The Synthesis Of CO₂-Based Cyclic Oligourea And Polyurea Thermoplastics

With the increase of fossil fuels consumption and Earth's temperature,CO₂ is considered to be the main greenhouse gas.However,as a green,cheap and abundant Cl resource,CO₂ can replace the highly traditional toxic raw materials such as phosgene and isocyanates,and its effective utilization and conversion of polyurea materials with diverse structures and excellent properties are significant and promising in sustainable developments,environmental protection and polymeric materials.The effective and conversion of CO₂ is one of the important ways to achieve the grand goals of“peak carbon dioxide emissions" and "carbon neutral",which are great concerns in the field of chemistry.In our previous research,a series of polyurea materials can be synthesized by polycondensation of CO₂ and amino-terminated diamines directly.Surprisingly,a kind of cyclic oligourea was detected to form during the polycondensation of linear pre-oligourea with CO₂ in the presence of N-methyl-2-pyrrolidinone(NMP)solvent.This encouraged discovery has aroused our great interest in the formation,structures and properties of cyclic polyurea.As is known to us,the synthesis and separation of cyclic polymers are always difficulties in polymer fields.Therefore,the synthesis,separation,purification,structure and properties of cyclic polyurea oligomers are systematically studied in this dissertation.The structural characteristics and properties of the cyclic polyurea were discussed and compared to the linear counterparts in detail.And the relationships between cyclic structures and properties were analyzed.Besides that,the synthesis and properties of CO₂-based thermoplastic polyurea materials with excellent performances were investigated,and the effects of hydrogen bonding and crystallinity on the properties of polyurea were revealed.The main contents and results of each chapter are as follows:(1)The separation,purification and properties of cyclic oligoureaBased on the structural differences of active terminal amino groups between linear and cyclic oligourea,the separation and purification of cyclic oligourea were achieved by using 4,4'-diphenylmethane diisocyanate(MDI)as a separation medium.Among the linear polyurea with high molecular weight was formed by the polymerization of MDI and the terminal amino group of linear polyurea,and the differences of solubilities between linear polyurea and cyclic oligourea in chloroform and ether solvent were used to achieve separation.Finally,cyclic oligourea with high purity(>99.4%)was obtained.According to the characterization results of Matrix-assisted laser desorption ionization-time of flight mass spectrometry(MALDI-TOF),Nuclear magnetic resonance(NMR),Cryo-EM,Thermogravimetry(TGA),Differential scanning calorimetry(DSC),X-ray diffraction(XRD)and solubility tests,the cyclic oligourea can be detected under Cryo-Electron Microscopy.And the cyclic oligourea from 4,7,10-trioxy-1,13-tridecanediamine(TTD)with semi-crystalline structure and high purity(>99.4%),the number average molecular weight(Mn,1040 Da),the melting point(Tm,106?),the maximum decomposition temperature(Td,max,302?),and the glass transition temperature(Tg,-10?)were confirmed.(2)Optimization of synthesis parameters of cyclic oligoureaA novel kind of cyclic oligourea was prepared via two-step polycondensation of TTD and CO₂ as carbonyl resource followed by effective separation and purification processes.The influences of CO₂ pressure depended on the molecular weight of prepolyurea.When the molecular weight of prepolyurea was smaller,the pressure effect was more obvious.With pressure increasing,the cyclization percentage increased first and then decreased.The concentration of prepolyurea has no obvious effect on the oligourea cyclization percentage.The cyclization percentage of oligourea increased and further stabilized as temperature and reaction time increased.When OL-2 was used as the prepolyurea(Mn,1687.2 g/mol),the cyclization percentage could reach 89.9%at 180? with the CO₂ pressure of 6 MPa for 6 h.