Nonisocyanate Biobased Poly(ester urethanes) with Tunable

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Non-isocyanate bio-based poly(ester urethanes) with tunable properties synthesized via an environment-friendly route Zhao Wang, Xing Zhang, Liqun Zhang, Tianwei Tan, and Hao Fong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00275 • Publication Date (Web): 27 Mar 2016 Downloaded from http://pubs.acs.org on March 29, 2016

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ACS Sustainable Chemistry & Engineering

Non-isocyanate bio-based poly(ester urethanes) with tunable properties synthesized via an environment-friendly route Zhao Wang,†,§ Xing Zhang,† Liqun Zhang,*,†,‡ Tianwei Tan,† and Hao Fong*,§ †

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical

Technology, 15 Bei-San-Huan East Road, Beijing 100029, China ‡

Key Laboratory of Beijing City for Preparation and Processing of Novel Polymer Materials,

Beijing University of Chemical Technology, 15 Bei-San-Huan East Road, Beijing 100029, China §

Department of Chemistry and Applied Biological Sciences, South Dakota School of Mines

and Technology, 501 East Saint Joseph Street, Rapid City, SD 57701, United States

Corresponding Author Professor Hao Fong, Ph.D. *E-mail: [email protected]. Tel.: (+1) 605-394-1229. Fax: (+1) 605-394-1232.

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Abstract: The objective of this study was to explore an environment-friendly route to synthesize

non-isocyanate

based

poly(ester

urethanes)

of

poly(1,10-

bis(hydroxyethyloxycarbonylamino) decane-co-dicarboxylic acid) (PBDA) from renewable materials/resources.

In specific, 1,10-bis(hydroxyethyloxycarbonylamino) decane (BHD)

was first synthesized from ethylene carbonate and decamethylene diamine via the melt ringopening reaction. Subsequently, the PBDAs with tunable properties were synthesized from BHD and five di-carboxylic acids (including three bio-based ones of oxalic acid, sebacic acid, and itaconic acid) via the melt polycondensation reaction. The structural, physical, and mechanical properties of PBDAs were characterized by FTIR, NMR, XRD, GPC, TGA, and mechanical testing machine; additionally, the environmental stability was evaluated upon measuring the water absorption amount in deionized water and the degradation percentage in phosphate buffer saline. The results indicated that the PBDAs possessed reasonably good properties, thus could potentially be used for engineering applications; moreover, since their macromolecular chains/backbones contained ester and urethane groups (which would usually result in excellent cytocompatibility), it was envisioned that these PBDAs might also be suitable for some biological and/or biomedical applications.

Keywords: non-isocyanate based poly(ester urethanes), bio-based polymers, environmentfriendly synthesis, sustainable materials, green chemistry


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INTRODUCTION Polyurethanes (PUs) are industrially important polymers with numerous applications;1−5 traditionally and most commonly, a PU is synthesized from a di- or polyisocyanate with a dior polyol.6−10 Because isocyanates are highly toxic and petroleum-dependent, non-isocyanate based PUs (NIPUs, particularly those made from renewable materials/resources) have attracted growing interests to mitigate the health and environmental concerns.11−19 A NIPU can be synthesized from a di-cyclocarbonate and a di-amine;20−35 after the polymerization, each repeating unit of the resulting NIPU macromolecules possesses a hydroxyl group.36,37 Hence, the hydrophilicity of these NIPUs is considerably higher than that of conventional PUs.38,39 Such a situation makes these NIPUs suitable for some applications (e.g., as coating and adhesive materials);40 however, it also limits/hinders other applications such as engineering materials.14,41

Although research endeavors demonstrated that some NIPUs

made from plant oils could have reasonably high mechanical properties, those NIPUs were molecularly crosslinked thus could not be re-processed/re-used.42,43

Another route for making a NIPU without hydroxyl groups is via the trans-urethanization reaction from a phosgene-free monomer of carbamate and a di- or polyol, and the resulting NIPU possesses similar structure and characteristics to conventional PUs.44−50 One type of the carbamate monomer is bis-hydroxycarbamate, which can be made from a cyclocarbonate and a di-amine. Subsequently, the bis-hydroxycarbamate can further react with a di- or polyol via the trans-urethanization reaction to synthesize a NIPU.51,52 Nevertheless, the NIPUs prepared through this route typically had low average molecular weights of a few 3 ACS Paragon Plus Environment


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thousand; thus their mechanical properties were presumably low. Note that the mechanical properties of those NIPUs were not evaluated in the reported studies.

The goal of this study was to design and synthesize innovative NIPUs with reasonably high mechanical and physical properties via an environment-friendly route.

