Subscriber access provided by RYERSON UNIV
Article
Facilely Recyclable Cu(II) Macro-complex with Thermo-Regulated Poly (ionic liquid) Macro-Ligand: Serving as a Highly Efficient ATRP Catalyst Bingjie Zhang, Lan Yao, Xiaodong Liu, Lifen Zhang, Zhenping Cheng, and Xiulin Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01961 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 22, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 12
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Sustainable Chemistry & Engineering
Facilely Recyclable Cu(II) Macro-complex with Thermo-Regulated Poly (ionic liquid) Macro-Ligand: Serving as a Highly Efficient ATRP Catalyst Bingjie Zhang, Lan Yao, Xiaodong Liu, Lifen Zhang,* Zhenping Cheng* and Xiulin Zhu Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis, Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren-ai Road, Suzhou 215123, Jiangsu Province, China. * Email:
[email protected] (Z. P. Cheng), or
[email protected] (L. F. Zhang) ABSTRACT: Copolymer poly (ionic liquids) (PILs) are the fascinatingly new polymerized polyelectrolytes which can provide the properties of ionic liquid with others combination into one. In this work, a thermo-regulated random copolymer PIL (PILL) with the side chains of both ionic liquids and ATRP ligands was designed and synthesized to form a macro-complex with CuBr2 and serve as ATRP catalyst to establish a thermo-regulated phase separated catalysis (TPSC) system and further applied in a typical ICAR (initiators for continuous activator regeneration) ATRP process for the catalyst separation and recycling in situ for the first time. This novel TPSC system could simultaneously recycle transition metal catalyst and ligand easily just by changing temperature from polymerization temperature to room one. Additionally, even if the highly efficient recyclable PILL with Cu catalyst was separated facilely and reused for ten times in situ, it didn’t sacrifice the controllability over polymerization nearly. Furthermore, after polymerization and a TPSC process, the metal catalyst residual in polymer solution phase kept just about 1.5 ppm, indicating highly efficient transition metal catalyst recycling efficiency.
KEYWORDS: Copolymer PILs; Macro-ligand; Atom transfer radical polymerization (ATRP); Thermo-regulated phase separated catalysis (TPSC); Catalyst recycle; Living radical polymerization
INTRODUCTION In the research field of polymer chemistry, atom transfer radical polymerization (ATRP) catalyzed by a transition metal (e.g., copper)/ligand complex, is well known as one of successful reversible deactivation radical polymerizations (RDRPs) that provides a versatile tool and a simple way for synthesizing well-defined polymers with diverse complex architectures.1-6 In this polymerization system, the transition metal and ligand (small organic molecules usually) play key role for a successful ATRP process. From green and economic viewpoint, the development of an atom efficient and cost-effective ATRP process will bear substantial significance to the large-scale use of ATRP technique. Theoretically speaking, the used transition metal and ligand can be recycled since they just serve as a catalyst complex in an ATRP process. Till now, there have been some intelligence strategies to separate and recycle the transition metal catalyst from the polymerization systems, which were well summarized by the reviews of our group7 and others.8,9 For example, we developed a series of methods based on thermo-regulated ligands of small organic molecules, such as thermo-regulated phase transfer catalysis (TRPTC) ATRP,10-13 diffusion-regulated phase-transfer catalysis (DRPTC) ATRP,14 and thermo-regulated phase separated catalysis (TPSC) ATRP,15-17 to separate and recycle transition metal catalyst in situ. However, these ligands of small organic molecules are easily lost in the separation and recycle process, which result in out of control over polymerizations or necessary supplement of additional ligands in next recycle experiments. Actually, for a general ATRP process, the feed of ligand is usually much larger than the amount of transition metal catalyst (the feed molar ratio of ligand to metal catalyst is usually 1: 1-3: 1) and the price of ligand is much expensive than the latter. Therefore, the pursuit of reuse of ligand is much urgent for a practical ATRP technique. Unfortunately, it is surprising that the reuse of ligand method is almost a neglectful topic now. This may be caused by the fact that the conventional ligands are small organic molecules, which are hard or high-cost to recycle them from the polymerization systems. How can we do about this issue? In our recent research, we designed a thermo-regulated macro-ligand, a random copolymer of octadecyl acrylate (OA) and MA-Ln (2-(bis(pyridin2-ylmethyl)amino)ethyl acrylate), POA-ran-P(MA-Ln), to realize the simultaneous separation and recycle both transition metal
1
ACS Paragon Plus Environment
ACS Sustainable Chemistry & Engineering
