Preparation And Aqueous Degradation Properties Of High Molecular Weight Oxalate-Based Polyesters

The severe marine plastic waste pollution has attracted widespread concern.Oxalate-based polyesters have been revealed to show non-enzymatic hydrolytic capability in water(seawater)environment,making them to be promising alternatives of traditional polyesters.For the purpose of obtaining high molecular weight oxalate-based polyesters,the conditions of high reaction temperature,long reaction time,and high catalyst loading were commonly employed.However,the oxalic acid-based monomers used in literature(oxalic acid,dimethyl oxalate,diethyl oxalate,etc.)have poor thermal stability.The decomposition or sublimation of monomers,as well as the decomposition of oxalate units on polymer chain during polymerization process led to the reduction of molecular weights.Furthermore,high temperature and long reaction time consume high energy.Additionally,high catalyst residue often results in larger yellow indexes and potential toxicity of the products.These factors severely limit the production and application of polyoxalates.In this thesis,oligo(ethylene oxalate)(OEO)was exploited as monomer to prepare oxalate-based polyesters due to its excellent thermal stability and volatility of the corresponding by-product ethylene glycol.1,6-Hexamethylene glycol was first employed as monomer to synthesis high-molecular-weight poly(hexylene oxalate).The polymerization temperature,time and catalyst loading were 150 ~oC～180 ~oC,2.5 h～4 h,and<300 ppm,respectively,all of which were lower than those reported in literatures.The approach can be applied to prepare a variety of high-molecular-weight aliphatic oxalate-based polyesters with methylene number(n)of 6,8,10,and 12.Notably,the polymerization can also be completed at 190 ~oC for ca.3 h without catalysts.It was found that a much faster reaction rate was obtained when the molar ratio of the repeat unit of OEO to the diol was 1:0.9.The polymerization was finished at 170 ~oC for only 1.5 h,where the reaction conditions were far better than those reported in literatures.This result provided an example for large-scale preparation of aliphatic polyoxalates.The weight-average molecular weight(M_w)of these polymers was higher than 50 k Da,and their color was nearly white with a yellow index of smaller than 10.Their degradation performance in fresh water(seawater)was adjustable.Their mechanical properties were close to those of LLDPE and PBS.This method can be also used to prepare aromatic polyoxalates under the condition of 180~oC～210 ~oC for ca.3 h.Poly(butylene terephthalate-co-butylene oxalate)with M_w of 1.1×10~5g/mol and polydispersity(PDI)of 1.75 was successfully synthesized.Meanwhile,the molar content of oxalic acid groups of the polymer was 43%,which was close to the feeding ratio(40%).The content of ethylene group was only 6 mol%.The polymer showed a T_m of 141.8 ~oC,a T_g of 16.7 ~oC,a tensile strength of 34.5 MPa,a Young’s modulus of 660 MPa and an elongation at break of 183%,similar to those of poly(butylene succinate)(PBS).The polymer also exhibited excellent water(seawater)degradation properties.In addition,it was found that the addition of oxalic acid inhibited the side reaction of ethylene glycol etherification when ethylene glycol was used as a monomer.The reduced content of diethylene glycol units effectively improved its crystalline properties and mechanical properties.