Tailoring of block copolymers based on the stoichiometric control of the end-functionality of telechelic oligomers and the utilization of large-scale fractionation by phase fluctuation chromatography: a synthetic strategy for the preparation of end-functionalized poly(L-lactide)-block- poly(oxyethylene)

An AB diblock copolymer of poly(L-lactide) (PLLA) and poly(oxyethylene) (PEG) with a cinnamate terminal in the PEG block was prepared by the copolymerization of L-lactide and partially end-modified PEG followed by fractionation. The first step was the terminal modification of PEG with cinnamoyl chloride (CC), in which the degree of cinnamoylation of the hydroxyl terminals of PEG was roughly controlled by the feed ratio of both reactants. The resultant PEG cinnamate was subjected to copolymerization with L-lactide to produce a mixture of unreacted PEG dicinnamate (C-PEG-C), the diblock copolymer (PLLA-PEG-C), and the triblock copolymer (PLLA-PEG-PLLA) corresponding to the three components of the PEG cinnamate. This mixture was separated by phase fluctuation chromatography (PFC) to obtain PLLA-PEG-C in sufficient purity. This process, involving the stoichiometric control of the terminal reaction of telechelic oligomers and the utilization of PFC for fractionation, can be an efficient method for synthesizing end-functionalized diblock copolymers from readily available telechelic oligomers

Phase fluctuation chromatography of diblock copolymer of poly(ethylene glycol) and poly(L-lactide) for fractionation by the block length ratio

Phase fluctuation chromatography was used to separate a diblock copolymer PEG-PLLA by its block length ratio on a preparative scale, where PEG is poly(ethylene glycol) and PLLA is poly-(L-lactide). A concentrated solution of the copolymer was injected into a column packed with surface-modified porous silica particles. Carboxymethyl-modified porous silica, in cooperation with size exclusion, separated the copolymer in a decreasing order of lactate content. To reverse the elution order, we grafted the surface of porous silica with PLLA chains by polymerizing L-lactide with surface silanol as initiator. A column packed with porous silica with long PLLA chains separated the copolymer in the opposite order against the size exclusion effect. Both separations resulted in fractionation primarily with respect to the length of PLLA block, indicating a broader length distribution of the PLLA block compared with the PEG block. The mass distribution of the separated fractions suggests a long tail at the high end of the PLLA block length. There was no evidence of correlation between the two block lengths

Composition analysis of poly(ethylene glycol)-poly(L-lactide) diblock copolymer studied by two-dimensional column chromatography

We used two-dimensional column chromatography to analyze the composition of a sample of presumably a diblock copolymer of poly(ethylene glycol) (PEG) and poly(L-lactide) synthesized from monomethoxy-terminated PEG. The first dimension of the separation is phase fluctuation chromatography to prepare fractions that contain various components of the copolymer in different ratios. The second dimension is size-exclusion chromatography, NMR, and HPLC at the critical condition of PEG. The PEG initiator has small amounts of diol-terminated dimeric components. We found that the copolymer sample contains a triblock copolymer and low-molecular-mass components in addition to the main part of the diblock copolymer. The SEC chromatograms show that the main part consists of two components with distinct peak lengths for the PLLA block. The low-molecular-mass components have a broad distribution in chemical composition. Phase fluctuation chromatography enriched the triblock copolymer and the diblock copolymer with the longer PLLA block in early fractions when the column was packed with carboxymethyl-modified porous silica. When the porous medium was PLLA-grafted silica, size exclusion dominated, but the low-molecular-mass components were separated according to their chemical composition. © 2002 Elsevier Science B.V. All rights reserved

Structure of a hydrogel generated by stereo-complexation of PLLA/PDLA : Effect of combination of block copolymers with different types

The AB diblock copolymers, Me-PEG-PLLA and Me-PEG-PDLA, were prepared by the ring-opening copolymerization of L-lactide and D-lactide, respectively, in the presence of Me-PEG. Since each of the resultant AB diblock copolymers contained a considerable amount of triblock copolymer, PLLA-PEG-PLLA or PDLA-PEG-PDLA, high osmotic pressure chromatography was conducted to separate off the triblock copolymers. The purified diblock copolymers were then examined for hydrogel formation

Hydrogel formation between enantiomeric B-A-B-type block copolymers of polylactides (PLLA or PDLA: A) and polyoxyethylene (PEG: B); PEG-PLLA-PEG and PEG-PDLA-PEG

A mixed suspension of the enantiomeric B-A-B triblock copolymers, polyoxyethylene-block-poly(L-lactide)-block-polyoxyethylene (PEG-PLLA-PEG) and poly-oxyethylene-block-poly(D-lactide)-block-polyoxyethylene (PEG-PDLA-PEG), was found to induce reversible gel-to-sol transition depending on the polymer concentration and temperature. The storage and loss moduli of the gel formed at lower temperature were much higher than those of the gel prepared from the corresponding ABA-type triblock copolymers because of the higher polymer concentration in the former. Although the stereo-complexation of the PLLA and PDLA blocks occurred at higher temperature also in the B-A-B copolymers, it was not responsible for the gelation of the mixed suspension. The PEG chains, involved in the helix formation of the PLLA and PDLA, should form helices with opposite helical senses to aggregate and lead the gelation of the system