Phloretic acid as an alternative to the phenolation of aliphatic hydroxyls for the elaboration of polybenzoxazine

Acerina Trejo-Machin ab, Pierre Verge *a, Laura Puchot a and Robert Quintana a
aLuxembourg Institute of Science and Technology (LIST), Esch-sur-Alzette L-4362, Luxembourg. E-mail: pierre.verge@list.lu
bUniversitat Politècnica de Catalunya (UPC), EEBE, Campus Diagonal Besòs, Eduard Maristany 10-14, 08930 Barcelona, Spain

Received 2nd August 2017 , Accepted 15th September 2017

First published on 18th September 2017


3-(4-Hydroxyphenyl)propanoic acid, so-called phloretic acid (PA), is a naturally occurring phenolic compound which can be produced by the hydrogenation of p-coumaric acid or synthesized from phloretin, a by-product of apple tree leaves. It is explored herein as a renewable building block for enhancing the reactivity of –OH bearing molecules towards benzoxazine (Bz) ring formation instead of phenol. PA is used to bring phenolic functionalities by its reaction with model molecules (ethylene glycol, as well as two polyethylene glycol molecules with molar masses corresponding to 400 g·mol−1 and 2000 g·mol−1) via solvent-free Fischer esterification. These phenolic groups are further reacted with a bio-based amine (furfurylamine) to form almost 100% bio-based benzoxazine end-capped molecules. Very interestingly, the whole synthesis of Bz monomers from PEG400 and PEG2000 does not require a solvent or purification, and their polymerization led to a set of materials with thermal and thermo-mechanical properties suitable for a wide range of applications. These results show that renewable phloretic acid is a sustainable alternative to phenol to provide easily the specific properties of benzoxazine to aliphatic –OH bearing molecules or macromolecules. This novel approach paves the way towards a multitude of applications given the large number of –OH bearing compounds in materials science.


1. Introduction

Polybenzoxazines (PBz) are a relatively new class of phenolic type thermosets with outstanding properties such as near-zero shrinkage upon polymerization,1 high char yield, high glass transition temperatures,2 low water absorption3 and excellent electrical and mechanical properties. In addition, curing does not usually require external catalysts.4 The versatility of the design of their chemical structure is an additional outstanding asset since it allows the development of solutions at the macromolecular scale to tackle their drawbacks, like difficulty in processability, brittleness,5 and high curing temperatures.6 Several strategies to overcome these shortcomings include the synthesis of benzoxazine monomers having additional functionality,7,8 the elaboration of composites,9,10 blending with a high performance polymer11,12 and anchoring benzoxazine (Bz) onto polymer backbones.13–16

Adding benzoxazine functionalities to polymers is a recent challenge benefiting from the design versatility of Bz monomers. The resultant oligomers possess properties specific both to benzoxazine, such as a high cross-linking density, and to the polymer backbone like processability, flexibility or film formation. End-chain,17–19 side-chain13,14,20 as well as main-chain21–23 Bz polymers have been reported and reviewed,24 underlining the importance of this topic. As a result, benzoxazine has been anchored onto a wide variety of polymers like polystyrene,14 polyvinylchloride,13 polybutadiene,25 polypropylene oxide,19 poly(ε-caprolactone),18 cellulose20 and even onto macromonomers like lignin.26 Lignin is particularly attractive for this topic since it is a bio-based polyphenol, composed of three phenolic monomers. However, two of these three basic building blocks, i.e. coniferyl alcohol and sinapyl alcohols,27 are ortho substituted and cannot be involved in the formation of Bz monomers. Recently, Lehnen et al. tackled this issue by enhancing the reactivity of organosolv lignin by grafting phenols, aiming at increasing the number of reactive phenolic groups.28 Then Ishida et al. demonstrated the adequacy of this approach by using this functionalized lignin to synthesise Bz monomers with aniline or propargyl amine to form PBz.26 A clear enhancement of the cross-linking density was achieved as well as the formation of lignin-based materials. This approach is efficient and could be considered for the functionalization of a wide range of (bio)polymers or materials bearing hydroxyl groups. However, this strategy suffers from a clear drawback: phenol is used while a wide propensity of the efforts spurred on benzoxazine chemistry during the past few years has focused on the use of renewable resources to provide greener alternatives.

3-(4-Hydroxyphenyl)propanoic acid, or phloretic acid (PA), is a naturally occurring phenolic compound, whose synthesis involves an enzymatic or chemical treatment of phloretin, a by-product from apple tree leaves.29 The hydrogenation of the aliphatic side chain p-coumaric acid leads to its production. PA was explored hereby instead of phenol for enhancing the reactivity of hydroxyl (–OH) bearing molecules towards benzoxazine ring formation. Indeed, the ortho positions of PA are not substituted and it is ideal for the design of benzoxazine monomers as previously reported by other authors.30 In addition, PA bears on its para position a propionic acid side chain suitable for esterification. The absence of a C[double bond, length as m-dash]C double bond on its side chain is also an asset: being more thermally stable than its unsaturated congeners, coumaric and ferulic acids, PA does not degrade upon polymerization.30 In this work, we evidenced that phloretic acid can be used as a building block to bring phenolic functionalities onto –OH containing model molecules (ethylene glycol, as well as two polyethylene glycol molecules with molar masses corresponding to 400 g mol−1 and 2000 g mol−1) via a solvent-free Fischer esterification. These phenolic groups were further reacted with a bio-based amine (furfurylamine)31 to form benzoxazine end-capped molecules, almost 100% bio-based. Very interestingly, the whole synthesis of Bz monomers from PEG400 and PEG2000 did not require a solvent or purification, and their polymerization led to a set of materials with thermal and thermo-mechanical properties suitable for a wide range of applications.

