On the copolymerization of monomers from renewable resources: l-lactide and ethylene carbonate in the presence of metal alkoxides

Materials

All materials were purified, stored and used in dry nitrogen atmosphere. Toluene (POCh), benzene (POCh) and 1,4-dioxane (POCh) were fractionally distilled from sodium/potassium and benzophenone after color change to navy blue, and then stored over dried 4 Å molecular sieves. EC (Aldrich) was dried over P₄O₁₀, then fractionally distilled under reduced pressure and crystallized from dry methylene chloride. Methylene chloride (POCh) for the latter purpose was fractionally distilled from CaH₂ onto dried 4 Å molecular sieves. (3S)-cis-3,6-Dimethyl-1,4-dioxane-2,5-dione l-LA (Aldrich) was crystallized from dry 2-propanol, then from toluene and finally dried at 30 °C under vacuum; it was stored at 4 °C under nitrogen. 2-Propanol (POCh) and 1-dodecanol (Fluka) were dried over CaH₂ and then fractionally distilled (1dodecanol – under reduced pressure). Ethanol (POCh) and 1butanol (POCh) were fractionally distilled from Mg/I₂ and stored over 4 Å molecular sieves. Polyethylene glycol monomethyl ether (PEO, 350 Da, Fluka) was conditioned under reduced pressure at 70 °C for several hours and then stored in dry nitrogen. Pyrogallol (1,2,3-trihydroxybenzene, Aldrich) was crystallized from ethanol/benzene (1:1), dried under vacuum and stored under nitrogen. Tin (II) 2-ethylhexanoate [Sn(Oct)₂, Aldrich], diethylzinc (Aldrich) and triethylaluminum (TEA, 25 wt. % in toluene, Adrich) were used as supplied. Methylene chloride (POCh, pure) and methanol (POCh, pure) for purification of crude polymers were used as supplied.

Procedures

Preparation of metal alkoxides was carried out in 250 mL three-neck round-bottom flasks equipped with a magnetic stirrer, addition funnel, dry-ice condenser coupled with a gas burette and nitrogen adapter replaceable with stopper. Dry-ice/acetone bath was used as a cooling medium. Diethylaluminum ethoxide was obtained according to the method described in [39]. Aluminum trialkoxide of PEO was obtained in the reaction of TEA (25 wt. % in toluene, 50 mmol, 27.1 mL) and PEO (1.5 M in toluene, 150 mmol, 100.0 mL) which was added dropwise (1:3 ratio). The reaction was carried out in toluene at 0 °C until ethane evolution finished and then the mixture was heated for 2 h under reflux. The solvent was removed under vacuum [¹H NMR in CDCl₃ δ: 3.26 (s, 3H), 3.43 (m, 2H), 3.53 (m, 26H) ppm, ²⁷Al NMR 4.97 ppm]. Ethylzinc alkoxides of PEO and dodecanol were obtained in reactions of equimolar quantities of ZnEt₂ (2 M in 1,4-dioxane, 50 mmol, 25.0 mL) and PEO (1 M in 1,4-dioxane, 50 mmol, 50.0 mL) or dodecanol (2.5 M in 1,4-dioxane, 50 mmol, 20.0 mL), respectively, carried out in 1,4-dioxane at 15 °C. The solvent was removed under vacuum. [¹H NMR in CDCl₃of EtZnOC1₂H₂₅ δ: 0.69 (q, 2H), 0.92 (t, 3H), 1.30 (m, 18H), 1.60 (t, 3H), 1.78 (m, 2H), 3.88 (t, 2H) and of EtZnPEO δ: 0.55 (q, 2H), 1.52 (t, 3H), 3.10 (s, 3H), 3.33 (m, 2H), 3.44 (m, 24H), 3.92 (t, 2H).] ZnEt₂/pyrogallol was obtained analogically to that of zinc alkoxides, however, the ratio of ZnEt₂ (2 M in 1,4-dioxane, 60 mmol, 30 mL) to pyrogallol (0.5 M in 1,4-dioxane, 20 mmol, 40 mL) was 3:1 [¹H NMR in CDCl₃ δ: 0.30 (m, 6H), 1.40 (m, 9H), 6.60 (m, 3H)]. Reactions of Sn(Oct)₂with alcohols (which are equilibrium ones) took place in situ in further reaction systems of the polymerization or model reaction with EC.

