Poly(urethane/malonamide) dendritic structures featuring blocked/deblocked isocyanate units

Abstract

We have used 4-isocyanato-4′-(3,3-dimethyl-2,4-dioxoazetidino)diphenylmethane and diethylenetriamine as building blocks to synthesize novel poly(urethane/malonamide) dendrons possessing terminal methyl ethyl ketoxime (MEKO) units (blocked isocyanategroups). Heating the MEKO-containing dendrons regenerated the terminal isocyanategroups. Subsequently, the regenerated isocyanategroups would react with any compound with active hydrogens. In one example, the dendrons with the deblocked isocyanates further reacted with stearyl alcohol (C18-OH) to form the corresponding dendrons presenting C18 moieties. This deblocking strategy allows replacement of reactive exterior groups with desired functionality for the construction of dendritic macromolecules.

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Introduction

Dendritic macromolecules such as dendrimers, dendrons, dendronized polymers, and hyperbranched polymers are among the most exciting molecular architectures to have been developed over the past 20 years;^1–3 they have led to the fabrication of new classes of materials having promising applications in various fields, ranging from biochemistry to microelectronics.^4–7 Typically, the preparation of divergent or convergent dendrimers is complicated by the number of synthetic steps and by their unpredictable reactivity at higher generations. Therefore, the use of dendrimers with blocked peripheral functional groups and their subsequent deblocking to obtain dendrimers with a diverse number of moieties has become a new concept for accelerating the applications of dendritic materials.^8,9

In previous studies, we employed the distinctive doubly functionalized molecule 4-isocyanato-4-(3,3-dimethyl-2,4-dioxoacetidino)diphenylmethane (IDD) to synthesize a series of dendrons in high yield under mild conditions.^10–12 The isocyanategroup of IDD is the more reactive of its two functional groups; it reacts readily with compounds possessing amino, hydroxyl, or other groups possessing active hydrogen atoms. Although the azetidine-2,4-dione unit is less reactive, it is more selective and reacts only with aliphatic primary amines, forming malonamide linkages, under ambient conditions.¹³ Using IDD, we have prepared series of dendrons presenting phenyl,^10–12,14decyl,¹⁵ and stearyl groups,¹⁶ and azo chromophores (disperse red 1, DR1)^17–19 at the exterior. Although these syntheses were successful, the complicated nature of the synthetic route was somewhat painstaking. To circumvent this dilemma, it is sensible to post-functionalize the dendrons with certain blocked functional groups in periphery.

For the building block structure, the highly reactivity isocyanategroup of IDD should be reacted firstly. According to the literature,^20–22 any urethane functional group can be considered as a “blocked isocyanate” that can be regenerated through thermal splitting; subsequently, the regenerated isocyanate can react with compounds featuring active hydrogen atoms to form new functional groups. This principle is applied in several heat-curable systems, such as powder coatings and heat setting adhesives.^8,9 The typical blocking agents used to form the blocked isocyanategroups are phenol, caprolactam (CL), and methyl ethyl ketoxime (MEKO).⁸ Among these blocking agents, oxime derivatives are more widely used because they require lower deblocking temperatures relative to those of alcohol, phenol, or caprolactam derivatives. Moreover, oxime derivatives are readily available through the reactions of ketones or aldehydes with hydroxylamine. In addition, because oximes exhibit high reactivity toward isocyanategroups, the blocked products are readily formed without the need for catalysts.

In this study, we reacted (Scheme 1) MEKO with IDD to form the blocked-isocyanate dendron structure [G0.5]-MEKO, which we then reacted with diethylenetriamine (DETA) through the azetidine-2,4-dionegroup, selectively with the primary amino groups. The remaining secondary amino group was then further reacted with IDD to afford the next generation of dendron. After sequential and alternative incorporation of the IDD and DETA into the growing dendrons, we obtained a series of blocked dendrons, ranging from [G0.5]-MEKO to [G2.5]-MEKO. Using an appropriate deblocking temperature, we regenerated the isocyanategroups for further reactions with stearyl alcohol (C18–OH) to produce (Scheme 2) a series of dendrons presenting C18groups on their exterior surfaces. This deblocking strategy for replacement with desired exterior functionality might accelerate the applications of dendritic macromolecules.

