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Journal of Macromolecular Science, Part A: Pure and Applied Chemistry Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lmsa20
Microwave Assisted Synthesis of Poly(lactic acid) and its Characterization using Size Exclusion Chromatography a
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Pankil Singla , Rajeev Mehta , Dusan Berek & S. N. Upadhyay
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School of Chemistry and Biochemistry, Thapar University, Patiala, Punjab, India
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Department of Chemical Engineering, Thapar University, Patiala, Punjab, India
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Polymer Institute of the Slovak Academy of Sciences, Bratislava, Slovakia
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Department of Chemical Engineering & Technology, IT-BHU, Varanasi, UP, India Version of record first published: 03 Oct 2012.
To cite this article: Pankil Singla , Rajeev Mehta , Dusan Berek & S. N. Upadhyay (2012): Microwave Assisted Synthesis of Poly(lactic acid) and its Characterization using Size Exclusion Chromatography, Journal of Macromolecular Science, Part A: Pure and Applied Chemistry, 49:11, 963-970 To link to this article: http://dx.doi.org/10.1080/10601325.2012.722858
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Journal of Macromolecular Science, Part A: Pure and Applied Chemistry (2012) 49, 963–970 C Taylor & Francis Group, LLC Copyright ISSN: 1060-1325 print / 1520-5738 online DOI: 10.1080/10601325.2012.722858
Microwave Assisted Synthesis of Poly(lactic acid) and its Characterization using Size Exclusion Chromatography PANKIL SINGLA1, RAJEEV MEHTA2∗, DUSAN BEREK3, and S. N. UPADHYAY4 1
School of Chemistry and Biochemistry, Thapar University, Patiala, Punjab, India Department of Chemical Engineering, Thapar University, Patiala, Punjab, India 3 Polymer Institute of the Slovak Academy of Sciences, Bratislava, Slovakia 4 Department of Chemical Engineering & Technology, IT-BHU, Varanasi, UP, India 2
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Received May 2012, Accepted June 2012
Poly(lactic acid) (PLA) has been synthesized catalytically under vacuum by microwave (MW) irradiation using stannous octoate (SnOct2 ) as catalyst. The polymerization is carried out at 180◦C up to 30 min. PLA with a molar mass of 104 g.mol−1 and a yield over 97% was produced in 20 min by the ring-opening polymerization of L-lactide using (SnOct2 ) as catalyst under microwave irradiation with a power level of 180 W. The structural investigations are done by NMR and FTIR. The average molar mass of PLA is determined by means of size exclusion chromatography, (SEC). The characterization is done using three different columns. The polymerization rate is much faster with microwave heating than conventional heating. Microwave irradiation gives rapid energy transfer and high-energy efficiency, hence, a faster reaction rate. Keywords: Poly(lactic acid), microwave irradiation, ring-opening polymerization, size exclusion chromatography, vacuum
1 Introduction Lactic acid is obtained from inexpensive raw materials, such as starch, glucose, and oligosaccharides through the biotechnological route (1). Such poly(lactic acid) (PLA) is a biodegradable and biocompatible material, which can be degraded into non-toxic end-products. PLA has high strength and can be used as fiber, food packaging material, surgical suture, drug delivery device, etc. (2–4). In view of these applications, the interest in developing appropriate technology for PLA synthesis has increased steadily. PLA can be prepared either by ring-opening polymerization (ROP) of lactide (LA) or by direct polycondensation of lactic acid (5) Through the poly-condensation route, it is difficult to prepare PLA with high molar mass, and as result, the product polymer does not have wide practical use due to its poor mechanical properties. The widely accepted route to prepare high molar mass PLA is through the ring opening polymerization. Both the methods, however, need long reaction time and should be carried out under high vacuum or an inert gas environment to complete the polymerization with high conversion (4–7).
