Chinese Chemical Letters  2016, Vol. 27 Issue (5): 681-684   PDF    
Facile preparation of antibacterial polymer coatings via thiol-yne click photopolymerization
Chuan Zhoua, Yi-He Lia, Zhen-Hua Jianga, Kwang-Duk Ahnb, Tian-Jiao Hua, Qing-Hua Wanga, Chun-Hua Wanga     
a Department of Chemistry and Biology, College of Science, National University of Defense Technology, Changsha 410073, China ;
b Polymer Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
Abstract: Novel antibacterial polymer coatings were prepared by a facile thiol-yne click photopolymerization of 1-propargyl-3-alkyl-1,3-diazanyl-2,4-cyclopentadiene bromide ([PAIM]Br) and tetra(3-mercapto-propionate) pentaerythritol (PETMP) (2:1 molar ratio) using 2,2-dimethoxy-2-phenylacetophenone (DMPA) as initiator. The antibacterial activity of the coatings was tested against Staphylococcus aureus (ATCC 292130) and Escherichia coli (ATCC 25922) by the dynamic shake method. The evaluation results revealed the antibacterial polymer coatings exhibited excellent inhibitory activity against S. aureus and E. coli, especially for S. aureus.
Key words: Antibacterial polymer     Thiol-yne     Click chemistry     UV-curing     Coatings    
1. Introduction

Infection caused by pathogenic microorganisms on material surfaces has been a long-term problem for public health. Using antibacterial polymer coatings to protect material surfaces is an effective antibacterial method [1, 2]. In recent years,antibacterial polymers with biocidal groups covalently bonded to their chains have attracted a tremendous amount of attention due to their advantages with reduced residual toxicity,increased efficiency and selectivity,and prolonged active lifetime [3]. However,most antibacterial polymers that include quaternary ammonium [4],phosphonium salts [5],N-alkylimidazolium [6] or N-alkylpyridinium [7] as pendant groups to the polymer chain exist in the form of chain and require complicated design and application procedures. In order to achieve certain potentially practical applications,it is necessary to process the polymers into films [8]. The reaction of thiol-yne belongs to the click reaction family [9, 10],which possesses remarkable advantages,such as mild reaction conditions,high reaction rate,functionality tolerance and atom economy [11]. Furthermore,it has also the additional feature that two thiol groups could react with one ethynyl group via a wellknown two-step processes [12]. These advantages of thiol-yne click reaction make it promising to prepare functional polymers with unique properties [13]. In addition,UV-curing is an important technique in the field of coatings for its advantages such as highspeed processing,low energy consumption,and environmental friendliness by avoiding solvent exposure [14, 15]. In view of the above mentioned merits and in continuation of our research [16],we report here the novel preparation of antibacterial polymer coatings by utilization of thiol-yne click photopolymerization. Exposure time of UV-curing reaction is determined by FT-IR and FT-ATR spectra. The thermomechanical properties and thermal stability of these films are characterized by DMA and TGA. The glass-transition temperatures (Tg) and antibacterial properties are discussed.

2. Experimental

All chemicals were purchased from commercial vendors and were used as received without further purification unless otherwise mentioned. 1H NMR spectra were recorded in DMSO-d6 (J & K) using TMS as internal standard on an Agilent 400 MHz. The NMR spectra were analyzed and processed using MestReNova-6.1.1-6384 software. FT-IR spectra were recorded on a Bruker TENSOR 27 spectrometer using KBr window and performed at room temperature. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra wererecordedona Thermo Nicolet IR100 spectrometer with a diamond ATR crystal. Thermogravimetric analysis (TGA) was performed under a controlled atmosphere of N2 with a Mettler TGA/DSC 1 instrument between 25 ℃ and 600 ℃ at a heating rate of 10 ℃/min. A TA Instruments Q2000 DSC equipped with a liquid nitrogen cooling accessory was used to investigate the glass transition temperature (Tg) of film samples between 8 mg and 12 mg. The midpoint value of the glass transition was reported after three heating and cooling cycles at 10 ℃/min. Dynamic mechanical properties were evaluated using a TA Instruments Q800 DMA with a frequency of 1 Hz,strain rate of 0.05%,and heating rate of 3 ℃/min. Tg were taken as the midpoint from DSC scans and as the peak maximum in tan δ plots from DMA.

