Abstract
This work presents the results of research on the preparation of bioresorbable functional polyestercarbonates containing side carboxyl groups. These copolymers were synthesized in two ways: the classic two-step process involving the copolymerization of L-lactide and a cyclic carbonate containing a blocked side carboxylate group in the form of a benzyl ester (MTC-Bz) and its subsequent deprotection, and a new way involving the one-step copolymerization of L-lactide with this same carbonate, but containing an unprotected carboxyl group (MTC-COOH). Both reactions were carried out under identical conditions in the melt, using a specially selected zinc chelate complex, with Zn [(acac)(L)H2O] (where: L-N-(pyridin-4-ylmethylene) phenylalaninate ligand) as an initiator. The differences in the kinetics of both reactions and their courses were pictured. The reactivity of the MTC-COOH monomer without a blocking group in the studied co-polymerization was much higher, even slightly higher than L-lactide, which allowed the practically complete conversion of the comonomers in a much shorter time. The basic final properties of the obtained copolymers and the microstructures of their chains were determined. The single-step synthesis of biodegradable polyacids was much simpler. Contrary to the conventional method, this made it possible to obtain copolymers containing all carbonate units with carboxyl groups, without even traces of the heavy metals used in the deprotection of the carboxyl groups, the presence of which is known to be very difficult to completely remove from the copolymers obtained in the two-step process.
Highlights
At present, there is an incessantly growing interest in the issue of the synthesis and application of biodegradable polymers [1,2]
Polymers containing functional carboxylic groups are appealing due to their chemical activity and their ability to form covalent bonds with reactive groups of active agents including drugs. In this respect, such functional polymers represent very promising materials for use as carriers of active agents in the drug and protein delivery systems that are currently being successfully used in pharmacy and cosmetology
Zinc (II) acetylacetonate monohydrate (Avantor Performance Materials Poland S.A., Gliwice, Poland), Pd/C palladium catalyst (Merck Sp. z o.o., Warszawa, Poland), N,N dimethylformamide (99.8%) (Chempur, Piekary Slaskie, Poland), ethyl chloroformate (97%), (Alfa Aesar, Heysham, Lancashire, United Kingdom), potassium hydroxide (99.8%) (Merck Sp. z o.o., Warszawa, Poland), L-phenylalanine, 4-pyridine carboxaldehyde, methanol anhydrous (99.8%), benzene anhydrous (99.8%), chloro-form anhydrous (99%), tetrahydrofuran anhydrous (99.9%), 2,2-Bis(hydroxymethyl) propionic acid (98%), and benzyl bromide (98%) were purchased from Merck Sp. z o.o., Warszawa, Poland
Summary
There is an incessantly growing interest in the issue of the synthesis and application of biodegradable polymers [1,2]. (LA/MTC-Bz)0: initial feed molar fraction of L-lactide and MTC-Bz; Conv: total conversion of copolymerization; (LA/MTC-Bz)N: molar ratio in copolymer; Mw: weight-average molecular mass, determined by GPC and calibrated with polystyrene standards; Ð = Mw/Mn: molecular mass dispersity; LLL and LM: average lengths of carbonate microblocks, determined with NMR; Tg: glass transition temperature; Tm and ∆Hm: melting point and heat of fusion of crystalline phases; *—the presence of insoluble fraction, n/d *—not determined, material only partially soluble in CHCl3: R—chain randomness.
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