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The synthesis of cyclic hydroxy-phosphonate bearing polybutene using ROMP
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The synthesis of cyclic hydroxy-phosphonate bearing polybutene using ROMP Ilay Ceren Cetinkaya, Tarık Eren
⁎
Yildiz Technical University, Davutpasa Campus, Faculty of Arts and Science, Department of Chemistry, D-2023 Esenler/Istanbul, Turkey
A B S T R A C T
The synthesis of cyclic hydroxy-phosphonate bearing polybutenes using ring-opening metathesis polymerization (ROMP) has been accomplished. Reacting cyclooct5-ene-1,2-diol with diethyl chlorophosphate gave a new monomer, cyclic hydroxy-phosphonate functional cyclooctene (IV). A kinetic study of the ROMP of (IV) using Grubb’s second generation (G2) and third generation (G3) catalysts was carried out using 1H NMR spectroscopy. The G2 catalyst was more efficient than G3 and gave ~85% conversion over 1 h at ambient temperature. Kinetic constants are also calculated and reported. At the same time, cyclooctadiene (COD) was introduced as the co-monomer in a co-polymerization route, and the characterization and thermal properties of the resulting product were investigated using NMR, GPC, TGA and DSC. Homopolymer IV gave a 37% char yield at 600 °C under nitrogen. Upon increasing the phosphorus content in the co-polymer also enhanced the thermal properties. This polymer has also been explored for Cu purification under both liquid-liquid and solid liquid extraction.
1. Introduction Phosphorus-containing polymers have gained a lot of research attention, mainly in the biomedical field [1]. Phosphonates bearing polymers are one of the important intermediates used in the synthesis of a variety of bioactive products, such as aminophosphosphonates, aminophosphonic acids, hydroxyalkyl phosphonates, amidophosphates and nucleoside H-phosphonates [2]. The literature mainly describes the relevant studies carried out on these materials for various applications including dentistry [3], regenerative medicine, tissue engineering [4] and drug delivery [5–9] due to their good hemocompatibility, biocompatibility and protein adsorption resistance. For example, α-hydroxy phosphonates are important biologically active compounds, which are widely used in pharmaceutical applications such as enzyme inhibitors, antibacterial agents and insecticides [10–15]. Another field of application is to be used as catalyst. A cyclic phosphoric acid derivative has been used as a chiral Brønsted acid and efficient catalyst for the synthesis of a variety of α-phosphonate derivatives in good yield [16–20]. However, one should in keep in mind that the acidic proton in phosphoric acid may be exchanged with metal ions, such as Ca2+ present in the silica gel during the purification of catalyst [19]. However, the presence of a small amount of metal contaminant in phosphonic acid can also catalyze the reaction. Polystyrene-supported phosphoric acid has been applied as a catalyst to prepare enantio-enriched compounds in good yield [21]. Various cyclic organic phosphoric acids have been used as catalysts in the polymerization of caprolactone [22]. The overall conversion (96%) and
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polydispersity index (PDI) (1.13) was improved. Immobilized phosphoric acid catalysts have been proven robust and capable of being repeatedly recycled. Phosphorus-containing polymers have also been mainly used in flame retardants, fuel cell membranes and metal extractions. Nowadays, the use of brominated aromatic flame retardants in a wide range of products, such as textiles and electronics, is now limited due to the environmental concerns in regard to the release of hydrogen halide containing gas upon their combustion. Therefore, the development of new flame retardants is required. Non-leaching from the polymer matrix and inherent flame retardancy without the addition of any flame retardant additives is one of the growing strategies used in this field. The incorporation of phosphorus molecules into the polymer chains via polymerization, surface modification or blending results in enhanced homogeneity and flame retardancy overtime due to the non-leaching of the flame retardant from the polymer matrix [23–28]. Along with the development of advanced influent proton-conducting fuel cell membranes, phosphorus-based polymers have been combined with sulfonic acid materials for advanced thermal and chemical featuries [3,29–31]. Phosphonic acid bearing polymers can be used at temperatures above the boiling point of water. Phosphonic acid groups display amphoteric character, which results in a high degree of proton self-dissociation under anhydrous conditions [32]. The third important feature of phosphonic acid type compounds have been shown applications in the separation and recycling of metals [33,34]. Phosphonic acid based polymer were investigated for the removal of monodivalent and trivalent cations under different experimental conditions
Corresponding author at: Yildiz Technical University, Chemistry Department, 34220 Esenler, Istanbul, Turkey. E-mail address: [email protected] (T. Eren).
https://doi.org/10.1016/j.eurpolymj.2019.109318 Received 19 August 2019; Received in revised form 19 October 2019; Accepted 21 October 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Ilay Ceren Cetinkaya and Tarık Eren, European Polymer Journal, https://doi.org/10.1016/j.eurpolymj.2019.109318
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should form (Z)-diethyl (8-hydroxycyclooct-4-en-1-yl) phosphate (V). However, the cyclic hydroxyl phosphonate (IV) was formed instead of the expected acyclic adduct. Different reaction conditions were used in the synthesis of compound IV including 5 d at room temperature (25 °C) in dichloromethane; 6 h at 60 °C in tetrahydrofuran; and 2 h at 90 °C in toluene. The highest yield was found upon heating at 90 °C in toluene and these reaction conditions were applied in the synthesis of IV. The proposed mechanism for the formation of the cyclic hydroxyl-phosphonate is given in Scheme 3. Initially, diol III attacks diethyl chlorophosphate with the loss of chloride to give the corresponding phosphonate ester that undergoes a second nucleophilic substitution reaction with the second hydroxyl group to give the 5-membered ring. The last step is the transformation of the cyclic phosphonate ester to the corresponding phosphonate via an Arbuzov reaction [54]. The methanolysis of methylethylene phosphate takes place has been observed to exclusively occur under basic conditions [55]. It has also been reported that nucleophilic displacement in the five-membered ring is more rapid than that found in the six-membered and its open-chain analogous [56]. The driving force for the five-membered ring formation is attributed to thermodynamic favorability with the