Dendritic poly(ether-imide)s: synthesis, characterization, and modification

Dendritic poly(ether–imide)s: synthesis, characterization,

and modification

C.-M. Leu

, C.-F. Shu

*, C.-F. Teng

, J. Shiea

Department of Applied Chemistry, National Chiao Tung University, Hsin-Chu, Taiwan, 30035, ROC b

Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, Taiwan, 80424, ROC

Received 29 June 2000; received in revised form 28 July 2000; accepted 3 August 2000

Abstract

A convergent approach to the preparation of dendritic macromolecules with an aromatic ether–imide inner structure and modifiable benzyl ether chain ends is described. The structural component that constitutes the branching regions was derived from 1-(4-aminophenyl)-1,1-bis(4-hydroxyphenyl)ethane. The key steps in ether–imide dendron synthesis involve an aromatic nucleophilic displacement of an activated nitro group with phenoxide leading to the ether linkage and cyclocondensation of the amino group with 3-nitrophthalic anhydride, to give rise to the imide ring containing an activated nitro functionality. The dendritic wedges are then coupled to a polyfunctional core to complete the synthesis. The benzyl ether chain ends can be selectively removed by catalytic hydrogenolysis to produce a dendrimer with phenolic chain ends. Further modification of the chain ends is accomplished by etherfication of the phenolic groups. Structures of these dendritic poly(ether– imide)s were confirmed by 1H NMR, 13C NMR and FTIR spectroscopy as well as by mass spectrometry. Properties such as the glass transition temperature and solubility of the dendrimer are greatly affected by the nature of the end groups.䉷 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Dendritic poly(ether–imide)s; Glass transition temperature; Solubility

1. Introduction

Dendritic macromolecules are characterized by a central polyfunctional core, from which monomeric units with branching points arise successively. This results in a nearly entanglement-free, highly branched spherical structure, the periphery of which consists of a large number of chain ends. Because such compounds have unique highly branched polymeric structures and well defined 3D architectures, it would appear that they have great potential for chemical applications. As a result of this, dendritic macromolecules have received considerable attention in recent years [1–45]. Two different synthetic strategies are currently adopted for synthesizing dendritic macromolecules, namely the divergent [46,47] and convergent growth approaches [48– 50]. In the divergent approach [46,47], synthesis starts from the center of the dendrimer and proceeds “outwards”, via the coupling of building blocks. This approach provides for regular branching around a core molecule. In the convergent

approach [48–50], construction begins at what will even-tually become the periphery of the final macromolecules and proceeds “inwards”, followed by the attachment of dendritic wedges to a polyfunctional core. Since the synth-esis of dendritic macromolecules is based on the interactive strategy, control of the architectural properties is maintained at each synthetic step. The synthesis of well defined, highly branched macromolecules with tunable interior/surface features and a controlled molecular size has sparked consid-erable interest, in anticipation of generating functional materials with desirable properties.

One of the intriguing prospects of the dendritic architec-ture is the large number of chain-end functional groups that can be tailor-made to yield dendritic molecules with unique chemical and physical properties. In this paper, we report the synthesis and characterization of ether–imide dendri-mers with modifiable benzyl ether groups located at the peripheries. The preparation of dendritic poly(ether– imide)s utilized the convergent strategy, involving the synthesis of dendritic wedges, which contained ether– imides as the structural components and benzyl ether as the chain-end groups, followed by their attachment to the core component. Structural components that constitute the branching regions of the dendritic wedges were derived 0032-3861/01/$ - see front matter䉷 2000 Elsevier Science Ltd. All rights reserved.

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* Corresponding author. Tel.:35-712121 ext. 56544; fax: ⫹886-35-723764.

from 1-(4-aminophenyl)-1,1-bis(4-hydroxyphenyl)ethane (2) and 3-nitrophthalic anhydride, while the core was derived from 1,1,1-tris(4-hydroxyphenyl)ethane. To modify the chain ends of the dendritic macromolecules, a protection–deprotection approach was employed. The benzyl group at the chain ends can be selectively removed by hydrogenolysis [51,52], thus generating the phenolic-terminated poly(ether–imide)s. Reaction of the peripheral phenolic groups with n-hexanol via the Mitsunobu reaction [53] led to dendrimers with n-hexyl ether chain ends. Such transformations of the terminal functionalities were shown to dramatically alter the thermal and solution behavior of the various surface modified dendrimers.

