Detection of Living Anionic Species in Polymerization Reactions Using Hyperpolarized NMR

Abstract

Intermediates during the anionic polymerization of styrene were observed using hyperpolarized NMR. Dissolution dynamic nuclear polarization (DNP) of monomers provides a sufficient signal-to-noise ratio for detection of (13)C NMR signals in real time as the reaction progresses. Because of its large chemical shift dispersion, (13)C is well-suited to distinguish and characterize the chemical species that arise during the reaction. At the same time, incorporation of hyperpolarized small-molecule monomers is a unique way to generate polymers that exhibit a transient signal enhancement at the active site. This strategy is applicable despite the decay of the hyperpolarization of the polymer due to rapid spin-lattice relaxation. Real-time measurements on polymerization reactions provide both mechanistic and kinetic information without the need for stable isotope labeling of the molecules of interest. These capabilities are orthogonal to currently established methods that separate synthesis and analysis into two steps, making dissolution DNP an attractive method to study polymerization reactions.

In Situ Determination of Tacticity, Deactivation, and Kinetics in [rac-(C2H4(1-Indenyl)2)ZrMe][B(C6F5)4] and [Cp2ZrMe][B(C6F5)4]-Catalyzed Polymerization of 1-Hexene Using 13C Hyperpolarized NMR

“…In the absence of catalyst deactivation, such as in a living polymerization (Scheme 1a), the monomer consumption follows a pseudo-first-order rate law. Considering the spin up and spin down concentration, the monomer signal evolution in the hyperpolarized NMR experiment can be described by 16,18 …”

Chemistry and biochemistry of13C hyperpolarized magnetic resonance using dynamic nuclear polarization

“…39 Modification of substrates by HP [1,1′- 13 C 2 ] acetic anhydride has demonstrated the transfer of hyperpolarization by chemical means, 40 while HP styrene has allowed direct visualization of living anionic intermediates during polarization. 41 For this last set of probes, the main emphasis is identifying special properties of DNP-NMR, and ways the technology itself can expand.…”

Hybrid polarizing solids for pure hyperpolarized liquids through dissolution dynamic nuclear polarization

“…D-DNP is generally performed in moderate magnetic fields (B 0 = 3.35 or in this study, 6.7 T) and followed by rapid dissolution of the frozen sample with a burst of superheated water to give highly polarized solutions. Applications include detection of intermediates in chemical reactions (3)(4)(5), protein folding in real time (6), and detection of cancer by monitoring abnormal rates of metabolic reactions in humans (7). PAs with narrow EPR lines, such as trityl radicals, are usually used for the direct polarization of 13 C nuclei (2).…”

“…However, for quantitative and rapid quenching, ascorbate must be used in excess, and the remaining ascorbate in solution may affect the analyte or sensitive components present in the NMR or MRI system, such as enzymes (5,27,28). Furthermore, the presence of potentially noninnocent additional products arising from the paramagnetic PAs is obviously undesirable for in vivo MRI experiments.…”