Synthesis and characterization of photo-crosslinkable linear segmented polyurethanes based on coumarin

1 Synthesis and characterization of photo-crosslinkable linear segmented polyurethanes based on coumarin Rubén Seoane Rivero1, Rodrigo Navarro2, Pilar Bilbao Solaguren1, Koldo Gondra Zubieta1, José María Cuevas1, Angel. Marcos-Fernández2* 1 Gaiker Centro Tecnológico, Parque Tecnológico Edificio 202, 48170 Zamudio, Spain, Tel.: +34 94 6002323, Fax: +34 94 6002324 2 Instituto de Ciencia y Tecnología de Polímeros (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain, Tel.: +34 915622900, Fax: +34 915644853 *Corresponding author: [email protected] Abstract Linear segmented polyurethanes with 10% weight of coumarin groups within the soft segment, the hard segment or equally distributed between both segments were successfully prepared. These polyurethanes, based on polycaprolactones, isophorone diisocyanate, butanediol and a dihydroxylated coumarin monomer, were completely amorphous and homogeneous with a single phase morphology. In non-irradiated polyurethanes Tg value varied linearly with hard segment content that in turn was the main factor influencing the mechanical properties. Photo-dimerization at 354 nm produced crosslinked materials with little change on the Tg and outstanding mechanical properties, as high as 54 MPa, better than that of any other coumarin-containing polymer described. Keywords Coumarin; ring-opening polymerization; polycaprolactone; segmented polyurethanes; photodimerization This is the accepted manuscript of the article that appeared in final form in European Polymer Journal 92 : 263-274 (2017), which has been published in final form at https://doi.org/10.1016/j.eurpolymj.2017.05.016. © 2017 Elsevier under CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) 2 INTRODUCTION There are many studies in which light has been utilized as a trigger for the preparation of smart materials.[1-4] Light has the advantage that light-triggered processes can be halted and resumed “on demand” by turning off and on the excitation light.[5] Several light-sensitive groups or chromophores that can undergo a reversible photo-induced reaction can be used in these materials such as antracenes,[6] coumarins,[7] or cinnamic acid derivatives.[8] These light-sensitive materials have been proposed for many applications among them shape memory polymers,[8] hydrogels,[9] surface patterning,[10] drug release,[11,12] light stabilization and surface sizing of paper,[13] cellular imaging,[14] photovoltaics,[15] oil recovery,[16] micro-phase stabilization of block copolymers[17] and intraocular lenses.[18]. These photo-reactive groups should be part of the polymer structure without compromising the desired properties, which often means a limited content. [19,20] Coumarin molecules can undergo reversible photo-induced reactions; thus, when irradiated at >300 nm a [2+2] cycloaddition reaction to give rise to a cyclobutane ring takes place, and when irradiated at 254 nm a photo-scission reaction renders the original coumarin structures as shown in Figure 1.[7,21-23] Figure 1. Photo-dimerization/photo-cleavage reactions of coumarin molecules 3 Coumarin monomers have been included in the backbone of different types of polymers, including polyethers,[21] poly(meth)acrylates,[23,24] polyesters,[25] silicones[26,27] and polyurethanes.[19] Polyurethanes are considered excellent materials because of their good physical properties. This is the reason why this material is widely used in different and numerous applications. When linear polyurethanes are prepared, usually they are composed of a difunctional flexible macroglycol of relatively long chain, a diisocyanate and a short chain diol referred as chain extender. The final linear polyurethane has a segmented structure where the flexible chains of the macroglycol constitute the so-called soft segments and the relatively rigid and polar chains produced by the reaction of the diisocyanate and the chain extender constitute the so-called hard segments.