Chemical Upcycling of Poly(3-hydroxybutyrate) (P3HB) toward Functional Poly(amine-alt-ester) via Tandem Degradation and Ring-Opening Polymerization

The upcycling of plastic waste into value-added products is a promising strategy to reduce plastic pollution and create a sustainable style for polymeric materials. We report here the synthesis of a novel N-heterocyclic lactone, 5-methyl-N-Boc-1,4-oxazepane-7-one (MeOxPBoc) and corresponding poly(amine-alt-ester) through upcycling of the poly(3-hydroxybutyrate) (P3HB) biopolymer. Specifically, tens of grams of MeOxPBoc can be easily prepared from P3HB waste in a satisfactory yield. Well-controlled ring-opening polymerization (ROP) of MeOxPBoc was achieved to give an amorphous poly(amine-alt-ester) P(MeOxPBoc) with a targeted molecular weight, narrow molecular weight dispersity, and well-defined terminal groups in toluene using stannous 2-ethyl hexanoate (Sn(Oct)2) as a catalyst. The obtained P(MeOxPBoc) could be depolymerized back to its pristine monomer either in trimethylbenzene at 120 °C using Sn(Oct)2 as a catalyst or in bulk at 150 °C in the presence of ZnCl2 as the catalyst. After simple purification by recrystallization, the recycled MeOxPBoc could be repolymerized to produce virgin-quality polymers. Remarkably, the obtained P(MeOxPBoc) can be converted to the corresponding cationic polymer by simply removing the pendent t-butoxycarbonyl protecting group. This study provides a new insight to address the end-of-use problem of plastic by upcycling the plastic waste to value-added functional poly(amine-alt-ester) with a closed-loop life cycle.