Cationic End-Functional Polyethylene via Catalyzed Chain Growth

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Cationic End-Functional Polyethylene via Catalyzed Chain Growth: Synthesis, Mass Spectrometry, and Applications Byron Helmut Staudt, Jannik Wagner, and Philipp Vana* Institute of Physical Chemistry, Georg-August-University Göttingen, Tammannstr. 6, D-37077 Göttingen, Germany

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ABSTRACT: We report the controlled synthesis of polyethylene (PE) via catalyzed chain growth (CCG) carrying a cationic end-group. CCG and subsequent iodine treatment give rise to both low dispersed PE and a high degree of end-group functionalization. The end-group can be further substituted by 3-dimethylamino-1-propanol leading to a well-defined cationic functionality, as validated by NMR spectroscopy. The positive charge enables matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) and electrospray ionization mass spectrometry (ESI-MS) of these PE chains with an excellent signal-tonoise ratio. Moreover, the charged PE acts as a phase transfer catalyst (PTC) in a nucleophilic substitution with excellent yields even in comparison to well-established conventional PTCs. This new PE-based catalyst is completely recoverable by simple filtration due to its temperature-dependent solubility. On the basis of this powerful phase-transfer ability, we prepared charged PE loaded by an anionic dye via the extraction of the dye from aqueous to organic phase. Atom transfer radical polymerization (ATRP) initiator and acrylate-based end-groups were also introduced to the polymer system resulting in a positively charged PE-based macroinitiator/macromonomer, expanding the array of applications of this new type of polymer. Further block copolymerization of the ATRP functionalized PE with n-butyl acrylate was conducted successfully.

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INTRODUCTION Polyethylene (PE) and polypropylene (PP) are the most widely used thermoplastics in industry due to their low cost, good mechanical properties, and thermal resistance.1,2 However, the chemical modification of these very hydrophobic polyolefins, such as the incorporation of polar groups, is still a challenging topic.3,4 Recently, the catalyzed chain growth (CCG) or coordinative chain transfer polymerization, in which a transition metal catalyst is combined with a main group organometallic species that acts as chain transfer agent (CTA), was introduced.5 This method provides an easy access to endgroup-functionalized PE. Based on this approach, several endgroups such as iodine, hydroxyl, epoxide, azide, or amine have been successfully incorporated into PE.6−9 In addition, CCG is a controlled polymerization technique and allows the synthesis of narrowly distributed polyolefins.10−13 Today, soft ionization mass spectrometry techniques such as electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are well-established methods in polymer analysis providing fast access to accurate molar masses.14 Unfortunately, ionization of polyolefins is still difficult due to the lack of polar groups. One well-established route to overcome this limitation is the introduction of coordinative polar or vinylic groups providing access to ionization by metal ion adducts in MALDI mass spectrometry.15,16 Another approach that is mainly based on traditional polymerization techniques (anionic and Ziegler−Natta type) is the covalent incorporation of charged groups.17−19 One well© XXXX American Chemical Society

investigated and facile synthesis route is based on vinylterminated polyethylene. There, bromination and subsequent ionization via a nucleophilic substitution using triphenylphosphine were applied.17,19 In addition, permanently charged endgroups in PE enable not only MALDI but also ESI mass spectrometry.20,21 However, to the best of our knowledge, up to now no successful ESI mass spectrum of PE via the metal adduct ionization has been reported. Besides mass spectrometric analysis, charged end-groups of PE synthesized via anionic polymerization or oxidation of commercially available PE provide access to certain applications. This kind of PE can be used as a recyclable phase transfer catalyst (PTC). Because of its insolubility at ambient conditions, PE can easily be recovered by filtration after the reaction has occurred above the solution temperature.22−24 In the present work, we report an easy synthetic route to permanently positively charged PE via the combination of CCG and an efficient nucleophilic substitution. This system provides PE that can easily be analyzed both by ESI and MALDI mass spectrometry. Furthermore, we demonstrate the application of this material as PTC as well as the possibility to permanently attach negatively charged compounds to PE. Further functionalization reactions of the charged PE chains, e.g., the introduction of an atom transfer radical polymerization Received: August 6, 2018 Revised: September 27, 2018