[28,29] When incorporated in a polyurethane chain, the coumarin monomer has been introduced as a chain end [20,30,31] or as a chain extender.[19], in both cases with the coumarin moiety linked to the isocyanate monomer by a polar urethane group, and therefore with the coumarin units were within the hard segments, and only recently we described for the first time a polyurethane with the coumarin within the soft segment.[32] This linear polyurethane, formulated without chain extender, produced after coumarin dimerization a tough elastomeric material with the best mechanical properties of all the coumarin-containing polymers described in literature to that date. In this work we report the preparation of linear segmented polyurethanes with coumarin molecules within the soft segment, within the hard segment or equally distributed in between both segments. These polyurethanes were formulated with chain extenders and therefore, different from our previous work [32], the resulting polyurethanes have a (AB)n segmented structure with alternating soft segments (A: flexible chains of the macroglycol) and hard 4 segments (B: polar chains produced by the reaction of diisocyanate+chain extender). In this way, the effect of having the coumarin molecules within the soft segment, within the hard segment or distributed in both segments, on the reactivity of the coumarin molecules when irradiated and the difference in the resulting physical properties after irradiation, can be studied. It is expected that depending on the location of coumarin molecules the effect will be very different. A coumarin diol monomer was used as initiator of the ring opening polymerization of ε-caprolactone to obtain a coumarin-containing polycaprolactone (PCL) diol of approximately 1000 g·mol-1 molecular weight to avoid PCL crystallization. This PCL diol was combined with another short PCL diol, isophorone diisocyanate, butanediol and the coumarin diol monomer in the appropriate ratios to produce the segmented polyurethanes with a 10% weight of coumarin in their structure. The photochemical, thermal and mechanical properties of these polyurethanes were evaluated. EXPERIMENTAL SECTION Materials Polycaprolactone diol of nominal molecular weight 530 g·mol-1 (PCL527, molecular weight 527 g·mol-1 as calculated by 1H-NMR[33]) was supplied by Sigma Aldrich Química S.L (Madrid, Spain). PCL527 was vacuum dried at 90 °C for 3 h and stored in a desiccator until used. 2,2-Bis(hydroxymethyl)propionic acid, 2,2-dimethoxypropane, p-toluenesulfonic acid monohydrate, 1,3-dicyclohexylcarbodiimide, resorcinol, ethyl acetoacetate, 2-bromoethanol, anhydrous pyridine, 1,4-dioxane, ethyl acetate, sodium bisulfite, Dowex H+ resin, - caprolactone, and stannous octoate (SnOct2) were supplied by Sigma Aldrich Química S.L (Madrid, Spain) and used as received. Acetone, dichloromethane, concentrated sulphuric acid, ethanol, N,N-dimethylformamide, potassium carbonate, magnesium sulfate and 1,2-dichloroethane were supplied by Scharlau (Barcelona, Spain) and used as received. 5 1,4-butanediol (BD) was purchased from Sigma Aldrich Química S.L (Madrid, Spain). BD was dried over magnesium sulfate, distilled and stored blanketed with nitrogen until use. Isophorone diisocyanate (IPDI) was a gift by Evonik Industries. Experimental techniques Solution NMR spectra were recorded at room temperature in a Varian Unity Plus 400 instrument (Palo Alto, CA, USA) using deuterated chloroform (CDCl3) or deuterated dimethylsulfoxide (DMSO-d6) as solvent. Spectra were referenced to the residual solvent signals at 7.26 ppm and 2.50 ppm for proton spectra and 77.0 ppm and 39.5 ppm for carbon spectra, respectively. Irradiations were carried out in a crosslinker supplied by Ultra-Violet Products (Upland, CA, USA) equipped with two sets of 5 x 8 watts lamps with emission maxima at 354 nm and 254 nm. UV experiments were performed in a Perkin Elmer Lambda 35 UV/Vis spectrometer (Waltham, MA, USA). Absorbance of the thin films was measured from 450 to 210 nm. Raman spectroscopy measurements were carried