l-lysine Efficiently Polymerizes with CO2 under Mild Conditions for Biocompatible Fluorescent Polyurea

Abstract

Polymerization of CO₂ into polyurea under mild pressure and temperature is still a challenge, especially when using diamine from natural feedstocks. Herein, l-lysine is directly used as a diamine without derivatization and protection for capture and copolymerization of CO₂ under mild temperatures and pressures (0.1–0.5 MPa, 60–90 °C) with the assistance of superbase 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Water-soluble lysine-polyurea (L-PU) has been synthesized. The weight-average molecular weight of L-PU reaches 3 × 10⁴ g/mol, indicating the efficient reaction of lysine and CO₂. In addition, NMR studies confirmed that the unprotected carboxyl acid group (−COOH) in L-PU remains more than 80%, and the polymer may be a candidate to replace nonbiodegradable polyacrylate acid (PAA). The free carboxyl acid group also provides a change to further derivatives of the L-PU. Interestingly, the water-soluble L-PU showed efficient fluorescence performance with quantum yield (QY) up to 20%, which may be attributed to the nonconventional intrinsic fluorescence of nonconjugated aliphatic polyureas containing heteroatoms. It allows the L-PU to be used for cell bioimaging, and in vitro studies reveal that the L-PU is biocompatible and can be efficiently endocytosed by cells with bioimaging ability, indicating that it may be a potential drug delivery system for imaging-guided treatment for cancer tissue.