Monomer Equilibria

The polymerization of THE is an equilibrium polymerization. It fits the equation that relates the enthalpy of polymerization, AH, and entropy of polymerization at 1 Af, to the equilibrium monomer concentration, [Af as a function of the absolute temperature, T, where R is the gas constant... 

In the copolymerization of five- and six-membered oxacyclic monomers, the effective monomer concentration in the propagation reaction decreases because only the monomer in excess of equilibrium is available for copolymerization. However, it is not easy to determine the equilibrium monomer concentration in a copolymerization system. The following equilibrium is expected to exist in the copolymerization of THF. 

Cationic polymerization of cyclic acetals generally involves equilibrium between monomer and polymer. The equilibrium nature of the cationic polymerization of 2 was ascertained by depolymerization experiments Methylene chloride solutions of the polymer ([P]0 = 1.76 and 1.71 base-mol/1) containing a catalytic amount of boron trifluoride etherate were allowed to stand for several days at 0 °C to give 2 which was in equilibrium with its polymer. The equilibrium concentrations ([M]e = 0.47 and 0.46 mol/1) were in excellent agreement with that found in the polymerization experiments under the same conditions. The thermodynamic parameters for the polymerization of 1 were evaluated from the temperature dependence of the equilibrium monomer concentrations between -20 and 30 °C. 

Increasing the temperature may also increase the rate of depropagation resulting in equilibrium monomer concentrations well above an acceptable residual monomer concentration (Sawada (1976)). It seems there exists an optimum temperature of polymerization that will reduce the time of polymerzation but will avoid the problems of depolymetization and of initiator depletion. 

Figure 1 shows the effect of initiator concentration on optimum temperature and optimum time. It is noticed that increasing the initiator concentration hardly affects the optimum temperatures. However, optimum time decreases considerably from 297 minutes (lo = 0.03 mol/L) to 99 minutes (Iq = 0.15mol/L). As is well known, and shown in Figure 2, equilibrium monomer concentration (M, ) increases with temperature. If temperature is increased further, the monomer concentration can not be reduced to the desired final level because of high values. The initiator concentration should be chosoi taking into account the cost of the initiator and the savings due to reduced time of reaction. An initiator concentration Io=0.10 mol/L that resulted in t,=128 minutes was chosen for further simulation studies. 

Figure 2. Effect of Temperature on equilibrium monomer concentration. (-AH) = 55 KJ/mol.

In this paper we formulated and solved the time optimal problem for a batch reactor in its final stage for isothermal and nonisothermal policies. The effect of initiator concentration, initiator half-life and activation energy on optimum temperature and optimum time was studied. It was shown that the optimum isothermal policy was influenced by two factors the equilibrium monomer concentration, and the dead end polymerization caused by the depletion of the initiator. When values determine optimum temperature, a faster initiator or higher initiator concentration should be used to reduce reaction time. 

M , = equilibrium monomer concentration Mo = initial monomer concentration (mol/L)... 

Liquid TIBA exists as an equilibrium monomer-dimer mixture. For example, at 40° TIBA is 16% associated. In hydrocarbon solution, the dimer is more extensively dissociated with increasing dilution (ISO). 

An alternative formulation of the phase-transfer DCC concept was reported in 2008 by the Sanders group [75]. In this case, thiol monomers were dissolved in water on either side of a U-tube containing chloroform (Fig. 1.23). After allowing the system to reach equilibrium, monomer distribution was identical in both aqueous solutions, and mixed species (e.g., 51) were observed in the chloroform layer. 

Increased pressure increases the ceiling temperature and decreases the equilibrium monomer concentration according to... 

Fig. 7-1 Determination of the equilibrium monomer concentration [M]c for the (CiHs O1 (Blvi) initiated polymerization of tetrahydrofuran in dichloroethane at 0°C. After Vofsi and Tobolsky [1965] (by...

Fig. 7-4 Temperature dependence of the equilibrium monomer concentration in THF polymerization by (JjNj PFg. After Dreyfuss and Dreyfuss [1966] (by permission of Wiley-Interscience, New York).

Busfield, W. K., Heats and Entropies of Polymerization, Ceiling Temperatures, Equilibrium Monomer Concentrations, and Polymerizability of Heterocyclic Compounds, pp. 295-334 in Chap. II in Polymer Handbook, 2nd ed., J. Brandrup and E. H. Immergut, eds., Wiley-Interscience, New York, 1989. 

PTHF does not behave ideally in solution and the equilibrium monomer concentration varies with both solvent and temperature. Kinetics of THF polymerizations fit equation 2, provided that the equilibrium monomer concentration is determined for the conditions used. 

Dainton and Ivin have related the equilibrium monomer concentration, Me, to temperature in the equation ... 

By using the value of kp at 20° C and the value of the equilibrium monomer concentration, the rate constant calculated for the depropagation reaction, ka, was 4.67 X 10-2 sec-1 at 20° C. An activation energy of 19.4 kcal/mole and a pre-exponential factor of 1.65 x 1013 sec-1 were calculated for the depropagation reaction. 

The equilibrium ratio polymer/monomer is the same in the base catalysed polymerization as in the hydrolytic process (43, 94) and so is the equilibrium content of cyclic oligomers in both processes (43). Using the extremely fast basic polymerization it was made possible to follow the equilibrium in the range of very low temperatures, where it was hardly possible before. It was found that in polymers which were prepared below their melting point, the monomer content is essentially lower than would be expected from the extrapolation of the temperature depend-ence of the equilibrium monomer content obtained at higher temperatures (94) (fig. 5). This indicates that the crystaline fraction of the polymer does not participate in the equilibrium. It is possible to establish the concentration of the amorphous part independently, e. g. 

The existing equilibrium monomer concentrations have been measured by several authors (9,17, 30, 31) for anionic polymerizations (from the type described by Szwarc (24) at different temperatures. The thermodynamic terms AH and AS° can be calculated from these data according to Equation 3. Table I shows the results of these calculations and a value for AH which was measured calorimetrically (20). (The values... 

The ROMP of cycloheptene can lead mainly to dimer or to high polymer depending on the conditions. The equilibrium monomer concentration is temperature-dependent271. 

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