One-Pot Synthesis of Block Copolymers by Orthogonal Ring-Opening

Mar 17, 2016 - ... School of Chemical Engineering, University of New South Wales, Sydney .... PET-RAFT polymerisation: towards green and precision pol...
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One-Pot Synthesis of Block Copolymers by Orthogonal RingOpening Polymerization and PET-RAFT Polymerization at Ambient Temperature Changkui Fu, Jiangtao Xu,* Mitchell Kokotovic, and Cyrille Boyer* Centre for Advanced Macromolecular Design (CAMD) and Australian Centre for NanoMedicine (ACN), School of Chemical Engineering, University of New South Wales, Sydney, NSW 2052, Australia S Supporting Information *

ABSTRACT: Well-defined poly(ε-caprolactone)-b-poly(methyl acrylate) (PCL-b-PMA) block copolymers were synthesized at ambient temperature by one-pot combination of diphenyl phosphate (DPP)-catalyzed ring-opening polymerization (ROP) and photoinduced electron/energy transferreversible addition−fragmentation chain transfer (PET-RAFT) polymerization. Full orthogonality of PET-RAFT polymerization and DPP-catalyzed ROP was confirmed by kinetic studies, which allowed facile synthesis of PCL-b-PMA block copolymers without a specific polymerization sequence. The resulting PCL-b-PMA block copolymers synthesized by either sequential or simultaneous ROP and PET-RAFT polymerization showed remarkably low molecular weight distributions (≤1.15), indicating that both ROP and PET-RAFT polymerizations proceeded in a controlled manner. In contrast to previous synthetic methods to prepare block copolymers, this facile one-pot method allows for rapid synthesis of block copolymers controlled via visible light.

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thermal polymerizations, the light-mediated CRP can be performed at ambient temperature in a neat and efficient way. Featuring temporal and spatial control, light-mediated CRP is capable of attracting numerous applications in elegant polymer synthesis,5j,n,s,6 bioconjugation,7 self-assembly,8 and surface modification,5c,9 etc. Our group has developed a novel visible-light-mediated CRP, namely, photoinduced electron/ energy transfer-RAFT (PET-RAFT) polymerization, for welldefined polymer synthesis.5g Through comprehensive research, we can now achieve smart control over polymer synthesis and architectures of various monomer families through the use of the PET-RAFT technique.10 This is achieved through the thoughtful selection of photoredox catalysts from those available, which cover a range of wavelengths from blue to red/near-infrared light11 and are effective at low light intensities and ambient temperature. The PET-RAFT polymerization can be conducted in bulk, solution,5g dispersion,12 and even miniemulsion,13 exhibiting great versatility, robustness, and oxygen tolerance.10 Due to its mild reaction conditions and versatility, we envisaged that PET-RAFT polymerization would be an excellent platform for the combination of other controlled polymerization techniques in the one-pot preparation of polymers with various functionalities and architectures.14

he one-pot synthetic methodology, a concept from organic chemistry, has been shifted to polymer chemistry for rapid synthesis and postmodification of polymers.1 The combination of different and compatible controlled polymerization techniques performed in one-pot reactions to prepare polymers avoids purifying intermediates, which saves time and resources and thus provides facile access to polymers with various functionalities and architectures.2 For instance, ringopening polymerization (ROP) has been combined with controlled radical polymerization techniques, such as nitroxide-mediated polymerization or reversible addition−fragmentation chain transfer (RAFT) polymerization to one-step prepare block and graft copolymers, reported, respectively, by Hedrick, Howdle, Barner-Kowollik, and others.2b,g,3 To achieve this goal, orthogonality and compatibility of these polymerization techniques are required. However, for most controlled polymerization techniques, the conditions required are different and often contradictory.4 For instance, Giacomelli and coworkers2p demonstrated the combination of thermal initiated RAFT and ROPs for the synthesis of polystyrene-b-poly(εcaprolactone) block copolymers using a one-pot polymerization. The authors successfully showed the preparation of copolymers; however, a relatively high polydispersity indice (PDI) was observed. The authors attributed this to the possible incompatibility issue of thermal initiated RAFT polymerization with ROP conditions. Recently, light mediated controlled radical polymerization (CRP) has emerged as a new and attractive polymerization technique for polymer synthesis.5 Compared with traditional © XXXX American Chemical Society

