A Simple and Efficient Approach toward Photosensitive Biobased Aliphatic Polycarbonate Materials

Fatty acids were used as precursors for the synthesis of photo-sensitive polycarbonate materials. In order to avoid multi-step reactions, a simple and straightforward methodology toward the synthesis of photo-sensitive monomers has been developed. Hence, a fatty acid-based cyclic carbonate bearing an unsaturation was synthesized and subsequently polymerized in a controlled manner (Ɖ=1,07) by organo-catalyzed ring-opening polymerization (ROP). A thio-cinnamate derivative was then readily synthesized via a one-pot reaction and grafted onto the polycarbonate backbone by thiol-ene reaction. The content of photo-responsive cinnamoyl moiety grafted on the polycarbonate was tunable with the reaction time. Such functionalized polycarbonates could be cross-linked (by UV irradiation at 365 nm) and de-cross-linked (irradiated at 254nm) and exhibit versatile properties ranging from rather tough materials to elastomeric networks with respect to the content of the photo-sensitive cinnamoyl moiety grafted on the polymer. Aliphatic polycarbonates are well known for their specific features such as low Tg, resistance towards hydrolysis, biocompatibility and biodegradability. Their synthesis can be achieved via different routes but such polymers are mainly synthesized through ring-opening polymerization (ROP) of cyclic carbonate monomers. Under suitable conditions, the polymer chain length, dispersity and microstructure as well as the nature of end-groups can be controlled. The advantage of designing functional polycarbonates over the traditional PTMC stems from the modulation of their physico-chemical properties for specific needs, thereby broadening and improving their performance characteristics. Functional polycarbonates can be synthesized either upon direct polymerization of functional monomer or upon chemical modification postpolymerization. Moreover, because of environmental concerns and also in view of searching novel functionalities, the use of building blocks from renewable resources such as vegetable oils is of great interest. In this purpose and adapted from the work of Venkataraman and coll., fatty acid-based cyclic carbonates were synthesized as precursors of original aliphatic bio-based polycarbonates. In addition, polycarbonate networks exhibiting elastomeric properties are desirable thanks to a large number of applications in the biomedical area, especially in the emerging field of soft-tissue engineering or drug delivery. Thus, several research groups have investigated various cross-linking methods affording polycarbonate materials. However, the main drawback of such crosslinked materials is their inability to be reshaped or recycled. As a consequence, new self-healing materials involving reversible cross-linking reactions have been developed. Such materials could be of interest in many fields such as protective coatings, biomedical applications, piping and electronics. Among all stimuli that can be employed to activate a reversible transformation, light is one of the most used. In the midst of photosensitive moieties, the photo-reactive cinnamate group dimerization, occurring through the [2+2] photochemical cyclo-addition reaction, was implemented for polymerization reactions and for cross-linking purposes. Using the photo-sensitive cinnamate group, the reversibility of the cross-linking was, for instance, exploited for the synthesis of self-healing polymers and tunable shape-memory materials. However, a challenge in the field of self-healing polymers is their ease of synthesis. The photo-sensitive moieties are mainly brought through copolymerization of the functional monomer, the latter needing several synthetic steps. Herein, a simple and efficient post-functionalization method involving a thiol-ene coupling is developed towards photo-sensitive aliphatic polycarbonate networks. After synthesizing a bio-based aliphatic polycarbonate bearing pendant unsaturations, several cinnamatecontaining polycarbonates were easily prepared and reversibly cross-linked through UV irradiation. The influence of the cinnamate content on the network properties was also studied.

