Synthesis and Characterization of Linear-Dendron-like PCL PEO

Synthesis and Characterization of Linear-Dendron-like PCL PEO

(Parte 1 de 3)

Synthesis and Characterization of Linear-Dendron-like Poly(ε-caprolactone)-b-poly(ethylene oxide) Copolymers via the Combination of Ring-Opening Polymerization and Click Chemistry

Chong Hua, Song-Ming Peng, and Chang-Ming Dong*

Department of Polymer Science & Engineering, School of Chemistry and Chemical Technology, Shanghai Jiao Tong UniVersity, Shanghai 200240, P. R. China

ReceiVed April 17, 2008; ReVised Manuscript ReceiVed June 20, 2008

ABSTRACT: A new class of linear-dendron-like poly(ε-caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) copolymers with unsymmetrical topology was synthesized via controlled ring-opening polymerization (ROP) of ε-caprolactone (CL) followed by a click conjugation with azide-terminated PEO (PEO-N3). The dendron-like PCL terminated with a clickable alkyne group (Dm-PCL, m ) 0, 1, 2, and 3) was for the first time synthesized from the ROP of CL monomer using a propargyl focal point dendrons Dm with primary amine groups as the initiators and stannous octoate as catalyst in bulk at 130 °C. Then, the linear-dendron-like Dm-PCL-b-PEO copolymers were obtained by the click conjugation of Dm-PCL with PEO-N3 using PMDETA/CuBr as catalyst in DMF solution at 35 °C. Their molecular structures and physical properties were in detail characterized by

FT-IR,NMR, MALLS-GPC,DSC, and WAXD.Both DLS and TEM analysesdemonstratedthat the biodegradable micellesand vesicleswith differentsizes (less than 100 nm) self-assembledfrom these Dm-PCL-b-PEO copolymers in aqueoussolution,and both the PEO compositionand the linear-dendron-likearchitectureof copolymerscontrolled the morphology and the average size of nanoparticles. To the best of our knowledge, this is the first report that describes the synthesis of linear-dendron-like PCL-b-PEO block copolymers via the combination of ROP and click chemistry. Consequently, this provides a versatile strategy not only for the synthesis of biodegradable and amphiphilic block copolymers with linear-dendron-like architecture but also for fabricating biocompatible nanoparticles with suitable size for controlled drug release.

Introduction

Dendritic polymers such as dendrimers, dendrons, dendronized polymers, and hyperbranched polymers have attracted much attention in the past decades because their compositions and architectures can be controlled by size, shape, chain flexibility, and surface functionality in the nanoscale region, which enable the fabricationof various micro/nanoscaledevices and scaffolds for biomedical diagnosis, drug delivery, tissue engineering, nanoelectronics, and catalysis.1-5 Moreover, the merger of dendrimers/dendrons with traditional linear polymers created not only the novel macromolecular architectures (e.g., linear-b-dendrimer/dendron block copolymers, dendronized polymers)but also the biomimeticnanostructuresfor biomedical studies.6-9

As a U.S. Food and Drug Administration approved biomedical polymer, biodegradable poly(ε-caprolactone) (PCL) and PCL-based biomaterials have been increasingly investigated for pharmacological and biomedical applications.10,1 However, PCL and its relatedbiomaterialsusuallypresenteduncontrollable biodegradation rate and undesirable biological response to cells and/or tissues because of high crystallinity,strong hydrophobicity, and lack of bioactive functions.12-14 Fortunately, these drawbacks might be tackled through the adjustment of polymer hydrophilicity-hydrophobicitybalance,the bioconjugationwith bioactive proteins/peptides and/or polysaccharides, and the control of branched macromolecular architecture.15-17 Because of the biocompatibility, the stealth property of poly(ethylene oxide) (PEO) shelled nanoparticles in vivo, and the ability to decrease protein absorption, the block and/or graft copolymerization of PEO with aliphatic polyesters (e.g., PCL, polylactides and copolymers) provided a facile strategy to improve the physical, degradation, and drug release properties of biodegrad- able polyesters.For example,amphiphiliccopolymerscontaining both aliphatic polyesters and PEO components were intensively investigatedfor fabricatingmicellarand vesicularnanostructures and thermosensitive hydrogels.18-23 However, these conventional polymericmicelles/vesiclesare not stable in physiological conditions because of their relatively higher critical micelle concentration, which hindered their clinical applications in biomedicalfields.24-26 In addition,the injectablethermosensitive hydrogels are usually generated with a high polymer weight percent in solution, while it is important to fabricate hydrogels with quick response to stimuli at a lower gelation concentration for tissue engineering.27,28 Thus, the rational design of dendritic PEO-containing copolymers provides a promising tool to generate unimolecular micelles/vesicles and high-performance hydrogels.24-27

