ОРГАНИЧЕСКАЯ ХИМИЯ ORGANIC CHEMISTRY
DOI 10.31489/2019Ch4/8-17 UDC 661.746.2:544.56
G.Ya. Guba1, A.O. Gusar1, E.A. Mamaeva1, V.А. Popova1, I.R. Dolgov1, A.A. Bakibayev2, R.R. Ahmedjanov3
1Tomsk Polytechnic University, Russia;
2National Research Tomsk State University, Russia
3Tomsk State Pedagogical University, Russia (E-mail: [email protected])
Polymerization of lactic acid oligomers under microwave irradiation
The kinetics of the polymerization of lactic acid oligomers (LAO) was studied for the first time at various mi- crowave irradiation powers (MW) in the presence of Sn(Oct)2/benzyl alcohol (C6H5CH2OH) catalyst and toluenesulfonic acid (TSA). LAO was polymerized in a multimodal reactor in a vacuum while sparging with nitrogen at powers of 130, 280, 360 W. The optimal concentrations of catalysts and initiator were determined, which represent 0.03 % Sn(Oct)2/C6H5CH2OH and 0.3–0.4 % TSA by weight of LAO. It was established that the polymerization of LAO in the presence of TSA proceeds already at 130 W. As the MW power increases, the rate of LAO polymerization increases. When Sn(Oct)2/C6H5CH2OH was used as a catalyst, the LAO polymerization process proceeds at a noticeable rate at 280 W, and after 25 min the molecular weight was about 20,000 Da. During the polymerization of LAO in the presence of catalysts at 360 W, lactide forms along with the formation of polylactic acid (PLA). Using the methods of IR and NMR spectroscopy, the structure of the obtained substances was studied. The synthesized PLA samples are optically pure; the optical rotation angle is 156–158 degrees×ml/dm×g. It has been suggested that during LAO polymerization under MW conditions, the so-called «non-thermal microwave effect» is observed, that is, at the same temperature (215 °C), but at different powers (280 W and 360 W), the LAO polymerization proceeds at different rates.
Keywords: microwave irradiation, lactic acid, polylactic acid, lactic acid oligomers, NMR spectroscopy, IR spectroscopy, specific optical rotation, catalyst.
Introduction
PLA is one of the most promising biodegradable polymers used for various biomedical applications be- cause of its biocompatibility and biodegradation in the human body into non-toxic metabolites [1–5].
Tin octanoate is recognized as the most effective catalyst for the synthesis of PLA. In this case, com- pounds containing hydroxyl groups, such as alcohols [2, 6, 7], initiate the ring opening polymerization pro- cess.
With traditional methods for the synthesis of PLA stringent conditions are necessary: high vacuum, long polymerization time and the consumption of large amounts of energy.
MW is able to accelerate chemical reactions 10–100 times. The wide possibilities offered by MW in chemistry are of great interest in the study and application of the effects of exposure to microwave radiation.
It is believed that MW has firstly, the ability to cause rapid and significant heating of organic compounds and secondly, to activate reagent molecules and especially, cause their dissociation into ions and free radicals.
The MW photon at a frequency of 2.45 GHz has the energy equal to 9.6×10–4 eV. This is not enough even to break the hydrogen bond [8, 9].
Ре по зи то ри й Ка рГ У
To date considerable experience has been accumulated on the use of MW to accelerate the synthesis of PLA [10–19].
The works [3, 6, 17–18, 20–23] show the results of studies of the synthesis of PLA under MW condi- tions by polymerization of lactides. The rate of polycondensation of lactic acid (LA) under MW conditions depends on the volume of reacting substances [24], pressure [19–20], and the monomer/catalyst ratio [25].
The polymerization of PLA with the opening of the lactide ring was successfully carried out using the conventional and microwave methods in [26]. The lactide polymerization reaction time under MW condi- tions was reduced from 6 hours to 20 minutes, the molecular weight (Mw) of the PLA was 11000 Da and under ordinary conditions the Mw of the PLA was 2000 Da.
