Ring-opening polymerization of monosubstituted oxiranes in the presence of potassium hydride: determination of initiation course and structure of macromolecules by MALDI-TOF mass spectrometry
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Several monosubstituted oxiranes were polymerized with suspension of potassium hydride (KH) in tetrahydrofuran (THF) at room temperature. This heterogeneous process resulted in polyethers with various starting groups depending on the kind of monomer. The macromolecules formed in ring-opening polymerization of monosubstituted oxiranes were analyzed by Matrix Assisted Laser Desorption/Ionization - Time of Flight Mass Spectrometry (MALDI-TOF MS). It was stated, that initiation of propylene oxide (PO) polymerization with KH proceeded via three ways, i.e. cleavage of oxirane ring in the β-position, monomer deprotonation and deoxygenation. Potassium isopropoxide, potassium allyloxide and potassium hydroxide were the real initiators. The main reactions, which occur in the initiation step, depend on the type of monomer used. In the case of allyl glycidyl ether (AGE) and phenyl glycidyl ether (PGE) deprotonation of the monomer did not occur. During initiation of glycidyl ethers oxirane ring was opened and also linear ether bond between glycidyl group and oxygen atom was cleaved under influence of KH. Interestingly, formation of new kinds of macromolecules was observed in the systems containing glycidyl ethers, which do not possess mers of the monomers used. Mechanisms of the studied processes were presented and discussed. Carbon-13 Nuclear Magnetic Resonance (13C NMR) was used as supporting technique for analysis of the obtained polymers. Number average molar masses of the polymers (Mn) determined by Size Exclusion Chromatography (SEC) were about two times higher than calculated ones. It indicated that half of used KH did not take part in the initiation step.
KeywordsPotassium hydride Anionic ring-opening polymerization Oxiranes MALDI-TOF spectrometry Polyethers
Polyethers are important class of synthetic polymers, which are applied in many areas of industry and medicine. They are synthesized by ring-opening polymerization of oxiranes [1, 2, 3]. The most useful is anionic polymerization of ethylene oxide (EO) and propylene oxide (PO) due to wide utility of the polymers obtained [1, 4]. Some of expected application are their use as impact modifiers, surfactants, dispersants, fuel additives, wetting agents, lubricants, rheological modifiers, biomedical application, adhesives [5, 6, 7], de-emulsifiers  and components in the fabrication of block copolymers and polyurethane elastomers or foams [3, 4, 9]. Initiators most frequently used for polymerization of monosubstituted oxiranes, e.g. PO are potassium hydroxide [10, 11, 12] and potassium salts of alcohols, glycols or glycerol [13, 14, 15]. In some cases cation complexing agents, e.g. coronand 18-crown-6 were applied for activation of chain growth centers [14, 16, 17]. Other salts for example as potassium potasside K+(15-crown5)2K− [18, 19] or potassium hydride K+H−/activated with various crown ethers [20, 21, 22, 23] were also used for initiation of the polymerization of many oxiranes. Among saline hydrides KH appeared to present the optimum balance of potential reactivity and potential considerations . The solvents, which are most suitable for reactions of KH are ethers, especially tetrahydrofuran (THF). All reactions, in which KH is extremely reactive as a base proceed at the crystal surface apparently. Prolonged stirring of KH with THF, followed by decantation, reveals no detectable dissolved hydride . However, many of the potassium salts produced by use of KH are moderately to highly soluble in THF. In the previous work  it was shown that the initiation and propagation reactions take place on the surface of the potassium hydride. The initiation occurs as long as the monomer is present in the reaction mixture. The cavity size of 18-crown-6 is most suitable to form 1:1 complex with potassium cation, which can exist both in solution and on hydride surface. Other ligands as 15-crown-5 or 12-crown-4 form in the solution sandwich composed of 2:1 complex. However, the existence of such complexes on the surface of solid phase is not possible . In this case 1:1 complex could be exclusively formed. In consequence, the effects related to heterogeneity of polymerization influence the reaction rate and number average molecular masses of PPOs. Then, Stolarzewicz et al.  proposed the course of PO polymerization mediated with KH. The authors observed, that in this system potassium isopropoxide and unexpectedly, potassium hydroxide were formed, which became the real initiators of the polymerization. The course of the polymerization of other oxiranes was not studied. Therefore, in this work we decided to determine in details the mechanism of propylene oxide polymerization initiated with KH and, comparatively, also other oxiranes, i.e. allyl glycidyl ether (AGE) and phenyl glycidyl ether (PGE).Especially interesting are glycidyl ethers, because of the fact that these monomers possess two kinds of ether bonds, i.e. cyclic and linear ones. The latter could be also cleaved under influence of hydride anion resulting in new kings of macromolecules. All processes were carried out in THF solution at room temperature under argon atmosphere. SEC, 13C NMR and MALDI-TOF techniques were used for analysis of the obtained polymers.
