Tunable Polymers Obtained from Passerini Multicomponent Reaction

Jul 15, 2013 - Subsequent free radical polymerization yielded polyacrylates with tunable properties depending on the used components for the Passerini...
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Tunable Polymers Obtained from Passerini Multicomponent Reaction Derived Acrylate Monomers Ansgar Sehlinger, Oliver Kreye, and Michael A. R. Meier* Laboratory of Applied Chemistry, Institute of Organic Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany S Supporting Information *

ABSTRACT: A new strategy to obtain functionalized acrylate monomers is introduced using the Passerini three-component reaction (Passerini-3CR). This straightforward one-pot synthesis is characterized by excellent atom economy and structurally diverse products. By using acrylic acid and a variety of aldehydes and isocyanides, a set of several acrylate monomers was synthesized. Subsequent free radical polymerization yielded polyacrylates with tunable properties depending on the used components for the Passerini reaction. For instance, by varying the aldehyde component from acetaldehyde to heptanal, control over the glass transition in the final polymer was achieved. Moreover, for highly polar acrylate monomers a thermoresponsive behavior (upper critical solution temperature; UCST) was observed in methanol and/or ethanol.

1. INTRODUCTION Multicomponent reactions (MCRs) are useful tools for the total synthesis of natural products and are often applied in combinatorial and medicinal chemistry.1 Some of the most popular MCRs are the isocyanide-based multicomponent reactions (IMCRs), especially the Ugi four-component reaction (Ugi-4CR) and the Passerini three-component reaction (Passerini-3CR).2 The Passerini-3CR, discovered in 1921, involves the addition reaction of an isocyanide (isonitrile), a carboxylic acid, and an oxo component (aldehyde or ketone), yielding, in a one-pot reaction, an α-acyloxycarboxamide.3 Worth mentioning is a 100% atom economy of this reaction.4 In combination with its straightforward application in the lab and the often observed high yields, the Passerini reaction can also be a valuable tool for green chemistry.5 In addition, the reaction can be performed in aqueous medium and is sometimes even accelerated in such an environment.6 The use of multicomponent reactions has only recently been transferred to the field of polymer chemistry. A first example was described in 2003 by Wright et al., who synthesized a set of Ugi-4CR monomers containing norbornenyl moieties that were polymerized via ring-opening metathesis polymerizations (ROMP).7 In 2011, we prepared fatty acid derived α,ω-diene monomers via the Passerini-3CR8 and polymerized them applying the acyclic diene metathesis (ADMET) polymerization.9 In addition, we demonstrated that the Passerini-3CR is also an efficient polymerization method.8 Afterward, Li et al. described a similar approach to obtain structural diverse poly(ester amide)s via Passerini-3CR by polymerizing diacids and diisocyanides as bifunctional monomers with different aldehydes.10 Furthermore, they introduced functional side groups to the Passerini products, which provided a platform for postpolymerization modifications or precursors for polymerization.11 Besides, the same research group showed © 2013 American Chemical Society

sequence control in the synthesis of poly(ester amide) segments by stepwise application of the Passerini-3CR.12 Recently, we demonstrated that the Ugi-4CR is also a very suitable method for the synthesis of highly diverse α-aminoacyl amide monomers bearing α,ω-diene moieties.13 Structurally diverse polyamides were obtained from these α,ω-diene monomers by ADMET polymerization9 and also applying the thiol−ene addition polymerization.14 Another monomer synthesis of hemilactides was realized by the Passerini-3CR.15 These substrates have been incorporated into poly(α-hydroxy acid) copolymers via ring-opening polymerization (ROP). Moreover, IMCRs are useful tools for the syntheses of dendrimers. In 2011, Wessjohann et al. showed the divergent synthesis of peptoidic and peptidic dendrimers by multiple iterative Ugi-4CRs and Passerini-3CRs.16 Recently, Rudick et al. demonstrated a convergent approach to achieve dendrimers.17 This group showed that functionalized precursor dendrons can be combined via Passerini-3CR. Moreover, Tang et al. demonstrated the synthesis of functional macromolecules with well-defined structures by a transition-metal-catalyzed three-component polycoupling of alkynes, aldehydes, and amines.18 Furthermore, Choi et al. introduced a coppercatalyzed multicomponent polymerization including diynes, sulfonylazides, and diamines for the synthesis of poly(Nsulfonylamidines).19 The same multicomponent reaction was also shown to be useful in a postpolymerization modification of an acetylene functionalized polymer.20 Furthermore, a palladium-catalyzed multicomponent approach was presented by Arndtsen and co-workers.21 The coupling of imines, diimines, and di(acyl chloride)s yielded in π-conjugated imidazole-based Received: May 30, 2013 Revised: July 4, 2013 Published: July 15, 2013 6031

