Synthesis of well-defined, amphiphilic poly(ethylene glycol)-b-hyperbranched polyamide

Authors


Correspondence to: T. Yokozawa (E-mail: yokozt01@kanagawa-u.ac.jp)

Abstract

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Condensation reaction of poly(ethylene glycol) (PEG) with a carboxyl group at one chain end and hyperbranched polyamide (HBPA) with a hydroxyl group at the focal point was performed in the presence of condensation agent at ambient temperature, yielding PEG-b-HBPAs with defined molecular weight and very narrow molecular weight distribution. PEG-b-HBPA formed micelles in water.

INTRODUCTION

Amphiphilic linear-dendrimer and linear-hyperbranched block copolymers have received considerable attention in recent years, because these block copolymers have potential applications for drug delivery, as biomaterials, and for molecular templating.[1-12] The former block copolymers containing dendrimer show lower values of critical micelle concentration (CMC), compared with those of low-molecular-weight surfactants such as sodium dodecyl sulfate.[13] Furthermore, the size, morphology, and CMC value of micelles can be tuned by changing the length of the linear polymer and the dendrimer generation number in the block copolymers,[14, 15] but it is often tedious to obtain high-generation dendrimers because of the need for multistep reactions, including protection and deprotection as well as purification, at each generation. On the other hand, hyperbranched polymers can be prepared by one-step polymerization of ABm (m ≥ 2) type monomers, without the numerous protection, deprotection, and purification steps.[5, 16-20] However, hyperbranched polymers are generally synthesized by step-growth polymerization and therefore possess uncontrolled molecular weight and broad molecular weight distribution. Thus, it is a challenge to synthesize well-defined linear-hyperbranched block copolymers. A few examples of linear-hyperbranched block copolymers with low polydispersity have been reported.[21-26] Frey and coworkers[21-24, 26] introduced the synthesis of linear-hyperbranched block copolymers based on a short, multifunctional block that was used for hypergrafting of glycidol and AB2- as well as AB3-type carbosilane monomers, respectively. We have synthesized hyperbranched polyamides (HBPA) with controlled molecular weight and low polydispersity by means of chain-growth condensation polymerization (CGCP) of AB2 monomers with a core initiator, and applied this polymerization method to obtain well-defined linear-hyperbranched polyamide diblock copolymers, using a convenient one-pot monomer addition method.[27-30] Furthermore, CGCP of AB2 monomers enables us to introduce functional groups into the core initiator unit of HBPA. In this article, we describe synthesis of well-defined, amphiphilic linear-hyperbranched block copolymers consisting of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic HBPA by means of two approaches: first, CGCP of AB2 monomer from PEG macroinitiator [Scheme 1(a)] and second, condensation reaction of PEG with a carboxyl group at one chain end and HBPA with a hydroxyl group on the core initiator unit [Scheme 1(b)]. The solubility and preliminary aqueous solution properties of PEG-b-HBPA were also investigated.

Scheme 1.

Syntheses of PEG-b-HBPA by (a) polymerization of 1 with PEG macroinitiator 2 and (b) condensation reaction of PEG-COOH 4 and HO-HBPA 3.

EXPERIMENTAL

Materials

A solution of lithium 1,1,1,3,3,3-hexamethyldisilazide (LiHMDS, Aldrich; 1.0 M solution in THF), dehydrated ethanol (Wako), dehydrated acetone (Wako), dehydrated N,N-dimethylformamide (DMF) (Kanto), dehydrated tetrahydrofuran (THF) (Kanto) and dehydrated dichloromethane (CH2Cl2) (Kanto) were used as received without purification. The AB2 monomers 1, low-molecular-weight initiator, diethyl 5-{N-methyl-N-[3,5-bis(trifluoromethyl)benzoyl]} aminoisophthalate (5), 4-(tert-butyldimethylsilyloxymethyl)benzoic acid (8), and 5-aminoisophthalic acid diethyl ester were prepared according to the literature.[27, 28] PEG macroinitiator 2, PEG-COOH 4, diethyl 5-{N-methyl-N-[4-(tert-butyldimethylsilyloxymethyl)benzoyl]}aminoisophthalate (10), TBS-HBPA (11), and HO-HBPA 3 were synthesized as described in the Supporting Information.