Recently, the Huisgen 1,3-dipolar cycloaddition between azides and alkynes catalyzed by copper (Cu+) ions, termed as “click chemistry” by Sharpless and colleagues, is proved to be a versatile method for complex polymers/materials syntheses and modifications, e.g., block and graft copolymers, star and dendritic polymers, and cross-linked networks and gels, demonstrating high efficiency and tolerance of functional groups under benign conditions.29-3 For example, Emrick et al. reported the synthesis of PEO- and peptide-grafted aliphatic polyestersby click chemistry.34 Jerome et al. synthesizeda novel ε-caprolactone-terminatedPEO macromonomerand established a platform for the preparation of comblike biodegradable biomaterials.35 As an extension of this synthetic effort, Jerome’s group in detail investigated the functionalization, grafting of PCL, and the synthesis of tadpole-shaped PCL-g-PEO copolymers via the combinationof clickchemistryand ROP.36-38 Very recently, Baker et al. synthesized a clickable polyglycolide and the thermoresponsivepolyglycolide-g-PEO copolymers.39 In this article, using commercial and biocompatible PEO as the precursor, a versatile strategy to prepare the linear-dendron-

* Corresponding author: phone 86-21-54748916; Fax 86-21-54741297; e-mail cmdong@sjtu.edu.cn.

10.1021/ma800857d C: $40.75 2008 American Chemical Society Published on Web 08/29/2008

like PCL-b-PEO blockcopolymerswith unsymmetricaltopology was successfully developed via the combination of controlled ROP and click chemistry, as shown in Scheme 1. The propargyl focal point dendrons Dm terminated with primary amines (i.e., propargylamine denoted as D0, Dm, m ) 0, 1, 2, 3) were first synthesized and used for initiating the controlled ROP of CL monomer, generating the dendron-like PCL homopolymers having 1, 2, 4, and 8 branches (i.e., Dm-PCL). These dendronlike Dm-PCL homopolymers were then conjugated with azide- terminated PEO (i.e., PEO-N3) to produce the targeted lineardendron-like PCL-b-PEO copolymers by click chemistry. To the best of our knowledge, this is the first example to describe the synthesis of linear-dendron-like PCL-b-PEO with unsymmetrical topology via the combination of controlled ROP and click chemistry.34-39

Experimental Section

Materials. ε-Caprolactone (CL, Aldrich) and toluene were distilledfrom CaH2. Poly(ethyleneglycol)methylether(Mn ) 5000,

Aldrich) was dried at 50 °C in vacuo overnight, and its purity was 100% within the error of 1H NMR measurement. Copper(I) bromide, 1,6-diphenyl-1,3,5-hexatriene (DPH), propargylamine, N,N,N′,N′′,N′′-pentamethyldiethylenetriamine(PMDETA),stannous octoate (SnOct2), and toluene-4-sulfonyl chloride were purchased from Aldrichor Acrosand used as received.Ethylenediamine(A.R.) and methyl acrylate (A.R.) were purchased from Sinopharm Chemical Reagent Corp. (Shanghai) and distilled under reduced pressure before use.

Methods. 1H NMR and 13C NMR spectroscopy was performed on a Varian Mercury-400spectrometer.Tetramethylsilanewas used as an internal standard. The actual molecular weights (Mn,LLS) and polydispersities(Mw/Mn) of polymers were determined on a Waters 717 plusautosamplergel permeationchromatograph(GPC)equipped with Waters RH columns, a refractive index detector, and the DAWN EOS (Wyatt Technology) multiangle laser light-scattering (MALLS) detector at 30 °C, THF as the eluent (1.0 mL/min). The differential scanning calorimetry (DSC) analysis was carried out using a Perkin-Elmer Pyris 1 instrument under nitrogen flow (10

Scheme 1. Synthesis of Linear-Dendron-like Dm-PCL-b-PEO Block Copolymers via the Combination of Controlled ROP and Click Chemistry Using the “Arm-First” Method

Scheme 2. Synthesis of Linear-Dendron-like Dm-PCL-b-PEO Block Copolymers via the Combination of Click Chemistry and Controlled ROP Using the “Core-First” Method

Table 1. Synthesis of the Dendron-like Dm-PCL (m ) 0, 1, 2, and 3) Homopolymers from the Controlled ROP of CL Monomer in Bulk at 130 °Ca entryb [CL]/[amine of Dm]

(mol/mol) Mn,LLSc Mw/Mnc Mn,NMRd yielde (%) a [CL]/[SnOct2] ) 500/1, polymerization time ) 24 h. b The subscript number represents the degree of polymerization of PCL branch, which was determined by 1H NMR spectrum (e.g., D0-PCL in Figure 2A). c Both the actual molecular weight (Mn,LLS) and the polydispersity (Mw/Mn)o fD m-

PCL were determined by the MALLS-GPC technique. d Mn,NMR was determined by 1H NMR spectrum in Figure 2. e The yield of Dm-PCL was determined gravimetrically. f The molecular weight of D0-PCL24 and D2-

PCL7 sampleswas lessthanthe limitof MALLS-GPC-determinedmolecular weight.