2 Experimental

2.1 Materials

3-(4-Hydroxyphenyl)propionic acid (98 wt%, phloretic acid, PA), furfurylamine (≥99 wt%, FU), polyethylene glycol (average Mn: 400, PEG400, and 2000, PEG2000), ethylene glycol (EG), paraformaldehyde (PFA) and p-toluene sulfonic acid monohydrate (≥98.5%, TsOH) were purchased from Sigma-Aldrich Chemicals. Dioxane was also supplied from Aldrich. All chemicals were used as received without any purification.

2.2 Measurements

Nuclear magnetic resonance spectroscopy (NMR). Proton nuclear magnetic resonance (1H NMR) spectra were recorded using an AVANCE III HD Bruker spectrometer operating at a proton frequency of 600 MHz. Carbon nuclear magnetic resonance (13C NMR) spectra were recorded using the same spectrometer and operating at a proton frequency of 150 MHz. The samples were dissolved in either deuterated chloroform (CDCl3) or deuterated dimethyl sulfoxide (DMSO-d6), depending on their solubility. The spectra were referenced relative to tetramethylsilane (TMS).
Fourier transform infrared spectroscopy (FTIR). The Fourier transform infrared spectroscopy (FTIR) analyses were conducted on a Bruker TENSOR 27 (Ettlingen, Germany) in the attenuated total reflection (ATR) mode using a diamond crystal. The background and sample spectra were recorded at 4 cm−1 spectral resolution across the 4000–400 cm−1 range.
Mass spectrometry analyses (MALDI-TOF/MS). Matrix assisted laser desorption ionization-mass spectrometry (MALDI-MS) experiments were conducted using a Bruker Autoflex III mass spectrometer (Bruker Daltonics, Leipzig, Germany) equipped with a frequency-tripled Nd-YAG laser (l = 355 nm) operating at a pulse rate of 50 Hz. For MALDI analyses, the samples were subjected to solvent-free preparation. α-Cyano-4-hydroxycinnamic acid (HCCA) was selected for the analysis of the benzoxazine monomers, and used as received from Sigma-Aldrich.
Differential scanning calorimetry (DSC). Differential scanning calorimetry (DSC) thermograms were recorded by means of a Netzsch DSC 204 F1 Phoenix apparatus operating under an inert atmosphere (nitrogen) with a linear heating ramp from 20 to 250 °C at a 10 °C min−1 rate.
Thermo-gravimetric analysis (TGA). Thermo-gravimetric analysis (TGA) was performed using a Netzsch TG 409 PC Luxx device operating either in synthetic air (nitrogen: 80 mL min−1 and oxygen: 20 mL min−1) or under nitrogen flow with a heating ramp from room temperature to 800 °C at a 10 °C min−1 rate.
Rheological measurements. Rheo-kinetic measurements were performed using an Anton Paar Physica MCR 302 rheometer equipped with a CDT 450 temperature control device with a disposable aluminium plate–plate (diameter: 25 mm, measure gap: 0.35 mm) geometry. The polymerization measurements were recorded in the oscillation mode at an imposed 1% strain amplitude (γ) and a frequency (f) of 1 Hz. A heating ramp of 20 °C min−1 was applied to reach the temperature of 180 °C. Gelation points were measured at 180 °C whereas the sample deformation was ramped linearly from 1% to 0.2% to remain within the instrument limitation and to maintain a linear viscoelastic behaviour as the moduli increase over several orders of magnitude upon curing. The gelation point corresponds to the time needed at this temperature to reach the crossing of the storage and loss moduli.

An additional post-curing step was added at 190 °C for 30 min. Subsequently, a normal force of 0 N was applied to allow automatic gap control and minimize the stress from thermal contraction or expansion of the solidified sample. The temperature was then decreased at a rate of 10 °C min−1 to 25 °C. After 20 min of stabilization at room temperature the thermal properties of the cured sample were investigated. Therefore, the sample was heated at a rate of 2 °C min−1 up to 50 °C above the observed mechanical relaxation temperature (Tα), kept at that temperature for 5 min, and cooled at a rate of 2 °C min−1 (γ = 0.2% and f = 1 Hz). The Tα was measured from this last step.