2. Experimental section

2.1. General directions

The CH3ONa/CH3OH solution was prepared by

dissol-ving sodium metal in absolute MeOH. THF was distilled from a sodium diphenyl ketyl solution just prior to use. Other starting materials and reagents were used as obtained from commercial suppliers. Nuclear magnetic resonance (NMR) spectra were recorded on a Varian Unity 300 MHz or a Bruker-DRX 300 MHz spectrometer. Infrared spectra were obtained on a Nicolet 520 FTIR Spectrometer. Mass spectra were obtained on a JEOL JMS-SX/SX 102A Mass Spectrometer. Differential scanning calorimetry (DSC) was performed on a SEIKO SSC 5200 DSC using a heating/ cooling rate of 10⬚C min⫺1. Samples were scanned from 30 to 300⬚C and then cooled to 30⬚C and scanned a second time from 30 to 300⬚C. Glass transition temperatures were determined from the second heating scan. Size-exclusion chromatography (SEC) was carried out on a Waters chro-matography interfaced with a Waters 410 Differential refractometer. Three 5-mm Waters styragel columns (300× 7.8 mm) connected in series in decreasing order of pore size (105, 104, and 103A˚ ) were used with DMF/0.05 M LiBr, as eluant, and poly(methyl methacrylate) standard samples were used for the calibration. Matrix-assisted laser desorption/ionization (MALDI) mass spectra were recorded on a linear time-of-flight mass spectrometer (TOF-MS) (HP G2025A) equipped with a pulsed nitrogen laser (337 nm). Minimum laser power was used to desorb the analyte from the sample solution. An aliquot of 1mL of the sample (1 mM) was briefly vortexed with 10ml of matrix (100 mM), followed by the addition of 1mL of satu-rated ethanolic solution of KCl or AgNO3. One microliter of

the resulting solution was transferred to the sample probe and dried under reduced pressure. The measurement was performed in positive ion mode using either 2,5-dihydroxy-benzoic acid or retinoic acid, as the matrix. The observed molecular masses were calibrated against bovine insulin and bradykinin standards, and the instrument gave m/z values

within 0.5% resolution. Analytical TLC was performed on commercial Merck plates coated with silica gel GF254. Silica gel for column chromatography was Merck Kieselgel 60 (70–230 mesh). The synthesis of compounds 1 and 2 has been reported elsewhere [54].

1,1-Bis(4-benzyloxy)phenyl-1-(4-nitrophenyl)ethane (3). A mixture of 1 (10.0 g, 29.8 mmol), benzyl bromide

(10.2 g, 59.6 mmol), K2CO3 (12.4 g, 89.6 mmol), and

18-crown-6 (1.5 g) in anhydrous acetone (100 ml) was heated at reflux with vigorous stirring under nitrogen for 5 h. The resulting mixture was poured into water (300 ml) and extracted with EtOAc (3× 150 ml). The combined extracts were dried over MgSO4and the solvent removed in vacuo to

give 3 (14.9 g, 97.0%). 1H NMR (CDCl3)d 2.07 (s, 3H), 4.96 (s, 4H), 6.81 (d, 4H, Jˆ 8.8 Hz), 6.89 (d, 4H, Jˆ 8.8 Hz), 7.18 (d, 2H, J ˆ 8.2 Hz), 7.23–7.30 (m, 10H), 8.01 (d, 2H, Jˆ 8.2 Hz).13C NMR (CDCl3)d 30.5, 51.6, 70.0, 114.3, 123.0, 127.5, 128.0, 128.6, 129.5, 136.9, 140.1, 146.2, 157.2, 157.4. MS (m/z): obs. 515.2090; calcd. 515.2096 for C34H29O4N.

[G1]-NH2. To a mixture of 3 (14.9 g, 28.9 mmol) and

iron powder (8.0 g) in ethanol (300 ml), 12 N HCl (15.0 ml) was added dropwise, followed by heating at reflux for 5 h. The reaction mixture was poured into water (300 ml) and extracted with EtOAc (3× 200 ml). The combined extracts were dried over MgSO4and the solvent

was removed in vacuo to give [G-1]-NH2(13.9 g, 99.1%).

1_{H NMR (CDCl} 3)d 2.08 (s, 3H), 5.02 (s, 4H), 6.60 (d, 2H, Jˆ 8.7 Hz), 6.85 (d, 4H, J ˆ 9.0 Hz), 6.86 (d, 2H, Jˆ 9.0 Hz), 7.00 (d, 4H, J ˆ 9.3 Hz), 7.28–7.43 (m, 10H). 13C NMR (CDCl3)d 30.7, 50.5, 69.9, 113.8, 114.5, 127.5, 127.9, 128.5, 129.4, 129.6, 137.1, 139.7, 142.2, 144.0, 156.7. MS (m/z): obs. 485.2357; calcd. 485.2355 for C34H31O2N.

