Time-resolved EPR in the sub µµµµ s time-scale

The SP-PLP-EPR method described withing this thesis is based on 100 kHz modulation of the magnetic field strength during measurement (in order to increase S/N). The field

146 CLOSING REMARKS AND OUTSTANDING CHALLENGES FOR SP-PLP-EPR modulation frequency however restricts the time-resolution to 500 ns. Time resolution is further decreased to a few µs by co-addition of several acquisitions per data-point on the time-axis in order to increase S/N. The given time resolution within SP-PLP-EPR is adequately high for the resolution of single propagation steps which refer to a propagation time t_p = (k_p·c_M)⁻¹ of at least 2 µs (for MA bulk at 80 °C). The time resolution of SP-PLP-EPR thus allows for detailed investigations into chain-length-dependent termination rate coefficients as outlined in chapters 5.3.2, 5.3.3, and chapter 6. Reaction steps associated with faster rate as compared to propagation are however hardly resolved by the given EPR setup.

Time-resution of the EPR detection may be increased beyond µs to even 2 ns by using a modulation-free so called time-resolved (TR)-EPR set-up^[185] The original TREPR set-up described by Forbes^[185] which requires additional timing electronics (boxcar signal averager) may be further improved by using fast state-of-the-art microwave bridges for transient-EPR in conjunction with a cavity of low resonance quality. The setup has already been planned and installed in the new EPR laboratory at the University of Göttingen. Time-resolution in the range of ns allows for studying extremely fast reaction steps which occur in the initial phase of free-radical polymerization. Such steps are: (i) Initiation, i.e. post-fragmentation of primary decomposition products of the photo-initiator and subsequent addition of the fragments to the first monomer unit with referring rate coefficient ki, (ii) addition of the second monomer unit which refers the first “real” propagation step associated with kp(1) and chain-length dependent propagation, (iii) fast transfer processes occurring in the initial phase of novel controlled radical polymerization such as RTCP (iv) inhibition and retardation periods in the very beginning of RP (v) transfer processes to radical species occurring during the polymerization of N-containing monomers and other monomers where detection of radical species via field-modulated PLP-EPR conditions fails (NVP, NVI, styrene, maleic acid etc.).

A unique feature is associated with non-field-modulation-free highly time-resolved TREPR spectroscopy of photo-initiated polymerizations: S/N is often found to be increased as compared to normal EPR conditions caused by the chemically induced dynamic electron spin polarization (CIDEP) phenomeon. There are several CIDEP mechanisms, each is capable of producing polarizations that are up to 100 times larger than the Boltzmann signal which is detected via field-modulated SP-PLP-EPR.^[185] Spin populations created by photoinduced radical formation refer to non-equilibrium states and relaxate into the Boltzmann distribution within a few 100 ns. The CIDEP relaxation has to be taken into account within the kinetic analysis of the time-resolved EPR signals. The signal

CLOSING REMARKS AND OUTSTANDING CHALLENGES FOR SP-PLP-EPR 147 enhancement by spin polarization may however be useful for providing mechanistic information i.e. by identification of short-lived radical types.

148 ACKNOWLEDGEMENTS

13 Acknowledgements

Prof. Michael Buback is greatfully acknowledged for his interest, his encouragement, his support and his persistent help in professionional and personal improvement. Thanks to Prof.

Phillip Vana for collaboration in the field of RAFT polymerization and for being co-referee for the present thesis.

I am very greatfull to my parents Kornelia and Wolfgang Barth for continuous support and encouragement.

I like to express my gratitude to the many collaborateurs from other universities for fruitful exchange during the past years: Prof. Greg Russell (Christchurch, New-Zeeland), Prof.

Robin Hutchinson (Kingston, Canada), Dr. Igor Lacik (Bratislava, Slowakia), Prof. Sabine Beuermann (Potsdam, Germany), Prof. Thomas Junkers (Hasselt, Belgium), Prof. Barner-Kowollik (Karlsruhe, Germany), Dr. Inga Woecht (Clausthal-Zellerfeld, Germany), Rebekka Siegmann (Potsdam, Germany).

Many thanks to the collaborateurs from industry, Dr. Klaus-Dieter Hungenberg (BASF SE, Ludwigshafen) and Dr. Pascal Hesse (BASF SE, Ludwigshafen), for their interrest, helpful discussions and stimulation from the industrial-perspective.

Collaborateurs from the University of Göttingen is gratefully acknowledged: Dr. Claudia Stückl, Nils Wittenberg, Jens Schrooten, Nicolay Soerensen, Joachim Morick, Sebastin Primke, Sebastian Smolne, Stephan Samrock and others. Special grate is expressed to Wibke Meiser and her co-workers Hendrik Kattner and Alana Schlieper for collaboration within the EPR investigations on the RAFT polymerization mechanism.

Help from the institute staff: Dr. Hans-Peter Vögele, Dr. Markus Hold, Marion Diegmann, Sandra Lotze, Heike Rohmann, Ute Friesen-Lippke and others is greatefully acknowledged.

I like to acknowledge my colleges from the AK Buback and the AK Vana for the friendly athmosphere, special thanks to Bastian Ebeling, Arne Wolpers, Annika Groschner, Arne Heins, Timo Scheffer, Frank Behrend, Julian Strohmeier and others for the relaxed times during the workspaces.

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