Synthesis and Characterization of Aliphatic Polyesters and Poly(Amino Acids)

The synthesis of poly-D,L-lactide (PDLLA), and PCL was carried out in bulk by ring-opening polymerization (ROP) of the corresponding monomers (D,L-lactide, and ε-caprolactone). The reaction conditions were optimized to obtain polymers with similar molecular weight characteristics and intrinsic viscosities. Polymerization was carried out at 130 °C for 4 h under vacuum for PDLLA and for 21 h under argon atmosphere for PCL. The monomer to tin (II) octoate molar ratio in the case of PDLLA synthesis was 2500. PCL was synthesized by adding methanol as a co-initiator to the reaction system, and the ratio of [monomer]:[Sn(Oct)₂]:[MeOH] was 5000:1:2. The resulting polymers were dissolved in a minimum amount of chloroform, precipitated into methanol, filtered, washed 2 times with methanol, and dried in vacuum at ~150 Pa. PDLLA and PCL yields were 83 and 57%, respectively.
Poly(L-lysine) (PLys) and random poly(amino acids), e.g., poly((L-lysine)-co-(L-leucine)) (P(Lys-co-Leu)) and poly((L-lysine)-co-(L-phenylalanine)) (P(Lys-co-Phe)), used to modify the surface of films based on aliphatic polyesters with amino groups were obtained at the Institute of Macromolecular Compounds RAS according to the previously developed protocol [39 (link),40 (link)]. The synthesis was carried out by ROP of pre-synthesized N-carboxyanhydrides (NCA) of the corresponding α-amino acids. n-Hexylamine was used as the initiator. The molar ratio of monomer to initiator was 50. The molar ratio between ε-benzyloxycarbonyl-L-lysine (Lys(Z)) NCA and the hydrophobic amino acid NCA was 4 in the case of L-leucine NCA and 8 in the case of L-phenylalanine NCA.
Weight average molecular weights (M_w) and dispersity (Ð) of all synthesized polymers were determined by size-exclusion chromatography (SEC) using a Shimadzu HPLC system (Tokyo, Japan) consisting of a pump LC-10AD VP, refractometric detector RID-10A, column oven CTO-20A, system controller SCL-10A VP (Canby, OR, USA) supplied with a Rheodyne 725i injection valve (Rohnert Park, CA, USA) and two Agilent PLgel MIXED-D columns (5 µм, 7.5 × 300 mm, Agilent Technologies, Santa-Clara, CA, USA). LC Solution Shimadzu software (version 1.25, Kyoto, Japan) was used to collect and process data.
Molecular weight characteristics of aliphatic polyesters were determined at a mobile phase flow rate of 1.0 mL/min and 40 °C using tetrahydrofuran as an eluent and polystyrene standards with molecular weights ranging from 2000 to 450,000 (Agilent Technologies, Santa Clara, CA, USA and Waters, Milford, MA, USA) for calibration. M_w and Ð for PDLLA and PCL were 137,000, 1.7, and 123,000 and 1.6, respectively. In the case of poly(amino acids), the analysis was carried out for protected (co)polymers using 0.1 M solution of lithium bromide in N,N-dimethylformamide as a mobile phase and poly(methyl methacrylate) standards with molecular weights ranging from 17,000 to 250,000 for calibration at a mobile phase flow rate of 0.7 mL/min and 40 °C. Their molecular weight characteristics were as follows: M_w = 38,500 and Ð = 1.1 for PLys(Z); M_w = 26,600 and Ð 1.3 for P(Lys(Z)-co-Leu); M_w = 36,300 and Ð = 1.4 for P(Lys(Z)-co-Phe). Deprotection of poly(amino acids) was carried out with the use of a 5.25% solution of trifluoromethanesulfonic acid in trifluoroacetic acid for 4 h. The complete deprotection was confirmed by ¹H NMR spectroscopy (AVANCE AV-400 NMR spectrometer, Bruker, Karlsruhe, Germany) by the disappearance from the spectra of signals corresponding to the methylene group (5.0 ppm) and the aromatic fragment (7.1 ppm) of Z-protective groups (-O-CH₂-C₆H₅).
The viscosity of a series of aliphatic polyesters solutions at various concentrations in chloroform was measured using an Ostwald’s capillary viscosimeter. Intrinsic viscosities calculated from the obtained data were 1.1 and 1.3 dL/g for PDLLA and PCL, respectively.