New insights into poly(lactic-co-glycolic acid) microstructure: using repeating sequence copolymers to decipher complex NMR and thermal behavior

Abstract

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 and LLG (L = lactic, G = glycolic) with controlled and varied tacticities were synthesized by assembly of sequence-specific, stereopure dimeric, trimeric, and hexameric segmer units. Specifically labeled deuterated lactic and glycolic acid segmers were likewise prepared and polymerized. Molecular weights for the copolymers were in the range M(n) = 12-40 kDa by size exclusion chromatography in THF. Although the effects of sequence-influenced solution conformation were visible in all resonances of the (1)H and (13)C NMR spectra, the diastereotopic methylene resonances in the (1)H NMR (CDCl(3)) 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.

Figure 1

Comparison of complex sequences prepared by segmer assembly with simpler PLA and PLGA microstructures prepared by ring-opening polymerization.^-,

Figure 2

(Top) Full spectrum of poly LG; (Bottom) expansions of selected regions for poly LG, L_racG and GLGL_R. ¹H NMR spectra at 600 MHz in CDCl₃.

Figure 3

Example of tetrad stereosequence encoding for poly LG (all isotactic) and GLGL_R (all syndiotactic).

Figure 4

(Top) Full spectrum of poly GLG; (Bottom) expansions of selected regions for poly GLG, GL_racG and GLG_d2. ¹H NMR spectra at 600 MHz in CDCl₃.

Figure 5

2D HMBC ¹H-¹³C correlation NMR spectrum for poly GLG in CDCl₃ (700 MHz, ¹H; 175 MHz ¹³C). The detailed cross peaks correspond to 3-bond correlations between the L-carbonyl with the G^C methylenes and the G^O carbonyl with the L-methine.

Figure 6

Example of octad-level stereosequence encoding for poly L_RL_SGL_SL_SG. The central tetrads for each type of unit are defined as including one distant and two adjacent relationships.

Figure 7

¹H NMR spectra of L^O-variable LLG polymers at 700 MHz in CDCl₃. Comparisons of the expansions of selected regions for poly LLG, L_racLG, L_d,racLG, L_RLGLLG, and L_RLG.

Figure 8

¹H NMR spectra of L^C-variable LLG polymers at 700 MHz in CDCl₃. Comparisons of the expansions of selected regions for poly LLG, LL_racG, LL_d,racG, LLGLL_RG, and LL_RG.

Figure 9

¹H NMR expansion for the upfield diastereotopic proton of the glycolic methylenes of poly L_racLG (full spectrum in Figure 7). The level of sensitivity for sequence ranges from tetrad (sii, iii) to hexad (sss) to octad (iss) depending on the core tetrad sequence.

Figure 10

2D HMBC ¹H-¹³C correlation NMR spectra at 700 MHz in CDCl₃ for poly LLG (A), L_RLGLLG (B), L_RLG (C), L_racL_racG (D) and L_racLG (E).

Figure 11

¹³C NMR spectra of L^O-variable LLG polymers at 175 MHz in CDCl₃. Comparisons of the expansions of selected regions for poly LLG, L_racLG, L_d,racLG, L_RLGLLG, and L_RLG.

Figure 12

¹³C NMR spectra of L^C-variable LLG polymers at 175 MHz in CDCl₃. Comparisons of the expansions of selected regions for poly LLG, LL_racG, LL_d,racG, LLGLL_RG, and LL_RG.

Figure 13

Glycolic methylene region of a mixed ¹H NMR spectrum for mixed sample (1:1:1) of poly LG, GLG and LLG at 600 MHz in CDCl₃.

Figure 14

Glycolic methylene region of poly L_racL_racG at 700 MHz in CDCl₃.

Figure 15

MALDI TOF patterns for poly LG (top) and poly GLG (bottom).

Figure 16

Simulated NMR spectra highlighting the challenges inherent in comparing a standard with perfect stereosequence control to a sample with multiple resolved sequences. Although all sequences share the same “ ” central tetrad and exhibit similar chemical shifts, the shifts of the nearly hexad-resolved sequences create a pattern that does not show an easily interpretable correspondence with the standard.

Scheme 1

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