Article pubs.acs.org/Macromolecules
Oxidation Degradable Aliphatic Polycarbonates with Pendent Phenylboronic Ester Fang-Yi Qiu, Mei Zhang, Fu-Sheng Du,* and Zi-Chen Li* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center for Soft Matter Science and Engineering, College of Chemistry & Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *
ABSTRACT: Functional polyesters and poly(carbonate)s (PCs) with controlled and on-demand degradation properties have great advantages for biomedical and pharmaceutical applications. Herein, we report a new kind of aliphatic PC that possesses the feature of oxidation promoted degradation. Two six-membered cyclic carbonates (C1 and C2) containing phenylboronic ester group have been synthesized from serinol (1) and 2-aminomethyl-2-methylpropane1,3-diol (2), respectively. Both monomers could undergo wellcontrolled ring-opening polymerization (ROP) catalyzed by an organic base, but the 5,5-disubstituted C2 has the character of equilibrium ROP and a much slower rate than the monosubstituted C1. The copolymerization of C1 or C2 with trimethylene carbonate and its derivative leads to a series of copolymers. Two series of amphiphilic block copolymers (BPC1s and BPC2s) have been prepared from C1 and C2 using poly(ethylene glycol) as the macroinitiator. They are able to form nanoparticles with the diameters less than 150 nm. The H2O2-triggered decomposition of C1, C2, and their corresponding noncyclic model compounds was studied by 1H NMR, showing the consecutive process of oxidation, 1,6elimination, release of CO2, and intramolecular isomerization or cyclization. The degradation of the block copolymer nanoparticles, investigated by 1H NMR, GPC, laser light scattering (LLS), and Nile Red fluorescence, can also be accelerated drastically by H2O2 following the similar mechanism but is affected by steric hindrance of the polymer chain and heterogeneous microenvironment inside the nanoparticles. The results of 1H NMR and LLS reveal that the nanoparticles of BPC1 and BPC2 exhibited different degradation profiles, with a slightly faster degradation rate for BPC2. Of particular interest, BPC2 nanoparticle is sensitive to as low as 0.02 mM H2O2.
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INTRODUCTION Reactive oxygen species (ROS) are generated in most of the biological systems and confirmed as key players in various physiological processes.1,2 However, overproduction of ROS (oxidative stress) is usually associated with various pathological disorders including the inflammatory diseases and cancers. Therefore, oxidative stress is becoming an important biomarker for the development of advanced detection probes, imaging agents, and site-specific delivery systems.3−7 In the past decade, ROS-responsive polymers containing different oxidationsensitive motifs have drawn an increased attention due to their capability as promising smart carriers for nanomedicines.8−17 Although these polymers can be synthesized by different approaches, most of the backbone degradable polymers under ROS triggering are prepared by step-growth polymerization. The molecular weight and topological structure of the ROS-responsive backbone degradable polymers are not easily controlled.18−22 Recently, poly(amino acid)s or aliphatic polyesters with pendent thioether groups were prepared by the well-controlled ring-opening polymerization (ROP) of functional cyclic monomers.23−25 However, relatively high concen© XXXX American Chemical Society
tration of H2O2 is necessary to trigger the disassembly of these thioether-containing polymeric nanoparticles.26,27 Self-immolative linkers have been utilized for developing small molecule prodrugs as early as in 1980s.28 In recent years, self-immolative dendrimers and polymers that undergo a fast and complete degradation through a cascade of self-immolative reactions with the cleavage of a triggering group have been extensively studied because of their potential applications in molecular probes, signal amplification, and drug delivery.29−32 The self-immolative processes are based on the fast 1,4-/1,6-/ 1,8-elimination or the intramolecular cyclization, or a combination of both. The self-immolative strategy has also been used to construct backbone degradable polyesters and poly(ester−amide)s, whose degradation is greatly different from the general hydrolysis mode for most biodegradable polymers and can be accelerated by the intramolecular cyclization.33−36 Among various triggering groups, phenylReceived: August 28, 2016 Revised: November 24, 2016
