Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Non-Isocyanate Polythiourethanes (NIPTUs) from Cyclodithiocarbonate Telechelic Polyethers Elise Vanbiervliet,† Steṕ hane Fouquay,‡ Guillaume Michaud,§ Fred́ eŕ ic Simon,§ Jean-François Carpentier,*,† and Sophie M. Guillaume*,† †
Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes), UMR 6226, F-35000 Rennes, France BOSTIK S.A., 420 rue d’Estienne d’Orves, F-92705 Cedex, Colombes, France § BOSTIK, ZAC du Bois de Plaisance, 101, Rue du Champ Cailloux, F-60280 Venette, France Downloaded via UNIV OF SOUTHERN INDIANA on July 25, 2019 at 13:50:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Commercially available poly(propylene glycol) (PPG) and poly(tetrahydrofuran diglycidyl ether) (PTG) α,ωend-capped with epoxide functions have been chemically modified into α,ω-bis(cyclodithiocarbonate) (DTC) telechelic analogues. The neat stoichiometric reaction of PPG/PTG-Epoxide2 with CS2 and LiBr enabled to prepare up to 300 g of the corresponding PPG/PTG-DTC2 functional polyethers. The subsequent one-pot, one-step aminolysis of the cyclodithiocarbonate end-groups of the polyethers using di- or polyamines, namely triethylene glycol diamine (Jeffamine EDR-148) and/or polyethylenimine (Lupasol), smoothly afforded the poly(mercaptothiourethane)/polyethers, as non-isocyanate polythiourethanes (NIPTUs) featuring pendant primary thiol groups. Ultimate cross-linking of the NIPTUs upon oxidation with MnO2 or through the Michaël reaction of the pendant thiol functions with di- or triacrylates, afforded cured NIPTU materials. Detailed macromolecular and thermomechanical characterizations of the (cross-linked) polymer materials, including NMR, FTIR, and Raman spectroscopies, MALDI-ToF mass spectrometry, DSC analyses, and tensile and rheology tests, are reported.
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INTRODUCTION Polyurethanes (PUs), first discovered by Otto Bayer in 1937, nowadays account for nearly 5 wt % of the total worldwide polymer production.1,2 PUs form the most important family of commodity polymers thanks to their tailor-made properties, with applications ranging from rigid and flexible foams to coatings, adhesives, sealants, elastomers, fibers, and biocompatible plastics for many industries such as aeronautics, automotive, packaging, sports/leisures, textile, and medical ones.3,4 Industrial PUs are traditionally produced at low cost from the polyaddition of diols with diisocyanates in the presence of a catalyst. The toxicity and reactivity/sensitivity of isocyanates to moisture, along with the toxicity and safety hazard inherent to phosgene from which they are derived, have prompted research toward greener, more sustainable and environmentally friendly routes to PUs.5−10 To this end, strategies based either on renewable resources, such as vegetable oils, wood starch, or sugars, or on isocyanatefree reagents have been developed over the past two decades.11−20 Among these, the most largely explored and most promising approach aims at developing non-isocyanate (and thus phosgene-free) PUs (NIPUs), upon the polyaddition reaction of di- or polyfunctional cyclocarbonates with di- or polyfunctional amines. Such NIPUs are more accurately © XXXX American Chemical Society
polyhydroxyurethanes (PHUs), which arise from the ringopening of five-, six-, seven-, or eight-membered cyclocarbonates upon aminolysis, resulting in the formation of urethane repeating units bearing primary or secondary hydroxyl groups in the β-position.11−20 The state-of-the-art on PHUs synthesis, as investigated notably by Caillol and Boutevin,21−24 Cramail and Grau,25−28 Detrembleur,29−32 Endo,33−39 Guillaume and Carpentier,40−43 Hillmyer,44−47 Kebir,48−51 Keul and Möller,20,52−54 Mülhaupt,55−58 Torkelson,59−62 Sardon,63−66 and others,67−77 addresses the synthesis of cyclocarbonate monomers (essentially five- but also six-membered ones), their subsequent reaction with di- or polyamines, the catalysis of the reaction, spectroscopic characteristics, and thermal features of the PHUs; on the other hand, very few investigations on the structure−properties relationship of the polymer materials have been reported. In spite of a few encouraging results such as mild and/or catalyst-free operating conditions and high molar mass values for PHUs therefrom (Mn,SEC up to 68100 g mol−1),41−43 the aminolysis/polyaddition reaction most often Received: April 5, 2019 Revised: June 27, 2019
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DOI: 10.1021/acs.macromol.9b00695 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. General Strategy toward the Synthesis of Cross-Linked PPG/PTG NIPTU Materials from PPG/PTG-Epoxide2, JEFFAMINE EDR-148, Lupasol, and the 50:50 mixture Lupasol FG/EDR-148.
