Letter pubs.acs.org/macroletters
Polyethylene‑g‑Polystyrene Copolymers by Combination of ROMP, Mn2(CO)10-Assisted TEMPO Substitution and NMRP Mustafa Ciftci,† Mustafa Arslan,† Michael Buchmeiser,‡ and Yusuf Yagci*,†,§ †
Department of Chemistry, Istanbul Technical University, Maslak, TR-34469, Istanbul, Turkey Institute of Polymer Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70550 Denkendorf, Germany § Center of Excellence for Advanced Materials Research (CEAMR) and Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah, 21589, Saudi Arabia ‡
S Supporting Information *
ABSTRACT: The synthesis of polyethylene-graf t-polystyrene copolymers by a multistep “grafting from” approach is described. In the first step, a bromo-functional polyethylene (PE-Br) was synthesized via ringopening metathesis polymerization (ROMP) of cis-cyclooctene (COE) and quantitative hydrobromination. Subsequent irradiation of PE-Br under visible light in the presence of dimanganese decacarbonyl (Mn2(CO)10) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) resulted in the formation of TEMPO-substituted polyethylene (PETEMPO). Polystyrene (PS) chains were then grown via nitroxide mediated radical polymerization (NMRP) from the PE-TEMPO precursor to give desired PE-g-PS copolymers in a controlled manner. The intermediates at each step and final graft copolymers were characterized by 1H NMR, FT-IR, GPC, and DSC analyses.
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free radical polymerization9 or free radical promoted cationic polymerization.10 Segmented copolymers are generally synthesized by employing more than one polymerization mode. Such combination of different polymerization techniques is known as mechanistic transformation, in which a polymer chain synthesized by a certain mechanism is functionalized for the initiation of another polymerization mechanism.11 The main advantage of this methodology is that it offers a straightforward route to obtain block and graft copolymers that cannot be made by a single polymerization mode.12 Many different transformation approaches combining conventional free radical systems with ring opening,13 anionic,14 activated monomer,15 cationic,16 free radical promoted cationic,17 and condensation polymerizations,18 but also, ring-opening metathesis polymerization with vinyl insertion polymerization19 have been established in our laboratories. With the recent progress in living/controlled polymerizations, the concept has been further extended to controlled polymerization methods for the synthesis of well-defined segmented copolymers with great opportunities in functionality, flexibility, and diversity.11 Although not fully controlled, photoinitiated polymerizations involving both free radical20 and cationic21 modes have also often been used in such mechanistic transformations. Such light-induced processes represents not
n recent years, much attention has been focused on controlled radical polymerization (CRP) techniques due to their ability to produce macromolecules with predetermined molecular weight, narrow molecular weight distribution, various architectures, and useful end-functionalities.1 Historically, nitroxide-mediated radical polymerization (NMRP)2 together with atom transfer radical polymerization (ATRP)3 and radicaladdition−fragmentation transfer (RAFT)4 have been the first and most widely used CRP techniques for the synthesis of polymers with different topologies due to their simplicity and applicability to broad range of monomers. Polyolefins are the largest class of thermoplastic polymers representing broad industrial applications including disposables, agriculture, appliances, electronics, construction, communication, and so on.5 On the other hand, polyolefins are incompatible with almost all other polymers due to the low surface energy arising from the lack of chemical functionality and high crystallinity. Usually, block and graft copolymers with a polyolefin segment are used as compatibilizers.6 However, desired modifications can only be achieved by the incorporation of polar functionalities since polyolefins have an inert chemical structure.7 Many different techniques for the fabrication of polyolefins with polar functionalities have been developed over the years.8 Recent reports from the authors’ laboratory are the typical examples on the polyolefin modifications facilitated by such functional groups. Accordingly, various polyolefin based graft copolymers were obtained by the combination of either ring-opening metathesis polymerization (ROMP), hydrobromination and dimanganese decacarbonyl (Mn2(CO)10)-mediated © XXXX American Chemical Society
Received: June 16, 2016 Accepted: July 22, 2016
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DOI: 10.1021/acsmacrolett.6b00460 ACS Macro Lett. 2016, 5, 946−949
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ACS Macro Letters Scheme 1. Synthesis of Poly(COE-TEMPO) Using Mn2(CO)10
only the advantage of being applicable at low temperatures, especially at room temperature, but also to produce reactive sites at defined positions in the macromolecule due to the selective absorptivity of certain chromophore groups. 22 Although, various photoinitiator systems23 have been employed for both modes, Mn2(CO)10 in conjunction with organic halides appears as an ideal photoinitiating system24 for the preparation of polymers with various topologies.23b,25 Many different applications of Mn2(CO)10 chemistry, including promotion of cationic polymerization,26 mechanistic transformation,27 graft copolymerization,9,10 atom transfer radical polymerization,28 iodine degenerative transfer polymerization,29 preparation of telechelics,30 and hyperbranched polymers,31 have been reported and reviewed.23b As part of our research program to develop new approaches for the preparation of polyolefin-based materials, we herein report controlled grafting of styrene from polyolefins by combination ROMP and NMRP. Thus, a bromo-functional polyolefin was synthesized by ROMP of COE and subsequently hydrobrominated. Then, TEMPO moieties were substituted with some of