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Supramolecular encapsulation of redox-active monomers to enable free-radical polymerisation
Authors:Stefan Mommer  Kamil Soko&#x;owski  Magdalena Olesi&#x;ska  Zehuan Huang  Oren A Scherman
Institution:Melville Laboratory for Polymer Synthesis, Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW UK,
Abstract: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 cucurbitn]uril (CBn]) macrocycles. The presented strategy relies on pre-encapsulation of the viologen monomers within the molecular cavities of the CBn] 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 CBn]-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.

Extended polymeric structures based on redox-active species are of great interest in emerging technologies related to energy conversion and storage.

Polyviologens are redox-active polymers based on N-substituted bipyridinium derivatives which have emerged as promising materials for energy conversion and storage.1–5 Their physicochemical properties can be adjusted through copolymerisation of the redox-active viologen monomers.6–8 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,9–13 as supramolecular polymers,14–16 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 oxygen-sensitive.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.23–25 Accessing higher molar masses has been possible via post-polymerisation modification,26–28 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 cucurbitn]uril (CBn]) macrocycles have lead to a variety of functional materials through host–guest chemistry.31–34 Moreover, the redox chemistry of viologens can be modulated through complexation with CBn].35–38 Specifically, CBn] (n = 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 −20 kcal mol−1 when encapsulated in CB7].39Consequently, we envisioned that incorporating CBn]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 CBn], co-polymerisation of the N-styryl-N′-phenyl viologen monomer 12+ and N,N-dimethylacrylamide (DMAAm) only occurs at high initiator loadings (>0.5 mol%, Fig. 1a), leading to low molecular weights and high polydispersity. Using our synthetic approach, 12+ is efficiently copolymerised with DMAAm in the presence of CBn] (n = 7, 8) macrocycles resulting in control of the polymer molar mass across a broad range, 4–500 kDa (Fig. 1b). Finally, CBn] 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.Open in a separate windowFig. 1Schematic representation of the investigated polymerisation. (a) Conventional free radical polymerisation either completely fails to copolymerise redox-active monomers (low initiator loading) or delivers copolymers with limited molar masses and high dispersities (high initiator loading). (b) CBn]-mediated protection suppresses interference of viologen monomers with radicals formed through the initiation process facilitating copolymerisation. The molar mass of the resulting copolymers is readily tunable via the amount of present CBn] macrocycles and the CBn] is post-synthetically removed via competitive binding to yield the final copolymer with desired molar mass. Cl counter-ions are omitted for clarity.Recent studies on symmetric aryl viologens demonstrated 2 : 2 binding modes with CB8] and high binding constants (up to Ka ∼ 1011 M−2).40,41 Incorporation of polymerisable vinyl moieties, in combination with the relatively static structure of their CBn] host–guest complexes, was postulated to allow polymerisation without unfavorable side reactions. The asymmetric N-styryl-N′-phenyl viologen monomer 12+ prepared for this study (Fig. S1a and S2–S13) displays a linear geometry and was predicted to bind CBn] (n = 7, 8) in a 2 : 1 and 2 : 2 binding fashion (Fig. S1b).40,42 Binding modes between CBn] (n = 7, 8) and 12+ were investigated through titration experiments (1H NMR and ITC) which confirmed the formation of 1·(CB7])2 and (1)2·(CB8])2 (see Fig. S25 and S26). 1H NMR titration of CB7] with 12+ demonstrates encapsulation of both aryl moieties (including the vinyl group) through upfield chemical shifts of the respective signals (Fig. 2a). Similar upfield shifts were observed for CB8] (Fig. 2c). Different para-aryl substituents (vinyl vs. hydrogen) resulted in either head-to-tail or head-to-head (1)2·(CB8])2 dimers (Fig. S1b and S26), a previously reported phenomenon.43 Nonetheless, the reversible nature of the complex renders the vinyl group temporarily available for copolymerisation. In the presence of CB8], 12+ yields polymer molar masses of up to 500 kDa as its complexation is more robust. ITC data confirmed binding stoichiometry, with binding constants of Ka = 2.64 × 106 M−1 for 1·(CB7])2 and Ka = 9.02 × 1010 M−2 for (1)2·(CB8])2 (Table S2, Fig. S29a and b).Open in a separate windowFig. 