the use of a di-/triacrylate in a Michaël type reaction. Detailed macromolecular and thermo-mechanical characterizations of these original NIPTUs by NMR, FTIR, and Raman spectroscopies, mass spectrometry (MS), DSC analyses, and tensile and rheology tests, are reported.
only proceeds at high temperature, providing PHUs of molar mass rarely exceeding 20000 g mol−1. In fact, the inherent limited reactivity of five-membered cyclocarbonates currently prevents the industrial implementation of this carbonate/ amine polyaddition route to provide PHUs as commercially viable alternatives to conventional thermoplastic PUs. Given, in particular, the poor reactivity of the five-membered cyclocarbonate moiety, other syntons more prone to react with amines have been considered. In this regard, five-membered cyclothiocarbonates were shown by Endo and co-workers to react under mild conditions (30 °C) with diamines, as the result of a larger/more favorable ring strain, as compared to the five-membered cyclocarbonate analogues.78−80 Thus, nonisocyanate poly(mercaptothiourethane)s (NIPTUs) were formed from the polyaddition of bifunctional five-membered cyclodithiocarbonates (DTCs) with diamines. Such NIPTUs are then exempt of hydroxyl groups but display thiourethane functions resulting from the regioselective nucleophilic addition at the S−C(S) bond rather than the C(S)−O bond. DTCs thus display a better selectivity toward the formation of the primary thiol (100%) than the one reached from fivemembered cyclocarbonates (ca. 60−70% of secondary hydroxyl groups).78,79,81−83 These pendant thiol functions along the polymer backbone can further get oxidized into disulfide linkages, resulting in inter- and intramolecular crosslinked polymer materials with improved chemical and mechanical strength.78,79,82−87 In this work, we report the synthesis and characterization of new NIPTUs for adhesive applications, derived from the aminolysis of DTC telechelic prepolymers based on polyethers, namely poly(propylene glycol) (PPG) and poly(tetrahydrofuran diglycidyl ether) (PTG). The α,ω-bisDTC telechelic PPGs/PTGs was first synthesized from the α,ωdiepoxide telechelic PPG/PTG parent polymers,88 and aminolysis using various primary amines was next implemented (Scheme 1). The resulting poly(mercaptothiourethane)/ polyethers were then cross-linked upon oxidation or through
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EXPERIMENTAL SECTION
Materials. All catalytic experiments were performed under air. PPG/PTG-Epoxide2 was commercially available from EMS-GRILTECH (Grilonit F 704 and Grilonit F 713; Figures S1 and S2). Complete characterization of these prepoymers (1H and 13C{1H} NMR, FTIR, SEC, viscosity, epoxide content) is reported in the Supporting Information (Table S1; Figures S3−S6). CS2 (caution: toxic, highly volatile, f lammable liquid with low ignition temperature; handle accordingly), JEFFAMINE EDR-148 (triethylene glycol diamine; Huntsman; primary amine content = 13.48 mequiv g−1; Table S2, Scheme 1, Figure S7) Lupasol (polyethylenimine featuring a definite ratio of primary, secondary, and tertiary amino groups; BASF; Table S2, Scheme 1 and Figure S7), trimethylolpropane triacrylate (TMPTA, SARTOMER; Table S3), tri(propylene glycol) diacrylate (TPGDA, purified through a silica column, SARTOMER; Table S3), and all other reagents (Aldrich, unless otherwise mentioned) were used as received (unless otherwise stated). Instrumentation and Measurements. 1H (500, 400 MHz) and 13 C{1H} (125, 100 MHz) NMR spectra were recorded on Bruker Avance AM 500 and AM 400 spectrometers at 23 °C in CDCl3. Chemical shifts (δ) are reported in ppm and were referenced internally relative to tetramethylsilane (δ 0 ppm) by using the residual 1 H and 13C solvent resonances of the deuterated solvent. The molar mass values of the PPG/PTG samples were determined by 1H NMR analysis in CDCl3 (Mn,NMR) from the integral value ratio of the signals of end-groups’ hydrogens (typically δ 3.12 ppm (methine HbPPG/PTG); 5.22, 5.17 ppm (methine Hb′PPG/PTG)) to internal ether hydrogens (δ 1.14 (methyl Hf,f′PPG), or δ 1.59, 1.55 (methylene He,e′PTG)) (Figure 1, Figures S3, S5, and S11). The number average molar mass (Mn,SEC) and dispersity (ĐM = Mw/Mn) values of the polyether samples were determined by size exclusion chromatography (SEC) in THF at 30 °C (flow rate = 1.0 mL min−1) on a Polymer Laboratories PL50 apparatus equipped with a refractive index detector and a set of two ResiPore PLgel 3 μm B