The polymerizations of l-LA with EC were carried out in glass pressure ampoules sealed by a screw with gasket on coupling, in dry nitrogen atmosphere. Solutions of l-LA (3.5 M in 1,4-dioxane, 20 mmol, 5.7 mL) and EC (3.0 M in 1,4-dioxane or toluene, 20 mmol, 6.7 mL) were placed in polymerization ampoules using glass syringes. Then, the solution of a respective catalytic system in 1,4-dioxane or toluene (0.4 mmol of metal species, usually 0.5 mL of 0.8 M solution) was added by a glass syringe. When all the components were added, the ampoule was placed in an oil bath at 120 °C. After a desired time, the ampoule was cooled, degassed, opened and methylene chloride was added in order to dissolve the reactants. The standard procedure consisted in shaking the organic solution once with diluted hydrochloric acid to wash out the catalyst residue. Then, the organic phase was washed with water three times and dropped into stirred methanol to precipitate the polymers. The products were dried under vacuum at 50 °C for 2 days. The yields of copolymers were calculated from the masses of methanol non-soluble fractions. For some experiments, after addition of methylene chloride the crude solution was dropped into stirred methanol. Then, the precipitate after redissolution in methylene chloride was purified as described before. The methanol-soluble fraction was initially concentrated on a rotary-evaporator to eliminate the solvents. The oily mixture of products was fractionally distilled under reduced pressure yielding the fractions of low molecular mass compounds (mainly dioxane and EC, but no lactide was observed) and nonvolatile oligomers.

Model reactions of metal alkoxides with EC were carried out in 100 mL two-neck round-bottom flasks equipped with a magnetic stirrer, dry-ice condenser coupled with a bladder and nitrogen adapter replaceable with a stopper. An oil bath was used as a heating medium. The reaction of Et₂AlOEt (1.25 M in toluene, 2.5 mmol, 2.0 mL) with EC (1.5 M in toluene, 2.5 mmol, 1.7 mL) at molar ratio of 1:1 was carried out at 120 °C for 15 h. After that time the reaction mixture was cooled and from a sample of it the solvent was removed under reduced pressure. Reaction of Al(PEO)₃ (2.5 mmol, 2.69 g) with EC (1.5 M in toluene, 15 mmol, 10 mL) with Al/EC molar ratio of 1:6 was carried out at 120 °C. After 1 h the reaction mixture became a gel and after another 5 h some liquid appeared. After 12 h the reaction was completed, cooled and the gel and liquid phases were separated. The solvent was removed under reduced pressure from both fractions. Reactions of EtZnOC1₂H₂₅ (1.5 M, 2.5 mmol, 1.7 mL) or EtZn-PEO (1.0 M, 2.5 mmol, 2.5 mL) with EC (2.5 M in 1,4-dioxane, 2.5 mmol, 1.0 mL) with Zn/EC molar ratio of 1:1 were carried out at 120 °C for 6 h. After that time the reaction mixtures were cooled and the solvent was removed under reduced pressure from the samples. Other samples were hydrolyzed by shaking with an aqueous solution of HCl and with water. The reaction of Sn(Oct)₂ (0.4 M in 1,4-dioxane, 2.0 mmol, 5.0 mL) activated by 1-butanol (1.2 M in 1,4-dioxane, 20 mmol, 16.7 mL) with EC (2.5 M in 1,4-dioxane, 2.0 mmol, 0.8 mL) with Sn/EC molar ratio of 1:1 and Sn/butanol molar ratio of 1:10 were carried out at 120 °C for 48 h. After cooling and concentration under reduced pressure, a precipitate isolated. The solid was washed twice with hexane and methylene chloride and analyzed by FT IR.

Analyses

Relative molecular mass and molecular mass distribution were determined by ESI MS or GPC techniques. ESI MS spectra were measured on Mariner spectrometer. Gel Permeation Chromatograph (RI detection) was equipped with a Jordi gel DVB column, Lab Alliance isocratic pump (1 mL/min), SFD refractive index detector RI-2000F and Degasys degasser DG-2410. GPC analyses were carried out in THF or toluene as a solvent at 20 °C. The relative molecular masses were determined with polystyrene standard calibration. ¹H NMR spectra of solutions were recorded on a Varian Mercury spectrometer (400 MHz) in CDCl₃ or C₆D₆ as solvents at room temperature. ¹³C NMR MAS spectra of solids were recorded on a Bruker spectrometer. FT-IR spectra of samples as KBr disks were recorded on a Bio-Rad 165 spectrometer. DSC thermograms were recorded on Perkin Elmer Pyris 1 DSC in the range from -50 to 200 °C by heating/cooling/heating cycles at 10 °C/min. TG thermograms were recorded on a PC MOM Derivatograph in the range of 25 to 800 °C by heating at 10 °C/min (air). Micrographs were recorded on a LEO 1530 SEM in the form of films conditioned at 150 °C for 2 h, followed by sputtering of graphite layer.