2Supramolecular complexation of 12+ and CBn]. 1H NMR spectra of 12+ at (a) χCB7] = 2, (b) χCBn] = 0 and (c) χCB8] = 1 in D2O. Cl counter-ions are omitted for clarity.The free radical copolymerisation of 12+ and DMAAm (M] = 2 M), in the absence of CBn], was based on optimised DMAAm homopolymerisations (Fig. S14 and S15) and full conversion was confirmed by 1H NMR spectroscopy (Table S1 and Fig. S16). 12+ was maintained at 1 mol% relative to DMAAm and by varying the radical initiator concentration molar masses of up to 30 kDa with broad dispersities (Đ = 11.4) were obtained (Fig. S17). Lower initiator concentrations (<0.25 mol%) limited polymerisation (Mn = 3.7 kDa) and size exclusion chromatography elution peaks exhibited extensive tailing, suggesting that 12+ engages in radical transfer processes.To verify our hypothesis that CBn] macrocycles can modulate the redox behavior of 12+, FRP of 12+ and DMAAm was conducted with varying amounts of CBn] (n = 7, 8) (Fig. 3, S18 and S20). Full conversion of all monomers including their successful incorporation into the polymer was verified via1H NMR spectroscopy and SEC (Fig. S18 and S21–S23). Using CB7], the molar mass of the copolymers was tunable between Mn = 3.7–160 kDa (Fig. 3b and S21a). Importantly, in the presence of CB8], a broad range of molar masses Mn = 3.7–500 kDa were accessible for 0 < χCB8] < 1.2 (Fig. S20 and S21b). Increasing the CBn] (n = 7, 8) concentration caused dispersity values to converge to Đ = 2.2 (χCB8] = 1.2, χ is the ratio of CBn] to the redox-active monomer, Fig. S20). The copolymers were purified by addition of adamantylamine (competitive binder) prior to dialysis to deliver CBn]-free redox-active copolymers (Fig. S23).Open in a separate windowFig. 3(a) In situ copolymerisation of DMAAm with 12+ and CB7]. (b) Molar mass and dispersity vs. amount of CB7] in the system. Fitted curve is drawn to guide the eye. Cl counter-ions are omitted for clarity.The range of molar masses obtainable through addition of CBn] (n = 7, 8) correlated with the measured Ka (Fig. 3b and S20). Binding of 12+ to CB8] was stronger and therefore lower concentrations of CB8] were required to shift the binding equilibrium and mitigate disruption of the polymerisation. Dispersity values reached a maximum at χCB7] = 0.6 or χCB8] = 0.3, suggesting 1+˙ is only partially encapsulated. Consequently, higher CBn] 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 12+.The redox-active monomer 12+ can engage with propagating primary radicals (P) to either be incorporated into the growing polymer chain (Pm–12+˙) or to abstract an electron deactivating it (Pm). This deactivation likely occurs through oxidative termination producing 1+˙ (energetic sink), inactive oligo- and/or polymer chains (Pm) 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.44–47 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).48The process of retardation can, however, be successfully suppressed, when monomer 12+ is encapsulated within CBn] macrocycles. Herein the formation of 1·(CB7])2 or (1)2·(CB8])2 results in shielding of the redox-active component of 12+ 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+˙ through supramolecular encapsulation minimises both π and σ dimerisation of the emerging viologen radical species,39 preventing any further reactions that could impact the molar mass or polydispersity of the resulting polymers.Cyclic voltammetry (CV) and UV-Vis titration experiments were conducted to provide insight into the impact of CBn] on the redox behavior and control over FRP of 12+. Excess of CBn] (n = 7, 8) towards 12+ resulted in a complete suppression of electron transfer processes (Fig. S31 and S32). Initially, 12+ shows a quasi-reversible reduction wave at −0.44 V forming 1+˙ (Fig. 4a). Increasing χCB7], this reduction peak decreases and shifts towards more negative potentials (−0.51 V, χCB7] = 1) accompanied by the formation of 12+·(CB7])1. A second cathodic peak emerges at −0.75 V due to the increased formation of 12+·(CB7])2. At χCB7] = 2, this peak shifts to −0.80 V, where it reaches maximum intensity, once 12+·(CB7])2 is the dominating species in solution. When 2 < χCB7] < 4, the intensity of the reduction peak decreases and the complexation equilibrium is shifted towards the bound state, complete suppression of the reduction peak occurs at χCB7] = 4. Similarly, the oxidation wave intensity is reduced by 95% at χCB7] = 4 causing suppression of potential oxidative radical transfer processes (Fig. 4c).Open in a separate windowFig. 