Keywords
Recyclable ionic liquid - Nano zero-valent iron - PhotoRDRP - Diblock copolymer -
Sustainable method
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Recyclable ionic liquids are used as solvents for polymerization, instead of conventional
hazardous organic solvents.
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Recyclable nanocatalysts enabled excellent control over polymerization.
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Remarkable temporal control was achieved through “ON–OFF” experiments.
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Well-defined narrow dispersed polymers and diblock copolymers were synthesized.
Introduction
The overutilization of organic solvents raises concerning environmental issues. Inhalation
or exposure to high concentrations of organic solvents can result in many health problems,
and these solvents are also extremely combustible, toxic, and irritating. The disposal
of organic solvents contributes to air pollution [1], and burning these dangerous solvents poses major problems. Therefore, reducing
the use of organic solvents is necessary although different processes rely on different
solvents. A sustainable approach can be achieved through the use of recyclable ionic
liquids (ILs) as solvents [2]. The presence of a range of anions and cations is a defining characteristic of ionic
liquids. Among the most commonly employed organic cations in ILs that contain nitrogen
are pyridinium and imidazolium. Bromides, tetrafluoroborate, chloride, iodide, and
hexafluoro phosphate are some common anions [3]. These organic equivalents have a number of peculiar features, such as low volatility,
effective dissolution, high flash point, non-volatility, extreme heat stability, and
high polarity [4], [5]. These features of ILs have gained their interest recently and may serve as alternative
solvents in various reactions. It is remarkable that the ILs can be recycled and used
as green solvents to develop circularity in the system [6]. Significantly, ILs have emerged in several fields, including catalysis [7], organic synthesis [8], electrochemistry [9], analysis [10], material chemistry [11], and polymerization.
Although there is a wide variety of polymerization techniques accessible, reversible
deactivation radical polymerization (RDRP) technologies have greatly improved over
the last several decades [12]. They have functioned as a possible technique for the development of stimuli-responsive
polymers [13], [14] and have amazing command over molecular mass, architecture, and functions [15]. The various RDRP techniques comprise atom transfer radical polymerization (ATRP)
[16] and many more [17]
[18]
[19]
[20].
Zero-valent transition metal-catalyzed RDRP procedures are well-proven methods for
achieving effective control over molecular mass and chain end fidelities [21]. Designing externally triggered RDRP has expanded the application of standard RDRP
procedures, and it has lately received attention from photo-irradiation [22]
[23]
[24], electrochemistry [25], and pressure [26]. Interestingly, photoinduced RDRP methods are appealing as they offer spatiotemporal
control, lower polymerization temperature, and tolerance to functional moieties and
hence are favored over other polymerization techniques [27], [28]. Although the RDRP procedures are well recognized, certain aspects still need to
be developed. Lately, an emphasis has been given to circular processes. Some areas
of interest include recoverable, reusable catalysts and reducing the usage of conventional
transition metal catalysts, as it impart coloration [29].
It is also essential to select a suitable solvent. Recently, Cu(II) was used as a
catalyst for ATRP of methyl acrylate (MA), which produced a well-defined polymer after
120 min at 60 °C in dimethyl formamide (DMF) as the solvent [30]. Another report depicts the synthesis of PMA using copper-mediated RDRP in a series
of solvents, including acetone, DMF, and DMSO at 110 °C, yet the conversion was found
to be 82% [31]. Various organic solvents, including acetone and DMF, have been utilized in these
reported techniques at increased temperatures. These conditions are not economically
sustainable and require various conditions thus making the method costly. Reusable
solvent and catalyst-mediated photoRDRP are potent solutions to these problems. Reusing
reaction components will help improve process competency and pave the way for a circular
economy. This can be easily achieved by IL as a solvent in transformations including
polymerization [32].
