Polymerization of Cyclic Esters Initiated by Carnitine and Tin (II) Octoate

Low-molecular weight poly(ε-caprolactone), polylactides and copolymers of ε−caprolactone and lactides were obtained by the polymerization of cyclic esters in the presence of a carnitine/SnOct2 system. Their structures were proven by means of MALDI−TOF, IR and NMR studies. Effects of temperature, reaction time and carnitine dosage on the polymerization process were examined.

Aliphatic polyesters are typical biomaterials, commonly used in medicine and pharmacy because of their good biocompatibility and lack of toxicity. The majority of the products are composed of homo-

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and copolymers of lactides (LA, LLA) and ε-caprolactone (CL) [1,2,14,15]. Aliphatic polyesters are usually prepared by ring-opening polymerization (ROP) of the relevant cyclic monomers (e.g. D,L-, L,L-lactide, ε-caprolactone; abbreviations: LA, LLA, CL, respectively). PLA, PLLA and PCL have been successfully synthesized by ring opening polymerization in the presence of cationic or anionic initiators, as well as coordinating and enzymatic catalysts . The tin octoate (SnOct 2 ) is probably the most often used catalyst in the polymerization of cyclic esters.
L-Carnitine (L-CA) is a hydrophilic amino acid derivative, naturally occurring in human cells. The compound is biosynthesized endogenously in the kidneys and liver from lysine and methionine, but it can also be delivered with red meat and dairy products of the diet. L-Carnitine plays an essential role in the transfer of long-chain fatty acids into mitochondria for beta-oxidation. Furthermore, L-carnitine binds acyl residues and helps in their elimination, decreasing the number of acyl residues conjugated with coenzyme A (CoA) and increasing the ratio between free and acylated CoA. Carnitine deficiency is a pathologic metabolic state in which carnitine concentrations in plasma and tissues are lower than the levels required for normal functioning of the organism [39].
Recently, we found that natural amino acids are satisfactory initiators for ROP of cyclic esters [34]. In the present paper, we describe a new effective synthesis of low-molecular weight aliphatic polyesters. It involves the ring opening polymerization of D,L-, L,L-lactide, ε-caprolactone in the presence of a L-carnitine/SnOct 2 system. We believe that thus obtained polymers can be practically applied as effective drug delivery systems.

Results and Discussion
The homo-and copolymerization reactions of CL, LA and LLA were carried out in the presence of the CA/SnOct 2 (2:1) system at 120-160°C. The molar ratio of CA to a given monomer was 1:25, 1:50 or 1:100. Reaction conditions, yields and average molecular mass values of polyesters are summarized in Table 1 Table 3. Main absorption bands of the synthesized polyestres (spectrum recorded from a KBr pellet).