Measurements

1H and 13C NMR spectra were obtained on JEOL ECA-600 and ECA-500 instruments. The internal standards for 1H and 13C NMR experiments in CDCl3 were tetramethylsilane (0.00 ppm) and the midpoint of CDCl3 (77.0 ppm), respectively. IR spectra were recorded on a JASCO FT/IR-410. Column chromatography was performed on silica gel (Kieselgel 60, 230–400 mesh, Merck) with a specified solvent. The Mn and Mw/Mn values of polymers were measured on a Shodex GPC-101 (eluent: THF; calibration: polystyrene standards) equipped with Shodex UV-41, Shodex RI-71S, and Wyatt Technology DAWN EOS multiangle laser light scattering (MALLS, Ga-As laser, λ = 690 nm) detectors and two Shodex KF-804-L columns.

Polymerization of 1 with PEG Macroinitiator 2

LiCl (0.160 g, 3.77 mmol) was placed in a round-bottomed flask equipped with a three-way stopcock, and dried at 250 °C under reduced pressure. The flask was cooled to room temperature under an argon atmosphere, and then charged with 1.0 M LiHMDS in THF (0.720 mL, 0.720 mmol). The flask was cooled to −30 °C under an argon atmosphere with stirring. PEG macroinitiator 2 (0.126 g, 0.0248 mmol) was charged into a pear-shaped flask, azeotropically dehydrated with dry toluene three times, and then dried under reduced pressure. Dry THF (1.0 mL) was added to the pear-shaped flask under dry nitrogen. The solution of 2 was added to the round-bottomed flask containing LiHMDS under dry nitrogen, then a solution of 1 (0.168 g, 0.669 mmol) in dry THF (4.0 mL) was added dropwise over about 40 min at −30 °C with stirring under dry nitrogen. The mixture was stirred at −30 °C for 1 h, and then the reaction was quenched with saturated aqueous NH4Cl. The whole was extracted with CH2Cl2. The organic layer was washed with water, dried over anhydrous MgSO4, and concentrated under reduced pressure. The residue was dissolved in CH2Cl2, and the solution was added to ether. Insoluble material was collected by filtration, washed with ether, and dried in a desiccator to give PEG-b-HBPA as a white solid (0.175 g, 67%).

Condensation Reaction of PEG-COOH 4 and HO-HBPA 3b

To a solution of HO-HBPA 3b (0.1053 g, 0.0391 mmol) in dry DMF (2.0 mL) was added PEG-COOH 4 (0.1000 g, 0.0196 mmol), 4-(dimethylamino)pyridine (DMAP) (0.0064 g, 0.052 mmol), and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDCI) (0.0090 g, 0.047 mmol) at ambient temperature. The mixture was stirred for 20 h, and then the reaction was quenched with water. The whole was extracted with CH2Cl2. The combined organic layers were washed with 1 M HCl and brine, dried over anhydrous MgSO4, and concentrated in vacuo. The residue was dissolved in CH2Cl2, and the solution was added to ethyl acetate/ether (3/5, v/v). Insoluble material was collected by filtration and washed with ethyl acetate/ether (3/5, v/v). This precipitation purification was performed twice. The obtained product was dried in a desiccator to give PEG-b-HBPA as a white solid (0.0946 g, 62%, Mn(MALLS) = 7960, Mw/Mn = 1.03).

