2,4-Dinitrobenzenesulfonic acid in an efficient Brønsted acid-catalyzed controlled/living ring-opening polymerization of ε-caprolactone

Abstract

The ring-opening polymerization of ε-caprolactone (ε-CL) using benzyl alcohol (BnOH) as initiator and 2,4-dinitrobenzenesulfonic acid (DNBA) as catalyst in acetonitrile at room temperature with a [ε-CL]₀/[BnOH]₀/[DNBA]₀ ratio of 40/1/1 has been investigated. The polymerization proceeded to obtain poly(ε-caprolactone) (PCL) with controlled molecular weights. In addition, ¹H NMR, SEC, and MALDI-ToF MS measurements demonstrated the initiator residue at the polymer chain end. Furthermore, propargyl alcohol, 5-hexen-1-ol, 2-hydroxyethyl methacrylate, 1,3-propanediol, and pentaerythritol were used as functional initiators to successfully obtain end-functionalized and α,ω-dihydroxy telechelic polyesters. The block copolymerization of PCL and PVL or PTMC provided conditions to afford well-defined poly(ε-caprolactone)-block-poly(δ-valerolactone) (PCL-b-PVL) and poly(ε-caprolactone)-block-poly(trimethylene carbonate) (PCL-b-PTMC).

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Introduction

Aliphatic polyesters are attractive biodegradable and biocompatible polymers used in pharmaceuticals and medicine.^1–4 In the past, polyesters were mainly prepared by ring-opening polymerization (ROP) using metal-based catalysts;^5–11 however, residual metals limited their wider applications. Consequently, much effort was focused on metal-free catalysis. Metal-free catalysts, called organocatalysts, are developing rapidly.^1,12 In particular, using organocatalysts to synthesize well-controlled macromolecular architectures has been studied extensively. Since the first report in 2001, Hedrick and Waymouth have used 4-dimethylaminopyridine¹³ for living ROP of lactide. Later, various organic base catalysts were developed, such as N-heterocyclic carbene,^14–17 thiourea/amine,^18,19 guanidine,²⁰ phosphazene,^21,22 1,5,7-triazabicyclo [4.4.0] dec-5-ene,^23,24 and 1,8-diazabicyclo[5.4.0]-undec-7-ene.^25,26 the ROP of cyclic esters.

Comparatively, Brønsted acid catalysts have not been widely reported. Pioneered in 2000 by Endo and co-workers using HCl·Et₂O-catalyzed in ROP of lactones,^27,28 acid catalysts such as methanesulfonic acid,^29,30 trifluoromethane sulfonic acid,^29,31 diphenyl phosphate,^32–34 and triflimide,^35,36 catalyze the ROP of cyclic esters and have attracted more and more attention. To further extend the scope of the acid catalysis system, our group reported several efficient acid catalysts for the controlled/living ROP of cyclic esters.^37–40

Previously, efficient catalytic performance in Hosomi–Sakurai,^41,42 Ritter,⁴³ Friedel–Crafts,⁴⁴ and enantioselective protonation⁴⁵ reactions was shown for 2,4-dinitrobenzenesulfonic acid (DNBA) as Brønsted acid. Additionally, DNBA is commercially available, inexpensive, low in toxicity, and easy to store and handle. As reported by Córdova in 2004,⁴⁶ the advantages of organic catalysts are low cost and low toxicity, but the ROP of CL required high temperature (120 °C). In contrast, the ROP in the current study was conducted at room temperature. In accordance with the excellent properties of DBNA, we decided to evaluate it as catalyst in the ROP of ε-CL. DNBA is a powerful organocatalyst for controlled/living ROP of ε-CL, which promoted rapid polymerization. A strong electron-withdrawing group strengthens the acidity of Brønsted acids;⁴⁷ the structural components of DNBA comprise a benzene ring connected with two nitro-groups and a sulfonic group. It is likely the strong electron-withdrawing group (NO₂) that provides the organic acid catalyst with high reactivity. To date, as far as we know, DNBA has not yet been applied in ROP. In this article, we describe (1) the characterization and optimization of DNBA for controlled ROP of ε-caprolactone, (2) the controlled/living nature of the DNBA-catalyzed ROP of ε-CL by ¹H NMR, SEC, and MALDI-ToF MS analyses, (3) various initiators used to initiate the polymerizations and produce end-functionalized and α,ω-dihydroxy telechelic polymers (as shown in Scheme 1), and (4) the synthesis of diblock copolymers consisting of PCL with PVL or PTMC.

