The influence of initiator and macrocyclic ligand on unsaturation and molar mass of poly(propylene oxide)s prepared with various anionic system
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Anionic polymerization of propylene oxide was carried out in the presence of two groups of potassium salts activated 18-crown-6 (18C6), e.g. alkoxide salts (CH3OK, i-PrOK, t-BuOK, CH3OCH2CH(CH3)OK, KCH2O) and other salts (CbK, Ph3CK, Ph2PK, Ph3HBK, KK, KH, and [(CH3)3Si]2NK) in THF at room temperature. Application of various initiating systems results in polyethers which are different in level and kind of unsaturation represented by allyloxy, cis- and trans-propenyloxy, as well as vinyloxy starting groups. In the presence of selected initiator, i.e. t-BuO−K+ unsaturation increases markedly by addition of 18C6 or C222. During the initiation step oxirane ring-opening and direct deprotonation of the monomer occur simultaneously involving in some cases also the ligand. All initiators opens oxirane ring in the β-position except i-PrOK, which opens it in the β- and α-position. The mechanisms of the reactions were discussed.
KeywordsAnionic polymerization Poly(propylene oxide) ROP Potassium salts Unsaturation Macrocyclic ligands
List of initiators utilized as initiators in the present study
Propylene oxide (Aldrich) was dried over CaH2 and finally distilled at 307 K (34 °C). Anhydrous tetrahydrofuran (THF) (Across Organics) was kept over CaH2 and distilled at 339 K (66 °C). A 35 wt% dispersion of potassium hydride (KH) in mineral oil (Aldrich) was mixed with n-pentane in a dry argon atmosphere and then decanted. That procedure was repeated three times followed by a threefold washing with dry tetrahydrofuran. Then, the solvent was evaporated in vacuum. The KH present was determined by a standard gas law calculation of the hydrogen liberated after treating 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) and cryptand C222 (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8,8,8]hexacosane) (Merck) were used for synthesis without purification. Anhydrous methanol (Aldrich) and 2-hydroxymethyl-18-crown-6 (18C6-methanol) (Aldrich) were used for synthesis without purification. Other reagents, i.e. i-propanol, propylene glycol methyl ether, carbazole, triphenylmethane, potassium t-butoxide (1.0 M solution in THF), potassium bis(trimethylsilyl)amide, potassium triphenylborohydride (0.5 M solution in THF) and potassium diphenylphosphide (0.5 M solution in THF) all from (Aldrich) were also used without purification (Table 1).
Propylene oxide polymerization
All syntheses were performed at 20 °C in a 50 cm3 reactor equipped with a magnetic stirrer and a Teflon valve enabling substrates delivery and sampling under argon atmosphere. Potassium methoxide was obtained in the reaction of potassium hydride with methanol dissolved in tetrahydrofuran containing 18C6. The initial concentration of the monomer was equal to 2.0 mol/dm3 and the initial amount of potassium methoxide was 0.70 g/dm3 (0.1 mol/dm3). Potassium hydride (0.08 g, 2.0 mmol), 18C6 (0.53 g, 2.0 mmol) and tetrahydrofuran (17.2 cm3) were introduced into the reactor and then methanol (0.08 cm3, 2.0 mmol) was added by use of microsyringe, the reaction mixture was stirred during 2 h until all hydrogen (44.7 cm3) was evolved. It resulted in a fine dispersion of anhydrous potassium methoxide in the ether medium. Other salts, i.e. potassium i-propoxide, potassium propylene glycoxide methyl ether, carbazylpotassium and triphenylmethylpotassium  were synthesized the in similar way. Potassium potasside was prepared by dissolution of metallic potassium in THF containing 18C6 [25, 26]. All the systems were used as the initiators when propylene oxide (2.8 cm3, 2.3 g, 40 mmol) was introduced into the reactor. The reaction mixture was then stirred for about 2 weeks. After complete conversion of the monomer (99%) methyl iodide was added to transform alkoxide active centers into the methoxy end groups. After the potassium iodide precipitate had been separated, the solvent was evaporated at 20 °C yielding a viscous liquid polymer. The concentration of monomer during the polymerizations was monitored by the 1,4-dioxane method . The yields of the reactions were 95–97%.