To fulfill the

requirement of being renewable/sustainable, most of the chemicals used in this study were bio-based. In specific, 1,10-bis(hydroxyethyloxycarbonylamino) decane (BHD) was first synthesized from ethylene carbonate (EC) and decamethylene diamine (DDA) via the melt ring-opening reaction.

EC was selected because it is commercially available and

inexpensive; additionally, EC could be prepared from ethylene oxide and CO2, while the consumption of CO2 would be helpful to mitigate the greenhouse effect.53 DDA was selected because it could be readily made from castor oil.54 After the synthesis of BHD, five biobased NIPUs of poly(1,10-bis(hydroxyethyloxycarbonylamino) decane-co-dicarboxylic acid) (PBDA) with tunable properties were further synthesized via the melt polycondensation reaction by using five different di-carboxylic acids, as depicted in Figure 1. In specific, three bio-based di-carboxylic acids including oxalic acid (OA), sebacic acid (SA), and itaconic acid (IA) were studied;55−58 additionally, two synthetic di-carboxylic acids of hexanedioic acid (HA) and terephthalic acid (TA) were also studied to tailor the structures and properties of PBDAs. It is envisioned that, this study might represent a new approach/strategy for the synthesis of bio-based NIPUs with reasonably high properties.


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Figure 1. A scheme depicting the synthesis route for making BHD and then five PBDAs. The inset pictures showing the natural products for making bio-based chemicals, the prepared BHD powder, and the dumbbell-shaped specimens made of PBDAs, respectively.

EXPERIMENTAL SECTION

Materials

Ethylene carbonate (EC), oxalic acid (OA), hexanedioic acid (HA), sebacic acid (SA), itaconic acid (IA), and terephthalic acid (TA) were purchased from Alfa Aesar in the United States. All of these chemicals were in analytical grade. Decamethylene diamine (DDA) was purchased from Beijing HWRK Chemical Co. in China. Note that the as-received DDA was purified through recrystallization for 3 times before being used; after purification, the purity of DDA was very high as evidenced by the 1H NMR spectrum (Figure S1).

Synthesis of BHD 5 ACS Paragon Plus Environment


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BHD was synthesized from EC and DDA via the melt ring-opening reaction, and the reaction was carried out in a 100 mL three-neck flask. Prior to the reaction, EC and DDA (with the molar ratio being 2.05/1) were thoroughly mixed in the flask; thereafter, the reaction system was slowly heated to 80 °C (the temperature above the melting points of reactants). Upon melt reaction for 30 min, the materials turned into a solid; and it appeared that the reaction stopped. Finally, the reaction system was kept for 3 h at 110 °C (the temperature that is a few degrees above the melting point of BHD, the DSC result is shown in Figure S2). It is important to note that the amount of EC was slightly in excess, and this was to ensure that DDA would be completely reacted. After the above synthesis, a crude product of white solid was obtained. The crude product was then ground into powder followed by being placed at 40 °C for 24 h in a vacuum oven; upon this process, the residual EC was evaporated/removed, and the purified BHD was obtained and ready for the FTIR, 1H NMR, and MS characterizations.

FTIR data (cm−1) acquired from BHD (Figure S3): 3462 & 3421 (ν, O−H), 3332 (ν, N−H), 3048 (νs-trans, N−H), 2925 (νas, CH2), 2851 (νs, CH2), 1688 (ν, C=O), 1535 (δ, N−H), 1141 (ν, C−O−C), and 1049 (ν, C−OH). The bands centered at 1688 (ν, C=O) and 1535 (δ, N−H) indicated that the urethane group was formed. 1

H NMR data (400 MHz, CDCl3-d, ppm) acquired from BHD (Figure 2A): 4.23 (t, 2H,

CH2CH2OH), 3.82 (q, 2H, CH2OH), 3.19 (q, 2H, CH2NH), 1.50 (t, 2H, CH2CH2NH), and 1.30 (s, 2H, CH2).


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MS date acquired from BHD (Figure S4): HRMS (ESI) m/z: [M + H+]+, C16H33N2O6 (with the calculated value of 349.441) was found at 349.2344; [M + Na+]+, C16H32N2O6Na (with the calculated value of 371.4308) was found at 371.2160. Note that the percentage of 349.2344 peak was 100%, indicating that the prepared BHD contained almost no impurity.

