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“Click” Chemistry As a Promising Tool for Side-Chain Functionalization Of ohio2/yma-yma/yma-yma/yma99907/yma4137d07z xppws 23:ver.6 5/20/08 15:15 Msc: ma-2008-00189z TEID: kbg00 BATID: ma7b03 Macromolecules XXXX, xx, 000 A 1 “Click” Chemistry as a Promising Tool for Side-Chain 2 Functionalization of Polyurethanes 3 David Fournier and Filip Du Prez* 4 Department of Organic Chemistry, Polymer Chemistry Research Group, Ghent UniVersity, Krijgslaan 5 281, S4-bis, B-9000 Ghent, Belgium 6 ReceiVed January 26, 2008; ReVised Manuscript ReceiVed May 1, 2008 7 ABSTRACT: Linear polyurethanes (PUs) having alkyne groups located along the backbone have been synthesized 8 by reacting two different alkyne-functionalized diols with a diisocyanate compound. 1H NMR and FTIR proved 9 the presence of these functional groups and the ability for the introduction of an elevated and controllable amount 10 of functional groups that do not interfere with the PU chemistry. TGA measurements demonstrated that the 11 incorporated alkyne diol in the PU materials strongly improves the final char yield. In the second part of the 12 work, the copper catalyzed Huisgen 1,3-dipolar cycloaddition was undertaken between the alkyne-functionalized 13 PUs and a variety of azide compounds such as benzyl azide and different fluorinated azide compounds, resulting 14 in side-chain functionalized PUs with varying degree of functionalization. 1H NMR spectra clearly indicated the 15 quantitative yields of the “click” reaction. 16 Introduction incorporate different functional groups such as sulfonate or 57 10 amine. 58 17 Since polyurethanes (PUs) have been discovered by Bayer 1 18 in the 40s, many academic and industrial research groups from Second, the functional group can be directly introduced during 59 2 19 all over the world have enlarged this field. PUs are mostly, the process (one-pot procedure) by using a functional building 60 20 but not only, based on the reaction between diisocyanatates, block. Nevertheless, careful attention should be paid toward the 61 21 diols and polyols. The wide range of starting compounds leads inertness of the introduced functional groups during the poly- 62 22 to the synthesis of materials with different and unique properties. urethane process to avoid secondary reactions, making the 63 23 The targeted applications are numerous such as in the field of materials unusable for the desired final applications. For 64 24 automobile, medicine, comfort, buildings, paintings, coatings, instance, the introduction of amino groups into PU requires 65 3 25 adhesives and packaging. Additionally, the chemistry of protection/deprotection steps. Recently, the preparation of PUs 66 26 polyurethanes allows for the synthesis of foams (flexible and bearing pendant amino groups starting from HDI, poly(- 67 27 rigid) or thermoplastic polymers, depending on their synthetic caprolactone) (PCL), and a modified poly(ethylene glycol) 68 11 28 conditions. Nowadays, thermoplastic PUs have found a growing (PEG) has been described. The approach consists in the 69 29 interest since the mechanical, thermal and chemical properties reaction of dihydroxyl PEG with NH2-protected aspartic acid 70 30 can be tailored by the choice and composition of the starting (Asp) leading to a prepolymer PEG-Asp-PEG. Then, the 71 31 compounds. Actually, much research is focused on the degrad- consequent reaction between HDI, PCL and the amino-protected 72 32 ability and the recycling of such materials due to their intense PEG-Asp-PEG allowed for the formation of PU with a low 73 4 33 production. More recently, much attention is shown for the loading of pendant amino groups after an ultimate deprotection 74 12 34 development of functional polyurethanes because it is expected step. At the same time, Endo et al. prepared PU bearing 75 35 that they lead to applications that are outside of the PU market, hydroxyl groups in the side chain (PHUs) by reacting a 76 36 thus giving rise to additional functionality to the end materials. bifunctional cyclic carbonate with a diamine, after which the 77 37 To introduce functionalities within the PU materials, few hydroxyl groups were converted into functionalized urethane 78 38 possibilities can be considered. One route to obtain functional groups with the help of functionalized isocyanates. In medicine, 79 39 polyurethanes is the use of monofunctional compounds (alcohol functional PUs are widely studied since it was established that 80 40 or isocyanate), which lead to polymer chain terminations with PU shows relatively good in Vitro blood compatibility in 81 13 41 the functional group at the chain end and to a reduced molecular comparison to other polymeric films. For example, Kim et 82 14 42 weight. The end-chain modification of the PU can also be al. synthesized heparin-immobilized polyetherurethanes con- 83 43 performed after its formation. For instance, hydroxyethyl taining pendant ester groups from a