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DOI: 10.1021/acs.macromol.6b01883 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules boronic acid or ester represents a unique one because it is highly sensitive to oxidation of H2O2, forming phenol and boronic acid.37 When the phenylboronic acid/ester moiety is connected to a suitable linker, after formation of phenol derivatives upon triggering by H2O2, sequential self-immolative reactions will occur. This feature has enabled the development of a wide range of ROS-responsive fluorescent probes,3,38 prodrugs,39,40 polymeric imaging agents,41,42 and biomolecule responsive hydrogels,43 using arylboronic acid/ester as the caging groups. Aliphatic poly(carbonate)s (PCs) represent an important family of biodegradable polymers owing to their good biocompatibility and biodegradability. Although PCs can be prepared by different methods, the ROP of cyclic carbonates is the most promising approach to afford PCs with predictable molecular weight, low polydispersity, controlled topological structure, and versatile functionalities.44−46 Aliphatic PCs, in particular poly(trimethylene carbonate) (PTMC) and its derivatives, have shown great potential for biomedical and pharmaceutical applications such as tissue engineering, drug/ gene delivery, and antimicrobial, in which the degradation rate of the polymer is one of the key factors influencing the performance.47−49 The degradation profiles of PCs can be tuned by copolymerization with other cyclic monomers such as lactide or ε-caprolactone,50,51 changing the hydrophilic/hydrophobic balance,52,53 or incorporating catalytic groups.54−56 However, considering the advantage of the environmentally responsive polymers, the triggered degradation of functional PCs by various endogenous or exogenous stimuli is preferred for their applications in the spatiotemporally controlled delivery or imaging systems. In our recent letter, we report a new type of degradable PC prepared by the ROP of a serinol-derived monosubstituted six-membered cyclic carbonate (C1) containing a phenylboronic pinacol ester. The oxidation-responsive PCs are stable against hydrolysis but degrade quickly via the consecutive oxidation, 1,6-elimination, releasing CO2, and intramolecular cyclization under triggering of H2O2. They have low cytotoxicity and are able to load hydrophobic guests.57 To expand the family of this new kind of oxidation-responsive aliphatic PC, in this article, we have synthesized another monomer of six-membered 5,5-disubstituted cyclic carbonate (C2) (Scheme 1). The purposes of this work are (1) to explore the effect of structure of the monomers on their ring-opening polymerization and copolymerization with other cyclic monomers, (2) to prepare two series of oxidation-responsive block copolymers, PEG-b-PC1 (BPC1) and PEG-b-PC2 (BPC2), and (3) to study comparatively the oxidative degradation profiles of the block copolymers and demonstrate the influence of polymer structure on the self-immolative degradation.
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Scheme 1. Synthesis of Functional Cyclic Carbonates Derived from the Amino-Substituted 1,3-Diolsa
a
Reagents and conditions: (i) (Boc)2O, TEA, THF/water (3:2, v/v), 0-25 °C, 6 h; (ii) ethyl chloroformate, TEA, THF, 0−25 °C, 5 h; or triphosgene, pyridine, CH2Cl2, −78 to 25 °C, 6 h; (iii) trifluoroacetic acid, CH2Cl2, 25 °C, 2 h; (iv) 4-hydroxymethylphenylboronic pinacol ester, triphosgene, ethyldiisopropylamine, pyridine, THF, 0−25 °C, 4 h; (v) 4-hydroxymethylphenylboronic pinacol ester, triphosgene, pyridine, THF/DMF, 0−25 °C, 2 h.
