Advances in the Synthesis of Copolymers from Carbon Dioxide, Dienes, and Olefins.

Abstract

Carbon dioxide (CO2) has long been considered a sustainable comonomer for polymer synthesis due to its abundance, easy availability, and low toxicity. Polymer synthesis from CO2 is highly attractive and has received continuous interest from synthetic chemists. In this regard, alternating copolymerization of CO2 and epoxides is one of the most well-established methods to synthesize aliphatic polycarbonates. Moreover, binucleophiles including diols, diamines, amino alcohols, and diynes have been reported to copolymerize with CO2 to give polycarbonates, polyureas, polyurethanes, and polyesters, respectively. Nevertheless, little success has been made for incorporating CO2 into the most widely used polyolefin materials.Although extensive studies have been focused on the copolymerization of olefins and CO2, most of the attempted reactions resulted in olefin homopolymerization owing to the endothermic property and high energy barriers of CO2 insertion during the chain propagation process. In this Account, we show how this challenge is addressed by taking advantage of a metastable lactone intermediate, 3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one (EVP), which is produced from CO2 and butadiene via palladium catalysis. Homopolymerization of EVP furnishes CO2/butadiene copolymers with up to 29 wt % of CO2 content. This reaction strategy represents a breakthrough for the long-standing challenge of inherent kinetic and thermodynamically unfavorable CO2/olefin copolymerization. A new class of polymeric materials bearing repeating bicyclic lactone and unsaturated lactone units can be obtained. Importantly, one-pot copolymerization of CO2/butadiene or terpolymerization of CO2/butadiene/diene can be achieved to afford copolymers through a two-step reaction protocol. Interestingly, the bicyclic lactone units in the polymer chain can undergo ring-opening through hydrolysis and aminolysis, while reversible ring-closing of the hydrolyzed or aminolyzed units was also achieved simply by heating.Over the past few years, more and more studies have utilized EVP as an intermediate to synthesize copolymers from olefins, butadiene, and CO2. Recently, we successfully incorporated CO2 into the most widely used polyethylene materials via the direct copolymerization of EVP and ethylene. Taking advantage of the bifunctional reactivity of EVP, we were able to access two types of main-chain-functionalized polyethylenes through palladium-catalyzed coordination/insertion copolymerization and radical copolymerization. Besides polyethylenes, CO2 was also incorporated into poly(methyl methacrylate), poly(methyl acrylate), polystyrene, polymethyl acrylate, polyvinylchloroacetate, and poly(vinyl acetate) materials via radical copolymerization of EVP and olefin monomers. The EVP/olefin copolymerization strategy provides a novel avenue for the synthesis of highly versatile copolymers from an olefin, CO2, and butadiene.