Studying the Polymerization Initiation Efficiency of Acetophenone-Type Initiators via PLP-ESI-MS and Femtosecond Spectroscopy

Abstract

The initiation efficiency and the excited state dynamics of the three triplet radical photoinitiators MMMP (2-methyl-4[prime or minute]-(methylthio)-2-morpholinopropiophenone), benzoin (2-hydroxy-1,2-diphenylethanone, Bz), and 4-methyl benzoin (2-hydroxy-2-phenyl-1-(p-tolyl) ethanone, 4MB) are investigated via a trifold combination of pulsed laser polymerization and subsequent electrospray ionization mass spectrometry (PLP-ESI-MS), femtosecond transient absorption (TA) spectroscopy, and density functional theory (DFT) methods. A quantitative analysis of the femtosecond TA data suggests higher radical formation capabilities for MMMP and 4MB compared to Bz based on the inherent photophysical properties of the three initiators. MMMP shows significantly higher complexity of the relaxation pathways due to a partial excitation into higher singlet states as well as extended triplet lifetimes. However, Bz shows - based on a detailed end group analysis - the highest initiation efficiency for a polymerization of MMA at a wavelength of 351 nm compared to both MMMP (Bz : MMMP corresponds to 1 : 0.63) and 4MB (Bz : 4MB corresponds to 1 : 0.86). The current study evidences that the combination of PLP-ESI-MS and TA experiments allows for arriving at quantitative initiation efficiencies of identical radical fragments originating from disparate source molecules. However, the quantitative initiation evaluation of different fragments originating from disparate source molecules requires additional information regarding the fragments' reactivity towards vinyl bonds.

Thomas Wolf

Staff Scientist

My research is focused on discovering structure-function relationships in ultrafast photochemistry to better understand and eventually control this type of reactions.