The crucial roles of guest water in a biocompatible coordination network in the catalytic ring-opening polymerization of cyclic esters: a new mechanistic perspective.

The ring-opening polymerization (ROP) of cyclic esters/carbonates is a crucial approach for the synthesis of biocompatible and biodegradable polyesters. Even though numerous efficient ROP catalysts have been well established, their toxicity heavily limits the biomedical applications of polyester products. To solve the toxicity issues relating to ROP catalysts, we report herein a biocompatible coordination network, CZU-1, consisting of Zn4(µ4-O)(COO)6 secondary building units (SBUs), biomedicine-relevant organic linkers and guest water, which demonstrates high potential for use in the catalytic ROP synthesis of biomedicine-applicable polyesters. Both experimental and computational results reveal that the guest water in CZU-1 plays crucial roles in the activation of the Zn4(µ4-O)(COO)6 SBUs by generating µ4-OH Brønsted acid centers and Zn-OH Lewis acid centers, having a synergistic effect on the catalytic ROP of cyclic esters. Different to the mechanism reported in the literature, we propose a new reaction pathway for the catalytic ROP reaction, which has been confirmed using density functional theory (DFT) calculations, in situ diffuse reflectance IR Fourier transform spectroscopy (DRIFTS), and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS). Additionally, the hydroxyl end groups allow the polyester products to be easily post-modified with different functional moieties to tune their properties for practical applications. We particularly expect that the proposed catalytic ROP mechanism and the developed catalyst design principle will be generally applicable for the controlled synthesis of biomedicine-applicable polymeric materials.

Perfluoroadamantane and its negative ion.

As a general rule, saturated hydrocarbons are unable to bind an electron, i.e., their electron affinities are negative, but the corresponding perfluorinated molecules can have significant electron affinities, especially in the case of branched and ring systems. Four different density functional theory (DFT) methods in conjunction with double-zeta plus polarization function augmented diffuse function basis sets (DZP++) have been employed to study the equilibrium geometries, electron affinities, and vibrational frequencies of the adamantane (C10H16) and perfluoroadamantane (C10F16) molecules. Three types of neutral-anion separations reported are the adiabatic electron affinity, the vertical electron affinity, and the vertical detachment energy. The adiabatic electron affinity predicted at the DZP++ B3LYP level of theory for adamantane is, as expected, negative (-0.58 eV), while that for perfluoroadamantane is distinctly positive, namely, 1.06 eV (or 1.31 eV after correction for zero-point vibrational energies).

The Perfluoroadamantyl Radicals C10F15 and Their Anions.

The optimized geometries, electron affinities, and harmonic vibrational frequencies of perfluoroadamantyl radicals (C10F15) have been obtained using four carefully calibrated density functional theory methods in conjunction with diffuse function augmented double-ζ plus polarization (DZP++) basis sets. There are two C10F15 isomers with close energies. With the DZP++ B3LYP method, the C3v isomer (1-C10F15) lies energetically above the Cs isomer (2-C10F15) by 0.086 eV (2.0 kcal/mol), while the anionic 1-C10F15(-) isomer is predicted to lie below 2-C10F15(-) anion by 1.00 eV (23.0 kcal/mol). The DZP++ B3LYP method predicts the ZPVE-corrected adiabatic electron affinity for the C3v isomer (1-C10F15) to be 4.16 eV, and that for the Cs isomer (2-C10F15) is 3.10 eV. These EAad values are significantly larger than that (1.31 eV) of the parent molecule perfluoroadamantane (C10F16). For the 1-C10F15 radical, the C*-C bond length is shortened by 0.043 Šupon removal of F from the C10F16 molecule. The C*-C bond distance for the 1-C10F15(-) anion is 0.068 Šshorter than that for C10F16. Similarly, for 2-C10F15 the C*-C distance is 0.053 Šshorter than for C10F16, while re(C*-C) for the anion is 0.061 Šshorter than for C10F16.