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DOI: 10.1021/acs.macromol.8b01691 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Applied Synthetic Route of the Amino and Ammonium End-Group Functionalized PE

Blockcopolymerization Using ATRP Functionalized Polyethylene with BA. For the block copolymerization the following procedure was performed. The ATRP macroinitiator (1.0 equiv, 10 wt %) in dry toluene, dry anisole (10%), PMDETA (1.0 equiv), and CuBr (1.0 equiv) were mixed in a vial and degassed with argon. Dry and degassed n-butyl acrylate (100 equiv) was added, and the solution was heated to 100 °C in a heating block under vigorous shaking. After 2 h the polymer was precipitated in methanol and filtered off. The precipitate was redissolved in toluene at 90 °C, precipitated in methanol, filtered, washed several times with methanol, diluted hydrochloric acid, and dried under vacuum at 70 °C. Synthesis of Cyanodecane Using Different PTC. In a typical experiment, a solution of bromodecane (1.0 equiv) in octane (∼0.7 M) and a solution of sodium cyanide (4.0 equiv) in water (∼2.7 M) were mixed. To the two-phase system the respective phase transfer agent (0.02 equiv) was added, and the solution was heated to 100 °C. After cooling the solution to room temperature, the organic phase was dried under reduced pressure. Characterizations and Measurements. 1H NMR spectra were recorded using a Varian Unity 300 spectrometer (300 MHz) at a temperature of 80 °C in deuterated toluene with a delay of 15 s. The chemical shifts (δ /ppm) were referenced to the residual of the toluene signal. High temperature size-exclusion chromatography (HT-SEC) analysis was conducted on an Agilent G1888 net-work headspace autosampler, an Agilent 1260 pump, an Agilent 1322A degasser, and a PSS 246 interface. The applied equipment includes a polefin 10 μm precolumn and three polefin separation columns (103, 105, and 106 Å) was operated at 150 °C using 1,2,4-trichlorobenzene as eluent. A two-channel Q 4 IR detector was used. The HT-SEC was calibrated using PE reference standards with molar masses in the range 340−126000 g mol−1. All samples were dissolved in the eluent (3 mg mL−1) at 160 °C for at least 1 h prior to the measurement. ESI-MS measurements were performed with a Synapt G2 HDMS (Waters Corporation) mass spectrometer either in the sensitivity or in the resolution mode. The mass spectrometer was calibrated using a methanol solution of sodium iodide (100 mg mL−1). Samples of PE were prepared by dissolving PE in dichloroethane (5 mg mL−1) at 90 °C and precipitating the solution by cooling to room temperature. The suspension was then filtrated through a 0.45 μm syringe filter to obtain a clear saturated solution of PE. This solution was diluted with acetonitrile (1/1 by volume), and in the case of the tertiary amine terminated PE 0.1 wt % of acetic acid was added. This mixture was sprayed into the mass spectrometer using the following parameters: the ESI source temperature was 120 °C, the capillary voltage was 5.0 kV, the desolvation temperature was 150 °C, the source offset voltage was 80 V, and the cone voltage was 40 V. The scan time was 0.5 s, and each presented spectrum is the sum of 120 scans. MALDI-MS measurements were performed using an autoflex speed mass spectrometer (Bruker) in the reflectron mode to gain high resolution. DCTB and HABA proved to be the best matrices for polyethylene. The samples were prepared by mixing a solution of matrix (40 mg mL−1 in THF) and polyethylene (5 mg mL−1 precipitated in toluene at room temperature after dissolving at 90 °C) in a ratio of 4/1. In case of the tertiary amine terminated PE small amounts of acetic acid were added to the mixture to ensure protonation of the amine group. 1 μL of the mixed solution was hand-spotted on the stainless steel MALDI target plate. The MALDI instrument was calibrated with a

(ATRP) initiator followed by a block copolymerization, were also performed.

