Catechol moiety offers a versatile platform in the preparation of functionalized polymers, but it is not usually compatible with catalysis in polymerizations. To address these challenges, we suggest employment of one Brønsted base in masking the activity of catechol moiety and to modulate the polymerization. Based on this strategy, the ring-opening polymerization (ROP) of sarcosine N-carboxyanhydrides (Sar-NCA) was carried out using dopamine hydrochloride as an initiator and triethylamine as a Brønsted base. PSar with predicted molecular weights (Mn,NMR=3.7 kg mol−1) and narrow dispersities (Đ<1.13) was prepared. Catechol initiator was successfully linked to PSar end as confirmed by MALDI-ToF MS. Subsequently, copolymerization of N-butyl glycine N-carboxyanhydrides (Bu-Gly-NCA) from the PSar in one-pot produced catechol end-functionalized amphiphilic polysarcosine-block-poly(N-butyl glycine) diblock copolypeptoids (cat-PSar-b-PGlyBu). Further, cat-PSar-b-PGlyBu enabled the aqueous dispersion of manganese oxide nanoparticles which was attributable to the anchor of the diblock copolymers onto the surface of the nanoparticles. The strategy for catechol masking and polymerization mediating by one Brønsted base offered a new avenue into the synthesis of catechol-ended block copolymers.
Catechol moiety is a key functionality in many chemical processes such as polyphenol antioxidants , synthetic anchors to solid surfaces , , and adhesive materials , . The adhesive proteins secreted by mussels were adhered virtually to all surfaces under their pristine environments , , . Typical catechol amino acid, 3,4-dihydroxyphenylalanine, is responsible for these adhesions , . Consequently, various attempts have been made to prepare catechol functionalized polymers , , . However, the sensitivity of catechol moiety to oxidation unveils less control over molecular weights of the designed polymers . It also easily coordinates with metals in the catalysis  and forms stable complexes , ,  which interfere with the polymerization process . To overcome these difficulties, three strategies toward the conjugation of catechol end-functionalized polymers were developed (Scheme 1, routes a, b, and c) , , . All the employed routes were either to protect the sensitive catechol groups (routes b and c), or to prepare polymers before conjugating them with catechol end-groups (route a). In an endeavor to explore wider scope of organocatalysis in ring-opening polymerization (ROP) , , we envisioned a new strategy to prevent the interference of catechol initiator (1) and concurrently mediating the polymerizations from the initiator by one Brønsted base (Scheme 1d). In a dual-duty mode, the Brønsted base (B) coordinated with the catechol initiator via H-bonding (2) and mediated polymerizations to afford block copolymers (3) initiated from 2.
Based on this polymerization strategy, catechol-ended amphiphilic diblock copolypeptoids were prepared (Scheme 1d, 3). Previously, polysarcosine (PSar) based amphiphilic block copolymers were successfully prepared , , , . PSar, the simplest polypeptoid with N-methyl glycine, or sarcosine (Sar), as the repeating unit, was known for its excellent water solubility . Polypeptoids or poly(N-substituted glycine)s were structural mimics to natural polypeptides . Unlike polypeptides, the substitution on the amide back-bone of polypeptoids interrupted H-bonding between polymer chains and offered possible functionalization , . Zhang first demonstrated the preparation of block copolypeptoids from N-heterocyclic carbene mediated ring-opening polymerization(ROP) of N-substituted N-carboxyanhydride (R-NCA) . Afterwards, Luxenhofer et al. reported the synthesis of amphiphilic polypeptoids with backbone degradability . Nevertheless, polypeptoids functionalized with catechol groups have not yet been investigated to the best of our knowledge. The possibility of catechol group in interference of metal catalysts, and the vulnerability of catechol to redox, acid, base, and other polymerization conditions challenged direct incorporation of catechol in polymer chain end. A Brønsted base mediated incorporation of catechol moiety to amphiphilic block copolypeptoids is illustrated in Scheme 2. The tertiary amines as Brønsted base (B) could play dual-role comprising: (i) mask the activity of catechol moiety, and (ii) activate the ammonium salt by deprotonation (Scheme 2). The intramolecular H-bonding masked the activity of catechol moiety when B coordinated with one of the acidic hydrogen. Additionally, B modulated the ROP of Sar-NCA and copolymerization of N-butyl glycine N-carboxyanhydrides (Bu-Gly-NCA).
