Bio-based polymer networks by thiol-ene photopolymerization of allylated l -glutamic acids and l -tyrosines

Accepted Manuscript Bio-based polymer networks by thiol-ene photopolymerization of allylated Lglutamic acids and L-tyrosines Shima Aoyagi, Toshiaki Shimasaki, Naozumi Teramoto, Mitsuhiro Shibata PII: DOI: Reference:

S0014-3057(18)30074-0 https://doi.org/10.1016/j.eurpolymj.2018.02.027 EPJ 8301

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

12 January 2018 16 February 2018 19 February 2018

Please cite this article as: Aoyagi, S., Shimasaki, T., Teramoto, N., Shibata, M., Bio-based polymer networks by thiol-ene photopolymerization of allylated L-glutamic acids and L-tyrosines, European Polymer Journal (2018), doi: https://doi.org/10.1016/j.eurpolymj.2018.02.027

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Bio-based polymer networks by thiol-ene photopolymerization of allylated L-glutamic acids and L-tyrosines

Shima Aoyagi, Toshiaki Shimasaki, Naozumi Teramoto and Mitsuhiro Shibata*

Department of Life and Environmental Sciences, Faculty of Engineering,

Chiba Institute of Technology, 2-17-1, Tsudanuma, Narashino, Chiba 275-0016, Japan

*Corresponding author. Tel.:+81 47 478 0423; fax:+81 47 478 0423. E-mail address: [email protected] (M. Shibata) URL: http://www.le.it-chiba.ac.jp/env-org/index.html (M. Shibata)

1

ABSTRACT Triallylated L-glutamic acid and L-tyrosine (A3E amd A3Y) and their tetraallyl derivatives (A4E and A4Y) were synthesized by reactions of L-glutamic acid and L-tyrosine with allyl bromide

in

the

presence

of

potassium

carbonate,

respectively.

Thiol-ene

photopolymerizations of the allylated amino acids and a pentaerythritol-based tetrathiol (S4P) at allyl/SH 1/1 in the presence of photo-initiators produced photo-cured products (pA3E-S4P, pA3Y-S4P, pA4E-S4P and pA4E-S4P).

Furthermore, thermal post-curing after photo-curing

of allylated amino acid/S4P mixtures containing photo-initiators and a thermal initiator produced photo and thermal cured products (ptA3E-S4P, ptA3Y-S4P, ptA4E-S4P and ptA4E-S4P).

Although the thermally post-cured products became dark in color, all the cured

films were flat and had smooth surfaces.

The FT-IR analysis revealed that allyl and thiol

conversions increased by the thermal post-curing for all the curing systems. The glass transition temperature (Tg), 5% weight loss temperature (Td5), tensile strength and tensile modulus of each thermally post-cured product were higher than those of the corresponding photo-cured product, reflecting the increase of crosslinking density by the thermal post-curing. Also, A4E- and A4Y-based cured products exhibited higher Tgs, Td5s, tensile strengths and moduli than the corresponding A3E- and A3Y-based cured products, respectively.

Especially,

ptA4Y-S4P exhibited the highest Tg (24.4 °C), Td5 (319 C), tensile strength (18.2 MPa) and tensile modulus (1037 MPa) among all the cured products.

Keywords:

allylated

glutamic

acid;

allylated

tyrosine;

photopolymerization; thermal post-curing; bio-based polymer network

2

tetrathiol;

thiol-ene

1. Introduction The utilization of renewable resources for polymer synthesis has become an important issue due to the price fluctuation and risks against the future depletion of petroleum resources [1-5]. Furthermore, it is expected that some of renewable resources-derived polymers (i.e., bio-based polymers) have potential biocompatibility and biodegradability necessary for the applications in biomaterials and ecofriendly materials [6].

Recently, radical-mediated

thiol-ene “click” reactions attracted a great deal of attention as an efficient method for the production of bio-based polymer networks [7-11]. initiated by traditional thermal conditions

The radical thiol–ene reaction can be

with common azo

species such as

2,2’-azobis(isobutyronitrile) (AIBN) or via photochemical methods, with or without added photo-initiator. Thiol-ene reactions combine the advantages of the formation of uniform polymer networks, rapid crosslinking, delayed gelation, reduced shrinkage and insensitivity to oxygen. Furthermore, as the reaction is initiated by the formation of primary radicals by the decomposition of an initiator and subsequent production of a thiyl radical by the abstraction of a hydrogen radical from the thiol group, monomers containing functional groups (OH, CO2H, NH2 etc.) with a hydrogen atom that is less easily abstracted than the hydrogen atom of a thiol group can be polymerized without protecting the functional groups [11]. In past studies, several research groups reported on the syntheses of bio-based polymer networks by thiol-ene reactions [12-24]. For example, Li et al. reported thiol-ene thermosets based on filly bio-based poly(limonene carbonate) [12]. Kristufek et al. reported isosorbide-based crosslinked polycarbonate elastomers by thiol-ene photopolymerization [15]. Colucci et al. reported keratin-based photo-cured coatings by thiol-ene polymerization [18].