release of strain during hydrolysis and thus, has a favorable entropy of activation. According to the Baldwin rule, ring closure processes related with the stereochemical requirements of the transition state. We envisaged that triethylamine might play a crucial role in the synthesis. Mono-epoxide COD (II) [39] and (Z)-cyclooct-5-ene-1,2-diol [51] (III) were synthesized and characterized according to the literature. Fig. 1 shows the 1H, 13C and 31P NMR spectra recorded for the cyclic phosphorus-containing monomer (IV). The olefinic hydrogen (Ha) was observed at δ = 5.59 ppm (Jab = 11.0 Hz). The protons (eCHdeOe) on the cyclic phosphonate are observed at δ = 4.36 ppm (dt) and the hydrogen of the eOH group indicated by the letter Hf was observed at δ = 4.00 ppm. The eCH2e protons in the cyclooctadiene ring indicated by the letter Hb appear at δ = 1.16 ppm and the remaining eCH2e protons (Hc and Hc′) close to the cyclic phosphonate ring were observed at δ = 1.66 and 2.16 ppm, respectively due to the close proximity Hc′ to the phosphonate center. The peak at δ = 1.18 ppm belongs to the floating water peak. The 13C NMR spectrum of IV shows four carbon signals. The carbon atoms in the cyclic olefin (C1) and the carbon atom (eCCHOe) (C4) were observed at δ = 129.29 and 83.83 ppm, respectively. The allylic carbon atom (eCCHOe) (C2) appears at δ = 32.01 ppm and the eCH2e carbons in the ring (C3) were observed at δ = 23.69 ppm. The 31P NMR spectrum shows a phosphorus peak at δ = 16.85 ppm (Fig. 1, inset spectrum). In the MS Q-TOF analysis of compound IV purified on silica gel, two major m/z peaks at 227.04481 and 204.05546 and one minor m/z peak at 409.11712 were observed, which indicate [IV + Na]+, [IV]+and [2 IV + H]+, respectively. Therefore, IV purified on silica gel involved at least Na. We did not purify the metal salt via washing with HCl solution and compound IV was used in the subsequent polymerization reaction. Afterwards monomer IV and COD were converted into their homoand co-polymers via ROMP using Grubb’s 2nd generation catalyst (G2). The homopolymer of IV with a theoretical molecular weight of 25,000 g/mol (PolyIVa) and 50,000 g/mol (PolyIV)b were investigated. Two series of co-polymers based on IV and COD were synthesized with varying the composition as poly(IV-COD)a and poly(IVCOD)b with a theoretical molecular weight of repeating units of 25,000:25,000 g/mol and 25,000:50,000 g/mol, respectively. Increasing the molecular weight of polyIV resulted in polymers, which were insoluble in a wide range of common solvents. It was also observed that the theoretically calculated molecular weight of the homo-/co-polymers were lower value than the targeted molecular weight (Table 1). We argue that the ROMP of the cyclic ring is a typical equilibrium-controlled polymerization that deviates from the theoretical molecular weight (Mw) and results in the formation of low-Mw oligomers. However, deactivation of catalyst via phosphonic acid chelation can not be ruled out. The ratio of the number of repeating units
[35]. It was demonstrated that increasing the phosphonate ligand groups cause an increase in the maximum retention capacity through the electrostatic interaction between the polymer backbone and metals ions. Polymers derived via ring-opening metathesis polymerization (ROMP) that possess pendant chelating functional group have been used to achieve metal ion binding [36–38]. For example, Sessler and coworkers reported polymers bearing pendant picolinic acid that have been synthesized via ROMP and further have been used for selective Cu (II) extraction [38]. Along with the development of functional group tolerant and highly active catalysts, olefin metathesis reactions comprising ROMP have recently become of great importance [6,7,39–46]. In order to prepare linear polymers with controllable molar mass, architecture and mechanical and thermal properties, ROMP has been proven as a powerful method. ROMP involving cyclic olefins, such as cyclooctene (COE) and cyclooctadiene (COD), can be used to prepare a variety of polyalkanamers that have extensive potential in a diverse range of fields [6,9,47]. Hydrogenation of poly(cis,cis-cyclooctene (PCOE) and poly(cis,cis-1,5-cyclooctadiene (PCOD) yields the polyethylene structure. Functional groups in the cis-cyclooctene and cis,cis1,5-cyclooctadiene monomers result in the unique properties in the assynthesized functionalized polyethylene materials [48]. To date, the most widely used catalysts for ROMP are ruthenium complexes [49,50]. Ruthenium-based catalysts, such as G2 and G3, were designed by Robert H. Grubbs and co-workers (Scheme 1) [3,51]. More efficient controllability in the polymerization and living features are observed when using the third generation Grubbs (G3) catalyst due to the enhanced initiation of the catalyst [52,53]. Polymeric hydroxy functional cyclic phosphonate is missing in the literature. Furthermore, there are many functionalities that can be obtained via the hydroxyphosphonates intermediate. Compound IV (Scheme 2) possesses multi-functional properties in which the presence of phosphonic acid provides both a Brønsted acidic site (eOH) and Brønsted basic site (]O) to simultaneously activate the electrophile and nucleophile, and a 8-membered ring olefinic structure that can undergo ROMP. In addition, functional groups bearing hydroxy phosphonate in the 8-membered monomer can give an opportunity to prepare a range of architectures after the co-polymer synthesis via ROMP. Consequently, multi-functional polymers bearing hydroxy phosphonate can be tailored using this synthetic strategy with applications in polymeric catalysis, flame retardants and metal chelation. Herein, we extend this approach to the synthesis of cyclic hydroxyphosphonatebearing polybutenes via ROMP (Scheme 2). 2. Results and discussion COD (I) was initially converted to the mono-epoxide, (9-oxa-bicyclo [6.1.0]non-4-ene) (II) using m-chloroperbenzoic acid [39]. The epoxyfunctionalized cyclooctene was then transformed into (Z)-cyclooct-5ene-1,2-diol (III) using aqueous sulfuric acid [51]. This compound was then reacted with diethyl chlorophosphate to obtain the 8-membered cyclic phosphonate functionalized compound (IV) that can undergo the metathesis reaction. We expected that the reaction of III and diethyl chlorophosphate
Scheme 1. The G2 and G3 catalysts used in this study. 2
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Scheme 2. The synthesis of cyclic phosphorus-containing monomer (IV) and the polymerization reaction using ROMP.
Scheme 3. The possible pathway for the formation of the five-membered ring formation in the cyclic phosphorus-containing monomer.
3
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Fig. 1. The 1H (upper) and
13
C NMR (lower) spectra of monomer IV. The inset is the corresponding
31
P NMR spectrum of IV.