Synthesis of PBDAs

The following was the procedure to synthesize five PBDAs: First, the equimolar amounts of BHD and each of the five di-carboxylic acids were put into a three-neck flask; and the mixture was then stirred at 180 °C for 3 h in nitrogen flow (thus the generated water could be removed). Subsequently, the system was kept at 200 °C for ~1 h under the reduced pressure of 300 Pa until the Weisenberg effect appeared;59 thereafter, the system was naturally cooled down to room temperature to obtain a PBDA. Note that 180 and 200 °C were adopted for the melt polycondensation reaction, because BHD was thermally stable when the temperature was below ~220 °C (as shown in Figures S5A and S5B). Hereafter, the PBDAs synthesized from BHD and di-carboxylic acids of OA, HA, SA, IA, and TA are denoted as PU-BHD-OA, PU-BHD-HA, PU-BHD-SA, PU-BHD-IA, and PU-BHD-TA, respectively.

PU-BHD-OA: The color of this PBDA was light yellow. IR (cm−1): 3302 (ν, N−H), 2925 (νas, CH2), 2857 (νs, CH2), 1725 (ν, C=O, ester), 1651 (ν, C=O, urethane), 1530 (δ, N−H), 1456 (δ, C−N−H), 1247 (ν, C−O, ester), 1045 (ν, C‒O, urethane).

1

H NMR (ppm): 4.82

(−NH), 3.80–4.50 (–C(O)–O–CH2–CH2–O–C(O)NH–), 3.10–3.75 (–CH2–NH), 1.66 (–CH2– CH2–NH), 1.30 (–CH2).



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PU-BHD-HA: The color of this PBDA was light yellow. IR (cm−1): 3302 (ν, N−H), 2926 (νas, CH2), 2855 (νs, CH2), 1735 (ν, C=O, ester), 1634 (ν, C=O, urethane), 1542 (δ, N−H), 1454 (δ, C−N−H), 1257 (ν, C−O, ester), 1036 (ν, C−O, urethane).

1

H NMR (ppm): 4.82

(−NH), 3.80–4.50 (–C(O)–O–CH2–CH2–O–C(O)NH–), 3.10–3.75 (–CH2–NH), 2.50–2.80 (CH2–C(O)–O–), ~1.80 (–CH2–CH2–C(O)–O–), 1.66 (–CH2–CH2–NH), 1.30 (–CH2).

PU-BHD-SA: The color of this PBDA was white. IR (cm−1): 3302 (ν, N−H), 2924 (νas, CH2), 2853 (νs, CH2), 1737 (ν, C=O, ester), 1631 (ν, C=O, urethane), 1548 (δ, N−H), 1465 (δ, C−N−H), 1257 (ν, C−O, ester), 1042 (ν, C−O, urethane). 1H NMR (ppm): 4.82 (−NH), 3.80– 4.50 (–C(O)–O–CH2–CH2–O–C(O)NH–), 3.10–3.75 (–CH2–NH), 2.50–2.80 (CH2–C(O)–O– ), ~1.80 (–CH2–CH2–C(O)–O–), 1.66 (–CH2–CH2–NH), 1.30 (–CH2).

PU-BHD-IA: The color of this PBDA was white. IR (cm−1): 3310 (ν, N−H), 2925 (νas, CH2), 2857 (νs, CH2), 1730 (ν, C=O, ester), 1691 (ν, C=C), 1634 (ν, C=O, urethane), 1548 (δ, N−H), 1460 (δ, C−N−H), 1247 (ν, C−O, ester), 1050 (ν, C−O, urethane).

1

H NMR (ppm):

5.08 (=CH2), 4.82 (−NH), 3.80–4.50 (–C(O)–O–CH2–CH2–O–C(O)NH–), 3.10–3.75 (–CH2– NH), 2.50–2.80 (CH2–C(O)–O–), 1.66 (–CH2–CH2–NH), 1.30 (–CH2).

PU-BHD-TA: The color of this PBDA was yellow. IR (cm−1): 3317 (ν, N−H), 3079 (ν, =C−H), 3060 (ν, =C−H), 3035 (ν, =C−H), 2926 (νas, CH2), 2855 (νs, CH2), 1720 (ν, C=O, ester), 1636 (ν, C=O, urethane), 1553 (δ, N−H), 1456 (δ, C−N−H), 1264 (ν, C−O, ester), 1025 (ν, C−O, urethane). 1H NMR (ppm): 7.90−8.30 (hydrogen atoms on benzene ring), 4.82 (−NH), 3.80–4.50 (–C(O)–O–CH2–CH2–O–C(O)NH–), 3.10–3.75 (–CH2–NH), 1.66 (–CH2– CH2–NH), 1.30 (–CH2).

13

C NMR (ppm): 170–155 (−COO−CH2−), 150–120 (carbon atoms 8 ACS Paragon Plus Environment

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in benzene ring), 70–55 (–C(O)–O–CH2–CH2–O–C(O)NH–), 50–35 (–CH2–NH), 35–20 (– CH2).