Ortiz et al.

reported thiol-ene photopolymerization of hexathiolated squalene and polyallyl compounds [19].

We reported polymer networks by thiol-ene photopolymerizations of allylated

trehalose, eugenol, gallic acid and pyrogallol with various polythiol compounds [21-25].

3

Also, we reported the preparation and properties of amino acid-incorporated polymer networks by thiol-ene thermal polymerizations of triallylated L-alanine and triallylated L-phenylalanine (A3A and A3F) with pentaerythritol tetrakis(3-mercaptopropionate) (S4P)

[26].

However, we could not prepare flat films with smooth surfaces by the photo-initiated

thiol-ene polymerizations of A3A/S4P and A3F/S4P, because of the generation of many wrinkles. Photo-curing methods have gained increased interest due to their low energy consumption, high curing speed and low emissions of volatile organic compounds.

In this

study, we succeeded to prepare flat films with very smooth surfaces by the photo-initiated thiol-ene polymerizations of tri- and tetra-allylated L-glutamic acids (A3E and A4E), and triand tetra-allylated L-tyrosines (A3Y and A4Y) with S4P (Scheme 1).

Furthermore, we

investigated the effect of thermal post-curing after photo-curing of the allylated amino acids and S4P. Our attention is focus on the influence of the difference in amino acids, degrees of allylation and curing conditions on the thermal and mechanical properties.

Scheme 1.

Syntheses of A3E, A4E, A3Y and A4Y, and the chemical structure of S4P.

4

2. Experimental 2.1. Materials L-Glutamic acid (Glu, E) and allyl bromide were purchased from Tokyo Kasei Kogyo Co. Ltd.

(Tokyo, Japan). L-Tyrosine (Tyr, Y) and pentaerythritol-based tetrathiol (S4P, pentaerythritol tetrakis(3-mercaptopropionate)) were purchased from Sigma-Aldrich Corporation (St. Louis, MO, USA).

As photo-initiators, 1-hydroxycyclohexyl phenyl ketone (IRGACURE ® 184,

mp. 45-49 °C, UV/VIS absorption peaks in methanol: 246, 280, 333 nm) and phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide (IRGACURE ® 819, mp. 127-133 °C, UV/VIS absorption peaks in methanol: 295, 370 nm) were purchased from Toyotsu Chemiplas Corporation (Tokyo, Japan).

Di-tert-butylperoxide was purchased from Tokyo Chemical

Industry Co. Ltd. (Tokyo, Japan). Potassium carbonate and N,N-dimethylformamide (DMF) were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan) and Wako Pure Chemical Industries, Ltd. (Osaka, Japan), respectively.

All of the commercially available reagents

were used without further purification.

2.2. Allylation of L-glutamic acid Potassium carbonate (30.4 g, 220 mmol) was added to a solution of Glu (7.36 g, 50.0 mmol) in DMF (150 mL), and the resulting mixture was stirred for 10 min. Allyl bromide (26.6 g, 220 mol) was dropwise added over a period of 30 min at room temperature. addition, the mixture was stirred for 24 h at 40 °C and filtered.

After the

Dilute hydrochloric acid was

added to the filtrate, and the product was extracted with ethyl acetate/hexane 3/1 (v/v, 50 mL3) three times.

The organic layer was washed with water, dried over sodium sulfate and

concentrated in vacuo to produce a mixture of incompletely allylated compounds (9.49 g) as a brown liquid. The isolated crude product was abbreviated as A3E, because the degree of substitution (DS) of allyl groups was evaluated to be approximately 3 by the 1H-NMR method.

5

A3E: ESI-MS (negative mode, m/z) calcd. for C14H21O4N 267.1471, found [M‒H]+ 266.1409. A3E (8.01g, 30 mmol) was again reacted with allyl bromide (7.26 g, 60.0 mmol) and potassium carbonate (8.29 g, 60.0 mmol) in a similar manner to the above to give A4E as a brown liquid (7.64 g) in 59% yield. A4E:

13

C-NMR (, ppm, in CDCl3) 173.0, 172.4, 136.5,

132.3, 132.2, 118.5, 118.3, 117.3, 65.2, 65.0, 60.6, 53.4, 30.9, 24.6; ESI-MS (positive mode, m/z) calcd. for C17H25O4N 307.17845, found [M+H]+ 308.1848.

