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Keywords:

  • block copolymers;
  • dual initiator;
  • one-step synthesis;
  • reversible addition fragmentation chain transfer polymerization (RAFT);
  • ring-opening polymerization

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES

A range of well-defined block copolymers were synthesized using 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanylpentanol (CDP) as a dual initiator for reversible addition-fragmentation chain transfer (RAFT) polymerization and ring-opening polymerization (ROP) in a one-step process. Styrene, (meth)acrylate, and acrylamide monomers were polymerized in a controlled manner for one block composed of vinyl monomers, and δ-valerolactone (VL), ε-caprolactone (CL), trimethylene carbonate (TMC), and L-lactide (LA) were used for the other block composed of cyclic monomers. Diphenyl phosphate was used as a catalyst for the ROP of VL, CL, and TMC, and 4-dimethyamino pyridine for the ROP of LA. These catalysts did not interfere with RAFT polymerization and the synthesis of various block copolymers proceeded in a controlled manner. CDP was found to be a very useful dual initiator for a one-step synthesis of various block copolymers by a combination of RAFT polymerization and ROP. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES

The synthesis of block copolymers using dual initiators has attracted considerable attention due to the limited number of synthesis steps and the possibility of a one-step process.1–10 The dual initiators have two functional groups that can perform two mechanistically different polymerizations without intermediate transformation steps. To obtain the well-defined block copolymers using dual initiators via a one-step process, there should be no interactions between the initiating groups, catalysts or initiators, and monomers.11–13 In addition, the two polymerization reactions should proceed in a controlled manner at the same reaction temperature. In practice, most of block copolymers synthesized using dual initiators are prepared by sequential reaction steps due to these limitations.

Dual initiators combining reversible addition-fragmentation chain transfer (RAFT) polymerization and other polymerization techniques are more useful for obtaining block copolymers with a variety of components because RAFT polymerization is applicable to a wide range of vinyl monomers.14,15 The major obstacle to the one-step synthesis of block copolymers using dual initiators for RAFT polymerization and ring-opening polymerization (ROP) is the interference between the ROP catalysts and RAFT polymerization reactions. In our previous study,16 the one-step synthesis of well-defined poly(alkyl methacrylate)-b-poly(δ-valerolactone) (PAM-b-PVL) and PAM-b-poly(ε-caprolactone) (PAM-b-PCL) block copolymers was conducted at 30 °C for the first time using 4-cyano-1-hydroxypent-4-yl dithiobenzoate (ACP-RAFT) as a dual initiator for RAFT polymerization and ROP. Diphenyl phosphate (DPP) was used as the ROP catalyst.17 The catalyst did not interfere with RAFT polymerization and the two polymerization reactions proceeded in a controlled manner. On the other hand, ACP-RAFT was not broadly applicable to other vinyl and cyclic monomers. The polymerization of acrylate and styrene monomers using ACP-RAFT was not well controlled, and ACP-RAFT was hydrolyzed during the ROP of lactide with basic catalysts. Dual initiators that are more applicable to a wide range of monomers and reaction conditions should be developed to obtain various block copolymers using dual initiators for RAFT polymerization and ROP using a one-step process. In the present study, 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanylpentanol (CDP), a hydroxyl-functionalized trithiocarbonate RAFT agent, was synthesized and tested as a dual initiator for the one-step synthesis of a range of block copolymers. Trithiocarbonate RAFT agents provide good control of the polymerization of various vinyl monomers.15 More importantly, they have good hydrolytic stability. A range of block copolymers containing one block, which was derived from vinyl monomers of styrene, (meth)acrylate, and acrylamide monomers, and the other block, which was derived from cyclic monomers of δ-valerolactone (VL), ε-caprolactone (CL), trimethylene carbonate (TMC), and L-lactide (LA) were polymerized successfully using CDP. DPP was used as a catalyst for the ROP of VL, CL, and TMC, and 4-dimethyamino pyridine (DMAP) was used as a catalyst for the ROP of LA. DMAP is a very effective catalyst for the ROP of LA in solution and melt.18,19 These catalysts did not interfere with RAFT polymerization and the synthesis of various block copolymers proceeded in a controlled manner. CDP was found to be a useful dual initiator for the one-step synthesis of a range of block copolymers.

