Ring opening polymerization of styrene oxide initiated with potassium alkoxides and hydroxyalkoxides activated by 18-crown-6: determination of mechanism and preparation of new polyether-polyols

Styrene oxide (Aldrich) was dried over CaH₂ and then distilled at 467 K (194 °C). Anhydrous tetrahydrofuran (THF) (Acros Organics) was kept over CaH₂ and distilled at 339 K (66 °C). Potassium hydride (KH) (Aldrich) was purified according to the procedure described by Brown [17]. A 35 wt% dispersion of KH in mineral oil (Aldrich) was mixed with n-pentane in a dry argon atmosphere and then decanted. It was repeated three times followed by a threefold washing with dry THF. Finally, THF was evaporated in vacuum. The KH present was determined by a standard gas law calculation of the hydrogen liberated after treatment with 2-butanol (1.0 H₂ = 1.0 KH). The resulting solution was titrated to a phenolphthalein end point. Very little excess (<1%) of total base over hydride base (from gas evolution) indicated small hydrolysis of the original KH sample. Coronand 18C6 (1,4,7,10,13,16-hexaoxacyclooctadecane) (Merck), methanol, i-propanol, 1-phenylethanol, 1-methylpropanol, 2-methylpropanol, dipropylene glycol, 2,2,6,6-tetrakis(hydroxymethyl)cyclohexanol and potassium t-butoxide in THF solution (1.0 mol/dm³) (all from Aldrich) were used as received for synthesis without further purification.

All syntheses were performed at 20 °C in a 50 cm³ reactor equipped with a magnetic stirrer and a Teflon valve enabling substrate delivery and sampling under argon atmosphere. Monopotassium salts were obtained in the reaction of potassium hydride with an appropriate amount of alcohol or dipropylene glycol (DPG) dissolved in THF containing 18C6 at 20 °C. The initial concentrations of the monomer were 2.0 or 5.0 mol/dm³ and the initial concentration of the initiator was 0.1 mol/dm³. For example, potassium hydride (0.08 g, 2.0 mmol) and THF (15.0 cm³) were introduced into the reactor and then 18C6 (0.53 g, 2.0 mmol) and methanol (0.064 g, 2.0 mmol) were added by use of syringe. The mixture was stirred for 1.5 h until all hydrogen (44.7 cm³) was evolved. It resulted in a suspension of pure anhydrous potassium methoxide activated ligand in the ether solution. That system was used as the initiator, when styrene oxide (4.6 cm³, 3.7 g, 40 mmol) was introduced into the reactor. The reaction mixture was then stirred for several days. After almost complete conversion of the monomer the reaction mixture was neutralized with HCl/H₂O system (0.1 mol/dm³, 70 cm³) and transferred to the separator containing chloroform (70 cm³). After shaking, two layers were formed, i.e., an inferior polyether layer and a superior layer containing water and the potassium salt. These layers were separated and the superior layer was removed. After three washes with fresh water, polyether was obtained by chloroform and water distillation in vacuum at 100 °C.

In another experiment, tripotassium salt of 2,2,6,6-tetrakis(hydroxymethyl)cyclohexanol (TKHMCH) activated by 18C6 was prepared in a similar way. The initial concentration of the monomer was 2.0 or 4.0 mol/dm³ and the initial concentration of the initiator was 0.02 mol/dm³. Potassium hydride (0.048 g, 1.2 mmol), THF (5.0 cm³) and 18C6 (0.32 g, 1.2 mmol) were introduced into the reactor. Then TKHMCH (0.088 g, 0.4 mmol) was dissolved in boiling THF (10.0 cm³) and the solution obtained was dropped into the system at 20 °C. After mixing for 1.5 h, hydrogen (26.8 cm³) was evolved. Finally, styrene oxide (4.6 cm³, 3.7 g, 40 mmol) was added and the reaction mixture was stirred for several days until complete monomer conversion. The separation of polymer obtained was performed as described previously. At the stage of shaking with the HCl/H₂O system, some additional interlayer was formed that may contain polyether with fully converted OH groups or in equilibrium with the polyether with some rest OK groups. The presence of monomer during polymerization was monitored by chromatographic method. After ending of the polymerization, all reaction mixtures were homogeneous.