Received: February 11, 2016 Accepted: March 7, 2016

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DOI: 10.1021/acsmacrolett.6b00121 ACS Macro Lett. 2016, 5, 444−449

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ACS Macro Letters

Scheme 1. One-Pot Synthesis of PCL-b-PMA Block Copolymer by Combining DPP-Catalyzed ROP of CL and PET-RAFT Polymerization of MA

Figure 1. (a) Conversions of MA and CL versus polymerization time; (b) ln([M]0/[M]t) of MA and CL versus polymerization time; (c) GPC traces of polymers obtained at 4 h, 8 h, and 12 h; (d) Mn,RI of polymers versus polymerization time in the synthesis of PMA-b-PCL block copolymers via one-pot sequential PET-RAFT polymerization of MA and DPP-catalyzed ROP of CL.

In this communication, we demonstrate, for the first time, the preparation of block copolymers in one pot and at ambient temperature by combining diphenyl phosphate (DPP)catalyzed ROP of ε-caprolactone (ε-CL) and iridium-catalyzed PET-RAFT polymerization of methyl acrylate (MA) using low intensity visible light (λmax = 460 nm, 0.7 mW/cm2). In our approach, we demonstrated that the iridium catalyst is compatible with the diphenyl phosphate (DPP)-catalyzed ROP condition and allows a spatial and temporal control of the polymerization using visible light. As shown in Scheme 1, three synthetic routes toward synthesis of polyCL-b-polyMA (PCL-b-PMA) block copolymers are comprehensively implemented: (i) to conduct PET-RAFT polymerization of MA first, sequentially followed by ROP of CL; (ii) to conduct ROP of CL first, sequentially followed by PET-RAFT polymerization of MA; (iii) to conduct ROP of CL and PET-RAFT polymerization of MA simultaneously to form PCL-b-PMA block copolymer. In all three synthetic routes, well-defined PCL-b-

PMA block copolymers with low molecular weight distributions (PDI < 1.15) are successfully prepared. More importantly, no intermediate purifications are essentially required in all three synthetic routes, indicating full orthogonality of DPP-catalyzed ROP of CL and PET-RAFT polymerization of MA. To the best of our knowledge, such one-pot polymerization to prepare block copolymers by combining two mechanistically distinct, but fully orthogonal, polymerization techniques without needing to consider the sequence of polymerizations has not yet been reported. In comparison with the seminal work of Goto, Kaji, and co-workers,5e which shows the first example of a combination of UV-initiated ROP and visible-light-mediated iodine transfer polymerization for the synthesis of diblock copolymer using a sequential reaction (i.e., MMA first, and then δ-valerolactone), in our approach, we demonstrate that the diblock copolymer could be prepared via a simultaneous or sequential approach (without the need of specific order). Both simultaneous and sequential approaches result in the synthesis 445

DOI: 10.1021/acsmacrolett.6b00121 ACS Macro Lett. 2016, 5, 444−449

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ACS Macro Letters

Figure 2. (a) Conversions of CL and MA versus polymerization time; (b) ln([M]0/[M]t) of CL and MA versus polymerization time; (c) GPC traces of polymer obtained at 3 h, 5 h, 7 h, and 11 h; (d) Mn,RI of polymers versus polymerization time in the synthesis of PCL-b-PMA block copolymer via one-pot sequential DPP-catalyzed ROP of CL and PET-RAFT of polymerization of MA.

