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Synthesis of High Molecular Weight Polyglycolide in Supercritical Carbon Dioxide

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Synthesis of High Molecular Weight Polyglycolide in Supercritical Carbon Dioxide Mathematisch-Naturwissenschaftliche Fakultät Christian Schmidt | Marc Behl | Andreas Lendlein Sabine Bauermann Synthesis of high molecular weight polyglycolide in supercritical carbon dioxide Suggested citation referring to the original publication: RSC Adv. 4 (2014), pp. 35099–35105 DOI http://dx.doi.org/10.1039/C4RA06815G Postprint archived at the Institutional Repository of the Potsdam University in: Postprints der Universität Potsdam Mathematisch-Naturwissenschaftliche Reihe ; 284 ISSN 1866-8372 http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-99439 RSC Advances View Article Online PAPER View Journal | View Issue Synthesis of high molecular weight polyglycolide in supercritical carbon dioxide† Cite this: RSC Adv.,2014,4,35099 Christian Schmidt,ac Marc Behl,b Andreas Lendlein*ab and Sabine Beuermann*ac Polyglycolide (PGA) is a biodegradable polymer with multiple applications in the medical sector. Here the synthesis of high molecular weight polyglycolide by ring-opening polymerization of diglycolide is reported. For the first time stabilizer free supercritical carbon dioxide (scCO2) was used as a reaction medium. scCO2 allowed for a reduction in reaction temperature compared to conventional processes. Together with the lowering of monomer concentration and consequently reduced heat generation compared to bulk reactions thermal decomposition of the product occurring already during polymerization is strongly reduced. The reaction temperatures and pressures were varied between 120 and 150 C and 145 to 1400 bar. Tin(II) ethyl hexanoate and 1-dodecanol were used as catalyst and Received 8th July 2014 À initiator, respectively. The highest number average molecular weight of 31 200 g mol 1 was obtained in Accepted 29th July 2014 5 hours from polymerization at 120 C and 530 bar. In all cases the products were obtained as a dry Creative Commons Attribution 3.0 Unported Licence. DOI: 10.1039/c4ra06815g white powder. Remarkably, independent of molecular weight the melting temperatures were always at www.rsc.org/advances (219 Æ 2) C. 1. Introduction Supercritical carbon dioxide (scCO2) possesses several favorable characteristics as reaction medium for polymer Polyglycolide (PGA) is of high interest for medical applications synthesis. It is a chemically inert, non-toxic, and environmen- such as implants or drug release systems.1–4 Compared to other tally benign gas, which can be easily separated from the product 5–7 dilactone based polyesters PGA degrades fast. Therefore a by reducing the pressure. Further, CO2 is well soluble in most This article is licensed under a relatively high molecular weight is required to guarantee a polymers resulting in polymer swelling, which changes the sufficient residence time of the PGA for the therapy. However, mechanical and physical properties of the material. Therefore, the thermal decomposition of PGA starts already below the CO2 can reduce the melting temperature (Tm) and the glass melting transition (Tm) of the PGA. When diglycolide is poly- transition temperature (Tg) of polymers and monomers. The 5 merized in the melt, the Tms of the oligo- and polyglycolide latter is caused by the plasticizing action of CO2. For example, increase rapidly with increasing molecular weights. Accordingly the Tg of poly(methyl methacrylate) (PMMA) is lowered by up to 5,8,9 the reaction temperature needs to be raised as well to avoid 60 C at 100 bar in a CO2 atmosphere. Supercritical carbon solidication of the reaction mixture. Here it needs to be dioxide was used as reaction medium for a wide variety of considered that the polymerization of diglycolide is a strong monomers encompassing conventional (meth)acrylates, uo- exothermic reaction. An additional challenge to be met is the roolens, and cyclic monomers leading to biodegradable – processing of the PGA into the intended application relevant polymers.10 22 bodies. The solubility of PGA in common solvents, which are It was hypothesized that scCO2 could be used as reaction suitable for biomaterials, is low and can only be achieved by medium to keep the PGA soluble during polymerization and by processing at low temperatures at high pressures. Otherwise this avoid high reaction temperatures whereby high Mn can be thermal degradations occurs, which becomes apparent by a achieved. In addition, scCO2 has proven to be an excellent brown coloring of the PGA. process medium for particle formation.23,24 Since particles of well-dened size are essential for the controlled drug delivery a combined process combing synthesis and shaping appears to be highly attractive. aInstitute of Chemistry, University of Potsdam, Karl-Liebknecht Str. 24-25, 14476 Previously, ring-opening polymerizations (ROP) of cyclic Potsdam, Germany bInstitute of Biomaterial Science, Helmholtz-Zentrum Geesthacht, Kantstraße 55, monomers were carried out in scCO2 as reaction medium using 19,22 14513 Teltow, Germany tin(II) ethyl hexanoate (SnEH2) as catalyst. The polymeriza- cInstitute of Technical Chemistry, Clausthal University of Technology, Arnold- tion mechanism for the cyclic diester diglycolide as monomer Sommerfeld-Str. 4, 38678 Clausthal-Zellerfeld, Germany following a coordination-insertion mechanism. First, SnEH2 † Electronic supplementary information (ESI) available. See DOI: reacts with dodecanol to form the catalytic active tin(II) alkoxide. 