Characterization Methods

A Tensor 27 Fourier transform infrared (FTIR) spectrophotometer (Bruker, Germany) equipped with a Smart Orbit diamond attenuated total reflection (ATR) accessory was employed for the studies. An FTIR spectrum was acquired by scanning a sample for 32 times in the wavenumber range of 600−4000 cm−1 at the resolution of 4 cm−1. All of the spectra were baseline-corrected and then normalized by using the methyl band centered at 1463 cm−1. 1

H and

13

C nuclear magnetic resonance (NMR) spectra were acquired from a Bruker

AV400 NMR spectrometer (Bruker, Germany) at the frequency of 400 MHz by using CDCl3d as solvent for BHD while sulfuric acid-D2 as solvent for PBDAs. In the 13C NMR spectra, the chemical shift values were referenced to the chemical shift value of sulfuric acid-D2 (δ = 11.2 ppm). The 13C NMR spectra were obtained with the spectral width of 200 ppm and the resolution of 4 Hz.

Gel Permeation Chromatography (GPC) was employed to determine the number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn, also known as polydispersity index (PDI)) of each PBDA.

A PL GPC-220 instrument (Polymer

Laboratories, the United States) with a DRI detector was employed for the GPC studies; the temperature was set at 150 °C, and 1,2,4-trichlorobenzene was selected as the solvent.



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Electrospray ionization mass spectrometry (ESI-MS) analysis was carried out by using a Waters XEVO G2 Q TOF mass spectrometer (Waters, the United States). A BHD sample was first dissolved in a mixture solvent of acetonitrile/methanol (with v/v ratio being 2/1), and the solution was then injected into the ESI source by using the syringe pump (which was equipped with instrument) at the rate of 5 µL/min. The ESI source of LCQ was operating at 3 kV, and the temperature of capillary heater was set at 350 °C. Nitrogen was used as the nebulizing gas. During the ESI-MS/MS experiments, the ions of interest were isolated monoisotopically in the ion trap and collisionally activated.

Thermogravimetric analysis (TGA) of a PBDA was conducted by using a STARe system TGA/DSC1 thermogravimeter (Mettler-Toledo International Inc., Switzerland) equipped with a cooling water circulator. The TGA thermograms were obtained under flowing nitrogen (20 mL/min) at the scanning rate of 10 °C/min in the temperature range of 30−550 °C. All of the samples were accurately weighed, and the weight of each sample was ~10 mg. The onset of degradation temperature (T5%) was determined from the TGA curve at the weight loss of 5%.

Differential scanning calorimetry (DSC) experiments were performed on a DSC1 STARe system (Mettler-Toledo International Inc., Switzerland) equipped with a liquid nitrogen cooling system. The temperature was first increased to 200 °C at the rate of 10 °C/min, and then kept at 200 °C for 5 min to eliminate the thermal history of a sample; thereafter, the temperature was decreased to -20 °C at the rate of 10 °C/min. Finally, the DSC data were recorded in the temperature range from -20 to 220 °C at the heating rate of 10 °C/min. Note that the nitrogen flow was set at 150 mL/min. The glass transition temperature (Tg), melting 10 ACS Paragon Plus Environment

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temperature (Tm), and heat/enthalpy of melting (Hm) were determined by the STARe software, while the crystallinity (Xc) values of PBDAs were calculated from Eq. (1):

Xc =

∆H m ×100 ∆H m0

(1)

where Hm0 is the melting heat/enthalpy of 100% crystalline polymer.60

X-ray diffraction (XRD) was employed to examine the crystalline structures of PBDAs. XRD studies were carried out by using a Rigaku RINT X-ray diffractometer, and the X-ray tube was operated at 40 kV and 200 mA by using the Cu−Kα radiation (λ = 0.154 nm). The XRD patterns were recorded in the 2θ range of 5−50° at the scanning speed of 5°/min.

Mechanical properties of PBDAs were evaluated at 20 ± 2 °C by using a CMT 4104 mechanical testing machine (Shenzhen SANS Testing Machine Co., China) equipped with a 50 N load cell. The dumbbell-shaped specimens (with length, width, and thickness being 65, 3.14, and 1 mm, respectively) were prepared via the vacuum pressing method, and the specimens were tested at the crosshead speed of 10 mm/min until breakage.

Young’s

modulus was calculated from the linear part of initial slope. For each PBDA sample, five specimens were prepared and tested to acquire the average value and the associated standard deviation.

Water absorption of a PBDA was determined from the weight change of a film specimen before and after the immersion in deionized (DI) water at room temperature for varied time periods. In specific, an accurately weighed film specimen was placed into a 20 mL vial filled with DI water; after a certain time period, the specimen was taken out and immediately wiped 11 ACS Paragon Plus Environment


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with tissue paper followed by being accurately weighed again. Thereafter, the specimen was put back into the vial. The water absorption percentage was calculated from Eq. (2):

Water absorption (%) =

Wt − W0 ×100 W0

(2)

where Wt is the weight of a specimen after being immersed in DI water for a certain time period, while W0 is the specimen’s initial weight.

