Entropy change lactams

The anionic polymerization of 58 shows typical equilibrium polymerization behavior. From the temperature dependence of the equilibrium monomer concentration, the thermodynamic parameters for the polymerization of 58 in dimethyl sulfoxide were evaluated to be AHSS = -23.8 1.5 kJ/mol and ASss = — 71.5 + 4.2 kJ/mol deg (subscript ss refers to a solution state). [67] The ceiling temperature for 1 mol/L solution is about 60 °C. The enthalpy change in the polymerization of 58 is considerably larger than those for monocyclic lactams, pyrrolidone and piperidone, but no quantitative comparison of these data can be made, because the reported data refer to different experimental conditions. The significant entropy decrease in the polymerization is ascribable to the presence of the six-membered tetrahydropyran ring in the repeating unit. 

Substituents affect both the heat and entropy of polymerization, mainly through conformational effects [58—61]. Steric repulsions between substituents as well as between atoms of the monomer unit and substituents usually do not change the enthalpy of the cyclic lactam. Only... 

Similarly, substitution only changes the entropy of the lactam very slightly, but decreases considerably the entropy of the linear monomer unit inside a polymer chain by restricting its rotation. In this way, substitution shifts the enthalpy and entropy of polymerization in favour of the cyclic structure and decreased polymerizability. 

As any ring-opening polymerization, the polymerization of lactams is characterized by competition between the intermolecular reaction resulting in a linear polyamide and the intramolecular reaction of cyclization. Thus, if thermodynamically feasible, as indicated by a negative value of the free-energy change of polymerization AGp, the conversion of lactam to the linear polyamide can be realized if an appropriate reaction path exists for a reasonable rate in a polymer-monomer equilibrium characterized by a preponderance of linear macromolecules. The corresponding equilibrium monomer concentration [M]g is related to the standard enthalpy and entropy ASp of polymerization and to... 

In addition to the aspects discussed thus far, the polymerizability of lactams is also affected by the presence of heteroatoms in the ring moiety [19-21] and by substituents. The latter affect both enthalpy and entropy of polymerization mainly due to changes in the conformation on conversion of the cyclic structures into linear ones. The overall effects depend markedly on number, size, location, and nature of the substituents [22-26]. 

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