Copolymerization of l-LA and EC

A majority of EC and l-LA copolymerizations in the presence of various tin, aluminum and zinc derivatives were carried out in a toluene or 1,4-dioxane solution at 120 °C for 48 h at a 1:50 metal to lactide mole ratio. At these conditions, at an equimolar amount of both monomers in the monomer feed, copolymers containing 3–5 mol. % of carbonate m.u. were obtained. In the presence of the most active catalysts the yield of these products calculated with respect to the sum of both monomers lies generally in the 30–50 % range, and the degree of conversion of the lactide to the macromolecular products is 50–80 % (Table 1).

The system obtained in the reaction of ZnEt₂ and pyrogallol (Scheme 2) appeared to be the most effective one in the insertion of carbonate m.u. from among the studied catalysts. It allowed for 5.2 % of carbonate m.u. to undergo insertion at 53 % conversion of l-LA to a high-relative molecular mass polymer.

Scheme 2

Synthesis of ethylzinc derivative of pyrogallol as ROP catalyst (association omitted).

More detailed studies carried out for this system show that at 120 °C the lactide undergoes rapid homopolymerization and already after 0.5 h the conversion of this monomer exceeds 80 % and the product obtained is characterized by high relative molecular mass (Table 2, no. 1). However, it contains only ca. 0.5 mol. % carbonate m.u. The carbonate m.u. content in the products insoluble in methanol increases with elapsing reaction time, which is, however, accompanied by a decrease in the polymer relative molecular mass and appearance of a large number of oligomers soluble in methanol. On the basis of the relative intensity of signals in the ¹H NMR spectra it can be estimated that these latter ones contain about 20 mol. % of carbonate m.u. (Fig. 1).

Fig. 1

¹H NMR spectrum of the products of l-LA and EC copolymerization (Table 2, no. 7), fraction soluble in methanol.

Figure 2 presents the ESI MS spectrum of the fraction of oligomers (obtained after removal of volatile products under vacuum). Macrocycles composed of 1 carbonate m.u. and 3–9 lactic acid (LAc) m.u. are the main products (C_n, D_n). These products result from the back-biting type intramolecular transesterification. They are probably formed as a result of an attack of a primary alkoxide terminated with a carbonate m.u. onto LAc m.u. (Scheme 3a). However, the alternative reaction consisting in the attack of the secondary center terminated with a LAc m.u. onto carbonate m.u. cannot be excluded. The attack of active centers terminated with LAc m.u. onto its own chain built of only LAc m.u. resulting in the formation of cyclic oligomers containing 6–8 molecules of this acid (A_m, B_m) is also possible. Two populations of signals of considerably smaller intensity are also present in the spectrum. The first one can be explicitly assigned to cyclic oligomers containing 2 carbonate m.u. and 3–7 LAc m.u. (E_x+y). The second population can, however, correspond to the linear LAc oligomers bonded through the carbonyl or ethylene glycol groups (F_a+b). The formation of linear carbonate oligomers requires the attack of 2 active centers terminated with LAc m.u. onto the carbonate monomer or m.u.. (As it will be shown in the further part of the paper, such a reaction with the monomer is possible and leads to the formation of ethylene glycol derivatives.)

Fig. 2

Positive ion mode ESI MS spectrum of oligomeric products of the l-LA and EC copolymerization (Table 2, no. 7).

Scheme 3

(a) Inter- and (b) intramolecular transesterifcation of PLA chains with primary alkoxide.

A considerable amount of intramolecular transesterification products in the final process step indicates also a high probability of intermolecular reactions, in which the active centers terminated with carbonate m.u. attack the LAc m.u. in another chain (exchange of segments, Scheme 3b). Such reactions result in the regeneration of the centers terminated with lactide m.u., which can further react with EC. Thus, it is possible to enrich the high relative molecular mass fraction with carbonate m.u. (Table 2, no. 9).

When the copolymerization is carried out at above 120 °C, decarboxylation of carbonate m.u. additionally proceeds in the system, leading to the formation of oxyethylene m.u. (Table 2, no. 5 and 6).

DSC thermograms (2^nd heating) of l-LA homopolymer and its copolymers comprising 8.3 (Table 2, no. 7) and 12.4 (no. 6) mol % of carbonate m.u. are shown in Fig. 3a,b and c, respectively.

