Tulips: A Renewable Source of Monomer for Superabsorbent

Article pubs.acs.org/Macromolecules

Tulips: A Renewable Source of Monomer for Superabsorbent Hydrogels Jozef Kollár,† Miroslav Mrlík,‡ Daniela Moravčíková,† Zuzana Kroneková,† Tibor Liptaj,§ Igor Lacík,† and Jaroslav Mosnácě k*,† †

Polymer Institute, Slovak Academy of Sciences, Dubravska cesta 9, 845 41 Bratislava, Slovakia Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Trida T. Bati 5678, 760 01 Zlin, Czech Republic § Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinskeho 9, 812 37 Bratislava, Slovakia ‡

S Supporting Information *

ABSTRACT: A new class of superabsorbent hydrogels was synthesized by copolymerization of acrylamide with sodium 4hydroxy-2-methylenebutanoate (SHMB). SHMB is obtained by saponification of α-methylene-γ-butyrolactone (MBL) which is a renewable monomer from tulips. Radical polymerization of SHMB was studied for the first time. The extent of side products through the ring closure depended on the pH used during the polymerization. Reactivity ratios for copolymerization of SHMB with acrylamide were estimated. Prepared hydrogels with various composition were investigated for their swelling, mechanical and thermal properties, and morphology as well as cytotoxicity. The achieved degree of swelling of the hydrogels was up to 82 000% and significantly exceeds the capacity of classical superabsorbent hydrogels made of acrylamide and salts of acrylic acid. The degree of swelling as well as mechanical and thermal properties of the hydrogels could be tuned by SHMB content.

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INTRODUCTION Natural products are widely studied during the past years with increasing demand to find new products for replacing petroleum-based raw materials.1,2 Low molecular weight compounds isolated from plants that can be used as monomers can be considered as a special class of renewable products. They offer opportunities for preparation of new types of polymers with a wide range of structural complexity and variety of properties, either by their homopolymerization or more often by copolymerization with other renewable or petroleum-based monomers.3−5 Renewable products, such as cellulose, chitosan, or itaconic acid, were also used for production of novel class of environmentally friendly superabsorbent hydrogels.6−8 Superabsorbent hydrogels are widely used in many applications for personal care and drug delivery systems as well as in biomedicine or in agricultural sector because they are able to absorb a large amount of water. α-Methylene-γ-butyrolactone (MBL), known also as Tulipalin A, is found in the form of glycoside (Tuliposide A) in relatively high concentrations (0.2−2 wt %) in various parts of tulips.9 In addition, MBL can be produced also from biomass sugar-based itaconic anhydride5 or by biosynthesis from pyruvate and acetyl coenzyme A.10 MBL consists of a five-member lactone ring and an exocyclic carbon−carbon double bond, and therefore it can serve as a dual monomer enabling both radical polymerization and ringopening copolymerization. There are just few papers reporting the ring-opening copolymerization, likely due to the low © XXXX American Chemical Society

propensity of the relatively stable ring of MBL to polymerize.11,12 Contrary to that, the reactivity of the MBL exocyclic double bond is high and thus the MBL has been successfully polymerized through the double bond by various polymerization techniques, such as free radical polymerization,13,14 reversible-deactivation radical polymerizations,4,15,16 grouptransfer polymerization,17 coordination polymerization with metallocene,18 and non-metallocene complexes19 and silyliumcatalyzed living anionic-addition polymerization.20 Poly(αmethylene-γ-butyrolactone) (PMBL) exhibits a high refractive index of 1.54021 and a high glass-transition temperature of 195 °C.13 MBL units present in various copolymers and blends render good optical properties, resistance to heat, weathering, scratching, and solvents.3 Polymers with various topology and composition including linear triblock copolymers, star copolymers, and grafted polymers were prepared.4,16,22 Recently, emulsion copolymerization of MBL with acrylic acid in the presence of cross-linker was used to prepare polymer particles.23 Subsequent saponification with sodium hydroxide led to partial opening of the MBL ring and superabsorbent properties of resulting material. In this work, the open form of MBL, i.e., sodium 4-hydroxy2-methylenebutanoate (SHMB), was prepared and used for the first time as a monomer in radical polymerizations. CopolyReceived: March 4, 2016 Revised: May 18, 2016

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DOI: 10.1021/acs.macromol.6b00467 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 1. Experimental Conditions of Radical Polymerizations and Corresponding Monomer Conversions entry

