Application of cesium hydroxide monohydrate for ring opening polymerization of monosubstituted oxiranes: characterization of synthesized polyether-diols

Cesium hydroxide monohydrate (CsOH·H2O) activated by cation complexing agents, i.e., 18C6 or C222 was applied as initiator of monosubstituted oxiranes polymerization. Propylene oxide (PO), 1,2-butylene oxide (BO), styrene oxide (SO) and some glycidyl ethers were used as monomers. All processes were carried out in tetrahydrofuran solution at room temperature. Such polymers, as PPO-diols, PBO-diols and PSO-diols, are unimodal and have molar masses Mn = 2000–5100. Their dispersities are rather high (Mw/Mn = 1.17–1.33). Moreover, PPO-diols and PSO-diols are not contaminated by monools with unsaturated starting groups. Poly(glycidyl ether)s are, in general, polymodal. For example, poly(isopropyl glycidyl ether)-diols are bi- or trimodal, whereas poly(allyl glycidyl ether)-diols possess two or even six fractions. Molar masses of main fraction are 4200–6400, and the second fraction is much lower, namely 600–2600. Dispersities of some fractions are very low (Mw/Mn = 1.01–1.07). Polymodality of polymers obtained was discussed in terms of the formation of two or more species propagating with different rate constants.


Introduction
Polyethers are an important class of synthetic polymers, which have many applications, e.g., as impact modifiers, surfactants, de-emulsifiers, dispersant agents, fuel additives, lubricants, biomedical materials or adhesives [1][2][3][4][5]. Especially interesting are polyether-and copolyether-diols or triols prepared from oxiranes, i.e., ethylene oxide (EO), propylene oxide (PO) or 1,2-butylene oxide (BO), which are applied for fabrication of polyurethane elastomers or foams [6]. Many initiators were used for anionic ring-opening polymerization of oxiranes, e.g., potassium hydroxide [7,8] or potassium alkoxides [9][10][11][12][13][14][15]. In industry, the most frequently used are KOH/1,2propylene glycol or glycerol systems, which allow to obtain PPO-diols or PPO-triols with M n = 2000-6000 at 110 °C [6,16]. The polymers possess low unsaturation due to excess of hydroxylic compound used, which strongly limited chain transfer reaction to monomer. On the other hand, PPOs with M n = 9000 and high unsaturation were prepared in the polymerization initiated with anhydrous KOH in THF solution at room temperature [8]. It was proposed, that in this case deprotonation of the monomer by initiator and active centers of growing chains occur to a wide extend. KOH, RbOH and CsOH are effective catalysts, while LiOH and Na OH are ineffective [17,18]. Unsaturation of PPOs depends strongly on the nature of alkoxide cation. The relative order of alkoxide reactivity in the transfer reaction with the monomer is: Li + > Na + > K + > Rb + > Cs + [19,20]. This effect is rather unexpected and needs some comments. Basicity of hydroxides and alkoxides increases from Li + to Cs + [21]. Deprotonation of PO occurs in the complex involving monomer and active ion pair of growing chain [6,10]  In our opinion, decreasing of deprotonation observed from Li + to Cs + could be explained by increasing of ionic radii of the metal cation (from 60 pm for Li + to 169 pm for Cs + ), which causes an increase in steric hindrance. On the other hand, in the polymerization initiated with potassium alkoxides and other salts activated coronand 18-crown-6 (18C6), formation of big K + 18C6 complexes results in direct deprotonation of the monomer and high unsaturation [15] (Scheme 2).
It was also worth noting that PPOs synthesized in the presence of KOH/ H 2 O/18C6 [8] or t-BuOK/t-BuOH/18C6 [14] systems have very low unsaturation and M n due to chain transfer reaction to hydroxylic compound.
The aim of the present work was characterization of polyether-diols prepared by ring opening polymerization of several monosubstituted oxiranes initiated with cesium hydroxide monohydrate (CsOH·H 2 O). This reagent was useful for synthesis of dicesium 1,2-propanedioxide, which was applied for some oxiranes copolymerization [22]. However, till now, it has not been used for initiation of oxiranes polymerization. Propylene oxide, 1,2-butylene oxide, styrene oxide, isopropyl glycidyl ether, allyl glycidyl ether and phenyl glycidyl ether were chosen as monomers for the study. Polymerizations were carried out in THF solution at room temperature. The influence of initial monomer concentration and kind of ligand complexing counterion (L), i.e., coronand 18C6 or cryptand C222 on molar mass ( M n ), dispersity ( M w / M n ) and modality of the prepared polymers were studied and discussed. 13 C NMR, MALDI-TOF and SEC techniques were used for analysis of polymers.

