Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Structure−Property Relations in Oligomers of Linear Aliphatic Diisocyanates Max Widemann,† Piet J. Driest,‡ Patrizio Orecchia,§ Frederik Naline,∥ Florian E. Golling,‡ Andreas Hecking,‡ Christoph Eggert,‡ Raul Pires,‡ Karsten Danielmeier,‡ and Frank U. Richter*,‡ †
Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany Covestro Deutschland AG, Kaiser Wilhelm Allee 60, 51365 Leverkusen, Germany § Technische Universität Berlin, Straße des 17. Juni 115, 10623 Berlin, Germany ∥ Cesson, France
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‡
S Supporting Information *
ABSTRACT: Oligomerization reactions of aliphatic diisocyanates, exclusively involving NCO groups are discussed. The reactivity and selectivity of these reactions are dependent on chain length, reaction conditions, and the catalyst employed. The resulting oligomers (“dimers” and “trimers”) sometimes deviate considerably from expectation with respect to the structure and properties. The synthesis and characterization of well-known derivatives such as uretdiones and isocyanurates are discussed, and special attention is devoted to the recently developed iminooxadiazinedione chemistry. Finally, a new class of compounds is introduced, the tricyclic diiminooxadiazinones. KEYWORDS: Polyisocyanates, Desmodur eco N, Isocyanurate, Iminooxadiazindione, Uretdione, Diiminooxadiazinone
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INTRODUCTION Polyurethanes (PU) are specialty polymers that can be tailor made for a large variety of different applications by selection of the appropriate building blocks: a more or less limited series of di- or polyisocyanates on the one side and largely varying types of reaction partners containing two or more X-H groups (X = O, S, NR) on the other side.1−3 For coatings, adhesives, and sealants, aliphatic polyisocyanates are often preferred over the aromatic ones, and the use of monomeric diisocyanates is rather the exemption. Higher molecular weight polyisocyanates with very low vapor pressure and thus a favorable ecological profile are used in the vast majority of applications.4 The term “polyisocyanate” is used in this paper for all products deriving from monomeric diisocyanates, OCN-R-NCO, R = alkylene, possessing at least twice the molecular weight of the corresponding monomer. For a very prominent class of polyisocyanates, the so-called “trimers”, see Figure 1.5 The market success of aliphatic polyurethanes is founded on the excellent durability of the coated material. Beyond that, the current focus lies not only on the general increase in lifetime and improved properties of a coating but also sustainability aspects inherent to the starting materials. The same holds true for properties of the final products relevant under ecological aspects such as carbon footprint, very low monomer content, and robustness to monomer formation upon storage.6,7 Hence, a more holistic approach is taken by looking at PU raw materials from a cradle-to-grave point of view, considering all © XXXX American Chemical Society
Figure 1. Diisocyanate trimerization in commercial products, the “real” trimer content (tris-isocyanatoalkyl-isocyanurate, n = 1) typically varies between less than 20% and more than 80% in the mixture of higher molecular weight oligomers (n > 1).
aspects relevant under those terms. Quite recently, this has led to the development of “trimers” made from biobased pentamethylene diisocyanate (PDI) furnishing comparable performance and stability to respective products derived from petrochemical-based hexamethylene diisocyanate (HDI).8,9 While no biobased HDI exists at present, the shorter-chained tetramethylene and trimethylene diisocyanates (BDI and GDI, respectively, abbreviations chosen with reference to the “butylene” and “glutaryl” backbone) may be available in the future from renewable resources, either from the corresponding amino acids (lysine → PDI; ornithine → BDI) or C3feedstock chemicals such as glycerine or glutaric acid, respectively. 1,3-Disocyanato-3-methylbutane (MBDI) was Received: February 20, 2018 Revised: June 24, 2018 Published: July 6, 2018 A
