Supramolecular encapsulation of redox-active monomers to enable free-radical polymerisation

Extended polymeric structures based on redox-active species are of great interest in emerging technologies related to energy conversion and storage. However, redox-active monomers tend to inhibit radical polymerisation processes and hence, increase polydispersity and reduce the average molecular weight of the resultant polymers. Here, we demonstrate that styrenic viologens, which do not undergo radical polymerisation effectively on their own, can be readily copolymerised in the presence of cucurbit[n]uril (CB[n]) macrocycles. The presented strategy relies on pre-encapsulation of the viologen monomers within the molecular cavities of the CB[n] macrocycle. Upon polymerisation, the molecular weight of the resultant polymer was found to be an order of magnitude higher and the polydispersity reduced 5-fold. The mechanism responsible for this enhancement was unveiled through comprehensive spectroscopic and electrochemical studies. A combination of solubilisation/stabilisation of reduced viologen species as well as protection of the parent viologens against reduction gives rise to the higher molar masses and reduced polydispersities. The presented study highlights the potential of CB[n]-based host–guest chemistry to control both the redox behavior of monomers as well as the kinetics of their radical polymerisation, which will open up new opportunities across myriad fields.

Polyviologens are redox-active polymers based on N-substituted bipyridinium derivatives which have emerged as promising materials for energy conversion and storage. [1][2][3][4][5] Their physicochemical properties can be adjusted through copolymerisation of the redox-active viologen monomers. [6][7][8] The resultant materials are stable, water soluble and exhibit fast electron transfer kinetics. Polyviologens have been commonly fabricated through step-growth polymerisation in linear and dendritic architectures, [9][10][11][12][13] as supramolecular polymers, [14][15][16] networks, 6,17,18 and covalent organic frameworks. 19,20 Alternatively, anionic/cationic or metathesis-based polymerisations are used to avoid interference of radical-stabilising monomers with the radical initiators, however, these techniques are highly water-and/or oxygensensitive. 21,22 When free-radical polymerisation (FRP) is conducted in the presence of viologen species, its reduction can cause a depletion of active radicals and thus disruption of the polymerisation process. Despite varying solvents, comonomers and initiator loadings, the direct FRP of viologen-containing monomers remains therefore limited to molar masses of 30 kDa. [23][24][25] Accessing higher molar masses has been possible via post-polymerisation modication, [26][27][28] which has impacted the electrochemical properties of the resultant materials. 29,30 Alternative strategies to access higher molar masses of redox-active polymers and control their polymerisation are highly desirable.
Incorporation of cucurbit[n]uril (CB[n]) macrocycles have lead to a variety of functional materials through host-guest chemistry. [31][32][33][34] Moreover, the redox chemistry of viologens can be modulated through complexation with CB[n]. [35][36][37][38] Specically, CB[n] (n ¼ 7, 8) can tune the redox potential of pristine viologens and efficiently sequester monoreduced viologen radical cations, avoiding precipitation in aqueous environments. Further to this, we recently demonstrated that the viologen radical cation is stabilised by À20 kcal mol À1 when encapsulated in CB [7]. 39 Consequently, we envisioned that incorporating CB[n]s as additives prior to polymerisation could (i) overcome current limits in accessible molar masses, (ii) increase control over FRP of viologen-based monomers through encapsulation and (iii) enable separation of radical species avoiding aggregation.
Here, we demonstrate a new approach to control FRP of redox-active monomers leading to high molar masses and decreased dispersity of the resultant polymers. In absence of CB[n], co-polymerisation of the N-styryl-N 0 -phenyl viologen monomer 1 2+ and N,N-dimethylacrylamide (DMAAm) only occurs at high initiator loadings (>0.5 mol%, Fig. 1a Cite this: Chem. Sci., 2022, 13, 8791 All publication charges for this article have been paid for by the Royal Society of Chemistry resulting in control of the polymer molar mass across a broad range, 4-500 kDa (Fig. 1b). Finally, CB[n] are successfully removed from the polymer via competitive host-guest binding and dialysis. Spectroscopic and electrochemical studies revealed that solubilisation/stabilisation of the reduced species and/or shielding of the redox-active monomers from electron transfer processes was responsible for this enhancement.