[G1]-NO2. A mixture of [G1]-NH2(13.9 g, 28.6 mmol)

and 3-nitrophthalic anhydride (5.5 g, 28.5 mol) in AcOH (200 ml) was refluxed under nitrogen for 6 h. The mixture was cooled to 25⬚C and added to water (600 ml). The result-ing precipitate was filtrated and purified by column chroma-tography (EtOAc/CH2Cl2 1:40) to give [G1]-NO2(13.1 g,

69.6%).1H NMR (CDCl3)d 2.16 (s, 3H), 5.05 (s, 4H), 6.89 (d, 4H, Jˆ 8.7 Hz), 7.03 (d, 4H, J ˆ 8.7 Hz), 7.20–7.50 (m, 14H), 7.97 (dd, 1H, Jˆ 7.2,8.1 Hz), 8.15 (d, 1H, Jˆ 8.1 Hz), 8.21 (d, 1H, J ˆ 7.2 Hz). 13C NMR (CDCl3) d 30.6, 51.2, 69.9, 114.0, 123.2, 125.6, 127.3, 127.5, 127.9, 128.5, 128.7, 129.4, 129.6, 133.6, 135.6, 136.9, 141.0, 145.2, 150.2, 156.9, 161.9, 164.8. MS (m/z): obs. 660.2260; calcd. 660.2260 for C42H32O6N2.

[G2]-NH2. A mixture of 2 (2.0 g, 6.6 mmol) and CH3ONa

(26.0 ml, 13.3 mmol, 0.51 M in CH3OH) was stirred under

nitrogen at 25⬚C for 1 h, and the CH3OH was then removed

in vacuo. The solid was further dried at 60⬚C in vacuo for 2 h. To the resulting solid was added anhydrous DMF and [G1]-NO2 (8.7 g, 13.2 mmol). The mixture

was stirred at 60⬚C for 12 h, and then added to water. The precipitate was isolated by filtration and purified by column

chromatography (EtOAc/CH2Cl2 1:50) to give [G2]-NH2 (6.2 g, 61.3%). 1H NMR (CDCl3) d 2.16 (br, 9H), 5.03 (s, 8H), 6.66 (d, 2H, Jˆ 7.8 Hz), 6.89 (m, 10H), 7.03 (m, 12H), 7.11–7.24 (m, 10H), 7.28–7.44 (m, 24H), 7.62 (m, 4H).13C NMR (CDCl3)d 30.7, 51.2, 69.9, 114.0, 114.8, 117.8, 118.8, 119.4, 123.1, 125.7, 127.5, 127.9, 128.5, 129.2, 129.4, 129.7, 130.3, 133.9, 134.0, 136.0, 137.0, 138.6, 141.2, 144.2, 146.2, 149.3, 153.0, 155.0, 156.9, 165.1, 166.8. MS (m/z): obs. 1531.5934; calcd.1531.5921 for C104H81O10N3.

[G2]-NO2. This was prepared from [G2]-NH2 (7.99 g,

5.22 mmol) and 3-nitrophthalic anhydride (1.05 g, 5.44 mmol) using the same procedure described for

[G1]-NO2. The crude product was purified by column

chromato-graphy (EtOAc/CH2Cl2 1:50) to give [G2]-NO2 (5.3 g,

59.5%). 1H NMR (CDCl3) d 2.18 (s, 6H), 2.23 (s, 3H), 5.02 (s, 8H), 6.86 (d, 8H, Jˆ 8.1 Hz), 7.02 (d, 8H, Jˆ 8.7 Hz), 7.06 (d, 4H, J ˆ 7.8 Hz), 7.11–7.24 (m, 12H), 7.25–7.50 (m, 26H), 7.63–7.68 (m, 4H), 7.96 (dd, 1H, Jˆ 7.8,8.1 Hz), 8.14 (d, 1H, J ˆ 8.1 Hz), 8.20 (d, 1H, Jˆ 7.8 Hz). 13C NMR (CDCl3) d 30.7, 51.2, 51.8, 69.9, 114.0, 118.0, 119.0, 119.5, 123.3, 125.7, 125.9, 127.5, 127.9, 128.5, 128.9, 129.0, 129.2, 129.4, 129.7, 130.3, 133.6, 134.0, 135.8, 136.1, 137.0, 141.3, 145.0, 145.4, 149.1, 149.4, 153.4, 154.8, 156.9, 161.9, 164.8, 165.1, 166.7. MS (m/z): obs. 1707.5883; calcd.1707.5905 for C112H83O14N4.