DOI: 10.1021/acssuschemeng.8b00758 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
a total of 37.8 g (0.32 mol) of trimethylsilyl azide (TMS-N3). After onset of the reaction, indicated by gas evolution and a temperature increase to boiling, external heating is no longer necessary, and the addition of TMS-N3 is adjusted to a rate that gentle reflux is maintained. Caution: it is essential to keep the internal temperature above 45 °C in order to avoid accumulation of hazardous concentrations of the explosive intermediate acid azide(s)! After complete addition of TMSN3 (2−3 h), the mixture is stirred for another 4 h at a 60 °C oil-bath temperature until no more gas evolution is observed. Thereafter, at 780 mbar, MTBE, TMS-Cl, and excess TMS-N3 are distilled off, and 75−100 g of diphenyl ether (Ph2O) was added simultaneously, dropwise to the distillation vessel. After complete removal of the lowboiling components, a fast trap-to-trap distillation at 3 mbar and ca. 110 °C oil-bath temperature (head temperature ca. 85 °C) follows. Prior to this fast trap-to-trap distillation, the collector was charged with a few milliliters of Ph2O in order to prevent spontaneous polymerization of the GDI obtained. In a typical run, 80.3 g of a colorless liquid containing 23.3% GDI in Ph2O was obtained (0.15 mol, 96% yield). The reaction product was analyzed by 1H NMR, 13C NMR. 1H NMR (signals for Ph2O omitted): δ [ppm] 0.77 (quin, 2H, 3JHH = 6.4 Hz, CH2CH2NCO), 2.41 (t, 4H, 3JHH = 6.4 Hz, CH2NCO). 13C{1H}-NMR: δ [ppm] 31.7 (CH2CH2NCO), 39.7 (CH2NCO), 122.8 (NCO). Upon storage at room temperature, yellowing occurs over time, and all further experiments with 4 have been executed with freshly redistilled, colorless GDI/Ph2O solutions uniformly adjusted to 20 wt % of 4. Depending on storage time, varying amounts of polymeric material12,13 remain as distillation residue. 1,3-Diisocyanato-3-methylbutane (MBDI) 5. Here, 100 g (0.62 mol) of 2,2-dimethylglutaric acid were placed in a two-necked roundbottomed flask equipped with a magnetic stirring bar, a dropping funnel, and a reflux condenser connected to a stirred flask containing 150 mL of a 50% solution of NaOH in water for HCl neutralization. Then, 298 g of SOCl2 (2.50 mol) were slowly added under vigorous stirring. After complete addition, the mixture was stirred at 100 °C in an oil-bath and left to react for 20 h. The reflux condenser was replaced by a distillation bridge, and excess SOCl2 was distilled off at 150 °C under normal pressure (head temperature ∼80 °C). Subsequently, the mixture was heated to 120 °C at about 0.5 mbar, and the product was trap-to-trap distilled and the distillate fractionated again through a 40 cm Vigreux column at about 0.5 mbar furnishing 98.9 g (0.50 mol, 80% yield) of 2,2-dimethylglutaryldichloride (boiling point 105 °C at 0.5 mbar). The dichloride was converted to the diisocyanate as described above for GDI with the exemption that no Ph2O addition was necessary in the distillation step, and the product is obtained as a colorless liquid with unlimited storage stability in the absence of moisture at room temperature (boiling point 88 °C at 0.5 mbar). The reaction product was analyzed by 1H NMR, 13C NMR. 1H NMR: δ [ppm] 0.71 (s, 6H, CH3), 0.98 (m, 2H, CH2CH2NCO), 2.58 (m, 2H, CH2NCO). 13C{1H}-NMR: δ [ppm] 29.8 (CH3), 38.7 (CH2NCO), 43.8 (OCNCH2CH2), 56.4 (C(CH3)2), 122.7 (CH2NCO), 123.7 (C(CH3)2NCO). Synthesis and Characterization of Oligomers of linear aliphatic diisocyanates. Bis(6-isocyanatohexyl) uretdione 6a. The lowest molecular weight representative of HDI-uretdione 6a (as drawn in Figure 2) was isolated from the commercially available HDI-
chosen as a model compound for this study containing two nonequivalent isocyanate groups.
