Determination of the Mode of Termination in Radical Polymerization via Mass Spectrometry

Abstract

We comprehensively explore the use of mass spectrometry (MS) for quantitative determination of the mode of termination, i.e., the extent of disproportionation versus combination, in radical polymerization. Development of the underlying kinetic theory forms a major portion of our endeavors. What emerges from this theory is that the ratio of polymer from disproportionation to combination is a function of chain length and of fundamental kinetic parameters such as the rate of initiation and the rate coefficients for propagation and termination. Therefore, all this information must be at hand in order to determine λ, the fraction of termination by disproportionation. We implement our findings using electrospray−ionization MS results for poly(methyl methacrylate) that we synthesized at 85 °C in benzene employing bis(3,5,5-trimethylhexanoyl) peroxide as initiator, chosen because it has the advantage of yielding only one primary radical species. Our results are well fitted by theory and are found to vary with chain length and initiator concentration as predicted. We also address the issue that signal intensities from macromolecular MS techniques may not give the true molecular weight distribution of a sample because of variation of ionization efficiency with species mass. We devise a way of obtaining λ from data for hypothetical combination and disproportionation species of identical mass. Our final result is λ = 0.63 ± 0.10, suggesting that disproportionation is not as rampant in methyl methacrylate systems as commonly thought.

Termination Rate Coefficients for Radical Homopolymerization of Methyl Methacrylate and Styrene at Low Conversion

“…For the fraction of termination by disproportionation we used λ = 0.63 for MMA [36] and λ = 0.1 for STY. [37] Of course the value of λ should vary with temperature, [37] however there is no reliable information in the literature on this variation, [36,37] and therefore we have simply used…”

Decisive Reaction Steps at Initial Stages of Photoinitiated Radical Polymerizations

“…[9][10][11][12][13] Reversible steps in "classical" radical polymerizations have been established in the case of depolymerizations (decompositions) [14][15][16][17] and during chain growth (also characterized by the ceiling temperature). [4,[18][19][20][21][22][23] Another important feature of photoinitiated radical polymerizations is the formation of benzaldehyde derivatives even at early stages of the polymerization. [24] Here we address mechanistic aspects of the reversibility and formation of aldehydes by using 1 H CIDNP (chemically induced dynamic nuclear polarization) spectroscopy [25] to follow the very early steps of the radical polymerization.…”

Determining Initiator Efficiency in Radical Polymerization by Electrospray‐Ionization Mass Spectrometry

“…The other peak pairs support this primary data, but show more pronounced scattering. This is most likely due to the fact that for this secondary information the molecular weight distribution from combination and disproportionation are interfering [24] and errors of different sources are hence amplified.…”

“…Peaks from disproportionation, however, must not be ratioed with those from combination and vice versa, as these originate from different underlying chain-length distributions. [24] As the method rests on referencing to an initiator of known efficiency, it is difficult to evaluate a potential monomer conversion dependence of f. The efficiency is known to slightly drop in the initial monomer conversion range and shows a significant fall-off at high monomer conversions. [3,6,25] These effects are governed by the restriction of motion of the primary radicals by surrounding polymer.…”