In this contribution we highlight ongoing developments directed towards the efficient industrial production of two of the most promising polyesters derived from renewable resources, namely polylactic acid (PLA) and polyethylene furanoate (PEF).
As a newcomer with strong market growth potential, PLA started to replace, on a local basis, its petroleum-based equivalents in commodity applications such as packaging, thermoforming, injection molding, fibers and 3D printing. With an estimated worldwide production capacity of 200 kta in 2015 and a forecast to reach 800 kta by 2020, it is considered as one of the leading bio-based polymers after polyolefins.1,2
The preferred route for the industrial production of PLA is the ring-opening polymerization of lactide catalyzed by tin octoate. The reaction mechanism is characterized by a reversible propagation-depropagation (back-biting) equilibrium, resulting in the formation of a polymer resin containing 3-5wt% of residual monomer, which must be finally devolatilized3 to avoid loss of material performances and to meet market requirements.
We first discuss with experimental results the importance of catalyst deactivation to perform an efficient devolatilization, in turn leading to a thermally stable resin and to an economically viable process.
Since these aspects are critical for the successful commercialization of the material, an experimental investigation was carried out to screen several compounds in terms of their ability in complexing and/or deactivating the polymerization catalyst in small scale batch reactions. In fact, despite the large body of literature focused on catalysts, reaction mechanism and kinetics of the propagation (forward) reaction,4-7 little is still known on how to deactivate the catalyst and slow down or block the (backward) depropagation reaction. The best performing candidates were then selected and tested on a continuous 1,000 tons per year production plant to proof their reliability for the sustainable manufacturing of low residual monomer PLA. Samples collected from the plant evidenced that a material with superior optical properties is obtained once the catalyst is effectively deactivated.
In the second part we present a new approach for the industrial production of PEF, which, thanks to its superior barrier properties, is considered a promising substitute of the oil-based PET.8,9 The traditional polymerization route to high molecular weight PEF involves a polycondensation/transesterification step followed by a solid state polymerization step. To overcome the typical drawbacks of this approach, namely the very long reaction time, we started investigating a novel synthetic pathway involving a cyclization of PEF oligomers followed by a living ring-opening polymerization, thus mimicking the PLA process.
We report preliminary laboratory results which support the viability of our approach and discuss its potential benefits as well as challenges in view of its scale-up to industrial production and to the commercialization of furan based polyesters.
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