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EXPERIMENTAL SECTION

Materials. Ethylene (99.9%) and argon (99.999%) were purchased from Linde AG. Toluene (HPLC grade) was purchased from Sigma-Aldrich, degassed and stored over a molecular sieve. Butyloctylmagnesium (20 wt % in heptane) was purchased from Chemtura Europe GmbH. Tetrahydrofuran (Sigma-Aldrich) was distilled over CaH2 and stored over a molecular sieve. The catalyst (C5Me5)2NdCl2Li(OEt)2 was synthesized similarly to the literature.25 3-Dimethylamino-1-propanol, 1-methylamino-2-ethanol, iodine (doubly sublimated), 2-bromopropionyl bromide, pyridine, tetraphenylphosphonium bromide (TPPB), n-butyl acrylate, tetrabutylammonium bromide (TBAB), tetrabutylammonium iodide (TBAI), anisole, tetraoctylammonium bromide (TOAB), octane, N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), sodium cyanide, acetonitrile (MeCN), trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB), 1-bromodecane, dichloroethane (DCE), 2-(4-hydroxyphenylazo)benzoic acid (HABA), CuBr, and methacryloyl chloride were purchased from Sigma-Aldrich and, if not stated otherwise, used without further purification. Synthesis of Iodo End Functionalized Polyethylene. In comparison to the literature,7 the polymerization of ethylene was performed in a glass reactor at 80 °C. The reactor was filled with 400 mL of dry toluene, degassed using a vacuum pump, and afterward saturated with ethylene with a constant pressure of 2.5 bar. Butyloctylmagnesium and the neodymium catalyst in toluene (ratio [Mg]/[Nd] = 200:1) were added, and the reaction was conducted for 4 h. Afterward, the solution was cooled to 10 °C, iodine (10 wt %, 5 equiv/[Mg]) in THF was added, and the solution was stirred for 12 h. The polymer was precipitated in methanol, filtered, and washed several times with methanol. The collected polymer was dried under vacuum at 70 °C for 18 h. The iodo end-group functionalization was determined to at least 83% by 1H NMR. Synthesis of Ammonium/Tertiary Amine Functionalized Polyethylene. In a typical experiment, iodo end-functionalized PE (1.0 equiv) was dissolved in toluene (10 wt %) at 90 °C and stirred for 1 h. Afterward, an excess (at least 3.0 equiv) of the nucleophilic hydroxylamine (3-dimethylamino-1-propanol or 1-methylamino-2ethanol) was added, and the solution was stirred for 48 h at 90 °C. The solution was cooled to room temperature, and the polymer was precipitated, filtered, and washed several times with methanol. The collected polymer was dried under vacuum at 70 °C for 18 h. Synthesis of ATRP or Macromonomer Functionalized Charged Polyethylene. The corresponding hydroxy functionalized charged polyethylene was dissolved (1.0 equiv, 10 wt %) and degassed in dry toluene at 90 °C. To the solution either triethylamine or pyridine (1.5 equiv, dry, degassed) was added, followed by the dropwise addition of 2-bromopropionyl bromide or methacryloyl chloride (5.0 equiv) for 30 min. The reaction was allowed to stir at 90 °C for 2 h, cooled to room temperature, and quenched with methanol. Afterward, the polymer was precipitated in methanol, filtered, washed several times, and dried under vacuum at 70 °C for 18 h. B

DOI: 10.1021/acs.macromol.8b01691 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules poly(methyl methacrylate) (PMMA) sample of a known molecular mass as a sodium adduct species. Each spectrum is the sum of 500 laser shots and is shown without editing. The laser energy was adjusted for each sample to obtain a good mass resolution and high intensity signals.