RESULTS AND DISCUSSION

Polymerization of 1 with PEG macroinitiator 2 ([1]0/[2]0 = 31) was first conducted in the presence of LiHMDS and LiCl in THF at −30 °C, which is the optimized condition for polymerization of 1 with low-molecular-weight bifunctional initiator 5, diethyl 5-{N-methyl-N-[3,5-bis(trifluoromethyl)benzoyl]}aminoisophthalate. However, PEG macroinitiator 2 and/or products were precipitated during addition of 1 to the mixture of 2 and LiHMDS, resulting in a polymer with a broad molecular weight distribution (Mw/Mn = 1.86) [Fig. 1(a)]. To suppress precipitation, the polymerization temperature was raised from −30 °C to −10 °C, but precipitation still occurred to afford a polymer with a broad molecular weight distribution (Mw/Mn = 2.62) [Fig. 1(b)]. When the polymerization was performed at 0 °C, the polymerization proceeded homogeneously, but the polydispersity became broader (Mw/Mn = 2.83) [Fig. 1(c)]. In previous work,[27, 28] we found that LiCl decreases the polydispersity of HBPA because of suppression of self-condensation of the monomers owing to LiCl-assisted stabilization of the amide anion of the monomer. Therefore, the observed formation of block copolymer with broad polydispersity from PEG-macroinitiator 2 might be accounted for by coordination of LiCl to the PEG chain of 2, resulting in failure to stabilize the amide anion of 1. However, the effect of LiCl on the polydispersity of HBPA had been observed in the polymerization of 1 with 5 at −30 °C.[27, 28] Accordingly, we checked the polymerization of 1 with 5[27, 28] at 0 °C, and obtained HBPA with low polydispersity (Mw/Mn = 1.22) (Supporting Information Fig. S1), indicating that LiCl worked well for decreasing the polydispersity of HBPA even at 0 °C. Consequently, we concluded that our first approach to PEG-b-HBPA by means of CGCP of AB2 monomer with PEG macroinitiator did not work, even in homogeneous polymerization conditions, because LiCl cannot stabilize the generated amide anion of 1 in the presence of the PEG macroinitiator, which has strong affinity for LiCl.

Figure 1.

GPC profiles of products obtained by polymerization of 1 with PEG macroinitiator 2 in the presence of LiHMDS and LiCl in THF at (a) −30 °C, (b) −10 °C, (c) 0 °C.

We next tried condensation reaction of PEG-COOH 4 and HO-HBPA 3 [Scheme 1(b)]. PEG-COOH 4 was prepared by the reaction of poly(ethylene glycol) monomethyl ether and succinic anhydride.[31] HO-HBPA 3 was synthesized via the following two steps. The CGCP of 1 with tert-butyldimethylsilyl (TBS)-protected initiator 10 was performed in the presence of LiHMDS and LiCl in THF at −30 °C, yielding TBS-protected HBPA (TBS-HBPA) 11 with low polydispersity (Mw/Mn = 1.11), then the TBS group in the initiator unit was removed by treatment with tetrabutylammonium fluoride (TBAF) at 0 °C to give HO-HBPA 3a (Mn(MALLS) = 3680, Mw/Mn = 1.14) (Scheme 2) [Fig. 2(a)].

Figure 2.

GPC profiles of (a) HO-HBPA 3a, (b) block copolymer from PEG-COOH 4 and HO-HBPA 3a, (c) HO-HBPA 3b, (d) block copolymer from PEG-COOH 4 and HO-HBPA 3b.

Scheme 2.

Syntheses of HO-HBPA 3.

The polymer end groups of PEG-COOH 4 and HO-HBPA 3a were confirmed by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry with dithranol as a matrix in the presence of sodium trifluoroacetate or potassium trifluoroacetate as a cationizing salt. The mass spectrum of PEG-COOH 4 contains one series of peaks, which corresponds to the K+ adduct of 4 (Supporting Information Fig. S2). For example, the exact mass peak of a single isotope of the 113-mer of 4 with K+ is expected to appear at 5146.39 Da, and indeed, a peak was observed at 5146.66 Da. The mass spectrum of HO-HBPA 3a contains only one series of peaks due to the Na+ adduct of 3a (Supporting Information Fig. S3). For example, the exact mass peak of a single isotope of the 13-mer with Na+ is expected to appear at 3090.02 Da, and in fact a peak was observed at 3089.13 Da, as shown in the magnified spectrum in Supporting Information Figure S3.

