Macromolecular engineering in functional polymers via ‘click chemistry’ using triazolinedione derivatives

Abstract

It has been almost two decades since the concept of ‘click’ chemistry was anchored in the monumental field of chemistry. Later, numerous chemical approaches have been implemented that exhibit features kindred to ‘click’ chemistry toolbox. Unlike the synthesis of organic compounds involving typical purification procedures, the modular and orthogonal ‘click’ concept substantially embraces the material research community with delineating innumerable macromolecular architectures. In polymer chemistry, there are various types of ‘click’ reactions like copper (I) catalyzed alkyne-azide (CuAAC), strain promoted alkyne-azide cycloaddition (SPAAC), Diels-Alder, Alder-ene, thiol-ene, thio-bromo, etc., are used to prepare different functional polymers. Among the various ‘click’ reactions, recently, the ultrafast ‘click’ modification based on different 1,2,4-triazoline-3,5-dione (TAD) derivatives has gained tremendous attention in the broad platform of polymer research. Similar to singlet oxygen, the heterocyclic TAD reagents undergo ‘click’ conjugation within a concise timescale. Following the uncovering of the conventional routes for synthesizing TADs, few spellbinding categories of research have been carried out to develop different functional polymers for diverse applications. The perspective of this review is to cover the recent fascinating outcomes from TAD based ultrafast ‘click’ modification of macromolecules. This review highlights the present state-of-the-art of synthesis of new TAD molecules and their use in designing different macromolecular systems with remarkable features based on ultrafast TAD ‘click’ chemistry.

Introduction

It has been 100 years since polymer science has started its initial journey when Herman Staudinger published the first article on the concept of "Macromolekül" in 1920 [1,2]. Now polymer chemistry has been a well-recognized sub-discipline in chemistry.

Now-a-days, polymers are being used not only in day-to-day life, but also in advanced applications in smart and strategic materials for which new functional polymers with well-defined functionality and tailored molecular weights are essential. In the quest for synthesis of new functional polymers via one-pot modification using easy and simple reaction pathway, since the inception of the ‘click’ concept in the year 2001, this consistent approach has served an immense inspiration to several research disciplines, starting from organic synthesis to redesigning macromolecules [3], [4], [5].

The introductory concept of such a highly specific approach relies on a few characteristics (Fig. 1). A wide range of organic reactions follows the criteria of ‘click’ chemistry toolbox, viz., azide-alkyne [6], [7], [8], thiol-ene [9], [10], [11], Diels-Alder (DA) [12], [13], [14], [15], [16], Alder-ene (AE) [17], [18], [19], thio-bromo [20,21], etc. The individual reactant blocks are separately prepared and congregated orthogonally, employing a specific ‘click’ reaction with relatively less content of byproducts (can be easily removable via crystallization or distillation). The straightforward criteria make ‘click chemistry’ differ from traditional synthetic chemistry. The latter often involves multi-step reaction routes, which are associated with inevitable chromatographic purifications.

The significance of the ‘click’ concept was originally foreseen to synthesize biologically active molecules. Eventually, it gained a remarkable recognition in macromolecular designing platform, which is affiliated with bulk-scale synthesis, lack of byproducts, and facile purification techniques [22], [23], [24], [25]. Furthermore, in 2011, a group of renowned polymer chemists congregated to define criteria for the ‘click’ designation for a polymer-based reaction [26]. Numerous review articles are available based on the utilization of ‘click’ chemistry in designing well-defined macromolecular systems [14,[27], [28], [29], [30]]. Moreover, the rapidly growing interest of ‘click chemistry’ has been immensely utilized in developing biomaterials [31,32], hydrogels [33,34], composites [35], modification of industrial polymers [36], etc.

Since the pioneering inception of its synthetic strategy by Cookson in 1962 1,2,4-triazoline-3,5-dione (TAD) derivative has been extensively implemented in different polymer systems [37]. Unlike the TAD-analogous maleimide ‘clicked’ reactions [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], the TAD based ‘click’ modification (with specific electron-rich dienes or enes) is accomplished within a relatively short timescale under ambient condition. Nevertheless, the execution of fast reaction completion of TAD derivatives was originally tested in elastomers by Saville et al. followed by Butler and Stadler in the later 20^th century [50]. Importantly, a few selective TAD based cycloaddition reactions (with specific reactive functionalities) are shown to be reversibly exchangeable at higher temperatures, which has been successfully executed in developing recyclable, self-healable, and shape-memory polymers [51,52]. Recently, Du Prez and his coworkers have developed and employed multifunctional TAD derivatives to design interesting macromolecules (associated with different applications) viz., self-healing polymers (SHPs), shape-memory polymers (SMPs), recyclable thermosets, bio-based polymers, sequence-defined polymers, etc. based on DA or AE ‘click’ chemistry [19,53].

Since the last few decades, a considerable number of reviews have been published on the utilization of different ‘click’ systems in macromolecular systems [16,[54], [55], [56], [57]]. On the contrary, only a limited number of reviews are available on post-modified polymers based on TAD ‘click’ conjugation [50,58]. However, there are a few notable reviews based on the implementation of TADs in organic synthesis [59], [60], [61]

Recently, the relevant synthetic procedures of TADs and their precursors have been comprehensively reviewed by Du Prez and coworkers [53]. Their article provided interesting examples illustrating the potential of development of functional polymer materials via post-polymerization modification using TAD derivatives. Till 2015–16, although the TAD derivatives were widely implemented in the field of elastomers, its exploitation in other polymers viz., polymethacrylates, polyurethanes, biopolymers, etc., was surprisingly seldom. Henceforth, the TAD based ‘click’ modifications become highly appealing to the research community in designing application-oriented polymers. This has been reflected in the increasing rate of publications (especially since 2015), which is delineated in this review article.

Herein, the growing interest of implementation of multifunctional TADs in adapting a wide range of macromolecular systems based on different ‘click’ chemistry has been discussed comprehensively. Importantly, the significance of theoretical approach has also been briefed in this review to find out the plausible reaction pathway and reaction intermediates during the ‘click’ reaction using TAD molecules. In addition to describing the current state-of-the-art, this review will prepare the material research community to anticipate novel ideas on executing well-defined macromolecules based on efficient TAD ‘click’ conjugation.