Ma [mmol]

Ia [mmol]

BISa [mmol]

M/H2O (w/w)

T [°C]

pH

time [h]

convb [%]

1 2 3 4 5 6 7 8 9 10 11 12

MBL/AM 4/6 SHMB 10 SHMB 10 SHMB 10 SHMB 10 SHMB/NaCl 10/10 SHMB/NaCl 10/20 SHMB/AM 2/8 SHMB/AM 4/6 SHMB/AM 6/4 SHMB/AM 6/4 SHMB/AM 6/4

V-50 0.02 V-50 0.02 ACVA 0.02 ACVA 0.02 V-50 0.02 V-50 0.02 V-50 0.02 V-50 0.02 V-50 0.02 V-50 0.02 V-50 0.02 V-50 0.02

0.1

20/80 30/70 30/70 30/70 30/70 30/70 30/70 15/85 15/85 15/85 15/85 15/85

50 55 75 75 55 55 55 50 50 50 50 50

7 7 7 7 5 7 7 7 7 7 7 7

4 16 6 16 16 16 16 4 4 4 4 4

95 10 5 7 10 10 10 >97 91 59 59 59

0.1 0.1 0.1 0.05 0.15

note

c d e

f f f f f

M, I, MBL, AM, SHMB, V-50, ACVA, and BIS stand for monomer, initiator, α-methylene-γ-butyrolactone, acrylamide, sodium 4-hydroxy-2methylenebutanoate, 2,2′-azobis(2-methylpropionamidine) dihydrochloride, 4,4′-azobis(4-cyanovaleric acid), and N,N′-methylenebis(acrylamide), respectively. bBased on 1H NMR spectra. c∼1 mol % of MBL was formed. d∼4 mol % of MBL was present in solution + insoluble polymer was formed. e∼5 mol % of MBL was formed. fConversion determined from polymerizations performed under the same conditions apart from the absence of BIS. a

white gel that was insoluble in water. The polymer sample was frozen and lyophilized overnight. ATR-FTIR of resulted gel is shown in Figure S1. Saponification of Poly(acrylamide-co-α-methylene-γ-butyrolactone) Gels. Poly(acrylamide-co-α-methylene-γ-butyrolactone) gel (1 g) was placed in a 25 mL round-bottom flask, 20% sodium hydroxide solution (10 mL) was added, and reaction mixture was heated at 95 °C for 5 h. Initially a white polymer sample formed a transparent gel after 2 h. The polymer was frozen and lyophilized overnight. ATR-FTIR of resulted hydrogel is shown in Figure S2. Homopolymerization of Sodium 4-Hydroxy-2-methylenebutanoate. α-Methylene-γ-butyrolactone (10 mmol) was stirred in aqueous solution (2 mL) of NaOH (1.2 equiv of MBL) at 95 °C for 2 h. The solution was neutralized with 1 M HCl to pH = 7, eventually to pH = 5, and initiator 0.02 mmol of V-50 (Table 1, entries 2 and 5) or ACVA (Table 1, entries 3 and 4) was added. Reaction mixture was diluted with water to form 15 wt % monomer solution. The solution was injected into glass tube, purged with nitrogen for 10 min, and sealed off. Polymerization was carried out at 55 and 75 °C for 6 or 16 h (Table 1, entries 2−5). Sodium 4-hydroxy-2-methylenebutanoate (SHMB): 1H NMR (300 MHz, D2O): 5.75 ppm 1H (d), J = 1.2 Hz (C−H); 5.35 ppm 1H (d), J = 1.2 Hz (C−H); 3.65 ppm 2H (t), J = 5.0 Hz (−CH2−O); 2.45 ppm 2H (t), J = 5.0 Hz (−CH2−C) (see Figure S3a). 13C NMR (100 MHz, D2O): δ 176.5 (⟩CO), 143.1 (⟩C), 121.5 (CH2), 60.6 (−CH2−O), 35.5 (−CH2−) (Figure S3b). ATR-FTIR (cm−1, Ge): 1638 w, 1558 s, 1422 m, 1398 m, 1045 w (Figure S4). 1H NMR spectra of polymerization mixtures after 16 h of homopolymerization of SHMB in D2O at (a) pH = 7, T = 55 °C; (b) pH = 7, T = 75 °C; and (c) pH = 5, T = 55 °C are shown in Figure S5. Degree of Ionization of Sodium 4-Hydroxy-2-methylenebutanoate. The degree of ionization α was determined by neutralization of fully ionized SHMB aqueous solution using 1 M hydrochloric acid. Corresponding pH values were measured by EcoScan Ion 6 m, Eutech Instruments Europe B.V., Netherlands. Reactivity Ratios for Copolymerization of Acrylamide with Sodium 4-Hydroxy-2-Methylenebutanoate. Solution of monomers AM and SHMB at the appropriate ratios ( f SHMB = 0.14, 0.31, 0.32, 0.46, 0.53, 0.63, 0.76, and 15 wt % monomer solution), initiator V-50 (0.2 mol % of total monomer content), and D2O (1 mL) were injected into NMR tube, purged with nitrogen for 10 min, and sealed. The NMR tube was then immersed into 50 °C water bath for 1 min followed by placing the tube into the NMR sample chamber of instrument with heating element thermostated to 50 °C. The tube was allowed to heat for 2 min prior to taking the first scan; during this time the instrument was shimmed to achieve desired homogeneity of the magnetic field. The scans were taken every 30 s for up to 40 min.