2.1. Synthesis of N-alkyl derivatized yne monomers 3a-d

N-alkyl derivatized yne monomers were synthesized via the quaternization reaction represented in Scheme 1. Alkyl imidazoles 2a-d were prepared following the method in literature with modifications [17]. An example synthesis procedure for 3a is given as follows. 1-Hexylimidazole 2a and 3-bromopropyne (mole ratio 1:1) were added to anhydrous ethanol. The synthesis was carried out in a round bottom flask equipped with a stirrer and a condenser at 50 ℃ for 24 h. Then,solvent was removed using a rotary evaporator. The crude product was washed three times with ether and dried in a vacuum oven at 50 ℃ for one day to obtain yne monomer 3a. This same procedure was used to synthesize modified yne monomers 3b-d.

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Scheme. 1. Synthetic route of antibacterial polymer coatings.

1-Propargyl-3-hexyl-1,3-diazanyl-2,4-cyclopentadiene bromide ([PHIM]Br) 3a: This compound was obtained in 90% as a white solid,m.p. 91-92 ℃. 1H-NMR (DMSO-d6,400 MHz,Fig. S1 in Supporting information): δ 9.41 (s,1H,-N-CH-N-),7.91 (s,1H,- N-CH-CH-N-),7.87 (s,1H,-N-CH-CH-N-),5.26 (d,2H,-CH2-C≡),4.23 (t,2H,-CH2-N-),3.88 (s,1H,≡CH),1.80 (m,2H,-CH2-CH2-),1.27 (broad,6H,-CH2-CH3),0.86 (t,3H,-CH3).

1-Propargyl-3-octyl-1,3-diazanyl-2,4-cyclopentadiene bromide ([POIM]Br) 3b: This compound was obtained in 92% as a yellow viscous oil. 1H-NMR (DMSO-d6,400 MHz,Fig. S2 in Supporting information): δ 9.41 (s,1H,-N-CH-N-),7.91 (s,1H,-N-CH-CH-N-),7.87 (s,1H,-N-CH-CH-N-),5.26 (d,2H,-CH2- C≡),4.23 (t,2H,-CH2-N-),3.88 (s,1H,≡CH),1.80 (m,2H,-CH2- CH2-),1.25 (broad,6H,-CH2-CH3),0.86 (t,3H,-CH3).

1-Propargyl-3-decyl-1,3-diazanyl-2,4-cyclopentadiene bromide ([PDIM]Br) 3c: This compound was obtained in 88% as a slight yellow solid,m.p. 54-55 ℃. 1H-NMR (DMSO-d6,400 MHz,Fig. S3 in Supporting information): δ 9.40 (s,1H,-N-CH-N-),7.90 (s,1H,-N-CH-CH-N-),7.86 (s,1H,-N-CH-CH-N-),5.25 (d,2H,- CH2-C≡),4.22 (t,2H,-CH2-N-),3.87 (s,1H,≡CH),1.79 (m,2H,- CH2-CH2-),1.24 (broad,6H,-CH2-CH3),0.86 (t,3H,-CH3).

1-Propargyl-3-dodecyl-1,3-diazanyl-2,4-cyclopentadiene bromide ([PDODIM]Br) 3d: This compound was obtained in 86% as a white solid,m.p. 80-81 ℃. 1H-NMR (DMSO-d6,400 MHz,Fig. S4 in Supporting information): δ 9.37 (s,1H,-N-CH-N-),7.89 (s,1H,- N-CH-CH-N-),7.85 (s,1H,-N-CH-CH-N-),5.24 (d,2H,-CH2-C≡),4.21 (t,2H,-CH2-N-),3.87 (s,1H,≡CH),1.79 (m,2H,-CH2-CH2-),1.24 (broad,6H,-CH2-CH3),0.86 (t,3H,-CH3).

2.2. Preparation of antibacterial polymer coatings via thiol-yne click photopolymerization

All of the N-alkyl derivatized yne monomers were readily miscible in PETMP thiol monomer (2:1 molar ratio) with DMF and mild sonication,resulting in optically clear,homogeneous liquid mixtures. Then 2,2-dimethoxy-2-phenylacetophenone (DMPA) (0.05 eq. to each thiol moiety) as initiator dissolved in DMF was added to the above mixtures,followed by mild sonication. The thiol-yne formulations were coated onto a round glass pane by means of a film applicator and then irradiated on a conveyor exposure to give pale yellow and transparent photocured films with a usual diameter of 22 mm and thickness of 20-35 mm. The conveyor UV-curing system employed was an Exposure Model RW-UVAT201-20 (RunWing Co.,Ltd.,Shenzhen,China) equipped with a 2 kW Hg-lamp,and the applied conveyor speed was 3 m/min for 20 s exposure in one run. The resulting cross-linked samples were soaked in deionized water for 24 h to remove DMF and trace amount of residual monomer from the networks and dried in a vacuum oven at 37 ℃.

3. Results and discussion

Appropriate exposure time for the UV-curing reaction can be determined by tracking the infrared characteristic absorption peak of sulphur and alkynyl (Fig. S5 in Supporting information). The characteristic peak of thiol and alkynyl appeared in 2557 cm-1 and 2125 cm-1,respectively. With increasing exposure time,the characteristic peak of thiol and alkynyl decreases. Photopolymerization is almost finished within 100 s. The Fourier transform attenuated total reflection infrared spectrometry (FT-ATR) characterization of four antimicrobial films also confirm that the characteristic peak of thiol and alkynyl (with a dotted box around it) vanish completely within 100 s of irradiation (Fig. 1).

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Fig. 1. ATR-FTIR spectra of four antibacterial films from 6, 8, 10, 12 carbon alykyl yne imidazolium monomers.

Thermogravimetric analysis (TGA) was used to characterize the thermal stability of the thiol-yne networks. Fig. 2 shows the TGA thermograms for the thiol-yne networks as a function of alkyl chain length. All four antibacterial films with different alkyl chain lengths showed parallel thermogravimetric curves. These polymers have a small amount of weight loss before 200 ℃,which may be caused by the residual water in the polymer. Besides,the initial onset of degradation for the thiol-yne networks was observed around 250 ℃,which is higher than antibacterial polymers containing quaternary ammonium [18, 19].

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Fig. 2. TGA of antibacterial films from 6, 8, 10, 12 carbon alykyl yne imidazolium monomers.

Experimental DSC scans are provided in Fig. S6 in Supporting information and experimental DMA tan d and storage modulus (E') plots are provided in Fig. 3 for the N-alkyl derivatized thiol-yne networks. Tg values as a function of alkyl chain length are shown in Fig. 4. The N-alkyl derivatized thiol-yne networks show a slight decrease followed by a slight increase in Tg via DMA as n increases; this behavior is consistent with certain reports on linear polymers [20]. Glass transitions by DSC are all lower than the DMA values which is due to the continuous nature of the glass transition [21]. However,the change in Tg for the N-alkyl derivatized thiol-yne networks is within 10 ℃ for both DSC and DMA methods,indicating that the alkyl chains in the cross-linked network do not strongly affect the glass transition. Shown also in Fig. 3b are E' plots for the N-alkyl derivatized networks. Rubbery storage modulus values at Tg + 40 ℃ for the PETMP-[PAIM]Br systems did not follow a particular trend with increasing alkyl chain length. This result suggests that the cross-link density,upon which the rubbery storage modulus is dependent,is not predictably affected by the length of the alkyl chains for this series of thiol-yne networks. As for the four antibacterial films,the tan δ significantly broadens and becomes asymmetrical compared with previous reports [13],indicating the formation of a more heterogeneous polymer network. The heterogeneity is due to the formation of nonregiospecific products caused by a sequential addition of thiol to alkyne reaction [10].