Degradation of a PBDA was evaluated by measuring the weight change of a film specimen upon being immersed in phosphate buffer saline (PBS) at 37 °C for varied time periods. In specific, an accurately weighed film specimen was placed into a 20 mL vial filled with PBS; after a certain time period, the specimen was taken out and then rinsed with DI water for 3 times followed by being dried in a vacuum oven at 40 °C for 24 h. Thereafter, the specimen was weighed again, and the degradation percentage was calculated from Eq. (3):

Weight loss (%) =

Wm − W0 ×100 W0

(3)

where Wm is the weight of a specimen after being immersed in PBS solution for a certain time period, while W0 is the specimen’s initial weight.

RESULTS AND DISCUSSION

Synthesis of BHD


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To synthesize NIPUs via the melt polycondensation reaction has never been reported before; prior to the synthesis of NIPUs (more specific, PBDAs), the monomer of BHD was first synthesized via the melt ring-opening reaction and then characterized by FTIR (Figure S3), 1

H NMR (Figure 2), and MS (Figure S4). These results indicated that BHD was successfully

synthesized and its purity was high. The reaction dynamics was studied to determine the optimal reaction time between the reactants of EC and DDA. In specific, a series of samples were collected after varied reaction durations; and the samples were then characterized with 1

H NMR spectroscopy. As shown in Figure 2, with prolonging the reaction time, the signals

at 1.62 ppm (NH2−(CH2)10−NH2) and 4.54 ppm (−CH2−CH2− in EC) decreased. After the reaction for 3 h, the signal of NH2−(CH2)10−NH2 disappeared, indicating that DDA was consumed. Note that the signal of −CH2−CH2− in EC could still be identified, because excess amount of EC was added to ensure that DDA could be completely reacted; after the purification of crude product, residual EC would be evaporated/removed, as evidence by 1H NMR spectrum. The purified BHD was a white powder, and the reaction yield was ~86%.



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Figure 2. 1H NMR spectra of BHD samples prepared under different conditions.

Synthesis of PBDAs

Five di-carboxylic acids were studied to synthesize PBDAs with varied structures and properties. In particular, (1) OA, HA, and SA are similar in molecular structures while have different numbers of methylene (−CH2−) groups between two carboxyl (−COOH) groups; in consequence, the distance/length between neighboring urethane/ester groups in the resulting PBDA macromolecules are adjustable.

(2) IA has a pendant C=C double bond in its

molecule, which is expected to hinder crystallization and decrease Tg of the resulting PBDA. (3) TA is molecularly rigid due to benzene ring; hence, the PBDA made from TA may have higher Tg and thermal stability. Based upon our previously reported research 59 and the TGA result of BHD (Figures S5A and S5B), PBDAs were synthesized via the two-step melt polycondensation method in this study. The reaction was stopped when the product started to climb the stirring shaft, a phenomenon known as the “Weissenberg effect”.37 The reaction durations and the number average molecular weights (Mn) of five PBDAs are shown in Table 14 ACS Paragon Plus Environment

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1. The Mn values of four PBDAs were similar (i.e., between 24400 and 28700), while the Mn value of PU-BHD-TA was distinguishably lower at 13900. When TA was used, it was observed that the viscosity of reaction system was considerably higher; presumably, this might be attributed to the rigid benzene ring in TA molecule. Note that the high viscosity made PU-BHD-TA easy to climb on the stirring shaft; as a result, the reaction might have been terminated at relatively low degree of polymerization.

Table 1. The reaction durations as well as Mn and PDI values of five PBDAs.

Reaction duration (h)

Mn (g/mol)

Mw/Mn (PDI)

PU-BHD-OA

3.6

26200

3.18

PU-BHD-HA

3.9

24400

3.17

PU-BHD-SA

4.2

28700

3.94

PU-BHD-IA

24.0

24500

3.18

PU-BHD-TA

3.5

13900

2.18

Figure 3. FTIR spectra acquired from five PBDAs.



