EQUILIBRIUM RING CONCENTRATIONS AND STATISTICAL CONFORMATIONS OF POLYMER CHAINS .I. OLIGOMERIC DIMETHYLSILOXANES

The molar cyclization equilibrium constants Kx of dimethylsiloxanes [(CH3)2SiO]x have been measured in the bulk polymer to within ±5 per cent at 110°C (x=4-40) and 145°C (x=4-30). Values at 110°C are compared with those of Brown and Slusarczuk for an equilibrate containing 0.22 g. ml-1 siloxane in toluene, and it is concluded that K4-K10 increase markedly with solvent dilution whereas K11-K40 remain constant. Using the Jacobson and Stockmayer equilibrium theory of macrocyclization, the dimensions of dimethylsiloxane chains with 40-80 chemical bonds in the bulk polymer at 110°C are deduced. They are found to be similar to those in Brown and Slusarczuk's equilibrate and in two θ-solvents of different cohesive energy density. Dilution effects in dimethylsiloxane systems are contrasted with predictions of the Jacobson-Stockmayer theory, and the experimental molar cyclization equilibrium constants of the smallest siloxane rings are discussed in terms of the statistical properties of the corresponding oligomeric chains using Flory, Crescenzi and Mark's rotational isomeric state model of poly(dimethylsiloxane). © 1969.