Scheme 1 Ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) using 2,4-dinitrobenzenesulfonic acid (DNBA) as the organocatalyst and various alcohols (R–OH) as initiators.

Experimental

Materials

Both ε-caprolactone (ε-CL; 99%, Sinopharm Chemical Reagent) and δ-valerolactone (δ-VL; 99%, Sinopharm Chemical Reagent) were distilled over CaH₂ in an inert environment. Trimethylene carbonate (TMC) was synthesized via a conventional method.⁴⁸ Acetonitrile (CH₃CN; >99% water content, <0.001%) was dried over 3 Å molecular sieve pellets for 48 h before use. Benzyl alcohol (BnOH; 99%, Acros) was refluxed over CaH₂ prior to distillation. DNBA (Tokyo Kasei Kogyo Co., Ltd. (TCI)) was dried prior to use. Both 5-hexen-1-ol (98%, Energy Chemical) and 2-hydroxyethyl methacrylate (99.5%, J&K Scientific Co.) were distilled under reduced pressure before use. Next, 1,3-propanediol (99%, Arocs) was azeotropically distilled with toluene. Pentaerythritol (99%, Arocs) was dried over P₂O₅ under high vacuum. Propargyl alcohol (99%, Alfa Aesar) was distilled over CaH₂. Triethylamine (98%, Sinopharm Chemical Reagent Co.) was used as received.

Characterizations

The number-average molecular weight (M_n,NMR) and monomer conversion were determined from the ¹H NMR spectra in CDCl₃ on a Bruker ARX-250 spectrometer at 300 MHz at ambient temperature. Size exclusion chromatography (SEC) was performed in tetrohydrofuran (THF) using an SSI 1500 pump equipped with Waters column (5 μm, 300 × 7.8 mm) at a flow rate of 0.7 mL min⁻¹ at 25 °C, and a Wyatt Optilab rEX differential refractive index (DRI) detector with a 658 nm light source. Polystyrene was used as the standard to determine the number-average molecular weight (M_n) and molecular weight distributions (M_w/M_n) of the polymers. All SEC data were processed using Wyatt Astra V 6.1.1 software. Matrix-assisted, laser desorption/ionization time-of-flight mass spectra (MALDI-ToF MS) was performed using a mass spectrometer (ultraflextreme; Bruker Co.) with Smartbeam/Smartbeam II modified Nd:YAG laser. The MALDI-ToF mass spectra represent averages over 500 laser shots at a 25 kV acceleration voltage. The polymer sample was dissolved in CHCl₃ at a concentration of 5 mg mL⁻¹; the matrix 2,5-DHB (2,5-dihydroxybenzoic acid) was dissolved in aqueous (1%, 10 μL) solution of trifluoroacetic acid and acetonitrile (volume ratio = 70/30).

