A versatile strategy for the synthesis of block copolymers bearing a photocleavable junction

Abstract

Various block copolymers in which the blocks are held together by a photocleavable junction have been synthesized via a one-pot simultaneous ATRP–CuAAC “click” reaction process, and their easy photocleavage has been demonstrated.

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Block copolymers are one of the most important classes of synthetic materials used for creating nanostructures.¹ Indeed, block copolymers have the ability to self-assemble into various nanostructures both in bulk and in solution (e.g. micelles, vesicles). Part of this interest stems from the fact that these materials can be used as precursors for generating hollow structures. Prominent examples are hollow micelles² (also called nanocages) and nanoporous polymeric materials.³ The underlying strategy for obtaining these hollow nanostructures consists of the self-assembly of the block copolymer into the desired structures (micelles or phase separated copolymer in the solid state) followed by the removal of one of the blocks of the copolymer . This second step is usually achieved by degrading the sacrificial block, often under harsh conditions. An alternative strategy is based on supramolecular interactions, such as hydrogen bonding or metal–ligand complexes, which are used to hold the two blocks together. The creation of the hollow structures then only requires the breaking of “weak” interactions to release the sacrificial block.^4–6 A drawback of this strategy, especially in the case of hydrogen bonds, is the rather limited conditions under which self-assembly can be performed since the supramolecular interactions have to be preserved. A last approach relies on block copolymers bearing a cleavable covalent junction between the blocks.^7–12 Despite the rather limited number of reported examples, this approach is highly promising since it combines the advantages of both methods discussed above. The conditions for performing self-assembly are not restricted since there are no weak interactions involved, and the sacrificial block can be removed under mild conditions by selectively addressing the junction between the blocks, typically by using either a chemical stimulus (pH, redox) or photoirradiation.

Here, we report on a versatile strategy to synthesize diblock copolymers where the two blocks are held together by a photocleavable covalent junction. Using light as stimulus offers two main advantages: it is highly selective and it does not require the reagents to diffuse to the cleavable junction as is the case for a chemical stimulus. Very few examples of such photocleavable block copolymers have been reported up to now.^7,9 Moreover, they were rather limited in the number of accessible chemically different copolymers , or required long irradiation time with intense light to cleave the junction.^7,9 Our approach, depicted in Scheme 1, is based on a one-pot simultaneous atom transfer radical polymerization (ATRP)–copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction, allowing for the synthesis of a wide range of different copolymers . Both ATRP and CuAAC, one type of “click” reaction, are indeed now well established as powerful synthetic tools in materials science.^13,14 Moreover, the junction between the blocks contains an o-nitrobenzyl ester derivative, an easily photocleavable group well-known in organic chemistry and peptide synthesis.¹⁵ Finally, to the best of our knowledge, only one example of a one-pot simultaneous ATRP–CuAAC “click” reaction has been previously reported.¹⁶

Scheme 1 A) General strategy for the synthesis of block copolymers with a photocleavable junction between the blocks by one-pot simultaneous ATRP–CuAAC “click” reaction procedures. B) Products obtained after photoirradiation of such copolymers .

The core of our strategy is an ATRP initiator which bears the o-nitrobenzyl ester photocleavable junction and the alkyne group for the CuAAC “click” reaction. This initiator was synthesized in two steps from the commercially available diol1 (Scheme 2).

Scheme 2 Synthesis of the functionalized ATRP initiator 3.

To demonstrate the versatility of our approach several diblock copolymers with various chemical structures and compositions were synthesized. The first step is the preparation of the azide-functionalized blocks (see ESI† ). Two types of polymer were used as first block, polystyrene (PS) and poly(ethylene oxide) (PEO). Two PSs were synthesized by ATRP, and the terminal bromide was substituted by an azide by reaction with sodium azide. The terminal hydroxyl group of a commercially available PEO was converted into an azide moiety in two steps by first performing a tosylation and then the substitution with sodium azide. The characteristics of the different blocks are summarized in Table 1.

