Double stranded polymers

Preparations of many double-stranded polymers were reported. For instance, polypyrrolones were formed from 2,3,5,6-tetraaminobenzene and various dianhydrides or tetracarboxylic acid compounds [213]. Following are two examples  

7 Step-Growth Polymerization and Step-Growth Polymers 

Different ladder polyquinoxalines were prepared as weU. One example is shown below [233]  

Some ladder polyquinoxalines were found to be stable in air at 460°C and in nitrogen up to 683°C. 

Not all attempts at formations of ladder polymers yielded completely cyclized fused ring structures. For instance, an attempt to form a polymer from tetraaminonaphthalene with naphthalene tetracarboxylic acid dianhydride failed to yield complete cyclizations [222]. 

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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 structure-based nomenclature rests upon the selection of a preferred CRU [1, 12] of which the polymer is a multiple the name of the polymer is the name of this repeating unit prefixed by poly . The unit itself is named wherever possible according to the established principles of organic nomenclature [3]. For double-strand polymers, this unit usually is a tetravalent group denoting attachment to four atoms. Since some of these attachments may be double bonds, the unit may be hexavalent or octavalent. Table 6 lists some examples. 

Note Partial ladder (imperfect ladder, block ladder) polymers [5], in which the sequence of rings is interrupted and a divalent constitutional repeating unit can be identified, are not double-strand polymers. They are named as single-strand polymers. 

On the other hand, occasional irregular deviations from ideality can be ignored in constructing graphical representations for double-strand polymers and in assigning corresponding names. 

At times, it is also useful to identify the starting monomers especially for such complex polymers as double-strand polymers, the synthesis of which is often a multi-step reaction involving condensation, cyclization and crosslinking. 

Kuhn element Number of repeat units needed so that the most probable distance between the first and the growing end is back at the first unit again, ladder polymer Double-stranded polymer for added stability. 

A double-strand polymer is composed of double-strand macromolecules, the structure of which comprises constitutional units connected in such a way that adjacent constitutional units are joined to each other through three or four atoms, two on one side and either one or two on the other side of each constitutional unit. 

The role of zinc in the enzymes listed in Table 12 is very often that of a strong Lewis acid, in which substrates are coordinated, polarized and hence activated. In other cases, zinc may play a regulatory, structural or template role. Zinc may also have a structural function in other biological molecules, for example in the unwinding and subsequent rewinding of the double-stranded polymers involved in replication and transcriptional processes. There is also evidence for a role for zinc in the stabilization of membranes and cell walls.463 The high concentrations of zinc in certain snake venoms reflect the presence in the venom of proteolytic enzymes and hemorrhagic toxins that all require zinc for activity.464... 

A. Rajca, A. Safronov, S. Rajca, C. R. Ross, II, J. J. Stezowski, Biphenylene Dimer. Molecular Fragment of a Two-dimensional Carbon Net and Double-stranded Polymer, J. Am. Chem. Soc. 1996, 118, 7272-7279. 

DECO calculations were carried out next for a double stranded polymer, using canonical B-form carbonyl geometries created by program MacroModel [21]. Without any adjustable parameters, the agreement between observed and computed spectral features was outstanding. Details of these results will be discussed along with the experimental data in Section 5. 

Generally, two ways have been pursued to overcome these drawbacks. The first is to suppress the strong interchain interactions which cause the poor solubility and processibility [16]. These interchain interactions are drastically reduced when the distance between the polymeric backbones is increased, for instance via the introduction of bulky side-groups. The second possibility is to separate the formation of the first and second chain of the double-stranded polymer, and to process the materials at the stage of the soluble single-stranded intermediates. These precursors can be converted into the (insoluble) ladder polymers during the final step that is a favored thermal process carried out in the solid state. 

DNA (deoxyribonucleic acid) A complex molecule found in the chromosomes of almost all organisms that comprises the genetic code. DNA is a double-stranded polymer of nucleotides. The two strands are held together by hydrogen bonds between base pairs of nucleotides. The four nucleotides in DNA contain the bases adenine (A), guanine (G), cytosine (C), and thymine (T). 

It allows us to consider all the results obtained as a novel approach to the synthesis of ladder ( double strand ) polymers. 

Since double-stranded polymers generally have higher stability than singje-stranded ones, Kondo et al. [51a, b] have synthesized phenoxasilin-containing polyamides 33. Unfortunately these proved to be hardly soluble in amide solvents at all and were even insoluble in concentrated H2SO4. Nevertheless, they were more thermally stable (Tj in the range 477-506 °C) compared with their corresponding open-chain polymers. 

Ladder polymers are also referred to as double-chain or double-strand polymers because, unlike other polymers, the backbone consists of two chains. Cleavage reactions in single-chain polymers cause a reduction in molecular weight that ultimately results in a deterioration in the properties of the polymer. For this to happen in a ladder polymer, two bonds will have to be broken in the same chain residue, which is a very unlikely occurrence. Therefore, ladder polymers usually have exceptional thermal, mechanical, and electrical properties. 

Double-strand polymers possess constitutional repeating units bonded at four positions. The relationships of these positions to each other is given by two pairs of numbers, written in italics and separated by a colon. Examples are... 

Figure 2-7. Ordered networks. 0, Adamantane as an example of a cage polymer (type 0) I, Cyclized and dehydrogenated l,2-poly(butadiene) as an example of a ladder or double-strand polymer (type 1) 2, graphite as a layer or parquet polymer (type 2). ( ) Carbon-to-hydrogen bonds (— and =) carbon-to-carbon bonds.

With 1-types it is possible to distinguish between bridge and spiro structures. Theoretically, spiro chains may be classed as single-chain polymers (see Section 2.2.3). 1-Type structures with bridging links are also known as double-strand polymers, or, because their structure is reminiscent of a ladder, they are also known as ladder polymers. Ladder polymers generally have good thermal stability, since, in contrast to linear chain polymers, the breaking of one main chain bond does not lead to a lower molar mass. 

W. De Winter, Double strand polymers. Revs. Macromol. Sci. 1, 329 (1966). 

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