Abstract
Theoretical molar cyclization equilibrium constants K x for dimethyl siloxane cyclics [(CH 3) 2SiO] x in undiluted poly(dimethyl siloxane) equilibrates are calculated for values of x in the range 7 < x < 13. The Jacobson and Stockmayer theory is used for the calculations and K x values are computed by finding the statistically weighted fraction of the 3 2 x−3 conformations [defined by the Flory, Crescenzi and Mark (FCM) rotational isomeric state model of poly(dimethyl siloxane)] that have terminal atoms in close proximity for ring formation. The FCM model gives theoretical K x values in good agreement with experiment for x = 8, 9, 10 but yields a K 11 value that is nine times greater than that found experimentally. This result is discussed in terms of the molecular structure of dimethyl siloxane chains. The effect on the calculated K x values of modifying the FCM model by assigning a smaller value to the SiOSi bond angle is explored.
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