2.3. Allylation of L-tyrosine Potassium carbonate (30.4 g, 220 mmol) was added to a solution of Tyr (9.06 g, 50.0 mmol) in DMF (150 mL), and the resulting mixture was stirred for 10 min. Allyl bromide (26.6 g, 220 mol) was dropwise added over a period of 30 min at room temperature. the addition, the mixture was stirred for 24 h at 40 °C and filtered.

After

Dilute hydrochloric acid

was added to the filtrate, and the product was extracted with ethyl acetate/hexane 3/1 (v/v, 50 mL3) three times.

The organic layer was washed with water, dried over sodium sulfate

and concentrated in vacuo to produce a mixture of incompletely allylated compounds (12.1 g) as a brown liquid. The isolated crude product was abbreviated as A3Y, because the DS was evaluated to be approximately 3 by the 1H-NMR method. A3Y: ESI-MS (negative mode, m/z) calcd. for C18H23O3N 301.1679, found [M‒H]+ 300.1616. A3Y (9.03 g, 30 mmol) was again reacted with allyl bromide (7.26 g, 60.0 mmol) and potassium carbonate (8.29 g, 60.0 mmol) in a similar manner to the above to give A4Y as a brown liquid (9.82 g) in 77% yield. A4Y:

13

C-NMR (, ppm, in CDCl3) 172.3, 157.2, 136.4,

133.5, 132.1, 130.7, 130.4, 118.3, 117.6, 117.3, 114.5, 68.9, 64.8, 64.0, 53.6, 35.1; ESI-MS (positive mode, m/z) calcd. for C21H27O3N 341.1992, found [M+H]+ 342.2055.

2.4. Thiol-ene photopolymerization

6

A typical procedure for the thiol-ene photopolymerization of A4E and S4P is as follows: a mixture of A4E (1.93 g, 6.28 mmol), S4P (3.07 g, 6.28 mmol) and photo-initiators (total 150 mg, 3.0 wt% of the total weight of reactants, IRGACURE ® 184 /IRGACURE ® 819 = 3/1 (w/w)) [24] and acetone (5 mL) was stirred for 1 h to form a homogeneous solution. The solution was poured on a poly(methylpentene) culture dish with an outside diameter of 90.5 mm and was dried at room temperature for 1 h. The culture dish was cooled for 10 min in a freezer, and then photo-irradiated for 60 s  10 times at 1 min intervals at room temperature to yield a photo-cured A4E/S4P film with the allyl/SH ratio of 1/1 (pA4E-S4P) (thickness: ca. 0.9 mm). A SPOT-CURE SP-7 (250 W light source, wavelength 240-440 nm, Ushio Inc., Yokohama, Japan) device equipped with a uniform-radiation optical unit was used for UV curing (irradiation distance 200 mm, irradiation intensity 60.6 mW cm‒2). Also, photo-cured A3E/S4P, A3Y/S4P and A4Y/S4P films with the allyl/SH ratio of 1/1 (pA3E-S4P, pA3Y-S4P and pA4E-S4P) were prepared in a similar manner to pA4E-S4P.

2.5. Thiol-ene photo and thermal polymerization A typical procedure is as follows: A mixture of A4E (1.93 g, 6.28 mmol), S4P (3.07 g, 6.28 mmol), photoinitiators (total 150 mg, 3.0 wt% of the total weight of reactants, IRGACURE ® 184 /IRGACURE ® 819 = 3/1 (w/w)), di-tert-butylperoxide (150 mg, 3.0 wt% of the total weight of reactants) and acetone (5 mL) was stirred for 1 h to form a homogeneous solution. The solution was poured on a poly(methylpentene) culture dish with an outside diameter of 90.5 mm and was dried at room temperature for 1 h. The obtained mixture was photo-irradiated in a similar manner to pA4E-S4P, and then thermally cured at 100 °C for 4 h and furthermore at 130 °C for 20 h in an electric oven to yield a photo and thermal cured A4E/S4P film with the allyl/SH ratio of 1/1 (ptA4E-S4P) (thickness: ca. 0.9 mm). Also, photo and thermal cured A3E/S4P, A3Y/S4P and A4Y/S4P films with the allyl/SH ratio of 1/1

7

(ptA3E-S4P, ptA3Y-S4P and ptA4Y-S4P) were prepared in a similar manner to ptA4E-S4P.