Table 1 Synthesis of the different azide-functionalized first blocks and of the block copolymers bearing the photocleavable junction

Polymer ^a	M n, SEC (g mol^−1)	PDI
a The numbers in subscript are the average degrees of polymerization determined from SEC and/or ^1H NMR. hν symbolizes the photocleavable o-nitrobenzyl ester junction.
PEO113–N3	5 000	1.04
PS52–N3	5 400	1.08
PS97–N3	10 100	1.06
PEO113-hν-PS119	25 300	1.14
PEO113-hν-PS133	30 100	1.15
PEO113-hν-PS176	37 700	1.17
PEO113-hν-PtBA305	63 100	1.15
PS52-hν-PMMA278	37 300	1.25
PS97-hν-PMMA33	15 700	1.12
PS97-hν-PMMA100	24 200	1.10

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The obtained azide-functionalized blocks were then engaged in the one-pot simultaneous ATRP–CuAAC “click” reaction procedure to synthesize several block copolymers , as summarized in Table 1. The one-pot reactions were performed with CuBr and PMDETA or Me₆-TREN ligands as catalytic system for both the ATRP and the CuAAC “click” reaction, see Scheme 3 for a typical example. The click reaction was very efficient (yield higher than about 85% as estimated by ¹H NMR by comparing the polymer signals, chain ends and/or main chain, and the signals of the junction, triazole and/or –CH₂– group next to the triazole) even in the case of the (meth)acrylate monomers for which short polymerization times (2 h) were used. The small amounts of residual homopolymers, i.e., non-coupled blocks, were easily extracted by washing with selective solvents of the blocks to be removed. The formation of block copolymers with narrow polydispersity index (PDI) was evidenced by SEC which shows a clear shift towards higher molecular weight (Fig. 1).

Scheme 3 Example of experimental conditions for the one-pot simultaneous ATRP–CuAAC “click” reaction.

Fig. 1 SEC traces of the starting azide-functionalized PEO block, of the PEO-hν-PScopolymer , and of this copolymer after irradiation at 300 nm for 15 min.

The next step was to demonstrate the photocleavage of the obtained block copolymers by UV irradiation. Solutions of the different block copolymers in dichloromethane were irradiated with a UV lamp having its maximum emission at 300 nm. The evolution of the absorption spectrum with the irradiation time was similar for all copolymers , and is illustrated in Fig. 2 for the PEO₁₁₃-hν-PS₁₇₆copolymer . The intensity of the band located at 308 nm, associated to the nitro-aromatic moiety, decreases with the irradiation time to reach a minimum after 15 minutes, while additional bands appear between 350 and 500 nm due to the formation of the nitroso compound (see Scheme 1). To further prove the quantitative cleavage of the copolymer into its two constituent blocks, the products obtained after irradiation were analyzed by SEC. Fig. 1 shows the SEC traces of the PEO₁₁₃-hν-PS₁₇₆copolymer before and after photoirradiation and of the starting azide functionalized PEO block as reference. After irradiation, the peak associated to the copolymer is no longer observed, but two new peaks are visible on the chromatogram, corresponding to the dissociated PS and PEO blocks. The difference in intensity between the two peaks originates from the block copolymer composition, PS being the majority block. This was ascertained by a SEC analysis of an equimolar mixture of a PEO 5 kg/mol and a PS 19 kg/mol. These results clearly indicate that the nitrobenzyl ester junction holding the two blocks together can be efficiently cleaved under mild conditions.

Fig. 2 Evolution of the UV-vis spectra of the PEO₁₁₃-hν-PS₁₇₆copolymer according to the irradiation time at 300 nm.

In summary, we have developed an easy and versatile strategy to prepare block copolymers where the blocks are held together by a photocleavable junction. The strategy is based on a one-pot simultaneous ATRP-CuAAC “click” reaction and relies on a functional initiator bearing the photocleavable moiety and an alkyne as anchoring group for the CuAAC “click” reaction. Thus, in one step, the second block is grown by ATRP and clicked to the azide-functionalized first block. By this approach, a large variety of block copolymers can be prepared thanks to the vast number of polymers accessible by ATRP, and to the easy introduction of the azide moiety as a chain end on many polymers. The photocleavage of the copolymers occurs under mild conditions and is quantitative. Moreover, both blocks bear a functional group after photocleavage (a carboxylic acid and an aldehyde, see Scheme 1) and this gives the opportunity to perform further chemistry. As an example, these functional groups could be used to functionalize the hollow structures (nanocages or nanoporous films) formed from these copolymers .

Acknowledgements

The authors thank the ESF program STIPOMAT for financial support. CAF is Research Associate of the FRS-FNRS. JMS thanks the FRIA for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details on the synthesis and characterization of the building blocks and of the copolymers . See DOI: 10.1039/b9py00218a

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