![]() ![]() In condensation polymerizations, the entropy of reaction is more subtle. In general, this entropic problem in entropy gives rise to the term "ceiling temperature", which is the temperature above which polymerization of a particular monomer will not occur. Of course, in these cases there tends to be a maximum extent of polymerization before the process bottoms out in an equilibrium between polymer and monomer. Nevertheless, there are certain polymerizations (such as ring-opening polymerization of cyclic esters) that are frequently run at elevated temperatures because of a significant barrier to the reaction. The entropy factor begins to dominate the equation only at higher temperatures, because the entropy contribution scales with temperature (in Kelvin) in the expression for free energy.Īs a result, thermodynamically, chain polymerizations should be run under cooler conditions in order to maximize the degree of polymerization. However, because entropy changes are generally rather small compared to enthalpy changes, polymerization proceeds despite this handicap. Polymerization is always entropically disfavored, regardless of the circumstances. ![]() That means that a negative entropy change translates into a positive, and unfavorable, free energy change. If W f < W i, then ΔS < 0 because the natural log of a fraction is a negative number.įree energy correlates positively with enthalpy and negatively with entropy. In which k B is Boltzmann's constant and W is the number of microstates available in the final or initial state. Consequently, the entropy change associated with polymerization is always negative. Because a large number of monomers are being enchained together in a macromolecule, the number of degrees of freedom is always decreasing, regardless of the specific polymerization process under consideration. Thermodynamically, there are always two factors to consider in a reaction: enthalpic contributions and entropic contributions.Įntropic changes in chain polymerizations tend to be qualitatively similar in different cases. Polymerization is the process of taking individual monomers and enchaining them into a macromolecule. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |