One-Step Synthesis of Block-Graft Copolymers via Simultaneous
Reversible-Addition Fragmentation Chain Transfer and Ring-Opening
Polymerization Using a Novel Macroinitiator
Temel €
Ozt€
urk,
1Mehmet Nuri Atalar,
1Melahat G€
oktas¸,
1Baki Hazer
2 1Department of Chemistry, Kafkas University, 36100 Kars, Turkey
2Department of Chemistry, Bulent Ecevit University, 67100 Zonguldak, Turkey Correspondence to: T. €Ozt€urk (E-mail: temelozturk@msn.com)
Received 3 December 2012; accepted 22 February 2013; published online 26 March 2013 DOI: 10.1002/pola.26654
ABSTRACT:One-step synthesis of block-graft copolymers by reversible addition-fragmentation chain transfer (RAFT) and ring-opening polymerization (ROP) by using a novel initiator was reported. Block-graft copolymers were synthesized in one-step by simultaneous RAFT polymerization of n-butylmethacry-late (nBMA) and ROP of e-caprolacton (CL) in the presence of a novel macroinitiator (RAFT-ROP agent). For this purpose, first epichlorohydrin (EPCH) was polymerized by using H2SO4 via cationic ring-opening mechanism. And then a novel RAFT-ROP agent was synthesized by the reaction of potassium ethyl xan-thogenate and polyepichlorohydrin (poly-EPCH). By using the RAFT-ROP agent, poly[CL-b-EPCH-b-CL-(g-nBMA)] block-graft copolymers were synthesized. The principal parameters such as monomer concentration, initiator concentration, and poly-merization time that affect the one-step polypoly-merization reaction were evaluated. The block lengths of the block-graft
copolymers were calculated by using1H-nuclear magnetic res-onance (1H NMR) spectrum. The block length could be adjusted by varying the monomer and initiator concentrations. The characterization of the products was achieved using 1H NMR, Fourier-transform infrared spectroscopy, gel-permeation chromatography, thermogravimetric analysis, differential scan-ning calorimetry, elemental analysis, and fractional precipita-tion (c) techniques. VC 2013 Wiley Periodicals, Inc. J. Polym.
Sci., Part A: Polym. Chem. 2013, 51, 2651–2659
KEYWORDS: block copolymers; block-graft copolymer; block length; cationic polymerization; fractional precipitation; graft copolymers; macroinitiator; one-step polymerization; reversi-ble-addition fragmentation chain transfer polymerization (RAFT); ring-opening polymerization
INTRODUCTIONIn recent years, the one-step process has been successfully used for the synthesis of block and graft copolymers using different techniques, which thus has several advantages over other popular methods. Because of the applic-ability of at least two transformation steps simultaneously, side reactions that lead to homopolymer formation are mini-mized.1–15 Barner-Kowollik and coworkers1 carried out the synthesis of poly(2-hydroxyethyl methacrylate-g-e-caprolac-tone) graft copolymers through the one-step combination of ring-opening polymerization (ROP) and reversible addition-fragmentation chain transfer (RAFT) polymerization in the presence of cyanoisopropyl dithiobenzoate and tin(II)2-ethyl-hexanoate using toluene as the solvent. Furthermore, various copolymers containing styrene,2–11 chloromethyl sty-rene,10 butadiene,4 N-isopropylacrylamide,12,13 lactide,6,11,12,14 e-caprolactone (CL),7,8,13
10-methylene-9,10-dihydroanthryl-9-spirophenylcyclopropane,5 ethylene oxide,6,7 hydroxyethyl methacrylate,7,8methyl acrylate,14hydroxyethyl acrylate,14 1,3-dioxepane,9,15 tetrahydrofuran (THF),10 1,5-cyclooctadiene,4 and methyl methacrylate15 monomers were synthesized by a
combination of the RAFT and ROP methods. RAFT polymeriza-tion represents the most recently developed controlled radical polymerization method and is a powerful technique for macromolecular synthesis of a broad range of well-defined polymers. The versatility of the method is demonstrated by its compatibility with a very wide range of monomers and reac-tion condireac-tions. Reversible chain transfer involves homolytic substitution or addition fragmentation, or some other transfer mechanisms.16–26
Macrointermediates such as macroinitiators, macromonomers, and macrocrosslinkers have been widely used for preparing various block/graft copolymers and their networks via a radi-cal initiated process.27–40Although block and graft copolymers have many similar characteristics, graft copolymers have a branching chain structure attaching polymer units to another polymer backbone.41,42 Block and graft copolymers that pro-vide specific combinations of physical properties are the most suitable materials for various purposes.43 Block or graft copolymers are one of the most important polymeric materials
used in technological applications and theoretical research because of their exceptional properties based on the micro-phase separation.44–64The synthesis of block copolymers usu-ally requires efficient controlled/“living” polymerization.65 This work is an extension of the recent studies carried out by the authors of this article involving the one-step synthesis of block copolymers through simultaneous free-radical polymeriza-tion/RAFT and ROP processes.28,38,39,44In this study, the synthe-sis of a novel macroinitiator (RAFT-ROP agent) obtained by the reaction of polyepichlorohydrin (poly-EPCH) with the potassium salt of ethyl xanthogenate, is reported. Poly-EPCH was synthe-sized by cationic ring-opening mechanism. Poly[CL-b-EPCH-b-CL-(g-nBMA)] block-graft copolymers were synthesized using this novel RAFT-ROP agent by the simultaneous ROP and RAFT polymerization of the reactants in one-step. Block-graft copoly-mers synthesized could be used to prepare with the desired segment ratio by changing the polymerization conditions. EXPERIMENTAL
Materials
The potassium salt of ethyl xanthogenate, THF, toluene, and H2SO4were supplied by Merck and used as received.
