Organocatalytic Synthesis Of Chalcogen-Rich Polymers And Mechanism Study

The use of organocatalysis to achieve controlled polymerization has received increasing attention in polymer chemistry.The core problem is how to develop efficient and selective organocatalysts for polymerization.This thesis describes the homo-and copolymerization of chalcogen-containing（oxygen,sulfur,and selenium）monomers,such as carbonyl sulfide（COS）,carbon disulfide（CS₂）,epoxides,γ-butyrolactone（γ-BL）,andγ-selenobutyrolactone（γ-SBL）.Based on the understanding of the polymerization mechanism,many effective organocatalyzed polymerization pathways are constructed,affording controlled（co）polymerizations of these monomers and a variety of polymers rich in oxygen,sulfur,and selenium.These obtained polymers include polymonothiocarbonate,polythioether,polyether,（selenium-containing）polyester,and block copolymers.This thesis clarifies the unique advantages of organocatalysis in the synthesis of chalcogen-containing polymers.First of all,based on the previous research,this thesis has developed new synthetic methods and new catalytic systems for sulfur-containing polymers.Through the strategy of inhibiting the oxygen/sulfur exchange reaction,a cooperative organocatalytic system is developed to catalyze the copolymerization of COS and epoxide without metal and boron.Using a bicomponent metal-free catalyst system composed of an organic base and a thiourea derivative,the living copolymerization of COS with many epoxides is achieved with high activity,affording the polymonothiocarbonate with fully alternating chain structure,complete regioselectivity,and high number-average molecular weight（M_n）of 98.4 kg/mol.The copolymerization mechanism is studied by investigating the kinetics of the copolymerization process and the binding constant of the catalytic system,which discloses the cooperative mechanism of thiourea derivatives as hydrogen bond donors to selectively activate epoxides and organic bases to selectively activate COS.A trace amount of water in the system,or an additional initiator such as benzyl alcohol,removes protons by an organic base to form an anion-initiated species to initiate the copolymerization.This organocatalytic system completely inhibits the oxygen/sulfur exchange reaction between COS and epoxide and obtains a polymonothiocarbonate with a well-defined structure.By using the oxygen/sulfur exchange reaction,a new synthetic route from COS and epoxide to aliphatic polythioether is established,achieving the one-pot synthesis of polythioether in a closed system.This route consists of two steps.First,a cycloaddition reaction of COS with an epoxide occurs to form a five-membered cyclic monothiocarbonate.When thiourea is added,the rate of the cycloaddition is significantly increased.Then,the cyclic thiocarbonate is decarboxylated and ring-opened to form an episulfide,which is polymerized to form a polythioether.The obtained polythioether has a mercapto group at one end and a head-to-tail structure content of 96%.Many organic bases and more than ten epoxides can obtain corresponding polythioethers.The glass transition temperature range is from-58 to53°C,and the refractive index is as high as 1.64.The above-mentioned strategy of synthesizing polythioether is successfully applied to CS₂,realizing a new method of synthesizing polythioether from CS₂ and epoxides.In an open system,through the one-pot coupling reaction of CS₂ and epoxide,the chemically selective production of aliphatic polythioether and COS is achieved.The prepared polymer has a unique thioether unit and a terminal structure of a mercapto group.This method can be implemented in the air,using a variety of common organic bases as catalysts（initiators）,and is suitable for a variety of epoxides.Notably,CS₂ and epoxides are low-cost chemicals that can be obtained in large quantities in industry.Therefore,this method provides a new and practical strategy for the large-scale synthesis of polythioether.Secondly,when studying the thiourea/organic base-catalyzed COS/epoxide copolymerization reaction,the developed cooperative organocatalytic system is successfully applied to catalyze the ring-opening polymerization（ROP）of lactones and epoxides and the controllable synthesis of block copolymers,affording a series of oxygenated polymers with well-defined structures.Considering the problem of low-molecular-weight products and transesterification side reactions in the ROP ofγ-BL,a new bifunctional organic catalyst system composed of a phosphazene base of P4 and a（thio）urea derivative is developed to synthesize high-molecular-weight and linear PγBL.