A Process for Well-Defined Polymer Synthesis through Textile Dyeing

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A Process for Well-Defined Polymer Synthesis through Textile Dyeing Inspired Catalyst Immobilization Yingying Chu, Nathaniel Corrigan, Chenyu Wu, Cyrille Boyer, and Jiangtao Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03726 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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ACS Sustainable Chemistry & Engineering

A Process for Well-Defined Polymer Synthesis through Textile Dyeing Inspired Catalyst Immobilization Yingying Chu,a Nathaniel Corrigan,a Chenyu Wu,a Cyrille Boyer,a* and Jiangtao Xua* a

Centre for Advanced Macromolecular Design and Australian Centre for Nanomedicine,

School of Chemical Engineering, UNSW Sydney, High Street, Kensington, NSW 2052, Australia E-mails: [email protected] (J. X.); [email protected] (C. B.).

ABSTRACT Industrialized textile dyeing technology has inspired a scalable and low-cost process for immobilization of a photoredox catalyst onto commercial cotton threads to prepare unique heterogeneous catalyst composites; these composites are capable of regulating photoinduced controlled/“living” radical polymerization in batch and flow reactors. Free-base porphyrin (e.g.,

tetraphenylporphyrin

(TPP)),

an

efficient

photocatalyst

for

photoinduced

electron/energy transfer – reversible addition-fragmentation chain transfer (PET-RAFT) polymerization of acrylamides, was attached to the cotton thread through a method analogous to reactive dye chemistry, to produce composites with excellent photocatalytic activity. Separation and recovery of the catalyst functionalized composite was realized through simple washing with solvents, which resulted in negligible catalyst leaching and maintenance of catalytic performance over multiple polymerization cycles. Polymerization was also successfully carried out in solvents where TPP has poor solubility, demonstrating the versatility of this approach. By taking advantage of the robustness and flexibility of cotton

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thread, the immobilized catalyst was fitted in a continuous flow reactor to prepare welldefined polymers through a flow process.

Keywords: cotton thread; photoredox catalyst; heterogeneous catalysis; PET-RAFT polymerization; tetraphenylporphyrin; continuous flow reactor


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INTRODUCTION Catalysis plays a vital role in many industries including fine chemicals, pharmaceuticals, and energy.1-4 As one of the key technologies for environmentally and economically benign “green chemistry”, catalysis allows chemical reactions to be conducted under mild conditions, which reduces waste and energy input, and increases product yields.5-7 In industry, heterogeneous catalysis is preferred to homogeneous catalysis because the used catalysts, which are generally expensive and less environmentally friendly, can be easily separated and recycled from final products.5, 8-9 More importantly, heterogeneous catalytic reactions can be accomplished in a cleaner and scalable way by merging continuous production through flow process.10-12 Therefore, catalyst immobilization plays an important role in heterogeneous catalysis and is currently attracting increasing attention. By immobilizing catalysts on a solid matrix, the soluble catalyst stays in solid phase in the reaction solution, enabling facile separation, recovery and recycling of the catalyst as well as the fast isolation of the desired reaction products.13-19 Furthermore, insoluble catalysts could be well dispersed in the reaction medium through immobilization to avoid undesired aggregation.20-23 Production cost can also be reduced as supported catalysts can be easily recycled.24-25 In controlled/“living” radical polymerization, especially atom transfer radical polymerization (ATRP), transition metal complexes were largely employed as catalysts.26-27 To recover and recycle the catalyst, and prevent coloration of the final polymer caused by the transition metals, supported catalysts have been comprehensively investigated.28-29 Silica gel,30-33 polymeric materials,32, 34-35 and nanoparticles36-37 have been studied as the supporting matrix for catalyst immobilization, through either physical adsorption or covalent attachment. More recently, porous materials, including metal-organic frameworks38 and melamine based porous system39 were reported for catalyst incorporation, and showed good catalytic performance in 3 ACS Paragon Plus Environment


ATRP. Furthermore, our recent work reported metalloporphyrins supported on cellulosic materials, and eosin Y supported on silica nanoparticles, as highly efficient heterogeneous catalysts for photoinduced electron/energy transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization.40-45 Despite the promising catalytic performance and recyclability of the immobilized catalysts, the supporting materials and immobilization methodologies are still associated with high preparation costs. The separation process often involves complicated centrifugation and filtration, thus hindering the potential industrial applications. Textile dyeing technologies, through which fabrics are imparted with different colors using dyeing chemistry to manufacture the final textile products, provided an inspiring idea for catalyst immobilization. Firstly, fabrics produced from cotton, polyamide, or polyester are promising carriers for catalyst immobilization due to the following advantages: (1) good mechanical properties, excellent durability, high chemical resistance, smooth surfaces, and good processability;46-47 (2) the intrinsic flexibility of fabrics makes them capable of being fitted in any reactor geometry and being separated easily without leaving any residues; (3) pore diffusion, which will influence the reactivity of the catalyst, can also be avoided due to the smooth fabric surface;48 (4) pendant groups on the surface of the polymeric fabrics, for instance, hydroxyl groups on the surface of cellulosic fabrics, or amino groups on the wool fabrics, are powerful moieties for anchoring catalysts;49-50 (5) manufacturing costs for fabrics are much lower than other solid supports, which is especially true for cotton considered as a nearly inexhaustible natural source for fabric production.51-52 Therefore, a variety of bioactive molecules and organocatalysts have been extensively immobilized on fabrics made of cotton,53-54 wool,55 polyester,56-57 poly(ethylene terephthalate),54, 58-59 polyvinyl alcohol,60 and polyamide 6.6.46, 54, 59


