Living Ring-Opening Polymerisation for the Synthesis of Biodegradable, Biocompatible Cyclic Polymers

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Poly(ester)s are an important class of macromolecule and are utilised in many industries from biomedical to microelectronics as a result of the biocompatibility and biodegradability of the materials. Furthermore, the derivation of these materials from renewable resources has led to applications as an environmentally friendly alternative to poly(olefin)s. Living polymerisation of cyclic ester monomers such as lactide, the cyclic diester of lactic acid, have given access to a wide array of possible polymer structures and topologies with linear, branched and star-shaped structures having been previously reported with living polymerisation techniques allowing the precise control over the chain length, conformation and functional group incorporation. This variety in structural composition can have a profound effect on the physical, mechanical and thermal properties of the bulk polymer as well as affecting the interaction of polymer chains, with only small changes in the make up of the polymer required to have large effects on the bulk material. In fact, while the 'tying' of polymer chain ends to form cyclic polymer architectures is only a minimal conceptual change, the physical properties of a macrocyclic polymer would be expected to differ markedly from its linear counterparts as a result of the physical constraints on conformational freedom and overall dimensions. Furthermore, the observation of cyclic DNA molecules, underlines the occurrence and importance of these structures in nature and demonstrates further possible structures accessible from these macrocycles such as catenates, supercoils and permanent knots.Synthetic cyclic polymers have been shown to display a variety of different physical properties than their linear analogues leading to potential new applications and processing techniques. However, whereas the use of discrete highly functional narrowly dispersed linear polymers to build larger structures has been well studied, the construction of cyclic polymers into more complex macromolecular nanostructures has received little attention, in part a result of the often complicated syntheses and elaborate purification procedures. Molecular construction with small cyclic molecules highlights the possibilities for these structures from polymeric catenanes and rotaxanes, with potential applications as molecular machines, to organic nanotubes, potentially applicable as polymeric drug delivery agents.