100 MHz ¹³C nuclear magnetic resonance (NMR) spectra were recorded in CDCl₃ at 25 °C on a BrukerAvance 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 satisfactory, but in order to observe the signals of the polymer chain ends 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 Prominence 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. Dithranol was used as a matrix. Spectrophotometer measurements were performed on the FTIR ATR device (Shimadzu IR Prestige) equipped with diamond ATR crystal purified prior to measurement with isopropanol. Data were analyzed using LabSolutions program.

Data characterizing the polymers obtained are presented in Table 1. They involve molar masses, dispersities and unsaturation of PSOs.

Figure 1 shows the characteristic signals of ¹³C NMR spectra for the starting groups of the polymers obtained in the presence of three initiators, i.e., potassium methoxide (Fig. 1a-I1), potassium i-propoxide (Fig. 1b-I2) and potassium t-butoxide (Fig. 1c-I3).

Analysis of ¹³C NMR spectrum indicated that initiation of SO polymerization with potassium methoxide (I1) (Fig. 1a) occurs by a different way than it was reported in the literature [2‐4]. Methyl signal of the starting group CH₃OCH₂CH(Ph)O– (59.21 ppm) is observed in the spectrum after quenching with HCl/H₂O resulting from SO ring opening in the β-position under influence of the initiator. A hypothetical second signal of starting group CH₃OCH(Ph)CH₂O– (at about 57.0 ppm) is absent in the spectrum which eliminates the possibility of monomer ring opening also in α-position. However, such a signal was detected in the spectrum of PSO quenched with CH₃I as the signal of the end group. The results obtained clearly indicated that in the initiation as well as propagation step the oxirane ring opened exclusively in the β-position.

A similar effect was observed in the polymerization initiated with potassium t-butoxide (I3) (Fig. 1c). However, in the presence of potassium i-propoxide (I2) the oxirane ring opens in the β- or α-position (~1/1). In ¹³C NMR spectrum of the polymer, two signals of CH₃ group are present (Fig. 1b). We proposed that these two signals were derived from two kinds of starting i-propoxy groups, i.e. (CH₃)₂CHOCH₂CH(Ph)O– (22.21 ppm) and (CH₃)₂CHOCH(Ph)CH₂O– (22.05 ppm). Further splitting only of the latter one, may be due to the closer vicinity of its methyl groups to the asymmetric carbon –CHPh–. It was interesting that replacement of one CH₃– group in potassium i-propoxide by the greater CH₃CH₂– group partly conserves the observed phenomenon. In the presence of potassium 1-methylpropoxide (I5) oxirane ring also opens in the β- or α-position (~1/0.2). Figure 2 shows the aliphatic part of the ¹³C NMR spectrum of PSO obtained with the mentioned initiator. According to notation in Fig. 2, the predominating signals of the starting group formed during β-opening (carbons 1, 2 and 3) may be found at 9.73, 29.07, and 19.14 ppm, respectively, while those from α-opening (carbons 4, 5 and 6) at 9.88, 32.04, and 22.10, respectively. Splitting of all the mentioned signals into two lines may be due to the presence of asymmetric methine carbon in these groups as it was already observed in our earlier work on PPO [18]. The presence of additional minor signals in this region may indicate some side reactions; however, the source of these reactions is yet unknown.

However, replacement of one CH₃– group by Ph– one in potassium 1-phenylethoxide (I6) or introduction of –CH₂– group between i-Pr– and –O⁻ groups in potassium 2-methylpropoxide (I4) results in the ring opening during the initiation exclusively in the β-position (Fig. 3). Opening in the α-position should lead to creation of a second signal for methyl group, shifted of about 0.4 ppm but no such signal is observed (Fig. 3a).

The results obtained indicate, that the structure of initiator used influences the direction of oxirane ring-opening in anionic polymerization of styrene oxide (Scheme 2).

In the unsaturated region of ¹³C NMR spectra of the obtained polymers two weak signals were observed at 133.02 and 159.66 ppm (Fig. 4). It was proposed that these signals derived from carbons of unsaturated starting groups CH₂=C(Ph)–O– (quaternary carbon at 159.66 ppm). Their formation by chain transfer to the monomer was presented on Scheme 3.

