Team:TU Darmstadt/Project/PLGC
Contents
PLGC
Abstract
Due to the fact that Poly(lactide‑co‑glycolide‑co‑caprolactone) (PLGC) is similarly constructed to Poly(lactic‑co‑glycolic‑acid) (PLGA), the way of manufacturing is the same. PLGC is also synthesized via an anionic ring‑opening polymerization[1]. Both the mechanism and the problems during the synthesis of both polymers are identical. Contrary to that, the mechanical characteristics, which are explained in the background, and the degradation behavior, differ a lot. The degradation behavior is an important polymer characteristic for all applications of PLGA and PLGC. Therefore, we will focus on the structural differences, which lead to different characteristics.
To analyze the synthesized PLGC, we used, similar to PLGA, GPC and NMR‑spectroscopy. After this, we used our purified polymer to manufacture PLGC nanospheres.
Figure 1: Structure of PLGC
Synthesis of PLGC
The synthesis of PLGC terpolymers resembles the synthesis of PLGA. Stannous‑octoate and 1‑octadecanol are forming the initiator part, reacting with lactide, glycolide and ε‑caprolactone monomers to form a polymer chain[1]. During our time in the laboratory, two set‑ups of PLGC were processed through anionic ring‑opening polymerization. For the synthesis, the monomers were put in a water free reaction vessel, melted and reacted with the initiators. The method is described in detail in the method book.
Degradation of PLGC
Triggered by the usage of ε‑caprolactone, two competing effects occur. As described in the background, PLGC mostly exists in its rubbery state, because of its low glass temperature (Tg)[1]. Due to that, the ε‑caprolactone makes the polymer chain more flexible, allowing water molecules to diffuse more easily to the ester bonds in those chains, hydrolyzing them. On the other hand ε‑caprolactone lowers the polarity of the chains making them less hydrophilic. Due to this, the water molecules are hindered to come near the ester bonds to hydrolyze them[2].
By containing a small mole fraction of the ε‑caprolactone in the chain, the degradation of PLGC proceeds faster than the degradation of PLGA. However, with the mole fraction of ε‑caprolactone increasing, the polymer becomes more and more non‑polar. This causes the degradation time to increase compared to the degradation time of PLGA.
Results and Discussion
Yields
The calculation of yields for all synthesized PLGC polymers was performed identical as for PLGA polymers. Therefore, equation 1 from the PLGA page was used. Table 1 contains the resulting values.
Table 1: Used amounts of monomers and yields of synthesis (total and relative).
| Polymer | m (lactide) [g] | m (glycolide) [g] | m (ε‑caprolactone)[g] | Total Mass [g] | Total Yield [g] | Relative Yield [%] |
|---|---|---|---|---|---|---|
| PLGC (I) | 1.685 | 0.483 | 0.240 | 2.412 | 0.38 | 15.75 |
| PLGC (II) | 6.68 | 5.72 | 1.17 | 13.78 | 0.494 | 3.58 |
Analysis through GPC
The expected molecular weight of PLGC (I) (ratio 71 %/19 %/10 %) was determined to be 891,076 g/mol, while the GPC result shows a molecular weight of 12,224 g/mol, as seen in figure 2. this indicates that the polymer chains are shorter than expected. For the PLGC (II) (ratio 46 %/44 %/10 %) the GPC result shows a molecular weight of 6,818 g/mol, while its expected molecular weight was determined to be 2.16*106 g/mol, which means that oligomers were produced instead of polymers.
Figure 2: GPC of PLGC (I).
Similar to the PLGA results, the chains are shorter, because of the insufficient magnetic stirring device. The mixture is not stirred well enough at higher viscosity, so the growth of chains is affected while the mixture solidifies. As in the case of PLGA, this can be avoided by a more sufficient stirring mechanism.
But this effect does not completely explain why the molecular weight of PLGC (II) is so low compared to the expected weight. It seemed to be problematic to shut down the reaction and restart it after a pause. The end of the chains seemed not to be reactivated, so the chain growth stopped during the pause. This leaves the conclusion, that an anionic polymerization can not be reactivated after it was shut down once.
A sufficient stirring device could not be implementated in our laboratory set up without losing the condition of a waterfree environment. Furthermore, the reactions must not be shut down, before they are completed, to guarantee the production of polymer.
