Ladder-type structures

U. Scherf, in Handbook of Conducting Polymers, 2nd edn, (Eds. T. Skolhcim, J.R. Reynolds), Marcel Dekkcr, New York 1997, Chapter 14 (Conjugated Ladder-Type Structures), p. 363. 

Another type of multimonomer has been synthesized and examined by Juntas. Selecting an appropriate dilution, concentration of initiator, and temperature, even if initiation is random, the polymerization leads to the ladder-type structure of the product as shown in the example of template polymerization of multimethacrylates according to following reaction ... 

We can assume that on average for one macromolecule of product obtained, there are X units with ladder-type structure, Y unreacted groups with double bonds, and Z points of crosslinking. The scheme of copolymerization of multimethacrylate with acrylic acid and a possible structure of the product obtained are presented in Figure 5.2. 

On the basis of these examples, we can see that using templates connected by covalent bonds, we can produce a new class of copolymers - semi-ladder block copolymers -with blocks of ladder-type structure. By hydrolysis, the template can be removed and block copolymers with defined length of block can be obtained. Such synthesis by conventional copolymerization is very difficult and sometimes impossible. 

Covalent bonding of acrylic or methacrylic monomer to the template leads to multifunctional monomers (multimonomers).If monomer units are connected by covalent bonds within the frame of the template and polymerization proceeds according to the zip mechanism , a product with ladder-type structure can he expected. The structure of products obtained depends on the competition between the reactions proceeding on the template and the reaction between groups belonging to different macromolecules (templates). Template homopolymerization in this case can he represented by the scheme given in Figure 9.1. 

Production of materials in which the daughter polymer and the template together form a final product seems to be the most promising application of template polymerization because the template synthesis of polymers requiring further separation of the product from the template is not acceptable for industry at the present stage. Possible method of production of commonly known polymers by template polymerization can be based on a template covalently bonded to a support and used as a stationary phase in columns. Preparation of such columns with isotactic poly(methyl methacrylate) covalently bonded to the microparticulate silica was suggested by Schomaker. The template process can be applied in order to produce a set of new materials having ladder-type structure, properties of which are not yet well known. A similar method can be applied to synthesis of copolymers with unconventional structure. 

The base pairs are stacked on top of one another, with the plane of the base pairs being perpendicular to the length of the duplex. This is shown diagramatically in the ladder-type structure in Fig. 7-2. 

Question In the duplex, ladder-type structure shown in Fig. 7-2, why are the two chains orientated in opposite directions ... 

A triple anion complex containing enolate, amide, and halide functionalities can be isolated from the mixture of n-butyl bromide, hexamethyldisilazane, TMEDA, Bu Li and pinacolone (Bu COMe). The resulting solution of LiBr, LiN(SiMc3)2, LiOC(Bu )=CH2, and TMEDA produces crystals of Li4(/.t4-Br)( u-OC(Bu )=CH2)2(M-N(SiMe3)2)(TMEDA)2, which, instead of forming a ladder-type structure, consists of a planar butterfly of four lithium atoms bonded to a //4-Br the stability of this arrangement has been studied with semi-empirical (PM3) and ab initio HE/ LANL2DZ computations. ... 

Several structural types are based on the combination of two units of (13). The edge-to-edge combination of (13) yields a ladder -type structure (19). Of course there are various combinations of solvent donor ligands and/or chelate donors possible in (20) and (21). The face-to-face combination of (13) can produce a relatively cubic infrastructure (22) as previously seen in the enolates (5) and (0). Distorted variations of the cube (22) are observed, such as (23), or alternatively as another variation (24), with opposite square faces offset from one another (by nearly 90 in 24). Such variations may be described as a tetrahedron within a tetrahedron . It is noteworthy that the cube (22) can be derived from the ladder (21) simply by decreasing the appropriate internal bond angles to about 90 as indicated by the sequence (tf formulae (13) — (19) — (21) — (22). An advantage of the closed cubic structure (22) over the ladder (19) is the additional coordination of the terminal metal cations (M) to a third anion. The cube (22) is most frequendy observed with four-coordinate metal cations, as in (25) and not in its unsolvated form (22) (Scheme 3). 

The reaction of optically active, helicene derivative 121 first with o-phenylenediamine and then with Ni(OAc)2 led to a helical polymer (Mn 7000) (122) having a unique ladder-type structure with Schiff base moieties immersed in the main chain (Figure 15) 206 -phg polymer showed red-shifted absorptions with respect to nickel salophene, the parent compound for the polymer, supporting the formation of a long conjugation system. Intense CD bands were reported for the polymer. 

Once the first double-ladder-type compound had been established it was apparent that systematic study of this class of compounds should be conducted and a number of questions in context with the structures (A), (B), and (C) shown in Scheme 2.10.12 be addressed. Thus, we were and still are interested in studying the influence the identity of the spacers Z, the organic substituents R, and the electronegative substituents X and Y have on which structure is acmahy observed in solution and in the solid state, and whether these structures can be inter-converted. Furthermore, we looked at the possibility of extending the concept from double to multiple ladder-type structures and made the first attempts to assemble tetraorganodistannoxanes with double ladder-type structures to give supramolecular networks. 

---