Ceiling temperature depolymerization

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... 

Because of its low ceiling temperature, depolymerization of PEtG occurs at room temperature. That is why a termination step using an adequate agent is necessary. 

The addition of radicals and, in particular, propagating radicals, to unsaturated systems is potentially a reversible process (Scheme 4.46). Depropagation is cntropically favored and the extent therefore increases with increasing temperature (Figure 4.4). The temperature at which the rate of propagation and depropagalion become equal is known as the ceiling temperature (rc). Above Tc there will be net depolymerization. 

Interestingly, it should not be assumed that a polymer will be useless above its ceiling temperature. A dead polymer that has been removed from the reaction media will be stable and will not depolymerize unless an active end is produced by bond cleavage of an end group or at some point along the polymer chain. When such an active site is produced by thermal, chemical, photolytic, or other means, depolymerization will follow until the monomer concentration becomes equal to [M]c for the particular temperature. The thermal behavior of many polymers, however, is much more complex. Degradative reactions other than depolymerization will often occur at temperatures below the ceiling temperature. 

This reaction is referred to as end capping or end blocking. The result is that reactive carbanion or carbocation centers do not form and depolymerization does not occur at the ceiling temperature of the polymer. The polymer chains are end blocked from depolymerization. The effective ceiling temperature is increased considerably above the ceiling temperature. Acetic anhydride is the usual capping reagent. 

Aldehyde Copolymer Self Developing Electron-beam Resists. The ceiling temperature for the copolymerization of aliphatic aldehydes is usually below 0°C and the copolymers are easily depolymerized into monomeric aldehydes above 150°C under vacuum. This depolymerization into monomers also occurs on electron-beam or X-ray exposure as evidenced by combined gas-liquid partition chromatography-mass spectrometry. As a result, the copolymers of aldehydes behaved as self-developing positive resists and almost complete development was accomplished without any solvent treatment. Electron-beam exposure characteristics of the aliphatic aldehyde copolymers studied here are... 

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. 

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.)... 

The 1,1-disubstitution of chlorine atoms causes steric interactions in the polymer, as is evident from the heat of polymerization (see Table 1) (24). When corrected for the heat of fusion, it is significantly less than the theoretical value of —83.7 kJ/mol (—20 kcal/mol) for the process of converting a double bond to two single bonds. The steric strain apparendy is not important in the addition step, because VDC polymerizes easily. Nor is it sufficient to favor depolymerization the estimated ceiling temperature for poly(vinylidene chloride) (PVDC) is about 400°C. 

The polymerization of ketones was studied for theoretical reasons. Acetone has an unusually low ceiling temperature and yileds high-molecular-weight polymers only at low temperatures (ca. 70 K) its polymer depolymerizes at temperatures of about 100 K. 

The most relevant early work in the context of this study is the radiation induced depolymerization of poly(phtalaldehyde) [9]. In this case, depolymerization is due to a ceiling temperature phenomenon whereby radiation induced cleavage of the polymer causes it to revert fully to monomer. Poly(phtalaldehyde) is a material with a very low ceiling temperature which is only rendered stable at room temperature through the device of capping its chain-ends after low temperature polymerization, thereby preventing its spontaneous degradation when heated. 

The ceiling temperature is therefore defined as the temperature at which the rates of propagation and depolymerization are equal. For that reason, is a threshold temperature above which a specific polymer cannot exist. Representative values of for some common monomers are given in Table 14.23. 

This can give a comparative measure of the ceiling temperature, T, in polymerization (which is important for the onset of thermal depolymerization as discussed in Section 1.4.1). The thermodynamic ceiling temperature is rarely achieved in practice, both because of the requirement for a closed system and also owing to the onset of other degradation reactions such as crosslinking. 

Thus the values shown in Table 1.8 are for standard conditions and represent just one of a series of ceiling temperatures for various monomer concentrations above which polymer formation is not favoured. Thus, in a bulk polymerization reaction the ceiling temperature may change with conversion in such a way that complete conversion is not achieved. For example, if methyl methacrylate is polymerized at 110°C the value of [M]c calculated from the above equation is 0.139M and this will be the monomer concentration in equilibrium with the polymer. The polymer, when removed from the monomer, will have the expected ceiling temperature as given in Table 1.8 and will depolymerize only if there is a source of free radicals to initiate the depolymerization (Section 1.4.1)... 

A widely studied commercially available polymer is poly(methylmethacrylate) (Fig. 14.2). It is a polymer that can be completely depolymerized by heating above the ceiling temperature (7c). It is possible only to achieve 100% monomer as product by laser irradiation with a C02 laser (2 9.6 or 10.6 pm) [71]. About 1% monomer can be detected in the ablation products after irradiation with 248 nm laser light, and about 18% monomer can be produced with 193 nm [71,72]. 

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