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EXPERIMENTAL SECTION
Materials and Methods. All percentages, unless noted otherwise, are to be understood to mean percent by weight. Mol% figures were determined by 1H NMR spectroscopy and always relate, unless specified otherwise, to the sum total of the NCO conversion products. The measurements were determined on the Bruker DPX 400 or DRX 700 instruments on about 5% (1H NMR) or about 50% (13C NMR) samples in dry C6D6, unless noted otherwise, at a frequency of 400 or 700 MHz (1H NMR) or 100 or 176 MHz (13C NMR). The reference employed for the ppm scale was small amounts of tetramethylsilane in the solvent with a 1H NMR chemical shift 0 ppm. Alternatively, the C6D5H present in the solvent was used as the reference signal: 1H NMR chemical shift 7.15 ppm, 13C NMR chemical shift 128.0 ppm. 15N NMR was referenced externally to liquid ammonia (0 ppm). Dynamic viscosities were determined at 23 °C using the MCR 501rheometer (Anton Paar) in accordance with DIN EN ISO 3219:199410. By measurements at different shear rates, it was ensured that the flow behavior of the polyisocyanate mixtures discussed here corresponds to that of ideal Newtonian fluids. The shear rate data can therefore be omitted. The NCO content was determined by titration in accordance with DIN EN ISO 10283:2007−11. The residual monomer contents were determined by gas chromatography in accordance with DIN EN ISO 10283 using an internal standard. GC-MS was performed using an Agilent GC6890, equipped with a MN 725825.30 Optima-5-MS-Accent capillary column (30 m, 0.25 mm i.d., 0.5 μm film thickness) and a mass spectrometer 5973 as detector. The carrier gas was helium, at a flow rate of 2 mL/min. Column temperature was initially 60 °C for 2 min, then gradually increased to 360 °C at 8 °C/min. For GC-MS detection, an electron ionization system was used with an ionization energy of 70 eV. Injector temperature were set at 250 °C. Size exclusion chromatography (SEC) was performed with tetrahydrofuran as the elution solvent according to DIN 556721:2016-03. X-ray structure elucidation was performed on an Oxford diffraction Xcalibur equipped with a CCD-area detector (Model Ruby), Cu Kα source, and Osmic mirrors as a monochromator at 106−107 K. The program CrysAlis Version 1.171.38.43 (Rigaku 2015) was used for data collection and reduction. SHELXTL version 6.14 (Bruker AXS, 2003) was used for the structure solution. Details of the structure refinement and solution were placed in the Supporting Informations. All reactions were conducted under a nitrogen atmosphere unless stated otherwise using standard Schlenk-line techniques. Solvents were always refluxed over diphenylmethane diisocyanate (∼1%) and freshly distilled prior to use. The diisocyanates 1 and 2 (HDI and PDI, respectively), Desmodur N3400, and Desmodur N3900 are products of Covestro AG, D-51365 Leverkusen. Monomeric diisocyanates were vacuum distilled prior to use. 4 and 5, although also accessible by gas-phase phosgenation,11 were made for this study by the Curtius rearrangement of the corresponding acid dichlorides as described below. As GDI obtained by this method is not long-term storage stable as a neat liquid, solutions in diphenyl ether have been used which were always freshly redistilled prior to use. MBDI can be stored as a neat liquid. All other chemicals have been purchased from Sigma-Aldrich. Synthesis and Charaterization of Linear Aliphatic Diisocyanates. Trimethylene Diisocyanate (GDI) 4. To a magnetically stirred solution of 26.2 g (0.16 mol) of glutaric acid dichloride (97%; distilled twice prior to use through a 40 cm Vigreux column in order to obtain a colorless liquid) in ca. 45 mL of methyl-tert-butylether (MTBE) placed in a three-necked round-bottomed flask equipped with a thermocouple, reflux condenser and dropping funnel was added dropwise at 50 °C internal temperature (external heating by oil-bath)
Figure 2. Lowest molecular weight representatives of typical polyisocyanates discussed in this paper: (a) R = −(CH2)6NCO for HDI derivatives and (b) R= −(CH2)5NCO for PDI derivatives. B
DOI: 10.1021/acssuschemeng.8b00758 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Atom numbering according to Figure 7: 1H NMR δ [ppm]: 1.18 (s, 6H, C(7)H3/C(8)H3), 1.37 (s, 6H, C(13)H3/C(14)H3), 1.69 (m, 2H, C(2)H2), 1.76 (m, 2H, C(10)H2), 3.76 (m, 2H, C(3)H2), 3.88 (m, 2H, C(9)H2). 13C{1H}-NMR δ [ppm]: 29.3 (C7/8), 30.1 (C13/ 14), 32.0 (C2), 39.2 (C3), 39.7 (C9), 40.1 (C10), 52.3 (C1), 56.6 (C11), 122.6 (C12), 137.8 (C6), 144.8 (C5), 147.0 (C4). 15N [ppm] taken from15N−1H-HMBC-NMR: δ [ppm] 52.4 (N4), 112.9 (N2), 130.5 (N3), 216.6 (N1). 3,4,9,10-Tetrahydro-2H,6H,8H-dipyrimido[2,1-b:1′,2′-e][1,3,5]oxadiazine-6-one 11 (Decarboxylation Product of 9). Compound 11 was obtained by heating 1,8 g of 9 to 220 °C, then gradually increasing to 250 °C until CO2 evolution ceased. The resulting residue was recrystallized from methylene chloride/n-hexane (mp 178 °C) and analyzed by 1H NMR, 13C NMR, and X-ray diffraction. The single crystal used for X-ray analysis was obtained as described above for 10. Atom numbering according to Figure 8: 1H NMR: δ [ppm] 1.06 (tt, 4H, 3JHH = 5.9 Hz, C(3)H2/C(8)H2), 3.12 (t, 4H, 3JHH = 5.7 Hz, C(4)H2/C(7)H2), 3.18 (t, 4H, 3JHH = 6.0 Hz, C(2)H2/C(9)H2). 13 C{1H}-NMR: δ [ppm] 20.6 (C3/8), 41.2 (C2/9), 43.9 (C4/7), 140.7 (C5/6), 146.4 (C1). X-ray Data. X-ray data of 10a and 11, SEC for 6a, 7a/8a, 6b, 7b/ 8b, and NMR spectra of 4, 5, 6a, 7a/8a, 6b, 7b/8b, 9, 10, and 11, as well as GC-MS of crude reaction mixtures of 4 → 9 and 5 → 10a are provided in the Supporting Information.