General procedure for polymerization of ε-caprolactone

All reactions were conducted in a glove box at room temperature. The ε-caprolactone (ε-CL) (0.213 mL, 2 mmol, 40 equiv.) was mixed into acetonitrile ([ε-CL]₀ = 1.0 mol L⁻¹) using benzyl alcohol (5.2 μl, 0.05 mmol, 1 equiv.) as the initiator. DNBA (0.0124 g, 0.05 mmol, 1 equiv.) was then dissolved in the reaction solutions. After 4 h, the monomer conversion reached 95.2%, which was determined by ¹H NMR. At the end of the polymerization, an excess of triethylamine was added to stop the reaction. The polymer was dissolved in the minimum amount of dichloromethane and separated out from cold methanol, and then dried in a vacuum drying oven. Conversion follows: 95.2%, yield, 61.0%; ¹H NMR (CDCl₃): δ (ppm), 1.39 (m, 2H × n, (–CH₂CH₂CH₂CH₂CH₂–)_n), 1.63 (m, 2H × n, (–CH₂CH₂CH₂O–)_n), 1.68 (m, 2H × n, (–COCH₂CH₂CH₂–)_n), 2.31 (t, 2H × n, J = 7.3 Hz, (–OCOCH₂CH₂–)_n), 3.65 (t, 2H, J = 6.6 Hz, CH₂CH₂OH), 4.06 (t, 2H × n, J = 6.6 Hz, (–CH2CH2O–)_n), 5.12 (s, 2H, ArCH₂O), 7.23–7.39 (m, 5H, aromatic); M_n,_NMR ≈ 4580 g mol⁻¹, M_w/M_n = 1.16.

Block copolymerization of ε-caprolactone and δ-valerolactone or trimethylene carbonate

All reactions were conducted in a glove box and at room temperature. The ε-caprolactone (ε-CL) (0.213 mL, 2 mmol, 40 equiv.) was mixed into acetonitrile ([ε-CL]0 = 1.0 mol L⁻¹). Benzyl alcohol (5.2 μl, 0.05 mmol, 1 equiv.) was used as the initiator. The DNBA (0.0124 g, 0.05 mmol 1 equiv.) was then dissolved in the reaction solutions. After 6 h (we extended the reaction time for full conversion of monomers), 40 equiv. of δ-VL (0.181 mL, 2 mmol) was then added to the reaction chamber, and start the block copolymerization to obtain PCL-b-PVL under the same conditions. An excess of triethylamine was used to quench the reaction. The polymer was dissolved in a small quantity of CH₂Cl₂ and isolated by cold methanol. Yield, 53%. ¹H NMR (CDCl₃): δ (ppm), 1.38 (m, 2H × n, (–CH₂CH₂CH₂CH₂CH₂–)_n), 1.63 (m, 2H × (n + m), (–CH₂CH₂CH₂O–)_n), 1.68 (m, 2H × n, (–COCH₂CH₂CH₂–)_n), 2.31 (t, 2H × (n + m), J = 7.3 Hz, (–COCH₂CH₂–)_n), 3.65 (t, 2H, J = 6.5Hz, –CH₂CH₂OH), 4.07 (t, 2H × (n + m), J = 6.6 Hz, (–CH₂CH₂O–)_n), 5.12 (s, 2H, ArCH₂O), 7.23–7.39 (m, 5H, aromatic). M_n,NMR ≈ 8140 g mol⁻¹, M_w/M_n = 1.16.

Block copolymerization of ε-caprolactone and trimethylene carbonate (PCL-b-PTMC) was carried out with the similar reaction conditions. Yield, 57.0%. ¹H NMR (CDCl₃): δ (ppm), 1.39 (m, 2H × n, (–CH₂CH₂CH₂CH₂CH₂–)_n), 1.66 (m, 4H × n, (–COCH₂CH₂CH₂–)_n), 1.93 (n, 2H, (–OCH₂CH₂OH)), 2.01 (m, 2H × m, (–OCH₂CH₂–)m), 2.31 (t, 2H × n, J = 6.8 Hz, (–COCH₂–)_n), 3.75 (m, 2H, –CH₂OH), 4.07 (m, 2H × n, (–OCH₂CH₂CH₂O–)_n), 4.25 (t, 4H × m, J = 6.4 Hz, (–OCH₂CH₂CH₂O–)m), 5.12 (s, 2H, ArCH2O), 7.24–7.38 (m, 5H, aromatic). SEC (THF): M_n,_NMR ≈ 8480 g mol⁻¹, M_w/M_n = 1.15.

