Rapid synthesis of new block copolyurethanes derived from l-leucine cyclodipeptide in reusable molten ammonium salts: novel and efficient green media for the synthesis of new hydrolysable and biodegradable copolyurethanes

Abstract

This study concerns the synthesis of novel multi block polyurethane (PU) copolymers containing cyclodipeptide, taking the advantage of ionic liquids (ILs) under microwave irradiation. For this, L-leucine anhydride cyclodipeptide (LACP) was prepared and then a new class of poly(ether-urethane-urea)s (PEUUs) was synthesized in molten ammonium type ILs. ILs were used as reaction media and PUs were prepared via two-step polymerization method. In the first step, 4,4'-methylene-bis-(4-phenylisocyanate) (MDI) was reacted with LACP to produce isocyanate-terminated oligo(imide-urea) as hard segment (NCO-OIU). Chain extension of the aforementioned pre-polymer with polyethyleneglycol (PEG) of molecular weights of 1000 (PEG-1000) was the second step to furnish a series of new PEUUs. These multiblock copolymers are thermally stable, soluble in amide-type solvents, hydrolysable and biodegradable. PEUUs prepared in ILs under microwave irradiation showed more phase separation and crystallinity than PEUUs prepared under conventional method. The protocol presented here has the merits of environmentally benign, simple operation, convenient work-up, short reaction time and good yields without using volatile organic solvents, and catalysts. Ammonium type reaction media were air and water stable, and relatively cheap, which makes them suitable for application. The results demonstrate that they can be easily separated into water and reused without losing activity. Reusability of tetrabutylammonium bromide as reaction media makes the method a cost effective and environmentally benign method under microwave irradiation. Thus, we could prepare environmentally friendly polymers via environmentally benign method.