Theoretical and experimental studies of the initiator influence on the anionic ring opening polymerization of propylene oxide

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Journal of Molecular Structure 879 (2008) 40–52 www.elsevier.com/locate/molstruc

Theoretical and experimental studies of the initiator influence on the anionic ring opening polymerization of propylene oxide Gabriel Cendejas a, Ce´sar Andre´s Flores-Sandoval a, Nelson Huitro´n a, Rafael Herrera b, Luis S. Zamudio-Rivera a, Hiram I. Beltra´n c, Flavio Va´zquez a,* a

c

Instituto Mexicano del Petro´leo, Programa de Ingenierı´a Molecular, Eje Central La´zaro Ca´rdenas 152, San Bartolo Atepehuacan, Me´xico, D.F. 07730, Mexico b Universidad Nacional Auto´noma de Me´xico, Facultad de Quı´mica, Me´xico, D.F., Mexico Departamento de Ciencias Naturales, C.N.I., UAM-Cuajimalpa, Av. Pedro A. de los Santos 84, San Miguel Chapultepec, Me´xico, D.F. 11850, Mexico Received 29 March 2007; received in revised form 9 August 2007; accepted 13 August 2007 Available online 2 September 2007

Abstract In this work, the influence of three different initiators (KOH, KOH dissolved in ethanol and the potassium salt of ethylene glycol) on the propylene oxide polymerization was studied by experimental and theoretical methods. A first series of reactions was carried out to establish the adequate thermal conditions for a minimal monomer transfer during the polymerization. The formation of end insaturations (main consequence of the monomer transfer interference) in the poly(propylene oxide) chains was studied by spectroscopic methods. Furthermore, a second series of poly(propylene oxide)s was prepared by using the mentioned initiators, and characterized by size exclusion chromatography. The initiator efficiency to create active centers in every reactive system was determined from the molecular weight and the conversion data obtained. Experimental results were elucidated by using quantum chemical calculations at density functional theory level, involving thermo-chemistry parameters, and the simulation of the infrared, and 13 C nuclear magnetic resonance spectra. This method led to studying the addition of up to ten propylene oxide unit, resulting into important energetic tendencies and regioselectivity, being compared to the physicochemical data of products obtained. These correlations meant further understanding of the reaction course and the type of products obtained, depending on the nature of the initiator.  2007 Elsevier B.V. All rights reserved. Keywords: Propylene oxide; Anionic ring opening polymerization (AROP); DFT; Initiator; Regioselectivity

1. Introduction Despite the substantial progress achieved within the last two decades in the polymeric materials area, one of the first synthetic methodologies for the preparation of a wide variety of homo-, hetero-, co-, ter,- and polymeric materials is anionic ring opening polymerization (AROP), it cannot indeed be considered a closed field. Although the number known of cyclic monomers used as starting materials, for the AROP reaction is rather limited, this procedure often *

Corresponding author. Tel.: +52 55 9175 6380; fax: +52 55 9175 840. E-mail address: [email protected] (F. Va´zquez).

0022-2860/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2007.08.023

creates unique possibilities, such as a controlled synthesis of macromolecules with various carbon and hetero-atoms set in regularly repeated moieties. Another synthetic point is the possibility to obtain equal moieties in the head and tail positions of the macromolecular entity, leading to further functionalization or coupled polymerization reactions. One of the most important categories of such macro molecules, is that constituted by the polymers derived from propylene oxide (PO). Polypropylene oxides (PPOs) have a great demand due to the different applications these materials can offer. Some of the expected applications are enclosed within their use as impact modifiers, surfactants, de-emulsifiers, dispersant agents, fuel additives, wetting

G. Cendejas et al. / Journal of Molecular Structure 879 (2008) 40–52

41

enhanced application properties. A specific example is the use of low molecular weight PPOs as intermediaries of the preparation of de-emulsifiers for breaking water/ petroleum emulsions [9–12]. However, the AROP from PO differs from that of ethylene oxide (EO) mainly in one aspect: for the former one, there is a higher amount of the starting material going through undesirable chain transfer reactions, thus, an elimination product is obtained. Therefore, this side reaction yields PO oligomers with a large fraction containing unsaturated end-groups (Fig. 2) [9,13–17]. It must be outlined that the occurrence of transfer reactions decrease the number of hydroxyl functions attached to the macromolecules, and causes a concomitant loss of potential reactive sites for the desired PPO transformation [9,16]. The transfer of the anion to the monomer during the PO polymerization also produces an increment of the polydispersity from the resulting material. In the case of polymerizations performed in a solvent phase, transfer reactions to the solvent molecules, have also been reported [14,18–24]. On the other hand, quantum chemical studies have been widely employed in polymer science to explain the poly-

agents, lubricants, rheological modifiers, biomedical applications, adhesives and intermediaries in the manufacture of urethane elastomers or block copolymers [1–7]. The PO moiety may be polymerized in a wide range of synthetic procedures, the type of product and molecular weight varies; depending on the preparation system used. Alkali metal hydroxides are often applied as initiating species in the PO bulk polymerization. It has been observed that KOH, RbOH and CsOH are more efficient initiators than NaOH and LiOH [8]. The latter tendency has been thoroughly reported due to the higher solubility from the former three hydroxides in the liquid raw monomer [4,5]. Regarding stated properties and further applications for PPOs, the PO polymerization mechanism is carried out, as shown in Fig. 1. As previously mentioned, hydroxyl terminal groups, present a very interesting, and promising reactive potential, as they represent a target group for further chemical modifications with a wide variety of specific substrates (for example, the substitution of the hydroxyl groups with aromatic and aliphatic amines, other amino containing building blocks, cyanide functional groups, and so on) leading to obtaining materials with new and

+ M

O O Initiation step:

+

MO R

RO

Me

O

M

Me O

+ O

+ M

Me

+

Propagation step:

Me

O RO

Me

Me RO O

Where: Tail

Head RO

Me

Fig. 1. Polymerization mechanism of PO (R = H, C2H5 and –OC2H5).