Scheme 1 Synthesis of the MEKO-containing dendrons.

Scheme 2 Synthesis of the C18-containing dendrons.

Experimental

Materials

IDD was prepared according to a method described in the literature.^10–19MEKO (Acros), DETA (Acros), and C18–OH (Showa) were used as received. Tetrahydrofuran (THF, Tedia), N,N-dimethylacetamide (DMAc, Tedia), and N,N-dimethylformamide (DMF, Tedia) were purified through distillation under reduced pressure over CaH₂ and then stored over 4 Å molecular sieves.

Measurements

¹H NMR spectra were recorded using a Varian Gemini-200 FT-NMR spectrometer. Fourier transform infrared (FTIR) spectra were recorded using a PerkinElmer Spectrumspectrometer. Variable-temperature infrared (VTIR) spectra were recorded using a PerkinElmer Spectrum One FTIR spectrometer operated at a heating rate of 10 °C min⁻¹ (from room temperature to 150 °C). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed using a Seiko SSC-5200 apparatus operated at a heating rate of 10 °C min⁻¹ under a N₂ atmosphere. Thermal degradation temperatures (T_d) were taken as the temperatures at which 5% weight losses occurred. Fast atom bombardment mass spectrometry (FABMS) was performed using a JEOL JMS SX/SX 102A mass spectrometer equipped with a standard FAB source. Gel permeation chromatography (GPC) was performed using a Waters apparatus equipped with a Waters Stygel columns and a refractive index (RI) detector, with THF as an eluent (polystyrene calibration). Elemental analysis was performed using a Heraeus CHN-OS Rapid Analyzer.

MEKO-containing dendrons

[G-0.5]-MEKO. A solution of IDD (2.44 g, 7.61 mmol) and MEKO (0.547 g, 6.28 mmol) in dry THF (10 mL) was stirred at 60 °C under a N₂ atmosphere for 6 h. The resulting solution was poured into hot cyclohexane to give a white solid, which was collected and dried under vacuum at 60 °C; yield: 83%. IR (KBr, cm⁻¹): 1856 (C=O), 1743 (C=O). ¹H NMR [DMSO-d₆, δ (ppm)]: 1.06 (t, J = 3.6 Hz, 3H, CH₃), 1.38 (s, 6H, CH₃), 1.96 (s, 3H, =CCH₃), 2.29 (m, H, =CCH̲₂CH₃), 3.88 (s, 2H, ArCH₂Ar), 7.10–7.63 (b, 8H, ArH), 9.53 (s, 1H, NH). Anal. (C₂₃H₂₅N₃O₄): Calcd. C 67.80%, H 6.80%, N 10.31%; Found C 67.61%, H 6.56%, N 10.33%. MS (FAB): m/z 407 [M⁺]. GPC (THF): polydispersity index (PDI) = 1.03; number-average molecular weight (M_n) = 391; weight-average molecular weight (M_w) = 405.

[G-1]-MEKO . DETA (0.256 g, 2.48 mmol) was added slowly to a stirred suspension of [G-0.5]-MEKO (2.08 g, 5.11 mmol) in dry THF (15 mL) and the stirred for 4 h. The mixture was poured into water, and washed with EtOAc to give [G-1]-MEKO (66%). IR (KBr, cm⁻¹): 1740 (C=O), 1650 [C=O(NH)]. ¹H NMR [DMSO-d₆, δ (ppm)]: 1.06 (t, J = 3.6 Hz, 6H, CH₃), 1.35 [s, 12H, C(CH̲₃)₂CONH], 1.94 (s, 6H, =CCH₃), 2.27 (m, 4H, =CCH̲₂CH₃), 3.11 (m, 8H, (CH̲₂)₂NHCO), 3.77 (s, 4H, ArCH₂Ar), 7.19–7.51 (b, 16H, ArH), 7.94–9.15 (b, 7H, NH). Anal. (C₅₀H₆₃N₉O₈): Calcd. C 65.41%, H 6.92%, N 13.73%; Found C 63.97%, H 6.99%, N 13.40%. MS (FAB): m/z 919 [M⁺].