2.3 General synthesis of phenol-terminated oligomers (PA-(P)EG)

Phenol-terminated oligomers were obtained by reacting polyethylene glycol or ethylene glycol with phloretic acid without a solvent at 110 °C overnight in a 100 mL round bottom flask with magnetic stirring. The reaction was catalysed by 0.5 wt% p-toluene sulfonic acid (Scheme 1). PA-PEG400 and PA-PEG2000 were synthesised from 1 g (6 mmol) of phloretic acid (PA) and 1.2 g (3 mmol) of PEG400 or 6 g (3 mmol) of PEG2000, respectively. PA-EG was synthesised from 4 g (24 mmol) of phloretic acid and 0.67 g (12 mmol) of ethylene glycol.
image file: c7gc02348k-s1.tif
Scheme 1 General scheme for the synthesis of phenol-terminated oligomers, PA-EG (n = 1), Bz-PEG400 (n = 9) and Bz-PEG2000 (n = 45).
PA-PEG400. 1H NMR (CDCl3, 600 MHz, 298 K) δ (ppm) = (assignment, experimental integration, theoretical integration). δ = 2.6 (CH2–C[double bond, length as m-dash]O [a], exp 4.41 H, th 4 H), 2.8 (CH2*–CH2–C[double bond, length as m-dash]O [b], exp 4.12 H, th 4 H), 3.61 (CH2–O [2], exp 38 H), 4.19 (CH2–O–C[double bond, length as m-dash]O [1], exp 4.24 H, th 4 H), 6.75 (–CH*[double bond, length as m-dash]C– [d], exp 4.12 H, th 4 H), 7.01 (–CH*[double bond, length as m-dash]C–CH2– [c] aromatic proton reference 4 H).
PA-PEG2000. 1H NMR (CDCl3, 600 MHz, 298 K) δ (ppm) = (assignment, experimental integration, theoretical integration). δ = 2.6 (CH2–C[double bond, length as m-dash]O [a], exp 4.5 H, th 4 H), 2.85 (CH2*–CH2–C[double bond, length as m-dash]O [b], exp 4.37 H, th 4 H), 3.6 (CH2–O [2], exp 205.5 H), 4.2 (CH2–O–C[double bond, length as m-dash]O [1], exp 4.24 H, th 4 H), 6.75 (–CH*[double bond, length as m-dash]C– [d], exp 4 H, th 4 H), 7.3 (–CH*[double bond, length as m-dash]C–CH2– [c] aromatic proton reference 4 H).
PA-PEG400 and PA-PEG2000. 13C NMR (CDCl3, 150 MHz, 298 K) δ (ppm) = 30 (CH2–CH2* [b]), 36 (CH2*–CH2 [a]), 64 (Ph-O–CH2 [1]), 69 (O–CH2–CH2* [1′]), 70 (O–CH2– [2]), 115 (–C–CH*[double bond, length as m-dash]C– [d]), 129 (CH[double bond, length as m-dash]CH*–C– [c]), 132 (C*–CH2–CH2 [4]), 155 (–C–OH [5]), 173 (C[double bond, length as m-dash]O [3]). FTIR (cm−1): 3333 (OH– stretching), 2869 (C–H stretching), 1731 (C[double bond, length as m-dash]O stretching from the ester), 1614–1516 (C[double bond, length as m-dash]C stretching vibrations of the ring), 1250–1000 (C–O stretching from the ester).
PA-EG. 1H NMR (CDCl3, 600 MHz, 298 K) δ (ppm) = (assignment, experimental integration, theoretical integration). δ = 2.55 (CH2–C[double bond, length as m-dash]O [a], exp 2.1 H, th 2 H), 2.7 (CH2*–CH2–C[double bond, length as m-dash]O [b], exp 1.9 H, th 2 H), 4.2 (CH2–O–C[double bond, length as m-dash]O [1], exp 1.8 H, th 2 H), 6.6 (–CH*[double bond, length as m-dash]C– [d], exp 2 H, th 2 H), 6.9 (–CH*[double bond, length as m-dash]C–CH2– [c] aromatic proton reference 2 H). 13C NMR (CDCl3, 150 MHz, 298 K): δ (ppm) = 30 (CH2–CH2* [b]), 36 (CH2*–CH2 [a]), 62 (Ph-O–CH2 [1]), 115 (–C–CH*[double bond, length as m-dash]C– [d]), 130 (CH[double bond, length as m-dash]CH*–C– [c]), 131 (C*–CH2–CH2 [4]), 156 (–C–OH [5]), 173 (C[double bond, length as m-dash]O [3]). FTIR (cm−1): 3339 (OH– stretching), 2953 (C–H stretching), 1708 (C[double bond, length as m-dash]O stretching from the ester), 1610–1510 (C[double bond, length as m-dash]C stretching vibrations of the ring), 1300–1000 (C–O stretching from the ester).

