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DOI: 10.1039/C5RA22279F (Paper) RSC Adv., 2016, 6, 11400-11406

A unique cooperative catalytic system carrying metallic iron and 2-hydroxyethyl 2-bromoisobutyrate for the controlled/living ring-opening polymerization of ε-caprolactone†

Xiu-Juan Shanga, Wen-Hua Zhang*a and Jian-Ping Lang*ab
aCollege of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, People's Republic of China. E-mail: jplang@suda.edu.cn; whzhang@suda.edu.cn
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 210032, People's Republic of China

Received 24th October 2015 , Accepted 17th January 2016

First published on 20th January 2016

Abstract

We herein report an interesting cooperative catalytic system containing iron powder and 2-hydroxyethyl 2-bromoisobutyrate (HEBiB), which can efficiently catalyze the ring-opening polymerization of ε-caprolactone (CL) under facile conditions, forming poly(ε-caprolactone) (PCL) with appreciably high molecular weight (Mn), narrow molecular weight distribution and high yield. The 1H NMR and GPC (gel permeation chromatography) measurements of the resulting PCL clearly indicate the presence of the initiator residue at the chain end. Optimization reactions, wherein iron powder or HEBiB alone, the combinative use of iron powder and benzyl alcohol (BnOH), iron powder and alkyl bromides in the absence/presence of benzyl alcohol (BnOH) are introduced, reveal that iron and alkyl bromides collectively initiate the reaction and define the molecular weight of PCL, while the presence of the hydroxyl group leads to higher product yields. The effects of metal contaminants, hydroxyl initiators, solvents, reaction temperatures and reaction times are also investigated. The kinetic and chain extension experiments support the controlled/living nature of the HEBiB/Fe-catalyzed ROP of CL.

Introduction

Ring-opening polymerization (ROP) of lactones and lactides is an essential industrial process to produce aliphatic polyesters such as poly(ε-caprolactone) (PCL) and poly(rac-lactide) (PLA). These polyesters have good hydrolyzability, mechanical properties, biocompatibility and miscibility with other polymers, making them leading candidates as drug delivery vehicles in the biomedical field, scaffolds in tissue engineering, adhesives, microelectronics, and packaging.1–5 Organometallic catalysts of Al,6–13 Fe,14–17 Zn,18–26 Sn,27–29 among several others,30–40 have been used in these conversion processes.2,3,41–43 The difficulty of catalyst removal and the inherent toxicity associated with certain types of catalysts are the major barriers in the commercialization of PCLs and PLAs.44

Among the active catalysts for ε-caprolactone (CL) polymerization, iron-based catalysts are particularly attractive owing to the abundance, low toxicity and biocompatibility of iron.45 Attempts have been made to use ferric oxide,46,47 iron porphyrins,48 iron lactates,49 monocarboxylic iron derivatives,50 ferric alkoxides and aryloxides,45 and iron N-heterocyclic carbene (NHC–Fe) compounds16 as catalysts for the polymerization of lactones. Anhydrous or hydrated Fe(III)/Fe(II) halides are also directly used to initiate the ROP of CL.44,51 But use of ferric halides results in lower degree of control over the molecular weight and polydispersity index (PDI), which is likely due to the hydrolyzation of FeCl3 into different active species “Fen(H2O)m(OH)x”.52,53 Ferric alkoxides are also attractive for their high catalytic activity but confined by their extremely hydrolytic sensitivity.

In exploiting cheap and efficient catalysts for CL polymerization,53–56 we found that a mixture of iron powder (≥98% metal purity) and hydroxybromide 2-hydroxyethyl 2-bromoisobutyrate (HEBiB) could efficiently catalyze the ROP of CL under facile conditions (toluene, 110 °C, 12 h), forming PCL with appreciably high molecular weight (Mn), narrow PDI and high yield (Mn = 23[thin space (1/6-em)]560 g mol−1, PDI = 1.23, yield = 99.6%). Metallic iron and other metals such as Cu, Zn and Mg have been reported in the atom transfer radical polymerization with activators generated by electron transfer (AGET ATRP).57–61 While there is no literature precedence of the CL polymerization using in situ generated catalyst from metallic Fe, and little is known on the reaction of metallic Fe and HEBiB, we set out to gain a better understanding of the catalytic process of this ubiquitous reaction by identifying the dominating factors in this system. In this article, we describe a cooperative, cost-effective and highly efficient catalytic system containing Fe powder and HEBiB and its excellent catalytic performance in the CL polymerization.