dx.doi.org/10.1021/ma401125j | Macromolecules 2013, 46, 6031−6037

Macromolecules

Article

1653.4, 1552.4, 1497.5, 1453.2, 1403.8, 1362.1, 1323.9, 1290.4, 1255.5, 1223.6, 1175.5, 1127.9, 1054.7, 1024.9, 982.6, 960.9, 932.3, 920.3, 802.4, 745.3, 731.2, 700.4, 683.1, 665.9 cm−1. Tm = 111 °C (water). 1-(tert-Butylamino)-1-oxooctan-2-yl acrylate 4b. Colorless solid, yield (1.82 g, 68%); Rf = 0.42 (n-hexane/ethyl acetate = 4:1). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 0.83 (t, J = 6.2 Hz, 3 H, CH3), 1.15− 1.40 (m, 8 H, 4 CH2), 1.31 (s, 9 H, 3 CH3), 1.68−1.93 (m, 2 H, CHCH2), 5.09 (t, J = 5.8 Hz, 1 H, OCHCO), 5.82 (br, 1 H, NH), 5.93 (dd, J = 10.4, 1.1 Hz, 1 H, CH2CHCO), 6.19 (dd, J = 17.3, 10.4 Hz, 1 H, CH2CHCO), 6.48 (dd, J = 17.3, 1.1 Hz, 1 H, CH2CHCO). 13C NMR (CDCl3, 75 MHz): δ (ppm) = 14.09, 22.58, 24.68, 28.72, 28.97, 31.65, 31.89, 51.29, 74.51, 127.88, 132.05, 164.88, 168.95. FAB of C15H27NO3 (M + H+ = 270.4). HRMS (FAB) of C15H27NO3 [M + H]+ calcd 270.2069, found 270.2067. IR (ATR) ν = 3307.3, 2957.0, 2926.4, 2858.6, 1728.0, 1659.1, 1619.2, 1552.8, 1454.6, 1402.8, 1363.6, 1260.2, 1220.9, 1187.6, 1120.0, 1076.5, 1055.6, 989.7, 963.3, 932.5, 813.4, 724.9, 648.8, 481.5 cm−1. Tm = 54 °C (water). 1-(tert-Butylamino)-3-methyl-1-oxobutan-2-yl acrylate 4c. Colorless solid, yield (1.69 g, 74%); Rf = 0.49 (n-hexane/ethyl acetate = 4:1). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 0.94 (d, J = 6.9 Hz, 6 H, 2 CH3), 1.34 (s, 9 H, 3 CH3), 2.25−2.40 (m, 1 H, CH(CH3)2), 5.02 (d, J = 4.3 Hz, 1 H, OCHCO), 5.75 (br, 1 H, NH), 5.94 (dd, J = 10.4, 1.2 Hz 1 H, CH2CHCO), 6.20 (dd, J = 17.3, 10.4 Hz, 1 H, CH2CHCO), 6.49 (dd, J = 17.3, 1.2 Hz, 1 H, CH2CHCO). 13C NMR (CDCl3, 75 MHz): δ (ppm) = 16.92, 18.83, 28.74, 30.67, 51.35, 78.40, 127.84, 132.10, 164.98, 168.35. FAB of C12H21NO3 (M + H+ = 228.1). HRMS (FAB) of C12H21NO3 [M + H]+ calcd 228.1600, found 213.1601. IR (ATR) ν = 3273.1, 3086.3, 2966.7, 2934.1, 1720.0, 1656.0, 1560.1, 1455.4, 1406.6, 1390.4, 1363.8, 1294.3, 1266.6, 1224.5, 1193.4, 1126.4, 1110.7, 1059.6, 986.3, 972.3, 941.3, 838.0, 809.8, 757.9, 713.6, 663.3, 582.5, 477.5, 432.0, 408.3 cm−1. Tm = 87 °C (water). 1-(tert-Butylamino)-1-oxobutan-2-yl acrylate 4d. Colorless solid, yield (1.43 g, 67%); Rf = 0.35 (n-hexane/ethyl acetate = 4:1). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 0.92 (t, J = 7.5 Hz, 3 H, CH3CH2), 1.35 (s, 9 H, 3 CH3), 1.83−1.97 (m, 2 H, CH2), 5.10 (t, J = 5.6 Hz, 1 H, OCHCO), 5.84 (br, 1 H, NH), 5.93 (dd, J = 10.4, 1.2 Hz 1 H, CH2CHCO), 6.19 (dd, J = 17.3, 10.4 Hz, 1 H, CH2CHCO), 6.48 (dd, J = 17.3, 1.2 Hz, 1 H, CH2CHCO). 13C NMR (CDCl3, 75 MHz): δ (ppm) = 8.92, 25.05, 28.70, 51.30, 51.38, 127.84, 127.94, 131.54, 132.04, 164.82, 168.53, 168.72, 169.40. FAB of C11H19NO3 (M+ = 213.3). HRMS (FAB) of C11H19NO3 [M]+ calcd 213.1365, found 213.1364. IR (ATR) ν = 3306.9, 3080.3, 2971.7, 2938.0, 1723.2, 1695.5, 1635.5, 1551.2, 1454.9, 1399.5, 1363.0, 1295.9, 1268.8, 1243.9, 1225.0, 1191.4, 1132.8, 1107.2, 1091.2, 1054.9, 989.1, 967.3, 940.2, 905.0, 812.2, 756.0, 675.4, 642.5, 502.5, 477.5, 457.8, 406.6 cm−1. Tm = 92 °C (water). 1-(tert-Butylamino)-1-oxopropan-2-yl acrylate 4e. Colorless solid, yield (1.71 g, 86%); Rf = 0.45 (n-hexane/ethyl = acetate 2:1). 1 H NMR (CDCl3, 300 MHz): δ (ppm) = 1.35 (s, 9 H, 3 CH3), 1.47 (d, J = 6.8 Hz, 3 H, CH3), 5.16 (q, J = 6.8 Hz, 1 H, OCHCO), 5.90 (br, 1 H, NH), 5.93 (dd, J = 10.4, 1.0 Hz, 1 H, CH2CHCO), 6.17 (dd, J = 17.3, 10.4 Hz, 1 H, CH2CHCO), 6.48 (dd, J = 17.3, 1.0 Hz, 1 H, CH2CHCO). 13C NMR (CDCl3, 75 MHz): δ (ppm) = 17.88, 28.73, 51.26, 71.06, 127.90, 132.13, 164.72, 169.48. EI of C10H17NO3 (M+ = 199.1). HRMS (EI) of C10H17NO3 [M]+ calcd 199.1208, found 199.1207. IR (ATR) ν = 3287.3, 3078.2, 2969.1, 1730.5, 1657.4, 1551.2, 1480.2, 1453.6, 1405.1, 1360.1, 1309.3, 1283.8, 1263.5, 1224.8, 1192.8, 1138.6, 1099.4, 1052.2, 1033.7, 985.7, 970.2, 929.2, 901.2, 875.7, 803.6, 702.2, 652.0, 479.2, 453.6, cm−1. Tm = 74 °C (n-hexane/ ethyl acetate). 1-(Cyclohexylamino)-1-oxopropan-2-yl acrylate 4f. Slightly yellow solid, yield (2.24 g, quant); Rf = 0.48 (n-hexane/ethyl acetate = 2:1). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 0.99−1.98 (m, 10 H, 5 CH2), 1.48 (d, J = 6.8 Hz, 3 H, CH3), 3.65−3.84 (m, 1 H, NCH), 5.23 (q, J = 6.8 Hz, 1 H, OCHCO), 5.90−6.04 (m, 1H, NH), 5.91 (dd, J = 10.4, 1.3 Hz, 1 H, CH2CHCO), 6.17 (dd, J = 17.3, 10.4 Hz, 1 H, CH2CHCO), 6.47 (dd, J = 17.3, 1.3 Hz, 1 H, CH2CHCO). 13C NMR (CDCl3, 75 MHz): δ (ppm) = 17.99, 24.83, 25.55, 33.04, 48.00, 70.86, 127.84, 132.22, 164.73, 169.31. FAB of C12H19NO3 (M + H+ = 226.0).

