Carbodiimide Ring-Opening Metathesis Polymerization

Abstract

Controlled incorporation of nitrogen into macromolecular skeletons is a long-standing challenge whose resolution would enable the preparation of soft materials with the scalability of man-made plastics and functionality of Nature's proteins. Nylons and polyurethanes notwithstanding, nitrogen-rich polymer backbones remain scarce, and their synthesis typically lacks precision. Here we report a strategy that begins to address this limitation founded on a mechanistic discovery: ring-opening metathesis polymerization (ROMP) of carbodiimides followed by carbodiimide derivatization. An iridium guanidinate complex was found to initiate and catalyze ROMP of N-aryl and N-alkyl cyclic carbodiimides. Nucleophilic addition to the resulting polycarbodiimides enabled the preparation of polyureas, polythioureas, and polyguanidinates with varied architectures. This work advances the foundations of metathesis chemistry and opens the door to systematic investigations of structure-folding-property relationships in nitrogen-rich macromolecules.

Figure 1

A. Ribbon diagram of chymotrypsin with nitrogen atoms highlighted. B. Examples of synthetic polymers with nitrogen-rich backbones utilized in industry and academia.^−,, C. State-of-the-art synthesis of polyCDIs and associated side-reactions.^, D. Cross-metathesis precedents and two proposed mechanisms involved.⁻ E. Summary of the present work.

Figure 2

A. Polymerization of M1 and M2, with key hydrogen atoms labeled. B. ¹H NMR spectra of crude polymerization reaction mixtures for M1 and M2 at 3 and 6 h, respectively, in perdeuterated tetrahydrofuran (THF-d₈); key resonances are assigned, and % monomer conversions are indicated at the bottom right side of the spectra; std = 1,3,5-tritert-butylbenzene. C. Semilogarithmic plot of monomer concentration versus time in the polymerization of M1 and M2 ([monomer]:[1] = 200:1, [monomer] = 0.5 M, temperature = 23 °C, Figures S2 and S3). D. Evolution of M_n and Đ, measured by gel permeation chromatography with multiangle light scattering (GPC-MALS) as a function of monomer conversion for the polymerization of M1 and M2 ([monomer]:[1] = 200:1, [monomer] = 0.5 M, temperature = 23 °C, Figures S4 and S5). E. Experimental DP and Đ, as measured by GPC-MALS vs theoretical based on variation in [monomer]:[1] loadings and near-terminal conversions for the polymerization of M1 and M2 (Figures S6–S9, Tables S1 and S2); * = data for high-MW fraction only (Figure S8).

Figure 3

A. Proposed mechanism for CDI ROMP. B. Reaction coordinate diagram for M1 based on DFT computations using a PBE-D2 functional with a split basis set: 6-311+G(d,p) for nitrogen, 6-311G(d,p) for carbon and hydrogen, and LANL2DZ with included ECP augmented with one f-polarization function (0.938) for Ir and CPCM solvation model for THF. Computed structures of key intermediates 7 and 8 are shown on the right, with hydrogen atoms omitted for the sake of clarity.

Figure 4

A. Derivatization of polyM1 with various nucleophiles; conversion is indicated adjacent to reaction conditions, and isolated yields are indicated in parentheses. B. AFM micrograph of spin-casted (from DCM) polyM1-BB. Inset: Enhanced images of single polyM1-BB bottlebrush in worm-like conformations. C. TGA data (10 °C/min) of polyM1 derivatives. D. DSC data (10 °C/min, second heating) for polyM1 derivatives.