CHAPTER 8: Chemosynthesis of Poly(ε-Lysine) via Ring-opening Polymerization of Cyclic Lysine

Poly(ε-lysine) is an uncommon cationic homopolymer and has many potential high-value applications. Due to its significant antimicrobial activity and nontoxicity to humans, poly(ε-lysine) is now industrially produced by a fermentation process as an additive, e.g. for food and cosmetics. However, the biosynthetic route can only make polymers with a molecular weight of about 3 kDa. Here, we propose the use of bases for the ring-opening polymerization (ROP) of cyclic lysine (ε-lactam) monomer towards poly(ε-lysine). Among the evaluated bases, NaH and t-BuP₂ were found to be the most effective for the polymerization of ε-lactam monomer, affording poly(ε-lysine) bearing pendant 2,5-dimethylpyrrole protecting groups with a number average molecular weight of up to 45 kg mol⁻¹. Moreover, poly(ε-lysine) was prepared by the removal of the 2,5-dimethylpyrrole protecting groups. Finally, a pilot-scale study was demonstrated to obtain poly(ε-lysine) with M_n up to 10 kg mol⁻¹. The new development in the ring-opening polymerization route and M_n improvement for poly(ε-lysine) have created new chances for industry. In particular, the low cost of lysine may help to produce low-cost poly(ε-lysine), providing a new solution that can overcome the cost problem, which has puzzled the poly(ε-lysine) industry since its birth.