Cyclo-depolymerisation of poly(ethylene naphthalene-2,6-dicarboxylate) and ring-opening polymerisations of the cyclic oligomers obtained

Commercial poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) was cyclo-depolymerised by treatment of a suspension in 1,2-dichlorobenzene at 180 °C with 3 mol% of di-n-butyltin oxide as a catalyst. The cyclic oligomers, produced in up to 93% yield, were characterised by gel permeation chromatography, 1H NMR spectroscopy and mass spectrometry. The products contained cyclic oligomers ranging from the dimer up to at least the nonamer, with the trimer being the most abundant. Careful column chromatography of the mixture allowed pure samples of the cyclic trimer and tetramer to be isolated. Ring-opening polymerisation of the mixture of cyclic oligomers could be achieved by heating the mixture with a catalytic amount of di-n-butyltin oxide at 350 °C for 10 min. These interconversions of PEN and the corresponding cyclic oligomers form the basis of a potential method to recycle this valuable commercial polymer. Cyclic oligomers were similarly prepared from commercial poly(ethylene terephthalate) (PET) and these were then homopolymerised and copolymerised with the cyclic oligomers from PEN.