Synthesis of l-Lactide via Degradation of Various Telechelic

Article pubs.acs.org/IECR

Synthesis of L‑Lactide via Degradation of Various Telechelic Oligomeric Poly(L‑lactic acid) Intermediates Wen-Jun Yi,† Li-Jun Li,† Zhen Hao,† Min Jiang,† Chang Lu,§ Yi Shen,§ and Zi-Sheng Chao*,†,‡ †

College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, P. R. China College of Materials Science and Engineering, Changsha University of Science & Technology, Changsha, Hunan 410114, P. R. China § Department of Spine Surgery, The Second Xiangya Hospital of Central South University, Changsha, Hunan 410011, P. R. China ‡

ABSTRACT: Hydroxyl-terminated telechelic oligomeric poly(L-lactic acid), namely t-OPLLA-(OH)n (n = 2, 3, 4), and carboxyl-terminated telechelic oligomeric poly(L-lactic acid), namely t-OPLLA-(COOH)2, were respectively synthesized by the copolymerization of L-lactic acid with comonomers 1,4butanediol, glycerol, pentaerythritol, and succinic acid, while the normal oligomeric(L-lactic acid), namely OPLLA, was obtained through the self-polymerization. The effects of the type and amount of comonomer as well as the molecular weight of starting oligomer on both the generation of L-lactide and the molecular weight of residual oligomer were examined. It was found that the selectivity to L-lactide for the degradation of t-OPLLA-(OH)n (91.7−93.56%) was slightly larger and that for the t-OPLLA-(COOH)2 was almost unchanged, relative to that for the degradation of OPLLA (90.3%). Both the yield of crude lactide and reaction rate for the degradation of t-OPLLA-(OH)n were higher, and those for t-OPLLA-(COOH)2 were lower than for the OPLLA. An as high as 92.7 wt % crude lactide yield had been achieved for the degradation of t-OPLLA(OH)4, being the maximum level ever reported in the literature. With increasing the reaction time, the molecular weight of residual oligomer increased throughout for the degradation of OPLLA, and it first increased and then decreased for the degradations of telechelic OPLLA. The increase in the molecular weight of starting oligomer was unfavorable to the generation of L-lactide and also increased the molecular weight of residual oligomer. The above results indicated that the efficiency for the generation of L-lactide from t-OPLLA-(OH)n was obviously higher, which increased with increasing the amount of terminal hydroxyl group, and that from t-OPLLA-(COOH)2 was lower than that from OPLLA. Possible reaction networks for the degradations of OPLLA and telechelic OPLLA were proposed. tional petroleum-based polymers.6,7 The competitive price, commercial availability of lactic acid, and good properties of PLA enable PLA to become the first mass-produced biobased polymer. There are two routes to synthesize PLA, i.e., the direct polycondensation of lactic acid and the ring opening polymerization (ROP) of lactide.8−13 While the direct polycondensation route acquired a limited application due to the low molecular weight and large polydispersity index (PDI) of the generated PLA,11 the ROP route had constituted the dominant way in the commercial manufacturing of PLA with a molecular weight greater than 100 000.14 The ROP of lactide is known to involve three steps, i.e., the condensation polymerization of lactic acid to oligomeric PLA (OPLA), the depolymerization degradation of OPLA to lactide, and the ring opening polymerization of lactide to PLA. Relative to the first and

1. INTRODUCTION Most of the currently existing polymers are nonbiodegradable and are manufactured from the petroleum-based raw materials, being confronted with the cost and environmental problems. As the petroleum shortage and environmental protection situation are becoming increasingly severe, it is highly desired to develop the biodegradable polymers from renewable resources. Nowadays, many synthetic biodegradable polymers have emerged. Among them, poly(lactic acid) (PLA) appears to be the most attractive one and is receiving much more attention. On the one hand, lactic acid, as the building block of PLA, is industrially manufactured with low cost and on a large scale from the biomass fermentation of a renewable agricultural source, with its production being in a rapid growth in recent years.1 On the other hand, PLA has shown many good properties, e.g., biodegradability, biocompatibility, bioresorbability, mechanical property profile, thermoplastic processability, and compostability.2−4 These properties decide that PLA could be applied in the surgical suture, drug delivery, and tissue engineering areas.5 Besides, PLA is viewed as one of the most suitable and promising alternatives to replace conven© 2017 American Chemical Society

Received: Revised: Accepted: Published: 4867

October 23, 2016 March 16, 2017 March 18, 2017 March 18, 2017 DOI: 10.1021/acs.iecr.6b04082 Ind. Eng. Chem. Res. 2017, 56, 4867−4877

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Industrial & Engineering Chemistry Research

when an OPLA with higher molecular weight was employed in the degradation, the resultant lactide possessed a lower optical purity.19 When an OPLA with sufficient low molecular weight was employed in the degradation, it might be evaporated directly from the reactant in the molten state without any cyclization.29 Therefore, there was an optimal molecular weight range of OPLA, usually being from hundreds to thousands Da, in consideration of both the yield and optical purity of lactide.19 Nevertheless, the molecular weight of OPLA would vary with the progress of degradation, due to the presence of side reactions, such as intermolecular and intramolecular transesterifications, and this might affect obviously the generation of lactide. To the best of our knowledge, the clarification of the variation in the molecular weight of OPLLA and telechelic OPLLA during degradation and its effect on the formation of lactide have not been reported in the literature up till now. In the present paper, a series of hydroxyl- and carboxylterminated telechelic OPLLAs were synthesized by the copolymerization of L-lactic acid with various multifunctional comonomers, and they were further employed in the degradation to L-lactide. It was found that the increase in the quantity of the terminal hydroxyl accelerated the reaction rate for the formation of lactide, and this could be attributed, for the first time, to the inhibition by the terminal hydroxyl on the overgrowth of the molecular weight of residual oligomers of lactic acid.

