Green and controlled synthesis of short diol oligomers from polyhydroxyalkanoate to develop fully biobased thermoplastics

Polyhydroxyalkanoates (PHAs) are a biodegradable and biobased family of bacterial polyesters with different architectures. Among them, the poly-3-hydroxybutyrate (PHB) is one of the most conventional and easily obtained by biotechnologies from different bioresources. However, this thermoplastic polyester known since more than 50 years presents until now only very limited applications due to a certain number of drawbacks such as poor mechanical properties, high degree of crystallinity, and low thermal stability. One major way to valorize this bacterial polyester could be to develop controlled building blocks for the synthesis of new generation of biobased polymers. In this way, short linear PHB diols oligomers (oligoPHB-diol) with controlled low molar masses have been successfully obtained from a new synthetic pathway using reactive solvents in a green way. Different transesterification reaction parameters were investigated to control the PHB molar mass decrease, such as the catalyst and reactive solvent contents, the temperature and the reactive solvent type. Obtained oligomers were then analyzed in detail and further incorporated into fully biobased thermoplastics. In this work, thermoplastic polyurethanes (TPU) were elaborated as an example of second-generation biobased thermoplastics. To obtain TPUs with high biobased content, a dimer fatty acid-based diisocyanate, the dimeryl diisocyanate, and another biobased diol, the isosorbide, were used. Two series of TPUs were prepared with (i) varying the isosorbide/oligoPHB-diol ratio, and (ii) different oligoPHB-diol molar mass. Materials were fully evaluated for their structural, thermal and mechanical properties, as well as their crystalline behavior. A large range of properties were obtained for these innovative thermoplastics which could open a large range of applications with a green and sustainable approach.