From the NMR spectra (Figure 2B,C) can be extracted that the substitution reaction proceeded quantitatively since the signal of CH2−I has completely disappeared. The 1H NMR spectra of the products confirm the successful end-group modification as shown in Figure 2D. In all cases the broad multiplet at δ = 1.1−1.5 ppm belongs to the polymer backbone. The characteristic singlet of the methyl group of the tertiary amine appears at δ = 2.05 ppm (Figure 2B), and the signal of the two methyl groups next to the charged nitrogen is shifted to δ = 3.4 ppm in the case of the ammonium (Figure 2C) due to the higher electron-withdrawing properties. In both cases the methylene groups adjacent to nitrogen and oxygen atoms are shifted downfield compared to the singlet of the nitrogen-bound methyl groups. Characterization via Mass Spectrometry. To show the potential of the charged end-groups, we performed mass spectrometric analysis using MALDI-MS and even ESI-MS that is usually hindered by the lack of polar binding sites in polyolefins.14 Sample preparation is a crucial step in MALDIMS and strongly impacts the spectrum quality.26,27 Therefore, the matrix, the solvent, and the ratio of matrix to polymer have to be optimized carefully. We found that DCTB and HABA are suitable matrices for the investigated PE. In both cases PE-I did not produce any signal. To achieve a fine dispersion of PE within the matrix solution, PE was first dissolved in toluene at 90 °C. To ensure protonation of PE-N, a small amount of acetic acid was added. Afterward, the solution was cooled to room temperature, and the resulting suspension was immediately mixed with the matrix solution in THF. We obtained very intensive polymer signals showing very high signal-to-noise (S/N) ratios due to the careful adjustment of the laser power. Figure 3 shows the MALDI mass spectra of an ammonium (Figure 3A) and tertiary amine (Figure 3B) terminated PE. The PE-N+ produces a clear spectrum of high intensity without the appearance of any observable background peaks (S/N ratio up to 3000). Contrarily, the protonated tertiary amine end-group functionalized PE (PE-NH+) produces more background signals and a decreased S/N ratio. As shown in the inset (Figure 3A), all peaks are separated by a distance of 28 g mol−1 corresponding to one monomer unit (C2H4). The isotopic pattern is resolved distinctively in all cases, and the experimentally determined masses and isotopic patterns fit very well to the calculated ones (for details see Figure S1 and Table S2). The maximum of the molar mass distribution (Mp) in the MALDI mass spectrum for the ammonium and amine terminated PE is 636 and 664 g mol−1, respectively. However, the SEC chromatogram provides a Mp of ∼850 g mol−1. This discrepancy between both methods may be explained by mass discrimination effects in mass spectrometry that shift masses of polymers to lower values. Several reasons for those shifts are discussed in the literature. In general, higher molecular weight chains are more difficult to evaporate, and their transport efficiency as well as their detector sensitivity is lower.28,29 Additionally, the sample preparation may also contribute to the mass discrimination effect as mentioned before.26,27 Furthermore, SEC is a relative method, and the calibration was done with PE bearing two −CH3 as end-groups and with standards of partially high dispersity. The end-groups may have some influence on the hydrodynamic volume in our case since the molecular weights of the produced PE chains are relatively low. In addition to MALDI-MS, we were also able to perform ESIMS, which is more challenging due to the requirement of a

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RESULTS AND DISCUSSION Synthesis and Characterization via NMR and SEC. The amino and ammonium end-group functionalization of PE was conducted as shown in Scheme 1. Polyethylene was synthesized according to the CCG procedure using a neodymium precatalyst ((C5 Me5 ) 2 NdCl2 Li(OEt) 2) and butyloctylmagnesium acting as CTA. The resulting PE Grignard type intermediate (PE−Mg−PE) was converted in situ using molecular iodine in a highly efficient reaction yielding polyethylene iodide (PE-I).6 The attachment of a hydroxy-bearing amine was carried out in a nucleophilic substitution. The collected PE-I was reacted with 3dimethylamino-1-propanol or 1-methylamino-2-ethanol in toluene for 2 days to produce the respective ammonium (PE-N+) or tertiary amine (PE-N) terminated PE. At least a triple excess of the respective amine was required while the reaction was carried out at 90 °C to keep polyethylene in the dissolved state. The iodine and ammonium terminated PE was analyzed by HT-SEC (Figure 1).