third steps, the second step was usually conducted at an appreciably high temperature, and it usually suffered from a few side reactions, e.g., hydrolysis by trace amounts of water, random main-chain scission, intermolecular and intramolecular transesterification, cis-elimination, or enolization.15,16 This resulted in the generation of byproducts such as lactic acid, meso-lactide, D-lactide, acyclic, and cyclic oligomers with molecular weights different from the original OPLA, and thus reducing largely both the quality and yield of lactide.1 However, the removal of these impurities in crude lactide before the ROP step was a process with big complexity of operation, large consumption of time, and high costs of both solvent and energy, while the presence of impurities was known to affect largely the quality of PLA, such as the crystallinity, mechanical properties, and some other properties.17,18 Therefore, the production of lactide from OPLA was considered to be the most important step in the overall process for PLA production.1,8,19 In the literature, the degradation of OPLA to lactide was usually performed under high temperature (≥200 °C) and reduced pressure (1−20 kPa) over a catalyst.18−21 Among various catalysts, such as the salt or oxide of Mg, Ca, Al, Zn, Sn, Ti, or Zr, stannous octanoate (Sn(Oct)2) was usually considered as the most efficient one in promoting both the yield and quality of lactide.20,22 The increase in the temperature promoted the intrinsic reaction rate for the degradation of OPLA as endothermic reaction.23 However, a too high temperature usually also led to a larger degree of racemization of lactide, due to the accelerated rate of the deprotonation of αproton in lactide caused by a weak basic substance such as water or the ester-semiacetal tautomerization occurring in a lactate unit in the OPLA chain, and this decreased both the yield and optical purity of lactide.1,24,25 Since the degradation of OPLA to lactide is an equilibrium reaction,1 the decrease in the pressure favored the removal of the generated lactide and water25 from the reactor and also the elimination of water from crude lactide, and, thus, increased not only the yield of lactide but also the optical purity of lactide. Besides the above factors, the yield and quality of lactide were also reported to be affected by the molecular weight and the end group of OPLA.21,26,27 The formation of lactide was considered as a result of unzipping depolymerization via the back-biting reaction, i.e., the attacking of carbonyl carbons in penultimate lactic units by the terminal groups in the OPLA chain.22,28 In this reaction, L,L-lactide or D,D-lactide, dependent on the configuration of lactic acid for the formation of OPLA, was selectively generated for hydroxyl as the terminal group, while meso-lactide was obtained for carboxyl as the terminal group. Shen et al.26 studied the degradations of the poly(L-lactic acid) oligomer as well as hydroxyl- and carboxyl-terminated poly(L-lactic acid) oligomers, i.e., OLLA, OLLAPOH, and OLLACOOH, and found that the yield of lactide had an order of OLLAPOH > OLLA > OLLACOOH. Inkinen et al.27 studied the degradations of poly(L-lactic acid) oligomers (OPLLA) and a series of telechelic OPLLA oligomers with different amounts of hydroxyl and carboxyl terminal groups. It was found from the TGA determination that the carboxyl termination increased the thermal stability of OPLLA. Besides, both the onset temperature of degradation and the weight of residue were lower, but the depolymerization rate of hydroxylterminated OPLLA was higher than for OPLLA; however, the above parameters for the carboxyl-terminated OPLLA were inversed, relative to OPLLA. Researchers also reported that,

2. EXPERIMENTAL SECTION 2.1. Materials. The materials were as follows: reactants: Llactic acid (90 wt % aq.), succinic acid, 1,4-butanediol, glycerol, and pentaerythritol; catalyst: stannous octanoate (Sn(Oct)2); and solvents: ethanol, chloroform, and dichloromethane. All these chemicals were of analytical purity and were purchased from Sinopharm Chemical Reagent Co., Ltd. (China), and they were used as received without further purification. 2.2. Preparation of Telechelic Oligomeric Poly(L-lactic acid). Oligomeric Poly(L-lactic acid). L-Lactic acid aqueous solution (60 g) and Sn(Oct)2 (0.5 g) were first mixed in a single-necked flask under magnetic stirring and reduced pressure (ca. 51.3 kPa) and then heated at 130 °C for 1 h to remove water. After that, the mixture was heated from 130 °C to 180 °C and evacuated from 51.3 kPa to 3.0 kPa within 5 h, according to a pressure/temperature-programmed procedure, i.e., first raising the temperature by 10 °C and reducing the pressure by ca. 10 kPa within 10 min and then holding the temperature and pressure for 50 min, and then repeating the above steps from the new higher temperature and lower pressure points. This led to the gradual polymerization of Llactic acid, generating oligomeric poly(L-lactic acid), namely, OPLLA. The product mixture containing OPLLA was directly employed in the subsequent reaction for the synthesis of Llactide. Telechelic Oligomeric Poly(L-lactic acid). The same procedure as above for the synthesis of OPLLA was performed, except that a calculated amount of the comonomer was introduced into the mixture of Sn(Oct)2 and L-lactic acid at the beginning. The employment of different comonomers, i.e., 1,4butanediol, glycerol, pentaerythritol, or succinic acid, resulted in various functional group-terminated telechelic oligomeric poly(L-lactic acid) products, namely t-OPLLA-(OH)2, tOPLLA-(OH)3, t-OPLLA-(OH)4, and t-OPLLA-(COOH)2. The product mixtures containing the above telechelic oligomers 4868

DOI: 10.1021/acs.iecr.6b04082 Ind. Eng. Chem. Res. 2017, 56, 4867−4877

Article

Industrial & Engineering Chemistry Research ⎛1 + q=⎜ ⎝1 −

were all directly employed in the subsequent reactions for the synthesis of L-lactide. 2.3. Synthesis of Crude Lactide. The above product mixtures were respectively heated to 210 °C under stirring and reduced pressure (ca. 0.2 kPa) to perform the thermal degradation of oligomer or telechelic oligomers of L-lactic acid. It took usually at least 35 min to accomplish the degradation reaction, as evidenced by the observation that there was no more liquid to be distilled from the flask. The crude lactide was distilled and weighed, and the yield was calculated by using eq 1 Yield (wt %) =

Weightactually obtained crude lactide Weight theoretically generated lactide

SL (%) = SD (%) =

× 100 (1)

q × SD / L 1+q SD / L 1+q

× 100

(6)

× 100

(7)

where SD/L was the selectivity to the according to eq 4.

