Tough and Thermally Recyclable Semiaromatic Polyesters by Ring-Opening Polymerization of Benzo-thia-caprolactones

Thermally recyclable semiaromatic polyesters with good mechanical properties are prepared by ring-opening polymerization (ROP) of benzene-fused caprolactones. Three constitutional isomers of benzo-thia-caprolactones (BOTO-I, BOTO-II, and BOTO-III) were synthesized. Their organocatalytic ROPs were systematically investigated and correlated with their structures. The diphenyl phosphate (DPP)-catalyzed ROP of BOTO-II is controllable and can reach high monomer conversion. BOTO-I has a high equilibrium monomer concentration ([M](e) = 0.15 M) at 30 degrees C, whose ROP can only be achieved with 1,5,7-triazabicyclo[4.4.0] dec-5-ene (TBD) as the catalyst, but with poor control. BOTO-III can be polymerized with TBD or a binary catalytic system of 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) and 1-(3,5-bis(trifluoromethyl)phenyl)-3-cyclohexylthiourea (TU) with poor control. Polymers P(BOTO-I) and P(BOTO-II) are thermally stable up to 270 degrees C, but P(BOTO-III) starts to decompose at 180 degrees C. These three polymers are semicrystalline materials with slow crystallization rates. P(BOTO-I) displays the highest T-m of 146 degrees C and a T-g of 59 degrees C. The mechanical properties of these P(BOTO)s were examined by uniaxial tensile tests. Preliminary data indicates that P(BOTO-I) is a tough polymer with superior comprehensive material properties as compared to other reported thermally recyclable polyesters. Catalytic chemical recycling of P(BOTO)s in solution and bulk were examined. Depolymerization in solution is not selective, generating both monomers and oligomers, while the bulk thermal depolymerization selectively gives corresponding monomers in good yield and high purity. Given its good mechanical properties, excellent thermal recyclability, and repolymerizability of the recycled monomer, P(BOTO-I) can be considered as a new type of closed-loop recyclable plastic.

Automated summary

Thermally recyclable semiaromatic polyesters were prepared by ring-opening polymerization of benzene-fused caprolactones. Three constitutional isomers of benzo-thia-caprolactones were synthesized and their organocatalytic ROPs were systematically investigated. The mechanical properties, thermal stability, and depolymerization behavior of these polymers were studied, showing promising results for potential closed-loop recyclable plastics.