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

Terpolymerization reactions with a mixed-monomer feedstock of epoxides, CO2, and beta-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 zeroto 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 degrees 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.