Results and discussion

Effect of metal purity

The purity of FeCl3 was reported to play a crucial role in iron-catalyzed cross-couplings, leading to arylated amides, phenols, thiols, and alkynes.62 These findings activate us to consider the effects of the purity of iron powder on the CL polymerization. In this work, there are metal contaminants Cu or Cu2O in ≥98% Fe and >99.998% Fe used for the reaction. Therefore we have to assess the metal purity on the reaction outcome. As shown in Table 1, the use of higher purity iron powder (99.998%) afforded similar polymer to that of ≥98% iron powder (entries 1 and 2). Iron powders with varied purity in the presence of Cu or Cu2O all initiated the polymerization (entries 3–6), affording PCL with similar Mn, PDI and yield. Cu or Cu2O alone could not initiate the CL polymerization (entries 7 and 8). Thus, it is conclusive that the purity of iron powder has negligible impact on the CL polymerization.
Table 1 CL polymerization by different metal sources and their combinations in the presence of HEBiB
Entrya Metal Mnb (g mol−1) PDIb Yieldc (%)
a Reaction conditions: [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][metal]0[thin space (1/6-em)]:[thin space (1/6-em)][HEBiB]0 = 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1; reaction time: 24 h; temperature: 110 °C; solvent: toluene.b Determined by GPC in THF, calibrated to a polystyrene standard.c Yield: weight of polymer obtained/weight of monomer used.d [Fe]0[thin space (1/6-em)]:[thin space (1/6-em)][Cu]0 = 10[thin space (1/6-em)]:[thin space (1/6-em)]1.e [Fe]0[thin space (1/6-em)]:[thin space (1/6-em)][Cu2O]0 = 10[thin space (1/6-em)]:[thin space (1/6-em)]1.
1 Fe(≥98%) 21[thin space (1/6-em)]660 1.23 100
2 Fe(99.998%) 22[thin space (1/6-em)]430 1.24 100
3d Fe(≥98%)/Cu 18[thin space (1/6-em)]760 1.28 100
4d Fe(99.998%)/Cu 18[thin space (1/6-em)]480 1.34 98.7
5e Fe(≥98%)/Cu2O 17[thin space (1/6-em)]810 1.33 100
6e Fe(99.998%)/Cu2O 23[thin space (1/6-em)]100 1.26 100
7 Cu — — NA
8 Cu2O — — NA

