Caryophyllene as a Precursor of Cross-Linked Materials

This paper aims at the synthesis of a new type of elastomers from caryophyllene. The adopted strategy was to cross-link the polycaryophyllene, which was synthesized by ring-opening metathesis polymerization (ROMP). The polycaryophyllene obtained showed M n = 2 x 10 4 g.mol -1 (Đ = 1.5) with a glass transition temperature (T g ) of -35 °C. On the first hand, thermal crosslinking was performed in the presence of organic peroxides or sulfur system. On the second hand, thiol-ene coupling initiated by UV-light at room temperature was also investigated as an alternative pathway to cross-link the polycaryophyllene. The materials obtained were analyzed by TGA, DSC, and DMA. The T g of cross-linked polycaryophyllene could be easily modulated from -35 °C to a range between -25 and 10 °C by changing the type of cross-linking agent. The curing process led to the improvement of thermal stability ranging from 200 °C to around 340 °C. Finally, the network storage modulus varied from 1 to 100 MPa at room temperature. Abstract This paper aims at the synthesis of a new type of elastomers from caryophyllene. The adopted strategy was to cross-link the polycaryophyllene, which was synthesized by ring-opening metathesis polymerization (ROMP). The polycaryophyllene obtained showed M n = 2 x 10 4 g.mol -1 (Đ = 1.5) with a glass transition temperature ( T g ) of -35 °C. On the first hand, thermal crosslinking was performed in the presence of organic peroxides or sulfur system. On the second hand, thiol-ene coupling initiated by UV-light at room temperature was also investigated as an alternative pathway to cross-link the polycaryophyllene. The materials obtained were analyzed by TGA, DSC, and DMA. The T g of cross-linked polycaryophyllene could be easily modulated from -35 °C to a range between -25 and 10 °C by changing the type of cross-linking agent. The curing process led to the improvement of thermal stability ranging from 200 °C to around 340 °C. Finally, the network storage modulus varied from 1 to 100 MPa at room temperature.


Introduction
New strategies to develop materials from renewable feedstock have been continuously sought as greener alternatives to commercial oil-based materials. There is a significant interest in polymeric materials, taking into consideration their wide range of applications, especially for elastomers. The elastomers market has witnessed significant growth in the global production in recent years, with an estimated increase in revenue from current US$ 80.73 billion to close to US$ 104.17 billion by 2026. 1,2 The adoption of vulcanized thermoplastics (TPV) in various application sectors has led to this development.
Among several categories of thermoplastics, the natural rubbers (NR) emerge as a major players. The NR segment plays an important role in the TPV global market since the NR is one of the most used thermoplastic thanks to their versatility. 2 Due to their properties such as high thermal stability, good chemical resistance, high tensile strength, low shrinkage, and greater design flexibility, they can be employed as devices and/or tools for automotive, medical, consumer goods, industrial, and other applications. 3,4 In the framework of looking for an alternative to oil-based consumer goods, an important number of research groups have concentrated their studies and described the synthesis of vegetable-oil-based thermoplastics, such as polyester, polyurethanes, polyamides, and many other bio-based polymers. [4][5][6][7][8][9][10] Terpenes and terpenoides have drawn considerable attention as potential starting precursors in the synthesis of elastomers. [11][12][13][14][15][16][17] Studies on the synthesis of elastomers from pinene, myrcene, and limonene [18][19][20][21] can be readily found in the scientific literature and some commercially available resins and adhesives (e.g. Piccolyte®) derived from polypinenes are already reported. However, there is little exploitation of sesquiterpenes by the scientific community. Among molecules defined as sesquiterpenes, caryophyllene and humulene stand out, since these molecules are made of interesting structural elements. These molecules are found in many essential oils such as rosemary, cannabis sativa, hops, and mostly clove oil. [22][23][24] The cheapest and most abundant sesquiterpene, the caryophyllene, is a versatile and significant molecule due to a bicyclic and a cyclobutane rings as well as unsaturated bonds present in its chemical structure. 23 Thanks to them, the caryophyllene can be easily modified by attaching different moieties to its backbone chain. Moreover caryophyllene is, to the best of our knowledge, the only readily available bio-based molecule that can be polymerized by ring-opening metathesis polymerization (ROMP) leading to polycaryophyllene (PCar). 25 PCar exhibits two carbon-carbon double bonds in its structure which can be further used to cross-link it.
Keeping in mind this context, the present research has aimed to the cross-linking of polycaryophyllene by using different routes and investigating the thermo-mechanical properties of the newly produced cross-linked bio-based elastomer. Two main pathways have been considered to reach three dimensional cross-linked polycaryophyllene: i) thermal pathways: in the presence of organic peroxides [benzoyl peroxide (BPO) or dicumyl peroxide (DCP)], or by classical vulcanization based on sulfur (SS) and ii) via UV light initiated thiol-click reactions at room temperature (see diagram in Fig.1). As far as we know, this is the first study about the cross-linking of polycaryophyllene. To confirm the crosslinking, the gel fractions were quantified by extraction in THF at 50 °C for 48 hours.
The gel fraction appears to be limited for BPO cross-linking at 65% since even an increase of the curing temperature to 180 °C does not improve the gel fraction (see Table S1).
Obtaining a higher amount of gel fraction with DCP compared to with BPO was already identified in other studies. 26,27 Indeed, DCP is the most frequently used cross-linking agent since it is an organic peroxide which is more selective to vinyl moieties than benzoyl peroxide. 28,29 The ability of cumyloxy radical from DCP to react with non-activated vinyl moieties in a free-radical cross-linking pathway via H

