Connecting 16- with 4-Membered Rings: Ring-Opening Polymerization of Bio-Based ω-Pentadecalactone with Amino-Alkoxy-Bis(phenolate) Yttrium Initiators and Copolymerization with β-Butyrolactone

Abstract

Ring-opening polymerization (ROP) of bio-based lactones is an effective way to overcome limited fossil resources. The 16-membered, musk-derived ω-pentadecalactone (PDL) occurs naturally as an ω-hydroxy fatty acid. As PDL is considered a macrolactone, controlled ROP of this transoid lactone is mostly obtained via enzymatic routes. Herein, amino-alkoxy-bis(phenolate) yttrium amido and alkoxide catalysts are reported for controlled metal-catalyzed ROP of PDL. PDL polymerizations are conducted at elevated temperatures from 60 to 100 °C to produce poly(ω-pentadecalactone) (PPDL) with number-average molecular weights of up to 134.5 kg mol−1 and moderate polydispersities (1.5 to 2.1). Utilizing isopropanol as a chain transfer agent (CTA) lead to higher initiator efficiencies and low polydispersities while still showing full monomer conversions with molar masses tunable by the amount of CTA. Block copolymerizations of PDL are performed via sequential addition. Four-membered, cisoid, and rather reluctant, racemic β-butyrolactone (BBL), which can be synthesized from carbon dioxide, is chosen as the second block. After determining that PDL has a lower coordination strength to the yttrium center and has to be added as the first monomer, successful connection between PPDL and poly(3-hydroxybutyrate) (PHB) blocks is shown via SEC and DOSY NMR spectroscopy. Different ratios of PDL and BBL are chosen to obtain PPDL-b-PHB polymers with different block length and molar masses. Powder X-ray diffraction measurements and DSC measurements prove the preservation of crystallinity in the block copolymers.