Macrocycles from the Photochemical Coupling of Preassociated Terminal Blocks of Block Copolymers

Abstract

Cyclization of long polymer chains is difficult because polymer chain ends have a low probability to contact one another and intramolecular cyclization has to compete with interchain coupling. To minimize intermolecular coupling, macrocycles are prepared traditionally under high dilution conditions, which limit the amount of polymer obtainable per volume of solvent. Reported in this paper is a new methodology for synthesizing polymer macrocycles. Instead of using polymers with one pair of reactive groups at the ends of a polymer chain, we use a block copolymer, poly[(2-cinnamoyloxyethyl methacrylate)-ran-(2-trifluoroacetoxyethyl methacrylate)]-block-poly(solketal methacrylate)-block-poly(tert-butyl acrylate)-block-poly(solketal methacrylate)-block-poly[(2-cinnamoyloxyethyl methacrylate)-ran-(2-trifluoroacetoxyethyl methacrylate)] or P(CEMA-r-TFAEMA)-b-PSMA-b-PtBA-b-PSMA-b-P(CEMA-r-TFAEMA), with reactive P(CEMA-r-TFAEMA) end blocks to increase the efficiency of end coupling. In our method, a micellar solution is first prepared in a solvent selectively poor for the end CEMA units. This micellar solution is then slowly pumped into a solvent reservoir or reactor under constant stirring and irradiation. In the reactor, where the polymer concentration remains low throughout the preparation for its conversion into macrocycles, the micelles dissociate quickly into end-associated rings or unimolecular micelles, and the rings then get covalently linked photochemically. Since the TFAEMA units in the end blocks are soluble in the solvent used and probably segregate preferentially on the surface of the “balls” formed from the aggregation of the end CEMA units, they help deter the chemical coupling of different macrocycles. Using this methodology, we can prepare large macrocycles in high purity and at high concentrations.