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Living supramolecular polymerization based on reversible deactivation of a monomer by using a ‘dummy’ monomer† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c9sc02151e

New method of living supramolecular polymerization is demonstrated. Spontaneous nucleation of a reactive monomer is suppressed by using a ‘dummy’ monomer. Addition of seeds can initiate supramolecular polymerization in a chain-growth manner.


Introduction
Most supramolecular polymerizations occur spontaneously with a negligible activation barrier for initiation. In other words, a monomer is highly "reactive" toward polymerization. Thus, controlling supramolecular polymerization has remained a signicant challenge. 1 On the other hand, a similar issue for covalent bond-based polymerization has been successfully resolved even for radical species, by taming radical polymerization into living radical polymerization (LRP). Though there are several approaches for LRP, almost all the current methods are based on a common principle, in which a reversible and dynamic equilibrium generating a so-called dormant state is coupled with the polymerization reaction (Fig. 1a). 2,3 The coupled equilibrium decreases the effective concentration of the reactive species (i.e., the propagation sites), thereby allowing for kinetic control of the concurrent polymerization.
In the last decade, mechanistic insights into supramolecular polymerization have been signicantly enriched, 1,4-11 and have recently led to the development of living supramolecular polymerization (LSP). [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] It is intriguing that both LRP and LSP rely on reversible and dynamic equilibrium coupled with the polymerization pathway. So far, the formation of metastable aggregates (Fig. 1b) [12][13][14][15][16][17] and intramonomer hydrogen bonds (Fig. 1c), [18][19][20][21][22][23] and photoisomerization (Fig. 1d) 24 have been shown to be effective in generating the dormant states in LSP. In this context, the difference between LRP and LSP is also noteworthy. In LRP, the reactive polymer terminus is deactivated (or "capped") in the dormant state, while in LSP, the spontaneous nucleation of monomers is kinetically suppressed, because of which the monomer itself needs to be deactivated. Inevitably, the monomer design is crucial for achieving LSP, which has limited the scope of the existing methods of LSP for synthesis of other functional supramolecular polymers. In fact, it has been shown that a subtle structural modication in the monomer inuences the energy landscape of the self-assembly and oen disables the monomer for participating in the LSP. 13,20,31 Herein, we show a new concept that would be more tolerant in terms of the monomer design, in comparison with the other methods reported so far. In the present approach, we use a 'dummy' monomer that is incapable of one-dimensional (1D) supramolecular polymerization but can entrap a "reactive" monomer into the dormant state owing to the structural similarity (Fig. 1e).

Impact of monomer design on supramolecular polymerization
The structures of the porphyrin monomers used in this study are shown in Fig. 2. As reported previously, 3Me can be processed in a manner consistent with LSP via the formation of offpathway aggregates as the dormant species (Fig. 1b). 12 3Me initially forms a metastable off-pathway aggregate but transforms into a thermodynamically stable supramolecular polymer aer a lag time (details are given in Fig. S2 †). These two aggregates have different porphyrin stacking modes: the former is the J-aggregate while the latter is the H-aggregate. This timedependent transformation allowed for kinetic control of supramolecular polymerization. It should be noted that in practice, a lag time of about an hour is required to perform LSP conveniently.
Despite the structural similarity, the self-assembly of 2Me was distinct from that of 3Me. Cooling a hot methylcyclohexane (MCH) solution of 2Me did not form the metastable J-aggregate but immediately resulted in heavily bundled supramolecular polymers as precipitates ( Fig. 3a-c). The difference between the self-assembly of 2Me and 3Me is attributable to the even-odd effect of the alkylene spacers. 32,33 We note that Würthner and coworkers 20 have previously found the length of the alkylene spacers to inuence the LSP in a different mechanism (Fig. 1c). These results clearly exemplify the criticality of the monomer design in achieving LSP.
To further gain insights into the supramolecular polymerization of 2Me, we studied the temperature-dependent changes in its absorption spectra (Fig. 3d). Upon cooling (À1.0 K min À1 ), the Soret band underwent a red-shi with the concomitant appearance of isosbestic points (373 to 318 K), thus suggesting the formation of J-aggregates. 12 This process was analyzed by the isodesmic model, 1 which yielded thermodynamic parameters comparable with those of the J-aggregate formation of 3Me ( Fig. S3 †). However, further cooling of the solution below 318 K resulted in the appearance of a blue-shied Soret band, suggesting the formation of H-aggregates. 12,13 This process was accompanied by a concomitant increase in the base line due to light scattering by the precipitates, thereby leading to a deviation of the isosbestic points. The complex aggregation behavior as well as the precipitation hindered further mechanistic studies to estimate the thermodynamic parameters.
The turnaround temperature at which the J-aggregates began converting to H-aggregates was dependent on the cooling rate (Fig. 3e). This behavior indicates that the formation of the Haggregate of 2Me is under kinetic control to a certain extent. However, even when a hot solution of 2Me was quenched in an ice bath, spontaneous nucleation of the H-aggregate could not be prevented. Due to the absence of a lag time, the LSP of 2Me was not feasible (Fig. S4 †).
Kinetic trapping of 2Me by using a dummy monomer, 3Ph To address this issue, we conceived a solution based on our previous ndings regarding a mixture of 3Me and 3Ph. 31 3Ph has a similar structure to 3Me, and likewise, forms a Jaggregate; however, the steric hindrance of the side chains  prevents the face-to-face stacking of the porphyrin plane (i.e., Haggregate supramolecular polymerization). When a hot solution of 3Me and 3Ph in MCH was cooled down, 3Me and 3Ph coassembled into a J-aggregate. We found that this coassembled J-aggregate was metastable, and the system showed time-dependent narcissistic self-sorting to the H-aggregates of 3Me and J-aggregates of 3Ph aer a lag time. 31,[34][35][36] Interestingly, this lag time was dependent on the ratio of 3Me to 3Ph; the smaller the ratio, the longer was the lag time. Considering this, we envisaged that if 2Me and 3Ph could co-assemble to form a Jaggregate owing to the structural similarity, the spontaneous nucleation of the 2Me H-aggregate would be retarded, thus enabling kinetic control of the supramolecular polymerization of 2Me.
To verify this concept, we studied the temperaturedependent changes in the absorption spectra of 2Me in the presence of 3Ph (Fig. 4a, molar ratio of 2Me : 3Ph ¼ 50 : 50). As shown in Fig. 4b, the formation of J-aggregates followed a sigmoidal cooling curve down to room temperature. As such, 2Me was trapped in the J-aggregate in the presence of the 'dummy' monomer, 3Ph (Fig. S5 †). We infer that the J-aggregate is not crystalline and is tolerant to co-assembly with monomers having different structures. 35 As in the case of the 3Me/3Ph system, 31 the J-aggregate consisting of 2Me and 3Ph was also metastable and showed time-dependent spectral changes (Fig. 4c). When we xed the total concentration of the porphyrins but changed their mixing ratio, the net changes in absorbance with time (Abs t¼0 À Abs t ) were dependent on the proportion of 2Me (Fig. 4d). This result suggests that 3Ph was not involved in the transformation and remained as a J-aggregate, namely, time-dependent narcissistic self-sorting (Fig. S6 †). Furthermore, a larger proportion of 2Me resulted in a shorter lag time. These results are consistent with those of our previous study. 31 The resultant lag time (49 min for 2Me : 3Ph ¼ 70 : 30) was shorter than that of the 3Me/3Ph mixture (555 min for 3Me : 3Ph ¼ 70 : 30), because of the higher nucleation propensity of 2Me as compared with 3Me; nonetheless, it was long enough for the LSP to proceed further.

