Redox and thiol–ene cross-linking of mercapto poly(3-caprolactone) for the
preparation of reversible degradable elastomeri...
spectroscopy. Finally, as our main objective is to prepare elas-
tomeric materials, mechanical properties of the cross-lin...
2.7 Synthesis of poly(a-sulfanyl-hexanethiol-3-caprolactone-
co-3-caprolactone) (PCL-HDT)
In a typical experiment, 7% iodi...
to the vicinal proton of the iodine, and at 4.05 ppm corre-
sponding to the non-substituted methylene group as shown in
eq...
and with a commercially available PCL as starting material. This
approach presents an interesting and efficient alternative...
thiol groups with the PETAE triene. The reaction was carried out
under classical conditions in the presence of AIBN as rad...
around À60
C, a melting temperature (Tm) around 60
C and a
melting enthalpy DHm around 80 J.gÀ1
.31
As a result of the l...
Acknowledgements
The authors thank the Erasmus Mundus Program for Youssef
Bakkour fellowship.
Notes and references
1 L. S....
of 8

polym chem

Published on: Mar 4, 2016
Source: www.slideshare.net


Transcripts - polym chem

  • 1. Redox and thiol–ene cross-linking of mercapto poly(3-caprolactone) for the preparation of reversible degradable elastomeric materials† Benjamin Nottelet,* Guillaume Tambutet, Youssef Bakkour and Jean Coudane Received 20th June 2012, Accepted 20th July 2012 DOI: 10.1039/c2py20436c A novel thiol-functionalized PCL (PCL-HDT) was synthesized following a convenient two-step procedure. Taking advantage of the pendant thiol group, degradable elastomeric materials have been prepared from PCL-HDT by redox or thiol–ene reaction. Elastomers were characterized by HRMAS NMR spectroscopy to confirm the formation of disulfide or thioether cross-links. The thermal and mechanical properties of elastomers have been assessed by DSC, DMA and tensile tests. Disulfide containing elastomers (EMSS) and thioether containing elastomers (EMTE) exhibited improved mechanical properties with ultimate strains up to 220%. The stability of the mechanical properties at temperatures close to body temperature was confirmed by DMA with G0 z 200 MPa and G00 z 15 MPa. Finally, the reversibility of the disulfide formation and breaking has been evaluated, and confirmed the potential of these degradable elastomers as biomaterials. 1. Introduction Aliphatic polyesters have been widely investigated in recent years for their potential as biodegradable biomaterials for biomedical applications.1,2 In particular, degradable elastomeric materials to be used as scaffolds for tissue engineering are highly desirable.3–5 However, aliphatic polyesters lack pendant reactive groups, including hydroxyl, amine and carboxylic groups, classically used to enlarge polymer functionalities. Synthesis and polymer- ization of functional lactones is a strategy of choice to overcome this limitation,6–8 but ROP procedures often require severe reaction conditions, which prevents the use of thermally labile functionalized monomers. Post-polymerization reaction repre- sents an interesting alternative. Our group developed, a few years ago, a methodology based on the anionic chemical modification of poly(3-caprolactone) (PCL). This strategy was largely exploited to synthesize various functionalized PCLs bearing lateral small functional groups (iodine, carboxylic acid) or side macromolecular chains [poly(vinylpyrrolidone), poly(dimethy- laminoethyl methacrylate), poly(L-lysine), etc.].9–12 Considering the potential of thiol groups, it is of great interest to introduce pendant mercapto groups on the PCL backbone. Although some efforts have been made on the preparation of thiol-functionalized polyesters, only a few have been prepared to date, with mainly end-functionalized polymers used for nano- particles decoration.13–15 In parallel, polycondensations of thiol- containing precursors have also been investigated. For example, the enzymatic polycondensation of dimethyl 2-mercaptosucci- nate with hexane-1,6-diol was reported by Matsumura et al.16 However, only low molecular weight polyesters were obtained. Scandium catalysts have also been used for the polycondensation of diols and thiomalic acid to prepare ‘‘RAFT-gel’’.17 Both approaches led to polyesters having one mercapto group per repeating unit. In the present work, we were interested in using the anionic chemical modification to generate new PCL