out by a Renishaw inVia Laser microRaman Spectrometer (Wotton-under-Edge, UK). A laser beam with wavelength of 785 nm served as the excitation light. The testing area on the film was about 1 µm2. The thermal transitions of the samples were analyzed by DSC on a Mettler Toledo DSC 822e calorimeter (Schwerzenbach, Switzerland) equipped with a liquid nitrogen accessory. Disc samples cut from films weighing approximately 6 mg were sealed in aluminium pans. Samples were heated, from 25 °C to 120 °C at a rate of 10 °C min−1, cooled to -90 °C at the maximum cooling rate of the instrument, maintained for 10 minutes at this temperature and re-heated from −90 °C to 120 °C at a rate of 10 °C min−1. Glass transition temperatures (Tg) were taken as the midpoint of the transition. Tensile properties were measured in a MTS Synergie 200 testing machine (Eden Prairie, MN, USA) equipped with a 100 N load cell. Type 4 dumbbell test pieces (according to ISO 37) were cut from the samples. A cross-head speed of 5 mm min-1 was used. Strain was measured from cross-head separation and referred to 10 mm initial length. A minimum of 5 samples were tested for each material. Synthesis of 2,2-bis(hydroxymethyl)propionate of 7-hydroxyethoxy-4-methyl-coumarin (DHEOMC diol) 6 The synthesis of this coumarin diol has already been reported by our group.[32] Its structure can be seen in Figure 2. Figure 2. Chemical structure of the monomer DHEOMC diol. Synthesis of 2,2-bis(hydroxymethyl)propionate of 7-hydroxy-4-methyl-coumarin (HMC diol) The synthesis of the HMC diol followed the same procedure than the synthesis of the DHEOMC diol (Supplementary information in reference 32) and it is shown in Figure 3. Details on the synthesis and characterization can be found in the Supplementary Data file. Figure 3. Synthesis procedure for HMC diol 7 The chemical structure of the HMC diol was confirmed by nuclear magnetic resonance (NMR) analysis. Synthesis of coumarin-polycaprolactone diols Polymerization was carried out following a general procedure for the ring opening polymerization of -caprolactone.[34] As an example, the synthesis of the coumarincontaining polycaprolactone diol of approximately 1000 g·mol-1 molecular weight based on DHEOMC diol is described. In a 25 mL round-bottomed flask previously dried, -caprolactone (CL, 78.9 mmol, 7.9 g), DHEOMC diol (9.57 mmol, 3.22 g) and SnOct2 (0.1% weight respect to the monomer weight) were charged and heated with magnetic stirring at 120 °C for 24 h. When hot, a vacuum pump was connected (final pressure approximately 100 mbar) to remove unreacted -caprolactone for 2 hours. Pump was disconnected and the product let to cool at room temperature. A solid was obtained with almost quantitative yield. The molecular weight of this coumarincontaining polycaprolactone diol (PCLcoum) was calculated by 1H-NMR following the methodology already described [32] and a value of 1231 g·mol-1 was obtained. With this calculated molecular weight, the amount of coumarin (DHEOMC) in the coumarin-containing polycaprolactone diol (PCLcoum) is 27.3% weight DHEOMC/weight PCLcoum. Synthesis of coumarin-polycaprolactone polyurethanes The synthesis of all polyurethanes was carried out using 1,2-dichloroethane as solvent, with approximately a 5% molar excess of diisocyanate respect to the total molar content of diols in the reaction. As it will be discussed later, it was not possible to prepare a polycaprolactone diol from HMC diol, and only polyurethanes with HMC diol within the hard segment were prepared. Six polyurethanes derived from HMC diol with 5, 10 and 15% weight of HMC diol in the final polyurethane and 1:3 or 1.1 molar ratio of BD:PCL527 were synthesized by reacting PCL527, IPDI, BD and HMC diol. In Table 1, the ratio of the reactants used in the synthesis is listed. In the case of DHEOMC diol, three polyurethanes were prepared with a 10% weight of DHEOMC diol in the final polyurethane and a 1:1 molar ratio of BD:PCL (PCL being PCL527, PCLcoum or PCL527+PCLcoum) by reacting PCL527 and/or PCLcoum, IPDI, BD and