n-Butyl-methacrylate (nBMA), 2,20-azobisisobutyronitrile (AIBN),
N,N-dimethylformamide (DMF), dichloromethane, EPCH, dibutyltin dilaurate (DBDTL), and CaSO4 were received from
Sigma-Aldrich and used as received. Methanol and ethanol were received from Birpa A.G. and used as received. Diethyl ether and petroleum ether were supplied from Carlo Erba A.G. and used as received. CL was received by Alfa Aesar and dried with anhydrous CaSO4, then fractionally distilled. All other
chemicals were reagent grade and used as received. Instrumentation
The molecular weights and molecular-weight distributions were measured with a Polymer Labs PL-GPC 220 gel-perme-ation chromatography (GPC) with THF as the solvent. A cali-bration curve was generated with four polystyrene green standards: 2960, 50,400, and 696,500 Da, of low polydisper-sity. Fourier-transform infrared spectroscopy (FTIR) spectra were recorded using a Nicolet-520 model FTIR Spectrometer.
1
H-nuclear magnetic resonance (1H NMR) spectra of the samples in CDCl3 as the solvent, with tetra methylsilane as
the internal standard, were recorded using a Bruker Ultra Shield Plus, ultralong hold time 400 MHz NMR spectrometer. Thermogravimetric analysis (TGA) of the obtained polymers was carried out under nitrogen using a Perkin Elmer Pyris 1 TGA and Spectrum thermal analyzer to determine thermal degradation. Differential scanning calorimetry (DSC) mea-surement was carried out under nitrogen by using a Perkin Elmer Diamond series thermal analysis system. Dried sample was heated at a rate of 10 C/min from 260 to 170 C under nitrogen atmosphere. The elemental analyses of the samples were carried out on a LECO CHNS 90 instrument. Synthesis of Poly-EPCH by Cationic Polymerization Briefly, 20 mL of CH2Cl2and 5 g of 98% H2SO4were placed
into a flask equipped with a magnetic stirrer, and
subsequently argon was purged into the tube through a nee-dle. To this system, 48 g of EPCH was added over 4 h peri-ods. After this time, the flask was opened and the sample was poured into 1 L of water to remove the inorganic mate-rials and then the organic phase was separated. After drying with CaSO4,the solvent was removed by using a rotary
evap-orator. A viscous liquid, poly-EPCH, was obtained.66 Synthesis of the Macroinitiator (RAFT-ROP Agent) A fixed quantity (3.00 g) of poly-EPCH was reacted with 13.00 g of the potassium salt of ethyl xanthogenate in 30 mL THF at 40C for 120 h. The solution was poured into 50 mL distilled water, and filtered to remove the unreacted xan-thate. The solvent was removed by a rotary evaporator. The RAFT-ROP agent was precipitated in cold diethyl ether and dried under vacuum at room temperature for 3 days. One-Step Polymerization
Specified amounts of the RAFT-ROP agent, nBMMA, CL, AIBN, DBDTL (catalyst for ROP of CL), and DMF (as solvent) were charged separately into a Pyrex tube, and subsequently ar-gon was purged into the tube through a needle. The tube was tightly capped with a rubber septum and was dropped into an oil bath thermostated at 90 C for fixed
tempera-tures. After the polymerization, the reaction mixture was poured into an excess of methanol to separate poly[CL-b-EPCH-b-CL-(g-nBMA)] block-graft copolymer. The polymers were dried at 40 C under vacuum for 3 days. The yield of the polymer was determined gravimetrically.
