Thermodynamics of Ring-Opening Polymerization

The stability of the ring structure as well as the stability of the resultant linear polymer determines the polymerizability of cyclic monomers. Thermodynamic factors, therefore, are of paramount importance in ring-opening polymerizations [233]. Actually, the polymerization of many bond-strained ring monomers is favored thermodynamically. Thus, for instance, DH, for three-membered cycloalkanes is 113.0 kJ/mole and DS is 69.1 J/mole C. It was shown (Sawada) that in three and four-membered ring structures, the change in enthalpy is a major factor in determining DF, the change in free energy. For three-membered cycloalkanes DF is 92.0 kJ/mole, while for four membered ones it is 90.0 kJ/mole. The entropy change, DS is a major factor in polymerization of f ive-membered cyclic monomers. The six-membered ring monomers that are relatively strain free are very hard to polymerize. An exception is trioxane, whose DH is close to zero. On the other hand, the enthalpy and entropy factors contribute about equally to the free energy change of larger rings. This means that with increases in temperature DF becomes less and less negative and above certain temperatures some large cyclic monomers will not polymerize. The transanular strain in seven- and eight-membered rings contributes to their polymerizability. Presence of substituents in cyclic monomers has a negative effect on the thermodynamic feasibility to polymerize. On the other hand, thermodynamic feasibility alone does not determine whether a cyclic monomer will polymerize. The entropy changes do not show much dependence of on angle strain. They are susceptible, however, to configurational influences. Sawaada [233] writes the entropy change of polymerization as a function of the probability of ring closure:

∆Sp = -blnP-a

where P is the probability of ring closure and a and b are constants. The probability of ring closure for a chain with n repeating units can be taken as a function of the probability that the chain ends will come together. This probability is usually expressed as a radius of gyrations, (r2), the root square distance of end to end. The entropy change for three-membered rings would have a large negative value. For larger rings the negative value would be less, because the end to ends would be further apart. Statistical mechanics treatment has shown that the entropy change of ring closure is [223]:

∆Sr = Rln (PV/2_xV_sN)

where P is the probability of ring closure or the fraction of chain ends that will come together and close to form ring structures, V is the total volume of the system, Vs is the volume of a constrained skeletal atom prior to bond breaking, x is the number of monomer units in the ring, and N is the Avogadro’s number.

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مواضيع ذات صلة

Polyethylene and Related Polymers

Ring-Opening Polymerizations by a Free Radical Mechanism

Polymerization of Trioxane

Ring-Opening Polymerizations of Cyclic Acetals

Polymerization of Oxepanes

The Propagation Reaction

The Initiation Reaction

The Propagation Reaction

The Initiation Reaction

Polymerization of Oxetanes

Steric Control in Polymerizations of Oxiranes

Polymerization by Coordination Mechanism