[G-1.5]-MEKO. A solution of IDD (2.09 g, 6.53 mmol) and [G-1]-MEKO (5.00 g, 5.44 mmol) in dry THF (15 mL) was stirred at 70 °C for 6 h. The solvent was evaporated and the residue purified through column chromatography (SiO₂, EtOAc). After drying under vacuum at 60 °C for 6 h, the product was obtained as a white powder (67%). IR (KBr, cm⁻¹): 1860 (C=O), 1740 (C=O), 1663 [C=O(NH)]. ¹H NMR [CDCl₃, δ (ppm)]: 1.02 (t, J = 3.6 Hz, 6H, CH₃), 1.14 (s, 6H, CH₃), 1.39 [s, 12H, C(CH̲₃)₂CONH], 1.94 (s, 6H, =CCH₃), 2.30 (m, 4H, =CCH̲₂CH₃), 3.32 (m, 8H, (CH̲₂)₂NHCO), 3.85 (s, 6H, ArCH₂Ar), 7.02–7.68 (b, 24H, ArH), 8.14–9.05 (b, 7H, NH). Anal. (C₅₀H₆₃N₉O₈): Calcd. C 66.92%, H 6.43%, N 12.44%; Found C 66.47%, H 6.48%, N 12.09%. MS (FAB): m/z 1238 [M⁺]. GPC (THF): PDI = 1.03; M_n = 1424; M_w = 1471.

[G-2]-MEKO . DETA (0.203 g, 1.97 mmol) was added slowly to a stirred solution of [G-1.5]-MEKO (5.00 g, 4.04 mmol) in dry THF (25 mL) and the stirred for 6 h. The solvent was evaporated and the residue purified through column chromatography (SiO₂, EtOAc) to give [G-2]-MEKO. After drying under vacuum at 60 °C for 6 h, the product was obtained as a light-yellow powder (44%). IR (KBr, cm⁻¹): 1705 (C=O), 1658 (C=O). ¹H NMR [DMSO-d₆, δ (ppm)]: 1.06 (m, 12H, CH₃), 1.35 (s, 36H, CH₃), 1.95 (s, 12H, =CCH₃), 2.27 (m, 8H, =CCH̲₂CH₃), 3.32 (b, 24H, (CH̲₂)₂NHCO), 3.78 (s, 12H, ArCH₂Ar), 7.08–7.47 (b, 48H, ArH), 7.91–9.48 (b, 19H, NH). Anal. (C₅₀H₆₃N₉O₈): Calcd. C 66.1%, H 6.68%, N 13.57%; Found C 65.67%, H 6.78%, N 13.09%. GPC (THF) PDI = 1.06; M_n = 2680; M_w = 2840.

[G-2.5]-MEKO. A solution of [G-2]-MEKO (5.00 g, 1.94 mmol) and IDD (0.744 g, 2.33 mmol) in dry THF (25 mL) was stirred at 70 °C for 6 h. The solvent was evaporated and the residue purified through column chromatography (SiO₂; EtOAc/hexane, 1 : 1). After drying under vacuum at 60 °C for 6 h, the product was obtained as a light-yellow powder (42%). IR (KBr, cm⁻¹): 1855 (C=O), 1740 (C=O). ¹H NMR [DMSO-d₆, δ (ppm)]: 1.10 (m, 12H, CH₃), 1.35 (b, 42H, CH₃), 1.95 (s, 12H, =CCH₃), 2.29 (m, 8H, =CCH̲₂CH₃), 3.31 (b, 24H, CH̲₂NHCO), 3.80 (b, 14H, ArCH₂Ar), 7.10–7.51 (b, 56H, ArH), 7.91–9.49 (b, 19H, NH). Anal. (C₁₆₁H₁₈₇N₂₇O₂₅): Calcd. C 66.67%, H 6.50%, N 13.04%; Found C 65.11%, H 6.55%, N 12.27%. GPC (THF) PDI = 1.08; M_n = 2923; M_w = 3159.