interaction of the column material with the hydroxy phosphonate in the polymer structure. A large deviation was observed between the theoretical and experimental molecular weights, as shown in Table 1. For example, the target molecular weight for the poly(IV-COD)a co-polymer was 50,000 g/mol, however, it was observed to be ~10,000 g/mol. We assume that backbiting and cross-metathesis causes an uncontrolled polymer distribution. However, end group analysis was also used to determine the Mn value of the polymers (Table 1). The relative integration of the peaks corresponding to the repeating units and the styrenic end group of the catalyst observed at δ = 7.3 ppm was used to calculate the Mn of the polymers. Proton attached to cyclic phosphonate at 4.1 ppm was used to calculate the molecular weight of the molecular weight of the poly(IV)a homopolymer. The calculation is applied
(m and n, Scheme 2) and molecular weight of the polymers were calculated using 1H NMR spectroscopy. For example, the hydrogen present in the five-membered ring (Hd, Fig. 1) observed at δ = 4.3 ppm indicates two protons, while the olefinic region possesses both double bonds coming from COD and monomer IV. (See supplementary information for further details). It was found that the ratio of monomers in copolymer series poly(IV-COD)a and poly(VI-COD)b were found to be 4 and 1.13, respectively (Table 1). The deviation observed between the theoretical and experimental values was due to the reactivity and conversion of each monomer during the metathesis reaction. The molecular weight distribution and the polydispersity index of the polymers were determined using THF-GPC against linear polystyrene standards (Fig. 2). The polydispersity indices were in the range of 1.06–1.7. However, analysis may not be accurate due to the 4
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Table 1 The synthesis and characterization data forthe homo-and co-polymers obtained from the ROMP of hydroxy phosphonate-bearing monomer IV. ma
Polymer
poly (IV)a poly(IV-COD)a poly(IV-COD)b
mb
– 110 400
53 100 95
Mole ratio (m/n)c
– 0.53 0.26
Mole ratio (m/n)d
– 0.13 0.23
Mn,
GPC
3884 10,041 32,184
PDI
1.06 1.54 1.73
Mn,
NMR
10,820 32,315 62,595
P% content
cis/trans %
(Theoretical)
(Determined via NMR)
15.39 9.91 7.36
15.81 9.90 3.52
16/84 19/81 7/93
ma: Number of repeating units for compound IV. mb: Number of repeating units for COD. Mole ratio (m/n)c: Determination of molar ratio using theoretical calculations. Mole ratio (m/n)d: Determination of molar ratio using 1H NMR spectroscopy. Mn, NMR (g/mol): Determination of the number average molecular weight using 1H NMR spectroscopy. Mn, GPC (g/mol): Determination of the number average molecular weight using GPC (g/mol). PDI: Determination of the polydispersity index using GPC (PDI = Mw/Mn). cis/trans %: Determination of the cis and trans ratio using 1H NMR spectroscopy. The target number average molecular weight using theoretical calculation is poly (IV)a = 25,000 g/mol; poly(IV-COD)a = 50,000 g/mol; poly(IV-COD) b = 75,000 g/mol. P%(determined via NMR) = [(ma × 31 g/mol) × 100]/Mn,NMR. poly( IV-COD)b
55
IV (228 g/mol) and COD (108 g/mol) g/mol to calculate observed molecular weight (Table 1). For example, the molar ratio observed for each repeating unit of poly(IV-COD)a was then multiplied by the molecular weight of monomers to evidence for Mn,NMR = (204,16 × 100) + (108,18 × 110) = 32,315 g/mol. In addition, the average olefin geometry in the polymer backbone was evaluated using 1H NMR spectroscopy and observed to be mainly the transisomer (~80–93%). In poly(IV)a, the high amount of trans-isomer gives an olefinic coupling constant of 15.3 Hz and the small amount of cisisomer gives an olefinic coupling constant of 11.0 Hz [57]. IR spectrum of each polymer indicated cis and trans repeat units. Strong absorption olefinic out of plane C-H bending displayed at 960–980 cm−1 and cis olefins at 665–730 cm−1 (see Supplementary Information Fig. S11). The olefinic region of the 13C NMR spectra of these polymers also provide evidence for assigning the microstructure. Stereorandom homo and copolymerization with a possible repeat units head-to-head (HH), head-to-tail (HT), and tail-to-tail (TT) scenario lead to multiplicity or broadening in the olefinic region (e.g., see Fig. S12). The carbon atom to which the cyclic phosphorus attached observed at around 82 ppm. We obtained predominantly trans-isomers as a result of the favorable catalyst metal center. The substitution on the cyclooctene ring may effect the conformation of the ring and influence the orientation of catalyst during the propagation step. Favorable trans geometry suggest that propagating polymer directed away from the cyclic phosphonate ring substituted cyclooctene. However, it should also be kept in mind that regiospecificity may also influenced by the interchain metathesis as well. The regio-and stereoselectivity play an important role in the polymer properties such as the thermo-mechanical, glossy and properties, etc. It has been reported that the lower the trans content in the polymer backbone lowers the melting transition and crystallinity of the
poly(IV)a poly(IV-COD)a
Refractive Index (mv)
50 Mn : 32184 PDI: 1.731
45 Mn :10041 PDI: 1.537
40
35 Mn :3894 PDI: 1.059
30 20
22
24
26
28
30
Retention Volume (ml) Fig. 2. GPC graph for the polymers.
according to the following formula:
DP = [(Ihydroxyphosphonate /1)/(Istyrenendgroup /5)] = 53 Mn,NMR = DP × 204.160 (Mw of repeating unit). and the observed molecular weight was 10,820 g/mol. Calculation of molecular weight of the copolymers follows first the calculation of degree of polymerization of monomer IV and then subtracting the olefinic region signals. The molar ratio observed for each repeating unit of copolymer series was then multiplied molecular weight of monomer
Fig. 3. The efficiency of the G2 and G3 catalysts in the ROMP as monitored using 1H NMR spectroscopy. 5
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constant observed as 0.055 min−1. The presence of the N-heterocyclic carbene and the 3-bromopyridine ligand greatly increase the initiation rate and enhance the activity of G3 [63]. It is assumed that monomer IV is not completely incorporated into the G3 catalyst. The unsuccessful polymerization was attributed to a strong coordination between the hydroxy phosphonate group and Ru-center, which results in insufficient co-ordination between the olefin group and metal center. The second hypothesis is the complexation of the labile bromopyridine ligand on G3 to the hydroxyl phosphonate group, which produces a base-acid complex. This leads to a phase separation in the polymerization that prevents coordination between the catalyst and monomer. It is noteworthy that the hydroxylphosphonate groups present in monomer IV do not affect the catalytic activity of G2 as much as G3. The thermal properties of the as-synthesized polymers were investigated using DSC and thermogravimetric analysis (TGA). The effects of the phosphorus functionality and quantity on the observed thermal transition were analyzed. We report the results of second heat cycle of polymers in DSC analysis. All the polymers were likely to be amorphous as no melting temperature (Tm) was observed in the DSC spectra (see supporting info Fig. S13). Only the poly(IV-COD)b co-polymer was observed to be semi-crystalline, which indicated a melting transition at ~ 1.5 °C. The co-polymer series, poly(IV-COD)a and poly(IV-COD)b, were also analyzed. Decreasing the monomer IV content to 3.5 wt% in the backbone of the poly(IV-COD)b polymer increased the movement between the chain segments and flexibility and formed a Tm in the co-polymer. DSC indicated that no other distinctive phase transitions occurred. TGA was carried out under a nitrogen atmosphere in the temperature range of 25–600 °C. The influence of the phosphorus content on the thermograms was investigated and the result shown in Fig. 5. Table 1 shows the P content in the polymer series, which was calculated via end group and repeating group analysis using 1HNMR spectroscopy. As expected, the char yield increases with an increase in the phosphorus content, which is also in accordance with the work of Eren and coworkers [64]. Thermal resistance of homopolymer was the highest (~33% char) due to the presence of high level P content in the polymer backbone. The poly(IV-COD)b co-polymer containing 3.52 wt% phosphorus started to decompose at 280 °C with the remaining amount of carbon waste being ~ 4%. The poly(IV-COD)a co-polymer containing 9.90 wt% phosphorus started to decompose at 420 °C, leaving about 30% residue at the end of cycle. All of the polymers degrade at ~ 480 °C. To understand the mechanism, bare poly(cyclooctadiene) referred as poly(COD) was also synthesized. Almost all the polymer was completely decomposed during the TGA and gave 2% char at
Grubbs 2 Grubbs 3
100
Conversion (%)
80 60 40 20 0 0
10
20
30
40
50
60
Time (min.) Fig. 4. The total monomer (IV) conversion study of the for ROMP reaction using the G2 and G3 catalysts.
