An Entropy-driven Ring-opening Metathesis Polymerization Approach Towards Main-chain Liquid Crystalline Polymers

Since the discovery of liquid crystal(LC)in the nineteenth century,explorations of methods for endowing polymers with liquid crystallinity have attracted widespread attention.Due to their excellent thermal stabilities and mechanical properties,liquid crystalline polymers(LCPs)have shown promising applications in electro-optic storage devices,engineering plastics,high modulus fibers,composite materials and so on.Generally,LCPs can be classified into two categories:main-chain liquid crystalline polymers(MCLCPs)and side-chain liquid crystalline polymers(SCLCPs),according to mesogenic groups either embedded in or laterally attached onto the macromolecular backbones.Taking advantage of the rigid mesogenic backbone structures,MCLCP materials compared with SCLCPs,usually possess higher mechanical strengths,higher glass phase transition temperatures,and higher LC-to-isotropic phase transition temperatures.The known synthetic methods of MCLCPs have been rather limited.Most of the previously reported MCLCPs were prepared using step-growth polycondensation reactions,such as polyesterification,polyamidation,palladium-catalyzed coupling reactions,hydrosilylation polyaddition,thiol-ene polyaddition and acyclic diene metathesis polymerization(ADMET).However,the nature of step-growth polycondensation endows the corresponding MCLCPs with relatively low molecular weights and broad molecular weight distributions.In this thesis,an entropy-driven ring opening metathesis polymerization(ED-ROMP)approach is for the first time applied in producing a MCLCP by polymerizing a macrocyclic olefin monomer containing one cyclohexanecarboxylic acid phenyl ester group as the mesogenic core and one icos-10-enedicarbonyl group as the flexible aliphatic chain.In comparison,Yamaguchi macrolactonization and ring-closing metathesis(RCM)methods are used to synthesize the macrocyclic olefin monomer respectively,while ADMET and ED-ROMP methods are applied to obtain the main-chain liquid crystalline polymers.Although the macrolactonization method can prepare a pure macrocyclic monomer,the overall reaction yield is only 4%,which is an unsatisfied result.By applying RCM approach in producing a macrocyclic monomer,this drawback can be addressed.Although oligomeric macrocycles appear inevitably in RCM process,they are not obstacles for ED-ROMP which is an equilibration procedure.Thereafter,an ED-ROMP protocol is applied in producing a MCLCP by polymerizing the macrocyclic olefin monomer for 3 hours,providing the desired polymer with high molecular weight,while the polymer prepared by ADMET in 3 hours only shows low molecular weight.The polymerization results of ED-ROMP obviously outweigh those of ADMET.The properties of polymers are studied in detail by a combination of 1H-NMR,GPC,TGA,POM,DSC and 1D WAXS.The results show that ED-ROMP approach can prepare MCLCPs with much higher molecular weights in a shorter reaction time.Moreover,manipulating polymer molecular weight is feasible by adjusting the molar ratio of Grubbs catalyst loading to the macrocyclic monomer.All the polymers prepared by ED-ROMP have excellent thermal stabilities and show an enantiotropic LC phase.Meanwhile,the mesomorphic behavior of the MCLCP is similar to smectic B(SmB)phase.Increasing polymer molecular weight could broaden the mesomorphic temperature range of the MCLCP.