D /L

blend, calculated

3. RESULTS AND DISCUSSION 3.1. Structure of Telechelic and Oligomeric OPLLA. Figure 1 shows the reactions for the formations of OPLLA and

Weight crude lactide Weight catalyst × Timedegradation

(5)

where b was the fraction of meso-lactide, relative to the sum of meso-lactide and D/L blend. Therefore, the selectivities to Llactide and D-lactide (i.e., SL and SD, respectively) could be respectively obtained through eqs 6 and 7:

The space−time yield (STY) of crude lactide was calculated by using eq 2: STY (g/gcat·min) =

2 1 − 2b ⎞ ⎟ 1 − 2b ⎠

(2)

2.4. Characterization. The structure of specimen was determined over a Varian Inova-400 1H NMR spectrometer, using CDCl3 as the solvent and TMS as an internal reference. Before the determination, the specimen was purified by dissolving in dichloromethane, followed by precipitating in an excess amount of cold methanol and vacuum drying. The viscosity (η) of specimen was determined over an Ubbelohde viscometer at 30 ± 0.1 °C, using chloroform as the solvent. The viscosity-average molecular weight (Mv) of specimen was calculated according to the Mark−Houwink equation (eq 3, corrected by John R. Dorgan):30 [η] = K × M v α

(3)

where K = 0.0131 mL/g and α = 0.777. The characterization of crude lactide was conducted over a Varian Model Saturn 2200/CP3800 GC/MS spectrometer. Two CP-Wax 52CB fused silica capillary columns (15 m × 0.32 mm) were linked, respectively, to the mass detector for qualitative analysis and the flame ionization detector (FID) for quantitative analysis. Before determination, the specimen was dissolved in a solution of dichloromethane and methanol (volume ratio = 1:1). It was identified that the crude lactide consisted of predominantly lactides (meso-lactide, D-lactide, and L-lactide) and a small amount of impurities (mainly lactic acid, lactoyllactic acid, and lactoyllactoyllactic acid). The distribution of various components in the crude lactide, i.e., the selectivity to various components, was calculated by area normalization method, based on GC chromatogram, which could be expressed as follows (eq 4): Si (%) =

Ai × 100 ∑ Ai

Figure 1. Reactions involved in the formation of OPLLA and telechelic OPLLA.

various telechelic OPLLA. The structures of these compounds have been determined by 1H NMR, which are shown in Figure 2. The OPLLA shows four peaks at δ = 1.45 (weak), 1.57 (strong), 4.36 (weak), and 5.16 (strong) ppm, which can be respectively attributed to the proton of the −CH3 group (a) next to the terminal hydroxyl, the proton of the −CH3 group (c) on the main chain of the oligomer, the proton of the >CH− group (b) next to the terminal hydroxyl group, and the proton of the >CH− group (d) in the main chain of the oligomer, respectively. This result confirms that the OPLLA possesses the normal structure of poly(lactic acid). The above-mentioned four peaks are all present for the t-OPLLA-(OH)2, t-OPLLA(OH)3, and t-OPLLA-(OH)4. Besides, the t-OPLLA-(OH)2 also shows two additional weak peaks at 2.71 and 4.26 ppm, which are attributed to the protons of −CH2−CH2− (f) and −OCH2CH2− (e) groups, respectively; the t-OPLLA-(OH)3

(4)

where Si and Ai respectively refer to the selectivity and chromatographic peak area of component i in the crude lactide. However, the chromatogram peaks for D-lactides andL-lactides were highly overlapping, exhibiting only a D/L blend peak, and this enabled the selectivities to D-lactides and L-lactides to be hardly directly determined. Fortunately, the ratio of L-lactide to D-lactide (namely, q) could be evaluated by the method, based on probability law, as reported in the literature,31 which was as follows (eq 5): 4869


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Figure 2. 1H NMR spectra of various oligomers.

shows an additional two peaks at ca. 4.36 and 5.19 ppm, which can be assigned to the protons of −CH2−O− (h) and >CH− O− (g) groups, respectively; the t-OPLLA-(OH)4 shows an additional peak at 4.18 ppm, being caused by the proton of the −CH2−O− group (i). It should be mentioned that the signals for the >CH− (b) and −CH2−O− (h) groups, which both appear at ca. 4.36 ppm, are overlapped, due to multiple splitting. The same phenomenon also occurred for the signals of the >CH− (b) and −OCH2CH2− (e) groups, which appear at 4.36 and 4.26 ppm, respectively. It confirms that the tOPLLA-(OH)2, t-OPLLA-(OH)3, and t-OPLLA-(OH)4 are constructed by the easternization of one molar 1,4-butanediol, glycerol and pentaerythritol with two, three and four molar oligomeric poly(lactic acid), respectively. The t-OPLLA(COOH)2 shows both peaks at 1.57 (−CH3; c) and 5.16 (>CH−; d) ppm as those for the t-OPLLA-(OH)2, t-OPLLA(OH)3, and t-OPLLA-(OH)4; however, no peaks at 1.45 (−CH3; a) and 4.36 (>CH−; b) ppm are identified, due to the absence of the terminal hydroxyl group in this telechelic OPLLA. It indicates that the t-OPLLA-(OH)4 is constructed by the esterification of two molar oligomeric poly(lactic acid) with one molar succinic acid. The above 1H NMR results are consistent with those reported in the literature26,38 and confirm the formation of copolymers through the copolymerization of L-lactic acid with different comonomers. Accordingly, one molar t-OPLLA(OH)2, t-OPLLA(OH)3, and t-OPLLA(OH)4 contains two, three, and four molar hydroxyl groups, respectively, while one molar t-OPLLA(COOH)2 contains two molar carboxyl groups. 3.2. Effect of the Terminal Group on the Degradation of Telechelic OPLLA. Figure 3 shows the yield of crude lactide from the degradation of telechelic OPLLA as a function of comonomer usage (molar percentage) in the synthesis batch for telechelic OPLLA. The degradation reaction was conducted at 210 °C, and under 0.2 kPa and it was lasted until no more lactide was generated. One can see that the yield of crude lactide generated from OPLLA (zero usage of the comonomer)