O O

+ M

Me O

Me Transfer step:

O

Me

+ O

RO

Me Me

+

+ O M

RO

Fig. 2. Formation of polymers with unsaturated end-groups as a consequence of a monomer transfer reaction.

42

G. Cendejas et al. / Journal of Molecular Structure 879 (2008) 40–52

merization mechanism. In this sense, for AROP from PO, it is of significant importance, to investigate the polymerization mechanism, to explain the performance of the initiator moiety, and the structure of the polymer obtained. Very recently, the cationic and anionic polymerization mechanisms of ethylene and propylene oxide have been studied by using quantum chemical methods [25]. Zhang et al. observed that the butoxy anion mainly attacks the less hindered carbon of propylene oxide (secondary carbon), to produce a new secondary alkoxide. This attack is performed through a SN2 process. However, the authors only reported the addition of two oxide molecule, and for studies regarding stereochemistry of the resulting polymeric molecules, as well as for other physicochemical characteristics, these were preliminary results. As far as we know, no theoretical studies were found to explain the influence of the hydroxyl, ethoxide or dianionic structure of initiator moieties when employed to start the AROP from PO. In this work, the influence of KOH (KOH/PO), KOH dissolved in ethanol, (KOH/EtOH/PO) and the potassium salt of ethylene glycol, (KEG/PO) as individual initiators of the PO polymerization was studied through experimental and theoretical methods. A first series of reactions was carried out to establish the adequate thermal conditions for a minimal monomer transfer, during the polymerization. The formation of end insaturations (main consequence of the monomer transfer interference) in the PPO chains was studied through spectroscopic methods. Afterwards, a second series of PPOs was prepared by using the mentioned initiators, and characterized by size exclusion chromatography (SEC). The initiator efficiency to create active centers in every reactive system was determined from the molecular weight and conversion data obtained. The experimental results were contrasted to those obtained from quantum chemical calculations employing DFT at the B3LYP/631G+(d) level of theory [26–28]. This method led to studying the addition of up to ten propylene oxide units [29] providing important energetic tendencies, being compared to physicochemical data of the products obtained. These correlations resulted into further understanding of the reaction course, and the type of products obtained, depending on the nature of the initiator. The results of the current research are following stated. 2. Materials and methods

2.2. Synthetic procedure 2.2.1. Preparation of di-potassium ethyleneglycolate (KEG) Ethylene glycol (0.064 mol) was dissolved in 80 mL of methanol and then mixed with 0.129 mol of potassium hydroxide. The solution was heated at 80 C during 2 h of stirring, hence 1 mL of dry benzene was added and the system was switched to the vacuum, to remove the water and solvent to lead the di-potassium glycolate as raw initiator. After this preparation step, the filling of the reactor with monomer was achieved. The remaining steps were performed as in the KOH initiator synthesis. 2.2.2. Polymerizations The reactions were performed in a glass reactor (Parr) with digital stirring rate control, as well as pressure and temperature controls. The drain of the reaction vessel with a nitrogen inlet ensured the required anhydrous atmosphere. In order to prepare a first series of PPOs by bulk polymerization, a constant amount of monomer and a quantity of finely powdered KOH were introduced into the reactor. The initiator content was varied to obtain PPOs with different molecular weights. The set up of the reactor consists on three fundamental steps, (i) filling of the vessel with the starting mixture, consisting of initiator and monomer; (ii) drain and pressurization of the reactor with nitrogen to achieve an inert atmosphere; and (iii) controlled heating during the reaction course. The polymerizations were carried out at temperatures of 80 and 100 C. A second series of reactions was performed at 70 C in order to study the influence of the initiation process on the PO polymerization. For some of these experiments, the KOH was previously predisolved in 10 mL or 20 mL of ethanol (an excess of alcohol is necessary to solubilize the initiator). In the case of the polymerization initiated by KEG, the synthesized salt of ethylene glycol once prepared was introduced directly into the reactor. All polymerization formulations within this scheme are summarized in Table 1. The reactions were terminated with a stoichiometric amount of a 85% (v/v) phosphoric acid solution. Later, the polymeric material was separated in the organic fraction, by extraction with a solvent mixture containing 70 mL of hexane and 70 mL of bidistilled water. The final conversions were determined through gravimetric measurement of non-volatile material.

2.1. Materials Propylene oxide (PO), Aldrich, 99%, (±)-methyloxirane) was used in the synthesis. Pellets of anhydrous KOH (Fermont, 99%), phosphoric acid (Aldrich, 85%), ethylene glycol (J.T. Baker, 99%) tetrahydrofuran anhydrous (THF, Mallinckrodt, 99.8%), ethanol (Sigma–Aldrich), methanol (Sigma–Aldrich) and hexane (Sigma–Aldrich) were used according to receipt.