[G1]3[C]. A mixture of

1,1,1-tris(4-hydroxyphenyl)-ethane (250 mg, 817mmol) and CH3ONa (4.9 ml,

2.5 mmol, 0.50 M in CH3OH) was stirred under nitrogen

at 25⬚C for 1 h. The CH3OH was removed in vacuo, and

the resulting solid was dried at 60⬚C in vacuo for 2 h. To this solid, anhydrous DMF (8 ml) and [G1]-NO2 (1.68 g,

2.55 mmol) were added. The mixture was stirred at 60⬚C for 12 h, and then poured into water. The precipitated solid was isolated by filtration and purified by column chromato-graphy (EtOAc/CHCl3 1:60) to give [G1]3[C] (753 mg,

42.9%). 1H NMR (CDCl3) d 2.13 (s, 9H), 2.21 (s, 3H), 5.02 (s, 12H), 6.86 (d, 12H, Jˆ 9.3 Hz), 7.01 (d, 12H, Jˆ 8.7 Hz), 7.05 (d, 6H, J ˆ 9.0 Hz), 7.15 (m, 9H), 7.18 (d, 6H, Jˆ 8.4 Hz), 7.30–7.43 (m, 36H), 7.64 (m, 6H).13C NMR (CDCl3)d 30.7, 51.2, 51.5, 70.0, 114.1, 118.0, 119.0, 119.5, 123.4, 125.7, 127.5, 127.9, 128.5, 129.3, 129.7, 130.2, 134.0, 136.1, 137.0, 141.2, 145.3, 149.4, 153.4, 154.8, 156.9, 165.1, 166.7.

[G2]3[C]. This was prepared from

1,1,1-tris(4-hydroxy-phenyl)ethane (100 mg, 327mmol), CH3ONa (2.0 ml,

1.0 mmol, 0.50 M in CH3OH), and [G2]-NO2 (1.80 g,

1.06 mmol) using the procedure described for [G1]3[C].

The product was purified by column chromatography (EtOAc/CHCl3ˆ 1:50) to give [G2]3[C] (530 mg, 30.7%). 1_{H NMR (CDCl} 3) d 2.13 (s, 18H), 2.22 (s, 12H), 5.01 (s, 24H), 6.86 (d, 24H, Jˆ 8.9 Hz), 7.02 (d, 24H, J ˆ 8.9 Hz), 7.06 (d, 18H, Jˆ 8.9 Hz), 7.13–7.25 (m, 45H), 7.29–7.43 (m, 78H), 7.63 (m, 18H).13C NMR (CDCl3)d 30.7, 51.2, 51.5, 51.8, 69.9, 114.0, 117.9, 118.9, 119.6, 123.2, 125.7, 125.9, 127.5, 127.9, 128.5, 129.2, 129.7, 130.3, 134.0, 136.0, 137.0, 141.2, 145.2, 145.4, 148.3, 149.4, 153.3, 154.9, 156.9, 165.1, 166.8.

[G2]3[C]-(OH)6. 10% Pd/C (50 mg) was added to a

solu-tion of [G1]3[C] (500 mg, 311mmol) in a mixed solvent of

EtOH (4 ml)/ THF (4 ml). The mixture was flushed three times with hydrogen to remove oxygen and then stirred vigorously at 25⬚C under hydrogen for 48 h. The reaction mixture was filtered and the filtrate was evaporated to give

[G2]3[C]-(OH)6(350 mg, 93.6%). 1H NMR (DMSO-d6)d 2.05 (s, 9H), 2.20 (s, 3H), 6.68 (d, 12H, Jˆ 8.6 Hz), 6.86 (d, 12H, Jˆ 8.6 Hz), 7.11–7.20 (m, 18H), 7.24 (d, 3H, Jˆ 8.4 Hz), 7.32 (d, 6H, J ˆ 8.5 Hz), 7.67 (d, 3H, Jˆ 7.2 Hz), 7.80 (dd, 3H, J ˆ 7.8,7.9 Hz), 9.29 (s, 6H). 13 C NMR (DMSO-d6) d 30.3, 50.6, 51.3, 114.6, 118.0, 119.0, 123.8, 126.5, 128.6, 129.2, 129.3, 130.2, 133.9, 136.9, 139.2, 145.0, 149.8, 153.2, 153.7, 155.3, 164.7, 166.4.

[G2]3[C]-(OH)12. 10% Pd/C (20 mg) was added to a

solu-tion of [G2]3[C] (200 mg, 47.5mmol) in a mixed solvent of

EtOH (1.5 ml)/ THF (3 ml). The mixture was reacted according to the procedure for [G2]3[C]-(OH)6.The reaction

mixture was filtered and the resulting filtrate evaporated to give [G2]3[C]-(OH)12(130 mg, 81.8%).1H NMR

(DMSO-d6)d 2.03 (s, 18H), 2.20 (s, 12H), 6.66 (d, 24H, Jˆ 8.6 Hz), 6.84 (d, 24H, Jˆ 8.6 Hz), 7.12–7.23 (m, 54H), 7.25 (d, 9H, Jˆ 8.2 Hz), 7.30 (d, 12H, J ˆ 8.5 Hz), 7.39 (d, 6H, Jˆ 8.2 Hz), 7.65 (m, 9H), 7.80 (m, 9H), 9.26 (s, 12H). 13C NMR (DMSO-d6) 30.3, 30.8, 51.0, 51.8, 114.8, 114.9, 118.4, 119.4, 119.5, 124.1, 125.3, 126.9, 127.2, 129.0, 129.5, 130.2, 130.6, 134.2, 137.3, 139.5, 145.2, 148.7, 150.1, 153.5, 154.0, 155.5, 155.7, 165.0, 165.1, 166.7, 166.8.