Figure 3. Yield of crude lactide as a function of comonomer usage. The degradation reaction was conducted at 210 °C, and under 0.2 kPa and it was lasted until no more lactide was generated. Comonomer: succinic acid (●); 1,4-butanediol (▲); glycerol (▼); pentaerythritol (◇).

is 68.2 wt %, and those generated from telechelic OPLLA with polyols as comonomers, i.e., t-OPLLA-(OH)n (n = 2, 3, 4), are higher but that with succinic acid as the comonomer, i.e., tOPLLA-(COOH)2, is lower than that from the oligomer OPLLA. The yields of lactide generated from t-OPLLA-(OH)2, t-OPLLA-(OH)3, and t-OPLLA-(OH)4 are all increased and then decreased with increasing the usage of polyol, and that from t-OPLLA-(COOH)2 decreases all along with increasing the usage of succinic acid. The maximal yield of lactide can be determined as 92.4 wt %, 91.0 wt %, and 91.7 wt % at the usage of 2.2 mol %, 1.6 mol %, and 1.1 mol % for 1,4-butanediol, 4870


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Industrial & Engineering Chemistry Research Table 1. Results for the Degradations of OPLLA and Telechelic OPLLAa selectivity (%)c prepolymer

comonomer

b

molar ratio, comonomer/lactic acid

t (min)

yield (wt %)

L

D

0

60

66.1

90.3

0.03

M

I

STY (g/gcat min)

3

6.7

1.19

OPLLA

none

t-OPLLA-(COOH)2

succinic acid

2.0/120 1.3/120

60 60

61.1 62.4

90.3 90.3

0.06 0.09

4.8 5.8

4.8 3.8

1.1 1.12

t-OPLLA-(OH)2

1,4-butanediol

2.0/120 1.3/120

50 60

85.6 74.8

92.5 91.7

0.04 0.04

3.7 4

3.8 4.3

1.85 1.35

t-OPLLA-(OH)3

glycerol

1.3/120 1.0/120

35 35

90 89.5

93.5 91.7

0.03 0.02

3.1 2.7

3.4 5.6

2.78 2.76

t-OPLLA-(OH)4

pentaerythritol

1.3/120 1.0/120

35 35

92.8 90.1

93.5 92.7

0.02 0.02

2.7 2.6

3.8 4.7

2.86 2.78

Conditions for degradation reaction: pressure = 0.2 kPa, temperature = 210 °C. bPeriod of time that the reaction lasted until crude lactide being no longer generated. cL = L-lactide, D = D-lactide, M = meso-lactide, and I = impurities (including, mainly, lactic acid, lactoyllactic acid, and lactyllatic acid). a

cyclic oligomers of lactic acid, respectively. The generation of lactide from t-OPLLA-(COOH)2 can be a result of its reaction with some impurities, such as water and lactic acid, which may involve the first dissociation of ester bonds to the smaller tOPLLA-(COOH)2 and/or OPLLA and then the further unzipping depolymerization of OPLLA.29 However, the rate for the non-catalyzed dissociation of ester bond is expected to be lower than the Sn(Oct)2-catalysed unzipping depolymerization of OPLLA. The above reasons account for the lower yield of lactide and the higher content of byproducts from the degradation of t-OPLLA-(COOH)2. The decrease in the yield of lactide beyond the maximum as increasing the usage of polyol comonomer may be due to the fact that the t-OPLLA(OH)n (n = 2, 3, 4) molecule possesses a branched structure with polyol unit being situated in the center, as shown in Figure 1, which corresponds to 1−3 lactate units, have been replaced for n = 2 and 3. Therefore, the formation of lactide through unzipping depolymerization would stop with only one or two lactate units being left on each branch of t-OPLLA-(OH)n, which cannot be further converted to lactide. The greater the usage of comonomer for the synthesis of t-OPLLA-(OH)n, the more lactate units are left unconverted to lactide and, in turn, the lower the yield of lactide. Table 1 shows the results for the degradations of OPLLA and telechelic OPLLA to lactide. The degradation reaction was conducted at 210 °C and under 0.2 kPa, and it continued until no more lactide was generated. One can see that both the yield of crude lactide and selectivity to L-lactide from various reactants have an order of t-OPLLA-(OH)4 > t-OPLLA-(OH)3 > t-OPLLA-(OH)2 > OPLLA > t-OPLLA-(COOH)2, but the corresponding reaction time is reversed, relative to the above order. Accordingly, the rate and STY for the formation of lactide from various prepolymers both follow an order of tOPLLA-(OH)4 > t-OPLLA-(OH)3 > t-OPLLA-(OH)2 > OPLLA > t-OPLLA-(COOH)2. Furthermore, the increase in the usage of polyol as comonomer leads to an increase in the yield and STY of crude lactide and selectivity to L-lactide but a decrease in the reaction time; however, the increase in the usage of succinic acid as comonomer shows the inverse effects. From Table 1, one can also see that the crude lactides generated from various prepolymers are all composed of Llactide as the predominant component and small amounts of D-

