Terpolymerization of β‑Butyrolactone, Epoxides, and CO2: Chemoselective CO2‑Switch and Its Impact on Kinetics and Material Properties

• Published in: Macromolecules 2019-11, Vol.52 (21), p.8476-8483
• Main Authors: Kernbichl, Sebastian, Reiter, Marina, Mock, Josef, Rieger, Bernhard
• Format: Article
• Language: English
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.9b01777

Terpolymerization reactions with a mixed-monomer feedstock of epoxides, CO2, and β-butyrolactone (BBL) at two different CO2 pressures are presented. The Lewis acidic zinc complex BDICF3–Zn–N­(SiMe3)2 1 is able to catalyze both the ring-opening polymerization (ROP) of BBL and the ring-opening copolymerization of epoxides and CO2. The carbon dioxide concentration thereby displays an attractive tool for the chemoselective tailoring of the incorporation of both monomer types to either a block or a statistical configuration. A high CO2 pressure (40 bar) leads to a block structure, whereas 3 bar CO2 allows the two catalytic cycles, ROP of BBL and ring-opening copolymerization of cyclohexene oxide and CO2, to proceed with similar rates. This results in a statistical polymerization behavior. Reducing the CO2 pressure from 40 to 3 bar involves a change in the reaction order of CO2 from zero- to first-order dependency. The statistical polymerization pathway offers a promising route to terpolymers with one mixed-glass transition temperature that can be adjusted in a range between 5 and 115 °C. Terpolymers in block structure show two segregated glass transitions. This phase separation was also confirmed via atomic force microscopy. Referring to the mechanical behavior of the resulting terpolymers, a decrease of the Young modulus for both the block and the statistical structure compared to the very brittle poly­(cyclohexene carbonate) is observed due to the incorporation of soft poly­(3-hydroxybutyrate) (PHB). An enhanced elongation at break is revealed for the block structure when the molecular weights exceed 100 kg/mol. The biobased monomer limonene oxide is also successfully terpolymerized with CO2 and BBL. Interestingly, the block structure shows a tunable stress–strain behavior depending on the amount of PHB in the terpolymer.