merization of SHMB with acrylamide (AM) at various ratios in the presence of cross-linker yielded hydrogels with superior degree of swelling and comfortable handling.

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EXPERIMENTAL SECTION

Materials. Acrylamide (AM, >98%), α-methylene-γ-butyrolactone (MBL, 97%), acrylic acid (AA, >99%), N,N′-methylenebis(acrylamide) (BIS, 99%), deuterated water (D2O, 99.9%), 2,2′-azobis(2-methylpropionamidine) dihydrochloride (V-50, 97%), 4,4′-azobis(4-cyanovaleric acid) (ACVA, >98%), and poly(ethylene imine) (PEI, Mn = 2000 g mol−1, branched polymer, 50 wt % aqueous solution) were purchased from Sigma-Aldrich, USA, and were used as received. Hydrochloric acid (35%), analytical grade, was purchased from Centralchem, Slovakia, and sodium hydroxide from AFT Bratislava, Slovakia. 3-(4,5-Dimethyldiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was purchased from Calbiochem (Merck Millipore, Darmstadt, Germany); dimethyl sulfoxide (DMSO) was from Sigma-Aldrich (Weinheim, Germany); Dulbecco’s Modified Eagle Medium (DMEM), fetal bovine serum (FBS), streptomycin, penicillin, and L-glutamine were purchased from Gibco (Life Technologies, Grand Island, NY). Ultrapure water was obtained using Ultrapure Water System NW Series (Heal Force Bio-Meditech Holdigs, Ltd., China). NMR Spectroscopy. 1H NMR were recorded in D2O on a Varian Mercury 300 instrument at 298 K at a working frequency of 300 MHz. Chemical shifts are reported in ppm with reference to the internal standard trimethylsilylpropanoic acid (TSP). 13C NMR were recorded in D2O on a Varian MR400 at 298 K. Determination of AM and sodium 4-hydroxy-2-methylenebutanoate (SHMB) in copolymer composition was perform by an in situ NMR technique using Varian/Agilent VNMRS 600 MHz spectrometer equipped with an indirect triple resonance HCN probe. ATR-FTIR Spectroscopy. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) measurements were performed with an FTIR NICOLET 8700 spectrometer (Thermo Scientific, UK) using a single bounce ATR accessory equipped with a Ge crystal. For each measurement, the spectral resolution was 2 cm−1 and 64 scans were performed. The spectra were characterized with abbreviations for determination of relative intensities of signals as s = strong, m = medium, and w = weak. Synthesis of Poly(acrylamide-co-α-methylene-γ-butyrolactone) Gels. Acrylamide (6 mmol), α-methylene-γ-butyrolactone (4 mmol), cross-linker BIS (0.1 mmol), and initiator V-50 (0.02 mmol) were placed in a tube, water (4.6 mL) was added, and the system was bubbled with nitrogen for 10 min and sealed off. The polymerization mixture was then heated at 50 °C for 4 h (Table 1, entry 1) to form a B

DOI: 10.1021/acs.macromol.6b00467 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. Representative 1H NMR spectrum of copolymerization mixture of acrylamide with sodium 4-hydroxy-2-methylenebutanoate in D2O. The conventional Mayo−Lewis method24,25 was used to determine the reactivity ratios rAM for acrylamide and rSHMB for SHMB at low conversions