EXPERIMENTAL

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES

Materials

Styrene, methyl acrylate (MA, Junsei, 99.0%), methyl methacrylate (MMA, Junsei, 99%), butyl acrylate (BA, Aldrich, ≥99%), butyl methacrylate (BMA, Aldrich, 99%), and tert-butyl methacrylate (t-BMA, Aldrich, 98%) were passed through a neutral alumina column, dried over calcium hydride, distilled under reduced pressure, and degassed by three freeze-pump-thaw cycles. VL (Acros, 99%), CL (Aldrich, 99%), anisole (Junsei, 98.0%), and 1,4-dioxane (Aldrich, ≥99.0%) were dried over calcium hydride, distilled under reduced pressure, and degassed by three freeze-pump-thaw cycles. N-isopropylacrylamide (NIPAM, TCI, >98%) was recrystallized twice from a mixture of acetone and hexane (1/1, v/v). Stearyl methacrylate (SMA, Aldrich) was dissolved in toluene, and passed through a neutral alumina column. The solvent was removed under reduced pressure. 2–2′-Azoisobutyronitrile (AIBN, Junsei, 96%) was recrystallized twice from methanol. LA (Aldrich) and TMC (Aldrich) were recrystallized twice from ethyl acetate. 1-Dodecanethiol (Aldrich, 98%), sodium hydride (Aldrich, 60% dispersion in mineral oil), carbon disulfide (Aldrich, 99.9%), iodine chip (Aldrich, ≥99%), tetrahydrofuran (THF, TCI, anhydrous, 98%), diethyl ether (Duksan, 99.0%), ethanol (Duksan, 95.0%), 4,4-azobis(4-cyano-1-pentanol) (ACP, Wonda science, 95%), DPP (Aldrich, ≥99%), and DMAP (Aldrich, 99%) were used as received.

Characterizations

The molecular weights (MWs) and MW distributions of the polymers produced were determined by gel permeation chromatography (GPC, Young Lin SP930D solvent delivery pump) coupled with an RI detector (RI 750F) and three columns (GPC KF-804L, KF805 and KF-805L, Shodex). The eluent used was THF at 35 °C at a flow rate of 1.0 mL min−1. Poly(methyl methacrylate) (PMMA) standards were used for calibration. For poly(N-isopropylacrylamide)-b-poly(L-lactide) (PNIPAM-b-PLA), the MWs and MW distributions were determined by GPC coupled with an RI detector and two columns (GPC KD-806 M x 2, Shodex). The eluent used was 0.01 M LiCl/DMF at 40 °C with a flow rate of 1.0 mL min−1. PMMA standards were used for calibration. The monomer conversion was determined using a gravimetric method and the block polymer composition was determined by 1H nuclear magnetic resonance (1H NMR, Varian VXR-Unity NMR 400 MHz) spectroscopy in CDCl3.

Synthesis of CDP

Bis(dodecylsulfanylthiocarbonyl) disulfide was first synthesized as described elsewhere.20 Bis(dodecylsulfanylthiocarbonyl) disulfide (2.92 g, 0.0053 mol) and ACP (2.00 g, 0.0079 mol) in ethyl acetate (100 mL) were heated under reflux for 24 h. After removing of the volatiles under vacuum, the crude product was washed with water and purified by column chromatography using hexane/ethyl acetate = 5:5 (v/v) as the eluent. The product was orange color oil (2.04 g, 48% yield).

1H NMR (CDCl3) δ = 0.89 (t, 3H, CH3); 1.28 (br, s, 18H); 1.68 (m, 2H); 1.89 (s, 3H, CH3); 1.98–2.32 (m, 4H, CH2CH2); 3.38 (t, 2H, CH2S); 3.71 (t, 2H, CH2O). 13C NMR (CDCl3) δ = 217.4 (C[DOUBLE BOND]S), 119.4 (CN), 61.3 (CH2OH), 46.9 (CH2S), 36.7 (C), 35.6 (CH2C), 31.7, 29.4, 29.3, 29.2, 29.1, 28.9, 28.7, 27.7, 27.5, 24.6, 22.5, 20.1 ([BOND]CH2[BOND], [BOND]CCH3) 13.9 (CH3CH2).

