Green synthesis of poly(succinimide) in ionic liquids as precursor for poly(aspartic acid)

• L-aspartic acid was converted to poly(succinimide), a poly(aspartic acid) precursor. • Ionic liquids as sustainable alternative for o-phosphoric acid to synthesize polysuccinimide. • High molecular weights up to ∼ 47,000 g/mol achieved. • Ionic liquids are recoverable and reusable with limited activity loss. Poly(aspartic acid) (PASP) is a biobased, biodegradable, biocompatible, and non-toxic material. PASP and its derivatives are widely used in water treatment systems, cosmetics, and biomedical applications, among others. PASP can potentially substitute other water-soluble fossil-based polymers, such as poly(acrylic acid). Biobased L-aspartic acid can be polymerized to the intermediate polysuccinimide (PSI), which can be hydrolyzed to PASP or modified with primary amines to materials with new properties. The conventional synthesis of high-molecular-weight polysuccinimide requires a large amount of acid catalyst (e.g., phosphoric acid) or organic solvents, generating a significant amount of acidic and organic waste. We propose substitution of the acid catalysts with ionic liquids, which can simultaneously play the role of solvent. The highest molecular weight PSI (M w ≈47,000 g/mol) was obtained with (SO 3 H)bmimHSO 4 ionic liquid. Moreover, ionic liquids can be recovered after the reaction and reused for further polymerization. Although the activity of the recycled ionic liquid declined compared to the virgin one, PSI with M w ≈30,000 g/mol was obtained after recycling. It is possible to scale up this process to an industrial level by reusing the ionic liquid and blending it with a fresh portion to maintain its activity.