oligomers that exhibit tunable HOMO−LUMO band gaps by modifying the building blocks. Inspired by our own previous results and the recent developments in this developing field, our aim was to apply the highly efficient Passerini-3CR for the synthesis of a set of acrylate monomers. The use of acrylic acid 1 in combination with different isocyanides 2a−e and aldehydes 3a−e yielded acrylate monomers bearing structural diverse amide and α-alkyl moieties. After their synthesis, these acrylate monomers were polymerized in a free radical polymerization,22 and the properties of the resulting new types of polymers were investigated in detail. Besides a possible control of the thermal properties of the synthesized polymers depending on the components used for the MCRs, some polymers showed a thermoresponsive behavior (UCST).

2. EXPERIMENTAL SECTION 2.1. Materials. The following chemicals were used as received: tertbutyl isocyanide 2a (98%, Aldrich), cyclohexyl isocyanide 2b (98%, Aldrich), pentyl isocyanide 2c (98%, Aldrich), benzaldehyde 3a (≥99%, Aldrich), heptaldehyde 3b (95%, Aldrich), isobutyraldehyde 3c (≥98%, Aldrich), propionaldehyde 3d (97%, Aldrich), acetaldehyde 3e (≥99%, Fluka), benzylamine (99%, Aldrich), n-butyl formate (97%, Aldrich), triethylamine (>99%, Aldrich), phosphorus(V) oxychloride (phosphoryl chloride, 99%, Aldrich), 4-aminobutyric acid (>98%, Aldrich), thionyl chloride (>99%, Aldrich), trimethyl orthoformate (99%, Aldrich), diisopropylamine (>99%, Aldrich), sodium bicarbonate (>99%, Aldrich), sodium sulfate (>99%, Acros Organics), sodium carbonate (99%, Aldrich), sodium chloride (>99.5%, Aldrich), silica gel 60 (0.035−0.070, Aldrich), cerium(IV) sulfate (>99%, Aldrich), phosphomolybdic acid (>99%, Aldrich), sulfuric acid (96%, Acros Organics), lithium chloride (>98%, Aldrich), and chloroform-d (CDCl3, 99.8 atom % D, Armar Chemicals). Acrylic acid 1 (99%, Aldrich) was freshly distilled before use in Passerini-3CRs,23 and 2,2′azobis(2-methylpropionitrile) (AIBN, >98%, Aldrich) was recrystallized from methanol. Benzyl isocyanide 2d was synthesized from benzylamine via the standard two-step procedure described by Ugi and co-workers.24 Methyl 4-isocyanobutyrate 2e was synthesized according to a procedure mentioned in several publications.13,16 All solvents were used without any kind of purification. Only water was used in a deionized form. 2.2. General Procedure for Synthesis of Acrylates via Passerini-3CR. Freshly distilled acrylic acid 1 (721 mg, 686 μL, 10.0 mmol), acetaldehyde 3e (440 mg, 560 μL, 10.0 mmol), and 10 mL of dichloromethane were mixed in a round-bottom flask. Subsequently, the appropriate isocyanide 2a−2e (10.0 mmol) was added under stirring. After 24 h of vigorous stirring at room temperature, the reaction mixture was concentrated under reduced pressure. The residue was either ready for polymerization (see 4f and 4g) or further purified by silica gel column chromatography (nhexane/ethyl acetate = 2:1). Alternative Procedure in Deionized Water. Freshly distilled acrylic acid 1 (721 mg, 686 μL, 10.0 mmol), the respective aldehyde 3a−3d (10.0 mmol), and 20 mL of water were mixed in a round-bottom flask. Subsequently, tert-butyl isocyanide 2a (831 mg, 1.13 mL, 10.0 mmol) was added under stirring. After 3 h of vigorous stirring at room temperature, the product was precipitating as a white solid. Direct filtration and high vacuum drying yielded the pure Passerini product. 2-(tert-Butylamino)-2-oxo-1-phenylethyl acrylate 4a. Colorless solid, yield (1.06 g, 41%); Rf = 0.30 (n-hexane/ethyl acetate = 4:1). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 1.36 (s, 9 H, 3 CH3), 5.87−6.00 (m, 1 H, NH), 5.94 (dd, J = 10.3, 1.1 Hz, 1 H, CH2CHCO), 6.05 (s, 1H, OCH), 6.23 (dd, J = 17.3, 10.4 Hz, 1 H, CH2CHCO), 6.51 (dd, J = 17.3, 1.1 Hz, 1 H, CH2CHCO), 7.30−7.52 (m, 5 H, 5 Ar−H). 13C NMR (CDCl3, 75 MHz): δ (ppm) = 28.8, 51.7, 75.8, 127.5, 127.7, 128.8, 129.0, 132.5, 135.9, 164.5, 167.3. FAB of C15H19NO3 (M+ = 261.9). HRMS (FAB) of C15H19NO3 [M + H]+ calcd 262.1443, found 262.1441. IR (ATR) ν = 3288.6, 3070.0, 2974.0, 2548.5, 1724.1, 6032