The prepared catechol end-functionalized amphiphilic diblock copolypeptoids were used for the surface modification of magnetic nanoparticles. Recently, manganese oxide nanoparticles (MnO-NPs) have been reported as new T1 contrast agents in magnetic resonance imaging . However, oleate-capped MnO-NPs are water insoluble and toxic, which drives the search for biocompatible coating materials. Additional coatings of MnO-NPs enabled excellent water dispersion and biocompatibility. Catechol containing polymers including ethylene glycol , glycerols , and peptides  have been reported for synthesizing water soluble and biocompatible metal oxide NPs. Frey et al. pointed out that a single catechol unit makes a difference for solubilization of MnO-NPs in the hay stacked linear and hyperbranched polyglycerols . Catechol bearing hydrophilic polymers are used so far for synthesizing water soluble MnO-NPs . The functionalization of MnO-NPs is based on the high binding affinity of catechol moieties to the surface. The first example of a biocompatible, (iron oxide nanoparticles-thermoresponsive), hybrid material with a polypeptoid brush using grafting from technique was reported . In this study, the prepared MnO-NPs are coated by catechol end-functionalized amphiphilic polysarcosine-block-poly(N-butyl-glycine) copolymers (cat-PSar-b-PGlyBu).
In this contribution, a strategy of one Brønsted base masked the activity of catechol moiety and modulated the polymerization of N-substituted N-carboxyanhydrides was proposed. To demonstrate the workability of this one base two-function strategy in ROPs, dopamine hydrochloride and triethylamine (TEA) are selected as initiator and Brønsted base, respectively. In general, we report (1) Brønsted base mediated ROP of Sar-NCA using catechol initiator and the copolymerization with Bu-Gly-NCA to afford amphiphilic PSar-b-PGlyBu diblock copolypeptoids; (2) catechol end-functionalized amphiphilic PSar-b-PGlyBu diblock copolypeptoids enabled aqueous dispersion of MnO-NPs. The phase transition of MnO-NPs capped with cat-PSar-b-PGlyBu is demonstrated in hexane/water binary solvent system.
Tetrahydrofuran (THF) was purchased from Sigma-Aldrich and distilled from Na/K. DMF was purchased from Sigma-Aldrich, dried over CaH2, and subsequently distilled in vacuo onto molecular sieves (3 Å). Dichloromethane (99.5%, DCM) was distilled over CaH2 under an argon atmosphere. Sar-NCA  and Bu-Gly-NCA  monomers were prepared following the modified literature procedure. Dopamine hydrochloride and other substances were used as received. Manganese chloride, MnCl2·4H2O (TCI, 98%), sodium oleate (TCI, 98%) and 1-octadecene (TCI, 95%) were used to synthesize oleate-capped MnO-NPs.