The processes of direct polymerization of LA under MW conditions are attractive as energy-saving process- es, since they avoid the labor-consuming and energy-consuming stage of lactide synthesis [3, 12, 14, 26, 27].
Direct condensation of LA is carried out in three stages: removal of free water, polycondensation with the formation of oligomers and condensation of oligomers to high molecular weight polymers in the melt. On the first and third stages, water removal is critical to the reaction rate. For the second stage, the rate is deter- mined by the chemical reaction [12, 28].
The authors of [12, 26] studied the polycondensation of 85 % LA in the presence of a SnCl2/TSA binary catalyst under MW conditions in a monomode reactor. MW with Mw 16000 Da was obtained by carrying out the reaction for 30 min at a pressure of 30 mmHg. When only TSA is used as a catalyst, LAO with Mw less than 1000 Da is formed. This polymer can also be converted to high molecular weight PLA by melt po- lycondensation [12].
The effect of MW is currently debatable [6, 11, 25, 27, 29–31]. No data were found on increasing the reaction rate due to an increase in activation energy in the MW field in the literature.
The authors of [25], while studying the polymerization of lactide in a toluene solution, expressed the opinion that microwave irradiation did not cause any changes in the polymerization kinetics, which made it possible to consider the absence of a specific «non-thermal» effect during microwave synthesis.
Authors [30] studied the kinetics of caprolactam polymerization under MW conditions. As a result, a specific «non-thermal» effect was suggested.
Authors [31] investigated the polycondensation of LA under MW conditions in xylene in the presence of catalysts at a power of 100–400 W and pointed to the existence of a specific «non-thermal» effect induced by an electric field.
In [32], a lactide-pullulan copolymer was synthesized in a monomode microwave reactor and it was found that at the same reaction temperature (80 °C) with increasing irradiation power from 50 to 200 W, the yield of the final product increased. The authors suggested the existence of a specific «non-thermal» effect.
An analysis of the literature data showed that, despite the large number of publications on the synthesis of PLA under MW conditions, the results obtained are disparate, performed under different conditions and in various microwave reactors [29]. Although it can be said unequivocally that under MW conditions, the time for the synthesis of PLA decreases by 10–15 times.
In this work, the influence of the MW power, the time and nature of the catalysts on the polymerization of LAO under the MW conditions in a multimode reactor was investigated.
Experimental
LAO obtained according to the method described in [33] by evacuation for 25 min at 280 W of an 80 % aqueous solution of D(+)˗acid of the PURAK 80 brand (country of origin is the Netherlands) was used as the object of study. Sn(Oct)2, TSA applied qualifications chemically pure, C6H5CH2OH analytical grade was used as initiator.
The IR spectra of the starting compounds and reaction products were recorded on an Agilent Resolu- tions Pro IR spectrometer.
The 1H NMR spectra of the synthesized LAO and PLA were recorded using a Bruker AVANCE AV 300 Fourier spectrometer (Germany), an operating frequency of 300 MHz, and a deuterated CDCl3 solvent.
The molecular weights of the obtained samples were determined by the viscometric method using a Ubbelode viscometer. Chloroform was used as a solvent. The molecular weights of PLA were also deter- mined by gel permeation chromatography on an Agilent 1200 instrument. Chloroform was used as a solvent, the eluent velocity was 1 ml/s.
To determine the angle of optical rotation (degrees×ml/dm×g), an AR-300 polarimeter was used, the tube length was 1 dm, and the solvent was chloroform, λ = 589 nm.
Ре по зи то ри й Ка рГ У
Thermal studies of PLA sample LAO was polymerized under m
Figure 1. Schem The resulting 10 g LAO was TSA) were added in an amount of 0 lyst/initiator 1:1). They were placed mmHg at various powers 130–360 W
It is known that Mw of PLA de ratio of catalyst:cocatalyst, tempera ence of the molecular weight of PL out at 280 W for 25 min. The h 0.03 mass.%.