C18-containing dendrons

[G-0.5]-C18. A mixture of [G-0.5]-MEKO (0.305 g, 0.750 mmol) and C18-OH (0.262 g, 0.969 mmol) was heated at 130 °C for 5 h under vacuum in a 100 mL flask. The product was washed thoroughly with hot MeOH to give [G-0.5]-C18 in 74% yield. ¹H NMR [CDCl₃, δ (ppm)]: 0.81 (t, J = 3.6 Hz, 3H, CH₃), 1.19 (b, 30H, CH₂), 1.40 (s, 6H, CH₃), 1.50 (m, 2H, NHCOOCH₂CH̲₂), 3.86 (s, 2H, ArCH₂Ar), 4.09 (t, J = 6.6 Hz, 2H, NHCOOCH̲₂), 6.46 (s, 1H, NH), 7.00–7.69 (b, 8H, ArH). Anal. (C₃₇H₅₄N₂O₄): Calcd. C 75.21%, H 9.21%, N 4.74%; Found C 75.16%, H 9.56%, N 4.17%. MS (FAB): m/z 590 [M⁺].

[G-1.5]-C18 . [G-1.5]-MEKO (0.309 g, 0.250 mmol) and C18-OH (0.166 g, 0.614 mmol) were heated at 130 °C for 5 h under vacuum in a 100 mL flask. The product was washed thoroughly with hot MeOH to give [G-1.5]-C18 in 72% yield. ¹H NMR [DMSO-d₆, δ (ppm)]: 0.83 (t, J = 3.6 Hz, 6H, CH₃), 1.21 (b, 60H, CH₂), 1.34 (s, 18H, CH₃), 1.57 (m, 4H, NHCOOCH₂CH̲₂), 3.33 (b, 8H, (CH̲₂)₂NHCO), 3.77 (s, 6H, ArCH₂Ar), 4.01 (t, J = 6.6 Hz, 4H, NHCOOCH̲₂), 7.03–7.91 (b, 24H, ArH), 7.91–9.48 (b, 7H, NH). Anal. (C₇₈H₁₃₇N₉O₁₁): Calcd. C 72.58%, H 8.60%, N 7.85%; Found C 71.29%, H 8.35%, N 8.30%. MS (FAB): m/z 1604 [M⁺].

[G-2.5]-C18 . [G2.5]-MEKO and C18-OH (feed molar ratio of 1 : 4.8, 1 : 8.0, and 1 : 16; Table 3) were heated at 130 °C for 5 h under vacuum. The product was washed thoroughly with hot MeOH; ¹H NMR spectra were recorded to determine the graft ratio of the product dendron. MALDI-TOF: m/z 3655 [M + Na⁺] for feed molar ratio at 1 : 16 in 28% yield.

Results and discussion

Synthesis and characterization of MEKO-containing dendrons

Using a convergent route (Scheme 1), we prepared a series of novel blocked dendrons, from G0.5[MEKO] to G2.5[MEKO], presenting MEKO units as blocking agents on their exterior surfaces. [G-0.5]-MEKO was obtained from the addition reaction between the isocyanategroup of IDD and the OHgroup of MEKO, forming a urethane linkage. Subsequently, [G-1]-MEKO was readily obtained from [G-0.5]-MEKO through a ring opening reaction of its azetidine-2,4-dione unit with DETA. Because of the selectivity of the azetidine-2,4-dione functional group,¹³ the focal point (the secondary amino group from the DETA unit) remained active for reaction with IDD to generate the next generation of dendrons. After sequential and alternative incorporation of the IDD and DETA onto the growing dendron molecules, we obtained a series of MEKO-containing dendrons. Notably, we performed the growth reactions of these dendrons with IDD and DETA under mild conditions without any added catalysts. We chose MEKO as the blocking agent for the isocyanategroups because it can be deblocked at elevated temperatures to regenerate the reactive functionality.⁸ Using the novel MEKO-containing dendrons as model materials allows the possibility to prepare dendrons presenting various functionalities on their exterior surfaces.