2.6. Measurements Proton and carbon nuclear magnetic resonance (1H- and 13C-NMR) spectra were recorded on a JEOL JNM-ECA500 (500 MHz) using CDCl3 and tetramethylsilane as a solvent and an internal standard. Fourier transform infrared (FT-IR) spectra were recorded at room temperature in the range from 4000 to 700 cm–1 on a Shimadzu (Kyoto, Japan) IRAffinity-1S by the attenuated total reflectance (ATR) method. scans at a resolution of 4 cm–1.

The IR spectra were acquired using 50

Degree of swelling (Ds) was measured by dipping a film (10

× 10 × 0.9 mm3) in DMF at room temperature for 24 h according to the following equation: Ds (wt%) = 100(w1–w0)/w0; where w0 is the initial weight of the film and w1 is the weight of the swollen film after dipping.

In addition, the film dipped in acetone for 24 h was dried at

40 °C in a vacuum oven for 24 h, and the gel fraction by the acetone extraction was calculated by the equation: Gel fraction (wt%) = 100w2/w0 ; where w2 is the weight of the dried film. The differential scanning calorimetry (DSC) measurements were performed on a Perkin-Elmer Diamond DSC (Waltham, MA, USA) in a nitrogen atmosphere. The samples were heated from −50 °C to 100 °C at a heating rate of 20 °C min−1 to determine the glass transition temperature (Tg). The 5% weight loss temperature (Td5) was measured on a Shimadzu TGA-50 thermogravimetric analyzer at a heating rate of 20 °C min−1 in a nitrogen atmosphere. Tensile testing of the rectangular plates (length 50 mm, width 10 mm, thickness 0.9 mm) was performed at room temperature using a Shimadzu Autograph AG-1 based on the standard method for testing the tensile properties of plastics (JIS K7161:1994, ISO527-1). The span length was 25 mm, and the testing speed was 10 mm min−1. Five specimens were tested for each set of samples, and the mean value and standard deviation were calculated.

8

3. Results and discussion 3.1. Synthesis and characterization of allylated glutamic acids and tyrosines

Figure 1. Overall and partially expanded 500 MHz 1H-NMR spectra of A4E in CDCl3.

The

values given in parentheses are integral values of each 1H-signal.

The reaction of Glu (E) with allyl bromide at the molar ratio of 1/4.4 in the presence of 4.4 molar equivalent of potassium carbonate against Glu produced A3E whose average DS is approximately 3. Although we added more allyl bromide and potassium carbonate, the DS did not increase. Therefore, A3E was again reacted with allyl bromide at the molar ratio of 1/2.0 in the presence of 2.0 molar equivalent of potassium carbonate against Glu produced A4E in an overall yield of 59% (Scheme 1). 1

H-NMR spectra of A4E in CDCl3.

Figure 1 shows overall and partially expanded

The 1H-signals of two O-allyl groups for A4E were

observed at  5.92 (m, 2H, H-b), 5.32 and 5.30 (ddd, 2H, H-c, Jcb = 17.0 Hz, Jcd = 3.3 Hz, Jca = 1.8 Hz), 5.23 and 5.21 (ddd, 2H, H-d, Jdb = 10.6 Hz, Jdc = 3.3 Hz, Jda = 0.9 Hz), and 4.58 9

ppm (m, 4H, H-a). The 1H-signals of two N-allyl group for A4E were observed at  5.70 (m, 2H, H-f), 5.16 (brd, 2H, H-g, Jgf = 17.2 Hz), 5.08 (brd, 1H, H-h, Jhf = 10.0 Hz), 3.33 (ddt, 2H, H-e, Jee’ = 14.0 Hz, Jef = 4.6 Hz, Jeg, Jeh = 1.4 Hz) and 3.03 ppm (dd, 2H, H-e’, Je’e = 14.0 Hz, Je’f = 7.9 Hz). The 1H-signals of the Glu core of A4E were observed at  3.49 (dd, 1H, H-i, Jjj’ = 9.8 Hz, Jji = 5.6 Hz), 2.44 (m, 2H, H-k) and 1.99 ppm (m, 2H, H-j). The 1H-NMR spectral analysis revealed that A4E is a tetraallylated derivative of Glu.

Figure 2. Overall and partially expanded 500 MHz 1H-NMR spectra of A4Y in CDCl3. The values given in parentheses are integral values of each 1H-signal.

Similar allylation reaction of Tyr (Y) produced A3Y whose average DS is approximately 3. The A3Y was again allylated to produce A4Y in an overall yield of 77% (Scheme 1).