The increasing of catalyst conc er, further catalyst concentration in Moreover, samples changed the col that catalyst concentration increase s
Sn(Oct)2
Figure 2. Dependence of M C6H5CH2OH is an effective ini Mw to Sn(Oct)2, the PLA increases the absence of C6H5CH2OH, the po lyst: initiator is 1:2 and the reaction mass becomes black-brown. The me known that alcohols presence incre Sn(Oct)2.
It could be suggested that C6H5
The kinetic curves of LAO po ure 4. The dependence of Mw at an during the course of the reaction for
es were performed on an SDT Q600 V20.9 Build microwave irradiation according to the general sch
me for the synthesis of PLA (catalyst — Sn(Oct)2, TSA loaded into a 100 ml glass heat-resistant flask 0.03–0.3 %, together with the initiator (C6H5CH2 d in a microwave reactor and sparged with nitrog W.
Results and Discussion
epends on a number of factors: the nature and am ature and polymerization time. Figure 2 shows th LA on the concentration of the catalyst. The pol
ighest Mw of PLA was achieved at a cataly centration to ω = 0.03 mass.% leads to molecular
ncrease provokes the reverse process of molec lor to darker one. Taking into account these fact stimulates polymer destruction process.
2/C6H5CH2OH = 1:1; Power = 280 W; τ = 25 min Mw PLA on ω of the catalyst during LAO polymerizati
tiator of Sn(Oct)2 in the synthesis of PLA. Upon and reaches a maximum at a ratio of 1:1, and th lymerization process is practically not observed.
n takes more than 15 minutes, the polymer is de echanism of PLA at Sn(Oct)2 and alcohols are de eases the reaction rate of cyclic LAO polymeriz
5CH2OH forms intermediate complex. The optima lymerization in the presence of Sn(Oct)2/C6H5CH n irradiation power of 130 W (205 °C), Mw LA
30 min, the Mw LAO is only 1500 Da.
d 20 instrument.
heme shown in Figure 1.
A)
and catalysts (Sn(Oct)2,
2OH, in the ratio of cata- gen at a pressure of 200
mount of the catalyst, the he curves of the depend- lymerization was carried yst concentration (ω) of r weight growth. Howev- cular weight decreasing.
ts, it could be suggested
ion under MW
addition of C6H5CH2OH hen decreases (Fig. 3). In When the ratio of cata- egraded and the reaction escribed in [15, 17]. It is zation at the presence of
al ratio is 1:1.
H2OH are shown in Fig- AO slowly increases. So,
Ре по зи то ри й Ка рГ У
Power = 280 W; τ = 25 min
Figure 3. Dependence of Mw of PLA on the molar ratio of Sn(Oct)2/C6H5CH2OH during LAO polymerization under MW conditions
Catalyst — Sn(Oct)2/C6H5CH2OH; Power: 1 — 130 W; 2 — 280 W; 3 — 360 W Ratio catalyst/initiator/LAO = 3/3/10000 (mass.%)
Figure 4. Kinetic curves of LAO polymerization the presence of a catalyst
At an MW power of 280 W, the sample heats up during the first 10 minutes and the process of polycondensation of low molecular weight LAO occurs with the removal of water. With an increase in the process time, the molecular weight of LAO sharply increases. Presumably, at this period of the process, the LAO polymerization stage is limiting with the opening of the cycle after 15 minutes the polymerization pro- cess slows down, which is apparently due to the fact that the diffusion stage of the process becomes limiting.
It should be noted separately that the temperature of the LAO reaction mass reaches 215 ºC both at 280 W and 360 W [33].
At a power of 360 W, the molecular weight of the polymer increases sharply and reaches its maximum value of about 20000 Da in 12 minutes.
With an increase in the process time, the resulting polymer is degraded and Mw LAO sharply decreas- es, and a white crystalline compound is formed at the outlet of the reactor, which was identified as lactide using IR spectroscopy.
Thus, we can assume that in this case the so-called «non-thermal microwave effect» is observed, that is, under the same conditions at the same temperature (215 ºC), but with different capacities (280 W and 360 W), the LAO polymerization proceeds at different rates.
LAO polymerization in the presence of TSA was performed at powers of 130 W, 280 W, and 360 W.
