Ceiling temperature

For most chain polymerizations there is some temperature at which the reaction becomes a reversible one, that is, the propagation step (Eq. 3-15) should be written as an equilibrium reaction [Joshi and Zwolinski, 1967 Sawada, 1976], 

The equilibrium position for the monomer-polymer equilibrium in Eq. 3-174 will be dependent on the temperature with increased temperature, resulting in a shift to the left, since the forward reaction is exothermic. The reaction isotherm 

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 he 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 hy thermal, chemical, photolytic, or other means, depolymerization will follow until the monomer concentration becomes equal to [M] 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 ceihng temperature. 

For most free-radical polymerization reactions, there are some elevated temperatures at which the chain-growth process becomes reversible and depropagation takes place  

In the above equation AF° is the free energy of polymerization of both, monomer and polymer, in appropriate standard states [88]. The standard state for the polymer is usually solid (amorphous or partly crystalline). It can also be a one molar solution. The monomer is a pure liquid or a one molar solution. The relationships of monomer concentration to heat content, entropy, and free energy are shown by the following expression. This applies over a wide range of temperatures [5]. 

In the above equation, T. is the ceiling temperature for the equilibrium monomer concentration. It is a function of the temperature of the reaction. Because the heat content is a negative quantity, the concentration of the monomer (in equilibrium with polymer) increases with increasing temperatures. There are a series of ceiling temperatures that correspond to different equilibrium monomer concentrations. For any given concentration of a monomer in solution, there is also some upper temperature at which polymerization will not proceed. This, however, is a thermodynamic approach. When there are no active centers present in the polymer structure, the material will appear stable even above the ceiling temperature in a state of metastable equilibrium. 

The magnitude of the heat of polymerization of vinyl monomers is related to two effects (1) Steric strains that form in single bonds from interactions of the substituents. These substituents, located on the alternate carbon atoms on the polymeric backbones, interfere with the monomers entering the chains. (2) Differences are in resonance stabilization of monomer double bonds by the conjugated substituents [70]. 

Most 1,2 disubstituted monomers, as stated earlier, are difficult to polymerize. It is attributed to steric interactions between one of the two substituents on the vinyl monomer and the (3-substituent on the ultimate unit of the polymeric chain [94]. A strain is also imposed on the bond that is being formed in the transition state. 

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The thermodynamic ceiling temperature (26) T for a polymerization is computed by dividing the AfTp by the standard entropy of polymerization, The T is the temperature at which monomer and polymer are in equHibrium in their standard states at 25°C (298.15 K) and 101.3... 

These reactions are usehil for the preparation of homogeneous difunctional initiators from a-methylstyrene in polar solvents such as tetrahydrofuran. Because of the low ceiling temperature of a-methylstyrene (T = 61° C) (26), dimers or tetramers can be formed depending on the alkaU metal system, temperature, and concentration. Thus the reduction of a-methylstyrene by sodium potassium alloy produces the dimeric dianionic initiators in THF (27), while the reduction with sodium metal forms the tetrameric dianions as the main products (28). The stmctures of the dimer and tetramer correspond to initial tail-to-tail addition to form the most stable dianion as shown in equations 6 and 7 (28). 

The stoichiometric reaction of y -diisopropenylbenzene [3748-13-8] with two moles of j -butyUithium in the presence of triethylamine has been reported to produce a useful, hydrocarbon-soluble dilithium initiator because of the low ceiling temperature of the monomer (78,79) which is analogous in stmcture to a-methylstyrene however, other studies suggest that oligomerization occurs to form initiators with functionahties higher than two (80). 

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. 

The sulfonyl chloride group is the cure site for CSM and determines the rate and state of cure along with the compound recipe. It is less stable than the Cl groups and therefore often determines the ceiling temperature for processing. The optimum level of sulfonyl chloride to provide a balance of cured properties and processibiUty is about 2 mol % or 1—1.5 wt % sulfur at 35% Cl. It also undergoes normal acid chloride reactions with amines, alcohols, etc, to make useful derivatives (17). 

Since amines initiate cyanoacrylate polymerization, the monomer cannot be isolated directly, because a polymer is generated immediately after formation of the monomer. An acid is then added to the polymer, and heat (140-180°C) is applied to the reaction mixture. Because of the relatively low ceiling temperature of the polymer, the pure monomer can be isolated, in greater than 80% yield, by the thermal reversion of the polymer back to the free monomer [4,5]. 

As has been mentioned earlier, polycyanoacrylates possess a relatively low ceiling temperature, because the polymer will thermally revert back to monomer by an... 

This method was first applied by McCormick27 and by Bywater and Worsfold11 to the system a-methylstyrene/poly-a-methyl-styrene, and the free energy, entropy and heat of polymerization as well as the ceiling temperature were determined. Similar studies concerned with the system styrene/polystyrene are being carried out in our laboratories. 

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. 

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). 

There have heen many studies on the polymerizability of a-substituted acrylic monomers.3jU35 33S It is established that the ceiling temperature for a-alkoxyacrylates decreases with the size of the alkoxy group. 35 However, it is of interest that polymerizations of a-(alkoxymethyl)acrylates (67)3 15 and a-(acyloxymethyl)acrylates (68)and captodative substituted monomers (69, 70) 39 appear to have much higher ceiling temperatures than the corresponding a-alkylacrylates methyl ethacrylate, MEA). For example, methyl a-... 

Propagation reactions in radical polymerization and copolymerization arc generally highly exothermic and can be assumed to be irreversible. Exceptions to this general rule arc those involving monomers with low ceiling temperatures (Section 4.5.1). The thermodynamics of copolymerization has been reviewed by Sawada.85... 

Copolymerizations of other monomers may also be subject to similar effects given sufficiently high reaction temperatures (at or near their ceiling temperatures - Section 4.5.1). The depropagation of methacrylate esters becomes measurable at temperatures >100 °C (Section 4.5.1).96 O Driseoll and Gasparro86 have reported on the copolymerization of MMA with S at 250 °C. 

One way to measure the thermodynamics of the reaction is by the ceiling temperatures, Tc (the temperature at which the free energy of reaction 56 is zero). Bowmer and O Donnell74 found that G(S02) increases with decreasing Tc showing the importance of the thermodynamic factor. Although kinetic factors can explain also the increase in G(S02) with temperature, they do not explain the observed correlation with Tc. 

Conversion is limited by propagation-depropagation equilibrium, due to low ceiling temperature... 

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. 

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