Ceiling temperature values

The 1,1-disubstitution of chlorine atoms causes steric interactions in the polymer, as is evident from the heat of polymeri2ation (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 apparentiy is not important in the addition step, because VDC polymeri2es easily. Nor is it sufficient to favor depolymeri2ation the estimated ceiling temperature for poly (vinyhdene chloride) (PVDC) is about 400°C. 

With most common monomers, the rate of the reverse reaction (depropagation) is negligible at typical polymerization temperatures. However, monomers with alkyl groups in the a-position have lower ceiling temperatures than monosubstituted monomers (Table 4.10). For MMA at temperatures <100 °C, the value of is <0.01 (Figure 4.4). AMS has a ceiling temperature of <30 °C and is not readily polymerizable by radical methods. This monomer can, however, be copolymerized successfully (Section 7.3.1.4). 

Thermal Effects in Addition Polymerizations. Table 13.2 shows the heats of reaction (per mole of monomer reacted) and nominal values of the adiabatic temperature rise for complete polymerization. The point made by Table 13.2 is clear even though the calculated values for T dia should not be taken literally for the vinyl addition polymers. All of these pol5Tners have ceiling temperatures where polymerization stops. Some, like polyvinyl chloride, will dramatically decompose, but most will approach equilibrium between monomer and low-molecular-weight polymer. A controlled polymerization yielding high-molecular-weight pol)mier requires substantial removal of heat or operation at low conversions. Both approaches are used industrially. 

The rate expressions and values, mechanisms, and the activation energies for the condensation reactions forming polymers are similar to those of small molecule reactions. Reaction rate increases with temperature in accordance with the Arrhenius equation. Average DP also increases as the reaction temperature increases to the ceiling temperature where polymer degradation occurs. Long chains are only formed at the conclusion of classical polycondensation processes. 

Progress in the polymerization of the carbonyl linkage did not result until there was an understanding of the effect of ceiling temperature (Tc) on polymerization (Sec. 3-9c). With the major exception of formaldehyde and one or two other aldehydes, carbonyl monomers have low ceiling temperatures (Table 5-13). Most carbonyl monomers have ceiling temperatures at or appreciably below room temperature. The low Tc values for carbonyl polymerizations are due primarily to the AH factor. The entropy of polymerization of the carbonyl double bond in aldehydes is approximately the same as that for the alkene double bond. The enthalpy of polymerization for the carbonyl double bond, however, is appreciably lower. Thus AH for acetaldehyde polymerization is only about 29 kJ mol-1 compared to the usual 80-90 kJ mol-1 for polymerization of the carbon-carbon double bond (Table 3-14) [Hashimoto et al., 1076, 1978],... 

For negative AHP and ASP, (most commonly the case encountered in addition polymerization) AFP becomes positive above a certain critical temperature, Te = A HpjA Sp, known as the ceiling temperature" of the system. The value of Te depends on the concentrations of the monomer and of the polymer as well as on the nature of the solvent, if the latter is present in the system. Of course, the high polymer cannot be formed above the ceiling temperature. For any monomer-polymer system, the process which converts pure liquid monomer into a crystalline polymer has the maximum ceiling temperature. 

It was once thought that the Tc of THF was very low, in fact near room temperature (23). However, in recent years, as catalyst systems have been improved and more intensive studies have been carried out, the presumed Tc has risen first to 60—70° (18,24) and finally to 85 2° C (25, 26, 27). The lower values were probably the result of working with systems where a true monomer-polymer equilibrium was not obtained. Possibly also, careful enough techniques were not used in the isolation of the lower molecular weight polymers obtained near the ceiling temperature. Precipitation in water cannot always be used because low molecular weight PTHF s are partially soluble in water. 

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. 