This reaction proceeded by deprotonation of monomer methine group with growing chain end resulting from the presence of acidic H-atom in PhCH group. However, the lack of the signals in ¹³C NMR spectrum at about 108 and 146 ppm of vinyl carbons in hypothetical starting group,

Ph–CH=CH–O– indicated, that reported in the literature deprotonation of methylene group in SO molecule did not occur. Signal of carbonyl group was also absent in the spectrum. Unsaturation of polymers increases with initial concentration of the monomer (Table 1). Chain transfer to the monomer is responsible for this phenomenon (Scheme 2). However, unsaturation of polymers depends also on the kind of initiator used. The lowest unsaturation has PSO obtained with CH₃O⁻ (~5%), the highest one (~19%) exhibits polymer prepared with PhCH(CH₃)O⁻ . It means, that the second source of unsaturation is deprotonation of the monomer by initiator.

Molar masses of synthesized polymers (M _n  = 1700–4800 Da) are higher than those reported in the literature for polymers prepared in toluene solution at 80 °C (M _n  = 980–1200 Da) or in bulk at 100 °C (M _n  = 2400–2900 Da). Analysis of the polymers by MALDI-TOF spectroscopy confirmed the formation of macromolecules with unsaturated starting groups. Figure 5 shows for example MALDI-TOF spectrum of PSO obtained with (CH₃)₂CHO⁻ .

Dispersity of polymers is relatively low, i.e., M _w/M _n  = 1.07–1.15, indicating high rate of cation exchange reaction. MALDI-TOF analysis of polymer 3 further confirmed its chemical structure (Fig. 5).

Three main series of signals were observed in the spectrum. The first series at m/z 1056.3–2133.9 represents macromolecules possessing isopropyloxy starting group and OH end group. For example, signals at m/z 1178.1, 1538.7 and 1897.6 represent macromolecules containing 9, 12 and 15 mers of styrene oxide (M _n (calcd.) = 1180.6, 1541.0 and 1901.5 respectively). They form adducts with K⁺ ions. The second series at 1040.3–2117.9 represents macromolecules with the same structure, but forming adducts with Na⁺ ions. However, the third series of peaks at m/z 1117.7–1839.0 represents macromolecules possessing unsaturated starting group and OH end group. They form adducts with K⁺ ions. For example, signals at m/z 1239.4, 1479.8 and 1839.0 represent macromolecules containing 9, 11 and 14 mers of styrene oxide (M _n (calcd.) = 1240.6, 1480.9 and 1841.3 respectively).

Data concerning the PSOs with several hydroxyl end groups were presented in Table 2. They were synthesized in processes initiated with monopotassium salt of dipropylene glycol (K-DPG) (I7) or tripotassium salt of 2,2,6,6-tetrakis (hydroxymethyl) cyclohexanol (3K-TKHMCH) (I8) (Scheme 4).

Figure 6 shows ¹³C NMR spectrum of macrodiol obtained at low initial concentration of monomer, i.e., [SO]_o = 2.0 mol/dm³.

Signals at 17.1 ppm derived from CH₃– groups of initiator. Signals of terminal groups CH₂ CH(Ph)OH which should arrive at about 73.8 ppm cannot be precisely identified since they fall in the region of the strong main chain signal, nevertheless, a sharp signal at 73.65 ppm could be observed in this broad band. However, signals of starting groups derived from initiator, i.e., –CH₂CH(CH₃)OH which should arrive at about 66.6 ppm as well as signals of unsaturated starting groups were not observed in the spectrum.

Figure 7 presents SEC chromatogram recorded for polymer 12 (p12) obtained with initiator I8 and polymer 13 (p13) obtained with initiator I8.

For polymer 12 (p12) two main fractions are present as the product of the synthesis, while for polymer 13 (p13) three fraction can be separated.

