Synthesis and Ring-Opening (co)Polymerization of Lactones Derived from the Cotelomerization of Isoprene, Butadiene, and CO2

Abstract

Herein, we report ring-opening polymerizations and copolymerizations of substituted δ-lactones derived from isoprene, butadiene, and CO₂. While the telomerization of CO₂ with butadiene to form a disubstituted δ-lactone is well-established, the similar telomerization of isoprene with CO₂—or cotelomerization of isoprene and butadiene with CO₂—have been less studied. Our initial efforts focused on identifying the factors that govern yield and selectivity in the cotelomerization of isoprene, butadiene, and CO₂. The most effective cotelomerization/hydrogenation reaction sequence was scaled-up, leading to mixtures of two isoprene-butadiene coupled lactones (3-ethyl-6-(prop-1-en-2-yl)tetrahydro-2H-pyran-2-one (EtPeP), and 3-ethyl-6-methyl-6-vinyltetrahydro-2H-pyran-2-one (EtVMeP) and the butadiene homocoupled lactone, 3-ethyl-6-vinyltetrahydro-2H-pyran-2-one (EtVP). The ratios of these three lactones varied depending on telomerization conditions and purification methods employed. Stepwise syntheses of pure EtVMeP and EtPeP via alternate routes were also carried out. The pure lactones as well as lactone mixtures were subjected to organocatalyzed ring-opening (co)polymerization (ROP) using triazabicyclodecene (TBD), yielding CO₂-based copolymers with molar masses (Mn) ranging from 5.5 to 12.7 kDa and narrow dispersities (Đ = 1.3). Increasing the proportion of EtPeP relative to EtVP led to a notable increase in the glass transition temperature (Tg) of the copolymers, reaching -20.5 °C. While EtPeP underwent successful ring-opening polymerization, reactions with EtVMeP resulted in termination of the polymerization owing to the formation of a non-nucleophilic tertiary alkoxide chain end. Thus, small amounts of EtVMeP can have a deleterious effect on copolymerizations of lactone mixtures derived from cotelomerization. These results motivate further development in the selective synthesis of EtPeP via cotelomerization of butadiene and isoprene with CO2.