Mechanism of ε-caprolactone polymerization in the presence of alkali metal salts: investigation of initiation course and determination of polymers structure by MALDI-TOF mass spectrometry

ε-Caprolactone (Aldrich) was heated over CaH₂ for 24 h at room temperature and then distilled under reduced pressure in argon atmosphere prior use. Anhydrous tetrahydrofuran (THF) (Acros Organics) was kept over CaH₂ and distilled at 66 °C. Potassium hydride (KH) was purified according to the procedure described by Brown [23]. 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, the solvent was evaporated in vacuum. Methanol, isopropanol, carbazole, NaH, Ph₂PK, Ph₃HBK, Ph₃CH, t-BuOK (the last as 1.0 M solution in THF), 12C4, 15C5, 18C6, DCH24C8 and C222 (from Aldrich) were used without purification.

MeOK, i-PrOK, CbK and Ph₃CK were synthesized in the reaction of appropriate substrate with KH-activated 18C6 in THF solution at 20 °C [24]. All syntheses were carried out in a 50 cm³ reactor equipped with a magnetic stirrer and a Teflon valve enabling substrates delivery and sampling under argon atmosphere. During the reactions gaseous hydrogen was evolved. For example, KH (0.08 g, 2.0 mmol), 18C6 (0.53 g, 2.0 mmol) and THF (15.0 cm³) were introduced into the reactor and then methanol (0.064 g, 2.0 mmol) was added by the use of microsyringe. The reaction mixture was then stirred for 1 h until all H₂ (44.7 cm³) was evolved. This resulted in a fine dispersion of pure anhydrous potassium methoxide in the ether medium. That system was used as the initiator when ε-caprolactone (4.56 g, 40 mmol) was introduced into the reactor. Thus, the initial concentration of the monomer was 2.0 mol/dm³ and monomer/initiator molar ratio was 20/1. The reaction mixture was then stirred for several hours. After complete conversion of the monomer, methyl iodide (0.5 cm³, 8.0 mmol) was added to it as a quenching agent. After the separation of potassium iodide–crown complex precipitate from the system by filtration, THF was evaporated from the solution in vacuum at 50 °C, yielding white solid polymer. In all polymerizations, the concentration of the monomer during the process was monitored by SEC technique. The final conversion was ~ 99% after few minutes to 2 weeks. The yields of the reactions were 96–98%. Some of the processes were heterogeneous (e.g., initiated KH- or NaH-activated 12C4), others were homogeneous (e.g., in the presence of potassium alkoxides).

100 MHz, ¹³C NMR spectra were recorded in CDCl₃ at 25 °C on a Bruker Avance 400 pulsed spectrometer equipped with 5-mm broadband 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 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 with dithranol used as a matrix.

Potassium alkoxides, i.e., MeOK, i-PrOK and t-BuOK, were used for initiation of ε-CL polymerization. Analysis of the polymers obtained by ¹³C NMR gave information about final groups, present in the polymer chains after treating with MeI as quenching agent. Further data concerning polymer chemical structure were obtained by the use of MALDI-TOF technique. Figure 1 shows, for example, MALDI-TOF spectrum of the polymer prepared in the presence of t-BuOK.

Three main series of signals are observed in the spectrum. The series of very weak peaks, i.e., from m/z 889.1 to m/z 1800.5, represents macromolecules with t-butoxy starting groups and methoxy end groups. They form adducts with hydrogen ion. For example, the peaks at m/z 889.1, 1117.3 and 1459.9 represent macromolecules A with 7, 9 and 12 ε-caprolactone units (M_calc = 888.1, 1116.4 and 1458.8), respectively. These macromolecules form in the polymerization initiated with t-BuOK according to the reaction presented in Scheme 1.

¹³C NMR analysis of the polymer reveals strong signals characteristic for carbons derived from polymers of ε-CL. They are following signals, which correspond well with the literature data [25]: δ (in ppm): 24.6, 25.6, 28.4, 34.1, 64.2 (OCO(CH₂)₅), 173.5 (COO). Moreover, Fig. 2 shows weak signal of (CH₃)₃CO– starting group (at 28.1 ppm) and signal of –OCH₃ end group (at 63.0 ppm).

Then, it was proposed that the main series of signals in Fig. 1, i.e., from m/z 815.7 to m/z 1271.6, represents macromolecules without t-butoxy starting group, which form in the polymerization initiated with organopotassium compound 2 resulting from deprotonation of the monomer with initiator (Scheme 2). For example, the peaks at m/z 815.7, 1043.4 and 1271.6 represent macromolecules B with 7, 9 and 11 lactone units (M_calc = 814.0, 1042.3 and 1270.6), respectively, and one methoxy end group. They form adducts with hydrogen ion.

Third series of signals was observed in the spectrum at m/z 831.5–1971.8. For example, the peaks at m/z 831.5, 1400.0 and 1857.4 represent branched macromolecules with 7, 12 and 16 monomer units (M_calc = 828.0, 1398.0 and 1855.0), respectively, and two methoxy end groups, which form proton adducts. It was proposed that macromolecules 3 react mutually due to the presence of lactone ring, which can open by acyl-oxygen bond cleavage in the reaction with alkoxide end group. Macromolecules C with three arms form in the process (Scheme 3).