of well-defined block copolymers, indicating excellent compatibility and orthogonality of these two polymerization techniques without the formation of byproducts. The current method provides a facile, versatile, and efficient way of preparing block copolymers, making block copolymer synthesis more easily accessible and industrially applicable. As shown in Scheme 1, modified chain transfer agent containing a hydroxyl group, 2-hydroxyethyl 2-(((butylthio)carbonothioyl)thio)propanoate (HEBCP), is used to initiate a ROP and PET-RAFT polymerization. The hydroxyl group of HEBCP is the initiator for DPP-catalyzed ROP, whereas the trithiocarbonate group mediates PET-RAFT polymerization. For synthesis of PMA-b-PCL block copolymer by Route 1, PET-RAFT polymerization of MA was conducted first, followed by ROP of CL. For PET-RAFT polymerization of MA, Ir(ppy)3 was used as the photocatalyst and toluene as solvent. A ratio of [HEBCP]:[MA]:[Ir(ppy)3] = 1:100:0.001 was used in the polymerization. The PET-RAFT polymerization was conducted at ambient temperature under blue light (λmax = 460 nm). As shown in Figure 1a, the PET-RAFT polymerization of MA proceeded smoothly and achieved 73% conversion of MA in 8 h. Ln([M]0/[M]t) value also increased linearly with time, indicative of a well-controlled PET-RAFT polymerization process (Figure 1b). A PMA homopolymer with Mn,RI of 6 430 g/mol was obtained after 8 h of polymerization (Figure 1d). The blue light was then switched off, followed by the addition of 50 equiv of CL and 1 equiv of DPP with respect to HEBCP to activate ROP. After addition of CL and DPP for 6 h, the conversion of CL was observed to reach 96% (Figure 1a). The linear, first-order kinetics in the ROP of CL showed a well-controlled ROP process (Figure 1b). The GPC trace of polymer after addition of CL and DPP also shifted to lower retention time, indicating an obvious increase in molecular weight (Figure 1c). Throughout the whole polymerization, the molecular weights of polymers increased

continuously with polymerization time, and the polydispersity indices (PDIs) of polymers remained very narrow ranging from 1.08 to 1.18 (Figure 1d). This evidence indicates the successful formation of well-defined block copolymers. A PMA-b-PCL block copolymer with Mn,RI of 11 350 g/mol and PDI of 1.11 was obtained after a total of 14 h of polymerization. The structure and composition of PMA-b-PCL block copolymers were characterized by 1H NMR spectroscopy, which confirmed the successful synthesis of block copolymers (SI, Figure S1). For synthesis of PCL-b-PMA block copolymer by Route 2, DPP-catalyzed ROP of CL was conducted first followed by subsequent PET-RAFT of MA. Unlike Route 1, all chemicals were mixed together prior to the start of polymerization with a ratio of [HEBCP]:[MA]:[CL]:[Ir(ppy) 3 ]:[DPP] = 1:100:50:0.001:1. The polymerization was conducted in the absence of light and at ambient temperature. As shown in Figure 2a, DPP-catalyzed ROP of CL was observed to proceed very fast with conversion of CL reaching 88% in 5 h. A linear first-order relationship between ln([M]0/[M]t) and polymerization time can be seen during the ROP of CL, showing excellent control over polymerization (Figure 2b). Meanwhile, no polymerization of MA was observed according to the 1H NMR spectrum of the reaction mixture (SI, Figure S2). PETRAFT polymerization is capable of being switched “on/off” under visible-light irradiation, which enables instant manipulation of polymerization. Thus, at the time of 5 h, the reaction mixture was irradiated under blue light to start PET-RAFT polymerization of MA. The polymerization of MA was successfully conducted as indicated by a kinetic study. The conversion of MA reached 56% after irradiation under blue light for 6 h (Figure 2a). The PET-RAFT polymerization of MA also exhibited controllable character as reflected by the linear first-order polymerization kinetics (Figure 2b). The GPC traces of polymers produced after addition of the MA block shifted to lower retention time, indicating a continuous increase 446

DOI: 10.1021/acsmacrolett.6b00121 ACS Macro Lett. 2016, 5, 444−449

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ACS Macro Letters

Figure 3. (a) Conversions of CL and MA versus polymerization time; (b) ln([M]0/[M]t) of CL and MA versus polymerization time; (c) GPC traces of polymer obtained at 1 h, 2 h, 3 h, and 5 h; (d) Mn,RI and Mn,theo of polymers versus polymerization time in the synthesis of PCL-b-PMA block copolymer via one-pot simultaneous PET-RAFT polymerization of MA and DPP-catalyzed ROP of CL.