Contributions of hydroxyl and bromide

Experiments using either hydroxyl initiator R′OH (R′ = PhCH2, (CH3)2CH or H) or alkyl bromides R′′Br (R′′ = CH3CH2OCOC(CH3)2 or C(CH3)3) to surrogate HEBiB (HORBr) were carried out to assess the contributions of the hydroxyl and bromide functionalities within HEBiB (Scheme 1). As shown in Table 2, iron powder (entry 8) or HEBiB alone (entry 10), or the combinative use of iron powder and benzyl alcohol (BnOH, entry 9) provided no observed product. In the absence of BnOH, the combination of iron powder and alkyl bromides such as ethyl-2-bromoisobutyrate (EBiB) and 2-bromo-2-methylpropane (BMP) yielded the anticipated PCL but with moderate to low yields (entries 1 and 2). The presence of both the alcohol (benzyl alcohol) and alkyl bromides (EBiB or BMP) in the systems gave PCL with favorable yields (entries 3 and 6). In all the active systems, the molecular weights (Mn) determined by GPC showed good agreement with their theoretical values (Mtheo). These results indicate that both hydroxyl and bromides in HEBiB are indispensable in promoting the CL polymerization. Notably, compared to the combinative use of Fe/HEBiB (Table 1, entry 1), the introduction of BnOH (Table 2, entry 7) provided a favorable yield of PCL but at the expense of PDI. Additional hydroxyl initiators like isopropanol and water were used to investigate the effect of R′OH on the CL polymerization (entries 4 and 5). All the listed hydroxyl initiators are suitable for the CL polymerization in this system, yielding the product with moderate to high yields depending on the nucleophilic character and steric feature of the hydroxyl initiator.24
image file: c5ra22279f-s1.tif
Scheme 1 Ring-opening polymerization of CL in the presence of Fe powder and alkyl bromides (R′′Br, e.g., EBiB or BMP) initiated by hydroxyl initiators (R′OH, e.g., BnOH).
Table 2 CL polymerization initiated by Fe powder coupled with different alkyl bromides (R′′Br) EBiB, BMP and HEBiB in the absence/presence of hydroxyl initiators (R′OH)
Entrya R′OH R′′Br Mnb (g mol−1) Mtheo (g mol−1) PDIb Yieldc (%)
a Reaction conditions: [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][Fe]0[thin space (1/6-em)]:[thin space (1/6-em)][R′′Br]0[thin space (1/6-em)]:[thin space (1/6-em)][R′OH]0 = 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1; reaction time: 24 h; temperature: 110 °C; solvent: toluene; BnOH: benzyl alcohol; i-PrOH: isopropanol.b Determined by GPC in THF, calibrated to a polystyrene standard.c Yield: weight of polymer obtained/weight of monomer used.d Mtheo = yield (%) × ratio [CL]0/[R′′Br]0 × 2 × M (CL).e Mtheo = yield (%) × ratio [CL]0/[R′OH]0 × M (CL).f No Fe was included.
1d 0 EBiB 19[thin space (1/6-em)]680 16[thin space (1/6-em)]000 1.09 70.1
2d 0 BMP 15[thin space (1/6-em)]250 13[thin space (1/6-em)]830 1.06 60.6
3e BnOH EBiB 8310 8750 1.10 76.7
4e i-PrOH EBiB 14[thin space (1/6-em)]540 12[thin space (1/6-em)]970 1.09 56.8
5e H2O EBiB 9660 12[thin space (1/6-em)]490 1.11 54.7
6e BnOH BMP 13[thin space (1/6-em)]070 10[thin space (1/6-em)]500 1.13 92.0
7e BnOH HEBiB 10[thin space (1/6-em)]080 11[thin space (1/6-em)]320 1.39 99.2
8 0 0 — — — NA
9 BnOH 0 — — — NA
10f HEBiB   — — — NA

Mechanism studies

As shown in Table 3, the Mn from 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2/3–3 CL/HEBiB/Fe ratios (Fe powder was surplus) resemble that from 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1/2 CL/HEBiB/Fe ratio, and double that calculated. The presumption was evidenced by the results from 100[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]1 and 100[thin space (1/6-em)]:[thin space (1/6-em)]3[thin space (1/6-em)]:[thin space (1/6-em)]1 CL/HEBiB/Fe ratios (Fe powder was completely consumed). Consequently, the theoretical molecular weight of the prepared polymers catalyzed by HEBiB/Fe depends on the molar ratio of monomer CL to the consumed Fe powder.
Table 3 CL polymerization in the presence of different ratios of CL/HEBiB/Fe
Entrya Ratio Mnb (g mol−1) Mcalcd (g mol−1) 2Mcalcd (g mol−1) PDIb Yieldc (%) Iron Relation
a Reaction condition: temperature: 110 °C; reaction time: 24 h; solvent: toluene; ratio = [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][HEBiB]0[thin space (1/6-em)]:[thin space (1/6-em)][Fe]0.b Determined by GPC in THF, calibrated to a polystyrene standard.c Yield: weight of polymer obtained/weight of monomer used.d Mcalc = yield (%) × 100 × M (CL).
1 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0.5 20[thin space (1/6-em)]680 10[thin space (1/6-em)]890 21[thin space (1/6-em)]780 1.23 95.4 Moderate Mn ≈ 2Mcalc
2 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2/3 24[thin space (1/6-em)]390 11[thin space (1/6-em)]410 22[thin space (1/6-em)]830 1.22 100 Surplus
3 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 21[thin space (1/6-em)]660 11[thin space (1/6-em)]410 22[thin space (1/6-em)]830 1.23 100
4 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 19[thin space (1/6-em)]790 11[thin space (1/6-em)]410 22[thin space (1/6-em)]830 1.24 100
5 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]3 25[thin space (1/6-em)]340 11[thin space (1/6-em)]410 22[thin space (1/6-em)]830 1.19 100
6 100[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]1 13[thin space (1/6-em)]530 11[thin space (1/6-em)]410 22[thin space (1/6-em)]830 1.41 100 Moderate Mn ≈ Mcalc
7 100[thin space (1/6-em)]:[thin space (1/6-em)]3[thin space (1/6-em)]:[thin space (1/6-em)]1 10[thin space (1/6-em)]810 11[thin space (1/6-em)]380 22[thin space (1/6-em)]760 1.37 99.7