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 12
catalyst and ligand in an ICAR (initiators for continuous activator regeneration) ATRP system suitable for hydrophilic monomers.18,19 It is well known that ionic liquids (ILs) are the salts with melting points below 100 °C, which are one of three green solvents (the others are supercritical CO2 and water), consisting of organic cations and organic or inorganic counter anions and have unique physical properties such as low vapor pressure, negligible volatility, low-melting-point, thermal stability and so on.20-23 Polymerized ionic liquids (PILs) can be obtained by designing ILs monomer and subsequent polymerization easily.24-27 The PILs can be divided into polycation backbone PILs,28 polyanion backbone PILs,29 polyzwitterionic PILs30 and copolymer PILs.31,32 Polycation PILs obtain flourishing development among varies of PILs due to their vast application in all walks of life. Generally speaking, cationic PILs can divided into representative types of ammonium,33 pyrrolidinium,34 imidazolium,35,36 and phosphonium cations.37 Furthermore, copolymer PILs are a newer class of materials mainly deriving from polycation PILs, and have drawn much attention because they combine two polymers in one unique ionic liquid materials. Combining the advantages of macro-ligand and PILs, we designed a recycled thermo-regulated copolymer PIL with covalent attachment of ATRP ligands, which can form a macro-complex with CuBr2 to establish a facile separation and recycle system of both ATRP catalyst and ligand simultaneously via a TPSC strategy in situ. To obtain the copolymer PIL, an active hydrophilic ligand monomer 2-(bis(pyridin-2-ylmethyl)amino)ethyl acrylate (MA-Ln) was selected for its good complexation with copper-based catalyst. Additionally, the less expensive and environmental-friendly PEG38,39 was chosen as a thermo-regulated fragment and anchored onto the most popularly applied imidazolium-based IL monomer for facilitating to realize homogeneous polymerization and heterogeneous separation just by changing the temperature.18 The corresponding IL monomer was synthesized in our lab and we named it MPEG350-MA-IL. Then a random copolymer, termed as thermo-regulated PIL macro-ligand (PILL), was obtained as shown in Scheme 1 by a free radical copolymerization of the two functional monomers. The resultant PILL and benzene was selected as thermo-sensitive phase and organic solvent, respectively, to construct a TPSC system and this system was applied in ATRP catalyst and ligand recycling. It is worth noting that this novel polymerization system provided a facile method for efficient separating and recycling of PILL/catalyst in situ with almost no any lose of transition metal catalyst and macro-ligand of ATRP.
Scheme 1. Synthetic pathway of the thermo-regulated PIL macro-ligand (PILL).
EXPERIMENTAL SECTION Materials. Methyl methacrylate (MMA, +99%), styrene (St, +99%), methyl acrylate (MA, +99%) and butyl acrylate (BA, +99%) purchased from Shanghai Chemical Reagents Co. Ltd. (Shanghai, China) were purified by passing through a neutral alumina column to remove inhibitors. 2,2′-Azobis (isobutyronitrile) (AIBN, 97%, Shanghai Chemical Reagents Co. Ltd) was recrystallized from ethanol and dried at room temperature under vacuum before use. Monomethoxy poly (ethylene glycol)-350 (MPEG 350, number-average molecular weight 350 g/mol, J&K ), p-xylene (99%) and o-xylene (99%, Alfa Aesar), pyridine-2-carboxaldehyde (98%, J&K), ethyl-2-bromo-2-phenyl acetate (EBPA) (97%, J&K), sodium triacetoxyborohydride (97%, Energy Chemical), 2aminoethanol (+99%, Energy Chemical), acryloyl chloride (97%, Energy Chemical) and 2-bromoethanol (+96%, Adamas) were used as received. Copper bromide (CuBr2, analytical reagent), acetic acid glacial (analytical agent), tetraethylammonium bromide (+97%), triethylamine (N(Et)3, analytical reagent), methanesulfonyl chloride(analytical reagent), saturated sodium bicarbonate, benzene (analytical reagent), toluene (analytical agent), methanol (analytical agent), methylene dichloride (analytical agent), tetrahydrofuran (THF, analytical reagent), cyclohexane (analytical reagent), n-hexane (analytical reagent) and all other chemicals were obtained from Shanghai Chemical Reagents Co. Ltd. (Shanghai, China) and used as received unless mentioned. The liquid ionic monomer, the ligand monomer and thermo-regulated macro-ligand poly (ionic liquid) were synthesized separately according to the procedure shown in Schemes 1-3. Synthesis of the Thermo-regulated Macro-ligand Poly (ionic liquid) (PILL). The ligand monomer 2-(bis(pyridin-2ylmethyl)amino)ethyl acrylate (MA-Ln) was anchored with a thermo-regulated imidazolium-based ionic liquid monomer (MPEG350-MA-IL) by free radical polymerization. The detail procedure was shown as follows. H2N Cl O
2
OH H
N N
O
O
O
N
1 OH
N N
N
2: MA-LN
Scheme 2. Synthetic pathway of the ligand monomer MA-LN. Synthesis of MA-Ln: the ligand monomer MA-Ln was prepared by the reported method18 as shown in Scheme 2.