The dependence of Mw of PLA on ω TSA is presented in Figure 5.
This dependence shows that with an increase in the catalyst concentration from 0.05 to 0.3 %, an in- crease in Mw is observed. The optimal ω TSA is in the range of 0.3–0.4 mass.%.
Ре по зи то ри й Ка рГ У
Power = 280 W; τ = 30 min
Figure 5. Dependence of Mw of PLA on ω TSA upon polymerization of LAO under MW conditions
The kinetic curves of LAO polymerization in the presence of TSA are presented in Figure 6. With an increase in reaction time, an increase in Mw of LAO is observed. However, Mw PLA grows unevenly at 280 W. When carrying out the reaction up to 15 minutes, a weak growth of the LAO chain is observed, low mo- lecular weight oligomers are formed. In the range from 15 to 30 minutes, a sharp increase in the Mw of the polymer occurs, and then the chain grows in a linear relationship.
1 — 130 W; 2 — 280 W (ω TSA 0.3 mass.%)
Figure 6. Dependence of Mw of the PLA sample on time in the conditions of MW
During the polymerization of LAO at a power of 360 W, volatile cyclic lactides are formed in parallel with the formation of PLA, which are white crystals deposited on the walls of the flask and the condenser.
Physicochemical properties of the synthesized samples and experimental conditions are presented in Table.
T a b l e Synthesis conditions and physicochemical properties of synthesized LAO samples
Catalyst Power,
W t, оС τ, min Mwa,
Da Mwb,
Da Mnb,
Da PD
25
[ ]α589 degrees×ml/
dm×g Sn(Oct)2/ C6H5CH2OH 130 205 35 1700 – – – – Sn(Oct)2/ C6H5CH2OH 280 215 25 19900 19230 12820 1,50 –156
TSA 280 215 35 9350 9940 5402 1,84 –157
Sn(Oct)2/ C6H5CH2OH 360 215
8 3000 –
12 19500 –158
17 10300 –152
25 6000 The sample darkened, lactide formed
TSA 360 215 10 Lactide formed
——————————
Note. t, оС — temperature; τ — reaction time, min; Mwa, Da — calculated using a viscometric method; Mwb, Da — determined by NMR spectroscopy; Mnb, Da — determined by gel permeation chromatography; [ ]α25589 — optical rotation angle.
Ре по зи то ри й Ка рГ У
Therefore, it could be suggested that MW power determines the kinetics of LAO polymerization in the presence of TSA.
The data obtained confirm the presence of a «non-thermal microwave effect» during LAO polymeriza- tion under MW conditions, since at the same temperature but diverse capacities, LAO polymerization pro- ceeds at different rates by various mechanisms at 280 W and 360 W.
The optical rotation angle of the synthesized samples is 156–158 degrees×ml/dm×g, i.e., the synthe- sized samples are optically pure. Figure 7 shows the IR spectra of PLA samples synthesized under MW con- ditions.
1 — τ = 20 min after polymerization;
2 — τ = 30 min at 280 W in the presence of 0.3 mass.% TSA;
3 — τ = 30 min at 280 W in the presence of 0.03 mass.% Sn(Oct)2/C6H5CH2OH Figure 7. IR spectra of LAO obtained at a power of 280 W
The interpretation of the absorption bands of the IR spectra is given below: 3500–3300 (υОН), 2997 (υasCH3), 2947 (υsCH3), 2882 (υCH), 1760–1727 (υC=O), 1452 (δasCH3), 1388–1348 (δsCH3), 1368–1360 (δ1CH+δsCH3), 1315–1300 (δ2CH), 1270 (υCOC), 1215–1185 (υasCOC), 1130 (rasCH3), 1100–1090 (υsCOC), 1045 (υC-CH3), 960–950 (r CH3 + υCC), 875–860 (υC-COO), 760–740 (δC=О).
In the IR spectra, intense absorption bands of carbonyl groups C=O (1757 cm-1) and absorption bands of C-O-C bonds of 1188 cm-1, PLA characteristic are observed.
