Sequence structure control of CO<sub>2</sub>-based copolymer and the relationship between structure and properties

Abstract

<p indent=0mm>Carbon dioxide-based (CO₂-based) copolymer is an attractive environment-friendly material with excellent biodegradability and using non-toxic, abundant and cheap carbon dioxide gas as one of its raw materials. Since CO₂ is generated as a waste gas in many economic activities leading to global warming, chemical fixation of CO₂ can be a developing and effective mean to reduce evironmental and energy pressures. Among all the CO₂-based copolymers, the aliphatic polycarbonates obtained by the copolymerization of CO₂ and epoxides are the most widely studied materials due to their excellent biocompatity, transparency as well as water and oxygen barrier properties, especially poly(propylene carbonate)(PPC). However, undesirable thermal and mechanical properties limit their practical applications. For examples, PPC has a low <italic>T</italic>_g (35–40°C) close to room temperature, which is easy to bond into blocks during long-term storage and transportation after granulation, leading to the difficulty in subsequent processing. Much efforts have been devoted to tailor the structures of CO₂-based polycarbonates to obtain desirable peoperties profile. A feasible approach is to control the configurations of synthesized polycarbonates, which can greatly improve the thermal peoperties. Another effective way is to inrtoduce a third monomer into the CO₂/epoxides copolymerization to endow copolymers with tunable properties by easily changing the monomers and controlling polymer units. Herein, this article summarizes the methods for controlling the sequence structure of CO₂-based copolymers. Four kinds of CO₂-based copolymers are included: (1) Fully alternating polycarbonates with <italic>regio</italic>- and <italic>stereo</italic>-chemistry configurations. In this part, the efforts on the stereochemistry control of polycarbonates are highlighted and the crystallinity of polycarbonates are emphatically introduced. Isotactic polycarbonates have the higher glass transition temperature than the amorphous one and some of them are crystalline. Even though no cystallinity can be observed in isotactic PPC, the <italic>T</italic>_g was 35–40°C higher than the atactic one. (2) Terpolymers prepared from the copolymerization of CO₂ with different epoxides are introduced. Since the activity of different epoxides copolymerized with CO₂ varied greatly, especially the electron-withdrawing and electron-donating epoxides, the control on the compositions and connetions of CO₂/different epoxides terpolymers is a big challenge. Highly effective metal-based catalysts were designed to sucessfully synthesize defined random terpolymers. Moreover, block polycarbonates can only be synthesized by subsequent monomer additions which requires the completely removal or consumtion of the first monomer. CO₂-besed polycarbonates with high added value are desirable today. Hence, the efforts on the post-functionalization of polycarbonates are also highlighted. (3) Terpolymers of CO₂, epoxide and cyclic anhydride. The difference in the rates of CO₂/epoxides and cyclic anhydrides/epoxides copolymerization determines the microstructure of CO₂/epoxides/cyclic anhydrides terpolymers. Therefore, copolymers with random/gradient or block structures can be easily prepared by choosing the right catalysts. Note that, some attempts to improve copolymers’ properties by introducing the crosslinked sturctures were also successful. (4) Terpolymers of CO₂, epoxide and cyclic ester. Random poly(ester-carbonate)s can be generated via one-pot/one-step strategy by using heterogenous catalysts. However, synthesis of block CO₂/epoxides/cyclic esters terpolymers usually requires multi-step operations. The good news that one-pot chemical selective polymerization of mixed monomers can be fufilled by designing the catalysts, which can tune the sequence structures of copolymers by a switch. Multiblock terpolymers are also presented. Finally, the thermal, thermal, mechanical and degradation properties of CO₂-based terpolymers are reviewed in each part to varify the relationships between structures and properties. In a summary, inserting a third monomer into the polycarbonate backbone through terpolymerization is a robust tool to tune polymers’ performances. The development directions of CO₂-based copolymers are prospected in the end.