Some of the key synthetic advances of the last 10 years are noted, and unsolved problems and important research directions are identified.

Control of Chain Architecture

Topology

Chain topology controls many critical properties (e.g., crystallization, solubility, and rheological behavior) of polymeric systems. The development of linear polyethylene via the Ziegler and Phillips processes in the 1950s provides the most important example, wherein "straightening of the chain" resulted in a rise in Tm of ~30°C and significant improvements in strength and toughness. Control of topology in this case led directly to a new class of materials now sold in quantities of millions of tons per year.

Current synthetic methodology affords a high level of control of chain structure, and syntheses of linear and branched chains, stars, rings, and combs have been achieved. A striking recent advance has been the preparation of hyperbranched—or dendritic—macromolecules, in which iterative branching steps lead to structures in which segment density grows rapidly as one proceeds radially from the molecular "core" (Figure 4.1). Dendritic polymers of narrow molecular weight distribution have been prepared by laborious stepwise synthesis, while "one-pot" methods have been developed for polydisperse samples. In some instances, remarkable improvements in solubility (in comparison with linear analogues) have been reported for hyperbranched chains, and there has been plausible speculation that dendritic polymers may have useful sequestration and reactivity properties. Commercial applications for these materials have not emerged as of this writing.

Polymeric rotaxanes, in which the chain backbone is threaded through a series of macrocycles, have also been reported recently. In this arrangement, the macrocyclic "rotors" are not covalently attached to the "axle" but instead are constrained by bulky end groups from topological dethreading. It has been suggested that controlled, reversible switching of rotor positions on the axle might provide a basis for functional molecular devices (Figure 4.2).

Even given these advances, there is no shortage of intriguing and important topological issues yet to be addressed, and it appears likely that attention will shift to molecules of even greater complexity. Preliminary reports of two-dimensional ribbonlike polymers have appeared, as have descriptions of synthetic DNAs with the topological character of a cube. Particularly intriguing is the prospect of exploiting both covalent and non-covalent interactions, to provide control not only of topology, but also of the molecular geometry over large length scales in real space.



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