FTIR spectra were recorded for polymers P10–P13 using ATR technique (Fig. 8). For synthesised polymers the main chain is composed of alkane-like fragments (–CH₂–). Hence stretching of –C–H reflects bands at 3000–2850 cm⁻¹ is visible with bending/scissoring vibration of C–H in the range 1470–1450 cm⁻¹ and rocking ones of C–H at 1370–1350 cm⁻¹ and 750–720 cm⁻¹. Presence of aromatic phenyl ring is manifested with C=C–H stretching bands located in the range above 3000 cm⁻¹ (3100–3000 cm⁻¹). The absorption within the range of 1600–1585 cm⁻¹ and 1500–1400 cm⁻¹ is caused by stretching bands of –C=C– in aromatic rings. Low intensity bands at 3492 cm⁻¹ represent stretching vibration of associated –O–H, while the peak at 1318 cm⁻¹ is attributed to O–H bending vibration. Complementary C–O stretching band is visible in range 1260–1050 cm⁻¹ with 1060 cm⁻¹ stretching band of C–O for primary alcohol. It is overlaid with ether C–O–C band located at 1095 cm⁻¹ characteristic for alkyl-substituted ether.

MALDI-TOF analysis of polymer P10 further confirmed its chemical structure (Fig. 9).

Two series of signals were observed in the spectrum. The first series at m/z 1119.5–4055.3 represents macromolecules possessing central part –OC₃H₆OC₃H₆O– derived from initiator and two OH terminal groups. For example, signals at m/z 1360.4, 1840.6 and 2432.2 represent macromolecules containing 10, 14 and 19 mers of styrene oxide (M _n (calcd.) = 1358.7, 1839.3 and 2440.1), respectively. They form as adducts with Na⁺ ions. The second series at m/z 1135.5–4071.3 represents macromolecules with the same structure, but forming adducts with K⁺ ions. For example, signals at m/z 1618.1, 1974.1 and 2447.9 belong to macromolecules with 12, 15 and 19 mers of styrene oxide (M _n (calcd.) = 1614.5, 1975.1 and 2456.1).

Based on the results obtained we proposed the course of ROP for SO (Scheme 5).

Rapid cation exchange reaction between macromolecules 10 and 11 is responsible for activation of the former resulting consequently in the lack of starting HOCH(CH₃)CH₂– group. Deprotonation of hydroxyl groups present in polymer chains with alkoxide groups is very important reaction. It makes possible growth of chains in two directions and does not influence the concentration of active centers and polymer chains, i.e., does not influence M _n of polymers. Moreover, this reaction strongly limits chain transfer reaction to the monomer and thus avoids decreasing of M _n .

In the last series of experiments tripotassium salt of TKHMCH (I8) was applied for SO polymerization. Figure 10 presents ¹³C NMR spectrum of the polymer obtained at [SO]_o = 4.0 mol/dm³.

In this spectrum the small signals at about 76.3 and 74.6 ppm can be ascribed respectively to the first two CH₂ carbons in the –CH₂–O–CH₂–CH(Ph)– beginning part of the polymer chains connected to the initiator ring and indicating formation of the polymer, however, the carbon signals of the initiator ring itself are too weak to be observed. Nevertheless, MALDI-TOF analysis of the polymer confirmed its chemical structure (Fig. 11).

Two series of signals were shown in the spectrum. It was proposed that the first series of peaks at m/z, for example 897.8, 1137.5 and 1975.8 represents macromolecules having central part derived from initiator as well as four PSO arms with OH end groups and one arm with OK group. These macromolecules contain 5, 7 and 14 mers of SO, respectively, and form adducts with K⁺ ion (M _n (calcd.) = 898.2, 1138.5 and 1979.6, respectively). The second series of the signals at m/z 1002.1, 1122.2 and 1960.4 represent macromolecules with the same structure which contain 6, 7 and 14 mers (M _n (calcd.) = 1001.3, 1122.4 and 1963.4, respectively) and form adducts with Na⁺ ion.

We proposed that the polymerization course is similar to that initiated with (I7), but in this case star-shaped macromolecules are formed (Scheme 6).

It is worth noting, that polymers obtained with salt (I8) consist of two or three fractions detected by the SEC method. This phenomenon can be explained by a strong tendency during polymerization to the formation of various macromolecular aggregates containing OK and OH groups with different reactivities [19].