Mechanochemical Post-Polymerization Modification: Solvent-Free

Letter Cite This: ACS Macro Lett. 2018, 7, 561−565

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Mechanochemical Post-Polymerization Modification: Solvent-Free Solid-State Synthesis of Functional Polymers Nuri Ohn and Jeung Gon Kim* Department of Chemistry and Institute of Physical Science, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896 Republic of Korea S Supporting Information *

ABSTRACT: Mechanochemical postpolymerization modification is reported herein. The fast and efficient synthesis of a library of macromolecules with functional diversity and structural uniformity was realized without a solvent by means of a high speed ball-milling technique. A series of polymers prepared from 4-vinylbenzaldehyde (4-VBA) underwent solid-state Schiff base formations with a series of amines and amine derivatives. The efficient mixing and energy delivery provided by the collisions between balls not only promoted rapid imine formation but also eliminated the need for a chemical solvent, which is highly desirable for green chemical synthesis.

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important merit of postpolymerization modification. As a result of the remarkable advances in modern chemical synthesis, the boundaries of the postpolymerization modification have rapidly expanded.23−26 Most polymer modifications are conducted in solution and the solvent-free or solid-state postpolymerization has not been explored.22 Considering the narrow solvent compatibility of many polymeric materials and the green chemical features of reducing the solvent input, we envisioned that the use of a mechanochemical method is an ideal technique for achieving the highly efficient and green functionalization of polymers (Scheme 1). In this report, the mechanochemical postpolyme-

echanochemical synthesis is a chemical transformation induced by means of mechanical force and has a long history in chemical synthesis.1−4 Recently, a group of chemists revisited this area and discovered new opportunities in organic,5,6 inorganic,7 organometallic,8 pharmaceutical,9 and supramolecular chemistry.10 In addition to being an alternative to thermo-, photo-, or electrochemical activation, mechanical force-induced transformations show unexpected reactivity and selectivity that conventional approaches cannot achieve.11 Moreover, the exclusion of solvents fulfills many aspects of green chemistry, such as a high atom economy and reduced environmental effects.12,13 For polymer chemistry, however, mechanochemical force has been regarded as the power of destruction.14−16 Many studies have utilized shearing, stretching, grinding, or ultrasonic treatments to alter or degrade macromolecules, while the opposite approach, mechanochemical polymerization, has had only a limited success. Recently, notable advancements were made that provide a better understanding of mechanochemistry. The research groups of Swager and Bochardt utilized a ballmilling technique to synthesize poly(phenylenevinylene)s,17 poly(azomethien)s,18 and polyphenylenes.19 Our group also accomplished the mechanochemical polymerization of lactic acid to give high molecular weights by overcoming chain degradation.20 The team led by Hernández demonstrated interesting examples of the use of milling as well as twin-screw extrusion for the synthesis of oligopeptides.21 Based on those achievements, we turned our attention to another polymer synthetic method, postpolymerization modification.22 The alteration or functionalization of a polymer after polymerization allows libraries of functional macromolecules with identical topologies and chain lengths to be created, enabling the facile and precise study of structure−property relationships. The installation of chemical groups that are not compatible with the polymerization method is another © XXXX American Chemical Society

Scheme 1. Environmentally Friendly Post-Polymerization Modifications: Mechanochemical Approach

rization modification of aldehyde-containing polymers using ball-milling promoted Schiff base synthesis with various amines is described. A simple procedure constructed the libraries of functional polymers without a use of any solvent or extra reagents, ensuring a high level of green chemistry. Received: March 1, 2018 Accepted: April 17, 2018

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DOI: 10.1021/acsmacrolett.8b00171 ACS Macro Lett. 2018, 7, 561−565

