Fabrication of cyclic poly(ethylene brassylate) from ethylene brassylate: a cost-effective biobased seventeen-membered macrocyclic monomer

Research on polyester rings has primarily focused on six- to seven-membered cyclic monomers, while macrocyclic monomers remain largely unexplored. Therefore, ethylene brassylate (EB), a seventeen-membered biobased macrocyclic lactone costing less than $10 kg−1 was selected for this project. Linear poly(ethylene brassylate) (l-PEB) was synthesized using propargyl alcohol as the initiator and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as the catalyst. Under optimized conditions l-PEBs with molecular weights (MWs) ranging from 1.4 to 17.5 kDa with dispersities between 1.27 and 1.53 were obtained, as determined by size exclusion chromatography (SEC). Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI MS) identified α-propargyl-ω-hydroxy-PEB as the major product, together with minor α-propargyl-ω-propargyl-PEB and α-hydroxy-ω-hydroxy-PEB byproducts. The major product was azidified to α-propargyl-ω-azido-PEB and subsequently cyclized by copper(i)-catalyzed azide–alkyne cycloaddition (CuAAC). During azidification, the dihydroxy byproduct converted into a diazide derivative, while the dialkyne byproduct remained unreacted. CuAAC of the minor byproducts produced cyclic dimers, while combinations of all three linear precursors generated small amounts of higher-order cyclic polymers. Cyclic PEBs with molecular weights of 1400, 2000, 2500, and 4800 Da were successfully synthesized. The structures of the linear, azidified, and cyclic PEBs were confirmed by MALDI MS, SEC, nuclear magnetic resonance (NMR), and Fourier-transform infrared spectroscopy (FTIR).