Photo-Induced Polymerization Reactions: Quantitative Information via Mass Spectrometry and Femtosecond Pump-Probe Absorption Studies

• Applicants: Professor Dr. Christopher Barner-Kowollik; Professor Dr. Andreas-Neil Unterreiner
• Subject Area: Preparatory and Physical Chemistry of Polymers
• Term: from 2009 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 158927932

• Final Report Year: 2016

Final Report Abstract

The project focused on fusing femtosecond spectroscopy experiments with pulsed laser initiated polymerization coupled with (high resolution) mass spectrometry to elucidate the behavior of photoinitiator systems as a function of their substitution patterns and the type of employed monomer species. Specifically, it was the aim to establish how the absorptivity at the irradiation wavelength correlates with the initiation performance of the initiator systems. Overall, we clearly and unambiguously established that (i) Absorptivity is a poor measure for reactivity: The extinction coefficient should be – for the studied benzoin-type systems – close to 60 L mol^-1 cm^-1. Smaller values may represent dark singlet states that can result in conical intersection. However, higher values are no inevitable sign for a good photoinitiator, but can – on the contrary – imply transitions from nπ* to ππ* excited states, having an inappropriate character for radical formation. (ii) After the excitation into a singlet state a fast intersystem crossing (ISC) process in the time range of 2-4 ps ensures the sufficient formation of photoinitiator species in triplet states. (iii) As a further selective criterion for a good photoinitiator, we observed a time frame of maximum 100 ps for the formation of radicals which derive from the initiators’ triplet state. (iv) Radicals can vary immensely in their reactivity towards monomer double bonds. Thus, a photoinitiator that fulfils all the above criteria, yet generates a high amount of unreactive radicals, will most likely not possess a high initiation efficiency. On the other hand, a photoinitiator that generates highly reactive radical fragments but in only very low numbers, will presumably not perform with a high initiation efficiency.

Publications

• "Quantitative Comparison of Photoinitiators using PLP-ESI-MS and Femtosecond Pump-Probe Spectroscopy" Precision Polymer Materials (P2M) 25th to 28th of August 2013, Ghent, Belgium
  E. Frick, H. A. Ernst, D. Voll, T. Wolf, A. Unterreiner, C. Barner-Kowollik
  
• "Investigation of Three Selected Photoinitiators via Femtosecond Transient Absorption Spectroscopy, TD-DFT Calculations and PLP-ESI-MS" 113th General Assembly of the German Bunsen Society for Physical Chemistry, 29th to 31st of May 2014, Hamburg , Germany
  H. A. Ernst, E. Frick, D. Voll, T. J. A. Wolf, C. Barner-Kowollik, A.-N. Unterreiner
  
• "Studying the Polymerization Initiation Efficiency of Acetophenone-Type Initiators via PLP-ESI-MS and Femtosecond Spectroscopy" Polym. Chem. 2014, 5, 5053-5068
  Frick, E.; Ernst, H. A.; Voll, D.; Wolf, T. J. A.; Unterreiner, A.-N.; Barner-Kowollik, C.
  
• "Enlightening the Mechanism of Copper mediated PhotoRDRP via High– Resolution Mass Spectrometry" J. Am. Chem. Soc. 2015, 137, 6889-6896
  Frick, E.; Anastasaki, A.; Haddleton, D. M.; Barner-Kowollik, C.
  
• "Structure-Reactivity Relationship of Photoinitiators studied by PLP-ESI-MS and Femtosecond Spectroscopy" Photopolymerization Fundamentals 2015, 13th to 16th of September 2015, Boulder, Colorado, USA
  E. Frick, C. Schweigert, H. A. Ernst, Y. Liang, A. Lauer, B. B. Noble, A.-N. Unterreiner, M. L. Coote, C. Barner-Kowollik
  
• "Ultrafast Dynamics of o-Nitrophenol: An Experimental and Theoretical Study" J. Phys. Chem. A, 2015, 119, 9225- 9235
  Ernst, H.; Wolf, T. J. A.; Schalk, O.; Gonzáles-García, N.; Boguslavskiy, A. E.; Stolow, A.; Olzmann, M.; Unterreiner, A.-N.
  
• "On the Polymerization Efficiency of Benzoin-type Photoinitiator Molecules by fs-TA Spectroscopy, PLP-ESI-MS and TD-DFT" 115th General Assembly of the German Bunsen Society for Physical Chemistry, 5th to 7th of May 2016, Rostock, Germany
  C. Schweigert, E. Frick, B. B. Noble, H. A. Ernst, Y. Liang, A. Lauer, M. L. Coote, C. Barner-Kowollik, A.-N. Unterreiner
  
• "Toward a Quantitative Description of Radical Photoinitiator Structure-Reactivity Correlations" Macromolecules 2016, 49, 80-89
  Frick, E.; Schweigert, C.; Noble, B. B.; Ernst, H. A.; Lauer, A.; Liang, Y.; Voll, D.; Coote, M. L.; Unterreiner, A.-N.; Barner-Kowollik, C.