We used FTIR spectroscopy, ¹H NMR spectroscopy, elemental analysis, GPC, and FAB-MS to confirm the chemical structures of the MEKO-containing dendrons. In the FTIR spectrum of [G-0.5]-MEKO, the signal of the isocyanategroup (2260 cm⁻¹) of IDD was absent, but those of the azetidine-2,4-dionegroup (1856 and 1743 cm⁻¹) remained intact. The subsequent ring opening reaction of [G-0.5]-MEKO with DETA gave [G-1]-MEKO, featuring a secondary amino group at its focal point. The signals at 1856 and 1743 cm⁻¹ disappeared completely, revealing that the ring opening reaction had occurred; in addition, a new signal emerged at 1650 cm⁻¹, corresponding to the C=O group of the malonamide linkage. The ¹H NMR spectra were in good agreement with the expected structures of the MEKO-containing dendrons and the elemental analysis data were close to the calculated values. In addition, the FABmass spectra of the dendrons from [G-0.5]-MEKO to [G-1.5]-MEKO (Fig. 1(a)) revealed molecular weights that were in agreement with the calculated values. Notably, however, the MALDI-TOF mass spectra of [G-2]-MEKO and [G-2.5]-MEKO did not contain signals for molecular ions having the correct theoretical molecular weights, possibly because of fragmentation of the MEKOgroups.²³ We performed GPC with THF as the eluent to determine the molecular weight distributions (M_w/M_n) of the MEKO-containing dendrons; the retention time decreased upon increasing the dendron generation (Fig. 2). The PDIs of [G-0.5]-MEKO, [G-1.5]-MEKO, and [G-2.5]-MEKO were 1.03, 1.03, and 1.08, respectively, indicating that they were nearly monodisperse. In addition, the retention time at 20 min was the THFsolvent peak. On the other hand, the small bump around 14.5 min next to the peak of [G-2.5]-MEKO indicates the presence of a higher molecular weight macromolecule. The possible cause for 3% impurity might be due to the amino group of excessive DETA reacting with the terminal urethane linkage of [G-2.5]-MEKO. A similar amination reaction had been addressed in our lab previously.²⁴ This side reaction led to the coupling of two mono-[G-2.5]-MEKO to form a higher molecular weight of dimer-[G-2.5]-MEKO. Taken together, these characterization data confirm the successful formation of the MEKO-containing dendrons.

Fig. 1 FAB mass spectra of (a) [G-1.5]-MEKO and (b) [G-1.5]-C18.

Fig. 2 GPC traces of (a) [G-0.5]-MEKO, (b) [G-1.5]-MEKO, and (c) [G-2.5]-MEKO.

Table 1 lists the solubilities of the MEKO-containing dendrons. All of them exhibited excellent solubility in highly polar solvents (DMAc, DMF); most were also soluble in THF, 1,4-dioxane, and acetone. This excellent solubility was probably due to the highly branched nature of the dendrons. Notably, however, [G-1]-MEKO and [G-2]-MEKO exhibited poor solubility in 1,4-dioxane and acetone, possibly because the absence of the IDD moieties encouraged noncovalent packing.^18,19

Table 1 Solubility of the MEKO-containing dendrons

Sample^a	DMAc	DMF	THF	1,4-Dioxane	Acetone	Toluene	MeOH
a Qualitative solubility was determined by attempting to dissolve 10 mg of sample in 1 mL of solvent. +: Soluble at room temperature. + −: Soluble when heated at 60 °C. −: Insoluble even when heated at 60 °C.
[G-0.5]-MEKO	+	+	+	+	+	+	+
[G-1]-MEKO	+	+	+	+ −	+ −	+ −	+ −
[G-1.5]-MEKO	+	+	+	+	+	+ −	+
[G-2]-MEKO	+	+	+	+ −	+ −	−	+ −
[G-2.5]-MEKO	+	+	+	+	+	−	−