deriva- tives with a few mercapto groups distributed along the polymer backbone, and to use these polyesters for the preparation of degradable elastomeric materials. Advantageously, the reactivity of thiol groups can be exploited to obtain polymeric cross-linked matrices. First, thiol can react with nonactivated double bonds following the thiol–ene chem- istry strategy, which has entered the realm of click chemistry.18–20 Secondly, redox reactions can be used to yield disulfide from thiol groups. Cross-linked materials have previously been prepared following these two strategies for biomedical and environmental applications.21–24 However, and to the best of our knowledge, only one example of reversible material based on degradable polyester segments bearing thiol groups has been reported so far by Matsumura et al.16 In this paper, we report on the two-step synthesis of a new mercapto-functionalized PCL, namely poly(a-sulfanyl-hex- anethiol-3-caprolactone-co-3-caprolactone) (PCL-HDT). Redox and thiol–ene conditions are then studied to prepare two types of elastomeric materials from the PCL-HDT precursor. Charac- terisation of cross-linked insoluble PCL networks was performed by high-resolution magic angle spinning (HRMAS) NMR Max Mousseron Institute of Biomolecules (IBMM), Artificial Biopolymers Group, UMR CNRS 5247 University of Montpellier 1, University of Montpellier 2, Faculty of Pharmacy, 15 Av. C. Flahault, Montpellier, 34093, France. E-mail: benjamin.nottelet@univ-montp1.fr; Fax: +33 4-67-52-08-98; Tel: +33 4-11-75-96-97 † Electronic supplementary information (ESI) available: HRMAS spectra, DSC thermograms and DMA curves. See DOI: 10.1039/c2py20436c 2956 | Polym. Chem., 2012, 3, 2956–2963 This journal is ª The Royal Society of Chemistry 2012 Dynamic Article LinksC<Polymer Chemistry Cite this: Polym. Chem., 2012, 3, 2956 www.rsc.org/polymers PAPER Publishedon25July2012.DownloadedbyUniversitedeMontrealon11/11/201515:50:39. View Article Online / Journal Homepage / Table of Contents for this issue
  • 2. spectroscopy. Finally, as our main objective is to prepare elas- tomeric materials, mechanical properties of the cross-linked materials are evaluated and the reversibility of the disulfide bonds containing material is investigated. 2. Materials and methods 2.1 Materials PCL (Mn ¼ 36 500 g.molÀ1 , PDI ¼ 1.7), iodine (I2, $ 99.8%), LDA (2 M in THF/n heptane/ethylbenzene), 1,6-hexanedithiol (HDT, 99.5%), pentaerythritol triallyl ether (PETAE, 70%) and 2-mercaptoethanol ($99%) were obtained from Aldrich (St. Quentin Fallavier, France). NH4Cl was obtained from Fluka (St. Quentin Fallavier, France), anhydrous Na2S2O3 from Acros Organics (Noisy-le-Grand, France), MgSO4 and potassium carbonate (K2CO3, >99%) from Prolabo (Paris, France), diethyl ether (Et2O), dichloromethane (CH2Cl2), N,N-dimethylforma- mide (DMF), dimethyl sulfoxide (DMSO), toluene and methanol (MeOH) from Riedel de Ha€en (St. Quentin Fallavier, France). All were used as received. 2,20 -Azobis(isobutyronitrile) (AIBN, 99%) was obtained from Fluka (St. Quentin Fallavier, France) and was recrystallized from methanol prior to use. Tetrahydro- furan (THF) from Acros Organics (Noisy-le-Grand, France) was distilled on benzophenone/sodium until a deep blue color was obtained. 2.2 NMR spectroscopy 1 H and 13 C NMR spectra were recorded using an AMX300 Br€uker spectrometer operating at 300 MHz and 75 MHz, respectively. Deuterated dimethyl sulfoxide or chloroform was used as solvent. Solid 13 C spectra were recorded using an AMX300 Br€uker spectrometer operating at 75 MHz. The HRMAS NMR measurements on the swollen networks were carried out on a Bruker Avance III 600 spectrometer, operating at 600.13 MHz for proton and 150.96 MHz for carbon, equipped with a 4 mm 1 H/13 C/2 H HRMAS gradient probe. The polymers were immersed in a solvent (CDCl3) at room temper- ature for 24 h. The samples (ca. 3 mg) were packed into a 4 mm HRMAS rotor (90 mL sample volume). The spectra were acquired at a temperature of 303 K. In all experiments, the samples were spun at 6 kHz. Gradient selected 1 H/13 C hetero- nuclear single quantum coherence (HSQC) spectra were recorded using the topspin Br€uker software (topspin 2.1). 