Thermodynamics of step and addition polymerization reactions

A few brief comments are merited about the thermodynamics of polymerization reactions [42,43]. In principle, all polymerization reactions are reversible. However, the reversibility of the propagation step is very dependent on there being a reaction mechanism available for the reverse process. In the majority of polymerization reactions, the depropagation step is either not possible or other side reactions occur which dominate under conditions where reversibility might be expected. Thus, the ability to study thermodynamic equilibria in a polymerization process is restricted to relatively few polymerization systems even though thermodynamic behaviour is not a function of the precise nature of the propagating species in, say, chain polymerization processes. 

The heat of polymerization depends on the nature of bond breaking and bond making in the polymerization process. For olefins, vinyl, and diene monomers, a double bond is broken to give two single bonds with the net release of energy, i.e. polymerization is exothermic. The heat of reaction in such systems is very similar in all cases, varying somewhat with the nature of the substituents on the double bond and with resonance effects where these 

For polymerization of any type to occur, the free energy change for the polymerization process should be negative. From the second law of thermodynamics the magnitude of the free energy change in a chemical reaction is a function of enthalpy and entropy contributions to the process and the temperature at any given pressure, as shown in equation (1.2). 

The polymerization of ring compounds is particularly interesting as ring strain has a considerable effect on the heat of polymerization and hence on the overall capability of ring monomers to polymerize, from a thermodynamic 

Almost all polymerizations are accompanied by a reduction in volume of the polymerization system since polymers are, in general, more dense than the monomers from which they are formed. Increasing pressure generally enhances polymerization but considerable pressures are needed in order to observe any significant effect on the position of monomer-polymer equilibria. 

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