Upon mixing CL, HEBiB and Fe with ratio of 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 (iron in excess), the mixture gradually changed into pale yellow, yellow, and finally orange-red within approximately 10 min, 20 min and 1 h, respectively. The orange-red color maintained throughout the reaction. In the exploration experiment of mechanism, no blood-red color (the characteristic color for [Fe(SCN)6]3−) was observed when the reaction mixture was injected to a 1 mol L−1 oxygen-free aqueous KSCN solution, which supports the diagnostic of Fe(II) and also excludes the formation of Fe(III) from the reaction. Furthermore, excess Fe powder could prevent the oxidation of Fe(II) to Fe(III). According to the above analysis, the plausible mechanism for the ROP of CL catalyzed by the combination of HEBiB and Fe powder is depicted in Scheme 2. Similar to the preparation of Grignard reagents from the reaction of an organic halide and metallic Mg,63 as well as the activation of alkyl halides by metallic Cu in reversible deactivation radical polymerization,60,61 mixing iron and HEBiB (R′′Br and/or R′OH) yields an organo-iron compound (R′′FeBr) in situ. Compound R′′FeBr subsequently reacts with another HEBiB (R′OH) to produce a ferrous alkoxide (R′OFeBr). Polymerization initiation and propagation proceed via a coordination–insertion mechanism, wherein the metal-alkoxide (R′OFeBr) attacks the CL carbonyl carbon to open the ring, simultaneously generating a new ferrous alkoxide for the next catalytic cycle.2,23,34,45,64


image file: c5ra22279f-s2.tif
Scheme 2 Proposed mechanism for the ring-opening polymerization of ε-caprolactone in the combination of R′OH/R′′Br/Fe.

Effect of temperature

Subsequently, the influence of temperature was investigated. As listed in Table 4, as the reaction temperature increased from 80 °C to 120 °C, the polymerization rate expectedly speeds up. When the reaction temperature is raised up to 120 °C, the PDI of the prepared polymers broadened a lot due to high temperature (Table 4, entries 5 and 10). Compromising the conversion and PDI, 110 °C and reaction time of 24 h were optimized for further study.
Table 4 CL polymerization in the presence of Fe/2-hydroxyethyl 2-bromoisobutyrate (HEBiB) at different temperatures for different reaction times
Entrya t (h) T (°C) Mnb (g mol−1) Mtheoc (g mol−1) PDIb Yieldd (%)
a Reaction conditions: [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][Fe]0[thin space (1/6-em)]:[thin space (1/6-em)][HEBiB]0 = 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1; solvent: toluene.b Determined by GPC analysis in THF, calibrated to a polystyrene standard.c Mtheo = yield (%) × ratio [CL]0/[HEBiB]0 × 2 × M (CL).d Yield: weight of polymer obtained/weight of monomer used.
1 12 80 — — — NA
2 12 90 6130 8900 1.20 39.0
3 12 100 12[thin space (1/6-em)]530 16[thin space (1/6-em)]410 1.19 71.9
4 12 110 23[thin space (1/6-em)]560 22[thin space (1/6-em)]740 1.23 99.6
5 12 120 27[thin space (1/6-em)]360 22[thin space (1/6-em)]830 1.24 100
6 24 80 — — — NA
7 24 90 10[thin space (1/6-em)]170 18[thin space (1/6-em)]220 1.22 79.8
8 24 100 14[thin space (1/6-em)]580 22[thin space (1/6-em)]690 1.14 99.4
9 24 110 21[thin space (1/6-em)]660 22[thin space (1/6-em)]830 1.23 100
10 24 120 20[thin space (1/6-em)]520 22[thin space (1/6-em)]830 1.33 100