On curves 2 and 3 (Fig. 7), belonging to the samples of synthesized PLA, the intensity of the absorption bands in the region of 3600–3500 cm-1, characterizing the vibrations of the OH groups, significantly de- creased. The peaks in the region of 1759 cm-1, which belong to the stretching vibrations of the C=O bond, become more symmetrical. The band in the region of 1188 cm-1, related to the stretching vibrations of the –C–O– bond, becomes more pronounced. Figure 8 shows the 1H NMR data of PLA samples synthesized un- der MW conditions.
Chemical shifts 1H NMR LAO and PLA are presented below:
Chemical shift in ppm Structural assignments 5.2 1H, m, –CH(CH3)–
4.4 1H, m, –CH(CH3)OH, end group)
1.6 (3H, d, –CH3)
Ре по зи то ри й Ка рГ У
1 — OLA obtained by irradiating MW in vacuum for 20 minutes;
2 — PLA, obtained from OLA in the presence of TSA for 30 minutes at a power of 280 W;
3 — PLA obtained from OLA in the presence of Sn(Oct)2 for 25 minutes at a power of 280 W Figure 8. 1H NMR spectra
The proton signal of the multiplet of the methine group of the terminal group –CH(CH3)OH LAO is 4.4 ppm, the chemical shift of the proton of the multiplet of the methylene group of the PLA is 5.2 ppm. By the ratio of signal intensities of methine groups in the region of 4.4 and 5.2 ppm we can judge the depth of the polymerization process of LAO to PLA.
Intensive chemical shifts of 4.4 ppm are observed on curve 1 (Fig. 8), which suggests the presence of a significant number of terminal hydroxyl (and, accordingly, carboxyl) groups in LAO. In the spectra of PLA samples obtained by the polymerization of LAO in the presence of TSA and Sn(Oct)2 (see Fig. 8). Chemical shifts at 4.4 ppm are observed. Other shifts are absent.
Figure 9 shows the DSC thermogram of a PLA sample synthesized at 280 W for 25 min in the presence of 0.09 mass.% Sn(Oct)2. The absence of a pronounced melting peak at 180–190°С suggests that the polymer mainly has an amorphous structure.
τ = 20 min in the presence of Sn(Oct)2
Figure 9. DSC-TGA spectra of PLA synthesized under MW conditions
Ре по зи то ри й Ка рГ У
Conclusions
The optimal concentrations of catalysts and initiator for the synthesis of PLA were determined, which are 0.03 % Sn (Oct)2/C6H5CH2OH and 0.3–0.4 % TSA by weight of LAO.
It was found that with an increase in the power of MW, the rate of LAO polymerization in the presence of catalysts increases. When Sn(Oct)2/C6H5CH2OH is used as a catalyst, the LAO polymerization process proceeds at a noticeable rate at 280 W, and after 25 min Mw is about 20000 Da. The synthesized PLA sam- ples are optically pure; the optical rotation angle of the synthesized samples is 156–158 degrees×ml/dm×g. It has been suggested that during LAO polymerization under MW conditions, the so-called «non-thermal mi- crowave effect» is observed, that is, at the same temperature (215 °C), but with various capacities (280 W and 360 W), the LAO polymerization proceeds at different rates by diverse mechanisms. During the polymerization of LAO in the presence of catalysts at 360 W, a small amount of lactide is formed along with the formation of PLA.
References
1 Masutani, K. & Kimura, Y. (2014). PLA Synthesis. From the Monomer to the Polymer, in Poly (lactic acid) Science and Technology: Processing, Properties, Additives and Applications. Polymer Chemistry Series, 12, 1–36.
2 Nakatsuka, T. (2011). Polylactic acid-coated cable. Fujikura Technical Review, 40, 39–45.
3 Madhavan K., Nampoothiri, Nimisha Rajendran, Nair, & Rojan Pappy, John. (2010). An overview of the recent develop- ments in polylactide (PLA) research, Bioresource Technology, 101(22), 8493–8501.
