Macrocyclic polymers: Synthesis, purification, properties and applications

Abstract

Cyclic polymers present a topology that differ significantly from their linear counterparts due to their circular structure and therefore, the lack of chain ends. These simple characteristics are responsible for important unique properties (e.g. lower intrinsic and melt viscosity, lower hydrodynamic volumes, slower degradation profiles, increased blood circulation times and more selective bioaccumulation) thanks to which the cyclic polymers are today a vanguard in macromolecular chemistry. However, the preparation of cyclic polymers with high topological purity and in large quantities is challenging, therefore demanding the continuous development of synthetic methods. Advances in organic chemistry and the development of new catalytic systems has allowed the field of cyclic polymers to expand enormously in recent decades. In this review, ring closure and ring expansion polymerization strategies are described along the milestones that have characterized the evolution of cyclic (bio)polymer chemistry over the years. We also focus on very recent advances and state-of-the-art techniques for the synthesis and purification of cyclic polymers and provide an overview of properties and applications of cyclic polymers in different fields.

Introduction

The synthesis of well-defined macromolecular architectures has been the focus of much research in polymer chemistry. Both the physical and chemical properties of a material depend on its molecular characteristics, such as molecular weight, polydispersity, functional groups and macromolecular architecture. The development of synthetic routes that allow the control of molecular characteristics has led to the preparation of increasingly complex materials including polymeric brushes and stars, dendrimers, hyperbranched structures, networks and cyclic polymers. Extensive studies have been conducted to link physical properties (e.g. glass transition temperature, melt and crystallization temperatures, intrinsic and melt viscosities, thermo-responsiveness and rheological properties) and chemical properties (e.g. reactivity, chemical stability and solubility) to the covalent structure of a material.

The end groups of a polymer have a great influence on many of the aforementioned properties. Therefore, the absence of end groups makes the cyclic polymers unique. And not only that, the absence of end groups makes a cyclic polymer to have lower conformational degree of freedom and more compact coil conformation compared to their linear analogs. They are characterized by many unique physical properties such as reduced melt viscosity, reduced entanglement, increased T_g at medium-low molecular mass, and lower hydrodynamic volume [1,2]. A significant effort has been made to demonstrate their usefulness in medicine, nanotechnology and material science. In particular, the use of cyclic polymers in surface coating [3,4], crosslinked networks [5], and as platforms for drug and gene delivery [6] has been addressed. The difficulty in producing a large quantity of pure cyclic polymers has made these materials of little interest to industry, for the time being. However, great efforts are being made to improve synthetic techniques towards the production of high-purity cyclic polymers, as well as towards the implementation of new purification techniques [7,8].

Cyclic polymers have been prepared by different synthetic strategies. Ring-chain equilibration was the first reported method to synthesize cyclic polymers [9]. The formation of cyclic poly(decamethylene adipate) [10] in the polycondensation reaction of adipic acid with decamethylene glycol was used to validate previous theory developed by same authors, Jacobson and Stockmayer [11]. They included the formation of rings in the theories of distribution of molecular species in polycondensates and postulated that the fraction of rings increases with dilution and molecular weight [11]. The formation of cyclic oligo and poly(dimethyl siloxane) was also studied within the concepts of ring-chain equilibration [12,13]. Through the years, many excellent reviews and books have been published describing progress in the area of ring-chain equilibration [9,[14], [15], [16]–17], including some controversy around the Jacobson and Stockmayer theory [18] and new concepts [19].

The ring closure (RC) and ring expansion polymerization (REP) techniques are nowadays more attractive than the ring-chain equilibration for the synthesis of well-defined macrocyclic structures due to their versatility and purity of the obtained products. The RC strategy relies on intramolecular coupling of the end-groups of a previously synthesized linear precursor. The use of predesigned linear polymers as precursors for the preparation of cyclic polymers allows high control over the molecular characteristics of the obtained rings. REP allows the synthesis of “specific” cyclic polymers via the formation of an initial ring that expands upon the incorporation of monomer units through a weak labile bond (e.g. organometallic or electrostatic). At the end of the reaction, the catalyst is either retained or expelled from the macrocycle. We say “specific” because only the right combination of monomer and catalyst would produce the desired cyclic structure. Excellent reviews [7,20–30] and books [2,31–34] describe the most important aspects of both RC and REP methods, with special emphasis on catalytic and chemical processes, physical and chemical properties, and potential applications of cyclic polymers. However, the rise of cyclic polymers in the polymer community and the increasing literature in this area demand for constant updates. Therefore, we considered important to make an overview of the most recent advances in the design and synthesis of cyclic polymers by compiling already known concepts of RC and REP and recent synthesis examples.