DHEOMC diol. In these polyurethanes, the coumarin molecules were included in the soft segment (PCLcoum), in the hard segment (DHEOMC diol) or equally distributed in both 8 segments (PCLcoum+DHEOMC diol). In Figure 4, the three DHEOMC containing polyurethanes with the location of the coumarin molecules within the structure are schematically represented. Figure 4. Synthesis procedure for polyurethanes with HEOMC within the soft segment (PU DHEOMC 10% SS), within the hard segment (PU DHEOMC 10% HS) and equally distributed in both segments (PU DHEOMC 5+5% SS+HS) In Table 2, the ratio of the reactants used in the synthesis is listed. As an example, the synthesis of the polyurethane with the coumarin within the soft segment is described. In a 25 mL round-bottomed flask, 1.140 g (2.16 mmol) of PCL527, 1.83 g (1.49 mmol) of PCLcoum (containing 0.50 g of DHEOMC), 0.332 g (3.68 mmol) of BD and 1.70 g (7.65 mmol) of IPDI were dissolved in 20 mL of 1,2-dichloroethane. Two drops of SnOct2 catalyst were added and the stirred solution heated at 80 °C for 3 h, followed by 24 h stirring at ambient temperature. The resulting polymeric solution was casted into a leveled glass and the solvent evaporated at room temperature for 48 h. Polymeric films of different thickness were prepared from this material. 9 Table 1. Amounts of reactants (g) in the synthesis of HMC diol containing polyurethanes (calculated for 100 g of final polymer) POLYURETHANE PCL527 diol IPDI BD HMC diol BD(1:3)HMC 5% 55.2 36.6 3.20 5.00 BD(1:3)HMC 10% 50.0 37.2 2.80 10.0 BD(1:3)HMC 15% 44.25 38.2 2.55 15.0 BD(1:1)HMC 5% 44.8 42.5 7.70 5.00 BD(1:1)HMC 10% 40.3 42.8 6.90 10.0 BD(1:1)HMC 15% 35.5 43.4 6.10 15.0 Table 2. Amounts of reactants (g) in the synthesis of DHEOMC diol containing polyurethanes (calculated for 100 g of final polymer) POLYURETHANE PCL527 diol PCLcoum diol IPDI BD DHEOMC diol PU DHEOMC 10% SS 22.8 36.6* 33.95 6.65 - PU DHEOMC 5+5% SS+HS 31.6 18.3** 38.3 6.8 5.00 PU DHEOMC 10% HS 40.6 - 42.5 6.9 10.0 * Contains 10.0 g of DHEOMC diol ** Contains 5.00 g of DHEOMC diol Synthesis of model polycaprolactone polyurethanes without coumarin A series of model polyurethanes based on PCL527, IPDI and BD were prepared with different BD:PCL527 ratios following the same procedure used for the coumarin containing polyurethanes. Details on the synthesis can be found in the Supplementary Data file. 16 The results for the measurement of the mechanical properties in tension of the segmented polyurethanes can be found in Table S3 in the Supplementary Data file. Mechanical properties depend mainly on the Tg of the material. When the material has a low Tg, tensile strength is very low and strain is very high, and when Tg is above ambient temperature, the material reaches good tensile strength values and strain is reduced. When the tensile strength and tensile strain for all polymers are plotted vs Tg value, Figures 7 and 8, it is clear that a certain Tg value the change from a very soft material to a tough material takes place. In the figures, the change takes place in between 15 and 20 °C (the discontinuous vertical line marks the value of 18 °C). 0 10 20 30 40 50 020 40 60 80 100 Stress / MPa Tg / ºC Figure 7. Tg vs tensile stress for the segmented polyurethanes: non-coumarin polyurethanes (empty circles), HMC diol derived polyurethanes (filled squares) and DHEOMC diol derived polyurethanes (crossed squares) 17 0 500 1000 1500 2000 2500 3000 020 40 60 80 100 Strain / % Tg / ºC Figure 8. Tg vs tensile strain for the segmented polyurethanes: non-coumarin polyurethanes (empty circles), HMC diol derived polyurethanes (filled squares) and DHEOMC diol derived polyurethanes (crossed squares) This trend is consistent with the expected behavior for homogeneous single-phase linear materials for which the mechanical properties are dictated by the rigidity of the chain that is determined by the Tg. Effect of photo-dimerization/photo-cleavage of coumarin units in the physical properties Photo-dimerization/photo-cleavage of coumarin units Photo-dimerization/photo-cleavage of the coumarin molecules were measured in the DHEOMC derived segmented polyurethanes with the coumarin within the soft segment (PU DHEOMC 10% SS), within the hard segment (PU DHEOMC 10% HS) or equally distributed between both segments (PU DHEOMC 5+5% SS+HS). 