2.3 Size exclusion chromatography The molecular weights of the polymers were determined by size exclusion chromatography (SEC) using a Viscotek GPCMax autosampler system fitted with two Viscotek LT5000L mixed medium columns (300 Â 7.8 mm), a Viscotek VE 3580 RI detector and a Viscotek VE 3210 UV/VIS detector. The mobile phase was THF at a flow rate of 1 mL minÀ1 and at 30 C. Typically, the polymer (10 mg) was dissolved in THF (2 mL) and the resulting solution was filtered through a 0.45 mm Millipore filter before injection of 20 mL of filtered solution. Mn and Mw were expressed according to the calibration using poly(styrene) standards. 2.4 Differential scanning calorimetry Differential scanning calorimetry (DSC) measurements were carried out under nitrogen at a 5 C minÀ1 rate on a Perkin Elmer DSC 6000 Thermal Analyser. The samples were heated from 20 C to 110 C and the temperature was held for 5 min at 110 C before cooling to À50 C. This temperature was then held for 5 min before the final heating ramp to 200 C. The melting temperature (Tm), glass transition temperature (Tg) and melting enthalpy (DHm) were measured from the second heating ramp while the crystallization temperature (Tc) and enthalpy (DHc) were measured on the cooling ramp. 2.5 Tensile tests and dynamic mechanical analyses Sample plates were prepared by compression of the polymer in a stainless steel mould for 10 min at 100 C and 1 ton using a Carver press (4120). For tensile mechanical tests, plates (20 Â 7 Â 1 mm) were analysed at room temperature in an Instron 4444 at a 1 mm minÀ1 crosshead speed rate. Each sample was analysed in triplicate and Young’s modulus (E, MPa), ultimate stress (sbreak, MPa), ultimate strain (3break, %), yield stress (syield, MPa), and elastic limit (3yield, %) were expressed as the mean value of the three measurements. E was calculated using the initial linear portion of the stress–strain curves. Dynamic mechanical analyses (DMA) were conducted on the sample plates (10 Â 5 Â 1 mm). Perkin Elmer DMA7 was used in the temperature or frequency scan mode. Temperature scans were run from 23 to 40 C at a 1 C minÀ1 rate, with a 1 Hz frequency, while frequency scans were run from 0.1 to 40 Hz, at 37 C. 2.6 Synthesis of poly(a-iodo-3-caprolactone-co-3- caprolactone) (PCL-I) Poly(a-iodo-3-caprolactone-co-3-caprolactone) (PCL-I) with a 7% iodine content was prepared as described elsewhere.25 Briefly, PCL (70 mmol, 8 g) was dissolved in anhydrous THF (300 cm3 ) in a reactor equipped with a mechanical stirrer and kept at À70 C under argon atmosphere. A solution of LDA (35 mmol, 17.5 cm3 ) was injected with a syringe through a septum and the mixture was kept at À70 C for 20 min. Iodine (35 mmol, 8.9 g) was dissolved in THF before addition and the reaction was carried out for 25 min. The reaction medium was then hydro- lyzed with 100 cm3 of a 4 M aqueous solution of NH4Cl and the pH was adjusted to ca. 7 by using HCl 37%. The copolymer was extracted with dichloromethane (2 Â 100 cm3 ). The combined organic layers were washed three times with a solution of Na2S2O3, dried on anhydrous MgSO4 and filtered. The solvent was partially evaporated under reduced pressure and the concentrated solution was treated with an excess of cold methyl alcohol. The precipitated copolymer was washed with methyl alcohol and dried under vacuum. PCL-I with Mn ¼ 10 400 g.molÀ1 and PDI ¼ 2.2 was obtained in 80% yield. 1 H-NMR spectral data were as follows. 1 H-NMR (300 MHz, CDCl3) d: 4.27 (t, 1H, COCHI), 4.04 (t, 2H, COCH2), 3.63 (t, 2H, CH2CH2OH), 2.28 (t, 2H, CH2O), 1.98 (q, 2H, CHICH2), 1.62 (q, 4H, CH2CH2CH2), 1.36 (q, 2H, CH2CH2CH2). This journal is ª The Royal Society of Chemistry 2012 Polym. Chem., 2012, 3, 2956–2963 | 2957 Publishedon25July2012.DownloadedbyUniversitedeMontrealon11/11/201515:50:39. View Article Online
  • 3. 2.7 Synthesis of poly(a-sulfanyl-hexanethiol-3-caprolactone- co-3-caprolactone) (PCL-HDT) In a typical experiment, 7% iodinated PCL-I (1 g, 8.1 mmol) was dissolved in a solution of K2CO3 (78 mg, 2 eq. with respect to C–I bonds) in 15 mL of DMF. Argon was bubbled in the medium for 30 minutes with constant stirring. HDT (0.09 mL, 2 eq. with respect to C–I bonds) was added under argon and the reaction was run at room temperature for 24 hours under inert atmo- sphere. After the reaction, K2CO3 residues were filtered before concentration of the filtrate under vacuum. The resulting viscous solution was poured into an excess of cold MeOH to form a white precipitate. The polymer was dried under vacuum (0.87 g). PCL-HDT with Mn ¼ 15 700 g.molÀ1 and PDI ¼ 3.9 was obtained in 92% yield. 