DOI: 10.1021/acs.macromol.9b00695 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. 1H NMR spectrum (500 MHz, CDCl3, 25 °C) of PPG-DTC2 (∗: δ 0.06 ppm residual grease; n = ca. 9).
Figure 2. 13C{1H} NMR spectrum (125 MHz, CDCl3, 25 °C) of PPG-DTC2 (n = ca. 9). MIXED-E 300 × 7.5 mm2 columns. The polymer samples were dissolved in THF (2 mg mL−1). All elution curves were calibrated with 12 monodisperse polystyrene standards (Mn range = 580− 380000 g mol−1). Mn,SEC values of polymers were uncorrected for their possible difference in hydrodynamic volume in THF vs polystyrene. MALDI-ToF mass spectra were recorded at the CESAMO (Bordeaux, France) on a Voyager mass spectrometer (Applied
Biosystems) equipped with a pulsed N2 laser source (337 nm, 4 ns pulse width) and a time-delayed extracted ion source. Spectra were recorded in the positive-ion mode by using the reflectron mode and with an accelerating voltage of 20 kV. A freshly prepared solution of the polymer sample in THF (HPLC grade, 10 mg mL−1), a saturated so lution of trans -2-[3-(4-tert-butylphenyl)-2-methyl-2propenylidene]malononitrile (10 mg, DCTB) in THF (1 mL, HPLC grade), and a MeOH solution of the cationizing agent (NaI, C
DOI: 10.1021/acs.macromol.9b00695 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. MALDI-ToF mass spectrum of PPG-DTC2 (DCTB matrix, Na cationizing salt); see the top zoomed-in region and the corresponding middle and bottom simulations for PPG-DTC2 (n = 7). 10 mg mL−1) were prepared. The three solutions were combined in a 10:1:1 v/v of matrix-to-sample-to-cationizing agent. The resulting solution (1−2 μL) was deposited onto the sample target and vacuumdried. FTIR spectra of the polymers were acquired (16 scans) with a resolution of 4 cm−1 on a Shimadzu IR-Affinity-1 equipped with an ATR module (23 °C). Kinetic monitoring was performed upon measuring the area of the targeted bands every 30 s (16 scans). Raman spectra of the polymers were acquired using a Kaiser Optical System RXN2 hybrid Raman spectrometer equipped with a 785 nm excitation line. Differential scanning calorimetry (DSC) analyses were performed with a Setaram DSC 131 apparatus calibrated with indium, at a rate of 10 °C min−1, under a continuous flow of helium (25 mL min−1), using aluminum capsules. The thermograms were recorded according to the following cycles: − 70 to 120 °C at 10 °C min−1; 120 to −70 °C at 10 °C min−1; − 70 °C for 5 min; − 70 to 120 °C at 10 °C min−1; 120 to −70 °C at 10 °C min−1. The rheological behavior of the cross-linked NIPTUs was studied by using a rotational rheometer (ARES-G2) (TA Instruments, USA) equipped with a stainless steel 40 mm plate−plate geometry. Temperature cooling was controlled via a thermocube circulation system. A rotational flow sweep was performed to measure the viscosity of the copolymers at 23 °C over a shear rate range of 1−100 s−1. An oscillation frequency sweep was performed to monitor the modulus change in response to frequency ranging from 0.1 to 100 Hz, with a fixed strain at 10% at 23 °C. It should be noted that both the amplitude and frequency sweeps give the storage modulus (G′) and the loss modulus (G″) of the copolymers, yet under different environments. An oscillation temperature sweep was used to determine the gelation temperature of the systems, with a controlled ramp rate of 5 °C min−1, from −50 to +40 °C. The experiment was performed at 10% strain. The sol−gel transition point was monitored from the point at which G′ and G″ intersect. Mechanical properties of tensile bar H-shaped samples (length × width × thickness = 75 × 12 × 0.3 mm3) prepared from films made of (cross-linked) PPG/PTG NIPTUs by using compression molded sheets were determined by tensile and rheology tests according to ISO37 NORM.
