Polymers based on lactic acid deserve great attention because they decompose by hydrolysis in the human body into nontoxic metabolites. Among the many applications found for these polymers in medicine, it is worth mentioning: a fracture fixer [1,2], surgical cord for the inner lesions suture [2–6], various implants [7,8] and material for target therapy or controlled release of medications [9–20]. The traditional method of poly(lactide) (PLA) synthesis required rigorous conditions: a high vacuum, long polymerization times and the consumption of great quantities of energy, using metal or metal oxide as a catalyst to speed up the reaction and minimize the pyrolysis by reducing the temperature [21–29].

Polymerization of lactide has the same basic approach:

– Re-crystallization of lactide monomer in order to eliminate possible impurities;

The poly(lactide) synthesis is carried out by ring-opening polymerization according to the scheme given in the Figure 1.

The difficulties of poly(lactide) synthesis can be successfully overcome by microwave heating. Microwave radiation has numerous advantages compared to conventional heating: homogeneous heating of the whole volume of the reaction mixture, high transfer energy per unit of time, improved yield, the possibility of the process acceleration and synthesis without using great quantity of the solvent. Polymerization assisted by microwave provides a new approach for enhancing polymer properties as well as economic advantages through energy saving and accelerated product development. Animated by numerous successes in the organic synthesis field, the use of microwaves enables a drastic reduction of polymerization time, to only 5–30 minutes, while obtaining polymers of high molar mass at the same time. For example, super absorbing resin was synthesized by using microwaves in only six minutes [30].

The power of microwaves of 150 W has been used in the synthesis of poly(D,L-lactide), which is several times lower than the power used in other polymer synthesis processes, using up to 800 W [31].

The homogeneous nature of microwave heating eliminates local overheating at the reaction walls, which can lead to side products. Therefore, microwave-irradiated reactions are not only faster, but proceed with higher purity and, consequently, higher yields. In an industry where time is money, the dramatic rate acceleration and increased purity and yields of microwave assisted reactions make them attractive for high-produced polymers.

To develop a technique of microwave-assisted polymerization of D,L-lactide, to efficiently and easily prepare poly(D,L-lactide) with high molecular weight, the ring opening polymerization of D,L-lactide by microwave irradiation under atmosphere was investigated. Both lactic acid and its oligomer are polar molecules, so they can absorb microwave energy to increase the temperature.

This work concerned the tin(II) 2-ethylhexanoate initiated synthesis of poly(D,L-lactide). Figure 2 shows the temperature and the applied power of the reaction mixture as dependent on the reaction time. D,L-Lactide readily absorbs the microwaves, having as a result a fast temperature increase in the first 80 seconds. After the start of the reaction, heat is released due to the exothermic effect of the polymerization reaction (since almost all of the initiator is included in the reaction). The temperature (140 °C) rises above the appointed value (100 °C) although the microwave radiation is automatically switched off, as the program of the microwave reactor is set at maintaining the temperature at 100 °C. The applied power of 150 W at the beginning of the reaction becomes zero after reaching the appointed temperature (in about 1.5 minutes). In the sequel of the polymerization reaction, power of only about 20 W is applied to maintain the temperature, in pulses of several tens of seconds with intermittent pauses of similar duration. The graph shows that the beginning of the polymerization occurs at 70 °C, that the absorption of microwaves decreases with the increase of the polymer content in the reaction mixture and that the highest absorption is that of pure monomer.

The change of reaction mixture temperature from the period of 3–20 minutes shows that the microwave absorption is still present and that there is still some monomer in the reaction mixture to be polymerized, hence the temperature leaps from the polymerization reaction exothermicity. Namely, every intermittent temperature leap follows the automatic turning off of microwave radiation. After 20 minutes the temperature change becomes insignificant and it is maintained constant only through the microwaves absorption, but there are no more temperature leaps or exothermal processes in the reaction mixture, indicating that the monomer conversion into polymer is complete.

