Stereoregular Polymerization Of CO₂ And Epoxides With Electron-withdrawing Group

The alternating copolymerization of carbon dioxide?CO₂?and epoxides to produce polycarbonate has the advantages of environmental-friendly,high atomic economy,inexpensive raw materials,and availability.Previous studies have focused on epoxides with electron-donating groups?such as propylene oxide?PO??,and very limited reports regards studies on epoxides with electron-withdrawing group.The main reason is that the electron-withdrawing group's electron effect and steric hindrance effect lead to poor regioselectivity of the reaction,and thus no high isotactic polycarbonate can be obtained.Therefore,a highly regioselective ring opening of styrene oxide?SO?and its derivatives with electron-withdrawing group was achieved by designing a catalyst to obtain highly isotactic polycarbonates and to obtain polymers.The thermodynamic properties were studied.Highly regioselective copolymerization of CO₂ and enantiopure terminal epoxides with an electron-withdrawing group,styrene oxide and its derivatives,was achieved by using binary catalyst systems of multichiral SalenCo???X complexes and an ionic quaternary salt,PPNY?PPN=bis?triphenylphosphine?iminium,X=Y=2,4-dinitrophenoxide?,in which epoxide ring-opening predominantly occurred at methylene C-O bond,affording isotactic CO₂copolymers with up to 98%head-to-tail connectivity and 97%enantioselectivity.Thermodynamic study of this highly stereoregular polymer found that the stereoregular poly?styrene carbonate?with 98%isotacticity is a typical semicrystalline material,possessing a melting temperature?T_m?of 137.3? and an enhanced glass transition temperature?T_g?of101.2?.The isotactic 3-methoxystyrene oxide/CO₂ copolymer is also a semicrystalline polymer with a T_m of around 90?,while the corresponding isotactic 3-chlorostyrene oxide/CO₂ copolymer appears amorphous feature,possessing a T_g of 82.1?.Mixing equivalent isotactic?R?-and?S?-poly?styrene carbonate?s,we found that it is the first time to discover the interaction between two opposite-configuration isotactic polycarbonates from the terminal epoxides,The strong interlocked interaction between isotactic?R?-and?S?-poly?styrene carbonate?s affords the crystalline stereocomplexes with a T_m of 164.1?,about 27? higher than that of the enantiopure isotactic polymers.Surprisingly,isotactic?R?-or?S?-3-chlorostyrene oxide/CO₂ copolymer is typical amorphous polymeric material;however,upon mixing both enantiomers together,an interlocked interaction between polymer chains of opposite configuration occurs,affording the crystalline stereocomplexes with a T_m of 147.3?.