Reversible polymerization, thermodynamics

Thermodynamic and Kinetic Parameters for Reversible Polymerization (Oosawa s Law) 46... 

ATP HYDROLYSIS LINKED TO ACTIN POLYMERIZATION PERTURBS THE THERMODYNAMICS OF REVERSIBLE POLYMERIZATION... 

Interestingly, there have been repeated attempts to view such gradual structural changes due to chemical equilibria as phase transitions as well [262]. Instructive examples for such an analysis are living polymers — for example, in demixing solutions of polymers [263] and in sulfur [264], where the reversible polymerization process has been treated as a second-order phase transition. The experimental evidence for such an interpretation is, however, at best weak [265], and classical association models [266] describe the thermodynamic properties equally well. 

In addition to the physical state of reactants, it should be remembered that the ideal behavior is encountered only in the gaseous state. As the polymerization processes involve liquid (solution or bulk) and/or solid (condensed or crystalline) states, the interactions between monomer and monomer, monomer and solvent, or monomer and polymer may introduce sometimes significant deviations from the equations derived for ideal systems. The quantitative treatment of thermodynamics of nonideal reversible polymerizations is given in Ref. 54. 

Some readers will be interested in the fact that Huang and Wang [75] in 1972 presented a newer theoretical treatment of the reaction kinetics of reversible polymerization in which this classic derivation of Dainton and Ivin is a special case. The thermodynamics of equilibrium polymerizations have recently been reviewed by Sawada [76]. 

The values for the thermodynamic parameters in the formation of polymers can be used for the characterization of depolymerization reactions. The formation of monomers in a polymer decomposition reaction (depolymerization) is relatively common (see Table 2.1.1). Depolymerization can be considered a reverse polymerization, the two reactions having equal absolute values for the heats of reaction but with opposite signs. Therefore, the heats of polymerization can be used for the thermodynamic characterization of pyrolytic reactions with formation of monomers (kinetic factors are also very important in pyrolytic reactions as further shown in Section 2.3). 

The results of the above polymerization thermodynamics calculations for parylene are compared to similar data for typical addition poljuners in Table 2. The Tc quantifies the stability of the polymer only with respect to reversion to monomer. When PPX is thermally degraded (ca 500°C), a mixture of degradation... 

Polymer Thermodynamics Blends, Copolymers and Reversible Polymerization... 

Sharma, K. R. Polymer Thermodynamics Blends, Copolymers and Reversible Polymerization, CRC Press, Boca Raton, FL, 2011. 

High molecular weight polymers or gums are made from cyclotrisdoxane monomer and base catalyst. In order to achieve a good peroxide-curable gum, vinyl groups are added at 0.1 to 0.6% by copolymerization with methylvinylcyclosiloxanes. Gum polymers have a degree of polymerization (DP) of about 5000 and are useful for manufacture of fluorosiUcone mbber. In order to achieve the gum state, the polymerization must be conducted in a kineticaHy controlled manner because of the rapid depolymerization rate of fluorosiUcone. The expected thermodynamic end point of such a process is the conversion of cyclotrisdoxane to polymer and then rapid reversion of the polymer to cyclotetrasdoxane [429-67 ]. Careful control of the monomer purity, reaction time, reaction temperature, and method for quenching the base catalyst are essential for rehable gum production. 

Lack of termination in a polymerization process has another important consequence. Propagation is represented by the reaction Pn+M -> Pn+1 and the principle of microscopic reversibility demands that the reverse reaction should also proceed, i.e., Pn+1 -> Pn+M. Since there is no termination, the system must eventually attain an equilibrium state in which the equilibrium concentration of the monomer is given by the equation Pn- -M Pn+1 Hence the equilibrium constant, and all other thermodynamic functions characterizing the system monomer-polymer, are determined by simple measurements of the equilibrium concentration of monomer at various temperatures. 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

Tethering may be a reversible or an irreversible process. Irreversible grafting is typically accomplished by chemical bonding. The number of grafted chains is controlled by the number of grafting sites and their functionality, and then ultimately by the extent of the chemical reaction. The reaction kinetics may reflect the potential barrier confronting reactive chains which try to penetrate the tethered layer. Reversible grafting is accomplished via the self-assembly of polymeric surfactants and end-functionalized polymers [59]. In this case, the surface density and all other characteristic dimensions of the structure are controlled by thermodynamic equilibrium, albeit with possible kinetic effects. In this instance, the equilibrium condition involves the penalties due to the deformation of tethered chains. 

In conclusion, phase transfer catalyzed Williamson etherification and Wittig vinylation provided convenient methods for the synthesis of polyaromatics with terminal or pendant styrene-type vinyl groups. Both these polyaromatics appear to be a very promising class of thermally reactive oligomers which can be used to tailor the physical properties of the thermally obtained networks. Research is in progress in order to further elucidate the thermal polymerization mechanism and to exploit the thermodynamic reversibility of this curing reaction. 

Note 1 Thermodynamic processes that produce reversible changes in the physical structure of a polymeric material are termed physical aging. 

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