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Introduction

Yan Xia

Stanford University, Department of Chemistry, Stanford, USA

Ladder polymers are double- or multiple-stranded polymers in which the adjacent monomeric units are connected by two or more bonds [1]. The repeat units of ladder polymers feature conformationally flexible or rigid, conjugated or nonconjugated rings; alternatively, the two strands in a ladder polymer could be held by noncovalent interactions, including hydrogen bonding, metal-ligand coordination, ion pairing, or van der Waals force. Ladder polymers represent a unique macromolecular architecture in that all other architectures are single stranded. While much less common than single-stranded structures, the concept of ladder polymers dates back to the early history of macromolecular science. Staudinger, recognized as "the father of macromolecular chemistry," first proposed the possibility of forming ladder-type polymers almost a century ago [2]. He hypothesized that ladder-type poly(cyclopentadiene) could be formed via repeated cycloaddition of cyclopentadiene, although this process is thermodynamically unfavorable.

Active pursuit of ladder polymers first flourished in the 1960s, driven by the expectation of improved thermal, chemical, photochemical, and mechanical stability compared to their linear polymer analogues. Ladder polymers can be generally synthesized by direct ladder polymerization or by "zipping up" a single-stranded precursor polymer via reactive pendants or by complexation or linkage of two polymer strands (Scheme 1.1) [3-7]. Early syntheses explored both strategies, zipping up linear, conformationally flexible precursor polymers or multifunctional polycondensation to form heterocycles. But those exploratory attempts have all resulted in insoluble, intractable, and, in some cases, pyrolyzed materials, making structural analyses of these assumed ladder polymers a considerable challenge. For example, while the first synthesis of ladder polysiloxane was reported in 1960 [8], its structure was not rigorously characterized, and the chemistry was more complex than originally believed (with uncontrolled stereochemistry in siloxane formation) [9]. Decades later, only ladder-type oligosiloxanes up to five fused siloxane rings have been isolated and characterized [10, 11]. An early review from Overberger and Moore covered the early designs and synthetic endeavors toward ladder polymers, along with discussion of several limitations and challenges in the field [3].

Scheme 1.1 Common strategies for ladder polymer synthesis.

Scheme 1.2 First synthesis of a soluble, unambiguously characterized ladder polymer via Diels-Alder polymerization.

The first unequivocally characterized, soluble ladder polymers were reported in 1989 by Schlüter via the Diels-Alder reaction between bisquinones and in situ generated bisfurans as monomers (Scheme 1.2) [12]. Diels-Alder reactions are indeed well suited for ladder polymerization due to the concerted cycloaddition to form ring structures. In the following decade, a number of creatively designed monomers were applied to Diels-Alder polymerizations [13-19]. The resulting ladder polymers exhibited rigid hydrocarbon backbones and were soluble in organic solvents when substituted with flexible alkyl groups, allowing complete spectroscopic and chromatographic analysis of the polymers. Interestingly, some of these nonconjugated ladder polymers can also be aromatized to form conjugated ladder polymers [20, 21].

In the 2000s and 2010s, McKeown and Budd achieved the polycondensation of tetrafluoro-dicyanobenzene and biscatechols via double nucleophilic aromatic substitution, as well as that of bisanilines via Tröger's base formation, to form a new type of ladder polymer that is soluble in organic solvents without the necessity for long alkyl substituents (Scheme 1.3) [22, 23]. These polymers generate abundant microporosity in the solid state due to the frustrated packing of their rigid and contorted macromolecular chains, and are thus given the name "polymers of intrinsic microporosity (PIMs)." PIMs represent the most recent breakthrough in ladder polymer development and have attracted broad attention as the next-generation membrane materials for chemical separations, particularly gas separations. Many variations on the ladder or spiro-ladder backbone structures as well as modifications of functional groups have been pursued to tune the molecular transport properties in PIM materials. In 2014, Xia and coworkers reported a catalytic ladder polymerization using norbornadiene and dibromoarenes as monomers (Scheme 1.3) [24]. This new polymerization also resulted in contorted rigid ladder polymers with abundant microporosity. The bromoarene structures and positions of bromo substituents can determine the backbone configuration, which has been found to greatly impact the separation performance of the resulting polymer membranes [25, 26].