UV light
Organic peroxide Classical vulcanization with sulfur system Dithiol DCP BPO abstraction may be a possible explanation of its higher reactivity compared to BPO. 30 Additionally, the cumyloxy radical can undergo β-scission resulting in radical fragments (methyl radical and acetophenone) with high mobility. 31 Furthermore, the unsaturated bonds are not easily accessible by benzoyloxy radicals due to the steric hindrance around double bonds as the increase of formation of cross-linked networks. 24 The SEC analysis of the BPO soluble fraction testified to the presence of oligomers (see Fig. S7, Mn=320 g.mol -1 , Đ=1.5) which indicated that some polymer backbone cleavage also takes place during the crosslinking. In the case of the sulfur system, the soluble fraction is mainly constituted of the accelerator and the catalyst used. Finally, the IR analysis of the materials obtained (see Fig. S8) shows that whatever the cross-linker, the vibration characteristics the carbon-carbon double bond (i.e. =CH2 bending 887 cm -1 , C=C stretching at 1645 cm -1 , C-H stretching at 3115 cm -1 ) are still present indicating a crosslinking by hydrogen abstraction mechanisms.
In order to confirm the occurrence of cross-linking reactions, on-line rheology measurements were carried out, they are shown in Fig.2. The visco-elastic analysis also provided an estimation of the initial temperature of the cross-linking process. As expected, polycaryophyllene in the absence of a crosslinker behaves as a liquid for temperatures up to 160°C with the loss modulus (G'') higher than the storage modulus (G'). Classically, as the temperature increases, both moduli continuously decrease and no cross-linking reaction takes place at 160°C since no change in moduli was observed over time.
In the presence of DCP, a shoot up in moduli is clearly observed with a gel point (intersection point) at 160°C. Similar curves were obtained for the curing of polycaryophyllene with BPO and SS (see with gel points occurring at 118°C and 160°C respectively and are in agreement with DSC online curing curves (Fig S11-12).
After the gel point, both loss and storage moduli reached constant plateaus and no more significant changes are observed. The magnitude of storage modulus increased up to 0.6, 0.5, and 0.5 kPa and the loss modulus to 1.2 MPa, 40, and 60 kPa respectively for polycaryophyllenes cross-linked by DCP, BPO, and sulfur system.
Concerning the UV cross-linking by thiol-ene coupling, the soluble fraction remains between 0% and 11% depending of the dithiol used (see Table 1). Interestingly, when the crosslinking is performed with 0.5 eq. of thiol per carbon-carbon double bond (db), a slight improvement of the gel fraction is observed (see Table S2). This may be to the presence of free dithiol in the soluble fraction. IR analysis of the polycaryophyllene obtained with 0.5 eq. of thiol per db shows no regioselectivity of the thiol-ene addition since the bands which are characteristic of the two carbon-carbon double bonds decrease (see Fig. S13).
For 1 eq. of thiol per db, double bonds appear to have totally disappeared whatever the dithiols used (see To obtain further information about cross-linked polycaryophyllenes, TGA and DSC measurements were performed in order to investigate their thermal properties, as shown in Fig. 3, Fig. 4 and Table 1.  According to TGA (see Fig 3 and Fig S15-S16), the crosslinking of polycaryophyllene increases its thermal stability. Indeed, the temperature of 10% of degradation (T10%) for polycaryophyllene is around 200 °C. After cross-linking, the T10% take place at temperature ranging from 310 to 340 °C.
DSC measurements were performed to evaluate the effect of newly formed cross-linked structures on the glass transition temperatures (Tg) of polycaryophyllene (See Fig. 4 and Fig. S17).
The Tg of cross-linked polycaryophyllene increased significantly compared to non-cross-linked polycaryophyllene (Tg = -35°C). The curing by DCP provided more rigidity to polycaryophyllene (Tg = 12 °C) than the one by sulfur system (Tg = -9 °C) or DDT (Tg = -6 °C). Moreover the Tg of the network gradually increases with the chain length of the dithiol from -22°C for EDT to -6°C for DDT.

