Ring-opening polymerisation of ɛ-caprolactone catalysed by Brønsted acids

Two new Brønsted acids [2,2′-ethylidene-bis (4,6-di- tert -butylphenol)] phosphoric acid (EDBPPOOH) and (3,3′,5,5′-tetra- tert -butylbiphenyl-2,2′-diol) phosphoric acid (TBPO-POOH) were synthesised and fully characterised by 1 H NMR and 13 C NMR spectra and mass spectra. The ringopening polymerisation (ROP) of ɛ -caprolactone ( ɛ -CL) catalysed by the two Brønsted acids proceeded at 110°C without a solvent or at ambient temperature in toluene. Experimental results indicated that the two Brønsted acids were efficient catalysts for the ROP of ɛ -CL with moderate number-average molar mass ( M n) and narrow polydispersity indices (PDI). The catalytic activity of TBPO-POOH is higher than EDBP-POOH in the ROP of ɛ -CL. After benzyl alcohol was added, it was able to accelerate the polymerisation process. The polymerisation can also occur with the addition of water with a monomer/catalyst/initiator mole ratio of 100: 1: 1. The living polymerisation was ascertained by the linear relationships of the M n vs. monomer conversion, then it was further confirmed by a second-feed experiment of a double monomer producing double M n. A kinetic study of the relationships between monomer concentration and time revealed a first-order dependence on monomer concentration in the polymerisation. End-group analysis of 1 H NMR spectra and electrospray-ionisation mass spectra suggests that the two Brønsted acids are capable of catalysing and initiating the ROP of ɛ -CL.