However, the equilibrium monomer concentrations of disubstituted alkenes is measurable. The equilibrium constants for dimerization, tri-merization, and polymerization of a-methylstyrene have been determined as a function of temperature under anionic conditions [12] similar values should be obtained under cationic conditions. Unfortunately, the equilibrium position can t be determined directly under cationic conditions due to the irreversible side reactions of isomerization and indan and spirobiindan formation (Section II. A). The equilibrium monomer concentrations of isobutene and isopropenyl vinyl ethers should also be relatively high, albeit lower than those of a-methylstyrenes. However, the true equilibrium can t be reached with these monomers due to irreversible side reactions, and reliable data are therefore not available. Nevertheless, the ceiling temperature of isobutene polymerization is apparently between 50 and 150° C. 

This equation describes the relation between two parameters characterizing the reversible polymerization ceiling temperature and equilibrium monomer concentration. For any temperature there is a certain value of [ML [Eq. (48)]. If the starting concentration of monomer is lower than this value, i.e., [M0] < [ML, the polymerization will not proceed. If [M]0 is higher than [ML, polymerization will proceed until concentration of monomer reaches [ML (i-e., consumption of monomer will be equal to [M]0 - [ML). 

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. 

The polymerization of this 1,4 epoxide has been widely studied [115] and the polymer produced, polytetramethylene oxide or polytetrahydro-furan, has found commercial application. The most characteristic feature of the polymerization is the occurrence of a ceiling temperature within the range of normal laboratory experiments, 85°C. In any polymerization the propagation reaction is normally shown as a reaction going 100% to completion although in fact this reaction, and the other elementary steps, are really chemical equilibria. Some reactant, usually monomer in the case of propagation, is present at the end of the reaction although the value of the... 

In this connection it has to be stressed that the ceiling temperature concept should be applied to the polymerization of lactams very carefully. At elevated temperatures, the reversible transacylation reactions are accompanied by decomposition of the polymer and/or monomer and by other side reactions. With respect to the low ASp values (as compared to vinyl monomers), very high ceiling temperatures can be expected for most lactams. Therefore, the calculated values of lie high above the decomposition temperature of the polymer and monomer except for five-, six- and substituted seven-membered lactams. In addition, for very large rings the heat of polymerization approaches zero and the polymerization entropy becomes positive, so that the value of T<, = AHp/ASp becomes meaningless. 

The ceiling temperature T can be considered the upper temperature at which a pyrolytic process will reach equilibrium. It may be seen, therefore, as a recommended temperature for pyrolysis. However, in practice, the application for macromolecules of the above relations is not straightforward. The theory was developed for ideal systems (sometimes in gas phase), and although in principle this theory should hold true for any system, its application to condensed phases or polymeric materials may be accompanied by effects difficult to account for (phase change, melting, cage effect [2], etc.). The reaction rate could also be low at calculated Tq values. For this reason, temperatures 50° C or 100° C higher than Tq must frequently be used as practical values of the temperature used in pyrolysis. 

A major limitation of a-methylstyrene in free-radical polymerizations is its very low ceiling temperature of 61 °C.347 As a result, AMS is utilized commercially only in radical copolymerization. Nonetheless, it is among the most active CCT monomers with Cc = 9 x 105 at 50 °C for 9a as CCT catalyst.348 This value is relatively unchanged at 40 °C. This high value reflects the low kp = 1.7 M 1 s 1 so that kc = 5 x 105 M-1 s 1. 

For the situation of polymerization in a closed system, it is necessary to consider the position of the equilibrium in the propagation reaction as given by the value of K, the equilibrium constant. This shows (Odian, 1991) that the ceiling temperature depends on the monomer concentration ... 

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)... 

It is obvious from the above definition of the ceiling temperature that Tc will depend on the monomer concentration in the system. If polymerization is performed at a temperature T and [M] is greater than [M]g at that temperature, as calculated from Eq. (6.189), then the polymerization would proceed until [M] falls to the value of [M]g. Conversely, for a given [M]e, a temperature satisfying Eq. (6.189) is the ceiling temperature (Tc)... 

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