Synthesis and Characterization of [l]-Lactide−Ethylene Oxide Multiblock Copolymers

Abstract

[l]-Lactide ([l]-LA)/ethylene oxide (EO) ring-opening copolymerizations were successfully carried out by using various catalysts including isobutylaluminoxane (IBAO), in situ AlR3·0.5H2O systems (R = ethyl, isobutyl) and Sn−Al bimetallic catalysts. Analysis of products by 1H NMR showed that methanol insoluble copolymer fractions had multiblock structures. The multiblock segment length and molecular weight of the copolymers were regulated by a variation in the reaction temperature, reaction time, reaction medium, and the catalyst structure. An increase in the reaction temperature was used to obtain shorter segment block lengths. Bulk reactions at elevated temperatures gave shorter block lengths than those of corresponding polymerizations conducted in solution (xylene). Differential scanning calorimetry (DSC) results showed two melting transitions corresponding to poly(ethylene oxide) (PEO) and [l]-polylactide ([l]-PLA) crystalline phases. The melting temperature and enthalpy of fusion for the phase-separated [l]-PLA crystalline phase was “tailored” by modulating the copolymer composition and the [l]-PLA block length. Blends with PLA were prepared by substituting poly(ethylene glycol) (PEG) with a high EO content [l]-PLA/EO multiblock copolymer. The idea explored was that the multiblock copolymers would be expected to leach into aqueous environments at a slower rate than PEGs. Substitution of the [l]-PLA/EO copolymer in place of PEG resulted in important increases in the film modulus and yield strength without loss in elongation at yield, break stress, and elongation at break. Thus, we demonstrated a versatile route to important new multiblock [l]-PLA/EO copolymers which have excellent potential to be useful for a wide range of biomedical applications including bioresorbable implant materials and tissue engineering. Furthermore, the synthetic methods developed herein provide routes which will be useful in “fine-tuning” product physicomechanical properties and degradation rates.

Synthesis and properties of multiblock copolymers consisting of poly(L-lactic acid) and poly(oxypropylene-co-oxyethylene) prepared by direct polycondensation

“…Chen et al observed that the multiblock copolymers of L-lactide and ethylene oxide exhibited two melting endotherms at different temperatures, depending on if the PLLA segments had short or long block lengths. 21 In this study, the block lengths of PLLA were not determined, but the observation of two similar endotherms for the crystal fusion of PLLA may be attributed to the distribution of the segment length of PLLA.…”

“…This obstacle may be remedied by using the multiblock copolymerization technique. In spite of the difficulties in controlling the molecular weight and the composition of the produced polymers, Chen et al prepared PLLA-PEG multiblock copolymers by the copolymerization of L-lactide and ethylene oxide while Penco et al prepared a similar copolymers by the chain extension method, which involves a reaction between oligo(lactic acid) and PEG bis(chloroformate) 21,39 In this study, a much simpler method of copolycondensation was developed to synthesize multiblock copolymers with a high molecularweight. In this method, oligo(L-lactic acid) which was prepared by the thermal dehydration of L-lactic acid was subjected to copolymerization with PN in the presence of a dicarboxylic acid by which the hydroxyl/carboxyl ratio was adjusted to unity.…”

Synthesis and characterization of poly(ethylene glycol) grafted poly(L-lactide)

“…H NMR spectrum of PLLA-g-PEG 5 and the resulting peak assignment. The peak assignments were performed with homo and hetero COSY (not shown) and DEPT (not shown), and the results of previous studies on random and diblock PLLA and PEG copolymers 10,13) . It was shown that the peak appearing at 4.35 ppm corresponds to the methine proton at the ester-ether linkage.…”

“…One of the promising approaches to overcome these problems is to introduce hydrolyzable segments or branched units into PLLA 10,11) . Since poly(ethylene glycol) (PEG) is water-soluble and shows favorable biocompatibility, many researchers focused on the preparation of PLLA combined with PEG which enhances the hydrophilicity 12,13) . They showed that versatile properties could be obtained from block copolymers comprising polylactide and PEG, and proposed the possibility of its use as a tailor-made polymer for biomedical application.…”

Synthesis and Characterization of PLA-Based Aliphatic-Aromatic Copolyesters: Effect of Diols

“…The incorporation of flexible water soluble polyethers, such as poly(ethylene glycol) and poly(propylene glycol) or their respective monomers, to form random and block copolymers, enables variation of hydrophilicity, biodegradation rate, and of course the final mechanical properties (Chen, 1997;Stevels, 1996). In addition, poly(orthoester) and poly(anhydride) have been reported as bioerodible materials, and thus the PLA copolymers with corresponding monomers have shown surface erosion behavior (Zhu, 1997;Schwach-Abdellaoui, 1999).…”