Chemoselective Ring-Opening Polymerization of Bio-Renewable α-Methylene-γ-Butyrolactone via an Organophosphazene/urea Binary Synergistic Catalytic System Toward a Sustainable Polyester

Despite the great potential of bio-renewable a-methylene-g-butyrolactone (MBL) to produce functional recyclable polyester, the ring-opening polymerization (ROP) of MBL remains as a challenge due to the competing polymerization of the highly reactive exocyclic double bond and the low strained five membered ring. In this contribution, we present the first organocatalytic chemoselective ROP of MBL to exclusively produce recyclable unsaturated polyester by utilizing a phosphazene base/urea binary catalyst. We show that delicate chemoselectivity can be realized by controlling the temperature and using selected urea catalysts. Experimental and theoretical calculations provide mechanistic insights and indicate that the kinetically controlled ROP pathway is favored by using urea with stronger acidity at low temperatures.