Results and discussion

Ring-opening polymerization of ε-caprolactone using benzyl alcohol and 2,4-dinitrobenzenesulfonic acid

In order to evaluate the activity of 2,4-dinitrobenzenesulfonic acid, we used DNBA as the catalyst and benzyl alcohol as the initiator in the ROP of ε-caprolactone in CH₃CN (1.0 mol L⁻¹) at room temperature with [ε-CL]₀/[BnOH]₀/[DNBA]₀ = 40/1/1 (Table 1, Run 4). The monomer conversion reached 95.2% after 4 h, which was determined from the ¹H NMR spectrum. To find the optimization of DNBA-catalyzed ROP of ε-CL reaction conditions, different solvents were carried out in DNBA-catalyzed ROP of ε-CL. (Table 1, Run 1–4). Toluene and THF were used as the solvent, while no polymer was separated out; thus, they acted as poor solvents for DNBA-catalyzed ROP of ε-CL. DNBA could be partly soluble in CH₂Cl₂ (completely dissolved at the end of polymerization); thus moderate monomer conversion (45.7% after 12 h) took place. The results demonstrated that CH₃CN was a suitable solvent for DNBA-catalyzed ROP of ε-CL. In this paper, we report experiments for ROP of CL using other organocatalysts (diphenyl phosphate (DPP), triflimide (HNTf₂) and trifluoromethanesulfonic acid (HOTf)) from other catalytic systems to compare catalytic performance, at room temperature with the [M]₀/[I]₀/[C]₀ ratio of 40/1/1 to afford PCL, as shown in Table 1 (Runs 4–6). All results demonstrated that 2,4-dinitrobenzenesulfonic acid (DNBA) was superior for the ROP of ε-CL.

Table 1 ROP of ε-caprolactone (ε-CL) catalyzed by 2,4-dinitrobenzenesulfonic acid (DNBA) with benzyl alcohol (BnOH) as the initiator^a

Run	[M]/[I]	Solvent	Time (h)	Conv.^b (%)	Mn,calcd^c (g mol^−1)	Mn,NMR^b (g mol^−1)	Mw/Mn^d	Mw^e (g mol^−1)
a [M]0 = 1.0 mol L^−1; room temperature.b Determined by ^1H NMR in CDCl3.c Calculated from ([M]0/[BnOH]0) × conv. × (Mw of ε-CL) + (Mw of BnOH).d Determined by SEC in THF using polystyrene standards.e Determined by SEC in THF using polystyrene standards and correction factors.^30,49,50f Catalyzed by diphenyl phosphate (DPP); [M]0 = 1.0 mol L^−1; room temperature; in toluene; [M]0/[I]0/[C]0 = 40/1/1.g Catalyzed by triflimide (HNTf2); [M]0 = 1.0 mol L^−1; in CH2Cl2; [M]0/[I]0/[C]0 = 40/1/1.h Catalyzed by trifluoromethanesulfonic acid (HOTf); [M]0 = 1.0 mol L^−1; in CH2Cl2; [M]0/[I]0/[C]0 = 40/1/1.
1	40	CH2Cl2	12	45.7	2190	—	—
2	40	Toluene	12	0	—	—	—
3	40	THF	12	0	—	—	—
4^f	40	Toluene	8	96.3	4500	4460	1.10	2509
5^g	40	CH2Cl2	12	87.3	4100	4330	1.16	3472
6^h	40	CH2Cl2	7	96.7	4520	4260	1.21	3978
7	40	CH3CN	4	95.2	4450	4580	1.16	4231
8	80	CH3CN	8	97.9	9050	9190	1.12	6673
9	100	CH3CN	14	92.1	10 620	11 180	1.14	9979
10	160	CH3CN	20	89.0	16 360	16 580	1.11	3978

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According to ¹H NMR spectra of the obtained PCL (Fig. 1), the peaks of initiator (BnOH) were observed; they appeared in the range of 7.23–7.39 ppm (A) and 5.12 ppm (B). The peaks for the polymer chain (PCL) were observed in 2.31 (C), 1.62–1.70 (D + F), 1.39 (E), 4.05 (G), and 3.65 (H) ppm, respectively. These results demonstrated that the obtained PCL was initiated from BnOH.