1H NMR spectra were recorded on a Bruker ARX-250 spectrometer with 400 MHz at room temperature and the data were analyzed by MestreNova software. Melting points of Sar-NCA monomer was determined using a melting point apparatus. Size exclusion chromatography (SEC) was performed in N,N-Dimethylformamide (DMF) using an SSI 1500 pump equipped with Waters column (5 mm, 300×7.8 mm) at a flow rate of 0.07 mL min−1 at 25°C, and a Wyatt Optilab rEX differential refractive index (DRI) detector with a 658 nm light source. DMF containing 0.05 mol L−1 LiBr was used as the eluent. Both the column and the detector temperature were kept at 25°C. All data were analyzed by Wyatt Astra V 6.1.1 software. The number average molecular weight (Mn) and molecular weight distributions (Mw/Mn) were determined at dn/dc values of 0.0987. Matrix assisted laser desorption/ionization time-of-flight mass spectra (MALDI-ToF MS) were performed on a mass spectrometer (ultraflextreme; Bruker Co.) with Smartbeam/Smartbeam II modified Nd: YAG laser. Mass spectra of 500 shots were collected for the spectra at a 25 kV acceleration voltage. The polymer sample was dissolved in CHCl3 at a concentration of 5.0 mg mL−1, the matrix 2,5-DHB (2,5-dihydroxybenzoic acid) was dissolved in solution of trifluoroacetic acid and acetonitrile (volume ratio=70/30) in water (1.0%, 10 μL). The self-assembly of block copolymers was analyzed by Dynamic Light Scattering (DLS) Malvern Instrument Zetasizer Nano S90 with a He-Ne laser (633 nm, 4 mW) at room temperature. The sample solutions were prepared in distilled water with a concentration of 5.0 mg mL−1. To observe the size and morphologies of the aggregates, transmission electron microscopy (TEM) measurement was performed using JEM-1200 TEM instrument operating at 120 kV. A drop of MnONPs@cat-PSar-b-PGlyBu solution in ethanol was placed on a carbon-coated copper grid followed by solvent evaporation at room temperature.
General procedure for the synthesis of polysarcosine (PSar50)
Sar-NCA (0.2647 g, 2.3 mmol) was dissolved in 2.3 mL dry DMF. Followed by the addition of dopamine hydrochloride (8.72 mg, 0.046 mmol) as initiator and 25 μL of TEA (0.18 mmol) as catalyst. The reaction mixture was stirred at room temperature under inert atmosphere (N2 flow) and continuous stirring. The polymerization was quenched in Et2O and the product was dried under vacuum oven. 1H NMR (500 MHz; D2O): δ=2.89–3.3 (150H, CH3-), 4.20 (104H, -CH2-CO-), 6.3–6.5 (3H, C6H3O2H2).
Synthesis of polysarcosine50-block-poly(N-butyl glycine)25 diblock copolypeptoids
Initially, Sar-NCA (0.2647 g, 2.3 mmol) was dissolved in 2.3 mL DMF. Dopamine hydrochloride (8.72 mg, 0.046 mmol) and 25.08 μL TEA (0.18 mmol) were added to the flask. The reaction was carried out under nitrogen flow. After 24 h, some portion of the reaction mixture was precipitated in diethyl ether and kept in vacuum oven for further characterization. To the flask containing PSar, Bu-Gly-NCA  (0.133 g, 1.15 mmol) was dissolved in 0.87 mL DMF and added to the flask containing the first block. The reaction mixture was stirred for additional 24 h under N2 flow. Then, the polymerization reaction was terminated by diethyl ether. 1H NMR (400 MHz; DMSO-d6): δ=6.57 (3H, C6H5O2), 4.19–3.88 (108H, C4H9-N-CH2-CO- and CH3-NH-CH2-C6H3(OH)2), 3.27 (36H,CH3-CH2-CH2-), 3.03–2.79 (159H, CH3), 1.44 (25H, CH3-CH2-CH2-CH2-), 1.24 (29H,CH3-CH2-C2H4), 0.88 (37H, CH3-C3H6-).
Functionalization of MnO nanoparticles
The synthesis and functionalization of oleate capped MnO-NPs was carried out following literature report . cat-PSar-b-PGlyBu (30.0 mg) was dissolved in chloroform (20.0 mL). The oleate-capped MnO-NPs (10.0 mg) were dissolved in chloroform (5.0 mL) and methanol (5.0 mL) followed by dropwise addition to the polymer solution. The solution was stirred overnight at room temperature and subsequently concentrated to 10.0 mL under reduced pressure. The product was obtained by the addition of hexane (10.0 mL) followed centrifugation (6 min, 6000 min−1) and redissolved in 5.0 mL acetone. The precipitate was washed three times and dried in a vacuum oven. The aqueous solubility of the cat-PSar–b-PGlyBu functionalized MnO-NPs were elucidated in hexane/water solvent systems.