[G1]3[C]-(OR)6. To a solution of [G1]3[C]-(OH)6

(100 mg, 62.3 mmol), 1-hexanol (120 mg, 1.17 mmol), and PPh3 (300 mg, 1.14 mmol) in dry THF (4 ml) under

nitrogen, disopropyl azodicarboxylate (0.24 ml) was added dropwise. The mixture was stirred at 25⬚C for 24 h and then added to methanol. The precipitate was purified by column chromatography (hexane/CH2Cl2 1:6) to give [G1]3

[C]-(OR)6(120 mg, 92.3%). 1H NMR (CDCl3)d 0.86 (t, 18H, Jˆ 8.6 Hz), 1.27 (m, 24H), 1.46 (m, 12H), 1.79 (m, 12H), 2.12 (s, 9H), 2.21 (s, 3H), 3.91 (t, 12H, Jˆ 8.6 Hz), 6.77 (d, 12H, Jˆ 8.7 Hz), 7.00 (d, 12H, J ˆ 8.7 Hz), 7.05 (d, 6H, Jˆ 8.7 Hz), 7.13–7.20 (m, 15H), 7.30 (d, 6H, J ˆ 8.4 Hz), 7.63 (m, 6H). 13C NMR (CDCl3)d 14.0, 22.6, 25.7, 29.3, 29.7, 30.7, 31.6, 51.1, 51.5, 67.9, 113.7, 118.0, 119.0, 119.5, 123.3, 125.6, 129.2, 129.3, 129.6, 130.3, 134.1, 136.1, 140.8, 145.4, 149.6, 153.4, 154.8, 157.2, 165.2, 166.8.

[G2]3[C]-(OR)12. This was prepared from [G2]3

[C]-(OH)12 (100 mg, 23.8 mmol), 1-hexanol (90 mg,

0.88 mmol), PPh3 (230 mg, 0.87 mmol) and diisopropyl azodicarboxylate (0.18 ml) using the same procedure as for [G1]3[C]-(OR)6. The crude product was purified by

column chromatography (Benzene/CH2Cl2 1:8) to give [G2]3[C]-(OR)12 (115 mg, 92.7%). 1H NMR (CDCl3) d

0.88 (t, 36H, Jˆ 8.6 Hz), 1.42 (m, 24H), 1.74 (m, 24H), 2.12 (s, 18H), 2.22 (s, 12H), 3.91 (t, 24H, Jˆ 8.6 Hz),

6.77 (d, 24H, Jˆ 8.7 Hz), 6.99 (d, 24H, J ˆ 8.7 Hz), 7.07 (m, 18H), 7.13–7.23 (m, 45H), 7.31 (d, 12H, Jˆ 8.7 Hz), 7.38 (d, 6H, Jˆ 8.7 Hz), 7.62 (m, 18 H).13C NMR (CDCl3) d 14.1, 22.7, 25.8, 29.4, 29.8, 30.8, 31.7, 51.2, 51.9, 68.0, 113.8, 118.0, 118.9, 119.1, 119.7, 123.2, 124.1, 125.8, 126.0, 129.4, 129.7, 129.9, 130.5, 134.1, 136.1, 140.9, 145.3, 149.7, 153.4, 155.0, 157.3, 165.3, 166.8, 166.9.

3. Results and discussion

3.1. Synthesis

The general synthetic procedure for the poly(ether– imide) dendrons is shown in Scheme 1. An acid-catalyzed

condensation reaction of p-nitroacetophenone with excess phenol gave compound 1 [55]. The reduction of the nitro group of 1 with H2, catalyzed by Pd/C, yielded compound 2.

