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A four-pulse version of the pulsed double electron electron resonance (DEER) experiment has been applied to a series of TEMPO diradicals with well-defined interradical distances ranging from 1.4 to 2.8 nm (see picture). The new pulse sequence allows broad distributions of electron-electron distances to be measured without dead-time artifacts.

CuCI, TMEDA, 3,5-bis-(fert-butyi)phenylacetylene (0.01 eq.), 1 ,2-dichlorobenzene, molecular sieve 4,&, 02.65 'C, 3 h, 90 %

The homopolymerization of methyl methacrylate (MMA) in the presence of 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) is described and compared to the TEMPO-mediated polymerization of styrene. Furthermore, the formation of block copolymers between MMA and styrene from polystyrene chains with TEMPO fragments as end groups is examined. The degree ofblock formation is estimated by GPC analyses. ESRexperiments are performed on the polystyrene chains to study the scission behavior of the TEMPO fragments.

Conformations of end-labeled alkanes in a rigid polymer matrix were investigated using the pulsed double electron-electron resonance (DEER) method. To this end, dipolar spectra of a series of aliphatic dicarbonic acids with 8−20 methylene units and chain ends bifunctionalized with nitroxide radicals were recorded in a polystyrene matrix. The biradicals are derived from hindered amine light stabilizers (HALS), used as polymer additives. Analysis of the dipolar spectra allows the determination of the end-to-end distances of the oligomer biradicals. The experiments show that the “flexible” alkane chains are almost completely stretched. This observation is consistent with the results of force field calculations.

The generalized hyperfine sublevel coherence transfer pulsed ESR experiment is described that consists of a nuclear coherence generator, a first evolution period, a nonselective microwave π pulse, a second evolution period, and a nuclear coherence detector. Several incrementation schemes to perform one-dimensional experiments are discussed, namely, a four-pulse electron spin echo envelope modulation scheme for measuring combination frequencies, two types of nuclear coherence transfer echo experiments (DEFENCE) to record dead time free electron spin echo envelope modulation spectra of disordered systems, and a new hyperfine spectroscopy experiment for the direct measurement of hyperfine frequencies. The one-dimensional experiments can be combined to four two-dimensional schemes. In addition to hyperfine sublevel correlation spectroscopy (HYSCORE) where two nuclear frequency dimensions are correlated, three new two-dimensional experiments are introduced that correlate the hyperfine with the nuclear frequency, the combination frequency with the nuclear frequency, and the combination frequency with the hyperfine frequency. With a properly selected two-dimensional incrementation scheme the digital resolution and signal-to-noise ratio of the spectra can be improved and artifacts can be suppressed. The predicted features of the one- and two-dimensional experiments are verified experimentally for both ordered and disordered systems using echo and coherent Raman beat detection.

A new concept for controlled radical polymerization in the presence of stable free radicals is presented. Due to irreversible side reactions of free polymer radicals, the amount of dead polymer chains and, consequently, the concentration of stable free radicals increases during the polymerization reaction ( Figure 1). Therefore, only monomers with the ability to constantly thermally initiate radical polymerization (e.g., styrene) to capture the excess counter radicals are susceptible to this type of polymerization. In contrast, we present a self-regulating process in controlled radical polymerization, which allows controlled radical polymerization of monomers with no spontaneous initiation. The increase of stable free radicals in our approach is prevented by slow decomposition of the counter radicals and by simultaneous formation of new initiating species. To prove this concept, two triazolinyl radicals (4 and 7) with different thermal stabilities have been studied. Their difference in stability shows a remarkable effect on their use as additives for the controlled radical bulk polymerization of styrene and MMA. The procession of the polymerization in the presence of spirotriazolinyl 7, a stable radical, again strongly depends on the initiating properties of the monomer. While the polymerization of styrene is very well controlled, polymerization of MMA in the presence of spirotriazolinyl 7 could only be accomplished to very low conversions. In contrast, triazolinyl 4, a less stable radical, is additionally able to effectively control the polymerization of MMA at comparably low temperatures.

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