For PLGC (I) we purified 100 % of our product and calculated the yield as shown in table 1. Since PLGC (II) was further used for nanosphere synthesis, the actual total yield could not be determined. Therefore, the yield of PLGC (II) is a lot lower than for PLGC (I).
The added ε‑caprolactone does not cause a change in viscosity in the reaction mixture. Since PLGC synthesis is the same as PLGA synthesis, the same problems occur. Yields tend to be low, as well.
NMR-spectroscopy
1H‑NMR spectroscopy was used to determine the monomer ratio. The CH‑group of lactide was normalized to 1. As signals of ε‑caprolactone overlap with the ones from lactide and glycolide, a 2D-NMR spectrum was recorded. In our case, we recorded a COSY spectrum, which shows the interactions of H‑atoms with a distance of three bonds to each other which allows a better differentiation between the individual protons of the molecules. The signals of α‑ and ε‑methylene groups can be distinguished clearly, since their proximity to the oxygen atoms deshields the ε‑methylene groups the strongest. Therefore, the ε‑methylene signal shows a shift of δ=3.66‑3.73 ppm. This signal is also used for the calculation of the monomer´s ratio. For lactide we used the methyl protons with a shift of δ=1.44‑1.69 ppm, and for glycolide the methylene protons with a shift of δ=4.69‑4.93 ppm. All peaks are shown in the following NMR‑spectrum (Figure 3).
Figure 3: 2D-NMR of PLGC (I).
The spectrum of our synthesized PLGC is shown. All integrals are normalized to the CH‑group of lactide. The important peaks are 1.44‑1.69 ppm, illustrating the methyl protons of lactide, 3.66‑3.73 ppm for the ε‑caprolactone´s methylene group and 4.69‑4.93 ppm for the glycolide methylene group.
We gained integrals, which were inserted into equation 1 to calculate the monomer ratio.
The speed at which the monomers are incorporated into the polymer vary as well. Generally, glycolide is inserted faster than lactide, while the incorporation of caprolactone is the slowest[3]. Table 2 compares the relative monomer amounts used for synthesis with the relative amounts of incorporated monomers.
Table 2: Mole fractions of inserted monomers compared to the mole fractions in the produced polymer. The ratio of ε‑caprolactone varies, even if it´s own concentration is initially identical, but the amounts of lactide and glycolide vary.
| Polymer | Inserted monomer ratio (L/G/C) [%] | Monomer ratio after synthesis (L/G/C) [%] |
|---|---|---|
| PLGC (I) | 71/19/10 | 47.6/47.6/4.8 |
| PLGC (II) | 46/44/10 | 46.2/51.2/2.6 |
In PLGC (II) the relative amount of ε‑caprolactone monomer was decreased by 74 %, compared to the initial relative amount of free ε‑caprolactone.
Outlook
PLGA and PLGC were produced using an insufficient magnetic stirring mechanism, which was not able to stir the reaction mixture at higher viscosity up to the end of the reaction. That led to a low conversion, which makes the results hardly reproducible. To avoid that, the reaction vessel needs to be designed in a way, which provides a continuous stirring all time throughout the reaction. To improve the reproducibility a higher performance stirring mechanism like mechanical stirring combined with a sufficient vacuum and protective gas atmosphereneeds to be incorporated into synthesis device. Those conditions are not achievable with the laboratory equipment, which we can afford. With both conditions given, we would be able to get better reproducible results and more constant chain lengths. With polymers produced this way we would be also able to add additives and produce for example composite materials out of them.
References
- ↑ 1.0 1.1 1.2 Synthesis, Properties, and In Vitro Hydrolytic Degradation of Poly(d,l-lactide-co-glycolide-co-𝜀-caprolactone).
- ↑ Biodegradable poly(lactide-co-glycolide-co-ecaprolactone) block copolymers – evaluation as drug carriers for a localized and sustained delivery system .
- ↑ Yodthong Baimark and Robert Molloy, Synthesis and Characterization of Poly(L-lactide-co-ε-caprolactone) Copolymers:Effects of Stannous Octoate Initiator and Diethylene Glycol Coinitiator Concentrations.