The adaptability of the photoRDRP system and the use of ILs as a solvent are highlighted
in this study, opening new possibilities for polymerization with greater temporal
control. Our group has been working in the field of photoRDRP, and in some of our
previous work, we have limited to only a single type of ionic liquid system [33], [34]. This work additionally focuses on the IL-enabled recyclable catalyst-aided polymerization
of MMA and illustrates the impact of chain length in imidazolium-based ILs. We have
synthesized and investigated a variety of ILs with diverse chain lengths, ranging
from ethyl to decyl, in the photoRDRP of MMA. Remarkably, even after numerous cycles
of photoRDRP, the catalyst maintained its activity with respect to controlling molecular
mass and dispersity. Moreover, well-defined diblock copolymers are produced by using
the synthesized poly(methyl methacrylate) (PMMA) homopolymer. The poly(methyl methacrylate)-block-poly(methyl methacrylate) (PMMA-b-PMMA), poly(methyl methacrylate)-block-poly(tert-butyl acrylate) (PMMA-b-PTBA), and poly(methyl methacrylate)-block-poly(tert-butyl methacrylate) (PMMA-b-PTBMA) are the diblock copolymers with lower Đ values (≤1.20). Additional features of photoRDRP, like temporal control, have also
been investigated through “ON” and “OFF” studies. These experiments demonstrate remarkable
control over polymerization and have produced a new, environmentally friendly way
of polymerization.
Results and Discussion
The effect of substituted alkyl chains on the activity of IL as a solvent in polymerization
was studied, and a variety of ILs have been produced following the reported method
[35]. In this study, the alkyl chain of ILs varied from ethyl (C2) to decyl (C10) and resulted in ILs named 1-ethyl-3-methylimidazolium bromide (EMIMBr), 1-butyl-3-methylimidazolium
bromide (BMIMBr), 1-hexyl-3-methylimidazolium bromide (HMIMBr), 1-octyl-3-methylimidazolium
bromide (OMIMBr), and 1-decyl-3-methylimidazolium bromide (DMIMBr), respectively (Figure
S2), and characterized via IR, 1H, 13C, HSQC and HMBC NMR spectrum along with LCMS spectra (Figure S3–S11, S13, respectively).
Initially, all ILs, that is, EMIMBr, BMIMBr, HMIMBr, OMIMBr, and DMIMBr, were used
in photoRDRP (UVA light-induced λ
max ≈ 352 nm) of nZVI mediated RDRP of MMA ([Figure 1]). The produced ILs were viscous with rising chain length, leading to an increase
in polymerization time from 45 to 240 min in the case of EMIMBr to DMIMBr, respectively
(P1–P5, Table S1). Another interesting observation was that when the carbon in the
alkyl chain was <4, it resulted in poor control over polymerization (P1–P5, Table
S1).
Figure 1 Representative scheme of the polymerization of nZVI-catalyzed photoRDRP of MMA in
IL and synthesis of the diblock copolymer.
A preliminary optimization of the nZVI-catalyzed photoRDRP of MMA in BMIMBr was carried
out considering the above observations, using a UVA light (λ
max ≈ 352 nm) photoreactor (P2, Table S1). The photoRDRP in the BMIMBr system was found
to produce controlled and well-defined PMMA. So, taking this observation into account,
further well-defined PMMA is achieved using Me6TREN and EBiB as ligand and initiating units respectively ([Figure 1]). The synthesized homopolymer was characterized using 1H NMR ([Figure 4A]) and ATR-IR (Figure S14), respectively, which also validates the formation of PMMA.
The linear dependence of ln([M][M]0/[M]) vs. time ([Figure 2A]) confirms that the nZVI-induced polymerization possesses all the features of the
RDRP approach, as is obvious from the kinetics investigation. It is evident from the
linear rise of M
n vs conversion ([Figure 2B]) that successful synthesis of narrow dispersed PMMAs (≤1.20) with varying M
n (from 1600 to 11,000 g mol−1, Figure S1, P11–P15, Table S2) was achieved.
Figure 2 Kinetic plots of (A) conversion vs. time, ln([M]0/[M]) vs time, and (B) evolution of molar mass (M
n) and dispersity (Đ) with monomer conversion.
Temporal control and catalyst recyclability are the two areas of interest in heterogeneous
photocatalysis; in this case, the polymerization method also shows IL recyclability.
To achieve this, the investigation of temporal control involved turning “ON” and “OFF”
the UV irradiation to either activate or deactivate the polymerization ([Figure 3A]), and there was a linear rise in ln([M]0/[M]) vs. total UVA light exposure over the period. It was discovered that the polymerization
stopped when the UVA radiation was turned “OFF” ([Figure 3A]). All the traits of photoRDRP processes are exhibited by this system, this system
exhibits remarkable temporal control ([Figure 3B]). Additionally, the green IL is separable and reusable in the reaction mixture.