Figure 1. HT-SEC analysis of PE-I (black curve) and PE-N+ (red curve). All spectra were recorded in 1,2,4-trichlorobenzene at 150 °C.

In both cases the characteristic values, i.e., the peak maximum (Mp = 850 g mol−1) as well as Mw = 780 g mol−1 and Mn = 650 g mol−1, are almost identical, indicating that the ionic group does not significantly interact with the SEC column. The calculated difference between the iodine and the ammonium end-group is only 23 g mol−1 and therefore cannot be resolved by the SEC analysis. The low dispersity (Đ = 1.19) confirms the good control of the polymerization system using the diorganomagnesium CTA. To validate the efficiency of the reaction and get detailed information about the end-group structure, NMR spectroscopy was performed. Figure 2A−C shows the respective NMR spectra recorded in deuterated toluene at 80 °C. The iodo end-group functionalization of PE was determined to be at least 83% by comparing the signal of the methylene group next to the iodine (CH2−I) at δ = 2.80 ppm and the methyl end-group at δ = 0.88 ppm (Figure 2A). C

DOI: 10.1021/acs.macromol.8b01691 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. 1H NMR spectra of iodo (PE-I), tertiary amine (PE-N), and ammonium terminated (PE-N+) polyethylene; all spectra were measured in deuterated toluene at 80 °C.

homogeneous solution with an appropriate polar solvent at room temperature. Neverthelessaccording to our experienceESI-MS results are more reproducible due to this homogeneous condition. Therefore, an initial solution of PE in DCE at 90 °C was precipitated by cooling to room temperature and afterward filtrated through a syringe filter (0.45 μm). The filtrate was diluted with varying amounts of MeCN whereas a mixture of 1/1 by volume of DCE and MeCN provides the best results. In the case of PE-N, 0.1 wt % acetic acid was added to ensure protonation. The obtained clear solutions of PE-N+ and PE-NH+ produce spectra (Figure 4) with low background and S/N ratios of ∼3000. However, in comparison to MALDI, a smaller number of chain lengths are detectable, and the higher molecular weight peaks have disappeared. Furthermore, smaller Mp values were observed. Similar observations were obtained before.21 The filtration step may have an influence on the detectable mass distribution due to their lower solubility of long polymer chains. In addition, we assume that the mass discrimination effects mentioned before are more prominent in the ESI instrument compared to the MALDI. Interestingly, the Mp of PE-N+ is 28 g mol−1 higher than the one of PE-NH+ which fits perfectly to the mass difference of their respective end-groups. The results above demonstrate that the introduction of a charged group is an excellent way to expand mass spectrometric analysis in the field of polyolefins. On the one hand, this offers the estimation of the molar mass and, on the other hand, the exact confirmation of the end-group Phase Transfer Properties. To demonstrate the advantage of this permanently charged PE, two different approaches were applied. In the first approachbased on a previously reported procedure for other type of polymers30,31the anionic dye methyl orange (MO) was diluted in water and then transferred into two vials. To both samples either pure toluene (blank sample) or a solution of PE-N+ in toluene was added. Initially, the anionic dye remained only in the lower, aqueous phase of both vials. To ensure a complete dissolution

Figure 3. MALDI mass spectrum of polyethylene terminated with PEN+ (A) or PE-NH+ (B). The inset in (A) shows a magnification of two adjacent peaks and with their respective isotopic pattern and their experimental masses as well as the deviation relative to the calculated masses.