Fractional Precipitations of the Polymers
Fractional precipitations of the polymers were carried out according to the procedure cited in literature.58,59 Vacuum-dried polymer sample (approximately 0.5 g) was dissolved in 5 mL of THF. Petroleum ether was added drop wise as a nonsolvent to the solution with stirring until turbidity occurs. At this point, 1–2 mL of nonsolvent was added to complete the precipitation. The precipitate was removed by filtration. The solvent was THF and the nonsolvent was pe-troleum ether. In this solvent–nonsolvent system, the c val-ues were calculated as the ratios of the total volume of nonsolvent (petroleum ether) used for the first fraction to the volume of solvent (THF) used.
c value 5Volume of Nonsolvent; mLðpetroleum etherÞ Volume of solvent; mLðTHFÞ
The nonsolvent addition into the filtrate solution was contin-ued according to the same procedure mentioned above to determine the c value for the second fraction if there is. RESULTS AND DISCUSSION
Synthesis of Poly-EPCH
Poly-EPCH with hydroxyl groups in the chain end was pre-pared by the cationic polymerization of EPCH using H2SO4
as catalyst. Cationic polymerization of EPCH was carried out by a slow addition of a solution of H2SO4to EPCH. Growing
terminated poly-EPCH. The yield of the product was 30 wt %. The FTIR spectrum of poly-EPCH in Figure 2(a) shows 3500 cm21for AOH groups, 2850 cm21for aliphatic ACH
2
and ACH groups, 1099 cm21 for AOAC groups, 704 cm21 ACl groups. The 1
H NMR spectrum of poly-EPCH in Figure 1(a) shows 1.2 ppm forACH2Cl protons, 3.1 ppm forAOH
protons, 3.7 and 4.7 ppm forAOCH2andAOCH protons. Mn
value of poly-EPCH obtained GPC was 2849 g mol21. The first line in Scheme 1 contains the basic outline for the syn-thesis of poly-EPCH.
Synthesis of the RAFT-ROP Agent
The novel dual initiator, the RAFT-ROP agent with two ethyl xanthogenates and two hydroxyl groups, was synthesized by the reaction of the potassium salt of ethyl xanthogenate with poly-EPCH obtained by cationic polymerization of EPCH. The yield of the product was approximately 45 wt %. The basic outline for the synthesis of the novel macroinitiator is shown at second line in Scheme 1. The FTIR spectrum of the dual initiator in Figure 2(b) also indicates the characteristic sig-nals of AC@S at 1738 cm21 and AOH at 3350 cm21. The FIGURE 1 FTIR spectrum of poly-EPCH (a), and the novel RAFT-ROP agent (b), poly(CL-b-EPCH-g-nBMA-b-CL) block-graft copolymer (c).
signal of AC@S (1738 cm21) at the spectrum of the RAFT-ROP agent [Fig. 2(b)] are not observed at the spectrum of poly-EPCH [Fig. 2(a)]. The1H NMR spectrum of the initiator in Figure 1(b) shows the 1.2 ppm forACH3protons of ethyl
xanthogenate group, 1.4 ppm for ACH2 protons of
poly-EPCH, 3.1 ppm forAOH protons of poly-EPCH, 3.7 ppm and 4.7 ppm for AOCH and AOCH2 protons of poly-EPCH, 4.7
ppm for AOCH2 protons of ethyl xanthogenate group. The
results of the elemental analysis show 36.08 wt % C, 4.97 wt % H, and 5.70 wt % S. The results of the elemental analy-sis agreed with theoretical values. By using 5.70 wt % S obtained from elemental analysis and Mn value (2849 g
mol21) of poly-EPCH obtained from GPC, it is calculated the number of the xanthogenate groups which is available for modification with chloromethyl groups. It is approximately found two xanthogenate groups per chain. Mnvalue of
poly-EPCH obtained GPC is a relative value.