Effect of solvent

The solvent contribution on the CL polymerization was also investigated (Table 5). The CL polymerization did not take place in C2H5OH, CH3OH, DMF, THF, and cyclohexanone, which may be due to the low solubility of the resulting PCL or their coordination with the active Fe species. However, it proceeded smoothly in toluene, CH2Cl2, CHCl3, dioxane, as well as in the absence of any solvent. As suggested by the above discussions, the CL polymerization may also take place by a combination of iron powder/halogenated solvents (CH2Cl2 or CHCl3), regardless of the presence of the hydroxyl initiator (entries 8 and 9). The Mn of the obtained polymers also showed good agreement with Mtheo.
Table 5 CL polymerization in the presence of Fe/2-hydroxyethyl 2-bromoisobutyrate (HEBiB) in different solvents
Entrya Solvent Mnb (g mol−1) Mtheod (g mol−1) PDIb Yieldc (%)
a Reaction conditions: [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][Fe]0[thin space (1/6-em)]:[thin space (1/6-em)][HEBiB]0 = 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1; reaction time: 24 h; reaction temperature: 110 °C.b Determined by GPC analysis in THF, calibrated to a polystyrene standard.c Yield: weight of polymer obtained/weight of monomer used.d Mtheo = yield (%) × ratio [CL]0/[Fe]0 × 2 × M (CL).e HEBiB absent.f HEBiB absent while the hydroxyl initiator benzyl alcohol present, Mtheo = yield (%) × ratio [CL]0/[Fe]0 × M (CL).
1 CH3OH — — — NA
2 C2H5OH — — — NA
3 THF — — — NA
4 DMF — — — NA
5 Cyclohexanone — — — NA
6 CH2Cl2 22[thin space (1/6-em)]060 22[thin space (1/6-em)]830 1.25 100
7 CHCl3 17[thin space (1/6-em)]430 21[thin space (1/6-em)]070 1.22 92.3
8e CHCl3 20[thin space (1/6-em)]180 20[thin space (1/6-em)]320 1.28 89.0
9f CHCl3 14[thin space (1/6-em)]410 11[thin space (1/6-em)]410 1.21 100
10 Dioxane 11[thin space (1/6-em)]820 11[thin space (1/6-em)]190 1.23 98.0
11 Toluene 21[thin space (1/6-em)]660 22[thin space (1/6-em)]830 1.23 100
12 0 23[thin space (1/6-em)]990 22[thin space (1/6-em)]830 1.21 100