polymer [58]. The effect of the microstructure is described below while discussing the differential scanning calorimetry (DSC) analysis. In addition, monitoring the ROMP of IV with the G2 and G3 catalyst using 1H NMR spectroscopy (CD2Cl2, 25 °C) was performed. Fig. 3 show that the disappearance of the cyclic olefins at δ = 5.60 ppm and the appearance of new signal at δ = 5.38 ppm corresponding to the acyclic double bond resonance of the polymer. Integration of the olefinic signal at δ = 5.60 ppm was carried out using the TMS signal as a Ref. [59]. It was observed that < 10% conversion was obtained after 30 min. via using the G3 catalyst. In contrast, G2 gave an almost 80% conversion in 30 min and an overall 85% conversion after 1 h. The total monomer conversion during the ROMP is presented in Fig. 4. In particular, it has been reported that the initiation efficiency of G3 is much faster than G2 [60]. G3 generally promotes a controlled living polymerization with high ki/kp ratios. G2 usually displays a slow initiation rate and high propagation rate resulting in a broad PDI. We calculated the kinetic constants of each reaction to investigate the efficiencies of the each catalyst. Kinetic constant of the observed rate constant (kobs) can be defined as kobs = kp × [active center] where kp is the corresponding kinetic constant [61]. Active center can be defined as bearing catalyst and growing polymer chains. Table 2 shows the observed propagation constants. It was observed that propagation constants higher for G2 than G3 under all concentration range used in the experiment. Observed propagation rate constant (kobs) in the presence of G2 ranging from 0.039 to 0.062 min−1 were determined in deutorochloroform which lower than the reported values for cis-cyclooctene [62]. The propagation rate constant decreased gradually when the monomer concentration is raised in the presence of G3 catalyst are through that increasing the phosphonic acid content retard the speed of the reaction dramatically. In spite of that, when using G2 catalyst there is no well correlation between rate constant rate and concentration doubled, rate constant drops from 0.062 to 0.039 min−1. However, when the four times higher concentrated solution was used rate
poly(IV-COD)a
[M]/[I]
kobs (min
2a 2b 2d 3a 3b 3d
30 60 120 30 60 120
0.062 0.039 0.055 0.030 0.020 0.010
poly(IV)a
char yield (%)
80
Table 2 Observed propagation constants under different conditions for the ROMP of compound IV in CCl3D (M = monomer, I = initiator, 2 = G2, 3 = G3) ([M] = 0.9 M). Entry
poly(IV-COD)b
100
poly(COD)a
60 40
33% 24%
20
−1
)
4% 2%
0 0
100
200
300
400
500
600
temperature (°C) Fig. 5. The TGA results obtained for the as-synthesized polymers prepared using the G2 catalyst. 6
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Ersin Acar (Bogazici University, Istanbul, Turkey) for his insightful discussions.
600 °C. The flame retardant function of phosphorus content is related to the condensed and vapor phase [65,66]. The condensed phase is related with the amount of char formed as a result of the formation of polyphosphonic acid, which catalyzes aromatization at high temperature. However, vapor phase retardation is related with the release of phosphorus containing radicals and gases such as·PO·and·PO2·derivatives, which contribute to the extinction of the flame. We expect that hydroxyl phosphonate functionalized polybutene retards both the condensed and vapor phase. However, further analysis including the limiting oxygen index (LOI), UL94 and combustion calorimetry tests should be investigated to analyze the flame retardancy behavior of these polymers. Metal chelation efficiency of phosphonic acid bearing polymer was also investigated. Extraction of Cu(II) ions was conducted in solid liquid and liquid liquid-extraction. At the end of each reactions, the amount of metal ion in the water phase was investigated by inductively coupled plasma mass spectrometry (ICP-MS). Extraction studies were carried out in 20 mL vial for 24 h at room temperature. In the liquid-liquid phase extraction, the water-phase copper ions were mixed with the dichloromethane phase homopolymer poly(IV) for 24 h. The initial concentration of copper ion was determined to be 1 mM and 0.025 g of homopolymer was dissolved in dichloromethane. In the solid-liquid extraction study, the same amount of polymer was added to the copper solution at a concentration of 1 mM as the solid and reacted under the same conditions. In the solid-liquid extraction test, the polymer was adsorbed to 7.24 ± 0.21% Cu (II). However, the amount of copper passing to the dichloromethane phase was 6.17 ± 0.18% in liquid-liquid extraction. The polymer-free dichloromethane phase was investigated by the same experimental conditions and it was observed that the 0.54 ± 0.02% Cu (II) ion observed in dichloromethane phase. For the Cu chelation experiment, a commercial polymer poly(styreneco-maleic anhydride) (SMA) was also used to extract copper ions from aqueous solution as a standard for comparison [67–69]. Under the same experimental conditions, SMA retained 6.82 ± 0.19% Cu ions and consequently extracted less Cu ions than poly(IV).
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3. Conclusion In conclusion, we have reported the synthesis of cyclic phosphonates bearing polycyclooctene using ring-opening metathesis polymerization (ROMP). The method involves the derivatization of 1,5-cyclooctadiene (COD) with a phosphonate functionality (monomer IV) and the synthesis of its homo-polymer and co-polymer with 1,5-cyclooctadiene (COD).The efficiency of Grubb’s second generation catalyst (G2) was higher than Grubb’s third generation catalyst (G3) during the polymerization of the monomer (IV). The phosphorus content enhanced the thermal properties of the resulting polymers. More importantly, the chemical reactivity of the phosphonate-bearing polymers with well-defined architectures is useful in various derivatization reactions and their application. This new polymer could be used toward metal ion extractions and might be used in the purification of Cu radioisotopes. In addition, the phosphorus-containing polymers will serve as an example for self-flame retardant polymers. The formation of high molecular weight polymers and investigations into their regio-and stereoselectivity as well as controlling the molecular weight by adding a chain transfer agent such as cis-4-octene are currently in progress. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgment This work was supported by TUBITAK (Project No. 114Z666) and EU COST Action 1302. The authors would also like to thank Assoc. Prof. 7
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The synthesis of cyclic hydroxy-phosphonate bearing polybutene using ROMP Ilay Ceren Cetinkaya, Tarık Eren
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Yildiz Technical University, Davutpasa Campus, Faculty of Arts and Science, Department of Chemistry, D-2023 Esenler/Istanbul, Turkey
A B S T R A C T
The synthesis of cyclic hydroxy-phosphonate bearing polybutenes using ring-opening metathesis polymerization (ROMP) has been accomplished. Reacting cyclooct5-ene-1,2-diol with diethyl chlorophosphate gave a new monomer, cyclic hydroxy-phosphonate functional cyclooctene (IV). A kinetic study of the ROMP of (IV) using Grubb’s second generation (G2) and third generation (G3) catalysts was carried out using 1H NMR spectroscopy. The G2 catalyst was more efficient than G3 and gave ~85% conversion over 1 h at ambient temperature. Kinetic constants are also calculated and reported. At the same time, cyclooctadiene (COD) was introduced as the co-monomer in a co-polymerization route, and the characterization and thermal properties of the resulting product were investigated using NMR, GPC, TGA and DSC. Homopolymer IV gave a 37% char yield at 600 °C under nitrogen. Upon increasing the phosphorus content in the co-polymer also enhanced the thermal properties. This polymer has also been explored for Cu purification under both liquid-liquid and solid liquid extraction.
