, complex, and functional polymer stractures. ADMET is a step-growth polycondensation reaction that proceeds under mild conditions, whereby any molecule that can be functionalized with two terminal olefin groups has the potential to become an ADMET monomer. This, in turn, allows an almost Hmitless possibility to create interesting and useful polymer structures. Recently, ADMET has been used to synthesize functionalized PEs, silicon-containing elastomers, conductive polymers, and many other exotic and interesting macromolecules. Yet, this area of research is by no means exhausted rather, this simple and elegant reaction will continue to provide the means to explore the basic stracture-property relationships of complex functional materials. 

As has already been mentioned, divinyl silicon derivatives, similarly to monovinyl-substituted silicon compounds, are also completely inert to productive homometathesis, particularly as far as acyclic diene metathesis (ADMET) polymerization is concerned. However, we have shown in earlier reports that in the presence of ruthenium, rhodium and cobalt complexes containing or generating M-H and/or M-Si bonds, divinyl-substituted silicon compounds undergo de-ethenated (poly)condensation to yield a mixture of oligomers and cyclic unsaturated siloxanes, silazanes and caibosilanes, as shown in Scheme 2 [21-29]. 

As a continuation of our earlier work on silicon-containing polymers, [6-9] and in light of the mentioned features of ADMET polymerization, we have applied this methodology in the synthesis of covalent organometallic polymers containing germanium and tin. 

SILYLATIVE COUPLING POLYCONDENSATION (SCP) VS. ADMET POLYMERIZATION OF DIVINYLSUBSTITUTED SILICON COMPOUNDS... 

Acyclic Diene Metathesis (ADMET) Polymerization of dialkenylsubstituted silicon compounds (except of divinylderivatives) can provide a universal route toward synthesis of well-defined linear organosilicon polymers. Under some conditions the ring closure of the substrate occurs in the presence of metallacarbene (Ru, Mo, W, Re) to yield silicon-containing cyclic olefins efficiently. 

The first attempt at ADMET polymerization of a silane diene, specifically dimethyldivinylsilane, was unsuccessful [20]. This was presumably due to the unfavorable steric interactions between the trisubstituted silicon atom and the adjacent olefin, analogous to results obtained by Schrock and coworkers with vinyltrimethylsilane [8b]. The reaction of Schrock s [W]l catalyst with... 

Polymerization data are summarized in Table 1. These results indicate that ADMET polymerization of monomer (1) and (2) using electrochemically reduced WCle-based catalyst proceeded with good selectivity to the silicon containing polymer (3) and (4) [8], The GPC analysis of these polymers showed Mn values of 9100 polymer (3) and 4500 polymer (4). The polydis-persity indexes are 2.28 and 2.05, respectively, which well fit the step-growth mechanism of the ADMET polymerization. 

The optimum conditions for ADMET polymerization of silicon contaning dienes (3) and (4) are molar ratio of monomer/catalyst 120 1 and about 48 h reaction time. 

Matloka, P.P.and Wagener, K.B. (2006) The acyclic diene metathesis (ADMET) polymerization approach to silicon containing materials. Journal of Molecular Catalysis A Chemical, 257,89-98. 

Sigma, Pi Conductive Hybrid Silicon Polymers The facile nature of ADMET polymerization leads to other opportunities for producing clean polymer structures. This chemistry occurs at room temperature or even lower if preferred, and as a consequence, it became evident that questions might be answered regarding structure performance behavior. The objective of this section is to describe the... 

The ADMET step condensation polymerization of carbosila- and carbosiloxa-dienes has generated a new class of unsaturated silicon-containing polymers when monomer structure/reactivity relationships, as described earher for the polar functionalities, are obeyed (equation 22). 

Metathesis of 1,9-decadiene and cyclooctene with trialkoxy- and trisiloxy-substituted vinylsilanes in the presence of Grubbs catalyst, carried out in appropriate conditions, leads to the formation of bis(silyl)diene with a high yield. Similar processes performed with divinyl-substituted siloxane lead to the formation of silicon-containing polymers (via ADMET copolymerization and tandem ROM/CD polymerization), thus opening a new convenient route to synthesis of unsaturated organosilicon copolymers. 

The Synthesis of Polycarhosilanes. Silicon-based polycarbosilanes do not exist naturally. A number of synthetic methods have been used to prepare them, but only now can the synthesis of polycarbosilanes, via ADMET chemistry, be realized (72). Our initial work began with the monomers, dimethyldivinylsilane and diphenyldivinylsilane. The attempted condensation of these monomers in a typical ADMET scheme did not lead to the formation of polymer. The explanation for the nonreactivity of these monomers can be attributed to steric interactions which preclude the formation of the required metallacyclobutane as illustrated in the polymerization cycle shown in Figure 3. This observation has precedence considering Schrock s work with vinyltrimethylsilane as illustrated in Figure 7 (13). Here it becomes evident that the steric interaction required to form the metallacyclobutane is inhibited by the... 

Typical of all carbosilane and carbosiloxane polymerizations is the fact that the trans stereochemistry predominates over the cis with respect to the olefin that is in the backbone. This trans arrangement arises from the polymerization mechanism leading to the connection of olefin units. This is observed to be true, not only for silicon polymers, but for all ADMET polymers made to this point, regardless of whether functionality is present in the backbone. 

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