Easy access to ultra-high molecular weight polylactones using a bismuth catalyst

Abstract

A facile method for the preparation of ultra-high molecular weight of polylactides and poly(ε-caprolactone) with relatively narrow polydispersity index using BiPh2Br catalyst is presented. Ultra-high molecular weight PLLA (Mw = 1.75 × 106 g/mol, Đ = 2.0, yield = 84 %) and PDLA (Mw = 1.58 × 106 g/mol, Đ = 2.0, yield = 84 %) having unprecedented molecular weights were synthesized by ring-opening polymerization (ROP) of the L- and D- lactides without any monomer purification and any co-initiators. Ultra-high molecular weight PCL having an unprecedented molar mass of 1.58 × 106 g/mol has also been achieved by ROP of ε-caprolactone without any monomer purification and co-initiators. Effects of initiator concentration, catalyst concentration, polymerization temperature and time on the molecular weight of poly-L-lactide are also investigated. The catalyst activation reaction mechanism in the absence of a co-initiator has been further studied by MALDI-TOF-MS, low and high-temperature 1H NMR and Fourier-Transform Ion-Cyclotron-Resonance Mass Spectrometry, elucidating the activation mechanism of the BiPh2Br catalyst precursor by the water from the monomer and revealing the formation of two active species: BiPh2OR and BiPh(OR)Br. DFT calculations reveal a high energy demand (25 kcal/mol) for the chain transfer to water compared with the ROP of the complex used, supporting the evidence for the synthesis of UHMW polymers. The resulting polymers have been characterized by DSC, GPC and MALDI-TOF-MS; MALDI-TOF-MS proves the presence of linear as well as macrocyclic structures for the molecular weight range detected.