Polymerization kinetics

The kinetic experiments and the chain extension experiment were carried out to confirm the HEBiB/Fe-catalyzed ROP of CL. Fig. 1 shows the semi-logarithmic plot of ln ([CL]0/[CL]t) versus time for the CL polymerization in the combination of HEBiB/Fe. The linearity of the plot indicates that the propagation was first-order kinetics with respect to the monomer concentration, and the straight line missing origin indicates the possibility of an incubation period of the catalyst, and agrees well with the proposed mechanism. From the slope of the plot, the value of the apparent rate constant (K) was found to be 0.124 h−1. The molecular weight is also plotted versus conversion in Fig. 2. The molecular weight increases linearly with respect to the monomer conversion as an evidence of controlled nature of the polymerization. The molecular weight distribution remained narrow (PDI = 1.14–1.23, Table 6) during the polymerization process. It also shows very good agreement between experimental and theoretical molecular weight even after high monomer conversion. However, we have to admit that the catalytic activity of this polymerization system is lower than that using the Zn complex for the ROP of CL at 25 °C (Mn = 11[thin space (1/6-em)]956 g mol−1, PDI = 1.12, conversion = 98%, 6 h).24
image file: c5ra22279f-f1.tif
Fig. 1 Kinetic plot for CL consumption versus time in the combination of Fe/HEBiB in toluene at 110 °C.

image file: c5ra22279f-f2.tif
Fig. 2 Plots of molecular weight and PDI versus conversion for the solution polymerization of CL in the presence of Fe/HEBiB.
Table 6 CL polymerization in the presence of Fe/2-hydroxyethyl 2-bromoisobutyrate (HEBiB) for different reaction times
Entrya t (h) Mnb (g mol−1) Mtheod (g mol−1) PDIb Yieldc (%)
a Reaction conditions: [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][Fe]0[thin space (1/6-em)]:[thin space (1/6-em)][HEBiB]0 = 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1; reaction temperature: 110 °C; solvent: toluene.b Determined by GPC analysis in THF, calibrated to a polystyrene standard.c Yield: weight of polymer obtained/weight of monomer used.d Mtheo = yield (%) × ratio [CL]0/[HEBiB]0 × 2 × M (CL).
1 2 — — — NA
2 4 5110 4630 1.14 20.3
3 6 7760 10[thin space (1/6-em)]800 1.21 47.3
4 8 8810 12[thin space (1/6-em)]080 1.20 52.9
5 10 13[thin space (1/6-em)]080 17[thin space (1/6-em)]490 1.19 76.6
6 12 23[thin space (1/6-em)]560 22[thin space (1/6-em)]740 1.23 99.6
7 24 21[thin space (1/6-em)]660 22[thin space (1/6-em)]830 1.23 100

The chain extension experiment also confirmed the controlled/living nature of the HEBiB/Fe-catalyzed ROP of CL. Fig. 3 shows GPC traces for the chain extension experiment. A PCL with Mn, GPC = 13[thin space (1/6-em)]770 g mol−1 and PDI = 1.41 was first obtained from the polymerization with [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][HEBiB]0[thin space (1/6-em)]:[thin space (1/6-em)][Fe]0 = 50[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 and the monomer conversion of 100% (Table 7, entry 2). A post-polymerization was then carried out by the subsequent addition of 50 equiv. of CL to afford a PCL with Mn, GPC = 18[thin space (1/6-em)]280 g mol−1 and PDI = 1.33 (Table S1,† entry 3), indicating that the chain end group of PCL possessed a living nature.


image file: c5ra22279f-f3.tif
Fig. 3 The GPC traces of first poly(ε-caprolactone) (PCL) sequence (black) and post-polymerization (red) catalyzed by the combination of Fe/2-hydroxyethyl 2-bromoisobutyrate (HEBiB).
Table 7 CL polymerization in the presence of Fe/2-hydroxyethyl 2-bromoisobutyrate (HEBiB) with different ratios (R) of [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][HEBiB]0[thin space (1/6-em)]:[thin space (1/6-em)][Fe]0
Entrya Ratio Mnb (g mol−1) Mwb (g mol−1) PDIb Yieldc (%) Mtheod (g mol−1)
a Reaction conditions: reaction time: 24 h; reaction temperature: 110 °C; solvent: toluene.b Determined by GPC analysis in THF, calibrated to a polystyrene standard.c Yield: weight of polymer obtained/weight of monomer used.d Mtheo = yield (%) × Ratio × 2 × M (CL).e Reaction time: 28 h.
1 25[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 9790 12[thin space (1/6-em)]530 1.28 81.5 4650
2 50[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 13[thin space (1/6-em)]050 17[thin space (1/6-em)]400 1.33 92.8 10[thin space (1/6-em)]590
3 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 21[thin space (1/6-em)]660 26[thin space (1/6-em)]700 1.23 100 22[thin space (1/6-em)]830
4e 200[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 33[thin space (1/6-em)]310 38[thin space (1/6-em)]310 1.15 100 45[thin space (1/6-em)]660