glycerol, and pentaerythritol, respectively, which corresponds theoretically to the content of hydroxyl group of 4.4 mol %, 4.8 mol %, and 4.4 mol % in the three telechelic oligomers. Therefore, the comparable maximal yields of lactide (ca. 91.7 wt %) have been obtained from the t-OPLLA-(OH) n containing approximately equal amounts of hydroxyl groups (ca. 4.4 mol %). It is indicated that the presence of the carboxyl group in the telechelic oligomer retards and that of the hydroxyl group in the telechelic oligomer promotes the generation of lactide; moreover, the increase in the content of the hydroxyl group increases the activity of the telechelic oligomer for the generation of lactide. In fact, one can find from Figure 3 that, for the same usage of the comonomer, the yield of lactide generated from various telechelic oligomers has an order of t-OPLLA-(OH)4 > t-OPLLA-(OH)3 > t-OPLLA(OH)2 > t-OPLLA-(COOH)2, and for the same yield of lactide, the usage of the polyol comonomer employed in the formation of hydroxyl group-terminated telechelic oligomers exhibits an inverse order as above. It is generally accepted that the generation of lactide is a result of the back-biting reaction of terminal hydroxyl group in PLA chain.12 The reaction involves the activation of carbonyl oxygen of PLA chain by catalyst, e.g., Sn(Oct)222,28,33 and simultaneously the attack of terminal hydroxyl group of PLA chain at the positively charged carbonyl carbon. In the case where the carbonyl carbon is situated at the penultimate position from the terminal end of PLA chain, lactide would be generated. Through the continuous back-biting reaction, lactide would be generated due to the unzipping depolymerization of PLA. The terminal carboxyl group of PLA may suppress the rate of depolymerization by shifting the equilibrium between active −OH and the Sn(Oct)2 complex to the inactive Sn(Oct)2 side.32 This accounts for the above sequential decrease in the yield of lactide generated from t-OPLLA(OH)n, OPLLA and t-OPLLA-(COOH)2 and also the consecutive increase in the lactide with increasing the value of n in t-OPLLA-(OH)n. Since the ineffective coordination with catalyst, the degradation of carboxyl-group-terminated OPLLA (i.e., t-OPLLA-(COOH)2) may suffer from, to more extent, a few side reactions, such as the random thermal breakup of the PLA chain and the intramolecular transesterification, which lead to the generation of vinyl-terminated smaller OPLLA12 and 4871


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Industrial & Engineering Chemistry Research Table 2. Effect of Temperature on the Degradations of OPLLA and Telechelic OPLLAa selectivityc (%) prepolymer

comonomer

molar percentage of comonomer

temp (°C)

tb (min)

yield (wt %)

L

D

M

I

STY (g/gcat min)

OPLLA

none

0

210 240 260

60 55 45

66.1 72.6 82.6

90.3 89.5 50.0

0.03 0.09 0.25

3.0 3.7 7.1

6.7 6.7 42.7

0.95 1.14 1.59

t-OPLLA-(COOH)2

succinic acid

1.67 mol %

210 240 260

60 55 45

61.1 65.3 76.3

90.3 88.8 79.3

0.06 0.04 0.07

4.8 5.7 4.5

4.8 5.5 16.1

0.88 1.03 1.46

t-OPLLA-(OH)3

glycerol

1.67 mol %

210 240 260

35 30 25

90.5 94.7 97.4

93.7 92.0 67.9

0.02 0.03 0.12

3.0 3.4 5.7

3.3 4.6 26.3

2.23 2.73 3.37

a

Condition for degradation reaction: pressure = 0.2 kPa. bPeriod of time that the reaction lasted until crude lactide being no longer generated. cL = L-lactide, D = D-lactide, M = meso-lactide, I = impurities (including lactic acid, lactoyllactic acid, and lactyllatic acid).

Table 3. Effect of Viscosity-Average Molecular Weights (Mv) Of OPLLA and Telechelic OPLLA on Their Degradationa selectivityd (%) prepolymer

comonomer

molar percentage of comonomer

L

D

M

I

Mv1e (× 103)

Mv2f (× 103)

66.1 58.0 49.5

90.3 89.5 84.7

0.03 0.06 0.30

3.0 4.7 9.3

6.7 5.7 5.7

3.5 5.0 6.5

10.2 13.2 27.8

t (min) yield (wt %)

OPLLA

b

60 60c 65c

t-OPLLA-(COOH)2

succinic acid

1.67 mol %

60b 60c 65c

61.1 58.0 41.4

90.3 90.8 90.0

0.06 0.06 0.1

4.7 4.5 5.3

4.9 4.8 4.6

3.8 6.7 7.4

6.20 12.60 20.70

t-OPLLA-(OH)3

glycerol

1.67 mol %

35b 60b 65c

90.5 75.20 51.44

93.6 91.4 90.3

0.02 0.05 0.08

3.0 5.3 6.4

3.4 3.3 3.2

3.4 4.6 5.3

2.60 3.39 5.57

a Conditions for degradation reaction: pressure = 0.2 kPa; temperature = 210 °C. bPeriod of time that the reaction lasted until crude lactide was no longer generated. cPeriod of time that the reaction lasted while crude lactide was still being generated. dL = L-lactide, D = D-lactide, M = meso-lactide, I = impurities (including lactic acid, lactoyllactic acid, and lactyllatic acid). eMv1 = viscosity-average molecular weight of prepolymer. fMv2: = viscosityaverage molecular weight of residual polymer.

