Ring-chain competition

Paffen TFE, Ercolani G, de Greef TFA, Meijer EW. Supramolecular buffering by ring—chain competition. rim Chem Soc. 2015 137 1501-1509. 

GENERALISATION OF RING-CHAIN COMPETITION KINETICS TO EMBRACE THE POST-GEL REGION SMOOTH CONTINUATION OF THE RATE CURVE THROUGH THE GEL POINT. 

The ring-chain competition model (Appendix A) postulates one intermolecular rate constant which applies to any pair of suitable free functionalities located on separate molecules, and one intramolecular rate constant k. Any suitable pair of free functionalities, located on the same molecule, reacts with rate proportional to Here z is the number of atoms in the ring... 

Irzhak et al. [36, 37] contributed two papers dealing with ring-chain competition in non-ideal KC polymerizations. The consequences for the number and mass distributions of the reaction products were discussed. Furthermore, six publications should be mentioned [38 3] reporting on the diffusion control of the rates of cyclization reactions. Yet, the course of KC polycondensations was not discussed. Furthermore, it should be mentioned that several authors have published... 

Finally, it should be noted that in none of the aforementioned publications the consequences of the ring-chain competition for the Carothers equation was discussed, although it is evident that this equation in its original form is not applicable anymore, when significant amounts of cyclics are formed, regardless, if in KC or TC step-growth polymerizations. 

In general [for a review, see (92KGS851)], this type of equilibrium actually proceeds via the scheme ring-chain-ring, but with a very small concentration of the intermediate open-chain tautomer at equilibrium, which prevents its detection by the methods used for the investigation of these equilibria. The two cyclic tautomers may be formed either by the competitive intramolecular addition of two different nucleophilic groups XH to one multiple bond, or by the same process, but of one nucleophilic group to the two different polar multiple bonds. 

Heterochain polymers produced by ring-opening polymerization contain the hetero-atoms in the main chain as well as in the monomer and the polymer chain competes with the monomer for the reaction with the propagating species. This competition leads to polymer transfer and back-biting reactions during the polymerization. Heterochain polymers are also susceptible to depolymerization by the ionic active species which are easily formed during processing. 

Optimization of the valence and dihedral angles yields planar cyclic structures for the 3- to 5-ring intermediates in contrast to a chair conformation for that of the 6-ring. In the cases of n = 4, 5, 6 the oxygen atom is placed almost in the plane of the three C-atoms directly bonded to it. Therefore, an intramolecular solvation of the cationic chain end by methoxy groups which are bonded to the polymer backbone is preferred in the gas phase. The calculations show that for a non-polar solvent such as CH2C12 a decrease in stability of the cyclic intermediates exists. But this decrease does not result in a total break of the intramolecular solvation (Table 13). An equilibrium between open chain and cyclic intermediates must only be taken into account in more polar solvents, due to the competition of intra- and intermolecular solvation. 

Entry 5 is an example of the use of fra-(trimethylsilyl)silane as the chain carrier. Entries 6 to 11 show additions of radicals from organomercury reagents to substituted alkenes. In general, the stereochemistry of these reactions is determined by reactant conformation and steric approach control. In Entry 9, for example, addition is from the exo face of the norbornyl ring. Entry 12 is an example of addition of an acyl radical from a selenide. These reactions are subject to competition from decarbonylation, but the relatively slow decarbonylation of aroyl radicals (see Part A, Table 11.3) favors addition in this case. 

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