18 A thin polyurethane film of less than 2 microns thickness was cast from a chloroform solution onto the external wall of a quartz cell to study the reaction kinetics of the coumarin irradiation, and a thick polyurethane film of approximately 150 microns was cast onto a levelled sililated glass and used for the rest of the studies. Films were exhaustively dried under vacuum for complete solvent removal. The films were irradiated with a set of 354 nm lamps to produce coumarin photo-dimerization, and with a set of 254 nm lamps to produce the photo-cleavage reaction. The photo-reversibility of the synthesized polyurethane was characterized in the thin films by UV. In Figure S11 in the Supplementary Data file, the UV spectra of the photo-dimerization and photo-cleavage reactions can be seen for the polymer PU DHEOMC 5+5% SS+HS. During dimerization, the UV peak with maximum in absorbance at 319 nm in the polyurethane UV spectrum related to the conjugated π-system decreased due to the disappearance of the double bond in the coumarin with formation of a cyclobutane ring by [2+2] cycloaddition (figure in the left side), and during photo-cleavage (figure in the right side), double bond restored and absorption peak with the maximum at 319 nm recovered. No significant differences in photo-dimerization/photocleavage were found in between the coumarin-containing polyurethanes. The kinetics was similar irrespective of the location of the coumarin molecules. When the photo-dimerization/photo-cleavage cycles were repeated at the optimum times obtained (90 minutes for photo-dimerization and 90 seconds for photocleavage), Figure S12 in the Supplementary Data file, a certain hysteresis was found. Photoconversion and photo-cleavage lost efficiency from cycle to cycle, as already found by other authors and was attributed to the equilibrium between the dimer and cleaved moieties.[11,19,21,24,25] and or the formation of irreversible structures.[20] It can be noted that the loss of efficiency was quite significant for photo-cleavage and it was much lower for photo-dimerization, as already observed by other authors.[21,37] We have found the same behavior for other polycaprolactone based polyurethanes containing coumarin units.[30,32] Photo-dimerization/photo-cleavage conversions in the thick films could not be measured by UV due to saturation, and Raman spectroscopy was used.[20] It has already been proved for similar polycaprolactone based polyurethanes containing coumarin units that conversion measured by UV and by Raman is the same.[30] Quantification was done by ratioing the peak related to the double bond at 1617 cm-1 and the peak at 1442 cm-1 related to methylene groups.[32] Photo-dimerization conversion after 240 minutes irradiation at 354 nm was approximately 75% for the three polyurethanes, conversion similar to the conversion obtained for a linear polyurethane without chain extender;[32] after 10 minutes irradiation at 254 nm 19 dimerization conversion lowered to approximately 50% for the three polyurethanes, thus photo-cleavage was not very efficient at the experimental conditions used; and after 240 minutes irradiation at 354 nm, photo-dimerization conversion increased again to 71-74%, almost the value of the first irradiation. As for the thin films, the kinetics of photodimerization/photocleavage is similar for the three polyurethanes in thick films and again no significant differences are found due to the location of the coumarin molecules within one segment or the other. Effect on the thermal properties After irradiation, a single Tg was again obtained. In table 4, the Tg values for the three segmented polyurethanes after consecutive irradiation at 354 nm, 254 nm and 354 nm are shown. Table 4. Tg value for the DHEOMC diol containing