Threading slippage

Scheme 2. Rotaxane synthesis combining different meth- threading-followed-by-stopperingthen (2) slippage...

Scheme 12a-d There are several general methods to prepare rotaxanes a threading followed by capping b clipping c slippage d snapping... 

Rotaxanes are synthesized by two basically different methods (Figure 3) threading (or slippage ), and clipping [47], In the threading strategy a macrocycle is threaded onto a molecular axle at first then the stoppers are attached to the ends of the axle. In a variant, the molecular axle is end-blocked at one extremity prior to... 

Figure 3 The three methods of rotaxane construction (a) threading and stoppering (h) slippage (c) clipping and cyclization.

Figure 4 Thermodynamic profile of the reaction equilibrium in the case of threading (full line) and slippage (dashed line), (a) Strongly negative AG" (b) weakly negative AG°.

In this scheme and the following, Z is a precursor to the stoppers (case of threading stricto sensu), or the stoppers themselves (case of slippage). The equilibrium of scheme 1 can be described by the following equations ... 

The relevant thermodynamic and kinetic parameters are displayed diagrammatically in Figure 4. From the point of view of kinetics, AC is significant only in the case of slippage. From the point of view of thermodynamics, if AH° 0, the threading is controlled by entropy this is the case of the so-called statistical threading. If AH° < 0, the threading is controlled by enthalpy the threaded species is thermodynamically much more stable than the separated components. In this latter case, the equilibrium of scheme 1 can be rewritten as shown in scheme 2 ... 

Figure 8 Statistical threading (temperature-controlled slippage). Preparation of [2]-rotaxane (10) [38],...

Harrison [4, 33, 34] and later, Schill and coworkers [38] had explored temperature-controlled slippage. Another approach was to control the size of the stoppers by complexation. Stoddart and coworkers [90] could thread macrocycle (27) onto dumbbells ended by aza crown ethers. The slippage process was inhibited by addition of an alkali cation. Conversely, complexation of the aza-crown after the slippage process inhibited unthreading. 

The threading and slippage methodologies were particularly useful for the preparation of the higher order [3]- and [4]-rotaxanes. For example, in the latter case, the reaction produced a mixture of [4]-rotaxane, three-station [3]-rotaxane, and three-station [2]-rotaxane [61]. The yields of the various rotaxanes could be fortunately controlled by the molar ratios of the reactants. 

Figure 15 Threading and slippage controlled by Tt-donor/n-acceptor interactions. Preparation of [3]-rotaxane (33) containing two different rings [60, 98],...

In this the weft is threaded successively above and below each warp thread, and then inversely in the following pass. It is the oldest and most common textile weave and is the firmest, most stable construction, providing porosity and minimum slippage. Strength is uniform in both directions if identical amounts of fibres are used. See Figure 3.1. 

An increase in the values of these factors increases the fabric resistance to seam slippage. Seam slippage depends on weave, fabric raw material, type of seam, stitch density, and sewing thread tension. 

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