The latter compound contained an aminophenyl group and two phenolic groups, and served as the building block utilized for the synthesis of the ether–imide dendrimer. Compound 1 was reacted with benzyl bromide in the presence of K2CO3 and 18-crown-6 to yield the benzyl

ether derivative 3, which was subsequently reduced with iron to produce [G1]-NH2. The addition of the aminophenyl

group of [G1]-NH2to 3-nitrophthalic anhydride, followed

by cyclodehydration, led to the corresponding phthalimide

[G1]-NO2, [56] which contained an activated nitro

function-ality located at the focal point of the growing dendritic wedge. The reaction of [G1]-NO2with the phenoxides of

O2N OH OH N O O NO2 H2N OH OH O2N O O H2N O O O O O2N O N O O NO2 N O O N O O O O O O O O H2N N O O N O O O O O O O O

1

2

3

[G1]-NO

[G2]-NH

[G2]-NO

[G1]-NH

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(v)

2 gave the next-generation ether–imide [G2]-NH2, [57]

which on a second cyclocondensation with 3-nitrophthalic anhydride resulted in the second-generation phthalimide dendron [G2]-NO2. Structures of the phthalimide dendrons

and intermediates synthesized were confirmed by1H NMR,

13_{C NMR, and high-resolution mass spectrometry. In the}1_H

NMR spectrum, the dendritic fragment [G2]-NH2, which

contained an aminophenyl group at the focal point, shows an AB quartet at 6.66 and 6.89 ppm corresponding to the aromatic protons of the aminophenyl group, and resonances for the aromatic protons of the ether-substituted phthalic rings were located at 7.28–7.44 and 7.62 ppm. The dendritic wedge [G2]-NO2, which contains a 3-nitrophthalimide ring

at the focal point, exhibits additional resonances for the aromatic protons of the nitro-substituted phthalic ring at 7.96, 8.14, and 8.20 ppm, while the resonances correspond-ing to the protons of the aminophenyl group disappeared. These significant differences in resonances allow the focal point group to be identified, the dendrons to be character-ized, and their purity to be determined. A comparison of the integration values for these groups to the values for other resonances confirms the generation number as well. 13C NMR also provides complementary information. The reso-nances assigned to the carbonyl groups of the ether-substi-tuted phthalic rings for [G2]-NH2 appeared at 165.1 and

166.7 ppm, while additional resonances corresponding to the carbonyl groups of the nitro-substituted phthalic ring appeared at 161.9 and 164.8 ppm, in the case of [G2]-NO2

[56].

The dendritic wedges obtained were then coupled to a polyfunctional core to yield the dendrimers. Reactions of phenoxide nucleophiles of the core molecule 1,1,1-tris(4-hydroxyphenyl)ethane with nitro-substituted phthalimide dendrons [G1]-NO2 and [G2]-NO2 gave the ether–imide

dendrimers, [G1]3-[C] and [G2]3-[C], respectively, as

shown in Scheme 2. The benzyl ether groups of ether– imides were selectively removed by catalytic hydrogenoly-sis without affecting the ether–imide functionalities. Hydro-genolysis of [G1]3[C] and [G2]3[C] was performed in a

mixed solvent of EtOH/THF under an atmosphere of hydro-gen using 10 wt% Pd/C as the catalyst to hydro-generate dendritic ether–imides [G1]3[C]-(OH)6 and [G2]3[C]-(OH)12,

respectively, containing 6 and 12 phenolic groups at their periphery. The phenolic-terminated dendrimers were then subjected to chain-end modification via the Mitsunobu reac-tion by the reacreac-tion of its phenolic groups with 1-hexanol in the presence of PPh3 and diisopropyl azodicarboxylate in

dry THF to give the corresponding ether derivatives

[G1]3[C]-(OR)6and [G2]3[C]-(OR)12, respectively.

3.2. General characterization of the ether–imide dendrimers

The characterization of the dendrimers was accomplished by a combination of techniques including 1H NMR, 13C NMR, IR, mass spectrometry, and size exclusion

chromato-graphy. The attachment of the dendritic wedges to the core molecule was readily followed by the disappearance of the resonances for the protons of the nitro-substituted phthalic ring. Fig. 1 shows the 1H NMR spectra of dendrimers

[G1]3[C] and [G2]3[C], which have similar features. The

peak assignments are based on the peak position of the dendritic intermediates and the auxiliary of 2D (H,H)-COSY spectra. The methylene resonance for the benzyl groups at the periphery of the dendritic structure appeared as a sharp singlet at 5.02 ppm. Resonances associated with the aromatic protons of the phthalimide rings in different layers were indistinguishable. These results are consistent with the highly symmetrical structures of ether–imide dendrimers. Integration of these areas and comparison with each other confirms not only the generation number, but also the number of phthalimide rings on the dendritic macromolecules. Information from13C NMR spectra added further support for the proposed structure. FTIR spectro-scopy also provided complementary evidence for the struc-tures, showing characteristic imide carbonyl absorptions at 1722 and 1776 cm⫺1 for [G1]3[C] and at 1724 and

1782 cm⫺1for [G2]3[C] [58].

