Ceiling temperature table

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],... 

There have been many studies on the polymerizability of cx-substituted acrylic monomers.It is established that the ceiling temperature for a-alkoxyacrylates decreases with the size of the alkoxy group. However, it is of interest that polymerizations of a-(alkoxymethyl)acrylates (67) and a-(acyloxymethyl)acrylates (68) and captodative substituted monomers (69, 70) appear to have much higher ceiling temperatures than the corresponding a-alkylacrylates (e.. methyl ethacrylate, MEA). For example, methyl a-ethoxymethacrylate readily polymerizes at 110 °C whereas MEA" has a very low ceiling temperature (Table 4.10). However, values of the thermodynamic parameters for these polymerizations have not yet been reported. 

Table 2. Entropies, Enthalpies, and Ceiling Temperatures for the Polymerization of Various Monomers at 25°C (298.15 K) and 101.3 kPa (1...

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. 

In the copolymerization of isopropenylferrocene with a-methyl-styrene at 0°C, using varying molar ratios of isopropenylferrocene and a-methylstyrene, traces of polymer formation were obtained only at a 30/70 ratio of the two monomers, as shown in the data in Table III. Because a-methylstyrene has a much lower ceiling temperature than styrene, we also decided to use styrene as a comonomer under conditions similar to those employed with a-methylstyrene. The reaction temperature for the copolymerization with a-methylstyrene was 20°C. 

The heat of reaction for vinyl polymers affects the thermal stability of the polymer during extrusion, and the thermal stability is related to the ceiling temperature. The ceiling temperature is the temperature where the polymerization reaction equilibrium is shifted so that the monomer will not polymerize, or if kept at this temperature all the polymer will be converted back to monomer. From thermodynamics the equilibrium constant for any reaction is a function of the heat of reaction and the entropy of the reaction. For PS resin, the exothermic heat of reaction for polymerization is 70 kj/gmol, and the ceiling temperature is 310 °C. Ceiling temperatures for select polymers are shown in Table 2.5. 

Table 2.5 Ceiling Temperatures and Heats of Reaction (Exothermic) for Select Polymers...

Table 2. Heats of polymerization determined from ceiling temperatures and from calorimetry...

Because of the general lack of quantitative thermodynamic (ceiling temperature Tc) and kinetic (kp, kp/k[J 5) data for the polymerization of the captodative olefins, it is impossible to draw firm conclusions about the importance of electronic factors on their polymerizability. If we compare them with other 1,1-disubstituted olefins by replacing the heteroatom O, S, or N by a CH2 and check the polymerizability of the resulting olefins, we find that the latter are in fact also difficult to polymerize as shown in Table 8 [77], Only methyl acrylates and methacrylates give high polymers easily. The polymerizability decreases rapidly with the steric hindrance of the substituent. 

Table 3 Enthalpies, Ceiling Temperatures, and Equilibrium Monomer Concentration for Bulk Polymerizations of Selected Monomers...

The ceiling temperatures listed in Table 3 refer to bulk conditions, whereas the equilibrium monomer concentrations refer to polymerizations performed at 25° C. As discussed in Section F, the equilibrium monomer concentration varies with the polymerization temperature, just as the ceiling temperature depends on the monomer concentration. Table 4 demonstrates that the equilibrium monomer concentration of a-methylstyrene... 

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. 

From the materials viewpoint, it is of Interest to examine the thermodynamics of radlolysls of polymers. Since gamma radlolysls data is readily available, polymers can be compared as in Table X. Note that materials having a large heat of polymerization tend to crosslink under radlolysls. The same polymers are thermally resistant to degradation. Degrading polymers have low ceiling temperatures ( 150°C) and low heats of polymerization. 

Table 1.9. Enthalpy and entropy changes for conversion of monomer to polymer, and the corresponding ceiling temperature...

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)... 

---