Living Alternating Copolymerization of Epoxide and Phthalic Anhydride

Author:Li Heng

Supervisor:zhao jun peng


Degree Year:2019





Living polymerization are widely utilized to construct macromolecular architecture owing to the synthesized polymers featuring controlled molar masses,low dispersities,and controlled structures.Chain-growth ring-opening polymerization of cyclic esters is part of the commonly methods for polyester synthesis.However,it generally suffers from a limited variety,low stability,and much higher costs of monomers.One other hand,ring-opening alternating polymerization(ROAP)of epoxide with cyclic anhydride has showed advantages for enriched monomer sources,atom economy,potential of being controlled,and so on.Thus more attention have been given to this area in the past two decades.Metal complexes have been the leading catalysts investigated and used for ROAP because of the high catalytic efficiency,and chemo-/regio-/stereo-selectivity,but catalysts are often complex in structure and the actual living ROAP has not been achieved.Organocatalytic polymerization has become an appealing research area and a powerful arsenal for polymer synthesis in recent years.The efficiency and selectivity of organocatalysis are same even beyond metal complexes catalysis in many cases.Therefore,we are focusing on the ROAP of commercially available monomers,epoxides and phthalic anhydride(PA).Aiming to achieve high efficiency and selectivity of living copolymerization and exploring the mechanism under this simple phosphazene bases,reveals the structures-properties relationship of the alternating copolymers.This organocatalytic system was also used for one-step sequence-selective terpolymerization.The main results are as follows:(1)Living ring-opening alternating polymerization of PA with ethylene oxide(EO)catalyzed by simple organocatalyst.A major success has been made on organocatalytic ROAP of PA and EO by use of a simple phosphazene base(t-BuP1).Polyesters with perfectly alternating sequence distribution,controlled molar masses,and low dispersities(DM<1.1)are obtained.The ROAP exhibited a distinct living nature so that block,non-linear,end-functional structures and postpolymerization modification of the alternating polyesters are readily achieved.Solvent,excess EO,and catalyst can be effortlessly recovered and reused.The ideally suitable basicity of t-BuP1 is considered crucial for the high selectivity and neatness of the ROAP,which engenders a self-buffering mechanism rendering the carboxy and hydroxy terminals appropriately active to allow chain growth in an strictly alternating manner and complete avoidance of side reactions even at full conversion of the anhydride.(2)Living ring-opening alternating polymerization of PA with substituted epoxide catalyzed by simple organocatalyst.Based on our previously study,alcohol-initiated ROAP of PA and a variety of mono-,di-,and trisubstituted epoxides has been performed with a weak phosphazene base(t-BuP1)as the catalyst.Each product exhibits perfectly alternating sequence distribution,controlled molar mass(Mn up to 124 kg mol-1),and low dispersity(?M<1.15,mostly).Full conversion of PA can be reached in 0.5 to 24 h depending on the substituent of the epoxide,the targeted degree of polymerization,and the amount of t-BuP1used when the reactions are conducted under solvent-free conditions at 100°C with a small excess of the epoxide.Glass transition temperature of the polyester ranges from-14 to 135°C.The living nature of the ROAP allows one-pot construction of well-defined block-alternating copolymers through sequential addition of two epoxides.Statistical-alternating copolymers have also been synthesized by copolymerization of PA and two mixed epoxides.Thus,the structural diversity of aromatic alternating polyesters synthesized by this simple organocatalysis has been largely enriched.(3)Sequence-selective terpolymerization and one-step synthesis of(AB)nCm block copolymers using a simple organocatalyst.Although ideally way for delivery block copolymers from mixed monomers is the sequential arrangement of monomer units in a polymer chain,limitations are usually posed by the reactivity ratios of the monomers and the incompatibility of their polymerization mechanisms.Using a simple non-nucleophilic organobase as the catalyst,we have achieved sequence-selective terpolymerization from a mixture of PA,an epoxide,and rac-lactide(LA).1H NMR and in suit ATR-RI spectroscopy describable that alcohol-initiated alternating copolymerization of PA and epoxide occurs first and exclusively because PA is substantially more active than LA for reacting with base-activated hydroxyl.When PA is fully consumed,LA polymerizes from the termini of the first block while excess epoxide stays intact because of the mild basicity of the catalyst.The two polymerizations thus occur tandemly both in chemoselective and controlled manners so that a well-defined aromatic-aliphatic block copolyester is generated in this one-step synthesis.The effectiveness and versatility of this approach are demonstrated by the use of ethylene oxide and several monosubstituted epoxides as well as mono-,di-,or tetrahydroxy initiators.This one-step block terpolymerization catalyzed by simple organocatalyst was realized for the first time.It is expected that the insights gained here can be translated into other organocatalytic/metal-free strategies for sequence-selective/controlled polymerization and simplified preparation of materials with tailed properties.