4Mechanism of the CBn]-mediated (n = 7, 8) strategy for the controlled copolymerisation of redox-active monomer 12+. (a) Cyclic voltammogram with varying amounts of CB7]. (b) UV-Vis titration of 12+ with varying amounts of CB7]. (c) Intensity decay of the oxidation peak at −0.27 V and change in absorption maximum of 1+˙ at 590 nm vs. χCB7]. (d) Electron transfer processes of 12+ to generate 1+˙ and 10. (e) Reduction of 12+ resulting in precipitation of 10. (f) Stabilisation of 1+˙ through encapsulation with CB7]. (g) Protection of 12+ from redox processes through CB7]-mediated encapsulation.The concentration of 1+˙ can be monitored using UV-Vis (Fig. 4b and S34).49 Absorbance at 590 nm (λmax) vs. χCB7] was plotted and the concentration of 1+˙ increases, reaching a maximum at χCB7] = 4 (Fig. 4c). When χCB7] > 4, a decrease in concentration of 1+˙ was observed. We postulate the following mechanism: at χCB7] = 0, 12+ is reduced to produce high concentrations of 1+˙ that partially disproportionates to form 10, which precipitates (Fig. 4e and S34). When 0 < χCB7] < 4, increasing amounts of green 1+˙ are stabilised through encapsulation within CB7] suppressing disproportionation (Fig. 4c (cuvette pictures), Fig. 4f). For χCB7] > 4, 12+ is protected from reduction through encapsulation (Fig. 4g).To further demonstrate applicability of this strategy, we chose another viologen-based monomer 22+ for copolymerisation (Fig. 5a). As opposed to 12+, CB binds predominantly to the styryl moiety of 22+ (Fig. S27 and S28).50 ITC data showed that 22+ binds CB7] in a 1 : 1 fashion with a binding affinity of Ka = 2.32 × 106 M−1 (Fig. S30 and Table S2). Monomer 22+ was also analysed via CV and showed three reversible reduction waves at −0.91 V, −0.61 V (viologen) and 0.40 V (styrene). Similar to 12+, excess CB7] selectively protects the molecule from redox processes, while the vinyl moiety remains accessible (Fig. 5a, S33c and d). For CB8], only partial suppression of electron transfer processes was observed (Fig. S33e and f). We therefore chose CB7] as an additive to increase control over FRP of 22+ (Fig. 5b). Copolymerisation of 22+ (1 mol%) and DMAAm (M] = 2 M) at χCB7] = 0 resulted in Mn = 28 kDa. When χCB7] = 0.1, 0.2 or 0.3, Mn increased gradually from 124 to 230 and 313 kDa, respectively, demonstrating the potential of this strategy for FRP of redox-active monomers. Higher percentages of CB7] led to copolymers with presumably higher molar masses causing a drastic decrease in solubility that prevented further analysis. Investigations on a broader spectrum of such copolymers, including those with higher contents of 22+ are currently ongoing.Open in a separate windowFig. 5(a) Cyclic voltammogram of viologen-containing monomer 22+ and its complexation with CBn] (n = 7, 8) at a concentration of 1 mM using a scan rate of 10 mV s−1 in 0.1 mM NaCl solution. (b) Molar mass and dispersity of 22+-containing copolymers vs. equivalents of CB7]. Cl counter-ions are omitted for clarity.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 CBn] macrocycles (n = 7, 8) for the copolymerisation of styrenic viologen 12+, a broad range of molar masses between 3.7–500 kDa becomes accessible. Our mechanistic investigations elucidated that the redox behavior of monomer 12+ is dominated by either CBn]-mediated stabilisation of monoradical cationic species or protection of the encapsulated pyridinium species from reduction. In the stabilisation regime (χCB7] < 4), 12+ is reduced to form the radical cation 1+˙, which is subsequently stabilised through CB7] encapsulation. Upon reaching a critical concentration of CB7] (χCB7] > 4), the system enters a protection-dominated regime, where reduction of 12+ is suppressed and the concentration of 1+˙ diminishes. The resulting copolymers can be purified by use of a competitive binder to remove CBn] 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.
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