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M. Cypryk


Kinetics of condensation of oligodimethylsiloxanediols catalysed by protic acids

Polimery 1995, No 7-8, 421


DOI: dx.doi.org/10.14314/polimery.1995.421

Summary

The kinetics of the condensation of oligo(dimethylsiloxanediols)s H(OSiMe2)nOH, n - 3, 4, 7, has been studied in CH2Cl2 in the presence of CF3SO3H as catalyst. The aim of this study was a deeper understanding of the mechanism of the condensation of silanols in acid-base inert media in order to achieve a better control of the process and, in particular, of the formation of cyclic by-products. Complex kinetic dependencies show the importance of the association processes in the mechanism of catalysis by the acid, involving the formation of hydrogen complexes of the acid with silanol and water. The catalytic activity of CF3SO3H decreases with the increase in silanol groups concentration in the system. The reactivity of siloxanediols decreases with the chain length up to at least 9 siloxane units in the chain. Comparison of the rates of cyclisation and of linear growth of oligosiloxanediols as a function of the chain length shows that the classical mechanism of inter-intra catalysis involving basic assistance of a silanol group in the condensation act becomes less import ant in the case of longer oligomers (n > 7). The assistance provided by siloxane oxygen from the chain dominates in this case. This effect causes the change of the kinetic law of cyclisation. Consequently, the yield of the cyclic by-products in the condensation of longer oligosiloxanediols strongly decreases with the initial SiOH concentration as expected for the classical scheme of competition of intermolecular condensation and cyclisation.


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