Silicones step-growth polymerization

Many thermoset polymers of major commercial importance are synthesized by step-growth polymerization, as the case of unsaturated polyester, polyurethanes, melamines, phenolic and urea formaldehyde resins, epoxy resins, silicons, etc. In these systems, the crosslinking process, which leads to a polymer network formation, is usually referred to as curing. 

Figure 3.8 Step-growth polymerization mechanism for silicones.

Apart from the hydrolysis route to polysiloxanes there have been efforts to prepare appropriate difunctional silicon derivatives that can be used for step-growth polymerization. For example, diaminosilanes are good syn-thons for condensation with diorganosilanediols. This method is quite effective because of the labiUty of the Si-N bond. Thus, the reaction of Me2Si(NMe2)2 with diphenylsilanediol affords a random copolymer [26] (see Eq. 6.20). The randomization occurs because of the catalytic action of the liberated amine to cleave Si-O bonds. 

Most types of polymerizations can be performed in liquid and supercritical C02. The two major types of polymerizations, chain-growth and step-growth, have been demonstrated in C02. Reviews in the literature (Canelas and DeSimone, 1997b Kendall et al., 1999) have described numerous polymerizations in C02, many of which will not be discussed in this chapter. Since only amorphous or low-melting fluoropolymers and silicones show appreciable solubility at relatively mild temperatures and pressures (T< 100 °C, P<400 bar), only these two classes of polymers can be synthesized by a homogeneous polymerization in C02. All other types of polymers, including semicrystalline fluoropolymers and lipophilic or hydrophilic polymers, must be made by heterogeneous methods, such as precipitation, dispersion, emulsion, and suspension, since the polymers are insoluble in C02 (when T< 100 °C and P<400 bar). Some semicrystalline fluoropolymers and hydrocarbon polymers can be dissolved at more extreme temperatures and pressures and are discussed in Chapter 7 of this book. 

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. 

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. 

ABSTRACT. Polysilanes, (-SiRR -)n, represent a class of inorganic polymers that have unusual chemical properties and a number of potential applications. Currently the most practical synthesis is the Wurtz-type coupling of a dihalosilane with an alkali metal, which suffers from a number of limitations that discourage commercial development. A coordination polymerization route to polysilanes based on a transition metal catalyst offers a number of potential advantages. Both late and early metal dehydrogenative coupling catalysts have been reported, but the best to date appear to be based on titanocene and zirconocene derivatives. Our studies with transition metal silicon complexes have uncovered a number of observations that are relevant to this reaction chemistry, and hopefully important with respect to development of better catalysts. We have determined that many early transition metal silyl complexes are active catalysts for polysilane synthesis from monosilanes. A number of structure-reactivity correlations have been established, and reactivity studies have implicated a new metal-mediated polymerization mechanism. This mechanism, based on step growth of the polymer, has been tested in a number of ways. All proposed intermediates have now been observed in model reactions. 

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. 

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