Fig. 1 ¹H NMR spectrum (CDCl₃, 300 MHz) of poly(ε-caprolactone) ([ε-CL]₀/[BnOH]₀/[DNBA]₀ = 40/1/1) CH₃CN, rt, [M]₀ = 1.0 mol L⁻¹, 4 h.

Moreover, in order to confirm the controlled/living nature of the polymerizations, we carried out ROPs of ε-CL by varying the [ε-CL]₀/[BnOH]₀ ratio from 40 to 160 (Table 1, Runs 4–7). The results showed that the obtained PCLs of predicted M_{n (NMR)} agreed with calculated ones by the initial ratio of [ε-CL]₀/[BnOH]₀ and the monomer conversions. For example, in the 40-mer (Table 1, Run 4), the values of 4580 g mol⁻¹ detected by ¹H NMR were a near match to calculated values of 4450 g mol⁻¹. In addition, molecular weights of polymers were higher than 16 000 g mol⁻¹, with simultaneous polydispersity (M_w/M_n) values of PCLs ranging from 1.19 to 1.12 via SEC analysis.

Furthermore, the kinetics and postpolymerization experiments provided convincing evidence of DNBA-catalyzed ROP of ε-CL in controlled/living nature. As shown in Fig. 2, we explored [ε-CL]₀ = 1.0 mol L⁻¹ and [ε-CL]₀/[BnOH]₀/[DNBA]₀ = 40/1/1; a linear relationship between monomer conversion and molecule mass values of PCLs was observed (Fig. 2(A)). The plots of ln([M]₀/[M]) versus reaction time also fit a strict linear relationship (Fig. 2(B)). Fig. 3(A) shows the SEC traces for chain extension experiments; the first polymerization proceeded with [ε-CL]₀/[BnOH]₀/[DNBA]₀ = 40/1/1 in CH₃CN at room temperature without quenching, the monomer conversion reached 97% after 6 h, and PCL with M_n,_NMR = 4310 g mol⁻¹, M_w/M_n = 1.19. Additional ε-CL (40 equiv.) was added to the second polymerization, obtaining PCL with M_n,_NMR = 9030 g mol⁻¹, M_w/M_n = 1.17, indicating the chain end group of PCL with living nature. Next, Fig. 3(B) shows the SEC traces of varying [ε-CL]₀/[BnOH]₀ from 40 to 160, and PCLs with low polydispersities. All results indicated characteristics of controlled/living nature.

Fig. 2 (A) Molecular weight (M_n) and polydispersity (M_w/M_n) versus the monomer conversion of ε-CL (theoretical M_n (solid line)) calculated from 108.13 (M_w of BnOH) + conv. × ([M]/[I]) × 114.07 (M_w of ε-CL). (B) Kinetic plots for the polymerization of ε-CL ([ε-CL]₀/[BnOH]₀/[DNBA]₀ = 40/1/1).

Fig. 3 (A) SEC traces of the first PCL sequence (solid line) and postpolymerization (dashed line). (B) SEC traces of PCLs with various [M]₀/[I]₀ ratios of 40 (a), 80 (b), 100 (c), 160 (d). Eluant, THF; flow rate, 0.7 mL min⁻¹.