2.4 Synthesis of end-chain benzoxazine monomers (Bz-(P)EG)

End-chain benzoxazine monomers were synthesised from the abovementioned phenol-terminated oligomers, furfurylamine and paraformaldehyde at 70 °C for 72 hours (Scheme 2). Bz-PEG400 and Bz-PEG2000 were synthesised without a solvent, from PA-PEG400 and PA-PEG2000 respectively, with 0.53 mL (6 mmol) of furfurylamine and 0.383 g (12 mmol) of paraformaldehyde. Bz-EG was synthesised from PA-EG in dioxane and reacted with 2.12 mL (24 mmol) of furfurylamine and 1.53 g (48 mmol) of paraformaldehyde.
image file: c7gc02348k-s2.tif
Scheme 2 Synthesis of end-chain benzoxazine monomers, Bz-EG (n = 1), Bz-PEG400 (n = 9) and Bz-PEG2000 (n = 45); n′ = 8–100.
Bz-PEG400. 1H NMR (CDCl3, 600 MHz, 298 K) δ (ppm) = (assignment, experimental integration, theoretical integration). δ = 2.6 (CH2–C[double bond, length as m-dash]O [a], exp 4.21 H, th 4 H), 2.8 (CH2*–CH2–C[double bond, length as m-dash]O [b], exp 4.16 H, th 4 H), 3.63 (CH2–O [2], exp 35 H), 3.9 (N–CH2 [h], exp 3.33 H, th 4 H), 3.98 (Ar–CH2 [g], exp 3.22 H, th 4 H), 4.2 (CH2–O–C[double bond, length as m-dash]O [1], exp 3.87 H, th 4 H), 4.85 (O–CH2–N [f], exp 3.17 H, th 4 H), 6.25 (–CH[double bond, length as m-dash]C–O– [i], exp 1.9 H, th 2H), 6.3 (–CH*[double bond, length as m-dash]CH–O– [j], exp 2.03 H, th 2 H), 6.7 (–CH[double bond, length as m-dash]C–O– [e], exp 1.84 H, th 2 H), 6.78 (–CH*[double bond, length as m-dash]C– [d], exp 1.46 H, th 2 H), 6.9 (–CH*[double bond, length as m-dash]C–CH2– [c], aromatic proton reference 2 H), 7.4 (–O–CH[double bond, length as m-dash] [k], exp 1.8 H, th 2 H).
Bz-PEG2000. 1H NMR (CDCl3, 600 MHz, 298 K) δ (ppm) = (assignment, experimental integration, theoretical integration). δ = 2.6 (CH2–C[double bond, length as m-dash]O [a], exp 6.3 H, th 4 H), 2.85 (CH2*–CH2–C[double bond, length as m-dash]O [b], exp 6.23 H, th 4 H), 3.62 (CH2–O [2], exp 364 H), 3.9 (N–CH2 [h], exp 3.8 H, th 4 H), 3.96 (Ar–CH2 [g], exp 3.91 H, th 4 H), 4.2 (CH2–O–C[double bond, length as m-dash]O [1], exp 6.44 H, th 4 H), 4.82 (O–CH2–N [f], exp 3.89 H, th 4 H), 6.22 (–CH[double bond, length as m-dash]C–O– [i], exp 2.2 H, th 2H), 6.3 (–CH*[double bond, length as m-dash]CH–O– [j], exp 2.26 H, th 2 H), 6.7 (–CH[double bond, length as m-dash]C–O– [e], exp 2 H, th 2 H), 6.76 (–CH*[double bond, length as m-dash]C– [d], exp 2.65 H, th 2 H), 7 (–CH*[double bond, length as m-dash]C–CH2– [c], aromatic proton reference 2 H), 7.4 (–O–CH[double bond, length as m-dash] [k], exp 2.1 H, th 2 H).
Bz-PEG400 and Bz-PEG2000. 13C NMR (CDCl3, 150 MHz, 298 K), δ = 30 (CH2 [b]), 36 (CH2 [a]), 48 (N–CH2*–C [h]), 50 (N–CH2*–C [g]), 63 (Ar–O–CH2 [1]), 69 (O–CH2–CH2* [1′]), 70 (O–CH2 [2]), 82 (N–CH2–O [f]), 108 (CH[double bond, length as m-dash]CH*–C [i]), [double bond, length as m-dash]110 (CH*[double bond, length as m-dash]CH–O [j]), 116 (CH[double bond, length as m-dash]CH*–C–O [e]), 119 (CH[double bond, length as m-dash]C*–CH2 [5]), 127.3 (–C–CH*[double bond, length as m-dash]C– [d]), 127.6 (CH[double bond, length as m-dash]CH*–C– [c]), 132 (C*–CH2–CH2 [4]), 142 (O–CH*[double bond, length as m-dash]CH [k]), 152 (CH[double bond, length as m-dash]C*–O [6]), 154 (–C–O [7]), 175 (C[double bond, length as m-dash]O [3]).

FTIR (cm−1): 2871 (C–H stretching), 1731 (C[double bond, length as m-dash]O stretching from the ester), 1614–1516 (C[double bond, length as m-dash]C stretching vibrations of the ring), 1226 (C–O–C stretching), 1250–1000 (C–O stretching from the ester), 936 (ring vibrations), 884 and 861 (out-of-plane wagging), 741 (mono-substituted ring).

Bz-EG. 1H NMR (CDCl3, 600 MHz, 298 K) δ (ppm) = (assignment, experimental integration, theoretical integration). δ = 2.06 (CH2–C[double bond, length as m-dash]O [a], exp 2.81 H, th 2 H), 2.03 (CH2*–CH2–C[double bond, length as m-dash]O [b], exp 3.1 H, th 2 H), 3.8 (N–CH2 [h], exp 2.04 H, th 2 H), 3.9 (Ar–CH2 [g], exp 2.01 H, th 2 H), 4.25 (CH2–O–C[double bond, length as m-dash]O [1], exp 2.29 H, th 2 H), 4.75 (O–CH2–N [f], exp 2.06 H, th 2 H), 6.3 (–CH[double bond, length as m-dash]C–O– [i], exp 1.06 H, th 1 H), 6.4 (–CH*[double bond, length as m-dash]CH–O– [j], exp 1.06 H, th 1 H), 6.7 (–CH[double bond, length as m-dash]C–O– [e], exp 1.02 H, th 1 H), 6.8 (–CH*[double bond, length as m-dash]C– [d], aromatic proton reference 1 H), 6.9 (–CH*[double bond, length as m-dash]C–CH2– [c], exp 1.01 H, th 1 H), 7.6 (–O–CH[double bond, length as m-dash] [k], exp 0.99 H, th 1 H).

13C NMR (CDCl3, 150 MHz, 298 K), δ = 30 (CH2 [b]), 36 (CH2 [a]), 48 (N–CH2*–C [h]), 50 (N–CH2*–C [g]), 73 (O–CH2 [1]), 82 (N–CH2–O [f]), 109 (CH[double bond, length as m-dash]CH*–C [i]), 111 (CH*[double bond, length as m-dash]CH–O [j]), 117 (CH[double bond, length as m-dash]CH*–C–O [e]), 120 (CH[double bond, length as m-dash]C*–CH2 [4]), 127.6 (–C–CH*[double bond, length as m-dash]C– [d]), 127.9 (CH[double bond, length as m-dash]CH*–C– [c]), 133 (C*–CH2–CH2 [3]), 142 (O–CH*[double bond, length as m-dash]CH [k]), 152 (CH[double bond, length as m-dash]C*–O [5]), 152.2 (–C–O [6]), 174 (C[double bond, length as m-dash]O [2]).