The molecular structures of synthesized polymers were confirmed by FTIR methods. FTIR spectrum of the monomer D,L-lactide (Figure 3) shows bands at 2,915.32 and 2,950.57 cm⁻¹ from symmetric and asymmetric valence vibrations of C-H, respectively. Bands at 2,925.22 and 3,004.12 cm⁻¹ originate from symmetric and asymmetric valence vibrations of C-H from CH₃, respectively. In the FTIR spectrum of the monomer D,L-lactide, bands also appear at 1,267.27 cm⁻¹ (asymmetric valence vibrations of C-O-C in the lactonic ring), 1,099.83 cm⁻¹ (symmetric valence vibrations of C-O-C in the lactonic ring), 1,770.43 cm⁻¹ (cyclic dilactone C = O valence vibration), 1,445.11 and 1,386.96 cm⁻¹ (asymmetric and symmetric bending vibration of C-H from CH₃, respectively) and 930.9 cm⁻¹ (COO ring breathing mode).

In the FTIR spectrum of poly(D,L-lactide), with the monomer/initiator ratio of 1/5,000, obtained for 15 minutes, bands were present at 2,831.82 and 2,945.82 cm⁻¹ from symmetric and asymmetric valence vibrations of C-H, respectively (Figure 4). At 2,881.28 and 2,996.37 cm⁻¹, bands were from symmetric and asymmetric valence vibrations of C-H from CH₃. Asymmetrical valence vibrations of C-O-C of the aliphatic chain were shifted at 1,187.45 cm⁻¹, and symmetrical valence vibrations of C-O-C of the aliphatic chain 1,090.16 cm⁻¹, compared with bands at 1,276 and 1,099 cm⁻¹, which appeared in monomer D,L-lactide, Figure 5. Accompanying bands at 1,757.33 cm⁻¹ (valence vibration of C═O of aliphatic ester), 1,455.41 and 1,383.37 cm⁻¹ (asymmetric and symmetric bending vibration of C-H from CH₃, respectively), 1,271.15 cm⁻¹ (the overlap C-H bending vibration and C-O-C stretching vibration) were also detected.

The GPC curves of synthesized polymer are shown in Figures 6 and 7. Unreacted monomer was found in the profile for poly(D,L-lactide) synthesized by microwaves, but with increasing reaction time a decreased quantity of unreacted monomers was observed (retention time 5.33 minutes), Figure 6. Figure 7 shows GPC curves for poly(D,L-lactide)s polymerized at the same time with different monomer/initiator ratios. As expected, it can be concluded that with decreasing initiator content, the molecular mass of obtained polymers increases. Peaks from unreacted monomers decreased with decreasing initiator content.

Table 2 shows the values of mean molar masses M_n, M_w and polydispersivity index Q, for poly(D,L-lactide) synthesized using microwaves as the function of polymerization time and monomer/initiator mol ratio [M]/[I]. The values of mean molar mass and the polydispersivity index were observed to increase with the increase of reaction duration. These values were also increased with the increase of monomer/initiator ratio.

Figure 8 shows the SEM image of poly(D,L-lactide) spheres obtained by spraying a fine fog of poly(D,L-lactide) solution in tetrahydrofuran into the water solution of poly(vinyl alcohol) with intensive stirring.

Such microspheres can be used as polymer matrices for the production of devices for controlled release of medicinal substances, since the diameter of the microspheres is appropriate for phagocytosis by macrophages.

The reduction of poly(D,L-lactide) synthesis duration and energy consumption by using microwaves enables a more economical production. The microwave synthesis requires more energy during the first few seconds only to obtain uniform and intensive initiation. The polymerization reaction course is readily supervised by monitoring the temperature of the reaction mixture by infra-red sensors and the applied power of microwave radiation. poly(D,L-lactide) could be synthesized effectively by microwave-assisted ring opening polymerization using tin(II)2-ethylhexanoate as a initiator. This microwave assisted polymerization was much faster than the literature data for polymerization heated by a conventional oil bath under similar reaction conditions. The monomer/initiator ratios had a strong influence on the molecular masses and polydispersity of obtained polymers. A higher monomer/initiator ratio resulted in polymers with higher molar masses and lower polydispersity, Q. At a reaction temperature of 100 °C, the prolonged microwave irradiation time showed a significant effect on the increasing polymer molar mass.

From SEM imagine of microspheres it was concluded that the technique provides uniform sized spheres. The size of the obtained microspheres was about 50 μm.