dx.doi.org/10.1021/ma401125j | Macromolecules 2013, 46, 6031−6037

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HRMS (FAB) of C12H19NO3 [M + H]+ calcd 226.1443, found 226.1441. IR (ATR) ν = 3277.7, 3093.7, 2931.7, 2852.8, 1725.0, 1656.2, 1560.1, 1449.3, 1400.4, 1289.9, 1271.3, 1251.5, 1190.9, 1140.8, 1095.0, 1037.1, 989.8, 968.8, 891.4, 862.3, 811.1, 671.3, 437.6, 401.6 cm−1. Tm = 61 °C (dichloromethane). 1-Oxo-1-(pentylamino)propan-2-yl acrylate 4g. Clear oily product, yield (2.12 g, quant); Rf = 0.44 (n-hexane/ethyl acetate = 2:1). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 0.89 (t, J = 6.7 Hz, 3 H, CH3), 1.19−1.39 (m, 4H, 2 CH2), 1.43−1.57 (m, 2H, CH2), 1.50 (d, J = 6.8 Hz, 3 H, CH3), 3.19−3.32 (m, 2 H, NCH2), 5.29 (q, J = 6.8 Hz, 1 H, OCHCO), 5.93 (dd, J = 10.2, 0.8 Hz, 1 H, CH2CHCO), 6.02−6.14 (m, 1H, NH), 6.18 (dd, J = 17.3, 10.4 Hz, 1 H, CH2CHCO), 6.49 (dd, J = 17.3, 0.9 Hz, 1 H, CH2CHCO). 13C NMR (CDCl3, 75 MHz): δ (ppm) = 13.98, 17.93, 22.32, 28.98, 29.19, 39.27, 70.76, 127.76, 132.18, 164.72, 170.20. EI of C11H19NO3 (M+ = 213.2). HRMS (EI) of C11H19NO3 [M]+ calcd 213.1365, found 213.1363. IR (KBr) ν = 3303.6, 3098.3, 2958.0, 2933.6, 2872.4, 1731.6, 1661.4, 1543.1, 1456.5, 1406.7, 1374.1, 1293.6, 1260.5, 1189.2, 1145.3, 1096.0, 1041.5, 984.0, 895.0, 809.6, 675.5 cm−1. 1-(Benzylamino)-1-oxopropan-2-yl acrylate 4h. Colorless solid, yield (1.73 g, 74%); Rf = 0.24 (n-hexane/ethyl acetate = 2:1). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 1.55 (d, J = 6.8 Hz, 3 H, CH3), 4.49 (d, J = 5.7 Hz, 2 H, NCH2), 5.36 (q, J = 6.8 Hz, 1 H, OCHCO), 5.91 (dd, J = 10.4, 1.1 Hz, 1 H, CH2CHCO), 6.15 (dd, J = 17.3, 10.2 Hz, 1 H, CH2CHCO), 6.29−6.52 (m, 1H, NH), 6.47 (dd, J = 17.3, 0.9 Hz, 1 H, CH2CHCO), 7.14−7.46 (m, 5 H, 5 Ar−H). 13C NMR (CDCl3, 75 MHz): δ (ppm) = 17.75, 42.86, 70.50, 127.29, 127.32, 127.53, 128.50, 131.99, 137.86, 164.70, 170.34. FAB of C13H15NO3 (M + H+ = 234.0). HRMS (FAB) of C13H15NO3 [M + H]+ calcd 234.1128, found 234.1130. IR (ATR) ν = 3263.3, 3101.3, 3032.8, 2981.6, 2918.7, 1718.7, 1658.9, 1617.9, 1569.2, 1495.6, 1453.7, 1434.4, 1407.3, 1355.5, 1324.3, 1272.3, 1253.5, 1191.0, 1142.6, 1096.5, 1080.7, 1065.3, 1051.3, 1031.0, 1000.8, 965.8, 885.0, 853.5, 815.0, 736.6, 698.7, 672.9, 591.2, 513.8, 474.3, 449.5 cm−1. Tm = 67 °C (n-hexane/ethyl acetate). Methyl 4-(2-(acryloyloxy)propanamido)butanoate 4i. Brown liquid, yield (2.27 g, 94%); Rf = 0.17 (n-hexane/ethyl acetate = 2:1). 1 H NMR (CDCl3, 300 MHz): δ (ppm) = 1.43 (d, J = 6.9 Hz, 3 H, CH3),1.70−1.86 (m, 2 H, CH2), 2.31 (t, J = 7.1 Hz, 2 H, COCH2), 3.16−3.31 (m, 2 H, NCH2), 3.60 (s, 3H, COOCH3), 5.19 (q, J = 6.8 Hz, 1 H, OCHCO), 5.87 (dd, J = 10.4, 1.3 Hz, 1 H, CH2CHCO), 6.14 (dd, J = 17.3, 10.4 Hz, 1 H, CH2CHCO), 6.44 (dd, J = 17.3, 1.3 Hz, 1 H, CH2CHCO), 6.49−6.74 (m, 1H, NH). 13C NMR (CDCl3, 75 MHz): δ (ppm) = 17.90, 24.26, 31.44, 38.79, 51.73, 70.59, 127.74, 132.15, 164.73, 170.51, 173.96. FAB of C11H17NO5 (M + H+ = 244.1). HRMS (FAB) of C11H17NO5 [M + H]+ calcd 244.1185, found 244.1183. IR (KBr) ν = 3314.3, 3093.7, 2952.6, 1730.8, 1665.6, 1543.3, 1440.0, 1408.2, 1371.5, 1294.2, 1261.7, 1190.3, 1097.3, 986.1, 875.9, 810.6, 676.1 cm−1. 2.3. General Polymerization Procedure. 2.00 mmol of the corresponding acrylate monomer 4a−4i and 1.0 mol % of AIBN (3.3 mg, 0.02 mmol) were dissolved in ethyl acetate (2.20 mL). Then, the reaction mixture was degassed with argon for 10 min, and afterward the polymerization was performed at 70 °C for 6 h. After this period of time, the solution was slowly dropped into cold diethyl ether. The precipitated polymers were separated by filtration and dried in vacuum to obtain polyacrylates P1−P9. Polymer 1. Polyacrylate derived from monomer 4a. Colorless solid, yield (392 mg, 75%). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 1.22 (s, 9 H, 3 CH3), 1.41−2.19 (m, 2 H, CH2 backbone), 2.19−3.03 (m, 1 H, CH backbone), 5.47−6.00 (m, 1 H, OCHCO), 6.00−6.76 (br, 1 H, NH), 7.00−7.79 (m, 5 H, Ar−H); Tg = 123 °C. Polymer 2. Polyacrylate derived from monomer 4b. Colorless solid, yield (423 mg, 79%). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 0.72− 1.00 (m, 3 H, CH3), 1.00−2.15 (m, 12 H, 5 CH2 and CH2 backbone), 1.33 (s, 9 H, 3 CH3), 2.15−2.87 (m, 1 H, CH backbone), 4.47−5.18 (m, 1 H, OCHCO), 5.90−6.92 (br, 1 H, NH); Tg = 59 °C. Polymer 3. Polyacrylate derived from monomer 4c. Colorless solid, yield (337 mg, 74%).1H NMR (CDCl3, 300 MHz): δ (ppm) = 0.72− 1.10 (m, 6 H, 2 CH3), 1.32 (s, 9 H, 3 CH3), 1.51−2.89 (m, 4 H,

