Highly Efficient And Controlled Ring-opening Polymerization Catalyzed By Ionic Organocatalysts

In this thesis,a novel class of bifunctional ionic organocatalysts have been developed and well utilized to actualize highly active and chemoselective ring-opening polymerization of cyclic esters,and to obtain aliphatic ester with controlled molar mass and structures.The catalysts can be prepared by a simple dehydration reaction between tetra-n-butyl ammonium hydroxide and N,N’-disubstituted（thio） ureas at elevated temperature under vacuum,and used in cooperation with hydroxy initiators.The catalytic activity and selectivity can be optimized by variation of the N-substituents of the（thio） urea.The independence of initiator and catalyst allows versatile design and construction of diverse polyester-based macromolecular structures.When the structure and loading of catalyst is appropriately chosen,polymerization of cyclic ester can proceed in a fast and selective manner.Typically,polymerization of L-lactide can reach ～100% monomer conversion in 1～2 min,with the turnover frequency（TOF）being as high as 120,000 h^-1.Poly（L-lactide） with near-perfect isotacticity,widely tunable molar mass(4-130 kg mol^-1),and low dispersity can be readily obtained in this way.The molar mass and low dispersity of polyester can be largely sustained when the reaction time is purposely and substantially extended,which further demonstrates the selectivity of catalyst for monomer enchainment over side reactions such as macromolecular transesterification.In addition to the hydrogen bond donor-acceptor type synergetic activating and controlling effect,our results have provided new insights into the reaction mechanisms.For example,catalyst derived from symmetric urea exhibits remarkably higher catalytic activity than the asymmetric analogue.We consider this effect a consequence of catalyst-assisted proton transfer which prevails because of the structural symmetry of urea and facilitates ring opening of cyclic ester.For the above-mentioned ionic organocatalysts,the catalytic activity originates primarily from the basic（anionic）moieties.Subsequently,we have studied another type of organocatalyst,silylated sulfonimide,in which the Lewis acidic moiety is the main origin of the catalytic activity.Owing to the stability of sulfonimide anion,N-Si bond in the catalyst structure is readily cleaved to generate a silyl cation which can effectively activate cyclic ester and afford hydroxyl-initiator ring-opening polymerization.Our results have shown that the catalytic activity of silylated sulfonimide is distinctly higher than the Br?nsted acidic sulfonimide at both room temperature and high temperature.Efficient and controlled polymerization of ε-caprolactone can be achieved at 120℃ with as low as<100 ppm of catalyst.In summary,the results in this thesis provide important information for designing organocatalytic ring-opening polymerization with high efficiency and high selectivity.