lactide, meso-lactide, and impurities, including mainly lactic acid, lactoyllactic acid, and lactoyllactoyllactic acid. The selectivities to L-lactide, D-lactide, meso-lactide, and impurities are, respectively, 91.7−93.5%, 0.02−0.04%, 2.7−3.7%, and 3.4−5.6% for the t-OPLLA-(OH)4, t-OPLLA-(OH)3, and tOPLLA-(OH)2, while they are 90.3%, 0.03−0.09%, 3.0−5.8%, and 3.8−6.7%, respectively, for the OPLLA and t-OPLLA(COOH)2. The above results confirm that, relative to OPLLA, the telechelic OPLLA obtained by employing polyol as a comonomer is more active and effective but that obtained by employing succinic acid as a comonomer is inactive and ineffective for the generation of lactide, particularly L-lactide, while the promoting and retarding effects manifested by hydroxyl group-terminated and carboxyl group-terminated telechelic OPLLA, respectively, become larger as the contents of hydroxyl and carboxyl groups in the corresponding telechelic oligomers increase. It should be addressed that both the yield of crude lactide (92.8 wt %) and selectivity to L-lactide (93.5%) from the degradation of telechelic hydroxyl-terminated OPLLA in the open system employed in our work are much higher than those in the closed system reported in the literature34 (14 wt % yield of crude lactide and 90.6% selectivity to L-lactide). Besides, the period of time for the degradation of telechelic hydroxyl-terminated OPLLA in this work lasts only for 35 min

until crude lactide being no longer generated, while ca. 60−120 min were required to achieve a 89.6−90.7% yield of crude lactide in the literature reports.20,21 3.3. Effect of Temperature on the Degradations of OPLLA and Telechelic OPLLA. Table 2 shows the effect of temperature on the degradations of OPLLA and telechelic OPLLA. The degradation reaction was conducted under 0.2 kPa and it continued until no more lactide was generated. One can see that, for all of the prepolymers, the elevation of degradation temperature causes the obvious increase in the yield of crude lactide and the shortening reaction time, and thus, the increase in the STY of crude lactide; however, it also largely brings a decline in the selectivity to L-lactide and elevation of the selectivity to impurities. The above variations in the degradation of various prepolymers become much more distinct when the temperature is above 240 °C. This may be due to the fact that the generation of L-lactide involves the coordination activation of OPLLA by a catalyst and the subsequent unzipping depolymerization of OPLLA via the back-biting of a terminal hydroxyl group; as the temperature increases, the coordination between OPLLA and catalyst is shifted toward the left, and accordingly, OPLLA is suffered from an increased extent of noncatalytic thermal pyrolysis, resulting in more byproducts. 4872


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3.5. Variation of the Molecular Weight of Residual Oligomer during the Degradations of OPLLA and Telechelic OPLLA. Figure 4 shows the effect of degradation

3.4. Effect of the Molecular Weight of Starting OPLLA and Telechelic OPLLA on Their Degradations. Table 3 shows the effect of the molecular weight of OPLLA and telechelic OPLLA on their degradations. The degradation reaction was conducted at 210 °C under 0.2 kPa. One can see that, with increasing the molecular weight of prepolymer, both the reaction time and selectivity to byproducts increase and both the yield of crude lactide and selectivity to L-lactide decrease. This can be due to the fact that, on the one hand, the increase in the molecular weight of prepolymer increases both the amount of the methyl group next to the carbonyl group and the acidity of the α-proton in this type of methyl group, the extent of the enolization of OPLA, and, thus, the racemization of lactide becomes larger;19 on the other hand, as the molecular weight increases, the viscosity of OPLA increases, leading to the heat and mass transfers becoming poor, thus increasing the side reactions.19,29 From Table 3, one can also find that, for the degradations of OPLLA and t-OPLLA-(COOH)2, the molecular weight of residual oligomer is obviously larger than that of the prepolymer, and the increment becomes larger as the molecular weight of prepolymer increases; for the degradation of t-OPLLA-(OH)3, the molecular weight of residual oligomer increases as the molecular weight of t-OPLLA-(OH)3 increases. As is known, there may exist the esterification and intermolecular transesterification between two OPLLA molecules, because of the competition with the back-biting reaction of hydroxyl group during the degradation of OPLLA and telechelic OPLLA, and these two competing reactions generate the oligomer with the increased molecular weight. Besides, it has been identified that the copolymerization of lactic acid and other monomers, such as polyol or polycarboxylic acid, was usually accompanied by the self-polymerization of lactic acid to generate OPLLA.27,35 It is expected that, the larger the molecular weight of telechelic OPLLA, the more OPLLA would be concomitantly generated, because of the decreased mass and heat transfers. Therefore, the esterification and intermolecular transesterification between two OPLLA and the intermolecular transesterification between OPLLA and telechelic OPLLA may contribute to variations in the molecular weight of residual oligomer with the increasing molecular weight of prepolymer. From Table 3, one can also see that a relatively smaller molecular weight of t-OPLLA-(OH)3 (3.4 × 103−4.6 × 103) leads to a slightly decreased molecular weight of residual oligomer (2.6 × 103−3.39 × 103) and a relatively larger molecular weight (5.3 × 103) of t-OPLLA-(OH)3 to a slightly increased molecular weight of residual oligomer (5.57 × 103). This phenomenon for the degradation of t-OPLLA(OH)3 is different from that of OPLLA and t-OPLLA(COOH)2. This can be due to the fact that the terminal hydroxyl group promotes and the terminal carboxyl group inhibits the coordination of carbonyl group with catalyst and, thus, the formation of lactide from the degradation PLA,28,36 and accordingly the rate of the back-biting depolymerization has exceeded greatly that of the intermolecular transesterification for the degradation of t-OPLLA-(OH)3 with relatively smaller molecular weight; However, in the case of tOPLLA-(OH)3 with relatively larger molecular weight, the coordination of carbonyl group and catalyst is ineffective, because of the poor mass and heat transfers. Therefore, the contribution of molecular transesterification increases significantly, relative to that of back-biting reaction, leading to the larger molecular weight of residual oligomer than that of tOPLLA-(OH)3.

