Silicon catalysts ring-opening polymerization

Mechanism of Equilibration. The generally accepted mechanism for the base-catalyzed ring-opening polymerization of cyclosiloxanes involves attack of the basic catalyst at the silicon atom (15). It has been proposed, and generally accepted, that the active species is a partially dissociated siloxanolate anion (13). In the results presented in this chapter, significant differences in reaction rates were observed as the corresponding cation of the siloxanolate species was varied. The more rapid disappearance of D4 and aminopropyldisiloxane in the presence of these catalysts increased in the following order ... 

Polymer structures are often described by using the capital letters M (mono-), D (di-), T (tri-), and Q (quadruple-) indicating the functionality of the respective monomer units. Thus linear trimethylsilyl terminated silicone fluids are characterized by M(D)xM. Linear polyorganosiloxanes can be produced either from cyclic organosiloxanes by ring-opening polymerization, which is promoted by both anionic or cationic catalysts [3, 4], or from oligomeric hydroxyterminated dimethylsiloxanes by polycondensation in the presence of acid catalysts [5, 6],... 

Temple K, JaMe F, Sheridan JB, Manners 1 (2001) The nature of the active catalyst in late transition metal-mediated ring-opening polymerization (ROP) reactions mechanistic studies of the platinum-catalysed ROP of silicon-bridged [l]ferrocenophanes. J Am Chem Soc 123 1355-1364... 

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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