Results and discussion
Dopamine hydrochloride initiator has two acidic sites comprising the catechol moiety and the ammonium salt (-CH3CH2NH3Cl) (Scheme 3, 1). Notably, the pKa value of the ammonium salt in H2O was estimated to be 8.59 and catechol moiety contains one intramolecularly bonded hydrogen with pKa value about 14.08 in H2O and one free hydrogen with pKa value about 11.16 in H2O . The free hydrogen of catechol moiety is much more acidic than that of intramolecularly bonded hydrogen. In this contribution, a Brønsted base mediated ROP of Sar-NCA using catechol initiator was suggested. The Brønsted base coordinates with the free acidic hydrogen which result in the formation of strong intramolecular H-bonding (Scheme 3, 2). Most importantly, the Brønsted base plays a dual role that is (1) prevent the intervention of catechol moiety (2) control the activity of the primary amine in the polymerization process. The ROP of Sar-NCA was carried out using dopamine hydrochloride initiator and triethylamine as Brønsted base. Initially, 1.0/2.0 ratios of dopamine hydrochloride initiator and TEA was used for ROP Sar-NCA such that 1.0 equiv. of TEA is supposed to react with the ammonium salt to generate the active amine end latter on the other 1.0 equiv. TEA react with free hydrogen of catechol moiety. The primary amine in dopamine was used to initiate the ROP of Sar-NCA.
A Brønsted base (B) is used to mask the activity of catechol via intramolecular H-bonding and controlled the ROP of Sar-NCA (Scheme 3). Briefly, about 1.0 equiv. of B first react with the ammonium salt to provide active amine (2) and conjugate acid (BH+). Latter, another 1.0 equiv. of B coordinates with the free acidic hydrogen from catechol moiety (1) which result in the formation H-bonding between catechol moiety (3) and the Brønsted-base (B). The ROP of Sar-NCA takes place following normal amine mechanism , . The primary amine in dopamine (3) as a nucleophile attacks the electron deficient carbonyl carbon of Sar-NCA monomer followed by decarboxylation of the carbamic acid to yield PSar (4). The secondary amine in PSar terminus as macroinitiator triggers the ROP of Bu-Gly-NCA monomer. Followed by the formation of catechol end-functionalized amphiphilic cat-PSar-b-PGlyBu diblock copolymers (5). To obtain cat-PSar-b-PGlyBu with free catechol moiety (6), (5) treated with 1.0 mol L−1 HCl (aq.).
Catechol functionalized polysarcosine
Ring-opening polymerization of α-amino acid N-carboxyanhydrides (NCAs) is well established method to prepare polypeptides , , . The strategy showed success in polypeptoid synthesis from NCAs of N-substituted glycine , . The incorporation of catechol moiety to PSar-based amphiphilic block copolypeptoids demands a precise control over the molecular weights of the designed polymers. Previously, primary amine hydrochlorides were used to control the ROP of NCAs through protonation of NCA anions , . Thornton et al. reported the polymerization of NCAs using dopamine hydrochloride initiator with slow rate . Later, Schlaad et al. disclosed a controlled ROP of NCAs with relatively enhanced rate using a primary amine/tertiary amine catalytic system . Herein, Brønsted base mediated ring-opening polymerization strategy was suggested to overcome catechol interference and modulate the polymerization. We disclosed TEA mediated ROP of Sar-NCA by dopamine hydrochloride initiator to obtain catechol end-functionalized polysarcosine with fast polymerization rate , , . In the initiation step, the ROP of Sar-NCA triggered by nucleophilic attack of primary amine on carbonyl carbon at C5 (Scheme 4) result in the formation of carbamic acid followed by decarboxylation to afford PSar with secondary amine terminus , .