Fig. 3

DSC thermograms of PLA (a) and copolymers of LA with EC containing 8.3 (b, Table 2, no. 7) and 12.4 (c, no. 6) mol % of carbonate m.u.

The internal plasticizing effect is clearly demonstrated by the decrease in glass transition temperatures of copolymers by the value of ΔT_g =3.7 (8.3 mol % of carbonate m.u.) and 12.4 (12.4 mol. % carbonate m.u.) °C. T_g of the copolymer can be adjusted by the amount of carbonate units. Moreover, the presence of 8.3 or more mol % of carbonate m.u. in the copolymer chain strongly affects the ability to non-isothermal melt- and cold crystallization, leading to fully amorphous materials. This phenomenon is confirmed by SEM images (Fig. 4), which show the surfaces and fractures of strained at room temperature pure (a, b) and EC-plasticized (c, d, Table 2, no. 6) PLA samples. Unmodified PLA sample exposes the regular grain-like structures of crystalline domains (on the surface) organized into semi-continuous formations (at the fracture) with sharp edges at the fracture lines of that brittle material. On the other hand, the PLA copolymer comprising 12.4 mol % carbonate m.u. exhibits plain, irregular surface in non-stretched areas and plastic fracture with distorted edges in stretched/broken areas. The same sample was examined as well by means of thermogravimetry (Fig. 5b). In comparison to PLA (a), it starts to decompose at 241 °C which is by ca. 20 °C lower temperature than that for PLA. The decomposition seems to be a single step and no residual ashes can be found for both samples.

Fig. 4

SEM micrographs of the surfaces (a, c) and fractures (b, d) of strained at room temperature l-LA homopolymer (a, b) and l-LA–EC copolymer (c, d, Table 2, no. 6) samples.

Fig. 5

Thermogravimetric curves for PLA (a) and l-LA-EC copolymer (b, Table 2, no. 6).

Reaction of EC with metal alkoxides

In order to confirm that the initiators used by us can react with EC, the reaction course of this monomer with a number of zinc and aluminum alkoxides as well as a mixture of tin 2-ethylhexanoate and butanol was studied. The reactions were carried out at 120 °C in a toluene or 1,4-dioxane solution.

Monoalkoxides of the general formula EtZnOR obtained in the reaction of diethylzinc and dodecanol (R = C₁₂H₂₅) or polyethylene glycol monomethyl ether [R = (OCH₂CH₂O)_7,2CH₃] were used as zinc derivatives. Due to the presence of long substituents in the alkoxide group, these compounds are well soluble and the reaction mixtures remain homogeneous. In both systems symmetric linear carbonates ROC(O)OR result from the reaction, which shows that EC reacts with two alkoxide molecules (Scheme 4). This reaction is accompanied by the elimination of the ethylene glycol derivative.

Scheme 4

Alcoholysis of EC with metal alkoxide.

Figure 6 shows the ¹H NMR spectrum of the mixture formed in the reaction of EC and dodecyl derivative at an equimolar amount of reagents. On the basis of the relative intensity of signals of the starting materials and products it can be estimated that the degree of EC conversion is ca. 40 %. In this spectrum, besides the signals characteristic of the reagents, symmetric carbonate and ethylene glycol derivative (singlet at δ 3.8 ppm), still three very weak signals at δ 2.3 (quartet), 3.8 (triplet) and 4.2 ppm (triplet) are present, which indicate the possibility of ring opening also as a result of alkylation of the carbonyl group. However, such a reaction is much less probable than the attack of the alkoxide group, which in the system studied leads to transesterification.

From the hitherto studies on the transesterification of five-membered cyclic carbonates it is known that at elevated temperature the alkoxide anions can attack all the carbon atoms in the ring [34, 40]. As a result of the attack onto the carbonyl group (Scheme 5a) an alkoxide center is formed and the cyclic orthocarbonate is probably the intermediate stage of this reaction. An attack onto the alkylene carbon atom (Scheme 5b) leads to the formation of a carbonate center, which at elevated temperature or after hydrolysis may undergo decarboxylation yielding hydroxyether derivatives.

Fig. 6

¹H NMR spectrum of the EC and ethylzinc dodecanoxide reaction mixture.

Scheme 5

Ring opening of EC with metal alkoxide leading to (a) carbonate group retention or (b) decarboxylation.

No unsymmetric carbonates or hydroxyethers were found in the reactions with selected zinc alkoxides, which should be formed as a result of EC ring opening by one alkoxide group. Therefore, it can be assumed that these reactions are characterized by very small equilibrium constants. In the absence of l-LA or PLA, the formed in the reaction alkoxide centers terminated with a carbonate m.u. (Scheme 5a) react with the second alkoxide molecule yielding thermodynamically stable symmetric carbonates.