the bromide groups by using Mn2(CO)10 to give TEMPO-functionalized polyolefin analogs that were used as macroinitiator for NMRP of styrene. Successful application of Mn2(CO)10 for the substitution of TEMPO moiety32 prompted us to employ this system for the preparation of polyethylene based graft copolymers in a controlled manner. Accordingly, a brominated polyolefin, poly(COE-Br) was prepared via the ROMP of COE in the presence of a chain transfer agent and subsequently hydrobrominated as described previously.6d Then TEMPO groups were attached to the PE main chain by the use of Mn2(CO)10 in a similar way as described by Wang et.al.32 In this process, photochemically generated •Mn(CO)5 radicals abstract bromine atoms to give PE radicals, which were then coupled with TEMPO to give poly(COE-TEMPO) (Scheme 1). Optical properties of Mn2(CO)10, TEMPO, and the irradiation system in the visible range were investigated by UV−vis spectroscopy. As can be seen in Figure S1, only Mn2(CO)10 strongly absorbs visible light, whereas the other components of the reaction mixture were fully transparent. In the second step, the obtained poly(COE-TEMPO) was used as a precursor for the NMRP of styrene to give the corresponding graft copolymer PE-g-PS (Scheme 2). Depending on the polymerization time, copolymers with different PS
Scheme 2. Graft Copolymerization of Styrene from Poly(COE-TEMPO) by NMRP
chain lengths were obtained (Table 1). Moreover, deliberately used excess TEMPO did not significantly affected the overall grafting efficiency. Table 1. Grafting of Styrene from Poly(COE-TEMPO)a via NMRP polymer
t (h)
conv.b (%)
Mn (g mol−1)
PDI
graftingc (%)
PE-g-PS(1) PE-g-PS(2)
2 6
41 72
134000 198000
1.50 1.57
60 87
Mn (poly(COE-TEMPO)) = 68200 g mol−1, PDI = 1.77. Determined gravimetrically. cPercentage of bromoalkyl groups participating in TEMPO substitution and starting graft copolymerization were determined by 1H NMR analysis.
a b
Functionalization of the precursor polymer and the graft copolymerization processes were confirmed by spectral analyses. In the NMR spectrum of the PE-TEMPO, the broadening compared to the precursor PE-Br between 1 and 2 ppm might be attributed to the overlapping of the methyl protons from TEMPO moiety. Additionally, the residual signals of the CHBr between 4.06 and 4.10 ppm indicate that some of the bromine atoms did not take part in the TEMPO substitution process. As can be seen from the spectra of the graft copolymer, the appearance of new aromatic signals in 947
DOI: 10.1021/acsmacrolett.6b00460 ACS Macro Lett. 2016, 5, 946−949
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ACS Macro Letters addition to the CHBr signal confirms the presence of a PS grafted chain. The styrene content (mol %) in the graft copolymer was also calculated from the 1H NMR spectra by using the following equation: St(mol%) =
b × 100% 5a + b
where a represents the integrated peak area for the CHBr hydrogens of the poly(COE-Br) segment at 4.04 ppm, while b signifies the total integrated peak area of the five aromatic protons in styrene moieties of the grafted PS (Figure 1). Figure 3. DSC results of poly(COE-Br), poly(COE-TEMPO), PE-gPS(1), and PE-g-PS(2).
The TGA curves for the precursor polymer and the final graft copolymer are presented in Figure S2. Poly(COE-Br) followed a two stage degradation process in which the first step might be attributed to the degradation of −CH2Br, while the second step corresponds to the degradation of the main chain. However, the weight loss of poly(COE-TEMPO) started at lower temperatures due to the decomposition of the TEMPO moieties attached. The observed further decompositions may arise from the degradation of −CH2Br groups. PE-g-PS represents small weight loss at low temperatures related with TEMPO and bromide groups, while the main loss appears at higher temperatures corresponding to PE and PS segments. In summary, we have demonstrated for the first time that by using manganese carbonyl photochemistry TEMPO moieties can be incorporated to polyolefins from where the initiation of NMRP of styrene can easily be accomplished in a controlled manner. The ROMP induced halogenation, TEMPO functionalization by photochemical halide side chain activation and NMRP may lead to new possibilities for polyolefin modifications. Mn2(CO)10/TEMPO protocol may also be applicable in radical coupling reactions which are in great demand in functionalization and modification processes.
Figure 1. 1H NMR spectra of poly(COE-Br), poly(COE-TEMPO), and PE-g-PS.
Figure 2 shows the GPC curves of the precursor polymer and the final graft copolymers. As can be seen, GPC traces of both
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00460. All experimental procedures and additional spectral data (PDF).
Figure 2. GPC traces of poly(COE-TEMPO), PE-g-PS(1) and PE-gPS(2).
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AUTHOR INFORMATION
Corresponding Author
of the grafted copolymers shift to the higher elution volumes indicating the increase in the molecular weight after the graft copolymer formation. The thermal behavior of the graft copolymer was also investigated and compared with that of the precursor polymers (Figure 3). Poly(COE-Br) possessed a glass transition peak at −27 °C, which is in good agreement with literature.33 While PE-g-PS(1), due to long PS chains, did not exhibit notable glass transitions assignable to PE segments, in the case of PE-g-PS(2) each polymer segment displayed its characteristic Tg value. These results provide further evidence for the successful grafting process.
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Istanbul Technical University, Research Fund, for financial support. One of the authors (M.C.) would like to thank TUBITAK for the financial support by means of a graduate program. M.R.B. wishes to thank the Deutsche Forschungsgemeinschaft (DFG, BU 2174/14−1) for financial support. 948
DOI: 10.1021/acsmacrolett.6b00460 ACS Macro Lett. 2016, 5, 946−949
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ACS Macro Letters
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