Metathesis Synthesis,Morphology,and Dielectric Energy Storage Properties Of Block And Cyclic Polymers Containing Ladderphane Structure

Polymeric ladderphane is a class of step-like structure comprising multiplayer of linkers covalently connected to two backbones,of which unique chain propagation andwell-definedstructurehaspotentialapplications in molecular synthesis technology and polymer material science.In this paper,the block copolymers containing ladderphane polyacetylene,polynorbornene,and polynorbornene-block-polyacetylene,were successfully synthesized via the third generation Grubbs catalyst（Ru-III）-initiated metathesis cyclopolymerization（MCP）,ring-opening metathesis polymerization（ROMP）,and tandem metathesis polymerization,and the influence of self-assembly nanostructures and dielectric energy storage property was studied.Due to self-assembled core-shell nanostructure,triazolinedione（TAD）molecules reacted with block copolymers in a controlled manner.Based on the unique chain growth of ladderphane,the high molecular weight cyclic copolymers with well-defined ring size and ring number were successfully synthesized.Ionic block copolymer PBNPI-（b-PFNP）₂ was synthesized by ROMP,which consisted of one pyrrolidine ionized ladderphane PBNPI chain and two PFNP chains containing N-3,5-difluorophenyl pendant group.The ionic block copolymer could self-assemble into a unique tree ring-like nanostructure in the selective solvent of THF,in which the ionic domain was isolated between the insulating chain segment,and the maximum breakdown strength increased to 200 MV m^-1.Due to the strong dipole,ionic,and interfacial polarizations,the dielectric constant of copolymer was 17,and the stored/released energy densities were 1.31/1.21 J cm^-3.In contrast,the triblock copolymer PBNPI-（b-PTNP）₂ containing the N-3,5-bis（trifluoromethyl）biphenyl pendant with high dipole moment had a higher stored/released energy densities of1.77/1.60 J cm^-3 because of the stronger dipole polarization;when the N-3,5-bis（trifluoromethyl）phenyl group was coupled to perylene bisimide,and the dipole moment of ionic block further enhanced,the dielectric constant and tored/released energy densities of PBNIF-（b-PTNP）₂ inceased to 25 and 2.60/2.38 J cm^-3,respectively,and the increment was higher than the former,which indicated that increasing the dipole moment of conductive ionic block was more beneficial to improve the dielectric energy storage of material.The triblock copolymer PBHPF-（b-PTNP）₂ was synthesized by tandem MCP-ROMP reaction,which consisted of one ladderphane PBHPF chain with polyacetylenebackboneandtwoPTNPchainscontaining N-3,5-bis（trifluoromethyl）biphenyl pendant group.It was found that the nano-interfacial polarization of block copolymer film containing the core-shell structure enhanced due to the conductive nanodomain,resulting in the high dielectric constant of 26,low dielectric loss of 0.03,high stored/released energy densities of2.89/2.75 J cm^-3,and high charge-discharge efficiency of 95.2%.On this basis,the bis（double-stranded）block copolymers PBNIF-b-PBHPF-（b-PTNP）₂,containing the ionic-conjugated hybrid conductive segments（HCS）with narrow bandgap and the insulating segments with wide bandgap,were synthesized by tandem metathesis polymerizations.The copolymers exhibited enhanced dielectric constant of 33-28accompanied with low dielectric loss of 0.055-0.02 at 10²-10⁶ Hz,and thus greatly increased stored energy density of 9.95 J cm^-3 was achieved at the electric field of 370MV m^-1,which is significantly higher than that of the commercially BOPP(about 1.6J cm^-3 at 400 MV m^-1).In addition,by doping with I₂,the dielectric constant values of the HCS-contained block copolymer can further reach to 36.5-29.0 with low dielectric loss of 0.058-0.026,and the stored energy density maintained at high level of 8.99 J cm^-3 under the applied electric field of 300 MV m^-1.The block copolymer PTNP-b-PSHD with polynorbornene-block-polyacetylene backbone was synthesized by tandem ROMP-MCP,and self-assembled into the core-shell nanostructure in the selective solvent,which could be modified by TAD in a controlled manner by tuning the TAD feeding amount,enabling the Alder-ene reaction with polynorbornene backbone to occur firstly in the shell,and then the cascade Alder-ene and Diels-Alder reactions on the polyacetylene backbone bearing five-membered rings in the core.The modified block copolymers incorporating varied amount of urazole moieties exhibit enhanced dielectric constant from 16.2 to 20.3 and lowered dielectric loss from 0.031-0.013 to 0.009,and the stored/released energy densities increased to 9.95/7.70 J cm^-3,which provides a new idea of the selective post-functionalization of nanostructures in solution for regulating the polymer structure and properties.Blocking-cyclization technique referred to as the third route to synthesize cyclic polymers with well-defined ring size（the repeat unit was 40-400）and ring number（1-3）by alternant ROMP of di-and monofunctional monomers in a one-pot process,where the linear intermediates bearing two single-stranded blocks are efficiently cyclized by the cyclizing unit of propagated ladderphane to generate corresponding mono-,bis-,and tricyclic polymers,and the well-defined ladderphane structure plays a crucial role in forming the cyclic topology.Monocyclic polymer is further modified via Alder-ene reaction and the cyclic molecular topology is clearly demonstrated.The diversity features of cyclic polymers are comprehensively revealed.This strategy has broken through the limitations of previous two cyclizing routes,and indeed opens a facile and popular way to various cyclic polymers by commercial Grubbs catalyst and conventional metathesis polymerization.