Figure 4. Effect of degradation time on the Mv of residual oligomers. The degradation reaction was conducted at 210 °C, and under 0.2 kPa. [Legend: OPLLA (■), t-OPLLA-(COOH)2 with 1.67 mol % usage of succinic acid (●), t-OPLLA-(OH)2 with 1.67 mol % usage of 1,4butanediol (▲), t-OPLLA-(OH)3 with 1.11 mol % usage of glycerol (▼), and t-OPLLA-(OH)4 with 0.83 mol % usage of pentaerythritol (◇).]

time on the viscosity-average molecular weight (Mv) of residual oligomer during the degradation reaction. One can see that the molecular weight of residual oligomer increases throughout the degradation of OPLLA. It is known that a competition exists between the back-biting reaction and the side reactions, such as the intermolecular transesterification of OPLLA and the esterification of two OPLLA molecules, which leads to the generation of lactides and oligomers with the increased molecular weight, respectively. With the progress of degradation, more residual oligomers with higher molecular weights are generated, leading to poor mass and heat transfers and, thus, the coordination of oligomer and catalyst. Therefore, the backbiting reaction is inhibited and the intermolecular transesterification of oligomer is predominant, leading to the increase in the molecular weight of residual oligomer. For the degradations of t-OPLLA-(OH)n (n = 2, 3, 4) and t-OPLLA(COOH)2, the molecular weight of residual oligomer increases first, passing a maximum, and then decreases with prolonging the degradation time. Since a certain amount of OPLLA can be also formed via the self-polymerization during the copolymerization of lactic acid and monomer, the intermolecular transesterifications among the OPLLA and/or the telechelic OPLLA lead to an increase in the molecular weights of residual oligomers in the early stage of degradation. However, with the progress of degradation, the OPLLA is gradually consumed and it contributes less to the variation in the molecular weight of residual oligomer. In the case of t-OPLLA-(OH)n (n = 2, 3, 4), the unzipping depolymerization via the back-biting of hydroxyl group emerges as the predominant reaction, and therefore, the molecular weight of residual oligomer becomes smaller and smaller, with prolonging the degradation time. In the case of tOPLLA-(COOH)2, the unzipping depolymerization via the back-biting, is inhibited because of the presence of carboxyl and 4873


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Figure 5. Network for the degradation of OPLLA.

Figure 6. Network for the degradation of t-OPLLA-(OH)2.

molecular of residual oligomer for the degradation of t-OPLLA(COOH)2 than for the degradations of t-OPLLA-(OH)n, as can be seen in Figure 4. 3.6. Mechanism on the Degradation of Telechelic OPLLA. From the above results, one can find that the behavior for the degradation of OPLLA is very different from that for

the absence of hydroxyl group, and, thus, the side reactions, such as the random thermal breakup of PLA chain and the intramolecular transesterification, which lead to the generation of smaller vinyl-terminated smaller OPLLA12 and cyclic oligomers of lactic acid, respectively, constitute the main reactions. The above reason also accounts for the slightly larger 4874


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Figure 7. Network for the degradation of OPLLA-(COOH)2.

lower than in the case of OPLLA, because of the shorter length of branch OPLLA chain, are generated. However, if the terminal hydroxyl group and carbonyl carbon, being involved in the back-biting reaction, are respectively situated on different branches of the OPLLA chain, a new t-OPLLA-(OH)2 with an elongated and a shortened lengths of branch OPLLA chains being connected respectively to the two ends of −O(CH2)4O− group are generated. The above-mentioned intermediate products of back-biting reaction can be further subjected to the similar reactions as described in the case of pure OPLLA. It is obvious that the possible cyclic oligomer intermediate in the case of t-OPLLA-(OH)2 can be smaller than in the case of pure OPLLA, since the length of the branch chain in the telechelic OPLLA is shorter than the pure OPLLA chain. This results in not only a reduced reaction time and reduced molecular weight of residual oligomer, but also both the higher yield of crude lactide and the larger selectivity to L-lactide. In the case of tOPLLA-(COOH)2 (Figure 7), the absence of a terminal hydroxyl group but the presence of a terminal carboxyl group retards the coordination between the carbonyl oxygen and the catalyst and, thus, the back-biting degradation reaction. However, because of the presence of some impurities, such as water and others, the starting t-OPLLA-(COOH)2 can be dissociated into the smaller t-OPLLA-(COOH)2 and OPLLA, which is subjected to the similar reactions as mentioned above for the pure OPLLA so as to generate lactide. Besides, tOPLLA-(COOH)2 can be also subjected to the thermal breakup, leading to the generation of smaller vinyl-terminated OPLLA and cyclic oligomer of lactic acid as byproducts. In any way, the complexity associated with the Route one for the degradation of t-OPLLA-(COOH)2 leads to not only the longer reaction time but also both the lower yield of crude lactide and selectivity to lactide. Route Two. This route starts from the intermolecular transesterification39 of oligomeric PLA. It involves the random exchange between the ester groups of two oligomeric PLA molecules, which leads to the generation of both a smaller and larger homologue (see Figures 5−7), and the subsequent

