Ladderphanes: A New Type of Duplex Polymers

Abstract

A polymeric ladderphane is a step-like structure comprising multiple layers of linkers covalently connected to two or more polymeric backbones. The linkers can be planar aromatic, macrocyclic metal complexes, or three-dimensional organic or organometallic moieties. Structurally, a DNA molecule is a special kind of ladderphane, where the cofacially aligned base-pair pendants are linked through hydrogen bonding. A greater understanding of this class of molecules could help researchers develop new synthetic molecules capable of a similar transfer of chemical information. In this Account, we summarize our studies of the strategy, design, synthesis, characterization, replications, chemical and photophysical properties, and assembly of a range of double-stranded ladderphanes with many fascinating structures. We employed two norbornene moieties fused with N-arylpyrrolidine to connect covalently with a range of relatively rigid linkers. Ring opening metathesis polymerizations (ROMP) of these bis-norbornenes using the first-generation Grubbs ruthenium-benzylidene catalyst produced the corresponding symmetrical double-stranded ladderphanes. The N-arylpyrrolidene moiety in the linker controls the isotactic selectivity and the trans configuration for all double bonds in both single- and double-stranded polynorbornenes. The π-π interactions between these aryl pendants may contribute to the high stereoselectivity in the ROMP of these substrates. We synthesized chiral helical ladderphanes by incorporating asymmetric center(s) in the linkers. Replication protocols and sequential polymerization of a monomer that includes two different polymerizable groups offer methods for producing unsymmetical ladderphanes. These routes furnish template synthesis of daughter polymers with well-controlled chain lengths and polydispersities. The linkers in these ladderphanes are well aligned in the center along the longitudinal axis of the polymer. Fluorescence quenching, excimer formation, or Soret band splitting experiments suggest that strong interactions take place between the linkers. The antiferromagnetism of the oxidized ferrocene-based ladderphanes further indicates strong coupling between linkers in these ladderphanes. These polynorbornene-based ladderphanes can easily aggregate to form a two-dimensional, highly ordered array on the graphite surface with areas that can reach the submicrometer range. These morphological patterns result from interactions between vinyl and styryl end groups via π-π stacking along the longitudinal axis of the polymer and van der Waals interaction between backbones of polymers. Such assembly orients planar arene moieties cofacially, and polynorbornene backbones insulate each linear array of arenes from the adjacent arrays. Dihydroxylation converts the double bonds in polynorbornene backbones of ladderphanes into more hydrophilic polyols. Hydrogen bonding between these polyol molecules leads to self-assembly and produces structures with longitudinally staggered morphologies on the graphite surface.

Comparison of Lipoic and Asparagusic Acid for Surface‐Initiated Disulfide‐Exchange Polymerization

“…The peptide‐like domain around the NDI in 1 is placed to enhance the self‐organization of the SOSIP and to surround the final π‐stack in 1 P with a hydrogen‐bonding network. The strained disulfides from asparagusic acid at two termini of 1 account for the disulfide exchange polymerization into the ladderphane16 architecture 1 P as discussed in the introduction. The benzaldehyde hydrazones at the other two termini template for post‐SOSIP TSE.…”

Norbornene derivatives from a metal-free, strain-promoted cycloaddition reaction: New building blocks for ring-opening metathesis polymerization reactions

“…However, ring‐opening metathesis polymerization (ROMP) has also been widely exploited in this regard. Because of its functional group tolerance, ROMP has seen wide use for the preparation of a broad range of functional polymeric materials …”

Azobenzene‐Incorporated Single‐ and Double‐Stranded Polynorbornenes: Facile Synthesis and Diverse Photoresponsive Property

“…Interactions between pendants or linkers via π–π stacking during the course of polymerization might enable the reaction to proceed chemoselectively. Luh has reported that the space occupied by each monomeric vinylcyclopentane moiety in poly(norbornene dicarboximide) was somewhat larger than those for typical π–π stacking, but could fit nicely when dicarboximides were reduced to pyrrolidine moieties . Therefore, monomers M1 and M2 were designed because not only can these compounds be easily synthesized (a classical Williamson coupling reaction) but also the endo ‐fused pyrrolidene ring with aromatic substituents could generate a favorable environment for the π–π interactions, which would lead to a coherent array of the pendants or linkers during the course of ROMP.…”

“…A polymeric ladderphane is defined as a ladder‐shaped polymer constituted of multiple layers of linkers covalently connected to two or more polymeric backbones . Structurally, it can be considered as a multiple stranded polymer with a linear array of cofacially oriented linkers.…”

“…Unlike most ladder polymers where each repeat unit is connected by two bonds, the linkers in covalently bonded ladderphanes are orthogonal to the polymeric strands and can be planar aromatic, macrocyclic metal complexes, or 3D organic or organometallic moieties. [11b,16] Luh and co‐workers reported a series of single‐, double‐, and triple‐stranded polynorbornenes by ruthenium‐catalyzed ring‐opening metathesis polymerization (ROMP) of the corresponding monomers bearing a range of planar aromatic pendants or linkers and further studied the photophysical properties of these polymeric ladderphanes . Each of the monomeric unit in these polymers spans around 5.5 Å, and strong interactions between the linkers in these polymers readily take place as confirmed by the significant diversification in performances in comparison with those for the corresponding monomers.…”