Figure

2 shows overall and partially expanded 1H-NMR spectra of A4Y in CDCl3. The 1H-signals of one allyl ester group for A4Y were observed at  6.05 (m, 1H, H-b), 5.40 (ddd, 1H, H-c, Jcb = 17.1 Hz, Jcd = 2.4 Hz, Jca = 1.8 Hz), 5.27 (ddd, 1H, H-d, Jdb = 10.7 Hz, Jdc = 3.6 Hz, Jda = 1.2 Hz), and 4.53 ppm (m, 2H, H-a).

The 1H-signals of one N-allyl group for A4Y were 10

observed at  5.70 (m, 2H, H-f), 5.14 (ddd, 2H, H-g, Jgf = 17.1 Hz, Jgh = 2.7 Hz, Jge = 1.8 Hz), 5.08 (brd, 2H, H-h, Jhf = 10.2 Hz), 3.38 (ddt, 2H, H-e, Jee’ = 13.9 Hz, Jef = 4.5 Hz, Jeg, Jeh = 2.3 Hz) and 3.09 ppm (dd, 2H, H-e’, Je’e = 13.9 Hz, Je’f = 6.9 Hz). The 1H-signals of one allyl ether group for A4Y were observed at  5.86 (m, 1H, H-n), 5.23 (ddd, 2H, H-o, Jon = 17 Hz, Jop = 3 Hz, Jom = 2 Hz), 5.19 (ddd, 2H, H-p, Jpn = 11 Hz, Jpo = 3 Hz, Jpm = 1 Hz) and 4.55 (m, 2H, H-m).The 1H-signals of the Tyr core of A4Y were observed at  7.05 (d, 2H, H-k, Jkl = 8.3 Hz), 6.82 (d, H-l, Jlk = 8.3 Hz), 3.68 (dd, 1H, H-i, Jij = 6.9 and 3.8 Hz), 3.38 (ddt, 2H, H-e, Jee’ = 13.9 Hz, Jef = 4.5 Hz, Jeg,eh = 2.3 Hz), 3.09 (dd, 2H, H-e’, Je’e = 13.9 Hz, Je’f = 6.9 Hz), 3.00 (dd, 1H, H-j, Jjj’ = 13.9 Hz, Jji = 8.1 Hz) and 2.83 (dd, 1H, H-j’, Jj’j = 13.9 Hz, Jj’i = 6.9 Hz). The 1H-NMR analysis revealed that all the carboxy, phenolic hydroxy and amino groups of A4Y are allylated.

Scheme 2.

Main components of A3E and A3Y.

Although we could not assign all the proton signals of A3E and A3Y which are mixtures of incompletely allylated compounds, the main components of A3E and A3Y were isolated by silica gel column chromatography in purification yields of 79% and 80%, respectively. The 1

H-NMR spectral analysis revealed the main components of A3E and A3Y are

N-allylamino-bis(allyl carboxylate) and N,N-diallylamino-(allyl carboxylate) derivatives, respectively (Scheme 2, for their 1H-NMR spectra, see Figures S1 and S2 in Supplementary Material).

The fact that the N-monoallyl derivative was obtained in spite of the presence of 11

a slight excess of allylation reagents for the main component of A3E may be attributed to a steric hindrance between the N-allyl and O-allyl groups. Also, regarding the fact that the oxygen atom of phenol moiety of Tyr was not allylated for the main component of A3Y, it is inferred that the phenolic hydroxy group with the lowest acidity has the lowest nucleophilic reactivity in the allylation reaction using a weakly basic potassium carbonate, considering that pKa values of carboxy, -ammonium and phenolgroups of Tyr are 2.20, 9.21 and 10.46, respectively [27]. The presence of allyl and ester groups for the allylated amino acids were also confirmed by the FT-IR spectral analysis, as is discussed in the following section. The chemical structures of A3E, A4E, A3Y and A4Y characterized by the 1H-NMR spectral analysis were confirmed by the

13

C-NMR and electrospray ionization mass spectroscopy

(ESI-MS) analyses (see experimental section and Figures S3-S6 in Supplementary Material).

3.2. Thiol-ene polymerizations of A3E, A4E, A3Y and A4Y with S4P A mixture of allylated amino acid (A3E, A4E, A3Y or A4Y), S4P and photoinitiators (IRGACURE®-184/819 = 3/1 (w/w)) at the allyl/thiol ratio of 1/1 on a culture dish was cooled for 10 min in a freezer set on ‒18°C. The culture dish was taken out of the freezer, and immediately photopolymerized under a condition of room temperature to produce a crosslinked pale yellow flat film (pA3E-S4P, pA4E-S4P pA3Y-S4P or pA4Y-S4P) with smooth surfaces as is shown in Figure 3 and Figure S7 (see Supplementary Material). When the mixture was simply photo-cured at room temperature without cooling in a freezer, we could not get a flat film due to the generation of many wrinkles. It is considered that an increase of viscosity of the liquid mixture and a mild photo-curing reaction by cooling the sample prevented the formation of wrinkles due to heterogeneous curing shrinkage.

