Purification of the synthesized polymer with ethanol

To gain more information on the nature of the substances leading to the bimodal molecular weight distribution of the polymer and hopefully to establish a method for the separation of the low and high molecular weight fractions of the polymer another approach was tested. In contrast to high molecular weight PLA low molecular PLA is soluble in ethanol.

AU

Figure 4: SEC chromatograms detected by the RI detector of samples collected during extraction of the polymer with ethanol.

a: Original polymer; b: Ethanol fraction collected at 20 °C; c: Ethanol fraction collected at 70 °C; d: Ethanol fraction collected at 77 °C

Chapter 4 Synthesis and Characterization of PEG_xPLA_y In a first attempt the polymer was extracted with ethanol at different temperatures, the supernatant was collected and volatiles were removed from the samples, which were subsequently analyzed by SEC (Figure 4).

Starting at 20 °C every 10 °C samples were collected. During the fist steps of the experiment no polymer could be detected in the ethanol fraction. Between heating from 50 °C to 60 °C the solvent became turbid, but still no peak could be observed in the corresponding chromatogram. The analysis of the sample drawn at 70 °C hints at traces of a low molecular weight polymer being eluted at about 17 min and in the chromatogram of the sample last collected at 77 °C polymer is clearly detected at about 17 min, which correlates well with the low molecular weight fraction of the original polymer.

A method was found to remove some of the low molecular weight fraction of the polymer.

However, this method is not quite satisfying. As the substance was applied as a solid some of the smaller chains might be enclosed in polymer particles thus escaping extraction.

Hence, in another experiment a solution of the polymer in acetone was prepared and ethanol added until the solution separated in two phases. The supernatant, which contained more solvent and less polymer, and the residue, which was highly viscous, were collected separately and analyzed by SEC (Figure 5).

Time (min)

Figure 5: RI chromatograms of the substances collected in the phases generated by adding ethanol to an acetone solution of the polymer.

a: Original polymer; b: Residue; c: Supernatant

Chapter 4 Synthesis and Characterization of PEG_xPLA_y

Chromatograms displayed in Figure 5 clearly show that a separation of the substances leading to the bimodal distribution of the molecular weight of the polymer could be achieved in parts. The residue is enriched with long chain polymers compared to the mass distribution of the original polymer. In contrast, the supernatant contains mainly a portion of the small molecular weight fraction. To learn more about the composition of the different samples they were analyzed by ¹H-NMR techniques.

(ppm)

Figure 6: ¹H-NMR spectrum of MePEG2PLA20 collected in CDCl3 with TMS as internal standard.

1H-NMR spectra of the various polymer samples showed all a similar signal pattern (Figure 6), differing only in the intensity of the signals. The signal stemming from the methyl group of the MePEG (Ha) could be detected at 3.38 ppm and the methylene repetitive unit of the MePEG (Hb) caused a distinct signal at 3.64 ppm. The triplet observed at 1.56 ppm originated from the methyl group of the PLA (Hc) and the multi-peaked signal at 5.17 ppm was attributed to the proton at the optically active center of the lactic acid monomer (Hd).

Chapter 4 Synthesis and Characterization of PEG_xPLA_y Table 1: Integrals of the ¹H-NMR signals corresponding to the protons of MePEG2PLA20. The signal of the terminal methyl group of the MePEG was used as standard and set at 3.00.

Proton Original Polymer Residue Supernatant

Ha 3.00 3.00 3.00

H_b 167.22 161.74 172.83

H_c 758.59 957.32 533.43

Hd 243.85 307.26 134.44

For comparison of the integrals calculated from the protons corresponding to the different polymers, the signals originating from Ha were used as standard and the integrals set at 3.00 for each polymer (Table 1). As expected no remarkable differences in the ratios of Ha to Hb, the MePEG protons, could be seen for the different polymers (original polymer, residue, supernatant). Calculating the molecular weight of the MePEG chain by dividing the values of the H_b proton signals by four, the number of protons of the monomer, and multiplying the resulting value by 44, the molecular weight of a monomer, values between 1.8 kDa and 1.9 kDa are obtained, which are in good agreement with the theoretical value of 2.0 kDa. In contrast, the signals stemming from H_c and H_d, which are protons attributed to the PLA chain, differ very distinctly in their ratio to the PEG protons. The molecular weights of the PLA chains can be calculated by multiplying the value of the H_d signal by 72, the molecular weight of a PLA monomer, or by first dividing the value of the Hc signal by three and subsequent multiplication by 72. This results in molecular weights of 18 kDa for the original polymer, 22 kDa to 23 kDa for the residue and 10 kDa to 13 kDa for the supernatant. So, the separation of the low and high molecular weight chains could be confirmed by a second method.

Chapter 4 Synthesis and Characterization of PEG_xPLA_y

Figure 7: Thermograms of ethanol treated polymers (original polymer, supernatant residue) obtained by modulated DSC measurements for determination of the glass transition of the polymers.

To confirm NMR and SEC results the polymers were analyzed by MDSC (Figure 7). For calculation of the glass transition, the reversing heat flows of the second heating were used.

Having a higher molecular weight, the residue shows a distinctly higher glass transition temperature compared to the low molecular weight polymers of the supernatant. Glass transition of the original polymer is settled between these two values, which correlate to the length of the polymer chains.

Considering the results of SEC, DSC and especially NMR analysis, the conclusion can be drawn that low and high molecular weight fractions differ only in the length of the PLA chain and not in their composition. Therefore, the assumption that the shorter fraction might be