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ACS Macro Letters The condensation of primary amines with aldehydes or ketones is a simple chemical reaction and is generally fast and high yielding with minimal generation of waste (only water). Structural control of the amine moieties could modulate the reversible nature of the condensation reaction, or subsequent reduction could generate an irreversible strong amine linkage. Considering the many desirable features described above, the postpolymerization reaction between aldehyde containing polymers and primary amines has been studied in many applications.27−32 From the perspective of mechanochemistry, the formation of imines is known to be an outstanding example of a mechanochemical transformation.33−36 Thus, we chose the Schiff base formation reaction as a model of mechanochemical postpolymerization modification. According to the protocol of Wooley and co-workers synthesizing 4-vinylbenzaldehyde (4-VBA) and performing its controlled polymerization,27 we synthesized a series of polymers having 4-vinylbenzaldehyde units. The initial study was conducted with a random copolymer composed of styrene (57%) and 4-VBA (43%) (Mn = 6.80 kg/mol, Mw/Mn = 1.23), and benzyl amine (2 equiv relative to aldehyde; Table 1, entry 1). Both the polymer and the amine were weighed into a 10 mL stainless container with three stainless balls of 7 mm diameter. Without any additive, the vessel was sealed and placed into a mixer mill (MM-400, Retsch) and vibrations were applied for 30 min. Comparison between 1H NMR spectra of the aldehyde polymer and the product showed complete imine formation (Figure 1). In infrared spectra, disappearance of the signal at 1698 cm−1 (CO) and formation of a new peak at 1643 cm−1 (CN) support successful Schiff base formation as well. Regardless of the reversible nature of the Schiff base synthesis reaction, a ball-milling condition exhibited high efficiency without a water removal agent or apparatus.37 To add practicality, we tested the use of solid amine derivatives. Liquid amines are prone to oxidation and are inconvenient to handle, especially low-molecular-weight amines. Among the several techniques available for achieving amine solidification, the solid ammonium carbamate salts developed by Lee and co-workers were tested.38 The solid benzyl ammonium carbamate obtained from dry ice and benzyl amine was mixed with the copolymer under high-speed ballmilling conditions and the same level of reactivity was achieved (entry 2). Another primary hexyl ammonium carbamate exhibited good reactivity, giving >99% hexyl group attachment to the polymer (entry 3). Next, a series of sterically demanding amines were applied. A secondary cyclohexyl ammonium carbamate gave complete conversion (entry 4), and isopropyl ammonium carbamate exhibited 92% imine formation (entry 5). Elongation of the reaction time pushed the reaction to >99% conversion (1 h, entry 6). The more-substituted tert-butyl ammonium carbonate showed only 42% installation on the polymer chain (entry 7). The size of the substituent clearly influenced the efficiency. Adamantyl amine (solid) gave a similar conversion of 39% (entry 8). Aromatic amines were also examined (entries 9−12). The electronic effects of the substituents were significant. Electronneutral aniline (entry 9) and electron-donating 4-methoxyaniline (entry 10) exhibited near-quantitative imine formation, while the weakly withdrawing bromine substituent slightly hindered the reaction (94%, entry 11). Strongly electronwithdrawing 4-nitroaniline gave no imine formation due to poor nucleophilicity of the amine (entry 12).

Table 1. Mechanochemical Post-Polymerization Modification of Poly(stryrene-co-4-vinylbenzaldehyde)a

a

Conditions: Polymer P1 (0.10 g, 0.32 mmol aldehyde), amine (2 equiv, 0.64 mmol), or ammonium carbamate salt (1 equiv, 0.32 mmol) in a 10 mL stainless-steel jar with three stainless-steel balls (7 mm in diameter). bDetermined by 1H NMR spectroscopy. cDetermined by GPC calibrated with polystyrene standards in tetrahydrofuran (THF) at 40 °C. dVibration frequency = 20 Hz. eReaction time = 1 h. f Na2CO3 (2 equiv) was added. 562

DOI: 10.1021/acsmacrolett.8b00171 ACS Macro Lett. 2018, 7, 561−565

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ACS Macro Letters Table 2. Mechanochemical Post-Polymerization Modification of Poly(4-vinylbenzaldehyde)a

Figure 1. 1H NMR comparison of the parent polymer and the product after ball-milling with benzyl amine (Table 1, entry 1).

Another type of solidified amine, an ammonium chloride salt, was tested as well. Quantitative postpolymerization modification was achieved, but Na2CO3 was required for neutralization. Other reactive groups that are useful for further functionalization were also installed. Both furfuryl and propargyl amines were attached to the parental polymer chain with high efficiency (entries 14−15). A methoxy amine acid salt showed moderate reactivity (entry 16) while other amine derivatives such as sulfonamide and hydrazine did not react at all with the aldehyde under the given ball-milling conditions. In all cases, expected molecular weight gain was observed with functional group attachment. A poly(4-VBA) homopolymer was synthesized and reacted with selected amines and their derivatives (Table 2). Poly(4VBA) possesses a higher functional group density than the random copolymer of styrene and 4-VBA, resulting in more sensitivity to steric changes. While primary amine derivatives, benzyl ammonium carbamate and β-alanine ethyl ester hydrochloride showed nearly complete reactions under the same conditions (entries 1 and 2), the secondary isopropyl ammonium carbamate showed a slower installation rate. Even after an hour of vibration, only 44% of the aldehyde was converted to imine (entry 3). Anilines were installed with high efficiencies with an hour of vibration (entries 4 and 5). Selective modifications of a block copolymer were conducted. The diblock copolymer, polystyrene-block-poly(4-VBA) underwent solvent-free mechanochemical modification (Table 3). Only the 4-VBA block showed chemical transformations, while the polystyrene block remained intact. Interestingly, sterically demanding isopropyl ammonium carbamate exhibited an unexpected high installation efficiency (96%, entry 3) with the block copolymer. Next, we investigated the stability of aldehyde and imine polymers under high-speed ball milling conditions (Table S1 and Figure S1). The collision energy among balls and a vessel is high enough to degrade or alter polymer chains.17,20 The random copolymer of styrene and 4-VBA used in Table 1 was subjected to 30 Hz vibration condition for an hour and the formation of a high molecular weight chain coupling product was observed with new signals in 1H NMR spectra. In contrast, the product, imine polymers, did not exhibited notable degradation or chain coupling. The difference in chemical reactivity between aldehyde and imine might reflect their stability toward mechanical force. Since the Schiff base formation is much faster than a reaction between aldehydes,

