Application of matrix-assisted laser desorption ionization time-of-flight mass spectrometry in pulsed laser polymerization. Chain-length-dependent propagation rate coefficients at high molecular weight: An artifact caused by band broadening in size exclusion chromatography?

The combination of matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-ToF-MS) and size exclusion chromatography (SEC) combined with pulsed laser polymerization has been used to evaluate propagation rate coefficients (k(p)) in bulk free-radical polymerization for the systems methyl methacrylate and styrene. By varying laser frequencies, the relation between polymer chain length and observed propagation rate coefficient (k(p)(obs)) has been investigated in detail. It has been found that deviations between MALDI-ToF-MS and SEC at higher molecular weights are the result of instrumental effects in SEC. Moreover, using a model and taking into account experimental studies on the propagation of oligomeric species, it has been inferred that the relationship between k(p)(obs) and polymer chain length is the result of chain-length-dependent behavior of the true k(p) in the oligomeric range, therewith excluding any chain-length-dependent behavior at higher molecular weights.