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We used DSC and TGA to measure the melting temperatures (T_m), glass transition temperatures (T_g), and thermal degradation temperatures (T_d) of the MEKO-containing dendrons; Table 2 summarizes their thermal properties. Dendrons [G-0.5]-MEKO and [G-1]-MEKO melted at 113 and 76 °C, respectively. As the generation increased, the dendrons became more amorphous, exhibiting only glass transitions in the range from 66 to 82 °C. The values of T_g decreased upon increasing the dendron generation, possibly because of the greater difficulty the dendrons encountered when undergoing molecular packing of their highly branched structures. In DSC thermograms, we found that the value of T_m of [G-0.5]-MEKO was close to its deblocking temperature; the other MEKO-containing dendrons also exhibited endothermic peaks close to 130 °C, indicating the occurrence of deblocking reactions.^8,9 The values of T_d were all in the range between 176 and 226 °C.

Table 2 Representative data for the MEKO- and C18-containing dendrons

Sample	Formula	M w	T m	T g	T d	Yield (%)
	M w
a Theoretical molecular weight. b Determined using FAB-MS. c Baseline shift in the second heating DSC trace (heating rate: 10 °C min^−1) under N2. d Temperature at which 5% weight loss occurred (heating rate: 10 °C min^−1). e Replacement yield. f Not detectable.
[G-0.5]-MEKO	C23H25N3O4	407	113	N.D.^f	188	83
	407
[G-1]-MEKO	C50H63N9O8	919	76	N.D.	176	66
	918
[G-1.5]-MEKO	C69H79N11O11	1238	N.D.	82	226	67
	1238
[G-2]-MEKO	C142H171N25O22	N.D.	N.D.	76	201	44
	2580
[G-2.5]-MEKO	C161H187N27O25	N.D.	N.D.	66	203	42
	2900
[G-0.5]-C18	C37H54N2O4	590	108	N.D.	271	74^e
	591
[G-1.5]-C18	C97H137N9O11	1604	N.D.	64	287	72^e
	1605

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To further determine the deblocking temperature, we recorded variable-temperature IR spectra of the [G-0.5]-MEKO dendron from 25 to 150 °C at a heating rate of 10 °C min⁻¹ (Fig. 3). For temperatures of up to 130 °C, the characteristic isocyanate absorption appeared at 2260 cm⁻¹. This deblocking temperature is consistent with those reported previously.^8,9,25

Fig. 3 VTIR spectra of [G-0.5]-MEKO recorded at various temperatures.

Synthesis and characterization of C18-containing dendrons

According to the literature^8,9,25 and our experimental findings described above, we performed deblocking of the MEKO-containing dendrons at 130 °C in the presence of C18-OH, which reacted with the free isocyanategroups of the deblocked dendrons to form novel dendrons presenting C18 moieties at their exterior surfaces. Note that we performed these deblocking reactions only with the dendrons ([G-0.5]-MEKO, [G-1.5]-MEKO, and [G-2.5]-MEKO), because their azetidine-2,4-dione functionality was unreactive toward the free isocyanategroups. Moreover, the deblocking process was sensitive to the presence of moisture, which would lead to the formation of reactive hydroxylamine and butan-2-one from the deblocked MEKO monomer.²⁵ Therefore, we performed deblocking reactions under dry conditions, applying a vacuum to remove MEKO, and any other compounds generated during the deblocking process, and, thereby, minimize the extent of side reactions.

Scheme 2 displays the C18-containing dendrons that we derived from the MEKO-containing dendrons. To ensure high yields of their corresponding C18-containing dendrons, we reacted [G-0.5]-MEKO and [G-1.5]-MEKO with stearyl alcohol at molar ratios of 1 : 1.2 and 1 : 2.4 (Table 3), respectively. Fig. 1 presents FAB mass spectra of [G-1.5]-MEKO and [G-1.5]-C18; the signal for the molecular ion of [G-1.5]-MEKO at m/z 1238 was absent in the spectrum of [G-1.5]-C18, which featured a signal for its molecular ion at m/z 1604. Mass spectra confirmed the structures of the two dendrons [G-0.5]-C18 and [G-1.5]-C18, which we had synthesized in high yields (Table 2).

