Activation energies anionic polymerizations

The direction of temperature effects in anionic polymerizations is conventional, with increased temperature resulting in increased reaction rates. Observed activation energies are usually low and positive. This apparent simplicity disguises complex effects, however, and the different ion pairs and free ions do not respond equally to temperature changes. Overall activation energies for polymerization will be influenced indirectly by the reaction medium because the choice of solvent shifts the equilibria of Eq. (9-1). 

In general, the activation energies for both cationic and anionic polymerization are small. For this reason, low-temperature conditions are normally used to reduce side reactions. Low temperatures also minimize chain transfer reactions. These reactions produce low-molecular weight polymers by disproportionation of the propagating polymer ... 

The term catalyst is often misused in anionic polymerization. These mechanisms require the use of initiators that differ from catalysts in that they are not regenerated at the end of the reaction. The similarity between initiators and catalysts is that they both create a situation that permits polymerization via a reduction in the activation energy of the process. 

Once the robotic system and procedure passed the optimization and reproducibility tests for a certain type of reaction, the researcher has the chance to move on to the most delightful part of a high-throughput experimentation workflow that is to follow the reaction kinetics of the reaction by withdrawing several samples under comparable conditions. The characterization of these samples allows the determination of the apparent rate constants and activation energies in a very reproducible way. As an example, the anionic polymerization of St in cyclohexane initiated by i-BuLi under different reaction conditions was investigated. Several samples were withdrawn during the reaction into small vials which were prefllled with 25 pL of... 

Table 1 Values of the activation energy reported in the literature for the propagation reaction of the anionic polymerization of styrene (St) in different solvents...

Based on these results, the activation energy of the anionic polymerization of St in cyclohexane was determined as 63 2kJ moP [55]. The results obtained were comparable to the literature results obtained with other solvents and are summarized in Table 1. 

The kinetics of anionic ring opening polymerization of caprolactam initiated by iso-phthaloyl-bis-caprolactam and catalyzed by caprolactam-magnesium-bromide satisfactorily fit Malkin s autocatalytic model below 50 percent conversion. The calculated value of the overall apparent activation energy for this system is 30.2kJ/mol versus about 70kJ/mol for Na/hexamethylene-l,6,-bis-carbamidocaprolactam as the initiator/catalyst system. 

With Na+, as a cation, the activation energies for the anionic polymerization of acrolein and propylene sulfide (11) are approximately the same. On the other hand, with Li+, it is impossible to compare the acrolein activation energy with the same monomer or another polar monomer because no result is found in the literature. Moreover, for the acrolein polymerization, (Raj u+) lower than ( > +). 

In some cases, the heat source can include both the heat of polymerization and the heat output of crystallization of the newly formed products. This is the case in anionic activated e-caprolactam polymerization. This dual heat source must be included in the energy balance equation. As was discussed above, the temperature dependence of the crystallization rate is somewhat complicated. Nevertheless, the propagation of the heat wave is analogous to other well-known cases of wave propagation from consecutive reactions. 

Various modes of termination of anionic polymerization can be visualized. The growing chain end could split out a hydride ion to leave a residual double bond. This is, however, a high activation energy process and has not as yet been reported in the cases where alkali metal cations are present. It is important in systems involving Al—C bonds, however (73). A second possibility is termination through isomerization of the carbanion to an inactive anion. Proton transfer from solvent, polymer, or monomer would also cause termination of the growing chain. Lastly, the carbanion could undergo an irreversible reaction with solvent or monomer. The latter three types have been shown or postulated as termination or transfer reactions. 

Reaction (55) exhibits a higher activation energy than the addition of the caprolactam anion to acyllactam. An addition of acyllactam as activator strongly reduces the induction period and makes possible polymerization at lower temperatures. 

Karpov er af. (1977) studied the polymerization in the presence of a number of anionic emulsifiers below the critical micelle concentration. High molecular weight polymers with low concentrations of impurities were obtained at high rates. The overall rate was found to be about 0.5 order with respect to the dose rate with an activation energy of 5 kcal/moI, in reasonable agreement with those reported by Barriac et al. (1976). 

The values both in tetrahydrofuran and tetrahydropyran are of the same order of mj itude as those of polystyrene compounds at 25°C. Only the lithium compound of a-methylstyrene has an appreciably higher than that of styrene. The free anion rate coefficient, 830 1 mole" sec" (extrapolated, 25°C A = 1.5 x 10 , E = 7.2 kcal mole" ) [145] is smaller than for styrene, as are the ion-pair coefficients, but the ratio between the two is roughly the same. A major factor producing low rates appears to be a higher activation energy than is found for styrene polymerization. 

The activation energy of anionic propagation in the homopolymerization of styrene was determined to be about 1 kcal. per mole. This value refers to the reaction proceeding in tetrahydrofuran solution. The activation energy for the same reaction in dioxane was reported (1, 2) to be 9 3 kcal. per mole. This is one of many examples which stresses the importance of a solvent in ionic polymerization. 

Since kp for the anionic homopolymerization of styrene in tetrayhdrofuran solution is / 600 liters per mole second and the respective activation energy is only 1 to 2 kcal. per mole, the entropy of activation is substantially more negative (by about 14 eu.) than AS for a radical polymerization of styrene. It is likely that the additional decrease in the entropy of activation is due to immobilization of the counterion in the transition state in the middle between the last unit of the growing end and the new unit being added, i.e. 

This polymerization goes so quickly because the anionic intermediate is highly resonance stabilized by the carbonyl and the cyano groups. A stable intermediate suggests a low activation energy which translates to a fast reaction. 

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