The

above mixtures were photo-cured in the presence of di-tert-butylperoxide, and then thermally post-cured at 100 °C for 4 h and then 130 °C for 20 h to produce ptA3E-S4P, ptA4E-S4P,

12

ptA3Y-S4P and ptA4Y-S4P. Although the thermally post-cured films became dark in color compared with the corresponding photo-cured films, these films were also flat and had sooth surfaces (Figure 3 and Figure S7, see Supplementary Material).

Figure 3. Photographs of pA4E-S4P, ptA4E-S4P, pA4Y-S4P and ptA4Y-S4P.

Figure 4 shows FT-IR spectra of pA4E-S4P, ptA4E-S4P, pA4Y-S4P and ptA4Y-S4P compared with those of A4E, A4Y and S4P.

In the FT-IR spectra of A4E, the bands ascribed

to allyl C=C stretching (νC=C) and =C–H out-of-plane bending (δ=C-H) vibrations were observed at 1643 cm−1 and 991, 922 cm−1, respectively. Also, A4Y exhibited allyl C=C and δ=C-H bands at 1641 cm−1 and 991, 922 cm−1, respectively. S4P displayed a clear absorption band due to stretching vibration of S-H group (SH) at 2569 cm−1 in a longitudinally enlarged spectrum at the wavelength region from 2500 to 2700 cm−1.

Regarding absorption bands

other than allyl and thiol groups, ester C=O stretching vibration (νC=O) bands were observed at 1728~1738 cm−1 for all the reactants (A4E, A4Y and S4P), and benzene ring νC=C bands were observed at 1610 and 1510 cm−1 for A4Y.

In the FT-IR spectra of pA4E-S4P,

ptA4E-S4P, pA4Y-S4P and ptA4Y-S4P, the allyl C=C and SH bands were almost nonexistent, 13

indicating that the thiol-ene polymerization certainly proceeded.

Since S4P had absorption

bands with medium intensities at a wavenumber region from 920 to 990 cm−1, where allyl δ=C-H bands should be observed, all the cured resins showed weak bands at the same wavenumber region. Therefore, the change of absorbance of the allyl C=C and SH bands was used to evaluate the conversion of thiol-ene reaction. The FT-IR spectra of pA3E-S4P, ptA3E-S4P, pA3Y-S4P and ptA3Y-S4P were similar to those of pA4E-S4P, ptA4E-S4P, pA4Y-S4P and ptA4Y-S4P, respectively, indicating that the thiol-ene polymerization certainly proceeded (Figure S8, see Supplementary Material).

Figure 4. FT-IR spectra of pA4E-S4P, ptA4E-S4P, pA4Y-S4P and ptA4Y-S4P compared with those of S4P, A4E and A4Y.

Figure 5 shows the change of absorbance at the wavenumber regions of 2700-2500 cm−1 and 1800-1500 cm−1 for FT-IR spectra of A4E/S4P and A4Y/S4P mixtures before curing and their cured products. Also, the corresponding FT-IR spectra of A3E/S4P and A3Y/S4P are 14

shown in Figure S9 (see Supplementary Material). As a reference peak which does not change by the thiol-ene reaction, the ester C=O band was used. The conversions of thiol and allyl groups by the thiol-ene reaction were evaluated by relative absorbance values of SH and allyl C=C bands at 2570 cm−1 and 1645 (1641) cm−1 relative to the ester C=O band at 1732 cm−1. Table 1 summarizes allyl and thiol conversions of all the cured products. For all the cured products, the thiol conversions were higher than the allyl conversions, suggesting that a side reaction (for example, oxidative thiol-coupling reaction) may have occurred to some extent besides the thiol-ene reaction. The allyl and thiol conversions of thermally post-cured products were higher than those of the photo-cured products, indicating that thiol-ene reaction further progressed by the thermal post curing.

Also, the fact that the allyl conversion

(69%) of pA4Y-S4P was lower than that (82%) of pA3Y-S4P may be caused by a lower reactivity of the allyloxybenzene moiety of A4Y, considering that the phenol moiety of the main component of A3Y is not allylated.

Figure 5. The FT-IR analysis for determining the conversion of thiol-ene reaction for curing reactions of A4E/S4P and A4Y/S4P. 15

Table 1 also summarizes the values of gel fraction and Ds of the cured films.