NCO-prepolymer and 84 44 (meth)acrylate has been employed for the modification of a diethyl bis(hydroxymethyl)malonate. The obtained ester-func- 85 45 NCO-functionalized polyurethane leading to UV-cross-linkable tionalized PU was then chemically modified to obtain either 86 5,6 46 polyurethane (meth)acrylate (PUA). The UV-cured PUA heparin or PEG-heparin as side groups. Another way that was 87 47 coatings received considerable attention for their resistance to largely explored consists of the functionalization of PUs by 88 7,8 48 accelerated weathering or exposure to light for instance. A hydrogen removal of the urethane group (i.e., NH) of the 89 49 similar strategy led to the elaboration of amino-functionalized material using for example, a drastic treatment based on sodium 90 15–19 50 PUs starting from the reaction between an excess of hexam- hydride (NaH). In general, the chemical modification of 91 9 51 ethylene diisocyanate (HDI) and propylene glycol. The isocy- such functional polymers can also suffer of a certain lack of 92 52 anate functions in the PU, located at the end-chains, were efficiency since the reactivity of functional groups may be 93 53 successively hydrolyzed with triphenylsilanol and water. These affected by the structure of the polymer and also by the 94 54 so-obtained amino-functionalized PUs may be afterward used efficiency of the chemical reactions used. For the past few years, 95 9 55 for coatings, adhesives, sealants. Surface modification of this efficiency was intensively studied in the area of polymer 96 20 56 isocyanate-functionalized PUs was also reported in order to chemistry since, in 2001, Sharpless and co-workers introduced 97 innovative approaches, named “click” chemistry, allowing 98 * Corresponding author. E-mail: fi[email protected]. quantitative reactions. Among the listed reactions, Huisgen 1,3- 99 10.1021/ma800189z CCC: $40.75 XXXX American Chemical Society PAGE EST: 8.4 ohio2/yma-yma/yma-yma/yma99907/yma4137d07z xppws 23:ver.6 5/20/08 15:15 Msc: ma-2008-00189z TEID: kbg00 BATID: ma7b03 B Fournier and Du Prez Macromolecules, Vol. xx, No. x, XXXX 100 dipolar cycloadditions between an azide and an alkyne com- and the reaction mixture was allowed to stir at 60 °C for 1 day. 168 101 pound have been widely explored due to, among others, its The mixture was then cooled to room temperature and a minimum 169 102 efficiency, versatility and inertness toward other functional of 5 extractions against ether was done. The combined ether layers 170 103 groups. The use of a copper catalyst leads to a tremendous were dried with magnesium sulfate and the solvent removed in 171 21,22 104 acceleration of the reaction at room temperature. Since the vacuum, yielding a yellowish powder that was further dried under 172 ) 1 105 first report of “click” chemistry in polymer chemistry by vacuum overnight (yield 95.0%). H NMR (300 MHz, CDCl3): 173 23 ) - - 106 Hawker, Sharpless and co-workers, the construction of well- δ 3.57 ppm (t, 2H, CH2 CH2 N3), 3.87 ppm (t, 2H, 174 - - - 175 107 V CH2 CH2 N3), 7.71 ppm (dd, 2H, Ar H), 7.84 ppm (dd, 2H, defined and complex macromolecular architectures ia “click” -1 -1 24–27 Ar-H). FTIR (ATR): ν(Ν ) ) 2111 cm , ν(CdO) ) 1710 cm . 176 108 chemistry has been a strongly growing field of research. 3 28 Synthesis of Zonyl-N . Zonyl-N was synthesized in two steps 177 109 Relating to the PU field, Qin et al. very recently reported the 3 3 from hydroxyl chain terminated fluorinated surfactant Zonyl-FSO- 178 110 elaboration of polyurethanes using the combination of 2,4- 100 (Mw ∼725 g/mol, with (CF2)n, n ∼ 8 and (CH2CH2O)x, x ∼ 8 179 111 toluene diisocyanate (TDI) and chromophore-functionalized 33,34 from 19F and 1H NMR analysis). In a 250 mL round-bottom 180 112 diols. Nevertheless, the authors first synthesized these diols by - flask, Zonyl (20 g, 1 equiv) was dissolved in freshly distilled THF 181 113 the 1,3-dipolar azide alkyne cycloaddition prior to their (100 mL). Then, freshly distilled triethylamine (4.22 mL, 1.1 equiv) 182 114 incorporation in the PU. These polymers were studied as organic was added and the temperature fixed at 0 °C. Methane sulfonyl 183 115 second-order nonlinear materials for their further application chloride was introduced dropwise (3.65 g, 1.15 equiv). After the 184 116 in the field of high-speed electro-optic devices. addition, the reaction mixture was allowed to stir at room 185 117 The aim of the present work was to develop an efficient way temperature overnight. Triethylamine salt was filtered off and the 186 118 for the elaboration of alkyne-containing PUs by the introduction filtrate was evaporated under vacuum. This intermediate compound 187 119 of alkyne diols during the PU process. Because of the resistance was not further purified and in a second step, it was dissolved in 188 120 of the alkyne group toward the usual reaction conditions and ethanol (100 mL) containing sodium azide (2.69 g, 1.5 equiv).
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