RAFT Polymerization of MMA with CDP for a Kinetic Investigation

The RAFT polymerization of PMMA was carried out at a feed ratio of [MMA]0/[LA]0/[CDP]0/[AIBN]0 = 50/50/1/0.2 at 60 °C. MMA (3 mL, 28.0 mmol), CDP (0.217 g, 0.564 mmol), LA (4.04 g, 28.0 mmol), and AIBN (0.0184 g, 0.112 mmol) were dissolved in 4 mL of 1,4-dioxane. The mixture was purged with nitrogen for 30 min and stirred at 60 °C. During polymerization, 1 mL of the reaction mixture was withdrawn periodically to monitor the level of monomer conversion and MW. The resulting PMMA was precipitated in an excess of cold methanol, filtered, and dried under vacuum for 24 h.

ROP of LA with CDP for a Kinetic Investigation

The ROP of LA was performed at a feed ratio of [MMA]0/[LA]0/[CDP]0/[DMAP]0 = 50/50/1/4 at 60 °C. MMA (3 mL, 28.0 mmol), CDP (0.217 g, 0.564 mmol), LA (4.04g, 28.0 mmol), and DMAP (0.274 g, 2.24 mmol) were dissolved in 4 mL of 1,4-dioxane. The mixture was stirred at 60 °C. During polymerization, 1 mL of the reaction mixture was withdrawn periodically to monitor the level of monomer conversion and MW. The resulting PLA was precipitated in an excess of cold methanol, filtered, and dried under vacuum for 24 h.

One-Step Synthesis of PMMA-b-PLA with CDP for a Kinetic Investigation

The one-step synthesis of PMMA-b-PLA was performed at a feed ratio of [MMA]0/[LA]0/[CDP]0/[AIBN]0/[DMAP]0 = 50/50/1/0.2/4 at 60 °C. MMA (3 mL, 28.0 mmol), LA (4.04 g, 28.0 mmol), CDP (0.217 g, 0.564 mmol), AIBN (0.0184 g, 0.112 mmol), and DMAP (0.274 g, 2.24 mmol) were dissolved in 4 mL of 1,4-dioxane. The mixture was purged with nitrogen for 30 min and stirred at 60 °C. During polymerization, 1 mL of the reaction mixture was withdrawn periodically to monitor the level of monomer conversion and MW. The resulting PMMA-b-PLA was precipitated in an excess of cold methanol, filtered, and dried under vacuum for 24 h.

One-Step Synthesis of Various Block Copolymers with CDP

For one-step synthesis of the PLA-based block copolymers, vinyl monomer, LA, CDP, AIBN, and DMAP were dissolved in 4 mL of 1,4-dioxane. Table 2 lists the corresponding feed ratios. The mixture was purged with nitrogen for 30 min and stirred at 60 °C for 12 h. The resulting polymers were precipitated in an excess of cold methanol, filtered, and dried under vacuum for 24 h. The one-step synthesis of PVL-, PCL-, and PTMC-based block copolymers were performed at a feed ratio of [vinyl monomer]0/[cyclic monomer]0/[CDP]0/[AIBN]0/[DPP]0 = 100/100/1/0.2/0.5. The reaction mixture was purged with nitrogen for 30 min and stirred at 60 °C for 7 h. The resulting product was precipitated in an excess of cold methanol, filtered, and dried under vacuum for 24 h.