4 Astrid J.R., Lasprilla, Guillermo A.R., Martinez, Betânia H., Lunelli, & André L., Jardini. (2012). Polylactic acid synthesis for application in biomedical devices — A review. Biotechnology Advances, 30, 321–328.
5 Gupta, A., & Kumar, V. (2007). New emerging trends in synthetic biodegradable polymers. European Polymer Journal, 43, 4053–4074.
6 Li, H., Liao, L., & Liu, L. (2007). Kinetic Investigation into the Non-Thermal Microwave Effect on the Ring-Opening Polymerization of ɛ-Caprolactone. Macromol. Rapid Communications, 28, 411–416.
7 Manoj B., Gawande, Sharad N., Shelke, Radek, Zboril, & Rajender S., Varma. (2014). Microwave-assisted chemistry: syn- thetic applications for rapid assembly of nanomaterials and organics. Accounts of Chemical Research, 47, 1338−1348.
8 Berdonosov, S., Berdonosova, D. (2000). Microwave radiation in chemical practice. Chemical Technology, 3, 2–8.
9 Nakamura, T., Nagahata, R., Suemitsu, S., Takeuchi, K. (2010). In-situ measurement of microwave absorption properties at 2.45 GHz for the polycondensation of lactic acid. Polymer, 51, 329–333.
10 Giachi, G., Frediani, M., Rosi, L., & Frediani, P. (2011). Synthesis and Processing of Biodegradable and Bio-Based Poly- mers by Microwave Irradiation. Microwave Heating. Ed. by Dr. Usha Chandra, Florence, 181–206. DOI: 10.5772/23692
11 Komorowska-Durka, M., Dimitrakis, G., Bogdał, D., Stankiewicz, A., & Stefanidis, G. (2015). A concise review on micro- wave-assisted polycondensation reactions and curing of polycondensation polymers with focus on the effect of process conditions.
Chemical Engineering Journal, 264, 633–644.
12 Sosnika, A., Gotelli, G., & Abrahamb, G. (2011). Microwave-assisted polymer synthesis (MAPS) as a tool in biomaterials science, How new and how powerful. Progress in Polymer Science, 36, 1050–1078.
13 Kempe, K., Becer, R., & Ulrich, S. Schubert. (2011). Microwave-Assisted Polymerizations: Recent Status and Future Per- spectives. Macromolecules, 44, 5825–5842.
14 Wiesbrock, F., Hoogenboom, R., & Ulrich, S. Schubert (2004). Microwave-Assisted Polymer Synthesis: State-of-the-Art and Future Perspectives. Macromolecular Rapid Communications, 25, 1739–1764.
15 Zhang, C., Liao, L., & Gong, S. (2007). Recent developments in microwave-assisted polymerization with a focus on ring- opening polymerization. Green Chemistry, 9, 303–314.
16 Manoj B., Gawande, Sharad N., Shelke, Radek, Zboril, & Rajender S., Varma. (2014). Microwave-Assisted Chemistry: Syn- thetic applications for rapid assembly of nanomaterials and organics. Accounts of Chemical Research, 47, 1338−1348.
17 Mallakpour, S., & Rafiee, Z. (2008). Application of microwave-assisted reactions in step-growth polymerization: a review.
Iran Polymer Journal, 12, 907–935.
18 Nagahata, R., Sano, D., Suzuki, H., & Takeuchi, K. (2007). Microwave-assisted single-step synthesis of poly(lactic acid) by direct polycondensation of lactic acid. Macromolecular Rapid Communications, 4, 437–442.
19 Nagahata, R., Momotake, M., Takeuchi, K., & Nakamura, T. (2012). US Patent 8,242,186 B2.
20 Pandey, A., & Pranesh B., Aswath. (2009). Microwave Synthesis of Poly (L-lactic acid). Journal of Biomaterials Science 20, 33–48.
21 Jing, S., Peng, W., Tong, Z., & Baoxiu, Z. (2006). Microwave irradiated ring-opening polymerization of d,l-lactide under at- mosphere. Journal of Applied Polymer Science, 100, 2244–2247.