The ROP of Sar-NCA monomer was performed in DMF at 25°C using dopamine hydrochloride initiator and TEA catalytic system. The rate of polymerization of R-NCA monomers by dopamine hydrochloride without the addition of a catalyst is quite slow and no polymer peak appeared in 1H NMR spectrum after 24 h (Fig. S3). After 96 h reaction time, monomer conversion was less than 10.0% and product was not observed in diethyl ether (Et2O). Afterwards, dopamine hydrochloride/TEA catalytic system were used for ROP of Sar-NCA to enhance rate of polymerization of the monomer. The reaction was terminated by Et2O after 24 h. The product was clearly observed and kept in vacuum oven for further investigation. The presence of TEA significantly enhanced the polymerization rate of Sar-NCA. The degree of polymerizations (DPs) and the molecular weights were calculated from 1H NMR spectra by comparing the integral values of aromatic protons (δ=6.5–6.8 ppm) with methylene protons (δ=3.7–4.5 ppm) of PSar (Fig. 1). The monomer conversion reached 99.9% after 24 h as confirmed by 1H NMR and the monomer peak was no longer available. The 1H NMR of the sample polymers were in accordance with the proposed structure of the repeating unit and DPs.
To prepare well-defined polypeptoids, the polymerization reaction was carried out at various monomer to initiator feed ratios. Low molecular weight PSar (Mn,SEC=1.1 kg mol−1) with narrow dispersity (Đ<1.1) and monomodal SEC traces obtained at M/I/TEA=20/1/1.1 (Fig. S4). However, as the M/I ratios increased to 30, 50, 75 and 100, keeping I/TEA same, the SEC traces are found to be unreliable and a shoulder appeared. Even though, the conversion reached 99.9% and Mn,NMR nicely agreed with Mn,theo but Mn,SEC values obviated (Table S1). In a quest to obtain high Mn,SEC, the ROP of Sar-NCA was carried out at different ratios of TEA.
The polymerization was done at M/I/TEA=50/1 by varying the ratios of TEA to obtain relatively close Mn,SEC to the predicted one. As the ratio of TEA increased from 1.1 to 2.1, the Mn,SEC changed dramatically from 1.6 kg mol−1 to 2.0 kg mol−1. Further increase in TEA ratios to 2.5, 2.7, 2.9 and 3.1 reveals high Mn,SEC. Table 1 entry 3 displays close agreement in Mn,SEC 3.4 kg mol−1 as the TEA ratio increased to 2.3. Besides, high Mn,SEC=4.3 kg mol−1 and Mn,NMR=4.1 kg mol−1 obtained at ratios I/TEA of 1/2.9 which is higher than the predicted Mn,theo (Table 1 entry 6). Hence, the results indicate that TEA worked a dual role including to mask catechol and control the activity of active amine-end. The traces of 50/1/2.3 are better than that of 50/1/1.1 which confirms the catechol masking. Mn,SEC is in close agreement with that of Mn,NMR for the polymerization which takes place at the initiator to catalyst ratio of 1/2.3 (Fig. 2d). The polymerization is not well controlled at higher TEA ratios i.e. 2.9 and 3.1 which result in broad SEC traces with shoulder.
|Entry||M/I/TEA||Samplea||M n,theo b (kg mol−1)||M n,NMR c (kg mol−1)||M n,SEC d (kg mol−1)||Đ d (Mn/Mw)|
a,cDPs determined by 1H NMR spectrometry in DMSO-d6; Conversion=99.9%; bMn,theo=Mw(Dopa.HCl)+[Sar-NCA]0/[Dopa.HCl]0×Mw (sarcosine unit)×conv.; dDetermined by SEC in 0.1 mol L−1 LiBr–DMF, using a calibration curve constructed with PMMA standards.
Catechol initiator was successfully attached to PSar end and the mass differences (∆m/z) between individual signal peaks attributed to the molecular weight of the Sar repeating unit which is 71.04 g mol−1  (Fig. 3). The results suggested that dopamine hydrochloride initiated the ROP of Sar-NCA to afford catechol end PSar. TEA coordinates with the acidic hydrogen of catechol initiator via H-bonding interaction. We reasoned that the Brønsted base coordinated with the free acidic hydrogen via H-bonding. As a result the other intramolecularly bonded hydrogen gets stronger in such a way that catechol masked throughout the polymerization. Water and unknown end groups also initiated the monomer. The presence of other peaks of the PSar is due to impurities and side reactions.