Concerning control and molecular mass in polymerization, (P19–P22, Table S4) show
that the process was continued for up to three cycles without any evident loss in
activity. Furthermore, in the above processes, nZVI, the magnetically separable catalyst,
was used. It is noteworthy to mention that there has been no decrease in the catalytic
activity (regarding control and molecular mass during polymerization (P23–P26, Table
S5)).
Figure 3 Represents temporal control with sequential “irradiation ON” (blue area) and “irradiation
OFF” (white area) for polymerization of MMA: (A) ln([M]0/[M]) vs radiation time of UVA-light and (B) M
n and Đ vs monomer conversion.
Polymerization did not occur in the reactions performed in the absence of the ligand
(Me6TREN), catalyst (nZVI), or initiator (EBiB) (P8–P10, Table S1). This emphasizes that
the polymerization process requires the presence of all components, i.e., the catalyst,
ligand, and initiator. Interestingly, a different set of investigations using 2,2′-bipyridine
(bpy) or N,N,N′,N″,N″-pentamethyl diethylenetriamine (PMDETA) produced an ill-defined
polymer and a sluggish polymerization (P6–P7, Table S1). Concurrently, the temporal
control investigation demonstrates that UVA irradiation (λ
max ≈ 352 nm) is an additional essential factor for polymerization. The mechanistic study
depicts ([Scheme 1]) that iron gets oxidized to Fe(II) and forms Fe(II)Br/L
n
complex. The introduction of MMA results in excellent control over polymerization,
which is attributed to the complex Fe(II)(Me6TREN)Br2. The mechanistic study was confirmed by the theoretical study was reported in our
earlier work [34].
Scheme 1 Proposed mechanism for the nZVI-catalyzed photoRDRP in IL.
A variety of block copolymers have also been synthesized to further illustrate the
active chain end of the produced homopolymer as a macroinitiator. The produced block
copolymers are namely PMMA25-b-PTBMA50-Br, PMMA25-b-PTBA50-Br, and PMMA25-b-PMMA50-Br diblock copolymers ([Figure 1], Table S3). Using 1H NMR, the synthesized PMMA25-b-PMMA50-Br copolymer was characterized [36] ([Figure 4B]) and the ATR-IR spectra (Figure S14) depict the formation of block copolymers.
Figure 4
1H NMR spectra of PMMA25-Br (A), PMMA25-b-PMMA50-Br (B), PMMA25-b-PTBA50-Br (C), and PMMA25-b-PTBMA50-Br (D).
The obtained PMMA25-b-PTBA50-Br copolymer was characterized through 1H NMR [37] ([Figure 4C]), and the ATR-IR spectra diblock copolymer shows characteristic signals (Figure
S14). This also validates the synthesis of PMMA25-b-PTBA50-Br. The prepared PMMA25-b-PTBMA50-Br copolymer was characterized through 1H NMR spectra [36] ([Figure 4D]), and the ATR-IR spectra (Figure S14) depict the formation of copolymers. The 1H NMR spectra of block copolymers show peak broadening and merging due to the randomness
of polymeric chain arrangement and a wide range of local chemical environments for
each proton [38]. The synthesis of several diblock copolymers demonstrates that the chain end is
active. It has been demonstrated that the BMIMBr system is capable of synthesizing
well-defined diblock copolymers.
Conclusions
In summary, we emphasize the widespread use of ILs in reversible-deactivation radical
polymerization (RDRP) methods to address the problems caused by hazardous solvents,
hence creating new opportunities for polymerization. Furthermore, nZVI-catalyzed polymerization
of MMA in the IL system has been described here; to investigate in detail, the ILs
have various alkyl substitutions ranging from ethyl to decyl. Surprisingly, The IL
with butyl substituent has been found to be promising in terms of control and molecular
mass. Another peculiar feature depicted by IL systems is recyclability, which is achieved
here, along with the reuse of magnetically separable nZVI catalysts. Moreover, the
polymerization showed a notable ability to regulate time by merely turning “ON” and
“OFF” the UVA radiation. We have effectively demonstrated the chain end fidelity through
the preparation of different diblock copolymers, i.e., PMMA25-b-PMMA50-Br, PMMA25-b-PTBMA50-Br, and PMMA25-b-PTBA50-Br. This simple IL-mediated method enables photo-controlled precision in the manufacturing
of customized polymers through a sustainable system inducing circularity.