In order to provide further evidence, MALDI-ToF MS measurements were undertaken to prove that the DNBA-catalyzed ROP of ε-CL was initiated by BnOH. As shown in Fig. 4, the MALDI-ToF MS analyses of PCL with monomers to initiator ratios of 40, in which PCL had a molecular formula of molar mass M = 108.13 (M_w of BnOH) + n × 114.07 (M_w of ε-CL) + 23 (Na⁺) or M = 108.13 (M_w of BnOH) + n × 114.07 (M_w of ε-CL) + 39 (K⁺), indicating the BnOH residue at the chain end. In addition, the mass differences between two adjacent peaks comprised a ε-CL unit. Those results strongly implied that BnOH initiated the polymerization, and emerged in a living manner without backbiting, transesterification or other undesirable side reactions. In addition, although the monomer (ε-CL), the catalyst (2,4-dinitrobenzenesulfonic acid), the initiator (BnOH) and the solvent (acetonitrile) were subjected to a rigorous drying process before use, concomitant loss of H₂O also occurred. From the MALDI-ToF MS spectrum, another two series of undesired peaks were initiated by H₂O. Obviously, measured values of water-initiated peaks were equal to theoretical values (as shown in Fig. 4). According to a report by Penczek and co-workers, the Brønsted acid-catalyzed ROP of cyclic esters is an activated monomer mechanism (AM) leading to well-defined polyesters;⁵¹ we have proposed a similar AM mechanism as shown in Scheme 2.

Fig. 4 MALDI-ToF MS spectra of the obtained PCL ([ε-CL]₀/[BnOH]₀/[DNBA]₀ = 40/1/1, conversion = 95.2%, M_n,_NMR = 4580 g mol⁻¹, M_w/M_n = 1.16).

Scheme 2 Activated monomer mechanism for DNBA-catalyzed ROP of ε-CL using BnOH as the initiator.

Syntheses of end-functionalized and α,ω-dihydroxy telechelic poly(ε-caprolactone)

For the purposes of evaluating DNBA-catalyzed polymerizations with the controlled/living nature characteristic, we used DNBA-catalyzed ROP of ε-CL with propargyl alcohol, 5-hexen-1-ol, 2-hydroxyethyl methacrylate, 1,3-propanediol, and pentaerythritol as initiators for providing end-functionalized and α,ω-dihydroxy telechelic polymers, which were confirmed by ¹H NMR spectra (Fig. S1–S5†). The obtained polymers with the alkyne group could serve for the click reaction,^52,53 and polymers with the methacrylate group as macromonomers, and polymers with alkene group could be used for further modifications.^52,54 Table 2 lists the results for obtained poly(ε-caprolactone), and the PCLs initiated by propargyl alcohol, 5-hexen-1-ol, 2-hydroxyethyl methacrylate, pentaerythritol, and 1,3-propanediol; the values of M_n,NMR of 4390, 4250, 4410, 4490 and 4460 g mol⁻¹ correspond to the M_n,calcd of 4510, 4540, 4610, 4190 and 4620 g mol⁻¹, respectively. Meanwhile, the M_w/M_n was at the range of 1.10–1.17 (Table 2), and showed narrow distributions. Thus, we demonstrated that DNBA was an efficient organocatalyst to synthesize well-defined macromolecular architectures.

Table 2 Syntheses of end-functionalized and α,ω-dihydroxy telechelic poly(ε-caprolactone) by the DNBA-catalyzed ROP of ε-CL using various initiators^a

Run	Initiator	Conv.^b (%)	Mn,calcd^c (g mol^−1)	Mn,NMR^b (g mol^−1)	Mw/Mn^d	Mw^e
a At room temperature; solvent, CH3CN; time, 10 h; [M]0/[I]0/[DNBA]0 = 40/1/1; [M]0 = 1.0 mol L^−1.b Determined by ^1H NMR in CDCl3.c Calculated from ([M]0/[I]0) × conv. × (Mw of ε-CL) + (Mw of initiator).d Determined by SEC in THF using polystyrene standards.e Determined by SEC in THF using polystyrene standards and correction factors.^30,49,50f HEMA = 2-hydroxyethyl methacrylate.
1	Propargyl alcohol	96.7	4510	4390	1.11	2497
2	5-Hexen-1-Ol	97.2	4540	4250	1.13	2531
3	HEMA^f	98.1	4610	4410	1.15	4119
4	Pentaerythritol	98.3	4190	4490	1.17	3763
5	1,3-Propanediol	90.1	4620	4460	1.10	2508

