Monomer driven design of aromatic-aliphatic polyesters built through ring-opening polymerisation

Abstract

The growing environmental impact of plastic waste has led to extensive research into the life-cycle of polymeric materials. Reduction in the build-up of commodity plastics, such as poly(ethylene terephthalate), has been a hot topic. However, recycling poly(ethylene terephthalate) requires energy intensive conditions and further puri cation to obtain the starting material phthalic acid after polycondensation. Advances in monomer design have expanded the range of biodegradable polymers accessible to mimic the properties of commercial plastics, whilst giving the advantage of greener recycling methods. Benzodioxepinones with varying meta-substituents were subjected to ring-opening polymerisation, using an aluminium salen catalyst to a ord a series of novel aromatic-aliphatic polyesters. The polymerisation, catalytic degradation, and thermal behaviours of the polyesters were explored along with their crystallographic structure. The characteristic degradation of poly(2-(2-hydroxyethoxy))benzoate back to its cyclic monomer, through exploitation of the monomer-polymer equilibrium, outweighed the poor thermal properties (Tg (27 C)) of the polyester. The unique degradability and thermal properties of this polyester gave potential as use in copolymers and blends with poly(lactic acid). Copolymerisation with poly(lactic acid) improved the thermal properties of poly(2-(2-hydroxyethoxy)benzoate) and introduced UV-vis absorbing properties to poly(lactic acid). The copolymers were shown to catalytically degrade, via the use of aluminum salen complexes, and enzymatically degrade, via proteinase K. Altering the electron donating and withdrawing properties of the meta-substituents from hydrogen has been shown to tune the rate of degradation and the thermal properties and stability of the polyester. The polymerisation and depolymerisation kinetics showed faster rates for electron withdrawing groups with selective depolymerisation back to their cyclic monomers in 10 minutes at 110 C. The thermal characterisation showed varying glass transition temperatures from 29.7 C to 60.7 C, with electron withdrawing groups exhibiting higher values. An increase in the melting temperatures and the thermal degradation activation energies of the polyesters were also observed. The tunability and characteristics of this class of aromatic-aliphatic polyesters gives insight into replacing commodity plastics with polyesters that have the ability to be recycled selectively back to their monomers, thus minimising the amount of starting material required to reprocess the plastic.