Table 1 Formulations for PO polymerizations using three different initiators Reaction number

1

2

3

4

5

6

T (C) PO (g) KOH (g) C2H5OH (g) KEG (g)

70 90.00 2.00 – –

80 90.00 2.00 – –

100 90.00 2.00 – –

70 90.00 2.00 8.16 –

70 90.00 2.00 16.32 –

70 90.00 – – 2.00

G. Cendejas et al. / Journal of Molecular Structure 879 (2008) 40–52

2.3. Characterization 2.3.1. FTIR spectroscopy The synthesized PPOs were characterized by FTIR with a Brucker Tensor 27 spectrometer showing in all cases, the C–O characteristic band at the interval of 1100–1050 cm1. 2.3.2. NMR spectroscopy The 1H and 13C NMR spectra were obtained from 150 mg of each of the samples dissolved in CDCl3. The NMR experiments were performed in a Varian NMR spectrometer model Mercury-BB at 200 MHz. The chemical shifts were referenced to tetramethylsilane (1H, d = 0.0 ppm, and 13C, d = 0.00 ppm). 2.3.3. Size exclusion chromatography The molecular weight distribution of the synthesized polymers was determined by Size Exclusion Chromatography (SEC), using an Agilent 1100 series Chromatograph consisting on a 5 lm column of Plgel and employing tetrahydrofuran (THF) as eluent. The flow rate was 1 mL min1 at 30 C (determined with the refraction index detector attached to the equipment). The synthesized polyethers were dissolved in THF at a concentration of 1.5 mg mL1. The average molecular weights Mn, Mw and the polydispersity index (I) were calculated from the SEC data. Calibration was made with a polystyrene standards kit. The average molecular weights and polydispersity index of a series of PPO standards purchased from Polymer laboratories (Mn = 580, 1270, 2960, 7200, 21,000, 50,400, 113,300, 325,000 and 696,500 g mol1) were determined by SEC to check that the calibration with PS is appropriated [30]. 2.4. Theoretical methodology Semiempirical and DFT calculations were made in this work. In the first case, the PM3 calculations were run in the Hyperchem software v. 6.0. A Polak-Ribiere (conjugate ˚ mol) were gradient), and RMS gradient of 0.01 kcal/(A employed to reach the minimal energy structure. DFT calculations are implemented in the Gaussian 98 software [31]. Energies were obtained by using Becke’s exchange (B), Lee, Yang and Parr (LYP) correlation, within the hybrid functional (B3LYP) approach [28,32] and using the 6-31G+(d) basis set [26,33]. The 13C NMR shielding tensors were calculated by using the methodology of Gauge Independent Atomic Orbitals (GIAO) [34–37] at B3LYP/6-31G+(d) level. The isotropic shielding values defined as riso ¼ 1=3ðr11 þ r22 þ r33 Þ (rii being the principal tensor components) were used to calculate the isotropic chemical shifts d with respect to TMS (dX = rTMS  rX) [38–40]. Furthermore, the IR spectra were obtained at the same theory level. The transition structures were searched by using the QST3 method [41]; then the resultant conformation was

43

optimized under the transition state (TS) key work. Furthermore, the frequency analysis was performed on the transition state with the correspondent basis set. The Zero-point energy correction (ZPE) was performed for all the calculations. The sense of the reaction was deducted through the thermochemical parameters. An alternative method to obtain the reaction path has been described by Grubmu¨ller [45]. Finally, the solvation energies in ethanolic solution with dielectric constant (e) of 24.55 were calculated by using the Polarizable Continuum Model (PCM) [42]. The solvation Gibbs free energy (DGs) was obtained at B3LYP level, with the same basis set. 3. Results and discussion 3.1. Experimental results 3.1.1. First series, KOH/PO system at T = 80 and 100 C A first series of PPOs was synthesized at 80 and 100 C. Both reactions were performed at a constant initial composition of KOH/PO of 2/90 g/g. The results of these polymerizations are shown in Table 2. The initiator efficiency, defined here as the quotient of the experimental and theoretical concentration of active centers, was calculated by employing the equation below [8]. f ¼

x½M0 MW M ½I0 M n

All the polymers were characterized by FTIR (an example is shown in Fig. 3). This spectroscopic technique confirmed that the AROP procedure occurred through the expected polymerization route in the temperature reaction interval. The absorption band at 3460 cm1, corresponding to the vibration stretching of the (OH) in the polyether, must be remarked as characteristic signal. Bands near 2930 and 2970 cm1 were also observed corresponding to the stretching modes of the (CH3). The bands near 2850 and 2880 cm1 are relative to the (CH2) stretching modes. A very strong signal was detected at 1100 cm1 being attributed to the (C–O) stretching mode characteristic of a polyether. Afterwards, the PPOs synthesized by AROP using KOH as initiator, were characterized by 13C NMR (see Fig. 4). The set of signals led to the detection of a double bond in the polymer chains prepared at different temperatures, Table 2 Reaction time, conversion and average molecular weight of PPOs obtained by AROP initiated with KOH at different temperatures (KOH/ PO = 2/90 wt./wt.) Temperature (C)

Time (h)

Final conversion (%)

Mn (g mol1)

I

Initiator efficiency (%)

80 100

20 18

45 45

2800 3000

1.40 1.57

0.28 0.29

44

G. Cendejas et al. / Journal of Molecular Structure 879 (2008) 40–52

spread when the polymerizations were performed at lower temperatures. It must be then concluded that the effects of the monomer transfer reaction decreases with the temperature.