Synthesis
All syntheses were carried out at 20 °C in a 50 cm 3 reactor equipped with a magnetic stirrer and a Teflon valve enabling substrates delivery and sampling under argon atmosphere. In the first series of polymerizations of propylene oxide, the initial concentration of the monomer was equal to 2.0 mol/dm 3 and the initial amount of CsOH·H 2 O (0.34 g, 2.0 mmol) and THF (17.0 cm 3 ) was introduced into reactor and then 18C6 (0.53 g, 2.0 mmol) was added. The mixture was stirred during 30 min. That system was used as the initiator, when the monomer (2.8 cm 3 , 2.4 g, 40.0 mmol) was introduced into the reactor. The reaction mixture was then stirred during 170 h. After this time complete conversion of the monomer occurred and the reaction mixture was neutralized with HCl/H 2 O system (0.1 mol/dm 3 , 50 cm 3 ) and transferred to the separator containing chloroform (70 cm 3 ). After shaking during 5 min, two layers were obtained, i.e., interferior polyether layer and superior layer containing water and the cesium salt. There layers were separated, and the superior layer was removed. After three washings with distilled water, polyether was obtained by evaporating of chloroform and water in vacuum at elevated temperature. In the next series, polymerizations were performed in the presence of other ligand C222 and higher initial concentration of the monomer, i.e., 5.0 mol/dm 3 . Similar procedure was applied for polymerization of other oxiranes. The concentration of monomer during the polymerizations was monitored by the 1,4-dioxane method [23]. The final conversions were ~ 99%. The yields of the reactions were 97−99%. All studied processes were heterogeneous and part of initiator remained at the end polymerization.

Measurements
100 MHz 13 C NMR spectra were recorded in CDCl 3 at 25 °C on a Bruker Avance 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. To obtain a good spectrum of the polymer main chain exhibiting its microstructural details, about 3000 scans were sufficient but in order to observe the signals of the polymer chain terminal groups more than 10,000 scans were necessary. 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 THF as a solvent. Polstyrenes were used as calibration standards. MALDI-TOF spectra were recorded on a Shimadzu AXIMA Performance instrument. Dithranol was used as a matrix.

Results and discussion
It was observed, that CsOH·H 2 O is practically insoluble in THF at room temperature and does not initiate polymerization of oxiranes. Therefore, all processes were carried out with this initiator, which is activated by macrocyclic ligands complexing Due to the ratio of the ionic cesium cation radius to the 18C6 cavity radius ≈ 1.2 [24], it is theoretically possible to exist in equilibrium two kinds of complexes, i.e., flat (1/1) and sandwich (1/2) ones [25]. Cryptand C222 is able to form exclusive and inclusive complexes being in equilibrium, but in THF the formation of the inclusive complex was hindered by the strong cation-anion interaction [26]. Molar masses ( M n ) of PPO-diols (1-4), PBO-diols (5-8), PSO-diols (9-12) and their dispersities ( M w / M n ) are collected in Table 1.

Scheme 4 Equilibrium involving complexation of initiator by ligand
It was suggested, that initiation occurs exclusively in liquid phase, i.e., it is mediated by the part of initiator, which is soluble in the reaction mixture. It causes increase of M n . Polarity of the medium depends on the kind and initial concentration of the monomer, which also influences M n of polymers. Moreover, chain transfer to water results in decrease of M n . Polymers synthesized in the presence of C222 has a little lower M n than obtained with 18C6 at the same monomer concentration. It may result from slight increase in initiator concentration in the reaction mixture containing C222 ligand. Relatively high dispersity of the polymers obtained ( M w / M n = 1.17-1.34) results from chain transfer reaction to water and slow solubilization of initiator during the polymerization. It was established, that PPO-diols and PSO-diols obtained in this work do not contain macromolecules with unsaturated starting groups. Signals in unsaturated region, i.e., at 116.7 and 134.9 ppm for PPO-diols as well at 133.2 and 159.7 ppm for PSO-diols were not observed in 13 C NMR spectrum. FTIR analysis of PPOs and PSOs did not indicate signals of C=C bonds in their spectra neither at 1600-1680 (C=C bond stretching) or at 3300-3200 cm −1 (sp2 C-H bond stretching). However, such signals were detected in other systems with K + counterion [8,26,27]. It indicates, that the presence of Cs + ions and high amount of water completely (2) PAGE-diol (24) eliminate unsaturation in these polymers, due to chain transfer reaction to water. In the studied systems, the rate of this reaction is evidently much more higher, than the rate of chain transfer to monomer. Polyether-diols (1-12) are unimodal. Unexpectedly, polymers synthesized from glycidyl ethers are, in general, bimodal (13-24) ( Table 2). One of them, i.e., (24) consists even six fractions.
It is worth noting, that dispersities of fractions (a) are lower than (b). Dispersities of some fraction are extremely low (1.01-1.05). Figure 1 shows SEC chromatograms of two exemplary polyether-diols. Basing on the results obtained, the course of polymerization is proposed in Scheme 5. Formation of two or more fraction in the polymerization of some oxiranes needs some comments. It is generally accepted, that in the polymerization process bimodality exclusively appears, when there are two species, propagating with different rate constants and that these species do not exchange fast enough [28]. According to this statement, it was proposed, that in the studied systems various kinds of anionic centers with different reactivity in growing polymer chains should be formed and be responsible for polymodality of some polymers (Scheme 6).
On the other hand, the heterogenicity of the system (transfer of an initiator from solid state to liquid followed by its ionization) may be consider as a reason of polymodality. However, unimodality observed in some polyethers indicates, that formation of two or more fractions in other polymers depends also on the kind of monomer. Explanation of this phenomenon needs further investigations. propagating with different rate constants, which do not exchange quickly enough; • Some of prepared polyether diols could be useful for fabrication of new polyurethane elastomers due to their relatively high M n, low dispersity, and lack of unsaturation.

Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of interest.
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Scheme 6
Various reactive species formed in anionic polymerization of monosubstituted oxiranes initiated with CsOH·H 2 O (ligand was omitted)