CH(CH3)2, CH2 and CH backbone), 4.27−5.09 (m, 1 H, OCHCO), 5.68−6.82 (br, 1 H, NH); Tg = 111 °C. Polymer 4. Polyacrylate derived from monomer 4d. Colorless solid, yield (278 mg, 65%).1H NMR (CDCl3, 300 MHz): δ (ppm) = 0.78− 1.01 (m, 3 H, CH3), 1.33 (s, 9 H, 3 CH3), 1.48−2.12 (m, 4 H, CH2, CH2 backbone), 2.21−2.88 (m, 1 H, CH backbone), 4.57−5.10 (m, 1 H, OCHCO), 5.97−6.89 (br, 1 H, NH); Tg = 96 °C. Polymer 5. Polyacrylate derived from monomer 4e. Colorless solid, yield (266 mg, 67%). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 1.33 (s, 9 H, 3 CH3), 1.35−1.45 (m, 3 H, CH3), 1.58−2.15 (m, 2 H, CH2 backbone), 2.20−2.69 (m, 1 H, CH backbone), 4.69−5.08 (m, 1 H, OCHCO), 6.03−6.70 (br, 1 H, NH); Tg = 104 °C. Polymer 6. Polyacrylate derived from monomer 4f. Colorless solid, yield (387 mg, 86%). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 0.85− 2.21 (m, 12 H, 5 CH2 and CH2 backbone), 1.42 (m, 3 H, CH3), 2.21− 2.82 (m, 1 H, CH backbone), 3.53−3.96 (m, 1 H, NCH), 4.73−5.23 (m, 1 H, OCHCO), 6.36−7.27 (m, 1H, NH); Tg = 122 °C. Polymer 7. Polyacrylate derived from monomer 4g. Colorless solid, yield (345 mg, 81%). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 0.87 (t, J = 5.9 Hz, 3 H, CH3), 1.03−2.19 (m, 11 H, 3 CH2, CH3 and CH2 backbone), 2.20−2.79 (m, 1 H, CH backbone), 2.87−3.49 (m, 2 H, NCH2), 4.76−5.35 (m, 1 H, OCHCO), 6.60−7.55 (m, 1H, NH); Tg = 51 °C. Polymer 8. Polyacrylate derived from monomer 4h. Colorless solid, yield (352 mg, 75%). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 0.79− 1.59 (m, 3 H, CH3), 1.29−2.16 (m, 2 H, CH2 backbone), 2.16−2.80 (m, 1 H, CH backbone), 3.86−4.49 (m, 2 H, NCH2), 4.64−5.21 (m, 1 H, OCHCO), 6.73−7.80 (m, 6 H, NH and 5 Ar−H); Tg = 85 °C. Polymer 9. Polyacrylate derived from monomer 4i. Colorless solid, yield (408 mg, 84%). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 1.27− 1.56 (m, 3 H, CH3), 1.58−2.18 (m, 4 H, CH2 and CH2 backbone), 2.24−2.44 (m, 2 H, CH2COO), 2.24−2.65 (m, 1 H, CH backbone), 3.06−3.41 (m, 2 H, NCH2), 3.65 (s, 3 H, CH3), 4.81−5.23 (m, 1 H, OCHCO), 6.94−7.66 (m, 1 H, NH); Tg = 30 °C. 2.4. Characterization. NMR spectra were recorded on a Bruker ADVANCE DPX spectrometer operating at 300 MHz for 1H and at 75 MHz for 13C measurements. CDCl3 was used as solvent, and the resonance signal at 7.26 ppm (1H) and 77.16 ppm (13C) served as reference for the chemical shift δ. Polymers were characterized on a SEC System LC-20A (Shimadzu) equipped with a SIL-20A autosampler, RID-10A refractive index detector in THF (flow rate 1 mL/min) at 50 °C. The analysis was performed on the following column system: main-column PSS SDV analytical (5 μm, 300 mm × 8.0 mm, 10000 Å) with a PSS SDV analytical precolumn (5 μm, 50 mm × 8.0 mm). For the calibration narrow linear poly(methyl methacrylate) standards (Polymer Standards Service PPS, Germany) ranging from 1100 to 981 000 Da were used. The melting point (Tm) of the monomers was determined using the automated melting point system Optimelt MPA 100 (Stanford Research Systems) at a heating rate of 4 °C/min. Thermal properties of the prepared polymers were studied via differential scanning calorimetry (DSC) with a Mettler Toledo DSC stare system operating under nitrogen atmosphere using about 5 mg of the respective polymer for the analysis. The glass transition Tg was recorded on the second heating scan by using the following method: starting from −70 to 150 °C (heating rate of 20 °C/min), cooling from 150 to 75 °C (cooling rate of 20 °C/min), and heating from −75 to 150 °C (heating rate of 10 °C/min). Determination of the UCST of some thermoresponsive polymers was carried out on a Varian Cary 100 Bio UV/vis spectrometer equipped with a temperature controller. Spectra were recorded at a wavelength of 500 nm and a heating rate of 0.7 °C/min. Infrared spectra (IR) were recorded on a Bruker Alpha-p instrument in a frequency range from 3998 to 374 cm−1 applying KBr and ATR technology. Fast atom bombardment (FAB) mass spectra were recorded on a Finnigan MAT 95 instrument. The protonated molecule ion is expressed by the term: [(M + H)]+. All thin layer chromatography experiments were performed on silica gel coated aluminum foil (silica gel 60 F254, Aldrich). Compounds were visualized by irradiation with a UV-lamp or by staining with Seebach solution 6033

