Polysiloxane equilibration reactions

The synthesis and equilibration reaction kinetics involved in the preparation of aminopropyl-terminated polysiloxanes has been studied most extensively because of the utility of the amino-terminated species as components of a large number of segmented copolymers such as imides, amides, and ureas. 

Significant differences were observed in the rate of incorporation of D4 and l,3-bis(3-aminopropyl)disiloxane for similar concentrations of potassium, tet-ramethylammonium, and tetrabutylphosphonium siloxanolate catalysts. The rate differences affected the reaction times that were required to obtain a completely equilibrated reaction mixture with the desired molecular weight. The potassium catalyst required excessively long reaction times or high concentrations before sufficient incorporation of the aminopropyldisiloxane was realized. The tetramethylammonium and tetrabutylphosphonium catalysts were much more efficient for the preparation of controlled-molecular-weight aminopropyl-terminated polysiloxane oligomers. 

The aim of the present work was the synthesis of alternating copolymers of tetramethyldisiloxane and different diolefinic molecules via hydrosilylation polyaddition followed by an equilibration reaction with SiH-containing polysiloxanes (PMHS) and poly(dimethylsiloxane) (PDMS) (see Schemes 1 and 2). 

In equilibration reactions (Scheme 2) with SiH containing polysiloxanes and poly(dimethylsiloxane) catalyzed by (PNCbL the amount of SiH per polymer chain can be adjusted to the desired values. 

Depolymerization of poly(dimethylsiloxane) was postulated by Gmbb and Osthoff in 1955, but it was Scott in 1946 who first determined the equilibrium concentrations of D4, as well as of D3 and several larger maaocycles (a few months earlier, the equilibration reaction between cyclic and linear polysiloxanes was reported by Wilcock ). 

The acidic or alkaline medium hydrolysis of organosilanes generally results in a mixture of polysiloxanes with different linear and cyclic structures. The equilibration reactions that occur within structurally different silicones during the synthesis could play a major role in the physicochemical properties of the final polymer. The spectroscopic techniques are helpful in investigating the inherent characteristics of the chemical entities [2]. 

A solvent at this step is omitted since this may result in a misleading homogeneity at an early stage of equilibration. If a statistical distribution of the functio nalized siloxy-units is not achieved the properties of the resulting polysiloxane are unfavorable. After the reaction the mixture is diluted with 20 volume percent of water and shaken for another hour. The two phases are separated by centrifugation, the polysiloxane is diluted with 1 volume of ether and extracted with water until the test for sulfate is negative. The solvent is removed in vacuo. 

The first arises from the susceptibility of Si—O—Si linkages to undergo scission and bond re-formation by the action of strong bases (equations 26a and 26b). These processes can lead to a randomization ( shuffling or equilibration) of siloxane units between polysiloxane chains and hence to a broadened distribution of MW s and often to the liberation of polysiloxane homopolymer, either during synthesis or on subsequent contact with bases. Care has therefore to be taken in the selection of basic initiators or catalysts for use in polymerization reactions in this series. 

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