Fig. 3. FTIR spectrum of PPOs synthesized by anionic bulk polymerization using KOH as initiator (KOH/PO = 2/90 wt/wt, T = 80 C).

they were specially evidenced when the PO was polymerized at 100 C. This double bond was produced by the stated transfer reaction at one of the terminal hydroxyl groups. Three main signals must be remarked: two from an olefinic CH2 carbon (at 116.9 and 135 ppm) and a third signal from an OCH2 carbon at 72.4 ppm. These three resonance signals are consistent with the assignment to an allyl ether end group. The 13C NMR signals appeared

3.1.2. Second series, KOH/PO, KOH/EtOH/PO and KEG/ PO systems at T = 70 C A second series of reactions was carried out in order to determine the influence of the initiation system; two additional kinds of process were included: KOH pre-dissolved in ethanol and the potassium salt of ethylene glycol. All the polymerizations were performed at 70 C in order to reach a minimal interference of the monomer transfer reaction. The polymers prepared with KOH again displayed high molecular weight polydispersity and low initiator efficiency (see Table 3 and Fig. 5). The 13C NMR spectrum of these materials, only showed very small signals associated to the double bond produced by monomer transfer reaction (Fig. 4). In contrast to the previous synthesis, the use of KOH pre-dissolved in ethanol led to obtaining low molecular weight polymers. The polydispersity index of these oligomers was smaller than that of polymers prepared with KOH. However, in the case of the polymerization containing pre-dissolved KOH, a simple calculation of the initiator

Fig. 4. 13C NMR spectrum at 50 MHz of PPOs synthesized by AROP initiated with KOH at 100 C. The signals corresponding to the double bonds at the chain end for PPOs synthesized at 100, 80 and 70 C are compared.

Table 3 Reaction time, conversion and average molecular weight of PPOs obtained by AROP at 70 C using, KOH, KOH/C2H5OH solution and KEG salt as initiators Initiation system

Time (h)

Final conversion (%)

Mn (g mol1)

I

Initiator efficiency

KOH KOH/C2H5OH KEG

20 18 16

33 70 85

2600 683 3100

1.3 1.16 1.10

0.32 –a 0.88

a

It was not estimated due to solvent transfer.

G. Cendejas et al. / Journal of Molecular Structure 879 (2008) 40–52

45

assigned to the methyl carbons and those around 65– 69 ppm to terminal groups in the polymer. In the case of signals around 73–77 ppm, they could be associated either to methyne or methylene carbons. The peaks at 116.9 and 135 ppm corresponding to the double bonds at the end of the polymer chains did not show up.

KOH

KOH/C2H5OH

3.2. Theoretical results

KEG

0

2

4

6

8

10

12

14

16

Elution volume (ml) Fig. 5. SEC of PPOs synthesized with KOH, KOH/C2H5OH and KEG as initiators at T = 70 C.

efficiency revealed values higher than 1, indicating an interference of active centers different from those produced by the KOH. This could be an evidence of a transfer reaction with the ethanol. An additional polymerization of PO, at the same conditions of the KOH/EtOH/PO reaction, but increasing the ethanol content in the reactor, revealed a sensitive decrease of the molecular weight (Mn = 383 g mol1, I = 1.12). It is now apparent that there is a transfer reaction with the solvent during the polymerization. However, the spectroscopic characterization by 13C NMR of the oligomers did not show the signals corresponding to the double bond, produced during a transfer with the monomer (see Fig. 6). A low average molecular weight and a narrow polymer distribution were determined by SEC when the PO polymerization was started with the salt of ethylene glycol and potassium (see Fig. 5). These results indicate that short PPO chains were synthesized because a high/great number of initiation centers activated the monomer molecules, during the early stages of the polymerization. The calculation of the initiator efficiency (f = 0.88) also confirmed a high/ great initiation rate during the reaction, leading to reaching high monomer conversions. The FTIR spectrum of the PPO obtained from the polymerization, initiated by KEG initiator also displayed at 3460 cm1 the absorption band corresponding to the (OH) stretching in the polyether, similar to that in Fig. 3. In addition, the 13C NMR spectra revealed that the signals belonging to the olefinic CH2 carbon, at 116.9 and 135.0 ppm, and that from the OCH2 carbon, at 72.4 ppm, are not present. It may consequently be concluded that the formation of a double bond, at the end of the polymer chains was banned; hence, there is no evidence of monomer transfer reactions during this AROP process. Fig. 6 shows the 13C NMR spectra from the different PPOs dissolved in chloroform at 20 C and the assignment of the main signals. The peaks at 17–18 ppm can be