The condensation reaction of PEG-COOH 4 and 2 equiv of HO-HBPA 3a was then performed in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDCI) and 4-(dimethylamino)pyridine (DMAP) as a well-known condensing system[32] in DMF at ambient temperature for 19 h. The reason why we used an excess amount of 3a is that isolation of PEG-b-HBPA by precipitation in the presence of an excess of 3a was expected to be easier than from an excess amount of 4, because the solubility of PEG-b-HBPA would be quite similar to that of 4, rather than that of 3a. Indeed, excess HO-HBPA 3a in the reaction mixture could be easily removed by precipitation in ethyl acetate/ether = 3/5 (v/v), yielding PEG-b-HBPA with the expected molecular weight and a very narrow molecular weight distribution (Mn(MALLS) = 8530 (Mn(calcd) = 8780), Mw/Mn = 1.04) in a moderate yield (61%) [Fig. 2(b)]. The 1H NMR spectrum of the obtained PEG-b-HBPA in CDCl3 showed both PEG and HBPA signals, and the integral ratio of the methylene proton c on carbon adjacent to the carbonyl group to the benzyl proton d is 4/2 [Fig. 3(a)]. The molecular weight estimated by the 1H NMR was 9090, which was close to the above value determined by MALLS. These results indicated that well-defined PEG-b-HBPA was obtained by the condensation reaction of 4 and 3a. Furthermore, condensation reaction of 4 and 3b (Mn(MALLS) = 2690, Mw/Mn = 1.13) [Fig. 2(c)] also afforded well-defined PEG-b-HBPA (Mn(MALLS) = 7960 (Mn(calcd) = 7790), Mw/Mn = 1.03) after purification by precipitation in the same cosolvent [Fig. 2(d)].

Figure 3.

1H NMR spectra of PEG-b-HBPA (Mn(MALLS) = 8530, Mw/Mn = 1.04) in (a) CDCl3, (b) D2O at 25 °C.

The obtained PEG-b-HBPA (Mn(MALLS) = 8530, Mw/Mn = 1.04) was insoluble in nonpolar solvents such as hexane and ether, but soluble in various organic solvents such as CH2Cl2, acetone, DMF, and DMSO. Furthermore, PEG-b-HBPA was soluble in water. Accordingly, the 1H NMR spectrum was taken in D2O to examine the aggregation state of PEG-b-HBPA in water. It was different from the spectrum in CDCl3, and only signals of the PEG unit were observed; signals of the HBPA unit were not detected [Fig. 3(b)]. This observation implies that PEG-b-HBPA formed micelles in water, with PEG in the corona and HBPA in the core. A detailed study of micelle formation is under way.

CONCLUSIONS

We have investigated the synthesis of well-defined, amphiphilic poly(ethylene glycol) (PEG)-b-hyperbranched polyamide (HBPA) by two methods, that is, CGCP of AB2 monomer with PEG macroinitiator, and condensation of PEG and HBPA. The polymerization of 5-(methylamino)isophthalic acid ethyl ester 1 with PEG macroinitiator 2 afforded PEG-b-HBPA with a broad molecular weight distribution, probabaly because PEG coordinates to LiCl, which is requisite for CGCP of 1 to afford HBPA with low polydispersity. By contrast, condensation of PEG with a carboxyl group at one end group and HBPA with a hydroxyl group at the focal point yielded PEG-b-HBPA with defined molecular weight and narrow molecular weight distribution. The obtained PEG-b-HBPA was soluble not only in various organic solvents, but also in water. The 1H NMR spectrum of PEG-b-HBPA in CDCl3 showed signals of both the PEG and HBPA units, whereas that in D2O only showed signals of the PEG unit, implying that PEG-b-HBPA forms micelles in water, with HBPA segregated in the core. Further studies are in progress, aimed at synthesis of PEG-b-HBPA with various N-alkyl chains and examination of the micellization process.

Acknowledgments

This work was partially supported by a Scientific Frontier Research Project Grant from the Ministry of Education, Culture, Sports, Science and Technology, Japan.