Macromolecules, Vol. 41, No. 18, 2008 Linear-Dendron-like PCL-b-PEO Copolymers 6687

mL/min). All samples were first heated from 0 to 90 °Ca t1 0 °C/ min and held for 3 min to erase the thermal history, then cooled to 0a t1 0 °C/min, and finally heated to 90 °Ca t1 0 °C/min. Wideangle X-ray diffraction (WAXD) patterns of powder samples were obtained at room temperature on a Shimadzu XRD-6000 X-ray diffractometer with a Cu KR radiation source (wavelength ) 1.54 Å). The supplied voltage and current were set to 40 kV and 30 mA, respectively. Samples were exposed at a scan rate of 2θ ) 4° min-1 between 2θ ) 5° and 40°. The mean size of nanoparticles was determined by dynamic light scattering (DLS) using a Malvern Nano_S instrument (Malvern, UK). The solution of nanoparticles was performed at a scattering angle of 90° and at 25 °C. All the measurements were repeated three times, and the average values reported are the mean diameter ( standard deviation. Transmission electron microscopy (TEM) was performed using a JEM-2010/ INCA OXFORD TEM (JEOL/OXFORD) at a 200 kV accelerating voltage. Samples were deposited onto the surface of 300 mesh Formvar-carbon film-coated copper grids. Excess solution was quickly wicked away with a filter paper. The image contrast was enhanced by negative staining with phosphotungstic acid (0.5 wt %).

Preparation of Propargyl Focal Point PAMAM Type

Dendron D1. The propargyl focal point PAMAM type dendrons with methyl ester terminal groups were recently designed for synthesis of symmetrical and unsymmetrical PAMAM dendrimers by Lee et al.40,41 The propargyl focal point PAMAM type dendrons with primary amine groups (D1, D2, and D3) were synthesized using a protocol similar to that described by Lee et al., as shown in Scheme 1A. A solution of propargylamine (denoted as D0, 311 µL, 4.6 mmol) in methanol (1 mL) was added dropwise to a cooled (ice-water bath), stirred solution of methyl acrylate (1.6 mL, 18.0 mmol) in methanol (2 mL) over 30 min. The reaction mixture was stirred vigorously for1ha t0 °C and then for an additional 24 h at room temperature under a nitrogen atmosphere. The reaction solution was evaporated, and then the residue was dried in vacuo at 35 °C to give the methyl ester-terminated dendron D0.5 (1.02 g,

6H).

A solution of D0.5 (951.5 mg, 4.2 mmol) in methanol (4 mL) was added dropwise to a cooled, stirred solution of ethylenediamine (3.4 mL, 50.4 mmol) in methanol (4 mL) over 45 min. The reaction mixture was stirred vigorously for1ha t0 °C and then for a further 48 h at room temperatureunder a nitrogenatmosphere.The reaction solution was evaporated, and then the residue was dried in vacuo at 35 °C to give the amino-terminated dendron D1 (1.17 g, 9% yield). 1H NMR (400 MHz, CDCl3): δ ) 1.46 (s, 4H), 2.24 (t,1H), 2.38 (t, 4H), 2.83 (q, 8H), 3.29 (q, 4H), 3.43 (d, 2H), 7.30 (s, 2H).

Preparation of Propargyl Focal Point PAMAM Type

Dendron D2. D2 was synthesized from D1 (600.0 mg, 2.1 mmol) using the same method as successive Michael addition of primary amines with methyl acrylate and amidation of methyl ester groups with a large molar excess of ethylenediamine(96% yield). 1H NMR of D1.5 (400 MHz, CDCl3): δ ) 2.18(t, 1H), 2.37 (t, 4H), 2.42 (t, 8H), 2.53 (t, 4H), 2.74 (t, 8H), 2.83 (t, 4H), 3.27 (q, 4H), 3.45 (d,

2H), 3.6 (s, 12H),7.10 (s, 2H). 1H NMR of D2 (400 MHz, CDCl3): δ ) 1.51 (s, 8H), 2.20 (t, 1H), 2.34 (p, 12H), 2.50 (t, 4H), 2.71 (t,