polyurethanes after irradiation at 354 nm, 354 nm + 254 nm and 354 nm + 254 nm + 354 nm Tg / °C POLYURETHANE Nonirradiated Irradiated at 354 nm Irradiated at 354 nm + 254 nm Irradiated at 354 nm +254 nm + 354 nm PU DHEOMC 10% SS 1.1 (0) 4.9 (77) 4.1 (51) 5.1 (73) PU DHEOMC 5+5% SS+HS 14.6 (0) 20.7 (73) 15.8 (51) 20.8 (71) PU DHEOMC 10% HS 20.0 (0) 24.2 (75) 23.8 (46) 25.1 (74) In parenthesis, photo-dimerization conversions (in %) measured by Raman Crosslinking produced by the dimerization of the coumarin units increased the Tg value for all the segmented polyurethanes, but the increase was relatively low (6.1 °C at most) taking into account that approximately 75% of the coumarin units have reacted. Photo-cleavage at 254 nm reduced Tg as a consequence of the reduction of crosslinking, and re-irradiation at 354 nm increased again Tg due to the increase in crosslinking, reaching almost the same value obtained after the first photo-dimerization reaction. Although in the second irradiation at 354 nm the conversion reached for dimerization was slightly lower respect to the first irradiation, the Tg value obtained was systematically slightly higher. This could be explained if it is 20 assumed that the secondary reactions leading to the irreversible structures that limit the photocleavage conversion produce some extra crosslinking. From the values in Table 4, it can be deduced that polyurethane with the coumarin units within the soft segment PU DHEOMC 10% SS will remain a soft material because Tg is always well below ambient temperature, polyurethane with the coumarin units within the hard segment PU DHEOMC 10% HS will remain a relatively rigid material because Tg is always above the critical value represented in Figures 6 and 7, and polyurethane with the coumarin units within both segments PU DHEOMC 5+5% SS+HS has a Tg below the limit that goes just above the limit after photodimerization thus will be a soft material that could increase its rigidity significantly. Effect on the mechanical properties The non-irradiated material was soluble in chloroform whereas the irradiated materials were only swelled in chloroform, showing the crosslinking produced by coumarin dimerization. . Table 5. Mechanical properties for the DHEOMC diol containing polyurethanes after irradiation at 354 nm, 354 nm + 254 nm and 354 nm + 254 nm + 354 nm Stress / MPa Strain / % Tensile Toughness / kJ·m-3 POLYURETHANE Nonirradiated Irradiated at 354 nm Irradiated at 354 nm + 254 nm Irradiated at 354 nm +254 nm + 354 nm PU DHEOMC 10% SS 0.64 ± 0.11 >2500 ≈ 10 ± 3 53 ± 11 1190 ± 110 148 ± 18 37 ± 8 1320 ± 80 134 ± 19 51 ± 10 1240 ± 120 160 ± 20 PU DHEOMC 5+5% SS+HS 1.1 ± 0.3 2500 ± 600 19 ± 3 30 ± 3 1040 ± 50 111 ± 13 29 ± 3 1330 ± 20 120 ± 10 29 ± 6 1180 ± 120 119 ± 7 PU DHEOMC 10% HS 17 ± 2 580 ± 80 59 ± 12 54 ± 3 700 ± 50 210 ± 20 33 ± 4 530 ± 140 120 ± 30 43 ± 5 600 ± 200 160 ± 60 As it can be seen on Table 5, non-irradiated PU DHEOMC 10% SS and PU DHEOMC 5+5% SS+HS, with Tg values below 15 °C, were soft and presented very low mechanical properties. Non-irradiated PU DHEOMC 10% HS, with a Tg value of 20.0 °C was more rigid and showed good mechanical properties. Crosslinking due to photo-dimerization produced tough materials (see Figures 9 and 10 and Figure S13 in the Supplementary Data file), elastomeric-like in the 21 case of the softer polyurethanes. This improvement on the mechanical properties cannot be due to the increase in the Tg that is very limited. 