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Fig. 3. Plots of (a) tan δ vs. temperature and (b) storage modulus vs. temperature for different length of alkyl chain in PETMP-[PAIM]Br networks.

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Fig. 4. Glass transition temperatures via DSC and DMA.

The antimicrobial nature of the N-alkyl derivatized thiol-yne networks was evaluated by the dynamic shake method [22]. The films on the circular glass (diameter: 22 mm) were shaken with 10 μL bacterial suspension containing Staphylococcus aureus (ATCC 292130) (5.2 × 107) and Escherichia coli (ATCC 25922) (3.6 × 107) in a 50 μL conical tube for 4 h at 37 ℃. The supernatant was then aliquoted and diluted appropriately,and colony forming units (CFUs) were determined. Fig. 5 shows the number of surviving cells after contact with the film samples with different alkyl chain lengths. The results showed that biocidal ability of antimicrobial films increased with increasing alkyl chain length against both S. aureus and E. coli. Alkyl chain with 12C is much more powerful than those with 6C,℃,or 10C,possibly because the 12C alkyl chain more easily makes holes in the cell membranes and kills microbes [23]. In particular,these films showed better inhibitory activity against S. aureus than E. coli,which was consistent with previous literature reports [23]. This is due to the fact that in Grampositive bacteria,a negatively charged teichoic acid is attached to the phospholipid bilayer which protrudes outwardly across the peptidoglycan layer and sometimes introduces additional negative charge to their surfaces,which favors interactions with the positively charged antibacterial polymer,killing the bacteria.

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Fig. 5. Antimicrobial activity of four films with alkyl length of 6C, 8C, 10C, 12C with respect to the control tests without films against S. aureus and E. coli.

4. Conclusion

In summary,novel antibacterial polymer coatings were prepared via thiol-yne click photopolymerization. The biological tests have shown that the crosslinked films are endowed with strong antimicrobial activity against both S. aureus and E. coli. Antimicrobial activity increases with alkyl chain length and is greater for S. aureus than for E. coli. Further investigations on the chemical properties and biological activities of these antimicrobial films are well on their way.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2016.01.002.