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DOI: 10.1021/acs.macromol.8b01691 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. ESI mass spectrum of polyethylene terminated with an ammonium group (A) or a tertiary amine group (B).

of the polymer and therefore to reach a high flexibility of the charged PE end group, the vials were heated to 90 °C. After shaking the vials at 90 °C the MO was immediately transferred into the organic phase in the case of the PE-N+ containing sample while the MO remained in the aqueous phase in the blank sample (Figure 5).

Figure 5. A two-phase system of (l.h.s.) charged polyethylene in toluene, an anionic dye, and water, and (r.h.s.) toluene, an anionic dye, and water after shaking.

Figure 6. Dyed PE-N+ and mixed MO@PE before (A, B) and after (C, D) solvent treatment; for details see the text.

The driving force for this ion exchange of the iodide anion (the counterion of the PE-N+) and the anionic MO (Scheme S3) can be assigned to the strong solvatation of NaI in the aqueous phase in combination with the more hydrophobic methyl orange anion that is transferred quantitatively into the toluene phase. To prove the efficiency and strength of the ionic interaction between PE-N+ and methyl orange, the polymer containing yellowish hot toluene phase (Figure 5, left) was carefully removed using a glass pipet, and the solvent was removed in vacuum to yield solid and orange PE (PE-N-MO, Figure 6A). For comparison, commercially available, nonionic polyethylene was dissolved in hot toluene and mixed with an aqueous solution of MO and TBAB that works as a phase transfer agent. Because of the phase transfer the organic phase was also dyed with methyl orange, and therefore could be pipetted, and the solvent was removed in vacuum to give a yellowish PE (MO@PE, Figure 6B). Both solid and colorized PE were treated with different solvents like water, ethanol, and THF. The MO@PE immediately bleached out since the dye is only loosely incorporated into the PE (Figure 6D), whereas the PE with the ionic MO exhibits excellent stability against the applied solvents without any leakage of the dye (Figure 6C). From these observations it can be concluded that the anionic compound is entirely fixed to the PE-N+ due to the strong ionic interactions. In a double-check experiment the described procedure was repeated with the protonated tertiary amine (PE-NH+). Remarkably, in this case the ion exchange did not take place, and the organic phase remained colorless while the aqueous

phase persisted orange. This emphasizes the requirement and advantage of the permanently charged PE. The anionic dye MO was used as a model system, and these experiments show the possibility of a very stable incorporation of an anionic compound into PE. This knowledge may be transferred to other anionic compounds such as antibiotics for antibacterial properties of food packaging or nonleaking plasticizers and flame retarders.32,33 PE-N+ in Phase Transfer Catalysis. To explore a further application of permanently charged PE, its usage as a phase transfer catalyst (PTC)as described earlier for charged polyethylene synthesized via anionic polymerization22was investigated in detail. We focused on the comparison of PE-N+ with different well-established conventional phase transfer catalysts. As a model reaction, a nucleophilic substitution of sodium cyanide in water with bromodecane in octane was performed using 2 mol % of the respective PTC (Scheme 2). In addition to PE-N+, the reaction was also performed using TPPB, TBAB, TBAI, or TOAB as PTC (structures of the PTCs are shown in Figure S4) under identical conditions. A blank sample without any PTC was also tested. The resulting Scheme 2. Applied Model Reaction Using Various Phase Transfer Catalysts

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DOI: 10.1021/acs.macromol.8b01691 Macromolecules XXXX, XXX, XXX−XXX

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principle two different end-groups were introduced via an esterification reaction in accordance with a literature procedure (Scheme 3).34,35

yields of cyanodecane are given in Table 1. The yields were determined by comparing the integrals of methylene groups Table 1. Resulting Yields (Determined by 1H NMR) of Cyanodecane Using Various PTCs PTC

reaction time (h)

conversion (%)

blank TPPB PE-N+ TBAB TBAI TOAB PE-N+

4, 18 4 4 18 18 18 18

0 0 20