Preparation And Properties Of Recyclable Polyesters By Strong Base/Urea-Catalyzed Ring-Opening Polymerization Of ?-Caprolactone

As one of the three major synthetic polymers,plastics have been widely used in many fields such as garment production,automobile manufacturing,food packaging,medical devices,etc.Since the plastics waste is difficult to degrade in the environment,the mass production and disposal of plastics has caused serious environmental pollution.It has attracted growing interests to produce sustainable polymers from renewable resources,among which the chemically recyclable polymers with a closed loop life cycle is the most attractive solution.In this thesis,chemically recyclable polymers with well-defined structures,including homopolymers(P?CL)and block copolymers(P?CL-b-PLLA,PLLA-b-P?CL-b-PLLA),were successfully prepared from commercially bio-based monomers using a binary catalytic system composed of strong base and urea.The thermal,mechanical and recyclable properties of the obtained polymers and copolymers were investigated.The tensile testing results show that the homopolymer has poor mechanical properties due to its low crystallinity.In contrast,the triblock copolymers behave as thermoplastic elastomers(TPEs)with good elastic recovery,tensile strength,and elongation,which are comparable to some commercial TPEs.The thesis consists of two following sections:1.Ring-opening polymerization of bio-based?-caprolactone(?-CL)was achieved with a binary catalytic system consisting of KOMe and urea.The effects of the monomer concentration,the type of catalyst and the feeding ratio on the polymerization were investigated.A rapid“living”/controlled polymerization of?-CL at room temperature to produce well-defined P?CL with narrow molecular weight distribution was achieved when KOMe/U3 was used as the catalyst.The enthalpy change and entropy change of the ring-opening polymerization of?-CL is-12.8 k J mol^-1 and-40.6 J mol^-1 K^-1,respectively.The Gibbs free energy of the polymerization is calculated to be-0.7 k J mol^-1 at 25°C.The ceiling temperature of polymerization is 40°C at an initial monomer concentration of[M]₀=1 mol L^-1.P?CL can be depolymerized back to pristine monomers,which successfully established the circular life cycle.2.Block copolymers were synthesized using the one-pot method by sequential addition of the monomers.The well-defined structures of obtained block copolymers(P?CL-b-PLLA,PLLA-b-P?CL-b-PLLA)were carefully examined with NMR spectroscopy(~1H NMR,¹³C NMR,DOSY)and size-exclusion chromatography(SEC).The thermal properties of the block copolymers were investigated using thermogravimetric analysis(TGA).The mechanical properties of the block copolymers were investigated using tensile testing.The tri-block copolymer(TPE-3)behaved as a thermoplastic elastomer.The tensile testing results showed that the elongation at break of the sample was 755%and the tensile strength was 13.4 MPa.The cyclic tensile testing results showed that the elastic recovery is 93.1%,the resilience was 70.8%and the residual strain was 6.3%.The depolymerization and recycling of the triblock copolymer were also achieved.