Synthesis of molecular objects: Two‐dimensional polymers

Abstract

AbstractThroughout this century polymer science has studied the linear chain and its architectural derivatives which include familiar forms such as the branched chain and the three dimensional network. Other derivatives with unique properties have been investigated more recently and include macromolecular rings, dendrimers, stars, combs and ladders. The objective of this work is to depart from the focus on linear chains and explore the “bulk” synthesis and properties of polymer molecules that can be considered molecular objects. Ideal molecular objects should be macromolecules with well defined shapes that persist as systems transform reversibly from solids to melts or solutions. The limited access to conformational space which is required in order to define and maintain shape in liquid and solid states of the system is an unusual molecular characteristic in common polymers. Our ability to create such objects through bulk reactions of reasonable scale will undoubtedly extend the current boundaries of polymer science and technology. Shapes that are particularly interesting are those not common in the conformational space of linear chains, for example, two‐dimensional polymers shaped as plates and macromolecular bundles shaped as cylinders or parallelepipeds. The molecular object to be discussed in this lecture is a rigid and anisotropic two‐dimensional polymer with planar dimensions greater than its thickness and a shape‐granting skeleton built by covalent bonds. We have so far developed three different strategies for their synthesis, all involving systems in which reactive oligomers organize spontaneously into the necessary planar assemblies to form the object. In one strategy molecular recognition driven by homochiral interactions plays a key role in the formation of two‐dimensional polymers.1 A different methodology relies on entropy‐driven nanophase separation in rodcoil block molecules in which a rigid segment is covalently bonded to a flexible one sharing the same backbone. The third strategy involves the folding of oligomers into hairpin structures which self assemble into two‐dimensional liquids. The lecture will also describe examples of unique properties that could be achieved in materials containing these rigid two‐dimensional objects. These examples will include bulk materials with self‐organized surfaces and also remarkably stable nonlinear optical properties.