Competitive copolymerization gives access to gradient copolymers with simple one-step and one-pot strategies. Due to the living nature of the sulfonyl-aziridine polymerization, gradient copolymers can be obtained with low dispersities and adjustable molar masses. The combination of different sulfonyl activating groups allowed to fine-tune the reactivity difference of the comonomers and thus an exact adjustment of the gradient strength. Sulfonyl-activated aziridines are to date the only monomer class providing access to gradient copolymers with reactivity ratios ranging from (1 ≤ r1 ≤ 2; 1 ≥ r2 ≥ 0.5) for statistical or soft gradient copolymers to block copolymers (r1 ≥ 20, r2 ≤ 0.02), only by adjusting the electron-withdrawing effect of the activation groups: the reactivity ratios were calculated by different models for a library of eight comonomers. This library was further used to classify between hard, medium, and soft gradients. From the data obtained from the monomer library, it was possible to predict polymerization rate coefficients (kp) for aziridines, which were not prepared so far: correlation of the shifts in the 13C NMR spectra, the Hammett parameters and secondary parameters such as calculated lowest unoccupied molecular orbital (LUMO) levels of the monomers and the natural charge at the electrophilic carbon, etc., were used to predict (co)monomer reactivity and the resulting gradient strength. We believe that our findings allow us to access tailored gradient copolymers with a controlled monomer sequence distribution depending on the chemical control of comonomer reactivity. With these systematic data on activated aziridines, also more complex copolymer structures can be predicted and prepared. Such materials might find application as linear polyethylenimine derivatives to act as functional polyelectrolytes, or predesigned compatibilizers and surface-active gradient copolymers by a predictable one-step copolymerization.
Gleede, T., Markwart, J. C., Huber, N., Rieger, E., & Wurm, F. R. (2019). Competitive Copolymerization: Access to Aziridine Copolymers with Adjustable Gradient Strengths. Macromolecules, 52(24), 9703–9714. https://doi.org/10.1021/acs.macromol.9b01623