FTIR (cm−1): 2953 (C–H stretching), 1708 (C[double bond, length as m-dash]O stretching from the ester), 1610–1510 (C[double bond, length as m-dash]C stretching vibrations of the ring), 1300–1000 (C–O stretching from the ester), 931 (ring vibrations), 885 and 872 (out-of-plane wagging), 735 (mono-substituted ring).

3 Results and discussion

3.1 Molecular characterization of phenol-terminated oligomers

Phenol-terminated oligomers were characterized by 1H NMR, FTIR and MALDI-TOF/MS. FTIR spectra of PA-PEG400, PA and PEG400 are displayed in Fig. 1. The formation of the ester bond is revealed by the shift of the characteristic peak of the carboxylic group of phloretic acid from 1698 cm−1 to 1731 cm−1 indicating that the esterification of the carboxylic group has occurred. The absorption band at 1096 cm−1 is attributed to the C–O–H stretching of the PEG backbone. Peaks of the stretching vibrations of the aromatic ring present in phloretic acid, C[double bond, length as m-dash]C stretching, were observed at 1516, 1599 and 1614 cm−1. In addition, the characteristic peaks of C–O stretching of an ester are observed between 1000 and 1250 cm−1. Similar results were obtained for PA-PEG2000 and PA-EG confirming the esterification of the hydroxyl groups in both cases.
image file: c7gc02348k-f1.tif
Fig. 1 FTIR spectra of PA-PEG400, phloretic acid and polyethylene glycol.

The 1H NMR spectrum of PA-PEG400 is depicted in Fig. 2. The peak at δ = 4.2 ppm, corresponding to the protons from CH2–O–CO [1], revealed the formation of the ester link. This peak shifted from 3.6 ppm in PEG400 to 4.2 ppm in PA-PEG400. Similar observations were made on PA-EG and PA-PEG2000 and the success of the phenolation of aliphatic hydroxyls via solventless esterification was evidenced (Fig. S1 and Fig. S2).


image file: c7gc02348k-f2.tif
Fig. 2 1H NMR spectrum (in CDCl3) of PA-PEG400.

For all PA-(P)EG, the amount of phenol-terminated groups was determined by the integration ratio between the peak at 4.2 ppm [1] and the peak at 3.6 ppm [2], the last one corresponding to the global methylene groups from the backbone, and the procedure is shown in Scheme 3. In the case of PEG400, the values of the integration were 4.24 H and 38.15 H respectively, leading to a degree of phenolation of 90%. Similar results were obtained for PA-EG and PA-PEG2000, with 85 and 93% respectively.


image file: c7gc02348k-s3.tif
Scheme 3 Calculation of the degree of phenol-terminated PEG/EG from the ratio of the integral area of 1H NMR.

The chemical structure of the synthesised PA-PEG400 was also characterised by MALDI-TOF/MS. The corresponding spectrum is displayed in Fig. 3 and compared to the spectrum of PEG400. A shift of the distribution of the characteristic pattern of PEG of about 298 m/z can be observed indicating that the esterification occurred at both ends of the PEG backbone leading to a di-telechelic molecule. Mono-functionalization also occurred, as indicated by the shift of 149 m/z to higher m/z of the characteristic pattern of PEG. In this case the mono-telechelic molecule was synthesised. Some traces of unreacted PEG also remained in the product. It is noteworthy that the characteristic pattern of PEG is clearly identifiable on these spectra by the gap of 44 m/z between adjoining peaks, corresponding to the molar mass of one repetitive unit of PEG.


image file: c7gc02348k-f3.tif
Fig. 3 MALDI-TOF/MS spectra of PEG and PA-PEG400.

3.2 Molecular characterization of end-chain Bz monomers

The chemical structure of synthesized end-chain benzoxazine was characterised by 1H NMR and FTIR.

Fig. 4 shows the FTIR spectrum of Bz-PEG400 compared to that of PA-PEG400. The presence of some new peaks such as those from ring vibration and C–O–C stretching at 936 and 1226 cm−1, respectively supports the successful synthesis of benzoxazine monomers. Remarkably, the O–H stretching band centred at 3333 cm−1 entirely disappeared indicating the successful formation of the oxazine ring. Characteristic peaks attributable to the furan ring are observed at 741 cm−1 which correspond to the mono substituted ring, and at 884 and 861 cm−1 due to out-of-plane wagging.


image file: c7gc02348k-f4.tif
Fig. 4 FTIR spectra of Bz-PEG400 and PA-PEG400.

The 1H NMR spectrum of end-chain benzoxazine from PEG400 is displayed in Fig. 5. The formation of the Bz monomers was revealed by the presence of peaks corresponding to the protons from O–CH2–N [f] and Ar–CH2–N [g] groups. These characteristic groups were observed at δ = 4.85 ppm and δ = 3.9 ppm respectively. In addition, the signals corresponding to the furan ring were also observed [k, j, i] at δ = 7.4, 6.3 and 6.25 ppm. Similar results were obtained for Bz-EG and Bz-PEG2000 that evidenced the success of the synthesis (Fig. S3 and Fig. S4).


image file: c7gc02348k-f5.tif
Fig. 5 1H NMR spectrum (in CDCl3) of Bz-PEG400.