1. Introduction Phosphorus-containing polymers have gained a lot of research attention, mainly in the biomedical field [1]. Phosphonates bearing polymers are one of the important intermediates used in the synthesis of a variety of bioactive products, such as aminophosphosphonates, aminophosphonic acids, hydroxyalkyl phosphonates, amidophosphates and nucleoside H-phosphonates [2]. The literature mainly describes the relevant studies carried out on these materials for various applications including dentistry [3], regenerative medicine, tissue engineering [4] and drug delivery [5–9] due to their good hemocompatibility, biocompatibility and protein adsorption resistance. For example, α-hydroxy phosphonates are important biologically active compounds, which are widely used in pharmaceutical applications such as enzyme inhibitors, antibacterial agents and insecticides [10–15]. Another field of application is to be used as catalyst. A cyclic phosphoric acid derivative has been used as a chiral Brønsted acid and efficient catalyst for the synthesis of a variety of α-phosphonate derivatives in good yield [16–20]. However, one should in keep in mind that the acidic proton in phosphoric acid may be exchanged with metal ions, such as Ca2+ present in the silica gel during the purification of catalyst [19]. However, the presence of a small amount of metal contaminant in phosphonic acid can also catalyze the reaction. Polystyrene-supported phosphoric acid has been applied as a catalyst to prepare enantio-enriched compounds in good yield [21]. Various cyclic organic phosphoric acids have been used as catalysts in the polymerization of caprolactone [22]. The overall conversion (96%) and
⁎
polydispersity index (PDI) (1.13) was improved. Immobilized phosphoric acid catalysts have been proven robust and capable of being repeatedly recycled. Phosphorus-containing polymers have also been mainly used in flame retardants, fuel cell membranes and metal extractions. Nowadays, the use of brominated aromatic flame retardants in a wide range of products, such as textiles and electronics, is now limited due to the environmental concerns in regard to the release of hydrogen halide containing gas upon their combustion. Therefore, the development of new flame retardants is required. Non-leaching from the polymer matrix and inherent flame retardancy without the addition of any flame retardant additives is one of the growing strategies used in this field. The incorporation of phosphorus molecules into the polymer chains via polymerization, surface modification or blending results in enhanced homogeneity and flame retardancy overtime due to the non-leaching of the flame retardant from the polymer matrix [23–28]. Along with the development of advanced influent proton-conducting fuel cell membranes, phosphorus-based polymers have been combined with sulfonic acid materials for advanced thermal and chemical featuries [3,29–31]. Phosphonic acid bearing polymers can be used at temperatures above the boiling point of water. Phosphonic acid groups display amphoteric character, which results in a high degree of proton self-dissociation under anhydrous conditions [32]. The third important feature of phosphonic acid type compounds have been shown applications in the separation and recycling of metals [33,34]. Phosphonic acid based polymer were investigated for the removal of monodivalent and trivalent cations under different experimental conditions
Corresponding author at: Yildiz Technical University, Chemistry Department, 34220 Esenler, Istanbul, Turkey. E-mail address: [email protected] (T. Eren).
https://doi.org/10.1016/j.eurpolymj.2019.109318 Received 19 August 2019; Received in revised form 19 October 2019; Accepted 21 October 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Ilay Ceren Cetinkaya and Tarık Eren, European Polymer Journal, https://doi.org/10.1016/j.eurpolymj.2019.109318
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should form (Z)-diethyl (8-hydroxycyclooct-4-en-1-yl) phosphate (V). However, the cyclic hydroxyl phosphonate (IV) was formed instead of the expected acyclic adduct. Different reaction conditions were used in the synthesis of compound IV including 5 d at room temperature (25 °C) in dichloromethane; 6 h at 60 °C in tetrahydrofuran; and 2 h at 90 °C in toluene. The highest yield was found upon heating at 90 °C in toluene and these reaction conditions were applied in the synthesis of IV. The proposed mechanism for the formation of the cyclic hydroxyl-phosphonate is given in Scheme 3. Initially, diol III attacks diethyl chlorophosphate with the loss of chloride to give the corresponding phosphonate ester that undergoes a second nucleophilic substitution reaction with the second hydroxyl group to give the 5-membered ring. The last step is the transformation of the cyclic phosphonate ester to the corresponding phosphonate via an Arbuzov reaction [54]. The methanolysis of methylethylene phosphate takes place has been observed to exclusively occur under basic conditions [55]. It has also been reported that nucleophilic displacement in the five-membered ring is more rapid than that found in the six-membered and its open-chain analogous [56]. The driving force for the five-membered ring formation is attributed to thermodynamic favorability with the release of strain during hydrolysis and thus, has a favorable entropy of activation. According to the Baldwin rule, ring closure processes related with the stereochemical requirements of the transition state. We envisaged that triethylamine might play a crucial role in the synthesis. Mono-epoxide COD (II) [39] and (Z)-cyclooct-5-ene-1,2-diol [51] (III) were synthesized and characterized according to the literature. Fig. 1 shows the 1H, 13C and 31P NMR spectra recorded for the cyclic phosphorus-containing monomer (IV). The olefinic hydrogen (Ha) was observed at δ = 5.59 ppm (Jab = 11.0 Hz). The protons (eCHdeOe) on the cyclic phosphonate are observed at δ = 4.36 ppm (dt) and the hydrogen of the eOH group indicated by the letter Hf was observed at δ = 4.00 ppm. The eCH2e