One-Step Synthesis of Poly[CL-b-EPCH-b-CL-(g-nBMA)] Block-Graft Copolymers
The one-step polymerization of a vinyl monomer and a lac-tone initiated by the RAFT-ROP initiator is shown in Scheme 2. This process creates four new active sites—two sites on an equal number of hydroxyl groups for the ROP reaction and two on the thiocarbonate group for RAFT polymeriza-tion. During this one-pot synthesis, RAFT polymerization of nBMA is carried out simultaneously as the ROP of CL pro-ceeds, to yield the block-graft copolymer. The effects of poly-merization time, initiator concentration, and monomer concentration on the copolymerization in the presence of the RAFT-ROP agent by the application of simultaneous RAFT and ROP processes have been studied. The results of the po-lymerization of nBMA and CL are shown in Tables 1–3. The conversion of the monomer was between 21.78 and 71.40 wt %. The FTIR spectrum of poly[CL-b-EPCH-b-CL-(g-nBMA)] block-graft copolymer are shown in Figure 2(c).The signal at 1723 cm21corresponding to AC@O, and AC@S of the copolymer appears in the FTIR spectra. This signal diminishes at the FTIR spectrum of the RAFT-ROP agent as corresponding toAC@S of the agent. Typical 1H NMR spec-tra of the block-graft copolymers in Figure 1(c) shows the FIGURE 2 1H NMR spectra of poly-EPCH (a), and the novel
RAFT-ROP agent (b), poly(CL-b-EPCH-g-nBMA-b-CL) block-graft copolymer (c).
1.0 ppm forACH3protons of ethyl xanthogenate group and
poly-nBMA, 1.2 ppm for ACH2 protons of poly-EPCH, 1.5,
1.6, 1.8, and 2.4 ppm for aliphaticACH2protons of poly-CL
and poly-nBMA, 2.5 ppm forACH2ACOA protons of poly-CL,
3.4 ppm for AOH protons for poly-CL, 3.6 ppm for AOCH2
protons for poly-CL, 4.2 ppm and 4.7 ppm for AOCH2
pro-tons of ethyl xanthogenate group and poly-EPCH.
The effect of the polymerization time on the one-step block-graft copolymerization is presented in Table 1.
Polymerization time dependence of Mn on the one-step
copolymerization is shown in Figure 3. For polymerizations of longer durations, polymers of higher molecular weights are obtained. Longer polymerization times cause higher poly-mer yields; these results being in good agreement with those stated by Heidenreich and Puskas67 for the RAFT polymer-ization. Higher amounts of the RAFT-ROP agent cause a higher polymer yield (Table 2). Interestingly, the value of Mn
can only decrease if new chains are generated which, how-ever, is not in accordance with a controlled polymerization. Increased amounts of initiator in the reaction mixture lead to the formation of a higher number of active centers. Conse-quently, increased numbers of growing macroradicals are formed in the system. Hence, it may be expected that they have shorter poly-nBMA and poly-CL segments, which is con-firmed by a decrease in the molecular weights of the block-graft copolymers, as shown in Table 2. The same situation was also observed in our previous articles.36,44 Dependence of RAFT-ROP agent concentration on Mn for the one-step
copolymerization is shown Figure 4. Increasing the amount of monomers also causes an increase in both the yield and the molecular weights of the copolymers as expected (Table 3). Dependence of nBMA concentration on Mnfor the
copoly-merization is shown Figure 5. The Mw/Mnvalues of the poly
[CL-b-EPCH-b-CL-(g-nBMA)] block-graft copolymers are between 2.02 and 2.74 (Table 1–3). Because of the branched structure, more than one propagating center initiates the polymerization, and the Mw/Mn values of the block-graft
copolymers are relatively higher than expected. All GPC chro-matograms were unimodal and indicated more the molecular SCHEME 2 Reaction pathways in the synthesis of the
block-graft copolymer, poly(CL-b-EPCH-g-nBMA-b-CL).
TABLE 1 The Effect of the Polymerization time on One-Step Block-Graft Copolymerization
Code RAFT-ROP Agent (g) CL (g) nBMA (g) Time
(min) Yield (g) Conv. (%) ca Mn,GPC Mw/Mn
Decomp. Temp. (C) Poly-EPCH/ Poly-CL/Poly-nBMA Segment (mol/mol/mol) PA-2 0.125 1.506 1.519 50 0.687 21.78 0.30 81,078 2.38 315 0.50/0.48/8.14 PA-3 0.104 1.530 1.519 75 0.785 24.87 0.28 85,582 2.46 355 0.50/1.63/32.66 PA-8 0.105 1.546 1.510 305 1.244 39.30 0.28 101,473 2.74 322 0.50/1.90/23.04 AIBN 5 1.3 3 1023 g; DBTDL 5 6.32 3 1024 g (1.00 3 1026 mol); polym. temp.5 90C; DMF 5 3 mL. a
Nonsolvent (petroleum ether, mL)/solvent (THF, mL).