Aliphatic polycarbonates are well known for their specific features such as low Tg, resistance towards hydrolysis, biocompatibility and biodegradability. 1 Their synthesis can be achieved via different routes but such polymers are mainly synthesized through ring-opening polymerization (ROP) of cyclic carbonate monomers. 2 Under suitable conditions, the polymer chain length, dispersity and microstructure as well as the nature of end-groups can be controlled. The advantage of designing functional polycarbonates over the traditional PTMC stems from the modulation of their physico-chemical properties for specific needs, thereby broadening and improving their performance characteristics. Functional polycarbonates can be synthesized either upon direct polymerization of functional monomer or upon chemical modification postpolymerization. 3,4 Moreover, because of environmental concerns and also in view of searching novel functionalities, the use of building blocks from renewable resources such as vegetable oils is of great interest. 5 In this purpose and adapted from the work of Venkataraman and coll., 6 fatty acid-based cyclic carbonates were synthesized as precursors of original aliphatic bio-based polycarbonates.
In addition, polycarbonate networks exhibiting elastomeric properties are desirable thanks to a large number of applications in the biomedical area, especially in the emerging field of soft-tissue engineering or drug delivery. 7-12 Thus, several research groups have investigated various cross-linking methods affording polycarbonate materials. [13][14][15][16][17] However, the main drawback of such crosslinked materials is their inability to be reshaped or recycled. As a consequence, new self-healing materials involv-ing reversible cross-linking reactions have been developed. [18][19][20][21] Such materials could be of interest in many fields such as protective coatings, biomedical applications, piping and electronics. 19 Among all stimuli that can be employed to activate a reversible transformation, light is one of the most used. 22,23 In the midst of photosensitive moieties, the photo-reactive cinnamate group dimerization, occurring through the [2+2] photochemical cyclo-addition reaction, was implemented for polymerization reactions and for cross-linking purposes. 24 Using the photo-sensitive cinnamate group, the reversibility of the cross-linking was, for instance, exploited for the synthesis of self-healing polymers 25-27 and tunable shape-memory materials. 28 However, a challenge in the field of self-healing polymers is their ease of synthesis. The photo-sensitive moieties are mainly brought through copolymerization of the functional monomer, the latter needing several synthetic steps. [24][25][26] Herein, a simple and efficient post-functionalization method involving a thiol-ene coupling is developed towards photo-sensitive aliphatic polycarbonate networks. After synthesizing a bio-based aliphatic polycarbonate bearing pendant unsaturations, several cinnamatecontaining polycarbonates were easily prepared and reversibly cross-linked through UV irradiation. The influence of the cinnamate content on the network properties was also studied. Scheme 1. Global strategy to access cinnamate-containing aliphatic polycarbonates from fatty acid derivative i: TBD, 80°C, 7h; ii: ethyl chloroformate, triethylamine, THF, RT, overnight; iii: DBU/TUS, BnOH, DCM, RT, 4h; iv: toluene, reflux, 2h; v: v70, DCM, 40°C.
The fatty-acid based cyclic carbonate was obtained in two steps. First is the formation of the amide functionalized NH-Und-1,3-diol by coupling the methyl undecenoate and 2-amino-1,3-propanediol. This amidation is performed in bulk at 80°C, under nitrogen to remove the methanol by-product, in the presence of 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD) as an organo-catalyst (Yield: 70%., see Figs. S1-S4). The second step is the intramolecular cyclization giving the corresponding 6membered ring cyclic carbonate NH-Und-6CC by using ethylchloroformate in THF in the presence of triethylamine as a base (Yield: 55%., see Figs. S5-S8).  Fig S9). The cinnamate-SH moiety was first synthesized through the direct and simple reaction of cinnamic acid chloride with the 2-mercaptoethanol as illustrated in Erreur ! Source du renvoi introuvable.. This reaction was performed in stoichiometric conditions with complete conversion and very high (yield: 95%, see Figs. S10-S13). The next step towards the design of photo-responsive polycarbonate material is the grafting of Cinnamoyl-SH on P(NH-Und-6CC). As above-mentioned, the thiol-ene coupling reaction was used to link these two entities. Nevertheless, since Cinnamoyl-SH is sensitive to UV irradiation, a radical initiator which decomposes thermally (V70) was used to initiate the thiol-ene reaction as depicted in Erreur ! Source du renvoi introuvable..
Syntheses of polycarbonates bearing different amount of pendant cinnamate moieties were carried out ( Table 1). The cinnamate content was controlled by the reaction time, see Fig. S15. As expected, an increase of the polymer molar mass values associated to a decrease of the Tg (23°C to -20°C) was observed with the cinnamoyl content grafted on the polymer, see Fig. S16. Such a decrease is ex-plained by the flexibility introduced by the sulphur atom to the polymer chains.
Polycarbonate networks were obtained by UV crosslinking using the reactivity of the cinnamate moieties.
With UV light at λ = 365 nm, the cinnamate group switches from the trans to the cis conformation. When two cis cinnamate groups react together, they undergo a [2+2] cyclo-addition reaction leading to the formation of a cyclobutane ring (Erreur ! Source du renvoi introuvable.). This reaction was used to cross-link all cinnamate-functionalized cx-P(NH-Und-6CC).
The curing kinetics was studied using UV−vis spectrophotometry by monitoring the absorption maximum at 280 nm, which relates to the double bonds adjacent to carbonyl group of the cinnamate functionality ( Figure 1-(a)). As seen in the UV−vis spectrum, a decrease in intensity of the peaks at 280 nm was observed within 6 h, meaning that almost 100% of the cinnamate groups have undergone the [2+2] cyclo-addition reaction. The fast initial absorbance decrease is characteristic of the polymer cross-linking through irradiation because the concentration of cinnamate groups is higher in the initial stages. As the cross-linking increases, concentration and mobility of the chains decreases leading to a plateau.

Scheme 2. Photo-reversible cross-linking of a cinnamate containing bio-based polycarbonate
Subsequently, upon exposing the cross-linked polycarbonate to 254 nm light irradiation, the de-cross-linking of the polycarbonate material occurs. The de-cross-linking reaction is relatively fast but the absorbance doesn't reach its former value before crosslinking indicating that the de-cross-linking is not complete probably due to the poor power of the UV lamp at this wavelength. In order to have a total decross-linking, an appropriate UV lamp must be used, allowing a proper irradiation below 300 nm. Nevertheless, Figure 1 demonstrates the reversible feature of the photo-mediated crosslinked polycarbonate.