The results of the ROPs of CL with varying [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][HEBiB]0[thin space (1/6-em)]:[thin space (1/6-em)][Fe]0 ratios from 25[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 to 200[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 (Table 7) also indicate that the polymerization system proceeds in a controlled/living fashion. The molecular weights of the obtained PCLs generally increased as the ratio increased (Fig. 4). And by using ICP-AES, the residual Fe content in the final PCL was determined to be ca. 750 ppm, which was comparable to those (1–1000 ppm) in the final polymers (e.g. polylactide (PLA), PCL) obtained by Sn(II), Sb(II), Pb(II), Bi(III), Fe(II), Ti(II), Ti(III), Mn(II), Mn(III), or Ge(II)-containing catalysts,65,66 but slightly higher than the residual tin content in the final PCL obtained by the traditional tin(II) catalyst [Sn(Oct)2] (Oct = 2-ethylhexanoate).67


image file: c5ra22279f-f4.tif
Fig. 4 The GPC traces of the obtained poly(ε-caprolactone) (PCL) with various [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][HEBiB]0[thin space (1/6-em)]:[thin space (1/6-em)][Fe]0 ratios (R) in toluene at 110 °C.

Chain end analysis

The chain end of PCL initiated by Fe/HEBiB was analyzed by 1H NMR spectroscopy. The methylene protons can be clearly identified at 4.06 ppm (triplet), 1.65 ppm (multiplet), 1.38 ppm (multiplet), 2.31 ppm (triplet). The signals of 3.65 ppm (triplet), 4.28 ppm (triplet) and 1.81 ppm (singlet) (Fig. S1†) correspond to the methylene protons attached to the terminal hydroxyl group, methylene and methyl protons from HEBiB, respectively. The chain ends of PCL initiated by BnOH and catalyzed by iron powder along with different bromides such as BMP, EBiB and HEBiB give 1H NMR signals at 5.12 ppm (singlet) and 7.35 ppm (multiplet) (Fig. S2†), corresponding to the protons derived from methylene and aromatic protons of BnOH.

Experimental

Materials

ε-Caprolactone (CL, >99%), ethyl 2-bromoisobutyrate (EBiB) and 2-bromo-2-methylpropane (BMP, >98%) were purchased from J&K Scientific Ltd. (Beijing, China). CL was dried over CaH2 for 48 h and distilled under reduced pressure. 2-Hydroxyethyl 2-bromoisobutyrate (HEBiB) was synthesized as described.68 Iron powder (99.998% metal purity) was purchased from Alfa Aesar (China) Chemical Co. Ltd. Iron powder (≥98% metal purity), benzyl alcohol (BnOH) and other chemicals were obtained from Shanghai Chemical Reagents Co. Ltd. (Shanghai, China) and were used as received unless mentioned.

Characterization

The molecular weight (Mn, Mw) and polydispersity index (Mw/Mn, PDI) values of the resulting polymers were determined using a TOSOH-HLC-8320 gel permeation chromatograph (GPC) equipped with a refractive index detector, using TSK gel SuperMultiporeHZ-M columns (4.6 mm I.D. × 15 cm × 2) with measurable molecular weights ranging from 103 to 10 × 105 g mol−1. THF was used as the eluent at a flow rate of 0.35 mL min−1 at 40 °C. GPC samples were injected using a TOSOH plus auto sampler and calibrated with polystyrene standards purchased from TOSOH. The 1H NMR spectra in CDCl3 were recorded at ambient temperature on a Varian UNITY plus-400 spectrometer and the chemical shifts were referenced to the TMS signal. The ICP-AES data was obtained on a Varian 710-ES inductively coupled plasma-atomic emission spectrometry.

