Tripartite hydrogen-bonding as a driving force for high-concentration cyclization of poly(l-lactide)

Area of Science:

• Polymer Chemistry
• Macromolecular Science
• Organic Synthesis

Background:

• High molecular weight poly(L-lactic acid) (PLLA) cyclization is challenging.
• Existing methods often lack efficiency or practicality for high molecular weights.
• Cyclic polymers offer unique properties compared to their linear counterparts.

Purpose of the Study:

• To develop an efficient three-step strategy for the cyclization of high molecular weight PLLA.
• To investigate the mechanism involving a persistent tripartite complex.
• To synthesize and characterize cyclic PLLA (cPLLA) under practical conditions.

Main Methods:

• A three-step synthetic strategy was employed.
• Utilized a persistent tripartite complex stabilized by hydrogen bonding and ion pairing.
• Selective intramolecular transesterification at a benzylic ester initiated ring closure.
• Scanning probe microscopy (SPM) was used for nanostructure characterization.

Main Results:

• Achieved efficient cyclization of high molecular weight PLLA at a 0.5 M concentration.
• Successfully synthesized cyclic PLLA with a molecular weight (Mn) of approximately 26,000 g mol⁻¹.
• SPM imaging confirmed the formation of nanorings with diameters matching the calculated contour lengths of the cyclic PLLA chains.

Conclusions:

• The developed three-step strategy provides a synthetically practical and efficient method for producing high molecular weight cyclic PLLA.
• The persistent tripartite complex is crucial for driving the cyclization reaction.
• The formation of well-defined nanorings demonstrates successful macrocycle formation.