Dimethylsiloxanes, ring-chain equilibria

Carmichael, J. B. Gordon, D. J. Isackkson, F. J., Dilution Effects on Dimethylsiloxane Ring-Chain Equilibria. J. Phys. Chem. 1967, 71, 2011-2015. 

The effect of pressure on rmdiluted oligo(dimethylsiloxane) ring-chain equilibria was studied by Wright.The author formd that at 3500 atm, the weight content of smaller cydics D where D denotes the dimethylsiloxane unit and x = 4, 5, 6,... 

As predicted by theory, the position of the ring/chain equilibrium was found to be independent of the nature of the redistribution catalyst employed (acid or base) (4,13,24- ) and of the specific inert solvent used (26). Russian authors (4,27) equilibrated mixtures of eyelosiloxanes comprised of dimethylsiloxane (75 mole %) and either trifluoropropylmethyl, cyanoethylmethyl, or cyanopropylmethyl siloxane (25 mole %) in acetone at a siloxane repeating unit concentration of 0.833 moles/A. They measured the dipole moments of the respective cyclosiloxanes, [(CH3)9SiO]3[Si(CH3)R0j>, to be 2.76 for R = trifluoropropyl, 3.45 for R cyanoethyl, and 3.58 in the case of R cyanopropyl. The equilibrium weight fraction of rings,... 

The ring-opening polymerization of D4 is controlled by entropy, because thermodynamically all bonds in the monomer and polymer are approximately the same (21). The molar cyclization equilibrium constants of dimethylsiloxane rings have been predicted by the Jacobson-Stockmayer theory (85). The ring—chain equilibrium for siloxane polymers has been studied in detail and is the subject of several reviews (82,83,86—89). The equilibrium constant of the formation of each cyclic is approximatdy equal to the equilibrium concentration of this cyclic, Kn [(SiRjO) J. Thus the total concentration of cyclic oligomers in the equihbrium is independent of the initial monomer concentration. As a consequence, the amount of linear polymer decreases until the critical dilution point is reached, at which point only cyclic products are formed. 

An understanding of the thermodynamics of ring-chain equilibration reactions is important in Inorganic Chemistry where such reactions abound (sse, for example. Allcock (65), Gee (66), Gimblett (67), and Moedritzer (65)). Two such reactions are found (i) in polyphosphazene chemistry (69) where a polymerisation-depolymerisation equilibrium can be established (70) and (ii) in silicon-sulphur chemistry, where the analogue of the poly(dimethylsiloxane) system has been studied (71) and shown to contain cyclics such as the dimer ( (013)2 SiS)2 the trimer ((CH3)2SiS)3, as well as small amounts of chain molecules of limited chain length. 

The remarkable ease with which poly(dimethylsiloxane) enters into ring-chain equilibration reactions prompted the investigations described in this section, as well as those outlined in Section IIL(C). Both investigations were concerned with the question as to whether it is possible to study the statistical conformations of chain polymers by the equilibrium cyclic concentration method, if the polymer itself cannot undei o ring-chain equilibration reactions. The polymers chosen for study were n-paraffinic and polystyrene chains. In each case, dimethylsiloxane linkages were incorporated into the molecules and ring[Pg.56]

This illustrative study of ring-chain equilibria in a paraffin-siloxane system demonstrates that the conformational characteristics of chain molecules can be investigated provided suitable labile groups are incorporated into the molecular structures. It should prove within the capabilities of chemists to synthesise a wide variety of molecules suitable for study by the equilibrium cyclic concentration method using the dimethylsiloxane (or other siloxane) linkage as a labile linkage, incorporated into the molecular structure, throu whicJi to effect ring-cJiain equilibration reactions. 

Scales, L. E. Semiyen, J. A., Equilibrium Ring Concentrations and the Statistical Conformations of Polymer Chains Part 14. Calculation of the Concentrations of Medium Sized Cyclics in Poly(dimethylsiloxane) Equilibrates. Polymer 1976,17, 601-604. 

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