The free energy of formation of the amide bond in polyamides

Abstract

AbstractThe 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.From data on polymers of <‐caprolactam equations have been deduced giving the relationship between k1 and k2 and the temperature T, and from these the heat of formation Δk, the entropy ΔS, and the free energy ΔF for the polyamide bond in polyaminocaproic acid have been calculated. Assuming that the values for nylon 66 (produced from hexamethylenediamine and adipic acid) are not greatly different, equations are obtained for the chain length n of nylon 66 in terms of the temperature T and the partial pressure of water, and also for n as a function of T in the presence of an aqueous phase.The reaction between carbon dioxide and a diamine is discussed, and it is shown that for the formation of a polymethyleneurea by this reaction k1 is probably much smaller than for the polyamides, so that conditions are less favourable to the formation of high polymers.It is shown that the inability of α‐amino‐acids to polymerize directly is due to the large amount of ionic work that has to be done in adding a single amino‐acid to a peptide chain. Tripeptides and higher peptides should be able to polymerize.A comparison is made between polyesters and polyamides. The formation of the amide group is accompanied by a large increase in entropy, 20–30 cal./mole degree, which is probably due mainly to the release of oriented water molecules held by the charged carboxyl and amino groups. Polyesters are formed by interaction between uncharged groups, so that the increase in entropy will be much smaller, and the conditions less favourable to polymer formation.