8H), 2.81 (p, 12H), 3.26 (m, 12H), 3.42(d, 2H), 7.63 (t, 4H), 7.89

Preparation of Propargyl Focal Point PAMAM Type

Dendron D3. D3 was synthesized from D2 (813.0 mg, 1.1 mmol) using the same method as successive Michael addition of primary amines with methyl acrylate and amidation of methyl ester groups with a large molar excess of ethylenediamine(94% yield). 1H NMR of D2.5 (400 MHz, CDCl3): δ ) 2.18 (t, 1H), 2.37 (t, 4H), 2.42 (t,

8H), 2.53 (t, 4H), 2.74 (t, 8H), 2.83 (t, 4H), 3.27 (q, 4H), 3.45 (d,

2H), 3.6 (s, 12H),7.10 (s, 2H). 1H NMR of D3 (400 MHz, CDCl3): δ ) 1.51 (s, 8H), 2.20 (t, 1H), 2.34 (p, 12H), 2.50 (t, 4H), 2.71 (t,

8H), 2.81 (p, 12H), 3.26 (m, 12H), 3.42(d, 2H), 7.63 (t, 4H), 7.89

Preparation of Azide-Terminated PEO (PEO-N3). Both poly(ethylene glycol) methyl ether (Mn ) 500; 1.0 g, 0.2 mmol) and toluene-4-sulfonyl chloride (381.0 mg, 2 mmol) were com- pletely dissolved in CH2Cl2 (10 mL) under a N2 atmosphere. Triethylamine (278 µL, 2 mmol) was added dropwise to the above solution at ice-water bath, and then the resulting solution was stirred for 24 h at room temperature. The reaction solution was centrifuged and precipitated into 80 mL of diethyl ether, and then the powder was dried in vacuo at 25 °C to give the monotosylated PEO (PEO-Ts, 968.0 mg, 94% yield). 1H NMR of PEO-Ts (400

MHz, CDCl3): δ ) 2.4 (s, 3H), 3.37 (s, 3H), 3.46 (t, 2H), 3.54 (t, 2H), 3.64 (s, 450H), 3.82 (t, 2H), 4.16 (t, 2H), 7.35 (d, 2H), 7.81(d,

2H). Thus, sodium azide (223.0 mg, 3.4 mmol) was added to a solution of the obtained PEO monotosylate (881.0 mg, 0.17 mmol) in dry DMF (10 mL) under a N2 atmosphere, and the reaction mixture was stirred vigorously at room temperature for 24 h. DMF solvent was removed under reduced pressure, and then the product was dissolved in 80 mL of dichloromethane. The mixture was extracted sequentially with NaCl (5 wt %) solution and distilled water,dried with anhydrousNa2SO4, and then precipitatedin diethyl ether to yield 687.2 mg of PEO azide (PEO-N3, 80% yield). 1H

NMR of PEO-N3 (400 MHz, CDCl3): δ ) 3.37 (s, 3H), 3.39 (t, 2H), 3.46 (t, 2H), 3.54 (t, 2H), 3.64 (s, 450H), 3.82 (t, 2H).

Synthesis of Dendron-like PCL Terminated with Clickable

Alkyne Group (Dm-PCL). The dendron-like PCL homopolymers terminated with clickable alkyne groups (Dm-PCL) were synthesized from controlled ring-opening polymerization of CL monomer using propargyl focal point PAMAM type dendrons (Dm, m ) 0,

1, 2, and 3) as initiators and SnOct2 as catalyst in bulk at 130 °C.

A typical example follows: 7.3 mg (9 µmol) of the SnOct2 catalyst in dry toluene was added to the melt mixture of the D0 initiator

(7.7 µL, 113 µmol) and CL monomer (1.03 g, 9.02 mmol), where the exhausting-refilling process was carried out for three times using a Schlenk line. The polymerization mixture was stirred moderately in bulk at 130 °C for 24 h. Then, the resulting product was dissolved in 5 mL of CH2Cl2 and poured dropwise into 50 mL of cold methanolunder vigorousstirring.The precipitatewas filtered and dried in vacuo at 40 °C to give 955.0 mg of D0-PCL sample

Synthesis of Linear-Dendron-like PCL-b-PEO Copolymers

(Dm-PCL-b-PEO) via Click Chemistry. A typical procedure for the synthesis of linear-dendron-like PCL-b-PEO copolymers was started with the feed ratio of reagents [PEO-N3]/[Dm-PCL]/[CuBr]/ [PMDETA] ) 1.1/1/1.1/1.1. The click coupling reaction between flask with 2 mL of DMF as solventand CuBr/PMDETAas catalyst. After 24 h, the polymersolutionwas then precipitatedin ethyl ether. The resulting copolymers were purified by solvent extraction using 10 mL of cold methanol (about 10 °C) to completely extract the

68 Hua et al. Macromolecules, Vol. 41, No. 18, 2008

(Parte 1 de 3)

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