protons in the cyclooctadiene ring indicated by the letter Hb appear at δ = 1.16 ppm and the remaining eCH2e protons (Hc and Hc′) close to the cyclic phosphonate ring were observed at δ = 1.66 and 2.16 ppm, respectively due to the close proximity Hc′ to the phosphonate center. The peak at δ = 1.18 ppm belongs to the floating water peak. The 13C NMR spectrum of IV shows four carbon signals. The carbon atoms in the cyclic olefin (C1) and the carbon atom (eCCHOe) (C4) were observed at δ = 129.29 and 83.83 ppm, respectively. The allylic carbon atom (eCCHOe) (C2) appears at δ = 32.01 ppm and the eCH2e carbons in the ring (C3) were observed at δ = 23.69 ppm. The 31P NMR spectrum shows a phosphorus peak at δ = 16.85 ppm (Fig. 1, inset spectrum). In the MS Q-TOF analysis of compound IV purified on silica gel, two major m/z peaks at 227.04481 and 204.05546 and one minor m/z peak at 409.11712 were observed, which indicate [IV + Na]+, [IV]+and [2 IV + H]+, respectively. Therefore, IV purified on silica gel involved at least Na. We did not purify the metal salt via washing with HCl solution and compound IV was used in the subsequent polymerization reaction. Afterwards monomer IV and COD were converted into their homoand co-polymers via ROMP using Grubb’s 2nd generation catalyst (G2). The homopolymer of IV with a theoretical molecular weight of 25,000 g/mol (PolyIVa) and 50,000 g/mol (PolyIV)b were investigated. Two series of co-polymers based on IV and COD were synthesized with varying the composition as poly(IV-COD)a and poly(IVCOD)b with a theoretical molecular weight of repeating units of 25,000:25,000 g/mol and 25,000:50,000 g/mol, respectively. Increasing the molecular weight of polyIV resulted in polymers, which were insoluble in a wide range of common solvents. It was also observed that the theoretically calculated molecular weight of the homo-/co-polymers were lower value than the targeted molecular weight (Table 1). We argue that the ROMP of the cyclic ring is a typical equilibrium-controlled polymerization that deviates from the theoretical molecular weight (Mw) and results in the formation of low-Mw oligomers. However, deactivation of catalyst via phosphonic acid chelation can not be ruled out. The ratio of the number of repeating units
[35]. It was demonstrated that increasing the phosphonate ligand groups cause an increase in the maximum retention capacity through the electrostatic interaction between the polymer backbone and metals ions. Polymers derived via ring-opening metathesis polymerization (ROMP) that possess pendant chelating functional group have been used to achieve metal ion binding [36–38]. For example, Sessler and coworkers reported polymers bearing pendant picolinic acid that have been synthesized via ROMP and further have been used for selective Cu (II) extraction [38]. Along with the development of functional group tolerant and highly active catalysts, olefin metathesis reactions comprising ROMP have recently become of great importance [6,7,39–46]. In order to prepare linear polymers with controllable molar mass, architecture and mechanical and thermal properties, ROMP has been proven as a powerful method. ROMP involving cyclic olefins, such as cyclooctene (COE) and cyclooctadiene (COD), can be used to prepare a variety of polyalkanamers that have extensive potential in a diverse range of fields [6,9,47]. Hydrogenation of poly(cis,cis-cyclooctene (PCOE) and poly(cis,cis-1,5-cyclooctadiene (PCOD) yields the polyethylene structure. Functional groups in the cis-cyclooctene and cis,cis1,5-cyclooctadiene monomers result in the unique properties in the assynthesized functionalized polyethylene materials [48]. To date, the most widely used catalysts for ROMP are ruthenium complexes [49,50]. Ruthenium-based catalysts, such as G2 and G3, were designed by Robert H. Grubbs and co-workers (Scheme 1) [3,51]. More efficient controllability in the polymerization and living features are observed when using the third generation Grubbs (G3) catalyst due to the enhanced initiation of the catalyst [52,53]. Polymeric hydroxy functional cyclic phosphonate is missing in the literature. Furthermore, there are many functionalities that can be obtained via the hydroxyphosphonates intermediate. Compound IV (Scheme 2) possesses multi-functional properties in which the presence of phosphonic acid provides both a Brønsted acidic site (eOH) and Brønsted basic site (]O) to simultaneously activate the electrophile and nucleophile, and a 8-membered ring olefinic structure that can undergo ROMP. In addition, functional groups bearing hydroxy phosphonate in the 8-membered monomer can give an opportunity to prepare a range of architectures after the co-polymer synthesis via ROMP. Consequently, multi-functional polymers bearing hydroxy phosphonate can be tailored using this synthetic strategy with applications in polymeric catalysis, flame retardants and metal chelation. Herein, we extend this approach to the synthesis of cyclic hydroxyphosphonatebearing polybutenes via ROMP (Scheme 2). 2. Results and discussion COD (I) was initially converted to the mono-epoxide, (9-oxa-bicyclo [6.1.0]non-4-ene) (II) using m-chloroperbenzoic acid [39]. The epoxyfunctionalized cyclooctene was then transformed into (Z)-cyclooct-5ene-1,2-diol (III) using aqueous sulfuric acid [51]. This compound was then reacted with diethyl chlorophosphate to obtain the 8-membered cyclic phosphonate functionalized compound (IV) that can undergo the metathesis reaction. We expected that the reaction of III and diethyl chlorophosphate
Scheme 1. The G2 and G3 catalysts used in this study. 2
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Scheme 2. The synthesis of cyclic phosphorus-containing monomer (IV) and the polymerization reaction using ROMP.
Scheme 3. The possible pathway for the formation of the five-membered ring formation in the cyclic phosphorus-containing monomer.
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Fig. 1. The 1H (upper) and
13
C NMR (lower) spectra of monomer IV. The inset is the corresponding
31
P NMR spectrum of IV.
