Lijun Qiao

Controllable synthesis of a narrow polydispersity CO2-based oligo(carbonate-ether) tetraol

A CO2-based oligo(carbonate-ether) tetraol was synthesized in a controlled manner by immortal copolymerization of carbon dioxide (CO2) and propylene oxide (PO) in the presence of 1,2,4,5-benzenetetracarboxylic acid (btcH4) catalyzed by using a zinc–cobalt double metal cyanide (Zn–Co–DMC) catalyst. The number average molecular weight (Mn) of the tetraol was in a good linear relationship with the molar ratio of PO and btcH4 (PO/btcH4), and hence can be precisely controlled. Besides, the rapid chain transfer in immortal copolymerization afforded the tetraol with a narrow polydispersity index (PDI) of 1.08 at a Mn of 1400 g mol−1. Notably, the weight fraction of the byproduct propylene carbonate (WPC) was reduced to as low as 4.0 wt%, which is the lowest Wpc ever reported for the synthesis of branched polyols. The structure of the oligo(carbonate-ether) tetraol was confirmed, providing new evidence for the effect of the acidity (pKa1 value) of the chain transfer agent (CTA) on the initial catalytic mechanism. The acid only acts as the CTA directly participating in the copolymerization via the chain transfer reaction when its pKa1 value is higher than that of adipic acid (pKa1 = 4.43). However, when its pKa1 value is lower than that of succinic acid (pKa1 = 4.2), it acts as the initiate-transfer agent, which first initiates PO homopolymerization to an oligo-ether polyol, and then the in situ formed polyol acts as a new CTA for the copolymerization.

Cheap and fast: oxalic acid initiated CO2-based polyols synthesized by a novel preactivation approach

Oxalic acid, the cheapest dicarboxylic acid, was used as an effective initiator to synthesize polyols by copolymerization of CO2 and propylene oxide over a zinc–cobalt double metal cyanide catalyst. Generally, reaction times as long as 255 min were observed for complete PO conversion, due to the existence of the free carboxylic acid group of oxalic acid. To overcome this disadvantage, we proposed a novel preactivation approach by formation of oxalic acid based oligo-ether-diol in advance. About 4.75 PO monomers were initiated at 80 °C, which was independent of time and oxalic acid amount; the diol then acted as a chain transfer agent for the following copolymerization. Under the optimal conditions the reaction could proceed to completion in 150 min, which was a remarkable reduction in reaction time compared to the previous reaction time of 255 min. Notably, the resulting CO2-based diol was stable up to 190 °C, indicating that oxalic acid may be applied as an effective initiator for this copolymerization.

A whole-procedure solvent-free route to CO2-based waterborne polyurethane by an elevated-temperature dispersing strategy

An elevated-temperature dispersing (ETD) strategy was developed to disperse prepolymers at 80 °C for producing waterborne CO2-based polyurethane (CO2-WPU) in a whole-procedure organic solvent-free route. The obstacle of high viscosity challenging the traditional prepolymer-dispersing process was overcome, and the concern of significant NCO loss at high temperature was eliminated because prepolymer dispersion ensured >90% NCO retention. The particle size of the typical CO2-WPU emulsion lay between 45 nm and 70 nm, and it showed excellent stability even after centrifugation for 30 min at 3000 rpm. The dried CO2-WPU film showed not only good mechanical performance with a tensile strength of 54.3 MPa and elongation at break of 641%, but also excellent hydrolysis resistance owing to the unique structure of the CO2-polyol. This ETD strategy showed clear feasibility for commercial polyether or polyester oligomerols. It avoids the cooling step and solvent-removal procedure required in a traditional prepolymer-dispersing process, thereby greatly shortening the preparation period and reducing energy consumption.

Toughening of amorphous poly(propylene carbonate) by rubbery CO2-based polyurethane: transition from brittle to ductile

Amorphous poly(propylene carbonate) (PPC) is brittle at room temperature, but the studies related to the toughening of PPC is rare. Herein, two types of polyurethane (PCO2PU) synthesized from a CO2-based diol and toluene diisocyanate were used as rubbery particles to toughen PPC. The notched impact strength of PPC increased from 20.8 J m−1 to 54.2 J m−1 at a PCO2PU loading of 20 wt%, comparable with that of neat nylon 6, and reached 228.3 J m−1 at a PCO2PU loading of 30 wt%, 10.9 fold that of neat PPC and even higher than bisphenol A polycarbonate. Matrix yielding as well as cavitation was observed during the impact process, which was responsible for the increase of impact strength. Moreover, the toughening efficiency was related with the carbonate content of PCO2PU, and the transition of fracture behavior from brittle to ductile occurred when the PCO2PU with a weight average diameter of 0.20 μm was uniformly dispersed in PPC substrate.