Monomers, i.e. propylene oxide, allyl glycidyl ether and phenyl glycidyl ether (all from Aldrich, Poland) were dried over CaH2 and destilled at 306 K (33 °C), 427 K (154 °C) and 518 K (245 °C), respectively. Anhydrous tetrahydrofuran (THF) (Acros Organics, Poland) was kept over CaH2 and distilled at 339 K (66 °C) and redistilled over Na/K alloy prior to use. Potassium hydride (KH) (Aldrich, Poland) was purified according to the procedure described by Brown . The KH present was determined by a standard gas law calculation of the hydrogen liberated (1.0 H2 = 1.0 KH). Very low excess (<1%) of total base over hydride base (from gas evolution after stirring with 2-butanol) indicated little hydrolysis of the original KH sample.
All syntheses were carried out at room temperature in a 50 mL reactor equipped with a magnetic stirrer and a Teflon valve enabling substrates delivery and sampling under argon atmosphere. For example, potassium hydride (0.08 g, 2.0 mmol) and tetrahydrofuran (17.0 mL) were introduced into the reactor and then propylene oxide (2.8 mL, 2.3 g, 40 mmol) was added. The reaction mixture was stirred during several days until all monomer was conversed. During the polymerization gaseous product was evolved. After ending of the process some unreacted KH was present in the reactor. Then, CH3I as quenching agent was added to the system and stirred during 10 min. After separation of solution from precipitated KI polymer was obtained by evaporation of solvent in vacuum. The concentration of monomer during polymerization was monitored by the 1,4-dioxane method . Chromatographic method indicated the presence of hydrogen in gaseous phase and propylene in liquid phase. The final conversion was ~99% and the yield of PPO obtained was ~98%. In the next experiments other polyethers, namely PAGE and PPGE were synthesized in the same conditions at initial concentration of monomer [M]o = 2.0∙103 mmol/mL and initiator [I]o = 100 mmol/L.
100 MHz 13C NMR spectra were recorded in CDCl3 at 25 °C on a BruckerAvance 400 pulsed spectrometer equipped with 5 mm broad band probe and applying Waltz16 decoupling sequence. Chemical shifts were referenced to tetramethylsilane serving as an internal standard. In order to obtain a good spectrum of the polymer main chain exhibiting its microstructural details, about 3000 scans were satisfactory, but in order to observe the signals of the polymer chain ends more then 10,000 scans were necessary. Number average molar masses and dispersities of polymers were obtained by means of size exclusion chromatography (SEC) on a Shimadzu Prominance UFLC instrument at 40 °C on a Shodex 300 mm × 8 mm OHpac column using tetrahydrofuran as a solvent. Polystyrenes were used as calibration standards. Matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) spectra were recorded on a Shimadzu AXIMA Performance instrument. The mass spectrometer operated in linear mode. The laser power was optimized to obtain a good signal-to-noise ratio after averaging 250 single-shot spectra. Dithranol was used as a matrix (10.0 mg/mL) and polymer samples were dissolved in tetrahydrofurane (2.0 mg/mL) producing clear, homogenous solutions. By using a pipette 0.5 μL of sample solution and 0.5 μL of matrix solution were applied onto a stainless-steel target plate then air dried at room temperature for several minutes. Data were acquired in continuum mode until acceptable averaged data were obtained and were analyzed using Shimadzu Biotech Launchpad program. Analysis of hydrogen was conducted by the GC technique on a 2,4 m long stainless stesl column packed with Al2O3 (0.02–0.03 mm). and deactivated with 5% K2CO3 using an INCO 505 gas chromatograph with flame ionization detector. Propylene was determined with a Chromatron GCHF chromatograph at ambient temperature on a 2 m metal column filled with modified alumina.
Results and discussion
Characterization of polyethers synthesized with use of potassium hydride in THF; [monomer]o = 2.0∙103 mmol/mL and [initiator]o = 100 mmol/L.
At conversion of the monomer above ~70% polymerization proceed markedly slower. It caused increasing of reaction time, especially in the case of PO. Results of polymers analysis by MALDI-TOF and 13C NMR techniques, which allows to propose mechanism of initiation step of the polymerization are presented below.
Polymerization of propylene oxide
In order to confirm possibility of such reaction model polymerization of PO initiated with potassium allyloxide was carried out. In this system macromolecules with allyloxy, cis- and trans-propenyloxy starting groups were formed. After complete monomer conversion KH was added to the reaction mixture and mixed during several hours. The system was then treated with CH3I. MALDI-TOF analysis of the polymer obtained did not indicate the formation of macromolecules F. Thus, reaction presented on Scheme 4 can be excluded.
Polymerization of glycidyl ethers
In the MALDI-TOF spectrum (Fig. 5) one series of weak signals can represent macromolecules J, which form adducts with potassium ions. For example, signals at m/z 3191.0, 3306.1 and 3879.2 belong to macromolecules possessing 27 mers of AGE and 1 mer of PO, respectively (Mcalc = 3194.9, 3309.0 and 3879.7).
