Copolymerization Reactions Of Epoxide And Cyclic Ester Catalyzed By Organic Superbase

In recent year,organocatalytic polymerization has emerged and blossomed into an appealing topic in synthetic polymer chemistry.Small-molecule organic catalysts have provided significant opportunities to develope new polymerization reactions and new macromolecular structures,and have contributed to a series of new breakthoughs in achieving highly efficient and selective,simplified and sustainable(green)polymerizations.Among others,the merits of organocatalysis have been well demonstrated in the copolymerization of mixed type monomers,i.e.monomers carrying different polymerizing moieties.The copolymers of mixed type monomers are featured by the main-chain structures that are distinct from the corresponding homopolymers and copolymers of the same type of monomers(carrying different substituents).This is crucial for enriching and tailoring the properties and functions of polymers.Besides,copolymerization of mixed type monomers has been a significant and effective means to turn non-homopolymerizable compounds into polymers,so as to expand the variety of(renewable)monomers.In the thesis,we have developed two copolymerization reactions of epoxide and cyclic ester and the corresponding new synthetic approaches to ether-ester random and alternating copolymers,taking advantage of the special catalytic activity and mechanism of non-nucleophilic organic superbases.The structure-property relationship of the obtained copolymers has also been investigated.The main contents are summarized below.1)A“non-copolymerization”approach to ether-ester random copolymers.Ring-opening polymerization of epoxide was carried out with hydroxyl-terminated poly(?-caprolactone)as the initiator and a phosphazene superbase as the catalyst.Under such a strongly basic condition,hydroxyl group participated in both ring opening of epoxide and transesterification reaction,and the latter was much faster than the former.The synergetic effect led to substantial disordering of the sequence distribution of the two monomeric units.As a result,the monomeric units derived from epoxide were randomly“embeded”in the polyester chain,so that an ether-ester random copolymer was finally yielded.The random copolymer exhibited good thermal stability with decomposition temperature lying between polyester and polyether,and could be degraded into oligomers or small molecules by base-promoted alcoholysis.As the feed ratio of epoxide to polyester increased,the product gradually evolved from random copolymer to multiblock-like polyether segregated by ester units.Therefore,the method can be utilized for chemical modification of polyesters as well as for synthesis of degradable polyethers.2)Tuning the enzymatic degradability of poly(?-caprolactone)via ether-ester random copolymerization.The“non-copolymerization”method mentioned above was used to react poly(?-caprolactone)with ethylene oxide or monosubstituted epoxides by the catalysis of a phosphazene superbase.A series of ether-ester random copolymers with different pendent groups and compositions were hence obtained.The hydrophilicity,crystallinity,and enzymatic degradability of the copolymers were inverstigated by static water contact angle,differential scanning calorimetry,and quartz crystal microbalance with dissipation,respectively.Compared with poly(?-caprolactone),random copolymer with ethylene oxide exhibited higher degradability owing to the enhanced hydrophilicity and decreased crystallinity.As the composition of ethylene oxide units increased,degradation of the copolymer became faster first and then slightly slowed down.For the copolymers derived from mono-substituted epoxides,pendent groups showed a significant impact on the enzymatic degradation.Larger pendent group led to lower crystallinity of the copolymer and thus higher degradation rate.But further enlarging the pendent group could cause a lower degradation rate which was probably attributed to the high surface hydrophobicity and/or the steric hindrance of the copolymer structre.3)Ring-opening alternating copolymerization of epoxide and phenolic lactone.3,4-dihydrocoumarin is a typical phenolic lactone which derives from largely occurring natural resources.Unlike?-caprolactone,it is unable to undergo homopolymerization.In this study,we have found that 3,4-dihydrocoumarin undergoes ring-opening copolymerization with epoxides when a hydroxy compound and a phosphazene superbase were used as the initiator and catalyst,respectively.Nuclear magnetic resonance spectroscopy and mass spectrometry revealed that the obtained products were ether-ester copolymers with strict alternating sequences of the two monomeric units,and the molar mass could be,in part,controlled by the feed ratio of monomer to initiator.When relatively low degree of polymerization was targeted,the product was demonstrated to feature a linear structure and near-complete end-group fidelity.When high degree of polymerization was targeted,the product was composed of both linear alternating copolymers with higher molar mass and cyclic ones with relatively lower molar mass caused by intramolecular transesterification.Thermal analysis results showed that the semi-aromatic ether-ester alternating copolymers had good thermal stability and higher glass transition temperatures as compared with the homopolymers of the corresponding epoxides(polyethers).4)Construction of complex polymer structures based on ring-opening alternating copolymerization.Polymer structure is a decisive factor for their properties.Construction of polymer structures in controlled manners has always been one major goal that directs the researches on polymerization.Phosphazene-catalyzed ring-opening polymerization conforms well to the ideal in-situ activation-initiation-propagation mechanism.Therefore,structures of the polymers synthesized by phosphazene catalysis can be well controlled and flexibly regulated by simply varying the initiator,which is unachievable for most metal-catalyzed ring-opening alternating copolymerizations.In this part,this particular feature of phosphazene catalysis is well utilized for constructing more complex ether-ester alternating copolymer structures.For example,ring-opening alternating copolymerization of 3,4-dihydrocoumarin and epoxide was performed with a small-molecule triol as the initiator,affording a three-arm star-alternating copolymer.Using poly(4-hydroxystyrene)as a multifunctional macroinitiator,we have conducted graft-alternating copolymerization and successfully obtained a complex graft copolymer with a polystyrene backbone and ether-ester alternating copolymer side chains.Using a poly(ethyene oxide)diol as the initiator,we have achievd a(CB)_mA_n(BC)_m type triblock terpolymer.The structure-property relationship of these polymers with complex structures was preliminarily investigated.