Terpene-derived polyester-block-polycarbonates using metal-free catalysis

Sustainable materials are a large research focus due to depleting resources and increasing environmental pollution. Therefore, bio-derived building blocks from renewable feedstocks are being pursued as an alternative to fossil-derivatives for producing polymers. Here, we have prepared a series of polyester and polyester-block-polycarbonate copolymers from a metal-free ring-opening copolymerization (ROCOP) of various terpene-derived cyclic anhydrides, epoxides (cyclohexene oxide (CHO) and propylene oxide (PO)), and carbon dioxide (CO2) under neat (solvent-free) conditions. Cyclic anhydrides were prepared via microwave-assisted Diels Alder reactions between terpenes and maleic anhydride (up to 90% yield). The ROCOP catalyst system consisted of triphenylborane (BPh3) with bis(triphenylphosphine)iminium chloride (PPNCl) and, after complete conversion of anhydride, resulted in polymers with moderate molecular weights (Mn up to 21 kDa), and good dispersities (between 1.05 and 1.75). Thermal analysis resulted in a single glass transition temperature (between 1.5 C and 101 C) indicating amorphic materials. 13C{1H} NMR spectroscopy revealed high stereoregularity with respect to substituent methyl groups and di-ester units and 2D DOSY NMR spectroscopy confirmed a single polymer chain was formed by sequential copolymerizations. Residual olefinic groups in the polymer backbone provides opportunity for post-polymerization functionalization and, therefore, further tailoring of macroscopic properties.