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES

In the present study, a range of block copolymers containing one block derived from vinyl monomers and the other block derived from cyclic monomers was synthesized by CDP in a one-step process. Scheme 1 shows one-step synthetic routes to PMMA-b-PLA and PMMA-b-PCL block copolymers using CDP. DMAP was used as a catalyst for the ROP of LA, and DPP was used for the ROP of CL. Before attempting one-step synthesis of various block copolymers, kinetic investigations of the RAFT polymerization of MMA and the ROP of LA using CDP were first carried out at 60 °C at feed ratios of [MMA]0/[LA]0/[CDP]0/[AIBN]0 = 50/50/1/0.2 and [MMA]0/[LA]0/[CDP]0/[DMAP]0 = 50/50/1/4, respectively. Figure 1 shows kinetic plots of the RAFT polymerization of MMA and the ROP of LA. The RAFT polymerization of MMA proceeded after an induction period of ∼4 h. Linear increases in ln([M]0/[M]) versus the reaction time were observed up to ∼70% conversion, and the polydispersity indices (Mw/Mn) of the PMMA and PLA obtained showed very low values. This suggests that both the RAFT polymerization and ROP proceeded in a controlled manner. The apparent propagation rate constants for the RAFT polymerization of MMA and ROP of LA were determined to be 1.93 × 10−5 and 1.79 × 10−5 s−1, respectively. The conversions of MMA and LA reached 68.3 and 62.5%, respectively, after 16 h.

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Figure 1. Kinetic plots for (a) RAFT polymerization of MMA and (b) ROP of LA using CDP at 60 °C.

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Scheme 1. One-step synthesis of PMMA-b-PLA and PMMA-b-PCL block copolymer using CDP as a dual initiator.

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Kinetic investigations of the one-step synthesis of PMMA-b-PLA block copolymer was performed at 60 °C at a feed ratio of [MMA]0/[LA]0/[CDP]0/[AIBN]0/[DMAP]0 = 50/50/1/0.2/4. Figure 2 shows kinetic plots of the one-step synthesis of PMMA-b-PLA block copolymer using CDP at 60 °C as well as the changes in the mole fraction of each monomer block with the polymerization time. 1H NMR spectroscopy was used to determine the mole fraction of each monomer block in the PMMA-b-PLA block copolymers by comparing the integration ratio of the methyl peaks for PMMA and the methylene peaks for PLA at 3.6 and 5.2 ppm, respectively (Fig. 3). A plot of ln([M]0/[M]) versus reaction time of each monomer showed a linear relationship. The GPC traces of the PMMA-b-PLA block copolymers were monomodal (Mw/Mn < 1.10) and clearly shifted toward the higher MW region with increasing reaction time (Fig. 4). This suggests that both the RAFT polymerization and ROP proceeded independently in a controlled manner. The trithiocarbonate unit in CDP was found to be stable to hydrolysis under the one-step polymerization condition. The apparent propagation rate constants for the RAFT polymerization of MMA and the ROP of LA were 2.32 × 10−5 and 1.76 × 10−5 s−1, respectively. Interestingly, in contrast to that expected from Figure 1, the mole fractions of the PMMA block in the PMMA-b-PLA block copolymers were higher than those of the PLA block from the initial stage of polymerization. The higher mole fractions of the PLA block were expected due to the induction period in the RAFT polymerization of MMA. On the other hand, a significant shortening of the induction period and an increase in the apparent propagation rate in the RAFT polymerization of MMA were observed, resulting in higher mole fractions of the PMMA block than those of the PLA block. Bian et al.21 reported a similar phenomenon from the RAFT polymerization of NIPAM at 100 °C using star-shaped PCL macro-RAFT agents (SPCL-RAFTs). The RAFT polymerization rate of NIPAM using SPCL25-RAFT was higher than that using SPCL10-RAFT. The subscripts represent the number of repeating units of each PCL arm. They suggested that dormant species were more difficult to form from the propagating chain radicals generated by the fragmentation of SPCL25-RAFT intermediate radicals than those generated from SPCL10-RAFT because of their lower diffusion rate, resulting in a higher concentration of propagating chain radicals and a higher polymerization rate. In the one-step synthesis of PMMA-b-PLA block copolymer, the length of the R group in CDP was initially increased by the ROP of LA. As the length of the R group increased, propagating chain radicals generated by the fragmentation of RAFT intermediate radicals would become more difficult to diffuse. This may result in a shortening of the induction period and an enhanced polymerization rate of MMA in the one-step synthesis of PMMA-b-PLA block copolymer. In practice, the reactivity of the propagating radicals was not changed by increasing the PLA chain length.

