Energetics of propagation reactions

The energetics of any propagation reaction can in the first instance be described by a simple Arrhenius relationship 

Substitution of the appropriate rate values yields a ratio of about 10, extremely close to that obtained from chemically initiated polymerization. 

Studies of the anionic polymerization of various monomers [39] have yielded data for both free ion growth and ion pair propagation. Indeed compared with the situation in cationic systems, data from anionic polymerizations are rather elegant. Swzarc et al. [108] have deduced a value of 6.5 x lO 1 mole sec for the free anionic propj ation coefficient of styrene in tetrahydrofuran at 25°C, with [109] 6 kcal 

Comparison of the data for free cationic propagation of isobutene, styrene, isobutyl vinyl ether and AT-vinylcarbazole is also useful. Elementary electronic arguments would predict a reactivity sequence as follows  

Observed values of rate coefficients do not depend simply on energy (enthalpy) terms but are also controlled by entropy effects, reflected in the pre-exponential factor, Ap. 

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