Removal of the benzyl ether groups was performed by catalytic hydrogenolysis and was easily monitored by the disappearance of the resonances corresponding to the benzyl groups in the 1H NMR spectra. The absence of a resonance at ca. 5.00 ppm for the methylene protons of the benzyl group clearly demonstrates that no residual benzyl groups at the chain ends of the ether–imide dendri-mers were presented. Fig. 2 shows the1H NMR spectra of

[G1]3[C]-(OH)6and [G2]3[C]-(OH)12, which contain 6 and

12 phenolic groups, respectively at their peripheries. The aromatic protons of the terminal phenol groups gave rise to an AB quartet at 6.66 and 6.84 ppm. Resonances corre-sponding to the protons of the phthalimide rings in different layers were indistinguishable and appeared at 7.25, 7.69 and 7.80 ppm. The ratio of integration of these areas correlated well with the expected structures. Concerning the chain-end modification of the phenolic-terminated dendrimers via Mitsunobu etherfication, the resonance for a-CH2 of the

alkyl end groups was observed at 3.91 ppm. The percent functionalization of the terminal groups was determined by comparing the integration ratio of the a-CH2attributed

to the alkyl end groups versus those from the aromatic units. The use of excess reagents (3.0 equiv. per phenol group) resulted in complete (95–100%) functionalization, indicat-ing that the hydroxyl groups at the chain ends are readily accessible to the reagents in solution.

Size-exclusion chromatography (SEC) proved to be useful in analysing the purity and polydispersity of the dendrimers. Since the ether–imide dendrimers were prepared by stepwise, controlled processes, each compound was predicted to be monodisperse. Table 1 illustrates the results of SEC measurements that were calibrated against narrow-dispersity poly(methyl methacrylate) standards to give Mn and polydispersity data of the dendrimers. The

O O N O O N O O N O O OR OR OR OR O O N O O O N O O N O O RO OR RO OR O O N O O N O O N O O OR RO OR RO O O O N O O OR OR N O O O OR RO N O O _O RO RO [G1]3[C] [G1]3[C]-(OH)6 R = H R = n-C6H13 [G1]3[C]-(OR)6 [G1]-NO2 (i) R = benzyl (ii) (iii) [G2]-NO2 (i) [G2]3-[C] [G2]3[C]-(OH)12 R = H R = n-C6H13 [G2]3[C]-(OR)12 R = benzyl (ii) (iii)

molecular weights of the ether–imide dendrimers were underestimated by a factor of 1.3–2.5. This factor increases with the growth of generations. Due to the highly branched, compact structures of these dendritic macromolecules in comparison with the coiled conformation of the linear poly-mer standards, SEC measurements tended to underestimate their true molecular weight [59,60]. The SEC chromato-grams of the second-generation dendritic poly(ether– imide)s are shown in Fig. 3. All the ether–imide dendrimers gave narrow, single-peak symmetrically shaped chromato-grams and showed no evidence of an unresolved shoulder. This result revealed that these dendritic macromolecules

were pure and had a narrow distribution of molecular weights.

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) is an attractive alter-native to mass analysis of the ether–imide dendrimers. This technique provides higher mass determinations for intact molecular ions of nonvolatile species and is particularly useful for structural characterization of high molecular weight dendrimers [61–68]. The method is based on the laser desorption of molecules dissolved in a suitable matrix. Results of MALDI-TOF-MS spectrometry of the ether– imide dendrimers are shown in Table 1. Fig. 4 shows the

R N O O O O O R a b c d e f g h i j _{k l} O N O O N O O O N O O O O O O O R R b c d e f g h i j a k l m n o p _{q r s}

[G1]

[C]

[G2]

[C]

Fig. 1. 300-MHz1_{H NMR spectra in the region of 6.8–7.7 ppm for [G1]}

3[C] and [G2]3[C] in CDCl3. ⴱ — signal due to CDCl3.

Table 1

SEC and MALDI-TOF-MS data of the ether–imide dendrimers

Dendrimer Mw(calcd.) Mw(SEC) PDIa Molecular ion obs. Massb(calcd.) Mass (measured)

[G1]3[C] 2148 1328 1.05 [M⫹ K]⫹c 2186.6 2186.8 [G2]3[C] 5289 2574 1.08 [M⫹ K]⫹c 5328.2 5324.7 [G1]3[C]-(OH)6 1607 1223 1.02 [M⫹ K]⫹c 1645.8 1644.6 [G2]3[C]-(OH)12 4208 2738 1.07 [M⫹ K]⫹c 4246.7 4253.4 [G1]3[C]-(OR)6 2112 1105 1.02 [M⫹ K]⫹d 2219.6 2225.4 [G2]3[C]-(OR)12 5218 2028 1.08 [M⫹ K]⫹d 5325.4 5323.4 a _{Polydispersity data.} b _{Weighted isotopic average.} c _{Matrix: 2,5-dihydroxybenzoic acid.} d _{Matrix: retinoic acid.}

MALDI spectra of the second-generation ether–imide dendrimers. The dominant signals observed are either potas-sium or silver adducts of the molecular ions, depending on the matrix used. These peaks occurred at m/z values con-sistently within 0.2% of the theoretical values. Thus, the MALDI spectra data provided additional support for the identity of these ether–imide dendrimers. It could also be concluded that these dendrimers are pure and monodisperse.