0 10 20 30 40 50 60 0500 1000 1500 Non irradiated 354 nm 354+254 nm 354+254+354 nm Stress / MPa Strain / % Figure 9. Stress-strain curves for the segmented polyurethane with the coumarin within the soft segment (PU DHEOMC 10% SS) before irradiation (thin continuous line), after irradiation at 354 nm (thick continuous line), after irradiation at 354+254 nm (dotted line) and after irradiation at 354+254+354 nm (discontinuous line) 22 0 10 20 30 40 50 60 0 100 200 300 400 500 600 700 800 Non irradiated 354 nm 354+254 nm 354+254+354 nm Stress / MPa Strain / % Figure 10. Stress-strain curves for the segmented polyurethane with the coumarin within the hard segment (PU DHEOMC 10% HS) before irradiation (thin continuous line), after irradiation at 354 nm (thick continuous line), after irradiation at 354+254 nm (dotted line) and after irradiation at 354+254+354 nm (discontinuous line) The increase in mechanical properties due to coumarin dimerization (see Table 5) was outstanding, especially for the polyurethane with the coumarin within the soft segment, PU DHEOMC 10% SS, that increased its tensile strength by a factor of 82, from 0.64 to 53 MPa, and its tensile toughness by a factor of approximately 15. For the polymer with the coumarin within the hard segment, PU DHEOMC 10% HS, tensile strength and tensile tougness increased approximately three-fold, and for the polymer with the coumarin within both 23 segments, PU DHEOMC 5+5% SS+HS, tensile strength increased by a factor of 27 and tensile toughness by a factor of approximately 6. And in all polyurethanes, tensile strain increase was achieved maintaining a high tensile strain, 700 to 1190 %. These excellent mechanical properties for the dimerized materials are much better than the properties described in literature for any coumarin-based polymer. Properties for coumarin containing polymers based on PEG [19,20] or silicone [26] with a coumarin content higher than 10% weight and conversion on the dimerization reaction higher than 77% reached a maximum value for tensile stress of 3.6 MPa and 3.8 MPa respectively. Polyurethane dispersions based on PPG diol, IPDI, BD and a coumarin diol, with 10% weight of coumarin, reached a maximum of 2 MPa tensile strength.[37] Coumarin-containing polyurethanes based on polycaprolactone previously described by our group,[30,32] with a 10% weight of coumarin content in the polymers, reached tensile stress values of 5.4 MPa for the branched materials and 8 MPa for the linear material, and recently described polyurethanes based on PCL530 or PCL2000, HDI and DHEOMC diol as chain extender, gave a maximum tensile strength of approximately 43 MPa for PCL530 with a 15% weight content of coumarin,[38] still below the values achieved by two of the new segmented polyurethanes presented here. The soft elastomeric polyurethane with the coumarin within the soft segment, PU DHEOMC 10% SS, has even better mechanical properties than unfilled crosslinked natural rubber, that presents tensile stress values up to approximately 30 MPa with tensile strain values up to approximately 900%.[39,40] From these results it is difficult to state if the location of the coumarin within one segment or the other is advantageous. Due to the difference in hard segment content of the polyurethanes the mechanical properties of the non irradiated materials are very different. It would be necessary to prepare a polyurethane with the same hard segment content than PU DHEOMC 10% HS but with the coumarin within the soft segment or a polyurethane with the same hard segment content than PU DHEOMC 10% SS but with the coumarin within the hard segment to directly compare the effect of the location of the coumarin. What is true is that despite the location of the coumarin molecules, the irradiated materials have an outstanding improvement in the mechanical properties produced by the chemical crosslinking. For all the irradiated branched materials, when irradiated at 254 nm mechanical properties decreased as a consequence of photo-cleavage of coumarin units, which decreased the crosslink density of the material. However the dimerization efficiency at the experimental conditions used was limited as shown by Raman measurements (dimerization conversion decreased from approximately 75% to 50%) thus the mechanical properties after photo- 24 cleavage were still good and far from the initial properties of the non-irradiated materials. Reirradiation at 354 nm increased again the crosslink density by photo-dimerization of coumarin units and mechanical properties recovered. CONCLUSIONS Two coumarin diols were used as initiators in the ring