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Address correspondence to: Rajeev Mehta, Department of Chemical Engineering, Thapar University, Patiala 147004, Punjab, India. Email:
[email protected]
Microwave, (MW) irradiation offers a number of advantages over conventional heating. It is a well-known method for heating and drying materials and is widely utilized in home kitchen and industry for these purposes. Many researchers have used microwave heating in chemical synthesis because of its speed, high efficiency and uniform heating (8–11). The water-soluble poly(methyl vinyl etherco-maleic anhydride) copolymer-bovine serum albumin bioconjugates were synthesized in the presence of 1-ethyl3-(3-dimetilaminopropyl) carbodiimide hydrochloride as crosslinking agents via microwave-assisted and conventional methods and characterized by size-exclusion chromatography and high-performance liquid chromatography. The reaction time for the synthesis can be significantly reduced by the application of MW heating in place of conventional heating. MW-assisted radical polymerization of styrene was carried out by Zhu and coworkers with a faster polymerization rate than conventional heating (12, 13). Few reports are available in the literature on MW-assisted ROP of LA. Liu et al. were the first to use MW heating for polymerization of d,l-lactide (DLLA) (14). The polymerization rate was found to be much faster than that observed with conventional heating. Peng and coworkers(15) polymerized d,l-lactide in the presence of Sn(Oct)2 under MW irradiation at atmospheric pressure. Effects of the different heating media, monomer purity, Sn(Oct)2 concentration, and application of reduced pressure on the molar mass and
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964 the polymer yield were studied. Gong and coworkers polymerized l-lactide in the presence of Sn(Oct)2 by using poly(ethylene glycol) and methoxy poly(ethylene glycol) as the macro-initiator (16, 17). The ROP was performed in a monomode MW reactor and poly(l-lactide)block-poly(ethylene glycol)-block-poly(l-lactide) tri-block copolymers and methoxy poly(ethylene glycol)-blockpoly(l-lactide) di-block copolymers were obtained. In both cases, a 20 min MW irradiation at 100◦ C led to high conversion (mostly greater than 90%) with different monomer/initiator ratios and poly(ethylene glycol) of different molar masses. Use of MW irradiation to synthesize PLA by direct polycondensation of lactic acid has resulted in the significant acceleration of the polymerization rate (18). By employing a binary catalyst SnCl2 /p-TsOH, PLA with an Mw of 16 Kg.mol−1 was obtained within 30 min under reduced pressure of 30 mm Hg. PLA/PEG copolymers was obtained by melt polycondensation of L-lactic acid and poly(ethylene glycol) undermicrowave irradiation (19). The effects of reaction conditions including catalyst kinds, mLA/mPEG feed ratios, etc, on the polymerization reaction were investigated. Nikolic and coworkers synthesize PDLA (poly(D,L-lactide)) using tin(II) 2-ethylhexanoate initiated ring-opening polymerization (ROP) takes over 30 hin bulk at 120◦ C (20). The use of microwave makes the same bulk polymerization process with the same initiator much faster and energy saving, with a reaction time of about 30 min at 100◦ C. Here, the poly(lactide) synthesis was done in a microwave reactor, using a frequency of 2.45 GHz and maximal power of 150 W. An excellent review of the effect of microwave radiation on the chemistry of materials has been given by Jin (21). According to size-exclusion chromatography and high-performance liquid chromatography results, the bioconjugates synthesized in the microwaveassisted method are more stable and efficient than the conventional method. The reaction time is shortened from 17 h to 15 minby means of the microwave-assisted method. At present, size exclusion chromatography, SEC dominates determination of molar mass averages and dispersities of synthetic polymers. SEC is an excellent method, which has extensively influenced development in polymer science and technology. Though SEC is simple, fast, precise and repeatable, it suffers from several experimental pre-requisites, and suffers from inherent limitations, drawbacks and pitfalls (22). As a result, the molar mass data acquired are often non-reproducible and have low accuracy. In general, obtaining exact molar masses of synthetic polymers accurately may represent a challenge. Among the often not-fulfilled requirements for a GLP SEC analysis, one can mention uncontrolled flow rate variations and absence of necessary column recalibrations. The data processing problems include poor base-line stability, its incorrect setting, and improper identification of peak limits. The dependences of molar mass vs. retention volume (calibration curves) are often unavailable for polymers under study. To cope up with the latter problem, either absolute detectors
Singla et al. are applied or universal calibration is utilized. The most common approach is based on the direct employment of polystyrene calibration dependences. The values thus obtained should be denoted as “polystyrene equivalent molar masses”. They provide valuable information on the tendencies in molar mass evolution but they do not represent absolute values. The important limitations of SEC include solubility, detectability, aggregation and interactivity of samples. The latter two issues contribute to the decreased accuracy of SEC results, as well. Poly(lactide)s are rather poorly soluble in the most common SEC mobile phase, tetrahydrofuran, (THF). The solubility of PLA in THF decreases with polymer molar mass and it also varies with difference in molecular topology. It increases in the order D