Experimental Section
Materials and methods
The monomers were procured from sigma Aldrich. Methyl methacrylate (MMA, 99%), tert-butyl methacrylate (TBMA, 98%), tert-butyl acrylate (TBA, 99%) were passed through the basic column. The precursors 1-methylimidazole
(C4H6N2, 99%), 1-bromoethane (C2H5Br, 98%), 1-bromobutane (C4H9Br, 99%), 1-bromohexane (C6H13Br, 98%), 1-bromooctane (C8H17Br, 99%), 1-bromodecane (C10H21Br, 98%), ethyl α-bromoisobutyrate (EBiB, 98%), and deuterated chloroform (CDCl3, >99.8%) were utilized as obtained. Tris [2-(dimethylamino)ethyl]-amine (Me6TREN, >98%, TCI Chemicals), along with methanol (CH3OH, Merck), tetrahydrofuran (THF, 99%), chloroform (CHCl3, Merck), and diethyl ether (C4H10O, Merck), were used as received. The nZVI catalyst is synthesized through a previously
reported method [39]. 1H NMR spectra were recorded using Bruker 600 MHz NMR spectrometer. IR spectra were
performed using the Perkin Elmer Spectrum 100 instrument in ATR mode. Molar masses
(M
ns) and dispersities (Ð
s) of the prepared polymers were assessed with triple-detection GPC, in DMF (comprising
0.1 wt % of LiCl), which is used as the eluent and narrow linear poly(methyl methacrylate)
standards are used for instrument calibration.
General procedure for synthesis of ionic liquids
The ILs were synthesized following an earlier reported procedure [35]. In this procedure, 1-methyl imidazole and respective alkyl halide namely 1-bromoethane
to 1-bromodecane were added in equimolar proportion in a round bottom flask. The system
is refluxed at 70 °C for 36 h. After this, a thick viscous liquid is obtained and
is purified using diethyl ether to obtain pale yellowish thick liquids. 1-ethyl-3-methylimidazolium
bromide (EMIMBr), 1-butyl-3-methylimidazolium bromide (BMIMBr), 1-hexyl-3-methylimidazolium
bromide (HMIMBr), 1-octyl-3-methylimidazolium bromide (OMIMBr), 1-decyl-3 methylimidazolium
bromide (DMIMBr) are the IL synthesized. The characteristics of BMIMBr are mentioned
as follows.
IR: 3650–3300, 3200–3000, 3000–2700, 1660–1400, 1640–1580, 1460–1330, 1190–1160 cm−1.
1H NMR: 10.25–9.85, 7.61, 7.49, 4.37–4.32, 4.15, 1.96–1.86, 1.43–1.36, 0.99–0.94 ppm.
13C NMR: 137, 124, 122, 50–47, 38–35, 33–30, 21–19, 14–11 ppm.
LCMS: 140 m/z (molecular ion peak + H+).
General procedure for homopolymerization and block polymerization
In an nZVI-mediated photoRDRP of MMA (P2, Table S1), a vessel containing nZVI (12 mg,
0.11 mmol), Me6TREN (30 μL, 0.11 mmol), BMIMBr (445 mg, 2.17 mmol) EBiB (16 μL, 0.11 mmol), and purged
MMA (1 mL, 11.03 mmol) were sealed under inert atmosphere. The reaction mixture was
irradiated under a UVA photoreactor (λ
max ≈ 352 nm). Upon completion of the reaction, the mixture is diluted with THF, and
the catalyst is recovered using a bar magnet. The polymer was isolated by precipitation
in chilled methanol and dried under a vacuum. A similar procedure is followed with
EMIMBr, HMIMBr, OMIMBr, and DMIMBr.