polymers synthesized during polymerization were monomodal and clearly shifted to higher molecular weight with increasing reaction time (Figure 3c). The molecular weights of polymers increased continuously, while the PDIs of polymers remained narrow (≤1.15) throughout the whole polymerization process (Figure 3d). Furthermore, the molecular weights of polymers (Mn,RI) were close to their theoretical molecular weights (Mn,theo) which was indicative of excellent control over both polymerization processes. After 5 h, a PCL-b-PMA block copolymer with Mn,RI of 11 960 g/mol and PDI of 1.15 was obtained. The structure and composition of PCL-b-PMA block copolymer obtained from one-pot simultaneous PET-RAFT polymerization of MA and ROP of CL was characterized by 1H NMR spectroscopy (SI, Figure S5). The characteristic peaks at δ 4.15, 2.32, 1.68, and 1.42 ppm corresponding to PCL and characteristic peak of the methyl group of PMA at 3.68 ppm can be clearly seen on the spectrum. The methene group of HEBCP at 3.38 ppm can also be clearly observed. According to the comparison of the integrals of peaks at 4.15, 3.68, and 3.38 ppm, the PCL-b-PMA block copolymer contained 52 repeating units of CL and 67 repeating units of MA. The molecular weight of the block copolymer was calculated as 11 970 g/mol which was close to the theoretical molecular weight of 12 120 g/mol. The temporal control of PET-RAFT polymerization in the presence of ROP was demonstrated by exposing the reaction mixture to an alternating light “ON” and “OFF” environment. When the light was on, the PET-RAFT polymerization proceeded as expected (SI, Figure 6a). However, in the absence of light (light “OFF”), the PET-RAFT stopped. Whether the light was on or off, the DPP-catalyzed ROP of CL proceeded continuously, showing great orthogonality with PET-RAFT polymerization (SI, Figure 6a and 6b). The on/off feature of PET-RAFT polymerization offered a simple way to

of molecular weight (Figure 2c). The molecular weight of polymer increased from 6240 g/mol to 11 020 g/mol after irradiation under blue light for 6 h (Figure 2d). The molecular weight distributions throughout the whole process remained narrow, ranging from 1.11 to 1.24. The successful formation of block copolymers was confirmed by 1H NMR spectroscopy (SI, Figure S3). It is worth to note that that after beginning the PET-RAFT polymerization the ROP of CL continued, with conversion of CL increasing from 88% to 98% (Figure 2a). This indicated that PET-RAFT polymerization would not interfere with DPP-catalyzed ROP. The two polymerizations were seemingly compatible, which inspired us to further explore the synthesis of PCL-b-PMA block copolymers via one-pot simultaneous PET-RAFT polymerization and ROP. For synthesis of PCL-b-PMA block copolymer by Route 3, the one-pot simultaneous PET-RAFT polymerization of MA and DPP-catalyzed ROP of CL to/from PCL-b-PMA block copolymers was conducted under irradiation of blue light at ambient temperature with a ratio of [HEBCP]:[MA]:[CL]: [Ir(ppy)3]:[DPP] = 1:100:50:0.001:1. 1H NMR spectroscopy was used to monitor the polymerization process. The ester group of CL at 4.25 ppm and methyl group of MA at 3.79 ppm decreased, while the ester group of PCL at 4.15 ppm and the methyl group of PMA at 3.68 ppm increased gradually (SI, Figure S4), indicating the PET-RAFT polymerization of MA and ROP of CL occurred successfully. Figure 3a shows the time−conversion curve in one-pot polymerization. The ROP of CL proceeded slightly faster than PET-RAFT polymerization of MA. Both the PET-RAFT and DPP-catalyzed ROP polymerizations resulted in high monomer conversions. After polymerization for 5 h, the conversion of CL was 93%, while the conversion of MA reached 75%. The kinetic curves of PETRAFT polymerization and ROP showed a linear dependence of ln([M]0/[M]t) on the polymerization time, indicating both polymerizations proceeded in a controlled manner despite the presence of each other (Figure 3b). The GPC traces of 447