1 H-NMR spectral data were as follows. 1 H-NMR (300 MHz, CDCl3) d: 4.05 (t, 2H, CH2O), 3.63 (t, 2H, CH2CH2OH), 3.18 (t, 1H, COCHSCH2), 2.66 (t, 2H, CH2CH2SH), 2.51 (t, 2H, COCHSCH2), 2.28 (t, 2H, COCH2), 1.74–1.55 (q, 4H, CH2CH2CH2; q, 4H, SCH2CH2(CH2)2CH2- CH2SH), 1.50–1.26 (q, 2H, CH2CH2CH2; q, 4H, S(CH2)2CH2- CH2(CH2)2SH). 2.8 Synthesis of a cross-linked PCL-based elastomeric material by redox reaction (EMSS) Poly(a-sulfanyl-hexanethiol-3-caprolactone-co-3-caprolactone) was cross-linked according to a procedure described elsewhere.16 In a typical experiment 370 mg of PCL-HDT were dissolved in 2.5 mL of DMSO in a reaction flask that was placed in a ther- mostated oil bath at 70 C. The reaction was carried out for 20 hours under air. An insoluble gel formed and was soaked 5 times in large amounts of CH2Cl2 to remove residual DMSO and non- cross-linked polymer chains. After drying under vacuum, 210 mg of a yellow solid was recovered. Cross-linked polymers were characterized by HRMAS NMR spectroscopy. 1 H NMR (600 MHz, CDCl3) d: 4.00 (t, 2H, CH2OCO), 3.60– 3.55 (t, 2H, CH2CH2OH), 3.10 (t, 1H, COCHSCH2), 2.60 (t, 2H, S(CH2)5CH2SS), 2.50 (t, 2H, SCH2 (CH2)5SS), 2.25 (t, 2H, COCH2), 1.80 (q, 1Ha, COCHSCH2; m, 2H, SCH2CH2(CH2)4- SS), 1.70–1.45 (q, 1Hb, COCHSCH2; q, 4H, CH2CH2CH2; q, 4H, SCH2CH2(CH2)2CH2CH2SS), 1.45–1.20 (q, 2H, CH2CH2CH2; q, 4H, S(CH2)2CH2CH2(CH2)2SS). 13 C NMR (150 MHz, CDCl3) d: 64 (CH2OCO), 63 (CH2CH2OH), 47 (COCHSCH2), 39 ((CH2)5CH2SS), 34 (COCH2), 31 (CHSCH2(CH2)5SS), 29 (COCHSCH2), 25–28 (COCH2(CH2)3CH2O; SCH2(CH2)4CH2S)). The gel fraction of the cross-linked PCL-HDT was determined from the weight remaining after washing using eqn (1). gel fraction ð%Þ ¼ Wd W0 Â 100 (1) where Wd is the weight of the dried cross-linked sample and W0 the initial PCL-HDT weight. The weight swelling ratio at equilibrium (Qwe) was calculated based on the initial dry samples weights according to eqn (2). Qwe ¼ Ws W0 Â 100 (2) where Ws is the final weight of the swollen sample weighed at equilibrium, i.e. when the solvated weight remained constant after soaking in CH2Cl2, and W0 the initial dry weight of the sample. The weight swelling ratio extrapolated to t ¼ 0 (Qw0) was obtained by soaking the sample in CH2Cl2 for defined short periods of time and extrapolating the plot giving the sample weight as a function of time. For reversibility studies, disulfide bonds of EMSS were reduced following a procedure described elsewhere.24 In a typical reac- tion, 0.2 g of EMSS were soaked in 1 mL of CH2Cl2. 0.5 mL of 2- mercaptoethanol was added and the mixture was mechanically stirred at 37 C for 72 hours. 2.9 Synthesis of a cross-linked PCL-based elastomeric material by thiol–ene reaction (EMTE) Poly(a-sulfanyl-hexanethiol-3-caprolactone-co-3-caprolactone) was cross-linked according to a procedure described elsewhere and modified to our systems.20 PCL-HDT (500 mg, 4.1 mmol), PETAE (26 mL, 0.1 mmol, 0.5 eq. with respect to thiol) and AIBN (38 mg, 0.15 mmol, 0.5 eq. with respect to alkene) were added in a Schlenk flask and solubilized in the minimum amount of toluene to solubilize all components. The mixture was degassed via three freeze–pump–thaw cycles. The Schlenk flask was heated at 100 C for 8 hours. An insoluble gel formed and was soaked 5 times in large amounts of CH2Cl2 to remove residual toluene, PETAE and non-cross-linked polymer chains. After drying under vacuum, 350 mg of a yellow solid was recovered. Gel fraction and swelling ratio were calculated as described above. Cross-linked polymers were characterized by HRMAS NMR spectroscopy. 1 H NMR (600 MHz, CDCl3) d: 5.90 (s, 1H, CCH2OH), 5.20 (s, 2H, OCH2C; s, 2H, CCH2OH), 4.10 (t, 2H, CH2OCO), 3.80–3.70 (t, 2H, CH2CH2OH), 3.55 (t, 2H, SCH2CH2CH2O), 3.30 (t, 1H, COCHSCH2; m, 2H, SCH2CH2CH2O), 2.75 (t, 2H, (CH2)5CH2S; t, 2H, SCH2CH2- CH2O), 2.65 (t, 2H, COCHSCH2), 2.40 (t, 2H, COCH2), 1.95 (q, 1Ha, COCHSCH2; m, 2H, CHSCH2CH2), 1.9–1.6 (q, 1Hb, COCHSCH2; q, 4H, CH2CH2CH2; q, 4H, SCH2CH2(CH2)2CH2CH2S), 1.50–1.26 (q, 2H, CH2CH2CH2; q, 4H, S(CH2)2CH2CH2(CH2)2S). 13 C NMR (150 MHz, CDCl3) d: 71 (SCH2CH2CH2O), 70 (OCH2C; CCH2OH), 64 (CH2OCO), 62 (CH2CH2OH), 46 (COCHSCH2), 38 ((CH2)5CH2S; SCH2CH2CH2O), 34 (SCH2- CH2CH2O; COCH2), 31 (CHSCH2(CH2)5S), 29 (COCHSCH2), 24–28 (COCH2(CH2)3CH2O; SCH2(CH2)4CH2S)). 