22 Bakibaev, A., & Guba, G. (2015). Polymerization of lactiс acid using microwave and conventional heating. Procedia Chem- istry, 15, 97–102.
23 Nagahata, R., Kunii, K., Soga, H., Sugimoto, S., & Takeuchi, K. (2010). Large-scale polycondensation of lactic acid using microwave batch reactors. Organic Process Research Development, 14, 781–786.
Ре по зи то ри й Ка рГ У
24 Nikolic, L., Ristic, I., Adnadjevic, B., Nikolic, V., Jovanovic, J., & Stankovic, M. (2010). Novel microwave-assisted synthe- sis of poly (D,L-lactide): The influence of monomer/initiator molar ratio on the product properties, Sensors, 10, 5063–5073.
25 Singlaa, P., Kaura, P., Mehtaa, R., Berekb, D., & Upadhyayc, S. (2012). Ring-opening polymerization of lactide using mi- crowave and conventional heating. Procedia Chemistry, 4, 179–185.
26 Yamada, S., Takasu, A., Takayamab, S., & Kawamurac, K. (2014). Microwave-assisted solution polycondensation of L-lactic acid using a Dean-Stark apparatus for a non-thermal microwave polymerization effect induced by the electric field. Polymer Chemistry, 5, 5283–5288.
27 Lei, H., Wang, P., & Yuan, W. (2009). Microwave-assisted synthesis of poly (L-Lactic Acid) via direct melt polycondensation using solid super acids. Macromolecular Chemistry and Physics, 210, 2058–2062.
28 Fangchao, Song, & Linbo, Wu. (2011). Synthesis of high molecular weight poly (L-lactic acid) via melt/solid polycondensation: intensification of dehydration and oligomerization during melt polycondensation. Applied Polymer Science, 120, 2780–2785.
29 Fimberger, M., & Wiesbrock, F. (2016). Microwave-assisted synthesis of polyesters and polyamides by ring-opening polymerization. Advances in Polymer Science, 274, 149–182.
30 Ramier, J., Renard, E., & Grande, D. (2012). Microwave-assisted ring-opening polymerization of d,l-lactide: A probe for the nonexistence of nonthermal microwave effects. Macromolecular Chemistry and Physics, 7, 784–788.
31 Manoj B., Gawande, Sharad N., Shelke, Radek, Zboril, & Rajender S., Varma. (2014). Microwave-assisted chemistry: syn- thetic applications for rapid assembly of nanomaterials and organics. Accounts of Chemical Research, 47, 1338−1348.
32 Tanga X., Huanga J., & Xub L., et al. (2014). Microwave-assisted rapid synthesis, characterization and application of poly (d,l-lactide)-graft-pullulan. Carbohydrate Polymers, 107, 7–15.
33 Guba, G., Gusar, A., Bakibayev, A., Tazhbayev, Ye., & Minayeva, Ye. (2019). Dehydration/polycondensation of lactic acid under microwave irradiation. Bulletin of the Karaganda university. Chemistry series, 1(93), 8–16.