Preparation of pentaethylene glycol methyl vinyl ether (8)
KH (1.0 g, 25 mmol) and THF (100 cm3) were introduced into the reactor. Then, diethylene glycol methyl ether (3.0 g, 25 mmol) was added dropwise by microsyringe. The course of the reaction was monitored by measuring the amount of hydrogen liberated. After 2 h of stirring potassium diethylene glycoxide methyl ether in THF was prepared (solution 1). In separate experiment potassium ethylene glycoxide vinyl ether was synthesized in the reaction of KH (1.0 g, 25 mmol) in THF (100 cm3) with ethylene glycol vinyl ether (2.2 g, 25 mmol) in the same manner. Then, 2-bromoethyl ether (5.8 g, 25 mmol) was added to the reactor. After 5 h of stirring, solution 1 was introduced to the system. After another 2 h of stirring, the potassium bromide precipitate was separated by decantation. The product present in the solution was distilled in a Kugelrohr apparatus; the fraction boiling at 120 °C, 0.16 mbar, consisted of pentaethylene glycol methyl vinyl ether. 13C NMR (acetone-d 6 ):δ 151.7 (OCH=); 86.5 (CH2=); 67.1–71.0 (OCH2, 6 signals); 58.9 (CH3). MS: m/e (rel intens) 219 (M-59, 0.1); 201 (0.2); 175(0.5); 147(0.5); 133(9); 117(7); 103(20); 87(18); 73(28); 59(100); 45(96); 43(45).
100 MHz 13C NMR spectra were recorded in CDCl3 at 25 °C on a Bruker Avance 400 pulsed spectrometer equipped with 5 mm broad-band probe and applying Waltz 16 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 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. Poly(propylene glycol)s were used as calibration standards. MALDI-TOF spectra were recorded on a Shimadzu AXIMA Performance instrument. Dithranol was used as a matrix. GS–MS analysis was run on a 30 m long DB1701 fused silica capillary column, using a Varian 3300 gas chromatograph equipped with a Finnigan MAT 800 AT ion trap detector. The methylated product (8) was identified by comparing its mass spectra and retention times with that of authentic compound. Diethylene glycol dimethyl ether was used as the internal standard for the yield measurements.
Results and discussion
Influence of the kind of initiator on PPO unsaturation
Active anionic centers in initiators molecules and polymers growing chains have characteristic basicity (b) and nucleophilicity (n). Basicity is responsible for deprotonation of the monomer, whereas nucleophilicity for its ring opening. Hence, the value of b/n ratio of initiator can be estimated from unsaturation level in the final polymeric substance. In all studied polymerisation, the basicity/nucleophilicity (b/n) ratio of alkoxide anion in growing chain end is obviously the same. As propagating species are the same for all chains, the results seem to indicate that the differences come from side reactions occurring only at the beginning (except the known transfer to monomer occurring during polymerisation). B/n ratio of initiator 3 is very similar to that of growing chain end. Thus, the latter should be capable of undergoing transfer reaction with the monomer resulting in the polymer unsaturation. However, b/n ratios of initiators 4, 5, 9, 10, 12 and 13 are markedly greater than that of the growing chain end. They differ from each other due to their different structure and influence on increase of polymer unsaturation. In the case of other initiators unsaturation of polymers is lower due to the fact that their b/n ratios are lower than that of growing chain end (1 and 6). B/n ratios of 2, 7 and 8 are near to b/n of 3. Thus, the polymers have unsaturation in wide range, that is, from 10.1 mol% for 6–86.6 mol% for 13 . Similar effect was observed earlier by Kricheldorf et al.  in the polymerization of β-butyrolactone initiated with potassium salts of alcohols, phenols, carboxylic acids or amino acids, some of them being activated dibenzo-18C6. The high yield of trans-crotonate groups resulting from polymerization initiated with potassium t-butoxide or potassium 4-chlorothiophenoxide proves that direct deprotonation of the monomer by initiator does occur. Signals of initiators were not found in 1H NMR spectra of the polymers obtained. It means that these initiators react exclusively as nonnucleophilic bases. Of particular interest is the formation of crotonate groups on addition of potassium benzoate (nucleophilic base). In this case b/n ratios of initiator and growing chain end are the same and chain transfer to the monomer takes place. The only experiments which did not yield crotonate groups were the polymerization initiated with potassium benzylxanthogenate or potassium N-t-butoxycarbonyl-l-alanine in bulk.