Scheme 1.3 Nonconjugated microporous ladder polymers (PIMs).

Conjugated ladder polymers have also sparked considerable interest, owing to the expected enhanced electron delocalization in their planar p-configuration, which may lead to improved optical nonlinearity, carrier mobility, and other optoelectronic properties [4, 5, 7]. However, the ultra-strong interchain p-p interactions between two conjugated ladder polymers cause insolubility, posing significant challenges in their characterization and processing. To overcome this issue, flexible alkyl side chains are typically installed and need to be optimized to both bestow solubility and maintain favorable packing.

Conjugated ladder polymers can be synthesized via polycondensation or by backbone ladderization of linear conjugated polymers [7]. In 1966, Van Deusen reported the first conjugated ladder polymer, poly(benzimidazobenzophenanthroline) (BBL), via polycondensation with chemistry derived from related dye syntheses (Scheme 1.4) [27]. BBL is typically insoluble but can be dissolved in moderately strong acids and n-doped to be conductive, leading to early organic electronic applications of conjugated ladder polymers [28, 29].

Scheme 1.4 Van Deusen's synthesis of BBL, the first reported conjugated ladder polymer.

Ladder-type poly(para-phenylene)s have been widely explored and are synthesized by different annulation reactions of the adjacent pendant substituents of linear poly(para-phenylene)s to ladderize the conjugated polymer backbone (Scheme 1.5) [5, 30]. Ladder-type poly(para-phenylene)s exhibit strong photo- and electroluminescence, as well as high charge carrier mobilities, make them promising materials for use in light-emitting diodes and solid-state lasers [5].

Scheme 1.5 Conjugated ladder polymers by zipping up linear conjugated precursor polymers.

Another impressive type of ladder conjugated oligomer/polymer is triply fused porphyrin ladders with up to 12 porphyrin units, which were synthesized via cyclodehydrogenation from linear porphyrin oligomers (Scheme 1.6) [31]. The porphyrin molecular ladders showed strong absorption in the IR region as a result of much more extended p-conjugation and intramolecular electronic coupling compared to the linearly linked porphyrin oligomers.

Scheme 1.6 Fused porphyrin ladder oligomer.

Graphene nanoribbons (GNRs) are a special class of conjugated ladder polymers that have emerged in the last two decades (Scheme 1.7). Significant advances have been made in controlling the width, topology, edge structure, and substituents of GNRs in order to tune their bandgap and electronic properties [32, 33]. In addition, heteroatom doping of the aromatic frameworks has emerged as another promising strategy to alter the electronic properties of GNRs [34].

Scheme 1.7 Graphene nanoribbons with different edge morphologies.

The most versatile strategy for GNR solution synthesis involves designing linear polymer precursors, which are often synthesized via cross-coupling or Diels-Alder polymerizations, followed by global intrachain cyclodehydrogenation to planarize the polymers (Scheme 1.8) [33]. In addition to solution synthesis, GNRs have been synthesized on metal surfaces under ultrahigh vacuum conditions [37]. This procedure typically involves surface-assisted dehalogenative polymerization, followed by surface-assisted cyclodehydrogenation at elevated temperatures. On-surface synthesis has not only enabled new atomically precise GNR structures that are often inaccessible or uncontrolled via solution synthesis but also allowed molecular visualization of such GNRs with atomic resolution. This approach, however, requires expensive and complicated instrumentation and a high purity of monomers and is limited in scale.

Scheme 1.8 Examples of graphene nanoribbon synthesis via Diels-Alder polymerization [35] or Suzuki polymerization [36] followed by cyclodehydrogenation.

DNA can be considered a naturally existing ladder polymer,...