uretdione mixture Desmodur N 3400 by repeated thin-film distillation in high vacuum, as described in ref 8 and ref 14, examples 3 and 6. The product was analyzed by SEC, 1H NMR, titration, and rheology. SEC: ∼98% purity (area-% of main signal), ∼1.5% 7a/8a, 0.1% 1, others. 1H NMR: ∼99 mol %, 0.5 mol % isocyanurates, 0.2 mol % iminooxadiazinediones, others. NCO content 24.8%, viscosity 28 mPas. Tris(6-isocyanatohexyl) iminooxadiazinedione/isocyanurate mixture 7a/8a. The lowest molecular weight representative admixture of HDI−trimers 7a/8a (as drawn in Figure 2) was isolated as described for 6a starting from the commercially available HDI(iminooxadiazinedione/isocyanurate) mixture Desmodur N 3900. The product was analyzed by SEC, 1H NMR, titration, and rheology. SEC: ∼95% purity (area-% of slightly broadened main signal, 7a and 8a almost coelute), ∼0.3% HDI-pentamers, ∼2.4% 6a, others. 1H NMR: ∼40 mol % iminooxadiazinediones, ∼57 mol % isocyanurates, and ∼3 mol % uretdiones. NCO content 24.6%, viscosity 430 mPas. Bis(5-isocyanatopentyl) uretdione 6b. The lowest molecular weight representative of PDI-uretdione 6b (as drawn in Figure 2) was isolated as described for 6a starting from a distillation residue of PDI production. The product was analyzed by SEC, 1H NMR, titration, and rheology. SEC: ∼99% purity (area-% of main signal), ∼0.9% 7b/8b, 0.1% 2, others. 1H NMR: ∼98 mol %; ∼1 mol % isocyanurates, others. NCO content 27.0%, viscosity 25 mPas. Tris(5-isocyanatopentyl) iminooxadiazinedione/isocyanurate mixture 7b/8b. A mixture of PDI-(iminooxadiazinedione/ isocyanurate)s was synthesized by an “asymmetrical trimerization” reaction starting from 2, analogous to the commercial production of the HDI analogue Desmodur N 3900, by methods described in ref 14. From the obtained reaction mixture, the lowest molecular weight representative admixture 7b/8b (as drawn in Figure 2) was isolated as described for 6a. The product was analyzed by SEC, 1H NMR, titration, and rheology. SEC: ∼98.6% purity (area-% of slightly broadened main signal, 7b and 8b almost coelute), ∼1% PDIpentamers, ∼2.3% 6b, 0.1% 2, others. 1H NMR: ∼39 mol % iminooxadiazinediones, ∼56 mol % isocyanurates, and ∼4 mol % uretdiones, others. NCO content 26.8%, viscosity 556 mPas. 3-(3-Isocyanatopropyl)-7,8-dihydro-2H,6H-pyrimido[2,1-b][1,3,5]oxadiazine-2,4(3H)-dione 9 (Bicyclic Iminooxadiazinedione of 4). Ten grams of a solution of 4 in Ph2O (∼16 mmol 4) was stirred at 120 °C, and 0.32 g of tri-n-butylphosphine was added. Stirring was continued for another 17 h at 120 °C, and all volatiles were removed at reduced pressure (p ∼ 1 × 10−1 mbar,