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Fig. 3 depicts the FTIR spectra acquired from five PBDAs. In the spectra, the bands centered at 3300, 1720, 1635, 1540, 1250, and 1100 cm−1 are attributed to the N−H stretching vibration, C=O (in ester groups) stretching vibration, C=O (in urethane groups) stretching vibration, N−H bending vibration, C−N/C−O stretching vibration (in ester groups), and C−O−C stretching vibration, respectively. The presence of these bands indicated that both ester and urethane groups were formed. From the FTIR spectra of PU-BHD-OA, PU-BHDHA, and PU-BHD-SA, it was evident that the band intensity of ester C=O group (1720 cm−1) decreased with the increase of methylene groups in di-carboxylic acids of OA, HA, and SA; and this was consistent with the variation of length between two adjacent ester groups in the respective PBDAs. In the spectrum of PU-BHD-TA, the band centered at 700 cm−1 is attributed to the out-of-plane vibration of benzene rings; while the band centered at 1250 cm−1 is attributed to the in-plane vibration of benzene rings. In the spectrum of PU-BHD-IA, the three bands from 1540 to 1720 cm−1 are respectively attributed to C=O (in both ester and urethane groups) stretching vibrations and C=C stretching vibration; additionally, the band centered at 1640 cm−1 is attributed to pendant C=C double bonds in the IA units.


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Figure 4. 1H NMR spectra acquired from five PBDAs. Macromolecular structures of PBDAs were further characterized by 1H NMR spectroscopy. Due to excellent solvent resistance of the synthesized PBDAs (i.e., these five PBDAs could not be dissolved in common solvents such as tetrahydrofuran, chloroform, ethanol, acetone, and toluene), only sulfuric acid-D2 (HDSO4-d2) could be used as the solvent for 1H NMR characterizations. As a result, the acquired 1H NMR spectra of some PBDAs were not very clear because the solution viscosities of those samples were relatively high. Nevertheless, some important information could still be extracted from the 1H NMR spectra. As shown in Figure 4, the signals in the chemical shift (i.e., δ) ranges of 2.50–2.80, 3.80–4.50, and ~1.80 ppm were respectively attributed to those hydrogen atoms in the methylene groups that between ester group and urethane group (a & d) and the hydrogen atoms of –CH2–CH2–COO (b); the presence of these signals indicated the formation of ester groups. The signals with 17 ACS Paragon Plus Environment


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the δ values being at 3.10–3.75, 1.66, and 4.82 ppm were attributed to the hydrogen atoms in the methylene groups (e) adjacent to –NH group, the hydrogen atoms of –CH2–CH2–NH (f), and the hydrogen atom of −NH, respectively; and these signals indicated the formation of urethane groups.

In other words, the high temperature during the melt polymerization

reaction did not lead to the breakage of original urethane groups in BHD; while the PBDAs were successfully synthesized via the melt polycondensation reaction. Note that the ratio of the signals/peaks in the regions of a, e, and d was determined at 1:1:1, which was in agreement with PBDA structures. For PU-BHD-TA and PU-BHD-OA, there was no signal in the region of a; and this was simply because of no methylene group in the structures of OA and TA. The signal with the δ value of 5.08 ppm was attributed to the hydrogen atoms of =CH2, and the signals in the δ range of 7.90−8.30 ppm were attributed to the hydrogen atoms in benzene rings. Since PU-BHD-TA could not be well dissolved in HDSO4-d2, its 1H NMR spectrum did not show the peaks of e and f, which were associated with urethane groups; while the macromolecular structure of PU-BHD-TA could be validated by its

13

C NMR

spectrum (Figure S6). Based upon the FTIR and NMR results, the formations of five PBDAs could be confirmed.

Properties of PBDAs

Thermal Behavior and Crystallization Behavior

To further understand the structure-property relationships, DSC and XRD were employed to study the thermal behavior and crystallization behavior of PBDAs. Figure 5A shows the DSC thermograms of five PBDAs. In general, the Tg of a polymer is determined by the 18 ACS Paragon Plus Environment

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factors including the chain stiffness, molecular symmetry, presence of a side group (as well as its size and flexibility), molecular weight, degrees of branching/crosslinking, and intermolecular interactions;61 while for these PBDAs made from five different di-carboxylic acids, chain stiffness and intermolecular interactions would be the major factors. It was evident that the Tg values of five PBDAs were lower than that of common polyesters such as poly(ethylene terephthalate). As shown in Table 2, the Tg values of PU-BHD-OA, PU-BHDHA, and PU-BHD-SA decreased from 5 to -20 °C with increasing the number of methylene groups in the respective di-carboxylic acids; this was due to the length/distance between neighboring urethane groups (note that urethane groups are more rigid than methylene groups) and the intermolecular interactions (such as hydrogen bonding) among these urethane groups. The presence of crystallization peak in the DSC curve acquired from PU-BHD-OA indicated that the crystallization speed of PU-BHD-OA was relatively slow. The Tg value of PU-BHD-TA was 53 °C, which was distinguishably higher because of two reasons including (1) the presence of rigid benzene rings in its macromolecular chain/backbone and (2) the shorter length/distance between neighboring urethane groups. For PU-BHD-IA, the pendant C=C double bonds in IA units would hinder the close packing of macromolecules, thus reduce the intermolecular interactions; as a result, this PBDA exhibited low Tg value.

