The degree of polymerization of a polyamide is determined by the equilibrium between carboxyl, amino, and amide groups, and water molecules. Equations are deduced for the relationship between the equilibrium constant k₁, the number-average chain length n and the partial pressure of water. Allowance is made for the presence of a cyclic monomer, whose amount is determined by a second equilibrium constant k₂. For monomers capable of forming 8-membered or larger rings k₂ approaches 0, and for those capable of forming 5- or 6-membered rings k₂ approaches 1, and no polymer is formed. For 7-membered rings k₂ has such a value that cyclic monomer and polymer can exist together. When the cyclic monomer is not miscible with the polymer, polymer formation is possible from 5- and 6-membered rings, e.g. diketopiperazine (dioxopiperazine), which polymerizes to polyglycine.

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partial pressure of water polyamide between carboxyl amino and amide 6membered rings eg diketopiperazine numberaverage

A. B. Meggy,.The free energy of formation of the amide bond in polyamides. 4 (4),154-159.

A. B. Meggy,"The free energy of formation of the amide bond in polyamides" 4

A. B. Meggy,"The free energy of formation of the amide bond in polyamides"