Figure 1. a) Cross-linking kinetic under 365 nm and b) decross-linking kinetic under 254 nm in solution (DCM)
All functional polycarbonates were cross-linked by the [2+2] photochemical cyclo-addition of cinnamoyl units to form cross-linked polycarbonates (CL-cx-P(NH-Und-6CC)). First, to evaluate the network cross-linking density, swelling tests have been performed. It can be noted from Table 2 that very low gel content (7%) was observed when only 10 mol.% of cinnamoyl units were grafted to P(NH-Und-6CC). The poor photoreactivity in a solidstate coated film explains this low gel content. However, the gel content increases significantly when the pendent cinammoyl content increases in the polymer to almost reach 100%. At the same time, the swelling ratio decreases when the cross-linking increases. This feature shows that the cross-linking density can be controlled by adjusting the polycarbonate cinnamoyl content. After crosslinking, thermal and mechanical chacterizations of these networks were investigated by differential scanning calorimetry (DSC, Figs. S16-17), thermogravimetry (TGA, Fig.  S18), tensile tests experiments (Fig. S19) and dynamic mechanical analysis ( Fig. S20-21). All the results are summarized in Table 2. Such cross-linked materials demonstrated slight increase in Tg with an increase of crosslinking density. More interestingly, mechanical properties of the networks strongly depend on the cross-linking density. Indeed, CL-c10-P(NH-Und-6CC) and CL-c30-P(NH-Und-6CC) are transparent and flexible materials at room temperature as shown in Figure 2. However, when high cinnamoyl content was used, the materials became more rigid. Consequently, the fully functionalized crosslinked polymer was very brittle. In addition, the influence of the cross-linking density on the polycarbonate mechanical properties can be clearly seen. CL-c10-P(NH-Und-6CC) displays very low Young modulus calculated from the initial slope of the stress−strain curve (1.3 MPa) while CL-c100-P(NH-Und-6CC) exhibits a Young modulus 1 000 times higher. In between these values, the higher the cross-linking density, the higher the Young modulus. Max stress at break values follows the same trend. However, the elongation at break follows opposite tendency. Therefore, the mechanical properties of the polycarbonate networks are significantly affected by the cinnamoyl content grafted on P(NH-Und-6CC).
The elastic modulus measured above Tg by DMA exhibited an increase up to 9.5 MPa for the most highly crosslinked polycarbonate. Moreover, in agreement with the results obtained by DSC, we observe that the Talpha increases with the increase of the cross-linking density. In conclusion, several cinnamate-containing polycarbonates were synthesized via the simple and controllable thiol-ene reaction between P(NH-Und-6CC) and thiofunctionalized cinnamate moiety. Photo-reversible polycarbonate networks were prepared thanks to the photoinduced [2+2] cyclo-addition reaction between two cinnamoyl groups.

Experimental Section
Materials.
All products and solvents (reagent grade) were used as received except otherwise mentioned. The solvents were of reagent grade quality and were purified wherever necessary according to the methods reported in the literature. Flash chromatography was performed on a Grace Reveleris apparatus, employing silica cartridges from Grace. Cyclohexane: ethyl acetate and dichloromethane: methanol gradients were used as eluents depending on the products. The detection was performed through ELSD and UV detectors at 254 nm and 280 nm.

Characterization.
1 H and 13 C-NMR spectra were recorded on Bruker Avance 400 spectrometer (400.20 MHz or 400.33 MHz and 100.63 MHz for 1 H and 13 C, respectively) by using CDCl3 as a solvent at room temperature, except otherwise mentioned.
Two-dimensional analyses such as 1 H-1 H COSY (COrrelation SpectroscopY), 1 H-13 C HSQC (Heteronuclear Single Quantum Spectroscopy) and were also performed. Size Exclusion Chromatography (SEC) analyses were performed in THF (25°C) on a PL GPC50 and with four TSK columns: HXL-L (guard column), G4000HXL (particles of 5 mm, pore size of 200A, and exclusion limit of 400000 g/mol), G3000HXL (particles of 5 mm, pore size of 75A, and exclusion limit of 60000 g/mol), G2000HXL (particles of 5 mm, pore size of 20 A, and exclusion limit of 10000 g/mol) at an elution rate of 1 mL/min. The elution times of the filtered samples were monitored using UV and RI detectors and SEC were calibrated using polystyrene standards. Differential scanning calorimetry (DSC) thermograms were measured using a DSC Q100 apparatus from TA instruments. For each sample, two cycles from -80 to 100°C at 10°C.min -1 were performed and then the glass transition and melting temperatures were calculated from the second heating run. Thermogravimetric (TGA) analyses were performed on TGA-Q50 system from TA instruments at a heating rate of 10 °C.min -1 under nitrogen atmosphere from room temperature to 600°C. Dynamic mechanical analyses (DMA) were performed on RSA 3 (TA instrument). The sample temperature was modulated from -50 °C to 150 °C, depending on the sample at a heating rate of 4°C.min -1 . The measurements were performed in a tension mode at a frequency of 1 Hz, an initial static force of 0.1 N and a strain sweep of 0.04%. Photo-crosslinking were performed using a UV lamp