interaction of the column material with the hydroxy phosphonate in the polymer structure. A large deviation was observed between the theoretical and experimental molecular weights, as shown in Table 1. For example, the target molecular weight for the poly(IV-COD)a co-polymer was 50,000 g/mol, however, it was observed to be ~10,000 g/mol. We assume that backbiting and cross-metathesis causes an uncontrolled polymer distribution. However, end group analysis was also used to determine the Mn value of the polymers (Table 1). The relative integration of the peaks corresponding to the repeating units and the styrenic end group of the catalyst observed at δ = 7.3 ppm was used to calculate the Mn of the polymers. Proton attached to cyclic phosphonate at 4.1 ppm was used to calculate the molecular weight of the molecular weight of the poly(IV)a homopolymer. The calculation is applied
(m and n, Scheme 2) and molecular weight of the polymers were calculated using 1H NMR spectroscopy. For example, the hydrogen present in the five-membered ring (Hd, Fig. 1) observed at δ = 4.3 ppm indicates two protons, while the olefinic region possesses both double bonds coming from COD and monomer IV. (See supplementary information for further details). It was found that the ratio of monomers in copolymer series poly(IV-COD)a and poly(VI-COD)b were found to be 4 and 1.13, respectively (Table 1). The deviation observed between the theoretical and experimental values was due to the reactivity and conversion of each monomer during the metathesis reaction. The molecular weight distribution and the polydispersity index of the polymers were determined using THF-GPC against linear polystyrene standards (Fig. 2). The polydispersity indices were in the range of 1.06–1.7. However, analysis may not be accurate due to the 4
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Table 1 The synthesis and characterization data forthe homo-and co-polymers obtained from the ROMP of hydroxy phosphonate-bearing monomer IV. ma
Polymer
poly (IV)a poly(IV-COD)a poly(IV-COD)b
mb
– 110 400
53 100 95
Mole ratio (m/n)c
– 0.53 0.26
Mole ratio (m/n)d
– 0.13 0.23
Mn,
GPC
3884 10,041 32,184
PDI
1.06 1.54 1.73
Mn,
NMR
10,820 32,315 62,595
P% content
cis/trans %
(Theoretical)
(Determined via NMR)
15.39 9.91 7.36
15.81 9.90 3.52
16/84 19/81 7/93
ma: Number of repeating units for compound IV. mb: Number of repeating units for COD. Mole ratio (m/n)c: Determination of molar ratio using theoretical calculations. Mole ratio (m/n)d: Determination of molar ratio using 1H NMR spectroscopy. Mn, NMR (g/mol): Determination of the number average molecular weight using 1H NMR spectroscopy. Mn, GPC (g/mol): Determination of the number average molecular weight using GPC (g/mol). PDI: Determination of the polydispersity index using GPC (PDI = Mw/Mn). cis/trans %: Determination of the cis and trans ratio using 1H NMR spectroscopy. The target number average molecular weight using theoretical calculation is poly (IV)a = 25,000 g/mol; poly(IV-COD)a = 50,000 g/mol; poly(IV-COD) b = 75,000 g/mol. P%(determined via NMR) = [(ma × 31 g/mol) × 100]/Mn,NMR. poly( IV-COD)b
55
IV (228 g/mol) and COD (108 g/mol) g/mol to calculate observed molecular weight (Table 1). For example, the molar ratio observed for each repeating unit of poly(IV-COD)a was then multiplied by the molecular weight of monomers to evidence for Mn,NMR = (204,16 × 100) + (108,18 × 110) = 32,315 g/mol. In addition, the average olefin geometry in the polymer backbone was evaluated using 1H NMR spectroscopy and observed to be mainly the transisomer (~80–93%). In poly(IV)a, the high amount of trans-isomer gives an olefinic coupling constant of 15.3 Hz and the small amount of cisisomer gives an olefinic coupling constant of 11.0 Hz [57]. IR spectrum of each polymer indicated cis and trans repeat units. Strong absorption olefinic out of plane C-H bending displayed at 960–980 cm−1 and cis olefins at 665–730 cm−1 (see Supplementary Information Fig. S11). The olefinic region of the 13C NMR spectra of these polymers also provide evidence for assigning the microstructure. Stereorandom homo and copolymerization with a possible repeat units head-to-head (HH), head-to-tail (HT), and tail-to-tail (TT) scenario lead to multiplicity or broadening in the olefinic region (e.g., see Fig. S12). The carbon atom to which the cyclic phosphorus attached observed at around 82 ppm. We obtained predominantly trans-isomers as a result of the favorable catalyst metal center. The substitution on the cyclooctene ring may effect the conformation of the ring and influence the orientation of catalyst during the propagation step. Favorable trans geometry suggest that propagating polymer directed away from the cyclic phosphonate ring substituted cyclooctene. However, it should also be kept in mind that regiospecificity may also influenced by the interchain metathesis as well. The regio-and stereoselectivity play an important role in the polymer properties such as the thermo-mechanical, glossy and properties, etc. It has been reported that the lower the trans content in the polymer backbone lowers the melting transition and crystallinity of the
poly(IV)a poly(IV-COD)a
Refractive Index (mv)
50 Mn : 32184 PDI: 1.731
45 Mn :10041 PDI: 1.537
40
35 Mn :3894 PDI: 1.059
30 20
22
24
26
28
30
Retention Volume (ml) Fig. 2. GPC graph for the polymers.
according to the following formula:
DP = [(Ihydroxyphosphonate /1)/(Istyrenendgroup /5)] = 53 Mn,NMR = DP × 204.160 (Mw of repeating unit). and the observed molecular weight was 10,820 g/mol. Calculation of molecular weight of the copolymers follows first the calculation of degree of polymerization of monomer IV and then subtracting the olefinic region signals. The molar ratio observed for each repeating unit of copolymer series was then multiplied molecular weight of monomer
Fig. 3. The efficiency of the G2 and G3 catalysts in the ROMP as monitored using 1H NMR spectroscopy. 5
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constant observed as 0.055 min−1. The presence of the N-heterocyclic carbene and the 3-bromopyridine ligand greatly increase the initiation rate and enhance the activity of G3 [63]. It is assumed that monomer IV is not completely incorporated into the G3 catalyst. The unsuccessful polymerization was attributed to a strong coordination between the hydroxy phosphonate group and Ru-center, which results in insufficient co-ordination between the olefin group and metal center. The second hypothesis is the complexation of the labile bromopyridine ligand on G3 to the hydroxyl phosphonate group, which produces a base-acid complex. This leads to a phase separation in the polymerization that prevents coordination between the catalyst and monomer. It is noteworthy that the hydroxylphosphonate groups present in monomer IV do not affect the catalytic activity of G2 as much as G3. The thermal properties of the as-synthesized polymers were investigated using DSC and thermogravimetric analysis (TGA). The effects of the phosphorus functionality and quantity on the observed thermal transition were analyzed. We report the results of second heat cycle of polymers in DSC analysis. All the polymers were likely to be amorphous as no melting temperature (Tm) was observed in the DSC spectra (see supporting info Fig. S13). Only the poly(IV-COD)b co-polymer was observed to be semi-crystalline, which indicated a melting transition at ~ 1.5 °C. The co-polymer series, poly(IV-COD)a and poly(IV-COD)b, were also analyzed. Decreasing the monomer IV content to 3.5 wt% in the backbone of the poly(IV-COD)b polymer increased the movement between the chain segments and flexibility and formed a Tm in the co-polymer. DSC indicated that no other distinctive phase transitions occurred. TGA was carried out under a nitrogen atmosphere in the temperature range of 25–600 °C. The influence of the phosphorus content on the thermograms was investigated and the result shown in Fig. 5. Table 1 shows the P content in the polymer series, which was calculated via end group and repeating group analysis using 1HNMR spectroscopy. As expected, the char yield increases with an increase in the phosphorus content, which is also in accordance with the work of Eren and coworkers [64]. Thermal resistance of homopolymer was the highest (~33% char) due to the presence of high level P content in the polymer backbone. The poly(IV-COD)b co-polymer containing 3.52 wt% phosphorus started to decompose at 280 °C with the remaining amount of carbon waste being ~ 4%. The poly(IV-COD)a co-polymer containing 9.90 wt% phosphorus started to decompose at 420 °C, leaving about 30% residue at the end of cycle. All of the polymers degrade at ~ 480 °C. To understand the mechanism, bare poly(cyclooctadiene) referred as poly(COD) was also synthesized. Almost all the polymer was completely decomposed during the TGA and gave 2% char at
Grubbs 2 Grubbs 3
100
Conversion (%)
80 60 40 20 0 0
10
20
30
40
50
60
Time (min.) Fig. 4. The total monomer (IV) conversion study of the for ROMP reaction using the G2 and G3 catalysts.