Г.Я. Губа, А.О. Гусар, Е.А. Мамаева, В.А. Попова, И.P. Долгов, А.А. Бакибаев, Р.Р. Ахмеджанов
Қысқа толқынды сəулелендіру жағдайында
сүт қышқылы олигомерлерін полимерлеу
Сүт қышқылының олигомерлерінің (СҚО) полимерленуінің кинетикасы алғаш рет микротолқынды сəулеленудің (МТС) əртүрлі қуаттарында, Sn(Oct)2 / бензил спирті (C6H5CH2OH) катализаторы жəне толуолсульфоқышқылы (ТСҚ) қатысуымен зерттелді. СҚО полимеризациясы мультимодальды реактордағы вакуумда азотпен барботаждау жəне 130, 280, 360 Вт қуаттылықтарда жүргізілді. 0.03 % Sn(Oct)2/C6H5CH2OH жəне СҚО салмағы бойынша 0,3–0,4 % ТСҚ болатын катализаторлар мен сокатализаторлардың оңтайлы концентрациясы анықталды. СҚО салмағынан 0,03 % Sn(Oct)2/ C6H5CH2OH жəне 0,3–0,4 % ТСҚ тұратын катализаторлар мен сокатализаторлардың оптималды концентрациясы табылды. ТСҚ қатысуымен СҚО-ны полимерлеу 130 Вт-тан басталатындығы анықталды. МТС қуатының жоғарылауымен СҚО полимерлену жылдамдығы артады. Sn(Oct)2/ C6H5CH2OH катализатор ретінде қолданғанда, СҚО полимеризациясы 280 Вт жылдамдықпен жүреді, ал 25 минуттан кейін молекулалық массасы шамамен 20000 Dа болды. СҚО полимерлеу процессі кезінде катализаторлардың қатысуымен 360 Вт қуаттылықта полисүт қышқылдың (ПСҚ) түзілуімен бірге лактид түзіледі. ИҚ жəне ЯМР спектроскопия əдістерінің көмегімен алынған полимерлердің құрылымы зерттелді. Синтезделген ПСҚ үлгілері оптикалық таза, оптикалық бұрылу бұрышы 156–158 градус×мл/дм×г тең. Болжам бойынша, МТС жағдайында СҚО полимеризациясы кезінде
«жылусыз микротолқынды эффект» деп аталатын, яғни бірдей жағдайларда, бірдей температурада (215 ºC), бірақ əртүрлі қуаттылықта (280 жəне 360 Вт) СҚО полимерленуі əр түрлі жылдамдықпен жүреді.
Кілт сөздер: микротолқынды сəулелендіру, сүт қышқылы, полисүт қышқылы, сүт қышқылы олигомерлері, ЯМР спектроскопиясы, ИҚ спектроскопиясы, меншікті оптикалық айналу, катализатор.
Г.Я. Губа, А.О. Гусар, Е.А. Мамаева, В.А. Попова, И.P. Долгов, А.А. Бакибаев, Р.Р. Ахмеджанов
Полимеризация олигомеров молочной кислоты
в условиях микроволнового облучения
Впервые изучена кинетика полимеризации олигомеров молочной кислоты (ОМК) при различных мощностях микроволнового облучения (МВО) в присутствии катализатора/cокатализатора Sn(Oct)2/бензиловый спирт (C6H5CH2OH) и толуолсульфокислоты (ТСК). Полимеризацию ОМК про- водили в мультимодальном реакторе в вакууме при барботировании азотом при мощностях 130, 280,
Ре по зи то ри й Ка рГ У
360 Вт. Определены оптимальные концентрации катализаторов и сокатализаторов, которые составля- ют 0,03 % Sn(Oct)2/C6H5CH2OH и 0,3–0,4 % ТСК от массы ОМК. Установлено, что полимеризация ОМК в присутствии ТСК протекает уже при 130 Вт. С повышением мощности МВО скорость процес- са полимеризации ОМК увеличивается. При использовании в качестве катализатора Sn(Oct)2/C6H5CH2OH процесс полимеризации ОМК с заметной скоростью протекает при 280 Вт, и че- рез 25 мин молекулярный вес составлял около 20000 Dа. При проведении процесса полимеризации ОМК в присутствии катализаторов при 360 Вт, наряду с образованием полимолочной кислоты (ПМК), происходит образование лактида. С применением методов ИК- и ЯМР-спектроскопии изучено строе- ние полученных полимеров. Синтезированные образцы ПМК являются оптически чистыми, угол оп- тического вращения равен 156–158 градусам×мл/дм×г. Высказано предположение, что при полимери- зации ОМК в условиях МВО наблюдается так называемый «нетепловой микроволновой эффект», т.е.
в одинаковых условиях при одинаковой температуре (215 ºС), но разных мощностях (280 и 360 Вт) процесс полимеризации ОМК протекает с различной скоростью.
Ключевые слова: микроволновое облучение, молочная кислота, полимолочная кислота, олигомеры молочной кислоты, ЯМР-спектроскопия, ИК-спектроскопия, удельное оптическое вращение, катали- затор.
