Pseudostationary Polymerization: Improving the Determination of the Rate Constant of Propagation

Pulsed-laser initiated polymerization (PLP) leads to chain length distributions with characteristic extrapeaks. The low molecular weight side points of inflection L_LPI are located near to (multiples) of a specific chain length L₀ which is equal to the product of pulse separation t₀ and propagation frequency k_pM], i.e. rate constant of propagation k_p times monomer concentration M], allowing a direct determination of k_p. However, Poissonian broadening inherent in the polymerization process as well as Gaussian broadening due to axial dispersion caused by the size exclusion chromatographic (SEC) device leads to a shift of L_LPI as compared to L₀ – its extent depending on the experimental parameters chosen – which in turn causes an error up to 10–20% in the rate constants evaluated. Fortunately, comparison of the experimental peak width with some sort of theoretical peak width yields several types of correction factors and furthermore master-correction functions which are able to reduce the remaining error on average by at least a factor of 10.