Catechol functionalized polysarcosine-block-poly(N-butyl glycine)
The switch of N-substituents from methyl to n-butyl turned hydrophilic PSar into hydrophobic poly(N-butyl-glycine). The synthesis of end-chain modified polypeptoids can be achieved by different ways using functional initiators , monomers , and postpolymerization reactions , , . In this work, a functional initiator dopamine hydrochloride was employed to prepare catechol end-functionalized amphiphilic diblock copolypeptoids. The Brønsted base, triethylamine, controls the activity of the active amine end. Thereafter, PSar with secondary amine terminus as macroinitiator triggers the ROP of Bu-Gly-NCA (Scheme 5, 1) to offer catechol end-functionalized amphiphilic diblock copolypeptoids (2).
The polymerization of Bu-Gly-NCA was carried out at 50°C under N2 flow. After 48 h, the final product was precipitated in diethyl ether. The molecular structures of the block copolymers were confirmed by 1H NMR in DMSO-d6. In the 1H NMR spectrum (Fig. 4), the proton resonances at 4.04–4.54 ppm and 2.88–3.14 ppm are assigned to the methylene proton and methyl proton of the PSar. The molecular structures of the amphiphilic polypeptoids and their corresponding monomers are in accordance with the literature report . Mn,NMR of the obtained block copolypeptoids are in agreement with that of the theoretical values.
However, the molecular weights of diblock copolypeptoids, Mn,SEC ar are not in agreement with the Mn,theo and Mn,NMR (Table 2 and Fig. S6). This may be due to the impurities and side reaction, the initiation of Bu-Gly-NCA monomer by the secondary amine macroinitiator is slower and resulted in relatively lower Mn,SEC of the second block . Further optimization of Bu-Gly-NCA and addition of the Brønsted base are suggested to control the polymerization. Overall, the substituents of N-alkyl glycines can be derivatized by changing the R group in the amide backbone for click and other postpolymerization reactions.
|Entry||M/Bu-Gly-NCA/I/TEA||Samplea||M n,theo b (kg mol−1)||M n,NMR c (kg mol−1)||M n,SEC d (kg mol−1)||Đ d (Mn/Mw)|
a,cDPs determined by 1H NMR spectrometry in D2O; bTheoretical Mn=Mw(Dopa.HCl)+DP(Sarcosine)×Mw(sarcosine unit)+DP(Bu-Gly-NCA)+Mw(Bu-Gly unit); dDetermined by SEC in 0.1 mol L−1 LiBr–DMF, using a calibration curve constructed with PMMA standards.
Amphiphilic N-substituted polypeptoids have shown the ability to disperse a hydrophobic model compound in aqueous media . Catechol initiator was successfully incorporated to amphiphilic PSar-b-PGlyBu diblock copolymers. The prepared amphiphilic PSar-b-PGlyBu block copolypeptoids bearing catechol moiety were soluble in water due to their aqueous self-assembly thereby further used for surface functionalization of MnO-NPs , , . The final product cat-PSar-b-PGlyBu with free catechol end was released by its acidification with 1.0 mol L−1 HCl (aq.) (Scheme 5, 3). Furthermore, the prepared amphiphilic diblock copolypeptoids with free catechol moiety were used for aqueous solubility and MnO-NPs surface coatings.
Aqueous dispersion of manganese oxide nanoparticles
Monodisperse MnO-NPs are prepared by thermal decomposition of manganese-oleate complex. Herein, we employed grafting technique to functionalize the pre-made MnO-NPs by catechol end-functionalized amphiphilic PSar-b-PGlyBu diblock copolypeptoids. Catechol was used as anchoring group to the surface of the MnO-NPs. The diblock copolypeptoids provide further stability and aqueous dispersion. Since the density of the hydrophobic segment is much lesser than that of the hydrophilic, the NPs coated by block copolymers can self-assemble in aqueous solution. Aqueous dispersability of olaete capped MnO-NPs and catechol end-functionalied PSar-b-PGlyBu are shown in Figs. 5 and S8. Figure 5a-left and 5C display the solubility of oleate-coated MnO-NPs in hexane and its corresponding TEM image, respectively. Figure 5a-right shows the aqueous phase transition of MnO-NPs coated by catechol-ended functionalized amphiphilic PSar-b-PGlyBu diblock copolypeptoids.