3.2.1. HO initiator (System A) Firstly, the monomer activation with HO (system A) is described. In this system the intermediate RO1 (see Fig. 1) has an energy value of 268.8703 Hartrees and a dihedral angle at the O1–C1–C2–O2 fragment of 164.84. The energy value was corrected for zero-point energy (ZPE). From now on all energies have included the ZPE correction. After the formation of RO1, as mentioned above, another PO molecule was added. Nevertheless, due to the fact that a racemic mixture of PO was employed, regarding the methyl groups, the new intermediate formed could have the methyl groups in syn- (Aa, Fig. 7), anti-position (Ab, Fig. 7) or a mixture of both. Such syn or anti orientation were defined depending on the relative direction of the methyl groups regarding an imaginary straight-line through O1–Oi (i = 3–11) positions. The propagation step during the PO polymerization was subsequently studied. Table 4 lists the energy values for the addition of PO monomers, to obtain the product with syn or anti orientation. As it may be observed from this table, the anti addition is preferred to the syn addition. Furthermore, when the chain is growing, a series of consecutive syn additions would cause a curvature in the chain, while with a succession of anti addition the chain remains almost linear (Fig. 8). This curvature could be due to electronic effects of non-bonded fragments (steric hindrance); as a Newman projection through C2–C4 atoms is observed, the methyl groups are in gauche disposition when a syn addition is produced (Fig. 9a), whereas after an anti addition the methyl groups present an antiperiplanar disposition (Fig. 9b). Due to the fact that PO was employed as a racemic mixture, stated above, products with a combination of syn and anti additions could be obtained. Thus, a polymer chain obtained from the subsequent reaction of five PO monomers was considered to include all the possible combinations. For doing it, the above stated way, an imaginary line was traced from O1 to O6 (Fig. 10). Then, the hydrogen atoms remaining up to the line are labeled as U, while those lying below the line are labeled as D. Table 5 shows all the addition possibilities. As can be observed the difference among energy values for all the possibilities are very similar; however, the gap value shows that a polymerization with the methyl groups in a syn orientation is preferred. On the other hand, due to basic reaction conditions, and to the acid character of hydrogen from the initial alcohol group in the polymer, having an atomic charge of 0.1788

46

G. Cendejas et al. / Journal of Molecular Structure 879 (2008) 40–52

Fig. 6.

13

C NMR spectra of PPOs synthesized by AROP using KOH, KOH/C2H5OH and KEG as initiators, at T = 70 C.

Fig. 7. Syn- (Aa) and anti- (Ab) conformations for PO polymerization initiated by OH- (intermediate RO1).

G. Cendejas et al. / Journal of Molecular Structure 879 (2008) 40–52

47

Table 4 Energy values for the syn- and anti-propagation during the AROP of PO using HO as initiator (intermediate RO1) PO unitsa

2 3 4 5 6 7 8 9 10 a

DE (EAa  EAb)

Energy (Hartrees) Aa

Ab

460.9339 654.9936 848.0537 1041.1137 1234.1725 1427.2329 1620.2924 1813.3519 2006.4119

460.9352 654.9984 848.0610 1041.1236 1234.1867 1427.2471 1620.3121 1813.3730 2006.4381

0.0013 0.0048 0.0074 0.0099 0.0142 0.0142 0.0197 0.0210 0.026

Number of added propylene oxide molecules.

(obtained through the ESP analysis population), this atom could be removed to form a dianionic species (see Fig. 11). In this case, the dianionic compound has an E+ZPE value of 268.1304 Hartrees, being higher in energy than the RO1 intermediate. Although for energetic stability, compound RO1 is preferred to a dianionic species, the calculations were carried out taking into account the possibility of formation of this compound. The dihedral angle for the fragment O1–C1–C2–O2 in the dianionic species is higher (3.49 more than that found for intermediate RO1) due to the electronic repulsion of two oxygen atoms with negative charge. This PPO chain has two different oxygen atoms: one bonding to a primary carbon atom (Fig. 7), and the other one bonding to a secondary carbon atom. Both atoms could act as nucleophilic species. However, due to the steric hindrance, the primary oxygen should be a better nucleophile. Calculations for this case were performed in the following way, first: (a) growing of the living chain for both sides (system Ac); (b) propagation only from the primary

Fig. 9. Newman projection through C2–C4 atoms. (a) syn additions; (b) anti additions (PO polymerization initiated by HO).

Fig. 10. PPO with a polymerization degree Xn = 5, obtained from a polymerization initiated with HO. An imaginary line may be traced from O1 to O6. The UUUDD system is shown here (U: up, D: down).

oxygen (system Ad) and (c) an increasing of the number of monomers, only at the secondary oxygen (system Ae). The three possible directions of the growth of the dianionic species are shown in Fig. 11. For the three systems both syn- and anti-possible conformations were calculated; Table 6 shows the energetic values obtained. However, the generation of the dianionic

Fig. 8. Polymerization of 10 PO units activated with HO- after a series of subsequent: (a) syn additions, showing a curvature of the chain and (b) anti additions, remaining as a linear chain.

48

G. Cendejas et al. / Journal of Molecular Structure 879 (2008) 40–52

Table 5 Energy values for the possible arrays of a PPO chain with five monomers (system A5) System

Arraya

Energy (Hartrees)

Gap (E5syn  E5*)

Gap (E5anti  E5*)

A5a A5b A5c A5d A5e A5f A5g A5h A5i A5j A5k A5l

UUUUD UUUDU UUDUU UDUUU UUUDD UUDDU UDDUU UUDDD UDDDU UDDUD UDUDD UDDDD

1041.1151 1041.1181 1041.1186 1041.1184 1041.1159 1041.1184 1040.8812 1041.1163 1041.1186 1041.1212 1041.1213 1041.1164

0.0014 0.0044 0.0049 0.0047 0.0022 0.0047 0.2354 0.0026 0.0049 0.0075 0.0076 0.0027

0.0086 0.0055 0.0050 0.0052 0.0077 0.0052 0.2424 0.0073 0.0051 0.0024 0.0023 0.0073

Polymerization initiated by HO (intermediate RO1). a UUUUU = A5syn, UDUDU = A5anti.