3. Results and discussion 3.1 Synthesis of PCL-I and PCL-HDT precursors In designing suitable macromolecular derivatives for evaluating the potential of thiol-functionalized PCLs for the preparation of elastomeric materials, PCL-I was chosen as an intermediate towards PCL-HDT (Scheme 1). As a consequence, the first step consisted of preparing PCL-I by anionic chemical modification under conditions already defined by our group in a previous work.25 Parameters of the activation step with LDA and substitution step with iodine have been optimized to yield poly- mers with ca. 10% molar substitution ratios in a one pot and time saving reaction starting from commercially available PCL. The chemical structure was characterized by 1 H-NMR (Fig. 1a). The substitution degree (SD) was calculated by comparison of the integrals of the resonance peaks at 4.27 ppm, corresponding 2958 | Polym. Chem., 2012, 3, 2956–2963 This journal is ª The Royal Society of Chemistry 2012 Publishedon25July2012.DownloadedbyUniversitedeMontrealon11/11/201515:50:39. View Article Online
  • 4. to the vicinal proton of the iodine, and at 4.05 ppm corre- sponding to the non-substituted methylene group as shown in eqn (3). SD ð%Þ ¼ I4:27 I4:05=2 Â 100 (3) A 7% substitution was obtained. The molecular weight of PCL-I was characterized by SEC analysis in THF. PCL-I with molec- ular weight Mn ¼ 10 400 g.molÀ1 and PDI ¼ 2.2 was obtained. These values were compared with the ones of the commercial PCL used as a starting material (Mn ¼ 36 500 g.molÀ1 and PDI ¼ 1.7) (Fig. 2). A molecular weight decrease between neat commercial PCL and PCL-I was observed, however, it is note- worthy that this is classically observed with the anionic activa- tion of polyester, as a consequence of hydrolysis and back-biting side reactions.26 In the present case, and although remarkable in terms of influence on the final molecular weight, this corre- sponded to a hydrolysis limited to only 0.87% of the ester groups initially present in the polymer chain. In the second step, PCL-I was reacted with 1,6-hexanedithiol to make the nucleophilic substitution of iodine. PCL-HDT was obtained in a 92% yield. After reaction, the polymer chemical structure was again characterized by 1 H-NMR (Fig. 1b). The substitution degree was calculated by comparison of the integral of the resonance peak at 4.05 ppm corresponding to the non- substituted methylene group on the polymer backbone, and of the combined integrals of the resonance peaks at 3.47 ppm, corresponding to the vicinal proton of the sulphur on the poly- mer backbone and between 2.46 and 2.70 ppm corresponding to vicinal methylene groups of the sulphur. Using eqn (4), a 6.5% substitution degree was found, which corresponds to a 90% yield for the reaction. SD ð%Þ ¼ ðI3:47 þ I2:46À2:70=4Þ I4:05 (4) The molecular weight of PCL-HDT (Mn ¼ 15 700 g.molÀ1 and PDI ¼ 3.9) was characterized by SEC analysis. Taking into account the substitution ratio and the molecular weight of the grafted HDT, no overall change of Mn was expected. The observed increase of Mn is due to some cross-linking side reac- tion. Bridging of two PCL-I chains reacting with the same HDT molecule occurs during the substitution reaction, thus leading to both increase of Mn and PDI. This is confirmed by the PCL- HDT chromatogram that exhibits small shoulders (Fig. 2). Considering the initial SD and the molecular weight increase, a maximum of 18% of the thiol groups are involved in the cross- linking side reaction. However, it should be noted that despite this limited cross-linking, PCL-HDT is freely soluble in organic solvents such as THF or CH2Cl2. It is also noteworthy that the proposed strategy allows the preparation of sulfhydryl functionalized polyesters with molec- ular weight around 15 000 g.molÀ1 in a simple two-step strategy Scheme 1 Synthesis of PCL-based elastomeric materials. Reaction conditions: (i) THF, LDA, À70 C, 30 min/I2, À70 C, 20 min; (ii) DMF, K2CO3, HDT, RT, 24 h; (iii) DMSO, 70 C, 20 h; (iv) toluene, PETAE, AIBN, 100 C, 8 h. This journal is ª The Royal Society of Chemistry 2012 Polym. Chem., 2012, 3, 2956–2963 | 2959 Publishedon25July2012.DownloadedbyUniversitedeMontrealon11/11/201515:50:39. View Article Online