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(mixture of phosphomolybdic acid hydrate, cerium(IV) sulfate, sulfuric acid, and water).

DCM proved to be a generally suitable solvent for these Passerini reactions. As observed by thin layer chromatography, most reactions showed good conversion of starting materials, and therefore simple evaporation of the solvent and unreacted compounds yielded the products in excellent yields (see 4f and 4g, Table 1). If no pure product was obtained, silica gel column chromatography was performed. Very good results could be achieved in most cases, giving the desired compounds in high isolated yields (compare entries 4b−4i in DCM, Table 1). Only entry 4a resulted in lower yields, most likely due to the reduced reactivity of aromatic aldehydes in Passerini-3CRs. The Passerini-3CR is known to be accelerated in water due to the hydrophobic effect, and thus we also tried this for our monomer synthesis.6,27 We observed that 3 h were sufficient to finish the reaction. Isolation was straightforward in this case, since the product precipitated during the reaction and simple filtration gave the pure product. Unfortunately, this method was not applicable to all monomers but provided excellent results for monomers 4a−4d. Thus, this alternative method shows advantages in terms of shorter reaction times and an easier purification processes. Further investigations demonstrated that a higher concentration of the reaction mixture resulted in significantly higher yields. A quick screening revealed that a 0.5 M solution showed the best results (Table 2). Other variations that are described in the literature, such as the use of ultrasonication or addition of salts, were not successful in our hands.6,28

3. RESULTS AND DISCUSSION 3.1. Synthesis of Acrylate Monomers via Passerini3CR. The use of acrylic acid 1 in IMCRs is only rarely described. Recently, Martens et al. described the Passerini-3CR of 1 with allyl ketones and several isocyanides, followed by ringclosing metathesis (RCM) to obtain α,β-unsaturated lactones.25 A few publications reported that 1 finds applications in Ugi4CRs for the construction of structurally diverse heterocycles via postmodification of the acrylate group.26 However, acrylates derived by IMCRs were not used in radical polymerizations yet. This new approach offers manifold opportunities, introducing desired functional groups to the acrylate monomer in a straightforward and simple fashion. The synthesis of acrylate monomers was realized in an equimolar ratio of freshly distilled acrylic acid 1,23 different aliphatic as well as aromatic aldehydes 2a−2e, and isocyanides 3a−3e by stirring for 1 day at room temperature. The reactions were carried out in dichloromethane (DCM) or in water (Scheme 1). Scheme 1. Passerini-3CRs of Acrylic Acid, Different Isocyanides, and Aldehydes to Yield Structurally Diverse Acrylate Monomers

Table 2. Concentration Optimization of Aqueous Passerini3CR for the Synthesis of 4c at Room Temperature and 3 h Reaction Time concentration [mol/L]

remark

0.1 0.2 0.2 0.2 0.2 0.25 0.33 0.5 0.5 0.5 1.0

yield [%]

1 M LiCl 2 M LiCl ultrasonication

ultrasonication 24 h

41 58 53 55 73 62 71 74 73 74 70

In summary, the aqueous Passerini-3CR demonstrates the method of choice if the product precipitates during the reaction, as it was the case for monomers 4a−4d. Otherwise, Table 1. Results of Passerini Reactions Yielding 4a−4i (Compare also Scheme 1)

a

product

R1

R2

solvent

yield [%]

Tm [°C]

Rf

4a 4b 4c 4d 4e 4f 4g 4h 4i

-t-Bu (2a) -t-Bu (2a) -t-Bu (2a) -t-Bu (2a) -t-Bu (2a) -c-Hx (2b) -(CH2)4CH3 (2c) -Bn (2d) -(CH2)3CO2Me (2e)

-Ph (3a) -(CH2)5CH3 (3b) -i-Pr (3c) -Et (3d) -Me (3e) -Me (3e) -Me (3e) -Me (3e) -Me (3e)

water/DCM water/DCM water/DCM water/DCM DCM DCM DCM DCM DCM

41/12 68/45 74/89 67/77 86 100 100 74 94

111 54 87 92 74 61 (l) 67 (l)