buffers (PB, pH 7.4, 50 or 500 mM) were prepared from NaOD (40 wt % in D2O, Alfa Aesar) and deuterated phosphoric acid (85 wt % in D2O, Alfa Aesar), to which sodium 2,2-dimethyl-2-silapentane-5sulfonate (DSS, J&K Chemical Ltd.) was added as an internal standard (0.03 wt %). 4-Hyroxymethylphenylboronic pinacol ester was synthesized following the reported procedure.58 Other solvents and reagents were purchased from Beijing Chemical Reagent Co. and used as received. Measurements. 1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer. The degradation kinetics of the monomers, the model compounds, and the copolymer nanoparticles were studied at 37 °C by monitoring their time-dependent 1H NMR spectra. The FT-IR spectrum of the monomer (mixed with KBr) was measured on a Bruker Vector22 spectrometer. Electrospray ionization mass spectrometry (ESI-MS) characterizations were performed on a Bruker APEX-IV Fourier transform mass spectrometer in the positive ion mode. Elemental analysis was performed on an Elementar VARIO EL equipment (Germany). Gel permeation chromatography (GPC) measurements were performed on the equipment consisting of a Waters 1525 binary HPLC pump, a Waters 2414 refractive index detector, and three Waters Styragel columns (HR1, HR2, and HR3) at 35 °C with THF as eluent (1.0 mL/min). A series of narrow dispersed polystyrenes were used for calibration; molecular weight and its dispersity were calculated using Millennium 32 software. Thermal gravimetric analysis (TGA) was carried out using a Q600-SDT thermogravimetric analyzer with nitrogen purging rate set at 100 mL/min. Measurements were conducted from room temperature to 600 °C at a heating rate of 20 °C/min. Differential scanning calorimetry (DSC) was performed on a TA Q100 differential scanning calorimeter at a heating rate of 10 °C/ min under a nitrogen atmosphere of 50 mL/min. Data of the endothermic curves were recorded from the second scan and analyzed with a TA Universal Analysis software. Fluorescence spectra of NR were measured on a Hitachi F-7000 fluorescence spectrometer equipped with a temperature controller. Both the excitation and emission slit widths were 5.0 nm. The emission spectra were collected from 560 to 700 nm at an excitation wavelength of 545 nm. Transmission electron microscope (TEM) photographs of the nanoparticles were taken on a JEM-2100 equipment at a 200 kV acceleration voltage. Synthesis of Monomers C1 and C2 and Model Compound 3. Monomer C1 was synthesized following the reported procedure.57 Monomer C2 was synthesized according to path B in Scheme 1. Linear model compound 3 was prepared by the ring-opening reaction of C2 with methanol using DBU as a basic catalyst. The detailed
EXPERIMENTAL SECTION
Materials. 3-Methyl-3-oxetanemethanol (J&K Chemical Ltd.), benzylamine (Sino Pharm.), palladium (5% on carbon, Type 87L, dry, Alfa Aesar), di-tert-butyl dicarbonate ((Boc)2O, J&K Chemical Ltd.), triphosgene (J&K Chemical Ltd.), Nile Red (NR, Aldrich), hydrogen peroxide (H2O2, 30 wt %, Beijing Chemical Works), and poly(ethylene oxide) monomethyl ether with molecular weight of 5000 (mPEG5k, Fluka) were used as received. 4-Methoxybenzyl alcohol (4-MeOBnOH, Aladdin Chemistry Co. Ltd.) was recrystallized from n-hexane and chloroform (v/v = 5:1). 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU, J&K Chemical Ltd.) was distilled over CaH2 under reduced pressure. Anhydrous dichloromethane and THF were distilled over CaH2 after refluxing for 10 h. Deuterated phosphate B
DOI: 10.1021/acs.macromol.6b01883 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Table 1. Copolymerization of C1 and C2 with TMC and Compound 1B convb (%)
monomer a
entry
M1
1
C1
1b
2
C2
1b
3
C1
TMC
4
C2
TMC
M2
time (min)
M1
M2
DPc (M1 + M2)
M̅ n,NMRb (kg/mol)
M̅ n,GPCd (kg/mol)
ĐM d
5 10 20 10 60 120 10 60 180 360 60 180 360
46 85 >99 99 >99 >99 32 66 88
33 67 92 41 >99 >99
