Thermodynamics depolymerization

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. 

Cationic polymerization of cyclic acetals generally involves equilibrium between monomer and polymer. The equilibrium nature of the cationic polymerization of 2 was ascertained by depolymerization experiments Methylene chloride solutions of the polymer ([P]0 = 1.76 and 1.71 base-mol/1) containing a catalytic amount of boron trifluoride etherate were allowed to stand for several days at 0 °C to give 2 which was in equilibrium with its polymer. The equilibrium concentrations ([M]e = 0.47 and 0.46 mol/1) were in excellent agreement with that found in the polymerization experiments under the same conditions. The thermodynamic parameters for the polymerization of 1 were evaluated from the temperature dependence of the equilibrium monomer concentrations between -20 and 30 °C. 

Second, linear chain polymers are thermodynamically unstable at elevated temperatures. Entropic influences favor a breakdown to small molecules either by random fragmentation or by depolymerization. The latter process involves a reversion of the polymer to monomer or small molecule rings. Depolymerization to small rings is a feature common to many inorganic polymers at temperatures above 200-250°C. 

Polymerization occurs very quickly and the process is controlled via kinetic effects rather than thermodynamic ones. The net result is that the molecular weight distribution of the product does not match the thermodynamically stable one. If the chains were not capped with monofunctional phenols, the polymer chains would depolymerize, allowing the monomers to rearrange themselves at elevated temperature to approach the thermodynamically stable... 

Claims of perpetual motion create moments of mirth and consternation for those knowledgeable in the laws of thermodynamics. Yet, is it only hyperbole when a responsible journal such as the European Plastics News [1] proclaims that depolymerization of polyethylene terephthalate (PET) can be repeated indefinitely The second law of thermodynamics brings us back to reality. The depolymerization of PET does not operate at 100% yields, but does offer the opportunity for near-stoichiometric recovery of the monomers used to make the polyester. With high yields of potentially valuable monomers, the commercial potential for polyester depolymerization to regain feedstocks must be considered. 

Sulfur is present in the petrochemicals derived from once-living matter as it is present in certain amino acids. Because of its removal from industrial waste, sulfur has been stockpiled and is available at a low price in large amounts. While the stable form of sulfur at room temperature is cyclooctasulfur (Sg), linear polysulfur is formed on heating. Unfortunately, the thermodynamically stable form of sulfur is the cyclooctasulfur monomer and the polymer undergoes depolymerization after sometime. 

Zn(R-dtp)2 complexes have been characterized and their thermal stabilities investigated 173,184,190,297-299,301-305) Zn(R-dtp)2 compounds are thermally degraded to volatile olefins and non-volatile residues and this serves as the basis for gas chromatographic determination of the compounds 304,30s) Several papers describing pyrolyses of Zn(R-dtp>2 complexes have discussed mechanisms for formation of olefins, sulfides, and other products 173,184,190,298,299, 304) Dakternieks and Graddon i8s,283)35 mentioned earlier, have reported thermodynamic measurements for depolymerization and adduct formation reactions of zinc, cadmium and mercury R-dtp compounds. 

Polymers of formaldehyde were found recently in interstellar space by N. Wickramasinghe [Nature, 252, 462 (1974)]. It is well known that polyformaldehyde is thermodynamically unstable already at not very high temperatures (close to room temperature), but it should be stable versus depolymerization near absolute zero. Therefore the formation of poly-oxymethylene near absolute zero is not a thermodynamic but a kinetic problem. 

Bryant, W. M. D. Free energies of formation of fluorocarbons and their radicals. Thermodynamics of formation and depolymerization of polytetrafluoroethylene. J. Polymer Sci., in press. 

Conversely, if the polymer could be made by some other route (for example, by macromolecular substitution), it might be stable at moderate temperatures where the rate of depolymerization is very slow, but would depolymerize to the cyclic trimer or tetramer when heated to higher temperatures. In fact, this behavior is found for uncross-linked polymers such as [NP(OPh)2] , that appear to be kinetically stabilized at moderate temperatures, but are sufficiently destabilized thermodynamically by the bulky aryloxy side groups that they depolymerize when heated above 150-200 °C. 

Rhombic sulfur is a brittle, crystalline solid at room temperature. Heating to 113 °C causes it to melt to a reddish-yellow liquid of relatively low viscosity. Above approximately 160 °C, the viscosity increases dramatically because of the free-radical polymerization of the cyclic molecules into long, linear chains.6,8 14 30 47-51 At this point, a degree of polymerization of approximately 105 is obtained. If the temperature is increased to above approximately 175 °C, depolymerization occurs, as evidenced by a decreasing viscosity. A similar type of depolymerization occurs with the polysiloxanes discussed in Chapter 4. In thermodynamic terms, the negative -TAS term overcomes the positive AH term for chain depolymerization. (The temperature at which the two terms are just equal to one another is called the ceiling temperature for the polymerization.)... 

Fig. 15 Low-temperature molar specific heat of tetragonal filled triangles), orthorhombic (dots), and depolymerized C60 (open symbols), plotted as Cp/T3. Reprinted with permission from A Inaba, T Matsuo, A Fransson, and B Sundqvist, Lattice vibrations and thermodynamic stability of polymerized C60 deduced from heat capacities , J. Chem. Phys. vol. 110 (1999) 12226-32 [105]. Copyright 1999 American Institute of Physics...

This equation permits the calculation of equilibrium constants for polymerization-depolymerization from copolymer composition data extrapolated to zero Mi feed. The agreement between equilibrium constants calculated in this manner from free radical copolymerizations and those obtained from anionic homopolymerizations is shown in Table II, and again emphasizes the thermodynamic character of this work. 

When molten sulfur is heated above 159°C, preferably to 200°-250°C, and then rapidly quenched to about — 20°C, a translucent elastic dark-brown material (plastic sulfur) is obtained (28). Plastic sulfur, which is a mixture of amorphous S8 rings and amorphous polymeric sulfur, is thermodynamically unstable (28). It undergoes embrittlement, especially above —10 °C, because of the rapid crystallization of octameric sulfur to orthorhombic sulfur (28). Also polymeric sulfur depolymerizes and crystallizes to orthorhombic sulfur slowly at ambient temperature and rapidly above 90°C (28). 

The occurrence of these reactions is always determined by thermodynamic factors. Oxirane has a large ring strain. Its polymerization around room temperature exhibits AGp<0. For 1,4-dioxane under the same conditions, AGp > 0. In other words, polyoxirane will split off 1,4-dioxane because the Gibbs energy of its depolymerization is negative. Actually the polymer should depolymerize completely. That this is not the case, is caused by kinetic factors. Termination of depolymerization need not coincide with termination of polymerization. 

In the polymerization of dialkyldichlorosilanes with sodium in refluxing toluene, propagation and a concurrent back-biting reaction to cyclic material could give the range of products found if the products are kinetically, instead of thermodynamically, determined (12). No evidence for depolymerization has been found for the reaction in toluene solution. 

Timasheff s preferential interaction mechanism also explains the influence of solutes on the degree of assembly of multimeric proteins. Preferentially excluded solutes tend to induce polymerization and stabilize oligomers since the formation of contact sites between constituent monomers serves to reduce the surface area of the protein exposed to the solvent. Polymerization reduces the thermodynamically unfavorable effect of preferential solute exclusion. Conversely, preferential binding of solute induces depolymerization because there is greater solute binding to monomers than to polymers. 

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). 

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