The degree of end-capping of Bz-PEG was determined by using the integration ratio between the peak of the methylene group of O–CH2–N ([f] δ = 4.85 ppm) and the peak between 3.5 to 3.8 ppm from the methylene groups of the PEG backbone [2]. The values of the integration were 3.17 H and 34.67 H respectively. The procedure followed to calculate the degree of end-capping is similar to the one shown in Scheme 3 obtaining a value of 75%. The same procedure was applied to Bz-PEG2000 obtaining 50% degree of end capping of benzoxazine. In the case of Bz-EG, the degree of end-capping was 77% considering that in the final product opened benzoxazine was also found (appearance of a peak around 3.6 ppm).32 The observed behaviour might be explained by the lower reactivity of phenol terminated PEG with extended chain lengths, as observed similarly on benzoxazine prepared with long aliphatic diamines as bridging groups.33

3.3 Thermal properties of (Bz-(P)EG)

Thermal properties of the synthesised Bz-end-capped molecules were studied by DSC as shown in Fig. 6. The DSC thermograms showed an exothermic peak, in the range of 180–240 °C, assigned to the thermally activated ring-opening of the Bz monomer. The onsets ranged from 165 °C to 200 °C, for Bz-EG and Bz-PEG2000, respectively (Table 1, column 3). This range of temperature, as well as the temperature required to reach the maximum of the exothermic peak, are in accordance with the values reported in the literature for conventional benzoxazine monomers, between 160 and 220 °C.6 It can be noticed that this exothermic peak has shifted from 214 °C, recorded for Bz-EG, to around 230 °C for Bz-PEG400 and Bz-PEG2000. The lower temperature obtained for Bz-EG could be explained by the presence of residual products that could have catalysed the polymerization.34 In addition, it is worth noting that the lower amount of benzoxazine groups in Bz-PEG400 and Bz-PEG2000 compared to that in Bz-EG is also an important reason to explain the increase of their polymerization temperatures.35 This behaviour is related to a dilution effect, also characterized by the significant decrease of the polymerization enthalpies of Bz-PEG400 (60 wt% Bz) and Bz-PEG2000 (20 wt% Bz) compared to that of Bz-EG (90 wt% Bz) (Table 1, column 5). The monomers were also analysed by TGA (Fig. S5).
image file: c7gc02348k-f6.tif
Fig. 6 DSC thermograms (N2, 10 °C min−1) of end-chain benzoxazine monomers.
Table 1 Thermal properties of end-chain benzoxazine monomers obtained from DSC and TGA analyses
Sample T m[thin space (1/6-em)]a (°C) T exo,o[thin space (1/6-em)]b (°C) T exo,m[thin space (1/6-em)]c (°C) ΔHexo[thin space (1/6-em)]d (J g−1/kJ mol−1) T 5%[thin space (1/6-em)]e (°C)
a Maximum temperature of the melting endotherm by DSC. b Onset temperature of the exotherm by DSC. c Maximum temperature of the exotherm by DSC. d Enthalpy of the exotherm by DSC. e Temperature of 5% weight loss under a N2 atmosphere by TGA.
Bz-EG 165 214 88/53.2 166
Bz-PEG400 180 230 57.9/55.5 262
Bz-PEG2000 52 200 230 5.9/16.5 299


The thermal stability of the Bz-end-capped molecules was measured to check if by-products were released or not during the first steps of polymerization as has been reported several times.36–41 The values are reported in Table 1 (column 6). The temperature of 5% weight loss of Bz-PEG400 was found to be 262 °C confirming that no early thermal degradation of the monomer occurs during the curing as the Texo,o is situated around 180 °C. Similar behaviour was highlighted for Bz-PEG2000 with T5% = 299 °C and Texo,o = 200 °C. However, in the case of Bz-EG, the T5% and Texo,o are closer, around 165 °C.

3.4 Rheo-kinetic characterization of (Bz-(P)EG)

Rheological analysis was performed on Bz-end-capped molecules to evaluate their polymerization reactivity. The recorded thermograms are displayed in Fig. 7 showing the evolution of the storage modulus G′ and the loss modulus G′′ at 180 °C. The observed behaviour of end-chain benzoxazine upon heating is typical of thermosets. At the beginning of the isothermal step, the material behaves like a liquid, as G′′ is above G′, meaning that the material is in its molten state. The start of the cross-linking is evidenced by the significant increase in both G′ and G′′. The gelation times of Bz-EG and Bz-PEG400, corresponding to the crossover point of G′ and G′′, was observed to occur around 2 and 3 min (Table 2, column 1), respectively. In the case of Bz-PEG2000, the gelation point was found after 25 min indicating a lower reactivity of the material in comparison to those of Bz-EG and Bz-PEG400. However, the G′ and G′′ values of Bz-PEG2000 were low. Even after one additional hour of curing at 190 °C, G′ just reached 100 Pa implying that the material that did not behave as a thermoset due to presumed low cross-linking density.
image file: c7gc02348k-f7.tif
Fig. 7 Isothermal rheology monitoring of Bz-EG, Bz-PEG400 and Bz-PEG2000 at 180 °C. Filled markers and continuous line: storage modulus (G′) and empty markers and dashed line: loss modulus (G′′). Gelation points are indicated by dotted lines.
Table 2 Thermal properties of polymerized end-chain benzoxazines obtained from rheology and TGA analyses
Sample t gelation (min) η R.T*/η50*[thin space (1/6-em)]b (Pa s) T α[thin space (1/6-em)]c (°C) T 5%,N2/T5%,Air[thin space (1/6-em)]d (°C) Char yielde (%)
a Gelation point at 180 °C by rheo-kinetic characterization. b Complex viscosity at room temperature and 50 °C by rheo-kinetic characterization. c Mechanical relaxation temperature, maximum of tan(δ) by rheo-kinetic characterization. d Temperature of 5% weight loss under nitrogen and air by TGA. e Under nitrogen at 800 °C by TGA.
Bz-EG 2 9.5/0.63 112 265/258 52
Bz-PEG400 3 9.9/0.85 27 333/316 40
Bz-PEG2000 28 -/0.96 −45 340/313 0