References
[1] K.G. Neoh, E.T. Kang. Combating bacterial colonization on metals via polymer coatings:relevance to marine and medical applications. ACS Appl. Mater. Interfaces 3 (2011) 2808–2819
[2] M. Kim, C. Song, D.K. Han, et al. Allylimidazolium salt based antibacterial polymer coatings produced by thiol-ene photocuring. React. Funct. Polym. 87 (2015) 53–60
[3] E.R. Kenawy, S.D. Worley, R. Broughton. The chemistry and applications of antimicrobial polymers:a state-of-the-art review. Biomacromolecules 8 (2007) 1359–1384
[4] E.H. Westman, M. Ek, L.E. Enarsson, L. Wågberg. Assessment of antibacterial properties of polyvinylamine (PVAm) with different charge densities and hydrophobic modifications. Biomacromolecules 10 (2009) 1478–1483
[5] E.R. Kenawy, Y.A.G. Mahmoud. Biologically active polymers, 6-synthesis and antimicrobial activity of some linear copolymers with quaternary ammonium and phosphonium groups. Macromol. Biosci. 3 (2003) 107–116
[6] U. Mizerska, W. Fortuniak, J. Chojnowski, et al. Polysiloxane cationic biocides with imidazolium salt (Ims) groups, synthesis and antibacterial properties. Eur. Polym. J. 45 (2009) 779–787
[7] V. Sambhy, B.R. Peterson, A. Sen. Antibacterial and hemolytic activities of pyridinium polymers as a function of the spatial relationship between the positive charge and the pendant alkyl tail. Angew. Chem. Int. Ed. 47 (2008) 1250–1254
[8] G. Reiter, S. Napolitano. Possible origin of thickness-dependent deviations from bulk properties of thin polymer films. J. Polym. Sci., B:Polym. Phys. 48 (2010) 2544–2547
[9] A.B. Lowe, C.E. Hoyle, C.N. Bowman. Thiol-yne click chemistry:a powerful and versatilemethodology formaterials synthesis. J.Mater. Chem. 20 (2010) 4745–4750
[10] B.D. Fairbanks, T.F. Scott, C.J. Kloxin, et al. Thiol-yne photopolymerizations:novel mechanism, kinetics, and step-growth formation of highly cross-linked networks. Macromolecules 42 (2009) 211–217
[11] H.C. Kolb, M.G. Finn, K.B. Sharpless. Click chemistry:diverse chemical function from a few good reactions. Angew. Chem. Int. Ed. 40 (2001) 2004–2021
[12] O. Türünç, M.A.R. Meier. A novel polymerization approach via thiol-yne addition. J. Polym. Sci., A:Polym. Chem. 50 (2012) 1689–1695
[13] J.W. Chan, J. Shin, C.E. Hoyle, C.N. Bowman, A.B. Lowe. Synthesis, thiol-yne "click" photopolymerization, and physical properties of networks derived from novel multifunctional alkynes. Macromolecules 43 (2010) 4937–4942
[14] G. Gozzelino, A.G. Dell' Aquila, D. Romero. Hygienic coatings by UV curing of diacrylic oligomers with added triclosan. J. Coat. Technol. Res. 7 (2010) 167–173
[15] R. Liu, J.C. Zheng, R.X. Guo, et al. Synthesis of new biobased antibacterial methacrylates derived from tannic acid and their application in UV-cured coatings. Ind. Eng. Chem. Res. 53 (2014) 10835–10840
[16] Y.H. Li, K.D. Ahn, D.P. Kim. Synthesis and properties of UV curable polyvinylsilazane as a precursor for micro-structuring. Polym. Adv. Technol. 26 (2015) 245–249
[17] S. Khabnadideh, Z. Rezaei, A. Khalafi-Nezhad, et al. Synthesis of N-alkylated derivatives of imidazole as antibacterial agents. Bioorg. Med. Chem. Lett. 13 (2003) 2863–2865
[18] T. Seyidoglu, U. Yilmazer. Modification and characterization of bentonite with quaternary ammonium and phosphonium salts and its use in polypropylene nanocomposites. J. Thermoplast. Compos. Mater. 28 (2015) 86–110
[19] K. Chen, X.D. Zhou, X.R. Wang. Synthesis and application of a hyperbranched polyester quaternary ammonium surfactant. J. Surfactants Deterg. 17 (2014) 1081–1088
[20] P. Wisian-Neilson, L. Bailey, M. Bahadur. Modification of poly(methylphenylphosphazene) to increase hydrophobicity. Macromolecules 27 (1994) 7713–7717
[21] W. Brostow, R. Chiu, I.M. Kalogeras, A. Vassilikou-Dova. Prediction of glass transition temperatures:binary blends and copolymers. Mater. Lett. 62 (2008) 3152–3155
[22] H.K. He, B. Adzima, M.J. Zhong, et al. Multifunctional photo-crosslinked polymeric ionic hydrogel films. Polym. Chem. 5 (2014) 2824–2835
[23] A.C. Engler, N. Wiradharma, Z.Y. Ong, et al. Emerging trends in macromolecular antimicrobials to fight multi-drug-resistant infections. Nano Today 7 (2012) 201–222