  • 5. and with a commercially available PCL as starting material. This approach presents an interesting and efficient alternative to the strategies described in the literature including enzymatic polycondensations or protection–deprotection strategies leading to limited molecular weights (Mn # 6000 g.molÀ1 )16 or consid- erable polyester degradation,27,28 respectively. 3.2 Synthesis of PCL-based elastomeric materials PCL-HDT was used to produce elastomeric materials following two strategies. In the first approach, disulfide bonds were tar- geted. Disulfides are known to be reversibly formed under redox conditions and appeared to be of interest in the frame of designing degradable elastomers. In the second approach, we chose to take advantage of thiol functionalities of PCL-HDT to prepare materials by the thiol–ene strategy which was success- fully applied in the past for the preparation of cross-linked polymeric matrices.21 A disulfide containing PCL-based elastomeric material (EMSS) was easily prepared by the oxidation of sulphydryl groups under mild conditions. Thiol pendant groups of PCL-HDT were oxidized to disulfides in air. Although a gel was formed after a few hours, the reaction was maintained for 20 hours. Despite this longer reaction time, the gel fraction was low (56% of cross- linked chains). With PCL-HDT being substituted with 6.5% of pendant thiol groups, intramolecular cross-linking might explain this result. It is our belief that the relatively low thiol content of PCL-HDT, combined with intramolecular cross-linking, are responsible for the observed low gel fraction. This should be compared with the 76% obtained by Kato et al. with poly- (hexanediol-2-mercaptosuccinate), which exhibits one thiol per monomer unit.16 Another explanation is the relatively high molecular weight of the PCL-HDT pre-polymer that decreases thiol accessibility, especially when cross-linking has started, compared to a lower molecular weight and highly substituted polymer used by the later group. A PCL-based elastomeric material obtained by thiol–ene reaction (EMTH) was prepared by the reaction of the pendant Fig. 1 1 H-NMR spectra of (a) PCL-I and (b) PCL-HDT (CDCl3; residual water at 1.5 ppm; residual MeOH at 3.5 ppm). Fig. 2 SEC chromatograms of PCL, PCL-I and PCL-HDT (THF, 1 mL minÀ1 ). Table 1 Physico-chemical characterization of PCL-based materials Sample Substitution (%) Mn g.molÀ1 PDI Mc,theo (g.molÀ1 ) Mc,exp (g.molÀ1 ) Tm/DHm ( C/J.gÀ1 ) Tc/DHc ( C/J.gÀ1 ) Tg ( C) PCL-I 7 10 400 2.2 / / / / / PCL-HDT 6.5 15 700 3.9 / / 50/66 25/À68 À46 EMSS / / / 1720 1550 49/31 19/À35 À47 EMTH / / / 1720 1500 48/35 23/À38 À44 Fig. 3 Typical tensile stress–strain curves for PCL (black line), EMSS (dark grey line) and EMTH (light grey line). 2960 | Polym. Chem., 2012, 3, 2956–2963 This journal is ª The Royal Society of Chemistry 2012 Publishedon25July2012.DownloadedbyUniversitedeMontrealon11/11/201515:50:39. View Article Online
  • 6. thiol groups with the PETAE triene. The reaction was carried out under classical conditions in the presence of AIBN as radical precursor and led after 8 hours to an insoluble gel. The gel fraction was calculated and found to be equal to 70%. Weight swelling ratios at equilibrium (Qwe) and extrapolated at t ¼ 0 (Qw0) were evaluated for both materials. Using eqn (2), Qwe of 4900% and 3100%, and Qw0 of 226% and 225% were calculated for EMSS and EMTH, respectively. These values were further used to calculate the molecular weight between cross- links ( Mc,exp) according to the Flory–Rehner equation (eqn (5)), Mc;exp ¼ Vso  dp  Fp 2 À Fp 1=3 ln À 1 À Fp Á þ Fp þ cFp 2 (5) where Vso is the molar volume of the solvent (64.341 mL.molÀ1 for CH2Cl2), Fp is the volume fraction of the polymer in the swollen gel equal to 1/Qw0 and c is the Flory solvent–polymer interaction parameter (c z 0.64).29 Mc,exp of 1550 g.molÀ1 and 1500 g.molÀ1 were calculated for EMSS and EMTH, respectively. These values are in good agreement with the theoretical molec- ular weight between cross-links Mc,theo ¼ 1720 g.molÀ1 calcu- lated by taking into account the molecular weight of the polymer precursor and the substitution degree (Table 1). Due to their cross-linked nature, the chemical characterization of the elastomeric materials could not be done via classical solution NMR. Instead, we took advantage of the high-resolu- tion magic angle spinning (HRMAS) technique which has proved to be pertinent for gels.30 In this way, a 1 H NMR line- width similar to liquid samples can be reached. 