The spectrum reveals also the presence of hydroxyl end groups via its methine carbon signal [–CH(CH3)OH at about 65.6 ppm)] and methoxy end groups (–OCH3 at about 56.6 ppm).
The ability of crowned active ion pair 1′ in growing chain to deprotonation of monomer indicates that explanation of chain transfer inhibition by complexation of counterion given by Ionescu  is contrary to our results.
However, the source of vinyloxy starting groups formed in some systems is quite different. We assumed that they were created in the reaction of initiator with the ligand. Mechanism of this reaction will be discussed in “Mechanistic aspects of initiation of PO polymerization”.
Transfer reaction causes decrease of M n. It indicates that some amount of potassium salts is inactive even if they are soluble in THF, probably due to the formation of ionic aggregates.
Effect of macrocyclic ligands on the PPO unsaturation
The influence of the presence and kind of macrocyclic ligand on unsaturation and molar masses of PPOs was determined previously by us for polymerization initiated with anhydrous KOH . Unsaturation is very high in the systems without ligands and does not depend on initial monomer concentration. At low initial monomer concentration, i.e. 2.0 mol/dm3 unsaturation increases in the presence of ligands, i.e. 18C6 and C222.
effect of initial monomer concentration,
influence of ligand presence,
kind of ligand used.
Together with the increase of initial monomer concentration, distinct increase of polymer unsaturation was observed. As can be seen from Fig. 4 the investigated PPOs unsaturation strongly depends also on the presence and kind of macrocyclic ligands. In the polymerization performed with 18C6 and (4b) an increase of unsaturation as well as isomerization was observed. This effect was much more distinct when C222, a stronger ligand for K+, was applied for activation of the initiator. In this case (4c) 42% macromolecules have unsaturated starting groups, mainly cis-propenyloxy. Evidently, in the presence of the ligands the acceleration of monomer deprotonation (which depends on basicity of anions) occurs simultaneously with the acceleration of monomer ring opening (which depends on nucleophilicity of anions). However, the acceleration of the first reaction is greater than the acceleration of the second one. Isomerization of allyloxy groups depends on basicity of anions because it is ionic reaction. The highest isomerization in the system containing C222 can be explained by the highest basicity of chain growing centre.
Very high isomerization is probably connected with the presence of high amount of polar solvent, i.e. tetrahydrofuran in the system. Similar effect was previously observed for isomerization of simple allyl ethers in polar ether solvents as 1,2-dimethoxyethane or dimethylsulfoxide . The effect of the ligand on isomerization was also found at higher initial concentration of the monomer (4a′–4c′). Isomerization diminishes markedly and allyloxy groups even prevail in the systems containing 18C6 or C222 in this case. It presumably results from decrease of polar solvent concentration in the system. At the highest initial monomer concentration isomerization is strongly limited and does not occur in the systems containing the ligands (4a″–4c″).
The results obtained in the present work indicate, that the effect of ligand on unsaturation and molar mass of PPOs prepared with t-BuO−K+ in the absence of alcohol is opposite to similar systems, in which potassium alkoxide/alcohol mixture was used for initiation [8, 22]. The presence of the ligand should accelerate propagation as well as deprotonation of monomer and alcohol. Presumably, in this case the acceleration of chain transfer to alcohol is greater than the acceleration of chain transfer to monomer, resulting in decrease of polymer unsaturation.
