Polyether Synthesis by Bulk Self-Condensation of Diols Catalyzed by Non-eutectic Acid Base Organocatalysts

Abstract

Polyethers constitute a well-established class of polymers covering a wide range of applications from industrial man-ufacturing to nanomedicine. Nevertheless, their industrial implementation is limited to short chain aliphatic polyethers such as poly-ethyeleglycol (PEO or PEG), polypropyleneglycol (PPG) or polytetramethylenglycol (PTMG) produced by the ring-opening polymerization of the corresponding cyclic ethers. Herein we report a sustainable and scalable approach for the preparation of medium and long chain aliphatic polyethers by the melt self-polycondensation of aliphatic diols in the presence of non-eutectic acid base mixtures as organocatalyst. These organocatalysts were prepared by forming stoichiometric and non-stoichiometric complexes of methanesulfonic acid (MSA) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as confirmed by NMR spectroscopy and DFT calcula-tions. The non-stoichiometric 2:1 and 3:1 MSA:TBD molar complexes showed superior thermal stability. These non-eutectic acid base mixtures were tested in the bulk-self condensation of 1,6-hexanediol leading to telechelic hydroxy-poly(oxyhexane). The optimized polymerization conditions involved the use of MSA:TBD (3:1) catalyst in a three step polycondensation process at 130 oC - 180 °C and 200 °C respectively. These conditions were applied to the synthesis of a wide range of aliphatic polyethers with a number of methylene units ranging from 6 to 12 units and molecular weights between 5,000 and 22,000 g mol-1. The aliphatic polyethers were highly semicrystalline with melting temperatures ranging from 55 to 85 °C. The synthesis approach was extended to the prepa-ration of value-added copolymers from different lenght chain diols and different functionality, giving rise to different copolymer architectures from linear copolyethers to polyether thermosets. Altogether, this straightforward polymerization strategy enables the access to medium-long chain and cross-linked aliphatic polyethers using easily prepared and recyclable organocatalysts.