Wave number in cm -1 Group and band
Poly(ε-caprolactone) 2943 (υ as CH 2  Insertion of the carnitine fragment into the polymer chain was confirmed by the proton NMR spectral analysis. The peaks at 2.83 (-CH 2 COOH), 3.51 ((CH 3 ) 3 N + -) and 3.43 (-CH 2 N + -) ppm were observed in all products obtained by homo-and copolymerization of CL, LA and LLA in the presence the carnitine/SnOct 2 system.
Composition of the CL and LA (PCLLA) copolymers was deduced from the 1 H-NMR spectra. The CL content in the copolymer of CL and LA exceeded the CL feed ratio for PCLLA (amounts to 56-58 mol %). Probably, CL is the most active co-monomer in this reaction.
The MALDI-TOF spectra of PCL contain double peaks, each component corresponding to a separate spectrum series. The most prominent series of peaks is characterized by a mass increment of 114 Da, which is equal to the mass of the repeating unit in the poly(ε-caprolactone) (Figure 3). It is assigned to PCL terminated with a hydroxyl group (residual mass: 57 Da, K + adduct) (A). The second series of peaks also corresponded to poly(ε-caprolactone), terminated with a hydroxyl group (residual mass: 40 Da, Na + adduct) (B).
In the MALDI-TOF spectra of PLA there are also two series of peaks. The main series corresponds to PLA molecules, terminated with a hydroxyl group (residual mass: 41 Da, Na + adduct), and the second series of smaller peaks corresponds also to PLA terminated with a hydroxyl group (residual mass: 57 Da, K + adduct). In the MALDI-TOF spectrum of PLA both populations of chains of even and odd number of lactic acid m.u. can be observed. The odd number of acid m.u. shows that under the reaction conditions the polymer chain undergoes intermolecular transesterification (leading to an exchange of segments), which is a typical phenomenon for the polymerization of lactides [18]. The molecular mass of PCL, PLA and PLLA is dependent on the monomer/carnitine molar ratio ( Table 1). The influence of the monomer/carnitine feed ratio on the molecular weight of polyesters was studies at three levels (25:1, 50:1, 100:1). As shown in Table 1, the PCL products were obtained with M n (from GPC) of 1800, 3800 and 6600 Da for PCL-2, PCL-3 and PCL-5, respectively. For PLA, M n (from GPC) amounts to 1700, 3200 and 4600 Da for PLA-1, PLA-2 and PLA-5, respectively. It was found that the molar mass of the polyesters increased with the monomer/carnitine feed ratios. On the other hand, according to M n of polyesters, the PCL and PLA conversion had tendency to decrease with the increasing monomer/carnitine feed ratio. For PCL-2, PCL-3, PCL-5, PLA-1, PLA-2 and PLA-5 the corresponding monomer conversion values were 85%, 71%, 67%, 62%, 53% and 36%, respectively. The reaction yield was determined by the weight method. The homo-and copolymerization reactions of CL, LA and LLA were repeated twice for each combination. The results were in good agreement with one another (reproducibility of them was about 5-10%). Both, the conversion and molecular mass of the polymers increased, when the reaction temperature was raised from 120 to 160ºC.
The The molecular mass values averaged over those of the obtained polymers were roughly in agreement with the theoretical molecular weights calculated from the feed ratio of the monomer to carnitine as well as the number average molecular mass determined from MALDI-TOF and GPC.
Finally, it should be mentioned, that the carnitine/SnOct 2 system was quite effective in the polymerization of ε-caprolactone, L-lactide and rac-lactide. The yield of PCL was in the range of 62-93 %, and for PDLA in the range of 30-68 %. Relevant kinetic and mechanistic studies are underway. They will be presented in the next paper.

Polymerization Procedure
Polymerization of homo-and copolymers of cyclic esters were carried out in the same way. Monomers (CL, LA, LLA) and CA were placed in 10 mL glass ampoules under an argon atmosphere. The reaction vessels were then left standing at the required temperature in a thermostated oil bath for the appropriate time (Table 1). When the reaction was complete, the cold product was dissolved in dichloromethane, the obtained solution was washed with methanol and dilute hydrochloric acid (5% aqueous solution) under vigorous stirring. The latter operation was repeated three times. The isolated powdery or oily polymer was dried in vacuum for 72 h. Purity of the isolated polymers was tested by 1 H-NMR.

Measurements
The polymerization products were characterized by means of 1 H-and 13 C-NMR (Varian 300 MHz), and FT-IR spectroscopy (Perkin-Elmer). The NMR spectra were recorded in CDCl 3 . The IR spectra were measured from KBr pellets. Relative molecular mass values and molecular mass distributions were determined using MALDI-TOF and gel permeation chromatography (GPC). The MALDI-TOF spectra were measured in the linear mode on a Kompact MALDI 4 Kratos analytical spectrometer using a nitrogen gas laser and 2-[(4-hydroxyphenyl)diazenyl] benzoic acid (HABA) as a matrix. Molecular mass values and molecular mass distributions of polymers were determined at 308 K on a Lab Alliance gel permeation chromatograph equipped with Jordi Gel DVB Mixed Bed (250 mm x 10 mm) columns and a refractive detector, using THF or chloroform as eluent (1 mL/min). The molecular mass scale was calibrated with polystyrene standards.