Determination of the DTC Content in PPG/PTG-DTC2. To perform an accurate stoichiometric polyaddition reaction between PPG/PTG-DTC2 (Figures 1−3, S10−S15) and the di- or polyamine so as to obtain high molar mass polymers, the DTC content in the polyethers was determined (mmol g−1 or mequiv g−1). The method consisted in a titration using benzylamine (Scheme S1). A known amount of PPG/PTG-DTC2 (mPPG/PTG‑DTC2; typically 1 g) was reacted with a known amount of benzylamine (nbenzyamnine,initial) for 10 min at 50 °C. After complete reaction of the DTC functions, as monitored by the complete disappearance of the methine signal (δ 5.22/5.17 ppm) in the 1H NMR spectrum (Figure 1 and Figure S11), the unreacted benzylamine was titrated by using an aqueous HCl solution (nHCl), thereby giving the amount of residual benzylamine (nbenzylamine,final) and the corresponding amount of consumed benzylamine (nbenzylamine consumed). The amount of DTC having reacted was determined from this latter value (nDTC = nbenzylamine consumed). Finally, the content in DTC in the PPG/PTG-DTC2 was calculated from DTC content = nDTC/mPPG/PTG‑DTC2 (mequiv g−1). The DTC content thus determined in PPG-DTC2 (2.51 ± 0.3 mequiv g−1) and PTGDTC2 (2.59 ± 0.3 mequiv g−1) was in agreement with the supplier’s data (3.0−3.3 and 2.3−2.6 mequiv g−1, respectively; Tables S1 and S4; Figures S1 and S2). Polyaddition of a PPG/PTG-DTC2 with a polyamine (without curing). In a typical procedure, PPG-DTC2 (1.00 g, 2.5 mmol(DTC).g−1, as determined from titration), a polyether diamine (JEFFAMINE EDR-148) (0.180 g, 13.5 mmol(NH2).g−1; Figures S7 and S8; Table S2), were charged in an aluminum cup. The resulting mixture was rapidly stirred manually using a wooden spatula (1 min) at 23 °C to get a homogenous mixture. The reaction proceeded up to 2.5 h (Table 1). All resulting polymers were isolated as yellow films, insoluble in all common organic solvents (THF, CHCl3, DMSO...). The polymers were characterized by rheology, DSC, FTIR and Raman spectroscopies (Tables 1−2, Figures 4, S16−S21). Polyaddition of a PPG/PTG-DTC2 with a Polyamine in the Presence of MnO2 as Oxidizing Agent. In a typical procedure, PPG-DTC2 (1.00 g, 2.5 mmol (DTC) g−1, as determined from titration), a polyether diamine (JEFFAMINE EDR-148) (0.18 g, 13.5 mmol (NH2) g−1; Figures S7 and S8; Table S2), and MnO2 8 wt % (9 mg) were charged in an aluminum cup (Schemes 1 and S5). The D