that of telechelic OPLLA; besides, the degradation of telechelic OPLLA is also affected obviously by the terminal group. We have summarized the possible reactions involved in the degradations of OPLLA and terminated telechetic OPLLA, with hydroxyl- and carboxyl-terminated telechetic OPLLA, i.e., t-OPLLA-(OH)2 and t-OPLLA-(COOH)2, being taken as the examples of the latter. These reactions constitute three routes for the degradation of oligomeric PLA, as shown in Figures 5, 6, and 7. Route One. This route starts from the intramolecular transesterification of oligomeric PLA. In the case of pure OPLLA (Figure 5), the carbonyl carbon is first activated due to the coordination of the carbonyl oxygen to the Sn(Oct)2 catalyst and then it is attacked by the terminal hydroxyl group.22 This back-biting reaction leads to the generation of a cyclic OPLLA and a smaller linear OPLLA, which are respectively denoted as (OCH(CH3)CO)m(OCH(CH3)CO)2 and H(OCH(CH3)CO)nOH. The cyclic OPLLA can be either lactide (m = 0) or byproduct (24 > m > 0)37 that is hard to be degraded.23,24,38 Since a 6-membered cyclic compound is thermodynamically more stable than its larger homologue, the selectivity to lactide is obviously higher than that to the cyclic byproduct. The smaller linear OPLLA is further subjected to the back-biting reaction mentioned above until the lactate unit number is smaller than four (n < 4), generating the easily distilled byproducts (lactic acid, lactoyllactic acid, and lactyllatic acid). In the case of tOPLLA-(OH)2 (Figure 6), the above back-biting reaction starts from both the terminal hydroxyl groups of the branch OPLLA chains connected respectively to the two ends of the −O(CH2)4O− group, unlike the case of pure OPLLA that starts from the solely terminal hydroxyl group and also the terminal carboxyl group is unfavorable to the coordination between carbonyl oxygen and catalyst. If both the terminal hydroxyl group and carbonyl carbon, being involved in the back-biting reaction, are situated on the same branch OPLLA chain in t-OPLLA-(OH)2, the smaller cyclic OPLLA and linear OPLLA, of which the molecular weights being both much 4875


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operation for the synthesis of L-lactide from the degradation of oligomeric PLA.

reactions are similar as mentioned above in Route One. Because of the formation of the larger oligomeric PLA, the viscosity of the reactant system in Route Two increases and, thus, the heat and mass transfers become poor, relative to the case of Route One. The rate for the intermolecular transesterification is expected to be lower than the back-biting reaction that is accelerated under the action of catalyst. Since the terminal hydroxyl group promotes and the terminal carboxyl group retards the coordination of oligomeric PLA, the contribution of intermolecular transesterification to the degradation of oligomeric PLA can be different for OPLLA and the telechetic OPLLA and shows an order of t-OPLLA(COOH)2 > OPLLA > t-OPLLA-(OH)2. This affects obviously the performances for the degradations of OPLLA, t-OPLLA(COOH)2, and t-OPLLA-(OH)2 (e.g., the reaction time, the yield of crude lactide, the selectivity to L-lactide, and the molecular weight of residual oligomer). Route Three. This route starts from the esterification between two oligomeric PLA molecules.40 The esterification generates an obviously larger oligomeric PLA molecule than the starting ones, and it occurs for OPLLA (Figure 5) but not for OPLLA-(OH)n (n = 2, 3, 4) (Figure 6) and t-OPLLA(COOH)2 (Figure 7), since the latter two do not contain simultaneously the terminal hydroxyl and carboxyl groups. The subsequent reactions of the larger OPLLA generated from the esterification is similar to those described in Route One. Because of the formation of the larger OPLLA, the viscosity of reactant system in Route Three increases and, thus, the heat and mass transfers become poor, relative to the case of Routes One and Two. This constitutes another important reason that the performance for the degradation of OPLLA (e.g., the reaction time, the yield of crude lactide, the selectivity to Llactide and the molecular weight of residual oligomer) is very different from those of t-OPLLA-(COOH)2 and t-OPLLA(OH)2. Based on all of the above results and discussions, it can be concluded that the addition of polyols largely promotes the generation of L-lactide, because of the elevation in the yield of crude lactide and the selectivity to L-lactide, the shortening in the reaction time, and also the decrease in the molecular weight of residual oligomer. This helps to increase greatly the efficiency for the production of lactide and, therefore, is of significant importance.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +86-731-88713257. E-mail: [email protected]; [email protected]. ORCID

Zi-Sheng Chao: 0000-0001-9248-1507 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21376068), Program for New Century Excellent Talents in University, the Ministry of Education of P. R. China, and the Program for Fu-Rong Scholar in Hunan Province, P. R. China.

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REFERENCES

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4. CONCLUSIONS The comparative studies on the degradations of OPLLA and telechelic OPLLA, i.e., t-OPLLA-(OH)n(n = 2, 3, 4) and tOPLLA-(COOH)2, show clearly that the employment of tOPLLA-(OH)n is more efficient and that t-OPLLA-(COOH)2 inefficient to the generation of L-lactide, relative to that of OPLLA. It can be due to the fact that the formation of L-lactide is a result of the unzipping depolymerization of oligomeric PLA via the back-biting terminal hydroxyl group on the carbonyl group that is activated by coordinating with catalyst. The increase in the amount of terminal hydroxyl group in oligomeric PLA favors the coordination of carbonyl group with catalyst and, thus, promotes the unzipping depolymerization to lactide but reduces the side reactions; however, the cases are just reverse for the increase in the amount of terminal carboxyl group in oligomeric PLA. Particularly, the introduction of more hydroxyl groups in oligomeric PLA also decreases the molecular weights of both the intermediate and residual oligomers. This is of significant importance in the practical 4876


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Synthesis of l-Lactide via Degradation of Various Telechelic

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