Diethylaluminum ethoxide (Et₂AlOEt) and aluminum trialkoxide containing long oxyethylene substituents Al(OR)₃ [R = (OCH₂CH₂O)_7,2CH₃] were used as aluminum derivatives. In the reaction with this latter compound a precipitate of insoluble alkoxide isolates after a certain time. It is suggested on the basis of its ¹³C NMR MAS spectrum (Fig. 7) that it is formed as a result of replacing part of the OR substituents by difunctional ethylene glycol m.u.. The ¹H NMR spectrum of the products in the liquid phase (Fig. 8) shows that it contains a symmetric carbon with oxyethylene substituents. Thus, the reaction proceeds similarly as with the zinc derivative comprising the same alkoxide substituent. The presence of three reactive substituents at the aluminum atom causes the formation of insoluble crosslinked ethylene glycol derivatives.

Fig. 7

¹³C NMR MAS spectrum of the insoluble product of the EC and Al(PEO)₃ reaction.

Fig. 8

¹H NMR spectrum of the soluble EC and Al(PEO)₃ reaction mixture.

In the reaction with Et₂AlOEt a very complicated mixture of products is formed. Figure 9 presents the ¹H NMR spectrum of the reaction mixture after removing toluene. Completely reliable assignment of particle signals is not possible in this case. However, it can be noticed that in these reactions linear carbonates are not the main reaction products, since the relative intensity of signals characteristic for methylene groups in such linkages (in the δ 4.2–4.4 ppm range) is small. On the other hand, intensive signals at δ 3.5–4.0 ppm are present, which allows to suggest that EC has been converted into orthocarbonates. Such derivatives can result from the addition of the alkoxide group to the carbonyl group (see Scheme 5a) or alkylation of this group without ring cleavage. We believe that the stability of these products results from small dimensions of the ethoxy and ethyl groups, but this hypothesis requires further verification.

Fig. 9

¹H NMR spectrum of the EC and Et₂AlOEt reaction mixture.

In the reaction of EC and initiators comprising tin 2-ethylhexanoate and butanol, a mixture of linear carbonates and 2-ethylhexanoic acid esters are formed. On the basis of ESI MS spectra of this mixture it can be explicitly found that 1-hydroxy-3,5-dioxanonane-4-one [m/z = 185 (M/Na⁺), 201 (M/K⁺)] is one of the products. Its formation indicates the EC ring opening as a result of addition of one alkoxide group according to Scheme 5a. It is believed that the detection of this product in the system is possible due to protonation of the alkoxide terminated with carbonate m.u. by butanol or 2-ethylhexanoic acid present in the system, which favorably shifts the equilibrium of the reaction presented in Scheme 5a. The alkoxide formed in this reaction undergoes also subsequent transesterification reactions resulting in the formation of dibutyl carbonate [m/z = 202 (M/Na⁺), 371 (2M/Na⁺), 387 (2M/K⁺)] and dibutyl dicarbonate BuOC(O)OCH₂CH₂OC(O)OBu [m/z = 301 (M/K⁺)]. The insoluble precipitate of the organic tin derivative is formed in these reactions, besides liquid products. An analysis of IR and ¹³C NMR MAS spectra of this product shows that it contains 2-ethylhexanoate groups (ν_COO- 1643 and 1573 cm^-1, δ: CH₃ 12.88, 14.42; CH₂ 23.38, 27.31, 30.30; CH 50.00; C=O 184.35 ppm) as well as oxyethylene groups (ν_C-O 1118, 1047 cm^-1, δ OCH₂ 63.86 ppm). Such type of derivatives can be formed in the already described transesterification processes leading to the elimination of the ethylene glycol m.u.. However, it was found that this product is formed also in the reaction of EC with commercial tin 2-ethylhexanoate in the absence of butanol, and also in the initial stage of EC and lactide copolymerization when the participation of transesterification processes is very small. It is believed that partial hydrolysis of the carboxylate with the formation of Sn-OH bonds, which initiate the EC decomposition with CO₂ evolution, is the reason for this (Scheme 6).

Scheme 6

Decomposition of EC initiated with tin derivative.

The results obtained show that the initiators used are capable of a nucleophilic attack on the carbonyl carbon atom in EC, which results in the formation of alkoxide active centers terminated with a carbonate m.u.. However, these products are thermodynamically unstable and undergo further transformations leading to the elimination of ethylene glycol derivatives.