opening polymerization of εcaprolactone to obtain short chain coumarin-containing polycaprolactone diols. The coumarin diol with aliphatic-aromatic ester group, HMC diol, was not stable in the polymerization conditions and produced branched and mono-functional species whereas the coumarin diol with aliphatic ester group, DHEOMC diol, produced polycaprolactone diols with the expected structure. Three linear segmented polyurethanes with 10% weight coumarin molecules within the soft segment, within the hard segment and equally distributed between both segments, were successfully prepared with high molecular weight. These polyurethanes, based on short chain PCL, short chain coumarin-containing PCL, isophorone diisocyanate, butanediol and DHEOMC diol, presented an amorphous homogeneous single-phase morphology with a single T g . Mechanical properties of the non-irradiated polyurethanes depended mainly on the T g value thus the two polyurethanes with a T g below 20 °C were soft and weak and the polyurethane with a T g of 20 °C was more rigid with good tensile stress and high tensile strain. Photo-dimerization of the coumarin molecules by irradiation at 354 nm produced crosslinked materials and photo-cleavage at 254 nm decreased crosslinking. Efficiency was lost from photo-dimerization/photo-cleavage cycle to cycle, more significantly in the photo-cleavage than in the photo-dimerization. No significant difference in the kinetics of photodimerization/photo-cleavage due to coumarin molecules location was found Photo-dimerization transformed the weak and soft linear polyurethanes with the coumarin within the soft segment or equally distributed between both segments in crosslinked tough elastomeric materials, and increased almost three times the tensile strength of the polyurethane with the coumarin within the hard segment. The mechanical properties improvement was due to the chemical crosslinking produced by photo-dimerization and not to the increase in Tg that was small. Photo-cleavage decreased the mechanical properties as a consequence of the decrease in crosslinks and re-photo-dimerization increased again the 25 properties by restoration of the crosslinks. Mechanical properties of the irradiated materials were superior to coumarin containing materials described in literature. Acknowledgements The authors would like to thank the Ministry of Economy and Competitiveness (MINECO) for the financial support of this work within the framework of the Plan Nacional de I+D+I through the research projects MAT2013-48059-C2 and MAT2014-52644-R and the INNPACTO project IPT-2012-0324-420000. Also, this work has been supported by programme Technology Centres Foundation Iñaki Goenaga and the Regional Government of Madrid, Spain (S2013/MIT-2862). REFERENCES [1] V. Amendola, M. Meneghetti, J. Mater. Chem. 22 (2012) 24501-24508. [2] M. Behl, A. Lendlein, Soft Matter 3 (2007) 58-67. [3] T. Ikeda, J. Mamiya, Y. Yu, Angew. Chem., Int. Ed. 46 (2007) 506-528. [4] H.Y. Jiang, S. Kelch, A. Lendlein, Adv. Matter. 18 (2006) 1471-1475. [5] D. Habault, H. Zhang, Y. Zhao, Chem. Soc. Rev. 42, (2013) 7244-7256. [6] S.D. Bergman, F. Wudl, J. Mater. Chem. 18, (2008) .41–62 [7] S. R. Trenor, A. R. Shultz, B. J. Love, T. E. Long, Chem. Rev. 104 (2004) 3059-3077 [8] L. Wu, Ch. Jin, X. Sun, Biomacromolecules 12 (2011) 235–241. [9] Y. Zheng, M. Micic, S.V. Mello, M. Mabrouki, F.M. Andreopoulos, V. Konka, S.M. Pham, R.M. Leblanc, Macromolecules 35 (2002) 5228-5234. [10] E. A. Chamsaz, S. Sun, M. V. S. N. Maddipatla, A. Joy, Photochem. Photobiol. Sci., 13 (2014) 412-421. [11] D. Kehrloesser, R.-P. Baumann, H. C. Kim, N. Hampp, Langmuir 27 (2011) 4149–4155. [12] J. Qiao, F. Mitschang, S. Agarwal, Biomacromolecules 12 (2011) 3684−3691. [13] G. Zhang, H. Zheng, G. Liu, P. Wang, R. Xiang, BioResources 11 (2016) 373-384. [14] A. Yamada, Y. Hiruta, J. Wang, E. Ayano, H. Kanazawa, Biomacromolecules 16 (2015) 2356-2362. [15] S. Banerjee, R. Tripathy, D. Cozzens, T. Nagy, S. Keki, M. Zsuga, R. Faust, ACS Appl. Mater. Interfaces 7 (2015) 2064-2072.