The melting point (Tm) and degree of crystallinity (Xc) could also be adjusted upon varying the di-carboxylic acid units in PBDAs. For PU-BHD-OA, PU-BHD-HA, and PU-BHD-SA, the Tm and Xc values became higher with increasing the number of methylene groups in the respective di-carboxylic acid units (as shown in Table 2); and this was because the longer methylene segment would make the resulting PBDA macromolecule more flexible thus easier 19 ACS Paragon Plus Environment


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to rotate and/or undergo conformational rearrangements. As shown in Figure 5A, no melting peak could be identified in the DSC thermogram of PU-BHD-TA, because the rigid benzene units in PU-BHD-TA made this PBDA hard to crystalize. In the thermogram acquired from PU-BHD-IA, there were a cold crystallization peak around 20 °C and a broad melting peak around 65 °C, suggesting that the crystallization speed of this PBDA was slow. As described before, the pendant C=C double bonds in IA units would reduce the intermolecular interactions and/or make the macromolecules more difficult to crystalize. As shown in Figure 5B, the XRD pattern of PU-BHD-IA had no diffraction peak, indicating that the prepared PU-BHD-IA was structurally amorphous; whereas it is necessary to note that PUBHD-IA could slowly crystallize upon heating/annealing. The XRD patterns of PU-BHDOA, PU-BHD-HA, and PU-BHD-SA were consistent with their DSC results; there were two diffraction peaks centered at the 2θ values of 20.3° and 23.3°, indicating that these three PBDAs possessed the α crystalline phases. With more methylene groups, the intensity of these diffraction peaks increased, indicating that the degree of crystallinity was improved. From the acquired DSC and XRD results, it was evident that the thermal behavior and crystallization behavior of PBDAs could be tailored upon the variation of di-carboxylic acid units in their macromolecules.


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Figure 5. DSC thermograms (A) and XRD patterns (B) acquired from five PBDAs.

Table 2. Thermal characteristics of five PBDAs extracted from DSC results.

Tg (°C)

Tm (°C)

Hm (J/g)

Hm0 (J/g)

Xc (%)

PU-BHD-OA

5

97

-20.35

141.64

14.37

PU-BHD-HA

-6

122

-31.48

159.24

19.77

PU-BHD-SA

-20

140

-67.31

173.00

38.91

PU-BHD-IA

-15

80

-21.58

139.00

15.53

PU-BHD-TA

53

167

-4.62

165.12

2.80

Mechanical Properties

The PBDA of PU-BHD-TA was brittle; presumably, this was because its macromolecules were relatively rigid and its average molecular weight was relatively low. In consequence, PU-BHD-TA could not be processed into dumbbell shaped testing specimens; thus its



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mechanical properties were not able to evaluate. The representative stress versus strain curves of other four PBDAs are shown in Figure 6. For PU-BHD-OA, PU-BHD-HA, and PU-BHD-SA, the Young’s modulus, shore A hardness, and yield stress became higher with increasing the number of methylene groups in their di-carboxylic acid units; and this was probably due to the increased degree of crystallinity. For example, the crystallinity of PUBHD-SA was the highest at 38.91%; and its tensile strength was also the highest at 17.9 MPa. On the other hand, the Young’s modulus, yield stress, and shore A hardness of PU-BHD-IA were the lowest; whereas its elongation at break could reach a high value of 235%. As shown in Table 3, the mechanical properties of PBDAs could also be tailored upon varying their dicarboxylic acid units.

Figure 6. Representative stress versus strain curves of PBDAs.


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Table 3. Mechanical properties of PBDAs.

Young’s modulus (MPa) PU-BHD-OA

Strain at break (%)

Shore A hardness (HA)

69 ± 3

10.0 ± 0.5

132 ± 11

92.7 ± 0.4

92 ± 2

13.7 ± 0.6

58 ± 8

93.9 ± 0.5

180 ± 1

17.9 ± 0.5

35 ± 3

95.5 ± 0.5

46 ± 1

9.0 ± 0.2

235 ± 15

88.6 ± 0.3

PU-BHD-HA PU-BHD-SA

Tensile strength (MPa)