General procedure for the ROP of ε-caprolactone

A typical polymerization procedure with the molar ratio of [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][HEBiB]0[thin space (1/6-em)]:[thin space (1/6-em)][Fe]0 = 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 was described as follows. A mixture was obtained by adding iron powder (6 mg, 0.1 mmol), HEBiB (14 μL, 0.1 mmol), CL (1.1 mL, 10 mmol) and distilled solvent (1 mL, toluene or others mentioned above) in a dried Schlenk ampoule, and degassed by three freeze–pump–thaw cycles to remove the dissolved oxygen in the reaction system. Then, the ampoule was sealed and transferred into an oil bath, held by a thermostat at the desired temperature (90 °C to 120 °C) to polymerize under stirring. After the desired polymerization time (2 h to 24 h), the ampoule was cooled to room temperature. Afterwards, it was opened and dissolved in THF (4 mL), and precipitated into a large amount of methanol/1 M HCl (6/1, v/v, 250 mL). The polymer obtained by filtration was washed and dried under vacuum at 30 °C to a constant weight. The monomer conversion was determined gravimetrically, that is, weight of polymer obtained divided by weight of monomer used.

In the polymerization procedure with the molar ratio of [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][HEBiB]0[thin space (1/6-em)]:[thin space (1/6-em)][Fe]0 = 25[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1, the reaction mixture was obtained by mixing iron powder (12 mg, 0.2 mmol) with HEBiB (28 μL, 0.2 mmol), CL (0.55 mL, 5 mmol) and toluene (0.5 mL). In the polymerization procedure with the molar ratio of [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][HEBiB]0[thin space (1/6-em)]:[thin space (1/6-em)][Fe]0 = 50[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1, the reaction mixture was obtained by mixing iron powder (12 mg, 0.2 mmol) with HEBiB (28 μL, 0.2 mmol), CL (1.1 mL, 10 mmol) and toluene (1 mL). In the polymerization procedure with the molar ratio of [CL]0[thin space (1/6-em)]:[thin space (1/6-em)][HEBiB]0[thin space (1/6-em)]:[thin space (1/6-em)][Fe]0 = 200[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1, the reaction mixture was obtained by mixing iron powder (6 mg, 0.1 mmol) with HEBiB (14 μL, 0.1 mmol), CL (2.2 mL, 20 mmol) and toluene (2 mL). Other steps were the same as above.

In the polymerization procedure using iron powder, hydroxyl initiators like benzyl alcohol, isopropanol, water, and alkyl bromides (R′′Br) like EBiB and BMP, the reaction mixtures were obtained by mixing 0.1 mmol iron powder, with 0.1 mmol initiator, 0.1 mmol R′′Br, 10 mmol CL and 1 mL toluene. And other operations and post-treatment were the same as above.

Conclusions

In the work reported herein, metallic iron and a dual functional HEBiB were used as an efficient and cooperative catalyst system for the ROP of CL under various reaction conditions to produce PCL with high molecular weight, narrow PDI and high yield. The kinetic experiments confirm the controlled/living nature of this polymerization process. Use of metallic iron as the stable pre-catalyst alleviates the often tedious catalysts syntheses. The present work may spur broad interest in exploring the direct use of metallic elements in other catalytic applications. Our preliminary data also suggest that the metal scope can be extended to metallic Zn, and these results will be reported in due course.

Acknowledgements

The authors thank the financial supports from the National Natural Science Foundation of China (21373142, 21401134 and 21531006), the State Key Laboratory of Organometallic Chemistry of Shanghai Institute of Organic Chemistry (2015kf-07), the “333” Project of Jiangsu Province (no. BRA2012139), and the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors would also like to thank the useful suggestions of the editor and the reviewers.

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Footnote

† Electronic supplementary information (ESI) available: 1H NMR spectra and post-polymerization data. See DOI: 10.1039/c5ra22279f
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