O-

O-

O-

-O

-O

-O

Fig. 11. Possible directions of propagation for a dianionic species formed after the activation of the AROP of PO initiated with HO.

species produces new intermediates with higher energy than the systems Aa and Ab, shown in Fig. 7, the formation of such dianionic species is feasible, due to reaction conditions. Another inconvenience related to this kind of system is that when a water molecule is formed in the reaction medium, during the propagation step, this causes a termination step of the polymerization process. Thus, the polymer obtained is a complex mixture deriving from the intermediates mentioned above. The theoretical modeling of the PO polymerization activated by HO revealed that PPO chains grow at different propagation rates, exhibiting different lengths, being easily correlated to the high polydispersity index determined by SEC (see Tables 2 and 3). The relatively low stability of the anionic species HO, regarding the EtO values, might be associated to the poor conversions observed when the

polymerization was initiated with KOH. Furthermore, as mentioned above, the formation of a dianionic species reduces the yield of polymerized monomer, due to water formation. 3.2.2. EtO- initiator (System B) The anionic living polymerization of PO initiated by EtO- (intermediate RO2, obtained from a KOH/C2H5OH mixture) was also studied. When this nucleophile attacks one PO monomer, the intermediate RO2 (Fig. 12) was obtained. This intermediate has an E+ZPE value of 347.4389 Hartrees. About to the dihedral angle value for the O1–C1–C2–O2 fragment (165.56), the introduction of the ethyl fragment did not modify significantly the conformation of the system. The addition of more PO units to the intermediate RO2 was the following step, in a similar way to that in HO case. Only two possibilities of polymerization, regarding methyl (similar to hydroxyl group) were considered: a series of consecutive syn additions (system Ba) or a series of consecutive anti additions (system Bb). Energy values obtained for both series are listed in Table 7. In the same way as for the system A, the anti addition is here preferred to the syn addition. However, the presence of methyl moiety provokes that the energy of the system be lower than that from system A. Furthermore, due to the absence of the proton acid from the alcohol group, no dianionic species could be formed under this system. The modeling of the propagation in the PO polymerization started by EtO led to forecasting a favorable and regular growing of the PPO chains and the obtaining of a monodisperse polymer population. However, the ethanol

Fig. 12. Structure of the intermediate RO2 obtained during the PO activation with EtO (system B).

Table 6 Energy values for the Ac (both sides growing), Ad (propagation from primary oxygen) and Ae (propagation from secondary oxygen) systems in the reaction of a dianionic species and PO units (polymerization initiated by HO) PO units*

2 3 4 5 6 7 8 9 10

Energy (Hartrees) Acsyn

Acanti

Adsyn

Adanti

Aesyn

Aeanti

– 654.3573 – 1040.5007 – 1426.6309 – 1812.7606 –

– 654.3446 – 1040.5021 – 1424.5297 – 1811.8914 –

461.2678 654.3572 847.4325 1040.5020 1233.5669 1426.6308 1619.6953 1812.2208 2005.8137

461.2679 654.3588 847.4366 1040.5079 1233.5781 1426.6449 1618.5239 1812.7747 2005.8394

461.2619 654.3491 847.4245 1040.4944 1233.5599 1426.6248 1619.6877 1812.7492 2005.8134

461.2619 654.3542 847.4328 1040.5050 1233.5736 1426.6406 1619.7062 1812.7708 2005.8359

G. Cendejas et al. / Journal of Molecular Structure 879 (2008) 40–52 Table 7 Energy values for the Ba and Bb systems in the polymerization reaction of PO initiated by EtO (intermediate RO2) PO unitsa

2 3 4 5 6 7 8 9 10 a

Energy (Hartrees) Ba

Bb

540.5035 733.1631 926.6252 1119.0466 1312.7454 1505.8046 1698.8655 1891.9252 2084.9633

540.5071 733.1665 926.6326 1119.0422 1312.7535 1506.8177 1698.8796 1891.9460 2084.9806

DE (EBa  EBb) (Hartrees) 0.0036 0.0034 0.0074 0.0078 0.0081 0.0131 0.0141 0.0173 0.0208

Number of PO units.

present in the medium releases a great number of H+, producing an undesirable anion transfer to the solvent and the breakage of the propagating polymer chains. 3.2.3. –OCH2CH2O– initiator (System C) On the other hand, for the case of –OCH2CH2O– initiator (intermediate RO3), four polymerization ways were considered. The first one was a symmetric growth during a series of consecutive syn addition (Ca). The second one was a symmetric propagation following anti additions (Cb). The third one is a growing through only one side through a sequence of syn additions (Cc). The last case implies a one side growing, obeying a series of anti additions (Cd) (Fig. 13). The energy values for both systems are listed in Table 8. Anti addition is also preferred under the syn addition. However, regarding the values of Table 8, a symmetrical propagation is preferred than an asymmetric propagation, although the higher gap is approximately of 0.0084 Hartrees (5 kcal mol1); therefore, under the reaction conditions, an asymmetric propagation could be observed. Another important point to stress out is that the calculation from the energetic value when two PO units are added reveals that system C is more stable than systems A or B (–POPO–O–CH2–CH2–O–POPO– > CH3–CH2–

