Enthalpy depolymerization

These techniques help in providing the following information specific heat, enthalpy changes, heat of transformation, crystallinity, melting behavior, evaporation, sublimation, glass transition, thermal decomposition, depolymerization, thermal stability, content analysis, chemical reactions/polymerization linear expansion, coefficient, and Young s modulus, etc. 

The entropy term is negative so that it is the enthalpy or energy term that drives the polymerization. At low temperatures, the enthalpy term is larger than the TASp term so that polymer growth occurs. At some temperature, called the ceiling temperature, the enthalpy and the entropy terms are the same and AGp = 0. Above this temperature depolymerization occurs more rapidly than polymer formation so that polymer formation does not occur. At the ceiling temperature depolymerization and polymerization rates are equal. The ceiling temperature is then defined as... 

Equation 3 shows that for a given monomer concentration [M]eq at temperatures above a critical value Tc the rate of the depolymerization step becomes greater than the rate of the polymerization step and dominates the reaction. The critical temperature Tc is called ceiling temperature (22, 23). (AH is the enthalpy of polymerization, and AS° is the entropy of polymerization at the monomer concentration [M] = 1 mole/liter.) The concentration of the monomer at equilibrium [M]eq is identical to the equilibrium constant K, which is defined by the rate constants kp and kd. 

The activation enthalpy for initiation of depolymerization was approximately 75 kJmol-1. This low value rules out spontaneous cleavage of Si—O or Si—Si bonds... 

Depolymerization is a special case of thermal degradation. It can be observed particularly in polymers based on a, a -disubstituted monomers. In these, degradation is a reversal of the synthesis process. It is a chain reaction during which the monomers are regenerated by an unzipping mechanism. This is due to the low polymerization enthalpy of these polymers. For the thermal fission of polymers with secondary and tertiary C-atoms, higher energies are required. In these cases elimination reactions occur. This can be seen very clearly in PVC and PVAC. 

Spiro orthoesters (92, R = Me, Ph, and H) show typical equilibrium polymerization behavior at or below ambient temperature. [92] The poly(cyclic orthoester)s derived from 92 depolymerize to the monomers, although they have sufficient strains to be able to undergo ring-opening polymerization. The polymerization enthalpies and entropies for these three monomers were evaluated from the temperature dependence of equilibrium monomer concentrations (Table 5). The enthalpy became less negative as the size of the substituent at the 2-position in 92 was increased H < Me < Ph. This behavior can be explained in terms of the polymer state being made less stable by steric repulsion between the bulky substituents and/or between the substituent and the polymer main chain. The entropy also changed in a similar manner with the size of the substituents. 

Fig. 1. Enthalpy change for the polymerization and depolymerization of exo- and endo-2-methyl-7-oxabicyclo[2.2. l]heptanes...

Using the enthalpy and entropy of polymerization data for methyl methacrylate from Table 6.13, calculate the depolymerization rate constant of poly(methyl... 

In case the reverse of the polymerization, the depolymerization, is significant, a more complicated kinetics describes the chain reactions. Figure 3.30 shows the scheme. If rates of the forward and reverse reactions become the same, equilibrium is reached. The equilibrium temperature is called the ceiling temperature, T (at a given concentration or vapor pressure [A]). Standard thermodynamics applies to this equilibrium (see Chap. 2). On depolymerization, the entropy of the system, S, increases because the number of molecules increases. With a positive AS, a T must exist at sufficiently high temperature, since one can write AG = AH - TAS, where AG is the Gibbs function or free enthalpy and AH, the enthalpy of the reaction. 

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