SUPRAMOLECULAR CHEMISTRY

Living supramolecular polymerization Greater control is achieved over the chain growth and properties of dynamic materials

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ost polymers that we encounter, like those used in grocery bags and soda bottles, are formed from small molecules (monomers) held together by strong covalent bonds. In supramolecular polymerization, monomers bond through weaker reversible interactions, such as hydrogen bonds (H-bonds). Supramolecular polymerization usually proceeds through step-growth mechanisms (1), where both ends of each monomer are reactive and many smaller oligomers form before long polymers appear. To overcome this problem, a method developed for covalent polymers known as living polymerization has been adopted for supramolecular polymers to achieve better control and uniformity of chain growth and dispersity. Living polymerization is a type of chaingrowth polymerization in which monomers undergo polymerization only upon reacting with an initiator to generate an active center. The active site regenerates with each monomer addition; it propagates along the polymer strand before transferring the active center to another polymer strand or terminating via mutual coupling. In fact, the last two steps of chain transfer or termination are essentially removed in a living polymerization process. The last monomer unit on a polymeric strand remains active until deliberately terminated, so adding more monomer—or a different monomer—resumes the reaction. Properties like the degree of polymerization (number of monomers in the chain), the chain conformation, and its lifetime (of the propagating chain) can therefore be efficiently controlled if a chain-growth polymerization is realized in such dynamic supramolecular systems (2). An early attempt on controlled supramolecular polymerization was made by Manners, Winnick, and co-workers (3) by assembling polyferrocenyldimethylsilane block copolymers in hydrocarbon solvents. Addition of a fresh feed of the polymer in a good solvent like tetrahydrofuran caused the nanosized cylindrical micelle seeds to grow up to micrometers in length. Very recently, they have

1

Chemical Sciences and Technology Division, CSIR–National Institute for Interdisciplinary Science and Technology (CSIRNIIST), Trivandrum 695019, India. 2Academy of Scientifc and Innovative Research (AcSIR), CSIR-NIIST, Trivandrum 695019, India. E-mail: [email protected]

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By Rahul Dev Mukhopadhyay1,2 and Ayyappanpillai Ajayaghosh1,2

N F

NH N B N F F

5 μm

C

B

Amide group Porphyrin Alkyl chains

Monomer reservoir 1J-agg

1Mono(in hot solvent) Step 1 Cooling

+

Step 3 Heating

Pre-assembled seed

Step-growth pathway

1H-agg(aliquot) 1J-agg(nanoparticle) Step 5 A few hours

Step 2 Several days Cooperative pathway

Seeded polymerization Step 4 Heating

1H-agg

1H-agg(nanofber) Formation of micelles and nanofibers. (A) Laser scanning confocal microscopy image (scale bar, 5 µm) of selfassembled polyferrocenyldimethylsilane block copolymer micelles functionalized with fluorescence tunable BODIPY (boron-dipyrromethene) derivatives. Molecular structures responsible for different emission colors are represented; n-Bu, n-butyl (6). (B) Supramolecular seeded growth of nanofibers from molecule 1 proceeds slowly after first forming nanoparticles, but when these nanoparticles interact with existing nanofibers (C), the process is much faster (7).

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INSIGHTS | P E R S P E C T I V E S

prepared hierarchical [one-dimensional (1D) or 3D] multiblock comicelle structures by addition of different block copolymers to the preformed micellar seeds, which were stable both in solution and in solid state (4). “Living crystallization”–driven block copolymer self-assembly was also used to prepare functionalized block copolymers (see the first figure, panel A) with segments of different emission colors (5, 6). Later, Sugiyasu, Takeuchi, and co-workers showed that systems undergoing self-assembly following a nucleation–elongation mechanism coupled with a kinetically controlled pre-equilibrium process results in living supramolecular polymers (7). These authors observed that a porphyrin-based molecule initially forms that “wrong product,” the for-

A

nanofibers of the organogelator as seeds. Although the above reports have taken us a few steps closer to the concept of living supramolecular polymerization, unimolecular control on chain growth remained elusive until recently. Miyajima, Aida, and co-workers (9) reported an interesting property of a nonplanar bowl-shaped corannulene molecule appended with five amide-functionalized thio alkyl chains (see the second figure, panel A). This C5-symmetric molecule did not undergo self-assembly via intermolecular H-bonding in methylcyclohexane (MCH) because the formation of intramolecular H-bonds between the five amide chains is more conformationally feasible. However, upon heating, this metastable cagelike monomer opened up to undergo a sponta-

