Macrocycles. 21. Role of Ring−Ring Equilibria in Thermodynamically Controlled Polycondensations

A new hypothesis is presented saying that an ideal thermodynamically controlled polycondensation is characterized by the following three aspects: first, the ring−chain equilibria automatically include ring−ring equilibria which become decisive for the thermodynamical properties of the system at high conversions. Second, at 100% conversion, all reaction products are cycles. Third, the chain growth is limited by the thermodynamical properties of the ring−ring equilibrium. The following polycondensations were discussed: base-catalyzed transesterification of ethyl cholate and combinations of Bu 2 Sn(OMe) 2 with α,ω-poly(ethylene glycol)s, with poly(tetrahydrofuran)diols or with two oligosiloxane diols. Moreover, equilibrations of oligo- and polysiloxanes and polycondensations of Bu 2 Sn bisacetate with various aliphatic dicarboxylic acids are discussed. Furthermore, polycondensations of pentaerythritol and its oligoether derivatives with Bu 2 Sn(OMe) 2 yielding spirocycles are discussed. It is demonstrated by a variety of chemical or analytical methods that Sn-containing polymers were never formed and that cycles were the only reaction products. These results clearly support the above formulated modifications of the Jacobson−Stockmayer theory and of the Carothers−Flory theory.