Study On High-efficiency Ring-opening Polymerization Of Low Strain Five-membered Carbonates

Five-membered cyclic carbonates represent one of important downstream chemical products for the CO2 capture and utilization(CCU).The production of cyclic carbonates is mainly derived from the atom-economic epoxides/CO2 cycloaddition reaction or copolymerization process due to the "back-biting" reaction.Currently,the annual output of cyclic carbonates from CO2 is around 80 Kt and has been increasing rapidly.Although cyclic carbonates have found widespread applications in organic synthesis,pharmaceuticals production,materials sciences and Li-ion batteries,there is a severe surplus with intensive efforts are directed to realizing the peak carbon dioxide emissions and carbon neutrality.On the other hand,cyclic carbonates are viewed as desirable chemical raw materials,benefiting from their multifaceted traits involving less toxicity,high boiling point,high flash point,good ionic conductivity and wide extensive sources.However,the low tension of five-membered ring makes them thermodynamically stable,therefore limiting their applications in chemistry domain,especially in polymer science.Although several literatures have reported the ring-opening polymerization(ROP)of ethylene carbonate(EC)and propylene carbonate(PC),only a mixture of oligomers and substantial unidentified by-products yielded,leading to a low-efficient atomic utilization.Also,the proposed polymerization mechanism is somewhat vague,which hinders the further developments of cyclic carbonate polymerizations.Therefore,revealing the definite mechanism of five-membered cyclic carbonates polymerization and realizing their high atom-economic polymerization represent a charming but formidable challenge far from being realized.In this thesis,to realize the high atom-economic polymerization of low strain five-membered cyclic carbonates,a detailed investigation on PC ROP and comprehensive analysis of the produced oligomers and small molecular by-products was conducted in priority.Subsequently,a careful mechanism of PC ROP was clarified.Furthermore,we proposed a novel strategy of in situ capture of the intermediate,for the first time,realizing the atom-economic polymerization of low strain five-membered cyclic carbonates.Also,a creative utilization of five-membered cyclic carbonates for precise and and atom-economic construction of block polyesters was uncovered.The detailed results are summarized as follows:1.The clear mechanism of PC ROP was revealed.Firstly,structural elucidations of the whole products including oligomers and by-products derived from KOH-catalyzed PC ROP were carried out by a combination of NMR matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy(MALDI-TOF MS)and gas chromatography-mass spectrometry(GC-MS)technologies.On the basis of detailed analysis of products,it was concluded that PC was firstly decomposed into the key intermediate PO,accompanied by decarboxylation(release of CO2).Subsequently,the in situ released PO was either isomerized into a series of small molecular by-products under high temperature,or participated in the copolymerization of PO/PA to give oligo(propylene oxide-co-propylene carbonate).More importantly,the in situ released PO was detected,the confirming the veracity of our proposed mechanism of PC ROP.The aforementioned results not only deepened our understanding of the mechanism of PC ROP,also provided a theoretical basis for the subsequent exploration of atom-economic utilization of low strain five-membered cyclic carbonates.2.On the basis of the deeper insight into PC ROP,a novel strategy of in situ capture of the intermediate PO by reactive cyclic anhydrides was proposed.This strategy could exclude all side reactions and transformed PC into polyesters selectively with high molecular weights and almost 100%yields.There were a variety of advantages for the creative PC/PA copolymerization system,including:ⅰ)slow-release of PO ensuring a perfectly alternating structure and excluding the side reactions;ⅱ)quantitative and fast transformation of PC;ⅲ)visualization of polymerization process by a CO2 pressure gauge.through tandem polymerizations,PC is fully transformed into polyesters and polycarbonates concurrently,thus achieving PC utilization with a high atom-economy for the first time.The university study demonstrated that the strategies of in situ capture and cyclic carbonates/cyclic anhydrides copolymerizations were vigorous for both commonly used five-membered carbonates such as EC,PC and six-membered analogues such as trimethylene carbonate(TMC).Therefore,a new approach for high atom-economic utilization of cyclic carbonates standed out.3.On the basis of the above-mentioned unique mechanism of in situ epoxide release from corresponding carbonates,we proposed a novel strategy of temperature-programmed switchable terpolymerization of epoxides/carbonates/anhydrides based on the protection and deprotection chemistry of CO2,allowing for the precise construction of block polyesters in an atom-economic manner.The key prime of this strategy lied in the initial CO2 chemoprotection of epoxides into cyclic carbonates.On the one hand,there existed vast gaps in polymerization temperatures between epoxides and cyclic carbonates for the copolymerization with cyclic anhydrides,which not only provided an "intelligent temperature switch" for the controlled transformation of different polymerization cycles,also ensured the precise construction of block polyesters without detectable tapering structures.On the other hand,cyclic carbonates could exert stable cages to provide both good solvents for the polymerization systems and reactive monomers for building the block polymers concurrently,therefore avoiding the additional solvents utilization or excessive monomers addition.Also,there was no need to alter the active propagating chain end because of the mechanism compatibility of the two polymerization cycles,whereas commercial organocatalysts such as bis(triphenylphosphine)iminium chloride(PPNCl)were vigorous in the two polymerization systems.Consequently,the multifaceted traits mentioned above render our strategy with typical merits of AM such as high atom-economy,precise construction,intelligent control and high efficiency,etc.