Dynamic Nuclear Polarization (DNP) has recently emerged as a key method to increase the sensitivity of solid-state NMR spectroscopy under Magic Angle Spinning (MAS). While efficient binitroxide polarizing agents such as AMUPol have been developed for MAS DNP NMR at magnetic fields up to 9.4 T, their performance drops rapidly at higher fields due to the unfavorable field dependence of the cross-effect (CE) mechanism and AMUPol-like radicals were so far disregarded in the context of the development of polarizing agents for very high-field DNP. Here, we introduce a new family of water-soluble binitroxides, dubbed TinyPols, which have a three-bond non-conjugated flexible amine linker allowing sizable couplings between the two unpaired electrons. We show that this adjustment of the linker is crucial and leads to unexpectedly high DNP enhancement factors at 18.8 T and 21.1 T: an improvement of about a factor 2 compared to AMUPol is reported for spinning frequencies ranging from 5 to 40 kHz, with ε_H of up to 90 at 18.8 T and 38 at 21.1 T for the best radical in this series, which are the highest MAS DNP enhancements measured so far in aqueous solutions at these magnetic fields. This work not only breathes a new momentum into the design of binitroxides tailored towards high magnetic fields, but also is expected to push the application frontiers of high-resolution DNP MAS NMR, as demonstrated here on a hybrid mesostructured silica material.

The ring-opening polymerization of cyclic ketene acetals (CKAs) by controlled radical mechanisms represents an alternative route for the synthesis of aliphatic polyesters. For the first time, 5,6-benzo-2-methylene-1,3-dioxepane (BMDO) and 2-methylene-4-phenyl-1,3-dioxolane (MPDL) were homopolymerized by nitroxide mediated polymerization (NMP), from the commercially available SG1-based BlocBuilder MA alkoxyamine. Various experimental conditions (i.e., reaction temperature, nature of solvent, and nature of the alkyl initiating radical) were varied to determine the optimized conditions in terms of polymerization kinetics and living character of the final polymer. Chain-end extensions from either PS-SG1 or PBMDO-SG1 were also performed in order to furnish PS-b-PBMDO and PBMDO-b-PS, respectively, thus demonstrating the synthesis of block copolymers comprising a CKA block. In order to have a better insight into the polymerization mechanism, the occurrence of side reactions was analyzed by ³¹P NMR spectroscopy and ESI-MS. It was supposed that the ketal-based macroradical could be irreversibly trapped by nitroxide and thus the corresponding macroalkoxyamine decomposed by CO–N bond dissociation. DFT calculations as well as PREDICI modeling were also undertaken in order to support this hypothesis.

The current study reports on the kinetic analysis of the free-radical polymerization of several seven-membered cyclic ketene acetal monomers and in particular 2-methylene-1,3-dioxepane (MDO) and 5,6-benzo-2-methylene-1,3-dioxepane (BMDO). Such monomers are known to undergo a complete ring-opening to afford polyester chains by a radical pathway. The influence of experimental conditions (e.g., nature and concentration of the initiator, temperature, solvent, etc.) on the kinetics and molar mass distribution was studied. Whereas it was shown that BMDO cannot be polymerized using peroxides due to H-abstraction, the experimental results also demonstrated that the polymerization kinetics were strongly driven by the nature of the propagating radical, with reactivity order: MDO > BMDO > 4,7-dimethyl-5,6-benzo-2(chloromethyl)-1,3-dioxepane (MeBMDO). The copolymerization of MDO and BMDO (1 : 1) was also investigated. The incorporation of both monomers was identical but the polymerization kinetics were governed by the monomer with the lowest reactivity. The experimental results were finally rationalized by the combination of DFT calculations of the propagation rate constants with PREDICI modelings.

The sensitivity of NMR spectroscopy is considerably enhanced by dynamic nuclear polarization (DNP). In DNP polarization is transferred from unpaired electrons of a polarizing agent to nearby proton spins. In solids, this transfer is followed by the transport of hyperpolarization to the bulk via ¹H‐¹H spin diffusion. The efficiency of these steps is critical to obtain high sensitivity gains, but the pathways for polarization transfer in the region near the unpaired electron spins are unclear. Here we report a series of seven deuterated and one fluorinated TEKPol biradicals to probe the effect of deprotonation on MAS DNP at 9.4 T. The experimental results are interpreted with numerical simulations, and our findings support that strong hyperfine couplings to nearby protons determine high transfer rates across the spin diffusion barrier to achieve short build‐up times and high enhancements. Specifically, ¹H DNP build‐up times increase substantially with TEKPol isotopologues that have fewer hydrogen atoms in the phenyl rings, suggesting that these protons play a crucial role transferring the polarization to the bulk. Based on this new understanding, we have designed a new biradical, NaphPol, which yields significantly increased NMR sensitivity, making it the best performing DNP polarizing agent in organic solvents to date.

The radical ring-opening polymerization (rROP) of cyclic ketene acetals (CKAs) by free radical or controlled radical mechanisms attracts considerable research interest as it presents an alternative route for the synthesis of aliphatic polyesters. These monomers can undergo radical addition to their C=C double bonds which subsequently leads to propagation by ring opening. CKA/vinyl monomer copolymerization appears to be an elegant method to produce partially or fully degradable copolymers depending on the proportion of the ester functionality incorporated into the copolymer backbone. Although this approach seems promising, some important limitations still remain. Owing to DFT calculations, we are now able to understand the reactivity of CKAs and common vinyl monomers. Indeed, the calculations confirm that cross-addition is not a key parameter for the copolymerization and the reactivity ratios are linked to the homopolymerization rate coefficients of the comonomer pair. In particular, it was demonstrated that trifluoromethyl vinyl acetate (CF₃VAc) should provide alternating copolymers. These structures were confirmed experimentally with reactivity ratios close to 0 for the MDO/CF₃VAc system (MDO = 2-methylene-1,3-dioxepane). A solid understanding of the reactivity of CKA monomers which allows for the tunable incorporation of main-chain functionalities into copolymers would open up exciting prospects in the field of degradable materials.