49

O–POPO– > HO–POPO–). For example, when the energy value was compared for Ab and Bb regarding Cd for 10 PO units, the last one is 153.1716 and 74.6291 Hartrees (96,115 and 46,830 kcal mol1) more stable than Ab and Bd, respectively. Therefore, it may be correlated to an easier activation monomer by each initiator. In addition, in the case of the polymerization initiated by dipotassium glycolate, the lower gap value means that there is no preference for a specific growing paths (symmetrical or unsymmetrical), which allows obtaining a monodisperse polymer population. This theoretical result was confirmed by the SEC measurements. 3.3. Transition states On the other hand, the transition states (TS) were obtained from the first PO addition with the three initiators employed in this work. To carry it out, the structures in the TS were searched by using the QST3 method. A frequency analysis was performed on the transition state structure. Fig. 14 shows the TS structures obtained, while the geometric parameters for the TS structures are listed in Table 9.

Table 8 Energy values of the Ca, Cb, Cc and Cd configuration systems obtained during the polymerization reaction of PO initiated by -OCH2CH2O(intermediate RO3) PO unitsa

1 2 3 4 5 6 7 8 9 10 a

Energy (Hartrees) Ca

Cb

Cc

Cd

– 615.0712 – 1001.2162 – 1387.3454 – 1773.4704 – 2159.5934

– 615.0712 – 1001.2201 – 1387.3565 – 1773.4867 – 2159.6188

421.9634 615.0618 808.1377 1001.2065 1194.2722 1387.3368 1580.3999 1773.4618 1966.5236 2159.5849

421.9717 615.0640 808.1428 1001.2147 1194.1931 1387.3494 1580.4162 1773.4800 1966.5481 2159.6097

Number of PO units.

Fig. 13. Propagation patterns of the PO polymerization initiated by –OCH2CH2O– (intermediate RO3). Symmetric propagation: (a) addition syn and (b) addition anti. Asymmetric propagation: (c) addition syn and (d) addition anti.

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G. Cendejas et al. / Journal of Molecular Structure 879 (2008) 40–52

Fig. 14. Transition structure obtained from QST3 methodology for: (a) initiated by HO, (b) initiated by CH3CH2O–, and (c) initiated by –OCH2CH2O–.

TS structures are very similar to all products, as can be noticed in Fig. 14. As mentioned above, the attack of the anions to the epoxide molecule is performed through a SN2 process. However, the nucelophile (Nu), in our case the anion species, is attacking an angle lower than 180, mainly due to the strain in the three-membered ring, impeding the reaching of this angle, another factor is the steric hindrance of the initiators. KEG shows the higher dihedral angle value, regarding the C2–C1–O1 fragment, steric and electronic effects, due to the presence of a negative oxygen atom provoked this increased. On the other hand, thermo chemistry parameters obtained at 298.15 K in gas phase, are listed in Table 10. The calculated DHr and DGr values show a preference of the initiators, the KEG is the best initiator for the polymerization reaction. Furthermore, the thermochemical parameters show a concordance with the conversion values experimentally obtained, shown in Table 3. On the other hand, the effect of the solvent (ethanol) in the reaction was taken into account by obtaining the Gibbs

free energy of solvation (DGs), which was calculated by using the PCM method. Therefore, the DGs value was of 22.34 kcal/mol, which is 0.84 kcal/mol lower than in gas phase. This decrease in the DG value could be due to a possible solvation effect of the ethoxide molecule or for the formation of hydrogen bonding. Both possibilities could provoke a minor disposition of the negative oxygen atom to attack the PO molecule. Moreover, the DG6¼ is s 1.37 kcal/mol lower than the one obtained in gas phase (Table 10), due to the fact that the solvent helps to support, in the best possible way, atomic charges present in the transition state of an SN2 reaction. Taking into account the above mentioned, a population analysis in the TS was performed by using the Merz-Singh–Kollman (MK) scheme [43,44]. The atomic charge values for the C1, C2, O1 and O2 in gas phase are: 0.0287, 0.9719, 0.5309, and 1.0965, whereas in solution they are: 0.0139, 0.8422, 0.5964, and 1.2846. The solvent provokes a decrease of the atomic charge value in the carbon atoms, while the oxygen atoms show a more negative value.

