Linear polycatenanes

The principle of the second synthetic approach to polycatenanes, i.e. stepwise polycondensation, has been proposed by Shaffer and Tsay, but not experimentally demonstrated [42, 43], This approach has the advantage over multifunctional polycondensation that a linear polymer is formed before cyclization (Scheme 7). However, the second step, which consists of the cyclization of n macrocycles along the polymer chain 19, is likely, again, to give rise to an undefined network, containing some rotaxane and catenane units 21, similar to the multifunctional polycondensation approach. 

In this section the unprecedented oligocatenanes, i.e. the [5]- and [7]catenanes 30 and 31 and the scarce experimental approaches to high molecular-weight linear polycatenane 9 have been presented. No synthetic Olympic network 32 has been reported to date, although their DNA analogs are known. The next section is dedicated to a new type of macromolecular architecture, structurally related to polycatenane 9, i.e. poly[2]catenanes. 

Scheme 29. Unreported macromolecular architectures containing defined topological bonds polycatenane 9, linear poly[3]catenane 74, poly[2]catenane network 75, multicatenane network 76, rigid polymeric catenane 77, polymeric trefoil knot 78, and polyknot 79.

The ring topology is the potential to form unique polymer structures. Like linear polymers, cyclic polymers not only can be branched or cross-linked, but also can form non-covalently linked structures based on their loop topology. These are referred to as topological polymers, including rotaxane, catenane, threaded rings, and rings threaded by network chains. Recently, much attention has been paid to how their particular properties not only differ from linear polymers, but also how they differ from a component of an interlocked polymer system, such as polycatenanes and polyrotaxanes. 

Fig. 8 Thermodynamic self-assembly of a polycatenane. Formation of a linear poly[2]catenane containing covalent, topological, and...

As illustrated by types C and D in Figure 17.1, side-chain polycatenanes are polymers that contain catenane subunits within their pendant groups, and which are expected to possess different properties compared to the main-chain polycatenanes. However, due to similar synthetic problems being encountered as for the preparation of linear poly[ ]catenanes, only poly[2]catenane-type side-chain polycatenanes have been reported to date. 

Endo et al. reported a synthesis ofpoly(l,2-dithiane) (PDT) [151,152] that possibly contained polycatenane structures (Scheme 17.24). A melt polymerization [153] of 1,2-dithiane 84, without initiators, yielded the polymer 85, the NMR analysis of which confirmed the presence of large cyclic structures, which were further verified using by mass spectroscopy and photodegradation analysis. In addition, dynamic viscoelastic measurements showed that the molten state of the polymer 85 has a rabbery plateau, while the Tg of PDT decreased with increasing molecular weight. These observations, which differed from those obtained with the linear PDT analog, provided further evidence for the formation of polycatenane structures. In taking... 

Over the past few decades, much attention has been focused on polycatenanes, which consist of mechanically interlocked structures that have novel topologies and, not unexpectedly, display somewhat different properties than do commonly used, conventional polymers. The linear polycatenanes (type A in Figure 17.1) are aesthetically perfect, and are expected to possess maximized effects of topologically bonded structures on properties. However, the synthesis of such linear polycatenanes remains one of the most difficult and as-yet unachieved synthetic goals in polymer science. Due to the relatively easy preparation of bifunctionaUzed [2]catenanes, success in the directed synthesis of polycatenanes has been mainly limited to the poly[2]catenanes, which contain essential mechanical hnkages. Nonetheless, some progress has been made recently towards creating polymeric catenanes and polycatenane networks. 

Figure 31 Polycatenanes (a) summary of types (i) linear, (ii) main-chain, and (iii) pendant (b) Sauvage s main-chain poly([2]catenane) and (c) Stoddart s pendant poly([2]catenane).

An interesting possibility for the formation of linear polycatenanes is the statistical synthesis technique proposed by Greek researdiers Tte tedmique consists... 

The polymers were characterized to have cyclic structures containing polycatenane stmctures in contrast to the linear stmaures obtained in the presence of thiol-containing chain transfer compounds. 

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