All the

cured products exhibited very high gel fractions (94.0~98.4%) as a proof of the formation of polymer network.

The gel fraction of each thermally post-cured product was a little higher

than that of the corresponding photo-cured product in agreement with the progress of thermal curing reactions. Also, the Ds of each thermally post-cured product was lower than that of the corresponding photo-cured product in agreement with the increase of crosslinking density. The fact that A3Y- and A4Y-based cured products exhibited lower Ds values than the corresponding A3E- and A4E-based cured products should be attributed to a more hydrophobic character of the oxybenzyl moiety of A3Y and A4Y than that of the propionic acid moiety of A3E and A4E. Table 1. The values of thiol-ene conversion, Ds, gel fraction and Td5 for cured films. Conversion of

Conversion of

Gel fractiona

Ds b

Td5

thiol group (%)

allyl group (%)

(wt%)

(wt%)

(°C)

pA3E-S4P

94

82

94.0

132

254

ptA3E-S4P

97

87

95.5

85

290

pA3Y-S4P

92

82

95.6

78

299

ptA3Y-S4P

95

83

98.4

60

317

pA4E-S4P

90

87

94.0

75

267

ptA4E-S4P

96

88

95.5

70

292

pA4Y-S4P

93

69

95.6

60

302

ptA4Y-S4P

97

75

98.4

48

319

Sample

a

The sample were extracted with acetone for 24 h.

b

The samples were dipped in DMF for 24 h.

16

3.3. Properties of cured films

Figure 6. DSC curves of all the cured products.

Figure 6 shows DSC charts of all the cured products.

Every cured product showed only one

Tg at a temperature region from ‒4 to 25 °C, suggesting that the cured product is amorphous and homogeneous. The Tg of each thermally post-cured product was higher than that of the corresponding photo-cured product, reflecting the increase of crosslinking density by the thermal post-curing.

Furthermore, A4E- and A4Y-based cured products exhibited higher Tg

values than the corresponding A3E- and A3Y-based cured products, respectively, in agreement with the fact that the former samples have higher crosslinking density than the latter ones. The Tg values (21.3 and 24.4 °C) of ptA3Y-S4P and ptA4Y-S4P were much higher than those (‒1.4 and 4.4 °C) of ptA3E-S4P and ptA4E-S4P, respectively, reflecting that the oxybenzyl moiety of A3Y and A4Y is more rigid than the propionic acid moiety of A3E and A4E. Similarly, the Tg (‒1.5 °C) of pA3Y-S4P was a little higher than that (‒4.4 °C) of pA3E-SP. However, the Tg (0.11 °C) of pA4Y-S4P was slightly lower than that (2.2 °C) of pA4E-SP. 17

This result should be caused by the fact that pA4Y-S4P exhibited the lowest allyl conversion (69%) among all the cured products. Table 1 also summarizes the Td5 values evaluated by TGA measurements.

The Td5 of

each thermally post-cured product was higher than that of the corresponding photo-cured product.

A4E- and A4Y-based cured products exhibited higher Td5s than the corresponding

A3E- and A3Y-based cured products, respectively. These trends of Td5 values should be caused by the difference in crosslinking densities in a similar manner to the trends of Ds and Tg. Also, A3Y- and A4Y-based cured products displayed much higher Td5s than A3E- and A4E-based cured products, respectively.

We have previously reported that the Td5 (282 °C)

of a thermally cured A3F/S4P (tA3F-S4P) was much higher than that (258 °C) of a thermally cured A3A/S4P (tA3A-S4P) [26]. These results are reasonably explained by the reason that the Tyr and phenylalanine frameworks containing aromatic rings are more thermally stable than the aliphatic Glu and alanine frameworks.

Furthermore, the fact that the Td5s (292 and

319 °C) of ptA4E-S4P ptA4Y-S4P are much higher than those (258 and 282 °C) of tA3A-S4P and tA3F-S4P suggests that the incorporation of side chains of amino acids into the polymer network is effective to improve the heat resistance. Figure 7 shows tensile properties of all the cured products. The tensile strength and modulus of each thermally post-cured product were higher than those of the corresponding photo-cured product.

Especially, the tensile strengths and moduli of ptA3Y-S4P and

ptA4Y-S4P were much higher than those of pA3Y-S4P and pA4Y-S4P, respectively, attributable to the fact that the former networks are in glassy state at room temperature, while the latter is in rubbery state, as is obvious from their Tgs.

