Recent progress on the synthesis of cyclic polymers via ring‐expansion strategies

Abstract

Cyclic polymers are the simplest topological isomers of linear macromolecules, but exhibit properties that differ from linear chains in ways that remain imperfectly understood. The difficulty of synthesizing appropriately pure and high molecular weight cyclic samples has hindered experimental studies. Ringclosure methods, while versatile, are inherently limited in the range of molecular weights that can be achieved. Ringexpansion methods are a much more promising strategy toward obtaining high molecular weight cyclic polymers. The current review focuses on recent developments in ring-expansion polymerization strategies toward the synthesis of high molecular weight cyclic polymers. Significant progress in the last decade has made the synthesis of cyclic polymers possible by a variety of methods, such as ruthenium-and tungsten-catalyzed ringexpansion metathesis polymerization, organocatalytic and Lewis acid-catalyzed zwitterionic polymerization, RAFT and nitroxidemediated radical polymerization, among many others. While the study of cyclic polymers has long been hampered by synthetic challenges, the recent resurgence of interest in this field presents an exciting opportunity for chemists. V C 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2892-2902 KEYWORDS: cationic polymerization; cyclic polymers; cyclopolymerization; Grubbs catalyst; macrocycles; macrocyclic polymerization; metathesis polymerization; ring-expansion polymerization; zwitterionic polymerization INTRODUCTION Most of our knowledge of the physical and dynamic properties of polymers derives from investigations of linear chain macromolecules. Cyclic polymers, topological isomers of linear chains, differ from their linear congeners by only one bond, and yet this topological difference leads to behavior that, in many respects, remains poorly understood. Early heroic synthetic efforts to generate and investigate high molecular weight cyclic macromolecules 1-4 were compromised by the dramatic influence of even minor amounts of linear contaminants on the properties (esp. rheological properties) of cyclic chains. 1,[5][6][7] The synthesis of cyclic macromolecules of sufficient quantity and purity has proven to be a formidable challenge and has imposed a significant limitation on the study of their behavior as a function of molecular weight. Until recently, only a limited number of different classes of cyclic polymers have been prepared and investigated. Nevertheless, these pioneering studies have shown that cyclic polymers behave in a way that can be quite different from their linear counterparts despite their nearidentical chemical compositions. Cyclic polymers, for instance, cannot reptate or entangle 3,5,7,8 in the same way that linear polymers do and have a smaller radius of gyration in solution, leading to lower solution viscosities. 1,6

Effects of Chain Ends on Thermal and Mechanical Properties and Recyclability of Poly(γ‐butyrolactone)

“…Other than changing the composition of the main chain through copolymerization, other approaches could be utilized to modify homopolymer P γ BL. For example, it is known that the overall properties of the polymers can be significantly affected by the polymer topology . In this context, the decomposition temperature ( T d , defined by the temperature of 5% weight loss in the thermogravimetric analysis [TGA] curve) of the linear P γ BL was around 220 °C, while the cyclic P γ BL with similar M n exhibited a significantly higher T d of around 300 °C .…”

Cyclic Polyester Formation with an [OSSO]‐Type Iron(III) Catalyst

“…[4] Indeed, av ery high degree of control over the polymerization process has been achieved, which offerst he possibility to obtain polymers with predictable molecular weights, compositions,a nd microstructures. Actually, the number of reports in which the production of cAPEs by means of aw ell-defined metal complex is described is limited, [24] and mosti nvolve the use of Al- [25][26][27] and Zn-based catalysts. [6] Different approaches have been proposed for the production of cyclic polymers, [7] andnotable examples have been reported in literature.…”

“…[8] However,t he formation of cyclic polyesters remains a challenge. Actually, the number of reports in which the production of cAPEs by means of aw ell-defined metal complex is described is limited, [24] and mosti nvolve the use of Al- [25][26][27] and Zn-based catalysts. Indeed,t he biodistribution of cyclic aliphatic polyesters (cAPEs)i sd ifferent from that of linear APEs, [10] which offers an opportunity to develop new drug deliverys ystems.…”

“…Indeed,t he biodistribution of cyclic aliphatic polyesters (cAPEs)i sd ifferent from that of linear APEs, [10] which offers an opportunity to develop new drug deliverys ystems. Actually, the number of reports in which the production of cAPEs by means of aw ell-defined metal complex is described is limited, [24] and mosti nvolve the use of Al- [25][26][27] and Zn-based catalysts. Fe complexes activei nt he ROP of cyclic esters are limited, [11][12][13][14][15][16][17][18][19][20][21][22][23] and no examples have been reported that produce cAPEs.…”

Functional Macromolecular Systems: Kinetic Pathways to Obtain Tailored Structures

“…[15][16][17][18] In recent years another new class of synthetic copolymers was developed with very precise positioning of comonomers along a polymer chain using advanced reversible addition fragmentation transfer polymerization (RAFT) techniques. [19] Moreover, besides linear chains also other macromolecular topologies can be synthesized like bottle brushes, [20,21] cycles, [22,23] combs, [24] stars, [24][25][26][27] etc. Starting from basically any of these polymers further polymers can be obtained via a large number of well-defined post modification reactions, with "click chemistry" being a rather popular one.…”