10.1039/c4ra06815g This journal is © The Royal Society of Chemistry 2014 RSC Adv.,2014,4, 35099–35105 | 35099 View Article Online RSC Advances Paper The alkoxide coordinates monomer and successive ring- white powder. Monomer conversions were determined opening of the cyclic monomer occurs. Then, additional gravimetrically. monomer units are coordinated and inserted between tin and the previously added monomer unit leading to chain growth. The polymerization is terminated by adding an excess amount Characterization of ethyl acetate, leaving an acetyl terminal group in the polymer. Differential scanning calorimetry (DSC). The thermal prop- The goal of this work was to explore whether high molecular erties of the polymers were analyzed with a DSC 1/500658/200W weight PGA may be obtained using supercritical carbon dioxide STARe System from Mettler Toledo equipped with a FRS5- as reaction medium and SnEH2 as catalyst. Since no informa- sensor and nitrogen cooling. For each sample two heating and tion are available for ROP of diglycolide in non-stabilized one cooling cycles were measured. Only the last heating cycle systems under high-pressure conditions several monomer and was analyzed to determine the thermal properties of the mate- catalyst concentrations as well as pressures and temperatures rial. All samples were analyzed using a constant heating and À were explored. cooling rate of 10 K min 1 in a temperature range from À60 C to 280 C. 2. Experimental section Elemental analysis (EA). Elemental analyses were performed with a Vario EL III (Elementar) instrument with an error limit of Materials Æ0.3% for carbon and hydrogen and Æ0.5% for oxygen was Diglycolide (purity 99.9%, Sigma, Munich, Germany) was used. recrystallized from a minimum amount of dry toluene. Tin(II) Fourier transform-infrared spectroscopy (FT-IR). FT-IR À ethyl hexanoate (purity 95%, Sigma), 1-dodecanol (purity 99.8% spectra were recorded with a resolution of 2 cm 1 and 32 scans Fluka), toluene (purity 99.9%, Sigma), ethyl acetate (purity 99%, using a Bruker Vertex 70 spectrometer with a globar source, a VWR) were used as received. CO2 (purity 99.998%) was obtained KBr beam splitter and a DTGS detector. All samples were from Air Liquide (Dusseldorf,¨ Germany). Creative Commons Attribution 3.0 Unported Licence. measured as KBr tablets. MALDI-TOF mass spectroscopy (MALDI-TOF). MALDI-TOF Typical polymerization procedure spectra were attained using an Ultraextreme mass spectrom- eter (Bruker) in the positive linear and/or reection mode and PGA synthesis under scCO2 conditions was performed in an optical high-pressure cell (RGT 601, material no. 2.4668, Arbed trans-2(3-(4-tert-butylphenyl)-2-methyl-2-propenylidene)-malo- Saarstahl) with an inner volume of 6 mL. A detailed description nitrile (DCTB in KTFA/THF) as a matrix. Ions were generated by is given in ref. 25. The cell was closed on one side and purged a nitrogen laser emitting at 337 nm and were accelerated at 20 À with argon before diglycolide (500 mg, 4.4  10 3 mol) as well kV. The detector voltage was 2.4 kV, and the mass spectra were À7 averaged from 500 to 6500 laser shots. as a solution of tin(II) ethyl hexanoate (0.05 mg, 1.2  10 mol) This article is licensed under a À and 1-dodecanol (0.18 mg, 9.7  10 7 mol) in toluene (0.1 mL) Size-exclusion chromatography (SEC). Molecular weight were quickly transferred into the cell. Aerwards the cell was distributions were measured with an SEC setup consisting of an sealed, CO was added and the required pressure was adjusted Agilent 1200 HPLC pump with a refractive index (Agilent) 2 À1 using a manual high-pressure generator. To control tempera- detector with a ow rate of 1 mL min and hexa- ture a resistive heating element (CGE Asthom) and a PID- uoroisopropanol (HFIP, Fluorochem, 99%) containing potas- controller (Eurotherm 815) were used. The reaction was sium tri uoroacetate (0.05 M) (Fluka, 99%) as eluent. Poly(methyl methacrylate) (PSS) standards were used for cali- quenched by releasing the pressure followed by quickly opening the cell to allow for fast removal of the product. The product is bration. All analyses were performed at 35 C with three – ˚ m  obtained as a white powder, as shown in Fig. 1. The crude columns (PSS PFG 1000 A, 7 m particle size, 8.0 300 mm; – ˚ m  – product was isolated and subsequently puried by Soxhlet PSS PFG 300 A, 7 m particle size, 8.0 300 mm and PSS PFG ˚ m  extraction with ethyl acetate for 4 hours, which resulted in a 100 A, 7 m particle size, 8.0 300 mm). Due to poor solubility of the samples with the highest molecular weights and crys- tallinity (marked with an asterisk in Table 1) Mn and PDI of these samples reported represent only the soluble fraction of the material.
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