DOI: 10.1021/acsmacrolett.6b00121 ACS Macro Lett. 2016, 5, 444−449

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ACS Macro Letters Table 1. Characteristics of PCL-b-PMA Block Copolymers Obtained in This Work entry [MA]:[CL] feed ratio 1 2 3 4f 5f

MA/CL conversion (%)a

[MA]:[CL] theoretical ratiog

[MA]:[CL] experimental ratioh

Mn,RIb

Mn,NMRc

Mn,theod

PDIe

73/96 56/98 76/93 84/92 81/99

73/48 56/49 76/46 84/92 40/99

78/51 60/45 78/45 86/91 42/101

11 350 11 020 11 960 16 050 14 510

12 630 10 400 11 970 17 850 15 200

12 260 10 680 12 120 17 770 15 010

1.11 1.11 1.15 1.08 1.10

100:50 100:50 100:50 100:100 50:100

Determined by 1H NMR b,eDetermined by THF GPC. cCalculated by the equation: Mn,NMR = MWHEBCP + m × MWMA + n × MWCL. dCalculated by equation: Mn,theo = MWHEBCP + α1 × DPMA × MWMA+ α2 × DPCL × MWCL, where m and n represent the number of MA and CL units in the PCL-b-PMA block copolymers, respectively, and α1 and α2 represent the conversion of MA and CL, respectively. DPMA and DPCL represent the targeted degree of polymerization of MA and CL, and MWHEBCP, MWMA, and MWCL represent the molecular weights of HEBCP, MA, and CL, respectively. fThe polymerization was conducted for 8 h. gTheoretical ratio of [MA]/[CL] in polymer calculated by conversion. hExperimental ratio of [MA]/[CL] in polymer calculated by 1H NMR. a,c

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ACKNOWLEDGMENTS CB acknowledges the Australian Research Council (ARC) for his Future Fellowship.

regulate the block length ratio of block copolymers in one-pot polymerization. The block lengths of block copolymers can be also easily adjusted by varying the feed ratios of MA and CL. Well-defined PCL-b-PMA block copolymers with different block lengths of PMA and PCL were prepared using this simple one-pot, simultaneous PET-RAFT polymerization and DPP-catalyzed ROP technique (Table 1, entries 3−5; SI, Figures S7−S10). In summary, we have developed, for the first time, a facile one-pot method to prepare PCL-b-PMA block copolymers by utilizing DPP-catalyzed ROP and iridium-catalyzed PET-RAFT polymerization. Due to the full orthogonality and compatibility of ROP and PET-RAFT polymerization, PCL-b-PMA block copolymers were synthesized at room temperature by either one-pot sequential PET-RAFT polymerization of MA followed by ROP of CL, one-pot sequential ROP of CL followed by PET-RAFT polymerization of MA, or one-pot simultaneous PET-RAFT polymerization of MA and ROP of CL. The polymerization kinetics of the three synthetic routes was thoroughly investigated by NMR and GPC analysis and demonstrated the well-controlled manner of both DPPcatalyzed-ROP and PET-RAFT polymerization. To the best of our knowledge, this is the first example showing the three synthetic routes to prepare a series of well-defined PCL-b-PMA block copolymers with remarkably low molecular weight distributions (PDIs ≤ 1.15) using a visible-light-mediated polymerization. Considering the facile nature and versatility of the current method, the combination of PET-RAFT and ROP appears an attractive method for the synthesis of polymers with complex architectures and advanced properties.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00121.

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REFERENCES

Experimental details, NMR spectra, GPC traces (Figures S1−S10) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. 448

DOI: 10.1021/acsmacrolett.6b00121 ACS Macro Lett. 2016, 5, 444−449

Letter

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DOI: 10.1021/acsmacrolett.6b00121 ACS Macro Lett. 2016, 5, 444−449