PU-BHD-IA

Environmental Stability

For practical applications, the environmental stability of a polymer is very important. In this study, the environmental stability of PBDAs was evaluated upon measuring the water absorption amount in deionized (DI) water and the degradation percentage in phosphate buffer saline (PBS). Figure 7A shows the water absorption results of five PBDAs. After 10 days in DI water, the water absorption of five PBDAs appeared to reach the maximum and/or equilibrium; in other words, the water absorption amounts did not have significant variations upon prolonging the time period. The water absorption amount of PU-BHD-TA was ~4 wt.% after being immersed in DI water for 50 days, which was slightly higher than the amounts of other four PBDAs; and this was probably due to lower molecular weight and the concomitant more end groups of hydroxyl (−OH) and/or carboxyl (−COOH). It is interesting to note that the water absorption amounts of other four PBDAs were no more than 2 wt.%, and the water 23 ACS Paragon Plus Environment


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absorption amount of PU-BHD-SA was even less than 1 wt.% (probably due to higher crystallinity). Overall, the relatively low water absorption amounts of five PBDAs (when the equilibrium was reached) suggested that their environmental stabilities might be reasonably good.

In addition to low amounts of water absorption, the degradation percentages of five PBDAs in PBS were also low, as shown in Figure 7B. The degradation percentage variations of five PBDAs were generally in agreement with their water absorption results, further confirming that the prepared PBDAs might be environmental stable.

Figure 7. (A) Results on water absorption of PBDAs after being immersed in DI water for varied time periods, and (B) results on degradation percentage of PBDAs after being immersed in PBS for varied time periods.

Thermal Stability


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TGA analysis was employed to study the thermal stability of PBDAs, and the results are shown in Figure 8A. In general, there were two stages of degradation for all of the five PBDAs, which could also be seen from the DTG curves (Figure 8B). During the first-stage degradation (with the temperature of maximum degradation rate being Td1), the value of weight loss became smaller with increasing the number of methylene groups in di-carboxylic acids of OA, HA, and SA (as shown in Table 4), suggesting that the first-stage degradation could be mitigated by the increase of the length/distance between neighboring ester and urethane groups.

In other words, the first-stage degradation was primarily due to the

breakages of ester and urethane bonds. On the other hand, the second-stage degradation (with the temperature of maximum degradation rate being Td2) could be attributed to the breakages of methylene and other bonds. The differences on weight loss of these two degradation stages were in agreement with the molecular structures of PBDAs.

Figure 8. TGA thermograms (A) and DTG curves (B) acquired from five PBDAs.



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Table 4. Degradation parameters of five PBDAs extracted from TGA results.

Td1 (°C)

Weight loss (%)

Td2 (°C)

Weight loss (%)

Residual (%)

PU-BHD-OA

325

31

430

66

2

PU-BHD-HA

320

26

435

69

3

PU-BHD-SA

302

13

461

84

2

PU-BHD-IA

323

26

449

72

1

PU-BHD-TA

293

12

463

80

6

CONCLUSIONS

In this study, non-isocyanate bio-based poly(ester urethanes) of PBDAs with tunable properties were synthesized from BHD and five di-carboxylic acids (including three biobased ones of OA, IA, and SA) via an environment-friendly route. The properties of PBDAs could be tailored upon varying the di-carboxylic acid units in their macromolecules. The Mn values of these PBDAs were in the range of 13900−28700 g/mol; their mechanical properties varied from brittle to soft/flexible, and their degrees of crystallinity were different as well. All of the five PBDAs exhibited excellent resistance to water (and other common chemicals), and their environmental stabilities were reasonably high. Therefore, these PBDAs could potentially be utilized for engineering applications.


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AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected]. Tel.: (+86) 10-6442-1186. Fax: (+86) 10-6443-3964.

*E-mail: [email protected]. Tel.: (+1) 605-394-1229. Fax: (+1) 605-394-1232. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This research was supported by the National Natural Science Foundation of China (Grant Numbers: 50933001, 51221002, 51273005, 81171682, and 81330043). H. Fong and L. Zhang would also like to acknowledge the State Key Laboratory of Organic-Inorganic Composites at the Beijing University of Chemical Technology (Grant Number: 201501003).

Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org/.

1

H NMR spectrum of purified DDA

(Figure S1), DSC thermogram (Figure S2), FTIR spectrum (Figure S3), mass spectrum (Figure S4), and TGA thermograms (Figure S5) of purified DDA, and 13C NMR spectrum of PU-BHD-TA (Figure S6).



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For Table of Contents Use Only

Manuscript title: Non-isocyanate bio-based poly(ester urethanes) with tunable properties synthesized via an environment-friendly route

Names of all authors: Zhao Wang, Xing Zhang, Liqun Zhang, Tianwei Tan, and Hao Fong

Brief (~20 word) synopsis: An environment-friendly route has been explored to synthesize non-isocyanate based poly(ester urethanes) with tunable properties from renewable materials/resources.

Table of Contents Graphic


Nonisocyanate Biobased Poly(ester urethanes) with Tunable

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