1 H and 1 H/13 C HSQC HRMAS confirmed the formation of disulfide and thio- ether bonds as a result of the cross-linking (ESI, Fig. S1–S3†). In particular, 1 H/13 C spectra showed coupling of peaks at 2.60/39 ppm and 2.50/31 ppm for EMSS corresponding to the vicinal methylenes of the disulfide ((CH2)5CH2SS) and of the thioether (CHSCH2(CH2)5SS) groups. The same was observed for EMTH with couplings at 2.75/38 ppm and 2.65/31 ppm. The 13 C chem- ical shift from ca. 25 ppm for the starting CH2–SH hexanethiol to ca. 38 ppm for CH2–S–S and CH2–S–CH2 confirms the cross- linking reaction. 3.3 Thermo-mechanical properties and reversibility of PCL- based elastomeric materials 3.3.1 Thermal properties. Thermal properties were investi- gated by DSC. Typical thermograms are given in the ESI, Fig. S4† and the results are listed in Table 1. Thermograms vertical scales have been adjusted for clarity reasons. PCL is a semi-crystalline polyester with a glass transition temperature (Tg) Table 2 Mechanical properties of PCL-based materials Sample Young’s modulus (E, MPa) Elastic limit (3yield, %) Ultimate stress (sbreak, MPa) Ultimate strain (3break, %) Storage modulus (G0 , MPa) Loss modulus (G00 , MPa) 25 C 37 C 25 C 37 C PCL 50 Æ 5 6.5 Æ 1 0.8 Æ 0.3 22 Æ 3 91 Æ 10 94 Æ 14 10.0 Æ 1.0 9.8 Æ 0.7 EMSS 135 Æ 7 28 Æ 2 9.0 Æ 1.4 223 Æ 35 220 Æ 13 182 Æ 3 16.0 Æ 0.6 15.0 Æ 0.6 EMTH 110 Æ 14 23 Æ 3 6.6 Æ 0.7 130 Æ 25 170 Æ 17 147 Æ 19 16.0 Æ 0.7 14.0 Æ 1.3 Fig. 4 Storage and loss moduli of PCL, EMSS and EMTH at 25 C (light grey bars) and 37 C (dark grey bars). Fig. 5 Reversibility of EMSS swollen in CH2Cl2 (a) before reduction and (b) after reduction. This journal is ª The Royal Society of Chemistry 2012 Polym. Chem., 2012, 3, 2956–2963 | 2961 Publishedon25July2012.DownloadedbyUniversitedeMontrealon11/11/201515:50:39. View Article Online
  • 7. around À60 C, a melting temperature (Tm) around 60 C and a melting enthalpy DHm around 80 J.gÀ1 .31 As a result of the lower molecular weight and of the long alkyl chains of the pendant thiol group, PCL-HDT has a lower Tm (50 C). The presence of few cross-links in PCL-HDT, as discussed in Section 3.1, induced an increase of the glass transition to À46 C and DHm was found to be equal to 66 J.gÀ1 . After cross-linking, whatever the strategy used, elastomeric materials still were semi-crystalline with DHm ¼ 31–35 J.gÀ1 . This lower value indicates a decrease of crystallinity, as expected for cross-linked polymers whose chains have less mobility for crystalline rearrangement. However, small PCL chains between cross-links, with Mc,exp z 1500 g.molÀ1 are still long enough to induce crystallization. PCL oligomers with low molecular weight (Mn ¼ 1200 g.molÀ1 ) are known to crys- tallize.32 In the present case, this crystallization results in semi- crystalline materials with for EMSS Tm ¼ 49 C and Tg ¼ À48 C and for EMTH Tm ¼ 48 C and Tg ¼ À44 C (hardly visible). Considering a melting enthalpy of 142 J.gÀ1 for a 100% crys- talline PCL,33 PCL-HDT, EMSS and EMTH have crystallinities of 46%, 22% and 25%, respectively. To summarize, cross-linking did not induce significant changes of Tm and Tg for the elasto- meric materials but was accompanied with a decrease of crys- tallinity by a factor 2. 3.3.2 Tensile mechanical properties. Mechanical properties have been evaluated on plates obtained by compression. Tensile mechanical tests have first been carried out to evaluate the elastomeric character of the cross-linked materials. Typical stress–strain curves are given in Fig. 3. For comparison, a commercial PCL with a molecular weight Mn ¼ 12 700 g.molÀ1 and PDI ¼ 1.7 was used as a control. This PCL was chosen for its molecular weight similar to the one of the PCL-I and PCL-HDT precursors. As expected, cross-linking had drastic effects on the mechanical properties of the PCL-based materials. Young’s modulus was doubled from 50 to ca. 110–130 MPa, but more interestingly, the elastic limit was increased from 6.5% for PCL to ca. 25% for the elastomeric materials (Table 2). One should note that, considering the strict ASTM definition of elastomers, our materials, with a uniaxial elastic limit of 25% (inferior to 100%), cannot be labeled as elastomers. However, even if improper, this denomination is largely accepted in the literature and, with this comment in mind, will be further used in this work for clarity reasons. Ultimate