When chiral initiators were used, polymerization occurred only when the configuration of the stereogenic center of the monomer matched with that of the initiator. The monomer (M) with achiral side chains consisted of an asymmetric center at the corannulene core that formed a racemic mixture through a bowl-to-bowl inversion process. The chiral initiators (I) could differentiate between the enantiomers of M and undergo polymerization with selective handedness. This property was used to optically resolve a racemic mixture of monomers MR and MS by using either IR or IS as an initiator (see the second figure, panel B). Supramolecular living polymerization is a step closer to the precision synthesis of complex architectures and presents us

Living polymerization without termination: I--M--M--M--M--M--M--M--M--M--M--M--M H-bond acceptor

H-bond acceptor

H-bond donor

Monomer (M) Initiator (I)

B

1:1 I-M complex

Concept of optical resolution IR I: S MS

MR

IR: S IS: S

mation of so-called “J-aggregates” that lead to metastable organic nanoparticles. After several days, these nanoparticles transformed into stable “H-aggregates” with nanofiberlike morphology via a cooperative mechanism (see the first figure, panel B). When an aliquot of the nanofibers was added to a solution of the nanoparticles, which serve as a reservoir for the porphyrin monomers, nanofibers formed much more rapidly (within a few hours) through a living polymerization process (see the first figure, panel C). Recently, a clever molecular design by Würthner and co-workers (8) allowed a prolonged lag time in the self-assembly process of a perylene bisimide–based organogelator by locking the molecule in an inactive conformation via induced intramolecular H-bonding. A living polymerization process was achieved by introducing preassembled 242

O O

N Me

N Me O N Me

M: S MR: S MS: S

O N H O N H O N H

neous 1D self-assembly. Once such assembly formed, it continued to grow with a fresh supply of the metastable monomer even at low temperatures (9). Further work by Miyajima, Aida, and co-workers used corannulene molecules functionalized with methyl-substituted amides as the initiator of the chain-growth polymerization because these molecules remain in an open conformation in the absence of intramolecular H-bonds and serve as suitable proton acceptors when a caged corannulene molecule approaches (10). Polymerization was initiated only from one face of the monomer and promoted growth only in a particular direction. The resulting polymers were robust enough to remain stable even at a 10-fold dilution, so it was possible to analyze them by size exclusion chromatography.

Unimolecular control of chain growth. (A) Unimolecular control in a supramolecular living chaingrowth polymerization with initiator I and monomer M. (B). Schematic representation of optical resolution process is shown. The different M and I derivatives are represented below; Me, methyl (10). [Figures reproduced with permission from (6), (7), and (10)]

with a broad canvass to work toward newgeneration functional materials. Controlled living polymerization in systems like triarylamines (11) or donor-acceptor molecules remains challenging with respect to the design of organic electronic devices with optimum light-conversion efficiency. ■ REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

T. F. A. de Greef, E. W. Meijer, Nature 453, 171 (2008). O. W. Webster, Science 251, 887 (1991). X. Wang et al., Science 317, 644 (2007). H. Qiu et al., Science 347, 1329 (2015). Z. M. Hudson et al., Nat. Chem. 6, 893 (2014). Z. M. Hudson et al., Nat. Commun. 5, 4372 (2014). S. Og et al., Nat. Chem. 6, 188 (2014). S. Ogi et al., J. Am. Chem. Soc. 137, 3300 (2015). J. Kang et al., J. Am. Chem. Soc. 136, 10640 (2014). J. Kang et al., Science 347, 646 (2015). V. Faramarzi et al., Nat. Chem. 4, 485 (2012).

10.1126/science.aac7422

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17 JULY 2015 • VOL 349 ISSUE 6245

Published by AAAS

Living supramolecular polymerization Rahul Dev Mukhopadhyay and Ayyappanpillai Ajayaghosh Science 349, 241 (2015); DOI: 10.1126/science.aac7422

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