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Furthermore, reactive dye chemistry used in textile dyeing technology is a robust and versatile methodology which can be applied to catalyst immobilization. Composed of a dye part, a bridging part, and reactive sites, reactive dyes can be covalently bonded to fabrics (Figure 1A).61-62 Among the reactive groups including triazine, vinysulfone, quinoxaline, pyrimidine and acrylamide, triazine and vinylsulfone reactive dyes are the most popular.61, 6367

Due to the high reactivity of triazine, it has been utilized to modify the surface of cellulosic

fibers to improve its reactivity,68 diffusion properties,69 antibacterial properties,70 and printability.71 For instance, by using this well-established triazine involved reactive dye chemistry, Glover and coworkers have immobilized nanostructures on fabrics to improve the adsorption abilities towards ethylene and ammonia.72



Figure 1. Reactive dye chemistry in textile dyeing technology (A) inspired the catalyst immobilization process in this study (B); (C) Proposed mechanism for PET-RAFT polymerization catalyzed by TPP-TA-N@cotton thread under green light irradiation.

In this contribution, we envisioned the modification of catalysts with triazine chemistry, inspired by the approach used for the preparation of commercial reactive dye, thus, a unique reactive photocatalyst with porphyrin scaffold was synthesized (Figure 1A and B). In comparison to our previous work, where DCC coupling was employed to immobilize carboxylic acid-functionalized TPP onto support,42 in this paper, we decided to functionalize TPP with triazine to allow the straightforward immobilization of the catalyst onto cotton textiles with the ease of chemical processes and higher scalability. The prepared TPP-cotton composite was capable of catalyzing the PET-RAFT polymerization (Figure 1C) of N,Ndimethylacrylamide (DMA), using 2-cyano-2-propyl dodecyl trithiocarbonate (CPDTC, SI, Scheme S1) as RAFT agent (vide infra). The catalytic activity of the immobilized catalysts was thus investigated for both batch and flow PET-RAFT polymerizations.

RESULTS AND DISCUSSION Photocatalyst immobilization The textile dyeing process includes the following three major steps: (a) preparation (pretreatment) of textile materials with aqueous alkaline substances and detergents; (b) application of color to the textile materials (dyeing) using mainly synthetic organic dyes, which is performed in specific steps and generally at elevated temperatures; (c) finishing, which involves treatments with chemical compounds aimed at improving the quality of the


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fabric.61, 67, 73-75 In our work, the photocatalyst immobilization was performed using a similar procedure to the industrial textile dyeing process. Firstly, the white cotton thread which is commercially used for sewing fabrics was immersed in aqueous NaOH solution (0.375 M) at 60 °C for 2 h, followed by washing and soaking in deionized water. Subsequently, the cotton thread was washed and soaked in different organic solvents (dimethyl sulfoxide (DMSO) and N,N’-dimethylformamide (DMF)) to expose reactive hydroxyl groups on the surface, as well as to remove fine particles and possible contaminants introduced during the manufacturing process. Secondly, a reactive photocatalyst was prepared by standard porphyrin synthesis and triazine chemistry to produce a triazine functionalized porphyrin, followed by catalyst immobilization through the reaction of triazine functionality and hydroxyl groups present on cotton thread. Scheme 1 shows the synthetic chemistry of reactive photocatalyst (TPP-TA-2Cl). 5-(Ohydroxyphenyl)-10, 15, 20-tri-(p-phenyl)porphyrin (TPP-OH, SI, Figure S2) was synthesized using a one-step synthesis where pyrrole, 4-hydroxyl benzaldehyde and benzaldehyde were mixed in propionic acid at 140 °C for 4 h, as reported in our previous publication.42-43 After purification by column chromatography, the phenolic alcohol in TPPOH was reacted with 2,4,6-trichloro-1,3,5-triazine (or cyanuric chloride) in tetrahydrofuran (THF) in the presence of weak base (N,N-diisopropylethylamine, DIPEA) at 0 °C. After 30 min, selective reaction of only one C-Cl bond (C 2) with phenolic alcohol was observed to generate TPP-TA-2Cl in high yield. The absence of reaction of the other two C-Cl bonds (C 4 and 6) was attributed to the reduced reactivity after the first addition. The chemical structure of TPP-TA-2Cl was confirmed by 1H and 13C NMR (SI, Figure S3 and S4). After drying, the cotton thread was placed in a DMF/water solution containing TPP-TA-2Cl and potassium carbonate (K2CO3) at 60 °C overnight. The cotton thread was collected and washed with DMSO to remove unreacted catalyst and K2CO3. Upon attachment of the photocatalyst, the 7 ACS Paragon Plus Environment


white cotton thread became light pink (Scheme 1), confirming the successful immobilization of the catalyst on cotton, i.e., TPP-TA-Cl@cotton thread.