polymer [58]. The effect of the microstructure is described below while discussing the differential scanning calorimetry (DSC) analysis. In addition, monitoring the ROMP of IV with the G2 and G3 catalyst using 1H NMR spectroscopy (CD2Cl2, 25 °C) was performed. Fig. 3 show that the disappearance of the cyclic olefins at δ = 5.60 ppm and the appearance of new signal at δ = 5.38 ppm corresponding to the acyclic double bond resonance of the polymer. Integration of the olefinic signal at δ = 5.60 ppm was carried out using the TMS signal as a Ref. [59]. It was observed that < 10% conversion was obtained after 30 min. via using the G3 catalyst. In contrast, G2 gave an almost 80% conversion in 30 min and an overall 85% conversion after 1 h. The total monomer conversion during the ROMP is presented in Fig. 4. In particular, it has been reported that the initiation efficiency of G3 is much faster than G2 [60]. G3 generally promotes a controlled living polymerization with high ki/kp ratios. G2 usually displays a slow initiation rate and high propagation rate resulting in a broad PDI. We calculated the kinetic constants of each reaction to investigate the efficiencies of the each catalyst. Kinetic constant of the observed rate constant (kobs) can be defined as kobs = kp × [active center] where kp is the corresponding kinetic constant [61]. Active center can be defined as bearing catalyst and growing polymer chains. Table 2 shows the observed propagation constants. It was observed that propagation constants higher for G2 than G3 under all concentration range used in the experiment. Observed propagation rate constant (kobs) in the presence of G2 ranging from 0.039 to 0.062 min−1 were determined in deutorochloroform which lower than the reported values for cis-cyclooctene [62]. The propagation rate constant decreased gradually when the monomer concentration is raised in the presence of G3 catalyst are through that increasing the phosphonic acid content retard the speed of the reaction dramatically. In spite of that, when using G2 catalyst there is no well correlation between rate constant rate and concentration doubled, rate constant drops from 0.062 to 0.039 min−1. However, when the four times higher concentrated solution was used rate
poly(IV-COD)a
[M]/[I]
kobs (min
2a 2b 2d 3a 3b 3d
30 60 120 30 60 120
0.062 0.039 0.055 0.030 0.020 0.010
poly(IV)a
char yield (%)
80
Table 2 Observed propagation constants under different conditions for the ROMP of compound IV in CCl3D (M = monomer, I = initiator, 2 = G2, 3 = G3) ([M] = 0.9 M). Entry
poly(IV-COD)b
100
poly(COD)a
60 40
33% 24%
20
−1
)
4% 2%
0 0
100
200
300
400
500
600
temperature (°C) Fig. 5. The TGA results obtained for the as-synthesized polymers prepared using the G2 catalyst. 6
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Ersin Acar (Bogazici University, Istanbul, Turkey) for his insightful discussions.
600 °C. The flame retardant function of phosphorus content is related to the condensed and vapor phase [65,66]. The condensed phase is related with the amount of char formed as a result of the formation of polyphosphonic acid, which catalyzes aromatization at high temperature. However, vapor phase retardation is related with the release of phosphorus containing radicals and gases such as·PO·and·PO2·derivatives, which contribute to the extinction of the flame. We expect that hydroxyl phosphonate functionalized polybutene retards both the condensed and vapor phase. However, further analysis including the limiting oxygen index (LOI), UL94 and combustion calorimetry tests should be investigated to analyze the flame retardancy behavior of these polymers. Metal chelation efficiency of phosphonic acid bearing polymer was also investigated. Extraction of Cu(II) ions was conducted in solid liquid and liquid liquid-extraction. At the end of each reactions, the amount of metal ion in the water phase was investigated by inductively coupled plasma mass spectrometry (ICP-MS). Extraction studies were carried out in 20 mL vial for 24 h at room temperature. In the liquid-liquid phase extraction, the water-phase copper ions were mixed with the dichloromethane phase homopolymer poly(IV) for 24 h. The initial concentration of copper ion was determined to be 1 mM and 0.025 g of homopolymer was dissolved in dichloromethane. In the solid-liquid extraction study, the same amount of polymer was added to the copper solution at a concentration of 1 mM as the solid and reacted under the same conditions. In the solid-liquid extraction test, the polymer was adsorbed to 7.24 ± 0.21% Cu (II). However, the amount of copper passing to the dichloromethane phase was 6.17 ± 0.18% in liquid-liquid extraction. The polymer-free dichloromethane phase was investigated by the same experimental conditions and it was observed that the 0.54 ± 0.02% Cu (II) ion observed in dichloromethane phase. For the Cu chelation experiment, a commercial polymer poly(styreneco-maleic anhydride) (SMA) was also used to extract copper ions from aqueous solution as a standard for comparison [67–69]. Under the same experimental conditions, SMA retained 6.82 ± 0.19% Cu ions and consequently extracted less Cu ions than poly(IV).
Appendix A. Supplementary material Experimental details, including the monomer and polymer synthesis, NMR, FTIR and DSC spectra can be found in the online version. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.eurpolymj.2019.109318. References [1] T. Wolf, T. Steinbach, F.R. Wurm, A library of well-defined and water-soluble poly (alkyl phosphonate)s with adjustable hydrolysis, Macromolecules 48 (12) (2015) 3853–3863. [2] K.D. Troev, Chemistry and Application of H-Phosphonates, Elsevier, 2006. [3] L. Xiao, H. Zhang, T. Jana, E. Scanlon, R. Chen, E.W. Choe, L. Ramanathan, S. Yu, B. Benicewicz, Synthesis and characterization of pyridine-based polybenzimidazoles for high temperature polymer electrolyte membrane fuel cell applications, Fuel Cells 5 (2) (2005) 287–295. [4] S. Sethuraman, L.S. Nair, S. El-Amin, M.-T. Nguyen, A. Singh, N. Krogman, Y.E. Greish, H.R. Allcock, P.W. Brown, C.T. 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3. Conclusion In conclusion, we have reported the synthesis of cyclic phosphonates bearing polycyclooctene using ring-opening metathesis polymerization (ROMP). The method involves the derivatization of 1,5-cyclooctadiene (COD) with a phosphonate functionality (monomer IV) and the synthesis of its homo-polymer and co-polymer with 1,5-cyclooctadiene (COD).The efficiency of Grubb’s second generation catalyst (G2) was higher than Grubb’s third generation catalyst (G3) during the polymerization of the monomer (IV). The phosphorus content enhanced the thermal properties of the resulting polymers. More importantly, the chemical reactivity of the phosphonate-bearing polymers with well-defined architectures is useful in various derivatization reactions and their application. This new polymer could be used toward metal ion extractions and might be used in the purification of Cu radioisotopes. In addition, the phosphorus-containing polymers will serve as an example for self-flame retardant polymers. The formation of high molecular weight polymers and investigations into their regio-and stereoselectivity as well as controlling the molecular weight by adding a chain transfer agent such as cis-4-octene are currently in progress. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgment This work was supported by TUBITAK (Project No. 114Z666) and EU COST Action 1302. The authors would also like to thank Assoc. Prof. 7
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