Premium
On the (im)possibility of evaluating correct individual rate constants of chain propagation k p and chain termination k t by combining k p 2 / k t and k p / k t data for chain‐length dependent termination
Author(s) -
Olaj Oskar Friedrich,
Kornherr Andreas,
Zifferer Gerhard
Publication year - 2000
Publication title -
macromolecular theory and simulations
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.37
H-Index - 56
eISSN - 1521-3919
pISSN - 1022-1344
DOI - 10.1002/(sici)1521-3919(20000301)9:3<131::aid-mats131>3.0.co;2-7
Subject(s) - chain (unit) , chemistry , chain propagation , chain termination , reaction rate constant , constant (computer programming) , polymerization , thermodynamics , kinetics , physics , quantum mechanics , radical polymerization , organic chemistry , computer science , programming language , polymer
On the basis of simulated data two ways of evaluating individual rate constants by combining k p 2 / k t and k p / k t ( k p , k t = rate constants of chain propagation and termination, respectively) were checked considering the chain‐length dependence of k t . The first way tried to make use of the fact that pseudostationary polymerization yields data for k p 2 / k t as well as for k p / k t referring to the very same experiment , in the second way k p 2 / k t (from steady state experiments) and k p / k t data referring to the same mean length of the terminating radical chains were compared. In the first case no meaningful data at all could be obtained because different averages of k t are operative in the expressions for k p / k t and k p 2 / k t . In spite of the comparatively small difference between these two averages (≈15% only) this makes the method collapse. The second way, which can be regarded as an intelligent modification of the “classical” method of determining individual rate constants, at least succeeded in reproducing the correct order of magnitude of the individual rate constants. However, although stationary and pseudostationary experiments independently could be shown to return the same k t for the same average chain‐length of terminating radicals within extremely narrow limits no reasonable chain‐length dependence of k t could be derived in this way. The reason is an extreme sensitivity of the pair of equations for k p / k t and k p 2 / k t towards small errors and inconsistencies which renders the method unsuccessful even for the high quality simulation data and most probably makes it even collapse for real data. This casts a characteristic light on the unsatisfactory situation with respect to individual rate constants determined in the classical way, regardless of a chain‐length dependence of termination. As a consequence, all efforts of establishing the chain‐length dependence of k t are recommended to avoid this way and should rather resort to methods based on inserting a directly determined k p into the equations characteristic of k p 2 / k t or k p / k t , properly considering the chain‐length dependent character of k t .