°C
The mechanical properties of cross-linked polycaryophyllene were evaluated by dynamic mechanical analyses (DMA), displayed in Fig. 5 and Fig. S18.
The DMA curves show a typical behavior for cross-linked materials with a glassy plateau before the alpha transition temperature (T) followed by a rubbery zone after this point.  Table   1) or even DDT cross-linking (4.6 MPa).

Conclusions
In the present work, caryophyllene was polymerized by ROMP and the polymer obtained was crosslinked using different routes: organic peroxides, sulfur system or dithiol. Rheological analyses confirmed the occurrence of cross-linking reactions in agreement with the extractions by THF results. Swelling experiments showed that the polycaryophyllene was entirely cross-linked by DCP or dithiols with an insoluble fraction over 90%. Cross-linking by sulfur system resulted in a gel content of 72%.
The cross-linked materials were in-depth analyzed by TGA, DSC and DMA. Materials were obtained with Tg between -22°C and 12°C with improved thermal stability compared to non-cross-linked polycaryophyllene (T10% > 300°C). At room temperature (20°C) storage modulus of the cross-linked polycaryophyllene were between 1 MPa to 100 MPa. Based on the achieved results, they could potentially find applications as replacement of synthetic rubbers.

iv) Characterizations
NMR spectra were recorded using a Bruker AC-400 NMR, at room temperature, in deuterated chloroform (CDCl3).
Fourier Transform Infrared (FT-IR) spectra were performed on a Bruker VERTEX 70 spectrometer equipped with diamond crystal (GladiATR PIKE technologies) for the attenuated total reflection (ATR) mode. The spectra were acquired from 400 to 4000 cm -1 at room temperature using 32 scans at a resolution of 4 cm -1 .
Polymer molar masses were determined by size exclusion chromatography (SEC) using tetrahydrofuran (THF with 250 ppm of BHT as an inhibitor) as an eluent and trichlorobenzene as a flow marker. Measurements in THF were performed on a ThermoFisher Scientific Ultimate 3000 system equipped with Diode Array Detector, Wyatt light scattering detector and RI detector. The separation was achieved on three Tosoh TSK gel columns: G4000HXL (particles of 5 mm, pore size of 200 Å, exclusion limit of 400 000 g mol -1 ), G3000HXL (particles of 5 mm, pore size of 75 Å, exclusion limit of 60 000 g mol -1 ), G2000HXL (particles of 5 mm, pore size of 20 Å, exclusion limit of 10 000 g mol -1 ), at flow rate of 1 mL min -1 . The injected volume was 20 µL. Columns' temperature was held at 40 °C. Data were recorded and processed by Astra software from Wyatt. SEC was calibrated using polystyrene standards.
The rheology measurements were performed on an Anton Paar MCR302 controlledstress rheometer and frequency was modulated at 6.28 rad.s -1 with a shear strain () variation of 1%. The measurements of G' and G'' were performed as a function of temperature from 30°C to 160 °C at a rate of 10°C.min -1 . The polycaryophyllene and cross-linker were previously homogenized and then placed between two plates of 8 mm diameter. Finally, the temperature was maintained at 160 °C for 30 minutes.
Thermogravimetric analysis (TGA) thermograms were obtained using a TGA Q500 apparatus from TA instruments. Samples (10 mg) were heated from room temperature to 700 °C at a rate of 10 °C min -1 under nitrogen atmosphere.
Differential scanning calorimetry (DSC) measurements of samples (5 mg) were performed using a DSC Q 100 apparatus from TA Instruments over temperature range from -120 °C to 240 °C, in a heating-cooling mode of 10 °C min -1 . The analyses were carried out in a nitrogen atmosphere using aluminum pans. Glass transition temperatures and melting points were obtained from the second heating runs. 4 Dynamic mechanical analysis (DMA) measurements were carried out using a RSA3 apparatus from TA Instruments equipped with a liquid nitrogen cooling system. The thermomechanical properties of the samples (width  5 mm; thickness  2 mm and length of the fixed section  10 mm) were studied from around -50 °C to 60°C at a heating rate of 3 °C min −1 . The measurements were performed in tensile test at a frequency of 1 Hz, a strain sweep of 0.03% and an initial static force of 0.1 N.