Siloxane polymers redistribution reactions

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... 

In many cases, these cyclic siloxanes have to be removed from the system by distillation or fractionation, in order to obtain pure products. On the other hand, cyclic siloxanes where n = 3 and n = 4 are the two most important monomers used in the commercial production of various siloxane polymers or oligomers via the so-called equilibration or redistribution reactions which will be discussed in detail in Sect. 2.4. Therefore, in modern silicone technology, aqueous hydrolysis of chloro-silanes is usually employed for the preparation of cyclic siloxane monomers 122> more than for the direct synthesis of the (Si—X) functional oligomers. Equilibration reactions are the method of choice for the synthesis of functionally terminated siloxane oligomers. 

The process of synthesizing high-molecular-weight copolymers by the polymerization of mixed cyclics is well established and widely used in the silicone industry. However, the microstructure which depends on several reaction parameters is not easily predictable. The way in which the sequences of the siloxane units are built up is directed by the relative reactivities of the monomers and the active chain-ends. In this process the different cyclics are mixed together and copolymerized. The reaction is initiated by basic or acidic catalysts and a stepwise addition polymerization kinetic scheme is followed. Cyclotrisiloxanes are most frequently used in these copolymerizations since the chain growth mechanism dominates the kinetics and redistribution reactions involving the polymer chain are of negligible importance. Several different copolymers may be obtained by this process. They will be monodisperse and free from cyclics and their microstructure can be varied from pure block to pure random copolymers. 

The reaction of PHMS to yield PMMS-type comb polysiloxanes is essentially quantitative for methoxypoly(ethylene glycol)s of up to 500. Substitution yields diminish for longer glycols, as indicated by the presence of residual Si-H groups in the IR and NMR spectra. Si NMR spectra and GPC data revealed that PM MS-8 is contaminated with nearly 25% cyclic products and that it also contained a considerable number of branched trisiloxy units. Both cyclic products and trisiloxy units are the result of redistribution processes common in nucleophilic siloxane reactions. The hydrosilylation reaction yielding the PAGS polymers is also quantitative, but no cyclic products are formed. 

Side reactions or postcuring reactions are possible in the formation of silicone networks. In most cases, the silicon-bound hydride is in stoichiometric excess to enhance reaction rates. Disproportionation reactions involving terminal hydride groups have been reported (222). A major side reaction consumes Si—H to give redistributed siloxane in the resulting polymers and gaseous silane as a by-product. 

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