Cationic chain polymerization activation energy

Solvent polarity and temperature also influence ihe results. The dielectric constant and polarizability, however, are of little predictive value for the selection of solvents relative to polymerization rates and behavior. Evidently evety system has to he examined independently. In cationic polymerization of vinyl monomers, chain transfer is the most significant chain-breaking process. The activation energy of chain transfer is higher than that of propagation consequently, the molecular weight of the polymer increases with decreasing temperature. 

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 ... 

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. 

A comparison of the observed propagation rate constants for styrene polymerization with different alkali metal counterions is shown in Table 2. Poly(styryl)sodium was presumably associated into dimers since kinetic orders of one-half were observed for the rate dependence on the active chain-end concentration. Poly(styryl)potassium exhibits intermediate behavior dependence on chain-end concentration was one-half order at higher concentrations, but first order at low concentrations. Poly(styryl)rubidium and poly(styryl)cesium exhibit first-order dependencies on chain-end concentrations which is consistent with unassociated chain ends in cyclohexane. The counterion dependence is K+ > Rb+ > Cs+ Li+ in cyclohexane and K+ > Na+ > Li+ in benzene. The interpretation of these results is complicated by the fact that the complex observed rate constants ( obs) reflect both the fact that the dissociation constant for the dimers increases with increasing cation size (no association for rubidium and cesium) and also the fact that the requisite energy associated with charge separation in the transition state would be less for the larger counterions. 

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