Several series of signals were observed in the spectrum. The main series at m/z 1200 to 6200 reveals signals of macromolecules K, which form adducts with potassium ions. For example, signals at m/z 1255.4, 3057.1 and 4560.9 belong to macromolecules containing 8, 20 and 30 mers of PGE, respectively (Mcalc = 1256.7, 3059.1 and 4561.1 respectively). The second series reveals weak signals of macromolecules, which belong to macromolecules L, containing one mer of PO. For example, macromolecules represented by signals at m/z 3264.4, 5220.1 and 6124.1 contain 21, 34 and 40 mers of PGE, respectively (Mcalc = 3267.4, 5220.0 and 6121.2 respectively). The third series reveals weak signals of macromolecules M containing starting PhO- group. For example, macromolecules represented by signals at m/z 1199.0, 1347.9 and 3001.3 contain 7, 8 and 19 mers of PGE, respectively (Mcalc = 1199.5, 1349.7 and 3001.9 respectively). Macromolecules K and M form adducts with potassium ions.
13C NMR analysis of PPGE confirmed the presence of CH3 group in macromolecules, i.e. at 17.4, 22.2 and 58.2 as it was observed previously for PAGE. The lack of carbon signal in the unsaturated region of the spectrum indicated that deprotonation of the monomer by KH did not occur.
It was assumed that main series at m/z 751.3, 975.0, 1198.8, 1422.5 and 1646.0 reveals signals of cyclic macromolecules formed by polymerization of glycidol initiated with potassium glycidoxide. They contain one mer of glycidoxide, methyl group and 9, 12, 15, 18 and 21 mers of glycidol (Mcalc = 755.0, 977.0, 1199.0, 1421.0 and 1643.0 respectively) and form adducts with H+ ions. The second series at m/z 897.5, 1122.0, 1344.7 and 1568.8 reveals signals of macromolecules with one mer of glycidoxide, methyl group and 11, 14, 17 and 20 mers of glycidol (Mcalc = 903.0, 1125.0, 1347.0 and 1569.0 respectively) and form adducts with H+ ions. The third series of signals at m/z 789.1, 864.0, 940.8, 1013.0, 1088.0, 1165.5, 1237.8, 1308.3, 1383.5, 1454.7, 1536.7, 1609.1 and 1685.1 belong to cyclic macromolecules containing one mer of glycidoxide, methyl group and 9 to 21 mers of glycidol (Mcalc = 793.0, 867.0, 941.0, 1015.0, 1089.0, 1163.0, 1237.0, 1311.0, 1385.0, 1459,0, 1533.0, 1607.0 and 1681.0 respectively) an form adducts with potassium ions. Similar series of signals were observed in MALDI-TOF spectra of PPGE. However, the course of this process has been not explained because the source of glycidol is unknown. It might be formed by hydrolysis of potassium glycidoxide. It is also worth noting, that postulated macromolecules were not detected by MALDI-TOF technique in the polymerization of AGE and PGE initiated with potassium hydroxide or potassium t-butoxide or other anionic species in the same conditions. We would like to continue the developing of our research in the future, taking into account also other oxiranes, mainly various glycidyl ethers in order to explain the influence of the kind of substituent on the initiation course in the polymerization mediated with KH, also in the presence of crown ethers. That discussion is not complete and needs to be expanded with additional experiments.
All monomers undergo ring-opening in the β-position
Deprotonation of monomer occurs exclusively with propylene oxide
Cleavage of linear ether bonds is observed in both glycidyl ethers
Deoxygenation of monomer takes also place, except phenyl glycidyl ether
In the case of glycidyl ethers additional reactions occur leading to macromolecules, which do not possess mers of monomer used; this phenomenon needs further studies.
The work was supported by Institutes’ own funds.
Compliance with ethical standards
Conflict of interest
The authors declare that there is no known conflict of interest concerning the given work.
- 3.Ionescu M (2005) Chemistry and Technology of Polyols for polyurethanes. Rapra Technology Limited. Shawbury, Shrewsbury, Shropshire, p 235Google Scholar
- 5.Ulrich H (1999) Ullmann’s encyclopedia of industrial chemistry, vol A9. 6th edn. Wiley, New YorkGoogle Scholar
- 16.Ding J, Heatley F, Price C, Booth C (1991) Use of crown ether in the anionic polymerization of propylene oxide—2. Molecular weight and molecular weight distribution Eur Polym J 27:895–899Google Scholar
- 18.Grobelny Z, Stolarzewicz A, Neugebauer D, Morejko-Buż B (2002) Structure of poly(propylene oxide) obtained in the presence of K−, K+(15-crown-5)2.Eur. Polym J 38:1065–1070Google Scholar
- 26.Siggia S (1963) Quantitative organic analysis via functional groups. J. Wiley, New York, p 241Google Scholar
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