Macrocyclization under thermodynamic control. A theoretical study and its application to the equilibrium cyclooligomerization of .beta.-propiolactone

Abstract

A general treatment of macrocyclization reactions occurring under thermodynamic control is presented. The fundamental quantities on which the treatment is based are the effective molarities of the cyclic oligomers and the equilibrium constant for the intermolecular model reaction between monofunctional reactants (Kinter). Four typical cases have been considered, namely, addition and condensation of a monomer of the type A-B, addition of A-A, and addition of A-A + B-B. A critical comparison with the classical theory of Jacobson and Stockmayer is presented.It is shown that the phenomenon characterized by the critical monomer concentration (cut-off point) is a limiting phenomenon which would occur only for infinitely large values of Kin,,,. The treatment has been successfully applied to the DOS/DTC-induced cyclooligomerization of /3-propiolactone in CDC13 solution that yields well-behaved ringchain equilibrates closely adhering to the theoretical model. Best fit of the experimental product distributions to the general equations gave the equilibrium constant (Kinter) of the intermolecular model reaction, as well as the effective molarities (EMi) for the cyclic oligomers from trimer to octamer. The EMi values decrease in proportion of the -2.5 power of the oligomerization degree, thus providing a strong indication that the oligomeric polylactones are essentially strainless. The extremely low value of Kin,,, (2.5) is responsible for the absence of a cut-off point, which is usually present in ring-chain polymeric equilibrates.

Controlling supramolecular polymerization through multicomponent self‐assembly

“…Ercolani has extended the Jacobson and Stockmayer theory 113 for cyclic and linear species in step-growth covalent polymers in order to correlate K inter and K intra to weight and number averaged degrees of polymerization. 114 Already in the first report of UPy based supramolecular polymers, Sijbesma and Meijer showed that the concept of chain stopper can be successfully applied to self-assembled macromolecules. 3 By dissolving a small amount of a low-molecularweight bifunctional UPy1-C 6 -UPy1 in chloroform a solution with a high specific viscosity g sp was obtained (Fig.…”

Supramolecular Buffering by Ring–Chain Competition

“…19 In applying the theory to describe experimental systems, the assumption of strainless rings is not always met.…”

“…19 Thus, in effect, component buffering is caused by competition between ring formation by UPy-UPy association and end-capping by UPy-NaPy binding. This competition also explains why the onset of the buffering plateau -and concurrently UPy-NaPy binding -occurs at a much higher concentration than the binding of monotopic UPy and NaPy (K UPy-NaPy = 5 .…”

Scope and Limitations of Supramolecular Autoregulation

“…5,6 Ditopic supramolecular compounds have the ability to form either rings (K intra ) or linear polymers (K inter ), where the stability of the rings can be defined by the effective molarity (EM = K intra / K inter ). 7,8 Since the EM represents a local concentration, the formation of intramolecular contacts, i.e. rings, is more favorable at low concentrations, while polymerization is favored at high concentrations.…”