The gelation points obtained for each of the materials increased with the decreasing amount of Bz present in the material as expected (Table 2, column 2). Interestingly, the Bz-EG and Bz-PEG400 gelation times are far below the values reported for relevant bis-phenol-A/aniline Bz cured at 190 °C which has been reported to be around 40 min.34 It is noteworthy that Bz-EG and Bz-PEG400 have striking features due to their inherently low complex viscosity at room temperature, 9.5 and 9.9 Pa s respectively, highlighting their ease of processability.42

3.5 Thermal properties of polybenzoxazine

After an additional 30 min post-curing time at 190 °C, the thermal properties of the cured samples were investigated (Fig. 8). Bz-EG shows the mechanical relaxation temperature (Tα) of about 112 °C, considering the maximum of the loss factor (tan[thin space (1/6-em)]δ) curve. For Bz-PEG400 and Bz-PEG2000, Tα was found at lower values of temperature due to the decrease in the amount of benzoxazine monomer with the increase of chain length, at 27 °C and −45 °C, respectively. Finally, the thermal stability of polybenzoxazine was studied by TGA under a nitrogen atmosphere as shown in Fig. 9. On the TGA spectrum of poly(Bz-PEG400) a one-step degradation occurring with the onset at 333 °C and with a Tmax of 415 °C is observed. This step is mainly attributed to the degradation of the main chain itself and the char yield obtained was 40%. The char yield values obtained for poly(Bz-(P)EG) are shown in Table 2 (column 6) and are in agreement with those reported for other polybenzoxazines.43 TGA of poly(Bz-PEG2000) shows similar T5% and Tmax but the char yield is 9% due to the higher influence of the PEG backbone. A higher char yield was obtained for poly(Bz-EG), 52%. However, the material was less thermally stable showing the temperature of 5% weight loss at 265 °C. It is noteworthy that the polymerized end-chain Bz exhibited much more thermal stability than the linear precursors, ethylene glycol (T5% 240 °C) and polyethylene glycol (T5% 200 °C). These end-capped polymers also show marked improvement in char yield over the starting molecules, 0% char yield under N2 at 800 °C for EG and PEG, showing values similar to those described previously in the literature.13,14
image file: c7gc02348k-f8.tif
Fig. 8 Temperature dependence of the storage modulus (G′) and the loss factor (tan[thin space (1/6-em)]δ) upon cooling. Filled markers: storage modulus (G′) and empty markers: loss factor (tan[thin space (1/6-em)]δ).

image file: c7gc02348k-f9.tif
Fig. 9 TGA (N2 atmosphere, 10 °C min−1) of poly(Bz-EG), poly(Bz-PEG400) and poly(Bz-PEG2000).

4 Conclusions

Phloretic acid was reacted with ethylene glycol and two polyethylene glycols (molar masses: 400 g mol−1 and 2000 g mol−1) via solvent-free Fischer esterification to obtain phenol-terminated molecules. Then, they were reacted with furfurylamine and paraformaldehyde to form benzoxazine-end-capped molecules. The chemical structures of the three model molecules were characterized by FTIR, MALDI-TOF/MS, 1H and 13C NMR. For each of them the degree of end-capping with Bz was calculated and found to be 77%, 75% and 50% for Bz-EG, Bz-PEG400 and Bz-PEG2000 respectively.

Thermal behaviour and stability of each Bz end-capped monomer were characterized by DSC and TGA, evidencing their suitability for the elaboration of thermoset materials. Indeed, the high thermal stability of the monomers allowed their polymerization, triggered from 165 °C for the most reactive model molecule Bz-EG. Rheo-kinetic measurements were performed to determine the gelation time of each monomer at 180 °C, highlighting a lengthening of the gelation time (from 1 min to 25 min) as well as a lowering of the mechanical relaxation temperature (from 108 °C to −50 °C), with increasing backbone chain length of the model molecules.

In conclusion, this work highlighted the suitability of phloretic acid to act as an efficient and green alternative to phenol to enhance the reactivity of –OH bearing molecules towards the synthesis of benzoxazine. It is noteworthy that the whole synthesis of Bz monomers from PEG400 and PEG2000 are almost 100% bio-based and do not require solvent or purification, and their polymerization led to a set of materials with thermal and thermo-mechanical properties suitable for a wide range of applications. Given the large number of –OH bearing compounds existing in materials science, this alternative is expected to find multiple application opportunities such as coatings or composites.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are warmly thankful to Régis Vaudemont and Benoit Marcolini for the thermal characterization as well as Reiner Dieden for his kind and precious help for NMR characterization.

