Efficient Synthesis And Cyclic Molecular Topology Of Cyclic Polymers Containing Ladderphane Structure

Cyclic polymers are a class of macromolecular materials with cyclic structures and do not contain end groups.In comparison with their linear analogues,cyclic polymers exhibit lots of unique properties as a result of the absence of chain ends,such as smaller hydrodynamic volumes and radius of gyration,lower melt viscosity,higher glass-transition temperature.In recent years,cyclic polymers containing different shapes have attracted more attention due to their unique endless topological structures.However,compared with other types of polymers,cyclic polymers have been less studied due to the limited synthetic methods.In this paper,the well-defined polynorbornene-based bicyclic,tetracyclic and 4-arm mono-cage polymers were first synthesized via ring-opening metathesis polymerization（ROMP）of tetrafunctional monomer TNBP based on blocking-cyclization technique.In addition,a novel cyclic hanging multicyclic polymers were synthesized by combining the blocking-cyclization technique and the ring-expansion.Finally,the conjugated segments-based monocyclic,bicyclic polymers with the same monocyclic size and bicyclic polymers with different monocyclic sizes were prepared by tandem MCP-ROMP reaction.The structure and morphology of the above cyclic polymers are characterized by transmission electron microscope（TEM）and atomic force microscope（AFM）techniques.The research results are summarized as follows:The well-defined bicyclic polymers,c-[PTNBP_n-（b-PTNP_m）₄-（b-PBNP_k）₂]are prepared by sequential ROMP based on blocking-cyclization technique.Herein,a novel tetrafunctional norbornene derivative,1,3,6,8-tetra（4-（N-4-phenoxyl undecyl norbornene dicarboximide）benzoate）pyrene（TNBP）,was designed as the initial monomer to establish four living ends-contained PTNBP_n ladderphane,which was utilized as a versatile motif to readily construct a type of well-defined bicyclic polymers,c-[PTNBP_n-（b-PTNP_m）₄-（b-PBNP_k）₂],by the ROMP-based blocking-cyclization technique in a simplified three-step feeding procedure.The topological differences between bicyclic polymers and 4-arm star polymers were investigated by means of NMR（¹H NMR,2D ¹H NOESY NMR）,GPC,intrinsic viscosity（[η]）,mean square radius（<R_g²>）.Bicyclic polymer is further post-modification via aminopropylisobutyl polyhedral oligomeric silsesquioxane（POSS-NH₂）transesterification to obtain the modified bicyclic polymer c-{PTNBP-[b-P（TNP-co-POSSNI）]₄-（b-PBNP）₂}and the molecular topology could be observed clearly in TEM image.Importantly,cyclic polymers exhibit higher tensile strengths,higher glass transition temperatures,and better dielectric properties.Based on the synthesis of bicyclic polymers,we designed and synthesized tetracyclic polymers and 4-arm mono-cage polymers.The tetracyclic polymer c-{PTNBP_n-[（b-PTNP_m）₄-（b-PBNP_k）₂]₂}are prepared by successive ROMP via the further alternating addition of monofunctional monomer N-3,5-bis（trifluoromethyl）biphenyl-norbornene-pyrrolidine（TNP）and difunctional monomer bis（norbornene pyrrolidine phenyl perylene imide）（BNP）from bicyclic c-[PTNBP_n-（b-PTNP_m）₄-（b-PBNP_k）₂]with four living chain ends,benefiting from the diversified blocking-cyclization technique and tetrafunctional monomer TNBP.More surprisingly,the mono-cage polymer cage-[PTNBP_n-（b-PTNP_m）₄-（b-PTNBP_k）₂]with four-chains can be efficiently prepared,when the cyclization unit is replaced from the original bifunctional monomer BNP to the tetrafunctional monomer TNBP and keep TNBP at the low concentration(2×10^-4 mmol/m L),which was conveniently realized the topology switching between bicyclic polymers and cage polymers.Compared with the corresponding 4-arm star and bicyclic polymers,the tetracyclic and 4-arm mono-cage polymers are all behaved smaller molecular weights,lower[η],and higher T_g,which is consistent with the fact that they have a more compact topology.A novel cyclic hanging multicyclic polymers c-{PBNP-b-[P（TNP-co-（c-PPMT））]₂-b-PBNP}are synthesized by combining the blocking-cyclization technique and the ring-expansion method.Firstly,a cyclic side chain-contained macromonomer N（c-PPMT）is prepared via ring-expansion,which was randomly copolymerized with monofunctional monomer TNP,and then bifunctional monomer BNP was added as a cyclization unit in a one-pot,finally a cyclic hanging multicyclic polymers was obtained.Compared to the monocyclic c-[PBNP-（b-PTNP）₂-b-PBNP]with the same block ratio,the novel polymer c-{PBNP-b-[P（TNP-co-（c-PPMT））]₂-b-PBNP}have a larger molecular weight(M_n=232.6 kg mol^-1 vs M_n=182.5 kg mol^-1),which preliminarily indicates that the successful ROMP of N（c-PPMT）.Meanwhile,NMR analysis showed that there are six N（c-PPMT）units on each single ring.In addition,cyclic polymer c-{PBNP-b-[P（TNP-co-（c-PPMT））]₂-b-PBNP}displayed excellent thermal stability and higher T_g.The introduction of N（c-PPMT）units into the monocyclic polymer provides an opportunity to directly observe the cyclic morphology,which is clearly demonstrated by AFM and TEM analysis.Monocyclic polymers c-[PBHPI-（b-PEHD_m）₂-（b-PBHPI）]are prepared by metathesis cyclopolymerization（MCP）-based blocking-cyclization process in an alternating addition of difunctional BHPI,monofunctional EHD,and difunctional BHPI monomers.Similarly,bicyclic polymers c-[PTNBP-（b-PEHD_m）₄-（b-PBHPI）₂]are obtained from TNBP by tandem ROMP-MCP.The topological differences between monocyclic,bicyclic polymers and the corresponding linear polymers are demonstrated by means of NMR,GPC,attenuated total reflectance-infrared spectroscopy（ATR-IR）,[η],<R_g²>.Excitingly,due to the rigidity of the polyacetylene backbone and the higher electron density of the conjugated double bonds,a regular monocyclic and bicyclic topology are clearly observed by TEM technique.Finally,UV-visible spectrophotometer（UV-vis）and cyclic voltammetry（CV）characterizations indicate that the conjugated polymers have lower lowest unoccupied molecular orbital（LUMO）and highest occupied molecular orbital（HOMO）energy levels and narrowed bandgaps（E_g）,which would be expected have potential application in polymer solar cells as the non-fullerene acceptor materials.Based on the synthesis of the conjugated segments-based monocyclic polymers c-[PBHPI-（b-PEHD_m）₂-（b-PBHPI）],the asymmetric bicyclic polymer c-{PBHPI-[（b-PEHD_m）₂-b-PBHPI]-[（b-PEHD_k）₂-b-PBHPI]}with different monocyclic sizes is synthesized by adjusting the ratio of the monofunctional monomer 2-ethyl-hexyl-1,6-heptadiyne（EHD）to Grubbs 3nd generation catalyst（Ru-III）.The structure of bicyclic polymer is characterized and confirmed by NMR,GPC,ATR-IR,and Raman techniques,which displayed a lower[η]and<R_g²>due to its more compact topology.Importantly,the visualized bicyclic topology was directly observed by TEM examination,the average outer diameters of the large and small single rings are 27 nm and 15 nm,respectively,and the size of the large ring is about twice than that of the small ring,which is not available by other methods to obtain cyclic polymer with different size for each single ring.