0.30a 0.42a 0.49a 0.35a 0.45b 0.48b 0.44b 0.24b 0.17b

TLC (n-hexane/ethyl acetate = 4:1). bTLC (n-hexane/ethyl acetate = 2:1), (l) = liquid at room temperature. 6034

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DCM is a very good, but less sustainable, alternative. After these optimization studies, a small set of acrylate monomers (4a−4i) was synthesized. These monomers contain different aliphatic chains (derived from acetaldehyde ranging to heptaldehyde; 4b−4e). Moreover, aromatic acrylates 4a derived from benzaldehyde 3a and highly polar substances 4e−4i applying acetaldehyde 3e (see Table 1, also indicated by the retention factor Rf of performed TLCs) were generated. In most cases, the Passerini products were obtained as colorless solids, except for acrylate monomers 4g and 4i, which are highly viscous substances. 3.2. Free Radical Polymerization and Characterization of the Polyacrylates. The different Passerini products 4a−4i were polymerized via free radical polymerization applying AIBN as thermal initiator.22 The experiments were performed at 70 °C in ethyl acetate for 6 h (Scheme 2). Figure 1. SEC traces of polymers P2, P3, and P5.

Scheme 2. Radical Polymerization Procedure of Passerini Products 4a−4i

1.0 Hz, 1 H, CH2transCHCO) disappeared completely, and broad signals in the range of 1.50−3.00 ppm were observed and assigned to the polymer backbone (Figure 2). Next, the thermal properties of all polymers were investigated. Only glass transition temperatures (Tg), but no melting points (Tm), were observed for all polymers. The lowest glass transition was found for P9 (30 °C), probably because the methyl butyrate moiety acts as plasticizer. In contrast, polyacrylates P1 and P6 show a much higher glass transition (Figure 3). This fact is explained by the bulky and inflexible benzene or cyclohexyl moiety. Thus, the glass transition can be easily tuned by applying different components in the Passerini-3CR. Considering the variation of the aldehyde component from methyl to hexyl (P2−P5) side chains (in combination with a fixed tert-butyl group derived from the corresponding isocyanide compound), P2 possessed the lowest Tg at 59 °C, followed by P4 at 96 °C and P5 at 104 °C. Only P3 does not follow this trend, but this observation could be explained again by the bulky (branched) isopropyl moiety. Hence, due to the diversity of the applied monomers, the glass transition of such polymers can be easily tuned. In addition, we have tested these polyacrylates for a possible thermoresponsive behavior, since they contain hydrogen bond donors and acceptors and are thus structurally similar to the well-known thermoresponsive PNIPAM.29 Therefore, we prepared solutions of 1.0 wt % of polymers in water, ethanol, and methanol. All these solvents permit the formation of hydrogen bonds, which is the prerequisite for a UCST or LCST behavior. For these experiments, the solubility of P1−P9 in the

After precipitation, polymers P1−P9 reached molecular weights in range between 26 kDa (P8) and 98 kDa (P3), and good yields were observed (Table 3). Some representative GPC traces are shown in Figure 1. The wide range in the observed molecular weight is most probably a result of the different solubility of the polyacrylates in ethyl acetate and optimization with different solvents might provide higher molecular weights. However, for this first investigation and since high molecular weights were obtained in all cases, we preferred to perform all polymerizations under the same conditions. Polymers P6−P9, derived from the more polar Passerini products 4f−4i, showed highly viscous reaction mixtures in ethyl acetate, which might be the explanation for the observed polydispersity indices (PDIs). The values are much higher than these of P1−P5, especially the PDI of 5.96 for the most polar polymer P9. The polymer formation was also successfully shown by 1H NMR analysis. The typical acrylate signals at approximately 5.93 (dd, J = 10.4, 1.0 Hz, 1 H, CH2cisCHCO), 6.17 (dd, J = 17.3, 10.4 Hz, 1 H, CH2CHCO), and 6.48 ppm (dd, J = 17.3,

Table 3. Results of the Free Radical Polymerization of Monomers 4a−4ia

a

polymer

monomer

Mn [g/mol]

PDI

yield [%]

Tg [°C]

UCST

water

EtOH

MeOH

P1 P2 P3 P4 P5 P6 P7 P8 P9

4a 4b 4c 4d 4e 4f 4g 4h 4i

43 100 59 550 98 500 56 100 88 250 45 500 29 150 26 100 45 500

2.11 2.11 2.51 2.60 2.25 3.40 4.16 3.31 5.96

75 79 74 65 67 86 81 75 84

123 59 111 96 104 122 51 85 30

no no no no no yes no yes yes

ns ns ns ns ns ns ns ns ns

s s s s s s s 55−74 6−19

s ns s s s 6−27 s ns −37 to −20

Reaction conditions: ethyl acetate as solvent, 70 °C, 1 mol % AIBN, 6 h reaction time; s: soluble; ns: not soluble. 6035

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showed an UCST in ethanol in a range of 55−74 and 6−19 °C, respectively (Figure 4). Furthermore, P6 possessed a UCST at

Figure 4. (top) Pictures of the visual UCST analysis of P9 in ethanol: turbid polymer solution at 6 °C and clear solution at 19 °C. (bottom) Temperature-dependent absorption of P9 in ethanol. Both determinations were performed at a concentration of 1.0 wt %.

Figure 2. Representative 1H NMR (300 MHz) spectra of 4b and the corresponding polymer P2 in CDCl3.

6−27 °C and P9 at very low temperatures of −37 to −20 °C in methanol. All pictures of the analyses can be found in the Supporting Information. Thus, these first investigations show that the Passerini-3CR can be used to obtain thermoresponsive polymers. In the future, the tuning of their cloud point temperatures by systematic variation of their side chains and the transfer to water-soluble polymers should be possible.