Notes and references

  1. H. Ishida and H. Y. Low, Macromolecules, 1997, 30, 1099–1106 CrossRef CAS.
  2. S. B. Shen and H. Ishida, Polym. Compos., 1996, 17, 710–719 CrossRef CAS.
  3. H. Ishida and D. J. Allen, J. Polym. Sci., Part B: Polym. Phys., 1996, 34, 1019–1030 CrossRef CAS.
  4. H. Ishida, in Handbook of Benzoxazine Resins, Elsevier, Amsterdam, 2011, pp. 3–81 Search PubMed.
  5. T. Takeichi, T. Kawauchi and T. Agag, Polym. J., 2008, 40, 1121–1131 CrossRef CAS.
  6. X. Ning and H. Ishida, J. Polym. Sci., Part A: Polym. Chem., 1994, 32, 1121–1129 CrossRef CAS.
  7. H. Ishida and S. Ohba, Polymer, 2005, 46, 5588–5595 CrossRef CAS.
  8. B. Kiskan and Y. Yagci, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 1670–1676 CrossRef CAS.
  9. T. Agag and T. Takeichi, Polymer, 2000, 41, 7083–7090 CrossRef CAS.
  10. B. Kiskan, A. L. Demirel, O. Kamer and Y. Yagci, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 6780–6788 CrossRef CAS.
  11. J. Huang, W. Du, J. Zhang, F. Huang and L. Du, Polym. Bull., 2008, 62, 127 CrossRef.
  12. S. Kirschbaum, K. Landfester and A. Taden, Macromolecules, 2015, 48, 3811–3816 CrossRef CAS.
  13. B. Kiskan, G. Demiray and Y. Yagci, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 3512–3518 CrossRef CAS.
  14. M. Ergin, B. Kiskan, B. Gacal and Y. Yagci, Macromolecules, 2007, 40, 4724–4727 CrossRef CAS.
  15. H. Kimura, A. Matsumoto, H. Sugito, K. Hasegawa, K. Ohtsuka and A. Fukuda, J. Appl. Polym. Sci., 2001, 79, 555–565 CrossRef CAS.
  16. B. Koz, B. Kiskan and Y. Yagci, Polym. Bull., 2011, 66, 165–174 CrossRef CAS.
  17. M. Nakamura and H. Ishida, Polymer, 2009, 50, 2688–2695 CrossRef CAS.
  18. B. Kiskan and Y. Yagci, Polymer, 2005, 46, 11690–11697 CrossRef CAS.
  19. A. Yildirim, B. Kiskan, A. L. Demirel and Y. Yagci, Eur. Polym. J., 2006, 42, 3006–3014 CrossRef CAS.
  20. T. Agag, K. Vietmeier, A. Chernykh and H. Ishida, J. Appl. Polym. Sci., 2012, 125, 1346–1351 CrossRef CAS.
  21. Y. Shibayama, T. Kawauchi and T. Takeichi, High Perform. Polym., 2014, 26, 60–68 CrossRef.
  22. T. Takeichi, T. Kano and T. Agag, Polymer, 2005, 46, 12172–12180 CrossRef CAS.
  23. B. Kiskan, Y. Yagci and H. Ishida, J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 414–420 CrossRef CAS.
  24. Y. Yagci, B. Kiskan and N. N. Ghosh, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 5565–5576 CrossRef CAS.
  25. M. Kukut, B. Kiskan and Y. Yagci, Des. Monomers Polym., 2009, 12, 167–176 CrossRef CAS.
  26. G. J. Abarro, J. Podschun, L. J. Diaz, S. Ohashi, B. Saake, R. Lehnen and H. Ishida, RSC Adv., 2016, 6, 107689–107698 RSC.
  27. R. W. Whetten, J. J. MacKay and R. R. Sederoff, Annu. Rev. Plant Physiol. Plant Mol. Biol., 1998, 49, 585–609 CrossRef CAS PubMed.
  28. J. Podschun, B. Saake and R. Lehnen, Eur. Polym. J., 2015, 67, 1–11 CrossRef CAS.
  29. A. Picinelli, E. Dapena and J. J. Mangas, J. Agric. Food Chem., 1995, 43, 2273–2278 CrossRef CAS.
  30. M. Comí, G. Lligadas, J. C. Ronda, M. Galià and V. Cádiz, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 4894–4903 CrossRef.
  31. P. Froimowicz, C. R. Arza, L. Han and H. Ishida, ChemSusChem, 2016, 9, 1921–1928 CrossRef CAS PubMed.
  32. A. Chernykh, J. Liu and H. Ishida, Polymer, 2006, 47, 7664–7669 CrossRef CAS.
  33. D. J. Allen and H. Ishida, J. Appl. Polym. Sci., 2006, 101, 2798–2809 CrossRef CAS.
  34. H. Ishida and D. J. Allen, J. Appl. Polym. Sci., 2001, 79, 406–417 CrossRef CAS.
  35. B. Kiskan and Y. Yagci, J. Polym. Sci., Part A: Polym. Chem., 2014, 52, 2911–2918 CrossRef CAS.
  36. L. Dumas, L. Bonnaud, M. Olivier, M. Poorteman and P. Dubois, Eur. Polym. J., 2014, 58, 218–225 CrossRef CAS.
  37. L. Dumas, L. Bonnaud, M. Olivier, M. Poorteman and P. Dubois, Eur. Polym. J., 2015, 67, 494–502 CrossRef CAS.
  38. M. Poorteman, A. Renaud, J. Escobar, L. Dumas, L. Bonnaud, P. Dubois and M.-G. Olivier, Prog. Org. Coat., 2016, 97, 99–109 CrossRef CAS.
  39. L. Dumas, L. Bonnaud, M. Olivier, M. Poorteman and P. Dubois, J. Mater. Chem. A, 2015, 3, 6012–6018 CAS.
  40. L. Puchot, P. Verge, T. Fouquet, C. Vancaeyzeele, F. Vidal and Y. Habibi, Green Chem., 2016, 18, 3346–3353 RSC.
  41. N. K. Sini, J. Bijwe and I. K. Varma, Polym. Degrad. Stab., 2014, 109, 270–277 CrossRef CAS.
  42. T. Agag, S. Geiger, S. M. Alhassan, S. Qutubuddin and H. Ishida, Macromolecules, 2010, 43, 7122–7127 CrossRef CAS.
  43. B. J. Howlin and I. Hamerton, in Advanced and Emerging Polybenzoxazine Science and Technology, ed. P. Froimowicz, Elsevier, Amsterdam, 2017, ch. 9, pp. 131–145 Search PubMed.

Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7gc02348k

This journal is © The Royal Society of Chemistry 2017