However, analysis of polymer 4b by MALDI-TOF technique allow to detect two additional kinds of macromolecules, i.e. E and F.
Mechanistic aspects of initiation of PO polymerization
Part of 13C NMR spectrum of the polymer obtained in the polymerization with i-PrO− Open image in new window is shown in Fig. 6b. In the region for the methyl signal of i-propoxy starting group (CH3)2CHO– quite unexpectedly, two kinds of i-propoxy groups were detected at 22.01 and 22.15 ppm. We assumed, that these two signals could come from two different chain beginnings, formed during initiation by potassium i-PrO− Open image in new window , i.e. R1, (CH3)2 CHO–CH2–CH(CH3)O– (22.01 and 72.09 ppm, respectively) and R2, (CH3)2 CHO–CH(CH3)–CH2O– (22.15 and 72.13 ppm, respectively). The nucleophilic attack of initiator would concern less substituted carbon. Recently, we observed similar phenomenon in the polymerisation of styrene oxide in the presence of the same alkoxide . In both systems two signals of (CH3)2CHO– group were shown in the spectrum. The signals come from two different chain beginnings formed during initiation. However, in the polymerisation of 1,1-dimethyloxirane (isobutylene oxide) only one signal of (CH3)2CHO– group was detected (at 22, 21 ppm), which indicates ring-opening exclusively in the beta position. Hence one may conclude that steric reasons are responsible for the observed effect.
Three series of peaks with peak-to-peak increment equal to the molar mass of the monomer (58.08 g/mol) are shown in the spectrum. The series from m/z 1012.0 to 1942.2 represents macromolecules with Ph3HB– starting groups and –CH3 end groups. For example, the peaks at m/z 1070.5, 1534.2 and 1884.0 (marked with stars) represents such macromolecules possessing 14, 22 and 28 monomer units (M calc = 1071.2, 1535.9 and 1884.3, respectively). The second series from m/z 1026.2 to 1956.2 represents macromolecules having (CH3)2CHO– starting groups and –OH end groups as adducts with potassium ion. For example, the peaks at m/z 1374.7, 1666.0 and 1898.0 (marked with triangles) represents such macromolecules having 22, 27 and 31 monomer units (M calc = 1376.9, 1667.3 and 1899.7, respectively). The third series from m/z 1042.1 to 1911.6 represents macromolecules with (CH3)2CHO– starting groups and –CH3 end groups as adducts with potassium ion. For example, the peaks at m/z 1100.9, 1506.1 and 1854.3 (marked with circles) represents such macromolecules possessing 17, 24 and 30 monomer units M calc = 1100.5, 1507.1 and 1855.6, respectively).
Organopotassium intermediate, i.e. glycidylpotassium 7 formed after deprotonation of monomer is extremely unstable and decomposes immediately by oxirane ring-opening in the α-position giving 3′.
Unsaturation of PPOs prepared in the presence of different potassium salts activated 18C6 is high and depends strongly on the kind of initiator, changing from 10.1% for potassium triphenylmethane to 86.6% for potassium hydroxide.
The source of unsaturation (starting allyloxy group) is direct deprotonation of the monomer with initiator and active chain end; unexpectedly, in some systems additional kind of unsaturation (vinyloxy groups) was found, which results from deprotonation of 18C6 with initiator.
Unsaturation of PPOs synthesized with selected initiator, i.e. potassium t-butoxide increases markedly after its activation with 18C6 and especially C222; this effect is opposite to that observed earlier in the polymerization initiated with alkoxide/alcohol mixtures after addition of complexing agent.
Isomerization of allyloxy groups to cis-propenyloxy ones depends on the kind of initiator, ligand and initial monomer concentration; the highest isomerization was observed in the polymerization initiated with potassium alkoxides and low initial monomer concentration.
During the initiation of the polymerization oxirane ring of PO opens exclusively in the β-position, except the systems containing potassium i-propoxide; in this case ring-opening occurs unexpectedly in the β- and α-position (~1/1).
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Conflict of interest
The authors declare that they have no conflict of interest.
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