2
ACS Paragon Plus Environment
Page 3 of 12
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Sustainable Chemistry & Engineering
1 H NMR spectrum of 1 in CDCl3. 1H NMR (400 MHz, CDCl3, δ): 8.56 (2 H, t, N-CH-CH), 7.62 (2 H, q, CCH-CH-CH), 7.37 (2 H, t, C-CH-CH), 7.16 (2 H, q, NCH-CH-CH), 5.00 (1 H, s, -CH2-OH), 3.98 (4 H, s, C-CH2-N), 3.70 (2 H, q, CH2-CH2-OH), 2.92 (2 H, q, CH2-CH2-N). 1 H-NMR spectrum of 2 in CDCl3. 1H NMR (400 MHz, CDCl3, δ): 8.56 (2 H, t, N-CH-CH), 7.62 (2 H, q, CH-CH-CH), 7.37 (2 H, t, C-CH-CH), 7.16 (2 H, q, CH-CH-CH), 6.43 (1 H, t, CH2-CH-), 6.13 (1 H, t, CH2-CH-CO), 5.85 (1 H, t, CH2-CH-), 4.32 (2H, q, O-CH2-CH2), 3.95 (4 H, s, C-CH2-N), 2.95 (2 H, q, CH2-CH2-N). The related 1H NMR spectra were shown in Figures S1-S2 (Supporting Information).
Scheme 3. Synthetic pathway of the thermo-regulated ionic liquid monomer MPEG350-MA-IL. Synthesis of MPEG350-MA-IL: The synthetic pathway of MPEG350-MA-IL was shown in Scheme 3. A MPEG350 was selected to synthesize intermediate 3 and 4 according to the reported method.40 Meanwhile, compound 5 was prepared by the reported literature.41 The function monomer MPEG350-MA-IL was synthesized as follows: under argon atmosphere, a solution of 200 mL of dry MeCN dissolving with 4 (40.00 g, 0.1 mol) and 5 (19.69 g, 0.11 mol) was added into a three-necked flask and refluxed for 72 h at 45 °C. The MeCN was removed using rotary evaporator at moderate temperature (T≤30 °C) and the concentrated filtrate was dissolved with a small amount of methanol and washed with anhydrous ether (4 ×100 mL). After dried in vacuum the ionic liquid monomer MPEG350-MA-IL was obtained as an orange viscous oil (46.3 g, 80%). 1
H-NMR spectrum of 3 in CDCl3. 1H NMR (400 MHz, CDCl3, δ): 4.38 (2 H, q, SO3-CH2-CH2), 3.76 (2 H, q, CH2-CH2-OCH2), 3.65 (26 H, m, O-CH2-CH2), 3.54 (2 H, q, CH2-CH2-OCH3), 3.38 (3 H, s, -OCH3), 3.09 (3 H, s, SO3-CH3). 1
H NMR spectrum of 4 in CDCl3. 1H NMR (400 MHz, DMSO-d6, δ): 7.55 (1 H, s, N-CH-N), 7.04 (2 H, t, N-CH-CH-N), 4.11 (2 H, q, N-CH2-CH2), 3.75 (2 H, q, CH2-CH2-OCH2), 3.64 (26 H, m, O-CH2-CH2-O), 3.54 (2 H, q, CH2-CH2-OCH3), 3.37 (3 H, s, OCH3). 1
H NMR spectrum of 5 in CDCl3. 1H NMR (400 MHz, CDCl3, δ): 6.43-6.49 (1 H, t, CH2-CH-), 6.11-6.20 (1H, m, CH2-CH-OC-), 5.87-5.91 (21 H, t, CH2-CH-), 4.47 (2 H, q, O-CH2-CH2), 3.56 (2 H, q, CH2-CH2-Br). 1
H NMR spectrum of 6 in DMSO-d6. 1H NMR (400 MHz, DMSO-d6, δ): 9.24 (1 H, s, N-CH-N), 7.84 (2 H, t, N-CH-CH-N), 5.98-6.38 (3 H, m, CH2-CH-CO2), 4.37-4.56 (6 H, m, N-CH2-CH2, CH2-CH2-CO2), 3.76 (2 H, m, CH2-CH2-O), 3.51 (26 H, m, OCH2-CH2-O), 3.42 (2 H, q, CH2-CH2-OCH3), 3.33 (3 H, s, -OCH3). The corresponding 1H NMR spectra were shown in Figures S3-S6 (Supporting Information). Synthesis of the PILL: In a dried Schelenk tube with a stir bar, MPEG350-MA-IL (5.78 g, 0.01 mol), MA-Ln (0.298 g, 0.001mol), AIBN (0.164 g, 0.001mol) and 10 mL of DMSO were added, and the solution was thoroughly bubbled with argon for about 15 min to eliminate the dissolved oxygen. Then the mixture solution was reacted at 80 °C for 72 h and the orange solution became a brawn one. The solution was dissolved with a small amount of methanol and washed with ether (5×50 mL) then dried in vacuum (5.12 g, 82%). The PILL was determined by GPC with Mn,GPC = 26300 g/mol and Mw/Mn = 1.46. Typical Procedure for the TPSC-Based ICAR ATRP of MMA and Catalyst Macro-complex Recycling. A typical procedure of TPSC-based ICAR ATRP