Living supramolecular polymerization of 2Me
As an initiator for LSP, we prepared seeds of the supramolecular polymer (i.e., H-aggregate) of 2Me by sonication, with a number average length (L n ) of 134 nm (details are given in the ESI, see Fig. S7 †). The addition of the seeds to a solution of coassembled 2Me/3Ph J-aggregates (50 : 50 molar ratio) (e) Plots of the change in absorbance of 2Me at 436 nm as a function of temperature obtained with different cooling rates. The temperature was changed from 373 to 283 K at a rate of (i) À10 K min À1 , (ii) À1 K min À1 , and (iii) À0.1 K min À1 . eliminated the lag time and initiated the supramolecular polymerization (Fig. 5a). The logarithm of the apparent polymerization rate, log(Àd(Abs 558 )/dt), and the amount of the added seed were proportional with a slope of 0.97, thus indicating a rst-order polymerization with respect to the concentration of the seed (Fig. 5b). 12 With a xed amount of the seeds, the net changes in absorbance with time (Abs t¼0 À Abs t ) were proportional to the proportion of 2Me in the co-assembled 2Me/3Ph J-aggregates, which indicated that only 2Me was involved in the LSP (Fig. 5c and d). Correspondingly, the polymerization rate (Àd(Abs 558 )/dt) was also dependent on the proportion of 2Me (Fig. 5d, red circle). In this way, the supramolecular polymerization of 2Me, which was otherwise spontaneous, could be kinetically controlled in the presence of the 'dummy' monomer, 3Ph. Importantly, this seeded supramolecular polymerization process was repeatable as in the case of our previous studies (Fig. S8 †). 12,13 Aer the LSP, we obtained a clear solution without any precipitate (Fig. S9 †). This was spin-coated on a highly oriented pyrolytic graphite (HOPG). Observation under atomic force microscopy (AFM) revealed the presence of both 1D supramolecular polymers and J-aggregate nanoparticles, consisting of 2Me and 3Ph, respectively (Fig. 6a). Statistical analysis for over 100 objects revealed that the number average length (L n ) and weight average length (L w ) of the obtained supramolecular polymers were proportional to the ratio of the concentrations of 2Me in the seed and 2Me in the co-assembled J-aggregate ( Fig. 6b and c, and S10 and S11 †). The polydispersity index (PDI: L w /L n ), the distribution of the length, of the thus-obtained supramolecular polymer of 2Me was 1.36, which was much smaller than the value predicted based on a thermodynamic equilibrium (2.0). 1,12 Remarkably, supramolecular polymers obtained in this way did not bundle, which clearly contrasts the structures obtained under thermodynamic control (Fig. 3c). This observation would imply that the strong propensity of precipitation of 2Me (Fig. 3a-c) originates from heterogeneous secondary nucleation through lateral interactions among the one-dimensional supramolecular polymers. 37 Thus, LSP allows control over the nucleation-elongation process, leading to   control over structures and properties (such as crystallinity) of supramolecular polymers at higher hierarchy.

Conclusions
In conclusion, we demonstrated a new LSP method for a monomer that spontaneously nucleated without a lag time. We used a 'dummy' monomer that was incapable of 1D supramolecular polymerization but could entrap an active monomer into the dormant state. We note that 2Me is still a unique monomer in that it undergoes biphasic self-assembly (Fig. 3d). 10,11,31 Nevertheless, we believe that the present study shows a new possibility of creating a dormant state based on coupled equilibria, which is the key to expanding the scope of LSP and its further applications.

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