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Synthesis diblock copolymers of ε-caprolactone and δ-valerolactone or trimethylene carbonate

According to the chain extension experiments, we concluded that the chain end of obtained PCL had a living nature. Indeed, we also explored the copolymerization of ε-CL with two different monomers (δ-VL and TMC) producing PCL-b-PVL and PCL-b-PTMC, which were versatile as biodegradable materials, and it also illustrated that the hydroxyl group at the chain end possessed further polymerization ability. We carried out the first polymerization with [ε-CL]₀/[BnOH]₀/[DNBA]₀ = 40/1/1 after the monomer was consumed completely without quenching, and then added another monomer (δ-VL, 40 equiv.) for the second polymerization. In addition, the reaction with TMC (40 equiv.), the second monomer to synthesize diblock copolymers (PCL-b-PTMC), was carried out with the same method. The obtained copolymers were determined by ¹H NMR spectra (ESI, Fig. S6 and S7†). The SEC traces then illustrated that the molar mass of the obtained PCL-b-PVL shifted from 4270 to 8140 g mol⁻¹, and the polydispersity was slightly narrowed from 1.18 to 1.16; and the molar mass of PCL-b-PTMC shifted from 4560 to 8480 g mol⁻¹, and the polydispersity varied from 1.16 to 1.15, as shown in Fig. 5. All results indicated that DNBA was an efficient catalyst in synthesizing well-defined diblock copolymers of ε-CL with δ-VL and TMC.

Fig. 5 (A) SEC traces of first sequence of poly(ε-caprolactone) (PCL; dashed line) and poly(ε-caprolactone)-block-poly(δ-valerolactone) (PCL-b-PVL; solid line). (B) SEC traces of first sequence of poly(ε-caprolactone) (PCL; dashed line) and poly(ε-caprolactone)-block-poly(trimethylene carbonate) (PCL-b-PTMC; solid line).

Conclusions

In this work, we used DNBA as an efficient Brønsted acid organocatalyst for ROP of ε-caprolactone (ε-CL) and BnOH as the initiator to produce well-defined poly(ε-caprolactone) (PCL). The controlled/living nature of the polymerization was confirmed, and the obtained polymers demonstrated narrow polydispersity even at high molecular weights. Moreover, we used various initiators (propargyl alcohol, 5-hexen-1-ol, 2-hydroxyethyl methacrylate, 1,3-propanediol, pentaerythritol) to produce end-functionalized and α,ω-dihydroxy telechelic PCL. We also synthesized well-defined block copolymers of PCL-b-PVL and PCL-b-PTMC. DNBA as a commercially available, inexpensive and shelf-stable acidic catalyst may be applicable in broad range of ROPs; we will extend DNBA catalysis in diverse cyclic esters.

Acknowledgements

This work was supported by the National High Technology Research and Development Program of China (2011AA02A202); Doctoral Program of Higher Education of China (20123221110009); and Priority Academic Program Development, Jiangsu Higher Education Institutions. We would like to thank Dr Hailong Liu of Nanjing Normal University for conducting the MALDI-TOF MS experiments and Dr Zhe Song of China Pharmaceutical University for NMR analyses.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: 1H NMR for end-functionalized and α,ω-dihydroxy telechelic poly(ε-caprolactone) and block copolymerization; and calculation details of ε-CL conversion. See DOI: 10.1039/c4ra09579k

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