DOI: 10.1021/acs.macromol.9b00695 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules resulting mixture was rapidly stirred manually using a wooden spatula (1 min) at 23 °C to get a homogeneous mixture. The reaction proceeded over 3−80 min (Table 1). All resulting polymers were isolated as black films, insoluble in all common organic solvents (THF, CHCl3, DMSO, etc.). The polymers were characterized by rheology, FTIR, and DSC analyses (Table 2 and Figures 6 and S22). Polyaddition of a PPG/PTG-DTC2 with a Polyamine in the Presence of an Acrylate as Cross-Linking Promoter. In a typical procedure, PPG-DTC2 (1.00 g, 2.5 mmol (DTC) g−1 as determined from titration), a polyether diamine (JEFFAMINE EDR-148) (0.18 g, 13.5 mmol (NH2) g−1; Figure S7 and Table S2), and TPGDA (0.38 g, 6.6 mmol (acrylate) g−1; Table S3) were charged in an aluminum cup. The resulting mixture was rapidly stirred manually using a wooden spatula (1 min) at 23 °C to get a homogeneous mixture. The reaction proceeded over 3−143 min (Table 2). All resulting polymers were isolated as yellow materials, insoluble in all common organic solvents (THF, CHCl3, DMSO, etc.). The cross-linked polymers were characterized by rheology, FTIR, and DSC analyses. Preparation of NIPTUs Films for Mechanical Tests. All films of NIPTUs were prepared according to the following typical procedure. PPG-DTC2 (1.00 g, 2.5 mmol (DTC) g−1), a polyether diamine (JEFFAMINE EDR-148) (0.18 g, 13.5 mmol (NH2) g−1; Figure S7 and Table S2), and TPGDA (0.38 g, 6.6 mmol (acrylate) g−1; Table S3) were charged in an aluminum cup. The resulting mixture was rapidly stirred manually using a wooden spatula (1 min) at 23 °C to get a homogeneous mixture. The mixture was layered onto a glass support, and H-shaped pieces were cut out from the resulting film. The thermo-mechanical properties of (cross-linked) PPG/PTG NIPTUs were then determined by DSC, tensile tests and rheology (Table 3, Figures S22−S33).
and methylene (δCa′ 36.4−36.3 ppm) signals were clearly shifted (Figures 1 and 2, Figures S3−S6, S10, and S11). No residual CS2 (120 >120
143 ± 70c 18 ± 6
80 ± 7 3 ± 0.3
1.15 1.11
0.55 0.52
αgel corresponds to the branching coefficient required to reach the gel point; αgel = 1/(f − 1) with f = weight-average functionality. bCalculated according to r × α2gel = 1/[(f PPG/PTG‑DTC2 − 1) × ( famine − 1)],94,95 with f PPG‑DTC2 = 1.76, f PTG‑DTC2 = 1.81, f EDR‑148 = 2, and famines mixture = 5.44, as determined from f = (amine content (in %) × Mn)/1000 (Mn: supplier data; Table S2). cThe standard deviation most likely originated from reproducibility issues inherent to the preparation procedure of the NIPTUs and the possibly inhomogeneous mixture recovered (refer to the Experimental Section). a
Figure 4. Zoom-in regions of the FTIR monitoring of the PPG-DTC2/EDR-148 reaction over 30 min (see Figure S17 for complete spectra).