Table 3 Feed molar ratios and graft ratios for MEKO-containing dendrons

Sample^a	Feed molar ratio		Graft ratios (%)^b
	[G-n]-MEKO : C18-OH	MEKO : C18-OH
a Deblocking process took place at 130 °C. b The molar ratios of C18 : MEKO were determined by ^1H NMR.
[G-0.5]-MEKO	1 : 1.2	1 : 1.2	100
[G-1.5]-MEKO	1 : 2.4	1 : 1.2	100
[G-2.5]-MEKO	1 : 4.8	1 : 1.2	25–50
	1 : 8.0	1 : 2.0	75
	1 : 16	1 : 4.0	Close to 100

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Table 2 summarizes the thermal properties of [G-0.5]-C18 and [G-1.5]-C18. Whereas [G-0.5]-C18 melted at 108 °C, [G-1.5]-C18 was amorphous and exhibited only a glass transition at 64 °C. Not surprisingly, the DSC data were different from those obtained for the MEKO-containing dendrons, but consistent with the thermal properties described previously in the literature for related compounds.¹⁶ The thermal degradation temperatures (T_d) of [G-0.5]-C18 and [G-1.5]-C18, determined using TGA, were 271 and 287 °C, respectively.

For [G-2.5]-MEKO, when we performed the deblocking process in the presence of C18-OH at a molar ratio of 1 : 4.8, the large number of MEKO moieties at the periphery resulted in crosslinking phenomena occurring during the deblocking process at 130 °C. This could be attributed to the large number of regenerated isocyanategroups which will form complicated reactions with multiple side products such as isocyanatedimerization or trimerization, particular when a large excess of C18-OH is absent.^8,9 From the ¹H NMR spectrum of the resulting soluble powder [Fig. 4(a)], we determined the grafting ratio (from integration of the signals of the azetidine-2,4-dione and C18methylene units at 1.35 and 1.21 ppm, respectively) to be in the range 25–50%. To enhance the grafting ratio, we reacted [G-2.5]-MEKO with higher molar ratios of C18-OH (Table 3). Fig. 4(b) and 4(c), respectively, display the resulting ¹H NMR spectra. Upon increasing the amount of the C18-OH, the grafting ratio was enhanced, up to almost 100% at a 1 : 16 ratio of [G-2.5]-MEKO to C18-OH. It is important to note that the grafting ratios for dendrons prepared from [G-2.5]-MEKO reached less than 75% in the cases with the molar ratios of [G-2.5]-MEKO : C18-OH at 1 : 4.8 and 1 : 8. We conclude that steric hindrance played an important role affecting the grafting ratio of this high-generation dendron, and that its large number of regenerated isocyanategroups could undergo side reactions in the absence of a large excess of C18-OH.

Fig. 4 ¹H NMR spectra of the dendrons prepared from [G-2.5]-MEKO and C18-OH at feed molar ratios of (a) 1 : 4.8, (b) 1 : 8.0, and (c) 1 : 16.

Conclusion

We have prepared a series of MEKO-containing dendrons, based on IDD building blocks, through a facile convergent synthetic scheme that does not involve protection/deprotection processes. The MEKO blocking agent for the isocyanategroups could be removed at a relatively low deblocking temperature (130 °C), allowing ready conversion to dendron structures presenting multiple isocyanategroups on the exterior surface. Reactions of the MEKO-containing dendrons with C18-OH at this deblocking temperature provided another series of dendrons presenting C18 units at their peripheries. We obtained the dendrons [G-0.5]-C18 and [G-1.5]-C18 in high yields (>70%), but, because of the effects of steric hindrance and crosslinking during the deblocking process, the higher-generation dendron [G-2.5]-C18 comprising 100% grafting ratio was formed with a low yield (28%). This approach appears to be effective for the construction of dendrons presenting various functional groups on their exterior surfaces.

Acknowledgements

We thank the National Science Council of Taiwan and the Ministry of Education of Taiwan, under ATU plan, for financial support.

Notes and references

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