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Figure 2. Kinetic plots for the one-step synthesis of PMMA-b-PLA block copolymer using CDP at 60 °C and changes in the mole fraction of each monomer block with the polymerization time.

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Figure 3. 1H NMR spectrum of the PMMA-b-PLA block copolymer (Mn,GPC = 12,400 g mol−1, Mw/Mn = 1.10).

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Figure 4. Evolution of GPC traces of the PMMA-b-PLA block copolymer with the polymerization time.

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The compositions of the PMMA-b-PLA block copolymers could be controlled by adjusting the reactant feed ratio. Table 1 lists the one-step synthesis results of PMMA-b-PLA block copolymers using CDP at various feed ratios at 60 °C for 16 h. At all feed ratios, PMMA-b-PLA block copolymers with narrow MW distributions were obtained, and their compositions were dependent on the reactant feed ratio. In practice, the mole fractions of the PMMA and PLA block in PMMA-b-PLA were more dependent on the MMA and DMAP concentrations, respectively. The one-step synthesis of various block copolymers containing the blocks derived from vinyl and cyclic monomers was performed using CDP. Table 2 lists the results of the one-step synthesis of various PLA-based block copolymers using CDP and DMAP at 60 °C for 12 h. For the block derived from vinyl monomers, styrene, MMA, BMA, t-BMA, SMA, and NIPAM were polymerized in a controlled manner. Table 3 lists the one-step synthesis results of a range of block copolymers using CDP and DPP at 60 °C for 7 h. DPP was used as an acid catalyst for the ROP of VL, CL, and TMC. For the block derived from vinyl monomers, MMA, BMA, t-BMA, BA, and MA were polymerized. NIPAM could not be polymerized via a one-step process in the presence of DPP. All block copolymers in Tables 2 and 3 had very narrow MW distributions, indicating that their one-step synthesis had proceeded successfully in a well-controlled manner. The mole fraction of each unit in the block copolymers was determined by 1H NMR spectroscopy (Supporting Information Figs. S1–S10). At the same feed ratio, their compositions appeared to be dependent on the reactivity of each monomer. Consequently, CDP was found to be a very useful dual initiator for the one-step synthesis of a range of block copolymers via a combination of RAFT polymerization and ROP.

Table 1. One-Step Synthesis of PMMA-b-PLA Block Copolymers Using CDP at 60 °C for 16 h
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Table 2. One-Step Synthesis of PLA-Based Block Copolymers at 60 °C for 12 h
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Table 3. One-Step Synthesis of Various Block Copolymers at a Feed Ratio of [Vinyl Monomer]0/[Cyclic Monomer]0/[CDP]0/[AIBN]0/[DPP]0 = 100/100/1/0.2/0.5 at 60 °C for 7 h
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CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES

The kinetics of the one-step synthesis of PMMA-b-PLA block copolymers using CDP was examined. The RAFT polymerization of MMA proceeds at an enhanced polymerization rate due to the ROP of CL occurring simultaneously. The one-step synthesis of various block copolymers was performed successfully using CDP. Styrene, MA, MMA, BA, BMA, t-BMA, SMA, and NIPAM were polymerized in a controlled manner for one block composed of vinyl monomers, and VL, CL, TMC, and LA for the other block composed of cyclic monomers. DPP was used as a catalyst for the ROP of VL, CL, and TMC, and DMAP was used for LA. These catalysts did not interfere with RAFT polymerization, and the synthesis of the block copolymers proceeded in a controlled manner. CDP was found to be suitable for a wide range of monomers and reaction conditions for the one-step synthesis of various block copolymers.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES

This study was supported by INHA UNIVERSITY Research Grant and the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean Government (MEST) (R11-2005-065) through the Intelligent Textile System Research Center (ITRC).

REFERENCES AND NOTES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. EXPERIMENTAL
  5. RESULTS AND DISCUSSION
  6. CONCLUSIONS
  7. Acknowledgements
  8. REFERENCES AND NOTES