Cyclosiloxanes kinetic studies

The mechanism of anionic polymerization of cyclosiloxanes has been the subject of several studies (96,97). The first kinetic analysis in this area was carried out in the early 1950s (98). In the general scheme of this process, the propagation/depropagation step involves the nucleophilic attack of the silanolate anion on the sUicon, which results in the cleavage of the siloxane bond and formation of the new silanolate active center (eq. 17). 

The lack of reliable information on kinetic parameters for the copolymerization of important cyclosiloxanes with ring sizes exceeding three is a serious deficiency. Industrial and routine laboratory syntheses rely almost entirely on such monomers, and in many cases, the processes are copolymerizations. An understanding of these constants would give valuable information on copolymer microstructure and its control. Unambiguous studies... 

The anionic polymerization of cyclosiloxanes was examined In benzene and toluene with lithium cryptates as counterions. Only one type of active species Is observed In the case of LI + [211] thus, the kinetics of the propagation and of the by-product cyclosiloxanes formation can be studied In detail for the first time. The reactivity of cryptated sllanolate Ion pairs toward the ring opening of D3 Is greatly enhanced compared to that of other systems. 

Chojnowski and Mazurek16 have studied the reaction of phenyldimethyl silanolates with cyclosiloxanes under conditions where siloxane bond redistribution reactions involving the polymer and the catalyst are completely suppressed. For the reaction of sodium phenyldimethyl silanolate (I) with 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane (II) they find that the rate of disappearance of I in an excess of II follows first-order kinetics. The observed first-order rate constant k varies with the initial... 

Anionic and cationic homopolymerizations of cyclosiloxanes in stable monomer dispersions, with particle sizes typically less than 1 pm, were first studied extensively in an industrial environment [94,95], due to their practical interest, inherent reactivity and straightforward preparation. Working in an aqueous medium may have been motivated by the peculiar kinetics (compared to bulk polymerization)... 

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