Polymerization enthalpies

In estimating the enthalpy of polymerization, the physical state of both starting monomer and polymer must be specified. Changes in state are accompanied by ethalpy changes. Therefore, they also affect the level of the polymerization enthalpy. The AfT forN ylylene previously mentioned is apphcable to the monomer as an ideal gas. To make comparisons with other polymerization processes, most of which start with condensed monomer, a heat of vaporization for N ylylene is needed. It is assumed herein that it is the same as that for N ylene, 42.4 kJ /mol (10.1 kcal/mol). Thus the AfT of the hquid monomer -xylylene is 192.3 kJ/mol (46.0 kcal /mol). 

The reaction enthalpy of this process was calculated to be 121 kl/mole. This value is much higher than the pure polymerization enthalpy (90 kJ/mol [2]). 

Starting with the n = 4 case, the desired polymer can be obtained by polymerization of either cyclobutene (16, n = 4) or butadiene. Using the cyclobutene polymerization enthalpy from Reference 16 and of the enthalpy of formation of monomer from Pedley, we find the enthalpy of formation of [— CH=CH—(CH2)2—] is 12 kJmol-1. We conclude that the enthalpy of hydrogenation is —116 kJmol-1. 

A value for the polymerization enthalpy of 21.5 kcal/mole can be used to estimate percent conversion and rates for N-substituted maleimide/vinyl ether and maleic anhydride/vinyl ether copolymerizations. A value of 18.6 kcal/mole can be used for the enthalpy of polymerization of acrylate monomers to convert heat evolution data to percent conversion. Since the molar heats of polymerization for N-substituted maleimide vinyl ether copolymerization and acrylates vary by less than 20 percent, the exotherm data in the text are compared directly. 

Depolymerization is a special case of thermal degradation. It can be observed particularly in polymers based on a, a -disubstituted monomers. In these, degradation is a reversal of the synthesis process. It is a chain reaction during which the monomers are regenerated by an unzipping mechanism. This is due to the low polymerization enthalpy of these polymers. For the thermal fission of polymers with secondary and tertiary C-atoms, higher energies are required. In these cases elimination reactions occur. This can be seen very clearly in PVC and PVAC. 

The polar group effect on the initial melting point and on the Tg of the crosslinked network has been shown by structure modification as exemplified in Fig. 31. It should be mentioned that the highest Tg observed with the nitrile group is obtained with a lower crosslinking density as can be concluded by comparison between the polymerization enthalpies (AH Ar2=186 kj mol 1/AH Ar3= 109 kj mol-1) [97]. Some other relationships are also discussed. For example, introduction of CF3 decreases the melting point of the BMIs and decreases the permittivity of the corresponding network [98]. 

Spiro orthoesters (92, R = Me, Ph, and H) show typical equilibrium polymerization behavior at or below ambient temperature. [92] The poly(cyclic orthoester)s derived from 92 depolymerize to the monomers, although they have sufficient strains to be able to undergo ring-opening polymerization. The polymerization enthalpies and entropies for these three monomers were evaluated from the temperature dependence of equilibrium monomer concentrations (Table 5). The enthalpy became less negative as the size of the substituent at the 2-position in 92 was increased H < Me < Ph. This behavior can be explained in terms of the polymer state being made less stable by steric repulsion between the bulky substituents and/or between the substituent and the polymer main chain. The entropy also changed in a similar manner with the size of the substituents. 

Studies by many authors, e. q. on copolymerizations of styrene with a-methylstyrene (characterized by low Tc, 334 K), appear to agree with the ideas of Lowry. Some author claim, however, that even copolymerization of this pair can be described by the simple copolymerization equation [221], Johnston and Rudin are of the opionion that the depropagation reaction is not as important in this case because only short sequences of a-methylstyrene are produced. The formation of short blocks is accompanied by relatively high polymerization enthalpy. They are thermodynamically more stable than the homopolymer and have a higher Tc. Only at considerably higher copolymerization temperatures (with the pair styrene—a-methylstyrene > 420 K) does the depropagation effect become important. 

Photo DSC experiments conducted in a ciear coating at L 400 nm - where only the substituted thioxanthone Z (Ri - Rg R3 H R4 = C00(CH2CH20)8 H) absorbs -demonstrates the increase of the poiymerization efficiency [25]. Under exposure, the mixture of Z + fi leads to a considerabie polymerization enthalpy whereas, in the presence of Z or S alone, the exothermic signal remains very small. The same is true when using a laser light at X, - 440 nm [26] for the excitation of a mixture S+fi- No polymerization occurs in the presence of or . The relative reactivity of fi at 363 nm and -t- at 440 nm shows a 35 1 ratio, thus defining a low quantum efficiency of excitation transfer. 

Polymerizations with positive polymerization enthalpies and entropies are very rare. Since the term —TASp becomes less negative with decreasing temperature, a floor temperature, below which no polymerization is possible, occurs. 

Only the translational and external rotational entropy components are lost in the polymerization of olefins. The loss in translational entropy, of course, is independent of constitution. The loss in external rotational entropy is also independent of the monomer constitution since the bond moments and moments of inertia of most monomers are of about the same magnitude. The internal rotational and vibrational entropy components are indeed different from monomer to monomer, but their absolute values are quite small (Table 16-7). Thus, the standard entropy ASl is practically independent of constitution in the case of compounds with olefinic double bonds (Table 16-8). Differences in the ceiling temperature are, in practice, caused by the polymerization enthalpy alone. 

Conformational (steric) effects and ring strains contribute to strain energies. It is the van der Waals radii, and not the atomic radii, which are decisive in these cases. For example, fluorine has a smaller van der Waals radius than hydrogen, since the larger fluorine atomic mass leads to lower vibrational amplitudes. Thus, hydrogen produces a greater steric effect than fluorine the polymerization enthalpy of tetrafluoroethylene is more negative... 

Table 16-9, Bond Energies of Multiple and Single Bonds and the Contribution 2Ea — En to the Polymerization Enthalpy AHpm Calculated from These Values...

Another well-developed mean-field model, known as the free association model, was developed by Dudowicz, Douglas and Freed and is based on the mean-field Flory-Huggins incompressible lattice model. ° Douglas and co-workers incorporated two temperature-independent parameters - polymerization enthalpy AHp and entropy ASp -along with parameters describing the flexibility of the polymer and a solvent-monomer interaction parameter (x). The model allows the calculation of various temperature-dependent properties, such as the number-average DP, constant volume specific heat (Q,), and osmotic pressure, and predias similar temperature-dependent behavior with the van der Schoot treatment. 

The polymerization enthalpy AHZp is negative and the entropy of polymerization AS p is positive. In this case, AG p is always negative. The polymerization is possible at any temperature. Experimental examples are unknown. 

Table 16-13, The Polymerization Enthalpies AHZp)xx of Various Monomers...

Heat of polymerization (enthalpy of polymerization). The difference between the enthalpy of 1 mol of monomer and the enthalpy of the products of the polymerization reaction. Addition polymerizations are exothermic, values ranging from about 35 to lOOkJ/mol, and removal of this heat is an important aspect of reactor design. The reported value of an enthalpy of... 

For long polymer chains the enthalpy and entropy changes in the propagation reaction are effectively those of the overall polsrmerization reaction (113,422). The polymerization enthalpies AHp of most free radical polymerizations are negative, with typical values of —30 to —100 kJ mol as can be seen in Table 8. 

Table 8. Standard Polymerization Enthalpies AH° and Polymerization Entropies AS° of Various Monomers for the Reaction of Liquid Monomers to Condensed Polymers ...

---