Scheme 1. (A) Preparation of catalyst immobilized cotton thread, TPP-TA-N@cotton thread; (B) Two catalysts used in control experiments.

Lastly, the immobilized catalyst on cotton thread was reacted with secondary amine to remove unreacted C-Cl bonds in triazine moieties. This step is very important as unreacted CCl groups are capable of generating radicals, which has been confirmed by both the homogeneous and heterogeneous polymerization of DMA in the presence of CPDTC, using TPP-TA-2Cl (SI, Table S1) and TPP-TA-Cl@cotton thread as catalyst under green light irradiation. In heterogeneous polymerization, radical generation by TPP-TA-Cl@cotton thread caused the growth of polymer chains and deposition of polymer aggregates on the thread, which was visualized by scanning electron microscopy (SEM, SI, Figure S7 (E~H)), resulting in physical separation of catalysts and a consequent decrease in catalytic performance. Therefore, piperidine was used to substitute excess C-Cl bonds in the triazine moieties under basic conditions. TPP-TA-Cl@cotton thread was washed with THF and then submerged in THF solution containing piperidine and DIPEA at 80 °C for 24 h. Finally, the


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catalyst immobilized cotton thread, TPP-TA-N@cotton thread, was collected and washed with DMSO to remove excess piperidine and other side products for further investigation.

Figure 2. SEM images of original cotton thread (A, B, and C), TPP-TA-N@cotton thread (D, E, and F), and TPP-TA-N@cotton thread after 5 cycles of PET-RAFT polymerization of DMA (G, H, and I) with different scale bars (1.0 mm for A, D and G; 100 µm for B, E and H; and 10 µm for C, F and I).

To assess the structural stability of TPP-TA-N@cotton thread against organic solvents, we immersed the composite in different solvents, including DMSO, dichloromethane (DCM), chloroform (CHCl3), DMF, N,N-Dimethylacetamide (DMAc) and THF. After two days of soaking, all the tested solvents remained colorless (SI, Figure S8), suggesting that the catalyst was permanently bonded to cotton thread with negligible catalyst leaching (SI, Figure S10). More importantly, despite frequent physical squeezing and chemical soaking, 9 ACS Paragon Plus Environment


the fibrous microstructures of the thread remained intact during the whole process of catalyst immobilization as visualized by SEM (Figure 2A-I), showing the robustness of such materials. To address the concern of any physical absorption and complexation between porphyrin and cotton thread, a control experiment to treat cotton thread with TPP using the same procedure in the absence of any reactive functional groups. The cotton thread remained white, after collecting and washing twice with DMSO. This result undoubtedly suggested that there was no specific adsorption or complexation of TPP with cotton thread. To quantify the amount of conjugated catalyst on the cotton thread, we conducted highly efficient metalation of TPP-TA-N@cotton thread by immersing the composites in a DCM/DMF (1/1, v/v) mixed solution containing zinc acetate; gentle stirring for 6 h yielded the fully metallated ZnTPP-TA-N@cotton thread. The amount of Zn in the porphyrin center was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES), through which the amount of the conjugated catalyst was calculated to be 50.2 µg/mg (Wcatalyst/Wcotton thread, SI, Equation S1).

Homogeneous PET-RAFT polymerization catalyzed by control catalysts Following the synthesis of the TPP-TA-N@cotton thread, a control experiment was designed to assess the effect of the triazine group on catalytic activity of the functionalized TPP, and also confirm that deactivation of all C-Cl bonds by piperidine is able to prevent radical generation. A control photocatalyst, TPP-TA-2N (Scheme 1B), was synthesized by the reaction of TPP-TA-2Cl and piperidine to deactivate the C-Cl bonds in reactive triazine, and was subsequently employed as catalyst for PET-RAFT polymerization of DMA in the presence of CPDTC under green light irradiation (λmax = 530 nm, 0.45 mW/cm2). The other control experiment was performed with TPP (Scheme 1B) as photocatalyst. In a typical


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homogeneous polymerization of DMA, the amount of catalyst was 50 ppm (relative to monomer concentration), the molar ratio of [DMA]:[CPDTC] = 200:1 was applied. After nitrogen degassing, the sealed reaction vessels were placed under light. Monomer conversion was measured by

1

H NMR while number-average molecular weights (Mn) and

polydispersities (PDIs) were determined by gel permeation chromatography (GPC). As shown in Table 1, control experiments in the absence of RAFT agents using either TPP or TPP-TA-2N as photocatalyst resulted in negligible monomer conversions (

A Process for Well-Defined Polymer Synthesis through Textile Dyeing

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