Ladder polymer, defined

Structure-based and source-based nomenclature rules have been extended to regular double-strand (ladder and spiro) organic polymers [7]. A double-strand polymer is defined as a polymer the molecules of which are formed by an uninterrupted sequence of rings with adjacent rings having one atom in common (spiro polymer) or two or more atoms in common (ladder polymer). 

The above examples of a stepwise synthesis of a ladder polymer involve the formation of single-stranded polymers via polymerization of suitable monomers to functionalized precursors. These consist of substituted poly(ethylene)- or (polyacetelyne)-type macromolecules, from which attempts are made to carry out a defined polymer-analogous cyclization reaction. 

The following example describes the first successful synthesis of a soluble and structurally defined ladder polymer by the stepwise route. It illustrates the synthetic potential of the classical route to ribbon-type macromolecules. 

The synthetic sequence to methylene-bridged poly(phenylene)s 71 represents the first successful employment of the stepwise process to ladder-type macromolecules involving backbone formation and subsequent polymer-analogous cyclization. As shown, however, such a procedure needs carefully tailored monomers and reaction conditions in order to obtain structurally defined materials. The following examples demonstrate that the synthesis of structurally defined double-stranded poly(phenylene)s 71 (LPPP) via a non-concerted process is not just a single achievement, but a versatile new synthetic route to ladder polymers. By replacing the dialkyl-phenylenediboronic acid monomer 68 by an iV-protected diamino-phenylenediboronic acid 83, the open-chain intermediates 84 formed after the initial aryl-aryl cross-coupling can te cyclized to an almost planar ladder-type polymer of structure 85, as shown recently by Tour and coworkers [107]. 

Conj ugated Ladder Polymers. Since the 1930s double-stranded, ladder-type polymers have been prepared in a multistep process with limited success of cyclization (191,192). Other routes have also been explored such as those for poly(acrylonitrile) (193,194), poly(l,2-butadiene), poly(3,4-isoprene) (195), or poly(butadiyne)s (196). These materials were found to be poorly soluble and unworkable, with a considerable number of defects in the structure (incomplete cyclization, cross-linking, radical sites). The first successful synthesis of a ladder polymer with a completely defined structure was accomplished in 1991 by Sherf and Mullen (197). The first step was the AA/BB-t5q)e polycondensation of an aromatic diboronic acid with a substituted 2,5-dibromo-l,4-dibenzoylbenzene to give a single-stranded precursor PPP-type polymer, followed by cyclization to the ladder structure (Fig. 8). Several other examples exist that have resulted in ladder-type structures. These include angular polyacene (198,199), Diels-Alder polyaddition of AB-type diene-dienophiles (200), AA/BB-type Diels-Alder polyaddition of a bisdiene and a bisdienophile (201), thienylene imits (202),... 

The reaction sequence described here represents the first synthesis of a structurally defined, soluble band polymer using a multistep process. In addition, it is actually the first known conjugated ladder polymer of defined molecular structure. 

The polymer-analogous cyclization is accompanied by a remarkable change in the absorption properties. The colorless intermediate is converted to the deep yellow planar ladder polymer connected with this is a distinct bathochromic shift of the longest wavelength absorption maximum. The polymer possesses an absorption band with well-defined vibrational fine structure and a sharp absorption edge of the 0-0 transition (see Table 14.4 and Fig. 14.3). 

The wealth of results from the last few years presented in this chapter impressively demonstrate the advance and the enormous dynamism of polymer research in the area of conjugated ladder polymers. Generally, a definite trend toward structurally well defined and easily worked materials can be ascertained. A particularly high level of interest appears from the side of potential applications (e.g., the use as emitter substances in polymer LEDs), from a direction in which structural accuracy or a minimum number of structure defects and the purity of the substances are absolute requirements for attaining optimal values. Apart from this, the availability of several powerful synthetic strategies for conjugated ladder polymers are extremely promising for the developments achieved hitherto and those anticipated in the future. 

We define a ladder polymer as one constructed of an uninterrupted series of rings coimected by sterically restrictive links around which rotation cannot occur vdthout bond rupture. Some illustrative examples are shown here ... 

The incorporation of substructures into ladder polymers provides molecular properties with well-defined configuration and conformation. The difficulties, however, in synthesizing ladder-type polymers have been reported [50,51] For the synthesis of soluble ladder-type PPP, first, PPP precursor 51 with functional ketone groups was prepared by Pd-catalyzed coupling reaction of dihexyl-substituted aromatic boronic acid 29 with didecyl-substituted aryl bromide 50 in 60-80% yields. The values determined by GPC were 6100-9200 [52]. 

Fahnenstich U., Koch K. H., Pollmann M., Scherf U., Wagner M., Wegener S., Muellen K. Design of Novel Strnctnrally Defined Ladder-Type Polymers Makro-mol. Chem., Macromol. Symp. 1992 54/55 465—476... 

The topochemical polymerization of 1,3-diene monomers based on polymer crystal engineering can be used not only for tacticity but also for the other chain structures such as molecular weight [ 102], ladder [84] or sheet [ 103] structures, and also polymer layer structures using intercalation reactions [ 104-107]. Some mechanical and structural properties have already been revealed with well-defined and highly or partly crystalline polymers [ 108-111 ]. A totally solvent-free system for the synthesis of layered polymer crystals was also reported [112]. 

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