with the molar ratio of [MMA]0:[EBPA]0:[CuBr2]0:[PILL]0:[AIBN]0 = 200:1:1:4:0.8 was conducted as follows. In a dried 5 mL ampoule with a stir bar, CuBr2 (10.4 mg, 0.047 mmol), PILL (348 mg, 0.188 mmol), EBPA (8.3 µL, 0.047 mmol), MMA (1.0 mL, 9.4 mmol), AIBN (6.2 mg, 0.04 mmol) and benzene (3.0 mL) were added, then thoroughly the ampoule was bubbled with argon for about 15 minutes to eliminate the dissolved oxygen and flame-sealed. The ampoule was transferred into an oil bath keeping at 70 °C. After a desire polymerization time, the ampoule was taken out and cooled with cool water at room temperature. After the solution completely separated into two phases, the organic phase (upper layer) with dissolved polymers was transferred by simple decantation and dissolved in 4 mL of THF then precipitated from a large amount of n-hexane (~200 mL) and filtrated. The obtained polymers were dried under vacuum until constant weight at 30 °C, and the monomer conversion was determined gravimetrically. The lower layer (PILL complexed with CuBr2) was transferred into another dried ampoule with fresh EBPA
3
ACS Paragon Plus Environment
ACS Sustainable Chemistry & Engineering
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 12
(8.3 µL, 0.047 mmol), MMA (1.0 mL, 9.4 mmol), AIBN (6.2 mg, 0.04 mmol) and benzene (3.0 mL) for catalyst/macro-ligand recycling experiment via ICAR ATRP. And the next procedures were same as described above. Typical Procedure for Chain Extension of PMMA. A typical chain extension polymerization procedure of PMMA with the molar ratio of [MMA]0:[PMMA]0:[CuBr2]0:[PILL]0: [AIBN]0 = 200:0.5:1:4:0.8 was as follows: a predetermined quantity of PMMA ( 0.024 mmol) was added in a dried 5 mL ampoule with a stir bar, then corresponding amount of CuBr2 (0.047 mmol), PILL (0.188 mmol), MMA (9.4 mmol), AIBN (0.04 mmol) and benzene (3.0 mL) were added. The rest of the procedure was the same as the typical procedure for the TPSC-based ICAR ATRP described above. Characterization. The number-average molecular weight (Mn,GPC) and molecular weight distribution (Mw/Mn) values of the resultant polymers were determined by a TOSOH HLC-8320 gel permeation chromatograph (GPC) equipped with a refractive-index detector (TOSOH), using a TSKgel SuperMP-N guard column (4.6 × 20 mm) and two TSKgel SupermultiporeHZ-N columns (4.6 × 150 mm) with measurable molecular weights ranging from 5 × 102 to 5 × 105 g/mol. THF was used as the eluent at a flow rate of 0.35 mL/min and temperature of 40 °C. The GPC samples were injected using a TOSOH plus autosampler and were calibrated with PMMA standards purchased from TOSOH. The 1H NMR spectra of the obtained polymers were recorded on an INOVA 400 MHz nuclear magnetic resonance (NMR) instrument using CDCl3 and DMSO-d6 as the solvents and tetramethylsilane (TMS) as an internal standard. Cu elemental analysis was carried out by inductively coupled plasma (ICP) using a Vista MPX. Matrix assisted laser desorption/ionization time of-flight (MALDI TOF) mass spectra were acquired on an UltrafleXtreme MALDI TOF mass spectrometer equipped with a 1 kHz smart beam-II laser. The instrument was calibrated prior to each measurement with external PMMA at the molecular weight under consideration. The compoundtrans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propenylidene]-malononitrile (DCTB, Aldrich, + 98%) served as the matrix and was prepared in CHCl3 at a concentration of 20 mg/mL.