(3)The effects of molecular weights and structures of prepolyureaFour kinds of TTD prepolyurea with different molecular weights(OL-1,OL-2,OL-3 and OL-4)were synthesized by controlling the reaction conditions at first,the corresponding molecular weight were 697.8,1687.2,3232.1 and 6324.0 g/mol,respectively.The higher molecular weight of TTD prepolyurea,the higher the cyclization percentage.when OL-3 or OL-4 was used as the prepolyurea,the cyclization percentage was up to 98.8%.The prepolyurea with different structures(OUas-148 and OUas-204)were synthesized by controlling the reaction conditions from 1,2-bis(2-aminoethoxy)ethane and 1,4-butanediol bis(3-aminopropyl)ether,respectively.The cyclization percentage of prepolyurea from ether diamines monomer with short-chain were much higher than those of TTD prepolyurea under the same reaction conditions,among them the cyclization percentage of OUas-148 was close to 100%.(4)The effects of solventsOL-3 prepolyurea was selected as reactant for post-polymerization,and a variety of organic solvents were estimated,including strong polar aprotic solvents with amide bond in their backbone(NMP,DMF,DMAc,NMA,DMI,DMPU,HMPA),polar solvents without amide bond(H2O,HFIP),polar solvents only containing carbonyl group(acetone)and mid-polarity solvents(CHCl3,methanol).The cyclization percentage of prepolyurea in the strong polar aprotic solvents with amide bond in their backbone were higher than that in the mid-polarity solvents.The promoting effects of solvents on the cyclization process of oligourea are as follws.On the one hand,the solvents created the "pseudo high dilutution" environment,which promoted the coupling of terminal amino groups of oligourea;On the other hand,the amide bond in the backbone of solvent molecules can form hydrogen bonding with nitrogen or hydrogen atoms in the backbone of oligourea,which promoted the transacylation process further.(5)The cyclization mechanisms of prepolyureaWhen TTD monomer was used as raw material,the pre-and post-polymerization were combined into "one-pot" process,no cyclic oligourea was observed.Compared with the post-polymerization process without NMP solvent,or without CO₂ or 8 MPa N2,when 8 MPa CO₂ and 10 mL NMP were present at the same time,the cyclization percentage increased to 80.3%.By comparing and analyzing the product distributions in/without the presence of CO₂ and solvent in the post-polymerization process,and combining with the specific effects of process parameters on the cyclization percentage,the cyclization mechanisms of prepolyurea varied with the reaction system.When there is CO₂ in the system,the end amino groups of linear oligourea and CO₂ formed carbamates,and then the intramolecular transacylation reaction occurred.Finally,the new linear and cyclic oligourea were formed.Oppositely,an intramolecular or intermolecular amino group exchange reaction may be occurred to form linear or cyclic oligourea.(6)The preparation and properties of thermoplastic polyureaA series of thermoplastic polyurea were prepared by two-step polycondensation of CO₂ and two aliphatic diamines(1,12-diaminododecane(DAD)and 1,6-hexanediamine(HMDA)).The mechanical properties of polyurea can be altered by adjusting the ratio of DAD to HMDA.The polyureas were formed by the single diamine of DAD or HMDA and CO₂,which have regular hydrogen bonds and high crystallinity,and the corresponding polyurea(DAD100 and HMDA100)were brittle materials.For the polyurea(DAD100-x HMDAx(37?x?63))formed by the reaction of mixed diamines of DAD and HMDA with CO₂,the regular hydrogen bonds co-existed with irregular ones,so the crystallinity is poorer.Compared to DAD100 and HMDA100,the corresponding polyurea material(DAD100-xHMDAx(37?x?63))is thermoplastics.The stress-strain tests showed that when the content of HMDA was 50%(DAD50HMDA50),Young's modulus,tensile strength and the elongation at break were 918 MPa,62.1 MPa and 267%,respectively,and exihibited strong toughness;when the content of HMDA was 63%(DAD37HMDA63),the Young's modulus,tensile strength and the elongation at break were 1247 MPa,63.2 MPa and 54%,respectively,and exihibited strong stiffness.In addition,a series of polyurea materials were prepared by introducing aliphatic diamines(decanediamine,octanediamine,hexanediamine and butylene diamine)with different chain lengths to replace HMDA while DAD content was 50%.It was further verified that the strength of hydrogen bonding and the regularity of crystallinity were the key influencing factors of polyurea materials properties.