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Lewis acid mediated polymerization of poly(dimethylsiloxane) polymers : investigating reaction kinetics using both NMR spectroscopy and cyclic voltammetry

Apedaile, Alistair and Liggat, John and Parkinson, John and Nikiforidis, George and Berlouis, Leonard and Patel, Mogon (2012) Lewis acid mediated polymerization of poly(dimethylsiloxane) polymers : investigating reaction kinetics using both NMR spectroscopy and cyclic voltammetry. Journal of Applied Polymer Science, 123 (5). pp. 2601-2608. ISSN 0021-8995

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Abstract

Bulk condensation polymerization of (dimethylmethoxy)-m-carborane and (dichlorodimethyl)-silane occurs in the presence of an Mx+Clx Lewis acid catalyst. In the literature, FeCl3 is commonly used as the catalyst of choice but little is known about the activation energy and entropy of this polymerization. By monitoring using 1H-NMR the reaction of a methoxy-terminated poly(dimethylsiloxane) and (dichlorodimethyl)silane the rate determining step in the FeCl3 catalyzed system is determined. The activation energy was calculated to be +43.6 kJ mol-1 and the entropy of the reaction was also calculated. The calculated large entropy of reaction indicates that the transition step is highly ordered. The formation of the electrophile intermediate species in the first step of the reaction has also been investigated using cyclic voltammetry. To the cyclic voltammetry data Randles-Sevcik fits have been applied to the oxidation peaks to determine the diffusion coefficients for the oxidation of Fe2+ to Fe3+ . Also, the initial prediction of a reversible reaction Step 1 was shown to be incorrect as the normalized reduction peak maxima increase with scan rate, indicative of an electron transfer-chemical reaction mechanism.