4. CONCLUSIONS The Passerini-3CR was demonstrated as a highly efficient and useful tool for the synthesis of acrylate monomers in DCM as well as water. Furthermore, the one-pot reaction enables the formation of a set of monomers in a straightforward fashion.30 Nine different Passerini products were polymerized for the first time in a free radical polymerization yielding polyacrylates possessing molecular weights up to 98.5 kDa. The fixed scaffold in combination with the manifold variety of the amide and αalkyl moiety offered promising opportunities in controlling the properties of the corresponding polyacrylate. It was shown that, by introducing different functional groups, the polarity and glass transition can be easily influenced. Most interestingly, a thermoresponsive behavior of the very polar polymers could be observed in three cases: P6, P8, and P9 showed an UCST in alcoholic solutions. Further investigations and monomer tuning in this direction will allow the development of a new class of tunable thermoresponsive polymers that can, of course, also be incorporated into block copolymer architectures via controlled radical polymerization techniques.

Figure 3. DSC curve of P1, P3, and P6, which show the highest glass transitions.

corresponding solvent was examined by cooling the solutions to −78 °C (0 °C in case of water) and slowly heating the mixture to the boiling point of the solvent. To observe a possible LCST/UCST behavior, we visually determined the temperature at which the solution was completely clear or turbid. Because of a lack of proper instruments operating at lower temperatures (i.e., below 0 °C) or near the boiling point of the solvent, determination of the cloud points by UV/vis was only possible for P6 and P9. The results of both measurements (visual and UV/vis) were in very good agreement. In water, all studied polyacrylates were insoluble. Interestingly, P8 and P9 6036

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(15) Rubinshtein, M.; James, C. R.; Young, J. L.; Ma, Y. J.; Kobayashi, Y.; Gianneschi, N. C.; Yang, J. Org. Lett. 2010, 12, 3560−3563. (16) Wessjohann, L. A.; Henze, M.; Kreye, O.; Rivera, D. G. WO Patent 134,607, 2011; European Patent 2563847, 2013. (17) Jee, J.-A.; Spagnuolo, L. A.; Rudick, J. G. Org. Lett. 2012, 14, 3292−3295. (18) Chan, C. Y. K.; Tseng, N.-W.; Lam, J. W. Y.; Liu, J.; Kwok, R. T. K.; Tang, B. Z. Macromolecules 2013, 46, 3246−3256. (19) Lee, I.-H.; Kim, H.; Choi, T.-L. J. Am. Chem. Soc. 2013, 135, 3760−3763. (20) Kakuchi, R.; Theato, P. ACS Macro Lett. 2013, 2, 419−422. (21) Siamaki, A. R.; Sakalauskas, M.; Arndtsen, B. A. Angew. Chem., Int. Ed. 2011, 50, 6552−6556. (22) (a) Odian, G. Principles of Polymerization, 4th ed.; John Wiley & Sons: Hoboken, NJ, 2004. (b) Barner-Kowollik, C. Macromol. Rapid Commun. 2009, 30, 1961−1963. (c) van Herk, A. M. Macromol. Rapid Commun. 2009, 30, 1964−1968. (d) Ahmad, N. M.; Charleux, B.; Farcet, C.; Ferguson, C. J.; Gaynor, S. G.; Hawkett, B. S.; Heatley, F.; Klumperman, B.; Konkolewicz, D.; Lovell, P. A.; Matyjaszewski, K.; Venkatesh, R. Macromol. Rapid Commun. 2009, 30, 2002−2021. (23) Armarego, W. L. F.; Chai, C. Purification of Laboratory Chemicals, 6th ed.; Butterworth-Heinemann: Weinheim, 2003. (24) Two-step procedure for the synthesis of isocyanides from primary amines: (a) Ugi, I.; Fetzer, U.; Eholzer, U.; Knupfer, H.; Offermann, K. Angew. Chem., Int. Ed. 1965, 4, 472−484. (b) Nunami, K.; Suzuki, I. M.; Yoneda, N. Synthesis 1978, 840−841. (c) Obrecht, R.; Herrmann, R.; Ugi, I. Synthesis 1985, 400−402. (25) Schwäblein, A.; Martens, J. Eur. J. Org. Chem. 2011, 4335−4344. (26) (a) Paulvannan, K. J. Org. Chem. 2004, 69, 1207−1214. (b) Oikawa, M.; Naito, S.; Sasaki, M. Heterocycles 2007, 73, 377−392. (c) De Silva, R. A.; Santra, S.; Andreana, P. R. Org. Lett. 2008, 10, 4541−4544. (27) Pirrung, M. C.; Sarma, K. D. Tetrahedron 2005, 61, 11456− 11472. (28) Pirrung, M. C.; Sarma, K. D.; Wang, J. J. Org. Chem. 2008, 73, 8723−8730. (29) (a) Gil, E. S.; Hudson, S. M. Prog. Polym. Sci. 2004, 29, 1173− 1222. (b) Roy, D.; Brooks, W. L. A.; Sumerlin, B. S. Chem. Soc. Rev. 2013, DOI: 10.1039/C3CS35499G. (30) Webster, D. C.; Meier, M. A. R. Adv. Polym. Sci. 2010, 225, 1− 15.

ASSOCIATED CONTENT

S Supporting Information *

Isocyanide syntheses, 1H and 13C NMR spectra of 4a−4i, 1H NMR spectra of the polymers as well as UCST pictures. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.A.R.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Rebekka Schneider for experimental support. A.S. is thankful for a scholarship from the Carl-Zeiss-Stiftung.

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REFERENCES

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dx.doi.org/10.1021/ma401125j | Macromolecules 2013, 46, 6031−6037