Furthermore, A4E- and

A4Y-based cured products exhibited higher tensile strengths and moduli than the corresponding A3E- and A3Y-based cured products, respectively. These trends of tensile properties can be reasonably explained by the structure-properties relationship elucidated

18

from Ds, Tg and Td5 values. Also, all the A3Y- and A4Y-based cured products showed higher tensile strengths and moduli than the corresponding A3E- and A4E-based cured products, except that the tensile modulus (5.35 MPa) of pA3Y-pS4P was a little lower than that (5.54 MPa) of pA3E-S4P. This result may be caused by the fact that the thiol and allyl conversions of pA3Y-S4P were slightly lower than those of pA3E-S4P (Table 1). The tensile strength and modulus (18.2 and 1033 MPa) of ptA4Y-S4P were much higher than those (9.48 and 406 MPa) of the previously reported tA3F-S4P [26], reflecting that the former has a higher crosslinking density than the latter.

Furthermore, the elongation at break (21.4%) of

ptA4Y-S4P was higher than that (15.9%) of tA3F-S4P, indicating that ptA4Y-S4P is a tough amino acid-incorporating polymer network.

Figure 7. Tensile properties of all the cured products.

19

4. Conclusions The allylation reactions of L-glutamic acid and L-tyrosine with excess allyl bromide generated incompletely allylated amino acid mixtures with average DS of ca. 3 (A3E and A3Y), which were again allylated to produce tetraallylated amino acids (A4E and A4Y), respectively. Photo-curing of A3E/S4P, A3Y/S4P, A4E/S4P and A4Y/S4P in the presence of photo-initiators produced pA3E-S4P, pA3Y-S4P, pA4E-S4P and pA4Y-S4P.

Furthermore,

thermal post-curing of photo-cured A3E/S4P, A3Y/S4P, A4E/S4P and A4Y/S4P in the presence of photo-and thermal-initiators gave ptA3E-S4P, ptA3Y-S4P, ptA4E-S4P and ptA4Y-S4P, respectively.

All the cured products could be obtained as flat films with smooth

surfaces by cooling the monomer mixtures in a freezer set on ‒18 °C before photo-curing. The FT-IR analysis revealed that allyl and thiol conversions increased by the thermal post-curing for all the curing systems. The thermal post-curing was very effective to improve the Tg, Td5, tensile strength and tensile modulus of each photo-cured product.

Also, the cured products

of tetraallylated amino acids exhibited higher Tg, Td5, tensile strengths and moduli than those of the corresponding triallylated amino acids, respectively.

Especially, ptA4Y-S4P exhibited

the highest Tg (24.4 °C), Td5 (319 C), tensile strength (18.2 MPa) and tensile modulus (1037 MPa) among all the cured products.

Biodegradability and biocompatibility of these

materials are currently under consideration. The methodology in this study is expected to be applied to the preparation of various amino acid-incorporated polymer networks which are applicable to biomaterials such as cell culture base materials and drug delivery materials, optical resolution membranes and environmentally benign coating materials.

Acknowledgements We gratefully acknowledge financial support from the Chiba Institute of Technology.

20

Appendix A. Supplementary material Supplementary data associated with this article can be version. Figure S1.

found,

in the

online

. Overall and partially expanded 500 MHz 1H-NMR spectra of the main

component of A3E in CDCl3. Figure S2.

Overall and partially expanded 500 MHz 1H-NMR spectra of the main

component of A3Y in CDCl3. Figure S3.

500 MHz 13C-NMR spectrum of the main component of A3E in CDCl3.

Figure S4.

500 MHz 13C-NMR spectrum of A4E in CDCl3.

Figure S5.

500 MHz 13C-NMR spectrum of the main component of A3Y in CDCl3.

Figure S6.

500 MHz 13C-NMR spectrum of A4Y in CDCl3.

Figure S7.

Photographs of pA3E-S4P, ptA3E-S4P, pA3Y-S4P and ptA3Y-S4P.

Figure S8.

FT-IR spectra of pA3E-S4P, ptA3E-S4P, pA3Y-S4P and ptA3Y-S4P compared

with those of S4P, A3E and A3Y. Figure S9. The FT-IR analysis for determining the conversion of thiol-ene reaction for curing reactions of A3E/S4P and A3Y/S4P.

21

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24

Graphical abstract

25

Highlights

►Tri- and tetra-allylated glutamic acids and tyrosines were prepared from glutamic acid and tyrosine.

►Thiol-ene photopolymerization of allylated amino acids gave flat films with smooth surfaces.

► The thiol-ene conversion increased by the thermal post curing.

► Thermal and mechanical properties were much improved by the thermal post curing.

►The cured products of allylated tyrosines exhibited the best thermal and mechanical properties.

26