Series of signals derived from macromolecules A and B were absent in the spectrum of poly(ε-CL) prepared in the presence of t-BuOK activated by 18C6. Signals of macromolecules C were observed in the spectrum, as well as new series of signals at m/z 845.7–2803.3 (Fig. 3).

For example, the peaks at m/z 845.8, 1758.3 and 2445.6 represent more branched macromolecules with 7, 15 and 21 monomer units (M_calc = 842.0, 1755.0 and 2439.9), respectively, and three methoxy end groups which form adducts with hydrogen ion. Thus, we proposed that macromolecules 3 can also react with 4 giving branched macromolecules D (Scheme 4).

It is also worth noting that cyclic oligomers were not found in all studied systems. Similar phenomenon was observed in the polymerization initiated with other potassium alkoxides. However, in ¹³C NMR spectrum of the polymer prepared with the presence of MeOK weak signal of CH₃OCO– starting group (at 51.4 ppm) was shown (Fig. 4). It results from acyl-oxygen bond cleavage by initiator in this case.

Table 1 presents molar masses and dispersities of prepared polyesters. The ring-opening/deprotonation ratio values were calculated by comparison of intensities of the MALDI signals of peculiar macromolecules structure along with confirmation based on the ¹³C NMR data. Such protocol is proper as the chosen mechanism of initiation leads to formation of the same type of macromolecules with the same susceptibility to ionization.

It is worth noting that M_n of the polymers and their dispersities increase, in general, from (1) to (6). It could be consisted with the formation of branched macromolecules, especially in the systems (5) and (6) with highest M_w/M_n values. Exemplary SEC chromatogram of polymer (6) is shown in Fig. 5.

In the next step of the studies, it was observed that polymerization of ε-CL occurs also easily in the presence of KH. During the process no gaseous products were evolved. The reaction mixture was very viscous and almost homogeneous. After quenching with MeI, the polymer obtained was analyzed by MALDI-TOF technique. MALDI-TOF spectrum reveals two series of signals, which indicate the formation of two kinds of macromolecules (Fig. 6).

It was proposed that first series of signals, i.e., from m/z 815.7 to m/z 1385.6, represents macromolecules with aldehyde starting group and methoxy end group. For example, the peaks at m/z 815.7, 1043.2 and 1385.6 represent macromolecules with 7, 9 and 12 monomer units (M_calc = 816.0, 1044.3 and 1386.7), respectively. They form adducts with hydrogen ion. ¹³C NMR analysis of the polymer confirmed the presence of O=CH– (at 207.21 ppm) and –OCH₃ (at 62.95 ppm) groups (Fig. 7).

We suggested that in the first step of the process KH reacts with the monomer by addition to carbonyl group or direct nucleophilic ring opening in the acyl-oxygen position (Scheme 5) and gives product 5. It becomes the real initiator of the polymerization resulting in macromolecules E after methylation.

The second series of signals, i.e., from m/z 831.7 to m/z 1401.6, represents branched macromolecules with three terminal groups, i.e., one aldehyde group and two methoxy groups. For example, the peaks at m/z 831.7, 1059.3 and 1401.6 represent macromolecules with 7, 9 and 12 monomer units (M_calc = 832.0, 1060.3 and 1402.7), respectively. They form adducts with hydrogen ion.

It was proposed that during the polymerization part of macromolecules 6 possessing alkoxide and aldehyde terminal groups react mutually giving macromolecules F after methylation (Scheme 6).

Similar effect was observed in the polymerization carried out in the presence of crown ethers 12C4 or DCH24C8 which form weak complexes with K⁺ counterion. However, after activation of KH with strong ligands, i.e., 15C5, 18C6 or C222, MALDI-TOF spectrum of the methylated polymers reveals signals of new macromolecules. No signals of macromolecules E were found in the spectrum. One series of the signals was already observed in the previous spectrum and represents macromolecules F. However, the second series of signals represents macromolecules formed in other reaction. It was found in separate experiment that during the polymerization hydrogen was evolved from the reaction mixture. We proposed that hydrogen was a product of the monomer deprotonation by KH activated by strong ligand. Thus, in these systems KH behaves in the initiation step not only as nucleophile but also as a base. Potassium lactone enolate forms as the product of monomer deprotonation and initiates ε-CL polymerization. The reaction course is similar to that observed for t-BuOK (Schemes 2 and 3).

It was observed that NaH without ligand and with 12C4 or DCH24C8 does not initiate ε-CL polymerization. However, in the presence of strong ligands, i.e., 15C5, 18C6 or C222, polymerization occurs easily. In the systems with 15C5 or 18C6, initiation takes place exclusively by ring opening of the monomer and macromolecules A and B were formed, whereas in the presence of C222 hydrogen was evolved, which indicated that also deprotonation of the monomer occurred (Table 2).

Molar mass and dispersity of the polymers obtained in the presence of alkali metal hydrides (7–15) were markedly higher in comparison with those prepared with other alkali metal salts (1–6 and 16–19).

Summarizing, alkali metal salts used in this work for ε-CL polymerization react in the initiation step mainly by deprotonation of the monomer as strong bases (5, 6 and 19), as nucleophilic bases (9, 12, 13 and 15) or nucleophilic reagents, which open monomer ring in the acyl-oxygen position (7, 8, 10, 11, 14 and 16–18). This effect depends on the nature of initiator, i.e., its nucleophilicity/basicity ratio. A broader representation of alkali metal salts like lithium and sodium ones for ε-CL polymerization is planned in the future studies.