3.3. Characterization of physical properties

Previous experimental and theoretical studies have shown that the glass transition temperature (Tg) of dendritic

macro-molecules depends mainly on the internal monomer units and the polarity and number of the end groups [69–71]. The

Tgs of these dendritic ether–imides, as determined by

differ-ential scanning calorimetry (DSC), are presented in Table 2. For dendrimers with identical terminal functionality, Tg

Fig. 2. 300-MHz1H NMR spectra in the region of 4.9–7.9 ppm for [G1]3[C]-(OH)6and [G2]3[C]-(OH)12in DMSO-d6.

Table 2

Effect of the functionality of the chain ends on the thermal and solution properties of the dendritic poly(ether–imide)s. Solubility:⫹, soluble; ⫹ ⫺ , partially soluble;⫺, insoluble

Dendrimer Tg(⬚C) Solubility in

Toluene CH2Cl2 CHCl3 THF DMF DMSO CH3OH/ 1 N NaOH(aq)a

[G1]3[C] 120 ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫺ ⫺ [G2]3[C] 159 ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫺ ⫺ [G1]3[C]-(OH)6 190 ⫺ ⫺ ⫺ ⫹ ⫹ ⫹ ⫹ [G2]3[C]-(OH)12 210 ⫺ ⫺ ⫺ ⫹ ⫹ ⫹ ⫹ [G1]3[C]-(OR)6 76 ⫹ ⫹ ⫹ ⫹ ⫹ ⫺ ⫺ [G2]3[C]-(OR)12 93 ⫹ ⫹ ⫹ ⫹ ⫹ ⫺ ⫺ a _{Volume ratio 1:1.}

increases with increasing molecular weight. Within the same generation, the value of Tg for the ether–imides

increases with increasing chain-end polarity. For the second-generation dendritic ether–imide, an increase in the end group polarity from n-hexyl ether to benzyl ether to a hydroxyl group results in an increase in Tgfrom 93 to

159 to 210⬚C. The relatively high Tgof [G2]3[C]-(OH)12is

probably a reflection of the hydrogen-bonding capability of the phenolic terminal groups.

Because of their highly branched structures, these dendri-tic ether–imides have enhanced solubility in organic solvents and are highly soluble in solvents such as THF and DMF. However, the different end groups also lead to differences in solubility, as shown in Table 2. The ether-terminated dendrimers are soluble in relatively nonpolar solvents such as toluene, CH2Cl2 and CHCl3, but are

insoluble or partially soluble in DMSO. In contrast, the phenolic-terminated dendrimers are soluble in DMSO but insoluble in relatively nonpolar solvents. In addition, the phenolic-terminated dendrimers are soluble in a mixed solvent of NaOH(aq)/CH3OH due to the acidic nature of

phenolic groups.

4. Summary

Dendritic ether–imide macromolecules with modifiable chain ends were synthesized by the stepwise convergent growth approach using 1-(4-aminophenyl)-1,1-bis(4-hydro-xyphenyl)ethane 2 as the building block. The synthetic sequence for the ether–imide dendrons was based on (i) the aromatic nucleophilic substitution of an activated nitro group with a phenoxide nucleophile to form the ether link-age [57], and (ii) the condensation of the aminophenyl group with nitro-substituted phthalic anhydride, followed by ring-closure to yield the imide ring, which contained the reactive nitro functionality [56]. Through aromatic nucleophilic

displacement, the dendritic wedges with an activated nitro group were coupled to the polyfunctional core, 1,1,1-tris(4-hydroxyphenyl)ethane, to give the dendritic poly(ether– imide)s. The subsequent selective removal of the benzyl ether chain ends by catalytic hydrogenolysis yielded dendri-mers with phenolic chain ends. Further modification of the chain ends was readily accomplished by Mitsunobu etherfi-cation [53]. The synthetic strategy employed here provides a clear demonstration of the protection–deprotection approach for the modification of the chain ends of dendritic macromolecules. The effect of the chain-end functional group on the physical properties of these ether–imide dendrimers was also investigated. The nature of the end groups was found to strongly influence the thermal and solution behavior of the various surface-modified dendri-mers. We are currently extending the use of compound 2 as the structural component in the one-step synthesis of hyperbranched poly(ether–imide)s [72] as well as in the preparation of poly(ether–imide) dendrimers via the ortho-gonal coupling approach [73,74].

Acknowledgements

We wish to acknowledge the National Science Council (ROC) (NSC 89-2113-M009-001) for financial support.

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