PTG-DTC2 remained in the reaction medium after 19 min (i.e., 1 min after the experimentally determined αgel of 18 min; Figure S18 and Table 1), and consequently 47% of the PTGDTC2 prepolymer was consumed, which is commensurate to the calculated value of 52% (Table 1). Further Raman analyses of the surface of the PPG/PTG NIPTUs mixture evidenced the presence of both thiol functions (S−H at 2574 cm−1) and disulfide bridges (S−S at 504 cm−1; arising from oxidation in air)97 bewteen NIPTU chains, suggesting incomplete crosslinking of the NIPTUs (Figure S21). It appears therefore that the reaction of DTC moieties from PPG/PTG-DTC2 with a primary amine is effective at room temperature in the absence of solvent. A multifunctional amine (i.e., a mixture of primary amines) is required for a sol−gel transition to occur, then resulting in solid PPG/PTG NIPTU materials; on the other hand, formulations involving EDR-148 only do not give such solid materials. To promote the formation of disulfide bridges from pending thiol functions into highly cross-linked materials, a curing agent (oxidizing agent or acrylate) was next added to the fomulations. Note that all curing reactions described therafter were performed under ambient conditions since the NIPTU materials are aimed essentially at DIY applications. Synthesis of Cross-Linked PPG/PTG NIPTUs Promoted by MnO2 Oxidation. The oxidation of −SH groups
over the PPG analogue. In EDR-148 reactions, regardless of the polyether, and although the reaction was (almost) completed as evidenced by FTIR (vide inf ra), the absence of any experimentally observed gel point was corroborated by the calculated αgel value greater than 1, which implied polyether and amine conversions exceeding 100% to reach a gel point, i.e., an experimentally essentially unreachable gel point. FTIR enabled also to qualitatively monitor the progress of the aminolysis of DTC through the disappearance of the strong υCS (O−C(S)−S) absorption band at 1192 cm−1, concomitant with the appearance of the new band at 1515 cm−1 from the resulting NIPTU.96 The PPG-DTC2/primary amine(s) polyaddition went to completion within ca. 30 min (although the NIPTU material recovered was not solid), whereas the reaction was a little longer with PTG-DTC2 (Figure 4; Figures S17 and S20). For the PPG/PTG-DTC2/ Lupasol/EDR-148 aminolysis, the predicted αgel value of 0.55/ 0.52, respectively (Table 1), suggested that both the polyether and amine(s) should reach half-conversion (namely 55% and 52% for PPG/PTG-DTC2, respectively, as determined from conversionPPG/PTG‑DTC2 = αgel × 100) at the gel point and, consequently, that the formation of a solid PPG/PTG NIPTU should not require complete reagents consumption (Figures S17−S20). FTIR monitoring of the polyaddition experiment supported these calculated αgel values. For instance, 53% of F
DOI: 10.1021/acs.macromol.9b00695 Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. Raman spectra of PTG NIPTU prepared from PTG-DTC2/Lupasol/EDR-148/MnO2 (blue trace) and without MnO2 (red trace) (Table 1).
in PPG/PTG NIPTUs induces the formation of −S−S− disulfide bridges, eventually resulting in intermolecularly crosslinked polyether materials (Scheme 1). Besides (adventitious) air or oxygen that are seldom used, typical −SH oxidizing agents include organic ones (p-benzoquinone dioxime, cumene hydroperoxide) as well as inorganic ones (PbO2, MnO2, KMnO4, ZnCrO4, Na 2Cr2O7, CaO2, BaO2, Na2O 2, or Na2B2O4(OH)4).98−104 With regard to bicomponent mastics, MnO2 is most commonly used for providing light/UV-resistant −S−S− cross-linked polymer materials with improved mechanical properties.100−101 Although less effective than PbO2 or Na2Cr2O7, MnO2 yet remains cheaper and much more environmentally friendly, and it was thus selected for this study (Scheme S3). To promote the cross-linking, addition of MnO2 (8 wt %−as based on the standard industrial preliminary evaluation procedure) to the PPG/PTG-DTC2 and Lupasol/EDR-148 mixture gave the corresponding −S−S− cross-linked PPG/ PTG NIPTUs (Scheme 1 and Table 1).93 Raman monitoring of the reaction clearly showed the complete disappearance of −SH groups (no signal at 2574 cm−1) while S−S disulfide bridges appeared at 504 cm−1 (Figure 5). The gel point determined by rheology was significantly