The range of molar masses obtainable through addition of CB[n] (n ¼ 7, 8) correlated with the measured K a (Fig. 3b and S20 †). Binding of 1 2+ to CB [8] was stronger and therefore lower concentrations of CB [8] were required to shi the binding equilibrium and mitigate disruption of the polymerisation. Dispersity values reached a maximum at c CB [7] ¼ 0.6 or c CB [8] ¼ 0.3, suggesting 1 + c is only partially encapsulated. Consequently, higher CB[n] concentrations can enable FRP with lower initiator concentrations (0.10 mol%, Fig. S19 †), which demonstrates the major role of complexation to modulate electron accepting properties of 1 2+ . Fig. 3 (a) In situ copolymerisation of DMAAm with 1 2+ and CB [7]. (b) Molar mass and dispersity vs. amount of CB [7] in the system. Fitted curve is drawn to guide the eye. Cl À counter-ions are omitted for clarity. The redox-active monomer 1 2+ can engage with propagating primary radicals (P mc ) to either be incorporated into the growing polymer chain (P m -1 2+ c) or to abstract an electron deactivating it (P m ). This deactivation likely occurs through oxidative termination producing 1 + c (energetic sink), inactive oligoand/or polymer chains (P m ) and a proton H + , causing retardation of the overall polymerisation. Oxidative terminations have been previously observed in aqueous polymerisations of methyl methacrylate, styrenes and acrylonitriles that make use of redox initiator systems. [44][45][46][47] Another example by Das et al. investigated the use of methylene blue as a retarder, with the primary radical being transferred to a methylene blue electron acceptor via oxidative termination, altogether supporting the outlined mechanism of our system (extended discussion see ESI, Section 1.4 †). 48 The process of retardation can, however, be successfully suppressed, when monomer 1 2+ is encapsulated within CB[n] macrocycles. Herein the formation of 1$(CB [7]) 2 or (1) 2 $(CB [8]) 2 results in shielding of the redox-active component of 1 2+ from other radicals within the system, hampering other electron transfer reactions. This inhibits termination and results in extended polymerisation processes leading to higher molar mass polymers through mitigation of radical transfer reactions. Moreover, suppressing the formation of 1 + c through supramolecular encapsulation minimises both p and s dimerisation of the emerging viologen radical species, 39 preventing any further reactions that could impact the molar mass or polydispersity of the resulting polymers.
In conclusion, we report a supramolecular strategy to induce control over the free radical polymerisation of redox-active building blocks, unlocking high molar masses and reducing polydispersity of the resulting polymers. Through the use of CB[n] macrocycles (n ¼ 7, 8) for the copolymerisation of styrenic viologen 1 2+ , a broad range of molar masses between 3.7-500 kDa becomes accessible. Our mechanistic investigations elucidated that the redox behavior of monomer 1 2+ is dominated by either CB[n]-mediated stabilisation of monoradical cationic species or protection of the encapsulated pyridinium species from reduction. In the stabilisation regime (c CB [7] < 4), 1 2+ is reduced to form the radical cation 1 + c, which is subsequently stabilised through CB [7] encapsulation. Upon reaching a critical concentration of CB [7] (c CB [7] > 4), the system enters a protectiondominated regime, where reduction of 1 2+ is suppressed and the concentration of 1 + c diminishes. The resulting copolymers can be puried by use of a competitive binder to remove CB[n] macrocycles from the product. This strategy was successfully translated to a structurally different redox-active monomer that suffered similar limitations. We believe that the reported strategy of copolymerisation of redox-active monomers will open new avenues in the synthesis of functional materials for energy conversion and storage as well as for applications in electrochromic devices and (nano)electronics.

Data availability
Data for this paper, including NMR, UV-Vis, CV and ITC are available at https://doi.org/10.17863/CAM.85780.

Conflicts of interest
There are no conicts to declare.