TABLE 2 The Effect of the Amount RAFT-ROP Agent on One-Step Block-Graft Copolymerization
Code RAFT-ROP Agent (g) CL (g) nBMA (g) AIBN (g) Yield (g) Conv. (%) ca Mn,GPC Mw/Mn Decomp. Temp. (C) Poly-EPCH/Poly-CL/ Poly-nBMA Segment (mol/mol/mol) PD-1 0.051 1.530 1.503 0.0005 0.917 29.72 0.26 160,059 2.35 321 0.50/0.24/7.88 PC-2 0.084 1.518 1.508 0.0008 1.032 33.16 0.24 135,883 2.44 346 0.50/0.92/17.35 PD-3 0.150 1.518 1.515 0.0015 1.173 36.82 0.30 105,721 2.24 344 0.50/1.72/3.96 PC-4 0.209 1.529 1.535 0.0020 1.388 42.36 0.22 88,283 2.05 336 0.50/1.40/17.49 PD-5 0.251 1.523 1.512 0.0025 1.413 42.93 0.26 61,262 2.10 343 0.50/2.82/4.70 Polym. time 5 270 min; DBTDL 5 6.32 3 1024
g (1.00 3 1026
mol); polym. temp.5 90C; DMF 5 3 mL.
a
weight values of block-graft copolymers than that of RAFT-ROP agent. For example, Figure 6 shows the unimodal GPC curves of block-graft copolymers (PD series in Table 2) and poly-EPCH.
The polymer composition of the copolymers was calculated using the integral ratios of the signals corresponding to the AOCH2groups of poly-EPCH (d 5 4.2–4.7 ppm), theACH3of
poly-nBMA (d 5 0.9–1.0 ppm), theAOCH2groups of poly-CL
(d 5 3.8–3.9 ppm). Interestingly, the poly-nBMA and poly-CL contents increased as the polymerization time increased for the copolymers. In general, poly-nBMA segments of the block-graft copolymers are more than EPCH and poly-CL segments the block-graft copolymers. When we used excess CL (4 g) in the one-step copolymerization, poly-CL content creased and poly-nBMMA content decreased (Table 4). Using RAFT-ROP agent, ROP of CL was carried out at 480 min in Table 4. At ROP of CL, the yield could not determinate at low polymerization time and was fairly low at 480 min. As shown in Table 4, nBMA segments at the RAFT poly-merization of nBMA were higher than poly-CL segments at the ROP of CL using RAFT-ROP agent. These results indicate that the one-pot reaction can be used to prepare block-graft copolymers containing the desired segment ratio by changing the polymerization conditions.
Thermal Analysis of Poly[CL-b-EPCH-b-CL-(g-nBMA)] Block-Graft Copolymers
Thermal analysis of the samples was carried out by taking TGA, and DSC curves. The mass loss in TGA of the block-graft copolymer (PA-2) was 99.3% at 350 C. TGA showed interesting properties of the block-graft copolymer indicating continuous weight loss starting from 265 C to nearly 395 C with a derivative at 315 C . Thermal properties of the
polymers were shown Figure 7. All samples exhibited glass transition temperatures (Tg). The reported Tg values were
obtained from the second heating curves. Tg value of the
block-graft copolymer (PB-2) was 126 C (Fig. 8). The only
one glass transition was observed. It could be concluded cor-responding homopolymers was mixture. Similarly, TGA showed that in the block-graft copolymers, nBMA, poly-EPCH, and poly-CL blocks had not the individual decomposi-tion temperatures (Td) as listed in Table 1–3. One main
indi-vidual Td of the block-graft copolymers was observed as
shown in Figure 7. This can be attributed to the high misci-bility of the polymerizable methacrylate groups of poly-nBMA with poly-EPCH and poly-CL moieties of the copoly-mers. The same situation (the observation of only one glass transition) can also be seen our previous articles.27,37,40 Tg
value reported in the literature for homo poly-EPCH, for FIGURE 3 Dependence of polymerization time on Mn for the
one-step copolymerization.