Synthesis of PMMA25-Br homopolymer was carried out via nZVI-mediated photoRDRP according to the above-described
method using EBiB (P2, Table S1) as the initiator. After the completion of the reaction,
nZVI was removed by a bar magnet, and the pure polymer was obtained by precipitation
into methanol and dried under a vacuum. The GPC profile of the prepared PMMA and block
copolymers (Figure S15) also shows the narrow dispersity.
IR: 2954, 1730, and 1139 cm−1.
1H NMR: 3.6–4.1, 2.2–2.4 and 1.6–2.0 ppm.
Synthesis of PMMA25-b-PTBMA50-Br diblock copolymer was carried out via nZVI-catalyzed photoRDRP according to the
previously described method using PMMA-Br (P16, Table S3) as the macroinitiator. After
the completion of the reaction, nZVI was recovered using a bar magnet, and the pure
polymer was obtained by precipitation into methanol and dried under a vacuum.
IR: 2954, 1730, and 1139 cm−1.
1H NMR: 3.6–4.1, 2.2–2.4 and 1.6–2.0 ppm.
Synthesis of PMMA25-b-PTBA50-Br diblock copolymer was carried out via nZVI-catalyzed photoRDRP according to the
above-described method using PMMA-Br (P17, Table S3) as the macroinitiator. After
the completion of the reaction, nZVI was recovered using a bar magnet, and the pure
polymer was obtained by precipitation into methanol and dried under a vacuum.
IR: 2954, 1730, and 1139 cm−1.
1H NMR: 3.6–4.1, 2.2–2.4 and 1.6–2.0 ppm.
Synthesis of PMMA25-b-PMMA50-Br diblock copolymer was carried out via nZVI-catalyzed photoRDRP according to the
above-described method using PMMA-Br (P18, Table S3) as the macroinitiator. After
the completion of the reaction, nZVI was recovered using a bar magnet, and the pure
polymer was obtained by precipitation into methanol and dried under a vacuum.
IR: 2954, 1730, and 1139 cm−1.
1H NMR: 3.6–4.1, 2.2–2.4 and 1.6–2.0 ppm.
General procedure for kinetics and ON/OFF
The polymerization reaction was carried out following the above-mentioned procedure
under UVA irradiation (λ
max ≈ 352 nm), to validate the kinetics of the polymerization, aliquots were withdrawn
using a degassed syringe at periodic time intervals (5 – 60 min) during the course
of the reaction and diluted with THF to assess the evolution of (i) the conversion
of monomer and (ii) molar masses (M
ns) and dispersity values (Đs
) by gravimetry and SEC, respectively. Keeping the other components (nZVI, Me6TREN, IL) constant and altering the [MMA]0/[EBiB]0 feed ratio, a series of PMMAs of various molar masses (DP = 15 –100) were synthesized
and precipitated in methanol. The UVA irradiation was turned “ON/OFF” (the reaction
mixture was held in the dark phase (OFF state), after which UVA irradiation was again
turned ON) for every 5 min in the initial 15 min of photoRDRP, followed by an interval
of 15 min. The “ON/OFF” experiments were performed over many cycles, with aliquots
being removed from each “ON” and “OFF” cycle to determine the monomer conversion as
well as the molecular mass and dispersity.
General procedure for recyclability studies
Under consistent reaction conditions as used for homopolymerization, the recycled
ILs were utilized for subsequent reactions. The ILs were removed by dissolving the
reaction mixture in methanol, after the first reaction of the photoRDRP. The PMMA-free
IL (500 mg) was then used for the subsequent photoRDRP of MMA for multiple cycles
and the purity of IL is confirmed through 1H NMR (Figure S12). The crude product was dissolved in THF, and the catalyst was isolated
by simply holding it against a bar magnet after the first set of photoRDRP of MMA
in ILs. The recovered catalyst (approx. 2 mg) was washed with methanol and THF twice,
dried, and then employed as a catalyst for the following series of photoRDRP of MMA.
Bibliographical Record
Amul Jain, Bhanendra Sahu, Nikhil Ingale, Sanjib Banerjee. Toward a Greener Tomorrow:
Sustainable Synthesis of Well-defined Polymers in Ionic Liquids via Recyclable Nanocatalyst-Mediated
Photopolymerization. Sustainability & Circularity NOW 2025; 02: a25297304.
DOI: 10.1055/a-2529-7304