shorter than the one measured in the absence of MnO2 (80 ± 7 min (PPG) and 3 ± 0.3 min (PTG) vs 143 ± 70 min and 18 ± 6 min, respectively; Table 1), evidencing, as expected, a more favorable sol−gel transition in −S−S− cross-linked NIPTU materials. However, the latter were recovered as black materials (versus yellow without the use of MnO2), thereby possibly limiting their industrial applications. Other cross-linking agents such as di-/ triacrylates were therefore evaluated. Synthesis and Monitoring of the Characteristics of Cross-Linked PPG/PTG NIPTUs as Promoted by Di-/ Triacrylates. The Michaël addition of thiol functions onto dior triacrylates23,88,105 has been exploited as another approach to promote the formation of cross-linked PPG/PTG-based NIPTUs. Tri(propylene glycol) diacrylate (TPGDA) and trimethylolpropane triacrylate (TMPTA) were selected for this purpose. These di- and trifunctional acrylate monomers are valuable for their low volatility and fast curing response and are commonly used in the manufacture of plastics, adhesives,
acrylic glue, and anaerobic sealants. Given that the thiol/ acrylate Michaël addition proceeds faster than the primary amine/acrylate reaction (Scheme S4),23,88,105 the di- and triacrylates were introduced either in stoichiometric amount (acrylate/DTC/NH2 = 1:1:1) or in default (acrylate/DTC/ NH2 = 0.1:1:1) in the PPG/PTG-DTC2/amine(s) mixture (Scheme S5 and Table 2). The formation of the NIPTU materials was monitored at room temperature by FTIR spectroscopy on 20 different formulations, and the sol−gel transition was assessed by rheology. The recovered materials revealed insoluble in common organic solvents, precluding NMR or SEC investigations, and were analyzed by DSC as well as by tensile and oscillation rheology tests.93,106 Both the di- and triacrylate additives were found to significantly affect the gel point. The gel point was observed within 3 min for the Lupasol/EDR-148 mixture, regardless of the polyether or di/triacrylate (Table 2, formulation nos. 3−5, 12−15, 18, and 20). FTIR monitoring of, for instance, the PPG NIPTU formation showed the disappearance of the υCS at 1192 cm−1 followed by a plateau beyond 3 min, consistent with gelification of the reaction medium, as illustrated in Figure 6 (Table 2, formulation nos. 3 and 5). As also visually observed, the reaction did not proceed much further, and large amounts (ca. 70−90%) of unreacted PPG-DTC2 remained in the gel at 3 min. After 30 min, the typical amount of remaining unreacted polyether-DTC2 measured within the mixtures of PPG/PTG-DTC2/Lupasol-EDR-148/TPGDA,TMPTA featuring a reasonable gel point (i.e. < 3 min), amounted to ca. 30% and ca. 90% (Table 2, formulation nos. 3 and 5; Figure 6), suggesting the better efficiency of the triacrylate vs diacrylate to promote an effective cross-linking of the NIPTU materials. As expected, the higher the functionality of the acrylate reagents, the lower their conversion had to be to reach the sol−gel transition (e.g., αcalc,gel#3 = 0.28, α calc,gel#5 = 0.20, that is, a theoretical conversion in PPG-DTC2 of 28% and 20%, respectively; formulation nos. 3 and 5). Also, in combination with EDR-148, a too low amount of acrylate (≤0.1 equiv) did not enable to reach the gel point within 2 h, regardless of the polyether (formulation nos. 6, 7, 9, 16, and 17). Although the thiol−di-/triacrylate reaction proceeded successfully from the PPG/PTG NIPTUs, all these formulations yet revealed G
DOI: 10.1021/acs.macromol.9b00695 Macromolecules XXXX, XXX, XXX−XXX
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Figure 6. FTIR monitoring of the evolution of the υCS band at 1190 cm−1 during the room temperature (23 °C) reaction of PPG-DTC2/ Lupasol-EDR-148/TMPTA (Table 2, formulations #3, #5 over 30 min (left) ; zoom from 0 to 5 minutes (right).
Table 2. Characteristics of the Cross-Linked PPG/PTG NIPTUs Prepared from the Neat Polyaddition of PPG/PTG-DTC2 with Amine(s) in the Presence of a Diacrylate (TPGDA) or Triacrylate (TMPTA) at Room Temperature without Any Solvent (Scheme 1) Lupasol/EDR-148
PPG-DTC2
PTG-DTC2
EDR-148
acrylate
−
(equiv)
0
0.1
1
0.1
1
0
0.1
1
0.1
1
formulation no. gel pointa (αgel) (min) Tgb (°C) formulation no. gel pointa (αgel) (min) Tgb (°C)
#1 143 −37 #11c 18 −25
#2 46 −35 #12
