Abstract
Biodegradable triblock copolymers based on poly(ε-caprolactone) (PCL) and poly(lactic acid) (PLA) were synthesized via ring-opening polymerization of L-lactide followed by reversible addition–fragmentation chain-transfer (RAFT) polymerization of poly(methyl vinyl ketone) (PMVK) as a photodegradable block, and characterized by FT-IR and 1H NMR spectroscopy for structural analyses, and by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) for their thermal properties. Porous, biodegradable PCL-b-PLA microspheres were fabricated via the oil/water (O/W) emulsion evaporation method, followed by photodegradation of PMVK blocks by UV irradiation. The macro-chain transfer agent (CTA) synthesized by reacting a carboxylic-acid-terminated CTA—S-1-dodecyl-S′-(a,a′-dimethyl-a′′-acetic acid)trithiocarbonate (DDMAT)—with a hydroxyl-terminated PCL-b-PLA block copolymer was used to synthesize well-defined triblock copolymers with methyl vinyl ketone via RAFT polymerization with controlled molecular weights and narrow polydispersity. Gel permeation chromatography traces indicated that the molecular weight of the triblock copolymer decreased with UV irradiation time because of the photodegradation of the PMVK blocks. The morphology of the microspheres before and after UV irradiation was investigated using SEM and videos of three-dimensional confocal laser microscopy, showing a change in their surface texture from smooth to rough, with high porosity owing to the photodegradation of the PMVK blocks to become porous templates.
Highlights
In the FT-IR spectrum of the PCL-b-poly(lactic acid) (PLA) block copolymer, as shown in Figure 1b, two types of carbonyl groups arose from both PCL and PLA, exhibiting strong FT-IR peaks at approximately 1758 and 1733 cm−1, respectively
The methyl peak of the PLA block was confirmed at 1.56 ppm in the 1 H NMR spectrum of PCL-b-PLA (Figure 2b)
Biodegradable triblock copolymers were successfully synthesized via ring-opening polymerization of L-lactide onto PCL, followed by reversible addition–fragmentation chain-transfer (RAFT) polymerization of methyl vinyl ketone (MVK), and used to fabricate porous, biodegradable microspheres via the oil/water single emulsion–evaporation method and photodegradation of poly(methyl vinyl ketone) (PMVK) blocks by UV irradiation at a wavelength of 312 nm
Summary
Biodegradable aliphatic polyesters such as PCL and PLA, endowed with semicrystallinity and biodegradability due to their potentially hydrolyzable ester bonds, have been widely used as scaffolds in tissue engineering, drug delivery systems, packaging, and biomedical applications, because of their controlled biodegradability, biocompatibility, and good physical properties [1,2,3]. It has been reported that, compared to PLA, the degradation period of PCL can range from several months to a few years, and its biodegradability is dependent on its molecular weight, degree of crystallinity, and degradation conditions. PCL has been considered for use as a drug delivery matrix, owing to its favorable permeability to drugs and thermal properties, it is known as a relatively biostable polymer compared to PLA, resulting in its limited use for targeted therapeutic applications requiring a controlled degradation rate. Branched polymers—including fourarmed star biodegradable polymers—have been reported to be more efficient for higher drug loading and controlled drug delivery, due to their suitability for the formation of porous structures [8,9,10]
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