RESULTS AND DISCUSSION Selection of Solvent for TPSC-Based ICAR ATRP System. Firstly, the PILL (Mn,GPC = 26300 g/mol, Mw/Mn = 1.46) was confirmed with 1H NMR spectroscopy. From Figure 1, it can be seen that the monomer’s double bond was disappear at δ = 5.84-6.47 ppm and a new group of chemical shifts were found in δ = 2.65-2.95 ppm (g in Figure 1) assigning to polymer backbone hydrogen bond which indicated the PILL was obtained successfully. Additionally, the molar ratio of integral value of the peaks f belongs to MA-Ln and e to MPEG350-MA-IL was 2:17, that is to say the molar ratio of ligand moiety and the thermo-regulated ionic liquid moiety in the PILL was about 1:6. Subsequently, the resultant PILL and benzene was selected as thermo-sensitive phase and organic solvent, respectively (Table 1), to construct a TPSC-based ICAR ATRP system with the molar ratio of [MMA]0:[EBPA]0:[CuBr2]0:[PILL]0:[AIBN]0 = 200:1:1:2:1. As displayed in Figure 2, the homogeneous polymerization and heterogeneous separation in situ could be easily realized by shifting temperature from 70 oC to room temperature.
Figure 1. 1H NMR spectrum of poly (ionic liquid) (PILL) with DMSO-d6 as solvent and tetramethylsilane (TMS) as internal standard.
Table 1. Selection of solvents couple with PILL (174 mg) for successful TPSC-based system Solvent (2.0 mL) p-Xylene o-Xylene Toluene Benzene Cyclohexane n-Hexane
25 °C
70 - 90 °C
25 °C
I I I I M I
S S S M M S
I I I I M I
4
ACS Paragon Plus Environment
Page 5 of 12
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Sustainable Chemistry & Engineering
I: Two solvents are immiscible at investigated temperature; S: two solvents are slightly miscible at investigated temperature；M: two solvents are miscible at investigated temperature.
Figure 2. Photographs of MMA in PILL/benzene biphasic system for TPSC-based ICAR ATRP.
In order to optimize the polymerization conditions, a series of polymerizations were conducted (Tables S1-S4, Supporting Information). The polymerization rate was significantly enhanced with a better control by using more PILL as shown in Table S1. It is speculated that more PILL provided more thermosensitive fragments to conduct homogeneous reaction with benzene and more ATRP ligand sites to facilitate the complexation with CuBr2, which facilitates the enhancement of polymerization rate. The amounts of benzene, catalyst and reducing agent were also examined (Tables S2~S3, Supporting Information) and optimized. The optimal conditions are as follows: [MMA]0:[EBPA]0:[CuBr2]0:[PILL]0:[AIBN]0 = 200:1:1:4:0.8, VMMA = 1.0 mL, Vbenzene = 3.0 mL and polymerization temperature = 70 °C. In addition, different molecular weights of the polymers could be adjusted by changing the molar ratios of monomer to initiator (Entries 1 to 3 in Table S4, Supporting Information). Polymerization Kinetics of MMA. In consideration of a relatively low viscosity reacting circumstance and a high polymerization rate with better control, we finally chose the molar ratio of [MMA]0:[EBPA]0:[CuBr2]0:[PILL]0:[AIBN]0 = 200:1:1:4:0.8 and 3.0 mL benzene to further investigate the polymerization kinetics of MMA. Four recycling polymerization kinetic plots followed approximately first-order kinetics with time as shown in Figure 3 (a), indicating that constant concentration of propagating species existed throughout the polymerizations up to high monomer conversion (>95%). Additionally, as shown in Figure 3 (b), although at higher monomer conversion the Mn,GPCs were slightly deviated from the theoretical values, linear increasing evolution of Mn,GPC with monomer conversion was observed and the molecular weight distributions were still under control (Mw/Mn