Table 9 Geometrical parameters of the TS structures obtained by using the QST3 method

3.4. NMR and IR simulation

Geometrical Parameters

Initiators KOH

KOH/CH2H5OH

KEG

Bond length (A˚) C1–O1 C1–O2 C2–O2

1.4237 2.3934 1.3133

1.4301 2.3791 1.3164

1.4164 2.3758 1.3281

Bond angle () C2–C1–O1 O2–C2–C1

114.3224 112.2847

109.3440 110.5969

123.2188 110.5777

Dihedral angle () O2–C2–C1–O1

171.7690

165.7276

165.8811

The 13C NMR shielding tensors for the products obtained from each initiator employed were calculated by using the methodology of Gauge Independent Atomic Table 10 Thermochemical parameters for the PPO’s synthetized Initiator

KOH KOH/C2H5OH KEG

Thermochemical parameters DHr (kcal/mol)

DGr (kcal/mol)

DG„ (kcal/mol)

30.77 38.71 63.76

18.95 23.18 48.89

3.55 6.87 5.38

G. Cendejas et al. / Journal of Molecular Structure 879 (2008) 40–52 Table 11 Theoretical

51

13

C chemical shifts for PPO synthesized by AROP using KOH, KOH/C2H5OH and KEG initiators

Initiator

13

Theoretical

Experimental

KOH KOH/C2H5OH KEG

76.1, 75.6, 75.4, 73.3, 73.2, 67.8, 65.4, 16.4, 16.3 75.8, 74.9, 74.5, 73.9, 73.2, 68.6, 66.1, 65.4, 17.6, 17.3, 16.5 75.9, 75.5, 75.1, 73.7, 73.2, 70.1, 67.0, 64.6, 17.6, 17.4

76.0, 75.8, 75.6, 73.6, 73.2, 67.6, 66.0, 18.2, 18.0 75.8, 75.2, 75.0, 73.6, 73.4, 68.8, 66.2, 65.6, 17.6, 17.4, 16.2 75.6, 75.4, 75.2, 73.6, 73.2, 70.2, 67.4, 65.6, 17.6, 17.4

C NMR dcalc (ppm)

Table 12 Calculated IR band for PPO synthesized by AROP using KOH, KOH/C2H5OH and KEG initiators Initiator

Wavenumber (cm1) Theoretical

Experimental

KOH KOH/C2H5OH KEG

3516.5, 3080.3, 3031.6, 1427.9, 1375.6, 1054.9 3495.8, 2980.9, 2810.7, 1500.6, 1377.7, 1076.9 3526.5, 3083.2, 3024.4, 1513.3, 1371.4, 1082.7

3461, 2971 2867, 1453 1374, 1094

Orbitals (GIAO) [34–37] at B3LYP/6-31G+(d) level. The isotropic shielding values were used to calculate the isotropic chemical shifts d with respect to TMS (dX = rTMS  rX). Table 11 shows the theoretical 13C chemical shifts (ppm) for PPOs synthesized by AROP using KOH, KOH/C2H5OH and KEG. The calculated d values were obtained at B3LYP level, using the 631G+(d) basis set. The absolute r(13C) of 184.1 ppm reported by Jameson was employed to obtain the calculated chemical shift [39]. The calculated chemical shift values are very similar to the experimental values (Table 11), except for the signals in 16.4 and 16.3 ppm when KOH was employed, where the difference is approximately 1.8 ppm. This difference is lower, and could be considered within margins errors. In the 13C NMR spectra simulation, the solvent effect was not considered with the use of C2H5OH, because the product experimentally obtained was purified before obtaining the 13C NMR spectra, eliminating all the solvent present in the reaction. On the other hand, the IR spectra were simulated to be compared to the FTIR spectrum of Fig. 3. In Table 12 calculated and experimental IR bands for the three initiators are listed. These bands were obtained at the same theoretical level than those from the shielding tensors. The three IR spectra show a good similarity with the experimental values. However, the C–H symmetrical tension band and C–H asymmetrical tension band, show higher discrepancy regarding experimental values, percent error ranges from 0.3 to 7.2. Nevertheless, this discrepancy is acceptable, calculated spectrum fairly represents the experimental one. It is worth mentioning, as in the case of the 13C NMR spectrum simulation for C2H5OH that the FTIR was obtained with no solvent, hence, the IR simulation was in gas phase. 4. Conclusions Three different potassium salts were compared as initiators of the anionic polymerization of PO. A first series of polypropylene oxides were synthesized using KOH, a very

simple initiator. The reactions were carried out by a bulk process at different temperatures in order to determine a convenient temperature range to minimize the transfer of the monomer. This side reaction was evidenced from the NMR signal corresponding to the double bond at the end of the polymer chain. It was also observed that all the polymers obtained by this bulk process resulted polydisperse mainly due to a limited solubility of the KOH in the raw monomer. The DFT calculations revealed for this system that different propagation patterns are possible, which explains the high polydispersity of the polymer population obtained when the polymerization is initiated by KOH. A second series of polymerizations were performed at a low temperature (70 C) with other two different initiators: potassium ethanolate (present in the mixture of KOH and an excess of ethanol) and dipotassium ethylenglycolate. In the first case the ethanol propitiated the solubility of the formed initiator in the monomer and reaching higher conversions at shorter times. The polymer prepared by this way exhibited a low polydispersity, but the calculation of the initiator efficiency evidenced a transfer reaction to the solvent (only polymers with a very low molecular weight were obtained). The theoretical modeling of the system revealed also a regular growing of all the propagating chains, which coincides with the low polydispersity detected by SEC characterization. The employment of the dipotassium ethylenglycolate as initiator of the PO polymerization allowed reaching high conversions at shorter times and synthesizing very monodisperse polymers. The spectroscopic characterization of this kind of materials did not show evidence of transfer reactions in the system. The high efficiency of the KEG as initiator and the monodispersity of the synthesized polymers may be explained in terms of the most energetic favored configurations, with regioselectivity, due to the regular propagation patterns revealed by the DFT calculations in the resulting system; furthermore, the DHr and DGr values support the statement mentioned.

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