TABLE 3 The Effect of the Amount of the Monomer on One-Step Block-Graft Copolymerization
Code
RAFT-ROP
Agent (g) CL (g) nBMA (g) Yield (g) Conv. (%) ca M
n,GPC Mw/Mn Decomp. Temp. (C) Poly-EPCH/Poly-CL/ Poly-nBMA Segment (mol/mol/mol) PB-4 0.102 1.503 2.054 1.527 41.72 0.26 63,945 2.02 344 0.50/4.41/4.80 PB-5 0.116 1.551 2.544 1.997 47.39 0.24 79,057 2.13 349 0.50/0.27/4.17 PB-6 0.102 1.525 3.038 2.148 46.01 0.22 106,747 2.26 – 0.50/0.03/0.95 PB-7 0.111 1.515 3.503 2.670 52.03 0.20 94,581 2.18 – 0.50/0.04/0.97
Polym. time: 150 min; AIBN 5 1.3 3 1023
g; DBTDL 5 6.32 3 1024
g (1.00 3 1026mol); polym. temp. 5 90C; DMF 5 3 mL.
a
Nonsolvent (petroleum ether, mL)/solvent (THF, mL).
FIGURE 4 Dependence of RAFT-ROP agent on Mnfor the one-step copolymerization.
homo poly-CL, and homo poly-nBMA as 15,68 272,69,70and 20 C,71
respectively. Tgvalues of the block-graft copolymers
changed to the value which was more than the values of homo poly-EPCH, poly-CL, and homo poly-nBMA. The only one glass transition temperature value for the samples shows the miscible nature of the related homopolymers. Fractional Precipitation
Fractional precipitation experiments also provided the evi-dence for formation of copolymers. The gamma values (c) of poly[CL-b-EPCH-b-CL-(g-nBMA)] block-graft copolymers were FIGURE 5 Dependence of nBMA on Mn for the one-step
copolymerization.
FIGURE 6 GPC chromatograms of poly-EPCH, and PD series in Table 2.
TABLE 4 ROP and RAFT Polymerization of RAFT-ROP Agent
Code RAFT-ROP Agent (g) CL (g) nBMA (g) AIBN (g) DBTDL (g) DMF (mL) Time (min) Yield (g) Conv. (%) Mn,GPC Mw/Mn Poly-EPCH/Poly-CL/ Poly-nBMA Segment (mol/mol/mol) PE-1 0.112 – 1.512 0.004 – 3 150 1.041 64.13 44,589 2.17 0.50/–/31.55 PE-2 0.110 – 1.520 0.006 – 3 300 1.361 83.52 37,932 2.06 0.50/–/59.24 PG-1 0.177 1.537 – – 0.0006 3 480 0.123 7.20 – – 0.50/0.76/– PG-2 0.131 1.515 – – 0.0006 – 480 0.502 30.31 – – 0.50/1.86/– PG-3 0.146 1.565 – – 0.0006 1 480 0.301 17.61 129,427 1.59 – PF-1 0.175 4.000 1.015 0.009 0.0006 3 75 3.011 57.90 70,766 2.33 0.50/0.11/1.01 PF-2 0.158 4.014 1.054 0.005 0.0006 3 150 3.511 67.20 109,545 2.38 0.50/0.12/0.98 PF-3 0.112 4.061 1.036 0.006 0.0006 3 300 3.721 71.40 98,288 2.42 0.50/0.13/0.96 Polym. temp.5 90C.
FIGURE 7 TGA curves of the block-graft copolymers (PD-1, PC-2, PD-3, PC-4, PD-5 in Table 1).
between 0.18 and 0.30, as shown in Tables 1–3, when the solvent was THF and the nonsolvent was petroleum ether. In this solvent–nonsolvent system, the c-values were found to be 0.64–0.68 for homo EPCH, 0.28–0.32 for homo poly-nBMA, and 1.02–1.20 for homo poly-CL. The c values of the block-graft copolymers were different from that of homo poly-EPCH and homo poly-CL, and were close to that of homo poly-nBMA. It could be explain the poly-nBMA seg-ment is dominant. Fractional precipitation behavior gives an evidence for the formation of block copolymer.
CONCLUSIONS
A novel macroinitiator, a RAFT-ROP agent, was synthesized. A set of one-pot synthesis and RAFT and ROP polymerization conditions of block-graft copolymers, poly[CL-b-EPCH-b-CL-(g-nBMA)], were evaluated. The proposed procedure for the preparation of block-graft copolymers is simple and efficient. The block length can be adjusted by varying the monomer and initiator concentrations. Basically, controlling the poly-merization parameters such as RAFT-ROP agent concentra-tion and polymerizaconcentra-tion time, RAFT-ROP initiators can be promising materials to obtain block-graft copolymers.
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