Sequence-Controlled Copolymers Prepared via Entropy-Driven Ring-Opening Metathesis Polymerization

Abstract

A new general synthetic approach to sequenced macromolecules was developed and applied to the synthesis of polymers comprising lactic acid (<b>L</b>), glycolic acid (<b>G</b>), and ε-caprolactone (<b>C</b>)-derived monomer units. The new method employs entropy-driven ring-opening metathesis polymerization (ED-ROMP) to prepare copolymers with embedded sequences and controlled molecular weights. Cyclic macromonomer precursors were prepared by ring-closing metathesis of ethylene glycol (<b>Eg</b>)-linked sequenced oligomers bearing terminal olefins. ED-ROMP of the resulting macrocycles using Grubbs' second generation catalyst yielded <b>poly(CL-Eg-LC-Oed)</b>, <b>poly(CLL-Eg-LLC-Oed)</b>, <b>poly(LGL-Eg-LGL-Oed)</b>, and <b>poly(LGL-Eg-LGL-Hed)</b> (<b>Oed</b> = octenedioc acid; <b>Hed</b> = hexenedioc acid). Hydrogenation produced the saturated sequenced copolymers. Molecular weight was well-controlled and could be adjusted by varying the monomer-to-catalyst ratio. <i>M</i>_ns of 26-60 kDa were obtained (dispersities = 1.1-1.3). The methodology proved general for three different sequences and two olefinic metathesis groups.