Catenanes periodic

The plectonemic nature of the DNA double-helix makes it a tractable molecule for experiments in molecular topology. This is a very rich vein for the exploration of the topological properties of matter. In this chapter, we have tried to illuminate some of the techniques by which the single-stranded topology of DNA can be directed in synthetic molecules. Catenanes and knots, periodic braids, and Borro-mean rings are available from simple protocols, and it is to be hoped that the physical properties associated with complex topologies (Moffatt 1990) will become available through the medium of DNA constructions. 

Two-terminal devices might seem more natural for the molecular-scale systems than three-terminal ones because of the technological difficulties in manipulating small structures. Furthermore, chemical assembly of molecular devices usually results in a periodic structure. This observation resulted in the idea to have a two-terminal switch, electronically reconfigurable, where a relatively high voltage (e.g. —2V or +2V in [62], which uses a 2-catenane-based molecule) (Fig. (7a)) is applied to close or open the switch, but a relatively low voltage to read (M). 1 V) [60]. These molecular switches [62], a mono-layer of rotaxane molecules, are not field-activated but can be described as small electro-chemical cells, which are characterized by... 

Entangled systems are extended arrays, more complex than their constituents, that are comprised of individual motifs forming, via interlocking or interweaving, periodic architectures infinite in at least one dimension. Simple interdigitation is not considered here. As previously stated, most of the entangled arrays can be considered regularly repeated infinite versions of finite molecular motifs like catenanes, rotaxanes and pseudo-rotaxanes. 

Mark and Semiyen, in a series of papers, have studied the mechanism and the effect of trapping cyclics in end-linked elatomeric networks [100-103], Sharp fractions of cyclics of polyfdimethylsiloxane) (PDMS), varying in size from 31 to 517 skeletal atoms, were mixed with linear chains for different periods of time and the linear chains were then end-linked using a tetrafunctional silane. The untrapped cyclics were extracted to determine the amount trapped. It was found that while cyclics with less than 38 skeletal atoms were not at all trapped, for n>38, the percentage of cycUcs trapped increased with size, with 94% trapped in the case of the cychc with 517 skeletal atoms. In effect, the system of trapped cycUcs in the end linked PDMS network is a polymeric catenane. It is thus possible to control the elastomeric properties of the network by incorporating the appropriate sized cyclics. This study has been extended to cyclic PDMS in poly(2,6-dimethyl-l,4-phenylene oxide) [104,105] and cyclic polyesters in PDMS [106]. 

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