（Thio）Urea derivatives can buffer the chain growing oxygen anion,which reduces its basicity and nucleophilicity,thereby inhibiting the side reaction of intramolecular transesterification in the ROP ofγ-BL.The process shows a high apparent rate constant（about 39 times that of P4 alone）,a catalytic frequency（TOF）of up to 125 h^-1,and a linear structure of PγBL with a M_n of up to 64.3 kg/mol（approximately three times the catalysis of P4 alone）.The obtained high-molecular-weight PγBL shows significantly improved mechanical properties.These（thio）urea derivatives with electron-donating substituents are commercially available and inexpensive.In the mechanism,the properties of the propagating anion can be regulated by a Lewis acid via a non-covalent bond.Based on the idea of regulating the propagating anion,using triethylborane（TEB）combined with a phosphazene base as the catalyst system,a variety of alkyl alcohols can effectively trigger the ROP of PO.TOF is as high as 7500 h^-1,and the M_n of PPO measured by GPC can reach 156 000 g/mol（M_w/M_n<1.1）.Alkoxy-terminated and double-terminal hydroxyl PPO can be prepared.TEB functions in two ways.On the one hand,it combines with the propagating anion via the O→B coordination bond,which reduces its basicity and nucleophilicity and inhibits the side reactions of chain transfer to the monomer.On the other hand,TEB can active epoxide,thus improving the reaction rate.This catalytic system is also suitable for ROP of various epoxides,including ethylene oxide and glycidyl ether epoxide.By studying the cooperative catalysis mechanism of the hydrogen-bonded and organoborane catalysts,a universal dual-active cite method is developed toward the synthesis of polyester-polythiocarbonate block copolymers.In the alcohol-initiating polymerization,the TBD/TEB bicomponent organocatalysts generate two types of active cites,i.e.,chain-extending alkoxide active species dynamically interact with TEB and TBD via hydrogen bond and O→B coordination bond,respectively.First,TBD active site selectively catalyzes ROP of cyclic esters to obtain polyester-segments.Then by introducing COS gas at a set time,the TEB active center selectively catalyzes the alternating copolymerization of COS and epoxides.This method can be extended toδ-VL,ε-CL,rac-LA,and four common epoxides,resulting in a variety of polyester-polythiocarbonate block copolymers.A 9-block copolymer with 5 unique sequences is prepared by a sequential monomer-feeding method.The obtained polyester-polythiocarbonate block copolymer is a new polymer with potential biocompatibility and biodegradability.Finally,based on synthesizing sulfur-containing polymers and understanding the mechanism of cooperative organocatalytic polymerization,this thesis successfully realizes the alternating copolymerization ofγ-SBL and epoxides to selenium-rich polymers.γ-SBL can be synthesized fromγ-BL,but it is difficult to perform ROP due to lower ring tension.Considering this problem,we have developed a method of alternating copolymerization of epoxides andγ-SBL,achieving the organocatalytic synthesis of polyester containing selenium ether in the main chain.Organic bases are used to catalyze this copolymerization at room temperature.The alternating degree of the obtained selenium-containing polyester is>99%,and the content of the head-to-tail structure is>99%.Each chain link contains an ester bond and a selenium ether bond,and the end groups of the product are controllable.The obtained selenium-containing polyester has an initial thermal decomposition temperature of 275°C,a glass transition temperature as low as-59°C,good dissolution in common solvents,and good ionic conductivity.In summary,this thesis mainly adopts cooperative organocatalytic methods,including regulating the properties of growing chain anions and activating monomers,to prepare polymonothiocarbonate,polythioether,polyester,polyether,selenium-containing polyesters,and corresponding block copolymers.These research results innovate the new system of organocatalyzed polymerization of chalcogen-rich monomers.