a

Conditions: Polymer (0.10 g, 0.76 mmol aldehyde), amine (2 equiv, 1.6 mmol), or ammonium carbamate salt (1 equiv, 0.76 mmol) in a 10 mL stainless-steel jar with three stainless-steel balls (7 mm in diameter). bDetermined by 1H NMR spectroscopy. cDetermined by GPC calibrated with polystyrene standards in tetrahydrofuran (THF) at 40 °C. dNa2CO3 (2 equiv) was added. eReaction time = 1 h.

this postpolymerization modification brought functionality as well as stability. The reaction rates of the solvent-free ball-milling and solution conditions were compared (Figure 2). The direct mechanochemical transformation of solid substrates means that the chemical reaction is performed without dilution. Upon proper mixing and input of energy, a kinetically maximum rate is expected for mechanochemistry. The reaction between the poly(4-VBA) homopolymer and the solid p-anisidine was compared with that of its congeners in solution (room temperature and 50 °C). The solvent-free ball-mill reaction reached the completion within an hour of vibration: however, the solution mixture at room temperature exhibited only marginal imine formation, even after several hours. An increase in the temperature to 50 °C still resulted in slower imine formation than that in the mechanochemical polymer modification. The increase in the internal temperature that occurs during ball-milling from the collision between balls or exothermic nature of reaction might accelerate imine formation: however, the marginal rate enhancement of the solution reaction by the increase in temperature supports the high concentration effect on the fast mechanochemical reaction. Additionally, solvent-free reactions above the melting temperature of p-anisidine (57 °C), where a liquid substrate could behave as a solvent, were examined (Table S2). At 70 °C, the mixture of polymer and molten p-anisidine (2 equiv) were placed in a glass vial with vigorous stirring. In case of poly(4VBA), a large amount of p-anisidine (190 mg to 100 mg polymer) provided a good mixing environment, resulting in comparable conversion to ball-milling (88% in 60 min). 563

DOI: 10.1021/acsmacrolett.8b00171 ACS Macro Lett. 2018, 7, 561−565

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ACS Macro Letters

In conclusion, we demonstrated solvent-free mechanochemical postpolymerization modification. A solid-state reaction using high-speed ball milling successfully mediated the condensation of aldehyde polymers and amine derivatives. The vibration of the balls in a container provided sufficient mixing and input of energy to facilitate the modification of polymers, allowing solvent to be excluded. Other types of mechanochemical postpolymerization modification reactions are currently under investigation.

Table 3. Mechanochemical Post-Polymerization Modification of Polystyrene-block-poly(4-vinyl benzaldehyde)a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00171. Detailed experimental procedure and 1H NMR and FTIR data (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jeung Gon Kim: 0000-0003-1685-2833 Notes

The authors declare no competing financial interest.

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a

Conditions: Polymer (0.10 g, 0.18 mmol aldehyde), amine (2 equiv, 0.36 mmol), or ammonium carbamate salt (1 equiv, 0.18 mmol) in a 10 mL stainless-steel jar with three stainless-steel balls (7 mm in diameter). bDetermined by 1H NMR spectroscopy. cDetermined by GPC calibrated with polystyrene standards in tetrahydrofuran (THF) at 40 °C. dNa2CO3 (2 equiv) was added.

ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF-2012M3A7B4049677).

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REFERENCES

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Figure 2. Reaction rate comparison between ball-mill and solution reactions. (ball-milling (●), RT in CDCl3 (■), and 50 °C in CDCl3 (◆)).

However, the reaction with the random polymer (42% aldehyde content, Table 1, SM) exhibited low efficiency and inhomogeneity due to a decrease of liquid p-anisidine (78 mg to 100 mg polymer). While ball-milling reaction showed effective activation and mixing (98% conversion at 10 min), the solvent-free reaction gave poor conversions as well as considerable deviations depending on sampling locations. This study clearly shows that ball-mill provides not only chemical activation energy but also efficient mixing. 564

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DOI: 10.1021/acsmacrolett.8b00171 ACS Macro Lett. 2018, 7, 561−565