Sequence Engineering: Fine Tuning Polymer Properties at the Microstructural Level

Sequence, which Nature uses to spectacular advantage, has not been fully exploited in synthetic copolymers. To investigate the effect of sequence and stereosequence on the physical properties of copolymers a family of complex isotactic, syndiotactic and atactic repeating sequence poly(lactic-co-glycolic acid) copolymers (RSC PLGAs) were prepared and their NMR and thermal behavior was studied. The unique suitability of polymers prepared from the bioassimilable lactic and glycolic acid monomers for biomedical applications makes them ideal candidates for this type of sequence engineering. Polymers with repeating units of LG, GLG, LLG, LLLG and GLLG (L = lactic, G = glycolic) with controlled and varied tacticities were synthesized by assembly of the corresponding sequence specific, stereopure dimeric, trimeric, tetrameric and hexameric segmer units. Specifically labeled deuterated lactic and glycolic acid segmers were likewise prepared and polymerized. Although the effects of sequence-influenced solution conformation were visible in all resonances of the 1H and 13C NMR spectra, the diastereotopic methylene resonances in the 1H NMR (CDCl3) for the glycolic units of the copolymers proved most sensitive. An octad level of resolution, which corresponds to an astounding 31-atom distance between the most separated stereocenters, was observed in some mixed sequence polymers. Importantly, the level of sensitivity of a particular NMR resonance to small differences in sequence was found to depend on the sequence itself. Thermal properties were also correlated with sequence.\ud Functionalized RSC PLGAs were also prepared by the introduction of a benzyl-ether substituted monomer, (S)-3-benzyloxy-2-hydroxypropionic acid, derived from serine. A series of dimeric and trimeric based copolymers were assembled with controlled and varied tacticities as well as a sequenced heptamer and decamer copolymer. Deprotection of the hydroxyl groups was accomplished by catalytic hydrogenolysis to yield highly functionalized, hydrophobic polyesters. The NMR spectra for all of the copolymers were consistent with sequence and stereosequence retention. Progress towards the development and incorporation of another functional monomer derived from malic acid was also investigated. \u