The self-assembly nature of cat-PSar-b-PGlyBu played a key role for the dispersion of MnO-NPs in water with no precipitation (Fig. 5a right). DLS results demonstrated the aggregate behavior of the cat-PSar-b-PGlyBu into nanostructures in aqueous solution with average hydrodynamic diameter of 64 nm (Fig. 5b). In addition, TEM image displays the formation of monodisperse cat-PSar-b-PGlyBu capped MnO-NPs with average size of about 12 nm (Fig. 5d). The size difference might be attributed to the solvation shell  along with the MnO-NPs surface. Overall, the resulting MnO@cat-PSar-b-PGlyBu nanocomposites have showed self-assembly driven excellent aqueous dispersability ,  and stabilization.
A Brønsted base performed dual-function in preventing catechol interference and mediating the ring-opening polymerization. The Brønsted base coordinated with one of the acidic proton of catechol moiety, conferred intramolecular H-bonding interaction, kept catechol moiety intact in the subsequent synthesis of amphiphilic diblock copolypeptoids. The same Brønsted base worked in a second role to modulate the ROP of Sar-NCA and Bu-Gly-NCA. To confirm this polymerization strategy, the ROP of Sar-NCA was carried out using dopamine hydrochloride as initiator and triethylamine (TEA) as the Brønsted base. Subsequently, the copolymerization of Bu-Gly-NCA in one-pot provides cat-PSar-b-PGlyBu amphiphilic diblock copolymers. Triethylamine does a dual role in masking catechol moiety and modulating the activity of the amine chain-end in ROPs of Sar-NCA and Bu-Gly-NCA. Polypeptoids with predicted molecular weight (Mn,NMR=3.7 kg mol−1) and narrow dispersities (Đ<1.13) were obtained. However, Mn,SEC seriously deviated from the observed Mn,NMR at initiator to TEA ratio [I]/[TEA] of 1/1.1. Increasing the ratios of [I]/[TEA] to 1/2.3 offered Mn,SEC=3.4 kg mol−1 with dispersity of Đ=1.01, which reasonably agreed with the Mn,NMR. Moreover, Mn,SEC (from 1.6 to 4.3 kg mol−1) agreed with Mn,NMR. Mn,NMR of cat-PSar and cat-PSar-b-PGlyBu diblock copolypetoids are in close agreement with that of theoretical values (Mn,NMR). Catechol initiator was successfully incorporated to polysarcosine as confirmed by MALDI-ToF analysis. The mass differences (∆m/z) between individual signal peaks are attributed to the molecular weight of the sarcosine repeating unit (71.04 g mol−1). The prepared amphiphilic diblock copolypeptoids with free catechol moiety unveiled self-assembly driven dispersion of MnO-NPs in water. DLS result demonstrated the aggregation nature of cat-PSar-b-PGlyBu capped MnO-NPs into nanostructures in aqueous solution with the average hydrodynamic diameter of 64 nm. The obtained MnO@cat-PSar-b-PGlyBu nanostructures may serve as contrast agent in medical imaging. Overall, this Brønsted base mediated polymerization strategy can be extended to the synthesis of variety of catechol-based functional polymers.
A collection of papers presented at the 17th Polymers and Organic Chemistry (POC-17) conference held 4–7 June 2018 in Le Corum, Montpelier, France.
Funding source: National Natural Science Foundation of China
Award Identifier / Grant number: U1463201
Award Identifier / Grant number: 21522604
Funding source: Natural Science Foundation of Jiangsu Province
Award Identifier / Grant number: BK20150031
Funding statement: This work was supported by the National Key Research and Development Program of China (grant no: 2017YFC1104802), National Natural Science Foundation of China (grant no: U1463201, 21522604), Natural Science Foundation of Jiangsu Province, China (grant no: BK20150031), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), and the project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Top-Notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP).
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