stress and strain were also strongly increased with a 10 fold increase for sbreak passing from 0.8 to ca. 6.6–9 MPa, and a 6 to 10 fold increase for 3break reaching up to 220% for EMSS. When comparing the elastomeric materials, no significant differences can be found between EMSS and EMTH. Only a higher ultimate deformation for EMSS should be noted with 220% compared to 130% for EMTH. This difference prob- ably results from the nature of the cross-linking with EMSS having bimolecular disulfide cross-links compared to the trimo- lecular cross-links expected in EMTH. 3.3.3 Dynamic mechanical properties. Beside tensile mechanical tests, the materials have also been assessed by dynamic mechanical analyses. As PCL is degradable and widely used in the biomedical field, we were interested in evaluating the PCL-based materials under two temperature conditions including room temperature and physiological temperature. Temperature scans were carried out at 1 Hz between 23 C and 40 C (see ESI, Fig. S5–S6† for typical curves). As can be seen in Fig. 4, the loss modulus (G00 ) was similar for the two elastomeric materials with values around 15 MPa and a limited 10% decrease when increasing the temperature from 25 C to 37 C. The differences were more pronounced for the storage modulus (G0 ) with values of 220 MPa for EMSS and 170 MPa for EMTH at 25 C. As expected, raising the temperature to 37 C decreased G0 for both materials with a 20% decrease for EMSS (180 MPa) and a 12% decrease for EMTH (150 MPa) (Table 2). To complete this study, frequency scans were also run at 37 C in the range 0.1 Hz to 40 Hz. However, no significant change of moduli was observed in this range (data not shown). Similarly to the Young’s modulus, the storage modulus of EMSS was ca. 20–30% higher compared to the one of EMTH. This difference may be due to the higher mobility of the 3 ether bonds in EMTH compared to the mobility of the single disulfide bond in EMSS. 3.3.4 Reversibility of Ess. Cleavage of disulfide bonds under reductive conditions is well known especially for proteins con- taining cystein moieties. In the present work, we were interested in checking the capacity of our disulfide containing EMSS to be reversible. Reduction was done via the reductive agent 2-mer- captoethanol (ME) as it was found to be effective on disulfide containing cellulosic materials.24 The reaction was carried out at 37 C for 72 hours. Contrary to what was observed for cellulosic material, after 24 hours no change was observed despite a lower disulfide bonds concentration. The reduction time was increased to 72 hours and a gel–sol transition was observed (Fig. 5). SEC analysis of the resulting polymer chains was carried out to evaluate the molecular weight of the isolated chains after reduction. No degradation of the PCL backbone was observed: Mn ¼ 19 000 g.molÀ1 and PDI ¼ 2.3 instead of Mn ¼ 15 700 g.molÀ1 and PDI ¼ 3.9 for the starting PCL-HDT (see ESI, Fig. S7†). This slight molecular weight increase can be attributed to a few residual uncleaved disulfide bonds. These findings confirm the capacity of disulfide elastomeric materials to be reversibly cleaved under redox conditions, without degradation of the PCL backbone, which might be of interest for future applications. 4. Conclusions Two types of degradable PCL-based elastomeric materials have been obtained from the new PCL-HDT precursor. This thiol- functionalized PCL was readily prepared from PCL in two steps, thanks to an initial anionic activation simple strategy. Redox and thiol–ene reactions were conducted with this precursor to yield disulfide (EMSS) and thioether (EMTH) containing elastomeric materials. HRMAS NMR spectroscopy confirmed the chemical structure of the cross-linked compounds. All mechanical prop- erties of the cross-linked polymers were increased compared to neat PCL, with doubled Young’s moduli and four fold increases of the elastic limits. EMSS was found to be superior for all properties when compared to EMTH. In addition, the demon- strated reversibility of the disulfide bond in EMSS might be an advantage for future applications as biomaterial. 2962 | Polym. Chem., 2012, 3, 2956–2963 This journal is ª The Royal Society of Chemistry 2012 Publishedon25July2012.DownloadedbyUniversitedeMontrealon11/11/201515:50:39. View Article Online
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